GB2164941A - Process for precipitating cellulose carbamate - Google Patents

Abstract

A process for precipitating cellulose carbamate from alkali solution comprises contacting the solution with a solution containing inorganic salt, whereby the cellulose carbamate precipitates. The carbamate solution is spun in a solution containing one or several inorganic salts selected from the group consisting of carbonate, bicarbonate, chloride, sulphate, sulphite and bisulphite.

Description

SPECIFICATION Process for precipitating cellulose carbamate The present invention concerns a procedure for precipitating cellulose carbamate from an aqueous alkali solution.

Cellulose carbamate is an alkali-soluble cellulose derivative which is a compound of cellulose and isocyanic acid. It is possible to produce an alkali solution of cellulose carbamate which can be spun into fibre and film form using a precipitation bath which precipitates the cellulose carbamate from the solution.

For the precipitating bath a solution containing sulphuric acid has normally been used. Cellulose carbamate is stable in acid conditions and it is therefore not decomposed when precipitated.

When the alkali solution of carbamate is contacted with sulphuric acid, the cellulose carbamate is precipitated and at the same time, as the sodium hydroxide is neutralized, sodium sulphate is formed.

Thus, the cellulose carbamate spinning process produces sodium sulphate as a by-product, which being low in price embarrasses the economy of the process. Another, more remarkable, drawback is that the acid neutralizes sodium hydroxide, which cannot easily returned to the process. The sodium hydroxide accounts for a significant part of the raw material costs of the process. It would therefore be desirable to develop a precipitation method in which no undesirable by-products are produced. Such a precipitation process would be particularly desirable in which there would be no need at all to use mineral acid and in which at least a substantial part of the sodium hydroxide could be recovered in an economical way.

All the same, the process should meet certain requirements as regards the fibre or film quality.

For instance, the initial strength of the fibre being precipitated should be sufficient so that the product that is produced could tolerate the mechanical strains which it is subjected to in the treatment. In fibre manufacturing, for instance. Stretching the fibre in the spinning step is essential for achieving the desired strength properities.

As taught by the invention, it has been found that the cellulose carbamate can be precipitated from an alkali solution satisfactorily, as regards fibre or film production, using a precipitation solution which instead of the conventional sulphuric acid contains an inorganic salt.

Therefore, a process is achieved by the teachings of the invention in which it is possible to precipitate cellulose carbamate in a solution which contains no mineral acid at all. In the process of the invention also no salts are produced as by-products from which NaOH cannot be economically regenerated. Furthermore, by the teachings of the invention a process is achieved in which the sodium hydroxide used for dissolving carbamate can easily be recovered and reused.

The procedure of the invention for precipitating cellulose carbamate from alkali solution is characterized in that the precipitation is carried out in a solution containing an inorganic salt.

The use of a salt solution as precipitating bath enables the sodium hydroxide contained in the spinning solution to be recovered and reused. In contrast, when sulphuric acid is used, the sodium hydroxide is converted into sodium sulphate and an equivalent quantity has to be introduced in the process. Sodium hydroxide accounts for a significante part of the raw material costs; therefore, it is possible with the aid of the invention to improve substantially the exonomy of the process.

Another essential advantage of the process of the invention is the feasibility of easy recycling and recovery of the chemicals. Particularly advantageous is then the use of carbonate solution because in that case the recovery and recycling of the sodium hydroxide used for dissolving the carbamate can be advantageously arranged. In principle, it is only necessary to take care that the sodium hydroxide content of the precipitating bath does not become too high. This is achieved in that from the precipitating bath an equivalent of sodium hydroxide is withdrawn and used for preparing cellulose carbamate solution. In principle, no sodium carbonate is consumed in the process, nor any other salts used in the process.

In the process of the invention, the salts contained in the precipitating solution are inorganic salts. Altmost all salts have a precipitating effect at least to some extent, although the best results have been achieved using carbonates, bicarbonates, chlorides and sulphates. Also acid salts such as bisulphates can be used efficiently as precipitates. The best results, regarding both fibre properties and recovery of chemicals appear to bo attainable when carbonate, sodium carbonate in particular, is used for said salt.

The required salt quantity depends, for instance, on which salt is used and on its solubility properties at the temperatures employed. Too low salt content has the consequence that the carbamate does not precipitate fast enough, whereby the strength properties are impaired. When sodium carbonate is used, the appropriate salt content in the precipitating solution is 10-30% by weight, preferably over 20% by weight, most advantageously about 25% by weight. When chlorides are used for salt, the appropriate chloride content in the precipitating solution is in the range 1-30% by weight, preferably in the range 20-30% by weight. When sulphates are used, the appropriate sulphate content in the precipitating solution is in the range 1-30% by weight, most appropriately in the range 20-30% by weight.The quantity of salt is normally regulated to be such that the precipitation takes place fast enough, so that the fibre or film being produced can be spun and stretched.

In addition to an inorganic salt, the precipitating bath may also contain sodium hydroxide. The sodium hydroxide content must not become too high because then the precipitating capacity of the bath is lowered. Consequently, in a continuous process, it is necessary continuously to remove sodium hydroxide at the same rate as it enters together with the spinning solution being precipitated. It has been observed in the invention that the precipitating bath may contain sodium hydroxide up to 6% by weight, preferably at most 4% by weight, before the precipitating capacity is too much impaired.

In the process of the invention, both fibres and films can be produced from cellulose carbamate, applying the conventional processes and means of the viscose method. When producing fibres, the carbamate solution is spun through spinnerettes into a precipitating bath, and when producing films, the solution is pressed through a slit-like nozzle into the precipitating bath. Since in the procedure of the invention no acid at all is used, cheaper materials can be used in the apparatus.

In the procedure of the invention, the carbamate solution being precipitated contains typically 4-15% by weight cellulose carbamate, and the appropriate viscosity of the spinning solution is as a rule in the range 5-50 Ns/m2. The appropriate sodium hydroxide concentration is typically in the range 5-12% by weight.

On completed precipitation, the after-treatment of the fibres or films, such as washing, drying and other steps, can be accomplished using procedures and means known in themselves in the art, which are not objects of the present invention.

The regeneration of the precipitating bath may be accompiished in a number of different ways.

Appropriate methods have been disclosed, for instance, in the Finnish patent applications No.

843813 and 843814 of the same applicant, which have been filed concomitantly with the present patent application.

In the following examples, the procedure of the invention is illustrated more in detail. In the examples cellulose carbamate was used which had nitrogen content 2.1% and DP about 300.

Solutions A-D were prepared from the cellulose carbamate, their compositions being presented in Table I below. The solutions were filtered and deaired by vacuum treatment. The solutions thus obtained were used in the spinning experiments.

Table I Solution Cellulose NaOH ZnO Sodium carbamate content, content, carbonate content, content, % % % % ------------------------------------------------------------- A 7.1 9 - B 6.5 9 0.9 C 6.5 9 - D 6.8 9 - 2 E 7.4 9 1 - Example 1 Solution E was spun at room temperature into a precipitating bath containing 8% sulphuric acid and 20% sodium sulphate. The spinning was caried out in a laboratory spinning apparatus having a nozzle with 100 holes of 50 um size. The physical characteristics of the fibre thus obtained were as follows: Titer 1.1 dtex, strength (air-conditioned) 2.2 cN/dtex, elongation 8%, and wet module 11.4 cN/dtex.

Example 2 Solutions A-D were spun at room temperature in a laboratory spinning apparatus having a nozzle with 100 holes of 50 um size. The properties of the precipitaging solutions and the physical characteristics of the fibres obtained are presented in Table II.

Table II Solu- Carbamate solution Precipitant solution Characteristics of the carbamate fibres Cellulose-NaOH ZnO Na2CO3 Na2CO3NAOH Temperature Titre Strength (air- Elongation Wet module carbamate conditioned) % % % % % % C dtex cN/dtex % cN/dtex A 7,1 9 - - 22,5 0,1 20 2,1 2,1 8 9 A 7,1 9 - - 20 - 20 2,3 1,9 9 4,7 B 6,5 9 0,9 - 20 - 20 1,5 2,0 8 6,4 B 6,5 9 0,9 - 24 0,4 72 3,0 1,8 7 5,1 C 6,5 9 - - 20 3,4 20 1,7 2,0 7 11,4 D 6,8 9 - 2 23 3 20 1,0 1,8 10 E 7,4 9 1 - 25 - 20 1,1 2,0 7 8,7 The test show that with the procedure of the invention comparable results are obtained to those in the case of the product in Example 1, produced by acid precipitation.

Example 3 A carbamate solution was prepared containg 10% by weight cellulose carbamate and 9% by weight NaOH. The precipitation took place at 42-44"C in a similar laboratory spinning apparatus as in Examples 1 and 2. The precipitating bath contained 26% sodium chloride. The physical characteristics of the fibre obtained were as follows: Titre 2.7 dtex, strength 1.5 cN/dtex, elongation 7%, and wet module 8.1 cN/dtex.

Example 4 A carbamate solution was prepared containg 7.4% by weight cellulose carbamate and 10% by weight sodium hydroxide. From the solution, fibre was spun with a pilot plant spinning apparatus, the spinnerette used therein having 100 holes of 50 um size. The spinning took place at room temperature into a precipitating bath containing 25% by weight sodium bisulphate. The characteristics of the fibres obtained were as follows: Titre 2.8 dtex, strength 2. 1 cN/dtex, elongation 4%, and wet module 14 cN/dtex.

Example 5 A carbamate solution was prepared containg 6.5% by weight cellulose carbamate and 9% by weight NAOH and 0.9% by weight ZnO. From the solution, fibre was spun at room temperature into a water solution, containing 15,4% by weight Na2CO3 and 3.1% by weight NaHC03. The characteristics of the fibres obtained were as follows: Titre 0.99 dtex, strength 1.06 cN/dtex, elongation 11% and wet module 12.4 cN/dtex.

Example 6 A carbamate solution according to example 5 was prepared and fibre was spun from the solution to a water solution containg 12.9% by weight Na7CO3 and 4.4% by weight NaHC03.

The characteristics of the fibres were as follows: Titre 0.99 dtex, strength. C.86 cN/dtex, elongation 7% and wet module 12.5 cN/dtex.

Example 7 A cellulose carbamate solution was prepared containing 6.6% by weight cellulose carbamate and 9% by weight NaOH. From the solution fibre was spun at room temperature to a water solution containing 7.0% NaHC03. The characteristics of the fibres of obtained were as follows: Titre 1,08 dtex, strength 0.74 cN/dtex, elongation 11% and wet module 6.7 cN/dtex.

Claims (5)

1. A process for precipitating cellulose carbamate from an alkali solution, characterized in that the solution is contacted with a solution containing an inorganic salt, whereby the cellulose carbamate precipitates.

2. Procedure according to claim 1, characterized in that the carbamate solution is spun into a solution containing one or several inorganic salts selected from the group consisting of carbonate, bicarbonate, chloride, sulphate, sulphite and bisulphite.

3. Procedure according to claim 1 or 2, characterized in that the pdrecipitating solution contains sodium hydroxide maximally 0-6% by weight.

4. Procedure according to any one of the preceding claims, characterized in that the precipitating solution contains sodium carbonate maximally 30% by weight and sodium hydroxide maximally 6% by weight.

5. A process for precipitating cellulose carbamate substantially as hereinbefore described by way of example.