Frequently Asked Questions (FAQ)
It is crucial that the characteristic properties of polyurethane in combination with material-driven design are understood for the successful application of polyurethane elastomers. Here you will find the answers to some frequently asked questions:
frequently asked questions:
WHAT IS THE CHEMICAL STRUCTURE OF PUR?
Polyurethanes are elastomers which are most commonly formed by reacting with a typical urethane group (NH-C-O) linking the molecular units.
This group is the result of the chemical reaction between a hydroxyl group (-OH) mainly built from linear polyols with an isocyanate group (NCO). Adding a cross-linker (from a chemical point of view, this is a chain extender linking the monomeric components to build macromolecular chains) finally leads to the creation of a regular molecular structure. This is the structure that is responsible for the outstanding mechanical properties of polyurethanes and especially their elasticity.
Polyurethanes are therefore somewhat related to rubber elastomers, however, they are superior to rubber as they can be tuned to offer a wide range of special features: from soft elastomers with rubber-elastic properties up to rigid or hard types of plastic.
Depending on the kind and type of components used in the synthesis of polyurethane, asma is able to design the properties of the final material and to tailor them to the particular field of application.
Depending on the type of polyol, there are two main groups of polyurethanes: polyester polyurethane (asma brands: asmaprene A, BE, Asmathane) and polyether polyurethanes (asma brands: asmaprene L, Baytec Reaktiv).
Additionally, various types of isocyanates (MDI, TDI, NDI etc.) and chain extenders or cross- linkers resp. (diols, triols, amines) are used to design the particular properties of each polyurethane.
IS PUR DANGEROUS TO HEALTH AND ENVIRONMENT?
Polyurethanes normally do not contain any fillers or toxic components. On completion of the chemical reaction, all the constituents/substances are chemically sealed. Under normal conditions of use, as with most other plastics, the material does not present a hazard to health.
WHAT IS THE DIFFERENCE BETWEEN VULKOLLAN AND PU?
„Vulkollan“ is the registered trademark of a compact, unfoamed cast polyurethane designed by Covestro AG, Leverkusen. Bayer is also considered to be the inventor of polyurethane. „Vulkollan“ is the first polyurethane formulation that was used industrially and, since the 1960s, has had huge success in a number of applications.
„PU” or „polyurethane” nowadays is a general term for all types of cast polyurethanes except „Vulkollan”.
All these materials have one thing in common: they are processed in different variations of casting procedures and methods.
The production of „Vulkollan“ as well as the use of all the raw materials and components are prescribed in detail. It may only be manufactured and sold under this name by producers that have been licenced by Covestro.
The polyol component is a linear polyester and naphtylen diisocyanate (D15) as well as butandiol serve as cross-linkers. The result is a homogeneous, cross-linked, high-density material, which does not contain any extractable additives like plasticizers. asma offers the typical hardness range of „Vulkollan“ from 65° to 95° Shore A.
The family of „polyurethanes” stands for a wide variety of formulations, types, components, and manufacturers providing a wide range of hardness levels or property profiles.
Within the group of „polyurethanes” there are different trade names and terms for nearly every application. „Vulkollan” is famous for its well-balanced properties, excellent performance and the outstanding dynamic load capacity.
„Vulkollan“ products, if no color is added, typically come in various shades of brown; the color, however, has no influence on their outstanding performance. For more detailed information, please contact asma or refer to Covestro’s special „Vulkollan“ brochure.
Depending on the chemical structure, „polyurethanes” look milky white to honey tone in colour as the darkening effect is usually less intensive than with „Vulkollan”.
Due to the complex processing procedures and the high cost of its raw materials, products made from „Vulkollan” tend to be more expensive than those made from standard „polyurethane”.
WHAT ARE THE ADVANTAGES OF PUR COMPARED WITH RUBBER?
Polyurethanes differ from natural or synthetic rubber in a number of special features, such as:
- outstanding wear- and abrasion resistance
- high tensile strength
- good damping and resilience
- high tear resistance
- good electrical properties
- high resistance to mineral oil and fuel
- high rebound elasticity over the full hardness range
WHAT IS THE MEANING OF "MATERIAL HARDNESS" AND HOW IS IT MEASURED?
Hardness is a key indicator and distinctive feature that is used to characterize the properties of the various types of polyurethane. It is measured using a Shore durometer, a hardness measuring device, that is also used in the rubber industry. This instrument measures the depth of indentation of a spring-loaded needle (rod) in the material to be tested. „Shore A“ is used for testing soft to medium hard elastomers (up to approx. 96° Shore A) by means of a truncated cone, „Shore D“ is used for harder polyurethanes by means of a conical indenter.
AT WHAT TEMPERATURES CAN PUR BE USED?
Depending on the type, polyurethanes can be used in temperature ranges down to approx. minus 20°C. Below 0°C the elasticity modulus (E-modulus) increases (and seems to be harder). However, despite this heightened stiffness the material will not break. Bending tests have shown that even at low temperatures the deformability remains very high.
At low temperatures, the damping capacities of polyurethanes remain very high, as the material will warm up under the dynamic load and will therefore quickly leave the critical (low) temperature range.
The upper temperature limit for polyurethane products in long-term usage is around 80°C. Short term exposure (a few hours) to up to 130°C is tolerable. Special types (as for instance our „Asmaprene D“) are designed to withstand temperatures of up to 150°C depending on the application. In choosing the appropriate material, it is also important to consider the mechanical stress (which leads to additional heat generation within the material).
IS PUR FLAMMABLE?
Polyurethanes are highly flame-retardant; however, they start to melt at an ignition temperature of about 350°C.
Special additives are available to improve the material’s fire-retardant features. This, however, will have a negative effect on the mechanical properties and will decrease them, depending on the quantity of additive used, by 10 to 20%.
WHERE DOES THE EXCELLENT RESISTANCE OF POLYURETHANES COME FROM?
High tensile strength, elasticity and breaking elongation are fundamental properties for the excellent resistance to wear of polyurethanes. The components are able to yield and rebound „under attack“. That‘s why especially cast polyurethane elastomers are so well-known for their outstanding abrasion resistance.
In practice, there are a lot of different factors to be considered in the choice of the appropriate material: a detailed analysis of the kind of abrasion (impact or sliding), of the source of abrasion (grain size, material, surface finish), and of the ambient conditions (climate/temperature, chemical influences) is therefore required.
WHAT ABOUT THE CHEMICAL RESISTANCE?
Generally speaking, polyurethanes offer excellent values in terms of resistance to oxygen, ozone, and UV light. They also offer good resistance to mineral oil, fuel, and are still quite resistant to various solvents and benzene.
Problems might however arise, particularly with polyester polyurethane, when it is exposed to water, strong solvents, acids/bases, and especially at higher temperatures.
Dry chemical substances usually do not affect polyurethane elastomers. Vegetable fat or oil might affect the polyurethane molecular structure.
Organic solvents will cause swelling, depending on the intensity and duration of exposure.
In many cases, the PUR-product will not be destroyed and will, after drying, regain its original shape.
It is important to note that the use of polyurethanes might be an appropriate option even if the conditions limit their resistance, i.e. if the overall balance of mechanical and chemical features is satisfactory, or if specific modifications in a process are advantageous (high hardness, low temperatures, short operating time versus long chemical exposure).
The best method to find out whether a product is appropriate for use with a certain medium is to carry out a test under realistic conditions or to simulate these conditions as accurately as possible (e.g. swelling test).
WHAT IS "HYDROLYSIS"?
Hot water or steam may affect the molecular structure of polyurethane.
This instability occurs because of their special chemical structure and all polyurethane elastomers can be more or less affected by hydrolysis with ester-based types (like „Vulkollan“) being considered as particularly vulnerable.
Elastomer products affected by hydrolysis develop a waxy, brittle surface.
If the PUR product is intended for applications in humid environments, we suggest the use of polyether polyurethanes like „Asmaprene L“ or „Adiprene“ which are far more resistant to hydrolysis.
IS PUR SUITABLE FOR DYEING?
Polyurethanes can be dyed in a wide range of shades. It is, however, necessary to consider the influence of the original natural colour of the raw material (usually a transparent honey tone to milky-white) as well as the typical browning effect of standard polyurethanes. Therefore, strong standard colours are very common.
In particular „Vulkollan“ changes its colour under the influence of sunlight from light brown (almost white just after casting) to dark brown, almost black. This would affect any colour added. Therefore, „Vulkollan“is usually offered in its „natural“ tone.
The colour change under exposure to light has no negative impact on the general properties and quality of polyurethanes.
For particular optical applications requiring higher lightfastness we offer special formulations (aliphatic PUR).
WHAT ABOUT ELECTRICALLY CONDUCTIVE PUR?
Like most plastics, polyurethanes are insulators, so electric current cannot pass plastic surfaces.
Surface resistance as well as volume resistance is at the giga-ohm level (109 ohm).
It is therefore necessary to take into account the possibility of electrostatic charging on polyurethane surfaces when they get in contact with other non-conductors (especially plastic foil).
In some cases it might be necessary to develop a certain discharge capability (e.g. for sheet carriers); we therefore offer special anti-static or electrical discharging products (equipped with anti-static additives, for instance for ATEX applications).
HOW DOES THE COATING AND MOULDING OF PU PRODUCTS WORK?
Polyurethane products are mainly cast products; other methods like spraying, vacuum casting, or injection moulding (only for thermoplastic polyurethanes (TPU)) can also be applied.
In the low pressure casting process, chemically reactive polyurethane formulations, consisting of the main components polyol, isocyanate and cross-linkers, are blended by hand or using machinery after pre-treatment and then poured into open moulds. In these heated moulds, the material reacts and changes into a solid state. The product is then removed from the mould and stored in tempered post-curing ovens overnight.
Use of light-weight and cheap moulds (due to low or no pressure)
High flexibility in processing (wide range of different material components can be combined in many different ways)
Customized product properties
Production of both low quantities or series
Labour and energy intensive production (difficult to automatize)
Limits in the design (undercuts, shrinking)
Some interesting variations are rotational casting (the highly reactive liquid is poured directly onto the rotating roll core), centrifugal casting (casting into rotating drums or moulds), and vacuum casting. The vacuum casting process is similar to the low pressure casting process; the only difference is that all steps are carried out under vacuum. This method combines the advantage of low pressure casting with the possibility to produce high-precision products, complex geometries and thin walls without trapped air bubbles and pin holes.
Use of simple moulds (no pressure applied)
High flexibility in processing
Customized material properties
Production of difficult designs and geometries (undercuts, thin walls)
Compact material quality
Limit of formulations that can be processed under vacuum
Highly complex technical process (even more complex than low pressure casting)
Suitable only for small quantities or very special, complex parts
Injection moulding is used for thermoplastic polyurethanes (TPU), where granulated material is melted under high temperature and injected into special moulds under high pressure.
High level of automation (unmanned operation)
Precision finish and high surface quality (even undercut, complex designs)
High precision and repeatability
Complex and expensive moulds and machines (to withstand the high processing pressures), Reduced flexibility in terms of material variety
Problems with adhesion to metal
In spray coating no moulds are used. A highly reactive type of polyurethane is sprayed directly onto the (metal) surface to be coated. Within a few seconds, the polyurethane coating turns into a solid, compact and wear-resistant plastic coating.
Quick and simple method to produce wear-resistant coatings on big surfaces, especially in the construction and steel industry.
Unappealing surface (suitable only for technical/functional applications)
Inferior properties in comparison with polyurethanes made in casting procedures
Only suitable for a few purposes
In mechanical processing steps such as cutting, milling, turning, grinding, etc., semi-finished goods (e.g. rods, plates and tubes) are produced using special tools and machinery to produce high-precision parts.
Low or limited investment in moulds or equipment
Limited application as not all polyurethanes, especially soft materials, are fit for machining
IS "VULKOLLAN" SUITABLE FOR INJECTION MOULDING?
„Vulkollan“ is a chemically reactive polyurethane which has to be processed as a liquid within a very limited reaction time (pot life time) – see also “Low Pressure Casting”
Since „Vulkollan“ is not available as thermoplastic granulated material it cannot be treated in a thermoplastic process like injection moulding.
WHAT IS THE DIFFERENCE BETWEEN CAST PUR AND TPU/TPE?
Cast polyurethanes are chemically reactive systems poured as a liquid into the (mostly heated) open moulds where they immediately react to become a solid state, whereas thermoplastics are „finished“ polyurethanes (granulate) which are then melted under high temperature and injected under high pressure into (mostly cooled) moulds (the material must be „frozen“ to become solid again).
Please also refer to the chapter „Manufacturing Methods“.
HOW SHOULD PU-PARTS BE STORED?
PUR products should be stored in dry, aerated storage rooms at normal temperatures, and should be protected against direct sunlight. Also see „Hydrolysis“ and „Colour Change“.
IS PUR RESISTANT TO UV?
In general, polyurethanes are resistant to UV, but changes in colour and surface quality may occur (also see „Colour Change“).
When PUR products are planned to be used in outdoor applications with exposure to intensive UV light, we recommend the use of formulations with additional UV protection or light-stable systems (e.g. crystal-clear or transparent polyurethane systems for optical products).
HOW TO CLEAN PU-SURFACES?
As with most plastics, we recommend the use of gentle detergents only. Short contact with solvents such as acetone or MEK should be no problem (e.g. wiping), whereas longer exposure to or storing in solvents or cleaners must be avoided (see “Chemical Resistance”).
IS PUR HAZARDOUS WASTE?
After use polyurethane products can be disposed of just like normal waste (it is not hazardous!) or may be burned in waste incineration plants (they are a valuable fuel).
WHAT IS THE MAXIMUM WALL THICKNESS OF PUR PRODUCTS?
Cast polyurethane products can be manufactured in almost any size and wall thickness, since no pressure is applied during processing and the chemical reaction (from liquid to solid) takes place in the mould and during the following post-curing process (tempering).
Please refer to „Manufacturing Methods“ for more details.
WHAT IS THE DELIVERY TIME OF THESE PRODUCTS?
For the proper execution of the chemical processes it is absolutely necessary to strictly observe processing parameters such as temperature and time (also see „Low Pressure Casting“).
To obtain the desired properties, the post-curing (tempering) and storage phases are most important. In this case thick walls or large parts may take several weeks until the necessary state is achieved.
That is why customized polyurethane products unfortunately are not available within a few days. However, if you have any particular requirements, please contact us – we will certainly find a solution.
IS THERE "FOOD-SAFE" PU?
Generally speaking, polyurethane elastomers can be used without concern for products that are briefly in contact with food.
For polyurethanes intended for unrestricted contact with food (long-term, humid foodstuff), we offer special FDA-approved formulations.
WHAT IS "DAMPING"?
Absolute damping is defined as the energy of a spring element that is converted into heat (hysteresis loss) instead of being released. Relative damping is defined as the ratio of energy loss to energy required for deformation.
HOW DOES PUR REACT TO DYNAMIC LOAD?
When polyurethanes are exposed to dynamic load, heat is generated. In case of inadequate heat discharge the material or its components might become damaged. This is particularly the case with the combination of high loads and high speed (high frequency), and could cause problems. This influence must be calculated in advance.
Here’s a rule of thumb: it is better to use hard material with thin walls than soft materials with thick walls. Even if the energy used – and thus the generation of heat – is the same in both cases the parts with thin walls are better suited to dissipate the heat.