Epoxy (left): a two-part adhesive. Difunctional epoxy-resin monomers (amber) carry reactive epoxide rings; the amine hardener (blue) carries N–H hydrogens on 2–3 arms. Each N–H opens an epoxide ring and forms a permanent C–N bond, so the trifunctional amine becomes a branch point — repeated across the mixture this stitches every monomer into one covalently bonded 3D network (thermoset). The reaction is thermally activated: raising temperature increases the collision rate and the fraction of molecules with enough energy to react, so cure finishes far faster hot than cold.
Cyanoacrylate (right): "superglue" cures by anionic chain polymerization, not by mixing two parts. Trace hydroxide ions / adsorbed moisture on the bonded surface act as an initiator, attacking the alkene of a cyanoacrylate monomer and generating a carbanion that immediately attacks the next monomer — a self-propagating chain that zips shut in seconds, which is why the glue "sets" almost instantly on contact with a slightly damp surface.
rate(T) = rate(25°C) · exp[(T−25)/12] (epoxy, illustrative Arrhenius scaling)
crosslink density ↑ => modulus ↑, elongation-at-break ↓
- Crosslink density — fraction of reactive sites already bonded into the network; only epoxy is a true 3D crosslinked network, cyanoacrylate is mostly long linear/lightly branched chains.
- Modulus vs. elasticity — a real trade-off in cured thermosets: a denser network resists deformation (high modulus) but has less chain mobility left to stretch before breaking (low elongation).
- Temperature — only shown for epoxy here because its cure is a bimolecular reaction with a real activation energy; cyanoacrylate's ionic chain reaction is already so fast at room temperature that the difference is not the controlling factor for a hobbyist bond.