Biodegradable starch / polyvinyl alcohol composites produced by thermoplastic injection containing cellulose extracted from soybean hulls (Glycine max L.)

Soybean hull is an agro-industrial residue of extensive generation in the soybean oil-producing industry, where approximately 5% of the processed volume of soy beans is converted into hulls. The main use for soy hulls is animal feed, being of great importance to find more ways of consumption for the hulls. This research proposed a noble application for soy hulls: using the hulls as a source for extracting cellulose fibers. Cellulose fibers were extracted from the hulls by alkaline treatment, ground (< 600 µm) and added to thermoplastic starch (TPS). Polyvinyl alcohol (PVA) was also added to the materials produced in order to promote better compatibility between the components. Ten formulations were produced, containing between 4 and 19 wt% of cellulose, and 2 controls. Half of the formulations contained 6 wt% PVA. Cellulose fibers and soybean hulls were characterized by FTIR-ATR and SEM, and the analyzes showed partial purification of the cellulose fibers, with a purity of 92 wt%. The composites were produced by extrusion followed by injection, and characterized by stress-strain, FTIR-ATR and dimensional stability after injection (shrinkage and warpage) tests. Cellulose fibers did not increase the tensile strength (TS) of the composites, but reduced the deformation capacity by 62% (to 19% of fibers) compared to the control. Cellulose fibers together with PVA increased the TS by up to 22% (to 19% fibers and 6% PVA in the composition), making the material more mechanically resistant. For this same formulation, the deformation capacity was reduced by a maximum of 75%, and the modulus of elasticity was increased by a maximum of 230% (the composite became more resistant and rigid). Cellulose fibers reduced the shrinkage value of the specimens, reducing the shrinkage value to less than 1% for the formulations containing 19% cellulose fibers. The cellulose fibers associated with 6% PVA had a synergistic action in the shrinkage of the specimens, further reducing the shrinkage in length (0.4% length shrinkage for 15% cellulose fibers; 0.1% length shrinkage for 19% cellulose fibers). The addition of cellulose fibers and 6% PVA to TPS made the composites more rigid and mechanically reinforced, and also with greater dimensional stability after injection.