ORIGINALS ORIGINALARBEITEN

Mechanical and physical properties of thermally modified Scotspine wood in high pressure reactor under saturated steamat 120, 150 and 180 �C

Lauri Rautkari • Juhani Honkanen •

Callum A. S. Hill • Daniel Ridley-Ellis •

Mark Hughes

Received: 9 August 2013 / Published online: 30 September 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Scots pine sapwood and heartwood were ther-

mally modified under saturated steam at 120, 150 and

180 �C in a high pressure reactor. Mechanical properties

such as dynamic and static modulus of elasticity (MOE),

static modulus of rupture (MOR), Brinell hardness and

impact toughness were evaluated. The static MOE for

sapwood did not decrease substantially (approximately

1 %), not even with a high mass loss of more than 12 %,

when the wood was modified at 180 �C. Static MOE of the

wood increased approximately 14 %, when modified at

150 �C. Surprisingly, MOR increased by 15 %, when

modified at 150 �C with mass loss of 2.3 %. Whereas

impact strength and hardness decreased somewhat, when

modified at 180 �C. Moreover, high anti-swelling effi-

ciency values were obtained (60 % for sapwood and 52 %

for heartwood) when modified at 180 �C.

Mechanische und physikalische Eigenschaften von in

einem Hochdruckreaktor unter Sattdampfbedingungen

bei 120, 150 und 180 �C thermisch modifiziertem

Kiefernholz

Zusammenfassung Kiefernsplintholz und –kernholz

wurde in einem Hochdruckreaktor unter Sattdampfbe-

dingungen bei Temperaturen von 120, 150 und 180 �C

thermisch modifiziert. Die mechanischen Eigenschaften

dynamischer und statischer Elastizitatsmodul (MOE),

statische Biegefestigkeit (MOR), Brinell Harte und

Schlagzahigkeit wurden bestimmt. Der statische

Elastizitatsmodul von Splintholz nahm auch bei einem

hohen Masseverlust von uber 12 %, wenn das Holz bei

einer Temperatur von 180 �C modifiziert wurde, nicht

wesentlich ab (ca. 1 %). Der statische Elastizitatsmodul

nahm bei einer Modifizierung bei 140 �C um ca. 14 % zu.

Uberraschenderweise stieg die Biegefestigkeit bei ein-

er thermischen Modifikation bei 150 �C und einem

Masseverlust von 2,3 % um 15 % an. Die Schlagzahigkeit

und die Brinell Harte nahmen bei einer thermischen

Modifikation bei 180 �C ein wenig ab. Dagegen wurde bei

dieser Temperatur ein hohes Quellresistenzvermogen

erzielt (Splintholz 60 % und Kernholz 52 %).

1 Introduction

Solid wood thermal modification has been widely studied

over the years and comprehensively reviewed (Hill 2006;

Navi and Sandberg 2012). In general, thermal modification

reduces hygroscopicity and water absorption, thus

increasing dimensional stability and decay resistance.

However at the same time, some of the mechanical prop-

erties (especially toughness) are reduced due to the

unwanted cell wall degradation in the wood. Several dif-

ferent thermal modification methods have been developed,

with the main differences concerning processing conditions

L. Rautkari (&) � J. Honkanen � M. Hughes

Department of Forest Products Technology, School of Chemical

Technology, Aalto University, P.O. Box 16400, 00076 Aalto,

Finland

e-mail: lauri.rautkari@aalto.fi

C. A. S. Hill

Norsk Institut for Skog og Landskap, P.O. Box 115, 1431 As,

Norway

C. A. S. Hill

JCH Industrial Ecology Limited, Bangor LL57 1LJ, UK

D. Ridley-Ellis

Forest Products Research Institute, Edinburgh Napier University,

10 Colinton Road, Edinburgh EH10 5DT, UK

123

Eur. J. Wood Prod. (2014) 72:33–41

DOI 10.1007/s00107-013-0749-5

(process steps, steam, nitrogen, wet, dry, open or closed

system, etc.). Thermally modified wood properties differ

and cannot always be compared because of high variation

in the conditions associated with the different processes.

However, in a typical thermal modification process (Metsa-

Kortelainen et al. 2006, Metsa-Kortelainen and Viitanen

2010; Hill et al. 2012) under superheated steam, in an open

system, the relative humidity (RH) is not controlled (or

cannot be accurately controlled), the wood material is dried

completely and it has to be re-conditioned to a specific

moisture content (MC) before end-use. This additional

processing step means not only increased production costs,

but also increased energy consumption. Some of the

modification processes have been performed in a closed

system in a high pressurised steam environment (Willems

2009; Dagbro et al. 2010), where the RH can be controlled

up to saturation. However, earlier studies (Torniainen et al.

2011; Borrega and Karenlampi 2008, 2010; Ding et al.

2011) have shown that thermal degradation under super-

heated steam (at atmospheric pressure) is slower than under

saturated steam atmosphere (at high pressure).

The mass of the wood material decreases due to the

thermal degradation in the wood cell wall polymers (hemi-

celluloses, cellulose and lignin) during the thermal modifi-

cation. The degradation concerns mainly the hemicelluloses

which have lower thermal stability than the other compo-

nents. As hemicelluloses are also the most hydrophilic of

these polymers, their degradation leads to more hydrophobic

wood. Due to the loss of thermally labile polysaccharides,

the relative lignin content is increased during the process.

The lignin is also the most thermally stable cell wall com-

ponent and it has been proposed that new crosslinks are

formed in the lignin network during the process (Tjeerdsma

et al. 1998), which might influence the dimensional stability;

although this phenomenon is probably process dependent

(Hill 2006). The polysaccharide degradation can also affect

the semi-crystalline regions of the cellulose, with reports

that the crystallinity index is increased (Sivonen et al. 2002),

especially in humid process conditions (Dwianto et al.

1996). The increase in crystallinity can also be explained by

increased mobility of the cellulose chains in the presence of

moisture and high temperature (Bhuiyan et al. 2000) and the

crystallite size might increase as well. However, it should be

noted that loss of amorphous hemicelluloses will also result

in an apparent increase in crystallinity (Hill 2006). It is

known that the presence of humidity during thermal modi-

fication accelerates the process due to acetic and formic acid

formation catalysing degradation of the accessible and

thermally labile polysaccharide components (Sundqvist

et al. 2006; Torniainen et al. 2011). Although there is a

considerable body of literature on thermally modified wood,

the influence of process conditions and especially the role of

moisture in the process are far from being understood. The

challenge at the present time is to reduce the undesirable

properties of wood (e.g. dimensional stability, susceptibility

to decay), whilst preserving as far as is possible the desirable

properties, especially toughness and mechanical strength. It

is clear that the presence of moisture in the thermal modi-

fication process is a key variable and its influence must be

understood.

The aim of this study was to evaluate mechanical

properties of thermally modified Scots pine (Pinus sylves-

tris L.) under saturated steam at different temperatures. The

mechanical properties were static MOE (modulus of elas-

ticity) and MOR (modulus of rupture) evaluated by 4-point

bending, dynamic MOE using an ultrasonic non-destruc-

tive method, Brinell hardness and impact strength.

Although, the determination of the dynamic MOE using the

ultrasonic method is widely used for other materials, it has

not been applied to thermally modified wood, so far as the

authors are aware. Anti-swelling efficiency (ASE) and

maximum volumetric swelling (S %) of the modified wood

was also determined.

2 Materials and methods

2.1 Wood material

Samples of kiln dried Scots pine (Pinus sylvestris L.)

sapwood (n = 36) and heartwood (n = 18) obtained from

Southeast Finland with average oven dry density of 500 kg/

m3 and 490 kg/m3, respectively with an initial kilned

moisture content of 14.5 % was used in this study. Paired

specimens of clear wood (reference ? modified) speci-

mens with dimensions of 25 9 25 mm2 and 300–450 mm

in length were cut from the middle of the board. The annual

rings were orientated horizontally to the face of the spec-

imens as far as possible for sapwood and heartwood.

2.2 Thermal modification

The specimens were thermally modified in a saturated

steam atmosphere in a high pressure reactor for 3 h. The

specimens were modified at three different temperatures

120, 150 and 180 �C with steam pressures of 2.0, 4.8 and

10.0 bars, respectively. Prior to the modification, the

specimens had been oven dried in a controlled manner first

at 40 �C, then 70 �C and finally 103 �C for 24 h each.

2.3 Static MOE and MOR

The specimens for static bending test were cut to

18 9 18 9 350 mm3 (R 9 T 9 L) and conditioned at RH

65 % and 20 �C until equilibrium. Only sapwood was

evaluated because the heartwood had numerous defects and

34 Eur. J. Wood Prod. (2014) 72:33–41

123

it was impossible to get a clear 350 mm long specimen.

The static MOE and MOR were measured according to EN

408 (2011) in a conditioned atmosphere (RH 50 %, 20 �C).

Prior to the experiments, the specimens were kept in a

sealed plastic bag. The 4-point bending test was carried out

using universal testing machine (Zwick 1475) combined

with an MTS Premium Elite controller. The loading

direction was perpendicular to the grain using a loading

speed of 10 mm/min.

2.4 Dynamic MOE

The specimens for measurement of dynamic MOE by

ultrasonic time-of-flight were cut to 18 9 18 9 100 mm3

(R 9 T 9 L) from the static bending specimen’s edges and

conditioned at RH 65 % and 20 �C until equilibrium. The

samples were measured using a Proceq Pundit Lab Plus

(Switzerland) device with firmware version 2.0.5 and

Punditlink software version 2.4.0. Measurements were

made with 54 kHz transducers using a thin sheet of nitrile

rubber for contact of each transducer to the sample surface.

A total of thirty measurements of time-of-flight in the

longitudinal direction were made for each specimen and

averaged in quick succession for each sample to a precision

of 0.1 ls with an automatic trigger, after having first

adjusted the amplitude and gain for optimum operation.

Dynamic MOE was calculated from the Newton–Laplace

equation (Eq. 1) as in a previously reported study that

employed the ultrasonic time of flight method on wood

(Haines et al. 1996).

MOEdynamic ¼q� l2

t2ð1Þ

where q is specimen’s current density (at RH 65 %, 20 �C),

l is the length of the specimen and t is the time of flight. A

total of 54 reference samples (36 sapwood and 18 heart-

wood) were compared with the 36 paired samples of

modified sapwood (12 each at 120, 150 and 180 �C) and 18

paired samples of thermally modified heartwood (6 each at

120, 150 and 180 �C).

2.5 Impact strength

The impact strength was evaluated according to standard

EN ISO 179-1 (2010). Two specimens were cut from each

sample piece, from modified heartwood and sapwood and

from their corresponding (unmodified) references. Unnot-

ched specimens were cut to size of 4 9 10 9 80 mm3

(R 9 T 9 L) and conditioned at RH 65 % and 20 �C until

equilibrium, prior to the experiments. The pendulum was

chosen so that the absorbed energy was between 10 and

80 % of the available energy of the impact, as described in

the standard EN ISO 179-1 (2010).

2.6 Hardness

Hardness measurements were conducted using a Zwick

Z050 universal testing machine and a 20 kN load cell

equipped with 10-mm indenter. The hardness measurements

were measured according to EN 1534 (2000), except the

indentation force used was 500 N rather than 1,000 N. Two

indentations were performed on the radial and tangential

direction of the samples. Two indentations on radial surface

and two on the tangential surface were performed per spec-

imen and averaged, with the indentation depth measured

automatically by the testing machine rather than measuring

indentation diameter manually. More details of this meth-

odology are explained in detail by Rautkari et al. (2011,

2013b). Maximum load was reached in 15 s and maintained

for 25 s, the load was released over a period of 15 s. The

Brinell hardness HB (N/mm2) was calculated using Eq. 2.

HB ¼F

p � D � hmaxð2Þ

where HB is Brinell hardness, F is applied load, D is the

diameter of indenter, and hmax is the maximum depth of the

indentation after unloading. The elastic behaviour of the

surface was evaluated within this measuring procedure.

The surface elasticity ee was then calculated using Eq. 3.

ee ¼hmax � he

he

ð3Þ

where maximum depth hmax of indentation and the elastic

deformation he of the indentation were measured. The

elastic deformation he was measured when the load reached

zero during unloading. An example is presented in Fig. 1.

2.7 Swelling behaviour

The maximum swelling and anti-swelling efficiency (ASE)

was evaluated from the oven dried (103 �C, 24 h) modified

and untreated specimens with dimensions of 25 9 25 9

7 mm3. The oven dried specimens were water soaked for

7 days and the dimensions were measured prior and after the

water soaking. The maximum swelling was based on volume

and calculated using Eq. 4 (Rowell and Ellis 1978; Hill 2006).

Sð%Þ ¼ 100� Vw � Vd

Vd

ð4Þ

where Vd is the volume of dried specimen and Vw is the

volume of water soaked specimen. The anti-swelling

efficiency (ASE) was calculated using Eq. 5 (Rowell and

Ellis 1978; Hill 2006).

ASEð%Þ ¼ 100� Su � Sm

Su

ð5Þ

where Su is the swelling of water soaked untreated wood

and Sm is the swelling of water soaked modified specimen.

Eur. J. Wood Prod. (2014) 72:33–41 35

123

3 Results and discussion

The moisture content after the modification process, mass

loss, density loss and equilibrium moisture content (EMC)

of modified wood at a relative humidity (RH) of 65 % are

presented in Table 1. The MC of the thermally modified

wood after the process is typically 0 % or nearby, if the

modification is performed under atmospheric pressure and

therefore the process includes also a moisturising phase,

which can be rather time consuming (Metsa-Kortelainen

et al. 2006). Unlike the thermal modification under atmo-

spheric pressure, the MC after modification under high

pressure saturated steam does not dry the wood, as shown

in Table 1. The MC of the modified wood varied between 5

and 18 %, depending on the processing pressure. This

means that the final remoistening phase is not needed and

the modified wood does not necessarily need any further

conditioning before end-use. The mass loss of modified

wood was rather small when modified at 120 �C, 0.6 and

1.2 % for heartwood and sapwood, respectively. The

higher mass loss for the heartwood was most probably

caused by higher extractives content and their evaporation

(Rautkari et al. 2012; Vainio-Kaila et al. 2013). The mass

loss was unsurprisingly increased with increased tempera-

ture, but the magnitude was higher than expected. An

earlier study (Metsa-Kortelainen et al. 2006) found a mass

loss of 1.8, 4,1, 6,7 and 10.0 % for Scots pine sapwood

modified at 170, 190, 210 and 230 �C, respectively under

superheated steam (low relative humidity) and slightly

higher for heartwood. This means that under saturated

steam, the thermal degradation is much faster compared to

modification at atmospheric pressure. In this study, also

density loss, measured from dry wood, decreased in a

similar manner to mass loss, but by a smaller extent, which

means that the thermally modified wood shrinks during the

process caused by the degradation of the cell walls. The

EMC at RH 65 % of the untreated heartwood was lower

than sapwood, as expected and was lower with an increase

in the processing temperature.

3.1 Mechanical properties

Mechanical properties of untreated and thermally modified

wood are presented in Table 2. Static and dynamic moduli

of elasticity (MOE) were measured with 4-point bending

according EN 408 (2011) prior to using an ultrasonic non-

destructive method. Surprisingly, static MOE was not

decreased significantly, even with the samples modified at

180 �C, when the mass loss was as high as 13.9 and 12.2 %

for heartwood and sapwood, respectively. Static MOE

increased by approximately 14 %, when the wood was

0

100

200

300

400

500

600

0.00 0.50 1.00 1.50 2.00

Lo

ad [

N]

Indentation depth [mm]

Plastic deformation Elastic deformation

Fig. 1 An example of a loading and unloading curve from the Brinell

hardness measurement. Elastic deformation represents the difference

between maximum depth of the penetration of the indenter and

recovered deformation. Brinell hardness was calculated using max-

imum force obtained during the experiments (app. 500 N) and the

penetration depth after unloading (see more details in Eq. 2)

Abb. 1 Beispiel einer Be- und Entlastungskurve bei der Messung der

Brinell Harte. Die elastische Verformung entspricht der Differenz

zwischen maximaler Eindringtiefe des Eindruckkorpers und der Ruck-

verformung. Die Brinell Harte wurde aus der erzielten Hochstkraft

(ca. 500 N) und der Eindringtiefe nach Entlastung berechnet (siehe

Gl. 2)

Table 1 EMC at RH 65 %, MC after modification and mass and

density loss during the modification for untreated and thermally

modified Scots pine sapwood and heartwood

Tab. 1 Gleichgewichtsfeuchte (EMC) bei rel. Luftfeuchte von 65 %,

Holzfeuchte nach Modifikation und Masse- sowie Dichteverlust von

unbehandeltem und thermisch behandeltem Kiefernsplint- und

–kernholz. Standardabweichung in Klammern

Sample MC after

modification

(%)

Mass loss

(%)

Density

loss (%)

EMC at

RH 65 %

(%)

Reference

Heartwood – – – – – – 10.8 0.4

Sapwood – – – – – – 11.7 0.6

120 �C

Heartwood 6.0 0.6 0.6 0.4 0.1 2.9 9.3 0.5

Sapwood 8.0 1.2 0.3 0.1 -0.3 1.1 10.4 0.2

150 �C

Heartwood 8.9 2.6 2.3 0.2 2.0 1.4 7.6 0.0

Sapwood 7.8 0.6 2.3 0.1 1.7 1.5 8.4 0.1

180 �C

Heartwood 10.7 4.4 13.9 1.3 8.8 2.0 5.5 0.2

Sapwood 9.1 2.8 12.2 0.9 9.3 1.1 6.0 1.1

Standard deviation in parenthesis

36 Eur. J. Wood Prod. (2014) 72:33–41

123

modified at 150 �C and decreased by approximately 1 %

when modified at 180 �C. An earlier study (Metsa-Korte-

lainen and Viitanen 2010) showed a 1 % increase in MOE

for Scots pine sapwood when modified at 230 �C under

superheated steam and with a mass loss of 10 %. This

means that somewhat similar bending properties can be

obtained under saturated and superheated steam, but when

using saturated steam the temperature can be decreased

from 230 to 180 �C. Furthermore, in the earlier study

(Metsa-Kortelainen and Viitanen 2010), the treatment

duration was much longer than in this study (3 h), but this

is hardly comparable, because the initial MC was different.

The MOE values obtained using the dynamic and static

methods showed different behaviour, in that the decrease in

dynamic MOE was slightly greater than that recorded using

the static method. This shows that properties can vary

depending on the test method used. Moreover, a relatively

good linear correlation was found between static and

dynamic MOE for untreated specimens (both sapwood and

heartwood) (Fig. 2). Bending strength, modulus of rupture

(MOR) decreased much more (30 %) as a result of thermal

degradation (mass loss 12.2 %) with wood modified at

180 �C, whereas an earlier study (Metsa-Kortelainen and

Viitanen 2010) showed a 5 % decrease when the mass loss

was 10 %. Surprisingly, in this study, MOR increased by

15 % when the wood was modified at 150 �C (mass loss

2.3 %), whereas in the earlier study, MOR decreased by

5 % when the wood was modified at 170 �C giving a mass

loss of 1.8 %.

It is very well known that thermally modified wood

exhibits a significant decrease in toughness when compared

to unmodified wood. Toughness can be measured using

several methods. Earlier studies measured the toughness

using an impact bending method (Korkut et al. 2008;

Korkut and Budakcı 2009), impact strength using hammer

method (Boonstra et al. 2007) and brittleness measured

from the load–deflection curve from static bending (Phu-

ong et al. 2007a). In this study the toughness was evaluated

using the hammer method EN ISO 179-1 (EN ISO 2010)

for plastics, determination of Charpy impact properties.

The results showed unsurprisingly decreased toughness up

to 31 % for heartwood modified at 180 �C. An earlier study

by Boonstra et al. (2007) found a large decrease of

toughness, up to 80 %. In that study wood was modified

using hydrothermolysis at 165 �C for 30 min followed by

Table 2 Mechanical properties of untreated and thermally modified Scots pine sapwood and heartwood

Tab. 2 Mechanische Eigenschaften von unbehandeltem und thermisch modifiziertem Kiefernsplint- und –kernholz. Gemittelte Werte. Stan-

dardabweichung in Klammern

Scots pine

Sapwood

MOE(Dynamic)

(GPa)

MOE(Static)

(GPa)

MOR(Static)

(MPa)

Impact strength

(kJ/m2)

Brinell hardness

(N/mm2)

Reference 17.0 (2.6) 16.3 (2.7) 82.3 (12.1) 13.8 (2.9) 11.6 (1.8)

120 �C 16.3 (2.4) N/A (N/A) N/A (N/A) 13.0 (2.1) 10.8 (1.9)

150 �C 18.0 (1.9) 18.6 (1.8) 94.5 (10.1) 9.0 (1.7) 12.0 (1.0)

180 �C 14.8 (2.8) 16.1 (2.8) 57.2 (21.6) 10.1 (2.5) 9.4 (2.5)

Scots pine heartwood MOE(Dynamic) (GPa) Impact strength (kJ/m2) Brinell hardness (N/mm2)

Reference 16.6 (3.2) 12.1 (3.3) 11.4 (2.4)

120 �C 17.0 (4.0) 10.7 (2.4) 10.9 (2.3)

150 �C 17.8 (2.8) 11.6 (3.2) 10.7 (1.7)

180 �C 15.7 (4.2) 8.3 (5.3) 9.9 (2.3)

The values are averaged. Standard deviation in parenthesis

8 10 12 14 16 18 20 228

10

12

14

16

18

20

22

R2 = 0.80

MO

ED

ynam

ic [G

Pa]

MOEStatic [GPa]

Fig. 2 Correlation between static and dynamic MOE for the 36

untreated sapwood samples. The range of values is typical for Scots

pine clear wood and the correlation between static and dynamic

values is similar to that obtained in other studies

Abb. 2 Korrelation zwischen statischem und dynamischem Elasti-

zitatsmodul (MOE) der 36 unbehandelten Splintholzprufkorper. Die

Werte sind typisch fur fehlerfreies Kiefernholz und die Korrelation

der statischen und dynamischen Werte entspricht den Ergebnissen

anderer Studien

Eur. J. Wood Prod. (2014) 72:33–41 37

123

what was referred to as curing at 180 �C for 6 h in dry

conditions (i.e. as used in the commercial PLATO process),

unfortunately the mass loss was not mentioned. However, it

seems that when using saturated steam the toughness

decreases less than the earlier studies had shown.

Brinell hardness was evaluated according to EN 1534

(2000), except that the indentation depth was measured and

not the diameter. Moreover, a lower load (500 N) was used

than that recommended in the standard (1000 N), because

when using 1000 N the indenter (10 mm) would have

penetrated too deep (more than 5 mm) and incorrect results

would have been obtained. No major changes were

observed in the hardness, only wood modified at 180 �C

showed a decrease for sapwood and heartwood as shown in

Table 2. In an earlier study, Rautkari et al. (2013a) found

negligible changes in the Brinell hardness for thermally

modified Scots pine sapwood when modified under super-

heated steam at 200 �C for 3 h. In this study, the decrease

in hardness with wood modified at the higher temperature

is caused by the cell wall degradation which makes the cell

wall more brittle which is manifested also as a decrease of

the impact strength. However, Brinell hardness correlated

linearly rather well with the density (Fig. 2). The surface

elasticity was aslo analysed as described earlier (Rautkari

et al. 2011, 2013a) at the same time as the Brinell hardness

(Fig. 3). The surface elasticity is an important value, which

reveals how the indentation recovers from maximum depth

of the intender (i.e. a fully elastic material recovers 100 %,

whereas a fully plastic material recovers 0 %). Rautkari

et al. (2011) showed that a composite of untreated hybrid

poplar and high density densified hybrid poplar recovers up

to more than 70 %. In this study of untreated and thermally

modified wood, elastic recovery was more than 40 %,

except with the wood modified at 180 �C where recovery

was slightly decreased from this value.

3.2 Swelling behaviour

The anti-swelling efficiency (ASE) of thermally modified

Scots pine sapwood (a) and heartwood (b) are presented in

Fig. 4. Unsurprisingly, the highest ASE values were

obtained from specimens modified at 180 �C and lowest

for wood modified at 120 �C. The highest ASE values of

60 % for sapwood and 52 % for heartwood were extremely

high. In an earlier study, Welzbacher et al. (2007) found

approximately 40 % ASE value for Norway spruce, when

modified at 240 �C for 3 h in an oil heat treatment process

(mass loss approximately 12 %, as in this study). In this

study, when modified at 120 �C, the negative ASE value

for heartwood reveals that the modified wood was swelling

more than the untreated one. However, the negative ASE

values were insignificant. The specimens modified at

150 �C showed somewhat increased ASE values of 29 and

25 % for sapwood and heartwood, respectively. In an

earlier study by Seborg et al. (1953), they found even

higher ASE values up to 75 %, when wood was modified in

a closed system in saturated steam at 300 �C, but weight

loss was as much as 45 % and most probably the

mechanical properties would have been severely decreased.

In this study, some of the heartwood specimens have a

somewhat diagonal grain orientation to one measuring face

and this might have an influence on the measuring accuracy

Fig. 5.

The maximum swelling (S %) of untreated and ther-

mally modified wood is presented in Fig. 6. The maximum

swelling represents the change in dimension of oven dry

specimens to water soaked dimensions. The swelling of

untreated specimens was unsurprisingly highest at 15.1 and

14.0 % for sapwood (a) and heartwood (b), respectively.

The difference between Fig. 6a, b was most probably

caused by higher extractive content of the heartwood

(Vainio-Kaila et al. 2013). The lowest swelling was found

for specimens modified at 180 �C, 7.1 % for sapwood and

5.5 % for heartwood. An earlier study by Stamm (1935)

showed that swelling is increased by the EMC and the

density of the wood. The equilibrium moisture content is

dependent, at least to some extent, on the hydroxyl group

content and their accessibility. Therefore, it might be

possible that the cell wall accessibility has been decreased

dramatically by thermal degradation (Phuong et al. 2007b)

and that lignin cross-linking has occurred. However, Rau-

tkari et al. (2013b) proposed that there has to be an addi-

tional mechanism other than the extent of available

hydroxyl groups to control the EMC.

350 400 450 500 550 600 650 7006

8

10

12

14

16

18

R2 = 0.86

Brin

ell h

ardn

ess

[N/m

m2 ]

Density [kg/m3]

Fig. 3 Correlation between density and Brinell hardness of untreated

and thermally modified Scots pine sapwood and heartwood. Each

value represents an average (n = 4) of each specimens hardness value

Abb. 3 Korrelation zwischen Rohdichte und Brinell Harte von

unbehandelten und thermisch modifiziertem Kiefernsplint- und

-kernholz. Die Brinell Harte entspricht dem Mittelwert aus 4

Messungen je Prufkorper

38 Eur. J. Wood Prod. (2014) 72:33–41

123

Reference 120 °C 150 °C 180 °C

b

0%

10%

20%

30%

40%

50%

60%

Ela

stic

rec

ove

ry

Reference 120 °C 150 °C 180 °C

a

Fig. 4 Averaged elastic recovery for untreated (n = 36 and thermally modified (n = 12) sapwood (a) and untreated (n = 18) and thermally

modified (n = 6) heartwood (b) specimens. (Four indentations per specimen were performed and averaged)

Abb. 4 Durchschnittliche elastische Ruckverformung von (a) unbehandelten (n = 36) und thermisch modifizierten (n = 12) Splintholzprufkorp-

ern sowie (b) unbehandelten (n = 18) und thermisch modifizierten (n = 6) Kernholzprufkorpern. (Vier Eindruckversuche je Prufkorper.

Gemittelte Werte)

-20%

-10%

0%

10%

20%

30%

40%

50%

60%

70%

AS

E

120°C 150°C 180°C 120°C 150°C 180°C

Fig. 5 Anti swelling efficiency

of thermally modified Scots

pine sapwood (a) and

heartwood (b) (n = 6). Error

bars are standard deviation

Abb. 5 Quellresistenzvermogen

von thermisch modifiziertem

a Kiefernsplintholz und

b Kiefernkernholz (n = 6).

Fehlerbalken geben die

Standardabweichung an

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

Sw

ellin

g

Reference 120°C 150°C 180°C

a

Reference 120°C 150°C 180°C

b

Fig. 6 Maximum volumetric swelling from oven-dried to water soaked specimens for untreated (n = 18) and thermally modified (n = 6)

sapwood (a) and heartwood (b). Error bars are standard deviation

Abb. 6 Maximale Volumenquellung zwischen Darrzustand und Wasserlagerung von unbehandelten (n = 18) und thermisch modifizierten

(n = 6) a Splintholzprufkorpern und b Kernholzprufkorpern. Fehlerbalken geben die Standardabweichung an

Eur. J. Wood Prod. (2014) 72:33–41 39

123

4 Conclusion

The results show that thermal modification under saturated

stem atmosphere can be performed at lower temperatures

and with shorter duration to obtain similar or even better

mechanical properties than when modified under super-

heated steam. This finding is in contrast to earlier studies. It

can be concluded that the temperature could be reduced to

150–180 �C rather than 180–210 �C as is currently used in

typical thermal modification processes when superheated

steam is used. High anti-swelling efficiency values were

obtained of 60 % for sapwood and 52 % for heartwood in

this study and it is proposed that this results, at least to

some extent, from a decrease in accessible hydroxyl

groups.

Acknowledgments Technical assistance of MSc Stefan Lehneke,

financial support of the Finnish Foundation for Technology Promo-

tion (Tekniikan edistamissaatio) and the Puumiesten Am-

mattikasvatussaatio are gratefully acknowledged.

References

Bhuiyan MTR, Hirai N, Sobue N (2000) Changes of crystallinity in

wood cellulose by heat treatment under dried and moist

conditions. J Wood Sci 46:431–436

Boonstra MJ, Van Acher J, Tjeerdsma BF, Kegel EV (2007) Strength

properties of thermally modified softwoods and its relation to

polymeric structural wood constituents. Ann For Sci 64:679–690

Borrega M, Karenlampi PP (2008) Effect of relative humidity on

thermal degradation of Norway spruce (Picea abies) wood.

J Wood Sci 54:323–328

Borrega M, Karenlampi PP (2010) Hygroscopicity of heat-treated

Norway spruce (Picea abies) wood. Eur J Wood Prod

68:233–235

Dagbro O, Torniainen P, Karlsson O, Moren T (2010) Colour

responses from wood, thermally modified in superheated steam

and pressurized steam atmospheres. Wood Mat Sci Eng

5:211–219

Ding T, Lianbai G, Liu X (2011) Influence of steam pressure on

chemical changes of heat-treated Mongolian pine wood. Biore-

sources 6:1880–1889

Dwianto W, Takana F, Inoue M, Norimoto M (1996) Crystallinity

changes of wood by heat or steam treatment. Wood Res

83:47–49

EN 1534 (2000) Wood and parquet flooring—determination of

resistance to indentation (Brinell)—Test method. CEN—Euro-

pean Committee for Standardization, Brussels

EN 408 (2011) Timber structures. Structural timber and glued

laminated timber. Determination of some physical and mechan-

ical properties. CEN—European Committee for Standardization,

Brussels

EN ISO 179-1 (2010) Plastics. Determination of Charpy impact

properties. Part 1: non-instrumented impact test. CEN—Euro-

pean Committee for Standardization, Brussels

Haines DW, Leban JM, Herbe C (1996) Determination of Young’s

modulus for spruce, fir and isotropic materials by resonance

flexure method with comparisons to static flexure and other

dynamic methods. Wood Sci Technol 30:253–263

Hill CAS (2006) Wood modification—chemical, thermal and other

processes. John Wiley and Sons Ltd., Chichester

Hill CAS, Ramsay J, Keating B, Laine K, Rautkari L, Hughes M,

Constant B (2012) The water vapour sorption properties

thermally modified and densified wood. J Mat Sci 47:3191–3197

Korkut S, Budakcı M (2009) Effect of high-temperature treatment on

the mechanical properties of Rowan (Sorbus aucuparia L.)

wood. Drying Technol 27:1240–1247

Korkut S, Akgul M, Dunbar T (2008) The effects of heat treatment on

some technological properties of Scots pine (Pinus sylvestris L.)

wood. Bioresour Technol 99:1861–1868

Metsa-Kortelainen S, Viitanen H (2010) Effect of fungal exposure on

the strength of thermally modified Norway spruce and Scots

pine. Wood Mat Sci Eng 1(13):23

Metsa-Kortelainen S, Antikainen T, Viitaniemi P (2006) The water

absorption of sapwood and heartwood of Scots pine and Norway

spruce heat-treated at 170�C, 190�C, 200�C and 230�C. Holz

Roh Werkst 64:192–197

Navi P, Sandberg D (2012) Thermo-hydro-mechanical processing of

wood engineering sciences. EPFL Press, Lausanne

Phuong LX, Shida S, Saito T (2007a) Effect of heat treatment on

brittleness of Styrax tonkinensis wood. J Wood Sci 53:181–186

Phuong LX, Takayama M, Shida S, Matsumoto Y, Aoyagi T (2007b)

Determination of the accessible hydroxyl groups in heat-treated

Styrax tonkinensis (Pierre) Craib ex Hartwich wood by hydrogen-

deuterium exchange and 2H NMR spectroscopy. Holzforschung

61:488–491

Rautkari L, Kamke FA, Hughes M (2011) Density profile relation to

hardness of viscoelastic thermal compressed (VTC) wood

composite. Wood Sci Technol 45:693–705

Rautkari L, Hanninen T, Johansson L-S, Hughes M (2012) A study by

X-ray photoelectron spectroscopy (XPS) of the chemistry of the

surface of Scots pine (Pinus sylvestris L.) modified by friction.

Holzforschung 66:93–96

Rautkari L, Laine K, Kutnar A, Medved S, Hughes M (2013a)

Hardness and density profile of surface densified and thermally

modified Scots pine in relation to degree of densification. J Mat

Sci 48:2370–2375

Rautkari L, Hill CAS, Curling S, Jalaludin Z, Ormondroyd G (2013b)

What is the role of the accessibility of wood hydroxyl groups in

controlling moisture content? J Mat Sci 48:6352–6356

Rowell RM, Ellis WD (1978) Determination of dimensional stabil-

ization of wood using the water-soak method. Wood Fiber Sci

10:104–111

Seborg RM, Tarkow H, Stamm A (1953) Effect of heat upon the

dimensional stabilization of wood. For Prod J 3:59–67

Sivonen H, Maunu SL, Sundholm F, Jamsa S, Viitaniemi P (2002) Mag-

netic resonance studies of thermally modified wood. Holzforschung

56:648–654

Stamm AJ (1935) Shrinking and swelling of wood. Ind Eng Chem

21:401–406

Sundqvist B, Karsson O, Westermark U (2006) Determination of

formic-acid concentrations formed during hydrothermal treat-

ment of birch wood and its relation to colour, strength and

hardness. Wood Sci Technol 40:549–561

Tjeerdsma BF, Boonstra M, Pizzi A. Tekely P, Militz H (1998)

Characterisation of thermally modified wood: molecular reasons

for wood performance improvement. Holz Roh Werkst

56:149–153

Torniainen P, Dagbro O, Moren T (2011) Thermal modification of

birch using saturated and superheated steam. In: Proceedings of

the 7th meeting of the Nordic-Baltic Network in Wood Material

Science & Engineering (WSE), Oslo, Norway

Vainio-Kaila T, Rautkari L, Nordstrom K, Narhi M, Natri O, Kairi M

(2013) Effect of extractives and thermal modification on

40 Eur. J. Wood Prod. (2014) 72:33–41

123

antibacterial properties of Scots pine and Norway spruce. Int

Wood Prod J. doi:10.1179/2042645313Y.0000000038

Welzbacher CR, Brischke C, Rapp OR (2007) Influence of treatment

temperature and duration on selected biological, mechanical,

physical and optical properties of thermally modified timber.

Wood Mat Sci Eng 2:66–76

Willems W (2009) A Novel economic large-scale production

technology for high-quality thermally modified wood. In:

Proceedings of the 4th European Conference on Wood Modi-

fication, Stockholm, Sweden

Eur. J. Wood Prod. (2014) 72:33–41 41

123