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Wood-Plastic Composite Fabrication
Wood-plastic composite (WPC) is a very promising and sustainable green material that is produced for its ability to be durable without containing toxic chemicals. The first WPC was discovered in Italy in the 1970s and, since, has become popular in North America. The physical and mechanical properties of WPCs depend heavily on the interaction of the plant and polymer fibers within the material. In order to improve the energy efficiency of this fabrication process, the current compounding method and forming processes used in WPC fabrication will be studied to determine ways of alleviating each process’s limitations.
- 1 Wood-Plastic Composites and Sustainability
- 2 The WPC Material Compounding Process
- 3 WPC Forming Processes
- 4 Improving Efficiency in WPC Fabrication
- 5 Economic Analysis of Implementing Wood-plastic Composite Products
- 6 Applications to Current Industries
- 7 References
Wood-Plastic Composites and Sustainability
Most of the composites available on the market today are produced with a high durability to ensure product longevity. Unfortunately, in order to make these products, companies have traditionally used non-biodegradable polymers and fibers, which were usually made from non-renewable resources. With an increasing number of composite applications, how these composites are disposed of after their intended life is becoming critical. A typical composite, which is made up of two dissimilar materials, is not easily recycled or reused. This inability to recycle has lead to the incineration or disposal of these composites, which has been very expensive and has increased pollution.
Greenpeace groups and NGOs have long been researching and pushing the use of “green-composites” to improve the performance, weight and cost of composite products. One of these “green” composites is wood-plastic composite. In WPCs, biodegradable fibers are used as a replacement for inorganic fibers, such as aramids and glass. The use of wood fibers instead of inorganic fibers make WPCs more energy efficient as it allows for left-over wood products (like wood flour, wood scraps, old furniture, hemp fibre, bamboo...) to be reused in their production, which cuts down on the expense of their disposal. While traditional composites typically have to be burned or left at the landfill after being used, WPCs can be either neutrally CO2 burned or reused as acoustic or thermal insulators (due to there hollow cellular nature).
Recently there have been plenty of development on the use of larger amounts of recycled materials that are used in production of Wood-Plastic-Composites. The methods to reuse and recycle the material are developed and ready for putting into practical use. Of course there is extra energy needed for taking back a material and remoulding it into new pieces, but lifetime of products is long and the number of times the same material can be remoulded is right now assumed to be as many as with aluminium or tin cans. One new promising company / product is Polyplank. This company use only wood-fibre that is waste from sawmills. They claim that sawing or drilling in their material is as easy as with wood and that same tools can be used. But drawbacks is that they want to keep their recycling process secret, so it seems this would only take place at their own plant. (resulting in long transports, if they do not decide to later licence the process to others.) But they use their own waste material to heat the factory.
The WPC Material Compounding Process
The first step in wood-plastic composite fabrication, called compounding, blends organic plant fibers with an inorganic thermoplastic. The percent of wood fiber used in this processing step is very important as it directly affects the tensile strength and Young’s modulus of the product produced. Graph 1 and Graph 2 show how the relative tensile strength and relative tensile modulus of the WPC produced changes when the amount of wood fiber is varied, respectively.
Note: Each trendline in both Graphs show the effect of different wood fiber percentages on thermoplastic matrices of different chemical composition.
|Graph 1: WPC Relative Tensile Strength vs. Wood Fiber Percentage in a Thermoplastic Matrix. ||Graph 2: WPC Relative Tensile Modulus vs. Wood Fiber Percentage in a Thermoplastic Matrix. |
With the effect the amount of wood used in a WPC has on its mechanical properties known, how the compounding process occurs can now be analyzed. During this process, wood fiber and a thermoplastic are heated up to molten temperatures in an intensive shear mixer, to allow for equal dispersion of each in the new composite. Once the newly formed composite is sufficiently mixed, it is cooled and pelletized for use in one of the three forming processes. The compounding process has traditionally been very energy inefficient as creating high shear forces within the materials to ensure equal dispersion requires a large energy input.
The major limitation encountered when trying to improve energy efficiency during the compounding process is the poor compatibility of plant fibers with the thermoplastic matrix. This poor compatibility stems from trying to combine hydrophilic plant fibers with a hydrophobic polymer. The hydrophilic nature of these fibers causes the wood to swell during mixing and shrink during solidification, creating large aggregates and voids to form in the respective matrices.
The formation of these aggregates and voids, shown in Figure 2, can be directly related to both the hydrophilic nature of plant fiber and molecular diffusionW. The following equations, which represent Fick’s first law of diffusion and the diffusion coefficient, can be studied to determine if any process properties can be changed to improve energy efficiency.
J = -D(δc/δx) (D = diffusion coefficient; δc/δx = concentration gradient)
D = Doexp(Ea/RT) (Do = maximum diffusion coefficient; Ea = activation energy;
R = gas constant; T = temperature)
In the above equations, it can be seen that the diffusion/flux of particles throughout a material depends on temperature. Due to the large size of the swelled wooden fibers and the relatively low process temperature (normally around 160C), it is probable that the wood fibers do not have enough energy to diffuse, which is why large aggregates form. In order to increase the energy available to these particles, one might suggest that the temperature of the mixing process should increase. However, this cannot occur because plant fibers cannot handle excessively large temperatures.
The maximum temperature of the mixer, during compounding, cannot reach a temperature of higher than 200C due to the adverse effects higher temperatures have on the wood fibers. When the plant fibers are exposed to temperatures above 200C, thermal degradation occurs in the fibers leading to both physical and chemical changes. These changes include the creation of an odor, change in color, high porosity, low density and deterioration of mechanical properties.
Hence, in order to increase the energy efficiency of the compounding process, the non-uniform distribution, hydrophilic nature and temperature restrictions on the plant fibers must be improved so that less energy is required to form a well-mixed wood-plastic composite.
WPC Forming Processes
The second step of wood-plastic composite fabrication involves forming the new composite from a pelletized compound using compression molding, extrusion or injection molding.
Compression Moldingcompression moldingW. The disadvantage of using this process in the manufacture of WPCs is that any part created takes a general form. This means that additional manufacturing processes, which tend to be wasteful and expensive, are required to produce a finished product. Due to these material and energy inefficiencies during production, the compression molding process is not widely used and therefore will not be studied in this article.
Currently, utilized as the industry standard and an improvement upon the compression molding process in wood-plastic composite forming, the extrusionW process can take place in either a single-screw or a twin-screw extruder.
Single Screw Extrusion
The original extruder used in wood-plastic composite forming was the single-screw extruder. During the single-screw extrusion process, friction is developed on the surface of the screw and barrel, which forces the material to flow from the hopper to the die, as shown in Figure 4. The speed at which the material moves down the barrel is directly related to the friction force produced by the screw as well as the screw’s thread diameter and rotational speed. The force of friction in the barrel is represented in the following equation.
Ff = L/D (where L = barrel length and D = screw diameter)
Despite this friction force being essential to moving the material down the barrel of the extruder, the creation of this force requires a large amount of energy and can have ill effects on the part's properties. In order to improve upon the material and energy efficiencies of the extrusion process, the twin-screw extruder will be discussed as a more efficient improvement on this single-screw extruder.
Relative to the compression molding and extrusion processes, the injection moldingW process, shown in Figure 5, is new to the field of wood-plastic composite fabrication. In the injection molding process, material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. The two main challenges faced in the production of WPCs using this process are inconsistent material properties and supply from the hopper. Unfortunately, due its unpopularity, the injection molding process will not be discussed in this article. However, it should be stated that the few limitations found in this forming process are very similar to those seen in the compounding process, which will be discussed in this article.
Improving Efficiency in WPC Fabrication
The two main inefficiencies in the wood-plastic composite fabrication process have been the inability of an organic fiber to mix with an inorganic thermoplastic and the large amount of energy required by the extrusion process. In order to increase the efficiencies of compounding and forming, these limitations must be improved upon.
Fiber and Thermoplastic Compatibility Improvements in Compounding
To improve the compatibility of the wood fibers with the inorganic thermoplastic, the limitations causing to this poor compatibility must be improved.
The first limitation of using wood fibers in a thermoplastic composite is their hydrophilic nature. In order to improve this limitation, the wood fibers should be treated with a hydrophobic chemical, like PPgMA. This treatment will limit the amount of water absorbed by the wood fibers during the compounding process and therefore, lead to less voids being formed in the material. This decrease in void population will allow for the maximization of the WPC’s material properties.
The third and final limitation is the low processing temperature allowed in the compounding process. High temperatures in the compounding and forming processes are unable to be reached due to the thermal degradation that occurs in the wood fibers at these temperatures. In order to be able to increase the temperature of these processes, the wood fibers can be grafted to monomers, which provide a protective coating around the wood fibers and allow the temperature to rise above the current maximum of 200C.
Efficiency Improvements allowed through implementing these changes
The implementation of these three ideas will allow for the compounding process temperature to be higher and the amount of force input into the fibers to be equal or smaller, while at the same time reduce the amount of time the WPC must be mixed. This reduction in mixing time will increase the energy efficiency of the compounding process.
Improving the Extrusion Process
A new type of extruder that can be used in WPC forming, to improve the energy efficiency of this forming process, is the intermeshing twin-screw extruder, shown in Figure 7. The main advantage of this twin-screw extruder, over the single-screw extruder, is the increased materials and energy efficiency gained during forming.
In an intermeshing twin-screw extruder with the co-rotating screws, the extruded material is more evenly mixed and spends less time in the extrusion barrel than its single-screw counterpart. This decrease in dwell time is caused by an increase the material’s speed down the barrel, which is allowed due to the elimination of back pressure. Back pressureW is present in single-screw extruders because of the friction forces created by the screw. The utilization of two screws in this intermeshing extruder eliminates backpressure by displacing the material using the screws themselves and not friction. This increase in material speed allows for a smaller energy input during forming and lower material temperature rise during extrusion, which produces better material properties in the final product and increased energy efficiency. Furthermore, this design allows for increase materials efficiency as no compounding process is required when this forming process is used, which means no material can be lost between processes.
Despite the improvement in energy and materials efficiency, the twin-screw extruder is a very expensive upgrade from the single-screw extruder, causing many manufacturers to be hesitant towards using it. The economics that play into upgrading to this more efficient machine is discussed below.
Economic Analysis of Implementing Wood-plastic Composite Products
Since wood-plastic composites are generally used in industrial settings, it is not known what the prices of wood plastic composite materials would be to the public. However, in very general terms, it was found that the cost of implementing a wood plastic composite material instead of the usual wood or traditional composite materials is about 2X to 3X more expensive.
In addition to the WPC product prices, the cost of implementing improvements in the manufacturing process can be analyzed. Due to the inexpensive nature of the products used to improve the compounding process, only the improvement to the extrusion process will be discussed. The cost of upgrading from a single-screw extruder to a twin-screw extruder was the most readily available. A new twin-screw extruder costs ~$200,000USD, while a new single-screw extruder costs ~$8,000. The difference in these prices can be mainly attributed to the twin-screw extruder being a newer technology than the single-screw extruder. Due to this large difference in price, many manufacturers have decided to stay with the single-screw extruder and just improve the compounding limitations.
Applications to Current Industries
Typically, fiber reinforced polymeric composites have been used for a variety of structural applications because of their high specific strength and modulus compared to metals. Current applications of WPCs, shown in Figure 8, lay in the automotive, construction, marine, electronic and aerospace industries.
The first major application of wood-plastic composites was in the construction industry, where it was used as flooring. From experiments conducted in the late 1970s, it was determined that WPCs contained many material characteristics that made it superior to wood as a flooring material. These characteristics included a lower rate of moisture absorption, improved fire resistance and better hardness and compression properties. Since this initial application, WPCs have been used in this industry to construct handrails, bows, knives and various other martial arts weapons.
The most popular current application of WPCs has been in the automotive and aerospace industries. In these industries, WPCs are used to improve the mechanical strength and biodegradability of parts, while also to reduce exterior noise, material weight and energy consumption. Recent automotive companies to employ WPC parts include Daimler Chrysler, Mercedes-Benz, Volkswagen, Audi, BMW and Ford.
The different WPC-materials gaining interest recently for uses that replaces chemically impregnated wood in construction for house, picket fences, and especially in applications like decking where it is placed close to ground level and around swimming pools/hot tubs. 
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