What soles are made of: Your guide to footwear tech lingo
From leather and polyurethane to carbon fiber and kevlar, we explain the science behind footwear's go-to materials.
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Here’s our layman’s guide to today’s footwear tech lingo for your inner scientist — whether you’re working the sales floor or updating inventory for next season.
The more technical outdoor footwear gets, the more its product descriptions resemble alphabet soup. PVC, EVA, PU, TPU … what’s the difference?
Even leather, the figurehead of the hiking boot industry, comes with its own set of jargon. The “top grain” is the tighter, tougher external surface of the animal’s skin. Roughing up the top grain will produce abrasion-resistant Nubuck, while taking the sander to the inside of the hide yields suede, Nubuck’s cheaper, stretchier, more absorbent cousin. Leather is breathable, develops softness and character over time, and has a classic look many customers prefer, but comparatively cheap, low-maintenance synthetics draw many buyers away.
Polyurethane (PU) is one of the most common polymers used in shoes, and for good reason — it’s lightweight, durable, and waterproof. PU is also remarkably oil-resistant, which makes it a great ingredient in boots designed for construction or industrial use.
Thermoplastic polyurethane (TPU) is a subset of PU. TPU can be re-melted and remolded even after it’s solidified, which makes it easier to work with and opens up an array of processing possibilities.
PU comes in two flavors: polyester polyols and polyether polyols. The polyester system is cheaper, and it withstands high temperatures well. Polyester will break down in wet environments, though, so shoemakers marketing to tropical adventurers should opt for polyether. Polyether polyols are more abrasion-resistant and impervious to the discoloration and breakdown caused by microbes that thrive in a well-loved shoe. Polyether systems also maintain their flexibility better at colder temperatures.
Cons? PU is somewhat UV-sensitive, and without UV-blocking pigments or additives, PU will grow yellow and brittle after long exposure to sunlight. However, its durability means it takes centuries to decompose completely — a concern for environmentally-conscious buyers. And though PU is typically a good intermediate between plastic and rubber, it tends to be on the tougher, stiffer side and is therefore better suited to hardwearing soles or support pieces like shanks, heel plates, and midsoles.
PU can be aerated and solidified into a soft foam, but the cushiest midsoles tend to be ethyl vinyl acetate (EVA). EVA has more spring to it than PU, which makes it ideal for trail runners. Like TPU, EVA can be compression-molded (CM), and CM EVA marries cushion with a glove-like fit. However, repeated impact will compress EVA layers much faster than the denser, heavier polyurethanes, which take the lead where longevity is concerned.
Polyvinyl chloride (PVC) is a strong, inexpensive polymer often used as a component in uppers and outsoles. It’s most famous for the plastic piping of the same name, but additives turn that sturdy plumbing staple into a flexible, inexpensive rubber substitute. Unfortunately, the same phthalates used to soften the PVC can leach out, and research has linked them to developmental defects in children. It’s also more expensive to recycle PVC than to produce new material, so this talent for ending up in landfills makes disposal an environmental concern. While PVC is a perfectly legal shoe ingredient, many companies — like Nike, Patagonia, and KEEN — have pledged to phase it out.
Knits like Nike’s Flyknit and Adidas’s Primeknit offer a greener alternative to the minimalist buyer. The outcome is a lightweight shoe that looks like it’s been crocheted onto the sole. Different weaves provide a gradient of support across the shoe. And unlike sheets of plastic, woven polymers let feet breathe. Manufacturers weave these shoes from the ground up in one location. That means lower shipping costs, reduced likelihood of cheap foreign labor, and less material waste.
Most knits are Nylon, which is self-lubricating, strong, and flexible but susceptible to chemical and UV breakdown. Nike supplements Nylon with Vectran, a component of helicopter rescue hoist cables and astronaut safety tethers. In the future, knits could incorporate wool, Kevlar, or even steel threads.
Kevlar (of bulletproof vest fame) has rigid, rod-like molecular structures that allow a degree of molecular organization higher than that of polymers. The resulting material can withstand much higher pressures than Nylon or polyester, even point pressure from the trail’s sharpest rocks. No challenge for something that’s bulletproof. Salomon uses Kevlar laces on its trail runners, and Zamberlan USA takes advantage of the material’s abrasion resistance on hiking shoes and its 6000 Denali RR mountaineering boots.
Not strong enough? Give Dyneema a try. UHMwPE (ultra high molecular weight polyethylene) is 40 percent stronger than Kevlar by weight. This abrasion resistant, chemical-resistant, UV-resitant polymer prodigy even floats on water. It’s the same material used in climbing harnesses and webbing, and mountaineering outfitter Mammut broke into the sneaker market by putting it in their trail runners. The trade-off? Dyneema isn’t cheap. And while it’s self-lubricating like Nylon, it has none of Nylon’s elasticity.
Carbon fiber is yet another industrial grade material that’s slowly making its way into footwear. It’s light and durable, and it withstands torsion well: a boot with a carbon-fiber support system is armored against turned ankles. Another benefit? The name is a lot easier to remember than UHMwPE.