*why* does diaper elastic wear out?

[schmamy]

I'm wondering: when the elastic in old dipes gets all crunchy and "shot" - is that because they're old and just worn out, or is it more likely to happen simply in between kids, when they're in storage and not being used?

I need to replace all my covers, because the elastic is shot in all six of them and it's too much of a pain to replace, unlike pocket dipes. I bought them used and they served me well...then sat in storage for the better part of two years. As I look at buying covers to replace these, I feel nervous about buying used ones...will the elastic last or will it soon be shot like my old ones?

Not sure if anyone can answer this or not...but wondered if elastic wears out more from a lot of use, or from *lack* of use.

[Peridot]

I do think that rubber tends to dry rot in storage. But I don't know why.


Urine also breaks down elastic, and if you pail your covers with your diapers, the elastic will wear out more quickly.

[Lady TS]

I think elastic just breaks down with age/heat exposure/urine.

[Auroras mom]

All of the above: The polymer slowly degrades over time through repeated mechanical stress (wearing), thermal stress (washing, storage), hygral fatigue (washing), and also photodegradation (uv radiation causing polymer decomposition). Elastic just has a finite life. :shrug3 Anything you can do to reduce the stresses, the better, but it simply won't last forever.

[Quiteria]

I think used are ok if they worked recently, not stored forever.

[ThreeKids]

Think of a nice and stretchy elastomer as a bowl of spaghetti (long molecule chains), with some occasional strands that connect parallel noodles (cross-links of the same molecules, but shorter) holding it into a 3D mass.

Cross-links continue to form spontaneously throughout the elastomer's life. Stretching use gives the energy to drive such a process so a cross-link formed under that condition will resist the elastomer returning to its original length. This is why you don't store your perm curlers with the rubber band stretched out - it will "freeze" that way and never spring to a shorter length. For diaper elastic under a lot of use, each "freeze" will be at a different position and only account for a few cross-links so this kind of damage is harder to pinpoint, you'll just notice it getting stretched out over time.

However, cross-links formed when something's in storage are all formed with the elastomer at the same amount of stretching. So, the cross-links tend to build up at the stored level of stretching and the elastomer becomes brittle. This is why a trailer tire left parked for a year will form a hard spot and break up when someone tries to pull the trailer.

So, your question of whether storage or use is worse? I'd say it would depend on the particular elastomer. One that's very stable at it's designed length and more driven to form cross-links during use will be more damaged by use. One that doesn't need an extra bit of energy to form cross-links will become more brittle in storage, but may not be a suitable material, anyhow, so it may still be damaged by use.

UV light is especially good at forming cross-links because it specifically drives the electronic processes that cause the spaghetti strand to break and seek out a new link.

The moral of this story is not to sew elastic in where it is stretched in storage. Use a stronger elastic or more bands if you need more pull.