Hiking boots and Hydrolysis.
Why do some Hiking boot midsoles crumble?
This slightly self-indulgent article is one for the footwear nerds and anyone who has ever found that the cushioned, middle bit of their favourite boots has started to crumble into a kind of foamy, plasticky dust.
The midsole is the layer that sits between the leather or fabric upper, which contacts the foot, and the rubber sole unit which contacts the ground. Generally speaking, hiking boots and trek shoes are made with three different kinds of midsole depending on price, expected usage and manufacturing capability.
The most basic is also the most traditional. A layer of rubber, thickness varies, is glued directly onto the upper. This has the advantage of simplicity and low cost. It is favoured in boots where the manufacturer is trying to work to a budget and some things have to be trimmed to keep the price in line with market expectations. It may mean that a very high quality upper, featuring good materials and fit can be sold at a competitive price as the cost of producing and fitting the sole is low. Don’t mistake these items for poor boots. They are made with good, sometimes excellent materials and hardware just without the high-end bells and whistles that feature on top of the range products.
This boot features a rubber only sole unit. Durable but less comfy.
Probably the most common material used across the footwear world, from high altitude boots to flip-flops, is EVA. Ethyl vinyl acetate is the soft foam usually found in running shoes. It can be made in any density to provide different amounts of stability or cushioning underfoot depending on the function of the footwear. Hoka One One Running shoes for example have extremely soft midsoles for maximal cushioning but limited stability. Hi-Tec Enduro boots on the other hand, have a much denser EVA midsole so that they provide better support on uneven terrain.
EVA works best in lightweight hiking boots made for trails and less extreme hillwalking. The lightness is good but the foam breaks down over time thus reducing both the shock absorption and the stability. For occasional walkers or those doing trails like the Camino or the Royal Canal Way they are often the best combination of price, performance and comfort. If you walk a lot however, especially if you are backpacking on rougher ground, they will tend to wear down faster. This will bother some people more than others. As someone who started out in stiff, heavy 1970s boots I really like the lightweight stuff available now and am prepared to go light and work a bit harder to stay upright. You take your pick.
This EVA soled boot is light and comfy but less durable for regular backpackers.
A super light, super cushioned running shoe with an EVA sole.
The daddy of all midsole materials though is Polyurethane. PU midsoles maintain their stability and shock absorption properties for hundreds of thousands of steps. They maintain their shape and form virtually unchanged through their expected lifespan. This is the material found on all of the top end mountain walking boots on the market and a growing number of mid-range models as well. It is more expensive but better performing through greater use and a wider range of conditions.
Sounds great. I’ll take a pair.
Not so fast.
There are one or two minor negatives to be borne in mind before emptying the wallet for that lovely new mountain boot.
Polyurethane for footwear is manufactured in one of two types. There is either a Poly Ether or a Poly Ester version.
Poly Ether based PU generally gets used in better quality work boots. Ether based PU is good at resisting attack and degradation from the most common workplace chemicals and solvents like diesel, battery acids and cleaning agents.
Ester based PU, on the other hand is much better at resisting the kind of microbial attacks that come from bacteria found in such sources as animal manure and peat bogs and is also good at dealing with the everyday hassles of outdoor gear such as actinic damage from UV light whilst providing an appropriate amount of shock absorption and stability.
It is however virtually impossible to guarantee that work boots will never come under microbial attack or that hiking boots won’t ever encounter damaging chemicals. The farmer in steel toe boots will encounter both on a regular basis. Hillwalkers walk across messy forecourts to put diesel into vehicles so bear in mind that the type of PU midsole used is the best option rather than the perfect option. They are all good, they’re just not magic. The laws of chemistry still apply.
Hydrolysis and lifespan of footwear.
The big threat to polyurethane midsoles of whichever type is something called hydrolysis. This is a degradation over time, which causes the PU material to dry out, harden and become porous and absorbent to water. The ultimate result is a brittle, crumbly, chalky midsole which literally separates from the upper. The sole may fall off and the boot is utterly unusable until properly repaired.
Hydrolysis happens from the inside and so the effects may not be apparent until catastrophe strikes. The core of the sole unit breaks down unseen, and by the time the user notices it is time to replace either the sole or the entire boot.
The expected average lifespan of a PU midsole would generally be somewhere between five and ten years.
This includes all the time involved since the midsole was manufactured. Even in these days of “Just in time” manufacturing it is not impossible that a midsole gets sent from the factory to the shoemaker but then sits around for three months before being used on a boot. Shipping from shoemaker to distributor is anything from four days to three more months and the distributor may hold that pair in stock for a while before sending it out to a shop. How long it sits on a shelf in a store will vary depending on the shop’s footfall, the amount of footwear they hold and how well they rotate the stock.
It is not unusual for the first year of the midsole’s lifespan to have passed before the boot ever sees a hill. To get an idea of how old a new boot can be, check the date of manufacture of the boot where possible. It is often on the size label but some companies use a production code rather than a simple date so this is not always an option. Apart from a date there really is no easy way to know.
Dealing with Hydrolysis.
Hydrolysis is exacerbated by heat so if you do the majority of your hiking in Southern India or Morocco then you can expect a slightly reduced lifespan. For most Irish hillwalkers extreme heat is not a problem except at home. Store your boots out of the light in a cool, well ventilated place.
Clean your boots. I know that when you get home after a long day out on the hill a cool beer or nice cup of tea is a much more attractive proposition than washing and waxing your boots but, limiting the amount of animal manure and reducing the time it spends on your footwear is enormously beneficial.
Don’t keep them for good wear. The more you use your boots the less of an issue hydrolysis is. Buying two pairs you like and keeping one in a cupboard for a few years is an absolute waste of your money. They are made to be worn. Stick them on your feet and get out there.
Repairs are possible. If the worst has happened and your good boots are now unusable then consider a full resole from a shoe repairer using the original or similar sole unit. Any cemented construction boot can be fully resoled and will come back as new. No Irish company offers this service unfortunately but some UK companies do a fantastic job and using Parcel Motel or a similar service eliminates the high cost of cross border postage.