bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of the Vulcanization of High Elastic Polymers. I. The Vulcanization of Natural Rubber with Thiuram Disulfides. Part 1
Authors: Scheele Walter , Lorenz Otto , Dummer Wilhelm
Published: February 2011

The present paper, which is concerned with the clarification of the mechanism of thiuram vulcanization, although the studies are still in progress, nevertheless leads us to the following fundamental facts. 1. In the vulcanization of natural rubber by thiuram disulfides in the presence of excess zinc oxide, the zinc dialkyldithiocarbamate at any stage is the principal, and probably even the sole, reaction product that can be extracted with organic solvents. 2. There are no experimental proofs that make probable the presence of thiuram monosulfide in the extracts. 3. The maximum attainable conversion in vulcanization of any particular tetralkylthiuram disulfide to the corresponding zinc dialkyldithiocarbamate amounts to about 66 per cent, and this is independent of the temperature. 4. The maximum conversion of the thiuram disulfide is practically independent of its concentration in the rubber, provided that the differences in the concentrations involved are not excessive. 5. The extracts of thiuram vulcanizates contain free thiuram disulfide in addition to the zinc dithiocarbamate. The concentration of the thiuram disulfide decreases rapidly with the time of vulcanization and approaches zero asymptotically. This is exemplified by the results with tetrapropylthiuram disulfide. 6. It is shown to be probable that the vulcanization continues to proceed slowly during extraction of the vulcanizate with boiling solvents, so that the analytical results of the hot extracts are not such as to throw light on the kinetics of thiuram vulcanization. 7. The decrease of concentration of the free thiuram disulfide in the vulcanizates evidently takes place more rapidly than the formation of the zinc dithiocarbamate. It is, therefore, probable that the vulcanization of rubber by thiuram disulfides can be separated into two partial processes. There is a rapid reaction of the rubber with the thiuram disulfide, which is presumably “vulcanization-inert”, and there is an additional, slower reaction which involves a splitting off of dithiocarbamic acid from a reaction product between rubber and thiuram disulfide and which leads to the vulcanization. 8. All exhaustively extracted vulcanizates contain sulfur. Apparently the sulfur content in vulcanizates of short reaction times is greater than in those of longer periods of vulcanization. 9. The formation of the zinc dithiocarbamate as sole reaction product in the thiuram vulcanization can hardly be explained otherwise than as a dehydrogenation of the rubber, concerning the mechanism of which we still know, to be sure, scarcely anything. It is reasonable to conceive of a reaction of the disulfides with the α-methyl groups, in which chain radicals may be formed which give rise to cross-linking over C—C bonds. The investigations are being continued and a further communication will appear in the near future.
Stress Relaxation in Rubber I. Evaluation of Antioxidants
Authors: Robinson H. W. H. , Vodden H. A.
Published: February 2011

Conventional evaluation techniques for rubber antioxidants are very time consuming. A method of evaluation based on stress relaxation due to oxidation at elevated temperatures, and an apparatus developed for the study of this phenomenon in a large number of natural rubber stocks, are described in this article. Results are compared with those obtained using conventional air- oven and oxygen bomb aging tests. The apparatus is suitable for testing six rubber samples simultaneously, and uses unbonded resistance wire strain gauges for the measurement of stresses in the samples. The stress vs. time curves are recorded automatically on a roll chart recording potentiometer. It has been found that the curves approximate to exponential decay curves, and, as such, can be characterized by a single parameter—the half life. This parameter, used as a measure of antioxidant efficiency, can be correlated with the loss of tensile strength for samples aged in a conventional air oven. The use of the half-life period as a measure of antioxidant efficiency leads to a rapid and easily interpreted method of evaluation for antioxidants in natural rubber. Good reproducibility and discrimination are features of the method.
Stress Relaxation in Rubber II. Simultaneous Oxygen Absorption
Authors: Baxteh S. , Potts P. D. , Vodden H. A.
Published: February 2011

In any serious study of antioxidant action, it is essential that the oxygen absorption rate of a protected rubber be determined and the changes in the network system caused by the oxygen be assessed. This information is normally obtained from separate oxygen absorption and aging studies. A method is described in this article which enables simultaneous measurements of oxygen absorption and stress relaxation to be made on a given rubber sample. A number of antioxidants have been compounded with natural rubber and, from the results, an estimate is made of the molecules of oxygen absorbed per rubber chain cut. This is found to vary with the antioxidant, although some difficulties of interpretation are pointed out. Variations of deactivating effect among the antioxidants shows clearly that any attempt to understand oxidative degradation in terms of oxygen absorption or decay in physical properties alone is not possible. The method provides a new technique that is of value in any basic study of antioxidant action in elastomers.
Gel Structure. VI. Preparation of Gels and Globular Structures from Rubbers by Vulcanization of Solutions
Authors: Zubov P. I. , Zhurkina Z. N. , Kargin V. A.
Published: February 2011

1. The mechanical properties of gels prepared from solutions of natural rubber and synthetic rubber were studied. 2. It was established that gels of synthetic rubber (polybutadiene rubber) have a wide range of relaxation periods. Gels of natural rubber (smoked sheet) behave like ideally elastic substances, following Hooke's law in the change of rate of deformation by 1000 times. 3. The viscosity of solutions of natural rubber and of polybutiadiene rubber in the presence of sulfur monochloride was studied. 4. We observed that sulfur monochloride sharply decreases the viscosity of natural rubber solutions and has almost no influence on the viscosity of synthetic rubber solutions.
Identification of Curing Agents in Rubber Products. Ultraviolet Absorptiometric Analysis of Selective Solvent Extracts
Authors: Kress K. E. , Mees F. G. Stevens
Published: February 2011

A method has been devised for identification of organic compounds that accelerate the vulcanization of rubber products. Identification of these active trace materials aids product improvement and quality control. Identification is made through ultraviolet spectrophotometric absorbance curves over the 220 to 380 mµ region on the aqueous alkali or acid extracts, or on liquid-liquid ethyl ether extracts of the aqueous solutions. Thiazole, thiuram, thiocarbamate, amine, and guanidine classes of commercial accelerators are regularly identified in 2 grams of uncured or cured rubber products, with less than 4 hours' elapsed time. Interference of common softeners and antioxidants is usually negligible. The method is more rapid than chromatographic procedures and more sensitive and specific than spot-test methods. Quantitative results may be obtained if calibration work is undertaken.
Studies of the Vulcanization of High Elastic Polymers. V. Vulcanization of Natural Rubber by Tetramethylthiuram Monosulfide and Sulfur
Authors: Scheele Walter , Bielstein Georg
Published: February 2011

The vulcanization of natural rubber (pale crepe) by tetramethylthiuram monosulfide and sulfur (1 mole monosulfide per gram-atom of sulfur) has been studied in the temperature range from 90° to 150° C. The following results were obtained : 1. During vulcanization, the concentration of the thiuram monosulfide and also of the sulfur decreases. 2. In accordance with a first-order reaction, about 66 mole-per cent of zinc dithiocarbamate is formed (calculated on the amount of thiuram monosulfide added). Thus the same relationships were found as in vulcanization with thiuram disulfides. 3. The vulcanization of rubber with thiuram monosulfide and sulfur is, therefore, nothing else than vulcanization by thiuram disulfide, whose quantitative formation precedes the actual vulcanization reaction. 4. In no stage of vulcanization could thiuram disulfide be detected with certainty in the extracts of the vulcanizates. It must be assumed, therefore, that it reacts with the rubber instantly after it is formed. 5. The velocity constants derived for the dithiocarbamate formation at the various temperatures both from the experimental results and from the graphical presentations have been given, and their temperature dependence has been discussed. At lower temperatures, the influence of the prior interaction of the monosulfide with sulfur makes its influence on the energy relationships felt. 6. It is once more emphasized that in thiuram vulcanization it cannot be a question of a vulcanization with sulfur. In this connection we take a decided stand also with regard to the results of the more recent investigations of Craig and his coworkers.
The Mobility of Sulfur Bonds in Soft Rubber and Ebonite
Authors: Blokh G. A. , Chuprina L. F.
Published: February 2011

Much attention has been devoted to the problem of the sulfur and oxygen bonds in vulcanized rubber in numerous studies of both Soviet and foreign scholars. In the Soviet Union, the works of a large group of investigators, including B. A. Dogadkin, A. D. Zai˘onchovskii˘, P. A. Rebinder, A. P. Pisarenko, A. S. Kuzminskii˘, and others, have dealt with this problem. The nature of the sulfur structures formed in soft vulcanized rubbers depends on a number of factors, including the nature of the accelerators. It was shown in one work that sulfur vulcanization in the absence of sulfur-bearing accelerators leads to the formation of relatively weak polysulfide bonds. We assume that here a group of different polysulfide bonds is formed at the expense of the passage of octatomic ring sulfur structures into open linear sulfur radicals. For vulcanized rubbers which contain sulfur and diphenylguanidine in their recipe, the group of polysulfide bonds evidently constitutes an excellent criterion of their characteristic vulcanization structures. A number of rubber products, including cable and insulation rubbers, which do not contain sulfur as a vulcanizing agent are also known. In this case vulcanization takes place at the expense of the decomposition of tetramethylthiuram disulfide (thiuram vulcanization). Here stronger intermolecular monosulfide sulfur bonds are formed at the expense of the atomic sulfur, which is liberated during the decomposition of thiuram:
Oxidation of Organic Sulfides. III. A Survey of the Autoxidizability of Monosulfides
Authors: Bateman L. , Cunneen J. I.
Published: February 2011

The reactivity of a variety of organic monosulfides towards molecular oxygen has been investigated at temperatures from 45° to 75°. Formally saturated sulfides by themselves are inert, allylic acyclic sulfides react initially much faster than comparable unsaturated hydrocarbons, and allylic and vinylic cyclic sulfides are still more reactive. Oxygen absorption is promoted by light, by the free-radical catalyst αα′-azoisobutyronitrile, and by certain metallic compounds, but not by peroxides. The azo compound is sufficiently active to catalyze the oxidation of saturates sulfides having C—H bonds adjacent to the sulfur atom, while aryl alkyl sulfides also respond to photocatalysis. A notable feature of these autoxidations is the self-inhibition or strong retardation which generally becomes apparent at small oxygen uptakes. Certain mechanistic details which can be deduced from the experimental findings are discussed.
Graft Polymers Derived from Natural Rubber
Authors: Bloomfield G. F. , Merrett F. M. , Popham F. J. , Swift P. Mc L.
Published: February 2011

Graft polymers result when vinyl monomers are polymerized in the presence of natural rubber, either in solution or as latex, and some of the polymeric chains become attached to the rubber molecules. The properties of the natural rubber can be widely modified according to the nature and the amount of the grafted polymer. The polymer-modified natural rubber appears to be produced by direct growth of polymer chains on to rubber molecules rather than by a transfer reaction involving the rubber. Graft polymers of styrene and methyl methacrylate with natural rubber can be compounded and cured to give light-colored articles of good tensile strength, and rubber-methyl methacrylate graft polymers have outstanding flex-cracking and fatigue resistance.
Filler Reinforcement and Tear Resistance in the Light of Thixotropy
Authors: Houwink R. , Janssen H. J. J.
Published: February 2011

Rubber mixes containing reinforcing fillers may be regarded as being thixotropic, with a very long recovery time. The explanation of this is found in the destruction and the reconstruction of filler-rubber linkages and filler-filler linkages, which adhere to each other as a result of a continuous spectrum of energies of attraction. It is assumed that the rubber-filler complex becomes oriented during the stretching of filled mixes. With unfilled mixes, the rubber molecules are able to spring back again, when the stress is released, into the position which is mostly favorable from the point of view of entropy. As a consequence of the introduction of potential energies by the incorporation of reinforcing fillers, the rubber molecules have their mobility adversely affected, as a result of which a definite orientation may be maintained when the stress is released. For instance, after prestretching of these mixes, a significant increase of the resistance to continued tearing at right angles to the direction of elongation and a decrease parallel to this direction are found. This partial increase of the tear resistance, which is not encountered with unfilled mixes, will presumably contribute, as does crystallization, to reinforcement. This increase indicates that the tearing forces in a stretched mix are distributed in one direction over a large surface area. A series of associated phenomena are explained.
Mastication of Rubber. II. Interpolymerization of Natural Rubber and Neoprene on Cold Milling
Authors: Angier D. J. , Watson W. F.
Published: February 2011

The softening of elastomers on cold milling results from scission of the polymer molecules by the applied shearing forces. The ruptured chains are free radicals, which can undergo mutual combination, interaction with oxygen and various additives, and branching (grafting) on to other polymer molecules. A general method of producing graft and block interpolymers between elstomers is therefore indicated, namely, to cold-mill the polymers together in the absence of small molecules which can terminate the polymeric radicals in order that the radicals may cross-terminate or graft onto the polymer molecules of the other type. A survey of several pairs of the commercially important elastomers, natural rubber, butadiene-styrene, Neoprene, and butadiene-acrylonitrile, has shown that cold milling does effect interlinking. Detailed results for the rubber-Neoprene system are reported in this communication. Experimental verification of polymer interlinking was obtained from the solubility properties of the milled elastomers. Cold milling of Neoprene under nitrogen produces gel, whereas of natural rubber does not, but the milling of mixtures gives gels containing natural rubber. Also, the solubilities and precipitation of the milled mixtures cannot be accounted for by these properties of the individual polymers. Finally, Neoprene-natural rubber mixtures, after and not before cold-milling, can be cross-linked by magnesium oxide, with rubber bound into the vulcanizate.
X-Ray Diffraction Studies of Crystallization in Elastomers
Authors: Alexander Leroy E. , Ohlberg Stanley , Taylor G. Russell
Published: February 2011

In general, extension of an elastomer results in a degree of preferred orientation of the molecular chains composing the amorphous phase. Therefore the amorphous fraction of a partially crystalline elastomer must be related to the integrated intensity of the amorphous diffraction halo rather than to the intensity at any one azimuth. A noteworthy exception is natural rubber, for which simple meridional measurements suffice. A Geiger-counter apparatus, with beam monitor and temperature-controlling accessories, is described for making accurate measurements of the x-ray intensities scattered at any azimuth and at small or moderate Bragg angles. Measurements of crystallinity in natural rubber are in essential agreement with the findings of previous workers. When polybutadiene is extended at room temperature, molecular orientation occurs, but little if any crystallization. Measurements at lowered temperatures show that the crystalline fraction becomes appreciable at about 0° C and that it increases with further reduction of temperature and with increasing extension ratio. Preferred orientation of the crystalline regions in extended polybutadiene has been measured quantitatively with the object of providing jointly with birefringence measurements a value of the birefringence of a single crystal of polybutadiene.
Sulfur Exchange Reactions in Vulcanized Rubber
Authors: Tarasova Z. , Kaplunov M. , Dogadkin B. A.
Published: February 2011

In one of our previous works we have shown that vulcanization structures contain mono-, di- and polysulfide bonds between the molecular chains of the rubber. The concentration, proportion, and distribution of these various bonds depend on the type of rubber and the conditions of vulcanization, but mainly on the nature of the curatives. Inasmuch as the vulcanization structures determine in large degree the physical, chemical, and mechanical properties of the vulcanizate, the different methods which enable us to determine the types of sulfide bonds in the vulcanizate are very important. Among the chemical methods applied for this purpose are the reactions of sodium sulfite and of methyl iodide with the vulcanizate. Of great interest in research on the nature of sulfur bonds in vulcanizates is the method of exchange reactions with radioactive sulfur; since sulfur, as numerous experiments have shown, diffuses easily in rubber, thus insuring the possibility of carrying out exchange reactions without having to get the vulcanizate into solution.
The Mechanism of Oxidation of Synthetic Rubbers
Authors: Kuzminskiĭ A. S. , Degteva T. G. , Lapteva K. A.
Published: February 2011

1. Adequately reproducible results were obtained, and a complete balance was achieved between substances entering the reaction and the products of oxidation. 2. It is shown that the oxygen which enters into the reaction at all stages of oxidation is retained mainly in the form of peroxides, acids, and ethers. 3. The kinetics of polymer oxidation, and also the accumulation of intermediate reaction products, were studied at various temperatures. 4. It is shown that the cessation of oxidation occurs long before the exhaustion of the polymer's double bonds; the reasons for this phenomenon are revealed. 5. The decrease of unsaturation in the process of oxidation at different temperatures was studied. 6. It is shown that the process of oxidative decomposition is accompanied by polymerization. 7. It is shown that oxidation extends mainly to the double bonds in the principal chains, while polymerization is confined mainly to the double bonds in the side chains. 8. A diagram of the oxidative mechanism is given, including both of the directions of reaction (decomposition and polymerization).
Determination of Sulfur in Rubber Vulcanizates
Authors: Zimmerman E. W. , Hart V. E. , Horowitz Emanuel
Published: February 2011

A combustion method for rubber which determines the sulfur evolved at 480° to 500° C is compared with the fusion and the zinc-nitric acid methods. In the combustion method an interaction occurs between sulfur and fillers present in the rubber. The effect of particular fillers with respect to this interaction is discussed. A value representative of the organically bound sulfur in an extracted specimen can be obtained by the combustion method when no reaction takes place between the sulfur and the fillers during combustion. The fusion method yields results in good agreement with the total sulfur added in compounding the rubber samples. Except in the presence of barium compounds, the zinc-nitric acid method likewise determines total sulfur. When barium compounds are present, barium sulfate is formed during the oxidation, and low values are obtained for the total sulfur.
Absorptiometric Microdetermination of Total Sulfur in Rubber Products
Authors: Kress K. E.
Published: February 2011

The 1.0 to 2.5 per cent of sulfur normally present in rubber products is oxidized with concentrated nitric acid-bromine reagent, followed by perchloric acid in the presence of excess lead nitrate. Sulfur as lead sulfate is precipitated and washed with acetone. The lead sulfate is dissolved in 50 per cent hydrochloric acid, and absorbance of the lead chloride complex is recorded at 270 mµ. Sulfur is calculated on the basis of the measured lead content of the precipitate. The high sensitivity puts the method in the micro-range. An experienced analyst can analyze 40 to 50 samples a day. Precision and accuracy are comparable to those of the conventional barium sulfate gravimetric method at the low sulfur concentrations normally found in rubber products.
Electrical Conductivity Experiments with High-Abrasion Furnace Black-Loaded Natural Rubbers
Authors: Humphreys N. C. H.
Published: February 2011

Considerable differences may exist between volume resistivities of vulcanizates prepared from a single mixing. Large differences between mixing effects can also occur. Contact resistance effects caused by bonded brass electrodes are shown to be negligible over a considerable range of bond strengths. The profound effects of compressive flexing on the resistance of vulcanizates is demonstrated. A formula containing sufficient HAF black to place the vulcanizates nominally in the electrically antistatic range was employed throughout the work.
Pull-Through Adhesion Test
Authors: Wood J. O.
Published: February 2011

A modified form of the pull-through test is described, followed by a series of experiments, the first three of which demonstrate the precision of the method. Further experiments assess the effect of various factors on the pull-through force. It is found that specimen age has little effect. The tension applied to the cord during construction also produces a small but appreciable effect, amounting to a decrease of 1.3 per cent for a two-fold increase of the tension, but a greater effect, amounting to a 7 per cent increase of the pull-through force, is obtained when the rate of loading is doubled. The pull-through force is also shown to depend on the shape of the mounting device. The twist in a rayon cord does not give a significant effect, though the variability exhibited by the conventionally dipped cord is high. Temperature increase during test reduces the pull-through force; linear plots of the logarithm of the pull-through force against the reciprocal of the absolute temperature are obtained. The cord length embedded in the rubber is shown to bear a hyberbolic relationship to the pull-through force, confirming the theoretical relationship obtained analytically. Finally the pull-through test is considered in terms of the product.
Problems Concerned with the Physical Testing of Vulcanizates
Authors: Kainradl Primus
Published: February 2011

A critical consideration shows that the test methods in current use for vulcanizates often fail to make possible a reliable decision about different rubber compounds out of a series of development formulations with respect to their evaluation for subsequent use. The tester by himself is not in a position to bring about an improvement in this unsatisfactory state of affairs, since this cannot be accomplished by the improvement of existing methods alone or by the creation of a new apparatus. A closer cooperation between the fields of testing and application is necessary.
Determination of Sulfur by Wet Combustion with Perchloric Acid
Authors: Bethge Per Olof
Published: February 2011

Difficulties in the determination of sulfur in wood and pulp samples by the conventional method of Grote and Krekeler have been overcome by applying a wet combustion technique, using perchloric and nitric acids. The sulfur is reduced to hydrogen sulfide by boiling with a reduction mixture, and determined iodometrically. The method gives quantitative yields for pure organic compounds, and is suitable for a great variety of samples with sulfur contents from 0.01 to 100 per cent. Sample weights can be varied from 9 mg. to 5 grams.
Ameripol SN—A Cis-1,4-Polisyoprene
Authors: Horne S. E. , Kiehl J. P. , Shipman J. J. , Holt V. L. , Gibbs C. F. , Willson E. A. , Newton E. B. , Reinhart M. A.
Published: February 2011

The important observation made by Katz in 1925, using x-ray diffraction technique, showed that the mere stretching of crude or soft vulcanized Hevea rubber caused at least a portion of the polymer to undergo crystallization. This and the subsequent experiments of Hock emphasized the point that successful attempts to synthesize a polymer resembling Hevea rubber would have to take regularity into account. The situation is complicated in isoprene polymers by the variety of ways the monomer can enter the growing polymer chain. Four isomeric forms of polyisoprene are shown in Figure 1. Since head-to-tail orientations must be considered because of the asymmetry of the isoprene molecule, there are eight possible arrangements of the units that can occur in polymerized isoprene. Sodium, potassium, emulsion, and alfin polyisoprenes have been shown to contain all four forms of the repeating units. All these uncured polymers have poor tack, and their pure-gum vulcanizates show poor physical properties. They do not crystallize when stretched or cooled. It has long been recognized that polymers of conjugated hydrocarbon dienes would have to be very regular in structure before their properties would approach those of Hevea rubber. Looking back over the last fifteen to twenty years, there has been a slow development, almost imperceptible at first, of catalyst systems which produce polymers that in some degree show an oriented structure. This orientation could result if the monomer, during the propagation phase of the polymerization, is held in a definite position with respect to the growing chain, as by cybotactic forces where polymerization is initiated at the melting point of the monomer, or by adsorption of the monomer onto a solid surface—perhaps to the surface of the catalyst itself. The work reported here deals primarily with the properties of a rubber made by directed polymerization.
A Comparison of the Dynamic Properties of Natural Rubber and GR-S
Authors: Papas L. J. , Shufler S. L. , DeWitt T. W.
Published: February 2011

It is well known that high molecular weight, uncross-linked, amorphous polymers show a characteristic behavior with respect to the dependence of the dynamic modulus on frequency and temperature. Generally these materials, at high frequencies and (or) low temperatures, are glasslike, with a modulus of approximately 1010 dynes/cm.2. As the frequency is lowered or the temperature raised, this modulus drops rather quickly to the region of 106 to 107 dynes/cm.2, and remains in this neighborhood for a while to form somewhat of a plateau, the length of which depends on the molecular weight. With a further lowering of frequency or rise of the temperature, the modulus again drops off in a fashion which is strongly dependent on the molecular weight and molecular-weight distribution and the material enters a plastic or liquid-like region. This general behavior seems to be characteristic of long-chain molecules in general, and differences in structure are more likely to be reflected in differences of detail, and in the temperature ranges in which the afore-mentioned regions occur, than in any considerable divergence from the general pattern. A comparison between the dynamic properties of the two important elastomers, GR-S and natural rubber, is here reported. In preliminary work in this laboratory, it has been observed that GR-S shows an appreciably higher level for the plateau region than natural rubber. It was felt that this and other possible differences between the two materials would merit exploration, particularly for any implication that the results might have for the long-standing problem of the origin of the tendency of GR-S to show a greater development of heat under repetitive stressing than natural rubber.
Chemical Relaxation of Stress in Polyurethan Elastomers
Authors: Offenbach J. A. , Tobolsky A. V.
Published: February 2011

The chemical stress-behavior of polyester urethan elastomers and a polyether elastomer have been determined through a temperature range of 90° to 130° C. Since both the polyether and poylester had approximately the same rates of stress decay, it was necessary to exclude ester-ester interchange as a reaction mechanism, and to regard the known thermal lability of the urea and urethan groups as indicative of possible sites of network scission. A likely possibility for the weak link is the trisubstituted urea (biuret) cross-linkage.
The Photoelastic Examination of Rubber Articles
Authors: Angioletti A.
Published: February 2011

After a short introduction to the photoelastic method and its applications to the examination of transparent planar structural models, some methods of examining plane models representing sections of actual rubber articles are indicated. In many cases it is advisable to place the model between parallel glass plates with their inner faces lubricated and at a distance apart equal to the thickness of the model itself. In other cases free models can be examined. In no case, however, is the law governing the birefringence of transparent materials within a range of small deformations, e.g., less than 1 per cent, equally applicable to greater deformations, e.g., up to 100 per cent. But in this range recourse can then be had to a law, based essentially on theoretical considerations and described by Treloar, which is an extension of the general law and which has been verified experimentally. Some special cases are considered where the law is of general application to the quantitative evaluation of the state of deformation at various points in a model under examination.
Accidental Factors Involved in the Fatigue Breakdown of Rubber Articles
Authors: Angioletti A.
Published: February 2011

Poor dispersion of compounding ingredients, and the presence of granules and of impurities which in some way get into vulcanized rubber and thereby disturb the normal distribution of stresses in an article and thus cause local stress concentration, are regarded as accidental factors in the fatigue breakdown of vulcanized rubber. This problem was studied by developing, by means of the photoelastic method, an adaptable schematic representation of the phenomena. Although only a qualitative evaluation of the local disturbance of the system of stresses caused by these factors is possible, in the particular case of granules, it has been found that the closer a granule is to the outside surface of the rubber article, the greater is the disturbance of the system of forces. Furthermore, the local stress concentration is greatest when there is no adhesion between the granules and the surrounding vulcanized rubber. Finally the stress concentration caused by a small cut or incision made at various locations on a rubber test-strip under tension is considered. This case has been included because fatigue cracks first become manifest in this form.
Mechanical Activation of the Oxidation of Vulcanizates by Static and Dynamic Deformation
Authors: Kuzminskiĭ A. S. , Lyubchanskaya L. I.
Published: February 2011

The deformation conditions to which rubber parts are subjected have a considerable effect, and in many cases the decisive effect, on their service life. One of us made a qualitative study which established the direct influence which the mechanical stresses created in a vulcanizate by dynamic deformation have on the rate of oxidative reactions. In the present work, a comparison was made between the effects of static and of dynamic deformation on the oxidation and breakdown of vulcanizates with differing structures. Dogadkin and Tarasova established that vulcanizates with tetramethylthiuram disulfide form mainly monosulfide bonds. In vulcanizates containing diphenylguanidine, the presence of a large number of polysulfide bonds was noted. In a thermal vulcanizate, a spatial network with —C—C— bonds was formed. All of the above vulcanizates were found to have approximately the same network densities, as determined from their equilibrium moduli. In order to remove the free ingredients remaining after vulcanization (free sulfur, accelerators, and phenyl-2-naphthylamine), all the vulcanizates before testing were extracted with acetone for 50 hours, in a stream of nitrogen, and were then dried in a vacuum. In experiments involving an antioxidant, the latter was introduced into the purified vulcanizate in a benzene solution. The thickness of the vulcanizate samples studied ranged from 0.2 to 0.25 mm. The work on dynamic deformation was carried out in a specially constructed apparatus, with a compensating mechanism for regulating the permanent set created under fatigue. It should be noted that in all the vulcanizates this accumulation of permanent set through fatigue was observed even in the first few cycles of the testing. The total amount of accumulated permanent set in a vulcanizate, from the time the testing began until the break, did not exceed 20–25 per cent of the original length of the sample. For the determination of the fatigue lives of vulcanizates, parallel tests were made on twenty-five samples. A detailed investigation was made of the influence that the type of cross-links in the vulcanizates has on the fatigue lives of the latter.
The Viscosity and Structure Viscosity of Rubber Solutions and Emulsions
Authors: Schurz J.
Published: February 2011

It is well known that the increase of viscosity of a colloidal solution over that of the pure solvent is produced by the frictional resistance to flow of the dispersed particles. The amount of this viscosity increase depends to a great extent on the shape of the particles. If these are spheres, only the ratio of the volume of the solute to the solvent is involved (Einstein's law), while in the other extreme case, namely with fiber molecules, the particle size plays such an important role, as is known, that one can calculate the particle weight of the dispersed phase from the viscosity (Staudinger's law, Mark-Houwink equation). But not only is the viscosity of a colloidal solution influenced by the particle form, but also its structural viscosity depends on it to a great degree. It is easy to understand this. For if the structure viscosity and, consequently, the decrease of viscosity with increasing velocity gradient, result from the fact that the hydrodynamic forces in the flowing solution, on the one hand, orient and distort the dispersed particles in the moving stream, and, on the other hand, oppose an eventual intermolecular mutual effect or aggregate formation of the particles, it is easily seen that the flow forces will be able to manifest these effects all the more readily and markedly the more extended and the stiffer the dispersed particles are. Thus it can be expected that, in general, in spherical particle suspensions no structural viscosity will appear, but that this will manifest itself the more strongly the more asymmetric the particles. With rubber it is easily possible to realize two extremely different dispersion forms. Rubber emulsions, as they exist in the various latexes, are typical definite spherical suspensions, with rather large particles of very uniform size. In rubber solutions, on the other hand, say in toluene or benzene, there is a macromolecular distribution. The dispersed particles are the individual kinky fiber molecules of the rubber, which are known to have a more or less elongated form. In fact these two types of rubber dispersions behave so differently that each of them can be called a typical representative of its class.
Slipping of Molecules during the Deformation of Reinforced Rubber
Authors: Houwink R.
Published: February 2011

It is proposed that, during the deformation of reinforced rubber, the molecules slide along the surface of the filling material, accompanied by breaking of bonds. However, new bonds of similar type are soon formed (reversible breaking of bonds). It is assumed that, in the quiescent state following deformation, the original bonds are eventually restored (disappearance of the Mullins effect); the increase of the entropy is supposed to be the effective source of energy. A model of reinforced rubber is shown in Figure 5.
Mechanism of Formation of Zinc Dimethyldithiocarbamate (ZnDMDC) in Tetramethylthiuram Disulfide (TMTD) Vulcanization
Authors: Craig David
Published: February 2011

Scheele and Bielstein recently have commented at length on the copolymerization (CP) theory of vulcanization which we proposed. They find the theory untenable with respect to TMTD vulcanization as well as with that using equivalent amounts of tetramethylthiuram monosulfide (TMTM) and sulfur. They feel that our presentation lacked in clarity and, therefore, that they did not understand it fully. Scheele and coworkers report that 66 per cent of the TMTD, or of the mixture of equivalent amounts of TMTM and sulfur, reacts to form ZnDMDC according to the first-order rate law during vulcanization in the presence of excess zinc oxide. This finding is a notable contribution. In particular, it suggests the possibility of sulfur-oxygen interchange as a feature of the overall reaction, which we may write:
Protective Action of Antioxidants of the Phenolic Type on the Aging of Natural Rubber
Authors: Yarmolenka N. F. , Novikava E. N.
Published: February 2011

Oxidation of natural rubber in solution by atmospheric oxygen at 100° C in the presence of antioxidants and iron palmitate was studied. On the basis of the relative viscosity of the oxidized rubber solutions, the following conclusions can be drawn. 1. Addition of 1 per cent of certain antioxidants (pyrogallol, pyrocatechol, hydroquinone, 2-naphthol, resorcinol, Neozone D, and Oxynone) retards the oxidation of rubber in solution. However, this quality of antioxidant is not sufficient to stabilize the solution. 2. Comparison of the effectiveness of antioxidants (according to the data of relative viscosity) showed that the best antioxidant is pyrogallol, followed by pyrocatechol, hydroquinone, and Oxynone. The weakest are Neozone D and especially resorcinol. 3. Pyrogallol A, which is not shown in the table, does not protect a 1 per cent rubber solution from deterioration, showing a relative viscosity in solution of 4.08 at the beginning of the experiment and 1.40 after 7 hours, which is equal to that of the unprotected rubber solution. 4. The cresol and xylenol fractions of peat phenols were also used as antioxidants. The addition of 1 per cent of peat fractions to a rubber solution retards the drop in relative viscosity of oxidized solutions, but less than does the standard antioxidant, Neozone D. 5. The structure viscosity of solutions prepared from natural-rubber films which were stored 400 days, both with and without antioxidants, was measured. 6. Antioxidants of the phenol type protect rubber during storage in the form of films in darkness in air; in this case the best antioxidant is pyrogallol, and the next best pyrocatechol. 2-Naphthol is more active than hydroquinone. 7. When rubber is stored for a long time in air in darkness in the presence of certain antioxidants of the phenol type, its solubility decreases considerably; this shows the polymerizing action of some phenols on rubber. 8. Pyrogallol has a particularly strong polymerizing effect on rubber films. After 40 days of storage in air, these films dissolve in benzene in 25–30 days; if they are stored longer, their solubility is still less.
Mastication of Rubber. IV. Polymerization of Vinyl Monomers by the Cold Mastication of Rubber
Authors: Angier D. J. , Watson W. F.
Published: February 2011

The rupture of natural and synthetic rubber molecules into free radicals by the shear imposed during cold mastication under nitrogen initiates the polymerization of monomers incorporated in the rubber. Experimental results on the polymerization of a range of monomers is reported. The polymer formed is mainly attached to the rubber, forming with it either a soluble interpolymer or a two-component gel depending on the monomer used. Results on the co-polymerization of two monomers are also presented and interpreted.
Mechanical Method of Determination of the Vitrification Temperature of Rubberlike Polymers
Authors: Marei A. I.
Published: February 2011

1. A mechanical method has been developed for the determination of the vitrification temperature of rubberlike polymers, based on measurement of the compression deformation of polymer samples in the vitrification temperature region under a constant stress. 2. An apparatus for the determination of the vitrification temperature of polymers by the mechanical method is described. 3. The vitrification temperatures of a series of polymers have been determined. The agreement of results of the determination of the vitrification temperature of the polymers by mechanical and by other methods is shown.
Thermodynamics of Crystallization in High Polymers. Gutta-Percha
Authors: Mandelkern L. , Quinn F. A. , Roberts D. E.
Published: February 2011

The melting temperatures of two of the crystalline modifications of gutta-percha and their mixtures with either methyl oleate or tetradecane as diluent were determined by x-ray diffraction and dilatometric methods. It was found that the melting temperatures of the two forms in the pure polymer were 64 and 74°, respectively. From these studies the heat of fusion per repeating unit of the high melting form was deduced to be 3070 ± 250 cal./mole, while the heat of fusion for the low melting modification was found to be about one-half to three-fourths this value. An analysis indicates the low melting form is a metastable one. The heats and entropies of fusion of each of the forms of gutta-percha are appreciably greater than those of the monomorphic as-hydro-carbon, natural rubber. The difference in the entropies of fusion of the two polymers persists even when allowance is made for the significant contribution of the volume change accompanying fusion.
Viscoelastic Behavior of Polyisobutylene under Constant Rates of Elongation
Authors: Smith Thor L.
Published: February 2011

A variety of methods has been used to study the viscoelastic properties of polymeric materials. These methods include the response to sinusoidal stress (dynamic measurements), stress relaxation, and creep under constant load and constant stress. The present investigation was made to determine whether or not the viscoelastic properties of rubberlike materials over an extended time scale could be obtained from stress-strain curves measured at different strain rates and temperatures. Polyisobutylene of high molecular weight was selected for study, since its viscoelastic properties have been investigated extensively in a cooperative program sponsored by the National Bureau of Standards. From the data obtained, Marvin has derived the distribution functions of relaxation and of retardation times over a time scale of 10−10 to 107 sec. These functions show quantitatively a change in properties from liquidlike to rubberlike to glasslike with decreasing time scale. The equilibrium stress-strain curves for lightly crosslinked rubber and other elastomers are closely linear for elongations up to 100 per cent. The non-equilibrium (viscoelastic) stress-strain curves for similar and noncrosslinked elastomers might be expected to be linear viscoelastic, as a first approximation, at temperatures above the glass transition, provided the strain and the strain rate are not excessively large. Nonlinear viscoelastic effects are usually pronounced for materials in their glasslike state and at large strains.
Radiation Stability of Elastomers
Authors: Bopp C. D. , Sisman O.
Published: February 2011

Ordinary elasticity, high elasticity, and viscous flow are the mechanisms of deformation of solid polymers. Qualitatively, the effect of radiation on the mechanical properties of polymers can be described by considering the effect of crosslinking and cleavage on these mechanisms. Crosslinking tends to impede viscous flow. The equilibrium value of Young's modulus for high elasticity is directly proportional to the concentration of crosslinks, but for ordinary elasticity the sensitivity of Young's modulus is less for more rigid materials. Cleavage reduces the yield stress for viscous flow, and decreases the equilibrium value of Young's modulus for high elasticity. Young's modulus for ordinary elasticity is again insensitive for the more rigid materials. The changes in the mechanical properties of irradiated elastomers depend on the rates of crosslinking and cleavage. These rates are governed both by the polymer's chemical structure and by the strength of the van der Waals forces. For uncrosslinked polymers, the change of Young's modulus is a measure of the crosslinking rate in the case of less rigid materials. Sensitivity to chain cleavage has been correlated with chemical structure for the materials studied.
Rate of Adsorption of High Polymers on Carbon Black in Relation to Their Molecular Weight
Authors: Yurzhenko A. I. , Maleev I. I.
Published: February 2011

The nature and rate of adsorption of high-molecular substances on carbon black is of interest in a number of instances, particularly in the development of adsorption methods of fractionation of high-molecular substances, but also in the use of carbon black as a filler for rubber ; the number of papers on the adsorption of high-molecular compounds is small. The effect of the molecular weight of different polymers on the rate of adsorption of high-molecular substances from their dilute solutions at C=0.1–0.25 per cent was studied in this work. Samples of polyisoprene, polystyrene, emulsion polymerization at a temperature of +5° and +50°, and also a plant sample of SKS-30 rubber, were the objects of the investigation. Moreover, a series of polystyrene fractions was obtained by the method of fractional precipitation from methyl ethyl ketone and butyl acetate, using methanol as the precipitant. For all the polymers and their fractions the characteristic viscosity was determined, from which the molecular weight value of the polymer was determined. Carbon black of the Ukhtinsky and Dashavsky factories was used as the adsorbent, the specific surface of which was determined by the adsorption of acetic acid from a benzene solution.