The degree of dispersion of carbon black in rubber is known to be a determining factor in the physical properties of the composite. It is important, therefore, to have a reliable, quantitative technique for assessing carbon black dispersion. It has been known for some time that the degree of dispersion of carbon black is reflected in its electrical resistivity. A resistivity instrument, developed by B. Boonstra, has been redesigned and constructed for use in this study. Factors that influence resistivity (i.e., black type, volume loading, and mixing time) are examined. Physical property and dynamic mechanical measurements are made, and their relationship to black dispersion is investigated. Light microscopy is also used to evaluate dispersion, and these data are compared to the resistivity results.

Recent developments in nonlinear finite element methods (FEM) and mechanics of composite materials have made it possible to handle complex tire mechanics problems involving large deformations and moderate strains. The development of an accurate material model for cord/rubber composites is a necessary requirement for the application of these powerful finite element programs to practical problems but involves numerous complexities. Difficulties associated with the application of classical lamination theory to cord/rubber composites were reviewed. The complexity of the material characterization of cord/rubber composites by experimental means was also discussed. This complexity arises from the highly anisotropic properties of twisted cords and the nonlinear stress—strain behavior of the laminates. Micromechanics theories, which have been successfully applied to hard composites (i.e., graphite—epoxy) have been shown to be inadequate in predicting some of the properties of the calendered fabric ply material from the properties of the cord and rubber. Finite element models which include an interply rubber layer to account for the interlaminar shear have been shown to give a better representation of cord/rubber laminate behavior in tension and bending. The application of finite element analysis to more refined models of complex structures like tires, however, requires the development of a more realistic material model which would account for the nonlinear stress—strain properties of cord/rubber composites.

SynopsisWide-angle x-ray relaxation studies were performed on films of low-and high-density polyethylene (PE). The samples were rapidly stretched by a pneumatic stretcher and held a t a fixed length. The rate of crystal reorientation was monitored by measuring diffracted intensity changes using a count-rate meter with output displayed on a storage oscilloscope as well as a digital counter-timer. Experiments were completed a t several temperatures and activation energies comparable to those reported for the relaxation in P E were obtained.

A comparison of the three techniques shows that while all can discriminate dispersions, their useful ranges are different. The electrical resistivity method is relatively insensitive at the early stages of mixing but has a high sensitivity at the intermediate and latter stages. This method is directly influenced by the dispersed black. There are limitations, however, to its utilization. The method works only within a narrow range of volume loadings (35–75 phr) and can only be used for small and intermediate particle size carbon blacks. One advantage that the resistivity technique has is that uncured rubber is tested. Elimination of the vulcanization step makes the resistivity technique an attractive quality control test. The Phillips rating and surface roughness are not limited by the volume loading of filler or the nature of the filler. Each technique works quite well at intermediate dispersions and only loses sensitivity at very good dispersions. These methods differ from the resistivity in that they are sensitive to the undispersed filler. The surface analysis is a more objective and quantitative technique than the Phillips rating. The surface analysis technique can have a microcomputer interfaced to store data and calculate results. This reduces the test time to a matter of minutes. The Phillips dispersion rating is made on vulcanized rubber so that there is added sample preparation which leads to a longer analysis time. The surface analysis has been reported for both vulcanized and unvulcanized rubber; however, the unvulcanized samples present special problems in cutting and do not show as strong a correlation with other dispersion ratings.