Hydroxylated Natural Rubber Effect on Crystallinity and Mechanical Properties of PLA

Abstract

This work focuses on the effect of functionality of hydroxylated natural rubber (HNR) and its loading on crystallinity and mechanical properties of polylactic acid (PLA). HNR with hydroxyl groups of 9.09% by mole was prepared from natural rubber latex via epoxidation reaction and then the epoxy rings were cleavaged to produce the hydroxyl groups by using heating at the temperature of 70 °C. HNR4 with the hydroxyl groups of 15.25% by mole was prepared from ONR4 latex with M̅n of 3.94 × 103 g/mol and M̅w of 5.06 × 103 g/mol. PLA was then blended with the obtained HNR or HNR4 with different weight ratios of PLA:HNR = 70:30 and 60:40. It was found that hydroxylated natural rubber increased the crystallinity of molded PLA sheet from 7.54% to 18.20% for molded PLA:HNR = 70:30 sheet, to 25.16% for molded PLA:HNR = 60:40 sheet and to 24.38% for molded PLA:HNR4 = 70:30 sheet. Morphological analyses revealed the appearance of co-continuous phase in the molded PLA:HNR4 = 70:30 clearer than the molded PLA:HNR = 60:40 and molded PLA:HNR = 70:30 sheets. As a result, it clarified that the higher functionality of hydroxyl groups based on lowering the molecular-weight enhanced the compatibility between PLA and HNR. On the other hand, the mechanical properties of PLA:HNR4 = 70:30 were found to be the highest tensile strength (14.46 MPa), elongation at break (13.62%) and modulus (4.10 MPa) though PLA:HNR = 60:40 gave the highest impact strength of 163.98 J/m. This was believed that the hydroxylated natural rubber enhanced the crystallinity and impact strength of PLA through the interface interaction and/or chemical bonds between hydroxyl groups of HNR and carboxyl terminal groups of PLA.