Gases for pneumatic tyres – safer and more economical
Ever since the Michelin brothers first fitted pneumatic rubber tyres to a motor car in 1895, it has become impossible to imagine our motoring existence without them. They provide comfort and safety when driving. In motor racing, they are sometimes the deciding factor for victory or defeat. Improvements to aerodynamics may perhaps save a tenth of a second per lap in Formula 1; the correct tyres can change everything. Aircraft and Formula 1 pilots fill their tyres with a special gas rather than air, and so too do demanding drivers. Gases also play an important role in the manufacture of tyres.
During the production of a tyre, up to 30 sorts of rubber are blended together to create different mixtures with proportions that depend on whether they are to be used to make summer or winter tyres, treads or side walls. The individual rubber layers are assembled layer by layer with the other components, such as textile fabrics or steel belts, and are then provisionally bonded with one another by powerful rollers. Only when vulcanised in the heating press do the layers finally merge to become an inseparable unit. It is here that the tyre also gets its tread, and the previously mouldable rubber mass transforms into the stable, but still elastic, material that we know in the form of car and bicycle tyres. Nitrogen (N2) plays an important role in this critical step. In the heating press, nitrogen can be used in two separate processing steps. In the first of these, the so-called bladder is inflated with nitrogen. This is a kind of balloon that is located in the middle of the heating press and which presses the tyre blank into the mould with a pressure of about one bar. Steam was formerly used for this step. But it is safer with nitrogen since the press is still open at this phase. As a consumable item, the bladder is regularly replaced; however, any damage that it might suffer before replacement could lead to an uncontrolled release of steam and endanger the workers. Compressed air is out of the question due to the risk of ignition. Inert nitrogen offers the best possible protection here.
In the second step, the press is closed and the bladder is filled with hot steam at 15 to 20 bars. This supplies the heat and pressure that are needed for vulcanisation, but it is replaced with nitrogen after a short time because steam would heat up again too much under pressure. However, heat and pressure must be kept within an optimal range if the desired quality is to be achieved. The larger the tyre is, the longer the process takes. For conventional car tyres, it requires about 10 minutes; for the tyres used on heavy goods vehicles, which can be up to four metres across, it can take a whole day. The use of nitrogen guarantees a uniform distribution of heat over the entire period.
The improved pressure stability and uniform heat distribution during vulcanisation that results from the use of nitrogen reduce the cycle time while extending the service life of the bladder. This allows higher rates of production as well as lower production and maintenance costs.
Following on from the hot N2, very cold CO2 comes into play when removing excess rubber from the press mould. The use of carbon dioxide dry ice allows the moulds to be cleaned without disassembly and with minimal loss of time. Cryogenic dry ice pellets are “shot” at the moulds from a blasting machine, such as the ASCOJET from Asco Carbon Dioxide Ltd, at up to 300 metres per second. Thermal shock and impact energy completely strip away the contaminants. The pellets change into the gaseous state, leaving behind a clean and dry surface. The tyre mould of the heating press has small venting holes into which the rubber is forced during vulcanisation. This is what gives rise to the small nubs that are found on new tyres. When a new car is fitted with tyres for the first time, the tyres are not only deburred, but also “de-nubbed”. The method of choice for both: The protruding bits of rubber are made brittle with cryogenic nitrogen and are then simply brushed off.
With or without nubs, tyres roll optimally only when they are at the correct pressure. Here, too, nitrogen – or a mixture of nitrogen and argon – is clearly superior to simple compressed air. Tyres that are filled with nitrogen maintain their pressure better and hence retain their optimal rolling properties for longer. This helps to save fuel and, furthermore, prevents oxidation of the inner wall of the tyre, which gives the tyre a longer service life. In Formula 1, on vehicles transporting dangerous goods, on vehicles used in tunnels or mines and on commercial aircraft, the use of nitrogen to fill tyres is a safety requirement. When a jet lands, the tyres are accelerated to about 260 kilometres per hour in an instant as it touches down and thus become hot very rapidly. In order to prevent them from being able to ignite from inside, they are filled with nitrogen.
Gases are also used when recycling tyres. In order to reclaim the valuable raw materials from old tyres, they must first be reduced to small pieces. Cold grinding is used to achieve the best quality here. The ground material is cooled and embrittled with liquid nitrogen. A very fine grain can be achieved by grinding in this way. The raw materials in the tyres – rubber, metal and plastic fibres – can then be sorted into their different types, and some can be reused for tyre production.
Old tyres in which the load-bearing structure, the carcass, has remained undamaged can be reused by replacing the tread. This is a particularly common practice for old lorry tyres. The first step of retreading is to inspect the carcass. If it is in good condition, the old tread is stripped away, producing rubber “buffings” in the process. The new tread can then be laid down. There are two procedures for this: the cold and the hot retreading processes. In the cold retreading process, a pre-vulcanised tread is attached to the carcass and is then bonded with the carcass under pressure in an autoclave at about 110 degrees Celsius. An alternative to this is the hot retreading process. In this process, the new tread is laid down in the form of strands of extruded rubber mass and is subsequently vulcanised with nitrogen in a heating press. This fuses the rubber mass and the carcass into a single unit. Nitrogen offers the same advantages here as it does for the production of new tyres.