Why Big Babies Leak, Part 3 - Shear

We've been there. We've heard it. The dreaded Attends 'pop.' It means that one of the gazillion tapes on our adult diaper just came loose. Time to break out the "magic eagle diaper tape" and gripe about physics. Physics is, as you recall, unfair.

In part one, we saw how real babies have the benefit of a high surface area to volume ratio. An adult diaper has to absorb more per square inch than a baby diaper. Anything that doesn't get absorbed instantly will dribble toward the seals at the leg or waist. In part two, we saw how babies get better seals because of their small, uniformly rounded curves. Getting good seals around lumpy, boney, or flabby adults is harder. Trying to overcome this requires tension.

This is where the tapes come in. The catch is that the tapes aren't on the edges, where the gathers are. The force from the gathers doesn't line up with the force in the tapes. This creates a shear load. If the diaper's plastic backing were thick, like a traffic sign, then it would convey shear loads well. (That is why many traffic signs just need one post.) A diaper made out of such thick plastic might need only one pair of tapes. However, it would be extremely uncomfortable. Somewhere between a Molicare and a hip abduction splint[DEF], perhaps.

Adult and baby diapers use a thin, plastic backing. This thin backing doesn't convey shear loads well, so the tapes need to be close to the edges. On adults, the tapes for the leg gathers need to be low, near the legs. The tapes for the waist gathers need to be high, near the waist. On babies, everything is close together, so only one pair of tapes is needed.

The tapes in the middle might have been intended support the weight of the diaper, reducing the load on the other two pairs of tapes. In practice, it merely ensures that any baby oil or powder that didn't adhere to the meaningful tapes is removed from your fingers.

The inability of thin plastics to transmit shear loads is yet another example of how physics is unfair. Babies only need one pair of tapes. And because they are babies, those two tapes don't need to be under as much tension to get a good seal. And because they are babies, the diapers will be less dependent on seals. And because they are babies, they'll be cute even if they do leak. And because they are babies they can be changed anytime and carry their stuffy anywhere. Life is so unfair.

Take care

~BitterGrey

Email BitterGrey Level: General | Status: Finished | Last Update: 9 May 2009| First Posted: 5 May 2009