DE1173077B - Process for the production of cellulose triacetate - Google Patents

Description

Process for the preparation of cellulose triacetate Acetylation of cellulose-containing material, such as cotton linters, wood pulp and the like, with assistance small amounts of sulfuric acid is known.

The German patent specification 243 581 (1912) already recommends at the esterification of cellulose from this catalytic ability of sulfuric acid Make use of it by being in the presence of less than 0.2% by weight of pure cellulose works on sulfuric acid. This was in contradiction to the previous one Belief that only significantly larger amounts of mineral acid have the desired effect could produce. However, the process is only used on very pure cellulose material, such as filter paper, pure cotton and the like, or on already digested "hydrocellulose" demonstrated and for this reason alone remained without practical application. A pre-treatment stage is not provided, and the treatment temperature should be above 100 "C. It has been shown in practice. that in this process during the Verestelung a much too extensive degradation of the molecular length occurs, so that the product of is of little value.

Observe a certain amount of progress ei st proposals that include a pre-treatment stage with the help of sulfuric acid in small quantities.

For example, the German patent specification 499 212 (1921) an impregnation of the cellulose in the first stage with a mixture of acetic acid and very little sulfuric acid, but that continues later in the acetylation considerably more sulfuric acid has to be added. This procedure resulted still with partial degradation of the cellulose into non-uniform products, although there is already an advance over the earlier mentioned in the patent specification itself Method according to the American patent 733 729 meant.

Also the method according to the USA patent specification 1 503 604 (1922), in which the cellulose is pretreated with dilute sulfuric acid alone and this Should be removed again before the acetylation, could not in practice push through.

In a monograph "Cellulose acetate" published in 1933 Krueger then introduced the previously known methods for the pretreatment of the Cellulose together before acetylation (loc. Cit., Pp. 125 to 132). From work it can be seen that at this point in time it was already known in principle, the pretreatment with acetic acid or glacial acetic acid and a little sulfuric acid as a catalyst, then the actual acetylation with acetic anhydride in the presence of Acetic acid is carried out.

In the numerous literature references cited in this work and in it The procedure described is, however, insofar as quantities are given at all, the amount of sulfuric acid used is so high that, if it remains in the cellulose, causes undesired degradation of the molecular structure during acetylation; especially when working at relatively high temperatures. This also applies to newer known methods, e.g. B. for that in the U.S. Patent 2,487,892.

Since the use of higher acetylation temperatures, however, from different It is highly desirable for reasons to be based on the procedure that is known in its basic features for the acetylation of cellulose in the presence of sulfuric acid as a catalyst designed so that z. B. technically also with acetylation at the boiling point flawless products are obtained.

The method according to the invention solves this problem in a surprising manner Way. It has been found that one can easily and quickly become one Particularly stable, highly viscous cellulose triacetate comes in when you consider the quantity range for the addition of sulfuric acid within very specific limits and in the pretreatment stage a completely uniform distribution of the sulfuric acid over the acetylated Well achieved. One can then without prejudice in the second stage, i. H. at the actual Apply acetylation, temperatures of 600 C and higher and still get first class and Highly viscous products which are completely resistant to the effects of heat and which are excellent suitable for melt spinning.

The method of the invention for making durable highly viscous cellulose triacetate through uniform impregnation of cellulose materials with aqueous, highly concentrated acetic acid or glacial acetic acid and with a low Amount of sulfuric acid in the first stage and subsequent acetylation with acetic anhydride in the presence of acetic acid in the second stage is characterized in that cotton lint or wood pulp with highly concentrated aqueous acetic acid, which can be replaced up to half by acetic anhydride, or glacial acetic acid and such an amount of sulfuric acid that 0.02 to 0.75 0 / o of the dry weight of the Cellulosic materials corresponds to about 1 hour or more at about 25 to 40 "C pretreated and the so evenly impregnated cellulose materials with a Mixture of acetic anhydride and acetic acid briefly at temperatures above 60 C acetylated until the triacetate stage is reached.

It was found that an equivalent to the above Amount of sulfuric acid when introduced into the cellulose in the manner described is able to catalyze the esterification so effectively that easily clear, essentially gel-free solutions arise. This can result from the small the amount of sulfuric acid present increases the esterification temperature above 60 "C, without an inadmissible degradation of the cellulose molecules and a resultant Causing loss of viscosity. It is actually possible to get such high temperatures to use that the esterification can be completed in a few minutes, z. B. in 5 to 15 minutes without causing an unacceptable loss of viscosity. This is of course a great advantage, as it creates the possibility is to carry out the esterification as a continuous process.

The sulfuric acid is due to the addition of glacial acetic acid or more highly concentrated aqueous acetic acid evenly distributed throughout the cellulose; possibly some of the acetic acid, but preferably not more than half, can pass through Acetic anhydride can be replaced. The acid mixture can be sprayed onto the cellulose which is then stirred or circulated during the entire pretreatment, or the cellulose can be immersed in the acid mixture. These operations are carried out at 25 at 400 C temperature. The cellulose on which the acid mixture has been sprayed on or has been immersed in the acids, must have sufficient time be left to allow the pretreatment acids to penetrate evenly to ensure all parts of the cellulose. Generally this is penetration complete after one hour, but the pretreatment can also last for a longer period of time Time to be stretched.

The uniform penetration of the cellulose with the pretreatment acids can be accelerated by first placing the cellulose in an aqueous liquid, z. B. water or a solution of aqueous acetic acid is soaked that already may contain some of the sulfuric acid. After soaking, the cellulose can squeezed or squeezed to remove some of the liquid, before the actual pre-treatment begins.

In a similar way, the cellulose after the pretreatment before the Acetylate are squeezed to remove the proportions of acetic acid and sulfuric acid in the cellulose to the value according to the invention, d. H. to 0.02 to 0.75 ovo preferred number 0.02 to 0.50 / ,, of the cellulose dry weight to reduce.

It can be seen that the viscosity of the cellulose triacetate obtained depends directly on the rate of acetylation. The amount by which the temperature of the acetylation mixture increases, of course, depends on a number Factors including the amounts of acetic acid and any present Anhydride in relation to the amount of cellulose, the presence or absence of small amounts of water in the pretreated cellulose when mixed with the acetylating liquid is brought into contact, as well as the degree of any during the acetylation applied cooling or heating and the amount of sulfuric acid present. The viscosity of the product can therefore be controlled to a certain extent, z. B. by choosing the starting temperature of the acetylating liquid so that the temperature rises by itself to the appropriate level during the acetylation.

The relatively high acetylation temperatures according to Invention come to application, bring important advantages among other things At these temperatures the cellulose triacetate becomes lighter from the acetic acid dissolved than at the previously used temperatures and solutions are achieved, which have a lower viscosity at the concentration concerned.

As a result, it is possible to use considerably less acetic acid than before, but still a clear, gel-free solution with considerable savings is achieved by acetic acid. The proportion of liquid in the acetylation mixture can be so low that the mixture forms a wet fibrous mass before acetylation of cellulose is present, in contrast to the previously customary slurry of cellulose in an acetylating liquid.

Another advantage is the ability to perform acetylation The boiling point of the acid, a very simple procedure to remove superfluous, in the acetylation generated heat, if any, to dissipate and to remove the Keep the acetylation temperature constant. The acetic acid vapors that are formed can be fed to a return condenser and returned to the after condensation Acetylator are recycled, or if there was enough acetic acid originally is present, some or all of the vapor from the acetylator can be a separate Container and condenser are fed, reducing the load on the acetic acid recovery system is significantly reduced.

Should work at temperatures around the boiling point of acetic acid it is generally advisable to use sulfuric acid as a catalyst, whose concentration does not exceed 0.1% to avoid any possibility of degradation to avoid the cellulose.

In general, it is neither a precooling of the acetylating liquid Cooling of the acetylation mixture, for example by means of a heat exchanger, is also required. In general, one must initially heat the acetylation mixture even from the outside add, especially if less than 0.1% sulfuric acid is used.

There is a particular advantage of the method according to the invention in that the products are immediately extremely heat-stable, so that a separate No need for stabilization treatment. The cellulose triacetate can be obtained from the acetylation solution be precipitated as soon as the acetylation is complete, e.g. B. by adding Water or dilute acetic acid. After the cellulose triacetate washed and is dried, it generally still has a sulfate content of at most about 0.015% and is suitable for the production of fibers without further treatment or movies. However, if desired, the cellulose triacetate can remain in solution and still to be matured, e.g. B. up to acetone solubility, preferably at temperatures above 100 ° C, in particular at around 100 to 1500C.

The following examples serve to illustrate the process according to of the invention, the proportions being parts by weight.

Example I 15 parts of ground wood were at a temperature of 25 "C In water containing sulfuric acid at a concentration of 0.023 ° / o for 1 hour, soaked, whereupon the wood pulp is washed out first and then in 98 above was soaked in aqueous acetic acid. After about 1 hour, the bulk of the Drain the liquid and squeeze out the pulp until it is about three times that its dry weight in liquid.

This completed the pretreatment. The pre-treated wood pulp was then divided into about 215 parts of a mixture of acetic anhydride containing 96% anhydride and acetic acid, stirring the mixture until clear Solution had formed. The reaction conditions were varied so that the temperatures could rise to different heights in the different rows.

The triacetate was made from the clear solution by mixing with dilute Acetic acid precipitated in a known manner and the precipitate washed and dried. Three typical series gave the numbers appearing in Table 1. In all cases the cellulose triacetate had good heat resistance, although none has been subjected to a separate stabilization treatment of some kind.

Table 1 Maxunale Maximum acetylation viscosity of Acetylation time of cellulose triacetate temperature 110 "C 1.5 dl / g for 5 minutes 80 "C 35 minutes 2.1 dl / g 60 "C 345 minutes 2.6 dl / g The intrinsic viscosity was calculated on the basis of the relative viscosities of dilute solutions of cellulose triacetate (0.1 to 0.5%) in 90: 10 (volume) mixtures of methylene chloride and methanol, the cellulose triacetate concentration units being »grams completely dry triacetate per deciliter of solvent «were used.

Example II 50 parts of ground wood were sprayed over with 150 parts of glacial acetic acid, of the 0.025 part sulfuric acid (i.e. 0.05%) the weight of the cellulose) and shaken for 2 hours at room temperature. The pre-treated cellulose were then about 260 parts of a mixture containing 96% acetic anhydride of acetic anhydride and acetic acid added, the mixture under a reflux condenser was held and stirred until a clear solution had formed. The achieved The temperature was 116 "C and the solution became clear after 30 minutes. The cellulose triacetate was then precipitated, washed and dried. It had its own viscosity of 1.4 dl / g and excellent heat stability.

Example III Three rows were made under the conditions of the example II carried out, with the exception that the pretreatment liquid 0.25 parts of sulfuric acid (i.e. 0.5% by weight of the cellulose). The result of these series is listed in Table 2.

Table 2 Maximum | Acetylation genes Viscosity of Acetylation time ß cellulose triacetate temperature 70 "C 41 minutes 1.5 dl / g 88 ° C 9 minutes 1.2 dl / h 95 "C 6 minutes 0.8 dl / g The heat resistance of all of these products was good.

Example IV 50 parts of cotton were mixed with 150 parts of glacial acetic acid, the containing either 0.1 or 0.25 parts sulfuric acid, sprayed over and for 1 hour shaken at room temperature. The pretreated cotton was then about 260 Parts 96 or 600 / 0ges acetic anhydride (with the remainder in both Cases of acetic acid) are added, stirring the mixture until the solution had cleared, whereupon the cellulose triacetate precipitated, washed and dried. The results are shown in Table 3.

Table 3 Amount of Acetic Acid- Maximum sulfuric acid (based on anhydride- acetylie- acetylie- concentration time Cellulose) temperature 0.50 in 960 / »76" C 29 minutes 0.2 0.66 0.16 "C 27 minutes 0.5 ° / o 60 ° / o 81 "C 35 minutes 0.2 ° / o 60 ° /. 105 "C 23 minutes In either case, the heat resistance of the product was good.

Example V Wood pulp (40 parts) was in water for 1 hour from Soaked 25 "C, washed out once with glacial acetic acid, expressed to about four times its original weight, into four times the amount (of the original weight) glacial acetic acid, which contained 0.04 part sulfuric acid, of 25 "C introduced and after I hour expressed to four times its original weight.

The resulting pretreated wood pulp was turned into a large Amount of 96% acetic anhydride (the remainder consists of acetic acid) at 25 ° C, soak for 5 minutes and then four times as much its original weight expressed and left standing. The temperature of the mass rose to 87 "C, a clear solution had formed after 240 minutes, from which the cellulose triacetate produced was precipitated, washed and dried. The intrinsic viscosity of the cellulose triacetate was 1.3 dl / g, with its heat resistance was good.

The following examples and comparative tests show the improvements which are opposed by the inventive production of melt spinning masses the prior art (Example VIa and VIIa) can be achieved. The percentages for H2SO4 refer to the dry weight of the cellulose as 1000/0.

Example VIa (comparative test according to the prior art) 1. Pretreatment: Spraying 1 part of cellulose with 1 part of glacial acetic acid (without H2SO4); Maintained at 36 "C for 1 hour.

2. Acetylation: With 7.8 parts of acetic anhydride in 12 parts of acetic acid with 80 /, H2S04; Temperature rise to 50 "C.

3. Neutralize the acid with magnesium acetate.

The result of melt spinning: Brown threads of low tensile strength and low elongation at break.

Example VIb (method according to the invention) 1. Pretreatment: spraying of 1 part cellulose with 1 part glacial acetic acid containing 0.2 ° ío H2SO4; 1 hour at 36 "C held.

2. Acetylation: With 4 parts of acetic anhydride in 2 parts of glacial acetic acid (without further addition of H2SO4); Temperature rise to 92 "C.

3. Neutralization: Not done.

Result with melt spinning: As good as undyed threads, tear resistance 1.3den; Elongation at break 38% ».

Example VITa (comparative experiment) This procedure corresponded approximately the method according to US Pat. No. 2,487,892.

1. Pretreatment: Spraying 1 part of cellulose with 3 parts of 99 ° / Oiger Acetic acid, containing 0.30 /, H2SO4; Maintained at 309C for 2 hours.

2. Acetylation: With 3.3 parts of anhydride containing 0.70 / 0 H2SO4, Initial temperature 28 "C, increase to 63" C after 20 minutes.

3. Neutralization with magnesium acetate.

The result of melt spinning: Yellowish threads with tear resistance of only 0.6 g / den and an elongation at break of 10%.

Example VIIb (method according to the invention) 1. Pretreatment: spraying of 2 parts of cellulose with 1 part of 990% of acetic acid, containing 0.1% of H2SO4.

2. Acetylation: With 3 parts of acetic anhydride in 2 parts of acetic acid (without additional H2SC) 4); 60 minutes at 60 to 83 "C.

3. Neutralization: Not done.

Result with melt spinning: As good as undyed threads, tear resistance 1.2 g / den; Elongation at break 330/0.

Claims (1)

Claim: Process for the production of permanent, highly viscous Cellulose triacetate through uniform impregnation of cellulose materials with aqueous, highly concentrated acetic acid or glacial acetic acid and with a small amount Sulfuric acid in the first stage and subsequent acetylation with acetic anhydride in the presence of acetic acid in the second stage, d u r c h e -k e n n z e i c h n e t that one can use cotton linters or wood pulp with highly concentrated aqueous Acetic acid, up to half of which can be replaced by acetic anhydride, or Glacial acetic acid and such an amount of sulfuric acid as 0.02 to 0.750 / »of the dry weight that corresponds to cellulosic materials, about 1 hour or more at about 25 to 40 C pretreated, and the so evenly impregnated cellulose materials with a Mixture of acetic anhydride and acetic acid briefly at temperatures above 60 "C acetylated until the triacetate level is reached.