There is a growing interest to increase the portion of bio-based components in various consumables. We have previously discussed about the challenges to incorporate microfibrillated cellulose (MFC) into composite materials with hydrophobic matrixes, such as PLA. Today we will take a step even further and see how cellulose fibrils can support the development of more environmentally friendly tires with high performance and durability.
The tire industry has for years developed tires which have lower fuel consumption and lower CO2 emissions as a result of lower rolling resistance. Despite the development in the industry, it is evident that the used tires create a huge recycling challenge as only in Europe 300 000 000 tires are annually removed from use.
Could MFC turn out be the key component for the manufacture of greener tires?
The building blocks of a Tire tread
Let us first take a look what a typical tire tread is made of. The major components of a tire tread are rubber (both natural and synthetic), fillers (carbon black and silica), silanes and additives. The ratio between natural and synthetic gum depends on the end use of the tire. The amount of natural gum is higher in the tires which are targeted for heavy use, such as trucks and tractors, as it gives more mechanical resistance to the tire. The amount of fillers is around 30% of the tire tread, meaning that in theory one could be able to increase significantly the bio-based portion of the tire by substituting fillers with polymers from renewable sources, such as MFC. This would decrease the environmental burden and carbon footprint of tires considerably.
Greener tire tread development
A first step to greener tires was taken in 1992 when Michelin introduced their green tire technology, based on silica filler and bifunctional silane, instead of traditional carbon black. The technology allowed to reduce the rolling resistance and at the same time increase the wet grip of the tire without sacrificing other properties. This was the first time that it was possible to go beyond the boundaries of the so-called magic triangle, which is a balance between rolling resistance, wet grip and abrasion resistance. It has to be noted that the “greenness” of these silica based tires is based on the lower fuel consumption, not on the use of renewable materials.
Goodyear took a step even further, as they introduced in 2001 a tire containing a corn starch based filler. In addition to lower rolling resistance and improved wet performance, the CO2 emissions were decreased by 7.7 g/km
We have previously compared fumed silica and MFC as well as described how the properties of a gel coat can be improved by using MFC instead of fumed silica.
MFC to replace silica in tire tread
Rubber feels oily and as a first thought it sounds impossible to mix highly hydrophilic MFC into rubber. The natural rubber, which mainly consisting of cis-1,4-polyisoprene, is however collected as a liquid suspension from the rubber trees. The rubber is coagulated from the liquid by acids, filtered from water, formed into sheets and dried. Therefore it should be in fact quite easy to introduce MFC into the natural rubber mixture. Yano et al. described in their patent application how MFC can be incorporated into tires. They mixed MFC derived from kraft pulp with natural rubber, and after high shear mixing, they coagulated the resulting mixture with aqueous formic acid. After drying, the master batch was mixed with rest of the tire components and vulcanized. The physical evaluation of the material indicated good driving stability and low rolling resistance.
It is also known that silica increases the wet grip of tire by increasing the hydrophilicity of the tire. As MFC also has a huge amount of free hydroxyl groups it can be assumed that it would function in a same way as silica. Also the hydroxyl groups on MFC would allow the use of bi-functional silanes for increasing the compatibility with the rubber, the same way as for silica.
What will the Future bring?
As it is only less than 30 years since the introduction of the silica technology and the story of MFC has just started, I'm quite optimistic that during the following years the incorporation of MFC into tires will become reality. The commercial availability of MFC is no longer a limiting factor for the development and the chemical nature of MFC is so similar to silica, that the compatibility should not be an issue either. Together with the synthetic rubbers, derived from renewable biomass, we are a step closer to a completely bio-based tires.
The future will tell how and in which form MFC will find its way to the tires eventually.
Editor's note: This post was originally published in March 2017. We've revamped and updated it for accuracy and comprehensiveness.
Otto Soidinsalo works as a technical application manager at Borregaard. He has a Ph.D. in organic synthesis from the University of Helsinki and his working experience ranges from organic synthesis, cellulose ethers and its applications to nanocrystalline cellulose and microfibrillated cellulose.
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