Some Additives Commonly Used in Wood Plastic Composites

Coupling agent: it can produce strong interfacial bonding between plastic and wood flour surface; At the same time, it can reduce the water absorption of wood powder and improve the compatibility and dispersion of wood powder and plastic, so the mechanical properties of the composite are significantly improved. Commonly used coupling agents mainly include isocyanate, cumene peroxide, aluminate, phthalate, silane coupling agent, maleic anhydride modified polypropylene (man-g-pp), ethylene acrylate (EAA). Generally, the added amount of coupling agent is 1wt% ~ 8wt% of the added amount of wood powder. For example, silane coupling agent can improve the adhesion between plastic and wood powder, improve the dispersion of wood powder and reduce the water absorption. However, alkaline treatment of wood powder can only improve the dispersion of wood powder, but can not improve the water absorption of wood powder and its adhesion with plastic. It should be noted that the maleate coupling agent and the stearate lubricant will have mutually exclusive reactions, and when used together, the product quality and output will be reduced.

Plasticizer: for some resins with high glass transition temperature and melt flow viscosity, such as hard PVC, it is difficult to process when compounding with wood powder. Plasticizer is often added to improve its processing performance. The molecular structure of plasticizer contains 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 mutual attraction of polar genes. The insertion of its long non-polar molecules weakens the mutual attraction of polymer molecules, thus making processing easy. The plasticizers commonly used in wood plastic composites are dibutyl phthalate (DOS) and so on. For example, adding plasticizer DOP to PVC wood flour composite can reduce the processing temperature, reduce the decomposition and smoke of wood flour, and improve the tensile strength of the composite decreases and the elongation at break increases with the increase of plasticizer DOP content.

Lubricants: wood plastic composites 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 selection of internal lubricant is related to the matrix resin used. It has good compatibility with the resin at high temperature and has certain plasticizing effect, reducing the cohesion energy between molecules in the resin and weakening the mutual friction between molecules, so as to reduce the melt viscosity of the resin and improve the melt fluidity. The external lubricant actually acts as the interface lubrication between resin and wood powder in the plastic molding process. Its main function is to promote the sliding of resin particles. Usually a lubricant has both internal and external lubricating properties. The lubricant has certain influence on the service life of the die, barrel and screw, the production capacity of the extruder, the energy consumption in the production process, the surface finish of the product and the low-temperature impact performance of the profile. The commonly used lubricants are: zinc cholate, ethylene double fatty acid amide, polyester wax, stearic acid, lead stearate, polyethylene wax, paraffin, oxidized polyethylene wax, etc.

During the use of colorants in wood plastic composite materials, the solvable substances in wood powder are easy to migrate to the surface of the products, making the products decolour and turn 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 wood plastic composites. It can make products have uniform and stable color, and decolorization is slow.

Foaming agent: wood plastic composite has many advantages, but due to the combination 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 wide use. However, the foamed wood plastic composite can passivate the crack and effectively prevent the crack expansion due to its good cell structure, thus significantly improving the impact resistance and ductility of the material and greatly reducing the density of the product. There are many kinds of blowing agents, and there are mainly two kinds commonly used: endothermic blowing agent (such as sodium bicarbonate NaHCO3) and exothermic blowing agent (Azodicarbonamide AC). Their thermal decomposition behavior is different and have different effects on the viscoelasticity and foaming morphology of polymer melt. Therefore, appropriate blowing agents should be selected according to the use requirements of products.

The application of UV stabilizers and other UV stabilizers also develops rapidly with the improvement of the quality and durability of wood plastic composites. It can make the polymer in the composite material not degrade or decrease the mechanical properties. Hindered amine light stabilizers and ultraviolet absorbers are commonly used. In addition, in order to maintain good appearance and performance of composite materials, it is often necessary to add fungicides. The selection of fungicides should consider the variety and amount of wood powder, fungi in the use environment of composite materials, water content of products and other factors. For example, zinc borate can prevent corrosion but not algae.