Polyurethane was introduced in the footwear sector in the late 60s, and since then it has become a very popular material for shoe manufacturers, since this material allows to obtain quality shoes, comfortable and durable, as well as light, at a low price.

Its appeal lies in the high amount of footwear that is manufactured globally every year. The “World Footwear Yearbook” published that, in 2013 alone, 22 trillion pairs of shoes were manufactured worldwide.

Thanks to polyurethane technology, various designs can be obtained, either from molding as units and their subsequent combination with the body of the shoe, or directly molded into the shoe. Polyurethane for shoe soles stands out for its lightness and resistance to abrasion, providing interesting mechanical properties.

Polyurethane soles are the result of a reaction between an isocyanate and a polyol system. Its hardness is imparted by hard segments in the polymer matrix and its density is the result of CO2 generated by the reaction of the isocyanate and water, as well as by the presence of small amounts of an expanding agent during mixing, as is the case of polyether systems.


Within the footwear sector, polyurethanes are found in a wide range of types. They can be used in shoes and sports boots, business and fashion shoe soles, as well as high quality safety shoes.
Low density to compact polyurethane systems are used for midsoles and outsoles.

The density will depend on the type of sole and its hardness:

Polyurethane type Hardness Density
Polyester Midsoles



280-450 g/l

900-1100 g/l

500-600 g/l

Polyether Monodensity soles 450 – 600 g/l

PU systems are based on both polyol polyester and polyether technologies. These technologies are suitable for different end uses and mean different processing characteristics.

The systems based on polyester offer greater chemical and abrasion resistance, while polyether systems have a superior resistance to hydrolysis.

Among the differences between the two production techniques is the processing temperature: polyether-based systems are liquid at room temperature, while polyesters require approximately a temperature of 45°C.

Nowadays, polyether and polyester systems are equally valued in the footwear industry, so it is shoe sole producers that make the choice according to their production requirements, being polyester the most used in safety footwear due to its chemical resistance to agents such as oils or solvents. Polyether, on the other hand, is ideal for hydrolysis-resistant soles and to withstand high humidity conditions, such as those found in tropical climates.

Advantages of polyurethane soles:

  • Low weight providing thermal and abrasion resistance.
  • Flexible soles are produced that enhance the comfort of the user by correctly absorbing the impacts that occur when walking, compared to other materials such as EVA, PVC, nylon or leather.
  • Versatility of finishes: Easily customizable with pigments or auxiliary elements.
  • Very moldable material: Polyurethane can be manufactured with different densities to adapt to different types of footwear, whether work or fashion.

A manufacturer of shoe soles usually has a very standardized process and puts its soles through strict quality tests, in which, for example, the parameters in the following table are measured:

Property Unit
Tension Mpa
Tear kN/m
Elongation %
Flexion %
Abrasion Mg


When soles are painted or are to be adhered, the manufacturer usually does a washing or “degreasing” process to remove any remnants of the release agent that may have been left superficially on the sole and that could interfere with adhesion with the paint.


A fairly common process is to insert demolded soles into an automatic washing machine with a hot perchlorine jet. In this process, the soles are exposed to perchlorine at 120ºC in a rotating tub, over a 10 to 15-minute cycle, to eliminate any grease or dirt impurities that could interfere with the bonding or finishing process.


Current market:

  • Asia is responsible for the manufacture of 87% of the world’s footwear, with China being the leading country, producing 2 out of 3 pairs that are produced globally.
  • However, there is currently a tendency for footwear manufacturers to relocate Chinese production to other areas such as Vietnam, Indonesia and Taiwan.
  • In Europe it is more difficult to compete on price and the market is dominated by companies that manufacture specialized footwear, such as safety footwear.
  • The same happens in the United States, where the production of military footwear must also be mentioned.



The release agents used for the production of polyurethane soles are classified according to the process and material to be demolded:

  • Monodensity or Bidensity soles.
  • Polyester or Polyether soles.


Currently the largest consumption is of solvent-based products, although there is increasing interest in the use of water-based release agents, whether ready for use or concentrates to dilute.

Choice should also be made according to the wanted type of finish of the sole: Matte, satin or glossy.

The main properties of a good release agent must be:

  • High demolding power.
  • Provide a good finish to the sole in terms of gloss or matte and uniformity.
  • Low fouling of molds and mold holders.
  • Versatility for different sole designs.


Release agents for Monodensity soles:

Solvent-based release agents for monodensity soles consist of a dispersion of anti-adhesion agents in an aliphatic or isoparaffin type solvent base.

Lately, chlorinated solvents and the like have been discontinued for toxicological reasons, since they could affect the health of workers.

The concentration of active matter can vary greatly, from 2 to 60%. Being near the highest point the ideal concentration that allows a quick application of the release agent in rotating carousels, good demolding and surface finish of the sole produced, and at the same time, emitting the least amount of VOC’s.

Water-based release agents are an emulsion, that is, they consist of anti-adhesion agents and additives emulsified in water by means of surfactant systems, which can be non-ionic, cationic or anionic in nature.

For this reason they are more critical in terms of transport and storage, being water based, their viscosity can vary depending on temperature, and in no case can they be subjected to temperatures below 0 °C, since this would cause a freezing process that could destabilize the emulsion, breaking it and making it unrecoverable.

A similar effect can occur if the product is subjected to high temperatures (> 50 °C), for example during transport under sunlight in countries with warm or tropical climates.

For their application it is recommended to use an airbrush gun with a 0.3 mm nozzle, as in the case of solvent based ones, but using an air pressure of 2.5 to 3 bars to aid drying.

The temperature of the mold should not be less than 50ºC to aid drying, as it is also very important to maintain an application distance of 20 to 30 cm between the gun and the mold, in order to form a uniform film on the surface that makes demolding easier.

Finally, in the case of water base it is important that between the application of the release agent and the injection / casting of polyurethane a minimum of 1 minute passes, otherwise it may be convenient to use an air gun to speed up the drying and reduce the times of the carousel cycle.


Release agents for bidensity soles:

Bidensity footwear consists of an external polyurethane sole and a midsole, both molded and joined with one another during the production process. The reason is to ensure that the outer sole has good resistance to abrasion or against oils, and an inner sole that guarantees greater comfort to the user. In this way, comfort and safety are combined in a single sole.

These release agents are widely used for the safety footwear industry, where an impeccable finish and above all a good bond of soles is an indispensable requirement.

Here, solvent-based or water-based release agents are used that contain an active material composed of special non-stick materials that create a homogeneous, stable and consistent film on top of the mold, facilitating demolding and reducing consumption. These formulations achieve that the residue that remains on the mold after several releases is minimal, thus lengthening the cleaning cycles.

These release agents facilitate the bonding and gluing between two soles that come from two injections of different density, producing soles with a satin finish and little or no greasy feel.

In the case of solvent based, they work perfectly in a wide range of temperatures, and, unlike the release agents for monodensity soles, these are usually formulated with high boiling point solvents as a carrier for the active material, thus obtaining less flammable products and drying a little slower making part of the solvent to act as an oil facilitating the lubrication of the sole in the mold.

In water bases, the factor to be taken into account is that the temperature of the molds is adequate to facilitate drying, together with a good spray to obtain a good result.

In order to obtain a perfect atomization and distribution of the active ingredients on the mold, as for monodensity soles, it is recommended that they be applied by means of aerographic equipment with a 0.3 mm nozzle.

If their electrostatic version is used, in addition to maintaining the properties of the original product, a saving of 30% in consumption is obtained, as well as the corresponding reduction of VOC’s emitted to the atmosphere.


New factors that are currently taken into account regarding the choice of a release agent:

  • Quantity of VOC’s and FOG.
  • Flammability, with all that entails regulation regarding the facilities where it is used (ATEX), danger according to flash point (Class) and aspects related to transportation and storage.
  • Toxicity of the products involved in its formulation and their implications in the work environment, with particular reference to tin compounds and solvents.
  • Final impact on the environment.


Reduction of VOC’s, TLV’s and DNEL:

To completely eliminate direct emission VOC’s, the best solution is to change from a solvent-based release agent to a water-based one.

Although this solution is still not very integrated in shoe producers, there is an upward trend to manufacture with water-based release agents, especially if consumption is high and there may be both economic and environmental savings.


Hybrid release agents

Between the solutions based on solvents and those based on water, there is one more option that is known as hybrid demolders.
Hybrid demolders or CO-SOLVENT MIX 50:50 demolders are release agents in which the main material, that is, the carrier of active ingredients, is half water, half solvent (approximately).

In terms of functionality and finishing of the demolded part they are like a solvent base, with the main disadvantage that at least half of their formulations contain VOC’s.

Even so, it is a good intermediate solution for footwear manufacturers that are currently producing their soles with a solvent-based release agent and do not want to take the step to change to water based due to the inconveniences and change in the way of working that entails.

The change from solvent to hybrid base implies an improvement in certain aspects, such as price reduction, reduction of VOC’s emitted to the atmosphere and better working environment in the production line.


Electrostatic release agents

If the producer of soles or footwear wants to reduce consumption and VOC’s without changing the chemical nature of their release agent, electrostatic release agents are a possible solution.

The technical principle under which these types of release agents operate is as follows: Specific electrostatic spray guns are used. They are connected to a negative pole, while the working mold is connected to a positive pole. When applying a high voltage current, an electrostatic field is generated between the electrode at the tip of the gun and the mold. When the product is atomized, the negatively charged particles move towards the surface of the mold under the influence of the electrostatic field.

The result is that, as opposing loads are attracted, the mold attracts the release agent, efficiently covering its entire surface, avoiding unnecessary losses and thus providing a decrease in consumption.

Any solvent-based release agent can be converted to electrostatic, guaranteeing a solution of sufficient, adjusted and adequate conductivity that meets the needs of the client or the application. Nowadays electrostatic solutions do not present strong odours or additional toxicity, and they allow to work electrostatically with solvents known in the sector as classes I, II or III.

The advantages of the use of electrostatic release agents are, therefore: a significant reduction of the consumption, of 30 to 50%, by avoiding the loss of material that does not enter the mold as a result of the dirtiness of the external parts (mold supports, floors, etc.) and a significant reduction of emitted VOC’s, improving the work environment.

The considerations to take into account are the greater difficulty in controlling their application through robotic systems (although manual application is possible) and the need for specific equipment and pistols, larger and heavier than usual, which means greater initial investment in equipment.


Tin-Free release agents:

In recent years both footwear manufacturers and mold release agent suppliers have been working to replace organotin compounds with other organometallic compounds, but with some limitations regarding their effectiveness.

Most of the organometallic compounds that have been used for a long time are based on tin and include, as the most used, dibutyltin dilaurate (DBTDL).

It is possible to replace DBTDL as a catalyst with other non-organotin substances using specific knowledge acquired in the sector, as well as with strong collaboration and testing with customers, in order to adjust a custom made product that meets all the requirements.


Auxiliary products:

For the footwear market based on polyurethane, there is a range of auxiliary products such as mold cleaners, from the most toxic but effective containing NMP and NEP to cleaners without risk to health and free of NMP.

The specialty chemical company Concentrol has decades of experience in the demolding industry. A pioneer in its commitment to research and development, Concentrol offers the market tailored solutions for each client using the latest technologies and knowledge of the sector to achieve maximum efficiency and performance. Contact us for more information:


Huntsman –  PU review April 2015