bluuebunny/crossref_metadata_2025 huggingface dataset:
Vulcanization Characteristics of Natural Rubber
Authors: Stiehler Robert D. , Roth Frank L.
Published: February 2011

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 vulcanization to determine their relative effects on the parameters and the classification of the rubbers. For this work, four Hevea rubbers that covered a wide range in both rate of cure and ultimate stiffness of the vulcanizate were chosen. The rubbers were smoked sheet, pale crepe, sprayed latex, and Tensorub. The sample of smoked sheet was from a bale of No. 1 grade rubber, taken at random. The same was true for pale crepe. The sample of sprayed latex rubber was prepared by the U. S. Rubber Co. plantation by spray-drying once-creamed formaldehyde-preserved latex. The sample of Tensorub was produced by Socfin Co., Ltd., in Malaya and was selected to represent a high-quality smoked sheet.
The Vulcanization of Thick Sheets of Rubber
Authors: Bott E. C. B.
Published: February 2011

Schmidt's method for the determination of temperatures attained by various thicknesses of solid sheets during heating and cooling is adapted for the calculation of the amounts of vulcanization obtained for flat sheets of soft rubber and of the extra time needed to obtain a full vulcanization in the center of the sheets. The uniformity of vulcanization of the central portion of the sheets is investigated.
Direct Determination of Oxygen in Rubber. Comparison between Isotopic and Schütze-Unterzaucher Methods
Authors: Kirschenbaum A. D. , Streng A. G.
Published: February 2011

The values obtained by the isotope method for the direct determination of oxygen in rubber agree very nicely with the values obtained for the gain in weight on oxidation. The values obtained by the Schütze-Unterzaucher method, reported herein, are all low. This study indicates that the Schütze-Unterzaucher may give low oxygen values for compounds which form non-vaporizable carbon residues, such as rubber.
Viscometric Detection of Branching in Polymers. II. Branching in Poly(Butadiene-Co-Styrene) as a Function of Polymerization Temperature
Authors: Cragg L. H. , Fern G. R. H.
Published: February 2011

Previous experience with polystyrene, normal and cross-linked, and with GR-S, gave strong support to our belief that, in careful viscosity measurement and evaluation of either of the slope constants β and k′, a useful and convenient means is available for the detection and measurement of branching in soluble high polymers. Moreover, since the measurements are made on separate fractions, information can be obtained regarding the relation between the size of polymer molecules and the degree to which they are branched. In the work reported here, this method was applied in a study of the effect of the temperature of polymerization on branching. It is now well known, of course, that, by reducing the temperature of polymerization, desirable changes of the physical behavior of emulsion polymers may be effected. This observation led to the development of “cold rubber” as an improvement over GR-S for use in tire tread stocks. Many efforts have been made to explain this superiority on the basis of changes in molecular structure. Several effects of lowering the temperature of polymerization have been noted : a narrowing of the molecular-weight distribution, a slight decrease of the proportion of the 1,2-isomer, a more marked increase of the proportion of the trans-form of the 1,4-isomer, and a decrease of the extent of chain branching. Of these various effects, the one most difficult to detect and to measure quantitatively is chain branching.
The Effect of Comonomer on the Microstructure of Butadiene Copolymers
Authors: Foster Frederick C. , Binder John L.
Published: February 2011

The microstructure of butadiene-styrene copolymers having a wide range of styrene contents has been determined by infrared absorption. The results demonstrate that the percentage of trans-1,4-addition increases, the 1,2-addition decreases, and the cis-1,4-addition decreases as the styrene content is increased. Similar measurements of five other butadiene copolymer systems indicate that all these vinyl monomers, acrylonitrile, methacrylonitrile, methylvinyl ketone, vinylpyridine, and α-methylstyrene, change the microstructure in the same direction as styrene, differing only in the magnitude of their effect. A theoretical explanation, consistent with the experimental results obtained, is given for the change in microstructure with comonomer content.
Self-Inhibition in the Oxidation of Butadiene-Acrylonitrile Rubbers
Authors: Kuzminskiĭ A. S. , Popova E. B.
Published: February 2011

Because of their valuable technical properties, copolymers of butadiene and acrylonitrile rubbers have found wide use in the rubber industry in the production of oil-resistant and heat-resistant products. Investigations have established that, during the copolymerization of butadiene with acrylonitrile, the latter enters principally at the 1,4-position of the main chain. This is observed most clearly with relatively large concentrations of acrylonitrile. Our own work showed that the double bonds in the main hains of the rubber molecules are the most active points in the chain. It indicated also that butadiene-acrylonitrile rubbers should oxidize at a fast rate. However, in reality they show great chemical stability. The present study is devoted to an explanation of the nature of oxidation of butadiene-acrylonitrile rubbers. Both the free and inhibited oxidation of copolymers containing, respectively, 20, 29.6 and 35 per cent of acrylonitrile (CKH-18, CKH-26, and CKH-40) was studied. Before oxidation, the rubbers were purified by extraction and double precipitation from benzene solutions. The rubber films were prepared and oxidized with molecular oxygen according to methods described previously.
Chlorination of Natural Rubber in Solution with Gaseous Chlorine
Authors: Ramakrishnan C. S. , Raghunath D. , Pande J. B.
Published: February 2011

The chlorination of rubber solutions by gaseous chlorine was followed by isolating the partially chlorinated products and preparing their ozonides. The ozonides were hydrolyzed, and the acids and aldehydes formed on hydrolysis were determined. By a comparison with the amounts of acids and aldehydes obtained from ozonides of unreacted rubber, the amount of residual isoprenic double bonds present was found. The loss of double bonds attending the introduction of chlorine atoms into the molecule of rubber indicates four definite stages in chlorination : (1) initial substitutive attack by chlorine, with concomitant cyclization, resulting in a loss of one double bond between two isoprenic units, (2) substitution, (3) additive reaction, and (4) essentially substitution. Chlorination of aged rubber solutions differs from the above in that the cyclization reaction (stage 1) seems to be absent.
Carbon Black Particle Size
Authors: Leigh-Dugmore C. H.
Published: February 2011

Carbon black particle diameters tend to be distributed log-normally. This discovery has led, first, to simplified methods for the calculation of mean diameter and specific surface from electron microscope measurements; second, to a demonstration that the observed differences between estimates of surface area from electron-microscope measurements and from low-temperature nitrogen adsorption measurements cannot be entirely explained by supposing the finest particles not to be resolved by the electron microscope, thus adding further support to the porosity explanation; and third, to the argument that any two size characteristics, such as mean particle diameter and specific surface, are entirely sufficient to describe particle size and its distribution and, consequently, their influence on vulcanizate properties.
A Study of the Goodrich Flexometer with Synthetic Polymer Compounds
Authors: Labbe B. G.
Published: February 2011

The relation between the testing temperature (100, 158, and 212° F) and the hysteresis-temperature rise of the synthetic polymers at the cures used was found to behave as a linear function. The slopes of the lines seem to indicate that the relative order of efficiency of the vulcanizates varies at different temperatures of test and cure levels. Frequency variations at 212° F ranged from 980 to 1800 flexures per minute. The temperature rise is a linear function of the frequency for all compounds tested. The increase of temperature rise with an increased rate of flexing is approximately uniform for all compounds (about 2° F per 100 f.p.m.) at the cure levels studied. The state of cure is shown to influence the rate of change of the temperature rise with the change in frequency. The relation of the degree of deflection to the temperature rise characteristics of the rubber samples at 100 and 212° F was studied at 10, 17.5 and 25. per cent deflections. An error of deflection of one percentage unit will cause a .3° to 4° F error in the 10-minute temperature rise value. As the state of cure or the temperature decreases, the error in the temperature rise increases. A change of load has less effect on the temperature rise and set values than any of the other conditions investigated. Most of the lack of reproducibility among laboratories is probably due to failure to adjust precisely the throw (deflection of the specimen) of the test apparatus which is being used.
The Mechanism of the Action of Aromatic Thiols on Rubber Solutions
Authors: Kheraskova E. P. , Gamayunova A. P.
Published: February 2011

1. The effect of aromatic thiols on the kinetics of the change in viscosity of natural-rubber solutions on heating was studied. 2. Aromatic thiols accelerate the oxidation of rubber. 3. In the process of heating rubber solutions, thiols are oxidized to the corresponding disulfides. The rate of oxidation of the rubber depends on the rate of oxidation of the thiol. 4. The mechanism of the action of thiols is explained by the presence of an accompanying reaction, whereby oxidation of the thiol induces oxidation of the rubber.
Birefringence of Rubbers
Authors: Saunders D. W.
Published: February 2011

It has been shown, both theoretically and experimentally, that in simple elongation of a sample of pure-gum vulcanizate, an approximately linear relationship between the tensile stress t and the birefringence (n1−n2) obtains, provided the strain is not sufficiently large to produce crystallization. We may write: (n1−n2)=Ct, where C, the constant of proportionality, is termed the stress coefficient, and is theoretically independent of the degree of cross-linking (vulcanization) of the vulcanizate, and should be identical for all pure rubbers.
The Preparation of Cyclized Rubber from Natural-Rubber Latex
Authors: van Veersen G. J.
Published: February 2011

The hydrochlorination of rubber in the form of latex which has been stabilized by surface-active agents, such as Emulphor-O, or by cationic agents such as Fixanol, shows that hydrogen chloride molecules diffuse very easily into the latex globules. Accordingly it was to be expected that, by replacing hydrogen chloride gas by hydrogen fluoride gas, which is a cyclizing agent of rubber, a cyclized rubber would be obtained. This line of reasoning was confirmed experimentally. As a result of an experimental study, it was proved that boron fluoride, BF8 also, brings about cyclization of rubber in latex, and van Amerongen has already shown that sulfuric acid likewise can be used as a catalyst. Since sulfuric acid is much easier to handle than either hydrogen fluoride or boron fluoride, it was used in most of the experiments in the present work.
Determination of Free Carbon in Cured Rubber Stocks
Authors: Kolthoff I. M. , Gutmacher R. G.
Published: February 2011

A method for the determination of free carbon in vulcanized rubber stocks is described. The sample is softened in boiling p-dichlorobenzene before treatment with tert-butyl hydroperoxide in the presence of osmium tetroxide. No difficulties are encountered in the filtration of the carbon black. The carbon black is washed on the filter with dilute nitric acid to remove acid-soluble inorganic fillers. The method has been successfully applied to natural rubber, GR-S, Butyl rubber, and Neoprene. No correction is necessary.
Theory on the Crystallization and Melting of Unvulcanized Rubber. II. Melting
Authors: Kikuchi Ryoichi
Published: February 2011

In this paper it will be attempted to explain the melting properties of rubber crystals discovered by Wood and Bekkedahl, with the same notations and definitions as in Part I.
The Effect of Chemical Changes in the Rubber Hydrocarbon on Its Oxidizability
Authors: Le Bras Jean , de Merlier Jacqueline
Published: February 2011

The hydrocarbon of rubber, which contains one ethylene linkage or double bond per isoprene unit, has such a structure that it is very sensitive to the action of oxygen, and the investigations of Farmer have shown clearly the part played by the α-methylene carbon atom in the process of oxidation. Since the rubber hydrocarbon can be made to react with compounds which unite directly with it, an attempt was made to find out what influence any changes of this kind might have on the oxidiazbility of rubber. The reagents chosen were saligenol and maleic anhydride. According to Cunneen, Farmer, and Koch, saligenol attaches itself to double bonds, with formation of a chromanic nucleus (I), the nature of which the present authors have examined and confirmed by ultraviolet spectrography of the compounds prepared. In the case of maleic anhydride, Delalande has shown that this compound and also related compounds, such as maleic imide, can combine according to the substitutive addition mechanism described by Alder, with the formation of a molecule of type (II). The reactions were carried out with exclusion of air, in accordance with the technique of Moureu and Dufraisse on benzene solutions of purified crepe rubber, and in the absence of a catalyst.
Tetramethylthiuram Disulfide Vulcanization of Extracted Rubber. I. Introduction and Compounding
Authors: Craig David , Juve A. E. , Davidson W. L.
Published: February 2011

(1) Large dosages of TMTM inhibit TMTD vulcanization or vulcanization with sulfur. (2) The desirability of up to about 1.5 phr zinc oxide for TMTD vulcanization was confirmed. If an unusually large amount of TMTD (10 phr) is used, a well vulcanized stock may be obtained without zinc activation. (3) Palmitic acid displays only a mild activating effect on TMTD vulcanization. (4) Zinc dimethyldithiocarbamate, a main product of TMTD vulcanization, is not an activator for a TMTD-zinc oxide recipe. (5) Dimethylammonium dimethyldithiocarbamate does not activate TMTD-zinc oxide formulations. (6) Zinc sulfide is an effective activator for TMTD vulcanization.
Tetramethylthiuram Disulfide Vulcanization of Extracted Rubber. II. Structure of Radioactive Tetramethylthiuram Disulfide
Authors: Craig David , Davidson W. L. , Juve A. E. , Geib I. G.
Published: February 2011

Tetra-substituted thiuram polysulfides are notable because of their action as “accelerators” in rubber vulcanization and also because of their ability to vulcanize rubber without added sulfur, especially in the presence of zinc oxide, which leads to the formation of zinc dimethyldithiocarbamate (ZnDMDC) and other substances. A study of the structure of tetramethylthiuram disulfide (TMTD) is reported in this paper. This has involved the introduction of radioactive sulfur. A corollary has been the similar study of ZnDMDC, which is an “accelerator” but not a vulcanizing agent. The synthesis of radioactive TMTD (i.e., TMTD*) was accomplished by heating 4.16 grams (0.02 mole) of tetramethylthiuram monosulfide (TMTM) and 1.28 grams (0.04 atom) of radiosulfur (S*8) to 120° in a stream of nitrogen for 30 minutes. By extraction with hot alcohol and benzene, a series of crystalline fractions was secured which totaled 3.38 grams, a yield of 70 per cent. This material melted at 150–156° C, with decomposition. The excess S*8 was recovered in a second experiment. Pyrolysis of the TMTD* prepared in the above manner was carried out at 160–230° C in a modified Claisen flask. There were produced S8*, tetramethylthiourea*, and CS2* in nearly theoretical yield. The radioactivities of the thiuram disulfide and of the pyrolysis products (the CS2* being converted to phenylhydrazinium phenyldithiocarbazate* or to 1-phenylthiosemicarbazide* for study), which are recorded in Table I, show that the tracer sulfur atoms distributed themselves uniformly throughout the various molecules.
The Swelling of Cross-Linked Amorphous Polymers under Strain
Authors: Treloar L. R. G.
Published: February 2011

The equilibrium swelling of vulcanized natural rubber in benzene and heptane was investigated experimentally as a function of the state of strain. Three types of strain were examined, i.e., simple elongation, 2-dimensional extension, and unidirectional compression. The swelling ratios were determined either from measurements of linear dimensions, or by weighing, precautions being taken to ensure uniformity of the strain over the measured region. Measurements of swelling ratios were reproducible to about 1 per cent and the swelling process was completely reversible (with respect to strain variations). Changes in swelling ratios of up to 50 per cent were observed, the swelling increasing under tensile stresses and falling under compressive stresses. The results were accurately accounted for by the author's extension of the Flory-Rehner statistical theory, using two independent parameters—one the μ-value in the Flory-Huggins equation for the free energy of mixing, which was obtained from vapor pressure data; the other, the degree of cross-linking, which was determined from the swelling of the unstrained rubber. It is concluded that, for good swelling agents, this theory is applicable to a homogeneous strain of the most general type.
Power Factor and Vulcanization
Authors: Waring J. R. S.
Published: February 2011

Over a wide range of curing systems electrical power factor was found to increase as combination with sulfur proceeded. The main experimental results were obtained at nine megacycles frequency and laboratory temperature, using a Q-meter for the electrical measurements. Some experiments were also carried out through frequency and temperature range. Analysis of the data shows that no simple conversion factor can change electrical power factor into combined sulfur, indicating that the electrical losses are affected by the type of combination with sulfur. The omission of zinc oxide causes a marked increase in the rate of change of power factor with combined sulfur. Dipole moments have been calculated from the slopes of these correlation curves, assuming monosulfide —S— when zinc oxide was not included in the mix and disulfide —S—S— when zinc oxide was present. Reasonable agreement has been obtained with experimental data on simpler compounds, e.g., dilauryl mono- and disulfides. The experimental technique has provided an analytical tool with possible applications both to problems of factory control of cure and to checking theories of vulcanization.
Thermoelastic Properties of Noncrystallizing Rubber
Authors: Bartenev G. M.
Published: February 2011

1. A method of measuring and obtaining the equilibrium isotherms of strain for vulcanized butadiene-styrene rubber is described. 2. It is shown that the component of the elastic force of the rubber which depends on the internal energy has a value of several per cent. 3. Calculation shows that the change in the volume of uncrystallizable rubber, at 300 per cent strain, has a value less than 0.1 per cent. 4. A comparison of the results obtained with the thermodynamic equations shows that the deformation of the rubber investigated can be considered strictly high elastic and entropic. 5. A comparison of the experimental data with the theory of high elastic deformation confirms the accuracy of the conclusions from the theory.
The Role of Intermolecular Forces in the Mechanism of High-Elastic Deformation. I. The Molecular Mechanism and an Equation for the Kinetics of High Elastic Deformation
Authors: Dogadkin B. A. , Bartenev G. M. , Reznikovskii˘ M. M.
Published: February 2011

1. The molecular mechanism of the relaxation of deformation of high-elastic polymers has been studied. 2. It is shown that the slow relaxation, which is typical of high-elastic polymers, may be best explained as a restoration process, which either partial or complete (depending on the degree of development of side chains in the molecular structure formed by the main valence chains) of the balanced configurations of the molecular chains. 3. It is shown that the rate of the relaxation process in this case is determined by the molecular activity of the particular polymer. 4. An approximate equation for the kinetics of high-elastic deformation which expresses qualitatively the mechanical properties of high-elastic polymers is proposed. 5. Hypotheses concerning the relation between the time of relaxation and the unbalanced stress are advanced. Equation (2) is derived as characteristic of this relation. 6. It is shown that the joint application of Equations (1) and (2) makes it possible to describe qualitatively the relaxation of stress at constant deformation.
Crystallization and Second-Order Transitions in Silicone Rubbers
Authors: Weir C. E. , Leser W. H. , Wood L. A.
Published: February 2011

In the course of an investigation to determine which rubbers might be suitable for use at low temperatures, interferometric measurements of the length-temperature relationships of silicone rubbers have been made. Crystallization was found between −60° and −67° C in Dow-Corning Silastic X-6160 and in General Electric 9979G silicone rubber, the latter of which contains no filler. Crystallization between −75° and −85° C was found in Silastic 250. Melting occurred over a range of temperature above the temperature of crystallization. The volume change on crystallization varied from 2.0 to 7.8 per cent. No crystallization or melting phenomena were observed in Silastic X-6073 between −180° and +100° C. All types of silicone rubber exhibited a second-order transition at about −123° C, the lowest temperature at which such a transition has been observed in a polymer. The coefficient of linear thermal expansion of silicone rubbers containing no filler was found to be about 40×10−5/degree C between −35° and 0° C.
Simple Method for Determining the Dynamic Coefficient of Friction of Vulcanized Rubber
Authors: Boonstra B. B. S. T.
Published: February 2011

A method for measuring the coefficient of friction at low speeds by means of a normal dynamometer for rubber testing is described. To this end a couple of molded rubber wheels are pulled over a piece of roadlike surface. At the same time the wheels are forced to rotate with a speed nonconcordant with the linear speed on the surface, so that a certain amount of friction occurs. The force necessary to turn the wheels over the surface is recorded on the dynamometer; the average is proportional to the average coefficient of friction. Preliminary experiments were carried out to prove the usefulness of the apparatus. A number of compounds of natural rubber, GR-S, and “cold” rubber were tested on four surfaces: asphalt, asphalt with rubber, concrete, and ground glass. Although the apparatus allows variation of load and of speed, the experiments were carried out at a speed of 100 cm. per minute and only with a load of 4 kilograms. On dry surfaces, the highest coefficient of friction was found with a natural-rubber compound on an asphalt material in which rubber powder had been dispersed. With most rubbers this surface showed a somewhat higher coefficient of friction than did the asphalt without rubber-powder, which in turn was better than the other two surfaces. On the average there was little difference between the various rubber compounds, though natural rubber seemed to yield the highest values. Natural-rubber pure gum compound did not show higher values than the carbon black mixes, but a vulcanizate with cyclorubber instead of carbon black was definitely better.
Failures of Rubber Insulation Caused by Soil Microörganisms
Authors: Blake John T. , Kitchin Donald W. , Pratt Orison S.
Published: February 2011

The failure of rubber insulation brought about by soil microörganisms can best be detected by a decrease of several decades in the insulation resistance. To study the mechanism of attack and the physical structure that makes rubber insulation vulnerable to such attack, it is necessary to have means of locating and marking the failed spots. Several methods of doing this have been described. The electrodeposition of copper in the micropores created by microbes affords the best insight into the physical structure of the faults. Two distinct modes of attack have been shown. The simpler one, which is characteristic of natural rubber, is a progressive eating away of the bulk insulation. The second, a more obscure type, creates micropores through the wall, but without visible surface erosion. This mode is typical of the synthetics, in which the base hydrocarbon or polymer, if freed of extraneous microbe food, does not support microbe growth. The imperfections consist of streaks of microbe food scattered in a random manner through otherwise inert material. Their distribution is affected by physical processing. These streaks are not hygroscopic and do not lead to failure in prolonged immersion in either hot or cold water. They do, however, tend to lower the d-c breakdown of samples after several days soaking in hot water. Thus with a given susceptible compound it is possible to predict which processing will produce the greatest stability in soil by breakdown tests on samples not exposed to soil. The breakdown tests relate to physical structure. To test the inherent resistance of a material to soil exposure, the simple visual fungus test is very convenient. The validity of this test as a criterion of stability has so far been confirmed by the routine resistance tests after accelerated laboratory soil exposure. It is not safe to predict the stability of a complete compound from the results of visual tests on the base material. The vulcanized insulation must be tested. Insulating compounds shown to be promising by the visual test should be given a thorough soil exposure test. The testing of preventives is a field in itself. It is not simple. Materials which are strongly antiseptic in water may not be so when mixed in a rubber compound. Many of them would be completely unsuitable in rubber insulation. Some preventives are effective in a compound before it is vulcanized but not afterwards. Some are completely inactivated by certain other necessary or desirable ingredients. Others may leach out in time and cease to protect the material. Oxidation inactivates some preventives. A nicely balanced compound is required to produce insulation with sufficient stability in soil, proper physical characteristics, and high electrical quality.
Optical Examinations of Anisotropy in Vulcanizates
Authors: Oberto Stefano
Published: February 2011

The anisotropic behavior of vulcanizates is revealed by differences in modulus, breaking load, and tear, swelling in solvents, and electrical properties, according to the direction of test. Anisotropy is attributed both to a state of tension in the rubber phase alone and to a preferential orientation of the elongated particles of certain fillers. It bas recently been recognized that anisotropy due to movement of the stock in the mold during the first phase of vulcanization is of very great importance. By submitting overflow slaba, sheets, and test-pieces to examination under transmitted polarized light, it is relatively easy to obtain optical evidence of the presence of anisotropy caused by movements during molding operations. The localization of the movements may make it desirable to examine with a microscope at low magnification microtome sections of the vulcanizates 50 microns thick. A transparent gum compound can be used, but more details can be seen if 1 per cent of carbon black is added. The transparent compound may be used under polarized light for the direct examination of samples a few millimeters thick or of sections of equal thickness taken from products of different dimensions. In injection-molded products, it is possible to tell from variations of double refraction when molding conditions favor scorching before the rubber has come to rest. If talc or some other ingredient with elongated strong double-refractive particles is introduced into the transparent compound, it is possible to observe the orientation of the particles cauaed by the processing or molding flow. In a calendered sheet, the talc particles lie almost parallel to each other, but toward the center of the sheet the orientation may be less marked or may occur along slightly different directions. In injection molding the tale clearly reveals flow direction and anisotropy. Under optical observation, in restricted areas, the difficulty of obtaining entirely isotropic rubber products is confirmed. Almost pure-gum compounds have been examined, but the anisotropy found will probably assume even greater importance in loaded mixings, especially those loaded with carbon black.
The Destructive Dissolution of Vulcanized Rubbers
Authors: Dogadkin B. , Tarasova Z.
Published: February 2011

In previous work the process of destructive dissolution of vulcanized natural rubber was described, and it was shown that, in the strict absence of oxygen, a vulcanizate cannot be dissolved in a hydrocarbon medium at temperatures up to 140° C. Only in the presence of molecular oxygen does heating bring about dissolution of vulcanized rubber in the solvent, under which conditions the process proceeds along this pattern: union of oxygen with the double bonds → disintegration of the molecular chains where oxygen has combined → dissolution. if the surface area of the vulcanizate remains constant, the process takes place at a constant rate which depends on the temperature and the partial pressure of the oxygen in solution. In the following, experiments are described, the results of which show that vulcanized synthetic polymers also undergo destructive solution. The experimental method was the same as that described previously. The vulcanizates were prepared from recipes adapted to the particular synthetic rubbers. As was expected, the rate of destructive solution depends on the molecular structure of the rubbers (see Figure 1).
Primary Creep and Stiffness of Vulcanized Rubbers
Authors: Cooper D. H.
Published: February 2011

The addition of two compounded blacks renders induction concerning the mechanism of either black uncertain. Generally speaking, however, the black in a specimen of given size may be thought of as replacing its volume of rubber, so, when under a particular load, the rubber remaining is more highly stressed. Because the rubber is highly stressed, the relative creep is increased, but because there is less of it, the actual creep is diminished. for a given shape of specimen, all soft compounds and gum stocks have the same curve of load-area vs. strain up to 20 per cent compression, 40 per cent tension, and 40 per cent shear; the same applies to medium hard stocks. The stiffness of any such specimen can be expressed in terms of a convenient reference level, such as the load-area at 10 per cent shear 1-minute strain. Over a wide range of conditions in which polymer, compound, cure, and geometry of loading have been varied, 10-minute creep remains proportional to 1-minute deflection for each particular geometry of loading, but in tension the ratio of 10-minute creep to 1-minute deflection is greater than in shear, in compression less—an effect which is generally more marked in soft than in hard compounds. Measurements of creep on specimens in which the ratio of diameter to height is greater than 1.6 were, mainly on account of the small deflections involved, not sufficiently accurate to be included. It is reasonable to assume, however, that creep is proportional to strain under other conditions than those considered here, such as in specimens of other shapes and specimens loaded in bending and torsion.
The Chemical Structure Responsible for the Deactivating Effect of Compounds Which Protect Rubber from Deterioration by Oxygen
Authors: Le Bras Jean , Le Foll Jacques
Published: February 2011

One of the present authors has already offered evidence which indicates the existence of a deactivating effect, whereby vulcanized rubber is protected against deterioration by oxygen. This effect is evident with such compounds as mercaptobenzimidazole (I), mercaptobenzoxazole, and ethylene-bis (N,N′-phenylthiourea) (II), and the phenomenon seems to be connected in some way with the presence in the molecule of a thiol group united to a nitrogen atom under such conditions that the possible tautomerism between the thion and thiol forms (III) is probably displaced toward the thiol form. We have completed these earlier experiments by a more systematic study, which has included an examination of the influence of cyclization, the nature of the ring, and hetero atoms.
The Chemical Constitution of Chlorinated Rubber
Authors: Allirot René
Published: February 2011

The results of the work described in the present paper indicate that, by the prolonged action of chlorine on polyisoprenes, chlorinated compounds corresponding to the composition (C10HxCl7)n are formed, irrespective of the molecular weight. The exact hydrogen content of these products can be calculated from determinations of the hydrogen chloride evolved in the reaction, and measurements to this end are in progress. Furthermore, determinations of the osmotic pressure and intrinsic viscosity, which have already been undertaken, should make it possible to determine with certainty the mean degree of polymerization of each fraction.
Polysulfide Liquid Polymers
Authors: Jorczak J. S. , Fettes E. M.
Published: February 2011

Polysulfide liquid polymers are a comparatively recent development conceived at the Thiokol Laboratories in 1943. The development was initiated by the problem of finding methods to reduce the molecular weight of a polysulfide rubber which was too tough to process on conventional rubber milling equipment. The problem was solved by reduction of a few of the disulfide links present in the polymer chain. It was soon found that the method was applicable to the preparation of polymers low enough in molecular weight to be liquids. The method produces dithiols of high purity which are extremely active in a wide variety of chemical reactions. Some formulations have been developed which depend on conversion from the liquid to rubber state at temperatures as low as 50° F in about 30 minutes. Most of the converting agents function through oxidation with hydrogen removal from the thiol and a linkage of sulfurs to reform the disulfide group. The converted polymers have the general properties of polysulfide polymers: good solvent resistance to a wide range of solvents, low diffusion rate of gases, good resistance to oxidation, ozone, and weathering, and a service temperature range from −65° to +250° F. (Some compounds can withstand intermittent temperatures as high as +350° F.) The low temperature properties are inherent in the polymer and do not depend on special compounding techniques.
Wax Blooming and Exposure Cracking
Authors: Best L. L. , Moakes R. C. W.
Published: February 2011

A brief résumé is given of the types of deterioration which cause cracking and similar forms of deterioration in vulcanized rubber and the possible remedies. A study has been made of the development of wax bloom on rubber and some of the factors which affect this development. The waxes constituting the blooms have been examined, as has the wax in the bulk of the rubber. The protection afforded by these wax blooms against ozone attack on various base mixes with both natural and synthetic rubber has been determined, using a standard technique for the ozone exposure.
Molecular Aggregation in Amorphous Polymers
Authors: Kasatochkin V. I. , Lukin V. V.
Published: February 2011

We have already established the existence of two types of packing of the links of neighboring molecular chains in amorphous rubber. The association of the links whose packing is characterized by a relatively dense distribution, similar to the packing of the molecules in low-molecular liquids, constitutes the liquid phase of amorphous rubber. In an x-ray diffraction diagram, this phase of the rubber shows an outer interference maximum caused by the intermolecular interference of the x-ray radiation which is coherently scattered by the adjacent links and corresponds to the average intermolecular spacing. Another phase of amorphous rubber, formed by the association of the segments of adjacent molecular chains, of quite disordered distribution, was called the gaseous phase. The links of the molecular chains which make up this phase of the rubber, because of their irregular distribution, scatter x-rays and show no intermolecular interference effect, so the scattering is like that in molecular gases. The dense background usually observed in diffraction diagrams of rubber, the intensity of which increases in the region of small scattering angles, can be explained by the effect of the independent scattering of the disordered links of the molecular chains of the rubber. It has the nature of scattering by a gas. The presence of such a type of molecularly disordered gaseous component is a peculiarity of the aggregation of molecules in amorphous high-molecular substances. Its existence is shown by the x-ray diffraction diagrams of amorphous high-molecular substances, in which, in the absence of low-molecular liquids, a background of independent scattering is always observed, besides the outer interference maximum which corresponds to the intermolecular spacing. The intensity I of the scattered (monochromatic) x-rays at any angle is, according to the theories developed, determined by the sum of the scattering intensities of the gaseous (Ig) and liquid (Il) phases of the amorphous substance.
Determination of Intrinsic Low Stress Properties of Rubber Compounds. Use of Inclined Plane Tester
Authors: Dunlap W. B. , Glaser C. J. , Nellen A. H.
Published: February 2011

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.
Some Properties of Vulcanized Rubber under Strain. Degree of Crystallization as Calculated from Temperature Coefficient of Elastic Tension
Authors: Boonstra B. B. S. T.
Published: February 2011

To elucidate the crystallization phenomenon in natural rubber and to investigate the applicability of thermodynamic calculation to measurements of the elastic tension as a function of temperature, it seemed necessary to check whether crystallization determined by x-ray analysis (and combined with density) lined up reasonably with the percentage of crystallization computed from the energy change found by applying thermodynamics to stretched vulcanized rubber) on stretching. Calorimetric measurements were desirable, as no accurate figures are available for the heat of crystallization of rubber crystallites. The heat of melting of rubber crystallites was determined to about 66 joules per gram, which is of the same order as that of isoprene. The spreading in the results was large; the determination is based on the degree of crystallization found by x-ray analysis of raw rubber. The heat of crystallization on stretching, found by thermodynamic evaluation of the elastic tension and its temperature coefficient, is combined with the value of 66 joules for the heat of melting of the pure rubber crystallites. The degree of crystallization calculated in this way agrees reasonably well with the direct x-ray measurements of Goppel and Arlman. Crystallization as determined by x-ray analysis and that responsible for the energy change on stretching are much the same. This also means that thermodynamic evaluation of the change of stress with temperature is justified if pufficient relaxation of stress has taken place.
Stoichiometry of Vulcanization with Sulfur
Authors: Zapp R. L. , Decker R. H. , Dyroff Margaret S. , Rayner Harriet A.
Published: February 2011

In the study of vulcanization with natural rubber and other polymers of relatively high chemical unsaturation, it has always been difficult to represent the vulcanization process molecularly because of the complexity of the sulfur-polymer reactions. To circumvent this difficulty, reactions of small olefin molecules with sulfur have often been studied to obtain this molecular insight. Some investigators have resorted to cross-linking the polymer with agents other than sulfur to characterize the network for comparisons with physical properties. By cross-linking rubber molecules with diazo compounds, which add quantitatively to the olefin bond, Flory has characterized the network so formed in a molecular manner and correlated the degree of cross-linking with physical properties. When correlating physical properties with degree of cross-linking in butadiene-styrene polymers, others have cross-linked the polymer as a final step in the polymerization process. When a polymer of low unsaturation is used, many of the experimental difficulties are eliminated or reduced, and a more reliable stoichiometric picture of the phenomenon can be obtained. The emphasis in this work is placed on the chemical combination of sulfur with polymer rather than on any correlation with physical properties, and rests upon the “dimensions” of swollen polymer networks as related to total combined and organically combined sulfur. The low unsaturation of Butyl rubber makes it possible to satisfy all the potential points of cross-linkage while still possessing a network that is soft and elastic. In actual practice, there probably is a small percentage of the reactive sites or points of unsaturation disposed in such a way that they cannot approach an active site in another molecule. Experimentally, however, one can obtain a “maximum” state of vulcanization where further vulcanization time or additional sulfur and accelerator do not contribute further to additional cross-linkage. This feature is utilized in the present investigations.
The Transformation of Thiols Compounded in Raw Rubber
Authors: Bras Jeanle , Montu Michel
Published: February 2011

During the last few years the rubber industry has made use of certain thiols, under the technical name of peptizing agents, which have the property of accelerating the plasticization of raw rubber during mastication. It is now known that this process of plasticization involves oxidation of the rubber, and that it does not take place in an atmosphere of an inert gas. Accordingly the present authors were induced, on the one hand, to follow the transformation of thiols during their participation in the mastication of rubber and, on the other hand, to observe their influence on the tendency of rubber to oxidize. In the first of these objectives, the analytical method utilized was ultra- violet absorption spectrography. To avoid pertubations in the spectra caused by the resins present in rubber, crepe rubber purified by acetone extraction was used in the experiments. The rubber was masticated at 100° C, and the thiol was added soon after the beginning of this mastication in the proportion of 5 per cent of the rubber. Samples were withdrawn at successive intervals of time, and the transformation products of the thiol, which were isolated by acetone extraction, were identified by their ultraviolet absorption spectra. In these experiments, chloroform solutions containing 0.5 gram per liter were employed.
Belt-Flexing Test of the Du Pont Type
Authors: Buist J. M. , Williams G. E.
Published: February 2011

In general, flexing tests are subject to large errors, some of which have been studied using a test of the du Pont belt type. An improvement in the standardization of the test conditions can be effected by using a suitably designed belt balance arm and regularly adjusting the length of the belt. The rate of cracking and also the reproducibility of the results is affected by the design of the test-piece and also by the type of fastener used. Although the error between the results on replicate test-pieces is extremely large, more important are the large errors due to lack of reproducibility of replicate molded slabs, and this variability probably arises during vulcanization. Extremely large differences can also occur between nominally identical repeat experiments. This may be due to differences in the judgment of the stages of cracking in different experiments. Graded photographs were found not to be of any value in improving the reproducibility of results between two different operators. Attention is drawn to the differences in types of cracking obtained with the du Pont and De Mattia test-pieces. It is pointed out that with the du Pont test-piece the cracking tends to develop in depth more than is the case with the De Mattia, and therefore it is more difficult to construct or use graded photographs with the du Pont test. Accepting the fact that variations in the disposition of the stages of cracking do occur between two different experiments, a method of treating flex-cracking results graphically is suggested. By this method random variations in results are ignored and the average flex-cracking results obtained. It is suggested that flexing properties may be defined by two characteristics, viz., (1) Crack initiation time, and (2) Crack propagation time, and an example of the method of determining these characteristics is given.