Thermoplastic Elastomers TPEs

For a long time elastomer materials were limited to thermoset type materials with permanent crosslinks, such as polyisoprene (natural rubber). However one major drawback with these materials was that they could not be recycled, which led to the development of thermoplastic elastomers (TPEs). One common example is styrene-butadiene-styrene (SBS). Since styrene and butadiene do not mix, the styrene breaks up when the SBS is heated allowing the material to be re-melted. TPEs offer considerable advantages over thermoset type elastomers  

The market for TPEs has also benefited from advanced processing techniques such as over-moulding (see Chapter 10). The ability to use TPEs either alone or in combination with other materials has enabled them to be used in applications such as 

The properties of thermoplastic elastomers that have made them so commercially successful are their lower modulus and flexibility. The ability to recover from stress and return to their original shape make them suitable for applications such as sealing rings etc. Of course thermosetting rubbers materials have long been available and it is these materials that TPES are replacing. 

Whilst newer material types have emerged in recent years, primary TPE types can be categorized into two generic classes, block copolymers (styrenics, copolyesters, polyurethanes and polyamides) or thermoplastic/elastomer blends and alloys (thermoplastic polyolefins and thermoplastic vulcanisates). These TPE types are known as two-phase systems as essentially, a hard thermoplastic phase is coupled mechanically or chemically with a soft elastomer phase. The result is a TPE that has the combined properties of the two phases. 

Thermoplastic elastomers usage has increased significantly in recent years and is expected to continue to rise. Worldwide, consumption was estimated at 1,400,000 metric tonnes/year in 2000. Some examples of common thermoplastic elastomers are shown in Table 2.5. As well as adhesion considerations, the suitability of a TPE will also depend on properties such as its hardness and compression ratio. Hardness can be defined as the resistance of the material to indentation and is usually measured on a durometer using a Shore Hardness scale. TPEs tend to be rated on a Shore A scale, the softest materials ranging from around 3 Shore A and the hardest to 95 Shore A. Out of interest, thermoplastic materials are measured on a different. Shore D scale. This gives an indicator of the differences in properties. TPE material grades are available with a range of hardness levels, however it must be considered that hardness is also a function of the thickness of the material and the substrate 

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Gun Propellents. Low sensitivity gun propeUants, often referred to as LOVA (low vulnerabUity ammunition), use RDX or HMX as the principal energy components, and desensitizing binders such as ceUulose acetate butyrate or thermoplastic elastomers (TPE) including poly acetal—polyurethane block copolymers, polystyrene—polyacrjiate copolymers, and glycidyl azide polymers (GAP) to provide the required mechanical... 

This unusual behavior results from unsolvated crystalline regions in the PVC that act as physical cross-links. These allow the PVC to accept large amounts of solvent (plasticizers) in the amorphous regions, lowering its T to well below room temperature, thus making it mbbery. PVC was, as a result, the first thermoplastic elastomer (TPE). This mbber-like material has stable properties over a wide temperature range (32,138—140). 

Block copolymers have become commercially valuable commodities because of their unique stmcture—property relationships. They are best described in terms of their appHcations such as thermoplastic elastomers (TPE), elastomeric fibers, toughened thermoplastic resins, compatibilizers, surfactants, and adhesives (see Elastot rs, synthetic—thermoplastic). 

The particular type of thermoplastic elastomer (TPE) shown in Figure 3 exhibits excellent tensile strength of 20 MPa (2900 psi) and elongation at break of 800—900%, but high compression set because of distortion of the polystyrene domains under stress. These TPEs are generally transparent because of the small size of the polystyrene domains, but can be colored or pigmented with various fillers. As expected, this type of thermoplastic elastomer is not suitable for use at elevated temperatures (>60° C) or in a solvent environment. Since the advent of these styrenic thermoplastic elastomers, there has been a rapid development of TPEs based on other molecular stmctures, with a view to extending their use to more severe temperature and solvent environments. 

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. 

Although rubber originally meant a natural thermoset material obtained from a rubber tree, with the development of plastics it identifies a thermoset elastomer (TSE) or thermoplastic elastomer (TPE) material. Different properties identify the elastomers such as strength and stiffness, abrasion resistance, solvent resistance, shock and... 

FIGURE S.1 Chemical structure of block copolymeric thermoplastic elastomers (TPEs) (a) styrenic, (b) COPE, (c) thermoplastic pol)oirethane, and (d) thermoplastic polyamide. 

FIGURE 5.2 Synthesis of polyisobutylene (PIB)-based star-block thermoplastic elastomer (TPE). (From Jacob, S. and Kennedy, J.P., Adv. Polym. Sci., 146, 1, 1999.)... 

FIGURE 5.3 Flow diagram for the production of polyethylene terepthalate (PET)-based thermoplastic elastomer (TPE). (From Papke, N. and Kargar-Kocsis, J., Polymer, 42, 1109, 2001. Courtesy of Elsevier.)... 

FIGURE 5.14 Elongation set properties of elastomers, thermoplastic elastomers (TPEs), and thermoplastics. (From Hofmann, W., Kunststoffe, 77, 767, 1987.)... 

FIGURE 5.18 Comparison of different thermoplastic elastomer (TPE) classes with corresponding cross-linked elastomers. (From Rader, C.P., Kunstst. Ger. Plast., 83, 111, 1993.)... 

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. 

In this part, we will discuss AFM images and nanomechanical data obtained in smdies of natural and synthetic rubbers, thermoplastic elastomers (TPE), and their vulcanized counterparts— thermoplastic vulcanizates (TPV). 

There are two approaches to the combining of scrap rubber and plastics. The initial interest was to use the cmmb in minor proportions to toughen the plastics improving impact strength and reduce the overall cost. A more recent interest is to develop a type of thermoplastic elastomer (TPE) wherein the mbber is the major component bonded together by thermoplastics, which can be processed and recovered as thermoplastics. 

FIGURE 38.10 Transmission electron micrograph of styrene-co-acrylonitrile/acrylonitrile butadiene mbber/ waste NBR (SAN/NBR/w-NBRybased thermoplastic elastomer (TPE). (Reprinted from Anandhan, S., De, P.P., Bhowmick, A.K., Bandy opadhyay, S., and De, S.K., J. Appl Polym. Sci., 90, 2348, 2003. With permission from Wiley InterScience.)... 

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. 

Thermoplastic elastomers (TPE), 9 565-566, 24 695-720 applications for, 24 709-717 based on block copolymers, 24 697t based on graft copolymers, ionomers, and structures with core-shell morphologies, 24 699 based on hard polymer/elastomer combinations, 24 699t based on silicone rubber blends, 24 700 commercial production of, 24 705-708 economic aspects of, 24 708-709 elastomer phase in, 24 703 glass-transition and crystal melting temperatures of, 24 702t hard phase in, 24 703-704 health and safety factors related to, 24 717-718... 

Global demand for thermoplastic elastomers (TPEs) is estimated at 2 million tonnes in 2005 and is forecast to increase by 6% per year. TPEs as a whole represent roughly 10% of elastomers or 1% of plastics. Consequently, economic statistics are rare and we can only make some assumptions concerning the consumption of each family ... 

Polyurethane multiblock copolymers of the type described by Eqs. 2-197 and 2-198 constitute an important segment of the commercial polyurethane market. The annual global production is about 250 million pounds. These polyurethanes are referred to as thermoplastic polyurethanes (TPUs) (trade names Estane, Texin). They are among a broader group of elastomeric block copolymers referred to as thermoplastic elastomers (TPEs). Crosslinking is a requirement to obtain the resilience associated with a rubber. The presence of a crosslinked network prevents polymer chains from irreversibly slipping past one another on deformation and allows for rapid and complete recovery from deformation. 

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