I work in a shoe repair facility (cobbler shop) and for quite some time we have had people come in with shoes that have soles that are literally crumbling. Sometimes just cracks, but sometimes to the point that you can set the thing on a counter for a couple hours and the soles have chunks spontaneously fall off and just turn to gritty dust. It’s not due to chemical exposure, mistreatment, overuse, or underuse. It seems chemical, like the material has a little timer in its molecules and when it goes off the stuff just ceases to exist in its former state
Anyhow, after some research it seems the common denominator is that these shoe soles are made of some sort of injection-molded polyurethane foam.
What the heck is going on here? Is this a normal feature of such a synthetic? What causes this break down? I’d like to understand the phenomena a little better, partly for myself, but also so I can explain to customers who are also interested in knowing what’s going on with this.
I can only extrapolate from a known issue: mattresses made with any form of polyurethane foam have a fixed and fairly short lifespan, especially vs. those made with latex foam.
The problem is that poly is inherently brittle and less elastic than latex, so every flex of the bubble walls contributes to breakdown. It doesn’t take long until enough bubbles are broken that the foam is collapsed and saggy. (Latex is vastly stretchier - think about the last condom you saw someone blow up to an insane size - and the bubbles take years to show significant degradation. Never buy another poly-foam mattress of any price or quality level again, kids.)
I’d suspect some equivalent; even if the sole material appears to be a solid it’s more likely a dense foam, and after a few hundred thousand flexings, the bubble walls are breaking. Why they would disintegrate on the counter is probably a function of the owner bringing them in just as they reach a critical point; a few more days of wearing would have had them disintegrate on the wearer’s feet.
If it’s the foam I think it is then it’s typical to see it become brittle over time. It will have a lot of additives that prevent discoloration and give it the initial flexibility to withstand the physical abuse a shoe takes. This reduces the amount of cross-linking of the polymer chains causing the material to degrade under physical stress. This stuff is different from the material used in couch cushions, it has much smaller bubbles in it to make it a flexible foam.
ETA: The manufacturers probably don’t mind the limited life of their product.
I had that happen to a set of Ecco boots. Hadn’t worn them for a year or so and the next time I put them on the soles fell apart in about two hours of wear. It was made worse by the fact that it was raining and I was walking on gravel.
There are academic papers on the subject " Microbial Enzymes Involved in Polyurethane Biodegradation: A Review"
That is, bacteria and fungi produce enzymes that break it down. That is why the clock starts ticking when they get dirty (eg soaked through with water or something. )
Maybe they chose polyurethane because it does break down, whereas rubber is the better choice for all other concerns.
I was an injection molding operator in a factory for some time.
Different materials (i.e. what was molded) had different properties. For example, “Estane,”[sup]TM[/sup] was flexible under cold conditions; “Estaloc”[sup]TM[/sup] was not. There were other materials that were molded, but the fact remains that they all had different properties and different costs.
My guess would be that somebody ordered a material for the shoe soles that was unsuitable for the conditions that the shoes would be used in. I have no idea why, but I’d bet cost played a part in the decision.
ETA: Just noticed that this is sort of a zombie. I’ll let my remarks stand, as they may be useful anyway.
What brand are they? I’ve had Ecco brand shoes for years and never had anything happen to the soles save wear and tear, which was considerably slower than normal rubber soles.
I wonder if the shoes are somehow off-brand copies of Ecco (or some other premium brand) and use lesser polyurethane that disintegrates.
