Polyetheramine hardeners cure by equivalent weight, not by the number in the grade name

Your epoxy floor line or spray polyurea system runs on one amine hardener, and the bill of materials names it by a producer’s grade code with a single approved source. The lead time keeps stretching, and purchasing asks the obvious question: does anyone else make this?

The molecule behind the code is a generic chemistry, a polyoxypropylene diamine or triamine, made by more than one producer. What you cannot buy from a second source is the code. The fix is to rewrite that line item as a chemistry specification that any capable producer can quote against and your laboratory can check on every lot.

Three variables decide what a polyetheramine does

A polyetheramine is a polyether chain whose ends have been converted from hydroxyl to primary amine groups by reductive amination. The chain belongs to the same flexible polyether family used in polyurethane polyols, while the amine ends let it cure epoxy resin and react with isocyanate.

On a polyoxypropylene (polypropylene glycol) backbone, each amine sits on a secondary carbon next to a pendant methyl group. That steric hindrance gives moderate, controllable reactivity: long pot life, low exotherm and thicker castings with less exotherm risk. The price is a slow cure at ambient temperature.

Three variables then set the cured properties, and you can predict a grade’s behavior from them before a sample arrives.

• Backbone. Polyoxypropylene is the less water-loving backbone (the lowest-weight diamine still mixes with water) and is the default for coatings, composites and elastomers. A polyethylene glycol backbone makes the amine water-soluble, which suits waterborne systems and costs the cured film moisture resistance.
• Functionality. A diamine carries four amine hydrogens: in epoxy it still crosslinks, while in polyurea it builds linear, extensible segments. A triamine, grown from a trifunctional starter such as glycerol or trimethylolpropane, carries six and adds a branch point that raises crosslink density, glass transition temperature (Tg) and chemical resistance while cutting elongation.
• Molecular weight. The longer the chain between amine ends, the softer and tougher the cured network, and the lower its Tg and solvent resistance.

Why does the number in the grade name not set your mix ratio?

The number in a polyetheramine grade name is a nominal average molecular weight. Your epoxy mix ratio follows a different value: the amine hydrogen equivalent weight (AHEW), the mass of hardener that carries one reactive amine hydrogen. Divide the AHEW by the resin’s epoxide equivalent weight and you have the hardener-to-resin ratio by weight. For a clean diamine the AHEW is close to a quarter of the molecular weight; for a triamine, close to a sixth.

The catalog record for the lowest-weight polyoxypropylenediamine shows the arithmetic: a molecular weight of 230 g/mol and an AHEW of about 57, the theoretical value for four amine hydrogens sharing a 230 g/mol chain; real lots run somewhat higher, so the acceptance test is an agreed range. When a certificate of analysis (CoA) drifts from that arithmetic, one of two things has changed. Either the average chain length moved, or the amination stopped short and left some chain ends as hydroxyl or secondary amine.

The second cause is the one a grade name hides. A hydroxyl end does not cure epoxy at ambient temperature, so it adds mass without adding amine hydrogens and raises the real equivalent weight. Run the line at the nameplate ratio and the network ends up short of amine: lower Tg, a softer surface and slower development of chemical resistance. That is why the acceptance test on every lot should be the total amine value and its primary-amine share, not the label.

Where each polyoxypropylene amine sits on the ladder

Read the table as a ladder, not a menu. Pick the lowest molecular weight that meets your flexibility and elongation target, because every step up is paid for in Tg and chemical resistance. When you need both, blend: a long diamine into a short-diamine cure for toughness, or a short triamine into a diamine cure to claw back Tg and solvent resistance. Every blend needs its own AHEW, calculated from the weight fractions and equivalent weights of its parts.

Epoxy: matching the amine to the job

Rigid, clear, chemically resistant work (flooring, castings, tooling) starts with the lowest-weight polyoxypropylenediamine. It is a pale liquid that gives low-color cures (put a numeric color limit on the CoA), and its low viscosity helps self-leveling and high filler loading. Because its AHEW is low, a small weight of hardener cures a lot of resin. The limitation is cure speed: a hindered amine cures slowly in a cold building, and full chemical resistance takes a post-cure or a faster co-curative.

A cold, humid slab can leave a greasy film that resists recoat. Plan the cure schedule around the coldest night the slab will see, not the afternoon it was poured.

For composites, infusion and laminating, the same diamine and the short triamine give long working time and good fiber wet-out. The triamine’s branch point lifts Tg and chemical resistance over a straight diamine cure, at some cost in toughness. Where a part must survive impact or thermal cycling, move up the ladder or blend in a long diamine as a flexibilizer, and keep the rigid hardener in the blend if the surface still needs to be hard.

Polyurea: the long amines do the structural work

In spray polyurea and reaction injection molding, the resin side is built from long polyoxypropylene diamines and triamines plus a short chain extender, and it meets an isocyanate side at the mix head. Primary amines react with isocyanate in seconds without a catalyst, so formulators tune gel time through the amine blend and the chain extender rather than with a catalyst. The long diamines keep the network extensible, while the triamines add branch points that raise tear strength and recovery. Tune the diamine-to-triamine ratio to trade elongation against modulus.

Two impurities matter more here than in epoxy. Water in the resin side reacts with isocyanate and releases carbon dioxide, which shows up as pinholes and foam in the film. Leftover hydroxyl ends from incomplete amination react with isocyanate far more slowly than amine ends, so they leave a slower, partly urethane network inside what was specified as a polyurea.

What to put on the purchase specification

A grade code tells a producer what you buy today; it does not tell your laboratory what to test. Write the line item so that any capable producer can quote it and every lot can be checked against it:

1. Chemistry: polyoxypropylene diamine or triamine, primary-amine terminated.
2. Nominal molecular weight as the description, not the acceptance test.
3. AHEW with a tolerance, or the total amine value behind it, on every lot.
4. Total amine value against total acetylatables (or hydroxyl value), your check on complete amination; the primary-amine share alone cannot see leftover hydroxyl ends.
5. Water content, capped tighter for a polyurea resin side than for an epoxy hardener.
6. Color, for clear floors and castings.
7. Viscosity and density, each at a stated test temperature.

The catalog lists 10 cSt and 0.948 g/mL for the lowest-weight polyoxypropylenediamine at 230 g/mol. Density is the value that converts a metering machine’s volume ratio into the weight ratio your stoichiometry uses.

Ask for a CoA on every lot that names the test method behind each value, and for written notice before any change to the process or the starter. Check the current Safety Data Sheet (SDS) before handling, sampling or blending any amine.

Qualifying a second source

Same chemistry and the same molecular-weight class do not guarantee identical behavior on your line, so treat a new source as a qualification, not a swap. Recalculate the mix ratio from the new lot’s AHEW. Then run pot life, peak exotherm and cure schedule side by side with your incumbent, and test the two or three end-use properties that gate your product: Tg, hardness, elongation or chemical resistance, depending on the job. Release the source when the results match within your own tolerances, not when the paperwork does.

Methodology: the molecular weight (230 g/mol), AHEW (about 57), viscosity (10 cSt) and density (0.948 g/mL) of the lowest-weight polyoxypropylenediamine come from the product catalog record. Everything else is general amine, epoxy and polyurea chemistry, stated without figures because no test data on a specific lot were used.