Volume 15, Issue 11, 01 November 1944
Index of content:
15(1944); http://dx.doi.org/10.1063/1.1707381View Description Hide Description
15(1944); http://dx.doi.org/10.1063/1.1707382View Description Hide Description
A new ``electrostaticmodulator'' for measuring the electrostatic charges on various materials is described in this paper. In this apparatus the electrostatic lines of force established between the charged specimen and the grid of an audiofrequency amplifier are cut or modulated at audiofrequency by a motor‐driven fan. The resultant alternating current voltage is amplified and measured on a meter in the output circuit. This device, in conjunction with a mirror surfaced metal plunger system for contacting the sample, has been used to measure the contact potential of various rubber and GR‐S compounds. Data are shown on both rubber and GR‐S compounds, and they bear out the formulation of an ``electrostaticcontact potential theory of reinforcement'' in which reinforcement is explained on the basis of contact potentials and resultant electrostatic forces set up between the rubber and the reinforcing agents. By the application of this theory, organic materials, which have a highly positive charge, such as polymerized Trimethyldihydroquinoline and Flectol H, have been found to increase the tensile of GR‐S pure gum type compounds as much as fivefold and to nearly double the tensile of high zinc oxide GR‐S compounds.
15(1944); http://dx.doi.org/10.1063/1.1707383View Description Hide Description
A method of measuring the speed of retraction of rubber is described which utilizes an electronic timing circuit with photo‐cell input. The results include measurements of the speed of retraction of Hevea and synthetic rubbers as a function of elongation, temperature, carbon black loading, and cure. They are discussed both from the standpoint of the new information which they give in regard to rubber structure and high elasticity as well as their practical application to evaluate quality and cure. High speed photographs are included which are of interest in showing the manner in which rubber retracts.
Some Relations Between Stress, Strain, and Temperature in a Pure‐Gum Vulcanizate of GR‐S Synthetic Rubber15(1944); http://dx.doi.org/10.1063/1.1707384View Description Hide Description
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.
15(1944); http://dx.doi.org/10.1063/1.1707385View Description Hide Description
The effect of the filler upon the modulus of compounded rubber has been calculated with the aid of some simplifying assumptions. From these calculations it is found that the increase in modulus, caused by the filler, is directly proportional to the volume loading, and is independent of the particle size of the filler. The stress system in the vicinity of a spherical filler particle has also been calculated. A series of experiments has been made to check these calculations. From the results of these experiments, it has been deduced that carbon black is flocculated in rubber; and that P‐33, Thermax, and Gilder's whiting are completely dispersed in rubber. These last‐named fillers give increases in modulus substantially in agreement with the calculations. It has further been found that Kadox and XX zinc oxide give unusually large increases in modulus. These are ascribed to alteration of the type of cure of the rubber matrix. Catalpo clay showed excessive modulus because of high calender grain.
15(1944); http://dx.doi.org/10.1063/1.1707386View Description Hide Description
A modification of the apparatus employed in measuring the Young's modulus of elastomers at low temperatures is described. The changes made in the apparatus permit the use of a simplified technique which can be adapted to routine testing procedure. The effects of various softeners on the low temperature bending moduli and the brittle point temperatures of stocks based on four butadiene‐acrylonitrile (B/A) type copolymers are given. It is shown that the softeners tested have the same relative effects in all four types and that a wide variation in low temperature properties is imparted to the stocks by the different softeners. Bending modulus curves and brittle point temperatures are given for typical test stocks based on Thiokol FA, and ``mass'' and ``emulsion'' polymerized polybutadiene. It is shown that continued exposure to low temperature affects the Young's modulus of some but not all typical vulcanizates. Data are presented showing this effect on an uncured Hevea gum stock held at 0°C. Certain softeners have been found to induce time effects in one of the B/A type stocks, which exhibits no crystallization, and hence no progressive stiffening in the absence of the given softeners. A method for measuring and evaluating creep under dead load at low temperatures is presented. Accompanying data indicate that in the case of those stocks tested, the creep constant defined reaches a maximum at a definite temperature which is a characteristic of the given stock.
15(1944); http://dx.doi.org/10.1063/1.1707387View Description Hide Description
15(1944); http://dx.doi.org/10.1063/1.1707388View Description Hide Description
Stress‐temperature relations at constant elongation have been studied in a pure‐gum vulcanizate of natural rubber. 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 crystallization of the rubber which occurs on stretching is a factor of considerable importance in the explanation of the results obtained. The compound contained 100 parts by weight of smoked sheet rubber, 2 parts of sulfur, 1 part of zinc oxide, and 0.5 part of zinc dibutyl dithiocarbamate (``Butyl zimate''), and it was vulcanized 40 minutes at 115°C. The stress‐temperature studies were made after the specimens were stretched and allowed to relax at a constant temperature of 25°C or 70°C for about two hours. The change of stress with time at the end of two hours was small enough to be without influence on later results, except when the flow was excessive or when the stretched rubber continued to crystallize, as it did at elongations between 150 percent and 500 percent at 25°C. At elongations of 150 percent and less, no evidences whatever of crystallization were found during the time of the experiments. At elongations of 500 percent and more, the crystallization which occurred during stretching and in the first few minutes thereafter was so complete that the stress soon became independent of time. At 70°C both the upper and lower limits of the range of elongations in which continued crystallization occurred were somewhat higher. Values of the stress after relaxation have been used to plot a stress‐strain curve. The stress‐temperature relations obtained by lowering the temperature to about −20°C were always linear and reproducible on successive runs of increasing and decreasing temperature, except when the flow was excessive or when the stretched rubber continued to crystallize. An additional period of relaxation was required if the rubber was heated above the temperature at which the first relaxation occurred. The slopes of the stress‐temperature lines and the values of the stress at any temperature depend upon the temperature at which relaxation was carried out. The stress intercepts at 0°K are small compared with the total stress for elongations up to 150 percent. At the higher elongations the intercepts take on increasingly large negative values, finally becoming approximately equal in magnitude to the total stress.