Desiccant Dryer Dew Point Calibration and Karl Fischer Titration Verification
Dew point monitoring alone fails to guarantee dry resin; coulometric Karl Fischer titration per ISO 15512 Method B is mandatory to prevent hydrolytic scission.
GAB Sorption Isotherm
Meaning
Mathematical models describe the relationship between the equilibrium moisture content of a material and the surrounding relative humidity at a constant temperature. For hygroscopic polymers, the GAB sorption isotherm represents the multi layer adsorption of water molecules onto polar groups within the polymer matrix. This model helps moulders predict how quickly dry resin pellets will reabsorb moisture when exposed to factory air.
It spans the entire range of relative humidity from dry air to saturated conditions.
Sorption Theory
Three parameters in the model account for the monolayer adsorption, the multilayer interactions, and the bulk water condensation. Applying the GAB sorption isotherm to materials like polyamide 6 or polyethylene terephthalate yields accurate curves that match experimental data up to ninety percent relative humidity. It provides a more comprehensive fit than simpler models that only apply to single layer adsorption on the polymer surface.
Drying Design
Drying systems rely on these curves to calculate the dewpoint required to dry resin to safe levels. Since polyamide retains water strongly, the GAB sorption isotherm shows that very low dewpoints are necessary to pull moisture out of the polymer matrix. Setting the drying temperature too low will fail to break the bonds between water and the polymer chains, even if the air is extremely dry.
Pellet Storage
Exposure of dried resin to high humidity environments results in rapid water uptake. This reabsorption occurs within minutes in humid molding halls, quickly rendering the drying process useless. Covering storage hoppers and using dry air blankets are necessary steps to preserve polymer dry state before molding.