High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers

A new method for the preparation of coatings based on renewable biomaterials such as castor oil and birch flour is suggested in this study. The introduction of birch flour in a polyurethane matrix synthesized from castor oil and oligomeric methylene diphenyl diisocyanate (MDI) leads to a more than doubled value of tensile strength and almost doubled strength of adhesion to steel at 20 wt.% loading. The composite with such level loading has tensile strength equal to 7.1 MPa at an elongation at break of 31%, with an adhesion to steel of 3.71 MPa. Hence, the use of such level loading allows for an increase in tensile strength of 887.5% in comparison with that of polyurethane based on neat (as received) castor oil, leading to a decrease in the value of elongation at break. The adhesion to steel of these composites increases by 185.5% in comparison with starting polyurethane. FTIR and SEM studies identified the mechanism of the reinforcement effect of birch fibers. This reinforcement is based on the good wetting of birch fibers by polyurethane with the formation of chemical bonds between them, and the cellulose and lignin components of wood fillers. As a result, we obtained cheap bio-based coatings with acceptable mechanical and adhesion properties.