**2. Natural fibers**

228 Some Critical Issues for Injection Molding

4. Thermoplastics typically perform poorly in long-term loading because linear polymer molecules exhibit a strong time and temperature dependent response. Addition of wood filler to the polymer matrix decreases creep response during loading (Bengtsson

5. The processing temperature of the cellulosic fibres in thermoplastics is limited due to the potential fibre degradation at higher temperatures. This limits their application with

6. Low impact strength is another disadvantages of WPCs, due to the presence of natural fibers in the polymeric matrix provides points of stress concentrations, thus providing

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.,

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,

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

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,

et al., 2005), but it is still a problem.

plastics of low melting temperatures.

sites for crack initiation and potential composite failure.

2009) are the most common examples of reported works in the literature.

2000; Yuan et al., 2004).Their results show enhanced polymer-matrix adhesion.

is depended on the chemical structure of the reagent.
