Two COAs, Same Grade, Two Monomer Numbers: Ask This One Question

Two certificates arrived in the same week for the same nominal grade, from two plants of the same supplier. One said 280 ppm residual acrylic acid. The other said 950 ppm. The buyer sent both to the sales contact with a one line email, and the reply was accurate but unhelpful: the two labs use different methods, and the numbers are not directly comparable.

Residual monomer is the single most misunderstood line on a polyacrylate COA. It is quoted with confidence, compared across suppliers, written into specifications, and frequently measured on a basis nobody states. Getting it right is mostly a matter of asking three questions before you argue about the figure.

What the number is measuring

Acrylic acid that did not polymerise stays in the product as free acid or as its salt. It is the main reason the polymer has an odour, it is the main toxicological concern for skin contact, and it is the number that regulators and hygiene customers look at first. It is not the same as total volatiles, and it is not the same as the pH of a slurry, though all three move together.

Typical industrial grades land between 200 and 1,000 ppm. Hygiene grades are usually specified below 300 ppm, and some food-contact routes go lower. The gap between 280 and 950 is real in performance terms, but only if both numbers were produced the same way.

The three questions that make numbers comparable

First, the basis. Is the figure reported on the polymer as received, or on a dry basis after subtracting moisture? A product at 6 percent moisture reports 6 percent lower on an as-received basis for the same absolute amount of monomer. That alone explains part of a 280 versus 950 gap, though not all of it.

Second, the method. Titration of an aqueous extract and high-performance liquid chromatography measure different things in different ways. Titration can pick up other acidic species and generally reads higher; chromatography separates and quantifies the specific molecule and generally reads lower, with a detection limit that matters at the low end. Neither is wrong. But a specification written as “below 300 ppm” without a method is a specification that two labs can both satisfy and both fail.

Third, the sample. A composite of twenty sacks, a grab sample from the top of a bulk bag, and a sample taken from the dryer outlet will not agree. Monomer distribution in a dried polymer is not perfectly uniform, and the surface of a granule carries more than the core because drying drives it outward.

What the number actually gates

For a hygiene or food-contact customer, the residual monomer figure gates the market. It sits alongside migration testing and bioburden, and a supplier who cannot hit the limit cannot sell into that channel at any price. That is why hygiene grades cost more: the extra washing and drying steps exist to remove monomer, and they cost energy and yield.

For an industrial customer, the figure gates almost nothing directly, but it correlates with two things you will notice. Odour in an enclosed space, which matters in litter, in waste solidification inside a building, and in any product that sits near people. And worker exposure at the bag dump, where the dust is the fraction with the highest surface monomer concentration.

The counter-intuitive finding is that chasing a lower number can hurt you elsewhere. More washing means more drying, and more drying means a harder, more brittle granule with more fines after conveying. A plant that switches to an ultra-low-monomer grade for an application that did not need it often finds the dust at the bag dump has gone up, not down.

Reading a COA without being fooled

Look for the method and the basis in the same block as the figure. If they are missing, the line is decoration. Look for a detection limit; a result reported as “less than 100 ppm” is not the same as 95 ppm and should not be averaged into a trend. And look at whether the figure is per lot or a typical value, because a certificate repeating a type-test result for three years is a datasheet with a date on it.

Ask for the last six lots in a table, not a single sheet. A supplier with a stable process shows 240 to 290 across six lots. A supplier with a washing problem shows 240, 260, 880, 310, 950, 280, and the average hides exactly what you need to see.

What to do with a high result

If a lot arrives above your limit, do not reject on the certificate alone. Re-test a retained sample with the method named in your specification, on the stated basis, and record both results. If the re-test confirms the figure, the decision is commercial: use it in an application that tolerates the number, return it, or blend it down with a low-monomer lot if your process allows and your customer agrees.

Then fix the specification so the next argument does not happen. Name the method, the basis, the sample preparation, and the reporting limit. A specification that says “residual acrylic acid below 300 ppm, dry basis, by the method agreed in the technical annex” is worth the extra page. Our note on reading a SAP certificate of analysis covers the other lines the same way.

One last thing: keep the retained sample sealed and dry. Monomer does not increase in storage, but moisture does, and a re-test on a damp sample eighteen months later will not reproduce the original figure.