Bio-based Blends of Wheat Gluten and Maleated Natural Rubber: Morphology, Mechanical Properties and Water Absorption

Wheat gluten (WG) is a complex protein derived from wheat. WG-based plastics are potentially used as substitutes for conventional petroleum-based plastics. However, the WG plastics are brittle and have high moisture absorption after being processed. In order to reduce stiffness and increase water resistance of the WG plastics, in this work, maleated natural rubber (NRMA) was blended with the WG. Grafting MA on the natural rubber (NR) is able to increase polarity of the NR, thereby increasing compatibility between the WG and NR. The NRMA was prepared by grafting maleic anhydride (MA) on natural rubber (NR) in molten state in an internal mixer at 120 °C with no initiator for grafting reaction. Under the grafting condition used in this work, it was found that the grafted MA content on the NR is about 1.78% determined by alkaline titration. Sulfur as a curing agent in the rubber phase was used to prepare WG/NRMA blends with various weight ratios (100/0, 90/10, 80/20, 70/30, 60/40 and 50/50). Mechanical properties, impact strength, water absorption and morphologies of the molded WG/NRMA blends compared with WG/NR blends were investigated. Incorporation of the NRMA into WG improved flexibility and water absorption resistance of the polymer blends with respect to the neat WG. Moreover, the WG/NRMA blends provided higher impact strength and mechanical properties, especially modulus and tensile strength than those of WG/NR, indicating more compatibility between the WG and NRMA comparing to NR, resulting in an improvement of mechanical properties in the WG/NRMA blends.