bluuebunny/crossref_metadata_2025 huggingface dataset:
The Nature and Action of Tackifier Resins
Authors: Aubrey Derek W.
Published: February 2011

A good tackifler resin appears to require a combination of high T9 at low molecular weight, and for this a highly condensed alicyclic structure appears to be desirable. Except for the rosin derivatives, the chemical structures of common tackifier resins are not fully resolved and some more definitive studies are needed. In addition, a high degree of miscibility of resin with rubber is important. Although some common tackifier resins cause phase separation over certain concentration ranges when blended with rubbers, this is probably exceptional, and it seems likely that most adhesive compositions are single phase in this respect. Where phase separation does occur, there is a need for fuller elucidation of the phase morphology and composition of the phases. The widely-held view that a two-phase morphology is necessary for the development of high tack appears to be incorrect. The few studies which have been carried out so far indicate that the surface energy of a rubber is modified only slightly by the incorporation of a tackifier resin. It is unlikely that this surface energy change can seriously affect the tack value as measured by the common form of probe test. The viscoelastic behavior of rubber-resin blends provides an adequate explanation for the phenomenon of tack. Over long response times, the low modulus (high compliance) of the blend compared with that of the rubber alone allows a high degree of intermolecular contact to be achieved during the bonding stage of a tack test. Over short response times, corresponding to the de-bonding stage of the test, the blend shows transition zone response with high energy dissipation and consequent high separation force. For further progress in our understanding, however, there is a need to distinguish clearly between the bonding and debonding effects of the resin and also for a more definitive study of the factors controlling the bonding stage.
Interfaces between Natural Rubber and Metals. Part I. A Study of the Interaction between Squalene and Steel Using XPS and SIMS
Authors: van Ooij W. J. , Rangarajan V.
Published: February 2011

1. XPS and SIMS in conjunction with a suitably chosen liquid model system form a powerful and highly complementary set of analytical tools for the study of the interaction between steel surfaces and unsaturated molecules; 2. We have presented evidence for an oxidative mechanism for the formation of a chemical bond between freshly abraded steel surfaces and squalene at 160°C. Such a bond can only be formed by unsaturated molecules on clean steel surfaces which exhibit a catalytic activity for the oxidation reaction. 3. This squalene approach appears to be very useful for the study and optimization of the bonding of 1,4-dienes to steel directly. The effects of the individual compounding ingredients on the rate of bond formation might conveniently be studied in this way in order to optimize compound formulations and surface preparations. The usefulness and limitations of a liquid model system such as squalene for the study of various aspects of natural rubber chemistry have been documented.
Effect of Oriented Platy Filler on the Fracture Mechanism of Elastomers
Authors: Eldred Roger J.
Published: February 2011

Through an investigation of tension fatigue failure modes, oriented platy fillers have been found to provide elastomers with a unique failure mechanism which allows the elastomer to resist destructive crack growth. Samples of chlorosulfonated polyethylene containing talc as a reinforcing agent failed in an interlaminar shear mode, parallel to the applied stress. This fracture pattern resulted from diversion of the classical Griffith-type crack which would normally proceed perpendicularly to the stress. The theoretical criterion for diversion requires the adhesive fracture energy, Gad, in the plane of the diverted crack to be about one-tenth of the cohesive fracture energy, Gco, in the plane of the Griffith crack. Measured values of the fracture energies determined by 180° peel (Gad) tests and trouser tear (Gco) tests showed that the talc-filled elastomer satisfied the criterion. However, the criterion was met, not by the introduction of planes of weakness in the lateral direction (thereby reducing Gad), but by a more than five-fold increase in the cohesive fracture energy over that measured for a control containing carbon black as reinforcing agent. Examination of the fractured surfaces by scanning electron microscopy showed that the increase in Gco was due to the many energy absorbing fracture paths down which the tear was diverted as it advanced through the elastomer. Besides exhibiting high tear energies, the elastomer was found to be highly insensitive to stress risers such as razor cuts. The combination of tear resistance and insensitivity to flaws leads to a prediction that elastomers containing oriented platy fillers will exhibit increased strength and durability. This first practical system affording crack diversion has special application to hoses and topologically similar shapes such as boots and sleeves.
Elastic Behavior of a Rubber Layer Bonded between Two Rigid Spheres
Authors: Gent A. N. , Hwang Y-C.
Published: February 2011

Finite-element methods (FEM) have been employed to calculate the stresses set up in a thin rubber layer, bonded between two rigid spheres, when small tensile or compressive deflections are imposed. Values of stiffness have been calculated for various spacings of the spheres, i.e., for various thicknesses of the rubber layer. They are in good agreement with earlier experimental measurements of compression stiffness and with the predictions of an approximate theoretical treatment. However, they are strongly affected by small departures of the rubber layer from complete incompressibility. The highest dilatant stress (in tension) is found to be set up on the central axis, near the bonded surfaces, where internal failure has been observed to occur in similar bonded layers. For moderately thick layers, the axial tensile stress in the center of the layer is substantially higher than the lateral stresses. This feature suggests that the initiation of failure in this location may not obey the same criterion as for an isotropic stress field, and that a crack, once formed here, will propagate as a tear across the axis of symmetry.
Tandem Mass Spectrometry (MS/MS) for Analysis of Additives in a Rubber Vulcanizate
Authors: Lattimer Robert P.
Published: February 2011

MS/MS appears to have considerable potential as a means for improved direct mass spectral characterization of organic additives in rubber compounds. In this report, we have shown how daughter-ion, parent-ion, and neutral-loss scans can improve the specificity for identification of organic additive components in rubber vulcanizates. The use of MS/MS to reduce the “chemical noise” in the direct analysis of complex mixtures is demonstrated. MS/MS should not be viewed as a technique that will replace other mass spectral methods in rubber applications. For example, desorption ionization methods, GC/MS, LC/MS and high-resolution mass measurements will still have important roles. MS/MS can serve as a supplemental tool for direct mixture analysis that is both rapid and powerful.
Interactions in a Miscible Polymer Blend
Authors: Roland C. M.
Published: February 2011

Characterization of blends of atactic poly(vinylethylene) with cis-1,4-poly-isoprene provides unique insights into the factors governing the properties of polymer mixtures. While studies of polymer blends have heretofore focussed on systems in which chemical reaction between the components enabled miscibility, any properties arising from the blending of PVE with PIP are not a result of greater cohesive energy density or densification. It is also interesting to consider the different crosslinking reactivity of the main-chain unsaturation in PIP versus that of the pendant vinyl group of PVE, which can give rise to interesting network properties. These will be the subject of a subsequent communication. While it is clear that the Flory interaction parameter for these blends must be non-negative, direct determination of its magnitude is best accomplished through measurement of small-angle neutron scattering. Currently deuterated polybutadienes of varying microstructure are being synthesized to enable such experiments to be carried out, and thereby allow the effect of vinyl level on the miscibility and phase behavior to be quantified.
Two-Dimensional Simulation of Injection Molding of Rubber Compounds
Authors: Isayev A. I. , Sobhanie M. , Deng J. S.
Published: February 2011

A general finite-element and finite-difference computer program has been developed for filling a thin cavity during injection molding of rubber compounds. This program employs a finite-element representation of the cavity in the planar dimensions and a finite-difference representation in the gapwise direction and is capable of accommodating any type of rheological equation and vulcanization model and incorporates both viscous heating and heat of vulcanization. A detailed example of an application of this program has been presented for filling a quarter-disk cavity with rubber compounds having a viscosity function obeying a modified Cross model and curing kinetics according to our recently proposed nonisothermal vulcanization model. The pressure, velocity, and temperature fields and melt-front locations have been calculated during the filling stage. The evaluation of state of cure distribution in the rubber moldings has been predicted in the postfilling stage. Comparisons of the prediction of the model with experimental data regarding pressure distribution, melt front position, and distribution of degree of cure in molded parts will be presented in the near future.
On the Role of Bound Rubber in Carbon-Black Reinforcement
Authors: Hamed Gary R. , Hatfield S.
Published: February 2011

A simple model of filler dispersion has been used to approximate particle spacing and the portion of rubber that is restricted as a result of particulate reinforcement. For N330 black, which has an equivalent radius of 20.2 nm, the critical concentration (in SBR of Mw=344 000) for coherent carbon-gel formation upon solvent immersion is 30 phr. At this level of filler, particle-particle spacing is approximately the diameter of the average SBR random coil. In the second part of the study, compositions in which portions of free rubber had been extracted were compared to conventional mixes at the same black concentrations. Conventional and extracted vulcanizates had similar cure behavior and resistance to deformation, however, extracted samples, with excessive bound rubber had reduced strengths, consistent with the presence of inherent flaws. Apparently, the extracted samples, which are torn apart upon mastication are unable to fully reknit together even after compression molding and vulcanization.
Synthesis and Properties of Ring Polybutadienes
Authors: Roovers Jacques
Published: February 2011

Much of polymer science is concerned with the linear joining of monomers and the properties of the new materials that emerge from this process. Of almost equal importance is the formation of tridimensional networks as the result of branching and crosslinking reactions. Ring formation has always been considered an unavoidable consequence of extensive crosslinking and, from the theoretical point, it is often neglected. Nevertheless, rings may contribute to the network properties by means of permanent entanglements. Indeed, rings alone can form special permanent networks without any chemical connectivity between the rings. The interest in flexible ring polymers has grown considerably in recent years with 32, 21, and 34 papers published in 1985, 1986, and 1987 respectively, dealing equally with experimental or theoretical aspects, although the theoretical and experimental papers do not always address the same problems. Most significant has been the work of Semlyen and his coworkers on ring poly(dimethylsiloxane) with molecular weights up to 30 000. This is not very much above the entanglement molecular weight of poly(dimethylsiloxane) (Me=12 000) but the low Tg and chemical inertness of the polymer made it ideally suited for ring-in-network entrapment studies.
Dilutability, a Potential Method to Analyze Polyurethane Elastomers
Authors: Oberth Adolf E.
Published: February 2011

Evidence presented in the foregoing study indicates a strong dependance of dilutability on the fraction of effective chains. There is some dependence on the level of crosslinking and, perhaps, on solvatibility of the polymeric network in question. Thus, it is possible that networks prepared from different prepolymers but with the same FEC might show some difference in dilutability. This is a difficult question to answer, as well-characterized prepolymers that could serve as a standard are not available. Dilutability should be particularly useful in quality-control type applications, where lot to lot differences of essentially the same prepolymer are assessed.
Detection of the Molecular Orientation in Nonvulcanized and Vulcanized Natural Rubber Compounds by the Low-Temperature X-Ray Method
Authors: Udagawa Y. , Ito M.
Published: February 2011

The low-temperature x-ray method can detect the orientation of NR molecules in both vulcanizate samples and nonvulcanizate samples of NR compounds. The presence of carbon black is important for causing the orientation. NR molecules orient in the direction of stretch-relaxation in the case of uniaxially fatigued vulcanizates or in the direction of shear in the case of nonvulcanizates prepared by various processing methods. The orientation of NR molecules in nonvulcanizates usually disappears when vulcanized, but a fairly large extent of molecular orientation remains, even after vulcanization, if there exist crosslinks in the oriented nonvulcanizate and the sample is prevented from shrinkage.
Determination of Crack-Growth Characteristics Using the Angled Testpiece
Authors: Muhr A. H. , Thomas A. G.
Published: February 2011

The anticipated advantages of the angled testpiece have been borne out with some important provisos. For sufficiently thin testpieces and high strain-energy release rates, it is possible to use the angled testpiece geometry and the machine described to determine the cyclic crack-growth characteristics without directly measuring the crack length or carrying out auxiliary elasticity measurements. The modifications to the theory for thicker test pieces (for which the buckling force is significant) should allow the technique to be applied, but one of the main advantages over the other techniques (the avoidance of auxiliary elastic measurements) is then lost. It has also become apparent that working at low strain-energy release rates with a machine of this sort calls for a very stable temperature. For this reason, the machine's capability to do such measurements has not yet been assessed. There is broad agreement between results from the various techniques for both time-dependent crack growth (angled, pure-shear, and trouser testpieces of SBR gum) and cyclic crack growth (angled, pure shear, and tensile testpieces of SBR gum and NR gum). However, although the fracture mechanics approach appears to be generally of considerable value in expressing the crack-growth properties of rubbers, giving results that are essentially independent of testpiece geometry, the latter feature is not invariably found. In the present work, significant discrepancies were found for NR gum at low values of T. Such discrepancies are being investigated further.
The Application of Swollen-State CW-1H NMR Spectroscopy to the Estimation of the Extent of Crosslinking in Vulcanized Polymer Blends
Authors: Loadman M. J. R. , Tinker A. J.
Published: February 2011

It has been shown that the measurement of peak broadening in the 1H NMR spectra of swollen gum vulcanizates can be used as an estimate of the degree of crosslinking in the vulcanizate. A technique for applying this measurement to the individual phases of a vulcanized blend has been developed and validated, first for a physical mixture of gum vulcanizates of NR and NBR and then for a vulcanized, gum blend of these two rubbers.
Mean-Field Calculations of the Dynamic Mechanical Properties of Heterogeneous Elastomer Blends
Authors: Mazich K. A. , Killgoar P. C. , Ingram J. A.
Published: February 2011

A method for calculating the dynamic mechanical properties of elastomer blends with co-continuous structures has been presented. The calculations are based on Kerner's packed-grain model for composite media. Comparisons of theoretical calculations with experimental data show that this model closely approximates the viscoelastic properties of blends with a co-continuous structure, at least in the glass-transition regions of the respective blend components. We have also shown that the storage modulus of co-continuous blends may be well-represented by a discrete-particle model. This result can be misleading, however, if the storage modulus alone is calculated and compared with experimental data. A comparison of viscoelastic data (log E′ and tan δ) with calculation clearly distinguishes the models and indicates that the packed-grain model is the correct representation of the structure of co-continuous blends. The agreement between theory and experiment reported above was principally found in the glass-transition regions of the respective components in the elastomer blend. We extended the comparison well into the rubbery region and found that the agreement between Kerner's mean-field theory and the experimental data was poor, particularly for the loss tangent. Different relaxation mechanisms (relaxations over greater periods of time) are available to the blend in the rubbery region of viscoelastic response, and these mechanisms are apparently not accounted for in the mean-field calculations.
Degradation Behaviors of Natural, Guayule, and Synthetic Isoprene Rubbers
Authors: Lin Sin-Shong
Published: February 2011

The degradation behavior of three isoprene based rubbers, NR, GR, and IR, under accelerated thermal and uv radiative environments have been investigated, and the extents of the degradation are compared. The oxidative degradation introduces oxygen atoms into the elastomer chains to form carbon—oxygen linkages via hydroxyl, ether, carbonyl, and carboxyl functional groups. Some polynuclear aromatic compounds are also found on the severely thermal-degraded surfaces. The secondary atomic constituents of the vulcanizates, such as nitrogen and sulfur, also oxidized during the aging. The formations of sulfate, sulfite, and zinc oxide are observed, but no direct oxidation of nitrogen to nitro or nitroso groups is detected. The atomic surface concentration of carbon decreases significantly (1/2 to 2/3), while that of oxygen increases substantially (2 to 3 times) during the oxidative aging test. The concentrations of minor elements, S and Zn, increase to a few percent more than two fold, but that of N increases only slightly. The surface enrichment of these elements is probably derived from segregation, migration, and oxidation. The element F is present with varying concentrations (less than 10%). The element is derived from remnants of the mold releasing agent containing fluorine. The flex cracking is observed on some severely thermally degraded specimens, but none is observed under the uv radiation. The formation of aromatic cyclic compounds from these linear chain elastomers is positively identified from the π−π* excitation peak in the XPS spectra. The thick, hardened laminated layers are the visual evidences of the formation. In short, NR is found to be slightly more stable than GR and IR in the present aging test. However, the thermal and uv radiation stabilities are strongly influenced by the rubber formulations. The presence of carbon black tends to nullify the differences existing between the degradation behavior of these vulcanizates. The antioxidant in the formulation is believed to be more effective than the characteristics and the origins of the gums in controlling the degradation behavior.
Alkyllithium-Initiated Polymerization of Trans-1,3,5-Hexatriene and Copolymerization with Styrene
Authors: Quirk Roderic P. , Bhatia Rajeev
Published: February 2011

The results reported herein established the complexity of the alkyllithium-initiated anionic homopolymerization, styrene copolymerization and styrene block copolymerization of 1,3,5-hexatriene in comparison to the controlled, predictable behavior observed with dienes. As a result of 1,6- and 1,2-enchainment, conjugated diene units are formed along the polymer backbone which can react with the living carbanionic centers to produce branched polymers as indicated in Equation (1). The general effect of solvent on microstructure, as indicated by infrared spectral analyses, is analogous to that of dienes in that 1,2-addition is increased in polar versus hydrocarbon solvents. The observation of branching reactions suggests that conjugated trienes may be useful as branching agents for copolymerizations or linking reactions analogous to the behavior of divinylbenzenes.
Bonding of Butyl and Nitrile Rubber by in situ Copolymer Formation
Authors: Chung M. H. , Hamed G. R.
Published: February 2011

We summarize and conclude the following: 1. The T-peel adhesion between uncrosslinked layers of IIR and NBR is rate and temperature dependent. Failure is interfacial at low rates; bulk cohesive tearing of the rubber occurs at intermediate rates, while interfacial failure returns at high rates. 2. Adhesion data could be reduced to mastercurves with experimentally determined shift factors which were in reasonable agreement with those calculated from the universal WLF equation with a Tg=−69°C. 3. At the lowest test rates, peel specimens containing in situ-formed copolymer had lower strength than those without the copolymer. This is probably due to the easy slippage of the segments of the low-molecular-weight copolymer in this regime. 4. At intermediate rates, in situ copolymer had no effect on peel strength; specimens failed by bulk cohesive tearing of rubber, whether or not the copolymer was present. 5. At high rates, peel specimens containing the copolymer exhibited improved strengths. The behavior is consistent with previous results on the autohesion of elastomers. Interdiffused chains of relatively low molecular weight are only capable of providing strong adhesion when the test speed is sufficiently rapid.
Long-Term Studies of Rubber Materials by Dynamic Mechanical Stress Relaxation
Authors: Björk Folke , Dickman Ola , Stenberg Bengt
Published: February 2011

The measurement of dynamic stress relaxation can reveal otherwise hidden weaknesses in rubber materials. The stress relaxation of the minimum of the restoring foce has the undesirable characteristic of being greater than the continuous stress relaxation. An increase in the amplitude of the deformation increases the decay in the minimum values of the varying retractive stress, and an increase in the prestrain makes it smaller. The normalized curve for continuous stress relaxation is found to lie somewhere between the normalized curves for the maximum and minimum of the retractive stress. The measurement of network density on specimens aged under continuous and dynamic stress relaxation indicate that the mechanism behind the degradation of the rubber network is the same in both cases. The relations between intermittent, continuous, and dynamic stress relaxation give a reasonable estimate of the dynamic stress relaxation from the other two when proper values of the coefficients are used. This relation says that results both of continuous and intermittent stress relaxation are important for predicting the life time of a dynamic seal. The relations offers an explanation of the differences between maximum and minimum values obtained during the measurement of dynamic stress relaxation and their relation to continuous stress relaxation. From these relations it can be concluded that a rubber intended for dynamic seals should contain neither reversion nor secondary crosslinking. Crosslink density should be high in order to overpower effects on crosslinking of factors in the surroundings.
Light Scattering from Gels
Authors: Patterson G. D.
Published: February 2011

The basic principles necessary to understand light scattering from gels are now well understood. The experimental techniques necessary to measure light scattering from gels are now highly developed. Yet there are many issues that need to be clarified in practice. The spatial correlations that exist in the polymer concentration in a gel need extensive further study. Gel samples with only intrinsic permanent concentration fluctuations need to be prepared. Defects due to dust, microsyneresis or other nonintrinsic inhomogeneities must be eliminated. Then careful measurements of R(q) should yield the necessary information about the microstructure of the gel. The value of the osmotic Poisson ratio is now well understood for gels at rest. Further work needs to be done on samples in partial states of swelling. More work appears promising in the study of critical concentration fluctuations in gels as well.
Thermoplastic Elastomers—Three Decades of Progress
Authors: Legge N. R.
Published: February 2011

In these three decades of progress, thermoplastic elastomers have risen in 1987 to a position of tenth in the order of commercial thermoplastic sales in the U.S.A., with a growth rate, 1986–1987, of 9.7%. It is very probable that the quantity shown for 1987 sales, 441 million pounds, is low, since it is well known that the largest producer of styrenic TPEs does not report offtake data. Much of the styrenic TPE goes to the adhesive industry, which also is very secretive in regard to materials consumption information. Thus, the 1986–1987 reported growth rate of 9.7% is on the low side. Another indicator of progress in the growth of TPEs has been illustrated by the number of product introductions from January 1986 to June 1987. During that period, TPEs led the major thermoplastics with the introduction of 270 new product types, and the nylons were a close second with 250. A third estimate of the explosive growth in TPEs may be seen in Table V which lists the number of manufacturers of TPEs in 1975, 1985, and 1987, increasing from 10 to 28 to 50. To summarize, the present thermoplastic elastomers, now high-volume commercial products, had roots in the chemistry and technology of polymers in the 1920's. Throughout the history of the “Roots” period one can detect precursor events from which several TPEs could have been foreseen. In each of the three decades of progress, major advances were made in the technology, physical properties, availability, and utilization of TPEs. The numbers of these increased in each succeeding period. Several paradigms appear in this review, for example: 1. The triblock styrene-diene A-B-A copolymers, morphology, and elastomeric character, in the first decade. 2. The copolyesters with (A−B)n morphology and greatly enhanced physical properties in the second decade. 3. The dynamically-vulcanized blends of EPDM and PP, followed in time by the concept of compatibilization to permit practical blends of NBR and PP in the third decade. Throughout these periods, growth was catalyzed by the favorable economics of manufacturing finished elastomeric products via low-cost thermoplastic processing techniques as compared with thermoset rubber processes. The reuse of scrap also provided a major incentive. In addition to these, the concept of component integration is now showing a path toward even more cost reduction incentives. New applicational areas continue to appear. One of these, blending relatively small amounts of TPEs with existing large volume thermoplastics, promises to provide extremely large offtakes of TPEs in the next decade. I am sure that the numbers of papers presented in symposia at meetings of the Rubber Division of the American Chemical Society confirm the continued explosive growth of TPEs we have seen in these past three decades.
Studies on Cure Synergism—Part I: Effect of Bis(Diisopropyl) Thiophosphoryl Disulfide and Thiazole-Based Accelerators in the Vulcanization of NR
Authors: Mandal Swapan Kumar , Datta Rabindra Nath , Basu Dipak Kumar
Published: February 2011

The effect of the synergistic combinations containing diisopropyl thiophosphoryl disulfide (DIPDIS) and a thiazole type of accelerator on the vulcanization of NR has been investigated. MBT, N-oxydiethylene-2-benzothiazole sulfenamide (OBTS), and N-oxydiethylene-2-benzothiazole disulfide (OBDS) represent the thiazole-based accelerators. It has been established from the hplc study that diisopropylthiophosphoryl benzothiazole disulfide (DIBDS) is formed in the synergistic systems along with other rubber additives during the early part of cure. In this exploratory study, it has been observed that DIBDS enhances the torque, modulus, crosslink density, and the tensile strength of the vulcanizates. Chemical characterization of the fine network structure of the NR vulcanizates indicates that synergistic combinations produce about 80% of the monosulfidic linkages. The resultant vulcanizates thus become stable to oxidative aging at 100°C for a considerable length of time. The vulcanizates obtained with DIBDS also produce the same results.
Differential Scanning Calorimetry Study of Natural Rubber and Epoxidized Natural Rubber and Their Binary Blends
Authors: Ng S. C. , Chee K. K.
Published: February 2011

In summary, the ENR exhibits a single Tg, whereas blending two different ENR samples of sufficiently high molecular weights would have no significant effects on their individual phase transition behavior in any event. This important finding implies that the epoxidation of NR basically leads to copolymers of random chemical structure and uniform composition, in line with the theoretical prediction of Scott. Moreover, the specific interactions presumably between the hydrogens of isoprene units and the oxirane oxygens of epoxidized isoprene moieties, are discovered to be strikingly weak, in the sense that the NR and ENR rubbers are positively immisicible.
Photocrosslinking of EPDM Elastomers. Photocrosslinkable Compositions
Authors: Hilborn J. , Rånaby B.
Published: February 2011

EFDM rubbers can be efficiently crosslinked using multifunctional monomers. Several factors must be taken into consideration: 1. Good solubility of the crosslinking agent in the rubber is essential. 2. It is favorable to have a mechanism of crosslinking which gives an alternating addition of rubber and crosslinking agent, as in the case of the stepwise radical thiol-ene system or alternating copolymerization of bis-maleimide with the olefinic rubber. These systems utilize each radical of the photoinitiator to form crosslinks, enhance the rate of crosslinking and prevent homopolymerization of the crosslinking agent. 3. A good match of the UV absorption of the photoinitiator, the UV transmission of the rest of the system, and the UV emission from the lamp is essential. 4. Since disproportionation of photoinitiator radicals in cage is an unwanted side reaction in the rigid rubber system, it is better to choose photoinitiators that mainly recombine to reform a photoactive molecule as before scission.
Effectiveness of New Alkyl-Aryl-p-Phenylenediamines Which Can Be Chemically Bound to Polymers—Model Study
Authors: Engels H-W. , Hammer H. , Brück D. , Redetzky W.
Published: February 2011

The new model compounds described above have all the structural characteristics of the most effective antioxidants, antiozonants, and antiflexcracking agents of the alkyl-aryl-p-phenylenediamine type. Whereas the SCH3 compounds chosen as migratable reference substances have patterns of effectiveness similar to that of 6PPD, attachment to the polymer largely immobilizes the SH compounds. The nonmigratable mercaptans afford less protection from surface degradation processes than the corresponding methylthioethers. The fact that the attached compounds afford almost no protection from ozone underlines the importance of diffusion as a prerequisite of an antiozonant effect. Although the bound molecules have a good antioxidant effect, their loss of effectiveness as a function of aging time is similar to that of the reference compound, which, being migratable, is subject to physical depletion. Under laboratory aging conditions that do not cause extraction, the concept of polymer attachment offers no prospect of proved long-term protection.
Postharvest Degradation of Guayule Rubber
Authors: Kstilai A. , Hamerstrand G. E.
Published: February 2011

Most guayule genotypes lose considerable amounts of rubber after harvest. The time that the harvested shrub stays in storage is an important factor controlling the amount of degradation. This study identifies genotypes which are more stable with respect to postharvest degradation of rubber quality and quantity. Genotypes carrying such an economically valuable trait provide for the development of high-rubber-yielding cultivars that prevent substantial losses between harvest and processing. Further studies are expected to throw light on the mechanisms of postharvest rubber degradation, in general, and on the genetic nature of this variation in particular. More genotypes must be exposed to more storage conditions in order to identify genotypes that may withstand the adverse effects of long storage. The observation that some genotypes continue to synthesize rubber after harvest, at least for a short period of time, is also encouraging. Such genotypes make it possible to handle the shrub between field and processing, within 1 to 2 weeks, with no rubber loss.
The Effect of Specimen Thickness on the Tearing Energy of a Gum Vulcanizate
Authors: Mazich Kenneth A. , Morman K. N. , Oblinger F. G. , Fan T. Y. , Killgoar P. C.
Published: February 2011

We have examined the effect of thickness on the critical tearing energy of a simple gum vulcanizate of SBR in pure shear. Laboratory experiments and finite-element calculations agree that the tearing energy that is measured with a pure-shear specimen increases with the thickness of the specimen. Laboratory measurements indicate that the deformation for crack growth in a pure-shear specimen increases with the thickness of the specimen. Finite-element calculations show that the energy available for release at a given deformation also increases with thickness in the range from t=1.4 mm to t=14 mm. Experiments show that the crtical tearing energy varies linearly with thickness in the range t=0.7 mm to t=2.7 mm. The effect of thickness on the tearing energy was also studied by calculating the J-integral at various points of the crack through the thickness of the pure-shear specimen. In general, the J-integral calculated at the surface of the specimen can be higher than the J-integral calculated at the center of the specimen for specimens that are sufficiently thick. The thickness effect measured in this work suggests that the “critical tearing energy” obtained from standard laboratory specimens may not be a true material property. For this reason, critical tearing energy that is measured on standard specimens may not be generally applied to predict failure in arbitrary elastomeric components.
Network Recovery from Uniaxial Extension: I. Elastic Equilibrium
Authors: Roland C. M.
Published: February 2011

The elastic response of rubber has a direct and controllable dependence on structure. The behavior of the various networks described herein is seen to be consonant with the well-established roles of spatial fluctuations of crosslinks and the inhibition of this Brownian motion by steric and entanglement constraints from neighboring chains. Although elasticity also governs to a large extent the nonequilibrium mechanical response, interpretation of the latter in terms of network structure is obscured by viscoelasticity. The junctions of real networks, having some freedom to reconfigure themselves away from strictly affine displacement, exhibit lower equilibrium stresses than predicted by the statistical model for affine chains. To the extent that this overestimation of the model and the compressive component of the retraction response mimic the effect of departures from equilibrium, some conformance of network behavior during retraction to Equation (1) may be realized. It is such a fortuitous circumstance that likely underlies the various proposals commending such a relationship. A measure of junction density is generally not obtainable from deformation experiments in which dissipative processes are active.
Network Recovery from Uniaxial Extension: II. The Origin of the Mullins Effect
Authors: Roland C. M.
Published: February 2011

The Mullins effect refers to the dissipation in crosslinked rubber of mechanical energy beyond that due to linear relaxation processes or irreversible structural changes. Physically, Mullins softening probably arises from adjustment of local imbalances in segment density and from contraction of the primitive path of network chain ends. The resulting more random chain configuration contributes to the relaxation of the stress, but at a sufficiently rapid rate that it is omitted from the experimentally measured linear relaxation spectrum. This omission is made apparent upon reversal of the applied strain. The mechanism underlying the hysteresis can be modeled empirically by the introduction of irreversibility into the damping function. Although the Mullins effect is a reflection of nonaffine network motion during deformation of rubber, it is independent of the extent to which the junctions of the network are affinely displaced at elastic equilibrium. Randomly crosslinked networks of widely varying structure exhibited comparable degrees of Mullins softening. Corroboration that the network chain ends are a major source of Mullins softening could be obtained by a similar study using end-linked networks. Such experiments on end-linked polytetrahydrofuran rubber are currently in progress.
The Influence of Interstitial Liquids on the Cohesive Strength of Carbon-Black Agglomerates
Authors: Horwatt Steven W. , Rwei Syang-Peng , Manas-Zloczower Ica , Feke Donald L.
Published: February 2011

The influence of interstitial liquids on the cohesive strength of carbon black has been investigated. Calculations of effective Hamaker coefficients for the wet materials predicted order of magnitude decreases from the Hamaker constants for the dry materials. Tensile-strength data showed slight increases over the Hamaker constants for the dry materials. It is clear that further research is needed to elucidate the effects of liquid content and wettability.
Coactivation of Delayed-Action Vulcanization. Part I. Polymeric, Nonmigratory Cure Coactivators for Accelerated Sulfur Vulcanization
Authors: Coran A. Y. , Ignatz-Hoover F. , Davis L. H.
Published: February 2011

Rubbery vinylpyridine-butadiene copolymers, containing 20–65% by weight of vinylpyridine monomer units, are effective coactivators of vulcanization for TBBS-accelerated sulfur-vulcanized SBR. In addition to emulsion SBR, the new co-activator has been evaluated in copositions of solution SBR, BR, NR, and various blends. The co-activator is active in all of the compositions which contain butadiene-derived synthetic rubber. This includes blends such as SBR/BR, solution-SBR/BR, SBR/NR, BR/NR, SBR/BR/NR, etc. There is little or no activity in which NR is the only polymer. The most efficacious copolymers contain between 30 and 60% 2-vinylpyridine. The incorporation of such a copolymer into an unvulcanized butadiene-derived rubber mix can give a substantial increase in the rate of crosslink formation with only a minimum loss of scorch resistance. Since the polymeric coactivators are very high in molecular weight, it can be at least tentatively concluded that they will not migrate from one component stock to another in a built-up multi-stock rubber article, either before or during vulcanization. Since the curing characteristics of a vinylpyridine-copolymer-containing TBBS-accelerated stock can be similar to those of TBBS-accelerated NR, it might be concluded that the new additives will solve some of the problems in balancing the cures of adjacent NR and SBR stocks in a multicomponent cured rubber article.
Effect of Gel and Nonrubber Constituents on the Extrusion Properties of Guayule Rubber
Authors: Montes Sergio A. , Ponce-Vélez M. A.
Published: February 2011

An experimental study on the effect of gel and nonrubber constituents on the extrusion behavior of guayule rubber (GR) has been carried out. At 140°C, gel increases the viscosity at low shear rates. When GR is properly stabilized, no peptizing effect is imparted by the resin. However, the resin acts as a moderate plasticizer as compared to paraffinic oil. At 120°C, the effect of gel is noticeable at high extrusion rates, increasing the viscosity, suggesting an increase in the tendency for the occurrence of flow-induced crystallization.
Rubber-Thermoplastic Compositions. Part VII. Chlorinated Polyethylene Rubber-Nylon Compositions
Authors: Coran A. Y. , Patel R.
Published: February 2011

Nylon resins and CPE rubber can be melt-blended to give compositions which have useful properties. If the rubber is cured by dynamic vulcanization, high strength oil resistant thermoplastic elastomers result. Cold milling of melt-mixed compositions results in nylon-reinforced rubber which can be mixed with curatives and press cured or statically vulcanized to give reinforced rubber compositions which resemble vulcanized rubber-short fiber composites. The reason for the high integrity of either the statically or the dynamically vulcanized composition appears to be that chemical bonds form between the rubber and plastic.
Effect of Crosslink Density on the Critical Flaw Size of a Simple Elastomer
Authors: Hamed G. R.
Published: February 2011

Tensile experiments have been carried out on a gum, amorphous EPDM elastomer. Samples with various crosslink densities and containing precuts of different lengths were tested. The energy, Wb, per unit volume, to fracture a specimen varied inversely with the length of the precut as predicted by simple tearing energy theory. Values of the inherent flaw size, Co, of the samples were determined by extrapolation of Wb to that value obtained when no intentional precut was present. Values of Co decreased with increasing crosslink density. This illustrates the decreasing ability of an elastomer network to blunt the crack tip of a stress-raising flaw as crosslinking is increased. It is suggested that this phenomenon is at least partly responsible for low values of Wb at high crosslink densities.
Stress Analysis of Adhesion Tests
Authors: Ridha R. A. , Crano R. N.
Published: February 2011

The ASTM D 2229-80 test specimen with a circular hole and a pad restriction was analyzed by 3D finite elements. Maximum shear stresses within the specimen are at the cord-rubber interface. Thus, failure is expected to initiate in the vicinity of the cord-rubber interface. This characteristic is similar to that seen in the MICA and TCAT tests; it represents an improvement over other pull-through tests including earlier ASTM tests. The computed tensile stresses normal to the cord indicate that friction forces will not be significant in this test (as they are in the TCAT test) and that debonding will propagate rapidly. In this regard, the D 2229-80 test is similar to the MICA test. Results of photoelastic analysis of the ASTM D 2229-80, MICA, and TCAT tests are in good agreement with the computed results. Maximum shear stress at the cord-rubber interface in the ASTM D 2229-80 test varies with the modulus of elasticity of the rubber. This variation becomes more pronounced with increasing rubber deformations (e.g., at higher loads and with softer rubber compounds). Thus, the cord pullout force will be dependent on the rubber modulus. From the above results we conclude the following: 1. The ASTM D 2229-80 test overcomes one of the serious drawbacks of earlier pull-through tests; failure is expected to initiate in the vicinity of the cord-rubber interface rather than elsewhere in the rubber. The new test's main deficiency is the dependence of the cord pullout force on the rubber modulus; in this regard, the MICA test offers a considerable advantage. 2. The ASTM D2229-80 test may be used in evaluating the adhesion of various steel cords embedded in a control rubber compound. 3. When changes are anticipated in the properties of the rubber compound, the MICA test offers a considerable advantage because the cord pullout force in the MICA test is independent of the rubber modulus.
Processability of Thermoplastic Olefin Elastomers: Vacuum Sheet Forming
Authors: Ho C. C. , Kontos E. G.
Published: February 2011

We have presented a method for evaluating the vacuum forming performance of polymers by comparing their forming window. The window is determined by (a) the sag of the heated sheet and (b) the material's ability to reproduce mold details. We also showed that the vacuum forming window of polypropylene homopolymer can be widened through modification with an olefinic thermoplastic elastomer. Compared to polypropylene homopolymer, TPE-modified polypropylene requires lower tensile stress to extend the sample at temperatures below the polypropylene melting point. In the melt stage. TPE modification enhances the melt viscosity of polypropylene. TPE modification increases significantly the low temperature impact of polypropylene. The flexural modulus becomes lower.
High Temperature Degradation of Silicone Rubber Compounds in a Silicone Oil Environment
Authors: Henry A. W.
Published: February 2011

We have found that silicone rubber compounds undergo a reduction in effective chain concentration when heated in an environment of high temperature silicone oil and that this rate of reduction is dependent upon the effective chain concentration to the 3/2 power. The 3/2 power relationship results from a slower than expected reaction rate due to the dilution influence of further reactions of already ineffective network chains. Based on our experimental findings, we believe that below 204°C, in a silicone oil environment, the predominate degradation of the siloxane network is due to the siloxane rearrangement reaction between silicone oil and the crosslinked network. The resultant bond breaking and reforming have an experimentally determined average activation energy of about 60 kJ/mole. In addition, based on our experiments, we have concluded that above 204°C, the higher activation energy of 105–125 kJ/mole observed for the two compounds containing little or no iron oxide is caused by a depolymerization mechanism which involves breaking of the network and formation of low molecular weight cyclic siloxane materials. With significantly more iron oxide present, as is evidenced in the RTV (Fe2O3 + Al2O3) material, the lower activation energy seen even above 204°C is believed to be caused by the continued catalysis of the siloxane rearrangment reaction by the iron oxide.
Bis(Triethoxysilylethyltolylene)-Polysulfide, a Scorch-Resistant Silane Coupling Agent for Mineral-Filled Elastomers
Authors: Pickwell R. J.
Published: February 2011

Bis(triethoxysilylethyltolylene)polysulfide offers greater scorch safety and reduced odor compared to mercaptosilanes, though its high molecular weight may require up to three times the mercaptosilane loading on a weight basis. This is highly dependent on the rubber formulation. The polysulfidedisilane can be made into stable dispersions on calcium silicate at concentrations up to 70 percent. Order-of-addition recommendations for the polysulfidedisilane are the same as those for mercaptosilanes. Different mix procedures should be checked for each formulation to ensure maximum silane effectiveness.
Structure and Biosynthesis Mechanism of Natural Cis-Polyisoprene from Goldenrod
Authors: Tanaka Yasuyuki , Sato Hisaya , Kageyu Akira
Published: February 2011

The chemical structure of naturally occurring cis-polyisoprene isolated from Solidago altissima was determined by 13C NMR spectroscopy. The alignment of the isoprene units in the polymer was estimated to be in the order: dimethylallyl terminal unit, three trans units, about 1000 to 2200 of internal cis units, and a cis allyl alcohol terminal unit. This demonstrates that the biosynthesis of cis-polyisoprene in this higher plant starts from trans,trans,trans-geranylgeranyl pyrophosphate.