Properties Of Rubber Compounds Research Articles

Determination of Intrinsic Low Stress Properties of Rubber Compounds. Use of Inclined Plane Tester

Abstract A hysteresis loop test made at low stress over a series of cures provides an extremely accurate measure of state of cure. The test is accurate and reproducible when made on test pieces which have been conditioned and are tested under controlled conditions of temperature and relative humidity. Furthermore, the type of test machine used and the autographic recording of the data practically eliminate the human element in testing. The accurate molding of the test pieces eliminates possibilities of error in gaging and compensating found frequently in the standard stress-strain test. The initial load data at 50, 100, and 150% elongation on the shortest cure in the series provide a scorch test which is accurate enough for practical factory control. The ratio of initial modulus at 50% elongation to modulus on recovery at 50% elongation provides a measure of hysteresis loss or heat build-up. Measurements made in the range of 50% elongation appear to be a more accurate measure of the properties of tire stocks, because the test range more closely approaches the conditions under which tire stocks operate in service. Finally, the use of this test provides a considerable saving in testing time.

Determination of Intrinsic Low Stress Properties of Rubber Compounds. Use of Inclined Plane Tester

Abstract A hysteresis loop test made at low stress over a series of cures provides an extremely accurate measure of state of cure. The test is accurate and reproducible when made on test pieces which have been conditioned and are tested under controlled conditions of temperature and relative humidity. Furthermore, the type of test machine used and the autographic recording of the data practically eliminate the human element in testing. The accurate molding of the test pieces eliminates possibilities of error in gaging and compensating found frequently in the standard stress-strain test. The initial load data at 50, 100, and 150% elongation on the shortest cure in the series provide a scorch test which is accurate enough for practical factory control. The ratio of initial modulus at 50% elongation to modulus on recovery at 50% elongation provides a measure of hysteresis loss or heat build-up. Measurements made in the range of 50% elongation appear to be a more accurate measure of the properties of tire sto...

Some Infrared Studies on the Vulcanization of Rubber

Abstract The two most effective ways of altering the physical and mechanical properties of natural rubber are vulcanization and the addition of carbon black. There is, however, little understanding of the structural changes brought about by these processes, in spite of extensive research by chemical and physical methods. For instance, in vulcanization it is not yet decided how the sulfur atoms are incorporated in the rubber polymer, what proportion are in bridges formed between the isoprene chains, whether such bridges are formed by —C—S—S—C— or —C—S—C— links, whether one double bond is broken for each sulfur atom incorporated, and so on. An ancillary problem is the effect of certain “accelerators” on the speed of vulcanization, about which even less is known. Again in the case of reinforcement of rubber by materials such as carbon black, there is controversy on the structural relationship of the carbon black and the rubber. Until such structural problems have been solved on the molecular scale there is little hope of controlling the macroscopic properties of rubber compounds in a truly scientific manner. This paper gives a preliminary account of the application of infrared spectroscopy to some of these problems.

Effect of Diameter and Surface Area of Carbon Black Particles on Certain Properties of Rubber Compounds

Abstract Carbon blacks can be grouped into different classes according to the way in which their fineness of division relates to different properties in rubber. Within any one class the principal properties vary in a regular manner with particle size. The normal class consists of the furnace carbons, Kosmos (Dixie)-40, Statex, the rubber-grade impingement carbons, and possibly, the color-grade impingement carbons. The subnormal classes consist of thermal carbons and acetylene and lamp blacks. Irrespective of the above classification, the properties which depend more on fineness of division than on other factors are rebound resilience, abrasion resistance, tensile strength and tear resistance. The lower limit of particle diameter for best tensile strength and tear resistance appears to be higher than that for abrasion resistance. B.S.I, hardness and electrical conductivity are properties which depend at least as much on other factors as on particle size. Stiffness (modulus) depends more on other factors than on particle size. Factors modifying the effects of particle size (or specific surface) include the presence of carbon-carbon structures and a reduction in strength of bond in rubber-carbon structures. Carbon black is thought to exist in rubber in four states: agglomerated, flocculated, dispersed, and bonded to the rubber molecules (the reënforcing fraction). Abrasion resistance is regarded as providing the only reliable measure of reënforcement.

Diffusion of Sulfur in Rubber. Relation to Vulcanization

Abstract A survey of the literature failed to disclose any previous attempt to determine the. rate of diffusion of sulfur through rubber. The present paper is intended to fill this gap and to show the importance of diffusion and solubility of sulfur in rubber on control of the vulcanization reaction. The vulcanization of rubber cannot be considered as a homogeneous chemical reaction since the reactants, i. e., sulfur, accelerators and activators, are seldom completely in solution or uniformly dispersed. The limited solubility of sulfur, many accelerators, and zinc soaps in rubber leads to the presence of these substances partly in solution and partly in the dispersed state during vulcanization. Local high concentrations of these substances occur in solution around the dispersed phase, owing to their increased solubility which results from the temperature rise during vulcanization, coupled with their slow rate of diffusion through rubber. This paper presents the results of a detailed study of the effects of solubility, diffusion, and crystallization of sulfur on heterogeneity of vulcanization. Results showing the effect of various methods of mixing and handling on the physical properties of certain rubber compounds are presented. The diffusion of sulfur into rubber to form a solution during vulcanization, followed by the separation of globular sulfur on cooling, and its transition into dendritic and finally into rhombic sulfur on standing, was observed under the microscope by Loewen and Endres. Several factors which influence the migration and crystallization of sulfur internally and as a surface bloom on rubber were studied by Rimpel, Endres, Twiss, Loewen, and Graffe. The absorption of sulfur by rubber at elevated temperatures was studied by Skellon and by Venable and Green, and the solubility of sulfur at various temperatures was investigated by Venable and Green, Endres, Kelly and Ayers, Loewen, Morris, and Williams.

Tensile Properties of Rubber Compounds at High Rates of Stretch

Abstract Stress-strain relationships and the work of extension for four rubber compounds were studied for a rate of stretching of the order of 1,000 per cent per second. This rate is sufficiently great so that the test may be considered to approach adiabatic conditions. A comparison of these data with similar data for a rate of stretching which lies in the range of speeds common to the usual routine tests shows that increased speed of stretching affects the observed tensile properties as follows: (1) the stresses' are increased at elongations up to about 500 per cent for the loaded compounds and up to 600 per cent for the pure-gum compound, the maximum increases ranging from 75 pounds per square inch for the pure-gum compound to 245 pounds per square inch for the clay compound; (2) the stresses in the carbon-black compounds are decreased slightly at elongations near rupture; (3) the work of extension to rupture is increased for the pure-gum and clay compounds and decreased for the carbon-black compounds, and (4) the work of extension for a given elongation is increased for all the compounds. The stresses and the work of extension at the higher speed reveal no information indicative of resistance to abrasive wear that cannot be gained from similar studies of data obtained in stretching the specimen slowly.

Tensile properties of rubber compounds at high rates of stretch

Abstract Stress-strain relationships and the work of extension for four rubber compounds were studied for a rate of stretching of the order of 1,000 per cent per second. This rate is sufficiently great so that the test may be considered to approach adiabatic conditions. A comparison of these data with similar data for a rate of stretching which lies in the range of speeds common to the usual routine tests shows that increased speed of stretching affects the observed tensile properties as follows: (1) the stresses' are increased at elongations up to about 500 per cent for the loaded compounds and up to 600 per cent for the pure-gum compound, the maximum increases ranging from 75 pounds per square inch for the pure-gum compound to 245 pounds per square inch for the clay compound; (2) the stresses in the carbon-black compounds are decreased slightly at elongations near rupture; (3) the work of extension to rupture is increased for the pure-gum and clay compounds and decreased for the carbon-black compounds, ...

The Use of the Pendulum in Rubber Testing

Abstract During the past few years considerable interest has been shown in the use of pendulum methods for the study of resilience properties of rubber compounds. We feel therefore that an account of the development and use of the pendulum method in the Dunlop Rubber Co.'s laboratories will prove of general use. In this country the period from 1918 witnessed a rapid extension in road transport, carried at that time almost exclusively on solid tires. With this development the performance of heavy vehicles improved considerably and tires were operated under the increasingly severe conditions, both with respect to speed and load. It is well known, that if a solid tire is operated under sufficiently severe conditions it is no longer able to dissipate the heat developed within it, and its temperature rises until eventually an explosion or the familiar “blow-out” occurs. This phenomenon, of course, results from the fact that rubber is not perfectly elastic. When transmitting energy it does not release all the energy supplied to it. Some of the energy is always absorbed and appears in the rubber in the form of heat; the faster the energy is supplied, the greater is the increase in temperature.

Dielectric Measurements in the Study of Carbon Black and Zinc Oxide Dispersion in Rubber

Abstract The dielectric constant, power factor, conductivity and d.c. resistivity of rubber compounds containing various types and quantities of zinc oxide and carbon pigments have been measured. It has been shown that the dielectric properties of rubber compounds having high loadings of zinc oxide depend on the particle size and purity of the zinc oxide used. The French process oxides with the smallest particle size were found superior to other grades. Water-soluble impurities in zinc oxide are shown to have a deleterious effect on dielectric properties, especially in the presence of moisture. The effect on dielectric properties of adding carbon black to a rubber compound has been shown to be dependent on the type and amount of black added, and on the nature of its dispersion in the rubber. The dielectric properties of rubber compounds containing “soft” black made by the thermal decomposition process are shown to be distinctly superior to, and widely different from, those of the same compounds containing equal amounts of channel process black. The general conclusion has been reached that the smaller the particle size and the better the dispersion of carbon pigments in the rubber, the greater will be the increase in the dielectric constant and conductivity, and the greater will be the decrease in resistivity.

The Effect of Pigment Particle Size on Some Physical Properties of Rubber Compounds

Abstract The importance of flow in rubber on the reinforcing properties of pigmented systems has been emphasized by Park. He suggests that: (1) in the presence of a finely divided pigment, the flow which occurs when a piece of rubber is stretched takes place in the capillary spaces between the pigment particles; (2) some modification of the laws of liquid flow may govern the behavior of rubber with reference to pigments embedded in it, and (3) the forces causing increased stiffness in pigmented rubber compounds are similar to those causing increased resistance to flow of liquids iii tubes of capillary dimensions. Thus increasing fineness of subdivision and the resulting fineness of capillary spaces between the particles should be accompanied by an increase in reinforcing properties. It would be desirable to study the actual stresses around pigment particles in rubber under strain, but so far no suitable microscopic set-up has beem devised. A few years ago the writer, resorting to analogies, measured the strains and stresses around large particles with the assumption that the strains would be relatively the same with small particles. For this study, holes of the desired size and shape were cut in strips of calendered but uncured rubber and fitted with pieces of an uncured semihard rubber compound. After vulcanization squares were marked on the tensile sheets as shown in Fig. 1.

The Effect of Pigment Particle Size on Some Physical Properties of Rubber Compounds

The stresses around large obstacles in strained rubber were studied with the assumption that the effects would be similar with actual pigment particles. The effects of variations in the size and shape of the obstacles on the stress concentration were investigated and an explanation offered for the stiffiening of rubber by pigments based on the change in slope of the rubber stress strain curve. The various average diameters of a series of zinc oxide samples were measured and their relationship with some physical properties of rubber compounds determined.

Heats of Combustion of Rubber and of Rubber-Sulfur Compounds

Abstract The work described in this paper is part of a series of investigations at the National Bureau of Standards to determine the properties of purified rubber and of compounds of purified rubber and sulfur. Values of the heat of combustion of rubber have been reported by a number of observers, and values of the heat of combination of rubber and sulfur, determined as the difference in the heats of combustion of vulcanized and unvulcanized rubber-sulfur mixtures, have been reported by several observers. The results obtained by the various investigators are not in satisfactory agreement. The values reported in the literature for the heat of combustion of rubber vary over a range of about 4.5 per cent, and the two recent sets of data on the heat of combination of rubber and sulfur differ radically, as may be seen from Fig. 2. It was thought that these differences might be due, in part at least, to differences in the composition of the rubber, and that more consistent results might be obtained by using purified rubber. The data presented in this paper were obtained on samples of purified rubber and on compounds of purified rubber and sulfur prepared in connection with an investigation of the electrical properties of rubber and rubber-sulfur compounds by Scott, McPherson, and Curtis.

Thermal Properties of Rubber Compounds. II. Heat Generation of Pigmented Rubber Compounds

Abstract THE first paper (1) of this series discussed thermal conductivity of rubber and a number of compounding ingredients which were measured using the electric current as the source of heat. In this article the fundamental factors controlling the generation of heat and the variations possible by pigmentation are being studied. Results obtained for pigmented rubber in the pendulum and flexometer will be discussed and correlated. In the writers' laboratory two machines have been used extensively in studying the temperature developed in rubber compounds subjected to distortion by compressive forces. The first of these is a flexometer described by Cooper (2), and the second a compression machine in which a rubber block 14 cm. (5.5 inches) in diameter and 9.53 cm. (3.75 inches) high is pounded with a definite load a specified number of times per minute. The laboratory test block used in the flexometer is in the shape of a frustrum of a rectangular pyramid, of which the base is 5.4 × 2.86 cm. (2.126 × 1.125 inches), the top 5.08 × 2.54 cm. (2 × 1 inches), and the altitude 3.81 cm. (1.5 inches). This block of rubber is compressed between two plates under definite load, one of the plates being stationary while the other travels in a circular motion of definite magnitude. After the sample has been placed in the machine, the moving plate is set to one side of the center. Both the loading and the amount of offset may be varied within wide limits. With this machine one may study either the temperature developed over a period of flexing or the time required to compress the sample a predetermined amount.

Thermal Properties of Rubber Compounds II. Heat Generation of Pigmented Rubber Compounds

Abstract THE first paper (1) of this series discussed thermal conductivity of rubber and a number of compounding ingredients which were measured using the electric current as the source of heat. In this article the fundamental factors controlling the generation of heat and the variations possible by pigmentation are being studied. Results obtained for pigmented rubber in the pendulum and flexometer will be discussed and correlated. In the writers' laboratory two machines have been used extensively in studying the temperature developed in rubber compounds subjected to distortion by compressive forces. The first of these is a flexometer described by Cooper (2), and the second a compression machine in which a rubber block 14 cm. (5.5 inches) in diameter and 9.53 cm. (3.75 inches) high is pounded with a definite load a specified number of times per minute. The laboratory test block used in the flexometer is in the shape of a frustrum of a rectangular pyramid, of which the base is 5.4 × 2.86 cm. (2.126 × 1.125...

The Impact Resiliometer

Abstract Original Purpose The writer's first conception of an instrument for quickly measuring the resilience of rubber samples arose in connection with the basic idea for and development of the present A.S.T.M. hardness tester, in an endeavor to provide a reproducible means of measuring and expressing those properties of rubber compounds superficially apparent to the average user. It seemed that if the resistance to pressure (indenting or flexing) and the pressure, or force, of recovery, of a sample were duplicated, the article produced would “feel” the same as the sample, and under normal conditions of use would act mechanically the same. Nature of Problem This problem of measurement, as is generally known, is complicated by the time-hysteresis characteristic of vulcanized rubber compounds, arising through a dual nature, and causing them to manifest both elastic and plastic properties. Any instrument which applies load momentarily measures the almost purely elastic properties, while one which applies a more or less sustained load measures elastic properties modified by the material's partially plastic nature. These ideas are expressed from commercial and industrial viewpoints without regard to their possible deviations from the strict definitions accepted in physical science.

Thermal Properties of Rubber Compounds I. Thermal Conductivity of Rubber and Rubber Compounding Materials

Thermal Properties of Rubber Compounds I. Thermal Conductivity of Rubber and Rubber Compounding Materials

Technical Application of Antioxidants

Abstract IT is unfortunate that the process of vulcanization, to which the commercial use of rubber is almost wholly due, makes it much more susceptible to oxidation. The deterioration that sets in as soon as rubber is vulcanized was first attributed to oxidation by Miller in 1865 but it is only during the last 20 years that a serious effort has been made to find means of preventing it. During this period it has been found that many chemicals retard the oxidation of rubber when incorporated with it prior to vulcanization. Phenolic compounds, primary and secondary amines, aminophenols and condensation products of aldehydes with amines are the chemical groups within which the most valuable antioxidants are found. Many amines and aldehydeamine condensation products are also vulcanization accelerators and some of the earliest known accelerators have a mild antioxidant effect. Para-phenylenediamine and para-aminodimethylaniline were used as accelerators long before the term antioxidant was coined and their favorable effect on the aging properties of rubber compounds has been a matter of common knowledge for more than ten years. Recent investigations have disclosed numerous compounds in these classes that have little or no accelerating effect and are far more effective in retarding oxidation than any of the known accelerators.

Effect of Various Types of Carbon Black on Certain Physical Properties of Rubber Compounds

Abstract The data presented herein show that in general the presence of oxygen on carbon black retards the rate of vulcanization in direct proportion to the amount of oxygen present and also decreases the maximum physical properties obtainable with a given amount of accelerator. The aging data show that the presence of this oxygen on the black increases the rate of aging as the amount of oxygen increases, but not in direct proportion to the per cent of this oxygen present. It can be concluded, therefore, that compounds which contain a small amount of oxygen, such as thermatomic, G black, or acetylene black, will give better aging stocks than compounds containing higher amounts of oxygen such as lampblack and standard channel blacks. No correlation could be found between the acetone extract, iodine adsorption, or oil adsorption, and the effect of these blacks on the rate of cure or aging.

Effect of Various Types of Carbon Black on Certain Physical Properties of Rubber Compounds1

Abstract The data presented herein show that in general the presence of oxygen on carbon black retards the rate of vulcanization in direct proportion to the amount of oxygen present and also decreases the maximum physical properties obtainable with a given amount of accelerator. The aging data show that the presence of this oxygen on the black increases the rate of aging as the amount of oxygen increases, but not in direct proportion to the per cent of this oxygen present. It can be concluded, therefore, that compounds which contain a small amount of oxygen, such as thermatomic, G black, or acetylene black, will give better aging stocks than compounds containing higher amounts of oxygen such as lampblack and standard channel blacks. No correlation could be found between the acetone extract, iodine adsorption, or oil adsorption, and the effect of these blacks on the rate of cure or aging.

Effects of Accelerated Aging upon Some Physical Properties of Hard Rubber Compounds

Effects of Accelerated Aging upon Some Physical Properties of Hard Rubber Compounds