Abstract
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.
Abstract
The plasticization of natural and synthetic rubbers by cold mastication results from the degradation of the macromolecules. In Part I experimental evidence was presented that such degradation is caused by mechanical rupture of primary carbon-carbon bonds in the polymer (R—R) to give polymer radicals (R · ) which are stabilized by oxygen to give degraded polymer molecules. Such a mechanism, invoking mechanical shear, had been advocated previously, e.g., by Staudinger, most explicitly by Kauzmann and Eyring, but required systematic mastications under nitrogen with a large variety of compounds (radical acceptors) to demonstrate more critically its consistency with experimental observations. However, Part I provided no direct chemical evidence supporting the mechanism by demonstrating reaction between the rubber radical and radical-acceptor molecule. Considering that only of the order of 0.1 per cent of radical acceptor is required for radical termination to give marked degradation, especially favorable reactions and sensitive analytical techniques are required for detecting the basic chemical reactions. The incorporation of radical acceptors into two types of elastomer on mastication has been demonstrated by the use of two radical-acceptor molecules and two distinct analytical techniques: (1) the incorporation of 1,1-diphenyl-2- picryl hydrazyl into natural rubber, using a spectrometric method, and (2) the incorporation of 1,1′-dinaphthyl [S35]-disulfide into GR-S, using a radiochemical method. The incorporation of the radical acceptors should be correlated with extent of degradation measured by decrease in molecular weight. Quantitative correlations have been obtained for both the above radical acceptors.
Abstract
The rupture of natural and synthetic rubber molecules into free radicals by the shear imposed during cold mastication under nitrogen initiates the polymerization of monomers incorporated in the rubber. Experimental results on the polymerization of a range of monomers is reported. The polymer formed is mainly attached to the rubber, forming with it either a soluble interpolymer or a two-component gel depending on the monomer used. Results on the co-polymerization of two monomers are also presented and interpreted.
Abstract
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.
Abstract
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.
Abstract
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.
Abstract
1. A mechanical method has been developed for the determination of the vitrification temperature of rubberlike polymers, based on measurement of the compression deformation of polymer samples in the vitrification temperature region under a constant stress. 2. An apparatus for the determination of the vitrification temperature of polymers by the mechanical method is described. 3. The vitrification temperatures of a series of polymers have been determined. The agreement of results of the determination of the vitrification temperature of the polymers by mechanical and by other methods is shown.
Abstract
The melting temperatures of two of the crystalline modifications of gutta-percha and their mixtures with either methyl oleate or tetradecane as diluent were determined by x-ray diffraction and dilatometric methods. It was found that the melting temperatures of the two forms in the pure polymer were 64 and 74°, respectively. From these studies the heat of fusion per repeating unit of the high melting form was deduced to be 3070 ± 250 cal./mole, while the heat of fusion for the low melting modification was found to be about one-half to three-fourths this value. An analysis indicates the low melting form is a metastable one. The heats and entropies of fusion of each of the forms of gutta-percha are appreciably greater than those of the monomorphic as-hydro-carbon, natural rubber. The difference in the entropies of fusion of the two polymers persists even when allowance is made for the significant contribution of the volume change accompanying fusion.
Abstract
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.
Abstract
A variety of methods has been used to study the viscoelastic properties of polymeric materials. These methods include the response to sinusoidal stress (dynamic measurements), stress relaxation, and creep under constant load and constant stress. The present investigation was made to determine whether or not the viscoelastic properties of rubberlike materials over an extended time scale could be obtained from stress-strain curves measured at different strain rates and temperatures. Polyisobutylene of high molecular weight was selected for study, since its viscoelastic properties have been investigated extensively in a cooperative program sponsored by the National Bureau of Standards. From the data obtained, Marvin has derived the distribution functions of relaxation and of retardation times over a time scale of 10−10 to 107 sec. These functions show quantitatively a change in properties from liquidlike to rubberlike to glasslike with decreasing time scale. The equilibrium stress-strain curves for lightly crosslinked rubber and other elastomers are closely linear for elongations up to 100 per cent. The non-equilibrium (viscoelastic) stress-strain curves for similar and noncrosslinked elastomers might be expected to be linear viscoelastic, as a first approximation, at temperatures above the glass transition, provided the strain and the strain rate are not excessively large. Nonlinear viscoelastic effects are usually pronounced for materials in their glasslike state and at large strains.
Abstract
1. A direct method has been proposed for the study of the elastic characteristics of rubber stocks under nonequilibrium deformation conditions. 2. This method has been successfully applied in the study of the elastic characteristics of “crystallizing” and “noncrystallizing” unloaded rubber compounds.
Abstract
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.
Abstract
Ordinary elasticity, high elasticity, and viscous flow are the mechanisms of deformation of solid polymers. Qualitatively, the effect of radiation on the mechanical properties of polymers can be described by considering the effect of crosslinking and cleavage on these mechanisms. Crosslinking tends to impede viscous flow. The equilibrium value of Young's modulus for high elasticity is directly proportional to the concentration of crosslinks, but for ordinary elasticity the sensitivity of Young's modulus is less for more rigid materials. Cleavage reduces the yield stress for viscous flow, and decreases the equilibrium value of Young's modulus for high elasticity. Young's modulus for ordinary elasticity is again insensitive for the more rigid materials. The changes in the mechanical properties of irradiated elastomers depend on the rates of crosslinking and cleavage. These rates are governed both by the polymer's chemical structure and by the strength of the van der Waals forces. For uncrosslinked polymers, the change of Young's modulus is a measure of the crosslinking rate in the case of less rigid materials. Sensitivity to chain cleavage has been correlated with chemical structure for the materials studied.
Abstract
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.
Abstract
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.
Abstract
1. A study of the effect of combined sulfur on the oxidation of swollen and unswollen polybutadiene vulcanizates shows that stocks vulcanized with different types of accelerators (tetramethylthiuram disulfide, diphenylguanidine, and zinc ethylphenyldithiocarbamate) form sulfur bonds which differ in their content of long polysulfide groups. 2. It is shown that sulfur liberated by the polysulfide bonds in vulcanizates reduces the speed of the oxidation reaction and the equilibrium concentration of stable peroxides. The sulfur content also governs the structure-forming process which develops when a raw polymer containing sulfur is oxidized at 90° C. The decomposition products of the polysulfide groups likewise lower the speed of oxidation in swollen vulcanizates at 150°. 3. When swollen vulcanizates are heated in the absence of oxygen at 150°, only the polysulfide bonds decompose. Heating unswollen vulcanizates under the same conditions, however, brings about the reverse effect, viz., structure formation. 4. Sodium sulfite and alcoholic alkali do not remove completely the polysulfide sulfur from vulcanizates. 5. In the process of vulcanization a rearrangement of the sulfur bonds takes place, and this is accompanied by a reduction of the polysulfide content.
Abstract
Fillers with little effect on abrasion resistance differ further from carbon blacks in having little or no tendency to introduce some form of strong-type linkage which could sitffen and strengthen the rubber at high extensions. Such linkages are here termed primary to distinguish them from secondary (weak-type) linkages, which have a range of lower strengths as revealed by breakage with applied stress, and contribute little to reinforcement. The abrasion resistance for a given filler concentration is much improved even by remarkably small numbers of primary linkages if the particles are sufficiently small; and it is comparatively insensitive to the number of primary linkages in the quantities normally obtained with carbon blacks. For instance, a considerable degree of reinforcement is obtained with the partially graphitized black known as Graphon, although this black shows drastically reduced capacity to form linkages and little change in particle diameter. The large differences in the wear resistance of vulcanizates containing different grades of carbon black must therefore be attributed mainly to the particle diameter itself rather than to the linkages formed. Moreover, primary linkages as reflected by stress-strain measurements could not explain the effect of particle diameter on reinforcement because they are unrelated to particle diameter. For good wear resistance the particles probably need to have macro-molecular dimensions, though small. To regard the effect of particle diameter in terms of the interfacial area for linkage formation is inconsistent with the above conclusions. For a given dispersion and concentration of filler it is suggested that reinforcement is most likely to find proper expression in terms of the number of linkages per particle and the number of particles. This is expressed mathematically in a tentative, empirical form of equation designed to fit general conceptions, and to correlate roughly the abrasion resistance with filler particle diameter and with primary linkages as reflected by stress-strain measurements. The equation implies that reinforcement increases with diminishing particle diameter until an optimum is reached, and thereafter decreases to become negligible for particles of molecular dimensions.
Abstract
The nature and rate of adsorption of high-molecular substances on carbon black is of interest in a number of instances, particularly in the development of adsorption methods of fractionation of high-molecular substances, but also in the use of carbon black as a filler for rubber ; the number of papers on the adsorption of high-molecular compounds is small. The effect of the molecular weight of different polymers on the rate of adsorption of high-molecular substances from their dilute solutions at C=0.1–0.25 per cent was studied in this work. Samples of polyisoprene, polystyrene, emulsion polymerization at a temperature of +5° and +50°, and also a plant sample of SKS-30 rubber, were the objects of the investigation. Moreover, a series of polystyrene fractions was obtained by the method of fractional precipitation from methyl ethyl ketone and butyl acetate, using methanol as the precipitant. For all the polymers and their fractions the characteristic viscosity was determined, from which the molecular weight value of the polymer was determined. Carbon black of the Ukhtinsky and Dashavsky factories was used as the adsorbent, the specific surface of which was determined by the adsorption of acetic acid from a benzene solution.
Abstract
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.
Abstract
Many finely divided solids have been added to elastomers to effect reinforcement of the cured stocks. It is generally agreed that the more finely divided the solid and the greater the degree of physical adhesion (wetting) between the surface of the filler and the polymer chains, the greater will be the general overall reinforcing effect. However, recent work on the mechanism of reinforcement has indicated that an important, if not limiting, property of the filler, in at least modulus reinforcement, is the immobilization of polymer segments on the solid surface. The immobilization is effected by the formation of strong chemisorptive bonds, if not covalent bonds, between the polymer and the filler. The application of this concept to carbon blacks has been discussed in detail by the authors, and similar conclusions have been reached by Blanchard and Parkinson and by Barton, Smallwood, and Ganzhorn. Up to the present time carbon blacks are unique as reinforcing agents in Hevea and GR-S type polymers. The numerous inorganic fillers which have been studied are generally inferior to the carbon blacks in one or more respects. The rubber chemist and technologist is chiefly interested in such properties as modulus, tensile strength, abrasion resistance, and resilience of the compounded stock. Tensile strength and resilience as commonly measured are primarily a function of the extent of surface, the particle-size distribution, and the degree of dispersion of the added pigment or filler. The chemical nature of the solid surface appears to be of secondary importance. Abrasion resistance is a difficult quantity to measure, especially in the laboratory. Therefore, in studying the mechanism of reinforcement and the reinforcing effect of different types of fillers, measurements have been confined to an evaluation of the so-called “equilibrium modulus”, from which an apparent work of retraction may be calculated. It has been found that the work of retraction is highly sensitive to changes in the chemical nature of the solid surface and the results may be interpreted in terms of the kinetic theory of elasticity.
Abstract
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.
Abstract
In the absence of air the absorption of water by a vulcanized crepe rubber tends to an equilibrium value in accordance with an osmotic mechanism of absorption. An equation for absorption as a function of time, derived by consideration of such a mechanism, represents the experimental data adequately. A relationship may be derived from the basic rate equation, correlating equilibrium absorption with ambient vapor pressure. This equation also gives good agreement with the experimental data. The divergence from the theoretical relationship found for absorption in the presence of air may be attributed to oxidative aging of the rubber. This effect is most marked at elevated temperatures, where the results of absorption under nitrogen are very much lower than those for absorption in the presence of air.
Abstract
The cracking of rubber vulcanizates when exposed to the atmosphere in a state of stress is a well-known phenomenon which has been studied extensively. A comprehensive review of the literature on this subject has been published by Newton. Following the lines of earlier work Newton showed conclusively that the cracking is produced by traces of ozone present in the atmosphere. The simultaneous presence of both stress and ozone is required for cracking to occur and the cracks that form are oriented with their lengths perpendicular to the direction of the stress. It has been reported, however, that similar cracking is produced when stretched rubber is exposed to free radicals produced by the thermal breakdown of organic peroxides. Unstretched vulcanizates do not crack when exposed to ozone, nor do they crack when stretched after exposure to ozone. Apart from the frosting effect described by Tuley, which is stated to require the presence of moisture and which may require the presence of small stresses, there seems to be no record in the literature of a visible effect being produced by the action of ozone on unstretched vulcanizates. The effect of ozone on unstretched vulcanizates has been investigated more completely and the results are reported in the present paper. Little is known of the mechanism of the reactions that lead to cracking of vulcanizates and this paper describes experiments designed to throw some light on this. The products of the reaction between ozone and solid rubber have been investigated by means of infrared spectroscopy. Allison and Stanley have previously described the infrared spectra of products formed from solutions of Hevea and various synthetic rubbers and Salomon and van der Schee have recently reported changes which occur in the infrared spectrum of a film of Hevea rubber when exposed to ozonized air.
Abstract
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.
Abstract
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.
Abstract
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.
Abstract
Studies of the reactions occurring during mastication and vulcanization of rubber must be regarded as still far from completed despite the many valuable experimental contributions. Although many theories have been developed on the mechanism of the processes to be discussed here, either based on the results of direct studies of the chemical processes or on the results of indirect physico-technological methods, one still has the impression that a thorough interpretation of all the relationships is not possible at present. Mastication and vulcanization are undoubtedly very difficult problems which are chosen by the experimenter mainly because of their importance. It is convenient to distinguish between those studies that deal with the elucidation of the reactions of the molecular chains of rubber during mastication, cyclization, and vulcanization, and those that deal with the performance of all the compounds which influence the course of these processes. At present we are interested in the second of these studies. In order to undertake the experimental work, it seemed to be desirable to have analytical methods by which we would be able to determine quantitatively the smallest amounts of accelerators, age resistors and, as well, the decomposition products of various of the rubber chemicals in question. We feel that the development of these methods is absolutely necessary for scientific studies of the kind mentioned and also desirable for the industrial laboratory. Many procedures for the analysis of rubber compounding ingredients, especially of vulcanization accelerators and age resistors, have been published; most of these, however, are for qualitative identification only. Besides precipitation and color reactions, physico-chemical methods are of considerable importance. Spectroscopic investigations of all kinds, and especially the characterization of substances by their ultrared, ultraviolet, and infrared spectra have been widely applied. Work on the application of polarography has become known. Chromatographic analysis has been put to use with marked success to separate compounds of interest and then to identify them by characteristic reactions.
Abstract
In recent years, the synthesis of polymers by reacting organic diisocyanates with compounds containing two or more active hydrogens has been studied extensively. Much of the pioneering work on the chemistry of organic polyisocyanates and their reaction products was carried out by Du Pont chemists. The products were fibers, elastomers, and plastomeric solids. In Great Britain, research of the Imperial Chemical Industries, Ltd., was directed toward the reaction of polyesteramides with organic diisocyanates to produce vulcanizable polymeric plasticizers, designated Vulcaprenes. In Germany, Bayer and co-workers developed the elastomer known as Vulcollan through their study of the reaction of polyester glycols with organic diisocyanates. Recently, interest in isocyanate-based addition polymers has been stimulated by the commercial availability of a number of polyisocyanates. An elastomer, Chemigum-SL polyester-urethan, was announced by the Goodyear Tire & Rubber Company in 1953. Both Vulcollan and Chemigum-SL are addition polymers of polyester glycols and organic diisocyanates, and both exhibit outstanding toughness and abrasion resistance relative to GR-S or natural rubber. Adiprene-B urethan rubber is a new addition polymer. It differs from other diisocyanate elastomers in that its polymeric segments are aliphatic polyethers rather than polyesters. Adiprene-B is a distinctive elastomer which combines in a single product high strength, abrasion resistance, solvent and ozone resistance, and excellent low-temperature properties.
Abstract
Adiprene-B urethan rubber is a high-viscosity thermoplastic elastomer that can be compounded and otherwise processed on conventional rubber manufacturing equipment. There are fundamental differences between Adiprene-B and the more common elastomers in processing which must be taken into consideration. 1. Processing temperatures should be less than 121° C to ensure smooth plastic flow and reduce the rate of scorching. 2. Processing temperature is proportional to Mooney viscosity, and both can be reduced by the use of plasticizers. 3. Moisture increases the rate of scorch, and it should be held to a minimum through all phases of processing. 4. Since stocks containing Adiprene-B increase in viscosity during storage, close scheduling of operations will ensure the maximum processable life.
Abstract
Butadiene-MVP copolymers and butadiene-acrylonitrile-MVP terpolymers are a new class of rubbers which can be rendered oil-resistant by reaction with organic halogen compounds during vulcanization. The rubbers respond normally to reinforcement by carbon black and are highly reinforced by mineral fillers such as clay. Oil resistance depends on MVP content of copolymers and MVP and acrylonitrile content of terpolymers. The type and quantity of quaternizing agent also regulate the properties and oil resistance of the vulcanizates. Excellent performance of the experimental rubbers was demonstrated after immersion in ketones, amyl alcohol, ethyleneglycol, acid, caustic soda, a 70/30 2,2,4-trimethylpentane-toluene mixture, and ester solvents. The terpolymers offer better resistance to aromatic hydrocarbons. The experimental rubbers are also superior to the butadiene-acrylonitrile control compounds in contact with hot diester lubricants, giving much higher tensile strength after exposure for 72 hours at 300° F or 48 hours at 158° F. The MVP-containing rubbers possess excellent resistance to flex crack growth. They are also considerably better than the controls in resistance to blowout under severe conditions of rapid cyclic deformation under heavy load at elevated temperatures.
Abstract
To ensure safety from skidding, attention has up to now been devoted to building rough surface roads, to the development of the proper vehicle construction with respect to this feature, and to the factor most directly involved, the tires. Special attention has been directed in connection with this latter phase to a much more open tread patterning and to the effect of decreasing tire inflation, both of which affect the life of the tire adversely. These steps neglected to take advantage of the physical effect of adhesion, which, without lowering the durability, now makes possible an enhanced contribution to the cohesive friction by the profile grooves which are of necessity retained to keep the weight down. The goal is, therefore, to provide the smooth surfaces of the tread pattern that come in contact with the road with the greatest possible physical gripping power, or adhesion. After illustrating the interfacial magnitudes with the help of a vector diagram, we shall survey the laws of boundary surface adhesion. Here the great influence of the liquid involved in wet friction becomes clear and the particularly favorable interfacial tension property of water can be assessed. Since skidding can occur only at the interfaces : rubber-water, or water-road, the requirement is as follows : both the greatest possible wetting power between rubber and water, and also between water and road surface, that is, hydrophilic properties in the rubber and hydrophilic road surfaces, in order to reduce the danger of skidding. Good insurance against skidding requires hydrophilic rubber and a hydrophilic road surface, for a tire that has been developed to be nonskidding holds on a hydrophilic road surface and skids on a hydrophobic road surface. A hydrophobic tire, on the other hand, skids on any wet road. Although considerable advances have been made with respect to safety from skidding since rubber tires were first developed for motor vehicles, with increase of speeds this problem demands our attention to a greater and greater degree. Safety from skidding can result only from the combined efforts of road and car builders, tire makers, and the chemists and physicists of all three groups.
Abstract
A series of carbon blacks of approximately constant surface area were prepared by heating a strongly acidic carbon black in N2 in the range of 200–1000° C. Their adsorption capacity for alkali and acid, catalytic behavior in H2O2 decomposition, and magnetic susceptibility have been studied and related to their effect on abrasion resistance when incorporated into a standard tire tread stock in the absence of antioxidants. It is suggested that there are, at least, two distinct chemical mechanisms by which carbon blacks can produce reinforcement, that is, by their (irreversible) action as polyphenol type antioxidants (acidic = low-temperature blacks), or in a catalytic (reversible) fashion through temporary stabilization of broken rubber chains on their surface (neutral = medium-temperature blacks).
Abstract
In order to survey the economics for a given country of driving with cold GR-S and natural-rubber tires, we proceeded as follows. With the usual average life of a tire as a starting point, the difference in life was calculated for the two types of rubber on the basis of relative wear resistance measurements. Based on the price of the tire this difference at each temperature is expressed in terms of money. There is obtained in this way, therefore, for the country selected, a curve showing price difference vs. temperature. This curve has to be further corrected because of the greater fuel consumption when the driving is done with cold GR-S tires. Even with equal tire life, it is certainly an advantage to save some fuel. Since our observations on fuel consumption showed no dependence on temperature, this correction only produces a parallel displacement of the curve of price difference vs. temperature. As an example, the resulting curve for West Germany is given in Figure 7 (lower diagram). With the aid of this curve every car driver in West Germany can calculate for himself what is the advantage or disadvantage of natural-rubber tires. For this he needs only to know how many kilometers he has gone at the different temperatures. The upper half of Figure 7 gives a view of the distribution of kilometer performance (mileage) of all private cars in West Germany. The percentage of the tire mileage at the different temperatures is given. In deriving these results, use was made of some statistical data on the traffic density in different months of the year. These data were from the Central Bureau voor de Statistiek in the Hague. Also the meteorological mean temperature values for definite hours of the day and months of the year were taken into account. The conclusion for West Germany is that, on the average, there is a distinct advantage in driving with natural-rubber tires. Conclusions for other countries can be drawn in a similar manner. They are given in very brief form in Figure 8. Surprisingly enough Figure 8 shows that GR-S is advantageous only in a very warm, i.e., tropical, climate. In all other countries, natural rubber shows more or less advantage. An estimate by J. C. Gerritsen, to whom we also are indebted for most of the preceding calculations, shows that over 70 per cent of the motor vehicles in the world can run more economically on natural-rubber tires.
Abstract
Systematic x-ray and electron diffraction studies of polymers of 1,3-butadiene prepared by different anionic catalysts have made it possible for us to identify two types of highly crystalline materials. One of us (G. N.) has already reported that our synthesis was carried out with the aid of different highly selective catalysts. The description of the preparation of these catalysts, which have made it possible for the first time to prepare these new polymers, will be separately published by one of us (G. N.) with L. Porri. Infrared analysis carried out by E. Mantica and D. Morero in our Institute has established that one type of these crystalline polymers obtained with heterogeneous catalysts represents a continuous l,4-trans arrangement of the monomer units. X-ray diffraction has shown that this material is dimorphous, having one crystalline modification that is stable at room temperature and another stable above 60° C. The x-ray melting point of a polymer having a molecular weight of the order of 100,000 is around 130° C. The low-temperature modification has an identity period of 4.9 A along the chain axis and a hexagonal packing with a period of a=4.54 A perpendicular thereto, but the symmetry of the crystals, as a whole, is lower. The high temperature modification seems to have the chains in helicoid form. The other crystalline polymer consists essentially of 1,2-monomer units; infrared studies have shown that more than 90 per cent of its double bonds are of the vinyl type and the residual 10 per cent represents a mixture of cis and trans. The higher the proportion of 1,2-units, the more crystalline is the material. Figure 1 shows the infrared absorption curves of two different typical samples of the product. In this article, we limit ourselves to describing the structure of the latter polymer, which, in our opinion, is the more interesting material and which we shall simply call 1,2-polybutadiene. We shall report on the structure of the two modifications of the l,4-trans-polybutadiene in another paper together with the study of the structure of other crystalline polydienes.
Abstract
Studies of the mechanism of biosynthesis of rubber in seedlings and cultured tissues of guayule have established that the carbon of the isoprenoid chain can be derived exclusively from acetate, and flax enzyme preparations have indicated that the branched chain acids, β-methyl crotonic acid and β-methyl-β- hydroxyglutaric acid are probably intermediates in the formation of the basic isoprenoid unit. However, the requirement for intact plants or tissue cultures has made detailed studies of the biosynthesis of rubber difficult. We have now observed that incubation of C14-labelled acetate results in the incorporation of radioactivity into rubber.
Abstract
Fresh Hevea latex is characterized by the presence of viscoids of greatly varying size (5–200 microns). When the viscosity of such a latex is determined with a viscometer of the rotary type the values found cover a wide range, having no clear relation to the initial DRC of the latex. Also, with the aid of a rotary viscometer, the structural viscosity of fresh or any other type of latex can be clearly demonstrated. When fresh Hevea latex is treated with ammonia in increasing amounts there is a sharp and regular drop in viscosity up to 0.05 per cent NH3. This runs parallel with a gradual and finally complete disintegration of the viscoids. Treatment with sodium sulfite in increasing concentrations up to 0.30 per cent yields a more slowly decreasing viscosity with end values approximately twice as high as with ammonia. Microscopic investigation shows a slow and only partial breakdown of the viscoids in the same area. Formaldehyde gave irregular results, but, in general, did not decrease or increase the initial viscosity considerably, neither could any influence be established on the size and shape of the viscoids. The viscosity of fresh Hevea latex is profoundly influenced by addition of distilled water giving a considerable increase up to 20 per cent and a sharp drop afterwards. Latex diluted with 20 per cent of water was microscopically found to be in a stage of pre-coagulation, with the immobilized rubber particles embedded in the viscoid complexes. It is concluded that the viscoids play a dominant role in viscosity phenomena of Hevea latex.
Abstract
This paper deals with the effect of dilution on the viscosity of sulfite or formaldehyde treated and ammoniated latexes, the relationship between viscosity and DRC for ammoniated field latex and concentrate and the influence of the tapping cycle on the viscosity of whole latex and white fraction latex. An explanation is given of the shape of the viscosity curve of water diluted fresh field latex. It is shown that sulfite in increasing concentrations shifts the maximum viscosity of dilution to regions of higher dilutions and at the same time lowers the viscosity value at the maximum. In formaldehyde treated latexes the maximum viscosity in the dilution curve still occurs at 15–20 per cent dilution, but the maximum viscosity value is considerably lower, especially at higher formaldehyde concentrations. Dilution of ammoniated field latex or concentrate yields a viscosity curve typical for a non-Newtonian liquid. The viscosity-DRC relationship for ammoniated field latexes shows wide discrepancies for individual samples and can not be used as a means for determining the DRC. Individual concentrate samples are in much closer agreement. The influence of the occurrence and removal of viscoids on these phenomena is discussed. A description is given of the influence of the tapping cycle on the viscosity of unpreserved fresh latex and the white fraction prepared therefrom, on the viscoid volume and on the DRC and TS of both whole latex and the white fraction. Finally, the possible role of viscoids in latex formation is discussed in connection with some recent investigations concerning their occurrence in sieve tubes and latex vessels in the Hevea tree.
Abstract
1. It is shown that the surfaces of particles of a synthetic latex stabilized with soap can be recharged by acidification or by the addition of an aluminum salt. 2. It is established that a necessary condition for this recharging is a low-solids concentration in the latex, which makes it possible for the particles to be recharged without the formation of any considerable amount of coagulum. 3. The opinion is expressed that recharging by acidification occurs through the adsorption of positively charged hydrogen ions by the particles of the latex; recharging by the addition of aluminum salts is due to the formation of an amphoteric aluminum hydroxide which is adsorbed by the surfaces of the particles.
Abstract
Working with ammonia-preserved natural-rubber latex, concentrated by centrifuging, the author shows that the slight fall in mechanical stability associated with the shipment of bulk supplies of latex can be explained by the fact that, during shipment, latex does not have access to the oxygen of the air. The mechanical stability of samples taken at the time of shipment increased in value when there was an appreciable air-space in the storage bottle, but fell when the bottle was full and air was excluded. These differences were accentuated when air was replaced by oxygen and was then possible to find marked differences not only in mechanical stability time but also in the volatile fat acid content of the latexes. The increase of volatile fat acid content of the samples stored in the absence of air was much greater than in the sample stored in the presence of oxygen. The fall of mechanical stability in the absence of air is ascribed to the formation of increasing quantities of salts, such as ammonium acetate. In the presence of oxygen, the rate of formation of salts of this type is very much less, and at the same time other changes take place which lead to a marked increase of mechanical stability.
Abstract
The viscosity values of different samples of latex diluted to different degrees were measured by means of three viscometers of different types, viz., the Brookfield viscometer, the Epprecht viscometer, and the French Rubber Institute viscometer. The results obtained are dependent on the type of apparatus, although within certain limits, the Epprecht and French Rubber Institute viscometers give approximately the same results. The values obtained with the Brookfield viscometer are distinctly different from those with other two viscometers. In spite of these differences, however, each of the viscometers has a field of use, which is indicated and discussed.
ADVANCEMENTS IN TYRE INNER LINER TECHNOLOGIES: A COMPREHENSIVE REVIEW OF MATERIALS AND PERFORMANCE CHARACTERISTICS
Enhanced Rebound Resilience: A Novel Approach
Basic Carbon black and Silica dual-filler control: application and mechanism analysis in solution polymerized styrene butadiene rubber composite materials
Thermo-Mechanical Devulcanized EPDM as a Functional Additive for PVC Composites: Effects on Mechanical Properties and Phase Compatibility
HISTORICAL PERFORMANCE OF CULTIVATED SCORZONERA TAU-SAGHYZ (MOUNTAIN GUM), A POTENTIAL ALTERNATIVE RUBBER-PRODUCING CROP
An Improved Low-Temperature Brittleness Test
Frictional Properties of Tread-Type Compounds on ICE
Flex Life of Passenger Tires. Relationship of Rayon Tire Cord Dip Pickup and Cord Adhesion
A Method of Measuring “Heat Embrittlement” of GR-S and Hevea Rubber Compounds
eISSN: 1943-4804
ISSN: 0035-9475