WO1998058102A1 - Cellulose microfibre - Google Patents

Abstract

The invention relates to a method for producing cellulose fibres. Cellulose is dissolved in an aqueous tertiary amino oxide giving a spinnable solution. This solution is spun into filaments using a spinneret, and the filaments are drawn through an air gap into a regenerating bath, in order to regenerate the dissolved cellulose. The method is characterized in that the solution used for spinning has less than 0.20 % volume cellulose, in particular less than 0.05 % volume in relation to the volume of the solution, with a molecular weight of at least 5x105, and the air in the air gap has a moisture content of less than 5g water/kg air.

Description

Cellulosic microfiber

The present invention relates to a cellulosic microfiber. The invention further relates to a process for the production of cellulosic fibers, wherein pulp or cellulose is dissolved in an aqueous tertiary amine oxide to form a spinnable solution, this solution is spun into filaments using a spinneret, and the filaments are drawn into a precipitation bath by stretching through an air gap be introduced to precipitate the dissolved pulp.

As an alternative to the viscose process, a number of processes have been described in recent years in which cellulose is dissolved in an organic solvent, a combination of an organic solvent with an inorganic salt or in aqueous salt solutions without the formation of a derivative. Cellulose fibers made from such solutions were given the generic name Lyocell by BISFA (The International Bureau for the Standardization of man made Fibers). BISFA defines a cellulose fiber as Lyocell, which is obtained from an organic solvent by a spinning process. BISFA understands "organic solvent" as a mixture of an organic chemical and water.

To date, however, only one process for the production of a cellulose fiber of the Lyocell genus has become established until industrial implementation, namely the amine oxide process. In this process, N-methylmorpholine-N-oxide (NMMO) is preferably used as the solvent. For the purposes of the present description, the abbreviation "NMMO" is used for the term "tertiary amine oxides", NMMO additionally representing the N-methylmorpholine-N-oxide which is preferably used today.

Tertiary amine oxides have long been known as alternative solvents for cellulose. From U.S. Patent 2,179,181 it is known, for example, that tertiary amine oxides can dissolve high-quality chemical pulp without derivatization and that cellulosic moldings, such as fibers, can be obtained from these solutions by precipitation. US Pat. Nos. 3,447,939, 3,447,956 and 3,508,941 describe further processes for the preparation of cellulosic solutions, cyclic amine oxides preferably being used as solvents. In all of these processes, cellulose is physically dissolved at an elevated temperature.

EP-A-0 356 419 by the applicant describes a process which is preferably carried out in a thin-film treatment apparatus in which a suspension of the comminuted pulp in an aqueous tertiary amine oxide is transported in the form of a thin layer and transported over a heating surface, the surface of this thin layer being subjected to a vacuum. When the suspension is transported over the heating surface, water is evaporated off and the cellulose can be brought into solution, so that a spinnable cellulose solution is discharged from the film spinner.

Methods for spinning cellulose solutions are e.g. known from EP-A-0 671 492 and EP-A-0 659 219 of the applicant. The spinning solution is extruded through a spinneret into filaments which are passed through an air gap into a precipitation bath in which the cellulose precipitates. The filaments are stretched in the air gap. This process is commonly known as dry / wet spinning.

Applicant's WO 95/02082 also describes a dry / wet spinning process. This method uses cooling air that has a temperature between 10 ° C and 60 ° C. The humidity of the cooling air supplied is between 20 g H 0 and 40 g H ^ O per kilogram.

WO 94/28218 generally proposes the use of spinnerets with 500 to 100,000 holes. The temperature of the Cooling air is between 0 ° C and 50 ° C. The person skilled in the art can see from this literature that the humidity is between 5.5 g H ₂ 0 and 7.5 g H_0 per kilogram of air. This creates a relatively dry climate in the air gap.

WO 96/17118 is also concerned with the climate in the air gap, it being determined that the climate should be as dry as possible, namely 0.1 g H_0 to 7 g H O per kilogram of air, with a relative humidity of less than 85%. 6 ° C to 40 ° C is suggested as the temperature for the cooling air.

Similar information can also be found in WO 96/18760, which has an air gap temperature between 10 ° C. and 37 ° C. and a relative humidity of 8.2% to 19.3%, which is 1 g H 0 to 7.5 g H ₂ 0 means per kilogram of air.

WO 96/21758 also deals with the climate to be set in the air gap, a two-stage blowing with different cooling air being proposed and blowing with less humid and cooler air in the upper region of the air gap.

EP-A-0 648 808 describes a method for forming a cellulose solution, the cellulosic components of the solution comprising a first component made of a cellulose with an average degree of polymerization (DP) of 500 to 2000 and a second component made of a cellulose with a DP of contain less than 90% of the DP of the first component in the range of 350 to 900. The weight ratio of the first to the second component should be 95: 5 to 50:50.

It is known that fibers with a titer of less than 1.7 dtex can only be produced with difficulty in the amine oxide process. The object of the invention is therefore to provide a process for the production of cellulosic fibers, pulp being dissolved in an aqueous tertiary amine oxide to give a spinnable solution, this solution being spun into filaments using a spinneret, and the filaments being stretched an air gap is introduced into a precipitation bath in order to precipitate the dissolved pulp, which method allows fibers with a titer of maximum or less than 1 dtex to be produced. These fibers are referred to as microfibers ("Lexikon der Textilveredelung", HK Rouette, Volume 2, page 1250 ff; 1995; Laumann'sche Verlagsgesellschaft, ISBN 3-87466-228-4). The invention has in particular the object to provide staple fibers of the Lyocell genus with a titer of maximum or less than 1 dtex.

This goal is achieved in that

(a) a solution is used for spinning which contains less than 0.20% by mass, in particular less than 0.05% by mass, based on the mass of the solution, of cellulose with a

5 has a molecular weight of at least 5x10, and

(b) the air in the air gap has a moisture content of less than 5 g water / kg air.

Cellulose molecules with a molecular weight of at least

5 5x10 are ui for the purposes of this description

Claims referred to as "long chain".

Surprisingly, it has been found that when measures (a) and (b) above are combined, microfibers with a titer of less than 1 dtex can be produced. When using the solution specified in measure (a), the moisture content obviously affects spinnability in that the lower the moisture content of the air in the air gap, the better it is. This is particularly evident when blowing the extruded filaments with dry air, i.e. air with a moisture content of 0%. Under these conditions, fibers with a titer of up to 0.30 dtex can even be produced. The same pulp cannot even be processed into fibers with a titer of 1.7 dtex when blown with moist air (more than 30 g H 0 / kg air).

A preferred embodiment of the method according to the invention is therefore characterized in that the filaments are blown with air immediately after they have left the spinneret and that this air has a moisture content of less than 5 g water / kg air.

In the process according to the invention, the tertiary amine oxide used is preferably N-methylmorpholine-N-oxide.

The invention also relates to the use of a spinnable solution of cellulose in an aqueous tertiary amine oxide which is less than 0.20% by mass, in particular less than 0.05% by mass, based on the mass of the solution, of cellulose with a molecular weight of at least 5x10, for the production of cellulosic fibers with a maximum titer of 1 dtex.

N-methyl-morpholine-N-oxide has proven itself best as a tertiary amine oxide.

The invention also relates to a cellulosic fiber of the Lyocell genus, which is characterized in that it has a titer in the range from 1.0 dtex to 0.3 dtex, in particular in the range from 1.0 dtex and 0.5 dtex, and preferably in Has a range of 1.0 dtex and 0.8 dtex. A preferred embodiment of the fiber according to the invention is in the form of a staple fiber with a length of in particular 1 cm to 10 cm.

In the process according to the invention it is of course possible to use all pulps which have a sufficiently small proportion of long-chain cellulose molecules, so that after dissolving in the aqueous amine oxide immediately before spinning they do not make up more than 0.20% by mass. Pulps with a higher proportion of long-chain molecules can only be used after the proportion of these molecules has been reduced, which can be achieved, for example, by irradiation with electron beams.

The invention is described in more detail below.

1. General method for determining the molecular weight profile of pulps

The molecular weight profile of a pulp can be obtained by means of gel permeation chromatography (GPC), the "Differential Weight Fraction" in [%] as ordinate against the molecular weight [g / mol; logarithmic plot].

The size "differential weight fraction" describes the percentage frequency fraction of the molecular weight fraction.

For testing by means of GPC, the pulp is dissolved in dimethyl acetamide / LiCl and chromatographed. Detection is carried out by means of refractive index measurement and so-called "MALLS" (= Multi Angle Laser Light Scattering) measurement (HPLC pump: Kontron; sample collector: HP 1050, Hewlett Packard; eluent: 9 g LiCl / L DMAC; Rl detector: Type F511, ERC; laser wavelength: 488 nm; increment dn / dc: 1.36 ml / g; evaluation software; Astra 3d, version 4.2, Wyatt; column material: 4 columns, 300 mm x 7 , 5 mm, filling material: PL Gel 20 μ.- Mixed - A, from Polymer-Laboratories; sample concentration: 1 g / 1 mobile solvent; injection volume: 40 μ.1, flow rate: 1 ml / min.

The measuring apparatus is calibrated using measures familiar to the person skilled in the art. The signal is evaluated according to Zimm, whereby the Zim formula may need to be set in the evaluation software.

The molecular weight profile for the pulp Viscokraft LV (manufacturer: International Paper) is shown as an example in FIG. The diagram of Fig. La shows that this pulp consists largely of molecules with a molecular weight of about 100,000 and that this pulp has practically no proportions with a molecular weight of greater than 500,000. A 15% cellulose solution of only this pulp (production see below) in an aqueous amine oxide (= spinning mass) thus corresponds to that which is used according to the invention.

For comparison, FIG. 1b shows the molecular weight profile of the pulp Alistaple LD 9.2 (manufacturer: Western Pulp). This pulp has a maximum molecular weight frequency of about 200,000 and the graph also shows that this pulp has a high proportion (about 25%) of molecules with a molecular weight greater than 500,000. A dope containing only 15% by weight of such pulp has about 4% (based on the mass of the solution; degradation not taken into account during the preparation of the solution) cellulose molecules with a molecular weight of more than 500,000 and thus does not correspond to those which invented - used according to the intended purpose.

2. Influence of radiation on the molecular weight profile

In order to show the favorable influence of irradiation on the molecular weight profile, the same pulp Alistaple LD 9.2 was irradiated in a 500 kV electron accelerator with a radiation dose of 20 kGy (inert gas: mixture of C0_ and N; temperature: 25 ° C). The proportion of long-chain molecules could be drastically reduced by the influence of the radiation, in a range that the Permitted production of cellulose solutions used according to the invention.

3. Preparation of the cellulose solution

The crushed pulp is suspended in aqueous, 50% NMMO, placed in a kneader (Type: IKA laboratory kneader HKD-T; manufacturer: IKA-Labortechnik) and left to impregnate for one hour. Subsequently, by heating the kneader with a heating medium heated to 130 ° C. and by reducing the pressure, water is evaporated until the pulp has completely dissolved.

4. Spinning the solution and determining the maximum take-off speed

A melt index device from Davenport used in plastics processing is used as the spinning apparatus. This device consists of a heatable, temperature-controllable steel cylinder, into which the spinning mass is filled. By means of a piston, which is loaded with a weight, the spinning mass is extruded through the spinneret attached to the underside of the steel cylinder, which has a hole with a diameter of 100 μm.

For the experiments, the spinning mass (cellulose content: 15%) introduced into the spinning apparatus is extruded through the spinning hole and passed over an air gap with a length of 3 cm into an aqueous precipitation bath, deflected, drawn off via a godet which is provided behind the precipitation bath, and thereby stretched. The spinning mass output through the nozzle is 0.030 g / min. The spinning temperature is 80 ° C to 120 ° C.

The minimum spinnable titer is used to simulate the spinning behavior. To do this, the maximum take-off speed (m / min) is determined. The take-off speed is like this increased for a long time until the thread breaks. This speed is noted and used to calculate the titer using the formula below. The higher this value, the better the spinning behavior and the finer the fiber obtained.

The titer that is given at the maximum withdrawal speed is calculated using the following general formula:

1.21 x K x A x 100 titer (dtex) = G x L

where K is the cellulose concentration in mass%, A is the spinning mass output in g / minute, G is the take-off speed in m / minute, and L is the number of spinning holes in the spinneret. For the examples below, the concentration of cellulose is 15%, A = 0.030 g / minute, and L = 1.

5. Setting the air humidity in the air gap

To adjust the humidity of the air in the air gap, air which is supersaturated with water vapor is cooled by a thermostat, adjusted to the respective humidity and blown at right angles to the entire length of the filament in the air gap.

Comparative Examples

In accordance with the above-described procedures, a spinning mass was produced with the (unirradiated) pulp Alistaple LD 9.2 (manufacturer: Western Pulp), whose molecular weight profile is shown in FIG. 1b, spun in the air gap at different humidities and thereby the maximum take-off speed or minimum spinnable titers determined. The results are shown in Table 1. In Table 1 and in the following tables, "Temp." for the temperature in ° C, "humidity" stands for the air humidity in the air gap in g water / kg air, and "max. take-off speed" stands for the maximum take-off speed in m / minute. The titer was calculated according to the above formula and has the unit dtex.

Table 1

Pulp Te p. Humidity ax. Trigger speed Titer

Alistaple LD 9.2

II 105 0 31 1.78

II 105 20 40 1.37

II 105 47 62 0.87

II 115 0 38 1.43

II 115 20 42 1.29

II 115 47 83 0.66

II 120 0 40 1.35

II 120 20 59 0.92

II 120 47 83 0.66

The results shown in Table 1 show the relationship that the maximum withdrawal speed and the minimum titer increase or decrease with increasing moisture in the air gap. This means that with increasing moisture, a thread with a smaller titer can be spun. However, fibers with a titer of less than 1 dtex can only be produced with difficulty and with extremely high humidity.

Examples according to the invention According to the procedures described above, the (unirradiated) Viscokraft LV pulp (manufacturer: International Paper Corp.), whose molecular weight profile is shown in FIG the minimum spinnable titer is determined. The results are shown in Table 2.

Table 2

Pulp Temp. Moisture max. Trigger speed Titer

Visco force LV (unirradiated) 105 0 156 0.35

105 20 176 0.31

105 47 43 1.27

115 0 176 0.31

115 20 99 0.55

115 47 63 0.86

120 0 170 0.32

120 22 83 0.66

120 47 52 1.05

The results shown in Table 2 show the relationship according to the invention that when the spinning mass according to the invention is used, the maximum take-off speed can be increased if the moisture in the air gap is reduced. This makes it possible to produce finer fibers. It is also possible to produce fibers with a titer of up to 0.3 dtex.

Similar good results could also be obtained with the irradiated pulp Alistaple LD 9.2 mentioned above. 6. Spinning behavior of cellulose solutions with varying proportions of long-chain molecules

According to the procedure described above, a dope with 15% by mass of a mixture of 30% Alistaple LD 9.2 and 70% Viscokraft LV was produced. The pulp mixture had a molecular weight distribution as in FIG. 1c immediately before spinning. The spinning mass was spun at a temperature of 120 ° C. at different humidities in the air gap. The result of these tests is shown in Table 3 below:

Table 3

Cellulose mixture moisture max. / Deduction speed Titer

(Alistaple / Viscokraft)

30/70 30 116 0.47

30/70 50 118 0.46

30/70 70 127 0.43

It can be clearly seen in Table 3 that, in contrast to a spinning mass with 15% of the Viscokraft pulp, there is no deterioration in the minimum achievable titer with an increase in the humidity in the air gap, but even a slight improvement. In comparison to a spinning mass with 15% of the pulp Alistaple, however, significantly lower titers can be achieved.

FIG. 2 shows the spinning behavior of cellulose solutions with varying proportions of long-chain molecules, the minimum titer (dtex) being the ordinate and the concentration of the respective cellulose solution to cellulose molecules with a molecular weight of at least 500,000 being plotted as the abscissa. The concentrations were determined immediately before spinning. The proportion of long-chain molecules was adjusted by adding appropriate amounts of Alistaple LD 9.2 to Viscokraft LV. The cellulose concentration of the solution was 15% by mass in all cases.

The spinning behavior was determined for each cellulose solution both at a humidity in the air gap of 30 g H-0 (curve "a") and at 0 g H "0 (dry) (straight line" b ").

2 can be seen:

that there is a connection between spinnability and concentration of long-chain molecules;

that in dry air in the air gap (straight line "b") the spinnability becomes almost linearly better as the concentration of long-chain molecules decreases;

that with moist air in the air gap (curve "a") the spinnability initially becomes increasingly better as the concentration of long-chain molecules decreases, but deteriorates again from a concentration of about 0.25 mass%, this deterioration starting from 0.05 mass -% is particularly pronounced.

FIG. 2 shows that two critical parameters have to be taken into account in the production of microfibers: the concentration of cellulose molecules with one molecule.

₅ lar weight of at least 5x10, and the moisture content of the air in the air gap.

Claims

Claims: A process for producing cellulosic fibers, wherein cellulose is dissolved in an aqueous tertiary amine oxide to form a spinnable solution, this solution is spun into filaments using a spinneret, and the filaments are introduced into a coagulation bath while being drawn through an air gap in order to add the dissolved cellulose fall, characterized in that for spinning a solution is used which is less than 0.20% by mass, in particular less than 0.05% by mass, based on the mass of the solution, with a cellulose 5 has a molecular weight of at least 5x10, and the air in the air gap has a moisture content of less than 5 g of water / kg of air. 2. The method according to claim 1, characterized in that the filaments are blown with air immediately after they have left the spinneret, and that this air has a moisture content of less than 5 g of water / kg of air. Method according to one of claims 1 or 2, characterized in that N-methylmorpholine-N-oxide is used as the tertiary amine oxide. Use of a spinnable solution of cellulose in an aqueous tertiary amine oxide which is less than 0.20% by mass, in particular less than 0.05% by mass, based on the mass of the solution, with a cellulose _{5 has a} molecular weight of at least 5x10 for Production of cellulosic fibers with a maximum titer of 1 dtex. Cellulosic fiber of the Lyocell genus, characterized in that it has a titer in the range from 1.0 dtex to 0.3 dtex. Cellulosic fiber according to claim 5, characterized in that the titer is in the range of 1.0 dtex and 0.5 dtex. 7. Cellulosic fiber according to claim 6, characterized in that the titer is in the range of 1.0 dtex and 0.8 dtex. 8. Cellulosic fiber according to one of claims 5 to 7, characterized in that it is present as a staple fiber