Dielectric elastomer-based transducers are rapidly gaining importance with the syntheses of new polymers that can potentially be used as dielectric materials. However, these materials are always prone to fracture in the presence of cracks and flaws. Failures originate from flaws (or notches), and a complete fracture may take place due to the propagation of cracks. The present work investigates the crack propagation behavior of two popular polymers, VHB 4910 and Ecoflex, that are widely used as dielectric elastomers. In this case, tensile loadings in laterally constrained boundary conditions are considered. The average crack propagation speed for Ecoflex is higher than that for VHB, implying that Ecoflex will fail earlier than that of VHB under similar conditions. Moreover, with increasing notch lengths at a fixed strain rate, the average crack propagation speed decreases appreciably but becomes constant for comparatively larger notches. The results also conclude that the average crack propagation speed and normalized crack tip diameter remain higher for VHB than for Ecoflex for larger normalized notch lengths. It is observed that the average crack propagation speed increases with strain rates, whereas the normalized crack tip diameter is independent of strain rates. Experimental results obtained here will provide a useful comparative insight to understand the failure behavior of two polymers widely used as dielectric elastomers.

This article presents the development of a new kind of magnetorheological elastomer blend made with natural rubber, acrylonitrile–butadiene rubber (NR-NBR), and electrolytic iron particles through solution mixing. The compressive stress and elastic modulus of the composites in the isotropic and anisotropic states of the filler were studied. A unique study of the filler distribution and filler orientation mechanism was proposed from the compressive properties and scanning electron microscopy. A strong improvement in the elastic modulus of the NR–NBR blend from isotropic to anisotropic change was achieved as compared with NR and NBR in single-rubber composites. The filler content in the anisotropic magnetorheological elastomers was optimized by measuring the field-dependent elastic modulus in the presence of an externally applied magnetic field. The blend rubber composites showed better sensitivity in the presence of a magnetic field than the NR and NBR composites did. The improvement might be due to the better filler orientation and strong adhesion of filler particles by the NR phase in the blend matrix. The new elastomer blends may have applications in active dampers, vibrational absorption, and automotive bushings.

ABSTRACT The strain rate dependence of uncured rubber is investigated through a series of tensile tests (monotonic, multistep relaxation, cyclic creep tests) at different strain rates. In addition, loading/unloading tests in which the strain rate is varied every cycle are carried out to observe their dependence on the deformation history. A strain rate–dependent viscoelastic–viscoplastic constitutive model is proposed with the nonlinear viscosity and process-dependent recovery properties observed in the test results. Those properties are implemented by introducing evolution equations for additional internal variables. The identified material parameters capture the experiments qualitatively well. The proposed model is also evaluated by finite element simulations of the building process of a tire, followed by the in-molding.

ABSTRACT In recent years, cellulose fibers have attracted considerable attention as biofillers for natural rubber (NR) composites. However, neat cellulose cannot be used as a substitute for conventional fillers due to its poor compatibility with NR. Therefore, a new surface treatment via maleic anhydride grafted to polyisoprene (MAPI) in solution was developed to improve the filler–matrix interaction. Different contents of carbon black (CB) and cellulose fibers (before and after modification) were used as a hybrid filler system to investigate the possibility of CB substitution in NR composites. First, contact angle, Fourier transformed infrared spectrometry (FTIR), and scanning electron microscopy (SEM) techniques were used to confirm the successful cellulose surface treatment. Second, morphological analysis, Payne effect, and swelling behavior of the rubber compounds in toluene confirmed the effect of cellulose treatment on improving the interfacial filler–matrix adhesion. Finally, the results showed that the composite filled with 20 phr modified cellulose and 20 phr CB (50% replacement of CB) exhibited even better results than the composite filled with 40 phr of CB, since the tensile strength was only 7% lower, but the elongation at break, tensile modulus at 100%, and storage modulus at 25 °C were respectively 35%, 24%, and 22% higher.

The development of ultra-high-performance tires that satisfy fuel efficiency, traction, handling performance, and abrasion resistance has gained significant importance in the tire industry. Solution SBR has been used as a raw material, owing to its useful characteristics (e.g., narrow dispersity controllable microstructure and chain-end functionalization). In a recent improvement, emulsion SBR (ESBR), a high-molecular-weight compound with narrow dispersity, has been reported for application in the tire tread compounds. In particular, S,S-dibenzyl trithiocarbonate (DBTC) reversible addition-fragmentation transfer (RAFT) ESBR has exhibited excellent abrasion resistance and fuel efficiency in unfilled and carbon black–filled vulcanizates. However, owing to the symmetrical structure of DBTC RAFT ESBR, the polymer chain was shortened by the reaction of a silane coupling agent with trithiocarbonate, leading to poor abrasion resistance and fuel efficiency in the case of silica-filled vulcanizates. In this study, benzyl (4-methoxyphenyl) trithiocarbonate (BMPTC), an asymmetric RAFT agent that promotes unilateral polymer growth, was synthesized and used in the polymerization of BMPTC RAFT ESBR. Chain cleavage was not observed. Upon application to silica-filled vulcanizates, BMPTC RAFT ESBR exhibited improved abrasion resistance (by 9%), improved fuel efficiency (by 20%), and improved wet traction performance (by 10%) compared with the DBTC RAFT ESBR.

Itaconic acid has been employed as a special facilitator to construct divalent metal ion based ionic crosslinking framework in the acrylonitrile butadiene rubber matrix. Readily accessible double bonds in itaconic acid could directly react with the elastomer to form effective covalent bonds. On the other hand, presence of easily dissociable protons in itaconic acid enables them to form ionic bonds that leads to an increase in crosslinking density of the vulcanizates. The synergistic effect of covalent crosslinking induced by peroxide and ionic crosslinking induced by metal carboxylate could effectively enhance the overall mechanical and dynamic mechanical properties of the rubber composites. In this study, three metal oxides, that is, zinc oxide, magnesium oxide, and calcium oxide, have been selected for this purpose. Tensile strength of nitrile rubber composites depends on the strength of ionic crosslinks, which in turn is influenced by the size of the alkaline earth metals, such as Mg, Ca, etc., and stoichiometric quantity of itaconic acid, which is at par in the formulation of this study. The novelty of this study is that the introduction of a dicarboxylic acid in combination with metal oxides enhances the crosslink density and tensile strength of nitrile rubber composites which could result from the metal organic framework.

The effects of the masticated state of isoprene rubber (IR) at the carbon black (CB) addition stage on subsequent mixing, microstructure, and physical properties in the case of a kneader with a characteristic large-diameter shaft are investigated by examining the mastication-time dependence. A sufficiently masticated IR shows a shorter black incorporation time, which results in an improved dispersion of CB and better physical properties. Observing the microstructure of a rubber compound using the atomic force microscope–based nanomechanical technique, poor CB dispersion is revealed for insufficient mastication. Specifically, large CB agglomerations surrounded by the interfacial rubber region with higher elastic modulus than that of a rubber matrix are formed. Such a large CB agglomeration, on the other hand, does not appear in rubber compounds with longer mastication times. The thickness of the interfacial region becomes shorter in these cases. These observations are further discussed by the concept of “rheological unit” introduced by Mooney et al. This study demonstrates that the microstructure of a rubber compound is highly heterogenous with rubber regions of different microscopic elastic moduli and that the microstructure has an influence on CB dispersion and the physical properties of rubber.

Effect of protein on vulcanization of NR, obtained from Hevea brasiliensis, was investigated by analyzing the crosslinking structure of the resulting vulcanizates prepared from untreated NR, deproteinized natural rubber (DPNR), and protein-free natural rubber (PFNR) by swelling methods and rubber-state NMR spectroscopy. The proteins present in NR were removed by three methods: deproteinization with enzyme, urea, or urea–acetone in the presence of sodium dodecyl sulfate. The amount of proteins present in NR, approximately 0.238 w/w%, was reduced to 0.000 w/w% by urea–acetone deproteinization, whereas it was reduced to approximately 0.003 and 0.019 w/w% by enzyme and urea deproteinizations, respectively. Hardness, swelling degree, and crosslinking structure depended on the amount of proteins. Changes in mechanical properties for the vulcanizates prepared from not only non-filler compounds but also carbon black–filled and silica-filled compounds were attributed to the amount of proteins.

Designing shape memory polymers (SMPs) based on thermoplastic vulcanizates (TPVs) is an essential research topic. An efficient SMP is designed with typical sea-island structured ethylene–methacrylic acid copolymer/nitrile–butadiene rubber (EMA/NBR) TPVs in which the heat-control switched phase performed by the EMA phase is related to the shape fixity ability. The results show that the heat-triggered SMPs exhibit surprising shape memory properties (shape fixity >95%, shape recovery >95%, and fast recovery speed <30 s at the switching temperature of 95 °C). Through X-ray diffraction characterization, it is seen that the shape fixity of TPVs is achieved mainly through ethylene crystallization. The switching temperature is largely determined by the melting temperature (98 °C) obtained by differential scanning calorimetery.

Fatigue crack growth behavior of carbon black–reinforced natural rubber is investigated. Rubber compounds of Shore A = 70 are prepared by varying the formulation loadings of a wide range of carbon black types based on their structure and surface area properties. The resulting fatigue crack growth behavior shows significant variation in β exponent values, depending on the properties of the carbon black. These variations are rationalized by considering the strain amplification of natural rubber by carbon black aggregates in the region of compound directly ahead of the crack tip. An assumption is made that little networking of the carbon black aggregates exists in this region of very high strain and that hydrodynamic calculations that consider occluded rubber can therefore provide realistic values for strain amplification. A reasonable scaling of power law crack growth parameters to calculated strain amplification factors is found, with the exponent, β, decreasing with increasing strain amplification. The implication here is that enhanced strain amplification promotes the formation of strain-induced crystallites in the crack tip region. Performance tradeoffs resulting from the crossover of crack growth data sets dependent on the carbon black type are discussed. Of practical significance is the fact that the strain amplification factors can be calculated directly from knowledge of carbon black type and loading in rubber formulations.

In this study, a phosphorus-containing flame retardant, polybis(4-hydroxypheyl)-2-(6-oxo-6-H-dibenzodibenzo [c,e][1,2] oxaphosphino-6-yl) methylene succinate (PHDO) was prepared by melt condensation between (6H-dibenz[c,e][1,2]oxaphosphorin-6-ylmethyl)-p-oxide-butanedioic acid (DDP) and 1,4-benzene dimethanol (PXG). Then, Fourier transform infrared spectroscopy (FTIR) and hydrogen nuclear magnetic resonance (1H NMR) were used to characterize the structure of this novel additive. It was added to silicone rubber (SR) in different proportions, and the flame retardant properties together with tensile behaviors of the SR blends were investigated. Results showed that the thermal stability was improved and the burning rate was slowed down after addition of this novel flame retardant. Vertical burning test, cone calorimetric evaluation, and limited oxygen index (LOI) measurement of the samples revealed that the SR with 15 phr of PHDO owned the best flame retardant properties and may pass UL-94 V-0 grade. This improved flame retardant performance may be ascribed to the formation of dense carbon layers, which effectively prevented the surface oxidation and inhibited combustion of the silicone matrix.

ABSTRACT Silica wet-masterbatch (WMB) is a well-known technique for manufacturing high-content, highly dispersed silica-filled compounds. Emulsion styrene–butadiene rubber (ESBR)/silica WMB offers several advantages, including excellent silica dispersion and reduced hysteresis, as compared with conventional dry masterbatch (DMB) compound. However, because of the residual emulsifiers in ESBR latex, it can exhibit a decrease in the crosslink density and reductions in its mechanical properties. Moreover, the abrasion resistance cannot be significantly enhanced because of the tradeoff between the improvement in silica dispersion and decrease in crosslink density. Accordingly, the objective of this study was to improve the silica dispersion and abrasion resistance of ESBR/silica WMB compounds by using liquid polybutadiene rubber (LqBR) extended WMB. In detail, three types of LqBR were emulsified to LqBR emulsions, and three types of LqBR extended WMBs were produced by co-coagulating ESBR latex, silane-modified silica, and the LqBR emulsion. A thorough characterization was conducted with emphasis on the silica content, cure characteristics, mechanical properties, abrasion resistance, and dynamic viscoelastic properties. Based on the results, silane-terminated LqBR extended WMB vulcanizate showed a 58% improvement in the 300% modulus, 48% reduced DIN abrasion loss, and a 23% improvement in dynamic properties.

ABSTRACT The mechanism of bromination of natural rubber (NR) was studied by solution-state 1H-NMR spectroscopy. The bromination of NR was carried out at 20–50 °C with N-bromosuccinimide as the brominating agent, and the kinetic study of bromination was conducted under nitrogen atmosphere at 30–50 °C for various reaction times. The influence of bromine atom substituent on the bromination rate constant (k) also was investigated. Bromine atom content was found to be dependent upon the reaction time, indicating first-order kinetics. The activation energy of bromination of NR, calculated from the reaction rate constants, was 19.3, 5.5, and 5.8 kJ mol−1 for bromine atom linked to carbon atom with methylene proton and methylene protons, respectively.

Carbon nanohorn (CNH)–filled elastomer hybrid nanocomposites were prepared based on NBR. Three different CNH types were analyzed, each featuring various characteristics such as aggregate structure, specific surface area, surface energy distribution, and electrical conductivity and resulting in different potentials regarding the properties of the developed elastomers. For the CNH types, a high tendency of agglomeration was observed in the pristine state, indicating the need for an effective strategy to break up the agglomerates during the mixing or the compounding procedure to realize their incorporation and sufficient dispersion in a polymer matrix. In addition to the melt mixing technology by means of an internal lab mixer, a discontinuous static and a continuous dynamic latex compounding process were used. Carbon nanotubes and a highly conductive carbon black (Printex) were used as hybrid fillers in the compounds mixed by melt mixing, whereas two different types of carbon black (Printex and Derussol) were also incorporated in the latex experiments. Hybrid nanocomposites with low content of CNHs (≤1 wt%) show an improvement in dynamic-mechanic and physical properties due to distinctive polymer–filler interactions. Dealing with higher amounts of CNHs leads to filler reagglomeration, resulting in deterioration of the elastomer properties. For the electric conductivity assessment, addition of CNH indicates no synergistic effects and no significant increase of the hybrid compounds, which is demonstrated in dielectric measurements, although pristine CNHs are conductive themselves. Elastomer compounds processed via the latex method show enhanced material performance by using the continuous dynamic latex compounding, which is mainly attributed to the dispersion of the hybrid filler.

ABSTRACT Mixing characteristics and mechanical loads of rubber-mixing rotors are considered to be the two most important factors in actual rotor design. For the design of highly reliable production mixers, there is a great need for a proper estimation method of mechanical load, such as radial force or rotation torque of the rotors. The mechanical load of tangential mixing rotors and surrounding flow are mainly discussed by using partially filled numerical flow simulation. Operational parameters of the mixing condition were set to be fill factor and rotor phase angle of two rotors rotating at an even speed. The Carreau model was applied to the shear rate dependence of viscosity. The volume-of-fluid method was used for free surface simulation. Both two-dimensional and three-dimensional simulations were carried out to discuss mechanical load and its fluctuation mechanisms. For the numerical results, radial force on rotors, pressure, and the velocity distribution around the rotors and their fluctuations are presented and discussed. It was found that the radial force of the rotors could be estimated using this kind of flow simulation, and the fluctuation phenomena could be explained by the movement of a high-pressure region between the front of the rotor wings and the chamber wall.

Recently, considerable attention has been paid to the development of new functionalized polymers to improve the fuel efficiency of vehicles by reducing the rolling resistance of tires to adhere to strict CO2 emission regulations. Accordingly, multifunctionalized (MF) reversible addition–fragmentation chain transfer (RAFT) emulsion styrene–butadiene rubbers (ESBR) were synthesized, in which chain-end and in-chain functionalization were performed simultaneously by introducing a third monomer (glycidyl methacrylate; GMA) using RAFT polymerization. Compared with GMA ESBR, in which GMA is introduced as a third monomer by conventional radical polymerization (CRP), there was an even distribution of GMA per chain in the MF-RAFT ESBR. After preparing the silica-filled compounds, vulcanizate structure analysis and mechanical property evaluation of the compounds were performed. The MF-RAFT ESBR prepared by RAFT polymerization exhibited superior in-chain functionalization efficiency compared with GMA ESBR prepared by CRP because of the even distribution of GMA and higher crosslink density. Consequently, MF-RAFT ESBR compound showed superior silica dispersion, abrasion resistance, and lower rolling resistance compared with the GMA ESBR compound.

ABSTRACT Uniaxial tension tests on dumbbells are routinely used to determine the stress–strain response of engineering materials. The simplest way to calculate strain is from grip displacement during extension, but this introduces significant error when dumbbells are gripped at the wider end sections to avoid the sample breaking prematurely in the grips. Mechanical and optical extensometers alleviate this problem by directly measuring strain in the gauge section. However, the equipment introduces significant additional hardware and software costs, and some experimental setups obstruct or prevent direct measurement of strain. The strain following systems also struggle both with the loss in mark intensity and changes of the shape of the marked point as the strain level is increased. To address these shortcomings, a novel analytical model to correct stress–strain data based on grip displacement is proposed. The model is implemented in Fortran and applied to hyperelastic materials which are assumed isotropic, but in principle the method is not restricted to elastomers. The model is validated with three studies on dumbbells: (i) a finite-element analysis for strains up to 660%; (ii) an experimental test with unfilled natural rubber up to 300% strain using a video extensometer; and (iii) a high temperature experimental test to fracture where the strain is corrected for a filled rubber. The model errors range from 2.2% to 3.1%, which is well within material and experimental uncertainties; hence, the model provides an accurate, inexpensive means of determining stress–strain behavior from grip displacement.

A new rheological methodology is used to quantify the kinetics and thermal activation of thixotropic recovery (flocculation) of uncrosslinked carbon black–reinforced emulsion SBR following high shears and over a range of annealing temperatures. A wide range of carbon black types are examined to determine the influence of aggregate morphology and surface area on compound flocculation. Several kinetic parameters are correlated with the carbon black aggregate structure and surface area, the results of which imply a transition in mechanisms controlling modulus recovery between shorter and longer recovery time scales. Thermal activation of flocculation is found to scale to the surface area and to the mean aggregate diameter of the carbon blacks following power law relationships. The thermal activation data for a subset of compounds with different carbon blacks prepared at different loadings collapses onto a single master line by rescaling the data to a parameter that is proportional to the theoretical interparticle force calculated for the idealized situation of two spherical particles in proximity. Three different van der Waals force models are evaluated, and in each case, an effective superposition of the thermal activation data is achieved. This indicates that the attractive force between aggregates plays a key role in the flocculation of carbon black in rubber, and this force can be traced back to the aggregate and primary particle sizes, interaggregate distances, and effective volume fractions. The activation energy for the viscosity of the unfilled, uncrosslinked SBR is similar to analogous values calculated for the thermal activation of flocculation. This coupling of energetics may be the result of creep/flow of rubber out of gaps between aggregates resulting from interaggregate attractive forces and any potential diffusive motion of the aggregates. Bound rubber data appear to contain information relating to aggregate packing, which could be exploited in future work to further explore the mechanism of flocculation.

ABSTRACT Hyperelastic materials can experience a large deformation process. A constitutive relation suitable for an entire region from small, moderate, to large deformations is of great importance for practical applications such as fracture problems. Treloar's data are first investigated, and the tension curve is divided into three regimes: small-to-moderate regime, strain-hardening regime, and limiting-chain regime. Next, the modeling theory of hyperelastic materials is introduced, and the tensile behaviors of basic energy functions are analyzed for different deformation regimes. Finally, a successive procedure is suggested to establish an entire-region constitutive relation and then applied to Treloar's data. The present constitutive relation can maintain the initial shear modulus while the experimental data are satisfactorily predicted. The present procedure is simple and feasible and hence applicable to other hyperelastic materials when their entire-region constitutive relations are studied based on experimental data.

To the best of our knowledge, for the first time, metal-organic framework (MOF), a porous reticular structure, has been tried as a reinforcing filler for rubber. A MOF synthesized by solvothermal reaction between 2-aminoterephthalic acid and aluminum chloride hexahydrate was characterized and incorporated as reinforcing filler in SBR. A comparative investigation on the properties of the well-dispersed, thermally stable nano-MOF composite (SBR-MOF) was carried out with reference to SBR–nano alumina composite (SBR-nAl). The SBR-MOF was mechanically more robust than SBR-nAl. The SBR-MOF showed 130% improvement in tensile strength over the pristine SBR composite and 50% better elongation at break than SBR-nAl at 10 phr loading. The thermal and dynamic mechanical properties of SBR-MOF are superior to SBR-nAl composite. The highly porous organic framework was favorable for the enhanced entanglement of polymer chains at the interface. The effectiveness of the organic framework on the dispersion and compatibility was evaluated by scanning electron microscopy. The dispersion studies substantially supported the overall property enhancement. To substantiate the superiority of MOF in the rubber matrix, the tensile properties of SBR-MOF were compared with SBR composites filled with nano silica, nano titania, as well as nano silica and nano alumina with a compatibilizer, thereby documenting a promising nanofiller for introduction into the rubber industry.