Morphological adaptation of expanded vermiculite in polylactic acid and polypropylene matrices for superior thermoplastic composites

Official URL: https://dx.doi.org/10.1002/pc.28108

The morphological transformation of expanded vermiculite (VC) during injection molding with brittle PLA and ductile PP polymers along with its positive and negative contribution to mechanical response is investigated. Polymer/VC mixtures with 30 wt. % VC are prepared by thermokinetic high shear mixing at 4000 rpm without any ex-situ exfoliation agents or compatibilizers. Obtained composite mixtures are then injection molded onto three-point bending and tensile test specimens. Performed mechanical tests suggested a significant increase in tensile (110%) and flexural modulus (112%) of PP30VC samples. Presented fractographic and morphological investigations suggested that the root cause of measured improved tensile (36%) and bending strength (26%) of PP is the fibrillation of VC associated with PP/VC interactions under high shear. A similarly increasing trend for tensile (147%) and bending modulus (137%) was observed for PLA30VC samples. Contrary to PP30VC, a decreasing pattern was present in the case of tensile (−40%) and bending strength (−13%) of PLA30VC. The root cause for such reduction is determined to be (i) in situ exfoliation of VC inside PLA matrix and transformation of VC into micrometer-sized platelets; (ii) evaporation of trapped water/crystalline water in the interlayer region of VC which caused in situ degradation of PLA during manufacturing. Highlights: Vermiculite-loaded PP and PLA composites are produced at high shear. Composites have superior modulus compared to neat polymers. Fibrillation of platelet vermiculite is observed in only the PP matrix.