Influence of post-cure temperature on final properties

[Fibre and resin guy]

Hello all, I am not coming from a chemical background so there is a phenomenon for which I cannot find a decent explanation.

I currently use thermoset polyurethane retort adhesives and coatings. These products have a fairly low fully reacted Tg (somewhere between 0 and 10°C) to remain flexible (elastomer domain) at ambient temperature.
Once gelation has been reached, cure kinetics drop dramatically so a few days postcure is needed.
Since the final Tg is lower than ambient temperature, some of these products can have an ambient temperature postcure for several days and will reach their optimal properties.

Yet there are also some products that require postcure temperature in the range of 50°C for several days, even if the final Tg of the system is well below ambient temperature to maximise final properties (heat and hydrolysis resistance, mechanical, etc).

I was assuming that for such systems as long as post-cure temperature was above the Tg, crosslinking would continue to increase, just slower. Apparently it isn't the case, so I was wondering apart from crosslinking, is there another phenomenon occuring that affects final properties and cannot be completed despite being above the Tg of the system ?

If people more familiar with this type of chemistry can help with this, I'd be happy to hear their thoughts !

Best regards.

 

[Compositepro]

Curing is a chemical reaction that stops when all the reactants are consumed, or when the molecules are no longer mobile enough to reach something with which to react. Two part systems are generally very reactive at low temperature, which is why they must be two part. Tg is the glass transition temperature, which means that the molecules have become so tightly linked together that they cannot move enough (by bending or rotation of the molecules) to react with other nearby molecules. Higher temperatures can induce more reaction. Once all the reactants are consumed, higher cure temperature has no effect.

Every chemical reaction rate slows assymptopocally to zero as the remaining reactant concentration approaches zero. In this case a higher temperature post cure can finish the cure faster.

 

[Fibre and resin guy]

Hello Compositepro,

Thanks for your insight, I think I understand a bit better what happens.

My initial undertsanding was that as long as you were above Tg (in the liquid or sol/gel state), molecular mobiltity was sufficient to have decent crosslinking kinetics. This is what can be found in litterature and is the case on all the epoxies I've encountered so far.

Apparently this is not always the case : some systems (typically some 2K PU elastomers) can still have a significant amount of unreacted reactants, be cured slightly above their fully reacted Tg, and yet crosslinking kinetics will be nil (due to a too low molecular mobility despite being in sol/gel domain I suppose). You will have to increase temperature much higher than the final Tg of the system to have sufficient molecular mobility in order to finish the reaction completely (this is how I understand it).

Happy to have your thoughts on this.

 

[Compositepro]

There are differing scales of molecular mobility. In a fluid a molecule can move though it's entire container until it finds something with which to react. All monomers must have at least two reactive sites in order to form and extend polymer chains. Reactive sites on extending chains are increasingly limited in mobility as the chain extends but small monomer molecules can still move and reach those sites. Large monomer molecules are less mobile and lose mobility quickly as they react. Mobility can be due to molecular diffusion through a bulk fluid or just the ability of a molecule to flex or rotate on its spine. The range of motion changes by orders of magnitude.