Diaper Secrets: Why your Absorbency Rating is never right

With everyone going crazy over the new BetterDry and the ensuing discussions over its absorbent qualities (mostly compared to its predecessor, the Comficare) the old subject of how to measure and quantify your favorite product’s absorption has come up again. Since I’ve mentioned it a couple of times already, now is as good a time as any to have a look at this quite taxing problem.

How much your diaper holds is determined by multiple factors, each single one alone complex enough to warrant its own article. The most obvious of those is naturally the actual absorbent pad. It consists of a mix of cellulose pulp and super absorber (SAP) plus any number of additional ingredients such as other fibers (cotton, cloth, synthetic polymers), smell inhibitors, perfume, skin protection and so on encapsulated in at least one layer of permeable tissue. Depending on the manufacturer, the percentages vary. On top of that pads come in different thicknesses, shapes and sizes. As if that weren’t enough, the manufacturing processes involved in producing them result in a different density and internal structure. Thus the ordering of the fibers affects the behavior as well.

With that in mind, now what’s the deal about all this absorption stuff? It’s obvious that there’s an inherent variability and not every product can be the same, so there has to be some way to objectify this. Since humankind likes to measure things ever since our progenitors wandered the surface of this Earth and may have wondered how far they traveled, there are several testing methods. Until a few years ago the most most common were:

  • Reference based testing methods whereby a pre-determined amount of product (so and so many grams or a 10 x 10 cm square or similar) is soaked under lab conditions in a beaker using a substitute liquid that simulates urine. After having let the excess drip off, this is weighed and the result extrapolated, meaning multiplied by the amount of actual product used in a diaper.
  • An extended version of the previous method, where a specific weight is simulated by assuming a certain pressure per area.
  • ISO 11948-1 (Rothwell test), which is basically the same as the first, only this time an actual readymade product is used in full.

Apparently they all have their point of failure built in already: They are abstract tests that don’t reflect actual use cases. Under real conditions, it’s near impossible to fill even the best diaper to the brim just like there is a difference in the amount of pressure a slim person will put on a product compared to an overweight one.

Anybody can see that this is not really useful beyond the technical testing and quality assurance that has to happen in a factory, so in recent years a new method has been proposed: ABL (absorption before leakage). This uses a specifically prepped dummy that gets a nappy wrapped around his bum and then through inbuilt tubes liquid is pumped out in intervals, while the whole thing is lying under pressure on some simulated bed at pre-defined angles until there is visible leakage. While it’s a better approach and gives more useful results, I still think it misses many critical aspects. Here’s why.

It’s still a static test that does in no way simulate the wear and tear a real product must endure. Even when in bed, people move around, which puts shear on different regions, effectively squeezing them out like a sponge. This is then multiplied exponentially when you add additional movements like sitting up or walking. In these situations another factor also comes into play a lot: the internal structure of the pad.

If it is a “bad” pad, it will suffer from leakage more quickly because liquid is only transported vertically from the inside away toward the outer side. As a result, you get puddles in one place and the rest of the pad stays dry (that is, BTW, also the reason why I don’t find wetness indicators too useful in most cases).

In reverse, a product that has decent horizontal transport will always have more safety reserve because even when squeezed out from one region, the liquid can take evasive action and move into other, dry areas. In particular this last point is something the manufacturers pay way too little attention to in my opinion.

So where does that leave us? The inconvenient truth is that, regardless of all those nice numerical values you read on the package of your favorite incontinence product, only your own testing can give you the experience to judge how much it will hold under realworld conditions during your daily routine. At best, the values can give you a base reference point.

ISO values can easily be halved to more properly reflect the actual conditions, though even that crude formula is far from exact. Even the worst pad has a fixed minimum absorbency and the larger a pad gets in area and weight does not necessarily mean it can absorb equally more liquid. The relation is not linear.

On the rare occasions where manufacturers give you an ABL value, things can in fact be even worse. You may need to subtract at least another third of the given value or add a considerable amount on top of it. This hearkens back to my point with that vertical/ horizontal thing and how the liquid flows inside the micro cavities along the fiber structure and where actually those little lumps of super absorber that stop the flow are.

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