Polymer-Plastics Technology and Engineering, 53: 1–8, 2014 Copyright # Taylor & Francis Group, LLC ISSN: 0360-2559 print=1525-6111 online DOI: 10.1080/03602559.2013.832854
Thermoplastic Hybrid Composites using Bagasse, Corn Stalk and E-glass Fibers: Fabrication and Characterization Alireza Ashori1, Amir Nourbakhsh2, and Ali Kazemi Tabrizi3 1
Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran 2 Department of Wood and Paper Science, Research Institute of Forests and Rangelands (RIFR), Tehran, Iran 3 Department of Wood and Paper Science and Technology, Karaj Branch, Islamic Azad University, Karaj, Iran
systems are those in which one kind of reinforcing material is incorporated in a mixture of different matrices[4–6], or two or more reinforcing and ﬁlling materials are present in a single matrix, or both approaches are combined. The incorporation of two or more cellulosic ﬁbers into a single matrix has led to the development of hybrid composites. The behavior of hybrid composites is a weighed sum of the individual components, in which there is more favorable balance between the inherent advantages and disadvantages. While using a hybrid composite that contains two or more types of ﬁber, the advantages of one type of ﬁber could complement what is lacking in the other. As a consequence, a balance in cost and performance could be achieved through proper material design. Hybrid composites have been developed by various researchers, combining ﬁbers with epoxy, PP, phenolic, polyvinyl ester, and polyurethane resins[10,11]. Owing to the increased environmental awareness, the usages of natural ﬁbers as potential replacement for synthetic ﬁbers in composite materials have gained interest among researchers throughout the world. Although a direct substitution of synthetic ﬁbers by cellulosic ﬁbers is not easily achieved, they offer several advantages as they are inexpensive, recyclable, available in abundance, have low density, and also possess low strength to weight ratio[12,13]. Moreover, they display a good set of mechanical properties, provide better working conditions and are much less abrasive than the common synthetic ﬁbers. All these aspects make their use very attractive to the manufacture of polymer matrix composites, making it an interesting product for countries with low wage. Based on our literature search, among the possible alternatives, the development of composites using agro-waste materials (including stalks of most cereal crops, rice husks, coconut ﬁbers, bagasse, maize cobs, peanut shells, and other wastes) is currently at the center of attention [4,16–18].
This article presents the application of agro-waste materials (i.e., bagasse and corn stalk ﬁbers) along with E-glass ﬁber in order to evaluate and compare their suitability as reinforcement for thermoplastic composites. The hybrid effect of glass and cellulosic ﬁbers on the tensile, ﬂexural, and impact properties of the composites were investigated. Water absorption and thickness swelling were also studied. In general, the hybrid composites presented superior properties compared to the control (without glass ﬁber) samples. Additionally, synergistic improvements in the physico-mechanical properties of composites were obtained with the addition of glass ﬁber. Overall trend shows that with the addition of agro-waste materials, tensile and ﬂexural properties of the composites were moderately enhanced. However, corn stalk ﬁbers showed superior mechanical properties due to their high aspect ratio and chemical characteristics. Addition of glass ﬁbers into the composites considerably enhanced tensile, ﬂexural, and impact properties without having signiﬁcant effect on the elongation at break. Morphological study also conﬁrmed the impact behavior of composites. Moreover, incorporation of glass ﬁber with agro-waste ﬁbers in PP matrix considerably decreased the water uptake and thickness swelling of the hybrid composites. Keywords Agro-waste materials; E-glass ﬁber; Fiber loading; Hybrid composite; Mechanical properties
INTRODUCTION In the last few decades, research interest has been shifting from polymeric composites to hybrid composite materials, as conventional composites often have limitations on increasing performance requirements in various environments. This new generation of composite materials now dominates the aerospace, automotive, construction, and sporting industries[1–4]. The word ‘‘hybrid’’ is of Greek– Latin origin and can be found in numerous scientiﬁc ﬁelds. In the case of polymeric composites, hybrid composites Address correspondence to A. Ashori, Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815-538, 1581944734 Tehran, Iran. E-mail: [email protected]
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However, natural (i.e., agro-waste) ﬁbers cannot replace synthetic ﬁbers in all their vast range of applications. Natural ﬁber composites suffer from low modulus, low strength, and poor moisture resistance compared to synthetic ﬁber composites such as glass ﬁber composites. Therefore, natural ﬁber composites are mainly restricted to upholstery applications rather than structural engineering applications. In order to overcome the weaknesses of natural ﬁber composites, agro-waste ﬁbers and glass ﬁber can be combined in the same matrix to produce hybrid composites that take full advantage of the best properties of the constituents, and thereby, an optimal, superior and economical composite can be obtained. Current research has established that the mechanical properties of the bagasse=glass and corn stalk=glass hybrid polypropylene (PP) composites such as tensile strength, tensile modulus, elongation at break, ﬂexural strength, ﬂexural modulus, and impact properties of hybrid composites are function of ﬁber type and ﬁber loading. The water absorption and thickness swelling behaviors of the hybrid composites were also studied. EXPERIMENTAL Materials Two types of agricultural residuals were investigated in this study: corn stalk ﬁber (CSF) and bagasse ﬁber (BF). The important chemical components and ﬁber morphology of cellulosic materials are given in Table 1. The ﬁbers were produced by reﬁner mechanical pulping process. Before the preparation of composites, all ﬁbers were oven-dried at 95 C for 24 h. Injection molding grade PP, with trade name V30S, was supplied by Arak Petrochemical Co. (Iran). The PP was in the form of pellets with a melt ﬂow index of 18 g=10 min and density of 0.92 g=cm3. Maleic anhydride functionalized polypropylene (MAPP) with a molecular weight of 52,000, acid number of 9 mg TABLE 1 Chemical constituents and morphological characteristics of the used materials Chemical components Cellulose (%) Hemicellulose (%) Lignin (%) Extractives (%)b Ash (%) Fiber morphology Length (mm) Aspect ratio (L=D) a
Corn stalk 46.5 32.5 14.2 3.6 1.2
(1.2) (1.1) (0.8) (0.4) (0.5)
1.01 (0.07) 64.1 (2.2)
Bagasse 55.3 18.8 21.0 2.9 1.9
0.96 (0.14) 42.2 (1.5)
Numbers in the parenthesis are standard deviations. Hot-water.
(.09) (1.8) (0.7) (0.4) (0.3)
KOH=g, and melting point of 158 C was provided by Eastman Chemical Products, Inc. The chopped strand E-glass ﬁbers (EGF) as reinforcement used in this work were supplied by Diba Co., Ltd (Shiraz, Iran). The ﬁber length was 8–10 mm, diameter 5–10 mm, and density 2.5 g=cm3. Sample Preparation Formulation of the mixes and abbreviation used for the respective mixes prepared are given in Table 2. Composites were produced in a two-stage process. In the ﬁrst stage, ﬁbers, MAPP, and PP pellets were premixed mechanically at various formulations, and the mixtures were then fed into a laboratory co-rotating twin screw extruder. The temperature proﬁle in the extruder was 165=170=175=175=180 C and the screw speed was set at 70 rpm. In the second stage, the extrudate in the form of strands were allowed to cool down to room temperature and then granulated using a CW Brabender Granulator. The resulting granules were dried at 105 C for 24 h before being injection-molded. Mechanical Property Tensile test was performed according to ASTM D638 using the Instron Universal Testing Machine (model 1186, USA). The standardized dumbbells of 150 10 4 mm were used. The gauge length was set at 60 mm with a crosshead speed of 5 mm=min. Tensile strength, tensile modulus, TABLE 2 Material formulations used to prepare the hybrid composites Code no.
BF1 BF2 BF3 BF4 BF5 BF6 BF7 BF8 BF9 CSF1 CSF2 CSF3 CSF4 CSF5 CSF6 CSF7 CSF8 CSF9
63 53 48 53 45 38 43 33 28 63 53 48 53 43 38 43 33 28
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
35 35 35 45 45 45 55 55 55 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 35 35 35 45 45 45 55 55 55
0 10 15 0 10 15 0 10 15 0 10 15 0 10 15 0 10 15
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and elongation at break were recorded and calculated by the instrument’s software. A ﬂexural test was performed according to ASTM D790 using the same machine. A crosshead speed of 8 mm=min was used. The ﬂexural strength and modulus value were recorded and calculated by the instrument’s software. The Izod notched impact test was carried out on samples with dimensions of 70 15 10 mm, using the Impact Pendulum Tester (Zwick model 1446, UK). The conventional V notched specimens were according to ASTM D256. The samples were rigidly mounted on a vertical position and were stricken using a pendulum with a force of 1 J at the center of the samples. All measurements were performed for ﬁve replicates of specimens and averaged to obtain the ﬁnal result. Physical Testing The water absorption and thickness swelling tests were carried out according to ASTM D570. Before testing, the weight and thickness of each composite sample were measured. Five specimens of each type of composite (treatment) were submerged in distilled water at 25 1 C. Samples were removed after 24 h, blotted to remove the excess water on the surface and immediately weighed=measured. Fracture Sample Analysis Studies on the morphology of the composites were carried out using a scanning electron microscope (SEM). SEM micrographs of the surfaces of impact fractured specimens were taken using Philips SEM model XL 30 (USA). Specimens were sputter coated with gold to a thickness of 20 nm in order to prevent electrical charging during the examination. An accelerating voltage of 20 kV was used to collect the SEM images. RESULTS AND DISCUSSION As can be seen from Figure 1, the three investigated ﬁbers are clearly distinguishable by differences in their compositions, and a different mechanical behavior can therefore be expected. PP was ﬁlled with various mixtures of agro-waste materials, E-glass and MAPP to produce hybrid composites. Mechanical and physical properties of the produced polymer composites are shown in Figures 1 to 3. The tensile and ﬂexural moduli of pure PP samples using the same conditions as the composites were 1252 MPa and 1130 MPa, respectively. Tensile Properties Figure 1a shows the variation of tensile properties of BF=glass and CSF=glass hybrid composites. The cellulosic ﬁbers loading of the composites are varied from 35 to 55 wt.% (Table 2). Generally, an increase in the tensile properties of the hybrid composites was observed with an increase in the content of cellulosic ﬁbers loading in
composites, indicating a considerable reinforcing effect from these ﬁbers. This is in accordance with the results reported by Karmarkar et al., who studied the properties of wood plastic composites. Their data show that the tensile strength of wood ﬁber=PP composites increases with increasing ﬁber content. The possible reasons proposed for this kind of behavior may be the improved interfacial adhesion between the matrix and ﬁbers. In addition, various parameters inﬂuence the mechanical properties of ﬁber-reinforced composites including the ﬁber aspect ratio, ﬁber-matrix adhesion, stress transfer at the interface and mixing temperatures. One of the most important parameters controlling the mechanical properties of short ﬁbers composite is the ﬁber length or more precisely its aspect ratio (ﬁber length= width). The corn stalk ﬁber-ﬁlled composites (with or without glass ﬁber) show superior strength compared to the bagasse ﬁber ﬁlled composites. The high aspect ratio of the corn stalk ﬁbers permits better stress transfer between the matrix and the ﬁbers. As can be seen from Table 1, corn stalk has higher ﬁber length and aspect ratio compared to the bagasse ﬁber. Another possible reason could be the incompatibility in the interfacial region between the bagasse ﬁber and PP matrix. This phenomenon is due to the highly hydrophilic nature of bagasse ﬁber and the even more hydrophobic PP matrix. The hydrophilicity of cellulosic ﬁbers arises from the hydroxyl group of lignin and cellulose[20,21]. Bagasse ﬁber has high cellulose content (55.3%) and lignin content (21%), but corn stalk ﬁber has lower cellulose and lignin content (Table 1). Thus, bagasse ﬁber is more hydrophilic due to its polarity caused by the free hydroxyl groups from the cellulose and lignin structures. This may lead to poor bonding quality between bagasse ﬁber and the matrix compared to corn stalk ﬁber. As can be seen from Figure 1a, the hybrid composite, made using 55 wt.% corn stalk ﬁber and 10 wt.% glass ﬁber, has the highest tensile strength and modulus values among the composites evaluated in this investigation. Similar behavior is also observed when the glass ﬁber content is increased in hybrid composites. The ﬁgure clearly shows that the tensile strength and modulus of hybrid composites are signiﬁcantly improved by increasing the glass ﬁber content. Composites having 10 wt.% of glass ﬁber exhibit the highest tensile properties among all the hybrid composites. In this case, the increase in the strength of the hybrid composites is mainly due to the high tensile strength (1.7–3.5 GPa) of glass ﬁbers. According to Sreekala et al., the properties of hybrid composites are mainly dependent on the properties of individual reinforcing ﬁbers, orientation, and arrangement of ﬁbers, the extent of intermingling of the ﬁbers and also the ﬁber-matrix adhesion. At 15 wt.% glass ﬁber loading, the increased population of long ﬁbers leads to agglomeration. The incompatibility in
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Comparison of mechanical properties of composites as function of the ﬁber type and ﬁber loading.
the interfacial region between the glass ﬁber and PP matrix also increased. Flexural Properties Generally, the ﬂexural properties slightly increased with incorporation of glass and agro-waste ﬁbers. However, the ﬂexural modulus of the composites is more dependent on the amount of glass rather than agro-waste ﬁbers, which could be due to the high modulus of glass ﬁber (66– 72 GPa). From the graphs of Figure 1b, it is observed that the ﬂexural properties increase as the glass ﬁber loading increases in hybrid composites. The highest ﬂexural strength and modulus is observed in hybrid composites with 45 wt.% weight fraction of ﬁber at 15 wt.% glass ﬁber loading. In ﬂexural loading, the composite samples are subjected to compression, tension and shear stresses. In a three-point ﬂexure test, failure occurs due to bending and shearing. The increased ﬂexural strength of the hybrid composites
with the loading of glass ﬁber is mainly due to the increased resistance to shearing of the composites as a result of the inclusion of rigid glass ﬁber. In addition, the high strength glass ﬁber layers are able to bear the applied compressive and tensile stresses subjected on the hybrid composites. This results in an increase in the ﬂexural strength of the hybrid composites. As expected, an increase in the cellulosic ﬁber loading up to 10 wt.% produces a corresponding increase in the ﬂexural strength and modulus values of the hybrid composites. The decrease in the ﬂexural properties at higher ﬁber loadings is due to the increased ﬁber-to-ﬁber interactions and dispersion problem, which results in low mechanical properties of composites. For the BF=glass PP composites, both ﬂexural strength and modulus values are found to have increased after further additions of up to 35 wt.%. The ﬂexural properties show good enhancement with increasing ﬁber loading due to the fact that the composites were
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able to withstand more loads when the population of the ﬁbers in composites increased. The ﬂexural modulus of corn stalk ﬁber composites increases with increase in ﬁber content. However, the bagasse ﬁber composites show less ﬂexural modulus compared to the corn stalk ﬁber composites. It is also noteworthy that the ﬂexural modulus of composites is extremely greater than the pure PP (1.15 GPa). Elongation at Break Figure 1c shows the variation of elongation at break with both bagasse and corn stalk ﬁbers loading. The values of elongation at break show a reduction with an increase of agro-waste ﬁber content in hybrid composites. With an increase in glass ﬁber over cellulosic ﬁber content, the hybrid composites show more reduction in elongation at break of composites. Among the hybrid composites, the incorporation of 15 wt.% glass ﬁber exhibits the lowest elongation at break of composites, whereas composites without glass ﬁber are found to have the highest values of these properties. This phenomenon was due to the fact that glass ﬁber is a low elongation ﬁber compared to the agro-waste ﬁbers. Thus, bagasse and corn stalk ﬁbers have high strain to failure characteristic compared to the low extensibility of glass ﬁber. Glass ﬁber, which has low elongation, fails ﬁrst whilst the agro-waste ﬁbers are able to withstand the applied stress. Impact Strength The effects of fracture on BF=glass and CSF=glass PP hybrid composites at various ﬁbers loading are shown in Figure 1c. In both agro-waste ﬁbers, impact strength was found to have decreased with an increase in the ﬁber content of composites up to 35 wt.%. Among the composites, the ones with 35% weight fraction of ﬁber loading show the highest impact properties. This value is contributed by the ﬁbers that are present in sufﬁcient amount that can provide the effective stress transfer between the ﬁber and matrix. Excellent dispersion of the ﬁbers also occurs at this composition. However, composites having ﬁber loading of over 35% weight fraction exhibit considerable reduction in impact properties among the other composites. At higher loadings (>35 wt.%), ﬁber to ﬁber contact increases in the composites. The inter ﬁber interaction decreases the effective stress transfer between the ﬁber and matrix. This contributes to the reductions in impact properties at higher ﬁber loading. Impact strength is deﬁned as the ability of the material to resist fracture under stress applied at high speed. The impact properties of composite materials are directly related to its overall toughness, which is highly inﬂuenced by the interfacial bond strength, the matrix and also ﬁber properties. In the case of effect of ﬁber type on the impact strength, corn stalk ﬁber has a superior effect
compared to the bagasse ﬁber. This phenomenon is due to corn stalk ﬁber’s low strength nature, irregular cross– section and the presence of ﬁber bundles. The bagasse ﬁber is unable to withstand the high load, which leads to fractures occurring before reaching its fracture strain level. As depicted in Figure 1c, it was found that impact strength increases with the addition of glass ﬁber in BF=PP and CSF=PP hybrid composites. Adding a small amount (15 wt.%) of glass ﬁber in CSF=PP composite resulted in about 20% increase in the impact performance of the hybrid composites. Abdul Khalil et al. also reported a similar trend. This result is due to the high energy absorption capability of the glass ﬁber. Morphological Study Figure 2 illustrates the SEM photomicrographs of the impact fractured surface of hybrid composites. It clearly shows the uniform distribution of ﬁbers in PP matrix (Figure 2a). In ﬁber-reinforced composites, ﬁbers play important role in the impact resistance of the composites as they interact with the crack formation in the matrix, acting as a stress transferring medium. The high bonding quality between the glass ﬁber and matrix creates a good interfacial region. This phenomenon results in an improvement in the ability of the composite system to absorb energy during fracture propagation, which enhances the impact resistance of hybrid composites. Figure 2b reveals the glass ﬁber and PP interface, showing a well-distributed dispersion and embedment of glass ﬁber in PP matrix, which suggests a good interface between the two. When the hybrid composites were impacted, the glass ﬁbers were able to resist the high impact load and absorb a signiﬁcant amount of impact energy through debonding of glass ﬁber as shown in Figure 2b. Thus, energy needed to initiate and propagate the crack increases. Moreover, the delamination at the glass ﬁber–bagasse ﬁber layer interface further contributes to the additional impact energy dissipated to the overall laminate through the ﬁber breakage in the bagasse layer. It is well known that the ﬁber pull out absorbs the impact energy far greater than the ﬁber breakdown. Fiber fracture is of an elasticplastic nature in hybrid composites. From the SEM images it was observed that the bulk of failure occurred due the ﬁber pull out from the matrix (Figs. 2c–2f). The interfacial interactions between the bagasse ﬁber and matrix were not strong enough to resist the ﬁber pull out during impact and as a result the energy consumed was small in magnitude. A poor interface between the wood ﬁber and matrix is visible in Figure 2e at larger magniﬁcation. A poor interface between bagasse ﬁber and PP was due to the existence of glass ﬁbers within the two surfaces, which did not allow much of the interaction among the two and made the ﬁber slippage easy during the pull out. This is visible in the image, in
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FIG. 2. SEM micrograph of impact fracture surface of composite types: (a) CSF2, (b) BF2, (c) CSF5, (d) BF5, (d) CSF8, (e) BF8.
Figure 2e, which was obtained from the pulled out ﬁber of the interface. Water Absorption Behavior Figure 3 depicts the extent of water absorption by composites with various agro-waste and glass ﬁbers loadings. It can be seen that the water absorption of composites increases with an increase in ﬁber loading. The composites ﬁlled with bagasse ﬁber show the highest values of water uptake. As the relative volume fraction of bagasse ﬁber is increased, the water absorption linearly increases. In this case, the increased number of hydroxyl groups is more pronounced in cellulose structures due to the high cellulose content (55.3%) compared to the corn stalk ﬁber (Table 1). These hydroxyl groups can hold the water molecules, via hydrogen bonding, within the ﬁber cell wall. As the loading of bagasse and corn stalk ﬁbers decreases, the amount of hydroxyl groups in the composites reduces,
resulting in low water uptake to the composites. Furthermore, the increased absorption of the hybrid composites at high volume fraction of agro-waste ﬁbers can be attributed to the poor compatibility between the ﬁbers and glass ﬁber and also between the used ﬁbers and the PP matrix. At higher volume fractions of natural ﬁber, the microlevel processing of the composites becomes difﬁcult and may lead to the ﬁber layering out, which creates microvoids and cracks within the composites. Thus, the water can easily penetrate and diffuse through the porous structure of the composites. This mechanism involves the ﬂow of water molecules along with the ﬁber–matrix interface, followed by diffusion from the interface into the matrix and ﬁbers. In addition, the water absorption of composites increases with an increase in soaking time. Figure 3 also shows that the increasing amounts of glass ﬁber in hybrid composites decreased the water absorption percentage of the composites. The water absorption
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Comparison of physical properties of composites as function of the ﬁber type and ﬁber loading.
properties of hybrid composites were found to be much less than that of the BF=PP and CSF=PP composites. By adding a small amount of glass ﬁber in hybrid composites, the water uptakes of the hybrid composites decrease from 30% to 60%. The addition of 15% weight fraction of glass resulted in hybrid composites having good resistance to water absorption. Thickness Swelling As mentioned earlier, the poor absorption resistance of the cellulosic ﬁbers is mainly due to the presence of polar groups such as hydroxyl and oxygen groups, which attract water molecules through hydrogen bonding. This phenomenon leads to a moisture buildup in the ﬁber cell wall (ﬁber swelling) and also in the ﬁber–matrix interface. This is responsible for the changes in the dimension of cellulosebased composites, particularly in the thickness and the linear expansion due to reversible and irreversible swelling of the composites. Figure 3 clearly shows that the thickness swelling behavior of hybrid composites signiﬁcantly decreases through an incorporation of glass ﬁber whilst a reverse trend is observed as the bagasse and corn stalk ﬁbers contents increased in the composites. Composites with 15 wt.% glass ﬁber show the lowest percentage in thickness swelling among the hybrid composites. In this case, the decrease in the percentage of thickness swelling is mainly due to the presence of glass ﬁber which has low tendency in water absorption characteristic compared to the high tendency in water absorption of the used agro-waste ﬁbers. Therefore, with the increase in the bagasse or corn stalk ﬁbers content, thickness swelling of the composites increases due to increase the percentage of water absorption into composites. The swelling of the ﬁber, places stress on the surrounding matrix and leads to microcracking, which would eventually cause the composite to fail catastrophically. As a consequence, the ﬁber–matrix adhesion is weak and the dimensional stability of composites particularly for outdoor applications will be greatly affected. Based on the graph
(Figure 3), it also can be seen that the thickness swelling values of composites increase with an increased water exposure time. By increasing the exposure time of composites to water, a signiﬁcant amount of water is absorbed resulting in the swelling of the ﬁber. CONCLUSIONS This article reports on the use of bagasse, corn stalk and glass ﬁbers as reinforcements in PP composites. The conclusions from this study are summarized as follows: 1. The incorporation of both used agro-waste materials and glass ﬁber into the PP matrix has resulted in the improvement of the mechanical properties of the composites. The enhancements of tensile and ﬂexural properties of the composites were higher with increase in loading of both corn stalk and glass ﬁbers. 2. The impact strength of hybrid composites are signiﬁcantly improved by increasing the glass ﬁber loading, while a reverse trend is observed as the bagasse or corn stalk ﬁbers contents increased in the hybrid composites. 3. Composites showed the highest mechanical performance at 55 wt.% of ﬁber loading. This is due to the excellent dispersion of the ﬁbers and good load transference occurring at this composition. 4. Among the hybrid composites, a moderate loading of glass ﬁber in composites showed the highest values of tensile and ﬂexural properties. 5. The mechanical properties of CSF=glass hybrid composites are found to be much higher than those of BF= glass composites. This may be a result of the good compatibility in the interfacial region between corn stalk ﬁber and the PP matrix, which improved the efﬁciency of the stress transfer mechanism in the matrix. 6. Incorporation of 15 wt.% weight fraction of glass ﬁber in hybrid composites gave the highest values of composite properties. All the improvements in the hybrid composite properties are mainly due to the high strength and modulus value of glass ﬁber than the inferior properties of the used agro-waste ﬁbers.
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