A Drum of Thickener, Not Absorber: Linear and Crosslinked Polyacrylate

The purchase order said “sodium polyacrylate, 25 kg”. The delivery was a drum of clear, viscous liquid, and the warehouse accepted it because the label matched the order. Three weeks later the plant was still waiting for a powder that absorbs 300 times its weight, and the supplier was pointing at the wording on the PO, which described exactly what had been shipped.

Both products are polyacrylate. One is a soluble chain that thickens water; the other is a network that absorbs it. They share a name, a chemistry family, and almost nothing else, and the difference is decided at the reactor by whether a crosslinker was added.

The structural difference, and why it matters commercially

A linear polyacrylate is a long chain with carboxylate groups along its length. In water it uncoils and the chains entangle, and the solution becomes viscous. It is used as a dispersant in detergents, a thickener in coatings, a scale inhibitor in water treatment, and a binder in some ceramics. It stays in solution and moves with the water.

A crosslinked polyacrylate has those same chains tied together at intervals. The result is one enormous molecule that cannot dissolve, only swell. It stays where you put it, which is exactly what you want in a diaper core, a cable tape, or a waste drum, and exactly what you do not want in a formulation that needs to be sprayed.

The price gap is large, usually a factor of two to four per kilogram in favour of the linear material, because the crosslinked product needs more drying, more careful neutralisation, and a screening step. That gap is one reason the wrong material gets quoted: on paper, the cheaper one looks like a bargain.

Three checks that settle it in twenty minutes

Stir 1 g into 500 ml of water for ten minutes. A linear product gives a clear or slightly hazy viscous liquid with no visible particles. A crosslinked product gives visible swollen particles suspended in water, and the liquid between them stays thin.

Pour the result through a 100 mesh sieve. The linear product passes through almost completely, with maybe a little gel on the mesh from incomplete dissolution. The crosslinked product leaves essentially all of its mass on the sieve.

Dry 20 ml of the liquid at 105 C overnight. The linear product leaves a continuous, often slightly tacky film that redissolves when you add a drop of water. The crosslinked product leaves a brittle, crazed film that swells but never goes back into solution. That film test is the one to photograph and keep on file, because it is hard to argue with.

Viscosity tells you which one you have been sold

Measure the viscosity of a 1 percent and a 2 percent solution. A linear thickener roughly doubles or triples the viscosity when you double the concentration; the relationship is steep and predictable. A crosslinked dispersion barely changes in viscosity, because what you are measuring is the water between the particles, not the particles themselves.

This catches a specific substitution that happens in the trade: a supplier blends a small amount of linear thickener with a cheap filler and sells it as an absorbent. The viscosity rises, the product looks like it is “working”, and the absorption is a fraction of what it should be. The sieve test catches it; the viscosity test explains it.

Wording that prevents the mistake

Write the specification around behaviour, not chemistry. “Crosslinked sodium polyacrylate, insoluble in water, free swell in 0.9 percent saline not less than 250 g/g, 30/60 mesh” is a specification a linear product cannot satisfy. “Sodium polyacrylate” alone is satisfied by both.

Add the physical form, the mesh, the moisture, and the residual monomer if it matters. Add the test methods, because a number without a method is an invitation to argue. And add a clause that any change in the manufacturing site or the crosslinker requires notification, because that is where substitutions usually enter.

If you already have a drum of the wrong material, the fastest resolution is the film test plus the sieve result, photographed, sent with the PO and the certificate. Most suppliers settle without argument once the evidence is in that form, because the alternative is a technical discussion they will lose.

Where both have a legitimate place

Some applications genuinely use both, in the same plant, for different jobs. A water treatment site might use linear polyacrylate as a dispersant in one process and crosslinked polymer to gel a sludge in another. Keeping them in identically labelled sacks is how a batch gets ruined at two in the morning.

Label by function, not by chemical name: “ABSORBENT — crosslinked” and “THICKENER — soluble” in different coloured print. Store them apart. Put the function on the batch sheet next to the item code.

Our note on checking SAP identity on arrival has a one-page version of the three tests, sized for a receiving bay with a balance and a sieve but no laboratory. It takes less time than signing for the drum.