Meaning
Thermosetting resins reach a point during curing where the increasing glass transition temperature of the network matches the actual processing temperature. The vitrification threshold marks the transition from a liquid or rubbery state to a glassy solid. Once this point is reached, the reaction rate drops because molecular motion is restricted.
Diffusion Transition
Chemical control gives way to physical constraints as the material freezes. At the vitrification threshold, the free volume in the resin disappears and the reactive groups can no longer move to find one another. Any further reaction must proceed through slow solid-state diffusion.
This transition is visible in heat flow data as a decrease in the rate of energy release.
Process Impact
Manufacturing cycles must be designed to account for this slowing of the reaction. If the vitrification threshold is reached too early, the material may never achieve full conversion within a reasonable time frame. Engineers often increase the temperature in stages to keep the resin in the rubbery state.
This ramping ensures that the reaction remains fast until the desired cross-link density is achieved.
Mechanical Development
Structural properties like stiffness and strength are locked in as the material passes this point. The vitrification threshold is often used to determine the demolding time for a part. If a part is removed from the tool before it has vitrified, it may lack the integrity to support its weight.
Monitoring this threshold is a part of optimizing production for composite aerospace components.
