The New Diaper Primer

Chapter 3: How Diapers Work

Cloth diapers can be made from any soft, absorbent fabric, but the best is cotton, a natural fiber. A cotton diaper absorbs liquid, effectively tying it up within the cotton fibers. Imagine, if you will, that a diaper consists of a large number of "diaper particles". The actual number doesn't really matter, let's say it will be in the millions, and these particles are individual cotton fibers. As you wet a diaper, particles become wet in the crotch area immediately next to your skin. Each particle can only hold a certain amount of liquid; then it passes the rest of the liquid along to the next particle. This process works in three dimensions; each wet particle passes excess liquid off in all directions to neighboring particles; they in turn do the same as the supply of liquid continues.

To picture this idea of absorption, think about (or even do) this experiment. Place a cotton handkerchief, a face cloth, and a folded hand towel on a waterproof surface such as a kitchen counter top or the bathroom vanity. I'm sure you can imagine that we now have a thin, medium, and a heavy-weight "diaper".

Dump a tablespoon of water into the center of the handkerchief and immediately pick it up. You will find the counter top will be pretty wet while the handkerchief will be mostly dry with just a wet spot in the middle.

Repeat this with the face cloth. The counter top will be much drier. A lot of the tablespoon of water got tied up just passing through the face-cloth thickness. In the case of the folded hand towel, you probably don't even need to look: the counter top will be dry. What are we demonstrating here? The handkerchief has fewer "particles" between the wetting and the waterproof surface so it could not tie up very much of the water. The face cloth provided more "particles" that grabbed water as it was going through; that meant less water would actually soak through to the countertop. In the case of the towel, the particles seriously out-numbered the available wetting, and no water even penetrated all the way through. The water supply stopped before all the particles were "used up".

This process is called absorption; it is the basic principle of how any diaper works, cloth or disposable. The absorbency is provided by cotton fibers in the case of cloth diapers and by cellulose fluff (padding) and also with Super Absorbent Polymer (SAP) gel crystals (small granules much like fine sand) in the case of most disposable diapers. The important point is that diapers work by absorbing - tying up liquid within the diaper. Liquid not so tied up within a diaper is free and can leak out onto clothes and furniture. A diaper's ability to quickly absorb free liquid is governed by its thickness; that is, how many "diaper particles" are there between the wetting source and the outside of the diaper? To be safe from leak worries, you have to accept the fact that you must wear enough diapering!

Now, let us take this same experiment a step further and introduce you to a related and important concept: wicking. A wick in an oil lamp is a piece of cotton fabric that extends from the burner area (where the oil is burning and producing light), down into the reservoir when it is immersed in the lamp oil. The wick conveys lamp oil from the reservoir up to the burner, against the force of gravity, even! Wicking is the ability of cotton (and some other fabrics) to pass liquid along from "particle" to "particle".

Returning to the absorption experiment, we invite you to start over with a dry handkerchief and face cloth. Again, dump a tablespoon of water onto the center of the handkerchief and the face cloth. Forget the towel, it's so thick that the basic absorbency tied up all the free liquid. Now, allow some time to pass. Note that the really soggy spot in the center of the handkerchief is spreading in all directions. After a while, say 5 to 10 minutes, you can probably pick that handkerchief up and not have free water on the counter top, just a wet to the touch countertop. And, the handkerchief will be totally wet all the way to every edge, whereas before, it was mostly dry and just had the really wet spot in the middle. For the face cloth, we can guarantee you won't have free water on the counter top under the face cloth.

What we're trying to demonstrate is that time allows cotton to do its natural "wicking," where the wet particles transfer the liquid to dry particles. This wicking characteristic is pretty much limited to cotton and some fabrics at this time. Disposable diapers fall behind cloth diapers primarily because of this lack of wicking ability. They can be great for absorption, but without wicking ability, they have problems handling subsequent wettings since they can't get that liquid transferred to dry parts of the diaper.

Now let us take a game metaphor. Consider the particles in a cloth diaper to be team players while the particles in a disposable diaper are all super stars that only care about their individual performance. The cotton particles keep passing the liquid along, even giving up liquid that they once held at their individual capacity. So a saturated area of the cloth diaper becomes un-saturated because there were dryer particles someplace else in the diaper. Cotton diaper particles are team players.

In contrast, the cellulose fluff in disposables, due to their characteristics and construction, cannot perform as effectively as cotton, so wicking through the fluff occurs only to a small degree. Now the SAP crystals are the greedy superstars. They absorb and hold a LOT of liquid but, they don't give it up. No passing along here! No team play! When a SAP particle grabs its maximum, it swells, of course, and then simply holds onto the liquid, forming a gel. Now, subsequent wettings provide additional liquid that the gel in the same immediate area. The particles are all non-team players and satisfied that they are holding their maximum allotment of liquid. That newly added free liquid has to work its way around and through the saturated gel to reach any dry part of the diaper. At some point that free liquid trying to reach a dry part of the diaper escapes the particle mass and leaks out a leg band so that you end up with wet pants and furniture. When you change that disposable diaper you are irritated to find a lot of it is still completely dry!

Back to our handkerchief, face cloth and towel. Adding a second tablespoon of water will probably be the end for the handkerchief; just no more particles available, so the water spreads and runs off the soaked handkerchief resulting in a major leak. The face cloth, you will find, can take several more tablespoons of water before it will run out of particles. And the towel? Well, we will grow old dumping tablespoons on it, trying to reach its limit. This limit is called saturation. When we reach saturation with any diaper, cloth or disposable, it will leak because subsequent wetting will produce more liquid than can be tied up (absorbed) in the diaper particles. That liquid is free and will find its way past leg bands and result in wet pants or a wet bed.

Let's return one more time to the experiment with the handkerchief and face cloth. Picture, if you will, adding multiple tablespoons of water to the face cloth. As you dump a second or third spoonful, note how the center gets saturated but then recovers over time (as wicking transfers the liquid to dryer parts). When you add the next one, you see the saturation occurs again, just in the center where you added the water and then wicks into the rest of the "diaper" as time passes. That momentarily saturated central spot becomes un-saturated after time. Wicking occurs and the particles have transferred liquid to other, outlying particles.

This wicking characteristic is typical for cloth diapers; it is the major reason for the superiority of cloth diapers in providing security. Disposables used to be far behind cloth because they lacked both absorbency and wicking capability. Recent advances have produced "premium" disposables for "improved" protection, but that's due only to greater absorbency. While a great improvement, disposables still come up short in their wicking ability. Without that, the disposable gets wet in the crotch area, then gets wetter and finally reaches saturation. Any subsequent wetting will result in free liquid that is not tied up in diaper particles (cellulose fluff and SAP crystals), and that free liquid will find its way out past the leg bands; you have a leak. When you change that disposable you will be disappointed to find MOST of it still dry, probably most of the padding in the rear and even some in the front. You paid for all that "protection" but did not get full value because disposables do not wick wetness from wet areas to dry areas.

Let us summarize this absorbency and wicking discussion. Picture what goes on in your cloth diapers. When you wet, the diaper particles in the immediate vicinity of the crotch get wet and even saturate. The particles absorb what they can and pass the excess moisture through in all directions. A subsequent wetting will repeat the process and the wetness will travel further this time, probably reaching your waterproof pants. The pants, like the counter top are waterproof and the liquid will spread between the interior of the plastic pants and more diaper particles and get conveyed (wicked) to other drier parts of the diaper. This continues with subsequent wettings until ALL of the diaper is wet....the front all the way up to the waistband, the rear all the way up to the waistband and sides. Because of wicking ability, ALL of a cloth diaper's absorbency can be utilized.

In the case of disposables, with very limited wicking ability, once the absorbency is used up in the crotch area, subsequent wettings can easily result in a leak even though most of the rest of the diaper is still dry. Another "problem" so to speak with disposables, is the nature of the crystals and the "stay dry feel" inner linings they use, tend to mask our perception of wetness. We won't feel "that wet" and so not change a diaper when we should. Suddenly, we may find a very disheartening and perhaps day-wrecking leak. You know the kind: wet pants, an ugly wet spot on the furniture! However, now that you understand how diapers work, you will be prepared and will experiment carefully and can even use disposables with better-than-average success in preventing leaks by watching your time and not going beyond your planned diaper change...NO MATTER WHAT.

For cloth diapers, you get a big break. They are very "forgiving," and they get wetter and wetter very predictably. You will sense the change in texture, and you can easily learn when you are getting into the danger zone. These characteristics allow you to ignore a change if you really can't do it, even when the clock says you should. You can put that change off a while until you can more conveniently deal with it. Your cloth diapers won't let you down and cause the wet pants/wet furniture heartache. We will delve into more detail in the "How much do I have to wear?" segment.

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