Epoxy Resin – Properties, Benefits and Applications

Epoxy resin – often simply called "epoxy" or "EP resin" – is a reactive synthetic resin consisting of two components: the actual resin (component A) and a hardener (component B). Only when both components are mixed in the correct ratio does the chemical reaction begin that cures the liquid material into a solid plastic.

Cured epoxy resin is a thermoset. This means it can no longer be melted or plastically deformed after curing. In return, it is dimensionally stable, hard and durable under load – ideal for fibre composites, bonding and coatings.

In fibre composites, epoxy resin forms the so-called matrix. It embeds reinforcement fibres such as woven glass fabric or non-crimp fabric, transfers loads into the fibres and protects them against environmental influences.

Epoxy resin cures through a so-called polyaddition: the molecules of resin and hardener bond directly with each other to form a three-dimensional network. This results in the most important difference to polyester resin:

• With polyester resin, a small amount of hardener (e.g. MEKP) starts the reaction. The amount of hardener mainly influences the speed.
• With epoxy resin, the hardener is a reaction partner. The mixing ratio must be observed exactly – more hardener does not speed up curing, it worsens the result.

The mixing ratio differs from system to system and is stated in the technical data sheet, for example 100:50 by weight for Epoxy Resin Resinpal 2301. Deviations lead to soft, tacky or brittle areas that no longer cure completely.

There are also a few points you should know: epoxy resin is usually more expensive than polyester resin, generally cures more slowly and requires an exact mixing ratio. Most epoxy resins also yellow under UV light unless they are UV-stabilised or protected by a top layer. A detailed comparison can be found in the article Epoxy Resin or Polyester Resin?

"Epoxy resin" is an umbrella term. Depending on the application, the systems are formulated differently – for example in viscosity, pot life, heat development and clarity:

• Laminating resin: low viscosity, good fibre wetting, for GRP components, repairs and boat building – for example Resinpal 2301 or Resinpal 2401.
• Casting resin: crystal clear, low in bubbles and designed for thicker layers – for river tables, jewellery and embedding. More in the article Casting Resin.
• Coating and topcoat systems: for resistant, smooth surfaces, e.g. the Epoxy Topcoat Resinpal 2405-H.
• Mould-making systems: epoxy gelcoats and laminating resins for precise, durable moulds – see Mould Making with Epoxy Resin.
• Bonding resins and fillers: paste-like, thickened systems for bonding, filling and repairs.

1. Prepare the substrate: The surface must be clean, dry and free of dust and grease. Sand smooth surfaces and remove the sanding dust.
2. Check the conditions: Material and room should be at the temperature stated in the data sheet. Avoid high humidity.
3. Weigh: Weigh resin and hardener exactly in the specified ratio. Only mix as much as you can process within the pot life.
4. Mix thoroughly: Stir slowly for at least two to three minutes, scraping the bottom and sides of the mixing cup. Unmixed residues at the edge are a common cause of tacky spots.
5. Process: Apply the resin quickly or wet out the reinforcement and roll out air bubbles with a laminating roller.
6. Let it cure: Maintain the temperature during curing, keep dust away and wait for the time stated in the data sheet before loading the part.

You can estimate how much resin you need for your laminate with the laminate calculator.

The pot life is the time during which the mixed resin remains workable. It depends heavily on the temperature and on the quantity mixed: the warmer it is and the larger the quantity in the cup, the faster the resin reacts.

Curing produces reaction heat (exotherm). In a large mixing cup or in layers that are too thick, the resin can heat up strongly, form bubbles and cracks or discolour. Laminating resins are therefore applied in thin layers, while casting resins are specifically developed for greater layer thicknesses. Always observe the maximum layer thickness stated in the data sheet.

Uncured epoxy resin and many hardeners irritate the skin and eyes and can cause allergic skin reactions. Once sensitisation has developed, it usually remains permanently. Therefore:

• Wear nitrile gloves and safety glasses, consistently avoid skin contact
• Ensure good ventilation
• Use a dust mask when sanding cured parts
• Observe the hazard warnings on the label and in the safety data sheet

Suitable protective equipment can be found in the Protective Clothing category.

• Wrong mixing ratio: "A little more hardener" does not speed anything up, it prevents complete curing.
• Mixed too briefly or unevenly: leads to tacky, soft spots.
• Too large a quantity at once: the resin heats up in the cup and becomes unusable.
• Processed too cold: curing slows down considerably or remains incomplete.
• Layer cast too thick: laminating resin is not casting resin – cracks, bubbles and discolouration are the result.

The ten most common mistakes and how to avoid them are described in the article Top 10 Mistakes When Working with Epoxy Resin.

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