Post cure hardening.

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#1

I am new to this site and think that I might be in heaven!! How sad is that?
I am not new to composites and I know the benefits of post curing. However I was asked a question yesterday I could not answer…
Why do resins (polyester in particular) get harder over a period of days once they have cured at ambient? Post curing is “tying up loose ends” of the molecular chains. However what reaction is going on as polyesters harden over a period of time? Are new covalent bonds being formed in the solid state, is the styrene relevant, what is going on on a molecular level?

Likewise when postcuring epoxies.

Is it like solid state transformations in metals? The analogy is poor as the bonding is different and easier to comprehend.

Anybody able to explain?

#2

Can nobody answer this question or point me in the right direction for an answer??

#3

I don’t have an answer for you but it could be that new covalent bonds form over time or that possibly the chains “settle” and the Van der Waals bonds become stronger. Your topic piqued my interest and I’ve been looking for a conclusive study but so far have been unable to find one.

#4

post curing is speeding up the normal process. epoxy’s and polyesters aren’t just “done” after 24 hours. Epoxy will harden up for about a week, polyester will take longer, and over the years polyester will change from somewhat flexible, to quit a brittle state. When heating up, I expect the molecules to form longer chains and better bonds, therefore the endproduct (resin) will be stiffer.
We noticed that after a initial cure, freezing the product, then placing it in sunlight, it will become stiffer much faster than one thats left just to cure over some time.

#5

Nice contraditio is that (warmcuring) epoxy cures into a brittle “B-stage” with high E-modulus, and postcures into a more flexible but stronger product after cure. (lower E modulus).