Conti tests the future

Technology centre in Hanover is the focus for some interesting work in the field of rolling resistance and related tire technologies. Patrick Raleigh reports

Typically, it takes about two years and a lot of hard work to develop a new tire and the process can involve many loops with customers, according to Dr Gerrit Bolz, head of process development, tire assembly at Continental.

Bolz was speaking at a press event in the German group’s Hanover technology centre, the main hub of a €250-million a year research & development effort employing around 12,000 people worldwide.

Currently, much of that R&D effort is around optimising the balance between wet grip and the levels of rolling resistance of tires, said Bolz, noting that the latter characteristic is currently a priority for car manufacturers.

“The vehicle makers are pushing us more in the direction of low rolling resistance rather than wear,” the Conti technology head commented. “The motivation for endurance comes from the market more than OE.”

But tire-development still comes back to achieving an optimum balance in tire performance, Bolz remarking: “Some tires on the market have very good rolling resistance, but we would not sell them due to low wet grip.”

Bolz said he did not expect huge steps forward in the future for braking distances and forecast that the biggest improvements would be in rolling resistance.

The Conti manager went on to highlight how processing conditions can often make the difference between a high-performance tire and an ordinary tire.

“If we cure the tire there is an optimum curing stage to have the best rolling resistance,” he explained. “While typically in a press there is one temperature, physically you want to have the same curing stage in a thick layer as in a thin layer.”

But as the tire tread is quite thick in can be under-cured, while the much thinner sidewall can end up over-cured.

“Tire makers go to the safe side but in doing this don’t achieve the full potential, particularly with regard to rolling resistance,” said Bolz.

In response, Conti has developed a technology to adjust the cure time or cure temperature to the thickness of the tire component and so improve rolling resistance.

“This is something we do not advertise: we have the technology to do so and we are extending this technology,” the process development chief added.

Bolz declined to give further details but said: “The inside of a press is usually made of metal so it is a big challenge to control temperature zones. Try to imagine how much effort it is to have different temperature zones in our presses and to adjust these.”

This focus on the relationship between processing and tire performance is “the direction we are going,” said Bolz who again emphasised the scope for improvements in rolling resistance.

“There are lots of possibilities and the effort is getting higher,” he said.

In terms of the tire itself, Bolz said Conti was focused on making tires thinner and thinner because rubber causes damping and so wastes energy.

“You can do it with rubber properties, but the easier thing to do is to reduce the amount of rubber,” said the Conti manager.

And, while noting the challenges of handling veryy thin materials in production, Bolz said he expected tires to become much lighter in weight over the next 10 years.

In the lab

Continental is developing new laboratory techniques to increase its ability to determine the performance of tires materials as they contact the road surface.

The main types of materials behaviour of interest to tire makers are more or less the hardness or stiffness of the compounds and the damping behaviour.

Minute irregularities in road surfacing mean the tire is being hit at frequencies of up to 100,000 Hertz depending on the surface and the velocity, a Conti lab manager explained.

Conventional dynamic test machines, however, measure compound behaviour at frequencies of around just 100 Hz. Results are then extrapolated to gauge performance at higher frequencies as a guide to performance parameters such as wet braking.

To extend the experimental range, tests have more recently been developed in which a rubber strip is put between two plates and shear forces applied with equipment at up to 1,000hz.

But Conti has taken this a step further with a new test – developed with test equipment supplier Metravib – that measures the response to impacts on small cylindrical samples, according to the lab manager.

“In this way, we can get up to 10,000 Hz, a range of real application for the compound,” he said. “This allows us to really get to the range that is important for tire performance, rather than making assumptions through extrapolation.”

After about a year of trialling, he said Conti is now “really evaluating this dynamic mechanical testing method for measuring stiffness and damping behaviour up to 10kHz.”

“Other companies can do this but it needs a lot of work,” added the manager. “It is not just plug-and-play: you have to spend time to do the right set-up for the test and evaluation to get reliable and meaningful results.”

Another fundamental property of polymers is glass transition temperature (Tg), which plays an important role in dynamic behaviour of tire materials.

The standard way to determine Tg is with differential scanning calorimetry (DSC) which measured sample temperature under quasi-static conditions. This standard approach typically measures at heating/cooling rates of several Kelvins per minute

But to better reflect condition in a rolling tire, Conti is now using a technique called flash DSC, which can measure at rates that are many orders of magnitude higher than the conventional technique.

“So, you are immediately heating up and cooling down the sample and can determine what happens to these phase transformations at 1,000 Kelvins per second,” explained the lab manager.

This, he said, enables test technicians to better “simulate what happens within the rubber compounds with very high increases in temperature in very short periods of time and when it cools down again.”

According to the Conti expert, flash DSC is particularly relevant for monitoring tire wear characteristics.

Conti introduced the testing techniques about a year ago and is now starting to get the first meaningful results, said the manager, adding that “as far as I know, we are the first tire maker to use flash DSC technology.”

Report based on article in the September/October issue of European Rubber Journal print magazine