bluuebunny/crossref_metadata_2025 huggingface dataset:
Vulcanization of Elastomers. 22. Thermal Vulcanization of Synthetic Rubbers. I
Authors: Scheele Walter , Stemmer Hans Dieter
Published: February 2011

In this work, the kinetics of the thermal vulcanization of Perbunan were studied with and without additives. The following results were obtained : 1. The pure thermal vulcanization of Perbunan is a very slow process which obeys a second order rate law. A limiting value of the crosslinking (reciprocal limit of equilibrium swelling) is reached, which limit is independent of the temperature. The activation energy is 23.3 kcal/mole. 2. The thermal vulcanization can be inhibited by hydroquinone but not by benzoquinone. 3. The thermal vulcanization of Perbunan can be considerably accelerated by MBTS, and other materials, and the reaction also follows a second order course. The activation starts suddenly after the expiration of an induction period, which decreases with increase in temperature. The activation energy is about 27 kcal/mole. 4. In a thermal vulcanization accelerated with MBTS, a portion of the MBTS is changed over into MBT ; the amount changed is independent of temperature. Perbunan takes up MBTS in the form of molecular fragments, in stable combination. 5. The reduction in MBTS (which falls to zero) and the increase in MBT follow a first order reaction and have the same activation energy which is also identical with the energy of activation of the accelerated crosslinking. The formation of MBT is the slower of the two reactions. 6. The rate constants for the decrease in MBTS and for the increase in MBT are independent of the starting amount of MBTS, and hence we consider that this is a unimolecular process (homolysis). 7. The rate constant for the second order crosslinking reaction increases with the square root of the initial benzothiazolyl disulfide content. 8. It is indicated that the above data must be explained, with the aid of experience in the realm of polymerization kinetics. The investigations are being continued.
The Tensile Strength of Elastomers According to Current Theories
Authors: Bueche F.
Published: February 2011

In summary, it is probably true to say that the basic mechanisms responsible for the tensile strength of rubbers are now mostly known. Some disagreement exists concerning the exact role played by crystallinity and filler action. Opinions concerning the way in which the fracture propagates through the sample also differ. It is clear that all the present theories concerning tensile strength are inadequate in many ways. Although the basic ideas involved may be quite correct, much refinement in these theories is still needed. They are only rather crude first approximations to a fully successful theory of tensile strength. It is hoped that this review has been successful in pointing out the successes and failures of current theories so that improvements may be made in them.
Analysis of Rubber
Authors: Burger V. L.
Published: February 2011

In this paper it has been assumed that a completely unknown vulcanized sample is in question for analysis. Although this occurs very rarely in practice it is thought that such an approach would bring out the more important factors in desirable sequence. Thus, although general sources of information are worth first consideration, one might well begin an analysis with identification and determination of elastomers and continue with analyses for sulfur in various forms, accelerators, activators, antioxidants, reinforcers, fillers, etc. In a later section some attention will be given to analyses of importance as rubber goods are being manufactured; that is, tests on crude rubber and some of the principal compounding materials.
The Preservation and Concentration of Hevea Latex
Authors: McGavack John
Published: February 2011

Some twenty years ago the author stated that, “in the simplest form latex at the time of tapping may be defined as an aqueous buffered neutral system containing water-soluble crystalloids and fatty acid esters, in which small rubber particles are dispersed by means of a vegetable protein—said protein operating on the basic side of its iso-electric point”. Since that time we have had more information about the crystalloids and about the fatty acid esters but essentially the definition still holds. He also went on to say, “it is desired to use this word ‘preservation’ in its broad sense, that is, the maintenance or improvement of the system in all of its aspects … by the term preservation we are including not only methods that prevent bacterial action or guard against outside infection, but also methods which involve the use of actual stabilizers for the improvement of mechanical stability. In the first case we deal with disturbances that occur within the material itself. In the second case we deal with disturbances that occur either within or without. It is essential that a preserving substance should prevent bacterial action or fermentation of the protein or like materials. It is also essential to maintain the original state of dispersion when any serious stress is put upon it”. In this article we will attempt to outline the more recent, broader aspects of preservation which are essential to the commercial use of natural latex.
Effect of Oxidation of Rubber on the Kinetics of Its Swelling
Authors: Gul V. E. , Khodzhaeva I. V. , Dogadkin B. A.
Published: February 2011

1. Swelling of rubber during reaction with oxygen is described by a kinetic curve. After a sharply rising segment, corresponding to the diffusion of the solvent, the curve has a practically linear segment, reflecting a slow change of the degree of swelling due to the oxidation processes which accompany swelling. 2. The swelling maximum of rubber in a polar solvent when the oxidation effect is disregarded, corresponds to the molecular weight, degree of dispersion, and other structural properties of the rubber. The slope of the linear segment of the kinetic curve of swelling can serve as a characteristic of the resistance of the rubber to oxidation.
The Effect of Ozone on Neoprene Vulcanizates and the Influence of Some Protective Agents and Fillers
Authors: Hartmann A. , Glander F.
Published: February 2011

In this investigation an attempt was made to show a certain analogy between the nature of the phenomenon of the crystallization of an undercooled melt and ozone crack formation in stretched natural or synthetic rubber, in order in this way perhaps to have some starting point for a more detailed experimental investigation of these interesting problems. At the same time the observed qualitative differences in the effects of ozone on natural rubber vulcanizates and on Neoprene vulcanizates might be explained. In the experimental part it is first shown, with the help of a simplified experimental method, that the rate of crack growth can be considerably reduced by the presence of protective agents. In addition, a great change in the temperature dependence of crack growth is observed under certain conditions. It can not yet be decided whether or not the rate of nuclei formation is similarly influenced. Furthermore, vulcanizates containing different fillers and different Neoprene contents have been examined.
Stress Relaxation of Vulcanized Rubber in Compression and Tension
Authors: Eller S. A.
Published: February 2011

Stress relaxation is an important consideration in determining the suitability of gasket materials for Navy door hatches, pipe lines, and similar applications. In view of this, the Material Laboratory, New York Naval Shipyard, has devoted considerable time and effort to the design of equipment to measure stress relaxation of rubber materials deformed in compression and in tension.
Reinforcing Effect of Condensation Polymers on Rubber in Connection with Their Particle Size
Authors: Houwink R. , van Alphen J.
Published: February 2011

It is still an open question to what extent the reinforcing effect of a filler in rubber is due to physical or chemical bonds. There is abundant evidence that the reinforcing effect with regard to modulus, tear strength, and abrasion resistance increases with decreasing particle size. Arguments have been brought forward, however, that below a certain particle size this reinforcing effect diminishes. In most of the experiments involved, carbon black has been used as a filler. In the present paper, data regarding the reinforcing effect of some condensation resins of different particle size are presented in order to bring more light on these questions.
Effect of Heat Treatment on Reinforcing Properties of Carbon Black
Authors: Schaeffer W. D. , Smith W. R.
Published: February 2011

The high degree of stiffness or modulus which reinforcing carbon blacks impart to rubber has often been associated with reinforcement. Modulus appears to be associated with the chemical nature of the carbon black surface ; when the carbon black surface is cleaned of combined oxygen and hydrogen, a drastic drop in modulus occurs, and this is not accompanied by an equally drastic decrease in tire road wear. Reinforcing and semireinforcing carbon blacks have been heat-treated at successive increments through a temperature range of 1000° to 2700° C. Treatment up to 1500° results in removal of all combined oxygen and hydrogen, followed by an increasing degree of graphitization at higher temperatures. These carbon blacks have been compounded in a standard natural-rubber compound and properties evaluated. Modulus is profoundly altered by the chemistry of the carbon surface. Electrical resistivity passes through a minimum at 1500° C. Scorchiness or premature vulcanization improves with removal of volatile matter. The degree of graphitization of the carbon has only a minor effect on rubber properties. A highly graphitized carbon black still imparts a high degree of resistance to abrasive wear to tire treads.
Studies of the Vulcanization of High Elastic Polymers. III. Vulcanization of Natural Rubber with Thiuram Disulfides. Part 3
Authors: Scheele Walter , Lorenz Otto , Dummer Wilhelm
Published: February 2011

The present paper, which relates again to the problem of the kinetics of thiuram vulcanization, has led to the following results : 1. The velocity constants kTD of the interaction of tetramethyl-, tetraethyl-, and tetrapropylthiuram disulfides with natural rubber, which proceeds according to a first-order reaction, are linear functions of the reciprocal of the absolute temperature. This is true also of the velocity constants kDC of the dithiocarbamate formation. 2. The activation energies of both reactions are independent of the particular chemical constitution of the thiuram disulfide involved. 3. The different thiuram disulfides vary with respect to their reaction with rubber only by the steric factor. 4. The reaction mechanism of thiuram vulcanization has been discussed in the light of the results of the kinetic investigation. These studies are being continued.
Mechanism of the Exchange Reaction of Elemental Sulfur with Mercaptobenzothiazole
Authors: Blokh G. A. , Golubkova E. A. , Miklukhin G. P.
Published: February 2011

As was shown in our preceding study, a rapid exchange of sulfur atoms takes place between elemental sulfur and 2-mercaptobenzothiazole at temperatures corresponding to the conditions of vulcanization of rubber. This indicates the existence of a reaction between the atoms, and also the formation during this reaction of some kind of intermediate compounds. On the basis of the experimental data, it was established that only one of the sulfur atoms of mercaptobenzothiazole enters into the exchange reaction. It was natural to assume, in view of the technical properties of derivatives of mercaptobenzothiazole and its own chemical properties, that the sulfur of the sulfhydryl group takes part in the reaction. In fact, as is shown in another study, 2-mercaptobenzoxazole (I) has a stronger accelerating effect during vulcanization than 2-aminobenzothiazole (II) or 2-oxybenzothiazole (III). Furthermore, it is known that the sulfur in the sulfhydryl group of mercaptobenzothiazole and its derivatives is considerably more mobile than the sulfur in the ring, and is easily converted into other substituents.
Oxidation of Organic Sulfides. IV. Autoxidation of Cyclohex-2-Enyl Methyl Sulfide
Authors: Bateman L. , Shipley F. W.
Published: February 2011

Cyclohex-2-enyl methyl sulfide readily absorbs up to about 0.3 mole of oxygen per mole at 55° to yield a complex mixture. Cyclohexenyl methyl sulfoxide, 6-methylthiocyclohex-2-enone (VIII), cyclohexenone, cyclohexenol, water, and dimethyl disulfide have been identified and estimated approximately quantitatively. A reaction mechanism consistent with these and other experimental data is presented and discussed. Several new keto-sulfides which were required as reference compounds have been prepared and characterized; the sulfide (VIII) and its 3-methylthioisomer (III) show marked antioxidant activity.
Temperatures of Vitrification and Fluidity of Natural Rubber of Different Molecular Weights
Authors: Tager A. , Iovlova M. , Kantor T. , Muzheva L.
Published: February 2011

The vitrification temperature of natural rubbers does not change with change of molecular weight, whereas the higher the molecular weight of the rubber, the higher is its fluidity temperature.
Stress Relaxation of Peroxide and Sulfur Vulcanizates of Natural Rubber
Authors: Berry J. P. , Watson W. F.
Published: February 2011

Direct chemical investigation of the degradation of polymeric networks is usually impracticable owing to the experimental difficulty of insolubility and to the fact that reaction at only a small proportion of network units is sufficient to cause marked alteration of network structure. Resort has, therefore, to be made to the measurement of physical changes brought about by the chemical reactions. Change of tension at constant extension is a useful measure for this purpose, since statistical elasticity theory predicts a direct proportionality between the tension and the number of chains supporting the stress. The present purpose is: (1) to describe a stress relaxometer possessing certain advantages over previous models; (2) to report on the stress relaxation of peroxide-crosslinked rubber, which can be considered from a chemical viewpoint to be the simplest possible rubber network; (3) to point out some complications in the relaxation behavior of sulfur vulcanizates, and (4) to interpret the shapes of stress relaxation curves.
The Plasticization of Butadiene-Styrene Rubber
Authors: Karmin B. , Bets B.
Published: February 2011

1. The kinetics of the plasticization of a butadiene-styrene copolymer on a cold laboratory mill was studied. It was established that, at temperatures of 20–30° C, the plasticity rises steadily and that, consequently, under these conditions a monodirectional destructive process takes place. 2. The kinetics of plasticization of a butadiene-styrene copolymer was investigated in a laboratory banbury at 140° C, both with and without a plasticization aid (chemical). Plasticization at a high temperature is accompanied by the simultaneous operation of the two reactions of destruction and structure formation proceeding at cross-purposes, and may be described by a kinetic equation of the type: P=P0 (1+a⋅m)(1−b⋅n) 3. Plasticized rubbers obtained by breakdown on a cold mill have a smaller capacity for recovery than those obtained by treatment in a boiler or a banbury at temperatures above 120° C. 4. Plasticized rubbers obtained by milling on a cold mill give stocks with higher tensile strength and higher relative elongation than rubbers plasticized by hot treatment. 5. The high recovery capacity of rubbers plasticized at elevated temperatures and the lowering of the physical and mechanical properties of vulcanizates of those rubbers are explained by the branched molecules which they form during structure formation.
Glass Transitions in Polymer-Plasticizer Systems
Authors: Breitman L.
Published: February 2011

A summary of the investigation with the three types of diluent can he made with reference to Table VI. Hydrocarbon oils are more effective low-temperature plasticizers the lower their temperature coefficient of viscosity. Dibutyl sebacate is representative of crystallizing diluents. Such compounds are ineffective low temperature plasticizers unless, (a) at equilibrium, favorable thermodynamic parameters permit their existence as liquids in the polymer, or (b) the plasticizers tend to exist as the nonequilibrium, supercooled, liquids in the transition temperature region of the polymer. Particularly desirable are liquids with low apparent activation energies of viscosity and high miscibility with polymer. The four ester plasticizers, tributoxyethyl phosphate, dibutyl phthalate, dioetyl adipate, and dioctyl sebacate, have temperature coefficients of viscosity corresponding to a virtually common activation energy of 4.5 kilocalories. Here, the magnitude of diluent-polymer interaction, as indicated by volume increase measurements, dictates the order of effectiveness of the ester.
Failure of Stretched Rubbers under the Influence of Ozone
Authors: Zuev Yu S. , Kuzminskiĭ A. S.
Published: February 2011

Cracks perpendicular to the axis of stretching rapidly appear on stretched or bent rubbers exposed to the atmosphere (in the case of loaded vulcanized polybutadiene in the spring and summer months, in a few days). The fundamental cause of this phenomenon is the action of the ozone which is always present in the atmosphere in concentrations of several millionths of a per cent. However, we still lack any clear idea of the mechanism of this phenomenon, which would make possible a rational determination of ways to protect rubber. Questions related to the mechanism of ozone cracking of rubbers are discussed in this article on the basis of new experimental data. Causes of the formation of cracks. The presence of a stress is an indispensable condition for ozone deterioration. It is believed that the role of the stress can be summed up as follows. 1. The rubber molecules, under the influence of stress, are activated with respect to ozone. 2. The stress modifies the relation between the processes of structure formation and destruction in favor of the latter. For example, the stress may facilitate rupture of the chains during regrouping of the initial unstable ozonide into isoozonide.
Chain Scission in the Oxidation of Hevea. II.
Authors: Bevilacqua E. M.
Published: February 2011

Oxidation of natural rubber in latex yields two molecules of carbon dioxide, one of formic acid and one of acetic acid for each rupture of the hydrocarbon chain. The yields of these products can be interpreted readily by reference to the structure of the primary oxidation products of 1,5-diolefins established by Bolland and Hughes.
Study of the Thermal Oxidation of Butadiene-Nitrile Elastomers
Authors: Kuzminskii˘ A. S. , Popova E. B.
Published: February 2011

1. Processes of destruction and structure formation take place during the thermal oxidation of butadiene-nitrile rubbers. These processes take place simultaneously, and are the result of absorption of oxygen by the rubber molecules. The dominant process, which governs the changes in the physical and mechanical properties of rubber, is structure formation. 2. An essential difference between the behavior of vulcanizates of SKN-18 on the one hand, and of SKX-26 and SKN-40 on the other, is shown by heat oxidation and aging. Depending on the length of the induction period of oxidation inhibited by phenyl-2-naphthylamine, the nature of the consumption of inhibitor, and the rate of change of the equilibrium modulus, relaxation constant, and standard physical-mechanical properties during aging, the behavior of SKN-18 is practically the same as that of other butadiene polymers containing an equal number of double bonds in the main chains of the molecules. 3. It was established that, during thermal oxidation of butadiene-nitrile copolymers, acrylonitrile chains of the rubber molecules undergo conversion. 4. The formation of a certain quantity of substances soluble in methyl and ethyl alcohol during oxidation of nitrile rubbers is revealed, and it is established that substances present in these byproducts can inhibit the oxidation of rubber. 5. The resistance of butadiene-nitrile rubbers to thermal inhibited oxidation and aging of vulcanizates, which is greater than that of other butadiene rubbers, is explained by the formation from the rubber itself, during oxidation, of substances which are, in combination with phenyl-2-naphthylamine, very strong inhibitors of the oxidation process.
Colorimetric Determination of Phenyl-2-Naphthylamine in Rubber
Authors: Menšík P. , Broulík D.
Published: February 2011

A colorimetric method was worked out for the determination of the amount of phenyl-2-naphthylamine in butadiene-styrene copolymers based on the coupling reaction of phenyl-2-naphthylamine with diazotized p-nitroaniline. The method was found practically applicable. It is accurate, and the procedure is simple.
Electron Microscopic Studies of Fillers in Rubber. III. Effect of Milling on the Dispersion of Fillers
Authors: Suito Eiji , Arakawa Masafumi , Hasegawa Hiroshi , Furusawa Yonemasa
Published: February 2011

In the study of types of fillers which have a marked effect on the properties of rubber, information as to how the filler particles are dispersed in rubber is a prerequisite. The authors have already reported on the state of dispersion of various fillers in vulcanized rubber, observed under an electron microscope by the replica method. How the dispersion of these fillers affects the properties of rubber is an interesting problem. Since, in the earlier work, filler particles were observed to orient themselves in certain directions, in this report the relationship between the state of dispersion observed under an electron microscope of filler particles in rubber milled in different ways and the resulting characteristics of the mixtures were examined.
Chromatographic Determination of Rubber in Plant Latexes
Authors: Bernal Dorado E.
Published: February 2011

Partition chromatography on paper makes possible the separation of rubber hydrocarbon from latex. This method offers a means which is both simple and rapid for determining the presence of rubber in plant latexes and for the subsequent estimation of its percentage in a given latex. The method also gives a fraction which is composed of the nonrubber components. In addition to this, the method makes possible a comparison of the partition coefficients of rubber hydrocarbon isolated from latexes of different plants. Experiments are now in progress, the purpose of which is the application of the chromatographic method to the rapid determination of plant rubbers. By its simplicity, this method unquestionably has certain advantages in the identification of rubber in any particular flora and in studies of this hydrocarbon in its various phases of the biological cycle of the plant.
Preparation and Use of Cyclized Rubber as a Stiffening Resin in Rubber
Authors: Janssen H. J. J.
Published: February 2011

The condition for miscibility of low-molecular liquids is that the cohesion-energy densities should be of the same order of magnitude. The permissible difference decreases sharply as the molecular weight is increased, and a solution of equal amounts of rubber and polystyrene separates into two layers at room temperature when the concentration rises above 2 per cent, whereas low-molecular aliphatic and aromatic hydrocarbons are completely miscible. The undiluted polymers can, however, be mixed to an apparently homogeneous state on the mill if the correct mixing temperature is chosen. Presumably the extreme length of the polymer molecules allows them to be caught and dispersed separately in the stress gradient between the rolls. That the mixture is not stable and tends to separate into the components on extension, even after vulcanization of the rubber, can be deduced from the bending of the stress-strain curve towards the strain axis—generally an indication of irreversible changes in structure for many materials. This tendency to irreversible separation, which is a disadvantage for any application, can be suppressed by the formation of primary bonds between the two polymers. There are at present three methods to produce such bonds : (1) covulcanization (if the added polymer is unsaturated); (2) mastication in an inert atmosphere; (3) graft polymerization. If a sufficient number of such bonds has been formed, the added polymer can no longer be extracted from the vulcanized compound by solvents, and the bend in the stress-strain curve has disappeared.
Oxidative Stress Relaxation of Natural Rubber Vulcanized with Di-Tertiary-Butyl Peroxide
Authors: Ore Svein
Published: February 2011

It has been shown by Farmer and Moore that natural rubber can be vulcanized with di-tert.-butyl peroxide (DTBP), Presumably the free radicals formed by the unimolecular decomposition of the peroxide abstract some of the more labile (e.g., α-methylenic) hydrogen atoms, leading to direct C—C crosslinks between the rubber molecules, with tert.-butanol and acetone as the main reaction products. This preliminary communication presents some of the results of an investigation of the oxidative stress relaxation of the following types of DTBP vulcanizates. (A) First grade pale crepe, DTBP, and carbon black (MPC) mixed on the mill and vulcanized in a press. The carbon black was added to minimize the deleterious effect of impurities. (B) Purified rubber vulcanized: (1) in aqueous heating media; (2) in the press; (3) in DTBP vapor.
Influence of Inhibitors on the Oxidation of Rubber Solutions
Authors: Novikava E. N.
Published: February 2011

1. The nature and distribution of substituent groups (NH2, N(CH3)2, CH3, OCH3, COOH) influences the relative activity of amines as rubber antioxidants. 2. Azo compounds fall in the following order of their inhibiting action on rubber oxidation: p-aminoazobenzene, dimethylaminoazobenzene-o-carboxylic acid, dimethylaminoazobenzene, azobenzene. 3. Phenylsemicarbazide is a more effective antioxidant than Neozone D. 4. Diazoaminobenzene and diphenylthiocarbazone do not inhibit the oxidation of rubber, but initiate it quite rapidly. 5. Sudan I and III inhibit the oxidation of rubber solutions, but less than does 2-naphthol.
The Hardness of Very Soft Rubbers
Authors: Blackwell R. F.
Published: February 2011

Hardness determinations are frequently required on very soft rubbers in the range of 5 to 30 (Shore). It is usual to make such measurements in the shop with a pocket-type tester; however this is not entirely satisfactory for laboratory work where a more precise reading is usually required, calling for a hardness tester of the dead-load type. Such a tester, moreover, is useful to keep a check on the accuracy of pocket testers. The British Standard dead-load hardness tester is not calibrated below 30° BS, but a method of extending the range downwards has been proposed, whereby the standard indentor of this tester is replaced by one of 0.25 inch diam.; unfortunately this method has the disadvantage of involving more than one revolution of the dial gauge needle, and a directly calibrated scale would be out of the question. There is, however, an alternative method in use at BRPRA, which gives hardness readings down to 4° in one revolution; and which replaces the standard added load of 535 grams by one of 70 grams. At present, a calibration curve is provided to convert from scale reading to hardness degrees, but it is intended to add a second scale in due course.
Oxidative and Nonoxidative Thermal Degradation of Rubber
Authors: van Amerongen G. J.
Published: February 2011

The mechanical properties of rubber often undergo a marked change on heating, with deterioration of its elasticity and strength as a result. This behavior is a problem not merely where rubber is used in surroundings of prevailing high temperature; arguments can be advanced to show that thermal deterioration may influence to some extent as seemingly remote a property as wear. The behavior of rubber exposed to heat is strongly influenced by the presence or absence of oxygen. When an ordinary rubber vulcanizate is heated to, say, 100° C in the presence of oxygen or air, it undergoes substantial changes in a comparatively short time. Yet the effects may be similar in the absence of oxygen at temperatures 60° to 100° C higher. The inference is that, although a given rubber heated in air to 100° C, for instance, may be stable owing to its high resistance to oxidation, it may become completely unstable when heated to 180° C on account of thermal decomposition. This paper is published to provide fuller details of the behavior of rubber when heated in the presence and absence of oxygen. The evidence was obtained from experiments with both unvulcanized and vulcanized rubber, differing in composition.
The Limiting Value of Dithiocarbamate Formation during the Vulcanization of Natural Rubber with Thiuram Disulfides
Authors: Scheele Walter , Lorenz Otto
Published: February 2011

This paper disproves an assertion by H. W. Zijp that a fundamental result of our investigations was wrong. The result in question is that the limiting value of dithiocarbamate formation (66 mole-percent of the thiuram disulfide added) is independent of the temperature and, in a certain range, is also independent of the concentration of thiuram disulfide, added. It is shown experimentally that a reaction of thiuram disulfide with rubber which may take place fortuitously during mixing cannot be of any influence on the limiting value of the dithiocarbamate formation.
Studies of Mastication
Authors: Lee K. O.
Published: February 2011

Acetone-soluble substances formed during the milling of rubber were divided by chromatographic analysis into three fractions. Infrared spectra of these fractions were identical with the spectra of corresponding fractions from heat-aged raw rubber. The significance of these results is discussed in conjunction with Pike and Watson's subsequent theory of cold mastication mechanism.
The Polymerization of Vinyl Monomers in Natural-Rubber Latex
Authors: Bloomfield George F. , Swift P. McL.
Published: February 2011

Practical methods are described for polymerizing methacrylic esters, styrene, and other vinyl monomers in natural-rubber latex. The larger rubber particles require an appreciable time to attain equilibrium with the monomer diffusing into them from a liquid monomer phase dispersed in the serum. Provided that substantial proportions of added surface-active substances are avoided, rubber-soluble monomers can be polymerized almost entirely within the rubber particles, and the modified latex then contains no separately emulsified free polymer. Such conditions favor combination of polymer with rubber. The addition of a sufficiently large amount of dispersing agent favors polymerization of emulsified monomer, with less involvement of the rubber. In this way there can be obtained mixtures of rubber and polymer from monomers whose polymerization is otherwise inhibited by the presence of polyisoprene hydrocarbons.
Crosslinking of Latex Rubber
Authors: Bevilacqua E. M.
Published: February 2011

Most of the properties of rubber are consistent with those expected of a high- molecular-weight linear polymer of isoprene. However, there are some features of its behavior that are inexplicable on this basis. They must be the result of the presence of small amounts of structures other than the regular head-to-tail chain of isoprene units. Bloomfield, in the first fundamental study of the properties of rubber from freshly tapped latex, found that the tree does not continue to build indefinitely a linear polymer, but that branching reactions occur in a rested tree. These reactions eventually convert individual latex particles into substantially single molecules of enormous molecular weight. Bloomfield also observed that small amounts of oxygen are intimately associated with the hydrocarbon, even when it is isolated directly from the tree with careful exclusion of atmospheric oxygen. Craig, Juve, and Davidson have found less certain indications of the presence of carboxyl groups, which, if they are present in the rubber in concentrations even approaching the amount indicated by their results, must be on side chains.
Characterization of Graft Polymers
Authors: Blanchette Joseph A. , Nielsen Lawrence E.
Published: February 2011

Since the earliest recorded attempts to modify rubber by the polymerization of monomers dissolved in rubber considerable work has been directed toward the preparation and characterization of copolymers other than those with random distribution based on the reactivity ratios of the two components. Block polymers consisting of A and B chains coupled to form a linear molecule have been reported by many investigators including Melville and coworkers and Woodward and Smets. Woodward and Smets used the different solubilities in selected solvents and their mixtures to separate block copolymers of styrene-methyl methacrylate and vinyl acetate-styrene from the two homo-polymers with which they occurred. Solution properties of the isolated materials were then investigated. This technique for isolation and characterization has been used with graft polymers in which branches containing all A are attached to a “backbone” of B units. Claesen and Smets prepared and separated vinyl acetate-polymethyl methacrylate and vinyl chloride-polymethyl methacrylate graft polymers from the homopolymer mixture by solubility differences and reported on the osmotic and viscometric behavior of the graft materials. Other investigators have studied conditions and techniques for preparing graft copolymers while attempting partial characterization of the materials produced by using solubility differences for isolation and the usual methods for property studies.
Chain Scissions in a Polymer Network
Authors: Horikx M. M.
Published: February 2011

Relations between degree of crosslinking and gel fraction in a polymer have been derived by several authors. An important parameter in these relations is the crosslinking index γ, the average number of crosslinks to which one original polymer molecule is linked. The relations have been applied to the determination of the degree of crosslinking in linear polymers, but they can be used as well to calculate the soluble fraction of the network in a crosslinked polymer that has been subjected to a degradation process. It will be shown in this paper that a simple relation exists between the soluble fraction of a network that has undergone scission and the effective number of chains in the gel fraction (as determined by swelling measurements). The fraction of soluble material in the degraded network is a measure of the number of scissions. This problem bears upon natural-rubber vulcanizates, which give increasing amounts of extractable material and increasing degrees of swelling on the absorption of oxygen.
Crystallization and the Relaxation of Stress in Stretched Unvulcanized Natural Rubber
Authors: Tobolsky A. V. , Brown G. M.
Published: February 2011

The first observation made in this laboratory in 1946 on the effect of crystallization on stress decay at constant extension was made on a vulcanized Neoprene GN gum stock. It was found that this rubber showed only slight stress decay after 100 hours at 35° C and 50 per cent extension. However, a complete decay of stress to zero stress was observed after only 50 hours at 0° C and 50 per cent extension. Furthermore, the sample after reaching zero stress began to increase in length (spontaneous elongation). This phenomenon was attributed to crystallization. Observations of spontaneous elongation, but no stress decay measurements, were previously reported for vulcanized and unvulcanized natural rubber and for vulcanized and unvulcanized ether polysulfide rubber. It was decided to make a careful study of the effect of temperature and elongation on the stress-decay curves of unvulcanized rubber (cast latex sheet) in the temperature region of crystallization. Some time after these studies were completed, two papers have appeared in which stress decay to zero stress in rubber vulcanizates due to crystallization were reported. In one of these a rather thorough study was made of the decay of stress and change in volume of a natural rubber vulcanizate at −26° C and maintained at four different extension ratios. Our own work on unvulcanized natural rubber was carried out at seven different temperatures in the crystallization region, and were made at five different extension ratios. Quite surprisingly, four of these are the same as were used by Gent. There are interesting differences as well as similarities between our work and the results reported for vulcanized natural rubber.
Dynamic-Mechanical Characteristics of Rubber Compounds
Authors: de Mey S. , van Amerongen G. J.
Published: February 2011

Since rubber articles are often exposed in service to small periodic deformations, great interest attaches to the dynamic-mechanical characteristics of rubber. It has been established that the conditions under which these characteristics are determined have a pronounced influence on the results obtained, so that the measurements must be undertaken under precisely specified conditions. A new test apparatus is described for measuring the dynamic-mechanical characteristics, with which measurements can be performed at any desired stress setting, frequency, temperature, and amplitude on the same samples, both for compression and for shear loading. The incompressibility of filler-free natural-rubber compounds has been demonstrated by measurements made on samples with different shape factors and with varied static initial stress, under compression and with shear loading. The temperature, frequency, and amplitude dependence of the dynamic-mechanical characteristics of different rubber compounds is discussed on the basis of a number of measurements. The maximum value of the loss factor, which occurs in the vicinity of the second-order transition point, appears at a higher temperature in GR-S (cold rubber), Vulkollan, and Butyl rubber than in natural rubber. There is a connection between this fact and the much greater frequency and temperature dependence of Butyl rubber compared to natural rubber in the vicinity of room temperature. A compound based on natural rubber and a styrene-butadiene (85/15) co-polymer shows two maxima in the loss factor. One of these is characteristic of natural rubber, the other of the polymer. The dynamic characteristics of filler-free rubber compounds are not very sensitive to amplitude. It is found that the marked amplitude dependence of reinforced rubber compounds cannot be accounted for by increased temperature or by any nonlinearity of the stress-strain curve. The influence of composition on the dynamic-mechanical characteristics of natural rubber has been tested for a number of compounds. It is established that the carbon black types can have a significant effect on the E′ modulus. At small amplitudes the magnitude is greater for a compound containing SAF or EPC carbon black than for one containing HAF carbon black. Natural rubber reinforced with Aerosil or aniline resin shows a small loss factor, while compounds vulcanized with Thiuram show a large one. The present study is part of a fundamental investigation on rubber carried out by the Research Division of the Rubber-Stichting in Delft under the direction of H. C. J. de Decker.
Oxidative Degradation of Swollen Vulcanizates
Authors: Degteva T. G. , Kuzminskiĭ A. S.
Published: February 2011

1. In the course of a study of the oxidative degradation of vulcanizates with different structures (a thermal vulcanizate, a vulcanizate with Thiuram, one with diphenylguanidine and, one with zinc ethylphenyldithiocarbamate) it was established that the swollen vulcanizate, regardless of the accelerator present, undergoes two simultaneous reactions—destruction and structure formation. 2. It was revealed that the dominance of the destructive process, in the oxidation of swollen vulcanizates, can occur only in the presence of antioxidants; and that only after a considerable consumption of antioxidant does the rate of structure formation equal or exceed the rate of destruction. 3. It was shown that the behavior of antioxidants in swollen vulcanizates is governed not only by the nature of the rubber, but also in large degree by the nature of the solvent.
Chain Scission in the Oxidation of Hevea. III. Effect of Temperature
Authors: Bevilacqua E. M.
Published: February 2011

When molecular oxygen reacts with raw Hevea rubber in latex at 90° C, two molecules of carbon dioxide and two molecules of “volatile acid” (one molecule of acetic acid and one molecule of formic acid) are produced for each apparent scission of the hydrocarbon chain, estimated from changes of solution viscosity. This corresponds to the complete destruction of one isoprene unit, and if the several hydrocarbon end groups are oxidized, requires a minimum of six molecules of oxygen per scission. Estimates of oxygen requirements for scission during the accelerated oxidation of vulcanized Hevea rubber much lower than this have been made. It has been suggested that the apparent high efficiency of scission in vulcanized rubber is the result of the predominance of scission at crosslinks over random cutting of the hydrocarbon chain. To investigate the less likely possibility that the mechanism of the reactions which leads to scission changes sharply with the rate of oxidation, the earlier estimates of yields of scissions and of volatile acids during the oxidation of Hevea latex at 90° C have been supplemented by measurements at 70° C and at 110° C.
Measurement of Dispersion in Black-Loaded Rubber
Authors: Leigh-Dugmore C. H.
Published: February 2011

The gross dispersion of carbon black (or of other particulate fillers) in rubber can be measured quantitatively by cutting thin sections and examining them under the light microscope. The method is very suitable for control of mixing processes either in the factory or the compounding laboratory.
Reaction of Natural Rubber with Hydrofluoric Acid
Authors: Tom D. H. E.
Published: February 2011

Within the scope of the research on natural-rubber derivatives carried out in the Rubber-Stichting laboratories, attention was also paid to the reaction of rubber with hydrofluoric acid. Very little work in this field has been achieved by other investigators. The results of their work are mainly to be found in a number of patents. These deal with the preparation of products incompletely described that undoubtedly must be considered as more or less completely cyclized rubbers. The fluorine content of these products is, generally speaking, very low and is often said to be lost on heating or storing. According to Nielsen, it would be possible to obtain products with 30 per cent fluorine when using gaseous hydrofluoric acid. This high percentage is tentatively explained by adopting the incorrect formula H2F2 for hydrofluoric acid. Elsewhere it is stated either that HF cannot be added to rubber or that the addition product is not stable.
Dilaminar Elastomeric Films
Authors: DeFries Myron G. , Schneider Louis , Fram Paul , Leonard Fred
Published: February 2011

While rapid strides have been made in the synthetic organic chemistry of rubberlike materials and in the elucidation of relationships between structure and mechanical properties of elastomers, it is generally not feasible by synthesis alone to construct a rubbery type of polymeric structure which is likely to fulfill all requirements for a given specific application. The tolerances imposed by synthetic methods are very close, and experience indicates that comparatively slight changes in reaction conditions, or monomer, soap, and catalyst concentrations, all may have a profound effect on the balance of properties desired. To aid in the achievement of desired properties, recourse is generally had to compounding. Thus, elastomers are compounded for a variety of reasons. For example, plasticizers are added for improved low-temperature properties, reinforcing agents for improved tensile, tear, and abrasion properties, vulcanizing ingredients for reduced plastic flow and improved elastic recovery, and antioxidants for providing resistance to aging. Furthermore, while optimum combination of synthesis and compounding efforts may achieve adequate bulk properties, the stock may still fail because of surface difficulties, e.g., permanent discoloration on contact with stains, and surface tack development. To overcome these difficulties, in some instances successful use has been made of surface coatings such as talc, waxes, or thin, adherent polymeric films.
Isotope Exchange between Sulfur Atoms in Vulcanization Accelerators in the Presence of Diphenylguanidine and Phenyl-2-Naphthylamine
Authors: Blokh G. A. , Golubkova E. A. , Kurskaya K. V.
Published: February 2011

In previous work it was shown that in the process of vulcanization a continual series of isotope exchange reactions takes place between the sulfur atoms in the various sulfur-containing ingredients of the vulcanizate. It was established that the sulfur atoms which enter into this isotope exchange include not only those which go into the structure of the molecule of accelerator, e.g., mercaptobenzothiazole, benzothiazolyl disulfide, and tetramethylthiuram disulfide, carbamates, and sulfenamides, but also the sulfur atoms which form the polysulfide bonds, R—Sm—R, and, in addition, those present in the form of secondary products formed in the vulcanization process (hydrogen sulfide and zinc sulfide). Evidently the exchange reactions between sulfur atoms in the substances present in an accelerator-type vulcanization, in which sulfur in a reactive form is liberated, and active radicals are formed from the accelerator and take part in structure formation in the rubber, are of prime importance. The formation of a vulcanization network in which the rubber, sulfur, and carbon black are chemically combined, is another result of industrial vulcanization.
Reaction of Elemental Sulfur and Mercaptobenzothiazole
Authors: Blokh G. A. , Golubkova E. A. , Miklukhin G. P.
Published: February 2011

One of the most important problems in the field of the physics and chemistry of rubber is that of vulcanization. Until now no single theory has been established, which elucidates the complex physico-chemical changes which occur during this process. Still more obscure has been the mechanism of the action of vulcanization accelerators, which, as is well known, not only reduce the time and the temperature of vulcanization, but also influence the physico-mechanical and chemical properties of the rubber. Most investigators have assumed that in the acceleration process a reaction with sulfur converts it to an active form which is capable of bringing about vulcanization at a lower temperature and at a greater rate, than with ordinary elemental sulfur in the absence of an accelerator. This point of view is based on the experimental fact that the vulcanization of rubber by sulfur dioxide and hydrogen sulfide, for example, which form sulfur in the nascent condition, proceeds rapidly even at room temperature. Investigators have also assumed that in the vulcanization process activation of sulfur in the presence of accelerators may occur by different mechanisms. It is possible that the accelerator, reacting with elemental sulfur, forms unstable intermediate compounds, which decompose with liberation of sulfur in an active form. The latter reacts with rubber, and the regenerated accelerator reacts again with elemental sulfur, etc. However, a different process is possible for the activation of elemental sulfur. By this second mechanism the unstable combination of accelerator and sulfur reacts directly with rubber without the formation of active sulfur. Both these mechanisms necessarily assume the formation of intermediate unstable combinations of the accelerator with sulfur. However, direct, experimentally-based demonstrations of such an interaction are lacking in the literature. There exist only theoretical hypotheses concerning the nature of the possible intermediate combination of the accelerator with sulfur. According to Ostromislensky's concepts, further developed by Bedford, such an intermediate compound has the character of a polysulfide. According to Bruni and Romani, this intermediate compound is a disulfide. As is well known, the disulfide theory was placed in doubt by Zaide and Petrov on the basis of data from the vulcanization of rubber in the presence of benzothiazolyl disulfide.