Manufacture of viscose and of articles therefrom

Details Manufacture of viscose and of articles therefrom Manufacture of viscose and of articles therefrom

2000-03-21

2003-03-25

Wilson, James O. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carbohydrates or derivatives

C536S060000, C536S061000, C604S374000

Reexamination Certificate

active

06538130

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the manufacture of viscose and of articles therefrom, particularly elongate members such as fibres and films.
BACKGROUND ART
The viscose process has been known for many years and is described for example in K. Goetze, Chemiefasern nach dem Viskoseverfahren, Scringer-Verlag, 3rd edition (1967) with considerable detail and in an article entitled “Cellulose” in Ullmann's Encyclopaedia of Industrial Chemistry, VCH Publishing, 5th edition, Volume A5 (1986) (in more general terms).
The conventional steps of the viscose process may be briefly summarised as follows. Dissolving-grade woodpulp (or some other suitable source of cellulose) is steeped in aqueous sodium hydroxide. Excess liquor is removed by pressing to yield alkali cellulose. The alkali cellulose is usually preripened in air for some hours in order to allow oxidative degradation to occur, thereby reducing the degree of polymerisation of the cellulose chains. The alkali cellulose is reacted with carbon disulphide (xanthated) to form cellulose xanchate, which is then dissolved in an aqueous liquor (dilute aqueous sodium hydroxide or water) to form the solution called viscose. The viscose is ripened by storage at or around ambient temperature for some hours, during which time chemical changes, including redistribution of xanthate groups, take place. Other operations, including filtration and deaeration, are also performed on the viscose during this time. In the manufacture of elongate members such as fibres and films, the ripened viscose is then extruded through a die into an aqueous acid bath (usually based on sulphuric acid), where it coagulates, and cellulose is regenerated from the cellulose xanthate. The extrusion process is commonly called spinning in fibre manufacture and casting in film manufacture. The regenerated elongate cellulose member is then washed free of impurities and dried.
In the viscose art, it is customary to express the carbon disulphide content of viscose in terms of the weight of carbon disulphide introduced into the xanthation step as a percentage of the weight of cellulose in the alkali cellulose (% CS
2
on CiA). This is sometimes (with less accuracy) referred to as the CS
2
content based on the cellulose in the viscose (% CS
2
on CiV). The percentage of cellulose in alkali cellulose (% CiA) may be assessed gravimetrically by acidification, washing with water and drying. The Percentage of cellulose in viscose (% CiV) may be assessed gravimetrically by regeneration in sulphuric acid, washing with water and drying. The alkalinity of alkali cellulose and of viscose (% SiA and % SiV respectively) may be assessed titrimetrically by a back-titration, in which the material to be analysed is first acidified with a known amount of sulphuric acid and the residual sulphuric acid is then estimated by titration against sodium hydroxide. Alkalinities measured in this manner are usually reported in terms of the corresponding weight percentage of sodium hydroxide. The weight ratio of alkali to cellulose in viscose is commonly called the soda/cellulose ratio or alkali ratio.
Viscose can be used for the manufacture both of standard (regular, conventional) viscose fibre and of a variety of specialised types such as industrial yard and modal fibres (including polynosic and HWM types), all such fibres being classed generically as viscose rayons. It can also be used for the manufacture of cellulose films and of articles such as food casings and teabags. Such specialised fibres generally demand the use of specialised viscoses, but the other products are generally made using standard (regular, conventional) viscose. In general, the recipes used in the manufacture of standard viscose reflect the desire to minimise costs while maintaining adequate product properties, whereas those used in the manufacture of the specialised types reflect the predominant desire to obtain specific product properties, cost being of secondary importance. It is well-known that product properties are controlled inter alia by viscose parameters at extrusion, including its cellulose content, alkali content, viscosity and degree of ripeness (degree of xanthate substitution, gamma value), and by bath parameters such as sulphuric acid content, sodium sulphate content, zinc sulphate content and temperature. These and other parameters are interdependent, and alteration of one may require alteration to one or more of the others if product properties are to be mainzained or optimised.
Efforts have been made over the years to reduce the cost of the viscose process by reducing the quantities of consumable chemicals, including carbon disulphide, sodium hydroxide and sulphuric acid, used therein. As well as the effects on product properties hereinbefore mentioned, it is well-known that the filterability of viscose is affected by (inter alia) the alkalinity of the alkali cellulose and the dissolving liquor and the amount of carbon disulphide used. It is well-known that filterability varies with the amount of carbon disulohide introduced into the xanthation step. The minimum acceptable amount for industrial-scale operation has reduced over the years from about 30-35% or more to about 25-30% CS
2
on CiA, consequent upon progressive improvements in woodpulp quality, equipment design, the use of chemical additives, and so forth. Similarly, it is well-known that, if the soda/cellulose ratio is permitted to fall below a certain value, then viscose filterability will be adversely affected. It has been suggested that the optimum soda/cellulose ratio is about 0.83, and indeed ratios of this order are used in the manufacture of some of the specialised types of fibre. In the manufacture of standard viscose, the minimum soda/cellulose ratio usable in industrial-scale operation has been found to be about 0.55, a value little changed over a number of years.
It is also well-known that viscose quality ,i.e. filterability) varies with the amount of hemicellulose present during key steps of its manufacture, notably steeping, xanthation and dissolving improved results can be obtained if pure caustic soda is used in the steeping and dissolving steps or if efforts are made to remove hemicellulose from the alkali cellulose before xantharion. Use of pure chemicals may be practical in the laboratory but not in the factory. Processes such as the “double-steeping” or SINI technique described in GB-A-1,256,790 which serve to remove hemicellulose from preripened alkali cellulose have failed to gain commercial acceptance. A considerable proportion of the hemicellulose dissolved during steeping can be incorporated into satisfactory products, e.g. by inclusion in the dissolving liquor, and it would be both commercially and environmentally unsound to discard this material.
DE-A-2,941,624 describes a process for the manufacture of viscose from woodpulp, wherein the woodpulp is irradiated with a dose of high-energy electrons amounting to 1-30 kGy prior to the steeping step and is then steeped in a solution containing less than 19 percent, preferably around 16 percent, by weight sodium hydroxide. This is said to have a number of advantages over conventional processes. The concentration of sodium hydroxide used in the steeping step can be reduced, with the result that savings can be made in sodium hydroxide, carbon distilohide and sulphuric acid consumption. A consequence of reduced carbon disulohide use is reduced environmental pollution arising from carbon disulphide and hydrogen sulphide liberated during regeneration and washing. With a suitably-chosen degree of irradiation, the preripening step can be omitted. Examples are given of the preparation of viscoses of good filterability using between 24 and 28 percent CS
2
on CiV, the product containing 8 percent by weight cellulose and 5.3 or 5.8 percent by weight alkali (soda/cellulose ratio 0.66 or 0.73). SU-A-1,669,916 describes the use of lesser amounts of CS
2
in such a process. Examples are given of the preparation of viscoses of satisfactory filterability using 9.3 to 23.10% CS
2

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