Do plastic/wood composites have a future ?

Date of publication 20 July 2020

Authors Vovk, Matej; Sernek, Milan

Sources Aluminium Trihydrate-filled Poly(methyl methacrylate) (PMMA/ATH) Waste Powder Utilization in Wood-plastic Composite Boards Bonded by MUF Resin. In : BIORESOURCES, vol. 15, n° 2, p. 3252–3269, 2020.




An alternative approach to producing a wood-plastic composite (WPC) from wood particles, aluminium trihydrate-filled poly(methyl methacrylate) (PMMA/ATH) waste powder, and melamine-urea-formaldehyde (MUF) resin is described. The surface of PMMA/ATH powder was modified with ureido- and amino-functional silane coupling agents at four different degrees of modification: 0.3%, 0.5%, 1.0%, and 2.0%. An X-ray photoelectron spectroscopy (XPS) analysis was executed on a silane-modified PMMA/ATH material, and the results revealed the presence of chemically bonded silanes on the PMMA/ATH surface. Contact angle measurements were also performed to calculate the surface free energies of the modified powders. Water contact angles of modified powders slightly decreased as surface free energy increased with the degree of surface modification. Mechanical tests of the composites showed that different degrees of surface modification had a significant influence on modulus of elasticity (MOE) and modulus of rupture (MOR) of the composite boards. However, there were no significant differences between the silanes used.

The SeaCleaners’ View :

Wood/plastic composite materials have been on the market for many years.
They are the subject of numerous innovations in terms of their composition and properties as well as their lifespan. The main challenge in the development of these materials is to replace natural raw wood in order to obtain increased properties, reduce costs and avoid additional chemical treatments. Indeed, natural wood of good quality is becoming more and more expensive. Its production requires cultivation over many years, a time scale that is not compatible with the current economic system. Forests, which are called “sustainably managed”, are in fact wild deposits, or long since planted, in which the removal of old trees is compensated by planting young trees for future generations. Foresters have turned to fast-growing species such as softwoods that provide sufficient wood qualities for indoor installations, but insufficient for outdoor use without appropriate and repeated chemical treatments. In addition, the mechanical properties of these woods are not up to the task of intensive use, such as a deck or terrace.

The technology of wood/plastic composites has become very advanced.
This article highlights the highly technical level of expertise surrounding the development of these composite materials. In a logic of financial profitability, this study shows that the margins realized on these materials and the sales volumes are sufficiently important to finance a high level of R&D. The key point is obviously the use of wood waste or poor quality wood associated with low cost polymeric materials. The result is a product with a low cost price and a high selling price justified by its technicality. It is interesting to note that the precautionary principle does not touch the minds of these scientists who use formaldehyde-based products whose use has been banned for joinery and interior fittings. The applications envisaged are certainly outside, but nevertheless there will be repeated contact with the skin of consumers, possibly children and animals. The inclusion of formaldehyde in the list of potentially carcinogenic products should motivate a technical choice towards other less harmful molecules, as well as for polymers of acrylic acid (PMMA) and aluminium.

An unmanaged end-of-life for these materials.
The approach of composite materials is very attractive to obtain high mechanical properties and durability, in general, in all fields. It is now known that their weak point is complex end-of-life management. Indeed, they are not recyclable as a raw material, and the separation of their elementary constituents is often very complicated if not impossible. In the specific case of plastic/wood composites, their lifespan extends over several years, which gives an impression of strength and safety to consumers. In fact, throughout their lifetime, the additives present in their composition will be released into the environment. The organic molecules will degrade under the combined action of water and UV light to form other more soluble molecules that will also diffuse into the environment. Finally, natural abrasion will lead to the formation of composite microparticles containing wood and plastic. The presence of wood will facilitate biological degradation, but the synthetic polymeric part will not be degraded, or very slowly. Microplastics will therefore be formed which will also diffuse into the environment. Finally, the end of life of the equipment will be managed, at best, in a sorting centre but, as this category of material is not recyclable, it will be either buried in a landfill or incinerated.

What if a less technically complex alternative was to use wood species resistant to degradation?
Teak, acacia, chestnut, red woods… are resistant to biological degradation and insect attack. They do not require chemical treatment and have a high durability, even in contact with the ground or under bad weather conditions. Wouldn’t it be interesting to inject all these financial resources applied to the research of composite materials into a system of reasoned cultivation with rapid rotation on naturally resistant wood species? This would have the advantage of being sustainable, having a reduced carbon footprint and a much lower impact on the environment, but also on the health of the users of this wooden equipment. The current cost of wood is linked to the structuring of the sector, which is not adapted to the demand of mass markets. Producers therefore confine themselves to market segments with very high added value that justify high selling prices.
Alternatives are possible. One example is the cultivation of chestnuts, a wood that is highly resistant to degradation and humidity, which has been used for centuries in constructions that need to last, for example as coverings in granaries to protect oak structures and stored grain. If the trees are cultivated for several decades in order to obtain wide trunks suitable for use in furniture, the costs are very high. In France, there are currently fast-rotating chestnut crops where the trees are cut to make panelling that is no more than 7 to 10 centimetres wide and therefore at a much lower cost. This is possible even with Western labour costs. It should be noted, by the way, that transport costs are reduced, as is the carbon footprint.
The other significant aspect of this approach is to allow end-of-life management by the users themselves. Indeed, the used structure made of natural wood can be used as fuel in fireplaces or adapted stoves. This would result in a virtual circle: no transport, a short circuit and better consumer responsibility.

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