Cure Terms Glossary
Gelation is process of gel formation. In the process of cure, the onset of gelation is the point when the mix is no longer capable of flow (the no-flow condition). At this point, viscous and elastic character are in balance (storage & loss moduli equal in value) and the gel point can therefore be regarded as the half-way house between purely viscous and purely elastic states. In molecular terms, gelation occurs when an infinite network first forms.
Gelation is critically
dependent on the functionality of the
starting mix - i.e., the higher the initial functionality, then the earlier
in the cure will gelation occur. The competition between intermolecular
(crosslinking) and intramolecular (cyclisation) complicates matters and
current theories focus on predicting a critical conversion, pc,
below which gelation is not possible. One such expression estimates the
critical conversion for gelation in stoichiometic cure effected by the
stepwise reaction of two types of mutually reacting groups (i.e. A + B
→ C) as:
pc2 = 1 / (φ1 - 1) (φ2 - 1)
where pc is a fractional conversion (i.e. 0 - 1), and φ1 & φ2 are the average functionalities of the starting molecules
Unlike vitrification, there is no change in change in reactivity of the system on gelation.