Shinzo Kohjiya - Reinforcement of Rubber Visualization of Nanofiller and the Reinforcing Mechanism

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This book is the most recent account on rubber reinforcement, highlighting nanofiller clustering in rubber matrix, in particular. It is needless to say that the rubber reinforcement has been recognized to be one of the most important research and developmental subjects among rubber chemists and engineers, together with vulcanization. On the one hand, the vulcanization is an indispensable precondition for rubber in order to make it serviceable as a stable elastic material. It was invented in 1839 by C. Goodyear when a rubber compound containing sulfur and white lead was by chance placed beside the heater which his wife, Clarissa, was using in the kitchen. This anecdote suggests that the kitchen was his laboratory, and he was not a professional technician but a poor backstreet inventor.

On the other hand, reinforcement of rubber by carbon black was reported by S. C. Mote in 1904 in the UK not as an invention but as a practice in rubber compounding process in combination with fiber cord, in order to manufacture automobile tires as a final product. In 1910, the Diamond Rubber Co. in Akron, Ohio, took out a license of this process, followed by the acquisition of the business by B. F. Goodrich in the USA. It is noticeable that the first mass-produced passenger car for ordinary people, i.e., Ford model T was commercialized in 1908. In accordance with the development of mass production of automobiles, the utilization of carbon black grew rapidly in the rubber industry, and black color soon became standard for tires.

Thus, due to the utmost technical importance of rubber reinforcement by loading carbon black, there have been published many, too many, technical reports and know-how to give a lucid and coherent explanation of reinforcing effect on rubber. In other words, the excessively abundant reports have failed to elucidate the exact reinforcing mechanism, which is to be of use in designing rubber compounds. Consequently, only one book entitled Reinforcement of Elastomers has been published so far focusing upon rubber reinforcement, in 1965. The book was edited by G. Kraus, and it has remained to be authoritative on rubber reinforcement for more than half a century. This surprising scanty of the dependable literatures of rubber reinforcement may be due to the extreme complexity of reinforcing mechanism, we have assumed.

The present book discloses a most recent trial to overcome this complexity by visualizing the three-dimensional (3D) image of nanofillers in rubber matrix, which has been enabled by applying a state-of-the-art technique using 3D transmission electron microscopy (3D-TEM), or alternatively called electron tomography, to rubber/nanofiller composites. Through the analysis of 3D images displaying nanofiller dispersion, we have successfully visualized the aggregation state of nanofillers in rubber matrix, from which we suggest the formation of a unique semiflexible nanofiller network. This result is reasonably assumed to be the most important factor for rubber reinforcement by filler loading. In the case of cross-linked natural rubber, self-reinforcement effect (without any filler loading) upon stretching is effectively functioning, and a new polymer crystallization mechanism, template crystallization, is suggested for the strain-induced crystallization responsible for the unique self-reinforcement.

From the meaning of the word reinforcement itself, engineering investigation based on fracture mechanics should be one of the approaches to the elucidation of rubber reinforcement. However, fracture mechanical studies on rubber materials have not been much successful so far, compared with those on metallic and inorganic materials. We hope that our results presented in this book may accelerate studies for establishing the fracture mechanical behaviors of rubber. In addition, reinforcement of rubber justifiably involves not only mechanical strength but also various functional properties: For example, the required functions of the tread rubber of pneumatic tires in driving include lots of dynamic properties such as traction, grip, rolling resistance, skid resistance, wear of rubber, and so on. Hence, only mechanically supporting the weight is absolutely insufficient for automobile and aircraft tires.