Mechanical properties of rice straw fiber-reinforced polymer ...

Fibers and Polymers 2011, Vol.12, No.5, 648-656

DOI 10.1007/s12221-011-0648-5

Mechanical Properties of Rice Straw Fiber-Reinforced Polymer Composites M. R. Ismail, Ali. A. M. Yassen*, and M. S. Afify1

Department of Radiation Chemistry, NCRRT,Atomic Energy Authority, Egypt Department of Chemistry, Faculty of Science, Al-Azhar University, Cairo, Egypt

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(Received September 17, 2010; Revised March 3, 2011; Accepted March 6, 2011)

Abstract: Rice straw fibers have been used as reinforcing fillers in different polymers, tow composites were prepared by

using polyvinyl alcohol and polystyrene polymers with different ratio of polymers. The pressed samples were subjected to various electron beam irradiation dose. The results indicated that, flexural strength, impact strength and modulus of elasticity increase with increasing the polymer ratio in the mix composition. It was found that the water absorption and thickness swelling percentages of rice straw fiber polystyrene composites decrease with increasing the polystyrene content in the same composition, while its values for rice straw polyvinyl alcohol composites improve. These are attributed to hydrophobic and hydrophilic characteristics of the tow composites. The improvement of physico-mechanical properties of both composites with the increase of electron beam irradiation dose are probably attributed to randomly initiated chain reactions, which started by the reactive centers created by electron beam. These observations were confirmed by IR spectrometer and SEM studies. Keywords: Flexural strength, Rice straw fibers, Composite, Thickness swelling

that the properties of particleboard produced from fine particles were better than those made from coarse particles. An increase in board density resulted in a corresponding improvement in the board properties. Agricultural legnocellulosic fiber (rice straw)-waste tire particle composite board were manufactured for use as insulation boards in construction using the same method as that used in the wood-based panel industry [7]. The waterproof water absorption and thickness swelling properties of the composite boards were better than those of wood particleboard. There boards can be used to prevent impact damage are easily modifiable and are inexpensive. They can be used as a substitute for insulation boards and other flexible material in construction. Other studies carried out on wood based fillers have been reported by a number of workers [8,9]. The most of the research works involving biocomposites have been devoted on the combination of lignocellulosic fillers and/or fibers with both amorphous and semi-crystalline thermoplastic matrices. The aim of the present work is to investigate the effect of polymer ratios as well electron beam irradiation dose of physicomechanical properties of rice straw fiber polymer composite.

Introduction

There is strong movement in research towards more highly engineered wood polymer composites with greater structural performance and more efficient design. Cellulosic fibers are effectively used as reinforcement for different thermoplastic [1]. The reactive radiation processing, applying monomers and oligomers of high reactivity, and electron beam (EB) as energy source, offers unique methods to bind together high strength cellulosic fibers and thermoplastic synthetic matrix. The siding will be manufactured from urban wood waste and recycled plastic film from shrink-wrap, bubble warp, and plastic grocery bags [2]. Advanced wood polymer composites are being investigated to replace treated timber currently used support pries and adsorb the shock of docking ships. Other products include pallets, flowerpots, shims, cosmetic pencils, grading stakes, tool handles, hot tub siding and office accessories [3]. The effect of blending Egyptian rice straw pulp with different pulp, namely, bagasse, reeds, cotton stalks, barley straw and wood wastes on physicomechanical properties of the finished hardboards was investigated [4]. Blending rice straw pulp with bugan caused the optimum improvement in the bending strength of the finished board. A nice able improvement in the thickness swelling was observed on the blending rice straw pulp with pulps from reeds and wood wastes. Ajiwe et al. [5] were prepared ceiling boards from agricultural wastes, such as rice husks and sawdust, and tested ceiling boards and commercial samples for moisture content, rate of water absorption, and tensile strength. The results confirmed that the boards produced were similar to standards. The effects of particle size and board density on reed and wheat particleboard properties were studied [6]. It was concluded

Materials and Method

Materials

Rice straw fibers collected from farm residue. However, before use it is cleaned and cursed into small pieces by locally made shatter machine. After grinding the fibers were sieved through sieve (No. 14), diameter fibers are range from 1.25 to 0.85 mm while its length ranged from 20 to 8 mm. Polyvinyl alcohol is white to cream colored powder; Sp. Gr. 1.27-1.31, M. wt app. 14,000 and it has been supplied by S. D. Fine lTD. Boisar, India. Polystyrene was a pure grade purchased from Aldrich chemical Co. It is in the form pellets and has an average M.Wt of 280,000.

*Corresponding author: [email protected] 648

Rice Straw Fiber/Polymer Composite

Fibers and Polymers 2011, Vol.12, No.5

Commercial trichloro-ethylene solvent used to dissolve the polystyrene polymer at a concentration of 15 %.

Preparation of Rice Straw Polymer Specimens

The rice straw fibers-reinforced polymer composites have been prepared by thoroughly mixing of the materials, in a variable speed two spindle mechanical mixer. The total weight of the mix was 250 gm, and the percentage of polymers to natural fibers was ranged 5, 10, 15, 20, and 25 % by weight. After mixing the samples were pressed in a steel mould of dimensions 16×16×0.8 cm, using an electric hot press type Carver-M-154. Hot pressing was performed at temperature 130 C for 15 min at 15000 psi then the sample cooled at the same pressure for 5 min. The pressed sample removed from the mould and kept for mechanical and physical testing. EB Irradiation of Composite Samples EB irradiation of composite samples was carried out after prepared at 10, 20, 30, and 40 kGy, respectively, in atmospheric air at ambient temperature using 1.5 MeV, 25 mA beam current, power of 37.5 kW, and variable scan width up to 90 cm. o

Physico-chemical and Mechanical Measurements

Static Bending The static bending properties were determined using mechanical testing machine type HT-9112, Hung-Ta Instruments, Taiwan. The test procedure of static bending was carried out according to the ASTM standards (D1037-11, 1987). Flexural strength (MOR) and flexural modulus (MOE) can be calculated from the following equations: Flexural strength (MOR) = 3PL/2bd (1) Flexural modulus (MOE) = P L /4bd y (2) Where b: width of specimen in cm; d: thickness of specimen in cm; L: length of span in cm; P: maximum load (kg/cm ); P : load at proportional limit (kg/cm ); y: center of deflection at proportional limit load in cm. Impact Toughness Measurements The impact toughness (IT) measurements have been carried out on the unnotched composite test samples. A pendulum impact apparatus PSW-4J (Gerhard Zorn Mechanische Werkestatten, Stendal, Germany), have been used in this test according to ASTM standards (D-256, 1987). The IT was calculated from the following equation: an = An/bh (3) Where an: the IT J/cm ; An: absorbed impact energy in Joule; b: width of the sample in its center in cm; h: height of the sample in its center in cm. 2

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Dimensional Stability Tests The thickness swelling and water absorption tests were conducted in accordance with ASTM D 570. Before testing, the weight and dimensions, i.e., length, width and thickness, of each specimen were measured. Conditioned samples of each type of composite were either soaked in distilled water at room temperature for 24±2 h. Samples were removed from the water, patted dry and then measured again. Each value obtained represented the average of six samples. Water absorption was calculated according to the following equation: WA (%) = 100(M − M )/M (4) Where WA is the water absorption in percentage, and M and M are the sample weights before and after immersion (g). The values of the thickness swelling (TS) in percentage were calculated using the following equation: TS (%) =100(T − T )/T (5) Where T is the initial thickness of the sample and T is the thickness of the wetted sample. Infrared Spectroscopy (IR) The specimens were ground and mixed with highly dried potassium bromide (KBr) 30 mg then pressed into a transparent disc. A FTIR spectrophotometer, Model SP2000, made by Pye Unicam was used over the range 400-4000 cm . SEM micrographs were taken with a JSM-5400 (Jeol/Japan). 2

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Results and Discussion

Effect of Rice Straw Fiber Polymer Ratio

Flexural Strength The variations of the flexural strength values of both rice straw/polyvinyl alcohol (RS-PVA) and rice straw/polystyrene (RS-PS) composites after being pressed at 140 C for 15 min are graphically presented in (Figure 1). The results indicate that, the flexural strength polyvinyl alcohol and polystyrene rice straw composites increases with the increasing of the polyvinyl alcohol and polystyrene polymers percentage in the composites. Studies of composites of sisal/eboxy [10] or Jute/polystyrene [11] showed that there is a linear relationship between the flexural strength and fiber loading and the composite flexural strength always exceeds that of the resin. However, randomly oriented coir fibers in a polyester matrix were shown to exhibit a flexural strength lower than that of the resin at all fiber loading [12]. In a study of oil palm emty fruit bunch fibers randomly oriented in phenolformaldehyde matrix, the flexural strength increased linearly up to a maximum at 38 % fiber loading. In this case, the flexural strength of the resin was only 11 MPa [13], composed with 50 MPa of the polyester resin [14]. Accordingly, with the increasing of the amount of latex polymer in the mix composition, a more of latex polymer diffused inside the hallow cell of the rice straw,this leads to a formation of the chemical bonds between the polymer latex and OH groups o

650 Fibers and Polymers 2011, Vol.12, No.5

Effect of polymer concentration on flexural strength for RS-polymer composite.

M. R. Ismail et al.

Effect of polymer concentration on impact strength for RS-polymer composite.

Figure 1.

Figure 2.

located at the surface of the natural fiber during the hot pressing of the mix composition. This is associated with a significantly enhancement of flexural strength of the final composite. It was also observed that, the flexural strength values of the rice straw polyvinyl alcohol composite is greater than its values of the rice straw polystyrene composite at any rice straw/polymer ratio. It was found that, the presence of (-OH) groups in the PVA structure leads to a more chemical hydrogen bonds between the polyvinyl alcohol and rice straw fiber during the hot pressing of the mix composition. This is accompanied by a strongly adhesion between the components of the rice straw fiber/polyvinyl alcohol as compact to the rice straw/poly styrene composite.

matrix are a major contributor to the observed toughness of the composite. This is facilitated by the surface smoothness and regular cross section of these fibers. For the natural fibers this mechanisam is not favored because of mechanical key ring between the fibers and the matrix [16]. Any enhancement in toughness due to the presence of these natural fibers, must rely upon the nature of the fiber matrix bond or the inherent toughness of the fibers themselves.. The presence of a weak fiber-matrix interface is able to account for a toughness composite that is itself formed from two brittle phases. The opening up of a new surface at the interface results in the absorption energy, diversion of cracks, and so force [17]. The nature of the interface region is thus of extreme importance in determining the toughness of the composite. If the fiber-matrix interfacial strength is too low, then poor stress transfer occurs and a weak composite results. Conversely, a strong interfacial bond allows for very efficient stress transfer but produces a composite exhibiting poor toughness properties [18]. According to the later speaking, with an increasing of the polymer percentage in the mix composition, both toughness and interfacial strength are significally improved, as a results of the formation of chemical bonds between the rice straw surface and the polymer latex during the hot pressing of the mix composition. These are associated with significally enhancement of the impact strength of the composite. The use of polyvinyl alcohol polymer showed a higher values of the impact strength of the rice straw polyvinyl alcohol polymer as compared to the rice straw/polystyrene polymer. The explanation in this case may be related to the presence of the hydroxyl groups in polyvinyl alcohol polymer

Impact Strength

The infulence of rice straw/polymer ratios on the impact strength values of rice straw fiber /polyvinyl alcohol and rice straw fiber/polystyrene composites after being pressed at pressing temperature 140 oC for 15 min is shown in (Figure 2). The experimental results indicated that, the impact strength values of the two composites increases with increasing the polymer percentage in the mix composition. It was also observed that for a given fiber/polymer ratio, the impact strength values of the rice straw/polyvinyl alcohol composite are higher than that its values of rice straw/polystyrene composite. It was reported that, the impact strength of the composite is affected by many factors including the toughness properties of the reinforcement, the nature of the interfacial region, and the frictional work involved in the pulling the fibers of the matrix [15]. In case of glass fiber reinforced composites, frictional losses as the fiber is pulled out of the



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structure these are leading to a formation of hydrogen chemical bonds between the polyvinyl alcohol and the rice straw fiber’s surface, and consequently, an enhancement of the toughness and interfacial strength of rice straw/polyvinyl alcohol composite. Modulus of Elasticity The effect of rice straw/polymer percentage on the elasticity modulus of the rice straw fiber/polyvinyl alcohol and rice straw fiber polystyrene composites prepared under the same conditions mentioned above is graphically presented in (Figure 3). The results, show that the modulus of elasticity of these composites are sharply increased up to 15 % of polymer, beyond, these percentage, modulus of elasticity of these composites slightly increase with the increase of the polymer content. Also, for a given rice straw fiber/polymer ratio, the modulus of elasticity of rice straw fiber/polymer ratio, the modulus of elasticity of rice straw fiber/polyvinyl alcohol composite is higher as compared to its values of rice straw fiber/polystyrene composite. The poor dispersion of the fibers and reduced interfacial adhesion to the polymer matrix may explain this reduction since; the effective transfer of stress between matrix and fiber requires an adequate interfacial bending [19,20]. In continuous fiber reinforced plastics it is the fiber that determines the basic property of the composite. The function of the resin is allowing the fiber to develop its full potential by transferring the stress from one fiber to another. The stress is transferred from the matrix to the fibers through the interfacial shear stress. This stress is concentrated at the fiber ends. With the increase in strain, these are the sites where the interface first fails and depending upon the fibers and matrix (shear failure of the matrix) begins [21].

The magnitude of this interfacial shear strength is determined by three factors: (a) the strength of the chemical bond between the fibers and matrix. (b) The function between the fibers and by the matrix; and (c) the shear strength of the matrix [22]. Accordingly, the increase of both polyvinyl alcohol and polystyrene in the mix composition leads to improve of the chemical bonds between the natural fiber and polymer; these is accompanied with the enhancement of the adhesion between the components and consequently reduce a numerous irregularly shaped minor voids or minor flaws in the composite structure. The presence of these minor flaws cause the transfer of stress from the matrix to fiber is stronger and the moduli of elasticity of both composites are improved. A strongly adhesion between the rice straw fiber and polyvinyl alcohol, resulting from the formation of chemical bonds via hydrogen bond formation, because these bonds formation, the modulus of elasticity of rice straw fiber/polyvinyl alcohol is higher as compared to its values of rice straw fiber/polystyrene composite. Water Absorption Percentage Figure 4 illustrates the influence of polymer contents on the water absorption percentages of both rice straw/polyvinyl alcohol and rice straw/polystyrene composites after using pressed at pressing temperature 140 oC and pressure 15000 psi. The experimental results indicate that, the water absorption percentage of rice straw/polyvinyl alcohol increase with increasing the polyvinyl alcohol content in the mix composite, while it behaves in opposite direction for rice straw fiber/ polystyrene composite. It was also observed that, the difference between water absorption percentages of rice straw/polyvinyl alcohol and rice straw-polystyrene were higher at lower

Effect of polymer concentration on MOE for RSpolymer composite.

Figure 4.

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Effect of polymer concentration on water absorption for RS-polymer composite.


polymer content, but at higher polymer contents, the difference between them were smaller. The amount of water absorbed by the composite is essentially dependent upon the rice straw fiber ratio in the mix composition. Since, the increase of polystyrene contents in the mix composition leads to a reduction in the water absorption percentages due to hydrophobic property of the polystyrene, as well as the decrease of rice straw natural fiber in the composite. While the enhancement of the water absorption percentages of rice straw/polyvinyl alcohol composite is mainly related to the hydrophilic characteristic of polyvinyl alcohol polymer, which is associated with a high water absorbed. The water absorption percentages of rice straw/polystyrene composite at 10 and 20 % polystyrene contents are 80.5 and 72.3 % respectively, while its percentages for the rice straw/ polyvinyl alcohol at the same polymer contents are 46.5 and 58.6 % respectively. These are also attributed to the reasons mentioned above. Thickness Swelling Percentage

The expermintal results of the variations of thickness swelling percentages of the rice straw fiber/polystyrene and rice straw fiber polyvinyl alcohol composites are graphically shown in (Figure 5). It was found that the thickness swelling percentages of rice straw fiber polystyrene composite decrease with increasing the polystyrene content in the mix composition, but, its percentages for rice straw fiber polyvinyl alcohol composite are increased with the increase of polyvinyl alcohol contents. Previous research showed that injection of steam into a resinless flackboard mat during pressing can substantially reduce the thickness swelling that occur when the mat is supmerged in water [23]. This study showed that a

Effect of polymer concentration on thickness swelling for RS-polymer composite. Figure 5.

M. R. Ismail et al.

number of factors including wood plasticization, “ligin flow” and chemical changes, may be responsible for the improvement in dimensional stability. Flakeboard mats bonded with 3 % isocyante were steam injection pressed the same relative improvements in thickness swelling observed for resinless mats were gleamed in the resin-bonded boards. Thickness swelling in boards submerged under water at atmospheric pressure for 24 h. was influenced by the vertical density gradient and/or board permeability [24]. The reduction of thickness swelling in rice straw fiber/polystyrene composites may be attributed to strong adhesion between the composite components and the hydrophobic property of polystyrene polymer composite, there are leading to reduce the permeability of water inside the rice straw fiber polystyrene composite. On contrary, the increase of the rice straw fiber polyvinyl alcohol, which is accompained by excess water absorped and caused a minocraks inside the composite. These are leading to a high swell of the rice straw fiber/polyvinyl alcohol composite. The higher difference in the thickness swelling of the two composites at higher polymer contents is mainly attributed to hydrophobic and hydrophilic characteristics of the two composites.

Effect of Electron Beam Irradiation Dose

The influence of electron beam irradiation dose on the flexural strengths of rice straw fiber/polymer composites is graphically presented in (Figure 6). The results show that, the flexural strength of composite increases with increasing electron beam radiation dose. Electron beam irradiation in the presence of a monomer is able to create radicals in both fibers and monomers, leading to the grafting of monomers on the cellulose fiber surface with same extent. Due to the

Effect of EB irradiation dose on flexural strength RSpolymer composite. Figure 6.



great number of reactions that take place during electron irradiation, a complex chemical structure is obtained on the fiber surface [25]. Czvikovszky [26] has been reported that, electron beam treatment of wood fiber/polypropylene created an active sites on both matrix polymer and fibers reinforcements, wood fiber and polypropylene bound together through reactive additives results in a composite which has not only a high modulus of elasticity, but also significantly higher flexural and tensile strengths and improved thermal tolerance over the conventional wood fiber polypropylene bends, and over the polypropylene itself. Similarly, the electron beam treatment creates reactive centers on the main components of the wood in different ratios [27]. The electron beam irradiation treatment of rice straw-polystyrene and rice straw-polyvinyl alcohol composites is a highly creating chemically reactive species. They are highely adhesive together, leading to improve in the flexural strength of the composite. With increasing the absorbed dose of electron beam irradiation, a more reactive species of the composite is formed and consequently, the chance of the chemical bonding formation are highly improved as well as the flexural strength of the rice straw polymer composite is also increased. The effect of electron beam irradiation dose on the impact strength of rice straw polymer composites is shown in (Figure 7). The results indicated that, strongly adhesion between composites constituents arise from the increased irradiation dose as results, to randomly initiated chain reactions started by the reactive centers created by the electron beam. These are leading to a strong chemical bonds between the components of the composites and consequently the impact strength of rice straw fiber/polymer composite improved. It was also observed that, the impact strength of rice straw polyvinyl alcohol composite values are higher

Effect of EB irradiation dose on impact strength RSpolymer composite. Figure 7.

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than those of rice straw polystyrene composite at the same irradiation dose.This is attributed to the presence of hydroxyl groups in polyvinyl alcohol which are joined to surface fiber to form hydrogen chemical bonds. The modulus of elasticity of the irradiated composites are also increased with increasing electron beam irradiation dose (Figure 8). This accompanied by improvements in the modulus of elasticity of the irradiated composites as compared to the unirradiated composites. The retardation of the diffusion of water into the composite is essentially to a higher rate of chemical rections between

Effect of EB irradiation dose on modulus of elasticity of RS-polymer composite. Figure 8.

Effect of EB irradiation dose on water absorption of RSpolymer composite. Figure 9.


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Figure 11(a, b).

Scanning electron microscopy for rice straw fiber.

Effect of EB irradiation dose on thickness swelling of RS-polymer composite. Figure 10.

the composite constituents during electron beam irradiation treatment. These are leading to a reduction of water absorption percentage of the irradiated composite (Figure 9). It was found that, the thickness swelling of rice straw fiber/ polyvinyl alcohol composites are reduced from 36.4 to 30 % of the rice straw fiber polystyrene composites are reduced from 41.8 to 16.0 %, while it is reduced from 36.4 to 30 % of the rice straw fiber polystyrene composites when both composites exposed to 10 KGy of electron beam irradiation (Figure 10). These are due to the presence of polystyrene in the rice straw fiber polystyrene composite which required a high electron beam irradiation dose to create reactive species. This leads to a porous structure of the rice straw fiber polystyrene composite and the diffusion of water into the composite in improved and the reduction of the thickness swelling of the rice straw fiber polystyrene composite are smaller as compared to its values of rice straw fiber polyvinyl alcohol composites. Scanning Electron Microscopy

Examination of the fracture surfaces of rice straw fiberpolymer composites by scanning electron microscope gave information about how polymer and electron beam irradiation dose affect the morphology of the composite. Figure 11(a), (b) shows scanning electron parameters, evaluated on the basis of structural contributions as well as similarity and analogy rules [28]. Microscopy of the rice straw fiber, considerable fiber pull-out is noticed together with fiber tearing. Some mall particles appeasing the surface of the fiber small amounts of voids are also appeared on the fiber surface.

SEM micrographs for (a) unirradiated RS-PVA composite and (b) irradiated RS-PVA composite to 40 kGy. Figure

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The morphology features of RS-PVA composites before and after irradiated are shown in Figure 12(a), (b). This micrograph is indicated that the rice straw fiber coated with polyvinyl alcohol and good adhesion between them. A small gas between the rice straw fiber and the polymer strands (Figure 12(a)). Figure 12(b) shows the morphology of irradiated rice straw fiber-polyvinyl alcohol composite. These is no significant difference of the micrographs of irradiated composite (Figure 12(b)) and unirradiated composite (Figure 12(a)), a small reduction of the voids present between the rice straw fiber and polyvinyl alcohol polymer which is leading to improve the mechanical properties of the irradiated RS-PVA composite as compared to unirradiated RS-PVA composite. The microstructure of the RS-PS composite before and after irradiated is shown in Figure 13(a), (b). Figure 13(a) shows good adhesion between the polystyrene matrix and the wood fibers surface, as these are voids around the rice straw fiber. The interfacial adhesion between rice straw fibers and polystyrene matrix is improved after electron beam irradiation (Figure 13(b)). The polystyrene polymer is covered the surface of rice straw fiber and a small amounts of voids also appeared. These confirmed the improvement of mechanical properties of irradiated rice straw fiberpolymer composites as compared to unirradiated composites.



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SEM micrographs for (a) unirradiated RS-PS composite and (b) irradiated RS-PS composite at 40 kGy. Figure 13.

Infrared Spectroscopic Analysis

FTIR spectrum for (a) rice straw, (b) RS-PS composite, and (c) irradiated RS-PS composite. Figure 15.

The IR spectra of rice straw fiber-polyvinyl alcohol polymer (RS-PVA) and irradiated rice straw fiber-polyvinyl alcohol polymer composite at 40 KGy are illustrated in Figure 14(a), (b), and (c) respectively. It can be clearly seen in Figure 14(a) that; IR spectra of rice straw fiber are characterized by a strong broad absorption OH band at about 3400 cm . The CH asymmetric vibration appears at 2940 cm ; the absorption band at 2350 cm is assigned to C ≡ N vibration. The absorption band appearing around 1650 cm is probably due to the C=C quinoid constitution [29]. The appearance of a broad absorption band around 1100 cm is assigned to the ether linkage C-O-C from lignin or cellulose or hemicellulose [30], or due to the stretching of the Si-Ocellulose and Si-O-Si bonds respectively [31-35]. The weak bands between 1400-1320 cm are probably due to stretching vibration of phenolic O-H groups. To obtain clear information about the molecular structure changes, the IR spectra of RS fiber will be compared with unirradiated and irradiated rice straw fiber polyvinyl alcohol polymer around 3400 cm , as result the stretching vibration of O-H groups was disappeared and replaced by a board absorption band after mixed with rice straw fiber. This band has a lower intense as compared -1

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to its intensity in IR spectra of rice straw fiber. Also, the two sharp bands around 2940 and 1450 cm of the C-H assynment stretching or C-H assynment bending were replaced by a broad band at 2940 cm with lower intense as compared to its intensity of the IR spectra of rice straw fiber (Figure 14(a)). A broad absorption band at 1100 cm of the rice straw polyvinyl alcohol composite also replaced the O-H stretching vibration of polyvinyl alcohol at 1370 and 1100 cm . This peak has a low intense as compared to its intensity of the IR spectra of rice straw fiber. These observations confirmed the obtained results of physico-mechanical properties and the reactions mechanisms discussed before, i.e. the intensity reduction of the main peaks of rice straw fiber and disappearance of the main peaks of polyvinyl alcohol in the rice straw fiber/polyvinyl alcohol composite are essentially related to the strong adhesion between the composite constituents. It was also observed that very little reducing of the main peaks of the composite after electron beam irradiation, as shown in (Figure 14(c)), this is mainly attributed to the effect of electron beam irradiation on the composite as discussed above. IR spectra of rice straw fiber polystyrene before and after electron beam irradiated are shown in Figure 15(b), (c). As seen in this figure the main peaks of polystyrene and rice straw fiber are over laid on each other (Figure 15(b)). This can be explained by the strong adhesion between the rice straw fiber and polystyrene polymer during the hot pressing of the mix composition, that can originate from the cross linking in ether linkage form and hydrogen bonding as discussed in the above. The development of the composite structure can be recognized with the evidences such as the appearance of the weak bands around 1450 cm , this may be attributed to the aromatic C=C skeletal vibration shown in (Figure 15(b), (c)). A slightly reduces in the peaks intensities of the irradiated rice straw polystyrene composite as compared to the unirradiated composite. -1

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FTIR spectrum for (a) rice straw, (b) RS-PVA composite, and (c) irradiated RS-PVA composite. Figure

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Conclusion

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The flexural strength and impact strength of polyvinyl alcohol and polystyrene rice straw composites increase with increasing the polyvinyl alcohol and polystyrene polymers percentage in the composites. Also, at a given polymer percentage, these properties of rice straw fiber polyvinyl alcohol had a high value than that those of rice straw fiber/ polystyrene composite. The modulus of elasticity of these composites are sharply increased up to 15 % of polymer, beyond, these percentage, modulus of elasticity of these composites slightly increase with the increase of the polymer content. Also, for a given rice straw fiber/polymer ratio, the modulus of elasticity of rice straw fiber/polymer ratio, the modulus of elasticity of rice straw fiber/polyvinyl alcohol composite is higher as compared to its values of rice straw fiber/polystyrene composite. The water absorption and thickness swelling percentages of rice straw/polyvinyl alcohol increase with increasing the poly vinyl alcohol content in the mix composite, while it behaves in opposite direction for rice straw fiber/polystyrene composite. It was observed that physico-mechanical properties of both composites improved with electron beam irradiation treatment.

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