Performance of bamboo flour/polypropylene foamed composite under accelerated weathering

In order to reduce the density and improve the toughness of wood-plastic composite (WPC), the foamed WPC was made through adding chemical foaming agent in this study. To fully utilize bamboo resources in China and reduce white pollution, the foamed composite with 54% PP and 13% HMSPP containing 33% bamboo powder and 1% modified azodicarbonamide (AC) foaming agent blends by weight was made by injection molding. Furthermore, the aging performance of bamboo powder/polypropylene (PP) foamed composites was studied in order to investigate the weathering mechanism of WPC and to strengthen its production and quality management and thus expand its application fields. The composites were exposed to 1 200 h accelerated xenon-arc radiation with water spray, the mechanical characteristics including bending performance, notched impact strength, and color change of composites were studied. The rheological behavior of composites with regard to frequency sweep ranges from 0.01 to 70 Hz at 195°C was observed. The surface morphology of composites with non-weathered and weathered for 1 200 h were investigated by ESEM and their chemical structures were analyzed by FTIR. The results showed that the mechanical properties of composites decreased significantly for weathering 300 h and decreased continuously with an increase of exposure time. The retentions of residual bending strength, flexural modulus and notched impact strengths were only 79.4%, 68.3% and 75.6% respectively. The weathering also resulted in significant color fading of the composites, especially for the first 900 h. The color began to change slowly within the next 300 h. After weathering for 1 200 h, the color change ΔE*, lightness change ΔL*, redness change Δa* and yellowness change Δb* were 49.0, 48.4, -5.9 and -4.9 respectively. The frequency sweep results indicated that the storage modulus, loss modulus and complex viscosity of composite weathered for 1 200 h decreased and the intersection value of energy storage modulus and loss modulus become lower, and the corresponding frequency was higher. These results indicated that the molecular weight distribution become wider and the molecular weight became lower. The ESEM observation revealed that the surface of weathered composite was no longer smooth; and the cracks and holes appeared and some bamboo fibers exposed. The FTIR analysis showed the new peak of C=O stretching vibration at 1717 cm-1 appeared and C=C absorption peak at 1 459 cm-1 strengthened and C=O stretching vibration peak at 1 059 cm-1 also strengthened. The FTIR result verified the photooxidation and photodegradation of composites for accelerated weathering.