This invention refers to a process for the preparation of alkali cellulose from a cellulosic raw material.
In the preparation from cellulosic raw material of fibres and films consisting of regenerated cellulose, or of cellulose ethers consisting, for instance, of carboxymethyl cellulose, the first processing stage is generally the preparation of alkali cellulose. The method of preparation according to known methods is as follows:
The cellulose pulp used as a raw material is treated in a solution of sodium hydroxide (NaOH) of sufficient concentration to change the cellulose to alkali cellulose. This process, or steeping, is carried out either by submerging the sheets of cellulose in a solution of NaOH or by making a slurry of cellulose in a solution of NaOH, such that the cellulose content of the suspension, or slurry, is 5%. The concentration of the NaOH solution used in the steeping is generally in the range 17 to 22% NaOH.
The excess NaOH solution is removed by pressing so that alkali cellulose is attained in which the NaOH content is 15 to 17%. The alkali cellulose obtained is flaked or shredded.
The alkali cellulose is aged by being kept exposed to the influence of atmospheric oxygen at a temperature between 20.degree. C. and 60.degree. C. During the aging the molecular chains of the cellulose break down in such a way as to reduce the average size of the molecules. The average size of molecules desired is determined by variation of the temperature and duration of the aging. The reaction can be accelerated by the use of catalysts, for example salts of cobalt or manganese.
Alternatively it is possible to add to the shredded cellulose only the quantity of NaOH solution required to obtain the desired composition of alkali cellulose directly.
The usual cellulose-containing raw material used is bleached sulphite pulp or pre-hydrolysed sulphate pulp. In particular in the preparation from alkali cellulose of cellulose xanthate solution or viscose, high quality requirements are set for the cellulosic pulp used. In the preparation of viscose it is especially important that the cellulose xanthate prepared from the alkali cellulose be as completely soluble as possible in order to avoid difficulties in the filtration of the viscose. This requirement is met only by pulps specifically manufactured for the viscose industry, or so-called dissolving pulps, which are either sulphite or prehydrolyzed sulphate pulps and have an alpha-cellulose content which is generally in excess of 90%.
Other pulps, in particular sulphate pulp for the paper industry, have not been suitable in the viscose industry, because in particular with paper-grade sulphate pulp, a viscose is obtained which is of exceptionally bad filterability, containing swollen gels and insoluble fibre particles. In the case of sulphate pulp this is caused by the fact that in the alkali cooking an inadequate quantity of structurally detached areas is formed in the walls of the cellulose fibres for the formation of alkali cellulose to begin. In an acidifying cooking stage as in sulphite cooking or in acidifying pre-hydrolysis, on the other hand, these areas are formed by the removal of hemicellulose by the influence of hydrolysis.
The pulp industry, which earlier used the sulphite method (acid cooking) more than the sulphate method (alkali cooking), has continuously moved more and more to the use of the sulphate method. No new sulphite pulp factories are constructed, and old sulphite pulp factories are closed down or converted to the sulphate method. The reasons for this are both environmental and economic.
In the preparation of high alpha-cellulose content sulphite pulp or, particularly, pre-hydrolysed sulphate pulp, a part of the alpha cellulose is lost, giving a lower pulp yield. This means that the price of the finished cellulose rises in proportion to the alpha cellulose content. If the alpha cellulose content is raised by 1%, the yield falls by about 3%. The present price difference is such that dissolving pulp is approximately 30% more expensive than the normal pulp used by the paper industry. For this reason it would be advantageous to use, for example, paper-grade sulphate pulp in the preparation of alkali cellulose if there were no technical obstacles. The advantages of sulphate pulp would also be the low resin content and small polydispersity, which are important in respect of the quality of viscose fibres.
According to Finnish Pat. Nos. 41543 and 41544 it is possible with the so-called double steeping method to prepare filterable viscose also with lower quality pulps, such as normal sulphate pulp. In the process represented in these patents the alkali cellulose is steeped again after the aging. The disadvantage arises in comparison with the normal method that twice the number of steeping presses are required, leading to higher capital and running costs.
By using various additives in the steeping lye an improvement in the reactivity of the alkali cellulose has been attempted, but this has met with only slight success. The use of various additives in the steeping lye is known and to a certain extent used in the industry. Additives are used inter alia to disperse the pulp resins, to improve shredability or to improve the sulphidizing (xanthation). However, for the preparation of alkali cellulose, dissolving pulp has always been used and the quantities of additives used has been small.
Surprisingly, however, it has now been established that by the use of a certain quantity of additives in the steeping lye the reactivity of the alkali cellulose can be increased to such an extent that even by steeping paper-grade sulphate pulp a sufficiently reactive alkali cellulose can be obtained for the preparation of viscose, which previously was not possible.
One object of the invention, therefore, is to provide a method by which the raw material basis of the viscose industry may be changed from expensive, specifically-refined dissolving pulp to ordinary paper-grade pulp, which is a completely different raw material from dissolving pulp.