(130c) Lignocellulosic-Fiber-Reinforced Thermoplastic Composites: Effect of Processing-Induced Stresses On Mechanical Properties

Natural fiber reinforced thermoplastic composites that use distillery or agricultural co-products such as distiller′s dried grains with solubles (DDGS) or sunflower hull (SFH) as reinforcements have emerged as substitutes to glass fiber reinforced composites. These composites exhibit uncompromised mechanical properties at a fraction of the material and processing costs of synthetically reinforced systems. A low-cost commodity thermoplastic matrix enables the incorporation of a wide range of lignocellulosic fibers, including short particulate-type fibers and long bast-type fibers.

In the present investigation, the lignocellulosic fibers, i.e., DDGS and SFH, obtained from vendors are subjected to an appropriate mechanical size-reduction to enable their incorporation into a thermoplastic matrix by melt blending. Following this operation, the fiber surface is treated with a compatibilizing agent, such as maleic anhydride-grafted polypropylene in the case of polypropylene, or graft-polymerization with acrylic acid in the case of poly(methyl methacrylate). The fibers are sequentially compounded with the thermoplastic polymer in a co-rotating twin-screw extruder, water-quenched and pelletized to a required size, and injection molded.

As the polymer/reinforcement undergoes high shear in both the extruder and injection molding under temperatures reaching or exceeding 200 °C, the initiation of thermal and oxidative degradation of the fibers can occur, often indicated by discoloration and odor of the resulting polymer blend. In addition to degradation in strength, the decomposition of the fiber constituents, i.e., cellulose, hemicellulose, lignin, and starch, may lead to an additional decrease in fiber/matrix interfacial strength. This deterioration can result in reduction of the overall composite strength due to the inability of the weakened fibers to transfer load through the matrix. Thermal stability, a function of the extent of thermal and oxidative degradation, is critical and varies with fiber constituents, processing stresses, and processing durations.

In this study, the effect of processing on the mechanical properties is parametrically analyzed. The overall thermal degradation is studied as a function of extruder screw speed and subsequent material dwell time. Static mechanical properties in tension and flexure are correlated with thermal analysis using dynamic mechanical analysis, and molecular structure using Fourier-transform infrared spectroscopy. Scanning electron microscopy is used for fractography and for studying the surface morphology.