Every absorbent hygiene product — period pants, sanitary pads, baby diapers, adult briefs, postpartum underwear — is built around the same three-part problem: get the fluid in fast, hold it under pressure, and keep the surface the wearer touches dry. The layer stack that solves it is broadly the same across categories, and understanding it is the single most useful piece of technical knowledge a buyer can carry into a supplier conversation.
The four functional layers
| Layer | Job | Failure symptom if weak |
|---|---|---|
| Topsheet | Accept fluid quickly, keep it away from skin | Wet feeling immediately after use, fluid pooling on the surface |
| ADL (acquisition / distribution layer) | Spread the surge across the core so one spot does not saturate | Leaks at the sides while the core is still partly dry |
| Core (SAP plus pulp) | Store fluid and hold it under body pressure | Leaks when the wearer sits or lies on the product |
| Backsheet | Contain fluid and provide the outer surface | Strikethrough, dampness on clothing or bedding |
Most quality failures are a mismatch between layers rather than a shortage of absorbent material. A buyer who adds SAP to fix a leak often makes the product more expensive without improving it, because the real constraint was distribution or acquisition.
Superabsorbent polymer (SAP)
SAP is a cross-linked polymer powder that absorbs many times its own weight in water and, critically, retains most of it under pressure. Two properties are usually tracked:
- Free swell capacity — how much liquid the particle takes up with nothing pressing on it. This number is large, easy to quote and frequently misused in marketing.
- Absorbency under load (AUL) — how much it retains while compressed, which is what actually happens when someone sits on a pad or sleeps on a diaper.
A high free-swell SAP with poor AUL looks impressive on a data sheet and underperforms in use. For any product where the wearer will be seated or lying down, AUL and the retention figure after centrifugation are the numbers worth asking for. Reading Absorbency Test Data explains how these measurements are made and what they do not tell you.
SAP also behaves differently in saline than in pure water, and differs again with urine ionic strength. Testing in deionised water flatters every material; testing in 0.9% saline is closer to real use.
Fluff pulp
Fluff pulp is thermally treated wood pulp that gives the core its bulk and its wicking structure. It moves fluid along the core and provides a scaffold so SAP particles stay distributed instead of settling into a dense band. Its contribution is often underestimated: a core with well-opened, evenly blended fluff distributes fluid far better than a core where pulp and SAP have separated during forming.
Because pulp adds bulk, the ultra-thin constructions that dominate modern period and panty products use much less of it, relying on a carrier layer to hold SAP in place. This is a genuine trade-off rather than a pure improvement: ultra-thin cores are more discreet and more comfortable, but they have less buffer against a sudden surge.
The ADL, and why it decides leak performance
The acquisition and distribution layer sits between topsheet and core. Its job is to take a fast, localised surge and spread it across the core's full area before the core can be overwhelmed at one point. Surfactant treatment of the nonwoven controls how quickly fluid passes through rather than beading on it; the pattern and thickness control how far it spreads.
When a buyer reports "it leaks at the leg but the middle is still dry", the ADL is usually the suspect. Remedies include a thicker or better-treated ADL, a wider core, or a repositioned core — not more SAP.
Elastic and containment
Fluid that reaches the edge of the core is stopped by geometry, not chemistry. Standing leg cuffs, elastic tension, and the core's lateral margin all determine whether the product contains what it absorbs. In period pants this is expressed as leg cuff height and the fit of the gusset; in diapers as cuff height and elastic decitex. These are cheap to specify and expensive to get wrong, because a leak complaint is rarely attributed by the customer to the cuff.
How to specify a core without specifying materials
Instead of dictating SAP grams and pulp grams, specify performance and let the factory engineer to it:
- Absorbency target at first, second and third dose, in millilitres, with the test method named.
- Rewet limit after a stated interval under a stated load.
- Strikethrough time for the first and repeat doses.
- Retention after centrifugation, if the product is worn while seated or lying down.
- Physical integrity after loading: no core splitting, no clumping.
This approach lets a supplier use its own material inventory and keeps you comparing products rather than recipes. It also makes quotations comparable across factories, since everyone is answering the same question. For how to turn those numbers into a purchasing decision, see Sourcing Checklist.
You can see how these layers come together in the disposable menstrual pants category, the adult diaper range and the sanitary pads range.