Performance Midsoles

Selection, Performance, Care, and What to Look For

Meindl has been building exceptional footwear for over 300 years only using premium materials to make our boots best in class. Whether that is using Gore-Tex® waterproof membranes, Vibram® outsoles, premium insulation, etc. to make sure our boots perform no matter the condition you are in. When purchasing boots these are all technologies you, the customer, have been educated on, and for good reason. Your feet deserve the best and your boots should not be a piece of equipment you have to worry about while at the top of the mountain. When doing your research on what the best boot is for you, how many of you have heard of the midsole and how important this layer of the boot is for your feet as well as the life of the boot? We feel by not educating our customers on the midsole, we are not giving you the proper tools to make the best decision on your next purchase.

What is a midsole? This is the layer of material between the inner and outer soles of a shoe. The midsole of your boot/shoe absorbs shock and adds a layer of protection, structure, and support between your feet and the ground.

When it comes to midsoles there are two main materials used when constructing high performance footwear, like you'll find at Meindl - EVA (ethylene vinyl acetate) and Polyurethane (PU). Each material is used for different reasons and purposes, and it all comes down to the end use for each specific boot. What is the terrain the boots are built for? What are the elements the boots will primarily be used in? Just to name a few. Like any material used there are the extreme benefits that each material provides, but there can be potential downsides as well.

Materials

EVA (ethylene vinyl acetate) is the most common material used in footwear midsoles. You will find EVA in all styles of footwear from alpine boots to walking shoes. EVA is a lightweight foam material made up of thousands of compressed bubbles. Each bubble is filled with gas that makes it lightweight as well as provides excellent cushioning and shock absorption for your feet. What makes EVA so popular in the footwear industry? EVA is extremely compressible and can be easily manipulated during the manufacturing process. Whether you need a high-density EVA for an alpine boot a low-density EVA for a walker you can use the same material.

Alpine Boots Pros of EVA

When it comes to using EVA there are some advantages and specific reasons why this material is used in performance footwear. EVA is more lightweight compared to PU as well as provides improved flexibility. This is what makes EVA perfect for light hiking and running shoes where you don't want that extra weight. In addition, EVA will absorb more shock from step impact relieving the stress on your joints as you are out on your hike.

Cons of EVA

As we stated earlier there can be disadvantages to every material used; with EVA the biggest downfall is a relatively short life span. EVA holds a memory, meaning after every step the material will compress and rebound. After thousands of steps, the ability for the EVA to rebound is reduced, resulting in your footwear not having the same cushion and support you have been accustomed to. As that material compresses over time there will not be noticeable defects to the midsole, but your footwear is going to become less supportive and less comfortable, to where they can feel "flat" on your feet. As the EVA loses its cushioning and support it is time to either replace your footwear or if boot allows, have the soles recrafted. Like all materials this is a natural process and the reason many recommend replacing shoes footwear using EVA more often, to ensure you have the proper support needed.

PU (polyurethane) is a synthetic plastic material commonly used by the best boot makers for midsoles in high performance trekking boots. Polyurethane is a much denser material compared to EVA or other materials used in hunting and hiking boots today. PU is most found in high performing boots (alpine boots), however is becoming more and more popular in mid-range boots as well. The majority of Meindl USA boots today use polyurethane due to the performance as well as how long these boots will last.

Pros of PU

Opposite of EVA the biggest advantage that PU offers is it does not hold a memory - meaning your midsoles will always rebound back to the same shape and provide the same cushion from day one through the lifespan of that boot. Also, with PU midsoles you are going to have a lot longer life on your midsoles and can get more years and miles with the same comfortable fit compared to an EVA midsole. The density and rigidness of PU provides more lateral support as well as removes almost all possibility of stone bruising in your feet. Simply put, polyurethane is the perfect material for technical boot midsoles. It is flexible, resilient, cushioning, and long lasting. When constructing a high-performance hunting/hiking boot the best midsole you can use for performance is PU.

Cons of PU

Just like EVA, PU over time can break down, the only difference is when this happens it is clearly visible to the point where the soles may come off, much more visible than EVA. This is a process called hydrolysis, a gradual deterioration and disintegration that can lead to the midsole detaching from the outsole or crumbling from the inside out. This is a process that occurs over years. With PU (and many other compound materials), moisture will find its way through the smallest of pores and chemical reaction, or breakdown, begins and microscopic cracks occur. Ultra-violet rays of the sun, extreme heat and other factors also encourage hydrolysis, hardening the PU to a point tiny cracks develop. To be clear this is part of the natural process, not a defect in material or manufacturing. When you think of an old set of tires that sit for years, you wouldn't expect those tires to be the same as the first day you got them. Same goes for your PU midsoles.

How to Extend the Life of Your Midsoles?

With both EVA and PU midsoles it is a natural aging process that results in the midsoles breaking down. As we stated with EVA, the midsoles hold a memory, meaning every step you take results in those midsoles compressing. Other than not wearing your boots/shoes (which is not an option) there is nothing to slow down the process of EVA compressing and becoming less supportive and comfortable.

Regarding PU midsoles, the best steps you can take to slow down and help prevent hydrolysis from taking place is by WEARING your boots. When wearing your boots, the midsole is getting a workout, flexing in the fresh air as you walk. With the air being worked in and out of the pores it is preventing any moisture from building up inside those midsoles, and therefore, helping prevent hydrolysis from occurring. When you are not wearing your boots, there are a couple critical steps to keep your PU midsoles from breaking down:

Store your boots in a dry, cool, ventilated space. Not near a heat source; not in a humid environment. And while in camp, keep them away from the campfire (or any heat source at home). Do not use a hair dryer, heated boot dryer or any heat source to hasten drying.
Keep your boots clean. Clean them regularly, especially after hard use in wet, muddy environments. And for Pete's sake, clean off any mud, manure, or other muck you may have encountered. You don't want to track that in the house anyway. Click here to learn more on how to clean and condition your boots.

Tips to check for Hydrolysis

With so many variables it is impossible to predict when hydrolysis will advance enough to cause soles to detach. We have well-worn boots we have used for more than a decade that show no outward signs of hydrolysis decomposition.

If you've had your boots for several years, you should inspect them (or have them inspected by an expert). A tell-tale sign that is easy to overlook is fine granular black specks on the floor or shelf where you store your boots. Like we said, easy to overlook because most hydrolysis occurs inside the midsole. Visible specks are a sign of advanced hydrolysis on the outer edges.
If your boots have not been used for years, we recommend you wear them on a test hike or for a few days of everyday wear before venturing far off the beaten path to determine if the soles are solidly attached or if any signs of hydrolysis damage become apparent.
Or you can perform a flexibility test by using a blunt pointed object, like a ballpoint pen with the ink tip retracted, key or similar object, into the midsole. DON'T use anything sharp, no knives, pointed pencils, etc. Push the blunt pointed object into the midsole, which may be easiest found on the heel wedge. The polyurethane should give under moderate pressure and rebound once pressure is lifted. If it has hardened, flexibility on the push in and on the rebound will be limited or nonexistent.

Once the process starts and your midsole begins to break down, there is nothing you can do to stop it. The good news is, Meindl Boots can be recrafted, and your well-broken-in boots can be given a new life. If you have any questions visit our FAQ page to help answer any additional questions. As always you can give us a call at 1-833-563-0101 or email us at info@meindlusa.com.