The symbiosis of plastic and wood can be aptly described as offering the best of both worlds. The materials combine the characteristics of plastic, such as durability and ease of care, with the visual and tactile advantages of wood. In the European market, Wood Plastic Composites (WPC) are used mainly in decking applications, i.e. panelling or floorboards. Since 2005 the European market for WPC has grown at an average annual rate of 35 percent. In the words of Michael Carus, Managing Director of the Nova Institute, Hürth, Germany: „After several years of double-digit growth in Europe we now see a single-digit figure of between 5 and 10 percent. The reason lies primarily in the drop in demand resulting from the economic crisis in Southern Europe.“

The year 2010 saw the production of some 220,000 tonnes of WPC in Europe, of which some 100,000 tonnes was produced in Germany; this material was also sold to industry in Europe, with about 50,000 tonnes going to the automotive industry and 167,000 tonnes being used for patio or deck flooring, fencing, and cladding. Ever more WPC materials are also being used to produce furniture, office and household articles such as tableware, compost bins, or toothbrushes, as well as small industrial components and casings. It is the firm opinion of Asta Eder, business consultant and WPC expert, of Composites Consulting, Vienna, Austria, that countries with high sales volumes of wooden products are also promising key markets for WPC.

Lack of Successful Product Marketing

Marketing is a key factor for the success of a material. „Before addressing the production process, the manufacturer should already have developed a marketing concept. Experience shows this to be the most difficult obstacle to overcome,“ maintains Thorsten Weber, Reifenhäuser Extrusion Technology, Troisdorf, Germany. And Carus also sees a need for action: „Deficiencies certainly exist here in marketing and public relations. Nor is there any support from the political arena – in stark contrast to the bio-fuels market. Moreover, the consumer is frequently unable to relate to the visual appearance. There is much still left to do.“
Nowadays WPC are used primarily in applications that emphasise product properties such as great rigidity and low shrinkage compared to pure plastics and superior durability and malleability compared to products made solely of wood. However, as prices for plastics rise, it will be a matter of just a few years before WPC pellets become cheaper than pure plastic pellets and can then conquer mass markets, according to a recent study undertaken by the Nova Institute. WPC are presently 20 – 30% more expensive than pure plastics.

As soon as the prices of PP and PE have exceeded a certain threshold (ca. €1.50/kg), WPC pellets will become less expensive than existing solutions owing to the significantly lower price of wood flour. „Large-scale markets will then open up“, Carus proclaims and Eder adds: „I see door and window production as one of the potential market segments for WPC in Europe.

The material has numerous applications in the automotive sector. „The EU automotive industry already uses about 50,000 tonnes of WPC per year and this amount could increase dramatically“, Carus believes. There are also examples of their use for interior furnishings. „The Ikea furniture company has several WPC products in its range“, according to Eder: „Shelves made of extruded WPC profiles are available from several manufacturers. In Asia WPC are used above all in living areas, for example as ceiling panels. In China interior doors are extruded from WPC, sometimes in full width. However, neither of these applications is to be expected in Europe.“

Purported Drawbacks of WPC

Consumers tend to view the claimed durability of WPC products with scepticism. The Nova Institute gives the following advice: A distinction is made between biotic (fungal) and abiotic (UV light) factors, which can exert individual effects on WPC material. However, newly developed additives can greatly reduce the effects of weathering and UV radiation on the materials, thus permitting manufacturers to guarantee their products for ten years or even longer. Microbial stability is largely assured. Carus adds: „Problems can arise predominantly with high proportions of wood, i.e. from 60 to 80 %. At lower proportions of 30 %, typical for injection moulding, the fibres are largely enclosed by the plastic. Moreover, the pressure applied during injection moulding is so high that the fibres become filled with plastic.“

Processing of WPC: Extrusion

In comparison with conventional wood materials, extrusion and injection moulding of wood-plastic composites offer an unprecedented variety of shapes at economically acceptable production costs. Extrusion machines are used to manufacture those products which belonging to the largest product category on the WPC market, viz. decking products. Major machine manufacturers include Battenfeld-Cincinnati, Krauss Maffei Berstorff, and Reifenhäuser Extrusion Technology.

„The greatest processing challenge is to combine various raw materials such as polymer, additives, pigments, and, of course, wood to create a suitable formulation. Yet it is important first of all to decide upon the processing method to be used, because not every formulation can be processed by every method, and secondly to decide upon the kind to product to be produced, indoor or outdoor. Choice of the appropriate die technology is another important point“, explains Thorsten Weber, Reifenhäuser, Troisdorf, Germany. In the last two years, Reifenhäuser has undertaken various steps to improve machine efficiency. „However, the most efficient machine is of little use if the end product is uneconomical with respect to its weight and processing speed.

This is the reason why we have bucked the current market trend towards solid profiles by developing a new profile which we will be showcasing for the first time at the K Trade Fair. This profile will be up to 50 % cheaper to produce than solid profiles“, according to Weber. But solid profiles offer other advantages. They can be cut diagonally without any problems and do not need any injection-moulded end caps, even when cut straight. Battenfeld-Cincinnati recently introduced its new Fiberex 114 parallel twin-screw extruder suitable for the production of solid WPC profiles on the occasion of the 9th Wood Plastic Composite Conference held at Vienna.

Significant machine wear and the low bulk density of the pellets represent further challenges in WPC processing. The composites are usually mixed immediately prior to extrusion. „WPC processing in a production environment is characterised by the interaction between material, extruder, and die and by wear protection to lessen the abrasive action of the WPC“, says Michael Finkenzeller, Product Manager Renewable Raw Materials at Krauss Maffei Berstorff, Munich, Germany: „Explicitly, it is vital to keep an eye on the wear behaviour of the processing unit during the initial phase.“ Krauss Maffei Berstorff protects the screws against wear with a tungsten carbide armour plating while the barrels are lined with a bi-metal bushing, thus ensuring a long service life of the processing unit. „In the course of the last twelve months we have increasingly observed that many processors are expressing greater interest in parallel twin-screw extruders rather than in conventional conical twin-screw extruder technology, owing to possible wear problems“, Finkenzeller expounds.

Yet Sonja Kahr, Product Manager, Division Construction, Battenfeld-Cincinnati, Vienna, Austria adds: „The same machine uptimes can actually be attained for conical and parallel machines, given the right quality of the feed material. We use a specially developed vacuum system for removal of the residual moisture present in natural fibres. The moisture poses different challenges on the degassing system from those encountered, for example, in PVC processing. WPC are friction-sensitive materials, with a narrower processing window than other plastics.“ And she continues: „A further problem is the low bulk density and the poor flowability of the raw material, which complicate metering and feed to the extruder.“ For example, long fibres are more difficult to meter and require specially adapted equipment for feeding them into the extruder, says the expert from Krauss Maffei Berstorff.

„Moreover, the extrusion process can influence fibre length. Long fibres also favour the formation of nests and agglomerates, which can lead to inhomogeneities in the end product.“ Attention is drawn here to the complex material preparation, including grinding and screening. „Another topic of importance is coextrusion in cases where existing lines are equipped with a coextrusion unit“, notes Sonja Kahr, Battenfeld-Cincinnati. Moreover, a distinct trend towards two-stage processing is to be observed. „This could substantially reduce the EUR h/kg ratio. Use of pellets as starting material makes higher outputs and greater process stability attainable“, Kahr confirms. And Finkenzeller continues: „We can also observe a certain degree of rethinking with regard to processing technologies.

In our experience, most processors formerly embraced direct extrusion, where deviations in the input parameters, such as wide variations in the residual moisture of the fibres, exerted a direct influence on the optical and mechanical properties of the starting material. Meanwhile, a general preference for two-stage extrusion has emerged, with a homogeneous material having a residual moisture of less than 1.5 % ensuring constant profile extrusion.“ Battenfeld-Cincinnati also sees scope for process optimisation in material preparation, since this provides the very basis for reliable processing. „In the field of profile extrusion – above all in the high-performance sector, i.e. in excess of 800 kg – appropriately adapted degassing concepts increase productivity“, Kahr adds. And Finkenzeller also sees further scope here: „To ensure almost ideal material efficiency it makes sense to grind the reject WPC material produced on start-up and return it to the process, or to process it separately as a secondary profile, e.g. for subconstruction.“

Processing by Injection Moulding

The range of possible products can be greatly extended by injection moulding. Large thick-walled components can be produced in very short cycle times by this method, according to WPC consultant Eder: „In particular, the Werzalit Company, Oberstenfeld, Germany has been successfully producing furniture-related products from S2 WPC pellets for over ten years.“ Moreover, WPC parts can be removed earler from the mould, thus shortening the cycle time, says Wolfgang Roth, Head of Application and Process Technology at Wittmann Battenfeld, Kottingbrunn, Austria: „These shorter cycle times permit an energy reduction per unit.“ And Carus predicts: „Use of wood flour instead of talcum as a filler for plastics leads to improved mechanical properties with regard to rigidity and dimensional stability on exposure to heat. A materials giant is slumbering here, waiting to be awoken.“ Processing costs fall dramatically compared to products produced by conventional means from wood.

„Injection moulding yields ready-to-use components in a one-step, non-cutting process“, says Peter Pokorny, Head of Application Engineering at Engel Austria, Schwertberg, Austria. Plastics processors wishing to process WPC on their injection moulding machines can convert their existing equipment. „That simplifies entry into the field and keeps investment costs low. The necessary modification concerns the plasticising unit. In order to reliably counteract colour changes of the raw material, WPC are processed with plasticising screws having a long compression zone and which operate at comparatively low compression. Furthermore, WPC processing requires corrosion-resistant steels for the screws, the hot runners, and the moulds. Since the moulded parts are usually large, this segment is dominated by Engel Duo machines“, explains Gerhard Bäck, Technology Development, Technology Centre for Lightweight Construction Composites, Engel Austria, St. Valentin, Austria.

Expert Jürgen Schray, AWT-Beratung, Arburg, Loßburg, Germany is of a
similar persuasion: „Only the compression ratio of the screw and the geometry of the non-return valve have to be adapted to match the filler content of the material. The barrel temperatures are set lower than in plastics processing to protect the natural fibres. We actually showcased such a machine making boxes from natural fibre and cellulose acetate at the Arburg Technology Days 2013.“ „In the processing of composites, attention should be focussed on materials handling, metering, material quality and moisture content, and the mould,“ Roth adds. Storage and drying require particular attention because the composites are hygroscopic. And Pokorny comments: „Since drying has to be performed at relatively low temperatures and takes correspondingly long, it is advisable to use ready-dried raw material sealed in plastic and to subject it to in-line drying; the moisture content is measured prior to processing. Any residual moisture still present can cause both ‘bloated‘ parts with surface defects and corrosive attack on metal components in contact with the melt.“

Professional Advice is Indispensable

Machine manufacturers provide some degree of support for plastics processors considering the use of WPC. And Carus urgently recommends that prospective users seek comprehensive information from competent consultants: „There already exists a considerable body of experience which should be put to use. In Germany we are fortunate that the world’s largest WPC congress takes place every two years at Cologne – accompanied by world’s largest WPC exhibition. The next event in the series is due in December 2013.“ „All the leading service providers and consultants will be present. European compounders, machine manufacturers, and also research institutions all offer consultancy services. My approach is prompted by market needs, in order that product development stays attuned to the market. I provide workshops and market data. I also offer support in the search for the right business partners“, is how Asta Eder characterises her range of consultancy services.

Autor

Über den Autor

Etwina Gandert Dr. Etwina Gandert is an editor of Plastverarbeiter. etwina.gandert@huethig.de