Study On The Aging Properties Of Wheat Straw/Pp Wood Plastic Composites

Wood-plastic composites were widely used in construction industry, furniture industry and packaging, with the advantages of renewable, environmental friendly and excellent mechanical characteristics. However, all sorts of environmental factors, including optical radiation, heat, oxidation, moisture, air pollution and microbial corrosion, caused the decrease of the mechanical property and surface quality and the changes of physical and chemical behaviors till the wood-plastic composite loose its use value. Grasping the change discipline of the aging properties had a great significance to improve the aging resistance performance and prolong the service life of wood-plastic composites. In this paper, filled with wheat straw powder modified by silicone hydride coupling agent, ultraviolet absorbent UV327, antioxidant1010and DSTP, wheat straw/PP wood-plastic composites were prepared by mixing compression molding. Composites were ageing by ultra violet (UV) accelerated weathering and natural weathering. The mechanical properties and color change of the original and ageing wheat straw/PP wood-plastic composites were investigated. The ageing mechanism of wood-plastic composite was analysed. Influence of three optical stabilizers on aging properties of wood-plastic composite was investigated through single factor experiment and the optimum formulation was obtained by the response surface methodology. The main conclusions can be drawn as follows:(1) The analysis of the influence of UV accelerated weathering on the properties of composites indicates that the maximum color change AE is27.56when the composites were ageing960h. After that the mechanical properties decreased rapidly. When the aging time was1200h, the flexural strength, tensile strength and impact strength of wheat straw/PP composites decreased by67.2%,47.89%and32.41%respectively. The analysis of infrared spectrum shows that hydroxyl groups of wheat straw fiber could accelerate the photo-oxidation aging of polypropylene. The cracks and defects were discovered on the surface of the aging composites. With ongoing of weathering time, the cracks stretched along the width direction and thickness direction. Moisture, heat and optical radiation caused the degradation of polypropylene and wheat straw fiber, which was responsible for the cracks and defects.(2) The analysis of the influence of natural weathering on the properties of composites indicate that the flexural strength, tensile strength and impact strength of wheat straw/PP composites decreased by41.3%,44.15%and32.45%respectively and the maximum color change AE is24.04when the natural weathering time was180d. The absolute values of chromatism Aa and Ab of wheat straw/PP wood plastic composites ageing by natural weathering increased by72.89%and90.76%, compared with composites ageing by UV accelerated weathering. The degree of flexural strength and color change△E of composites ageing by natural weathering for60d was equivalent respectively to UV accelerated weathering for480h-720h and240h-480h. The degree of mechanical performances and color change△E of composites ageing by natural weathering for120d was similar respectively to UV accelerated weathering for960h-1200h and720h-960h. The degree of ageing properties of composites ageing by natural weathering for180d was similar to UV accelerated weathering for1200h. Surface quality of the wood plastic composites ageing by natural weathering was affected by outdoor environment and worse than composites ageing by UV accelerated weathering.(3) The analysis of infrared spectrum of wheat straw ageing by natural weathering and UV accelerated weathering shows that the absorption peaks of chromophoric groups increased after the composites were ageing and the effect of natural weathering on wheat straw was obvious especially. It was found that the dominant oxidation products of the polypropylene ageing by UV accelerated weathering were ketone and the absorption peak of carbonyl group increased obviously, while hydroxyl groups of wheat straw fiber could accelerate the photo-oxidation aging of polypropylene.(4) The analysis of the influence of different ultraviolet absorbent and antioxidant contents on the aging properties of composite indicate that the mechanical performance of the wheat straw/PP composites’with the single addition of0.5%UV327,1.2%1010or1.6%DSTP were better. However, with the contents of ultraviolet absorbent and antioxidant increased, the mechanical properties descend gradually. The addition amount of UV327,1010and DSTP should be respectively in reasonable range of0.5%～1.5%, 0.4%～1.2%and0.4%～1.6%, but excessive additive took counteraction on the mechanical properties. The surface of composites with antioxidant was integrated and had no significant deficit and cracks compared with the composites without antioxidant ageing by UV accelerated weathering. Contact angle of composites ageing by UV accelerated weathering could be improved greatly with the additions of three kinds of optical stabilizer.(5) The optimum formulated proportion was optimized by the response surface methodology, using color change△E and flexural strength FS as the assessment index. The minimum color change is10.37when the contents of UV327,1010and DSTP were2.5%,1.2%and0.87%respectively. The maximum flexural strength is17.9MPa when the contents of UV327,1010and DSTP were0,0.4%and1.01%respectively.