Optimal mechanical properties of biodegradable natural rubber-toughened PHBV bioplastics intended for food packaging applications

Abstract

Abstract Incorporation of natural rubber (NR) into poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) through melt blending improves PHBV flexibility and toughness but sacrifices tensile strength, due to low NR modulus and insufficient compatibility between NR and PHBV. These unbalanced mechanical properties restrict NR/PHBV use in packaging applications. A response surface methodology (RSM), with a Box-Behnken design, was used to optimize the mechanical properties of PHBV/NR blends enhanced with synergistic use of TMPTA coagent and peroxide. Notched impact strength (toughness) of the blends increased with increasing coagent and peroxide, maximal at 15 wt. % rubber loading. Tensile strength increased with increasing coagent, decreased with increasing rubber content, but was independent of peroxide. In contrast, flexibility, which also was independent of peroxide, decreased with coagent and increased with rubber loading. The optimal PHBV/NR blend was obtained at 15 wt. % NR, 4.2 phr peroxide, and 3 phr coagent, with a tensile strength of 28.1 MPa, notched impact strength of 27.5 J/m, flex modulus (1% secant modulus) of 8,679 MPa. The blends degraded by ˜15% in 53 days in a lab-scale aerobic composting system at 58 °C. Trays made from the optimized PHBV/NR blend had water vapor permeability and sealability comparable to polypropylene (PP), and migration studies indicated that the trays were safe for food-contact applications.