Polyurethanes are elastomers that contain the so-called urethane group – NH – C – O – as a typical chemical reaction group.

This grouping is formed by the chemical reaction of a hydroxyl group (-OH), which usually originates from linear polyols, with an isocyanate group (NCO). With the addition of a crosslinker (in a chemical sense, actually a chain extender that connects the monomeric components to macromolecular chains), a regularly structured network is finally formed.

This is responsible for the excellent elastic properties. Thus, polyurethanes are related to various rubber elastomers, but in a number of properties, these materials, which are partly rubber-elastic and partly more horn-like (especially hard types), are superior to rubber.

Depending on the type and nature of the building blocks used, polyurethanes with different properties are created, and these properties can be influenced by the targeted selection of these polyurethane components.

Basically, we differentiate between, for example, polyester and polyether polyurethanes, depending on the type of polyol used.

In addition, various types of isocyanates are available (MDI, TDI, NDI, etc.) and, together with the available crosslinkers (diols, triols, amines), shape the property profile of the polyurethane material.

Polyurethanes usually contain no fillers or toxic components. After the chemical reaction is complete, all ingredients are chemically bound. For the user, as with most other plastics, there is no health hazard under normal use and application.

What is the difference between “Vulkollan” and “PUR”?
“Vulkollan” is a trade name for compact, unfoamed cast polyurethane from Covestro AG (formerly Bayer AG), Leverkusen. Bayer is considered the inventor of polyurethane. Since the 1960s, “Vulkollan” has also been the oldest industrially used polyurethane formulation and is still successfully used in numerous applications today.

In common parlance, “PUR” or “polyurethane” encompasses all other cast polyurethanes on the market.

Common to all these polyurethane systems is their processing in various types of casting methods.

The production of “Vulkollan” and the use of raw material components are precisely prescribed by Covestro. Only licensed processors are permitted to offer original Vulkollan. Linear polyesters are used as the polyol component, and diisocyanates and dialcohols are used as crosslinking products, resulting in a homogeneous, crosslinked, high-molecular material. No extractable components, such as plasticizers, are contained. Vulkollan types are available in hardness levels from 65° to 95° Shore A.

In the group of so-called “polyurethanes,” there is a multitude of different formulations, types, components, and suppliers, offering a wide range of hardnesses and property profiles.

For “polyurethanes,” there are special formulations with respective trade names for almost every application. “Vulkollan” in particular is known for its very balanced performance spectrum, but above all for its very good dynamic load capacity.

Without coloring, products made from “Vulkollan” have a characteristic brownish inherent color, which can appear in various shades without affecting the properties. See also Bayer AG’s special brochure on Vulkollan.

Depending on their chemical composition, “polyurethanes” are naturally colored from milky white to honey-colored; the darkening under the influence of light is generally less pronounced than with “Vulkollan.”

Finally, due to the complex processing and sometimes very expensive raw materials, products made from “Vulkollan” are often priced higher than conventional “polyurethanes.”

Polyurethane differs from rubber materials, based on both natural and synthetic rubber, by the following special characteristics:

• excellent abrasion resistance
• high elongation at break
• good damping
• high tear propagation resistance
• good electrical properties
• good resistance to gasoline and mineral oil
• high rebound resilience across the entire hardness range

Hardness serves as an identifier and distinguishing feature of the various polyurethane types. A Shore hardness tester is used, which is primarily employed in the rubber industry. The penetration depth of a spring-loaded needle is measured. In “Shore A” measurement, the needle is a truncated cone and is used for soft to medium-hard elastomers (up to approx. 96° Shore A); in “Shore D,” the needle is pointed for harder polyurethanes beyond this range.

Depending on the type, polyurethanes can be used down to approx. minus 20°C. Below 0°C, the modulus of elasticity increases (apparently higher hardness). Despite the increasing stiffness, however, there is no risk of breakage. Flexural fatigue tests on 3 mm thick plates showed that very high deformations are still possible at low temperatures.

At low temperatures, the damping of polyurethanes is very high. The resulting heating under dynamic stress means that the material moves away very quickly from the critical, very low temperatures.

The upper temperature limit for continuous stress is 80°C. Short-term peak loads (a few hours) up to 130°C are still permissible. Special types withstand temperatures up to 150°C depending on the application. The type of mechanical stress must be taken into account in each case (additional heat generation due to load energy introduced into the material).

The temperature specifications apply in dry environments. Additional chemical influences (moisture, fats, acids, etc.) can significantly reduce temperature resistance.

Polyurethanes are difficult to ignite and then burn with dripping from an ignition temperature of approx. 350°C.

Fire resistance can be improved through special additives. However, this causes the mechanical properties to decrease somewhat.

High strength, elasticity, and elongation at break of polyurethane materials are prerequisites for high wear resistance. The components can thus yield to the wearing material and spring back. This is why polyurethane elastomers in particular are known for their excellent abrasion resistance.

In practice, a multitude of influencing factors determine the success of an application. The most accurate possible analysis of the type of wear (impact or sliding wear), the wear partners (grain size, material properties, surface design), as well as the environmental conditions (temperature, chemical influences) form the basis for selecting the most suitable polyurethane type.

High strength, elasticity, and elongation at break of polyurethane materials are prerequisites for high wear resistance. The components can thus yield to the wearing material and spring back. This is why polyurethane elastomers in particular are known for their excellent abrasion resistance.

In practice, a multitude of influencing factors determine the success of an application. The most accurate possible analysis of the type of wear (impact or sliding wear), the wear partners (grain size, material properties, surface design), as well as the environmental conditions (temperature, chemical influences) form the basis for selecting the most suitable polyurethane type.

Polyurethanes are generally characterized by excellent resistance to oxygen, ozone, UV radiation, and by good resistance to pure mineral oils, gasoline, but also relatively good resistance to benzene and various solvents.

Water, acids, and alkalis increasingly cause problems with rising temperature, especially with polyester polyurethanes.

Dry chemicals usually cause no problems. Vegetable fats and oils can attack polyurethanes.

Organic solvents lead to swelling of polyurethane depending on the type and duration of exposure. In many cases, however, the elastomer is not destroyed and largely returns to its original form after drying.

In general, even with limited resistance of the polyurethane material, use may still be worthwhile if the overall balance of mechanical and chemical properties justifies it, or if certain modifications (higher hardness, lower operating temperatures, planned service life perhaps overall shorter than the chemical exposure time) provide the decisive improvement.

The best method of determining whether the article is suitable for use in a specific medium is to test it in actual use, or through test procedures that simulate the operating conditions as accurately as possible (swelling test).

Hot water or steam can attack or destroy the bonding structure of polyurethane.

This instability is due to the chemical structure. All polyurethane elastomers are more or less subject to the influence of hydrolysis, and the ester types in particular (such as Vulkollan) are considered especially susceptible.

Elastomers damaged by hydrolysis are recognizable by their waxy, brittle surface.

For planned applications in humid environments, we generally recommend using polyether polyurethane formulations (Adiprene, Asmaprene L-series), which are significantly more resistant to hydrolytic influences.

Polyurethanes can be colored in a variety of shades. However, the influence of the natural color (usually honey-colored transparent to milky-white) and darkening under light exposure must be considered. Therefore, a palette of strong standard colors is usually chosen. An exception is special light-stable polyurethane systems (aliphatic polyurethanes) for predominantly optical applications.

Vulkollan, in particular, discolors very strongly under the influence of light and thus significantly alters any potential coloring (which is why Vulkollan is almost exclusively offered in its natural color). The initially light beige natural color, especially in harder Vulkollan types, darkens and ultimately takes on a dark to black-brown tone.

Discoloration due to light has no influence on the material quality.

Like all plastics, polyurethanes are considered insulators, meaning that current conduction or discharge from plastic surfaces is not possible.

The surface resistance as well as the specific volume resistivity is in the range of 10⁹ Ohm.

Therefore, the potentially possible static charge of polyurethane surfaces in contact with other non-conductors (especially with plastic films) must be considered.

For special applications (film transport rollers, etc.), it is sometimes necessary to achieve a certain conductivity on polyurethane surfaces. For this purpose, we offer appropriately equipped antistatic or electrically conductive formulations (e.g., for ATEX applications).

Production usually takes place via casting, but also via spraying, vacuum casting, or injection molding (only possible with thermoplastic polyurethanes).

In the low-pressure casting process, chemically reactive polyurethane systems, consisting essentially of polyol, isocyanate, and crosslinker, are mixed in machines or manually after appropriate pre-treatment (temperature, vacuum) and poured pressure-free into open molds. In the (heated) mold, the polyurethane system reacts to form a solid workpiece, which is stored (tempered) in heating cabinets until final maturity after demolding.

Advantages: Low mold costs (no pressure), high flexibility in processing (various raw material components can be combined in many ways), customized product properties, large volumes can also be produced.

Disadvantages: Labor and energy-intensive production processes (limited rationalization for large quantities), not every geometry can be produced (undercuts, void formation).

Important special forms of the casting process include rotational casting (casting highly reactive formulations directly onto rotating roller cores), centrifugal casting (casting into a rotating drum), and vacuum casting (see below).

The vacuum casting process is technically similar to the low-pressure casting process, but with the difference that all processes are carried out under vacuum. Therefore, although there is no pressure during the casting process, complex geometries and thin wall thicknesses are molded with high precision.

Advantages: Low mold costs (no pressure), high flexibility in processing, customized product properties, complex geometries can also be produced (undercuts, thin wall thicknesses), absolutely compact, void-free material quality.

Disadvantages: Not every formulation can be processed under vacuum, technically even more complex than low-pressure casting (therefore usually only interesting for small quantities or very special, complex molded parts).

In the injection molding process, thermoplastic polyurethanes (TPU) in granular form are plasticized on injection molding machines and injected into special molds under high pressure.

Advantages: Highly automated (unmanned operation), detailed reproduction of surfaces (including undercuts and complex geometries), high repeatability.

Disadvantages: Complex molds and machines (high pressures), less flexibility in material selection, adhesion to metal problematic.
In the spraying process, a special, highly reactive polyurethane type suitable for this manufacturing method is sprayed directly onto the surface to be coated (metal) without a mold. Within a few seconds, the polyurethane layer solidifies and forms a compact, closed, and wear-resistant plastic layer.

Advantages: Quick and simple method for producing wear-resistant coatings for large-area applications, primarily in the construction and steel industries.

Disadvantages: Visually less appealing surfaces (for technical functional applications), lower property level than comparable polyurethane systems produced by casting, only a few special, highly reactive formulations are suitable for this process.

With mechanical processing methods, products are machined from prefabricated semi-finished products (plates, rods, tubes) using cutting tools.

Parts of high precision are thus created on turning, grinding, and milling machines.

Advantages: High dimensional accuracy, low to no investment in molds or fixtures required.

Disadvantages: Not every polyurethane type can be machined; especially soft polyurethanes are difficult to process.

As a cast polyurethane, Vulkollan is a reactive system that is processed in a liquid state within a narrowly defined reaction window (pot life)
– see also: Casting process.

Vulkollan is not available as a thermoplastic system and therefore cannot be processed with thermoplastic methods such as injection molding.

Cast polyurethanes are chemically reactive systems that react into a solid directly in the mold while still in the liquid phase. Thermoplastics, on the other hand, are already “finished” polyurethanes in granular form, which are melted under high temperature and pressure and then injected into the mold.

See also processing methods.

Ideally in dry, ventilated storage rooms at normal temperature, protected from direct sunlight.

See also the topics: “Hydrolysis,” “Color change.”

Polyurethanes are generally considered UV-resistant, but color shifts and surface changes can occur (see “Coloring”).

For planned outdoor applications with expected high UV radiation, we recommend using formulations with additional UV stabilizers or special systems (e.g., crystal-clear transparent polyurethanes for optical applications).

As with many plastics, we recommend using mild cleaners. Brief contact with solvents such as acetone or MEK is unproblematic (wiping), but intensive long-term treatment or storage in solvents must be avoided (see resistance).

After use, polyurethane waste can be disposed of normally (not hazardous waste!) or in suitable waste incineration plants (valuable fuel).

Cast polyurethanes can be produced in almost any size and wall thickness, as processing is pressure-free and the chemical reaction only takes place in the mold or during the subsequent post-curing process.

See also processing methods.

Chemical processes require strict adherence to process parameters such as temperatures and times, see “Low-pressure casting process.”
The tempering and storage times are particularly important for the final properties, and for large wall thicknesses, these can take several weeks until the plastic structure is fully matured.

Therefore, custom-made polyurethane parts are unfortunately usually not available within a few days.

However, with appropriate coordination between the customer and manufacturer, an alternative solution can usually always be found in emergencies.

By definition, absolute damping refers to the energy that is not released again by a spring element but is converted into heat (hysteresis loss). Relative damping is defined as the ratio of this loss energy to the applied deformation work.

When polyurethane materials are subjected to dynamic stress, the resulting heat generation must be considered, which, if inadequately dissipated, can often lead to material destruction within the component. High operating speeds in conjunction with high loads can cause problems here and must be calculated in advance.

A rule of thumb applies: It is better to use hard and thin-walled parts than thicker walls of lower hardness, because even if the energy applied and thus the heat generation is the same in both cases, the heat generated is better dissipated in the thinner parts.

Sustainability is a central component of our actions. We rely on energy-efficient processes, invest in modern infrastructure – for example, in renewable energies at our site – and continuously work to further optimize our operations. A special focus is on the responsible handling of materials: At asma in Weitra, waste is transformed into new valuable materials. The company is developing a process to reintroduce difficult-to-recycle polyurethane residues back into the production cycle. Our polyurethane solutions themselves also contribute to sustainability: They are particularly durable, resistant, and precisely adapted to the respective application. This extends maintenance intervals, conserves resources, and increases the lifespan of systems.

Polyurethane is only partially recyclable. However, asma is developing its own solutions to recycle production residues: In Weitra, difficult-to-recycle PUR residues are reintroduced into the production cycle using a special process.
Additionally, the durable properties of our products contribute to the sustainable conservation of resources.