Materials and Design 32 (2011) 2221–2227

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Materials and Design

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The effect of crosslinker on mechanical and morphological propertiesof tropical wood material composites

Md. Saiful Islam a,⇑, Sinin Hamdan a, Md. Rezaur Rahman a, Ismail Jusoh b, Abu Saleh Ahmed a

a Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Sarawak, Malaysiab Faculty of Resource Science & Technology, Universiti Malaysia Sarawak, 94300 Sarawak, Malaysia

a r t i c l e i n f o

Article history:Received 4 August 2010Accepted 11 November 2010Available online 19 November 2010

Keywords:E. MechanicalG. Scanning electron microscopyG. X-ray analysis

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.11.026

⇑ Corresponding author. Tel.: +60 149922251.E-mail address: msaifuli2007@gmail.com (M.S. Isla

a b s t r a c t

In this study, wood polymer composites (WPCs) based on five kinds of selected tropical wood species,namely Jelutong (Dyera costulata), Terbulan (Endospermum diadenum), Batai (Paraserianthes moluccana),Rubber (Hevea brasiliensis), and Pulai (Alstonia pneumatophora), were impregnated with methyl methac-rylate (MMA) and hexamethylene diisocyanate (HMDIC) monomers mixture in the ratio of 1:1 for com-posite manufacturing. All these tropical wood reacted with hexamethylene diisocyanate and crosslinkedwith MMA which enhanced the hydrophobic (restrained water) nature of wood. The vacuum-pressuremethod was used to impregnate the samples with monomer mixture. The monomer mixture loadingachievable was found to be dependent on the properties of wood species. Low loading was observedfor the high density wood species. Mechanical strength of fabricated wood polymer composites (WPCs)in term of modulus of elasticity (MOE) and modulus of rupture (MOR) were found to be significantlyimproved. The wood–polymer interaction was confirmed by Fourier transform infrared (FTIR) spectros-copy. Morphological properties of raw wood and WPC samples were evaluated by scanning electronmicroscopy (SEM) and XRD analysis and an improvement in morphological properties was also observedfor WPC.

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1. Introduction

Structural wood has always been, and continues to be, a veryimportant and versatile material with many uses because of itsvery aesthetically pleasing character. But wood has some draw-backs such as high moisture uptake, biodegradation, and physicaland mechanical property change with environmental factors [1].These troublesome inherent properties of wood can be minimizedby appropriate chemical treatment such as the formation of woodpolymer composite (WPC) [2]. The presence of hydrophilic hydro-xyl groups (AOH) in the wood components is the main factorresponsible for the negative properties. Wood attracts moisturethrough hydrogen bonding, making it physically unstable. Thephysical and mechanical properties of wood can be improved byusing an impregnation technique with suitable chemicals thatcan react with cell wall components [3]. The improvement in prop-erties of wood may also be enhanced by preparing wood polymercomposites (WPC) with different monomers [4–7]. ManufacturedWPC generally exhibits effective dimensional stability and excel-lent mechanical properties [8,9]. WPCs can also improve manyproperties of solid wood such as surface hardness, toughness, abra-

ll rights reserved.

m).

sion resistance, moisture exclusion and weather resistance. In gen-eral, the improvement in property can be attributed to the polymercontent, which is dependent on the type of wood, the polymer andprocessing technology applied. The main factors influencing theWPC properties of wood are density, moisture content, directionof the grain, and the physico-chemical composition of the cell wallof the wood. In the literature, it can be seen that the heavy hard-woods species gain lower amount of monomer than those fromthe medium, light hardwoods species [10]. The lower monomerloading of hardwoods can be ascribed by their some inherent prop-erties such as high density and specific gravity including internalvessel diameter, the number of vessel percent per unit area, andits high extractive. Therefore, it has been established that the prop-erties of WPC depend on the wood species and the physical andmechanical properties of composite increases with the increasesof monomer loading [11].

Recently, considerable interest has been manifested in woodimpregnation with a variety of monomers such as styrene, epoxyresins, urethane, phenol formaldehyde, methyl methacrylate(MMA), vinyl or acrylic and their combination to change the spe-cific properties of WPC [12,13]. However, it has been establishedthat most monomers do not form bonds with hydroxyl groups ofthe wood component. They simply bulk the void spaces withinthe wood structure [14]. It can therefore be deduced that if bond-

2222 M.S. Islam et al. / Materials and Design 32 (2011) 2221–2227

ing were to take place between the impregnated monomers andthe hydroxyl groups on the wood component, the physical andmechanical properties of WPC may be further improved. Hexam-ethylene diisocyanate (HMDIC) is a difunctional reagent whichhas two reactive functional groups and also has been widely usedas a wood adhesive, crosslinker and copolymer [15]. HMDIC mod-ification of wood relies on modifying the predominant wood com-ponents like cellulose, hemicellulose and lignin by reacting woodhydroxyl groups with a diisocyanate group to form wood–urethanederivatives [16]. Some researchers consider that the isocyantesalso react with accessible AOH groups according to the followingproposed chemical reaction [17]:

wood—OHþ R—N@C@O! wood—O—Cð@OÞ—NH—R

This reaction can also create new structures in the WPC thatinfluence morphology, crystallization, and mechanical, thermal,biological, and other properties of wood [18,19]. Many studieshave been carried out on physical, mechanical and morphologicalproperties of wood and WPC [20–24]. Little work, however, hasbeen devoted to Malaysian tropical light hardwood species andtheir chemical modification with the combination of two mono-mers [25].

In the present work, five species of selected Malaysian tropicallight hardwood species namely Jelutong, Terbulan, Batai, Rubber,and Pulai were utilized as starting materials keeping in mind thatthey are abundantly available in the local forest and have a mini-mal effect on the environment. The dwindling supplies and risingcosts of the heavy hardwoods, has created interest in the utiliza-tion of lower grade woods such as tropical light hardwoods, whoseusage can be extended by converting into WPC. The tropical lighthardwoods are classified according to density, strength and dura-bility properties. The physical and chemical properties of cellulose,hemicelluloses and lignin play a major role in the chemistry ofstrength. Generally these wood species are porous and contain cel-lulose (40–44%), lignin (18–25%) and hemicelluloses (15–35%)[26]. Other polymeric constituents present in lesser and often vary-ing quantities are starch, pectin, and ash for the extractive-freewood. The internal properties of these tropical woods are high ves-sel diameter (90–340 lm), medium of the number of vessel pres-ent per unit area (1–10%), high fiber length (800–1800 lm), andmedium cell wall thickness [27]. The major problem of using thesespecies is their high moisture uptake, biodegradation, and physicaland mechanical property change with environmental variations,which limit their use [28,29]. These effects are especially pro-nounced in tropical areas where wood suffers from exposure tosunlight and high hygroscopicity, causing swelling and deforma-tion. Chemical modification could be a promising new approachto obtain better products [30,31]. In order to overcome these prob-lems, wood species were impregnated with a combination ofMMA/HMDIC mixture where HMDIC was used as a crosslinker re-agent. Thus the aim of this research is to manufacture WPC byimpregnating some selected tropical light hardwoods with MMA/HMDIC (1:1 ratio) mixture and to investigate their mechanicaland morphological properties.

2. Experimental

2.1. Monomer solution

The monomer solution used for WPC production was MethylMethacrylate/Hexamethylene Diisocyanate (MMA/HMDIC, 1:1 ra-tio) mixture containing 2% benzyl peroxide catalyst as a polymer-ization initiator. MMA and HMDIC have a density of 0.942–0.944 g/mc3 and 1.046–1.047 g/mc3 respectively, and both are analyticalgrade products of Merck, Germany.

2.2. Specimen preparation

Five species of tropical woods (Jelutong, Terbulan, Batai, Rubberand Pulai), were felled and cut into three bolts of 1.2 m long. Eachbolt was quarter-sawn to produce planks of 4 cm thickness andsubsequently conditioned to air-dry in a room with relativehumidity of 60% and ambient temperature of 25 �C for one monthprior to testing. The planks were ripped and machined to 300 mm(L) � 20 mm (T) � 20 mm (R), 100 mm (L) � 25 mm (T) � 25 mm(R) and 39 mm (L) � 10 mm (T) � 4 mm (R) specimens for theThree Point Bending Test, Compression Parallel to Grain Test andWater Absorption Analysis respectively.

2.3. Density determination

All specimens were kept in the oven at 103 �C for 72 h beforedensity determination. Oven-dry density of each sample was thendetermined by using the Water Immersion Method [32]. The calcu-lation is as follow:

density ¼mass of wood=volume of wood ð1Þ

2.4. Manufacturing of wood polymer composites

All oven dried samples were placed in an impregnation vacuumchamber, evacuated with vacuum pressure of 75 mm Hg, and heldfor 10 min. The respective monomer system was introduced intothe chamber as the vacuum pressure was released. The sampleswere kept immersed in the monomer mixture solution for 6 h atambient temperature and atmospheric pressure to obtain furtherimpregnation. Samples were then removed from the chamberand wiped of excess impregnate. Samples were wrapped with alu-minum foil and placed in an oven for 24 h at 105 �C for polymeri-zation to take place. The samples were then measured for weightpercentage gain (WPG) using the Eq. (2):

WPGð%Þ ¼ ðWi �WoÞ=Wo � 100 ð2Þ

where Wo and Wi are oven dried weight of raw wood and monomermixture impregnated WPC samples, respectively.

2.5. Microstructural analysis

2.5.1. Fourier transform infrared spectroscopy (FTIR)The infrared spectra of the raw and fabricated WPC grounded

powder samples were recorded on a Shimadzu Fourier TransformInfrared Spectroscopy (FTIR) 81001 Spectrophotometer. The trans-mittance range of the scan was 400–4000 cm�1. The obtained spec-tra are described in the results and discussions section.

2.5.2. Scanning electron microscopy (SEM)The interfacial bonding between the cell wall polymer and

monomer mixture was examined using a Scanning Electron Micro-scope (JSM-6701F) supplied by JEOL Company Limited, Japan. Thespecimens were first fixed with Karnovsky’s fixative and then ta-ken through a graded alcohol dehydration series. Once dehydrated,the specimen was coated with a thin layer of gold before viewingon the SEM. The micrographs, taken at a magnification of 1000�and 2000�, are presented in the results and discussions section.

2.5.3. X-ray diffraction (XRD)In order to assess the morphological properties of WPC, XRD

analysis was applied. A PANalytical XRD diffractometer was usedwhere Cu Ka (k = 1.54 Å) radiation was employed with 2h varyingbetween 4� and 80� at 5�/min.

M.S. Islam et al. / Materials and Design 32 (2011) 2221–2227 2223

2.6. Mechanical test

2.6.1. Bending and compression testIn order to characterize mechanical properties of manufactured

composites, bending and compression tests were carried outaccording to ASTM D-143 (1996) [33] using a Shimadzu UniversalTesting Machine having a loading capacity of 300 kN. A cross headspeed of 2 mm/min was used during the test.

2.7. Water absorption test

To determine the water uptake, the Water Absorption Test ofthe raw wood and WPC specimens was carried out according toASTM D 570-99, 2002 [34]. Rectangular specimens were preparedhaving dimensions of 39 mm (L) � 10 mm (T) � 4 mm (R). Thespecimens were dried in an oven at 105 �C, cooled in a dessicatorcontaining silica gel and immediately weighed. A Denver Instronbalance was used for weight measurement. The dried andweighted specimens were immersed in distilled water for 7 daysat ambient temperature. The samples were then removed fromthe water and dried with a cotton cloth. The final weight of thespecimens was then taken. The increase in the weight of the spec-imens was calculated using the following equation:

Water absorptionð%Þ ¼ final weight� original weightoriginal weight

� 100 ð3Þ

2.8. Evaluation of results

The significant difference between untreated wood and woodpolymer composites (WPC) was determined using multiple t-testanalysis.

3. Results and discussion

3.1. Weight percentage gain (WPG%)

Fig. 1 illustrates the relation between density of wood speciesand weight percentage gain (WPG) of wood polymer composites(WPCs). After impregnation with monomer mixture the WPG forJelutong, Terbulan, Batai, Rubber, and Pulai was 50%, 35%, 55%,18% and 47% respectively. This result reveals that MMA/HMDmonomer mixture was successfully incorporated in all wood spe-cies, with the Batai wood samples gaining the highest percentageof monomer among the tested species. These results also indicatethat the amount of polymer that can be introduced into the woodis dependent on the density of wood species. This is expected be-

Fig. 1. Weight percentage gain (WPG%) of WPCs.

cause lower density wood species gain higher amounts of polymerand vice versa [10]. The hierarchies of density of these tropicalwood species are 380, 450, 455, 480 and 650 kg/m3 for Batai, Jelu-tong, Pulai, Terbulan and Rubber wood respectively as obtained inpresent research using Eq. (1). Generally, the higher weight per-centage gains of wood samples exhibit better improvement inproperties of WPC. Therefore, it can be deduced that the densityof tropical wood play a vital role in the formation of wood polymercomposite. The formation of WPC using low density and low qual-ity tropical wood species such as Batai, Jelutong, Pulai, and Terbu-lan are easier than high density one (i.e. Rubber wood). And thedevelop WPCs can be used as a replacement of good quality woodwhich allow to utilize different applications that include for exam-ple interior or exterior usage and structural or engineeringapplication.

3.2. Microstructural analysis

3.2.1. Fourier transform infrared spectroscopy (FTIR)Interaction between wood, MMA and HMDIC was confirmed by

FTIR spectrum analysis of the raw wood and WPC as shown inFig. 2. The FTIR spectrum of the raw wood clearly shows theabsorption band in the region of 3418 cm�1, 1736 cm�1 and2933 cm�1 due to OAH stretching vibration, C@O stretching vibra-tion and CAH stretching vibration respectively. These absorptionbands are due to the hydroxyl group in cellulose, carbonyl groupof acetyl ester in hemicellulose and carbonyl aldehyde in lignin[35]. On the other hand, the very strong OAH stretching absorptionband has been replaced by a much weaker absorption at3359 cm�1 and a new carbonyl absorption band was develop atthe region of 1688 cm�1. We believe that these changes are dueto fact that with isocyanate groups of HMDIC, virtually all the hy-droxyl groups have been replaced, and the new 3359 cm�1 absorp-tion is due to the carbamate NAH bonds as shown in Fig. 2 [36].Therefore, it can be confirmed that HMDIC reacted with wood fiberand produced wood-OAC(@O)ANHAR compound. It can also beseen from the Fig. 2, the carbonyl band at 1736 cm�1 was com-pletely disappeared and the absorption band of CAH group alsoshifted towards higher wave numbers (2918–2933 cm�1) withnarrow band intensity, which gave further evidence of the interac-tion and crosslink between wood, HMDIC and MMA. These resultsconfirmed that HMDIC interacted better between the wood andMMA, which significantly increased the hydrophobicity of WPC.

3.2.2. Scanning electron microscopy (SEM)Scanning electron micrographs (SEM) of raw wood and WPC are

shown in Fig. 3i and ii in a typical transversal section. The SEM inFig. 3i shows that the raw wood fiber surfaces are covered with anuneven layer, which is probably waxy substances and a number ofvoid/hole spaces, as reported previously [37]. The polymer filledsurface of WPC after impregnation could be explained by theremovable nature of the waxy substances on the surface of ligno-cellulosic materials and the ability to fill the void surfaces by theMMA/HMDIC [37,38]. Fig. 3ii shows clean and completely voidspaces filled polymer throughout the wood surface. This photo-graph also explains the high conversion of impregnated monomermixture to polymer for wood which was found in both the cell walland vessels of the wood. It is seen that the filling behavior of MMA/HMDIC is in interaction with the outside surface of the cell lumensas well, because monomer mixture is polymerized homogeneously.This is expected because HMDIC reacts with wood hydroxyl groupsthrough the isocyanate group which enhances the adhesion andcompatibility of polymer to the cell wall and vessels of the wood[39]. This result also suggests that the polymer formed a stronginterface with wood cell walls, accounting for the observed in-crease in mechanical strength.

Fig. 2. FTIR of raw wood and WPC.

(i)

(ii)

Fig. 3. SEM micrographs of (i) for raw wood and (ii) for WPC.

0

1000

2000

3000

4000

0 20 40 60 80 100

2 Theta

Inte

nsity

Fig. 4. Typical X-ray diffraction patterns of raw wood.

0

1000

2000

3000

4000

0 20 40 60 80 1002 Theta

Inte

nsity

Fig. 5. Typical X-ray diffraction patterns of WPC.

2224 M.S. Islam et al. / Materials and Design 32 (2011) 2221–2227

3.2.3. X-ray diffraction (XRD)The X-ray diffraction patterns of raw wood and WPC are given

in Figs. 4 and 5. As seen in Fig. 4, the patterns of raw wood fibersexhibit three well defined peaks (2h) at 15.1�, 22.8� and 34.7�.The 15.1�, 22.8� and 34.7� reflections correspond to the (1 1 0),(2 0 0) and (0 2 3) or (0 0 4) crystallographic planes, respectively[40]. These peaks are mainly of the crystal of native cellulose fiberand the remaining amorphous areas are due to lignin and hemicel-

luloses component in wood material. The effect of chemical mod-ification of lignocellulosic materials on their crystallinity hasbeen investigated by various researchers and reported an increase

M.S. Islam et al. / Materials and Design 32 (2011) 2221–2227 2225

or decreases the crystallinity after treatment [37,41]. When thecrystal content is high then one may observe some new peaks orpeak position at around 15.1� divided into two peaks, but whenthe fiber contain high amounts of amorphous material such as lig-nin, hemicellulose, and amorphous cellulose, these two peaks aresmeared, thus appearing as one broad peak (Figs. 4 and 5).

From Fig. 5 it is observed that there are some new peaks (2h) ofvarious intensities in the amorphous region 40–75.9�. The diffrac-tion patterns of WPC exhibits six new prominent peaks at 42.13�,43.61�, 49.23�, 50.96�, 72.65� and 75.81�. In addition, the cellulosecrystalline peak 15.1� was divided into two peaks at 11.60� and12.6�. These new peaks and divided of cellulose peaks may bedue to the strong interaction of HMDIC, wood and MMA and theformation of wood composites. The crosslinker HMDIC reactedwith cell wall hydroxyl groups of wood and formed wood-OAC(@O)ANHAR compound which make a rigid linking bridgewith wood fiber and MMA, thus enhancing the over all crystallinityof WPC. This result indicates that the manufactured WPC signifi-cantly increased the crystallinity of wood, as seen by otherresearchers [41,42].

3.3. Static bending (MOE/MOR) analysis

The modulus of elasticity (MOE) and modulus of rupture (MOR)of Jelutong, Terbulan, Batai, Rubber and Pulai raw wood and theirWPC are shown in Tables 1 and 2. The effects of MMA/HMDIC load-ing on the MOE and MOR of the raw wood and WPC were investi-gated. The MOE of WPC for Jelutong showed the highest increment,followed by Pulai, Batai, Terbulan, and Rubber respectively. FromTable 1, the MOE for all WPC were significantly increased fromtheir untreated one. The increase in MOE value of WPC comparedto the raw samples are due to the presence of crosslinker HMDICwhich reacts with cell wall hydroxyl groups and provides betterinteraction between the MMA and wood [43–45]. In the wood

Table 1Modulus of elasticity of raw wood and WPCs.a

Treatment Modulus of elasticity (GPa) t-Test groupingb

Jelutong (Raw) 5.31 ± 0.44 AJelutong (WPC) 7.22 ± 0.51 BTerbulan (Raw) 7.39 ± 1.15 CTerbulan (WPC) 8.63 ± 0.87 DBatai (Raw) 6.51 ± 0.57 EBatai (WPC) 7.89 ± 1.12 FRubber (Raw) 11.62 ± 1.05 GRubber (WPC) 12.20 ± 1.39 GPulai (Raw) 4.12 ± 1.80 HPulai (WPC) 5.36 ± 0.91 I

a Each value is the average of 10 specimens.b The same letters are not significantly different at a = 5%.

Table 2Modulus of rupture of raw wood and WPCs.a

Treatment Modulus of rupture (MPa) t-Test groupingb

Jelutong (Raw) 46 ± 5.71 AJelutong (WPC) 54.1 ± 2.92 BTerbulan (Raw) 60.5 ± 2.46 CTerbulan (WPC) 65.7 ± 1.94 DBatai (Raw) 55.4 ± 2.87 EBatai (WPC) 60.9 ± 1.79 FRubber (Raw) 105.8 ± 2.57 GRubber (WPC) 109 ± 1.39 GPulai (Raw) 37.8 ± 1.81 HPulai (WPC) 44.4 ± 2.75 I

a Each value is the average of 10 specimens.b The same letters are not significantly different at a = 5%.

samples, the polymer completely filled all void spaces of wood,thus enhancing the MOE value significantly. However, for Rubberwood, a small difference was found between raw wood and itsWPC because of its high density and little amount of monomerloading, as confirmed by other researchers [46]. It can be seen fromTable 2 that the modulus of rupture (MOR) also significantly in-creased for all species after composites manufacture, except forRubber, which is in agreement with previous research [47]. TheMOR of WPC for Jelutong (18.15%) showed the highest percentageincreases, followed by Pulai (17.37%), Batai (10.05%), Terbulan(7.97%), and Rubber (2.89%) respectively. The WPC of Rubber woodhad the lowest increment compared with other WPCs because ofits high density (650 kg/m3). This observation indicates that theMOR also depends on the wood properties [10]. Therefore, wecan conclude that an improvement in mechanical strength is oneof the important advantages of WPC over natural tropical lighthardwood.

3.4. Compressive test analysis

Table 3 shows the compressive strength parallel to the grain forWPC and raw wood samples. From Table 3, it is apparent that therewas a significant increase in compressive strength for all WPC of61–78%. Of the five wood species used, the highest increase incompressive strength was observed on Jelutong, followed by Pulai,Batai, Terbulan and Rubber respectively. Untreated wood speciesfail in compression because of the bulking of relatively thin cellwalls due to a long column type of instability. The presence ofpolymer in the cell wall enhances the lateral stability [48]. Thisis also expected because HMDIC has the ability to increase theadhesion and compatibility between the wood cell wall andMMA, thus forming a strong polymer coating on its surface. Thisenhances the lateral stability of the cell wall.

3.5. Water absorption

Table 4 demonstrates the result of water absorption tests. Allraw wood samples exhibited a higher percentage of water absorp-tion than the modified WPCs. This is expected because cell wallswith hydrophilic hydroxyl groups will absorb water to its surfacethrough the formation of hydrogen bonding [48,49]. As one cansee from Table 4, manufactured WPC significantly decreases waterabsorption over the raw wood species, with the WPC of Rubberhaving the least water absorption, followed by Jelutong, Pulai, Ba-tai and Terbulan respectively. The number of hydroxyl groups inthe raw wood increases the water absorption. However, HMDIC re-acts with OH groups of wood through its isocyanates groups andreduces the water absorption [50]. HMDIC also increases the de-gree of polymerization which reduces void spaces inside woodand causes less water absorption. Therefore, the reduction of mois-

Table 3Compressive strength of raw wood and WPCs.a

Treatment Compressive strength (GPa) t-Test groupingb

Jelutong (Raw) 2.85 ± 0.57 AJelutong (WPC) 5.1 ± 0.65 BTerbulan (Raw) 3.82 ± .63 CTerbulan (WPC) 6.28 ± 0.95 DBatai (Raw) 3.58 ± 0.83 EBatai (WPC) 6.0 ± 0.94 FRubber (Raw) 2.68 ± 0.83 GRubber (WPC) 4.34 ± 1.40 HPulai (Raw) 2.42 ± 1.17 IPulai (WPC) 4.14 ± 1.28 J

a Each value is the average of 10 specimens.b The same letters are not significantly different at a = 5%.

Table 4Water absorption in 7 days of raw wood and WPCs.a

Treatment Water absorption (%) t-Test groupingb

Jelutong (Raw) 232 ± 15.53 AJelutong (WPC) 60.84 ± 7.43 BTerbulan (Raw) 205 ± 20.94 CTerbulan (WPC) 82 ± 8.32 DBatai (Raw) 259 ± 27.99 EBatai (WPC) 81 ± 7.68 FRubber (Raw) 122 ± 26.84 GRubber (WPC) 59 ± 9.78 HPulai (Raw) 396 ± 46.47 IPulai (WPC) 79 ± 10.85 J

a Each value is the average of 10 specimens.b The same letters are not significantly different at a = 5%.

2226 M.S. Islam et al. / Materials and Design 32 (2011) 2221–2227

ture uptake is another important advantage of WPC through thetropical light hardwood species.

4. Conclusions

Significant improvements in mechanical and morphologicalproperties were obtained for all WPCs which were fabricated byimpregnation with MMA/HMD monomer formulation. Manufac-tured WPCs were confirmed from the FTIR spectrum where theabsorption band of 1736 cm�1 disappeared and weak absorptionband intensity of OH groups was shown. The SEM and X-ray dif-fraction results of WPC reveal a complete polymer covered surfacetexture and improved degree of crystallinity. The WPCs increasedthe MOE and MOR by 8–36% and 5–15% respectively, at 18–55%WPG. Furthermore, WPCs showed lower water absorption com-pared to the raw woods. The authors propose that HMDIC cross-linker increased the adhesion and compatibility of wood fiber tothe polymer matrix thus enhancing the degree of polymerizationand the degree of crystallinity of wood composite. This signifi-cantly increased the mechanical and morphological properties ofall selected tropical light hardwoods used in this study.

Since the high quality hardwoods is decline, many tropical lowquality light hardwood can be manufactured into WPCs to givehigh mechanical and morphological properties to enable their uti-lization for specific application.

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