Grams of SAP per Litre: A Dosage Table for Firm, Transport-Stable Gel

A gel pack that feels firm on the bench can still slump into a corner after six hours in a box. The difference is rarely the SAP grade and almost always the dosage — how many grams of polymer went into each litre of water. Too little and the fill never builds a network; too much and the gel becomes rubbery, traps air during filling, and freezes into a block that will not sit flat against a carton wall.

Start from grams per litre, then convert

Ice pack fills are usually quoted as a percentage by weight, which is convenient for a mixing tank but awkward on a production floor where the operator measures water in litres. The conversion is direct because a litre of water weighs 1 kg: 0.8 percent equals 8 g per litre, 1.2 percent equals 12 g per litre.

Working bands for a standard acrylic SAP in deionised or softened water at 20 °C:

• 0.5 percent (5 g/L): loose, pourable gel. Suitable for packs that will be frozen solid and never handled warm; will not hold shape above 15 °C.
• 0.8 percent (8 g/L): soft-set gel. Holds a pack shape, deforms under thumb pressure, sheds a little water after 30 days. Common for single-use chilled packs.
• 1.0 percent (10 g/L): firm gel. Holds shape at 25 °C, minimal free water, good for reusable packs and vertical placement.
• 1.3 to 1.6 percent (13 to 16 g/L): very firm to rubbery. Used where packs are stacked or where a pack must resist compression in a palletised load; harder to fill and slower to hydrate.

These figures assume water with low hardness and near-neutral pH. Tap water at 300 mg/L hardness or water carrying residual sanitiser will cut effective absorption by 15 to 30 percent, and the dosage has to rise to compensate. Always dose against the water you actually use, not the water in the datasheet.

Reading firmness back to dosage

Firmness is easier to measure than absorption capacity, and it correlates better with how a pack behaves in transit. Three bench tests are enough for most plants.

Thumb test. Press a 10 mm probe 5 mm into the set gel after 4 hours. A 1.0 percent fill resists and rebounds; a 0.6 percent fill dents and stays dented. Crude, but fast enough for line checks.

Incline test. Fill a cut-down pouch to 300 g, set it, and stand it at 45 degrees for 24 hours at 25 °C. Measure how far the gel has flowed from its original edge. Under 5 mm is good; over 15 mm means the pack will pool at one end of the box.

Freeze-thaw drip. Freeze a sample at −18 °C for 24 hours, thaw at 25 °C for 24 hours, and weigh the free liquid that separates. A well-dosed fill releases under 2 percent of its water; a starved fill releases 8 to 15 percent, which is the water that ends up in the bottom of a carton.

Why salt and glycol change the number. Any pack designed to work below −5 °C needs a freezing point depressant, and salt or glycol both reduce SAP absorption sharply. A fill that needs 10 g/L in pure water may need 14 to 18 g/L in a 10 percent propylene glycol mix, because the glycol competes for water and the ions collapse the polymer’s osmotic drive. Dose by trial at the actual glycol concentration, and check retention under load rather than free swell — a glycol-filled gel can look adequately thick and still release water under a 20 g/cm² load.

Salt is worse than glycol per unit of freezing point depression, and chlorides also stress the film. Where a low freeze point is essential, choose a grade with higher crosslink density and accept a lower absorption ratio; the pack will hold shape better at −18 °C even though the datasheet number looks modest.

Mixing sequence matters as much as the dose

Adding powder to water too fast produces lumps with dry cores that never hydrate, and the batch then reads soft even at the correct dose. Disperse the SAP into water under moderate shear at 300 to 600 rpm, add it over 60 to 90 seconds rather than dumping, and hold mixing for 15 to 20 minutes before filling. Let the batch rest 30 minutes to de-aerate; air trapped in a firm gel shows up as white flecks in the frozen pack and reduces contact with the carton.

Water temperature above 40 °C speeds hydration but degrades some thickener systems; below 10 °C, hydration can take two hours and the line will fill partially set gel. Aim for 20 to 25 °C and adjust dwell time rather than temperature.

Grade data covering absorption at different salinity levels is available on the SAP technical data page.