bluuebunny/crossref_metadata_2025 huggingface dataset:
The Structure of Neoprene. I. The Molecular-Weight Distribution of Neoprene Type GN
Authors: Mochel W. E. , Nichols J. B. , Mighton C. J.
Published: February 2011

Polychloroprene rubber (Neoprene Type GN) was fractionated by partial precipitation from dilute solution in benzene and the fractions were examined both osmotically and viscometrically in benzene solutions. The molecular-weight distribution curve for Neoprene Type GN based on osmotic pressure measurements shows a pronounced maximum at 100,000, but has a long extension to molecular weights of over one million, indicating the presence of branched or cross-linked material which is still soluble. The uniformity is somewhat less than that of sol natural rubber, while in shape the Neoprene distribution curve resembles more closely that of peptized natural rubber than fresh sol rubber. Observed variations in the slopes of the π/c vs. c and the ηsp/c vs. c curves also indicate the presence in solution of complex, branched and (or) cross-linked molecules. Calibration of the intrinsic viscosity-molecular weight relationship by osmotic pressure measurements gave good agreement with the equation: [η]=KMa, where K=1.46×10−4 and a=0.73.
Have Mechanical Stresses Any Effect on the Oxidizability of Rubber?
Authors: Le Bras Jean , Salvetti André
Published: February 2011

Subjection of rubber to mechanical stresses, whether static or dynamic, does not change its inherent oxidizability, at least within the limits of stress which were applied in the experiments described. Perhaps changes would have been observed if the range of stresses had been reached where crystallization phenomena became pronounced, for sufficient distortion or change in molecular state might have a certain influence on the oxidizability. For example, Williams and Dale, in a study of infrared absorption by rubber, pointed out that linear extensions greater than 400 per cent are necessary to bring about any appreciable increase in the vibration frequency of the C — C groups. However, the elongations employed in the present work cover, in general, the range of deformations to which vulcanized rubber products are normally subjected in service; hence there seemed to be no advantage in attempting to carry out the tests under more severe conditions. From another point of view, it would appear of interest to record the following observation. In all the oxidation tests under dynamic conditions, the test-specimens broke either by being cut through near the points where they were fastened, at random points where rupture began, or when the tensile strength was lowered too much by oxidation; but in no case was actual flexcracking observed. Although this is only negative evidence, the view of Eccher that ozone is indispensable to the formation of cracking would appear to be confirmed.
Aging of Elastomers. Comparison of Creep with Some Conventional Aging Methods
Authors: Throdahl M. C.
Published: February 2011

Subjection of elastomers to mechanical stresses results in unusually complicated behavior. Recent theoretical researches have shown that this behavior cannot be described satisfactorily by either of the classical theories of elasticity or viscosity. The general molecular theories which describe the behavior of elastomers have experimental verification manifested by three regions of temperature-stress relationship: (1) a low temperature region in which stiffening is observed, due to the stability of secondary bonds between network chains, (2) an intermediate temperature region in which the secondary bonds are so unstable that complete relaxation occurs before measurements can be obtained; the scission of primary valence bonds is occurring at such a slow rate that no measurable effects are obtained during the course of the usual laboratory experiment; and (3) a high temperature region in which the relaxation of stress with time is associated with a chemical reaction which, through breaking of primary-valence bonds in the network, severs the chains rapidly enough to be measured during the course of usual laboratory experiments. The high temperature region is that in which elastomers soften and (or) harden and finally lose their rubbery characteristics. Oxygen has been shown to be necessary for the chain-scission reaction. Several papers have described this fundamental experimental technique for the stress-relaxation and creep of different elastomers. Well known laboratory methods for artificially aging elastomers in oxygen and air bombs and in circulating air atmosphere have selected conditions somewhat arbitrarily. In exploratory searches for promising compounds to be used as antioxidants in elastomers and in the evaluation of well known antioxidants, it has often been found that the conventional methods of aging do not differentiate among several antioxidants. It is the purpose of this paper to describe an application of the previously described creep technique as a convenient and precise means of studying the relative performance of antioxidants and accelerators in Hevea and GR-S rubbers.
Chemorheology of Polysulfide Rubbers
Authors: Mochulsky Marcos , Tobolsky Arthur V.
Published: February 2011

Experimental results indicate that the socalled “cold flow” of polysulfide rubbers is almost certainly chemical rather than physical in nature. The term chemorheology has been adopted to describe this chemical type of plasticity. The experimental method employed in this investigation was the measurement of relaxation of stress in stretched rubber samples held at a constant elongation. The changes in relaxation rate produced by changing the molecular structure of the rubber (by cross-linking), by incorporating carbon black, by illuminating with ultraviolet light, and by treating the rubber with various chemical agents, such as sulfur, a thiol, and agents that destroy thiol groups, were studied by this method. From the results of the above experiments and from additional considerations, it is concluded that the chemical reaction responsible for cold flow is an intermolecular exchange reaction, and that this exchange reaction is probably an exchange between a terminal thiol group of one chain and a disulfide linkage of an adjacent chain.
Preparation of Highly Purified Hevea Rubber
Authors: Verghese G. T.
Published: February 2011

A new method is described by which rubber hydrocarbon of a high degree of purity can be obtained without resorting to any drastic chemical or mechanical treatment of the rubber. Rubber latex is treated with ammonium oleate, which displaces the proteins from the surface of the rubber particles. The displaced proteins, along with other nonrubber substances present in the serum, are removed by repeated creaming of the latex. Finally, the creamed latex is “solubilized” in n-hexane, and the rubber precipitated by adding acetone.
Elastomer-Resin Blends. Acrylonitrile Type Synthetic Rubber and Polyvinyl Chloride Resins
Authors: Young D. W. , Buckley D. J. , Newberg R. G. , Turner L. B.
Published: February 2011

1. 1,3-Butadiene-acrylonitrile copolymers were mill-mixed with benzothiazoyl disulfide, sulfur, litharge, and vinyl resins, such as Vinylite (VYNW), and Saran and cured to compounds with good tensile strength, modulus, hardness, solvent resistance, and blocking temperature. 2. Results show that higher acrylonitrile type of copolymers give cured Vinylite-rubber blends with higher tensile strength, higher 100 per cent modulus, and greater ultimate elongation to break than low acrylonitrile copolymers. 3. The low temperature properties of the cured blends improve as the acrylonitrile content of the synthetic rubber is reduced. 4. An effective cure at 287° F is obtained in 15 to 30 minutes by using 2 parts of accelerator and 2 parts of sulfur per hundred parts of 1,3-butadieneacrylonitrile type copolymer-Vinylite blends. Added amounts of sulfur, and accelerator did not improve the properties or decrease the cure time at 287° F. 5. Some of the cured blends studied are free of tackiness at temperatures as high as 230° F. 6. Some stabilizers for vinyls tested as well as sodium acetate can be used to activate sulfur cures in 1,3-butadiene-acrylonitrile copolymer-Vinylite blends to formulate light-colored transparent products.
Action of Organic Accelerators in Vulcanization of Rubber. Effect of Basicity
Authors: Kratz G. D. , Young H. H. , Katz Isadore
Published: February 2011

When organic compounds were first found to function as accelerators for the vulcanization of rubber, Peachey and others attributed their activity, at least in part, to the basicity of the substances employed. Patents, to mention but one, were issued stating that there was a specific order of basic strength beneath which accelerating activity would not occur. We have already shown that this is not necessarily true, since acceleration was observed with a weaker base than the patent would have permitted. However, the whole subject has been dormant for many years. The results reported here indicate that there is a real correlation between accelerating power and basic strength: In a closely related series of compounds e.g., aniline, methylaniline, dimethylaniline, the accelerating power, as measured by combined sulfur, is a linear function of the logarithm of the basic dissociation constant, Kb. This relationship has been observed most strikingly with o-, m-, and p-phenylenediamines, and with o-, m-, and p-toluidines. It has been observed also with aniline, diphenylamine, and triphenylamine, and with mono-, di-, and triphenylguanidines. Guanidine itself does not fall into line, but exhibits a behavior comparable with that of sodium hydroxide, which it approaches in basicity (approximately 103 to 104 times the phenyl-substituted compounds).
Effect of Moisture Variations on Curing Rate of GR-S
Authors: Rush Ian C. , Kilbank S. C.
Published: February 2011

Variations which are beyond experimental error have occurred in the rate of curing of GR-S control and production lot samples tested in this laboratory. It is believed that other laboratories in the synthetic rubber industry have encountered similar variations. Changes in the moisture content of the compounded stock at the time of curing have been suspected of causing the fluctuations in the rate of curing. Several papers on the effect of moisture on the rate of curing of GR-S have dealt largely with relatively high moisture contents. However, this investigation was undertaken to correlate the 300 per cent modulus with the moisture contained in the stock at the time of cure. Rupert and Gage observed that, in testing a large master batch of GR-S at intervals, variations in the rate of curing and modulus values beyond experimental error occurred without any apparent relation to time of standing. The same authors have shown that both GR-S raw polymer and compounded stock absorbs or adsorbs moisture from the air, the amount depending on the humidity of the air. Braendle and Wiegand have shown that the moisture content of rubber-grade carbon black during storage can vary from 0 to 4 per cent. Samples of black taken in this laboratory between December 1946 and March 1947 varied from 0.2 to 2.5 per cent moisture. Under the same conditions at which the black contained 1.0 per cent moisture, the other pigments (sulfur, zinc oxide and mercaptobenzothiazole) each contained roughly 0.7 per cent moisture. Two previous papers have shown that the moisture retained in a mixed batch varies in proportion to, but is less than, the moisture added to dry pigments. Both authors, however, used relatively inaccurate means to determine the water retained, and were interested in higher moisture contents than mixed batches would normally contain.
Carbon Black Dispersion in Rubber. Effect of Fat Acids
Authors: Morris Ross E. , Hollister Joseph W.
Published: February 2011

Whether or not stearic acid assists in dispersing carbon black in rubber has not been settled. Blake was the first to claim that fat acids are dispersing agents for carbon black. He postulated a mechanism, based on Langmuir's work, with monomolecular films of fat acids on water and calculated that the fat acids naturally present in Hevea rubber, which he stated to be present to the extent of 2 per cent, are just sufficient to form a monomolecular film on each particle of 30 volumes of channel black in 100 volumes of rubber. He assumed that this monomolecular film serves to facilitate wetting the individual particles with rubber and prevents agglomeration or flocculation of the particles. Later developments, however, have invalidated Blake's calculations. Blake assumed that the diameter of the average channel-black particle is 200µ, whereas electron photomicrographs have shown that the actual average particle diameter is 28µ. Therefore, 30 volumes of channel black have about seven times the surface area calculated by Blake. The fat acids in Hevea rubber, even if present to the extent of 2 per cent and entirely concentrated on the surface of the particles, are insufficient to cover completely the surface of this quantity of channel black.
Carbon Black-Loaded GR-S Stocks. Relationship between Reinforcement and Swelling Properties
Authors: Dannenberg E. M.
Published: February 2011

The fact that equilibrium swelling measurements do not show any strong interaction of a primary-valence type between filler and rubber does not exclude the possibility of weaker attractive forces, or such low concentrations of primary valence cross-linkages that they cannot be detected by the methods used. The results of this work indicate that carbon black—rubber systems should be considered as simple physical mixtures. It is hoped that this concept will clarify some of the confusion regarding the reinforcing effect of finely divided powders.
Effect of Carbon Blacks on Swelling of Neoprene GRM-10 Vulcanizates
Authors: Catton N. L. , Thompson D. C.
Published: February 2011

Reinforcement of elastomers with fillers has generally been measured by physical properties, such as tension stress-strain, tear resistance, hardness, and compression modulus. To a more limited extent, swelling in solvents has been recognized as associated with reinforcement. In natural-rubber vulcanizates it has been demonstrated that reinforcing fillers impart greater resistance to solvents and oils than do nonreinforcing types. Addition of the latter gives only the reduction in swelling attributable to elastomer dilution. In the case of Neoprene vulcanizates, Catton and Fraser reported that fillers function only as elastomer diluents and that those fillers commonly considered as of the reinforcing type impart no greater resistance to solvents than the nonreinforcing type. More recently, however, Buist and Mottram, in describing the effects of carbon blacks on the physical properties of natural rubber and Neoprene, reported that with both of these elastomers compounds containing thermal type carbon black gave slightly greater swelling in benzene than compounds containing equal loadings of other types of carbon black. With Neoprene, they reported good correlation between moduli and swelling characteristics.
An Improved Low-Temperature Brittleness Test
Authors: Smith E. F. , Dienes G. J.
Published: February 2011

An improved low-temperature brittleness tester, capable of testing five specimens simultaneously, is described. All machine specifications conform to A.S.T.M. Method D 746-44T. Data are presented which show that many elastomers do not possess a sharp brittle point but are characterized by a distribution of failures over a temperature interval. The improved brittleness tester makes it possible to carry out the necessary statistical study of the distribution of per cent failures versus temperature with a reasonable amount of work. A simple analysis of the resulting distribution curve is presented.
Relationship between Laboratory Abrasion Tests and Service Performance
Authors: Griffith T. R. , Storey E. B. , Barkley J. W. D. , McGilvray F. M.
Published: February 2011

In development work on GR-S commercial recapping compounds originated in 1943 by the Directorate of Mechanical Engineering, Department of National Defence, Ottawa, Canada, in which an attempt was made to correlate road performance with physical properties as determined in the laboratory, it was found that no relationship whatever existed between the results of road tests carried out under the supervision of that directorate and standard laboratory abrasion resistance tests carried out in the Canadian National Research Council Rubber Laboratory at Ottawa. In the laboratory test the sandpaper in the abrasion machine became coated with a smear of tacky viscous material which the air jet was unable to remove. Under these conditions the rubber tends to slide over the sandpaper surface, with relatively little actual abrasion of the rubber. The effect remains even after a considerable overcure of the sample. It was felt that the removal of the tacky viscous material from vulcanized GR-S by extraction might give more reliable abrasion resistance results, inasmuch as, on the road, rubber is constantly coming in contact with a new surface and such viscous material is thus being continually removed as it migrates to the surface of the rubber. From this point of view, then, the tread surface while being abraded on the road may be looked upon as extracted rubber and may be considered as conforming closely to the extracted laboratory specimen.
Flex Life of Passenger Tires. Relationship of Rayon Tire Cord Dip Pickup and Cord Adhesion
Authors: Murphy R. T. , Baker L. M. , Reinhardt R.
Published: February 2011

The tests described herein show that in woven rayon passenger tires, tire performance is improved when the quantity of dip pickup is reduced, even though there is a sacrifice in adhesion values.
Drying of Latex Rubber Deposits
Authors: Partridge E. G. , Hansen M. E.
Published: February 2011

In conclusion, it may be stated that the general principles of drying latex rubber deposits are much the same as for drying any other article, but there are conditions present that make drying more difficult. The porosity of the deposits decreases as drying proceeds; consequently the diffusion of the water through the deposits is progressively slower. Blisters can be formed between the deposits and the forms, but the formation of blisters can be controlled. During drying, flowable films such as those created by straight dipping can become wetter after being put into a drying oven. All these difficulties can be overcome by the proper design of ovens and the proper applications of fundamental drying principles.
The Hysteresis of Vulcanized Rubber
Authors: Clouaire Jacques
Published: February 2011

The present paper is based on an extensive investigation of the tensile properties of vulcanized natural rubber, carried out at the French Rubber Institute from April 1944 to December 1946. The purpose of this paper is to show how, in problems such as hysteresis, the correlation of certain experimental facts which apparently bear no relation to one another may lead to an explanation of the phenomenon involved. The hysteresis effect shown by both natural rubber and synthetic rubbers after stretching has been recognized for a long time, and it can be defined as the nonsuperposition of the strain and recovery curves of one complete cycle of deformation. It should be noted that this definition makes no reference to the causes of the phenomenon nor to the idea of loss of energy, which is not so clearly defined as generally believed. As a matter of fact, although the literature on the elasticity of rubber is remarkably extensive, little information is to be found on the underlying causes of hysteresis. The only real experimental work on the subject was carried out by Bouasse, who succeeded in throwing light on some of the complexities of the phenomenon. However, it appears that since that time little attention has been paid to his work. This may account for the fact that the idea of hysteresis being due solely to internal friction is still so widely accepted, although actually so completely incorrect.
Hysteresis and Heating in Vulcanized Rubber
Authors: Oberto S. , Palandri G.
Published: February 2011

Laboratory tests for evaluating energy losses due to hysteresis are analyzed. The importance of taking into consideration the thermal conductivity and cooling conditions of the rubber sample for the evaluation of heating due to hysteresis losses at temperature equilibrium is discussed. Two new test methods, based on forced resonance oscillations, are described, in which the rubber samples are deformed respectively in shear and in tension at frequencies of 20 to 200 cycles per second, with dynamic shear strain of 0.02 to 0.4 and dynamic tension strain of 1 to 5 per cent, and at a temperature of 20 to 110° C. Experimental results obtained with natural rubber and GR-S gum and tread stocks are presented. The hysteresis losses in gum stocks are found to follow the quadratic law versus dynamic shear strain, and the dynamic modulus G is found to be independent of shear strain. With tread stock the hysteresis losses are lower than would be the case with the quadratic law versus dynamic shear deformation, and the dynamic modulus G decreases with increasing dynamic shear strain. Neither the hysteresis losses nor the modulus depend on the shape or size of the specimen. The modulus is found to be independent of frequency, while the hysteresis losses are found to increase proportionally with frequency. As an application of the test method, results are presented of the effect of the quality and quantity of various types of carbon black on hysteresis heating of GR-S stocks.
A Study of the Increase in Tensile Strength of Vulcanized Rubber when it Begins to Age
Authors: Judeinstein Robert
Published: February 2011

In conclusion, the present study confirms the phenomenon of an increase in tensile strength at the beginning of aging. The magnitude of this increase depends on the nature of the acceleration and the state of cure; the tensile strength of an undercured sample increases much more than that of a sample vulcanized to its optimum state of cure, and the maximum tensile strength reached by the former is definitely higher than the maximum tensile strength reached by the latter. The phenomenon is therefore not simply postvulcanization. This increase in tensile strength takes place in the presence of oxygen and also in a vacuum as high as 10−3 mm. In the latter case, measurements of the T-50 value and combined sulfur give indications of an increase in network structure for a given percentage of combined sulfur. Oxygen is a contributing factor in increasing the network structure, but, in its presence, combination of sulfur is not so great and the maximum tensile strength attained is lower.
Permanent Set in Vulcanized Rubber
Authors: Mullins L.
Published: February 2011

The residual extension which remains after a sample of rubber has been stretched for some period, then released and allowed to recover, is popularly called permanent set. This set, however, is far from being permanent since it continuously decreases with the period of recovery; furthermore, after the rate of recovery has become exceedingly slow and is no longer readily observable, an increase in temperature will usually result in a sharp increase in the rate of recovery. It has been usual to identify this set with irreversible plastic flow, but it will be immediately evident that this can rarely be justified for, owing to incomplete high-elastic recovery, the measured value of set is a combination of both plastic flow and high-elastic deformation which has not completely recovered. Thus before any attempt is made to discuss the interpretation of the results of set tests, a study must be made of the significance of set. Treloar has investigated this phenomenon in raw natural rubber and has shown that entanglements or cohesional linkages may form while the rubber is stretched, and these oppose recovery; further, although van der Waals forces between the long-chain molecules largely control the rate and the amount of recovery, the crystallization of rubber produced by stretching may profoundly influence the set. On the other hand Tobolsky has studied the set which results from stretching rubber vulcanizates at high temperatures ; in such cases the amount of set is controlled by two processes which take place while the rubber is stretched; one of these involves the oxidative breaking of network chains, the other the oxidative cross-linking of network chains. Although these ideas are well founded, they do not provide a completely satisfactory basis for the understanding of set, and the purpose of this work is to extend these ideas and to explain the significance of the results of normal set tests ; in these tests rubber samples were extended at room temperatures to moderate elongations for relatively short periods of time. Most of the tests performed in this investigation were made on dumbbell shaped samples, which were extended by 200 per cent of their initial length for fifteen minutes at room temperature and then allowed to recover for one hour at room temperature; the residual extension was then noted and expressed as a percentage of the initial length. These tests will be referred to as normal set tests. In some tests various periods and temperatures of extension and recovery were used.
A Survey of Resilience and the Development of Resilient Compounds
Authors: Shaw Robert F.
Published: February 2011

A study was made of the methods and machines used in measuring resilience and classified as of the impact, free oscillation, forced oscillation, or other type: A search of the literature revealed that resilience is affected by numerous factors, such as temperature, duration of loading, magnitude of stress, method of applying stress, amplitude, frequency, and energy losses during measurement ; by shape and thickness of the sample, and by the state of cure, amount and type of plasticizer, amount and type of fillers, and the type of elastomer used. A series of Neoprene compounds was compounded, cured, and tested to determine the effect of various classes of carbon black on the resilience of the Neoprene elastomer, which is the most resilient commercially made rubber material. The resilience was found to decrease as the carbon black particle size decreased. A second series of compounds utilizing 15, 25, 35, 45, and 55 parts of medium thermal carbon black based on the Neoprene were tested, and the results show that resilience decreases with increase in loading. A comparison of the physical properties of these compounds with those of the currently used natural-rubber packing shows that the former are superior for replacing the natural rubber in mechanisms. The Neoprene compounds possess better tensile strength, elongation, resilience, aging, and oil resistant properties than the rubber compound, but are slightly inferior in low-temperature flexibility.
Energy of Resilience and Abrasion Resistance. A Comparison of Reclaimed and Crude Natural Rubber
Authors: Upham F. N.
Published: February 2011

When laboratory-machine tests on rubber were first introduced as a routine procedure, the only tests made were the elongation and tensile at break. Later, the tests were extended to the measurement of the amount of work done in stretching and breaking the test-specimen, and the computation was based on the area under the stress-strain curve.
Vulcanization at High Pressures
Authors: Wilkinson C. S. , Gehman S. D.
Published: February 2011

A technique was developed for vulcanizing cylinders of rubber 0.5 inch in diameter and up to 1 inch long at pressures as high as 150,000 pounds per square inch, using a small laboratory press (8-inch square platens). The specially designed mold was made of alloy steel according to general principles from the high pressure work of Bridgman. It was prestressed at 200,000 pounds per square inch. The temperature in the mold cavity was carefully calibrated with thermocouples to determine the equivalent length of cure of the test-specimens. Cylinders of GR-S tread stock were vulcanized for a range of cures at 1000, 20,000, 50,000, and 100,000 pounds per square inch. GR-S gum stock and Hevea gum and tread stocks were vulcanized for a range of cures at 100,000 pounds per square inch. Control specimens were cured in a regular type of mold, for which the pressure was not determined. All the volume compression during vulcanization is recovered when the pressure is released. This was determined by precise density measurements of the test-specimens. The electrical resistivity, dynamic modulus, internal friction, and resilience of the test-specimens were determined and the results are discussed.
A Method of Measuring “Heat Embrittlement” of GR-S and Hevea Rubber Compounds
Authors: Scholl Allen W. , Liska J. W.
Published: February 2011

This paper is a description of some studies of the phenomenon variously described in the literature as heat embrittlement, heat tenderness, heat shortening, etc. For purposes of discussion the term heat embrittlement will be employed in a general sense to denote the change in breaking elongation resulting from a change in test conditions, i.e., test temperature, cure, or aging. Inasmuch as the expression heat embrittlement is purely qualitative, no definite experimental procedure for its determination is implied, such as is implied, for instance, in the term ultimate tensile strength. Such lack of definiteness in terminology often leads to considerable confusion and the development of widely different test techniques for measuring the same phenomenon. Illustrative of the latter point are two heat embrittlement tests, one introduced by W. B. Wiegand and the other by R. B. Hobson. The first method is based on a measurement of the length of time which a stretched T-50 strip will withstand contact with an iron heated to 400° F without breaking. In the second method, samples of raw polymers are air-aged for specified periods of time, following which benzene solubilities are determined. The amount of residue remaining after extraction is employed as a measure of the heat embrittlement of the material. With the exception of Hobson's method, there appeared to be general acceptance of the idea that heat embrittlement was correlated in some manner with breaking elongation. It was accordingly decided in this study to measure breaking elongations directly, under various test conditions, and then to express the results in terms of heat embrittlement indices, each of which is defined mathematically. Although these definitions were originally derived on purely empirical bases, they appear to yield results capable of unique and useful interpretations, as will be pointed out later.
Properties of Ebonite. XXXVII. Electrical Properties of Synthetic Rubber Ebonites
Authors: Fisher D. G. , Mullins L. , Scott J. R.
Published: February 2011

Experiments were carried out to explore the possibility of making good electrical ebonites from various types of synthetic rubber. The ebonites produced were tested for permittivity and power factor over wide ranges of temperature and frequency. Thioplasts (Thiokols AZ and FA) apparently do not produce hard ebonitelike vulcanizates by the normal procedure. Neoprenes (GN and I) give ebonites, but with such high dielectric power loss as to be unsuitable for use as high-frequency dielectrics; moreover, if the mix contains zinc oxide, the ebonite has a very hygroscopic and therefore electrically unsatisfactory surface. Butadiene copolymers containing polar groups (butadiene-acrylonitrile types and Thiokol RD) give ebonites with high power loss, hence are not suitable for making high-grade electrical ebonites. Polybutadiene (Buna-85) and butadiene-styrene copolymers (GR-S, Hycar-EP, Buna-S) are much nearer to natural rubber as far as the radio-frequency (100 to 2,500 kc. per sec.) power loss of their ebonites is concerned. The GR-S ebonite examined was not so good as natural rubber at room temperature, but was superior above about 50° C. Buna-85 and Hycar-EP were superior to natural rubber over the whole temperature range; indeed, the high-styrene copolymers, as represented by Hycar-EP and Buna-SS, appear to be the best type of synthetic rubber for making ebonite with low power loss, especially at high frequencies and temperatures. The effects of changing temperature and frequency on permittivity and power factor are discussed. Attention is drawn to the big effect of temperature on power factor; this was less with polybutadiene and butadiene-styrene ebonites than with natural rubber ebonite, in keeping with the greater heat resistance of the former as judged by plastic yield tests. Comparison of the effects of rising temperature and decreasing frequency shows that these produce broadly similar effects on power factor, as would be expected on theoretical grounds, but that rising temperature superposes a second effect (an increase), presumably due to increased ionic conduction.
Photoelastic Applications in Rubber Technology
Authors: Hurry James A. , Chalmers Douglas
Published: February 2011

Photoelastic analysis has been shown to be a feasible and reliable method for anticipating design problems in rubber products. It lends itself qualitatively and quantitatively to the selection of materials, designs, and profiles for products, test-specimens, and certain processing equipment. The apparatus, method, and calculations are relatively simple and inexpensive, as our description and examples have shown. It is hoped that other rubber laboratories will explore its possibilities further. More widespread use and publications dealing with this method will, we believe, contribute to a better understanding of rubber product performance and to more effective and economical use of materials.
Determination of Tensile Strength of Natural Rubber and GR-S. Effect of Specimen Size
Authors: Higuchi Takeru , Leeper H. M. , Davis D. S.
Published: February 2011

Peirce's equation, which relates observed tensile strength of textile fibers with their length, was found to be applicable to rubberlike material if specimen volume is used in place of specimen length. Experiments in which the tensile strengths of GR-S and natural rubber compositions were determined for a range of specimen volumes yielded results in close accord with theory. A tenfold increase in the volume of the material resulted in a decrease of 308 and 339 and of 204 pounds per square inch in the observed tensile strength of GR-S and comparable natural rubber stocks, respectively. The numerical magnitudes of the slopes of the straight lines obtained when tensile strengths were plotted against the logarithms of the relative specimen volumes are shown to bear direct relationships to the homogeneity of the stocks under test. The use of a dumbbell sample with a constricted center should result in a relatively simple means of measuring quantitatively the degree of homogeneity of rubber compositions.
The Determination of Nitrogen in Crude Rubber
Authors: Van Der Bie G. J.
Published: February 2011

It was proved that various modifications of the Kjeldahl method for determining the nitrogen content of raw rubber (with phosphoric-sulfuric acid, sulfate-sulfuric acid, mercury, copper, selenium) do not yield greatly differing values. The use of selenium as catalyst is to be preferred, since the time of destruction is shortened considerably. A standard procedure was established. The hydrogenation method of Ter Meulen was applied to a sample of rubber powder (Pulvatex). Since this determination gave the same results as the various modifications of the Kjeldahl method, it is concluded that all these methods are reliable.
Rubber Metabolism in Plant Life. Development of a New Theory
Authors: Altman R. F. A.
Published: February 2011

Systematic analysis of the latex of Hevea brasiliensis yielded results which induced the author to study the metabolism of rubber more closely. As it was realized that the latex picture alone would not lead to sound conclusions about the formation and breakdown of rubber, analysis of the whole Hevea tree, from the highest leaflets down to the very root tips, seemed desirable. For practical reasons, however, such a profound investigation had to be reduced to the analysis of only the most important parts of the tree. The various analytical results obtained have forced the author to make a radical revision of some of the current concepts. For several reasons the synthesis of rubber probably takes place mainly in the green parts of the plant, and principally directly from carbon dioxide. The possibility of the formation of rubber from precursors (secondary formation) is still uncertain. Such formation, for example, must proceed in etiolated germinating plants. Freshly synthesized rubber is present in the plant in the liquid state. This liquid rubber is translocated through the plant in the form of a highly dispersed oil-in-water emulsion, in which the rubber droplets are surrounded by a layer of a phospholipoid complex. As an instructive illustration of this hypothesis the formation of latex is discussed. The process of polymerization results in the transformation of the liquid rubber into the solid phase'; this solid rubber is deposited everywhere in the plant. It appears most probable that, in Hevea brasiliensis, polymerization takes place mainly in the latex vessels. The breakdown of rubber, finally, is proved to be an oxidation process. For several reasons there is cause for believing that oxidation of rubber in the plant leads to the formation of those incrusting substances which are built up by C5-groups, i.e., pentosans, hemicelluloses, lignins, etc.
The Heat Sensitivity of Hevea Latex. III. The Process of Heat Sensitization of Latex Which Has Been Degraded Biochemically
Authors: Lepetit Francis
Published: February 2011

The present investigation shows that the heat sensitizing effect of powdered hog pancreas on degraded latex depends on a different principle from that of other methods, since it involves the mutual effect of two actions, a decrease in stability of the rubber-phase and an increase in ionic strength of the serum. For a given small increase in ionic strength, the greater the degradation of the stabilizing substances, the greater is the tendency toward coagulation. This makes it possible in practice to dispense with washing the coagulum. Since, in addition, it is possible to preserve heat sensitized mixtures for several weeks, it is obvious that this process meets most of the requirements for the manufacture of molded goods directly from latex.
Natural, Thermal, and Biochemical Degradation of Ammoniated Hevea Latex to Give Heat Sensitivity
Authors: Lepetit Francis
Published: February 2011

In most heat sensitizing processes ammonium salts are added to the latex, with consequent complications in manufacturing operations. A method is described which eliminates the use of ammonium salts and in which heat sensitivity of ammoniated latex in the presence of zinc oxide is obtained by natural aging, by heat treatment, or by biochemical methods. With natural aging or heat treatment, the sensitized mixture is stable at ordinary temperature provided the zinc oxide (or carbonate) is free from ionizable impurities. Biochemical degradation of the latex is readily obtained by adding trypsin. It produces sensitized mixtures which gel quickly and satisfactorily at ordinary temperature, and which are suitable for the manufacture of articles, even of great thickness in cold molds, the resulting coagulum needing no washing on removal from the mold.
Microgel, a New Macromolecule. Relation to Sol and Gel as Structural Elements of Synthetic Rubber
Authors: Baker W. O.
Published: February 2011

In connection with the rubber crisis of the war, useful fundamental controls and improvements of quality of synthetic rubber which would fit in with current engineering were urgently needed. Variations in the solubility of butadiene polymers and copolymers had been noted by many rubber researchers seemingly to reflect important and controllable variables in the polymerization process, such as “modification” and degree of conversion. Solubility also was thought by some workers to bear on processability. Agreement was neither definite nor general on these phenomena. Solubility behavior and gel fraction had been casually observed for thirty years in natural and synthetic rubbers studied in Europe. No record of engineering application was found. After the organization of the polymer research program in the Office of the Rubber Director, more intensive examination of the significance of sol-gel in controlling the uniformity of GR-S was begun. A proposal new to polymer technology was made, viz., that the gel fraction of synthetic rubber comprised a characteristic structure in the raw polymer which affected the physical properties and quality (as in tire performance) of the final vulcanizate.
Preparation and Properties of Rubberlike High Polymers. VI. Polymerization and Dimerization of Isoprene
Authors: Salomon G. , Boonstra B. B. S. T. , van der Meer S. , Ultée A. J.
Published: February 2011

The inhibitor and initiator action of thirty-three compounds on the bulk polymerization of pure isoprene has been studied. Picric acid was found to be the most powerful inhibitor at 100° C. The conclusion reached in Part I that the initiator activity of diazoaminobenzene is superior to all other compounds has been found to be valid also between 100–150° C. The heat polymerization of isoprene between 100–150° C in the presence of picric acid leads only to the formation of dimers. Three of the four possible isomers have been separated. The solid complex compound of diprene (I) with silver nitrate has been used to separate it from dipentene (II) and the hydrocarbon of Lebedev (III, IV) has been identified as a monochloride. The relative yields (I) : (II) : (III = IV) of 16:1:2 are in agreement with the relative availability and localization of π-electrons in isoprene. Observations on the properties of polyisoprene are in agreement with the more extensive work by d'Ianni. Some remarks on polymers from bulk polymerization are added. Isoprene acts as a powerful inhibitor on the polymerization of vinylidene chloride. This effect is independent of the method of polymerization.
Preparation and Vulcanization of Unsaturated Acrylic Elastomers
Authors: Mast W. C. , Fisher C. H.
Published: February 2011

Ethyl acrylate was copolymerized with small proportions of eleven dienes, eleven polyolefinic esters, and six polyolefinic ethers in an attempt to prepare olefin-containing acrylic elastomers that would vulcanize readily, yielding products having improved rubbery characteristics. In general, the resulting copolymers were insoluble in organic solvents, presumably because of cross-linkage. Acrylonitrile and dodecanethiol appeared beneficial in the copolymerization of ethyl acrylate with polyolefinic esters, but of questionable value in the diene polymerizations. The best vulcanizates from the standpoint of tensile strength and elongation were obtained from an ethyl acrylate-acrylonitrile-vinyl ether copolymer. Some preparations of this copolymer, however, had a tendency to pit and bubble during vulcanization. Isoprene, piperylene, and 2,3-dimethylbutadiene were more suitable for preparing vulcanizable ethyl acrylate copolymers than the other dienes studied; their vulcanizates had moderately high tensile strengths and elongations. Some of the dimethylbutadiene-ethyl acrylate copolymers were soluble. Crotyl acrylate and geranyl acrylate, when copolymerized with ethyl acrylate, yielded copolymers that gave vulcanizates having moderately high tensile strengths and elongations ranging from 300 to 400 per cent. The physical properties of the vulcanizates prepared from unsaturated acrylic copolymers were not superior to those of the chloropropyl acrylate and chloroethyl vinyl ether products described previously.
The Reaction of Sulfur and Sulfur Compounds with Olefinic Substances. VI. Factors Influencing Cyclic Sulfide Formation in the Reaction of Sulfur with Polyisoprenes
Authors: Bloomfield George F.
Published: February 2011

Cyclic sulfides are to a large extent secondary products of the reaction of primarily formed intermolecular polysulfides with polyisoprene hydrocarbon, and undue prolongation of the time of reaction of sulfur with polyisoprenes favors cyclic sulfide formation and so reduces the cross-linking efficiency of the sulfur. Various accelerating substances markedly increase the cross-linking efficiency, either by permitting shorter reaction times, or by promoting a cross-linking rather than a cyclizing reaction of the sulfur. Thiuram disulfide accelerators in the absence of sulfur bring about cross-linking, the linkages being of —C—S—C type rather than C—C.
Behavior of Different Fractions of Purified Hevea Rubber during Vulcanization
Authors: Verghese G. T.
Published: February 2011

Considerable data on the vulcanization characteristics of molecular fractions of ordinary (unpurified) natural rubber are available. There is, on the other hand, little information of any systematic work on the vulcanization of purified rubber and of its fractions. Pummerer and Pahl vulcanized the sol and gel fractions obtained from purified Hevea rubber, and also the purified whole rubber. But apart from a statement that whole rubber vulcanized much faster than the two fractions obtained from it, no details have been published. Vulcanization of purified whole rubber and of its sol and gel fractions was studied also by Smith and Holt. They concluded that the difference which they observed in the stress-strain behavior of the fractions and whole rubber was due to differences in the rubber which persisted through vulcanization. The present paper deals with a study of the vulcanization characteristics of different fractions of purified rubber prepared by a method described in a previous paper. Also, for comparative purposes a similar study was made of the corresponding fractions of unpurified rubber. As the difference in molecular weight of some of the fractions obtained by the above method was rather small, a grouping of the fractions was made as follows :
Theory on the Crystallization and Melting of Unvulcanized Rubber I. Crystallization
Authors: Kikuchi Ryoichi
Published: February 2011

Recently Wood and Bekkedahl have reported new experimental results on the crystallization and melting of unvulcanized rubber in the unstretched state at different temperatures. Their results are summarized as follows: (1). When a specimen of vulcanized amorphous rubber is kept at a constant temperature between about +15° and −50° C, it begins to crystallize and its volume gradually decreases, as seen in Figure 1. We shall denote the ordinate, “Decrease in volume (in per cent)” of Figure 1 by w (t), which is a function of time t for a constant temperature. (2). The arrows in Figure 1 indicate the estimated values for half the total decrease of volume at each temperature. Wood and Bekkedahl considered the reciprocal of this time to be a measure of the average rate of crystallization during the first half of the volume decrease, and it is plotted against temperature in Figure 2. We should like to call this time briefly the half-time, and denote it by th. (3). As the temperature of crystallized rubber is gradually raised, its volume increases suddenly in some region of temperature, and eventually reaches the value corresponding to amorphous state as shown in Figure 3. This phenomenon seems to be the melting of the crystal, but peculiarly the location and the width of this melting-region varies according to the previous crystallization temperature. (4). The range of melting does not depend on the extent of crystallization and is definitely determined by the temperature at which crystallization occurs. Figure 4 is derived from Figure 3 and shows the relation between the melting range and the temperature of crystallization. In the present paper the author tries to explain these results theoretically and consistently, with certain reasonable assumptions.