The physical properties of carbon black-filled rubbers are affected mainly by the distribution of filler, carbon black properties and polymer-filler interactions. A number of experimental methods and approaches for characterization of the dispersion state of filler in rubber-carbon black composites and an investigation of polymer-filler interactions are reviewed. The dispersion state characterization, electrical conductivity measurements, bound rubber evaluations, mechanical properties measurements, carbon black surface properties study, small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) spectroscopy are among the most important ones.

Characterization of dispersion state of filler and polymer-filler interactions in rubber-carbon black composites

Increasing utilization of the electrical properties of polymeric blends and composites has prompted our renewed interest in developing a general working relationship which can explain the electrical properties of polymer composites and blends in terms of processing characteristics, morphology, and compositions. Here, we restrict our attention to the following two-component systems: (1) two component systems with conductive particulate inclusions (e.g. carbon black) embedded in a continuous polymeric matrix, and (2) two component polymer blend systems with one conductive polymer (e.g., polyether copolymer) dispersed in another continuous polymeric matrix. The following processing aspects related to the electrical property of particulate filled composites are discussed: (1) critical concentration of rigid conductive fillers, ϕc, and (2) redistribution of conductive fillers upon processing. An equation based on the crowding factor of concentrated suspension rheology and Janzen's particle contacts percolation is proposed to describe the relationship between ϕc, and the maximum packing fraction of conductive fillers. The relationship is used to explain the influence of particle morphology on conductivity, and the conductivity difference in the high shear and the low shear region of a processed polymer composite part. Furthermore, some qualitative guidelines for blending a low conductivity polyether copolymer to achieve an overall balance of antistatic and mechanical properties of polymer blends are also discussed.

Electrically conductive polymer composites and blends

Flexible strain sensors based on conductive carbon black (CB) filled styrene-butadiene rubber were developed. The use of ionic liquid (IL) allows improvement of the filler dispersion, rubber-filler interaction and flexibility of the sample that finally enhances the piezoresistive performance and the sensibility. At filler loading close to the percolation threshold, the electrical conductivity increases by two orders of magnitude when the IL/CB ratio is increased from 0 to 1.5. In contrast to the use of normal plasticisers, the loss in piezoresistivity at low strains is overcome. The sensitivity at 2.5% strain using an IL/CB ratio of 1.5 is about 600% higher compared with the sample without IL. Also, the response consistency becomes better with higher IL/CB ratios. Moreover, the use of IL allows the composites to be deformed more easily while still providing high responsivity to small strains. This enables the construction of better flexible strain sensors with long-term stability.

An efficient highly flexible strain sensor: Enhanced electrical conductivity, piezoresistivity and flexibility of a strongly piezoresistive composite based on conductive carbon black and an ionic liquid

The rheological properties of an extrudable material can be measured by extruding a strand of the material, measuring its diameter continuously, to give a series of diameter values, interpreting the variations in the strand diameter so as to characterize the surface of the strand, and deriving rheological data from the surface characteristics of the strand. The variations in the strand diameter can be interpreted as to peak-to-peak amplitude, average diameter value, the frequency of significant extreme values, standard deviations from a mean value, and the like. By means of these mathematical characterizations, the rheological behavior of the extrudable material can then be derived. The scorch behavior of a curable elastomer can be measured. The heterogeneity of a rubber compound can be described, or the macromolecular properties of a polymer can be measured (such as "gel", or degree of cross-linking.)

Method and apparatus for measuring the rheological properties of an extrudable material

A series of carbon black filled SBR 1500 compounds (without curatives) were prepared in carefully controlled lab-mixing conditions so that filler content ranged from 0 to 50 phr, with the usual compounding ingredients. Carbon Black agglomeration and dispersion quality were assessed through an advanced microscopic technique with automated data treatment. Linear and nonlinear viscoelastic properties were evaluated using a closed cavity torsional rheometer, equipped for Fourier Transform rheometry experiments. In the linear viscoelastic regime, dynamic moduli mastercurves were derived from experiments in the 60−160°C temperature range. In the nonlinear viscoelastic regime, complex modulus and torque harmonics variation with strain amplitude were investigated at 100°C and frequency 0.5 and 1.0 Hz. Relationships between dispersion quality, carbon black loading and mixing parameters were investigated so that dispersion appears better when the filler loading is above a critical level that to some extent (but not exactly) corresponds to the theoretical percolation threshold (around 13%). Linear and nonlinear viscoelastic measurements demonstrate and quantify the role played by plasticizing compounding ingredients, that is, processing oil, stearic acid and other chemicals, with nearly no significant effect of mixing energy. At least 10−15 phr carbon black are necessary to recover the modulus loss associated with this plasticizing effect. Nonlinear results and particularly the torque harmonics reveal a number of details as to how the compounding ingredients do affect the viscoelastic behavior, with expectedly the carbon black playing the major role. By fitting results with mathematically simple models, all of the observed effects can be summarized in a rather limited number of parameters so that the effects of compounding ingredients can be studied in details. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

A thorough study on the relationships between dispersion quality and viscoelastic properties in carbon black filled SBR compounds

A stylus-type surface roughness tester has been applied to the study of carbon black dispersion in a number of different rubber systems. The method is based on a freshly cut rubber surface which is tracked by the stylus to provide a roughness trace on a strip chart. Carbon black agglomerates deflect the cut path because of their higher hardness relative to the surrounding matrix. Thus, surface roughness diminishes at increasing levels of dispersion. A quantitative index of dispersion quality may be derived from the frequency and average height of the roughness peaks. The method offers a distinct advantage over previous methods in its ability to provide precise dispersion ratings over a very wide range of rubber processing levels from the masterbatch to the final product. The technique is applicable to the analysis of unvulcanized rubber compounds and could be utilized as a factory quality control procedure. Direct interfacing of the surface analyzer to a programmable calculator would provide quan...

V. E. Chirico

Papers

Measurement of Carbon Black Dispersion in Rubber by Surface Analysis