Epoxy is a two-part thermoset in which a resin and a hardener react chemically, not by drying, to form a cured solid with tensile strength of 55-130 MPa, strong adhesion to metal, wood, ceramic and glass, and low shrinkage. The reaction needs the mix ratio on the label to be followed exactly, and it gives off heat that a large mass cannot lose. Hardener components can cause permanent skin allergy, so use gloves and ventilation every time.
Part of Thermosets
Epoxy systems are two-component resins, a resin and a hardener, used as structural adhesives, coatings and composite matrices.
The Reaction Needs The Right Proportion Of Each Part
The Advanced Materials article calls epoxy the highest-performance resin in the section, with strength of 55-130 MPa, far above polyester resin, excellent adhesion, low shrinkage during cure (less warping), and good chemical and water resistance. Its two parts react until one runs out. With the right ratio, both are used up and the network is complete. With too much hardener or too little, unreacted material stays in the part as a soft or sticky patch, and no amount of waiting cures it. See Thermosets for the general principle.
Worked example: a salvaged kit labelled with a resin-to-hardener ratio of 2 : 1 by weight needs 30 g of hardener for 60 g of resin (30 × 2). Mixing 75 g of resin needs 75 / 2 = 37.5 g of hardener, so using 30 g is 7.5 g short, or 20% less hardener than the reaction requires (30 / 37.5 = 0.80), and leaves resin uncured. Weigh on a scale. By-volume ratios are less exact.
Where The Resin Comes From
The Advanced Materials article describes the synthesis in outline: epichlorohydrin, from glycerol and hydrochloric acid, reacts with bisphenol A, from phenol and acetone, under alkaline conditions. It adds that this needs organic chemistry capability. This article does not give a procedure. Epichlorohydrin is toxic and bisphenol A a hazardous chemical, and the batch cannot be controlled without good measurement. The realistic source for a settlement is salvaged epoxy: hardware-store kits, boat-repair resin, sealed ampoules in electronics and old composite parts. Salvaged resin has a shelf life, so test a small mix first.
Getting A Good Bond
Cured epoxy sticks to a surface by chemical and mechanical grip. A weak interface means a weak joint, whatever the resin’s strength.
- Clean. Degrease and remove rust, oil and dust.
- Roughen. A scuffed surface gives more area. Use coarse abrasive, then wipe.
- Dry. Moisture interferes with the cure.
- Mix thoroughly. Scrape the sides and bottom of the pot so no unmixed streaks remain.
- Apply thin. A thin glue line is stronger than a thick one.
- Clamp lightly and wait. Let it cure at the right temperature. Warmth speeds it and cold slows it. A post-cure at moderate heat improves strength and temperature resistance.
Limits To Respect
- Heat. Epoxy softens above the temperature at which its chains move again. Do not use a common epoxy near a stove, an engine or a furnace.
- Sunlight. Long exposure chalks and yellows the surface. Coat it with a protective finish.
- Brittleness. Alone, cured epoxy cracks under impact, so for tough parts use it as the matrix in glass cloth as in Fiberglass.
- Thick pours. A thick mass overheats and cures fast. Pour in layers.
Hardener sensitises skin, and a hot mass can burn
Hardener components, especially amines, cause allergic skin reactions that become permanent with repeated exposure, and vapours irritate the lungs. Wear nitrile gloves and eye protection, work with ventilation, and do not use solvent on skin. Do not breathe sanding dust from cured epoxy. A large mixed batch in a deep pot can heat enough to smoke and burn skin, so mix small batches and set the pot outdoors on a non-flammable surface.
Common Mistakes
- Wrong ratio — a permanent soft patch.
- Rushing a cold cure — slow or incomplete.
- Skipping surface prep — the glue line fails first.
What’s Next
- Learn next: Phenolic Resins — another thermoset with a different strength
- Combine with: Fiber-Reinforced Composites — using epoxy as a matrix