The inner lining that retains air

he wheels on a car - or a motorcycle - can be visually striking, with their elaborately designed alloy wheels and black tires boasting a vast array of dynamically designed tread. The substance of a modern tubeless tire, however, does not stop with the visible elements, but includes a crucial and little-known feature: the inner liner. This thin layer covers the inside of the tubeless tire and performs several functions, the first of which is to prevent air leaks. The absence of an inner tube, which could be optimized to seal the tire against leaks, forced manufacturers to assign this function to the tire itself, and this is where the inner liner comes into the picture.

A vital element

The retention of gas, whether air or nitrogen, is crucial in maintaining the correct pressure, which is one of the variables, along with the state of the tread, that will most likely affect a tire’s performance. It is not difficult to maintain proper tire pressure, and yet more or less regularly we here news about many cars being found with tires unevenly worn or under inflated. A recent survey reported flat tires, damaged treads and other non-compliances in nearly 1 in 5 vehicles. Even in the U.S., the situation is far from ideal when 1 in 3 Americans do not know when tires need to be changed and 40% measure the tire pressure based on physical observation alone.

Airtight liners are needed now more than ever as maintenance intervals have lengthened and in some cars this is determined on the type of use. If service intervals have lengthened, there are fewer opportunities to check the tires, so inner liners have to be even more "airtight".

New demanding mobility

The correct pressure enhances also a vehicle’s efficiency, and when this is lower than it should be, the higher working temperature of the tire will shorten its life and increase friction, which translates in higher consumption and emissions. As hybrid and electric vehicles become increasingly popular, this aspect must be taken even more seriously as it could decrease a vehicle’s mileage.  With the changes to mobility patterns, current and future, there will be an increasing need for maintenance-free tires still able to deliver the lowest possible air leakage rates. Materials research is therefore even more intense, and ExxonMobil, for example, has developed its own compound. For decades now, halogenated isobutylene-isoprene is the polymer used in inner liners, abbreviated to halo-butyl, which is different from natural rubber and the Sbr and Br polymers used for the other elements.

Chemistry helps

The latter are rather porous, since their carbon-carbon bonds are angled at 109°, and therefore not very suitable to form an inner airtight layer. In isobutylene, the angle increases to 123°, and it can be said that the closer the angle between the carbon atoms is to 180°, the “flatter” the polymer chains are. This creates more polymer chains per unit volume hindering the escape of the gas molecules. However, a polymer made only with isobutylene does not create a consistent enough rubber, but with a small amount of isoprene a good polymerization can be achieved. The presence of halogens, such as bromine or chlorine then improves the adhesion of the liner to the inside of the casing. To improve workability, various fillers are added, some of which are similar to those used in the casing. However, since these have a low molecular weight, they are more permeable while isobutylene has a tendency to degrade with heat.

Made for liners

ExxonMobil's recipe stems from the use of different polymers: that is, isobutylene-co-paramethylstyrene brominated (Bimsm) without isoprene. The trade name of the compound is Exxpro. The structure is particularly compact and gas molecules escape with difficulty. The 3563 formulation, designed specifically for liners, is more workable and dimensionally stable. The carbon black used to improve its characteristics disperses much better, giving a better consistency. Other companies too are studying new formulas for inner liners, so we can be confident that tires suitable for new mobility patterns are on the way and will improve significantly over the years.

The production of "tubeless" tires is now well-established thanks to the use of liners with the task of retaining the pressure. One of the key properties of the liner is airtightness, which is essential to maintain inflation pressure over time, and therefore reduce the frequency of maintenance, and to protect the inside of the tire from oxidation in order to extend its useful life. Over the years, Bridgestone has developed increasingly airtight compounds to optimise the properties just described and reduce the thickness of the inner liners in order to reduce the weight of the tire and its rolling resistance.

Looking at a new mobility model

In the case of electric vehicles, there is a need to develop tires with low rolling resistance to maximize mileage. To this end, Bridgestone is developing airtight technologies both to increase pressure resistance and to reduce the thickness of the material in order to produce lighter tires. In this case, though, in addition to the challenge of materials, there are issues also in relation to working with thinner components. Shared or autonomous vehicles imply reduced vehicle maintenance. This is why the Japanese manufacturer is looking at working on "puncture solutions", which may or may not involve the use of inner liners.

Marco Vellone, pr & communication, after sales director

Inner liners are therefore quite interesting elements and to learn more about them we gathered some information from Marco Vellone, pr & communication, after sales director of Continental Italia. We also asked if innovations in mobility - electric cars and car sharing/hailing formulas - could influence the development of inner liners. Vellone confirmed how important inner liners are because all other tire components are quite permeable to gases. Air leaks are accentuated every time the tire is subjected to stress, for example when encountering potholes or hitting accidentally a sidewalk. The phenomenon, however, is natural and occurs, even if imperceptibly, each time the sidewalls flex during wheel rotation.

EVs are more exposed

Electric cars are more sensitive to this issue for a number of reasons. In fact, the need to have low rolling resistance clashes with an average weight that is higher than a conventional ICEV and a maximum torque that is delivered from the get go . This leads to higher inflation pressures in order to increase the stiffness of the sidewall and reduce micro-slippage due to the torque that arrives all at once. All these factors complicate the work of the inner liner.

Inflation pressure and EVs

Maintaining the correct pressure is important for electric and hybrid vehicles, as this will maximize mileage while reducing rolling resistance. At Continental, tests were performed on Renault Zoe, electric Ford Focus and Tesla Model S. Different cars with very different weight and power and 14 -16 - and 19-inch wheels respectively. Rolling resistance was then converted into CO2 equivalent emissions. Since EVs are very efficient, a reduction of one kilogram of CO2 equivalent emitted corresponds to a 3-4% increase in mileage.

Improved Inner liners for all kinds of tires

Continental is working not only on inner liners dedicated to electric vehicles. The German manufacturer, in fact, is equally hard at work on heavy-duty tires that work with much higher pressures and loads, as well as lighter vehicles. Continental's innovative Air Keep technology has a molecular matrix with a "labyrinth" shape that, by obstructing the passage of air, minimizes the permeability of the liner. These are fillers that act like three layers of an overlapping network with a slight rotation of each layer, thus creating "obstacles" in the path of the air molecules.

Proprietary formulas and controls

This increased “airtightness” is useful for all vehicles because it preserves good levels of safety, durability and energy sustainability for longer. Research continues but, for now, the established butyl rubber base is still the go-to solution. The most permeable components are those with a low molecular weight and in order to reduce them - while maintaining the butyl base, acceptable costs and good workability – experiments are being carried out involving materials that are protected by industrial secret. The improvement of inner liners is stimulated by the need for increased mileage. Some models determine maintenance intervals according to use, but it goes without saying that regular tire checks are a guarantee of safety. The issue becomes even more accentuated with electric vehicles, which require little maintenance, so the entire supply chain will need to come to grips with tire maintenance needs. The weight and strong torque place increased pressure and stress on tires and therefore maintenance must be relatively frequent, which collides with the reduced need for mechanical work and assistance.