Polymer processing thermoplastic elastomers

Keywords Dynamic vulcanization Polymer blends and alloys Reactive processing Thermoplastic elastomers Thermoplastic vulcanizates... 

Styrene-butadiene block copolymers belong to a new class of polymers called thermoplastic elastomers (TPE). Products made from these polymers have properties similar to those of vulcanized mbbers, but they are made from equipment used for fabricating thermoplastic polymers. Vulcanization is a slow and energy-intensive thermosetting process. In contrast, the processing of thermoplastic elastomers is rapid and involves cooling the melt into a rubberlike solid. In addition, like true thermoplastics, scrap from TPE can be recycled. 

In the last few decades polymers have attracted attention for their cost-effective, flexible solutions for different applications because of their high performance properties, light weight, and process ability in different industries. Polymers are commonly classified into three groups thermoplastics (meltable plastic), thermosets (crosslinked) and elastomers (often thermoset polymers), and thermoplastic elastomers (TPEs, which can be melted). 

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

As with all thermoplastic elastomers, the copolyesterethers can be processed as thermoplastics. They are linear polymers and contain no chemical cross-links, thus the vulcanisation step needed for thermosetting elastomers is eliminated and scrap elastomer can be re-used in the same process as virgin material (176—180). 

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

The classification given in Table 1 is based on the process, ie, thermosetting or thermoplastic, by which polymers in general are formed into usehil articles and on the mechanical properties, ie, rigid, flexible, or mbbery, of the final product. AH commercial polymers used for molding, extmsion, etc, fit into one of these six classifications the thermoplastic elastomers are the newest. 

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will eause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

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. 

Thermoplastic elastomers (TPES), as the name indicates, are plastic polymers with the physical properties of rubbers. They are soft, flexible, and possess the resilience needed of rubbers. However, they are processed like thermoplastics by extrusion and injection molding. 

Polymer structure and formulation. As an example, Woo et al. [7] measured OIT values for series of commercial PVC resins and polyester thermoplastic elastomers (TPEs). The researchers used the ASTM D3895-80 procedure, but substituted air as the oxidising gas instead of pure oxygen. A dependency on thermal processing history of the TPE film samples appeared to influence the measured OIT in the PVC study, chemically different chain ends affected polymer stability and hence OIT values. 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

The styrenic thermoplastic elastomers are the only type which are fully compounded in the manner of conventional elastomers. In this case, however, the addition of carbon black, or other fillers, does not give reinforcement. Additions of polystyrene, or high impact polystyrene, and oil are used to vary hardness and tear strength, and fillers can be used to cheapen the material. Other added polymers, e g., EVA, can be used to increase ozone resistance. These materials also require antioxidants for protection during processing and service life, and the poor UV stability restricts their use in outdoor applications. 

---