Solution-polymerized thermoplastic rubber

RESINS (Acrylonitrile-Butadiene-Styrene). Commonly referred to as ABS resins, these materials are thermoplastic resins which are produced by grafting styrene and acrylonitrile onto a diene-rubber backbone. The usually preferred substrate is polybutadiene because of its low glass-transition temperature (approximately —80°C). Where ABS resin is prepared by suspension or mass polymerization methods, stereospedfic diene rubber made by solution polymerization is the preferred diene. Otherwise, the diene used is a high-gel or cross-linked latex made by a hot emulsion process. 

Produced by a solution polymerization process, this material exhibited an ordered molecular structure with the styrene monomer located at the ends of the butadiene monomer chain. In addition, other monomers such as isoprene, ethylene, butylene, and others, could be added to the polymer chain, which further modified basic properties. These materials possess a continuous rubber phase for resilience and toughness, and a discontinuous plastic phase for solubility and thermoplasticity. A variety of different grades are also available for this type of SBR, with differences in molecular weight, differences in the types of monomers used, differences in structural configuration, and differences in the ratio of endblock to midblock. Both emulsion and solution polymerized grades of SBR are available as solvent-based and water-based adhesives and sealants. Block copolymers are extensively used for hot melt formulations and both water-based and solvent-based pressure sensitive adhesive applications. Today, SBR elastomers are the most popular elastomers used for the manufacture of adhesives and sealants. 

After the war when natural rubber became available again the consumption of styrene-butadiene rubber began to fall however, the trend was reversed in 1949 with the advent of a copolymer made at low temperature. This product gives a passenger-tyre rubber superior to natural rubber and styrene-butadiene rubbers have remained the most important of the large-tonnage rubbers (Table 20.2). In the early 1960s, solution polymerized styrene-butadiene rubbers became available. These rubbers show further improvements in tyre performance. In 1965, styrene-butadiene thermoplastic elastomers were introduced. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

The thermoplastic-rich phase may be separated in the course of polymerization (Sec. 13.4.2) or can be incorporated as a dispersed powder in the initial formulation (Sec. 13.4.3). A strong drawback of the in situ-phase separation for processing purposes is the high viscosity of the initial solution which results from the much higher average molar mass of the TP compared with the liquid rubbers. Also, for the same reason, the critical concentration crit has a smaller value (phase inversion is observed at smaller concentrations of modifier). 

Different polymers have different unique properties. To combine these unique properties of component polymers blending is an attractive means. There are a few methods to make polymer/polymer blends solution blending, melt extrusion, in situ polymerization, among others. Compatibility usually plays a major role in the development of properties. The blends prepared by melt mixing of thermoplastic materials and rubbers have met industrial needs in recent years. 

In the late forties, work began to improve the prcperties of a new thermoplastic blend. It had been revealed [2] in 1948 - 50 that poly(styrene-co-acrylonitrile), or SAN, could be blended with Buna N, a copolymer of butadiene and acrylonitrile, or Buna S, a copolymer of loutadiene and styrene, to get useful thermoplastics. These materials were impact resistant, with Izod impact values of 2 to 3 foot-pounds. The commercial use of these materials was hindered loy the lack of low tenperature impact strength. The rubber technologists of the narbon Division (as the Marsene Corporation had been named on assimilation into Borg-Warner) knew that polybutadiene remained "rubbery" at lower tenperatures than the copolymers cited above. However, blending experiments showed that polybutadiene and SAN were incompatible. The polymerization of SAN could be acconplished in solution, in bulk, or... 

The story of these systems begins as long ago as 1925. Ostromislensky [70] of Naugatuck Chemical filed a patent application for the polymerization of a natural rubber solution in styrene. This produced a tough white thermoplastic in the place of a brittle, glassy, transparent... 

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