Polymer testing answers questions about shoe quality

They say that “shoes make the man,” but what do you know about how shoes are made? Everyone knows that there are different kinds of shoes for different occasions. Going to work? Wear something comfortable but stylish. Jogging? You’re going to want to maximize support and reduce impact. Getting married? Fashion before function dictates that you’re probably going to look fantastic and feel like you’re walking on glass. But what goes into the anatomy of a good shoe? In order to answer that question, you have to start with the most important part: the sole.

People say she’s crazy she’s got plastic on the soles of her shoes?

The soles of a shoe can tell a lot about the person who wears them. In fact, orthopedic doctors can analyze the way you walk based on the wear on your sole. If you have a limp or you drag your foot, your doctor will be able to tell just from looking at your shoes. You can also tell how long someone has been wearing a pair of shoes based on the state of the soles. According to The Hanger Project, a blog all about shoes, you can tell when a shoe needs to be replaced when it starts to take on a spongy appearance. Based on the quality of your shoe’s soles, this can happen after anywhere between a few months to a few years.

Most shoe soles these days are made out of high quality, dense foam, which is composed of billions of tiny polymeric air cells. All of these cells support each other and provide the telltale spring that is associated with new shoes. Over time, these cells compress and lose their elasticity, which means they can no longer bounce back to their original form when you step on them. Shoes with higher densities of these air cells will take longer to wear out, which is why those cheap flip-flops you bought on vacation had entirely flattened out in the shape of your foot after only a week, while high quality running shoes will feel brand new for months after you buy them. At least that is the theory, but what about the science behind it? Is it even possible to increase the density of these air cells, and if so, does it lead to a higher quality shoe?

Counting air cells
That was exactly the question that was posed to our plastics testing experts here at PSI, by a shoe manufacturer. In order to answer it, we had to determine exactly how many air cells were in the sole of a shoe. We identified two steps in order to make this determination. First, we needed to pinpoint the volume of the sole, then we needed to identify the size of the cells in the sole.

Volume was the easy part. That has, more or less, been the same procedure since Archimedes had his eureka moment in the bathtub. Scanning the air cells in the foam, however, was significantly more challenging. In order to accomplish this, we needed to use scanning electron microscopy, but they first had to open the sole without damaging the air cells. Cutting the sole with something like a bandsaw would destroy the air cells by ripping them, thus throwing off any reading that could be gathered regarding their size. Tearing the foam would yield similar problems since the air cells would be disfigured by the process.

The solution? Freeze fracture. When frozen in liquid nitrogen, the air cells would shatter instead of pull apart. So, using a large blade and heavy force, the our materials scientists were able to prep the sample for image analysis with scanning electron microscopy in our polymer testing lab. Finally, we were able to get an accurate measurement of the air cells, and the answer may surprise you. There are as many air cells in a single shoe sole as there are people in the world, a little bit over seven billion.

Of course, when you’re going to the store to pick up some new duds, you’re not going to have time to freeze fracture the soles and count the air cells, but it’s interesting information nonetheless. At the very least, take some time to consider the density of the soles and give them a quick, non-scientific squeeze test to see if they seem like high quality.