**1. Introduction**

224 Some Critical Issues for Injection Molding

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> The composites and their constituent components and structures have to meet increasingly development during recent decades. Some important concerns such as increasing price of petroleum and the impending depletion of fossil fuels and the interest in reducing the environmental impact of polymers is leading to the development of newer materials that can reduce stress on environment. Current developments and likely future trends are covered across key areas of the natural fibers reinforced polymer industry, together with existing and potential opportunities for the innovative use of plastic and bio-based fibers products. The challenges facing the world, such as environmental requirements and the need for recycling of plastic materials, are also included. Hence, the attention is increasingly being given to the use of natural fibers as reinforcement filler in low melting thermoplastic matrix manufactured by conventional plastic process such as extrusion and injection molding process. Injection molding is one of the most widely used processes for manufacturing molded parts from reinforced thermoplastic materials. Short natural fiber reinforced composites can be processed into complex shaped components using standard thermoplastic injection molding equipment.

> Although the use of bio-based fillers is not as popular as the use of mineral or inorganic fillers, natural fiber-derived fillers have several advantages over traditional fillers and reinforcing materials such as low density, flexibility during the processing with no harm to the equipment, acceptable specific strength properties and low cost per volume basis.

> The worldwide markets show the increased demands for natural and bio-based fibers. In 1967, the USA demand for fillers by the plastic industry was 525,000 tons; filler use had grown to 1,925,000 tons by 1998 (Eckert, 1999) and the projected use of fillers by the USA plastic industry in 2010 is to 8.5 billion pounds, of which 0.7 billion pounds (8%) was estimated to be bio-based fibers. It has been also summarized major markets for natural fibers in plastic composites as fig. 1a, on a weight basis (Eckert, 2000). Based on Fig. 1a, the main application areas of bio-based fibers filled composites are the building products in which they are used in structural applications as fencing, decking, roofing, railing, cladding and siding, park benches and etc. as shown in Fig. 2.

> Most bio-fiber plastic additives are derived from wood that incorporated in Wood-plastic composites (WPCs). WPCs contain wood (fiber or flour) and polymer as matrix. The

Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 227

Fig. 3. Some applications of natural fibers in the automotive field.

0.8 million metric tons in 2016 (BCC Research Report, 2011).

said that:

weight of WPCs.

WPCs.

Fig. 4. Interior automotive parts made of wood and natural fiber reinforced plastics.

The WPCs global market for automative applications is estimated at 0.3 million metric tons in 2011 and is expected to increase at 17.1% compound annual growth rate to reach nearly

In China and other parts of Asia, wood and natural fiber reinforced plastics are beginning to experience considerable growth along with a wider variety of product offerings including pallets, doors, and architectural moldings. While other countries are manufacturing these products, international trade is still limited, in part due to the weight of the finished products. Despite all the advantages there are also serious concerns. For explanation of these, must be

1. the density of WPCs is almost twice that of solid lumber (Li and Matuana, 2003). The concept of creating cellular foamed structures has been shown to greatly reduce the

2. Exposure to ultraviolet (UV) radiation during outdoor use is of particular concern for

3. the main disadvantages of using natural fibers as fillers is their high water absorption or desorption when subjected to changes in the relative humidity of the environment.

Fig. 1. Application areas and countries share for wood- and natural fiber-thermoplastic composites.

Fig. 2. Applications of natural fiber reinforced plastics in building products.

elasticity of wood fiber is approximately forty times greater than that of neat polyethylene and the overall strength is about twenty times higher (Oksman and Selin, 2004). However, other natural fibers, such as flax or wheat straw are finding their way into the fiber/plastic industry.

WPCs represent one of the rapidly growing markets within the plastics industry. More than 1.5 million tons of WPCs are produced worldwide, with the bulk of composites manufactured in the U.S., as shown in Fig. 1b.

Based on Freedonia Group report on 2010, Demand for WPC and plastic lumber is projected to advance 9.2 per cent per year to US\$5.3 billion in 2013, creating a market for 3.3 billion pounds of plastic. WPC lumber will post more rapid gains than plastic lumber, advancing 10.5 per cent annually to US\$2.4 billion in 2013. Growth in demand for plastic lumber is forecast to rise 8.2 per cent per year to US\$2.9 billion in 2013. Demand for WPC and plastic lumber to reach \$5.3 billion in 2013 (Centre Magazine, 2010).

In the USA, the wood- and natural fiber-plastic composites market has been dominated by rail and decking products, while in Europe more emphasis has been placed on automotive applications. Applications for these composites include interior panels, headliners, dashboard; car roofs, seat panels, parcel shelves, and acoustic panels (Fig. 3,4).

Fig. 1. Application areas and countries share for wood- and natural fiber-thermoplastic

Fig. 2. Applications of natural fiber reinforced plastics in building products.

manufactured in the U.S., as shown in Fig. 1b.

lumber to reach \$5.3 billion in 2013 (Centre Magazine, 2010).

elasticity of wood fiber is approximately forty times greater than that of neat polyethylene and the overall strength is about twenty times higher (Oksman and Selin, 2004). However, other natural fibers, such as flax or wheat straw are finding their way into the fiber/plastic industry. WPCs represent one of the rapidly growing markets within the plastics industry. More than 1.5 million tons of WPCs are produced worldwide, with the bulk of composites

Based on Freedonia Group report on 2010, Demand for WPC and plastic lumber is projected to advance 9.2 per cent per year to US\$5.3 billion in 2013, creating a market for 3.3 billion pounds of plastic. WPC lumber will post more rapid gains than plastic lumber, advancing 10.5 per cent annually to US\$2.4 billion in 2013. Growth in demand for plastic lumber is forecast to rise 8.2 per cent per year to US\$2.9 billion in 2013. Demand for WPC and plastic

In the USA, the wood- and natural fiber-plastic composites market has been dominated by rail and decking products, while in Europe more emphasis has been placed on automotive applications. Applications for these composites include interior panels, headliners,

dashboard; car roofs, seat panels, parcel shelves, and acoustic panels (Fig. 3,4).

composites.

Fig. 3. Some applications of natural fibers in the automotive field.

Fig. 4. Interior automotive parts made of wood and natural fiber reinforced plastics.

The WPCs global market for automative applications is estimated at 0.3 million metric tons in 2011 and is expected to increase at 17.1% compound annual growth rate to reach nearly 0.8 million metric tons in 2016 (BCC Research Report, 2011).

In China and other parts of Asia, wood and natural fiber reinforced plastics are beginning to experience considerable growth along with a wider variety of product offerings including pallets, doors, and architectural moldings. While other countries are manufacturing these products, international trade is still limited, in part due to the weight of the finished products.

Despite all the advantages there are also serious concerns. For explanation of these, must be said that:


Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 229

bubbles in the product (Toriz et al., 2002), and poor resistance to weathering (Pavithran,

In this chapter, we focus on structure and composition of natural fiber and review some research on optimization of interfacial adhesion between natural fibers and thermoplastic matrix witch published during the past decades and then report the effect of various chemical modifications of wood fiber and contemporaneous effect of PP-g-MA coupling agent on the interfacial strength of wood polypropylene injection molded composites. The effects of chemical treatments and PP-g-MA coupling agent on the interfacial behavior were evaluated by mechanical properties and dynamic mechanical analysis and so determined

Natural fibers are subdivided based on their origins, coming from plants, animals or minerals. All plant fibers are composed of cellulose while animal fibers consist of proteins (hair, silk and wool). Natural fibres can be classified according to which part of the plant

The strength characteristics of fiber depend on the properties of the individual constituents,

Natural fibers exhibit considerable variation in diameter along with the length of individual filaments. Quality and other properties of fibers depend on factors such as size, maturity and processing methods adopted for the extraction of fiber (Mohanty et al., 2001). Properties such as density, electrical resistivity, ultimate tensile strength and initial modulus are related to the internal structure and chemical composition of fibers (Mohanty et al., 2001). The structure, microfibrillar angle, cell dimensions, defects, and the chemical composition of fibres are the most important variables that determine the overall properties of the fibres.

the fibrillar structure and the lamellae matrix (Joseph et al., 2000).

tensile strength, adhesion factor, storage modulus and loss factor.

1981).

**2. Natural fibers** 

**2.1 Characteristics of natural fibers** 

they are obtained from, as shown in Fig. 5.

Fig. 5. Classification of natural fibres.


But there is more important drawback in natural fiber-thermoplastic composites that limit its applications. The highly hydrophilic nature of natural fibers causes compatibility problems with the hydrophobic thermoplastics in composites. The mechanical properties of composites depend strongly on the interfacial adhesion between components. This can be maximized by improving the interaction and adhesion between the two phases in final composites. There are two approaches to improve the interfacial adhesion: polymeric matrix and lignocelluloses fiber modification. Different coupling agents have been used to modify the polymeric matrix and improve the interfacial strength and subsequently the mechanical properties of the products. Maleic anhydride grafted styrene-ethylene-butylene-styrene (SEBS-*g*-MA)( Oksman et al., 1998) and maleic anhydride grafted polyolefin such as HDPE-*g*- MA (Polec et al., 2010), PP-*g*-MA (Farsi, 2010), and LDPE-*g*-MA (Tasdemir et al., 2009) are the most common examples of reported works in the literature.

Another approach for enhancement of interfacial adhesion in natural fiber reinforced thermoplastic matrix is fiber treatment before mixing with polymer. Some of these treatments have physical nature and some of them are of chemical nature. Plasma and corona treatments of the fibers as physical methods have been reported in some papers (Gassan & Gutowski, 2000; Yuan et al., 2004).Their results show enhanced polymer-matrix adhesion.

Natural fibers have a good potential for chemical treatment due to presence of hydroxyl groups in lignin and cellulose. Reaction of hydroxyl groups can change the surface energy and the polarity of the natural fibers. Many studies have been undertaken to modify the performance of natural fibers. Different surface treatment methods such as alkali treatment (Chang et al., 2009), isocyanate treatment (Maiti et al., 2004,) acrylation (Huda et al., 2008), benzoylation (Mohanty et al., 2001), latex coating (Sreekala, 2000), permanganate treatment (Joseph, 2000), acetylation (Larsson-Brelid et al., 2008), silane (Bouza et al., 2008) and peroxide treatment (Sapieha et al., 1990) have been applied on the fiber to improve its strength, size and its shape and the fiber-matrix adhesion. The aforementioned methods have their own merits and demerits, e.g., alkali treatment improves the fiber-polymer adhesion due to the removal of natural and artificial impurities and changes in chemical composition of the fiber by removing the cementing substances like lignin and hemicelluloses. Generally, mechanism of the performance of these methods is different and is depended on the chemical structure of the reagent.

There are also several other challenges presented by natural fibers filled polymer composites such as large variability of mechanical properties (Sydenstricker et al., 2003) lower ultimate strength, lower elongation, problems with nozzle flow in injection molding machines, bubbles in the product (Toriz et al., 2002), and poor resistance to weathering (Pavithran, 1981).

In this chapter, we focus on structure and composition of natural fiber and review some research on optimization of interfacial adhesion between natural fibers and thermoplastic matrix witch published during the past decades and then report the effect of various chemical modifications of wood fiber and contemporaneous effect of PP-g-MA coupling agent on the interfacial strength of wood polypropylene injection molded composites. The effects of chemical treatments and PP-g-MA coupling agent on the interfacial behavior were evaluated by mechanical properties and dynamic mechanical analysis and so determined tensile strength, adhesion factor, storage modulus and loss factor.
