A new electrostatic field strength meter has been described which measures and records, not only the magnitude of electrostatic fields, but also their polarity. This device has been used in a study of the mastication of rubber, and has furnished data which indicate that rubber becomes less negative, and may actually become positive, as mastication proceeds ; this indicates in turn an oxidation of the molecules as well as an electronic rearrangement and orientation of the rubber molecular chains. Data on GR-S polymerized at different temperatures show entirely different electrostatic breakdown curves, which, with other physical characteristics, indicate unusual properties of the crude low-temperature polymers; e.g., high molar weight, long-chain molecules, already or easily oriented, which do not oxidize or break up appreciably on mastication. A partial correlation of Mooney viscosity and electrostatic contact-potential and field strength has been shown for rubber and GR-S which have been masticated on a rubber mill. The new electrostatic-field strength meter shows promise as a new tool which can be used in processing control, and also as a research instrument for obtaining fundamental information on mastication, pigment dispersion and incorporation, as well as an aid in forming new concepts concerning the structure of matter.

A new electrostatic modulator has been described for measuring the electrostatic charges on rubber, synthetic rubber, and various materials. In this apparatus the electrostatic lines of force established between the charged specimen and a stationary electrode plate connected to the grid of an audiofrequency amplifier are cut or modulated at audiofrequency by a small four-bladed, motor-driven fan. This produces in effect an alternating current voltage which can be readily amplified by the audio amplifier and quantitatively measured on a meter in the output circuit. This device, in conjunction with a mirror-surfaced metal plunger system for contacting the rubber samples, has been used to measure the contact potential of various rubber and GR-S compounds. Electrostatic contact potential data are shown for both rubber and GR-S compounds with and without organic and inorganic reinforcing agents, and these data bear out the formulation of an electrostatic contact potential theory of reinforcement in which the reinforcement of rubber and GR-S is explained on the basis of contact potentials and resultant electrostatic attractive forces which exist between the rubber and the reinforcing agents. By the application of this theory, organic materials which have a highly positive electrostatic charge, such as polymerized trimethyldihydroquinoline and Flectol-H (an acetone-aniline condensation product), have been found to increase the tensile strength of the pure-gum type of GR-S compounds as much as fivefold and nearly to double the tensile strength of high zinc oxide loaded GR-S compounds.

A comparative analysis of some of the factors involved in road tests on tires and laboratory flexometer tests indicates that not enough attention has been paid to heat dissipation and the role of the thermal conductivity of stocks in increasing heat dissipation. Flexometer tests currently used, in which samples are flexed between insulated plates, measure mainly heat generation. A modified St. Joe flexometer test is described, using metal-faced plates, which takes into account not only heat generation but heat dissipation, and should more nearly simulate road tests on tires when comparing GR-S (Buna-S) synthetic rubber stocks having different thermal conductivities. Using the new type test, mixtures of easy-processing channel black and zinc oxide are shown to have higher tensile strength and lower heat generation than soft blacks or mixtures of soft blacks and easy-processing channel blacks. The data given in this report indicate the need of further work along this line in the way of road tests on actual GR-S (Buna-S) tires containing zinc oxide in both tread and carcass.

This paper describes a new apparatus for measuring electrostatic contact potentials on various materials at elevated temperatures. In this apparatus, the electrostatic charge, acquired by rolling a steel ball down the surface of a rubber test specimen on a heated inclined plane, is measured when the ball drops into the cup of a suitable measuring device such as our electrostatic modulator. With this instrument the contact potentials of both rubber and GR-S was found to become highly negative at elevated temperatures. This apparent ``boiling off'' of electrons and resultant disruption of electrostatic attractive forces within the material is much greater for GR-S than for rubber and probably accounts for the much greater decrease in tensile of GR-S over rubber at elevated temperatures, and is further confirmation of the electrostatic contact potential theory of reinforcement. By the further application of this theory suitably dispersed compounding materials, which are in effect highly positive at elevated temperatures such as certain proteins, finely divided silica and sodium silicate, have been found to increase substantially the hot tensile strengths of GR-S compounds.

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