Thermoplastics and Short Fibers of Natural Origin

So-called WPCs are thermoplastic composites that combine wood flour with plastics, intended to offer the advantages of wood-like materials with the processing and part-design capabilities of thermoplastic polymers. 

Whilst finely grinding wood material may considerably hide the fiber aspect of wood particles, WPC exhibit properties which are typical of systems filled with short fibers of natural origin. As WPC is a rich subject in itself, only a few basic aspects will be discussed hereafter. It is obvious that not all thermoplastics can be used to prepare WPC. Indeed the polymer component melts or at least softens at or below the degradation point of the wood component, normally 200-220°C but must be rigid at normal usage temperatures (i.e., up to around 65°C). Practically, this reduces the choice to polypropylene, low-and high-density polyethylenes, polystyrene, and vinylics (essentially PVC). 

As such wood flour, used as a filler in thermoplastic composites, offers only modest, if any, reinforcement, but wood fibers can lead to superior composite properties and act more as reinforcing filler. Commercial wood flour is a by-product of the wood industry, often mechanically processed from waste materials such as planer shavings, chips, and sawdust, which are reduced to fine powders, with various grades available depending upon the particle size and the wood species. Wood (cellulose) fibers are produced through more or less complex defibrillation techniques, using raw materials from both virgin and recycled resources, and are different from natural fibers, such as jute, hemp, or sisal. 

Composites with wood material and polymer can be prepared in two ways. In the first, the wood fiber/flour is a reinforcing agent or a filler in a continuous thermoplastic matrix. In the second, the thermoplastic is a binder to the majority wood component. Only the first approach is discussed hereafter since a continuous thermoplastic matrix determines the processability of the composite material and is the necessary condition for processing WPC with conventional thermoplastic processing techniques, whilst certain adaptation of the equipment and operation may be required. 

Effect of (pine) wood flour on the mechanical properties of PP based composites. (Data from l.C. Caraschi, A.Lopes Leao. Mat. Res., Sao Carlos, Brazil, Oct./Dec.5 (4), 405-409,2002.) 

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