CA1189068A - Procedure for precipitating cellulose derivatives - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention provides a process for pre-cipitating cellulose carbamate from an aqueous alkali solu-tion. The solution is brought into contact with an aqueous solution of sulphuric acid containing one or several cations selected from the group consisting of Na, Al, Mg, Zn and Ca cations, whereby in the precipitation of cellulose carbamate the advantage is gained that the dry solids contents of the fibre can be made sufficiently high.

Description

The present invention relates to a process for pre-cipitating a cellulose derivative from an alkali solution.
In particular the invention relates to a process for preci-pitating cellulose carbamate from its alkali solu-tion, in -the form of fibre or film.

Applicant's Finnish patent No. 61033 issued May 10, 1982 and applicant's Canadian Patent applica-tion No.
394,900 filed January 26, 1982, disclose a process for pro-ducing an alkali solllble cellulose derivative from celluloseand urea at elevated temperature. The process is based on the fact that when urea is heated to i-ts melting point or to a higher temperature it begins to decompose in-to isocyanic acid and ammonia. Isocyanic acid in i-tself is not a par-ticularly stable compound and it tends to become trimerizedin-to isocyanuric acid. Further, isocyan:ic acid tends to react with urea, whereby biuret is formed. Isocyanic acid also reactes with cellulose, producing an alkali soL~lb:Le cellulose compound which is called cellulose carbamate. rL'he reaction may be written as follows:-Cell - OH -~ HNCO -7Cell - O - C - NH2 The compound -thus produced, cellulose carbamate, may be dried after washing and s-tored even over prolonged periods, or i-t may be dissolved in an aqueous alkali solu-tion, for manufacturing fibre for instance. From this solution may be manufactured cellulose carbamate fibres or films by spinning or by ex-truding, in like manner as in the viscose manufacturing process. The keeping quality and transportability of cellulose carbamate in -the dry s-ta-te afford a great advantage compared with the cellulose xanthate in the viscose process, which cannot be s-tored nor trans-ported, not even in solu-tion form.

If, for instance, continuous fibre or filament ~,~ .. . .

manufactured from cellulose carbamate suitable for textile use is required, it is necessary first to make a solution of earbamate in an aqueous solution of an alkali, e.g. of sodium hydroxide. Thereafter, it is possible to precipitate from this solution fibre or film in like manner as in manu-facturing viscose fibre cellulose is regenera-ted from an NaOH
solution of cellulose xan-thate. In -the viscose process, xanthate solution is as a rule spun in-to a preeipi-tation bath which contains dilute sulphuric acid and sodium sul-phate.

Although the precipi-tation of cellulose carba-ma-te in sulphuric acid resembles the precipi-tation process in the viscose method, i-t is to-tally differen-t in principle.
Cellulose carbamate is s-table in acid conditions and does not, therefore, decompose into eellulose on preeipitating, as in -the viseose proeess. ~lowever, eontaeting the a:Lkaline solution of the earbarnate with sulphurie aeid induees pre-eipitation of the eellu:Lose earbamate, ancl at the same~ timc?
sodium sulphate is formed as sodium hydroxide is neutralized.

When trying -to apply, in the preeipitation of eellulose earbamate from a solu-tion of sodium hydroxide, similar preeipitation condi-tions to those used in manufae-turing viseose, it was found that -the dry solids eontent of the preeipitated carbamate fibre remained far lower -than in the viscose proeess, whieh is far too low. This has -the further eonsequenee that -the initial streng-th of the fibre produet thus produeed was not suffieient to withstand the meehanieal stresses involved in the treatmen-t of the fibre.
The low dry solids eontent of the fibre also means that in the drying phase of the fibre mueh more water mus-t be evapora-ted and more energy eonsumed.

The present invention provides a proeess whieh in the preeipitation of eellulose earbamate a high enough dry solids content is achieved. In the process of the invention for precipitating cellulose carbamate from an alkali solu-tion, the solution is brought into contact with an aqueous solution of sulphuric acid containing one or several cations selected from the group: Na, Al, Mg, Zn and Ca ca-tions.

Suitable sources Eor supplying AL, Mg, Ca or Zn cations are the corresponding sulphates, al-though also other salts, for instance chlorides, may be used. Differences exist between different cations. It appears as if trivalent cations are more powerful -than bivalen-t or monovalen-t cations.
A highly desirable ca-tion source is aluminium sulphate, but results have also been a-ttained with magnesium sulpha-te, sodium aluminium sulphate - or alum - and wi-th zinc sulpha-te.
Calcium sulphate may also be used, although its solubility in dilu-te sulphuric acid is fairly low.

As in the viscose process, it is l:ikew:ise possib:le to regulate and con-trol the precip:itat:ion cond:itions oE cellu-lose carbamate by changing the process conclit:ions. Suchprocess variables are e.g. the compos:ition of the prec:ip:i-ta-tion ba-th, tempera-ture, time, carbama-te content of -the solution and its viscosity. The sulphuric acid content affects e.g. the carbama-te precipita-tion ra-te, while the cation conten-t affects -the dry solids content of -the fibre in the first place. The proper sulphuric acid content in the process of the invention varies as a rule between 2-20~ by weight and the quanitity oE Al, Mg, Ca or Zn sal-t be-tween 0.1-25~ by weight. In a continuous process, sodium sulpha-te -accumulates in the solution, and the precipitation solution may therefore be regenera-ted, as known in the art, by adding substances consumed during the process and removing sub-stances which accumula-te in excess quan-tities. I-t is thus ~' possible in -the process of -the inven-tion -to remove sodium sulpha-te, e.g. by evaporating wa-ter and precipitating, and to add sulphuric acid and aluminium sulphate or ano-ther sulphate.

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The quantity of sodium sulphate is normally kept between 3 and 30O by weight.

By the process of the invention both fibres and Eilms can be produced using conventional apparatus and pro-cesses of -the viscose method. When fibres are produced, the carbamate solution is spun through spinning orifices into the precipitation bath, and when films are produced, -the solution is pressed -through a slit orifice to become film.

In the process of the inven-tion, -the sodium hydr-oxide solu-tion of cellulose carbamate contains 4-15% by weight of cellulose carbamate. A proper viscosity for -the solution is as a rule between 50 and 500 P.

After precipi-tation, the after-treatment of the fibres or films such as washi.ng and dryin~ steps may be carried out using methods and apparatus known per se.
In the following Examples, an application of -the invention is illustrated in the spinning of cellulose car-bamate fibres, while it is obvious that the me-thod of the invention is no-t confined only to precipi-tating fibre bu-t it is equally suitable for manufacturing film-like products or other kinds of products by precipitating cellulose car-bamate from an alkali solution.

Example 1 Cellulose carbamate was madeby mixing 430 g of dry-beaten bleached pine cellulose (DP 510) and 35 g of Na2CO3 in cold xylene in a glass flask with reflux condenser.
The xylene was warmed to 139C, and 500 g of granular urea was added. Warming with agitation was con-tinued for two hours. Thereaf-ter, the xylene was distilled off in sub-atmospheric pressure, and the cellulose carbamate product was ~8~

washed wlth water. The nitrogen con-tent of -the carbamate was 3.6% by weight and DP was 365.

A cellulose carbamate solution was prepared by dissolving cellulose carbama-te produced in -the manner jus-t described in a 10% NaOH solution. The carbamate cont:ent of the solution was 5.8% and the ball viscosity, 53 s.

The c:Logging number oE the solu-tion was de-termined by -the method described in: H. Sihtola, Paperi ja Puu 44 (1962), No. 5, p. 295~800. The clogginq number of -the solu-tion was found to be 15,000. The solution was forced into a sulphuric acid solution through an orifice with 100 holes of 0.09 mm diame-ter.
The fibre strand produced by precipitation in the sulphuric acid solution was lifted onto a roll from the pre-cipitation solu-tion. The dry mat-ter content of the fibres was determined in two ways. In one case, the fibre strand was first dried betWeen blotting cardboards. r['herea~ter, the fibres were weighed, washed with water and once again weighed, and the dry solids contents was calculated. In the other case, the fibres were not dried between blo-tting papers before the first weighing.
In Table I have been s-ta-ted -the composi-tions of the precipitation solutions used, the precipita-tion -times and the dry solids conten-ts obtained.

TABLE I
Composition of Precipita-tion Dry solids content precipitation time(s) clried not dried solutions (% by w.) ~% by w.) 6% H2SO4 16 9.6 5.6 12% " 16 9.9 5.6 .~ ~

, . ' ' K~

(Table I cont'~) 10%112SO4 + 7% Na2SO4 16 8.9 5.2 12% " + 18% " 20 10.3 5.1 S 12% " -~ 30% " 20 8.7 5.5 20% " + 30% " 20 8.8 ~1.6 10% " + 10% A12(S4)316 13.4 10.1 10% " -~15% " 16 19.7 13.4 10% " ~ 10% " +15% Na2SO4 16 17.1 12.2 10% " + 10% " +15% " 8 16.8 11.7 This Example shows that cellulose carbamate is precipita-ted even in mere sulphuric acid solu-tion, bu-t that the dry solids conten-t remains low. In the same way behaves a precipitating solution containing sodium sulphate in addi---tion to sulphuric acid. Aluminium sulphate causes a sub-stantial improvement in the dry solids content of the E:ibre.

Example 2 Cellulose carbamate was manu~actured as follows:
430 g of dry-dispersed bleached sulphite cellulose (spruce/
pine) (DP 400) was impregnated with a solution containing 3.6 1 of liquid ammonia and 200 g of urea, a-t -45C for 3 hrs. Thereafter, the ammonia was allowed to evaporate at room temperature in a 15 litre reactor overnight. The reac-tor -temperature was raised -to be 135C for -three hours, whereafter the cellulose carbamate product (with N content

2.4%, DP 310) was washed with wa-ter and dissolved in 10%
NaOH solution. The carbamate content of the solution was 6.5% by weight and the clogging number, 13,000.

Precipi-tation tests were carried out in the manner `~
of Example 1, but using zinc and magnesium ca-tions. The results are stated in Table II.

~ 8~n8 TABLE II
Composition of Precipitation Dry matter content precipitation time(s) dried not dried SOlUtlOllS (~6 by w.) (% by w.) 10%~12SO4 + 5% MgSO4 16 13.2 10% " + 10~6 ~I 16 10.8 10% " + 5% ZnSO4 16 10.4 6.5 10% " + 5% " +15% Na2SO4 11 9 9 6.7 10% " -~ 5% " +25% " 11 10.7 6.4 9% " + 9% " +18% " 11 9.2 6.3 Example 3 Cellulose carbamate was manuEac-tured as in Example 2, with the exception tha-t the urea quantity was 300 g. The DP of the cellulose carbamate thus obtained was 320 and the nitrogen con-ten-t, 2.2~.

A solution was prepared of the cellulose carbamate as in Example 2. The carbamate con-tent of the solution was 6.5% by weight and the clogging number was 1045. The results of the precipi-tation tests are stated in Table III.

TABLE III
Composi-tion of Precipita-tion Dry mat-ter content precipitation -time(s) dried not dried solutions (% by w.) (% by w.) 5~ H2SO4 + 15% A12(S4)3 16 14.7 10.0 10% " + 15% " 16 20.0 15% " + 15% " 16 14.2 10.9 -~ample 4 Cellulose carbamate was made as in Example 3, with the exception that bleached sulphite cellulose (pine) was used as the starting material, the DP of which 380. 'rhe nitrogen content of the carbamate was 1. 7% and its DP, 290.
An NaOHsolution was made from th~ carbamate as in Example 3, and the clogging number was found to be 1900. The precipi-tation solutions -that were used are sta-ted in Table IV. The precipi-tation time was 11 minutes.
TABLE IV
Composition of Dry matter conten-t solu-tions (~ by w.) not dried 10% H2SO4 -~ 5% Al2(SO~)3 + 20% Na2SO4 12.7 9.9 10% " -~ 5% " -~ 25% " 17.0 11.9 10% " ~ 7% " -~ 20% " 1~. 5 12.0 10% " -~ 5% Nc~l(S04)2 17.1 l2.6 - :;1

Claims (4)

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

1. A process for precipitating cellulose carbamate from an aqueous alkali solution, in which the solution is brought into contact with an aqueous solution of sulphuric acid containing one or several cations selected from the group consisting of Na, Al, Mg, Zn and Ca cations.

2. A process according to claim 1, in which the sulphuric acid solution contains 2-20% by weight sulphuric acid, 3-30% by weight sodium sulphate and 0.1-25% by weight aluminium, magnesium, calcium or zinc sulphate.

3. A process according to claim 1, in which the alkali solution of the carbamate is pressed through a perforated or slit-like orifice into the sulphuric acid solution to the purpose of precipitating carbamate fibre or film.

4. A process according to claim 1, 2 or 3, in which the alkali solution of carbamate contains 4-15% by weight of cellulose carbamate.