bluuebunny/crossref_metadata_2025 huggingface dataset:
Vulcanization of Elastomers. 30. Kinetics of the Decrease of Sulfur Concentration during Vulcanization
Authors: Scheele Walter , Cherubim Martin
Published: February 2011

The present discussion deals with the explanation and critical analysis of the kinetics of the decrease in concentration of sulfur in accelerated and unaccelerated vulcanizations of natural and synthetic rubber. A marked distinction is made between the time law and the dependence on concentration of the rate of decrease of sulfur, since there is lack of agreement between them, both in accelerated and unaccelerated vulcanizations, which indicates the complex nature of vulcanization. In unaccelerated vulcanization, the sulfur concentration drops, at all temperatures and independent of the concentration, according to a time law with exponent n=0.6. On the other hand, the dependence on concentration of the initial rate of sulfur decrease proceeds according to a first order reaction. These relations make it seem probable that sulfur decrease involves a process which is catalyzed by reaction products. In accelerated vulcanization, the reduction of concentration of sulfur also is expressed by a time law with the exponent n<1, and this too is independent of temperature and concentration of reactants (sulfur and accelerator). However, the concentration dependence of the rate of sulfur decrease cannot be expressed by a power law, so that we find no order of reaction in the usual sense. We attempt to explain the characteristic trend of the initial rate of sulfur decrease with sulfur concentration at constant accelerator content, or with accelerator concentration at constant sulfur content, by assuming that the vulcanization takes place through intermediate compounds. The pertinent theoretical conceptions are disclosed and critically examined and the attempt is made to show the extent to which they may be brought into accord with the experimental data.
Relation of Structure to Properties in Polyurethanes. Crosslinking Studies
Authors: Cluff E. F. , Gladding E. K.
Published: February 2011

The present work was undertaken to assess the effect of crosslink structure on the properties of the elastic network derived from a linear polyetherurethane elastomer. In order to study this effect, a polyetherurethane was synthesized which contained two types of reactive sites suitable for establishing crosslinks— pendant hydroxyl groups for reaction with diisocyanate curatives and pendant double bonds for vulcanization with sulfur. Incorporation of both types of curing sites within the same linear polymer served to hold constant variables which might otherwise influence vulcanizate properties. Thus, the elastic networks formed by both curing systems were produced from the same polymer, and such factors as polymer molecular weight and molecular weight distribution, interchain forces and cure site distribution remained constant. Furthermore, the curing sites have been placed on pendant groups, well removed from the main polymer chain, in order to avoid degradation of the polymer by side reactions which may accompany sulfur vulcanization. It is recognized that common elastomers with internal unsaturation, such as SBR and natural rubber, can be crosslinked by more than one method (e.g., with sulfur, peroxides, or high energy radiation), but the extent and nature of side reactions which may occur is not known with certainty. There are possibilities of polymer degradation reactions with all of these curing systems, and the occurrence of such degradation reactions would cloud any conclusions concerning the relation between crosslink structure and vulcanizate properties.
The Vulcanization of Neoprene with N,N′-Dinitroso-p-Phenylene-Bis(Hydroxylamine) Salts, a New Class of Curing Agents
Authors: Martel Rock F. , Smith David E.
Published: February 2011

The reaction of nitric oxide with p-benzoquinonedioxime in the presence of sodium methoxide to form disodio-N,N′-dinitroso-p-phenylene-bis (hydroxylamine) (I) in 96% yield has recently been reported. When a water solution of I is treated with soluble metallic salts of Zn++, Ca++, Mg++, etc., the corresponding N,N′-dinitroso-p-phenylene-bis(hydroxylamine) salt is obtained. Stable organic salts can be isolated when the bases piperidine or aniline are employed in place of sodium methoxide. Adducts have also been obtained (see PDF for diagram) using piperazine and ethylenethiourea, but the structures of these materials have not been clarified.
Modern Role of the Electron Microscope in Rubber Research
Authors: Ladd W. A. , Ladd M. W.
Published: February 2011

Until recently, the main role of the electron microscope in rubber research was the determination of particle size and shape of reinforcing pigments and fillers. The electron microscope proved its value in its first industrial problem in 1940 when it showed the particle size of Micronex to be 28 mµ. In 1942 it established the correlation between particle size of carbon and rubber properties. The next contribution was the establishment of reticulate chain structure, in 1945, using stereo-photomicrography. The electron microscope led the way to the carbon gel concept when carbons producing different road wear results were shown to have equal particle size and structure. Micrographs of carbon gel were published in 1951. Studies of rubber latexes, various pigments and fillers were also applications of the electron microscope in which the determination of particle size and shape was involved. Today, improvements in resolution, development of new techniques and accessory equipment have greatly expanded the application of the electron microscope. This paper is concerned mainly with describing these new developments ; first, as a help to electron microscopists in the rubber field; second, to illustrate what can be done with the electron microscope, so that research and production men can recognize possible applications of this discerning tool to the solution of their problems. Electron microscopes have come a long way in the short 20-year period. Figure 1 shows the microscope used in our laboratories. This instrument is currently resolving detail as fine as 10 A or 1 mµ in routine daily operation. Its design makes possible the development of special holders and thereby increases its value as a research tool. Another piece of equipment that is as important in microscope research as the electron microscope itself is the evaporating unit shown in Figure 2. This is used for shadowing and forming replication films under vacuum, thus making possible studies of rubber surfaces and surfaces of pigments and other rubber ingredients. In shadowing, detail is made visible by coating the specimen with metal evaporated at an angle from the tungsten basket at A. Carbon can be evaporated by the arc at B, forming a replicating film or mold of the specimen. This will be discussed under “preparation”. Today, the proper preparation of specimens is the most important step in successful microscopy. The various preparations used are as follows.
Mechanochemical Reactions Leading to Reinforcement. II. Natural Rubber and Phenolic Resins
Authors: Ceresa R. J.
Published: February 2011

The reaction of polyfunctional metallic salts with natural rubber during mastication has been extended to gelation by phenolic resins. High gel contents are achieved by mastication with 1 to 2 per cent of resin in the absence of oxygen. By using polyphenols and phenol alcohols it has been shown that an average of at least three functional groups, —OH or —CH2OH, per molecule are required to achieve gelation. The presence of the gel network in the unvulcanized rubber gives an improvement in processing characteristics.
The Cepar (Cure, Extrusion, Plasticity, and Recovery) Apparatus. An Instrument for Measuring Processing Characteristics of Rubber Mixes
Authors: Conant F. S. , Claxton W. E.
Published: February 2011

The principal advantages of the Cepar apparatus over commercially available plastometers for making processability tests include the following: small sample size; rapidity of testing; the wide range of orifice sizes, driving pressures (loads), and temperatures readily available; versatile controls for incremental extrusions at timed intervals (for studying curing effects) and/or for continuous extrusions; and the feasibility of using this apparatus to evaluate flow characteristics of stiff or “boardy” compounds or even of cured compounds normally considered out of range for conventional tests. The apparatus, in its normal application, will yield values for instantaneous orifice deflection (elastic stiffness factor), the continued rate of extrusion, and hot recovery upon load removal. Tests can also be made upon the removed sample for such things as orifice swell, extruded length, etc. In addition to the applications cited in this paper, the apparatus has been used to study the problem of knitting of factory stocks, as a control instrument to select the best temperature for extrusion processes, and as an evaluation instrument for various compounding studies. It has been used also as an instrument to evaluate the degree of heat softening (up to temperatures of 600° F) occurring in high-temperature heat-resistant stocks, in studies correlating factory extruded tread center gage with Cepar swell measurements, and in tests for degree of polymerization in SBR produced by the continuous process.
Antioxidant Efficiency of p-Phenylenediamines in Natural Rubber Vulcanizates
Authors: Lorenz O. , Parks C. R.
Published: February 2011

The consumption of various p-phenylenediamines during the oxidation of natural rubber vulcanizates has been investigated in the temperature range of 80–120° C. Oxygen absorption was used to follow the oxidation. Diaryl-p-phenylenediamines were consumed mainly in a termination reaction during the constant rate region of the oxygen absorption, where five to six molecules of oxygen were absorbed per molecule of diamine consumed. The constant rate stage of the oxygen uptake was found to be about twice as long as the time of the constant rate consumption of the diamine, indicating that the reaction product also possessed antioxidant activity. The effect of the nature of the curing system, carbon black, structure of the diamine, initial concentration of the diamine, and temperature of oxidation were studied. Dialkyl- and alkyl-aryl-p-phenylenediamines were consumed predominantly by a direct, relatively fast reaction with oxygen, the rate being dependent on the structure of the diamine and also on the nature of the curing system. The oxidation products formed exhibited antioxidant activity, those of N-isopropyl-N′-phenyl-p-phenylenediamine being particularly effective.
Influence of Vulcanization on Adhesion to Non-Vulcanizing Polymers
Authors: Raevskiĭ V. G. , Voyutskiĭ S. S.
Published: February 2011

According to contemporary ideas the mechanisms of adhesion and co-vulcanization of rubbers have much in common. The phenomenon of co-vulcanization, like that of adhesion of polymers, includes the diffusion of macromolecules and of segments of them, which is particularly intensive in the initial period of vulcanization, when the crosslinks have not yet formed. The difference between covulcanization and adhesion lies solely in the fact that the process of interpenetration of the molecular chains in the former case is sooner or later broken off by the vulcanization process being superimposed upon it, securing the macromolecules or portions of them by strong chemical bonds. This leads to the formation of an adhesion joint considerably superior in strength to a joint in which the molecules diffusing through are attached only by weak intermolecular bonds. It is important to ascertain what is the influence of the vulcanization of elastomers upon their adhesion to polymers not capable of forming chemical bonds with them. This case is of immense practical importance, since unvulcanized rubbers are very often used in combination with vegetable and synthetic fibers which are not capable of vulcanizing. As adhesives we selected elastomers of varying molecular structure, polarity and degree of molecular mobility: butadiene-acrylonitrile rubber (SKN-26), butadiene-styrene rubber (SKS-30A), butadiene-styrene oil-extended rubber (SKS-30ARM-15) and butyl rubber. Using these rubbers we prepared stocks both containing and not containing vulcanizing groups (in the latter case the components of the vulcanizing groups were replaced by an equal quantity of inert filler—whiting). The amount of vulcanizing group was set at the optimum for each rubber. The substrates used were films containing no plasticizers—cellophane (cellulose hydrate) and Perfol' PK-4 (polycaprolactam). The adhesive was applied to the substrate by coating on a calender at the optimum conditions for each elastomer. The specimens produced in this way were plied up and subjected to vulcanization. Vulcanization was carried out in a Berstorff apparatus for continuous vulcanization at 143° C and a specific pressure ≈ 0.85 kg/sq cm. The time of cure was varied between 0 and 88 min.
Stress as a Reduced Variable: Stress Relaxation of SBR Rubber at Large Strains
Authors: Landel Robert F. , Stedry Paul J.
Published: February 2011

Stress relaxation measurements on SBR were carried out at temperatures from −5 to +60° C and at initial strains of up to 550%. The effects of strain and time were found to be factorable, so that the isochronal stress-strain curve may be written as a modified Hooke's law with a time-dependent modulus: S=E(t)eƒ(α), where ƒ(α) is an appropriate function of the strain. By defining a strain-reduced stress S*=S/∫(α), i.e., a strain-reduced modulus E*(t)=E(t)ƒ(α), it can be shown that Ferry's method of reduced variables may be extended to large deformations. An appropriate strain function was obtained from the empirical Martin-Roth-Stiehler equation as ƒ(α)=α−2 exp A(α−α−1) with A=0.40. Although it cannot yet be certain that A is truly a constant and the same for all elastomers, this equation has the advantage of being valid right out to the breaking strain.
The Protection of Transparent Vulcanizates against Aging in Sunlight
Authors: Dunn J. R. , Fogg S. G.
Published: February 2011

Care has to he taken in choosing antioxidants for use in transparent natural rubber vulcanizates since some amines commonly employed as protective agents against thermal degradation photosensitize aging in ultraviolet (uv) light. Further studies using the technique of stress relaxation, undertaken in order to find the most satisfactory type of antioxidant for protection against sunlight aging, have shown that, with additives introduced into extracted samples of peroxide vulcanizate by swelling from ethyl acetate solution, the best results were obtained using a mixture of a substituted bis-phenol and an absorber of uv light. Hitherto, nickel isopropylxanthate (NPX) has been regarded as the most potent protective agent against light; however, the rate of degradation in the presence of NPX was unchanged by the addition of uv absorber and the mixture of bis-phenol and absorber was markedly more effective. Furthermore, the nickel salt is green, whereas the bis-phenols are relatively nonstaining. The work described here is an evaluation of vulcanizates in which the phenol and absorber have been incorporated before cure. The phenol employed was 2,2′-methylene-bis(4-methyl-6-tert-butyl phenol) (Antioxidant 2246 of the American Cyanamid Co.) and the absorber was Tinuvin P (a product of undisclosed formula marketed by Geigy Ltd.), chosen since it protected better in the absence of antioxidant than two other commercial absorbers. Two types of transparent vulcanizate were examined, one being an accelerated sulfur cure and the other a dicumyl peroxide cure. The recipes were as follows:
Internal Rupture of Bonded Rubber Cylinders in Tension
Authors: Gent A. N. , Lindley P. B.
Published: February 2011

An unusual rupture process is described. It consists of the sudden appearance of internal cracks in bonded rubber cylinders at a well-defined and comparatively small tensile load. The cracks occur in all the vulcanizates examined, and in particularly weak rubbers are found to cause failure. Their appearance may also lead to marked changes in the load-deflection relationship. A theoretical treatment is presented on the basis of a proposed criterion for crack formation. It is assumed that they form when the negative hydrostatic pressure component of the applied stress reaches a critical value. It is shown by means of the theory of large elastic deformations that a critical value of the negative pressure exists at which any cavity in the rubber will burst, and the calculated value is shown to be in satisfactory agreement with experiment. Moreover, the theory successfully predicts the observed dependence of the cracking stress on the Young's modulus of the rubber, its virtual independence of the rubber strength and extensibility, and the general form of the observed variation of cracking stress and crack disposition with the thickness of the testpiece.
Structure of Filled NBR Vulcanizates by the Stretching Method (Mullins Effect)
Authors: Shvetsov V. A. , Novikov A. S. , Pisarenko A. P.
Published: February 2011

1. It is possible to use the equation of Blanchard for butadiene-nitrile rubbers with channel black and aluminum hydroxide. 2. It is shown that vulcanizates of butadiene-nitrile rubbers with aluminum hydroxide are subjected to more marked softening on stretching in comparison with vulcanizates containing channel black. 3. Ideas are put forward relating to the ratio of primary and secondary linkages in vulcanizates on the basis of SKN-26 with channel gas black and aluminum hydroxide.
Swelling Effect of Liquids on NBR (Nitrile Rubber)
Authors: Starmer Philip H. , Lufter Carl H.
Published: February 2011

Immersion tests in various polar and nonpolar solvents have been undertaken with a range of butadiene-acrylonitrile copolymers. For a particular liquid, it would appear that the volume change can be related to the acrylonitrile content of the rubber' by a Gaussian type curve which is described by three constants α, β and γ. The constant a corresponds to the nitrile content of the rubber which has maximum swelling in the liquid and would appear to be related in some way to the dielectric constant of that liquid. The constants β and γ control the value of maximum swelling. Investigation into the nature of the three constants tends to suggest that α and β are governed by molecular attraction due to the “dispersion forces”. On the other hand, γ seems to be connected with the electrostatic forces of attraction and is probably not a constant over the full range of nitrile content, but should be replaced by an expression governed by two other constants. Therefore, to describe the solvent power of a nonpolar solvent, two parameters are needed while four are required for a polar solvent.
Branching in Butadiene Rubber
Authors: Poddubnyi˘ I. Ya , Erenburg E. G.
Published: February 2011

1. By studying the intrinsic viscosities of fractions in an “ideal” solvent, the degree of branching of potassium-butadiene polymers was determined. It was found that a specimen obtained at 0° was already markedly branched; increasing the temperature of polymerization (to 40–60°) led to the synthesis of highly-branched polymers, containing one branching site per several hundred monomer units. 2. On the basis of quantitative data on the branching of the polymers studied, the difference between the activation energies of the chain growth and branching reactions was calculated for the polymerization of butadiene in the presence of potassium. At the same time it was found that the increase in the degree of branching of the macromolecules with the increase in temperature in the gas phase polymerization of butadiene was determined not only by the mentioned difference in the activation energies, but also to a considerable extent by the temperature dependence of the solubility of the monomer in the polymer. 3. It was shown that for the investigated system, fractionation of the branched polymers by means of fractional precipitation from dilute solutions yields fractions which differ in molecular weight but not in the degree of branching. 4. It is shown that the relation between the intrinsic viscosities of polymers of a given molecular weight but of a different degree of branching, remains constant in the switch from a good solvent to an ideal solvent.
Some Correlations in the Second Order Transition of Rubbers
Authors: Bartenev G. M. , Gorbatkina Yu A.
Published: February 2011

1. The thermal shrinkage was measured for three rubbers (SKS-30, SKN-18, SKN-40) in the temperature interval −180 to +50° at rates of cooling of 0.3 to 30°/minute. 2. The dependence of the glass temperature on the rate of cooling approximately obeys Formula (1), for which the constants C1 and C2 were determined, whereupon C2=0.03 C1, regardless of the type of rubber. 3. The activation energy of the glassing process for the rubbers studied, as indeed for other amorphous substances, is proportional to the glass temperature of the substance. 4. The width of the temperature interval of the glass region and the coefficient of expansion beyond the limits of the glass region are practically independent of the rate of cooling. 5. The greater the rate of cooling, the greater the change of length of the sample in the glass region and the higher on the temperature scale the same region is located.
Carbon Black—Latex Masterbatches
Authors: Janssen H. J. J. , Weinstock K. V.
Published: February 2011

The steady increase in production of black latex masterbatches over the past three years makes it appear probable that the process has become firmly established the “second time aroud”, and its potential value in various development projects promises even greater importance in the future. The following list of advantages and disadvantages of the process has been compiled from many of the articles to which we have already referred. In addition, statements by Drogin, Forrester, LaPorte, Malone, and Samuels are used.
Oxidation of TiCl3 in Hydrocarbon and Aqueous Media
Authors: Belonovskaya G. P. , Dolgoplosk B. A. , Chernova Zh D.
Published: February 2011

It was found that tho oxidation of TiCl3, with oxygen and isopropyl benzene hydroperoxide, is accompanied by the development of radical reactions leading to the degradation and crosslinking of polymers. The mechanisms of the processes for oxidizing TiCl3 were examined on the basis of the experimental data. It was shown that it is possible to prevent the development of crosslinking and degradation processes in the polymer, by making use of inhibitors.
A Compact Test Instrument for Determining the Indentation Failure of Elastomers
Authors: Livingston Daniel I. , Hildenbrand Leurene E.
Published: February 2011

There exists a considerable area in which conventional methods for the evaluation of physical properties of polymeric materials are either inconvenient or practically impossible. This is the case for very small specimens, irregularly-shaped regions, or composite structures of vulcanized elastomers. Here micro test procedures are indicated, and tests of local indentation are particularly appropriate; for when the indenter diameter is small compared to the dimensions of the test region, the results become independent of the dimensions, and little or no special preparation of the sample is required. We have extended the scope of indentation tests, from the measurement of stress-strain properties alone, to include the capability of securing measurements of the ultimate strength and indentation. These indentation failure properties should be pertinent to the service life of many fabricated articles. This paper describes the use of a compact instrument designed to perform indentation tests up to failure in a rapid and reliable fashion. A synchronous motor raises the test specimen against the indenter, a flat-ended cylinder of 0.014-inch diameter, until failure occurs, whereupon the machine stops automatically. Readings of a dial gage dynamometer and revolution counter on the platform drive are converted into puncture force and depth. Applications of the instrument to the evaluation of some typical reinforced rubbers have been illustrated, and the relationship of these measurements to other mechanical properties discussed.
Linear Thermal Expansion of Elastomers in the Range 300° to 76° K
Authors: Robbins R. F. , Ohori Y. , Weitzel D. H.
Published: February 2011

From the relations between Tg's of the component homopolymers with that of the resulting copolymer, one sees that the limit of the Tg of copolymers is that of the homopolymers. To obtain a low Tg one must avoid polymers with polar groups and/or bulky groups. Polar groups afford opportunities for intermolecular links which hinder rotation necessary for “rubberyness.” Bulky groups cause stiffness, and also serve as obstructions to contractions. On the other hand, the same reasons that polar and bulky groups raise Tg serve to lower overall thermal contraction, since αr is always greater than αg. Fillers can be used to lower contraction, and new ceramic fillers with negative coefficients of expansion might be combined with specific polymers to obtain a material with a desired contraction. The authors feel that the data presented here, together with design data available from commercial sources, should enable the design engineer to find many new uses for elastomers at cryogenic temperatures.
The Fraction of Effective Sulfur Crosslinking in High Sulfur-Natural Rubber Vulcanizers
Authors: Meltzer T. H. , Dermody W. J. , Tobolsky A. V.
Published: February 2011

Shear modulus measurements as a function of temperature for smoked sheet natural rubber vulcanized with sulfur in amounts between 2 and 34 per cent show the crosslinking efficiency of sulfur to be much lower in natural rubber than in SBR-1000, at least for the particular curing recipe here used. The ratio of crosslinks to total sulfur atoms computed from the rubbery plateau modulus is below 0.07 for all of the natural rubber mixtures. Ratios up to 0.20 were found in SBR mixtures. The shape and slope of the modulus-temperature curves of natural rubber vulcanizates are similar to those of SBR mixtures and are typical of crosslinked amorphous polymers. Consistent displacement of the curves toward higher temperatures and higher rubber modulus as the percentage of sulfur in the recipes is increased also follows the same general pattern found for SBR. Cyclization is concluded to be the primary cause of the low sulfur efficiency in crosslinking both polymers, although polysulfide formation in natural rubber may further reduce the ratio of crosslinks formed per atom of sulfur combined.
Predicting Elastomer-Fluid Compatibility for Hydraulic Systems
Authors: Beerbower A. , Pattison D. A. , Staffin G. D.
Published: February 2011

Elastomers used in sealing hydraulic and similar systems are frequently swelled or shrunk by prolonged contact with the fluids. The fundamental properties, especially the Hildebrand Solubility Parameter, of the fluid and elastomer components are related to these tendencies. Basically, the Solubility Parameter concept has been used to predict suitable elastomeric sealing materials for various fluids by avoiding close matches in solubility parameter. The present studies have shown that: (a) copolymer performance cannot always be predicted by averaging the properties of the homopolymers; (b) mixtures of solvents may not behave like an imaginary single fluid of average properties; and (c) chemical interactions, hydrogen bonding, and molecular shape factors introduce major complications. New correlations are proposed to enable the hydraulic engineer to obtain Solubility Parameters from the more common properties of heavy petroleum fractions.
The Permanence Properties of Flexible Urethan Foams
Authors: Steingiser Samuel , Darr W. C. , Saunders J. H.
Published: February 2011

The acceptance of urethan foam as a commercial product has hinged to an important extent on its permanence as a foam under all conditions of actual end use. Customer experience with commercial flexible urethan foams has confirmed the satisfactory service life of these materials. However, in the development of this industry, it was necessary to be able to predict the performance of experimental systems which later were to become fully commercial. Most often, this prediction had to be made very early, long before the materials were allowed to be put into end service. The common way to do this is by accelerated testing, wherein the time axis is condensed by stepping up the aging process well beyond any normal conditions. Considerable time and expense are saved in this kind of test; however, there is always the question that such an accelerated step-up of test conditions is not representative of actual use, and/or causes reactions in the sample which do not occur in normal use. It is important that these questionable features be reconciled before the intercomparability of accelerated and shelf-life testing is accepted.
Thermomechanical Study of Polyisobutylene by Stretching and Indentation
Authors: Fomenko B. A. , Volodin V. P. , Sidorovich A. V. , Kuvshinskii E. V.
Published: February 2011

1. Thermomechanical studies of polyisobutylenes of various molecular weights under conditions of uniaxial stretching showed that the thermomechanical behavior of PIB is analogous to the behavior of other linear polymers. 2. Comparison of thermomechanical curves obtained by the method of uniaxial stretching by a constant force, and by the method of indentation by dynamometric weights showed that, in spite of the qualitative similarity of the curves, the quantitative comparison is impossible because of the great difference in the nature of the strained state of the samples studied under the different methods. 3. It was shown that for polyisobutylene, just as for other amorphous polymers, deformation corresponding to the steep branch of the thermomechanical curve is of a highly elastic nature.
Representation of the Mechanical Damping Possessed by Rubberlike Materials and Structures
Authors: Snowdon J. C.
Published: February 2011

Discussed in detail is the manner in which the damping possessed by rubberlike materials and structures experiencing sinusoidal vibration may be represented by the ratio of the imaginary to the real part of a complex elastic modulus. Examples are given of the way in which the damping possessed by low- and high-damping rubbers depends on frequency. Equations that predict the response to vibration of such distributed systems as damped rods and beams, and a simple structure comprised of two damped beams and a lumped element of mass are derived; and representative computations of input impedance and transmissibility are presented.
The Relation between the Friction and Viscoelastic Properties of Rubber
Authors: Grosch K. A.
Published: February 2011

This paper describes a study of the friction of several types of rubber against hard surfaces over a wide range of temperatures and sliding velocities. The highest velocity did not exceed a few centimeters per second so that frictional heating was negligible. The results show that the friction increases with the sliding velocity to a maximum value and then falls. The application of the Williams, Landel and Ferry transform shows that the frictional behavior of a rubber sliding at various velocities and temperatures on a given surface can entirely be described by a single master curve and the glass transition temperature of the material. The master curve on a rough abrasive track shows, in general, two peaks—one of these occurs at a velocity related to the frequency with which the track asperities deform the rubber surface. This maximum is absent on a smooth track and thus reflects the deformation losses produced by the passage of the asperities over the rubber surface. The other peak occurs in general at much lower velocities; it coincides in position with the single maximum obtained on a smooth surface. Introduction of a fine powder (MgO) into the interface between the rubber and track eliminates this peak on both smooth and rough surfaces; it is therefore attributed to molecular adhesion. Comparison with the relaxation spectrum of the rubber gives a fundamental jump distance of the order of 60 A. It appears, therefore, that friction arises from adhesion and deformation losses, and that both are directly related to the viscoelastic properties of the rubber.
Effect of Deformation on the Crystallization of Supercooled Cured and Uncured Rubbers
Authors: Bukhina M. F.
Published: February 2011

1. The dependence of the polymer crystallization rates on temperature is considered on the basis of the general theory of crystallization kinetics for supercooled liquids. The coefficients in the equations relating the crystallization half time, τ½, to the degree of supercooling are calculated for natural rubber. 2. An approximate expression is obtained which relates the equilibrium melting temperature of deformed rubber with the mechanical stress applied when crystallization starts. 3. The acceleration of crystallization which is induced by deformation is shown to be basically associated with an increase of the equilibrium melting temperature. 4. The possibility of calculating the crystallization rate at all temperatures and stresses from the results of a small number of experiments is established.
Rheological Properties of Two Unvulcanized Rubber Compounds as Determined on an Extrusion and a Rotational Viscometer
Authors: Devine F. E. , Ross J. A.
Published: February 2011

The viscometer manufactured by A. Macklow-Smith Ltd., which operates at constant rate of extrusion, is a capillary instrument designed for the determination of flow properties of plastics. The instrument has been used for examining the rheological characteristics of rubber compounds and it has been possible to derive the shear stresses required to extrude material at various shear rates between 2 and 1050 sec−1. The general method of operation of the instrument for this purpose is discussed including the determination of preheat time before extrusion and the aspect of incipient vulcanization in the viscometer during extrusion. The effects of various lengths and diameters of dies on rheological parameters and entrance effects are discussed. An important aspect of the work is the observation of a piston height effect in the extruder barrel. Two different compounded polymers have been examined and in one case, the extrusion pressure was found to decrease with piston height whilst in the other highly elastic material, it was found that as the piston descended, the extrusion pressure increased to an equilibrium value. The importance of this effect in obtaining consistent shear stress data is discussed. Results of the work have been expressed in terms of apparent viscosity and also in terms of the parameters which describe a power law relating shear stress and shear rate within the capillary die. Rheological data have been obtained at various temperatures within the range of processing temperatures of the compounds. The effects on rheological properties of mastication and relaxation time after mastication have been examined. Results have been compared with figures obtained on conventional laboratory viscometers such as the Mooney Plastometer and the Wallace Rapid Plastimeter. A biconical, rotational viscometer has also been used at the lower range of shear rates and the results are compared with those obtained from the extrusion viscometer.
Relative Contributions of Viscoelasticity and Aging to the Relaxation of Cured Rubber
Authors: Thirion P. , Chasset R.
Published: February 2011

Relaxation in relatively stable pure gum natural rubber vulcanizates has been studied to determine the effects of viscoelasticity and aging, respectively, using a dark air oven. A quantitative analysis of experimental results shows that, in the case of a dicumyl peroxide vulcanizate and at 100° C, relaxation is caused by aging, except in its initial stages. Stress decreases as a linear function of time, in agreement with theoretical assumptions. Conversely, at 30° C, the influence of aging is negligible. At this temperature the difference between actual stress and stress extrapolated to infinite time, is proportional to a negative power of time. At intermediate temperatures, both phenomena occur simultaneously over a time interval ranging from 3 minutes to 150 hours.
Surface Area of Nonporous Carbon Blacks and Area of the Adsorbed Nitrogen Molecule
Authors: Voet Andries
Published: February 2011

Surface areas of completely nonporous carbon blacks of widely varying particle chain formation (structure) have been determined by means of the nitrogen adsorption method as well as by electron micrography. Accurately determined densities in helium were used as the basis of calculations in the latter approach. It was found that the ratio of areas measured by nitrogen adsorption to electron micrographically determined surface areas is greatly dependent upon chain formation. A higher structural build-up leads to a lower ratio, explained by the observation that fusion areas in carbon chains are necessarily, though erroneously, counted as surfaces in electron micrography. The ratio differs markedly from unity, however, in low structure blacks, where fusion areas are negligible. By accepting the area of a nitrogen molecule adsorbed in a monolayer as being equal to that in the solid state, 13.8 A2, the ratio becomes unity for nonporous low structure blacks. It appears likely, therefore, that all surface area data based on the area of a nitrogen molecule in the liquid state of 16.2 A2 are too high by about 15 per cent of presently accepted values.
Vulcanization. Part III. Rapid Methods for Characterizing Rubber Networks
Authors: Coran A. Y.
Published: February 2011

Equilibrium swelling determinations have long beea used as a means of estimating crosslink density, usually through the Flory-Rehner equation. Most of the reported methods for determining equilibrium swelling in rubber vulcanizates have required long periods of swelling before equilibrium was reached, due to the fact that thick specimens were used. The method presented here makes use of thin films as specimens for swelling, and equilibrium can be reached in periods of one to five minutes. In addition, the use of thin films makes possible the semiquantitative determination of the types of chemical bonds responsible for the crosslinks in rubber vulcanizates. In thin films the reaction of polysulfide crosslinks with lithium aluminum hydride (in benzene-tetrahydrofuran) is rapid. The determination of crosslink density after such a reduction is indicative of only those crosslinks which are not reducible by the hydride and the difference between crosslink densities before and after the reduction is a measure of the number of crosslinks of the polysulfidic type. The work described here has been limited to natural rubber gum stocks. The methods used are undoubtedly applicable to other types of stocks, though in some instances a certain amount of modification will be required.
Vulcanization. Part V. The Formation of Crosslinks in the System: Natural Rubber-Sulfur-MBT-Zinc Ion
Authors: Coran A. Y.
Published: February 2011

The response of the delay time and specific rate of vulcanization (treated as first order) to changes in starting concentrations of accelerator, sulfur and activator in the system: natural rubber-sulfur-MBT-Zn++ was defined by empirically derived relationships. The effects of compounding variables on the number and types of crosslinks formed in the vulcanizate were also studied. A reaction scheme was proposed to explain the role of soluble Zn++. The scheme is consistent with the fact that increases in the concentration of soluble Zn++ give slower specific rates of crosslink formation but more crosslinks. In the scheme, it was proposed that Zn++ chelates certain crosslink presursors.
Molecular Sieves in the Vulcanization of Rubber
Authors: Blokh G. A. , Neĭmark I. E. , Borodushkhina Kh. N. , Boguslavskiĭ D. B. , Shevchenko Yu G.
Published: February 2011

1. The effect has been studied of accelerators, introduced into a rubber mix in the absorbed state on molecular sieves, on the scorch resistance of the mixes at 110°–130° C. 2. Molecular sieves can be used to introduce ultra-accelerators (triethylammonium dibutyldithiocarbamate) and activators (ethanolamines) into rubber mixes, so permitting vulcanization to be accelerated without the danger of scorching during mixing. 3. Molecular sieves can be used to introduce gaseous ammonia into rubber mixes; ammonia considerably accelerates vulcanization and can replace, either completely or partially, such accelerators as MBTS or DPG.
Iodimetric Determination of Milligram Amounts of Rubber Hydrocarbon
Authors: Middleton K. R.
Published: February 2011

An iodimetric method is described for determining rubber hydrocarbon; it is based on the bromination of the rubber molecule, and is sensitive to 2 mg of rubber dissolved in benzene. By comparing it with a standard procedure, it has been shown that the proposed method will determine accurately the rubber hydrocarbon content of field latex. The precision with which solutions of pure rubber in benzene can be determined by the method corresponds to a coefficient of variation of about 0.5 per cent and rubber hydrocarbon in latex can be determined with a precision of about 1.5 per cent.
New Synthetic Rubbers from Copolymerization of Butadiene and ENE-YNE Diols
Authors: Nikitin V. I.
Published: February 2011

Of the large number of different industrial synthetic rubbers the foremost is butadiene rubber, first produced by the polymerization of butadiene by Lebedev. Nowadays, in order to improve the mechanical properties of the rubber the polymerization is carried out in conjunction with some copolymerizing additive: styrene, acrylonitrile, etc. Styrene rubbers (SKS) and nitrile rubber (SKN) have a number of quite high chemical and mechanical properties and are produced on a full industrial scale. Hydroxyl rubbers, i.e., those where the selected substance for copolymerizing with the butadiene is a hydroxyl-containing monomer, are able to offer a great deal to industry, since the presence of the hydroxyl groups considerably improves their mechanical properties. However, they do not figure among the available industrial grades of synthetic rubber; yet from the data from American scientists they have considerable value. Good results have been given by work effected by Nagibina and others in the Inst. Khim. Nauk SSSR on producing a rubber on the basis of the joint polymerization of butadiene with dimethyl vinyl acetylenyl carbinol under various conditions.
New Dispersible Carbon Blacks
Authors: Voet Andries
Published: February 2011

It has been shown that the process of pelletization of a carbon black carried out to permit easier transportation through bulk handling, leads to irreversible changes in properties. There are strong indications that the pelletization process of carbon black involves free radical reactions, leading to chemical particle bonding. The superior physical properties of vulcanizates reinforced with unpelletized blacks indicate that chemical particle bonding is undesirable, since it reduces interaction between carbon black particle and polymer. In order to overcome chemical bonding of carbon black particles during pelletizing, carbon blacks were prepared the particles of which were covered withat least a monolayer of an oleaginous material compatible with the polymer. It was found that the product obtained could be pelletized to form predominantly physically adhering particles. These carbon black compositions disintegrate spontaneously in solvents for the oleaginous liquids, in view of the dissolution of the pellet binding material. The new carbon black compositions are ideally suited for black solvent masterbatching. In addition, they also show improved physical properties upon dry mixing with polymers in conventional dispersing equipment. Finally, they provide for a simple, rapid and efficient way to incorporate processing oils into polymers in conventional mixing equipment.
Ultimate Tensile Properties of Elastomers. I. Characterization by a Time and Temperature Independent Failure Envelope
Authors: Smith Thor L.
Published: February 2011

The tensile stress at break (σb) and the associated ultimate strain (εb) of an elastomer depend on (1) the chemical and topological characteristics of the polymeric network, and (2) the test conditions under which rupture is observed. To separate these effects, the ultimate tensile properties can often be characterized by a “failure envelope” defined by values of σb and εb determined at various strain rates over a wide temperature range. Provided time—temperature superposition is applicable, such data superpose on a plot of log σbT0/T versus log εb, where T is the test temperature (absolute) and T0 is an arbitrarily selected reference temperature. The resulting failure envelope is independent of time (strain rate) and temperature and thus it depends only on basic characteristics of the polymeric network. To illustrate the characterization method, data on two styrene-butadiene gum vulcanizates, SBR-I and SBR-II, were analyzed. For SBR-I, values of σb and εb obtained over extensive ranges of strain rate and temperature superposed to give a failure envelope. Data at elevated temperatures also gave a reliable value for the equilibrium modulus. For SBR-II, data obtained at various temperatures under conditions of constant strain and constant strain rate yielded identical failure envelopes; this strongly suggests that the failure envelope is independent of the test method. A theoretical consideration of the time-to-rupture associated with different test methods showed that for given values of σb and εb the time-to-rupture from the following types of tests should increase in the order: constant strain < constant stress < constant strain rate < constant stress rate.
Tensile Strength of Gum and Reinforced EPR and SBR Vulcanizates
Authors: Dudek T. J. , Bueche F.
Published: February 2011

The tensile strength and elongation at break of EPR and SBR gum vulcanizates were determined as a function of network chain density. A large viscous contribution to the tensile force was found at low crosslink density, and the maximum in the tensile strength versus chain density curve decreased as more nearly equilibrium conditions were approached. The rate dependence of tensile strength of black-filled EPR was studied by creep rupture experiments. The temperature dependence of the tensile strength of filled EPR and SBR was studied and compared. The same limiting degree of cure was obtained for both filled and gum vulcanizates of EPR.
Level and Variation of Tensile Strength in Relation to Dispersion of Compounding Ingredients
Authors: May W.
Published: February 2011

Tensile strength—being a distinctive property—is sensitive to flaws in the rubber vulcanizate. A procedure is described by which the occurrence of flaws can be detected. These are interpreted in terms of dispersion of certain ingredients and the various possibilities are illustrated by examples of work carried out on styrenebutadiene rubber and polyisoprene rubber. It is shown how this procedure teads to better processing and thus to better product properties.