A guide for retailers and distributors on how to advise customers to avoid sole degradation

Richard Kushlick runs footwear education and training programmes for all tiers of the industry (https://footweareducation.co.za/).
Starting this month, he will write columns on various topical subjects for S&V.
In this issue, he covers a complaint which retailers often face from consumers – sole degradation in soles made from crepe rubber and polyurethane (PU).
Although, to the eye, they look the same, the technical factors behind the degradation are different, although the best advice to give customers is surprisingly similar. 

A guide for retailers and distributors on how to advise customers to avoid sole degradation

In the modern age of Google, Copilot, ChatGPT and a host of other AI assistants, it’s more important to have a better understanding and knowledge of the footwear being sold out of your store or warehouse. Yes, we have pseudo professors of everything nowadays, many of whom are all the better for having done the research. There are however those that tend to take this information to the next level, and with their vast knowledge, should be enrolling at MIT or Harvard. Jokes aside, the following series of short articles are there to ensure we who are responsible for selling footwear, can proactively act and advise before these processes manifest in your customers shoes.

The most commonly used materials in the manufacture of soles and midsoles, are rubber and polyurethane (PU). These materials are available in various blends and polymers, depending on the specifications required for the particular footwear type. I trust this brief explanation will suffice in your efforts to satisfy most of your consumer enquiries in this regard. Both Rubber and PU are subject to decay, which shouldn’t be confused with wear. Decay indicates a chemical change in the structure of the material, while wear is normally associated with abrasion and pressure. The advantages of Crepe Rubber and some latex polymers are generally associated with being abrasion resistant, durable and with excellent shock absorption properties; whilst PU has a better combination of shock absorption and cushioning, being slightly less durable and abrasion resistant. There is of course a weight / mass difference between these materials, rubber being heavier and denser in most cases. There is a distinct difference between rubber and PU degradation (PU Rot), in that Rubber can harden and crack over time, whilst PU goes through a chemical process, namely hydrolysis. Hydrolysis is a process where water molecules attack the chemical bonds in PU. Over time this softens the material and then causes it to become sticky, brittle, and eventually crumbly.

For these reasons of comparison, there are two theoretical examples that for reasons pertaining to this article are cement lasted shoes, one with a PU unit attached and the second pair with a Crepe unit attached. Simple enough. Both pairs were purchased on the same day at the same time at the same store. Let’s call it on sale, where the prices were so low and the salesperson so enthusiastic, that he couldn’t refuse. To further elaborate, the customer placed both pairs in a cupboard, as he / she / they / them intended to use the shoes later (3 to 6 months). What could possibly go wrong, safe to say that both pairs were brought back for a refund or claim, since the shoes hadn’t been worn in the six-month period since purchase, yet there was noticeable deterioration in the soles of both pairs. Had the salesperson been better informed, this situation can easily be avoided, however now that the damage is done, here’s what could have been done and printed on a leaflet in the box.

Shoe 1 – with crepe rubber sole. [Photo: https://nushoe.com/how-to-fix-cracked-rubber-sole/]

1. Oxidation of Natural Latex Polymers (Incl Crepe Rubber)

Mechanism: Over time, oxygen slowly reacts with the unsaturated bonds in natural rubber.
Result: Cross-linking of polymer chains → rubber hardens and loses elasticity. Accelerated by: Warm storage (above ~25 °C)
Poor ventilation (stale air, oxidative buildup)
Advice: Keeping shoes in a box in a cupboard is unwise. Better to store in an airy closet, where fresh air can circulate.

2. Loss or Migration of Natural Plasticisers & Resins

Mechanism: Crepe contains natural oils, resins, and proteins that give it softness and resilience. With age: Oils migrate to the surface and evaporate. Resins oxidise and stiffen.
Result: The sole dries out, feels harder, and loses spring.
Advice: Ensure the shoes are used on a regular basis (weekly). Movement ensures the shock absorbing properties and spring last much longer.

3. Light & Ozone Exposure

Mechanism: UV light breaks down proteins/resins, darkening and stiffening the crepe.
Ozone (from electric motors, fluorescent lights, etc.) causes chain scission and surface cracking.
Result: Even if not cracked, the crepe feels less responsive.
Advice: Store in natural shade light (not direct sunlight). This ensures the hardening process is greatly reduced.

4. Compression Set During Storage
Mechanism: If shoes are stored under their own weight (soles compressed against a surface or stacked), the rubber network slowly deforms.
Result: Permanent flattening and reduced rebound.
Advice: Store the shoes on a carpeted or cushioned surface.

5. Humidity & Microbial Factors
Mechanism: In slightly damp conditions, residual sugars and proteins in crepe can encourage mold / mildew, which excretes acids that further degrade latex.
Result: Stiff patches, loss of uniform resilience.
Advice: Regular use and a dry storage area will reduce degradation.

6. Infrequent Use
Mechanism: Rubber benefits from occasional flexing, which redistributes oils and keeps the network mobile.
Result: Shoes that sit unused most of the year age faster than those gently worn more regularly.
Advice: Very simple, wear the shoes regularly or at least flex the soles to ensure responsiveness.

Always Try to Do the Following for Crepe and Other Rubber Soled Shoes:
• Store in a cool, no direct sunlight (shaded), dry, ozone-free environment.
• Avoid stacking or compressing soles.
• Lightly flex or wear them every few months to redistribute oils.
• Store in cotton or kraft paper bags (not plastic, which traps migrating oils).

Worn Crepe Rubber Sole (difficult to see degradation, hardening or surface cracking - left) – Clear image of PU hydrolysis (crumbling sole material right)

Shoe 2 – with PU Unit Sole.

1. Heat
Mechanism: Increases molecular mobility of water and PU chains, accelerating ester bond cleavage.
Result: Faster onset of softening → sticky midsoles, loss of cushioning, then crumbling.
Advice: Store PU footwear in cool, stable environments; avoid hot attics, near heaters, or direct sunlight. All cool shaded cupboard is advisable.

2. Humidity
Mechanism: Provides the water molecules required for hydrolysis to occur.
Result: Continuous water uptake → bond breakdown, visible disintegration over months/years.
Advice: Use desiccants (silica gel, dry packs) in shoe boxes; keep storage rooms below 50% relative humidity. In humid environments, store shoes where there is free movement of cooler air.

3. Poor Ventilation
Mechanism: Traps moisture and acidic by-products inside boxes or cupboards, creating a self-catalytic loop.
Result: Hydrolysis accelerates, leading to sudden catastrophic crumbling when shoes are taken out after storage.
Advice: Store in well-ventilated spaces or breathable containers (cotton bags, vented boxes) instead of sealed plastic.

4. Lack of Use
Mechanism: Shoes not utilised, flexed or aired out → absorb moisture and breakdown products remain locked in the PU.
Result: Hydrolysis accelerates “silently”, so soles may collapse on first wear after years in storage.
Advice: Periodic use (even light wear) or at least occasional airing helps release trapped moisture and extend PU lifespan

Always Try to Do the Following for PU Soled Shoes:
• Store in a dry, cool, no direct sunlight (shaded) environment.
• Lightly flex or wear them every few months to reduce crumbling.