Thermoplastics hard-elastic

Example 5-23) and of ABS-polymers (made from acrylonitrile, butadiene, and styrene), whereby grafting occurs in situ at the beginning of the polymerization process. The formed graft copolymers act in two ways As emulsifiers during the polymerization process and, secondly, in the solid end product as compatibilizer between the thermoplastic hard phase and the rubber-elastic dipersed phase (already in concentrations below 3%). 

Rubberlike polymers include the thermoplastic elastomers (TPE) already mentioned in Chapter 1. They are mostly two-phase systems consisting of an elastic soft phase and a thermoplastic hard phase. The possible number of combinations is almost unlimited, which complicates their identification and nearly always necessitates expensive instrumental methods of analysis. In many cases the materials are block copolymers and, less frequently, blends. The following scheme provides an overview of the most important TPE types and can also be used as a guide for their qualitative analysis ... 

Figure 11-14. Schematic representation of the tensile stress aw as a function of strain e at constant temperature for an elastomer E, a partially crystalline thermoplast T, and a hard-elastic thermoplast HT. The ductile region is la-II-III. The necking effect shown below the diagram is typical of normal thermoplasts, but does not occur with elastomers or hard-elastic thermpolasts. The diagram is not drawn to scale for example, elastomers show a much larger elongation at break than do thermoplasts.

TPEs have a two-phase system, see Table 10, i.e., an elastic soft phase and a thermoplastic hard phase. They are produced as block copolymers or polyblends. 

Sequential coextrusion also offers the possibility of manufacturing a single part made with different regions of hardness, elasticity, strength, or heat deflection temperatures. This opens up a wide variety of applications. Sequential coextrusion can be used, for example, when the ends of a pipe must be flexible (for coimection purposes), but the middle section must be stiff and strong. Such a part, made out of thermoplastic elastomer (TPE), for example, can then be substituted for rubber parts. 

At present the most important resins of this group are the oil-soluble thermoplastics produced fi-oia forntaldehyde and substituted phenols, such as those containing phenyl, tertiary butyl, and tertiary amyl groups in the para position. These resins find use in arnishes and other coating compounds, where they are characterized by their hardness, elasticity, and resistance to exterior exposure. 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

The pseudocross-links, generated by the hard-segment interactions, are reversed by heating or dissolution. Without the domain crystallinity, thermoplastic polyurethanes would lack elastic character and be more gum-like in nature. In view of the outlined morphology, it is not surprising that many products develop their ultimate properties only on curing at elevated temperature, which allows the soft- and hard-phase segments to separate. 

One partieular form of thermoplastic polyurethane elastomers is the elastic fibre known as spandex fibre. Like the usual thermoplastic rubbers these materials consist of hard and soft segments but to qualify for the term spandex by the US Federal Trade Commission the polymer used should contain at least 85% of segmented polyurethane. The first commercial material of this type was introduced by Du Pont in 1958 (Lycra). Several other similar materials have since been introduced including Dorlastan (Bayer), Spanzelle (Courtaulds) and Vyrene (US Rubber). 

Thermoplastic, linear and saturated polyester give, depending on their chemical composition, hard or elastic and tacky bond lines they have relatively high melting viscosities bond lines are resistant against moisture, water and UV. 

Thermoplastic polyurethane (TPU) is a type of synthetic polymer that has properties between the characteristics of plastics and rubber. It belongs to the thermoplastic elastomer group. The typical procedure of vulcanization in rubber processing generally is not needed for TPU instead, the processing procedure for normal plastics is used. With a similar hardness to other elastomers, TPU has better elasticity, resistance to oil, and resistance to impact at low temperatures. TPU is a rapidly developing polymeric material. 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

Such soft-touch materials are usually TP Vs or thermoplastic elastomers (TPEs) which combine the moldability of thermoplastics in the melt state with elasticity, lower hardness, fracture resistance, and surface characteristics of elastomers. However, plastics and elastomers respond differently to mechanical stress. Hence, both rheological behavior and mechanical strength will to a large extent depend on the morphology of the blend which may change with change in the composition. 

Styrene-butadiene-styrene (SBS) block copolymers are adequate raw materials to produce thermoplastic mbbers (TRs). SBS contains butadiene—soft and elastic—and styrene— hard and tough—domains. Because the styrene domains act as cross-links, vulcanization is not necessary to provide dimensional stability. TRs generally contain polystyrene (to impart hardness), plasticizers, fillers, and antioxidants processing oils can also be added. Due to their nature, TR soles show low surface energy, and to reach proper adhesion a surface modification is always needed. 

The elasticity of thermoplastic polyurethane rubbers (which are also known as thermoplastic urethanes or TPUs) is a function of their morphology which comprises hard and soft phases. The hard phases consist of hydrogen bonded clusters of chain segments, which are linked by flexible chain segments that make up the soft phase. The hard blocks, which are the minor phase, exist as separate domains within a continuous matrix of the majority soft phase, as shown schematically in Fig. 25.9. 

Fillers are solid materials that are dispersed in plastics and elastomers. One distinguishes between inactive fillers that are used in the first place to make the plastics less expensive and active fillers (reinforcing fillers) that improve specific mechanical properties and thus effect a reinforcement . With the aid of these fillers, the elastic modulus, hardness, and thermostability are enhanced predominantly, whereas the impact strength of thermoplastic niaterials is re-... 

Unlike simple mixtures of polystyrene and polybutadiene such blends can be thermoplastically processed without phase separation ( splicing ) Furthermore, they can to a certain extent withstand mechanical impact without disintegration. This is because the above-mentioned graft polymers function also as compatibilizer at the borderline of the hard phase and the rubber-elastic dispersed phase (already at concentrations below 3%). 

The most suitable physical properties are likely to depend on the particular material, with plastics test methods being used for the harder elastomers (where the title elastomer may not even seem appropriate) and rubber methods for the less hard and more elastic materials. Where thermoplastic elastomers are to compete with conventional rubbers then clearly rubber test methods will be expected. On the other hand, where they are being compared to normal thermoplastics it would seem reasonable to use appropriate plastics test methods. 

Another example is even older the thermoplastic elastomers based on polyurethane their chains contain a number of blocks, altematingly hard and soft PU blocks, (elastic yarns, such as Lycra and Spandex ). 

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