Foaming Behaviors and Mechanical Properties of Injection-Molded Polylactide/Cotton-Fiber Composites

In this study, we investigated the effects of short shot size, lubricant, and cotton fibers on the foaming and mechanical properties of poly(lactic acid) (PLA). The short shot filling capacity influenced the expansion ratio and void fraction of PLA and PLA/cotton-fiber dramatically, a set shot size of 45 cm3 (80%) filled part presented a relatively uniform cell morphology and the improved impact strength, while 38 cm3 (70%) filled ones exhibited an inferior cell structure, low tensile strength, and impact strength. Because of the microsize and the difficult dispersion of the cotton fiber, the tensile properties of all fiber filled solid and foamed samples decreased. Compared to the solid injection molded neat PLA, an impact strength of 52 cm3 (100%) fully filled and 45 cm3 filled foaming neat PLA climbed from 32.48 to 42.36 and 47.40 J/m, respectively. Moreover, at the 45 cm3 filled level, the impact strength of 10 wt % cotton fiber added composite is 31.24 J/m, a 3.8% slight decrease was calculated compared with the solid neat PLA. The composite with a higher fiber filling content of 20 wt % resulted in a severe cell deformation and obvious less cell asymmetry, and a less impact strength of 28.65 J/m was obtained. The filling of the cotton fiber could slightly change the crystallization behavior of PLA through the injection foaming process, and the increasing of crystallinity was limited.