Low viscosity polyether diamine with a PEG core? Reactive content picks the grade

Picture a hydrogel line running on RS-ED600, the Mn ≈ 600 PEG-core polyetheramine. The development team wants more swell, so someone pencils in RS-ED900 and leaves the kilograms on the batch sheet unchanged. RS-ED900 carries 1.8 to 2.25 mmol/g of amine against 3.0 to 3.43 mmol/g for RS-ED600. The equal-mass swap puts fewer amine ends into the batch than the isocyanate side was balanced for. The isocyanate left without an amine partner reacts with water instead, and the cast gel comes off the line with bubbles and a swell nobody qualified.

Buyers searching for a low viscosity PEG-core diamine often land on these two grades. Reactive content and the polyethylene glycol segment each grade brings decide between them, not viscosity.

Is there a low viscosity polyether diamine with a PEG core?

Two of the RS polyetheramine grades have a PEG core: propylene oxide-capped polyethylene glycol diamine at Mn ≈ 600 (RS-ED600) and at Mn ≈ 900 (RS-ED900). Both are difunctional primary amines, specified as a colorless to pale yellow transparent liquid. Viscosity is not specified for either grade, so no figure makes one of them the low viscosity option. Reactive content and hydrophilic segment decide.

Both are aliphatic polyether diamines made from a propylene oxide-capped polyethylene glycol, so the chain is polyethylene glycol with propylene oxide units at the ends. That chain is why these grades exist. The propylene oxide cap sits at the chain ends and puts each nitrogen on a secondary carbon, so the amine stays primary but carries a methyl group next door. If the primary-versus-secondary distinction needs a refresher, the overview of amine types, properties and uses covers it.

Both grades carry CAS 65605-36-9. The EPA Substance Registry Services record names it “Oxirane, methyl-, polymer with oxirane, bis(2-aminopropyl) ether.” As retrieved October 2026, SRS lists the substance on the TSCA Inventory, the list EPA must compile, keep current and publish under TSCA section 8(b), 15 U.S.C. 2607(b).

One shared CAS number covers both grades, so receiving staff cannot use it to catch a grade swap. The wider family, including the propylene oxide-backbone diamines and the triamines, is mapped on the polyetheramine supplier hub. This piece covers only the two hydrophilic grades.

What the polyethylene glycol chain does in a polyamide

The polyethylene glycol chain is the reason to buy either grade: it makes the polymer hydrophilic as well as flexible. In polyamides, it can raise antistatic performance and moisture vapor transmission.

That property carries the buying decision. A propylene oxide-backbone diamine adds flexibility to a polyamide with little affinity for water. The ethylene oxide units in these two grades add both. In a breathable film or an antistatic fiber, the hydrophilic segment is the point of the purchase. In a polyamide that must hold its dimensions in humid service, the same segment is a liability.

Use the table as a screen and confirm on the lot. Only RS-ED600 is used for epoxy resin adducts. That does not rule RS-ED900 out, but epoxy work with the heavier grade needs your own qualification data behind it before the first production lot.

Choosing between the two on reactive content and hydrophilic segment

Both grades are made with a primary amine at each end of the chain, and conversion is never quite complete, which is one reason amine value is specified as a range. RS-ED900 spreads its amine ends over a longer chain, so it carries less amine per gram and more polyethylene glycol between its reactive ends. Each chain extension or crosslink it forms brings more hydrophilic segment into the polymer.

So the choice is a trade. RS-ED900 gives wider spacing between reactive ends and a looser network, and its longer polyethylene glycol chain is what makes it the more hydrophilic of the two. RS-ED600 gives tighter spacing and a stiffer network, and it covers the broader set of uses: it is the only one of the two used for epoxy resin adducts.

Decision rule: start from the hydrophilic content the polymer needs. If RS-ED600 delivers enough polyethylene glycol segment, buy it: its higher amine content gives the tighter network, and it also covers epoxy adducts if your plant makes them. Move to RS-ED900 only when you need more water uptake or moisture vapor transmission per reactive end than the lighter grade can carry. The RS-ED600 grade page and the RS-ED900 grade page show the same sales specification lines for a side-by-side check.

How should you set the charge from the lot COA?

Once the grade is chosen, the lot sets the charge. In the isocyanate hydrogels and polyamides these grades go into, each primary amine end reacts once: with one isocyanate group to form a urea, or with one acid group to form an amide. The measured amine value in mmol/g is therefore the equivalent content per gram for those reactions.

An epoxy adduct is the exception, because each primary amine end can open two epoxide groups. There the basis is the amine hydrogen equivalent weight (AHEW). For a diamine whose amine groups are all primary, AHEW in g/eq equals 1,000 divided by twice the amine value in meq/g, and for an amine titration meq/g is the same number as mmol/g.

The lot Certificate of Analysis (COA) is the document that should set the charge. A specification gives a range; the COA gives what is in the drum you are about to open. For a reactive diamine in a hydrogel, the difference between the low and high ends of the amine value range moves the equivalent balance of the batch.

Blending the two grades to land between them on hydrophilic content is a legitimate route, and it is where dosing errors creep in. Work in equivalents: multiply each lot’s mass by its measured amine value and add the two. A linear average of two equivalent weights overstates the blend’s equivalent weight, so dosing on it over-charges amine hydrogens. The trade-off is real: a blend buys an intermediate hydrophilicity at the price of two lots, two COAs and two charge calculations to track.

What to do on every lot of either grade:

1. Require the measured amine value on the COA, in mmol/g, as a measured figure for that lot rather than a restated specification range.
2. Hold the measured value against the grade’s specified range: 3.0 to 3.43 mmol/g for RS-ED600, 1.8 to 2.25 mmol/g for RS-ED900. A lot outside its range does not enter a batch.
3. Set the charge for the lot from that measured value in your batch record before the first charge, and note which COA it came from.
4. For a blend, recalculate the equivalents each time either lot changes.

The water limit to write into the purchase order

Both grades set water content at 0.35 max %. That is the number the purchase order should carry, word for word, for either grade.

Water matters most in the isocyanate hydrogels both grades go into. Water reacts with isocyanate, releasing carbon dioxide and forming urea. Any water in the amine takes isocyanate away from the network you designed and can leave bubbles in a cast gel. A polyethylene glycol chain also takes up moisture from air. A drum that met 0.35 max % when filled can drift above it after partial use with the bung loose.

Put the limit on the COA and confirm it with a Karl Fischer check on receipt, in a working medium buffered for amines. Set your own retest interval for an opened container. Decide how long an opened 200 kg drum may sit before it is retested, based on your plant humidity and draw-down rate. Plants feeding hydrogel lines from 1,000 kg IBC totes face the same question at a larger scale.

Where the same urethane system also runs polyether or polyester polyols, keep the water discipline consistent across both streams. A dry amine paired with a wet polyol buys nothing. The polyurethane polyols page covers the polyol side of that build.

What to write into your own specification

Viscosity is the requirement buyers search for, and neither grade specifies it. Name the parameter yourself: viscosity at a test temperature you choose, recorded as a measured value on early lots. Choose that temperature to match your coldest transfer point. The guide to reading a chemical technical data sheet explains how to separate typical values from sales specification lines when you write that requirement.

Color is the one cosmetic line both grades hold tightly, at 30 max Pt-Co. In a clear hydrogel or a light film, hold every lot to it and reject on it. In a pigmented polyamide it seldom decides anything.

The decision itself is narrow. If the polymer needs only the hydrophilic segment RS-ED600 can carry, buy the lighter grade and keep its wider coverage of uses. If it needs more polyethylene glycol between reactive ends, RS-ED900 is the grade to buy. The question to answer before the RFQ goes out is how much hydrophilic segment per reactive end your polymer actually needs.

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 CAS identity and TSCA Inventory listing are from EPA Substance Registry Services (retrieved October 2026) and TSCA section 8(b), 15 U.S.C. 2607(b). The reaction stoichiometry is stated as a method and used only to set or check a charge.