Effect of temperature and durations of heating on coir fibers

Biocomposites derived from polymeric resin and lignocellulosic fibers may be processed at temperatures ranging from 100 °C to 230 °C for durations of up to 30 min. These processing parameters normally lead to the degradation of the fiber's mechanical properties such as Young's modulus (E), ultimate tensile strength (UTS) and percentage elongation at break (%EB). In this study, the effect of processing temperature and duration of heating on the mechanical properties of coir fibers were examined by heating the fibers in an oven at 150 °C and 200 °C for 10, 20 and 30 min to simulate processing conditions. Degradation of mechanical properties was evaluated based on the tensile properties. It was observed that the UTS and %EB of heat treated fibers decreased by 1.17–44.00% and 15.28–81.93%, respectively, compared to untreated fibers. However, the stiffness or E of the fibers increased by 6.3–25.0%. Infra red spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were used to elucidate further the influence of chemical, thermal and microstructural degradation on the resulting tensile properties of the fibers. The main chemical changes observed at 2922, 2851, 1733, 1651, 1460, 1421 and1370 cm−1 absorption bands were attributed to oxidation, dehydration and depolymerization as well as volatization of the fiber components. These phenomena were also attributed to in the TGA, and in addition the TGA showed increased thermal stability of the heat treated coir fibers with reference to the untreated counterparts which was most probably due to increased recrystallization and cross linking. The microstructural features including microcracks, micropores, collapsed microfibrils and sort of cooled molten liquid observed on the surface of heat treated coir fibers from the scanning electron microscope (SEM) could not directly be linked to the effect of temperature and durations of heating although such features may have largely account for the lower tensile properties of heat treated coir fibers with reference to untreated ones.

Research highlights▶ Tensile strength and percentage elongation at break decrease as temperature and durations increase. ▶ The effect of the increase of temperature at the same duration of heating is probably the same to the effect of increasing the durations of heating at the same temperature, provided that the temperature is above the critical temperature for the beginning of thermal degradation of lignocellulosic fibers. ▶ Stiffness is less affected by temperature compared to tensile strength and percentage elongation at break. ▶ By exposing lignocellulosic fibers such as coir fibers to temperatures near 150 °C for durations not above 20 min that could probably be less detrimental to the mechanical properties of the fibers.