Plastic-wood composite material (WPC) is a new environment-friendly composite material that uses wood fiber or plant fiber in various forms as reinforcement or filler, and is compounded with thermoplastic resin (PP, PE, PVC, etc.) or other materials after pretreatment.

Plastic-wood composite materials and their products have the dual characteristics of wood and plastic. They have a strong sense of wood and can be made into different colors according to needs. They have many characteristics that wood does not have: high mechanical properties, light weight, moisture resistance, acid and alkali resistance, and easy cleaning. At the same time, they also overcome the disadvantages of high water absorption of wood materials, easy deformation and cracking, and easy to be eaten and mildewed.

Market status

Under the dual impetus of the national circular economy policy and the potential benefit demand of enterprises, the national "wood plastic craze" gradually rose.

According to incomplete statistics, there were more than 150 enterprises and institutions directly or indirectly engaged in the research and development, production and supporting of plastic wood in China in 2006. Plastic and wood enterprises are concentrated in the Pearl River Delta and Yangtze River Delta, with the eastern part far exceeding the central and western parts. Individual enterprises in the east are leading in technological level, while enterprises in the south occupy the number of products and? The absolute advantage of the field. The distribution of China's plastic wood industry is shown in Table 1.

There are tens of thousands of employees. The annual production and sales of plastic and wood products are close to 100000 tons, and the annual output value is about 1.2 billion yuan. The test samples of major technical representative enterprises in the industry have reached or exceeded the international advanced level.

Because the plastic wood material conforms to China's industrial policy of "building a resource-saving and environment-friendly society" and "sustainable development", it has developed rapidly since its emergence. Now it has penetrated into the fields of construction, transportation, furniture and packaging, and its radiation and influence are expanding year by year.

China's natural wood resources are decreasing? The market demand is increasing day by day. tremendous? Market demand and technological breakthroughs will inevitably broaden the scope of plastic and wood materials? Field. From? From the perspective of market demand, plastic wood is most likely to start large-scale expansion in the fields of building materials, outdoor facilities, logistics and transportation, transportation facilities, household goods, etc.

Existing problems

China's plastic-wood composite products have a single category and low added value, and are dominated by medium and low-grade products such as pallets and bedding boards. Compared with the United States, Europe and Japan, China still has a big gap in the research of wood-plastic composite materials, which is caused by the comprehensive impact of a variety of factors such as raw materials, equipment, processing technology, etc.

Raw materials

At present, the matrix resins used to manufacture wood-plastic composites are mainly PE, PVC, PP, PS, etc. Due to the high cost of new raw materials, waste and recycled plastics are generally used to replace them. The lack of strict sorting and testing methods for recycled plastics in China is likely to cause a small amount of other plastic wastes to be mixed into the selected plastic waste varieties, which cannot be subdivided. The waste wood material may not be a single wood, resulting in the difficulty in ensuring the quality of the waste due to its complex composition. In addition, there is still a lack of systematic research on the relationship between the selection and treatment of wood materials and the performance of final products.

Processing equipment

Due to the influence of metal material performance, processing accuracy, equipment manufacturing cost and? Due to the limitation of market price, there are still some problems in domestic plastic wood processing equipment, such as low processing output, short service life of screw, insufficient exhaust, etc.

Wood-plastic molding equipment has developed from the earliest single-screw extruder to the second generation of conical twin-screw extruder, which is widely used but has many problems. Due to the large amount of wood powder and plant fiber added, it is difficult to plasticize the resin wrapped in it. The equipment is required to have a gear box with high back pressure resistance, strong screw driving force and excellent plasticizing performance. However, the conventional conical twin-screw extruder screw is an integral screw with short length and diameter and short material residence time, which makes it difficult to realize the mixing, dispersion and uniform plasticization of materials. In addition, if the screw structure design is not reasonable, it is easy to cause the barrel and screw wear, and finally lead to cracking and fracture. This phenomenon is particularly prominent when producing large products.

Processing technology

The research on the influence of formula and processing parameters on the properties of wood-plastic composites needs to be strengthened. Although plastic wood composites have many advantages, they still have shortcomings, such as poor aging resistance, poor creep resistance, poor color consistency and durability, microbial erosion, and tensile strength significantly lower than ordinary wood products. These deficiencies need to be studied in depth from the formulation and technology to absorb foreign experience, and constantly improve the production technology and quality level of plastic wood products.

For example, formula and processing conditions are critical to the beauty and aging resistance of products. Because wood materials do not absorb ordinary mineral-based pigments, their color depends entirely on whether they are wetted by resin and whether wood fibers are dispersed in the polymer matrix. Inadequate dispersion will lead to visible white spots on wood materials without coloring. The wood particles on the surface are also not protected by the resin, and are easy to be wet and attacked by microorganisms, reducing the mechanical properties.

In addition, the scattered research force and insufficient funds are also one of the reasons that hinder the development of China's wood-plastic composites. Research and development institutions and plastic and wood enterprises lack communication, coordination and cooperation mechanisms, and the transformation of scientific research achievements is insufficient. The survey shows that China's research and development of wood-plastic composite materials has published about 200 journal documents, about 80 invention patents, and about 50 utility model patents since 2000, with a good momentum of development, but most of the achievements have not yet been industrialized. However, many plastic wood production enterprises are weak in technology and lack of innovation ability, and many technical problems have not been well solved.

New products

Recently, in the development of new products, various research and development institutions and wood-plastic enterprises have also made certain achievements, endowing wood-plastic composites with new functions. Foamed wood-plastic products

The micro-foaming of wood-plastic composites is one of the current research hotspots. The cellular density of the foamed material is 109~1015 cells/cm3, the cell diameter is 0.1~10.0Lm, and the cell size is much smaller than that of the traditional foamed material. These small bubbles can effectively prevent the expansion of the original crack in the material and make the crack tip blunt. Therefore, the impact strength, toughness, fatigue cycle, thermal stability and other aspects of performance have been significantly improved, and the existence of bubbles reduces the material density. Therefore, microcellular foaming of wood fiber composites can overcome the defects of physical and mechanical properties, and greatly improve the performance of the materials. For example, Langfang? DPCA Jinhai Plastic and Wood Technology Co., Ltd. has successfully developed the micro-foaming PVC/wood flour composite: based on the production of pure PVC micro-foaming materials, through the use of self-made composite surface treatment agent, processing rheology agent and foaming agent with the characteristics of absorbing heat balance, the processing equipment has been improved, and the secondary decompression adjustable molding mold has been designed and manufactured. When the wood flour content reaches 30%, The micro-foaming WPC material manufactured can fully meet the requirements of QB/T2463.1-1999 Rigid PVC Low-foaming Sheet.

Plastic-wood alloy

Most of the plastics used in wood-plastic composites are waste plastics, and it is difficult to accurately classify waste plastics. Due to the poor compatibility between different plastics, there are also great differences in processing technology. Therefore, how to mix all kinds of waste plastics to reduce costs is a research problem and research hotspot in the current wood-plastic composites processing industry.

The Institute of Plastic Machinery and Plastic Engineering of Beijing University of Chemical Technology proposed to prepare polymer-reinforced plastic-wood alloy by using anisotropic mixing technology. This technology uses two or more completely incompatible polymers to form a co-continuous phase structure, and obtains a new polymer wood material with high strength, high hardness and low cost. In addition, it can expand the selection of plastic and wood raw materials to a wider range, and make the current manufacturing of plastic and wood products with single-component waste plastics develop to the direction of using multi-component waste plastics to form plastic alloys.

GG high-efficiency reinforced hemp fiber/polypropylene composite

The composite made of natural hemp fiber reinforced polypropylene can compete with glass fiber reinforced polypropylene, and even replace PC/ABS and glass fiber reinforced PC/ABS.

Compared with glass fiber reinforced polypropylene and PC/ABS, hemp reinforced polypropylene has outstanding advantages: low density, the density of polypropylene containing 50% hemp fiber is only 1.08g/cm3; In addition to impact toughness, it has good strength and rigidity, especially high bending modulus; It can be processed with the original mold on the existing injection molding machine; There is no sharp edge during impact breaking, meeting the requirements of vehicle parts; It has higher heat resistance and better heat insulation and sound insulation characteristics; It is recyclable and environmentally friendly.

Development trend

With the industrialization of plastic wood composite products, application researchers have found the defects of plastic wood composite products, and the users have also put forward higher requirements for them: the outdoor products of garden landscape should be anti-aging and anti bleaching, and improve the service life; The interior doors and windows and decorative materials shall have good flame retardancy; The load-bearing plastic and wood products of the building foundation should have high mechanical properties, and need to be strengthened and improved in creep resistance. With the increase of energy saving voice, the construction industry also put forward higher requirements for the energy saving effect of wood-plastic composite products. Therefore, plastic wood composite products are developing towards functionalization and high added value: flame retardant, reinforced, anti-aging, creep resistant, thermal insulation, etc. In addition, the selection of plastic-wood composite materials is also developing towards diversification and variety, including raw wood flour and plastics.

In terms of molding technology, extrusion molding technology is the current mainstream method for the molding and processing of plastic-wood composite products, mainly used to develop profiles, pipes, plates, etc. With the diversification of plastic-wood products, such as plastic-wood multilayer composite pipes and plastic-wood micro-foaming floors, higher requirements are put forward for the extrusion process. Its research and development direction is multi-layer plastic-wood composite co-extrusion, coating co-extrusion, steel/plastic-wood co-extrusion, plastic/plastic-wood co-extrusion, etc.

Injection molding is mainly used for plastic and wood special parts and auto parts, etc. Its new development trend is large-scale product development and engineering plastics, such as ABS plastic and wood products.

The molding method and process are the new trend of the current research and development of plastic-wood composite products, such as the development of multi-layer multi-station intermittent, hot extrusion and cold pressing, and continuous rolling, especially the development of large plastic-wood products such as wide thick wall panels. The use of the molding process can greatly improve work efficiency, reduce production costs, and produce products that are difficult to achieve by other methods.