The resin can be used in WPC processing thermoset and thermoplastic. Thermosetting resin such as epoxy resin, phenolic resin. Thermoplastic resin is polyethylene(PE), polypropylene(PP) and polyvinyl chloride(PVC). But, as a result of wood fiber, poor thermal stability only processing temperature under 200℃ of thermoplastic resin is widely used as the matrix resin of WPC, especially polyethylene. The main evidence for the selecion of resin are: the intrinsic characteristic of resin, the product need, raw material availability, cost, and its familiar degree. Is mainly used for automobile products and items such as polypropylene, polyvinyl chloride(PVC) is mainly used for building doors and windows and cladding plate, etc. In the same process conditions, the resin of the MFR is higher, better overall invasive of wood powder, wood powder are more uniformly distributed, and the invasive and distribution of wood powder effect on the mechanical properties of the composites, especially the impact strength. According to statistics, currently on the market still is given priority to with PE wood plastic composite, about 65%, PVC wood plastic composite materials account for about 16%, about 14%PP wood plastic composite.
3.2 The selecion of the additives
Due to plant fiber polarity is very strong, with strong water imbibition, and most of nonpolar thermoplastic resin, has hydrophobic, so the compatibility between the two is poorer, interface bond is small. Often need to use proper additives to change the surface properties of resin and fiber, so as to improve the interface between the fiber and resin affinity. And high filling plant fiber dispersion effect is poor in the molten
Thermoplastic resin, often exist in state of aggregation, the melt illiquid, extrusion molding processing difficulties, need to add lubricant to improve liquidity to facilitate processing. At the same time, also need to join all kinds of resin additives to improve the processing performance and product performance.
3.2.1 The interface modifier
Interface modifier can make the resin and generate strong interface bonding between the fiber surface, and can reduce the water absorption of plant fiber, improve the compatibility and dispersity of plant fiber with resin, so can obviously increase the force of the composite material performance. Commonly used interface modifier mainly include: isocyanate, aluminum acid ester, titanate and silence coupling agent(such as gamma ammonia propyl silance three oxygen radicals), ethylene-acrylate copolymer (EAA), phenolic resin and maleic anhydride grafted polypropylene wax ( Licomont AR504). General interface modifier amount for the plant fiber 1~8wt%. Note that maleic acid salts interface modifier and stearate lubricant are repellent reaction happens, used together will lead to dro product quality and yield.
3.2.2 Lubricant
WPC is often necessary to add lubricant to reduce between the melt and processing machinery and melt the internal friction and adhesion, improve liquidity, promote the processing molding, improve surface quality of the product. Lubricant is divided into lubricant. External lubricant adhesion on the surface of melt or processing machinery, mold, forming lubrication interface, reduce the friction between the melt and processing machinery. In the choice of lubricant is associated with the matrix resin, it must have good compatibility with resin in high temperature, weaken the interaction force between molecular chains, promote the molecule chain movement, reduce the cohesive energy between the resin molecule. Lubricant on the service life of dies, material barrel, screw, extrude production can force, the energy consumption in the process of production, product surface can force, the energy consumption in the process of production, product surface finish and the profile of the low temperature impact performance will produce certain effect. Usually a kind of lubricant often have both inside and outside two lubricant performance. The commonly used lubricants are: zinc stearate, ethylene double fatty acid amide, polyester wax, stearic acid, lead stearate, polyethylene wax, paraffin wax, oxidized polyethylene wax, etc.
3.2.3 Plasticize
When the plant fiber and glass transition temperature and melt flow of high viscosity resin compound, often processing difficulties, often need to add the plasticizers to improve its processability. Plasticizer can increase the plasticity of polymer products, improve its flexibility, extensibility and processability. Wood powder composite materials such as PVC, add plasticizer can lower processing temperature, reduce the decomposition of plant fiber and smoke, But the effect of plasticizer on the mechanical properties of WPC, general with the increase of plasticizer content, the tensile strength of the composites decreased elongation at break increased. Plasticizer common molecular structure contains two polar and nonpolar groups, under the high temperature shearing action, it can enter the polymer molecular chain, through polar groups and the polar groups on the polymer are attracted to each other to form uniform and stable system, while it nonpolar part insert the attraction of the polymer molecules can be reduced, increased the activities of the polymer chain segment space, thus make the process easier. Commonly used plasticizer are: butyl phthalate(DBP), phthalate dioctyl(DOP), sebacic acid dioctyl ester（DOS), etc.
3.2.4 Uv stabilizer
In outdoor use of polymer materials, caused by the photodegradation of light cannot be ignored. With the improvement of quality and durability of WPC, the application of uv stabilizer get rapid development. It can greatly reduce the mechanical behavior of composite materials in the polymer degradation and decline. Commonly used have hindered amine light stabilizer and ultraviolet absorbent.
3.2.5 Colorant
In the processing of the use of WPC, plant fiber easy to migrate to soluble substance in the product surface, make the product decoloring, and eventually become gray. Used sometimes in certain circumstances, and will produce dark spots or rusty spot. Colorant can give the products color, beautiful, easy to identify and improve the weathering resistance. So,colorants in WPC production also have become more widely used. It can make the products have uniform and stable color, slow and decoloring. Ferro, companies in the United States has been industrialized production of a variety of colorants, and continue to improve.
3.2.6 antioxidants
Polymer materials exposed to air, under the action of oxygen oxidation reaction will happen, this kind of reaction usually occurs between the room temperature to the forming temperature, according to the typical chain mechanism of free radical, with automatic catalytic characteristics, so often called automatic oxidation reaction. Therefore, also often join in WPC antioxidant to inhibit or relieving oxidation speed automatically, prolong service life. The commonly used antioxidants such as aldehyde amine and ketone amine.
3.2.7 Anti bacteria agent
In order to make the composite materials can keep a good appearance and perfect performance, from microbes to the negative impact of the plant fiber is often necessary to add anti bacteria agent. Anti bacteria agent selection should consider the types of plant fiber, add quality, fungi in composite materials using the environment, many factors such as the water content of the product. Such as zinc borate can anticorrosive but cannot prevent algae.
4. WPC molding of equipment
4.1 Main extrusion molding of equipment
At present can be used in WPC extrusion molding of equipment mainly include single screw extrusion, twin screw extrusion, etc.
4.1.1 Single screw extruder
The conventional single screw extrusion is not suitable for wpc processing, this is because the single screw extrude and conveying of plasticizing capacity is weak. Delivery of the single screw extruder effect mainly by friction, due to the structure of wood powder and fluffy, not easy to feed, and wood powder filling make the polymer melt viscosity increases, increase the difficulty and cause the material stay in the cylinder for a long time, at the same time its exhaust the effect not beautiful, not to add workers higher moisture content of plant fiber. Therefore, the conventional single screw extruder in wood plastic composite extrusion is limited by the larger. Can be used in the processing of wpc single-screw extruder must use a specially designed screw, screw should have strong material transport and mixing plasticizing capacity, and often before extrusion to mix of refining grain material.
4.2.1 Twin screw extruder
At present the main processing equipment for wpc twin screw extruder, this is because the twin-screw extruder rely on the principle of positive displacement conveying material, no pressure backflow, charging easy, exhaust effect is good, can be fully ruled out of the wood powder can be volatile component, screw meshing with each other, the strong shearing action make the material mixing and plasticizing effect is better. Wood powder dosage is relatively low, the material in the double screw retention time is shorter, there will be no wood powder burning. Therefore, because of the twin screw extruder is good material, mixing effect, can use powder production of wpc. Twin-screw extruder can be divided into the same parallel twin-screw extruser and vision to the conical twin screw extruder.
4.2.1.1 Direction parallel twin-screw extruder
Synthetic parallel twin-screw extruder is often by double extrusion unit, separate dry wood powder and resin melt. Can be directly processed wood powder or plant fiber, after complete wood powder dry, then mix with molten resin continuous extrusion, this double order extruder is called wood with extruder. Although the double order extruder for wood powder dry, but for the raw material has certain requirements, water content of wood powder is commonly 4%~8%(mass fraction). A similar form is wood powder to join extruder main ingredient, extruder front for dewatering, swing device, then through the side feeder in plastic resins, additives, therefore relatively long extruder screw, screw length to diameter ratio(L/D) can be up to 44~48, two-thirds for in addition to water and to take off the swing. Plastic wood materials processing industry called synthetic parallel twin-screw extruder, “mixing”type equipment, high speed and high energy consumption is usually combined screw, adjustable screw length to diameter radio and configuration（Angle of kneading block and the blocks, the different kneading block combination method), set to take off the port. Davis in the United States-Standard, the Krupp W&P famous plastic machinery factory production this kind of synthetic twin screw extruder.
