bluuebunny/crossref_metadata_2025 huggingface dataset:
Upturn Effect in the Non-Newtonian Viscosity of Polymer Solutions
Authors: Burow S. P. , Peterlin A. , Turner D. T.
Published: February 2011

Previously consideration of the hydrodynamic consequences of the separation of the segments of a macromolecule in laminar flow has led to the prediction that a very long polymer molecule in an extremely viscous solvent should have a limiting viscosity number which increases with the gradient G or, more precisely, with the parameter β=M[η]0η0G=NkT which is defined below. Experiments in Cannon—Fenske and Single-Pass capillary viscometers reveal such upturns of viscosity in solutions of polymethylmethacrylate in Aroclor and polyisobutene in polybutene oligomers which, as predicted, are very sensitive to molecular weight (1 to 7×106) and solvent viscosity (η0 =1 to 100 poise). A satisfactory comparison with theory is presented using values of [η]=ηsp/c, extrapolated to c=0, for solutions of polymethylmethacrylate in Aroclor-in the concentration range 0.125 to 2.0×10−3 g/cm3.
Effect of Temperature on Ozone Cracking of Rubbers
Authors: Gent A. N. , McGrath J. E.
Published: February 2011

The rates of growth of single ozone cracks have been measured for vulcanizates of a series of butadiene—styrene copolymers, over a temperature range from − 5° C to 95° C. The rates appear to be determined by two mechanisms. At low temperatures, near the glass transition temperature, they are quantitatively related to the segmental mobility of the polymer. The principal rate-controlling step in this case is concluded to be movement of the polymer chains after scission to yield new surface. At high temperatures the rate approaches a limiting value of 10−3 cm/sec/mg of ozone/1. This is about 1/1000 of the maximum possible value when instantaneous reaction of one incident ozone molecule causes scission of one network chain.
Thermodynamics of Swelling. I. Thermodynamic Properties of Crosslinked Binary Systems
Authors: Rehage Günther
Published: February 2011

Part I of this article was devoted to the thermodynamic properties of binary-mixed phases, in which one component was a low-molecular-weight liquid and the other a polymer crosslinked by primary-valence bonds. Special attention was paid to the case where the macromolecular material is in a glassy state at the measurement temperature. When solvent concentrations are low, the mixed phases are also in the glassy state. If the freezing concentration is exceeded during isothermal absorption of liquid, elastic materials appear, stable as to form when swelled, which are designated gels. Like liquids, gels are in a state of internal equilibrium. When polymer components have primary-valence crosslinks, liquid take-up always leads to an equilibrium state (swelling maximum, saturated gel). The enthalpy-concentration diagram was investigated rather thoroughly. If the polymer is in internal equilibrium at the measurement temperature, then the heat of mixing (heat of swelling) arises only from interaction between molecules. On the other hand, if the pure polymer component is frozen, the negative heat of freezing is superposed upon the “genuine” heat of swelling, resulting from interaction. Even when the enthalpy of the mixed phase is an additive function of the values for the pure components, a heat of mixing appears which originates only from liberation of the heat of freezing. The sign of the limiting heat of dilution, i.e., the differential heat of dilution at the swelling maximum, determines the temperature dependence of the swelling equilibrium. In athermic systems, maximum swelling is independent of temperature. The osmotic swelling equilibrium was investigated thermodynamically. It was shown that swelling pressure in crosslinked systems corresponds to osmotic pressure in solutions. A survey of results from the statistical theory of crosslinked systems is presented using, as a basis, Flory's theory for an ideal-athermal gel. As with the thermodynamic treatment of low-molecular-weight systems, excess functions were also introduced here. For low-molecular-weight, liquid mixtures, the excess functions have reference to formulas for an ideal solution. For polymeric, crosslinked systems, however, the reference values suggested are the statistical equations for an ideal-athermal gel. Finally, limiting laws for infinite dilution are derived for a crosslinked, mixed phase.
Anisotropy of Thermal Conductivity in Stretched Elastomers
Authors: Hennig J.
Published: February 2011

Thermal conductivity, η⊥, of some crosslinked, stretched elastomers and of stretched, plasticized PVC was determined, normal to the direction of elongation ε, as a function of temperature, between − 10° C and +45° C. Elongation was kept constant during measurement. η⊥ is smaller than heat conductivity of the unstretched (isotropic) material and decreases as elongation is increased. Anisotropy of thermal conductivity is particularly marked (η⊥=λ0≅0.8 to 0.9 for ε=200 per cent)—assuming equal elongation—in the case of polymers whose chain molecules contain polar side groups (e.g., butadiene-acrylonitrile copolymers, polychloroprene, chlorosulfonated polyethylene) whereas in those with molecular chains which possess only non-polar side groups (such as natural rubber and polyisobutylene), hardly any anisotropy appears. As far as plasticized PVC is concerned, anisotropy decreases, at the same elongation, with the plasticizer content and disappears almost entirely when the plasticizer concentration is higher than 60% by weight, i.e., the ratio η⊥=λ0 approaches 1, and in fact all the more rapidly, the more effective the plasticizer. Dependence of anisotropy on time was not observed.
Vulcanization of Elastomers. 47. Vulcanization of Natural Rubber and Polybutadiene with Benzoyl Peroxide
Authors: Scheele Walter , Rohde Ernst
Published: February 2011

The present article reports experiments intended to elucidate the kinetics of benzoyl peroxide decompositions in natural rubber and polybutadiene, and of consequent reactions. Both in natural rubber and in polybutadiene and at all temperatures, peroxide decomposition is first order with respect to concentration as well as time. The decomposition rate is substantially higher in natural rubber than in polybutadiene, and independent of the constitution of the polymer chains of the polybutadiene (tacticity, vinyl side groups, and the like). A somewhat higher activation energy of peroxide decomposition was derived for polybutadiene than for natural rubber. Formation of benzoic acid and crosslinking both follow a first order time law; the rates for peroxide decomposition, formation of benzoic acid, and crosslinking are the same in natural rubber and polybutadiene. It seems probable that peroxide decomposition in polybutadiene is purely homolytic. Interaction between peroxide and natural rubber is assumed, resulting in an increase in the rate of decomposition. The benzoic acid yield is considerably higher in natural rubber than in polybutadiene, and diminishes in the latter with the increase in vinyl side groups. In natural rubber this yield is a function of temperature, but in polybutadiene it shows practically no temperature dependence. Benzoyloxy radicals are incorporated into the elastomers as benzoate groups, as confirmed by infrared spectroscopic determinations with cis-1,4 polybutadiene. An interpretation of the formal kinetics of the vulcanization of 1,5-polyenes is proposed, which, together with the experimental results, may be used as a basis for consideration of the reaction mechanism.
Effect of Curing Systems on Fatigue of Natural Rubber Vulcanizates
Authors: Cox W. L. , Parks C. R.
Published: February 2011

The fatigue life of natural rubber-HAF black vulcanizates showed maxima when plotted as a function of crosslink concentration as did other properties related to a tearing process such as tensile strength, crack growth, and tear strength. Accelerated-sulfur vulcanizates were superior to peroxide and nonelemental-sulfur cures; this can be attributed to an exchange of polysulfide crosslinks under stress. An effective antioxidant was essential for maximum fatigue resistance. Accelerated-sulfur systems, although having a higher original fatigue life than peroxide or nonelemental-sulfur cures, showed a rapid loss on accelerated aging in air. This would indicate that an oxidative effect was involved. Sulfur group analyses of the flexed samples showed an increase in the concentration of RSSxSR linkages but a decrease in the total polysulfide sulfur, Sx, with no change in the crosslink densities. This suggests that the polysulfide linkages not only underwent exchange during the fatigue process but also homolytic cleavage to polythiyl radicals. These radicals can add to double bonds and in the presence of oxygen initiate oxidation chains which would lead to main chain scission.
Comparison of Number—Average Molecular Weights Calculated by GPC and Membrane Osmometry
Authors: Ambler M. R. , Mate R. D.
Published: February 2011

Data are presented which show that when a polymer contains an appreciable amount of low molecular weight species below the diffusion limit of the osmometer membrane, the osmotic molecular weight, Mn, is generally higher than the Mn calculated from gel-permeation chromatography (GPC). Experiments were performed on samples of poly(vinylchloride) (PVC) and high-cis poly (butadiene) polymers. Osmotic data were obtained in the usual manner, while GPC data were obtained using the universal calibration approach. It was found that when all polymer species below approximately 10,000 molecular weight were excluded from the calculation of Mn by GPC, agreement in Mn was obtained between membrane osmometry and GPC. The data obtained suggest that the choice of Mn as measured by membrane osmometry in the calibration of the GPC should not be done casually, as the measured Mn may not reflect the “true” value of that sample, especially when the polymer sample contains an appreciable amount of low molecular weight material.
Effect of the Reference Chain Dimensions in the Molecular Theory of Viscoelastic Behavior of Polymer Networks
Authors: Ilavský M. , Hasa J. , Havlíček I.
Published: February 2011

Mooney's version of the molecular theory of polymer networks has been generalized to the case when the external strain applied in the isotropic state is different from that at network formation. As in the theory of equilibrium behavior of the polymer networks, this generalization allows inclusion in the viscoelastic functions of effects connected with the temperature dependence of internal energy of the chains and with the strain effect of the solvent. From viscoelastic functions thus generalized, it is possible to derive a relation for calculation of the monomeric friction coefficient. It also suggests the possibility of superposing data obtained at various temperatures, degrees of swelling, and condition of network formation.
The Application of the Molecular Probe Technique to a Study of Polymer Crystallization Rates
Authors: Gray D. G. , Guillet J. E.
Published: February 2011

Most experimental methods for following the changes in crystallinity of polymeric systems have involved measurement of volume changes, changes in absorption spectra, and changes in optical properties. The basis of the novel method reported here is that the bulk interaction between small concentrations of a “probe” molecule and a polymer substrate depends in a simple manner on the degree of crystallinity of the polymer sample. In a recent paper from this laboratory it was suggested that this bulk interaction may be measured by a gas chromatographic method. In this paper, the method is extended to measuring the rate of polymer crystallization; a linear and a branched chain polyethylene) were studied using decane as the “molecular probe”.
Temperature Dependence of Ultimate Strength and Elongation of Vulcanized Rubber
Authors: Furukawa Junji
Published: February 2011

The ultimate elongation of vulcanized rubbers attains its maximum at a temperature Tmax, which was found to be sensitively increasing with increasing sequence regularity of alternating BD-AN copolymers and with increasing length of hard block of BD-ST block copolymers. Two kinds of rupture are proposed: a brittle rupture at low temperature occurs when the chain is extended fully and a plastic rupture occurs when the loosening of the chain caused by bond scission becomes faster than elongation. In the former case αb ∝ 1/αη˙, whereas in the latter αb ∝ 1/ατ˙, η and τ being resistance to viscous slippage and bond scission, respectively. It is estimated that Tmax decreases with increasing the crosslinks as well as the amorphous part, similar to a melting point.
1,2-Polybutadiene Resin Co-Agents for Peroxide Cure of Rubber Compounds
Authors: Martin J. W.
Published: February 2011

It is clear that low MW 1,2-poly (butadiene) resins are useful components of a wide range of peroxide-cured rubber compounds because they can provide non-scorch plasticizing before cure and reinforcement after cure. The resins increase the crosslinking efficiency of EPDM and EPR compounds and provide properties consistent with high crosslink densities. In NBR the resins serve as plasticizers before cure and unextractable softeners after cure. Applications for the low MW 1,2-poly (butadiene) resins in rubber technology appear to include high-hardness compounds, easily processed compounds, electrical insulations, and mechanical goods. Because some of the favorable effects of the resin (viscosity and crosslinking) counteract some of the undesirable properties of low-cost fillers, highly extended rubber compounds may well benefit from the addition of these special resins.
Dependence of Compression Modulus on Poisson's Ratio
Authors: Moghe S. R. , Neff H. F.
Published: February 2011

The experimental results and the widely used empirical formula (E/EApp)=(1+βS2)−1+(E/B) for the compression modulus are compared with analytical solutions obtained by Moghe and Neff. It is shown that the empirical formula does not represent the data adequately, particularly for large and small shape factors. It is also shown that the variation in β in the empirical formula arises due to the inadequacy of the representation rather than the thixotropic behavior as claimed.
Pure Flexure of a Layered Orthotropic Shell-Elasticity Study of a Tire-Related Problem
Authors: Levinson M. , Demiray H. , Sheung S. C.
Published: February 2011

This paper presents the analysis of the plane strain, pure bending of a portion of a three-layered cylindrical shell which is taken as a simple model of a part of a belted pneumatic tire. The two inner layers, representing the body and belts, respectively, are modeled as orthotropic, incompressible materials whose properties can be found from the appropriate cord and matrix material propties in a manner given in an Appendix. The outer layer, representing the tread rubber, is considered to be an isotropic, incompressible material. We concern ourselves with the study of the stresses tending to separate the various layers. These stresses, for two particular cases, are found to be strongly dependent on the crown angles of the cords in the body and belt layers. The results are presented in graphical form with the separation stress as a function of the body crown angle and with the belt crown angle entering as a parameter.
Preparation and Activity of Polymerizable Antioxidants for Emulsion Rubbers
Authors: Kline R. H. , Miller J. P.
Published: February 2011

A number of compounds have been prepared which contain both an antioxidant function and a polymerizable function in the same molecule. These compounds, which include representatives of both amine and phenolic antioxidants, were designed to be used as comonomers in the preparation of emulsion rubbers. Oxygen absorption tests in SBR-1006 show that these compounds exhibit antioxidant activity typical of the class of antioxidant, i.e., amine or phenol, to which they belong.
Thermoelastic Properties of Rubberlike Networks and Their Thermodynamic and Molecular Interpretation
Authors: Mark J. E.
Published: February 2011

Thermoelastic measurements and their interpretation by means of rotational isomeric state theory provide a great deal of insight into both thermodynamic and molecular aspects of rubberlike elasticity. Furthermore, conformational energies obtained in part from thermoelastic studies can in turn be used in the interpretation and even prediction of a variety of configurationally dependent properties of chain molecules, as is shown in many of the theoretical investigations cited in Sections IV and VII of this review.
Fluoroelastomers
Authors: Arnold R. G. , Barney A. L. , Thompson D. C.
Published: February 2011

The importance of fluorine in polymer chemistry has been known since the discovery of poly (tetrafluoroethylene) in 1938. Highly fluorinated polymers are very stable and have remarkable resistance to oxidative attack, flame, chemicals, and solvents. This stability has been attributed to the strength of the carbon—fluorine bond compared to that of the carbon—carbon bond, to steric hindrance, and to strong Van der Waals forces. The synthesis of elastomeric polymers containing enough fluorine to impart a significant degree of stability was not achieved until the mid-1950's. Since then a multitude of fluoroelastomers have been reported, but, of these, only certain copolymers of vinylidene fluoride, and fluorosilicone polymers have become commercially important. Recently, however, perfluoroelastomers based on perfluoro (alkyl vinyl ethers) and tetrafluoroethylene have been shown to possess an even greater degree of high temperature stability than do the fluoroelastomers hitherto available. These perfluoroelastomers also are essentially inert to most chemicals and solvents. Copolymers of perfluoro (alkyl vinyl ethers) with partially fluorinated monomers also have been developed that give better flexibility at low temperature. These polymers, now at the developmental stage, appear to be the forerunners of a new generation of superior high-performance elastomers. The discussion that follows will concern firstly, the commercial fluoroelastomers; secondly, those that show commercial promise and might be considered semi-commercial or at the developmental stage but which are available only in limited developmental quantities; and thirdly, those reported in the literature that, so far at least, have been of research interest only. In this last category, polymers will not be included which contain so little fluorine that performance characteristics are not substantially enhanced. For example fluoroprene [poly(2-fluorobutadiene)] and copolymers with nonfluorinated, olefin monomers in which the latter predominate will not be included.
The Effect of Compounding Variables on the Rate of Heat and Smoke Release from Polychloroprene Foam
Authors: Stewart C. W. , Dawson R. L. , Johnson P. R.
Published: February 2011

The addition of certain hydrated fillers or char promoters to a neoprene latex foam recipe can result in significantly lower rates of heat and smoke release. The degree of improvement depends to some extent on the severity of the test conditions, with the greatest improvement being obtained for conditions which are encountered during the early stages of a fire.
The Preparation and Properties of meta-Carborane—Siloxane Elastomers
Authors: Peters E. N. , Hedaya E. , Kawakami J. H. , Kwiatkowski G. T. , McNeil D. W. , Tulis R. W.
Published: February 2011

A new synthesis of D2-carborane—siloxanes is described which overcomes many of the disadvantages of previous synthetic methods. Linear polymers with molecular weights up to 250,000 can now be prepared using silanol-silylurea condensation reactions. Elastomers with useful general properties and good retention of mechanical properties after heat aging in air at 315°C for 300 hr were produced. The property profile and ease of vulcanization suggest utility in high-performance sealant, gasket, O-ring, and coating applications.
Ozone Attack and Antiozonant Protection of Vulcanized Natural Rubber. A Surface Study by Attenuated Total Reflectance Spectroscopy
Authors: Andries J. C. , Ross D. B. , Diem H. E.
Published: February 2011

This work reports a direct surface analysis of ozonization of vulcanized NR compounds by ATR. On ozonization of a vulcanized NR compound without clay filler or AO a degraded rubber layer, which contains ozonides and carbonyl compounds, is formed on the surface. This reaction is considerably slower than that of raw NR. When clay filler is added, the first effect is a decrease of rubber on the surface and a corresponding increase in the surface concentration of clay. With further ozone exposure, ozonides and carbonyl compounds appear. When a compound containing N,N′-dioctyl-p-phenylenediamine AO is ozonized, a film is seen, which has an ATR spectrum essentially identical to that of the ozonized liquid AO. There is no evidence of ozonized NR products, or of complex reaction products between AO and ozonized NR. These results are consistent with a scavenger or protection-layer mechanism for antiozonant protection.
Scrap Tire Disposal Process
Authors: Crane Grant , Kay Edward L.
Published: February 2011

Experimental results are presented which indicate that scrap rubber vulcanizates, typical of today's tires, can be depolymerized to a product which is essentially a carbon black dispersion in oil. The depolymerization is free radical in nature and requires an efficient chain transfer agent to prevent hardening of the vulcanizate and to increase the rate of depolymerization. The product, termed “Depolymerized Scrap Rubber” (DSR), should be useful, particularly as a rubber compounding ingredient and as a fuel oil extender.
Antioxidant Activity of Depolymerized Scrap Rubber
Authors: Crane Grant , Fieldhouse John W. , Kay Edward L.
Published: February 2011

Depolymerized scrap rubber (DSR) shows stabilizer activity for solution SBR rubber. It appears that the stabilizer activity of conventional rubber reclaim is due, to a significant extent, to the hydrocarbon portion.
A Master Curve for Amorphous Elastomers Derived from Small and Large Deformations Using Various Instruments
Authors: Nakajima N. , Collins E. A.
Published: February 2011

Dynamic mechanical measurements, stress—strain measurements, and steady-shear measurements made over a range of temperatures and frequencies or deformation rates are used to characterize the viscoelastic properties of raw elastomers. The measurements involve both small and large deformations. It is shown that the results on either butadiene—acrylonitrile (NBR) or butadiene—styrene (SBR) can be reduced to a single master curve. The instruments and ranges covered included Instron stress—strain (0.2–20 in./min; 25–75°C), Instron capillary (100−104sec −1; 100°C), Rheovibron (110 Hz; 23–156°C), Rheometrics (4×10−2−2.6×102 sec−1; 100°C), MTS high speed tester (267-26 700%/sec; 25–97°C), steady-state Mooney (0.05–20 rpm; 25–150°C) and transient Mooney (0.05 rpm; 25–150°C).
Strain-Dependent Dynamic Properties of Carbon-Black Reinforced Vulcanizates. II. Elastomer Blends
Authors: Sircar A. K. , Lamond T. G.
Published: February 2011

The three-dimensional aggregated structure of carbon black in elastomer blends behaves in a similar fashion to that of the individual elastomers. The elastomer seems to act merely as a dispersing medium. The properties of the rubber reflect the structural effects of the filler superimposed upon the elastomer itself. The elastomer molecules no doubt retain their individual characteristics of rotation of bonds which govern the stiffness of the molecule. However, the superimposed carbon-black network exerts the predominant effect, as far as the low-strain dynamic characteristics are concerned. The polymeric medium seems to influence this interaction by determining the magnitude of agglomeration and distribution of black in the phases, but does not have visible influence on the overall characteristics of the carbon-black networks. In this respect blends of two elastomers behave as a single elastomer. The importance of the present work is that the strain-dependent dynamic properties of blends of elastomers are very similar to those of single elastomers. In tires and antivibration applications, the strain imposed is usually less than 10%. More and more blends of elastomers are being used for these applications. The filler structure and its breakdown at these strains have an important contribution to hardness, modulus, and hysteresis of these compounds.
Monomer Sequence Distribution in Ethylene—Propylene Rubber Measured by 13C NMR. 3. Use of Reaction Probability Model
Authors: Carman C. J. , Harrington R. A. , Wilkes C. E.
Published: February 2011

One can measure r1r2 for an ethylene—propylene rubber by determining composition and χ, the ratio of contiguous to isolated propylene units. Our previous determination of χ depended on measuring methine carbon resonances in a 13C NMR spectrum. The χ method based solely on methine peak area measurements is inaccurate. This is not because the χ method is theoretically unsound, but because it is impossible to accurately extract the methine areas, necessary to calculate χ from the complex 13C NMR spectra of ethylene—propylene rubbers. The presence of inverted propylene produces ambiguity in a portion of the methine region. This means the methine areas can be used only to calculate a maximum and minimum boundary for r1r2. Another difficulty in using only methine areas to determine χ arises at propylene levels less than 35 wt%, because the methine areas for contiguous propylene triads become very small and are overlapped by strong resonances from long methylene sequences. We have developed a reaction probability method which accurately accounts for all the resonance areas in a 13C NMR spectrum of an ethylene—propylene rubber instead of restricting the analysis to the tertiary carbon areas. The reaction probability method produces a complete calculated 13C NMR spectrum as a best fit to the observed spectrum. The deduced probabilities provide the following derived quantities: (1) “r1r2”, a measure of monomer sequence randomness, (2) the distribution of methylene sequence lengths, (3) composition, (4) amount of propylene inversion. The probabilities can be used to directly calculate “r1r2”. Or alternatively, the probabilities can be used to calculate the methine carbon areas including the separation of the ambiguous methine area into contiguous and isolated contributions. These areas calculated from the probabilities can then be used to determine χ and the resultant r1r2. The two values for r1r2 are in good agreement, lending credence to the probability analysis. The composition derived from probabilities agrees with other analytical methods such as proton NMR and infrared. The limitation of the method is that we cannot accurately determine the percent propylene inversion. We can only conclude that in the majority of the copolymers of 20 to 60 wt% propylene that we have studied, the concentration of inverted propylene is significant and is between 10 and 40% of the total propylene present. Propylene enriched in carbon-13 could be used to prepare copolymers and perhaps resolve this uncertainty.
Sequence Distribution of 1,4-Polybutadienes
Authors: Tanaka Yasuyuki , Sato Hisaya , Hatada Koichi , Teeawaki Yoshio , Okuda Hiroshi
Published: February 2011

The spin-decoupled 1H NMR and 13C NMR methods were employed for the determination of the cis-trans sequence distribution in polybutadiene. Poly(butadiene-2,3-d2) and cis-trans isomerized polybutadienes were used for the peak assignment and the resonance of methylene protons in cis-trans linkage could be described as an A1B2 system. The dyad and triad sequence distributions of cis-1,4 and trans-1,4 units were determined for isomerized, equibinary, and lithium catalyzed polybutadienes, and polycyclooctadienes. The accuracy of the measurements was also studied comparing the results with those by infrared method.
Use of Power Integrator and Dynamic Stress Relaxometer to Shorten Mixing Cycles and Establish Scale-Up Criteria for Internal Mixers
Authors: Myers F. S. , Newell S. W.
Published: February 2011

The need for increased production from existing internal mixers has required the reduction of cycle times. The shortening of the cycle also enables mixing with less energy. However, the shortened mixing cycle requires closer control. Our ability to successfully shorten mixing cycles and to combine two-step mixes into one-step has been the direct result of the consistent mixing and uniform stock made possible through energy controlled mixing. Energy controlled mixing has worked, because we have had the necessary tools, the Power Integrator and Dynamic Stress Relaxometer. The transition of a new recipe from research and development to production can be slow and difficult. We believe that energy controlled mixing can reduce the shakedown time required to introduce a new recipe into production. It can also allow production problems to be successfully scaled-down for solution in laboratory mixers.
Vulcanization of Butyl Rubber by p-Quinone Dioxime
Authors: Gan L. M. , Soh G. B. , Ong K. L.
Published: February 2011

The vulcanization of butyl rubber by p-quinone dioxime oxidized by red lead and tetrachloroquinone was investigated. The maximum physical effective crosslinking density of the vulcanizates appeared to be when p-quinone dioxime and the oxidizing agent were equimolar. The formation of one physical effective crosslink required one molecule of p-quinone dioxime. Chemical reactions are suggested for the vulcanization steps.
Thermorheological Properties of Carbon Black Filled Elastomers
Authors: Caruthers J. M. , Cohen R. E.
Published: February 2011

A nonisothermal constitutive equation for a thermorheologically simple, linear viscoelastic materials has been developed. The influence of temperature on both the retardation times and the limiting compliance difference has been considered. This general nonisothermal equation can be reduced to previously published results for the creep T-jump experiment. Isothermal creep response was measured for the filled vulcanizates at various levels of strain. At relatively low strains, the true compliance was observed to increase with strain, the initial increase depending upon the carbon black volume fraction and structure. This increase in compliance was attributed to a breakdown of the carbon black network. A reversal of this trend at much larger levels of deformation was associated with orientation of the rubber chains. The creep rate at 300 seconds depends primarily on the level of strain, independent of the carbon black structure or loading. The apparent activation energy was determined by creep T-jump at various levels of stress for two series of filled compounds and an unfilled gum elastomer. For the filled materials, the activation energy decreased as the level of applied stress was increased. For example, the apparent activation energy of a SBR-1500 vulcanizate filled with 80 phr of N347 carbon black decreased from 167 to 71 kJ/mole as the applied stress was increased from 0.041 to 1.88 MPa. The activation energy at low stresses is a function of the carbon black volume fraction and structure. The apparent activation energy of the unfilled gumstock was 54 kJ/mole, independent of the level of deformation. The apparent activation energy of the filled vulcanizates approached that of the gumstock as the stress increased. The T-jump results and the isothermal data suggest the presence of two deformation mechanisms for the filled materials: (1) deformation of the carbon black network with an apparent activation energy dependent upon the loading and carbon black structure and (2) deformation of the rubber matrix surrounding the carbon black aggregates. At small deformations the first process is the rate controlling viscoelastic process and at large deformations the second process dominates. At intermediate levels of applied stress both mechanisms contribute to the deformation of the filled compounds.
Effect of Curing Systems on Selected Physical Properties of Natural Rubber Vulcanizates
Authors: Studebaker Merton L.
Published: February 2011

Selected physical tests were made on natural rubber vulcanizates prepared with six levels of each of six different curing systems. These combinations produced different types of crosslinks which were subjected to analysis using a combination of swelling measurements and lithium aluminum hydride reduction analyses. The effect of crosslink type or length was reflected in stress-strain measurements. One might reasonably look to crystallization measurements on such vulcanizates for a more complete explanation of the effects of crosslink type on stress-strain measurements. The ability of polysulfide crosslinks to exchange to form new links as the sample is strained should also be examined. When subjected to stress, especially when hot, polysulfide crosslinks are more susceptible to rupture than monosulfide or carbon-to-carbon crosslinks. This crosslink rupture may or may not be compensated for by subsequent formation of new polysulfide or monosulfide crosslinks in a deformed state. Under suitable experimental conditions, as in our compression set measurements, no net change in total crosslinks may take place. Under sufficiently severe test conditions, as in our blowout tests, degradation is severe and total crosslinks measured by swelling decrease. Under some circumstances, it is possible to follow changes in both the quantity and type of crosslinks by the analytical methods used in this study.
Polymerization of Chloroprene II. Role of Dimers in Thermal Polymerization
Authors: Leeming P. A. , Lehrle R. S. , Robb J. C.
Published: February 2011

The results in this paper indicate that the bulk thermal polymerization of chloroprene at 35° is in fact a polymerization of chloroprene dimers, and the rate measurements indicate that little or no monomer adds during the propagation process. Since DPPH does not inhibit the dimerization process but inhibits polymerization, it is tempting to conclude that the polymerization is a free-radical addition process, and this idea is to some extent supported by the retarding influence of some other free-radical inhibitors. On the other hand certain features of the polymerization of bulk dimers are difficult to reconcile with a simple free-radical chain mechanism, and it is considered that it would be premature to postulate such a process. It seems likely that monomer participates in the initiation of chains, but there is no unambiguous evidence to suggest how this process might occur. In view of these problems a detailed discussion of possible reactions is hardly relevant at the present stage, and this must await the results of future experimental work.
Theory of Branching Processes and Statistics of Rubber Elasticity
Authors: Dobson G. R. , Gordon M.
Published: February 2011

Recent efforts to reformulate statistical theories of polymeric systems in terms of the theory of branching (“cascade”) processes, are here extended to calculations of statistical parameters for the theory of rubber elasticity, viz., the number and mean length of various forms of active network chains. New simple results are given for random ƒ-functional polycondensation; it is shown that such systems are of interest in rubber elasticity studies in the region just after the gel point, where the concentration of active network chains varies rapidly with conversion, while the mean length of these chains diverges at the gel point. General exact formulas are derived for random crosslinking of arbitrary primary distributions, without assuming that the mean primary chain length is necessarily large; examples are worked out for uniform (homodisperse) chains, random, self-convoluted random, or Poisson primary distributions. Calculations are extended also to the cross-polymerization type of vulcanization reaction attributed to the cure of polybutadiene with peroxides. The results suggest reasons for differences in technical performance observed with such rubbers.
Elastomer Behavior IV. Loop Structure of Elastomer Networks
Authors: Case L. C. , Wargin R. V.
Published: February 2011

A new theoretical treatment strongly indicates that an elastomer network actually consists of a system of fused, closed, interpenetrating loops of polymer chains. This interpenetrating loop structure restricts the movement of the chains and thereby affects the stress-strain behavior of the elastomer. Methods have been developed to enable the calculation of the number of effective crosslinks caused by loop interpenetrations (virtual crosslinks). The uniaxial stress-strain behavior of an elastomer predicted using our methods can be fitted almost perfectly to published experimental data by proper selection of chain parameters. Previous theoretical treatments gave only a qualitative fit to the experimental data for the stress-strain behavior of elastomers and were not capable of predicting the correct shape of the experimental stress-strain curve. The present treatment gives a nearly perfect fit for both stress as a function of strain at constant crosslink density, and stress as a function of crosslink density at constant strain, and thus represents a vast improvement.
Carbon Black Reinforcement in Pre-Swollen Rubbers
Authors: Cotten G. R.
Published: February 2011

The mechanism of carbon black reinforcement was studied by examining stress-strain properties in SBR 1500 vulcanizates cured while swollen, then de-swollen. Carbon black-polymer attachments started contributing to the modulus of rubber at much higher elongations (ca. 300 per cent for a swelling ratio of 4.0) than in normally cured, unswollen specimens. The modulus in pre-swollen rubbers is considerably lower than in normal vulcanizates but it can be increased by heating the same specimens 30 minutes at 155° C in their de-swollen state. The observed increase in modulus of vulcanizates containing graphitized and regular carbon blacks after such a treatment was found to be related to carbon black “activity” and loading. Bound rubber does not seem to function as a distinct phase contributing to reinforcement, but it can be used as a measure of the number of attachments of carbon black and polymer. Additional information on the molecular mechanism of the phenomenon was obtained through radioactive tracer analysis. It was also found that regular vulcanizates can be softened by heating swollen samples at elevated temperatures (150-170° C). This softening was related to carbon black “activity” and was fully reversible: the original modulus could be recovered by additional heating of de-swollen vulcanizates for a further thirty minutes at 155° C. The above observations are discussed in terms of mobile adsorption of polymer chains on carbon black surface and the steric effect of fixing carbon black particles into an already extended polymer network.
Oxidative Stress Relaxation of Natural Rubber Vulcanizates at High Strains
Authors: Bell C. L. M.
Published: February 2011

The effect of high strain on the oxidative stress relaxation of several natural rubber vulcanizates has been investigated. In peroxide and CBS accelerated vulcanizates, the rate of stress relaxation increases with increasing strain, and this increase appears to be due to an increase in the rate of oxidation of the network. TMTD and MBT vulcanizates showed marked premature failure at high strains and no oxidative stress relaxation measurement could be made. The tensile strength of a TMTD vulcanizate was at least 20 per cent higher in vacuum than in oxygen, due, it is believed, to stress-induced oxidative degradation at the tip of surface flaws in the rubber.
Reaction of Ozone with p-Phenylene-Diamine and Related Compounds
Authors: Layer Robert W.
Published: February 2011

An improved method for measuring the relative rates of reaction of materials with ozone is given. This method is used to evaluate the reactivities of new antiozonants. The similar reactivities of isomeric N-alkyl-p-phenylenediamines indicate that antiozonants may in large measure be active as a result of the physical nature of the ozone and antiozonant (and possibly rubber) reaction product. Proton magnetic resonance spectroscopy was used to evaluate the types of products formed on the ozonation of p-phenylenediamines. These results are compared with previously reported infrared work. Evidence for a one electron oxidation of p-phenylenediamines with ozone is also presented. This evidence is not conclusive. A correlation of the HOMO energies of N-substituted p-phenylenediamines with the reactivities of p-phenylenediamines towards ozone is attempted. Although a rather broad correlation is noted, an exact correlation is not found. On the basis of these results, it is felt that the reaction between phenylenediamines and ozone may be free radical in nature. But the general type of substitution on the phenylenediamine alters the products that are formed and possibly changes the rate-determining step of the reaction.
Technological Characteristics of Polyisoprenes with Different 1,4-cis Contents
Authors: Bruzzone M. , Corradini G. , Amato F.
Published: February 2011

This work deals with the physical properties of polyisoprenes with different cis content. Small changes in the structural purity of these polymers affect strongly their crystallizability and, consequently, influence cured elastomer properties. In order to evaluate quantitatively the influence of cis content on elastomer properties it is clearly not sufficient to compare properties of the natural and synthetic polymers, obtained with different catalytic systems, relying on the same curing recipe. The difference in vulcanization kinetics, molecular weight distributions, dispersion of the ingredients in the elastomer, and other minor variables can in fact overshadow the influence of cis content on properties of the final product. In order to achieve a clear distinction between the effect of cis content and the influence of other variables, we decided to compare physical properties of vulcanized products over a large range of crosslink densities, obtained with suitable changes of the curing system, allowing for three variables: sulfur, accelerator, and curing time. The accelerator is a sulfenamide, suitable both for natural and synthetic polyisoprenes. When all the results obtained with different curing systems and times are plotted on a single diagram, in terms of physical property against crosslink density (or modulus) substantial differences between different polyisoprenes are noticed. In the low crosslink density range properties are probably influenced by network flaws related to the elastomer molecular weight distribution. Beyond a certain limit of crosslink densities the effect of molecular weight distribution decreases as the network flaws, mainly due to the ends of the primary chains, become less and less important. In this crosslink density range, properties are related mainly to structural purity, which influences the crystallizability of the network chains and perhaps also the nature of the inter or intramolecular bonds formed by curing.
Radiation Induced Block or Graft Co-Polymers of Methyl Methacrylate with Isobutylene and Ethylene Propylene Rubbers
Authors: Pellon Joseph , Valan Kent J.
Published: February 2011

Attempts to prepare block or graft copolymers of isobutylene rubbers by radiation in the presence of methyl methacrylate gave low yields of copolymer with only 20 per cent of the rubber entering into the copolymer. The remainder of the rubber is degraded during the process. A single experiment describes the graft copolymerization of methyl methacrylate with ethylene propylene rubber in which 45 per cent of the rubber is copolymerized.
Degradation of Elastomers 4. Continuous Chemical Stress Relaxation of Natural Rubber Vulcanizates
Authors: Scheele Walter , Hillmer Karl-Heinz
Published: February 2011

Temperature dependence of the chemical stress relaxation in oxygen of a series of natural rubber vulcanizates, extracted and unextracted, with the same pretreatment was investigated. Decrease of tension to about 50 per cent of the original was followed, using the half lives as a measure of velocity. The following observations were made: 1) Relaxation velocity of unextracted thiuram vulcanizates increases with increasing sulfur content; the activation energy decreases. 2) The relaxation velocities of extracted thiuram vulcanizates are independent of sulfur content of the mixtures and significantly greater than those of unextracted samples. This is caused by the loss of dithiocarbamate on extraction. 3) Other natural rubber vulcanizates (dithiocarbamate, MBT and peroxide vulcanizates) give the same relaxation half lives after extraction. 4) The relative importance of random scission and crosslink scission was discussed.
Molecular View of Fracture in Amorphous Elastomers
Authors: Halpin J. C.
Published: February 2011

Perhaps the first question coming to mind is, what does the strength of a body represent? It is natural to suppose that fracture of an elastomer involves rupture of molecular chains which interesect the fracture plane. The theoretical strength should then be the sum of the strengths of individual ruptured bonds. Computation of strength in this way involves estimating the number and nature of bonds to be broken and forces required. Bueche performed such a computation and his estimated strength was at least a thousand times as great as that usually observed. In essence, analysis suggests that rupture occurs before the average chain bond in a vulcanizate is anywhere near the maximum breaking load. A preliminary explanation for this effect was advanced by Bueche, who suggested that a specimen ruptures by a process which concentrates high local stress upon a few chains in the fracture cross section. The chains rupture under this high stress throwing stress on neighboring chains. The increased load on neighboring chains causes additional chain ruptures which in turn contribute to a repetitious process which is propagated at an increasing rate until rupture of the specimen occurs.
Survey of the Theory of Rubberlike Elasticity
Authors: Krigbaum W. R. , Roe Ryong-Joon
Published: February 2011

The problem of rubberlike elasticity has been under intensive investigation, both theoretically and experimentally, for over thirty years. The resulting literature has become so voluminous that adequate coverage, even at the survey level, demands the selection of a small segment of the overall problem. For this reason our review will deal exclusively with the reversible aspects of rubberlike elasticity. Furthermore, the historical development of the theoretical treatment is admirably covered in a number of recent books. Hence, our objective will be to present a fairly detailed and comprehensive survey of the current state of theory of rubberlike elasticity, and to compare this with pertinent experimental observations.
Reinforced Thermoplastics
Authors: Haward R. N. , Mann J.
Published: February 2011

If a thermoplastic is placed under load, as in a normal tensile tester, the characteristic stress strain curve is of the form given in Figure 1. Whether the material is described as tough or brittle depends on the relation between the fracture stress and the yield stress. In many cases the apparently irreversible deformation which takes place on yield is described as flow, but this is seldom, if ever, the case. The extension which takes place consists in the orientation of the tangled molecular network in a manner similar to that which occurs during extension of rubber, but with characteristic hard thermoplastics the deformation is opposed by viscous forces so that most of the energy applied is irrecoverably lost as heat. Deformations of this type are largely reversed on heating—as shown in the case of high-density polyethylene in Figure 2. In order to modify a brittle polymer to make it tougher we need to alter the relation between deformation and fracture so that, at least under short time tests, deformation takes place before fracture so that enough energy can be absorbed to cope with shock conditions. For this purpose the yield stress must be lower than the fracture stress. Naturally the ideal way of achieving this would be to raise the fracture stress, as any reduction in yield stress will normally lead to a softer and less rigid product. The extent and limitations of this problem will now be examined in relation to the styrene plastics.