RESIN MOISTURE & DRYING

Hygroscopic vs. Non-Hygroscopic Resins

What does hygroscopic mean, and how does it affect resin moisture retention and drying?

Hygroscopic Resins

(i.e., Nylon, ABS, Acrylic, Polyurethane, Polycarbonate, PET, PBT,)

• Have a strong affinity to attract moisture
• Will absorb moisture onto their molecular structure if exposed to ambient air
• Internal moisture can not be removed with hot air alone

Water vapor surrounding a hygroscopic pellet is absorbed into the pellet. As the vapor pressure within the pellet increases to equal the vapor pressure surrounding the pellet, equilibrium occurs. This is referred to as, moisture equilibrium.

When an environment of hot, dry air surrounds a wet hygroscopic pellet, the vapor pressure surrounding the pellet is lower than the vapor pressure within the pellet. Consequently, the moisture within the pellet begins to migrate toward the area of low vapor pressure outside the pellet. Expose the pellet to the hot, dry atmosphere for a sufficient period and the pellet eventually reaches moisture equilibrium with the surrounding dry conditions. In other words, the pellet becomes dry.

 

Hygroscopic plastic pellets have a strong affinity to attract moisture and absorb it into the molecular structure of the pellet. Photo Credit: Novatec

 

Non-Hygroscopic Resins

(i.e., Polyethylene, Polypropylene, Polystyrene, PVC)

• Do not have an affinity for moisture
• Any moisture collected is adsorbed on the surface of the pellet
• Typical moisture collection is due to condensation
• Moisture is easily removed by passing a sufficient stream of warm air over the material

 

Non-hygroscopic polymers do not absorb moisture from the atmosphere into the pellet but may have moisture on the pellet's surface. Photo Credit: Novatec

 

How to Dry Crystalline vs. Amorphous PET for Optimal Processing

All PET (polyethylene terephthalate) 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 a moisture content of .005% or less (by weight) prior to processing to ensure molecular weight retention and optimum physical properties.

Virgin PET

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 the production of a good product.

Reground PET

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: See PET Desiccant Wheel Energy-Saver Dryer and Infrared Crystallizers/Dryers. Biaxially oriented bottle scrap, regardless of color, is crystalline and should be further crystallized prior to drying – for safety's 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.

Thermoformed PET

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.