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Polyethylene terephthalate, commonly known as PET, is a popular material for bottles and containers. PET may exist as a clear amor- phous material or in semi-crystalline state that is often, though not always, opaque milky white. The IV or (intrinsic viscosity) of the PET resin is probably its most important characteristic. The IV is an indication of the average length of the PET's polymer chains. The higher the IV, the longer the polymer chains and the higher the performance characteristics it will possess. IV retention of the PET during processing is critical; anything that may degrade or lower the IV during processing must be moni- tored and controlled, and the most common cause of IV degrada- tion is the presence of water in the PET. PET is extremely sensitive to absorbed water, which will cause hydrolytic degradation at elevated melt temperatures. Thus, the most important issue is proper drying prior to melt processing. This article will describe the basic fundamentals of predrying virgin PET pellets for rigid container applications. THE BASICS OF DRYING PET PET is hygroscopic: When exposed to the atmosphere it will readily absorb water vapor from the surrounding air. The migra- tion of water vapor into the PET granules occurs over time and will eventually slow and stop when the moisture content reaches a state of equilibrium with that of the surrounding air. At that equi- librium point, the water vapor within the PET granule will be evenly distributed throughout the pellet's interior. The water molecules are attracted to the polymer chains, forming a strong intermolecular bond (Fig. 1). That bond is what makes drying PET difcult. Even a small amount of moisture in PET will cause a chemical reaction (hydrolysis) when the polymer is heated above its melt temperature. The drying process involves four fundamental parameters: • Drying temperature, • Dewpoint, • Drying time, • Airfow. DRYING TEMPERATURE: Temperature is probably the important consideration in drying any hygroscopic polymer. When the tem- perature of the PET is increased, its molecules will begin to move about more vigorously, reducing the attraction the water mol- ecules have for the polymer chains, and allowing the water mol- ecules to escape the bonds, as illustrated in Fig. 2. The polymer temperature therefore has a very strong infuence on the difusion rate of the water molecules through the polymer. The water molecules can travel more freely through the polymer at higher temperatures, when the polymer's attraction for the water molecules has been reduced. Heating the PET pellets to the optimum recommended drying temperature is a prerequisite to good drying. Generally speaking, the higher the drying temperature, the more quickly and thor- oughly the PET will dry. There are, however, practical limitations to the range of drying temperatures acceptable for PET. If the drying temperature is too high, thermal degradation and/ or oxidation may occur. If the drying temperature is low, the PET will give up its moisture more slowly. Following the recommendations of your material supplier is the best and safest advice to follow. By Pete Stoughton Consultant The three fundamental principles to grasp are drying temperature, dewpoint, drying time, and airfow. PET is extremely hygroscopic, highly moisture sensitive, and one of the toughest challenges to dry. Here are the basic principles of doing it right for rigid packaging applications. Guidelines for Drying PET 50 DECEMBER 2014 Plastics Technology PTonline.com Tips and Techniques