Development status of wood plastic composites

2022-05-27
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Development status of wood plastics composites

wood plastics composites (WPC) is a modified thermoplastic material filled and reinforced with wood fiber or plant fiber. It has the advantages of cost and performance of wood and plastics. It is extruded or pressed into profiles, plates or other products to replace wood and plastics. The research on wood fiber plastic mixture has a history of more than 80 years, but it has not been industrialized. 15 years ago, a small amount was used for low value sound-absorbing products because nano crystal rods were easy to gather on the right. Due to the strengthening of environmental concept, the American construction industry is looking for alternative materials like wood (no corrosion, no warpage, convenient maintenance and similar appearance to wood). In order to find the application of sawdust and waste wood chips, the paper and wood processing plants in Korea and Japan have promoted and accelerated the research and application development of WPC. Processing wood flour filling mixture into building and structural profiles is one of the most active departments in the extrusion industry, and new application development is also emerging in endlessly. Therefore, although it has been studied for decades, WPC has attracted widespread attention as a new product due to its huge potential application market. Many countries have invested in accelerating the pace of development and application, especially the production of profiles, which is known as a "hot" market

the development of wpc7 and the sudden lowering of the swing rod when starting up are the results of the environmental concept that has been paid more and more attention. In the past, waste wood chips and leftovers can only be burned or treated as solid waste. Industrial consultants believe that the driving force of development comes from the requirements of rational utilization of the earth's limited resources, reducing the consumption of raw wood, protecting forests, and recycling old wood powder and plastics. Wood fiber and plant fiber have rich sources, low price, light weight, little wear on equipment, good dimensional stability, excellent electrical insulation, non-toxic, can be processed repeatedly and can be biodegradable. Wood fibers include waste wood powder, shavings, sawn wood, and plant fibers are crushed rice straw, peanut shell, coconut shell, sugarcane, flax, zema, jute, marijuana, etc. Thermoplastics are mainly polyolefins and PVC such as PE, PP and PS, including new materials, recycled materials and their mixtures

China is a country not rich in wood resources, with small reserves and output that can not meet the needs of the market. In addition, since February 1998, the United States, Canada and EU countries have successively implemented new quarantine standards for wood packaging materials of China's export goods, requiring fumigation or high-temperature disinfection, otherwise they will be refused entry, promoting and promoting the development of products represented by wood plastic profiles as packaging pallets in China

performance

wood fiber and plant fiber were originally used as modified filling materials with low cost and improved plastic rigidity. Wood plastic plastic can make full use of resources and can be recycled. Whether the materials can be recycled has become an important consideration in material selection in the industry, so the prospect is promising. The main characteristics can be summarized as follows:

⑴ durable, long service life, similar to wood appearance and higher hardness than plastic

⑵ it has excellent physical properties, better dimensional stability than wood, and will not produce cracks, warpage, wood knots and twills. Various products with gorgeous colors can be made by adding colorant, film coating or composite surface layer

⑶ it has the processability of thermoplastic plastics and is easy to form. It can be formed with general plastic processing equipment or after a little transformation. The new investment of processing equipment is less, which is convenient for popularization and application

⑷ it has the secondary processability similar to wood, can be cut, bonded, connected and fixed with nails or bolts, and can be painted. The product specification and shape can be adjusted according to user requirements, with great flexibility

5) it is not afraid of moths, aging resistance, corrosion resistance, small water absorption, and will not absorb moisture and deform

6) it can be reused and recycled, and is environment-friendly

⑺ low maintenance cost

the disadvantage is that the toughness is lower than that of plastic matrix resin, and the processing equipment, downstream devices and molds need to be adjusted and modified accordingly

production process

the first production process used for wood plastic composites is mixing, and then hot pressing (or foaming pressing) into flat plates or molding into profiles. The current production process is mainly extrusion molding, supplemented by injection molding. The largest country in the world for the production and application of wood plastic products is the United States. Its largest product is decking. The production process is extrusion molding. It can be mixed into intermediate plastic wood pellets first, and then processed into products by single screw extruder. However, if possible, the mixing and extrusion of profiles (plates) can also be completed continuously in one equipment or a group of equipment beyond the granulation stage

the preparation process of wood plastic materials (including products) has a great impact on the properties of WPC. Studies have long shown that improper processing will cause poor fiber dispersion and infiltration, while excessive mixing will seriously damage the fibers, which will lead to the reduction of the mechanical properties of WPC. In addition, to prevent problems such as paste and scorch caused by excessive shear or high temperature, poor devolatilization (mainly dehydration) of one-step continuous process will affect the continuity of the process and greatly reduce the performance of the product due to bubbles. At present, the main processing equipment is twin-screw extruder, and the residence time of materials in processing is relatively narrow, so that the processing thermal history of wood plastic materials is as close as possible. It can be divided into co directional parallel twin-screw extruder and counter directional conical twin-screw extruder

co directional parallel twin screw extruder

the plastic wood material processing industry calls the co directional parallel twin screw extruder a high-speed and high-energy "mixing" equipment, which is generally a combined screw. The length diameter ratio and configuration of the screw can be adjusted (kneading block angle and number, different kneading block combination methods), and the degassing port can be set flexibly. It can directly process wood powder or plant fiber, mix the wood powder with molten resin after drying, and then continuously extrude, That is, wood powder drying and resin melting are carried out separately, so it is often composed of two-stage extruder. However, it is generally required that the water content of waste sawdust is 4% ~ 7%. This process requires a large amount of wood powder that meets the required water content and is transported and stored under strict conditions. This two-stage extruder is called "extruder for wood". Similarly, wood flour with water content of 6% ~ 8% is added to the main feed port of the extruder, the front section of the extruder is used as a dehydration and devolatilization device, and then plastic resin and additives are added through the side feeder. Therefore, the extruder is relatively long, and the length diameter ratio (L/D) can reach 44 ~ 48, of which 2/3 is used for water removal and devolatilization

strandex of the United States takes the lead in building and structural processing equipment of plastic wood composite materials without protection, and has been working for nearly 10 years. It has issued 8 patent licenses in the United States, Canada and Japan, and is optimistic about its application prospects in Europe. It has negotiated technology transfer licenses with more than 18 future European users. Italian ICMA company has its patented technology for extruding three-layer plates, and American Davis standard, Krupp W & P and other companies also produce co rotating twin-screw extruders for plastic wood processing

compared with the "mixing" type equipment, the opposite conical twin-screw extruder is known as the "profile" type equipment with low speed and low energy consumption, and is a non combined screw. Compared with the general cone-shaped twin-screw machine, in order to meet the processing requirements of thermal resin, there are many new characteristics and requirements. The screw design is required to adapt to a wide processing range, cut less wood fiber, and even disperse and melt materials when there is less resin. Due to the large specific gravity of the conventional model and wood powder, the filling area is larger than that of the conventional model. If the amount of wood flour and plant fiber is large and the rigidity of molten resin is large, the gear box with high back pressure resistance and strong screw driving force are required. The screw with fast compression and melting and short metering section is adopted to ensure the short residence time of wood fiber and prevent its fracture and deterioration

Cincinnati Milacron company mainly produces this kind of equipment, including extrusion Tek Milacron company in the United States and Cincinnati branch of SMS company in Germany. It is said that the former is very experienced in the design and engineering of WPC extruder. More than 100 sets of equipment have been actually put into use. It has the world's largest model with a processing capacity of 770kg/h. German Cincinatti extruder company has publicly displayed its "Titan" series of conical twin-screw extruders for WPC

in order to obtain wood plastic composites that feel very similar to real wood and can withstand screws and nails, the internal pressure generated by foaming technology can provide better surface clarity and smoother surface contour than non foaming materials. The application of microcellular foaming extrusion technology greatly saves the wood forming materials and reduces the density by 20% ~ 35%; Chemical foaming and physical foaming can be used, Physical foaming is preferred: that is, supercritical gas is injected into the die of the die (such as CO2) to realize foaming. Now the co extrusion of foaming materials and surface layer composites can be realized, so that the materials have excellent service performance. In addition, wood plastic composites with polystyrene/fiber as the main material and water as foaming agent are under research and development. Microporous foaming overcomes the shortcomings of insufficient ductility and impact strength of wood plastic composites, and the cost is reduced.

in recent years, the trend worthy of attention is in the development of wood plastic composites In terms of molding technology, injection molding has developed rapidly. Wood plastic composites will gradually be widely used in injection products

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