Quick Facts: Underpad SAP Absorption
- Super absorbent polymer (SAP) absorbs up to 100–400 times its own weight in water, 30–60 times in saline.
- Underpad capacity is measured on the whole product, not the polymer alone — the reference method is ISO 11948-1:1996.
- Fluid is locked inside the polymer as gel, so rewet stays low even under body weight. Match rated capacity in mL to expected output between changes, not to pad size.
Super absorbent polymer (SAP) is the granular material inside an underpad’s core that turns liquid into gel within seconds of contact. This guide covers how SAP works and what care teams and purchasers should compare when selecting underpads. If you buy absorbent products for a care home, understanding SAP is the difference between pads that hold through a change cycle and pads that fail mid-shift.
What Is Super Absorbent Polymer (SAP)?
SAP is a cross-linked sodium polyacrylate — a polymer whose chain network swells instead of dissolving when it meets water. One gram of dry SAP holds 100 to 400 grams of deionized water, but only 30 to 60 grams of saline solution, which behaves much closer to urine. That gap is why the same pad can look different in a lab test and in daily use.
Underpads do not rely on SAP alone. The core blends SAP granules with fluff pulp, a wood-fibre material. The pulp wicks fluid sideways so the pad fills edge to edge, while the SAP stores the bulk of the liquid.
How SAP Works: Osmosis, Locking, and Low Rewet
Absorption happens in three stages. Capillary action pulls fluid through the surface layer and into the core. Osmotic pressure — the ion-concentration difference between the dry polymer and the liquid — drives water into the polymer network. Finally, the cross-linked chains expand and trap the water as gel.
Locking is the property that matters most in care. Once water is inside the polymer network, it does not easily come back out, even under load. A resident lying on a pad exerts steady pressure: a fluff-only core squeezes fluid back to the surface, while an SAP core holds it. Rewet tests apply a fixed load after saturation to measure how much liquid returns to the surface.
The 5-Layer Structure of a Medical Underpad
- Surface layer — a soft nonwoven that lets fluid pass quickly and stays dry against the skin.
- Acquisition layer — pulls liquid downward so one spot does not saturate.
- Distribution layer — channels fluid across the full core so SAP granules fill evenly.
- Absorption core — fluff pulp and SAP granules; this is where liquid is stored as gel.
- Waterproof backing — a polyethylene film that stops moisture reaching the bed.
Every layer has a job. A strong core with a slow surface feels wet because fluid pools; fast intake with a thin core overflows early.
SAP vs Fluff Pulp: Side-by-Side
| Property | SAP (polymer) | Fluff pulp (wood fibre) |
|---|---|---|
| Absorption speed | Steady intake; fills gradually | Fast wicking; soaks quickly |
| Capacity | 30–60 g/g in saline; up to 400 g/g in water | Roughly 5–8 g/g |
| Fluid locking | Turns liquid into gel, held inside the network | Held between fibres; can be squeezed out |
| Rewet under pressure | Low — gel resists compression | Higher — pressure pushes fluid back |
| Cost | Higher material cost | Lower material cost |
What to Check When Buying Underpads for LTC
Start with rated capacity in millilitres, not grams of SAP or pad size. A 60 × 90 cm pad rated at 1,600 mL suits a different care pattern than one rated at 2,400 mL. Ask how the number was produced — whole-product testing under ISO 11948-1:1996 gives a consistent figure to compare across brands.
Next, look at rewet. A pad can absorb plenty and still feel wet if the core releases fluid under pressure. Low rewet matters more than peak capacity for residents who stay in one position for hours. Then check the surface layer: soft, fast-drying nonwoven textures limit moisture contact with skin.
Related reading
FAQ: SAP and Underpad Absorbency
Is SAP safe when it sits against skin?
Medical-grade sodium polyacrylate has been used in diapers for decades. It is non-toxic in normal use, and the gel stays inside the core, separated from skin by the surface and acquisition layers.
How do I calculate the absorbency an underpad needs?
Estimate typical urine output between changes — roughly 200–400 mL per void — add margin for movement and multiple voids, then compare with the pad’s rated capacity in mL.
Can SAP leak out of an underpad?
No. The granules are mixed inside the core and held between the nonwoven layers and the waterproof backing. Even when saturated, the gel stays within the pad structure.
Why do two “SAP” pads absorb so differently?
SAP content by weight, the SAP-to-pulp ratio, and layer design all change real-world performance. Compare rated capacity in mL, measured the same way.
Underpads for Canadian Long-Term Care Facilities
CliniEco supplies medical underpads to long-term care homes, clinics, and distributor partners across Canada. The care range includes the CliniEco Medical Underpad 60×90 cm, 5-layer with SAP, vacuum-compressed 50-pack, the CliniEco Disposable Underpads, heavy absorbency 30×36 in, and a PLA biodegradable heavy-absorbency underpad, 50-pack for facilities working on waste reduction.
For bulk pricing, samples, or a spec sheet with ISO 11948-1:1996 test data, contact the CliniEco sales team. We quote within one business day and ship across Canada.
Related Reading
Explore more guides in this category:
- Adult Diapers vs Underpads: Choosing by Care Scenario
- Wetness Indicator Underpads: Do They Improve LTC Care?
- Underpad vs Adult Diaper: When Each Product Is Appropriate
Related reading: explore our long-term care and home care resources.
start with our underpad buying guide for absorbency, sizing and cost.
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