CA1189067A - Procedure for producing soluble cellulose derivatives - Google Patents

Abstract

Abstract of Disclosure The present invention concerns a procedure for producing cellulose carbonate from cellulose and urea. To avoid alkali treatment of the cellulose used as starting material with a view to decreasing the degree of polymerisation of the cellulose, the cellulose in web form is exposed to a radiation dose of preferably 0.5 to 10 Mrad from a radiation source, whereafter the cellulose is reacted with urea at elevated temperature.

Description

The present invention relates to a method for pro-ducing soluble cellulose derivatives. In par-ticular the invention relates to a method for producing cellulose com-pounds which can be dissolved in alkali and precipitated therefrom in the form oE film or fibre.

When producing regenerated cellulose, most often -the so-called viscose method is used for dissolving cellu-lose. In said me-thod alkali cellulose is first prepared and this is reacted with carbon disulphide to obtain cellu-lose xanthate. The cellulose xanthate may be dissolved nan alkali solution and regenereatd by precipitation in film or fibre form to become cellulose again. The carbon disul-phide used n the method is an extremely -toxic substance.
Endeavours have been made to find a substi-tuting chemical to replace it which would be economical enough in use and which would not cause similar environmental and healtll hazards as are caused by carbon disulphide. No commercia:L rnethods or processes to replace the viscose method are in use to date.

In applicant's Finnish pa-ten-t No. 61033 issued 10 May 1982 and applicant's Canadian Paten-t applica-tion No.
394,900 filed January 26, L982 are disclosed methods for manufacturing alkali soluble cellulose carbamate from cellu-lose and urea. When urea is heated to -the melting point or to a higher tempera-ture, it begins to decompose, forming isocyanic acid and ammonia. Isocyanic acid in itselE is not a particularly stable compound and tends -to become trimeri-zed into isocyanuric acid. Further, isocyanic acid tends to react with urea, whereby biuret is formed. Isocyanic acid also reacts with cellulose, producing an alkali solublecellulose compound, which is called cellulose carbama-te.
The reaction may be written as follows:-Cell - OH + E-INCO > Cell - O - C - NH2 ,, ............. . .. .c - . , . ... ,. . ,~

The compound thus procluced, tha-t is cellulose car-bamate, may be dried after washing and stored even for pro-longed periods or it may be directly dissolved in alkali solution. From this solu-tion may be manuEactured cellulose carbamate Eibres by sp:inning, in like manner as in the vis-cose fibre manufacturing process. The keeping quality of cellulose carbamate and its transportabili-ty in dry state afford a very grea-t advantage compared wi-th the cellulose xan-thogenate in the viscose method, which cannot be stored nor transported, not even in solution form.

The degree of polymerization of -the cellulose used as the starting material in the procedures of Finnish Patent No. 61,033 and Canadian Pa-ten-t application No. 39~,900 has a great significance regarding the end product. The higher -the degree of polymerization of the starting cellulose, the higher the viscosity of the alkaline solution oE the end pro-duct, and correspondingly, the lower the cellulose carbamate content of -the solution obtained ln dissolving -the end pro-duct. As regards the solubili-ty of the end product, it is also essential that -the degree of polymerization of -the cellulosic starting material is as uniform as possible. A
suitable DP of the cellulosic starting ma-terial is between 800 and ~00.
For decreasing the polymeriza-tion degree oE cellu-lose, an alkali treatment has been used in -tradi-tional methods. This involves -treating the cellulose in sheet or ;
fibre form wi-th an 18% alkali solution. Through the effect of air or oxygen, depolymerization of the cellulose is achieved. If the cellulose is in sheets, it is shredded at this phase in order to enhance the effect of air. If desired, the alkali treatment may be discontinued at the polymeriza--tion degree desired by washing wi-th water and drying.
Cellulose which has been treated in this manner and dry-comminuted is suitable for use as s-tarting material :in manu-facturing cellulose carbamate.

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llowever, the alkali treatment mentioned is a pro-cess involving several steps and conducive to higher cos-t and it includes very rnany drawbacks. In the method, it is necessary to add water and alkali to the cellulose. Before the next step, the substances mentioned have to be removed from the cellulose by a washing and drying process, which causes considerable extra cost.

The present invention provides a method for manu-facturing cellulose carbamate from cellulose and urea, in which the cellulose to be used as star-ting material need no-t be treated wi-th alkali to reduce -the degree of polymerization of -the cellulose.

In -the method oE the invention for producing cellulose carbamate from urea and cellulose, on ceLLuLose in web Eorm is directed from a radiation source, a ratl:iation dose of between 0.5 and lO Mrad, whereafter the ceLlulose is reacted wi-th urea at elevated temperature.
Said irradiation causes in the fibres of the cellu-lose web a reduction of their degree of polymerization in a most economical way. The irradiation is bes-t effected by conducting the cellulose in the form of a web past the radia-tion source. When an efficient radiation source is used, :
depolymerization takes place very rapidly, and the degree of polymerization is easily adjustab]e by changing the power or the irradiation time. The irradiation may either be carried out at the cellulose mill at the cellulose drying phase or equally at the first phase of the cellulose car-bamate process. Since -the irradia-tion takes place in sheet form, it is possible to trea-t the cellulose at the initial phase of the carbamate process in the form of a con-tinuous web, which simplifies the process. This is not, however, indispensable. Wha-t is essential is than when irradiation is used, the expensive and mul-ti-step alkali trea-tment is ~ ... ~ .. . . . . ......

totally eliminated.

When speaklng of irradiation, that part of the electromagnetic spectrum is usually referred to which has an energy level between 10 eV and 10 MeV and a wavelength correspondingly downward from -the wavelength of ultraviolet ligh-t. In the process according to the invention, that part of the elec-tromagnetic spec-trum in particular is used which is called the ionizing radia-tion range. This includes electron rays and gamma radiation. Electron rays are a particle radiation produced by charged electrons when they are accelerated in an electric field. Gamma radiation is electromagnetic radiation, or photons. As is well-known, ionizing radiation may ini-tate various chemical reactions such as polymerization, cross-linking, branching and decom-position.

In the method accord:ing to the irlventiorl, conven-tional apparatus may be uti:Lized to produce gamma radiation or an accelerated electron beam. Conventional gamma radia--tors in general comprise radia-tion shielding, -transportation apparatus for the material -to be irradiated and the radia-tion source itself, in which capacity cobal-t-60 is usually employed. To produce elec-tron radia-tion, i-t is common to employ a tungsten filament cathode to genera-te electrons, which are accelerated in vacuum by the aid of an elec-tric field to obtain the desired energy level, which most oE-ten is between 0.5 MeV and 4 MeV. As regards the invention, it is not essential what -type of apparatus is used -to produce the accelerated elec-tron beam or the gamma radiation. For this reason, their design and properties are not discussed here in more detail.

The cellulose web irradiated in the manner des-cribed above is subsequently contacted wi-th urea at elevated tempera-ture to form cellulose carbama-te. The urea may be , ;
. ~

~LB~7 added Eor instance by applying the method disclosed in Finnish Patent No. 61033 and Canadian Patent appLication No.
394,900. As taught in the latter method, urea is dissolved in liquid ammonia, and the cellulose fibres are -treated at a temperature lower or higher than the boiling point of ammonia. In the la-t-ter case, the treatment is carried out in a pressure vessel. In the method of -the presen-t invention, it is possible to use the same procedure with the only dif-ference that -the cellulose fibres are treated in ammonia solution in web form. If need be, the cellulose web is conducted through an ammonia ba-th suppor-ted by endless wires.

The amount of urea in the ammonia solution may be selected within a fairly wide range, depending on -the other process variables. Normally, -the sufEicient quan-tity of urea is betwen 15 and 120~ by weight, calculated on the cellulose weight. The quantity of urea to be selected in each case is depenedent inter a:lia on the reaction tempera-ture used and reaction time. The requislte impregnation period ranges from abou-t Eive seconds to several hours.

After the ammonia/urea trea-tment, -the ammonia is removed from the cellulose web in a sui-table manner. The urea remains in the cellulose, being homogeneously distri-buted. The ammonia is preferably recovered and reused. For evaporation, it is possible to use for ins-tance vacuum treatment and/or heating.

The ac-tual reaction between cellulose and urea is carried out a-t elevated temperature after removing the ammonia. The reaction time depends in-ter alia, on -the quantity of urea, on the conditions during the impregnation and on the method of hea-ting. As a rule, a tempera-ture between 110C and 200C is needed. The requisite reaction time usually varies from one hour to a few hours. The heating and reacting of cellulose and urea are preferably ?~

~.

carried out at substmospheric pressure, whereby the ammonia (g) that is formed in the reaction is rapidly evacuated from the reaction volume.

'I`he heat trea-tment may -take place either in a space such as a heating oven or by conducting the cellulose web treated with urea through a liquid bath which is at the requisi-te temperature. If needed, the web is conducted -through the heat treatment, suppor-ted by wires or equivalent.
If liquid is utilized, a liquid is selectecl which will not dissolve urea. The lower -the temperature a-t which -the liquid boils, the grea-ter the ease wi-th which it can be re-moved after the reaction, e.g. by evapora-ting. Suitable liquids are, for instance, aromatic or aliphatic hydrocar-bons wi-th a relatively low boiling point.

On comple-tion of the reaction, the end procluct is washed once or several times e.g. with methanol and clriecl.
Preferably, however, the end product :is washed with liquid ammonia. This has the advantage -that -the biurets which may have formed as a by-produc-t in the reaction may be further converted in-to urea and reused. The dried end product, -tha-t is cellulose carbamate, is stable in -the dry state, and i-t can be stored or transported as it is. This is a major advantage compared with -the viscose process, where the cellu-lose xantha-te produced with -the aid of carbon disulphide is not a stable compound which could be stored or transported for use elsewhere.

The cellulose carbamate manufactured by the method of the inven-tion may at any time whatsoever be dissolved in an aqueous solution of sodium hydroxide, of which i-t is possible to produce fibre and film by precipitating it in proper conditions.
The cellulosic starting ma-terial used in the method of the invention may be wood cellulose or cotton or it may consist of other natural or artificial fibres containing cellulose. The cellulose may enter the process in its in-herent state, or in bleached condition, as cellulose hydrate, as alkali cellulose or in a form trea-ted in another way, Eor instance with acids. Using conventional methods, a web is produced from the starting ma-terial, having a -thickness which may vary from 0.2 -to 2 mm, which corresponds to density per unit area of ~00 to 2000 grams per square metre. If the starting material is co-tton, it may well be e.g. in the form of fabric.

The invention will be fur-ther described in the fol-lowing Examples. When a cellulose solution in-tended to be spun into fibres is concerned, one of its most important characteristics is filtrabili-ty. Filtrability is described in the Examples by the so-called clogging number defined in the article: H. Sihtola, Paperi ja Puu 44 (1962), No. 5, p.
295-300. In the method a miniature filter is used, havinJ
3.8 cm2 effective area, the filter material being Macheey-Nagel MN 616 paper. The fil-trability is calculated by the formula:

KW20 60 = 1 . 10 (60 p ),

2 60 20 where P20 = cellulose quantity (in g) passing -through the filter in 20 min., P60 = cellulose quantity (in g) passing through the filter in 60 min., KW20 60 = clogging number-Example 1 80 grams of bleached sulphite cellulose wi-th DP 800 were irradiated in the form of a web, of -thic]cness 1.1 mm.
An electron accelerator with 400 keV voltage and 13.7 mA
current in-tensity was used for irradiation. The irradia-tion dose was 1-1.2 Mrad. The DP of the cellulose decreased under irradiation to be 360. The cellulose was bea-ten to powder in a hammer mill and impre~natecl at -35 to -~0C with 600 ml of liquid ammonia, in which had been dissolved 40 g of urea so that the proportion by weight of cellulose and urea became 1:0.5 and 1:1. The impregnation time was 3 hours. Thereupon the ammonia was allowed to evaporate a-t normal pressure.

In order to effect the reac-tion be-tween cellulose and urea, the cellulose samples impregna-ted with urea were placed in a hea-ting chamber, and treatment periods of various lengths and varyinq tempteratures were used. Upon comple-tion of -the reaction, the cellulose carbamate products were washed once with methanol, twice with water and one more time with methanol. The degree of polymerisation (DP) and nitrogen content of the cellulose carbama-te samples obtained were measured. ~or es-timating solubility and spiniability, the samples were dissolved a-t -5C in sodium hydroxide, of 10%
concentration. Viscosity, carbamate conten-t and clogging number of -the solutions obt~ined were measured. The resu:lts are stated in Table I in which the abbreviation AGU stands for anhydroglucose unit.

TABLU I

REACTION PRODUCT DISSOLVING TEST
Time Temper J AGU:UREA DP Nitroger Vi~co~ CCA Cloge1ng ~;
(h) a-ture content ity content number (~, r~' (~, (~, K 1 5~ 5 140 l r 5 230 l~ 7 69 8" 0 4100 6pO 140 1 0~5 245 1~4 116 8~2 ll~lO0 5~0 1/15 1 0~5 205 1~8 56 8~0 21~)0 5~5 145 l 0~5 245 l~S 113 ~0 1900 2~5 155 l l I 230 1~8 89 8~0 635 205 155 1: 1 235 2 ~ l 86 8~0 690

3,0 155 1: I 250 2, 3 l08 8~0 ~00 . .1 E~ample 2 A cellulose web comprising bleached sulphite cellu-lose (DP = 800), of thickness l.l mm was treated in an elect-ron accelerator (~00 kV, 13.7 mA), whereby the cellulose fibres received an irradiation dose of l Mrad. The DP of the cellulose was 350 after this treatment. Thereafter the web was immersed in liquid ammonia in which lO~ by weight urea had been dissolved; whereby -the molar propor-tions of urea and cellulose was 1:1.3 and -temperature -40C. The sheet im-pregnating time was 3 hrs. Thereaf-ter, the sheets were trans-ferred into room temperature and the ammonia was allowed -to evaporate.

The reaction be-tween cellulose and urea was effec-ted by immersing the sheets in a petroleum e-ther bath of 140C. The reaction periods were 4, 5 and 6 hours. The carbamate products obtained after reaction were washed once with methanol, -twice wi-th water ancl one more time wlth methanol.
From the cellu:Lose carbamate samples obtainecl, DP ancl nitro-gen content were measured. For assessiny solubility ancl spin-ability, the sheets were dissolved in lO~ sodium hydroxide.
From the solutions thus obtained, viscosi-ty, carbamate con-tent and clogging muber were de-termined. The results are presented in Table II:
TABLE II
R~ACTION PRODU ,T DISSOLV ~NG TEST
Time Temper- AGU:UREA DP i-trogen Vi9cog- CCA Clogging (h) ature ontent ity content` number (C) %) (g) (%) K

4.0140 1:1,3 250 1~1 91 7~7 5400

5,0140 1:1~3 250 1~2 80 7y7 2900

6,0140 1:1,3 250 1,1 119 7,7 20000 Example 3 Cellulose web was irradiated as in Example l, whereaf-ter the cell.ulose was beaten i.n a ball mill into fine powder. Also the impre~nation with a mixture of urea and :
ammonia was caried out as in Example 1, but using 1:0.75 cellulose/urea molar proportion.
.
The reaction between cellulose and urea was efEec-ted by immersi.ng the cellulose samples in a petroleum ether bath, the -temperature of which was 1~0C. The dissolving test was carried ou-t as in Example 1.

The results are stated in Table III. .

TABLE III
:.

REAC'rION PRODUCT I DISSOLVING TEST
, , _ Time Temper- AGU:UREA ~P Nitrogen Vi~co~- CCA Cloggi.ng (h) ature content ity content number (C) (~) (c) (~) K
, _ _ ~ _ ~ I I .
4,0 140 1:0,75 225 1~3 76 8.0 ~20 5,5 1~0 1:0,75 235 2,3 ~3 7~9 700 L6~5 L 1:0~75 240 L 2~ ~1 ~ 700 I :

Example 4 40 grams of bleached sulphite cellulose in sheet form was irradia-ted wi-th a gamma radia-tor in such manner that the irradiation dose absorbed by the cellulose fibres was -.
2 Mrad. The DP of the cellulose was thereafter 280. The shee-t was ground in-to powder in a ball mill and the powder was placed for 6 hrs. in ~50 ml of liquid ammonia (-~0 C) in which 15 g of urea had been dissolved. The ammonia was there-after allowed to evaporate a-t room tempera-ture and under ;
normal pressure.
The reaction between cellulose and urea was carried `

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out in a heating chamber at 145C. Vpon completion of -the reaction, the cellulose carbamate was washed with methanol, twice with water, and once again with methanol. The dis-solving -test was carried out as in Example 1. The results are stated in Table IV.

T~BLE IV
PEAC'r IONPRODUC'r DISSOLV ING TEST
. .
Time Temper-~P Nitrogen Visco~- CCA Clogging (h) ~ture content ity content number (C) _ (%) = (~) (O ~
3 145 2~l5 2.2 106 8,0 1170 3,5 145 : 250 2,4 116 8,0 950 4 1ll5 200 2,0 78 9,0 1100 4,5 ll~5 I 2ll0 2,4 106 ~,0 3700 145 250 2b6 102 8~0l~l~oO
3~5 145 245 1,6 100 8,0 ?3?30 4~0 145 . 235 1,8 95 8,0 820 4,5 145 245 2r1 92 8.0 4150 5,0 145 240 2~3 104 8,0 1100 5l5 145 Z45 ~,3 98 a ,o 890 Example 5 .
Cellulose shee-ts treated by gamma irradiation as in Example 4, having 1 1 mm -thickness, were impregnated with liquid ammonia at -40C in which was dissolved 10% urea. ~;

Thereafter, the ammonia was evaporated a-t room temperature and in normal pressure. The tes-t shee-ts were :, ;' 'l '' ;. ~, .

à7 then immersed in petrol ether of 180C for lO-60 min. Upon completion of the reaction, the sheets were washed twice with 600 ml methanol and water and finally once more with methanol.
The clogging number was determined in 10% NaOl3 at -5 C.
The resul-ts are stated in Table V.
TABLE V
, REAC'rION PRODUCT DISSOLVING TEST
, . ~
Time Temper- Urea, DE' Nitrogen Viscos- CCA Clogging (h) ature ~ of cel l content ity content number ( C) lulose ~ (~) (g) (~) K
15180 48 293 2~0 83 7~01400 10180 ll8 286 1,6 102 7001800 15180 28 341 1,7 66 6vO700 L601 180 _ 28 , 372 1,~3 L 77 6,0600 _ -Example 6 The test as ln Exc~le 5 was repeated, xylene being, however, used instead of petrol ether. The results are stated in Table VI.

TABLE VI

¦REACTION PRODUCT DISSOLVING TEST
Time Temper- Urea, DP ¦Nitroger Viscos- CCA Clogging (h) ature % of cel content ity content number (~) lulose (~) (g) ~ K

4~0 140 25 250 l,1 91 7~7 54~)0 ~ 14~ l~8 250 1 2 80 7,7 2900 - lla -- 12 - ~9~7 Example 7 A8 in Example 1~ bleached 8ulphite cellulo9e wa8 expo8ed to radia-tion, ~hereafter the DP of the cellulose wa8 434. The cellulo9e sheet8 were then impregnated with aqueous urea ~olution, which contained pota~siUm cyanate. After the impregnation, the 8heet~
were dried, and the urea content of the cellulo8e wa8 then 50% and the KOCN-content was 1%. Thereafter, the sheet8~were placed for 3 hour3 in a heating chamber with 140 C temperature. In the firc~t two ca8e8, nitrogen ga8 wa8 drawn through the oven, and air in the third. The characteri8tics of the product obtained are 8tated in Table VII.

TABLE VII

_ Nitrogen content Vi8cosity ~Clogging number 260 1,3 42,5 l970 372 1,2 58,7 1690 390 _ 0,8 47,2 4240 The clogging number was determined from a solution which contained 10% NaOH and 2% ZnO.

Example 8 A8 in Example 1, cellulo8e With DP approximately 800 Wa8 expo8ed to radiation. The DP of the cellulo8e decreased to the value 520.
Thereafter, the cellulose wa8 beaten in the ball mill into powder Which was 3uspended into xylene. The xylene was tran8ferred into a reactor to be at 140C, and urea at 38~ of the weight of the cel-lulo9e wa8 added. The cellulo8e carbamate product obtained wa8 8eparated from the xylene after three hour8 and washed with metha-nol and water. The product had DP 400, vi8C08ity 50.2 3, nitrogen content 1.~% and the clogging number 1310. The clogging number wa8 obtained from a solution which contained 10~ NaOH and 2~ ZnO.

Example 9 The cellulose sheets (5 x 5 dm2, DP = 186) treated with 40 g gamma radiation (8 l~rad) were dried at 105C, impregnated in liquid am-monia (3 h) with 10~ urea, the ammonia was allowed to evaporate at room temperature in normal pres~ure. The sheet~ were thereafter kept at 105C for 1 hour9 whereafter the sheets were weighed ( 48 g, urea content 20% of cellulose). Solvent petroleum was heated to 190C on an oil bath. The cellulose sheets trea-ted with urea were immersed in the solvent petroelum for 10 minutes. The temperature of the petroelum decrea~ed first to 175C but rose to 179C in the course of the reaction. The sheets were tran~ferred into cold solvent petroleum and wa3hed twice with 1 li-tre of methanol, three time~ with 1 litre of water (60C), and once again with methanol.

The DP of the cellulose carbamate thus obtained wa~ 146, its ni-trogen content 1.4~ and the c]ogging number 2346, measured at -5C
in 10~ NaOH solution (11.5~ carbamate).

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of producing cellulose carbamate from cellulose and urea, which comprises exposing cellulose in web form to a radiation dose of 0.5-10 Mrad, thereby causing depolymerization of said cellulose, and reacting the thus depolymerised cellulose with urea at elevated temperature to thereby form cellulose carbamate.

2. A method according to claim 1, wherein after irradiation the cellulose is treated in web form with liquid ammonia in which urea has been dissolved, the ammonia is evaporated, and the resulting cellulose is heated to react it with the urea to form cellulose carbamate.

3. A method according to claim 1, wherein after irradiation the cellulose is treated with liquid ammonia in which urea has been dissolved, the ammonia is evaporated, and the resulting cellulose is heated in a liquid fluid in which urea is substantially insoluble.

4. A method according to claim 3, wherein a hydrocarbon or mixture of hydrocarbons is used as said liquid fluid.

5. A method according to claim 2, wherein the treatment of cellulose with ammonia solution of urea takes place at a temperature below -33°C or in a pressure vessel at a temperature higher than the boiling point of ammonia.

6. A method according to claim 5, wherein the reaction of cellulose and urea is carried out under reduced pressure.

7. A method according to claim 6, wherein after the reaction of cellulose and urea, the reaction product is washed with liquid ammonia.

8. A method according to claim 7, wherein the radiation used is within the range of ionizing radiation of the electromagnetic spectrum.

9. A method according to claim 8, wherein electron or gamma radiation is used to effect radiation.

10. A method according to claim 9, wherein the cellulose is in web form during all treatment steps.