Rayon process

Rayon is made from cellulose, a naturally occurring polymer, consisting of D-glucose monomer units joined by 14 glucosidic bonds, which form an ether linkage by the elimination of water. 

In each cellulose unit, where n is the degree of substitution, there are (in+ 2) alcohol groups located at carbons 2, 3 and 6 of each anhydroglucose unit, with one —OH at each end of the cellulose chain. 

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Its early commercial success owed much to the flammabUity disadvantages of the Chardoimet process, but competition from the viscose process led to its decline for aU but the finest filament products. The process is stiU used, most notably by Asahi in Japan where sales of artificial sHk and medical disposable fabrics provide a worthwhile income. However, its relatively high cost, associated with the cotton fiber starting point, prevented it from reaching the large scale of manufacture achieved by the viscose rayon process. 

The recovery of caustic from hemiceUulose (qv) ia the rayon process was well estabUshed ia the 1930s (18), and is still used ia modem times (19) (see Pulp). Very few new iadustrial appHcations of dialysis emerged duriag the 1940—1980 period. More recendy, iaterest has reawakened ia isobaric dialysis as a unit operation for the removal of alcohol from beverages (20,21) and ia the production of products derived from biotechnology (22,23). 

Sodium Sulfate. Although considered a weak builder by some manufacturers, sodium sulfate (Na2S04) contributes little to detergent performance. It is commercially available from natural sources and as a byproduct from rayon processing. In dry mix and agglomerated type products, sodium sulfate is used to improve finished product flow characteristics. In spray-dried products, sodium sulfate acts as an inert filler and aids in density control. 

Another example in which a biocatalytic transformation has replaced a chemo-catalytic one, in a very simple reaction, is the Mitsubishi Rayon process for the production of acrylamide by hydration of acrylonitrile (Fig. 1.42). Whole cells of Rhodococcus rhodocrous, containing a nitrile hydratase, produced acrylamide in >99.9% purity at >99.9% conversion, and in high volumetric and space time yields [121]. The process (Fig. 1.42) currently accounts for more than 100000 tons annual production of acrylamide and replaced an existing process which employed a copper catalyst. A major advantage of the biocatalytic process is the high product purity, which is important for the main application of acrylamide as a specialty monomer. 

The first large scale application of ion exchange to effluent treatment was in the recovery of water, ammonia, and basic copper sulfate from the waste streams encountered in the cuprammonium rayon process. Originally a phenolic type condensation resin was employed, but more recently carboxylic acid acrylic-based exchangers have been introduced. A similar process exists for zinc recovery from the spinning acids of viscose rayon plants, except that in this operation a sulfonic acid resin is employed. 

The oldest and most widely practiced cellulose regeneration technology of the derivatizing solvent-type is the viscose rayon process. It is based, in part, on the discovery in 1857 by Cross, Bevin, and Beadle of the dissolution of sulfidized cellulose in alkali. Viscose fibers are by far the most important cellulose regenerates, amounting to an annual fiber production of 2.5 X 10 t worldwide [13,74]. 

Process diagram of the viscose rayon process (Adopted from Woodings [74])... 

The process of cellulose regeneration in the form of lyocell fibers is significantly simpler than that of the viscose rayon process. It is illustrated in O Fig. 16. A solution containing 14% cellulose, 10% water, and 76% NMMO plus stabilizers is extruded at a temperature slightly above 100 °C into an aqueous NMMO-bath from which cellulose is precipitated [74,75]. The extrusion has been described as a melt-spinning process that has recently made it possible to manufacture cellulosic self-bonded meltblown nonwovens as well [76]. 

Alternatives to alkaline solutions of carbon disulfide are combinations of formaldehyde and DMSO various alkyl silanes, and urea and alkali. The latter is currently being promoted for industrial applications. It is known under the term carbamate process. The carbamate process has advantages over the viscose rayon process while being able to use the same process technology. 

In the viscose rayon process, still used today, cellulose is digested in a concentrated solution of NaOH to convert the —OH groups to —O Na ionic groups. Reac-... 

Dialysis (5) Liquid Difference in diffusion rate across membrane Membrane Recovery of purified caustic soda from rayon process liquid, Vol. 7, p. 14... 

Basically, all methods for producing rayon filaments or fibers depend on solubilizing cellulose, then reshaping it into long-fibered products by extrusion through the small holes of a spinnerette, immediately followed by conversion into solid cellulose. Although there are a number of ways in which this can be done, the viscose rayon process is by far the most important and widely practiced. 

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. 

Figure 14.36. Block scheme of the viscose/rayon process, up to and including the spinner stage, showing the general process conditions and typical addition points for the surfactants...

Godet Roller used in the viscose rayon process to guide rayon tow, when using pairs rotating at different speeds, will impart differential stretch to fiber. 

Xanthate zan- that (1831) n. A sodium salt of a dithiocarbonic acid ester, in particular the one formed in the viscose-rayon process by the reaction between sodium hydroxide cellulose and carbon disulfide and having the structure shown below, called cellulose xanthate or viscose. The viscose is subsequently precipitated, filtered, extruded as... 

Na2S04 Rayon processing and pulp and paper processing... 

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