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 so 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. Lots of tension.

This is where the gazillion 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. They have all the luck.

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, these taps merely ensure 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

Last Update:20 March 2011  First:5 May 2009   Email BitterGrey   

Do you have Questions, Tips, Suggestions, or other feedback?

Kovu asks Do you know if there are adult diapers with two tapes (one on each side that is)?

There are few. Some places still list two-tape diapers, such as They arent truly practical for adults, so most places will only carry four or six-tape diapers. However, there are still the odd exceptions. One is Safe Comfort, that still lists SureCare diapers on their website. SureCare used to be made by a division of Tyco (according to the bag I found at a thrift store one day). Independent thrift stores are usually good places to find less-than-current diapers. According to GhostWulf, diapers with only two tapes have shown up at Sams club, but these might not have been the classic wingfold design.

LilJennie adds AB Universe.com sells two types of diapers, "Cushies" and "Super Dry Kids," that have one tape per side. I have tried the Super Dry Kids diapers, and the tape isn't like a typical diaper tape -- it's wider than it is long, basically like what you'd get if you took a two-tape-per-side diaper and bridged the gap between the tapes with more tape. They seem to have solved the problem that one-tape (per side) diapers pose for adult-size babies. And they've solved it in spades. Not only is there adhesive at both the top and bottom of each side, but it's really strong. This makes sense, because the adhesive force of a piece of tape increases with surface area, and instead of one small tape at the top and another at the bottom, they've got everything in between as well.

Not only is it difficult for these tapes to come loose accidentally, it's difficult to pull them loose deliberately. The only real problem they pose is in fitting -- you must get a diaper of the correct size for you, or you won't be able to get the entire extra-wide tape on the taping panel, where it adheres best and where it can be removed without tearing the outer plastic liner. And of course then there's the fact that the rest of the diaper isn't apparently designed all that well, because they always leak for me. Great look, excellent taping system, but wear something over them if you're going to wet them.