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The Influence of Elevated Temperatures The Influence of Elevated Temperatures

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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