Technical Microscopy in the Rubber Industry R. P. ALLEN’, The B. F. Goodrich Company, Akron, Ohio
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OR supplying information not obtainable by the usual methods the microscope is useful and often essential; not only will it disclose details which would otherwise be unseen, but by its unique methods of examination measurements of certain properties of materials can he readily secured
are described in an excellent summary by Chamot and Mason (10). Those methods invariably depend primarily on direct observation of the individual particles, but indirect methods are also useful. For example, Pohle (35)employed a “mass color effect” for determining the presence or absence of small particles; thin sections of rnbher compounds loaded with pigments containing a high proportion of “colloidal” particles, such fls channel blacks, exhibit a reddish-yellow transparency which may be distinguished easily from the neutral gray observed where the particles are largely noncolloidal. In the Goodrich laboratory also, this Tyndall phenomenon has been found useful. This effect was utilized quantitatively by Stutz and Pfund (37) and later by Stutz (36) alone in measuring the average particle size of zinc oxides. The methods of determining particle size may logically be divided into two classes, those applicable to particles above the limit of resolution of the microscope and those applicable below that limit, although few commercial pigments contain particles which in size tie entirely in one class or the other. GENERALASPECTSOF THE MEASUREMENT OF PARTICLE SIZE. I n 1923 Perrott and Kinney (33) discussed the factors of fundamental importance in the use of particle size measurements, From one set of measurements obtained by microscopic observation, these authors showed that an “average particle size” may he calculated with respect to “averagenumber, length, surface, or volume, and that the significance of an average particle size depends on which 3 average is chosen for *.._-.___. .... .. . . . . . use. For example, in Figure 1 is shown a diagrammatic repre21 a n ff tt h hn s ea nn tt oa tt ;i no nn o e powdered coal with which Perrott and Kinney worked and the 0 10 K) 30 40 50 60 several averages ohMICRONS tained, these ranging from the average of 3 microns based on numher to the 36 microns based on volume. In Figure 2 is shown diagrammatically the ap36 proximate distribution of particles in the FIGURE 2. DISTRIBUTION OF P A R sample of powdered TICLES IN POWDERED COAL
which are not otherwise obtainable. When one considers the value of the microscope to the rubber industry specifically, the study of the size of particles is usually the first and freauentlv the only application ESO which comes to 200 mind, although UI other uses have been 150 0 suggested and described (7, 8, 93, 100 2 95,37,35,38). The f present paper not so only describes the 30 of the importance IO microscope in deterAverage p a r * , c i e bllD ac
