US2473473A - Method of preparing regenerated cellulose - Google Patents

Description

Patented June 14, 1949 UNITED STATES ATENT OFFICE METHOD OF PREPARING REGENERATED CELLULOSE No Drawing. Application February 5, 1945, Serial No. 576,364

5 Claims. 1

This application relates to the preparation of regenerated cellulose products in which low viscosity cellulose is dissolved in a mixture of N202 and N204 at a temperature below the boiling point of N203. This solution is employed for the preparation of the regenerated-products.

Regenerated cellulose products have previously been prepared by converting cellulose to, for instance, the xanthate form and then coating out onto a film-forming surface. Nowhere in the prior art has it been previously recognized that nitrogen oxides are suitable for use for this purpose, forming regenerated cellulose sheets or filaments having good transparency.

Cellulose has previously been treated with N02 or its dimer N204, but in those cases the primary object was the enhancing of the reactivity of the cellulose and no dissolving of the cellulose was desired, or obtained. In other cases, cellulose has been treated with N02 and an oxidized or a nitrated product has been obtained.

One object of our invention is to provide a method of preparing regenerated cellulose using nitrogen oxides. Another object of our invention is to provide a method of preparing regenerated cellulose showing good clarity, using nitrogen oxides in which the final product contains less than 1% nitrogen and shows no oxidized characteristics. Other objects will appear herein.

We have found that cellulose having a viscosity 1 forming sheets of regenerated cellulose. We have found that filaments of regenerated cellulose may be prepared by forming a cellulose solution in nitrogen oxides as specified and then extruding the solution such as into a precipitating bath or into air to form filaments or fibers of rayon or synthetic yarn.

The cellulose which is dissolved in accordance with our process may be any low viscosity cellulose material, this feature of our invention being critical. If the cellulose material is of medium or high viscosity, such as, acetylation grade cotton linters, or the like, it is necessary that the viscosity be reduced in some manner. This may be accomplished by treating the cellulose with a mixture of acetic and sulfuric acids for a time. For instance, one part of the cellulose mixed with three parts of acetic acid and .09 parts of sulfuric acid, is allowed to mix for a sufficient time to reduce the viscosity of the cellulose to the desired point. Ordinarily, mixing for 4 hours at 100 F. is sufficient to obtain a viscosity below 200 centipoises. However, the time and temperature necessary depend upon the amount of viscosity reduction which is desired. Oftentimes, in the refining of cellulose materials, severe treatments are employed, and, in those cases, the refined cellulose may be employed directly without any preliminary treatment thereof. In some cases, we prefer to add sufiicient alkaline earth salts such as MgCOs, MgO or CaCOz: to effectively neutralize the catalyst present, in this example sulfuric acid. If desired, an alkali metal salt of a weak acid such as sodium bicarbonate may be employed for this catalyst neutralization.

The liquid in which the cellulose is dissolved should be one containing both N203 and N204, at a temperature not to exceed 37 F. Although in practical operation, a temperature within the range of 26-28 F. is quite suitable, nevertheless, temperatures lower than this may be employed, the criterion being that the solution remain liquid at the temperature used.

The dissolving mixture of N202 and N204 may be made in either of two ways:

1. By mixing the oxides themselves. Mixtures which are suitable may vary anywhere from a composition of 97% N204 and 3% N20: to 10% N204 and 90% N202. Cellulose which will not dissolve in either of these oxides by themselves will dissolve in any mixture of the two oxides within the range stated.

2. By incorporating a small amount of water in liquid N204 at a temperature not to exceed 30 F. In mixing water with the N204, it is necessary to use care so that the temperature is kept down, thus preventing the loss of N203. The amount of water which would be mixed with the N204 should be limited, the upper limit being not more than 20% including the water in the cellulose which is to be treated. Ordinarily, an amount of water not exceeding 10% of the N204 would be mixed therewith. Above this proportion, there is danger of causing oxidation of the cellulose, so that the desired product may not be obtained. In order to form a mixture suitable for dissolving cellulose, it is necessary that at least 0.5% of water be incorporated in the N204.

After forming the dissolving mixture of nitrogen oxides, low viscosity cellulose, as prescribed herein, is mixed therewith, ordinarily in the proportion of from 1% up to 810% and even as high as 20% of the dissolving solution, the proportion of cellulose dissolved depending upon its viscosity. For instance, with a cellulose having a viscosity of 2-12 centipoises, a solution on the order of 3-20% may be formed and will be useful for forming regenerated cellulose. With cellulose having a viscosity in the upper portion of the range given (150-200 centipoises), it is desirable to dissolve approximately 1-5% of cellulose in the dissolving solutign to obtain a solution of the best fluidity-tor use. It is usually desirable in practical operation in dissolving the cellulose to also employ an inert solvent either adding the solvent after the solution has taken place, or

diluting the solvent mixturewith the diluent.-

Acetic acid has been found to be well suitedfor this purpose and as much asl9. partsof acetic acid per part of N204 may be used. -I- I,owever, various other solvents, such as .methylenechloride, isopropyl ether, tetrachlorethane, chloroform, propylene chloride, trichloroethylene or ethylene chloride may be used.

To make regenerated cellulose sheeting inaccordance with our invention, the cellulose of the viscosity given is added to the-dissolving medium .Whichi isf kept ata temperature .of 26-37'F. or-be- ,low. lU pon stirring, thecellulose-almost immediately-dissolvedtand is coatedout directly at this temperature onto glassplates cooled to about .25,F. 'Ihe cooling prevents the solvents from evaporating too rapidly thereby preventingbub- -.b a t ye an ra l w' e gel f r t Thesetting or ,gellingmay be accomplished, for instance, by subjectingthe coating to a draft of air-fol a short time-suchfas -5 minutes. Dur-ing this inter-val, the coating sets or gels,.and may then .be zimmflrsed in Water. :After treatment therewiththe coating maybe-stripped from the plate andwashedto'remove all traces ofacid therefrom. Broadly, the formation ofthe regenerated cellulosesheet is accomplished-by coatins a solu r f rabl o in wh ch th in solvent h v lso, be used, o t aifi mri in .suri sa. cau n the coat n to an n was ..i ,to remove acid or other water-solublematerial therefrom. In cases where an inert diluent is vem lm'ed, set in o t o t n ta es pla more readily than-inthose caseswhere the sheet iscoated out, from asolution of the cellulose, using only nitrogen ,oxideswas solvents. If desired, the washings baths for. the regenerated cellulose sheet m'aybe aconnter, current system involving acetic acid and water. -.We have found that the-initial use. of an aceticacidbath facilitatesthe curing operation in forming thefinished sheet. It is also desirable in the finalwash to incorporate asmall amount of water-,solublematerial which acts as a plasticizer in the sheet, for instance, some watersoluble ,polyhydric material which acts as a humectant, such as glycerin or aiglycol.

Cellulose dissolved in mixtures of N2O3-.-N2O4 in accordancewith our-invention is found when regeneratedtherefrom not to-have been oxidized horis the nitrogen content thereof increased to any substantial extent. For instance, cellulose was dissolvedin a mixture of N203 and N204, and one-half of the solution-formedwas immediately regenerated by pouring the solution into distilled water. The remaining portion was held for '66 hours in aDryIce bath and was then regenerated. The materials were dried and gave the following analyses:

Time Carboxyl Nitrogen Hours Per cent Per cent l5 22 6 6 .23 .40

If solutions of cellulose in accordance with our invention are allowed to stand for prolonged periods of time, loss of nitrogen oxides occurs, depending upon the temperature at which the solution is kept. If the solution is kept at a temperature above 40 F., this loss is, fairly pronounced, andgelling will take placelwithina short time. If, however, the temperature is below 40 F., the

- loss of nitrogen oxides is more gradual depending sistant to wetting with the wash water.

upon the temperature at which the solution is kept. Therefore, if the solutions are to be kept for some time, it is desirable that it be at a tem-- DeraturebelowBOi such as 26-28" F. or below,

to avoid gelling.

If .a.so1utionhas gelled, it may be redissolved by, adding nitrogen oxides thereto, particularly N203 or a mixture of N204 with some water therein.

:The solutions of cellulose obtained in accordance with our invention are extremely clear and brilliant, and nofibers can .be discerned therein either in theeunlight for in polarized light. Any

dirt or insolublamatter ca'n'be easily-removed therefrom by filtering these dopes, :such as throughpglass filter, cloth. -Sheets formed from solutions in accordance with our invention give tensile strengths compa abletothat of sheets of cellulose regenerated from; the xanthate.

The following examples illustrate ,our invention:

Example 1 Sixty parts of cellulose having a cupramm'on-ium viscosity of 5 centipoiseswere added to a mixture of 402 parts of acetic acid, 18 parts of water, and 180 parts 0 5N204, all at a temperature of 28-30 'F. "The mass was, stirred while maintaining the restricted temperature until a uniform, brilliant, clear solution was formed. The solution was coated onto aglass plate toform a sheet having .013 inch thickness, and the film or sheet, allowed to stand in a draft of air for 5 minutes. 'The film was then immersed in glacial acetic acid for 5 minutes. The film was then washed free from acid with water and immersed in a 3% glycerin solution for 15 minutes. An alternative washing method is that of immersing the air-treated film directly in water, The wet film was dried on a smooth, metal surface to give a clear, .fiexible film of 001-0013 inchthickness. The dry product analyzed within the following ranges:

Percent Nitrogen -l .04-.06 Carboxyl 0.2-0 .5

Thesefilms when freshly coated are somewhat re- To-improve the susceptibility to water and aid in reducingthe washing time,;it-hasgbeen found advantageous to use wetting agents, such as -ordinary soap, TritonK-fiO, or Tergitol 4. As-far as can be determined, these wetting agents have no deleterious efiects on film regenerated'from nitrogen oxidesolutions.

Example H Two hundred fiftyparts'of-refined cotton linters having a moisture content of -8.-'l5 parts and an ash content of .03 part-wastreated with "750 parts of acetic acid and-16% parts of sulfuric acid for 4 'hours at F. in a Werner- 'Pfleiderer type mixer. This treatment reduced the c'uprammonium viscosity of the cellulose to well-below 200 centipoises. "Five hundred parts of the mixture thus obtained were added 'to 656-parts'of N2Q4,-66 parts-pf water, and'768 parts of acetic acid, all at -a--temperature of 28-30" F.

The mixture was stirred while maintaining the temperature at 28-30 F. until the cellulose dissolved and formed a clear green-blue viscous uniform solution. The solution was cast as a thin layer onto a glass plate, forming a film or sheeting of .013 inch. This film was allowed to stand in a draft of air for minutes and was then washed in water until free from acid and transferred to a water bath, containing suflicient magnesium carbonate to give an initial pH value of 9.3. The film or sheeting was then immersed in 3% glycerin solution and was dried. The 656 parts of N204, 66 parts of water, and 768 parts sheeting was tested by heating for 4 /2 hours at 160 F. using commerical sheeting as the control. The film or sheeting prepared in accordance with the above example exhibited only a light color and was clear, whereas the sample of commerical sheeting developed more color than did the formed sample.

Example III Nine hundred forty-six parts of cotton linters having a 2 cuprammonium viscosity of 7.8 centipoises was added to a mixture of 394 parts of water, 1000 parts Of N204, and 16,000 parts of acetic acid. The N204 employed here was a portion of the red-brown upper layer of an N204-HN03 equilibrium mixture, this means being employed to refine the N204. The temperature of the liquid mixture used and of the entire mass durin the reaction was maintained at 28-30 F. Upon stirring, a uniform green-blue viscous solution was obtained which was regenerated by casting into sheets. In this example as in the preceding one, the temperature was maintained at the reduced range over the entire course of the dissolving operation.

Emample IV Nine hundred forty-six parts of cellulose having a 2 /2 cuprammonium viscosity of 7.8 centipoises was added to a mixture having a temperature of 20-30 R, which mixture consisted of 159 parts of water, 8000 parts of N204 and 2000 parts of acetic acid. The temperature of 28-3 F. was maintained while stirring until a clear green-blue viscous uniform solution was obtained. This solution was cast into films, the films washed, plasticized and dried, to form clear, flexible foils.

Erample V Sixty parts of cotton linters having a 2 :cuprammonium viscosity of 7.8 centipoises was mixed with a liquid mixture previously cooled below 30 0., consisting of 402 parts of acetic acid, 180 parts of N204, and 18 parts of water. The mass was mixed while maintaining the temperature until a clear green viscous liquid was obtained. The solution was coated out in the form of films of .0011 inches and were analyzed for carboxyls and nitrogen contents. When analyzed, these films showed the following values:

Per cent Carboxyl 0.124 Nitrogen 0.18

Example VI Two hundred fifty parts of commercial cotton cellulose were treated with a mixture of 750 parts of acetic acid and 16.56 parts of 95% sulfuric acid for 4 hours at 100 F. in a Werner-Pileiderertype mixer, this treatment reducing the viscosity of the cellulose well below 200 centipoises. Four hundred parts of this treated material was mixed with a mixture of 450 parts of N204, 45 parts of water, and 705 parts of acetic acid and the mass was stirred until a clear green or blue solution was obtained. The solution was held for 24 hours at 26.28 F. and the cellulose was re generated by pouring the solution into a bath of methyl alcohol. The product was washed free of acid with 90% methyl alcohol and dried, the dried product analyzed 0.3% nitrogen. The same procedure was repeated using 400 parts of the preheated cellulose except that a mixture of 10 parts of magnesium carbonate and 200 parts of acetic acid was first added to the pretreated cellulose and mixed in prior to its treatment with the acetic acid solution of N204. The dried product analyzed 0.06% nitrogen.

The viscosity values for cellulose as used herein refer to the viscosity of the cellulose in a 2 cuprammonium solution as is ordinarily known and employed in the cellulose art for determining the viscosity of cellulose. This test is designated as Technical Association Method T-206-m-3'l in the literature on cellulose. The N204 which may be employed in dissolving cellulose in accordance with my invention may be obtained from the usual sources for obtaining oxides of nitrogen, such as by the action of nitric acid on metals, such as copper, the reduction of nitric acid, such as by arsenious acid, or the synthetic preparation of N02, such as by oxidizing ammonia or by synthesizing these oxides directly from nitrogen and oxygen. It is usually desirable to employ N204 of a refined type. The nitrogen dioxide gas formed by one of the reactions specified may be collected by passing it through a brine-cooled condenser into a refrigerated container or flask cooled with solid carbon dioxide.

N203, such as is suitable for use in forming dissolving mixtures in accordance with our invention, is conveniently prepared by treating commercial sodium nitrite with sulfuric acid. The sulfuric acid may be slowly mixed with the sodium nitrite, N203 gas coming off and being collected by means of a brine-cooled condenser and a refrigerated flask, such as mentioned for use in collecting N204. The N20: is thereby collected in the form of a bluish liquid.

It is desirable when cellulose is pretreated with a mixture of acetic acid and sulfuric acid to prepare it for treatment in our process that the cellulose be treated with the magnesium salt of a weak acid, such as magnesium carbonate, prior to the treatment with N204. We have found that this procedure reduces the amount of combined nitrogen introduced into the cellulose in solution in the process and Example VI is for the purpose of illustrating that efiect by repeating the same procedure only employing magnesium carbonate therein. Instead of magnesium carbonate, some other neutralizing magnesium compound may be employed, such as magnesia itself, its hydroxide or the salt of a weak acid.

We claim:

1. A method of preparing regenerated cellulose which comprises mixing together at a temperature not to exceed 37 F. cellulose having a viscosity not to exceed 200 centipoises, N204, an inert solvent compatible with N204 containing between 0.5 and 20% of water, and 05-20% of water, based on the weight of the N204, until solution of the cellulose occurs, followed by imparting a physical form to the solution and setting the so-formed solution.

2. A method of preparing regenerated cellulose which comprises mixing together at a temperaewe-.413

tnrernot to exceed .37 F. cellulose having :aw'ViS- cosityz-not to eXceedZOO centipoiseaNzOr, an inert solventcompatible withNzOl containing approxi- -mate1y.10% of Water, .based on.:the weight of the N204, until solution of the cellulose occurs, followed bysetting afilm of the solution upon afilmforrnin'g' surface.

.3. "Amethod of preparing regenerated cellulose which comprises mixing-together at a tempera- .the solution and setting the so-formed solution.

4. A method of preparing regenerated cellulose,

which: comprises mixing together at a temperaturenotto exceed 30 F. cellulose havinga viscosity not -.to exceed 200 centipoises, N204 acetic acid and 05-20% of water, based on the weight of- -the.-N2O4 until solution of the cellulose occurs followed :bysetting: a film .of: the solution: upon a film-forming surface.

5. ;-A' method oi preparing regenerated cellulose which comprises mixing.together:-at a temperature-nottdexceed 37 F. cellulose having avislcositynot to exceed 200 centlpoises, N204, acetic acidwandeapproximatel'y 10% of water, basedon the weight ofthe-NzOa-until solution of the celluloseoccurs, followed .by setting a filmxof the solution upon'a film-forming surface.

GORDON D..HIATT. CARLTON L. CRANE.

REFERENCES CITED .Name Date Staud Dec..12, 1'933 Number Certificate of Correction Patent No. 2,473,473. June 14,1949.

GORDON D. HIATT ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 13, Example II, strike out 656 parts of N 0 66 parts of Water, and 768 parts; line 45, Example IV, for 283 F. read 28-80 F.;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofl ice.

Signed and sealed this 15th day of November, A. D. 1949.

THOMAS F. MURPHY, 4

Assistant commissioner of Patents.