Solutions, solvent stripping polymer

Inversion ofMon cjueous Polymers. Many polymers such as polyurethanes, polyesters, polypropylene, epoxy resins (qv), and siHcones that cannot be made via emulsion polymerization are converted into latices. Such polymers are dissolved in solvent and inverted via emulsification, foUowed by solvent stripping (80). SoHd polymers are milled with long-chain fatty acids and diluted in weak alkaH solutions until dispersion occurs (81). Such latices usually have lower polymer concentrations after the solvent has been removed. For commercial uses the latex soHds are increased by techniques such as creaming. 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

As dimerization competes with polymerization, dimer formation very slightly reduces polymer yield. The toxicity and the smell of VCH are much more relevant. VCH is removed from the rubber solution together with residual monomer and solvent prior to the isolation of the rubber from the polymer solution. As VCH has a higher boiling point than BD and hexane an efficient stripping process has to be used in order to reduce VCH contents to environmentally friendly levels. 

All the solution processes require high efficiency in recovering the solvent. The most widely used process consists of termination of the polymerization and the addition of antioxidant to the polymer solution. The solution may be treated to remove catalyst residue and then transferred into an agitated steam stripping vessel in which unreacted monomer and solvent are flashed off, leaving the rubber as a crumb slurry in water. The water-crumb slurry then is dewatered and dried. The recovered monomer/solvent is recirculated to a... 

Hash devolatilization is a simple and effective method to remove the majority of solvent and unreacted monomers from the polymer solution. Product from the reactor is charged to a flash vessel and throttled to vacuum conditions whereby the volatile solvent and monomers are recovered and condensed. In the process, the polymer melt cools, sometimes considerably, due to the evaporation of volatiles. The polymer product is pumped from the bottom of the flash vessel with a gear pump or other suitable pump for viscous materials. Critical to operation of the flash devolatilization unit is prevention of air back into the unit that reduces stripping ability and potentially allows oxygen into the unit that can discolor products or pose a safety hazard if low autoignition temperature solvents are used. Often one flash devolatilization unit is insufficient to reduce the residual material to a sufficient level and thus additional units can be added in series [61]. In each vessel, the equilibrium concentration of volatile material in the polymer melt, is a function of the pressure and temperature the flash unit operates at, with consideration for the polymer solvent interaction effects described by the Hory-Huggins equation. Flash devolatilization units, while simple to operate, may be prone to foam development as the superheated volatiles rapidly escape from the polymer melt. Viscous polymers or polymers with mixed functionalities... 

Styrene-based thermoplastic elastomers (see Chapter 4) are sensitive to oxidation since they contain unsaturated soft segments. These elastomers are manufactured by solution polymerization process in aliphatic hydrocarbons. In order to prevent autoxidation during the finishing steps (stripping, drying), which manifests itself by a rise in melt flow index and discoloration of the raw polymer, antioxidant is added to the polymer solution before finishing. Hence the antioxidant has to be soluble in the polymerization solvent. 

Evaporation. In this method (Fig. 14), the polymer solution is pumped through a die, cast upon a stainless-steel belt, and transferred to a drying chamber to evaporate. The solvent is recovered (15). As the solvent evaporates, a gel forms. Further drying gives a stable film. The film is stripped from the belt and rolled up. 

Synthetic c/s-1,4-poly isoprene latices are also available. These are made by emulsification of polymer solutions followed by solvent stripping. 

Completed polymerizations can be terminated at -78°C by introducing a protic material such as methanol or acetic acid in which case the product will have isobutyrate ester termini. Alternatively the mixture can be warmed to room temperature whereupon spontaneous termination occurs to form cyclic ketone ends (2). With THF solvent, isolation of polymer is accomplished by precipitation in water, hexane or methanol depending upon the sorts of contaminants or by-products one wishes to remove or, the polymer solution can simply be stripped free of solvent. 

In the solution mode process illustrated in Figure 5.9, the solvent utilized must be an excellent solvent for polyethylene, as the polymer produced is completely dissolved in the solvent. Consequently, the first commercial plant employed cyclohexane as the solvent. Temperature of polymerization was 125-175°C at reactor pressures of 400-500 psig. The cyclohexane solvent is continuously fed to the reactor along with the silica-supported powder catalyst, ethylene and 1-butene. Polymer solution is continuously removed from the stirred autoclave to a holding tank where unreacted ethylene monomer and 1 -butene are easily flashed off and recycled to the reactor. The solid catalyst is removed from the solution by filtering and the polyethylene is isolated after the solvent has been removed by steam stripping. 

Because of the highly nonideal behavior of polymer-solvent solutions, polymer-vapor equilibrium relations account for the third major difference found in stripping operations with polymeric solutions. The appro-... 

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