Wood plastic composites have seen good growth particularly in USA over the last decade. Processability and compatibility of wood flour with polymers are the two major technical obstacles faced by the processors and the equipment ssuppliers while developing wood plastic composites.
The rapid expansion of the wood plastic composites industry over the past several years is largely based on the performance advantages they provide over competing wood materials. These advantages include better flexural and impact strength, better moisture resistance, less shrinkage, and improved weatherability. A key element behind these improvements is the additives incorporated into wood-filled plastic formulations. Perhaps the most important of these additives are coupling agents - chemicals that enhance the compatibility of the nonpolar plastic resin molecules with the highly polar cellulosic wood fillers. Coupling agents help transfer the inherent strength of cellulosic wood fibers to the surrounding plastic by improving the bonding between cellulose molecules and hydrocarbon-based polymers. They also aid in dispersion of the wood fillers.

Coupling agents are used in combination with other additives. Among them are lubricants, heat stabilizers, light stabilizers, colorants, biocides and foaming agents. Sometimes these additives have antagonistic effects - some lubricants, for example, tend to interfere with coupling agents. Development work in additives aims to keep such interference to a minimum. Another goal is to come up with multifunctional additives, such as a single compound that acts as both a coupling agent and a lubricant.

Polymers used in wood plastic composites are mostly polyolefins such as polyethylene (particularly HDPE) and PP. PVC is also used as well, particularly in windows, doors and house shingles. Styrenic polymers such as PS and ABS find occasional applications in wood composites. Composites contain both wood fibers and wood flour. The wood fillers make up anywhere from 50% to 70% of a composite. Coupling agents are generally added at levels of 1% to 3% of the weight of the wood fillers.

Maleated polyolefins are possibly the largest group of coupling agents is polyolefins. These consist mostly of PE or PP with maleic anhydride functional groups grafted onto the polymer backbones. Grafting is done with peroxide reagents and takes place at tertiary carbons in the polymer chain or at terminal olefinic groups. When the grafted polyolefins are melted with polymers of similar composition and then cooled, they crystallize into the base polymers, while the maleic anhydride groups react with the hydroxyl groups on the surface of cellulosic fibers to form strong covalent ester linkages. Maleated polyolefin additives are available in pellet form and can be added to standard extrusion or injection molding equipment.

Other coupling agents employed in wood-plastic composites include organosilanes, fatty acid derivatives, long-chain chlorinated paraffins, and polyolefin copolymers with acid anhydrides incorporated into the polymer backbones (instead of grafted).

Typically available in LDPE, HDPE and PP base polymers; maleated polyolefins can impart some dramatic improvements to wood-plastic composites. One maleated polyolefin coupling agent grade from Dupont (Fusabond) is said to increase the tensile strength of wood-polyethylene composites by 200% to 300% compared to PE composites without coupling agents.

Another product group of maleated polyolefin from Equistar has been reported to boost Izod impact strength of wood-plastic composites by up to 300%. The modulus of rupture of wood composites rises by 100% with the addition of 2% of this group of coupling agents from Equistar.

Similarly, a 3% loading of a maleated polyethylene coupling agent (Eastman Chemical's Epolene G-2608) can double the tensile strength and triple the impact properties of wood-polyethylene composites, compared to composites lacking the coupling agents.

For maximum effectiveness of maleated polyolefin coupling agents it is important to use the right lubricants in the formulation. According to research data from Equistar, zinc stearate lubricants have an antagonistic effect on the performance of maleated coupling agents. Performance of the maleated additives is largely retained, however, if a non-metal stearate lubricant is used.

One producer of polyolefin-based coupling agents, Dyneon, has developed a proprietary method of controlling the polymer architecture of these additives in ways that are said to maximize their performance. The technology, known as controlled-architecture materials ( CAM ), includes maleic anhydride and other functional groups. These coupling agents are incorporated into an additive package that includes lubricants. According to the company, 1% of the CAM additive package is capable of increasing the MOR in a PP-wood composite by up to 41%, while reducing extruder pressure by 50%. Reduction in edge tear, lower scrap rates and improved surface aesthetics are other claimed benefits of the new additive package.

Replacing grafted anhydride polyolefin coupling agents with copolymers in which the anhydride is one of the reactive monomers is a recent development in wood-plastic composites. Among the benefits of this approach is that less of the coupling agent is needed, Dupont has developed a grade of Fusabond W PC 567 D on copolymer of polyolefins. This grade can give good performance even at a low level of 0.5 %. Even at this level, however, it provides a threefold reduction in water absorption by HDPE composite with 55% wood over a 30-day period, compared to a composite with no coupling agent. Meanwhile, the new additive is reported to yield a twofold increase in strength of the composite, plus a corresponding improvement in its stiffness.

Organosilanes can be used to enhance compatibility of wood and plastic resins in two ways. Wood fibers can be treated directly with the silanes, thereby making them hydrophobic and thus compatible with the resin. The silanes can also be added as coupling agents during the compounding step, improving adhesion of wood particles and base resins. Reported benefits of organosilane coupling agents include enhanced tensile, flexural and impact strength, improved heat deflection temperature, and lowered moisture absorption.

Long-chain chlorinated paraffins (LCCPs) have also achieved success as coupling agents for wood-plastic composites; they also have lubrication properties. The addition of 5% to 7% LCCPs (based on total composite weight) increased the flexural strength of 60% wood composites by as much as 40%. One company, Dover Chemical, markets commercial products containing LCCPs and proprietary lubricants.

The products improve lubrication of wood-plastic composites during processing without reducing compatibilization between base resins and wood fillers. Benefits of these formulations include reduced power consumption by processing equipment, lower processing temperatures, and increased flexural strength of the composites.

Several lubricant systems are also claimed to improve the compatibility between base resins and wood fillers. Struktol's general-purpose system (TPW 101) is a mixture of zinc stearate and waxes intended to enhance processing characteristics of highly filled polyolefins, but is also designed to improve dispersion properties of the fillers. Another entry from the company (TPW 104) is a blend of aliphatic carboxylic acids and salts plus mono- and diamides. It is said to raise output of polyolefin-wood composites and improve their surface quality. Another lubricant (TPW 113), a blend of fatty acid esters, offers improved wetting of fillers in polyolefin-wood composites. One Struktol lubricant system is designed to improve the processing properties, surface characteristics and physical properties of PVC-wood composites. The formulation, TPW 012, is a proprietary blend of oleochemicals and waxes.
(Source Courtsey: Omnexus )