Anti-aging agents improving natural weathering properties of bamboo powder/polypropylene foamed composites

Abstract: Wood-plastic composite (WPC) has environmental and economical advantages and has been extensively applied and studied. Weathering causes negative changes in the surface appearance, mechanical properties and surface chemical properties of the WPC products. Thus, to explore the weathering mechanism of WPC, improve its durability and provide some data and guide for WPC production, the natural weathering performance of bamboo powder/polypropylene (PP) foamed composites was researched. The composites containing 33% bamboo flour, 1% modified AC (azodicarbonamide) foaming agent and different anti-aging agents were prepared by the injection molding. The composites were exposed to outdoor natural weathering from October 2012 to October 2013 in Fuzhou (26°08′N, 119°18′E), the southeast of China. The effects of natural weathering on the composites' color, flexural creep behavior and rheological behavior were investigated. Meanwhile, the effects of the antioxidant B225 and anti-ultraviolet agent UV770 on the composites' weathering performance were also studied. The B225 is a complex antioxidant agent using Tetra-methylene-β-(3, 5-di-tert-butyl-4-hydroxyphenyl)-propionate methan and Tns-(2, 4-di-tert-butyl)-phosphite, with a ratio of 1:1. The UV770 is bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate. The results showed that both the B225 and the UV770 could slow down the color fading and improve the color stability during weathering. After weathering for 12 months, the color change reduced from 22.8 for the composites without adding antioxidant to 14.2, 17.6 and 16.8 for the composites contained with 0.6% B225, 0.6% UV770, 0.3% B225 and 0.3% UV770, respectively. The B225 has more positive effect on color stability of the composites than UV770. According to authors' prior research, the influence of natural weathering on the composites' color was similar with the artificial accelerated weathering. The color change caused by the natural weathering for 12 months was equal to that by accelerated weathering of xenon-arc radiation with water spray for 0.4 months, and also was similar to that by the accelerated cycling of water immersion followed by freeze-thaw cycle at the temperature of -40－60℃ for 22 times. Due to the combined effect of the creep and long-term weathering, particularly at 75% stress level, the creep strain increased, and the flexural modulus and flexural strength declined. The composites containing the anti-aging agents also had higher creep resistance for their decreased creep strain and enhanced residual flexural properties. After creep test for 5400 s at 75% stress level, the control sample weathered for 12 months fractured but the retention rates of flexural modulus and flexural strength of composites with anti-aging agents were 92.2%-96.1%. The rheological results of composites with respect to frequency sweep ranging from 0.01 to 100 Hz at 190℃ indicated that the natural weathering decreased the storage modulus, loss modulus, complex viscosity and relaxation time of composites. The decreased crossover modulus, relaxation time and average molecular weight confirmed the photodegradation of composites caused by natural weathering. Adding anti-aging agents improved both the rheological properties of the unweathered composites and the rheological stability of the weathered ones. The B225 and UV770 had synergistic effect in terms of rheological behavior. The weathered composites contained with 0.3% B225 and 0.3% UV770 showed the highest rheological stability for its higher residual moduli and relaxation time. It is recommended that using these 2 types of anti-aging agents to prepare the WPC.