Processing and Moisture

Recognize that properties of designed products can vary, in fact can be destructive, with improper processing control such as melt temperature profile, pressure profile, and time in the melted stage. An important condition that influence properties is moisture contamination in the plastic to be processed. There are the hygroscopic plastics (PET, etc.) that are capable of retaining absorbed and adsorbed atmospheric moisture within the plastics. The non-hygroscopic plastics (PS, etc.) absorb moisture only on the surface. In the past when troubleshooting plastic's reduced performance was 90% of the time due to the damaging effect of moisture because it was improperly dried prior to processing. At the present time it could be at 50%.

All plastics, to some degree, are influenced by the amount of moisture or water they contain before processing. With minimal amounts in many plastics, mechanical, physical, electrical, aesthetic, and other properties may be affected, or may be of no consequence. However, there are certain plastics that, when compounded with certain additives such as color, could have devastating results. Day-to-night temperature changes is an example of how moisture contamination can be a source of problems if not adequately eliminated when plastic materials are exposed to the air. Moisture contamination can have an accumulative effect. The critical moisture content that is the average material moisture content at the end of the constant-rate drying period, is a function of material properties, the constant-rate of drying, and particle size.

Although it is sometimes possible to select a suitable drying method simply by evaluating variables such as humidities and temperatures when removing unbound moisture, many plastic drying processes do not involve removal of bound moisture retained in capillaries among fine particles or moisture actually dissolved in the plastic. Measuring drying-rate behavior under control conditions best identifies these mechanisms. A change in material handling method or any operating variable, such as heating rate, may effect mass transfer.

Drying Operations

When drying at ambient temperature and 50% relative humidity, the vapor pressure of water outside a plastic is greater than within. Moisture migrates into the plastic, increasing its moisture content until a state of equilibrium exists inside and outside the plastic. But conditions are very different inside a drying hopper (etc.) with controlled environment. At a temperature of 170°C (350°F) and -40°C (-iO°F) dew point, the vapor pressure of the water inside the plastic is much greater than the vapor pressure of the water in the surrounding area. Result is moisture migrates out of the plastic and into the surrounding air stream, where it is carried away to the desiccant bed of the dryer.

Target is to keep moisture content at a designated low level, particularly for hygroscopic plastics where moisture is collected internally. They have to be carefully dried prior to processing. Usually the moisture content is >0.02 wt%. In practice, a drying heat 30°C below the softening heat has proved successful in preventing caking of the plastic in a dryer. Drying time varies in the range of 2 to 4 h, depending on moisture content. As a rule of thumb, the drying air should have a dew point of -34°C (-30°F) and the capability of being heated up to 121°C (250°F). It takes about 1 ft3 min-1 of plastic processed when using a desiccant dryer.

The non-hygroscopic plastics collect moisture only on the surface. Drying this surface moisture can be accomplished by simply passing warm air over the material. Moisture leaves the plastic in favor of the warm air resulting in dry air. The amount of water is limited or processing can be destructive.

Determine from the material supplier and/or experience the plastic's moisture content limit. Also important is to determine which procedure will be used in determining water content. They include equipment such as weighing, drying, and/or re-weighing. These procedures have definite limitations based on the plastic to be dried. Fast automatic analyzers, suitable for use with a wide variety of plastic systems, are available that provide quick and accurate data for obtaining the in-plant moisture control of plastics.

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