bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship of Oxygen to the Activity of GR-S Reclaiming Agents
Authors: Bennett R. B. , Smith G. E. P.
Published: February 2011

1. An alkylphenol sulfide reclaiming agent was found to have little activity in the absence of oxygen ; conversely oxygen produced little activity without the reclaiming agent. Together, the oxygen and reclaiming agent showed exceptional activity in attacking a sulfur-cured GR-S gum vulcanizate to produce soluble, low molecular-weight fragments under relatively mild experimental conditions. 2. The solubilizing effects of oxygen with reclaiming oils and the alkylphenol sulfide reclaiming agent were produced without significant increase of the amount of combined oxygen in the acetone or chloroform insoluble portions of the vulcanizate. Large increases of oxygen content were produced quickly by subsequent exposure of the treated and extracted samples to an atmosphere of oxygen. 3. No significant quantity of combined sulfur was removed under experimental conditions involving an excess of liquid reclaiming oils and agents. However, a significant, constant amount of sulfur was removed under conditions in which only small amounts of the reclaiming oils and agents were carried on the surface of the ground vulcanizates. This loss of sulfur was independent of both oxygen and of reclaiming aid and appeared to depend solely on the type of stock and on the physical conditions employed.
A Study of Stiffness Testing of Elastomers at Low Temperatures
Authors: Conant F. S.
Published: February 2011

In light of the above discussion, it does not appear logical to choose a “best” test for determining low-temperature moduli. A study of the data as a whole indicates that the correlation between the two sets of values from the simple beam-method (laboratories C-1 and C-2) are closest, but there is nothing to show that they are the most valid. It should be emphasized that, in the temperature region of greatest technical importance, that is, where the rubber is changing from its ordinary flexible state to a hard stiff material, the different tests show good agreement. A low-temperature limit of serviceability would be close to −80° F for the Hevea gum and −60° F for the GR-S by each of the methods studied. Another conclusion that might well be reached by a study of the test methods and data presented is that the term “modulus”, as applied to rubberlike materials, is rather vague unless the method of obtaining that modulus is also given.
A Study of Hardness Testing of Elastomers at Low Temperatures
Authors: Labbe B. G.
Published: February 2011

The spring-actuated instruments are desirable, since instantaneous readings may be obtained. Unfortunately, neither of the Shore instruments (A or D) provides adequate results over the complete range of temperatures for the stocks herein studied. A hardness instrument embodying the indentor point of the Shore-A Durometer with a loading spring requiring 0 to 5 pounds for the scale range would probably be more suitable for low-temperature work. The Pusey and Jones instrument is somewhat difficult to operate at the low temperatures (−60° and −80° F) because of the manner of loading and because of friction between the moving parts. The Olsen instrument provided similar difficulties as far as frosting of the parts was concerned. Some trouble was experienced with this instrument even at −40° F. No results were reported for the Armstrong indentation equipment below −40° F because of the unavailability of conditions below that temperature. On the basis of the information furnished, it appears that the Material Laboratory Indentometer is the most satisfactory of the dead-weight load hardness testers for use at low temperatures.
Crystallization and the Relaxation of Stress in Stretched Natural-Rubber Vulcanizates
Authors: Gent A. N.
Published: February 2011

Experiments are described in which the decrease of volume and simultaneous relaxation of stress in crystallizing extended samples of vulcanized natural rubber were followed with time at a temperature of −26° C. The theoretical treatment of Avrami is extended and shown to predict to a first approximation the time functions governing crystal growth in stretched vulcanizates. A continuous transition from polyhedral to aciform crystal growth on increasing the extension is indicated The relaxation of stress is found to be approximately proportional to the degree of crystallization throughout the process, the factor relating them being given by the theory of Flory at high extensions, that is, for aciform crystal growth. The retarding influence of vulcanization is attributed to a banned volume mechanism, whereby an element of volume around each cross-link is prohibited to nucleation, and an approximate value for the banned volume has been obtained. The influence of irregular chemical groups and prior heating on the rate of crystallization is also discussed.
Migration of Antioxidants during Oven Aging
Authors: Schonlau W. J. K.
Published: February 2011

The aging characteristics of a natural-rubber vulcanizate containing no antioxidant, measured in a Geer oven at 70° C, are affected by migration of phenyl-β-naphthylamine from other vulcanizates aged in the same oven at the same time.
Crystallization in Butadiene-Styrene Copolymers
Authors: Wood Lawrence A.
Published: February 2011

From Figure 3 one draws the following significant conclusions: (1) Crystallization is not observed if the polymerization temperature is above 60° C. (2) For polymerization at 50° C, a small amount (2 to 6 per cent) of bound styrene inhibits crystallization completely. (3) For polymerizations at 5° C, the limit is at about 15 to 18 per cent bound styrene content. (4) At the lowest polymerization temperatures normally utilized, this limit is at about 30 per cent bound styrene. Direct experimental observation is in general accord with these conclusions.
Effect of Structurizing Agents on the Plasticization and Vulcanization of Butadiene-Styrene Rubber
Authors: Novikov A. , Galil-Ogly F. A.
Published: February 2011

Compounds of the type of p-anisidine and N,N′-diphenyl-p-phenylenediamine cause structural changes in SKS-30A rubber and its solutions, particularly during heating. Vulcanization accelerators, in particular, mercaptobenzothiazole, greatly accelerate these structural changes in rubber in the presence of p-anisidine or N,N′-diphenyl-p-phenylenediamine during both heat oxidative plasticization and vulcanization. The degree and nature of the structural changes in rubber vary according to the type of accelerator. The structurizing action of p-anisidine and of N,N′-diphenyl-p-phenylenediamine during vulcanization is manifested in the increase of the tensile strength, equilibrium modulus, and rebound resilience of vulcanizates, and the retention of high relative elongations. The agents cited accelerate somewhat the process of combination of sulfur with the rubber in the initial stage of vulcanization, without increase of combined sulfur content at the point of optimum vulcanization. The hypothesis is advanced that the influence of p-anisidine and N,N′-diphenyl-p-phenylenediamine on the observed structural changes in SKS-30A rubber during its heat plasticization and vulcanization is related directly to the character of the oxidation processes, and may be caused by the reaction of the oxidized segments of the rubber (peroxide radicals, stable peroxides, functional oxygen groups, etc.) In unvulcanized rubber, this leads to a loss of plasticity and solubility, and, by vulcanization, to the formation of additional cross-links, to an increase of tensile strength, and with retention of high elongation at rupture. Vulcanization accelerators, such as mercaptobenzothiazole, which activate the oxidation processes in the rubber, increase considerably the rate and extent of these structural changes.
The Interaction of Sulfur and Sulfur Compounds with Olefinic Substances. VII. Low-Temperature Sulfuration of Trialkylethylenes with Hydrogen Sulfide-Sulfur Dioxide and with a Sulfur-Zinc Dithiocarbamate System
Authors: Farmer E. H. , Ford J. F. , Lyons J. A.
Published: February 2011

The sulfuration of trialkylethylenes with hydrogen sulfide-sulfur dioxide at 0° C (Peachey process) results in disubstitutive cross-linking of the olefins, yielding dialkenyl tetrasulfides. At higher temperatures, substitutive-additive cross-linking occurs, and alkyl alkenyl polysulfides are formed. Dialkenyl tetrasulfides are similarly formed by causing the olefin to react with sulfur at room temperature in the presence of zinc oxide and zinc dibutyldithiocarbamate, low concentrations of hydrogen sulfide acting as a catalyst for this reaction. At higher temperatures, the reaction is also exclusively disubstitutive, a feature connected with the function of zinc compounds in influencing the cross-linking reaction. The sulfuration of olefins with tetramethylthiuram disulfide at 140° C shows a similar influence of zinc compounds.
Studies of Rubberlike Polymers by Nuclear Magnetism
Authors: Honnold V. R. , McCaffrey F. , Mrowca B. A.
Published: February 2011

The actual width of the proton resonance line in uncured natural rubber has been determined at room temperature to be 0.06 gauss. Curing of polymers increases the line width at a given temperature. The small increase in natural rubber is possibly compatible with a physical bonding rather than the usually assumed cross-linking. For a butadiene-styrene copolymer, the increase of line width due to cure is somewhat larger. Carbon-black loading increases the line width to a lesser degree than does cure. This is compatible with the concept of physical bonding between the blacks and the polymer chain molecules. Variations of line width caused by changes in chemical composition and copolymerization were also investigated. Polypropylene and polypropylene oxide of roughly the same average molecular weight are compared. The polypropylene oxide exhibits a greater degree of “rotation” about its C—O bonds than polypropylene does about its C—C bonds. Two butadiene-acrylonitrile copolymers also have been studied as a function of temperature. Finally, spin-lattice relaxation time vs. temperature studies are reported for a butadiene-acrylonitrile copolymer and for raw Butyl, over the temperature range from −50° to 70° C. Estimates of the magnitude of the barriers hindering “rotation” are made.
Some Osmotic Measurements of the Molecular Weight of a High Molecular-Weight Fraction of Rubber Hydrocarbon
Authors: Drake G. W.
Published: February 2011

Fractionation of the rubber hydrocarbon in temperate climates has usually resulted in high molecular-weight fractions, with a molecular weight of the order of one million. Bloomfield has shown that fresh latex contains a considerable proportion of hydrocarbon having an intrinsic viscosity (η) of 10 or over and, therefore, a molecular weight of well over 106. The fractionation technique used by Bloomfield in Malaya has now been applied by the writer to smoked sheet and to F rubber, working in the United Kingdom. No very high molecular-weight fractions were found in the smoked sheet, but the F rubber yielded a fraction of (η)=7.3 and a number average molecular weight 6×106, determined osmometrically. The average molecular weight of natural rubber when freshly prepared is probably well over a million, and includes a substantial portion having a molecular weight of several millions. By the time smoked sheet has reached temperate climates, the high molecular-weight portion has probably been converted to gel. F rubber, presumably because of its different method of preparation, retains the major part of its high molecular-weight material during prolonged storage.
Pressure Coefficient of Resistance of Conductive Rubber
Authors: Walker D. L.
Published: February 2011

The pressure coefficients of resistance of several black-loaded compounds of natural and synthetic polymers were measured at temperatures from 0° to 80° C. The effect is shown to be due to variations of the contact resistance between adjacent black particles.
Measurement of Steady Flow Characteristics of Rubber at High Rates of Shear
Authors: Bulgin D. , Wratten R.
Published: February 2011

A coaxial cylinder viscometer suitable for use with long-chain polymers and technical rubberlike materials is described. Special features of the apparatus are the use of thin films to reduce heating, the ease of obtaining end corrections, and the use of high pressures to eliminate slip and facilitate filling the shearing gap. Results with uncompounded rubber show that the shear stress converges to a value of approximately 2.5 dynes per sq. cm. at shear rates in the region of 100, and that this value is independent of temperature (90° to 150° C) and of the chain length of the polymer. Certain polymers do not show this convergence.
The Mooney Viscometer
Authors: Baader Th
Published: February 2011

The preparation and forming of test-specimens for the Mooney viscometer by compression between the stator and rotor is accomplished in a rather crude way, and certainly more care should be taken than is evidently considered necessary. Investigations have shown that, by the use of pieces of rubber stock of as nearly the same size as possible as the cavity between the stator and rotor, several sources of error and certainly disturbing factors can be avoided, or at least minimized. As a good example of such errors are the considerable internal stresses which affect the measurements. In Figure 1, the effects caused by internal stresses, which, after completion of a test and removal of the specimen from the rotor, cause shrinkage and swelling, are easily recognizable by a comparison, to scale, with a vulcanized specimen whose cross-section is the same as that of the unvulcanized specimen as it fills the cavity between the rotor and stator. The internal stresses result in a hardening of the test-specimen, which may increase the true Mooney viscosity by as much as 5 per cent.
Chlorination of Natural Rubber Solutions by Means of Phenyl Iododichloride
Authors: Ramakrishnan C. S. , Raghunath D. , Pande J. B.
Published: February 2011

The loss of unsaturation attending the chlorination of carbon tetrachloride solutions of unmasticated pale crepe and deproteinized rubbers by means of phenyl iododichloride was followed by quantitative ozonization of the chlorinated products, ranging in chlorine contents from 2.3 to 73.6 per cent. It is concluded from such studies that the reaction proceeds mainly by the additive reaction of the chlorine radicals produced by the thermal decomposition of phenyl iododichloride. The hydrogen chloride found in the preparation of these chlorinated products was determined. It has a value below 1 per cent up to about 15 per cent chlorine content and thereafter it increases. However, this value generally does not exceed 4 per cent. This indicates the occurrence of only a slight amount of substitutive reaction by chlorine. Cyclization also takes place, probably following an additive reaction of the chlorine radicals. The main difference between the deproteinized rubber and pale crepe rubber reactions is confined to the very early stages, wherein the nonrubber components have a directive influence in bringing about a slightly earlier onset of cyclization. It is also concluded that atmospheric oxygen has a negligible effect on the reaction.
Nonrubber Components of Ammonia-Preserved Latex. An Analytical Study
Authors: Tunnicliffe M. E.
Published: February 2011

Analyses of ammonia-preserved latex concentrates have been made in order to study some of the main features of composition of the nonrubber material at the time of using the latex, usually about three months after tapping on the plantations. The information thus obtained is supplementary to corresponding work carried out on fresh latex in Malaya. Latex, at the stage now being considered, contains no phospholipin as such; but its decomposition products, ammonium soaps, free choline, and combined phosphate (probably glycerophosphate) are found. There is protein in the serum and at the rubber interface, a small amount of free amino acid, and probably also substances of a polypeptide nature. The serum contains salts of volatile and nonvolatile acids, in some cases as ammonium salts, and also a considerable amount of quebrachitol. The rubber phase contains phytosteryl esters. From the analyses of an electrodecanted and a centrifuged concentrate prepared from a common field latex it is shown that the higher amounts of some, but not all, nonrubber components in the electrodecanted concentrate are in accordance with its having a smaller average rubber particle size. The excess of both protein and phospholipin breakdown products indicates the existence of a protein-phospholipin complex. Analysis of the serum remaining after electrodecantation and of the original field latex has enabled a tentative distribution of nonrubber material to be calculated between serum and rubber phases.
The Thermal Effect in the Elongation and Relaxation of Rubber
Authors: Ehrbar J. P. , Boissonas Ch G.
Published: February 2011

Samples of natural rubber and synthetic rubber were stretched, and the temperature rises were followed by a thermocouple. The heat evolution was found to correspond to an adiabatic process. The study of the dependence of the heat effect (the Gough-Joule effect) on time shows that, while stretched, rubber crystallizes slowly and the phenomenon can be followed experimentally for several hours. The final stage of crystallization is probably a first-order reaction, the velocity constant of which is nearly independent of the percentage elongation and which has a small temperature coefficient. On the contrary, fusion following release of the applied stress is relatively rapid.
Thermodynamics of Crystallization in High Polymers. Natural Rubber
Authors: Roberts Donald E. , Mandelkern Leo
Published: February 2011

The existence of an equilibrium melting temperature, T0m, at 28 ± 1°, for unstretched natural rubber has been established, using dilatometric methods. The lower melting temperatures previously observed are a consequence of the low temperatures of crystallization and the rapid heating rates employed. From melting point studies of mixtures of the polymer with low molecular-weight diluents, the heat of fusion per repeating unit, ΔHu has been evaluated as 15.3 ± 0.5 cal./g. The values of ΔHu and T0m have then been combined with data of other workers to obtain the following information concerning natural rubber: (1) The variation of melting temperature with applied hydrostatic pressure has been calculated from the Clapeyron equation to be 0.0465° C/atm. (2) The degree of erystallinity resulting from maintaining a sample at 0° until the rate of crystallization is negligible has been calculated, by three independent methods, to be in the range 26 to 31 per cent. (3) Analysis of the stress-strain-temperature relationship has indicated that crystallization is the cause of the large internal energy changes that are observed at relatively high elongations.
X-Ray Investigation of the Amorphous State of Rubber
Authors: Kasatochkin V. I. , Lukin B. V.
Published: February 2011

The x-ray diffraction pattern of amorphous rubber, which is an amorphous ring, resembles the pattern of low-molecular liquids. In our previous work, it was established that the diffraction pattern observed is due to the coherent scattering of only those segments of the molecular chains in which the aggregation is analogous to that of low-molecular liquids, and is determined by the presence of a pseudo-order. A large part of the links of the molecular chains, owing to the prevailing disorder, scatters the x-rays incoherently, like scattering by a gas. For one component of amorphous rubber, the concept of “liquid phase” was introduced, and, for the other, that of “gaseous phase”, thereby subdividing them according to the type of scattering of x-rays. Amorphous rubber, according to our data, contains a large number of chair segments which are characterized by a high degree of disorder. The presence of such a disordered molecular phase is a general and characteristic property of high-molecular substances, and is caused by natural obstacles in the dense packing of the large molecules. This characteristic of molecular aggregation is undoubtedly reflected in the physical-mechanical properties of polymers.
Molecular Weights of Emulsion Polydienes
Authors: Morton Maurice
Published: February 2011

In the polymerization of dienes there is always the possibility of a cross-linking reaction between the polydiene chains because of the residual double bonds in the chains. Such a reaction can eventually lead to the formation of “infinite networks” which are characterized by loss of solubility (gel). The statistics, mechanism, and kinetics of the formation of such networks during polymerization have been treated in recent years. From this treatment it becomes apparent that the onset of gel occurs when there is one cross-linked monomeric unit per weight-average primary chain. The latter term refers to the polymeric chain uncontaminated by any cross-links. Recent data have shown that the rate of the cross-linking reaction is relatively slow compared to the propagation reaction during a polymerization, and that polydiene chains can reach lengths of at least several thousand units without including a cross-linked unit. This permits the attainment of synthetic rubber of reasonably high molecular weight without the inclusion of an insoluble gel fraction. The emulsion polymerization system, which is used so extensively in the synthetic rubber industry, is governed by special considerations, as regards both the cross-linking reaction and the factors controlling the chain length. Because the bulk of the polymerization occurs within the polymer particles, which are kept more or less saturated with monomer, their concentration remains relatively constant as long as any free monomer phase exists (50 to 60 per cent conversion). Hence, unlike the case of bulk polymerization, the rate of the cross-linking reaction (as well as that of the polymerization reaction) remains constant up to 50 to 60 per cent conversion. In other words, in emulsion systems, the cross-linking reaction proceeds at a uniform but faster rate than in bulk systems during the first half of the polymerization, owing to the higher concentration of polymer at the locus of reaction. Beyond that point, the cross-linking rate increases as the polymer concentration increases.
Reaction of a Thiuram and Sulfur
Authors: Blokh G. A. , Sazonova L. P.
Published: February 2011

In a previous article the occurrence of an intensive interchange between atoms of elemental sulfur when present as vulcanizing agent and the sulfur atoms of the mercapto group of mercaptobenzothiazole as accelerator was shown by the isotopic method, i.e., by the application of tagged atoms of radioactive sulfur. Thus, the existence of a reaction between the accelerator and the vulcanizing agent during the vulcanization of rubber was established. The application of isotopes as tagged atoms is an effective new method of investigating the chemical structure of substances, their reactivities, and the mechanism of chemical reactions in which they take part. This method is being successfully developed in our country, particularly by the work of Brodskii˘ and his pupils. The present study is devoted to the problem of the reaction of tetramethylthiuram disulfide (hereafter called thiuram) and elemental sulfur. Studies in this field are of great scientific and practical interest, since the mechanism of the accelerating action of thiuram is not yet clear. We assume that the chemistry of the reaction is as follows. Thiuram is decomposed during vulcanization, with the formation of a dithiocarbamate and free active sulfur. Because of this, the thiuram can vulcanize rubber without the addition of elemental sulfur to the mixture. Such vulcanizates are particularly widely used in the cable industry.
Studies of the Mechanism of the Mastication of Rubber. VIII. The Reaction of Natural Rubber with 3,5-Dibromobenzoyl Peroxide
Authors: Imoto Minoru , Kiriyama Shigeru
Published: February 2011

The reaction of a natural-rubber solution with 3,5-dibromobenzoyl peroxide was carried out, and the following results were obtained. 1. Lowering of the viscosity of the rubber solution was observed. 2. Bromine contents in the degraded rubber were determined. 3. The molecular weight of the degraded rubber was determined. The reaction of rubber with DBPO is discussed on the basis of these observations.
Importance of the Polar Component of Mixed Fluids in the Swelling of Vulcanizates
Authors: Ermolenko I. N. , Mazel M. I. , Ermolenko N. F.
Published: February 2011

The problem of the relation of the swelling of vulcanizates to the nature of the liquid medium is very important in explaining the nature of vulcanization and determining the nature of the bond between the separate macromolecules in the vulcanizate. The relation of the swelling of vulcanizates to the molecular polarization of mixed liquids has been studied several times; however, the mechanism of the reaction of the polar component of a mixed liquid with the rubber molecules during swelling is still not very clear. According to contemporary theory, the swelling of polymers, particularly rubber, is thermodynamically analogous to the mixing of low-molecular liquids. Swelling is caused by a decrease of the free energy of the system: ΔG=ΔH−T⋅ΔS, which in the case of nonpolar polymers is due principally to an increase of entropy, T⋅ΔS, as a result of the separation of the molecular chains and their increased elasticity. For polar polymers, ΔH is the important factor. In this work rubber is regarded as a polymer in which the molecular chains are bound by nonequivalent bonds with varying amounts of energy, as appears in the swelling process, and not as a union of molecular chains surrounded by a uniformly strong field. It is evident from other studies that the latter assumption is not entirely correct. On the basis of a study of the protective action of antioxidants during the oxidation of rubber by oxygen, one of the authors regarded the mechanism of this action as the result of solvation of the antioxidant in the polar segments of the rubber molecule by the polar molecules.
Thermodynamics of the Swelling of Natural Rubber and Sodium-Butadiene Rubber in Binary Mixtures
Authors: Starobinets G. L. , Aleksandrovich Kh M.
Published: February 2011

1. The swelling of vulcanizates of natural rubber and sodium-butadiene rubber in the saturated vapors of binary systems of benzene and low-molecular alcohols (from methyl to butyl) at 20° and 50° C was studied in the entire range of composition of binary solutions. 2. The general form of the curves of isothermic equilibrium between the swollen polymer and binary solution were found to conform to Konovalov's law. 3. The absorption of a binary solution by both polymers has a very pronounced selective character. The selectivity of absorption of the solution is more pronounced, the greater is the density of energy of cohesion of the alcohol. In natural rubber, the capacity of selective absorption is more pronounced than in sodium-butadiene rubber. This is probably due to the larger concentration of oxidized groups in natural rubber. 4. The changes of the free energy, the heat content, and entropy for the passage of one gram-mole of alcohol from an infinitely dilute solution into the polymer were calculated from the experimental data. These calculations indicate that the reaction of the alcohol molecules with the polar groups of the polymer is dipole-dipole. 5. The passage from pure benzene to a swollen polymer of any particular composition is characterized by a large increase of entropy in comparison with the passage to a binary solution of the same composition. The relation in alcohol is the reverse. The increase of the entropy of benzene can serve as a measure of the elasticity of the polymer chains. A decrease of the entropy of alcohols indicates the content of free polar groups in the polymer phase. 6. The values of the partial heat content of the low-molecular components are positive; consequently, the decrease of free energy during their sorption by the polymer is due exclusively to an increase of entropy.
Performance of Carbon Blacks. Influence of Surface Roughness and Porosity
Authors: Dannenberg E. M. , Boonstra B. B. S. T.
Published: February 2011

Porous blacks of high surface area can be produced without changing particle size either directly, in the normal furnace black process, or indirectly, in the laboratory by the air oxidation of a normal commercial and essentially nonporous HAF black raw material. Blacks of high surface area produced by the direct process are characterized by a retardation of rate of vulcanization, lower rebound resilience, and high electrical conductivity. Laboratory-prepared products of high area show cure retardation, but their electrically conductive and resilience properties show relatively little dependence on the degree of porosity and surface area. Bound rubber measurements, modulus, tensile strength, and abrasion properties of the laboratory-oxidized samples indicate an increase of reinforcement with increasing surface area.
The Abrasion of Rubber
Authors: Schallamach A.
Published: February 2011

Theoretical results on the abrasion of rubber have been deduced from a few simple assumptions concerning the initiation of the surface damage. The abrasion is proportional to the normal load, independent of the particle size of the abrasive if the particles are polyhedral, and proportional to their mean radius of curvature if they can be approximated to hemispheres. The spacing of the abrasion pattern is proportional to the cube root of the normal load, proportional to the two-thirds power of the particle size on an abrasive with polyhedral particles, and directly proportional to the particle size on an abrasive with hemispherical particles. These predictions have been reasonably well confirmed by experiment.
Evidence for a Hydrochlorination Retarder within Natural-Rubber Latex Particles
Authors: Gordon Manfred , Taylor James S.
Published: February 2011

Kinetic evidence is presented for the occurrence in Hevea latex particles of a trace substance that retards the hydrochlorination reaction. No retardation effect is observed with synthetic polyisoprene latex. The chemical nature of the retarder in Hevea latex particles is not known, but it is likely to be a basic substance. Its retardation action reflects its power to deactivate the reactive precursor of the hydrochlorination reaction, which is interpreted as a form of ion pair (H+, Cl−). A constant amount of the precursor is deactivated by the retarder, irrespective of the temperature or the pressure of the hydrochlorination reaction.
Nature of Stark Rubber
Authors: Roberts Donald E. , Mandelkern Leo
Published: February 2011

The melting behavior and x-ray diffraction patterns of four different samples of stark rubber have been investigated. The melting temperatures, 39° to 45.5° C, are substantially higher than that observed for natural rubber crystallized by cooling. The x-ray diffraction patterns indicate that the crystallites in stark rubber are oriented. This observation can explain the higher melting temperatures. Thus, the previous assignment of an equilibrium melting temperature, 28° (±1°) C, to unoriented crystalline natural rubber is shown to be appropriate. Several different methods that have been used successfully in preparing stark rubber under controlled conditions in the laboratory are outlined.
Combination of Rubber and Carbon Black on Cold Milling
Authors: Watson W. F.
Published: February 2011

Although the structure of carbon black is imperfectly known, x-ray analysis has shown it to consist of layers of condensed rings of carbon atoms. Unsaturation and discontinuities in the layers are likely to provide sites for attack of free radicals, and thus make carbon black a radical acceptor of a special polyfunctional type. Assuming combination of rubber radicals and carbon black, a particle could terminate more than one sheared rubber chain. Furthermore, rubber chains attached to a carbon black particle could also undergo scission by shear and be terminated by combination with other particles. The result anticipated from this picture is a network of rubber and carbon black held together by chemical bonds. In rubber solvents, such a network would be insoluble and merely become swollen gel. Insolubilization of rubber on milling with carbon black has been reported on occasion, but the evidence is not sufficiently extensive to draw reliable conclusions as to its cause. This paper reports a systematic investigation of the occurrence of rubber-carbon black gel on cold milling. This gel has been shown to form, and the conditions for its formation and its properties have been interpreted as strong evidence for the above hypothesis of chemical-bond attachment of rubber and carbon black.
Determination of the Fatigue Resistance of Rubber Parts under Cyclic Shear Stress in Relation to Their Shape and Energy
Authors: Roelig Hermann , Fromandi Guido
Published: February 2011

Rubber is used far less frequently in tension than in compression for spring suspensions and vibration isolation. But for particularly soft springs, it is used in shear. For example, the compression modulus (at a deformation of 10 per cent of the height of the specimen) of a rubber cylinder 25 mm. in diameter and 25 mm. high, of 60° Shore hardness, may be 45 kg. per sq. cm., while its shearing modulus is only 7 kg. per sq. cm. Thus one may obtain a 6.5 times softer spring suspension by a shear mounting for the same volume of rubber. The use of rubber shear suspensions is facilitated by rubber-metal bonds, the quality of which today can be safely controlled technically in rubber factories. Thus we find shear suspensions in rubber-cushioned railway cars and automobile and aircraft construction. The exact design of such spring suspensions still is handicapped by lack of precise data on their fatigue strength, which can be obtained from dynamic tests. Such information would also facilitate the study of new elastomers and their comparison with those currently in use.
New Instruments for Measuring Hardness
Authors: Oberto Stefano
Published: February 2011

This report describes the characteristics of an instrument for measuring the hardness of rubber which has been designed in the laboratories of Pirelli S.p.A., and which is characterized by both precision and efficiency. The instrument meets the requirements for measuring hardness in International Rubber Hardness Degrees in accordance with the recommendation developed by the ISO/TC 45, the essential features of which are summarized. The usefulness of a hardness test for determining shear modulus, even when determined statically, and also the means of allowing, at least in part, for the associated after elastic effects are discussed. Finally, a durometer for determining hardness, using the same units, and of service when the thickness of the rubber sample is of the order of one millimeter, is described.
Rubber Hardness Testing
Authors: Scott J. R.
Published: February 2011

Dictionaries give a confusing array of explanations of “hard” and “hardness”. A hard material, according to the Oxford English Dictionary, is one “that does not yield to blows or pressure; not easily penetrated or separated into particles; firm and resisting to the touch …”. “Hardness” implies “difficulty of penetration, inflexibility, rigidity, stiffness …”. Even the earlier scientific workers on rubber were equally vague; thus, Dawson in 1927 regarded hardness as “an ill-defined little understood property” and considered that resistance to penetration—the practical criterion of hardness—“brings into play not merely forces associated with the surface cohesion, but also more deep-seated disturbances connected with elasticity, plastic deformation (or set), internal friction, resilience, and energy absorption”—truly an intractable mixture! The practical man, disregarding theoretical difficulties, has always judged hardness by some form of indentation test, if only with the thumb-nail or a pencil point, and out of such crude beginnings have grown the precise standardized tests of today.
Ozone Resistance of Butyl Vulcanizates
Authors: Edwards D. C. , Storey E. B.
Published: February 2011

A study of the cracking behavior of Butyl-rubber vulcanizates, exposed under static strain in an atmosphere containing 25 parts of ozone per 100 million parts by volume of air, is described. An account is given of sampling and testing procedures, including the development of a means of rating the severity of cracking in triangular strip specimens. The factors studied include polymer unsaturation, black and oil loadings, antioxidants, state of cure, and mixing variations. Cracking increases moderately with increasing raw polymer unsaturation, and is markedly reduced with increasing state of cure. Black loadings in excess of 40 parts per 100 parts of polymer are desirable. Plasticizers are generally detrimental to ozone resistance; the effect is proportaional to plasticizer loading, but varies considerably between plasticizers. The antioxidants tested were all of some benefit, but none offered complete protection at conventional loadings. Variations of laboratory mixing and processing techniques caused no substantial change in cracking behavior. By the careful choice of compounding ingredients and the use of a high state of cure, it is possible to prepare low-cost, highly resistant compounds without reliance on waxes or excessive antioxidant loadings.
Ozone Crazing of Biaxially Stressed GR-S Vulcanizates
Authors: Ossefort Z. T. , Touhey W. J.
Published: February 2011

Typical crazing of stressed GR-S vulcanizates can be produced by exposure of biaxially stressed specimens to ozone in the absence of light.
Correlation of Room-Temperature Shelf Aging with Accelerated Aging
Authors: Youmans R. A. , Maassen G. C.
Published: February 2011

In 1931, a series of compounds was mixed to obtain data for publication purposes. After the necessary data for use at that time had been obtained, the remaining vulcanized test slabs of each compound were tied together in a bundle and stored in a desk drawer without wrapping. The compounds used for this series of tests contained different percentages of antioxidant. After being stored for 22 years, the several stocks were analyzed to determine whether there had been any migration of antioxidant from stocks containing antioxidant to those containing no antioxidant. A definite migration was found; compounds that had originally contained no antioxidant definitely showed its presence. It was, therefore, decided in this work to use a compound that had originally contained antioxidant, so that the results would not be affected by the gradual acquisition of an antioxidant.
The Shrinkage of Mold Cured Elastomer Compositions
Authors: Juve A. E. , Beatty J. R.
Published: February 2011

The coefficient of expansion of elastomer compositions depends on the kind of polymer employed, the volume per cent of polymer plus all other organic materials, and the kind and the amount of pigment present. The specific effects of pigments in retarding the shrinkage are shown. A procedure is given by which the shrinkage of most soft rubber compositions of the principal rubbers may be calculated from the recipe. The major part of the shrinkage of elastomer compositions is due to the much greater thermal contraction of the composition than of the material from which the mold is made. This portion of the shrinkage depends on the thermal coefficient of expansion of the composition and the mold material and the temperature difference between the curing temperature and room temperature. A minor part of the shrinkage is due to a contraction of volume resulting form vulcanization. For normal soft rubber stocks with sulfur ratios of three parts or less and for normal Neoprene stocks, this factor may be neglected. For stocks having higher sulfur ratios, an allowance needs to be made for this factor.
Temperature Control during Mixing of Rubber Compounds
Authors: Roth Frank L. , Decker George E. , Stiehler Robert D.
Published: February 2011

A study was made of the effect of the surface temperature of the rolls of a 6 × 12-inch laboratory mill on the properties of GR-S compounds and means of controlling this temperature. It was found that the properties of the GR-S compound depend on the surface temperature of the mixing rolls. No practical method of controlling the temperature of conventional rolls was found because of the slow rate of heat transfer. Therefore three types of special rolls, including one designed at the National Bureau of Standards, were investigated. The heat transfer characteristics of the NBS rolls permitted automatic control of their temperatures to within 7° F during mixing of natural rubber and several synthetic elastomers, with only one controller being used. A mill with the special NBS rolls has been in operation for about a year, and has resulted in a marked improvement in the control of mixing of rubber compounds. The automatic temperature control has been much more convenient than manual control, and has saved considerable time, since the rolls do not have to be warmed by milling scrap rubber.
Influence of Salt Coagulants on the Properties and on the Aging of Rubber Products Made by the Dipping Process
Authors: Thorsrud A.
Published: February 2011

The limited time which we had for this work permitted us to give only an outline of the effects of different salts on processing and on the properties, notably aging, of rubber articles prepared by the dipping process. Nevertheless this work justifies the following conclusions. 1. Ammonium salts have a thickening effect on a latex mixture, the extent of which depends on the nature of the anion. 2. Ammonium salts and salts of bivalent cations influence the wetting of the form by the latex mixture and, in addition, they have an effect on the uniformity of the deposit. 3. The rate of vulcanization of rubber containing a salt is greatly retarded and, in general, this retardation increases with the content of the salt. 4. Tensile strength is considerably lowered, especially by salts of bivalent cations; on the other hand it is only slightly influenced by ammonium salts. 5. Cations have a much more pronounced effect on the rate of vulcanization than do anions. 6. In a general way all the salts have a deleterious effect on the aging of vulcanizates, although it is impossible to differentiate with certainty between the effects of the anion and the cation. However, it would seem that chlorides have a more deleterious effect than acetates. Nitrates are definitely less harmful. 7. The hygroscopicity of the salts influences the rate of vulcanization, but it does not play so important a role in the aging of latex products as one might be led to expect a priori. 8. The natural aging of test samples containing barium chloride, calcium chloride and calcium acetate is particularly poor. In addition, the present work confirms the fact that different results are obtained by different methods of aging and seems to make it advisable to replace calcium chloride by calcium nitrate. Nevertheless, the avoidance of inorganic salts of any kind is recommended. Finally it should be pointed out that the structure of rubber in a product which has been washed (microporous) is not the same as that of rubber which has not been washed, as in the present work; consequently one cannot apply these conclusions to latex products which have not been through the washing process.
Effect of Sulfuric Acid Coagulation on Properties of Natural Rubber
Authors: Best L. L. , Morrell S. H.
Published: February 2011

The data indicate that the use of sulfuric acid in normal amounts as a coagulant has no deleterious effect on the properties of the raw rubber or on vulcanizates made from the raw rubber. When four times the normal dosage of sulfuric acid was employed, there was a slight softening of the raw rubber, but all samples examined had properties within the normal range of plasticity associated with natural rubber. The tendency to scorch and the rate of vulcanization of rubbers do not appear to be affected appreciably by the method of coagulation: any differences which may exist are not of such a magnitude as to cause the production of abnormal rubbers. It would seem that the use of excessive quantities of sulfuric acid during the coagulation of latex may cause some deterioration to the rubber. The sole advantage in using excess would appear to be a reduction of the time needed to complete coagulation. It should be noted, however, that the main reason for the use of sulfuric acid is economic, and it does not seem logical that the producer would use more than is necessary and thereby increase his coagulation costs. If the producer were to use four times the normal dosage of sulfuric acid, the material costs would be about twice those for the normal quantity of formic acid.
The Significance of the Yellow Fraction in Spontaneous Coagulation
Authors: Paton F. J.
Published: February 2011

The characteristics of the heavier yellow fraction which is separated on centrifuging fresh latex have been described in the literature. This material is made up of a loose aggregate of particles which have a relatively high specific gravity. It can be dispersed in dilute salt solutions, but tends to coalesce into an irreversible precipitate when diluted with water. It is still not quite clear what happens when water is added to the yellow fraction. After diluting latex with water, the formation of a layer at the bottom of the tube no longer takes place, the flocculated rubber carrying the bulk of the aggregate to the top of the tube. The observed phenomena can be explained on the assumption that the yellow fraction is made up of a number of cells, and some evidence for the existence of membranes has been offered by Ruinen. On such an assumption, they can be accounted for as osmotic phenomena. However, this explanation is not entirely satisfactory, since volume shrinkage does not appear to take place in a hypertonic solution.
X-Ray Determination of Crystallinity in Deformed Rubber
Authors: Nyburg S. C.
Published: February 2011

A vulcanized natural rubber has been deformed in simple extension and in pure shear and the resulting diminution of x-ray intensity of the amorphous halo used as a measure of crystallinity. The effect of mode of deformation and attempts to reach equilibrium states by heat treatment at constant deformation are described. The values obtained for simple extension are in accord with those reported by Goppel and not with those of Field. The degree of crystallinity is greater in pure shear than in simple extension at the same principal extension ratio. Corrected values for the orientation of crystalline regions have been obtained, and a comparison is made between volume changes and x-ray measurements for simple extension.
Dynamic Mechanical Properties of a Number of Elastomers and Related Polymers from 4 to 250° K
Authors: Morgan R. J. , Nielsen L. E. , Buchdahl R.
Published: February 2011

Young's modulus and mechanical damping have been measured down to 4° K for cis- and trans-polybutadienes, cis- and trans-polyisoprenes, butyl rubber, and random SBR copolymers. Trans-polybutadiene has two large damping peaks below Tg, trans-polyisoprene one peak, while the cis-polymers and butyl rubber show either no damping peaks or only very small ones below Tg. The low-temperature damping peaks of the trans-polymers might be explained by a type of crankshaft motion and by a special motion of the trans-monomeric unit which involves a simultaneous vibration and bond angle deformation. The high Young's moduli of the polybutadienes compared to the polyisoprenes and the SBR copolymers are explained by the good molecular packing of the polybutadienes.