Debdipta Basu

Ionic Modification Turns Commercial Rubber into a Self-Healing Material

Invented by Charles Goodyear, chemical cross-linking of rubbers by sulfur vulcanization is the only method by which modern automobile tires are manufactured. The formation of these cross-linked network structures leads to highly elastic properties, which substantially reduces the viscous properties of these materials. Here, we describe a simple approach to converting commercially available and widely used bromobutyl rubber (BIIR) into a highly elastic material with extraordinary self-healing properties without using conventional cross-linking or vulcanising agents. Transformation of the bromine functionalities of BIIR into ionic imidazolium bromide groups results in the formation of reversible ionic associates that exhibit physical cross-linking ability. The reversibility of the ionic association facilitates the healing processes by temperature- or stress-induced rearrangements, thereby enabling a fully cut sample to retain its original properties after application of the self-healing process. Other mechanical properties, such as the elastic modulus, tensile strength, ductility, and hysteresis loss, were found to be superior to those of conventionally sulfur-cured BIIR. This simple and easy approach to preparing a commercial rubber with self-healing properties offers unique development opportunities in the field of highly engineered materials, such as tires, for which safety, performance, and longer fatigue life are crucial factors.

Rubber composites based on silane-treated stöber silica and nitrile rubber Interaction of treated silica with rubber matrix

Role of silane-treated stöber silica as reinforcing filler for nitrile rubber (NBR) has been studied. Stöber silica is synthesized by sol–gel method, and the surface of silica is modified with the treatment of silane-coupling agent viz. γ-mercaptopropyltrimethoxysilane (γ-MPS) in varying proportions. Average particle size of stöber silica of spherical shape in the range of 200 to 400 nm is evident from scanning electron microscopy (SEM). Surface modification of silica particle with silane-coupling agents decreases surface energy and reduces agglomeration of silica particles in rubber matrix. Stress–strain study and dynamic mechanical analysis of silica-filled composites are compared with the unfilled ones. Analysis of cross-linking density, mechanical properties, and storage moduli indicates a strong rubber–filler interaction in the silane-treated, silica-filled NBR composites. Silane treatment is found to be effective in uniform dispersion of silica in rubber matrix and in improving the mechanical properties of rubber composite. Different functionalities of organosilane at its both end improve the compatibility of silica with rubber matrix and offer better rubber–filler interaction.

Controlled growth of in situ silica in a NR/CR blend by a solution sol–gel method and the studies of its composite properties

Silica is grown in situ into a natural rubber (NR)/chloroprene rubber (CR) blend (at 40/60 ratio), by a solution sol–gel method, where the silica content in rubber blend is increased in a controlled manner exceeding the limit found for the same blend ratio in the soaking sol–gel method. Reaction conditions have been optimized to get adequate conversion of tetraethoxysilane (TEOS, a silica precursor) to silica. Rheological, thermal, mechanical and viscoelastic properties of all the composites are compared with those of the unfilled rubber blend at similar conditions. Thermal and mechanical properties of the composites are found to improve consistently as silica content in the composite increases owing to increased rubber–filler interaction as revealed in dynamical mechanical analysis (DMA). Further improvement in the properties is observed for a particular composite where a silane coupling agent ((γ-aminopropyl)trimethoxysilane, γ-APS) is used in the reactive sol–gel system during in situ generation of silica. This is attributed to the uniform distribution of silica in the rubber matrix and strong rubber–filler interaction, caused by bifunctionality of silane, as revealed by morphology and DMA studies respectively. The reinforcement effect of silica is evaluated by comparing the experimental results with theoretical values obtained from the Guth–Gold model and the modified Guth model. The present study supplements the in situ silica generation in NR/CR blend of 40/60 ratio, following the solution sol–gel method, to the earlier study involving the soaking sol–gel method where the maximum reinforcement was found for this composition.

Fire-safe and environmentally friendly nanocomposites based on layered double hydroxides and ethylene propylene diene elastomer

In this work we describe layered double hydroxide (LDH), known as naturally occurring hydrotalcite, based rubber composites that can serve as outstanding fire retardant elastomeric materials. The preparation and detailed characterization of these composites are presented in this study. The inherent slow sulfur cure nature of EPDM rubber is considerably improved by the addition of LDH as realised by the observation of a shortening of the vulcanization time and an improvement of ultimate rheometric torque. This behavior of LDH signifies not only the filler-like character of itself, but also offers vulcanization active surface properties of layered double hydroxide particles. A good rubber–filler interaction was also realised by observing a positive shift of the glass transition temperature of ethylene propylene diene rubber (EPDM) in dynamic mechanical analysis (DMA). The flame retardant property was studied by the cone calorimeter test. The cone calorimeter investigation with sulfur cured gum rubber compounds found a peak heat release rate (PHRR) value of 654 kW m−2. However, at a higher phr loading of Zn–Al LDH i.e., at 40 phr and 100 phr, the PHRR is diminished to 311 kW m−2 and 161 kW m−2, respectively. Thus, this present work can pave the way to fabricate environmentally friendly fire retardant elastomeric composites for various applications.

Stearate Modified Zinc-Aluminum Layered Double Hydroxides and Acrylonitrile Butadiene Rubber Nanocomposites

The aim of this investigation is to highlight the potentials of layered double hydroxides (LDH) and to serve as a replacement for zinc oxide and stearic acid from the basic rubber formulation. This will eventually result in about a 10× significant reduction of Zn2+ ion concentration in the final compound. The unique advantage of stearate ion-modified LDH is the delivery of zinc ions to accelerate and stearate ions to activate the vulcanization process. Furthermore, it can also reinforce the rubber matrix by virtue of its layered structure as nanofiller.

Nanostructured Ionomeric Elastomers

Driven by the desire to find an alternative way of vulcanizing elastomers without sulfur, researchers have widely explored ionic crosslinking techniques. The opportunity was taken to play with the functionality of the host polymer and its modification process to develop nanostructured ionic elastomers. Neutralization of polar elastomers by various divalent metal cations has been the route most employed for fabrication of this class of material. Ionic association or aggregation on the molecular level results in microphase separation of certain regions and, hence, enables easier processing. Thermally labile ionic domains introduced into the network make the entire material thermoresponsive and, therefore, it is possible to obtain reversible transition of dynamic mechanical properties. The unique network structure of these materials has led to outstanding physical properties that have not been achieved so far for conventional sulfidic networks. Consequently, many multifunctional and smart materials have been envisaged and designed using these systems. A detailed overview is provided on the various nanostructured ionic elastomers developed over the years. It would not be exaggerating to mention in the context of the discussion that nanostructured ionic elastomers will definitely open up new horizons in materials research.