A Beaker, a Stopwatch and One Afternoon: Qualifying a New SAP Supplier

The new supplier’s samples arrived on a Tuesday, the price was 18% below the incumbent, and the plant manager wanted to run production on Friday. There was no laboratory, no QC chemist, and no time. What there was: a beaker, a stopwatch, a 0.01 g scale, a sieve, and one afternoon.

That is genuinely enough. You cannot characterise a polymer fully with kitchen equipment, but you can separate a usable lot from a bad one, and you can do it before 40 tonnes are in the silo.

Test one: how fast it wets out

Weigh 1.00 g of SAP into a 250 ml beaker. Add 200 ml of 0.9% saline at 23 °C, stir once for five seconds with a glass rod, and start the clock. Look at the beaker every 15 seconds and note when there are no visible dry particles left and the mass has stopped expanding quickly.

Typical good material reaches that point in 60–150 seconds. Under 40 seconds often means a surface-treated grade that will hydrate too fast and clump in the hopper. Over four minutes means fines or a heavily post-treated surface, and your fill line will not keep up.

This number matters because it sets your minimum mixing time. If the polymer needs four minutes and your batch cycle gives it ninety seconds, you are shipping packs with dry cores whether or not anyone notices on the day.

Test two: how much it holds, under load

After 30 minutes, pour the beaker through a 0.5 mm sieve for two minutes and weigh the drained gel. Subtract 1 g and divide by 1 g. That is free swell in saline.

Then do the loaded version, which is the one that predicts transport performance. Put 150 g of the drained gel into a 100 ml syringe barrel with the plunger removed, place a flat plate on top, add a 2 kg weight (about 0.7 psi on a 60 mm diameter), and leave it for 10 minutes. Collect and weigh what comes out of the bottom.

Good gel packs release less than 6% of their mass under that load. Material above 12% will sag in a pallet, no matter what the free swell says. This is the test that catches the “high absorbency, low strength” grade that looks impressive on a datasheet.

• Free swell in saline: 200–280 g/g for most gel pack grades
• Release under 2 kg load: under 6% good, over 12% reject
• Wet-out time: 60–150 seconds
• pH of a 0.5% gel: 6.0–7.5
• Moisture: under 8%, measured by weight loss at 105 °C for 3 hours

Test three: what happens after five freezes

Fill three 200 g samples of the gel into small pouches or jars, freeze at −18 °C for 16 hours, thaw at 30 °C for 8 hours, and repeat five times. After the fifth cycle, tip each sample into a sieve and weigh the free water that runs off.

Fresh gel releases almost nothing. A gel that has broken down releases 8–20% of its water, and that water is exactly what ends up in your carton. If the sample sheds more than 10% by cycle five, it will not survive a reusable pack programme or a long frozen lane. Do not skip this because the first freeze looks fine; the damage accumulates.

The fourth test, which costs nothing

Smell the powder. A faint acrylic odour is normal. A sharp, solvent-like or ammonia note is not, and it usually means residual monomer or a process aid that was not stripped. In a pack that sits next to food for two days at 30 °C, that odour migrates into the film and then into the customer complaint inbox.

Then look at the powder under a bright light and rub a pinch between your fingers. Gritty, with visible hard specks, means agglomerated material that will never hydrate fully. Smooth and slightly cool means good.

Put the limits in the purchase order

The point of running these tests is not to become a laboratory. It is to convert them into acceptance criteria so that every subsequent lot is checked against a number instead of an impression. Write four lines into the PO: free swell in 0.9% saline at 30 minutes, release under 2 kg load, wet-out time, and free water after five freeze-thaw cycles. Add “certificate of analysis required per lot” and “retained sample held for 12 months”.

A supplier who pushes back on those four numbers is telling you something useful. One who sends the data without being asked is telling you something better.

Run the tests on the first three lots, then every fifth lot once the process is stable. Keep the retained samples in a sealed bag with the date, because when a complaint arrives in eight months the only way to answer it is to have the powder that made the pack. Bulk SAP powder supply is quoted with these four values per lot, which is what makes an afternoon of bench work repeatable.