WO2000012286A1

WO2000012286A1 - Production of cellulose films

Info

Publication number: WO2000012286A1
Application number: PCT/EP1999/006287
Authority: WO
Inventors: John Ryden; Stephen Peter Withers
Original assignee: Ucb, S.A.
Priority date: 1998-08-27
Filing date: 1999-08-27
Publication date: 2000-03-09
Also published as: GB9818561D0; AU5971099A

Abstract

A process for the production of a multi-layered cellulose film which comprises co-extruding through a single orifice into a regeneration bath two or more adjacent flows of solutions of cellulose or a cellulose derivative from which cellulose can be regenerated thereby to regenerate the cellulose in the respective flows and produce said multi-layered film, the position at which the respective flows are brought into contact with each other prior to co-extrusion through the single orifice being controlled by an adjustable separator. Apparatus is also provided for carrying out the process. The films produced have particularly high inter-ply strengths.

Description

Production of Cellulose Films

This invention concerns processes for the production of multi-layered cellulose films, and apparatus for use in such processes.

Cellulose films produced by the regeneration of cellulose from a variety of dopes consisting of cellulose derivatives or solutions of cellulose in a solvent are well known. Examples of cellulose derivatives which have been used for the purpose include cellulose xanthate and complexes with tetrammine-copper (II) hydroxide, and examples of solvents which have been used include aliphatic amine oxides, for example

N-methylmorpholine-N-oxide, and mixtures of lithium chloride and N,N-dimethyl acetamide. It has also been proposed to produce cellulose dopes by the steam explosion of cellulose followed by dissolution of the cellulose in aqueous sodium hydroxide solution.

Regeneration of the cellulose can then be effected using an appropriate regenerating agent, for example an acidic solution in the case of cellulose xanthate, tetrammine-copper (II) hydroxide or alkaline solutions of steam exploded cellulose, or merely water in the case of amine oxides and solutions in mixtures of lithium chloride and N,N-dimethyl acetamide.

Cellulose films in the form of flat or cylindrical mono-webs have been prepared by extruding a cellulose derivative or a cellulose solution into a bath of an appropriate regenerating agent, regeneration from cellulose derivatives to produce flat or tubular films having been carried out commercially for many decades. The resulting films have found many end uses, for example as packaging materials and as membranes, for example for dialysis in artificial kidneys or osmotic bags for the purification of water.

Regenerated cellulose films in the form of mono-webs inevitably have physical properties which depend on the nature of the regenerated cellulose in the single cellulose layer, and these properties are not necessarily desirable for certain end uses. It has therefore been common practice to modify the properties of regenerated cellulose films for particular end uses, for example by applying coatings which make the films heat sealable and/or modify the permeability of the cellulose. It has also been proposed to modify the cellulose itself so that it has different physical properties from that of unmodified regenerated cellulose. The problem with chemical modification of the cellulose itself is that it can reduce the strength of the regenerated cellulose films, thereby making them unsuitable for particular end uses.

It has also been proposed in GB1562069-A to produce tubular dialysis membranes having two or more layers by extruding separate flows of solutions of cellulose in tetrammine-copper (II) hydroxide through adjacent annular die orifices into a coagulation bath where the cellulose is regenerated to form a multi-layered tubular cellulose film, gas or a liquid being supplied to the interior of the cellulose tube as it is being formed to increase bonding between the respective layers as they are regenerated. Despite the use of pressure to promote bonding between the respective layers of the tubular film as the cellulose is being regenerated, inter-layer adhesion can be sufficiently low that delamination occurs. Furthermore, the process cannot be used for producing flat multi-layered regenerated films using slot dies.

US2141776 describes the production of tubular multi-layered cellulose films using an annular die through which two streams of viscose are coextruded. The streams of viscose are separated by a fixed division ring within the die before being combined immediately before coextrusion.

US3983201 describes the production of flat multi-layered films having a core layer of hydroxyalkyl cellulose and two outer layers of a lacquer composition, flows of the precursors for the respective layers being brought together at a fixed point within the apparatus prior to being coextruded into a coagulation bath.

According to the present invention there is provided a process for the production of a multi-layered cellulose film which comprises co-extruding through a single orifice into a regeneration bath two or more adjacent flows of solutions of cellulose or a cellulose derivative from which cellulose can be regenerated thereby to regenerate the cellulose in the respective flows and produce said multi-layered film, the position at which the respective flows are brought into contact with each other prior to co-extrusion through the single orifice being controlled by an adjustable separator.

The present invention enables multi-layered cellulose films to be produced which have layers which are strongly bonded to each other, inter-ply bond strengths of greater than 350joules/m² having been achieved, and they can be produced not only in the shape of tubular but also essentially flat films. Furthermore, particularly thin films can be produced which are difficult if not impossible to produce using adjacent orifices such as are proposed in GB1562069-A. It has also been surprisingly found that despite co-extruding the various layers simultaneously through a single orifice, mixing of the adjacent layers can be minimal by adjusting the conditions used to effect the co-extrusion. As will be appreciated, in order to minimise mixing of the flows it will in general be necessary adjust various parameters of the co-extrusion process such as the flow rates and viscosities of the respective flows, and also the geometry of the apparatus used to effect the co-extrusion.

The present invention also enables a wide variety of symmetrical and asymmetric films to be produced, and they can consist of two, three or more layers. For example, membranes can be produced with a base layer made from one type of cellulose, e.g. with a high molecular weight cut-off (MWCO - the molecular weight at which 90% of a solute will be prevented from passing through a membrane) , with one or two outer layers of a different type of cellulose, e.g. with a low MWCO. Symmetrical three-layered films can also be produced with thin outer layers produced from a relatively high tenacity cellulose with a relatively thick but lower strength inner layer, thereby providing the films with good strength whilst enabling production costs to be minimised. Since co-extrusion can enable particularly thin individual layers to be produced for films in accordance with the present invention, the core layer of three layered films can be made substantially thicker than the two outer layers, and indeed thicker than the combined thicknesses of the two outer layers.

Other regenerated cellulose film structures which can be produced in accordance with the present invention include those which include a highly opaque layer, for example using titanium dioxide as an opacifier, and at least one other layer on the opaque layer which provides the films with particular print characteristics which may themselves differ between the outer layers when the opaque layer is an inner layer of the films. Alternatively, the films can have one or two relatively thin outer layers containing an iriodin type pigment with the outer layers being next to a relatively thick layer containing a pigment which is complementary to the color of the iriodin type pigment.

Cellulose derivatives from which cellulose can be regenerated in accordance with the present invention include cellulose xanthate and complexes with tetrammine-copper (II) hydroxide, these usually being dissolved in aqueous alkali with regeneration usually being effected using an aqueous solution of an inorganic acid.

Examples of solvents which can be used to dissolve cellulose itself rather than as a regeneratable derivative include aliphatic amine oxides, for example N-methylmorpholine-N-oxide, and mixtures of lithium chloride and N,N-dimethyl acetamide. Cellulose dopes can also be used, for example those produced by the steam explosion of cellulose followed by dissolution of the cellulose in aqueous sodium hydroxide solution. Regeneration of cellulose from such solutions can then be effected using water or an aqueous solution of an acid.

Films can also be produced in accordance with the present invention with one outer layer of regenerated cellulose containing an antiblock agent in an amount which inhibits blocking and another outer layer which contains less or no antiblock agent and has good printability characteristics.

The present invention further provides apparatus for producing multi-layered cellulose film the apparatus comprising inlets for two or more flows of solutions of cellulose or a cellulose derivative from which cellulose can be regenerated, a single orifice through which the two or more flows can be coextruded, and adjustable separator means for separating the respective flows and controlling the position within the apparatus where the respective flows are brought together prior to their co-extrusion through the single orifice.

The single orifice of apparatus in accordance with the present invention can be a substantially linear slot, whereby substantially flat multi-layered regenerated cellulose films are produced. However, the single orifice can be in the form of a closed loop, and preferably in the form of a substantially annular slot, thereby enabling substantially tubular multi-layered regenerated cellulose films to be produced.

The separator means which separates adjacent flows within the apparatus is adjustable to enable the position within the apparatus where the respective flows come into contact with each other to be adjusted prior to their co-extrusion through the single orifice. This can be of particular importance in reducing mixing of the flows prior to their co-extrusion and also for improving the strength of the bond between adjacent layers in the resulting regenerated films.

The production of cellulose films in accordance with the present invention will now be described by way of example with reference to the accompanying diagrammatic drawings which illustrate an embodiment of apparatus in accordance with the present invention: -

Fig. 1 is a transverse section through apparatus for producing a cellulose film with two layers; and

Fig. 2 is a transverse section through apparatus for producing a cellulose film with three layers.

In the two Figures components having similar functions are indicated by the same reference numeral for ease of description.

The two accompanying drawings show a substantially conventional pourer feed arrangement for extruding cellulose xanthate viscose into a regeneration bath of aqueous acid to form a flat regenerated cellulose film, the apparatus having been modified to produce a multi-layered film in accordance with the present invention. In each case they have a pourer body 1 through which the viscose is extruded under pressure through a single orifice in the form of a slot between two pourer blades 2 and 3, the gap between the blades being adjustable in known manner using a plurality of differential micrometers for adjusting the gap between the blade 2 and the blade 3 along the length of the blades 2 and 3, one such micrometer being shown at 4.

Referring in particular to Fig. 1, a divider plate 5 between the two halves of the pourer body 1 forms two chambers 6 and 7 so that two separate pressurised flows of viscose from respective feeds 8 and 9 are separated whilst they flow through the pourer body 1, the two flows being combined in the space between the pourer blades 2 and 3. Mixing of the two flows of viscose as they are combined is reduced by positioning the divider plate 5 close to the gap between the pourer blades 2 and 3.

Referring to Fig. 2, two divider plates 10 and 11 serve to keep three flows of viscose from respective feeds 12, 13 and 14 apart as they pass through the chamber in the pourer body 1, the flows being combined into a single flow as they leave the slot between the pourer blades 2 and 3.

Regeneration of cellulose following coextrusion of the combined flows of cellulose xanthate viscose through the slot between the pourer plates 2 and 3 in Figs 1 and 2 is then effected in conventional manner in a bath of dilute sulfuric acid (not shown) .

Thereafter the regenerated cellulose film can be subjected to conventional treatments, for example washing and, if required, plasticising. Further treatments can also be effected, for example coating with one or more coating compositions, for example to modify the permeability of the films and/or to provide them with heat sealability.

As will be appreciated by those skilled in the art, although the production of films in accordance with the present invention has been particularly described with reference to the regeneration of cellulose from cellulose xanthate viscose, the films can be formed from other solutions from which cellulose can be regenerated, for example solutions of cellulose in lithium chloride/N, N-dimethyl acetamide mixtures, in tetrammine-copper (II) hydroxide or in amine-N-oxides, for example N-methyl-morpholine-N-oxide, or solutions of steam exploded cellulose in sodium hydroxide.

It has surprisingly been found that the adhesion between adjacent layers of regenerated cellulose of films produced in accordance with the present invention can be particularly high without significant mixing occurring between the adjacent flows of cellulose solution or cellulose derivative prior to regeneration of the cellulose after the combined flows have left the single orifice. More particularly, using the illustrated apparatus it has been found that the positioning of the divider plates 5 (or 10 and 11) relative to the pourer blades 2 and 3 can have a significant effect in this regard, delamination tending to occur if the divider plates 5 (or 10 and 11) end at relatively short distances from the orifice formed by the pourer blades 2 and 3, and mixing tending to occur if the divider plates 5 (or 10 and 11) end at relatively large distances from the orifice formed by the pourer blades 2 and 3. The necessary adjustments required to achieve this should be possible for a person of ordinary skill in the art using routine experiment.

As will be appreciated by those skilled in the art, the relative thicknesses of the respective regenerated cellulose layers of films produced in accordance with the present invention can in general be adjusted by adjusting the respective viscosities and/or the respective pressures of the streams of cellulose dopes which are brought together through a single orifice before regeneration. These factors can also be controlled to increase the bond between adjacent layers of regenerated cellulose in the films which are produced and to minimise mixing of the dopes at the interfaces between respective streams of the dopes. Although films in accordance with the present invention can be produced having two or more layers of substantially the same composition, for example to produce films in which pin holes which may be formed in one layer have a low risk of coinciding with pin holes which may form in an adjacent layer, thereby to reduce the risk of air holes of the multi-layered films, the present invention can with advantage be used to form multi-layered cellulose films having two or more layers of different compositions.

Differences in the compositions of the respective layers of the present invention can result from differences in the cellulose used to form the layers and/or any additives which may be present in these layers. For example, the cellulose in one or more of the layers can be an at least partially modified cellulose, e.g. to include chemically bound antithrombogenic agents when the films are to be used as dialysis membranes.

One or more of the layers of films of the present invention can, if desired, include one or more additives. Examples of additives which can be used include antiblock agents, antistatic agents, slip additives and pigments. As will also be appreciated, different combinations of additives can be included in the various layers according to the properties required for the films. For example, different layers can be pigmented with different types and/or different amounts of pigment to provide the films with different appearances when viewed from opposite sides, and the outer surfaces of the films can be provided with different physical properties, e.g. different slip and/or antiblock characteristics.

Furthermore, not only can multi-layered films in accordance with this invention have individual layers which are of substantially the same thickness as each other, the layers can be of substantially different thicknesses, for example to provide the films with particular physical properties. More particularly, three layered films can be produced having a central core layer which is thicker than the two outer layers, the core layer, for example, being pigmented, and either or both of the two outer layers optionally being non-pigmented.

Films with four or more layers can similarly be produced, and again the composition of the additional layers can be selected to confer particular properties on the resulting films.

In addition to the above possible differences in the respective layers of films in accordance with the present invention one or more of the layers can be made to have a different porosity from that of other layers of the films, for example by providing one of the layers with a more open pore structure, e.g. a central layer of a three layer film, than that of other layers, e.g. the outer layers of a three layer film. Alternatively, the outer layers can be made of a composition having a higher tenacity than that of an inner layer or layers, thereby enabling the strength of the films to be increased compared with that of films produced from the inner layer compositions alone.

The two outer surfaces of films in accordance with the present invention can also be provided with different print characteristics for each surface of the film. Other possibilities include asymmetric structures having a thin layer containing an injected iriodin type pigment combined with a thicker pigmented backing layer which serves to provide a film with desired aesthetic properties combined with reduced cost compared with use of an iriodin pigment throughout.

Claims

1. A process for the production of a multi-layered cellulose film which comprises co-extruding through a single orifice into a regeneration bath two or more adjacent flows of solutions of cellulose or a cellulose derivative from which cellulose can be regenerated thereby to regenerate the cellulose in the respective flows and produce said multi-layered film, the position at which the respective flows are brought into contact with each other prior to co-extrusion through the single orifice being controlled by an adjustable separator.

2. A process according to claim 1, wherein the said flows are of different thicknesses and the film produced comprises at least two layers of different thicknesses.

3. A process according to either of the preceding claims, wherein the orifice is substantially circular and the film produced is substantially tubular.

4. A process according to either of claims 1 and 2, wherein the orifice is substantially linear and the film produced is substantially flat.

5. A process according to any of the preceding claims, wherein the film produced comprises two layers.

6. A process according to any of claims 1 to 4, wherein the film produced comprises three layers.

7. A process according to any of the preceding claims, wherein solutions of cellulose in a solvent are used for each of the at least two flows and regeneration is effected using water.

8. A process according to claim 7, wherein the cellulose is in solution in an amine oxide or in a mixture of lithium chloride and N,N-dimethyl acetamide.

9. A process according to any of claims 1 to 7, wherein cellulose derivatives are used for each of the at least two flows.

10. A process according to claim 9, wherein the cellulose derivative is a cellulose xanthate, a complex of cellulose with tetrammine-copper (II) hydroxide, or steam exploded cellulose dissolved in aqueous sodium hydroxide.

11. A process according to claim 10, wherein the cellulose is regenerated from the cellulose derivative using aqueous sulfuric acid.

12. A process according to any of the preceding claims, wherein at least one of the said flows includes a particulate pigment or opacifying agent.

13. A process according to any of the preceding claims, wherein at least one of the said flows includes an antiblock agent .

14. A process according to any of the preceding claims, wherein at least one of the said flows includes an iriodin type pigmen .

15. Apparatus for producing multi-layered cellulose film the apparatus comprising inlets for two or more flows of solutions of cellulose or a cellulose derivative from which cellulose can be regenerated, a single orifice through which the two or more flows can be coextruded, and adjustable separator means for separating the respective flows and controlling the position within the apparatus where the respective flows are brought together prior to their co-extrusion through the single orifice.

16. Apparatus according to claim 15, wherein the single orifice comprises a substantially linear slot whereby substantially flat multi-layered regenerated cellulose films are produced.

17. Apparatus according to claim 15, wherein the single orifice forms a closed loop whereby substantially tubular multi-layered regenerated cellulose films are produced.

18. Multi-layered cellulose film produced by a process according to any of claims 1 to 14 or using apparatus according to any of claims 15 to 17.