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Investigation of acrylamide-modified melamine-formaldehyde resins as a compatibilizer for kenaf-unsaturated polyester composites.

INTRODUCTION

Natural plant fibers are renewable, biodegradable, of low density, and of low cost, and have a great potential for replacement of glass fibers for the production of fiber-reinforced composites (1-3). Natural fiber-reinforced composites (NFRCs) can potentially be used in automobiles, aircrafts, appliance components, and many other applications (4-6).

Kenaf is a fast-growing herbaceous annual plant, and can grow up to 4-5 m high and 25-35 mm in diameter in a growing season. Kenaf fibers are typically referred to those from the bast (bark) of the kenaf stem. Kenaf fibers are widely used for rope, twine, cloth, and paper and are gaining popularity as reinforcing materials for NFRCs (7-9).

Unsaturated polyester (UPE) is one of the thermosetting polymers that are extensively used as polymer matrix in NFRCs. It has good curing capability, good mechanical properties, low cost, and no byproduct during the curing reaction. These favorable characteristics contribute to the widespread application of UPE. UPE has a 40% market share of all thermosetting resins for composite applications (10).

There are two main issues of using kenaf fibers for kenaf--UPE composites: poor interfacial adhesion between hydrophilic kenaf fibers and hydrophobic UPE, and high water uptake of the kenaf--UPE composites. A number of approaches have been taken on improving the interfacial adhesion and reducing the water uptake. The treatment of kenaf fibers by electron-beam irradiation was shown to improve the mechanical properties of the kenaf--UPE composites, but did not reduce the water uptake (11). Surface coating and edge sealing of the kenaf--UPE composites reduced the water uptake, but did not improve the mechanical properties of the composites (12). Acetylation of kenaf fibers could reduce the water uptake of the resulting acetylated-kenaf--UPE composites by about 50% (13), but did not result in a significant increase in the strengths of the composites. Chemical modifications of kenaf fibers with alkali enhanced the flexural strength and flexural modulus, but did not reduce the water uptake of the kenaf--UPE composites (14). Chemical modifications of kenaf fibers with silane coupling agents improved the mechanical properties of the kenaf--UPE composites, but the water uptake of the resulting kenaf--UPE composites were not determined (15). The treatments of kenaf fibers with a combination of 1,6-diisocyanatohexane (DIH) and 2-hydroxylethyl acrylate (HEA) and with N-methylol acrylamide improved the mechanical properties and reduced the water uptake of the resulting kenaf--UPE composites (16), (17). However, an organic solvent, anhydrous ethyl acetate, had to be used for dissolution of DIH and HEA. Moreover, the kenaf--UPE composites from these two treatments still have lower mechanical properties and higher water uptake than those of glass-fiber-reinforced UPE composites. More research on further improvement of the mechanical properties and further reduction of the water uptake of the kenaf-UPE composites is needed.

Melamine--formaldehyde (MF) resins have a long history of being used for saturating paper. MF-saturated paper is strong and water resistant, and is widely used for laminating wood-based panels for furniture, kitchen counter tops, and many other applications. In this study, MF resins were modified with acrylamide. The acrylamide-modified MF resins were investigated as compatibilizers for improving mechanical properties and water resistance of the kenaf--UPE composites.

EXPERIMENTAL

Materials and Analytical Instruments

Kenaf fibers were purchased from Kenaf Industries (Raymondville, TX). Aropol 7030 resin (a mixture of about 60% UPE and 40% styrene) and LP-4016 (poly(vinyl acetate)) were provided by Ashland Chemical (Columbus, OH). Styrene, tert-butyl peroxybenzoate (TBPB), and melamine were obtained from Sigma--Aldrich (St. Louis, MO). Zinc stearate was purchased from Acros Organics (Morris Plains, NJ). Acrylamide was obtained from IT. Baker (Phillipsburg, NJ). Formaldehyde solution (37 wt%) was supplied by Fisher Chemicals (Fair Lawn, NJ). Ethylene glycol was obtained from Avantor Performance Materials (Phillipsburg, NJ). The hot pressing was performed on an automatic Benchtop Carver press (Carver, Wabash, IN). Flexural properties were obtained from a Sintech machine (MTS Systems, Enumclaw, WA). The fractured surfaces of the composites after the flexural test were analyzed with a FEI Quanta 600 scanning electron microscopy (SEM; Hillsboro, OR). The Fourier transform infrared (FTIR) spectra were obtained with a Nexus 470 spectrometer (Thermo Nicolet, Madison, WI), using a KBr pellet method.

Preparation of Kenaf Fiber Mats

Kenaf fibers (100 g, 2.5 cm in length) with a moisture content of 10% were fed into a LOUET drum carder for tearing apart fiber bundles and forming unidirectionally oriented kenaf fiber mats through a carding, layering and needle-punching process. The resulting fiber mats were cut by a paper cutter into five mats, with each mat having the dimensions of 200 mm X 200 mm X 10 mm. The fiber mats were stacked horizontally in an aluminum tray and oven-dried at 103[degrees]C for at least 20 h before use. The weight of five oven-dried fiber mats was 78 g.

Preparation of UPE Resin

Aropol 7030 resin (62.2 parts) and LP-40I6 (28.6 parts) were mixed together to form a solution. Styrene (4.8 parts) and zinc stearate (4.4 parts) were added to the solution, and the resulting mixture was mechanically stirred at room temperature for 2 h for forming a UPE resin.

Preparation of Melamine--Formaldehyde--Acrylamide (MFA) Solution

Melamine (3.8 g, 0.03 mol) was dissolved in formaldehyde solution (14.6 g, 0.18 mol HCHO) in a 500 ml flask. The solution was made slightly basic (pH = 7.5) with 10 wt% sodium hydroxide solution. The resulting mixture was stirred at a speed of 500 rpm at 50[degrees]C for 40 min. Acrylamide (6.4 g, 0.09 mol) and distilled water (287 ml) was then added to the melamine--formaldehyde mixture, and the resulting mixture was stirred at a speed of 500 rpm at 50[degrees]C for 30 min. The pH of the resulting reaction mixture was adjusted to 5 with 10 wt% sulfuric acid solution. The resulting MFA solution (312 g) contained 5 wt% total solids content with the melamine/formaldehyde/acrylamide molar ratio of 1:6:3. The MFA solutions with the melamine/formaldehyde/acrylamide molar ratio of 1/6/1 or 1/6/2 were prepared through changing the usage of acrylamide in the same procedure as for preparation of the MFA solution with the molar ratio of 1:6:3. The MFA solutions with the total solids content of 10 wt% or 15 wt% were prepared through changing the usage of distilled water in the same procedure as for preparation of the MFA solution with the solids content of 5%. The total solids content of each MFA solution is based on the sum of the dry weight of melamine, formaldehyde, and acrylamide divided by the overall weight of a MFA solution.

Treatment of Kenaf Fiber Mats with the MFA Solution

Each kenaf fiber mat was soaked in the previously described MFA solution (312 g) in a water bath at 19 [+ or -] 2 [degrees]c for 2 min, followed by oven-dry at 103[degrees]C for at least 20 h before use.

Hot Press Procedure for Kenaf--UPE Composites

UPE resin (78 g) and TBPB (1.2 g) were mixed by spatula for 1 min and the resulting UPE--TBPB mixture (15.8 g) was uniformly poured onto an oven-dried kenaf fiber mat that had been placed into a stainless steel mold with a dimension of 200 mm X 200 mm X 3 mm. The second mat was stacked above the first mat in a way that the kenaf fibers were oriented in the same direction, and the resulting UPE--TBPB mixture (15.8 g) was then uniformly poured onto the second mat. This process was repeated until a stack of five mats was made. The mold was placed onto the lower platen of an automatic bench-top Carver press, and pressed at 3.24 MPa for 10 min at room temperature allowing for thorough penetration of UPE resin into fibers. The mold was then pressed at 4.24 MPa, whereas the temperature was raised to 160[degrees]C. The hot pressing was maintained under a pressure of 4.24 MPa at 160[degrees]C for 20 min. The heating of the hot press was then turned off and two plywood boards were put at the top and bottom of the mold, respectively, to insulate the heat. The mold was pressed at 4.24 MPa for 100 min and then removed from the hot press and cooled at ambient environment. The weight of UPE resin for each composite board was 78 g, and the total weight of five oven-dried fiber mats was also 78 g. Thus, the resulting composite board had a fiber loading of 50 wt%.

The kenaf fiber mats without any treatment were used for making untreated-kenaf--UPE composites as control.

Determination of Flexural Properties of the Kenaf--UPE Composites

For the flexural test, each specimen had a rectangular dimension of 70 mm X 14 mm X 3 mm. The longitudinal direction of the test specimen was parallel to the longitudinal direction of the fibers. The three-point flexural test was performed on a Sintech testing machine in accordance with ASTM D 790-03, with a 50 mm span, crosshead moving downward at a speed of 5 mm/min. The flexural strength and flexural modulus were obtained from the test.

Water Uptake of the Kenaf--UPE Composites

Method 1: soaking the composite specimens in distilled water at room temperature. Each specimen had a dimension of 80 mm X 25 mm X 3 mm. All specimens were weighed and then soaked in water at room temperature in accordance with ASTM D 5229 M-04. At a predetermined time, the specimens were removed from water, wiped with tissue paper, weighed, and then put back to water for continued soaking. The water uptake was obtained from the weight gain divided by dry weight of the specimen.

Method 2: soaking the composite specimens in boiling water. Each specimen was cut into a dimension of 80 mm X 25 mm X 3 mm. All specimens were weighed and then soaked in boiling water for 4 h. The specimens were then removed from water, wiped with tissue paper, and weighed. The water uptake was obtained from the weight gain divided by dry weight of the specimen.

Characterization of Fractured Kenaf--UPE Composites with SEM

Specimens after the flexural tests were cut to small pieces for SEM imaging, the cross area of fractured surfaces having dimensions of 5 mm X 3 mm. The fractured surfaces of the specimens were coated with an Au-Pd film in a coater for 50 s before testing. The SEM images were obtained at an accelerating voltage of 10.0 kV.

Characterization of Untreated and MFA-Treated Kenaf Fibers with FTIR

Oven-dried kenaf fibers (8 g) were immersed into the MFA solution (40 g) for 2 min. The fibers with the solution were oven-dried all together at 103T for 20 h for forming MFA-treated kenaf fibers.

Untreated kenaf fibers and MFA-treated kenaf fibers (3 g) were wrapped with filter paper and then extracted with ethylene glycol in a Soxhlet extractor for 48 h, respectively. The extracted fibers were oven-dried at 80[degrees]C for 3 h and then cut into small pieces for FTIR characterization. FTIR spectra were recorded using a KBr pellet method and the number of scans for all three samples was 64.

Statistical Analysis

Data from flexural tests and water soaking tests were analyzed with two sample t-test method using R statistical software (Boston, MA). All p-values from tests were based on a 95% confidence interval.

RESULTS

Effects of Total Solids Content of the MFA Solution on Flexural Properties and Water Resistance of MFA-Treated Kenaf--UPE Composites

Compared with the control, that is, at 0% MFA, the MFA treatment at the 5 wt% total solids content significantly increased both the flexural strength and flexural modulus of the composites (Fig. 1). Increasing the solids content of the MFA solution from 5 wt% to 10 wt% significantly decreased the flexural strength, but did not significantly change the flexural modulus. The flexural strength and flexural modulus of the kenaf--UPE composites sharply decreased when the total solids content was further raised from 10 wt% to 15 wt%. Results from Fig. 1 demonstrated that the composites with 5 wt% total solids content of the MFA solution had the highest flexural strength and flexural modulus.

All water uptakes increased with an increase in the soaking time of below 50 days at each pre-determined soaking time and then flattened out when the soaking time was above 50 days (Fig. 2). The MFA treatment at the 5 wt% total solids content significantly lowered the water uptake of the composites over the control at each soaking time. Increasing the total solids content from 5 wt% to 10 wt% did not statistically decrease the water uptake. However, composites at the 15 wt% total solids content had a lower water uptake than those at the 5 wt% total solids content (Fig. 2).

The effect of the total solids content of the MFA solution on the water uptake of kenaf--UPE composites in boiling water is shown in Fig. 3. MFA-treated kenaf--UPE composites at the 5 wt% total solids content had a significantly lower water uptake than the control. Increasing the total solids content from 5 wt% to 10 wt% or 15 wt% did not decrease the water uptake.

Effects of Melamine/Formaldehyde/Acrylamide Molar Ratios on Flexural Properties and Water Resistance of MFA-Treated Kettaf--UPE Composites

Effects of the melamine/formaldehyde/acrylamide molar ratio on flexural properties of the composites are shown in Fig. 4. When compared with the control, the MFA treatment with melamine/formaldehyde/acrylamide molar ratios of 1/6/0, 1/6/1, and 1/6/2 did not improve the flexural strength. A significant increase in the flexural strength was observed when the molar ratio was changed from 1/6/0 to 1/6/1. However, the flexural strength did not significantly change when the molar ratio was further changed from 1/6/1 to 1/6/2. The kenaf--UPE composites at the 1/6/3 molar ratio of melamine/formaldehyde/acrylamide had a significantly higher flexural strength than the control. When kenaf fibers were treated with MF resin alone, that is, at the 1/6/0 molar ratio of melamine/formaldehyde/acrylamide, the flexural modulus of the resulting kenaf--UPE composites was significantly higher than that of the control. The flexural modulus further increased when the molar ratio of melamine/formaldehyde/acrylamide was changed from 1/6/0 to 1/6/1. However, the flexural modulus did not further increase when the molar ratio of melamine/formaldehyde/acrylamide was changed from 1/6/1 to 1/6/2 or 1/6/3. Thus, 1/6/3 was the optimum molar ratio in terms of enhancing the flexural properties.

Effects of melamine/formaldehyde/acrylamide molar ratios on water uptake of the composites at room temperature are shown in Fig. 5. All the water uptake increased with an increase in the soaking time below 40 days at each predetermined soaking time and then flattened out when the soaking time was above 40 days. When the molar ratio of melamine/formaldehyde/acrylamide was 1/6/0, the water uptake of the resulting composites did not significantly change over the control. Changing the molar ratio of melamine/formaldehyde/acrylamide from 1/6/0 to 1/6/1 significantly decreased the water uptake over the control. Treatment of the fibers with MFA at the molar ratio of 1/6/2 had no significant difference from that at the molar ratio of 1/6/1 in terms of the water uptake. Further addition of acrylamide, that is, changing the molar ratio from 1/6/2 to 1/6/3, significantly lowered the water uptake of the composites. Thus, 1/6/3 was the optimum molar ratio in terms of reducing the water uptake.

Effects of molar ratios on water uptake of the composites in boiling water are shown in Fig. 6. At the molar ratio of 1/6/0, the treatment did not significantly change the water uptake over the control. The treatment with MFA at the molar ratio of 1/6/1 dramatically lowered the water uptake over the control. Further increasing the acrylamide content, that is, changing the molar ratio from 1/6/1 to 1/6/2 and then to 1/6/3, did not further decrease the water uptake.

Effects of pH Value of the MFA Solution on Flexural Properties and Water Resistance of MFA-Treated Kenaf--UPE Composites

The effect of pH value of MFA solution on the flexural properties of the resulting composites is shown in Fig. 7. When compared with the control, the treatment at pH = 3 significantly lowered the flexural strength of the composites. The increase in the pH value from 3 to 5 significantly increased the flexural strength over the control. Further increase in the pH value from 5 to 7 and then to 9 (the pH was adjusted with 10 wt% NaOH solution) did not significantly increase the flexural strength of the composites. The flexural modulus significantly increased at pH = 3 over the control. The flexural modulus did not further increase when the pH value was raised from 3 to 5, 7, and 9.

The effect of the pH value on the water uptake of kenaf--UPE composites at room temperature is shown in Fig. 8. All the water uptake increased with an increase in the soaking time below 40 days at each predetermined soaking time and then flattened out when the soaking time was above 40 days. The MFA treatment at pH = 3 lowered the water uptake of the composites over the control. The water uptake at pH = 5 was much lower than that at pH = 3. However, the water uptake dramatically increased when the pH value was raised from 5 to 7 or 9. Results in Fig. 8 indicated that the pH = 5 was optimum for the treatment of kenaf fibers with the MFA in terms of reducing the water uptake (Fig. 8).

The effect of the pH value on the water uptake of kenaf--UPE composites in boiling water is shown in Fig. 9. The treatment at pH = 3 significantly decreased the water uptake when compared with the control. The water uptake did not significantly change when the pH value was raised from 3 to 5 and then to 7. However, the water uptake dramatically increased and reached the same level of the control when the pH value was raised from 7 to 9.

The untreated and MFA-treated kenaf fibers were first extracted with ethylene glycol for removal of chemicals that were not covalently bonded on the fibers. Melamine, formaldehyde, and acrylamide dissolved well in ethylene glycol. Therefore, the residual melamine, formaldehyde, and acrylamide, if there was any, should be completely removed after the extensive extraction with ethylene glycol. A separate experiment demonstrated that the reaction products of melamine, formaldehyde, and acrylamide were soluble in ethylene glycol. Thus ethylene glycol could also remove the reaction products of melamine, formaldehyde, and acrylamide that were not covalently bonded onto fibers. The extracted fibers were characterized with FTIR spectroscopy. MFA-treated kenaf fibers show strong peaks of C=0 functional groups of acrylamide at 1660 [cm.sup.-1] and strong peaks of N--H bending vibration at 1548 [cm.sup.-1] (Fig. 10). However, untreated kenaf fibers had weak peaks at 1660 [cm.sup.-1] and did not have peaks at 1548 [cm.sup.-1] (Fig. 10).

SEM images of fractured kenaf-UPE specimens are shown in Fig. 11. The fiber pull-out and voids were clearly observed on the surface of the untreated kenaf-UPE composites (Fig. 11a), indicating the poor interfacial adhesion between the kenaf fibers and the UPE matrix. For the MFA-treated kenaf-UPE composite, there were extensive fiber breaks and no fiber pull-out, and smaller and less gaps between the kenaf fibers and the UPE resins were observed (Fig. 11b), which indicated the good interfacial adhesion between the fibers and the UPE matrix. Comparison of Fig. 11a with b revealed that the treatment of kenaf fibers with MFA significantly increased the adhesion between the fibers and the UPE matrix.

DISCUSSION

The possible reactions in the preparation of MFA solution are proposed in Fig. 12. Each melamine molecule contains three amino groups that can potentially react with six formaldehyde molecules to form compound I (R=[CH.sub.2]OH in I) (Fig. 12). This is why the molar ratio of melamine/formaldehyde was set at 1/6. Each terminal hydroxyl group in I can be replaced by the amide group (CON[H.sub.2]) of acrylamide to form the compatibilizer II during the preparation of MFA and the treatment of kenaf fibers with the MFA solution. The possible reactions in the treatment of kenaf fibers with the MFA solution, and in the curing of kenaf--UPE composites are proposed in Fig. 12. The compatibilizer II contains multiple N-hydroxymethyl groups that can readily react with hydroxyl groups of kenaf fibers to form ether bonds (III) between the coupling agent II and the kenaf fibers (Fig. 12). The FUR characterization of MFA-treated fibers indeed confirmed that MFA was covalently bonded onto the kenaf fibers. The C=C bonds on the MFA-treated fiber surfaces could react with the unsaturated C=C groups of the UPE resin and styrene through a free-radical polymerization reaction during the hot pressing (Fig. 12). Therefore, MFA can form covalent linkages between the kenaf fibers and the UPE matrix, thus improving the interfacial adhesion between the kenaf fibers and the UPE matrix. The improved interfacial adhesion could improve the stress transfer from the UPE matrix to kenaf fibers, thus enhancing the flexural strength and flexural modulus. Results from Fig. 11 indeed confirmed that the MFA-treatment of kenaf fibers improved the interfacial adhesion.

The MFA resins are a viscous solution because the hydroxymethyl groups in MFA resins can self-condense to form polymeric materials, although the C[H.sub.2]OC[H.sub.2] ether linkages from the condensation are not stable in water. It was observed that the viscosity of the MFA solution increased along with the total solids content although specific viscosity data were not obtained. In the treatment of kenaf fibers with the MFA solution, it is ideal that MFA can freely penetrate the fibers where the hydroxymethyl groups react with hydroxyl groups of kenaf fibers to strengthen and stiffen the fibers and leave the C=C bonds of the acrylamide sticking out of the fiber surfaces. The MFA resins with a higher viscosity are less effective in penetrating the fibers and have a higher tendency to form a thick layer of a coating on the fiber surfaces than those with a lower viscosity, thus making them less effective in strengthening the fibers. The MFA resins with a higher viscosity could have a higher percentage of the C=C bonds of the acrylamide being buried inside the resins than those with a lower viscosity. The buried C=C bonds are not available for copolymerization with styrene and UPE resins. Therefore, the viscosity has to be sufficiently low for the good penetration of MFA resins and for formation of effective interfacial adhesion. These explanations appear to be consistent with the results shown in Fig. 1 that the 5 wt% total solids content had the highest flexural strength and flexural modulus. Treatment of kenaf fibers reduces the number of hydroxyl groups of kenaf fibers and forms water-repelling coatings on the fiber surfaces, which explains that MFA-treated kenaf--UPE composites all had much lower water uptake in water at room temperature and in boiling water than the untreated kenaf--UPE composites (Figs. 2 and 3). The improved interfacial adhesion also restricts the penetration of water into the kenaf fibers embedded in the UPE resins. It appears that the MFA resins with a higher total solids content tend to form a better water-repelling coating than those with a lower total solids content, which may explain that the water uptake of the MFA-treated kenaf--UPE composites slightly decreased along with the increase in the total solids content (Figs. 2 and 3).

Melamine--formaldehyde resins, that is, the 1/6/0 in Figs 4-6, can only react with kenaf fibers, but not with UPE resins because they do not contain the C=C bonds of acrylamide. Therefore, treatment of kenaf fibers with the MF resins does not expect to improve the interfacial adhesion between MF-treated kenaf fibers and UPE resins. The gaps and pores of kenaf fibers are already occupied by the MF resins, which restricts the penetration of UPE resins in kenaf fibers for formation of entanglements and mechanical interlocks. Because the flexural strength of kenaf--UPE composites is closely related with the interfacial bonding strengths between the kenaf fibers and UPE resins, the flexural strength of the MF-treated kenaf--UPE composites is expected to be similar to or even slightly lower than that of untreated kenaf--UPE composites, which is consistent with the results shown in Fig. 4 that the 1/6/0 had similar flexural strength to the control. It is well known that natural fibers become stiff and brittle after they are treated with MF resins. Because the flexural modulus of the kenaf--UPE composites is closely related to the stiffness of the kenaf fibers, the flexural modulus of the MF-treated kenaf--UPE composites is expected to be higher than that of untreated kenaf--UPE composites, which is consistent with the results in Fig. 4 that the 1/6/0 had a much higher flexural modulus than the control. When the hydroxylmethyl groups and the C=C bonds of the acrylamide are both present in the MFA resins, the MFA resins can stiffen the fibers and form strong bonding between the fibers and UPE resins, which explains why the composites at 1/6/1 had a higher flexural strength and a higher flexural modulus than those at 1/6/0 (Fig. 4).

It is known that MF-treated fibers absorb less water at room temperature than untreated fibers, which is consistent with the results that the MF-treated-kenaf--UPE composites (i.e., the 1/6/0 in Fig. 5) had a lower water uptake than the untreated-kenaf--UPE composites (i.e., the control in Fig. 5). However, the cured MF resins are not stable in boiling water, which may explain why the MF-treated-kenaf--UPE composites (i.e., the 1/6/0 in Fig. 6) had the same water uptake in boiling water as the untreated-kenaf--UPE composites (i.e., the control in Fig. 6). The interfacial adhesion in the MFA-treated-kenaf--UPE composites is strengthened by water-resistant covalent bonding. The improved interfacial adhesion greatly restricts the penetration of water to the kenaf fibers imbedded in the UPE resins, which may explain why all MFA-treated-kenaf--UPE composites had much lower water uptake in water at room temperature and in boiling water than the MF-treated-kenaf--UPE composites and the untreated-kenaf--UPE composites (Figs. 5 and 6). The 1/6/3 had slightly lower water uptake in water at room temperature and in boiling water than the 1/6/1 and the 1/6/2 probably because the 1/6/3 had a higher strength, that is, better interfacial adhesion, than the 1/6/1 and the 1/6/2.

It is still poorly understood that the flexural strength at the pH 3 was much lower than that of the control (Fig. 7). It is also not well understood that the water uptake at the pH 5 in water at room temperature and in boiling water were lower than that at the pH 7 or 9, whereas the flexural strength and flexural modulus at the pH 5 were comparable to those at the pH 7 or 9 (Figs. 8 and 9).

CONCLUSIONS

The novel compatibilizer based on melamine, formaldehyde, and acrylamide was successfully synthesized. MFA solution improved the compatibility and interfacial adhesion between kenaf fibers and UPE matrix, and was an effective compatibilizer in terms of enhancing the flexural properties and reducing the water uptake of the resulting kenaf--UPE composites. The total solids content, molar ratios of melamine/formaldehyde/acrylamide, and the pH value of the MFA solution had significant impacts on the flexural properties and water resistance of the composites. Composites with MFA treatment at the 5 wt% total solids content had the highest flexural strength and flexural modulus, as well as the lowest water uptake in boiling water. The MFA-treated-kenaf--UPE composites at the 15 wt% total solids content had the lowest water uptake at room temperature. The MFA-treated-kenaf--UPE composites at the 1/6/3 melamine/formaldehyde/acrylamide molar ratio had the highest flexural properties and the lowest water uptake. The pH = 5 of the MFA solution was optimum for the treatment of kenaf fibers in terms of enhancing the flexural properties and reducing the water uptake.

Correspondence to; Dr. Kaichang Li; e-mail: Kaichang.li@oregonstate.edu

DOI 10.1002/pen.234 18

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Xiaofeng Ren, Chunhong Li, Kaichang Li

Department of Wood Science and Engineering, Oregon State University, Corvallis, Oregon 97331
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Author:Ren, Xiaofeng; Li, Chunhong; Li, Kaichang
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Date:Aug 1, 2013
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