Advances in Elastomers I

Elastomers are becoming an inevitable part of day to day life. The materials based on elastomers have tremendous applications in almost all areas of life. The present chapter deals with a brief account on various types of elastomers, elastomeric based blends and IPNs (interpenetrating networks). Various classes of elastomers, and blends are addressed by giving importance to the interfacial compatibility of different phases. Topics such as immiscible rubber blends, rubber/thermoplastic blends (micro and nano structured), rubber-thermoset blends (micro and nano structured), interphase modification and compatibilization, interpenetrating polymer networks, micro and nanofillers in rubbers have been very briefly discussed. Finally the applications, new challenges and opportunities of these elastomeric based blends and IPNs are also discussed.

Elastomers are unique to polymers and exhibit extraordinary reversible extension with low hysteresis and minimal permanent set. They are the ideal polymers relieved of molecular interactions, crystallinity and chain rigidity constraints. The common elastomers have characteristic low modulus, though with poor abrasion and chemical resistance. Theoretical concepts have been established for their thermodynamics and kinetics and this knowledge has been applied to extending their properties by design of chemical and molecular structures, or by modification by control of crosslinking, blending or additions of fillers. This chapter reviews elastomer theory and the demanding range of properties expected. Natural rubber is the starting material for introduction of chemistries that introduce damping, abrasion resistance and higher modulus through copolymerization and interacting functional groups. Heteroatoms such as fluorine, silicon, oxygen and nitrogen are shown to extend properties and give chemical resistance. Thermoplastic elastomers move beyond typical cured systems due to formation of two-phase block copolymers. Finally modification by filler and blended systems is considered, followed by introduction to shape memory materials and a brief comment on the future trends. The unique and diverse properties and performance of elastomers continues to be a fascinating field for science and application.

Elastomers are notable as very special class of polymers due to their multifunctional applications. The superior mechanical properties, high flexibility, resilience and good viscoelastic behaviour make this class applicable in a wide range of technology and industry. Depending on the various properties and general applications elastomers are classified in to a number of categories. This particular chapter deals with a very important class of special purpose elastomers. The synthesis, structure, different properties, mode of vulcanization, processing and applications of most of the synthetic elastomers are discussed. Apart from providing a basic understanding about the materials, this chapter can facilitate wide information about the technical details and industrial importance of this class of rubbers.

Compounding is a unique requirement of the rubber, generally not found in other material. The performance properties can be controlled by properly selecting and adjusting various compounding ingredients. The stages of rubber product manufacturing are broken down into three primary classes: selection of compounding ingredients, mixing or compounding, and vulcanization techniques or final product manufacturing process. The present chapter gives a brief introduction of the all classes with their importance. By proper selection of the variables in each class, the properties can be manipulated from virtually incompressible with a bulk modulus some thousand times greater than shear modulus, to large extent impermeable to gases and liquids and with excellent recovering and abrasion resistance etc.

Generally elastomer processing involves two major steps. First one is the designing of a mixing formulation for a specific end-use and the second one is the production process by which rubber compound is transformed into final product. When designing a mixing formulation the compounder must take account not only of those vulcanisate properties essential to satisfy service requirements but also cost of the raw materials and the production process. There should always be a compromise between cost of production and quality of the product. This chapter is an attempt to deal with different processing techniques normally used in the rubber industry.

Most polymer blends are thermodynamically immiscible, leading to a phase-segregated morphology. Control of this morphology, including the domain sizes and interfacial regions, along with partitioning of compounding ingredients such as filler and curatives between the phases, provides opportunities for achieving properties that are otherwise unattainable. This chapter reviews fundamental aspects of phase-separated rubber blends, with a survey of the important literature on the topic.

Research on recycling, scarcely visible only a few decades ago, is now a very active, fastgrowing discipline, particularly focusing on wastes re-use as second raw materials. This chapter presents an overview on the state-of- art in recycling, the most recent technologies, and recent developments. Rubber and PET are the most frequently recycled polymers, and are particularly addressed to within this chapter. Recent results are presented on rubber/thermoplastic-based micro/nano blends, along with their manufacturing and characterization methods. There are described methods to obtain the rubber-PET composites, based on ground discarded tires as a matrix composites, using as fillers plastic materials (PET, HDPE, and LDPE) and inorganic oxides (CaO, ZnO, and fly ash). Based on the structural and output properties and the chapter outlines the role of various components in the polymer composites. It is demonstrated that inorganic materials in the polymer composites allow obtaining performances unrecorded by pure polymer composites. However, the control of the inorganic material (type, quantity, particle size, and molecular structure) dispersed in polymeric matrix is essential in achieving the expected performance. Using different recipes, the composites can be tailored for various indoor and outdoor applications, as building materials as paving slabs, as thermal and electrical insulators, etc.

Highly cross-linked thermosets which are susceptible to brittle failure can be effectively toughened by blending them with rubbers. However, if the materials are already cross-linked, then blending with rubber as it is done in the conventional way with thermoplastics, is virtually impossible. Thus it asks for an altogether different method to accomplish successful blending. Initially, miscible liquid rubbers in small amounts or preformed rubber particles are incorporated in the matrix of curing agent incorporated precured thermosets resins and then the whole mass is subjected to curing. The phase separation, in case of liquid rubber toughening depends upon the formulation, processing and curing conditions and incomplete phase separation may occur resulting in unwanted lowering of glass transition temperature. The phase separation in case of liquid rubber is based upon nucleation and growth. In case of preformed rubber particles, these difficulties are not encountered and the resulting morphology can be better controlled. However, the problem of proper dispersion of these particles in the themoset resins limits the use of this method. The improvement in fracture resistance occurs in either case due to dissipation of mechanical energy by cavitation of rubber particles followed by shear yielding of the matrix. Rubber particle size plays an important role in improving toughening and very small or very large sizes are undesirable. The toughenability increases with increase in inherent ductility of the matrix.

Blending of two or more elastomers is carried out for several purposes. The properties of an elastomer blend depend strongly on its state of compatibility and miscibility. In this chapter, recent advances on development of interphase modification and compatibilization of rubber-based blends are summarized. Current trends in compatibilization of rubber/rubber blends, TPEs and other rubber/thermoplastic blends, natural polymer blends, rubber-based blends with and without filler modification are discussed in detail. Finally, new challenges and opportunities of rubber-based blends are given.

Interpenetrating polymer networks (IPNs) are defined as combination of two or more polymers in network form with at least one of which is polymerised and/or crosslinked in the immediate presence of the others. IPNs are based on combinations of two or more polymers and are younger cousins to polymer blends, blocks and grafts. All these are members of a larger class of multicomponent polymeric systems, where as in IPNs, the polymers are crosslinked, thus providing a mechanism for controlling the domain sizes and reducing creep and flow. Though the idea behind IPN synthesis is to effect molecular level interpenetration of the polymer networks, most IPNs form immiscible systems with phase separation during some stage of synthesis. Aylsworth, in 1914 invented the first known IPN, but the term IPN was coined much later in 1960, by Millar who developed PS/PS IPNs to be used as ion exchange resin matrices (Aylsworth, US Patent 1, 111, 284, 1914), (Millar, J. Chem. Soc. 1311, 1960). The literature review shows that Sperling and coworkers at Lehigh university, USA followed by Frisch from University of Detroit and Frisch from Suny, Albany have made the most contributions to this research area. The current review on IPNs summarises the processing, properties and applications of IPNs, with special focus on some recent developments and trends.

As a most general definition, filler is a finely divided solid that is used to modify the properties of a material in which it is dispersed. From the inception of the rubber industry, fillers have a crucial role in either providing durability and performance or in reducing the price by decreasing the rubber partition in the compound. The fillers used in rubbers can be divided into two main groups such as black and non-black fillers. Besides the conventional micron size fillers, nanofillers recently have gained both academic and industrial importance. In this chapter, it is aimed to introduce the fillers used in rubbers. Their characteristics and their impact on properties of rubbers are discussed by giving examples from the recently published literature. In addition to the conventional ones, the new emerging nano-fillers and their added value to the rubbers are given in detail by highlighting some selected studies.

Magnetorheological elastomers (MRE) are smart materials whose modulus or mechanical performances can be controlled by an external magnetic field. In this chapter, the current research on the MRE materials fabrication, performance characterisation, modelling and applications is reviewed and discussed. Either anistropic or isotropic or MRE materials are fabricated by different curing conditions where magnetic field is applied or not. Anistropic MREs exhibit higher MR effects than isotropic MREs. Both steady-state and dynamic performances were studied through both experimental and theoretical approaches. The modelling approaches were developed to predict mechanical performances of MREs with both simple and complex structures. The sensing capabilities of MREs under different loading conditions were also investigated. The review also includes recent representative MRE applications such as adaptive tuned vibration absorbers and novel force sensors.

The chapter provides introduction to radiation processing of solid state materials, using commercially available sources of ionizing radiation, i.e., radio-isotopic and/or accelerated electron beam installations. Dosimetry is described as the method of controlling progress of changes in irradiated material. Distribution of doses in irradiated material is described, allowing proper processing of polymers. Basics of radiation chemistry of polymers is explained, in particular of elastomers. Radiation-induced crosslinking is most interesting reaction, but it can be accompanied by undesired phenomena like chain scission. Specific phenomena like energy transfer occur in radiation processing; therefore, composites of elastomers with components of different radiation characteristics may show unexpected results. Examples of selected cases are described in details. Comparisons between traditional methods of crosslinking with these using ionizing radiation allow consideration of introduction of the latter into industrial praxis.

A continuum finite-deformation model is described for the study of the isothermal electro-elastic deformations of electrostrictive elastomers. The model comprises general balance equations of motion, electrostatics and electro-mechanical energy, along with phenomenological invariant-based constitutive relations. The model is presented in both Eulerian (spatial) and Lagrangian (material) description. Specialization of the considered model is also presented for “Dielectric Elastomers”, which are a specific class of electrostrictive elastomers having dielectric properties independent of deformation.