Rubber products – and tyres are no exception - deteriorate over time, thus changing many of their physical and mechanical properties. This process is inevitable and can worsen the behaviour of tyres and affect safety. Several studies have been made on the subject, and among the findings, it was discovered that even spare tyres age even if not used. These findings show how tyre specialists can play an important role in ensuring the safety of motorists by carefully monitoring the age and condition of tyres.
Tyres age from within
One of the main culprits is rubber oxidation, i.e. the combination of oxygen with the polymer chains in the compound: research by the US National Highway Traffic Safety Administration (NHTSA) has shown, for example, that 'tyres degrade mainly from inside out, due to the presence of oxygen in the pressurised air inside the tyre, the effect of which is proportional to temperature'. NHTSA also identifies the four key elements that determine how quickly a tyre will age. First the inner liner, that has the specific purpose of retaining pressurised air. Generally made of butyl rubber, it obviously cannot be completely impenetrable and so some air will slowly seep through the casing of the tyre depending on the actual permeability of the liner. Another key factor closely linked to the first is air pressure: a tyre stored waiting to be fitted and inflated is only subjected to atmospheric pressure and therefore the degree of oxidization will be greatly reduced. In spite of this fact, inactivity is not a tyre's best friend because the pressure applied to the tyre when in use and the flexing of it due to rotation causes the oils contained in the compound to circulate and therefore lubricate the tyre, inhibiting the development of cracks on the outer edges. The fourth factor, probably the most insidious, is temperature, which we know could be very high in summer due, in part, to the prevalent warm weather of the season, but also to the contact with hot road surfaces. The mechanism of action is twofold: the high temperature increases both the permeability and the reactivity of oxygen, thus facilitating the passage of oxygen through the inner lining and its effect on the tyre.
Temperature, under scrutiny
Another NHTSA report, presented to the US Congress, showed a significant correlation between climate zones and tyre wear: warmer states showed a higher incidence of tyre-related accidents. E.T.R.T.O. - the European Tire and Rim Technical Organisation, which in 1964 took over the mission of the European Tyre And Wheel Technical Conference - also draws attention to the conditions under which tyres are used and stored. In the E.T.R.T.O Recommendations of September 2018, the link between tyre storage conditions and use, the maintenance of their performance and their durability are mentioned several times. With regard to storage, we can read for example that "The storage temperature should be below 35° C and preferably also below 25° C. At temperatures above 50° C, and especially if stock rotation is insufficient, accelerated deterioration may occur. Any direct contact with hot pipes and heaters should be avoided”.
Storage at very low temperatures "is not harmful in itself, but can cause the tyre to stiffen; so, tyre deformation during handling or assembly should be avoided unless the tyres are brought to room temperature beforehand". Other indications concern humidity and condensation: “tyres should be stored in a cool, dry and moderately ventilated place. If tyres are stored outdoors, they must be covered to protect them from water and humidity”. E.T.R.T.O. also recommends avoiding contact with ozone, which is produced not only by mercury-vapor lamps and electrical machines but also by photochemical reactions acting on exhaust gases. All sources of ultraviolet radiation must also be eliminated (some fluorescent lamps produce appreciable amounts of it), and solvents, fuels, lubricants, chemicals, acids, disinfectants and the like must be stored in a separate room. Additional recommendations concern deformation and stock rotation: tyres in stock must not be subjected to any deformation whatever the cause, and the duration of storage of new tyres must be kept to a minimum so that tyres stored first have priority in leaving the warehouse. These precautions also apply to items such as inner tubes, their protective flaps and valves; if possible these measures should also be applied during transport.
Keep an eye on the manufacture date
Even if tyres have been stored following these recommendations, a certain amount of degradation still occurs due to ageing, which happens faster for tyres that have been fitted as they are more exposed to harmful elements such as ultraviolet rays, ozone, high temperatures, fuel vapours and the like. Again, E.T.R.T.O. reminds us that 'tyres age even if they have not been used or have been used little. Tread and sidewall cracks, sometimes accompanied by casing deformation, are possible indications of ageing: old tyres should be checked by specialists to ensure that they can be used'. Tyres used on, for example, caravans and boat trailers, which are inactive for long periods of time, tend to age more quickly (see above) than regularly used tyres, and it is therefore important to lighten the load and shield them from direct light. Particular attention should also be “paid to spare tyres, which can age even if they are rarely or never used: in that case they should be used with great care and replaced as soon as possible”.
Finally, we report the position of Fabio Bertolotti, director of Assogomma, regarding a 9.90 euro surcharge per tyre requested by an Internet site to have tyres with a 2021 manufacturing date: "Tyres are not chosen on the basis of when they were produced, and even less so on the basis of their expiry date, which does not exist, but on the basis of their brand and their specific technical characteristics". The Assogomma communiqué also states that "given the same product, tyre life is mainly influenced by the conditions of use, i.e. the combination of driving style, external conditions and the type of vehicle" and also that "even before fitting, the life of a tyre is determined by the conditions in which it has been stored: its state of preservation is in fact directly related to its correct storage". So, what really counts for the life of a tyre, more than the year of manufacture, is the care to which it is subjected, before and after being fitted on the vehicle.
Making tyres more resistant
A study by John Baldwin and David Bauer reviewed various cases of tyre failure and included countermeasures to combat tyre ageing. One of the most basic solutions is to use antioxidants in the compound and to replace inflation air with Nitrogen, since most of the Oxygen responsible for the oxidation of the rubber comes from within the tyre. This practice is widely used in demanding applications such as aerospace, racing and off-road trucks. Nitrogen also has a lower permeability rate than oxygen and therefore improves inflation pressure retention. These advantages became clear during artificial ageing tests conducted in special ovens but are more difficult to identify on the road. The role of the inner liner, on the other hand, is clearer, and research by Waddel and others, found that switching to a 100 per cent halo-butyl rubber composition reduces permeability by 2.44 times and breaking test resistance by 60 per cent. A low permeability inner liner is even more useful for truck tyres, which reach inflation pressures of 8/9 bar. If inner liners with a high (if not total) halo butyl rubber content allow a significant improvement in gas permeability, even more promising results are possible with nanotechnology. Liners containing nano-clays, for example, greatly restricts their passage of gas molecules. Exxon Chemical and Yokohama, on the other hand, offer vulcanised liners made of rubber and polyamide instead of the traditional butyl rubber liner.