How Sanitary Pads Work: Cores, Topsheets and Choosing Right
Quick Answer
A sanitary pad works in layers: the topsheet quickly channels menstrual fluid into the absorbent core (pulp, SAP or foam), which stores it, while the impermeable backsheet and wings stop leaks. Choose the core by flow and thinness preference, the topsheet by skin sensitivity, and the wing/rear design by your movement and sleep habits.
Key Takeaways
- A pad works in layers: topsheet channels liquid, the absorbent core (pulp, SAP or foam) stores it, and the backsheet keeps it from leaking.
- Three core types exist: traditional pulp, high-SAP thin cores and foam cores like FlexFoam — each balances thinness, speed and cost.
- Topsheet choice (dry-mesh vs cottony) is the main comfort differentiator for sensitive skin.
- Wings, side guards and rear extension determine leak protection more than total pad length.
- Change every 2–3 hours during the day regardless of visible saturation, and choose pH-friendly materials for intimate health.
The Layer-by-Layer Anatomy
Every pad — from budget lines to premium — follows the same layered architecture:
- Topsheet: the layer against skin — either a dry-mesh perforated film (fast-drying) or a cottony nonwoven (soft, for sensitive skin).
- Acquisition/distribution layer: pulls liquid away from the surface and spreads it along the core.
- Absorbent core: pulp, SAP or foam — the storage layer (details below).
- Backsheet: impermeable film that stops leaks; breathable versions allow air exchange.
- Wings and side guards: fold around underwear and block side leakage.
Core Technology Compared
The core decides thinness, capacity and bunching behavior:
| Core Type | Thickness | Absorption Speed | Bunching Resistance | Typical Products |
|---|---|---|---|---|
| Traditional pulp core | Thicker | Fast (capillary action) | Moderate | Value pads |
| Pulp + SAP core | Medium | Fast + gel locking | Good | Mainstream pads (most brands) |
| High-SAP thin core | Very thin | Fast, locks under pressure | Good | Ultra-thin pads |
| Foam core (FlexFoam) | Very thin | Sponge-like, no bunching | Excellent | Always Infinity, premium pads |
Most mainstream pads blend pulp with SAP for a good balance. Foam cores like FlexFoam (Always Infinity) behave differently — absorbing like a sponge and staying thin.
Topsheet: The Skin-Facing Decision
If you have sensitive skin or experience chafing, the topsheet matters more than the core:
- Dry-mesh perforated film: quick-drying, good for heavy flow, but can feel plastic-like to some.
- Cottony nonwoven: soft and gentle, ideal for sensitive skin; pair with a skin-friendly pH formula.
- Organic cotton: lowest allergen risk, in lines like Always Pure and Kotex Natural.
Leak Protection: Wings, Guards and Shape
Total length is a poor predictor of leak protection. What matters:
- Wings secure the pad around underwear and block side leaks.
- Side guards / raised edges create a physical barrier.
- Rear extension on night pads protects when lying down.
Choosing by Flow and Lifestyle
- Light days: liners or day pads.
- Heavy days: longer pads with SAP cores, or foam-core pads.
- Night: night pads with rear coverage, or period pants for maximum protection.
For brand-level decisions, see our Always vs Kotex comparison, and for pad selection basics our how to choose sanitary pads guide.
Frequently Asked Questions
How does a sanitary pad absorb blood?
The topsheet channels liquid into the core, where pulp fibers wick it and SAP (if present) locks it into a gel. The impermeable backsheet stops leakage while allowing some air exchange.
What is the difference between pulp, SAP and foam cores?
Pulp cores are bulky but absorb fast; SAP-thin cores are thin and lock liquid under pressure; foam cores (like FlexFoam) absorb like a sponge without bunching. Premium thin pads combine pulp with SAP.
Do pads with SAP cause irritation?
SAP (sodium polyacrylate) is widely used and considered safe in pads; irritation is more often caused by fragrance, dyes or friction from the topsheet than by SAP itself.
Why do my pads bunch up?
Bunching happens when the core breaks apart during movement. Foam cores and embossed channels resist bunching better than loose pulp cores.
How often should I change a pad?
Every 2–3 hours during the day regardless of flow, and choose a longer overnight pad for the night. This keeps the area drier and reduces odor and irritation risk.
References
- JP CIA (industry) - feminine hygiene product materials — 行业媒体 (accessed 2026-08-02)
- CDC - Personal Hygiene — CDC (accessed 2026-08-02)
- EDANA - Absorbent Hygiene Products (AHP) Industry Overview — EDANA (accessed 2026-08-02)