Drying Parameters
Heat is the first fundamental drying parameter for all polymers.
Heat is the driving force in drying. If you don’t heat the pellet, it will not release its moisture.
Hygroscopic polymers have a strong attraction for water and the water molecules are bound to the polymer chains. Heat causes the molecules to move about more vigorously, weakening the forces that bind the water molecules to the polymer chains. Above a certain temperature, the forces that bind the water molecules to the polymer chains are reduced allowing free movement of the molecules for the drying process.
Non-hygroscopic resins don’t really adsorb moisture but moisture can collect on the surface of the pellets and applying heat is an important part of removing surface moisture.
Click here for more details on hygroscopic and non-hygroscopic resins.
Dew Point is the second fundamental drying parameter for drying hygroscopic polymers using most drying methods.
Dew Point is the temperature at which moisture in the air begins to condense. The low vapor pressure (dew point) of the dry air surrounding the pellets causes the freed moisture molecules to migrate to the surface of the pellet.
Drying Time is the third fundamental drying parameter.
Plastic pellets do not dry out instantaneously. First, you must heat them to allow the water molecules free movement. Then, you must allow time for the water molecules to defuse to the surface of hygroscopic pellets or for surface moisture to evaporate from the surface of non-hygroscopic materials.
Airflow is the forth, fundamental drying parameter.
Airflow is an important parameter because it carries heat or dry heated air from the dryer’s heaters to the material in the drying hopper. In the case of non-hygroscopic materials, you must force hot air over and around the pellets to remove surface moisture.
With hygroscopic materials, you must force low dew point heated air over the material to make the molecules of moisture disengage from the polymer chains to the surface of the pellets where that all-important airflow carries the moisture away.
The volume of drying air must be sufficient to develop and maintain the desired temperature profile within the drying hopper. If four hours of drying time is required, you must maintain the drying temperature for the four-hour level within the drying hopper. If the volume of airflow is reduced, the temperature profile will be reduced.
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