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.
Water Diffusion Coefficient
Meaning
Kinetic transport parameters measure the rate at which water molecules move through a solid polymer matrix under the influence of a concentration gradient. This property dictates the time required to dry or hydrate a polymer pellet before it can be safely processed. For engineering resins, the water diffusion coefficient determines the speed of the drying cycle and governs how quickly a moulded part will absorb atmospheric moisture.
Diffusion Mechanism
The transport of water molecules proceeds through the free volume between the polymer chains, with polar groups on the chains slowing the movement by forming hydrogen bonds. This makes the rate of diffusion highly dependent on the molecular structure of the specific polymer and its crystallinity. Amorphous regions allow faster diffusion than highly crystalline regions, which act as barriers to the moving water molecules.
Moulding Practice
Knowing this value allows moulders to calculate the minimum preheating time needed for thick-walled or highly hygroscopic materials. If the drying time is too short, the core of the pellets will remain wet, causing hydrolytic degradation and cosmetic defects during injection. This calculation is especially important when processing recycled materials, which may have different diffusion rates due to previous thermal history.
Temperature Response
Higher temperatures increase the diffusion rate by expanding the polymer structure and increasing the kinetic energy of the water molecules. This allows for much faster drying times but carries the risk of thermal degradation if the temperature exceeds the safe limits for the resin.