Chains versus networks
When every molecule has two reactive groups (functionality 2), they link end to end into long chains. Add molecules with three or more reactive groups, such as a triol, a triamine like T-5000, an HDI trimer or a multifunctional novolac epoxy, and chains branch and tie together into a three-dimensional network. The more junctions per volume, the higher the crosslink density.
What crosslink density changes
| Property | Higher crosslink density |
|---|---|
| Hardness and Tg | Increase |
| Chemical and solvent resistance | Increase (less swelling, slower diffusion) |
| Elongation and flexibility | Decrease |
| Impact and crack resistance | Often decrease (more brittle) |
| Cure shrinkage and stress | Often increase |
The formulator’s levers
- Functionality: swapping a diol for a triol, or a BPA epoxy for a novolac, raises density.
- Equivalent weight: long, high-equivalent-weight segments (like D-2000) space junctions far apart; short ones pack them tightly.
- Stoichiometry: off-ratio mixes leave dangling, unreacted ends and lower effective density. See mix ratio calculations.
- Reactive diluents: monofunctional diluents cap chains and reduce density.
- Degree of cure: cold or incomplete cure leaves a looser network than intended.
How it is checked
In the lab, crosslink density is estimated from swelling in solvent or from the rubbery modulus measured by dynamic mechanical analysis. In the field, a solvent rub test is a practical proxy for whether a reactive coating has cured into its intended network.