Question of the Day

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Each weekday Marshall Brain answers questions submitted by the readers of How Stuff Works in the Question of the Day section of HSW. You can also view previously answered questions in the HSW Question Archive. Here is today's question!

Question

When we change our baby's diaper, he often has what we refer to politely as "crystal butt". There are little clear crystals on his bottom. It seems like they come from his diaper. Do you know what these crystals are?

Answer

The crystals are probably coming from the "super absorbent layer" found in most disposable diapers. This layer consists of paper fluff and a chemical absorbent called sodium polyacrylate. Sodium polyacrylate is an amazing water absorber - it can absorb 200 to 300 time its weight in tap water (even more if the water is distilled) and hold it in a gooey gel.

Sodium polyacrylate is a polymer, meaning that it consists of chains of identical units (monomers). The monomer for sodium polyacrylate is -CH2-CH(COONa)- . It is a pretty cool polymer - shake the crystals out of a new diaper and add water to them (or buy the crystals at a garden center). Pretty amazing!

There is concern in certain circles about the use of sodium polyacrylate next to a child's skin. The concern is often used as an argument against disposable diapers. This diaper page talks about the issue briefly and will give you a sense of what people are concerned about.

Here are several interesting links:

• Polymer experiment
• AP Chemistry polymer project - polymer experiments
• Plant gel - The other common use for sodium polyacrylate is as a water-retaining gel in houseplants.
• Toxic shock syndrome and tampons - The diaper page mentioned above talks about TSS and and hints at a relation between TSS and sodium polyacrylate. This page offers a much clearer explanation of TSS.

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