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Why Doesn't Blood Show Through a Sanitary Pad? [Diaperman]





πŸ“Ί Watch the video on YouTube: https://youtube.com/shorts/IwqKVzQkyR8

Why doesn't blood show through a sanitary pad? A question everyone has wondered at least once, but felt too embarrassed to ask. In this episode, we dissect it through materials science. The secret is the same 3-layer structure as a diaper — ① a white topsheet (an opaque curtain) ② micro-holes for one-way passage plus anti-rewet backflow ③ a pulp-and-SAP absorbent core that locks blood into gel. From above, only a clean white surface remains. Here's why.

This video was generated by AI.

Same 3-Layer Structure as a Diaper

The basic structure of a sanitary pad and a diaper is identical. Memorize these three layers, which appear again and again in papers and patents:

LayerMaterialRole
① Topsheet (surface)White PP nonwovenSkin contact, liquid passage, hiding what's below
② Absorbent corePulp (fluff) + SAPAbsorbing and locking liquid
③ Backsheet (bottom)PE filmWaterproof — keeps clothes clean

The difference is in 'ratio and design'. A modern diaper core is typically 50% SAP (SAP/pulp 50/50), thin cores exceed 70%, and ultra-thin pulpless cores reach 100% (Fameccanica specs, P&G patent US7175910B2). In contrast, a sanitary pad's core is SAP:pulp = 10:90 by weight (Wiley, Macromolecular Materials and Engineering). Less than a tenth of a diaper's SAP. Why so little?

Topsheet: The Opaque White Curtain

The first secret is at the very top. The topsheet is a white, opaque PP nonwoven. Even with red below, all you see from above is a white surface. This isn't a design accident — it's a functional requirement: a hygiene product's surface must maintain a 'clean-looking' state for the user.

The curtain analogy is precise. A curtain blocks light (gazes) while letting air (liquid) through. Same with the topsheet: micro-holes quickly pass liquid downward only, while the downward gaze is blocked by the white fiber layer.

One-Way Passage and Rewet Blocking: The Direction of Holes

The second secret is how the liquid moves. Once through the topsheet's micro-holes, liquid descends into the absorbent core and does not come back up. In materials science, liquid returning to the surface is called 'rewet' — there's even an official test standard, EDANA WSP 80.10.

Pads additionally have a fluid distribution layer that spreads liquid sideways, widening the absorption area and preventing localized pooling. In other words, a pad's real job is less 'absorbing' than 'spreading'.

Why Spreading Is the Job: Menstrual Blood Is Highly Viscous

This design exists because of the fluid's properties. Urine is thin and voluminous (300–400 mL per void). Menstrual blood is a high-viscosity fluid. UniCharm's patent EP4331545A1 explicitly names menstrual blood's "high viscosity" and describes diffusion technology that spreads it through the topsheet and core.

FluidVolumePropertyDesign strategy
Urine300–400 mL/voidThinLocking design — SAP gel swelling 'locks' it away
Menstrual blood25–80 mL/cycleHighly viscousSpreading design — pulp disperses, SAP locks at points

Total menstrual volume is only 25–80 mL per normal cycle (average 30–40 mL, NHS reference). Small amount and sticky — so instead of loading up on high-capacity SAP, the structure lets pulp spread liquid along the pad's length while SAP locks it at key points. That is why the SAP ratio is a mere 10:90.

How Much Does SAP Absorb? The Fluid Changes Everything — 9x

A fun number: measured absorbency (FSC) of AA-based SAP is about 63x its own weight in saline (0.9% NaCl), but about 568x in distilled water (Wiley measurements). The fluid alone changes absorbency nearly 9-fold. The more ions in a fluid, the less SAP absorbs — consistent with UniCharm patent EP1829563A1's note that protein- and serum-rich menstrual blood lowers conventional SAP's absorbency and retention. When you see marketing claims like "absorbs X times its weight," always ask first: in what fluid?

Summary: The 3 Steps of No Show-Through

  1. Hide — the white opaque topsheet acts as a curtain over what's below
  2. Send down — micro-holes pass liquid downward only and block rewet backflow
  3. Lock in — the pulp + SAP core drinks the blood and fixes it as gel

Same 3-layer structure as a diaper — engineered for a different fluid.

This video was generated by AI.

References

#diaperman #diaper #shorts #viral #trending #fyp #sanitarypad #periodscience #hygiene

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