Polyurethane (PU) is a versatile synthetic polymer used in nearly every part of a shoe, from the cushioning midsole to the outer sole to the upper material that mimics leather. It’s made by reacting two chemical compounds (an isocyanate and a polyol) to form a flexible, durable material that can be tuned to be soft and spongy or firm and rugged depending on how it’s manufactured. If you’ve worn work boots, running shoes, or dress shoes with synthetic leather, you’ve almost certainly walked on polyurethane.
Where PU Shows Up in a Shoe
Polyurethane isn’t a single material with one job. Manufacturers use different formulations of it in different parts of the shoe, each optimized for a specific purpose.
In midsoles, PU appears as a microcellular foam, meaning it’s full of tiny air pockets that compress under your weight and spring back. This is the layer between your foot and the ground that provides cushioning. In outsoles, it takes a denser, tougher form designed to resist abrasion from pavement and provide grip. And in uppers, a thin polyurethane coating is applied over a fabric base (cotton, nylon, or another textile) to create what’s commonly sold as “PU leather” or synthetic leather, a material that looks and feels similar to animal hide at a fraction of the cost.
PU Leather vs. Real Leather
PU leather is made by laminating a layer of polyurethane resin onto a base fabric using high pressure and heat. The result is a smooth, uniform surface that can be textured to imitate the grain of genuine leather. It’s lighter, cheaper, and doesn’t require animal products, which makes it popular in everything from casual sneakers to dress shoes.
One practical advantage is water resistance. The polyurethane coating prevents the material from immediately absorbing water the way untreated leather does, though it’s not fully waterproof. Water can still seep through seams and stitching over time. PU leather also tends to be easier to clean, since you can wipe the surface without worrying about staining.
The tradeoff is breathability. Genuine leather allows more airflow, which matters if your feet run hot. PU leather has improved significantly over the years, but it still traps more moisture against your skin than full-grain leather.
How PU Compares to EVA
The other dominant cushioning material in footwear is EVA (ethylene-vinyl acetate), and the two fill different roles well. EVA is significantly lighter, which is why it dominates in running shoes and athletic footwear where minimizing weight reduces fatigue. PU is heavier but far more durable, making it the standard in work boots and shoes built for longevity.
The biggest difference over time is compression. EVA foam gradually flattens under repeated pressure, losing its cushioning as the tiny air cells collapse. PU foam resists this much better, maintaining consistent shock absorption even after months of heavy use. If you’re on your feet for long shifts on hard surfaces, PU midsoles hold their shape and support noticeably longer than EVA. For lighter daily wear or weekend runs, EVA’s weight advantage usually wins out.
Grip and Slip Resistance
PU outsoles perform surprisingly well on slippery surfaces. A study published in the journal Footwear Science tested shoes with polyurethane, thermoplastic polyurethane (TPU), and vulcanized rubber outsoles on steel and tile surfaces coated with glycerine and canola oil. The PU outsole had 61 to 125% higher friction on contaminated surfaces compared to rubber, making it the least slippery option tested. That’s a significant margin, and it helps explain why PU outsoles are common in restaurant, hospital, and industrial footwear where wet or greasy floors are a daily hazard.
TPU and Energy Return in Running Shoes
Thermoplastic polyurethane, or TPU, is a variation that can be melted and reshaped, unlike standard PU which permanently sets during manufacturing. This property allows TPU to be formed into expanded foam beads or plates that store and release energy with each stride. It’s the material behind many of the “super shoes” that have reshaped competitive running over the past several years.
Research comparing TPU midsole shoes to minimalist footwear found that runners used about 2.4% less oxygen at the same pace in the TPU shoes, a measure of running efficiency. Across multiple studies, TPU midsoles have improved running economy by roughly 1 to 5% compared to conventional foam. That range might sound small, but for competitive runners, even a 1% efficiency gain can translate to meaningful time savings over a marathon distance.
The Hydrolysis Problem
Polyurethane has one well-known weakness: it breaks down over time through a process called hydrolysis. Moisture in the air slowly reacts with the chemical bonds holding the polymer together, and eventually the material crumbles into a sticky, flaky mess. This can happen even if the shoes are sitting unworn in a closet.
The timeline varies depending on the specific PU formulation, storage conditions, and humidity levels, but most PU soles begin to degrade noticeably after five to ten years. Shoes stored in hot, humid environments break down faster. You’ve probably seen this if you’ve ever pulled an old pair of sneakers out of storage and found the soles disintegrating. The industry tests for this using accelerated aging methods (the SATRA TM344 standard, for example, exposes samples to warm, humid conditions over weeks to simulate years of natural aging).
This is worth knowing if you buy shoes to keep for years. PU-soled shoes have a shelf life even without wear. Rubber outsoles don’t have this problem, which is why vintage rubber-soled shoes can last decades while PU-soled shoes from the same era often can’t.
Quality Standards for PU Soles
Footwear manufacturers aren’t guessing when they formulate PU soles. The industry follows ASTM D3851, a specification that sets minimum physical property requirements for microcellular polyurethane soling materials. This standard covers density, tensile strength, elongation (how far the material can stretch before breaking), tear resistance, hardness, and abrasion resistance. Shoes that meet this standard have been tested to ensure the PU will hold up under real-world conditions, though not all manufacturers advertise whether their products comply.
Sustainability and Recycling
Traditional polyurethane is petroleum-based, and recycling it has historically been difficult because most PU formulations are thermoset plastics, meaning they can’t be melted down and reformed. That’s changing. Bio-based polyurethane made partly from plant-derived raw materials is gaining traction in the footwear industry, and major chemical suppliers are investing in recyclable PU formulations. TPU, because it’s thermoplastic, is inherently easier to recycle than standard PU since it can be remelted and reshaped.
These shifts are still early-stage for most consumer footwear. If sustainability matters to you, look for brands that specifically mention bio-based or recycled PU in their materials list rather than assuming “polyurethane” on a label means the shoe is recyclable.