Abstract
The scientific measurement of the rheological properties of raw or unvulcanized rubber is a problem that has required new methods in rheological laboratory technic. Hot, moderately milled, or masticated raw rubber exhibits to a very marked degree each of the three properties of elasticity, plasticity, and thixotropy. It is highly elastic (not perfectly elastic), in that if deformed and released quickly it can take a large deformation and still return to its initial form. It is highly plastic, in that under proper conditions it can be deformed to any given extent permanently and without rupture. It is highly thixotropic, in that its viscosity or stiffness is very considerably decreased by brief mechanical working or mastication, and the lost viscosity is largely regained again when the mastication ceases. Obviously such a wide and varied range in rheological properties cannot be measured in either of the two types of rheological instruments, the compression and the extrusion plastometers, commonly used for raw rubber. There is no way in either instrument to determine or correct for thixotropy; and the viscosity can only be calculated with considerable uncertainty and difficulty. For example, thixotropy has not been considered in either of the already very complicated analyses of parallel plate plastometry by Peek and by Scott. Peek and Erickson have attempted to analyze the effects of thixotropy and elasticity in capillary tube viscometry without obtaining a complete or generally useful solution. However useful the compression and extrusion plastometers may be in factory control work, they are inadequate as scientific research instruments. The rotating cylinder viscometer seems to be the only type of instrument that meets the requirements of the problem considered. It allows an unlimited deformation of the test material, the previous deformation history can be controlled at will, and the subsequent elastic recovery can be measured if desired. There are two serious deficiencies of the conventional cylindrical viscometer, as applied to raw rubber, in that, first, the rubber, being semi-solid, would be likely to slip on the moving surfaces; secondly, if the rubber did not slip but sheared as it should, it would soon roll and climb out of the viscometer against the force of gravity. These deficiencies can be corrected, first, by fluting or otherwise roughening the cylindrical surfaces so that they grip the rubber and prevent slipping; secondly, by adding means for compressing and holding the rubber in place. The combined use of these two devices in a factory control instrument, the rotating disc plastometer, has already been described; and a rotating cylinder viscometer with similar features was referred to in the same article. The cylindrical viscometer there referred to was used in the experimental researches described below. Since this instrument is so different in several respects from the conventional cylindrical viscometers and, furthermore, is designed to facilitate the measurement of thixotropy and elastic recovery as well as viscosity, it seems appropriate to give it a new name and call it a rubber rheometer.