The effect of the reinforcement agent on the notched Izod impact energy for the composite is also listed in the following table.

The average for five test specimens and their significant standard error is given for each property. Figures 12.7 and 12.8 graphically summarize the data listed in Table 12.3.

matrix. This behavior is observed for all matrix reinforcement combinations, al­though the rate in reduction of the tensile strength and Izod impact strength varied from case to case, depending on the reinforcement. Most of all plant fibers are hy­drophilic in nature with a moisture content enough high due to the presence of cel­lulose in cell structure. All these organic reinforcements generally have high aspect ratio, so, the efficiency of transmitting stress from matrix to these types of agents is quite poor. On the other hand, lignin increased the hard segments of composite the films, making the films less elastic and more brittle, which led to the impact strength decreasing. This explains why the impact strength decreased as the filler content reached 40% in weight. Poor interfacial bonding causes partially separated micro spaces between the filler and the matrix polymer, which obstructs stress propaga­tion, when tensile stress is applied, and induces decreased strength and increased brittleness but compatibilizing agent can solve partially this problem. This is what justifies the use of a coupling agent such as MAPP in all formulations. As the rein­forcement loading (50%) is higher, filler-filler agglomeration occurs and degree of weak interface regions between reinforcement particles and matrix become more and leads deterioration in tensile strength. As the degree of agglomeration increases, the filler- matrix interaction becomes poor, leading to the decrement in the tensile strength. Incorporation of both types of fillers (rice, wood and DDGS), generates a reduction in tensile strength of PP and its composites.

Figure. 12.7(b) shows that notched impact strength decreases in all the com­posites. This may be explained by the fact that the presence of reinforcing fillers ends within the body of the composite can cause crack initiation and subsequent failure. The reason is that the ends of reinforcing fillers act as notches and generate considerable stress concentrations, which could initiate micro cracks in the ductile PP matrix.

The impact test machine used in this study did not provide enough energy to break the neat PP because of the high flexibility of the PP matrix. By contrast, all specimens broke completely into two pieces. Introducing the reinforcement in the composites led to an increased stress concentration because of the poor bonding between the reinforcement (wood, rice or DDGS) and the polymer. As impact wave met different phases such as fiber, polymer, and voids in the cross machine direction, it would lose its energy as dissipation energy. Although crack propagation became difficult in the polymeric matrix reinforced with filler, the decrease in the impact energy observed was ascribed to fiber ends, at which micro cracks formed and fibers debonded from the matrix. These micro cracks were a potential point of composite fractures. Another reason for the decreased impact strength may have been the stiff­ening of polymer chains due to the bonding between the wood fibers and the matrix. For high-impact properties, in fact, a slightly weaker adhesion between the fiber and polymer is desirable, as it results in a higher degradation of impact energy and sup­ports the so-called fiber pullout56. In composites, the effect of the reinforcement is to increase the tendency to agglomerate, which generates a low interfacial adhesion

leading to the weakening of the interfacial regions. These agglomerates then act as sites for crack initiation. Poor interfacial bonding has been indicated in the literature as the major reason for the loss in strength and elastic modulus57.

Adding fillers also resulted in an increment of void content, which contributes to stress concentration, thus reducing strength. This behavior is consistent with what is observed in the impact tests that revealed a decrease in composites samples. The presence of numerous cavities is clearly visible in Fig. 12.4b (PWM) which has the lowest impact strength, this indicates that the level of interfacial bonding between the fibers and the matrix is weak and when stress is applied it causes the fibers to be pulled out from the matrix easily leaving behind gaping holes. These two properties are indicators of the plasticity of the material, and showed that the PP has a tendency for the occurrence of fracture with loading reinforcement. The flexural modulus of composites is influenced mainly by the adhesion between the matrix and disper­sion of reinforcing fillers inside it. The results for this mechanical property also supported the existence of a certain degree of miscibility in the composite plastics (Fig. 12.8).

From Fig. 12.8, all the compositions showed aflexural modulus higher than the pure PP. This increase in flexural properties was expected due to the improved ad­hesion between components in the blends. For some authors58, this is due to the restriction of the mobility and deformability of the matrix with the introduction of mechanical restraint. Many researchers59,60,61 have observed that the inclusion of wood fibers or lignocellulosic fibers into thermoplastics such as, polyethylene, or PP generally results in a decrease in tensile strength and elongation at break but an increase in Young’s modulus. This increase of flexural modulus can be attributed to the increase in volume fractions of high-modulus fibers in plastic composites62. When increasing the reinforcement, tensile and compression strengths constantly decreased. The presence of the wood or other reinforcement in the polymeric matrix augmented the polymer’s rigidity, increasing the value of the modulus in relation to the pure polymer. This phenomenon has also been reported by other research­ers who studied the effect of wood flour on mixtures of recycled polystyrene and polyethylene63,64.

and virgin polystyrene65. The Flexural modulus of composite with DDGS is sig­nificantly higher than all the other ones, may be, this is likely due to the oils being removed in the DDGS material. So, we can notice that the initial chemical treat­ments on DDGS certainly has a positive effect on the improvement of mechanical properties (FM and IS) that are higher than those of untreated wood. These results are in agreement with those of Julson et al61. The improved dispersion obtained from the composite with treated DDGS was also responsible for the highest flex­ural modulus. For none coupled composite (wood and PP only), the filler particles began to form aggregates. Direct physical bonds between filler particles are weak and, thus, easily broken during tensile loading, which explains the decrease in the flexural modulus (WPM). Compatibilizers can change the molecular morphology of the polymer chains near the fiber-polymer interphase. Yin et al.66 reported that the addition of coupling agent (MAPP) even at low levels (1-2%) increases the nucle — ation capacity of wood-fibers for polypropylene, and dramatically alters the crystal morphology of polypropylene around the fiber. When MAPP is added, surface crys­tallization dominates over bulk crystallization and a transcrystalline layer can be formed around the wood-fibers. Crystallites have much higher moduli as compared to the amorphous regions and can increase the modulus contribution of the polymer matrix to the composite modulus67. The flexural modulus of the composites can be correlated with the morphology of these ones. Composites whose surfaces are smoother and more homogeneous exhibit the greatest flexural modulus. The resul­tant increase in flexural modulus properties (Fig. 12.8) can be explained on the basis of improved wettability (compatibility) of the reinforcement fibers with the polymer matrix. The increased compatibility is obtained by reducing the polarity of the wood fiber surface nearer to the polymer matrix.

The mechanical results of this study show that loading of PP with these natural fibers leads to a decrease in tensile and impact strength of the pure polymer. On the other hand, the flexural modulus increases due to the higher stiffness of the fibers. The significant improvements in flexural properties of the blends composites made with MAPP and reinforcing fillers were further supported by SEM micrographs.

12.2 CONCLUSION

Wood fibers, rice husk and DDGS, which originate from renewable resources, are an interesting alternative to mineral fibers. All these samples with reinforcement exhibited a markedly heterogeneous and highly rough fracture surface with large voids, or cavities, around the filler particles due to the accumulation of stresses in the particle-matrix interface zone. This produced an adverse effect on mechanical properties such as tensile strength and impact resistance.

The SEM micrographs reveal that interfacial bonding between the treated filler and the matrix has significantly improved, suggesting that better dispersion of the filler into the matrix was achieved upon treatment of rice husk and DDGS. The ther­mal properties revealed the strong nucleation ability of the reinforcement flour and MAPP on PP crystallization. Crystallization of all the composites with the coupling agent MAPP only or with reinforcement began earlier compared to that of pure PP. This suggested that MAPP and organic reinforcement acted as nucleation agents and were responsible of the shift of crystallinity towards higher temperatures.

Tensile and impact strength exhibited a marked downward tendency as reinforce­ment was loaded. This is due to the weak interfacial adhesion and low compatibility between matrix and filler. The weak bonding between the hydrophilic lignocellu- losic agent and the hydrophobic matrix polymer obstructs the stress propagation and causes the decrease of these properties. These properties were not significantly affected by changing rice husk by DDGS. However, the flexural modulus for all composites with organic reinforcement is higher than the value for neat PP, as a consequence of the high modulus of cellulosic agent.

In summary, the use of organic reinforcements as fillers to polymer matrix com­posites proved to be a viable alternative. Reductions in tensile and impact strength properties reported with the addition of fillers may be tolerable to some applica­tions. Increment in flexural modulus is achieved in all cases. The development of alternatives for recycling rice husks and DDGS as reinforcements in polymer matrix composites is an important step to provide a good destination for these wastes and opens an opportunity to produce a new value added product. This development can permit reduction of production costs and less use of wood in composites based on polymer matrix, especially with the scarcity of wood across the world. This novel application of rice husk and DDGS for bio composites has significantly higher eco­nomic value than its traditional use as a feed stuff. The distillers’ dried grains with soluble (DDGS) from corn ethanol industry; the rice husk products show immense opportunities in engineering new green composites when integrated with thermo­plastics. The properties of PP composites can be adjusted by mixing different rein­forcements species for filler blend.

12.3 FUNDING

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.