Every epoxy formulation mistake that repeats across the industry starts in the same place: somebody used a percentage by weight where the chemistry demanded an equivalent. Epoxy equivalent weight (EEW) is the bridge between the two, and for monofunctional diluents and difunctional crosslinkers the arithmetic behaves in opposite directions. This article sets out the calculation and the four places it goes wrong.
What EEW Is
EEW = grams of material that carry one mole of epoxide groups, i.e. molecular weight divided by epoxide functionality. Its reciprocal, multiplied by 100, is the epoxy value in equivalents per 100 g:
Epoxy value (eq/100 g) = 100 / EEW
For a simple monoepoxide the EEW is the molecular weight itself. For a diepoxide it is half the molecular weight. For a resin that is a mixture of oligomers – a standard liquid DGEBA is not a single compound – the EEW is a measured average, quoted on the certificate and typically between 180 and 190 g/eq for the low-viscosity grade.
| Material | Functionality | MW | EEW | Epoxy value (eq/100 g) |
|---|---|---|---|---|
| 1,2-Epoxyhexane (C6) | 1 | 100.16 | 100.2 | 1.00 |
| 1,2-Epoxydecane (C10) | 1 | 156.26 | 156.3 | 0.64 |
| 1,2-Epoxydodecane (C12) | 1 | 184.32 | 184.3 | 0.54 |
| 1,2-Epoxytetradecane (C14) | 1 | 212.37 | 212.4 | 0.47 |
| Cyclopentene oxide | 1 | 84.12 | 84.1 | 1.19 |
| 1,5-Hexadiene diepoxide | 2 | 114.14 | 57.1 | 1.75 |
| 3,3′-Bi-7-oxabicyclo[4.1.0]heptane | 2 | 194.27 | 97.1 | 1.03 |
Cure Ratio for Amine Hardeners
The stoichiometric hardener weight per 100 g of epoxy component is:
phr = (amine equivalent weight x 100) / EEW
The amine equivalent weight is the amine’s molecular weight divided by the number of active hydrogens on nitrogen – the same figure that appears on a hardener certificate.
| Hardener | Amine eq. wt | phr with DGEBA EEW 185 | phr with C12 monoepoxide EEW 184.3 | phr with 1,5-hexadiene diepoxide EEW 57.1 |
|---|---|---|---|---|
| Diethylenetriamine (DETA) | 20.6 | 11.1 | 11.2 | 36.1 |
| Triethylenetetramine (TETA) | 24.4 | 13.2 | 13.2 | 42.7 |
| Polyether amine D-230 type | ~60 | 32.4 | 32.6 | 105.1 |
Look at what the diepoxide column does. The same hardener that needs 11.1 phr against a DGEBA needs 36.1 phr against a difunctional epoxide – three times as much – because each gram of the diepoxide carries 1.75 equivalents of epoxide rather than 0.54. Nothing about the hardener changed; only the epoxide density did.
Blends: the Reciprocal Mixing Rule
Real formulations are blends, and the blend EEW is not the arithmetic average of the component EEWs. It is the reciprocal of the sum of each component’s mass fraction divided by its EEW:
1 / EEWblend = (w1 / EEW1) + (w2 / EEW2) + …
With 80% DGEBA at EEW 185 and 20% C6 epoxide at EEW 100.2:
(0.8 / 185) + (0.2 / 100.16) = 0.004324 + 0.001997 = 0.006321, so EEWblend = 158.2 g/eq
and the DETA requirement becomes 20.6 x 100 / 158.2 = 13.0 phr rather than 11.1 phr. That is the entire mechanism by which a reactive diluent changes a formulation: not by “thinning” it, but by changing the number of epoxide equivalents per kilogram.
| Blend (80% DGEBA EEW 185 + 20%) | Blend EEW | DETA phr | Change vs undiluted |
|---|---|---|---|
| 1,2-Epoxyhexane (100.2) | 158.2 | 13.0 | +1.9 phr |
| 1,2-Epoxyoctane (128.2) | 169.9 | 12.1 | +1.0 phr |
| 1,2-Epoxydecane (156.3) | 178.4 | 11.6 | +0.5 phr |
| 1,2-Epoxydodecane (184.3) | 184.9 | 11.1 | 0.0 phr |
| 1,2-Epoxytetradecane (212.4) | 189.9 | 10.9 | -0.2 phr |
| 1,5-Hexadiene diepoxide (57.1) | 127.7 | 16.1 | +5.0 phr |
Anhydride Cure Uses the Same Logic
Anhydride hardeners are dosed per epoxide equivalent too, with the anhydride equivalent weight taken as molecular weight divided by the number of anhydride groups. In practice formulations run at somewhat less than the full stoichiometric equivalent – the exact ratio depends on the accelerator, the cure temperature and whether the system needs excess anhydride for esterification – so the calculation gives the starting figure and the bench work sets the final one. The important point is the same: the number is derived from EEW, not from a fixed percentage.
Where the Calculation Goes Wrong
| Mistake | What happens | Fix |
|---|---|---|
| Using weight percent as if it were equivalent percent | A 20% dilution with a low-EEW epoxide leaves the system badly under-cured, and the failure appears months later as poor chemical resistance | Recalculate blend EEW, then phr |
| Ignoring a reactive diluent entirely | Under-cured film at correct stoichiometry for the base resin alone | Include every epoxide-bearing component in the blend EEW |
| Treating a diepoxide as a “stronger diluent” | Hardener demand roughly triples; the mix gels hot and brittle | Diepoxides crosslink – dose them as crosslinkers |
| Using the theoretical EEW instead of the measured one | DGEBA is a mixture; a certificate EEW of 188 versus an assumed 185 moves phr by 2% | Use the batch EEW from the COA |
The last one deserves emphasis: for pure monoepoxides the calculated EEW is exact (MW divided by one), but for oligomeric resins it is an average that varies batch to batch. That is exactly why epoxy value is specified and measured on a certificate rather than assumed from a structure drawing.
Checking a Certificate
- EEW and epoxy value must be reciprocals. 184.3 g/eq means 0.543 eq/100 g. If both are quoted and they disagree, one of them is a typo or a different batch.
- GC purity and epoxy value are different measurements. A 96% GC assay with a 0.50 eq/100 g epoxy value is consistent (some of the “96%” is unreactive isomer); a 96% assay with 0.40 eq/100 g means ring opening.
- Measure what you buy. Titration of epoxy value takes twenty minutes and settles arguments about cure that no amount of GC data will.
Frequently Asked Questions
How do I convert epoxy value to EEW?
EEW = 100 / epoxy value. Epoxy value 0.54 eq/100 g gives 185 g/eq – which is why a DGEBA at EEW 185 and a C12 monoepoxide at EEW 184.3 behave so similarly in a blend.
Why does my diepoxide need three times the hardener?
Because it contains roughly three times the epoxide equivalents per gram. At EEW 57.1 it carries 1.75 eq/100 g against 0.54 for a C12 monoepoxide; phr scales directly with that ratio.
Can I use the molecular weight as the EEW?
Only for a pure monoepoxide. For anything with two or more epoxide groups, divide by the functionality; for oligomeric resins, use the measured certificate value.
Does water affect the epoxy value?
Slowly on its own, faster with acid present. Ring opening converts epoxide to glycol, the epoxy value falls, and the formulation becomes under-cured without any change to the recipe.
References and Disclaimer
- Molecular weights and formulas from PubChem records for the CAS numbers quoted; epoxide functionality from structure.
- Amine equivalent weights for DETA (20.6), TETA (24.4) and a D-230-type polyether amine (~60) as quoted on standard hardener data sheets.
- All phr and blend EEW values are calculated here from the formulae above and must be confirmed against the actual resin, hardener and cure schedule.
Disclaimer: technical information for formulators, not a specification or SDS. Cure ratios are starting points for bench work, not a released formulation.
PolyBlueChem supplies monofunctional and difunctional epoxides – C6 to C14 linear, cyclic and difunctional – with per-batch COA reporting assay and epoxy value so that the calculation above can be run on real numbers. Send the base resin EEW and the property you are trying to shift, and we will confirm the grade. See the C6 to C14 selection guide for the viscosity and storage trade-offs.