Development and Investigation of Bio-based Environmentally ...
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Development and Investigation of Bio-based Environmentally
Friendly Fire Retardant PLA Composites
Von der Fakultät für Maschinenbau der
Technischen Universität Chemnitz
Genehmigte
Dissertation
zur Erlangung des akademischen Grades
Doktor-Ingenieur
(Dr.-Ing.)
Vorgelegt
Von Dipl.-Ing. Pengcheng Zhao
aus Liaoning, VR China
Tag der Einreichung: 11.01.2019
Tag der Verteidigung:18.04.2019
1. Gutachter: Prof. Dr.-Ing. Michael Gehde
2. Gutachter: Prof. Dr.-Ing. Udo Wagenknecht
Bibliografische Beschreibung
Pengcheng Zhao
Thema
Development and investigation of bio-based environmentally friendly fire retardant PLA
composites
Dissertation an der Fakultät für Maschinenbau der Technischen Universität Chemnitz,
Professur Kunststoffe, Chemnitz, 2018
152 Seiten, 55 Abbildungen, 37 Tabellen, 221 Literaturzitate
Referat
In der vorliegenden Dissertationsarbeit wird auf die Thematik der Entwicklung von
Polymerwerkstoffen, basierend auf vollständig natürlichen Resourcen, eingegangen.
Die vorliegende Lösung beruht auf der Compoundierung von Polylactid mit
unterschiedlich modifizierten Vanillin. Ziel war es, flammschutzwirkende Komponenten
einzubringen und die Abhängigkeiten zwischen Zusammensetzung und Eigenschaften
aufzuklären. Dem liegt die Absicht zugrunde, optimale Werkstoffe zur Verfügung zu
stellen, die sich durch deutlich verbesserte flammhemmende und mechanische bzw.
thermo-mechanische Eigenschaften auszeichnen.
Die erzeugten modifizierten Vanillin-Derivate sowie deren Composite wurden
hinsichtlich der physikalischen und chemischen Struktur mittels REM, EDX, FTIR, NMR,
DSC, TGA, SEC und Zugversuch charakterisiert. Zur Bestimmung der flammwidrigen
Eigenschaften wurden UL-94 V, LOI und CCT durchgeführt. Es hat sich gezeigt, dass
System aus PLA und einem Vanillin-Phosphorsäure-Ester in Bezug auf werkstofflichen
Eigenschaften insgesamt die optimale Leistung aufwies. Die Materialen ergaben eine
verbesserte Zähigkeit und erheblich erhöht flammwidrige Eigenschaften. In einem
weiteren Schritt wurden MMT und APP, zwei kommerzielle Flammschutzmittel, mit dem
PLA/VP System kombiniert. Die daraus abgeleiteten Resultate bewiesen eine
synergistische Wirkung zwischen VP und MMT bzw. APP und führten zu besseren
Brandklassen bei LOI und UL-94 Brandtests.
Schlagworte
Polylactide, bio-basierend, umweltfreundlich, Vanillin, Flammschutzmittel, Polymer
Composite, Flammteste
Bibliografische Beschreibung
Pengcheng Zhao
Theme
Development and investigation of bio-based environmentally friendly fire retardant PLA
composites
Dissertation at the Faculty of Mechanical Engineering, Chemnitz University of
Technology, Professorship of Plastic Materials, Chemnitz, 2018
152 Pages, 55 Figures, 37 Tables, 221 Literatures
Abstract
The present work demonstrates the development of fully bio-based polymeric
composites. It was realized by the compounding of poly(lactic acid) and differently
modified vanillin. The aim of this work was to introduce flame retardant components into
PLA and to study the flame retardant mechanism. The intention of this approach is the
preparation of optimized PLA composites with significantly improved flame retardant,
mechanical as well as thermo-mechanical properties.
The modified vanillin and the PLA composites based on those vanillin derivatives were
characterized by means of SEM, EDX, FTIR, NMR, DSC, TGA, SEC and tensile test for
their physical and chemical structures. UL-94 V, LOI and CCT were carried out to
determine the corresponding flame retardant properties. The results showed that, the
PLA/VP system represented the best overall performance. The PLA/VP composite
exhibited increased toughness and significantly improved flame retardancy. In addition,
two commercialized flame retardants, MMT and APP, were introduced into the PLA/VP
system, respectively. It was suggested that there were synergic effects between VP and
MMT as well as APP. The combined used flame retardants resulted in an improved
classification in UL-94 and LOI tests
Keywords
Poly(lactic acid), bio-based, environmentally friendly, vanillin, flame retardant, polymer
composite, fire test
vi
Contents
1 Introduction .................................................................................................. 1
1.1 Fundamental of Biopolymer ................................................................ 1
1.2 Brief Overview of Polymer Combustion .............................................. 2
2 Synopsis of the Work ................................................................................... 4
3 Literature Review ......................................................................................... 6
3.1 Green Polymeric Material ................................................................... 6
3.2 Material Fundamentals ....................................................................... 7
3.2.1 Poly (Lactic acid) ....................................................................... 7
3.2.2 Vanillin ..................................................................................... 13
3.3 Combustion of Polymer .................................................................... 15
3.4 Flame Retardancy of Polymers ........................................................ 16
3.4.1 Basic Explanation of Polymer Flame Retardancy ................... 16
3.4.2 Classification of Flame Retardant ........................................... 18
3.4.3 Strategies of Developing Flame Retardant PLA ...................... 19
4 Experimental: Materials, Processing and Characterization Methods ......... 24
4.1 Material Description .......................................................................... 24
4.2 Synthesis section .............................................................................. 24
4.2.1 Synthesis of bis(5-formyl-2-methoxyphenyl) phenylphosphonate
24
4.2.2 Synthesis of bis(5-((E)-(allylimino)methyl)-2-methoxyphenyl)
phenylphosphonate............................................................................ 25
vii
4.2.3 Synthesis of bis(2-methoxy-5-((E)-((3-(triethoxysilyl)propyl)imino)
methyl)phenyl) phenylphosphonate .................................................... 26
4.3 Preparation of flame retardant PLA Composites ............................... 27
4.3.1 Melt Compounding .................................................................. 27
4.3.2 Extrusion.................................................................................. 28
4.3.3 Melt Spinning ........................................................................... 29
4.3.4 Injection Molding ...................................................................... 29
4.3.5 Compression Molding .............................................................. 30
4.4 Characterization Techniques ............................................................. 31
4.4.1 Structure Characterization ....................................................... 31
4.4.2 Gel Permeation Chromatography ............................................ 31
4.4.3 Morphological Characterization ............................................... 31
4.4.4 Rheological Characterization ................................................... 31
4.4.5 Thermal Properties Tests ......................................................... 32
4.4.6 Mechanical Properties Tests .................................................... 32
4.4.7 Flammability and Combustibility .............................................. 33
4.4.8 Mechanism Studies ................................................................. 36
5 Results and Discussion .............................................................................. 38
5.1 VP based PLA Composites ............................................................... 38
5.1.1 Structural Characterization ...................................................... 39
5.1.2 Morphology of VP and PLA/VP Composites ............................ 42
5.1.3 Thermal Decomposition Behavior ............................................ 43
viii
5.1.4 Thermal Crystallization Behavior ............................................. 49
5.1.5 Flammability ............................................................................ 52
5.1.6 Combustion Behavior .............................................................. 53
5.1.7 Rheological properties ............................................................ 57
5.1.8 Mechanical Properties ............................................................. 60
5.1.9 Conclusion .............................................................................. 66
5.2 VPA and VPS based PLA Composites ............................................. 67
5.2.1 Characterization of VPA and VPS ........................................... 68
5.2.2 Thermal Decomposition Behavior ........................................... 71
5.2.3 Flammability ............................................................................ 75
5.2.4 Combustion Behavior .............................................................. 76
5.2.5 Mechanical Properties ............................................................. 78
5.2.6 Conclusion .............................................................................. 79
5.3 PLA/VP Composite with Conventional Flame Retardants ................ 80
5.3.1 PLA/VP Composite with Montmorillonite ................................. 81
5.3.2 PLA/VP Composite with APP .................................................. 93
5.3.3 Conclusion ............................................................................ 106
6 Summary ................................................................................................. 107
6.1 Synthesis and characterization of VP ............................................. 108
6.2 Performance of PLA/VP composites .............................................. 108
6.3 Further modification of VP and VP used in combination with
commercialized flame retardant .............................................................. 109
ix
7 Outlook ...................................................................................................... 111
References ..................................................................................................... 112
List of Figures ................................................................................................. 133
List of Tables ................................................................................................... 137
Publications .................................................................................................... 139
x
Abbreviations
PLA Poly (lactic acid)
FR Flame retardant/ flame retardancy
MMT Montmorillonite
APP Ammonium polyphosphate
VP Bis(5-formyl-2-methoxyphenyl) phenylphosphonate
VPA Bis(5-((E)-(allylimino) methyl)-2-methoxyphenyl)
phenylphosphonate
VPS Bis(2-methoxy-5-((E)- ((3-(triethoxysilyl)propyl)
imino)methyl) phenyl) phenylphosphonate
PHA Polyhydroxyalkanoates
PCL Poly (ε-caprolactone)
PBS Polybutylene succinate
LA Lactic acid
PLLA Poly (L-lactic acid)
PDLA Poly (D-lactic acid)
ROP Ring opening polymerization
GA Glycolic acid
PHB Poly(β-hydroxybutyrate)
PBAT Poly(butylene adipate-co-terephthalate)
PEG Poly(ethylene glycol)
WPC Wood plastic composites
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LDH Layered double hydroxide
EG Expandable graphite
CNTs Carbon nanotubes
UV Ultraviolet
IFRs Intumescent flame retardants
POSS Polyhedral oligomeric silsesquioxane
mPLA Modified PLA
pHRR Peak of heat release
LOI Limiting oxygen index
DAB Diaminobenzidine
PMMA Poly (methyl methacrylate)
ATH Aluminum trihydrate
AHP Aluminum hypophosphite
HPAPC Hexa(phosphaphenanthrene
aminophenoxyl)-cyclotriphosphazene
HBPE Hyperbranched poly(phosphamide ester)
UL-94 The Standard for Safety of Flammability of Plastic
Materials for Parts in Devices and Appliances testing
PER Pentaerythritol
MEL Melamine
TPM Tris(2-hydroxyethyl) isocyanurate polyphosphate
VA Vanillin
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APTES 3-Aminopropyltriethoxysilane
TEA Trimethylamine
PPDC Phenylphosphonic dichloride
NMR Nuclear magnetic resonance
FTIR Fourier transform infrared spectroscopy
SEM Scanning electron microscopy
TGA Thermogravimetric analysis
DSC Differential scanning calorimetry
CCT Cone calorimetry Test
HRR Heat release rate
TTI Time to ignition
THR Total heat release
ML Mass loss
TSP Total smoke production
SPR Smoke production rate
CO Carbon oxide
CO2 Carbon dioxide
TG-FTIR Thermogravimetry- Fourier transform infrared
spectroscopy
DTG Differential thermal gravity
MFI Melt flow index
av. EHC Average effective heat of combustion
xiii
Symbols
Mw Molecular weight
Wt.-% Weight percent
Φ Diameter
Tx% the temperature at x% weight loss
Tmax the temperature at maximal weight loss rate
Ea Apparent activation energy
T Temperature
β Heating rate
R Ideal gas constant
E Activation energy
α Degree of conversion
r2 Goodness of fit
Tg Glass transition temperature
Tcc Cold crystallization temperature
Tm Melting Temperature
∆Hcc Cold crystallization enthalpy
∆Hm Melting enthalpy
Χc Weight fractional crystallinity
G’ Storage Modulus
G’’ Loss Modulus
xiv
Acknowledgment
The accomplishment of this thesis is the commencement of my research life
accompanied and supported by many kind-hearted people. I could not have
completed this thesis without the assistance from them. I extend my profound
gratitude to all of them.
First of all, I want to give the special thanks to Prof. Udo Wagenknecht, Prof.
De-Yi Wang and Dr. Ines Kühnert for providing me the opportunity at
Leibniz-Institute for Polymer Research Dresden and giving me necessary
support. I would like to express my sincere gratitude and indebtedness to my
supervisor Dr. Andreas Leuteritz for accepting me in his research group and also
for his support, encouragement, insightful comments and guidance throughout
this work.
I thankfully acknowledge Prof. Michael Gehde for giving the opportunity to join
his outstanding department in the faculty of mechanical engineering faculty of
Technical University Chemnitz.
My sincere thanks go to the colleague of IPF for their support and assistance in
the work. I also would like to thank Mrs. Sabine Krause, Dr. Hartmut Komber, Dr.
Mikhail Malanin, Mr. Holger Scheibner, Dr. Regine Boldt, Mrs. Maria Auf der
Landwehr and Mrs. Petra Treppe for their help in analysis work. I am truly
grateful to Mrs. Anna Ivanov, Mr. Christian Lehmann, Mr. Andreas Scholze and
Mr. Bernd Kretzschmar for the preparation of specimens.
Furthermore, I gratefully thank Mrs. Anne Hofmann for her assistance in the
administrative issues and Mrs. Anna Ivanov for the friendly office time. I would
like to express my sincere gratitude to Dr. Zhiqi Liu, Dr. Baobao Chang, Mr.
Yilong Li, Mr. Xueyi Wang, Dr. Jaime Alejandro Puentes-Parodi and Mr. Sajid
Naseem for those help and fruitful discussions.
For the financial support, I am truly thankful to the China Scholarship Council
xv
(Ph.D. Program Nr. 201308080080) and IPF scholarship.
Last but not the least, my sincere thanks would go to my family. In particular,
huge indebtedness to my parents for their unconditional support, and special
thanks to my beloved wife Yan Yan for her love and understanding. Their love,
encouragement and great confidence in me, help me go through the most
difficult time.
Introduction 1
1 Introduction
1.1 Fundamental of Biopolymer
The development of polymeric materials has a history of more than 150 years
since the first man-made polymer, which is based on nitrocellulose and known
as celluloid, was obtained in the 1860s by Parkes and Hyatt.[1] In the following
decades, numerous kinds of synthetic polymeric materials were invented and
began to replace conventional materials in various fields.
Figure 1-1: Plastic production since 1950[2]
Today, plastic materials are ubiquitous and one of the most versatile materials.
The applications of plastic are technically endless so that the consumption of
plastic is extremely huge. Usually, synthetic plastics or petrochemical plastics
consist of macromolecule with long carbon chain and are thus durable and high
resistant to moisture and chemical solvents, which are actually one of the best
properties of plastics. However, for short-term applications, this advantage is,
unfortunately, the biggest problem: the duration of degradation does not match
their expected useful life. Only a tiny part of the used products is recycled and
Introduction 2
reprocessed. The accumulation of plastic waste has become a serious
environmental and social issue.[3] According to statistic, the total amount of
plastic products manufactured since 1950 is more than 8000 million tons, and
60 % of those are discarded. Assuming the current developing rate and
regulations against plastic consumption is not changed, the number of total
amount of plastic waste will be increased to 25000 million tons by the year 2050,
and more than 12000 million tons will be discarded in the natural environment.[2]
On the other hand, the great demand for plastic products and the
overconsumption has caused more and more serious concerns regarding the
fossil resource depletion. Thus, biopolymer has attracted rapidly growing
attentions both in academy and industry. The application of biopolymers offers a
promising solution for the sustainable development of society.[4]
1.2 Brief Overview of Polymer Combustion
Most of the polymeric materials, both natural and synthetic, are highly
inflammable. The widespread use of polymeric materials leads to a dramatic
increase in fire risk in our day-to-day life. For example, 2 million accidental fires
are reported in Europe every year, which cause more than 4000 deaths and
70000 injuries as well as 120 billion € of economic losses. Thus, there are great
sociological and economic pressures on the whole society to produce plastic
products with certain flame retardancy.
Figure 1-2 presents the schematic diagram for the development of heat flux in a
burning room as a function of time. Additionally, the influence of proper flame
retardant (FR) is also illustrated.[5] The fire scenario can be basically divided
into three stages: ignition, development and fully developed fire. The ignition will
occur when the combustible materials exposed to open fire or high temperature
under the existence of oxygen. If the materials are not flame retardant, the
evolved heat will cause rapid development of the temperature in the room, and
Introduction 3
reach the so-called “flash over”. Then, the fire will be fully developed and the
temperature in the room might reach over 1000 °C. The fire may spread to the
whole building and cause structural damage. In comparison, for flame retardant
materials, the ignition and flash over could be significantly delayed and the
average escape time will be longer.
Figure 1-2: Schematic illustration of the heat flux vs time in a fire[5]
Synopsis of the Work 4
2 Synopsis of the Work
The ultimate goal of this thesis is to develop new environmentally friendly flame
retardant composites and study the impact of flame retardant on the properties
of PLA as well as the mechanism behind. In this work PLA was used as the
polymeric matrix and vanillin was selected as the raw material to synthesize
flame retardant. In particular, three different vanillin derivatives with functional
groups were synthesized and used to improve the flame retardancy of PLA.
Furthermore, two commercialized flame retardants, MMT (montmorillonite) and
APP (ammonium polyphosphate), were introduced into the flame retardant PLA
composites respectively to study the synergic effect between the vanillin
derivative and MMT/APP. Meanwhile, the thermal properties, micromorphologies,
combustion behavior and mechanical properties of these PLA composites were
investigated in details.
Chapter 3 is a brief review of the state of the art. The importance of developing
green polymer is first presented, followed by an overview of the PLA composites
reinforced by different biomass. The second part of the review is the basis of
polymer combustion and flame retardant fundamental as well as the state of art
of flame retardant PLA.
In Chapter 4, the raw materials and processing methods are described in detail.
The synthesis pathway and characterization techniques used in this thesis were
introduced.
In Chapter 5, all experimental results are illustrated and discussed, including the
synthesis and structure characterization of the vanillin based flame retardants:
bis(5-formyl-2-methoxyphenyl) phenylphosphonate (VP), bis(5-((E)-(allylimino)
methyl)-2-methoxyphenyl) phenylphosphonate (VPA) and bis(2-methoxy-5-((E)-
((3-(triethoxysilyl)propyl)imino)methyl) phenyl) phenylphosphonate (VPS). All the
Synopsis of the Work 5
flame retardants are used in PLA matrix to enhance the flame retardancy. In
addition, VP is used in combination with MMT and APP. The properties of all the
PLA composites are characterized and the relation of composition and
properties relation is studied.
The final Chapter 6 provides an overall conclusion and a brief outlook for
possible further work.
Literature Review 6
3 Literature Review
3.1 Green Polymeric Material
The history of use of polymeric materials can be traced to centuries ago when
mankind started to use natural polymer like cellulose and gum.[1] Since last
century the primary raw materials of polymeric materials have converted to fossil
resources when people learned the way to produce synthetic polymers and
attracted by the multifarious and excellent properties of synthetic plastics.
However, the massive manufacture and overconsumption of plastic production
are causing serious environmental issues and petroleum resource depletion is a
further challenge. With awareness of the need for sustainable development,
recent years have seen more and more researches focus on biopolymers.[6, 7]
Figure 3-1: Type of polymeric materials and examples[3]
Biopolymer denotes polymeric materials either bio-based or biodegradable. That
Literature Review 7
means, biopolymers should be able to be produced from renewable feedstocks
such as agro-resources or undergo complete decomposition into carbon dioxide
and water under various natural conditions. They are environmentally friendly
either at “the cradle”, at “the grave” or both.[3] Figure 3-1 illustrates the
classification of polymeric materials. The common biopolymer family includes
starch, chitosan, cellulose, polyhydroxyalkanoates (PHA), poly (lactic acid)
(PLA), poly (ε-caprolactone) (PCL), polybutylene succinate (PBS), etc.[8] Among
them, PLA is one of those polymers, which can also fulfill both criteria and is
considered as a promising replacement of conventional petroleum-based
plastics.[9, 10]
Besides biopolymer, biocomposites have attracted increasing attention.[11]
Many efforts have been done to develop biocomposites with improvement in
desired properties. For examples, many natural fibers and particles are used in
polymeric materials as a reinforcement, such as jute, flax, hemp, ramie, sisal,
kenaf, rice and wood powder.[12, 13] While some biomass can be used as a
modifier or functional additives in plastic, such as alginate, catechin, cellulose,
chitosan, cyclodextrin, lignin, phytic acid and starch.[6]
3.2 Material Fundamentals
3.2.1 Poly (Lactic acid)
3.2.1.1 Synthesis and Manufacture
PLA is a linear aliphatic polyester. Lactic acid (LA), also known as milk acid, is
the basic building block for PLA.[14] LA can be produced from biomass like corn,
sugarcane, sugar beet and cassava by fermentation. There are four unique
groups including hydroxyl and carboxyl groups in the structure of LA, therefore
LA can exist in different optically active enantiomers, LLA and DLA.[15] PLA can
be produced in homopolymers of poly(L-lactic acid) (PLLA) and poly(D-lactic
Literature Review 8
acid) (PDLA) or copolymer of poly(D,L-lactic acid) (PDLLA), depending on the
use of different enantiomers.[16] The synthesis procedure is illustrated in Figure
3-2. The ratio of D- and L-enantiomers can dramatically influence the properties
of PLA, such as glass transition temperature, crystallinity and melting point. The
most common synthetic techniques to produce PLA are polycondensation
(including solution polycondensation and melt polycondensation) and ring
opening polymerization (ROP).[14]
Figure 3-2: Synthesis of PLA[17]
Since the structure of LA contains both hydroxyl and carboxyl, the
polymerization can occur by self-condensation. The reaction can take place
whether under the presence of a solvent or by heating the monomer over critical
temperature. Some researchers have obtained PLA with a molecular weight (Mw)
between 20000 and 30000 by using polycondensation, some even produced
PLA with Mw as high as 600000. However, it is very difficult to use these methods
in industrial scale, while the equilibrium reactions are strongly affected by
various parameters. For example, the water generated by the reaction could
lead to degradation of long PLA molecular chains. Impurities caused by solvent
and side reactions are also unavoidable disadvantages. Besides, the solution
Literature Review 9
polycondensation technique consumes a large number of organic solvents,
which is also considered as potential pollutants to the environment.[18]
In contrast to polycondensation, ROP is considered to be an important and
effective technique to PLA with high molecular weight. By controlling reaction
conditions and application of catalysts, it is possible to control the ratio and
sequence of D- and LLA and also the molecular weight of the final product. The
ROP technique also has some drawbacks: it requires strict purity of the lactide
monomer; the use of metal-based catalyst limits the application in special fields
like medicine and food packaging. Hence, many researchers are making effort to
develop a new approach to solve these problems.[19] The advantages and
disadvantages of different PLA synthesis techniques are summarized in Table
3-1.
Table 3-1: Comparison of different PLA synthesis techniques[18]
Synthesis techniques Advantages Disadvantages
Solution
polycondensation
Easy to control, economical Low molecular weight,
impurities, pollution
Melt polycondensation Low molecular weight,
high temperature
Ring-opening
polymerization
High molecular weight High requirement on raw
material, high cost
New approaches High molecular weight,
efficient, no pollution, etc.
Under development
3.2.1.2 Modification and Application
PLA possess some promising features such as excellent mechanical and optical
Literature Review 10
properties. PLA is widely used in medical components, packaging materials,
automotive and electrical components as well as house holdings.[20, 21]
However, its broadened application field has been limited by the shortcomings,
such as brittleness and inflammability.[22] Many efforts have been done in order
to reinforce the mechanical properties of PLA without destroying the
biodegradability. In order to achieve these, numerous methods of modification
have been developed, involving copolymerization, physical blending and
composition. For example, the introduction of functional monomers into the
backbone of PLA or incorporation of functional additives like a nucleating agent,
plasticizer and flame retardant.
Chemical Modification
Some inherent properties such as brittleness, low thermal stability and
flammability pose considerable scientific challenges and restrict the application
field of PLA. The incorporation of strange monomers into the backbone of PLA is
one of the most common chemical modification strategies to overcome the
limitations. Extensive research has been done to synthesize PLA copolymers via
different techniques, such as polycondensation, cationic, anionic and
coordination insertion ring opening polymerization. Monomers with specific
properties are employed to enhance the versatility of PLA. For instance,
copolymer of LA and GA (glycolic acid) is widely used in bio-medicine as the
competitive suture; PLA-co-PCL copolymer shows adjustable plasticity
depending on the LA/CL ratio. Beside of copolymerization with monomers, other
techniques like block copolymer, branched copolymer and dendritic copolymer
of PLA were also successfully synthesized in the last decades.[14]
Blending
Physical blending with other polymers is another feasibility to render PLA
different properties.[23-25] Furthermore, it can also reduce the cost of the final
Literature Review 11
product, since the price of PLA is relatively higher than most petroleum plastics.
Considering the sustainability, PLA is usually blended with renewable and/or
biodegradable components, including PCL, poly(β-hydroxybutyrate) (PHB),
poly(butylene adipate-co-terephthalate) (PBAT), poly(ethylene glycol) (PEG),
chitosan and starch. [8, 26-30]
Composition
The daily increasing demand for polymeric materials inspirits the development of
high performance polymer, especially in the form of polymeric composites.
Despite PLA processes excellent mechanical properties, thermal plasticity and
biocompatibility, some improvements such as toughness are still necessary for
end-use applications.[17, 31, 32]
Using filler to reinforce polymeric material is very common in the production and
processing stages.[33-35] In the beginning, the reinforcement of PLA was mainly
focused on fibers, especially natural fibers due to the lower cost and weight
specific properties.[12, 36, 37] Natural fibers can be obtained from either plants
or animals. Wood plastic composites (WPC) are typical natural fiber based
polymer composites and attract growing interests.[38] Nonwood fibers can be
produced from different parts of plants, such as leaf, bast, seed, straw or fruit.
The primary advantage of leaf fiber is, they usually have excellent impact
properties. Examples of leaf fibers include sisal, henequen, palm, banana leaf
and pineapple leaf.[39-41] The main disadvantage of this kind of fibers is their
hydrophilic nature which causes poor interaction to hydrophobic polymeric
materials. The most common solution is to use a surface modifier like silane
coupling agent to enhance the interfacial adhesion.[42] Compared to leaf fibers,
bast fibers such as ramie, flax, kenaf, hemp, jute and abaca show better
adhesion to polymeric materials, therefore are used to enhance the mechanical
properties.[42-47] Leaf and bast fibers usually have long dimension thereby
Literature Review 12
polymeric composites based on such types of fibers possess good reinforced
performance and used to prepare structural applications. Fibers derived from
fruit and seed such as cotton, coconut and coir are usually used in the
non-structural application.[48, 49] Other examples for natural resources of fibers
include bamboo, red algae, silk and so on. Table 3-2 provides the comparison of
some fiber reinforced PLA composites.
Table 3-2: Comparison of different fibers reinforced PLA composites
Types Fiber Description Reference
Wood Wood flour Storage and flexural moduli increase with
filler volume fraction
[50, 51]
Leaf Sisal Surface treatment is necessary to improve
the interfacial adhesion
[41, 52]
Bast Ramie Act as reinforcing filler [42, 47]
Flax Increase stiffness and flexural strength [53, 54]
Kenaf Improve tensile and storage moduli [43-45]
Seed/
Fruit
Coconut Enhanced tensile strength and modulus [48]
Cotton Improve tensile and impact strength [49]
Straw Rice straw Enhance tensile strength and crystallinity [55]
Grass Bamboo Improve flexural and thermal properties [56]
Artificial Cellulose Improve storage modulus, poor adhesion
between untreated cellulose and matrix
[13, 57]
With the development of nanotechnology, nanoparticles such as montmorillonite
Literature Review 13
(MMT),[58-60] layered double hydroxide (LDH),[61] expandable graphite (EG),
cellulose whisker[62, 63] and carbon nanotubes (CNTs) have gained
tremendous attention. The nanocomposites show excellent properties because
the nanoparticles have a small size, huge specific surface area and many
modification feasibilities. For instance, the incorporation of MMT into PLA leads
to enhanced mechanical properties, degree of crystallinity and thermal
stability.[64-66] Depending on the different modifier, LDH can provide UV
absorption, corrosion resistance, degradability enhancement and flame
retardancy.[67-73] Extensive researches focused on PLA/CNTs composites
have been carried out. The results show that, like many other PLA based
composites, PLA/CNTs composites possess different improved performances in
mechanical properties, thermal properties, etc. In addition, one of the unique
advantages is, the introduction of CNTs into PLA can provide electrical
conductivity to the matrix.[74-76]
3.2.2 Vanillin
Vanillin (4-hydroxy-3methoxybenzaldehyde), is a major component of the extract
of vanilla beans and responsible for its flavoring properties. Therefore, vanillin is
one of the most commonly used industrial flavoring agent in foods, beverages,
perfumes, cosmetics and pharmaceuticals.[77-79] In a scientific aspect, vanillin
has been found to possess some important pharmacological properties such as
antibacterial and anti-inflammatory activities.[80-85]
The natural vanilla extract is a mixture of vanillin and hundred different
compounds. Besides, growing and harvesting of vanilla plants are high-cost
processes. Thus, vanillin obtained from natural extract contributes less than 1%
of the total vanillin production. Around 85% of the worldwide synthesized vanillin
is produced from petroleum-based intermediates.[86] However, only the vanillin
produced from natural materials can be recognized as “natural vanilla aroma”.
Literature Review 14
The global annual demand of vanillin as a flavoring agent is estimated to be
around 20000 tons. The increasing market has inspirited the development of the
techniques to produce artificial vanillin.[87-89] Recently, with the development of
technology, the problems of low yield and purity have been solved, promoting
the lignin-to-vanillin process again one of the most promising sustainable
methods.[90, 91] The proposed mechanism is described in Figure 3-3.
Figure 3-3: Proposed mechanism of the lignin-to-vanillin process[86]
Lignin is the second abundant renewable resource with around 50 million tons of
annual production, suggesting vanillin produced from lignin a feasible
sustainable technique. In fact, vanillin is currently the only aromatic compound
produced from lignin at industrial scale. For these reasons, extensive researches
have been tried to use vanillin as a renewable resource in material development.
Since vanillin has both aldehyde and hydroxyl groups in its structure, most of the
works are focused on the possibility to use vanillin as a building block for
different resins.[92-100] Others have tried to use vanillin directly in polymeric
matrix as functional additives due to its antioxidant or antimicrobial
properties.[101-105]
Literature Review 15
3.3 Combustion of Polymer
With numerous outstanding advantages that polymeric materials provided, there
is still an inescapable and vital drawback for nearly all polymers related to their
high flammability. The combustion of the polymer is a very complicated process
involving several stages: ignition, fire development and extinction. When
polymer exposes to sufficient heat, pyrolysis and degradation will occur. The
typical thermal decomposition process consists of several steps which could be
illustrated as following:[106]
Chain initiation RH −−−−> R· + H·
Chain propagation R· + O2 −−−−> ROO·
RH + ROO· −−−−> ROOH + HO·
Chain branching ROOH −−−−> RO· + HO·
2ROOH −−−−> ROO· +RO· +H2O
Chain termination 2R· −−−−> R-R
R· +HO· −−−−> ROH
2RO· −−−−> ROOR
2ROO· −−−−> ROOR + O2
As the result of the chain reaction, the polymer turns into combustible products
and volatile. These products will be ignited by the surrounding heat and release
more heat as feedback. Then a self-sustainable combustion is established,
promoting the thermal degradation of the polymer and increasing the release of
combustible products. The fire will spread to the whole surface of the polymer
and consume all the material with high velocity. The combustion behavior of
polymer is illustrated in Figure 3-4.
Literature Review 16
Figure 3-4: Mode of action of polymer in fire[107]
3.4 Flame Retardancy of Polymers
The three basic elements of fire are heat, fuel and oxygen. In order to stop a fire,
it is necessary to remove at least one of the three conditions from the fire. Since
the middle of last century, some basic idea of flame retardant has been proposed,
such as using additives to interrupt the thermal degradation of the polymer in
order to decrease the release of combustible products or applying an external
fire resistant coating to isolate polymer from oxygen. Today the modes of action
of flame retardants during the combustion of fire are considered mainly as gas
phase and condensed phase as well as the synergism.
3.4.1 Basic Explanation of Polymer Flame Retardancy
3.4.1.1 Gas Phase
In gas phase mechanism, there is interaction between flame retardant and
polymer, which interferes the chemical reaction in polymer combustion. There
are plenty of chemical reactions in polymer combustion. However, the most
important reactions that release free radicals, which propagate the chain
reaction, are followed:[107]
Literature Review 17
H• + O2 = OH• + O•
O• + H2 = OH• + H•
The main exothermal reaction which releases the most energy sustaining the
combustion is:
OH• + CO = CO2 + H•
Flame retardant (FR) acts in the gas phase can serve as radical scavenger
which interrupts the propagation reaction. The exothermal processes are thus
stopped, the system cools down, and the supply of combustible gases is
reduced and eventually completely suppressed. For instance, halogens exhibit
high efficiency as flame inhibitors. However, the application of
halogen-containing FRs is restricted in many areas due to health and
environment issues, and phosphorus-containing FRs, are getting more popular
as the replacement. The PO• radicals generated by FRs have been proved to
involve the destruction of hydrogen atoms, showing high efficient quenching
effect. Moreover, some FRs can release nonflammable gases such as water,
and dilute the concentration of combustible gas or lower the environmental
temperature.
3.4.1.2 Condensed Phase
The condensed phase mechanism postulates an interaction between flame
retardant and the polymer. The interaction between flame retardant and polymer
takes place usually at a lower temperature and leads to char formation, which is
probably the most important explanation for condensed phase mechanism. It
serves as the barrier between oxygen, heat and the polymer surface. Therefore,
the carbonaceous layer with good thermal stability and low thermal conductivity
has a good effect on preventing the conversion of polymer to combustible gas. It
is reported that a continuous and compact protective char layer contribute better
Literature Review 18
lowering the heat release. The carbonaceous layer can serve as a barrier which
can reduce both the heat and mass transfer. In particular, the intumescent flame
retardants (IFRs) are typical examples. IFRs can generate nonflammable gases
and expand the carbonized or vitreous layer. It is also been proved that the
combination of metal oxides with some conventional IFRs, the quality of the char
can be further improved.
3.4.2 Classification of Flame Retardant
The flame retardant industry has rapid development in the last decades, many
species of FR have been explored. The FRs can be classified into different
categories as following:[108, 109]
Halogenated FRs
Halogenated FRs are primarily based on chlorine and bromine and exhibit high
flame retardant efficiency. However, more and more halogenated FRs are
restricted due to environmental issues.[110]
Inorganic FRs
Inorganic FRs represent the largest fraction of the global flame retardant market.
This category includes metal hydroxides, silicon dioxide, antimony compounds,
boron compounds and other metal compounds.[73, 111-114] Some compounds
of this group, e.g. antimony oxides, must be combined with bromine or chlorine
to be effective. Others like metal hydroxides and silicon dioxide can be used
independently. However, a significant high loading is necessary to achieve a
satisfying flame retardant effect.
Nitrogen-containing FRs
A typical example of this category is melamine. It can transform into a
cross-linked structure during the combustion and release inert gas which can
Literature Review 19
dilute the combustible products.[115-119]
Phosphorus-containing FRs
These compounds can work both in the gas phase and condensed phase. On
one hand, PO• radicals are able to cut off the propagation reaction of the
polymer. On the other hand, phosphorus promotes the formation of char,
inhibiting the mass transfer and the pyrolysis process.[120-124]
Nanofillers
The development of nanocomposites technology has encouraged numerous
interest in the research of flame retardant polymeric materials because the
incorporation of nanofillers improves dramatically flame retardancy and various
other properties of polymer matrix at loadings as low as 3%-5%. In the last years,
various nanofillers with flame retardant effect based on different compounds
have been developed, such as montmorillonite (MMT), layered double hydroxide
(LDH), graphene, metal oxides, polyhedral oligomeric silsesquioxane (POSS),
sepiolite, halloysite etc.[113, 125-129]
3.4.3 Strategies of Developing Flame Retardant PLA
Originally, PLA was only used in short-term application such as packaging
materials, which have no requirement on flame retardancy. In recent years, with
the increasing concern of environmental issue and depletion of fossil resources,
PLA is considered as a promising solution and its application has rapidly spread
to other field like automotive and electrical components as well as house
holdings, for which the flammability a potential risk. Thus, improved fire
resistance of PLA based materials is an urgent task for our day-to-day safety.
The strategies are summarized as follows:
Literature Review 20
3.4.3.1 Chemical Copolymerization
The primary properties of polymeric materials are normally determined by the
chemical structure. In order to overcome the inherent limitation, it is a common
strategy to prepare a copolymer.[50, 130] Wang et al. prepared a
phosphorus-containing PLA by chain extending reaction.[131] The modified PLA
(mPLA) showed excellent flame retardancy and was used as FR for virgin PLA.
With 5 wt.-% loading, the compound could pass V-0 rating in the UL-94 test.
When the loading increased to 10 wt.-%, the peak of heat release (PHRR) value
decreased by around 20 % and the material exhibited a LOI (limiting oxygen
index) value as high as 35 %. Xiong et al. synthesized a copolymer of LA and
diaminobenzidine (DAB) via melt polycondensation.[132] Different reaction
conditions such as temperature, time, catalyst and the molar ratio of raw
materials were studied. The product, P(LA-co-DAB) showed better thermal
stability and higher char yield, suggesting a better flame retardant performance.
3.4.3.2 Blending Approach
The blending of other polymers can impart desired properties such as thermal
stability, ductility, flame retardancy, etc. to the matrix polymer. Kimura et al.
blended PLA with polybutylene succinate (PBS) for use in computer housings.
The blends showed improved fire performance. With the combined application of
FR, the materials exhibited V-2 rating in the UL-94 test, compared no rating of
virgin PLA.[133] To improve the fire performance, Bocz et al. blended PLA with
thermoplastic starch;[53] Teoh et al. prepared blends with PLA and poly (methyl
methacrylate) (PMMA).[134] The PHRR of materials was reduced due to the
charring effect of the strange polymer. Besides conventional plastics, polymeric
flame retardants have been developed and used in PLA.
Poly(bis(phenoxy)-phosphazene), a conventional FR with the trade name
SPB-100 was blended with PLA. High LOI value and V-0 rating were achieved,
Literature Review 21
as PHRR in cone calorimeter test decreased by more than 50 %. The main
mechanism of enhanced flame retardancy by blending was mainly related to the
improved charring effect, leading to easier formation of protective layer.[135]
3.4.3.3 Inorganic Flame Retardant Additives
The non-halogenated inorganic FRs include mainly metal oxide, hydroxide and
salt.[136-139] Aluminum trihydrate (ATH) is a widely used FR due to the low cost
and smoke production.[140] Nishida et al. prepared PLA/ATH composites by
solution compounding.[141] In this particular case, ATH was used as FR as well
as a catalyst to accelerate the depolymerization of PLA. The composites were
considered as recyclable feedstock. However, high loading of ATH (50-65 wt.-%)
was necessary to achieve a satisfactory flame retardant effect, which led to a
negative effect on the mechanical properties. Cheng et al. introduced MMT into
PLA/ATH composites to lower the ATH loading without losing satisfactory flame
retardancy.[142] With a total filler loading of 40 wt.-%, the PLA composites could
obtain V-0 rating in the UL-94 test.
Aluminum hypophosphite (AHP) is another well-known FR for polymeric
materials.[143, 144] Tang et al. studied the influence of AHP on the flame
retardancy of PLA. The PLA/AHP composite with 20 wt.-% loading passed V-0 in
UL-94 test and exhibited a LOI value as high as 28.5 %.[145] Zhou et al.
combined used AHP with other conventional FR in PLA. The composites
showed improved anti-dripping properties and more compact char layer after the
cone calorimeter test, suggesting a synergic effect.[146]
3.4.3.4 Organic Flame Retardant Additives
In recent years, many organic FR containing phosphorus, nitrogen and/or silicon
were synthesized and exhibited high flame retardant efficiency in PLA.[147-158]
Hexa(phosphaphenanthrene aminophenoxyl)-cyclotriphosphazene (HPAPC)
was synthesized by Jiang et al. and blended with PLA. At a loading of 10 wt.-%,
Literature Review 22
the composite passed V-0 rating in UL-94 and the LOI value was improved to
35.6 %.[159] Li et al. synthesized a hyperbranched poly(phosphamide ester)
oligomer (HBPE) and melt blended with PLA. HBPE was proved to be a highly
effective FR for PLA. With only 2 wt.-% of HBPE, the PLA composite exhibited a
LOI value as high as 33 % and passed V-0 rating in UL-94.[160] Similarly, Liao et
al. reported an efficient polymeric FR (PNFR) containing phosphorus, ester
groups and phenyl rings for PLA.[161] LOI value was improved to 31.5 % and
UL-94 V-0 was achieved with 3 wt.-% PNFR.
3.4.3.5 Intumescent Flame Retardant System
An intumescent flame retardant (IFR) system usually consists of an acid source
which yields acid species upon heat, a carbonization agent which decomposes
at the right temperature and a blowing agent which can expand the char
layer.[106] Char formers are mainly hydroxyl-rich compounds such as starch and
polyols. Examples for the acid source are ammonium salts and phosphates.
Blowing agents are compounds which can release gases upon heat like urea
and melamine. IFR is a widely used FR for various polymeric materials, such as
polyester, polyamide and polyolefin. A classic example for IFR is the combination
of ammonium polyphosphate (APP) and pentaerythritol (PER), in which acts as
a charring agent and acid source at the same time and PER as the carbonization
agent.[162, 163] Recently, many other compounds which can be used as IFR
have been synthesized and reported.[164-171] To prepare composites with
higher bio-based content, starch[172-174], lignin[175-179], β-cyclodextrin[180]
were used as charring agent. Song et al. prepared PLA/PEG/APP (polyethylene
glycol 6000) composites with considerable flame retardancy and toughness at
the same time.[181] Fontaine et al. reported highly fire resistant PLA composites
based on APP and melamine (MEL) incorporated with MMT.[182, 183] With 30
wt.-% of APP/MEL (5:1), PHRR was reduced by 87 %. Zhao et al. synthesized
another macromolecular charring agent tris(2-hydroxyethyl) isocyanurate
Literature Review 23
polyphosphate (TPM) and applied in PLA.[184] The composite with 25 wt.-%
APP/TPM (2:1) acquired an LOI value of 36.5 % and achieved V-0 in the UL-94
test without dripping behavior observed. PHRR was reduced by 68 %.
3.4.3.6 Use of Nanoparticles
Nanocomposite technology is a relatively new research interest. In recent years,
great efforts have been done to develop nanoparticles with different functions,
including a flame retardant effect. To date, many FR nanoparticles have been
used in PLA matrix, such as MMT,[142, 183, 185-188] LDH,[187, 189-191]
CNTs,[46, 47, 192, 193] EG,[135, 187, 194, 195] POSS[196-199] and many
other clay mineral.[200-205] Chow et al. and Liu et al. investigated the influence
of organo-modified MMT on the fire resistance of PLA.[185, 206] Improved
anti-dripping behavior and reduced PHRR were observed. The mechanism was
proposed to be the enhancement on the melt stability and the physical barrier
effect. Some researchers tried to combine other FR with MMT.[186-188] The
addition of MMT was proved to be able to promote the formation of char. CNTs
have been found to have a similar effect as MMT. Bourbigot et al. introduced
MWCNT into PLA by reactive extrusion. The composite showed an anti-dripping
effect and a lower burning rate in the LOI test.[192]
3.4.3.7 Combination of Flame Retardants
Sometimes it is necessary to improve more than one properties of the matrix,
thus more than one fillers or additives are needed.[207] For instance, natural
fibers which are common reinforcement for PLA, are usually combined used with
conventional FR like APP to achieve improvement in mechanical and fire
performance simultaneously.[42-45, 208, 209] Similarly, the organic-inorganic
hybrid can enable a better dispersion of inorganic fillers in the polymeric matrix
with desired properties.[178, 210]
Experimental: Materials, Processing and Characterization Methods 24
4 Experimental: Materials, Processing and Characterization Methods
4.1 Material Description
Various chemicals were used in synthesis and processing as follows: Poly (lactic
acid), Ingeo Biopolymer 6202D was purchased from NatureWorks LLC. Vanillin
(VA) and 3-Aminopropyltriethoxysilane (APTES) were purchased from TCI
Deutschland GmbH. Phenylphosphonic dichloride (PPDC, 90%), trimethylamine
(TEA), allylamine, sodium sulfate anhydrous, magnesium sulfate anhydrous,
tetrahydrofuran, ethanol and ethyl acetate were purchased from Sigma-Aldrich
Corporation. Modified montmorillonite with the trade name Cloisite® 30B was
supplied by BYK-Gardner GmbH. Ammonium polyphosphate (APP, degree of
polymerization ≥ 1000, density = 1.9 g/cm3, solubility ≤ 0.50 g/100ml (25 °C
in water)) was kindly provided by Budenheim company. All the materials were
used without further purification.
4.2 Synthesis section
4.2.1 Synthesis of bis(5-formyl-2-methoxyphenyl) phenylphosphonate
The synthesis path of bis(5-formyl-2-methoxyphenyl) phenylphosphonate (VP) is
shown in Figure 4-1. In detail, 0.2 mol Vanillin (VA) and 0.2 mol triethylamine
(TEA) were dissolved in 200 ml tetrahydrofuran in a three-neck flask with
magnetic stirring. Then, 0.1mol of PPDC, which dissolved in 100 ml of
dichloromethane, was added into the flask dropwise in 2 hours. The temperature
was kept at 0-5 °C with an ice bath during the addition. Then, after removing the
ice bath, the mixture was stirred for another 5 hours at room temperature,
orange solution and white precipitates were produced. The white precipitates,
trimethylamine hydrochloride salts, were filtered off. The resulting liquid was
condensed using rotary evaporation and again dissolved in ethyl acetate. Then
Experimental: Materials, Processing and Characterization Methods 25
the solution was washed with water for 4-5 times and subsequently dried with
sodium sulfate anhydrous for 2 hours. After filtration and removing the ethyl
acetate by rotary evaporation, the resulting syrup-like product was dried in a
vacuum oven for 24 h at room temperature. Finally, yellow powder,
bis(5-formyl-2-methoxyphenyl) phenylphosphonate (VP), was successfully
fabricated.
Figure 4-1: Synthesis path of VP
4.2.2 Synthesis of bis(5-((E)-(allylimino)methyl)-2-methoxyphenyl)
phenylphosphonate
Bis(5-((E)-(allylimino)methyl)-2-methoxyphenyl) phenylphosphonate (VPA) was
synthesized from VP and allylamine. In a 500 ml round-bottom flask, 4.26 g VP
(0.01 mol) and 2 g magnesium sulfate anhydrous were added in 200ml ethanol
with stirring and maintained at 78 °C under cooling reflux. Then, 1.14 g (0.02 mol)
allylamine, which was dissolved in 100 ml ethanol, was slowly added in the
above solution with vigorous stirring. After 8 hours, the solution was cooled to
room temperature and dried with magnesium sulfate anhydrous for 2 hours.
After removing the precipitate and the solvent, a dark red solid, VPA, was
acquired. The reaction route is shown in Figure 4-2.
Experimental: Materials, Processing and Characterization Methods 26
Figure 4-2: Synthesis Path of VPA
4.2.3 Synthesis of bis(2-methoxy-5-((E)-((3-(triethoxysilyl)propyl)imino)
methyl)phenyl) phenylphosphonate
The synthesis of bis(2-methoxy-5-((E)-((3- (triethoxysilyl)propyl)imino)methyl)
phenyl) phenylphosphonate (VPS) was similar to that of VPA. In a 500 ml
round-bottom flask equipped with a magnetic stirrer, a heating bath and cooling
reflux, 4.26 g VP (0.01 mol) and 2 g magnesium sulfate anhydrous were added
in 200ml ethanol. Then, 4.42 g (0.02 mol) APTES was dissolved in 50 ml ethanol
and slowly added into the solution. The temperature of the system was
maintained at 78 °C for 8 hours and cooled to room temperature. Then, the
solution was dried, filtered and condensed via rotary evaporation. The mixture
was moved into a vacuum oven to remove the rest solvent. The final product,
VPS, was obtained as a dark red viscous liquid. The synthesis path is shown in
Figure 4-3.
Figure 4-3: Synthesis Path of VPS
Experimental: Materials, Processing and Characterization Methods 27
4.3 Preparation of flame retardant PLA Composites
4.3.1 Melt Compounding
The flame retardant PLA composites were prepared with Plasticorder mixer
(chamber size: 50 or 30 cm3, Plastograph, Brabender, Germany) with a rotation
velocity of 60 rpm at 160°C using 6 min compounding cycle. The PLA granulates
and additives were dried in a vacuum oven at 40 °C overnight before processing.
According to the recipe, granulates and additives were mixed in sequence.
Information like torque, the temperature of the chamber and molten polymers
were monitored by computer. When the torque reached a plateau of minimum, it
indicated that the additives were homogeneously dispersed in polymer matrix.
Then the mixture can be collected from the chamber and granulated via an
electrical cutter. The recipes of flame retardant PLA composites processed by
plasticorder are shown in Table 4-1.
Figure 4-4: Mixing chamber of plasticorder (left) and extruder (right)
Experimental: Materials, Processing and Characterization Methods 28
Table 4-1: Recipe of Vanillin and Vanillin derivative based flame retardant PLA
Sample PLA
(wt.-%)
VA (wt.-%) VP (wt.-%) VPA
(wt.-%)
VPS
(wt.-%)
PLA 100 0 0 0 0
PLA10VA 90 10 0 0 0
PLA1VP 99 0 1 0 0
PLA2VP 98 0 2 0 0
PLA5VP 95 0 5 0 0
PLA10VP 90 0 10 0 0
PLA5VPA 95 0 0 5 0
PLA10VPA 90 0 0 10 0
PLA5VPS 95 0 0 0 5
PLA10VPS 90 0 0 0 10
4.3.2 Extrusion
Extrusion is one of the most common processing plastic methods to produce a
homogenous mixture of polymer and additives. The individual extruder
employed to obtain flame retardant PLA composites was a twin-screw extruder
(Paralleler Doppelschnecken-Extruder Process 11, Thermo Fisher Scientific)
with a screw diameter of 11 mm. Molten polymers were extruded through a 3
mm die with the temperature of 195 °C (Zone 1), and the temperature of the melt
with the value between 195 and 200 °C was determined via an external
thermometer. The extrudate was directly cooled by a water bath and
subsequently granulated via an electrical cutter. Default extrusion parameters
are described in Table 4-2.
Experimental: Materials, Processing and Characterization Methods 29
Table 4-2: Default extrusion parameters
Temperature (°C) Screw speed
(rpm) Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7
195 195 190 185 180 180 180 250
4.3.3 Melt Spinning
Melt spinning is one of the most economical and one of the simplest processing
technique of fiber manufacture. This method can be considered as a specialized
extrusion to create multiple continuous polymer filaments. Generally, polymer
granules are first heated above their melting point and then extruded through the
spinneret, where the melt is filtered to remove any un-melted, which could lead
to the weak point of the fiber. Afterwards, the fibers undergo hot drawing by roll
machine with specific winding speed to achieve fiber orientation. The winding
speed is a critical element to influence the tenacity and stiffness of the fibers.
Default processing parameters are described in Table 4-3
Table 4-3: Default spinning parameters
Temperature Nozzle Propulsion speed Winding speed
190 °C Φ=0.3 mm 10 mm/min 1000 and 2000 m/min
4.3.4 Injection Molding
Injection Molding is the most widely shaping process to convert pellets into
articles, especially into complex forms or those require high dimensional
precision for plastic materials. Samples for tensile and impact tests, LOI and
UL94 measurements were prepared using an injection machine (BOY 22 A HV,
BOY Machines, Inc., Germany) with a screw diameter of 18 mm and clamping
force of 220 kN. Default injection parameters are described in Table 4-4
Experimental: Materials, Processing and Characterization Methods 30
Table 4-4: Default injection molding parameters
Temperature (°C) Injection flow
(mm/s) Zone 1 (Nozzle) Zone 2 Zone 3 Zone 4
185 185 170 160 30
4.3.5 Compression Molding
Compression molding is commonly used for forming articles that do not have
complicated features, for example, it is very efficient to prepare different sheets
via hot compression molding machine because of the ease and low expense to
produce compression mold. In this particular work, this method was used to
prepare sample plates for cone calorimetry tests. Moderate amount of granules
were placed in the compression mold, which located between two metal sheets.
The sheets and mold were pre-heated at 195 °C for 3 minutes on the heating
plate of the compression machine. Then the bubbles were removed by manually
casting and releasing pressure vigorously. Subsequently, the pressure was
maintained at 50 kN for firstly 1.5 minutes at 195 °C, and then the mold and
material were cooled down with built-in circulating water cooling system under
the same pressure for 4 minutes. Finally, the resultant sample sheets were
removed from the mold.
Figure 4-5: Injection molding machine (left) and hot compression machine (right)
Experimental: Materials, Processing and Characterization Methods 31
4.4 Characterization Techniques
4.4.1 Structure Characterization
1H, 13C and 31P Nuclear Magnetic Resonance (NMR) measurements were
performed with a Bruker Avance III 500 spectrometer (Rheinstetten, Germany)
operating at 500 MHz. CDCl3 was used as the solvent and the solvent signal
was used for internal calibration (CDCl3: δ(1H)=7.26 ppm).
The Fourier transform infrared spectroscopy (FTIR) of the Vanillin and its
derivatives were obtained using the FTIR-spectrometer Vertex 80v (Bruker,
USA). The samples were directly measured using attenuated total reflection
(ATR)-objective (20x, Gecrystal) in FTIR-Microscope. The used spectroscopic
range was 4000-600 cm-1. The resolution was 4 cm-1.
4.4.2 Gel Permeation Chromatography
The molecular weights of PLA and processed PLA were determined by means of
size exclusion chromatography (SEC) equipped with HPLC pump (Serie 1200,
Agilent Technologies). Samples were dissolved with Chloroform and pre-filtered
to remove the unsolvable fillers. The flow rate was controlled at 1mL/min.
4.4.3 Morphological Characterization
Scanning electron microscopy (SEM) with energy dispersive X-ray analysis
(EDX) was observed via a microscope (Carl Zeiss SMT, Germany) to study the
morphological features of Vanillin and its derivatives and their dispersion in PLA
matrix.
4.4.4 Rheological Characterization
Rheological behavior of the composites was evaluated utilizing an ARES
rheometer (Rheometrics Scientific, USA) with the sample dimension
Experimental: Materials, Processing and Characterization Methods 32
diameter=20mm and thickness=2mm. Dynamic frequency sweep tests were
carried out at 170 °C with 10% strain in the frequency range of 0.1-100 rad/s.
4.4.5 Thermal Properties Tests
The thermogravimetric analysis (TGA) was carried out using a
thermogravimetric analyzer, TGA Q5000 (TA Instruments, USA) in the range
between room temperature and 800 °C at different heating rates under nitrogen
atmosphere as follows: 10 K/min as default single heating rate, Multiple heating
rates 10, 20, 30, 40 K/min measurements were done in order to study thermal
decomposition kinetics of PLA and PLA composites.
The differential scanning calorimetry (DSC) was performed using a differential
scanning calorimeter, DSC Q2000 (TA Instruments, USA) to study the thermal
behavior of PLA composites. The samples were firstly heated from room
temperature to 200 °C and keep the temperature for 3 minutes under a nitrogen
atmosphere to get rid of the thermal history. Then the samples were cooled
down to 25 °C. Finally, the samples were subjected to a second heating to
200 °C. The changing rate of temperature was kept at 10 K/min during all the
scanning.
Melt flow index (MFI) of composites was determined utilizing a Göttfert MI-4
according to DIN-ISO-1133-2005 at 170 °C with a weight of 2.16kg.
4.4.6 Mechanical Properties Tests
Tensile tests have been done with a Zwick 1456 (model 1456, Z010, Ulm
Germany) with cross head speed 1 mm/min according to DIN EN ISO
527-2/1BA/1 for tensile bars and with cross head speed 100 mm/min according
to ASTM D 2256 for filaments. For each composition at least 5 specimens were
tested to establish reproducibility. Prior to tests, all the specimens were stored in
a desiccator for at least 48 hours and then carried out at 23 °C and 50 %
Experimental: Materials, Processing and Characterization Methods 33
humidity.
Impact tests were performed on a pendulum impact tester (PSW 4J, Zwick,
Germany) to determine the Charpy impact strength according to ISO 179/1eU
with the specimen dimension of 80 x 10 x 4 mm3, and for each composition, at
least 10 specimens were tested to establish reproducibility. Prior to tests, all the
specimens were stored in a desiccator for at least 48 hours and then carried out
at 23 °C and 50 % humidity.
4.4.7 Flammability and Combustibility
The limited oxygen index (LOI) test was first proposed by Fenimore and Martin
in 1966 to evaluate the ease of ignition of materials by given their LOI value.
Figure 4-6 illustrates the schematic set up of the LOI measurement. The test
sample was vertically placed at the center of a glass chimney, with the inside
filled with a mixture of oxygen and nitrogen. The sample was inflamed with a
burner from the top. The LOI value is defined as the minimum oxygen
concentration [O2] in the mixture that either sustains the burning of the specimen
for 3 minutes or the flame reaches the mark 5 cm from the top of the specimen.
The LOI value can be expressed as follow:
𝐿𝑂𝐼 =[𝑂2]
[𝑂2] + [𝑁2]× 100%
In this work, the LOI tests were carried out using an Oxygen Index instrument
(FTT, UK ) with the specimen dimensions of 127 x 6.5 x 3 mm3, and the LOI
values were determined according to the international standard (ISO 4589).
Experimental: Materials, Processing and Characterization Methods 34
Figure 4-6: Schematic set up of LOI (left) and UL-94 V (right) instruments[211]
The set of UL-94 tests has been approved by “Underwriters Laboratories Inc.” to
determine the safety of flammability of plastic materials parts in devices and
appliances. It contains a series of flammability tests (94 HB, 94 V, 94 VTM, 94 5V,
94 HBF, 94 HF and radiant panel). The most commonly used test method is
UL-94 V for investigating the ignitability and flame spread when materials
exposed to a small fire. In the test 5 specimens are tested. The sample is
vertically fixed on sample clamp as shown in Figure 4-6. A burner flame with a 20
mm height and a power of 50 W is applied to the bottom of the specimen for 10
seconds. The after-flame time t1 (the time required for the flame to extinguish) is
recorded. After extinction, the burner flame is applied for another 10 seconds,
the after-flame time t2, after glowing time t3 (the time required for the fire glow to
disappear) and whether the dripping causes the cotton beneath the specimen to
ignite, are recorded. For each material, 5 specimens must be tested. Then
according to the standard shown in Table 4-5, the UL-94 classification of
materials can be determined as V-0, V-1 or V-2.
Experimental: Materials, Processing and Characterization Methods 35
Table 4-5: Classification of materials for the UL-94 V tests
Criteria Condition V-0 V-1 V-2
After flame time (t1 or t2) for each individual
specimen
≤ 10 s ≤ 30 s ≤ 30 s
Total after flame time (t1 + t2) for all 5 specimens of
any set
≤ 50 s ≤ 250 s ≤ 250 s
After flame and after glowing time (t2 + t3) for each
individual specimen after second flame application
≤ 30 s ≤ 60 s ≤ 60 s
Flaming dripping allowed No No Yes
Cotton ignited by dripping from any individual
specimen
No No Yes
Flame or glowing to the clamp No No No
In this work, the UL-94 V tests were performed according to ASTM D 3801,
using a horizontal vertical flame chamber, HVUL2 (ATLAS MTS, USA) with the
specimen dimension of 130 x 13 x 3.2 mm3.
Cone calorimetry Test (CCT) is one of the most effective bench scale polymer
fire behavior test methods.[212] It is based upon the principle of oxygen
consumed during the combustion process of the polymer. It is approved that the
heat released by a burning specimen is proportional to the total oxygen
consumption. The cone calorimeter is able to measure the gas flow, oxygen, CO,
and CO2 concentration and smoke density in the combustion products. Figure
4-7 illustrates the schematic set up of a cone calorimeter. During the test, a
sample sheet with a surface of 100 x 100 mm2 is wrapped with aluminum foil
covering the sides and bottom and located horizontally in the load cell. An
Experimental: Materials, Processing and Characterization Methods 36
external incident radiant flux can be applied from above by means of a
temperature controlled conical heater. The inflammable volatile products, which
are released by the degradation of the specimen, will be ignited by the electric
spark. The quantity of heat released per unit of time and surface area, heat
release rate (HRR), can be calculated on the basis of oxygen consumption of
13.1 MJ for 1kg of oxygen and expressed in kW/m2. The evolution of HRR over
time, particularly the peak of HRR (pHRR), is usually taken into account in order
to evaluate fire hazard. In addition, many further valuable parameters of the
combustion can also be recorded, such as the time to ignition (TTI), total heat
release (THR), mass loss (ML) of the sample, CO and CO2 production, total
smoke production (TSP), smoke production rate (SPR) and smoke toxicity index,
etc.[213, 214]
Figure 4-7: Schematic set up of a cone calorimeter[211]
4.4.8 Mechanism Studies
Thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) was
carried out using a thermogravimetric analyzer, TGA Q5000 (TA Instruments,
USA) coupled with Fourier transform infrared spectrometer, Nicolet iS50
Experimental: Materials, Processing and Characterization Methods 37
(Thermo Electron, USA) in the range between room temperature and 800 °C at a
heating rate of 10 K/min under nitrogen atmosphere. The FTIR spectra of
evolved gaseous products were recorded in the range of 4000-600 cm-1 with a
resolution of 4 cm-1 and averaging 8 scans.
Scanning electron microscopy (SEM) with energy dispersive X-ray analysis was
observed via a microscope (Carl Zeiss SMT, Germany) to study the
morphological features of char after cone calorimetry tests.
Results and Discussion 38
5 Results and Discussion
5.1 VP based PLA Composites
Vanillin (VA) is one of the most promising bio-based and bio-degradable raw
material for polymer synthesis. It also shows the potential to be used as a
functional additive. In this subchapter, the impact of vanillin on the properties of
PLA will be discussed. In addition, a novel vanillin derivate,
bis(5-formyl-2-methoxyphenyl) phenylphosphonate (VP), was synthesized
according to the procedure described in Figure 4-1 and characterized by FTIR
and NMR. VA and VP were mixed with PLA via plasticorder according to the
recipe shown in Table 5-1 and then injection molded to prepare different samples
with a suitable dimension for different measurements. The morphology of the
sample was first investigated using SEM with EDX. Then, the thermal
degradation behaviors were studied via TGA and DSC. Afterwards, the influence
of VA and VP on the mechanical properties of PLA were determined. Finally, the
flame retardant properties of PLA composites and the mechanism were
investigated via LOI, UL-94 V, CCT and TG-FTIR.
Table 5-1: Recipe of pure PLA and its composites
Sample PLA (wt.-%) VA (wt.-%) VP (wt.-%)
PLA 100 0 0
PLA1VP 99 0 1
PLA2VP 98 0 2
PLA5VP 95 0 5
PLA10VP 90 0 10
PLA10VA 90 10 0
Results and Discussion 39
5.1.1 Structural Characterization
5.1.1.1 FTIR Spectroscopy
In order to identify the desired structure as described in the experimental part,
VP and VA were characterized by FTIR, the FTIR spectra of VA and VP are
shown in Figure 5-1, and the assignment of the main vibrational modes for VA
and VP are listed in Table 5-2.
Table 5-2: Assignment of the main vibrational modes for VA and VP
Vibration VA VP
υ (C=O) 1660 1685
υ (O-H) 3138
δ (O-H) 1198
υ (C-OH) 1150
δ (CH3) 1427,1452 1421,1463
υ (C-C arom.) 1508,1584 1501,1590
υ (P=O) 1267
υ (P-O-C) 1146, 1214
υ (P-ph) 905, 1111
In the spectrum of VA, the strong intensity broad band centered at 3138 cm -1
could be assigned to OH stretching vibrations. The broad range of 2700-3500
cm -1, which disappears in the spectrum of VP, is due to the intermolecular
hydrogen bonding.[215] Weak bands observed at 3026 and 2947 cm-1 could be
attributed to CH3 asymmetric stretching vibrations. The weak band at 2864 cm-1
and very strong band observed at 1660 could be assigned to CH stretching
vibrations and C=O stretching vibrations of the aldehyde group respectively. The
strong bands at 1584 and 1508 cm-1 are attributed to CC stretching vibrations of
the benzene ring. The bands centered at 1452 and 1427 cm-1 could be due to
Results and Discussion 40
the asymmetric deformation vibrations of CH3 group. The bands observed at
1398, 812 and 731 cm-1 could be assigned to CH, O=C-H and C-C-CHO
deformations related to the aldehyde group, respectively. While the medium and
strong bands at 1198 and 1120 cm-1 are due to the CCH deformation of the
benzene ring. The bands observed at 1298 and 1150 could be attributed to
C-O-H deformation and C-O stretching vibrations related to the hydroxyl group.
And the band appears at 1024 cm-1 is due to the O-CH3 stretching vibrations. In
comparison of the spectrum of VA, a few new characteristic bands have been
obtained: the very strong band appears at 1267 cm -1 could be assigned to the
P=O stretching vibrations,[216] and the bands observed at 1111 and 905 cm-1
could be attributed to P-C stretching vibrations related to the benzene ring. The
FTIR spectra confirmed the desired structure as described in the experimental
part.
Figure 5-1: FTIR Spectra of VA (black) and VP (red)
5.1.1.2 NMR Spectroscopy
In order to further validate the successful synthesis of VP, 1H-, 13C- and 31P-
NMR were carried out and shown in Figure 5-2. In the case of 1H NMR, the
structure of VP was confirmed by the appearance of chemical shifts at 7.53, 7.63
Results and Discussion 41
Figure 5-2: 1H (top), 13C and 31P (bottom) NMR spectra of VP
and 8.05 ppm corresponding to label 5, 4 and 3 attributed to protons of the
benzene ring from phenylphosphonic dichloride (PPDC); the chemical shifts at
7.40, 7.44 and 7.45 ppm (labeled 8, 7 and 6 in Figure 5-2-a) are assigned to the
protons of the benzene ring of vanillin. The chemical shifts at 3.80 and 9.91 ppm
correspond to the protons from the methoxy and aldehyde groups of vanillin. In
the 13C NMR spectra of VP, chemical shifts at 123.71, 128.42, 132.27 and
133.38 ppm are attributed to the carbons from PPDC; the chemical shifts at
Results and Discussion 42
55.95, 110.96, 122.14, 124.90, 134.26, 144.60, 151.60 and 190.80 ppm are
assigned to the carbons from vanillin. The result of 31P NMR is also shown in
Figure 5-2(bottom). The chemical shift at 12.7 ppm is assigned to the phosphor
from PPDC, and the single signal indicates the high purity of the product. In
summary, all these NMR results mentioned above correspond well with the
desired structure from the experimental part, and further confirm the formation
and the high purity of VP.
5.1.2 Morphology of VP and PLA/VP Composites
The morphology of VA and VP, as well as flame retardant PLA composites based
on them, were observed via Scanning electron microscopy (SEM) coupled with
energy dispersive X-ray analysis (EDX). As shown in Figure 5-3, VA has a
rod-like shape with a smooth surface (left). However, the surface of VP is relative
rougher and some small particles clung to each other (right). The morphological
surface of VP is obviously different from that of VA, indicating the occurrence of
modification. And the EDX spectrum of VP with the presence of phosphorus
(blue) has further confirmed the chemical modification.
Figure 5-3: The morphology of VA and VP by SEM (left) and EDX (right)
The cross-section scans of PLA composites were also observed and the pictures
are demonstrated in Figure 5-4. Both samples showed dense plane with an
extremely smooth surface, indicating that both VA and VP are well miscible with
Results and Discussion 43
the matrix. The bright areas and dots seemed to be VA and VP dispersed in the
matrix. VA presented two types of dispersion in the matrix, micro-particles in
nanometer level and circles in roughly 50 μm. By contrast, VP showed uniform
nano-dots, revealing a highly improved dispersion. The reason for the
agglomeration of VA could be associated with its inter molecule hydrogen
bonding. However, the hydroxyl group in VP was removed by modification.
Hence, VP didn’t present inter molecule hydrogen bonding. The difference could
be also observed by FTIR (Figure 5-1).
Figure 5-4: SEM pictures of PLA/VA (left) and PLA/VP (right) composites
5.1.3 Thermal Decomposition Behavior
Thermogravimetric analysis (TGA) was employed to evaluate the thermal
decomposition behavior of VA and VP, as well as their effect on the thermal
stability of PLA and its composites.
Table 5-3: Data obtained from TGA plots of VA and VP
Sample T5% (°C) T50% (°C) Tmax (°C) Residue (wt-%)
VA 124 165 175 0
VP 286 348 343 24.5
Tx% was the temperature at x% weight loss of the samples. Tmax was the temperature at
the maximum rate of weight loss. Residue was the yield at 600 °C.
Results and Discussion 44
Firstly, the thermal decomposition of VA and VP under a nitrogen atmosphere
was evaluated via normal TGA with the heating rate of 10 K/min. The resulting
curves are shown in Figure 5-5. The temperature at which the weight loss of
sample was 5% can be considered as the initial decomposition temperature. The
relative thermal stabilities of samples were evaluated by comparing the
temperature at 50% weight loss, the temperature at maximum weight loss rate
as well as char residue at 600 °C. These detail data are summarized in Table
5-3.
Figure 5-5: TG (left) and DTG (right) plots of VA and VP
Like many other food additives, vanillin is relatively more sensitive to heat than
normal additives for thermoplastics. The TGA results showed that pure vanillin
started to lose weight at around 80 °C corresponding to a slight peak on the DTG
(differential thermal gravity) curve, and subsequently showed a T50% of 165 °C,
indicating that vanillin has a volatile nature. And this poor thermal stability leads
to the decomposition and evaporation of vanillin during the processing of its
polymer composites. In comparison with pure vanillin, the thermal stability of the
modified one, VP, was significantly improved, T5%, T50% and Tmax were shifted
from 124, 165 and 175 °C to 286, 348 and 343 °C, respectively. It showed an
improvement of more than 160 °C. Furthermore, the residue of VP at 600 °C in a
nitrogen atmosphere was around 30%. These properties make VP a
considerable additive for plastic.
Results and Discussion 45
The next step consisted in the evaluation of the effect of VP on the thermal
oxidative decomposition of PLA. The relative thermal stabilities of the samples
were compared by the value of T5%, T20%, Tmax as well as char residue at 600 °C.
TG and DTG curves of the samples are presented in Figure 5-6 and the data are
summarized in Table 5-4. As shown in Figure 5-6, there was only one
degradation step for the pure PLA ranging from 300 to 400 °C. As for the PLA/VA
composite, the degradation obviously consisted of two steps. The first step was
from around 150 to 300 °C and the weight loss in this stage was about 10%,
which corresponded to the loading of vanillin in the composite. It can be also
observed from the DTG plots of PLA/VA that the rate of weight loss in the
temperature range 160 to 300 °C was almost constant, indicating the weight loss
was only corresponding to the evaporation of vanillin. Comparing with the TG
plots from Figure 5-5, the beginning temperature was higher than the initiation of
the weight loss of VA. This should be due to that the PLA matrix was not melted
until 160 °C, and vanillin was completely released only when the sample was
heated above its boiling temperature (285 °C). Consequently, the thermal
stability of PLA was decreased after the introduction of VA.
Figure 5-6: TG (left) and DTG (right) plots of PLA and its composites
In the case of PLA/VP composites, based on the results in Table 5-4, the residue
at 600 °C remained at the same level, the differences were negligible. Besides,
the value of T20% of PLA1VP, PLA2VP, PLA5VP and PLA10VP were shifted from
Results and Discussion 46
344 °C of PLA to 350, 352, 357 and 357 °C, respectively, while Tmax shifted from
363 °C of PLA to 366, 369, 373 and 380 °C, respectively, indicating the thermal
stability of material was improved, even when the loading of VP was only 1%
and the improvement was stronger as the loading of VP rose. However, another
interesting result found in TG was that the value of T5% of the samples reached
the maximum when the loading of VP was 2%. It can be supposed that on one
hand, the introduction of VP could improve the thermal stability of PLA; on the
other hand, VP decomposed at a lower temperature than PLA. The
decomposition seemed to be not noticeable when the loading of VP lower than
2%. However, when the samples were further heated, VP could react with PLA
and enhance its thermal stability. Hence, T20% and Tmax shifted to a higher
temperature as the loading of VP increased.
Table 5-4: Data obtained from TGA plots of PLA and its composites
Sample T5% (°C) T20% (°C) Tmax (°C) Residue (wt-%)
PLA 326 344 363 0
PLA1VP 331 350 366 0.8
PLA2VP 333 352 369 0.2
PLA5VP 325 357 373 0.3
PLA10VP 324 357 380 0.7
PLA10VA 225 334 362 0.6
Tx% was the temperature at x% weight loss of the samples. Tmax was the temperature at
maximum rate of weight loss.
Furthermore, the thermal decomposition kinetics of PLA/VP composite was
studied by means of Flynn-Wall-Ozawa method. In this work, multiple heating
rate TGA, by which the heating rate was set as 10, 20, 30, and 40 K/min, were
performed to determine the necessary parameters. The fitted curves of lnβ
against 1/T at different conversion for PLA and PLA10VP (10 wt.-% VP) are
Results and Discussion 47
shown in Figure 5-7, and the resulting parameters, apparent activation energy
(Ea) as well as the coefficients of determination (r2) obtained from the fitted
curves of lnβ against 1/T are listed in Table 5-5. Generally, the basic
decomposition kinetic during the TGA measurement can be described via the
following equation:
𝑑𝛼
𝑑𝑇=
𝐴
𝛽𝑒(−
𝐸
𝑅𝑇)𝑓(𝛼)
Where α is the degree of conversion, T is the temperature in kelvins, β is the
heating rate, A is the pre-exponential factor, R is the ideal gas constant and E is
the activation energy. Using the Doyle’s approximation, the following expression
can be obtained:
lg 𝛽 = lg𝐴𝐸
𝑅𝑔(𝛼)− 2.315 − 0.4567
𝐸
𝑅𝑇
It can be seen that the fitted curves, lnβ plotted against 1/T, give straight lines
with slope m= -0.4567Ea/R during a series of measurements with different β at a
certain degree of conversion. Through this relationship, a series of Ea values can
be calculated corresponding to every α value.
Figure 5-7: Fitted curves of lnβ against 1/T at different α of PLA (left) and PLA10VP (right)
As shown in Figure 5-7, the fitted curves of lnβ against 1/T at different α of PLA
were almost parallel at different values of α, while the slope of fitted curves of
Results and Discussion 48
PLA10VP slightly increased with risen α value. Based on the calculated Ea
values from Table 5-5, both PLA and PLA10VP showed a similar decreasing
trend. The addition of VP raised the Ea value of PLA until the degree of
conversion reached 0.8. However, the enhancement decreased with rising α.
For example, when α was 0.1, Ea of PLA was 9 kJ/mol lower than that of
PLA10VP, while then difference reduced to only 0.6 kJ/mol when α was 0.7.
Furthermore, the difference shifted continually in a minor way, and when α
reached 0.9, Ea of PLA was already 3.1 kJ/mol higher than that of PLA10VP. It
can be concluded that the addition of VP had two opposite effect on the thermal
decomposition of PLA, which reach the equilibrium when α was between 0.7 and
0.8. On one hand, the thermal stability could be improved due to the antioxidant
effect of vanillin segment; further, thermal decomposition of VP could release
phosphonic acid, which could accelerate the decomposition of PLA. Therefore,
when more phosphonic acid was produced at high temperature, the reduction
effect on the thermal stability was stronger, and the composite decomposed
faster.
Table 5-5: Calculated values of Ea and r2 of PLA and PLA10VP at different α
PLA PLA10VP
Conversion (α) Ea (kJ/mol) r2 Ea (kJ/mol) r2
0.1 100.9 0.98994 108.9 0.98428
0.2 98.9 0.99052 109.1 0.99378
0.3 97.1 0.99224 104.6 0.9930
0.4 96.6 0.99475 103.9 0.99043
0.5 95.6 0.99268 100.1 0.99034
0.6 94.8 0.99492 96.7 0.98562
0.7 94.0 0.98914 94.6 0.98731
0.8 93.4 0.99201 93.0 0.98848
0.9 95.5 0.99233 92.4 0.98652
Results and Discussion 49
5.1.4 Thermal Crystallization Behavior
Different crystallization characteristics could affect various bulk properties of
polymer composites. Understanding the crystallization behavior of materials has
both analytical and practical importance. Differential scanning calorimetry (DSC)
was used to estimate the thermal characteristic of PLA and PLA/VP composites.
The samples were measured under a N2 atmosphere with 10 K/min
heating/cooling rate. In Figure 5-8, the comparison of the DSC thermographs of
the second heating between PLA and PLA/VP composites with various VP
loadings is presented. Additionally, Table 5-6 summarizes the data obtained from
this heating run and also the calculated degree of crystallinity of all the samples.
The thermal history of samples was removed by keeping the temperature at
200 °C for 3 minutes. Then the samples were cooled down to room temperature,
following immediately by the second heating, so that all the samples had the
same thermal history.
Figure 5-8: DSC plots of PLA and PLA/VP composites
The DSC curves of PLA and PLA/VP composites revealed similar thermal
crystallization behavior. The sample underwent following thermal events
successively with increasing temperature: the glass transition (characterized by
Results and Discussion 50
the glass transition temperature Tg), the exothermic cold crystallization
(characterized by the cold crystallization temperature Tcc and the cold
crystallization enthalpy ∆Hcc) and two steps endothermic melting process
(characterized by melting temperature Tm1 and Tm2 as well as the melting
enthalpy ∆Hm).
Table 5-6: Data obtained from DSC of PLA and PLA/VP composites
Sample Tg
(°C)
Tcc
(°C)
Tm1
(°C)
Tm2
(°C)
∆Hcc
(J/g)
∆Hm
(J/g)
Χc (%)
PLA 60.6 113.7 160.1 166.0 33.00 33.08 35.5
PLA1VP 59.5 115.3 159.6 165.4 34.02 34.07 37.0
PLA2VP 59.2 112.9 158.6 165.4 32.89 34.56 37.9
PLA5VP 56.8 111.7 156.3 163.9 32.63 33.56 37.9
PLA10VP 53.6 115.4 154.9 162.5 31.66 31.75 37.9
Based on the thermographs and the data from Table 5-6, it can be concluded
that the addition of VP altered the Tg of samples continuously to the minor
direction. As the loading of VP was 10 wt.-%, Tg of the sample was shifted from
60.6 °C of PLA to 53.6 °C. That was considered to be attributed to the
plasticization effect of VP. The small molecular VP enhanced the chain segment
mobility of PLA. Furthermore, at around 113 °C, PLA represented a sharp
exothermic peak, corresponding to the cold crystallization process during the
heating. Comparing with that, after the introduction of VP, the peaks of Tcc of the
samples showed a decreasing trend as the loading increased, suggesting the
incorporation of VP enhanced cold crystallization ability of matrix polymer.
Besides, the crystallization enthalpies of the samples decreased with rising
loading of VP, suggesting that VP had a nucleating effect and can accelerate the
crystallization speed of PLA. However, by more specific analysis, it was
Results and Discussion 51
observed the Tcc value decreased to lowest 111.7 °C with 5 wt.-% loading. When
the loading continued rose to 10 wt.-%, although the cold crystallization enthalpy
was lower, the Tcc shifted to higher 115.4 °C. Proposed reason for that was that,
higher loading of filler resulted in more agglomeration and hindered the cold
crystallization.[65]
Parallel to the shift of Tg, the Tm values of the samples were also altered to the
lower direction due to the plasticization effect of VP, and the shift rose with
increasing VP loading. All the samples showed double endothermic melting
peaks model, which has been reported for several semicrystalline polymers.
This bimodal peaks phenomenon was considered to be associated with the
different crystalline structures in the polymer. During the slow DSC scans, the
less perfect and unstable structure had enough time to melt and recrystallize into
the crystal with higher structural perfection, which melted at higher temperature.
According to the theoretical melting enthalpy of perfectly crystalline PLA, the
weight fractional crystallinity of the samples can be calculated based on the
following equation:
𝑥𝑐 =∆𝐻𝑚
(1−∅)∆𝐻𝑚∗ × 100%
Where Φ is the weight fraction of the filler in the composites, ΔHm is the melting
enthalpy calculated from the DSC curves and ΔHm* equals 93.1 J/g as
reference.[65, 217]
As shown in Table 5-6, the crystallinity of PLA represented an increasing trend
after the introduction of VP. However, it reached the highest value when the
loading of VP was 2 wt.-% and kept at the same value even when the loading
increased to 10 wt.-%. It could be explained by the good dispersion of VP in the
polymer matrix. In other words, when the filler dispersed well enough in the
polymer matrix, the particles amount in the matrix were very huge, which was
Results and Discussion 52
able to support the nucleation and promote the growth of crystals of the polymer
matrix.
Interestingly, the content of VP also affected the magnitude of the double melting
peaks of the samples. Firstly, with 1 wt.-% of VP, the first melting peak was
higher than that of PLA. It could be ascribed to the acceleration effect to the
crystallization of PLA. However, the low loading was only enough to accelerate
to form less perfect crystallization. When the loading of VP continued to increase,
the fraction of the first melting peak gradually decreased, suggesting that VP
contributed to form crystallization with higher perfection.
5.1.5 Flammability
In this work, LOI and UL-94 were applied to evaluate the flammability of PLA
composites. LOI is a quantitative method which shows the minimum oxygen
concentration of an oxygen/nitrogen mixture to maintain the combustion of the
sample, while the UL-94 measurements provide a qualitative classification
presenting extinguishment or upward spreading behavior of flame after the
ignition from bottom of samples, and the samples are classified by no rating, V-2,
V-1 and V-0, where the V-0 demonstrates the best flame retardant rating. The
results of LOI and UL-94 tests of PLA and PLA composites are summarized in
Table 5-7. Results showed that the LOI value of pure PLA was 21.4%, revealing
that PLA was an easily flammable material. It was also observed that during the
UL-94 tests, the samples were burned to the sample holder with intensive
burning dripping behavior and high velocity, and the cotton was ignited by the
droplets. With 10 wt.-% of VA, some of the samples can self-extinguish in about
10 seconds. However, the cotton under the samples was still ignited by the
flaming dripping, and LOI value decreased from 21.4% of PLA to 19.5%. Reason
for this result is that vanillin is flammable and volatile at high temperature, it can
be ignited more easily than PLA. By contrast, after the introduction of VP, the
Results and Discussion 53
samples showed a positive effect both in LOI and UL-94 tests. When the loading
of VP were 5 wt.-% and 10 wt.-%, the LOI values of the composites were
improved to 25.8% and 26.3%. Furthermore, both of the composites passed V-0
level in UL-94 tests, and the cotton was not ignited. It seemed that the
introduction of VA and VP has changed the melt flow behavior of PLA. The ease
of flow could take heat away in large scale. This could be the reason for the
better results of UL-94. Nevertheless, all the results suggested that VP can
provide good flame retardancy to PLA.
Table 5-7: LOI, UL94 and MFI results of PLA and PLA composites
Sample LOI (%) UL-94 Rating * MFI (g/10min) Ignition of cotton
PLA 21.4 No rating 6.0 Yes
PLA5VP 25.8 V-0 15.9 No
PLA10VP 26.3 V-0 59.5 No
PLA10VA 19.5 No rating 28.4 Yes
* Sample thickness of UL-94 tests was 3.2mm
5.1.6 Combustion Behavior
Cone calorimeter test (CCT) is one of the most efficient methods to investigate
the flame retardant properties of polymeric materials. The consumption of
oxygen is recorded to calculate the heat release of the measured samples.
Curves obtained from the CCT are presented in Figure 5-9 and Figure 5-10, the
corresponding characteristic parameters are collected in Table 5-8, including
time to ignition (TTI), peak of heat release rate (pHRR), total heat release (THR),
total smoke production (TSP), average effective heat of combustion (av. EHC)
and residual mass.
Results and Discussion 54
Table 5-8: Data obtained from CCT of PLA and PLA composites
Sample TTI
(s)
pHRR
(kW/m2)
THR
(MJ/m2)
TSP
(m2)
Residue
(%)
PLA 68±3 407±16 66.0±0.9 0.32±0.17 0.5±0.2
PLA10VA 64±5 396±2 64.9±0.7 0.57±0.19 0.9±0.3
PLA5VP 76±4 370±14 62.8±1.1 1.53±0.09 6.0±0.1
PLA10VP 79±2 292±28 62.6±1.7 3.33±0.18 7.7±0.5
Based on the results obtained from CCT, it can be seen that TTI of PLA was 68 s.
When pure PLA was ignited, it exhibited fiercely combustion and consumed all
the material, showing a total heat of 66.0 MJ/m2, with a pHRR value of 407
kW/m2. After the introduction of pure vanillin, the pHRR and THR value
decreased slightly to 396 kW/m2 and 64.9 J/m2, respectively. However, the TTI
was 64 s, which is 4 s earlier than pure PLA. It could be explained by the volatile
nature of Vanillin. When the material was melted by heat radiation, vanillin was
evaporated and then decomposed earlier than PLA into gaseous flammable
products. Hence, the specimen was ignited faster than pure PLA. In comparison,
VP and PLA demonstrated better cooperation towards flame retardancy. The
pHRR and THR value of PLA5VP were reduced to 370 kW/m2 and 62.8 J/m2,
respectively. When the loading of VP increased to 10 wt.-%, the pHRR and THR
value of sample decreased further to 292 kW/m2 and 62.6 J/m2, respectively. In
addition, TTI values of PLA/VP composites were delayed to 76 and 79 s
respectively.
Results and Discussion 55
Figure 5-9: HRR plots of PLA and PLA composites
As shown in Figure 5-9, the HRR curves of pure PLA, PLA5VP and PLA10VA
exhibit a similar pattern. After the ignition, the heat release increased fast and
continuously to the peak, suggesting that the flame spread rapidly into the
surface of the entire specimen. The broad peak indicated that the combustion
consumed all the material without any inhibition, corresponding to the slow
decreasing of HRR after the peaks. Interestingly, the HRR of PLA10VP grew
rapidly to the peak after the ignition and decreased immediately from 292 kW/m2
to around 200 kW/m2. The reason for that was supposed to be the
decomposition of VP, which simultaneously provide combustible gas products
and phosphor-containing segments. The combustible products were ignited and
contributed to the strong release of heat. And immediately the combustion was
suppressed by the phosphor-containing segments due to the radical capture
effect. As a consequence, the HRR of PLA10VP exhibited a sharp peak direct
after the ignition due to the evolved phosphor containing radical scavenger.
When all these products were consumed, the combustion behavior of the rest
specimen was similar like pure PLA, showing broad peak until all the material
was burned off. Hence, it can be concluded that the incorporation of VP can
Results and Discussion 56
significantly reduce and delay the heat release in PLA composites.
As mentioned before, although the introduction of both VA and VP can reduce
the THR of specimens in comparison with pure PLA, the reduction was not that
distinct. However, in Figure 5-10-(a) more obvious difference of the development
of THR could be found: the curve of PLA10VP was almost linear and its slope
was much lower than that of other curves, indicating that the combustion of
PLA10VP was much slower. Consequently, its THR at the same time after the
beginning of measurement was also much lower than that of other samples. This
result also matched with the curves of mass loss of the samples based on Figure
5-10-(b). The mass loss rate of PLA10VP kept almost constant and was much
slower than that of other samples.
Considering the few residues remained after the CCT (Table 5-8), it was unlikely
that those residues could act as a barrier or protective layer to reduce the heat
release. So the action in gaseous phase should mainly contribute to the flame
retardant property of the PLA composites. The release of gaseous products and
smoke were very important parameters to estimate the combustion behavior of
materials, especially for the action in gaseous phase. The total smoke
production (TSP) and the ratio of CO/CO2 are presented in Figure 5-10-(c) and
(d). It can be seen from Figure 5-10-(d), the first peak of pure PLA was around
0.03, while that of PLA5VP and PLA10VP was around 0.08 and 0.13 respectively.
It was considered to be associated with the radical scavenger effect of
phosphor-containing products evolved by VP, which lead to incomplete
combustion of the samples. The double peaks at the beginning of the curve of
PLA10VA could be explained by the earlier ignition of the volatile vanillin and its
acceleration effect to the ignition of PLA. The peaks at the end of the curves
could arise from the thermal degradation of the samples without the presence of
fire.
Results and Discussion 57
Figure 5-10: THR (a), Mass loss (b), TSP (c) and CO/CO2 ratio (d) plots of PLA and PLA
composites
5.1.7 Rheological properties
The rheological performance of PLA and PLA composites are presented in
Figure 5-11. The complex viscosity of neat PLA presented low dependency in
the lower frequency range. A Newtonian plateau was obtained in the range
0.1-10 rad/s, while a slight shear thinning behavior was observed at the higher
shear rate. The complex viscosity gradually decreased with rising VP loading,
indicating that VP possessed notable plasticization effect for PLA. For PLA10VP,
the complex viscosity was reduced by more than an order of magnitude. In
addition, the Newtonian plateau was extended to high frequency range, almost
no shear thinning behavior was observed. For all the samples, G’’ was overall
higher than G’, indicating that the samples were more viscous than elastic.
Comparing with neat PLA, both G’ and G’’ of composites were declined with
Results and Discussion 58
increasing VP loading. In case of PLA10VP, G’’ decreased by two over the entire
frequency range, while such notable difference in G’ only observed at the higher
shear rate. In summary, the lubricant effect of VP led to the decrement of
strength provided by the entanglements of PLA chain, thereby reducing the melt
viscosity and enhancing the processability of PLA. Since the dripping behavior
could be considered as a low shear rate phenomenon, the results of rheological
measurements further supported the results obtain in LOI and UL-94 tests.
Figure 5-11: Rheological properties of PLA and PLA composites: (a) Complex viscosity; (b)
Storage modulus; (c) Loss modulus
5.1.7.1 Analysis of evolved gaseous products
To further understand the decomposition and flame retardant mechanism,
TGA-FTIR was employed to study the evolved gaseous products. The 3-D
images and FTIR spectra of involved products for PLA and PLA10VP are shown
in Figure 5-12. The thermal degradation process of PLA has been intensively
studied by many researchers.[153, 154, 218] According to the results, the main
gas products from degradation products of PLA were water (3577 cm-1),
Hydrocarbons (2900-3000 cm-1), CO2 (2350 cm-1), CO (2114 and 2182 cm-1),
carbonyl compounds (1762 cm-1) and aliphatic ethers (1100-1250 cm-1). An
obvious decreasing at 1238 cm-1 (related to C-O stretching) for PLA10VP was
observed.
Results and Discussion 59
Figure 5-12: 3D-images of evolved gaseous products for (a) PLA (b) PLA10VP; (c) Total
absorbance vs time for PLA and PLA10VP; (d) Absorbance at 1238 cm-1 vs time for PLA
and PLA10VP (c) At Tmax for PLA and PLA10VP; (d) Zoom in on selected area. (Tmax
appeared at 47 min)
In addition, the total absorbance of PLA was also stronger than that of PLA10VP,
indicating that the incorporation of VP led to an overall reduction of evolved
gaseous products. Furthermore, as can be seen in Figure 5-12-(e), two bands at
1266 and 1284 cm-1 appeared, which could be assigned to P-C and P=O,
Results and Discussion 60
respectively. This could be the reason for the reduction of C-O containing
products evolved. The phosphorus-containing compounds were considered to
be able to strongly interact during the radical reactions especially at high
temperature, and thus reduce the flammability as it is demonstrated and
discussed in cone calorimeter results.[153]
5.1.8 Mechanical Properties
5.1.8.1 Effect of different fillers
The first selective criterion for a specific material in the application is the
mechanical performance. When developing a new functional additive for
polymeric materials (in this work, flame retardant), it is therefore also necessary
to evaluate the impact of incorporating this additive to other properties of the
final composites. The stress-strain curves of PLA and PLA Composites are
shown in Figure 5-13, and the results of Young’s modulus, tensile strength,
elongation at break and impact strength are summarized in Table 5-9.
Figure 5-13: Tensile test plots of PLA and PLA composites
It is widely known that PLA is a stiff thermoplastic polymer. Based on the results,
pure PLA exhibited a high rigidity (3700 MPa), a high tensile strength (57 MPa)
Results and Discussion 61
and a low elongation at break (3 %). Despite the introduction of VA improved
slightly the elongation at break to 4 %, some mechanical properties were notably
deteriorated: the Young’s modulus decreased to 2900 MPa while the tensile
strength declined to 42 MPa. In case of PLA/VP composites, comparing the
significant improvement of elongation at break of PLA/VP composites (11 % for
PLA5VP), the decreasing of tensile strength and Young’s modulus was
negligible. In contrast, the introduction of some molecular additives led to an
increase in the impact resistance of PLA. The impact strength of PLA10VA,
PLA5VP and PLA10VP was improved from 16 kJ/m2 of PLA to 17, 18 and 18
kJ/m2, respectively. This result can be explained by the lubricant effect of the
additive, which enhanced the chain mobility of PLA.
Table 5-9: Data obtained from tensile test of PLA and PLA composites
Sample Young’s
modulus
(MPa)
Tensile
strength
(MPa)
Elongation
at break
(%)
Impact
resistance
(kJ/m2)
PLA 3700±100 57±1 3±1 16±2
PLA5VP 3500±100 54±1 11±3 18±3
PLA10VP 3500±200 52±1 9±3 19±2
PLA10VA 2900±100 42±1 4±2 17±3
As the SEM results of the samples showed (Figure 5-4), due to the
intermolecular hydrogen bonding the phase size of VA was much bigger than
that of VP. So the plasticization effect of VP to PLA was better than that of VA.
Besides, the agglomeration caused stress concentration in composites. It can be
seen from the specimens after tensile tests that the microcracks of PLA/VA
specimens were more random and developed from the edge to the middle of
specimens, and most of the cracks crossed less than 1/4 of the specimen’s width
Results and Discussion 62
(shown in Figure 5-14). By contrast, the microcracks of PLA/VP specimens were
finer and most of them crossed the specimen’s width. The cracks developed
from the breakage to both ends of the specimens. This phenomenon indicated
that the stress transfer across specimens was much better in PLA/VP
composites due to the homogeneous structure. This was in good agreement
with the better dispersion of VP observed in SEM. Consequently, VP had a
plasticization effect, which caused decreasing of stiffness and improvement of
elongation at break. Besides, the agglomeration of filler could also cause
deterioration of various mechanical properties. Hence, as the loading of VP rose,
there was more chance that agglomeration formed, so the elongation at break of
samples decreased. Nevertheless, VP could significantly improve the elongation
of PLA with similar tensile strength.
Figure 5-14: Pictures of tensile bar after tests of PLA10VA (left) and PLA10VP (right)
5.1.8.2 Effect of different processing methods
PLA can be processed using almost all the nowadays processing methods, and
its properties can be altered depending on different processing conditions. In this
work, PLA and PLA composites were mainly mixed via plasticorder and extruder,
and then further converted into desired specimens for tests. To investigate the
Results and Discussion 63
influence of different processing techniques on the mechanical properties of PLA
and PLA/VP composite, the materials were prepared via different processing
methods. Figure 5-15 shows the stress-strain curves of original PLA (PLA O) and
processed PLA via plasticorder (PLA B) and extruder (PLA E), as well as those
of PLA10VP composite. It is well known that PLA undergoes shear forces and
thermal degradation which lead to change of molecular weight. So the alteration
of molecular weight of PLA was monitored via size exclusion chromatography
(SEC). The detail results are summarized in Table 5-10.
Based on the results, it can be seen that, after processing, the samples showed
decrease in molecular weight, from 125000 to 101000 for PLA B and to 109000
for PLA E, respectively. The reduction of molecular weight can be attributed to
thermal and mechanical unzipping and chain scission reactions caused by heat
and shear force, which are also responsible for an increase in chain segment
mobility.[10, 219] Thus PLA E showed an improvement in elongation at break.
Figure 5-15: Tensile test plots of PLA and PLA/VP composites processed via different
methods
On the other hand, a slight increase in tensile strength, from 55 to 57 MPa for
PLA B and to 61 MPa for PLA E, can be observed. By contrast, it is interesting to
Results and Discussion 64
find that, the Young’s modulus PLA B (3700 MPa) was higher than PLA E (3300
MPa). The different processing techniques are the most responsible for the
change of properties. Generally, at the same temperature and rotational speed,
the shear rate of the extruder is higher than that of plasticorder. On the other
hand, it takes more time for a plasticorder to achieve a similar dispersion of filler
as an extruder does. Consequently, the materials processed with plasticorder
undergo more thermal degradation, whereas those processed with extruder
suffer more mechanical load. In addition, the different cooling rate of the molten
polymer after compounding is a potential factor to affect the crystallinity of
samples. Compared to the water bath cooling system of extrusion, after mixing
with plasticorder, samples were air cooled to room temperature. Despite the
materials underwent other thermal history, the differences of both experimental
sets in terms of mechanical behavior as well as crystallization and degradation
were not very large.
Table 5-10: Parameters of PLA and PLA/VP composites processed via different methods
Sample Young’s
modulus
(MPa)
Tensile
strength
(MPa)
Elongation
at break
(%)
Impact
resistance
(kJ/m2)
Molecular
weight
(g/mol)
PLA O* 3600±200 55±1 11±1 16±1 125000
PLA B* 3700±100 57±1 3±1 16±2 101000
PLA E* 3300±400 61±1 9±2 16±1 109000
PLA10VP B 3500±200 52±1 9±3 19±2 90400
PLA10VP E 3400±100 61±2 13±3 17±3 107000
* O (Original) means as received, B (Brabender) means processed by Brabender
plasticorder, E (Extruder) means processed by mini extruder
An additional study highlighting the processability of PLA/VP system was
performed. One of the most challenging processes for multicomponent materials
Results and Discussion 65
is melt fiber spinning, as small changes in material properties like defects,
agglomerates and inadequate phase changes usually lead to problems in
spinnability.
Filaments of PLA and PLA/VP composites were prepared by means of melt
spinning with a take-up speed with 1000 and 2000 m/min, corresponding to titer
of 8 and 4 tex, respectively. Mechanical properties of fibers in bundles (10
monofilaments) were obtained and showed in Table 5-11. As listed, typical
mechanical properties for non-drawn PLA fibers were observed, tensile strength
between 20 and 25 cN/tex and elongation at break in the range of 50-100%. The
properties showed a take-up speed dependence due to the increased molecular
orientation with higher speed.
Table 5-11: Mechanical parameters of fibers for PLA and PLA/VP composites
Sample Titer
[tex]
Young’s
modulus
[cN/tex]
Tensile
strength
[cN/tex]
Elongation
at yield [%]
Elongation
at break [%]
PLA_1000 8.5 323.8±11.4 18.6±1.7 88.1±5.4 104.6±1.6
PLA_2000 4 454.5±7.0 22.9±0.7 46.8±2.5 57.8±2.2
5VP_1000 8 294.2±6.7 10.5±0.9 118.5±1.2 136.7±2.6
5VP_2000 4 446.5±32.7 17.7±1.6 52.5±6.7 64.2±1.6
10VP_1000 8 276.0±30.5 10.5±1.0 128.6±2.5 140.7±6.3
10VP_2000 3.8 312.9±12.4 12.5±5.0 61.9±2.2 70.3±1.9
20VP_1000 8 258.0±10.3 12.0±3.7 110.7±4.7 126.3±2.8
20VP_2000 3.5 321.7±19.5 12.5±1.2 51.8±8.1 64.8±1.1
* Sample name e.g. 5VP_1000 means fibers with 5 wt.-% VP and wound at 1000 m/min
Results and Discussion 66
Compared to neat PLA, all tested PLA/VP compositions generally showed a
distinct decrease of nearly 50% in maximum tensile strength, however with a
noticeable increase in strain behavior, significantly for lower take-up speeds. VP
seems to have a slight plasticizing effect, independently of the added VP content,
and particularly for fibers wound at low speed: neat PLA shows elongation at
break of about 105 % and PLA/VP values ranging from 125-140 %. For higher
speeds, here shown for 2000 m/min, the strain properties of the fibers seem to
be nearly independent of the added amount of VP compared to neat PLA
(elongation at break for all specimens about 60-70 %).
5.1.9 Conclusion
In this part, a novel bio-based flame retardant, VP was successfully synthesized.
VP exhibited remarkable flame retardant efficiency. At very low loadings (5 and
10 wt.-%), PLA/VP composites achieved V-0 rating in UL-94 tests and improved
notably the LOI value from 21.4 for PLA to 25.8 % for PLA5VP and 26.3 % for
PLA10VP. In addition, in the cone calorimeter tests, an obvious reduction of
PHRR value was observed for PLA10VP. The most important is, the introduction
of VP didn’t cause any deterioration of the mechanical properties like many
flame retardants, in contrast, the elongation at break of PLA was significantly
improved from 3 % to 9 % (PLA10VP), i.e. the relative increase was as high as
200 %. This results may indicate VP as a promising flame retardant also for
other polymer systems and offer new possibilities for bio-based multifunctional
polymeric composites. However, the dripping behavior of PLA composites by
burning is still unsolved. To enhance the anti-dripping property, further
modification of VP would be done in the next step. Further it could be shown that
the system PLA/VP had a very good processability even with highly demanding
processes like melt fiber spinning.
Results and Discussion 67
5.2 VPA and VPS based PLA Composites
In this subchapter, two derivatives of vanillin were synthesized based on VP
according to the procedure described in Figure 4-2 and Figure 4-3 and
characterized by FTIR and NMR. Crosslinkable groups were introduced into the
structure of VP, aiming to form protective layers during the combustion and
prevent the dripping behavior of PLA. It was expected that the double bond in
bis(5-((E)-(allylimino)methyl)-2-methoxyphenyl) phenylphosphonate (VPA) could
contribute or/and improve the char yield during the combustion, and the siloxane
structure in bis(2-methoxy-5-((E)-((3- (triethoxysilyl)propyl)imino)methyl)phenyl)
phenylphosphonate (VPS) was expected to be able to form a condensed layer
consisting of SiO2 and/or carbon which can isolate the material from heat and
oxygen. Structure of VPA and VPS were characterized by 1H NMR and FTIR.
Since the main target of this subchapter is to enhance the prevention of dripping
behavior of PLA, the study of properties of composites will be focused on the
flame retardancy. LOI, UL-94 and CCT were employed to evaluate the flame
retardant properties of composites. Thermal degradation behavior and
mechanical properties were investigated via TGA and tensile test. PLA
composites were prepared via plasticorder according to the recipe shown in
Table 5-12 and then injection molded to prepare different samples with a suitable
dimension for different measurements.
Table 5-12: Recipe of VPA and VPS based PLA composites
Sample PLA (wt.-%) VPA (wt.-%) VPS (wt.-%)
PLA5VPA 95 5 0
PLA10VPA 90 10 0
PLA5VPS 95 0 5
PLA10VPS 90 0 10
Results and Discussion 68
5.2.1 Characterization of VPA and VPS
5.2.1.1 FTIR Spectroscopy
VPA and VPS were characterized by FTIR in order to identify their chemical
structure, the FTIR spectra of VPA and VPS are shown in Figure 5-1, and the
assignment of the main vibrational modes for VA and VP are listed in Table 5-2.
In the spectrum of VPA (Figure 5-16), series of new bands appeared: the broad
band centered at 3187 cm-1 could be attributed to NH stretching vibrations and
its hydrogen bonding. The bands at 3061 and 3003 cm-1 could be assigned to
alkene CH stretching vibrations, and those at 2934 and 2835 cm-1 could be
corresponding to aliphatic CH stretching. The band at 1707 cm-1 could be related
to C=C stretching from the allylamine structure. The C=O stretching vibrations
for VP at 1660 cm-1 was replaced by the C=N stretching for VPA at 1641 cm-1. In
addition, the bands in the 1470-1420 cm-1 region could be attributed to alkene
CH bending vibrations.
Table 5-13: Assignment of the main vibrational modes for VPA and VPS
Vibration VP VPA VPS
υ (C=O) 1685
δ (C-CHO) 729
υ (C=N) 1641 1644
υ (N-H) 3187
υ (C=C) 1707
υ (Si-O-C) 1101,1072
υ (C-H) 3100-2800 3100-2800
δ (C-H) 1470-1420 1470-1420
υ (P=O) 1267 1226 1274
υ (P-O-C) 1196, 1146 1198,1158 1193,1160
υ (P-ph) 1111, 905 1124,919 1115,915
Results and Discussion 69
Figure 5-16: FTIR spectrum of VP (black) and VPA (red)
Figure 5-17: FTIR spectrum of VP (black) and VPS (red)
Compared to the spectrum VPA, several different new bands appeared in that of
VPS (Figure 5-17) due to the successful boning between APTES and VP. The
bands in the 3000-2800 cm-1 region should be attributed to aliphatic CH
stretching. The C=O stretching vibrations for VP at 1660 cm-1 was also absent
and replaced by C=N stretching at 1644 cm-1. The bands in the 1470-1420 cm-1
region could be assigned to CH bending vibrations. In addition, the strong bands
Results and Discussion 70
at 1101 and 1072 cm-1 should be corresponding to SiO asymmetric deformation
vibrations. The FTIR spectra confirmed the desired structure as described in the
experimental part.
5.2.1.2 NMR Spectroscopy
The chemical structures of VPA and VPS were further confirmed via 1H NMR. As
shown in Figure 5-18, the chemical shifts at 8.06, 7.59, 7.50, 7.46, 7.32 and 7.12
ppm (labeled 6, 7, 8, 9, 10 and 11) are assigned to the protons of the benzene
ring. The peak at 8.21 ppm (labeled 5) is attributed to the proton of the
carbon-nitrogen double bond. The chemical shifts at 6.05, 5.18 and 4.24 ppm
(labeled 1, 2 and 3) correspond to the aliphatic and alkene protons, which are
related to allylamine. The peak at 3.78 ppm (labeled 4) is assigned to the
protons from the methoxy group of vanillin.
Figure 5-18: 1H NMR spectrum of VPA
Based on the spectrum of VPS (Figure 5-19), several new peaks can be found
comparing to that of VP. The location of peaks corresponding to protons from
benzene rings has slightly changed, which are 7.91, 7.57, 7.48, 7.42, 7.30 and
Results and Discussion 71
7.09 ppm (labeled 8, 9, 10, 11, 12 and 13). The peak at 8.17 ppm (labeled 7) is
assigned to the protons from carbon-nitrogen double bond. The peak at 3.74
ppm (labeled 2) corresponds to the protons from the methoxy group. The peaks
at 3.83 and 1.24 ppm (labeled 1 and 5) are attributed to the protons from
siloxane group, and the peaks at 3.59, 1.81 and 0.67 ppm (labeled 3, 4 and 6)
are assigned to aliphatic protons, which are related to APTES.
Figure 5-19: 1H NMR spectrum of VPS
In summary, all these NMR results mentioned above correspond well with the
desired structure from the experimental part, and further confirm the formation
and the high purity of VPA and VPS.
5.2.2 Thermal Decomposition Behavior
The thermal degradation behavior of VPA and VPS as well as their PLA
composites were characterized via TGA at a heating rate of 10 K/min and a
nitrogen atmosphere. The TG and DTG plots are presented in Figure 5-20 and
the detail parameters are summarized in Table 5-14. TGA results of VP is shown
as a reference.
Compared to VP, the decomposition of VPA and VPS started at a lower
Results and Discussion 72
temperature. Both of them started to decompose before 100 °C, the T5% value of
VPA and VPS were 163 and 174 °C, indicating lower thermal stabilities at low
temperature than VP. The weight loss in this range (50 °C to 300 °C) could be
related to the decomposition of the carbon-nitrogen double bond. However, the
T50% value of VPA and VPS were significantly improved to 417 and 480 °C,
respectively. In addition, the maximal rate of weight loss decreased from 1.87 of
VP to 0.33 and 0.29 %/°C of VPA and VPS, respectively.
Table 5-14: Data obtained from TGA plots of VPA and VPS
Sample T5% (°C) T50% (°C) Tmax (%/°C) Residue (wt-%)
VP 286 348 1.87 24.5
VPA 163 417 0.33 32.3
VPS 174 480 0.29 43.5
Tx% was the temperature at x% weight loss of the samples. Tmax was the maximum rate
of weight loss. Residue was the yield at 600 °C.
Figure 5-20: TG (left) and DTG (right) plots of VP, VPA and VPS
The decomposition process is changed from one step to multiple steps. The rate
of weight loss from 300 °C to 500 °C decreases significantly. This suggests the
further modification of VP leads to the formation of a protection layer. In the case
of VPA, based on the chemical structure, the char layer generated by the first
Results and Discussion 73
decomposition step consists mainly of carbon. In the range from 300 °C to
500 °C, the weight of the sample was reduced from 85% to 38%, and the char
residue at 600 °C was 32.3 wt.-%. In case of VPS, it can be seen that the first
decomposition step was similar to that of VPA. However, the rate of weight loss
in further steps was lower than that of VPA. In the range from 300 °C to 500 °C,
the weight of the sample was reduced from 87% to 49%. The reason could be
that a ceramic layer consisting of Si-O-Si structure was formed and could
provide better protection than char layer of VPA. Furthermore, due to the
presence of the ceramic layer, VPS showed a char residue of 43.5 wt.-% at
600 °C.
Figure 5-21: TG (left) and DTG (right) plots of VPA and VPS based PLA composites
TG and DTG plots of VPA and VPS based PLA composites are shown in Figure
5-21. The detail data are summarized in Table 5-15. It can be observed that the
thermal degradation process of PLA composites occurred mainly in the
temperature range from 300 °C to 400 °C. With increasing concentration of VPA
and VPS, this process shifted to a higher temperature.
In the case of PLA/VPA composites, the T5% value of PLA5VPA and PLA10VPA
was 318 and 308 °C, the T20% value was 341 and 336 °C, respectively.
Furthermore, the Tmax value of amounted to 363 and 361 °C. It can be concluded
that the thermal stability of PLA/VPA composites was lower than pure PLA.
Results and Discussion 74
Based on the TGA results of PLA/VP composites, the main reason was amine
generated from decomposition of carbon-nitrogen double bond. It accelerated
the decomposition of PLA and overpowered the stabilization effect of vanillin
segment. However, it contributed to the char yield. The residue at 600 °C of
PLA5VPA and PLA10VPA varied to 2.5 and 4.6 wt.-%. It was in good agreement
with the residue of pure VPA and its concentration in composites, indicating that
VPA didn’t induce the char formation of PLA.
Table 5-15: Data obtained from TGA plots of VPA and VPS based PLA composites
Sample T5% (°C) T20% (°C) Tmax (°C) Residue (wt-%)
PLA 326 344 363 0
PLA5VPA 318 341 363 2.5
PLA10VPA 308 336 361 4.6
PLA5VPS 329 349 370 2.8
PLA10VPS 322 348 372 4.8
Tx% was the temperature at x% weight loss of the samples. Tmax was the temperature at
the maximum rate of weight loss. Residue was the yield at 600 °C.
In the case of PLA/VPS composites, the T5% value of PLA5VPS and PLA10VPS
was 329 and 322 °C, and the T20% value was 349 and 348 °C, respectively. This
suggested that VPS decomposed earlier than PLA. The first decomposition step
of VPS caused quick weight loss, so at a lower temperature, the higher the
concentration of VPS, the faster the samples loss its weight. As discussed in the
previous subchapter, the vanillin segment can reduce the thermal degradation
rate of PLA. Therefore, at a higher temperature, when the vanillin segments
were released, they depressed the thermal degradation of PLA. With rising VPS
concentration, increased the Tmax value of the composites. In addition, a ceramic
condensed layer of SiO2 could also protect the composite from heat and lower
the thermal degradation. Consequently, PLA5VPS and PLA10VPS showed a
Results and Discussion 75
Tmax value of 370 and 372 °C, respectively.
5.2.3 Flammability
The flammability of PLA/VPA and PLA/VPS composites was evaluated by LOI
and UL94 measurements. The results are listed in Table 5-16. It can be
observed that all the composites based on VPA and VPS passed V-0
classification in the UL-94 test. The LOI value of PLA5VPA, PLA10VPA,
PLA5VSP and PLA10VPS amounted to 29.8%, 30.5%, 24.5% and 25.0%,
respectively. Compared to PLA/VP composites, at the same concentration, the
LOI values of PLA/VPA composites increased significantly; by contrast, these of
PLA/VPS composites decreased slightly. This can be explained by the change of
melt flow rate at a high temperature of the composites. It is well known that, for
polymeric materials in a fire, dripping and flowing of the molten polymer can
enormously take the heat and degraded products away, so that the difficulty of
ignition or maintenance of burning of the material is elevated. In the case of the
LOI test, a higher concentration of oxygen is necessary to ignite the sample.
Table 5-16: LOI, UL94 and MFI results of PLA composites based on vanillin derivatives
Sample LOI (%) UL-94 Rating * MFI (g/10min) Ignition of cotton
PLA 21.4 No rating 6.0 Yes
PLA5VP 25.8 V-0 15.9 No
PLA10VP 26.3 V-0 59.5 No
PLA5VPA 29.8 V-0 61.0 No
PLA10VPA 30.5 V-0 109.2 No
PLA5VPS 24.5 V-0 7.2 No
PLA10VPS 25.0 V-0 9.6 No
* Sample thickness of UL-94 tests was 3.2mm
It can be concluded that the LOI and UL94 results of PLA/VPA and PLA/VPS
composites were affected by two factors. The first one is the MFI of materials.
Results and Discussion 76
Material with higher MFI drips faster and takes more heat away. So higher MFI
altered the flame retardancy in a positive direction. The second one is the ratio of
effective component in flame retardant. As discussed in the previous subchapter,
the phosphor-containing segment is most responsible to the enhanced flame
retardancy of PLA composites. However, compared to VP, the ratio of the
phosphor-containing segment in VPA and VPS decreased. So the modification
of VP altered the flame retardancy to the negative direction. In the case of
PLA/VPA composites, the extremely high MFI overpower the negative effect. So
the LOI values of these samples were significantly improved. In contrary, for
PLA/VPS composites, the LOI values decreased due to the negative effect.
5.2.4 Combustion Behavior
The curves obtained via cone calorimeter test are shown in Figure 5-22 (HRR)
and Figure 5-23 (THR and CO/CO2 ratio), the detail parameters are listed in
Table 5-17.
Table 5-17: Data obtained from CCT of PLA and PLA composites
Sample TTI
(s)
pHRR
(kW/m2)
THR
(MJ/m2)
Residue
(%)
PLA 68±3 407±16 66.0±0.9 0.5±0.2
PLA5VPA 70±5 406±17 62.1±1.3 0.8±0.1
PLA10VPA 68±6 478±74 62.4±0.8 2.6±0.5
PLA5VPS 62±3 397±27 63.0±0.2 7.7±1.0
PLA10VPS 68±9 417±18 64.0±0.5 5.6±0.8
The main purpose of further modification of VP was to introduce a protective
layer by condensation of Si-derivatives and thus improve the char yield of the
PLA composites, expecting it can form a ceramic layer and protect the
Results and Discussion 77
underlying material. Based on Table 5-17, the char residue of PLA5VPA,
PLA10VPA, PLA5VPS and PLA10 VPS was 0.8%, 2.6%, 7.7% and 5.6%,
respectively. The char yield of PLA composites was higher than that of neat PLA.
However, the heat release could not- be reduced. This result implied the char
layer was not sufficient and stable to protect the underlying material from heat.
Figure 5-22: HRR plots of PLA/VPA and PLA/VPS composites
Interestingly, it is observed that the pHRR values of PLA/VPA and PLA/VPS
composites increased with rising VPA or VPS concentration. In detail, the pHRR
of PLA5VPA, PLA10VPA, PLA5VPS and PLA10VPS were 406, 478, 397 and
417 kW/m2, respectively. The pHHR values of samples with 10 wt.-% of vanillin
derivative were even higher than that of pure PLA. Based on the THR curves, it
was detected, at the same time, the heat release of PLA composites was higher
than that of pure PLA, while the total heat release was lower. It could be also
observed from Figure 5-23, there was no notable difference of the first peaks of
CO/CO2 ratio between PLA and PLA composites. The reason for these results
could be inferred as follow. On one hand, the high release of CO was due to
incomplete combustion of the polymer. In this work, it was mainly caused by the
radical capture effect of the phosphor-containing segment. In comparison with
Results and Discussion 78
VP, the ratio of phosphor in VPA and VPS was lower. Hence, VPA and VPS were
less efficient to suppress the combustion of PLA. On the other hand, the
carbon-nitrogen double bond in the structure of VPA and VPS were relatively
less stable to heat. Combustible products could be released by its
decomposition process, which could accelerate the ignition or/and the
combustion of the composites. Consequently, the pHRR of PLA became higher
when the loading of VPA or VPS increased.
Figure 5-23: THR (left) and CO/CO2 ratio (right) plots of PLA/VPA and PLA/VPS composites
5.2.5 Mechanical Properties
The stress-strain curves of PLA/VPA and PLA/VPS Composites, as well as pure
PLA, are shown in Figure 5-24, and the parameters of mechanical properties are
summarized in Table 5-18. PLA is a highly rigid thermoplastic with a tensile
strength of 57 MPa, a Young’s modulus of 3700 MPa and an elongation at break
of 3.0 %. After the introduction of VPA and VPS, all the samples showed lower
elongation at break than pure PLA and broke without any yield behavior. The
composites could sustain hardly any deformation at all. The tensile strength
decreased with rising filler loading, while the Young’s modulus shows totally
opposite trend. The tensile strength of PLA10VPA and PLA10VPS was 40 and
51 MPa, while the Young’s modulus was 3500 and 3400 MPa, respectively. The
reason for that could be, some of the crosslinkable groups reacted with other
Results and Discussion 79
molecular and limited the molecular mobility of PLA.
Table 5-18: Data obtained from tensile test of PLA and PLA composites
Sample Young’s
modulus
(MPa)
Tensile
strength
(MPa)
Elongation
at break
(%)
Impact
resistance
(kJ/m2)
PLA 3700±100 57±1 3.0±0.6 16±2
PLA5VPA 3400±100 45±1 1.4±0.1 14±1
PLA10VPA 3500±100 40±4 1.5±0.1 12±1
PLA5VPS 3300±300 57±3 2.0±0.2 18±2
PLA10VPS 3400±200 51±2 1.7±0.1 15±3
Figure 5-24: Tensile test plots of PLA and PLA composites
5.2.6 Conclusion
In this part, two further modification of VP were realized. The products, VPA and
VPS, were characterized via 1H NMR, FTIR and TGA, and mixed with PLA via
plasticorder. The composites showed good performance in small-scale
flammability test (LOI and UL94). In addition, the char yield of the composites in
Results and Discussion 80
the cone calorimeter test was significantly improved, e.g. PLA5VPS showed a
residue of 7.7% compared to 0.5% of neat PLA. However, the pHRR value of
composites increased with rising filler loadings. When PLA containing 10 wt.-%
of VPA or VPS, the pHRR value of material was higher than that of pure PLA.
The reason for that could be, the decomposition of VPA and VPS released
combustible products and accelerated the combustion process. Moreover,
despite the Young’s moduli and tensile strength remained at an acceptable level,
the composites could hardly sustain deformation. It strongly limited the
application of these materials. Hence, other methods to enhance the
anti-dripping properties of PLA need to be found. Addition of inorganic filler into
PLA would be a promising way. It will be discussed in the next subchapter.
5.3 PLA/VP Composite with Conventional Flame Retardants
In the previous subchapter, PLA/VP composites showed promising performance
both on flame retardancy and mechanical properties. Furthermore, two modified
VP, VPA and VPS, were proved to be able to enhance the char yield of PLA
during the combustion. The results showed that the char layers generated by the
VPA and VPS were improved but insufficient to hinder the combustion of PLA.
Hence, using conventional flame retardant, which could catalyze char forming, is
a reasonable choice.
VA and VP were mixed with PLA via plasticorder according to the recipe shown
in Table 5-1 and then injection molded to prepare different samples with a
suitable dimension for different measurements. The morphology of the sample
was first investigated using SEM with EDX. Then, the thermal degradation
behaviors were studied via TGA and DSC. Afterwards, the influence of VA and
VP on the mechanical properties of PLA were determined. Finally, the flame
retardant properties of PLA composites and the mechanism were investigated
via LOI, UL-94 V, CCT and TG-FTIR.
Results and Discussion 81
5.3.1 PLA/VP Composite with Montmorillonite
Montmorillonite (MMT), also known as clay, is a mineral silicate with two
dimensional layered structure, which is used in various application areas,
including as flame retardant for plastics. In addition, polymer nanocomposites
based on MMT have shown remarkable improvement compared to pure
polymers, such as rigidity, crystallinity and so on. Furthermore, as a member of
the smectite family, MMT is literally powdered rock. It has no environmental
pollution or/and impact to nature and doesn’t destroy the biodegradability of the
biodegradable polymer when MMT is incorporated in polymers. On the other
hand, owing to the inorganic structure of pristine MMT, its compatibility with the
polymer is relatively low. Hence, the MMT used in this work, with the trade name
“Cloisite 30B”, is organically modified with methyl, tallow, bis-2-hydroxyethyl,
quaternary ammonium chloride, to enhance the compatibility with PLA. The
composites were prepared via a mini extruder according to the recipe in Table
5-19. The morphology of composites was observed using SEM. The thermal
degradation behaviors were studied via TGA and DSC. The flame retardant
properties and the mechanism were investigated via LOI, UL-94 and CCT. And
the mechanical properties were determined by tensile test.
Table 5-19: Recipe of PLA/VP/MMT composites
Sample PLA (wt.-%) mPLA* (wt.-%) MMT (wt.-%)
PLA5MMT 95 0 5
PLA10MMT 90 0 10
mPLA5MMT 0 95 5
mPLA10MMT 0 90 10
*mPLA was PLA/VP composite which contained 10 wt.-% VP
5.3.1.1 Morphology
The cross section of the tensile bars of PLA10MMT and mPLA10MMT were
Results and Discussion 82
observed using SEM, and the obtained micrographs are shown in Figure 5-25. It
was found that PLA/MMT has a relatively smooth surface with MMT
agglomerations randomly embedded inside. By contrast, the dispersion of MMT
in mPLA was better and the particle size was smaller. It is interesting to find that,
the amount of MMT in mPLA/MMT composite of this individual cross section
seems to be higher than that in PLA/MMT composite. Considering the smaller
particle size and the more homogenous dispersion, it could be concluded that,
the miscibility of MMT in mPLA was better than that in pure PLA.
Figure 5-25: SEM pictures of PLA/MMT and mPLA/MMT composites
5.3.1.2 Thermal Decomposition Properties
TG and DTG plots of PLA/MMT and mPLA/MMT composites are shown in
Figure 5-26 and the resulting data are summarized in Table 5-20. The T5% values
of PLA/MMT and mPLA/MMT composites decreased with increasing MMT
concentration. The decrements of mPLA/MMT composites were 39 °C and
45 °C compared to those of PLA/MMT composites when the concentration of
MMT at 5 wt.-% and 10 wt.-%. And T5% values of all composites were lower
compared to that of pure PLA. This could be explained by that, the organic
modifier of MMT is less thermally stable than PLA, and the presence of VP
further accelerated the decomposition progress.
Results and Discussion 83
Table 5-20: Data obtained from TGA plots of PLA and PLA composites
Sample T5% (°C) T20% (°C) Tmax (°C) Residue (wt-%)
PLA 326 344 363 0
PLA5MMT 324 346 366 2.9
PLA10MMT 318 347 366 5.9
mPLA5MMT 285 351 371 5.9
mPLA10MMT 273 351 370 7.9
Tx% was the temperature at x% weight loss of the samples. Tmax was the temperature at
the maximum rate of weight loss. Residue was the yield at 600 °C.
Figure 5-26: TG (left) and DTG (right) plots of PLA and PLA composites
It is known well that, the incorporation of clay into the polymer can enhance
overall thermal properties due to the superior heat insulation and mass transport
barrier effect. [107, 220] On the other hand, based on the result, the
concentration difference of MMT didn’t show notable influence on the T20% and
Tmax values of the composites. The T20% values of mPLA5MMT and
mPLA10MMT both amounted to 351 °C and Tmax values were 371 °C and
370 °C, respectively. The T20% values of PLA5MMT and PLA10MMT were
346 °C and 347 °C and Tmax values both amounted to 366 °C. The results
indicated that low concentration of MMT was already sufficient to act as an
insulator, and a small amount of VP could further suppress the thermal
Results and Discussion 84
decomposition of PLA, which came to a good agreement with the conclusion of
the previous results (Chapter 5.1.3). In addition, VP seemed to be able to
enhance the char yield of the PLA composites.
5.3.1.3 Thermal Crystallization Behavior
Figure 5-27 shows the DSC plots of PLA/MMT and mPLA/MMT composites, and
the corresponding data are represented in Table 5-21. The corporation of MMT
into MMT led to a slight decrement of Tg. It could be explained by the fact that
MMT could cause degradation of PLA, and PLA with lower molecular weight
showed correspondingly lower Tg. Different from Tg, the influence of MMT on Tcc
was more complicated. It has been clarified by various researches that MMT
could promote the nucleation of the polymer at low loading. However, when the
loading of filler increases, agglomeration will form, which could limit the mobility
of the molecular chain. For the PLA/MMT composites, the cold crystallization
peak became from sharp to broader with increasing MMT loading. The melting
enthalpy of the second melting phase also declined with rising loading. In the
case of PLA10MMT, there was only one melting peak left, which corresponded
to the less perfect crystalline structure. In, summary, despite the fact that higher
MMT loading increased the crystallinity of PLA, the aggregates still prevent PLA
from forming perfect crystallization.
Table 5-21: Data obtained from DSC plots of PLA and PLA composites
Sample Tg
(°C)
Tcc
(°C)
Tm1
(°C)
Tm2
(°C)
∆Hcc
(J/g)
∆Hm
(J/g)
Χc (%)
PLA 60.6 113.7 160.1 166.0 33.00 33.08 35.5
PLA5MMT 60.0 112.3 159.8 165.2 32.61 32.83 37.1
PLA10MMT 59.6 121.4 161.2 -- 32.91 33.71 40.2
mPLA5MMT 54.4 110.3 153.6 162.2 31.70 32.49 40.8
mPLA10MMT 53.9 106.9 153.0 162.2 28.24 31.18 41.3
Results and Discussion 85
In comparison, for mPLA/MMT composites, the increasing of MMT loading didn’t
have a notable effect on Tg and Tm of the matrix polymer. However, it did promote
the cold crystallization behavior, as evidenced by the decrement in Tcc and ∆Hcc
values. As discussed in the previous chapter, VP could function as a plasticizer
and nucleating agent for PLA, the lubrication and nucleating effect may
overpower the negative influence caused by the aggregates of MMT.
Furthermore, the appearance of VP could improve the dispersion of MMT in PLA
and reduce the forming of aggregates. Hence, for the samples mPLA5MMT and
mPLA10MMT, the bimodal melting peaks appeared at the same temperature
with a similar sweep, while Tg and Tm decreased with rising MMT loading.
Figure 5-27: DSC plots of PLA and PLA composites
5.3.1.4 Flammability
The results of LOI, UL94 and MFI tests of PLA/VP/MMT composites are listed in
Table 5-22. The LOI value of PLA5MMT and PLA10MMT was 20.9% and 23.6%,
while that of mPLA5MMT and mPLA10MMT amounted to 26.7% and 26.1%.
Results and Discussion 86
Table 5-22: Results of LOI, UL94 and MFI tests of PLA and PLA composites
Sample LOI (%) UL-94 Rating * MFI (g/10min) Ignition of
cotton
PLA 21.4 No rating 6.0 Yes
PLA5MMT 20.9 No rating 2.9 Yes
PLA10MMT 23.6 No rating 1.9 Yes
mPLA5MMT 26.7 V-0 21.7 No
mPLA10MMT 26.1 V-0 15.8 No
MMT is widely used as a flame retardant for polymeric materials, especially as
anti-dripping and char agent. Based on the MFI results, it could be found that the
incorporation of MMT could reduce the melting flow rate of PLA. Despite MMT
could introduce protection char layer into PLA, it also prevented the taking away
of heat by melting flow of polymer. Hence, at low loading (5 wt.-%), the sample
showed lower LOI value than pure PLA, while at higher loading (10 wt.-%), more
protection char layer was formed, and the LOI value increased. However,
regardless of the anti-dripping effect, both PLA5MMT and PLA10MMT presented
no rating in the UL-94 test and were burned out faster to the clamp compared to
PLA. Large pieces of burning materials were observed, and the cotton beneath
sample was ignited.
In the case of mPLA/MMT composites, the melting flowability of materials, which
was most responsible for the enhanced flame retardancy, was significantly
improved by the introduction of VP. The MFI of samples changed from 6.0
g/10min of PLA to 21.7 g/10min of mPLA5MMT and 15.8 g/10min of
mPLA10MMT, respectively. Higher MMT loading contributed to higher restriction
of dripping. Thus, the LOI value decreased with rising MMT content. However,
both mPLA5MMT and mPLA10MMT passed V-0 in UL-94 test, indicating
Results and Discussion 87
dramatic improvement on flame retardancy comparing to PLA/MMT composites.
5.3.1.5 Combustion Behavior
The combustion behavior of mPLA/MMT composites was evaluated utilizing a
cone calorimeter test and compared with that of PLA/MMT composites. The
important parameters obtained are listed in Table 5-23. The HRR plots of the
composites are represented in Figure 5-28, and curves of other major
parameters are shown in Figure 5-29. Comparing to PLA, the TTI of PLA5MMT
and PLA10MMT was earlier, from 68 s of PLA to 66 s and 63 s, respectively. TTI
of mPLA/MMT was also slightly lowered with rising MMT loading. The decreased
melting flowability caused by MMT should be responsible for that. Without the
protection of thermal stable structure, such as a stable char layer, the lower the
flowability, the faster the polymer was ignited.
Table 5-23: Data obtained from CCT of PLA/VP/MMT composites
Sample TTI
(s)
pHRR
(kW/m2)
THR
(MJ/m2)
TSP
(m2)
Residue
(%)
PLA 68±3 407±16 66.0±0.9 0.32±0.17 0.5±0.2
PLA5MMT 66±5 312±1 64.2±0.9 0.42±0.02 4.7±0.7
PLA10MMT 63±2 276±1 64.1±1.2 0.52±0.01 8.5±1.1
mPLA5MMT 67±3 383±11 64.4±1.6 2.57±0.02 3.7±0.9
mPLA10MMT 66±6 383±9 64.0±0.4 2.67±0.05 6.5±0.5
Based on Figure 5-28, it can be seen that the pHRR value of PLA5MMT,
PLA10MMT, mPLA5MMT and mPLA10MMT was reduced from 407 kW/m2 of
PLA to 312, 276, 383 and 383 kW/m2, respectively. The HHR curves of
PLA/MMT composites showed after the first peak broad plateau, suggesting
protection char layer was formed and the heat released rate was
correspondingly suppressed. In comparison, the HRR curves of mPLA/MMT
Results and Discussion 88
composites processed similar shape as PLA. This could be explained by the fact
that the presence of VP has significantly improved the melting flowability of PLA,
which elevate the difficulty to form char layer during the combustion of samples.
The curves of THR and mass loss of PLA composites are shown in Figure 5-29
(a) and (b). It could be found that the THR of PLA composites was only slightly
decreased compared to PLA. However, the shape of curves of PLA/MMT
composites indicated that in the interval between 150 s and 300 s, the heat
release was significantly declined, and the reduction increased with higher MMT
loading. Parallel to THR, the curves of mass loss of PLA/MMT composites
showed a similar pattern. For mPLA/MMT composites, comparing to PLA, the
differences in THR and mass loss were negligible. Interestingly, at same MMT
loading, the char residue was reduced after the introduction of VP into PLA/MMT
composites. This was also due to the enhanced melting flow rate. Char layers
were destroyed by the flowing of the molten polymer before stable protection
layer was formed.
Figure 5-28: HRR plots of PLA/VP/MMT composites
Figure 5-29 (c) and (d) show the curves of TSP and CO/CO2 ratio of PLA
composites, which give the information of the gaseous products evolved during
Results and Discussion 89
the combustion of samples. The TSP was increased with rising MMT loading,
which was due to the organic modifier in MMT. And the incorporation of VP was
responsible for the further dramatic improvement. The mechanism was already
discussed in the previous chapter. Another interesting difference between
PLA/MMT and mPLA/MMT composites was found in the curves of the CO/CO2
ratio. The first sharp peak corresponded to the ignition of samples. After that, for
PLA/MMT composites, the ratios were kept at a low level until the end of the
tests; in contrary, for mPLA/MMT composites, a second mountain like peak
appeared. This should be due to the release of the phosphor-containing
component evolved by the decomposition of VP and led to incomplete
combustion of PLA.
Figure 5-29: THR (a), Mass loss (b), TSP (c) and CO/CO2 ratio (d) plots of PLA composites
Results and Discussion 90
5.3.1.6 Possible Flame Retardant Mechanism
The digital photos and SEM pictures of the residue of samples after cone
calorimeter tests are shown in Figure 5-30 and Figure 5-31 respectively. A
notable amount of char can be observed. The quality of the char layer was
improved with increasing MMT loading both in PLA/MMT and mPLA/MMT
composites. PLA/MMT composites left thicker char with separated domains,
whereas the residue of mPLA/MMT composites was much less but cohesive.
Figure 5-30: Residue of samples after cone calorimeter test of PLA/MMT composites (top)
and mPLA/MMT composites (bottom) with 5 wt.-% (left) and 10 wt.-%(right) MMT
As observed in the SEM pictures, the residue of mPLA/MMT composites showed
foam-like microstructure with many pores, while that of PLA/MMT composites
was more compact. It was reported that the flame retardant mechanism of MMT
in PLA was mainly as a physical barrier. By increasing the melt stability of PLA,
the presence of MMT promoted the formation of char. However, the introduction
of VP improved the melt flow rate of the composites, allowing the gaseous
product evolved by combustion to escape easier from the melt, as evidenced by
Results and Discussion 91
the pores observed in SEM. Hence, the barrier effect of char was reduced.
Figure 5-31: SEM pictures of char layer after cone calorimeter test of PLA10MMT (top) and
mPLA10MMT (bottom) for surface (left) and inner (right) residue
5.3.1.7 Mechanical Properties
Figure 5-32 shows the stress vs strain curves of PLA and PLA composites, and
the primary parameters are listed in Table 5-24. The tensile strength of
PLA5MMT, PLA10MMT, mPLA5MMT and mPLA10MMT were 55, 57, 40 and 39
MPa, respectively. Their Young’s modulus was 4400, 4600, 3600 and 3800 MPa,
respectively.
Various researches have focused on the mechanical properties of clay
reinforced PLA composites. As reported, the incorporation of clay leads to
obvious improvement of Young’s modulus because the rigidity of inorganic fillers
is normally much higher than that of polymeric materials. Furthermore, due to
the high specific surface area of the nano-size particles and also the existence of
organic modifier, the adhesion between clay and matrix can be kept at a
relatively good level. Thus, for PLA/MMT composites, the tensile strength didn’t
Results and Discussion 92
decrease a lot. In the case of mPLA/MMT composites, antagonistic effects are
present. Tensile strength and Young’s modulus of mPLA were lowered due to the
plasticization effect of VP. However, the addition of MMT elevated again the
Young’s moduli of the mPLA/MMT composites.
Table 5-24: Data obtained from tensile tests of PLA/VP/MMT composites
Sample Young’s
modulus
(MPa)
Tensile
strength
(MPa)
Elongation
at break
(%)
Impact
resistance
(kJ/m2)
PLA 3300±100 61±1 6.0±2.1 16±1
PLA5MMT 4400±100 55±1 1.7±0.4. 16±1
PLA10MMT 4600±100 57±1 1.5±0.1. 14±2
mPLA5MMT 3600±100 40±1 4.9±0.4 19±4
mPLA10MMT 3800±100 39±1 4.3±0.6 13±2
Figure 5-32: Tensile test plots of PLA/VP/MMT composites
On the other hand, an unavoidable fact is the incorporation of inorganic fillers
apparently led to the deterioration of poor ductility of PLA. The elongation at
Results and Discussion 93
break declined from 6.0 % of PLA to 1.7% and 1.5% of PLA5MMT and
PLA10MMT, respectively. Interestingly, the elongation at break of mPLA5MMT
and mPLA10MMT was reduced to only 4.9% and 4.3%. A possible explanation
was that there was some interaction between MMT and PLA, which made the
material so rigid that the lubricant effect of VP could not enhance the chain
mobility of PLA that obvious anymore. Nevertheless, the improvement in
elongation at break was still notable when comparing to PLA/MMT composites.
5.3.2 PLA/VP Composite with APP
Ammonium polyphosphate (APP), with the chemical formula [NH4PO3]n(OH)2, is
an inorganic ammonia salt of polyphosphoric acid. Nowadays, APP is mostly
commercially produced and used as a flame retardant for polymeric materials,
where it performs as part of intumescent flame retardant (IFR). In addition, APP
consists of the elements of N, P, O and H, it can be also used as fertilizer. Hence,
the incorporation of APP into PLA will not destroy the biodegradability of material.
The composites were prepared by means of extrusion according to the recipe in
Table 5-25.
Table 5-25: Recipe of PLA/VP/APP composites
Sample PLA (wt.-%) mPLA
(wt.-%)
APP (wt.-%)
PLA5APP 95 0 5
PLA10APP 90 0 10
PLA20APP 80 0 20
mPLA5APP 0 95 5
mPLA10APP 0 90 10
mPLA20APP 0 80 20
*mPLA was PLA/VP composite which contained 10 wt.-% VP
Results and Discussion 94
The morphology of composites was characterized using SEM. The thermal
degradation behaviors were determined using TGA and DSC. The flame
retardant properties and the mechanism were investigated via LOI, UL-94 and
CCT. And the mechanical properties were evaluated by the tensile test.
5.3.2.1 Morphology
The cross section of the tensile bars of PLA20APP and mPLA20APP were
observed utilizing SEM, and the obtained micrographs are shown in Figure 5-33.
It was found that for both samples, APP has well dispersed in PLA matrix, taking
into account the size of APP of D50 = 5 µm. By contrast, in the same size of
section, there were more APP particles in mPLA20APP, despite the particle size
of APP in both PLA composites were similar, possibly indicating that the
dispersion of APP in mPLA20APP was better. Furthermore, the cross section of
APP particle in PLA20APP and mPLA20APP showed an interesting difference.
The cross section of APP in mPLA20APP was smoother. It could be contributed
to that during the thermal processing APP has reacted with VP.
Figure 5-33: SEM pictures PLA/APP and mPLA/APP composites
5.3.2.2 Thermal Decomposition Properties
TG and DTG curves of PLA and PLA composites are represented in Figure 5-34
and the corresponding parameters are summarized in Table 5-26. Different
decomposition phases are characterized by T5%, T20% and Tmax.
Results and Discussion 95
Table 5-26: Data obtained from TGA plots of PLA/VP/APP composites
Sample T5% (°C) T20% (°C) Tmax (°C) Residue (wt-%)
PLA 326 344 363 0
PLA5APP 332 350 367 3.7
PLA10APP 334 350 368 4.7
PLA20APP 332 348 366 6.4
mPLA5APP 288 330 368 5.1
mPLA10APP 288 329 364 6.9
mPLA20APP 292 330 364 7.8
Tx% was the temperature at x% weight loss of the samples. Tmax was the temperature at
the maximum rate of weight loss. Residue was the yield at 600 °C.
As shown in Table 5-26, the addition of APP slightly improved the thermal
stability of PLA, as reported in many results, this was contributed to its inorganic
nature and higher thermal stability than PLA.[221] However, the decomposition
of APP could lead to release of phosphoric acid and catalyze the thermal
degradation of PLA. There was a critical loading of APP, above which the
catalytic effect would overpower the improvement. For PLA/APP composites in
this study, the critical loading was possibly the critical concentration. T5%, T20%
and Tmax values decreased slightly when the concentration rose to 20%. In
contrast, the char residue at 600 °C increased continuously with rising APP
loading.
For mPLA/APP composites, the thermal decomposition process consisted of
four stages. Comparing the results of the previous study (subchapter-5.1.3), it
can be suggested that the first stage in the range between 200 and 300 °C could
be related to the reaction between VP and APP, and the second stage between
300 and 350 °C could be due to the decomposition of VP structure. The third
stage, which showed the maximal decomposition rate, should be corresponding
Results and Discussion 96
to the decomposition of the PLA matrix under the catalytic effect of APP. This
stage was also the most responsible to the char formation. In the fourth stage
between 400 and 500 °C, unstable char was decomposed. The loading of APP
didn’t have a notable influence to the first three stages. However, the fourth
stage showed a strong dependency on APP concentration. Furthermore, at
same APP loading, the char yields of mPLA/APP composites were higher than
that of PLA/APP composites, indicating a synergic effect between APP and VP.
Figure 5-34: TG (left) and DTG (right) plots of PLA/VP/APP composites
5.3.2.3 Thermal Crystallization Behavior
The DSC curves of PLA/APP and mPLA/APP are illustrated in Figure 5-35 and
Figure 5-36, the corresponding parameters are listed in Table 5-27. For
PLA/APP composites, there was no remarkable change in Tg value observed
after the incorporation of APP into PLA. For PLA5APP and PLA10APP, the cold
crystallization capability seemed to be lower than that of PLA. The Tcc valued
shifted to a higher temperature and the peaks became broader. In addition, there
was only one melting peak corresponding to less perfect crystallization structure.
Comparing PLA20APP with PLA10APP, the Tcc valued shifted to lower direction
with sharper cold crystallization peak, and the second melting peak for a
structure with higher perfection appeared again. Furthermore, the crystallinity of
samples increased to the maximum with 20% APP loading. All the results
Results and Discussion 97
suggested that the crystallization capability of PLA was enhanced with
increasing APP loading.
Table 5-27: Data obtained from DSC plots of PLA and PLA composites
Sample Tg
(°C)
Tcc
(°C)
Tm1
(°C)
Tm2
(°C)
∆Hcc
(J/g)
∆Hm
(J/g)
Χc (%)
PLA 60.6 113.7 160.1 166.0 33.00 33.08 35.5
PLA5APP 60.2 123.9 161.6 -- 34.15 34.18 38.6
PLA10APP 60.2 125.0 161.9 -- 33.00 33.37 39.8
PLA20APP 59.8 114.7 160.1 165.8 30.98 32.38 43.5
mPLA5APP 55.9 111.3 155.8 163.6 30.93 31.94 40.4
mPLA10APP 55.8 113.0 156.4 163.8 29.24 31.42 42.2
mPLA20APP 57.4 115.4 158.0 164.8 27.03 28.12 43.1
Figure 5-35: DSC plots of PLA/APP composites
Comparing with PLA, the Tg of mPLA5APP was about 5 °C lower due to the
plasticization effect of VP. When the APP loading increased, the Tg value was
altered to the higher direction, indicating that the chain mobility of PLA was
Results and Discussion 98
limited by the APP particles. In parallel with the change in Tg, Tcc and Tm also
increased gradually with rising APP loading, and the melting peak corresponding
to higher perfection structure shifted down at the same time. All the results
suggested a reduced crystallization capability. A possible explanation could be
that with increasing APP loading, more and more VP reacted with APP and lose
its lubricant effect. These deductions were in good agreement with the TGA
results.
Figure 5-36: DSC plots of mPLA/APP composites
5.3.2.4 Flammability
The LOI, UL-94 and MFI results of PLA, PLA/APP and mPLA/APP composites
are listed Table 5-28. It was observed that the addition of VP into PLA could
remarkably improve the flame retardancy of PLA. The LOI values of PLA5APP,
PLA10APP and PLA20APP amounted to 29.6%, 30.4% and 31.3%. And with the
existence of VP, LOI values of mPLA5APP, mPLA10APP and mPLA20APP were
further improved to 32.1%, 34.6% and 40.7%. By careful analysis, the LOI
values of mPLA/APP showed greater loading dependency of APP. This
suggested a synergized action between VP and APP. The MFI of both
composites systems declined when the loading of APP increased from 5 % to
Results and Discussion 99
Table 5-28: Results of LOI and UL-94 tests of PLA/VP/APP composites
Sample LOI (%) UL-94 Rating * MFI (g/10min) Ignition of cotton
PLA 21.4 No rating 6.0 Yes
PLA5APP 29.6 V-0 5.5 No
PLA10APP 30.4 V-0 5.2 No
PLA20APP 31.1 V-0 6.5 No
mPLA5APP 32.1 V-0 32.8 No
mPLA10APP 34.6 V-0 24.4 No
mPLA20APP 40.7 V-0 128.7 No
Figure 5-37: Pictures of samples after LOI test: (a) PLA5APP, (b) PLA10APP (c) PLA20APP
(d) mPLA5APP (e) mPLA10APP (f) mPLA20APP
10 %. It can be explained by the inorganic nature of APP. However, when the
loading further increased to 20%, the MFI values were again elevated.
Especially for the mPLA20APP, showed a huge increase from 24.4 g/10min of
mPLA10APP to 128.7 g/10min. Based on the results of TGA, this change could
be induced by thermal degradation of PLA. Another change observed in LOI
Results and Discussion 100
tests was the enhanced anti-dripping properties. As shown in Figure 5-37, the
PLA/APP composites still exhibited strong dripping behavior, even when the
loading of APP was at 20%. For mPLA/APP composites, the dripping behavior of
PLA was well hindered at 10% of APP loading, and there was almost no dripping
observed for the sample mPLA20APP, indicating that the combination of VP and
APP could contribute to char formation. In summary, the addition of VP in could
lead to further improved flame retardancy of PLA/APP composites.
5.3.2.5 Combustion Behavior
In order to understand the combustion behavior of PLA/APP and mPLA/APP
composites, cone calorimeter tests were carried out. The primary parameters
obtained in the tests are listed in Table 5-29. The HRR curves are represented in
Figure 5-38, and the THR, mass loss, TSP as well as CO/CO2 ratio are
illustrated in Figure 5-39. The TTI was slightly delayed after introducing of APP
due to its higher thermal stability than PLA. However, incorporation of VP into
PLA/APP composites led to earlier TTI. It could be due to the earlier start of
decomposition in combination with the rather high MFI value.
Table 5-29: Data obtained from CCT plots of PLA/VP/APP composites
Sample TTI
(s)
pHRR
(kW/m2)
THR
(MJ/m2)
TSP
(m2)
Residue
(%)
PLA 68±3 407±16 66.0±0.9 0.32±0.17 0.5±0.2
PLA10APP 71±5 357±13 58.4±1.0 1.92±0.02 9.8±1.2
PLA20APP 70±9 341±3 52.9±2.5 1.37±0.21 19.3±0.9
mPLA10APP 65±3 318±26 55.6±0.4 2.01±0.14 12.0±1.1
mPLA20APP 66±2 297±8 50.1±0.5 1.95±0.05 20.9±0.5
Results and Discussion 101
Figure 5-38: HRR plots of PLA/VP/APP composites
The pHRR of PLA10APP, PLA20APP, mPLA10APP and mPLA20APP were 357,
341, 318 and 297 kW/m2, respectively. Obviously, the incorporation of VP into
PLA/APP composites could further suppress the heat release of PLA during the
combustion. The curve shape of PLA/APP composites was similar to that of PLA
but with lowered pHRR. In comparison, for mPLA20APP, the HRR rose quickly
to pHRR and then went down rapidly. As reported in many investigations, during
combustion of polymeric materials containing APP, APP will decompose into
phosphoric acid and ammonia.[42, 162, 195] The phosphoric acid can react with
active groups to form unstable phosphate ester, which could turn into char upon
dehydration. A compact and expanded carbon foam can protect the polymeric
materials from fire by isolating the materials from heat and matter transfer. The
valley in the HRR curve of mPLA20APP suggested the formation of the
protective layer. As the sample further exposed to heat, the HRR increased
again. It was due to the fact that the established char layer was not stable
enough and was burned through as the test continued. Nevertheless, the heat
release rate was well suppressed by incorporating APP and VP together into
PLA.
Results and Discussion 102
THR is another parameter to evaluate the fire safety of the polymeric material. At
the end of the tests, the THR of PLA amounted to 66.0 MJ/m2. As the THR of
PLA10APP, PLA20APP, mPLA10APP and mPLA20APP declined to 58.4, 52.9,
55.6 and 50.1 MJ/m2 respectively. This indicated that the introduction of APP has
increased the incombustible component and also protected parts of the samples
from fire. Since the released heat was generated by the combustion of samples,
the study of mass loss of sample could also help to understand the combustion
process. It was observed, samples with 20 wt.-% APP loading presented as high
as 80%-100% more residue than those with 10 wt.-%. Whereas at the same
APP loading, the char yields of mPLA/APP composites were only increased by
1%-2% when compared to those of PLA/APP composites. The results revealed
that higher char quality of mPLA/APP composites was mainly responsible for the
lower THR values.
Figure 5-39: THR (a), Mass loss (b), TSP (c) and CO/CO2 ratio (d) plots of PLA/VP/MMT
composites
Results and Discussion 103
The information of gaseous products evolved during the combustion are
illustrated in Figure 5-39-(c) and –(d). It was observed that for PLA/APP
composites, low APP loading led to an increase in TSP due to the phosphorus
acid evolved by decomposition of APP. By contrast, at high APP loading,
charring occurred during the combustion, which exhibited smoke suppression
effect. As found in previous work, the incorporation of VP into PLA led to
significant improvement in TSP due to the incomplete combustion. In case of
mPLA/APP composites, incomplete combustion still existed, that was evidenced
by the continuous high level of CO/CO2 ratio shown in Figure 5-39-(d). However,
PLA10APP, mPLA10APP and mPLA20APP showed the same TSP values at the
end of combustion, suggesting the char layer with better smoke suppression
effect were formed in mPLA/APP composites.
5.3.2.6 Possible Flame Retardant Mechanism
The digital photos and SEM pictures of the residue of samples after cone
calorimeter tests are shown in Figure 5-40 and Figure 5-41 respectively. All the
samples showed expanded char layers which could act as a barrier to hinder the
transmission of fuel and oxygen. The amount and quality of residue increased
with rising APP loading. In fact, mPLA/APP composites showed more cohesive
char structure. For example, no obvious open hole was observed from the digital
photo of mPLA20APP. The microstructures of the outer and inner surface for
char residue were obtained to further understand the relationship between char
and flame retardancy of PLA composites. As observed, the inner structures of
both composites were similar. The outer surface of PLA20APP, however,
showed more pores than mPLA20APP. In addition, the amount of char residue of
mPLA/APP composites was higher than that of PLA/APP composites (Table
5-29), suggesting that there was synergic effect between VP and APP, which
contribute to the formation of char.
Results and Discussion 104
Figure 5-40: Residue of samples after cone calorimeter test of PLA/APP composites (top)
and mPLA/APP composites (bottom) with 10 wt.-% (left) and 20 wt.-%(right) APP
Figure 5-41: SEM pictures of residue after cone calorimeter test of PLA20APP (top) and
mPLA20APP (bottom) for surface (left) and inner (right)
Results and Discussion 105
5.3.2.7 Mechanical Properties
Figure 5-42 shows the stress vs strain curves of PLA and PLA composites, and
the primary parameters are listed in Table 5-30. Different from PLA/MMT
composites, the Young’s moduli of PLA/APP and mPLA/APP composites
decreased with rising APP loading. The higher the APP loading, the more
phosphorus acid would be generated during the thermal processing, the faster
the PLA would degrade.
On the other hand, it has been reported that APP demonstrated plasticization in
some polymeric matrix. This could be the explanation that the elongation at
break of PLA/APP composites was improved with rising APP loading, while the
tensile strength showed a declining trend. In addition, APP and VP seemed to
have synergic action on the plasticization effect. The sample mPLA20APP
exhibited an elongation at break as high as 29%, while its tensile strength was
kept at the same level as PLA20APP. This was a dramatic improvement for the
rigid PLA material.
Table 5-30: Data obtained from tensile tests plots of PLA/VP/APP composites
Sample Young’s
modulus
(MPa)
Tensile
strength (MPa)
Elongation
at break (%)
Impact
resistance
(kJ/m2)
PLA 3300±100 61±1 6±2 16±1
PLA5APP 3400±100 45±1 4±1 31±5
PLA10APP 3400±100 43±1 5±1 32±4
PLA20APP 3300±400 41±1 11±2 30±3
mPLA5APP 3500±100 42±1 6±1 21±4
mPLA10APP 3500±200 39±1 7±1 18±2
mPLA20APP 3300±100 37±2 29±3 20±3
Results and Discussion 106
Figure 5-42: Tensile test plots of PLA/VP/APP composites
5.3.3 Conclusion
In this subchapter, VP was introduced into two flame retardant PLA composites
systems: PLA/MMT and PLA/APP. For PLA/MMT composites, the introducing of
VP didn’t show improvement in the cone calorimeter test, but it did enhance the
flame retardancy in small scale tests like LOI and UL94. A remarkable
improvement was observed in ductility and processability of the composites. In
case of PLA/APP composites, introducing of VP led to significant improvement
of flame retardancy evidenced by increasing LOI values as well as better quality
of char residue acting as heat and matter barrier, which was the primary reason
for the decreased pHRR and THR values. Moreover, VP synergized with APP as
an excellent plasticizer for PLA.
Summary 107
6 Summary
The ultimate goal of this work was to develop environmentally friendly bio-based
functional polymer composites that would possess good flame retardancy and
fulfill the desired mechanical properties for conventional applications. PLA has
attracted more and more attention in the last decades due to its excellent
mechanical properties and is considered as a promising candidate to replace
petroleum-based plastics. But PLA does exhibit some disadvantage such as
high flammability and brittleness. In order to overcome those shortcomings, the
research has been done focused as follows:
The first motivation was to synthesize novel functional additives to improve the
flame retardancy and ductility of PLA. To execute that plan, phosphor-containing
compound was a suitable candidate, since many phosphor containing
compounds were highly efficient flame retardant or flame retardant intermediates.
Besides, some phosphor containing compounds had a lubricative effect when
used in a proper polymeric matrix. Vanillin, another bio-based resource, was
used as raw material of the functional additive. As consequence, VP was
successfully synthesized based on vanillin and phenylphosphonic dichloride.
The chemical structure of VP was confirmed by NMR and FTIR.
Next target was to use the synthesized VP as a flame retardant and plasticized
in PLA. Different processing methods, including melt blending, extrusion, hot
compression, injection molding and melt spinning were employed to process
PLA/VP composites. The thermal behavior, mechanical properties as well as
flame retardancy of composites were studied by various characterization
techniques.
The last goal was to prevent the dripping behavior of PLA during the combustion.
Two pathway, further chemical modification of VP and introduction of filler, were
suggested. By means of chemical modification, crosslinkable groups were
Summary 108
introduced into the structure of VP. VPA and VPS, were successfully synthesized.
By physical melt compounding, MMT and APP, two commercialized flame
retardant were incorporated into PLA/VP composites. The thermal behavior,
mechanical properties as well as flame retardancy of composites were evaluated
by various characterization techniques as well.
6.1 Synthesis and characterization of VP
Bis(5-formyl-2-methoxyphenyl) phenylphosphonate (VP) was synthesized based
on vanillin and phenylphosphonic dichloride. In the FTIR spectrum of VP, the
appearance of peaks assigned to P=O and P-O-C evidenced that vanillin was
successfully chemical linked to phenylphosphonic dichloride. Besides, the
results of 1H-, 13C- and 31P-NMR proved the high purity of the product. It was
observed via SEM that after the modification the surface of VP show higher
toughness and signal of the element phosphor was found by EDX. The melting
point of VP was determined by DSC. In comparison with vanillin, the thermal
stability of VP was significantly improved, allowing a processing temperature of
at least 280 °C.
6.2 Performance of PLA/VP composites
PLA composites with different loading of VP were prepared by means of melt
compounding and extrusion and then processed into tests specimen with
different dimensions utilizing hot compression, injection molding and melt
spinning. PLA and PLA composites based on vanillin were also prepared as
references.
The micromorphology, thermal behavior, flame retardancy, combustion behavior
and mechanical properties of selected samples were extensively investigated. In
comparison to vanillin, VP showed better miscibility in PLA. It was observed via
SEM that VP dispersed in PLA as nano-size particles. The thermal stability of
PLA was enhanced by the incorporation of VP, even at the loading 1wt.-%. And
Summary 109
the improvement increased with rising VP loading. Using multiple heating rate
TGA, it was found that the introduction of VP led to an elevation of activation
energy of the thermal decomposition of PLA. The Tg and Tm values of PLA
composites declined with increasing VP loading, indicating that VP could
improve the mobility of the molecular chains of PLA, showing a potential of VP
as a plasticizer for PLA. In the small-scale flame retardancy tests, PLA/VP
composites also exhibited excellent performance. The samples with 5 wt.-% VP
could pass V-0 level in UL-94 test and with 10 wt.-% VP showed an LOI value of
26.3%. In the cone calorimeter test, the sample PLA10VP reduced the pHRR
value by more than 25% compared to PLA. Finally, in the tensile test, PLA/VP
composites demonstrated good tensile strength and Young’s modulus as well as
significantly improved elongation at break.
6.3 Further modification of VP and VP used in combination with
commercialized flame retardant
In the following work, two different flame retardants, VPA and VPS, were
successfully synthesized based on VP. Despite the thermal stabilities of VPA and
VPS were unexpectedly not good as VP, PLA composites based on VPA and
VPS exhibited good flame retardancy in small-scale tests. Both PLA5VPA and
PLA5VPS passed V-0 level in UL-94 tests. And PLA with 10 wt.-% VPA showed
an LOI value as high as 30.5%. In cone calorimeter tests, both PLA/VPA and
PLA/VPS composites showed enhanced char yield after the measurements,
indicating that the carbon-carbon double bond of VPA and silanol group of VPS
did contribute to the formation of char. However, both PLA/VPA and PLA/VPS
composites exhibited no suppression effect of heat release and high brittleness
in mechanical test. The applications would be limited as small articles such as
pendant and decorations.
As the applications VPS and VPS couldn’t decrease the dripping behavior of
PLA during combustion, two commercialized flame retardants, MMT and APP,
Summary 110
were employed. Compared with PLA/MMT composites, with the introduction of
VP, the sample showed better flame retardancy small-scale tests. In particular,
the addition of 10 wt.-% VP allowing the samples passed V-0 level of UL-94 test,
while PLA5MMT and PLA10MMT showed “no rating”. And the LOI values of
samples with VP were higher than that of PLA/MMT composites. Furthermore,
addition of VP could slightly improve the ductility of PLA/MMT composites.
In case of PLA/APP composites, the incorporation of VP led to overall
improvement. For example, the sample mPLA20APP show an LOI value as high
as 40.7%, and remarkable intumescent behavior was observed both in LOI and
cone calorimeter tests. Both the pHRR and THR values were reduced by about
25% compared to PLA. Most surprisingly, notable synergic effect between VP
and APP was observed in tensile tests. The elongation at break of mPLA20APP
was improved to 28.2% compared 6.0% of PLA.
The results of this part of work implied various flame retardant mechanism of VP
or VP segment when they combined used with different components. In the
PLA/VP system, one of the main flame retardant effects of VP was to improve
the melt flowability of PLA to remove the heat and material from the burning area.
However, the introduction of MMT has limited this effect. Additionally, the
increased melt flowability elevated the difficulty of the formation of continuous
char layer, which was the main flame retardant mechanism of MMT.
Consequently, VP and MMT showed antagonistic effect on the flame retardancy
of PLA.
Comparing to that, distinct synergic effect was observed between VP and APP.
The appearance of VP resulted in significant increase of char yield in the fire
tests (LOI and CCT) for PLA/APP composites. Together they acted as an
intumescent flame retardant system and provided excellent flame retardancy. In
particular, mPLA/20APP presented the best flame retardant performance (LOI,
UL-94 and CCT) among all the specimens.
Outlook 111
7 Outlook
The work presented a novel pathway to prepare environmentally friendly flame
retardant PLA composites. Despite several achievements have been done in this
work, it also offers different possibilities for continuing researches.
The modification of vanillin allowed its application as a flame retardant for PLA
whose processing temperature was above 170 °C. This method could be used in
another polymeric matrix.
The modification of vanillin was towards the improvement of flame retardancy. It
could be otherwise modified focus on different functions.
Textile fabricated from PLA should be prepared and its properties should be
determined and studied.
The additives and fillers used in this work were considered and selected to keep
the biodegradability of PLA. The biodegradability of the PLA composited need to
be studied.
The presence and/or generation of water during thermal processing play a
determined role in the degradation of PLA.
References 112
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List of Figures 133
List of Figures
Figure 1-1: Plastic production since 1950 .......................................................... 1
Figure 1-2: Schematic illustration of the heat flux vs time in a fire ...................... 3
Figure 3-1: Type of polymeric materials and examples ...................................... 6
Figure 3-2: Synthesis of PLA.............................................................................. 8
Figure 3-3: Proposed mechanism of the lignin-to-vanillin process ................... 14
Figure 3-4: Mode of action of polymer in fire .................................................... 16
Figure 4-1: Synthesis path of VP ...................................................................... 25
Figure 4-2: Synthesis Path of VPA ................................................................... 26
Figure 4-3: Synthesis Path of VPS ................................................................... 26
Figure 4-4: Mixing chamber of plasticorder (left) and extruder (right) ............... 27
Figure 4-5: Injection molding machine (left) and hot compression machine (right)
......................................................................................................................... 30
Figure 4-6: Schematic set up of LOI (left) and UL-94 V (right) instruments ...... 34
Figure 4-7: Schematic set up of a cone calorimeter ......................................... 36
Figure 5-1: FTIR Spectra of VA (black) and VP (red) ....................................... 40
Figure 5-2: 1H (top), 13C and 31P (bottom) NMR spectra of VP......................... 41
Figure 5-3: The morphology of VA and VP by SEM (left) and EDX (right) ........ 42
Figure 5-4: SEM pictures of PLA/VA (left) and PLA/VP (right) composites ....... 43
Figure 5-5: TG (left) and DTG (right) plots of VA and VP ................................. 44
Figure 5-6: TG (left) and DTG (right) plots of PLA and its composites ............. 45
Figure 5-7: Fitted curves of lnβ against 1/T at different α of PLA (left) and
List of Figures 134
PLA10VP (right) ................................................................................................ 47
Figure 5-8: DSC plots of PLA and PLA/VP composites .................................... 49
Figure 5-9: HRR plots of PLA and PLA composites .......................................... 55
Figure 5-10: THR (a), Mass loss (b), TSP (c) and CO/CO2 ratio (d) plots of PLA
and PLA composites ......................................................................................... 57
Figure 5-11: Rheological properties of PLA and PLA composites: (a) Complex
viscosity; (b) Storage modulus; (c) Loss modulus ............................................ 58
Figure 5-12: 3D-images of evolved gaseous products for (a) PLA (b) PLA10VP;
(c) Total absorbance vs time for PLA and PLA10VP; (d) Absorbance at 1238
cm-1 vs time for PLA and PLA10VP (c) At Tmax for PLA and PLA10VP; (d) Zoom
in on selected area. (Tmax appeared at 47 min) ................................................ 59
Figure 5-13: Tensile test plots of PLA and PLA composites .............................. 60
Figure 5-14: Pictures of tensile bar after tests of PLA10VA (left) and PLA10VP
(right) ................................................................................................................ 62
Figure 5-15: Tensile test plots of PLA and PLA/VP composites processed via
different methods .............................................................................................. 63
Figure 5-16: FTIR spectrum of VP (black) and VPA (red) ................................. 69
Figure 5-17: FTIR spectrum of VP (black) and VPS (red) ................................. 69
Figure 5-18: 1H NMR spectrum of VPA ............................................................. 70
Figure 5-19: 1H NMR spectrum of VPS ............................................................ 71
Figure 5-20: TG (left) and DTG (right) plots of VP, VPA and VPS ..................... 72
Figure 5-21: TG (left) and DTG (right) plots of VPA and VPS based PLA
composites ....................................................................................................... 73
Figure 5-22: HRR plots of PLA/VPA and PLA/VPS composites ........................ 77
List of Figures 135
Figure 5-23: THR (left) and CO/CO2 ratio (right) plots of PLA/VPA and PLA/VPS
composites ....................................................................................................... 78
Figure 5-24: Tensile test plots of PLA and PLA composites ............................. 79
Figure 5-25: SEM pictures of PLA/MMT and mPLA/MMT composites ............. 82
Figure 5-26: TG (left) and DTG (right) plots of PLA and PLA composites ........ 83
Figure 5-27: DSC plots of PLA and PLA composites ........................................ 85
Figure 5-28: HRR plots of PLA/VP/MMT composites ....................................... 88
Figure 5-29: THR (a), Mass loss (b), TSP (c) and CO/CO2 ratio (d) plots of PLA
composites ....................................................................................................... 89
Figure 5-30: Residue of samples after cone calorimeter test of PLA/MMT
composites (top) and mPLA/MMT composites (bottom) with 5 wt.-% (left) and 10
wt.-%(right) MMT .............................................................................................. 90
Figure 5-31: SEM pictures of char layer after cone calorimeter test of
PLA10MMT (top) and mPLA10MMT (bottom) for surface (left) and inner (right)
residue ............................................................................................................. 91
Figure 5-32: Tensile test plots of PLA/VP/MMT composites ............................. 92
Figure 5-33: SEM pictures PLA/APP and mPLA/APP composites ................... 94
Figure 5-34: TG (left) and DTG (right) plots of PLA/VP/APP composites ......... 96
Figure 5-35: DSC plots of PLA/APP composites .............................................. 97
Figure 5-36: DSC plots of mPLA/APP composites ........................................... 98
Figure 5-37: Pictures of samples after LOI test: (a) PLA5APP, (b) PLA10APP (c)
PLA20APP (d) mPLA5APP (e) mPLA10APP (f) mPLA20APP ......................... 99
Figure 5-38: HRR plots of PLA/VP/APP composites ...................................... 101
Figure 5-39: THR (a), Mass loss (b), TSP (c) and CO/CO2 ratio (d) plots of
List of Figures 136
PLA/VP/MMT composites ............................................................................... 102
Figure 5-40: Residue of samples after cone calorimeter test of PLA/APP
composites (top) and mPLA/APP composites (bottom) with 10 wt.-% (left) and
20 wt.-%(right) APP ........................................................................................ 104
Figure 5-41: SEM pictures of residue after cone calorimeter test of PLA20APP
(top) and mPLA20APP (bottom) for surface (left) and inner (right) ................. 104
Figure 5-42: Tensile test plots of PLA/VP/APP composites ............................ 106
List of Tables 137
List of Tables
Table 3-1: Comparison of different PLA synthesis techniques ............................ 9
Table 3-2: Comparison of different fibers reinforced PLA composites .............. 12
Table 4-1: Recipe of Vanillin and Vanillin derivative based flame retardant PLA
......................................................................................................................... 28
Table 4-2: Default extrusion parameters .......................................................... 29
Table 4-3: Default spinning parameters ............................................................ 29
Table 4-4: Default injection molding parameters .............................................. 30
Table 4-5: Classification of materials for the UL-94 V tests .............................. 35
Table 5-1: Recipe of pure PLA and its composites ........................................... 38
Table 5-2: Assignment of the main vibrational modes for VA and VP ............... 39
Table 5-3: Data obtained from TGA plots of VA and VP ................................... 43
Table 5-4: Data obtained from TGA plots of PLA and its composites ............... 46
Table 5-5: Calculated values of Ea and r2 of PLA and PLA10VP at different α . 48
Table 5-6: Data obtained from DSC of PLA and PLA/VP composites .............. 50
Table 5-7: LOI, UL94 and MFI results of PLA and PLA composites ................. 53
Table 5-8: Data obtained from CCT of PLA and PLA composites ..................... 54
Table 5-9: Data obtained from tensile test of PLA and PLA composites ........... 61
Table 5-10: Parameters of PLA and PLA/VP composites processed via different
methods ........................................................................................................... 64
Table 5-11: Mechanical parameters of fibers for PLA and PLA/VP composites 65
Table 5-12: Recipe of VPA and VPS based PLA composites ........................... 67
List of Tables 138
Table 5-13: Assignment of the main vibrational modes for VPA and VPS ......... 68
Table 5-14: Data obtained from TGA plots of VPA and VPS ............................. 72
Table 5-15: Data obtained from TGA plots of VPA and VPS based PLA
composites ....................................................................................................... 74
Table 5-16: LOI, UL94 and MFI results of PLA composites based on vanillin
derivatives ........................................................................................................ 75
Table 5-17: Data obtained from CCT of PLA and PLA composites ................... 76
Table 5-18: Data obtained from tensile test of PLA and PLA composites ......... 79
Table 5-19: Recipe of PLA/VP/MMT composites .............................................. 81
Table 5-20: Data obtained from TGA plots of PLA and PLA composites ........... 83
Table 5-21: Data obtained from DSC plots of PLA and PLA composites .......... 84
Table 5-22: Results of LOI, UL94 and MFI tests of PLA and PLA composites .. 86
Table 5-23: Data obtained from CCT of PLA/VP/MMT composites................... 87
Table 5-24: Data obtained from tensile tests of PLA/VP/MMT composites ....... 92
Table 5-25: Recipe of PLA/VP/APP composites ............................................... 93
Table 5-26: Data obtained from TGA plots of PLA/VP/APP composites ........... 95
Table 5-27: Data obtained from DSC plots of PLA and PLA composites .......... 97
Table 5-28: Results of LOI and UL-94 tests of PLA/VP/APP composites ......... 99
Table 5-29: Data obtained from CCT plots of PLA/VP/APP composites ......... 100
Table 5-30: Data obtained from tensile tests plots of PLA/VP/APP composites
....................................................................................................................... 105
Publications 139
Publications
Journals
Pengcheng Zhao, Zhiqi Liu, Xueyi Wang, Ye-Tang Pan, Ines Kühnert,
Michael Gehde, De-Yi Wang and Andreas Leuteritz. Renewable Vanillin
Based Flame Retardant for Poly(lactic acid): a Way to Enhance Flame
Retardancy and Toughness Simultaneously. RSC Advances, 2018,
Submitted
Pengcheng Zhao, Zhiqi Liu, Michael Gehde, Ines Kuehnert and Andreas
Leuteritz. Effect of phosphorus-containing modified magnesium hydroxide
on the mechanical properties and flammability of PLA/MH composites. AIP
Conference Proceedings, 2017
Pengcheng Zhao, Andreas Leuteritz, Claudia Hinüber, Harald Brünig, Ines
Kühnert. Flame resistant and halogen-free fibers based on
phosphate-modified vanillin and poly (lactic acid), In preparation
Conferences
Poster
Pengcheng Zhao, Zhiqi Liu, Michael Gehde, Ines Kuehnert and Andreas
Leuteritz. Effect of phosphorus-containing modified magnesium hydroxide
on the mechanical properties and flammability of PLA/MH composites.
Europe Africa Conference 2017, Dresden.
Oral presentation
Pengcheng Zhao, Michael Gehde, De-Yi Wang, Ines Kuehnert and Andreas
Leuteritz. Design of vanillin based flame retardants for PLA composites.
Technomer 2017, Chemnitz.