What Materials Absorb Fastest?

Speed of Absorption Testing

Don’t hate me, but I’ve never had much of a problem with leaking cloth diapers. I like to attribute this to the following factors: (1) My baby has the perfect body type, (2) My impeccable instincts lead me to only the very best cloth diapers, (3) I’ve devised a fool-proof laundry routine, and (4) I always know precisely which insert or combination of inserts to use for every occasion.

I kid, I kid. I’m sure it’s mostly just dumb luck.

For parents who are having trouble with leaks, though, it can be a huge hassle. You’re dealing with a baby who should be sleeping but instead is awake, crying, and wet. You’re making frequent clothing changes for the baby (and possibly having to change your own clothes and the sheets as well). While doing all that extra laundry, you find yourself wondering, “Is the diaper repelling? Is it not fitting properly? Is my baby just an unusually heavy or fast wetter?”

If you have eliminated the possibility of repelling or a fit problem, you can experiment with different types of absorbent materials.

I’ve always heard microfiber described as a “fast-absorbing” material, but wasn’t inclined to believe it. It’s a synthetic material, and if you’ve ever thrown a microfiber insert in a bowl of water, you might have noticed that it will float happily on the surface of the water for quite some time before succumbing to the fathoms below. (Wait, what?! You mean to tell me that no one else does this for fun?!)

Floating Microfiber

“Microfiber Magic Carpet Ride” – cue music from the Aladdin soundtrack.

I decided it was high time I test the speed of absorbency of various cloth diaper materials for myself and then spread the word. You know, for the edification of cloth diapering parents everywhere. And to satisfy my own curiosity.

The testing method involved emptying a syringe of 50 mL of water into a tube that is anchored and weighted where it touches the insert (to simulate a good, snug fit on baby). I simply timed how long it took each insert to absorb the full 50 mL of water.

Testing Method

The lineup of test subjects from my own personal stash included two synthetic-fiber inserts and two natural-fiber inserts. The results? Yeah, totally proved my “slow-absorbing microfiber” theory wrong. Both microfiber inserts absorbed the water significantly faster than the natural-fiber inserts:

  • 19.1 seconds – bumGenius microfiber
  • 21.0 seconds – Rumparooz microfiber
  • 27.7 seconds – SoftBums bamboo
  • 30.0 seconds – Thirsties hemp
Speed of Absorption Test Results

Left to Right: Thirsties large hemp insert (55% hemp, 45% cotton), Rumparooz one-size 6r microfiber soaker (80% polyester, 20% polymide), SoftBums one-size bamboo pod (70% organic bamboo, 30% organic cotton), and bumGenius one-size microfiber insert (microfiber terry).

Now don’t go hating on the natural fibers just because they’re slower on the uptake. Natural fibers are more breathable, less prone to build-up problems, and often better for babies with sensitive skin. And for lucky folks like me who don’t have leak problems, speed of absorption doesn’t really matter. I just wanted to know!

What is your favorite type of insert and why?

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 MadeHow.com 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.


All Prefolds Are Not Created Equal

Whenever I hear someone say, “I’m not really sure if I’m going to like cloth diapers, so I’ll just start out with some inexpensive Gerber prefolds…” I cringe and think “…then you’re not going to like cloth diapers.” For some reason, Gerber prefolds are on the shelves in Targets and Walmarts galore; I’m sure it’s mainly because people use them as burp cloths.

So what’s the Diaper Wrecker to do? Waste $17 at Target on these “premium” Gerber prefolds to prove my point, that’s what!

Prefold Cover Image

Contents: cotton and polyester. Meh.

Gerber Prefold Packaging

After washing them up a few times with my regular diaper laundry they became pilly and not even a tiny bit softer than the way they felt straight out of the package. Plus, the fibers seemed pretty loosely woven, which seems contrary to their claim that these are “premium”. Premium compared to what? Leaves?

Gerber Prefold Close Up

I have, use, and love two different kinds of (real) prefolds – GroVia’s bamboo/organic cotton prefolds and the BabyKicks hemp/organic cotton prefolds. The GroVia prefolds are thick and super-soft and a little bulky. The BabyKicks are less bulky and they don’t look as pretty, but they have this awesome stretchiness that makes them so much easier to fasten on a baby. So I was curious how the three brands compare.

I present our contestants:

Prefold Absorbency Test

I did some quick absorbency testing to see how Gerber stacked up. Each prefold weighed dry, submerged in water until fully saturated, hung to drip off for 20 minutes, and then weighed wet. The difference between the dry weight and wet weight was calculated as the diaper’s absorbency. Now, this is only what I’d call “theoretical absorbency”. In reality, when worn by a baby, the diapers would leak at that capacity, due to the shifting pressure of baby’s weight. However, this theoretical absorbency is fine to use for comparison purposes since they’re all being tested the same way.

However, it’s not fair to compare the results of the straight-up absorbency of each prefold, because they’re all different sizes – and the Gerber was the smallest. Instead, you get a better understanding by looking at the ratio of absorbency (in grams) to area (in square inches). Gerber’s absorbency by area was almost half of BabyKicks’ and GroVia’s!

Comparison of Prefold Absorbency

And it’s not as if Gerber prefolds are so dirt cheap that the price justifies the limited absorbency. At $1.79 each, they’re only slightly cheaper than Cloth-Eez (a brand I used on my son as a newborn). GroVia prefolds range from $2.67 each to $5.33 each depending on size; BabyKicks start at $6.59 up to $8.79. However, those are just what I had on hand and use for my child. There are plenty of other quality prefolds available – what’s your favorite brand?