Plastics Technology

MAY 2012

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wood-plastic composites troubleshooting percentage of water that can be removed from the extrudate. If the amount of water in the wood fed to the extruder is double the recommended levels—e.g. 2% in counter-rotating extruders and 8% in co-rotating extruders—the maximum throughput of the extruder will be roughly halved. WPC profiles with excess moisture at the die develop localized bubbles, or "measles" under the "skin" of the profile. If the mois- ture level is high enough, the "skin" will separate almost entirely from the underlying profile as the water vaporizes and expands. Wood is a complex mixture comprised primarily of celluloses and lignin. This is important in WPC production because cellu- loses and lignin thermally degrade at temperatures above 190 C (374 F). Figure 1 shows the thermal gravimetric analysis (TGA) of wood fiber, which is a plot of the percent weight remaining of the wood fiber sample versus temperature. There are two significant mass-loss events in the TGA analysis: loss of water at 25 C to 125 C, accounting for 6.02% of the initial sample weight; and the decomposition of celluloses and lignin be- tween 190 C and 400 C, accounting for 73.86% of the sample weight. Figure 2 shows the temperature ranges for wood drying, slow decomposition of the celluloses and lignin, and rapid decomposi- tion of the celluloses and lignin. The difficulty this poses for the WPC producer is evident when considering the standard extrusion temperatures for the plastics commonly used in WPC, shown in the table on p. 34. These are the range of typical melt temperatures seen at the entrance to the extrusion die. The values in the table overlap almost completely with the temperature range labeled "Slow Decomposition" in Fig. 2. This means that the producer of WPCs is making a profile in which the constituent with the highest weight percentage in the formulation is likely to be undergoing thermally induced decomposition as it enters the extrusion die. Preventing this decomposition—or at best controlling it adequately—poses the greatest challenge in WPC extrusion. Common WPC quality problems arising from thermal degradation of wood are distortions in the extruded profile, known in the industry as "dog-boning," "swelling," and "ridging." Figure 3 shows the thermal gravimetric analysis of a medi- um-density fiberboard (MDF) sample. This fiber source was cheap, plentiful, and unsuitable for extrusion. Note that signifi- cant mass loss commences at 150 C, probably from the decom- position of a resin or glue used in the MDF process. THE ROLE OF RESINS Newcomers to WPC with a background in plastics tend to think "wood is wood." Newcomers from the wood industry tend to think "plastic is plastic." In reality, both the fiber and plastic properties are critical to extrusion success. PVC, polyethylene, and polypropylene are the most com- monly used plastics in WPC extrusion. PVC is suited to counter- rotating extruders with low (<50 rpm) screw speeds because it degrades when subjected to high shear or temperatures above the standard processing temperature. PE and PP are better suited for FIG. 1 Thermal Gravimetric Analysis of Wood Fiber 4.917% 1.103% 100 80 60 40 20 0 73.86% 5.670% 13.76% 0.6904% 100 200 300 400 500 600 Temperature, °C The curve shows the weight of sample remaining vs. tem- perature. It indicates two major mass-loss "events' typi- cal of wood fiber—the first representing loss of water as steam, and the second the thermal decomposition of the wood celluloses and lignin. co-rotating extruders because these polymers can withstand higher shear and temperatures than PVC. This allows co-rotat- ing extruders to reach throughputs in excess of 3000 lb/hr (1365 kg/hr) in a machine with a diameter of 103 mm. Because WPC is a temperature-limited extrusion process, the rough rule of thumb for scale-up is by the square of the diameter of the extruder. For example, given a known throughput of 1600 lb/hr on a 73-mm extruder, the expected throughput of a 103-mm extruder would be (103/73)2 x 1600, or 3185 lb/hr. PE is the most commonly used plastic for WPCs in the U.S. HDPE is used in WPC applications because it offers better physi- cal properties than LD or LLDPE. HDPE with a melt-flow index between 0.3 and 1 g/10 min, known as fractional-melt-flow grades, are commonly used for WPC decking. Fractional-melt- flow HDPE offers better physical properties than higher-flow HDPEs, but there is a limit on how low a melt flow is practical. HDPEs with very low melt flows, say less than 0.2 g/10 min, require higher specific energy to process in the extruder. This results in higher melt temperatures. Because the wood in WPCs degrades at temperatures above 190 C, to maximize throughput the WPC pro- ducer should choose a plastic with the highest melt flow index that consistently gives the required physical properties in the end product. VARIATION IN PLASTIC FEEDSTOCK Plastics are chemically simple, but complex with respect to chain length, chain configuration, molecular-weight distribution, crystal- linity, etc. These properties are seldom measured at WPC extrusion plants, but can have a dramatic impact on WPC production. Much of the plastic used in WPCs is recycled, and another significant fraction is wide- or off-spec virgin material. Because the properties of these streams are not tightly controlled, there PLASTICS TECHNOLOGY MAY 2012 33 Weight, %

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