1 the influence of elevated temperatures the influence of elevated temperatureson selected...
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The Influence of Elevated Temperatures The Influence of Elevated Temperatures
onon
Selected Properties of RubberwoodSelected Properties of Rubberwood
1,2 Sik H.S., 2 Sarani Z., 2 Sahrim Hj. A., & 1
Choo K.T.
1Forest Research Institute Malaysia (FRIM)
2 Universiti Kebangsaan Malaysia (UKM)
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Introduction Rubberwood - Hevea brasiliensis or rubber trees of Euphorbiaceae family. Plantation timber- harvested when latex production is no longer economically viable. Wooden furniture, mostly rubberwood, accounted for 80% of total furniture exports of 7.25 billion ringgit (USD 2.07 billion) in 2006.
Drying - Reduce the moisture content in freshly sawn timber / dry timber down to the equilibrium moisture content (emc) that it will attain in service.
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Elevated temperature drying of tropical
hardwoods is still unknown in Malaysia and other tropical hardwoods producing region.
Drying at elevated temperature is accomplished at dry-bulb temperatures of 100 °C or higher.
Currently, more than 95% of the drying mills in
Malaysia are based on conventional low temperature-heated system.
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Elevated-temperature Drying?
time and cost saving practice reduce drying time; allow “just-in-time” production that leads to lower
inventory cost and smaller plant sites; lower energy consumption and probably fewer
deformations enhancing the properties of throughput dried timbers
To improve the performance of drying operation, towards achieving a more energy-and-cost efficient system.
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Objectives
Investigate the influence of elevated temperatures on specific properties of tangential and radial sawn rubberwood compared to conventionally dried material in a laboratory experimental kiln
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Simulated Drying using Experimental Kiln : 60 ºC (control) 100 ºC 120 ºC 130 ºC 140 ºC 150 ºC
Initial moisture content : 62.36 – 64.13 % Monitoring of drying activities up to 24 hours for elevated
drying.
Methodology
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Physical properties Mechanical properties Timber stress at elevated drying temperatures Low molecular sugars content
Results and Findings
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quarter –sawn
W
T
flat sawn
W
T
Cross sectionCross section
Source : USDA Handbook
A. Physical Properties
Shrinkage in wood Wood Shrink – to achieve dimensional stability Shrinkage in Transverse direction :Shrinkage in Transverse direction :
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W
T
FIGURE 1. Shrinkage in Quarter Sawn Rubberwood
0
1
2
3
4
5
6
7
Temp. (°C)
Sh
rin
ka
ge
(%
)
Width 2.22 1.58 3.21 4.07 3.56 3.25
Thickness 4.65 2.75 6.53 6.24 4.19 4.00
60 100 120 130 140 150
Cross-section of a quarter –sawn timber
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FIGURE 2. Shrinkage in Flat Sawn Rubberwood
0
1
2
3
4
5
6
Temp (°C)
Sh
rin
ka
ge
(%
)
Width 4.12 3.79 3.81 3.75 3.69 3.45
Thickness 2.8 3.29 5.52 5.27 3.89 4.55
60 100 120 130 140 150
W
T Cross-section of a flat sawn timber
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Shrinkage in Longitudinal direction
tension wood normal wood
Drying Temperature
(°C)
Quatersawn Flatsawn
60 0.45 0.33
100 0.00 0.06
120 0.28 0.44
130 0.44 0.50
140 0.22 0.67
150 0.33 0.33
Excessive shrinkage along length
presence of tension wood.
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Specific Gravity
Specific gravity of rubberwood at green and after drying at different temperatures.
Temperature
(°C)
Specific Gravity
at Green
Specific Gravity
after Drying
Increase of Specific Gravity
(%)
60 0.61 0.63 3.28
100 0.63 0.67 6.35
120 0.62 0.66 6.45
130 0.62 0.67 8.06
140 0.61 0.68 11.48
150 0.60 0.67 11.67
1313
0
0.2
0.4
0.6
0.8
60 100 120 130 140 150Temperature (°C)
Spe
cific
Gra
vity
, S
G
0.00
4.00
8.00
12.00
Incr
ease
of S
G (
%)
SG at green SG after drying Increase of SG (%)
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Drying stresses
– the shell of a board undergoing drying initially shrinks more than the core compressive stress collapse.
Collapse in Wood ( Capillary tension)
Crucial, especially at elevated drying temperature
Rubberwood Timber y150 ºC 100 ºC
Typical honeycombing/ internal checks formation
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150 ºC60 ºC 120 ºC 140 ºC
Prong Test
Degree of pinched-in of all test pieces were generally in permissible range of 1 to ≤ 3mm after conditioned for 24 hours at room temperature
Pinched-In
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B. Mechanical Properties Compared to 60°C ( control) :
MOR was higher in 100°C, 120°C, 130°C MOE was higher in 100°C, 120°C, 130°C and
140°C
Compressive strength was higher in all elevated
temperatures (highest in 100 °C) Hardness value was higher in all elevated
temperatures (highest in 140 °C) Shear strength was higher in all elevated
temperatures (highest in 140 °C)
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Modulus of Rupture
0.00
40.00
80.00
120.00
60 100 120 130 140 150
Temperature (°C)
MO
R (
MP
a)
Increase of MOR values are significant in 100 °C- and 120 °C-dried samples
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Modulus of Elasticity
0.0
4000.0
8000.0
12000.0
60 100 120 130 140 150
Temperature (°C)
MO
E (
MP
a)
Increase of MOE values are insignificant; reduction is significant at 150°C
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0.00
20.00
40.00
60.00
60 100 120 130 140 150
Temperature (°C)
Co
mp
ressi
ve s
tre
ng
th (
MP
a) Compression perpendicular to grain
Increase in compression parallel to grain is significant at 95% confidence interval for all temperatures.
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Hardness Value
0.000
2.000
4.000
6.000
60 100 120 130 140 150Temperature (°C)
Har
dnes
s (K
N)
Increase in hardness values is significant at 95% confidence interval for all temperatures.
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Shear Strength
0.00
5.00
10.00
15.00
20.00
60 100 120 130 140 150
Temperature (C)
Shea
r Str
engt
h (M
Pa)
Increase in shear strength is only significant (P< 0.05) for 140 °C-dried samples
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C. Influence of temperature on the redistribution of LM sugars in rubberwood
Temperature
(°C)Air dry 60 100 130
1-3 mm 0.27 0.27 0.53 0.83
14-16 mm 0.42 0.71 0.53 0.23
Case : Core 0.4 : 1 0.25 : 1 1 : 1 3.6 : 1
Note : Air dry condition : ambient temperature at 25 – 32°C
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Conclusion The incremental shrinkages of rubberwood measured at elevated temperatures indicates the requirement for increase shrinkage allowances, or green sawn size target.
In tangential sawn, where drying stresses are more significant than in radial, more regular and positive increase of shrinkage in thickness is noticed.
Abnormal shrinkage in longitudinal direction was detected, due to the occurrence of tension wood, commonly found in rubberwood.
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The relative mechanical strength of rubberwood dried at elevated temperatures up to 130 °C increased when
compared to conventional-dried sawn.
Redistribution of hydrolysed low molecular sugar (LMS) during drying is more prominent at elevated temperature.
The incidence of collapse and honeycomb did not occur in elevated-temperature drying up to 150°C.
Rubberwood is able to withstand drying stresses at high temperatures, thus tolerable of drying at elevated
temperatures up to 150ºC.
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Thank youThank you