Cooling processes, polymerization

The second approach is thru the modification of the Compn B formulation and the elimination of cast charge defects, especially at the interface with the projectile base. Studies have centered primarily in the utilization of crystal control agents — polymeric materials, HNS, etc, the finish on the interior of the projectile, and on controlled loading cooling processes... 

Spherical beads that can be expanded into foam under the influence of heat or steam are produced directly by suspension polymerization in the presence of blowing agent. The term suspension polymerization describes a process in which water-insoluble monomers are dispersed as liquid droplets with suspension stabilizer and vigorous stirring to produce polymer particles as a dispersed solid phase. Initiators used in suspension polymerization are oil-soluble. The polymerization takes place within the monomer droplets. The kinetic mechanism of the suspension process is considered to be a free radical, water-cooled microbulk polymerization [1]. 

The application of computer calculations to DTA studies of the crystallization kinetics of polymers was described by Gornick (51). Calculations were made of the temperature of a polymeric sample during the cooling process using a Burroughs Mode 5500 computer. Morie et al. (52) used an IBM Model 1130 computer to prepare standard vapor-pressure plots of In P versus 1/T the vapor-pressure data being obtained from DTA or DSC curves. The heat of vaporization was calculated by the Haggenmacher method as modified by Fishtine. 

In a complex apparatus, Gimesch and Schneider [30, 119] studied the suspension polymerization of vinyl acetate. Their procedure involved equipment which automatically added tempered water to the reacting system as heat was evolved as a result of the polymerization process. Thus they maintained isothermal reaction conditions. The rate of reaction could be followed by recording the water uptake of the equipment with time. The heat of polymerization was also determined (found to be 23 kcal/mole which was considered a satisfactory check of the literature value which is scattered around 21.4 kcal/mole). From this work, a somewhat different mechanism of the suspension polymerization process emerges than the widely accepted concept of the water-cooled bulk polymerization of small particles. It was noted that with an increase in the initiator concentration, there was the expected increase in polymerization rate. With increasing stirring rate, the rate of polymerization decreased. Along with the suspension polymerization, there was always a certain amoimt of imdesirable emulsion polymerization. It was postulated that in the process, free radicals, formed in a monomer drop may be extracted into the aqueous phase where they may act on dissolved vinyl acetate by kinetic processes unique to this system and different from the conventional mechanism of suspension polymerization. 

Temperature and orientation are the most influential factors which impact secondary crystallization (the number of crystallites, their stmcture and size, and their spatial distribution and interaction). Therefore thermal processes (heating and cooling) during polymerization, storage, and PVC processing affect the resultant crystalline structure of PVC product. 

In Section 11.2, general principles of reactor safety and heat balance of reactors are presented, with an emphasis on specific aspects of polymerizations. Section 11.3 is devoted to safety-related thermodynamics and reaction engineering aspects of polymerization reactions. In Section 11.4, cooling of polymerization reactors is reviewed. The chapter is concluded by a section describing safety aspects of industrial processes, together with technical risk-reducing solutions. 

In industrial processing, polymeric materials usually crystallize during cooling. Their low thermal conductivity and diffusivity can result in temperature gradients across the product thickness, especially when the release of latent heat of fusion contributes to development of the temperature gradient. Moreover, during processing... 

The polymeric solutions in a certain range of their physical properties combine good points of both oil and water as quenching liquids and permit to control the cooling process over wide ranges of the process parameters. For the aims of heat treatment, a number of water-soluble polymers are used, e.g., PVA, PEO, PAA, polymetacryle acids (PMAA, PAA) and their salts, cellnlose compounds, etc. " " The optimal concentration range is 1... 

Several recent patents describe improvements in the basic belt process. In one case a higher soHds polymerization is achieved by cooling the starting monomer until some monomer crystallizes and then introducing the resulting monomer slurry onto the belt as above. The latent heat of fusion of the monomer crystals absorbs some of the heat of polymerization, which otherwise limits the soHds content of the polymerization (87). In another patent a concave belt is described which becomes flat near the end. This change leads to improved release of polymer (88). 

A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

Aqueous media, such as emulsion, suspension, and dispersion polymerization, are by far the most widely used in the acryUc fiber industry. Water acts as a convenient heat-transfer and cooling medium and the polymer is easily recovered by filtration or centrifugation. Fiber producers that use aqueous solutions of thiocyanate or zinc chloride as the solvent for the polymer have an additional benefit. In such cases the reaction medium can be converted directiy to dope to save the costs of polymer recovery. Aqueous emulsions are less common. This type of process is used primarily for modacryUc compositions, such as Dynel. Even in such processes the emulsifier is used at very low levels, giving a polymerization medium with characteristics of both a suspension and a tme emulsion. 

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