Meaning
Polymeric materials that absorb water from the environment must reach equilibrium before they are tested or put into service to ensure consistent performance. This preparation is called moisture conditioning and it involves exposing moulded specimens to a specific humidity and temperature for a designated period. The process accelerates the absorption that would otherwise occur slowly over months of exposure to ambient air.
It does not apply to hydrophobic polymers such as polyethylene or polystyrene which do not absorb water.
Material Treatment
Polyamides and other hydrophilic resins possess polar amide groups that attract water molecules from the surrounding atmosphere. This moisture conditioning process is typically performed in a humidity chamber or by submerging the parts in hot water. The treatment is complete when consecutive weight measurements show that the part has reached a stable weight, indicating that the core has been fully saturated to its equilibrium level.
Dimension Control
Water absorption causes polymer chains to push apart, resulting in a slight swelling of the moulded component. This volumetric expansion must be accounted for during the tool design phase to ensure that the finished parts meet assembly tolerances after moisture conditioning has occurred. Parts measured immediately after moulding will always be smaller than their final in-service dimensions.
Mechanical Change
Absorbed water acts as a plasticizer within the polymer matrix, which reduces the glass transition temperature and decreases tensile strength. The moisture conditioning treatment increases the impact resistance and flexibility of the part while reducing its stiffness. This change is particularly pronounced in polyamide parts, which can be brittle when dry but become tough and resilient once they have absorbed the correct amount of water.
Designing structural components using dry-as-moulded properties rather than conditioned values can lead to unexpected failures in the field.
