Straw/Nano-Additive Hybrids as Functional Fillers for Natural Rubber Biocomposites

Currently, up to 215 million metric tons of harvestable straw are available in Europe, 50% of the crops come from wheat, 25% from barley and 25% from maize. More than half of the production remains undeveloped. The overproduction of straw in the world means that the current methods of its management are insufficient. The article describes the production method and characterization of natural rubber biocomposites containing cereal straw powder modified with functional nano-additives in the form of carbon black, silica and halloysite nanotubes. The use of cereal straw in the elastomer matrix should contribute to obtaining a product with good mechanical properties while ensuring a low cost of the composite. In turn, the application of the mechanical modification process will allow the combination of specific properties of raw materials to obtain new, advanced elastomeric materials. As part of the work, hybrid fillers based on mechanically modified cereal straw were produced. The impact of hybrid fillers on mechanical, rheometric and damping properties was assessed. The flammability and susceptibility of the obtained biocomposites to aging processes were determined. The use of hybrid fillers based on mechanically modified straw allowed us to obtain a higher cross-linking density of vulcanizates (even up to 40% compared to the reference sample), and thus higher values of the rheometric moment during the vulcanization process of rubber mixtures (from approx. 10% (10 phr of filler) up to 50% (30 phr of filler) in relation to the unfilled system) and higher hardness of vulcanizates (by about 30–70%). The curing time of the blends was slightly longer, but the obtained composites were characterized by significantly higher tensile strength. The use of fillers in the elastomer matrix increased the modulus at 100, 200 and 300% and the elongation at break. Moreover, greater resistance of vulcanizates to the combustion process was confirmed.