In this blog post, we’ll explore the structure and mechanism of superabsorbent polymers (SAP), which enable diapers’ exceptional absorbency.
We’ve all worn diapers at some point. Diapers are a wonderful invention that helped keep us comfortable even during those days when we urinated at all hours. If you recall your own childhood or observe your younger cousins or other babies using diapers, you’ll notice that the surface of the diaper remains relatively dry even after urination. Furthermore, their exceptional absorbency—which prevents urine from leaking out easily despite the narrow space between the baby’s bottom and the diaper—is truly remarkable. Imagine if the same situation occurred while wearing regular underwear; the discomfort would be indescribable. The outstanding moisture-absorbing power that keeps babies comfortable lies within the small, white granules found inside the diaper when you tear it open. These granules are a type of superabsorbent polymer (SAP). Let’s take a closer look at how this material works to effectively absorb urine.
SAP traps and absorbs water using a structure similar to a fishing net—just as fish get caught in the mesh when a net is set to catch them. Let’s examine the structural characteristics of SAP in a bit more detail. As its name suggests, SAP is a type of polymer. A polymer is a macromolecule formed by the repeated linking of small monomer molecules. The typical structure of a polymer consists of identical molecules bonding together to form long chains, and SAP also has this chain structure. However, if the long chains were simply arranged side by side, water molecules would simply pass through them without being absorbed by the SAP. Unlike typical polymer molecular structures, SAP has a structural feature called cross-linking. Cross-linking connects adjacent chains to one another, forming throughout the structure to create a three-dimensional, net-like structure.
Simply having a net-like structure is not enough to effectively absorb water molecules. When catching fish, the fish get caught because they are larger than the holes in the net, but water molecules are much smaller than the spaces formed by SAP and can simply slip right through. Therefore, a mechanism is needed to draw water molecules into the net-like structure and keep them there. Just as anglers attach bait to a hook to attract fish, SAP contains highly polar functional groups with a strong affinity for water, which draw water molecules into the mesh structure. A functional group refers to a group of atoms that performs a specific chemical function; SAP contains charged functional groups such as carboxylate groups (COO⁻) and sodium ions (Na⁺).
A water molecule is a polar molecule composed of a positively charged (+) hydrogen atom and a negatively charged (-) oxygen atom. Because an electrostatic attraction acts between positive and negative charges, the hydrogen atom of the water molecule forms an attractive bond with the carboxylate group, and the oxygen atom forms an attractive bond with the sodium ion. Thanks to these electrostatic attractions, water molecules are strongly drawn toward the functional groups inside the SAP. In other words, the water in the baby’s urine is drawn into the internal space of the polymer—which forms a network structure—by these polar functional groups and is effectively retained there.
Ultimately, the baby’s urine is absorbed and trapped inside the SAP due to the electrostatic forces between charged molecules and the SAP’s network structure. The small, white SAP granules contained in diapers absorb moisture based on this principle and are generally known to be capable of absorbing hundreds of times their own weight in water. This outstanding absorbency is made possible by the cross-linked, porous structure of SAP. Without SAP—which is responsible for the diaper’s moisture absorption—today’s disposable diapers, which effectively manage a baby’s waste, would be difficult to produce. Behind the convenience of the diapers we use every day lies the little-known scientific principle of superabsorbent polymers.
