Solid State Polymerization Volatile Stripping Mass Transfer Limits in rPET
Solid state polymerization stripping rate is dictated by internal Fickian diffusion limits of condensation by-products and volatiles within rPET pellet cores.
Mass Biot Number
Meaning
Dimensionless transport equations describe the ratio of convective mass transfer at the surface of a particle to the internal diffusion rate. The mass biot number helps polymer engineers determine whether the rate of pellet drying is governed by surface evaporation or by the slow movement of moisture within the polymer chain. This ratio guides the optimization of drying hoppers for engineering resins like nylon, which are highly sensitive to moisture.
Dimensionless Ratio
A low value indicates that internal diffusion is rapid, meaning the concentration gradient inside the pellet is almost flat. Under these conditions, the mass biot number shows that the drying rate can only be increased by altering the gas flow or temperature to enhance surface transfer. Conversely, a high value suggests that the rate is limited by slow diffusion inside the pellet, making changes to external airflow ineffective.
Process Sensitivity
Processors use this dimensionless analysis to select the correct drying profile for virgin versus regrind materials. Regrind particles often have irregular geometries that alter the characteristic diffusion length, shifting the mass transfer mechanics.
Moulding Control
Controlling the mass transfer regime prevents the hydrolytic degradation of polymers during the melting phase. Accurate moisture removal ensures a stable melt viscosity during injection.