Elastomers, thermoplastic general characteristics

Compounded polymers prepared with thermoplastic elastomers (TPE) are prepared in much the same manner as thermoplastics, with some exceptions. Thermoplastic elastomers generally combine the flexibility and frictional behavior of mbber compounds with the practical forming considerations of thermoplastic materials. These are commonly encountered in automotive and appliance applications where non-slip surfaces are desirable. Since compounds in this class have elastomeric physical characteristics, this often requires that a twin-screw mixer makes use of... 

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. 

Reversible network structure is the single most important characteristic of a thermoplastic elastomer. This novel property generally arises from the presence of a phase-separated morphology in the bulk material which in turn is dictated by the molecular structure, often of a block copolymer nature. A wide variety of synthetic methods can, in principle, produce endless varieties of thermoplastic elastomers this fact coupled with the advantageous processing characteristics of these materials suggest that the use of thermoplastic elastomers will continue to grow in the 1980 s. 

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

PROPERTIES OF SPECIAL INTEREST In general, thermoplastic elastomers (TPE) provide the mechanical properties of rubber in combination with the processing characteristics of plastics recyclable Kraton D s are the lowest cost TPE (> 0.85 Ib ). 

The ABA triblocks which have been most exploited commercially are of the styrene-diene-styrene type, prepared by sequential polymerization initiated by alkyllithium compounds as shown in Eqs. (99-101) [263, 286]. The behavior of these block copolymers illustrates the special characteristics of block (and graft) copolymers, which are based on the general incompatibility of the different blocks [287]. Thus for a typical thermoplastic elastomer (263), the polystyrene end blocks (-15,000-20,000 MW) aggregate into a separate phase, which forms a microdispersion within the matrix composed of the polydiene chains (50,000-70,000 MW) [288-290]. A schematic representation of this morphology is shown in Fig. 3. This phase separation, which occurs in the melt (or swollen) state, results, at ambient temperatures, in a network of... 

The thermoplastic elastomer polyurethanes, TPU, may be of two general types partly crystalline elastic fibers (see preceding paragraph) or softer elastomers, depending on the relative length of the soft segment. Those polyurethanes based on polyester soft segments tend to be more resistant to hydrocarbons, while the polyether types are more resistant to hydrolysis but tend to swell more in aqueous environments (144). The block copolymer characteristics of polyurethanes are discussed further in Chapter 13. 

Polyester (PE) elastomer combines characteristics of thermoplastics and elastomers. They have excellent resistance to ozone and good resistance to general weathering. However, the polyester rubbers do not have satisfactory resistance to sunlight-induced aging unless they are formulated with appropriate additives. 

Chemical composition, type of the resin (thermoplastic, elastomer, thermoset, crystalline, amorphous, etc.), morphology, history (discovery, commercialization dates and names), general characteristics, etc.). 

The primary forming of plastics is generally a flow process. The individual macromolecules of thermoplastics, duroplastics, and elastomers must be mobile and capable of chain slipping. The duroplastics and elastomers do not crosslink or vulcanize until after forming, when they develop their characteristic crosslinked structure. 

Incorporation of chlorine atoms onto the polyolefin backbone then causes sufficient molecular irregularity to break up crystalline chain segments of the base resin. As the chlorine content is increased, the crystallites gradually disappear and, eventually, the thermoplastic material becomes amorphous and behaves as an elastomer because of the inherent flexibility of the polyethylene chain. Chlorosulfonated polyethylene resins made in slurry or fluidized beds generally have a more blocky chlorine distribution, both intramolecularly and intermolecularly, so that the same degree of amorphous characteristic is not always achieved. The increase in molar cohesion, by the addition of chlorine atoms, increases the polymer solubility parameter, and thus decreases its miscibility with paraffinic and aromatic oils. So, as chlorine content of the polymer increases, resistance to swelling effect of oil increases. 

There is a large literature on the influence of additives on particle-filled elastomers and thermoplastics. Many particle-additive systems are marketed to produce polymer compounds containing small particles, which have strong interparticle forces. Small molecule additives are used in compounds with very small polar particles. Larger particles (i.e., particles greater in size than 5 pm) generally do not require associated additives because their flow behavior is dominated by hydrodynamic factors as described in Section 2.3. Some small nonpolar particles do not have suitable additives. Carbon black has relatively weak interparticle forces (Section 2.4.3), and additives have not been found to significantly modify the flow and mechanical characteristics of its compounds. 

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