Low-Temperature Sintering of Stereocomplex-Type Polylactide Nascent Powder: From Compression Molding to Injection Molding

Stereocomplex (SC) crystallization between poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) exhibits tremendous potential to improve the properties of polylactide (PLA), but the practical processing of SC-type PLA (SC-PLA) is significantly hindered by the insufficient melt stability and serious degradation after complete melting of SC crystallites over 240 degrees C. In a previous work, powder metallurgy inspired low-temperature (180-210 degrees C) compression sintering is proposed to fabricate SC-PLA products from its nascent powder. However, its industrial application is still challenging mainly due to the low production efficiency (half an hour is required) as well as the weak ability to fabricate products with complex shapes. In this work, an effort to rapidly fabricate complex-shaped SC-PLA products using low-temperature injection molding technology is reported, where SC-PLA nascent powder is partially melted. To ensure appropriate powder flowability, small amounts of glyceryl monostearate are incorporated as a lubricant. The results show that, both the self-nucleation and flow-induced nucleation effects facilitate the SC crystallization across the interfaces between adjacent unmelted powder particles, which considerably shortens the sintering cycle to about 2 min. In this way, complex-shaped SC-PLA products (such as gears) with superb performance are successfully fabricated by injection molding for the first time.