The resin for wood-plastic products can use new materials such as PE, PP, PVC, PS, ABS, PMMA, etc., or use various waste recycled materials, or a mixture of the two. The used wood plastic molding products can also be reused after grinding. For the resin used in wood-plastic composites, due to the different melting point and molding temperature of the resin itself, the different performance requirements of the product, and the different molding process, the resin must be appropriately selected.
① Molding temperature of resin. Generally speaking, after adding wood flour to the resin, its fluidity will decrease, and the high temperature resistance will also be reduced. In the presence of oxygen, at about 200℃, wood flour will violently smoke, reducing weight by 5%~10% , The product has an obvious burnt smell of wood, the color becomes darker and black, and the performance is greatly reduced. Therefore, the resin used for wood-plastic composite materials should be molded at a temperature below 240°C, otherwise it will be extremely scorched. Therefore, the resin in wood-plastic composite materials should be resins with lower melting points and processing temperatures, which is why wood-plastic composite materials use more olefin resins.
②Product usage and performance requirements. According to different product uses, materials are required to have corresponding performance requirements. For polyolefin resins, HD-PE and MD-PE resins are mainly used for PE resins; PP has obvious properties due to its homopolymerization, block copolymerization and random copolymerization and other polymers with different polymerization types and different monomer components. The difference. For example, the flexural properties (flexural modulus, flexural strength) of PP homopolymers, block copolymers, and random copolymers are from high to low; while the impact properties are from block copolymers, random copolymers, and homopolymers. From high to low. As materials for construction and automobile parts, they generally require high flexural modulus and certain impact properties. Therefore, homopolymers with better rigidity or polypropylene resins blended with block copolymers and homopolymers are often used; If the product requires high toughness, the elongation at break must be relatively high. Block polypropylene copolymers need to be selected and even necessary toughening modifications are required.
③Forming process. Different molding processes have different requirements for the basic resin of wood-plastic composite materials. This is basically the same as the traditional molding process for resin performance requirements, but more stringent. Injection molding, such as the screw speed is too fast during pre-molding, especially high pressure and high speed during injection molding. When the material passes through the nozzle and the pouring system, due to the high flow rate, shear friction heat, and the temperature often exceed 240 ℃, the material will change color and lift. Foaming, or even scorching, a resin with better fluidity should be selected for this, and necessary adjustments should be made in the process. For extrusion molding, because the speed of the material passing through the narrow slit of the die is relatively low, the shear friction heat is less, and the temperature of the material will not be too high due to cooling or vacuum removal in the shaping part, so the choice is The resin with the same fluidity or slightly higher fluidity used for extrusion molding is sufficient. For PVC, the resin with the corresponding K value or polymerization degree should be selected according to different molding processes.
In short, for wood-plastic composite materials, resins with a melting point or viscous flow temperature below 200°C should be selected according to product performance requirements and molding processes to avoid thermal degradation of plant fibers during the molding process.