Elastomers
Characteristic of elastomers is that they have a glass temperature below ambient temperature; however, in contrast to thermoplastics, the most obvious property of cross-linked elastomers is their almost completely reversible elastic behavior. Some cross-l
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Elastomers 18.1 Permanently Cross-Linked Elastomers – 478 18.2 Properties – 479 18.3 Rubber Elasticity – 482 18.4 Elastomer Additives – 484 18.5 Processing – 485 18.6 Technically Important Elastomer Types – 486 18.7 Thermoplastic Elastomers – 487 18.8 Polyether Amide-12 Elastomers – 488 18.9 Liquid Crystalline Elastomers – 490
© Springer-Verlag Berlin Heidelberg 2017 S. Koltzenburg et al., Polymer Chemistry, DOI 10.1007/978-3-662-49279-6_18
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Chapter 18 · Elastomers
Characteristic of elastomers is that they have a glass temperature below ambient temperature; however, in contrast to thermoplastics, the most obvious property of cross- linked elastomers is their almost completely reversible elastic behavior. Some cross-linked elastomers can be stretched to up to seven times their original length and still return almost immediately to their original length. On account of this, this chapter is dedicated to this class of materials. On a molecular level, this reversible elasticity requires that the polymer chains in these materials have a high degree of mobility. Elastomers are therefore exclusively amorphous materials with low glass transition temperatures that lie below their service temperature range. To prevent the material flowing at these temperatures, the polymer chains need to be cross-linked. However, the cross-linking density must not be so high that the mobility of the chains becomes severely restricted. Elastomers are therefore wide-meshed, crosslinked macromolecular materials and can be distinguished from thermoplastics, which are linear or branched, and duromers, which are densely cross-linked. Elastomers are not fusible because of their cross-links and, if exposed to excess thermal stress, they decompose without melting. 18.1
Permanently Cross-Linked Elastomers
As is well known, polymers can be divided into different groups according to their architecture as linear, branched, or cross-linked structures (. Fig. 18.1). In addition, linear polymers, whose chains are longer than a certain (critical) length, tend to form entanglements. Permanently cross-linked elastomers can be obtained by irreversibly chemically linking neighboring chains to one another. A wide-meshed, cross-linked elastomer is also referred to as a rubber. It is important to distinguish between vulcanized (cross-linked) and non-cross-linked (unvulcanized) elastomers. The naming of elastomers distinguishes the various chemical structure of the main chain. An elastomer with a C=C double bond along the main chain is an R-elastomer, whereas an M-elastomer has a main chain of methylene units. Elastomers having both C and O atoms along the main chain are designated as O-elastomers and those having a siloxane main chain as Q-elastomers. T-elastomers are those with sulfur and carbon in the main chain and U-elastomers those with a carbon, oxygen, and nitrogen backbone. Typical examples of these different kinds of elastomers are shown in . Table 18.1. Some commercial products are listed in . Table 18.2.
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Branched
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