ORIGINAL: Remby
MJD;
Your second description of the two is what I see here. I am just as surprised as you stated you were yourself. Any way to add something to the resin to re-liquefy it ? It is about as thick as say, between thick hair gel VS a super ball.
Could exposure to air cause this?
No, there is nothing you can add to fix it. Once polymerization has occured, you cannot reverse it. The options for additives are:
1. More resin. Well, if you have good resin, then use that instead, right?
2. Reactive or non-reactive epoxy diluent. No point - these will reduce mechanical properties further, plus nobody has this around their workshop unless they formulate epoxy adhesives for a fun or a living. In which case they should already have some inventory of fresh resin to use instead.
3. Solvent. Don't use solvents in structural adhesives, except for specialized applications where you really know what you are doing.
Homopolymerization can cause epoxy resins to harden over time in storage and is a one-way street, as opposed to crystallizing which CAN be reversed with heat and agitation. With the amount of time most of us own epoxy (and the temperature in my workshop) crystallization is a more common problem, but old or crappy resins can indeed harden before their time.
Generally the cause of short storage life from homopolymerization is impurites in the resin, typically rogue metals (trace amounts of metals that act as polymerization catalysts). This relates directly to the quality of the base resin in the system - which in some cases is 100% of the A side in a non-formulated system. Resin that has polymerized notably will not deliver the same mechanical properties as fresh material - since some of the resin system has reacted already, the adhesion to the substrate will be reduced to some degree. In addition, the equivalent weight of the resin has changed meaning that mixing to original specified proportions will now result in a curative-rich mixture.
As to crystallization, that is indeed reversible, and usually about 120-125F is all that is needed. I typically sit the resin bottle in a hot water bath until it is up to temp, then stir. But I do use a microwave on occasion, but I pulse it for a few seconds, check/mix, then repeat to avoid local overheating. Crystallized resin recovered with moderate heat works just as well as it did before crystallization.
p.s. "formulated system" = an adhesive or laminating resin system involving more than simply downpacked resin (which is quite common), typically involving one or more of: epoxy resin, reactive or non-reactive diluents, rubber modifiers (tougheners), wetting agents, pigments, etc. Raw epoxy resins are not generally intended to be used as is for structural adhesive tasks, except in thin film applications (laminating) or composite layups where the reinforcing matrix takes care of the mechanical needs. But they are often marketed as such for economy (read: profit), and in many cases the mechanical properties of the unformuated adhesive exceed that of the substrate so it doesn't matter too much.
Switching gears to CA adhesives, one common cause of short life in the workshop or field is proximity of the CA bottle to the catalyst container (kicker). Typical CA catalyst is a couple of percent of active material in a large percentage of solvent. Before awareness and action regarding fluorocarbons and the environment, Freon 113 was used as the solvent because it was pretty much inert, and would flash off in seconds. But of course it is not used anymore, and other solvents have replaced it in this application. BUT, the key point is that the catalysts themself have a fairly high vapor pressure, and free molecules of it tend to float around in an invisible cloud around the parts that have been spritzed, and the container itself. As these find their way onto the spout of your CA bottle and into the headspace above the adhesive, they begin to catalyze it and eventually you end up with blocked spouts, crusty chunks on the spout, and eventually premature curing of the CA itself. You can greatly extend the service life of your CA adhesives by keeping the CA bottle well away from the spritzer bottle, the spritzing job itself, and the spritzed parts. Now, I don't suggest you run into a different room to spritz parts then wait 5 minutes before returning, let's ne practical. But awareness of this tendency could help you avoid some headaches. I learned this on the job nearly two decades ago and once I became aware of the phenomenon, my CA started lasting longer and longer and I had a lot less trouble with clogged spouts and early curing in the bottle.
MJD