Effect of crystallinity on the mechanical properties of starch/synthetic polymer blends

Crystallization behaviour of starch and maleated blends was studied at 50°C over a period of 20 weeks using wide angle X-ray diffraction (WAXS). The variation of mechanical properties (tensile and flexural) and stress relaxation behaviour of the blends stored at 50°C and -10°C were studied over the same period. The starch content in the blends was 70% by weight. The synthetic polyolefins used in the blends were two grades of ethylene-co-vinyl acetates (EVA) containing 28% and 18% VA, two grades linear low-density polyethylene (melt index of 40 and 20) and high density polyethylene. An increase in the tensile properties of all the blends was observed in the first 5 weeks for samples kept at both temperature conditions. Blend samples kept at 50°C had higher tensile strengths than the ones at -10°C. Flexural strength remained constant over the duration of time. Freshly moulded specimens relaxed faster than the samples aged at either temperature. X-ray diffraction patterns showed that the starch was completely melted and had lost its crystallinity. Also, starch blends with EVA did not show any crystalline structure. The crystallinity in the starch blends with polyethylene was mainly due to the crystallinity of the synthetic polymer. The X-ray patterns of pure synthetic polymers were not found to be different from their functionalized counterparts. Crystal intensity was found to decrease for all the polyethylene blends. The effect of crystallinity on the mechanical properties is discussed.