Effect of Carbon Black on Mechanical Properties

Once they have been shaped into a suitable object, rubber compounds are vulcanized in such a manner that the full development of their mechanical properties is achieved, without creep phenomena that are normally exhibited by all polymer materials when on their rubbery plateau. In other terms, vulcanization extends toward infinity the rubbery plateau of the relaxation modulus function G t). Furthermore, reinforcing fillers somewhat increase the magnitude of the modulus at a given time. The combination of vulcanization and reinforcing effects induces quite complex changes in material functions of polymers, as easily demonstrated through purposely simple calculations. 

Expected variation of modulus function during filler loading and vulcanization. 

As previously underlined (Chapter 3), CB reinforcement must be appreciated with respect to a balance of properties, some of them modified in antagonistic manner when filler loading is changed. This commands to consider first how specific mechanical properties are modified through CB addition. 

Effect of temperature and carbon black on tensile stress at break. 

A common explanation for the Mullins effect is that, between the first and the second extension, the structure of the material has changed and it is generally observed that the higher the CB loading, the larger this effect. TSS is not solely due to the presence of the filler, whilst the latter enhances the effect. When a vulcanized rubber sample, which has been stretched twice, is submitted to a thermal treatment (for instance several hours at 80°C), it recovers 

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Table 2-16. Example of the Effect of Carbon Black on Mechanical Properties of...

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