bluuebunny/crossref_metadata_2025 huggingface dataset:
Difunctional Initiator Based on 1,3-Diisopropenylbenzene: 7. Mechanical and Thermal Properties of New Butadiene-Based Thermoplastic Elastomers
Authors: Yu Y. S. , Dubois Ph , Jérôme R. , Teyssié Ph
Published: February 2011

Mechanical and thermal properties of SBS thermoplastic elastomers, in which S stands for polystyrene and B for polybutadiene, can be remarkably improved through end-capping by syndiotactic poly(methyl methacrylate) sequences. Furthermore, the upper service temperature of these sPMMA teleblock copolymers can also be enhanced by blending with isotactic PMMA so that the rubbery PBD soft segments are connected to high-melting stereocomplexed hard segments. The factors which affect the mechanical and thermal properties of these novel thermoplastic elastomers have been extensively investigated.
Improvement of Low-Temperature Flexibility of Hydrogenated Nitrile—Butadiene Rubber
Authors: Kobatake Takushi , Kodama Kazumi , Hayashi Sachio , Yoshioka Akira
Published: February 2011

Improvement of low-temperature flexibility of Hydrogenated Nitrile Butadiene Rubber (HNBR) is a requirement for industrial applications. In this study, the relationship between low-temperature flexibility and polymer structure—such as higher order structure and monomer unit distribution along a polymer chain—was examined for HNBRs having various nitrile contents, using X-ray diffraction, nuclear magnetic resonance, computer simulation, and other methods. It was found that the length of tetramethylene sequence in a polymer chain is the key factor for improving the low-temperature flexibility. Tetramethylene sequences having five or more than five continuously linked tetramethylene units (TM≥5) facilitate crystallization upon stretching, which leads to a poor low-temperature flexibility. Consequently, it is deduced that the TM≥5 content must be as low as possible to improve the low-temperature flexibility. Two methods were tried to decrease the TM≥5 content: (1) copolymerization with third monomers; and (2) addition of mercaptans. It has been confirmed that branched HNBRs made by each of the two methods have lower Tgs and improved low-temperature flexibility.
Reflectometry—A New Method of Measuring Filler Dispersion
Authors: Gerspacher M. , Nikiel L. , Yang H. H. , O'Farrell C. P.
Published: February 2011

A new method of measuring carbon black dispersion in rubber is presented. This method is based on the measurement of the light reflectivity from the rubber sample using standard equipment used in the evaluation of the tint strength of carbon black (ASTM D3265 procedure). This method can be applied for both uncured and cured samples; however, the uncured sample measurement has significant advantages in the simplicity of the sample preparation and obtaining important filler dispersion information of the compound. It is shown that samples with better dispersed carbon black reflect less light as compared to samples with poor carbon black dispersion.
The Influence of Matrix Viscosity and Interfacial Properties on the Dispersion Kinetics of Carbon Black Agglomerates
Authors: Yamada H. , Manas-Zloczower I. , Feke D. L.
Published: February 2011

The dispersion of carbon black agglomerates suspended in polydimethyl siloxane (PDMS) or polybutadiene (PBD) liquids has been studied. Agglomerates comprised of either a low-structure or a high-structure carbon-black were subjected to simple shear flow. Two characteristic length scales that affect the dispersion process are identified. One length scale (Lp) is a measure of the ease with which fluid can flow through the porous agglomerate structure. The second (δ) is a measure of the depth to which processing fluid has soaked into the agglomerate. Values of Lp were found to be independent of types or viscosity of the polymer but were sensitive to the structure and packing density within agglomerates. The parameter δ, which varies with time of immersion in the processing fluid, was found to be smaller at comparable immersion times for matrix fluids with high viscosity. Shearing experiments showed two kinetic regimes (fast and very slow dispersion). For each type of matrix liquid, there exists a critical packing density; agglomerates having a packing density lower than this critical value exhibit only fast erosion, but those with packing density higher than this critical value show dispersion kinetics that starts as fast but shifts to slow after a certain period of shearing. The critical packing density was higher in PBD than in PDMS for agglomerates of the low-structure carbon-black but was not significantly different for agglomerates of the high-structure carbon-black. The critical packing density was found to shift to higher values if higher viscosity PDMS was used. The dispersion kinetic regime for the different polymers liquids could be correlated with the value of δ/Lp. If δ/Lp remained less than a critical ratio, only fast erosion was observed. Measurement of fragment size distributions support the notion that fragments are produced primarily from the periphery of the agglomerate in the slow kinetic regime, but can be larger (up to the size of δ) for the fast kinetic regime.
Reinforcement of Acrylonitrile—Butadiene Rubber by Silica Generated in situ
Authors: Tanahashi H. , Osanai S. , Shigekuni M. , Murakami K. , Ikeda Y. , Kohjiya S.
Published: February 2011

In situ silica reinforcement was applied to the acrylonitrile—butadiene rubber (NBR) vulcanizates. The amount of in situ silica introduced in the NBR vulcanizates was limited due to the high polarity of NBR. The presence of γ-mercaptopropyltrimethoxysilane (γ-MPS) in the NBR vulcanizate increased the conversion of TEOS in the sol-gel reaction and resulted in the higher amount of in situ silica, compared to the system without γ-MPS. The obtained silica was very fine and dispersed very homogeneously. In situ sol-gel reaction of TEOS in the NBR vulcanizates mixed with a conventional silica (VN-3) was also carried out. The reinforcement efficiency in this system increased with the increase of the amount of mechanically mixed conventional silica. Interestingly, the hysteresis loss decreased by the in situ filling of silica.
Crack Growth in Twisted Rubber Disks. Part I: Fracture Energy Calculations
Authors: Aboutorabi H. , Ebbott T. , Gent A. N. , Yeoh O. H.
Published: February 2011

When a thin bonded rubber disk is subjected to repeated torsional strains by rotating one end-piece with respect to the other, an initial circumferential crack grows inwards. This is the basis of a new test method for determining rates of growth of a fatigue crack in rubber. Values of torsional stiffness K for a partially-cracked disk are computed here by FEA and corresponding fracture energies G are deduced in terms of the shear modulus μ, radius r of the still-uncracked portion, and rotation angle ϕ, assuming that the rubber compound is linearly elastic. Values obtained for G are shown to vary with r2, and thus decrease rapidly as the crack grows and the radius of the uncracked portion decreases. They are approximately the same as values obtained when the cracked disk is assumed to be equivalent to a solid disk having a slightly larger radius than the actual uncracked portion. For thin disks the correction in radius for K is approximately 0.22h, and for G approximately 0.33h, where h is the disk thickness. Measurements of crack growth rates dc/dn for a wide range of values of fracture energy G are described in Part II, the next paper in this issue.
Ultrasonic Devulcanization of SBR Rubber: Experimentation and Modeling Based on Cavitation and Percolation Theories
Authors: Yushanov S. P. , Isayev A. I. , Kim S. H.
Published: February 2011

Continuous ultrasonic devulcanization of styrene-butadiene rubber (SBR) is considered. Experiments are performed under various processing conditions. Two recipes of SBR with different amounts of polysulfidic linkages are utilized. Gel fraction and crosslink density of devulcanized rubbers are measured and a unique relationship between them is established. This relationship is found to be in agreement with the 3D percolation theory. Die characteristics with and without imposition of ultrasonic waves are determined. A modification of acoustic cavitation and flow modeling of ultrasonic devulcanization of SBR is proposed using a concept of effective viscosity characterizing the flow of vulcanized particles before devulcanization combined with a shear rate, temperature, and gel fraction dependent viscosity of devulcanized rubber. Velocity, shear rate, pressure, and temperature field along with gel fraction, crosslink density, and number of bonds broken are simulated. Predicted data on gel fraction, crosslink density, amount of poly-, di- and monosulfidic bonds, and pressure are found to be in qualitative agreement with experimental data.
Application of Reproducing Kernel Particle Method to Large Deformation Contact Analysis of Elastomers
Authors: Chen Jiun-Shyan , Pan Chunhui , Wu Cheng-Tang
Published: February 2011

A meshless formulation based on the Reproducing Kernel Particle Method (RKPM) is applied to the large deformation and contact analysis of elastomeric components. In this approach, a kernel estimate that ensures the completeness requirement of a linear field is introduced to construct the global Reproducing Kernel shape functions of the displacements. The Galerkin approximation of the variational equation is formulated using the Reproducing Kernel shape functions, and the discretization can be performed without the need of a structured mesh. In this paper, a RKPM formulation for rubber materials including frictional contact conditions is presented. The contact constraint equations and essential boundary conditions are formulated in the nodal coordinate using a direct transformation method. The global Reproducing Kernel shape functions expressed in a material description are used in the total Lagrangian formulation of hyperelasticity. By the use of the smooth Reproducing Kernel shape functions, the method is effective in dealing with large deformation and contact conditions in elastomeric applications.
Stem Length and Stem Length Distribution of Chain-Folded Lamellar Crystal Grown from Dilute Solution of cis-1,4-Polyisoprene by Ozonolysis-GPC Measurements
Authors: Kawahara Seiichi , Inomata Yoriko , Kakubo Takashi , Tanaka Yasuyuki , Hatada Kouichi , Ute Kouichi , Miyatake Nobuo
Published: February 2011

Lamellar crystal of natural rubber (NR), deproteinized natural rubber (DPNR), transesterified natural rubber (NR-TE) and synthetic cis-1,4-polyisoprene was grown from 0.2% (w/v) hexane solution at the temperature between −50 and −70 °C. The stem length and stem length distribution of the lamellar crystals were investigated by gel permeation chromatography (GPC) and supereritical fluid chromatography (SFC) of the ozonolysis products. The stem length (L) was detected as the number of isoprene units of ozonolysis products. A broader stem length distribution ranging from 2-mer to 13-mer in the single traverse element was found for the solution-grown crystal, suggesting an irregular structure of the lamellar crystal of the polymer. The stem length distribution of NR was narrowed by transesterification due to the decomposition of branching of NR. The equilibrium melting temperature was determined to be −17.5 °C from a plot of melting temperature versus inverse stem length, for the solution-grown crystal of transesterified DPNR.
The Effect of Bis(3-Triethoxysilylpropyl) Tetrasulfide on Silica Reinforcement of Styrene-Butadiene Rubber
Authors: Hashim A. S. , Azahari B. , Ikeda Y. , Kohjiya S.
Published: February 2011

Bis(3-triethoxysilylpropyl)tetrasulfide (TESPT) was found to affect the curing characteristics and the physical properties of styrene-butadiene (SBR) gum rubber and xin situ silica-filled SBR vulcanizates. Silica incorporated by the in situ sol-gel reaction of tetraethoxysilane with TESPT showed a much higher reinforcing efficiency than did conventional mechanical mixing and the in situ method without TESPT. The higher reinforcing efficiency is attributed to the formation of a silica-rubber network, which also changed the dynamic mechanical behavior of the SBR vulcanizates. Transmission electron microscopy observations showed in situ silica incorporation of very fine particles in comparison to the sol-gel process without TESPT.
FTIR Analysis of Silica-Filled Natural Rubber
Authors: Kralevich Mark L. , Koenig Jack L.
Published: February 2011

The interactions of silica filler in zinc activated, sulfur vulcanized cis-1,4-polyisoprene (IR) were characterized using Fourier Transform Infrared (FTIR)-Attenuated Total Reflectance (ATR) spectroscopy. The variables examined were increased levels of silica and quality of mixing of the batch. The increased silica level resulted in band broadening and a frequency shift to lower wavenumbers of the silica absorbance region (1250-1000 cm−1) due to a combination of physical and chemical adsorption of the rubber and parts of cure system on the silica surface. The peaks that appeared near 1040 and 1017 cm−1 were attributed to physical interactions between silica and the natural rubber. However some chemical bonding of the IR was present as a result of a free radical reaction that takes place during the milling process. Inefficient mixing had an affect comparable to adding less filler, according to the results above. That is, there was less “effective” filler in the poorly mixed samples because poor dispersion promotes agglomeration of the polar silica in the nonpolar hydrocarbon rubber.
Black and White Fillers and Tire Compound
Authors: Donnet Jean-Baptiste
Published: February 2011

The present stand on the understanding of the role of fillers in elastomer compounds shall be reviewed and discussed. Unpublished results of our laboratory are also introduced in this review paper, particularly concerning silica observed by atomic force microscopy and scanning probe microscopy and recent transmission electron microscopy results concerning furnace carbon black and its inception and growth.
Structurally Realistic Modeling of Elastomers
Authors: Burkhart Craig W.
Published: February 2011

This review covers the methods by which elastomers have been microscopically modeled and simulated since the mid-1960s. Of particular importance was the rotational isomeric state (RIS) methods of the Flory school in the 60s. His group was the first to attempt to use such statistical mechanical methods to determine observables such as the characteristic ratio, Cn. The early success of Flory's efforts were expanded in the early 1970s by members of his group in what appears to be the first true attempt to use molecular mechanics optimization techniques to obtain RIS statistical weighting matrices. Following these two pioneering breakthroughs, molecular mechanics techniques were refined and extended. Not much later, it became possible to perform temporal molecular dynamics simulations on flexible molecular liquids. Simulations of elastomer systems lagged somewhat; both single chains and elastomer amorphous liquids were not studied until the beginning of this decade. With the improvement in molecular forcefields and computational horsepower, it has become possible to perform simulations on relatively large-scale polymer systems. We track these developments, which include equation-of-state simulations and the simulation of oxygen diffusion in elastomers.
Contact-Mechanics-Based Studies of Adhesion between Polymers
Authors: Mangipudi V. S. , Tirrell M.
Published: February 2011

Contact mechanics deals with the deformation of solid bodies in contact. In recent years, significant advances have been made both in the theoretical and experimental areas of contact mechanics, especially in the area of soft solids, in relating the contact deformation to interfacial adhesion. On the theoretical front, new theories of contact mechanics have been developed to relate the interfacial force induced deformation to the thermodynamic work of adhesion both for elastic and viscoelastic solids. On the experimental front, several new techniques have been developed to measure the interfacial forces and the interfacial-force-induced deformations. These techniques have been used, with the aid of the theories of contact mechanics, to measure directly the surface and interfacial energies of a variety of polymers and other model surfaces. These experimental and theoretical developments have also been exploited to measure quantitatively the effect of interfacial chain diffusion on the adhesion of polymer interfaces.

We summarize the recent developments in the theories of contact mechanics, and their applications in the design and interpretation of experimental measurement of molecular level adhesion between elastomers, glassy and viscoelastic polymers. We also review the experimental and theoretical developments related to the role of chain diffusion on interfacial adhesion. Finally, we identify some potential new applications of contact-mechanics-based techniques in such emerging area of adhesion science as molecular level studies of adhesion of viscoelastic materials and biomaterials.
Specialty Elastomers for Automotive Applications
Authors: Hashimoto Kinro , Maeda Akio , Hosoya Kiyoshi , Todani Yoshihiro
Published: February 2011

Various elastomers are used for automotive components because of their physical properties, chemical properties, durability, etc. There are belts to transmit energy, seals to support radial or reciprocating parts, gaskets and O-rings to seal in oils and fuels, hoses to deliver liquid and gases and diaphragms to control them. Figure 1 shows one example of elastomers for gasoline engines reported by Akema and Yoshida. Rubbers are soft polymeric materials having generally 100 MPa of Young's modulus (shown in Figure 2) for the normally applied temperature range. They are widely used for the buffer position between high modulus materials such as metals, plastics, glasses, etc., in energy transmission, liquid delivery, and energy isolation positions. The materials are selected for their resistance to fuels, oils, and heat; as well as their cold flexibility and sealing ability. However, different materials are sometimes selected for the same device depending upon such factors as applied temperature, class of fuels and oils, and the engineering design of the car. The many problems confronting the automotive industry these days are shown in Table A-I. They are roughly classified into two areas: environmental and safety. Elastomers for automotive applications have been changing to solve many kinds of problems. Table A-II shows the trends of rubber materials being used for automotive parts. High performance rubbers, which have heat resistance, long life, low permeability, good abrasion resistance, and so on, are all being adopted.
Effect of Polymer-Filler and Filler-Filler Interactions on Dynamic Properties of Filled Vulcanizates
Authors: Wang Meng-Jiao
Published: February 2011

In the past few years, an enormous amount of work has been reported on the progress in the application of conventional fillers and the development of new products to improve the reinforcement of rubber, dynamic properties in particular. While all agree that the filler as one of the main components of the filled-rubber composite, has a very important role in improving the dynamic performances of the rubber products; many new ideas, theories, practices, phenomena, and observations about how and especially why the filler alters the dynamic stress-strain response have been presented. This, of course, suggests that not only is the real world of the filled rubber complex and sophisticated but also multiple mechanisms may be involved. However, it must be admitted that the possibility exists for explaining the effect of all fillers on rubber properties ultimately in similar and relatively nonspecific terms, i.e., the phenomenon related to all filler parameters should follow a general rule or principle. It is the author's belief that, with regard to the effect of filler on the dynamic properties of a given polymer and cure system, filler networking, both its architecture and strength, is the main (although not only) parameter to govern the behavior of the filled rubber. From the thermodynamic and kinetic points of view, filler network formation is especially related to filler-filler, polymer-filler, as well as polymer-polymer interactions. As mentioned in the introduction, this paper is an attempt to review the effect of filler characteristics on dynamic properties in connection with processing conditions and additives. Since the polymer-filler interaction is not only affected by filler, what is also important are the characteristics of polymers such as chemical composition, chain microstructure including molecular weight and molecular weight distribution, configuration, stereoregularity, monomer unit distribution and sequence, and their functionality. All of these have a substantial influence on the physical and/or chemical interactions with other additives and filler surfaces, as well as on the interaction between polymer molecules themselves, which would impact not only the filler network formation but also the viscoelastic response of the polymer matrix. In addition, the polymer network structure which was formed during vulcanization and characterized by crosslink density, crosslink structure, and chain modification, is equally important in contributing to the overall dynamic properties of the filled rubber. In this paper, only some of these aspects have been discussed relative to their effect on filler network formation. Further discussion of these parameters is beyond the subject of the present paper. However, some guidance to these important topics is given in a handbook that was recently published.
Crystallization and Strength of Natural Rubber and Synthetic cis-1,4-Polyisoprene
Authors: Gent A. N. , Kawahara S. , Zhao J.
Published: February 2011

Crosslinked samples of natural rubber (NR, SMR-5L) and a synthetic cis-1,4-polyisoprene (IR, Natsyn 2200, The Goodyear Tire & Rubber Company) were crystallized at −25 °C at various strains, and then slowly melted. Crystallization was somewhat slower for IR under all conditions, taking two to three times as long as for NR crosslinked to the same degree. The melting temperature was also a few degrees lower. These effects are consistent with the presence of a small fraction of non-cis material in the IR molecule. Measurements of tensile strength and tear strength were made over a wide temperature range. The results were closely similar for the two materials at low rates of stretching although the abrupt fall in tensile strength associated with a failure to crystallize occurred at slightly lower temperatures for IR. Tear strength at low speeds showed a steady fall on raising the temperature, with little difference between the two materials. However, at high rates of strain the IR materials appeared to have significantly lower tensile strength, and at high tear speeds the tear strength was markedly lower, about one-half of that for NR. Thus, the minor structural irregularities in IR appear to be of little consequence at low rates of strain, but cause significant weakening at high rates, when the lower rate of crystallization is inferred to become a critical factor.
Homogeneous Zinc(II) Catalysis in Accelerated Vulcanization I. Reaction-Stage Modeling and Cross-Link Formation
Authors: Nieuwenhuizen Peter J. , Timal Sandjai , van Veen Jeroen M. , Haasnoot Jaap G. , Reedijk Jan
Published: February 2011

This paper reports a novel approach for the study of the mechanism of accelerated vulcanization, namely, Reaction-Stage Modeling (RSM). By carefully studying the reactivity of relevant model compounds under selected conditions, detailed knowledge about a particular reaction stage of vulcanization can be obtained. Background, experimental details and synthesis of model compounds are described. An RSM study after cross-link formation in the thiuram- and dithiocarbamate type vulcanization has been performed, and the role of zinc compounds herein was investigated. In contrast to earlier studies, it has appeared that at 140 °C, cross-links form from cross-link precursors solely via disproportionation. Allylic substitution was not observed. Zinc compounds act as catalysts for disproportionation, but especially ZDMC can be regarded as an efficient, soluble molecular turntable for sulfur atoms.
High Flow TPO Compounds Containing Branched EPDM Modifiers
Authors: Dharmarajan N. , Kaufman L. G.
Published: February 2011

Polymer blends of ethylene-propylene elastomers and polypropylene plastics, referred to as thermoplastic olefins, are finding increasing use in automotive applications. The combination of attractive mechanical properties, low raw material cost and recyclability make these materials ideal substitutes for expensive engineering thermoplastics (polycarbonate/polybutylene terephthalate alloys) and nonrecyclable polyurethane systems. The primary application is in automotive bumper fascia. This paper describes the addition of long chain branched ethylene-propylene elastomers in thermoplastic olefin compounds containing a high flow polypropylene resin matrix. In such compounds, the modifier molecular architecture plays an important role in impact toughening. The results clearly indicate that linear modifiers such as traditional ethylene-propylene copolymers are ineffective in impact toughening, while long chain branched polymers provide enhanced impact resistance with a ductile failure mode in high flow polypropylenes.
Characterization and Physical Properties of New Isobutylene-Based Graft Copolymers
Authors: Tse Mun F. , Dias A. J. , Wang H-C.
Published: February 2011

A number of isobutylene-based graft copolymers with different compositions are compared to several commercial thermoplastic block copolymers, such as SIS, SBS and SEBS, in terms of morphology and viscoelasticity. The backbone of these graft copolymers is a terpolymer (BIMS) of isobutylene, p-methylstyrene and p-bromomethylstyrene, and the side chains are either polystyrene or poly(2,6-dimethyl-1,4-phenylene ether). Graft copolymer synthesis, statistics of graft formation, and stress-strain properties are described. Overall, these graft copolymers exhibit unique shear dependent viscosity effects, such as rapid thickening (quick setting) at low shear and lower viscosity (better processability) at high shear, compared to linear triblock copolymers. The rheological behavior of these graft copolymers could be a key advantage for high-shear calendering, extrusion, hot-melt spraying, and injection molding.
Tensile and Tear Behavior of Anisotropic Double Networks of a Black-Filled Natural Rubber Vulcanizate
Authors: Hamed G. R. , Huang M. Y.
Published: February 2011

Double networks of a black-filled natural rubber composition have been prepared by partially curing a sheet, stretching it, and then completing cure. Upon release, a double network retracts to a residual extension ratio, αr. Samples cut perpendicular to the stretch direction have stress—strain responses like the isotropic single network, while parallel samples have enhanced stiffness and tensile strength, and reduced extensibility. Tensile strength is rather weakly dependent on αr. Tear strengths of the double networks, determined using edge-cut strip specimens, exceed that of the single network for low αr. However, when αr is high, double networks have very low tear strengths. Consistent with previous studies, high tear strengths are associated with extensive longitudinal cracking.
Fatigue of Cord—Rubber Composites: II. Strain-Based Failure Criteria
Authors: Lee B. L. , Ku B. H. , Liu D. S. , Hippo P. K.
Published: February 2011

Fatigue failure mechanisms under low-frequency loading and their dependence on the strain properties were assessed for the rubber matrix composite of bias aircraft tire carcass reinforced by nylon cords as well as two model rubber composites reinforced by steel wire cables. Under cyclic tension at constant stress amplitude, these angle-plied, cord—rubber composite laminates exhibited localized damage in the form of cord—matrix debonding, matrix cracking, and delamination. The process of fatigue damage accumulation in the cord—rubber composite laminate was accompanied by a steady increase of cyclic strain (dynamic creep) and moderate temperature changes. The fatigue life was found to be linearly proportional to the inverse of the dynamic creep rate, i.e., the time required to increase cyclic strain by a unit amount. Regardless of the associated level of stress amplitude or fatigue life, the gross failure under low-frequency loading occurred when the total strain accumulation, i.e., cumulative creep strain, reached the static failure strain. The use of higher stress amplitude resulted in a decrease of fatigue life by simply shortening the time to reach the critical level of strain for gross failure. This observation indicates that the damage initiation and eventual structural failure of angle-plied, cord—rubber composite laminates are “ strain-controlled” processes. These critical strain properties appear to be controlled by the process of interfacial failure between the cord and matrix. Under static tension, the strain levels for cord—matrix debonding and gross failure of composite laminates showed no significant dependence on the level of carbon black loading of the matrix compound, despite the fact that carbon black loading strongly affected the modulus, strength and strain properties of the matrix. Also the number of debonding sites around the cut ends of cords increased at almost the same rate as the static strain increased regardless of the variation of matrix properties.
Poly(Dimethylsiloxane) Elastomers from Aqueous Emulsions: I. Preparations, and Characterization of the Curing and Aging Processes
Authors: McCarthy D. W. , Mark J. E.
Published: February 2011

Poly(dimethylsiloxane) (PDMS) elastomers were prepared from aqueous emulsions, in combination with one of several carefully chosen alkoxy silanes as a crosslinking agent, and either stannous-2-ethylhexanoate or dioctylitin dilaurate as catalyst. Conditions were generally chosen so as to generate silica/silicate phases within the elastomeric particles or at their surfaces, for improvements in mechanical properties of films to be cast from the emulsions. A series of such films was obtained from emulsions consisting of various combinations of these ingredients, and was then characterized, primarily by measurements of soluble fractions, and by extents of equilibrium swelling (as inverse measures of degrees of crosslinking). Of primary interest were experimental results clarifying the mechanisms for the PDMS crosslinking and for the subsequent “aging” of these materials. For all the systems, increase in concentration of crosslinking agent was found to decrease the fraction of soluble polymer present and to decrease the extent of swelling. The extent to which this occurred, however, varied significantly with the nature of the alkoxy silane crosslinking agent, its concentration, and the nature of the catalyst. The observed differences were used to propose mechanisms with regard to the role of the silanes in the formation of silica-like phases, as well as for the endlinking of the PDMS chains. The formation of this inorganic phase was found to have a large effect on the extraction and swelling results, particularly through its barrier effects on the surfaces of the emulsion particles. Aging effects were found to depend not only on the nature of the reactants and their concentrations in the preparation of the emulsions, but also on whether the aging had been carried out in the dry state or in the wet state. Two major effects were decreases in soluble polymer fraction and extent of swelling with time. These changes were associated with increases in degree of crosslinking, but opposing changes can arise from bond cleavages or reorganizations. It appeared that dry-state aging effects are greatest for systems with a low concentration of the precipitated inorganic phase, with the effects of aging being suppressed by the presence of the inorganic, filler-like phases.
Poly(Dimethylsiloxane) Elastomers from Aqueous Emulsions: III. Effects of Blended Silica Fillers and γ-Radiation-Induced Crosslinking
Authors: McCarthy D. W. , Mark J. E.
Published: February 2011

The morphologies of the reinforced elastomers prepared from emulsions were found to depend greatly on how the silica phases were introduced. More specifically, blending colloidal silica into the elastomer prior to film formation was found to yield a highly unusual elastomer in which the continuous phase was the silica and the dispersed phase the elastomer. This is the opposite of the usual case in which, for example, the silica is generated in-situ within the continuous elastomeric phase. These morphological differences were used to explain the different shapes of the stress—strain isotherms of these materials in simple elongation. Results on the crosslinking effects of gamma radiation demonstrated that this treatment represents another useful technique for optimizing the properties of these emulsion-derived materials.
Synthesis, Characterization, Physical and Processing Properties of New TPEs: Star-Blocks Comprising Multiple Polystyrene-b-Polyisobutylene Arms Radiating from a Polydivinylbenzene Core
Authors: Asthana Siddhartha , Majoros Istvan , Kennedy Joseph P.
Published: February 2011

The synthesis of the title star-block thermoplastic elastomers (TPEs) was accomplished by linking living polystyrene-b-polyisobutylene (PSt-b-PIB⊕) diblocks with divinylbenzene. Linking was fast and high yields of star-block polymers were obtained. The products were characterized by a variety of techniques. The physical and processing properties of the star-blocks have been investigated and they were found to exhibit superior tensile properties and lower melt viscosities to those of similar linear triblock copolymers.
Crosslink Density Distributions in NR/BR Blends: Effect of Cure Temperature and Time
Authors: Groves Sally A.
Published: February 2011

Crosslink distributions in gum natural rubber (NR)/cis-poly(butadiene) (BR) blend vulcanizates have been studied previously for cure states ranging from times shortly after the onset of cure to over-cure at 150 and 175 °C, respectively. The effects of lower cure temperatures on the crosslink distributions through the cure in NR/BR blend vulcanizates are considered here. The results of this and the previous work indicate that at temperatures above 140 °C the BR phase in gum NR/BR blends begins curing before and at a faster initial rate than the NR phase. Below 140 °C it is the NR phase which attains high crosslink densities first and maintains this increased level over BR until over-cure. The effect of cure temperature on crosslink distribution at maximum state of cure is also presented.
Particle Size Distribution of Carbon Blacks by Mercury Porosimetry
Authors: Leon y Leon Carlos A.
Published: February 2011

Carbon blacks are made up of near-spherical elementary colloidal particles chained together in small clusters known as aggregates. These unique morphological characteristics have complicated their particle size characterization by conventional techniques. In this study the potential of mercury porosimetry for the generation of primary particle size distributions (PPSD) of carbon blacks is investigated. Two literature theories (Mayer—Stowe and Smith—Stermer) were applied to derive PPSD curves from mercury porosimetry analyses of six ASTM standard reference blacks (series SRB A5-F5) covering a wide range of particle sizes. Good agreement was found between particle sizes expected from electron microscopy and those determined by mercury porosimetry. The advantages of the latter technique for the routine evaluation of continuous PPSD curves are discussed.
Friction of a Rubber Wedge Sliding on Glass
Authors: Extrand C. W. , Gent A. N. , Kaang S. Y.
Published: February 2011

The contact width, and hence contact area, for an elastic wedge pressed against a rigid flat surface appears to be proportional to the applied load per unit length. For a particular rubber sample, the reciprocal of the constant of proportionality, i.e., the mean normal pressure, was 130 kPa, i.e., about 7% of the tensile modulus E of the material. It was also independent of sliding speed over the range examined. Thus, a sharp wedge gave a relatively high loading pressure, independent of the applied load. As a result, the coefficient of friction was also independent of applied load over a wide range. The coefficient of friction was measured for a wedge of an unfilled natural rubber vulcanizate over wide ranges of sliding speed (50 µm/s to 100 mm/s) and test temperature (3°C to 63°C). It was found to increase with sliding speed and decrease with temperature over these ranges. The results at different temperatures were superposable using the WLF rate-temperature equivalence to create a master curve of friction vs. reduced sliding speed, rising from a value of about 1.5 at high temperatures and low speeds to about 5 at low temperatures and high speeds. Chlorination of a thin surface region reduced the coefficient of friction and its dependence on speed and temperature. It then became similar to that typically found for thermoplastics, 0.4 to 0.7. The geometry of sliding a flexible strip against a rigid curved surface caused high values of the apparent coefficient of friction to be obtained at relatively small departures from normal loading. In an extreme case, frictional seizure was observed when a high-friction sample contacted the glass surface at an angle of about 15° to the direction of loading. The apparent coefficient of friction then became indefinitely large. This same phenomenon of abnormally large frictional effects would be expected to occur in the case of conventional windshield-wiper blades, sliding over curved glass windshields.
Solid-State Carbon-13 Studies of Vulcanized Elastomers. VIII. Accelerated Sulfur-Vulcanized Butyl Rubber at 75.5 MHz
Authors: Krejsa M. R. , Koenig J. L.
Published: February 2011

The chemical microstructural changes of an accelerated sulfur cure of IIR with TMTD/ ZnO/sulfur have been elucidated. No reaction occurs in the isobutylene units. Isomerization occurs at the initial stages of vulcanization and reaches an equilibrium at 30–35 min. Sulfurization initially results in III and VI and is polysulfidic in nature. At cure times greater than 75 min, desulfurization of III and VI occurs and mono- and poly-sulfidic sulfurization giving IV and VII occurs. The reaction rate of III and VI is much greater than that of IV and VII. Equilibrium swelling measurements have been shown to correlate with rheometry measurements for this system and have shown that polysulfldic III and VI correlate with an increase in properties, while mono- and poly-sulfldic IV and VII correlate with reversion. No migration of the double bond, saturation, internal cyclization, or sulfurization resulting in V and VIII were observed.
Effect of Temperature and Oxygen on the Strength of Elastomers
Authors: Gent A. N. , Liu Ginger L. , Sueyasu T.
Published: February 2011

1. A linear relationship has been found to hold for SBR vulcanizates between the logarithm of the rate of steady tearing and the logarithm of air pressure, over the range 10 Pa to 100 kPa. The slope varied from 0.08 for a C-C crosslinked material to 0.18 for a polysulfide-crosslinked material. Similar effects of air pressure were found in the rate of crack propagation under intermittent loading. They are attributed to attack of oxygen (in air) on highly stressed bonds, either sulfidic or hydrocarbon. 2. Approximately linear relationships were found to hold between the logarithm of the rate of steady tearing and the reciprocal of test temperature for temperatures between 25°C and 130°C. Apparent activation energies were deduced from the slopes, of 135 kJ/mole for a polysulfide-crosslinked SBR compound, 105 kJ/mole for a C-C crosslinked SBR compound, and 85 kJ/mole for a sulfur vulcanizate of BR. For SBR, these values were independent of air pressure. 3. These activation energies are far too large to be accounted for by changes in segmental mobility (and hence in internal energy dissipation) with temperature, for simple viscoelastic materials at temperatures far above Tg. The discrepancies are attributed to a second temperature-dependent factor. It is suggested that the threshold tear strength T0, i.e., the strength under nondissipative conditions, is itself temperature dependent. 4. Estimates of the changes in T0 with temperature have been obtained from the observed discrepancies in temperature dependence of tear strength. They correspond to a decrease from about 100–250 J/m2 at 25°C to about 50 J/m2 at 130°C. These changes in T0 may reflect the stress-activated rupture of thermally-sensitive bonds.
New Metallic Coagents for Curing Elastomers
Authors: Costin Richard , Nagel Walter , Ekwall Raymond
Published: February 2011

Based on the data reported in this paper, it is evident that the zinc salts of acrylic acid and methacrylic acid are effective coagents for the peroxide curing of EPDM and NBR. They appear to be unique among coagents in their ability to maintain high elongation while increasing the hardness and tensile properties of these elastomers. Zinc diacrylate was found to be the most effective in providing high hardness and high tensile properties. Zinc dimethacrylate gave a slightly better balance of properties by providing the highest elongation for a given hardness or modulus value.
Chemical Reactions of an Epoxy-Functional Silane in Aqueous Solutions
Authors: Xue Gi , Koenig Jack L. , Ishida Hatsuo , Wheeler Don D.
Published: February 2011

When the coupling agent is dissolved homogeneously in water, the hydrolysis rate of the alkoxy group is influenced by the pH of the solution, the size of the alkoxy group, and to a lesser extent, the size of the organofunctional group directly attached to the silane atom. We have quantitatively studied the reactions of γ-GPS in aqueous solutions. In a dilute solution, the silanol group slowly condensed to siloxane at pH = 4. The epoxy group reacts with both water and the silanol group. The reaction rate of the epoxy group with water is influenced by the pH of the solution. The reaction of the epoxy group with water proceeds faster than with the silanol group due to the large excess of water present, although the rate constants of these two reactions are comparable.
Chemical Reactions of Resorcinol-Formaldehyde Latex and Coupling Agent
Authors: Xue Gi , Koenig Jack L. , Ishida Hatsuo , Wheeler Don D.
Published: February 2011

In this study, the chemical reactions of the epoxy silane subcoat with resorcinol formaldehyde latex (RFL) topcoat were studied using model compounds. The experimental evidence indicated that various reactions took place between the γ-GPS and RFL. The epoxy group may react with the resorcinolic alcohol, methylol, and the residual silanol. Some reactions are catalyzed by NaOH or pyridine derivatives. Among the reactants, resorcinolic alcohol is the most active component in the presence of alkaline materials. It also has the highest concentration in an ordinary application. Thus, the resorcinolic alcohol opening the epoxy ring can be considered as the major reaction between the epoxy coupling agent and RFL. The 2-alkylpyridine component of the latex acts as a catalyst for the reaction. The residual silanol may consume the epoxy group upon heating during the processing. Since the reaction rate constant is low as compared to the silanol self-condensation and the interfacial reaction, and, furthermore, the concentration of the residual silanol is low, it will not influence the bonding at the silane/matrix interface.
Properties of Highly Filled, Highly Plasticized, Semicrystalline Polymer Networks
Authors: Vargo Richard D. , Kelley Frank N.
Published: February 2011

1. Component reactivity of ingredients such as fillers and plasticizers is significant and is measurable by a technique developed during this work. 2. The undesirable syneresis problem common to these highly plasticized materials can be controlled through adjusting equivalence ratios. Syneresis can be controlled primarily by decreasing the crystallinity of the material. 3. Changing percent crystallinity with temperature is a very important variable controlling the physical properties, i.e., ultimate properties, tearing energy, and dynamic-mechanical response. 4. The tearing energy data did not display simple amorphous behavior, and, as such, could not be shifted using a reduced variables technique such as WLF shifting. All variables were needed to represent the data. Three dimensional plotting developed previously by von Merrwall et al. was utilized to represent the data. The resulting tear-energy data exhibit the normal viscoelastic effects of rate and temperature as well as the superposition of the effects of crystallinity on the tearing energy. A decrease in tearing energy with increasing temperature is primarily due to increasing crystallinity in the samples. Plasticizer decreased the tearing energy, while filler increased the tearing energy. Filler lessened the effects of temperature and plasticizer on tearing energy. 5. Ultimate property measurements using ring samples for these model propellants revealed that these materials did not behave in a simple thermo-rheological manner, since crystallinity effects are predominant in the tensile mode. Because of crystallinity and strain-induced crystallinity, the data could not be represented by a failure envelope as proposed by Smith. The presence of plasticizer has the effect of decreasing the tensile strength, while filler tends to increase the tensile strength for the plasticized systems. 6. A model is presented to explain the high strain-to-failure behavior of these systems. Further details of this work can be found in Reference 22.
Reaction Kinetics and Injection Molding of Liquid Silicone Rubber
Authors: Batch Gibson L. , Macosko Christopher W. , Kemp Daniel N.
Published: February 2011

The curing of liquid silicone rubber (LSR) in an injection mold was studied using a two-pronged attack. First, a reaction kinetic model of LSR polymerization was derived from the hydrosilation mechanism. The model was tested with reaction kinetics of different rubber formulations with varying concentrations of catalyst and inhibitor. It was found that inhibitor consumption follows a zero-order kinetic model, and the subsequent polymerization kinetics are first order in vinyl concentration and concentration of available catalyst. Then this kinetic model was applied to a heat transfer and curing analysis which predicts gel time in a cavity of known thickness and mold temperature. Because silicone rubber has a low heat of cure, reaction exotherm was neglected in the analysis. The gel conversion was predicted using the recursive method, and thermal diffusivity was determined by transient temperature measurements in a cured slab. The predicted gel times were in good agreement with experimental demolding data. Because of its simplicity and accuracy, this analysis can be a useful tool for formulation optimization and mold-temperature selection.