Commercial Styrenic Block Copolymers

The first commercial thermoplastic elastomers deriving their properties from an ABA block copolymer structure were poly(styrene-isoprene-styrene) and poly (styrene-butadiene-styrene) triblocks introduced in 1965 at an ACS Rubber 

The primary applications for SBCs are listed in Table 21.3. There are several characteristics that are general to their applications  

Because of their ability to blend with a wide variety of materials, SBCs are rarely used alone. In the applications described below, it is unusual for an SBC to make up more than 50% of a product. 

Because of their low polarity, SBCs have relatively weak adhesion to other materials and limited oil resistance, and are therefore generally used in applications where polarity is not important or is undesirable. 

Strength generally increases as the driving force for phase separation between the blocks, %N, increases in the order SIS SBS SEPS, SEBS, where N is the polymer molecular weight and x is the segmental interaction parameter. 

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If we move from data point A , our grand average commercial styrenic block copolymer having elastic properties, along the axes of the grid, we run into the boundaries discussed. 

Having roughly defined the practical boundaries of commercial styrenic block copolymers with useful elastomeric properties, why do we have so many choices available within those boundaries What key property change will drive a polymer manufacturer and/or polymer user to head southwest in Figure 21.5 What would drive a customer to ask for something more northeasterly, please Or, go as far east as possible and do not worry about melt processability for my application . In essence, why is not everyone totally content to make, and use, polymer A for every application ... 

As shown in O Fig. 15.1, uncross-linked adhesives can exhibit two maxima in peel with shear rate. At slow peel rates they will fail cohesively. When pulled sufficiently fast their failure becomes adhesive, the peel force drops, but then rises with peel rate - as with cross-Unked adhesives - until falling again when slip-stick failure occurs. Commercial styrenic block copolymer-based adhesives can exhibit high peel force cohesive failure at standard test speeds when bonded to high-energy substrates like polished steel. 

As more complex multicomponent blends are being developed for commercial appHcations, new approaches are needed for morphology characterization. Often, the use of RuO staining is effective, as it is sensitive to small variations in the chemical composition of the component polymers. For instance PS, PC, and styrene—ethylene/butylene—styrene block copolymers (SEES) are readily stained, SAN is stained to a lesser degree, and PET and nylons are not stained (158,225—228). 

More recent examples include end-functionalized multiarmed poly(vinyl ether) (44), MVE/styrene block copolymers (45), and star-shaped polymers (46—48). With this remarkable control over polymer architecture, the growth of future commercial appHcations seems entirely likely. 

Copolymer technology is progressing along two "fronts." First, new appHcations for copolymers are being found to increase the volume of materials that are already commercially available. One example of this is the rapid growth of styrenic block copolymers sold as asphalt (qv) and polymer modifiers over the past 10 years (Fig. 7). Another is the increased interest in graft and block copolymers as compatihilizers for polymer blends and alloys. Of particular interest are compatihilizers for recycled polymer scrap. 

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Global consumption of thermoplastic mbbers of all types is estimated at about 600,000 t/yr (51). Of this, 42% was estimated to be consumed in the United States, 39% in Western Europe, and 19% in Japan. At present, the woddwide market is estimated to be divided as follows styrenic block copolymers, 48% hard polymer/elastomer combinations, 26% thermoplastic polyurethanes, 12% thermoplastic polyesters, 4% and others, 9%. The three largest end uses were transportation, 23% footwear, 18% and adhesives, coatings, etc, 16%. The ranges of the hardness values, prices, and specific gravities of commercially available materials are given in Table 4. 

Trade names and suppHers of commercial thermoplastic elastomers of all types are given in Tables 5—7. Table 5. Trade Names of Thermoplastic Elastomers Based on Styrenic Block Copolymers ... 

Triblock copolymers of ABA type, where B is the central elastomeric block and A is the rigid end-block, are well-known commercially available polymers [7,8]. The chemical structures of some common TPEs based on styrenic block copolymers are given in Eigure 5.1. Synthesis of such ABA-type polymers can be achieved by three routes [9] ... 

Though living anionic polymerization is the most widely used technique for synthesizing many commercially available TPEs based on styrenic block copolymers, living carbocationic polymerization has also been developed in recent years for such purposes [10,11], Polyisobutylene (PlB)-based TPEs, one of the most recently developed classes, are synthesized by living carbocationic polymerization with sequential monomer addition and consists of two basic steps [10] as follows ... 

Other rubber systems have been commercially successful. Styrene block copolymers yield a HIPS product with a small particle size and provide high gloss. A mixed rubber system consisting of styrene-butadiene block rubber and/or ethylene-propylene diene modified (EPDM) rubber can be blended with the polybutadiene to form bimodal rubber particle size distribution for a... 

The synthesis of styrenic block copolymers (SBCs) has been discussed in a number of books and review articles concerning block copolymers [1] and anionic polymerization [2]. A comprehensive review of the field is beyond the scope of this chapter, the objective of which is to provide an overview of the technology, with particular emphasis on processes currently used for commercial production. 

Since shortly after its discovery by Szwarc et al. [5] in the mid-1950s, living anionic polymerization has been recognized as an ideal route to styrenic block copolymers [6]. To date, living anionic polymerization remains the only commercially important technology for SBC synthesis. The anionic polymerization of styrene and common dienes such as butadiene and isoprene satisfies the criteria outlined above, particularly when carried out in a hydrocarbon solvent and initiated by an appropriate lithium alkyl. 

If we look at the three panels of data in Figure 21.4, we can begin to appreciate the intractable nature of some of the styrene-hydrogenated rubber-styrene block copolymers. The right panel shows an SBS polymer (not hydrogenated) that is typical in its overall size and styrene content for a commercial polymer. Viscosity is plotted versus shear stress at two testing temperatures (200 and 220 CC) for this SBS polymer, and it serves as our reference point for the center panel. 

The resulting TPE can either be used alone or blended with aliphatic oil and polypropylene. In the former case a higher tensile strength and elongation at break are obtained in comparison with the commercially available styrene-hydrogenated butadiene-styrene block copolymers, especially at high temperatures. 

Several approaches to compatibilizing PPE blends with commercial polyolefins (polypropylene, etc.) have been reported in the literature [Lee, 1990 Kirkpatrick, 1989]. However, no commercial blends of PPE/polyolefins have been offered to date. CompatibUization and impact modification of PPE/polypropylene can be achieved by choosing a selected type of styrene-ethylene/butylene-styrene block copolymer and PPE of low molecular weight [Akkapeddi and VanBuskirk, 1992]. 

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