Post-Consumer Polyester Non-Aqueous Volatile Desorption Kinetics Baseline
Non-aqueous volatile desorption kinetics in post-consumer polyester depend on flake thickness, Arrhenius diffusion limits, and sweep gas partial pressures.
Mass Transfer Limit
Meaning
Physical constraint that determines the maximum rate at which a substance can move from one phase to another. In polymer processing, the mass transfer limit defines the speed of drying, degassing or the migration of additives into a food stimulant.
Boundary Layer
Resistance often occurs at the interface between the solid polymer and the surrounding fluid. This mass transfer limit is reached when the concentration at the surface cannot be cleared away fast enough to allow more molecules to escape. Increasing the velocity of the air or liquid can help overcome this barrier.
Process Design
Extruders are designed to maximize the surface area of the melt to bypass the restriction of the mass transfer limit. Mechanical agitation or the use of static mixers ensures that the concentration of volatiles remains uniform. If the system is limited by diffusion within the bulk plastic, increasing the temperature is the only effective solution.
Quality Outcome
When the mass transfer limit prevents the removal of moisture, the resulting part may suffer from structural weakness. Designers must calculate the time required for these processes to complete under various operating conditions. This value sets the minimum residence time in the devolatilization zone.