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Modifying polymers with additives and colorants has always been an important innovation issue in the plastics industry. In fibre-reinforced plastics, conventional inorganic fillers such as chalk, talcum, glass, aramid, and carbon, as well as natural fibres were used initially. Then, following decades of falling material prices and the wider availability of technical reinforcing fibres, natural materials were relegated to second place. However, in view of the increasing environmental awareness and the search for resource-saving alternatives, the situation has now changed fundamentally. The demand for natural fibres as a reinforcement for composites is on the rise.
(Published on January-February 2009 – JEC Magazine #46)
BEATE TREFFLER, TECHNICAL MANAGER WAXES, CLARIANT PRODUKTE (DEUTSCHLAND) GMBH
Wood Plastic Composites (WPC) are a special type of natural fibre-reinforced composite. Using wood fibres offers ecological, economic, and technological benefits. Compared to wood, WPC give designers more flexibility because the parts can be shaped three-dimensionally, and they also have better moisture resistance. Compared to plastics, WPC offer superior stiffness, and a significantly lower thermal expansion coefficient.
Wood, like any other organic material, has only limited thermal resistance. This makes polymers with a melting/processing temperature of under 200°C the best choice for WPC. In the case of a polymer with a higher melting temperature, the wood fibres would undergo thermal decomposition, even with short processing times. This rules out the use of many polyamide, polycarbonate and polymethyl methacrylate materials as a matrix for WPC. Worldwide, the matrix materials used are almost exclusively thermoplastics, such as polyethylene (65%), polyvinylchloride (16%), polypropylene (14%) and polystyrene (5%). In Europe, polypropylene is by far the most commonly used matrix material for WPC, at 82%.
For more convenient dosing, the wood component is often added in the form of pellets. Different types of loose fibres are also available on the market, differing in their fibre geometry and type of wood. Due to their ample availability and reasonable price, soft woods such as pine, spruce or cedar, or mixtures thereof, are the most commonly used for wood fibres in Europe. In other regions of the world, rice hulls and peanut shells, aloe hemp, cotton, rubber tree, oak, and maple are used. The wood content of wood fibre-reinforced plastics ranges between at least 50% to a maximum of 90%.
WPC are processed using standardised thermoplastic moulding methods, such as extrusion, injection moulding, or hot pressing, all of which require minor adjustments to fit the properties of each material. The standard methods must undergo several technological changes to suit WPC, for several reasons: the very low bulk density of the wood chips, which are incorporated into the polymer matrix in large quantities; large amounts of water vapour forming in the process because of the high equilibrium moisture content of wood (10%); and, last but not least, the difficulty of filling the highly viscous molten mass evenly into the profile tools.
Economics and applications
The WPC market has developed rapidly over the recent years, especially in the United States and Japan, where it recorded an annual growth rate of more than 25%. With an annual production of 70,000 metric tons, the European market is considerably smaller than the North American one, which accounts for over 700,000 metric tons annually. WPC applications with a very large market volume in the United States, such as decks, are still underdeveloped in Europe, including Germany. However, the European WPC industry is becoming more and more successful in finding suitable applications for WPC. Applications such as injection-moulded furniture or automotive components will give the production volume a tremendous boost.
To fulfil the high requirements of these applications, WPC needs to be improved on different levels. Improved impact resistance and UV stability, reduced water absorption and improved dimensional stability are just a few of the goals that the WPC industry is pursuing. Additives amount to only 2-4% of the total weight in a typical composite. However, it is the judicious selection of additives that gives WPC their superior profile. Depending on the individual application, almost all additives established in the plastics industry are also used as additives in WPC, including UV stabilisers, lubricants, impact modifiers, pigments, coupling agents, and odour absorbers. The Pigments & Additives Division of Clariant offers a comprehensive portfolio of special additives for WPC, which are continuously developed in close cooperation with WPC processors.
Coupling agents and compatibilisers
Interface adhesion is of special importance in natural fibrereinforced plastics, because only a limited fibre length is available in the composite for stress transmission. To maximise stress transmission between wood fibres and the plastic matrix, it is indispensable to add agents that improve bonding and compatibility, also referred to as “coupling agents”. Stress is transferred as shearing stress from the matrix to the fibres. If the fibre-matrix bond is weak, it will cause sliding friction of the fibre composite at the interface. This will lead to a shift between fibre and matrix, and eventually, under increasing stress, to fibre pullout and a premature failure of the composite. The quality of the fibre-matrix bond depends on the formation of both chemical and physical bonds (Van-der-Waal forces and entanglements).
Comprehensive tests have demonstrated that maleic anhydride (MAH)-functionalised polyolefins significantly improve the mechanical and physical properties of WPC. Polypropylenes grafted with MAH have non-polar areas and polar functional groups that react with the hydroxyl group of the wood, forming covalent ester bonds. The newly formed carboxyl groups (-COOH) can form hydrogen bonds with the free OH groups of the wood. The non-polar areas have a strong affinity with the hydrocarbons of the polyolefin chain. Acting as a connecting bond between the wood fibres and the polymer matrix, the coupling agents significantly increase the flexural strength and stiffness, dimensional stability and impact resistance of woodplastic composites. Bonding agents also reduce material creep and water absorption.
Under the trade name Licocene®, Clariant offers highperformance waxes produced with metallocene catalysts. The sandwich-like molecular structure of the metallocene catalysts can be modified as needed, making it possible to synthesise waxes that are perfectly tailored to a particular application. A newly launched product is the top-of-the-line Licocene® PP MA 7452 TP, a bonding agent grafted with MAH that generates excellent results in polypropylene compounds in terms of mechanical properties, water absorption, and processing behaviour. An economical alternative is Licocene® PP MA 6452 TP, which has a lower drop point than Licocene PP MA 7452 TP while also offering good results in polypropylene compounds. In polyethylene composites, Licocene® PE MA 4351 TP, a MAHgrafted polyethylene wax with high polarity, provides outstanding results. Its low melting viscosity, producing optimal wetting of the natural fibres with the coupling agent, supports the high effectiveness of the product.
Waxes or wax-like products used as lubricants are also crucial WPC additives. These additives usually consist of fatty acid derivatives, montanic wax esters, polar and non-polar polyolefin waxes, or amide wax, and fulfil diverse functions. Adding suitable lubricants has a positive impact on the surface quality of WPC components. Among other things, lubricants facilitate mould release, influence the flowability of the plastic molten mass, and serve as a compatibiliser between the fibre and the plastic. The use of an appropriate lubricant, such as a Licomont® ET 141, Licolub® H 12 or Licolub FA 6 TP, gives natural fibrereinforced composites a smooth, high-quality surface. The products also reduce the tool cavity pressure, which increases the throughput. And, finally, materials to which these additives have been added absorb less water and show less swell. To protect WPC components from exterior influences, light stabilisers and flame retardants are also usually added. Additional high-quality additives, such as pigments for attractive colouring and odour absorbers, make it possible to combine two highly different materials - wood and polymers - into a firstclass material with an excellent property profile. Current applications include outdoor flooring and stairway railing, furniture and high-quality automotive products. Continuous product improvements will open additional markets worldwide, help WPC become established as a material using mainly renewable resources, and open up new and interesting perspectives and unexpected possibilities.