From the outside, tyres appear unassumingly simple. But below the tread, they are deceptively complex composite products - resulting from a marriage of highly developed polymer material with metal, carbon black and other additives.
The technology now found in everyday bicycle, car, truck and aircraft tyres is a far cry from the seminal “elastic belt with air”, first patented by R.W. Thomson 175 years ago (see the patent here). Although initial uptake of the technology was slow, the first commercial pneumatic tyres rolled onto the scene nearly 40 years later, in 1888, at the hand of the now well-known Dunlop family.
Dunlop built upon Thomson’s ‘first generation’ tyre designs, and in doing so secured patent protection for the first composite tyres.
This is a formative example of IP driving innovation. Indeed, Dunlop’s composite tyres were far superior, by virtue of their construction from a strong, woven canvas belt laminated with a strip of supple rubber. Furthermore, incremental development in his own design, for example in the weave of the canvas belt (in particular, having threads running both diagonally across and around the circumference of the tyre), led to Dunlop building up a substantial patent portfolio.
However, the natural limitations of the rubber used to protect the canvas belt from the 19th century road surfaces meant that these early tyres were primarily used on lightweight “velocipedes” (i.e. bicycles). Translating the improved comfort and rolling resistance onto larger, heavier vehicles was reliant on improving the resilience of these natural rubbers.
One way to improve natural rubber is through vulcanisation. This process was developed by Goodyear in 1839, and results in a hardening of the natural rubber. The treatment exposes the rubber to sulfur gas, and in doing so the sulfur molecules displace weakly bound hydrogen atoms from the rubber’s polymer backbone. The sulfur molecules can therefore form cross-links between the polymer backbones, to reinforce and strengthen the rubber’s crystalline structure.
A more radical approach, however, was in the development of synthetic rubber. In fact, parallel to Dunlop’s tyre development during the early 20th century, Bayer were working on the synthesis of hard wearing synthetic rubbers. Using similar divinyl building blocks to those found in natural rubber, Fritz Hofmann of Bayer pioneered and patented the polymerisation of various divinyl monomers throughout 1910, including butadiene, isoprene, and other substituted butadiene species.
Bayer simultaneously filed applications directed to the process of polymerising butadiene and isoprene monomers to create a synthetic rubber. Filing multiple applications on the same day is a strategy which is still frequently employed today, in order that multiple applications to related subject-matter cannot anticipate each other.
Since the first generation synthetic rubbers were, at least superficially, identical to natural rubber, Bayer could not patent the synthetic rubbers themselves. Only later, once the process had been applied to new divinyl monomers, such as the substituted butadiene species, were the patents directed to the novel synthetic rubber per se.
The Goldilocks Copolymer
Unfortunately, these early synthetic alternatives were, in practice, no better than the natural rubber they sought to replace. One particular drawback was that the heavily substituted synthetic rubbers were impervious to vulcanisation, so could not be hardened using this well-established technique. It transpired that for vulcanisation to occur, the polymer backbone needed to contain an abundance of weakly bound allylic hydrogens (i.e. adjacent to a carbon-carbon double bond). Otherwise, the sulfur could not readily displace the hydrogens in the polymer backbone to form the strengthening crosslinks.
In essence, what was needed was a synthetic rubber which was both susceptible to vulcanisation (like the allylic hydrogen rich natural rubber), but which also yielded the tyre-specific performance enhancements demanded by the burgeoning automobile market.
This ‘Goldilocks’ combination came in the form of copolymeric synthetic rubber, in particular the styrene-butadiene rubber first developed and patented by Farbenindustrie of Germany in 1929.
Using two different monomers results in a final copolymer having a fusion of properties taken from each of the constituent monomers. In this case, the styrene monomer enhances the physical properties of the rubber itself and eases polymerisation through pi-pi stacking interactions between the styrene appendages. Meanwhile, the unsubstituted butadiene monomer provides the allylic hydrogens needed to facilitate vulcanisation.
This results in a polymer perfectly suited for use in tyres. Indeed, the same styrene-butadiene copolymer is still used today, with millions of tons being produced annually.
Exclusivity from Innovation
Copolymers also afford massive opportunities for customisation. Altering the monomer ratio, tacticity, and molecular weight distribution, to name but a few, allows the rubber’s properties to be tailored to withstand the demands facing a tyre in any application. This boundless variation provides rich picking for new patents, and as such countless improvements to the same basic styrene-butadiene polymer have been patented since the first disclosure over 90 years ago.
From the start, innovation in tyre development has been driven by IP - from the significant changes made by Dunlop to improve the seminal design of R.W. Thomson, to today’s creative, science-led improvements in copolymer compositions in what is now a dynamic and busy IP landscape.
This demonstrates the versatility of the patent system to reward both first steps in a new sector, as well as ongoing technical innovation in a well-established field.
Innovation in tyre rubber technology continues apace, driven by new demands placed on tyres by modern consumers.
For example, there is renewed interest in designing compositions that not only ease the manufacturing process and provide excellent performance when fitted to a car, but which also facilitate recycling after use.
A new frontier of development has also opened up exploring the possibility of tyres playing a far broader role in the driving experience. Tyres are uniquely placed to provide feedback on the driving experience - all being well, they are the only point of physical contact between a vehicle and the road. Recognising this, companies are looking beyond the tyre composition itself and rushing to create so-called “smart tyres”, featuring internet-enabled electronic sensors to provide feedback on car and driving performance.
Watch this space for future blogs on exciting developments in these areas.
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