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.
Moisture Extraction Kinetics
Meaning
Mathematical description of the speed at which water molecules migrate from the core of a polymer pellet to its surface during a heating cycle. Understanding moisture extraction kinetics is necessary for sizing desiccant systems for engineering resins that exhibit high affinity for water vapor.
Diffusion Gradient
Water moves from areas of high concentration in the center of the pellet to the low concentration at the surface. The rate of this movement is dictated by Fick’s laws of diffusion. A drier air stream creates a steeper gradient which pulls the moisture out faster.
Energy Requirement
Heat provides the kinetic energy needed for water molecules to break free from the polymer chains. Cold resin holds onto water much more tightly than hot resin. Pre-heating the material is the most effective way to improve the speed of the drying process.
Time Constraint
Removing moisture from the very center of a large pellet takes a specific amount of time that cannot be bypassed by simply increasing the air flow. If the production line runs faster than the moisture extraction kinetics allow, the resin will enter the screw while still containing water. This leads to hydrolytic degradation where the water reacts with the polymer melt and permanently weakens the part.