NO321195B1 - Process for making flat cellulose films and using the same - Google Patents

Description

The present invention relates to a method for the production of flat cellulose films and cellulose membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is cast in the form of a film using an extrusion nozzle which has an elongated extrusion gap, the solution is passed through an air gap and into a precipitation bath whereby a flat cellulose film is formed in the precipitation bath, characterized by the fact that the flat cellulose film is stretched in the transverse direction after introduction into the precipitation bath.

From US-A-2,179,181 it is known that tertiary amine oxides have the ability to dissolve cellulose and that molded cellulose bodies such as fibers can be recovered from these solutions as a result of precipitation/regeneration. A method for producing solutions of this type is known, for example, from EP-A-0,356,419. According to this publication, a suspension of cellulose is first prepared in an aqueous tertiary amine oxide. The amine oxide contains up to 40% by weight of water. The cellulosic suspension is transferred to the solution in a thin film processing apparatus.

From DE-A-28.44.163 it is known the production of cellulose fibers in an air gap arranged between a spinning nozzle and a precipitation bath for drawing at the nozzle. Drawing at the nozzle is necessary since stretching of filaments is made more difficult after contact with cast spinning solutions in the aqueous precipitation bath. In the precipitation bath, the fiber structure obtained in the air gap is fixed.

A method for the production of cellulose threads is further known from DE-A-28.30.685 in that a solution of cellulose is formed into filaments in a tertiary amine oxide in a hot state. The filaments are cooled with air and then introduced into a precipitation bath to precipitate the dissolved cellulose. The surface of the spun filaments is further wetted with water to reduce their tendency to stick together with neighboring filaments.

From DE-A-195.15.137 a method for the production of flat films is known in which a tubular film is first formed using a ring nozzle, after which the film is cut into flat films followed by washing and drying. In the production of tubular films, the extruded tube is extended in the air gap both in the drawing direction and in the transverse direction. This is obtained as a result of positive gas pressure inside the pipe. The disadvantages of this method lie in the complicated construction of the device used in the washing and drying process, which is more complex with tubular films than with flat films.

Methods of making tubular cellulose films are further well known from US-A-5,277,857 and EP-A-0,662,283. According to these known processes, a cellulose solution is formed into a tube via an extrusion nozzle with an annular extrusion gap, which tube is pulled over a cylindrical mandrel and introduced into a precipitation bath. In order for the extruded tube not to adhere to the surface of the mandrel, its surface is covered with a film of water so that the inside of the tube coagulates and slides over the cylindrical mandrel. According to EP-A-0,662,283, the tubular film is extended after washing by blowing in a gas.

DE-C-44.21.482 describes a blowing process for the production of oriented cellulose films by extruding the cellulose solution via a film blowing nozzle and an air gap into a precipitation bath. It is mentioned that stretching can be carried out in the transverse direction to the direction of introduction of the blown film via a gas pressure inside the blown film and that the relationship between the mechanical properties in the longitudinal and transverse directions can be adjusted.

A method and a device for producing cellulose films, in particular tubular films, is also known from WO-A-95/07811 (the applicant). According to this process, the dissolved cellulose is cooled before being brought into the precipitation bath by subjecting the heated solution to a gas stream immediately after extrusion.

From WO-A-97/24215, a method is known for the production of an oriented cellulose film in which the cellulose-containing solution is applied to a stretchable surface to which the solution adheres, the solution is then stretched by stretching the stretchable film and then unfolding.

From EP-B-0,494,851, a method is known for the production of a flat cellulose film in which the cellulose solution is passed through a nozzle or gap, and then passed through an air gap and then coagulated in a precipitation bath and the coagulated flat film is stretched in the longitudinal direction.

For cellulose membranes, especially in the form of flat membranes, i.e. membranes from a flat film, the permeability of membranes is an important property. In order to solve certain separation tasks, it is important to choose membranes with optimal permeability, pore size and pore structure for the respective separation purposes.

Dialysis membranes made from regenerated cellulose in the form of flat films, tubular films or hollow threads have been recognized for some time whereby the regeneration of cellulose can take place by means of the kuoxam process, the viscose process or by means of the hydrolysis of cellulose acetate. Depending on the process and process conditions used, membranes with different dialysis properties can be obtained.

US-4,354,938 describes, for example, a method for the production of dialysis membranes according to the viscose process, where a tubular molded membrane is stretched in the transverse direction between 40-120% by blowing it up with air before drying, which leads to a membrane with a uniform orientation in the longitudinal and transverse directions. When transferring the dried membranes in a wet state, the membranes produced in this way will undergo a shrinkage in the longitudinal and transverse direction of 0.5-10%. Ultrafiltration values are in the range of 2.5 ml/m2.h.mm Hg and 5.2 ml/m2.h.mm Hg at a wet thickness in the range of 184 µm to 45 µm.

In "Membranes and membrane processes" by E. Staude, 1992, VCH Verlagsges.m.b.H on page 19 it is described that the bi-axial stretching of finished cellophane membranes leads to an enlargement of the pores, monoaxial stretching will on the other hand lead to a reduction of the effective pore diameter.

However, the viscose process only offers limited possibilities for setting the membrane gene caps in a well-directed way. Further recycling of the chemicals that accumulate in this process, such as sodium sulphate and carbon bisulphide etc. is very expensive.

The present invention aims to provide a method for the production of flat cellulose films with improved mechanical properties. Furthermore, the purpose of the invention is to provide a method for the production of cellulose membranes in the form of flat membranes, whereby membranes can be obtained whose permeability can be optimized for individual separation tasks.

This purpose is achieved by a method for the production of flat cellulose films and cellulose membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is cast in the form of a film using an extrusion nozzle having an elongated extrusion gap, the solution is passed through a air gap down into a precipitation bath whereby the flat cellulose film is formed in the precipitation bath, and according to the invention in the flat cellulose film stretched in the transverse direction after introduction into the precipitation bath. Stretching in the transverse direction can thereby take place in the precipitation bath or at a later time. Stretching in the transverse direction is understood as stretching in a widthwise direction of the flat cellulose film.

It is known that cellulose films produced according to viscose processes can hardly be shaped after regeneration and they can only be stretched in the transverse direction to a limited extent. Surprisingly, according to the invention, it has been shown that in the case of films produced according to the amine oxide process, according to the characterizing part of claim 1, stretching in the transverse direction is also possible after precipitation of the film-like cast solution of cellulose. In this way, flat cellulose films with improved mechanical properties are obtained. The expensive inflation of a tubular cast

cellulose solution in the air gap is therefore not necessary.

Stretching of films in the transverse direction can be carried out according to well-known methods such as those used for thermoplastic films (for example described in Handbook of Plastics Extrusion II, extrusion plants, Hanser-Verlag, 1986, 261-269), for example with conveyor belts or by clamping devices which are attached to endless belts, respectively chains, by guiding the belts in divergent directions.

Preferably, the cellulose solution is extruded using an extrusion die having an extrusion gap with a length of at least 40 cm. However, cellulose solution can be extruded from an extrusion gap with a length less than 40 cm, leading to films with a smaller width.

According to a preferred embodiment of the present method, the flat cellulose film is stretched in the longitudinal direction of the air gap, preferably 0.2-5 times.

A further advantageous embodiment of the present method is characterized by the fact that the flat cellulose film is first washed after deposition and then stretched after washing.

It has surprisingly been found that flat cellulose films produced according to the present process in the washed state can be stretched in the transverse direction up to 3.5 times their original width.

According to a further preferred feature of the present method, the flat cellulose film is first washed after precipitation and dried and then the dry flat cellulose film is moistened and stretched, preferably by spraying water. It has surprisingly been found that flat cellulose films treated in this way can be stretched up to 3.5 times their original width in the transverse direction.

The present method is advantageous in that the stretching of the flat cellulose film up to 3.5 times its original width in the width direction means that the mechanical properties of the film can be adjusted within a range in the longitudinal and transverse directions.

Preferably, N-Methylmorpholine-N-oxide (NMMO) is used as tertiary amine oxide.

The above-described advantages of the present invention relate in particular to the production, according to the invention, of cellulose membranes in the form of flat membranes.

In this respect, the permeability of the membrane and thus the ultrafiltration rate (UFR) can be particularly affected by the choice of the rate at which the film-like cast solution is withdrawn from the air gap. It has thus been shown that a low draw-off speed increases the permeability and thus the ultrafiltration speed of the membrane. In contrast to this, transverse etching of the film after introduction into the precipitation bath will increase the permeability of the membrane. Thus, basic membrane properties can be controlled by choosing the withdrawal speed and transverse stretching of the film.

The invention also relates to the use of a cellulose film produced by the method above, as a packaging material, especially for foodstuffs, as a material for waste and carrier bags, as a film for agricultural use, as a film for diapers, as a substrate for connections, as an office film, as a household film or as a membrane for the separation of mixtures of constituents.

With the following examples, the invention will be described in more detail. The cellulose solutions used were prepared according to the method described in EP-A-0,356,419. In all the examples, the films were washed after precipitation of the cellulose and treated with glycerol (glycerol content in dried film approx. 15% by weight) and finally dried on a stretching frame, in which the films were fixed in the longitudinal and transverse directions. The properties reproduced for the examples were determined for dried films, where the tenacity (longitudinal and transverse) and elongation in the longitudinal and transverse directions were determined according to DIN 53457.

The ultrafiltration rate given in the examples is defined as time unit per volume of the permeate that is passed through the membrane wall in relation to the membrane area and the sample pressure.

V= volume of liquid (permeate)[ml]

t= time [t]

A= membrane area [m<2>]

p= test pressure [mm Hg]

The values for diffuse permeabilities were determined by extending the straight line obtained by plotting ln (ct/co) against time.

co = initial concentration

ct= concentration at time t

A= membrane area [cm<2>]

V= dialysis volume [cm<3>]

Pdiff.= diffusion permeability [cm/min]

t= time [min]

To directly compare different membranes, all permeabilities are converted to a wet thickness of 75 µm, i.e. the corresponding times to reach the balanced state were standardized to this thickness. For example, if a membrane with a thickness of 200 µm, and this balanced state for NaCl dialysis was reached after 100 hours, then this corresponds to a time of 100 x 75/200 = 37.5 hours for a membrane with a thickness of 75 um.

Example 1 (comparison)

A cellulose solution at a temperature of 85°C, containing 15.5 wt.% cellulose, 74.5 wt.% NMMO and 10.0 wt.% water, was extruded at a rate of 37.8 kg/h by means of an elongated extrusion nozzle, which had an extrusion gap with a length of 40 cm and a width of 300 µm, through an air gap of 20 mm into a precipitation bath comprising 80 wt% NMMO and 20 wt% water.

The film-like cast cellulose solution exited the nozzle at a rate of 4.2 m/min and was withdrawn at a rate three times the exit rate.

The obtained flat film had the following properties:

Example 2

Procedure was the same as in Example 1, except that the flat film was stretched 50% in the transverse direction on a stretching frame before drying.

The obtained flat film had the following properties:

Example 3

Procedure was the same as in Example 1, except that the flat film was stretched 75% in the transverse direction on a stretching frame before drying.

The obtained flat film had the following properties:

Example 4

Procedure was the same as in Example 1, except that the flat film was stretched 100% in the transverse direction on a stretching frame before drying.

The obtained flat film had the following properties:

Example 5

The procedure was the same as in Example 1, except that the flat film was stretched 125% in the transverse direction on a stretching frame before drying.

The obtained flat film had the following properties:

Example 6

The procedure was the same as in Example 1, except that the flat film was stretched 175% in the transverse direction on a stretching frame before drying.

The obtained flat film had the following properties:

Example 7 (comparison)

A cellulose solution with a temperature of 110 °C, containing 15.0 wt% cellulose, 74.5 wt% NMMO and 10.5 wt% water, was extruded at a rate of 37.8 kg/h using an elongated extrusion die , which had an extrusion gap with a length of 40 cm and a width of 300 µm, through an air gap of 20 mm in a precipitation bath comprising 80 wt% NMMO and 20 wt% water.

The film-like, cast cellulose solution exited the nozzle at a speed of 4.2 m/min and was withdrawn at the same speed. This means that the flat film was not stretched lengthwise in the air gap.

The obtained flat film had the following properties:

Example 8

The procedure was the same as in Example 7, except that the flat film was stretched on a stretching frame at 100% in the transverse direction before drying.

The obtained flat film had the following properties:

Example 9

The procedure was the same as in Example 7, except that the flat film was stretched on a stretching frame by 200% in the transverse direction before drying.

The obtained flat film had the following properties:

Example 10 (comparison)

A cellulose solution at a temperature of 85°C, containing 15.5 wt%, 74.5 wt% NMMO and 10.0 wt% water, was extruded at a rate of 37.8 kg/h using an elongated extrusion die, with an extrusion gap of a length of 40 cm and a width of 300 µm, through an air gap of 20 mm into a precipitation bath comprising 80 wt% NMMO and 20 wt% water.

The film-like cast cellulose solution exited the nozzle at a speed of 4.2 m/min and was withdrawn at the same speed. This means that the flat film was not stretched in the air gap in the longitudinal direction.

The obtained flat film had the following properties:

Example 11

The procedure was the same as in Example 10, except that the flat film was moistened again after drying on a stretching frame and stretched on this 100% in the transverse direction.

The obtained flat film had the following properties in the dry state:

Example 12

The procedure was the same as in Example 10, except that the flat film was wetted again after drying on a stretching frame and was stretched 200% in the transverse direction on the stretching frame.

The obtained flat film had the following properties in the dry state:

Example 13

A cellulose solution at a temperature of 85°C, containing 15.0 wt% cellulose, 74.5 wt% NMMO and 10.5 wt% water, was extruded at a rate of 37.8 kg/h using an elongated extrusion die , with an extrusion gap of 40 cm length and 300 µm width, through an air gap of 20 mm into a precipitation bath, comprising 80 wt% NMMO and 20 wt% water.

The film-like cast cellulose solution exited the nozzle at a speed of 4.2 m/min and was withdrawn at the same speed. This means that the flat film was not stretched in the longitudinal direction in the air gap.

The dry flat film was built up in water for 2 minutes and then stretched 25% in the transverse direction on a stretching frame.

The obtained flat film had the following properties in the dry state:

Example 14

The procedure was the same as in Example 13, except that the film was developed in water and stretched 75% in the transverse direction on a stretching frame.

The obtained flat film had the following properties in the dry state:

Example 15

The procedure was the same as in Example 13, except that the film was developed in water and stretched 100% in the transverse direction on a stretching frame.

The obtained flat film had the following properties in the dry state:

Example 16

The procedure was the same as in Example 13, except that the film was developed in water and stretched 250% in the transverse direction on a stretching frame.

The obtained flat film had the following properties in the dry state:

Example 17 (comparison)

A cellophane film produced according to the viscose process was wetted and dried on a stretching frame without stretching.

The obtained flat film had the following properties in the dry state:

Example 18 (comparison)

A cellophane film produced according to the viscose process was moistened and stretched 50% in the transverse direction on a stretching frame. It was not possible to achieve a higher transverse stretch than 50% before the cellophane film tore.

The obtained flat film had the following properties in the dry state:

Example 19

A cellulose solution with a temperature of 110°C, containing 14.2% by weight cellulose, 76.2% by weight NMMO and 9.6% by weight water was extruded using an elongated extrusion die, with an extrusion gap having a length of 40 cm and a width of 500 µm, at a rate of 75.6 kg/h through an air gap of 3 cm vertically into a regeneration bath comprising 98 wt% water and 2 wt%

NMMO.

The film-like cast cellulose solution exited the nozzle at a speed of 5.0 m/min and was withdrawn at three times that speed and stretched 50% in the transverse direction in the precipitation bath.

The obtained flat film exhibited the following properties:

Example 20

Cellulose solution with a temperature of 85 °C, containing 14.2 wt% cellulose, 76.3 wt% NMMO and 9.5 wt% water, was extruded using an elongated extrusion die, as an extrusion gap with a length of 4 0 cm and a width of 500 µm, at a rate of 75.6 kg/h through a 1 cm air gap and vertically into a precipitation bath comprising 98 wt% water and 2 wt% NMMO.

The film-like cast cellulose solution exited the nozzle at a speed of 5.0 m/min and was withdrawn at the same speed. After the precipitation bath, the flat film was stretched 100% in the transverse direction.

The obtained flat film exhibited the following properties:

Claims (8)

1. Process for the production of flat cellulose films and cellulose membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is cast in the form of a film using an extrusion nozzle having an elongated extrusion gap, the solution is passed through an air gap and into a precipitation bath whereby a flat cellulose film is formed in the precipitation bath, characterized in that the flat cellulose film is stretched in the transverse direction after introduction into the precipitation bath. 2. Method according to claim 1, characterized in that the flat cellulose film is stretched, preferably 0.2-5 times, in the longitudinal direction in the air gap. 3. Method according to claim 1 or 2, characterized in that the flat cellulose film is washed after deposition and stretched after washing. 4. Method according to claim 1 or 2, characterized in that the flat cellulose film is washed and dried after precipitation and that the dry flat film is moistened and stretched. 5. Method according to claims 1-4, characterized in that the flat cellulose film is stretched up to 3.5 times in the transverse direction. 6. Method according to one of claims 1-5, characterized in that N-Methylmorpholine-N-oxide is used as tertiary amine oxide. 7. Method according to any one of claims 1-6 for producing a cellulose membrane, characterized in that the permeability of the membrane is controlled by stretching the film in the transverse direction after it has been introduced into the precipitation bath. 8. Use of a cellulose film produced by the method according to claims 1-6, as a packaging material, especially for foodstuffs, as a material for waste and carrier bags, as a film for agricultural use, as a film for diapers, as a substrate for compounds, as a office film, as a household film or as a membrane for the separation of mixtures of components.