bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of Resistance to Abrasive Wear. VIII. Influence of Softeners in Tests with the Akron-Croydon and Du Pont Machines
Authors: Morley J. F.
Published: February 2011

These experiments indicate that softeners can influence abrasion resistance, as measured by laboratory machines, in some manner other than by altering the stress-strain properties of the rubber. One possible explanation is that the softener acts as a lubricant to the abrasive surface. Since this surface, in laboratory abrasion-testing machines, is relatively small, and comes repeatedly into contact with the rubber under test, it seems possible that it may become coated with a thin layer of softener that reduces its abrasive power. It would be interesting in this connection to try an abrasive machine in which a long continuous strip of abrasive material was used, no part of it being used more than once, so as to eliminate or minimize this lubricating effect. The fact that the effect of the softener is more pronounced on the du Pont than on the Akron-Croydon machine lends support to the lubrication hypothesis, because on the former machine the rate of wear per unit area of abrasive is much greater. Thus in the present tests the volume of rubber abraded per hr. per sq. cm. of abrasive surface ranges from 0.03 to 0.11 cc. on the du Pont machine and from 0.0035 to 0.0045 cc. on the Akron-Croydon machine. On the other hand, if the softener acts as a lubricant, it would be expected to reduce considerably the friction between the abrasive and the rubber and hence the energy used in dragging the rubber over the abrasive surface. The energy figures given in the right-hand columns of Tables 1 and 3, however, show that there is relatively little variation between the different rubbers. As a test of the lubrication hypothesis, it would be of interest to vary the conditions of test so that approximately the same amount of rubber per unit area of abrasive is abraded in a given time on both machines; this should show whether the phenomena observed under the present test conditions are due solely to the difference in rate of wear or to an inherent difference in the type of wear on the two machines. This could most conveniently be done by considerably reducing the load on the du Pont machine. In the original work on this machine the load was standardized at 8 pounds, but no figures are quoted to show how abrasion loss varies with the load. As an addition to the present investigation, it is proposed to examine the effect of this variation with special reference to rubbers containing various amounts and types of softener. Published data on the influence of softeners on the road wear of tire rubbers do not indicate anything like such large effects as are shown by the du Pont machine. This throws some doubt on the value of this machine for testing tire tread rubbers, a conclusion which is confirmed by information obtained from other workers.
Sulfur Linkage in Vulcanized Rubber. Acetone Extraction of Vulcanizates
Authors: Selker M. L. , Kemp A. R.
Published: February 2011

1. For the soft rubber stock SB1A, there is no evidence for the change: polysulfide → free sulfur on acetone extraction. 2. Evidence for the change: polysulfide → other sulfur links on acetone extraction is indicated by increased reactivity of extracted SB1A to methyl iodide. 3. Natural and GR-S ebonites lose about one-third of their combined sulfur on reaction with methyl idode at 24° C. Trimethylsulfonium iodide is found in large quantities. 4. Neither extraction experiments nor methyl iodide reactions offer evidence for occurrence of polysulfides in 32 per cent sulfur natural ebonite.
The Interaction between Rubber and Liquids. X. Some New Experimental Tests of a Statistical Thermodynamic Theory of Rubber-Liquid Systems
Authors: Gee Geoffrey
Published: February 2011

Experimental data are reported on the effects of vulcanization, extension, and temperature on the swelling of rubber. Although in qualitative agreement with a simple equation derived from a statistical theory of polymer solutions, these reveal quantitative discrepancies which are important in discussions of two-phase equilibria.
The Dynamic Characteristics of Neoprene Vulcanizates
Authors: Catton N. L. , Krismann E. H. , Keen W. N.
Published: February 2011

The utilization of the elastomeric spring goes back at least to the days of solid rubber carriage tires. The use of such materials to dampen vibration or absorb shock expanded rapidly despite the lack of engineering data and the inadequacy of testing methods for proper evaluation. Within the last twenty years these materials have been used in many dynamic applications, even though no appropriate means existed for the measurement of dynamic properties to determine their suitability for the particular service. Coincident with the practical application of rubber parts to vibration problems, engineers developed considerable data on the use of natural rubber in springs. Marked differences in performance were encountered when engineers were obliged to replace natural rubber with synthetic rubber in spring applications. Some engineers redesigned the rubber part used and obtained workable, although not entirely acceptable, performance. Design alone was not a complete solution to the problem, and it was necessary to call on the rubber technologist to produce vulcanizates having properties especially suitable for dynamic service.
Electrically Conducting Rubber
Authors: Lane K. A. , Gardner E. R.
Published: February 2011

In recent years the dangers and inconveniences arising from the presence of static electrical charges on rubber conveyor belts, rubber flooring, rubber-tired vehicles and the like, have aroused interest in the use of electrically conducting rubber as a means of minimizing the accumulation of static electricity. Conventional rubber compounds, which have electrical resistivities normally above 107 ohm-cm, and as high as 1014–1016 ohm-cm. for nonblack compositions, favor the accumulation of static charges. By using high loadings of channel black, the resistivity can be reduced considerably. If special types of carbon black are employed the resistivity can be reduced to a very low value ; in fact, the development of a compound with a resistivity of 1 ohm-cm. has been reported. A compound with a resistivity of about 10 ohm-cm., processible on ordinary factory-size rubber machinery, is described later in this paper. Rubber compounds with resistivities less than 107 ohm-cm. are generally grouped under the generic title of “electrically conducting rubbers”. The conduction of electricity through rubber-carbon black compositions is attributed to the ability of the carbon black to form chains of particles through the rubber. The formation of these chains depends on the particle size, crystal structure, and degree of dispersion of the black. The special types of black referred to above, termed conducting blacks, possess this ability for chain formation to an advanced degree. The work described below deals with the compounding of conducting rubbers, their application, and the methods used for testing. It appears under three main headings: measurement of resistivity; development of highly conducting rubber ; and development and testing of antistatic tires.
Properties of Ebonite. XXXV. Further Experiments on the Influence of Sulfur Ratio
Authors: Scott J. R.
Published: February 2011

1. As the percentage of sulfur in a rubber-sulfur ebonite is progressively increased, the yield temperature rises to a maximum and then falls again. There is thus an optimum proportion of sulfur for producing a heat-resistant ebonite. The cause of the reversion on exceeding this optimum has not yet been studied ; it is suggested, though without proof, that it may be due to the large amount of sulfur left uncombined. The swelling time (period required for the swelling in benzene to reach a given fraction of its maximum) likewise passes through a maximum at about the same sulfur ratio, and possibly for the same reason. 2. During prolonged vulcanization ebonite may become much harder and more heat-resistant, with little increase in combined sulfur. 3. Plastic yield temperature, hardness (as measured by Young's modulus), and absorption of petroleum hydrocarbons (petroleum ether, paraffin, transformer oil) are not determined solely by the combined sulfur content of the ebonite, but depend on how this has been attained, i.e. on the proportion of sulfur in the mix and the period of vulcanization. Increase of combined sulfur by prolonging vulcanization is more effective in producing the characteristic properties of ebonite (high yield temperature, hardness, resistance to petroleum hydrocarbons) than the same increase brought about by adding more sulfur. 4. In attempting to improve the heat-resistance of unloaded rubber-sulfur ebonites it is useless to increase the proportion of sulfur beyond the maximum now generally used, i.e. about 35 per cent of the mix, even when an accelerator is used to promote its combination with the rubber. The fact that ebonite can be made more heat-resistant without increase of combined sulfur, suggests a line along which improvement may be sought. 5. The results confirm the previous observation that well vulcanized ebonite is practically unaffected by oils of the petroleum (aliphatic) type, even at 90°– 100° C. The swelling action of such liquids, however, increases rapidly when the degree of vulcanization of the ebonite is reduced. The proportion of combined sulfur needed to confer good swelling resistance varies with the nature of the liquid, but can be deduced from the data recorded. There should be no difficulty in making an ebonite to give satisfactory service, so far as swelling resistance is concerned, in contact with oils, provided these consist of aliphatic hydrocarbons. As, however, aromatic hydrocarbons, e.g. benzene, toluene, xylene, have a strong swelling action, equally good resistance cannot be expected towards oils of appreciable aromatic content. Hydrocarbons of the cycloparaffin series (naphthenes) appear to be relatively innocuous.
Pyrolytic Depolymerization of Rubber into Isoprene
Authors: Boonstra B. B. S. T. , van Amerongen G. J.
Published: February 2011

The depolymerization of natural rubber into isoprene under the influence of high temperature was studied. Several methods, discontinuous and continuous, were tried on the principle of heating the rubber as rapidly as possible to cracking temperatures. The best method appeared to be the pressing of rubber by means of an extruder into a reaction tube of the desired temperature and pressure. After systematically varying the conditions for cracking, a yield of 58 per cent isoprene from crude rubber was obtained at a cracking temperature of about 750° C and a pressure of 10 mm. of mercury. Good isoprene yields were obtained in a rather wide range of temperatures between 675° and 800° C; however, the yield became rapidly less at higher pressures. At lower temperatures, e.g., 450° C, dipentene was the main cracking product. By studying the pyrolysis of isoprene itself in contrast to the pyrolysis of rubber, it became evident that low pressure has a twofold favorable influence—it causes quicker “evaporation” of rubber at lower temperature and shortens the contact time of pyrolysis products. When Hevea latex was dropped into a cracking tube, the yield of isoprene from the rubber it contained was about 52 per cent. Several synthetic rubbers and plastics were depolymerized under almost the same conditions as rubber.
Decomposition of Elastomers at High Temperatures
Authors: Skinner Glenn S. , McNeal James H.
Published: February 2011

All of the elastomer compositions tested from natural rubber, GR-S, butadiene-acrylonitrile copolymer, and Neoprene undergo an exothermic decomposition when rapidly heated. Increase in the heating rate enhances the abruptness and magnitude of the exothermic rise. Long heating at 190–200° C eliminates exothermic action. The rapid action also takes place in an atmosphere of nitrogen. Various compounding agents may modify this behavior, but they do not eliminate it. The exothermic action is therefore characteristic of the elastomers in the stocks. The only known structural characteristic common to all these elastomers is the residual double bond. The exothermic activity is, therefore, ascribed primarily to this source. This interpretation is further supported by the work of Midgley and Henne, which showed that the C—C bond in the 1–3 position to the residual double bond in crepe rubber is most readily cleaved. It is rendered still more probable by the evolution of products from the gum stocks tested, which require active hydrogen atoms for their formation. Very little hydrogen chloride is evolved before rapid action begins, and the rate of evolution increases sharply at the peak of exothermic action. Similarly, the butadiene-acrylonitrile copolymer yields hydrogen cyanide, and natural rubber yields saturated gaseous hydrocarbons. The evolution of smoke does not necessarily parallel the exothermic action. Smoke densities sufficient to cause 50 per cent extinction are always obtained before the beginning of the rapid rise in temperature. Complete extinction of the light in the apparatus occurs at a point between the beginning and the peak of the exothermic action. The smoke just above the sample of some stocks may clear completely during the exothermic action. Stocks containing zinc and magnesium oxides give colored smokes of a greatly increased order of density during the exothermic action. Phosgene cannot be detected in the smoke from Neoprene stocks. The outside heating temperatures at which the smoke and volatile gases flash are very similar for the four elastomers in the gum stocks. The initial flash occurs at the beginning of the exothermic rise. The smoke from Neoprene extinguishes the pilot light, while the other samples are ignited and continue to burn.
Frictional Properties of Tread-Type Compounds on ICE
Authors: Conant F. S. , Dum J. L. , Cox C. M.
Published: February 2011

The laboratory test method described is capable of indicating with a fair degree of accuracy the tractive ability of a tire tread compound on ice. Coefficients of both dynamic and static friction are important in determining the resistance to slide of a tire. The base polymer in a tread compound is the principal factor affecting the coefficients of friction. Among compounds based on different polymers, the coefficients of friction on ice are not closely related to either the stiffness or hardness of the stock or to its hysteresis. The coefficients of friction of stocks based on a single polymer, however, are greatly influenced by the type and amount of softener and by the type and amount of black.
The Structure of Neoprene. II. Determination of End Groups by Means of Radiosulfur
Authors: Mochel W. E. , Peterson J. H.
Published: February 2011

By use of radioactive sulfur it has been shown that sulfur-modified Neoprene, i.e., Neoprene Type GN, is essentially a copolymer of chloroprene and sulfur in the approximate ratio of 100 chloroprene units per sulfur atom. The sulfur units, possibly disulfides, are cleaved by an alkaline emulsion of tetraethylthiuram disulfide, so that a gel polymer containing combined sulfur can thus be converted to a soluble, plastic product. The mechanism of this cleavage reaction has not been completely elucidated as yet. Potassium persulfate, used as initiator for the polymerization of Neoprene, appears to be combined with the polymer in Neoprene Type GN in amounts equivalent to 8–17 polymer molecules formed for each sulfur-cpntaining initiator fragment. However, when dodecanethiol is used as modifier, a large proportion of the polymer molecules contain essentially no combined sulfur from the persulfate, and it appears that the true initiator is the RS free radical formed by reaction of the thiol with potassium persulfate. Dodecanethiol used as a polymerization modifier or chain transfer agent is combined in a greater amount than the one thiol per polymer molecule expected on the basis of simple chain transfer. High molecular-weight fractions contained 3–4 thiol sulfur atoms per polymer molecule and the results suggest that thiol addition to the double bonds of the polymer had occurred.
Modern Views on the Chemistry of Vulcanization Changes. III. Reaction of Sulfur with Squalene and with Rubber
Authors: Bloomfield George F.
Published: February 2011

The hexaisoprene, squalene (C30H50), is a promising hydrocarbon for inclusion in a comprehensive survey of sulfur-olefin reactivity and the mechanism of rubber vulcanization, since its molecular complexity is such as to render it more closely comparable with long-chain polyisoprenes than is the diisoprene dihydromyrcene, while its molecular weight is still sufficiently low to enable molecular distillation of the sulfurated reaction product to be accomplished without recourse to unduly elevated temperatures. It has been shown elsewhere that the behavior of squalene toward halogens resembles more closely that of rubber than does the behavior of dihydromyrcene. It is now found that the reaction of squalene with sulfur at the ordinary vulcanization temperature pursues a course very similar to that of dihydromyrcene while at the same time showing a close resemblance to rubber-sulfur vulcanization.
Kinetic Studies in the Chemistry of Rubber and Related Materials. III. Thermochemistry and Mechanisms of Olefin Oxidation
Authors: Bolland J. L. , Gee Geoffrey
Published: February 2011

A new estimate of the O—O bond strength in hydroperoxides is reported, based on a determination of the heats of oxidation of ethyl linoleate and linolenate. This value is employed, together with other known bond strengths and resonance energies, to estimate the heats of a number of alternative oxidation reactions of olefins. By considering the effects of the olefin structure on these heats of reaction, it is possible to see how structural differences can lead to the formation of different oxidation products. The discussion is confined to primary products of the oxidation chain, but, even so, a considerable diversity of behavior is shown to be possible. The final material may include simple monomeric hydroperoxides, polymers with varying amounts of oxygen incorporated in the chain, cyclic peroxides, and scission products. The very limited experimental evidence available falls generally into line with expectation.
Accelerated Aging of Vulcanized Rubber. Factors Determining the Relation between Oven and Bomb Aging
Authors: Newton R. G. , Scott J. R.
Published: February 2011

A study has been made of the influence of the following factors on the relative rates of tensile deterioration of natural rubber vulcanizates in the Geer oven and oxygen bomb, both at 70° C : period of vulcanization, sulfur ratio, nature of accelerator. Expressing this relation as the oven/bomb ratio, i.e., the ratio between oven and bomb aging periods that give the same deterioration, the following conclusions are drawn. (1) Varying the period of vulcanization of a given mix over a moderate range (2:3) around the optimum does not noticeably alter the oven/bomb ratio, except in unaccelerated mixes, where overvulcanization gave an abnormally low ratio, owing to the rubber perishing very rapidly in the oven. (2) The effect of varying the sulfur ratio within the usual limits for soft rubbers is small, and appears to result from the quicker vulcanization of the mixes richer in sulfur. (3) The nature of the accelerator profoundly affects the ratio, values ranging from 3 to 27 being found among the present mixes ; this effect is believed to be exerted through the influence of the accelerator both on rate of vulcanization and on the oxidizability of the vulcanizate. Theoretical considerations indicate that a rubber which vulcanizes quickly and (or) oxidizes slowly should give a low oven/bomb ratio, whereas one that vulcanizes slowly and(or) oxidizes quickly should give a higher ratio. The experimental data are generally in accord with this, and there is evidence that both rate of vulcanization and oxidizability can have an important effect, but the data do not enable the separate effect of each to be assessed exactly. The bearing of the results on the use of the oven and bomb tests is briefly discussed. It is shown that they demonstrate the fallacy of using a single factor for converting oven to bomb aging periods or vice versa, and that the relative natural aging resistances of widely different vulcanizates cannot be reliably assessed by either test, a limitation arising from the fact that the relative importance of oxidative decay and aftervulcanization varies from one type of mix to another, and that the temperature coefficients of these processes are not necessarily the same. These facts are very important in relation to the choice and application of accelerated aging tests for specification purposes, since these usually have to be applied to rubbers of unknown compositions, which are likely to vary widely in their relative response to different aging conditions.
Stress-Optical Properties of Rubber
Authors: Treloar L. R. G.
Published: February 2011

Considerable progress in the quantitative description of the mechanical properties of rubber has taken place in recent years by the consideration of a rubber as a network of statistically kinked long-chain molecules. A treatment of the optical properties of strained rubber on this basis, with the assumption that the links of the molecular chain are themselves optically anisotropic, has been developed by Kuhn and Grün for the special case of simple elongation, and by me for general homogeneous strain. In the latter case, the rubber has three principal refractive indices, n1, n2, and n3, corresponding to the three principal axes of the strain ellipsoid.
Two Methods of Protection against Oxygen and the Improvement Which Results from Their Combined Use. I. Observations on the Mechanism of Protection
Authors: Le Bras Jean
Published: February 2011

The observations which are recorded in the present paper represent an extension of the single case of litharge which has already been described. They show that, when small percentages of certain substances are added to rubber with a view to protecting the rubber from deterioration by oxygen, these substances are capable of directing the combination of oxygen with the rubber in different ways. This is shown by the fact that, as a result, a given percentage of combined oxygen does not lead to the same deterioration in physical properties. This difference in behavior can be explained logically on the basis of the antioxygenic theory by assuming that some agents act, not by retarding the rate of oxidation, but by deactivating the peroxides as soon as they are formed. By what term are these agents to be designated? First of all it should be recalled how an antioxygenic substance is defined. Every substance is an antioxygenic agent when it has the power, in small percentages, of retarding the rate of absorption of free oxygen by an autoxidizable substance. This definition obviously does not apply to a perfect deactivating agent, since the latter has no effect on the rate of absorption of oxygen, in spite of it too protecting rubber against deterioration by oxygen, and therefore being equally worthy, from the practical point of view, of being called an antioxygenic agent. However, this would only lead to confusion between the phenomenon itself and its effects. Furthermore, since commercial antioxygenic substances appear to show, to a greater or less degree, a combination of the two actions, one might consider designating them by some term which would embody both mechanisms. The word “antiaging agent” is not suitable, for it is too general and applies to cases where, in addition to oxygen, other influences such as light and repeated flexing play a part. There is, then, a problem in terminology to be settled, but this will have to be left unanswered provisionally until sufficient facts which have a more direct bearing on the case are available. Finally attention should be called to the useful effect which may be pictured as a possibility when the two types of protective agents which have been described act jointly. In other words, if the two mechanisms in question were to be superimposed, it would appear to be possible to improve considerably the resistance of rubber to deterioration by oxygen, since any oxygen which escapes the protective action of the true antioxygenic agent has its harmful effect reduced by the deactivating agent. To express it figuratively, it might be said that oxygen which has succeeded in overcoming the first obstacle opposing its action finds itself confronted with a new defense which puts the oxygen partially out of action. As shown by experiments carried out on this subject, which are described in the following paper of this series, this theoretical conclusion is actually borne out by the results of the experiments.
The Autoxidation of Rubber and Mechanisms of Protection against Oxygen
Authors: Le Bras Jean
Published: February 2011

The autoxidation of ethylenic hydrocarbons has been the object of various investigations, and various authors, among whom Engler and Stephens in particular might be mentioned, have studied the part played by peroxides. Up to 1936 it was generally believed that structurally these peroxides represent the fixation of one molecule of oxygen on the double bond. About this time, however, Criegee on the one hand, and Hock and Sehrader on the other, published papers almost simultaneously on the preparation and constitution of cyclohexene peroxide, in which they demonstrated that the latter can exist in the form of a hydroperoxide.Later these investigators offered further proof in favor of this view by extending the reaction to other hydrocarbons. These studies led to the important conclusion that ethylenic hydrocarbons have two sensitive points: the double bond itself, and the carbon in α-position to this double bond; hence there are two forms of the peroxide, the first of which retains its unsaturated character.
Effect of Temperature on Resilience
Authors: Mullins L.
Published: February 2011

The recent use of rubber components in extremes of climate and under a variety of service conditions has drawn attention to the importance of the knowledge of the physical properties of rubber vulcanizates at temperatures other than normal; the dearth of such knowledge has been emphasized by the enforced replacement of natural rubber by synthetic rubbers having reduced resistance to cold. When designing mechanical parts, the engineer must know first the conditions under which the part will be required to operate and, second, the physical properties under these conditions of the materials to be used. In a recent review Riesing has given illustrations of the importance of such knowledge; in particular he has shown that rubber mountings in an automobile may well be subjected to temperatures as high as 80 to 100° C, and although rubber mountings normally warm up during their operation, they may commence to function at extremely low temperatures, while there is a limiting temperature below which they fail even to warm up. The first part of this paper gives the results of the measurement of rebound resilience on a number of vulcanizates over a wide range of temperatures. Resilience is one of the important physical properties of a rubber vulcanizate, and in designing parts for shock or energy absorption, data on the resilience of the material are essential; for such applications a material with a low resilience is required, but as the energy absorbed manifests itself in the form of heat, the temperature rise of the absorber may control the permissible value of the resilience. In many other applications it is necessary that the material should have a high resilience and so absorb little energy. The resilience is normally determined by measuring the rebound of a ball or pendulum after impact on a sample of rubber; various other methods have been used, involving, either measurement of the decay in amplitude of the damped free vibration which results when a sample is deformed and then released, or measurement of the energy loss during sustained forced vibrations. Unhappily, the results from one test do not always show great similarity to the results from another test and, as a result, the engineer has to relay on empirical correlations with resilience tests conducted in a particular way, or on service behavior. The second part of this paper gives the lines along which an investigation is being conducted to illuminate the significance of these dissimilarities. The measurement of resilience over a range of temperatures has an added importance, since it throws some light on the structure of the rubber and on the processes taking place during deformation. Recent papers which have just become available in this country show that there has been interest and activity in this field in Germany during the war years.
Composition of Butadiene-Styrene Copolymers Prepared by Emulsion Polymerization
Authors: Meehan E. J.
Published: February 2011

The formation of a high-polymer molecule occurs by the successive additions of monomer molecules to a growing chain (chain propagation), the growing chain itself being made up of a number of monomer molecules with a radical derived from a monomer molecule on the end of the chain at which addition occurs. It was pointed out by Norrish and Brookman that chain propagation in the formation of a copolymer from two polymerizing monomers involves four different reactions; the growing chain may have a radical unit derived from either of two monomers at the end of the chain, to which either of two monomer molecules may add. In general it may be expected that the four growth processes will involve four different reaction-rate constants. This situation has been studied thoroughly from the theoretical point of view by Alfrey and Goldfinger, by Mayo and Lewis, by Wall, and by Simha and Branson. The theoretical treatment has been extended by Walling and Briggs to the copolymerization of an arbitrary number of monomers. A considerable amount of experimental data on copolymerizations of various pairs of monomers in homogeneous solution and the quantitative interpretation in terms of the concept given above have been presented. Prior to the comprehensive treatments of the composition of copolymers referred to above, Wall had shown that a relatively simple treatment could be given to the problem, on the assumption that only two rate constants are involved in the chain propagation, corresponding to the addition of either of two monomers to the growing chain This would mean that both radical types on the end of the growing chain would have the same relative reactivity with monomers. Wall has pointed out that such a copolymerization would be “ideal” in the same sense as a pair of liquids which follows Raoult's law. Mayo and Lewis, however, have stated that this “ideal” copolymerization behavior does not correspond to the experimental facts in the case of the bulk copolymerization of styrene and methyl methacrylate, or in the case of the copolymerization of maleic anhydride with certain ethylene derivatives.
Alkaline Materials as Accelerators for GR-S
Authors: Breckley Joseph
Published: February 2011

During the development of the national synthetic rubber program it was decided to utilize the research and development facilities of compounding material suppliers in an investigation to determine the best methods of formulating GR-S and other synthetic rubbers. This investigation was directed by the Compounding Research Branch of the Office of the Rubber Director, which then collected and distributed to the participating laboratories the large amount of data gained from the many tests carried on with different accelerators, fillers, and softeners in GR-S. Included in this information were indications that the curing rate of GR-S was affected to a considerable degree by inorganic oxides and alkaline materials. Consequently this phase of the problem was investigated still further in this laboratory, and some data were released through the same agency regarding the efficacy of magnesia, litharge, and red lead as activators for some organic accelerators. The results obtained encouraged further work to determine the effect of more strongly alkaline materials. It was discovered that ethanolamines were very effective activators for some organic accelerators; in fact, triethanolamine activates tetramethylthiuram monosulfide so strongly that a combination of the two materials promotes vulcanization of GR-S at room temperature.
Determination of Rubber Hydrocarbon by a Gravimetric Rubber Bromide Method
Authors: Willits C. O. , Swain M. L. , Ogg C. L.
Published: February 2011

Direct determination of rubber hydrocarbon in crude rubbers and in latices has been generally considered so inaccurate and unreliable that determination of rubber hydrocarbon by difference has been a common practice. The difference method usually suffices in the analysis of products derived from Hevea rubber, in which the nonrubber components have been well characterized or occur in low concentration. In the analysis of natural rubber products obtained from sources other than Hevea, such as guayule, kok-saghyz, and Cryptostegia, the situation is different. The chemical and physical properties of the nonrubber components of such products are largely unknown, and in many cases the nonrubber contaminants are present in excessive amounts, sometimes more than 50 per cent of the total. Since the rubber hydrocarbon analysis usually affords the most reliable means for estimating the rubber content of plants and for following the concentration and purification of rubber by mechanical and chemical processes, it appeared desirable to devise or adapt a method which would permit direct determination of this constituent in the presence of relatively large amounts of natural and added contaminants.
Variation in the Specific Heat of Rubber as a Function of Elongation
Authors: Mayor Andre
Published: February 2011

The changes in specific heat of several samples of rubber when stretched were measured. Unvulcanized rubber showed no change in specific heat when it was stretched up to 300 per cent. The specific heat of vulcanized rubber increased very slightly and in proportion to the elongation. With some of the samples, a bend in the curve appeared at about 200 per cent elongation at room temperature, in which case the linear increase in specific heat was about ten times at higher elongations. Supposedly this sudden increase in specific heat is related to the heat of crystallization.
Methods of Averaging Physical Test Results. Use of Median
Authors: Buist J. M. , Davies O. L.
Published: February 2011

Different testing laboratories use different methods of averaging test results, and it would facilitate comparison of results between different establishments if a uniform procedure could be agreed upon. To facilitate the correct choice the following methods have been compared: (1) Averaging all results within 10 per cent of the highest. (2) Averaging all results until the average is within 10 per cent of the highest. (3) Averaging the highest four results out of six, the highest three out of four. (4) Taking the median. (5) Method of Horrobin and Davies. Methods (1) and (2) are unacceptable on logical grounds, but there is little to choose between methods (3) and (4). The choice for routine use rests on simplicity of method and freedom from bias. Method (3) introduces a slight bias by always rejecting the low result and, therefore, it is recommended that the median be used either by averaging the middle two out of four results or by averaging the middle four out of six results. For five rings we take the average of the middle three and for four rings the average of the middle two. When we are interested in estimating the variations arising at each source in rubber testing, it is preferable to use the more elaborate method (5). This is considered the most logical of all methods, but it is not suitable for routine use.
Device for Evaluating Surface Cracking of GR-S
Authors: Throdahl M. C.
Published: February 2011

1. Dynamic flex-cracking may be studied indoors under conditions which yield results closely comparable to outdoor atmospheric exposure. 2. Although the photographs are not sufficient to show the depth of cracking, it was observed that the stocks of several compounds which showed the tendency to bloom and migrate to the surface cracked badly and to a depth equivalent to control stocks. By comparison, however, these same stocks when statistically exposed showed only slight tendency to crack in the usual exposure time of 100 hours. 3. Minimum concentration of additives for retarding the rate of surface cracking was found to be 0.25 per cent, but best results were obtained with 1 per cent. 4. Cracking always occurred at right angles to the direction of strain. Preliminary studies with this apparatus indicated that none of the specimens cracked when exposed in the looped but unstretched position (this was done by removal of the pin and stirrup) up to 50 hours of exposure, but some cracked slightly at 100 hours. Flexing in the laboratory without the source of ultraviolet light did not produce cracking of the specimen in 100 hours. The nature and severity of cracking were made the same for natural sunlight outdoor exposure in July as they were for artificially generated conditions of the laboratory except that approximately 3 weeks (roughly 60 exposure hours) were required. (This overall time was due to excessive rain, which necessitated coverage of the entire flexing mechanism during much of the time.) This may not be a fair comparison because of the rather contaminated atmospheric conditions in Nitro, W. Va. However, stocks of the same composition flexed on a revolving belt in Florida atmosphere cracked in the same manner as those shown here. Variation of degree of strain from 10 to 35 per cent did not produce significant differences in results. This is in agreement with the observations of Powell and Gough, who pointed out that the so-called critical stretch does not exist. Change in height of the light source—that is, the radiation intensity—over the short range did not produce significant changes in rate of cracking. In order that the surface temperature of the stock remains at 30–35° C, the minimum height was fixed at 50 cm. above the turntable.
Copolymers of Butadiene with Halogenated Styrenes
Authors: Marvel C. S. , Inskeep G. Esler , Deanin Rudolph , Juve A. E. , Schroeder C. H. , Goff M. M.
Published: February 2011

Butadiene has been copolymerized in a typical emulsion polymerization recipe with each of the monochloro-, monobromo-, monofluoro-, and dichlorostyrenes, with the mixed mono-, di-, tri-, tetra-, and pentachlorostyrenes, and with p-cyanostyrene and several of its derivatives. While 2,5- and 3,4-dichlorostyrene copolymerized with butadiene more rapidly than does styrene itself, 2,6-dichlorostyrene and the mixed tri-, tetra-, and pentachlorostyrenes copolymerized at a slower rate than did the control. 2,5-Dichlorostyrene entered the copolymer much more rapidly, and 2,6-dichlorostyrene and pentachlorostyrene much more slowly, then does styrene itself. The mono- and dihalogenated styrenes in general gave copolymers whose quality indexes were about equal to that of GR-S; o-chlorostyrene, 2,5- and 3,4-dichlorostyrenes, and p-cyanostyrene all gave copolymers with quality indexes higher than that of GR-S. The mixed tri- and tetrachlorostyrenes gave copolymers of inferior quality.
Physical Properties of Diene Polymers. Effects of Side Vinyl Groups and Other Structural Features
Authors: D'Ianni James D.
Published: February 2011

The detailed molecular structures of natural rubber, emulsion polyisoprene, polyisoprene prepared with an organometallic catalyst, and sodium polyisoprene were studied by comparison of data available from infrared absorption spectra, perbenzoic acid titration, refractive index, density, iodine number, chromic acid oxidation, and hydrochlorination. These polymers showed decreasing amounts of 1,4-addition content in the order listed. Similar data were collected where possible for emulsion polybutadiene, polybutadiene prepared with an organometallic catalyst, and potassium polybutadiene (Buna-85), as well as for GR-S and corresponding butadiene-styrene copolymers prepared with an organometallic catalyst and with sodium. These two groups also showed decreasing 1,4-addition content in the order listed. Correlation of structure inferred from the above data with physical properties of corresponding tread stock vulcanizates indicated that for diene polymers with decreasing amount of 1,4-addition content the brittle point rose, the rebound value decreased, and the tensile strength increased, provided comparison was restricted to polymers of approximately the same molecular weight range made with the same monomer. Natural rubber occupied a unique position because it was substantially a linear high polymer with the cis-configuration around all the double bonds. For the butadiene-styrene 75/25 copolymers the brittle point rose with decreasing amount of 1,4-addition content, but no satisfactory correlation could be obtained for the tensile strength and the rebound value, probably because of the predominant effect of the phenyl side groups.
Effect of Molecular-Weight Distribution on Physical Properties of Natural and Synthetic Polymers
Authors: Johnson B. L.
Published: February 2011

The molecular weights of rubberlike polymers are average values for mixtures of macromolecules which differ greatly in size. The heterogeneity of natural rubber was recognized as early as 1929 by Whitby. The fact that solutions of higher quality crude rubbers are more viscous than lower quality rubbers was observed by Axelrod in 1905. Other workers have separated rubber solutions into fractions of differing molecular weight and have studied the properties of the fractions. In the case of synthetic polymers, these heterogeneous mixtures of molecules have been characterized by fractional precipitation and construction of molecular-weight distribution curves. The particular type of distribution determined in this investigation is that obtained from weight average properties based on viscosity measurements of fractions of the polymers. Application of these methods to the characterization of natural polymers confirmed observations that a variety of molecular-weight distributions existed in the case of the natural polymers. The physical properties of the natural polymers having these diverse molecular-weight distributions were well known. Therefore a correlation of their physical properties and weight distributions seemed pertinent to an evaluation of the fractionation technique. Such a correlation had not been possible on early butadiene-styrene copolymers because of the similarity of the distributions then obtained, even under different polymerization conditions.
A Study of the Deactivating Effect
Authors: Hildenbrand René
Published: February 2011

It has already been shown by Le Bras that certain substances protect vulcanized rubber from deterioration by oxygen without retarding to any significant extent its rate of oxidation. In this case the phenomenon is different from a true antioxygenic effect, where protection is evident by a notable retardation of the rate of oxidation. The phenomenon was first manifest in a systematic comparison of a series of antioxygenic agents of different chemical types with respect to their influence on the oxidizability of a vulcanizate and the protection which they impart in the artificial accelerated aging of the vulcanizate. It was proved that, in conformity with the mechanism of the antioxygenic effect, there was in general a retardation of the rate of oxidation and protection against deterioration by oxygen at the same time. Only a vulcanizate containing mercaptobenzimidazole (a substance classed among antioxygenic agents, since it is known to increase resistance to accelerated aging) showed an anomalous behavior with respect to the rate of oxidation. Although this particular vulcanizate showed good resistance to artificial aging, its oxidizability was of the same order as that of the control vulcanizate containing no protective agent, which deteriorated very rapidly. In this case, consequently, the protection against deterioration by oxygen involved a mechanism quite different from that characteristic of the classic antioxygenic effect.
Chromatographic Analysis
Authors: Bloomfield G. F.
Published: February 2011

As a part of a general study of vulcanization reactions, the British Rubber Producers' Research Association has been concerned with the fate of accelerators during cure. The recent paper by Bellamy, Lawrie, and Press (Trans. Inst. Rubber Ind. 22, 308–324 (1947); Rubber Chem. Tech. 21, 195–219 (1948)), describing chromatographic methods of separating and identifying accelerators and their decomposition products, was therefore of interest. Mercaptobenzothiazole found in an eluate from an alumina column does not, however, necessarily show that benzothiazoyl disulfide originally present in a vulcanizate is converted to mercaptobenzothiazole during vulcanization.
The Mooney Viscosity and Gel Plasticity Relationship for GR-S-Xylene Gels
Authors: Driskell J. C. , Kracke R. D.
Published: February 2011

1. A new method for measuring plasticities of highly viscous GR-S-solvent gels is described. 2. The coefficient and the standard error of estimate for the regression of plasticities of 35 per cent GR-S-xylene gels on Mooney viscosity (ML 4/212) of the rubber have been calculated. 3. An explanation for an observed shift from the relationship established for Mooney viscosity and gel plasticity caused by hot or cold milling of the polymer is suggested.
Basic Reactions Occurring during Rubber Reclaiming. I. The Influence of Reclaiming Media, Antioxidant, and Defibering Agents on Vulcanized Natural Rubber at 195 Pounds Per Square Inch Gauge Pressure (196.6° C)
Authors: Le Beau D. S.
Published: February 2011

The influence of surrounding atmosphere, antioxidant, and defibering agents at 196.6° C on the hydrocarbon of rubber was studied by subjecting a laboratory prepared, pure-gum, natural rubber compound to reclaiming in the absence of any reclaiming oils or chemicals. Data obtained on the amount of acetone and chloroform extracts and on the unsaturation of the reclaims and the chloroform extracts could therefore be interpreted in terms of the effect of the factors mentioned above on the rubber hydrocarbon and not as the effect of any chemicals added to the rubber to promote reclaiming. It was found that the effect of the surrounding atmosphere during reclaiming is the predominant factor in the reaction. Acidic surrounding atmospheres result in a slow and rather uniform rate of molecular breakdown and in the formation of small amounts of oxidation products. Apparently no secondary oxidative attack occurs at the double bonds. An open steam atmosphere provides the greatest amount of oxidized products and also a high initial molecular breakdown, followed by a slow progressive increase in it as the reclaiming periods are lengthened. A decrease in the unsaturation value of the acetone extracted reclaim indicates that secondary oxidative attack at the double bonds occurs at long reclaiming intervals. Alkaline reclaiming atmospheres provide little if any oxidation products and result in a very high initial rate of molecular breakdown. Longer reclaiming periods do not change further the amount of molecular breakdown. The very small amounts of oxygen necessary to promote the reclaiming reactions, the dependence of the course of these reactions on the acidity or alkalinity of the surrounding reclaiming media, together with the high initial rate of molecular breakdown reactions under conditions where such rate would be speeded up, permit the tentative explanation of these reactions on the basis of hydroperoxidic chain reactions. Further experiments along this line are being carried out.