Meaning
Release of absorbed water molecules from the molecular structure of a polymer into the surrounding dry air represents the physical mechanism behind the drying process of hygroscopic resins. The rate of moisture desorption governs the time required to prepare resins like polyamide and polycarbonate for melt processing. This mechanism must be completed before molding to prevent thermal and hydrolytic degradation.
Drying Rate
Temperature increases the kinetic energy of the water molecules, allowing them to break their hydrogen bonds with the polymer chains and migrate to the pellet surface. This migration occurs more rapidly at elevated temperatures, which reduces the required pre-heating time.
Diffusion Control
Internal diffusion of water through the solid polymer limits the overall speed of the drying process. In thick-walled pellets, the moisture must travel a longer distance, making the process highly dependent on the diffusion coefficient of the specific resin.
Process Variable
Maintaining a low dew point in the drying air is necessary to sustain the concentration gradient that drives the moisture out of the polymer. If the surrounding air is not dry enough, a state of equilibrium is reached where the rate of water absorption equals the rate of release, which halts the drying process before the polymer achieves the required dryness.
