Styrenic block copolymers SBC

SBCs were developed in the US in the mid sixties. They are the largest volume thermoplastic elastomers, being polymers with rubberlike properties but able to be processed as thermoplastics. As a result of these properties, they present two advantages over conventional types vulcanisation is unnecessary and the scrap can be reprocessed. SBCs account for about 5 % of the total synthetic rubber production (see Table 8.1). The consumption within the EU is approximately 280000 tonnes/year. 

SBCs are made using anionically catalysed polymerisation techniques. They are composed of well defined polystyrene and poly-diolefin blocks. Two basic types of configurations, hnear and branched (or star) can be produced. The polystyrene and polydiene blocks are chemically incompatible and so a two phase system is formed. The hard polystyrene domains are linked to the rubbery polydiene phase and act as multifunctional cross-links. Below the glass transition temperature of polystyrene, SBCs act like vulcanised rubber whilst above it they act like thermoplastics. 

SBCs are used to produce modified bitumen for roofing and roads (43 %), footwear (43 %), adhesives (11%) and a wide variety of technical goods (3 %). 

Thermoplastic elastomers, in general, cannot compete with vulcanised rubber compounds where the highest level of physical properties are required. SBC also suffers from having a relatively low maximum operating temperature of about 70 °C. Despite this, it has achieved considerable market penetration. 

Styrenic block copolymers (SBC) are the largest volume TPE used today. Approximately 2 billion pounds were consumed worldwide in 2010. The reason that it has such a high consumption rate is because it is not only used by the rubber industry, but also by the adhesives industry in hot melt adhesives and as an additive with asphalt in road construction. The footwear industry also uses very large quantities of SBC, where it is the preferred rubber. 

Hard Glassy Domain-Soft Rubbery Domain-Hard Glassy Domain 

SBS for Styrene-Butadiene-Styrene SIS for Styrene-Isoprene-Styrene 

SEES for Styrene-Ethylene/Butylene-Styrene SEES for Styrene-Ethylene/Propylene-Styrene 

These styrenic block copolymers are manufactured through solution polymerization with special catalysts. The type of SBC being manufactured will determine the appropriate combination of the following monomer feedstocks. 

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Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

Currently, important TPE s include blends of semicrystalline thermoplastic polyolefins such as propylene copolymers, with ethylene-propylene terepolymer elastomer. Block copolymers of styrene with other monomers such as butadiene, isoprene, and ethylene or ethylene/propy-lene are the most widely used TPE s. Styrene-butadiene-styrene (SBS) accounted for 70% of global styrene block copolymers (SBC). Currently, global capacity of SBC is approximately 1.1 million tons. Polyurethane thermoplastic elastomers are relatively more expensive then other TPE s. However, they are noted for their flexibility, strength, toughness, and abrasion and chemical resistance. Blends of polyvinyl chloride with elastomers such as butyl are widely used in Japan. ... 

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. 

Let us first review the various thermoplastic elastomers used in automotive applications. These are styrenic block copolymers (SBCs), thermoplastic olefins (TPOs) (cross-linked... 

Styrenic block copolymer (SBC) thermoplastic elastomers are produced by Shell Chemical (Kraton ), Firestone Synthetic Rubber and Latex, Division of Bridgestone/Firestone (Stereon t), Dexco Polymers (Vector ), EniChem Elastomers (Europrene ), and other companies. SBC properties and processes are described for these four producers TPEs. 

The family consists of, in order of market volume, polybutadiene (or butadiene rubber, BR), solution styrene butadiene rubber (SSBR) and styrenic block copolymers (SBC). A further subdivision may be made into those polymers that require vulcanisation (BR, SSBR) and those that do not (SBC). The latter are also known as thermoplastic rubbers as they have rubbery properties below a certain temperature when they soften and may be processed like thermoplastics. 

The use of thermoplastic olefins or TPOs is growing at a faster rate than the styrenic block copolymers (SBC) just discussed, or the thermoplastic vulcanizates (TPVs), which are discussed in the next section. The TPO growth rate is currently about 10% annually worldwide. Unlike the SBCs, certain TPOs can sometimes be used at higher service temperatures. Unlike the TPVs (which are derived through dynamic vulcanization), the TPOs are usually used uncured or are only very slightly vulcanized, if at all. This means that the TPOs are usually less costly than the TPVs. The TPOs have been very successful in the automotive industry in replacing the soft polyvinyl chloride compounds used in car interiors. TPOs are commonly available in softness ranges from 60 Shore A to 60 Shore D. A major use for TPOs is in the manufacture of automotive fascias, where appearance is very important. 

Styrenic block copolymer (SBC) is a commercially important thermoplastic elastomer. The polymer is made of three separate polymeric blocks (see Section 3.2 for an explanation of block copolymers). At one end is a hard PS block, in the middle a long polybutadiene (or other elastomeric) block, followed by a second hard block of PS. These blocks are immiscible, so they form discrete domains of PS within a polybutadiene matrix. The separate domains are chemically connected. This is shown in Fig. 4.44, where one might notice that this looks a lot like HIPS, except that the continuous phase and hard discrete phase are switched in SBC and the domains are... 

Figure 4.44 The microscopic structure of styrenic block copolymers (SBC).

Instrumented probe test using a Texture Analyzer to record force versus time during debonding for a styrenic block copolymer (SBC) - based adhesive (dark lines] and a solution acrylic adhesive (gray lines] at thickness of 50 pm and 125 pm... 

Comparison of G and tan delta (G"/G ) versus temperature for a styrenic block copolymer (SBC) -based PSA, a solution acrylic PSA, and an emulsion acrylic PSA. Reprinted with permission from Hu and Paul (2009), copyright Taylor and Francis... 

OBCs are advantaged in their eapability to retain oil when compared to Polyolefin Elastomers of similar density and Ml, and can therefore deliver lower hardness compounds ( 20 Shore A). This is hypothesized to be due to the OBC block structure wherein the blocks of amorphous segments may allow for more swelling and oil incorporation. Oil-filled OBCs also show decreased compression set at both room temperature and elevated temperature, which is again attributed to their imique block structure. These combined benefits expand Ae application range of OBC compounds to include markets currently served by high performance thermoplastic elastomers, such as styrenics block copolymers (SBCs), thermoplastic urethanes (TPUs) and thermoplastic vulcanizates (TPVs). 

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