Correlating Dryer Air Dew Point with Karl Fischer Titration Values
Resin moisture correlates to air dew point only at thermodynamic equilibrium; diffusion kinetics and hopper dwell dynamics govern actual Karl Fischer values.
Sorption Isotherm
Meaning
Thermodynamic plots describe the relationship between the moisture content of a solid material and the relative humidity of the surrounding air at a constant temperature. This graphical representation is essential for designing and operating polymer drying systems, as it defines the theoretical limit of moisture removal under specific conditions. By analyzing a sorption isotherm, a moulder can determine the precise air dew point required to reach the target dryness for a given resin.
Equilibrium Mechanics
The curve shows how the polymer resin takes up or releases water depending on the partial pressure of water vapour in the air. At high humidity, the resin absorbs water rapidly, whereas in dry air, it releases moisture until it reaches a state of equilibrium. The shape of the curve varies depending on whether the polymer is polar, like nylon, or non-polar, like polycarbonate.
Dryer Performance
Modern dryers use these relationships to optimise energy use by adjusting the air flow and dew point based on the current resin type and moisture load. If the dryer fails to maintain the correct dew point, the resin will start to reabsorb moisture from the air, defeating the drying process and leading to defects like splay and brittleness in the moulded parts.
Polymer Behaviour
Ambient temperature shifts the isotherm, meaning that a hotter environment generally reduces the equilibrium moisture content but accelerates the absorption rate. This makes it necessary to control the ambient conditions in the storage and drying areas of the moulding plant.