What is epoxy resin, and what do resin and hardener do?

From aircraft to art

Epoxy resin wasn’t developed for jewelry or art. Its roots are in industry. It was first synthesized in the 1930s, with the goal of creating a high-strength, chemical-resistant, and waterproof material for technical applications. Its first uses were aircraft manufacturing, electrical engineering, and shipbuilding and boatbuilding. Only decades later did designers, furniture makers, and eventually artists discover the material for themselves.

The two main players: resin and hardener

In Part A, the resin, you’ll find what are called epoxy groups, also known as oxirane groups. These are tiny ring-shaped structures made of two carbon atoms and one oxygen atom. This ring is under constant strain, like a mousetrap about to snap shut. In systems that cure at room temperature, the hardener consists of amines. Think of them as perfectly cut ignition keys: only when they’re present does molecular cross-linking begin. Without them, the resin would stay liquid.

What happens when you mix them

Mixing Part A and Part B starts an exothermic reaction, which releases heat. The resin’s epoxy groups meet the hardener’s amine groups, the rings open, and the molecules link up into a three-dimensional network. Two clear liquids become a solid plastic called a thermoset. The process is permanent: cured epoxy resin can’t be melted or dissolved again, unlike thermoplastics.

Epoxy resin doesn’t dry, it cures

Curing happens in several stages:

• Liquid stage: you can pour and shape the resin. Fumes are at their peak, so a respirator, protective gloves, and good ventilation are especially important.
• Gel stage: the resin becomes as thick as honey. There are still a lot of fumes, so keep full protection on.
• Tacky stage: the surface is still sticky, and the material is still actively reacting and releasing fumes.
• Initial set: the piece is firm enough to touch but still delicate. No sanding or polishing yet.
• Full cure: only now does your piece reach its final strength and properties.

In lab tests at DIPON, samples reach their final Shore hardness only after 7 to 30 days, depending on the epoxy resin. The recommendation: keep a freshly poured piece out of living rooms and bedrooms for at least a week, even if it already feels solid.