Material Matters: Foam Will Eventually Crush Your Sole
We’ve all seen pictures of crumbling classics on Instagram. We’ve all seen NBA stars blow out their soles on the court. And it’s always a sad, sad sight indeed as those kicks head to the graveyard. What’s even sadder – and scarier – however, is this: it can happen to you, too. In fact, it will happen to you, too – and there’s absolutely nothing you can do about it.
Now, we know what you’re thinking: you take good care of your collection, right? Maybe you’re even the rare ‘head that cleans your kicks after every wear and stores everything in plastic drop-front boxes. Maybe you even go the extra mile to make sure your prized possessions are kept in the perfect area: up and off the floor, away from direct sunlight, and in dry and moderate conditions. You’re good, right? Your collection is going to last forever, right?
And you’re wrong because no matter how well you maintain your stock, there’s one thing you can’t account for: time.
Time, Why You Punish Me?
Blowouts – Zion’s PG 2.5s notwithstanding – generally don’t happen to newer kicks; they happen to old ones. The same goes for crumbling soles, too. And there’s a reason for this.
All materials have a shelf life of some sort and this is especially true of those used for something like footwear, which not only has to stand up to the elements, but also has to withstand constant pressure and impact. Some sort of decay and breakdown is only natural given the general wear and tear that a pair of shoes experience.
But there would seemingly be a big difference between, say, your everyday running shoes and your favourite pair of retro Jordans that you rarely wear, and keep in pristine condition. You would expect the running shoes to fall apart way before those AJ3s, right? Unfortunately, though, there is no difference. In the end, your kicks are eventually going to fall apart, no matter how well you take care of them. For sneakers, time does not heal all wounds.
And that’s because the shelf life of your shoes’ synthetics, and the foam-based soles in particular, isn’t particularly long.
PU, Boo Hoo
Synthetic materials have been the go-to for sneakers for decades, as brands across the globe have sought to create the lightest, highest performing designs possible. But all of that weight shedding and performance comes at the expense of the long-term shelf life of the shoes – especially with regards to the cushioning.
Polyurethane (PU) appears in just about every sneaker you can think of, and it’s popular for a bunch of reasons. Not only is PU lightweight and flexible, it’s also pretty good at absorbing shock and resisting abrasion. It’s even relatively slip-resistant. Regrettably, though, PU is particularly susceptible to both hydrolysis and oxidation, not to mention what’s scientifically referred to as ‘Environmental Stress Cracking’, or ESC. As a 2011 study on PU laid out:
‘Poly (ester) urethanes and poly (ether) urethanes, which are widely used for long-term applications, have been shown to degrade under hydrolytic conditions and in oxidative environments, respectively. In addition, ESC of polyurethanes is also another important way of polyurethane degradation. Degradation can lead to significant changes in the polymer mechanical properties, surface chemistry, and structure, leading to malfunction.’
So, unfortunately for your kicks, two of the most vital things on earth – water and air – are incredibly hard on PU. Even with optimal conditions, there’s really nothing you can do to stave off the PU’s eventual breakdown – or even the yellowing of the other synthetic materials on your kicks, for that matter.
As WIRED pointed out a few years ago, even museum-level conditions – 68 to 72 degrees Fahrenheit with 45 to 55 per cent relative humidity – can’t help: That level of humidity is still too high, and using a bunch of silica gel packs would have minimal effect.
And don’t think for a second that newer cushioning technologies are immune, either. While it’s generally believed that newer formulations like BOOST and React have a longer lifespan than their other foam-based predecessors, just how much longer that lifespan will be remains unclear. They are still synthetic foams, after all.
Solution: Rock ‘Em
As a Northwestern chemistry professor hilariously said in that same WIRED piece, there’s really only one solution: an airtight steel vessel filled with argon. Bummer.
So unless you truly just want something pretty to look at on a shelf or to show off to your friends (or if you’re cool with paying for sole swap after sole swap), you should probably just go ahead and rock ‘em, not stock ‘em.
This feature was originally published on August 2, 2019.