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Drying TechnologyAn International Journal
ISSN: 0737-3937 (Print) 1532-2300 (Online) Journal homepage: http://www.tandfonline.com/loi/ldrt20
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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