Thermoset elastomers deformation

Elastomers constitute a third class of polymers. Similarly to thermosets, elastomers have a network structure formed by crosslinking between the polymer chains. However, the number of links is less than in the case of thermosets which gives these materials elastic properties. Elastomers can be deformed by the application of external forces. When these forces are suppressed, the polymer recovers its original form. From a commercial point... 

Load Fracture. Thermoplastics, thermosets, and elastomers deform in different ways. In the case of ductile thermoplastics, as the load increases, the tangled, thread-like macromolecules become stretched, aligned, and in some cases will glide apart. The stretched areas will recoil like a spring, and this will be most pronounced at the highly stressed fracture site, which additionally warms up on... 

Dynamic mechanical analysis techniques permit measurement of the ability of materials to store and dissipate mechanical energy during deformation. DMA is used to determine the modulus, glass transition, mechanical damping and impact resistance, etc., of thermoplastics, thermosets, elastomers and other polymer materials. Information regarding the phase separation of polymers is also available by DMA [2]. In DMA, viscoelastic materials are deformed in a sinusoidal, low strain displacement and their responses are measured. Elastic modulus and energy dissipation are the measured properties. 

In most polymeric as well as nonpolymeric amorphous materials, the ability to undergo large-scale molecular motions imphes the fi-eedom to flow, so that the material becomes a fluid above Tg. However, in the special class of polymers commonly described as thermosets, covalent cross-links limit the ability to undergo large-scale deformation. Consequently, above Tg, the rigid thermosets become thermoset elastomers. Thermoset elastomers are very often referred to as cross-linked rubbers or more simply as rubbers for historical reasons. 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

Currently, thermoplastics account for less than 5% of the elastomeric closures for parenterals. Their limited resistance to heat deformation imder stress during autoclave sterilization is the main reason for this limited use. However, thermoplastics have two advantages over thermosets. First, they are chemically less complex and therefore less prone to interact with parenteral medications, and second, they may be manufactured by a simpler and more automated process. Thermoplastic elastomers have found use in baby bottle nipples and dropper bulbs that are not typically heat sterilized under compression. 

Polymers that, in contrast to thermosets, have a macromolecule structure with wide-meshed crosslinks are called elastomers. Their characteristic property is their not being flowable up to the temperature range of chemical decomposition, but they are rubbery-elastic and reversibly deformable, to a large extent independent of temperature (e.g., rubber products). 

It will be shown in Chapter 11 that the correlations developed in this monograph can be combined with other correlations that are found in the literature (preferably with the equations developed by Seitz in the case of thermoplastics, and with the equations of rubber elasticity theory with finite chain extensibility for elastomers), to predict many of the key mechanical properties of polymers. These properties include the elastic (bulk, shear and tensile) moduli as well as the shear yield stress and the brittle fracture stress. In addition, new correlations in terms of connectivity indices will be developed for the molar Rao function and the molar Hartmann function whose importance in our opinion is more of a historical nature. A large amount of the most reliable literature data on the mechanical properties of polymers will also be listed. The observed trends for the mechanical properties of thermosets will also be discussed. Finally, the important and challenging topic of the durability of polymers under mechanical deformation will be addressed, to review the state-of-the-art in this area where the existing modeling tools are of a correlative (rather than truly predictive) nature at this time. 

Keywords Additives, Compounding, Deformation of melts, Extrusion foaming. Flow properties, Influences of processing on properties. Injection molding. Plastic melts, Processing of elastomers and thermosets. Processing of fiber composites. Processing of thermoplastics... 

Elastomers are also sometimes known as rubbers. They are also irreversibly cross-linked by covalent bonds. The elastomer raw materials (rubber base) generally have higher molar masses than the thermoset raw materials. In addition, the cross-linked network produced by cross-linking (vulcanization, hardening) is not so densely cross-linked as in the case of the thermosets. For this reason, elastomers have a high segmental mobility above the glass transition temperature. They deform readily under stress above this temperature, but because of their cross-linked structure, they rapidly return to their initial state when the stress is released because the initial state corresponds to the state of maximum conformational entropy. 

There are three kinds of polymers used for making foams thermoplasties, thermosets and elastomers. Elastomers are a special kind of polymers eom-posed of linear chains with a few widely spaced crossUnks attaching each molecule to its neighbours. The main difference with respeet to thermoplasties and thermosets is their Tg, whieh is well below room temperature (—60 Due to the presenee of the spaced crosslinks, elastomers ean undergo deformations of up to 500%. However, when unloaded, the material returns to its original shape. ... 

A thermoplastic elastomer (TPE) is a rubbery material with properties and functional performance very similar to those of a conventional thermoset rubber, yet it can be fabricated in the molten state as a thermoplastic. ASTM D 1566 defines TPEs as a diverse family of rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and recycled like thermoplastic materials. Many TPEs meet the standard ASTM definition of a rubber, since they recover quickly and forcibly from large deformations, they can be elongated by more than 100 percent, their tension set is less than 50 percent, and they are sometimes insoluble in boiling organic solvents. Figure 4.35 indicates hardness ranges for various types of TPEs and conventional elastomers. 

Cross-linked polymeric liquid crystals offer a wide variety of unique and in-tere.sting properties. Because of the interaction between the mesogens and the network backbone in liquid crystal elastomers, mechanical deformations can align the director, and these materials are piezoelectric. Industrial applications of liquid crystalline thermosets are driven by additional properties such as toughness, a tunable coefficient of thermal expansion, ferroelectricity, and nonlinear optical properties. Reviews on this topic are given by Barclay and Ober [4] and by Warner and Terentjev [5]. 

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