Karl Fischer Moisture Testing of Recycled Polyethylene Terephthalate
Coulometric Karl Fischer testing with oven desorption at 170 °C isolates true water content in rPET below 50 ppm, preventing costly IV degradation during melt processing.
Moisture Absorption Kinetics
Meaning
Water vapor uptake over time by a polymer matrix depends on the diffusion coefficient of the material and ambient humidity. This process is governed by moisture absorption kinetics, which dictate how quickly dry molded parts absorb water from the surrounding air. Polar polymers like polyamide 6 are highly sensitive to this phenomenon, which alters their physical dimensions and mechanical properties.
Non-linear uptake dynamics mean that the rate of absorption slows down as the polymer approaches saturation.
Plasticization Effect
Absorbed water molecules locate between polymer chains and act as plasticizers by increasing chain mobility. This change, driven by moisture absorption kinetics, lowers the glass transition temperature and increases ductility while reducing tensile strength. Polyamide parts become tougher but softer as they reach equilibrium.
Understanding this behavior is essential because the mechanical performance shifts continuously during conditioning.
Dimensional Influence
Part dimensions increase as water molecules are incorporated into the amorphous regions of the polymer. The rate of this expansion is determined by moisture absorption kinetics, making it necessary to distinguish between dry-as-moulded and conditioned states. Measurements taken immediately after moulding will appear smaller than the final assembled size of the part.
Processing Prevention
Resin drying before moulding is necessary to prevent hydrolytic degradation of the polymer chains in the melt. Although moisture absorption kinetics describe post-moulding uptake, they also apply to the raw resin pellets stored in silos. Desiccant dryers are utilized by moulders to reduce moisture levels below the critical threshold before processing.