DD260187A3 - METHOD FOR PRODUCING CELLULOSE SOLUTIONS - Google Patents

Abstract

The invention relates to a process for the preparation of cellulose solutions in alkali liquors. The aim and task is to achieve an improved loop state by keeping the proportion of unresolved cellulose particles as low as possible and to recycle already formed gels into the solution state. The task was solved by heating the cellulose solution to a temperature of up to 373 K after cooling below 278 K, subsequently cooling again and then further processing it at room temperature. The cooling and warming can be repeated several times. The process is suitable for the production of films, films, foams or other shaped bodies of cellulose and for chemical reactions of the dissolved cellulose.

Description

Features of the invention

Surprisingly, it has been found that the gels spontaneously forming on cooling of cellulose solutions in the temperature range above the freezing temperature of the solution and below 278K and held as irreversible are again dissolved by heating up to 373 K under shear stress and another cooling to dissolution temperature. Depending on the degree of polymerization, the cellulose concentration and the solvent composition, heating may lead to precipitation of the cellulose. However, cooling this mixture under shear stress, preferably stirring, again, the cellulose is thereby redissolved. It turned out that the state of gel formation can also be greatly reduced from the outset if the dissolution time to gelation at low solution temperatures is arbitrarily prolonged by interim heating of the solution. This can be achieved by heating the cellulose solution again to preferably 293 to 333 K during cooling and cooling again to below 278 K shortly before gel formation. This process of cooling and heating can be repeated several times.

Due to the interval cooling and heating, the content of undissolved particles can thus be greatly reduced and the Lössenalität be improved.

As cellulose solutions are solutions in aqueous pure or mixed or provided with other solubility-added additives alkali solutions. The advantage of the method according to the invention is that the proportion of undissolved cellulose particles in the cellulose solution is so low that a good filterability is achieved. Furthermore, it is possible to dissolve already reclaimed cellulose again and further process at room temperature as a solution.

embodiments Example 1 (comparative example)

A spruce sulfite pulp from DP 793 is electron-beam degraded to a DP of 538 and thermally activated at 420 K for one hour. 5.77 parts of this pulp treated in this way are pulped in 94.23 parts of an aqueous solution containing 10% NaOH and 2% ZnO as zincate at room temperature and then cooled to 263 K in a vessel with a tempering jacket with stirring. After 35 minutes, a clear solution is obtained, which shows under the microscope but many unresolved fiber fragments and fibers in the stage of Kugelbauchquellung. If the dissolution process continues under cooling, the solution gels after another 10 minutes. Addition of an additional 46 parts of dissolving liquor and further stirring with cooling does not dissolve the gel again over the course of 3 hours. In the gel numerous undissolved cellulose particles are determined under the microscope.

Example 2:

3 parts of pulp according to Example 1 are mixed with 50 parts of an aqueous solution containing 10% NaOH, 2% ZnO as zincate and 6% urea, and dissolved according to Example 1 at 263 K. At this temperature, the solution gels after one hour. The gel is heated after changing the temperature control with stirring for 20 minutes at 313 K. Then the cloudy solution is cooled back to 263 K. The resulting clear solution is warmed to room temperature and is stable for 4 days.

Example 3:

A solution according to Example 1 from 3 parts of pulp and 50 parts of solvent is heated to 373 K for 15 minutes. During heating, the cellulose precipitates. This mixture is stirred for 10 minutes at 263 K and the precipitated cellulose thereby dissolved. The solution, warmed immediately to room temperature, can be stored for several hours.

Example 4:

5 parts of pulp according to Example 1 are dissolved in 95 parts of 10% sodium hydroxide solution at 263 K. At the moment of receiving a clear solution it gels. The gel is heated to 323K with stirring for 20 minutes and then cooled to 263K. A clear solution is obtained, which is immediately warmed to room temperature and is flowable.

Example 5:

A solution according to Example 1 is warmed to room temperature before gelling and after 5 minutes cooled again to 263 K for 10 minutes. This process is repeated 5 times, whereby the cellulose fragments visible microscopically in one drop of solution become less with each cold pass.

Example 6:

A solution is prepared according to Example 5 by circulating it in a tube reactor with cooling and heating zones until the desired degree of dissolution is achieved.

Example 7:

A spruce sulfite pulp degraded by electron beam irradiation to a DP of 394 is dried at 378 K for 16 hours and then thermally activated at 420K. 2 parts of this pulp are mixed with 98 parts of 17% sodium hydroxide solution containing 2% ZnO as zincate and 6% urea, and dissolved according to Example 5 at intervals.

Claims (2)

1. A process for the preparation of cellulose solutions in aqueous 5 to 20% sodium hydroxide solutions without or with generally conventional solubility-improved additives at cellulose contents between 2 to 12% and a cuoxam-DP of cellulose greater than 100 by cooling the cellulose solution in the temperature range above the freezing temperature of the solution and below 278K and then heating to room temperature, characterized in that the cellulose solution is heated after cooling under shear stress to a temperature up to 373 ⁰ K, subsequently cooled again and then further processed in a known manner at room temperature. 2. The method according to item 1, characterized in that the cooling and heating is repeated several times in alternation. Field of application of the invention The cellulose solutions prepared according to the invention are suitable for the production of films, films, threads and other form bodies made of cellulose and for spinning, coating and bonding of objects, but also for chemical reactions of the dissolved cellulose. Characteristic of the known technical solutions The production of cellulose solutions in caustic lye has long been known (Götze, K .: man-made fibers by the viscose process, Springer-Verlag Berlin / Heidelberg / New York 1967, pp. 350-351). Since the dissolving power of pure alkali solutions for cellulose is limited, especially regenerated cellulose or hydrolytically degraded cellulose were used as starting material (DE-PS 155745,418056,523271). The dissolving power of caustic soda can by additions of lithium hydroxide (DE-OS 3247776), zincate (Treiber, EN and Abrahamson, A .: The Paper 13 [1959] 253-262), urea, cyanamide and other carbonic acid derivatives (US Patent 1771461, GBPS 216475, AT-PS 101001, SU-US 950734, Kisperslak, EZ, Paksver, AB, Maljugin, Yu.Ja. and Nikitina, AO: Chim, volokna [1979] 6, 26-27), organic bases such as quaternary ammonium hydroxides (DE-PS 443095) alone, but also in mixtures with each other (DE-OS 3247776, Sarkov, VI and Amosov, VA: Sbomiktr VNII celljulozno-bum. Prom-sti 65 [1975] 119-25, Moscow, Lesnaya prom- st ') are improved. In addition, the dissolving power of the caustic soda or o.g. Mixtures can be increased by lowering the temperature. This makes it possible to dissolve the corresponding starting cellulose more completely or to produce more concentrated cellulose solutions or to use celluloses with a higher DP (DE-OS 3247776, DE-PS 523271, Kirperslak, EZ et al., Chim. Volokna [1979] 6, 26-27). , In addition to the mentioned degradation of the degree of cellulose polymerization or the falling over of the cellulose, other types of activation without or with little decrease of the DP can improve the dissolution of the cellulose in the alkaline media (GB-PS 216475, EP-PS 0147634). However, all of the alkaline systems mentioned have the disadvantage that these cellulose solutions tend to gel especially at higher degrees of polymerization necessary for the strength of the molded articles and / or higher cellulose concentrations (DE-PS 523271, DE-OS 3247776). This gelation takes place spontaneously at room temperature, but particularly rapidly at temperatures below room temperature and is irreversible (DE-PS 523271). The gels thus formed are not dissolvable by further cooling analogous to the dissolution process. It is therefore proposed after the dissolution of the cellulose in the cold, an immediate heating of the cellulose solution (DE-PS 523271, DE-OS 3247776). But always remain undissolved cellulose particles (DE-OS 3247776) and according to their own investigations cellulose fibers in the stage of Kugelbauchquellung back, which complicate the filtration and processing of the solution. On the other hand, however, the heating of the solution must be carried out in a controlled manner, since too high temperatures lead to coagulation of the solution (DE-OS 3247776, DE-PS 523271). Object of the invention The aim of the invention is to keep the proportion of undissolved cellulose particles so low in the production of cellulose solutions in alkali liquors that the filtration or further processing of the solution is substantially facilitated, and that already gelled and retarded solutions can be used again. Object of the invention The object of the invention is to influence the release process, which depends on the cellulose concentration, the degree of polymerization and the temperature, so that the resulting cellulose gels can be redissolved or the dissolution time can be extended to gel formation at low dissolution temperatures.