Styrene stability

Figure 12 The electron micrographs of the final particles and the variation of the monomer conversion with the time at different stabilizer concentrations in the dispersion polymerization of styrene. Stabilizer concentration (g/dL) (a) 0.5, (b) 1.0, (c) 2.0. The original SEM photographs were taken with 2600 x, 2000 x, and 2600 x magnifications for (a), (b), and (c), respectively, and reduced at a proper ratio to place the figure. (From Ref. 93. Reproduced with permission from John Wiley Sons, Inc.)...

Once this is accomplished, the polyester is discharged to a blending tank and blended with cold styrene. Stabilizers such as hydroquinone and /-butyl hydro-quinone can be added to prevent styrene polymerization. 

The polymerization of styrene in Winsor I-like systems by semi-continuous feeding of monomer stabilized by either DTAB, TTAB or CTAB has been systematically investigated by Gan and coworkers [69a]. Rather monodisperse polystyrene microlatexes of less than 50 nm with molecular weights of over one million were obtained at a polymer/surfactant weight ratio of 14 1. The Winsor I-like (micro)emulsion polymerization of styrene stabilized by non-ionic surfactant and initiated by oil-soluble initiators has also been reported very recently [69b]. The sizes of the large monomer-swollen particles decreased with conversion and they merged with growing particles at about 40-50% conversion. 

TABLE 5.4 Impact Styrene Stabilization ASTM D-1925 Yellowness Index After 80°C Oven Aging... 

Colloid Polymer Science 278, No.9, Sept. 2000, p.821-9 MICROEMULSION POLYMERISATION OF STYRENE STABILIZED BY SODIUM DODECYL SULPHATE AND DIETHYLENE GLYCOL MONOALKYL ETHER Chem C S Liu C W... 

Stabilization of the polymeric and monomeric components is required to avoid premature radical polymer formation of UP resin dissolved in styrene by heat, light, or metallic initiators. Insufficient stabilization causes an increase in viscosity that makes further processing impossible. Quinones and phenols in concentrations < 500 ppm are proven stabilizers for unsaturated polyester. The most important examples are hydroquinone, p-benzoquinone, tert-butyl-brenzcatechol and 2,6-di-tert-butyl-p-cresol [574]. For styrene, stabilization with 25 ppm p-tert-butyl-brenz-catechol is required [575]. Slight yellowing caused by the stabilizers is possible appropriate brighteners are recommended for optically demanding applications. 

Varela de la Rosa et al. [68-70] carried out emulsion polymerizations of styrene stabilized by sodium dodecyl sulfate and initiated by potassium persulfate at 50 °C. They proposed the following reaction mechanism to describe the conventional styrene emulsion polymerization system. 

Capek and Chudej [87] studied the emulsion polymerization of styrene stabilized by polyethylene oxide sorbitan monolaurate with an average of 20 monomeric units of ethylene oxide per molecule (Tween 20) and initiated by the redox system of ammonium persulfate and sodium thiosulfite. It is interesting to note that the constant reaction rate period is not present in this polymerization system. The maximal rate of polymerization is proportional to the initiator and surfactant concentrations to the -0.45 and 1.5 powers, respectively. The final number of latex particles per unit volume of water is proportional to the initiator and surfactant concentrations to the 0.32 and 1.3 powers, respectively. In addition, the resultant polymer molecular weight is proportional to the initiator and surfactant concentrations to the 0.62 and -0.97 powers, respectively. Some possible reaction mechanisms may explain the deviation of the polymerization system from the classical Smith-Ewart theory. Lin et al. [88] investigated the emulsion polymerization of styrene stabilized by nonylphenol polyethoxylate with an average of 40 monomeric units of ethylene oxide per molecule (NP-40) and initiated by sodium persulfate. The rate of polymerization versus monomer conversion curves exhibit two nonsta-tionary reaction rate intervals and a vague constant rate period in between. 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

Figure 13.19 Effect of KPS concentration on the evolution of particle size (a) and on the average number of radicals per particle (b) for conventional emulsion polymerization of styrene stabilized by sodium dodecyl sulfate.

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