By: Jonas Burke

Color consistency and durability are critical to wood-plastic composites, especially in outdoor applications. So WPC makers take great care to select ingredients and processing conditions that enhance mixing of colorants, wood flour, and other components. This is important both for aesthetics and weather-ability of the finished product.

Because wood does not absorb common mineral-based pigments, coloring of wood flour depends entirely on how well the wood is wetted by resin and the extent to which the wood fibers are de-agglomerated and dispersed within the polymer matrix. Inadequate dispersion will lead to visible white flecks of unpigmented wood flour. Wood particles at the surface are also exposed to the elements without protection from resin, which leads to moisture and microbial attack and loss of physical properties.

Colorants are typically selected for good dispersion in the matrix polymer of the WPC. So poor mixing in the extrudate indicates a problem either in operating conditions or in the ingredients that aid dispersion--mostly lubricants and process aids.

Troubleshooting should begin with checking barrel-zone temperatures, melt temperatures, motor amperage, and screw rpm. If these operating conditions are within specification, then the formulation and material interactions should be considered.

Check barrel temperatures
A conical twin-screw extruder, common in WPC extrusion, may have four barrel-temperature zones. Zones 1 and 2 usually have higher heat settings to overcome the insulating properties of the wood flour and help compress, melt, and mix. Zones 3 and 4 remove heat and control melt temperature. If Zones 1 and 2 are too cool, poor mixing can result.

Low melt temperature may prevent polymer from wetting the wood adequately and also make it difficult to disperse colorants effectively. Check for powder in the vent, as a large build-up of powder there is an indication that the temperature is too low. Try raising temperatures in Zones 1 and 2 by 5[degrees] F every half-hour and note the effect on color mixing.

Too high a melt temperature can also cause mixing problems by lowering the melt viscosity so far that inadequate shear stress is generated to disperse the wood. High temperatures can also cause thermal degradation of the polymer or wood, which can lead to color variations caused by yellowing. At any sign of thermal degradation, barrel temperatures should be reduced.

Check motor amperage
Amperage is an indirect measure of the amount of work being performed by the extruder. A feed rate that is too high or screw rpm that is too low will overfill the screw flights. That reduces mixing of colorant and dispersion of agglomerates by lowering the work done on the melt. It may show up as higher amperage draw on the motor. Alternatively, screw rpm may be too high for the feed rate, which will result in underfilling the screw flights. This can also cause poor mixing and may show up as lower motor amperage.

If there are large wood agglomerates in the extrudate and adjusting the feed rate and screw rpm doesn't improve mixing, that may indicate that unwanted fines from wood flour and other fillers are forming hard agglomerates that require more shear force to break up than the extruder screws can deliver. Compounders should check the effectiveness of their fines removal. Many compounders have pneumatic screening to remove fines from the powder blend.

Know your lubricants
Dry pigment powders such as iron oxides are often used to color WPC profiles. But dry powders are dusty and imprecise to feed, and they contaminate other product runs. Liquid colorants can also be messy, and they're expensive.

Thus, many compounders use pellet concentrates for more predictable feeding and easier handling. However, the melt-flow index (MFI) of the carrier resin and the lubricants used in the carrier may have unwanted rheological effects on the compound as a whole.

Concentrate suppliers rarely provide information on the MFI of the carrier resin and the lubricants it contains. But the MFI of the carrier resin can drastically affect WPC processing and product quality, even at the typical 3% loading. Concentrates with a high MFI work better with lubricants that are less lubricating, and vice versa.

Color masterbatches may contain lubricants that interact with the WPC's lubricant package and cause over- or under-lubrication, producing rough profile surfaces. And if the WPC formulation contains a coupling agent, the lubricants in both the composite and the color concentrate must be free of metallic stearates stearate any salt (soap), ester, or anionic form of stearic acid.