KOSA Medical · Materials & Structure

Disposable Underpad Material & Absorbent Core Structure Explained

How a disposable underpad is built layer by layer — nonwoven topsheet, ADL, fluff pulp and SAP core, and PE backing — and which layer actually decides absorbency, rewet and skin comfort.

Published August 12, 2026 · Updated September 8, 2026 · KOSA Medical

Key Takeaways

A disposable underpad is a five-layer system; changing one layer changes the whole performance profile.
Fluff pulp moves and stores fluid fast; SAP locks it in under pressure.
The ADL (acquisition layer) is what makes strike-through feel fast on a thin pad.
Backing film decides barrier strength, slip resistance and breathability — not absorbency.
Ask suppliers for core composition by weight, not just a total grammage figure.

Why the layer structure matters to buyers

Two underpads can carry the same grammage and the same nominal size and still behave completely differently on a ward. The reason is almost always the layer structure rather than the total weight. When you buy on weight alone you are buying a number that does not predict rewet, migration or how often linen has to be changed.

Understanding the stack lets you write a specification that suppliers can actually be measured against — which is the difference between a tender that is comparable and one that is not.

The five layers of a standard disposable underpad

Layer	Typical material	Job	What to specify
Topsheet	Hydrophilic spunbond / hot-air nonwoven	Keep surface dry, stay soft on skin	Basis weight, softness, wet-back value
Acquisition / distribution layer (ADL)	High-loft through-air bonded nonwoven	Pull fluid off the surface and spread it	Loft, strike-through time
Absorbent core	Fluff pulp, optionally blended with SAP	Store the fluid volume	Pulp grammage, SAP ratio and SAP type
Tissue wrap	Lightweight crepe tissue	Hold the core together, prevent shifting	Presence, basis weight
Backing film	PE film, breathable film or non-slip textured film	Stop strike-through to the mattress	Film thickness, breathability, slip resistance

Fluff pulp vs SAP — the part most tenders get wrong

Fluff pulp is a bulky cellulose fibre. Its job is speed: it wicks fluid away from the surface quickly and holds it in the capillary structure. It performs well under low output and low pressure, but under body weight much of what it absorbed can be pressed back out.

Superabsorbent polymer (SAP) is a cross-linked polyacrylate that turns liquid into a gel. It absorbs far more slowly than pulp but holds it under compression. That is why SAP-blended cores show better retention under load and lower rewet even when total capacity figures look similar.

The practical rule: for low-output day use, a fluff-dominant core is usually sufficient and cheaper. For night use, high-output patients, bariatric care or long dwell times, a SAP-blended core reduces linen changes and total cost per patient day even at a higher unit price.

Reading a core specification

Core type	Typical build	Best for	Trade-off
Fluff only	100% fluff pulp	Short procedures, light output, chair protection	Lower retention under load
Fluff + SAP blend	Pulp with 10–40% SAP	General ward, long-term care, night use	Higher unit cost
SAP-rich thin core	High SAP ratio, minimal pulp	Slim discreet products, transport, ambulatory use	Slower acquisition, needs a good ADL
Airlaid composite	Airlaid paper with SAP	Premium underpads, thin high-capacity formats	Highest material cost

Always ask for the pulp-to-SAP ratio and the total core grammage separately. "Total pad weight" hides both the film and the topsheet, so it cannot be used to compare two offers.

Backing, edges and the details that cause complaints

PE film backing gives the strongest fluid barrier. Standard choice for beds and procedure surfaces.
Breathable backsheets reduce heat build-up on long dwell times, but must be matched to the expected output or the barrier will be the weak point.
Non-slip / textured backing and adhesive corner tabs address the single most common complaint in long-term care: the pad moves.
Sealed versus cut edges — sealed perimeter edges reduce core leakage at the sides when a pad is saturated.
Embossing pattern affects both fluid spreading speed and perceived quality; a diamond embossing is typical for mid-range hospital pads.

Frequently Asked Questions

What is the difference between fluff pulp and SAP in an underpad core?

Fluff pulp is cellulose fibre that absorbs quickly and stores fluid in capillaries, but releases some of it under pressure. SAP is a superabsorbent polymer that absorbs more slowly but locks fluid into a gel that stays trapped under load. Most hospital-grade underpads blend both.

How much SAP should a medical underpad contain?

For general ward and long-term care use a core with roughly 10–40% SAP by weight is typical. Light-output chair protectors can use fluff only, while night or high-output formats benefit from a higher SAP ratio because retention under load matters more than acquisition speed.

Does a heavier underpad always absorb more?

No. Pad weight includes the backing film, topsheet and tissue wrap, none of which absorb. Two pads with identical grammage can differ substantially in capacity depending on the core composition, so specify core grammage and SAP ratio rather than total pad weight.

What is an ADL and is it worth paying for?

An acquisition and distribution layer is a high-loft nonwoven between the topsheet and the core. It pulls fluid off the surface quickly and spreads it across the core, which is what makes a thin pad feel dry. On thin or SAP-rich products it is essential rather than optional.

Should I choose a breathable backing for long-term care?

Breathable backsheets improve comfort during long dwell times by reducing heat and moisture build-up. Choose them where output is moderate and dwell is long, but verify the barrier rating for your worst-case output, because a breathable film is a weaker absolute barrier than solid PE.