Subject thermoplastic elastomers

N. R. Legge, G. Holden, and H. E. Schroeder, eds.. Thermoplastic Elastomers—H Comprehensive Keview Cad Hanser Vedag and Oxford University Press, Munich, New York, 1987. This reference and the next are recommended for readers who wish to obtain more detailed information on the subject. 

All three types of material have now been available for some years and it is probably also true that none have yet realised their early promise. In the case of the thermoplastic elastomers most of the commercial materials have received brief mention in earlier chapters, and when preparing earlier editions of this book the author was of the opinion that such materials were more correctly the subject of a book on rubbery materials. However, not only are these materials processed on more or less standard thermoplastics processing equipment, but they have also become established in applications more in competition with conventional thermoplastics rather than with rubbers. 

A manufacturer considering using a thermoplastic elastomer would probably first consider one of the thermoplastic polyolefin rubbers or TPOs, since these tend to have the lowest raw polymer price. These are mainly based on blends of polypropylene and an ethylene-propylene rubber (either EPM or EPDM) although some of the polypropylene may be replaeed by polyethylene. A wide range of blends are possible which may also contain some filler, oil and flame retardant in addition to the polymers. The blends are usually subject to dynamic vulcanisation as described in Section 11.9.1. 

Shape Change of Structural Entities. In many cases the growing anisotropy is not only a phenomenon of rotating structural entities, but also goes along with a deformation of the structural entities themselves. This case will be studied here. Only affine deformations shall be discussed. In practice, such processes are observed while thermoplastic elastomers are subjected to mechanical load, but also while fibers are spun. 

The dynamic melt viscosity measurements of select star blocks and a similar triblock were carried out on a rheometric mechanical spectrometer, RMS. Circular molded samples of 2 cm diameter and -1.5 mm thickness were subjected to forced sinusoidal oscillations. Dynamic viscosities were recorded in the frequency range of 0.01-100 rad/s at 180 °C. Figure 10 shows the complex viscosities of two select star blocks and a similar linear triblock. The plots showed characteristic behavior of thermoplastic elastomers, i.e., absence of Newtonian behavior even in the low frequency region. The complex viscosity of the star block... 

Boltzmann and cell dynamics simulations. Non-linear flows are experienced when processing block copolymer thermoplastic elastomers, and this subject is thus likely to attract industrial and academic interest. Molecular models will provide insight into the viscoelasticity at higher frequencies. 

Subcontractors to the auto industry are using considerable quantities of one-package PU enamels to finish plastic parts, for example, soft EPDM and thermoplastic elastomer front fascia and RIM and RRIM parts. These subcontractor operations are not yet subject to the stringent emission controls mandated by the EPA on auto assembly lines. 

Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO-1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1. 

The EP thermoplastic elastomers are distinguished from the crossUnked analogues, which are not thermoplastics since reforming is impossible. A very important thermoplastic elastomer is comprised of a blend of an EP copolymer with an ethylene-propylene-diene (EPDM) terpolymer. This latter material is, of course, a crosslinkable thermoset however, these materials can be processed as thermoplastics if the crosslinkable component is present at low enough concentration to be present as an isolated phase. Melt-processing causes the formation of chemical bonds within the isolated rubber phase, a process called dynamic vulcanization. A commercial example of this type of material is Santoprene [4] manufactured by Advanced Elastomer Systems. Other blends of noncrosslinkable TPEs with crosslinkable materials are used commercially. These materials are classified as elastomer blends and are the subject of Chapter 12. 

During processing block copolymers are subjected to flow. Eor example thermoplastic elastomers formed by po/y(styrene-butadiene-styrene) (PS-PB-PS) triblock copolymers, are moulded by extrusion. This leads to alignment... 

The materials were subject to a series of cyclic uniaxial tensile tests at room temperature and ambient humidity, designed to characterize features of their constitutive response relevant to their performance as thermoplastic elastomers, especially focusing on their stiffness and their deviations from purely elastic behaviour. 

Block polymers containing polydimethylsiloxane soft blocks have been the subject of considerable recent synthetic activity. In particular, polydimethylsiloxane-b-polystyrene polymers have received considerable attention as thermoplastic elastomers. For the most part, anionic polymerization methods have provided the most successful routes to the preparation of these block polymers. Among the most notable papers in this field are those of Dean, Saam et al, Juliano, and Bajaj and coworkers. Recently Chaumont and his coworkers have prepared polydimethylsiloxane-b-polystyrene polymers by the platinum catalyzed condensation polymerization of a,o)-vinyl terminated polystyrene oligomers with a,o)-hydrogen terminated polydimethyl-siloxanes. 

When products are subjected to dynamic loads where energy and motion controls are required use is made of thermoplastic elastomer (TPE) components. These products involve buildings (Fig. 2.11), bridges, highways, sporting goods, home appliances, automobiles, boats, aircraft, and spacecraft. 

TPE-E (polyester-based thermoplastic elastomers) are subject to UV-llght degradation in outdoor weathering and therefore have to be stabiiized for extended solar exposure. Carbon black provides the best protection. Without suitable stabilization, embrittiement as weii as gioss ioss occur under extended irradiation by UV iight [83]. 

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