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Leather is made of collagen protein polymer, which is used in the manufacture of a variety of products including footwear, automotive upholstery, garments, and sports equipment. Animal skins/hides are converted into leather using a series of chemical processes. Of them, the tanning process is the most important chemical process that converts animal skins into leather by stabilising collagen fibre so that they do not putrefy. However, it is a hazardous process because of the use of various toxic chemicals in tanning, re-tanning and fatliquoring treatments producing toxic effluent. Over the years many tanning treatments based on chromium sulphate, and vegetable and synthetic tannins in combination with heavy metals, have been developed but tanning with chromium sulphate (known as Cr-tanning) is still the most effective, cheap, and widely used tanning process in the leather industry. Although the development of various improved Cr-tanning methods highly reduced the chemical and water consumption in leather tanning, it is still under scrutiny because of the production of effluent containing a harmful level of Cr and there is strong evidence that when the treated leather is disposed into the environment, part of the released trivalent chromium is converted into carcinogenic hexavalent chromium. Many sustainable alternatives to Cr-tanning based on chemical and enzymatic crosslinking, various bio-derived polymers, enzymes, modified zeolites, and nanostructured materials have been developed over the years with limited success. The alternative methods are either not as effective as Cr-tanning, affect the dyeability and other functional and organoleptic properties of leather, and or are cost-prohibitive. In this comprehensive review article, various tanning methods used in industry or studied in the laboratory are critically reviewed, and their advantages and disadvantages are outlined. The consumption of tanning agents, total chemicals including various auxiliaries and fatliquoring agents, and water in tanning, and the tanning performance and mechanical properties of the processed leather are compiled and compared. The reaction mechanisms of novel tanning agents with leather collagens and the future directions to make leather tanning more sustainable are outlined. This review article will be a guide for academicians/researchers/manufacturers involved in leather processing to develop more sustainable leather materials. [ABSTRACT FROM AUTHOR]

Soft robots have better flexibility than their rigid counterparts thus offering greater adaptability to changing environments. These robots made from flexible materials are generally with low stiffness and have limited capability to perform tasks that require the application of large forces. Soft artificial muscles (AMs) with variable stiffness are promising solutions for soft robots to handle larger payloads. However, the existing actuators need to overcome their small range of stiffness variation and slow response. Further, it is difficult to integrate self‐sensing into existing actuators. Herein, a novel electric AM with variable stiffness and self‐sensing referencing smooth muscle contraction in nature is proposed. The AM consists of two pieces of soft anode mesh, a flexible cathode and a polyvinyl chloride (PVC) gel dielectric layer, where the cathode is also a resistive sensor. The stiffness variation is induced by the friction change caused by electrostatic adsorption. Compared with the existing PVC gel actuation technology, the new design integrated the dielectric and the cathode layers and combined the sensing function. Also, the rigid anode and cathode are replaced by the soft ones, respectively, which makes the AM suitable for soft robotics or wearable devices. [ABSTRACT FROM AUTHOR]

Filler reinforced rubber is widely used for engineering applications; therefore, a sound characterization of the effects of physical aging is crucial for accurately predicting its viscoelastic properties within its operational temperature range. Here, the torsion pendulum is used to monitor the evolution of the storage and loss modulus of carbon black filled samples for four days after a temperature drop to 30   ° C . The storage modulus presents a continuous increase, while the loss modulus generally displays a steady decrease throughout the four days that each test was conducted. The relationship of the recovery rates with the carbon black properties is also studied, analysing its dependency on the particle size and aggregate structure. The evolution of the recovery rate seems to depend linearly on the surface area while the carbon black structure appears to have a much weaker influence on the physical aging behavior for the set of compounds tested. The obtained results corroborate the presence of physical aging at room temperature for filler rubber materials and the ability of the torsion pendulum to monitor the storage and loss modulus change, providing pivotal data on the influence of physical aging on the viscoelastic properties of the material. [ABSTRACT FROM AUTHOR]

The Payne Effect (also known as the Fletcher–Gent Effect) has a fundamental impact on the behavior of filled rubber composites and therefore must be considered during their design. This study investigates the influence of carbon black (CB) surface area and structure on the observed Payne Effect and builds on the existing models of Kraus and Ulmer to explain this phenomenon. Dynamic strain sweeps were carried out on natural rubber (NR) compounds containing eight different grades of CB at equivalent volume fractions. The loss and storage moduli were modeled according to the Kraus and Ulmer equations, using a curve optimization tool in SciPy. Subsequent regression analysis provided strong correlations between the fitting parameters and the CB structure and surface area. Using this regression analysis, this work provides further insight into the physical meaning behind the Kraus and Ulmer models, which are phenomenological in nature. [ABSTRACT FROM AUTHOR]

Compounds incorporating hydrogenated acrylonitrile butadiene rubber (HNBR) are widely adopted in applications that operate under extreme conditions such as at high temperatures, often whilst in contact with oil and other chemicals. They also work effectively when subjected to high strains and high strain rates. The detailed mechanisms that allow these materials to perform so well under these specific conditions are not yet fully understood. In this study a high acrylonitrile (ACN) content HNBR is incorporated into four different rubber compounds that are subjected to different thermal ageing conditions to investigate their impact on fatigue crack growth resistance of the cured compound. The resulting compounds are subjected to detailed DSC, TGA and FTIR analysis to help understand the changes that take place in the compounds during ageing for the first time. Counterintuitively, in this work ageing under specific set of conditions can produce an increase in the fatigue crack resistance in some of the elastomer compounds and when this happens it is observed that this occurs when the HNBR compounds develop knottier tearing profiles. Analysis of the DSC, FTIR and TGA results confirm that during ageing there is a steady evolution of the some of the ingredients in the compound such as the processing oils and the plasticiser which appears to have a significant effect on the fatigue properties of the HNBR materials. [ABSTRACT FROM AUTHOR]

The influence of carbon black (CB) structure and surface area on key rubber properties such as monotonic stress-strain, cyclic stress–strain, and dynamic mechanical behaviors are investigated in this paper. Natural rubber compounds containing eight different CBs were examined at equivalent particulate volume fractions. The CBs varied in their surface area and structure properties according to a wide experimental design space, allowing robust correlations to the experimental data sets to be extracted. Carbon black structure plays a dominant role in defining the monotonic stress–strain properties (e.g., secant moduli) of the compounds. In line with the previous literature, this is primarily due to strain amplification and occluded rubber mechanisms. For cyclic stress–strain properties, which include the Mullins effect and cyclic softening, the observed mechanical hysteresis is strongly correlated with carbon black structure, which implies that hysteretic energy dissipation at medium to large strain values is isolated in the rubber matrix and arises due to matrix overstrain effects. Under small to medium dynamic strain conditions, classical strain dependence of viscoelastic moduli is observed (the Payne effect), the magnitude of which varies dramatically and systematically depending on the colloidal properties of the CB. At low strain amplitudes, both CB structure and surface area are positively correlated to the complex moduli. Beyond ~2% strain amplitude the effect of surface area vanishes, while structure plays an increasing and eventually dominant role in defining the complex modulus. This transition in colloidal correlations reflects the transition in stiffening mechanisms from flexing of rigid percolated particle networks at low strains to strain amplification at medium to high strains. By rescaling the dynamic mechanical data sets to peak dynamic stress and peak strain energy density, the influence of CB colloidal properties on compound hysteresis under strain, stress, and strain energy density control can be estimated. This has considerable significance for materials selection in rubber product development. [ABSTRACT FROM AUTHOR]

Fingertip‐mounted tactile displays of softness are needed for various virtual‐ or augmented‐reality applications such as surgical simulation, tele‐operation, computer‐aided design, 3D model exploration, and tele‐presence. Displaying a virtual softness on a fingertip requires the generation of quasi‐static large displacements at moderate forces (as opposed to high‐frequency small vibrations at high forces), via a deformable surface, to control both the contact area and the indentation depth of the skin. State‐of‐the‐art actuation technologies are unable to combine simple structure, low weight, and low size, as well as energy efficiency and silent operation. Here, the progress on the development of a non‐vibratory display of softness made of electroactive polymers is reported. It consists of a hydrostatically coupled dielectric elastomer actuator, shaped as a bubble interfaced to the fingertip, having a weight of 6 g. Prototypes can generate displacements up to 3.5 mm and forces up to 1 N. By combining this technology with a compact hand tracking sensor, a simple and cost‐effective virtual‐reality system is demonstrated. A psychophysical study engaging 15 volunteers in poke and pinch tactile tasks shows that users can properly distinguish between different stimuli rendered by the display, with an accuracy correlated to the perceptual difficulty of the tactile comparative task. [ABSTRACT FROM AUTHOR]

The possibility to electrically tune the optical transparency of thin membranes is of significant interest for a number of possible applications, such as controllable light diffusers and smart windows, both for residential and mobile use. As a difference from state-of-the-art approaches, where with an applied voltage the transparency can only increase or decrease, this paper presents the first concept to make it electrically tuneable to both higher and lower values, within the same device. The concept is applicable to any soft insulating membrane, by coating both of its surfaces with a circular transparent stretchable conductor, surrounded by a stretchable annular conductor. The two conductors are used as independently addressable electrodes to generate a dielectric elastomer-based actuation of the membrane, so as to electrically control its surface topography. We show that the optical transmittance can electrically be modulated within a broad range, between 25% and 83%. This approach could be especially advantageous for systems that require such a broad tuning range within structures that have to be thin, lightweight and acoustically silent in operation. [ABSTRACT FROM AUTHOR]

Smart heating devices with reliable self-regulating performances and high efficiency, combined with additional properties like mechanical flexibility, are of particular interest in healthcare, soft robotics, and smart buildings. Unfortunately, the development of smart heaters necessitates managing normally conflicting requirements such as good self-regulating capabilities and efficient Joule heating performances. Here, a simple and universal materials design strategy based on a series connection of different conductive polymer composites (CPC) is shown to provide unique control over the pyroresistive properties. Hooke's and Kirchhoff's laws of electrical circuits can simply predict the overall pyroresistive behavior of devices connected in series and/or parallel configurations, hence providing design guidelines. An efficient and mechanically flexible Joule heating device is hence designed and created. The heater is characterized by a zero temperature coefficient of resistance below the self-regulating temperature, immediately followed by a large and sharp positive temperature coefficient (PTC) behavior with a PTC intensity of around 106. Flexibility and toughness is provided by the selected elastomeric thermoplastic polyurethane (TPU) matrix as well as the device design. The universality of the approach is demonstrated by using different polymer matrices and conductive fillers for which repeatable results are consistently obtained. [ABSTRACT FROM AUTHOR]

Carbon black particulate reinforcement of rubber is examined in terms of linear viscoelasticity and the dynamics of the filler particle network. First, it is demonstrated that for the case of purely hydrodynamic reinforcement, the dynamics of the filled rubber are equivalent to those of the corresponding unfilled material. A breakdown in thermorheological simplicity is observed with the onset of filler networking in reinforced compounds. The dynamics of the filler network are initially examined by strain sweep/recovery experiments performed on uncrosslinked materials. The role of the surface activity of carbon black in defining the rate and magnitude of flocculation is explored and various models to describe this process are reviewed. The dynamics of carbon black filler networks in crosslinked materials are probed using small strain torsional creep experiments. Physical ageing (structural relaxation) of filled compounds at temperatures well above the glass transition temperature of the rubber matrix is observed and the ageing rate is found to scale with the level of filler networking in the various compounds. Physical ageing is the result of non-equilibrium, slow dynamics, which sheds light on the physical origin of the filler network. Furthermore, the implications of physical ageing of highly filled rubbers on typical linear viscoelastic time–temperature superposition experiments are discussed. [ABSTRACT FROM AUTHOR]

The strain and thermal dependence of the dynamic mechanical properties of carbon black filler networks underpin the performance of many rubber components. These effects are examined by varying the surface energetics of carbon black. This has a profound influence on the level of flocculation of the carbon black network in the final crosslinked compounds. Filler networks comprised of thermally deactivated carbon blacks are significantly more strain-sensitive - shifting the onset of the Payne effect to smaller dynamic strains. Using free vibration equipment to precisely probe the thermal sensitivity of the linear viscoelastic properties of the filled compounds, it is shown that carbon black deactivation results in carbon black networks which are more thermally sensitive than corresponding unmodified carbon black networks. Increased thermal dependence of the dynamic moduli results in the appearance of a secondary increase in tan δ as a function of increasing temperature well above the rubber Tg - which is not correlated with any thermal transitions in calorimetric experiments. Such effects are prevalent in the relevant literature for various rubber-filler combinations but their physical origins are often misinterpreted or unexplained. A rationalization of these effects based on the dynamics of the filler network is presented. [ABSTRACT FROM AUTHOR]

The process of particulate flocculation in natural rubber melts and subsequently crosslinked samples is investigated using carbon blacks of varying surface free energy, surface area and morphology. The surface free energies are varied via thermal treatment of the carbon blacks (graphitization). Reduction in surface free energy of the particulates accelerates the flocculation processes in the melt as measured by rheological experiments and reduces the percolation volume fraction threshold as determined by D/C conductivity measurements. The consequent effects on amplification of the small strain storage moduli of the crosslinked compounds are dramatic. Reduced polymer-filler interactions result in both an increased small strain modulus versus the unmodified carbon black-filled materials and an increased mechanical fragility of the fractal networks. Examination of the dynamic elastic moduli of crosslinked carbon black-filled samples loaded at volume fractions below the onset of network development reveal a significant temperature dependence isolated at the filler-rubber interface. [ABSTRACT FROM AUTHOR]

Small strain Young's moduli of natural rubber (NR)/organoclay nanocomposites were estimated using the Guth-Gold, Halpin-Tsai (HT), and Krieger-Dougherty (KD) models, and compared with experimental measurements of NR vulcanizates containing organo-montmorillonite (OM) or organo-sepiolite (OS). To account for the effect on modulus of the NR matrix of the vulcanization-active modifier in the organoclay, a matrix modulus correction (MMC) term was derived from the vulcanization parameters of the nanocomposites. The KD model gave a better empirical fit with the experimental data than the Guth-Gold model, with both giving good agreement with particle shape factors estimated from transmission electron microscope (TEM) images. The HT model gave the best fit with experiment for both types of nanocomposite, and use of the MMC term meant that the empirical shape factor was sufficiently close to that estimated from TEM images that the model could potentially be used to accurately predict the Young's moduli of NR/OM and NR/OS nanocomposites. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1621-1627, 2011 [ABSTRACT FROM AUTHOR]

The effect of silica nanofiller surface chemistry on compounded particle size and high strain particle dewetting in a semitransparent nanosilica-filled elastomer composite was determined using backscattered visible light and transmitted light, respectively. The integrated intensities of backscattered light from the samples were collected at various visible wavelengths for thin-film composites using ultraviolet-visible spectrometer with an integrating sphere. The data revealed strong Rayleigh-type scattering from compounded filler particles. Size information was extracted and found to broadly correlate with scanning electron microscopy image analysis of fracture surface. Incorporation of a siloxane surface treatment chemical during compounding resulted in a reduced average filler particle size in the cured composite. On extension of the samples, an optical transition was observed only in the filled composites. At high strains, the semi transparent samples displayed an abrupt drop in transparency becoming opaque. This was quantified using a simple light transmission-sample extension technique. Strain-induced crystallization was discounted as the cause for the transition by X-ray diffraction analysis. The onset yield stress for the optical transition was found to be filler surface-chemistry-dependent with the siloxane-treated filler exhibiting a greatly increased onset stress value. These observations were discussed and rationalized in terms of filler particle-matrix dewetting and cavitation at high strains. Matrix-filler dewetting was distinguished from matrix cavitation by comparison with Beer-Lambert behavior derived from unstrained samples. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011. [ABSTRACT FROM AUTHOR]

Elastomeric composites based on Ethylene-Propylene-Diene-Monomer (EPDM) filled with multi-wall carbon nanotubes (MWNTs) have been prepared, showing improved mechanical properties as compared to the pure EPDM matrix. The results have been discussed using the Guth model. The main focus of the study was on the electrical behavior of these conductive polymer composites (CPCs), in view of possible sensor applications. A linear relation has been found between conductivity and deformations up to 10% strain, which means that such materials could be used for applications such as strain or pressure sensors. Cyclic experiments were conducted to establish whether the linear relation was reversible, which is an important requirement for sensor materials. [ABSTRACT FROM AUTHOR]

This article discusses a study on the use of microstructural finite element models investigating the modulus of filled elastomer over a wider range of strains than has been possible previously. This study offered insight into rigid filler reinforcement and the filler interaction mechanisms. The study considered medium thermal (MT)-type carbon-black filled rubber behavior due to the large amount of experimental data available in the literature.

The introduction of dynamic bonds capable of mediating self-healing in a fully cross-linked polychloroprene network can only occur if the reversible moieties are carried by the cross-linker itself or within the main polymer backbone. Conventional cross-linking is not suitable for such a purpose. In the present work, a method to develop a self-healable and recyclable polychloroprene rubber is presented. Dynamic disulfide bonds are introduced as part of the structure of a crosslinker (liquid polysulfide polymer, Thiokol LP3) coupled to the polymer backbone via thermally initiated thiol-ene reaction. The curing and kinetic parameters were determined by isothermal differential scanning calorimetry and by moving die rheometer analysis; tensile testing was carried to compare the tensile strength of cured compound, healed compounds and recycled compounds, while chemical analysis was conducted by surface X-ray Photoelectron Spectroscopy. Three formulations with increasing concentrations of Thiokol LP-3 were studied (2, 4, 6 phr), reaching a maximum ultimate tensile strength of 22.4 MPa and ultimate tensile strain of 16.2 with 2 phr of Thiokol LP-3, 11.7 MPa and 10.7 strain with 4 phr and 5.6 MPa and 7.3 strain with 6 phr. The best healing efficiencies were obtained after 24 h of healing at 80 °C, increasing with the concentration of Thiokol LP-3, reaching maximum values of 4.5% 4.4% 13.4% with 2 phr, 4 phr and 6 phr, respectively, while the highest recycling efficiency was obtained with 4 phr of Thiokol LP-3, reaching 11.2%. [ABSTRACT FROM AUTHOR]

Modelling the viscoelastic behavior of rubber for use in component design remains a challenge. Most of the literature does not consider the typical regimes encountered by anti-vibration devices that are deformed to medium dynamic strains (0.5 to 3.5) at medium strain rates (0.5/s to 10/s). Previous studies have either focused on the behaviour at small strains and small strain rates or in fast loading conditions that result in low cycle fatigue or tearing phenomena. There is a lack of understanding of the dynamic response of natural rubber suspension components when used in real vehicle applications. This paper presents a review of popular viscoelastic nonlinear constitutive models and their ability to model the mechanical behaviour of typical elastomer materials such as Natural Rubber (NR) incorporating different PHR (Parts per Hundred Rubber, XX) of carbon black. The range of strain and strain rate are typical for the materials used in rubber suspensions when operating in severe service operating conditions, such as over rough terrain or over pot-holes. The cyclic strain is applied at different amplitudes and different strain rates in this medium strain range. Despite the availability of many models in the literature, our study reports that none of the existing models can fit the data satisfactorily over a wide range of conditions. [ABSTRACT FROM AUTHOR]