Processing resin that contain excess moisture can lead to problems with processing and final part quality. Too much moisture in resin can result in cosmetic or structural part defects – and in some cases, complete part failure.
While plastic pellets might appear dry, they might actually contain enough moisture to create quality issues with parts during production.
Moisture in resin is a function of many variables, from a material’s natural affinity to retain or reject moisture to the processing environment and equipment.
Hygroscopic polymers naturally attract moisture internally. Non-hygroscopic polymers don’t absorb moisture but retain it on the material surface.
Absorption of moisture from the atmosphere depends on multiple factors, including:
- Resin type
- Ambient conditions
- Seasonal weather changes
- Packaging/storage
- Transportation
- Manufacturing variables
Moisture on or in resin will turn to steam when the material is melted for processing. Steam gets trapped in the cooling melt. These microscopic steam pockets can cause defects in the finished product (e.g., blistering) if not outright structural damage.
The Effect of Ambient Conditions
Conditions in and around where the resin is stored and manufactured.
The atmosphere is made up of:
- Nitrogen: 78% to 80%
- Oxygen: 18% to 20%
- Water vapor: 0.01% to 2%
Moisture is picked up by resin through exposure to ambient conditions.
Hygroscopic polymers absorb moisture in the pellet, where it is weakly bound. The percentage of moisture absorbed is dictated by the molecular structure. So-called intragranular moisture – moisture that travels deep inside a pellet – must be dried before processing. Hot air alone will not achieve the proper drying required.
Non-hygroscopic polymers, while not having an affinity for moisture, will pick up surface moisture if exposed to high humidity. Moisture on the outside of pellets (e.g., condensation) can be easily removed.
Note that adding a component to a non-hygroscopic material like polyethylene or polypropylene can make it hygroscopic and require dehumidified air drying.
The relative humidity and temperature of the surrounding air, combined with the length of the time the polymer is exposed to the atmosphere, influences the amount of water vapor a polymer will absorb.
Another consideration is dew point – specifically low dew point air. The dew point is the temperature at which moisture in the air begins to condense. Before a resin is exposed to heat, humidity can be removed from the air that will circulate in the drying system. Humidity can be removed by passing air through a desiccant bed or wheel. When this super-dry air is circulated into the drying system, it combines with heating of the air to create a moisture-absorbing flow into the drying hopper and through the material.
Initial Moisture Content
The amount of moisture a polymer resin contains before processing varies greatly depending on not only their inherent characteristics but also storage conditions, seasonal changes and other factors.
Whether sitting in a warehouse, being transported or entering a different climate, resins face moisture from multiple sources.
Resin delivered from the same supplier can contain different levels of moisture despite sharing the same recommended drying time and temperature. In fact, the initial moisture content of the same type of resin arriving in separate containers might vary significantly from batch to batch.
Initial moisture content varies by region. In a dry region, a resin will exhibit lower initial moisture than in a damp environment.
When buying resins, initial moisture content plays a significant role as well. Plastic pellets might contain a significant amount of moisture when being sold, which has led the industry to advocate for moisture analyzers to give buyers sound information when purchasing.
