Crystalline vs. Amorphous PET
All PET materials are hygroscopic, so exposure to a humid environment will cause them to absorb moisture. This moisture causes a cleavage in the polymer chain during melt and results in a degradation of the physical properties of the product produced.
In almost all applications, you should dry PET to moisture content of .005% or less (by weight) prior to processing to ensure molecular weight retention and optimum physical properties.
Virgin PET is usually in a crystalline state (indicated by its white color) when it is purchased from a supplier. It must be dry prior to processing. Using conventional desiccant drying systems, you should dry PET at 300 - 350˚ F. at a dew point of -20 to -40˚ F. for a period of 4 to 6 hours. This reduces the moisture content to .005% or below, ensuring production of a good product.
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Regrind PET material is usually not crystalline, but amorphous and clear in color. A characteristic of amorphous materials is that they tend to soften and agglomerate at normal drying temperatures, forming clumps and adhering to the walls of a drying hopper. Crystallize amorphous material, prior to drying, to prevent this agglomeration from occurring. Agitating the material while heating it to a temperature of about 180˚ F will convert the material from its amorphous state to a crystallized state, ready for drying. We call this process crystallizing but technically, it is a re-crystallization of the material. Different formulations of PET have different rates of crystallization, ranging up to one hour. Note that if the regrind PET is white in color, it is probably already crystalline and may be dried as-is.
There are a couple of methods used for the crystallization/drying process.
Biaxially oriented bottle scrap, regardless of color, is crystalline and should be further crystallized prior to drying – for safety sake.
Blow Molded PET
Blow molders, producing a minimal amount of scrap often blend the amorphous scrap with crystalline virgin material and dry them together. As a rule of thumb, when the amorphous scrap is less than 20%, and it can be thoroughly blended with 80% or more crystalline virgin material, it can be dried without experiencing agglomeration problems.
Thermoformers producing more than 20% amorphous scrap and requiring the highest physical properties possible must crystallize the scrap prior to drying to ensure that agglomeration problems do not occur. The minimal web scrap produced by some thermoformers is usually the crystalline white color and it can be dried as-is. If a large percentage of the scrap being produced is clear, it is amorphous and must be crystallized prior to drying