Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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troubleshooting Wood-Plastics Composites Done Right Mixing and extruding wood fiber in a plastic matrix is technically demanding. It requires careful attention to formulation, drying, and extrusion temperatures. By Dr. Kirk Hanawalt, ENTEK Extruders Wood-plastic composite (WPC) products were first widely marketed in the early 1990s as weather-resistant decking. The product concept has evolved rapidly into railing, fencing, trim, automo- tive parts, and other applications. Adding wood to plastic to make WPC reduces the formula cost, raises the modulus of elasticity, and lowers the coefficient of thermal expansion. These benefits come at a cost, however: Extruding wood fiber in a plastic matrix is technically demanding. Wood is not the only natural fiber used in plastics composites, but it is the most common. Wood used for WPCs is ground, screened, and dried prior to extrusion. For decking and fence profiles, where a rough surface texture is acceptable or even desir- able, screening the wood fiber to 40 to 60 mesh results in good flow characteristics and ease of mixing into the plastic matrix. For railing and other profiles requiring a smooth finish, the wood is screened to 80 to 100 mesh. Fines that pass through a 120 mesh screen are not desirable: These flow poorly and are difficult to distribute evenly in the plastic matrix. Poorly distrib- uted wood fibers, called "wood spots," are a common quality 32 MAY 2012 PLASTICS TECHNOLOGY problem when the wood contains excessive fines or when the extruder is too worn to achieve good mixing. Ground wood fiber equilibrates at 8% to 12% moisture by weight, depending on the humidity. Residual water in the extrudate leaving the extrusion die causes swelling of the WPC profile, so it is necessary to remove essentially all of the water in the extrudate before it leaves the die and is exposed to atmospheric pressure. Extruders have devolatilization capability using atmospheric and vacuum venting, but the amount of water that can be removed from a WPC extrudate is diffusion-limited. For counter-rotating extruders, the highest production rates are achieved when the wood fiber is dried to 1% or lower moisture content before being fed into the extruder. Co-rotating extruders achieve better surface renewal of the extrudate than counter-rotating extruders and can tolerate higher wood moisture in the feed. The ideal wood mois- ture content is in the range of 2% to 4% by weight. Reducing the wood moisture below 2% in the feed for a co-rotating extruder can actually reduce the maximum throughput, because the evolu- tion of steam from the extrudate has a beneficial cooling effect . Counter- and co-rotating extruders can process wood fiber at moisture levels higher than 1% and 4% respectively, but at the expense of throughput. Reducing the throughput increases the maximum residence time in the extruder, which increases the

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