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.
Sorption Isotherm
Meaning
Graphical representations plot the equilibrium moisture content of a polymer against the relative humidity of the surrounding air at a constant temperature. Under a sorption isotherm, the relationship shows the material behavior under different storage conditions. This curve determines the maximum dryness of the resin.
Moisture Equilibrium
Each point on the curve represents the state where the rate of water absorption equals the rate of desorption. This state is highly dependent on the chemical structure of the polymer, particularly the presence of polar functional groups that attract water molecules. The shape of the curve provides info on whether the moisture is held as a monolayer or in multiple layers within the free volume.
Analytical Curve
Laboratory measurements are conducted by exposing polymer samples to controlled humidity levels until their weight stabilizes. The resulting plot typically shows a characteristic sigmoidal or linear shape depending on the nature of the polymer and the temperature of the test. These curves allow quality control teams to predict the moisture content of polymers stored under varying seasonal humidity levels.
Industrial Application
Engineers use these curves to calculate the required drying time and energy needed to bring the resin down to processing levels. This information helps in sizing the drying equipment and setting the operating parameters of desiccant dryers in the factory. Having this data prevents the over-drying of polymers, which can lead to thermal degradation, while ensuring that the resin is dry enough to prevent molding defects and maintain the structural performance of the parts.