FI61033C - FRAMEWORK FOR CLEANING OF ALKALINE CELLULOSE - Google Patents

Description

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Patent · och registerstyrelsen 'AmMun utltgd eeh utl. ) (72) Kurt Ekman, Porvoo, Jouko Huttunen, Porvoo, Olli Turunen,

Porvoo, Suomi-Finland (FI) (7M Forssin & Salomaa Oy (5M Method for the preparation of a time-soluble cellulose compound) - For the preparation of alkali-free cellulose compounds

The invention relates to a process for preparing an alkali-soluble cellulose compound. In particular, the invention relates to a process for preparing a cellulose compound which can be regenerated from an alkali solution in the form of a cellulose film or fiber.

In the manufacture of regenerated cellulose, the so-called a viscose process in which alkali cellulose is first prepared which is reacted with carbon disulphide to form a cellulose xanthogenate. The cellulose xanthogenate can be dissolved in an alkaline solution and regenerated by blending as a cellulose film or fiber. The carbon disulphide used in the process is a highly toxic substance. Attempts have been made to find a replacement chemical that is sufficiently economical to use and does not cause the same environmental damage as carbon disulphide. This has not been successful so far.

The object of the invention is to provide a process for the preparation of an alkali-soluble cellulose compound which can be regenerated from an alkali solution in the form of a cellulose film or fiber in the same way as a product obtained by a known viscose process. Another object of the invention is to provide a process for the preparation of a soluble cellulose compound which does not require the use of carbon disulphide or other chemicals which are equally harmful to the environment.

2 61 033

The process for preparing an alkali-soluble cellulose compound according to the invention is characterized in that the cellulose is heated in the presence of urea in a liquid hydrocarbon containing one or more of the following salts: sodium carbonate, potassium carbonate, sodium cyanate, potassium cyanate, and in which urea is substantially insoluble.

The preparation of cellulose derivatives by urea is previously known. U.S. Patent 2,134,825 discloses a process for preparing a soluble cellulose derivative from cellulose and urea. In the process, the cellulose fibers are impregnated with an aqueous solution of urea, the water is removed by evaporation, and the cellulose is heated to effect a reaction between the cellulose and the urea.

When urea is heated to melting point or higher, it begins to decompose into isocyanic acid and ammonia. Isocyanic acid as such is not a very stable compound, but tends to trimerize to isocyanuric acid. Furthermore, isocyanic acid also tends to react with urea to form biurea compounds. Isocyanic acid also reacts with cellulose to form an alkali-soluble cellulose compound called cellulose carbamate. The reaction can be represented as follows:

Cell - CH2 - OH + HNCO -a- Cell - CH2O - 0 - C - NH2

It is essential to the process of the aforementioned U.S. Patent 2,134,825 that the urea is introduced into the cellulose in the form of a solution, preferably a NaOH solution, and that the actual reaction takes place after removal of the solvent or water in the dry phase. Another less preferred option is to add urea to the cellulose as a melt and perform the reaction in the molten phase.

The product made according to U.S. Patent 2,134,825 is only partially alkali soluble. The solutions contain considerable amounts of insoluble fiber parts, which make filtration difficult and prevent the fibers from spinning through sufficiently small-hole nozzles. According to the patent, the aim has been to improve solubility by adding zinc salts to the sodium hydroxide solution. Poor solubility is probably due to performing the reaction in the dry phase. When water is evaporated out of the fibers, the urea is unevenly distributed in the fibers, tending to travel to the edge portions of the cellulose cake. As a result, the final product is inhomogeneous. In addition, when an aqueous solution of sodium hydroxide is used as the urea solvent, the sodium hydroxide remaining in the fibers after evaporation causes a sharp decrease in the degree of polymerization of the cellulose, which is undesirable for the quality of the final product.

In the process of the invention, the cellulose is heated in the presence of urea 3 61033 in a medium in which urea is substantially insoluble. At elevated temperatures, urea melts and is immediately absorbed into the cellulosic fibers. This is also observed in the fact that the cellulosic fibers swell strongly if the amount of urea is suitable. The reactions in the process of the invention take place mostly within the cellulosic fibers. In this case, no harmful urea migration occurs. For this reason, the resulting product is homogeneous and has good solubility in alkali. Furthermore, the medium prevents the isocyanic acid formed during the decomposition of urea from trimerizing into isocyanuric acid.

The medium in which the cellulose according to the invention is heated in the presence of urea is selected from a hydrocarbon in which the urea is substantially insoluble and whose boiling point is preferably higher than the temperature used in the reaction. The decomposition of urea to isocyanic acid takes place at a suitable rate between 130-150 ° C. Suitable media for use in the method of the invention include e.g. relatively high boiling aromatic hydrocarbons that do not dissolve urea. Xylene has proven to be a very suitable medium, but of course other compliant hydrocarbons can be used.

It has been found that the properties of the cellulose compound formed in the process of the invention are improved if certain salts, such as sodium or potassium carbonate or sodium and potassium cyanate, are used in the reaction. The exact mechanism of action of the salt is not known, but it is possible that when the salts dissolve in the molten urea, the urea is better able to penetrate the cellulose, i.e. swell the cellulose part. In this case, the isocyanic acid formed when the urea decomposes has a greater chance of reacting more evenly with the hydroxyls of the cellulose. Similarly, if the cellulose is impregnated with a saline or alkali solution prior to the reaction, the cellulose remains in a swollen state, allowing urea to penetrate the cellulose. The amount of salt can vary considerably, but a suitable upper limit has been found to be at most an equivalent amount of salt. The salt may be added to the suspension of cellulose and solvent alone before the addition of the urea or together with the urea as a powder or melt.

The cellulose starting material used in the process of the invention may be wood cellulose or cotton or other natural and man-made fibers containing cellulose. The cellulose may be as such or bleached, cellulose hydrate, alkali cellulose or otherwise, for example in acid-treated form. Furthermore, the cellulose used may be in the form of fibers, yarns, fabrics, films, sheets, and the like.

4 61033 The degree of polymerization of the cellulose used is important for the viscosity of the final product. If the starting material is ordinary wood cellulose or cotton, then the viscosity of the resulting soluble end product is high and consequently solutions with a relatively low cellulose content are obtained. When using cellulose with a somewhat reduced degree of polymerization, solutions with a correspondingly higher cellulose content can be prepared. The degree of polymerization of the cellulose used as a starting material can be adjusted, for example, by treating the desired length of time in 18% sodium hydroxide solution. Under the influence of air, depolymerization of the cellulose takes place, which can be stopped at the desired degree of depolymerization by washing with water and drying. The cellulose treated and dry-decomposed in this way is very suitable for use in the process according to the invention as a starting material.

The addition of urea to cellulose can take place in different ways in the process according to the invention. Preferably, the cellulose is first slurried in the spent hydrocarbon medium and urea is added to this mixture as either a powder or a melt. A suitable amount of liquid is 10-30 times that of cellulose. Sodium carbonate or other usable salt may also be added with the urea as a powder or dissolved in molten urea. The salt may also be added first to the medium used and then to the urea as mentioned above.

The invention is described in more detail in the following examples. The starting materials used in the examples were depolymerized and dry-decomposed pulp prepared from soluble pulp for viscose production with a DP of about 700 and an alpha-cellulose content of 94%.

The cellulose sheets were soaked in 18% NaOH solution. The lye solution was squeezed out of the sheets until they weighed 3.5 times the weight of dry cellulose. The alkali cellulose thus obtained was dry-decomposed with a propeller mixer and placed in a sealed bottle.

Cellulose depolymerized under the influence of air oxygen at 30 ° C. The depoly immersion time was adjusted so that the DP of the cellulose after treatment was at 450. The time required for this was about 17 hours. Neutralization with acetic acid, washing with water and drying in vacuo at 60-70 ° C were then carried out. The depolymerized cellulose thus obtained was used as a starting material in the examples.

In the manufacture of cellulosic fibers and films, one of the most important properties is the filterability of the cellulosic solution. The filterability described here is the so-called clogging number, was determined by the method described in H. Sihtola, Paper and Wood 44 ^ (1962): 5, pp. 295-300. The method uses a miniature filter, 5,61033. 2 with an effective area of 3.8 cm and the filter material is Machevey-Nagel MN 616. The filterability is calculated by the formula: v 1,, n4, 60 20.

K =. ! 0 (_---—), where "20,60 60 20 P20 = amount of cellulose solution passed through the filter in 20 minutes (g)

PgQ = amount of cellulose solution passed through the filter in 60 minutes (g) IC, = clogging number W20,60

Example 1

The derivative cellulose digested with alkali for DP taeo 413 was neutralized with acetic acid and washed with water as previously described. 20 g of the cellulose thus treated were impregnated with a 10% KOCN solution and compressed to a compression ratio of 2.5. The cellulose was then torn and allowed to air dry to dry at room temperature. The KOCN content of the cellulose was then 23.6%.

The cellulose thus treated was decomposed on a Warring-Blender dry disperser and suspended in 600 g of xylene. The temperature of the suspension in the reaction vessel was raised to 135 ° with an oil bath. About 100 ml of the water-xylene mixture was distilled off from the mixture under reduced pressure.

The reaction vessel was equipped with a condenser and 18 g of urea was added, decreasing the temperature by a few degrees. Ammonia began to escape through the condenser and at the same time the temperature rose back to 135 ° C. Heating was continued for 3 hours. The reaction mixture was filtered, the fibrous granular material was washed 3 times with water, 3 times with methanol and again 3 times with water at 60 ° C. The product was air-dried and analyzed. It had a DP of 344 and a nitrogen content of 3.5%. The product was dissolved in 10% NaOH solution at -5 ° C. An attempt was made to adjust the ball viscosity of the solution to about 50 seconds. A clogging number of 2470 was obtained from the solution by the above method. The cellulose content was 5.9%.

Organic carbamates are resistant to acid hydrolysis but readily decompose in alkali cooking. Similarly, the product obtained by the process of the invention does not decompose on boiling in dilute acids, but instead decomposes in dilute sodium hydroxide into cellulose, sodium carbonate and sodium cyanate, which can be detected by titration. The amount of nitrogen calculated on this basis was found to correspond closely to the theoretical amount of nitrogen in the carbamate groups.

Adsorption was also observed in the IR spectrum at approximately the same point as with organic carbamates, indicating that it is a carbamate compound.

Example 2

A comparative experiment was performed using the method of U.S. Patent 2,134,825 for preparing a urea-cellulose compound. 24.3 g of cellulose depolymerized as described above were impregnated with a solution of 25% urea, 5% sodium carbonate and the rest water. The mixture was filtered and compressed to a weight of 618 g and placed in an oven at 130 ° C for 2.5 hours.

Dissolution experiments were performed on samples taken from the middle and edge portions, which had been washed as in Example 1. Attempts were made to dissolve the samples in 10% NaOH solution. Visually, it appeared that the product was soluble, but microscopic examination showed that a large amount of insoluble fibers remained from the samples taken from both the middle and edge portions. For this reason, the clogging number could not even be determined. Nitrogen analysis gave 10.2% nitrogen at the edges and 7.5% at the interior.

The above shows that the product obtained by this method is inhomogeneous and only partially soluble. Based on the clogging number, the filterability of the solutions is so poor that it is practically not possible to spin the fibers from the solutions.

Examples 3-5

As in Example 1, experiments were performed in which the starting cellulose was impregnated with different concentrations of KOCN solutions and allowed to air dry to dry at room temperature. The continuation proceeded in the same manner as in Example 1. The results as well as the reaction conditions used are shown in Table 1.

Examples 7-9

As in Example 1, a series of experiments were performed in which the cellulose starting material was impregnated with 5% Na carbonate solution, 10% sodium cyanate solution and 5% potassium carbonate solution. Further work-up was carried out as in Example 1. The results as well as the conditions used are shown in Table 1.

7 61033 TABLE 1

Eg Salt Concentration Urea / Nitrogen DP Dissolution test% pulp% Pulps. Viscosity Clogging% (s) Chapter 3 KOCN 5 3 4.8 346 5.8 48 1378 4 "2.5 3 4.5 333 5.7 47 1768 5" 2.5 3 4.5 371 5.7 56 2160 6 1 2.5 3.3 380 5.0 53 11301 7 Na2CO3 5 4.5 3.9 372 5.6 46 3820 8 NaOCN 10 3 3.4 316 5.8 35 3620 9 K2CO3 5 4.5 4.1 380 5.8 51 7250

Examples 10-13

As in Example 1, a series of experiments were performed in which the cellulose starting material was not impregnated with saline but the salt was added as a powder with the addition of urea. Further work-up was carried out as in Example 1. The results as well as the conditions used are shown in Table 2.

TABLE 2

Eg Salt% pulp- Urea / Nitrogen DP From the leaching test leach pulp%.

^ Sellupit. Viscosity Clogging% ratio (s) 10 KOCN 10 2 3.8 358 5.7 43 5600 11 ·· 7.4 3 4.0 328 5.6 32 13700 12 NaOCN 50 3 3.5 322 5.5 48 18000 13 Na2CO3 25 3 3.8 360 5.4 51 4000 8 61033

Examples 14-16

The molten groove was made into a melt with the salt, which was added to the cellulose starting material as in Examples 10-13. Further processing as above. Results in Table 3.

TABLE 3

Eg Salt% pulp- Urea / Nitrogen DP From the lodge of the dissolution test pulp% Pulps. Viscosity Clogging% ratio (s) figure 14 KOCN 30 3 3.8 346 5.8 57 15000 15 "15 1.6 4.0 377 5.7 55 13700 16 Na2CO3 25 4.5 4.0 331 5.7 46 18700 - <I ______ - 1 -

Examples 17-19

The dry-decomposed bleached sulphate pulp (DP = 750) was suspended in 600 g of xylene, heated to 136 ° C and then co-ground with 24 g of urea and 20 g of salt. Further treatments as in Example 1. Results as well as conditions used in Table 4.

TABLE 4

Eg Salt Nitrogen DP Dissolution Test% Pulps. Viscosity Clogging (s) Chapter 17 KOCN 3.1 643 3.06 53.4 6690 18 NaOCN 2.6 652 3.03 46.8 9010 19 Na 2 CO 3 2.4 660 3.18 53.5 12220

Claims (6)

A process for preparing an alkali-soluble cellulose compound, characterized in that the cellulose is heated in the presence of urea in a liquid hydrocarbon containing one or more of the following salts: sodium carbonate, potassium carbonate, sodium cyanate, potassium cyanate, and in which urea is substantially insoluble. Process according to Claim 1, characterized in that the liquid hydrocarbon is xylene. Process according to Claim 1 or 2, characterized in that the heating of the cellulose is carried out at 130 to 150 ° C, preferably at 132 to 136 ° C. Process according to Claim 1, characterized in that urea is added to the mixture of cellulose and liquid hydrocarbon as a powder for impregnating urea into the cellulose. Process according to Claim 4, characterized in that at most an equivalent amount of one or more of the following salts is added with the urea: sodium and potassium carbonate and sodium and potassium cyanate. Process according to Claim 1, characterized in that the urea is added to the mixture of cellulose and liquid hydrocarbon as a melt.