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From a technical standpoint, drying non-hygroscopic materials such as polyethylene and polypropylene may not be necessary. However, doing so may be advantageous to your process. Here are details. Polymers of all kinds can be classified into one of two groups: hygroscopic or non-hygroscopic. Hygroscopic materials such as nylon, ABS, PC, and polyesters— to name just a few—will attract and absorb water mol- ecules into the plastic's interior structure. Non-hygroscopic plastics such as polyole- fins (polyethylene and polypropylene), on the other hand, will not absorb water molecules into the polymer structure. Because hygroscopic polymers attract and absorb water mole- cules from the atmosphere, they must be dried prior to processing to achieve consistent production results regardless of changes in the ambient temperature and humidity conditions that occur throughout the year. But what about polyolefins? Polyolefin processors molding and extruding products for automotive, medical, and certain other markets some- times predry materials to remove surface moisture and, in effect, "preheat" or soften the material to facilitate melting. But this is not a common practice. Still, the fact is that processing poly- olefins without predrying can lead to problems: • Although polyolefins are non-hygroscopic and will not attract moisture from the surrounding air, they will certainly pick up some surface moisture when exposed to high ambient humidity. While a very small amount of surface moisture may not cause By Pete Stoughton, Polymer Drying Services The fact is, processing poly- olefins without predrying can lead to problems. Why Drying Polyolefins Might Be Right for Your Process In a hot-air dryer, hot air will absorb and hold more water than cool air. As shown here, for example, ambient air at a temperature of 75 F with a dewpoint temperature of 60 F has a relative humidity of 59.62%, meaning it has absorbed over half of the water that it is able to hold at that specific temperature. FIG 1 Relative Humidity/Drying Temperature Drying Air is +60 F Dew Point % Relative Humidity Drying Temperature, F 60% 50% 40% 30% 20% 10% 0% 75 95 115 135 155 175 195 59.62 31.4 17.37 10.04 6.03 2.41 3.76 Filter Heater Hot-Air Drying System Single-Pass Air Circuit Drying Temperature 150 F to 180 F +60 F Ambient Air Dew Point 54 JANUARY 2018 Plastics Technology PTonline.com Tips and Techniques