Tire industry utilizes about 70% of rubber production, both natural and synthetic, the highest level compared to other industries. The industry is constantly developing raw materials, capacity and quality of products, technology and equipment. Moreover, there are more laws and regulations related to higher quality and more environmentally friendly products. Capacity includes wet grip, rolling resistance and tread wear resistance (picture 1)
Picture 1 “Magic tire triangle” is three characterizations that indicate capacity of tire tread.
Initiated in the European Union (EU) per EU Regulation No. 1222/2009, tire labeling grades tires based on level of energy efficiency or rolling resistance, road adhesion or wet grip, and level of noise measured outside of the vehicle in motion. The labeling system has already been adopted by Japan, South Korea, and the USA. China, which is among the world’s largest markets, is recently in the process of adopting the labeling system as well. Sample of tire labeling (in picture 2) vary from country to country – EU grades tires based on level of energy efficiency which depends on rolling resistance, wet grip, and level of noise whereas USA grading is based on level of energy efficiency, wet grip, and tread wear resistance which indicates durability and then Japan emphasizes level of fuel efficiency and wet grip. This development should be a wake up call to other competitors in the world’s market.
Picture 2 Samples of tire labeling: (a) EU; (b) USA; (c) Japan
A new tread formula that replaces carbon black, a sooty-by-product of the burning of oil that gives tires their blackness, with silica has proven to lower resistance hence increases fuel or energy efficiency. In 1992 Michelin of France signed patent right of the formula of silica and silane blend which, when compared to its carbon black formula counterpart, significantly lowers rolling resistance, improves wet grip, and reduces carbon dioxide emission. The formula was initially experimented with synthetic rubber tire then later with silica-reinforced natural rubber truck tires.
Since silica filler does not blend well with natural rubber compound, there is an issue of low bondage between rubber and silica surface. Silica particles are hydrophilic thus has considerable difficulty in dispersing in the hydrophobic rubber compound. In order to overcome this issue, silica is to be mixed with coupling agent like silane to increase blending. Elastomer Technology and Engineering, University of Twente, of the Netherlands, has long led a research effort on using silica to reinforce natural rubber compound in truck tires, which are divided into two directions: 1) blending silica with silane coupling agent 2) using epoxidized natural rubber as compatibilizer in place of silane as coupling agent. The research concludes the following:
1) Using Bis-(triethoxysilylpropyl) tetrasulfide (TESPT) coupling agent with silica proves that tire characterization depends on proper blending, rubber compound formulations, and mixing temperature. The temperature influences level of silanization between silanol units on silica surface and alkoxy group of coupling agent. Optimal silanization increases interaction between silica and rubber compound which in turns disperses silica evenly and efficiently reinforces silica into elastomer. The volume of TESPT, Diphenylguanidine (DPG) and Conventional precipitated silica compared to Highly dispersible (HD) silica in silica-reinforced natural rubber prove that proper tire formulation is imperative.
2) Using epoxidized Natural Rubber (ENR), both unaffected molecular weight and low molecular weight epoxidized natural rubber (ELMNWR), as compatibilizer in silica-reinfocrced NR improves interactions between epoxidized groups of the ENR and silanol-groups on the silica surfaces. This means that the ENR compatibilizer increases silica dispersion in the compounds and vulcanizates have better mechanical properties, when compared with a silica-reinforced compound without. However, it is found that vulcanizate is still inferior than using silane coupling. In order to solve this issue, replacing silane coupling agent with ENR in part can increase safety and environmentally friendly effect as it reduces alcohol or volatile substances produced by silanization.