Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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Standard Extrusion (Melt) Temperatures for Plastics Used for WPCs PVC Polyethylene Polypropylene 186°C - 206°C 183°C - 216°C 199°C - 246°C can be significant variation in the extrusion process over time. This variation can affect maximum throughput, yield, and scrap rates. Variations in process throughput of ±15% over the course of a few days are not uncommon. These variations can occur at the same frequency as plastic delivery to the WPC production plant. Measuring the melt-flow index of the incoming plastic gives an incomplete but still useful indication of the processing characteristics of the plastics. But even this relatively simple measurement is often lacking at WPC facilities. WPC production makes significant use of lubricants as a process aid. Lubricants in WPC have two broad functions. The first is minimizing shear heating of the extrudate to limit the melt temperature—known as "internal lubrication." The second func- tion is to overcome the no-slip boundary condition on the sur- faces of the profile die so that the profile has a smooth "skin"— known as "external" lubrication. Lubricant levels in PE-based WPC formulas are typically 3% to 5% by weight. PP-based for- mulas typically contain 1% to 3% by weight lubricant. In the largest single market for WPCs—outdoor decking —the mechanical properties of WPCs are important to both the install- er and end-user of the product. The product's resistance to bend- ing under a load, or modulus of elasticity, determines how much structure is needed to support the deck surface and the "feel" of the deck under the end-user's feet. The product's resistance to breaking under load, or modulus of rupture, is a matter of end- user safety and long-term satisfaction with the deck installation. Compared to 100% wood, WPCs have lower moduli of elasticity and rupture. Coupling agents have been developed to improve the mechanical properties of WPCs. The most common coupling agents are maleic anhydrides grafted onto a polyolefin backbone. These work by improving the adhesion of the plastic matrix to the surface of the wood fiber. The benefits of coupling agents must be weighed against increased formula cost, higher specific energy in the extrud- er, and reduced effectiveness of external lubrication in the profile die. The use of a capstock layer on WPCs raises the performance of the product in several ways. PE- and PP-based WPCs are relatively soft and have poor resistance to scratching. The wood fiber in the WPC is prone to attack by mold. Colorants, antioxidants, and anti-UV agents are expensive, especially when they are distributed equally throughout the WPC profile when only the surface of the product needs to be colored and protected. A capstock layer of a harder plastic with a concentrated package of additives mitigates all 34 MAY 2012 PLASTICS TECHNOLOGY FIG. 2 Thermal Gravimetric Analysis of Wood Fiber 4.917% 1.103% 100 80 60 40 20 0 DRYING 73.86% SLOW DECOMPOSITION 5.670% RAPID DECOMPOSITION 100 200 300 400 500 600 Temperature, °C Colored bands indicate temperature ranges where drying (water loss) and thermal decomposition occurred. This thermal behavior is typical for a wood-fiber sample. FIG. 3 Medium Density Fiberboard Sample: Drying and Decomposition 100 80 60 40 20 0 Thermal Gravimetric Analysis - MDF Product 3.393% 1.853% 29.80°C DRYING 64.35% 303.28°C SLOW DECOMPOSITION 9.967% RAPID DECOMPOSITION 100 200 300 Temperature, °C For this sample of medium-density fiberboard, colored bands indicate temperature ranges where drying (water- loss) and decomposition occurred. Because significant mass loss commenced at 150 C, this material was not usable for WPC extrusion. of these problems and greatly improves the performance of the WPC product. However, the benefits of capstocking are offset by the additional complexity and cost of the process. ABOUT THE AUTHOR Dr. Kirk Hanawalt is president of ENTEK Extruders in Lebanon, Ore. He started his career with ENTEK in 1995 as a process-control engineer and has also held the positions of director of environmental compliance, director of engineering, v.p. of worldwide sales, and v.p./COO. Contact: (541) 259-1068; e-mail: khanawalt@entek-mfg.com; entekextruders.com 20.35% 0.9642% 400 500 600 13.76% 0.6904% Weight, % Weight, %

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