1. How Much Water Can the Powder in a Diaper Absorb?
Inside a diaper, a high-absorbency polymer (SAP) is wrapped in cotton-like pulp. While dry, it looks like powder, but upon contact with water, it expands into a gel [5]. Manufacturers state that SAP can absorb and retain water at several hundred to a thousand times its own weight [1]. This figure represents the upper limit for water that does not contain salt.
In actual diapers handling urine, the absorption is explained as roughly 50 to 100 times the powder’s weight, and the gel is said to resist leaking even when pressed [4]. If we treat 1 mL of urine as approximately 1 g, then 1 g of SAP absorbs 50 to 100 mL of urine. To hold the volume of a 500 mL (about 17 oz) bottle, you would need 5 to 10 g of SAP, which weighs about as much as 5 to 10 one-yen coins. Note that this is a calculation estimate, not the exact amount found in a real diaper. How does a single grain of powder trap dozens of times its weight in water?
2. How Does a Water-Soluble Material Form a Net That Traps Water?
The main material in SAP, sodium polyacrylate, is naturally soluble in water. It forms a gel instead of dissolving because its molecules are linked together by "cross-linking," creating a mesh structure [8]. Because it is a net, it swells with water but does not dissolve. Imagine loose strings washing away in water; if you tie them at intervals, they form a net. The molecules in the powder form a similar invisible net. The absorbed water is trapped inside this mesh and is difficult to squeeze out.
Two forces drive water into the mesh. The first is osmotic pressure caused by the difference in ion concentration inside and outside the mesh. Because the concentration of ions differs, this pressure moves water into the mesh. The second force is the repulsion between ionized carboxyl groups. These negatively charged parts push against each other, causing the mesh to expand [2][3]. This osmotic pressure depends on the concentration difference between the inside and outside of the mesh. So, what happens if the water outside becomes more concentrated?
3. Why Does Salt in Urine Reduce the Powder's Absorption?
Acrylic-based SAP is generally not good at absorbing salt-containing solutions like urine. When the electrolyte concentration outside is high, the concentration difference between the inside and outside of the mesh decreases, weakening the absorption force [2]. The same source shows a graph where absorption drops as salt concentration rises, even for products that absorb about 500 times their weight in pure water [2].
When comparing the manufacturer’s "several hundred to a thousand times" with the industry association’s "50 to 100 times," it looks like a tenfold difference. However, these numbers come from different products and measurement methods. They are not a direct comparison of the same product in pure water versus urine. Therefore, we cannot say "urine reduces absorption by one-tenth." We can only say that in salt-containing liquids, the absorbable amount drops significantly compared to pure water. If absorption is lower, the diaper must rely on other materials besides the powder.
4. How Do Pulp and Polymer Share the Work in a Diaper?
The absorbent material in a diaper consists of SAP wrapped in cotton-like pulp, placed under an absorbent paper layer. This structure absorbs and retains urine [5]. The pulp primarily spreads the urine. The powder then absorbs the spread-out urine to form a gel, which is hard to release even under pressure [6][4]. By dry weight, pulp makes up 40 to 70% of the diaper, while plastics and SAP make up 30 to 60%. This is the ratio for the whole diaper, not just the SAP [5].
If the powder spreads urine well, less pulp is needed. In 2013, Sanyo Chemical Industries announced a new grade of super-absorbent resin with excellent urine diffusion. This allows for thinner diapers and increases effective absorption by more than 10% compared to conventional types [6]. However, this performance is specific to that company’s product, not a general figure for all SAP. When and where did this powder originate?
5. How Did Absorbent Materials Develop from Starch to Diapers?
Researchers at the USDA (United States Department of Agriculture) created an absorbent material by linking synthetic polymers to starch, capable of absorbing hundreds of times its weight in water. The name "Super Slurper" was reportedly given by a public relations officer at the department [7]. About 40 non-exclusive licenses were granted, allowing various companies to use the technology.
This research sparked the full-scale development of super-absorbent resins in the early 1970s. Publication dates vary between 1973 and 1974 in different sources, so "early 1970s" is the safest description [2][7]. In 1978, super-absorbent resin was first used in sanitary products [2]. In the same year, Sanyo Chemical Industries announced it had begun the world’s first commercial production, though this "world first" claim is from the company itself [1]. Adoption in diapers is listed as 1981 in Japan by Toagosei and 1983 by the Japan Hygiene Products Industry Association. Due to this discrepancy, "early 1980s" is used [2][4].
The Japan Hygiene Products Industry Association chronicle notes that after polymer absorbents were adopted, daily diaper use dropped from 7.7 to 5.5 sheets [4]. This is a reduction of about 30%. As of 2002, Toagosei reported that over 90% of SAP demand was for diapers [2]. Today, the main raw material is acrylic acid derived from petroleum-based propylene [1]. What happens to the powder after use?
6. What Happens When You Sprinkle Salt on the Swollen Gel?
It is explained that if you sprinkle salt on a swollen gel, water may move out [8]. Since water entered due to the concentration difference inside and outside the mesh, raising the outside concentration weakens the force pulling water in, and under certain conditions, water comes out.
Safety Note: If you have an unused diaper, ask an adult to help you cut it open with scissors. Take a small amount of the cotton-like mass and put it in a cup of water. Once it swells, sprinkle a pinch of table salt and observe. Handle the powder gently to avoid inhaling it, and do not put the water or gel in your mouth. In 1984, it was noted that products using SAP allowed adults to absorb one urination with a single sheet [4].
Recycling is a challenge. Divalent cations like calcium bind strongly to the carboxyl groups in SAP, hindering free swelling [2]. In 2020, Sanyo Chemical Industries announced a new SAP that dehydrates by more than 75% in about 5 minutes using calcium chloride treatment. This improves dehydration by about 30% compared to conventional products and addresses recycling issues for used diapers [1]. However, this is a new technology announcement, and its widespread adoption is not yet known. You might also find sodium polyacrylate in absorbent sheets under fish trays at supermarkets [2]. Next time, look under the tray to see where else this powder’s relatives are used.