Understanding the Science Behind How Baby Diapers Absorb Urine

Baby diapers, despite being thin, possess the remarkable ability to absorb multiple instances of a baby's urine without leakage. Regardless of how much the baby moves or exerts pressure, the urine remains contained within the diaper. This leads many mothers to wonder: what is the principle behind the absorption of baby urine by diapers?

Let's delve into the science behind it: How do baby diapers absorb urine?

Firstly, let's understand the structure of baby diapers. The high absorption and retention capabilities of baby diapers can be attributed to their absorbent materials and structural design. Baby diapers typically consist of four layers:

1. Top layer: Made of non-woven fabric, in direct contact with the baby's skin, it receives the urine and passes it on to the next layer.

2. Transfer layer: Also made of non-woven fabric, it evenly distributes the urine received from the surface layer to the absorbent core.

3. Absorbent core: Composed of a mixture of superabsorbent polymer (SAP) and fluff pulp fibers, or sometimes solely SAP, this layer is the most crucial functional layer responsible for absorbing and storing urine.

4. Bottom layer: Made of PE film or PE film combined with non-woven fabric, it is breathable yet waterproof, primarily functioning to prevent urine leakage.

Now, let's shed some light on the absorbent substance inside baby diapers, the superabsorbent polymer (SAP).

Many mothers mistakenly believe that baby diapers contain cotton for absorption. However, upon inspection, one would find small granular beads mixed with pulp fibers inside baby diapers, far superior in absorption rate and retention capacity compared to cotton.

SAP, resembling tiny beads with a diameter of about 0.2mm, has the remarkable ability to absorb hundreds to thousands of times its weight in water due to its high affinity for water molecules. This is because SAP is a low cross-linked or partially crystalline polymer with many hydrophilic groups. When SAP particles have a size of 100 to 120 mesh and a pH value of 6 to 8, their absorption capacity and rate are optimal.

In a typical baby diaper, the SAP in the absorbent core is uniformly mixed with pulp fibers.

Now, let's explore the process of urine absorption and retention in baby diapers.

When a baby urinates, the urine permeates through the layers of the diaper in a stepwise manner:

The top layer, in direct contact with the baby's skin, receives the urine.

The transfer layer guides the urine downwards, ensuring even distribution and absorption.

The SAP in the absorbent core absorbs and retains the urine.

The bottom layer prevents leakage.

Regarding the locking mechanism of water in diapers:

SAP, being a high-molecular-weight compound with numerous water-attracting atoms, easily absorbs water molecules, causing the SAP particles to swell and merge. Despite the considerable absorption, SAP does not dissolve in water, allowing it to retain water without leakage.

To illustrate, just like how water absorbed into cooked rice or kneaded into dough remains trapped without leaking, SAP effectively locks in moisture.

However, it's essential to note that the water retention capacity of SAP has its limits. Prolonged use without changing diapers may exceed the SAP's water-holding capacity, leading to diaper rash. Therefore, it's crucial to change diapers promptly based on wetness indicators and the baby's urination frequency.

Understanding the science behind how baby diapers absorb and retain urine ensures effective and comfortable diaper use for babies, providing peace of mind for parents.