Core Stability Under Shear: Why Some Diapers Clump and Others Don't

One of the most common complaints about diapers is clumping—the formation of hard, uneven lumps in the absorbent core after the diaper has been worn for some time. Clumping is not caused by poor absorption capacity; it is a mechanical failure. The core has been subjected to shear forces during wear—twisting, compression, and friction—that cause the fiber network to collapse and the SAP particles to migrate and agglomerate. Understanding how shear forces affect the core is the key to understanding why some diapers clump and others do not.

What Are Shear Forces in a Diaper?

When a diaper is worn, the core is constantly subjected to mechanical stress from body movement:

• Twisting: When the wearer rolls over, turns, or shifts position, the core experiences torsional shear, which can distort the fiber network.

• Compression: Sitting, lying down, or being held applies vertical pressure to the core, compacting the fibers and forcing SAP particles closer together.

• Friction: Walking, crawling, or sliding creates lateral friction between the core layers, causing fibers to shift and SAP particles to migrate.

These forces are not isolated; they occur simultaneously during normal daily activities. Over time, they can cause the core to lose its structural integrity, leading to clumping.