Dunlop Falken Tires Ltd (head office: Tokyo) of the Sumitomo Rubber Group unveiled "ENASAVE 97" at the 40th Tokyo Motor Show 2007. It is made of non-petroleum natural materials by 97%.
The tires will be released in May 2008, according to the company. The amount of petro-related materials was drastically reduced by using materials other than synthetic rubber for side walls and the inner liner.
The inner liner is a rubber sheet stuck on the inner surface of tires. It prevents the air leaking from the tire. The rubber sheet needs to be airtight. It was difficult to replace synthetic rubber with natural rubber, which is less airtight.
Because natural rubber has a molecular structure without branches on main chains, the air leaks through the material easily. The company paid attention to this point and improved airtightness of natural rubber by applying branches of epoxy radicals on its main chains.
The side walls on the sides of tires repeat distortion responding to the revolution of tires. This requires tires to be resistant to bending. Conventional material was a mixture of synthetic rubber and natural rubber, where islands of synthetic rubber are floating in the ocean of natural rubber. The islands of synthetic rubber prevented the cracks in the natural rubber ocean, realizing high resistance against bending.
If the material is replaced with natural rubber, generated cracks are likely to extend because it does not have islands. To solve this problem, the company created islands made of natural rubber and modified rubber. However, the rubbers do not blend easily. As a solution, the company blended vegetable oil and devised a mixing method.
The tread rubber is required to keep the rolling resistance low while maintaining high grip performance. The company developed a plant-derived modifier that improves flexibility of rubber. As a result, the company succeeded in creating a tire with high grip performance and low rolling resistance.
The rolling resistance is approximately 35% of "EC201," one of the general tires that contain synthetic rubber by more than 50%. The tire improves the fuel efficiency by approximately 7%, according to the company.
By increasing the rate of non-petroleum natural materials to 97%, CO2 emission during manufacturing of tires will be reduced by 17% compared with EC201. In addition, because 57% of the materials used are biomass, which absorbs CO2 in the process of growing, CO2 emission at the time of disposal will be reduced by 94% compared with tires made of petro-related materials.