DRAFT Polymers, fillers, interaction

Efforts to optimise polymer-filler interactions are helping to increase the sustainability and performance of rubber compounds:

Improved understanding of how polymers and fillers interact in rubber compounds is guiding R&D and commercial developments in the tire and rubber-products industries – programmes at three major suppliers indicate.

A particular focus is on modifying rubber polymers and the carbon black and silica-silane reinforcing filler systems to optimise the performance of tire compounds.

Birla Carbon is among the companies researching how surface modifications can enhance the dispersion and reinforcement effect of carbon black in tire-rubber applications.

To optimise its effectiveness, carbon black must be well dispersed in the rubber compound, Dr Lewis Tunnicliffe, lead scientist, product development at Birla set out at a recent IoM3 Rubber in Engineering Group technical meeting.

To be effective, the filler dispersion process requires a surface chemistry that promotes easy incorporation, Tunnicliffe explained at the event held 22 March in London.

But despite the best efforts to disperse carbon black, aggregates tend to re-agglomerate due to differences between the surface energy of the filler and polymer.

With functionalisation, however, the filler-polymer system can be tailored to achieve an optimised balance between stiffness and hysteresis in rubber compounds.

Among a range of carbon-black treatment options, Tunnicliffe focused in on the use of oxidation to achieve a polar filler-surface that interacts well with functionalised elastomers.

The Birla scientist presented the findings of tests on different tire rubbers, including an in-chain, carboxylic-group functionalised solution styrene butadiene (SSBR).

The enhanced polymer-filler interaction resulted in a large increase in bound rubber, leading to lower compound hysteresis and tire rolling resistance as well as improved wet traction.

Epoxidised rubber

Among the compounds being touted as sustainable alternatives to solution styrene butadiene rubber (SSBR) in tire compounds are those based on epoxidised natural rubber (ENR).

While these materials have been available to the tire industry for almost three decades, ENR compounds are still some way short of matching the performance of SSBR.

This could be set to change, believes Dr Joachim Bertrand, director of marketing and sales for rubber at Behn Meyer Europe, which supplies chemicals to the tire and rubber products industries.

In an interview for Tire Technology Expo, Bertrand explained Behn Meyer’s development of surface-modified silica grades to overcome the problems that have limited the use of ENR in tire applications.

While epoxidation helps to directly link natural rubber to silica, ENR compounds still fail to compete with synthetic-rubber tire compounds when it comes to abrasion resistance.

Moreover, the epoxy groups make the rubber less compatible with silica so that mixing is more difficult. And, adding silane coupling agents can lead to cleaving of the functional groups thereby increasing polarity of the polymer.

According to Bertrand, Behn Meyer’s latest surface-modified silica grades deliver much better compound properties, especially abrasion and moduli, while almost matching the wear performance of unmodified NR.

The new compound systems increase the effectiveness of reinforcing fillers in ENR compounds, “because they increase the network density”, he explained.

The products are said to offer new options to design tread compounds with NR and ENR, ranging from low- to high-damping behaviour and featuring acceptable wear performance.

Behn Meyer is also investigating ways to modify the bonding mechanisms to create special characteristics in ENR compounds, particularly for non-tire applications.

Meanwhile, Evonik Industries has developed a silica/silane technology for use in natural rubber compounds that could help reduce rolling resistance in tires.

Truck & bus tires

The company says its system based on Ultrasil 91000 GR precipitated silica reinforcing filler is particularly suitable for use in truck and bus tires – now a target for EU regulators seeking to reduce COs emissions.

According to Evonik, testing shows that the silica/silane technology could yield fuel savings of up to 8% versus traditional carbon-black reinforced NR compounds, while also improving braking performance.

“Silica/silane technology, especially when used in green tires for buses and trucks with high fuel consumption levels, is making an important contribution to climate protection,” said Jens Kiesewetter, Evonik’s head of technical service for rubber silica.

Despite having a high specific-surface area, Ultrasil 9100 GR is incorporated into and dispersed in the natural rubber compound relatively easily, according to Kiesewetter.

Evonik is currently expanding tire silica capacity at a plant in Adapazari, Turkey, including that for its Ultrasil 9100 GR precipitated silica reinforcing filler.

Designed to increase capacity by 40 kilotonnes per annum, the project is expected to be completed in late 2019.