bluuebunny/crossref_metadata_2025 huggingface dataset:
Force-Balance Capillary Rheometry at Low Shear Rates
Authors: Chalifoux J. P. , Meinecke E. A.
Published: February 2011

The validity and accuracy of force-balance rheometry have been demonstrated by comparing data with readily available methods. At low speeds of extrusion the entrance pressure drop accounts for a very small portion of the overall pressure drop. This situation permitted us to show the equivalence between τwa and the true τw derived from equation (5) or the slopes in the Fe−Lb/D graph. Bagley plots confirmed that the effective τw and results from bottom-top dies yield comparable values over a decade of shear rate. Using flow birefringence techniques, Han and Drexler observed an exit effect at shear rates lower than 10 sec−1, but the data obtained with this thermoplastic elastomer indicated that the velocity profile rearranges only when the exit plane is reached. The wall shear stress was also shown to be independent of L/D.
Dynamic Properties of SBS Block Copolymers
Authors: Yang R. M. , Meinecke E. A.
Published: February 2011

A recently developed mechanical test, consisting of a mechanical excitation where small sinusoidal strains are superimposed upon finite static deformation, was applied to a series of SBS block copolymers. The energy loss per cycle for unit volume of material showed a pronounced peak at those strains, where most of the stress-softening during a normal stress-strain cycle occurs. This peak seems to arise from a strain induced change in structure. The original rod-like structure seems to be changed into one resembling independently dispersed spheres, since the increase of the losses with increasing filler content in the plateau region following the peak can be explained with the help of the Einstein-Guth-Gold strain magnification factor. The loss peak is not observed immediately after a previously applied stress-strain cycle but reforms with time, while the stress-softening is permanent. This discrepancy cannot be explained at this time.
Influence of Compression upon the Shear Properties of Bonded Rubber Blocks
Authors: Porter L. S. , Meinecke E. A.
Published: February 2011

Rubber has a stress-strain response to compression-shear loadings that is the same as its stress-strain response to simple shear loadings. However, its load-deflection response to the compression-shear loading is not the same as its simple shear response. In determining the stress-strain relationship of the compression-shear loading from the load-deflection responses, three factors must be considered. First, the compression of the sample gives a lower rubber thickness. After calculating the strain, the lower thickness will give a higher strain than the original thickness at an equal deflection. Second, the compression gives a larger surface area due to bulging of the rubber. The higher area would result in a lower stress than the original area at an equal load. Third, the force that is necessary to compress the rubber block is stored in the rubber. When the rubber is sheared, the shear vector of the compressive force aides in deflecting the rubber. Therefore, the shear force vector would be added to the recorded load to determine the total force needed to shear the rubber. The resulting shear stress would be higher than the shear stress calculated by using the recorded load in calculating the shear stress. With all three factors accounted for, the shear stress-strain of the rubber is the same for the compressed part as it is for the uncompressed part. Therefore, the rubber's shear modulus, the slope of the shear stress-strain curve, has not been affected by the superimposed compression and remains an inherent property of the rubber. When designing a part to be used in a compression-shear application, one can use the shear and compression moduli normally obtained for shear and compression applications. The compression modulus would be used for determining the compressive spring rate and the amount of force used in lowering the shear spring rate. The shear modulus would be used to determine the shear rate by taking into account the geometry changes and the force due to compression.
Structure-Property Studies on a Series of Polycarbonate-Polydimethylsiloxane Block Copolymers
Authors: Tang S. H. , Meinecke E. A. , Riffle J. S. , McGrath J. E.
Published: February 2011

Block copolymers composed of hard and soft segments form an interesting class of materials ranging from thermoplastics to thermoplastic elastomers depending on their composition and/or the size of segments. These materials have attracted much attention in the past decade because by careful tailoring, polymers of desired properties can be obtained. Yet they are thermoplastic in nature and can be processed and even reprocessed thermally. Among the common elastomers that can be used as the soft segment in a block copolymer, polydimethyl-siloxane is of special interest due to its great thermal stability at elevated temperatures and high flexibility at low temperatures. Block copolymers containing polydimethylsiloxane as the soft segment and various thermoplastics such as poly(α-methylstyrene), polystyrene, and polysulfone, etc., as the hard segment, have been synthesized and studied. A group of randomly alternating block copolymers of bisphenol-A polycarbonate and polydimethylsiloxane have also been prepared by in situ polymerization of dichloro-terminated siloxane oligomers and bisphenol-A and phosgene. The properties of these block copolymers as well as those of the others have been discussed to some extent in a general review. This work reports the results of a study on the structure-property relationship of a series of perfectly alternating block copolymers of bisphenol-A polycarbonate and polydimethylsiloxane synthesized via different routes. They were prepared by silylamine-hydroxyl reaction. Slightly less than the stoichiometric quantity of siloxane oligomers was incrementally added to a hydrated solution of the polycarbonate in refluxing chlorobenzene. The reaction can be represented by the simple scheme:
Mechanisms of Ozonation of N,N′-di-(1-Methylheptyl)-p-Phenylenediamine
Authors: Lattimer R. P. , Hooser E. R. , Diem H. E. , Layer R. W. , Rhee C. K.
Published: February 2011

The ozonation products of a common rubber antiozonant, N,N′-di-(l-methylheptyl)-p-phenylenediamine (DOPPD), have been separated by liquid or gas chromatography. Molecular weights of about thirty LC separated components have been measured by field desorption mass spectroscopy. Elemental formulae have been determined by atomic composition mass spectroscopy. Other structural details have been elucidated by electron impact mass spectroscopy and attenuated total reflectance infrared spectroscopy. Two principal mechanisms appear to govern the ozonation of DOPPD. Amine oxide formation leads to observed nitrosoaryl and nitroaryl products. The second major mechanistic pathway is side chain oxidation. This leads to a number of low molecular weight components, including some that contain an amide moiety. A third (minor) mechanistic pathway involves a nitroxide radical intermediate and leads to the formation of a stable dinitrone species. The surface film formed on ozonation of a black loaded natural rubber sheet containing DOPPD has also been examined. The film contains appreciable quantities of unreacted DOPPD and many of the same low molecular weight components as observed in the ozonized liquid antiozonant. It is clear that DOPPD blooms to the rubber surface and acts as a scavenger for ozone. The results are consistent with a combined “scavenger-protective film” mechanism for antiozonant protection.
Analysis of Cured, Filled Elastomeric Compounds by 13C NMR
Authors: Werstler D. D.
Published: February 2011

This paper has discussed the study of cured, filled elastomers by a newly developed 13C NMR method. This method overcomes many of the shortcomings of other methods in that the 13C method is capable of distinguishing nonstandard or experimental polymeric components in an elastomeric blend, a definite advantage for the analyst. Other advantages are that the 13C data are primary, i.e., compositional calibration standards are not necessary once a polymer's 13C carbon resonances have been assigned; the data are accurate and reproducible; the filler (carbon black, TiO2, glass fibers) does not have to be removed prior to the analysis; and, the method is sensitive to low levels of polymer concentration. The superiority of 13C quantitative data over IR data has been demonstrated. Practical examples including nonstandard and/or unique elastomeric components have been discussed.
Ultrafine Recycled Rubber
Authors: Swor R. A. , Jensen L. W. , Budzol M.
Published: February 2011

A technical advance in the size reduction of cured scrap rubber has been achieved by a new process. The performance of products whose particle dimensions are 90% under 20 µm in diameter has been described in this study. When small panicle size rubber made by this process is used to extend high quality compounds, the degree of retention of key properties of the extended compounds surpasses that of compounds extended with conventional recycled materials. Many unexpected product and process improvement characteristics have been reported. In most cases, the properties of the extended compound are close to those of the unextended compound, even at high loading levels. Initial trials using this unique paniculate rubber at up to 30% concentrations in bias-ply passenger tire treads have shown no performance deficiencies. The addition of RR does not require any special processing techniques, mixing cycles, or equipment. Substantial cost savings are possible by extending virgin rubber compounds with ultrafine recycled rubber.
A Fatigue-Life Model of a Rubber Bushing
Authors: Oh H. L.
Published: February 2011

A major performance requirement for a rubber bushing is durability. The model for fatigue life of a bushing given in Equation (4) was derived using an energy balance concept (energy available versus energy required to extend a tear). This model agreed well with test results. A design procedure was developed from the model which gives the optimum insert geometry for maximum bushing life.
Test for Evaluating Extrusion Resistance for Oil Well Packer Applications
Authors: Shell Robert L.
Published: February 2011

A standard test has been modified such that the extrusion resistance of a cured compound can be determined. A simple visual rating system has been developed which can be used to quantify cured rubber extrusion resistance thus allowing easy comparison of the effect of various compounding materials. Twenty-three compounding ingredients have been investigated at various levels for their effect on extrusion resistance. Test results have been determined for several commercial oil well packer compounds. Thus compounds developed in the laboratory can be compared directly with known workable formulations. Excellent performance combined with improved processability is no longer just a dream but a real possibility. The modified annular orifice extrusion test is currently being used to develop superior compounds for the oilfield industry.
The Solubilization of Sulfur in the Latex Vulcanization Process
Authors: van Gils G. E.
Published: February 2011

Recent work has shown that optimum properties of rubber vulcanizates are obtained when the vulcanizing ingredients are very well dispersed in the rubber, preferably dispersed to the molecular state, in other words, dissolved in the rubber. The process by which the vulcanization of latex is carried out seems to contradict this. Here the vulcanization ingredients are present in the water phase, and vulcanization of the rubber particles can be accomplished by heating the latex. Apparently molecular dispersion in the rubber phase is not necessary. In the case of sulfur, if we assume that the sulfur particles have the same size as the latex particles and that the density of sulfur is 2 and that of rubber is 1, then only one sulfur particle per 100 rubber particles is needed to obtain a sulfur concentration of 2 phr. The question is, how the sulfur is transferred from that one particle to the 100 rubber particles. It can only be through the water phase. However, sulfur is not soluble in water nor in aqueous ammonia. The experiments described below show that sulfur can indeed “dissolve” in the water phase (serum), be it in very small amounts, and subsequently is transferred into the rubber phase at concentrations sufficient for curing.
Resistivity of Unvulcanized Compounds of Rubber and Carbon Black
Authors: Boonstra B. B.
Published: February 2011

1. A coaxial probe has been developed which allows measuring electrical resistivity of uncured carbon black-rubber compounds with good precision (mean coefficient of variation 4%). 2. For a given compound the resistivity correlates well with the standard dispersion rating and thus can be used for rapid monitoring of dispersion at various stages of processing. 3. Resistivity may also serve as a check on the loading and type of black in a rubber compound. 4. The resistivity of uncured compounds is higher by a factor of 2–10 than the corresponding vulcanizate value. 5. The logarithm of the resistivity appears to be a linear function of the reciprocal of the absolute temperature. The apparent activation energy of conductance for SBR compounds is 14.7–29.3 kJ/mol, the higher values being found for the better dispersions. The activation energies of EPDM compounds were found to be approximately 50% higher than those of SBR.
Properties of Polyether-Polyester Thermoplastic Elastomers
Authors: Lilaonitkul A. , Cooper S. L.
Published: February 2011

The viscoelastic properties of polytetramethylene oxide—polytetramethylene terephthalate block polymers are strongly influenced by phase separation of the 4GT hard blocks into crystalline domains. Thermal analysis reveals a single Tg which increases with increasing 4GT content. This suggests that short sequences of hard segments form a compatible interlamellar amorphous phase with the polyether component. The Gordon-Taylor equation was found to model Tg behavior accurately, provided that the crystalline polyester component was not included in the definition of the hard segment. The melting point of the polytetramethylene terephthalate blocks depends on the average block length of crystallizable segment. Incorporating non-crystallizing polytetramethylene 1,4-cyclohexanedicarboxylate into the hard segment reduces the 4GT melting point and degree of crystallinity. The morphological features of the copolymers depend on sample composition and fabrication procedure. The basic structure is spherulitic. Three different types of spherulite were observed: positive and negative spherulites, as well as spherulites which have their optical axis 45° to their radial direction. The different spherulite types are relatively stable; annealing the samples at elevated temperatures does not alter their morphology. Annealing does increase the degree of crystallinity somewhat and produces crystallites in equilibrium at the annealing temperature. Infrared dichroism studies reveal that, at low deformations, the hard segment lamellae orient as a whole in the stretching direction. This is refleeted by the initial negative orientation of the hard segments. At this stage of elongation, the deformation of the crystallites is nearly reversible. At higher strain levels, the lamellae are disrupted and the hard segments orient positively with a high degree of orientational hysteresis. The soft segments, however, orient almost reversibly in the stress direction at all strain levels studied. It is concluded that the extensive stress softening is brought about by plastic deformation of the crystalline hard segments.
A New Method for Measuring Adhesion of Vulcanized Rubber to Steel Cord at Elevated Temperatures
Authors: Dietrick M. I. , Emmert D. E.
Published: February 2011

The analysis of variance of the elevated temperature tests conducted in this study showed that the experimental sample holder gave slightly higher values than tests run in an oven with the standard sample holder. However, this difference was less than 1% of the average adhesion value. Student's t-tests of the individual unit-to-unit comparison rated the test procedures equivalent over a wide range of temperature and aging histories. The advantages of this experimental sample holder are: 1. lower equipment costs; 2. rapid means of testing steel cord adhesion at elevated temperatures; 3. ease and simplicity of operation; 4. adaptability of the experimental holder to various tensile testing equipment.
Effects of Diffusion in the Oxidative Degradation of Vulcanized Rubbers. I. Rate of Chain Scission in the Steady State
Authors: Ono K. , Kaeriyama A. , Murakami K.
Published: February 2011

The effects of diffusion in the oxidation of cis-1,4-polyisoprene vulcanizates were investigated by means of the stress relaxation method. It was assumed that the diffusion of oxygen is coupled with first-order oxygen consumption and that the rate of chain scission is proportional to the rate of oxygen consumption. The diffusion equation of this process was solved under the steady-state condition to give a simple relation between the rate of chain scission and the film thickness. The experimental results were in good agreement with the theoretical treatment. The true activation energy as well as the ratio of the rate of oxidation k to the diffusion constant D could be estimated.
Efficient and Semiefficient Vulcanization Systems Employing 2-(Morpholinodithio)-Benzothiazole Generated in Situ
Authors: Lawrence J. P.
Published: February 2011

The MBTS-N-(morpholinothio)phthalimide vulcanization system has been found to be effective in producing heat resistant semi-EV and EV cures in a variety of representative rubbers. When used as a direct replacement on a molar basis for 2-(morpholinodithio)benzothiazole (MDB), improvements in processing safety were found in all cases studied. In this respect it is most effective in NR, SBR, and NBR. The experimental system also generally gave slower-curing stocks. The heat resistance of the experimental stocks was found to be equal to or better than that of the MDB control stocks. Substantial improvements in heat resistance for both SBR and IIR unexpectedly resulted from the use of the MBTS—imide system. This result suggests that the nature of the crosslinks formed by the two vulcanization systems are to some extent different in these two elastomers. This, in turn, may reflect upon the mechanisms of crosslink formation.
Viscoelastic Characterization of Ethylene-Propylene-Diene Terpolymer Elastomers
Authors: Nakajima N. , Harrell E. R. , Collins E. A.
Published: February 2011

A practical range of frequency with the Rheometric mechanical spectrometer for the determination of G′ and G″ was shown to be from 2.5×10−2 to 2.5×102s−1, but one decade lower frequency may be reached routinely. The values G′ and G″ may be calculated from the steady-state viscosity curve obtained with the variable-speed Mooney rheometer. However, with samples having long-chain branching, the calculated G′ and G″ are somewhat higher, the difference being close in magnitude to the error of the measurements. With the variable-speed Mooney rheometer, the highest available shear rate is 16 s−1, about one decade less than that with the Rheometrics instrument. With high-molecular-weight rubbers, the range is further reduced, because of “slipping” or “fracturing” of the polymer. On the other hand, the Mooney rheometer enables one to extend the low-frequency range to 2×10−3s−1 very conveniently. The degree of long-chain branching may be estimated from the viscoelastic behavior by comparing complex viscosity and steady-state viscosity at the same time scale, both measurements being made with a Mooney rheometer at a slow speed with a single charge of sample.
The Thermal Diffusivity and Conductivity of Natural Rubber Compounds
Authors: Hands D. , Horsfall F.
Published: February 2011

New methods of measuring the conductivity and diffusivity of rubbers from ambient to 250°C have been developed. The method of measuring diffusivity described in this paper is simple to use and very much quicker than previous methods, the whole temperature range being covered in about half an hour. The accuracy is about 5%. Two disk-shaped samples are used, 48 mm diameter and 2 mm thick, but the geometry can be changed to accommodate samples of varying thickness. The method is suitable for anisotropic samples but, because the measurement is made over a small area, they must be homogeneous. Diffusivity and conductivity measurements have been made on a series of carbon-black-loaded natural rubber compounds. Both thermal properties depend on temperature, type of rubber, and the type and quantity of carbon black. Empirical equations have been obtained relating diffusivity and conductivity to temperature and black loading. These fit the data to better than 10%.
Static Friction of Smooth Clean Vulcanized Rubber
Authors: Roberts A. D. , Thomas A. G.
Published: February 2011

This picture of the origin of static friction for smooth rubber indicates that it is governed by the purely elastic consideration of the strain necessary to produce surface instability. The interfacial adhesion energy required to detach the rubber, which will be the true thermodynamic value at zero peel rate, does not appear to be the limiting factor. The rate effects observed, which persist to surprisingly low rates, are due to the viscoelastic behavior of the rubber. Once sliding has begun, the energy is lost in the peeling process, and this is then dominated by interfacial peel adhesion scaled by viscoelastic characteristics. At zero rate the value found here of F8/G=0.09 implies a static friction coefficient of about 0.6. However, if the normal load were reduced to zero, a finite contact area would remain, at least for smooth surfaces, due to surface attraction, and to express static friction as a coefficient would be meaningless in this case. It is believed that this investigation demonstrates the existence of a true static friction for smooth clean rubber and suggests that its magnitude is determined by the elastic deformation and consequent instability of the rubber.
The Thermal Transport Properties of Polymers
Authors: Hands D.
Published: February 2011

Values of thermal diffusivity and thermal conductivity are needed for heat-flow calculations, for the determination of structure-property relationships, and for material selection and comparison. However, all aspects are hampered by a dearth of reliable data and anything more than a superficial glance at the literature is apt to be discouraging for the uninitiated. Hardly any thermal diffusivity data exist, and the reported values of thermal conductivity show very large scatter. The present state of confusion can be seen, for example, in Figures 1 and 2, which show the reported thermal conductivity values for polystyrene and gum natural rubber. Not only do the values differ at some temperatures by more than 100%, and in the case of rubber by almost 300%, but different trends are indicated throughout the temperature range. Discrepancies of this size cannot be due to sample variations, and they give some indication of the experimental difficulties associated with thermal property measurements.
Prevulcanization Inhibitors
Authors: Trivette C. D. , Morita E. , Maender O. W.
Published: February 2011

Conversion of raw rubber to a usable product is basically a three-step process. Crosslinking ingredients, fillers, and other components are added, the compounded green stock is shaped into the desired form, and the form is heated (cured) to introduce crosslinks (covalent bonds) between the rubber polymer chains and produce a three-dimensional network with vastly improved properties. The crosslinked product no longer flows and thus cannot be reprocessed. The curing system is activated by heat, heat is required for and generated by processing, and faster processing is desirable for economic reasons. Thus, a basic conflict exists between the processing and curing stages. The balance is delicate because the compounder, again for best economics, wants the product to cure rapidly after it is processed. Modern curing systems provide a scorch delay time, during which heating produces no crosslinks, followed by a period of rapid crosslink formation. The compounder then adjusts process conditions to provide enough heat history to eliminate most of the scorch delay so that the cure time will be minimized. Since the compounder is balancing processing against cure, subtle and sometimes uncontrollable changes in factory equipment or operating parameters can produce unprocessable (scorched) rubber stocks. Such changes can be compensated for by making adjustments in the curing system or in the processing conditions. However, relationships between curing system and properties are complex, and considerable effort is generally required to develop a new formulation. Thus, the need exists for an additive which increases only the scorch delay time with minimal effects on other cure parameters and vulcanizate properties. Such compounds, called prevulcanization inhibitors, scorch inhibitors, or retarders, are in commercial use. A number of papers and patents describing the chemistry and use of these compounds have been published and are reviewed here.
Incorporation of Antioxidant Groups into Polydienes. II. Via Free-Radical Reactions of Phenol- or Amine-Substituted Sulfur Compounds
Authors: Weinstein A. H.
Published: February 2011

Methods for incorporation of effective antioxidant groups into both emulsion diene homo- or copolymers and stereospecific diene homopolymers via free radical reactions of sulfur-containing compounds were described. They involved either chain transfer reactions of phenol- or amine-substituted mercaptans or disulfides in emulsion diene polymerization systems or free-radical additions of phenol- or amine-substituted alkyl mercaptans to polydienes after polymerization. The latter reactions were applicable both to polydiene latexes and to solutions of stereospecific polydienes. The potential effectiveness of certain phenol- or amine-substituted mercaptans and disulfides to interact with polymer radicals was determined by measurement of chain transfer constants in bulk styrene polymerization. The efficiency of conferment of antioxidant activity upon polydienes by sulfur-containing compounds via chain transfer or addition reactions was determined by oxygen absorption studies of preextracted polymers. In some instances the relationship between the degree of oxidation resistance conferred upon a polymer with its concentration of chemically bound antioxidant function was determined by colorimetric or uv spectrophotometric analysis.
Sulfurless Crosslinking of Diene Rubbers with 2-R-4,6-Dithiol-S-Triazines
Authors: Nakamura Y. , Mori K. , Akaishi F.
Published: February 2011

A novel crosslinking formulation comprising a combination of 2-dibutyl-amino-4,6-dithiol-s-triazine (DB) with a radical agent gave a crosslinked polybutadiene in which 2-dibutylamino-s-triazine-4,6-dithio crosslinks were formed. The recommended radical agents are dibenzothiazyl disulfide, 1-(o-nitrophenylazo)-p-cresol, benzeneazo-α-naphthylamine, dicumyl peroxide, and 2,5-dimethyl-2,5-di-t-butylperoxyhexane. The crosslinking efficiency and aging resistance of the resulting vulcanizates were much improved compared to vulcanizates with a similar formulation without DB. The BR crosslinking formulation using 2-R-4,6-dithiol-s-triazines, where R was an antioxidant substituent, gave vulcanizates with aging resistant properties remaining after solvent extraction. Similar crosslinking effects were also confirmed on diene rubbers to yield vulcanizates with improved aging resistance. Typical formulations and the properties of the resulting vulcanizates are given for SBR, IR, NBR, NR, and EPDM.
Chemical Reactions between Thiocarbamylsulfenamides and Benzothiazylsulfenamides Leading to Cure Synergism
Authors: Krymowski J. F. , Taylor R. D.
Published: February 2011

A combination of N-oxydiethylenethiocarbamyl-N′-oxydiethylenesulfenamide, N-oxydiethylene-2-benzothiazylsulfenamide, and sulfur was found to be synergistic in acceleration of vulcanization. The combination reacts at 142°C to form a number of new covalent compounds, most of which eventually decompose to ionic species. The types of compounds produced and the kinetics of formation of these intermediate compounds explain the good scorch delay, high cure efficiency, and high thermal stability of the vulcanizates when combinations of these two sulfenamides are used as vulcanization accelerators.
Spectroscopic Study of Photosensitized Oxidation of Polyisoprene
Authors: Golub M. A. , Rosenberg M. L. , Gemmer R. V.
Published: February 2011

The microstructural changes which occur in cis- and trans-1,4-polyisoprenes and in squalene during photosensitized oxidation were investigated with the aid of infrared and proton and carbon-13 NMR spectroscopy. The singlet oxygenation of these isoprenic compounds resulted in allylic hydroperoxides with shifted double bonds, according to the expected “ene”-type process. In contrast to trans-1,4-polyisoprene and squalene, which displayed the three possible double bond shifts, cis-1,4-polyisoprene showed essentially two of the shifts (to di- and trisubstituted double bonds) and very little of the third (to exomethylene groups). A suitable measure of the extent of hydroperoxidation was afforded by the absorbance ratio, A3400/A1440≡A′. Similar correlations of A′ with oxygen uptake were obtained for the three isoprenic compounds, using chlorophyll or methylene blue as sensitizer. The use of rose bengal gave erratic results indicative of some autoxidation accompanying the hydroperoxide formation. The singlet oxygenation followed zero-order kinetics, the relative rates for cis- and trans-1,4-polyisoprenes being approximately 1.0:1.5.
Miscibility Studies on Heterogeneous Polymer Blends by Radioluminescence Spectroscopy and Particle Coarsening Measurements
Authors: Böhm G. G. A. , Lucas K. R. , Mayes W. G.
Published: February 2011

Interest in the subject of polymer miscibility has been stimulated by the investigation and common use by industry of heterogeneous polymer systems. Particularly, in recent years we have seen the development of materials, such as ABS, high-impact polystyrene, block copolymers, thermoplastic elastomer blends (TPR, etc.), and many more, which owe their unique properties to a certain critical degree of immiscibility of the polymeric constituents. This subtle difference in miscibility contributes to the formation of morphological features in the above-mentioned materials. More importantly, it governs the adhesion between the domains of the phase-separated polymeric composite. The latter property provides for stress transfer across the interface and thus is needed for the attainment of physical strength. Thus the questions posed by researchers are not so much concerned with whether two polymers are fully miscible on a molecular scale, a rare event indeed, but rather in the degree of miscibility of the two materials. In spite of recent advances made, the bulk miscibility of polymers cannot be predicted by theory. The lattice theory of Flory leads to unsatisfactory conclusions. It can, however, be used for an after-the-fact representation of experimental findings by use of an empirically determined concentration- and temperature-dependent polymer interaction coefficient. More insight and qualitative postulates on polymer miscibility are provided by the equation-of-state theory. It correctly predicts the marked influence of the degree of polymerization and of the thermal expansion and pressure coefficients and, more importantly, it anticipates the lower critical solution temperature observed in a number of polymer systems. However, the equations are complicated and contain many generally unknown, but experimentally accessible, parameters, and thus this theory too is of little help to the investigator seeking miscibility data for a specific pair of polymers.
Identification of Natural and Synthetic Polyisoprene Vulcanizates by the Carbon-14 Dating Technique
Authors: Chen H. Y.
Published: February 2011

The idea of using the radiocarbon dating technique for the differentiation of natural rubber from synthetic rubber is novel and has never been tested before experimentally. The experiments were designed to demonstrate the feasibility of the approach rather than to perfect the technique. With the limited data gathered here, it was shown that the radiocarbon dating technique furnishes results with accuracy quite satisfactory for the analysis of gum rubbers. For the cured samples, the accuracy is reduced mainly by the errors introduced from steps other than that of the radioactivity measurement. A further reduction of the accuracy results if the age of the natural rubber in the stock is not known. We conclude that the technique is feasible and yields results of an accuracy which is probably the best ever achieved by any known technique.
Scanning Electron Microscopy of Rubber Tear
Authors: Bascom W. D.
Published: February 2011

A relatively simple technique has been described which allows observation of the deformation zone at tear tips in rubber specimens. Investigation of various rubbers revealed qualitative information about the type of yielding that occurs in the deformation zone under different conditions. The initial appearance of the tear tip in nitrile, polybutadiene (both with and without carbon filler), and natural rubber suggests a process of stretching followed by rupture and then relaxation onto the tear walls. Only in the case of a fluoro-elastomer was there evidence of cavitation, although microcavitation (of sizes ≪1 µm) may be occurring in the other rubbers at much higher magnification. On the other hand, aging in laboratory air had the dramatic effect of producing fibrous and nodular networks in the tear tip material of all the rubbers except the fluoroelastomer. In the first few hours, the network structure developed as cavities (Figure 9), but after 24 h the network was fully developed across the tear. The aging process is very likely an ozone attack not only on the rubber under stress at the tear tip, but also on the rubber on the tear wall. It is quite possible that there were residual stresses in the material on the wall which facilitated attack. Chain scission appeared to be allowing the rubber to relax by viscous flow under the action of surface forces. These results point up the need to distinguish between failure controlled by pure stress and failure assisted by oxidation/ozonolysis in studies of slow tear growth in rubbers. The SEM technique also provides information about fillers. Their elemental composition can be determined by XES, and some qualitative information can be gained about their adhesion to the matrix and their effect on tear tip yielding.
Kinetic Effects Associated with the Glass Transition of Polyisoprene
Authors: Prud'homme J. , Goursot P. , Laramée A.
Published: February 2011

In this study we have found that the glass transition temperature of polyisoprene as determined by DSC is not a linear function of the heating rate. An equivalent study of polystyrene suggests that this behavior is general for all amorphous polymers. When the heat history of the polymer is fixed by linear cooling, Tg measured as the inflection point of the DSC curve is a function of the type q=C exp (−E/RTg), where E varies slightly with the rate of cooling before the measurement. Analysis of the individual curves according to the method of Ellerstein shows that this behavior is compatible with a kinetic model of the glass transition which implies a simple relaxation process with an activation energy only slightly above the value of E determined from the dependence of Tg upon the heating rate.
Guayule Rubber: Vulcanization Properties of Gum and Filled Stocks
Authors: Winkler D. S. , Schostarez H. J. , Stephens H. L.
Published: February 2011

Guayule rubber has been used both in the resinous and deresinated form for more than sixty years during periods of either scarcity or high prices of hevea rubber. During this time little or no efforts have been made to standardize the individual processes to a point where the products had specifications similar to the “Technically Specified Rubbers”. This lack of specifications consequently left users of the guayule rubbers with various opinions of the capabilities of this type of natural rubber in their commercial products. It was generally believed that the rubber was only good for low-grade products or where guayule was only used as a tackifier for other rubbers. Recent interest in the utilization of arid regions and the increasing need for less dependence on foreign sources has led the Mexican government to develop a modern guayule product. It was the purpose of this study to evaluate Mexican guayule rubber in cooperation with CIQA in an effort to determine processing and vulcanization characteristics of the products prepared in their pilot plant in Saltillo, Mexico.
Rubber Elasticity in the Range of Small Uniaxial Tensions and Compressions. Results for Poly(Dimethylsiloxane)
Authors: Erman B. , Flory P. J.
Published: February 2011

Results of uniaxial tension and compression experiments are reported on crosslinked poly(dimethylsiloxane) networks in the unswollen state over the range 0.5<α−1<1.2 , where α is the extension ratio. Curves representing the reduced force [ƒ]=ƒ(V0/V)1/3(α−α−2)−1 plotted against α−1 can be approximated by straight lines for 0.5<α−1<0.9, in agreement with the phenomenological Mooney equation. As α−1 approaches 1, however, they tend to level off and continue into the α−1>1 region with decreasing slope. These results are in agreement with the predictions of recent elasticity theories that incorporate the effect of junction-chain entanglements in the elastic free energy.
Rubber Heat Engines, Analyses and Theory
Authors: Farris R. J.
Published: February 2011

It has long been known that elastomeric solids could be used as the working “fluid” in engines designed to convert heat into mechanical work. In the past rubber heat engine cycles were not given serious consideration since energy alternatives were not in demand and the majority of the scientific community is unaware of their gas-like thermodynamic behavior. Consequently, past work has dealt with the subject primarily as a novelty or as a demonstrative proof of thermodynamic behavior. This paper provides an idealized mechanical and thermodynamic analysis of the rubber cycle and compares it to an equivalent cycle wherein a gas is the working fluid. Experimental data on a small rubber fiber engine are included which confirms the high power potential of these engines when they are designed using modern elastomeric fibers. These materials have remarkable properties and can respond rapidly to cyclic thermal disturbances. Power densities of roughly one watt/g of rubber have been attained using only a 30°C difference between the heat source and heat sink. Engine speeds well over 1000 rpm have also been attained when atmospheric pressure steam was used as the heat source. The analyses demonstrate that elastomers are ideally suited for energy conversion when only small temperature differences are available.
Influence of Carbon Black on Processability of Rubber Stocks. IV. Kinetics of Extrusion Shrinkage
Authors: Cotten G. R.
Published: February 2011

A careful examination of post-extrusion relaxation rates of SBR compounds containing various carbon blacks has shown that two distinct relaxation processes are taking place. The fast recovery region observed at times of less than 0.1 s after exit from the die is thought to be due to solid-like elastic recovery of the polymer imposed in the extensional flow region at the entrance to the die. The magnitude of that recovery increases with increasing surface area and decreasing structure of carbon black. The slow recovery region is observed at times longer than 1 s and caused by molecular orientation that occurs mainly during the extensional flow at the die entrance and maintained during the viscous flow through the die. Carbon black surface area and structure do not significantly affect the magnitude of the shrinkage in the slow recovery region. The observed two relaxation mechanisms may be simulated mathematically by two modified Maxwell elements in parallel, whose spring constants (and relaxation times) differ by a large factor.
13C NMR Analysis of Carbon-13 Enriched Ethylene Copolymers with Propylene
Authors: Brame E. G. , Holmquist H. E.
Published: February 2011

Carbon-13 nuclear magnetic resonance (13C NMR) has been used rather extensively in the characterization of ethylene-propylene copolymers and terpolymers. Chemical shift assignments on these copolymers were first made by Carman and revised only slightly by him recently. Despite all the work that has been done to assign the lines in the 13C NMR spectrum to the various sequence structures in ethylene-propylene copolymers, ambiguity still remains in some of the assignments, such as the assignment of the Sαβ line. It still is not known whether it is attributable entirely to two propylenes with one of them inverted, to an ethylene sandwiched between two propylenes with one of them inverted, or to an equal population distribution of these two structures. Carman and coworkers state that inversion of propylene in their copolymers is certain, but because of the large error in the calculation, the degree of inversion can be between 10 and 40% of the propylene content. His suggestion is that the Sαβ line is attributable to a combination of the two structure forms indicated above. Tanaka6 also makes this same suggestion in his 13C NMR work. He based his conclusion on some earlier infrared work. Most recently Randall describes a sequence length distribution method in which he estimates that the total amount of propylene inversions is the order of 20–30% for a concentration range of about 50–60 wt% propylene in copolymers with ethylene. Ray does not have to deal with the problem, as his 13C NMR spectra show no evidence of propylene inversions. His copolymers were prepared with an aluminum-titanium catalyst. Ours as well as Carman's were prepared with an aluminum-vanadium catalyst. Our reason for undertaking this program was to aid in the assignment of the Sαβ line. For this effort we prepared a series of ethylene-propylene copolymers that are carbon-13 enriched in both carbons of the ethylene, and we analyzed them by 13C NMR.
Elastomer Structures and ‘Cold Crystallization’
Authors: Bruzzone M. , Sorta E.
Published: February 2011

In a great number of applications an ideal elastomer should satisfy, to a certain extent, both of the following requirements: (1) nearly instantaneous crystallization upon application of strain (strain induced crystallization) and (2) slow or no crystallization when cooled at the temperature of maximum crystallization rate (cold induced crystallization). A noteworthy case of (2) is elastomer crystallization in a strained state. The connection between the points (1) and (2) has not been clearly understood up to now, but it is known that some crystallizable elastomers fulfil the requirements of both (1) and (2) better than others. From an experimental point of view, cold induced crystallization kinetics are substantially easier to measure than those of very fast strain induced crystallization. The phenomenon of cold induced crystallization in natural rubber, NR, has been known since the very beginning of elastomer technology and the tendency of natural rubber to crystallize by cooling has been overcome by crosslinking it with sulphur (vulcanization) without impairing its ability to crystallize by stretching (Goodyear, 1836). The synthesis of cis-polyisoprenes (IR) and cis-polybutadiene (BR) of different microstructural purity (different cis content) gave the possibility of changing the crystallization rate. It has also been reported that the very fast cold crystallization of trans-polypentenamer (TPA) could be reduced by lowering the trans content. The same fact had been observed earlier for trans-polychloroprene. There is a general agreement in postulating that the reduction of the crystallization rate, obtained either by cross-linking or by chain regularity reduction, can be linked with the lowering of the melting point. In both cases the low level of structural defects introduced in the chains does not affect the glass transition temperature in such a way as to vary the crystallization rate. The aim of this paper is to emphasize the importance of the variations of the glass transition temperature and melting point on the elastomeric cold crystallization rate and the way these may be used in planning new elastomer structures.
The Conversion Chemical Heterogeneity of Random Butadiene-Styrene Copolymers
Authors: Stejskal J. , Kratochvíl P.
Published: February 2011

The conversion chemical heterogeneity of random copolymers of butadiene and styrene with an average content 24 wt% of styrene is discussed quantitatively. With increasing weight conversion of copolymerization, the distribution of the chemical composition of the product becomes broader.
Effects of Sulfur on the Peroxide Cure of EPDM and Divinylbenzene Compounds
Authors: Fujio R. , Kitayama M. , Kataoka N. , Anzai S.
Published: February 2011

By introducing a small amount (0.15 part by weight) of sulfur into EPDM/DVB/DCPO compounds, Young's modulus and the rupture energy of the vulcanizates are markedly increased. The swelling equilibrium and dynamic mechanical behaviour show that an increase in crosslinking is achieved by the addition of sulfur. Moreover, electron microscopic study has revealed that vulcanizates from sulfur containing compounds have a better state of dispersion of the resinous poly-DVB phase than those without sulfur. When the amount of sulfur is too great (>0.20 part), Young's modulus of the vulcanizate is markedly reduced, mainly because of insufficient polymerization of DVB. Sulfur apparently acts as a retarder of DVB polymerization. Furthermore, it is proposed that some reaction products of peroxide and sulfur could be the initiator of this polymerization.
Model Compound Vulcanization—Part I. Basic Studies
Authors: Lautenschlaeger F. K.
Published: February 2011

Rapid, yet reliable model compound vulcanization techniques have been developed in order to establish a link between MCV and vulcanizate properties. The main features of the new approach are the determination of the sulfidic product yield and a more extensive analysis of the isomer distribution. An increase in reaction time, a decrease in the sulfur level or an increase in the accelerator level lead to similar changes in the isomer distributions. Increasing zinc oxide levels lead to an increasing vulcanization efficiency. The isomer distribution is significantly affected by concentration changes. This has not been previously observed. A good correlation between MCV results and the analysis of sulfidic rubber networks in actual vulcanisates was observed.