Most important it will help tire makers to deliver higher-performing tires. Not on its own, but in combination with other materials and technologies.
Styrene-butadiene rubber (SBR) was one of the first synthetic polymers to be developed back in the 1930s. It was made in an emulsion process (E-SBR). That was a good process for those times. Today, it is not enough for top-rated tires.
E-SBR offers limited control
The main problem with E-SBR is that there is limited control over the process. The different monomers (styrene and butadiene) combine together in ways which cannot be predicted.
This process results in a polymer which has a wide range of molecular weights and morphologies.
That was great for tires of the 1950s and 1970s which offered limited grip and poor fuel economy. It is not so good for tires which offer good fuel economy and long life.
Many generations of S-SBR
Latest versions of S-SBR offer accurate control of molecular size and shape.
More importantly, they have specific chemical groups located at pre-determined sites along the molecule. These groups improve interactions between the rubber and the filler (such as silica). They also help the different rubber molecules to interact.
These functional groups help to improve fuel economy and grip in both the wet and the dry. Future generations of S-SBR will also help to improve wear properties.
The first generation of S-SBR came out a decade ago and they offered better control of the chain length and the extent of branching, but little more.
Controlling molecular weight and shape means the resulting rubber behaves in a consistent way. Those early S-SBR materials did not have much impact on the properties of finished tires. The new materials did mean that processing in the factory was more consistent.
The next generation added functional groups at the chain ends. This is important because a significant part of rolling resistance originates where molecular chain ends are free to move within the rubber matrix. These loose ends can move around and that absorbs energy. That energy emerges as poor fuel economy.
The functional groups at the chain ends anchor the chain ends to a filler particle. The result is that the tire delivers better fuel economy in service.
Later generations also have functional groups along the backbone of the molecule.
A revolution in computer-aided design of molecules has had its effect on S-SBR. It is now possible to create within a computer, a molecule which has never appeared in nature. This design process can start with the desired characteristics and work back to the molecule.
With very careful control of the polymerisation process, that molecule can be turned into a reality within the reaction vessel.
Several companies who specialise in rubber for the tire industry are using these techniques to create customised molecules. Such customised molecules are not just for an idealised tread rubber. They can offer a specific combination of properties which are appropriate for a tire performing on ice at high speeds. Another molecule might be better for a heavy load-bearing tread operating for hours at a time over long highway distances.
These molecules need careful control during polymerisation. Two families of solution processes have developed. Continuous processes offer better productivity, but less control. Batch processes offer tight control, but are less efficient.
Batch processes are now being used for the most advanced molecules, but these are relatively expensive. The continuous processes are used where volumes are higher and these tend to be in the less specialised materials.
There is little difference in the processes used to make high-performance butadiene rubber (BR) and S-SBR. The plants which make BR can also make S-SBR and the other way around. This means some rubber companies operate swing plants. These can be used to make different types of material.
As a result, it is hard to estimate precise capacities of S-SBR around the world.
Countries around the world are introducing consumer labels for tires. Governments do this in an effort to reduce fuel consumption. Tire makers can achieve top ratings in fuel economy while maintaining performance in grip, life and other parameters only by using the very latest S-SBR materials.
The chart (courtesy of Lanxess) shows how different materials are required to achieve different label scores. The top A-grade can only be achieved with functionalised S-SBR as well as neodymium-catalysed BR and silica.
Consumers are influenced by the tire label. Tire makers are therefore trying to make tires with the best label scores. This means the demand for high-performance S-SBR materials is growing rapidly.
The introduction of a tire label in the EU triggered a wave of research into advanced materials. S-SBR technology advanced rapidly prior to the introduction of the label in Europe. This was driven by tire makers wanting the top label scores and by advances in computer power and molecular design capability.
As the United States and China also adopt tire labelling legislation, demand for S-SBR is expected to grow faster.
Who supplies S-SBR
Sumitomo Rubber Industries is also working with its sister company Sumitomo Chemical on custom-designed molecules. Sumitomo is also building a new S-SBR plant in Singapore.
In Europe, three companies supply S-SBR materials: Styron, Lanxess and Versalis. Lanxess is very strong in butadiene rubber but less so in S-SBR. However, as world leader in synthetic rubber, it can use its BR plants as swing plants for S-SBR as well.