Polyurethane soles for footwear: a look at global production

The Asia-Pacific market is the largest and the fastest growing in the manufacture and distribution of polyurethane soles for footwear. The high growth rate of the market in that region is due to the large footwear production of countries such as China, India, Indonesia and Vietnam. Currently it is forecast that the high demand for sports and casual footwear will further drive the demand for polyurethane soles and therefore it can be said that it is a growing market. There are other regions that stand out in the manufacture of this material, such as Brazil, Mexico, Turkey and Italy, among others.


Dual density footwear: comfort and safety in a single sole

Dual density footwear is made up of two parts: an external polyurethane sole called an outsole and a midsole. The two parts are moulded and bonded to each other during the same production process. As its name indicates, the sole is made up of two layers of polyurethane of different density and colour, so that the densest and most compact is on the outside and the intermediate and less dense is located between the outer sole and the shoe. While the outer sole has good resistance to abrasion or oils, among others, the inner sole has the mission of providing greater comfort to the user. With this combination, comfort and safety are united in a single sole.

For the manufacture of the outer sole, the polyurethane is poured or injected into the mould, and then a false lid called a dummy is used to close it. At the end of the outsole cure cycle, the cap is lifted and the midsole foam is poured or injected. Then another lid is used to secure the mould and a second cure cycle is performed.


The direct injection process: less time and more productivity

In direct injection processes, the upper part of the shoe, which is the textile part, is placed as a cover, and the polyurethane is injected under high pressure into the closed mould. In this way, the polyurethane foam cures and adheres permanently. This process eliminates the need for adhesives or seams, resulting in a significant improvement in time and productivity. In this case, problems in the injected polyurethane that involve rejection can lead to the loss of the entire shoe. 

Release agents for footwear

Release agents are used for the production of single and dual density soles, safety footwear, boots, insoles and shoe soles in direct injection processes.

There are two basic types of polyurethane shoe systems defined according to the polyol: polyester polyol and polyether polyol. It is important to know this parameter, since the formulation of the release agent is clearly different (polyester is a material much less attackable by some components of the release agent, which in the case of polyether would cause problems).

In the specific case of dual density soles, these release agents have to provide a good balance between release and bonding of the soles. First, the outsole is removed without touching it, the lid is lifted and another is placed to inject the second. This second one is injected onto the first, which has just been demoulded, so that any product residue that may remain on top of the first one does not hinder the gluing of the last one.

In the case of dual density soles, the release agents must also provide a flawless finish, and above all a good bond of the soles as an essential requirement.


The Concentrol range of release agents: properties and main lines

Concentrol offers release agents for manufacturers of shoe soles, safety footwear, military shoes and boots among others, and also for those who use direct injection processes.

The extensive range is made up of solvent-based, hybrid, co-solvent and water-based products. In addition, apart from having both ready-to-use and concentrates to dilute, we have tin-free and electrostatic-applied products. These are products ready to be used in a convenient way, and with the guarantee of minimum dirtying of the mould, although for when necessary, Concentrol also has different options of specific cleaners.

Specifically, we can highlight these two lines of release agents for their performance and efficiency:

LP ZBC-49 GVE, class II, solvent-based and with a satin finish.

– and LX ZBAC-01, water-based, to dilute and with a satin finish.

Among the many properties of the products, the following stand out: their high releasing power, the ability to perfectly and uniformly bond the midsole and the sole, their different flash points, their great adaptability to the parts defined in the design of footwear and the possibility of adjusting the surface finish depending on whether the customer prefers satin, matte or glossy.

The use of these Concentrol products provides great efficiency in the demoulding process and a substantial improvement in the productivity of the manufacturing chain.



Author: Àngel Rodríguez
Technical Department