PU soles have conquered the shoe industry—and the market keeps growing

What are polyurethane soles?

Polyurethane soles are an innovative, tailorable class of shoe components that are quickly becoming the preferred choice of footwear manufacturers around the world.

Thanks to their durability, flexibility, and comfort, polyurethane products are widely used for insoles, midsoles, outsoles, and more. You can find polyurethane soles in shoes of every type, from slippers and sneakers to work and safety shoes.

To understand what’s driving their takeover of the footwear manufacturing market, let’s take a look at the various types of PU soles and their benefits. For a differentiated picture, we’ll also cover their disadvantages and how they compare to other common sole materials.

Polyester vs. polyether PU soles

Polyurethane is produced through a reaction of isocyanates and polyols. Various polyols can be used in this process to adjust the molecular weight and material properties of the resulting polyurethane, which is most commonly either polyester PU or polyether PU.

Polyester PU soles come in densities ranging from 280 to 1,100 g/L and can be used as insoles, midsoles, and outsoles. They offer improved chemical, oil, heat, and abrasion resistance and are often used in safety shoes and sports shoes.

Polyether PU soles are typically around 450 to 600 g/L in density and are often used as monodensity soles. They can be processed at room temperature, have a superior finish, and are water resistant, making them good choices for street shoes and sandals.

PU insoles vs. midsoles vs. outsoles

Polyurethane formulations and production processes are often varied to produce materials suited to insoles, midsoles, or outsoles. Insoles and midsoles are generally lighter, more comfortable, and more flexible, while outsoles tend to be heavier and tougher.

Advantages of PU

Footwear needs to be comfortable, durable, and light, properties that PU soles offer in a convenient, cost-effective package. The many advantages of PU soles make them a great fit for most footwear applications.

Polyurethane soles offer excellent material properties:

• Durable: Since PU soles are tough and abrasion-resistant, they are commonly used in soles of every type. Their durability makes them a longer-lasting option than almost any other sole material.
• Lightweight: PU soles are extremely light and come in a variety of low densities ideal for insoles, midsoles, and outsoles. Even the higher-density polyurethane variants used in outsoles provide tougher material with less weight than other outsole options.
• Chemical & water resistant: Polyurethane is a tough and chemically resistant polymer. Depending on the needs of the application, polyurethane soles can be tailored for greater resistance to either water or chemicals and oils.
• Grip performance: PU outsoles provide excellent grip and prevent slippage, making them ideal for outdoor use in safety shoes, sports shoes, and work shoes.
• Well-established: As the most common sole material in use today, there are countless varieties of polyurethane to fit nearly any application, and the ideal methods of working with the material are widely known and well-established.

Shoes with polyurethane soles provide many benefits to their wearers:

• Comfortable and flexible: Lightweight PU soles are smooth, soft, and elastic, making them a great choice for insoles and midsoles as they conform to the foot. Heavier polyurethane variants are flexible, highly elastic, and widely used in outsoles for comfortable, breathable shoes.
• Cushioning: Thanks to their great elasticity and low density, PU soles offer excellent shock absorption and dampening for use in sports shoes and safety shoes.
• Cost-effective: Polyurethane is a great choice in production, as it is both less expensive to produce and easier to work with than other sole materials. Its durability also provides customers with longer-lasting, more cost-effective footwear.
• Versatile finishes: Polyurethane is available in a wide variety of finishes that enable complete freedom of design through customization options such as coloring, pad printing, and brushing.
• Health benefits: PU soles are hypoallergenic and breathable and are usable in footwear from sandals to sports shoes.

Disadvantages of PU

The main disadvantage of PU soles is their tendency to degrade over long periods of time. As the shoes are stored, moisture in the air reacts with the polymer chains that make up polyurethane in a process called hydrolysis.

This process slowly splits apart the polymer chains over time, making the material more brittle and less flexible. As a result, particularly old PU may disintegrate under stress. Storing shoes with PU soles in areas with high humidity and temperature will accelerate this process, while wearing them regularly will slow this process by exposing them to fresh air.

To increase their lifespan, shoes with PU soles should be stored in a cool, dry place and worn regularly. PU soles may deteriorate more quickly if not stored properly, but they can easily last 5 to 10 years before this occurs—enough that the shoes will usually need to be replaced for another reason long before hydrolysis becomes an issue. Additionally, polyether-based PU soles are a variation that offers greater hydrolysis resistance in applications where the lifespan of the shoe is particularly important.

PU vs. leather

Although leather soles can last longer if properly cared for, they require much more maintenance and will still deteriorate more quickly than PU soles due to natural wear and tear. PU is often the better choice as it is less expensive, more comfortable, lighter, and can even be styled to look like leather.

Sport shoes

Sport shoes have many requirements for their soles—they need to be durable, comfortable, breathable, light, flexible, shock-resistant, and slip-resistant. PU soles allow sports shoes to achieve all of these qualities at an affordable price without compromising in any areas of performance.

Slippers & sandals

PU soles are commonly used in both cheap and comfortable options, as well as high-end leisure footwear. Their wide variety of formulations, low density, and ability to be used as an insole, midsole, and outsole allow PU soles to fill almost every niche in casual footwear.

Direct injection process (DIP)

In direct injection, liquid polyurethane is pushed into a closed mold affixed to the shoe, creating a strongly bonded outsole. The pressure used in this process ensures excellent physical properties and a good grain structure.

Pouring

In pouring, liquid polyurethane is poured through a mixing head into an aluminum mold. The mold is then closed to allow the material to set and cool, producing a sole.

PU modification with KURARAY LIQUID RUBBER

To improve abrasion resistance and production efficiency, Kuraray offers KURARAY LIQUID RUBBER, a polyurethane additive ideal for use in both insoles and outsoles.

KURARAY LIQUID RUBBER provides a considerable boost to abrasion resistance when used as an additive in polyurethane formulations. Our internal tests show that before adding L-BR-307, PU has its surface abraded by 82 mg in the Taber abrasion test*. After adding L-BR-307, the PU product is only abraded by 28 mg—a significant improvement to a material that’s already the most durable on the market. The result is an ultra-tough polyurethane for long-lasting shoes, inside and out.

*Test method: JIS K7312, H-22 abrasive wheels, 1 kg load, 1000 rotations.

When used as an additive in polyurethane soles, KURARAY LIQUID RUBBER also improves demoldability. PU formulations that include KURARAY LIQUID RUBBER are easier to remove from their molds, increasing production speed, reducing product loss, and limiting energy waste to make your process smoother and more cost-effective.

KURARAY LIQUID RUBBER is already in use in PU sandals with combined insoles and outsoles, improving the quality and durability of the product for the customer and the efficiency of the manufacturer.