Polystyrene and Styrene Copolymers

Polystyrene Dispersions. On account of their glass transition temperature T of ca. lOO C, polystyrene dispersions do not form films at room temperature. These rigid polymers can only be applied with means of heat drying (e.g., to stiffen fabrics and nonwovens). Film formation is not required in agents used to protect floor coverings and paper coatings (plastic pigments) in this case polystyrene is therefore applied in the form of a dispersion at room temperature. 

Styrene Copolymer Dispersions. The and hardness of polystyrene can be adjusted over a wide temperature range by copolymerization of styrene with soft monomers such as butadiene and acrylate esters. Styrene-butadiene (SB) dispersions are quantitatively the most important. With a styrene-butadiene weight ratio of 85 15 the is ca. 80 C, at a ratio of 45 55 the T, is ca. — 25 C. On account of the cross-linking capability of butadiene, SB copolymers are not thermoplastics, but elastomers. Elasticity can be modified by controlling the molecular mass and degree of cross-linking. 

Styrene butadiene dispersions are generally stabilized with anionic, or anionic and nonionic emulsifiers. Carboxylation (incorporation of a small proportion of unsaturated carboxylic acids) of SB dispersions increases their stability and improves adhesion to various substrates. Almost all SB dispersions used in the coating sector are carboxylated. 

Styrene butadiene dispersions may undergo oxidative post-cross-linking via the double bond in the butadiene unit. Uncontrolled oxidation leads to embrittlement and, finally, to breakdown of the binder (chalking). The dispersions are thus unable to satisfy stringent requirements regarding color stability and UV resistance. 

In conventional exterior-use paints SB dispersions have largely been replaced by styrene-acrylate dispersions, and their use is now restricted to special applications (corrosion protection primers, wood primers, mortar modification) where low film permeability to gases, water vapor, etc., and complete resistance of the polymer to hydrolysis are necessary. In order to achieve a uniform surface and thus improve printability, paper and card are coated with paper-coating colors. Carboxylated SB dispersions are used in these paints as binders. 

In this chapter the homopolymer, polystyrene, is considered together with styrene-acrylonitrile copolymers, acrylonitrile-butadiene-styrene copolymers and styrene-a-methylstyrene copolymers. The important styrene-butadiene copolymers are described with other diene polymers in Chapter 18. The use of styrene in the cross-linking of unsaturated polyesters is described in Chapter 10. 

The bulk of commercial styrene is prepared from benzene by the following 

In the first stage, a Friedel-Crafts reaction is carried out by treating benzene with ethylene in the liquid phase at 90—100°C at slightly above atmospheric pressure. The catalyst is aluminium chloride (with ethyl chloride as catalyst promoter). A molar excess of benzene is used to reduce the formation of poly ethylbenzenes the molar ratio of reactants is generally about 1 0.6. The reactants are fed continuously into the bottom of a reactor whilst crude product is removed from near the top. The product is cooled and allowed to separate into two layers the lower layer, which consists of an aluminium chloride-hydrocarbon complex, is removed and returned to the reactor. The remaining ethylbenzene is then separated by distillation from polyethylbenzenes and benzene, which are recycled. 

Some ethylbenzene is obtained by direct recovery from catalytic reforming processes. In these processes aliphatic hydrocarbons are converted into mixtures of aromatic hydrocarbons from which ethylbenzene may be separated. 

The second stage of the styrene process involves the dehydrogenation of ethylbenzene. The reaction is carried out in the vapour phase at temperatures of 600—650°C over catalysts based on either ferric or zinc oxides with lesser amounts of other metallic oxides such as chromic, cupric and potassium oxides. The reaction is favoured by low pressure and in order to reduce the partial pressure of the ethylbenzene the feed is mixed with superheated steam before passage over the catalyst. Normally, a conversion of 35—40% per pass is achieved. The product is cooled and allowed to separate into two layers the aqueous layer is discarded. The organic layer consists of styrene (about 37%), ethylbenzene (about 61%) and benzene, toluene and tar (about 2%). The separation of styrene by distillation is difficult because of the susceptibility of the monomer to polymerization at quite moderate temperatures and because the boiling point of styrene (145°C) is rather close to that of ethylbenzene (136°C). It is necessary therefore to use specially designed columns and to add a polymerization inhibitor (commonly sulphur) before distillation and to distil under reduced pressure. In a typical process, a four-column distillation train is used. In the first column benzene and toluene are removed at atmospheric pressure in the second and third columns ethylbenzene is removed at about 35 mm Hg in the fourth column styrene is separated from sulphur and tar, also at about 35 mm Hg. Finally, an inhibitor is added to the styrene t-butyl catechol is preferred for this purpose rather than sulphur which leads to discoloration of the final polymer. Styrene is a colourless liquid with a characteristic odour. 

Styrene may be polymerized by means of all four techniques outlined in section 1.5, i.e. by bulk, solution, suspension and emulsion polymerization. Each of these methods is practised commercially, but solution polymerization is now the most extensively used. The four processes are described below. 

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Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Benzene is alkylated with ethylene to produce ethylbenzene, which is then dehydrogenated to styrene, the most important chemical iatermediate derived from benzene. Styrene is a raw material for the production of polystyrene and styrene copolymers such as ABS and SAN. Ethylbenzene accounted for nearly 52% of benzene consumption ia 1988. 

J. Scheirs, "Historical overview of styrene polymers," in J. Scheirs and D. Priddy, eds., Modem styrenic polymers Polystyrenes and styrenic copolymers, Wiley Series in Polymer Science, chapter 1, pp. 3-24. John Wiley, Chichester, 2003. 

Chain polymerization (addition reactions) polyoxymethylene, polymethyl methacrylate (PMMA), acrylic polymers, polystyrene and styrene copolymers, water-soluble polyamide... 

Of great importance are the ethylene derivatives with aromatic substituents. Styrene (vinylbenzene) is one of the monomers produced industrially in large volume. Polystyrene and styrene copolymers still belong to the important representatives of modern plastics and rubbers. Styrene can be polymerized by any of the known procedures. It has suitable physical properties, and therefore it is one of the most frequently studied monomers. It also... 

Modern Styrenic Polymers Polystyrene and Styrenic Copolymers. Edited by J. Scheirs and D. B. Priddy C) 2003 John Wiley Sons Ltd... 

M. Kuhlmann, BASF Internal Report Processing Polystyrene and Styrene Copolymers. 

This chapter discusses the dynamic mechanical properties of polystyrene, styrene copolymers, rubber-modified polystyrene and rubber-modified styrene copolymers. In polystyrene, the experimental relaxation spectrum and its probable molecular origins are reviewed further the effects on the relaxations caused by polymer structure (e.g. tacticity, molecular weight, substituents and crosslinking) and additives (e.g. plasticizers, antioxidants, UV stabilizers, flame retardants and colorants) are assessed. The main relaxation behaviour of styrene copolymers is presented and some of the effects of random copolymerization on secondary mechanical relaxation processes are illustrated on styrene-co-acrylonitrile and styrene-co-methacrylic acid. Finally, in rubber-modified polystyrene and styrene copolymers, it is shown how dynamic mechanical spectroscopy can help in the characterization of rubber phase morphology through the analysis of its main relaxation loss peak. 

Modern Styrenic Polymers Polystyrenes and Styrenic Copolymers... 

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