Morphology and properties of super-toughened bio-based poly(lactic acid)/poly(ethylene-co-vinyl acetate) blends by peroxide-induced dynamic vulcanization and interfacial compatibilization

Abstract

Super-toughened poly(lactic acid) (PLA)/poly(ethylene-co-vinyl acetate) (EVA) blends were prepared via 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (AD) induced dynamic vulcanization and in situ interfacial compatibilization. The effects of AD on the morphology and properties of PLA/EVA blends were studied using a Brabender torque rheometer, gel content test, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) thermogravimetric analysis (TGA) and mechanical properties test. The torque and gel content demonstrated that EVA and PLA was successfully vulcanized in the presence of free radicals obtained by the decomposition of the 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (AD). Additionally, the gel content results indicated that, compared with PLA, EVA is more aggressive with free radicals. The SEM revealed that a relatively uniform phase morphology and good interfacial compatibilization were achieved in the dynamically vulcanized PLA/EVA/AD blends. The interfacial reaction and compatibilization between the component polymers resulted in the formation of super-toughened PLA/EVA blended materials.

Introduction

The widespread use of traditional petroleum-based plastic and rubber has received considerable attention in recent years due to their environmental pollution [1], [2], [3], [4]. To overcome it, the development of new bio-degradability polymer materials to replace the petroleum-based plastic and rubber is proposed [5], [6], [7]. Poly(lactic acid) (PLA) is one of the most extensively studied bio-materials with advantages such as being bio-sourced, bio-degradable, excellent transparency, high strength and stiffness [8], [9].

However, the inherent brittleness of PLA is the major drawback that has restricted its broader use in applications that need plastic deformation at high stress levels [10], [11]. The brittleness notch sensitivity can be improved by copolymerization or blending with other flexible components [12], [13], [14], [15]. Among these approaches, blending PLA with flexible polymers is a practical and economical way to obtain toughened PLA-based products. Various biodegradable or nonbiodegradable polymers, such as poly-[(butylene succinate)-co-adipate] (PBSA) [16], poly(caprolactone) (PCL) [17], poly(butylene succinate) (PBS) [18], natural rubber (NR) [19], polyurethane elastomer prepolymer (PUEP) [7], poly(ether) urethane (PEU) elastomer [10] and ethylene-co-vinyl acetate (EVA) [20], [21], [22], have been blended with PLA, acting as toughening components. However, more often, these PLA-based materials prepared by simple blending suffer from poor impact resistance due to the phase separation and poor interfacial adhesion between the two immiscible components. How to enhance the interfacial adhesion or improve the compatibility between the different components in PLA-based blends is an important issue [13].

Poly(ethylene-co-vinyl acetate) (EVA) is a widely applied ethylene copolymer. With increasing vinyl acetate (VA) content, EVA copolymers change from a semicrystalline thermoplastic material into a rubber and again into an amorphous thermoplastic material. It is known that PLA is miscible with poly(vinyl acetate) (PVAc), and compatibility of PLA and EVA can be achieved by tuning the VA content of the EVA copolymer [23], [24], [25]. Ma et al. [24] studied the effect of VA content of EVA on the properties of PLA/EVA blends and found that EVA with a VA content of 50 wt% is an excellent toughening modifier for PLA.

Dynamic vulcanization refers to a process of selectively vulcanizing the rubber phase during its intimate melt mixing with a nonvulcanizing thermoplastic polymer, leading to a two-phase material in which particulate cross-linked elastomer phases are dispersed in a melt-processable plastic matrix [4], [7], [26], [27]. For PLA/EVA blends, in the presence of peroxide, the reactive melt blending involved simultaneous cross-linking of EVA and interfacial reactive compatibilization between PLA and EVA. Thus, dynamic vulcanization could provide a new route for improving the interfacial compatibility and properties of PLA/EVA blends.

In this study, a super-toughened bio-based PLA/EVA blended material was produced through a dynamic vulcanizing technique using 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (AD) as curing agent. Results of rheological and gel content measurements showed that a crosslinking reaction occurred between the EVA and PLA during the melt blending process. A product of the reaction compatibilized the PLA matrix and EVA phase in situ. To the best of our knowledge, a similar toughening route for super-toughened PLA/EVA blends through dynamic vulcanization has not previously been reported.