ARTICLE pubs.acs.org/IECR
Dimensional Stability and Dynamic Young’s Modulus of Tropical Light Hardwood Chemically Treated with Methyl Methacrylate in Combination with Hexamethylene Diisocyanate Cross-Linker Md. Saiful Islam,†,* Sinin Hamdan,† Ismail Jusoh,‡ Md. Rezaur Rahman,† and Zainal Abidin Talib§ †
Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia ‡ Department of Plant Science and Environmental Ecology, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia § Department of Physics, Universiti Putra Malaysia, Serdang, Selangor, Malaysia ABSTRACT: Chemical treatment is often used to improve the physical and mechanical properties of wood materials. In this study, wood polymer composites (WPC) were prepared from five types of tropical wood species by impregnating the woods with methyl methacrylate (MMA) combined with a cross-linker hexamethylene diisocyanate (HMDIC). Their dimensional stability and mechanical properties were then investigated. The impregnation of wood with monomer systems and polymerization was accomplished through vacuum-pressure and catalyst heat treatment method, respectively. The manufacturing of WPC was confirmed through FT-IR spectroscopy and X-ray diffraction (XRD) analyses. The dimensional stability of manufactured WPC in terms of volumetric swelling (S) and antishrink efficiency (ASE) was measured and found to be improved on treatment. In addition, the modified WPC had lower moisture absorption and higher water repellent efficiency compared to raw wood. The mechanical property of treated samples in terms of dynamics Young’s modulus (Ed) was also shown to improve. These improvements in properties were observed as more effective with MMA-HMDIC combination.
1. INTRODUCTION In the past few years, interest in chemical modification of solid wood has been increasing due to the extreme shortage of good quality hardwood in the world. This trend has inspired the researchers to search for an alternative way to develop higher quality wood from low quality samples. Wood is a renewable resource and one of the most preferred building materials because of its complex structure and its universal practicality. Generally, wood is a natural polymeric material, made up mainly of cellulose, hemicelluloses, and lignin. These three polymer components are the most important responsible factors for the physical, mechanical, and thermal properties of wood. The physical and chemical properties of wood are readily deteriorated by weathering effects such as light, water, temperature, biological organisms, and others.1 The main problems associated with wood for outdoor and indoor uses are its dimensional instability due to high moisture uptake, biodegradation, and decay by microorganisms.2-4 These defects are due to the presence of numerous hydroxyl groups (-OH) in the three major wood components and their various cavities. The -OH groups of wood attract water molecules through hydrogen bonding, thus making it dimensionally unstable, which in turn promotes physical, mechanical, and chemical properties changes. Chemical modification of cell wall polymer is an often-followed route to improve these inherent properties.5 More precisely, modification using suitable chemical treatments such as the formation of wood polymer composites (WPC) shows potential in improving wood properties.6-8 Impregnating wood with polymerizable monomer formulation and then polymerizing it in place produces a WPC. The WPC is more convenient as a product material compared to r 2011 American Chemical Society
plain wood as it is less susceptible to moisture-induced swelling, shrinking, and thermal degradation. Consequently, it has a longer life-span. Moreover, the WPC has a smoother surface structure. Recent considerable interest has been manifested in wood impregnation with a variety of monomers such as styrene, epoxy resins, urethane, phenol formaldehyde, methyl methacrylate (MMA), vinyl, and acrylic monomer to improve the negative properties in wood.9-11 WPC made with combinations of monomers like hexadiol, diacrylate, hydroxyethyl methacrylate, glycidyl methacrylate, and anhydride has been shown to improve the dimensional and thermal stability.12 However, it has been established that most monomers do not form bonds with hydroxyl groups of cell wall polymer. Since most monomers are nonpolar, there is little interaction between these monomers and the hydroxyl groups of the wood component. Poor chemical and physical interfacial interactions between the wood surface and chemical are two of the most important causes of bond failure. Therefore, the polymer component of the WPC simply bulks the wood structure by filling the capillaries, vessels, and other void spaces within the wood. They simply bulk the void spaces within the wood structure. It can therefore be deduced that if there is a bond between the impregnated monomers and the hydroxyl groups of wood, the dimensional stability and mechanical and physical properties of WPC may be improved further. It has been noted that adhesion and interaction between wood component and polymer can be enhanced by using varieties of Received: October 27, 2010 Accepted: February 8, 2011 Revised: January 24, 2011 Published: February 24, 2011 3900
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Industrial & Engineering Chemistry Research chemicals and cross-linking monomers such as alkoxysalin coupling agents, diazonium salt, sodium perchlorate, glycidyl methacrylate, trimethylolpropane triacrylate, trivinyl isocyanurate, and ethylene glycol dimethacrylate, etc.13-15 The crosslinking of material in wood samples provides better dimensional stability to the WPC.16 The properties of a wood composite are significantly improved by the addition of an isocyanate compound to the vinyl or acrylic monomer treating mixture. Hexamethylene diisocyanate (HMDIC) is a difunctional reagent which has two reactive functional groups. HMDIC modification of wood relies on modifying the predominant wood cell wall polymer by reacting wood hydroxyl groups with a diisocyanate group to form a wood-urethane derivative.17 Some researchers consider that the isocyanates also react with accessible -OH groups according to the following proposed chemical reaction. wood - OH þ R - NdCdO f wood - O - CðdOÞ - NH - R
However, the isocyanate group of HMDIC can be exploited for reaction with -OH groups in wood components and for copolymerization with vinyl or acrylic type monomers. This reaction can also create new structures in the WPC that can/may influence morphology, crystallization, and mechanical, thermal, biological, and other properties of wood. Many studies have been carried out on dimensional stability and mechanical and physical properties of various wood and their composites.18-20 Little work, however, has been devoted to Malaysian tropical light hardwood species and their chemical modification with a cross-linker reagent. In the present work, five species of selected tropical light hardwood species, namely jelutong, terbulan, batai, rubber, and pulai were utilized as starting materials as they are abundantly available in the tropical region. The major drawbacks of using these species are their high moisture uptake, dimensional instability, and high probability of biodegradation. These effects are especially pronounced in tropical areas where wood suffers from exposure to sunlight and high hygroscopicity which cause swelling and deformation. To overcome these problems and to improve the interaction and compatibility of polymer to the cell wall component of wood, the wood samples were impregnated with methyl methacrylate (MMA) and combined with a crosslinker monomer hexamethylene diisocyanate (HMDIC). The main purpose of this study is thus to determine the effect of MMA impregnation in the presence of cross-linking monomer HMDIC on the dimensional stability and mechanical properties of some selected tropical light hardwood composites.
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merization initiator. Methyl methacrylate and hexamethylene diisocynate had densities of 0.942-0.944 and 1.046-1.047 g/mm3, respectively. All chemicals were analytical grade products of Merck, Germany. 2.3. Specimen Preparation. Clear, defect-free planks were ripped and machined to 60 mm (L) 20 mm (T) 20 mm (R) samples for moisture absorption, water repellent efficiency (WRE), and dimensional stability tests and 340 mm (L) 20 mm (T) 10 mm (R) specimens for dynamic Young’s modulus test. 2.4. Manufacturing of Wood Polymer Composites (WPC). All oven-dried specimens were placed in an impregnation vacuum chamber at a vacuum pressure of 75 mmHg for 30 min. The respective monomer system and 2% benzyl peroxide catalyst as a polymerization initiator were introduced into the chamber as the vacuum pressure was released. The specimens were kept immersed in the monomer mixture solution for 6 h at ambient temperature and pressure to obtain further impregnation. These were then removed from the chamber and wiped of excess impregnate. Specimens were wrapped with aluminum foil and placed in an oven for 24 h at 105 °C for polymerization to take place. Weight percentage gain (WPG) of the samples was then measured using eq 1; WPG ð%Þ ¼
ð1Þ
where Wi and Wf are oven-dried weight of raw wood and fabricated WPC samples, respectively. 2.5. Fourier Transform Infrared Spectroscopy (FTIR). The infrared spectra of the raw and modified WPC grounded powder samples were recorded on a Shimadzu Fourier transform infrared spectroscopy (FTIR) 81001 spectrophotometer. The transmittance range of scan was 370-4000 cm-1. The obtained spectra are described in the Results and Discussion section. 2.6. X-ray Diffraction (XRD). To assess the morphological properties of WPC, XRD analysis was applied. A PANalytical XRD diffractrometer was used where Cu KR (λ = 1.54 Å) radiation was employed with 2θ varying between 4° and 80° at 5°/min. 2.7. Determination of Moisture Absorption. The raw samples and WPC samples were oven-dried at 103 °C for 24 h. They were then placed in a conditioning chamber at a temperature of 22 °C and a relative humidity of 65% for approximately 6 weeks. After stabilization, the weight of each sample was measured. The moisture content (MC) at equilibrium (E) was calculated as follows: EMC ð%Þ ¼
2. EXPERIMENTAL PROCEDURES 2.1. Wood Materials. Five species of tropical light hardwood namely jelutong (Dyera costulata), terbulan (Endospermum diadenum), batai (Paraserianthes moluccana), rubber (Hevea brasiliensis), and pulai (Alstonia pneumatophora) were collected from a local forest in Sarawak, Malaysia. All of the wood species were felled and cut into three bolts measuring 1.2 m in length. Each bolt was quarter sawed to produce planks of 4 cm thickness. The bolts were subsequently conditioned to air-dry in a room with relative humidity of 60% and ambient temperature of around 25 °C for 1 month prior to testing. 2.2. Monomer Solutions. The chemicals used for WPC production were methyl methacrylate (MMA) and methyl methacrylate/hexamethylene diisocyanate (MMA/HMDIC, 1:1 ratio) mixture containing 2% benzyl peroxide catalyst as a poly-
Wi - Wf 100 Wi
M2 - M1 100 M1
ð2Þ
where M2 is the weight of the raw wood at moisture absorption equilibrium, and M1 is the oven-dried weight of the raw or fabricated WPC sample. 2.8. Estimation of Dimensional Stability. Water repellent efficiency (WRE) and dimensional stability for both the fabricated WPC specimens and the raw wood samples were measured according to ASTM-1037 (1999).21 The weight gain and dimensional changes of each sample were determined by soaking the oven-dried specimens in a water bath at a temperature of 20 ( 1 °C for 24 h for each type of sample for 7 days. The weight and dimension of the specimens were measured before and after soaking. The water repellent efficiency (WRE) and antiswelling efficiency (ASE) were calculated as follows: 3901
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Figure 1. FTIR spectra of raw wood (I), MMA-treated WPC (II), and MMA-HMDIC treated WPC (III).
Wr - Wt WRE ð%Þ ¼ 100 Wr
3. RESULTS AND DISCUSSION ð3Þ
where Wr is the water absorption of raw wood sample and Wt is the water absorption of MMA and MMA-HMDIC treated WPC sample, which was calculated by eq 4: water absorption ð%Þ ¼
Wa - Wb 100 Wb
ð4Þ
where Wb is the initial weight of an oven-dried sample before water soaking and Wa is the weight after water soaking for 7 days. antiswelling efficiency, ASE ð%Þ ¼
Sr - St 100 St
ð5Þ
where Sr is the volumetric swelling coefficient of the raw samples and St is the volumetric swelling coefficient of the WPC samples. The volumetric swelling coefficients were calculated according to the formula S ð%Þ ¼
V2 - V1 100 V1
ð6Þ
where V2 is the volume of wood after soaking and V1 is the volume of wood before soaking. 2.9. Dynamic Young’s Modulus (Ed) Measurements. Dynamic Young’s Modulus (Ed) was measured using the free-free flexural vibration testing system. The details of this test can be obtained from refs 22 and 23. The Ed was calculated from the resonant frequency by using the following equation: Ed ¼ 4π2 f 2 l4 AF=Iðmn Þ4
ð7Þ
Where I = bd3/12, d is beam depth, b is beam width, l is beam length, f is natural frequency of the specimen, F is density, A is the cross sectional area, n is the mode of vibration, and m1 = 4.730. 2.10. Evaluation of Results. The test results were analyzed using analysis of One-Way ANOVA, and significant differences between raw wood and treated wood polymer composites (WPC) were determined by the Tukey Multiple Range Test.
3.1. Weight Percentage Gain (WPG). After impregnation with MMA and MMA-HMDIC, the weight percentage gains for jelutong, terbulan, batai, rubber, and pulai were 14%, 9%, 17%, 7%, and 11% and 50%, 35%, 55%, 18%, and 47%, respectively. This result revealed that MMA and MMA-HMDIC were successfully incorporated in the wood species, and that the MMA-HMDIC monomer system exhibited higher percentage gains compared to MMA in all five selected tropical wood species. 3.2. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of raw wood and MMA, MMA-HMDIC impregnated WPC samples are presented in Figure 1. The FTIR spectrum of the raw wood clearly showed the characteristic absorption band in the region of 3418, 1736, and 2933 cm-1 due to O-H stretching vibration, CdO stretching vibration and C-H stretching vibration, respectively.24 From spectrum II, it can be seen that the peak was at 1736 cm-1, which, due to carbonyl stretching vibration, partially disappeared upon impregnation with MMA. The position of the peak at 2933 cm-1 (O-H stretching) remained unchanged by incorporation of MMA. On the other hand, the particularly strong O-H stretching absorption band had been replaced by a much weaker absorption at 3359 cm-1 and a new carbonyl absorption band was developed at the region of 1688 cm-1 (Spectrum III) as a result of the interaction with MMA-HMDIC. These changes were due to the fact that with isocyanate groups of HMDIC, virtually all the hydroxyl groups had been replaced, and the new 3359 cm-1 absorption was due to the carbamate N-H bonds as shown in spectrum II.25 Therefore, it can be confirmed that HMDIC reacted with wood fiber and produced wood-O-C(dO)NH-R compound. It can also be seen from spectrum II that the carbonyl band at 1736 cm-1 had completely disappeared, and the absorption band of C-H group had shifted toward higher wave numbers (2918-2933 cm-1) with narrow band intensity, which gave further evidence of the interaction and cross-link between wood, MMA, and HMDIC. 3.3. X-ray Diffraction (XRD) Analysis. The X-ray diffraction patterns of raw wood, MMA, and MMA-HMDIC incorporated 3902
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Figure 3. Moisture absorption of raw wood, MMA, and MMAHMDIC treated WPC.
Figure 4. Water repellent efficiency of MMA and MMA-HMDIC treated WPC.
Figure 2. (a) Typical X-ray diffraction patterns of raw wood; (b) typical X-ray diffraction patterns of MMA-treated WPC; (c) typical X-ray diffraction patterns of MMA-HMDIC treated WPC.
WPC are given in Figure 2a-c. As seen in Figure 2a, the patterns of raw wood fibers exhibited three well-defined peaks (2θ) at 15.1, 22.8, and 34.7° which corresponded to the (110), (200), and (023) or (004) crystallographic planes, respectively.26 It was observed that there were three new peaks (2θ) at 45.5, 51.1, and 55.2° of low intensities in the amorphous region 45°-56° in the X-ray spectra in Figure 2b. These peaks may be due to the incorporation of MMA inside the wood fibers and the formation of wood composites.27 On the other hand, diffraction patterns of MMA-HMDIC impregnated WPC sample exhibited five new prominent peaks at 42.13°, 43.61°, 49.23°, 50.96°, and 75.81° as
shown in Figure 2c. These new peaks may be due to the strong interaction of HMDIC, wood, and MMA and the formation of wood composites. The cross-linker HMDIC reacted with cell wall hydroxyl groups of wood and formed wood-O-C(dO)NH-R compound which created a rigid linking bridge with wood fiber and MMA, thus enhancing the overall crystallinity of WPC. This result also indicated that manufactured WPC with HMDIC as the cross-linker significantly increased the crystallinity of wood, as ascertained by other researchers.27,28 3.4. Moisture Absorption. The moisture absorption of raw wood, MMA, and MMA-HMDIC impregnated WPC samples were measured. The measurements are illustrated in Figure 3. All raw wood samples exhibited a higher percentage of moisture absorption than the modified WPCs. This was expected because cell walls with hydrophilic hydroxyl groups will absorb moisture to its surface through the formation of hydrogen bonding.29 From the figure, it can be seen that MMA and MMA-HMDIC incorporated WPC samples showed much lower moisture absorption than raw wood samples. It is also apparent that MMAHMDIC impregnated WPC had the lowest (2.5-6%) moisture absorption compared to raw wood and MMA-treated WPC samples. Among the wood species used, the highest decrement of moisture absorption was observed in pulai, followed by batai, jelutong, terbulan, and rubber for both MMA and MMAHMDIC treated WPCs. The reduction in moisture absorption for the manufactured WPCs could be attributed to the impregnated polymer MMA and/or the MMA-HMDIC, which may block sorption sites in the interior of wood cell lumens and the 3903
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Table 1. Volumetric Swelling Coefficient (S%) of Raw Wood, MMA, and MMA-HMDIC Treated WPC Samplesa wood species and sample particulars jelutong
terbulan
batai
rubber
pulai
volumetric swelling coefficient (S %)
standard deviation
homogeneity group (HG)
raw
7.28
0.08
a
MMA/WPC
5.12
0.07
b
MMA-HMDC/WPC
2.13
0.09
c
raw
8.17
0.05
d
MMA/WPC
6.10
0.07
e
MMA-HMDC/WPC
2.73
0.05
f
raw
9.31
0.06
g
MMA/WPC MMA-HMDC/WPC
5.65 1.84
0.06 0.05
h i
raw
9.13
0.07
j
MMA/WPC
6.97
0.10
k
MMA-HMDC/WPC
3.76
0.07
l
raw
5.44
0.08
m
MMA/WPC
3.53
0.09
n
MMA-HMDC/WPC
1.10
0.06
o
Each value is the average of 10 specimens. The same letters are not significantly different at R = 5%. Comparisons were done within each wood species group. a
cell wall.30 The number of hydroxyl groups in the raw wood increases the moisture absorption.13 However, MMA-HMDIC reacts with OH groups of wood through its isocyanates groups and cross-link between wood, MMA, and HMDIC, which strongly reduced moisture absorption. As a result, this monomer combination system showed the least moisture absorption compared to the raw and MMA-treated WPC samples.31 3.5. Water Repellent Efficiency (WRE). Figure 4 illustrates the water repellent efficiencies (WREs) of MMA and MMAHMDIC impregnated WPC samples. The WREs of MMAHMDIC treated wood samples were significantly higher than that of MMA-treated WPC samples. The highest increment of WRE was again observed in pulai, followed by jelutong, batai, terbulan, and rubberwood. The improvement in WRE for MMA-HMDIC treatment has been expected because HMDIC reacts with numerous hydroxyl groups contained in wood components as stated earlier. Consequently, fewer water absorption sites remain, which also contribute to the reduction in water uptake.30,31 3.6. Dimensional Stability. 3.6.1. Volumetric Swelling Coefficient (S %). The results of volumetric swelling coefficient (S) of raw wood, MMA, and MMA-HMDIC treated WPC samples are given in Table 1. From Table 1, it is clear that all raw wood samples displayed the highest swelling measurements (5.449.31) compared to MMA and MMA-HMDIC impregnated WPC samples. Both MMA and MMA-HMDIC treatments significantly decreased the volumetric swelling coefficient in all five wood species. The reduction of S in WPC samples corresponded to the moisture excluding capacity of the treatment. The main reason for this trend has been explained earlier.32 The wood samples treated with MMA-HMDIC combination showed the least swelling compared to the raw wood and MMA-treated WPC samples. This result has also been expected due to cross-linking formed by the interaction between isocyanate groups of HMDIC with hydroxyl group of wood and MMA, respectively, which decreased water absorption and consequently reduced swelling.32,33 3.6.2. Antiswelling Efficiency (ASE %). The ASE of raw wood and WPC samples were determined and are illustrated in Figure 5. As indicated, wood samples treated with MMAHMDIC combination exhibited significant improvement in
Figure 5. Antiswelling efficiency of MMA and MMA-HMDIC treated WPC.
ASE compared to MMA-treated WPC samples. This result is evident because MMA-HMDIC has the ability to react with the hydroxyl group of wood component and also possesses crosslinking capability between MMA and wood.34 The highest increment of ASE was observed in pulai, followed by jelutong, batai, terbulan, and rubber. The ASE of the MMA-treated WPC samples was 23-40%, while the ASE of MMA-HMDIC treated WPC samples was 58-81%. These results indicated that MMA only bulked the cell wall and did not react fully with the wood component, whereas the presence of HMDIC increased the interaction between MMA and wood through its isocyanate groups. This particular interaction between wood, MMA, and HMDIC is considered to effectively improve ASE.35 The MMA-HMDIC treatments improved ASE significantly compared to the MMA-only treatments, which means that better dimensional stability could be obtained using MMA-HMDIC impregnated WPC samples. 3.7. Dynamic Young’s Modulus (Ed). The dynamic Young’s modulus (Ed) values for the raw wood, MMA, and MMAHMDIC treated WPC samples were measured from the freefree flexural vibration testing system and are illustrated in Table 2. As illustrated, the Ed of MMA-HMDIC treated WPC samples exhibited much higher values compared to that of the raw and MMA-treated WPC samples. It is also apparent that there was significant improvement of dynamic Young’s modulus for MMA-HMDIC treated WPC samples, whereas MMA-treated 3904
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Table 2. Dynamic Young’s Modulus (Ed) of Raw Wood, MMA, and MMA-HMDIC Treated WPC Samplesa wood species and sample particulars jelutong
terbulan
batai
rubber
pulai
Dynamic Youg’s modulus (Ed) (GPa)
standard deviation
homogeneity group (HG)
raw
6.61
0.18
a
MMA/WPC
6.72
0.17
b, a
MMA-HMDC/WPC
8.52
0.13
c
raw
7.92
0.12
d
MMA/WPC
8.02
0.13
e, d
MMA-HMDC/WPC
9.85
0.05
f
raw
7.4
0.15
g
MMA/WPC MMA-HMDC/WPC
8.19 10.32
0.10 0.15
h i
raw
15.33
0.35
j
MMA/WPC
15.53
0.23
k, j
MMA-HMDC/WPC
17.3
0.16
l
raw
4.77
0.12
m
MMA/WPC
6.74
0.19
n
11.64
0.24
o
MMA-HMDC/WPC
Each value is the average of 10 specimens. The same letters are not significantly different at R = 5%. Comparison were done within the each wood species group.
a
WPC samples showed no significant improvement for all species. These increments were 1.25-41.22% for the MMA treatment and 12.85 to 143.73% for the MMA-HMDIC treatment compared to the raw samples. The increase in Ed value for both treatments may be due to the incorporation of monomer or monomer mixture into wood fiber which either polymerized or formed cross-linking with wood cell wall polymer, thus increasing the modulus.11,18 Nevertheless, the sample treated with MMA-HMDIC showed the highest increment in Ed. This could be attributed to the presence of the cross-linker HMDIC which reacted with hydroxyl groups of the wood and provided better interaction between the MMA and wood. The increase of Ed in WPC compared to that in raw wood has also been reported by various researchers.22,23,34 Of the five wood species used, the highest increment of dynamic Young’s modulus was observed in pulai, followed by batai, jelutong, terbulan, and rubber for both MMA and MMA-HMDIC treatments. For rubber, though, a slight improvement was found for both treatments because of the wood’s high density and small amount of monomer loading, which was concurrent with other research findings.36
4. CONCLUSIONS It was interpreted that the presence of HMDIC as the crosslinking monomer significantly improved the properties of wood polymer composites prepared with MMA as the monomer. FTIR spectroscopy and X-ray diffraction confirmed the interaction between wood, MMA, and HMDIC. Significant improvements in water repellency and dimensional stabilities were obtained for the MMA-HMDIC treated samples over the MMA-treated and raw wood samples. The volumetric swelling in water for MMAHMDIC treated samples showed better results compared to both the MMA-treated samples and raw samples. An ASE value of 58-81% was obtained for MMA-HMDIC treated samples compared to that of 23-40% for MMA treated samples for all wood species. Furthermore, the mechanical property in terms of dynamic Young’s modulus (Ed) also demonstrated significant improvement in the WPC samples treated with MMA-HMDIC formulation. The WPC samples increased the Ed by 12.85143.73% at 18-55% WPG for MMA-HMDIC treatment
compared to the raw samples. The authors therefore propose that HMDIC as a cross-linker increased the interaction between wood, MMA, and HMDIC, thus significantly improving the properties of dimensional stability and dynamic Young’s modulus of all tropical light hardwoods used in this study. Owing to their superior dimensional stability and improved mechanical strength, MMA-HMDIC treated tropical light hardwood polymer composites are deemed suitable for specific applications.
’ AUTHOR INFORMATION Corresponding Author
*
[email protected].
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