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Polyurethane is a polymer commonly used for producing insoles, midsoles or outsoles in the footwear industry. This article is describing thermosetting polyurethane used for outsoles but there are also thermoplastic polyurethane (TPU) widely used as well with slightly different properties. PU is used since it´s generally lightweight, has good abrasion resistance and is very flexible. Both PU and TPU is also widely used in for coatings of materials, where TPU can be used without any backing.

The first polyurethan was developed during by chemists in Germany. Since then it´s undergone many improvements to the product we have today. PU are generally coming in two forms, either solid or a blowing agent is used to decrease the density to achieve a lighter material. This is know as cellular PU or foam rubber. Read more about foam rubber later in this article.

Generally PU is either used through a Direct Injection Process (DIP) or by pouring.

Chart of different sole materials and durability. Source: www.polyurethanes.org

Foam rubber

Wedged foam rubber sole from Dr Sole

Foam rubber is made by blowing an agent into the base material. This expands it by creating a cellular structure, it lowers the density and making it spongy and more light. With a less solid material the durability is affected negatively. Today the most common polymer to use is Polyurethane.

Foam rubber is sometimes referred to as rubber sponge, expanded rubber or blown rubber.

Foam rubber sole from Dr Sole

Solid PU vs Cellular PU (foam rubber)

Solid PU generally has a more shiny appearance than cellular PU which can be controlled by anti-shine agents. The big difference is in weight and abrasion resistance.

Left: Solid PU. Right: Cellular PU

PU Direct Injection

The direct injection process pushes liquid PU into a closed mold, creating an outsole around the shoe. This creates a very strong bond between the outsole and shoe. Due to the high extrusion pressure the PU gets a good grain structure and obtains good physical properties.

PU Direct Injection Process (DIP)

PU Pouring

By mixing Polyol and Isocyanate, with PU the right consistency and properties to pour the material into an open aluminum mould through a mixing head. The molds are closed, heated for the PU to set and cooled until opened.

There are two options of PU pouring machines:

Rotary machine or Conveyor type fabricated mould holders machine. where the mixing head is stationary, and the mould-holders move . It requires less workmanship but relatively big investment in molds and space. A rotary PU pouring machine typically have 32, 40, 60, 90 or 100 mold stations. It´s common that not all stations are having a mold but it´s depending on the current production need.

The banana machine has gotten it´s name from the shape of the mold rack. The mixer is moved manually where the molds are stationary. It´s generally a smaller machine but requires more manual support. It normally has 6, 12, 18, 24 and up to 32 mold stations.

Banana PU pouring machine

Resources

https://pinzashoes.wordpress.com//08/06/direct-injection-process-dip-and-pouring-method-for-pu-soling-onto-footwear/

https://www.linkedin.com/pulse/one-material-rule-them-all-polyurethane-footwear-servet-casabona/

With competitive price and timely delivery, Jianlida sincerely hope to be your supplier and partner.

https://en.wikipedia.org/wiki/Polyurethane

http://www.warrior.co.in/blog/pu-sole-one-material-to-rule-the-footwear-industry/#sthash.v9y3GJoW.dpbs

https://www.drsole.com/copy-of-light-rubber-sheet

https://goo.gl/zPf97N

About PU Sole and Its Advantages

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A few decades ago, the footwear industry was searching for an alternative to the conventional sole materials to create a more robust, stylish and sustainable sole. The quest saw new hope when the sole manufacturers had caused a disruption with an intriguing and innovative material - Polyurethane (PU).

The PU sole manufacturers have been endorsing and providing this splendid sole material for several years to create a plethora of footwear. The primary reason for its popularity is that the PU sole advantages the footwear manufacturers as well as the end users to a great extent with its amazing properties.

The polyurethane has been extensively utilized as a shoe sole material to develop robust and sustainable high-quality shoes for the applications requiring rough use such as trekking, sports, labour-intensive industries, etc. However, the PU sole manufacturers have made it possible now for them to be used as a sole material for causal and office footwear like formal shoes, sport shoes, safety shoes, sandals, sneakers, chappals etc. Moreover, the constant endeavours of the PU sole manufacturers and the continual demand for this material by the footwear industry, have now made the polyurethane quite affordable. The reasonable cost of the PU sole makes it suitable to be used in more types of footwear than its conventional counterparts.

How is PU Sole Made?

The polyurethane is produced by reacting an isocyanate prepolymer, such as methylene diphenyl isocyanate, and a polyol component in the presence of heat and an organometallic catalyst.

The liquid Polyurethane produced from the reaction can then be shaped into the required sole designs using injection moulding or pouring method. As the former has more merits, it is usually the most favourable method for the production of PU sole. By varying the diverse factors such as the types of the key or additive components and their contents, the density and hardness of polyurethane can be altered to produce the material with diverse characteristics.

To ensure that the PU sole advantages the footwear manufacturers with its use in a broad range of footwear, the PU sole manufacturers have developed mainly two versions of the polyurethane.

• Polyester: This version of the polyurethane is manufactured by using polyester polyols and offers exceptional resistance against heat and abrasion. Thus, it is used as a shoe sole material in the development of high-quality sports and safety shoes.
• Polyether: Produced using a blend of polyether polyols, the polyether polyurethane has a superior finish to its surface. It offers water-resistant qualities, making it ideal to be used in stylish general-purpose footwear such as street shoes and sandals.

PU Sole Advantages

The highly competent characteristics of PU sole advantage the footwear manufacturers and consumers alike and this has rightfully made it the soul of the footwear industry. Here are some of these advantages:

• Lightweight: There is hardly any who&#;d like to drag heavy feet around; thus, an airy material like the polyurethane often serves as the first choice as the sole for lightweight footwear.
• Chemical & Water Resistance: Polyurethane is known for its inert, robust and durable characteristics. Thus, the footwear manufacturers find PU sole advantageous enough to be used as a shoe sole material in high-quality safety shoes for various industries.
• Finest Grip: These soles exhibit excellent grip on even the most slippery surfaces, making them alluring enough to be used in nearly all kinds of footwear.
• Comfort & Durability: Due to its well-finished surface and leather-like elasticity, Polyurethane offers both these qualities in every type of footwear.
• Shock Absorption: For the applications where the consumer needs to stand or walk around for long, the footwear should have good shock absorption properties. Thus, the polyurethane serves as the perfect fit to be used as the sole material for such a purpose.

This article was originally published on Airysole&#;s Blog:
https://www.airysole.com/blog/about-pu-sole-and-advantages.html

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