Super Absorbent Polymers and Diaper Absorbency (Nerd Alert)

Are you curious how your cloth diapers stack up to disposables in terms of absorbency? It might seem like a simple task – weigh them dry, dunk them in water, weigh them wet, and calculate the difference as the weight of the water absorbed. But did you know that if you follow those steps using plain tap water you’ll be giving the disposable diaper far more credit than it actually deserves?

(I warned you in the post title: this information is a bit nerdy. My husband made fun of me for the “diaper lab” I rigged up in our kitchen to test this.)

When they first debuted, disposable diapers used fiber-based products – tissue paper, cotton, or fluff pulp – for absorbency. The absorbent capacity of these types of materials is limited, and much of it is lost under even moderate pressure. In 1984, the introduction of super absorbent polymers (“SAP”) began a race for manufacturers to engineer a slimmer disposable. This immediately resulted in the reduction of diaper bulk by 50%, and in the decades since then, disposable diapers have continued to shrink while their absorbent capacities have risen. Today, the absorbency of disposable diapers is primarily attributable to SAP, with fluff pulp responsible for helping spread the moisture more evenly throughout the core of the diaper so that the polymers can trap it.

So what is SAP? I’m certainly no chemist, so I asked my good buddy Wikipedia, which said: “Super absorbent polymers are now commonly made from the polymerization of acrylic acid blended with sodium hydroxide in the presence of an initiator to form a poly-acrylic acid sodium salt (sometimes referred to as sodium polyacrylate).”

Did you hear me, Wikipedia?? I said I’m no chemist! The easier version from explains, “These polymeric particles act as tiny sponges that retain many times their weight in water.”

And since a picture is worth a thousand words, here are some SAP crystals at work:


So the important takeaway for you and me is that SAP retains water. But babies don’t fill diapers with water. Urine contains salts and and other compounds that inhibit the SAP’s ability to absorb. For purposes of testing diaper absorbency, human urine can be simulated with a 0.9% saline solution.

Rumor has it that some cheaper brands of disposable diapers used to do unscrupulous supermarket demonstrations to con shoppers into believing that their diapers were just as good as the premium, name-brand diapers. Unbeknownst to the shoppers, however, the demonstration used a saline solution on the name-brand diapers and plain tap water on the cheap diapers, thereby skewing the results of the absorbency comparison.

Just how skewed would the results be? I tried this experiment on my GroVia Biosoaker inserts, which contain SAP. I fully submerged one insert in tap water and let it soak for 10 minutes. I fully submerged another insert in a 0.9% saline solution (dyed green) and let it soak for 10 minutes as well. The result:

Side by Side

The insert on the left absorbed 793 grams of tap water. The insert on the right absorbed 286 grams of saline.

So remember: if you want to see how your cloth diapers stack up to disposables, test the disposables with a mix of 9 grams of table salt per liter of water to get an fair picture of their absorbency.