Study Of Co-Extruded Hdpe Based Wood Plastic Composites With A Reinforced Shell Structure

The co-extrusion technology of polymer is a method which two or more extruders combined with one die to produce multiple-layer products, thus offering different properties between surface and bulk. Using this technology in wood plastic composites (WPCs) can design and prepare a production with core-shell structure from different materials. If adding some functional materials or agents into the shell structure during processing, it can be expected that the co-extruded WPC would obtain outstanding properties through this technology. Based on these conceptions, this paper firstly studied the preparation and properties of materials in core and shell structure, respectively, and then investigated the manufacture and properties of co-extruded WPCs with a functional shell structure, meanwhile, discussed the improvement of the functional shell structure on properties of co-extruded WPCs. The main content of this paper are shown in below.The research related to materials in core structure. Using high-density polyethylene (HDPE) as plastic matrix and four kinds of natural agricultural fibers (i.e., pine flour, bagasse fiber, rice straw, and rice husk) as infill, and prepare HDPE/natural fiber composites. The use of natural fiber enhanced the tensile and flexural properties of neat HDPE, but decreased the impact strength. The comprehensive mechanical properties of HDPE/natural fiber composites were significantly improved by maleated polyethylene (MAPE). The impact strength was further enhanced with the use of maleated triblock copolymer styrene-ethylene/butylenes-styrene (MASEBS). The dynamic mechanical analysis (DMA) showed that the composites obtained a significant reinforcement by the incorporation of natural fibers. The reduced storage modulus and increased loss tangent showed that MASEBS performed as a flexibilizer in HDPE/natural fiber composites.The research related to materials in a novel shell structure. Microfibrillar composites from HDPE and polyamide-6(PA6), which is an engineering plastic with a high melt point and high strength, were made through a reactive extrusion at the processing temperature of PA6in combination with hot stretching to form microfibrillar composites, and injection molding at the melt temperature of HDPE to form final composites. The scanning electron microscope observations demonstrated the PA6formed into microfibers and in-situ existed in the composite during extrusion, and it exhibited a regular orientation along the stretching direction in the presence of moderate coupling agents. Increasing the loading level of coupling agents caused a reduction in the diameter of PA6microfibers. Incorporation of PA6microfibers led to an improvement in both flexural and tensile properties as compared to the neat HDPE, but decreased the impact strength. The different coupling agents showed distinct contributions in the typical parameters of mechanical properties. The differential scanning calorimeter (DSC) analysis showed that the temperature of crystallization peak of PA6in the composites increased with coupling agents added, while its degree of crystallinity decreased. DMA results showed that the viscous feature is dominant over the elasticity in the coupled composites.The research related to adding reinforced materials into core and shell structure. The HDPE matrix and WPCs were reinforced by a small quantity of Kevlar fiber (KF). The KF was treated in NaOH solution firstly, and then mixed with HDPE through a twin-screw extruder after dried. The blends were secondly melting mixed in a thermo-kinetic high-shear mixer with or without wood flour to make itself disperse homogeneously in the composites. The mechanical properties of both HDPE and WPCs were remarkably enhanced by this kind of low density fiber with a small quantity used, especially flexural strength and tensile modulus. Fourier transform infrared spectroscopy (FT-IR) analysis demonstrated that the amide bond was destroyed after the hydrolysis of KF, and an esterification reaction happened between the hydrolysed KF and MAPE. DMA results showed KF enhanced the stiffness of HDPE, and the use of MAPE increased the viscous characteristic of HDPE/KF composites. Meanwhile, the incorporation of KF in WPCs increased the elastic characteristic of composites in the higher temperature range.The exploratory research related to a novel co-extruded WPC with a functional shell structure. The co-extruded WPCs with core-shell structure were manufactured using a pilot-scale co-extrusion line. The preparation of co-extruded WPCs and the influence of adding functional materials and agents into the shell structure on the properties of whole composites were investigated. The blend of virgin and recycled HDPE was used as the matrix of core structure, and the low-cost pine flour was used as infill. In the shell structure, HDPE was used as matrix, while PA6micro fibers as reinforced material and a small quantity of zinc borate (ZB) as fire retardant were added respectively and together. Compared with the WPCs without the shell structure, a much better flexural strength, flexural strain and impact strength were obtained in the co-extruded WPCs based on the decreased whole density of composites. The incorporation of20wt%PA6microfibers in shell structure significantly enhanced the flexural modulus and strength of co-extruded WPC. Selectively adding ZB (with a quantity fraction of10%in shell structure and a total quantity fraction of2%) into the shell structure reduced the peak of heat release rate and total heat release of co-extruded WPCs, while extended the time to ignition at the beginning of the test and markedly reduced the total smoke production of co-extruded WPCs during the whole process of combustion. The long-term water immersion test showed that the co-extruded WPCs exhibited much better water resistance than that of WPCs without the shell structure whether the edge of the test samples were sealed or unsealed.As a result, the shell structure only has a mall quantity and volume fraction in the co- extruded WPC, however, selectively added PA6microfibers into the shell structure significantly improved the mechanical property of the co-extruded WPC. Further added a small quantity of ZB into the shell structure improved the flame retardancy and smoke suppression of the whole material. It indicated that adding reinforced materials and functional agents into shell structure could become an effective method to improve the properties of the novel co-extruded WPCs with core-shell structure.