Two quotes, both specifying the same three things: particle size 0.5 to 1 mm, absorbency 300 g/g, potassium polyacrylate. One is 26 percent cheaper per tonne. On paper they are the same product, and a procurement officer comparing the two lines has no reason to prefer either.
They are not the same product. The number on the label is a single measurement taken under conditions that favour the seller, and there are at least five variables behind it that change what happens in a field. None of them appear in a quotation, and all of them are worth asking about.
Five things hidden behind one number
What liquid was the number measured in?
Absorbency in distilled water is the standard headline figure and the least useful one. The same polymer measured in a 0.9 percent saline solution typically reads 20 to 35 percent of the distilled figure, and in water at 2 dS/m it can fall further.
Ask for the saline figure. A supplier who has it has tested their product the way a customer will use it. A supplier who does not have it is quoting a lab curiosity.
Free swell or absorbency under load?
Free swell measures a granule swelling in a beaker with nothing pressing on it. In soil, a granule is confined by the surrounding matrix and has to swell against pressure. Absorbency under load, usually measured at 0.3 or 0.9 psi, is the figure that correlates with field performance in a planting hole.
A product with 300 g/g free swell and 18 g/g under load is a very different material from one with 260 g/g free swell and 32 g/g under load. The second will hold water where it is placed. The first may perform well in a pot and poorly in a clay backfill.
The mesh distribution behind the range
0.5 to 1 mm describes a range, not a distribution. A well-graded product has most of its mass in the middle of that range. A poorly graded one has 30 percent fines below 0.3 mm and 20 percent coarse above 1.2 mm, and it still passes the specification.
Fines are the problem. They blow away in a spreader, they clog screens and emitters, they hydrate into a paste that seals the soil surface, and they contribute almost nothing to water storage at depth. Ask for a sieve analysis by mass, and be suspicious of any quote that will not provide one.
Residual monomer and the question nobody asks
Residual acrylic acid monomer in a finished polymer is usually below 0.1 percent in a well-controlled process and can be several times higher in a cheap one. For agricultural use the relevant issue is not acute toxicity but consistency and the smell that comes with it, plus the fact that a high residual usually signals a poorly controlled reaction, which also shows up as inconsistent swelling between batches.
Certificates vary in what they report. If the material is going anywhere near food crops, ask for the residual monomer figure explicitly, and ask whether the same figure applies to the batch you are buying rather than the product family.
Crosslink density, which decides field life
Two products can both read 300 g/g on day one and diverge completely by August. A lightly crosslinked polymer swells enormously and then collapses under repeated wet-dry cycles, mechanical stress, and salt. A more heavily crosslinked one swells less and lasts longer.
The field test is simple and takes a month: put 5 g of each product in a jar of your own irrigation water, leave it in the sun, and measure the swollen volume at day 1, day 14 and day 30. A product that loses more than a third of its volume by day 30 is a product that will not last a season on your block.
The bid that won and then failed
A municipal supplier switched to a cheaper grade that met the same written specification: same particle range, same headline absorbency, same polymer type. The material went into 1,800 street trees irrigated with bore water at 1.8 dS/m. By the second summer, the granules recovered from the root zones were hard, glassy and roughly a third of their original swollen volume. Survival was 12 percentage points below the previous contract on the same streets.
Nothing in the specification had been breached, which is why the dispute went nowhere for eight months. The specification had asked for a number that did not describe the thing that failed.
How to compare two bids fairly
Add four lines to the specification: saline absorbency at a stated concentration, absorbency under load at a stated pressure, sieve analysis by mass, and a 30-day stability test in project water. Ask for the batch certificate against those lines, not the product brochure.
Then compare price per kilogram of usable water held at day 30, not price per tonne. The cheap bid usually stops looking cheap at that point, and when it does not, you have found a genuine bargain rather than a specification loophole.
If you have two quotes in front of you and the numbers look identical, the SAP powder product range lists the figures we publish for each grade and the test method behind them. Send both data sheets and we will tell you which three figures to ask each supplier for before you sign.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
