bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermodynamic Examination of the Polymorphic Transition of Trans-1,4-Polybutadiene. II. Experimental
Authors: Natta G. , Pegoraro M. , Szilágyi L.
Published: February 2011

Using actual fibers of trans-1,4-polybutadiene of high steric purity, it is possible to realize a thermodynamic cycle of the type described in Part I, and to measure the mechanical work obtainable. Reasons are indicated for the principal differences between cycles conducted with actual fibers and cycles calculated for ideal fibers. The lower efficiency of real cycles is also explained. A simple laboratory apparatus is illustrated which demonstrates the practical possibility of transforming thermal energy into mechanical work in this manner.
Infrared Spectra of Reaction Products of Natural Rubber with Alkylphenol Formaldehyde Resins
Authors: Nikitin Yu N. , Epshtein V. G. , Polyak M. A.
Published: February 2011

Infrared spectroscopic studies were carried out with reaction products of natural rubber with 101K tertiary butylphenol-formaldehyde resin without activators and in the presence of stannous chloride dihydrate and zinc oxide. No reduction was observed in unsaturated rubber content during reaction with resin. Reaction of resin with rubber probably takes place at the α-carbon atom and not at the double bonds. A reduction in the proportion of resin phenol hydroxyl is basically due to its participation in recombination reactions of radicals formed during breakdown of ether bridges of resin. At 150° C activators accelerate decomposition of resin ether bridges, but increase the stability of phenol hydroxyl. In the case of zinc oxide, phenol hydroxyl is sufficiently resistant even at 180° C.
Thin-Layer Chromatographic Method to Distinguish Natural Rubber and Synthetic Polyisoprene
Authors: Davies J. R.
Published: February 2011

With advent of increasing production of synthetic polyisoprene and subsequent use in commercial vulcanizates, there was an obvious need for a simple, reliable method to distinguish between natural rubber and synthetic polyisoprene, to be used in a research program of the Association. Present analytical methods, e.g., the Weber test, infra red spectroscopy etc., will only identify the polymers as cis-polyisoprene and will not differentiate between natural rubber and synthetic polyisoprene. Several methods have been published which depend on identification of a minor constituent of natural rubber not found in synthetic polyisoprene. Disadvantages of these methods are that they are time-consuming and present some uncertainty when dealing with rubber of unknown origin. Natural rubber contains about 1% w/w of extractable lipid materials consisting of phospholipids, sterols, tocopherols, tocotrienols, carotenes, squalene etc., whereas synthetic polyisoprene will contain only ingredients added during manufacture. A new approach to this problem is based on examination of a solvent extract of the rubber by thin-layer chromatography, in order to characterize and distinguish between the lipid components in natural rubber and additives in synthetic polyisoprene.
Considerations on Structure of Natural Rubber Vulcanizates
Authors: Westlinning H. , Wolff S.
Published: February 2011

Deep insight into the chemistry of vulcanization, its kinetics, and network structures have been provided by the elucidation of reactions of mono and diolefins with sulfur and accelerators, primarily at NRPRA, and the kinetic measurements of transformation of vulcanizing agents with 1,5-polyenes by Scheele and co-workers at the Kautschuk Institute der Technischen Hochschule, Hannover. It is now established that both accelerated and unaccelerated vulcanization of 1,5-polyenes with sulfur begins with formation of polysulfide crosslinks. As sulfur disappearance and formation of polysulfide proceed, secondary reactions participate increasingly. These involve a series of transformations of the crosslinks originally formed. This leads to a continual change in chain length of crosslinks, in their chemical constitution, and in structural features of the polymer. Figure 1 shows schematically the results of sulfur vulcanization. Crosslinks consist of mono, di, or polysulfide structures, depending on vulcanization time and temperature. As polysulfide bonds disappear, the polymer is modified by the appearance of monosulfidic ring structures and conjugated double bonds in the chain. Secondary reactions may change the crosslink density of the vulcanizate. There are three possibilities: shortening of the crosslink chain length can proceed (a) without change in, (b) with decreasing, or (c) with increasing, crosslink density. Which occurs must be determined for each system. For each polymer the density of crosslinking, its chemical structure, and the extent of polymer modification determine physical, chemical, and technological properties of the vulcanizate.
Effect of Temperature on Decay of Radicals Formed during Radiolysis of Solid Rubbers
Authors: Kozlov V. T.
Published: February 2011

Temperature-time curves for radical decay, when radicals formed in rubbers by low-temperature radiolysis (−196° C) are heated, have been studied by the EPR method. Analysis of results shows that radical decay processes begin much below Tg, with simple increase in the amplitude of thermal vibrations. Extent of decay is greater the lower the steric hindrance in the monomer unit, the order of this effect being SKD > SKS > KHK > SKI, NR, SKEP. On the other hand, contribution of the sudden change in the region of Tg in the processes of unfreezing of motion and decay of radicals is greater in rubbers with side chains. Results obtained permit determination of a number of important energy characteristics of the polymers, in particular molecular energy barriers, UT, activation energies for radical decay, ET and energies of thermal motion at which radical decay begins. These results show that the radical decay process at low temperatures begins before movement of macromolecules relative to one another and is dependent on a migration mechanism with very low energy of activation. This process consists in migration of “secondary” ions formed by capture of charges by free radicals. Schemes of radical decay processes involving a combination of reactions between radicals and “secondary” ions of radical origin are suggested.
Abrasion Resistance of Stereoregular Butadiene Rubber Treads as a Function of Tire Operating Conditions
Authors: Evstratov V. F. , Buiko G. N. , Sakhuovskii N. L. , Arenzon N. M. , Karmanova A. I.
Published: February 2011

In operating tires on Group A roads, the abrasion resistance of SKD/SBR and SKD/NR blends is substantially higher than that of SKS-30 ARKM-15. The abrasion index of butadiene rubbers increases with the severity of the test conditions, as in operating on mountain roads, with partial braking and acceleration. In operating tires on Group B and V roads (for example, in quarries), the abrasion index of SKD rubbers, particularly those with high SKD content, decreases considerably as a result of increased chipping and cutting of the tread pattern. In summer, the superiority of BR rubbers over SKS-30 ARKM-15 in abrasion resistance is less apparent than in winter. This is particularly characteristic for SKD/NR blends. For automobile tires of different types and sizes, the selection of composition and properties of treads in which SKD is used should be carried out in such a way as to take into account operating conditions.
Rubber Chemicals from Cyclic Amines. I. Preparation of Dithioamines, Derivatives of Dithiocarbamates and Thiazolethiols, and Related Compounds
Authors: D'Amico John J. , Morita Eiichi , Young Evan J.
Published: February 2011

The synthesis of dithioamines, derivatives of dithiocarbamates and thiazolethiols and related compounds derived from cyclic amines is reported. Novel or alternate methods for the preparation of 2-benzothiazolyl disubstituted dithiocarbamates and 2-(iminodithio)-benzothiazoles along with their possible mechanisms are discussed.
Rubber Chemicals from Cyclic Amines. III. Iminocarbodithioates as Accelerators and Curing Agents
Authors: Mortia Eiichi , D'Amico John J. , Young Evan J.
Published: February 2011

Evaluations of cyclic iminocarbodithioates as accelerators and vulcanizing agents are described. The starting amines include morpholine and 3-azabicyclo-[3.2.2]nonane and vary in ring size from tetra- to octamethylenimine. The chemical structure of these compounds is correlated with their cure characteristics and the kinetics of vulcanization. Basicity and ring size of the amine moiety govern the activity of the iminocarbodithioates.
Rubber Abrasion Test with Paper Tape
Authors: Bassi A. C.
Published: February 2011

The hypothesis of oxidative and fatigue abrasion phenomena as the major cause of tire tread wear in normal operation has led to development of a laboratory test machine for which low severity, obtained by using paper tape as abrading material, and full temperature control are outstanding characteristics. Good uniformity and reproducibility of abrasion are obtained if proper care is taken. Proof of the basic theory and of its applicability to such a machine has been obtained through a survey of the effects of antioxidants in compounds and of test temperature variation. Application of this test method to tire tread compounds has given fair results here it has been possible to establish testing conditions sufficiently related to those in road tests. When these latter include a large range of severities, comparisons are less favorable and further study is being made to try to accommodate such cases.
High Speed Tensile Testing of Tire Textiles
Authors: O'Neil K. B. , Dague M. F. , Kimmel J. E.
Published: February 2011

Techniques for testing tire-reinforcing materials in the laboratory under simulated operating conditions of high speed and high temperature are presented. The effects of these conditions on breaking strength and breaking energy of rayon, nylon, polyester, and fiber glass are compared. The tests include an individual cord impact test, and three dynamic tire tests using a pendulum, a ballistic plunger, and a resiliometer bruise. The value of each test is weighed against test results obtained from conventional testing methods and tire field performance based on vehicle fleet testing. A comparison is shown between the conventional slow speed tests, high speed tests and vehicle tests and shows that the high speed laboratory tests give a better correlation with tire bruise failures.
Impact Resistance of Automobile Tires
Authors: Lothrop E. W.
Published: February 2011

The construction and operation of a high speed plunger test machine to determine the impact energy required to produce failure of automobile tires are described. The equipment has been designed to operate over an energy range of 1500 to 15,000 in. lb at preselected velocities between 40 and 100 mph and at preselected temperatures between 75 and 300° F. These ranges are adequate to permit the testing of passenger tires over a range of conditions simulating those encountered in high speed driving. The test machine consists of a pneumatic gun for propelling the plunger at an inflated tire, mounted in a temperature controlled cabinet, and photoelectric devices for measuring plunger velocity. In a test the kinetic energy of the plunger is increased in increments, until failure occurs. The minimum kinetic energy required to cause failure of the tire fabric is taken as a measure of the carcass breaking energy. Data on rayon and nylon cord tires indicate that the breaking energy of a tire is dependent on both speed and temperature and that the relationship between energy, speed, and temperature depends on the cord-reinforcing material in the tire. As a result it is impossible to predict the relative impact resistance of tires under use conditions on the basis of laboratory test results obtained with the standard static plunger test, in which the plunger penetrates the tire at 2 in./min at room temperature.
Tire to Wet Road Friction at High Speeds
Authors: Allbert B. J. , Walker J. C.
Published: February 2011

The general problem of loss of tire grip on wet road surfaces is discussed. The initial section of the paper covers a description of testing techniques employed for the investigations reported. These include both road and indoor rig techniques developed to study limiting tire braking and cornering properties under a wide range of operating conditions. The level of friction between a tire and a wet road is primarily related to the ability to remove a water film from the ground contact area. The contact area is considered to be separated into three effective zones. Initially the ‘bulk’ of the film is displaced leaving a thin residual film to be penetrated at, or absorbed from, the interface before substantially dry contact can be established. The size of this dry area, at the rear of the contact zone, has an overriding control on the level of available friction and depends on the time occupied in displacing the water film in the frontal zones. An increase in the peripheral speed of the tire reduces the time available for water displacement and effectively shortens the area of actual ground contact. In the limiting condition the vertical load on the tire becomes entirely supported on the water film and the condition of aquaplaning exists. The requirements for this condition to occur are briefly discussed. Tread pattern and road surface design are shown to exert a considerable influence on the rate of water displacement from the contact area and hence on the relative sizes of the water supported zones. The forces developed in the dry zone at the rear of the ground contact depend on the frictional characteristics of the tread material. A section of the paper considers the wide scale effects of tread pattern, tread material, and road surface characteristics and discusses the nature of the interactions between these three variables. Some results of recent studies related to different types of tread pattern, specific pattern design features, and the effect of tire casing construction are included and future potential development of the tire contribution is reviewed.
Lubricated Friction of Rubber. I. Introduction
Authors: Bevilacqua E. M. , Percarpio E. P.
Published: February 2011

This review introduces a series of reports on a quantitative study of friction of rubber on wet surfaces. It was derived from concern over safety aspects of skidding on wet roads; this first paper deals with the relation between safety and traction. Subsequent papers deal with: A quantitative approach to characterization of road surfaces, identification of the surface features of importance, and estimation of their relative contributions to lubricated friction. Quantitative estimates of effects of properties of rubber materials on lubricated friction and an analysis of their relative importance in interaction with the significant features of the road surface. A quantitative basis for evaluation of wet skid resistance of roads, the choice of the rubber to be used in this evaluation, and methods of testing. An improved technique to measure the property of tread rubber important for wet skid resistance of tires. Identification and interpretation of the nature of friction on ice at low temperatures.
Lubricated Friction of Rubber. III. Components of Friction in Mating Surfaces
Authors: Percarpio E. P. , Bevilacqua E. M.
Published: February 2011

Friction is usually discussed in terms of contributions from two processes: deformation and adhesion. This terminology has been retained in discussions of lubricated friction of rubber although there is not apparent any clear mechanism by which adhesion might occur. Our study of wet friction leads us to suggest that the components making up frictional resistance to sliding might more appropriately be referred to as deformation and abrasive components. On this basis, there is an obvious mechanism for both processes, and some of the disagreements about interpretation of certain experimental results may be resolved. The basis for our interpretation is implicit in the literature but was originally expressed somewhat tentatively and has not been restated in recent discussions. Greenwood and Tabor pointed out that hysteretic losses make an important contribution to friction of rubber sliding on a lubricated surface and that this contribution is independent of asperity size or distribution so long as only the asperities support the load and the radius of curvature at the asperity tip is above a critical limit which is remarkably low. Further, the experiments of Sabey showed that it was difficult to obtain coefficients higher than 0.4 with spherical sliders. Mean average local pressures high enough to give high friction coefficients could be obtained only with sharp asperities. In the laboratory experiment these were cones. Greenwood and Tabor found that with these damage to the rubber becomes visible at just the semiangle at which the coefficient began to rise abruptly (Figure 1). It is entirely reasonable to call the process causing this damage abrasive friction.
Lubricated Friction of Rubber. VI. Portable Rebound Tester, with Particular Application to Wet Skid Resistance of Tires
Authors: Percarpio E. P. , Bevilacqua E. M.
Published: February 2011

An important conclusion from Part V of this series is that the wet skid resistance of a tire tread rubber is almost completely determined by its resilience and hardness, resilience being the more important variable. A useful feature of this relation is that both measurements are nondestructive and can be made, in principle, on a finished tire. Hardness is easily measured with commercially available instruments, such as the Shore Durometer, but a comparably useful instrument, one portable and suitable for use to measure resilience of finished and mounted tires, was not available. We have made and used such an instrument and have found additionally that it has features which make it useful for other purposes, notably measurement of resilience of very thin rubber specimens.
Measurement of Rolling and Sliding Friction Coefficients of Tread Compounds
Authors: Clamroth E. , Heidemann W.
Published: February 2011

We have described two methods to measure friction forces arising from sliding and rolling on a wet surface. In rolling friction we showed that energy of deformation of a braked rubber specimen could be estimated and separated from friction energy. Rolling friction shows a characteristic slip dependence for various compounds. The rise at low slip values can be quantitatively explained. Especially with polybutadiene vulcanizates, at higher slips the friction coefficient decreases; this is attributable to thickness of the water layer between specimen and friction surface and depends on ease of wetting with water. Wetting can be altered by addition of a wetting agent like Pril, and measured by the contact angle of a water drop on the rubber surface. In wet friction, the changes in the water layer outweigh the temperature and velocity dependent shifts expected for adhesion and deformation friction components. A sort of mixed friction occurs on wet surfaces. The true contact surface, that is, surface not covered with water, has a strong effect on the magnitude of friction forces. In general only the highest asperities of the friction surface extend above the water and are in direct contact with the rubber surface. It is therefore understandable that surface asperity has a strong effect on friction coefficients. Finally we have compared friction coefficients determined with our apparatus with those obtained using the Portable Skid Resistance Tester and with results of road tests. The Portable Skid Resistance Tester provides a single point method, which has good reproducibility and whose results correlate well with those on the road at a speed of about 48 km/hr. Our apparatus has the advantage that test conditions such as velocity, temperature, and load can be widely varied. In this way, various actual driving conditions can be approximated and one may seek to determine the mechanism of friction from dependence of friction coefficients on these parameters.
Calculated Deflected Contours and Load-Deflection Curves for Tires
Authors: Gehman S. D.
Published: February 2011

The tire contour equation is derived for a flexible tire body with radial cords and a complete circumferential deflection, corresponding to the case of a radial-ply tire or a tire inflated inside a constraining cylinder. Equations are derived to calculate the load on a single cord in the deflected region using parameters of the un-deflected contour. It is then shown how the reasoning and equations can be generalized for a tire body with any cord path having a complete circumferential deflection so that the load-deflection curve for a single cord can be calculated. A new approximation method is described so that the integrals involved in these calculations can be evaluated in terms of incomplete elliptic integrals of the first and second kind with any desired accuracy. Finally, a procedure is given for summing cord loads in the usual spot deflection of a tire so that the load-deflection curve for a flexible tire body can be calculated from contour parameters of the undeflected tire. An illustrative calculation is included. Although the load-deflection curve for a single cord is convex toward the load axis, that for the tire is concave because more cords are involved as deflection progresses. A calculated contour does not exist beyond a limiting deflection, at which, presumably, buckling starts above the bead.
Recent Advances in Glass Transitions in Polymers
Authors: Eisenberg A. , Shen M.
Published: February 2011

Since the publication of our review “Glass Transition in Polymers” in 1966, a number of interesting advances have been made in this field. The present article is intended to be an addendum to this review, reporting the progress that has been made by various workers during the past three years. In addition, a number of topics were not covered in the previous article due to the relatively incomplete understanding at that time. These will now be discussed. This is not because these topics are at present fully understood. Rather, we hope it will serve as a progress report to stimulate further interest in areas where further work is needed. The numbering systems and notations in this paper will follow those in the previous article for the sake of continuity.
Electron Microscopy of Rubber
Authors: Seward R. J.
Published: February 2011

In conclusion we must point out that the basic text book for this subject is C. H. Leigh-Dugmore's “Microscopy of Rubber”. Unfortunately parts of this excellent book need extending to cover modern developments some of which have been reviewed in this paper.
Theory of Adhesion, Mechanism of Bond Failure, and Mechanism for Bond Improvement
Authors: DeLollis N. J.
Published: February 2011

The various theories of adhesion are discussed, with mention of some aspects that are inconsistent with recent bonding developments. A mechanism for adhesive failure is proposed on the basis of thermodynamics and experimental phenomena. Surface treatments are described which significantly improve adhesion and which suggest that chemical bonds are necessary to achieve the ultimate in bond durability.
Internal Energy Contribution to the Elasticity of Natural Rubber
Authors: Shen Mitchel
Published: February 2011

It has recently been noted by a number of workers that the relative energy contribution, fe/f, to the elastic stress of rubberlike materials appears to depend on the extension ratio at which thermoelastic measurements were carried out. This apparent strain dependence is in contradiction to the free energy additivity principle of the statistical theory of rubber elasticity. In this paper we resolve this problem by determining fe/f from the temperature coefficient of shear moduli. The shear moduli were not directly determined from measurements of shear, but calculated from tensile elongation data. This method circumvents the difficulty encountered in directly obtaining the relative energy contribution from stress—temperature data. Both constant length and constant stress thermoelastic measurements were used to obtain fe/f. A series of natural rubber samples, crosslinked in the presence of various amounts of n-hexadecane, were investigated. It is found that the relative energy contribution to the elasticity of natural rubber is 0.18. Changes in inter-molecular interactions, brought about by the incorporation of diluents, produce no variation in the value of fe/f. This observation supports the hypothesis that the energetic stress in rubber elasticity is wholly attributable to intrachain energies of the network chains.
Large Elastic Deformations of Some Cord-Reinforcedrubber Shells
Authors: Charrier J. M.
Published: February 2011

A simple theoretical treatment of certain large elastic deformations of a symmetrical network of cords embedded in rubber is explained. The calculations are based on the main assumptions of thin, inextensible and incompressible cords and a rubber behaving like a Neo-Hookean solid. This model, although simple, respects the physical characteristics of certain cord reinforced components which would be designed for large deformations. Calculations for the simple extension of the composite material, the two-dimensional extension and its application to the inflation of tubes, are compared to experiments with reasonable success. Theoretical predictions are made for related problems of the behavior of tubes in contact with plates.
The Raman Spectra of Polybutadiene Rubbers
Authors: Cornell S. W. , Koenig J. L.
Published: February 2011

The Raman spectra of cis-1,4-, trans-1,4-, and 1,2-polybutadiene are presented. Analysis of the spectra of these model compounds, and the normal coordinate results, enable the Raman frequencies to be classified by configurational structure type. The Raman carbon—carbon double bond stretching vibrations can be used to describe uniquely the structure content in polybutadienes.
Studies in the Morphology and Vulcanization of Gum Rubber Blends
Authors: Gardiner J. Brooke
Published: February 2011

The solvent extraction of 1 mil films of cured gum rubber blends is an attractive semi-quantitative method for determining the extent of cure. Material balance and ir analysis give a good picture of which rubber is curing. Indirect evidence from this method indicates that massive curative diffusion is occurring from one zone to another. A theory based on curative diffusion, before or during cure, and different cure rates and curative solubility in the rubbers appears to explain most of the results. The importance of zinc oxide and stearic acid in addition to a sulfur cure for an NR/chlorobutyl blend cure has been established. Different cure packages can give considerably different results in overall cure. Some indication of a phase inversion in an NR/chlorobutyl system has been found.
Chemical Structures in CIS-1, 4-Polybutadiene Vulcanizates. Model Compound Approach
Authors: Gregg E. C. , Katrenick S. E.
Published: February 2011

We studied the vulcanization reaction of one curing recipe for cis-1,4-polybutadiene (CB). The curing recipe was zinc oxide (ZnO), diphenylguanidine (DPG), 2,2′-bis(benzothiazolyl) disulfide (MBTS), and sulfur and it produced only polysulfidic crosslinks. Devulcanization of this CB vulcanizate by phenyllithium solution permitted a comparison of the molecular weight of the CB before and after vulcanization. A small but measurable reduction of the molecular weight of the rubber occurred as a result of the vulcanization reaction. Reaction of the curing recipe in cis, cis-1,5-cyclooctadiene (COD) permitted the study of the fate of the curing chemicals in the vulcanizate because COD is an accurate chemical model for CB. This study showed that: 1. The vulcanization reaction consumes none of the DPG and none of the ZnO. 2. An insoluble coating forms on the ZnO and separates it from the vulcanization reaction during at least part of the reaction. 3. All of the MBTS reacts to form one compound: an S-alkyl derivative of 2-mercaptobenzothiazole attached to the rubber molecule at a point next to a thiacyclohexene ring. 4. All the sulfur reacts to form crosslinks and intramolecular rings. The rings are three-, five-, and six-membered thiacycloalkanes and five- and six-membered thiacycloalkenes. Some of the rings are isolated and some are next to crosslinks. 5. The three-membered rings are episulfide and they are very likely the key intermediate to the formation of the cyclic and crosslink structures. 6. The crosslinks are polysulfidic and occur singly and as clusters of two and four. The more crosslinks in a cluster, the less unsaturation in its vicinity. The four-link cluster has no unsaturation and occurs as two pairs with thiacyclopentane rings between. The sum of the single crosslinks and clusters of crosslinks in the model network agrees with the number of elastically effective chemical crosslinks found in the CB vulcanizate as measured by swelling in benzene. The elastically effective chemical crosslinks in the vulcanizate were calculated by the Flory-Rehner equation and the Moore-Watson calibration curve. This agreement proves that the theory of rubber elasticity in the form of the Flory-Rehner equation and the Moore-Watson calibration curve may be used to count the number of elastically effective chemical crosslinks in a sulfur vulcanizate. Three new chemical compounds and a new method for quantitatively titrating mercaptans are disclosed. The success of this technique recommends it for further study of sulfur vulcanization by other recipes and the study of other vulcanizate changes.
Pendent Accelerator Groups in Rubber Vulcanizates
Authors: Parks C. R. , Parker D. K. , Chapman D. A. , Cox W. L.
Published: February 2011

A rapid build-up of pendent accelerator groups in the initial stages of vulcanization of natural rubber has shown that they are the precursors to the formation of crosslinks. Larger amounts of attached accelerator fragments were found for efficient and so-called sulfurless systems than for conventional accelerator-sulfur systems especially at short cure times. Cleavage of disulfides and polysulfides in the pendent groups by hexanethiol in piperidine showed a build-up of di- and polysulfides in the initial stages corresponding to that of accelerator fragments. Analyses were made by liquid scintillation counting using 14C-labeled accelerators. Due to the necessity of using liquid samples, vulcanizates were degraded in xylene solution with tert butyl hydroperoxide using osmium tetroxide as a catalyst. The use of a dual vulcanization system of dicumyl peroxide followed by the addition of sulfur and labeled accelerator by swelling techniques made possible the determination of accelerator fragments at short cure times when their concentration was the greatest. Polysulfides were found to have a profound and detrimental effect on the hardening of SBR during accelerated aging in air. An increase in attached accelerator fragments was found to take place during aging which may contribute to additional crosslinking and hardening.
Relationships between the Structures of Natural Rubber Vulcanizates and Their Thermal and Oxidative Aging
Authors: Blackman E. J. , McCall E. B.
Published: February 2011

Accelerated sulfur NR vulcanizate networks are known to contain poly-, di- and monosulfidic crosslinks, sulfur-containing main chain modifications including cyclic sulfides and pendent accelerator groups, and extra-network material, primarily vulcanization residues. Both oxidative and non-oxidative aging characteristics of the vulcanizate can be improved by use of an efficient or EV curing system. These systems utilize either a high accelerator to sulfur ratio or an accelerator in conjunction with a sulfur release agent to insert predominantly mono- and disulfidic crosslinks, and produce a minimum of main chain modifications. The resulting vulcanizates have excellent aging properties, but fatigue life is poor in comparison to that of a conventional vulcanizate. With a compromise between the conventional and EV curing systems, namely use of intermediate accelerator to sulfur ratios or mixtures of sulfur and sulfur donor, vulcanizates with good fatigue properties and improved aging characteristics can be obtained. Since the most obvious difference between EV and conventional vulcanizate networks is the per cent of monosulfidic crosslinks, it has been assumed that monosulfide crosslinks are responsible for the poor fatigue resistance of EV systems. Recently, it was found that a zinc dimethyldithiocarbamate (ZDC) EV system gave an NR gum vulcanizate having fatigue resistance equal to that of a conventional sulfenamide accelerated compound. The main features of the network are a high percentage of monosulfide crosslinks and extensive main chain modification. In this paper, the aging and fatigue behavior of black filled substituted phenylenediamine protected NR ZDC accelerated systems is examined. The ZDC EV vulcanizate has imaged fatigue resistance equal to that of a conventional sulfenamide accelerated vulcanizate, but oxidative aging is very poor. By decreasing the ZDC/sulfur ratio, a vulcanizate structure equivalent in every measurable way to that of a conventional sulfenamide accelerated system is obtained. Unaged fatigue properties of the two compounds are similar. However, oxidative aging resistance of the ZDC compound is again very poor, and fatigue life of an air aged specimen is virtually zero. It is concluded that fatigue life is not necessarily a function of the crosslink type distribution. Accelerator residues and possibly main chain modifications play an important role insofar as oxidative and non-oxidative aging characteristics are concerned.
Complex Stress Relaxation in Rubber
Authors: Williams J. G. , Major T. J.
Published: February 2011

Rubber is frequently employed in engineering in states of complex stress where stress relaxation is an important factor. The mathematical analysis of the strain systems involved can be accomplished by numerical methods, and this is illustrated with a thick-walled cylinder problem. Data from simple tension and pure shear tests are used to predict the relaxation behavior of a cylinder in which the internal diameter is increased.
Modified Equations of Rubber Elasticity Applied to the Inflation Mechanics of a Thick-Walled Rubber Cylinder
Authors: Skala D. P.
Published: February 2011

The mechanics of the inflation of a thick-walled, gum rubber cylinder are predicted on the basis of the Neo-Hookean and Mooney theories of rubber elasticity. These predictions are compared to experimental results for two rubber cylinders of different geometries. The Mooney theory predicts pressures that are too high while the Neo-Hookean theory predicts pressures that are too low. Moreover, the Mooney theory fails to predict the observed instability (bulge of the tube). Data from the literature is used to show that gum rubber has different elastic properties in tension than in compression. This difference is allowed for in a modified description of rubber elasticity. The modified theory is then used to predict the mechanics of inflating thick-walled, rubber tubes. Agreement with experimental results is good.
Load-Compression Behavior of Flexible Foams
Authors: Rusch K. C.
Published: February 2011

The load-compression behavior of a foam reflects its geometric structure and the physical properties of the matrix polymer. Quantitative relations between these parameters have been established in the present study. Based on both theoretical analyses and experimental data obtained on a flexible polyurethane foam, it is shown that the compressive stress can be factored into the product of two terms: (1) a dimensionless function of the compressive strain, ψ(ε), calculated from experimental load-compression data and reflecting the buckling of the foam matrix; and (2) a factor, εEƒ, where Eƒ is the apparent Young's modulus of the foam (which is a function primarily of the modulus of the base polymer E0 and of the volume fraction of polymer, φ). Thus the compressive stress behavior of a foamed polymer is determined by E0, φ, and the matrix geometry, the latter described by the function ψ(ε). Using these established relations, it now is possible to delineate precisely the structural features a foam must possess—density, cell shape and size distribution, and modulus of the base polymer—to meet a given load-compression specification.
Thermodynamic Properties of Rubber Vulcanizates Based on Ethylene-Propylene Copolymers
Authors: Flisi U. , Crespi G. , Valvassori A.
Published: February 2011

The study of the stress-temperature curves of ethylene-propylene copolymers having different compositions revealed that ethylene-rich samples crystallize under stretching at room temperature. The results obtained suggest that crystallization is probably the outcome of two phenomena: orientation of the microcrystallites, undetectable by x-ray analysis, and true crystallization under stretching. These phenomena are accompanied by strong negative variations of internal energy; these variations, when occurring at low stretching ratios, cannot be distinguished from the variations of conformational internal energy. Therefore, when calculating the ratio ƒE/ƒ in samples containing equimolecular amounts of the two monomers, and even more in the ethylene-rich ones, at room temperature, the values may be affected by uncertainty due to the occurrence of crystallization. The calculation of ƒE/ƒ at high temperature, where the rubbery network follows Equation (2) perfectly, even in ethylene-rich samples, shows that this ratio is always negative and depends on the stretching ratio α, for any composition.
Degradation of Highly Elastic Polymers. VI. The Intermittent Chemo-Stress Relaxation of Crosslinked 1,5-Polyenes
Authors: Hillmer K.-H. , Scheele W.
Published: February 2011

Measurements of the continuous stress relaxation of thiuram vulcanizates and intermittent stress relaxation of both sulfur and C—C crosslinked vulcanizates of natural rubber were discussed. These measurements were made in the presence of either oxygen or nitrogen at various temperatures both with and without sample pretreatment. The following conclusions were drawn: 1. Determination of the rate of continuous tension loss is not suitable for measuring the oxidative degradation of vulcanizates from 1,5 polyenes according to a stress relaxation method. The reason for this is that rheological processes superimpose themselves on the chemical reactions initiated by oxygen, cause a reduction in tension, and affect both the form of the relaxation curve and also the rate of tension loss. This is particularly noticeable in the case of vulcanizates containing polysulfide bonds. 2. It was possible, using intermittent stress relaxation and by choosing suitable temperatures and frequencies, to eliminate the influence of rheological processes and thus to measure only the rate of tension loss due to oxidative degradation.
A Study on Heat History of Organic Accelerators in Rubber Stock (BR Base)
Authors: Imase I.
Published: February 2011

Rubber with some exceptions must generally undergo such processes as mastication, mixing, warming-up, extrusion, spreading, calendering, etc. prior to vulcanization under heat to obtain cured articles. Consequently the rubber matrix receives a heat history caused by mechanical frictional heat or the heat which cannot be avoided during these processes. On the other hand, when an uncured rubber compound, ready for vulcanization, containing such curing agents as sulfur, such activators as zinc oxide, and organic accelerators is heated during the processes or during storage between individual processes, each incremental effect of heat is accumulated with time. It is a well-known fact that this accumulation of heat can lead to the trouble of scorching, etc. As a cause for the trouble, organic accelerators seem to play the most important role. A few reports have been published on the action of accelerators under heat, but, to my knowledge, no report is available on the behavior of accelerators in rubber stocks, namely, on the change of the properties of uncured rubber compounds and on its influence on the properties of vulcanizates. This paper shall report these problems, though it describes only the results of the tests carried out under specific conditions.
Stress-Induced Birefringence of Swollen Polymer Networks
Authors: Gent A. N.
Published: February 2011

Stress-optical coefficients have been determined for lightly crosslinked samples of cis- and trans-polyisoprene, swollen to a high degree in a variety of nonpolar solvents. A strong dependence on the geometrical asymmetry of the solvent molecule has been observed. Solvents with long, straight molecules gave large values of the stress-optical coefficient whereas those with compact, symmetrical molecules gave minimum values. This effect is attributed to short-range orientational order, due to packing effects in a molecularly anisotropic medium. The large values obtained for unswollen polymers correspond to those in relatively anisotropic solvents, and thus indicate a significant degree of local order in the bulk phase. The minimum values are interpreted, using calculated optical anisotropies of chain repeat units, as measures of the stiffness of isolated chains.
Physics Does It in Rubber Research. Charles Goodyear Medal Address—1970
Authors: Gehman Samuel D.
Published: February 2011

From all this you can see that a physicist in rubber research can be a very busy person, both in theory and practice. Rubber science, already enriched by contributions from many disciplines, undoubtedly will continue to discover interests in some of the newer, advanced techniques in fields such as physics. I wish once more to express my appreciation for this opportunity to travel back upstream with you to glimpse a few memorable past scenes which fade again into the distance as we look to broader vistas and adventures ahead.
Surface Energy Effects for Small Holes or Particles in Elastomers
Authors: Gent A. N. , Tompkins D. A.
Published: February 2011

Expansion of a small spherical hole in a highly elastic solid is treated theoretically. Both elastic and surface energy terms are considered; the corresponding surface forces are assumed to be additive. The surface energy of the elastomer is assumed to be similar to that of simple liquids. Pressures or triaxial tensions required to inflate pre-existing holes to an indefinitely large size are calculated. Small holes require extremely large pressures, of the order of 1000 atm for holes of 10 A˚ radius. These results suggest a means of determining the distribution of hole sizes in elastomers and account, in principle, for experimental observations of cavitation processes. Detachment of the elastomer from a small rigid inclusion is treated in a similar way. The general absence of dilation or cavitation on stretching carbon black filled elastomers is thus accounted for solely in terms of the small size of these filler particles.
Volume Change Measurements of Filled Rubber Vulcanizates under Stretching
Authors: Shinomura T. , Takahashi M.
Published: February 2011

The stretching dilatometer of our own design was manufactured, and, using it, we investigated the effects of the types of fillers, the amount of fillers, and the curing time on the volume change phenomena of filled rubber vulcanizates under elongation. It was found that the Δυ/σr (the volume increase induced by the strain divided by the real stress) as a function of γ (the strain) pattern of carbon black loaded rubber vulcanizates was greatly different from that of a whitetex loaded rubber vulcanizate. This phenomenon was explained in the viewpoint of the superposition of the first type and the second type cavitations which had been mentioned in the previous paper. It was found that the critical strain γ0 is dependent upon the carbon black structures, and that the γ0 value decreases while the Δυ value increases with the amount of carbon blacks. It was also found that the Δυ value increases while the Δυ/σr value decreases with curing.
A Note on Stress Relaxation of Rubber Networks in the Vicinity of Room Temperature
Authors: Steiner G. , Tobolsky A. V.
Published: February 2011

The question of whether stress relaxation of natural rubber under normal usage at room temperature up to 100° C is caused primarily by chemical aging or physical phenomena connected with reversible changes is one which has been extensively studied. Thirion and Chasset investigated the relative effects of network relaxation and aging for dicumyl peroxide cured natural rubber in air and found network relaxation effects predominating below 55° C with aging becoming increasingly important at higher temperatures and longer times. In this study samples of natural rubber, cured by dicumyl peroxide, were relaxed both in air and in vacuum in an attempt to elucidate further the phenomenon of network relaxation. It was concluded that stress relaxation between 25° C and 100° C is much smaller in vacuum than in air, and that true equilibrium stresses are rapidly reached in vacuum.
A Mathematical Model for a Tire Sidewall
Authors: Clark S. K. , McIvor I. K.
Published: February 2011

A number of previous experimental studies on the cord load distribution in loaded and inflated bias-ply tires have revealed greatly different cord load fluctuation patterns between the crown and the side-wall regions of the tire. Typical among these is data taken from reference, where the cord load fluctuation measured at the crown center line of a typical bias-ply tire is as shown in Figure 1. The symmetry of this figure is easily explained by either bending or shell membrane considerations or by some combination of these. On the other hand, cord load fluctuations measured in the side wall of a bias-ply tire typically show fluctuations such as shown in Figures 2 or 3, also taken from the previously cited reference. Both of these latter figures show considerable asymmetry of the cord load as a function of position diagrams as the loaded tire is rolled through the contact patch. Bending of the side wall cannot account for such behavior as shown in Figures 2 or 3, since the changes in radius of curvature in the side wall region are completely symmetric about the center line through the contact patch, denoted by the zero degree point in Figures 2 and 3. Simple pressure vessel considerations are also not capable of explaining such diagrams, since changes in curvature in the side wall region are also completely symmetric and hence would result in symmetric stress distributions.
Influence of Chemical Nature on Flow Behavior of Polymers. I. Shear Rate and Temperature Dependence
Authors: Collins E. A. , Mass T. , Bauer W. H.
Published: February 2011

The flow behavior of low molecular weight liquid polybutadiene polymers was studied as a function of shear rate, temperature, and molecular structure. Measurements were made in the temperature range 15 to 50° C and shear rate range 1 to 15,000 sec−1 with a cone and plate and a capillary viscometer. Limiting low shear viscosity, flow activation energy, and critical shear rate marking onset of shear rate thinning were found to be strongly dependent upon the presence of functional groups such as hydroxyl, carboxyl, and nitrile. The modifying groups showed a greater effect than the molecular weight upon the properties measured.
Properties of a Trans-1,5-Polypentenamer Produced by Polymerization through Ring Cleavage of Cyclopentene
Authors: Haas F. , Nützel K. , Pampus G. , Theisen D.
Published: February 2011

Trans-1,5-polypentenamer can be produced with a high degree of uniformity by solution polymerization using complex catalysts of the Ziegler/Natta type. The state of order of the polymer indicates a rubber with good low temperature properties and crystallization behavior similar to that of natural rubber. There is therefore a fundamental difference between this material and other synthetic rubbers of the general purpose type. Trans-1,5-polypentenamer is easy to process, can be heavily loaded, and is therefore inexpensive to compound. Even when small amounts of the crosslinking agents are used, a high cross-linking yield is obtained and this makes it possible to produce vulcanizates that are resistant to dynamical stresses. As high green strength and good tack are present simultaneously, the material is suitable for use in the tire industry, particularly in carcass production.