Crystallization behavior of PLA in PLA/NR compared with dynamic vulcanized PLA/NR

บทคัดย่อ

PLA has been an interesting biopolymer widely used in various applications. It has been widely known that crystallization of PLA is rather slow. After melt processing, crystallinity of PLA could be lost. The study of PLA added with fibers, particles or even nano-particles has been studied extensively. Nonetheless, the suffering in some mechanical characteristic such as toughness has been encountered. Our research was aimed to enhance toughness of PLA by melt blending NR, and dynamic vulcanized NR into PLA using an internal mixer. PLA:NR and PLA:NR vulcanizate with the ratio of 90:10, 80:20, 70:30, 60:40 and 50:50 were studied. Crystallization behavior and thermal stability of the blend were investigated using DSC and TGA respectively. It was found that, for the former systems, at low content of NR, i.e. 10-30 %, rubber particles were found to be presented as spherical disperse phase in PLA. Meanwhile at higher content, such as 40-50 % NR, polymers were found as co-continuous. Having NR in PLA did not affect T_g of the plastic but could induced cold crystallization and subsequently the crystallinity of the plastic. This resulted in the change of melting temperatures of PLA. Surprisingly, at the temperature higher than T_m of PLA, exothermic behavior upon heating was found, at the content at 10-30 % of NR. This is the cross polymerization between NR and PLA molecule and cause the rapidest degradation temperature of polymer blend to be increased. In the case of PLA:NR vulcanizate, dispersed NR particles were also present in PLA continuous phase. But the particles were found larger at the higher content of NR. The effect of NR particle onto the crystallization of PLA was found to be similar to the previous case. However, the cross polymerization between two polymers was not found in DSC thermograms. This also resulted in the two distinct degradation steps in the blends and less stability of PLA upon TGA than the previous system.