Material PropertiesMorphology and properties of super-toughened bio-based poly(lactic acid)/poly(ethylene-co-vinyl acetate) blends by peroxide-induced dynamic vulcanization and interfacial compatibilization
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.
Section snippets
Raw materials
The PLA used in the study was commercial grade (PLA 4032D) obtained from NatureWorks LLC (Minnetonka, MN). Its glass transition temperature was Tg ≈ 60 °C, and its melting temperature was Tm ≈ 170 °C. Rubber grade EVA (Elvax® 250) with vinyl acetate (VAc) content of 28 wt% and melt mass-flow rate (190 °C, 2.16 Kg) of 25 g/10 min was supplied by DuPont Company. Chloroform, n-hexane and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (AD, purity of 93%) with a typical half-life time of approximately
Dynamic vulcanization of the PLA/EVA/AD blends
In the presence of free radicals obtained by the decomposition of the 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (AD), both PLA and EVA can form macro-radicals via hydrogen abstraction leading to a three-dimensional network structure. Simultaneously, PLA-g-EVA copolymers may also be produced at the interface between PLA and EVA. The typical chemical route employed in this study is shown in Scheme 1; complex reaction products could be obtained including cross-linked EVA, cross-linked PLA and
Conclusions
A commercial commodity copolymer, EVA, was used to toughen PLA by AD-induced dynamic vulcanization and interfacial compatibilization. Super-toughened PLA/EVA blends with moderate strength and stiffness were successfully prepared. The PLA reacted with EVA during melt-blending, in the presence of AD, which resulted in an improved interfacial compatibilization. The compatibility between PLA and EVA is improved with increasing AD content. Results of torque analysis during mixing and gel content
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