Response of Carbon Black-in-Oil to Low-Amplitude Dynamic Stress at Audiofrequencies

The results reported here demonstrate the feasibility of investigating the dynamic mechanical properties of carbon black-agglomeration networks over wide ranges of temperature and frequency by measurements on carbon black mixed with oil. From the data displayed in Figures 3, 4, 5, and 6, it is evident that the general levels of audiofrequency elastic compliance and modulus, J′ and G′, change more than two orders of magnitude as the temperature is varied between − 12.2 and 50.6°C; the general levels of loss compliance and loss modulus, J″ and G″, change almost as much. A comparison of measurements at 25.2°C made at the beginning and after the conclusion of the measurements at various temperatures (Figure 7) shows little change except for an increase in low frequency values of J″ which are tentatively ascribed to water absorption due to condensation within the sealed measurement apparatus at low temperatures. From this close agreement of before and after compliance values, it is concluded that the large effects of temperature change on the measured dynamic mechanical properties are reversible and essentially independent of thermal history and/or time. The general level of measured dynamic compliance and modulus of the sample of 50 parts by wt. of N299 carbon black in 100 parts by wt. of process oil are also close to those observed for this same carbon black in a cured tire stock for similar temperatures and amounts of black, although the frequency dependences are different. This result agrees with the measurements previously reported by Payne where room temperature values of G′ at 0.1 Hz for carbon black in paraffin oil and for carbon black-butyl rubber were about the same for the same proportion of carbon black. Thus, at low dynamic stress (or strain) amplitudes, the independent carbon-carbon agglomeration network can evidently influence the dynamic mechanical properties of a tire stock as much or more than the cured rubber matrix. The observed broad retardation/relaxation dispersions of compliance and modulus at each temperature clearly cannot be reduced to a common reference temperature by shifts along the frequency axis so that superposition to give composite functions of compliance or modulus over an extended range of frequency is not possible. However, approximate superposition of some compliance and modulus vs. frequency curves to a common reference temperature can be accomplished by vertical shifts indicating that temperature-magnitude reduction may be successful; such reduced curves are shown and treated more extensively in a separate article.