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Poly vinyl chloride, which is commonly abbreviated as PVC, 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. The thing should be considered in using PVC is safety and environmental issues. Mixing PVC with natural fibres is an interesting alternative. This could reduce the environmental inconvenience while conserving its advantages. The main challenge in the research on natural fibre/Polymer composites is the poor compatibility between the fibres and the matrix. The reinforcement effect of natural fibres in PVC matrix depends on the compatibility. When they are mixed with PVC, some natural fibres may acts as reinforcing materials while other natural fibres only act as filler, which are not contributing to mechanical strength improvement. However, generally the natural fibres give positive aspect to the stiffnes of the composites while decreasing the density. Hence, beside the environmental friendliness, the natural fibre reinforced PVC composites have a great stiffness to weight ratio compared to the original PVC.
Keywords: natural fibre, poly (vinyl chloride), composites
Polymers have substituted many of conventional materials, especially metals, in various applications due to the advantages of polymers over conventional materials. They are used in many applications because they are easy to process, high productivity, and low cost in combination of their versatility. However, for some specific uses, some mechanical properties, e.g., strength and toughness of polymer materials are inadequate. Various approaches have been developed to improve such properties. In most of these applications, the properties of polymers are modified using fillers and fibres to suit the high strength/high modulus requirements. Fibre-reinforced polymers have better specific properties compared to the conventional materials and find applications in diverse field, ranging from appliances to spacecraft (Saheb and Jog 1999)
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. 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. It is also uniquely responsive to functional additives which permit the generation of rigid and flexible products, useful in designed engineering application (Nass 1985 and Willoughby 2002)
On the other hand, there are several safety and environmental issues on PVC. Vinyl-chloride (VC) is reported can make serious health problem. The anaesthetic 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. The issues have provoked environmental groups to criticize concerning its mass utilization (Ayora et al. 1997).
Mixing PVC with natural fibres, as natural fibres/PVC composites, is an interesting alternative due to the "ecological friendliness" of natural fibre. This could reduce its inconveniences while conserving its advantages (Ayora, et al. 1997). Moreover, during the last decade natural fibres have recently attracted the attention of scientists and technologists because of the advantages that these fibres provide over conventional reinforcement materials, glass-fibres. The advantages are (Bledzki and Gassan 1999):
Natural fibres are a renewable raw material and that will be available continuously.
When they are subjected to a combustion process or landfill at the end of their life cycle, the released amount of CO2 of the fibres is neutral with respect to the assimilated amount during their growth.
The abrasive nature of natural fibres is much lower compared to that of glass-fibres, which leads to advantages with regard to technical, material recycling or process of composite materials in general.
The density of natural fibres is much lower compared to glass-fibres. It means there is potential advantage of weight saving.
In this review, the potential of PVC as natural fibre composite matrix and some efforts to improve the mechanical properties of natural fibres/PVC composites will be discussed.
Mechanical and Physical Properties of Poly (vinyl chloride)
PVC is a member of vinyl polymers. It has 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 colourless gas possessing a faintly sweet odour which can cause anaesthesia at high concentrations (Sarvetnick 1977).
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).
Many authors have reported the properties of PVC-natural fibre composites. So far, at least there are nine variants of natural fibres that have been used for filler/reinforcing materials. They are wood (Mengeloglu, Mauana and King 2000; Zhao, et al. 2006; Jiang et al. 2004) , bamboo (Ge, Li and Meng 2004), pine (Ge, Li and Meng 2004), rice straw (Kamel 2004), sisal (Djidjelli, et al. 2007), oil palm (Abu Bakar, Hasan and Yusof 2006), sugarcane bagasse (Zheng, et al. 2007), banana (Zainudin, et al. 2007), and coconut (Leblanc, et al. 2007).
Mechanical and Physical Properties of Natural Fibres
Natural fibres may be classified in two broad categories: Non-wood fibres and wood fibres. Non-wood fibres divided into (Mohanty, et al. 2005):
Straw, examples: corn, wheat, and rice straw;
Bast, examples: kenaf (Hibiscus cannabicus), flax (Linum usitatissimum), jute (Corchorus) , ramie (Boehmeria nivea) , and hemp (Cannabis sativa);
seed/fruit, examples: sisal (Agave sisalana), pineapple (Ananas comosus) leaf, and henequen (Agave fourcroydes) fibre;
grass fibres, example: bamboo fibre, switch grass (Panicum virgatum), and elephant grass (Erianthus elephantinus);
Natural fibres in the form of wood flour have also been often used for preparation of natural fibre composites. The characteristic values of natural fibres vary from one fibre to another. Some of the characteristic values are the cellulose content in the fibre, the degree of polymerization of the cellulose and the microfibrils angle fibres. Higher cellulose content, higher degree of polymerization and a lower microfibrillar angle will affect higher tensile strength and modulus. The variations in the characteristic value exhibit the variations in mechanical properties both along the length of an individual fibre and between fibres (Jayaraman 2003).
Natural fibres also have non-uniformity and variation of dimensions, even between individual plants in the same cultivation. To generate fibres suitable for specific end products, the various types of raw material are separated. Bast or stem fibres, for example, are mainly used in the textile or rope industries because of the length of the fibres. Bast straw is not separated into single fibres but into fibre bundles, which may contain thousands of single fibres. In contrast with it, wood is usually separated into single fibres or very small fibre bundles suiting the particular needs of the pulp, paper or board industries. Thus, there are a great number of challenges for selecting fibres in different dimensions and properties (Olesen and Plackett 1999).
Generally, the density of natural fibres is much less than that of E-glass fibre. The specific strength and specific modulus of natural fibres are comparable or even superior to E-glass fibres. Many natural fibres have higher specific modulus compare to E-glass fibres. Hence, there is an opportunity for using the natural fibres to replace the E-glass fibre (Drzal, et al. 2004)
The challenge now is how to make the natural fibres compatible enough with the PVC. 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 fibre, moisture absorption can be high. It leads to poor wet-ability by the polymers and weak interfacial bonding between fibres and hydrophobic polymers, as the matrix (Brouwer 2000). In order to develop composites with better mechanical properties, it is necessary to solve the problems by suitable treatments to enhance the compatibility between fibres and the matrices (Brouwer 2000 and Drzal et al. 2004).
Compatibility and Reinforcement Effect of Natural Fibre in Polymer Matrix
Development sugarcane bagasse reinforced PVC composite will face a major challenge in utilizing natural fibre as reinforcing material in synthetic polymer composite, which is compatibility issue. Due to the presence of hydroxyl and other polar groups on the surface and throughout natural fibre, moisture absorption can be high. It leads to poor wet-ability by the polymers and weak interfacial bonding between fibres and hydrophobic polymers, as the matrix (Drzal, et al. 2004).
Many efforts have been carried out to solve this problem. Generally, surface treatment should be carried out in order to enhance the compatibility. Alkali, acrylic acid, maleic anhydride, and other acids and anhydrides treatment are the example of the surface treatments (Kokta, et al. 1990a; Vilay, et al. 2008). Another surface treatment is a coupling agent addition. Poly [methylene poly (phenyl isocyanate)] (PMPPIC) has been reported as one of suitable coupling agents for natural fibre reinforced PVC composites. The -N=C=O group of isocyanate may undergoes a chemical reaction with -OH group of cellulose or it counterpart lignin to develop strong interface between PMPPIC and natural fibres. On the other hand, the non-polar benzene ring of PMPPIC can interact with PVC (Maldas, et al. 1989; Joseph, et al. 2002). Figure 1 shows hypothetical chemical structure of cellulose fibre-PMPPIC-PVC in the interfacial area.
Figure 1 Hypothetical chemical structure of cellulose-PMPPIC-PVC
Due to the compatibility issue, rule of mixtures fail to show reasonable agreement with most experimental tensile properties of natural fibre composites. The rule of mixture is usually used to describe the strength of unidirectional continuous fibre reinforced composites. It is assumed that uniform strain condition exist in both matrix and fibres (Kalaprasad, et al. 1997). In another word, the rule of mixture will agree the experiment result only if unidirectional continuous fibres are used and there is a good compatibility between fibres and matrix.
The stress transfer in a composite depends largely on fibre orientation, stress concentration at the fibre ends, fibre length, interfacial shear strength and compatibility between fibre and matrix, etc. Kalaprasad et al. (1997) reported that there are two models show very good correlation with experimental result of natural fibre reinforced polymer composites. They are Hirsch model and modified Bowyer and Bader model.
According to Hirsch model, modulus of elasticity and tensile strength are calculated using the following equations (Kalaprasad, et al. 1997):
where x is a parameter which determines the stress transfer between fibre and matrix.
The following equations are used for calculating modulus and tensile strength in modified Bowyer and Bader Model (Kalaprasad, et al. 1997):
(4) where k1 is fibre orientation factor and k2 is fibre length factor, which is depending on critical fibre length.
However, there is a possibility that fibre is only acting as filler, not as reinforcing phase as reported by Vázquez et al. (1999). The strength of untreated bagasse fibre-polypropylene composite decreased when fibre content increased. The strength of the composites was even lower than the matrix alone. In this case, all of formula (1) to (4) cannot be used. The application of Nicolais-Narkis equation is preferred:
where K is a parameter that depends on fibre/matrix adhesion.
Reinforcement Effect of Natural Fibre in PVC Matrix
Previous research reports represented in table 1 show that besides PMPPIC treated wood-PVC and benzoic acid treated sugarcane bagasse-PVC composites, the natural fibres act only as filler in PVC composites. Fibre content gives negative effect to the tensile strength of the composite. The PMPPIC treatment seems suitable for wood-PVC composite as it can change the fibre content reinforcement effect from negative to positive. It means, with PMPPIC treatment, the strength of composite is increasing with the increasing of the fibre content, as a result of chemical reaction between the -N=C=O group of isocyanate with -OH group of cellulose or it counterpart lignin:
-NCO + HO-Â®NH-COO-
Hence, the strong interface between fibres and PVC were developed as described in figure 1.
It can also be seen that generally the fibre content gives positive effect to the stiffness (E) of natural fibre PVC composites.
Table 1. Effect of fibre content to mechanical properties of natural fibre PVC composites
Fibre Content Reinforcement Effect*
Kokta et al. 1990a; Djidjelli et al. 2002; Ge et al (2004)
Maldas et al. 1989; Kokta et al. 1990b; Saheb and Jog 1999
Ge et al. 2004
Djidjelli et al. 2007
Abu Bakar et al. 2005
Abu Bakar et al. 2005
Crespo et al. 2008
Zheng et al. 2007
* + represents increasing of the property with the increasing of fibre content
- represents decreasing of the property with the increasing of fibre content
Applications and Ageing of Natural Fibre Reinforced PVC Composites
There are many possible applications of natural fibre reinforced PVC composites. It can be directly moulded into a final shape, or pelletised and sold to third party processors in extrusion, injection, or compression moulding (Jones, 2004). In recent years, the trend of natural fibre composites usage has moved from thermoset to thermoplastics, including PVC. Demand has taken off for several products, such as decking, window/door profiles, fencing/siding/railings, furniture, flooring, automotive interior parts, pallets/crates/boxes, and marine components (Kline & Company, inc., 2000). Some of those products are already being manufactured, whereas others are the positive outcome of feasibility studies (Rijswijk et al., 2003).
It is common issue that there are changes in polymer properties due to long-term ageing. However, it has been investigated that the elongation at break of PVC indoor profiles was not changed after 20 and 25 years of service life compared to the value when the profiles were new (Yarahmadi et al., 2003).
With the wide range possibility of applications and good property stability due to long-term ageing, natural fibre reinforced PVC is a potential material to be developed.
The use of natural fibre as reinforcing agent in PVC based composites were reviewed from viewpoints of status and recent development in general, structure and properties of natural fibre, fibre surface modifications, and physical and mechanical properties of natural fibre based PVC composites. Wood fibres have mostly used as reinforcements in PVC based composites. Due to the low density and high specific properties of these natural fibres, composites based on these fibres may have very good implications in the automotive and furniture industry. The use of natural fibres as a source of raw material in PVC plastic industry not only provides a renewable resource, but could also generate a non-food source of economic development for farming and rural areas. However, to obtain a good interfacial bond between filler and matrix, it is necessary by the addition of suitable modifier agent. Poly [methylene poly (phenyl isocyanate)] (PMPPIC) is one of suitable coupling agents can be utilized for natural fibre reinforced PVC composites. Once the interfacial bonding problem is solved, there is wide range possibility of applications.