If you’ve ever pulled out an old pair of sneakers and found the soles stiff, yellow, or even falling apart like cake, you’ve witnessed how sneaker materials break down over time. It’s more science than wear and tear.
1. From Flexible to Brittle
Soles are made from polymers — long chains of molecules designed to flex and cushion. Over time, oxygen, UV light, and heat break these chains into shorter ones, a process called depolymerisation.
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Long, flexible chains = soft and springy soles
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Short, broken chains = stiff, brittle soles
That’s why older sneakers often feel harder and more fragile, even if they haven’t been worn much.
2. Why Soles Get Slippery
As polymers oxidise, they form a thin, glassy film on the surface. This hardened skin reduces grip, making soles feel slick on smooth floors. At the same time, the oils and plasticisers that once kept the material flexible evaporate or chemically bind, leaving the sole drier and less able to “bite” the ground.
3. The Truth About Discoloration
That yellowing or browning isn’t caused by a new pigment forming — it’s an optical effect created by chemistry.
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Oxidation changes the surface composition, forming new chemical groups like carbonyls and quinones.
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These absorb and reflect light differently: fresh soles scatter light evenly (bright/clean), while aged soles absorb more blue light and reflect more yellow/brown.
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Microcracks and roughness scatter light unevenly, amplifying the effect.
So the color appears to change, but what’s really changed is the material’s composition and how it reflects light back to your eyes. This is also evident in mesh used for some shoe uppers and embellishments. (Jordan 4 cage)
4. When Soles Crumble Like Cake
Some midsoles, especially those made of polyurethane (PU), suffer from another process: hydrolysis. Moisture in the air attacks the polymer bonds, breaking them down from the inside out.
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At first, the PU hardens and loses its bounce.
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Then, as more bonds snap, the structure collapses into powdery chunks.
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That’s why neglected sneakers sometimes “cake” or crumble the moment you try to wear them.
Unlike yellowing or surface oxidation, PU crumbling is irreversible — once the polymer structure collapses, the sole can’t be repaired. (unless you sole swap)
Tips to reduce oxidation
Store Correctly
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Keep sneakers in a cool, dry environment (around 18–20°C).
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Avoid humid basements, hot attics, or sealed plastic bags.
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Use breathable sneaker boxes or cotton dust bags — let air circulate.
Limit UV & Ozone
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UV light speeds up oxidation and yellowing → store away from windows.
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Avoid areas near electronics or motors (ozone-producing) — ozone is especially harsh on rubber.
Rotate & Wear Them
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Occasional wear helps. Flexing the soles redistributes oils and additives, keeping them from stiffening.
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Pressure from walking or light activity can even encourage some polymer chains to rebind or realign, slowing brittleness.
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Deadstock pairs left untouched often degrade faster than sneakers that get gentle use.
Keep Them Clean
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Dirt, sweat, and grime can accelerate chemical reactions.
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Regular cleaning with a gentle sneaker solution helps preserve the surface and slow oxidation. (Our formula is available soon)
Support with Inserts
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Use shoe trees or stuffing to maintain shape. This reduces stress cracks in brittle soles.
Bottom Line
Aging soles don’t just get dirty — they chemically change.
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Depolymerisation makes them brittle.
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Oxidation makes them slippery and discolored.
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Hydrolysis makes some soles crumble like cake.
But with the right care — proper storage, occasional wear, cleaning, and support — you can slow these changes and keep sneakers soft, grippy, and looking fresh for years longer.
At Sneaker Pharm, we focus on not just cleaning, but preserving sneakers, so your collection lasts as long as you do.
Unfortunately when shoes reach this state there is little that can be done for the soles to keep them as is from factory. (Spare parts aren’t available m without donor shoes).
Ive spoken about it previously but I’d love champion a solution where brands can supply components to authorised repair centres and make spare soles available for popular silhouettes available for purchase to repair.
In the meantime we offer aftermarket solutions with sole swaps for Vibram products. The pinnacle industry leader of sole design and manufacturing. Silhouettes will change but that introduces a uniqueness to your shoes which we like to celebrate