Epoxies are one of the wonder materials of modern boating – strong and stable even in the harsh marine environment. Today we’re using epoxy to repair a broken strip of Iroko wood that came from a boat cockpit side.

Epoxy history

Prior to the invention of plastics in the 1900’s, humans had to make do with the limited range of materials that nature provides – wood, metal and stone. This all changed in 1909 when Leo Baekeland invented the world’s first synthetic hard mouldable plastic – Bakelite. Bakelite was an exciting new material – it could be shaped, is heat-resistant and also an electrical insulator so it quickly found its way into lots of household and industrial products.

The main trick behind Bakelite was figuring out how to make very long molecules (“polymers”) by joining together lots of identical subunits. In Bakelite, ring-shaped phenol molecules are joined together into a large 3d mesh. But it’s not easy persuading the molecules to hook together in this way – it needs carefully controlled (high) temperatures and pressures. We’ll return to this point when we talk about epoxy.

The success of Bakelite drove chemists to try making plastics from other kinds of molecules, leading to the invention over the next few decades of polystyrene, poly vinyl chloride (PVC) and polyethylene (polythene) all with different properties and uses.

Epoxy resins came along in the 1940s, and started to get used for boat building in the late 1960’s. There’s lots of different epoxy resins, but the thing that makes them “epoxy” is that their molecules contain an epoxide group, a little triangular group of atoms which, with a little bit of help, can hook up to the corresponding site on other molecules. The “little bit of help” comes in the form of a second chemical (“hardener”) such as phenol or aromatic amines. Different chemicals react at different rates and so West System sell a ‘fast’ hardener (“205”) and the safety data tells you what chemicals are actually in it, and a ‘slow’ hardener (“206”).

One of the remarkable things about epoxy is that the reaction occurs at normal room temperature and at everyday pressure. Bakelite had to be made in special machines, but you can mix up a batch of epoxy in the boat yard or in your garage and have it harden into solid in a few hours.

Like Bakelite, epoxy resin molecules also have lots of hydroxl (“OH”) groups which make them good at adhering to other substances.

Actual epoxy resin

Several companies market epoxy resin for boat builders. Today we’re using West System epoxy, but Blue Gee in the UK sell similar kits. West System sell a handy “mini pack” consisting of 250ml of resin and 50ml of hardener, some mixing tubs and measuring syringes and two different kinds of filler. The equivalent Blue Gee kit doesn’t have the syringes or fillers, but is nearly 40% cheaper.

The basic idea with epoxy is to thoroughly mix 5 parts resin to 1 part hardener (by volume or by weight) to give a runny liquid which after some time will harden into plastic. This “unthickened” epoxy is great for coating porous materials such as wood, but to change it into a something you can use as a glue or a gap-filling compound you need to add a filler material to thicken it up. West System provide a range of fillers to make the epoxy more of a thicker mayo or peanut-butter consistency for gluing or filling.

Let’s get back to our broken bit of wood. The first step is to mix up a suitable amount of resin and hardener – 5ml resin is more than enough for this job. Coat both sides of the wood with the unthickened epoxy to saturate the porous wood with epoxy.

Next, use the filler powder to thicken the rest of the epoxy so it’s about the consistency of mayo. There’s no magic recipe here, just add a few scoops and mix well to see how thick it’s getting.

Mixing up the epoxy with 403 microfiber filler

Then it’s just a case of spreading the mayo-consistency epoxy generously on one side of the wood and clamping gently together. You want to use enough epoxy that some squeezes out the side when you clamp the joint – this ensures that there’s no voids. But don’t clamp it excessively because otherwise you might squeeze all the epoxy out!

Curing

Now everything is in place, the magic starts to happen. All the molecules in the epoxy are wiggling around a bit because they’re warm. As the hardener molecules and resin molecules bump into each other, they react and start to join together into bigger and bigger clumps. This happens more quickly when its warmer, because the molecules are jiggling around more. After about 6 hours, the epoxy will be solid. But it’s still building up more and more links, and can take up to six days to develop full strength.