It occurs more than you would like.
Most Prepregs are fabrics impregnated with a partially cured resin mixture " B" staged.
This cure cycle is stopped by refridgerating the product.
As soon as the prepreg is taken out of the freezer this cure cycle starts again very slowly, if the stuff is left out of the freezer for too long it gets unusable because of this unwanted curing.
An exception to this, is the very high temp pre pregs that use a dispersed solid salt hardener, that require the resin to be brought to a temperature that melts the salt before curing can take place.
B.E.

This is a stupid idea. Boeing tried this approach using a room temperature curing film adhesive on the first 296 737 aircraft. The result was Aloha 243 where the fuselage lap splices failed. The final cause was multi-site cracking but that cracking would not have occurred if the bond had been effective.

The problem is handling and storage. If you freeze the item to prevent it going off before use, and then thaw it out, it cures while it is thawing. If you apply it frozen (as Boeing did) then atmospheric humidity will settle on the surface inhibiting bonding. The same would be true for a pre-preg.

Regards

Blakmax

First of all, I dont think any ideas are stupid. Perhaps bad, yes, but not stupid. I made close to a million dollars licensing what many called a "stupid idea" so you be the judge.

Second, I don't know much about FRP so that's why I posed the question. I understand that resins can cure in a number of different ways whether they be thermosets, thermoplastics, etc. Take super glue for example, it doesn't cure until it's exposed to the atmosphere which contains hydroxyl ions from moisture. What if you were to have a prepreg that was sealed in rolls (release paper on all sides), and you removed the release paper before bonding, to say a bridge or beam for retrofitting. If the gel time were long enough, one could mold the prepreg and let it cure in the atmosphere. Just a thought. It would sure beat having to mix epoxy and lather it on. It's a very messy process if you've never seen it done.

I think I am getting a better handle on what you want to do.
Have you investigated UV ( ultraviolet light) curing resins.
These are used for some bridge beam and column reinforcing applications. The material is applied at night under incandesent light, then when the sun comes up the UV in the sunlight cures the resin. there are some short comings to this method in that the reinforcing material has to be transparent.
B.E.

I have seen moisture curing polyurethanes used as a matrix material. Applications include in-situ pipe repairs etc..
There are some applications which call for cure during service. Deployable space booms and antenna for one - these require tight packaging during launch and then can be inflated or unfolded to form large structures which are more structural when cured.
Cure does have to be considered very carefully - UV cure already mentioned requires a transparent reinforcing medium and also can be prone to shadowing effects. Moisture cure systems require very careful handling/deploying as any exposure to the environment can result in cure when you don't want it.

This made me think of self-repairing composite materials. Interesting reading if nothing else, but essentially once cracked or agitated, new resin components are introduced (stored in hollow spheres, tubes, etc in the composite) wich then cures and re-bonds the damaged area.

Other than the expense of a lot of really big "FRAGILE" stickers, I'm not sure what major hurdles there would be in making the concept work with a pre-preg. Go over it with a rolling pin to release the hardener, form it, let it cure.

I'm not aware of any RT-cure pepregs, even with resins such as UV- or moisture-cure that put off their cure by a relatively unusual mechanism (those seem to be aimed at wet layup (often repair) or maybe infusion).

You imply you'd like a resin that will cure just with time at RT, possibly due to some sort of solvent/cure-inhibitor evaporating. There are plenty of resins that cure at RT (often under vacuum if used with a fibre reinforcement), usually two-part ones, mostly polyester or epoxy. You might get somewhere with impregnating some cloth with resin that takes seveal hours to go off and freezing the result, but it sounds most inconvenient (a frozen material that will largely cure while thawing out and have very inconstant short-lived properties even once back at RT—I dread to think of the complications of automatically laying down such material!).

An actual resin formulator might tell you why it isn't done if you want more detail, but Blakmax's (somewhat dismissive) reply probably summarises it.

Your reply says that it might be handy for composite reinforments on bridges, etc., a largely manual op. If the mess cost much actual money I'd have thought it would have been addressed some way. Mind you, these uses take time to mature. If that reinforcement's been going bigtime since 1994 after the LA earthquake (when it got major publicity) it's not that long. Maybe there just hasn't been enough money in it to interest the big process designers?