#14: Epoxy resin chemistry

Unless you have a hobby building surfboards, canoes or boats, you probably know very little about the chemistry of epoxy resins. Take the time to gain a little insight and it can save you time and money.


Starting with epoxy resins is kind of like starting a new relationship. If you take the time to learn and understand what makes epoxy tic, you are in for a long and prosperous life together. But, the second you take that resin for granted… BAM, its going to bite you where it hurts.

Mixing epoxy resin and hardener begins a chemical reaction that transforms the combined liquid ingredients into a solid. The time it takes for this chemical transformation is called cure time. As it cures, the epoxy passes from the liquid state, to a gel state, to a solid state.

The liquid state of the resin, after resin and hardener are mixed, is also known as pot life. The pot life is determined by several factors, including;

  • The temperature of the resin at the time of mixing.
  • (the warmer the resin, the shorter your pot life)
  • The type of hardener you use.
  • The volume of resin that you mix in one container.

It’s a common misconception that once resin and hardener are mixed, the curing process begins straight away, and the process happens as a nice steady transition from liquid to solid. This is not the case. Once you mix both components, it remains dormant for some time. This is your pot life. In layman’s terms it’s a bit like a General getting his troops ready to go into battle. The resin is just getting itself organised and it is during this period that you can effectively work with it.

The pot life calculations we provide are based on 20°C using a 200ml container. When you need larger volumes, say 20 litres for instance which will cure very quickly, it’s worth considering the following;

  • Use a hardener with a longer pot life.
  • Avoid working in warm conditions.
  • Mix a couple of smaller pails instead of one large – pour one at a time into your felt, and wring it out to reduce the size of the ‘slug’ of resin.

Once the resin starts to cure, gel time begins. The first thing you will notice is your liner starting to warm up. We try to limit ourselves to one big word per day so here it is – exothermic reaction. Exotherm just a fancy way of saying the resin begins to give off heat. The heat is caused by molecules in the resin getting themselves in order before they cross link and cure. It is this cross linking of the molecules that gives the epoxy resin its strength. So, now the soldiers have formed ranks and present a formidable barricade.

If your liner isn’t in the pipe by the time it starts to exotherm then it’s too late and the only place for you to install the liner is onto the ground, because once it starts to cure the process moves very quickly. The epoxy is no longer workable and will progress from a tacky, gel consistency to the firmness of hard rubber. At this stage you can still dent it with your fingernail.

Once the epoxy’s chemical reaction is complete, the mixture has cured to a solid state. At this point in epoxy chemistry, the product has reached about 90% of its ultimate strength, so the calibration tube or bladders can be removed. It will continue to cure over the next several days at ambient temperature.


If your liner begins to exotherm while it is still in your inversion drum, get it out as quickly as possible otherwise you’ll be removing hardened liner from your inversion drum with a 4″ grinder!


Curing epoxy generates heat. Several centimetres of mixed epoxy in a confined mass (such as a mixing cup) will generate enough heat to melt a plastic cup, burn your skin or ignite combustible materials if left to stand for its full pot life. For this reason, do not use foam or glass mixing containers or pour in confined spaces. If a pot of mixed epoxy begins to exotherm quickly move it outdoors. Avoid breathing the fumes. Do not dispose of the mixture until the reaction is completed and has cooled.


Pipe Core are Australia’s leaders in trenchless technology, supplies and training. For more information on Epoxy resins give us a call, drop us a line or come and see us at our factory in South Melbourne.