NO310665B1 - Process for producing cellulosic fibers and using bleached, chlorine-free cellulose - Google Patents

Abstract

The invention relates to a process for producing cellulose moulded articles of high brightness, the process being characterised by the following combination of measures: pulp is bleached with a bleaching agent which must not contain chlorine or any chlorine compound; the pulp once bleached is dissolved in an aqueous tertiary amine oxide to produce a mouldable cellulose solution; and the mouldable cellulose solution is worked into moulded articles.

Description

Procedure for the production of cellulose-containing fibers and the use of

bleached, chlorine-free cellulose.

Background

For some decades, efforts have been made to arrive at a method for the production of cellulose fibers that would replace today's widespread viscose method. As an interesting alternative, not least because of a better environment, a method of dissolving cellulose in an organic solvent, without derivatization, and extruding foils and other shaped bodies, e.g. fibers, of this solution. Such extruded fibers were given the trade name Lyocell by BISFA (The International Bureau for the Standardization of man made fibers). By organic solvent, BISFA means a mixture of an organic chemical and water.

It has been shown that a mixture of a tertiary amine oxide and water is a particularly suitable organic solvent for the production of cellulose-containing shaped bodies. IN

firstly, N-methylmorpholine-N-oxide (NMMO) is used as amine oxide. Other amine oxides are described in e.g. EP-A-0,553,070. A method for producing malleable cellulose solutions is known from, for example, EP-A-0,356,419. The production of cellulose-containing moldings using tertiary amine oxides is generally referred to as the amine oxide method.

EP-A-0,356,419 describes an amine oxide method for producing spinnable cellulose solutions, which, among other things, uses a suspension of cellulose in liquid aqueous N-methylmorpholine-N-oxide (NMMO) as starting material. This method consists in transferring the suspension to a formable solution continuously and in one step in a thin layer processing apparatus. The malleable solution is finally spun in a molding tool into filaments, e.g. a spinning nozzle, by passing the solution through an air gap, where it is stretched, and to a precipitation bath where the cellulose is precipitated. Finally, the cellulose fibers obtained are washed.

In order to prevent unwanted coloring of the fibers, which primarily originates from lignin and colored organic compounds, it is known to bleach the cellulose before processing. Here, the raw cellulose is reacted under precisely defined conditions with a number of bleaching agents, for example elemental chlorine, chlorine compounds such as hypochlorite and chlorine oxide, oxygen, peroxides and ozone, whereby the reaction takes place in a specific order with a combination of these chemicals, which is referred to as a bleaching sequence.

Classic bleaching methods begin with elemental chlorine, which, depending on the pH value, chlorinates or oxidizes to varying degrees. Hypochlorite and chlorine oxide oxidize the lignin and colored compounds. Between the individual reaction steps with these bleaching agents, an alkaline extraction is usually inserted to remove the lignin put in solution and the other compounds from the reaction medium. For an overview of the cellulose bleaching technique, reference can be made to R.P. Singh, The Bleaching of Pulp, TAPPI Press, Atlanta, USA.

Chlorine-containing bleaches produce, to varying extents, chlorinated products with cellulose bleaching that are very poorly degradable and are mainly discharged into drains. These methods are therefore questionable from an environmental point of view, chlorine-containing bleaches and above all elemental chlorine, are used to an ever smaller extent. Admittedly, the quality of the bleaching is also reduced since elemental chlorine has a high bleaching effect.

Of the chlorine compounds, chlorine oxide bleaches better than hypochlorite, but is however more expensive than the more frequently used hypochlorite.

In the known technique, bleaching methods are also used which are carried out entirely without chlorine compounds, e.g. with combinations of oxygen/peroxide. Cellulose bleached in this way is referred to as TCF (total chlorine free) cellulose as it has been bleached neither with elemental chlorine nor with chlorine compounds. In the literature, there is also mention of a cellulose which has not been bleached with elementary chlorine but with chlorine compounds, which is called ECF (elementary chlorine free) cellulose.

In connection with the present description, an ECF cellulose which has been bleached with hypochlorite is referred to as ECF hypochlorite cellulose.

In the known technique, the so-called degree of whiteness is a measure of bleaching intensity. From the viscose method, it is known that there is a relationship between the degree of whiteness of a bleached cellulose and the degree of whiteness of cellulose products produced from the same, in that cellulose with a higher degree of whiteness can generally be processed into fibers with a correspondingly higher degree of whiteness.

Purpose

The purpose of the invention is to provide a method for the production of cellulose-containing fibers with a high degree of whiteness, which with a similar initial degree of whiteness for the cellulose used and an otherwise similar method exhibits a higher degree of whiteness than cellulose-containing moldings produced with an ECF hypochlorite cellulose.

The invention

This purpose is achieved with a method for the production of cellulose-containing

fibers with a high degree of whiteness, in combination with

- bleach cellulose with a bleaching agent provided that the bleaching agent does not contain chlorine

or a chlorine compound,

- dissolving the cellulose bleached with the bleaching agent in aqueous tertiary amine oxide to provide a malleable cellulose solution, and

- process the malleable cellulose solution into cellulose fibres.

With the method according to the invention, cellulose fibers with a high degree of whiteness can be produced according to the dry/wet spinning method.

For the person skilled in the art, it is surprising that one can obtain TCF cellulose with a higher degree of whiteness than ECF hypochlorite cellulose with the same raw cellulose, and that a positive side effect occurs in that the fibers can be produced in this way with a particularly environmentally friendly method.

The invention also relates to the bleaching of fibres, which are produced according to the amine oxide method, with chlorine-free bleaching agents such as oxygen, ozone and particularly hydrogen peroxide.

A preferred embodiment of the method according to the invention is characterized by

that the fibers produced according to the invention are bleached with a bleaching agent that does not contain chlorine or chlorine compounds.

N-methyl-morpholine-N-oxide is particularly used as tertiary amine oxide.

The invention also relates to the use of cellulose bleached with a bleaching agent in the production of cellulose-containing moldings according to the amine oxide method, provided that the bleaching agent does not contain chlorine or chlorine compounds.

The invention is subsequently described in more detail with the help of an example.

Example

To produce a TCF cellulose, the cellulose material (type BuchensulfidZellstoff) was bleached first with the aid of an alkaline peroxide-enhanced oxygen extraction, then with ozone and finally with peroxide, according to known methods. The individual bleaching steps are described in e.g. R. P. Singh, The Bleaching of Pulp, TAPPI Press, Atlanta, USA and in EP-A-0,426,652.

The degree of whiteness of the TCF cellulose obtained was 90.6 according to ISO 3688. The cellulose showed a viscosity (Schnellkupferviskosität) according to Zellcheming ZM IV/30/62 of 19.8 Pas and an a-content of 90.9.

Finally, this TCF cellulose was processed in a known manner in a mixing container into a spinning pulp with a composition of 12% cellulose, 77% NMMO and 11% water. The viscosity of the spinning mass was 1630 Pa s (temperature: 90°C, shear rate: 0.1 s"').

The spinning stock was spun into fibers at 120°C through a spinning nozzle with spinning holes with a cross-sectional dimension of 100 micrometres, according to the known wet/dry spinning method (see for example EP-A-0,584,318). The degree of whiteness for the fibers produced according to the invention was approximately 50.

The CIELAB whiteness of the fibers was determined according to the following method: the color coordinates were determined according to P^, Ry, R^ with DIN 6174 and DIN 5033 with light type D65 and at 10° observation angle. From these color coordinates are calculated according to DIN 55981 (=tone deviation according to Gårtner/Griesser) and color printability in the red/green axis as follows:

CIE whiteness : W = Y + 800 (x0-x) + 1700(y0-y)

Color print (=color tone) : T = 900- (xO-x)-650-(YO-y)

(note: T negative = red, T positive = green)

y..norm color value for green sensitivity (=Ry...remission value green)

c0, y0...standard color value proportion for the white point (for D65/10<0> X0 = 0.3138; y0 = 0.3310 applies)

x, y...norm color value proportion for the samples; is calculated according to:

x = X/(X+Y+Z)

y = Y/(X+Y+Z)

z=l-x-y

norm color value X, Y, Z, which appears in these formulas, is calculated again from the remission values (=filter values) Rx, Ry, Rz with light type D65 and 10° observation angle according to the formula

X = 0.94811 Rx

Y = Ry

Z= 1.07304-Rz

Finally, these fibers were bleached with hydrogen peroxide (1.5 g H 2 O 2 , stabilizer: 0.2 g/l MgSO 4 , pH 10.5, flow ratio 1:20, 70°C, bleaching time: 3 minutes). The CIELAB whiteness of the bleached fibers was 57.

Comparison example

To produce an ECF hypochlorite cellulose, the same starting material (cellulose) as in the previous example was first bleached with the help of a peroxide-enhanced alkaline oxygen extraction, then with hypochlorite and finally with peroxide.

The degree of whiteness of the obtained ECF hypochlorite cellulose was 91.6 and therefore practically equal to the TCF cellulose prepared in the preceding example.

Finally, analogously to the example above, a spinning mass and fibers were produced. The produced fibers had a CIELAB degree of whiteness of around 39. It is therefore clear that the TCF cellulose according to the invention, bleached without any chlorine compounds, can be processed into fibers with a higher degree of whiteness than e.g. ECF cellulose bleached with hypochlorite.

Finally, the manufactured fibers were bleached as in the previous example. The bleached fibers had a CIELAB whiteness of around 54.

Claims (5)

1. Process for the production of cellulosic fibers with a high degree of whiteness, characterized by: - bleaching cellulose with a bleaching agent provided that the bleaching agent does not contain chlorine or chlorine compounds, - dissolving the bleached cellulose in aqueous tertiary amine oxide to obtain a formable cellulose solution, and - processing it malleable cellulose solution into fibers. 2. Method according to claim 1, characterized in that the malleable cellulose solution is processed into fibers with the dry/wet spinning method. 3. Method according to claim 2, characterized in that the fibers are bleached with a bleaching agent that does not contain chlorine or chlorine compounds. 4. Method according to claim 1 or 2. characterized in that N-methylmorpholine-N-oxide is used as tertiary amine oxide. 5. Use of cellulose bleached with a bleaching agent for the production of cellulose-containing moldings according to the amine oxide method, provided that the bleaching agent does not contain chlorine or chlorine compounds.