Modern Role of the Electron Microscope in Rubber Research

Abstract Until recently, the main role of the electron microscope in rubber research was the determination of particle size and shape of reinforcing pigments and fillers. The electron microscope proved its value in its first industrial problem in 1940 when it showed the particle size of Micronex to be 28 mµ. In 1942 it established the correlation between particle size of carbon and rubber properties. The next contribution was the establishment of reticulate chain structure, in 1945, using stereo-photomicrography. The electron microscope led the way to the carbon gel concept when carbons producing different road wear results were shown to have equal particle size and structure. Micrographs of carbon gel were published in 1951. Studies of rubber latexes, various pigments and fillers were also applications of the electron microscope in which the determination of particle size and shape was involved. Today, improvements in resolution, development of new techniques and accessory equipment have greatly expanded the application of the electron microscope. This paper is concerned mainly with describing these new developments ; first, as a help to electron microscopists in the rubber field; second, to illustrate what can be done with the electron microscope, so that research and production men can recognize possible applications of this discerning tool to the solution of their problems. Electron microscopes have come a long way in the short 20-year period. Figure 1 shows the microscope used in our laboratories. This instrument is currently resolving detail as fine as 10 A or 1 mµ in routine daily operation. Its design makes possible the development of special holders and thereby increases its value as a research tool. Another piece of equipment that is as important in microscope research as the electron microscope itself is the evaporating unit shown in Figure 2. This is used for shadowing and forming replication films under vacuum, thus making possible studies of rubber surfaces and surfaces of pigments and other rubber ingredients. In shadowing, detail is made visible by coating the specimen with metal evaporated at an angle from the tungsten basket at A. Carbon can be evaporated by the arc at B, forming a replicating film or mold of the specimen. This will be discussed under “preparation”. Today, the proper preparation of specimens is the most important step in successful microscopy. The various preparations used are as follows.