Cure Terms Glossary
Vitrification is the process of becoming vitreous - i.e. glassy. A glass is an amorphous material with molecular mobility restricted by intermolecular associations. The types of intermolecular association determine the stability of the glass. Strong ionic boding, as in the silicate glasses, provide structures which are stable at room temperature and above. With weaker associations the glassy state is more likely to be a sub-ambient phenomenon.
For high molecular weight polymers, intermolecular associations operate collectively and stable glasses can be obtained even through van der Waals forces if the molecular backbone is stiff enough: polystyrene is glassy up to 100°C. The softening temperature of the glass is called the glass transition temperature, Tg. Cooling a polymer below its Tg is a process of vitrification.
Covalent crosslinking provides another route to vitrification. Since glassy character is a function of backbone mobility, different polymers will reach the glassy state at different levels of crosslinking. Some may not reach it at all if the cure cannot reach the desired crosslink density. A rubber cure would be such a case - the ultimate crosslink density is limited by the formulation (ingredient type or level of curative). However, even rubbers will vitrify if the temperature is taken low enough.
The glassy state achieved by covalent crosslinking provides for materials with outstanding dimensional stability and resistance to creep. Elevated temperature performance is also good - quite possibly the glassy state may be stable up to the onset of degradation.
Where the cure is capable of vitrification, it is often a step beyond gelation. The various states attainable in vitrifying cures can be represented pictorially in a time-temperature-transformation cure diagram.
Vitrification effectively seals the fate of the cure, as there may be very little reaction beyond this point. For reactions in glasses, the concentrations of functional groups and the energetics of reaction count for little - what matters is whether there is sufficient mobility for the reacting groups to find one another. Rates of diffusion and the activation energy for diffusion are the dominant influences. The reaction is said to come under diffusion control. The result is that reaction rates can drop by several orders of magnitude with the onset of vitrification. Mobility is further reduced if the temperature is dropped. Thus if an elevated-temperature cure is taken to the point of vitrification, the cure is more or less terminated on cooling.
Conversely, if an elevated temperature cure which has reached vitrification is heated further (above the glass transition temperature), it is possible that the cure can be reactivated. The cure will progress until vitrification occurs at this higher temperature. Thus, for any vitrifying cure, crosslink density and Tg are inextricably linked: Tg increases with crosslink density.
The process of vitrification
during cure can therefore be interpreted as the Tg increasing until it
reaches the temperature of cure.