Thermoplastic Polymer Moisture Absorption Mechanics during Storage
Polymer moisture absorption follows Fickian kinetics, requiring hermetic packaging, Karl Fischer verification, and desiccant drying to prevent hydrolytic cleavage.
Molecular Weight Loss
Meaning
Polymer chain degradation during melt processing results in a reduction of the average size of the macromolecular chains. In molecular weight loss, the polymer chains are broken down into shorter segments. This degradation occurs when polymers are processed with excessive moisture.
Polymeric Cleavage
Water molecules present in the polymer melt attack the backbone linkages of condensation polymers at processing temperatures. This hydrolysis reaction splits the polymer chain, which creates shorter molecules with acid and amine end groups. The rate of this cleavage increases with higher temperatures and higher initial moisture concentrations.
Quality Defect
Molded parts made from degraded material show a sharp decline in tensile strength, impact resistance and fatigue life. This reduction in performance often occurs without any visible surface defects on the part, which makes it difficult to detect during standard visual inspections. The lack of structural strength can lead to catastrophic failure when the part is subjected to mechanical stress in the field.
Industrial Control
Monitoring the melt flow index of the processed polymer provides a method for detecting this internal degradation on the factory floor. An increase in the melt flow rate indicates a lower viscosity, which corresponds to the breakdown of the polymer chains. Quality control teams set strict limits on this change to ensure that the molded parts maintain their mechanical properties, and they use these measurements to audit the performance of the resin dryers.