Carbon black dispersion in rubber assessment methods and process studies

The degree of carbon black dispersion is a very important consideration in the manufacture of rubber both in quality control and basic research. A study has been made of various aspects of assessing dispersion and the dispersion process of carbon black in rubber. The main objectives of this work were: (1) To investigate the relationship between the light scattered at a fixed angle from the rubber surface and carbon black dispersion and hence develop and evaluate a new dispersion assessment system. (2) To determine the normal variations of industrial rubber mixing installations, and (3) To study the effect of internal mixing variables on black dispersion and other properties of rubber. To achieve these goals a carbon black dispersion assessment system based on an inverted Dark Field Reflected Light (D.F.R.L.) microscope was developed in three versions and successfully tested. The three versions differed in their degree of sophistication and automation and would be expected to find application ranging from routine quality control to research. The basic principle was that a rubber sample (cured or uncured) was cut with a new razor blade and the surface observed in a D.F.R.L. microscope. The light beam from the sample surface is sensed by a photometer and its intensity was shown to be related to black dispersion. In version III an automatic stage driven by two stepper motors was designed and fitted to the microscope to perform object plane scanning. The photometer and the stage were interfaced with an Apple II microcomputer providing the following functions; stage control, photometer control, data acquisition, statistical analysis, data storage and results output. The system was evaluated by taking measurements on several identically formulated compounds differing only in black dispersion. A general rubber goods and a tyre manufacturing installation were studied. Several production batches were sampled at various mixing stages and subjected to black dispersion assessment, cure and vulcanisate properties measurement. Analysis of variance of the results was accomplished with a statistical computer package designated GENSTAT Version 4.03. Factorial experimental designs and multivariate regression analysis techniques were used in studying the effect of mixing variables on black dispersion and other properties. The results are presented in the form of response equations and contour graphs are used to enable second order interactions to be readily identified.