THERMALLY CONDUCTIVE DURABLE STRAIN SENSORS FOR NEXT-GENERATION INTELLIGENT TIRES FROM NATURAL RUBBER NANOCOMPOSITES

A substantial knowledge gap persists in the material development of smart tires for future self-driving automobiles, which can increase both the vehicles' performance as well as the safety of the passengers. Due to the very high stiffness of conventional strain sensors compared to the softer rubber compound used as the tire tread material, an inaccurate representation of tire deformation characteristics is anticipated. Here, a comprehensive characterization of the electrical conduction and strain sensing behavior of a natural rubber (NR)-based commercial tire tread composite combining the reinforcement of a carbon black-conductive nanofiber dual filler system was carried out for the very first time. The incorporation of as low as 2 wt.% of carbon nanotubes (CNT) and graphite nanofibers (GNF) could increase the electrical conductivity of the control compound by two orders of magnitude compared to the control compound. The gauge factor observed was much higher than the value reported for metallic or polyvinylidene difluoride (PVDF) based stain sensors developed for this application. A 25% enhancement in thermal conductivity was also observed. Thus, the developed composites have the potential to be used as in situ strain sensors so that the problems of debonding and heating differences in the sensor–rubber interfaces in tires can be avoided in future.

Electron beam processing of rubbers and their composites

Electron beam (EB) processing of pristine and filled polymeric materials is considered as one of the most viable techniques in the development of three-dimensional (3D) network structures of polymeric or composite systems with improved physical and chemical properties. The grafting, or the crosslinking process induced by the merging of the macro free radicals generated during the electron beam modification without the aid of any chemical agent or heat, is responsible for the formation of the 3D networks in polymeric systems. Owing to its distinct advantages such as fast, clean and precise, electron beam (EB) radiation technology takes up a vital role in the crosslinking of polymeric compounds. However, during the course of electron beam treatment of polymers, two processes viz., crosslinking and chain scission take place simultaneously, depending on the level of radiation dose used for the processing. The present paper reviews the role of irradiation dose in the presence and absence of radiation sensitizer on the crosslinking and structure formation in a wide variety of soft matrices such as elastomers, latexes, thermoplastic elastomers and their respective filled systems. Notable improvements in mechanical and dynamic mechanical properties, thermal stability, processing characteristics, etc., of the EB processed elastomers and their composites are discussed elaborately in the paper. Specially, the property improvements observed in the EB processed pristine and filled rubbers in comparison to the conventional crosslinking technology are critically reviewed. The level of radiation dose inducing crosslinking in both pristine and filled rubbers, determined by calculating crosslink to scission ratio on the basis of Charlesby–Pinner equation is also discussed in the paper. Finally, the application aspects of electron beam curing technology with special emphasis to cable and sealing industries as developed by one of the authors are highlighted in the paper.

MECHANICAL PROPERTIES OF NATURAL RUBBER AND STYRENE–BUTADIENE RUBBER NANOCOMPOSITES WITH NANOFILLERS HAVING DIFFERENT DIMENSIONS AND SHAPES AT LOW FILLER LOADING

Reinforcement of rubber by nanofillers has been a topic of great interest in recent years. This work compares the reinforcing efficiency of nanofillers with different topologies such as spherical (carbon black and silica), fibrous (silicon carbide nanofibers and carbon nanotubes), and sheetlike (nanoclays, expanded graphite, and graphene) in two different diene rubbers (natural rubber [NR] and styrene–butadiene rubber [SBR]) at low loadings. Tensile strength improved by 88% in the case of NR and 57% in the case of SBR by the addition of just 3 phr of graphene nanoplatelets with high aspect ratio and surface area. An increase in the Mooney–Rivlin constant (C1) with filler loading variation was also observed for these filler systems in NR and SBR. The analysis of the composites using a tube model showed that the confinement of rubber chains due to the presence of fillers with a high aspect ratio gave rise to a lower tube diameter. The addition of nanofillers resulted in higher hysteresis losses, confirming their ability for higher energy dissipation. A higher Payne effect was observed in the composites due to the formation of a percolating filler network, which was accompanied by a weak strain overshoot in the loss modulus. Dynamical mechanical analysis of the composites showed a significant increase in the storage modulus of the composites at both low and room temperatures. The reduction observed in the tan δ was correlated with the crosslink density of the composites.

SYNTHESIS AND CHARACTERIZATION OF GRAPHENE SHEETS DECORATED WITH CARBON BLACK BY DIRECT PYROLYSIS OF A MOLASSES–CARBON BLACK MIXTURE AS A POTENTIAL VERSATILE FILLER FOR RUBBER

A mixture of molasses and carbon black was pyrolyzed in an inert atmosphere, which resulted in graphene of high quality, comprising of three to four layers on average with carbon black particles distributed over the graphene sheets. Molasses is the viscous dark colored slurry which is obtained at the last stage of refinement of sugar from sugarcane, in which sucrose is present as the major chemical component. Carbon black was also used as a substrate for the growth of graphene. The carbon black decorated graphene hybrid nanostructure was thoroughly characterized by different techniques and improved the failure properties of cured styrene butadiene rubber when incorporated into the rubber matrix. Bound rubber content increased by 50% with the hybrid filler compared to carbon black at 45 phr filler loading. The hybrid filler displayed 63% enhancement in the tensile strength at 2 phr filler loading and 86% increase at 45 phr filler loading, compared to the carbon black filled rubbers at the same loading. The vulcanizates containing the novel filler also exhibited improved abrasion resistance, ice traction, and wet traction and decreased rolling resistance compared to the carbon black filled systems. The new filler exhibited fair value of specific capacitance, 127 F/g when incorporated in an uncured rubber latex matrix. The hybrid filler synthesized, characterized, and studied thus can be classified as a versatile smart filler for rubber nanocomposites with a range of functionalities from mechanical reinforcement to electrochemical properties.

METAL-ORGANIC FRAMEWORK: A SMART REPLACEMENT FOR CONVENTIONAL NANOFILLERS FOR THE ENHANCEMENT OF MECHANICAL PROPERTIES AND THERMAL STABILITY OF SBR NANOCOMPOSITE

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.

WASTE MORINGA OLEIFERA GUM AS A MULTIFUNCTIONAL ADDITIVE FOR UNFILLED SBR COMPOUND

A natural waste (gum) of the drumstick tree, Moringa oleifera, was used for the first time as a sustainable multifunctional additive in an SBR compound. Improved cure rate with lower optimum cure time was obtained by using the gum as an accelerator activator. Tack strength of the M. oleifera gum–SBR compound was superior to both the control and the compound containing commercial phenol–formaldehyde resin at 5 parts per hundred of rubber loading. At different loadings, the gum acted as a plasticizer for the rubber and augmented processing by reducing the viscosity of the compound. The glass transition temperature of the compounds decreased by 2 °C compared with the pristine SBR. Moringa oleifera gum at any loading reduced the die swell of SBR. This study has relevance because the rubber industry is looking for feasible sustainable additives as alternatives to existing petroleum-based compounding ingredients.

Facile Synthesis and Characterization of Few-Layer Multifunctional Graphene from Sustainable Precursors by Controlled Pyrolysis, Understanding of the Graphitization Pathway, and Its Potential Application in Polymer Nanocomposites

The key feature of the present work is the dexterous utilization of an apparently destructive process, pyrolysis, for the synthesis of the most esteemed nanomaterial, graphene. This work is an attempt to synthesize graphene from nonconventional sources such as tannic acid, alginic acid, and green tea by a controlled pyrolysis technique. The precursors used in this work are not petroleum-derived and hence are green. A set of pyrolysis experiments was carried out at different temperatures, followed by a thorough step-by-step analysis of the product morphology, enabling the optimization of the graphitization conditions. A time-dependent morphological analysis was also carried out along with isothermal thermogravimetric studies to optimize the ideal pyrolysis time for graphitization. The specific capacitance of the graphene obtained from alginic acid was 315 F/g, which makes it fairly suitable for application as green supercapacitors. The same graphene was also used to fabricate a rubber-latex-based flexible supercapacitor film with 137 F/g specific capacitance. The graphene and graphene-based latex film exhibited room-temperature magnetic hysteresis, indicating their ferromagnetic nature, which also supports their spintronic applications.

PENETRATION RESISTANCE OF RUBBER VULCANIZATES

Aspects of penetration resistance of rubber compounds have been studied by developing a quasi-static test. The effects of indenter material and design, nature and dosage of fillers, and crosslinking density were investigated. Indenter material was found to have a negligible contribution to the penetration characteristics of the rubber compounds, whereas the conical indenter's shape and size of the tip were important. A change in the slope of the generalized penetration characteristic curve of the developed quasi-static test was considered to be the fracture initiation point. Although fracture initiation was early at higher carbon black loading, the overall penetration resistance was improved due to hysteresis, which was in accord with the impact energy method. This was a unique observation. The carbon black–filled sample was compared with the silica-filled vulcanizate. Surface morphology of the specimens penetrated at different energy levels was examined using scanning electron microscopy. A theoretical interpretation of the forces acting at the tip of the indenter and the energy requirement while penetrating a rubber compound against a conical indenter has been proposed. The initiation energy for penetration has inverse square root dependence on the Young's modulus of the compounds. The energy required for crack propagation in contrast, was directly proportional to the Young's modulus and also correlated with the hysteresis loss and frictional coefficient for the carbon black–filled vulcanizates.

Controlled Methodology for Development of a Polydimethylsiloxane-Polytetrafluoroethylene-Based Composite for Enhanced Chemical Resistance: A Structure-Property Relationship Study

Polydimethylsiloxane (PDMS) polymers are highly appreciated materials that are broadly applied in several industries, from baby bottle nipples to rockets. Momentive researchers are continuously working to understand and expand the scope of PDMS-based materials. Fluorofunctional PDMS has helped the world to apply in specialty applications. Efforts are taken to develop such siloxane-fluoropolymer composite materials with good thermal, solvent, and chemical resistance performances. We leveraged inherently flexible PDMS as the model matrix, whereas polytetrafluoroethylene (PTFE) was used as the additive to impart the functional benefits, offering great value in comparison to the individual polymers. The composites were made at three different mixing temperatures, that is, 0-35 °C, and different loadings of PTFE, that is, 0.5-8% (w/w), were selected as the model condition. A strong dependency of the mixing temperature against the performance attributes of the developed composites was noted. Mechanical and thermal stability of the composites were evaluated along with optical properties. X-ray diffraction demonstrated the change in the crystallite size of the PTFE particles as a function of processing temperature. Compared to the phase II crystallite structure of the PTFE, the fibrils formed in phase IV imparted a better reinforcing capability toward the PDMS matrix. A synergistic balance between higher filler loading and mechanical properties of the composite can be achieved by doping the formulation with short-chain curable PDMS, with 238% increment of tensile strength at 8 wt % PTFE loading when compared to the control sample. The learning was extended to check the applicability of doping such PTFE powder in commercial liquid silicone rubber (LSR). In the window of study, the formulated LSR demonstrated improved mechanical properties with additional functional benefits like resistance toward engine oil and other chemical solvents.

NATURAL RUBBER NANOCOMPOSITES BASED ON NEW FIBROUS NANOFILLERS WITH IMPROVED BARRIER PROPERTIES FOR USE IN TIRE INNERLINER APPLICATIONS

Hybrid nanocomposites were prepared by predispersion of new nanofibers such as aramid nanofibers, carbon nanotubes, silicon carbide nanofibers (SiC), cellulose nanofibers, and graphite nanofibers in natural rubber (NR) latex prior to melt mixing in an internal mixer to ensure the exquisite dispersion of nanofibers in NR. The competency of these nanofibers in reinforcing NR as well as enhancing its barrier properties has not been widely investigated. The fabricated nanocomposites showed enhanced curing as well as mechanical and dynamic mechanical properties. Morphology of the composites was analyzed through electron microscopy. The increase in tortuosity created by the presence of the hybrid filler system consisting of carbon black and nanofibers was studied using permeability models. At higher tearing energies, it was seen that the nanofiber-reinforced composites showed comparable crack growth properties; however, at lower energies, the fabricated composites exhibited higher crack propagation rates compared with the control compound when studied using a tear fatigue analyzer. The improved mechanical, dynamic mechanical, and barrier properties along with comparable fatigue crack growth properties offer an opportunity to apply these systems in high-end applications such as a thinner tire inner liner with a higher NR blend ratio, which can result in improved processability and reduced hysteresis, fuel consumption, and cost.

INFLUENCE OF FLUOROACRYLATE CURE SITE MONOMER ON THE THERMAL AND MECHANICAL PROPERTIES OF THE POLYACRYLIC ESTER ELASTOMER

The rubber industry is facing strong challenges in recent times because of imposed stringent standards on the performance of a product under adverse thermal and chemical applications. The choice of proper elastomer plays a significant role in imparting useful product performance. A new type of acrylic rubber with a fluoroacrylate cure site monomer was developed. Structural characterization, such as Fourier-transform infrared spectroscopy and nuclear magnetic resonance (NMR), suggested the presence of four different monomer units. Two-dimensional NMR spectroscopy analysis was also performed to support the assessment of the resonance peaks of coupled nuclei spins in the terpolymer. The newly developed acrylic rubber exhibited superior thermal and mechanical properties. Hexamethylenediamine carbamate in combination with zinc oxide (ZnO) was used as the curing package for the new elastomer. ZnO acts as an acid scavenger to avoid the micro-void formation. The new elastomer with a higher number of cross-link junctions resulted in superior mechanical and thermal properties as well as swelling resistance of the vulcanizate both with and without carbon black.

INFLUENCE OF DIANILINE CROSS-LINKING SYSTEMS ON THE STRUCTURE, CURING MECHANISM, AND PROPERTIES OF POLYACRYLICESTER ELASTOMER

To develop high-performance polyacrylicester (ACM) elastomeric components with higher scorch safety and superior thermal and mechanical properties, we replaced aliphatic diamine curatives with aromatic dianiline curatives. The influence of dianiline curatives 4,4′-(4,4′-isopropylidenediphenyl-1,1′-diyldioxy)dianiline, 4,4′-(hexafluoroisopropylidene)bis(p-phenyleneoxy)dianiline, and 4,4′-(1,1′-biphenyl-4,4′-diyldioxy)dianiline on the network structures and thermal, dynamic mechanical, and mechanical properties of ACM vulcanizates was investigated. The kinetics of vulcanization was analyzed for different dianiline curatives, with the use of rheometer curves. To understand the electronic properties and study the relation between chemical structure and reactivity, density functional theory was used. The time–temperature superposition principal was used to evaluate the activation energy for degradation of cross-linked samples. Finally, the curing mechanism of ACM in the presence of dianiline curative was studied with X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. These spectroscopic analyses suggested that the reaction mechanism took place via two steps: the first step was formation of the amide linkage and the second step was formation of imide linkages.

Selective Orientation of Needlelike Sepiolite Nanoclay in Polymer Blend for Controlled Properties

Sepiolite nanoclay needles have been selectively localized either in the natural rubber (NR) phase or in the carboxylated nitrile rubber (XNBR) phase of the XNBR/NR (50/50) blend prepared by the solution casting method. In a systematic manner, the role of the difference value between the interaction parameter of individual blend components (NR or XNBR)/solvent and the interaction parameter of sepiolite nanoclay/solvent in selectively localizing the sepiolite nanoclay to the NR phase or the XNBR phase of the XNBR/NR (50/50) blend has been explored. A higher percentage of sepiolite nanoclay resides in the dispersed NR phase when the difference value between the interaction parameter of NR/solvent and the interaction parameter of sepiolite nanoclay/solvent is lower than the difference value between the interaction parameter of XNBR/solvent and the interaction parameter of sepiolite nanoclay/solvent. On the other hand, a higher percentage of sepiolite nanoclay resides in the continuous XNBR phase when the difference value between the interaction parameter of XNBR/solvent and the interaction parameter of sepiolite nanoclay/solvent is lower than the difference value between the interaction parameter of NR/solvent and the interaction parameter of sepiolite nanoclay/solvent. It has been shown that by judiciously choosing different solvent combinations to prepare the blend and to disperse nanoclay, it is possible to fine-tune the difference value between the interaction parameter of individual blend components (NR or XNBR)/solvent and the interaction parameter of sepiolite nanoclay/solvent and dictate the selective localization of sepiolite nanoclay to the NR phase or the XNBR phase of XNBR/NR (50/50) blend. This study shows that it is possible to generate a rubber blend with controlled properties by selectively localizing needlelike sepiolite nanoclay in the dispersed phase or the continuous phase of the rubber blend prepared by the solution casting method.

EFFECT OF SILICA LOADING AND COUPLING AGENT ON WEAR AND FATIGUE PROPERTIES OF A TREAD COMPOUND

The effects of highly dispersible silica and the nature of silane in a tire tread cap compound were studied with particular reference to dynamic mechanical properties, abrasion resistance, side force coefficient, and fatigue crack growth (FCG) properties. The rubber matrix chosen was a blend of solution grade styrene butadiene rubber and polybutadiene rubber. Six different loadings of silica were used. Bistriethoxysilylpropyltetrasulfide (S) was taken as the coupling agent. In addition, the potential of two new generation silanes, 3-octanoylthio-1-propyltriethoxysilane (N) and 3-mercaptopropyl-di [tridecan-1-oxy-13-penta ethyleneoxideethoxysilane] (V) was also explored at 70 phr silica loading. Optimum properties were obtained at 50 phr loading of silica (S50). The tensile moduli for the compounds increased sharply with silica loading. Higher values of tan δ, indicating higher hysteresis, were obtained in compounds containing higher filler dosage. However, enhanced abrasion resistance and side force coefficient were observed at higher loadings of silica due to an increased reinforcement phenomenon. The crack growth exponent (β) was lowest for S50. Among the silanes tested, V showed a 22% drop in tan δ at 70 °C, 11% drop in abrasion loss, and an increase in FCG rate. N exhibited a lower FCG rate as compared with the silane S.

INFLUENCE OF NANOFILLER ON THERMAL DEGRADATION RESISTANCE OF HYDROGENATED NITRILE BUTADIENE RUBBER

Studies on the degradation of elastomers and their prevention have become increasingly important in recent years because of stringent environmental conditions in many industrial applications. The reactive atomistic simulation was executed on a hydrogenated acrylonitrile-butadiene rubber (HNBR40) model compound composed of 40 monomer units. The reactive simulation was used to study the decomposition behavior of HNBR40, to visualize different pyrolysis products, and also to analyze the degradation mechanism of HNBR40. Ethylene, propylene, and acrylonitrile were observed as dominant products at lower temperature, and 1-butene was found at higher temperature. Pyrolysis–gas chromatography–mass spectrometry was used to verify the decomposition products obtained from the prediction of atomistic simulation. In this study, nanofillers, especially nanoclays and nanosilicas, were used to prevent degradation significantly. Restricted degradation by the nanofiller-reinforced rubber prolonged the durability. Furthermore, the reactive simulation was performed to understand thermal decomposition characteristics of the model compound in the presence of the nanofiller. The initial decomposition temperature, the final degradation temperature, and the rate of degradation improved to a great extent on the addition of the model nanosilica compound as obtained from the simulation studies. Moreover, the lifetime of nanoclay- and nanosilica-reinforced hydrogenated acrylonitrile–butadiene rubber was calculated by using thermogravimetric analysis, and its useful lifetime was compared with that of the pristine polymer in the application temperature range of 150 °C.

SMART THERMOPLASTIC ELASTOMERS WITH HIGH EXTENSIBILITY FROM POLY (VINYLIDENE FLUORIDE) AND HYDROGENATED NITRILE RUBBER: PROCESSING–STRUCTURE–PROPERTY RELATIONSHIP

Novel smart thermoplastic elastomers (TPEs) with very high extensibility were prepared by blending polyvinylidene fluoride (PVDF) with hydrogenated nitrile rubber (HNBR) at an appropriate ratio, and their processing–structure–property relationship was investigated. Although the rubber phase was found to be dispersed in the matrix of PVDF for all compositions, morphology was shear sensitive and changed significantly after each processing step. Dropletlike structure was observed after the mixing in an internal mixer and compression molding, which changed to the lamellar structure after milling and injection molding. The compression molded sample exhibited very high extensibility (∼1600% elongation at break for 30/70 PVDF/HNBR blend) and a tensile strength of ∼6 MPa due to the formation of a strong interface. The elongation at break was much higher than any of the TPEs reported so far. Theoretical calculation of rubber particle size was also in agreement with the experimental observation. Dissipative particle dynamics simulation was run to capture the morphology, where HNBR chains were more sensitive to the shear force than the PVDF chains. The electromechanical sensitivity of the blend was 14.3 MPa−1, much better than the existing reported elastomeric actuator as well as pristine PVDF. Dynamic vulcanization gave further improvement in tensile strength and tension set properties.

HIGH-TEMPERATURE THERMOPLASTIC ELASTOMERS FROM RUBBER–PLASTIC BLENDS: A STATE-OF-THE-ART REVIEW

This article reviews different types of high-temperature thermoplastic elastomers and thermoplastic vulcanizates from rubber-plastic blends. Preparation, structure, and properties of these materials are discussed briefly. Strategies to further improve the high-temperature performance of these materials are presented herein. A synopsis of the applications of these high-performance materials in the automotive industry is reported, pointing out the gaps to motivate potential research in this field.