bluuebunny/crossref_metadata_2025 huggingface dataset:
Strain-Dependent Properties of Polymers II
Authors: Miller H. T. , Warnaka G. E.
Published: February 2011

Work is described to relate strain-dependent properties of polymers to time-temperature dependency. Methods for graphically shifting frequency-strain properties are shown. The paper also shows the development of frequency-strain equations similar to the well-known WLF equation. This type of equation has been written for many types of polymeric materials with excellent correlation with measured values. Correlation with experimental data for a number of polymers is shown. Relationships between temperature and strain have similarly been developed. These relationships are also illustrated and compared with experiment. Finally, the generalized frequency-temperature-strain dependence of the dynamic mechanical properties of homogeneous, amorphous polymers is discussed.
Stress Optical Behavior of Rubber Networks at Large Deformations
Authors: Smith K. J. , Puett D.
Published: February 2011

The birefringence of natural rubber networks at large deformations has been investigated experimentally and compared with the simultaneously determined stress—strain behavior. Our data is analyzed using a statistical theory of flexibly jointed chains, derived herein, which is believed to be more significant for the particular range of deformation used than the theories of Treloar and of Kuhn and Grün. In addition, the experimental data of Saunders is commented on in light of our theoretical development. We find that for network extensions exceeding those of the Gaussian region there is little correlation between the observed and theoretical behavior of the stress and birefringence (based upon the theory of flexibly jointed chains) and this lack of agreement is attributed to the fact that the statistical parameters needed for the description of the optical chain properties differ in magnitude from those required for the mechanical properties. Furthermore, by considering the points of incipient crystallization the strain behavior of the stress-optical coefficient is highly indicative of nonGaussian behavior rather than crystallization, and therefore yields strong support for the position that nonGaussian behavior does exist in rubber networks.
Diffusion in Amorphous Polymers
Authors: Paul D. R. , DiBenedetto A. T.
Published: February 2011

Diffusion and solubility coefficients are presented for He, Ne, Ar, and N2 in three ethylene propylene copolymers, two styrene butadiene copolymers and a cis-polybutadiene at five temperatures in the range 0–50° C. The data were obtained by using a slightly modified Dow Gas Transmission Cell and analyzed by a new modified time lag technique which properly accounts for the increase in pressure on the downstream side of the cell. Activation energies were calculated for the twenty-four diffusion systems and correlated with polymer intermolecular forces and gas molecule sizes by using a cell theory for amorphous polymers which employs a potential energy depth parameter, ε*, a range parameter, ρ*, the number of degrees of freedom per structural unit (center), 3Cm, and the number of centers per unit cell, n. The first three parameters were evaluated from thermodynamic data, while n was evaluated from the diffusion data. It is believed that n is an indication of the number of backbone carbon atoms active in polymer segmental motion. The values obtained here (5–9) agree well with existing values estimated from polymer viscous flow data. A correlation for the solubility data is shown and a linear free energy relationship is discussed.
Characterization of Block and Random Ethylene Propylene Copolymers by Differential Thermal Analysis
Authors: Barrall Edward M. , Porter Roger S. , Johnson Julian F.
Published: February 2011

DTA furnishes a reliable method for the analysis of physical mixtures and for multisegment and bisegment block copolymers of ethylene and propylene in cases where chain randomness is minor. If a secondary method of analysis which is independent of crystallinity is possible, e.g., use of C14-tagged samples, the DTA method can aid in elucidating the chain structure, including degree of copolymer randomness. Block copolymers and physical mixtures have been found to have thermograms with endothermal minima at approximately 138 and 163° C. The ratio of heights of the two minima has been found to be a function of the ethylene and propylene content for mixtures of homopolymers and for block copolymers without randomnness. Cooling thermograms of block copolymers have been demonstrated to have two exotherms in all cases where the block ethylene content exceeded 10 per cent; the propylene limit was not fixed. Mixed homopolymers do not exhibit two freezing exotherms. The total heat of fusion of the polymer has been found to be a usable measure of chain randomness and block nature. Conditions for observation of single and multiple transitions have been extended. The following conclusions can be drawn from Table I. Multisegment block copolymers are characterized by heats of fusion almost equal to or lower than corresponding physical mixtures. Bisegment block copolymers have heats of fusion equivalent to physical mixtures within experimental error. Cooling curves definitely indicate that samples 3–6 and 3–13 are block copolymers according to Ke's criterion of double exotherms. Sample 3–3 may be inferred to have a large amount of randomness between blocks from its low heat of fusion, which is comparable to that of polymers with known randomness. Samples 3–35 and 3–40 are both block copolymers by the double freezing point technique. The block copolymer identification and the high heat of fusion (almost as large as a comparable physical mixture) indicate long block length and only a few blocks. Very little randomness is present (compare data of Figure 2). Low heats of fusion consistent with short block segments in sequential arrangement are noted for samples 3–30 and 3–31. No confirmation of the block nature can be obtained from freezing point data due to low ethylene content. The displacement from curve in Figure 2 indicates noncrystalline segments in the chain. Multisegment block copolymers are identified from heat of fusion data and freezing point thermograms; note samples 3–32, 3–33, and 3–34. The location of the peak height ratios in Figure 2 indicates the presence of random copolymer with some crystallinity between blocks in all cases.
Stress Softening in Natural Rubber Vulcanizates III. Carbon Black Filled Vulcanizates
Authors: Harwood J. A. C. , Payne A. R.
Published: February 2011

This paper has confirmed the conclusions of the previous paper that the stress softening (Mullins effect) of a black-loaded vulcanizate is similar in magnitude to the stress softening of a gum rubber if the two vulcanizates are stretched initially to the same stress. The initial stress used in the present work was 180 kg/cm2, which is very near to the breaking stress of these vulcanizates. The similarity of the normalized stress-strain curves for all the vulcanizates, both gum and loaded with 60 phr of different types of black, suggests that the main difference between the stress-strain characteristics of a filled and a pure gum rubber, after the initial stressing cycle, can be accounted for by the strain amplification factor X. The more reinforcing blacks possess the higher X factors, i.e., they stiffen the rubber more than, for example, a fine thermal black. It is concluded that the black is acting mainly in a stiffening capacity due to the hydrodynamic effects of the degenerate carbon black networks. For sulfur crosslinked pure gum vulcanizates, in which the crosslinks are polysulfidic, the stress softening is partly associated with the breakage of polysulfide linkages. These reform in the extended condition and produce a real permanent set, but the major stress softening is attributed to the incomplete recovery of the crosslinked network to its initial random state due to network junctions or similar associations being displaced in a nonaffine way during extension. For example, junctions at the ends of chains which become fully extended at relatively low extensions will be displaced in this way. Thus when the rubber is subsequently strained, the network is already in a preferred disposition.
Recent Studies in the Aging of Natural Rubber
Authors: Bell C. L. M. , Cain M. E. , Elliott D. J. , Saville B.
Published: February 2011

Natural rubber, when properly compounded, produces articles having an unrivalled combination of properties such as strength, elastic modulus, and resilience. However, in common with the properties of all other polymers, these excellent qualities progressively deteriorate on contact with oxygen, especially at elevated temperatures. The increasing demand for elastomers capable of service at even higher temperatures requires more stringent efforts by research workers for the production of aging-resistant natural rubber, and, in general, two approaches are possible. On the one hand further developments may be sought in the efficiency of the protective systems used. This will necessitate a reasonable understanding of the mechanisms of antioxidant action, and recent reviews have shown the large amount of research effort being applied to this end. On the other hand, modification of the present vulcanization methods to give vulcanizates with inherently greater resistance to aging is a possibility, although progress in this direction requires a detailed knowledge of vulcanization mechanism and vulcanizate structure. The recent work of the Vulcanization Group at NRPRA has shed considerable light on both these aspects of natural rubber chemistry and improved the prospects of success in this direction. In this paper we report results of studies carried out in both the fields described above which have led to the development of technologically acceptable vulcanizates having improved aging resistance.
Thermal Stress Relaxation of Vulcanizates of Various Structures
Authors: Tarasova Z. N. , Dogadkin B. A.
Published: February 2011

Heating stressed vulcanizates in the absence of oxygen results in purely thermal degradation of the three dimensional network accompanied by reduction of the stress; this process is called thermal relaxation. Thermal relaxation of various rubbers (natural, polybutadiene, butadiene styrene, isoprene and carboxylated rubber) vulcanized with various vulcanizing mixtures and methods, including vulcanization by irradiation, is first order. The rate constants of thermal stress relaxation decrease with network density. The product of the rate constant and the initial number of crosslinks (q=N·K) which yields the number of bonds broken per unit time, is independent of the network density if degradation proceeds by rupture of the molecular chains. This product increases with the network density if relaxation is due to rupture of crosslinks. Hence, examination of the relationship between the relaxation rate and the network density is a way to find the points where the network is ruptured. Thermal relaxation of stress in natural rubbers vulcanized by irradiation is mainly due to rupture of the molecular chains. Here, relaxation is accompanied by a decrease of the network density because the polymer radicals formed are localized by reaction with the compounds of low molecular weight, by formation of ring structures and by disproportionation. Thermal stress relaxation in rubbers vulcanized with metal oxides is due to rearrangement of the salt crosslinks by exchange. Here, the molecular chains remain intact, and the number of crosslinks is not altered. Thermal relaxation in rubbers vulcanized with sulfur is due to preferential rupture and rearrangement of the weak polysulfide crosslinks. The new crosslinks contain a smaller number of sulfur atoms. In this case the number of crosslinks may either rise or fall, depending on the secondary reactions of the polymer radicals formed.
Labile Crosslinks in “Saturated” Elastomers as Measured by Stress Relaxation
Authors: Lyons P. F. , Lee T. C. P. , Tobolsky A. V.
Published: February 2011

Stress relaxation measurements have been made on ethylene propylene terpolymer (EPT) vulcanizates. The relative stability of the EPT backbone, compared with highly unsaturated elastomers, makes it possible to isolate the reactivity of the crosslink. The effects of type of cure, cure time and change of atmosphere on the stress relaxation characteristics have been studied.
Oil Extended Natural Rubber for Tire Treads
Authors: Grosch K. A. , Swift P. Mc L.
Published: February 2011

Large amounts of oil can be added along with reinforcing carbon blacks to natural rubber or natural rubber plus polybutadiene in high speed factory mixers. Unmasticated rubber is used, premastication cycles are eliminated, and mixing cycles are less than three minutes long. A new and important principle is established by trials on passenger cars that the wear rating of one tread compound relative to another is unequivocally determined by the tire surface temperature, over wide limits of absolute wear loss of any one compound. In consequence, accelerated wear tests can be combined with conventional car tests to give average annual service ratings of OENR or OENR-BR compounds which can be markedly better than similar compounds based on OESBR. The wet skid rating of one compound relative to another is independent of speed and road surface. On wet road surfaces the skid rating of OENR-based compounds is comparable to that of OESBR-based compounds. On dry ice at sub-zero temperatures the OENR-based compounds are much superior to the OESBR-based compounds. In many thousands of miles of passenger car tire testing groove cracking, tread separation, chipping or cutting has never been a problem.
Improvement of Impact Strength of Polystyrene by Compounding with Styrene-Grafted Ethylene Propylene Elastomeric Copolymers
Authors: Natta G. , Pegoraro M. , Severini F. , Dabhade S.
Published: February 2011

Ethylene propylene copolymers having good elastomeric properties lend themselves very well to improving impact strength of fragile polymeric materials, such as polystyrene. In order to obtain good results, the percentage of copolymer to be added should be chosen in such a way that the strength of the resulting mixture is as high as possible, but compatible, however, with the practically useful values of the hardness and of the modulus of elasticity, which decrease regularly with an increase in the percentage of elastomer. The best results, for an equal percentage of elastomer, are obtained when the polystyrene is mixed with polystyrene grafted on the ethylene propylene copolymer, rather than with the ethylene propylene copolymer alone. This we attribute to the improved dispersion of the elastomer in the polystyrene matrix that is found in case of the use of the graft polymer. Thus, formation of vitreous and elastomeric microphases, rather than of macrophases, occurs, and there is a better adhesion between these. The use of a graft polymer prevents, in practice, the phenomena of migration and sorting of the phases, and guarantees the maintenance over a period of time of a microphasic structure. The use of a saturated elastomer, such as that used by us, permits, additionally, an improvement in the resistance to aging, in comparison with the traditional materials consisting of mixtures of polystyrene and unsaturated elastomers.
Interactions of Elastomers and Reinforcing Fillers
Authors: Kraus Gerard
Published: February 2011

Use of reinforcing fillers in elastomers ranks as one of the two most important processes in rubber technology—only vulcanization can be considered to surpass it in its universal application. From the time of the earliest observations that certain pigments are capable of imparting reinforcement to natural rubber, particularly against wear, people have been concerned with interactions between filler surfaces and rubbers and their role in the mechanism of reinforcement. Progress, initially painfully slow, received a great boost about 30 years ago with the application of the electron microscope to establish the role of particle size. However, it very soon became apparent that particle size, while undoubtedly the single most important variable in rubber reinforcement, cannot account for all the effects of fillers in elastomers and that specific interactions between rubbers and fillers must also play an important part. Today the importance of these interactions is universally recognized, although much remains to be learned about the exact mechanisms by which they give rise to reinforcing effects.
Rubber Birefringence and Photoelasticity
Authors: Angioletti A. , Eccher S. , Polvara O. , Zerbini V.
Published: February 2011

The phenomenon of birefringence, discovered in 1669 by Erasmus Bartholin and later studied by Christian Huygens, is well known for its appearance in transparent crystalline solids. It is to be traced essentially to internal anisotropy of crystals for light propagation, so that under certain conditions a single wave front may give rise to multiple coherent waves which can cause double refraction or other phenomena such as the appearance of fringes varying in brightness and color due to interference of emerging wave fronts. Birefringence may be expressed as the difference of the velocities of propagation in various directions in the birefringent medium and particularly as the difference between the maximum and the minimum velocity or, alternatively, between the maximum and the minimum refractive index or even as the phase difference between emerging waves, given frequently as a number of wavelengths. While true double refraction phenomena may be observed with ordinary light when the difference n1−n2 between the refractive indexes is very high, the interference phenomena may be obtained only with polarized light, but may be observed even with very small differences between indexes. This is not the place to treat extensively general concepts like polarizer, analyzer, ordinary wave, extraordinary wave, uniaxial crystal, biaxial crystal, etc. which can be found in any good optics treatise. Birefringence is not in any way limited to crystalline media, possessing inherent structural anisotropy. It may also appear in bodies which are normally isotropic, when structural anisotropy is caused by external forces. Then birefringence is quantitatively dependent on force intensities, even if not always in an easily detectable way. This is called “accidental birefringence” or “stress-birefringence”. It was observed by Seebeck in 1813, and later studied in 1816 by Brewster for glass. It is observable in many transparent materials. Stress-birefringence is particularly conspicuous in macromolecular substances, including elastomers, vulcanized or not, where it is determined by the orientation of molecular links. This, of course, may not be ascribed altogether to stress, but sometimes also to partial crystallinity. Other phenomena of accidental birefringence may be observed in liquids or in solutions, especially of elastomers, when they are subjected to a velocity gradient, as when flowing through a capillary tube or, more commonly, when sheared between two coaxial cylinders.
Effect of Oxygen on Natural Rubber Latex
Authors: Bevilacqua E. M.
Published: February 2011

Natural rubber is exposed to oxygen from the time that trees are tapped through its vulcanization and final use. Reaction with oxygen at each stage in its history affects its properties. Extensive studies have been made of the effects in later stages, on raw and vulcanized rubber; much less work has been done on the oxidation of latex. Fresh latex is a dilute aqueous emulsion in which the non-volatile content is approximately 90 per cent rubber hydrocarbon. The rest is a complex mixture of compounds, most of which are normal constituents of plant sera. Rubber exists as high polymer in latex at tapping; in mature trees tapped for the first time, it is of high molecular weight and is extensively crosslinked. In normal commercial practice, when latex is withdrawn as fast as rubber is sythesized by the tree, rubber is essentially all linear high polymer of average molecular weight 105 to 106. As soon as a tree is tapped the latex begins to change. In the short period between tapping and coagulation the changes are small and not usually significant when latex is used for the production of dry rubber. Changes in preserved latex continue over a long time and are significant in its use. Latex of commerce is preserved with ammonia, which serves the dual function of being toxic and controlling pH in a range where latex stability is high. It is sometimes supplemented with another biocide because of the desire to keep ammonia level as low as possible for the convenience of the final user.
Stress-Strain Behavior of Rubber-Reinforced Glassy Polymers
Authors: Newman Seymour , Strella Stephen
Published: February 2011

The main objective of this work has been to develop some rational hypothesis for the toughness of rubber polyblends as defined by the energy to rupture in a tensile measurement. It is shown, both by induction and by direct experimental evidence, that the S/AN rigid phase yields and cold draws. It is thereby established that the toughness of a polyblend is a result of the large energy absorption involved in the cold drawing of the matrix. The presence of inherent flaws or cracks limits the strength and ductility of some glassy polymers, whereas others are known to cold-draw. It is believed, therefore, that the rubber phase in a polyblend acts principally to induce a yielding in the S/AN matrix. There are at least two factors which contribute to this yielding. (a) By limiting the crack growth, the rubber phase effectively strengthens the rigid phase, prevents premature fracture, and permits yielding. In this connection, it is believed that under a tensile strain, the difference in the Poisson's ratio of the rubber and matrix phases results in a triaxial stress field in the environment of the rubber particle. Under such a triaxial stress field, the rubber particle possesses a strength which is greater than in tension and which is sufficient to prevent crack propagation. (b) Secondly, the triaxial field will result in an increase in the free volume of the SAN matrix adjacent to the rubber particle. This, combined with other factors, causes an increase in free volume sufficient to lower the glass transition temperature of S/AN to the point where large-scale molecular motion or cold drawing is further favored.
A Measurement of Carbon Black Pellet Quality and its Significance
Authors: Atkins J. H. , Boonstra B. B.
Published: February 2011

A new instrument for measuring pellet quality is described which records the pellet strength of screened fractions of pellets. Samples large enough to be statistically representative can be measured in a relatively short period of time. The average pellet strength values, for each screened fraction, can be used to predict the incorporation time of the pellets into polymer. The maximum pellet strength values correlate with dispersion and by so doing, effect the stress—strain properties of the compounded elastomer.
Vulcanization Accelerator and Activator Complexes. 2. Chemistry of Amine and Zinc Carboxylate Complexes of Zinc and Cadmium Benzothiazolyl Mercaptides
Authors: Milligan Brian
Published: February 2011

The behavior of zinc benzothiazolyl mercaptide (ZMBT) and its complexes with amines and zinc carboxylates in effecting nucleophilic displacement reactions related to those which occur during accelerated sulfur vulcanization has been studied. Ligands influence the activity of ZMBT in nucleophilic displacement reactions (a) by increasing the solubility of ZMBT in the reaction medium and (b) by increasing the polarity of the Zn—S bond; this increase in polarity only enhances the intrinsic activity of the catalyst if a second zinc species (e.g., a zinc carboxylate) is present. Complexes of zinc carboxylates with ZMBT are very active catalysts for disulfide interchange reactions, but not for sulfur insertion reactions. In the latter, an amine is also necessary to act as a co-catalyst with traces of hydrogen sulfide in order to convert octa-atomic sulfur into a more reactive open-chain polysulfide species. A combination of ZMBT, an amine and a zinc carboxylate provides the best catalyst for effecting sulfur insertion reactions at high temperatures; the active species may be a ternary complex. Amine complexes of ZMBT, either in the absence or presence of a zinc carboxylate, are good accelerators for the vulcanization of cis-1,4-polyisoprene with sulfur.
Elastomers Resistant to Rocket Propellants
Authors: Green Joseph , Levine N. B. , Sheehan W.
Published: February 2011

Resistance of polymers to hydrazine-type fuels, Hybaline A-5, nitrogen tetroxide, and fluorine-containing oxidizers was investigated. Elastomers found suitable for application in hydrazine-type fuels and Hybaline A-5 environments are butyl and ethylene propylene rubbers. Carboxy-nitroso rubber is recommended for long term exposure to nitrogen tetroxide at 165° F while resin-cured butyl rubber is satisfactory for limited application. Fluorocarbon plastics and silicone rubber are most suitable for use with oxygen difluoride, and Teflon, polyethylene, and ethylene propylene rubber with a perchloryl fluoride-tetrafluorohydrazine mixture. Chlorine trifluoride is a very reactive oxidizer and Teflon, Kel-F 81, and uncured nitroso rubber gum are suitable only for limited application.
Elastomeric Seals for the Army's Lance Missile
Authors: McManus S. P. , Pikkn S.
Published: February 2011

The LANCE propulsion system requires dynamic and static seals which must operate under extremely severe conditions for short periods. By analysis and tests it has been found that resin-cured chlorobutyl rubber meets the requirements for low and high temperature operation and retains its properties when exposed to the LANCE propellants for short periods. The chlorobutyl rubber, however, is not a panacea. Its properties become increasingly poorer as exposure time increases. Since the need for high performance, versatile systems should continue indefinitely, it is hoped that research can produce elastomers to meet the growing need for unusual environments such as are described here. For successful accomplishment of this task, the rubber and chemical industry will have to meet the challenge to reach this goal.
Physical Properties of Two O-Ring Compounds after Exposure to Reactor Radiation
Authors: Lewis J. H.
Published: February 2011

Two commercial O-ring compounds, NBR and SBR, specially formulated for radiation resistance were irradiated by means of a nuclear reactor to six gamma-dose levels in the range from 4.9×108 to 15×1011ergs/gm(C). O-rings and compression-set buttons of both materials were irradiated in air at near room temperature; O-rings of NBR were also irradiated in oil. Tensile properties, hardness, and compression set were measured as a function of radiation dose. Fifteen O-rings and five compression buttons were used for each experimental condition to provide statistically valid results. Analytical procedures were used to compute molecular weight between crosslinks and network-chain ratio as a function of dose. The wide dose range provided an accurate assessment of degradation produced by radiation. The highest dose resulted in complete loss of useful mechanical properties. Properties most adversely affected at lower doses were ultimate elongation, hardness, and compression set. While compression set was 50% at a dose of 3×109ergs/gm(C), tensile properties of both materials remained relatively good at a gamma dose of 1.2×1010 ergs/gm(C).
Dynamic Mechanical Testing to Evaluate Radiation Induced Changes in Polymers
Authors: Teaeger R. K.
Published: February 2011

Dynamic mechanical testing is one of the most useful tools for evaluating effects of radiation on polymers. Present property-temperature spectra provide molecular and mechanical information. Property-time spectra are of more fundamental interest but present molecular theories or molecular descriptions of phenomenological theories are too limited to allow interpretations of the spectra. The application of dynamic testing to specimens in a radiation field is a new approach to radiation effects studies and has great potential for further understanding the interaction of radiation with polymers.
Fungus Resistance of Millable Urethans
Authors: Kanavel G. A. , Koons P. A. , Lauer R. E.
Published: February 2011

Sulfur cured polyether and polyester millable urethans inherently possess some fungal inertness. Although they do not support fungal growth, fungicides must be used therewith to provide greater resistance to fungal attack. Only a few of the fungicides tested appear to be practical in the host polyester compositions. Peroxide cured millable polyether and polyester urethans do not inherently possess fungal resistance. Therefore, the need for fungistatic chemicals is even greater.
Stress Softening in Rubber Vulcanizates. Part I. Use of a Strain Amplification Factor to Describe Elastic Behavior of Filler-Reinforced Vulcanized Rubber
Authors: Mullins L. , Tobin N. R.
Published: February 2011

Measurements of Young's modulus of vulcanized rubbers containing thermal carbon black show the predicted dependence on the volume concentration given by relationships derived for a suspension of spherical particles, for example, that due to Guth, Simha, and Gold which gives E=E0 (1+2.5c+14.1c2). A simple interpretation of the results is that the strain in the rubber is increased by the presence of filler so that the ratio of the average strain to the measured overall strain is given by the factor X=1+2.5c+14.1c2. This factor was used to analyze simple extension stress-strain data obtained at larger extensions. For this purpose the Mooney—Rivlin relation was used to describe the behavior of the rubber phase. Values of C1 independent of the volume concentration and in close accord with measurements of the equilibrium volume swelling of the rubbers were obtained. Values of λ* were also consistent with those of #5#. Analysis of stress—strain data obtained on rubbers containing smaller particle-sized carbon blacks is more complex. For these materials the relation due to Guth, viz., E=E0 (1+2.5c+14.1c2), was chosen. By the choice of suitable values of f, good agreement with the Mooney—Rivlin stress—strain relation was achieved at volume concentrations less than about 0.15.
Properties and Structure of Elastomers
Authors: Lal Joginder , Scott Kenneth W.
Published: February 2011

The glass transition temperatures of high molecular weight poly (vinyl n-alkyl ethers) decrease with increasing length of the n-alkyl group. On lengthening the n-alkyl group, 14 per cent of the specific volume increase is free volume. Branching or substitution in the alkyl group of the polymer increases the Tg value. A comparison of poly (vinyl n-alkyl ethers) and polymers of normal α-olefins shows that an ether group and a methylene group in the side chain are equivalent in influencing the glass transition temperature. We have varied systematically the side chain alkyl group in poly (vinyl alkyl ethers) and evaluated at 3 different temperatures the influence of these variations on the dynamic properties of the vulcanizates of these polymers. The relative position of the curves, relating dynamic resilience to dynamic modulus of these polymers, is generally in the order of their glass transition temperatures. The dynamic mechanical property data on poly (vinyl n-pentyl ether) and poly (vinyl 2-ethylhexyl ether), which have the same glass transition temperature, fall on a common curve characteristic of the temperature of measurement. Apparently, the Tg is a major factor in correlating the dynamic mechanical behavior of these elastomers. The size and shape of the alkyl group appear to be reflected primarily in their effect on the Tg. Polysulfidic crosslinks are not essential for the attainment of high tensile strength in natural rubber vulcanizates cured with a sulfur-diphenylguanidine system. Data for the samples which had lost significant amounts of polysulfidic crosslinks by reaction with triphenylphosphine fitted a curve of tensile strength as a function of 300 per cent modulus for the control samples.
Dynamic Mechanical Properties of Cross-Linked Rubbers. II. Effects of Crosslink Spacing and Initial Molecular Weight in Polybutadiene
Authors: Maekawa Etsuji , Mancke Ralph G. , Ferry John D.
Published: February 2011

The complex shear compliances of eight samples of polybutadiene crosslinked by cumyl peroxide and four samples crosslinked by sulfur have been measured over a frequency range from 0.2 to 2 cps at temperatures from − 6 to 45° C by a torsion pendulum. On four of the samples, measurements were extended by the Fitzgerald transducer from 45 to 600 cps at temperatures from − 71 to 55°. The vulcanizates had been prepared from polymers of two different molecular weights (180,000 and 510,000) with sharp molecular weight distribution; the physical crosslink density ranged from 0.57 to 2.68×10−4 mole/cm3, and the chemical crosslink density calculated following Kraus ranged from 0.22 to 1.49×10−4 mole/cm3. The mechanical data were all reduced to T0=298° K by shift factors calculated from the equation log aT=−3.64(T−T0)/(186.5+T−T0). In the transition zone of frequencies, the viscoelastic functions of the cumyl peroxide vulcanizates were closely similar, except for a shift toward lower frequencies with increasing crosslinking, corresponding to a small but unexpected increase in the monomeric friction coefficient. Cross-linking by sulfur caused a somewhat larger shift toward lower frequencies at a comparable crosslink density. In the rubbery zone, the sample with least cross-linking exhibited a substantial secondary loss mechanism at very low frequencies. The low-frequency losses are evident in all the samples, but their magnitude falls rapidly with increasing crosslink density as previously found for natural rubber. It also falls somewhat with increasing initial molecular weight, indicating a contribution from network strands with loose ends. The possible relation of the low-frequency losses to trapped entanglements is discussed.
Near-Infrared Analysis of Ethylene Propylene Copolymers
Authors: Bly R. M. , Kiener P. E. , Fries B. A.
Published: February 2011

A number of block and random copolymers of ethylene and propylene have been synthesized using C14 labeled ethylene. Using the radio-assay results of these materials, a near-infrared analysis for ethylene content was developed. The 2.275 micron (4396 cm−1) and 2.312 micron (4325 cm−1) near-infrared combination bands are useful for determining monomer ratio concentrations in the 0 to 40% ethylene range regardless of copolymer type. They are superior to the fundamental infrared bands because they are much less sensitive to slight differences in crystallinity. Other advantages include internal thickness calibration by relative peak height measurements on peaks of more nearly equal intensities than the fundamental bands and greater convenience in sample preparation techniques. The fundamental 720 cm−1 band intensity is a good indication of ethylene content in crystalline block copolymers but requires the accurate measurement of sample thickness for an analysis. The 968 cm−1 and 1163 cm−1 bands, ratioed with the the 720 cm−1 band, show definite correlation with monomer concentration but are less precise than the near-infrared data because of sensitivity to randomness and crystallinity.
Radiolytic Stress Relaxation of an Ethylene Propylene Copolymer
Authors: Yu Hyuk , Wall Leo A.
Published: February 2011

The kinetics of network chain scission induced by γ-rays has been studied by stress relaxation at constant elongation under vacuum and in air. The rate of chain scission of a 1:1 ethylene propylene copolymer was found to be independent of initial network chain density, and the initial rates were roughly the same in air and under vacuum. Parameters for crosslinking induced by γ-rays were obtained by sol extraction and by measurement of elastic properties of the resulting networks. Values of G for scission and crosslinking and gel dose, obtained by stress relaxation, by elastic properties, and by sol extraction, were found to agree with each other.
Bonding of Rubber to Carbon Black by Sulfur Vulcanization
Authors: Rivin D. , Aron J. , Medalia A. I.
Published: February 2011

As an approach toward understanding the interaction of rubber with carbon black, we have studied the chemisorption of a model olefin (2-methyl-2-octene) on carbon black. Some olefin becomes chemisorbed on simply heating these two materials together, and can be distinguished from physically adsorbed material by its resistance to removal by solvents at room temperature. The presence of an MBTS accelerated sulfur curing system causes additional chemisorption. The amount of olefin chemisorbed can also be increased by the use of larger amounts of sulfur but is decreased by an increase in MBTS, owing to competitive chemisorption of the MBTS. Sulfur is also chemisorbed, to a somewhat greater extent than the olefin, but is desorbed more readily than the olefin, suggesting that these two components are chemisorbed separately. The chemisorbed olefin is believed to be highly mobile on the carbon black surface, and indeed its stability is ascribed primarily to a loss of translational entropy associated with the rate-determining step in desorption. The curing system decreases the stability of the chemisorbed olefin, apparently due to the formation of localized bonds which decrease the translational entropy of the olefin.
Survey of Treadwear, Groove Cracking, and Carbon Black Properties
Authors: Studebaker Merton L.
Published: February 2011

Resistance to treadwear is dependent upon the particle size, ultimate composition and “structure” of the carbon black used for reinforcement. Under moderate service conditions, reinforcement, as measured by treadwear index, is dominated by particle size. Under severe test conditions, the “structure” of the carbon black makes a very important contribution to the treadwear index. Groove cracking is quite dependent on the nature of the elastomer. Reversals in the effects of the nature of the crosslink, carbon black structure and particle size on groove cracking have been observed when the elastomer is changed from natural rubber to SBR.
Thermal Analysis of Polyurethan Elastomers
Authors: Slade Philip E. , Jenkins Lloyd T.
Published: February 2011

Differential thermal analysis and thermogravimetric analysis were used to evaluate the thermal properties of several types of polyurethan elastomers. The composition of the polymers investigated was changed by varying the substrate, or soft segment, diisocyanate and extender. The data indicated that within the limits of the compositions under study, polyurethans were thermally stable to their melting points, however, above the melt they were susceptible to oxidation unless stabilized by the addition of an antioxidant. On the differential thermograms the absence of a pronounced endothermic peak at the melting point, characteristic of noncrystalline polymers, was observed. The decomposition of polymers made with 4,4′-diphenylmethane diisocyanate (MDI) occurred above 400° C and was at least a two step process, while the decomposition of polymers containing toluene diisocyanate (TDI) occurred below 400° C and appeared to be a one step reaction.
Angular Distribution of Intensity of Rayleigh Scattering from Comblike Branched Molecules
Authors: Casassa Edward F. , Berry Guy C.
Published: February 2011

The angular distribution function P(θ) for intensity of light scattered by a dilute solution of comblike branched molecules has been determined for three situations of some interest for evaluation of experimental data: (1) the molecules are identical with branches of equal length attached equidistantly along linear backbone chains; (2) the molecules are homogeneous in mass, with the same number of branches on each molecule, but the branches are distributed at random along the chain; (3) branches and main chains are still uniform, but the molecules are heterogeneous in mass with the number of branches per molecule distributed according to the binomial distribution and the branches in any molecule spaced randomly along the backbone. Examination of numerical results shows that the scattering functions for models (1) and (2) arc not very different. The function for case (3) is somewhat different from the others when the mean number of branches per molecule is small but they contain a large fraction of the mass of the molecule. Over a limited range of the pertinent variables (corresponding roughly to observations on typical vinyl polymers of molecular weights up to 106) all three functions agree quite well with P(θ) for homogeneous linear chains with the same mean square radius of gyration.
Influence of Test Conditions on Wet Skid Resistance of Tire Tread Compounds
Authors: Grosch K. A. , Maycock G.
Published: February 2011

This paper is primarily concerned with the skid behavior of tread compounds, and the extent to which testing parameters such as the type of road surface, the vehicle speed, or whether peak and sliding coefficients are considered, influence the skid behavior of one tread compound in relation to another. Whilst the actual braking coefficients depend profoundly on all these parameters the rating of two compounds is much less affected by them. Moreover, subject to the qualification given in the paper, the ranking of compounds will in many cases be predicted correctly by the RRL skid tester. The question arises how the rating of tread compounds will be affected by other testing methods. The three most commonly used are cornering, traction, and stopping distance tests. The first two are dominated by the peak value of the friction coefficient because the measurements are taken just before sliding occurs whilst the last reflects the sliding value of the braking coefficient. Since the rating is virtually the same for these two types of measurement it is likely to be independent of the measuring method. Although the ranking of compounds does not depend on the road surface, well drained road surfaces gave more reproducible results and larger ratings so that fewer readings are required on such a surface than on a flooded one. The following observations as to the nature of wet friction emerge from the present study. (a) Changes in coefficient of friction with tread compound and type of road surface texture confirm the findings of Miss Sabey, and Greenwood and Tabor, that the energy loss component of wet friction on coarse surfaces increases with the sharpness of the asperities, and thus with the pressure on the tops of the asperities. A mechanism for such energy losses based on elastic stored energy is suggested. (b) The decrease in the braking coefficient, as observed on well drained or dry road surfaces after the wheels have become locked, is explained by the frictional temperature rise in the area of contact. (c) It is suggested that in the case of the sliding tire, temperature increase in the contact region with increasing vehicle speed contributes to the observed fall in coefficient with increasing speed. This, together with the temperature dependence of the rubber properties, is used to explain small differences between the speed coefficients of various compounds in their rate of fall off with speed. (d) The mean braking coefficients of the compounds increase with their internal viscosity. It appears that the internal losses can be increased and therefore the skid resistance improved by incorporation of a heavy oil without an appreciable change in the glass transition temperature as measured by a torsion pendulum. This is strikingly demonstrated by the highly resilient natural rubber, whose skid resistance is greatly improved by oil extension without any significant loss in resistance to tire wear.
Incompatibility and Phase Formation in Solid Polymer Mixures and Graft and Block Copolymers
Authors: Bohn Lothar
Published: February 2011

Compatibility in the sense of a tendency to molecular mixture and mutual solubility is rare for two solid polymeric components. Thus, not only mixtures but also graft and block copolymers constitute two phase systems. Possibilities for identification of separate phases are discussed, including optical methods and determination of freezing and melting temperatures of the individual phases. Problems of preparation of two phase mixtures and the phase distribution arising thereby, including identification of disperse and continuous phases are discussed. Examples are given of little known polymer mixtures for which complete or partial compatibility exists.
Reinforcement of Fluoroelastomers with Halopolymers
Authors: Kaufman Martin H. , Gonzales John
Published: February 2011

This study demonstrated the utility of Teflon as a reinforcing agent. Elastomeric compositions prepared with Teflon powder by milling or extrusion have very large increases in tensile strength when measured parallel to the direction of shear, and high tear strength when measured perpendicular to the direction of shear. Teflon powder sheared in a fluoroelastomer matrix coalesces to form fibrils. The hypothesis that free radicals, and therefore branching, are produced when Viton and Teflon mixtures are sheared at ambient temperature is supported by the increase in slope of curves of ηsp/c as a function of c. However, the principal cause of the large improvement in tensile and tear strength is believed to be formation of oriented Teflon fibers and films in the elastomer matrix. The formation of these fibers and films would explain the rough surface of milled material as well as the poor consolidation of these materials when they are compression molded. Reinforcement by fibrillation does not occur with Kcl-F resin or fluorinated ethylene propylene.
Freezing and Relaxation of Elastooptical Effects in Polymers
Authors: Sheyanova A. I. , Sokolov S. I. , Shchegolevskaya N.
Published: February 2011

It is known that stress birefringence in optically isotropic, amorphous substances in the elastomeric state is firmly fixed by transition into the glass state. Only a small change is observed as a rule, upon reducing the load. This phenomenon is used in studying stress with polarized light and three dimensional models by the “freezing” method. The freezing process and its reverse, relaxation of birefringence, as complex processes characteristic of polymers, merit more detailed study. In our first investigations, two materials were used having three dimensional, network structures and practicable for stress studies by polarized light (1) epoxy resin E-40 set up by maleic anhydride and (2) an oligomeric copolymer (trademark MIRhM-IMASh) of poly(diethylene glycol sebacate fumarate) with styrene and methyl methacrylate and later, two linear polymers of polystyrene and poly-(methyl methacrylate), produced on a laboratory scale with benzoyl peroxide and completely stabilized by long, stepwise heating.