Material Selection of Outdoor PVC Decking Products
Polymers used in the processing of outdoor PVC decking can be thermosetting plastics and thermoplastics, thermosetting plastics such as epoxy resins, thermoplastics such as polyethylene (PE), polypropylene (PP) and polyoxyethylene (PVC). Due to the poor thermal stability of wood fiber, only the processing temperature in the 200℃ below the thermoplastics are widely used, especially polyethylene. 1. Plastic polymers are selected on the basis of their inherent properties, product requirements, availability of materials, cost and familiarity. Such as: polypropylene is mainly used in automotive products and daily life products, PVC is mainly used in building doors and Windows, paving board. In addition, the melt flow rate (MFI) of plastics also has a certain effect on the properties of composites. Under the same processing conditions, the MFI of resin is higher, the overall wetting property of wood powder is better, and the distribution of wood powder is more uniform. The wetting property and distribution of wood powder affect the mechanical properties of composites, especially the impact strength. 2. The additive with wood powder with strong water imbibition, and strong polarity, and thermoplastic mostly nonpolar, hydrophobicity, so the compatibility between the two is poorer, interface cohesive force is small, often need to use the appropriate additive to the surface of the modified polymer and wood powder, in order to improve the interface between the wood powder and resin affinity ability. Moreover, the high filling quantity of wood powder in the molten thermoplastic dispersion effect is poor, often exists in the form of a certain aggregation state, making the melt fluidity is poor, extrusion molding processing is difficult, need to add surface treatment agent to improve the fluidity to facilitate extrusion molding. At the same time, various additives should be added into the plastic matrix to improve its processing performance and the performance of the finished products, to improve the bonding force between wood powder and polymer and the mechanical properties of the composites. Common additives include the following: a) coupling agent can make the plastic and wood powder surface have a strong interface; At the same time, it can reduce the water absorption of wood powder, improve the compatibility and dispersion of wood powder and plastic, so the mechanical properties of composite materials are improved obviously. Commonly used coupling agent mainly includes: isocyanate, peroxide isopropyl benzene, aluminate ester, phthalate ester, silane coupling agent, maleic anhydride modified polypropyl agent (man-g-pp), ethylene – acrylate (EAA). Generally, the addition amount of coupling agent is 1wt% ~ 8wt% of the addition amount of wood powder. For example, silane coupling agent can improve the bonding force between plastic and wood powder, improve the dispersion of wood powder, and reduce the water absorption. It is important to note that the maleate coupling agent and the stearate lubricant will react with each other, resulting in a reduction in product quality and yield when used together. b) plasticizer for some glass temperature and melt flow viscosity of higher resin such as hardness PVC, wood powder composite processing difficulties, often need to add plasticizer to improve its processing performance. Plasticizer molecular structure contains two kinds of polar and non-polar genes. Under the action of high temperature shear, it can enter the polymer molecular chain and form a uniform and stable system through the mutual attraction of polar genes. However, the insertion of longer non-polar molecules weakens the mutual attraction of polymer molecules, thus making the processing easier. The plasticizer that is often added in wood plastic composite decking is dibutyl phthalate (DOS). For example, in outdoor PVC decking composite materials, the addition of plasticizer DOP can reduce the processing temperature, reduce the decomposition of wood powder and smoke, improve with the increase of DOP content, the composite material’s tensile strength decreases and the elongation at break increases. c) lubricants PVC decking often need to add lubricants to improve the melt fluidity and the surface quality of extruded products, the lubricants used are divided into internal lubricants and external lubricants. The choice of inner lubricant is related to the matrix resin used, it must have a good compatibility with the resin at high temperature, and produce a certain plasticizing effect, reduce the cohesion between molecules in the resin, reduce the mutual friction between molecules, in order to reduce the resin melt viscosity, improve melt fluidity. The outer lubricant actually plays the role of the interface lubrication between resin and wood powder in plastic molding, and its main function is to promote the sliding of resin particles. Usually, a kind of lubricant often has both inside and outside two kinds of lubricating properties. Lubricant has a certain influence on the service life of the mold, barrel and screw, the production capacity of the extruder, the energy consumption in the production process, the surface finish of products and the low-temperature impact performance of profiles. Commonly used lubricants are: hard cholic acid zinc, ethylene difatty acid amide, polyester wax, stearic acid, lead stearate, polyethylene wax, paraffin, oxidized polyethylene wax, etc. d) during the use of colorant in wood plastic composite materials, the solvent substances in wood powder are easy to migrate to the surface of the product, decolorize the product, and eventually become gray. Different products will also produce black spots or rust spots under certain use conditions. Therefore, colorants are also widely used in the production of outdoor PVC decking. It can make products have uniform and stable color, and fade slowly. e) foaming agent PVC decking has many advantages, but due to the composite of resin and wood powder, its ductility and impact resistance are reduced, the material is brittle, and the density is nearly 2 times larger than that of traditional wood products, which limits its extensive use. However, due to the good bubble structure, the foamed wood plastic composite decking can passivate the crack tip and effectively prevent the crack expansion, thus significantly improving the impact resistance and ductility of the material, and greatly reducing the product density. There are two main types of foaming agent is a lot of more phyletic, the commonly used, endothermic blowing agent, such as sodium bicarbonate NaHCO3) and exothermic foaming agent (azobisformamide AC), the thermal decomposition behaviour is different, the polymer melt viscoelastic and foaming form has a different effect, thus to select the appropriate foaming agent according to the requirements of the products used. F) applications of UV stabilizers and other UV stabilizers have also developed rapidly with the improvement of people’s requirements on the quality and durability of wood plastic composite decking. It can prevent the polymer from degradation or mechanical property degradation. Commonly used are blocked amine light stabilizer and ultraviolet absorbent. In addition, in order to maintain a good appearance and perfect performance of composite materials, it is often necessary to add anti-bacterial agent, the choice of anti-bacteria agent should consider the kind of wood powder, the amount of added, the fungus in the composite material use environment, the water content of the product and other factors. Zinc borate, for example, is preservative but not algae repellent. The production and use of wood plastic composite materials will not emit volatile substances harmful to human health to the surrounding environment, and the wood plastic composite products themselves can be recycled and reused, so outdoor PVC decking products are new green products, ecological self-cleaning, has a broad development prospect.