14.7.1 Techniques to Improve the Interface Adhesion

The biggest challenge in developing of material composites based on natural fibers is the incompatibility between the hydrophilic fibers and the hydrophobic matrix. This incompatibility leads to a poor homogenization between fibers and the

polymer chains (Nazrul et al. 2010); as a result, a poor adhesion between both com­pounds is often seen (Pickering et al. 2007; Ku et al. 2011). Most studies refer to the modification of the interface of natural fibers or polymer matrix, to ensure compat­ibility between the fiber and the matrix (Huu Nam et al. 2011; Arrakhiz et al. 2012a). The treatment parameters used is a factor that influences the properties and characteristics of the result composites. Therefore, appropriate treatment tech­niques and parameters must be carefully selected to produce an optimal composite product. These techniques can be divided into two categories: physical and chemi­cal methods.

The physical treatments do not alter the chemical composition of the cellulose fiber and they are expensive (Belgacem et al. 1995). Stretching, calendering, ther­mal treatments are considered as an example of physical treatment (Belgacem et al. 1996; Liu et al. 1998). Other types of physical treatment are also found in the litera­ture as the electric discharge (corona, cold plasma). For example the electrical discharge treatment can modify the fiber surface from hydrophilic to hydrophobic by changing the surface energy (Belgacem and Gandini 2005; Kato et al. 1999).

The use of chemical treatments is for removing the non-cellulosic components in the fiber surface or for adding functional groups to increase connection with

polymer chains. Several chemical techniques were considered to improve the adhesion interfacial such as, grafting compatibilizer groups, addition of a coupling agent or cleaning the fiber’s surface. The alkali treatment is one of the standard chemical treatments using sodium hydroxide to remove amorphous and non-cellu — losic components from the fibers’ surface. The estherification or etherification of the hydroxyl groups found on the fibers’ surface is a possible technique to graft a func­tional group (Arrakhiz et al. 2012a). The use of maleic anhydride modified polypro­pylene (MAPP) as a coupling agent is another pathway to enhance the interface adhesion between fibers and matrix (Arrakhiz et al. 2012c). Fiber surface treatment may also increase the strength of the fiber; reduce the water absorption and surface tension between fiber and matrix.

The use of maleic anhydride grafted polypropylene; SEBS-g-MA as compatibil — izer between fibers and matrix improves the water resistance of fibers and enhance the wettability of fibers in the polymer matrix, also the use of SEBS-g-MA create a strong ester bonds between polymer and hydroxyl groups of fibers. Figure 14.9 shows the FTIR curves of PP/Pine cone composites with and without coupling agent. The peaks at 1,703, 1,652, and 1,560 cm-1 were observed after addition of compatibilizers, these peaks are the main characteristic peaks of ester bonds formed. These formed ester bonds between matrix and hydroxyl groups of fibers enhance the thermal and mechanical properties of composites.

SEM micrographs analysis of fracture surfaces of composites with coupling agent (Fig. 14.10) confirm that the addition of coupling agent improves the interface adhesion between doum fibers by absence of decohesion zones (pull out fibers), and reduction of the fiber/fiber contact.

The comparative thermal analysis between the composites with compatibilizer and without compatibilizer is found in the literature Arrakhiz et al. (2012a, b, c). Table 14.1 illustrates the comparative thermal analysis of Doum and Hemp fibers and their composites with and without compatibilizer. It was seen that the compos­ites compatibilized exhibit a higher temperature degradation than composites without compatibilizer.

The improvement in the fiber-matrix adhesion enhances the mechanical prop­erties of composites (Elkhaoulani et al. 2013). Figure 14.11 shows the compara­tive tensile properties of the fibers as Pine cone, hemp and Doum. Addition of

fibers increases Young’s modulus, until it reached one maximum value at 25 wt.% (for all composites). On the other hand, the tensile strength of various composites compatibilized is stabilized at a high value, except PP/hemp which shows a slight decrease. The tensile strength properties are higher in the composites compatibil­ized than composites without compatibilizer, this is due to the good interfacial adhesion (fibers/matrix). The maleic anhydride molecules grafted to PP construct a strong ester bonds with the hydroxyl groups (OH) present on the fibers’ surface (Elkhaoulani et al. 2013).

A good dispersion and interfacial adhesion between the matrix and fibers are both critical factors for the resulting composites to achieve improved mechanical properties. Chemical treatment of the fibers’ surface was used to improve interfacial adhesion in the composite. Table 14.2 illustrates influence of chemical treatments and fictionaliza — tion on two composite systems with different fibers (Alfa and Coir), at 20 wt.%, and thermoplastics matrix (PP and HDPE). The chemical modifications used in this work, namely NaOH, etherification (C12 (Dodecane bromide)), Estherification (Palmitic acid N-succinimidyl), and silane (3-(trimethoxysilyl) propylamine) exhibit a different inter­action mechanism with both fibers and polymer matrices.

The results show that chemical treatments improve the mechanical thermal pro­prieties of fibers, leading to improvement in properties of composites reinforced with these treated fibers. Alkaline treatment shows higher values in terms of young’s modulus, also composites with fibers fictionalized with silane and C12 shows a sig­nificant tensile modulus when compared to raw fibers reinforced polymer.