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International Biodeterioration & Biodegradation 72 (2012) 108e113

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International Biodeterioration & Biodegradation

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Effect of coupling reactions on the mechanical and biological properties of tropicalwood polymer composites (WPC)

Md. Saiful Islam a,*, Sinin Hamdan a, Mahbub Hasan b, Abu Saleh Ahmed a, Md. Rezaur Rahman a

aDepartment of Mechanical and Manufacturing Engineering, Faculty of Engineering, University Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, MalaysiabDepartment of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh

a r t i c l e i n f o

Article history:Received 6 April 2010Received in revised form8 May 2012Accepted 8 May 2012Available online 28 June 2012

Keywords:Wood polymer compositesMechanical propertiesDecay resistanceCoupling reactionMethyl methacrylate

* Corresponding author. Tel.: þ60 82583232; fax: þE-mail addresses: msaifuli2003@yahoo.com

(Md.S. Islam).

0964-8305/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.ibiod.2012.05.019

a b s t r a c t

Wood polymer composites (WPCs) based on five types of selected Malaysian tropical light hardwoodspecies were prepared using a methyl methacrylate (MMA) and hexamethylene diisocynate (HMDIC)monomer mixture at a 1:1 ratio. Before being impregnated with MMA/HMDIC, the wood species werechemically pretreated with benzene diazonium salt for the coupling reaction with wood and to increaseadhesion and compatibility of the wood fiber to the polymer matrix. The monomer mixture (MMA/HMDIC) was impregnated into both the raw wood and diazonium salt pretreated wood specimens tomanufacture wood polymer composites (WPC) and pretreated wood polymer composites (PWPC),respectively. Microstructural analysis (scanning electron microscopy and Fourier transform infraredspectroscopy) and mechanical (three-point bending and compression) and biological (fungal decayresistance) tests were conducted. Comparisons were made among the properties of raw wood, WPC, andPWPC. The results reveal that PWPC yielded better mechanical and biological properties compared tountreated WPC and raw wood.

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1. Introduction

Recently, bio-based wood polymer composites (WPC) have beenreceiving considerable attention due to their low processing cost,problem-free biodegradation, and improved physical, mechanical,and biological performance. Wood is mainly composed of threepolymers, namely cellulose, hemicellulose, and lignin, with a minorproportion of extractives that is subject to biodegradation. Theseorganic polymers are readily deteriorated by environmental factors,such as light, water, temperature, and biological organisms (Hill,2006). Modifying wood is an often-followed route to improvethese properties. More precisely, modification using non-biocidalthermal, chemical, or resin treatments has been shown to havepotential to improve characteristics (Kamdem et al., 2002; Landeet al., 2004; Hakkou et al., 2006). Of late, wood has been treatedwith a variety of chemicals such as styrene, epoxy resins, urethane,phenol formaldehyde, methyl methacrylate (MMA), and vinyl oracrylic monomers (Yalinkilic et al., 1991; Brelid et al., 2000; Chaoand Lee, 2003; Islam et al., 2012a). However, it has been estab-lished that monomers and their mixtures do not form bonds with

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hydroxyl groups of the cellulose fibers. Instead, they simply fill thevoid spaces within the wood structure (Shane et al., 1995). Sincemost vinyl monomers are non-polar, there is little interactionbetween these monomers and the hydroxyl groups of the cellulosefiber. Poor chemical and physical interfacial interactions betweenthe wood surface and chemical are two of the most importantmechanisms of bond failure. Therefore, the polymer component ofthe WPC simply bulks up the wood structure by filling the capil-laries, vessels, and other void spaces within the wood. It cantherefore be deduced that if bonding was to take place between theimpregnated monomers and the hydroxyl groups in the cellulosefibers, the mechanical and biological properties of WPC may befurther improved. It has been noted that the interaction betweenhydrophilic wood fibers and polymer can be modified usingcoupling agents. Raw fiber has also been chemically treated withbenzene diazonium salt to increase its compatibility with thepolymer matrix (Haque et al., 2009). It has been established thatbenzene diazonium salt yields a diazo cellulose compound by thecoupling reaction with hydroxyl groups of cellulose fiber (Islamet al., 2012b).

In the present work, five species of selected Malaysian tropicallight hardwoods, namely jelutong (Dyera costulata), terbulan(Endospermum diadenum), batai (Paraserianthes moluccana),rubber (Hevea brasiliensis), and pulai (Alstonia pneumatophora)were utilized as starting materials, keeping in mind that they are

Md.S. Islam et al. / International Biodeterioration & Biodegradation 72 (2012) 108e113 109

easily obtainable in the local forests. The major drawbacks of usingthese species are their high moisture uptake and biodegradation;in addition, their physical and mechanical properties changewith environmental variations, which subsequently limit theiruse.

The hydrophilic nature of wood is the main factor responsiblefor moisture absorption, fungal attack, and deformation of theproduct. By reducing the moisture content of wood, fungal growthis inhibited (Goethals and Stevens, 1994; Donath et al., 2004, 2006;Hill et al., 2004; Mai and Militz, 2004; Mai et al., 2005). Thephysical, mechanical, and biological changes of wood can also beminimized by suitable chemical treatment, such as the formation ofWPC, which is a promising strategy to improve wood properties(Baysal et al., 2007).

In order to overcome the hydrophilic nature of wood and toimprove the adhesion and compatibility of polymers to the cellu-lose of wood, the wood samples were chemically pretreated withbenzene diazonium salt and then impregnated with an MA/HMDICmonomer mixture to yield wood polymer composites (WPC) andpretreated wood polymer composites (PWPC). Therefore, thisstudy examines the mechanical and biological properties of woodpolymer composites (WPC) pretreated with benzene diazoniumsalt.

2. Materials and methods

2.1. Materials

In this study, five kinds of tropical light hardwood species,namely jelutong (D. costulata), terbulan (E. diadenum), batai (P.moluccana), rubberwood (H. brasiliensis), and pulai (A. pneumato-phora), were collected from the local forest of Sarawak, Malaysia.Each specimen was treated with aniline (C6H5eNH2), sodiumnitrite (NaNO2), and hydrochloric acid (HCl). The monomermixturemethyl methacrylate (MMA)/hexamethylene diisocynate (HMDIC),at a 1:1 ratio, was used for the production of wood polymercomposites. The methyl methacrylate (MMA) and hexamethylenediisocynate (HMDC) had densities of 0.942e0.944 g mc�3 and1.046e1.047 g mc�3, respectively. All chemicals were AR gradeproducts of Merck, Germany.

2.2. Synthesis of benzene diazonium salt

Benzene diazonium salt was synthesized in the laboratory withaniline and sodium nitrite in the presence of a mineral acid at0e5 �C using the standard diazotization method (Ismail et al.,2002). The reaction scheme for synthesis of benzene diazoniumsalt is shown in Fig. 1. The prepared compound was used imme-diately after synthesis for the coupling reaction with wood species.

2.3. Specimen preparation

All wood species were felled and cut into three bolts, each withthe length of 1.2 m. Each bolt was quarter sawn to produce planksof 4-cm thickness and subsequently conditioned to air-dry ina room with relative humidity of 60% and ambient temperature ofaround 25 �C for three months prior to testing. The planks were

Fig. 1. The reaction scheme for synthesis of benzene diazonium chloride.

ripped and machined to 20 mm (L) � 20 mm (T) � 20 mm (R)specimens for decay resistance tests and ground into samples forFTIR tests.

2.4. Density determination

All specimens were kept in the oven at 103 �C for 72 h beforedensity determination. The oven-dry density of each sample wasthen determined by using the water immersion method (Bowyeret al., 2003). The calculation is as follows:

Density ¼ Mass of wood=Volume of wood (1)

2.5. Coupling reaction

The reaction of the diazo compound with cellulose or cellulosederivatives is known as the coupling reaction (Ibrahim, 2002). Alloven-dried raw wood specimens were placed in a benzene diazo-nium salt solution (5 �C) containing 5 L of 5% NaOH in a reactionvessel for 30 min during pretreatment. Specimens were thenremoved and soaked in cold acetone to arrest the reaction. Chem-ically modified wood species were subsequently extracted usingacetone:toluene (1:1) to remove un-reacted reagents, and subse-quently oven-dried at 105 �C for 24 h.

2.6. Manufacturing of wood polymer composites

For WPC and PWPC manufacturing, oven-dried raw wood andpretreated wood specimens were placed in an impregnationvacuum chamber at a vacuum pressure of 75mmHg for 10min. Therespective monomer system was introduced into the chamber asthe vacuum pressure was released. The specimens were keptimmersed in the monomer mixture solution for 6 h at ambienttemperature and atmospheric pressure to obtain further impreg-nation. Those were then removed from the chamber and wiped ofexcess impregnate. Specimens were wrapped with aluminum foiland placed in an oven for 24 h at 105 �C for polymerization to takeplace. The weight percentage gain (WPG) of the samples was thenmeasured using Eq. (2):

WPGð%Þ ¼ ½ðWi �WoÞ=Wo� � 100 (2)

where Wo and Wi are the oven-dried weight of raw wood andmonomer mixture impregnated WPC samples, respectively.

2.7. Microstructural analysis

The infrared spectra of the raw and modified wood polymercomposite specimens were recorded on a Shimadzu Fourier trans-form infrared spectroscopy (FTIR) 81001 spectrophotometer. Thetransmittance range of the scan was 370e4000 cm�1. The obtainedspectra are described in the Results and Discussions section. Theinterfacial bonding between the cell wall polymer and monomermixture was examined using a JOEL scanning electron microscope(SEM) (JSM-6701F). The specimens were first fixed withKarnovsky’s fixative and then taken through a graded alcoholdehydration series. Once dehydrated, the specimens were coatedwith a thin layer of gold before being viewed on the SEM. Themicrographs are presented in the Results and Discussions section.

2.8. Mechanical test

In order to characterize mechanical properties of manufacturedcomposites, bending and compression tests were carried outaccording to ASTMD-143 (1996) using a Shimadzu universal testing

Md.S. Islam et al. / International Biodeterioration & Biodegradation 72 (2012) 108e113110

machine having a loading capacity of 300 kN. A cross head speed of2 mm min�1 was used during the test.

2.9. Laboratory fungal decay resistance test

The decay resistance test was carried out using the StandardMethod of Accelerated Laboratory test (ASTM D2017, 2001) to testnatural decay resistance of wood. The specimens were air-dried,and, after conditioning to constant weight, were weighed in thelaboratory and transferred into a large, perfectly dark containermaintained at 20 � 1 �C and a relative humidity of 65 � 4%. Twotypes of common rot fungi (white-rot and brown-rot), Polyporusversicolor (L.ex. Fr.) (ATCC 12679) and Poria placenta (Fr.) Cook(ATCC 11538), were cultured and used to study the resistance ofPWPC andWPC against the decay. There were eight replications foreach specimen. The decay test was terminated after 12 wk whenthe reference blocks reached a weight loss of 60%. Mycelium wasbrushed off and test specimens were air-dried and again condi-tioned to constant weight. The weight was recorded for eachspecimen. Weight loss was determined for individual specimensusing Eq. (3).

%Weight loss ¼h�

Wo �Wf

�.Wo

i� 100 (3)

where Wo is the oven-dried weight of samples prior to exposure,and Wf is the oven-dried weight of samples after exposure tofungus.

2.10. Statistical analysis

Mechanical and biological test results were analyzed bya computerized statistical program composed of analysis of vari-ance (one-way ANOVA) and following the Tukey test at a 95%confidence level. Statistical evaluations were made on homoge-neity groups (HG), with different letters reflecting statisticalsignificance.

3. Results and discussion

3.1. Weight percentage gain (WPG)

Fig. 2 illustrates the weight percentage gain (WPG) of WPC andPWPC of jelutong, terbulan, batai, rubber, and pulai wood species.The WPG values of pretreated wood polymer composites werehigher than those of untreated wood polymer composites. Thevalues of WPG of PWPCs for jelutong, terbulan, batai, rubber, and

Fig. 2. Weight percentage gain of WPC and PWPC.

pulai wood were 51, 30, 27, 15, and 45%, respectively. On the otherhand, WPG for WPCs of jelutong, terbulan, batai, rubber, and pulaiwood were 45, 23, 20, 9, and 39%, respectively. These results indi-cate that after pretreatment, moreMMA/HMDICmonomer mixturewas impregnated in all wood species than without pretreatment.This particular result was expected because the coupling reactionwith OH groups of wood fiber in wood structure increased theadhesion and compatibility between wood fibers and polymer,resulting in higher WPG. It was found that out of the five species,batai wood specimens gained the highest percentage of monomermixture. This indicates that the amount of polymer that can beintroduced into wood is dependent on the properties of theparticular wood species. Again, this result had been anticipated bythe researchers, as low-density wood species gained higheramounts of polymer, while high-density wood species gainedlower amounts of polymer (Yap et al., 1990). The hierarchies ofdensity of these selected wood species are 380 kg m�3, 450 kg m�3,455 kg m�3, 480 kg m�3, and 650 kg m�3 for batai, jelutong, pulai,terbulan, and rubberwood, respectively, as obtained in the presentresearch using Eq. (1).

3.2. Microstructural analysis

3.2.1. Fourier transform infrared spectroscopy (FTIR)The coupling reaction of benzene diazonium salt with wood

fiber in raw wood at 0e5 �C and 75 mm Hg yielded wood-azoderivative. This is confirmed by the FTIR spectroscopic analysis ofthe raw wood and pretreated wood as shown in Fig. 3. The FTIRspectrum of the raw wood [Fig. 3; see (i)] clearly shows theabsorption band in the region of 3407 cm�1, 2917 cm�1, and1736 cm�1 due to OeH stretching vibration, CeH stretchingvibration, and C]O stretching vibration, respectively. Theseabsorption bands are due to hydroxyl group in cellulose, carbonylgroup of acetyl ester in hemicellulose, and carbonyl aldehyde inlignin (Ismail et al., 2002). On the other hand, FTIR spectra of pre-treated wood in Fig. 3(ii) clearly shows the presence of the char-acteristic NO group band in the region of 1512e1646 cm�1 and CeOstretching in the region of 1287e1006 cm�1. The peak at 1403 and1457 cm�1 is due to the eN]Nemoiety of the azo compound, andthe absorption band at 1309 cm�1 may be attributed to thesymmetric deformations of NO2 in the wood-azo compound(Covolan et al., 1997; Haque et al., 2009).

The formation of wood-azo derivative can be explained as beingdue to the presence of three hydroxyl groups in the cellulose

Fig. 3. FTIR spectrum of (i) raw wood and (ii) pretreated wood.

Md.S. Islam et al. / International Biodeterioration & Biodegradation 72 (2012) 108e113 111

anhydroglucose unit. One is the primary hydroxyl group at C6 andthe other two are secondary hydroxyl groups at C2 and C3. Theprimary hydroxyl group is more reactive than the secondary onesand the coupling reaction with carbon 2 and carbon 6 resulted inthe formation of wood-azo derivative.

3.2.2. Scanning electron microscopy (SEM)Fig. 4(i) shows the void spaces and uneven layer in the rawwood

fiber surface, which is removable with suitable chemical treatment(Sreekala et al., 2001; Zafeiropoulos et al., 2002). Fig. 4(ii) depictsthe micrograph of untreated WPC, while Fig. 4(iii) is the micro-graph of a benzene diazonium salt pretreated WPC. Fig. 4(ii) showsclean polymer stands throughout the wood fibers with remarkablegaps between the polymer and the cell wall, while (iii) shows nonoticeable gaps and a strong bond between the polymer and thecell wall. Furthermore, fibrous cellulose material adheres to thesurface of the polymer stands. It is thus deduced that the couplingreaction of diazonium salt with wood fiber surface increased theadhesion and compatibility of the polymer to the fibers of thewood.

3.3. Bend test analysis

The moduli of elasticity of the raw woods and their compositesare given in Table 1. The MOE of WPC and PWPC was higher thanthose of raw wood. From the table it can be seen that the MOE wassignificantly affected by pretreatment with diazonium salt. Thisresult is expected because the coupling reaction with fiber surfacein wood enhanced the adhesion and compatibility between woodfibers and the polymer, resulting in an improved MOE. The higherMOE ofWPC and PWPC compared to the rawwoodwas due to theirchemical modification, which is in accordance with findings byother researchers (Yildiz et al., 2005; Islam et al., 2011a). In thewood specimens, diazonium salt reacted with OH groups of woodand yielded wood azo derivative, thus enhancing polymer loading,polymerization inside wood, and increased MOE of PWPC. It is alsoapparent from Table 1 that the MOE of WPC and PWPC of thejelutong was highest; this wood was followed by pulai, batai, ter-bulan, and rubberwood, in descending order. However, for rub-berwood only a small increment was found for its WPC and PWPCdue to the high density of this species and a small amount of

Fig. 4. SEM micrographs of (i) untreate

chemical incorporation inside the cell wall, as discovered by otherresearchers.

The modulus of rupture (MOR) of the wood sample is shown inTable 1. The MOR of PWPC was higher than those of WPC and rawwood, a finding similar to previous research (Yalinkilic et al., 1999;Cai et al., 2007). As seen from Table 1, there was significantimprovement of MOR of PWPC for all wood species. These resultssuggest that the coupling reaction of wood fiber and benzene dia-zonium salt enhanced the interfacial bonding strength between thewood fibers and polymer. The MOR of WPC and PWPC was highestfor jelutong, followed by pulai, batai, terbulan, and rubber indescending order. The WPC and PWPC of rubberwood showed thelowest MOR because of its high density. This indicates, as statedearlier, that MOR also depends on the wood properties.

3.4. Compression test analysis

The compressive modulus values for the raw wood, WPC, andPWPC are summarized in Table 2. Here, it is seen that there wasa significant increase of compressive modulus for both WPC andPWPC of all species. Table 2 also shows that WPC and PWPC of allspecies exhibited much higher moduli compared to all raw woodspecimens. These increments were 61.94e78.95% for WPC and89.03e108.77% for PWPC over raw wood. It is also apparent thatPWPC had higher (11e20%) compressive moduli compared tothose ofWPC due to increased adhesion and compatibility betweenthe wood fiber and the MMA/HMDIC in treated wood. Among thefive wood species used, the highest increments of compressivemodulus were observed in jelutong, followed by batai, pulai, ter-bulan, and rubber respectively for both WPC and PWPC. Untreatedwood species failed in compression because of the bulking ofrelatively thin cell walls due to a long column type of instability.The chemical modification of raw wood puts a coating on the wallsthat thickens them, thus greatly increasing their lateral stability.This is also expected because MMA/HMDIC has the ability to fill thevoid spaces in the wood and the strong branched polymeric situ-ation inside the wood thus led to formation of wood polymercomposites with improved compressive strength. The increase ofcompressive modulus of WPC compared to those of raw wood wasalso reported by various researchers (Shane et al., 1995; Islam et al.,2011b).

d wood, (ii) WPC, and (iii) PWPC.

Table 3Weight losses of untreated wood, WPC, and PWPC after decay testing for 12 wk(SD ¼ standard deviation, HG ¼ homogeneity group).

Wood species Compositetypes

Weight losses (%)

White-rot fungi Brown-rot fungi

Mean SD HG Mean SD HGa

Jelutong Untreated 67.36 3.00 A 60.24 1.45 AWPC 15.42 0.70 B 14.10 0.80 BPWPC 12.50 0.88 C 12.30 0.58 C

Terbulan Untreated 60.60 2.67 D 65.28 1.22 DWPC 17.00 1.05 E 19.75 0.44 EPWPC 14.11 1.44 F 16.35 0.75 F

Batai Untreated 55.84 1.96 G 55.84 1.46 GWPC 16.48 0.97 H 19.48 0.72 HPWPC 12.20 1.50 I 16.35 0.75 I

Rubber Untreated 67.21 4.80 J 67.45 4.70 JWPC 24.10 1.20 K 24.10 1.00 KPWPC 19.72 2.08 L 22.10 1.39 K L

Pulai Untreated 53.80 1.87 N 60.41 1.56 NWPC 15.96 0.90 M 21.10 0.85 MPWPC 11.86 0.64 O 18.19 1.09 O

Mean value is the average of 10 specimens.aThe same letters are not significantly 481 different at a ¼ 5%. Comparisons weredone within each wood species group.

Table 1MOE and MOR of untreated wood, WPC, and PWPC (SD ¼ standard deviation,HG ¼ homogeneity group).

Wood species Compositetypes

Static bending properties, at 10% MCa

MOE (GPa) SD HG MOR (MPa) SD HGb

Jelutong Untreated 5.31 0.44 A 46 5.71 AWPC 7.22 0.51 B 54.1 2.92 BPWPC 8.10 0.57 C 56.5 1.27 B C

Terbulan Untreated 7.39 1.15 A 60.5 2.46 AWPC 8.63 0.87 B 65.7 1.94 BPWPC 9.43 0.64 B C 68.1 0.86 B D

Batai Untreated 6.51 0.57 A 55.4 2.87 AWPC 7.89 1.12 B 60.9 1.79 BPWPC 8.75 0.49 B C 63.3 0.76 B E

Rubber Untreated 11.62 1.05 A 105.8 2.57 AWPC 12.20 1.39 A B 109 1.39 BPWPC 12.96 0.72 B C 111 1.17 B F

Pulai Untreated 4.12 1.80 A 37.8 1.81 AWPC 5.36 0.91 A B 44.4 2.75 BPWPC 6.23 0.45 B C 46.8 0.92 B G

Mean value is the average of 10 specimens.bThe same letters are not significantly different at a ¼ 5%. Comparisons were donewithin each wood species group.

a Moisture content.

Md.S. Islam et al. / International Biodeterioration & Biodegradation 72 (2012) 108e113112

3.5. Fungal decay resistance test analysis

Weight losses due to fungal attack for raw wood, WPC, andPWPC are shown in Table 3. The results show that the raw woodwas severely attacked by both decay fungi (Polyporus versicolor andPoria placenta) with large weight losses. By contrast, WPC andPWPC showed significant and considerable resistance to both thebrown- and white-rot fungi compared to untreated wood species.The remarkably good decay resistance of PWPC can be explained byits high moisture exclusion efficiency and inhibition of mycelialspread (Yalinkilic et al., 1991). This higher resistance has beenexpected because benzene diazonium salt reacted with OH groupsin wood and yielded wood-azo compound, which made the PWPCsamples less hydrophilic.

From Table 3, it can be seen that the weight losses of raw woodand MMA/HMDIC monomer-only impregnated WPC were higherthan those of PWPC because monomer on its own does not formbonds with the OH groups of the wood fibers’ surfaces (Shane et al.,1995). Therefore, the PWPC gave the best results compared toWPC

Table 2Compressive modulus of untreated wood, WPC, and PWPC (SD ¼ standard devia-tion, HG ¼ homogeneity group).

Wood species Compositetypes

Compressive modulus (GPa), at 10% MCa

Mean SD HGb

Jelutong Untreated 2.85 0.57 AWPC 5.1 0.65 BPWPC 5.95 0.33 C

Terbulan Untreated 3.82 0.63 AWPC 6.28 0.95 BPWPC 7.03 0.22 C

Batai Untreated 3.58 0.83 AWPC 6.00 0.94 BPWPC 6.79 0.49 B C

Rubber Untreated 2.68 0.83 AWPC 4.34 1.40 BPWPC 4.99 0.26 B D

Pulai Untreated 2.42 1.17 AWPC 4.12 1.28 BPWPC 4.94 0.37 B E

Mean value is the average of 10 specimens.bThe same letters are not significantly different at a ¼ 5%. Comparisons were donewithin each wood species group.

a Moisture content.

and raw wood. The results also showed that generally all raw woodspecies were not resistant to decay exposure. However, benzenediazonium salt pretreatment enhanced the decay resistance anddecreased the weight losses due to both fungi for all wood species.As shown by the results, the PWPC of jelutong was highly resistantto both (white-rot and brown-rot) fungi decay exposure; it wasfollowed by terbulan, batai, pulai, and rubberwood. Hence, it can beconcluded that coupling reactions that occur during chemicalpretreatments were highly effective in improving decay resistance,a result that is in accordance with findings of previous researchers(Yalinkilic et al., 1998; Umit et al., 2005).

4. Conclusion

The present study shows that mechanical and biological prop-erties of all selected tropical wood species studied were signifi-cantly improved by pretreatment with benzene diazonium salt.Coupling reactions between diazonium salt and wood speciesyielded awood azo derivative, a finding that was confirmed by FTIRspectroscopic analysis. The significant effect of coupling reactionson the properties of PWPC can be explained by the behavior of themonomermixture, which adhered to the fiber surfaces of thewood,as seen under the scanning electron microscope. The mechanicaltests indicated that the PWPC led to significant improvements inMOE, MOR, and compressive modulus. The MOE, MOR, andcompressive modulus of diazonium salt pretreatment PWPC werehigher by 6e17%, 2e6%, and 11e20% than those of untreated WPC.Furthermore, the decay exposure of PWPCwas higher than those ofWPC and rawwood. The authors propose that coupling reactions aspretreatments enhanced the monomer loading and degree ofpolymerization inside wood, which significantly improved themechanical and biological properties of all the selected tropicallight hardwoods.

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