Zinc oxide as a new catalyst for poly(lactic acid)/maleated polypropylene reactive blending: an approach to enhance miscibility and mechanical properties

Abstract

To eliminate the defects of polylactic acid (PLA), maleated polypropylene (MAPP) as a toughener in the presence of a new exchange catalyst, ZnO, was applied. By keeping the amount of MAPP constant, samples with different amounts of ZnO nanoparticles (NPs) were prepared and studied by tensile, Izod, differential scanning calorimetry (DSC), melt flow index (MFI), water droplet contact angle (WDCA), transition electron microscopy (TEM), energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses. In comparison to neat PLA, mechanical tests showed that with a slight decrease in elastic modulus and tensile strength, elongation at break and impact strength values increased more than three times for the sample containing 0.125 wt.% of catalyst. DSC analysis indicated that with the catalyst present, the T_g values of both polymers became closer to one another. Additionally, the absence of cold crystallization in PLA and the reduction in T_m and crystallinity of both polymers are evident factors contributing to the best miscibility of the reactive blend. MFI evaluation displayed decrease in viscosity and WDCA measurement indicated the creation of more polar groups on the surfaces of samples containing ZnO. TEM, EDX and XRD analyses proved the appropriate distribution and dispersion of NPs within the blend matrix as well as obvious decrease in the crystallinity of the resulting alloy. All these results demonstrate the better performance of exchange (esterification and transesterification) reactions between the active groups of both polymers (with ZnO presence) and the formation of PLA–MAPP copolymers, resulting in better miscibility and mechanical properties.

Graphical abstract

Possible transesterification reactions during melt blending of PLA and MAPP.