Diffusion Control Integration for High Temperature Thermoset Kinetic Calculations during Mold Cure
Integrating diffusion control into thermoset kinetic calculations prevents overestimating late-stage mold cure rates near vitrification.
Rabinowitch Diffusion
Meaning
Chemical kinetic phenomena describe the reduction in reaction rates as a liquid system approaches a solid state. Rabinowitch diffusion specifically accounts for the moment when molecular movement is so restricted that reactants cannot collide. This effect becomes the dominant factor in the final stages of a thermoset reaction.
Proximity Constraint
Steric hindrance and increased viscosity prevent the free rotation of polymer chains. Under these conditions rabinowitch diffusion dictates that the time required for a reactive group to find a partner exceeds the time for the chemical step itself. This phase signifies the transition from a reaction controlled by chemistry to one controlled by physical transport.
Collision Frequency
Reaction speed depends on the number of successful encounters between active sites. As the network tightens the rabinowitch diffusion effect reduces these encounters to nearly zero. Industrial cycles often stall at this point if the mold temperature is too low to provide the energy needed for chains to move.
Temperature Dependence
Overcoming this physical barrier requires heat sufficient to expand the free volume of the material. If the process ignores rabinowitch diffusion the parts will remain under-cured and lack the necessary thermal stability. The model represents the theoretical limit of reaction rate in a diffusion-limited environment.