Solvent extrusion

Continuous Solvent—Extrusion Process. A schematic for a typical continuous process, widely used for making solvent propellant for cannons, is shown in Figure 7. This continuous process produces ca 1100 metric tons of single-base propellant per month at the U.S. Army Ammunition Plant (Radford, Virginia). Continuous processes have also been developed for double- and triple-base propellants and for stick as well as granular geometries. A principal aspect of these processes has been the extensive use of single- and double-screw extmders instead of the presses used in the batch process. 

A detailed study of the photophysics and photochemistry of 2,2-diphenyl-hexamethyltrisilane (73) (Scheme 7) has been made. In polar solvents (73) exhibits an intense fluorescence attributed to intramolecular charge transfer, and this solvent effect is also reflected in the photochemistry. In non-polar solvents extrusion of diphenylsilylene and 1,3-silyl migration to give (74) are the major photoreactions but in ethanol-hexane solvolytic cleavage of the... 

Finally, many polyphosphazenes have been studied as tbin films, solvent-extruded fibers, and nanofibers by solvent casting, spin casting, solvent extrusion, and electrospinning methods (Figures 7.2 and 7.5). These techniques have proved useful for semi-crystalline fluoroalkojyphosphazene polymers and for arylojy-substituted derivatives. 

Solventless Extrusion Process. The solvendess process for making double-base propellants has been used ia the United States primarily for the manufacture of rocket propellant grains having web thickness from ca 1.35 to 15 cm and for thin-sheet mortar (M8) propellant. The process offers such advantages as minimal dimensional changes after extmsion, the elimination of the drying process, and better long-term baUistic uniformity because there is no loss of volatile solvent. The composition and properties of typical double-base solvent extmded rocket and mortar propellant are Hsted ia Table... 

Extrusion Processes. Polymer solutions are converted into fibers by extmsion. The dry-extmsion process, also called dry spinning, is primarily used for acetate and triacetate. In this operation, a solution of polymer in a volatile solvent is forced through a number of parallel orifices (spinneret) into a cabinet of warm air the fibers are formed by evaporation of the solvent. In wet extmsion, a polymer solution is forced through a spinneret into a Hquid that coagulates the filaments and removes the solvent. In melt extmsion, molten polymer is forced through a multihole die (pack) into air, which cools the strands into filaments. 

The solvent used to form the dope is evaporated during the extrusion process and must be recovered. This is usually done by adsorption on activated carbon or condensation by refrigeration. For final purification, the solvent is distilled. Approximately 3 kg of acetone, over 99%, is recovered per kg of acetate yam produced. Recovery of solvent from triacetate extmsion is similar, but ca 4 kg of methylene chloride solvent is needed per kg of triacetate yam extmded. 

Dry processes which obviate solvent difficulties are now preferred and are similar to those employed with the major thermoplastics. They include the use of two-roll mills, internal mixers, extruders and extrusion compounders. The use of dry blend techniques similar to that used more recently with PVC have also been used. ... 

It is possible with solvents of a particular composition to determine quantitatively the level of stress existing in certain TP products where undesirable or limited fabricated-in stresses exist. The stresses can be residual (internal) stresses resulting from the molding, extrusion, or other process that was used to fabricate the plastic product. Stresses can also be applied such as bending the product. As it has been done for over a half century, the... 

When cycloaikylidine peroxides (e.g., 59) are heated in an inert solvent (e.g., decane), extrusion of CO2 takes place the products are the cycloalkane containing three carbon atoms less than the starting peroxide and the lactone containing two carbon atoms less" (the Story synthesis). The two products are formed in comparable yields, usually 15-25% each. Although the yields are low, the reaction is... 

Recently, Tsakala et al. (90) formulated pyrimethamine systems based on several lactide/glycolide polymers. These studies were conducted with both microspheres (solvent evaporation process) and implants (melt extrusion process). In vitro studies indicated that pyrimethamine-loaded implants exhibited apparent zero-order release kinetics in aqueous buffer whereas the microspheres showed an initial high burst and considerably more rapid drug release. In vivo studies in berghi infected mice confirmed that the microspheres did not have adequate duration of release for practical application. However, the implants offer promise for future clinical work as more than 3 months protection was observed in animals. 

Porous membranes have been prepared by leaching an additive from films and tubes of PCL (64,72). The procedure involves extrusion or casting blends of PCL and Pluronic F68, the latter being an FDA-approved oxyethylene-co-oxypropylene triblock copolymer. Treatment of the phase-separated blend with aqueous acetone or aqueous alcohols causes both swelling of the polymer and extraction of the Pluronic F68. The induced pore size and void volume may be controlled by the time, temperature, and solvent composition. 

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