SILICON DIOXIDE; THE THIRD MILLENNIUM'S "PHILOSOPHER'S STONE"?
Tire compounds are a mix of finely measured ingredients as well as one of the most closely guarded trade secrets. Silicon dioxide represents one of the most important ingredients in this “cocktail” and the object of ongoing studies and developments
A modern tire is an extremely technological component, the result of more than 160 years of research and experiments: in fact, we have to go back to 1855, when Charles Goodyear discovered how the addition of sulfur and its compounds improved the chemical-physical qualities of natural rubber. From that date, which probably marked the beginning of the modern rubber industry, tire compounds have evolved tremendously, mixing an impressive amount of additives and fillers to improve the characteristics of rubber, which was of natural origin in the first decades to be gradually mixed with synthetic polymers the following years.
A world of additives
Additives are a real quagmire, even for experts, either because of sheer numbers or because of unpronounceable names. These substances are accelerators, activators, antioxidant, anti-aging, inhibitors, and much more. Among the first we find mercaptobenzothiazole (MBT) and then benzoic acid, amines, phenols, alcohols, barium sulfate and, of course, the better-known carbon black and silica.
Also the main raw material, namely rubber, is far from being well defined, in the sense that both natural and artificial rubber, the latter being made up of various polymers, are used. Various types of rubber coexist in a tire and it is interesting to note that natural rubber is employed proportionally more in tires dedicated to commercial vehicles: 14% natural and 27% synthetic in light passenger vehicles while commercial vehicles can boast 27% natural and 14% synthetic rubber, a complete reversal.
In both types of tires, silica remains one of the main additives, something like a Philosopher's Stone, able to optimize seemingly contrasting features such as grip and wear resistance. The advantages of silica-based fillers have been appreciated since their introduction in the early 90's.
Taking center stage
The reputation of this material, after almost 30 years, is still unchanged and just think of the fact that in every self-respecting tire brochure or presentation references to this substance are always abundant and very detailed. This is the case of car tires, in which synthetic rubber takes the lion’s share; truck tires, on the other hand, which contain much more natural rubber, have not benefited from the advantages of silica-based additives. But there are signs that things could soon change, given that research efforts in this direction are multiplying.
There are very good reasons to use natural rubber in industrial tires, since artificial polymers such as synthetic poly-isoprene are pretty similar, from a chemical standpoint, to natural rubber but lack its resistance. The polymers that make them up are very similar but artificial ones do not have the proteins and phospholipids that reinforce, as in-born fillers, natural rubber. Furthermore, they also lack that process of stress-induced crystallization, which helps to improve the tensile strength of natural rubber.
This is especially important for tires moving 40-ton trucks or allowing an airliner to land at several hundred kilometers per hour. So far, the combination of natural rubber with silica-based fillers, able to guarantee a low rolling resistance, has proved almost impossible to obtain, but the whole industry is trying hard to solve this issue: improving the tire-road contact on commercial vehicles without compromising resistance is every manufacturer’s dream.
Silica, though, needs “help”
The Elastomer Technology and Engineering (ETE) research institutes at the Dutch University of Twente, led by Professor Anke Blume since 2014, is certainly at the forefront of research: she is quite convinced that, despite the difficulties, a workable solution on an industrial scale can be reached. Blume explains that there are significant differences between natural and synthetic rubber and these prevent the use, in natural rubber, of the same set of proteins and phospholipids and high-polar molecules (molecules which, though not ionized, show partial discharges caused by their non-symmetrical form).
This means that they "compete" with Silanes (a class of substances like hydrocarbons but with silicon atoms instead of carbon), substances used to join synthetic rubber and silica. Moreover, natural rubber lacks vinyl, which helps synthetic rubber to form strong connections with the filler. One solution is to optimize the use of natural rubber with an epoxy group: experiments led by Anke Blume show that a 20-30% of epoxidation can create good links with silica, although a small amount of silane must still be used. Epoxy groups have CH2-O-CH formulas and give their name to the resins used in special adhesives and compounds.
Problems are yet to be solved: mixing problems, for example, are far from being simple to overcome as finding a mix of materials that are reactive enough to be combined easily during curing but do not react so quickly, increasing viscosity, is a matter of difficult solution.
In any case, the path towards epoxidation of natural rubber looks quite promising: a study by the Malaysian Rubber Board (Malaysia is one of the leading producers of natural rubber) confirms the conclusions of the team led by Professor Blume and provides further details on the mechanism behind epoxidation, which increases the polarity of natural rubber making it more "compatible" with silica, which is also highly polar.
Tests performed by the Malaysian Rubber Board on an experimental compound have shown a better wear resistance, better grip on wet road conditions and lower rolling resistance, indicating that the positive action of silica, commonly found in automotive tires, could be transferred also to tires with a high content of natural rubber. Likewise the Sumitomo Group reached a similar conclusions, and therefore, we will very likely have heavy duty tires go “green” because their structure, which uses a higher rate of natural elements, will be enhanced by greater smoothness leading to greater fuel saving.