Abstract:

Crack growth resistance of rubber compounds is of vital importance for tire applications such as the safety of vehicle drivers and the life-time of tires. Two parts of related work will be presented to address the key issue. The first is how to monitor the cracking process, and the second is how to improve the crack resistance of tire materials with a simple processing strategy.
Under cyclic deformation, it is reported that rubbers of rich cracking morphology dissipate more energy and have low crack growth rate (dc/dn) . In this study, real time morphology monitoring at the crack tips of elastomers is achieved by the use of high-resolution high-speed camera and dynamic mechanical analyzer with crack growth testing function.
It is found that natural rubber (NR)’s cracking process has two regimes, and each has distinct dynamic morphology of the cracking tip. Interestingly, the two distinct dynamic morphologies correlated very well with the two distinct slops of the log-log plot of dc/dn and tear energy (T).  This behavior of two regimes with a transition point is so far only exists for NR. The styrene-butadiene rubber(SBR) and polybutadiene(BR) have only one kind of cracking morphology and one slop at the current testing range of the instrumentation. These dynamic “ligament” morphology during crack growth is resulted from the breakup of the ligament-like shape at the cracking tip. Meanwhile, the characteristic dimensions of all the cracking morphologies are proportional to the tear energy. When filled with carbon black, the crack growth rate is significantly reduced, and the transition point between two regimes of NR is shifted to higher tear energy; while the ligament dimensions of SBR and BR become smaller and more uniform. Results may suggest that the characteristic cracking tip morphology of NR is somewhat related to the strain induced crystallization. The effect of carbon black on ligament dimension of SBR and BR rubbers may be related to the alteration of stress-distribution and surface instability as a result of filler dispersion in the polymer bulk.
Block copolymer has been extensively used to reduce the domain size of immiscible polymers in order to improve the mechanical properties. To make sure that these polymeric surfactant molecules are actually stays at the interface of the polymer blends, many efforts have been focused on the design of molecular architecture and synthesis of block copolymers. With a very simple mixing strategy, a small amount of low molecular weight liquid diblock copolymer rubber has reduced the domain size of NR/BR, thus improves the cracking resistance, by making each and every diblock molecules stays at the interface of NR and BR. Amazingly, the crack resistance, the other mechanical properties, as well as processing characteristics are all improved.

Keywords: crack propagation, real-time monitoring, blending, compounding, morphology evolution, polydiene, natural rubber, liquid rubber, styrene-butadiene rubber

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Biography:

Zhongren Chen, Ph.D. (Caltech 1998)
2011-Sir Y.K.Pao Distinguished University Professor and Dean School of Materials Science and Chemical Engineering Ningbo University, Ningbo, China
2000-2011 Senior Research Scientist (Key Management Band) Bridgestone Americas Center for Research & Technology Akron, OH, USA
1999-2000 Postdoctoral Researcher Honeywell Electronic Materials, Sunnyvale, CA, USA
1998-1999 Postdoctoral Fellow in Chemistry Stanford University, Stanford, CA, USA
1992-1997 Ph.D. and M.S. in Chemical Engineering and Chemistry California Institute of Technology, Pasadena, CA, USA
Advisors: Kornfield and Grubbs (2005 Nobel Laureate)
1987-1992 Lecturer of Chemical Engineering
Beijing University of Commerce and Industry, China
1980-1987 M.S. and B.S. in Chemical Engineering (Polymer)
Zhejiang University, Hangzhou, China