High-impact polystyrene block rubbers

Styrene and butadiene also form copolymers known as high impact polystyrene, or rubber-modified polystyrene, when the content of butadiene is 10%. This type of material has excellent mechanical properties, and it is widely used in practice for the manufacturing of numerous objects, including parts for household appliances, furniture, etc. Rubber-modified polystyrene is commonly used as wood replacement and also for packaging. The synthesis of this material typically is done by dissolving polybutadiene in styrene monomer, followed by free radical polymerization achieved using a peroxide catalyst. This procedure leads to block or graft type copolymers. 

In a block copolymer, a long segment made from one monomer is followed by a segment formed from the other monomer. One example is the block copolymer formed from styrene and butadiene. Pure polystyrene is a transparent, brittle material that is easily broken polybutadiene is a synthetic rubber that is very resilient, but soft and opaque. A block copolymer of the two monomers produces high-impact polystyrene, a material that is a durable, strong, yet transparent plastic. A different formulation of the two polymers produces styrene-butadiene rubber (SBR), which is used mainly for automobile tires and running shoes, but also in chewing gum. 

Order-disorder transitions and spinodals were computed for linear multi block copolymers with differing sequence distributions by Fredrickson et al. (1992). This type of copolymer includes polyurethanes, styrene-butadiene rubber, high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) block copolymers. Thus the theory is applicable to a broad range of industrial thermoplastic elastomers and polyurethanes. The parameter... 

The potential problem of styrene taint in foods is well known and documented in the literature (Saxby 1996). Styrene (see Chapter 2) is the monomer that is polymerized to make polystyrene (PS) (also known as general purpose or GPPS grade). It is also commonly used with butadiene rubber (5-20 % w/w) as a block copolymer to form high impact polystyrene (HIPS). In addition there are less common copolymer grades such as acrylonitrile-butadiene-styrene (ABS) having a mixture of 25 %, 15-25 % and 50-65 % of each monomer respectively or a copolymer with acrylonitrile (styrene-acrylonitrile, SAN). 

Polystyrene (PS) in its atactic and syndiotactic forms is a brittle thermoplastic, even in an orientated state [4]. To improve the toughness of aPS, impact modification has been practised for a long time, either by polymerizing the styrene in the presence of a polybutadiene rubber leading to high-impact polystyrene, commonly called HIPS, or by blending the polystyrene with multi-block copolymers, mainly of the styrene-butadiene-styrene (S-B-S) type. 

In rubber-modified polymers like high impact polystyrene or acrylonitrile-butadiene-styrene (ABS) resins, the toughening effect of the dispersed rubber particles appears only in the presence of block or graft copolymers. These copolymers regulate the particle size of the rubber dispersion and achieve adhesion of the two phases. Hence, graft copolymers are of practical importance in polymer alloys. 

Kuboky and co-workers [40] used transmission electron microscopy (TEM) to study block and white crazes in high impact polystyrene (PS). They examined the mechanism of block craze formation and found that the rubber molecules were not necessarily diffused into the entire crazes. The length of the block crazes varied before they turned to white and in some cases only white crazes were generated from the rubber particles. TEM should thus be used with caution in examining stained rubber-toughened polymers to ensure that all crazes, including the white crazes, were considered for evaluation of the extent of the deformation behaviour [41]. 

These structures are illustrated in Figure 1.3. Through the leadership of Amos and others, polymer blends and grafts found uses as rubber-toughened plastics, which include high-impact polystyrene (HiPS) and acrylonitrile-butadiene-styrene (ABS) plastics. As further illustrated in Table 1.1, block copolymers containing a water-soluble block and an oil soluble block became important as surfactants through the work of Lunsted, while other block copolymers, composed of elastomer and plastic blocks, were useful as thermoplastic elastomers. 

Super-high-impact polystyrene can be made from blends of polystyrene with butadiene-styrene block copolymers. As illustrated in Figure 2.4, the morphology of rubber-containing domains is finer than the high-impact polystyrene structure shown in the upper left of Figure 2.3. The subject has been recently reviewed by Aggarwal and Marti and Riess. ... 

Proportion of Hard Phas . The tensile behavior of otherwise similar block copolymers with differing hard segment contents shows a family of stress-strain curves (4,6,7,24). As the hard segment content is increased, the products change from very weak, soft, rubber-like materials to strong elastomers, then to leathery materials, and finally to hard flexible thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Thermoplastic elastomers in general show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Isopropanol vapor was used to dissolve the matrix in polymer blends [245]. Williams and Hudson [246] etched microtomed blocks of high impact polystyrene so that the rubber particles protruded from the matrix. Later, Kesskula and Traylor [130] removed rubber particles from Hire and ABS polymers by dissolving the matrix in a cyclohexane solution of osmium tetroxide and extracting the dispersed phase for SEM. Olefin particles were removed from impact modified nylon and polyester [6]. Selective etching of the polycarbonate phase with triethyl-amine in a mixture with styrene-acrylonitrile copolymer (SAN) revealed the nature of the blend [247]. 

Butadiene (CH2=CH—CH=CH2) and styrene, two monomers listed in Table 25.1, can form a block copolymer called poly(styrene-butadiene-styrene) or SBS rubber [Figure 25.6(a)], in which there are alternating blocks of polybutadiene and polystyrene. They also form a graft polymer known as high-impact polystyrene (HIPS) [Figure 25.6(b)], in which chains of polybutadiene branch off of the main polystyrene chain. 

Natural polystyrene has a strong tendency to crack, and can only be used in most potential applications when modified to form high-impact or toughened polystyrene grades or ABS. It can be toughened by polybutadiene, butadiene-acrylonitrile copolymer rubber, or SBS or SEBS block copolymers. The rubber can be added before or after polymerisation of the styrene monomer. In the case of polybutadiene, two reactions take place styrene polymerisation, and graft copolymerisation of the styrene with the polybutadiene. A brief outline of this operation is available on the Dynasol website (www.dynasolesastomers.com). 

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