Wood-plastic composite materials (WPC: Wood plastic composite) are materials basically made up of two phases, a continuous plastic called matrix (includes PE , PP , PVC , etc.) and another reinforcement or filler made up of fiber or wood dust. In addition to wood and plastic fibers, WPC can also contain other filler materials (ligno-cellulosic and/or inorganic). WPCs are a subset of a broader category of materials called natural fiber plastic composites (NFPCs), which may also contain fiber-based fillers, such as cellulose fibers, peanut shells, bamboo, straw , etc.
the bakelite Invented in 1909 by Leo H. Baekeland, it is one of the first wood-plastic compounds as it is made up of fine wood particles to which phenol and formaldehyde resins are added. The vast majority of wood-plastic products currently manufactured incorporate the same products: on the one hand, plastic materials, in this case thermoplastics, such as high-density polyethylene (HDPE) and polyvinyl chloride (PVC), and on the other, powdered wood. This combination did not seem very promising, since on the one hand wood fibers are very short, absorb a lot of moisture and are less resistant than glass or carbon fibers; and on the other hand, thermoplastic materials are weaker than thermosetting materials (they soften with heat and deform under stress). Nevertheless, the results of the combination have been surprising. These composites, despite the fact that they use very cheap raw materials and their manufacturing process is of a low technological level, have a final market price higher than that of wood treated with chemical products (without considering the life cycle, and its greater durability ). The product began to be used for outdoor terraces, but has been making its way into fences, railings, handrails and façade elements.
Wood-plastic composites were first seen in the United States during the 1960s, primarily in window and door frame applications. The window and door industry, which has experience with wood, plastic and aluminum materials, looked for ways to use cheaper materials, hence sawdust seemed both light and cheap.
The country with the highest production of this material is the People’s Republic of China, although in India its research and development has increased in recent years, because the use of this type of material can reduce costs since all the wood waste is used. and recycled plastic. The density of the material can be controlled by means of foaming agents or by the type of process.
The main problem with these compounds is the process temperature which, in general, should not exceed 190°C. Another problem is the humidity of the wood, which is not compatible with most polymers and in some cases can cause degradation and loss of physical properties due to depolymerization.
Equipment used to extrude this material is commonly equipped with twin extrusion screws. For injection, the technology generally has few developments, although there are highly technical extrusion-injection processes (Injection molding composite) capable of combining the compounder and injection stage in a single process. Other processes are compression and thermoforming.
The color of the final piece depends largely on the color of the wood and the polymer. The color of the compound can be modified after processing with a lacquer or during processing with a color concentrate which, if properly designed, can present flow lines that resemble normal wood grain.
Wood fibers are randomly distributed within the matrix of plastic materials. The percentage of wood fibers varies depending on the product, from 70% and even higher percentages, up to recycled plastic products that do not contain wood fibers.
– Plastic materials: Thermoplastics (which soften when heat is applied) or thermosetting materials (which do not soften with heat once they have set) can be used.
Within the group of thermoplastics are the polyethylenes and polypropylenes that are used in many non-structural applications, such as bottles or containers; and in the group of thermosetting polyester and epoxy structural resins. Plastic waste can be reinforced or increased in size with natural fibers, glass fibers and other materials such as rubber.
– Wood: The percentage of wood, both in the form of powder and short fibers (less than 5 mm), normally varies from 50 to 80%. Wood flour is used to give body to the product, but it has a different mission than the fillers used in adhesives, since in this case its mission is reinforcement. The use of wood powder or other materials is not recommended due to the risk of explosion.
– Additives: The most common additives are lubricants, fire retardants, products that improve the wood-plastic bond, UV stabilizers, pigments, etc.
The mixture of raw materials is done in batches or by continuous processes and is forced through a laminator or an extruder or to be injected into the corresponding moulds. The vast majority of wood and plastic products are extruded, solid or hollow profiles.
Properties and characteristics
The properties to be evaluated depend on their use outdoors or indoors, but there is still no specification standard, although there is a first draft of the test standards to check the following properties:
- Physical: density, moisture content, etc.
- Mechanical: impact resistance, traction, etc.
- Durability: artificial aging, swelling and absorption, etc.
- Thermal: linear expansion, thermal conductivity, action of heat.
- Fire: oxygen limit, reaction to fire.
- Other properties
Wood-plastic products are less rigid than wood, their strength is also lower, they deform when subjected to loads, they soften under the action of heat and they are brittle at low temperatures. Its tensile and compressive strengths are similar to coniferous wood, but in most applications these properties are not relevant. In those applications that require greater rigidity, such as outdoor terrace flooring, the solution is to increase its cross-section or thickness or reduce the distance between supports.
Although warping does not occur and moisture absorption is lower than that of solid wood, in any case it is necessary to leave joints to absorb the dimensional changes caused by its higher coefficients of expansion (similar to those of aluminium).
Durability and aging
The action of the sun tends to darken it, which is why it is usually light gray. It has high resistance against the attack of fungi, larval cycle xylophagous insects, termites and marine xylophagous. Some products incorporate protectors to prevent the appearance of molds and the growth of fungi, although in the first prototypes for floors cubic rot, soft rot, cracks, fraying and weathering were produced. Subsequently, the products were perfected in this regard.
- Its slip resistance is greater than that of natural wood.
- Sometimes static electricity discharges may occur.
- They can be recycled.
- They are considered as ecological composite products.
- They are more expensive than natural and treated wood, but when considering their life cycle and their absence of maintenance, their final cost would be better.
WPCs are produced by thoroughly mixing wood particles and heated thermoplastic resin. The most common method of production is to extrude the material into the desired shape, although injection molding is also used. WPC can be produced from virgin or recycled thermoplastics, including HDPE, LDPE, PVC, PP, ABS, PS, and PLA. Polyethylene-based WPC is by far the most common. Additives such as colorants, coupling agents, UV stabilizers, blowing agents, foaming agents, and lubricants help tailor the final product to the intended application. Extruded WPC can be formed into solid and hollow profiles. A wide variety of injection molded parts are also produced, from car door panels to cell phone covers.
In some manufacturing facilities, the components are combined and processed in a pelletizing extruder, which produces granules of the material. The pellets are re-melted and shaped into the final shape.
Other manufacturers make the finished part in a single mixing and extrusion/injection step (IMC: injection molding composite) without going through pelletizing.
The extrusion of these compounds is the most used technique to manufacture profiles of this material, but it is also used to make the compound, this happens through a physical mixture, which in combination with coupling agents can form some chemical bonds or form emulsions. so that the material is stable and can later be injected or compression molded.
The extrusion of wood-plastic composites presents some technological challenges compared to traditional extrusion. The first problem present is the feeding of the material in the hopper, the wood dust and granules of plastic material are fed in the hopper, however the fine wood dust tends to conglomerate in the mouth of the hopper, which due to the action of heat and from moisture it sticks and does not enter the barrel and does not have contact with the spindle.
Extrusion can occur in equipment from:
- Double spindle
- Simple spindle
The material must be dried prior to injection and preferably dosed by robots, to ensure homogeneity during extrusion.
The speed with which the screw rotates determines the rate of material extruded per minute, however it should be noted that this rate may have a maximum and a higher speed would only consume more energy without providing benefits in the amount of material per unit of time.
The rate of material extruded per minute can be increased with flow agents, coupling agents, and foaming agents.
In general, the injection of wood and plastic compounds obeys the same technique used in injection molding, however it is necessary to take into account some important technical points such as the degradation temperature of the compound, since temperatures greater than 205°C can causing degradation problems and too low temperatures would prevent proper mixing of the materials.
The mold temperature can be kept slightly higher than traditional injection because the wood in the composite allows for better dimensional stability than the polymer alone, so less intensive cooling results in time and cost savings.
Compound flow into the mold cavity occurs faster when the mold temperature is high.
Compression molding is the most widespread method for the production of wood-plastic composites, multiple internal automotive parts (parts that are not exposed to view both inside and outside the car) are made of this compound and molded by compression, In this technique, natural fibers (such as jute or henequen) are commonly used in addition to wood flour.
The same machines that are used to work wood can be used. It does not present problems in nailing and screwing. Products containing few fibers can be welded together. Some types of products can be painted, varnished or sealed with silicone or acrylic products. The extruded profiles are obtained in a single operation and do not require or can be subjected to subsequent machining.
Advantages and disadvantages
WPC does not corrode and is highly resistant to rot, decay, and attack from salty seawater, even though it absorbs water into the wood fibers embedded in the material. They are easy to machine and can be shaped using conventional carpentry tools. WPC is often considered a sustainable material as it can be made using recycled plastics and the waste products of the wood industry. Although these materials continue the useful life of used and discarded materials, and have their own considerable half-life, wood-plastic composites are difficult to recycle again after use. One more advantage over wood is the material’s ability to be molded to meet almost any desired shape. Another important point of advantage of these materials is their lack of need for painting. They are manufactured in a variety of colors. Despite having up to 70% cellulose content (although 50/50 is more common), the mechanical behavior of WPC is more similar to pure polymers. This means that they have lower strength and stiffness than wood, and that they exhibit time- and temperature-dependent behavior. The wood particles are susceptible to fungal attack, although not as much as solid wood, and the polymer component is vulnerable to UV degradation. It is possible that strength and stiffness can be reduced by moisture absorption and freeze-thaw cycles, although testing is still ongoing in this regard. Some WPC formulations are also sensitive to staining from a variety of agents.
One of the main applications of this type of material is found in the coating of surfaces that are outdoors, being widely used in docks, terraces, and other surfaces near swimming pools or in recreation centers. The inherent advantages for these applications lie in greater resistance to moisture and insects, coupled with a low level of maintenance. Companies such as Louisiana Pacific Polymers and Trex in the United States are leaders in this type of application, offering a variety of references with different colors, finishes and physical properties.
Another common use of wood-reinforced plastics is in the manufacture of extruded profiles. These profiles can be used for the manufacture of profiles for doors and windows, games for children, chairs, or to replace railway sleepers. In the case of the manufacture of profiles, the possibility of making profiles foamed inside has begun to be explored. Although some manufacturers have already implemented this process due to the advantages it presents in saving raw material costs and reducing weight, it is still necessary to go deeper into the understanding of the manufacturing process.
On the other hand, wood-reinforced materials have entered the field of automobile manufacturing, displacing fiberglass-reinforced plastics in the production of some parts. This trend started in Europe, where such materials are used to make interior door panels, seat backs, headrests, fenders and storage trays. The trend has reached the United States, and the impact has been such that large manufacturers of fiberglass-reinforced materials such as Owens Corning have begun to experiment with natural fibers.
Wood-plastic composites are also widely used in the manufacture of pallets, platforms and cleats for footwear, nautical items, etc.