Two-part epoxy glue

Section 3 of 4

Epoxy glue comes in two tubes, or a double syringe. Each part stays runny for a year or more on its own. Mix them, and within minutes to hours they set into one of the strongest glues there is.

One part is the resin. In most epoxy glues it is bisphenol A diglycidyl ether, BADGE for short: two benzene rings with an epoxide at each end. An epoxide is a three-membered ring of two carbons and an oxygen. Its bonds are squeezed into a 60° triangle, like cyclopropane's, far from the 109.5° they sit at in methane, so the ring opens easily.

The other part is the hardener, usually a molecule with several amine groups. A common one is triethylenetetramine, TETA: a chain with an -⁠NH₂ group at each end and two -⁠NH-⁠ groups inside it, six N–⁠H bonds in all.

BADGE, the resin in epoxy glue, as a skeletal formula A skeletal formula drawn left to right: a three-membered epoxide ring of two carbons and an oxygen, a -CH2- group and an oxygen, a benzene ring, a carbon carrying two methyl groups, a second benzene ring, an oxygen, a -CH2- group and a second epoxide ring. O O O O
The resin, BADGE, C₂₁H₂₄O₄: a three-membered epoxide ring at each end.
Triethylenetetramine (TETA), C₆H₁₈N₄A common hardener for epoxy glue. A chain of four nitrogen atoms with two carbon atoms between each pair. The two end nitrogens are -NH₂ groups and the two inside ones are -NH- groups, so it has six N-H bonds.NHNHNH2NH2
The hardener, TETA, C₆H₁₈N₄: four nitrogens holding six N–⁠H bonds between them.

Opening the ring

A nitrogen's lone pair reaches for one carbon of the epoxide ring. The new N–⁠C bond forms, the C–⁠O bond of the ring breaks, and the hydrogen that was on the nitrogen moves to the oxygen, making an -⁠OH group. Nothing is given off: every atom of both molecules stays in the product, as in addition polymerisation.

An N-H of the hardener opens an epoxide ring of the resin Top: on the left, a nitrogen atom of the hardener with two hydrogen atoms; its bond to the right-hand hydrogen is amber. On the right, the end of a resin molecule: a three-membered epoxide ring of a CH2, a CH and an oxygen, with the CH2 to oxygen bond amber. An arrow labelled the ring opens points down. Bottom: the nitrogen is now bonded to the CH2 by a new amber bond, and the ring is open: the CH carries an -OH group, shaded, made from the ring's oxygen and the hydrogen that left the nitrogen. NHHH2CCHONHCH2CHOH hardener resin: epoxide ring the ring opens new N–⁠C bond new -⁠OH
An N–⁠H of the hardener opens an epoxide ring. The nitrogen bonds to the ring's CH₂ (amber), and the oxygen keeps its bond to the other carbon and picks up the hydrogen (shaded).

Each N–⁠H can open one ring, so one TETA molecule can join up to six resin molecules, and each resin molecule, with a ring at both ends, joins two hardener molecules. The links branch in every direction and tie all the molecules into one giant network. The new -⁠OH groups also make hydrogen bonds with the surfaces, which helps the grip.

Hardener and resin molecules joined into one network A legend shows a blue dot for a hardener molecule and a teal bar for a resin molecule. Below, fifteen blue dots in three rows are joined by teal bars, each bar running from one dot to another, so that every dot is linked to between two and six others and the whole drawing is one connected net. hardener resin molecule
Set epoxy as a network. Each resin molecule (teal) links two hardener molecules (blue), and each hardener molecule links several resins, so the links run in every direction.

A network of cross-linked chains is a thermosetting plastic, like a melamine plate: warm it and it keeps its form, and at high temperatures it chars. The two parts must be mixed in the ratio on the pack, so that the N–⁠H bonds and the rings match up. With too little hardener, rings are left unopened and the glue stays soft.

Quick check

Why does mixed epoxy glue set into one solid network that never melts?