Meaning
Rate-based analysis of the speed and mechanism by which water molecules are released from a solid polymer matrix during heating. Diffusion coefficients of water in the specific polymer and the geometry of the pellets govern the speed of the moisture removal. This study of moisture desorption kinetics determines how quickly a resin can be dried and which temperatures are most effective for that process.
Diffusion Mechanism
Water molecules move from the interior of the pellet to the surface, where they are carried away by dry air. This process is usually the slowest part of the drying cycle and cannot be rushed simply by increasing airflow. Heat provides the energy required for the water molecules to break their hydrogen bonds with the polymer chains and begin moving.
Rate Factors
Temperature has the most substantial impact on the speed of desorption, as higher thermal energy increases molecular mobility. However, exceeding the glass transition temperature can cause the pellets to stick together or begin to degrade. The initial moisture concentration also affects the rate, with the last few hundred parts per million being the hardest to remove.
Process Application
Moulders use these kinetics to set the residence time and temperature for their drying hoppers. A resin with slow desorption requires a larger hopper to ensure the material stays in the dry air stream long enough. Understanding these rates prevents the processing of wet resin, which would otherwise lead to hydrolytic degradation and part failure.
