What Changes: Thickness, Temperature, and SAP Gel State
Thickness: From Thin to Thick (and Back)
When dry, the core is compact and thin, consisting of fluff pulp fibers and SAP particles densely packed together. As liquid enters, the SAP particles absorb and swell, expanding to many times their original volume. This swelling forces the fluff pulp fibers apart, increasing the core's overall thickness. Depending on the SAP loading, a core can swell from 2–3 mm dry to 8–12 mm when fully saturated.
However, after the diaper is removed and pressure is released, the core partially recovers. The fluff pulp fibers act as a spring-like skeleton, providing some resilience. Over time — especially under the weight and movement of the wearer — the swollen gel deforms, and the thickness gradually decreases again. A core that maintains its thickness under pressure offers better comfort and less sagging.
Temperature: From Ambient to Warm
Immediately after urination, the liquid is at body temperature (approximately 37°C). The core warms rapidly as liquid is absorbed. However, the exothermic reaction of SAP absorbing water also generates a small amount of heat. While this warming is usually not perceptible, in high-absorbency products with rapid intake, the core may feel noticeably warm to the touch for a few minutes. As the core cools back toward ambient temperature, any dampness left on the surface feels colder to the skin, contributing to discomfort.
SAP Gel State: From Powder to Gel
The most dramatic change occurs at the molecular level. Dry SAP is a granular powder with a glassy, crystalline structure. Upon contact with liquid, it undergoes a phase transition: the polymer chains relax and unfold, trapping water molecules within their three-dimensional network. The gel becomes soft, slippery, and translucent. If the gel is too soft (low gel strength), it deforms easily and may flow under pressure, squeezing liquid back to the surface (rewet). If the gel is too rigid (high cross-link density), it resists swelling, reducing total capacity.
⭐What Doesn't Change: Core Shape, Fiber Skeleton, and Zonal Distribution
Core Shape
Despite the swelling, the overall footprint of the core — its length and width — remains largely unchanged. The core is anchored within the chassis of the diaper, and the edges are sealed or bonded. The swelling occurs primarily in the Z-direction (thickness), not in the X-Y plane. This ensures that the core continues to fit within the diaper's chassis without distorting the leg cuffs or waistband.
Fiber Skeleton
The fluff pulp fibers themselves do not dissolve or disappear. They remain as a supportive network, even when swollen. This fiber skeleton provides mechanical integrity, preventing the gel from shifting and clumping. It also acts as a conduit for liquid distribution, drawing fluid from the point of impact to the edges of the core. If this fiber skeleton is insufficient, the gel particles may migrate or clump, leading to a lumpy, uncomfortable diaper.
Zonal Distribution
Modern diapers are often designed with zoned SAP application — more SAP in the front (for boys) or more uniformly distributed. This zoning remains intact throughout the lifecycle. The SAP in the front zone swells more than the back zone, but the design ratio remains constant. This is by design: the zones are pre-set to match the typical pattern of liquid insult, ensuring that the entire core is used efficiently.
Baby Diaper
Baby Pull Up Pants
Adult Diaper
Sanitary Napkin
Wet Wipes
Tissues Paper
Disposable Cotton Soft Towel
Pet Diaper
