Two powders, both 300 g/g, both 0.2% moisture, both passing every incoming test you run. One makes a pack that survives a 12-box stack in a 38 °C trailer. The other makes a pack that sags into the corner of the box and weeps by hour twenty. The difference is not on the certificate of analysis, and it is not a manufacturing defect. It is how tightly the polymer network is tied together.
Crosslink density is the number of chemical ties between polymer chains per unit volume. It is the single most important structural variable in a superabsorbent, and it is the one most buyers never see quoted.
Why absorbency and strength trade off
Think of the network as a net with knots. Fewer knots means the mesh can stretch further, so each gram of polymer can swallow more water. It also means the mesh is floppy. Under its own weight, or under the weight of the packs above it, the water migrates out and the gel collapses.
More knots means a shorter, stiffer mesh. Absorbency drops, sometimes to 180 or 200 g/g, but the gel holds its shape, resists compression, and keeps water where it was put. For a gel pack that will be stacked, dropped and driven over a mountain pass, stiffness matters more than the headline number.
This is why a supplier advertising “highest absorbency in the industry” is often selling you a polymer that is wrong for a transport application. The number is real. It is just measured on a product designed for a different job, usually agricultural or hygiene, where the gel is never asked to carry a load.
The three figures that reveal it
You will not usually get crosslink density as a direct value. You can infer it from three numbers that are routinely available if you ask.
| Figure | What it shows | Typical range for gel packs |
|---|---|---|
| Free swell, 0.9% saline | Total capacity, no load | 200–280 g/g |
| Absorbency under load, 0.7 psi | Capacity while squeezed | 22–40 g/g |
| Soluble fraction | Uncrosslinked polymer that leaches out | under 8% |
The ratio between the first two is the tell. A powder at 280 free swell and 22 under load is loosely crosslinked: high capacity, poor retention. A powder at 210 free swell and 38 under load is tight: lower capacity, but it will still be a gel after a week in a trailer. Both can carry the same “300 g/g” marketing line.
Soluble fraction matters for a different reason. Those are chains that were never tied into the network. They stay dissolved in the water, raise viscosity slightly, and then migrate. In a pack they show up as a slippery film on the inside of the film, a sticky residue around the seal, and eventually as water that separates and sits at the bottom of the pouch.
A case where the higher number lost
A packer in the Netherlands switched to a cheaper, higher-absorbency grade to hit a cost target on a 1.2 kg pack. Free swell went from 240 to 330. In lab tests the new pack was firmer, because there was more bound water per gram and less free water. In the field, pallet rejects went from 0.9% to 4.1% within six weeks. The failure mode was always the same: packs in the bottom two layers of a 1.4 m pallet deformed, the gel sheared away from the film, and the pack looked half empty at delivery.
The lab test had been done on a single pack on a bench. It never applied a load. That is the trap: absorbency is measured in a bag hanging in water, and a pallet is nothing like a bag hanging in water.
What to put in the enquiry
Ask for free swell and absorbency under load at 0.7 psi in 0.9% saline, plus soluble fraction, plus the test method used. Ask for a retained sample so you can run a compression test on a filled pack: 20 kg on a 100 × 150 mm pack for 24 hours at 35 °C, then measure how much water has pooled. That single test predicts pallet performance better than any datasheet.
When you compare quotes, compare the pair of numbers, not one. A grade at 210/38 is usually a better buy for transport than one at 330/20, even at a higher price per kilo, because the dose that keeps it firm is lower and the reject rate is not a lottery. The SAP powder specification guide lists both figures for each grade rather than a single headline.
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.
