Study On The Aging Characteristics Of Wheat Straw Reinforced Recycled Polypropylene Composites

Using waste plastic and wheat straw to prepare wood plastic composites(WPC)plays an important role in developing low carbon economy of China.In this study,recycled disposable packaging bags(RPP-DB)and recycled woven bags(RPP-WB),were used as matrix and wheat straw(WS)was used as filler to prepare WS/RPP-DB and WS/RPP-WB composites.From morphology,crystal structure,functional groups,physical properties(color,weight,thermal stability,moisture absorption),mechanical properties and friction and wear properties,systematically studied the aging mechanism of WS/RPP composites under accelerated aging and outdoor weathering.In order to explore the relationship between the accelerated weathering and outdoor weathering,the gray system was introduced.The response surface methodology was applied to comprehensive analysis the interaction of antioxidants,calculated the optimum anti-aging formulation of WS/RPP composites and compared the anti-aging properties with commercial WPC.The effect of wheat straw characteristics on the performance of WS/RPP composite was also studied.The main results were as follows:(1)The aging process of WS/RPP-DB was gradually from surface to interior,while the internal collapse aggravated the aging process of WS/RPP-WB.The carbonyl index increased with aging time,and the lignin index decreased with aging time.The crystallinity increased at first and then decreased with aging time.Fading and weight change were phased and related with surface crack and fractures.Thermal analysis results indicated that RPP-DB was better than RPP-WB at protection WS,and also proved the aging process of WS/RPP-DB and WS/RPP-WB composites.The diffusion coefficient and maximum moisture absorption increased at first and then decreased with aging time.MOR and MOE decreased significantly after aging which was related with surface cracks and also affected by surface crack and fractures.The coefficient of friction and wear mass loss of WS/RPP composites increased with loading level and aging time.The primary wear mechanisms were fatigue and abrasive wear.(2)The aging process under outdoor weathering was similar to that under accelerated aging.The carbonyl index was influenced with climate factors and lignin index was decreased with aging time.The crystallinity of WS/RPP-DB increased at first and then decreased with aging time,while the crystallinity of WS/RPP-WB decreased all along with aging time.Fading increased with aging time.Weight change was affected by complex climate factors.The rate of decline in the mechanical properties of WS/RPP composites was very fast under outdoor weathering conditions,in which WS/RPP-WB lost almost all of the mechanical properties after 180 d.In outdoor environment WS/RPP composites were affected by solar radiation wavelength,total amount of radiation,precipitation and environmental pollution,which could exacerbate the aging process.Accelerated aging had lager gray correlation in simulating the mechanical changes under outdoor weathering.Only use time or radiation to analyze the correspondence between the two kinds of aging conditions was one-sided.(3)Antioxidants played a role in slowing aging process of composites.WS/RPP-DB was more sensitive to antioxidant than that of WS/RPP-WB.The interaction of 1010 and DSTP was non-uniformity synergy,and that of 1010 and UV-327 was enhancing synergy.The optimized formulation was 1010 1.56 wt%,DSTP 0.8 wt%,UV-327 0.67 wt%.The optimum formulation composites exhibited better performance on color change,mechanical,thermal stability and crystalline properties.Comparing with commercial WPC under the same conditions of accelerated weathering,the optimum formulations WS/RPP composites had good morphology,mechanical properties and their color change was close to commercial products,which indicated it had some practical value.(4)Cellulose could significantly improve the mechanical properties and water absorption of composites.Lignin could increase the fiber-matrix interfacial bonding,improve banding strength and flexural modulus and has a positive effect on the thermal stability and water resistance.After accelerated weathering,cellulose group had obvious cracks on surface,while the surface structure of lignin group was flat and complete.Adding cellulose had positive effect on preventing fading,while adding lignin would aggravate fading.Continuously increasing cellulose content would lead to significant decline in the mechanical properties.An appropriate amount of lignin could play a role in anti-oxidation and improve the mechanical properties after weathering.However,an excess amount of lignin would accelerate aging process of composites.The composites with 5%lignin and light stabilizer had better thermal stability and antioxidant properties after weathering.The composites filled with NaOH combined treated and acetic acid combined treated wheat straw fibers had good mechanical properties and anti-water absorption and anti-moisture absorption performance,followed by the composites filled with hydrothermal combined treated and steam explosion combined treated wheat straw fibers.After combined treatment,the surface of wheat straw became rough,so that the interfacial adhesion of straw fibers and PP matrix was improved.In the study of effects of different surface treatments on WS/RPP composites,NaOH,hydrothermal and steam explosion treatments had significant impact on the chemical composition of wheat straw fiber.The tribological behavior of PP composites filled with combined treated fibers excelled than that of the composites filled with coupling agent treated fibers.The main wear mechanism of the worn surfaces was abrasive wear and adhesive wear.It was found that the composites filled with hydrothermal combined treated and steam explosion combined treated wheat straw fibers had good tribological behavior.