Evaluation of melt-processed polyhydroxyalkanoates and zinc oxide nanocomposite films as flexible packaging materials

Abstract

Polyhydroxyalkanoates (PHAs) show great potential as renewable packaging materials due to their biobased and biodegradable nature along with high versatility. The copolymers poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(3-hydroxy-butyrate-co-3-hydroxyhexanoate) (PHBHHx) are the most promising because of increased flexibility and intermediate barrier properties, depending on the copolymer content. However, the relation between melt processing parameters and mechanical performance remains to be elucidated in order to optimize processing conditions. In addition, the incorporation of ZnO nanoparticles could further improve the required thermal stability and barrier properties. Therefore, the aim of this study is the incorporation of ZnO into a suitable PHA matrix, targeting sufficient flexibility while minimizing embrittlement by crystallization. In this study, process parameters such as extrusion and mold temperature, screw speed and cooling time were systematically varied in a full factorial design of experiments. The injection molded PHBV samples show increased tensile strength and Young's modulus under a critical set of mold temperatures. The improved mechanical properties are in parallel with increased crystallinity caused by lower cooling rates at high mold temperatures. However, the stretchability of PHBV with low valerate content (<3%) remains insufficient compared to PHBHHx, limiting its applicability to high stiffness and strength applications (e.g. cutlery). Therefore, the more flexible PHBHHx copolymer was selected as a matrix for the preparation of 1-10 wt.% ZnO/PHBHHx nanocomposite films via a combination of twin-screw extrusion and compression molding. As such, the feasibility of ZnO/PHBHHx nanocomposite films for flexible packaging with enhanced mechanical, thermal and optical properties was investigated.