Wood-Plastics Compatibilisers

Wood plastics composites have been successfully and rapidly developed in North America to produce a market of 690,000 tonnes, and they are now becoming more popular in Europe and Asia. Compatibilisers improve the water resistance and rot resistance and transform the mechanical properties, particularly the strength. They also improve the heat distortion temperature. This is because of the poor compatibility between polar cellulosic fibres and nonpolar polyolefins. In practice many manufacturers do not yet use compatibilisers because of their high cost. 

The preferred compatibilisers for wood-polyolefm composites at present are maleated polyolefins. Crompton Corp. supplies maleated LLDPE (Polybond 3109), maleated PP (Polybond 3200) and maleated PE (Polybond 3209) for the wood-plastics composite market. A polyethylene copolymer containing a polar co-monomer is also available. The 3109 compound is said to improve the notched Izod impact strength by 550% when used at the 5% dose level. 

Maleic anhydride-functionalised polyolefins feature in Solvay s Priex range of compatibilising resins, some of which have very low viscosities so that they can be emulsified in aqueous systems. They are used to compatibilise resins with wood fibres in the production of Wood-Stock wood-plastics composites for the automotive industry. (There is also another type of Priex product, consisting of ionomers designed to improve the mechanical properties of polyolefins.) 

Clariant offers a maleated PP imder the Hostamont name. Around the end of 2003 Clariant also introduced a maleated polypropylene wax called Licomont AR 504 as a coupling agent and compatibihser, also for wood and natural fibre composites. The viscosity was deliberately low to facilitate fibre impregnation. Benefits claimed were improvements in the toughness and elasticity of the products. 

ExxonMobil Chemical s Exxelor compatibilisers include maleated PP and maleated elastomeric ethylene copolymers. They improve the impact strength of wood plastics composites, but there needs to be at least 50% wood in the composition to see the full effect. They also improve the water absorption and allow the wood content to be increased from 50% to 60%, giving better mechanical properties with no net increase in materials costs. Some of these additives are also recommended for use in polymer blends such as PA/ABS or PA6/PA6-6. 

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DuPont Dow Elastomers has evaluated chlorinated polyethylene as a compatibiliser for PVC-based wood plastics composites. 

Compatibilisers make two largely incompatible polymers mix together to form a new blend or alloy. There has been a steep rise in demand for them in recent years, because polymer blends provide a fruitful way to tailor polymer properties to specific applications. They have also led to improved impact modifiers which, being polymeric, must form a blend whenever they are used. Some of the newer alloys, such as Rhodia s PA/ABS and Crompton s PP/SEBS, are highly impact-resistant. Compatibilisers are also used to improve the properties of mixed plastics waste and to compatibilise polymers with wood or mineral fillers. 

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