Poly(butylene adipate-co-terephthalate)/thermoplastic starch/zeolite 5A films: Effects of compounding sequence and plasticizer content

Highlights

• Compounding sequence and glycerol content affected PBAT/TPS/Zeolite 5A composite.

• Blending PBAT with TPS/Zeolite 5A improved the dispersion of TPS and Zeolite 5A.

• Increasing glycerol content reduced the viscosity and size of TPS dispersed phase.

• Mixing was improved by blending PBAT with TPS/Zeolite 5A at high glycerol content.

• Enhanced mixing improved tensile and oxygen barrier properties of PBAT/TPS blend.

Abstract

This work investigated the effect of the compounding sequence and the glycerol content on poly(butylene adipate-co-terephthalate)/thermoplastic starch/zeolite 5A (PBAT/TPS/Z5A) composites. The composite pellets and films were prepared by an extrusion process using a PBAT:TPS ratio of 60:40, Z5A loading of 3 wt%, and glycerol contents of 35 and 40 parts per hundred parts of starch (phs). Prior to blown film extrusion, the composite pellets were produced by two compounding sequences: sequence I (SI)–mixing PBAT with Z5A prior to blending with TPS; sequence II (SII)–mixing TPS with Z5A before blending with PBAT. The SII compounding sequence provided improved mixing between PBAT and TPS, leading to increased continuous phase region and a reduced TPS dispersed phase size. Increasing the glycerol content decreased the viscosity and size of the TPS dispersed phase and gave rise to a more uniform dispersion of the TPS domains and Z5A particles. Compounding Z5A via the SII sequence with a glycerol content of 40 phs effectively improved the mixing and the performance of the PBAT/TPS blend.

Introduction

Plastic waste is a growing problem worldwide, especially waste from single-use plastic packaging. To reduce the environmental impact of plastics, biodegradable polymers are being considered as alternative materials to replace conventional petroleum-based plastics. However, commercially available biodegradable polymers are generally more expensive than commodity plastics. Efforts have been made to blend biodegradable polymers with low-cost materials such as thermoplastic starch (TPS). Commercially available biodegradable polymers include poly(butylene adipate-co-terephthalate) (PBAT), polylactic acid (PLA), polycaprolactone (PCL), and poly(butylene succinate) (PBS). PBAT is an aliphatic–aromatic copolyester with high toughness and flexibility, resulting in mechanical characteristics similar to those of low-density polyethylene (LDPE) [1]. However, PBAT is currently more expensive than conventional plastics [2]. An alternative approach to creating biodegradable polymers could be to blend PBAT with a low-cost biodegradable material, such as TPS [1,[3], [4], [5], [6], [7]]. TPS is produced by the same processes used for petroleum-based plastics, but it still has limited applications due to its moisture sensitivity, poor tensile properties, and the occurrence of starch retrogradation [8,9]. Blending with PBAT could be an alternative to improve the performance and stability of TPS. However, immiscibility and phase separation between hydrophobic PBAT and hydrophilic TPS often occurs and leads to poor mechanical properties [1,3,4]. The incorporation of nano- and micro-inorganic fillers, such as zeolites and clays, might be an interesting approach to improve the mixing and mechanical properties of polymer blends [2,5,[10], [11], [12], [13], [14]].

Zeolites, which are nanoporous crystalline aluminosilicate materials, have been used as fillers to improve the physical and mechanical properties of polymers [15,16] and to enhance the mixing of polymer blends [12,13]. Djoumaliisky and Zipper [12] reported that adding 1–2 wt% of natural zeolite (clinoptilolite) into a polyolefin blend consisting of polypropylene, low-density polyethylene, high-density polyethylene, and polystyrene (PP/LDPE/HDPE/PS) resulted in improved Young's modulus and impact strength. Thipmanee and Sane [13] found that incorporating zeolite 5A (1–5 wt%) enhanced the dispersive mixing and tensile properties of a PE/TPS blend. Furthermore, improving the mixing of polymer blends can be achieved not only by incorporating a compatibilizer but also by tuning both the viscosity of the blend components and the compounding sequence. Some research groups have investigated the effect of glycerol on the properties of TPS-based blends [17,18]. Martin and Avérous [17] found that increasing the glycerol content in a PLA/TPS blend led to a decreased size of the TPS dispersed phase. Furthermore, Rodriguez-Gonzalez et al. [18] reported that increasing the glycerol content led to the decreased viscosity of TPS and improved the distribution of the TPS dispersed phase within the PE matrix. The effects of the compounding sequence and the chemical affinity between fillers and polymers on the mixing and properties of blend composites have been reported by As' habi et al. [19], Jarnthong et al. [20] and Thipmanee et al. [14]. Furthermore, an improved distribution of the dispersed phases (i.e., filler and polymer) effectively enhances the mechanical and thermal properties of polymer blends. Thus far, to the best of our knowledge, no studies have focused on the effects of both the plasticizer content and the compounding sequence on TPS-based blends.

The aim of this work was to improve the mixing of a PBAT/TPS blend by tuning the viscosity of TPS and optimizing the compounding sequence of zeolite 5A (Z5A). The effect of the glycerol content and the Z5A compounding sequence on the morphological, rheological, thermal, mechanical, and oxygen barrier properties of the PBAT/TPS blend were systematically investigated.