As follows from what was written in the previous part, the physical and mechanical properties of polyurethanes can be fundamentally different. Since I am still engaged in the production of auto parts from polyurethane monolithic elastomers, and this article is devoted to this, we will try to describe the “general” properties for this particular class of polyurethanes. It should be noted here that there are only a few manufacturers of basic polyurethane components in the world, therefore, there are practically no fundamental differences in physics and engineering for conscientious processors, and, say, American, Japanese, or Australian polyurethane does not differ much from ours in terms of physics and technology.
So, in general, we are interested in the following physical and mechanical properties:
- Hardness. The physical meaning is the ability of a material to resist the introduction of other material into it. In the case of auto parts, the hardness depends primarily on the deformation of the part when a force is applied to it. Objectively, the greater the hardness – the stronger the connection between the parts, the less vibration damping, the greater the load on the metal. Usually in the automotive industry rubbers with a hardness of 65-70 Shore A are used, this is largely due to the fact that the rubber sharply loses its tensile strength with increasing hardness. Manufacturers of polyurethane products can vary the hardness of the manufactured parts within a fairly wide range without significant damage to other properties. We produce color-coded parts: orange – 71A, green – 80A, blue – 87A, well, on request or in special products – up to 98A.
- Tensile force. Everything is clear from the name – the effort that must be made to break the material. According to this characteristic, polyurethane significantly outperforms almost all rubber brands. So, the maximum for some well-known brands of rubber is less than 36 N / mm2, for our polyurethane – 47 N / mm2. This is what allows polyurethane parts to withstand large loads without breaking, which in turn gives a longer resource.
- Effort to tear. It differs from the tensile force by the presence of a notch – a stress concentrator during stretching of the part. An extremely important indicator for more complex parts than bushings, especially those used in places not completely covered by metal. For most materials, including polyurethane, tensile strength is several times lower.
- Permanent deformation. The ability of a material to recover after prolonged constant loading in one direction. Usually, other things being equal, it is inversely proportional to hardness – the harder the material, the more difficult it is to crush it, but the worse it restores its original shape. All materials, including steel, have permanent deformation, but for elastomers it is extremely important: in a car that is standing in one place for a long time, all loaded silent blocks and bushings are deformed, the hole under the axle becomes oval, and the resulting slack can hardly be corrected in any way – the silent block / bushing get backlash.
- Abrasion resistance. There is also a double-sided medal: within each class of materials, the softer the sample, the less prone to abrasive wear. However, even the hardest automotive polyurethanes are more resistant to abrasives than rubber, and sand that gets inside the silent block will erase metal rather than polyurethane.
- Adhesion to metal. Many rubber auto parts are brazed to metal fittings. And here lies, perhaps, the biggest problem with polyurethane parts: rubber adhesion is often stronger, and most importantly, technologically more stable than that of polyurethane. There are a number of technological tweaks to treat metal parts and polyurethane for better adhesion, but they are all imperfect. Therefore, the company Polyurethane produces many silent blocks, including in the so-called pressed-in version, with non-welded metal parts.
- Frost resistance. The ability of a material not to lose elasticity at low temperatures. And just here, some polyurethanes significantly outperform rubbers: even fragile rubbers with a high content of plasticizers almost completely lose their elasticity at -40. We checked the anthers of our production in a container with “dry ice” – a solid phase of carbon dioxide, approximately -73 degrees Celsius (approximately because none of our thermometers can measure this anymore), the frozen anther, although with difficulty, could be squeezed and it was restored to its original size without cracks.
- Stability and durability under operating conditions. Most rubbers are unstable to fuels and lubricants, but most importantly, they degrade over time under normal conditions. This is primarily due to the release of unbound rubber modifiers and re-vulcanization of the rubber molecules themselves. Any car enthusiast has seen cracked bushings that have lost their elasticity, and knows that Chinese substitutes lose their properties much faster than the original parts. Polyurethanes are not afraid of fuels and lubricants and road chemicals, although the process of hardening goes on throughout its life, it is extremely slow (a couple of Shore A units in ten years), the only thing is that they are afraid of ultraviolet radiation without a dye, which, however, is not very critical for most auto parts.
As I said above, all these properties are to some extent “programmable” – by changing the composition of the mixture, introducing copolymers and additives, changing the reaction conditions, preparing the components and other tricks, you can significantly change them. What we are doing: at our production, at the same time, a dozen different polyurethanes are processed only in serial production, and for custom-made on individual orders, we promptly change the composition.
Manufacturers of base components of polyurethanes produce both base components and prepolymers (partially reacted components), have their own recommended compositions for different applications, but they work very slowly and clumsily with modifications, and not that they are reluctant to communicate, but everything is too slow for them. Slowly – this is when it takes six months to issue an invoice, and then it turns out that they forgot to turn on the catalyst, but without a catalyst it does not work, the catalyst is supplied in another six months, during which time the pilot batch becomes unusable … in short, everything is very difficult. Oh yes, I forgot to say: the lifetime of pure PU components is a few weeks, the lifetime of partially reacted ones is a few months. It is almost impossible to bring pure isocyanate, it will “go bad” even at customs.
That is why we work with a reliable, trusted supplier of prepolymers (partially reacted components). This somewhat reduces our freedom in material modifications, but otherwise it is identical to direct work with isocyanate, and most importantly – it gives us the opportunity to quickly edit the technological parameters of the components.