Renewable proteins from agricultural and animal sources as innovative multifunctional bio-fillers for rubber composites: A comprehensive review

There is a growing need to find novel, sustainable solutions for elastomer reinforcement. Many biomass polymers have been used for this purpose, including cellulose, starch, lignin and protein. Among these, proteins are an under-explored solution, although they are promising materials due to their abundance in agricultural and animal by-products and the multifunctional properties they can impart to composites. Proteins are naturally occurring components of Hevea Brasiliensis natural rubber, where many of its peculiar properties have been attributed to these biopolymers, such as strain-induced crystallization, improved resistance to thermo-oxidative aging and vulcanization accelerating effects. The production of 'artificial' rubber/protein composites using these biopolymers offers other advantages such as low cost, lightweight, biodegradability and high availability. This review focuses on wheat gluten, corn zein, keratin and collagen. The incorporation of such materials into elastomeric matrices is challenging due to their hydrophilicity; therefore, dispersion and interfacial adhesion need to be improved to obtain composites with improved properties. In this sense, several strategies have been addressed in the literature to solve this problem, including rubber and/or protein functionalization and different compounding techniques. The aim of this review is to describe and compare these strategies in order to provide a toolbox for optimal composite fabrication.