Kinetic Rate Modeling for Moisture Extraction in Polyesters
Polyester moisture extraction follows temperature-dependent Fickian diffusion, requiring dew points below minus forty degrees to prevent melt hydrolysis.
Equilibrium Moisture Content
Meaning
Steady state condition where a polymer neither gains nor loses water when exposed to a specific temperature and relative humidity. Attaining the equilibrium moisture content is a prerequisite for accurate dimensional measurement of hygroscopic resins like polyamide.
Environmental Saturation
Polymers like nylon 6 or nylon 66 absorb water from the air until their internal concentration matches the surrounding humidity. This process can take weeks or months depending on the thickness of the part. If the environment becomes drier, the part will lose water until a new balance is reached.
Dimensional Stability
Water molecules act as plasticizers that increase the space between polymer chains. Parts grow in size as they reach the equilibrium moisture content. A moulder must design the tool to account for this post-moulding expansion to ensure the final assembly fits correctly.
Weight Measurement
Testing the moisture level involves heating a sample and measuring the weight loss. If the sample was already at its equilibrium moisture content for the laboratory conditions, the reading reflects the moisture the part will hold during its service life. This data point helps engineers predict how the material properties like stiffness and impact resistance will change after the part leaves the factory.