Polymer Drying 101: The Basics
By Pete Stoughton

Many plastics, especially so-called engineering materials, are “hygroscopic.” As soon as the polymer is manufactured and exposed to the atmosphere, it begins to absorb moisture. Water vapor continues to migrate from the surrounding air until equilibrium is reached between the moisture content of the polymer and the surrounding air. This moisture absorption takes place on a molecular level. The natural attraction between polymer chains and water molecules is what causes hygroscopic resins, exposed to a humid atmosphere, to take up and retain water. It is also what makes drying them difficult.

Fortunately, the process can be reversed. A few fundamental factors govern this process:

In addition, there must be some mechanism that extracts the moisture released by the polymer during the drying process and, in most dryer designs, this is accomplished by a stream of dry air. Let’s look at each of these closely.

Polymer Temperature

The temperature of a hygroscopic polymer determines the rate at which water molecules move through it. Therefore, temperature is probably the most important consideration in typical drying applications. As the temperature of a hygroscopic polymer is increased, the molecules move about more vigorously and the attraction between the polymer chains and the water molecules is decreased. Thus, heating used in the drying process allows the water molecules to escape from the polymer chains. Generally, the higher the drying temperature, the faster the polymer will dry. There is, however, a practical temperature limit. If the polymer is exposed to a drying temperature that is too high, the pellets may stick together and bridge in the drying hopper. Some polymers, such as nylon, may oxidize and discolor at drying temperatures above those recommended by the material supplier.

Conversely, there is also a practical limit to how low drying temperatures can be and still be effective. The lower the drying temperature is, the longer it will take for a polymer to give up its moisture.

Dewpoint/Relative Humidity

Once heat has freed water molecules from their bond with the polymer, it is necessary to induce them to migrate out of the plastic pellet. This is accomplished by surrounding them by air that has a lower “vapor pressure” than the pellet. In most dryers, a stream of low-dewpoint (dry) air provides this low-vapor-pressure environment. The drier the air surrounding the pellet, the higher the vapor pressure inducing the water molecules out of the pellet.

Most conventional dryer designs use this dry air to convey water away from the polymer and, because hot air can hold much more moisture than cool air (it has a lower “relative humidity”), it is advantageous to heat the air. In fact, most dryers use hot dry air as the heat input to the polymer.

Drying Time

Plastic pellets do not dry instantaneously. It takes time to raise the temperature of the pellets and, once the pellets are subjected to lower vapor pressure conditions, it takes more time for moisture to diffuse and migrate to the surface. Resin manufacturers have defined how long this process takes for their particular resin types and grades. Remember, however, that effective drying time is the time the pellets are exposed to optimum drying conditions. The time the polymer is in the drying hopper at anything less than the recommended temperature and dew point cannot be considered drying time, and you run the risk on insufficiently drying the material. This, in turn, can be comparable to not drying the resin at all.