Viscose, process for

SINI Also known as the Double Steeping process. A variation of the viscose process for making regenerated cellulose fibers, in which the treatment with sodium hydroxide is done in two stages, at different concentrations. Invented by H. Sihtola, around 1976. 

Figure 5.12 Main reactions taking place in the viscose process for cellulose regeneration...

It is remarkable that neither of these workers appears to have investigated aqueous preparative methods, despite the fact that Cross and Bevan and their colleagues, from 1892 onward, had been developing the Viscose process for cellulose, and had, in 1907, applied similar reaction-conditions to starch. The formation of a starch viscose imder aqueous conditions was confirmed later by Ost and coworkers, who found that the solution decreased in viscosity on storage ( ripened ), like cellulose Viscose, but with no accompanying separation of starch. Various industrial uses for starch xanthate have since been suggested for example, as a frothing... 

Cellulose xanthate (SELL-you-lohs ZAN-thate) is a compound produced from cellulose in the viscose process for making rayon. The subscript n in the chemical formula indicates that many molecules chain together to make up the polymer. In this case, between 200 and 400 C6H 02 (0H)20CS2Na molecules combine to make a molecule of cellulose xanthate. 

British chemists Edward Bevan (1856-1921) and Charles Cross (1855-1935) develop the viscose process for making rayon. 

Other early polymer materials included Chardonnet s artificial silk, made by regenerating and spinning cellulose nitrate solutions, eventually leading to the viscose process for making rayon (see Section 6.10) still in use today. 

In 1892 Charles Cross, Edward Bevan and Clayton Beadle patented the viscose process for dissolving and then regenerating cellulose. The process was first used to produce viscose rayon textile fibres, and subsequently for production of cellophane film. 

On account of this, and with the great strides being made in chemistry, research was begun to find ways of making artificial yams and fibers. The first successful artificial yam was the Chardonnet artificial silk, a cel-lulosic fiber regenerated from spun nitrocellulose. Further developments lead to the cuprammonium process and then to the viscose process for the production of another cellulosic, rayon [23]. This latter viscose was fully commercialized by Courtaulds in 1904, although it was not widely used in mbber reinforcement until the 1920s, with the development of the balloon t3 e [24]. 

The RDX particle size distribution must be carefully controlled to produce castable slurries of RDX and TNT having acceptable viscosity. Several classes of RDX are produced to satisfy requirements for the various pressed and cast RDX-based compositions. A continuous process for medium-scale production of RDX has been developed by Biazzi based on the Woolwich process (79,151—154). 

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. 

CeUulose is the most abundant polymer, an estimated 10 t being produced aimuaUy by natural processes. SuppUes for the rayon industry can be obtained from many sources, but in practice, the wood-pulping processes used to supply the needs of the paper and board industries have been adapted to make the necessary speciaUy pure grade. Of the 3 x 10 t of wood used by the paper and board industry (13) in 1989, about 6 x 10 t were purified to provide the 2.5 x 10 t of dissolving pulp required by the viscose processes. 

The flow diagram for the viscose process is given in Figure 2. The sequence of reactions necessary to convert cellulose into its xanthate and dissolve it in soda used to be performed batchwise. Fully continuous processes, or mixtures of batch and continuous process stages, are more appropriate for high volume regular viscose staple production. 

Modified Viscose Processes. The need for ever stronger yams resulted in the first important theme of modified rayon development and culminated, technically if not commercially, ia the 0.88 N/tex (10 gf/den) high wet modulus iadustrial yam process. 

The Courtaulds Tencel Process. The increasing costs of reducing the environmental impact of the viscose process coupled with the increasing likelihood that the newer cellulose solvents would be capable of yielding a commercially viable fiber process led Courtaulds Research to embark on a systematic search for a new fiber process in the late 1970s. 

Antlblaze 19. Antiblaze 19 (Mobil), a flame retardant for polyester fibers (134), is a nontoxic mixture of cycHc phosphonate esters. Antiblaze 19 is 100% active, whereas Antiblaze 19T is a 93% active, low viscosity formulation for textile use. Both are miscible with water and are compatible with wetting agents, thickeners, buffers, and most disperse dye formulations. Antiblaze 19 or 19T can be diffused into 100% polyester fabrics by the Thermosol process for disperse dyeing and printing. This requires heating at 170—220°C for 30—60 s. 

Fig. 7. Approximate viscosity values for forming and processing methods (Pa-s x 10 = P).

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

Slurry Viscosity. Viscosities of magnesium hydroxide slurries are determined by the Brookfield Viscometer in which viscosity is measured using various combinations of spindles and spindle speeds, or other common methods of viscometry. Viscosity decreases with increasing rate of shear. Fluids, such as magnesium hydroxide slurry, that exhibit this type of rheological behavior are termed pseudoplastic. The viscosities obtained can be correlated with product or process parameters. Details of viscosity deterrnination for slurries are well covered in the Hterature (85,86). 

Commonly used heat-transfer surfaces are internal coils and external jackets. Coils are particularly suitable for low viscosity Hquids in combination with turbine impellers, but are unsuitable with process Hquids that foul. Jackets are more effective when using close-clearance impellers for high viscosity fluids. For jacketed vessels, wall baffles should be used with turbines if the fluid viscosity is less than 5 Pa-s (50 P). For vessels equipped with cods, wall baffles should be used if the clear space between turns is at least twice the outside diameter of the cod tubing and the fluid viscosity is less than 1 Pa-s (10... 

However, some semiaromatic nylons can give problems as a result of the high melt viscosity. A process for produciag polymers of hexamethylenediamine, adipic acid, terephthaUc acid, and isophthaUc acid has been developed, which iavolves vaporising the salt mixture ia a high temperature flash reactor followed by molecular weight iacrease ia a twia-screw extmder with efficient moisture removal (17). 

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