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The Effect of Wood Drying and Heat Modificationon Some Physical and Mechanical Properties ofRadiata Pine

René Herrera-Díaz, Víctor Sepúlveda-Villarroel, Natalia Pérez-Peña, LinetteSalvo-Sepúlveda, Carlos Salinas-Lira, Rodrigo Llano-Ponte & Rubén A.Ananías

To cite this article: René Herrera-Díaz, Víctor Sepúlveda-Villarroel, Natalia Pérez-Peña, LinetteSalvo-Sepúlveda, Carlos Salinas-Lira, Rodrigo Llano-Ponte & Rubén A. Ananías (2017): The Effectof Wood Drying and Heat Modification on Some Physical and Mechanical Properties of RadiataPine, Drying Technology, DOI: 10.1080/07373937.2017.1342094

To link to this article: http://dx.doi.org/10.1080/07373937.2017.1342094

Accepted author version posted online: 30Jun 2017.

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1

The Effect of Wood Drying and Heat Modification on Some Physical and

Mechanical Properties of Radiata Pine

René Herrera-Díaz1, Víctor Sepúlveda-Villarroel

2, Natalia Pérez-Peña

2, Linette Salvo-

Sepúlveda2, Carlos Salinas-Lira

2, Rodrigo Llano-Ponte

1, Rubén A. Ananías

2

1Chemical Engineering & Environmental Department, University of the Basque Country,

San Sebastian, Spain 2Research Group on Wood Drying & Heat Treatments, Wood

Engineering Department, Universidad del Bio-Bio, Concepcion, Chile

Abstract

In order to evaluate evolution of physical and mechanical properties due to drying and

heat modification, a load of radiata pine wood was selected and properties were measured

after each drying process. The results revealed interesting correlations between intrinsic

factors and properties; the values of density were highly dispersed after drying or thermal

treatment and uncorrelated with others parameters, but the minimum density values were

kept constant after heat-treatment. Moreover, weight loss (WL) and moisture content

(MC) were decreasing proportionally to the treatment intensity, due to wood-water

interactions, cell wall changes and thermal degradation of wood fractions. WL and MC

were reasonably correlated with the dimensional stability, improving the dimensional

stability after drying treatments but keeping the same order of anisotropy. Regarding the

wood stiffness (MOE), it was unaffected by the drying temperature, and the correlations

between MOE and MC or WL appear to be acceptable, and the values of MC or WL did

not adversely affect the MOE. However, the modulus of rupture was dropped during the

drying process, obtaining three differentiated groups with a decrease of around 59% after

thermal modification.

2

KEYWORDS: Heat-treatment, high temperature drying, kiln drying, thermal

modification, wood drying

INTRODUCTION

Wood drying is a fundamental step in processing wood products, where about 90% of

sawn timber is dried and 100% of the exported wood requires any type of drying

process[1]

. The international legislation ensures the fulfillment of the phytosanitary

requirements from importing countries regarding the wood products. In case of radiata

pine wood, it requires from 14 h to usually more than 24 h of drying when using artificial

drying schedules such as kiln-drying as well as in thermal modification methods[2]

. To

sum up, it is important to get a good drying quality as generates standardized wood

products, besides increasing its service life in different environments[3]

.

The wood industry is particularly interested in the development of improved drying

technologies that provide better energy efficiency, enhancing the drying quality and rate

while reducing the air emissions of conventional drying systems[4,5]

. Among the

technologies currently used, it is worth noting the conductive heating of wood by heating

plates, the convection heating of wood using superheated steam or hot air as well as the

dielectric heating of wood by radio-frequency for phytosanitary treatment[6]

. However,

these heating methods present some drawbacks regarding the transmission of heat to

wood when pressure is reduced at sub-atmospheric pressure; especially the rates of

convective heat transfer which are highly reduced under vacuum[7]

.

3

The wood quality for fast-growing conifers initially depends on the rotation time, in

which plantations with short rotations usually present higher amount of juvenile wood in

contrast to mature wood[8,9]

. This particularity alters the properties after drying, and

therefore, wood from short rotations usually present low stiffness (MOE), greater

longitudinal shrinkage and low hardness[10,11]

. Therefore, it is fundamental to follow

changes that occur in the physical-mechanical properties from green sawn timber to the

thermally modified wood, in order to know the factors that are significantly influenced by

the drying process and which alter the wood dimensions or its strength values, in this

particular case with radiata pine, due to its low dimensional stability compared to others

commercial species.

In this study the fast growing specie Pinus radiata was used for modeling the physical-

mechanical changes after each drying process by measuring the moisture content,

density, shrinkage, MOR and MOE from the same load, in order to find correlations

between properties and improvements as a result of drying process.

MATERIAL AND METHODS

Timber freshly sawn from radiata pine (Pinus radiata D. Don) was harvested in the

Chilean forests and supplied by CMPC-Maderas S.A. for this study. The initial load

supplied was 244 samples (25x100x3200mm) and 14 samples from this load were

randomly taken in order to measure the moisture content and density of the green

material.

4

Wood Drying Process

The initial load (244 samples) was uniformly piled and placed in layers of equal distance

which were separated using sticks of 20 mm thickness to let the air move through the

stack and distribute the weight vertically from top to bottom. Then, the load was air-dried

exposing by natural convection but with certain controls: the load was partially covered

to protect from precipitation and direct sun, an additional weight was used above the load

to avoid defects as it dries and the lumber stacked was over a concrete surface where

water could not pool. Finally, the load was air-dried during the warmer months (4 weeks)

to obtain standard conditions comparable to that found in sawmills. From this load 14

samples were randomly taken in order to characterize the air-dried material.

Subsequently, 230 pieces from the initial load were prepared to kiln drying in a pilot

scale kiln by stacking it using stickers of 20 mm thickness for each set of samples. The

kiln used was a flexible industrial prototype with capacity of 3.5 m3

designed to undergo

temperatures up to 250 ºC in presence of steam and with a flow speed between the

lumber pile up to 10 m/s (Fig. 1). The dry and wet bulb temperatures were monitored as

well as the temperature and moisture content of wood according to the setup and kiln

schedule. Specifically, the drying schedule executed was 100/70ºC (dry bulb/wet bulb)

and air flow speed of 6 m/s. After the drying process 14 samples were randomly taken to

measure the properties of the kiln-dried material.

Thermal Modification

5

After wood drying the kiln was loaded with the remaining 216 pieces and the load was

prepared in the same way as before. The temperatures and moisture content were

monitored according to the treatment with an air flow speed of 7 m/s and in presence of

steam to prevent oxidation reactions. The modification process began with a fast increase

of temperature to 100 °C, allowing the wood drying up to 3-4% of moisture content.

Subsequently, steam was sprinkled in order to avoid damage in wood and the temperature

was raised to 190/100ºC (dry bulb/wet bulb) and the heat-treatment temperature was

maintained for approximately 2 hours. The last stage was the cooling down and

stabilization of the samples during about 4 hours at controlled relative humidity until

room temperature (Fig. 2). The temperature gradient between surface and inner site of the

samples did not exceed of 15-20 °C with the purpose of retaining the wood quality. After

thermal modification 14 samples were randomly taken to characterize the heat-treated

material.

Physical And Mechanical Characterization

From each drying step (air-dried, kiln dried and heat treated wood) 14 samples were

randomly taken and three different density values were measured: anhydrous density

(Eq.1), basic density (Eq.2), and reference density (Eq.3), in accordance with the Chilean

normative NCh 176/2[12]

. The following equations were used for the calculations:

0

0

3m0

V

kg    ( )

m (1)

0

v

3m

V

kg   ( )

mb (2)

6

R

R

3m

V

kg      ( )

mR

(3)

In which m0 is the anhydrous mass of sample; V0 is the anhydrous volume of sample; Vv

is the the volume of sample in green state (over the fiber saturation point); mR is the mass

of sample at the current drying condition; VR is the volume of sample at the current

drying condition. The moisture content (MC) of samples was calculated based on the

oven-dry weight at 103 ± 2 ºC (NCh 176/1[13]

), using the following equation:

b a

a

W WMC 100 ( )

W    % (4)

Where Wb is the sample weight at the current drying condition and Wa is the sample

weight after oven-dry at 103±2 ºC. The dimensional changes in each plane (longitudinal

(L), tangential (T) and radial (R)) were expressed as volumetric shrinkage (Eq.5) and as

anisotropy coefficient (Ψ), which is the tangential to radial ratio (Eq.6) before (b) and

after (a) drying process. In addition, the fiber saturation point (FSP) was estimated

according to Eq. 7[14]

:

b aVS L T R L T R 100  (%)  (5)

b a

b a

(T T )Ψ

(R R ) (6)

b

VSFSP    100(%)

(0.09 ρ ) (7)

For the mechanical tests samples were cut (20 x 20 x 320 mm) and stored under standard

conditions (20 ± 2 ºC; 65 ± 3% RH) before performing the modulus of elasticity (MOE)

and module of rupture (MOR), calculated according to NCh /986-987 standards[14,15]

.

MOE and MOR were measured by using the universal testing system (Instron 4468) in

7

the three-point bending method with 10 kN of load cell and with a crosshead speed of 2.8

mm/min. The load was done at tangential direction of sample calculating MOR and MOE

as follows:

lim

2

3P lMOE (MPa)

2wh  (8)

max

2

3P lMOR (MPa)

2wh  (9)

Where Plim is the load at the limit of rupture, Pmax is the maximum load supported, l is the

free span distance between the centers of the two supports (110 mm), w is the width and

h is the height of the sample.

Data Analysis

The results of the measured properties were analyzed according to the characteristic

values of normality and homogeneity of variances UNE-EN 14358[16]

. In addition, a

multiple comparison procedure (ANOVA) was used to determine which means were

significantly different from others and the confidence levels were examined. In some

cases, the Bonferroni correction (BSD) was applied after rejecting the null hypothesis.

The software used for the statistical and graphing analysis was Origin 9.1.

RESULTS AND DISCUSSION

Following the progress of wood properties during the drying process from green state to

heat treatment, provides us with a range of information on the effects of drying and

possible correlations regarding the physical-mechanical interactions. When the wood is

green, the moisture content (MC) is very high and variable; reaching values up to 100-

8

120% within the same load[11]

. However, after air drying process, the MC was found

below the fiber saturation point (FSP=29.07%) but with a high standard deviation, and

values of MC between 40 to 10% within the same load. Subsequently, the kiln-dried

process stabilizes the MC, showing values around 10 to 8%. Finally, after thermal

modification the MC decreases and becomes narrow, reaching values around 6% (Table

1). Regarding the radiata pine wood density, the anhydrous density (ρ0), basic density

(ρb), and reference density (ρR) were calculated in order to know their ranges, differences

and tendencies. The average density after air-drying varies from 496 to 388 kg/m3 within

the following range: ρR > ρ0> ρb, but according to the Bonferroni correction (BSD at the

0.001 level) only the ρR was significantly different. Moreover after kiln-drying the

average density was slightly lower with narrower variability, from 455 to 399 kg/m3, and

with the same proportion (ρR > ρ0> ρb), but in this case the ρb was significantly different.

The density range found in literature for radiata pine was similar to that measured in this

work[16,17]

. After thermal modification the lowest mean density was found (from 459 to

396 kg/m3), with a narrower interval and lower variability within the range ρR > ρ0> ρb

but without significant differences between densities according to BSD. These results

showed that samples with lower densities were generally more resistant to heat-treatment

as density values were not lower but were close-fitting. The density values of radiata pine

after heat-treatment were similar to those reported elsewhere[18,19]

.

The MC calculated after each drying process was correlated with the density (ρR, ρ0, ρb)

and analyzed at 95% confidence interval (Fig.3). After air-drying process, the parameters

associated presented a positive correlation, with a moderate association between MC-ρR

9

(r=0.58) but a negative correlation with a moderate association between MC-ρ0 (r=-0.52)

and MC-ρb (r=-0.53). These results showed that when using either the anhydrous or green

volume to calculate density, the correlation with MC presented similar trends but

opposed to those obtained when density was calculated using reference values. The high

variability at air-drying conditions was related to the wood-water interactions that occur

close to the FSP in which the capillary water (free water) plays an important role as well

as the cell wall microporosity of each wood sample[20]

.

After air-drying process, the MC-density correlation was negative and moderate with ρ0

(r=-0.33) or with ρR (r=-0.32) but the association was weak when ρb (r=-0.19) was

correlated. The correlation MC-density decreased while MC was relatively constant after

kiln-drying, which implies that the free water was evaporated and moisture content was

kept below the FSP. Finally, after heat modification a weak or very weak MC-density

relationship was presented (r<0.20), in which MC decreased about 2% compared to kiln-

drying, but density was not an influencing factor. Thus, the decrease of MC does not

correlate with density values, and it depends on other physical changes such as the cell

wall thickness or microporosity[21,22]

.

One important effect of heat-treatment was the reduction of the hygroscopicity, which in

turn contributed to higher dimensional stability. Thus, the load presented a progressive

recovery of the volumetric shrinkage by more than 50% from air-drying to kiln-drying, to

finally show reduced dimensional changes (around 1%) after heat treatment (Table 1).

This effect is proportional to the decreasing number of accessible hydroxyls on the

10

cellulose, hemicelluloses and lignin due to thermal modification[20,22]

. All mean values

were significantly different (P<0.001, BSD) either in volumetric or single direction

(longitudinal, tangential and radial).

Despite the fact that a substantial volumetric stability was achieved after heat treatment,

the transversal to radial shrinkage ratio (Ψ) presented only a slight decrease towards the

unity from air-drying to heat treatment, although without statistical differences. The

relationship between shrinkage (tangential and radial) and moisture content was plotted

in Figure 4 where the association of factors is well defined and linearly strong (R2>0.96).

The results showed that radiata pine kept the same order of anisotropy either at high or

low moisture content, but improving the dimensional stability after drying treatments.

Thus, the anatomical aspects that influenced the wood shrinkage, such as the tracheid

structure, cell wall thickness, ray tissues, and the microfibril angles of the transversal

directions, were not significantly affected[23]

.

In addition, weight loss versus transverse shrinkage was plotted in Figure 5A, in which

the distribution in three groups correlated with each drying process is clearly highlighted

(R2>0.84). The group with air-dried samples is distributed within the quadrant containing

the higher weight loss and shrinkage range, proving that at this point the load was below

the FSP since wood only shrinks when the free water from the lumens and intercellular

spaces is evaporated, therefore, the wood load lost in average 50% of its weight. Figure

5B was set for the remaining groups in a reduced range (kiln-dried and heat-treated),

finding the weight loss of each group well differentiated and with acceptable correlation

11

of factors (R2>0.70). In the kiln-drying range it was found that wood loses moisture in the

form of bound water and thus decreases in weight and volume, but keeping the wood

anisotropic relationship. After heat treatment mass loss persisted, but in this case it was

due to the degradation of compounds that takes place at treatment temperatures,

especially the degradation of hemicelluloses, which in turn leads the anisotropic

differences towards unity as well as decreases the sorption sites and the MC. These

results are comparable to that found with similar species by other authors[24,25]

.

Regarding the mechanical properties, the wood stiffness (MOE) from air-dried to kiln-

dried increased about 30%, and from kiln-dried to heat treatment the variation was small

in magnitude and without significant differences according to the BSD correction (Table

1). These results shown that wood stiffness was unaffected by the drying temperature,

whereas the values were similar when drying at 100 °C or when thermally modified at

190 °C. However, the modulus of rupture (MOR) was significantly dropped during the

drying process, obtaining three differentiated groups (BSD at the 0.001 level), in which

the MOR was 18% lower after kiln drying followed by a decrease of around 59% after

thermal modification. The bending strength loss seems to be an effect of the degradation

of the hemicelluloses fraction and structural changes, since this fraction is the most

thermal-chemically sensitive, and at heat-treatment conditions some chemical reactions

are produced which could even affect the cellulose fraction, increasing its crystalline

proportion, and thus having a negative impact on MOR[22,26,27]

. The mechanical values of

this work were similar to those found in other studies[27,28,29]

.

12

Finally, the wood stiffness was correlated in function of basic density (ρb), MC and

weight loss, and their mean values were associated to the fit line and confidence ranges

(Fig. 6). Between MOE and ρb there was no clear correlation, showing a wide range of

densities for each drying step but without a particular trend, suggesting a low dependence

of the density with respect to the drying process (air-dried samples were out of 95%

confidence band). On the other hand, there appears to be an acceptable correlation

between MOE-MC, with a negative linear dependence (R2=0.49), thus, below the FSP

the MOE increased almost linearly with the decrease in moisture content, and the low

moisture content obtained after heat treatment did not adversely affect the MOE. The

correlation between MOE-weight loss presented a similar negative linear dependence

(R2=0.53), and according to some authors the lower MC could reduce the effect of weight

loss on bending strength[27,30]

.

CONCLUSIONS

The evolution of physical and mechanical properties due to wood drying and heat

modification revealed interesting correlations between intrinsic factors and radiata pine

properties. The values of density were highly disperse after drying or thermal treatment

and not categorically correlated with other parameters. In contrast, wood with low density

was generally more stable, since the minimum density values were kept constant after

heat-treatment. Moreover, weight loss and MC were decreasing proportionally to the

treatment intensity; in relation to the weight loss, the decrease after kiln-drying was due

to loses of bound water, and after heat treatment it was due to the degradation of the

hemicelluloses fraction and amorphous cellulose. Regarding the weight loss, it was

13

associated with changes in microporosity or in cell wall thickness. Both weight loss and

MC were reasonably correlated with the dimensional stability, improving the dimensional

stability after drying treatments but keeping the same order of anisotropy; thus, the

anatomical aspects that influenced the wood shrinkage were not significantly affected.

Regarding the mechanical properties, the MOE was unaffected by the drying

temperature, whereas the values were similar when drying at 100 °C or when thermally

modified at 190 °C. However, the MOR was dropped during the drying process,

obtaining three differentiated groups with a decrease of around 59% after thermal

modification. The correlations between MOE and MC or WL appear to be acceptable,

with a negative linear dependence in which the values of MC or WL did not adversely

affect the MOE.

ACKNOWLEDGEMENTS

The authors appreciate the financial support of the National Commission of Scientific &

Technological Research (Conicyt) of Chile (Fondequip EQM130236), the Basque

Government (scholarship of young researchers training, IT1008-16) and the support of

COST Action FP1407 through STSM-35419.

NOMENCLATURE

FSP Fiber saturation point (%)

h Height (mm)

L Longitudinal dimension (m)

l Free span distance between supports (mm)

14

MC Moisture content (%)

MOE Modulus of elasticity (MPa)

MOR Module of rupture (MPa)

m0 Anhydrous mass (kg)

mR Mass at the current drying condition (kg)

Plim Load at the limit of rupture (kN)

Pmax Maximum load supported (kN)

R Radial dimension (m)

T Tangential dimension (m)

Tdb Dry bulb temperature (°C)

Tw Wood temperature (°C)

Twb Wet bulb temperature (°C)

V0 Anhydrous volume (kg)

VR Sample volume (m3)

Vv Green volume (m3)

w Width (mm)

Wa Weight after oven-dry (kg)

Wb Weight at the current drying condition (kg)

WL Weight Loss (kg)

Greek symbols

ρ0 Anhydrous density (kg/m3)

ρb Basic density (kg/m3)

ρR Reference density (kg/m3)

15

Ψ Ratio tangential/radial (/)

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19

Table 1. Effect of drying and heat treatment on physical and mechanical properties.

Sample Density

[kg/m3]

MC

[%]

WL

[%]

VS

[%]

Ψ

[T/R]

MOE

[MPa]

MOR

[MPa]

ρ0 ρb ρR

Air-

dried

432.52

±45.19

*** 0(0-

b)

388.66

±38.66

***

0(0-b)

496.36

±40.39

***

1

22.61

±10.95

***

1

51.83

±5.80

***

1

10,17

±0.63

***

1

1.55

±0.36

7036.79

±1650.96

***

1

101.29

±4.80

***

1

Kiln-

dried

454.98

±45.85

***

0(0-R)

399.38

±45.34

***

1

474.19 ±

46.57

***

0(0-R)

9.16

±0.50

***

1

8.35

±0.42

***

0

4,41

±0.42

***

1

1.47

±0.33

9241.89

±1976.19

***

0

82.98

±5.55

***

1

Heat-

treated

437.88

±42.18)

***

0(0-b-R)

395.88

±40.18

***

0(0-b)

459.14

±43.18

***

0(0-R)

6.25

±0.60

***

1

5.88

±0.54

***

0

1,37

±0.33

***

1

1.37

±0.27

9653.65

±1456.63

***

0

48.97

±11.07

***

1

ρ0= Anhydrous density ρB= Basic density ρR= Reference density; MC= Moisture Content;

WL= Weight Loss; VS= Volumetric Shrinkage; Ψ= anisotropy coefficient; MOE=

Modulus of elasticity; MOR= Modulus of rupture; Significance of each set of values

means (one-way ANOVA): (***) significantly different, P<0.001; means comparisons by

Bonferroni test: (0) difference of the means is not significant at ***, (1) difference of the

means is significant at ***

20

Figure 1. Flexible kiln-dried prototype (Neumann, Modelo Lab-3.5e, Concepción, Chile).

21

Figure 2. Evolution of temperatures during thermal modification: Tdb: Dry bulb

temperature; Twb: Wet bulb temperature; Tw: Wood temperature.

22

Figure 3. Correlations between MC and measured densities from air drying to heat

modification.

23

Figure 4. Performance of moisture content versus shrinkage from air drying to heat

modification.

24

Figure 5. Weight loss and shrinkage in the transverse dimensions after drying and heat

treatment.

25

Figure 6. Wood stiffness correlated with basic density, moisture content and weight loss

from air-drying (A-D) to kiln drying (K-D) and heat treatment (H-T).

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