bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of Free Soap in Styrene Butadiene Latex
Authors: Lake M. , Rappaport G.
Published: February 2011

This method may find application in the determination of dialyzable substances in mixtures where the direct determination of such substances cannot be made. If the analyte of interest is a substance that dialyzes through without interferences, then a determination of the amount that dialyzes through in two separate dialysis times will allow for the calculation of the amount originally present. The cmc of a surfactant may readily be determined.
Rubber-Thermoplastic Compositions. Part II. NBR-Nylon Thermoplastic Elastomeric Compositions
Authors: Coran A. Y. , Patel R.
Published: February 2011

Thermoplastic elaslomeric compositions based on crosslinked NBR and nylon can be produced. A variety of nylons of differing melting points and NBR's of differing AN contents can be used in a broad range of proportions. The compositions can be further altered by the incorporation of fillers and plasticizers. The work indicates that a number of types of nylon-NBR based clastomeric materials, fabricable as thermoplastics and exhibiting good strength and excellent hot oil resistance, can be produced in a range of hardnesses.
An ESR Study of Oxidation in Uniaxially Deformed Polybutadiene and Polyisoprene
Authors: Mead W. T. , Porter Roger S. , Reed P. E.
Published: February 2011

The ESR spectra obtained from diene rubbers following mechanical deformation at cryogenic temperatures in a commercial purity nitrogen atmosphere are composite spectra arising from two or more radicals. The two radicals are thought to be an allyl radical resulting from main-chain rupture and a peroxy radical. The majority of the peroxy radicals observed in the composite spectra are formed as a result of oxygen in the test environment reacting with the allyl radicals formed shortly as chain rupture occurs. However, it appears that a small proportion of the peroxy radicals arise from oxygen incorporated into the rubber network during the processing stage before mechanical testing commences. Only if oxygen is rigorously excluded at all stages of processing and testing is the true spectrum of the primary allyl radical observed following mechanical degradation, showing that the chain rupture site is at the weakest bond, between the α- methylene groups. Oxygenated radical species incorporated during processing can be removed by heat treatment, provided the heat treatment is carried out in an oxygen-free atmosphere. The radical stability vs. temperature tests have shown that the peroxy radical decays at a temperature approximately 20 K lower than the allyl radical, which in turn decays rapidly in the vicinity of the Tg of the rubber. It appears that the anomalous increase in radical concentration near Tg, particularly noticeable in the case of mechanically deformed polymers, possibly arises from stored energy release on warming assisted by the presence of oxygen, oxygen being the major contributor to the process. However, the anomalous effect appears to be complex and other factors may contribute to it. It is generally concluded that the ESR technique offers a sensitive method of observing the oxidation of polybutadiene and other diene rubbers.
Changes in the Dynamic Modulus during Thermal Degradation of Polyisoprene Vulcanizates
Authors: Yano Shoichiro
Published: February 2011

Thermal degradation of IR is investigated in air and in N2 by dynamic mechanical measurements. In this dynamic method, the change in the relative dynamic modulus is found to be frequency dependent, but the data at various frequencies can be superimposed upon each other by shifting along the time axis. Changes in relative modulus as a function of time in air and in N2 can be expressed by Equation (1), which contains two exponential terms. The activation energies for k1 and k2 are 84 and 57 kj/mol in air, and 61 and 57 kJ/mol in N2, respectively.
Effect of Curing Temperature and Curing System on Structure-Property Relations of Rubber Blends
Authors: Bhowmick A. K. , De S. K.
Published: February 2011

In the present paper we have studied the effects of curing temperature (150° and 180°C) and curing system (conventional and efficient vulcanization) on 1) the technical properties, 2) polymer-filler interaction and 3) network structure of NR-BR, NR-SBR-BR, NR-SBR blends. On the basis of cure rate and interphase crosslinking, it has been suggested that the tendency to form mechanically compatible blends follows the order NR-BR > NR-SBR-BR > NR-SBR. It is the interplay of the network structure and polymer-filler interaction which controls the properties of the blend vulcanizates. The resilience, heat build-up and set properties depend more on the network structure, while the tensile, tear and fatigue resistance are influenced more by the polymer-filler interaction.
The Effects of Cross-Linking and Strain on the Glass Transition Temperature of a Polymer Network
Authors: Sharaf M. A. , Mark J. E.
Published: February 2011

The glass transition temperature Tg of an elastomer is of great importance with regard to its utilization since at this temperature and below, the material can no longer exhibit rubberlike behavior. In the present study, networks were prepared from atactic poly(vinyl acetate) and poly(isobutyl methacrylate), both types of networks being inherently non-crystallizable and therefore immune from complications associated with strain-induced crystallization. The values of Tg were obtained by dilatometry, differential scanning Calorimetry, the measurement of viscoelastic losses, and irreversible thermal shrinkage. For both types of networks, Tg was found to increase with increase in degree of crosslinking and with increase in elongation. These results suggest that the most important effect of crosslinking and network elongation is a decrease in the mobility or entropy of the network chains.
The Effect of Chain Length Distribution on Elastomeric Properties. III. Non-Random Networks Prepared with a Partially Vinyl-Selective Bis(Alkylperoxy)Diisopropylbenzene
Authors: Llorente M. A. , Mark J. E.
Published: February 2011

The reactant α,α′-bis(t-butylperoxy)diisopropylbenzene was used to crosslink PDMS polymers containing vinyl side groups with varying degrees of segregation along the chains. On the basis of the elongation moduli of the resulting networks, this reactant was found to be moderately selective for vinyl groups, falling approximately midway between the essentially non-selective benzoyl peroxide and γ-radiation and a highly selective silicon hydride crosslinking agent. As expected, a significant amount of the selectivity was lost at longer reaction time. Placing crosslinks in close proximity decreases the effective degree of crosslinking, and may affect the swelling response more than the modulus in elongation.
Modified Bitumens Containing Thermoplastic Polymers
Authors: Piazza S. , Arcozzi A. , Verga C.
Published: February 2011

The mechanical and viscoelastic behaviour of bitumen, both at high and low temperature, is noticeably improved by addition of thermoplastic polymers such as thermoplastic SBS rubbers and atactic polypropylene. These polymers can be used complementary to each other. Atactic polypropylene decreases the flow of bitumen at room temperature, thus reducing creep. The SBS block copolymer, instead, prevents excessive stiffening of bitumen at low temperature, reducing the brittleness. Characteristics of bitumen-polymer blends depend heavily upon bitumen quality, particularly upon its aromatic content and mixing process.
Numerical Evaluation of the State of Cure in a Vulcanizing Rubber Article
Authors: Prentice G. A. , Williams M. C.
Published: February 2011

In its present state this model should be effective as an aid in analyzing and optimizing cure cycles for vulcanizing articles. It is clearly necessary to have accurate thermo-physical data when simulating the SOC in thick articles. Additional improvements could be realized by more careful treatment of the heat transmitted into the rubber through the blades.
Vibration Isolation: Use and Characterization
Authors: Snowdon J. C.
Published: February 2011

This article is concerned with vibration isolation, with antivibration mountings (resilient isolators), and with the static and dynamic properties of rubberlike materials that are suited for use in antivibration mountings. The design of practical antivibration mounts incorporating rubber or coiled-steel springs is described in Refs. 1–27; pneumatic isolators (air mounts, etc.) are described in Refs. 5, 28–35. Throughout the literature, as here, attention is focussed predominantly on the translational (vertical) effectiveness of antivibration mountings. However, the two- and three-dimensional vibration of one- or two-stage mounting systems is addressed in Refs. 4, 10, 12, 36–56. Following a description of the static and dynamic properties of rubberlike materials, the performance of the simple or one-stage mounting system is analyzed, account being taken of the occurrence of second-order resonances in the isolator and in the mounted item. In the latter case, as likely in practice, the bulk of the mounted item is assumed to remain masslike, whereas the feet of the item are assumed to be nonrigid (multiresonant). Discussion is then given to the two-stage or compound mounting system, which affords superior vibration isolation at high frequencies. Subsequently, the powerful four-pole parameter technique is employed to analyze, in general terms, the performance of an antivibration mounting with second-order resonances (wave effects) when both the foundation that supports the mounting system and the machine are nonrigid. The universally adopted method of measuring mount transmissibility is then described, followed by an explanation of how transmissibility can also be determined by four-pole parameter techniques based on an apparatus used by Schloss. The four-pole measurement approach has not been exploited hitherto, but it is apparently feasible and valuable because it enables mounts to be tested under compressive loads equal to those routinely encountered in service.
Physical Characterization of Pivalolactone-Grafted Elastomers. I. General Morphology
Authors: Buck W. H.
Published: February 2011

The pivalolactone graft copolymers represent a new type of thermoplastic elastomer, and their morphology appears to be unique. A comparison of some of the properties of the three principal types of thermoplastic elastomer is given in Table III. Over a fairly wide composition range (10–40% polyPVL), the general morphology of the polyPVL graft copolymers remains the same: discrete crystallites in a continuous elastomeric phase. The poly(ethyl acrylate)-based grafts, having much higher values of HSL and SSL, tend to form spherulitic structures. These spherulites are composed, however, of discrete polyPVL crystallites. Clean phase separation, as indicated by invariance of Tg with composition, is partly due to the relative compatibility of the backbone and grafted side-chain polymers. Nevertheless, the extent of phase separation between hard and soft phases should always be greater for ABA block and polyPVL graft copolymers than for segmented copolymers because of the free end possessed by each hard segment in the first two cases. The melting temperature of the hard segments in polyPVL grafts depends primarily on the HSL, which can be varied easily during synthesis. The melting temperature of segmented and triblock copolymers cannot easily be changed without large changes in physical properties. The low initial (Young's) modulus of the polyPVL graft copolymers and some ABA triblock polymers is a consequence of a continuous elastomeric phase and a discrete hard segment phase. When the hard phase becomes cocontinuous with the soft phase, a high initial modulus more like that of a plastic than of an elastomer may result. Annealing of crystalline segmented and polyPVL graft copolymers causes improvement in such physical properties as compression set, creep resistance, and tensile strength. This is attributed tc relaxation of molded-in stress, accompanied by recrystallization of the hard segments. Since the glassy hard segments in ABA triblock polymers lack crystalline order, similar improvement in properties would not be expected. PolyPVL crystallites act as both crosslinks and reinforcing filler in these copolymers. The size of the crystallites depends upon both the length of the grafted chain (HSL) and the separation of chains along the elastomer backbone (SSL). Generally, large values of HSL and small values of SSL cause the largest crystallites to form. Based on experimental evidence from transmission electron microscopy, x-ray diffraction, thermal analysis, and solvent swelling, a morphological model has been developed.
Processability of Rubber and Rheological Behavior
Authors: White J. L.
Published: February 2011

The rheological and ultimate properties of unvulcanized rubber and compounds are described. Unfilled elastomers may be represented by the theory of viscoelastic fluids. Various experimental studies are described and interpreted. Filled polymer melts exhibit more complex behavior including yield values, reduced elastic memory, and thixotropy. The influence of these properties on several processing operations including milling/mixing and extrusion is critically discussed.
Temperature Buildup in Bonded Rubber Blocks Due to Hysteresis
Authors: Holownia B. P.
Published: February 2011

The comparison between theoretical and experimental results of the temperature distribution in bonded cyclindrical rubber blocks due to compressive cyclic loading was largely dependent on the value of Poisson's ratio. It was found that, for thin rubber blocks (D/h>6), the third significant figure in the value of v appreciably altered the temperature distribution, while for thick blocks (D/h<4), the same change in v had much smaller effect on the temperature distribution within the rubber block. The theoretical analysis used in the paper can easily be adapted for blocks of different geometries, and hence the temperature distribution within a desirable limit can be achieved by changing the geometry of the rubber block.
Another Look at S-Type Carbon Blacks
Authors: Haws J. R. , Cooper W. T. , Ross E. F.
Published: February 2011

Oxidized carbon blacks impart unique properties to rubber compounds. The most interesting aspects include modified curing characteristics, good resistance to tear, and the potential to promote good adhesion of rubber compounds to brass-plated metal. When these improvements are considered in conjunction with the wide range of surface area and structure available in carbon black, it is evident that oxidized furnace blacks provide combinations of properties not previously available.
Compatibilization of Rubber Blends through Phase Interaction
Authors: Bauer R. F. , Dudley E. A.
Published: February 2011

It can thus be concluded that phase heterogeneity in rubber blends almost always prevails and can be shown by means of microscopy. Compatibilization of these blends, as judged by the emergence of intermediate dynamic loss peaks, can be achieved through a vulcanization process which promotes crosslinks between the heterogeneous polymer phases. Such compatibilization through interphase crosslinking can occur even with polymers of significantly different solubility parameters. Interphase crosslinking can be recognized from swelling restriction in the differential swelling test but also from the development of intermediate dynamic loss peaks. The development of interphase crosslinks, therefore, results in the formation of a new copolymer system from the two mixed components which has new and unique properties. The requirements of cure systems which promote cocuring of polymer phases are not fully understood. Important factors to be considered are curative migration; however, a discussion of this subject would be beyond the scope of this presentation.
Determination of Functionality Distributions in Telechelic Prepolymers by Thin-Layer Chromatography
Authors: Min T. I. , Miyamoto T. , Inagaki H.
Published: February 2011

Functionality distributions of telechelic prepolymers have been determined by thin layer chromatography (TLC). Commercially available 1,2-polybutadienes having either carboxyl or hydroxyl groups were examined. TLC with p-xylene as the developer made it possible to separate the sample into a nonfunctional component and a mixture of mono- and difunctional components. Complete separation of the sample into the three components was achieved by selecting the developer and development procedure appropriately. Quantification of the chromatograms was performed successfully in a TLC apparatus equipped with a flame ionisation detector. A simple method for the determination of the functionality distribution was proposed.
Identification of Natural and Synthetic Polyisoprene Vulcanizates by Thermal Analysis
Authors: Sircar A. K.
Published: February 2011

1. NR and IR vulcanizates can be distinguished from one another by their DSC and DTG curves. 2. Thermal curves remain more or less unaffected by recipe changes. 3. Identification of blends of NR and IR is complicated since different IR's have different exotherm areas. 4. Natural and synthetic trans-polyisoprenes can be distinguished from one another, as well as from their cis counterparts, by their DSC and DTG curves. 5. Differences observed in the natural and synthetic polyisoprenes seem to be due to a higher amount of cyclized product, formed as a result of catalysis by the residual inorganic material in the synthetic product.
Degradation of a Vinylidene Fluoride-Hexafluoropropylene Copolymer in Neutral and Alkaline Aqueous Solutions
Authors: Bryan C. J.
Published: February 2011

During the preparation for the launch of Skylab I, it was noted that the poly(vinylidene fluoride co hexarluoropropylene) (Viton A®, FKM), bladder in the Apollo telescope mount coolant storage tank was perforated, and coolant water, buffered to a pH of 9.2 for corrosion control, was found in the gas pressurization side of the storage tank. Careful examination of the bladder and storage tank before disassembly revealed that the position of the hole in the bladder did not coincide with any protrusion or irregularity in the tank. After disassembly of the system, the bladder was found to have several cracks in the surface exposed to the coolant water in addition to the previously mentioned hole. Samples of the failed bladder were tested on an Instron, and the tensile strength and elongation were found to have decreased to 48% and 42.5%, respectively, of the values determined by the fabricator at the time of manufacture. In order to better understand the degradation of FKM in neutral and alkaline aqueous solutions, the rate and extent of change in the tensile strength, elongation, and per cent swell of FKM were studied as a function of temperature (50°, 70°, and 90°C), pH (7.0, 9.0, and 11.0), and time of exposure to the test solutions.
13C NMR Characterization of Copolymers Of 2-Chlorobutadiene-1,3 and 2,3-Dichlorobutadiene-1,3
Authors: Brame E. G. , Khan A. A.
Published: February 2011

Due to the facile free-radical polymerization of 2-chlorobutadiene-1,3 (chloroprene, CB), most experimental studies and commercial polymer preparations involve free-radical-initiated emulsion systems. From the reactivity ratios published in the literature and the calculated propagation rate of CB at 40°C, its rate is about 10 times faster than that of butadiene. Uniquely, among all the vinyl and diene monomers known, 2,3-dichlorobutadiene-1,3 (DCB) is more reactive than CB. Modification of the properties of polyCB through copolymerization is often relatively difficult. The comonomer most frequently used with CB is DCB, even though many other dienes and vinyl monomers have been tested. Copolymerization of DCB with CB modifies the polymer properties, as judged by evidence from increased crystallization resistance and the retention of building tack. The effect of DCB on the properties of the copolymer of CB and DCB is markedly influenced by the amount of comonomer present. Since no detailed studies have been reported on this monomer pair, we report here microstructure analyses by both 1H and 13C NMR on CB-DCB copolymers. However, it was the 18C NMR analyses which provided the major characterization of these copolymers, while the 1H NMR was only used for the analysis of their composition.
Crystallization Behavior as a Tool for Assessing the Degree of Dispersion of Elastomer Blends
Authors: Ghijsels A.
Published: February 2011

The present study has demonstrated the suitability of the DSC technique for studying the blending characteristics of unvulcanized unloaded BR-SBR blends. It has been shown that, especially during the initial stages of the blending process, BR-SBR blends can be conveniently characterized in terms of the index M, which can be derived from composition (heat of fusion) measurements on a number of spot samples taken at random from the blend. During the later stages of the blending process, moreover, additional information about the degree of blending can be obtained from the degree of interference of SBR with the crystallization of BR. Furthermore, it has been found that the BR-SBR systems investigated follow an exponential rate law of blending during the early stages of blending, which allows blending processes to be described quantitatively by a single parameter, the rate constant. The approach adopted appears to be useful for a suitable characterization of BR-SBR blends in terms of the degree of blending and also for assessing the influence of blending conditions and the performance of blending equipment. An additional advantage of this method is that it can easily be extended to the characterization of unvulcanized (probably also vulcanized) black-loaded compounds which are difficult to examine by other methods. Finally, it should be noted that this approach is not necessarily restricted to BR-SBR blend systems, but can also be applied to other elastomer systems, (e.g. butadiene rubber and natural rubber, natural rubber and isoprene rubber), blends of thermoplastic materials (e.g. polyethylene and polystyrene), or blends of elastomers and thermoplastics. The only limitation, however, is that one of the components of the blend must be able to crystallize under conditions where the other component, is not.
Analysis of Morphology Formation in Elastomer Blends
Authors: Tokita N.
Published: February 2011

Based on the assumption that an equilibrium particle size of dispersed phase will be reached when the breaking-down rate and the coalescence rate are balanced, a theoretical expression was obtained. The theory showed qualitatively that the equilibrium particle size becomes smaller when (1) the stress field is increased, (2) the interfacial tension between matrix and dispersed phase becomes smaller, and (3) the concentration of dispersed phase decreases. Qualitative verification of the theory was obtained by experimental examination of the NR-EPM blend system. In practice, in order to obtain a small particle size in a short time at above 20% volume fraction, the matching of rheological properties of the matrix and the dispersed phase is desirable. On changing from internal mixer to mill, the temperature became one of the most influential factors that control the particle size. Future work, such as the quantitative value of interfacial tension as a function of temperature, and macroscopic breaking energy measurement, etc., is necessary to confirm the theory quantitatively.
Effect of Adsorption on Reinforcement of Filled Polyurethane
Authors: Seto J.
Published: February 2011

The effective network chains, ns formed at the polyurethane-particle interface are brought about by physical adsorption of the polymer from solution. The relation between ns and adsorption, Γs, is given as Equation (3). The coefficient K is evaluated as 10. This value supports multicontact adsorption and agrees with random coil dimensions. The pretreating of powder by a surfactant (oleate) brings about neither adsorption nor network chain formation. The glass transition temperature increases with increasing amount of adsorption.
Reinforcement of Elastomers by Silica
Authors: Voet A. , Morawski J. C. , Donnet J. B.
Published: February 2011

The physical characteristics of silica pigments covering a wide range of properties were determined and related by regression analysis to the static and dynamic mechanical properties of SBR vulcanizates reinforced with these silicas. Three morphological parameters were found to determine the vulcanizates' characteristics: the external surface area by CTAB adsorption, the structural index, and the DBP absorption number. The adherence between silica and elastomer was studied by means of swelling in solvents, by determination of the volume increase on deformation, and by scanning and transmission electron micrography. It was found that dewetting, the formation of vacuoles on deformation, is a most important factor, limiting reinforcement by silica at loadings above 40 phr. Equally, at such loadings the pH of the silica has an important influence : the higher the acidity, the less the adherence between pigment particle and elastomer. Silane coupling agents not only increase adherence, but also improve the degree of dispersion of the silica, which is frequently characterized by the presence of large, partly unwetted particle clumps under conventional conditions of incorporation, thus severely limiting the strength properties of the vulcanizates. The curing system employed and the accelerators used are also of paramount significance for adherence.
Diisocyanate Vulcanization of 1-Chlorobutadiene-Butadiene Rubber
Authors: Yamashita S. , Tamuba M. , Terada J. , Kohjiya S.
Published: February 2011

TDI-d was found to react quantitatively with hydroxyl group in CB-BR under relatively mild curing conditions to give CB-BR vulcanizates, while AEP afforded vulcanizates having a certain amount of pendent AEP. The latter could be used to increase the network-chain density by postcuring with TDI. This crosslink formation was found to proceed almost quantitatively through the reaction of pendent AEP and hydroxyl groups with equimolar amounts of TDI in CB-BR under suitable curing conditions.
Creep of Rubber under Repeated Stressing
Authors: Derham C. J. , Thomas A. G.
Published: February 2011

Under conditions of high stresses and/or strains it is not possible to predict cyclic creep behavior by combining a knowledge of Boltzmann's superposition principle with a knowledge of the static creep. Although experiments here have been restricted to tensile measurements, the effect is likely to occur also in other modes of deformation. It may be of significance in applications where alternating stresses are applied to rubber components and indicates a further reason for keeping working stresses low and designing to avoid fully relaxed conditions.
Flammability of Elastomeric Materials
Authors: Fabris H. J. , Sommer J. G.
Published: February 2011

The use of synthetic polymeric materials in numerous consumer-oriented applications has increased rapidly over the past few decades. This trend has caused considerable concern regarding the responses of these materials to fire, and this concern has resulted in a flurry of activities by numerous agencies (private and government) directed toward better characterization of the flammability of manmade polymers as well as toward regulations to reduce any potential hazard these materials may present. As a consequence, the number of end uses for polymers requiring some degree of flame retardancy has risen sharply over the last several years. Much of the original action was in the field of plastics, and relatively little attention was paid to the flammability of elastomeric products. Even though the ignition and burning characteristics of rubbers were a matter of some concern in a few specific applications, the total volume of rubber used in these applications was rather small and, consequently, research on this subject lagged considerably behind the corresponding efforts spent in the field of plastics. The rapid increase in the use of elastomers in many consumer-oriented, nontire applications has, however, more recently caused greater emphasis to be placed on the flammability behavior of these polymer classes. The awakening interest in flammability characteristics of elastomer products is reflected in the sharp increase of published information on the subject over the last decades.
Incorporation of Antioxidant Groups into Polydienes. I. Via Chain Transfer Reactions of Symmetrical Dithiodiphenols
Authors: Weinstein A. H.
Published: February 2011

A number of substituted symmetrical dithiodiphenols and related monothio-and trithiodiphenols were prepared and characterized. Some of these dithiodiphenols showed chain transfer activity either in bulk styrene or in emulsion diene polymerizations. The fact that phenolic sulfide units were incorporated into polyisoprene and polybutadiene by use of one of these compounds, 2,2′-dithiobis(6-t-butyl-p-cresol), as chain transfer agent was confirmed by ultraviolet analysis. It was demonstrated that selfresistance to oxidation was incorporated into various diene homo- or copolymers, to varying degree, by use of some of these dithiodiphenols as chain transfer agents, on the basis of oxygen absorption tests on preextracted polymers.
Shear Degradation of Polyisobutene
Authors: Abbås K. B. , Porter R. S.
Published: February 2011

A polyisobutene of Mw 1.98×106, Mw/Mn 1.8, was extruded in an Instron capillary rheometer. Shear degradation occurred at high shear stresses, approaching melt fracture, and was more prominent at lower extrusion temperatures for tests at 60–140°C. The capillary was 51 mm long with a length/diameter ratio of 66.7 and a 90° entrance angle. Repetitive extrusions at constant shear rate caused a decrease in a molecular weight and a simultaneous narrowing of the molecular weight distribution. Extrudate expansion was measured after each successive capillary pass for tests at 80°C. Extrudate swelling correlated well with (Mz+1)Mz/Mw, except for the two first passes, where melt fracture was pronounced. The correlation with equilibrium extrudate expansion was almost as good for (Mz/Mw)3.7 (Mill's correlation) and for Mz+1 alone. The efficiency of bond rupture is low, with the energy required to rupture 1 mol of bonds being about 840 MJ at 80°C.
The Role of Morphology and Structure of Carbon Blacks in the Electrical Conductance of Vulcanizates
Authors: Verhelst W. F. , Wolthuis K. G. , Voet A. , Ehrburger P. , Donnet J. B.
Published: February 2011

The low electrical resistance of vulcanizates loaded with conductive carbon blacks in a given particle size range is cause by the low density of the particles, high structure, or both. In addition, the smaller the particle size of the black, the lower the resistivity. The same factors also cause high modulus and, in liquid dispersions, high viscosity. It is shown that acetylene black owes its lower resistivity in vulcanizates to its high structure. Vulcan XC 72 owes its even lower resistivity to both high structure and low particle density, while EC Black owes its lowest resisitivity to an extremely low particle density caused by the presence of a large number of hollow-shell particles, as indicated by electron micrography. It was shown that the resistivity of vulcanizates loaded with EC Black is practically unaffected by deformation up to 150%, in contrast to other conductive blacks at comparable resistivities. It has been observed that increasing the degree of loading leads to a lower limit of resistivity, about equal for all of the blacks. The various phenomena could be explained by the concept that the resistivity of carbon black loaded vulcanizates resides in the gaps between particles or aggregates and is determined by a process of electron tunneling.
Elasticity of Rubbery Networks in the Region of Zero Strain
Authors: Faṣina A. B.
Published: February 2011

1. Contrary to the widely held view of linearity through the origin of the Gaussian stress-strain plot (σ versus (Λ − Λ−2)), there does exist a curvature in the plot around the region of zero strain though the plot is continuous to the origin. 2. The inability of the Mooney-Rivlin expression to describe accurately the behavior of rubbery networks in the region of zero strain is not indicative of non-Gaussian character of the network, nor can it be ascribed to entanglements, or supramolecular organization in the network.
Carbon Black Morphology in Rubber
Authors: McDonald G. C. , Hess W. M.
Published: February 2011

Electron microscope image analysis of carbon blacks in specific rubber compounds has greatly expanded the range of useful applications for studies of this type. This dispersed carbon-gel procedure has improved the sampling and test precision at operating speeds that are now reasonably comparable to the simple colloidal procedures for characterizing carbon black. Improved models have been developed for deriving black surface area and intraunit occlusion capacity. The EM image analysis approach has been useful in applying certain principles of reinforcement theory, as well as in explaining rubber property differences that are attributable to carbon black variables. Studies on hysteresis at constant strain (E″) have indicated that the important black variables, in diminishing order of significance, are loading, structure (intraunit occlusion and anisometry), unit size, unit size distribution, and surface activity. For hysteresis at constant energy (resilience), the most important black variables appear to be black loading, unit size, unit size distribution, surface activity, and structure. In terms of tread wear resistance (moderate wear rates with SBR-BR), a somewhat different pattern of carbon black variables is apparent. At constant loading, the most important black properties appear to be specific surface area, surface activity, structure, and unit size distribution. At any given tread wear-surface area level, hysteresis can be lowered by broadening the unit size distribution and increasing the surface activity of the black.
Studies of Viscoelastic Relaxation of Some Nitrile Rubbers under Compressive Stress
Authors: Stenberg B. , Jansson J. F.
Published: February 2011

Stress relaxation in rubbers is usually supposed to be due to chemical aging phenomena of a mainly elastic material. Considerable physical viscoelastic processes can, however, be observed in the rubbery region, depending upon the type of rubber, crosslink density, type of crosslink, filler, and so on. Thus, in chemical stress relaxation experiments chemical and physical mechanisms are superimposed and can seldom be distinguished. Stress relaxation curves registered at different temperatures contain contributions from both types of mechanism. From a practical, as well as a theoretical, point of view it is therefore essential to find methods of distinguishing between the two relaxation processes. This would be possible if the relaxation curves were obtained during periods of time so short that the chemical relaxation can be neglected. The long-term physical relaxation is then obtained by shifting the curves by, for instance, the method of reduced variables. This technique has been utilized by Curro and Salazar. We now present an alternative procedure by which the physical viscoelastic stress relaxation behavior is determined from dynamic data. The physical relaxation curves are calculated from values of E′(ω) and E″(ω) obtained at different frequencies and temperatures. In this method of determining the physical relaxation, no change of sample is needed, nor is it necessary to allow the sample to relax between measurements. The stress relaxation behavior under compression of three nitrile rubbers has been studied, and it is shown that physical mechanisms dominate just above room temperature, while chemical mechanisms dominate at higher temperatures.
Mechanical Behavior of Lightly Crosslinked Polyurethanes
Authors: Dzierża S. , Janáček J.
Published: February 2011

Polyurethane elastomers are a numerous group of polymeric materials of wide practical application. They are usually formed by polyaddition of diisocyanates with hydroxyl-terminated polyesters or polyethers in the presence of low molecular weight diols or diamines as chain extenders. One may consider urethane elastomers to be block copolymers, consisting of moderately flexible long linear polyester or polyether segments and relatively stiff segments of aromatic and urethane groups. The length and structure of each block can be easily controlled. Crosslinking by an excess of diisocyanate can occur only at the stiff segments, and the number of branch points can also be controlled. The properties of these elastomers can be widely changed using components of different structures and varying their quantitative ratios. They are the results of a combination of segment flexibility, crosslinking, chain entanglement, orientation of segments, hydrogen bonding and other van der Waals forces, as well as rigidity of aromatic units. In the urethane systems, hydrogen bonding and other van der Waals forces, play a much more pronounced role than in familiar olefin-derived elastomers. Although polyurethane elastomers have very good mechanical properties at room temperature, their application is strongly limited by rapid deterioration of properties which takes place at elevated temperatures. The decay of mechanical properties of polyurethane is caused by the breaking of hydrogen and other secondary bonds, as well as by the presence of relatively weak crosslinks that make up their network. The properties of polyurethanes at elevated temperatures may, perhaps, be improved by forming additional crosslinks, besides the typical ones. Some efforts concerning this problem have been published. The aim of our study was to obtain and check the properties of polyurethane elastomers having unsaturated bonds, on which some additional crosslinks were expected to be formed in the presence of a suitable crosslinking agent.
Short-Fiber-Reinforced Elastomer Composites
Authors: O'Connor J. E.
Published: February 2011

The reinforcement of elastomers with short fibers results in composites with a wide variety of properties. The performance and properties are a function of fiber type, fiber content, fiber aspect ratio, fiber orientation, fiber dispersion, fiber-matrix adhesion, processing methods, and properties of the elastomer matrix. A composite with almost any desired property can be obtained by manipulation of these parameters. Of the five fibers studied in this work, glass and carbon are the poorest for increasing mechanical properties. The cellulose, aramid, and nylon fibers all reinforce elastomers to give composites of approximately the same magnitude in properties. Alignment of reinforcing fibers by milling creates a significant anisotropy in the composite properties. The degree of fiber alignment is best for glass, carbon, and cellulose fibers. The uniformity of fiber dispersion is again best for glass, carbon, and cellulose fibers. Aramid and nylon fibers tend to clump together and do not disperse easily. Fiber-to-matrix adhesion is a problem. No evidence of consistently good fiber-matrix adhesion is observed except for the precoated cellulose fibers. The interaction between fiber and elastomer can only improve with a coating or sizing that is compatible with both the fiber and its matrix. Adhesion-promoting bonding agents also improve fiber-matrix adhesion. However, each fiber and/or elastomer may be influenced differently by a bonding agent. Adhesion promoters specific to the type of composite being prepared must be sought in order to obtain optimum properties.
Theory of Elasticity of Polymer Networks. The Effect of Local Constraints on Junctions
Authors: Flory P. J.
Published: February 2011

The effects of restrictions on the fluctuations of network junctions imposed by neighboring chains are treated by resort to a model in which these restrictions are represented by domains of constraint. Whereas in a phantom network devoid of such constraints the displacements of junctions from their mean positions are invariant with strain, involvements with neighbors in a real network render them dependent on the strain. The observed stress consequently is increased above predictions for the equivalent phantom network. On the plausible assumption that the dimensions of the domains are transformed linearly (affinely) by the macroscopic strain, the constraints become less restrictive in the direction of a principal extension λt with increase in λt. Hence, the relative enhancement of the stress diminishes with the strain. The characteristic departures of the observed tension (ƒ)-extension (α) curve for elongation from the expression ƒ α kT(α−α−2) of previous molecular theories are qualitatively reproduced by the present theory, which also accounts for the approximate constancy of ƒ/(α−α−2) observed in compression and the decrease of this ratio with dilution.
Gas and Particle Emissions from Automobile Tires in Laboratory and Field Studies
Authors: Cadle S. H. , Williams R. L.
Published: February 2011

The purpose of this work was to determine the contribution which automobile tires make to air pollution. The gaseous hydrocarbon and sulfur compounds emitted in laboratory tests were identified. Although these hydrocarbons can participate in smog reactions, their mass emission rate is less than 0.1% of the current exhaust hydrocarbon emission rate. Hydrocarbons from tires are not measurable near a freeway. The particulate emitted from tires ranges in size from 0.01 to more than 30 µm, with the larger particles dominating the total mass. Measurements along a California freeway showed that most of the tire debris had settled within 5 m of the pavement edge. Airborne rubber concentrations were less than 0.5 µg/m3, or less than 5% of the total tire wear. These field measurements confirm the indoor emission pattern and verify that tire wear products are not a significant air pollution problem.
Influence of Carbon Black on Processability of Rubber Stocks. II. Extrusion Shrinkage
Authors: Cotten G. R.
Published: February 2011

Extrusion shrinkage is caused by molecular relaxation of polymer after exit from the die. The rate of molecular relaxation is extremely rapid and under the present experimental conditions, 80% of limiting extrusion shrinkage, L∞, is reached in less than 10 sec. The dependence of extrusion shrinkage L, on time t may be expressed by an empirical equation: L=L∞(1−kt−n) where k and n are constants. An increase in relaxation temperature increases values of L∞ and decreases n, while increase in shear rate decreases constant k. As found previously, extrusion shrinkage, L∞, is related to shear stress. Maintaining a good uniformity of product in factory extrusion may be accomplished by controlling extrusion pressure as well as by speeding up and controlling the process of molecular relaxation. The latter goal may be attained by maintaining the extrudate at an elevated temperature for about 2 min, and controlling that temperature within narrow limits. The results show the importance of defining extrusion conditions, including extrusion pressure and the temperature at which the extrudate is allowed to relax. The feasibility of rapid measurements (i.e., within ca. 1 min) of extrusion shrinkage using a laser beam detection system was demonstrated, and a good correlation was found with results obtained by conventional (weighing) method. The concept of occluded rubber and its effect on extrusion shrinkage was reexamined and found to provide a very satisfactory prediction of extrusion shrinkage of SBR compounds containing a wide selection of carbon blacks.
Looking at Rubber Adhesion
Authors: Roberts A. D.
Published: February 2011

Rubber has been employed in contact with a variety of surfaces for decades, yet the adhesional mechanisms involved are not fully understood. This article describes fundamental investigations carried out over the last decade. Emphasis is placed upon the use of optical techniques for looking directly at the contact area rubber makes with another surface. The interpretation is based upon a rate-dependent surface energy approach. In this way it is possible to predict the level of peel adhesion between surfaces, for example, when a ball rolls on smooth rubber. Other examples treated in the same way are the time for detachment of a ball from a smooth rubber track under gravity, its resilience when bounced on the track, and its friction when slid over the track. The influence of surface roughness, electrostatic forces, surface bloom, and humidity are considered, together with rubber compound variables such as glass transition temperature, crosslink density, and fillers. The central theme is that the adhesional mechanisms all physically depend upon the product of surface properties and bulk viscoelasticity of the solids in contact.
Nitrile Rubber Functionalized for Silica Reinforcement
Authors: Edwards D. C. , Sato K.
Published: February 2011

1. Conventional silica-reinforced NBR vulcanizates are characterized by stiffness at low elongation and low modulus at high elongations. These properties are associated with reticulate structure formation of silica particles, which occurs rapidly when compounds are heated to curing temperatures. 2. An NBR terpolymer, functionalized with about 0.07 mol of epoxy groups per kg polymer, interacts with silica when mixtures are subjected to combined heat and shear. Vulcanizate stiffness is greatly reduced, modulus at higher extensions is greatly increased, and mechanical properties in general are changed in the directions characteristic of carbon black reinforcement. 3. The interaction is similar in effect, but in principle much more economic, compared to that obtained with a mercaptosilane coupling agent. 4. Early experience indicates that the improvements are attainable to a large degree, although not optimally, in a conventional laboratory Banbury mixing operation.
Optimum Choice for Reference Temperature in Evaluation of Curing Cycles
Authors: Chehebar A. L. , Galli E. J.
Published: February 2011

An exact expression for the optimum reference temperature to be used in evaluating curing cycles is developed. This reference temperature minimizes the dependence of the equivalent cure time on the activation energy. Some examples are analyzed by means of analytical and numerical methods, and the computer program used is listed. The convenience of using the optimum reference temperature criterion is clearly demonstrated.
Compounding Bromobutyl for Heat Resistance
Authors: Timar J. , Edwards W. S.
Published: February 2011

Bromobutyl rubber is a good candidate for heat resistant applications, particularly when cured with zinc oxide in conjunction with sulphur donors or with dithiocarbamates plus active magnesium oxide. Several antioxidants provide added protection. When suitably compounded, bromobutyl compares well with resin-cured regular butyl in heat resistance and is much faster curing.