Frank L. Roth

Some Relations Between Stress, Strain, and Temperature in a Pure‐Gum Vulcanizate of GR‐S Synthetic Rubber

Stress‐temperature relations at constant elongation and at constant length have been studied in a pure‐gum vulcanizate of GR‐S. Such studies yield information useful for calculations involved in the theory of its elastic behavior, and furnish practical data regarding its tensile properties at different temperatures. The compounding recipe was: 100 parts by weight of GR‐S, 2 parts of sulfur, 1 part of zinc oxide, and 0.5 part of zinc dibutyl dithiocarbamate. The specimens were first held at constant length and constant temperature for a period of ½ hour to 2 hours, after which time the effects of relaxation of stress during the observation of stress‐temperature relations were negligible. The value of the stress after relaxation at each elongation was used to plot a stress‐strain curve. The stress‐temperature relations observed for temperatures below the relaxation temperature were linear and reproducible on successive runs of increasing and decreasing temperature. When the temperature was raised above the relaxation temperature straight lines were not obtained, since further relaxation occurred at the higher temperatures. The intercepts at 0°K for the lines obtained below the temperature of relaxation are useful in evaluating the internal energy changes. The intercepts of the lines representing the experiments at constant elongation were found to be negative. The absolute values were of the order of 10 percent of the stress after relaxation for the lowest elongations, and increased to almost 30 percent of the stress at the highest elongation.

Density Measurements on Synthetic Rubbers

Abstract For many purposes the density of a material need be known with an accuracy of only a few per cent. For many materials the density of one sample may differ from that of another by this amount, and no useful purpose may be served by making more precise measurements on individual samples. In natural rubber, the densities of different samples have been shown, in a compilation of twenty-one values, to lie with two exceptions between 0.905 and 0.919 gram per cc. at 25° C. The variations probably represent real differences in the samples and not accidental errors of observation. Since one can seldom know exactly the origin and subsequent treatment of a sample of natural rubber, there is little value in increasing the precision of measurement. Synthetic rubbers, on the other hand, can be regarded as usually produced under conditions which are much better controlled and known. It is logical, then, to measure the density with greater precision and to hope to be able to ascribe significance to its variation...

Strain Test for Evaluation of Rubber Compounds

Abstract Measurements of elongation of rubber vulcanizates at a fixed stress have been made with a precision much greater than can be obtained in the usual measurements of stress at a specified elongation. Such measurements form the basis of a strain test developed to characterize rubber vulcanizates in control and research testing. Statistical analyses show that the errors introduced in the actual strain measurements are negligible compared to those introduced by variations during compounding and curing, whereas the errors introduced by the usual measurements of stress at a specified elongation are of the same order of magnitude as those for compounding and curing. The high precision of strain testing has been used to detect variations within a single sheet of vulcanized rubber and variations among sheets cured from the same compounded batch. It has been possible also to determine with a single sheet its change in stiffness or modulus with age. The uniform treatment of specimens in the strain test makes ...

Vulcanization Characteristics of Natural Rubber

Abstract The present classification program for natural rubbers has been handicapped by the lack of adequate means to characterize their behavior on vulcanization. Several methods have recently been proposed for this purpose. Roth and Stiehler described the strain test from which three vulcanization parameters can be calculated. Two of these parameters have been combined by Schade. Thirion proposed a method for calculating two parameters from the customary measurements of stress at 600 per cent elongation. Gee and Morrell have recently suggested a procedure for determining two parameters from measurements of Mooney viscosity and stress at 100 per cent elongation. Workers at The B. F. Goodrich Company have proposed the use of the Mooney viscometer for determining two parameters. The present study was initiated to compare these methods for classifying natural rubbers. At the same time, an investigation was made of several conditions of treatment of the compound before vulcanization and of the temperature of...

Standard Materials for Rubber Compounding

Abstract When the synthetic rubber plants began operation in 1943, the Government took steps to establish a system of quality control that would assure that the rubber produced in 16 plants had essentially the same properties and could be used interchangeably by the manufacturers of rubber products. The most important characteristic for quality control was the curing behavior of the rubber. The measurement of this behavior required mixing the rubber with compounding ingredients, vulcanizing the mixed compound for several periods of time, and measuring the tensile properties of the vulcanizates. Early interlaboratory tests led the Government to an extensive standardization program in order to achieve the necessary quality control of production. In this program, it became obvious that the compounding ingredients as well as the methods of mixing, curing, and testing required standardization. Therefore, in 1944, the Rubber Reserve Co. established standard compounding ingredients for testing synthetic rubbers....

Temperature Control during Mixing of Rubber Compounds

Abstract A study was made of the effect of the surface temperature of the rolls of a 6 × 12-inch laboratory mill on the properties of GR-S compounds and means of controlling this temperature. It was found that the properties of the GR-S compound depend on the surface temperature of the mixing rolls. No practical method of controlling the temperature of conventional rolls was found because of the slow rate of heat transfer. Therefore three types of special rolls, including one designed at the National Bureau of Standards, were investigated. The heat transfer characteristics of the NBS rolls permitted automatic control of their temperatures to within 7° F during mixing of natural rubber and several synthetic elastomers, with only one controller being used. A mill with the special NBS rolls has been in operation for about a year, and has resulted in a marked improvement in the control of mixing of rubber compounds. The automatic temperature control has been much more convenient than manual control, and has s...

Strain Tester for Rubber

Abstract A tester for measuring the strain of rubber vulcanizates when subjected to a predetermined stress is described. The operation of the tester and a description of the apparatus for cutting and measuring the test specimens are presented. With this equipment routine determinations of a point on the stress-strain curve can be made with greater accuracy and precision than has hitherto been possible with the usual stress-strain equipment. This strain test also requires less labor than the customary stress tests.