Properties of accelerated weathering of tea stalk/polypropylene composites

Abstract: Wood plastic composite has gradually gained importance recently all over the world. The composite manufacturers continue to search for new and cheaper lignocellulosic materials or annual plant residues as an alternative to wood. Tea is the most popular beverage in the world. The worldwide consumption of tea is about 4.5 million tons annually, which is far more than that of coffee, beer, wine and carbonated drinks. It always ends up with lots of waste tea leaves, stalks and dusts daily during the process of tea production and consumption. The tea residue is made of fibrous biomass, which brings the increasing problems of solid waste and disposal in many countries. Studies have been conducted to determine potential uses of waste tea residues such as adsorbent for synthetic dyes and heavy metals, fertilizer, mushroom growing medium, energy source, livestock and poultry feed, and particleboard. The tea residue to use as organic filler in polymer composites has showed a great prospect because of its lignocellulose behavior and low cost, but there have been few researches about the work, and no researches about the outdoor application or weathering mechanism of polymer composites by using waste tea residues as organic filler so far. Because the chemical compositions and fiber morphology of tea fiber in the waste tea residue were quite different from that in wood residue, in order to make high-valued use of the waste tea residue, the wasted tea stalk/Polypropylene (TS/PP) composites were manufactured by open mill and injection molding machine, with maleic anhydride grafted polypropylene (MAPP) as coupling agent. At the same time aimed to explore the outdoor application and weathering mechanism of TS/PP composites, the effects of freeze-thaw cycles accelerating weathering on the mechanical properties, color change and thermal behavior of the composites were investigated, the surfaces morphology of composites were investigated by SEM (scanning electronic microscopy), and the chemical structures and weathering mechanism were also analyzed by FTIR (Fourier transform infrared spectrum) and TGA (thermogravimetric analysis) The results showed that the TS/PP composites exhibited great color change and large decrease in mechanical properties. The corresponding lightness change ?L*, redness change ?a*, yellowness change ?b* and the color change ?E were 16.41, 2.80, 9.03 and 18.93, respectively after freeze-thaw cycles for 12 times. The retentions of bending, notched impact strength, flexural modulus and hardness of the composites were 28.9%, 40.1%, 34.5% and 86.5%, respectively after freeze-thaw cycles for 12 times. The vicat softing point and the largest degradation temperature of the composites were decreased by 2.9 and 4℃ respectively. The IR (infrared spectrum) absorption peak corresponding to tea stalk became weaken, which indicated the degradation of lignin in tea stalk during the freeze-thaw accelerating weathering. But the largest degradation temperature and IR absorption peak corresponding to PP component in TS/PP composites changed little after freeze-thaw cycles. The SEM (scanning electronic microscopy) images indicated that the micro cracks and folds appeared first on the surfaces of weathered composites after freeze-thaw cycles; and then the width, depth and quantity of cracks were increased with the increasing of the number of freeze-thaw cycles. There even appeared cross-like cracks or holes on the surface of the TS/PP composites when weathered by freeze-thaw cycles for 12 times.