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Effect of coupling agents on the properties of bamboo fiber-reinforced unsaturated polyester resin composites a

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D.T. Tran , D.M. Nguyen , C.N. Ha Thuc & T.T. Dang

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Faculty of Material Science , Ho Chi Minh City University of Science , 227-Nguyen Van Cu Road, Ho Chi Minh City , Viet Nam Published online: 06 Jun 2013.

To cite this article: D.T. Tran , D.M. Nguyen , C.N. Ha Thuc & T.T. Dang (2013) Effect of coupling agents on the properties of bamboo fiber-reinforced unsaturated polyester resin composites, Composite Interfaces, 20:5, 343-353, DOI: 10.1080/15685543.2013.806100 To link to this article: http://dx.doi.org/10.1080/15685543.2013.806100

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Composite Interfaces, 2013 Vol. 20, No. 5, 343–353, http://dx.doi.org/10.1080/15685543.2013.806100

Effect of coupling agents on the properties of bamboo fiber-reinforced unsaturated polyester resin composites D.T. Tran*, D.M. Nguyen, C.N. Ha Thuc and T.T. Dang

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Faculty of Material Science, Ho Chi Minh City University of Science, 227-Nguyen Van Cu Road, Ho Chi Minh City, Viet Nam (Received 23 January 2013; accepted 8 May 2013) In this study, biocomposites were prepared by using an unsaturated polyester and Luong bamboo fibers (BFs). Fibers separated from bamboo body by mechanical method had size distribution mainly between 0.24 and 0.40 mm diameter and 1–2.5 cm length. Some factors having effect on properties of materials, such as fiber content, type, and concentration of coupling agents were also studied. As a result, composite materials showed good properties at 90 phr of BFs that were modified by 0.7% vinyl-functional silane coupling agent. Keywords: bamboo fiber; composite; unsaturated polyester; coupling agent

1. Introduction For many decades, composite materials have been usually made with organic matrices reinforced by synthetic reinforcement fibers, such as carbon or glass fiber. But the high cost and the environmental awareness of these synthetic fibers has limited their applications.[1] Meanwhile, natural fibers are very attractive because they are of low cost, low density, eco-friendly, available in high quantities, renewable, biodegradable, and show excellent mechanical properties.[2] When natural fibers are used as fillers in composite materials, products improve physical and mechanical properties. More recently, there have been many research works and issues on natural fiber-reinforced plastic composites to prepare eco-friendly biocomposites.[3–7] Among many various renewable resources, bamboo emerges as one of the largest species, which grows in all continents of the world, especially in Asia-pacific and South America. Bamboo grows very fast and could be exploited for applications after three years as compared with wood which takes almost more than 20 years.[8] Additionally, higher strength of bamboo fibers (BFs) as compared with other natural fibers [9] is also an attractive factor to explain why bamboo has been the subject of enormous interest in many areas, including bamboo-reinforced polymer composites in recent times.[8,10] However, only a few researches have been focused on the role of coupling agents on structure and properties of BF-reinforced composites.[11–14] For example, the effect of chemical modification by using aminopropyltrimethoxysilane for the BFs in the BF-filled polypropylene bio-composites were investigated by Sun-young Lee et al.[11] Results indicated the improvement of PP/fiber properties by silane modification. With Epoxy/ *Corresponding author. Email: [email protected] Ó 2013 Taylor & Francis

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bamboo cellulose fiber composites in presence of 3-glycidoxypropyl-trimethoxysilane (KH-560), results approved that the modification was positive to the mechanical behavior of the composites.[12] In the other research, BF-reinforced high-density polyethylene composites which were added by maleic anhydride (MAH) grafted polyethylene were prepared. The increase in mechanical properties also confirmed efficient fiber/matrix adhesion.[14] To follow closely those researches, our present work deals with the property improvement of composites based on an unsaturated polyester (UP) matrix and BF reinforcement by using several suggested coupling agents, such as vinyltrimethoxysilane (VS), MAH, acrylic acid (AA). We expect that these coupling agents, which simultaneously possess reactive functional groups and vinyl group, not only react with hydroxyl groups of cellulose of fibers to reduce the number of hydroxyl groups in the fiber/matrix interface but also participate in the curing process of polymer matrix. Therefore, they could perform as effective chemical bridges to promote adhesion between fibers and polymer, and finally lead to the increase in composite properties. Besides, poly(vinyl alcohol) (PVA), a biodegradable polymer, which has been proved as an effective factor for glass fibers and UP interfacial adhesion,[15] is also exploited as the coupling agent in this study. Some factors that affect the properties of materials were characterized by different means, such as Fourier transform infrared spectroscope, scanning electron microscope (SEM), flexural, and tensile tests. 2.

Experimental methods

2.1. Materials Bamboo used in this research belongs to the family of Bambusa stenostachyum. This type of bamboo is designated as bamboo Luong in this paper. The plants were collected in Phu An Bamboo Park, Binh Duong province, Vietnam. Unsaturated polyester, SHCP 268 BQT, having styrene concentration of 30% wt was obtained from Highpolymer Chemical Products Pte Ltd, Singapore. The free radical initiator was methyl ethyl ketone peroxide (Butanox M50) from Asia Akzo Nobel Co., Ltd, China, with cobalt naphthanate and dimethyl aniline as cocatalysts. VS (Silquest silane A-171) was provided by Synatech Fine Chemicals Co., Ltd, China. MAH and AA were provided by Sinochem Nanjing Corporation, China. Finally, PVA was supplied by Kuraray Co., Ltd, Japan. 2.2.

Experiment

2.2.1. Fiber extraction processing The node portions and the thin layer of exoderm and endoderm (bark) were removed from the bamboo stem. Rest of the hollow cylindrical portion of culms was taken for extracting fibers.[9] The cylindrical portions of the culms were peeled in the longitudinal direction to make strips of 1.0–3.0 mm thick, about 120–150 mm length, and 20–40 mm width. The strips were bundled and kept in water for 72 h to soften. A mechanical method, rolling mill technique (RMT), was used for the fiber separation.[16] Bamboo strips were executed in two stages along the parallel directions compared with axis. During the first stage, strips were rolled once with a distance between the two axes of 0.5 mm and a rate of 60 cycles per minute. During the second stage, axes were adjusted to a distance of 0.1 mm and then strips were rolled twice. After fibers were separated, they were exposed to sun for 48 h and then dried for 72 h at 80 °C to remove the remaining water.

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2.2.2. Fiber treatment process by NaOH solution The BFs that were separated from strips were treated by 1% wt NaOH solution at room temperature for 72 h. Then, the solution was neutralized by sodium carbonate to pH 7. BFs were taken out and washed several times by water. Afterwards, they were dried in oven at 100 °C for 3 h. 2.2.3.

Preparation of NaOH-treated fiber mat

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Ten grams of NaOH-treated BFs was uniformly laid on an aluminum substrate. The aluminum substrate then was pressed between plates of a hot press at 100 °C and pressure of 60 bars for 30 s to obtain fiber mat having 0.3 mm thickness. 2.2.4.

Modification of NaOH-treated fiber mat

To modify the NaOH-treated fiber surface by silane coupling agent, VS was dissolved into solvent system water:ethanol (5:95) and the solution was maintained at pH 4 by acetic acid for complete hydrolysis.[17] In case of PVA, PVA was dissolved in water. [18] MAH and AA were dissolved in water containing 1% H2SO4.[18] Solution of each coupling agent was sprayed on both sides of mats. Then, mats were pressed at 110 °C for 3 min and then stored in an oven at 80 °C. 2.2.5.

Making of composite material samples

The composite sheets (length 17 cm, width 17 cm, thickness 0.2 cm) were made by hand lay-up method according to following processing conditions. The selective mat layers were poured with an enough suitable quality of UP. The wetted fibrous mat layers superposed each other in the mold and then placed between the electrically heated plates of a hot press at 80 °C. Then, the mold was heated to 110 °C at a molding pressure of 60 bars and was kept under these conditions for 20 min to define the shape of the samples. Finally, samples were taken out and passed through post-cure process in an oven at 80 °C for 24 h. 2.2.6. Characterization methods Mechanical properties of the composites were evaluated by tensile and flexural testing. All specimens were kept in desiccator under vacuum for 24 h before measurement. Tensile and flexural tests were carried out according to ASTM standard D638 and D790, respectively by using QC-505B1 apparatus (Cometech Co., Taiwan) at 24 °C and a crosshead movement of 3 mm/min. In case of tensile test as well as flexural test, at least five specimens were measured for each composition to obtain average values of modulus and stress. Adsorbent infrared spectrum of BFs was recorded in the spectral range 4000–500 cm 1 with the resolution 2 cm 1 by using the Tensor 27 FT-IR spectrometer (Bruker Optics Co., Germany) at room temperature. In case of coupling agent-modified BFs, unreacted coupling agent was removed from fiber surface by Soxhlet process in acetone for 3 h (AA or MAH modified BFs) and in ethanol for 15 h (VS modified BFs) before measurement was started. Photomicrograph was obtained by a hand held digital photomicroscopy, Dino-Lite Microscope Model AM311, at a magnification of 200 times at a digital resolution of 640  480.

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SEM was used to observe fractured surface of composite specimen. The procedure was performed by using a Model JSM-6600 analyzer (JEOL Co., Japan). 3.

Results and discussion

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3.1. Characteristics of luong BFs 3.1.1. Effect of extraction technique on fiber size distribution The distribution of the length and diameter of the BFs after separation by a roll mill is shown in Figure 1. The results indicate that the fiber length distribution is from 0.5 to 7.5 cm and mainly concentrated in the range of 1–2.5 cm (Figure 1(a)). Meanwhile, diameter of the BFs is distributed from 0.16 to 0.54 mm and concentrated in the range from 0.24 to 0.40 mm (Figure 1(b)). The observed results can be understood by the mechanism of fiber separation of RMT. Strips are simultaneously exposed to both tearing and pressure stress along each fiber bundle. Therefore, bundles are separated from each other but are not broken. This resulted in BFs having narrow distribution of diameter and length. Generally, RMT is a suitable method to extract a large amount of BFs having quite homogeneous length and small diameter [16] to apply for composite materials 3.1.2. Effects of NaOH treatment on the fiber structure Structure of BFs, similar to other natural fibers, is composed of a few main components, such as cellulose, hemicellulose, lignin, and a small amount of several other organics, such as xylem, pentosan, wax, pectin, and inorganic minerals.[19] Hemicellulose contains several different sugar units and has a degree of polymerization 10–100 times smaller than the native cellulose.[20] Lignin is amorphous, highly complex, and mainly aromatic polymer of phenyl-propane units distributed throughout the secondary cell of fiber as a cell wall adhesive. These ingredients cover cellulosic parts and obstruct interactions between hydroxyl groups of cellulose and reactive functional groups of coupling agents. Therefore, the purpose of the NaOH treatment is to clean fiber surface and then help cellulose component effectively interact with coupling agent during modifying process. Besides, the NaOH treatment may also destroy strong hydrogen bonds between hydroxyl groups of cellulose, thus contributing in making hydroxyl groups more reactive.[21]

Figure 1.

Length and diameter distribution of BFs extracted by RMT method.

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Figure 2.

IR spectrum of (a) original BFs and (b) 1% NaOH-treated BFs in 72 h.

Figure 3.

Micrographs of longitudinal views of (a) original BFs and (b) 1% NaOH-treated BFs.

FT-IR spectrum of NaOH-treated BFs is shown in Figure 2(b). The peak appearing at 3200–3500 cm 1 is a characteristic vibration of the hydroxyl groups of cellulose and hardly changed at all when BFs are treated. Meanwhile, the C=O vibration of hemicellulose at 1732 cm 1 is almost disappeared. Moreover, the peak intensity of the symmetric vibration of CH3–O– groups of lignin at 1250 cm 1 is markedly reduced. In short, 1% NaOH treatment essentially removes significant amount of hemicellulose and a little part of lignin from fiber structure. The surface of the original and NaOH-treated BFs is also shown in Figure 3. The surface of the original BFs indicates that the cellulosic part is surrounded by lignin and some other components (Figure 3(a)), but surface becomes clean and glib in case of the NaOH-treated BFs (Figure 3(b)). These results again demonstrate that hemicellulose, lignin, and some other components are significantly removed by NaOH treatment.

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Figure 4.

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Effect of NaOH-treated fiber content on the mechanical parameters of composites.

3.2. Effect of NaOH-treated fiber content on the mechanical properties of composites From Figure 4(a), when NaOH-treated fiber content increases, both tensile stress and modulus of composite are improved and obtain maximum value at 90 phr of BFs. However, tensile strength decreases at higher fiber content. These results can be explained by the fact that the amount of UP is not enough to completely wet the BFs at high fiber content, thus causing the decrease in transferring applied load between UP matrix and BFs. Flexural strength of composite also behaves similar to the tensile strength as shown in Figure 4(b). According to the obtained mechanical parameters, the optimum NaOH-treated fiber content for making composite materials is 90 phr.

Figure 5. (90/100).

Effect of coupling agent content on the mechanical parameters of composites BFs/UP

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Figure 6. Adhesion mechanisms between UP matrix and BFs under effect of coupling agents (a) MAH, (b) AA and (c) VS.

3.3.

Effect of coupling agents on mechanical properties of composites

Figure 5 shows effect of coupling agent contents on the mechanical behavior of composites. Tensile strength is impressively enhanced at 0.7% of VS, 0.3% of MAH, and 0.3% of AA (Figure 5(a) and (b)). As seen in Figure 5(c) and (d), flexural strength also displays the tendency similar to tensile case. Flexural strength also gains the best value at 0.7% of VS, 0.3% of MAH, and 0.3% of AA. Meanwhile, mechanical properties of composites drop at all the contents of PVA. The enhancement of mechanical parameters of composites in presence of modifiers, such as MAH, AA, and VS could be explained by mechanisms as shown in Figure 6. [19,22,23] Some reactive functional groups, such as anhydride, carboxyl, and hydroxyl in structure of MAH, AA, and VS, respectively are expected to react with hydroxyl groups of cellulose of NaOH-treated BFs through etherification or esterification reaction in acidic environment. From there, coupling agents could be grafted on the surface of BFs as seen in Figures 7–9 and make BFs become more hydrophobic. Besides, VS, MAH, and AA also contain vinyl group that is possible to participate in curing process

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Figure 7.

FT-IR spectrum of (a) NaOH-treated BFs and (b) 0.3% MAH-modified BFs.

Figure 8.

FT-IR spectrum of (a) NaOH-treated BFs and (b) 0.3% AA-modified BFs.

of UP matrix. Therefore, these coupling agents act as good chemical bridges to enhance interfacial adhesion between UP matrix and BFs and lead to the improvement of mechanical parameters of composites. As shown in Figures 7–9, the IR spectrums of BFs modified by MAH (0.3%), AA (0.3%), and VS (0.7%), respectively approve that some new vibrations appear when compared with NaOH treated BFs. Of course, that is the consequence of reactions between fiber surface and coupling agents. IR spectrum of MAH-modified BFs (Figure 7(b)) shows the appearance of absorption at about 1726 cm 1 suggesting a significant increase in carbonyl C=O groups due to an ester bonding between fiber and MAH. The IR spectrum of the AA-modified BFs is shown in Figure 8(b). As expected, the most obvious evidence is an enhanced absorption at 1718 cm 1. This is also a specific feature of saturated ester C=O. Figure 9(b) shows appearance of absorption peaks at 1599 and 1164 cm 1 in case of VS-modified BFs. These absorptions are characteristic vibrations of

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FT-IR spectrum of (a) NaOH-treated BFs and (b) 0.7% VS-modified BFs.

Figure 10. SEM of composites at fiber content 90 phr: (a and b) UP/NaOH-treated BFs; (c and d) UP/7% VS-modified BFs. (b and d are higher magnification of a and c, respectively.)

the C=C and Si–O bonds, respectively. But vibration of Si–O may be from either Si–O–C or Si–O–Si bonds. In IR spectrum, the Si–O–Si and Si–O–C signals are generally detected at the same wavenumber positions, thus the existing of peak at 1164 cm 1 confirms the occurrence of the grafting reaction between hydrolyzed silane and BFs, and also indicates the existence of the polysiloxane networks deposited on the fiber surface. [24,25]

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PVA is not a good coupling agent. Different reasons may be proposed to explain this phenomenon. The first reason is that PVA may act like a plasticizer when it disperses into the matrix which becomes softer, and mechanical properties of composites decrease accordingly. The other possibility is that, during the drying of mats, PVA creates a thin film layer on the both sides of mats restraining the further impregnation of UP resin, leading to composites with high void contents. Effect of VS coupling agent is also reflected in the fracture surfaces of composites observed by SEM for NaOH-treated and 0.7% VS-modified BFs (Figure 10). We can see the smooth surface of BFs and an obvious partition between fiber surface and UP matrix in composites reinforced by NaOH-treated BFs (Figure 10(a)). This result demonstrates the poor interaction between matrix and filler. On contrary, BFs and UP phase well interact together in case of composite containing 0.7% VS-modified BFs (Figure 10(b)). Rough surface of BFs may be due to VS bridges connecting fibers and UP by forming chemical linkages (covalent bonds). 4. Conclusion The RMT is a good way to extract BFS because this is a simple and environment friendly method to produce a large amount of BFs having narrow distribution of size. Through mechanical testing, the NaOH-treated BF content, which is most effectively used to reinforce for UP, is at 90 phr. A great improvement of the tensile and flexural strength is obtained when coupling agents, such as VS, MAH, and AA, are applied for modifying surface of BFs at concentration of 0.7, 0.3, and 0.3%, respectively. Acknowledgements This study is financially supported by a grant from the Department of Material Science, Ho Chi Minh University of Science, Vietnam. We thank all participants for their enthusiasm and cooperation in the study.

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