NO141994B - PROCEDURE FOR THE PREPARATION OF A NITRIT TESTER OF CELLULOSE - Google Patents

Description

The invention relates to a method for production

of a nitrite tester of cellulose, whereby a hydrated cellulose,

before treatment with the nitrosating agent, contains 4 to 12% by weight

water and is reacted with dinitrogen tetroxide or nitrosyl chloride in the presence of a proton acceptor and a reaction solvent which is a swelling or solubilizing agent for the cellulosic reaction product. The water is substantially uniformly distributed in the cellulose reactant, and the reaction is carried out with stirring at a reaction temperature below 50°C, and the solvent is present in an amount sufficient to provide at least

3 parts by weight of solvent for each part of cellulose.

The invention relates to a method for production

of a nitrite tester of cellulose whereby the cellulose is obtained by treating hydrated cellulose containing 4% by weight or more of water distributed substantially uniformly throughout the cellulose, the cellulose being washed with a strongly polar aprotic solvent to reduce the water content of the cellulose to as little as less than approx. 4% by weight of the cellulose. The washed cellulose is then reacted with dinitrogen tetroxide or nitrosyl chloride in the presence of a proton acceptor and a reaction solvent which is a swelling or solubilizing agent for the cellulose-containing reaction product. The reaction is carried out with stirring at a reaction temperature of less than 50°C,

and the reaction solvent is present in an amount sufficient to provide at least 3 parts by weight of solvent for each part by weight of cellulose.

Starting cellulose is suspended in a suitable solvent which includes a swelling or solubilizing

agent for the polymeric reaction product and a proton acceptor. Loose-

Solvents which have been found suitable to serve as both proton acceptor and swelling or solubilizing agent include weak tertiary amine bases, and typical examples of such are pyridine, quinoline and isoquinoline, and also N,N-dialkyl-acrylamides e.g. N,N-dimethylformamide and N,N-dimethylacetamide (hereinafter referred to as DMF and DMAC respectively), and mixtures thereof. Suitable swelling or solubilizing agents are usually solvents capable of dissolving polymeric esters. Typical examples of such swelling or solubilizing agents are ethyl acetate, ethyl formate, benzene, acetone, methyl ethyl ketone and the like, and mixtures thereof. Compounds suitable as proton acceptors are those, as previously described, which are capable of providing both swelling or solubilization to the polymeric nitrite tester and also act as a proton acceptor.

The amount of solvent that can be used to suspend the cellulose is not critical and can be varied over a relatively wide range. But sufficient solvent should be used to avoid difficulties in processing the resulting viscous mixture. It has generally been found that the minimum solvent to polymer weight ratio is 3:1, i.e.

3 parts by weight of solvent for each part of polymer.

It is generally preferred that the solvent is capable of

both swelling or solubilizing the resulting cellulose nitrite tester and also acting as a proton acceptor. Using a single solvent, as opposed to using a mixture of a proton acceptor and a swelling or solubilizing agent, leads to a more economical process since it facilitates recovery and reuse of the solvent material. But if a mixture of a proton acceptor and a swelling or solubilizing agent is used, it is necessary that the mixture contains at least one mole of proton acceptor for each mole of nitrosating agent, e.g. dinitrogen tetroxide.

The invention relates to an improved method for producing the nitrite esters of cellulose. As described, the use of a nitrite ester intermediate of cellulose makes it possible to produce new cellulose esters, e.g. cellulose sulfate. or cellulose nitrate, whereby the properties of the final product can be attributed to the essentially uniform distribution of ester groups among the polymer units in the cellulose. Nitrosation of cellulose is described in US patent 3,702,843. It has been found that the nitrosation of cellulose with dinitrogen tetroxide or nitrosyl chloride and the subsequent sulfation, as previously described, can be improved if the cellulose is in an activated state or suitably activated. In accordance with the improved process, the cellulose is in hydrated form and contains from 4 to 12% by weight of water, and with the water substantially uniformly distributed in the cellulose reactant prior to treatment with the nitrosating agent.

By using hydrated cellulose as reactant, the nitrosation reaction can be carried out in a shorter time by using essentially stoichiometric amounts of the nitrosating reactant. This produces a more homogeneous reaction mixture and product

with greater clarity, and this leads to easier separation of the product from the reaction mixture and reduces the need for filtration.

A further aspect of the invention relates to a variation of the above-mentioned nitrosation process, whereby the cellulose is essentially uniformly hydrated. With this variation, a hydrated cellulose that can contain over approx. 4% by weight of water distributed substantially uniformly throughout the cellulose, washed with a strongly polar aprotic solvent to reduce the water content of the cellulose to less than 4% by weight. It has surprisingly been found that the cellulose remains in an activated state after washing with the aprotic solvent, even though the washed cellulose has a water content of less than 4% by weight. The washed cellulose can then be used by the previously described method for nitrosation with dinitrogen tetroxide or nitrosyl chloride.

On yet another side of the invention is that

have been found that the nitrite substituent groups present in the nitrite esters of cellulose are surprisingly labile. Thus, unlike other nitrites, the nitrite substituent groups on the cellulose can be easily removed to form alkyl nitrites during the formation of nitrite or sulfate esters of cellulose from cellulose nitrite esters and with alkyl nitrites as by-products in the formation of cellulose nitrate or cellulose sulfate esters.

The method in accordance with this invention comprises nitrosating cellulose suspended in a suitable solvent with dinitrogen tetroxide, nitrosyl chloride or mixtures thereof to give the corresponding cellulose nitrite test.

The nitrosating compound is used in the reaction mixture with a molar ratio between anhydroglucose or polymer unit in general and dinitrogen tetroxide or nitrosyl chloride of approx. 1:0.1 to 1:3, and this results in a D.S. of 0.1 to 3. Since the reaction is quantitative, D.S. falls. approximately together

with the molar amount of nitrosating agent used. If nitrosyl chloride is used with DMF or DMAC, a 2.5 to 3.0 times excess of nitrosating agent is needed to achieve these D.S. values. This can be said in another way, namely that one mole of dinitrogen tetroxide or nitrosyl chloride is required to replace one mole of hydroxyl radical in the cellulose, and if nitrosyl chloride is used with an N,N-dialkyl acid amide as proton acceptor, 2, 5-3.0 moles of nitrosyl chloride.

The maximum D.S. for cellulose is approx. 3 and for hemicellulose approx. 2. Thus, for cellulose, the molar amount of dinitrogen tetroxide required to achieve complete esterification is approx. 3 moles per mol with anhydrohexose unit and for hemicellulose approx. 2 moles per mole of anhydropentose unit. The same molar ratios are necessary for nitrosyl chloride to completely esterify each of the above-described classes of celluloses, if DMF or DMAC is not used as solvent, because then the amount must essentially triple. Nitrosating compound may be added in an amount greater than that required for complete esterification, and the only effect is an increased rate of esterification.

The nitrosation reaction is preferably carried out with constant stirring of the reaction mixture. It is preferred to cool the reaction vessel in an ice bath or the like since the reaction is moderately exothermic, and it is desirable to keep the temperature of the reaction mixture below 50°C.

If maximum esterification is desired, complete reaction is indicated by the formation of a clear solution or paste, while partial esterification is indicated by a swelling and/or partial dissolution of the product in the reaction mixture.

The cellulose nitrite tests are relatively sensitive products and decompose immediately after the addition of a protic solvent, e.g. water, methanol, ethanol, isopropanol or the like, in the presence of a mineral acid catalyst. This results in the regeneration of the cellulosic starting material.

Since the cellulose nitrite esters in this process have their primary use as intermediates in the production of other ester derivatives, e.g. polymeric nitrate and sulfate esters and the like, there is no need to isolate the nitrite esters since reaction

the mixture can be used in these processes, as described later.

But the cellulose nitrite testers can be isolated by neutralizing the reaction mixture by adding a base, e.g. selected from mono-, di- and trialkylamines, pyridine, alkali or alkaline earth metal hydroxides, carbonates, bicarbonates and the like. Addition of such a base is only necessary if an N,N-dialkyl acylamide has been used as proton acceptor, due to the fact that during the nitrosation with dinitrogen tetroxide or nitrosyl chloride, an equimolar amount of nitric acid or hydrochloric acid is formed. If a weak tertiary amine base is used, e.g. pyridine or quinoline,

as a proton acceptor, addition of a base is unnecessary since the acid formed is neutralized by the tertiary amine base and cannot serve as a catalyst for cleavage of the polymeric nitrite.

The neutralized or preferably slightly alkaline solution

ing is added to ice-cold water with stirring to separate the cellulose nitrites as a "fibrous material, which can be readily separated. The products having a D.S. considerably below the maximum may be swellable or even soluble in water, and in this case an alcohol is used instead of water.

The isolated product is relatively unstable, and for storage purposes it is preferred that it is solvated in a suitable solvent, e.g. benzene, ethyl acetate, ethylene dichloride, DMF, DMAC or the like, and is stored at a low temperature, preferably below 10°C.

The following examples illustrate specific preferred embodiments of this invention. All ratios in the following examples and also in the description and subsequent claims are weight ratios unless otherwise stated, and temperatures are expressed in degrees Celsius.

Example I

Cotton linter pulp was dried at 100 to 110°C in vacuum

over ^ 2°5 ^ ^ hours. A dried portion of 20 g was placed in a three-neck round-bottomed flask fitted with a calcium chloride tube, a strong stirrer and a dropping funnel, and 1 liter of DMF was added. A quantity of 30 g of N-O. was then introduced with stirring during approx. 30 minutes while the temperature in the mixture was kept below approx.

30°C when cooling in a cold water bath. The mixture thickened, but the reaction was incomplete even after several hours of mixing,

which was shown by the presence of turbidity and apparently unreacted fibers. Addition of an additional 3 g of N 2 O 3 did not result in any significant improvement.

By isolating and analyzing the resulting cellulose nitrite tester. an excess of pyridine was added to a small part of the mixture. The slightly alkaline mixture was added to ice water and the precipitate was collected, washed with ice water and squeezed while the temperature was maintained at ca. 0°C. The material was redispersed in distilled water in a closed Erlenmeyer flask and acidified with sulfuric acid. After stirring with a magnetic stirrer for approx. 1 hour, the mixture was neutralized, and the regenerated cellulose was separated, washed, dried and weighed. The filtrate was collected quantitatively, and the nitrite was determined by oxidizing with permanganate to nitric acid. The presence of nitric acid after the oxidation was ascertained by determining it as nitrone nitrate according to the method of Hick, described in Analyst, vol. 59, pp. 18-25 (1934). The degree of nitrosation was found to be approx. 2.7-2.8.

The cellulose nitrite reaction mixture prepared from 20g cellulose and 30g N2O^ was sulfated by slow addition (about 30 minutes) of a solution of DMF-SO^ complex (about 6 g SO^)

in DMF. The reaction was carried out under vigorous stirring and to the exclusion of moisture, and the temperature was kept below approx. 20°C. The mixture remained cloudy and still contained fibers even after mixing for approx. 1-2 hours. The reaction mixture was then transferred to a mixer, was diluted with approx. 500 ml of water and was adjusted to a pH of 7-8 by slow addition of sodium carbonate solution.

During the neutralization, the temperature of the mixture was kept below

20-25°C when adding ice. Sufficient isopropanol was then added to separate the product, and the product was expressed

set, washed twice with aqueous isopropanol, squeezed again,

washed in vacuum at 100°C and ground. The product had a D.S. of approx.

0.6 and a 1% aqueous solution had a viscosity of approx. 3000 cP.

(Brookfield viscometer, model LVT, 60 rpm) but was somewhat cloudy and contained fibres.

In another experiment, 20 g of anhydrous cotton-linter pulp was treated as described above, but only approx. 15

g N 2 O 4 to the nitrosation resulting in a D.S. of approx. 1.5

and in the subsequent sulphation, an amount of DMF-SO3 complex containing approx. 12 g SO 3 . The results were of a similar nature to those above. The cellulose sulfate had a D.S. on

about. 1.5 and a 1% aqueous solution had a viscosity of approx. 500 cp,

but was cloudy and contained fibers.

Substantially similar results were obtained when, instead of cotton-linter pulp, wood cellulose, cellulose from vegetable pulp or bagasse, or chemically treated and degraded cellulose, e.g. Whatman cellulose powder, was used. But it turned out that the reaction down Whatman cellulose powder proceeded more smoothly and that a solution of its final sulfated product was the least cloudy and contained a smaller amount of fibers than the solutions from the other cellulose types.

Replacement of DMF in the reaction medium with DMAC or pyridine or mixtures of these compounds did not significantly change the results. Nor was any significant difference observed when SO2 was used as a complex together with other solvents, e.g. DMAc, dioxane, DMSO and the like, or when NOCl was used instead of ^0^, but provided that the molar amount of NOCl was increased to 2.5 to 3 times as much.'

Example II

Cotton linter pulp as described in Example I was suspended in water and mixed in a Waring mixer for 2 minutes, squeezed, washed with DMF and squeezed again. The cellulose then had a water content of approx. 8-10%. A portion of 20g of this cellulose was then nitrosated with 30g of 2°4 as described above. The resulting solution was clear and did not contain fibers after a reaction time of 20-30 minutes, and no excess of

^ 2°4 f°r ^ ^ a quantitative turnover. The sulphation was carried out

as described above and resulted in products that formed perfect, clear solutions that did not contain fibers.

Similar results were obtained when the residual water content was 5% and 2% and when the cotton cellulose was replaced with wood cellulose or cellulose from bagasse or chemically treated cellulose.

When the amount of DMF-SO₂ complex added was such that it was calculated to produce a D.S. of below 0.3-0.4, the resulting product was insoluble but highly swellable in water.

Example III

Cotton linter pulp was hydrated by treatment with water in a Waring mixer as described above. The cellulose was then squeezed out and was divided into 4 parts. A portion was dried in vacuum at 30-40°C with continuous mixing down to a water content of approx. 20%. The other three parts were dried under the same ratio of 8-10%, 4-5% and 1-2% water respectively. For the last test, the temperature was increased somewhat. Amounts of 20 g of each of the parts were nitrosated as described above, with 30 g of N204« The reactions with the samples containing 8-10% and 4-5% moisture, proceeded smoothly and were complete after approx. 20-30 minutes during the formation of clear, viscous solutions. The cellulose that contained 20% moisture needed approx. 35 g of N 2 O 4

and formed a clear solution immediately after 5 g in excess had been added. The cellulose part which contained 1-2% moisture, be-

had a long mixing time and even after adding a moderate excess, it remained somewhat cloudy and contained fibres.

Subsequent sulphations using DMF-SO^ complexes which contained approx. 6 g of SO^ , resulted in each case in cellulose sulphates with a D.S. of approx.0.6 which produced glint-

clear solutions in water when celluloses with a moisture content of 20%, 8-10% and 4-5% were originally used. But solutions of the cellulose with the lowest moisture content were somewhat cloudy and appeared to contain fibers.

Similar results were obtained when cotton-linter pulp

was replaced with wood cellulose from vegetable pulp or bagasse, or when DMF was replaced with DMAc, or when instead of the DMF-SO2 complex a complex with dioxane was used,

DMAc, DMSO or the like.

In another identical series of experiments where portions of

20g of cellulose was nitrosated with approx. 20 g N204 (approx. 25 g N204 is needed for a cellulose containing approx. 20% moisture) and sulfated with DMF-SO^ complex which contained approx. 10-12 g SO3, products with D.S. values of 1.1-1.2 were obtained, while the other results were similar.

Nitrosation and subsequent sulphation proceeded in the same way

manner smooth and resulted in sulfated products that formed clear aqueous solutions when a commercial cotton linter pulp or a wood cellulose with a moisture content between 4 and 12% was used directly. If at one time just before the turnover 5% was set,

10% or 20% water to anhydrous cellulose, similar

the results as those obtained in Example I with anhydrous cellulose.

Example IV

Cotton linter mass (400 g) with a moisture content of approx. 5-6% was mixed with 2 liters of DMF in a double-planetary mixer under cooling and exclusion of air, and over a period of 30 minutes 600 g of N20d was added, resulting in a cellulose trinitrite tester.

Claims (1)

Process for the preparation of a nitrite tester from cellulose, whereby the cellulose is reacted with a nitrosating agent which is dinitrogen tetroxide or nitrosyl chloride, in the presence of at least one mole of a proton acceptor for each mole of nitrosating agent and a swelling or solubilizing solvent for the resulting cellulose nitrite, the reaction being carried out with agitation at a reaction temperature below 50°C and the solvent is present in an amount sufficient to provide at least 3 parts by weight of solvent for each part by weight of cellulose, characterized in that the cellulose, before treatment with the nitrosating agent> either contains 4-12% by weight of water which is substantially uniformly distributed throughout the cellulose, or the cellulose has a water content as low as less than 4% by weight of the cellulose, which is achieved by treating cellulose, which has a water content of 4% by weight or more calculated on the cellulose in all substantially evenly distributed in the cellulose, with a strong polar, aprotic solvent for reducing the water content in the cellulose.