Spinning stage

After the set ripening period, the viscose is pressurized with air through a final filtering stage and metered with a gear pump, by way of a pivoted rounder end (swan neek) to which a multi-holed precious metal (e.g., Au/Pt) spinneret is attached to the open end. The viscose is extruded horizontally into a lead lined viscose spin bath, through whieh spin bath liquor is eireulated at about 50°C. The size of the jet hole is about 0.05-0.1 mm diameter and the holes 

The viscose coagulates in the spin bath, initially forming a skin, then as the spin bath penetrates through into the center of the filament, all the viscose is converted to cellulose, the NaOH is neutralized by the acid spin bath and CS2 is liberated. Sodium trithiocarbonate, present in the viscose, decomposes to H2S and CS2. 

This combination of hot H2SO4 under reducing conditions, produces an extremely corrosive environment, hence necessitating the use of precious metals for constructing the spinnerets and lead lining for the material of the spin bath. 

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Over 90% of commercially produced silk is by the domesticated silkworm Bombyx mori. This is a monophagous insect with a diet that is restricted to the leaves of white mulberries, Moms alba and Moms indica. The production of silk requires the maintenance of mulberry plantations (moriculture), the rearing of silkworms to the cocoon spinning stage (sericulture) and finally the unravelling of cocoons (reeling). Each subsection of the production will be outlined below. 

Figure 14.37, shows a block scheme of the viscose/rayon process from the spinning stage to production of the final viscose/rayon material. In this process, two types of surfactant process additives are introduced, i.e. spin bath additives and lubricant finishing additives. 

On the other hand, small amounts of flame-retardant monomeric units are polymerized directly into the chain molecule of synthetic fibers such as poly(acrylonitrile) and poly(ethylene terephthalate). The flame retardants are bound so strongly into the chain that, in contrast to low-molar-mass substances, they cannot be removed by repeated washing of the textiles. To a limited extent, however, PAN, PET, and PP are also made fire resistant by addition of additives previous to or at the spinning stage. 

Increased pressure helps to suppress off-gassing at the spinning stage. 

Also, fibres pre-treated at the spinning stage are now available, requiring only RFL treatment, although as with polyester, high treatment temperatures are required to achieve the full levels of adhesion. 

The dry spinning stage is completed after the wash liquor has passed through the cake and the dry solids are removed by the hydraulically activated peeling device, down to the residual cake heel. The peeled solids are removed from the basket through a chute or by a saew conveyor. The residual heel left in the basket after a batch of solids has been removed serves as the primary filter medium for the next cycle, and so on, until it becomes impermeable and needs to be removed by washing with filtrate. 

Water is continuously added to the last extraction bath and flows countercurrenfly to filament travel from bath to bath. Maximum solvent concentration of 15—30% is reached in the coagulation bath and maintained constant by continuously removing the solvent—water mixture for solvent recovery. Spinning solvent is generally recovered by a two-stage process in which the excess water is initially removed by distillation followed by transfer of cmde solvent to a second column where it is distilled and transferred for reuse in polymer manufacture. 

It is possible to add modifiers or delustrants at the dissolving stage. However, modem viscose dope plants feed several spinning machines which are often expected to make different grades of fiber. It is therefore now more common to add the materials needed to make special fibers by injection close to the spinning machines. 

Application. In the past, the break strength of a synthetic yarn has averaged 34.6 lb. The first-stage draw ratio of the spinning machines has been increased. Production management wants to determine whether the break strength has changed under the new condition. 

An example of a typical turboexpander is shown in Fig. 29-46. Radial-flow turbines are normally single-stage and have combination impulse-reaction blades, and the rotor resembles a centrifugal-pump impeller. The gas is jetted tangentially into the outer periphery of the rotor and flows radially inward to the eye, from which the gas is jetted backward by the angle of the rotor blades so that it leaves the rotor without spin and flows axially away. 

Structure calculation algorithms in general assume that the experimental list of restraints is completely free of errors. This is usually true only in the final stages of a structure calculation, when all errors (e.g., in the assignment of chemical shifts or NOEs) have been identified, often in a laborious iterative process. Many effects can produce inconsistent or incorrect restraints, e.g., artifact peaks, imprecise peak positions, and insufficient error bounds to correct for spin diffusion. 

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