bluuebunny/crossref_metadata_2025 huggingface dataset:
Polyurea Dispersions for RIM Applications
Authors: Phillips B. A. , Taylor R. P.
Published: February 2011

PHD polyols have been demonstrated to be more effective agents for increasing the flexural modulus of high modulus RIM elastomers than normal poly(oxyethylene) polyols or polymer polyols. As expected for higher modulus RIM materials, tensile strength also increases, and impact strength and elongation decrease. The use of PHD polyols also effectively increases flexural modulus without the excessive brittleness or reactivity that would result from the use of higher concentrations of glycol or amine chain extenders, polyfunctional isocyanates or polyol crosslinking agents.
Model Compound Vulcanization—Part II. Comparison of Accelerators
Authors: Lautenschlaeger F. K. , Zeeman P.
Published: February 2011

The efficiency of each of 20 accelerators to convert sulfur into sulfidic products has been evaluated by Model Compound Vulcanization. The sulfidic products can be isolated and identified by gas-liquid chromatography. The sulfidic products can be separated into groups of mono-, di- and trisulfides, with several isomeric products within each group. This paper has reported on the analysis of such isomeric products which correspond to different types of sulfur crosslinks in natural rubber vulcanizates. The analysis of monosulfidic product yields and of the isomer distribution within the monosulfide group is sufficient to characterize an accelerator.
Model Compound Vulcanization—Part IV. Comparative Assessment of Sulfenamide and Triazine Accelerators
Authors: Lautenschlaeger F. K. , Edwards K. , Kirkham M. C.
Published: February 2011

The application of model compound vulcanization (MCV) to the rating of the relative efficiency of CBS (N-Cyclohexylbenzothiazole-2-sulfenamide) and an experimental triazine-based accelerator is demonstrated. The sulfur to accelerator ratios were established at which both accelerators lead to the same total sulfidic product yield. At these ratios, selected initial properties of natural rubber vulcanizates and their reversion resistance are similar. Hence, the increased reversion resistance of triazine-accelerated natural rubber vulcanizates is essentially due to their greater efficiency in converting sulfur into sulfidic crosslinks. A higher level of the less efficient CBS leads to similar results. This analysis shows that MCV simplifies the evaluation of accelerators.
Dithiodimorpholine-Based Accelerator System in Tire Tread Compound for High-Temperature Vulcanization
Authors: Bhowmick A. K. , De S. K.
Published: February 2011

A tire tread compound suitable for high-temperature vulcanization has been designed. Of the various mixes studied, the one containing dithiodimorpholine (1 phr), CBS (2 phr) and sulfur (0.5 phr) was found to be most suitable for high temperature vulcanization of natural rubber. The network structures of the vulcanizates have been determined, and the technical properties of the new tire tread compound have been compared with those of a conventional compound. It has been found that a DTDM-based compound is superior in technical properties, reversion resistance and ageing resistance. The polymer-filler attachment was found to be higher DTDM-based compounds than for conventional ones.
Control of Alfin Rubber Molecular Weight
Authors: Hansley V. L. , Greenberg H.
Published: February 2011

Control of molecular weight of alfin catalyzed diene polymers has been demonstrated for polymers of butadiene, isoprene, styrene, piperylene and combinations of these. The 1,4-dihydro derivatives of benzene and naphthalene are effective molecular weight moderators. The known microstructure of alfin polymers remains unchanged with the use of these agents. Molecular weight controlled alfin gums process readily on standard rubber machinery. Oil extenders or softeners, although not necessary, are easily incorporated. The feasibility of continuous processing has been demonstrated for the production of catalyst, moderators and polymers on pilot plant scale. A wide series of polymers and copolymers at predetermined molecular weights and compositions have been produced in the same pilot plant. Alfin polybutadiene with limited amounts of styrene is essentially a solution type SBR. Alfin copolymers with styrene and/or isoprene have undergone preliminary road tests in tires. Wear and heat build-up data indicate performance to be intermediate between SBR and polybutadiene.
Measurements of Friction of Elastomers by The Skid Resistance Tester
Authors: Bassi A. C.
Published: February 2011

A skid resistance tester was used as a laboratory instrument for testing dynamic friction coefficients between rubber vulcanizates and standard wet surfaces. Test conditions required to obtain satisfactory experimental results, are examined. The influence on skid measurements of the type of elastomer and amount of carbon black are considered. Skid resistance is correlated with hysteretic properties of the material; some results on the influence of surface phenomena, like water-rubber adherence are reported. Other measurements performed on oiled instead of water wet surfaces and on smooth ice are described.
Volatility of Antioxidants and Antiozonants II. Effect on Rubber Testing
Authors: Spacht R. B. , Hollingshead W. S. , Bullard H. L. , Wills D. C.
Published: February 2011

In the first paper of this series data were presented on the volatility and vapor pressure of eight antioxidants. The present work attempts to show how volatility-affects the performance of an antioxidant, particularly in accelerated aging tests. The usual accelerated tests are of two types: those using closed systems such as the air bomb, the oxygen bomb, and oxygen absorption, and those using open systems such as air oven aging and creep.
Vulcanization of Elastomers. 39. Vulcanization of Natural and Synthetic Rubbers by Sulfur and Sulfenamides. II
Authors: Scheele W. , Kerrutt G.
Published: February 2011

The dependence of the kinetics of vulcanization of natural rubber by sulfur in presence of N-cyclohexyl-benzothiazolylsulfenamide (CHBS) on temperature and for varied molar ratios of accelerator and sulfur was more exactly investigated. 1. Although the vulcanization of natural rubber accelerated with N-cyclohexyl-benzothiazolylsulfenamide with conventional experimental conditions is characterized by very long induction times, nevertheless—as also for other accelerated vulcanization reactions—sulfur decrease follows a time law with nt<1 and the activation energy, as in other cases, is calculated to be 29 kcal/mole. 2. The value nt=0.6 is found for the exponent of the time law, independent of temperature and concentration of reactants. 3. The concentration dependence of the rate of sulfur decrease under conditions of constant sulfur content and increasing sulfenamide concentration is found to be consistent with catalysis by an intermediate compound. 4. The actual accelerator may be essentially cyclohexylammonium-benzothiazoylmercaptide. 5. The temperature and concentration dependence of the induction periods for sulfur decrease and crosslinking were investigated. The course of crosslinking with reaction time, which is characterized by strong reversion, is discussed.
Vulcanization. Part VII. Kinetics of Sulfur Vulcanization of Natural Rubber in Presence of Delayed-Action Accelerators
Authors: Coran A. Y.
Published: February 2011

The scheme and treatment of the kinetics of scorch-delay vulcanization proposed in a previous paper were applied to natural rubber sulfur vulcanization accelerated by 2,2′-thiobisbenzothiazole (MBTS), 2-(N-cyclohexyl)benzothizaolesulfenamide (CBS), 2-(N-morpholinothio)benzothiazole (MOR), or 2-(N,N-diisopropyl)benzothiazolesulfenamide (DPBS). According to the scheme (see PDF for diagram) where the subscript x = a positive integer. The specific rates k1 and k2 and the ratiok4/k3′ were related to starting concentrations of accelerator, sulfur, and stearic acid. In general, an increase in stearic acid concentration or a decrease in accelerator concentration induces an increase in k1 or k4/k3′ but a decrease in k2. An exception to this is that, when the highly hindered sulfenamide DPBS is used as the accelerator, k1 does not change with changes in starting concentrations. It is interesting that k2 does not respond appreciably to changes in sulfur concentration when sulfenamides are used, unless the sulfur concentrations are extremely low. In contrast to this, k2 increases linearly with the square root of the sulfur concentration when MBTS is used. The changes in rates which occur with changes in the starting concentrations were attributed to the formation of chelates between zinc ion (brought into solution by stearic acid) and accelerator, intermediate reaction products, or crosslink precursors. The differences noted between the rates obtained with the three sulfenamides were attributed to differences in steric hindrance and stability.
Compounding Styrene-Butadiene Rubber with Shellac. II
Authors: Saxena K. K. , Banerjee S.
Published: February 2011

This study demonstrates that both shellac and its ester improve considerably most of the mechanical properties of SBR. Shellac-ester functions more favorably in this regard. Both these materials can be used as a reinforcing filler for SBR especially if the end product is not intended to be used in service demanding severe abrasion.
Infrared Studies of the Rubber-Filler System. II. Identification of Rubbers Compounded with Fillers by Infrared Absorption Spectra
Authors: Suito E. , Arakawa M.
Published: February 2011

Infrared spectra of vulcanized rubbers compounded with fillers were studied on thin sections of samples made by a low temperature microtome technique. By this method, identification of fillers as well as rubbers is possible. The infrared absorption spectra of rubbers compounded with several types of fillers are described.
Peroxide Crosslinking of Ethylene-Propylene Rubber
Authors: Loan L. D.
Published: February 2011

The crosslinking of ethylene-propylene rubber by means of cumyl peroxide has been followed in detail. The overall crosslinking efficiency has been found to be about 0.4 crosslinks per peroxide molecule, and this result has been shown by stress relaxation and permanent set experiments to result from a mixture of both scission and crosslinking reactions. The results obtained with two different rubbers are consistent with the view that scission results from abstraction of a tertiary hydrogen atom while crosslinking arises from attack at a secondary hydrogen. Such a scheme allows a relative reactivity of tertiary:secondary of 6:1 to be calculated. The action of sulfur in peroxide cures has been shown to be to introduce labile, presumably sulfur-containing, crosslinks. Such labile crosslinks explain the increased tensile strength of such vulcanizates. Allyl compounds are effective in increasing the crosslinking efficiency of the peroxide.
Light Scattering from Crystalline Superstructure in Stretched Natural Rubber
Authors: Yau W. , Stein Richard S.
Published: February 2011

The existence of fibrous structure in stretched crystalline elastomers has recently been reported by Walters. It has been suggested that this structure represents stress crystallinity. The low-angle light scattering from stretched, cured natural rubber has been under study in this laboratory, and it has been reported that the scattering pattern results from crystalline superstructure accompanying the stress-induced crystallization. This structure has a size of the order of microns and is undoubtedly related to the structure reported by Walters. Rubber samples studied were experimental sulfur-cured samples not containing any filler. Light-scattering studies were carried out by the laser photographic technique which has been described. It was found necessary to sandwich the elongated samples between microscope cover glasses using a silicone oil immersion fluid to prevent surface scattering from making an undue contribution. Scattering patterns obtained for stretched samples at room temperature in this way are presented in Figures 1 and 2 for VV polarization (polarizer and analyzer parallel to each other and to the stretching direction) and VH polarization (polarizer parallel the to stretching direction and analyzer perpendicular). HH and HV patterns were also obtained.
Nonhomogeneous Straining and Fracture Mechanism in a Filled Elastomer
Authors: Ronay Maria
Published: February 2011

This paper presents an experimental study of strain concentrations arising near the edges of elliptical cutouts in uniformly strained, flat strips of polyurethane rubber filled with a high volume percentage of an inorganic salt. The strain concentrations are studied under various conditions of uniform straining, for strains up to fracture initiation. The fracture mechanism at the root of the cutout is studied micrographically and it is found that the local fracture process is similar to that in unfilled elastomers, with consecutive rupture of highly deformed strands of the elastomeric material.
Fracture of Amorphous Polymeric Solids: Time to Break
Authors: Halpin J. C.
Published: February 2011

The failure theory of Bueche and Halpin is generalized and expanded to obtain a prediction of the time dependence of tensile strength and ultimate elongation. The model pictures rupture as propagation of tears or cracks within the material. The growth of a tear or crack is viewed as an ideally simple process in which the molecular chains at the tear tip stretch viscoelastically under the influence of a high stress concentration until they rupture. As a consequence, the failure process is a nonequilibrium one, developing with time and involving the consecutive rupture of the molecular chains. Substantial support for the theory is found by comparing the theoretical prediction with experimental results obtained for SBR and EPT. In addition, it is experimentally demonstrated that delayed and forced rupture experiments can yield qualitatively identical data for viscoelastic bodies. Also discussed is the basis for the approach to the nonlinear response of a viscoelastic body required here in the development of a fracture theory.
Fracture of Amorphous Polymeric Solids: Reinforcement
Authors: Halpin J. C. , Bueche F.
Published: February 2011

Although the tensile stress at break (σb) and the corresponding ultimate elongation (αb) of a reinforced elastomer depend markedly on temperature and test rate, characteristic time-to-break curves can be obtained utilizing the Leaderman-Tobolsky-Ferry superpositioning techniques. In addition, a distinctive failure envelope is obtained when the logσbT0/T is plotted against log(αb−1), where T and T0 (° K) are the test temperature and an arbitrary reference temperature. Delayed rupture experiments yield qualitatively the same results as forced rupture experiments. Furthermore, an analysis of the Leaderman-Tobolsky-Ferry shift parameters indicate a concomitant increase in the apparent activation energy with increasing reinforcement concentration. These observations are explicable by utilizing the failure theory of Bueche and Halpin. It is now apparent that failure is the result of the propagation of tears or cracks within the viscoelastic body and that the time and temperature dependence of the ultimate properties is directly related to the time-temperature dependence of the modulus. A preliminary picture of the salient molecular events is presented.
Cut Growth and Fatigue of Rubbers. I. The Relationship between Cut Growth and Fatigue
Authors: Gent A. N. , Lindley P. B. , Thomas A. G.
Published: February 2011

The relationship between the cut growth and fatigue failure of natural rubber vulcanizates under repeated loading is examined. The cut growth behavior has been investigated using several types of test piece, and the results are shown to be consistent when interpreted in terms of the tearing energy concept developed previously. The most comprehensive data have been obtained by measuring the growth of a small cut in the edge of a strip cycled in simple extension. It is found that the cut growth per cycle is approximately proportional to the square of the maximum tearing energy attained during the cycle. Using this relation, the fatigue life of a specimen containing a small cut is deduced from elasticity theory as a function of initial cut size and maximum strain. Experimental results give good agreement with theory. A similar strain dependence is found for the fatigue life of die-stamped dumbbell test pieces with no intentionally produced flaws; this is consistent with the mechanism of failure being cut growth from small flaws present in the specimens. Their effective size is estimated to be about 2×10−3 cm, which is compatible with the observed superficial imperfections of the cut edges.
Cut Growth and Fatigue of Rubbers. II. Experiments on A Noncrystallizing Rubber
Authors: Lake G. J. , Lindley P. B.
Published: February 2011

Tensile fatigue failure of a gum vulcanizate of noncrystallizing SBR can be accounted for by the growth of small flaws initially present in the rubber. Fatigue of crystallizing natural rubber was shown in Part I to be attributable to the same cause. Cut growth results are interpreted in terms of the tearing energy theory of Rivlin and Thomas. SBR exhibits cut growth under both static and dynamic conditions; in each case the rate is approximately proportional to the fourth power of the tearing energy. Variation of the dynamic cut growth rate with frequency can be explained by the summation of a time-dependent static component of growth and a cyclic component not dissimilar to that occurring in natural rubber. Fatigue failure, under both static and dynamic conditions, is predicted from the cut growth results. These predictions are found to account quantitatively for experimentally observed fatigue lives when a suitable value is assumed for the initial flaw size. Fatigue lives at different temperatures correlate well with cut growth results obtained by Greensmith and Thomas over the same temperature range. The results are compared to those obtained previously for natural rubber, and possible reasons for the differences in fatigue behavior of crystallizing and non-crystallizing rubbers are discussed.
Studies in the Thermodynamics of Polymer-Liquid Systems. I. Natural Rubber and Polar Liquids
Authors: Booth C. , Gee G. , Holden G. , Williamson G. R.
Published: February 2011

There is a great lack of reliable thermodynamic data on concentrated polymer solutions. This group of papers contributes further measurements, especially on systems consisting of a polar liquid and nonpolar polymer, and presents a review of the current situation. This first part records observations on the swelling of natural rubber in several polar liquids, using two different techniques. One of these is the conventional study of vapor pressures; the other is a novel procedure involving measurements of swelling and tension in stretched crosslinked samples. The results are compared with the Flory-Huggins equation, and show that as swelling proceeds and approaches saturation, there is a marked fall in the interaction parameter χ`. At the same time the entropy of dilution falls progressively further below the curves predicted by current theories.
Studies in the Thermodynamics of Polymer-Liquid Systems. II. A Reassessment of Published Data
Authors: Booth C. , Gee G. , Jones M. N. , Taylor W. D.
Published: February 2011

Part II contains a complete reassessment of all published data on polymer solutions, using a single method of analysis. Non-polar liquids in non-polar polymers show an entropy of mixing higher than predicted by current theories. The discrepancy is attributed to a degree of order in the solid polymer. Polar liquids in non-polar polymers follow the pattern described in Part I: this behavior is attributed to non-random mixing at concentrations approaching saturation.
Contribution to the Study of Ethylene-Propylene Copolymers by Infrared Spectroscopy. Distribution of Monomeric Units
Authors: Bucci Giovanni , Simonazzi Tonino
Published: February 2011

Elastomeric properties of ethylene-propylene copolymers depend not only on the composition but also on the copolymer microstructure, i.e., on the distribution of monomeric units along the macromolecular chain. According to Natta and coworkers the best polymer should be obtained when two monomeric units are randomly alternated in the macromolecule. The random alternation of the monomers is actually only partial because somewhat longer homosequences are formed. In such a case one deals with block polymers. On the basis of some theoretical considerations on the copolymerization process and also on the basis on the reactivity ratios of the two monomers, Natta and coworkers have calculated the distribution functions of sequences for various catalyst systems. Infrared spectroscopy has been widely used in connection with this problem though from different viewpoints and with different aims. The purpose of our work is to reanalyze the whole problem. We have focused our attention on the CH2 rocking band in the sequence (—CH2—)n, where n varies from 1 to 5 or more. Some disagreement exists between various authors on the assignment of these bands, especially in the case of the (—CH2—)3 and (—CH2—)4 sequences. We have then divided our work as follows: (a) assignment of infrared bands in the spectral region between 900 and 650 cm−1; (b) calculation of absorbances at various frequencies; (c) attempt at a numerical evaluation of the distribution of monomeric units.
Use of Sol-Gel Analysis to Estimate Chain Scission during Vulcanization. II. Peroxide Vulcanization of Synthetic cis-Polyisoprene
Authors: Bristow G. M.
Published: February 2011

Sol-gel analysis indicates that in the vulcanization of synthetic cis-polyisoprene by cumyl peroxide there are only 2–3 sites of scission per 100 crosslinked isoprene units, a value closely comparable with that observed for natural rubber.
Bonding of Fibrous Glass to Elastomers
Authors: Vanderbilt Byron M. , Clayton Robert E.
Published: February 2011

Elastomers can be bonded to fibrous glass using a chemical at the interface which is capable of reacting with both glass and rubber. Organic silanes have proven most useful for this purpose. Unsaturated silanes are very effective for bonding glass to peroxide-cured elastomers. Use of a nonconjugated diolefin as coupling agent between silane and elastomer greatly enhances the degree of bonding. Certain substituted alkyl silanes are effective when used with the right combination of rubber and curative. Bonding strengths at ordinary temperatures approach or exceed those of the respective vulcanizates. Glass-elastomer adhesions at elevated temperatures show promise. All commercial elastomers can be effectively bonded to fibrous glass.
Elastic Stability of Rubber Compression Springs
Authors: Gent A. N.
Published: February 2011

Experimental measurements are described of the critical (buckling) compressive loads for columns of rubber springs under various loading conditions. They are shown to be in good accord with the theoretical treatment of Haringx, in which the classical theory of buckling is modified to take into account the shear deflection of column elements. Measurements are also described of the shape of the buckled column and of the shear stiffness of columns under compressive loads smaller than the buckling load. They are found to be satisfactorily described by the theory also.
Elastomeric Polycarbonate Block Copolymers
Authors: Goldberg Eugene P.
Published: February 2011

Polycarbonate block copolymers were prepared by phosgenating pyridine solutions of polyether glycol-bisphenol-A mixtures. Copolycarbonates derived from poly(oxyethylene) glycols (Carbowaxes) were studied in detail for property-structure effects as a function of glycol molecular weight (1000–20,000) and copolymer composition (5–70 weight per cent or 0.3–10.0 mole per cent of a 4000 molecular weight glycol). Remarkable strength (>7000 psi) and snappy elasticity (>90 per cent immediate recovery) were observed at poly(oxyethylene) block concentrations greater than 3 mole per cent. These thermoplastic elastomers also exhibited high softening temperatures (>180° C) and tensile elongations up to about 700 per cent. Both Tg and softening temperature varied linearly with comonomer mole ratio over the composition range studied, with Tg displaying much greater polyether concentration sensitivity. It is suggested that the observed property effects result to a large extent from the variation in poly(bisphenol-A carbonate) block length that accompanies the changing of copolymer composition. An initial increase in flexural modulus (stiffness) was observed at low polyether concentrations (0–1 mole per cent). This phenomenon is considered to be related to similar modulus effects found in plasticized rigid thermoplastics at low plasticizer concentrations. A moderate degree of molecular order, due to bisphenol carbonate segments rather than the normally crystalline polyether, was detected by x-ray analysis. Elastomeric carbonate-carboxylate tetrapolymers were also prepared by partial replacement of carbonate with isophthalate, terephthalate or adipate linkages in polyether-bisphenol systems. The dramatic softening temperature depression observed in this class of polymers is attributed to the disruption of long bisphenol carbonate block sequences that exist in the simpler polyether glycol-bisphenol carbonate copolymers.
Mechanical Degradation of Polymers
Authors: Booth C.
Published: February 2011

A sample of polyisoprene having a narrow molecular weight distribution has been degraded mechanically in several ways and molecular weight distributions of the degradation products have been determined. From these data we conclude that chain scission occurs at a point far removed from the ends, but not necessarily near the center, of the molecules.
Reactions between Peroxide and Polydiolefins
Authors: van der Hoff B. M. E.
Published: February 2011

Reactions of cumyl peroxide with polyisoprenes and polybutadienes are compared. Hydrogen abstraction from polyisoprenes and recombination of polymer radicals yield a network with crosslink concentration equal to that of decomposed peroxide. The concentration of crosslinks in polybutadiene networks is much greater than that of decomposed peroxide. Values of the ratio of these two concentrations as high as 50 have been calculated from swelling measurements of the vulcanizates. The polybutadienyl radical attacks double bonds in the polymer; thus crosslinks are formed by propagation. Infrared analysis shows that propagation proceeds via unsaturation in the main chain as well as via double bonds of vinyl side groups. The experimental relation between crosslink concentration and antioxidant concentration is consistent with a reaction scheme based on termination of polymer radicals by antioxidant only.
Anionic Polymerization of Isoprene
Authors: Worsfold D. J. , Bywater S.
Published: February 2011

Polymerization of isoprene with butyllithium in cyclohexane has been studied. The initiation reaction was found to be complex; plots of conversion of butyllithium against time are sigmoidal. The order with respect to the initiator was found to be between one-half and one. The order of the isolated propagation reaction in polyisoprenyllithium was found to be one-quarter and was correlated with a fourfold association of active chain ends determined by dilute solution light-scattering measurements. The major effect of tetrahydrofuran on the reaction was the great increase of the initiation rate. The kinetic order changes slowly with the addition of small amounts of tetrahydrofuran, suggesting that the aggregates are broken down with difficulty. The microstructure of the polymer has been determined by NMR measurements and shows a steady transition from a largely 1:4 to a largely 3:4 structure as the polarity of the solvent is increased by the addition of tetrahydrofuran.
Mechanochemical Effects in Fatigue of Rubber
Authors: Reztsova E. V. , Chubarova G. V. , Slonimskii G. L.
Published: February 2011

1. It has been shown that the effectiveness of the action of antifatigue agents changes depending on the method of addition to the rubber. 2. The change in effectiveness of such agents points to the fact that there is significant connection between mechanochemical phenomena occurring during milling and fatigue of the polymer, which must be taken into account in manufacturing practice. 3. The effect of preliminary milling of the rubber with antifatigue agents on fatigue life of the compound apparently can be explained as resulting from consumption of antifatigue agents in the milling process, owing to mechanochemical reactions.
Thermal Degradation of Ethylene Propylene Rubber. C2-C6 Hydrocarbons as Thermal Decomposition Products of SKEP
Authors: Slobodin Ya M. , Mayorova V. E. , Smirnova A. M.
Published: February 2011

1. Degradation of SKEP was carried out. 2. The qualitative and quantitative compositions of C2-C6 hydrocarbons in the products of degradation were established by gas-liquid chromatography.
A New Tear Test for Rubber
Authors: Veith A. G.
Published: February 2011

Knowledge of rupture properties is required to characterize rubbers in laboratory evaluation adequately and to attempt to estimate service performance. The most commonly measured rupture property is tensile strength. Tear strength, although known to be important, is measured less often due to a general consensus that existing tests are inadequate. This paper describes a new, practical, tear test method based on theoretical work by workers at the Natural Rubber Producers' Research Association (NRPRA). Their work has shown that a characteristic tear strength may be measured independent of testpiece geometry. A testpiece has been devised that circumvents two deficiencies inherent in the “trousers” testpiece advocated by NRPRA. These are: appreciable leg extension, and failure of the tear to propagate down the central axis when tear tests are carried out on a wide variety of vulcanizates. These deficiencies are overcome by reinforcing the legs with fabric and by molding a longitudinal groove of a definite cross-sectional geometry into the testpiece. These alterations of the basic trousers test specimen allow almost any type of rubber to be tested successfully. Detailed descriptions of the testpiece, mold, method of test, and tear curve analysis are given, as well as results of a statistically planned evaluation program. Effects of crosslink density, carbon black content, processing oil content, and test temperature are determined for natural rubber and SBR. The influence of speed of testing in a rather restricted range is also included. When modified trousers tear strengths are compared with tear strengths from a common test, ASTM D-624, Die C, pronounced differences are apparent. These are pointed out and deficiencies in the Die C test discussed.
New Approach to Tire Durability Testing
Authors: Coddington D. M. , Marsh W. D. , Hodges H. C.
Published: February 2011

A new fundamental approach to tire durability testing is described based on dynamic temperature build-up. This has been made possible by development of a practical technique for continuously measuring the internal shoulder temperature of tires during actual road operation at speeds up to 80 mph. Utility of this technique has been demonstrated in a systematic characterization of effects of load, inflation, speed, and ambient temperature on tire running temperature for tires made from two widely different elastomers, SBR and butyl. Within the scope of the experimental study, correlation of tire durability to running temperature has been developed. The dynamic temperature measuring technique and the apparent relations between running temperature and durability should have broad application in the tire industry.
Determination of Molecular Weight Distributions of Elastomers with the Ultracentrifuge
Authors: Henderson J. F. , Hulme J. M. , Small R. M. B. , Williams H. L.
Published: February 2011

The ultracentrifuge has been used to obtain molecular weight distributions of cis-1,4-polybutadiene and styrene/butadiene rubber. When corrections are made for the effects of centrifugal pressure and solution concentration, distributions are obtained that are in good agreement with those determined by elution chromatography. Rapid comparisons of polymers are sometimes possible, especially if the polymers are chemically similar.
Molecular Weight Distributions of Elastomers—Comparison of Gel Permeation Chromatography with Other Techniques
Authors: Gamble Leon W. , Westerman Lowell , Knipp Ebnest A.
Published: February 2011

During the past decade interest in polymer fractionation for evaluating molecular weight distribution has increased. New polymers, such as polypropylene, high density polyethylene and many others have been subjected to extensive characterization, but some older polymers, including natural rubber have not. In recent years column fractionation has received a great deal of attention; a review has been made by Schneider. Most of the techniques of polymer fractionation, such as precipitation from solution and column fractionation, are quite lengthy and require days or even weeks to complete. A more rapid method for molecular weight distribution has been the goal of polymer chemists for years. The most promising of the rapid methods are turbidimetric and gel permeation chromatography by which analysis is accomplished in a few hours. This paper discusses application of gel permeation chromatography to fractionation of elastomers. In addition this technique is compared with data from column fractionation, light scattering and osmometry. Some of the assumed factors used in converting gel permeation data into molecular weights are in error. But by calibration of the gel permeation chromatograph with classical methods reliable data can be obtained.
Fatigue Failure in Nylon Reinforced Tires
Authors: Patterson Ralph G. , Anderson Robert K.
Published: February 2011

The principal mechanism leading to fatigue failure of nylon cord tires is progressive loss of cord strength from filament ruptures that occur at an angle of approximately 25 degrees to the fiber axis. Flexing causes cords to become weaker until residual strength is less than tensile forces in the tire, at which time the remaining filaments break with a blunt-ended tensile rupture. Individual cords in tires have stress environments determined by such factors as cord spacing, uniformity, and stiffness of the immediate flexing region, which appear to influence significantly their fatiguing rate. It is probable that these factors also operate in laboratory in-rubber fatigue tests, so that a high degree of precision in sample preparation and loading conditions is essential for such tests to give reproducible data. Measurement of cord strength loss was found to be a valid and useful means for following the progress of fatigue when it was recognized that cords under opposite sidewalls of tires lost strength at markedly different rates, which were not linearly proportional to test mileage. The direction of tire rotation and the cord angle in a given ply determined which half of the ply lost strength fastest. Strength loss was higher for the half of the cord that led into the load bearing region of the tire as it rotated. This presumably happened because of unsymmetrical stress conditions, but no measurement of cord stress was made. Fatigue failures of tires generally occurred at localized points and resulted from failure of a few cords that lost strength faster than the surrounding cords. The failures occurred in the innermost ply at “hinge points” opposite the end of the flipper fabric in the ply splice.
Microstructures of Polydienes Prepared from Alkyllithium
Authors: Hsieh Heney L.
Published: February 2011

The stereochemistry of polymerization of isoprene and butadiene by means of alkyllithium initiators was examined. The stereochemistry of polybutadiene and polyisoprene responded to the same variables in anionic polymerization. Micro-structure of both polymers is sensitive to initiator level, solvent type, and polymerization temperature, but is independent of monomer concentration, conversion and initiator structure. The heat activation energy of the reaction leading to 1,2 addition is greater than of that leading to trans-1,4 addition (1200 cal/mole). The exact sequence leading to stereospecific polymerization of dienes by means of alkyllithium is still not clear. However, it seems to involve orientation of the incoming monomer units by association and passage through a transition complex in which all bonds are largely covalent in character and within which polymerization occurs.
Melting Point of cis-1,4-Polybutadiene
Authors: Mitchell J. C.
Published: February 2011

Melting point values for cis-1, 4-polybutadiene thus far reported in the literature have not exceeded +1° C. Varying values have been reported, both because of sample variations (differing amounts and distributions of chain imperfections) and because of variations in experimental techniques used to measure the melting point (in particular, varying heating rates). Some workers have extrapolated their data to predict that the melting point of a perfectly regular (100 per cent cis) cis-1, 4- polybutadiene (Tm°) would be close to +1° C. These extrapolations have been used to arrive at heats of fusion for the polymer from Flory's theory for melting point depression due to copolymeric impurities. One such value has been used in another investigation to estimate extents of crystallization from thermodynamic data. We wish to report observation of melting points several degrees higher than the assumed Tm° value of +1° C. Our experiments show that these higher values are not due to orientation effects.
Determination of Effective Crosslink Density in Silicone Rubber
Authors: Seeley Robert D. , Dyckes George W.
Published: February 2011

As a result of the investigations reported here, a simple and precise method was evised to measure the compression-deflection of solvent-swollen silicone rubber. The method was found to be reliable, and the resultant data were used to calculate effective crosslink densities of solid and cellular silicone rubber compounds. Empirical equations were derived relating compression and deflection to effective crosslinking of solid and cellular silicone rubber swollen in toluene. The weak swelling action of MEK precludes the derivation of empirical relationships between compression, deflection, and effective cross-linking of the rubber. The investigation further showed that toluene is a better and more useful swelling agent than methyl ethyl ketone (MEK). The volume swelling ratios for toluene and MEK were determined. Toluene was found to be about 1.4 times more effective as a swelling agent than MEK. Limited laboratory physical test data show a reasonable correlation to νe/Vr data. (More testing will be required to establish definite relationships.) This investigation also showed that the method employed here for crosslinking determinations is suitable for cellular rubber, since the data obtained from toluene-swollen specimens agrees quite well with theoretical calculations. Finally, the Flory-Huggins interaction parameters for toluene and MEK were determined.
Viscosity and Molecular Weight of Masticated Styrene-Butadiene Rubber
Authors: Bhatnagar S. K. , Banerjee S.
Published: February 2011

Changes in the value of [η], [ηm], K′, (M) and μ of SBR masticated in the cold at 25.0 ± 5.0° C in presence of oxygen with the times of mastication are reported. An empirical relation has been developed between the intrinsic viscosity [η] and Mooney viscosity [ηm] which permits molecular weight of the rubber to be determined directly from Mooney viscosity. The value of g which appears in the Flory equation connecting true crosslink density with the physically determined one has been calculated for unfilled SBR.
Compounding an Ethylene Propylene Terpolmer Elastomer for Dynamic Applications
Authors: Baseden G. A.
Published: February 2011

Dynamic properties of vibration isolators made with EPT vulcanizates may be controlled, within limits, by choice of compounding ingredients in the vulcanizate. The choice of compounding ingredients will also affect efficiency of the isolator in preventing transmission of vibrations at low temperatures. Efficiency of vibration isolation may be calculated by measuring dynamic spring rates at the temperatures of operation.
Polymerization of Chloroprene in the Presence of Organometallic Compounds: The System Chloroprene-Butyllithium
Authors: Erusalimskii B. L. , Krasnoselskaya I. G. , Mazurek V. V.
Published: February 2011

In the polymerization of chloroprene in the presence of butyl lithium, a substantial role is ascribed to reactions of deactivation of growing chains, which prevents the possibility of attaining a high degree of conversion. Kinetic data indicate the anionic mechanism of the process.
Injection Molding of Rubber: Structure, Processing and Properties
Authors: Tsai Boh C.
Published: February 2011

The anisotropic behavior of an injection molded part from a disk mold is reported. It is concluded that orientation rather than state of cure is responsible for the anisotropic behavior. The slow strain recovery process of elongational deformation of the uncured rubber is the major factor in introducing orientation. This example is used to illustrate the viewpoint that the total injection molding process should be investigated from an integral approach which takes into account the interactions of parameters in various areas. Engineering concepts such as the unit operations approach have been gradually gaining acceptance in injection molding of rubber, and compound development continues to progress satisfactorily. However, the success of injection molding will largely depend on the skillful manipulation of material characteristics, design parameters, and operating variables under some unfavorable constraints to meet the cost and performance requirements.
Tear Strength of Stretched Rubber
Authors: Gent A. N. , Kim H. J.
Published: February 2011

Highly stretched rubber sheets can be split apart quite easily, indicating that the originally isotropic material becomes “fibrous” in character—i.e., much weaker for a tear running in the direction of extension than for one running at right angles to it. This anisotropy of strength is shown most clearly by elastomers which crystallize on stretching, for example, natural rubber compounds; it is not so evident in rubber compounds which do not undergo strain-induced crystallization. Also, it is apparently more pronounced for compounds containing reinforcing fillers, notably carbon black, which stiffen and strengthen elastomers. Indeed, carbon-black-filled natural rubber compounds show anisotropy of strength even after the extension is removed. The test sheet then regains its original dimensions more or less completely, but it remains softer for subsequent deformations in the direction of pre-stretching, and weaker for a tear running in this direction. Although unfilled natural rubber compounds and filled and unfilled noncrystallizing elastomers are also softened to varying degrees by prestretching, they do not appear to show anisotropy of strength to any significant degree when the prior extension is removed. This phenomenon is apparently unique to filled, strain-crystallizing, elastomer compounds. Differences in strength in different directions, induced either by a previous deformation or by a present one, are of considerable practical interest. They are probably responsible for the phenomenon of “knotty” tearing, when a running tear spontaneously deviates from the expected path, at right angles to the principal tensile stress, and turns instead into the stress direction. As a result, stress concentrations at the tear tip are greatly reduced, and the tear stops altogether until the applied stress becomes high enough for a new tear to break ahead. This stick-slip pattern of tearing is characteristic of filler-reinforced elastomers; indeed, it appears to be the principal mechanism of reinforcement because their tear strengths are not much greater than those of unfilled elastomers when knotty tearing is prevented. Thus, reinforcement by fillers is closely associated with, and may depend upon, anisotropy of tear strength in the deformed state.
The Ultimate Properties of Unswollen Polydimethylsiloxane Networks and Their Dependence on Crosslink Density and Dilution during Crosslinking
Authors: Rahalkar R. R. , Yu C. U. , Mark J. E.
Published: February 2011

Polydimethylsiloxane networks were prepared by means of γ-irradiation of the polymer, both in the undiluted state and in cyclohexane solutions. Seven polymer concentrations were employed, and radiation doses were varied at each concentration, thereby giving a series of networks varying in both the amount of diluent present during crosslinking and the degree of crosslinking. A total of 72 networks thus prepared were characterized, unswollen, with regard to their ultimate properties in uniaxial extension at 30°C. For networks prepared at constant polymer concentration, an increase in degree of crosslinking was found to give an approximately linear increase in ultimate strength, as measured by either the nominal stress or reduced stress at rupture. The accompanying decrease in the average length of the network chains caused a corresponding decrease in the maximum extensibility. At a constant crosslink density, decrease in the polymer concentration, in the system which had been used to form the network, generally decreased the ultimate strength. This decrease is possibly due to a change in the network chain length distribution arising from the presence of diluent during crosslinking.
Mechanism of Rubber-To-Brass Adhesion : Effect of Rubber Composition on the Adhesion
Authors: van Ooij W. J.
Published: February 2011

A new adhesion model for the bonding of sulfur-vulcanizable rubber compounds to brass has been presented and discussed. The adhesion is explained on the basis of an autocatalytic effect of a thin film of cuprous sulfide formed on the brass surface by the combined action of sulfur and vulcanization accelerator. This film transfers sulfur atoms to rubber molecules, which results in a high rubber surface polarity. This surface bonds to brass by physical interaction. Although minute amounts of cuprous sulfide are a prerequisite, an excess of this compound is detrimental to the adhesion, as thick films are brittle and nonadherent to the brass substrate. Consequently, all parameters which affect the reactivity of a brass surface for cuprous sulfide formation, have an effect on the adhesion. These parameters are brass copper content, brass surface composition, and rubber compound formulation. The action of the various individual rubber ingredients on the reactions at the interface can be simulated by measuring the rate of sulfidation of a standard type of brass in a paraffin solution containing a simple rubber curing system. In spite of the differences between the conditions during actual rubber vulcanization and sulfidation in paraffin, the latter approach yields information which can be used to explain, on a qualitative basis and within the limits of the adhesion model, certain well-known but hitherto unexplained effects, such as the action of cobalt complexes on the adhesion. The method can also be regarded as a simple test for the effect of rubber compounding ingredients on steel cord adhesion.