Suspension polymerization thermal stability

The synthesis of silica membranes has only recently been described. Silica forms sols and gels very easily both by the colloidal suspension and by the polymeric gel route. Its chemical resistance and its thermal stability in the presence of water vapor or metal impurities are not very good however. Larbot et al. (1989) have described the synthesis of silica membranes starting with a commercially available silica sol (Cecasol Sobret) in an aqueous solution at pH 8. 

Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stability of the polymer. 

The thermal stability of poly(vinyl chloride) is improved greatly by the in situ polymerization of butadiene or by reaction with preformed cis-1,4-polybutadiene using a diethyl-aluminum chloride-cobalt compound catalyst system. The improved thermal stability at 3-10% add-on is manifested by greatly reduced discoloration when the modified poly-(vinyl chloride) is compression molded at 200°C in air in the absence of a stabilizer, hydrogen chloride evolution at 180°C is retarded, and the temperature for the onset of HCl evolution and the peak decomposition temperature (DTA) increase, i.e. 260°-280°C and 290°-325° C, respectively, compared with 240°-260°C and 260°-280°C for the unmodified homopolymer, in the absence of stabilizer. The grafting reaction may be carried out on suspension, emulsion, or bulk polymerized poly(vinyl chloride) with little or no change in the glass transition temperature. 

Dialkylperoxides are used as high temperature catalysts for suspension and bulk polymerization as well as hardeners for unsaturated polyester resins and for cross-linking polymers because of their good thermal stability. While the liquid di-tert-butylperoxide is relatively volatile at application temperatures, dicumylperoxide is much less volatile and has the disadvantage of forming decomposition products with intense odors (acetophenone)(Chapter 13). 

In our work we have used the polymerization of vinyl chloride at pressures (P) below the saturation value (P ) as a way to produce polymers, subsaturaticn PVC (U-PVC), with increased amounts of defects. This system is also a model for the later stages in a conventional batch polymerization of vinyl chloride, i.e. after the pressure drop. With decreasing relative monomer pressure, P/P, the thermal stability of PVC deteriorates strongly (6-8. l8). In a series of investigations we have determined different structures in several U-PVC samples and, as a reference, in a series of fractions of a commercial suspension PVC (S-PVC) (6-8. 11). 

The technique of polymerizing emulsions of monomers to produce synthetic latexes which could be processed to produce rubbers similarly to natural rubber latex was developed in industrial laboratories in Germany, USA and the former Soviet Union fiom 1927 onwards. Thermally initiated polymerization of monomer dispersions had beat tried earlier, protective colloids were used to stabilize the monomer droplets these suspension polymerizations were very slow. 

To characterize the polymerization behavior of FA and to investigate how the presence of MMT influences this polymerization, FTIR spectra were collected before and during the resiniflcation process. The dispersion of the MMT in the PFA matrix is shown both directly and indirectly. The direct evidence consists of the XRD patterns of the FA-MMT suspension, which was used to monitor the process of intercalation and exfoliation of the MMT at various stages of resinifica-tion. The dispersion is indirectly evidenced in increased thermal stability of the MMT-PFA nanocomposite, as measured by TGA. The thermal stability is discussed and compared to the pure polymer and to the CW-PFA nanocomposites. In addition, the important differences between oxidative and nonoxidative degradation of the NaMMT-PFA nanocomposite is discussed, and a mechanism is proposed to explain the difference in terms of acid-catalyzed degradation. 

Apart from the fluoro monomers vinyl fluoride (VF), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE), only chlorofluoroethylene has found commercial use as homopolymer. It is applied as thermoplastic resin based on its vapor-barrier properties, superior thermal stability (Tdec > 350 °C), and resistance to strong oxidizing agents [601]. Chlorofluoroethylene is homo- and copolymerized by free-radical-initiated polymerization in bulk [602], suspension, or aqueous emulsion using organic and water-soluble initiators [603,604] or ionizing radiation [605], and in solution [606]. For bulk polymerization, trichloroacetyl peroxide [607] and other fluorochloro peroxides [608,609] have been used as initiators. Redox initiator systems are described for the aqueous suspension polymerization [603,604]. The emulsion polymerization needs fluorocarbon and chlorofluorocarbon emulsifiers [610]. 

Figure 10.2 Dehydrochlorination rate versus tertiary chlorine atoms + internal double bonds [72] fractionated commercial suspension PVC, polymerization temperature 55°C [73,74], OPVC polymerized at reduced monomer pressure, polymerization temperature 55 C, monomer pressure between 59-92% of the saturation pressure of vinyl chloride at 55 °C [73,74], bulk and suspension polymerization by lUPAC Sub-Group on Defects in the Molecular Structure of PVC and their relation to thermal stability. [Modified from [72].]...

The preferred polymerization medium was a saturated fluorocarbon or chlorofluorocarbon solvent, although an aqueous medium could also be used. The solvent system was used to control the reaction conditions and increase the polymerization reaction rate. The reaction medium could also improve melt processability and increase the thermal stability and chemical resistance of the polymer. The reaction could be carried out by bulk, solution, suspension, emulsion, or vapor phase polymerization regimes. Azo compounds, peroxy compounds, ultraviolet radiation, or high-energy ionizing... 

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