Problems To Be Solved in Wood-Plastic Materials
With the development of wood-plastic industry, people gradually realize that using waste plastic as the raw material in wood and plastic industry is an effective way to solve the recycling of waste plastics. Wood-plastic composites have a variety of mechanical properties better than wood and plastics, wood plastic products have better dimensional stability, low water absorption, will not produce warping, cracking and other phenomena like wood. Because the substrate added in wood plastic products is thermoplastic, so in the corrosion resistance and acid and alkaline and other aspects have incomparable advantages, its service life is also relatively long.
Recycled waste wood-plastic composite products can also be used as new fillers in the addition of new wood plastic products, its performance will not change much. Wood powder used in wood plastic products is often the wood preparation process produced by the scraps of wood chips, sawdust and some plant fibre substances such as peanut shells, coconut shells, sugar cane, linen, jute, cannabis, etc., in the raw materials problem wood plastic composites need the price of fibre is much lower than the price of wood raw materials. For plastics, due to the addition of wood powder, its mechanical properties have been greatly improved, such as wear-resistant relative to plastic on the average increase of about 35%, at the same time wood plastic composite products also have plastic easy to process and other characteristics, but wood-plastic materials also have their own limitations and problems.
1） The problem of interface compatibility
Interface compatibility has always been the primary hotspot in the research of wood-plastic composites. The wood powder is composed of cellulose, lignin, hemicellulose, extract and other parts, these components contain a large number of hydroxyl groups, resulting in strong surface polarity of wood powder, greater water absorption. For plastics, the current wood-plastic products market commonly used thermoplastic pe, pp, polystyrene (PS), PVC, these four commonly used plastics in addition to PVC are non-polar, even if the polarity of PVC is weaker than wood powder. This results in a very poor interface compatibility between wood powder and these thermoplastics. To change the interfacial compatibility of wood powder and plastics, the raw materials should first be pretreated-drying wood powder and plastic to reduce the proportion of water residues in composites. Secondly, it is to modify the treatment, the general modification method has two kinds of physical modification and chemical modification. The physical modification includes physical processing, alkaline treatment, discharge treatment, etc., the principle of physical modification is to reduce the hydroxyl ratio of wood powder surface, reduce the polarity of wood powder surface, chemical modification methods include coupling agent treatment, compatibility agent treatment, grafting modification and so on. At present, the most widely used coupling agent on the market is maleic anhydride grafted polymer copolymer, maleic anhydride grafted polymer copolymer can not only react with the plastic activity, but also with the hydroxyl chemical reaction on the surface of wood powder to form a covalent bond, reduce the polarity of the surface of wood powder. Coupling agent has played a good role in improving the interfacial compatibility of wood-plastic composites, and the wood powder and plastic can be combined organically to improve the mechanical properties of wood-plastic composite products. Nevertheless, the mechanical properties of wood-plastic composite products in some aspects from the structure of the material there is still a big gap.
2） Economic benefit problems
Wood plastic composite Products in addition to the necessary ingredients wood powder and plastic, but also need some additives, such as processing modifiers, colourants, plasticizers, compatibility agents, coupling agents, foaming agents and so on. This takes into account the issue of cost and performance balance. Reducing the proportion of thermoplastics and increasing the amount of wood powder in composites can reduce costs, but it may also lead to reduced performance. Therefore, the process should be diversified to improve the formulation, to achieve maximum performance and efficiency.
3） Classification and source problems of raw materials
The plastic currently recycled on the market is basically a mixture, which will make it difficult to reuse waste plastics. Therefore, in the recycling of plastics to broaden the type of plastic recycling, refinement of plastic varieties. For fibre, some manufacturers now use seasonal characteristics of straw, cotton poles, rice husk, etc., which should consider the storage of raw materials. Although these materials are much cheaper than wood fibres, there are many problems with storage.
4） Density issues
The density of wood-plastic composite products is generally twice times the density of wood products, the corresponding production costs of wood plastic products will also be increased. Now the general way to reduce the density of wood-plastic composites is microporous foaming technology. The main moulding methods of microporous foamed wood-plastic composites are continuous extrusion moulding and injection moulding. The application of foaming technology not only solves the problem of large density of wood-plastic composite products but also prevents crack amplification because of the passivation crack tip of the bubble hole structure, thus improving the mechanical properties of the material.
5） Production equipment and processing technology
At present, the methods of producing wood plastic products on the market are extrusion moulding method, pressing moulding method and injection moulding method. However, the equipment used or the general production equipment for the continuation of plastics, or the general production equipment for the continuation of wood, has resulted in many restrictions on the processing and performance of wood-plastic materials. In recent years, many wood-plastic manufacturers have carried out equipment improvement, or custom foreign specializing in the production of wood plastic equipment. For the processing process, the addition of wood powder will reduce the fluidity of raw materials, processing process is blocked, so generally add lubricants and processing modifiers to reduce the friction of the mixture, improve the fluidity of the meat mixture, reduce the damage to the equipment.
As a new green environmental protection material, wood-plastic composites can be used in almost all fields of wood products application, which provides a broad market space for wood-plastic composites. And most of the raw materials of wood-plastic composites use waste recycled plastics, waste wood powder, which also provides an economical solution for white pollution in China. But wood-plastic composites still have a lot of problems to explore and solve. Wood-plastic composites themselves have good mechanical properties and considerable social and economic benefits will become a very promising material.