Squeezed Flat: What a 12 kg Stack Does to a Wet Pad

Ask a supplier for absorption capacity and you will get a number from a beaker. A square of pad is dropped into deionised water at room temperature, left to swell freely, lifted, drained and weighed. The result might read 400 grams per gram. Put that same pad in a tray under a twelve-kilo stack in a chiller, and the usable figure can drop by a third before the first drop of product drip arrives.

Free swell is not the condition a pad works in. It works compressed, cold, and often for hours. The gap between the two numbers is where most pad specifications quietly fail.

Why compression costs capacity

A swollen SAP gel holds fluid in the spaces between polymer chains and in the void volume of the fibre matrix. Press down and the void volume collapses first, pushing free fluid back out of the pad. The gel itself also deforms, and at higher loads it can be forced into the pores, blocking them. The result is not a linear loss. A light load removes a little free fluid; a heavy, sustained load can remove thirty to fifty percent of what the pad held without pressure.

Temperature compounds it. A gel that is firm at 20 degrees becomes softer at 4 degrees, and softer gel deforms more under the same weight. This is why a stack that behaves in a warehouse trial can leak in a chiller.

A test you can run on a bench

You need a scale, a timer, a flat rigid plate and a known weight. Soak the pad to a set percentage of its rated capacity, say sixty percent, so you are measuring retention rather than rate. Weigh it. Place it on a flat surface, put the plate on top, add five kilos, and leave it thirty minutes at 4 degrees. Lift the weight and weigh the pad again. The difference is fluid the pad surrendered under load, and it will end up on the tray floor.

Run the same test at two kilos and at ten kilos and you have a curve. A pad that retains ninety percent at two kilos and sixty-five percent at ten tells you exactly what stack height it can survive. Most tray stacks in a chill room put between three and eight kilos on the bottom tray, so the ten-kilo figure is the one that matters least and the eight-kilo figure is the one to ask for.

What to put in the supplier questionnaire

Ask for retention under load, not just capacity, and ask for the load in kilos per pad area rather than an abstract pressure. Ask how long the load was applied, because a thirty-minute test flatters a pad that will sit for twelve hours. Ask whether the sample was tested at 4 degrees. A supplier who can answer all three is testing the product as it is used; one who cannot is quoting a beaker.

There is a second, cheaper defence. A pad that is supported by a ribbed tray floor keeps more of its void volume than one lying on a flat base, because the ribs take part of the load off the pad. If you cannot change pads, change the tray. In one poultry plant, swapping to a ribbed tray recovered enough capacity that the pad change interval went from six hours to nine, with no change to the pad itself.

Stack height is the number to control. Six trays of four kilos each is twenty-four kilos on the bottom pad, which is a different world from two trays. If the rack cannot be shortened, put the heaviest trays at the top and the lightest at the bottom, and note it in the thaw plan so the night shift does not rebuild the stack by habit.

Run the compression test on your current pad before the next price negotiation. If it loses more than a third of its capacity at your real stack weight, you are paying for absorbency that never reaches the tray. The SAP grades with higher gel strength are designed for exactly this condition.