When optimum curing temperature is advised as 20 degrees.....what happens if below? Does Epoxy still cure but just take Longer? Or will it kill the chemical process?

I can’t guarantee that my workshop will stay anywhere near 20, but can easily leave to cure for several days.

Yes and no. It will cure, but not to the state it would at 20°. If it is much colder, you could end up with an super brittle resin, that falls apart during the demould. "Postcuring" at 20° will result in a normal curing state.
The real problem is, that most epoxys become very thick below 20° wich makes air bubble free lamination almost impossible.

Build a 'hotbox' to maintain a consistent temperature. Best thing I ever did.

If all you're doing is trying to keep parts above 20 degrees, this can really be as simple as a few sheets of insulation foam taped together with heat reflective tape, and a small fan forced desktop heater poked through one of the walls. If you dont want to splash out on a PID controller for regulating the temperatures, then just flick the heater on for 30 minutes and then flick it off again, you'll be surprised how well the box holds the warmth in. Likely wont hold it overnight in really cold temperatures, but it will still help.

Hi, as the guys have suggested curing at a lower temperature for longer is not a great substitute for curing at or closer to the recommended 20'C. If you're looking at more like 15'C then you would be OK to just let it cure for longer but anything much less than that and you would start to see cured properties that are not what they would have been if the part had been cured at a more appropriate temperature. A rule of thumb for how temperature affects time is to double (or halve) values for every 10 degrees C increase or decrease. This means that at 10'C the resin would have twice the pot life and take twice as long to cure as it would at 20'C. At 30'C you would have half the pot life and it would cure in half the time. I wouldn't suggest using this rule to go much below the recommended cure but it's very handy when you're working in ambient temperatures above the quoted 20'C.

As Oakmont points out, the viscosity and handling of the resin will also be quite different and is likely to cause problems at lower temperatures and you should be aware that the lower the cure temperature, the lower the Tg (effectively the temperature tolerance) of the cured resin.

So, with all this said, it's definitely best to try to do your work in an environment as close as you can get to 20'C. We all know that this isn't always possible and you will still find that the resin cures (albeit slower) at 15'C or even 10'C but you will be compromising the mechanical properties and the behaviour of the resin if you stray too far from optimal conditions.

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

All great advice guys. Thank you. I think I will invest in some heating and insulation for each project. Or wait for summer!

Nah....heating and insulation it is.