Dosing polyetheramines: AHEW times 100 over EEW, and blends never average linearly

Two drums sit in the receiving bay under one purchase order. Both lot Certificates of Analysis (COAs) print CAS 9046-10-0. One gives an amine value of 454 to 488 mg KOH/g. The other gives 0.44 to 0.52 mmol/g. The units differ, and that is not the only difference.

The batch sheet carries the same parts per hundred resin it carried last month. If the second drum is the long-chain diamine, the two grades sharing that CAS differ in dose weight by a ratio of 16.7. A mix ratio written for the short chain will leave the epoxy soft.

The arithmetic that prevents this fits on one line. Parts of hardener per hundred parts of resin equals the hardener’s amine hydrogen equivalent weight (AHEW) × 100, divided by the resin’s epoxy equivalent weight (EEW).

The position here is narrow. Dose on the grade AHEW. Use the lot COA amine value only as a gate that accepts or rejects the lot. Dose any blend on the harmonic mean of its AHEWs. A formulator who recalculates every dose from the lot’s amine value can argue that a measured number beats a typical one. That argument is examined below. Inside the specification band it loses, because the grade AHEW sets the dose. Outside the band a recalculated dose is not the answer either: the lot is held and rejected.

What sets the dose of polyetheramines in an epoxy?

The dose of polyetheramines in an epoxy is the grade’s AHEW × 100 divided by the resin’s EEW. Polyoxypropylene diamine RS-D230 has an AHEW of 60 g/eq. The numerator is therefore 60 × 100 = 6,000. Divide 6,000 by the EEW on your resin’s lot COA, and the result is parts per hundred resin.

RS-D230 is the low-viscosity polyether diamine in the set, the grade often written as polyetheramine D-230. Its sales specification caps viscosity at 15 max mPa·s at 25 °C. RS-D230 is a polyether diamine with low viscosity and moderate reactivity in epoxy curing. Its amine groups are all primary. Values, packaging and the full specification sit on the RS-D230 grade page.

The triamine works the same way. RS-T403 has an AHEW of 81 g/eq, so its numerator is 81 × 100 = 8,100, again divided by the resin’s EEW. Every other graded diamine and triamine on the polyetheramine hub follows the same rule: multiply the grade AHEW by 100.

The resin side is where a precise hardener number gets wasted. Take the EEW from the resin lot’s own COA, not from a typical value on a brochure. A hardener dosed to the gram against a resin EEW copied from a catalog is still off by the gap between that catalog value and the drum.

One CAS number covers four diamines with AHEWs from 60 to 1000 g/eq

RS-D230, RS-D400, RS-D2000 and RS-D4000 all carry CAS 9046-10-0. The EPA Substance Registry Services record for CAS 9046-10-0 gives the EPA name “Polypropylene glycol bis(aminopropyl) ether”. It lists the substance on the TSCA Inventory, the list EPA compiles and publishes under TSCA section 8(b), 15 U.S.C. 2607(b) (EPA Substance Registry Services, retrieved October 2026).

That listing identifies the polymer family. It says nothing about chain length, so the CAS on a COA cannot tell you which dose basis applies.

The spread is large. Against the same resin, RS-D4000 at 1000 g/eq needs 16.7 times the weight of RS-D230 at 60 g/eq (1000 ÷ 60). Hold any lot whose COA shows the CAS without the RS grade code. Match the code and the grade AHEW to the batch sheet before the drum is opened.

– not specified; ask the desk.

RS-D4000 hides a second trap. It has two equivalent weights: an AHEW of 1000 g/eq and an equivalent weight with isocyanates of 2000 g/eq. Against epoxy, each primary amine nitrogen opens two epoxide groups, one per hydrogen. Against isocyanate, each amine group counts once. Put 2000 g/eq on an epoxy batch sheet and you charge twice the hardener the resin needs: an amine excess that leaves an under-crosslinked, plasticized network.

The isocyanate figure belongs to polyurea and polyurethane work, where amine equivalents are matched against isocyanate in the same way that polyether and polyester polyols are matched by hydroxyl equivalent. Label every equivalent weight on the batch sheet with the co-reactant it was written for.

Read the amine value in the grade’s own unit

The table shows the problem from the dock. RS-D230 reports mg KOH/g. RS-D400, RS-T403, RS-D2000 and RS-T5000 report meq/g. The rest report mmol/g. None of these is wrong, but two of them cannot be compared without a conversion.

The bridge is the formula weight of potassium hydroxide in PubChem. One milliequivalent of base corresponds to 56.1 mg KOH, so dividing an amine value in mg KOH/g by 56.1 gives meq/g. For an amine titration, one millimole of basic nitrogen is one milliequivalent, so meq/g and mmol/g are the same number. Convert every lot to meq/g before the comparison, then compare it against the grade’s specification band, written in the grade’s own unit.

From there, a lot check follows. For a grade whose amine groups are all primary, each nitrogen carries two reactive hydrogens. AHEW in g/eq is then 1,000 divided by twice the amine value in meq/g. Run it as a check on the lot. It does not replace the grade value.

Why not dose on that calculated figure, since it comes from the actual lot? Because an amine value titrates nitrogen, not hydrogen. The conversion assumes every nitrogen is primary. The sales specifications do not promise that: RS-D230 guarantees 95.0 min wt% primary amine, and RS-T403 guarantees 90.0 min wt%. Any nitrogen that is not primary carries fewer reactive hydrogens than the formula credits, so the calculated AHEW reads low.

The position here can be wrong in one direction. The grade AHEW is a typical value, and a lot at the edge of its band drifts from it. The decision rule handles both cases. If the lot’s amine value falls inside the specification band, dose on the grade AHEW. If it falls outside, hold and reject the lot before it reaches a batch.

RS-T5000 shows why a single titration line is not the whole picture. Alongside its amine value, RS-T5000 has acetylatables of 0.58 to 0.63 meq/g. Acetylation counts amine and residual hydroxyl ends together. The gap between the two lines is the part of the chain end that is not amine. That gap does not react with epoxy as an amine hydrogen does, but it will consume isocyanate. If RS-T5000 goes into a spray polyurea system, its stated application, read both lines on the COA.

Why does a secondary-amine grade count one hydrogen per nitrogen?

RS-SD2001 is N,N’-diisopropyl polyoxypropylene diamine. Each terminal nitrogen already carries an isopropyl group, which leaves one reactive hydrogen where a primary amine has two. The lot check therefore drops the factor of two: AHEW in g/eq equals 1,000 divided by the amine value in meq/g. No AHEW is specified for RS-SD2001, so the lot check from the COA is the only basis for this grade.

Its amine value is specified at 0.875 to 1.03 mmol/g, the same number in meq/g, and primary amine is capped at 5 max %. That cap matters for the arithmetic. Any residual primary nitrogen carries two hydrogens, so a lot near the cap holds slightly more hydrogen than the all-secondary formula credits, and its true AHEW sits slightly below the calculated one.

Read the primary amine line on every lot. When the lot is near the cap, expect the cure to run a little richer than the arithmetic predicts.

RS-SD2001 has relatively low reactivity, which suits applications that need a slower cure. Its applications are polyurea, polyurethane and reaction injection molding systems. In those systems a secondary nitrogen reacts with one isocyanate group, so its hydrogen count is the same whether the co-reactant is epoxide or isocyanate. RS-D4000 has no such symmetry and needs two equivalent weights. RS-SD2001 needs one.

Dosing a blend of two grades: the harmonic mean, never the average

Formulators blend a short-chain grade with a long-chain one for working properties. RS-D2000 is a co-reactant in epoxy systems that need increased flexibility and toughness, and pairing it with RS-D230 is the obvious case. The dose of the blend still follows the rule. The question is which AHEW to put in it.

Equivalents add, and weights do not. The blend AHEW is the reciprocal of a sum: for each grade, divide its mass fraction by its AHEW, add those terms, and take one over the total. Written out, blend AHEW = 1 ÷ Σ(wᵢ ÷ AHEWᵢ), with wᵢ the mass fraction of each grade.

This is a harmonic mean weighted by mass. A linear average of the two AHEWs always overstates it, because the heavy grade contributes weight far faster than it contributes hydrogens. Dose on the linear figure and the batch over-charges amine hydrogens relative to the epoxide present.

Build the harmonic-mean formula into the batch spreadsheet, and recompute it each time the blend ratio changes. Do not carry a blend AHEW forward from an earlier ratio. Choosing which grades to pair on properties is a separate decision, covered in the epoxy curing agent selection guide.

There is a genuine “it depends” here. The stoichiometric point is a basis, not an instruction. Some systems run deliberately off-ratio to shift flexibility or cure speed, and that can be the right call. It belongs in the formulation record as a stated offset from the computed point. It should not arrive as an unexplained drift that nobody can reproduce.

When a batch comes back tacky, the first thing to rebuild is the arithmetic, and the usual symptoms are covered in why epoxy won’t cure, or stays tacky or soft.

What goes on the batch sheet

• Grade and basis: the RS code, its AHEW (60, 118, 81, 505, 1000 or 250 g/eq for the graded set), and the co-reactant that AHEW applies to.
• Resin EEW: taken from the resin lot’s COA for every lot, not from a typical value.
• Amine value gate: the lot value converted to meq/g (mg KOH/g divided by 56.1), checked against the grade’s specification band; outside the band, hold and reject the lot.
• Primary amine line: read against the specification minimum, for example 95.0 min wt% on RS-D230 and 90.0 min wt% on RS-T403, or the 5 max % cap on RS-SD2001.
• Moisture line: read against the maximum, 0.30 max wt% on RS-D230. Water is weight you charge that carries no amine hydrogen.
• Blend AHEW: computed by the harmonic mean for every ratio change.

Whether to dose a lot on the grade AHEW or on its own amine value comes down to one line on the COA: does the amine value fall inside the grade’s specification band? If it does, the grade AHEW sets the dose. If it does not, no recalculation makes the lot the grade you qualified.

Methodology: each grade value is what the technical data sheet for that RS code states, in its units, typical where marked typical; the lot COA governs. The potassium hydroxide factor is from the PubChem compound record; CAS identities and TSCA Inventory status are from EPA Substance Registry Services, retrieved October 2026.