Thermoplastic Hybrid Composites using Bagasse, Corn Stalk and E ...

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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 ...
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[6]. 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[9]. 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[14]. 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]




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[14]. 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.

PP (wt.%)

MAPP (wt.%)

BF (wt.%)

CSF (wt.%)

EGF (wt.%)


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


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.[19], 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.[22], 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



FIG. 1.

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[12]. 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


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[14]. 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[14]. 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[23]. 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[22]. 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.[14] 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[22]. 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



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[22]. 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


FIG. 3.


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[14]. 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[9]. 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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