This article is part of FMC’s Recreation Transition campaign, a mission to encourage low-carbon recreation. Being mindful of the impacts your gear choices make is one of the ways to reduce your footprint; we encourage people to think about repair before replacement.
So why did my sole fall off?
Many midsoles (the part of the sole between the upper and outsole) are molded poly-urethane (PU).
Use of PU makes a lightweight, flexible sole which are not only shock absorbent but also extremely hardwearing, that’s why PU is ideal for boots.
One of the main technical challenges with polyurethane is overcoming an aging deterioration known as hydrolysis. Hydrolysis is the chemical breakdown of the PU polymer and the resulting physical breakdown or crumbling of the PU sole by the effect of water (usually in vapour form), occurring over a period of several years (even when unworn).
The process is accelerated by warmth and high humidity. It will therefore happen in confined spaces such as cupboards, wardrobes particularly if the boots are put away damp. In the most advanced state of hydrolysis, the PU sole will lose all its physical strength, thus cracking or crumbling.
So once its cracked, can the boots be repaired?
Yes! The repair is made by removing the damaged midsole completely and rebuilding another midsole from EVA (Ethylene – vinyl acetate) a co-polymer of ethylene and vinyl acetate. It is an elastomeric polymer that produces materials which have ‘rubber like’ in softness and flexibility. The material has good stress – crack resistance and waterproof properties.
The challenge for any cobbler is to clean and dispose of any old PU and then rebuild the midsole using a suitable EVA material. Several layers of EVA are used, cemented together to mould the new midsole then trimmed and applied before the new outsole is secured. The shank is also replaced at this point unless the boot has a thermo plastic shank which forms part of the insole for rigidity and flexibility.
The result is significantly more durable than the original mid-sole although there can be some loss of feel to the flexing of the boot compared to the original PU but this is more psychological than physical.