Agricultural proteins as multifunctional additives in ZnO‐free synthetic isoprene rubber vulcanizates

Corn zein and wheat gliadin protein are compounded into synthetic cis‐1,4‐polyisoprene rubber (IR) and sulfur‐cured in a zinc oxide (ZnO)‐free system. The curing kinetics and mechanical and morphological properties are compared to a ZnO‐activated or carbon black (CB)‐reinforced cure system. The proteins provide reversion resistance and reinforcement to IR at filler loadings as low as 1 part per hundred rubber (phr). The zein‐IR composites exhibit higher moduli, better filler–matrix adhesion, and less filler agglomeration/migration than gliadin‐IR because zein is more chemically compatible with IR. The gliadin‐IR composites have a lower percent set and hysteresis, indicating the formation of an elastic restoring gliadin network. Optimal properties are achieved at 2‐phr gliadin and 4‐phr zein. At gliadin loading >2 phr and zein loading >4 phr, the protein domain size increases and mechanical properties deteriorate. At equal filler loading, property improvements over CB‐IR are observed for one or both proteins. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136 , 48141.