Effect of thermal modification on physical and mechanical behavior of sunflower-polypropylene composites

The Use of agricultural plants wastes as an alternative forest resources in the cellulose industry can be recommended due to the severe shortage of wood raw material and restriction of forest harvesting. The aim of current research was to investigate the effect of thermal modification on applied properties of composite made from sunflower stem flour-polypropylene. Thermal modification of sunflower stems was done at temperatures of 160, 180 and 200°C for 30 minutes and mixed with polypropylene in ratio of 30wt%. Test samples were prepared with maleic anhydride-grafted polypropylene as coupling agent by injection molding method. FTIR spectroscopy of sunflower stem flour confirmed the chemically changes due to thermal modification. More integration and uniform distribution, and less cavities were observed in the scanning electron micrographs of failure surface. The removal of hemicellulose of sunflower stem flour improved the thermal stability of composite, which thermal gravimetric analyzer (TGA) confirmed it with increasing in modification temperature up to 180°C. Modification at 200°C decreased thermal stability compared to other temperatures. Thermal modification leads to significant decrease in water absorption and improvement in bending and tensile properties due to hemicellulose degradation, increasing the crystallinity, loss of polarity, increased compatibility between the two phases of composite and uniform distribution of fillers in polymeric matrix. Notched impact strength of modified composites decreased due to better adhesion between lignocellulosic filler and polymer that facilitate the crack development along the interface