FI69851C - FOERFARANDE FOER UTFAELLNING AV CELLULOSAKARBAMAT - Google Patents

Description

69851 1 Method for precipitating cellulose carbamate For the recovery of cellulose carbamate 5

The invention relates to a process for precipitating cellulose carbamate from an alkaline solution.

Cellulose carbamate is an alkali-soluble cellulose derivative which is a compound of cellulose and isocyanic acid. An alkaline solution can be prepared from the cellulose carbamate that can be spun into a fiber or film using a precipitation bath that precipitates from the cellulose carbamate solution.

15 A solution containing sulfuric acid is normally used as the precipitation bath. Cellulose carbamate is stable under acidic conditions and thus does not decompose upon precipitation. When the alkali solution of the carbamate is contacted with sulfuric acid, cellulose carbamate precipitates and at the same time sodium sulfate is formed upon neutralization of sodium hydroxide.

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The spinning process of cellulose carbamate thus produces sodium sulphate as a by-product, which at low cost strains the economics of the process. Another more significant drawback is that the acid neutralizes sodium hydroxide, which is not easily returned to the pro-25 session. Sodium hydroxide is a significant part of the raw material costs of the process. Thus, it would be desirable to develop a precipitation method that does not generate undesirable by-products. In particular, it would be desirable to have a precipitation process in which no mineral acid is required at all and in which at least a substantial portion of the sodium hydroxide is recovered economically.

The process should still meet certain requirements for fiber or film quality. For example, the initial strength of the fiber to be blended must be sufficient for the resulting product to withstand the mechanical stresses arising during processing. For example, in fiber fabrication, stretching of the fiber during the spinning step is essential to achieve the desired strength properties.

According to the invention, it has been found that cellulose carbamate can be precipitated from an alkali solution satisfactorily for fiber or film production by using a precipitating solution which contains an inorganic salt instead of the usual sulfuric acid.

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Thus, according to the invention, there is provided a process in which cellulose carbamate can be precipitated in a solution which does not contain any mineral acid. In the process according to the invention, salts are also not formed as a by-product, from which NaOH cannot be economically regenerated. Furthermore, according to the invention, there is provided a process in which the sodium hydroxide used to dissolve the carbamate can be easily recovered and reused.

The process according to the invention for precipitating cellulose carbamate from an alkali solution is characterized in that the solution is contacted with a non-acidic solution containing one or more inorganic salts selected from the group consisting of carbonate, chloride, sulphate, sulphite and bisulphite.

The use of brine as a precipitation bath allows the recovery and reuse of the sodium hydroxide contained in the spinning solution. Instead, when sulfuric acid is used, sodium hydroxide is converted to sodium sulfate and a corresponding amount has to be introduced into the process. Sodium hydroxide represents a significant part of the raw material cost, so that the invention can substantially improve the economics of the process.

Another essential advantage in the process according to the invention is the possibility of easy recycling and recovery of chemicals. The use of a carbonate solution is particularly advantageous in this case, since it is then possible to arrange the recovery and recycling of the sodium hydroxide used for dissolving the carbamate 30 in an advantageous manner. In principle, it is only necessary to ensure that the sodium hydroxide content of the precipitation bath does not become too high. This is accomplished by removing from the precipitation bath an equivalent amount of sodium hydroxide, which is used to prepare the cellulose carbamate solution. Sodium carbonate is in principle not consumed in the process, nor are the other salts used in the process.

3 69851 1 In the process according to the invention, the salts contained in the precipitation solution are inorganic salts. Almost all salts work at least to some extent in a precipitating manner, although the best results have been obtained with carbonates, chlorides and sulfates. Acidic salts such as bisulfites can also be effectively used as precipitants. The best results, both in terms of fiber properties and chemical recovery, appear to be achievable when carbonate, especially sodium carbonate, is used as the salt.

10 The required amount of salt depends on e.g. the salt used and its solubility properties at the temperatures used. Too low a salt content causes the carbamate not to precipitate fast enough, deteriorating the strength properties. When using sodium carbonate, a suitable salt content in the precipitation solution is 10-30% by weight, preferably more than 15% by weight, most preferably about 25% by weight. When chlorides are used as the salt, a suitable chloride content in the precipitation solution is between 1 and 30% by weight, preferably between 20 and 30% by weight. When sulphates are used, a suitable sulphate content in the precipitation solution is between 1 and 30% by weight, preferably between 20 and 30% by weight. The amount of salt is normally adjusted so that the precipitation takes place quickly enough to allow the resulting fiber or film to be spun and stretched.

In addition to the inorganic salt, the precipitation bath may also contain sodium hydroxide. The sodium hydroxide content must not become too high, as this will reduce the precipitation capacity of the bath. As a result, sodium hydroxide must be continuously removed from the precipitating solution in a continuous process at the same rate as that which comes with the spinning solution to be doped. According to the invention, it has been found that the precipitation bath can contain up to 6% by weight of sodium hydroxide, preferably at most 4% by weight, before the precipitation capacity deteriorates too much.

In the process of the invention, both fibers and films can be made from cellulose carbamate using conventional viscose process processes and equipment. In the preparation of fibers, the carbamate-35 solution is spun through spinning nozzles into a precipitation bath, and in the preparation of films, the solution is pressed through a slotted nozzle into a precipitation bath. Since no 4 69851 l of acid is used in the process according to the invention, cheaper materials can be used in the apparatus.

The dopable carbamate solution in the process of the invention typically contains 4-15% by weight of cellulose carbamate and a suitable spinning-5 solution generally has a viscosity of between 50-500 P. A suitable sodium hydroxide concentration is typically between 5-12% by weight.

After precipitation, the post-treatment of the fibers or films, such as washing, drying, etc., can be performed using methods and apparatus known per se, which are not the subject of the present invention.

Regeneration of the precipitation bath can be performed in several different ways. Suitable methods are described, for example, in Finnish patent applications Nos. 843813 and 843814 of the same applicant, which are filed simultaneously with this patent application.

The following examples further illustrate the method of the invention. The examples utilized cellulose carbamate with a nitrogen content of 2.1% and a DP value of about 300.

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Solutions A-D were prepared from cellulose carbamate, the compositions of which are shown in Table I below. The solutions were filtered and deaerated by vacuum treatment. The resulting solutions were used in spinning experiments.

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table 1

Solution Cellulose- NaOH- ZnO- Sodium- Carbamate- Concentration Concentrate Carbonate-30 Concentration Concentration

Of 07 στ <7 / o / o fo Λ> A 7.1 9 - 35 B 6.5 9 0.9 C 6.5 9 - D 6.8 9 - 2 F, 7.4 9 1 5 69851 1 Example 1

Solution E was spun at room temperature into a precipitation bath containing 8% sulfuric acid and sodium sulfate. Spinning was performed in 5 laboratory spinning devices with 100 50 μm holes in the nozzle. The physical properties of the obtained fiber were as follows: Titer 1.1 dtex, strength (air-conditioned) 2.2 cN / dtex, elongation 8% and wet modulus 11.4 cN / dtex.

10 Example 2

Solutions A-D were spun at room temperature in a laboratory spinner with a nozzle with 100 50 μm holes. The properties of the precipitating solutions and the physical properties of the fibers obtained are shown in Table II.

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7 69851 1 The experiments show that the process according to the invention achieves comparable results compared with the product of Example 1 carried out by acid precipitation.

5 Example 3

A carbamate solution was prepared containing 10 wt% cellulose carbamate and 9 wt% NaOH. Precipitation occurred at 42-44 ° C in a laboratory spinner similar to Examples 1 and 2. The precipitation bath contained 26% sodium chloride. The physical properties of the obtained fiber were as follows: Titer 2.7 dtex, strength 1.5 cN / dtex, elongation 7% and wet modulus 8.1 cN / dtex.

Example 4 15

A carbamate solution was prepared containing 7.4 wt% cellulose carbamate and 10 wt% sodium hydroxide. The fiber was spun from the solution with a pilot plant spinner with 100 nozzles with 50 holes. Spinning took place at room temperature in a precipitation bath containing 25% by weight of sodium bisulfur. The properties of the obtained fibers were as follows: Titer 2.8 dtex, strength 2.1 cN / dtex, elongation 4% and wet modulus 14 cN / dtex.

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Claims (3)

8 69851 A process for precipitating cellulose carbamate from an alkaline solution, characterized in that the solution is contacted with a non-acidic solution containing one or more inorganic salts selected from the group consisting of carbonate, chloride, sulphate, sulphite and bisulphite. 1 Claims Process according to Claim 1, characterized in that the precipitating solution contains at most 0 to 6% by weight of sodium hydroxide. Process according to Claim 1 or 2, characterized in that the precipitating solution contains at most 30% by weight of sodium carbonate and at most 6% by weight of sodium hydroxide. 15 20 25 30 35