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This article was downloaded by: [University of Cambridge]On: 08 October 2014, At: 00:28Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK
Polymer-Plastics Technologyand EngineeringPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lpte20
Swelling Behavior of Wood andWood-Plastic Composite (WPC)Mubarak A. Khan a & K. M. Idriss Ali aa Radiation Chemistry Laboratory , Institute ofNuclear Science and Technology, BangladeshAtomic Energy Commission , P.O. Box 3787, Dhaka,BangladeshPublished online: 22 Sep 2006.
To cite this article: Mubarak A. Khan & K. M. Idriss Ali (1992) Swelling Behaviorof Wood and Wood-Plastic Composite (WPC), Polymer-Plastics Technology andEngineering, 31:3-4, 299-307, DOI: 10.1080/03602559208017750
To link to this article: http://dx.doi.org/10.1080/03602559208017750
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P0LYM.-PLAST. TECHNOL. ENG., 31(3&4), 299-307 (1992)
SWELLING BEHAVIOR OF WOOD AND WOOD-PLASTIC COMPOSITE (WPC)
MUBARAK A. KHAN and K. M. IDRISS ALI
Radiation Chemistry Laboratory Institute of Nuclear Science and Technology Bangladesh Atomic Energy Commission P.O. Box 3787 Dhaka, Bangladesh
Abstract
Wood-plastic composites were prepared by y-radiation with five types of low-grade timbers of Bangladesh using methylmethacrylate as monomer. Water-swelling behavior of both wood and its WPC was studied. The water uptake by the WPC was found to be lower compared to the untreated wood. Higher tensile strength values were obtained with untreated WPC than with the corresponding timbers both before and after the water-swelling treatments.
INTRODUCTION
In recent years wood-plastic composites (wpc) received considerable atten- tion both in the literature and in industry. Water is a natural constituent of all parts of a living tree and in the xylem portion, water (moisture) commonly makes up over half the total weight; that is, the weight of water in green wood is normally equal to or greater than the weight of dry wood substance (Haygreen and Bowyer, 1989). When the tree dies or a log is processed into lumber, veneer, or chips, the wood immediately begins to lose some of its moisture to the surrounding atmosphere. If the drying continues long enough, the dimen- sions and the physical properties of the wood begin to undergo change. Some water will remain within the structure of the cell walls even after wood has been manufactured into a lumber, veneer, particle, or fiber products. The physical and mechanical properties, resistance to biological deterioration, and dimensional stability of the products are affected by the amount of water
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Copyright 0 1992 by Marcel Dekker, Inc.
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KHAN AND ALI
present and its fluctuation with time. It is reported (Khan and Rahman, 1991) that in moisture sorption isotherm studies of wood and wood-plastic composite, the equilibrium moisture content is lower with WPC than with the untreated wood. Bangladesh is a country of high humidity. Most of the wooden appliances and articles exhibit various dimensional instability during different seasons of the year. The objective of this study was to prepare WPC from low-grade wood native to Bangladesh and to observe the swelling behavior of both WPC and wood in order to study the extent of the utility of WPC products.
EXPERiMENT AL
Wood samples of sizes 6.2 X 0.8 X 0.4 cm were taken from kadom (An- throcephulus cadamba), simul (Salmalia malabarica), koroi (Samanea samane), mango (Mangifera indica), and debdaro (Polythia logifolia). Their respective densities are 0.36, 0.40, 0.44, 0.51 and 0.71 g/cm3.The samples were uniformly polished with suitable sandpapers and dried in an oven at 70 "C under vacuum (50 mm Hg) to remove free water. Methylmethacrylate (MMA-from E. Merck) was used as the main monomer component while methanol (10%) was used as the swelling agent.
Wood samples were impregnated under vacuum (50 mm Hg) with a solu- tion of the monomer (MMA) and swelling agent (methanol). The impregnated samples were encapsulated within polyethene bags and were irradiated with Co-60 gamma source (50 kCi) for 3 Mrad at 0.8 Mrad/h. Untreated monomer was removed from the irradiated samples by heating them at 60°C under vacuum (50 mm Hg). This was continued till a constant weight was achieved. Polymer loading (PL) an index for composite formation, was determined from the percentage of weight increase.
Weter Absorption
Treated and untreated samples were tied to rods and lowered into a static water bath so that they were completely immersed. Weights of the samples were recorded at different times. Samples were wiped with tissue paper to remove surface water before weighing. Finally the weight gain and the weight loss were calculated.
Weight G8in
Water absorption (i.e., weight gain) was determined by the following relation:
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SWELLING BEHAVIOR OF WOOD-PLASTIC COMPOSITES 301
where W, = % of weight gain; We = equilibrium weight after water treat- ment; Wo = oven dry weight before water treatment.
Weight Loss
Samples, so treated with water, were then dried at 105 "C for 20 h, after which they were cooled at the ambient temperature under normal conditions. Their final weighs were taken to determine the weight loss (i.e., corrosive effect on wood and its WPC); values were calculated by the following equation:
x 100 wo -4 W, = - WO
where W, = oven dry weight after water treatment; Wo = oven dry weight before water treatment; W, = % of weight loss.
Tenslle Strength
Tensile strength (TS) of treated and untreated samples was directly measured by the INSTRON Tensile Strength Machine (Model 101 1, UK) integrated with a personal complter (Amastrad PC 1640 HD20, UK).
RESULTS AND DISCUSSION
As the affinity of wood toward water (moisture) is responsible for some of the adverse properties of wood, it is important to study the effect of water uptake on wood and wood polyemr products.
Water Swelling
The water absorption profiles of treated and untreated samples are shown in Fig. 1. water absorption increased with soaking time and reached a constant moisture content (equilibrium) within about 720 h soaking time by all samples of different polymer loading (PL) values. The swelling (absorption) behavior due to water uptake can be analyzed using the equation Wtge and Peppas, 1987)
M = k tn
where M = moisture content (7% wet basis); t = water soaking time in hour; k and n are constants. Experimental data of the absorption were fitted in this equation by the regression method. Values of k and n, thus calculated, are given in Table 1.
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KHAN AND ALI
I I I I
KODOM 0 KODOM + MMA
0 DEBDARU + MMA DEBDARU
0 I 9 2 384 5 7 6 768
TIME tN H O U R S
FIG. 1. Water absorption capability of wood and wood-plastic composites.
Equilibrium moisture content decreased with increase of PL. This can be correlated for M o m by the equation
Me = 66.60 - 0.427 PL
where Me = equilibrium moisture content; r2 = 0.932 (r refers to regres- sion constant).
The preceding relation can also be adopted for s h u l , koroi, mango, and debdaro:
For simul: 4 = 65.01 - 0.469 PL with r2 = 0.972
For koroi: Me = 63.00 - 0.736 PL with r2 = 0.974
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SWELLING BEHAVIOR OF WOOD-PLASTIC COMPOSITES 303
TABLE 1 Values of k and n Woods and Their WPC
Wood PL k n r2
Kadom 0 26.43 0.31 0.82 Kadom + MMA 69 10.17 0.16 0.99
SimUl 0 21.30 0.18 0.99 Simul + MMA 53 5.00 0.32 0.96
Koroi 0 20.46 0.17 0.96 Koroi + MMA 48 5.87 0.27 0.95
Mango 0 18.52 0.17 0.99 Mango + MMA 49 6.77 0.20 0.87
Debdaro 0 12.58 0.17 0.99 Debdaro + MMA 26 6.60 0.23 0.96
For mango: Me = 55.95 - 0.581 PL with r2 = 0.993
For debdaro: Me = 48.27 - 0.834 PL with r2 = 0.977
The results are plotted in Fig. 2 for PL against the corresponding equilibrium moisture contents. The reduced water uptake with increase of PL is due to the reduction of hygroscopicity caused by replacement of OH groups with other, less polar components (monomer). Here monomer occupies the void spaces of wood and then crosslinks with the molecules of the cell walls.
Antishrinking efficiency (ASE) is a property used to measure the water ab- sorption ability of wood and can be calculated as:
ASE = 100 (W, - W,)/W,
where W, = maximum weight of untreated sample; W, = maximum weight of treated sample.
ASE values have been plotted against %PL in Fig. 3. Kadom has shown the maximum ASE per unit change of PL; mango is the next, followed by simul, koroi, and debdaro. This trend of ASE is similar to the trend of changes of tensile strength and Young’s modulus (Khan and Ali, in press).
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304 KHAN AND ALI
fY 2 0 - m 2 3 0 w
*
-
I I 1 I
*/o POLYMER L O A D I N G
FIG. 2. Equilibrium moisture content as a function of polymer loading (PL).
Percent weight gain of absorption by water by both reference (C) and WPC samples was determined. Thus, the ratio (R) of the percent gain by the WPC to the percent weight gain by the reference (R = WPC/C) is a measure for monitoring the absorption ability of water of the different wood samples. Results are given in Table 2. It is observed here that the WPC samples of all the five types of wood have shown less absorption tendency toward water, since values of R are less than unity. This signifies that the WPC product is a better material, to be considered in place of the corresponding wood for any particular application.
Similarly, weight loss of both WPC and reference samples by the corrosive effect of water was also determined. Results are given in the same table (Table 2). Low ratio values of WPC samples (less than the unity) indicate good
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SWELLING BEHAVIOR OF WOOD-PLASTIC COMPOSITES 305
80
6 0
40 w v)
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POLYMER LOADING
FIG. 3. Antishrinking efficiency (ASE) plotted against polymer loading of wood with monomer.
TABLE 2 Weight Gain and Weight Loss by Wood and WPC Subjected to Water Treatment
% Weight gain, Wg % Weight loss, W,
Wood Wood WPC R Wood WPC R
Kadom 71 46 0.64 1.10 0.81 0.73 Simul 67 40 0.69 1.75 0.85 0.48 Koroi 65 35 0.53 0.97 0.46 0.47
Mango 56 28 0.50 2.50 0.85 0.34 Debdaro 55 27 0.49 1.33 0.19 0.14
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306 KHAN AND ALI
TABLE 3 Tensile Strength of Wood and WPC Before
and After Water Treatment
T S w d TSwpc Wood mi) (W TS, TSw
Kadom 2.12 3.18 75 48 Simul 2.3 3.36 36 23 Koroi 2.84 3.69 50 25 Mango 3.43 4.11 89 61 Debdaro 5.10 5.61 65 42
Note. TS, = X TS loss by untreated samples; TS, = X TS loss by WPC.
resistance toward corrosion attack in water. Treated samples (wpc) exhibited far less corrosive effect by water. This may be attributed to the fact that the composites are formed by filling up the void spaces with polymer, which is generally inert toward water attack,
Tensile Strength
Tensile strengths (TS) of both wood and WPC before and after water treat- ment were determined and the results are presented in Table 3. TS values, percentage of TS loss, were determined by the formula:
TSb - TS, TSb
% TS loss = x 100
where TSb = tensile strength before corrosion; TS, = tensile strength after corrosion.
Consistent with the weight loss obtained during water attack, TS measurements have again demonstrated the fact that the composite materials are less susceptible to water corrosion than the corresponding wood samples.
Acknowledgment
The authors extend their thanks to the staff members of the Radiation Chemistry Laboratory, Institute of Nuclear Science and Technology (INST), for their
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SWELLING BEHAVIOR OF WOOD-PLASTIC COMPOSITES 307
cooperation during this work; and to those of the Co-60 source facility of the Institute of Food and Radiation Biology (IFRB). Thanks are also due to IAEA for providing chemicals and financial support for this work under grant BGD/8/008.
REFERENCES
Haygreen, J. G. , and Bowyer, J. L., Forest Product and Wood Science, 2nd ed., Iowa State University Press/AME, 1989, p. 155.
Khan, M. A., and Ali, K. M. I. , Studies of physicomechanical properties of wood and wood-plastic composite (WPC), J. Appl. Polym. Sci. (in press).
Khan, M. A., and Rahman, M. S., Moisture sorption isotherms of wood and wood- plastic composite (WPC), Polym. -Plust. Technol. Eng., 30(5&6), 435 (1991).
Ritge, P. L., and Peppas, J. , J. Controlled Resease, 5 , 37 (1987).
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