Cooling fiber processing

The process flowsheet for a cellulose acetate fibers process is shown in Figure 19.12. Solvent is removed from the fibers in a dryer by recirculating air. The air is cooled before it enters an absorber where the solvent is absorbed in water. The solvent-water mixture is separated in a distillation column and... 

In a Pasteur pipette loosely place a very small piece of cotton followed by 2.5 g of alumina. Add to the top of the pipette 1.5 mL of styrene and collect 1 mL in a disposable 10 x 75-mm test tube. Add to the tube 50 mg of benzoyl peroxide and a thermometer and heat the tube over a hot sand bath. When the temperature reaches about 135°C polymerization begins and, since it is an exothermic process, the temperature rises. Keep the reaction under control by cautious heating. The temperature rises, perhaps to 180°C, well above the boiling point of styrene (145°C) the viscosity also increases. Pull the thermometer from the melt from time to time to form fibers when a cool fiber is found to be brittle remove the thermometer. A boiling stick can be added to the tube and the polymer allowed to cool. It can then be removed from the tube or the tube can be broken from the polymer. Should the polymer be sticky the polymerization can be completed in an oven overnight at a temperature of about 85°C. 

At higher temperatures than 110°C the rigid PAM main chain acts as a stiff fiber and the Cj2 side chain/C2o eicosane mixture can be considered as a continuous matrix phase. Both phases are dispersed molecularly to foim a molecular composite, though it is only stable at higher temperature than 110°C, because upon cooling the process returns to its original state. 

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. 

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. 

Texturing. The final step in olefin fiber production is texturing the method depends primarily on the appHcation. For carpet and upholstery, the fiber is usually bulked, a procedure in which fiber is deformed by hot air or steam jet turbulence in a no22le and deposited on a moving screen to cool. The fiber takes on a three-dimensional crimp that aids in developing bulk and coverage in the final fabric. Stuffer box crimping, a process in which heated tow is overfed into a restricted oudet box, imparts a two-dimensional sawtooth crimp commonly found in olefin staple used in carded nonwovens and upholstery yams. 

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

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