The applications for wood flour are dictated by a number of important properties. On the positive side, as well as being regarded as sustainable, it is relatively low in density and gives some reinforcement, although not as much as wood fibers. Important limitations include:
- • Color. It can only be used for dark-colored products and can show color fade on outdoor exposure.
- • Thermal stability. This limits processing temperatures to about 200 °C, unless special precautions are taken.
- • Moisture content. It will rapidly pick up moisture from the atmosphere when dry and so must be processed immediately after drying in applications where moisture would affect processing.
- • Flammability. It is flammable which creates some hazards in transportation, storage, and processing; it can also decrease the fire rating of the host polymer.
- • Weathering. Water absorption, swelling, and discoloration can be issues.
- • Durability. Biological attack including mold and fungal effects can occur unless additives are used to prevent it.
These problems are least important in thermoset applications, explaining the early use of this material in such polymers. The processing temperatures here are low, and the moisture present can be tolerated in many instances. The limitations are far more important in thermoplastic applications.
Despite these issues, the main application today, and one that has seen rapid growth, is in what are known as wood polymer composites. These are combinations of wood flour in thermoplastic matrices, predominately polyethylene, but also using polypropylene and rigid PVC. About 80% of this market is in outdoor products, particularly decking. This market is most developed in North America. These applications frequently use reclaimed polymer as well, further increasing their green image. Estimates for the size of the market vary, but most agree that several hundred thousand tons of composite are used, with wood flour contents from 40 to 65% w/w.
When used in polyolefins, the wood flour gives useful increases in stiffness, partly due to the development of transcrystallinity, nucleated by the particle surfaces. There is usually a negative effect on unnotched impact, while the effect on notched impact can be either positive or negative depending on the source of the wood used. Processing is difficult due to high melt viscosity and low melt strength, and various additives are usually employed to allow economically acceptable extrusion rates. Coupling agents, such as maleated polyolefins, are also frequently employed to improve bonding between the polymer and the particles, and this can improve impact strength.
While the effect on composite properties varies significantly with the wood source, the trends are all similar and typical results are presented in Figs. 1 and 2. It should be noted that the relatively low specific gravity of wood compared with mineral fillers means that the volume loading is significantly higher than that of a mineral at equal weight level.
Figure 1 shows the effect on stiffness which is seen to increase significantly with filler loading. The effect on notched and unnotched impact resistance is presented in
Fig. 1 Effect of wood loading on the flexural modulus of a polypropylene homopolymer (adapted from C.M. Clemons 2010)
Fig. 2 Effect of wood loading on the impact resistance of a polypropylene homopolymer (adapted from C.M. Clemons 2010)
Fig. 2. While there is a modest increase in notched resistance, there is a marked drop in unnotched. This is explained by the notched property being controlled by crack propagation, while the unnotched is controlled by crack initiation. Particulate inclusions can have very different effects on these two properties, and the wood particles are clearly acting as flaws in the composite and thus lowering unnotched impact strength, an effect that is frequently seen with particulates. Fortunately, the notched figure is more important than the unnotched one in most cases.
In theory, wood polymer composites are themselves recyclable, although there are significant problems to doing this on an industrial scale. Some are those related to collection and variability common to nearly all recycling operations. The others are due to deterioration of both the polymer and wood flour during processing and also use in an outdoor environment. These include molecular weight degradation of the polymer and oxidative, light, and weather degradation of the wood. These issues can be overcome sufficiently by careful processing and addition of stabilizers, to at least allow some level of incorporation into a new product.