Regenerated celluloses

Use of low viscosity cellulose. Cellulose which has been degraded by acids or by other means may be used as a starting material. Hydrocellulose and cellulose regenerated from viscose have been used. 

Giasson et al. (117) give direct electron microscopy evidence for the helicoidal structure of films of the cellulose acetates and of cellulose regenerated from the GTA films by aqueous ammoniiun hydroxide. 

In the cellulose-regenerating process, sodium hydroxide is initially added such that approximately one hydrogen, believed to be predominately a mixture of the hydroxyl groups on carbons 2 and 3, is replaced by the sodium ion. This is followed by treatment with carbon disulfide forming cellulose xanthate, which is eventually rechanged back again, regenerated, to cellulose. This sequence is depicted in structure 9.6. 

An alternate procedure used in a few specialty applications is the cuprammonium process. This involves stabilization of cellulose in an ammonia solution of cupric oxide. Solubilization occurs by complex formation of cupric ion with ammonia and the hydroxyl groups of cellulose. Regeneration of cellulose, after formation of the desired products, is accomplished by treatment with acid. The main application of the cuprammonium process is for the synthesis of films and hollow fibers for use in artificial kidney dialysis machines. The cuprammonium process yields products with superior permeability and biocompatibility properties compared to the xanthation process. Less than 1% of all regenerated cellulose is produced by the cuprammonium process. 

Cellulose regenerated from the addition compound may demonstrate a certain degree of nitration (0.5—2.2% N). The properties of the compound are similar to those of hydrocellulose, eg, glittering fibers, increased hygro-scopicity and higher reactivity ... 

Cellulose regenerated by treatment with ammonium sulphide suffered extensive degradation on nitration in the presence of dinitrogen tetroxide. 

USSpecifications MlL-C-2G6(Cotton cellulose for use in expl) MIL-C-216 [Wood pulp(sulfite) cellulose for use in expls] and MiL-C-677 (Cellulose, regenerated, strip for use in primer vent seals of small arms ammo)... 

Wood pulp(For Use in Explosives) 9)JAN-C-677 Cellulose, Regenerated Strip(For Use in Ammunition) 10)US Military Specification MIL-C-20301, Cellulose Acetate(For Use in Propellants) 11) MlL-C-5537A(l), Cellulose Acetate ButyratefFor Use in the Manufacture of Organic Protective Coatings) 12)MIL-E-L 0853B(Ethylcellulose)... 

Cellulose, regenerated SCHWEIZER s reagent Organic solvents... 

Moreover, the same term was recently used for small cellulose patches formed as parts of open spin-coat films produced from low-concentration trimethylsilylcellulose solutions and by subsequent heterogeneous desilyla-tion of the primary formed silylcellulose layers under cellulose regeneration [5]. 

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

All the cellulose regeneration processes, with or without chemical modification, cause cellulose molecules to organise in a different crystalline form, called cellulose II and sometimes cellulose IV (especially in MODAL-HWM fibres)... 

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]. 

Complexity, environmental difficulties (especially with CS2 regeneration) and overall slow processing conditions (compared to melt-spun s)uithetic fibers) catalyzed the search for alternative cellulose regeneration processes. 

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]. 

The third type of cellulose regenerate with commercial significance operates on the basis of cuprammonium hydroxide (Schweizer s reagent). The process had significance for fibers in the first half of the 20th century, but it started to decline in the 1960s due to insurmountable obstacles in the recovery of copper ions. The main producer of cuprammonium fibers... 

The methods used for the cellulosic regenerated fibres are the same as those for cotton, with the exception that there is less colouring matter to remove and the severity of the process can be decreased. Thus 1 g per litre of available chlorine will always be sufficient and bleaching must be carried out cold with sodium hvpochlorite. A 015 to 0-3 per cent w/w hydrogen peroxide liquor is used and approximately lb per 100 lb of sodium chlorite would be required. The amount of alkali must be reduced when bleaching... 

An important process is the manufacture of regenerated cellulose applied to make fibers (e.g., rayon) and films (e.g., Cellophane). Solvents used classically in cellulose regeneration are a mixture of carbon disulfide and sodium hydroxide or ammoniacal copper solutions. More recent solvents include N-methylmorpholine-N-oxide and phosphoric acid [4]. The cellulose solution is extruded through nozzles into an acidic precipitation bath and is spun into fibers. Recently, the partly toxic and strongsmelling solvents have been replaced by ionic liquids which are even able to improve the solubility of the slightly soluble cellulose [10]. For example, 1 1 of l-butyl-3-methylimidazolium chloride dissolves lOOg of cellulose at 100°C [11],... 

---