Effect of temperature and frequency on the dielectric constant, power factor, and conductivity of compounds of purified rubber and sulphur

Abstract

The dielectric constant, power factor, and conductivity of purified rubber and of a series of its compounds with sulphur were determined at temperatures from -75°to 235°C.The dielectric constant and power factor were measured at five frequencies from 60 to 300,000 ~and the apparent conductivity at about 0.002 second and at one minute after the application of potential.The results of the measurements are expressed in both tabular and graphic form.At 25°C .and 1,000 ^the dielectric constant of purified rubber containing no sulphur was 2.37.With increasing sulphur content the dielectric constant increased to a maximum of about 3.75 at 11.5 percent sulphur, then decreased to a minimum of 2.70 at 22 percent sulphur and again increased to 2.82 at 32 percent sulphur.Under similar circumstances, the power factor increased from 1.6X10 -3 for the rubber alone to a maximum of 93.8X10 -3 for the compound containing 13.5 percent sulphur; it then decreased to about 4.0 X 10~3 at 20 percent sulphur, and again slowly increased to 5.1 X 10 -3 at 32 percent sulphur.The 1-minute conduc- tivity was 2.3X10 -17 mho/cm for the rubber alone.With 12 percent sulphur, it was only 0.5X10 -17 , while at 18 percent sulphur it passed through a sharp maxi- mum of 38X10 -17 , and then decreased to values between 1 and 1.5X10 -17 for compositions between 22 and 32 percent sulphur.Changes of the temperature or the frequency at which the measurements were made shifted the maxima and minima in these curves and modified their heights.For example, at -25°C , the maximum dielectric constant at 1,000-^was 2.8, and was obtained with a compound containing 4 percent sulphur, while at 145°C. the maximum was 4.5 and was obtained for a compound containing about 28 percent sulphur.Comparison of the results of this investigation on purified rubber with previous work done with crude rubber indicates that purification alters the values obtained for the electrical properties, but it does not modify the general manner in which these properties vary with changes in composition, temperature, or frequency.The results may have practical bearing on the selection of rubber compounds for specific uses and in pointing out the manner in which the properties of rubber are related to temperature or frequency. CONTENTS PageI. Introduction 174 II.Preparation of specimens 175 1.Purification of rubber 175 2. Mixing of rubber and sulphur 175 3. Electrodes 176 4. Molding of specimens 176 5. Vulcanization of specimens 177 III.Measurement of the dimensions of specimens 177 1. Measurement of thickness 177 2. Measurement of area 179 3. Change in dimensions with temperature 179 IV.Mounting of specimens for electrical measurement 180 1. Electrical connections to test specimens 180 2. Chamber for measurements at 25°C 180 3. Chamber for measurements at -75°to 235°C 181 4. Means for maintaining constant temperature 182 Bureau of Standards Journal oj Research [Vol.u Pafee V. Electrical measurements 183 1. Capacitance and power factor 183 2. Dielectric constant 185 3. Conductance 187 4. Conductivity 187 5. Accuracy of determinations 188 (a) Effect of errors in dimensional measurements 189 (6) Possible sources of error in electrical connections and measurements 189 (c) Independent determinations of dielectric constant-189 (d) Effect of the condition of electrode surfaces 190 VI.Values of electrical properties 190 1. Electrical properties at 25°C 191 (a) Dielectric constant 191 (6) Power factor 192 (c) Conductivity 193 2. Measurements at -75°t o 235°C 193 (a) Dielectric constant 196 (6) Power factor 200 (c) Conductivity 205 VII.Comparison of the electrical properties of compounds of purified rubber and sulphur with those of crude rubber and sulphur 207 VIII.Selection of rubber compounds for specific uses 208 IX.Conclusion 209 X. Acknowledgment 209 I. INTRODUCTIONThe authors have previously reported results 1 on the electrical properties of a series of rubber-sulphur compounds which were made from commercial rubber and were measured at 25°C .with 1,000 ~.The present paper reports results on the electrical properties of a similar series of compounds made from purified rubber and measured over a wide range of temperature and frequency.Sufficient data have been obtained to enable one to determine the dielectric constant and power factor for any composition of rubber and sulphur at any useful temperature and at any normal frequency.The results obtained in this paper do not entirely confirm the conclusions of the previous paper.When the electrical measurements reported in that paper were plotted as functions of the percent of combined sulphur, curves were obtained which showed changes in direction at compositions corresponding to simple, empirical formulas.From this evidence as well as from a consideration of changes in density with composition, it was inferred thatThe existence of a definite compound of empirical formula (C 5 H 8 )2S is indicated by all the properties which have been studied.Evidence for the other three compounds (CsHg^S, (C 5 H 8 ) 3 S, and (C 5 H 8 ) 4 S3 is less complete.Soon after that paper was published it was pointed out by Kimura, Aizawa, and Takeuchi 2 that the electrical properties are functions of the temperature, and by Kitchin 3 that they are functions of the fre- quency at which measurements are made.Kimura and Namikawa 4

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Affiliated Institutions

• United States Department of Commerce US

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