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DEWPOINT TEMPERATURE: The dewpoint temperature is the second fundamental drying parameter. This term describes the moisture content of the drying air. The dewpoint temperature is the "saturated air temperature" at which condensation will occur. For example, If you were to gradually reduce the air tempera- ture of the room in which you are sitting, you would eventually reach a temperature at which condensa- tion would occur. That is the saturated air temperature, or dewpoint. If you were to repeat that experiment with a sample of air from a dryer that has been dehumidifed to -40 F dewpoint, you would need to reduce the air's tempera- ture to -40 F before condensation would begin to occur. Another more common example: When you leave the house on an early summer morning and see "dew" on the grass it is because the temperature of the air fell below the saturated air temperature or dewpoint during the night and condensation occurred. How does the low-dewpoint air from a dryer remove the moisture from the PET granules in the drying hopper? First, the drying air is dehumidifed to a very low residual moisture content by fltering the air through a moisture-adsorbing desiccant This graph shows the importance of drying time. PET pellets give up their moisture quite rapidly in the frst hour of drying, but the rate of moisture removal slows dramatically, requiring another 3-4 hr of drying to reach an appropriate level of dryness for melt processing. % Moisture Content 0.1823 0.04 0.015 0.0075 0.0053 0.004 0.0035 0.0021 0.011 Initial 60 120 180 240 300 360 420 480 Drying Time, minutes 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 FIG 4 Moisture Loss Characteristics Material ............................. PET Pellets Description ........................ Virgin, Crystalline Pellets Drying Temperature .......... 340 F Dew Point .......................... -40 F Bulk Density ...................... 55 lb/ft 3 Ambient Humidity ............. 50% RH Method............................... Karl Fischer Analyzer ............................. Mitsubishi CA05 Analyzer Temperature ....... 205 C Guidelines for PE T FIG 1 FIG 3 Water molecules bond strongly to the PET polymer chains (Fig. 1). Hot drying air loosens the bonds between moisture and the resin, allowing the molecules to move freely through the pellet (Fig. 2). Because the drying air has low moisture content, water molecules within the heated pellet move toward to region of lower vapor pressure at the surface of the pellet (Fig. 3), where the moisture can be swept away by the stream of drying air. FIG 2 @plastechmag 51 Plastics Technology G u i d e l i n e s f o r P E T D R Y I N G P E T