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CHAPTER 2 LITERATURE REVIEW
There is no specified definition of the composite material. Daniel and Ishai (1994) define a composite as a material system consisting of two or more phases on a macroscopic scale, whose mechanical performance and properties are designed to be superior to those of the constituent materials acting independently. In the present context, composite is a multiphase material that is artificially made, as opposed to one that occurs or forms naturally, chemically dissimilar and separated by a distinct interface (Callister, 2007). According to Gay and Hoa (2007), a composite material currently refers to material having strong fibers surrounded by a weaker matrix material. The matrix serves to distribute the fibers and also to transmit the load to fibers.
Roesler et al. (2006) characterize composite materials by the following properties:
A strengthening second phase is embedded in a continuous matrix.
The strengthening second phase and the matrix are initially separate materials and are joined during processing - the second phase is thus not produced by internal processes like precipitation.
The particles of the second phase have a size of several micrometers at least.
The strengthening effect of the second phase is at least partially caused by load transfer.
The volume fraction of the strengthening second phase is at least approximately 10%.
These properties, however, cannot be used as definition for composites in further developments. Some of new composites (e. g., in nanotechnology) do not posses some of the list.
Generally composite material is considered to be a material that composed of at least two physically distinct phases of materials. One is term the matrix, which is continuous and surrounds other phase, the dispersed one. The dispersed materials can be in the form of particle as well as fiber. The fibers and particles usually act as reinforcements to carry load or to control strain, whereas the matrix acts as a bonding medium to transfer load and to provide continuity and structural integrity. The matrix also protects the fibers and retains them in position to form the desired shape for a finished article (Owen, 2000).
The advantages of composites compared to conventional materials are often related to the high ratios of stiffness and strength to weight (Swanson, 1997). For that reason, there are many applications of this material in aerospace and sporting goods. Composites also often have superior resistance to environmental attack that makes they are used extensively in the chemical industries marine applications besides being considered for infrastructure applications (Swanson, 1997). Composites materials are attractive because they are combination of material properties in ways not found in nature (Mohanti et al., 2005). The materials can be solution for many of modern technologies require materials with unusual combinations of properties that cannot be met by the conventional metal alloys, ceramics, and polymeric materials (Callister, 2007).
In term of the matrix, composite materials are divided into three main categories: polymer-matrix composites, metal-matrix composites, and ceramic-matrix composites. Technologically, the most important composites are polymer-matrix composites in which the dispersed phase is in the form of a fiber, called fiber-reinforced polymeric composites or simply fiber-reinforced polymer (Astrom, 1997).
Fiber-reinforced polymer or fiber-reinforced plastic (FRP) is a combination of fibers in a polymeric matrix material (Owen et al, 2000). Fibers are available in many geometry and architectures. They can be in form of short or long (continous) fibers, aligned or randomly oriented. In FRP, they are dispersed in two main categories of polymer as composite matrix: thermosets and thermoplastics.
There are significant differences between thermosets and thermoplastics. Some aspects must be considered in choosing the matrix.
Thermosets are polymeric materials that in their final state are insoluble, cannot be fused and degrade before melting (Owen, 2000). They are network polymers that become permanently hard during their formation and do not soften upon elevating temperature. There are chemical links between linear polymer chains, called cross-links that make a three-dimensional network. The network can be formed, either by addition reaction, without evolution of volatiles (e.g., epoxy resin, unsaturated polyester or addition polyimides) or by condensation reaction (e.g., phenolic resin) (Varma, and Gupta 2000).
One of their main advantages compared to thermoplastic is the starting process. They are processed starting from low viscosity and molecular weight. This allows products to be molded easier, using low pressures and low temperature, with lower processing costs (Boogh and Mezzenga, 2000). Generally, they are harder, stronger and have better dimensional stability compared to thermoplastics (Callister, 2007). Because of the mechanical properties, most high-performance composites have thermosetting matrices.
Opposite to the thermosets, in which the molecular chains are cross-linked by covalent bonds, the intermolecular chemical bondings of thermoplastics are weak. Hence, they can be overcome by thermal energy and the material reaches a softened or molten state (Michaeli and Koschmieder, 2000). When the temperature is raised, they generally pass a glass transition temperature, Tg, which corresponds to the transition from glassy to rubbery properties, and then melting before degrade (Boogh and Mezzenga 2000). It means thermoplastics can be reshaped or melted by heating.
In recent years thermoplastic polymers have gained importance as a matrix in composite parts. Compared to thermosets, thermoplastics have larger elongation at break and their good damping behavior provide a better impact resistance. With respect to processing, even the temperature have to be high, processing of thermoplastics is much faster. The polymer quickly solidifies by cooling, while thermosets have to be cured in a time consuming processes (Michaeli and Koschmieder 2000). Moreover, because it can be melted and re-solidified, thermoplastics are easy to be recycled.
There are, however, many challenges to use thermoplastics as composite matrix. Thermoplastics often have less tensile strengths and moduli compared to those of many thermosets. The combination of high melt vicosities and the lack of reactive groups make it difficult for a thermoplastic to wet and bond to reinforcing fibers. Sometimes it needs the addition of hybrid polymers, called compatibilizers, which can associate well both to the fibers and matrix. Thus, combining thermoplastic matrices with reinforcements and insuring good fiber wet-out and bonding is a significant technical challenge (Muzzy, 2000).
There are well over 50 types of thermoplastics based on chemical structure and more than 10 000 specific grades of thermoplastics available commercially. There are thermoplastic in the group of polyolefins (e.g polyethylene, polypropylene), styrenics (e.g polystyrene), vinyls (e.g, poly-vinylchloride), acrylics ( e.g polymethylmetacrylate), fluoropolymers (e.g polychlorotrifluoroethylene), Polyesters (polyethylene terephthalate), polyamides (e.g nylon 66), polyimides (e.g polyetherimide), Polyethers (e.g polyacetal), Sulfur-containing polymers (e.g polysulfone), and some additional thermoplastics, such as polyurethane (Muzzy, 2000).
One of the widely used thermoplastics is polyvinyl-chloride, which is commonly abbreviated as PVC. It is widely used because it is inexpensive, durable, and flexible. There are many uses of PVC. As a hard thermoplastic, PVC is used as building materials, pipe, plumbing, and many other applications. It also can be made softer and more flexible by the addition of plasticizer. The development of PVC composites, especially PVC/natural fiber composites, has been rising rather rapidly in several years. In facts, there are several inventions regarding PVC and wood/cellulosic composites have been patented.
In a composite, the fibers carry the primary loads and overcome the limitation of the matrix material, which usually have low modulus and poor crack propagation resistance (Owen 2000). Beside the fiber and matrix properties, the properties of FRP are strongly dependent on the architecture of the fibers, or the way the fibers are laid in the composites. Proper selection of the type, amount, and orientation of fibers is very important since it influences the following characteristics of a composite (Mallick, 1993):
Tensile strength and modulus
Compressive strength as well as fatigue failure mechanisms
Electrical and thermal conductivities
A fiber carries the load and the composite strength along the axis of the fiber (Mallick, 1993). Long fibers composite have superior tensile properties compared to the same fibers chopped into short length. (Mallick, 1993). Hence, there will be significant different mechanical properties, either between aligned and randomly oriented fibers composites or between long and short fibers composites. However, it is more complicated to produce either aligned or long fiber composites. For structural application, aligned and/or long fibers are recommended, whereas for non structural application, random and/or short fibers are recommended.
In the advanced development, reinforcing fibers are also available in the form of fabric. They can be woven, knitted, or braided. Fabric fibers can be used to produce three dimension composites (Astrom, 1997).
The most common fibers are glass, carbon (graphite), or aramid (aromatic polyamid). Today, glass fiber reinforced plastic (GFRP) composites are the higest-volume commodity materials among other reinforcement fibers (Dwight, 2000). Carbon fibers are specialty fibers for high performance application. There are at least two classes of carbon fibers according to their origin, polyacrylonitrile (PAN)-based carbon fibers (Shindo, 2000), and pitch precursor carbon fibers (Diefendorf, 2000). Aramid fibers are polymer-based fibers have superior strength, thermal stability and and chemical resistance (Yang, 2000). One of the famous aramid is KevlarTM, which is used for ballistic rated body armor fabric.
The other fibers, especially natural fibers, are also being developed. During the last decade natural fibers have attracted the attention of scientists and technologists on the advantages that these fibers provide over conventional reinforcement materials, such as glass-fibers (Bledzki and Gassan, 1999).
Natural Fibers and Their Properties
According to their origin, natural fibers can be classified as plant or vegetable fibers, animal fibers, and mineral fibers. The development of natural fiber composites, however, mostly regarded to only natural fibers. In this review, the word "natural fibers" means plant or vegetable fibers.
Natural fibers are lignocellulosic fibers with the basic components of cellulose and lignin. Cellulose is a natural polymer with the formula of (C6H11O5)n, consisting of D-anhydroglucose (C6H11O5) repeating units joined by 1,4-b-D-glycosidic linkages at carbon's, C1 and C4. It consists of a linear chain of several hundred to over ten thousands repeating units (n). Three hydroxyl groups contained in the repeating units have ability to hydrogen bond. The hydrogen bond plays a major role in directing the crystalline packing and also governs the physical properties of cellulose. Solid cellulose forms a microcrystalline structure with regions of high order, called crystalline regions and regions of low order, named amorphous regions. Cellulose is resistant to strong alkali (17.5 wt %) but is easily hydrolyzed by acid to water-soluble sugars. Cellulose is relatively resistant to oxidizing agents. The reinforcing efficiency of natural fiber is related to the nature of cellulose and its crystallinity (Maya and Sabu, 2008).
Lignin is a thermoplastic polymer compound contributed to the rigidity of the plant. It is a complex, three-dimensional copolymer of aliphatic and aromatic constituents with very high molecular weight. Lignin contains hydroxyl, methoxyl and carbonyl groups. It is believed that the structural units of lignin molecule are derivatives of 4-hydroxy-3-methoxy phenylpropane which has less affinity of water. The main difficulty in lignin chemistry is that no method has been established by which it is possible to isolate lignin in its native state from the fiber. The glass transition temperature of lignin is around 90oC and the melting temperature is around 170oC (Maya and Sabu, 2008 and Oelsen and Placket, 1999).
Figure 2.1 shows that natural fiber is actually a natural composite. It contains hollow cellulose fibrils held together by a hemicellulose and lignin as the matrix. The cell wall in a fiber is a heterogeneous membrane. Each fibril has a complex structure consisting of many layers. A thin primary wall that is the first layer deposited during cell growth encircling a secondary wall. The secondary wall is made up of three layers. The middle layer consists of a series of helically wound cellular microfibrils formed from long chain cellulose molecules (Jayaraman, 2003).
Figure 2.1: Structure of natural fiber (Rong et al., 2001)
The characteristic values vary from one fiber to another. Some of the characteristic values are the cellulose content in the fiber, the degree of polymerization of the cellulose and the microfibrillar angle. Higher cellulose content, higher degree of polymerization and a lower microfibrillar angle will cause higher tensile strength and modulus. The variations in the characteristic value exhibit the variations in mechanical properties both along the length of an individual fiber and between fibers. (Jayaraman, 2003)
Natural fibers also have non-uniformity and variation of dimensions, even between individual plants in the same cultivation. To generate fibers suitable for specific end products, the various types of raw material are separated. Bast or stem fibers, for example, are mainly used in the textile or rope industries because of the length of the fibers. Bast straw is not separated into single fibers but into fiber bundles, which may contain thousands of single fibers. In contrast with it, wood is usually separated into single fibers or very small fiber bundles suiting the particular needs of the pulp, paper or board industries. Thus, there are a great number of challenges for selecting fibers in different dimensions and properties (Olesen and Plackett, 1999).
Natural fibers may be classified in two broad categories: Non-wood fibers and wood fibers. Non-wood fibers divided into (Mohanty et al., 2005):
Straw, examples: corn, wheat, and rice straw
Bast, examples: kenaf, flax, jute, ramie, and hemp
seed/fruit, examples: sisal, pineapple leaf, and henequen fiber
grass fibers, example: bamboo fiber, switch grass, and elephant grass
The scheme of natural fiber classifications is shown in Figure 2.2.
Figure 2.2: Classification of natural fibers (Mohanty et al., 2005)
Natural Fibers offer several advantages over glass-fibers (Bledzki and Gassan, 1999, and Satyanarayana et al, 1990):
Natural fibers are renewable raw materials that will be available continuously, having low cost and low energy consumption.
When they are subjected to a combustion process or landfill at the end of their life cycle, the released amount of CO2 of the fibers is neutral with respect to the assimilated amount during their growth.
The abrasive nature of natural fibers is much lower compared to that of glass-fibers, which leads to advantages with regard to technical, material recycling or process of composite materials in general.
The density of natural fibers is much lower compared to glass-fibers. It means there is potential advantage of weight saving.
These fibers commonly have high work of fracture so that composites containing them are also expected to have high resistance to crack propagation.
They are non-toxic that safe to work with.
On the other hand, there are several problems in using natural fibers and these have to be solved in order to be competitive with glass. Natural fibers durability and strength are lower compared to glass fibers. When they are used as reinforcements of synthetic polymers, there is a major drawback of the application. Due to the presence of hydroxyl and other polar groups on the surface and throughout natural fiber, moisture absorption can be high. It leads to poor wettability by the polymers and weak interfacial bonding between fibers and hydrophobic polymers, as the matrix. In order to develop composites with better mechanical properties, it is necessary to solve the problems by suitable treatments to enhance the compatibility between fibers and the matrices (Brouwer, 2000 and Drzal, 2004). Besides the fibers-matrix, the major issues in development of natural fibers/polymer composites are thermal stability, moisture content, biodegradation and photo-degradation, and processing of polymer composites (Brouwer et al., 2000).
PVC and Its Natural Fiber Composites
PVC is a member of vinyl polymers. All of the member have the vinyl group (CH2=CH-). The other example of group members are polyethylene, polypropylene, polystyrene, polyvinyl acetate, polymethyl methacrylate and polyvinylidene chloride. Through common usage, the word vinyl generally refers to PVC and its copolymers. Vinyl chloride (VC) monomer, CH2=CHCl, is a colorless gas possessing a faintly sweet odor which can cause anesthesia at high concentrations (Sarvetnick, 1977).
Figure 2.3: chemical structure of VC and PVC
Although there is speculation that acetylene derived from coal may become the major hydrocarbon used, the source of hydrocarbon for VC has been ethylene in recent times. The production of VC also needs chlorine, produced mainly from common salt (NaCl). On a weight basis, chlorine accounts for 56.8% of the total weight. PVC is hence less affected by the cost of petroleum and natural gas then other polymer. Since the prices of petroleum and natural gas probably will be still rising in the future more rapidly than chlorine prices, PVC will still less expensive compared to other polymers (Nass, 1985).
Beside the cost of the raw materials, there are several reasons why PVC has the broadest range of application and its use has grown more rapidly than of other plastics. It is because PVC is easy to fabricate and it can last for long time. PVC has outstanding chemical resistance to wide range of corrosive fluids and offer more strength and rigidity than most of the other thermoplastics. Tensile stress of PVC is 40-60 MPa, its modulus of elasticity is 2-7 GPa with density of 1380-1410 kg/m3. PVC is also uniquely responsive to functional additives which permit the generation of rigid and flexible products, useful in designed engineering application (Willoughby, 2002).
The things should be considered in using PVC are safety and environmental issues. VC is reported to cause serious health problem. The anesthetic property of VC was recognized in early 1930s and has been investigated by several workers. At concentration much above 8 to 12%, it may kill animals rapidly. If an excess of VC is inhaled, the liver capacity is overwhelmed and allowing tumour formation to occur. It is also reported as carcinogen (Nass, 1985).
PVC is also currently suspected as a contaminant material. When it is processed, or when it decomposes, it produces some substances that can damage the atmosphere, for example hydrogen chloride and dioxins. This has provoked environmental groups to criticize concerning its mass utilization. Mixing PVC with natural fibers is an interesting alternative. This could reduce its inconveniences while conserving its advantages (Ayora et al., 1997).
Many researchers have investigated the properties of PVC-vegetable fiber composites. So far, at least there are nine variants of vegetable fibers that have been used for filler/reinforced materials. They are wood, bamboo, pine, rice straw, sisal, oil palm, bagasse, banana and coconut.
Kokta et al. (1990) evaluated the mechanical properties PVC composites reinforced by different wood species (softwood, spruce, hardwood, aspen, and birch). It has been reported that the tensile strength, elongation, and toughness decrease, while modulus increases linearly with the rise of fiber level in composites. There are also inferior performances compared to that of the original polymer. This can be described by the poor adhesion between untreated cellulose fibers and polymers. Nevertheless, there is significant improvement in their modulus. Among the species of wood, birch fiber rank first and aspen last as for the tensile strength and modulus properties as a composite. However, the elongation of aspen is better than any other species except birch.
Mengeloglu et al. (2000) examined the effects of impact modifier types and addition levels on the mechanical properties of rigid PVC/wood-fiber composites where they found that the impact resistance of rigid PVC/wood-fiber composites depends strongly on the type and content of impact modifier. In addition, with the suitable selection of modifier type and concentration, the impact strength of rigid PVC/wood-fiber composites can be extensively improved without degrading the tensile properties. They also found that methacrylate-butadiene-styrene and all-acrylic modifiers performed in a similar manner and were more effective and efficient in improving the impact resistance of rigid PVC/wood-fiber composites than the chlorinated polyethylene (CPE) modifier.
Different parameters regarding the performance of isocyanate as coupling agent have been discovered by Kokta et al. (1998) where better premixing time, e.g. 20 min (of polymer, fiber and coupling agent), leads to better mechanical properties of the composites. Chemical structure of isocyanate which provides better interactions to the thermoplastic, gives superior properties. Again, isocyanate can act as a promoter, or inhibitor, depending on the concentration of isocyanate used, for instance, at moderate concentration it promotes to maximum mechanical properties, while at higher concentration mechanical properties are deteriorated.
Shah et al. (2005) discovered two natural polymers, i.e. chitin and chitosan, as novel coupling agents for PVC/wood-flour composites. Addition of chitin and chitosan coupling agents to PVC/wood flour composites increased their flexural strength by approximately 20%, their flexural modulus by approximately 16%, and their storage modulus by approximately 33-74% compared to the PVC/wood flour composite without the coupling agent. Significant improvement in the composite performance was attained with 0.5 wt% chitosan and while 6.67 wt% chitin was used.
According to Zhao et al. (2006), after wood flour was modified by 1.5 h-1 silane (coupling agent), the impact strength and the tensile strength of wood flour-PVC composite were increased by 14.8% and 18.5%, respectively. Mechanical tests showed that the addition of organomodified montmorillonite (OMMT) did not enhance the untreated wood flour-PVC composites. However, adding 0.5% OMMT did improve the mechanical properties of the treated ones. The grafting improved the interfacial compatibility between components producing higher properties of the composites. Further adding of OMMT reinforced the composites, but too higher contents of silane and OMMT impaired some properties because of weak interfacial layer and higher concentrated stress.
Another treatment has been observed is with copper amine reported as by Jiang and Kamdem (2004). The significant improvement of flexural strength, flexural toughness and unnotched impact strength can be obtained by this treatment. It was found that flexural strength of PVC/wood flour can be improved up to 36%, the flexural toughness can be improved up to 40%, and the unnotched impact strength can increase up to 45%. These enhancements can be obtained by using 0.2% Cu contained treated wood flour at the 60 wt% PVC and 40 wt% wood flour. Mechanical properties of PVC/wood composites can be enhanced by combining wood with mica or glass fibers to form hybrid reinforcements (Maldas and Kokta, 1993). Ultraviolet light resistance and weathering dimensional stabilities of PVC/wood composites are superior to those of natural wood. Density reduction can be achieved through the microcellular foaming technique by using chemical blowing agents, such as azodicarbonamide and sodium bicarbonate, or physical blowing agents, such as carbon dioxide (Jiang and Kamdem, 2004).
Besides mechanical properties, the thermal and dielectric properties are also important to be observed. Djidjelli et al. (2002) reported that filler content of wood flour influence the thermal stability of wood flour/PVC composites. The decomposition onset temperature, measured using TGA, decreases with the increases of wood flour content. However, there is no significant influence on the glass transition temperature. Dielectric permittivity is reported to be increased as the wood flour content is increased (Djidjelli et. al., 2002). In term of recycling, there is no significant change appears until five cycles of internal recycling as reported by Augier et al. (2007). Therefore, it should be possible to recycle the composite internal waste for five times, without adding fresh material. Practically, the internal waste part does not exceed 20% of the entire blend. Hence, the internal recycling would not influence the mechanical properties of the recycled composite. There are some applications have been produced using PVC/wood composites i.e. insulation sheet of roof tile, sheets for construction site, and insulation sheets for cars, houses, and so on. In industrial world, Ochiai (1999) found there is still a problem regarding the production cost process where wood fiber must be completely dried and cut in very small pieces, which may be a factor to increase the price of this sheet.
Prachayawarakorn et al. (2006) investigated the effects of low density polyethylene (LDPE) content, compatibilizer type and rubberwood sawdust loading on the properties of the blend of PVC/LDPE composites. Their experimental findings suggested that as the LDPE content was increased the mechanical properties of PVC-LDPE blend progressively decreased due to poor interfacial adhesion. The continuity and compatibility between PVC and LDPE phases could be improved through three different types of compatibilizers which included chlorinated polyethylene (CPE) poly (methyl-methacrylate-co-butyl acrylate) (PA20) and poly (ethylene-co-methacrylate). The PA20 was found to be the most suitable compatibilizer for the blend. The decomposition temperature of PVC in the blend decreased with the loading of the PA20 and the wood sawdust. As the sawdust content was increased the tensile and flexural moduli increased with considerable decreased in the tensile, flexural and impact strength, a slight improvement being achieved if the PA20 was incorporated in the composite.
The use of untreated sawdust as a filler in PVC was examined by Sombatsompop et al. (2003) with the effects of sawdust content on structural and thermal changes, and rheological and mechanical properties being of main interest. The results revealed that the torque and die entrance pressure drop values during mixing were independent of sawdust particles up to 23.1 wt%. The extrudate swell monotonically decreased up to 33.3 wt% sawdust content. Smooth wood-like texture with controllable size of the extrudate could be obtained at sawdust content greater than 33.3 wt%. Tensile, impact, flexural and hardness properties of the PVC/sawdust composites considerably decreased with up to 16.7 wt% sawdust content before levelling off for higher sawdust loadings. The composites having sawdust higher than 16.7 wt% showed a benefit of cost savings. The decreases in the mechanical properties of PVC with sawdust are explained in association with the presence of moisture, interfacial defects between fiber and polymer, and fiber dispersions in the PVC matrix. The overall results in this work suggest that the properties of PVC/sawdust composites were strongly influenced by sawdust content up to 16.7 wt%. Beyond this value the effect of sawdust content on the properties was comparatively small (Sombatsompop, 2003).
Further study by Sombatsompop et al. (2004) investigated the mechanical and thermal properties and structural changes of PVC/wood sawdust composites were assessed with respect to the effect of moisture content, varying from 0.33 to 3.00 % by weight in the composite, for three different wood sawdust contents. The results suggest that at low moisture content, the tensile modulus decreased and elongation at break of the composites increased, the effect being reversed for high moisture content. Tensile strength decreased with increasing moisture content up to 1-2 %, and then unexpectedly increased at higher moisture contents. The effect of moisture content on flexural properties of the composite was similar to that on tensile properties. Impact strength of the composites was considerably improved with moisture content at low sawdust contents (16.7 wt %), and was independent of the moisture content at higher sawdust contents (28.6 and 37.5 wt %).
PVC/Bamboo and Pine Composites
It is reported by Ge et al. (2004) when compared with neat PVC resin, the introduction of both bamboo flour and pine flour significantly improved the stiffness of the unplasticized PVC composites although to some extent the composite's tensile strength decreasing. In addition, tensile tests showed that pine flour-filled composites exhibited better mechanical properties than those filled with bamboo flour with the same particle size at the same loading level. Moreover, experimental results indicated that both bamboo flour and pine flour additions showed no observably adverse effect on the thermal stabilities of these composites. PVC wood, which includes PVC foam and PVC/wood flour composite, shows improved performance over wood in the following properties: termite resistance, weathering aging, less moisture absorption, and ease of installation. It can be nailed, screwed, sawed, cut, and glued like wood by conventional tools without any special skills required. Although the bending strength of PVC wood is lower, it can be used for decorative applications, i.e., cornice, door, and siding (Chetanachan et al., 2001).
Djidjelli et al. (2007) reported the mechanical properties, water absorption and thermal stability of PVC/Sisal composites with and without maleic anhydride treatment where PVC and sisal fibers were blended and processed at a speed of 3000 rpm at 50oC. The specimens where then molded by compression molding with the pressure level up to 250 kN. It was found that the Young's Modulus was increased with the increasing of fiber content. Until the 20% of concentration, there is no effect of fiber amount to the Charpy impact strength. The specimens remain resistant, unbreakable, and ductile. Above 20% the samples become breakable and low values of impact energy was recorded as small as 0.9 and 0.36 Joule for the 20% and 30% composites respectively. The positive effect of fiber content comes to the hardness of the composites. Better hardness measured on untreated fiber compared to treated fiber. This result can be explained by the fact that the sisal fiber has a hollow central region which gave access to water penetration by capillary action. However, the water absorption decreased after anhydride treatment. The thermal stability is also increased with the increase of virgin sisal fiber content. Maleic anhydride treatment affect negatively to this property. It decreases the thermal stability very significantly.
PVC/ Oil Palm Empty Fruit Bunch (OPEFB) Composites
Abu Bakar et al. (2006) reported a study on oil palm empty fruit bunch (OPEFB) fiber reinforced PVC composites, where the PVC resin and the additives were dry blended and test specimens were prepared using a hot press. The analysis showed that the oil residue was successfully extracted from OPEFB fibers. Both the extracted and unextracted fibers decreased the fusion time and melt viscosity of unplasticized PVC. However, the extracted fiber was found to increase the fusion time of PVC as the fiber content increased from 10 to 40 phr. The impact and flexural properties of composites were not significantly affected by the fiber extraction. In other study conducted by Abu Bakar et al. (2005), it was found that the increase in OPEFB fiber content resulted in an improvement in flexural modulus at the expense of impact strength and flexural strength. The incorporation of OPEFB slightly enhanced the glass transition temperature but it decreased the thermal stability of the composites, evidenced by a decrease in decomposition temperature and a change in the degradation process from two to three stages.
Ratnam et al. (2007) modified the OPEFB/PVC composites by blending the PVC with epoxidized natural rubber (ENR) followed by irradiation of blend-electron beam test. The effect of irradiation on the tensile properties of OPEFB fiber reinforced poly (vinyl chloride)/epoxidized natural rubber (PVC/ENR) blends were studied. It was found that the irradiation increases tensile strength and modulus of elasticity while the elongation at break reduces. Even though the morphological studies revealed that an improvement in adhesion between fiber and matrix has achieved after methyl acrylate treatment, there are no significant effect to the mechanical properties.
PVC/Rice Straw/Husk Composites
Kamel (2004) studied the effect of fiber treatment, concentration of PVC, pressure and pressing temperature on the mechanical properties and water absorption of rice straw/PVC composites. Composites of rice straw comprising both PVC and a coupling agent offer superior properties compared to those made from only rice straw and PVC. The extent of improvement in the mechanical properties and dimensional stability of composites depend not only on the pre-treatment of rice straw, concentration of PVC and lignin but also on pressure and pressing temperature. It is reported that the maximum bending and tensile strength were obtained after 5 or 10% NaOH treatment at 80oC while the minimum value of both were obtained after H2O treatment at 130oC. The alkali treatment also result the positive effect to the water absorption. Alkali treated rice straw-PVC composites showed less water absorption compared to the untreated and water treated composites due to better fiber-matrix interface adhesion. When the addition of lignin conducted during the making of the composites, it decreased water absorption. It is also observed that mechanical properties of the composite improve with the addition of lignin. The maximum mechanical properties were reached when the amount of lignin was 7%. Processing pressure and pressing temperature affected to the mechanical properties and water absorption. Bending and tensile strength increased with increasing pressure as well as pressing temperature while the water absorption decreased.
In the recent study Crespo et al. (2008) developed PVC composite materials derived from rice husk fillers (a by-product of agri-food industry). They found that even though the mechanical properties (tensile and hardness) decreased, the resulting composite became more rigid with the increase of the filler amount regardless of any particle sizes (in their case 150, 500 and 1000Î¼m). Later on, it was also seen that smaller particles equipped the material with improved mechanical properties due to better dispersion of the particles within the PVC polymer matrix.
A research carried out by Zheng et al. (2007) on the effect of bagasse fiber content, benzoic acid content, and fiber treatment temperature. The fiber contents studied were 15%, 25%, and 35%, with the benzoic acid contents of 3%, 5%, and 10% whereas the fiber treatment temperature were 140oC, 150oC, and 160oC. Tensile strength reported to be more affected by content of bagasse fiber and content of benzoic acid compared to by the fiber treatment temperature. Content of bagasse fiber reported to be the most significant factor. The tensile strength clearly increased with increasing bagasse fiber and the content of the modifier. This phenomenon indicates that there is strong interface interaction between bagasse and PVC. There is also effective chemical reaction between benzoic acid and the hydroxyl group of cellulose in bagasse fibers. It was also found that positive strong effect was given by bagasse fiber content to the tensile modulus. Tensile modulus increased as the increasing of fiber content especially when the bagasse fiber content was 35%. Optimum value of benzoic acid content, where the tensile modulus reached the maximum value, was 5%. Maximum value of the modulus was obtained with the maximum content of fiber and the lowest treatment temperature. However, it was still below the modulus of untreated bagasse fiber/PVC composites. Noncooperative effect was given by bagasse fiber content to the impact strength of composites. Impact strength decreased as the content of fiber increased. There is no significant effect of benzoic acid content on the impact strength. On the other hand, the impact strength increased when the treatment temperature rose up to 150oC and then decreased as the temperature continued to ascend.
PVC/Banana Fiber Composites
Zainudin et al. (2007) studied the mechanical properties of banana pseudo-stem (BPS) fiber/unplasticized polyvinyl chloride (UPVC) composites with reference to the effect of filler loading. The mechanical properties show that the composites did not have good adhesion between filler and matrix; on the other hand, the filler insertion improved the flexural modulus and the material rigidity. The thermal stability of acrylic modified and unmodified BPS/UPVC composites has been studied. The results showed that the BPS filler degraded before UPVC matrix and the BPS/UPVC composites are more stable than both components. However as the amount of BPS filler is increased, the decreases in the thermal stability of BPS/UPVC composites took place. In the case of BPS filler the decomposition starts from 58oC. For neat UPVC, decomposition begins at 268oC. In the case of 10% BPS/UPVC composite, the degradation begins at the temperature of 279oC. As the amount of BPS filler is increased up to 40%, the decomposition temperature decreases down to 256oC. The thermal stability of acrylic modified BPS/UPVC composites was found to be higher than that of unmodified BPS/UPVC composites.
PVC/ Coconut Coir Composites
A series of molten state PVC/green coconut fiber (GCF) composites processed by melt mixing were studied by Leblanc et al. (2007) and found that PVC-GCF composites are heterogeneous materials that, in the molten state, exhibit essentially a nonlinear viscoelastic character, in contrast to pure PVC, which has a linear viscoelastic region up to 50-60% strain. The complex modulus increases with the GCF content but in such a manner that the observed reinforcement is at best of hydrodynamic origin, without any specific chemical interaction occurring between the polymer matrix and the fibers. As expected, PVC offers good wetting of GCFs, as reflected by the easy mixing and the rheological and mechanical properties. Fibers can be incorporated into PVC up to a 30% concentration without any problem, with the PVC/plasticizer ratio kept constant.
Previous work by Owolabi and Czvikovszky, (1988) used fibrous biomass byproducts for radiation-treated composite materials, where two different kinds of PVC have been compounded with chopped coconut fiber (coir). Pre-irradiation of coir has been applied together with some crosslinking additive to achieve chemical bond between PVC and fibrous biopolymer. The effect of addition of 10-50% coir to PVC on the processability was monitored by Brabender plastograph. Dynamical mechanical analysis (DMA) data as well as tensile and impact strength of these coir composites have not been found superior to that of the starting thermoplastics. Considering, however, coconut fiber as cheap filler, composites with acceptable tensile and impact strength could be produced with coir content as high as 30%.
Sugarcane Bagasse and Its Composites
Bagasse fibers are produced as residue of the sugarcane (saccharum) milling process. During the milling process, the sugarcane stalk is crushed to extract the sucrose. This procedure produces a large volume of residue, bagasse, containing both crushed rind and pith fibers (Reis, 2006; Vazquez et al., 1999). The chemical contents of bagasse fibers are cellulose (40%), natural rubber (24.4%), lignin (15.0%), sucrose (14%), ash (5%), protein (1.8%), glucose (1.4%), oils (0.6%), and acid (0.6%). (Vazquez et al,, 1999).
Sugarcane Bagasse can be utilized in paper industry and for animal alimentation but the main current use is as combustible in the sugarcane industry. However, their calorimetric value is relatively low compared to other fuels. For that reason, the availability of these kind of fibers, as a waste, are high (Vazquez et al,, 1999). Previous research on bagasse has suggested many approaches to converting bagasse into more value-added industrial products, such as liquid fuels, feedstocks, enzymes and activated carbon. Another prospective solution is the use of bagasse fibers for manufacturing material products (Reis, 2006). With their tensile strength (170-290 MPa) and modulus of elasticity (15-19 GPa) with low density (550 kg/m3), they has a potential to be used as natural fiber composite (Reis, 2006; Vilay et al., 2007).
Nowadays, there are several studies on the Bagasse-Fiber Reinforced polymer composites have been reported, either in thermoplastic or thermoset matrix. Beside PVC, there are at least four kinds of polymers have been studied as Bagasse-Fiber Reinforced composite matrix. They are polypropylene (PP), unsaturated polyester, poly(ethylene vinyl acetate) (EVA), and polyethylene (PE).
Bagasse-Fiber Reinforced Polypropylene Composites
Vazquez and his colleagues (1999) studied the effects of bagasse fiber content and the fiber treatment on the mechanical properties of polypropylene matrix composites. After different chemical treatments were performed on the fibers, they concluded that the best results were obtained with mercerization, which produces a highly fibrillated survace and mechanical adhesion to the matrix. Isocyanate treatment also reduce hydrophilic surface, leading to better fiber-matrix adhesion.
Tensile strength and elongation at break decrease when untreated bagasse fiber content increases, indicating there are poor adhesion between fiber and matrix. Fiber treatment improved the properties as a consequence of the increased interfacial adhesion. The fiber treatment also improved the composite creep behavior.
Bagasse-Fiber Reinforced Polyester Composites
A study reported by Vilay et al. (2008) showed the increasing of tensile and flexural properties of polyester when higher amount of bagasse fibers were loaded, even when the fibers were untreated. Fiber treatments, however, can improve the mechanical properties. Acrylic Acid treatment seems results better improvement of mechanical properties compared to alkali (NaOH) treatment. Lee and Mariatti (2008) stated that the rind fiber composites produced higher flexural and impact properties, and lowered water absorption rate compared to inner fiber composite
El-Tayeb (2007) found that sugarcane fiber reinforced polyester composite offers a good degree of wear resistance and friction coefficient comparable to glass fiber reinforced polyester when sliding against stainless steel. He concluded that sugarcane fiber had a strong potential to reinforce polyester and proved to be a quite competitive to glass fiber.
According to study of Sousa et al. (2004), the bagasse pre-processed at sugar and alcohol mills has useful characteristics of surface cleaning that favor its direct use as reinforcement in polyester matrix composites. It means that no further cleaning operation needs to be done before the use of the leftover bagasse in composites. Moreover, the smaller size of the fibers will result better properties of composites. With the use of smaller size fibers, there will bi maximum surface area available for stress transfer, and microstructures are more homogenous.
Bagasse-Fiber Reinforced Poly(Ethylene Vinyl Acetate) Composite
Stael et al. (2001) used bagasse directly obtained from sugar cane mills, after being processed to extract sugar and liquor. This "as received" material was dried at 80°C for 48 h and then was chopped and sieved. As a result, the incorporation of chopped bagasse reduces the deformation capacity of EVA polymer without significant effect of fiber length. in practice, it implies there is no need for sieving the bagasse pieces for chopped bagasse with sizes smaller than 30 mm.
Bagasse and EVA develop a good interface and, by adjusting the volume fraction of bagasse, the mechanical properties can be adjusted to reproduce mechanical properties of wood-based particle boards (Stael et al., 2000).
Bagasse Reinforced Polyethylene
Pasquini and colleagues (2007), filled Low Density Polyethylene (LDPE) with cellulose fibers from sugar cane bagasse. Chemical modification with octadecanoyl and dodecanoyl chloride acids was also conducted. By X-ray photoelectron spectrometry, it was clearly appeared that the chemical modification improved the interfacial adhesion between fibers and matrix. However, there was no improvement of mechanical performance observed. This was ascribed to the strong lowering of the degree of polymerization of cellulose fibers after chemical treatment. The chemical treatment made the fibers weaker.
Another study by Lei and colleagues (2007) stated the coupling agent of MAPE (Maleated Polyethylene) may increase the tensile strength, modulus, and impact strength of Bagasse reinforced Recycled High Density Polyethylene (RHDPE). However, it has no significant influence on the thermal degradation. They were two thermal degradation temperatures of composites. The first, appearing at lower temperature, was affected by the fibers, and the second was the thermal degradation of HDPE. The first stage of composite thermal degradation was higher than the fiber's.