BIAXIALLY ORIENTED CELLULOSIC FILMS

Abstract: Biaxially oriented cellulosic film is produced by stretching in sequence in one direction and then the other a web (20) of a solution of cellulosic material which is cast onto an extensible surface (3) to which the web adheres, and the film is stretched in at least one direction by stretching the extensible surface.

Description: BIAXIALLY ORIENTED CELLULOSIC FILMS This invention concerns cellulose films, and processes and apparatus for their production. Cellulose films are known, the films being produced by converting cellulose pulp into viscose, for example as cellulose xanthate, and then extruding the viscose through a slot die into a regenerating bath m which the extrudate is converted into a film of regenerated cellulose, for example by reaction of cellulose xanthate with a dilute mineral acid The films produced have desirable properties, particularly good physical properties, good optical properties, feei, twist and fold retention, and an ability to allow moisture vapour to pass through the films without perforating them. This has resulted in regenerated cellulose films being used extensively as packaging materials in addition to various uses in other industries Despite the good physical properties of regenerated cellulose films, they do have certain disadvantages compared with films made from synthetic polymers Thus although the water vapour permeability of regenerated cellulose films can be an advantage m some end uses, in many, if not most, it is a disadvantage Furthermore, the tensile strength of regenerated cellulose films is often relatively low when compared with that of synthetic polymer films, especially when the films are wet. This has led to a decline m the world wide market for regenerated cellulose films and a very significant increase in the market for synthetic polymer f lms . The present invention seeks to provide cellulose films with improved physical properties compared with prior art regenerated cellulose films. According to the present invention there is provided biaxially oriented cellulosic film, the orientation having been imparted by the sequential stretching of a web of a solution of cellulosic material in a solvent before removing solvent from the web to form the film. The films may have substantially balanced properties in the two directions. Alternatively the properties may be unbalanced as is the case of membranes. The bi-axial directions are preferably perpendicular to each other. Non-derivatised cellulose films in accordance with the present invention have particularly good physical properties when compared with regenerated cellulose films produced by the xanthate process. Non-derivatised cellulose is cellulose that is formed from a solution of cellulose in a suitable solvent, typically an amine-oxide solvent and is defined in US-A- 5,277,857. In particular, non-derivatised cellulose films in accordance with the invention have high tensile strengths in both the longitudinal and transverse directions. Substantially similar strengths to those of films produced by the viscose process can therefore be achieved with thinner films in accordance with the present invention. The present invention also provides a process for the production of an axially oriented cellulosic film, the process comprising extruding or casting a web of a solution of cellulosic material in a solvent therefor onto an extensible surface to which it adheres and stretching the web in at least one direction by stretching the extensible surface in at least said one direction. Preferably the cellulosic solution is extruded through a slot die. The cellulosic solution is extruded onto an expandable or extensible elastomeric surface to which it adheres and the web is stretched in a direction transverse to the direction of extrusion by stretching the extensible surface in said transverse direction to produce biaxially oriented film. The cellulosic material may be cellulose dissolved in a tertiary-amine-N-oxide solvent for cellulose, preferably N-methyl-morpholine-N-oxide. The concentration of cellulose in the solvent is preferably from 2 to 25 percent (w/w) , and more preferably from 4 to 18 percent (w/w) and typically about 15%. The degree of polymerization of the cellulose is preferably from 200 to 5000, and more preferably from 400 to 1000. Films in accordance with the present invention are preferably produced by extruding a flat web of a solution of cellulose in a tertiary-amine-N-oxide solvent through a slot die on to a surface to which the solution can adhere, the surface being moved away from the die more quickly than the web is being extruded, so that the web is stretched in the direction of extrusion. Stretching of the web in the transverse direction is then effected by stretching the surface to which the web has adhered transversely to the direction of extrusion of the web. Thereafter, the solvent for the cellulose is leached out of the biaxially stretched web to produce a film of biaxially oriented cellulose. Sequential stretching of the cellulose solution enables the degree of orientation in the two directions to be controlled independently of each other. Stretch ratios in the direction of extrusion will usually be at least 1.5:1, typically at least 3:1, although they can be considerably higher, for example up to at least 25:1. Stretch ratios in the transverse direction will likewise usually be at least 1.5:1, and again they can be considerably higher, for example at least 3:1 or more, e.g. at least 8:1. The overall area stretch ratio of the web will therefore usually be at least 2.25:1, but considerably higher stretch ratios can be achieved, for example up to 10:1 or more, e.g. up to 25 :1. Associated with increasing surface area of the web by stretching is a consequential reduction in the thickness of the web. This can enable particularly thin films to be produced. For example, an extruded web 150μm thick can be stretched to form a resultant film of 25μm or less, or a 25μm thick web can be stretched to form a film of 5-10μm or less, according to the degree of stretching in the longitudinal and transverse directions. Furthermore, the physical properties of the films in the longitudinal and transverse directions can be controlled by selecting the relative ratios of stretching in the longitudinal and transverse directions whilst providing the desired overall area stretch ratio. Although the thickness of the extruded web can be varied fairly widely, for example from 20 to lOOOμm, excessively thick webs can be difficult to control since they can become physically unstable between the orifice of the slot die and the surface on to which the web is being extruded, leading to breakage of the web. Furthermore thick stretched webs can be difficult to regenerate. Excessively thin webs tend to be too thin to allow significant increases in stretch ratio without web breakage or breakage of the resultant films. A web thickness of I50μm before stretching has been found suitable. Leaching of the tertiary-amine-N-oxide solvent from the biaxially stretched web to produce a biaxially oriented cellulose film can be effected using water or dilute solutions of the amine oxide in water. It is also possible to regenerate the film in other non-solvents, such as ethanol, to make porous regenerated materials. It has been found that the leaching process facilitates separation of the resultant film from the surface on which the web has been stretched. If desired, the web can be treated with one or more gas flows between the extrusion die and the surface on which the web is to be stretched transversely. Such treatments could include air or other gas, of varying temperature, optionally containing some level of moisture to produce a humidified environment The gas flow could be arranged into a plurality of flows of different composition and/or temperature to provide regions of different treatment The length of the gap between the die face and the web take-up can be varied, so that the extruded web travels through the conditioned atmosphere for different timescales. Such environments can optionally be extended to include treatment of the gel web whilst m contact with the extensible surface before immersion into the regeneration bath. According to another aspect of the invention there is provided apparatus for the production of oriented film, especially biaxially oriented film, the apparatus including an expandable surface onto which a web of polymer solution may be cast or extruded, the surface being expandable to stretch the web adhered thereto in at least one of said biaxial directions. Conveniently the expandable surface is an elastomeric surface, and is preferably an external elastomeric sleeve on an eccentric cylindrical drum having inclined rigid ends and a trapezoidal longitudinal cross-section, stretching of the sleeve being effected by rotation of the rigid ends with each other The apparatus may be used for cast film in general and is preferably used m combination with an extrusion device for extruding a substantially flat web of a solution of polymeric material on to the expandable surface The polymeric solution is preferably a cellulosic solution, preferably a solution of cellulose m amine oxide to produce a non-derivatised film. The apparatus may also include a solvent removal means for removal of solvent from the solution to produce the film Apparatus could also be used for other films, e.g. cellulose acetate, carboxymethyl cellulose, cellulose nitrate and other cellulose derivatives . Embodiments of apparatus in accordance with the present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic vertical section through an embodiment of the present invention with some parts omitted for clarity; Figure 2 is a plan of the apparatus of Figure 1 in the course of producing a film in accordance with the present inven ion; Figure 3 is a side view of the apparatus of Figures 1 and 2 with some parts omitted for clarity; and Figures 4 and 5 illustrate schematically alternative constructions for the drum shown in Figure 1. Referring to Figure 1, the apparatus includes a drum shown generally at 1 having rigid circular ends 2 connected by a resilient cylindrical sleeve 3 made of an elastomeric material. Suitable elastomeric material may comprise one of polyisobutylene, natural rubber, chloroprene, silicone rubber and styrene butadiene rubbers . Each of the circular ends 2 is mounted on an axial shaft 4,5, the shafts 4,5 being connected by a universal joint 6 or by some other joint means. The joint 6 allows the angle θ between the shafts to be varied, and the transfer of motion from one to the other is accomplished so that the angular velocity is constant during rotation. Such joints could include constant velocity joints, Hooke's joints or even gear boxes. By altering the respective mounting positions of the ends 2 along the length of the shafts 4 and 5, the drum 1 can be made to assume a substantially trapezoidal cross-sectional shape as shown in Figure 1. The shafts 4 and 5 are supported by self-aligning bearings 9 mounted in flanges 15 so that rotation of the shaft 5 causes the drum 1 to rotate The flanges 15 are formed on a housing 19 m which the drum is housed. Since the ends 2 are not parallel to each other, rotation of the shaft 5 first stretches the cylindrical sleeve 3 and then allows it to contract as a point on the surface of the sleeve 3 moves from the position indicated at "A" to the position indicated by "B", and then back to "A". The degree of stretch of the band 3 will vary according to the position of the cylinder ends 2 on the shafts 4 and 5 and the angle θ between the snafts Rotation of the shaft 5 is effected using a motor 7 via a universal joint 8. The ends 2 each comprise a pair of circular plates 21,22 made from a suitable material, e.g polymethyl methacrylate (perspex) or stainless steel, and are bolted together w th the end of the sleeve 3 held clamped between the plates 21 and 22. With reference also to Figure 2 the drum 1 is shown being used to produce a biaxially oriented cellulose film A web 20 of a solution of cellulose in N-methylmorpholme-N-oxide is extruded from an extruder 10 having a slot die orifice. The web 20 passes through an air gap 11 and then contacts the sleeve 3 to which it adheres. Preferably the die is located about 10 mm from the surface of the drum. The speed of rotation of the motor 7 is adjusted so that the peripheral speed of the sleeve 3 is faster than the rate of extrusion of the web 20, thereby stretching and orienting the web 20 in the direction of extrusion. A typical surface linear speed might be 60 meters/minute so that a drum of 1.33 meters diameter would rotate 14 revolutions per minute. The trapezoidal cross-section of the drum 1 causes the web 20 adhered to the sleeve 3 to expand transversely as the sleeve 3 itself expands on rotation of the drum l about the shafts 4 and 5. Biaxial stretching of the web 9 is thereby effected. The drum may have a length of 0.5 meters at its upper surface as shown, and a length of 1.5 meters at its lower surface. The stretching of the elastomeric sleeve 3 may lead to its concavity. This can be overcome by introducing compressed air into the drum 1 via the shaft 4, which may be hollow, and a rotational connector 25. With reference to Figure 3 the drum 1 is located in a bath 12 of water 13 or aqueous amine oxide solution in which the amine oxide does not exceed 70% by weight and is more typically about 25%. The amine oxide solvent is leached from the web 20 to leave a biaxially oriented cellulose film which readily separates from the sleeve 3 due to the film being less adhesive than the web 20 to the sleeve 3. The film is taken off the drum 1 passing over a pair of guide rollers 16 and 17. The bath 12 rests on a height adjustment mechanism 23, such as a scissor jack, which supports the bath at variable heights above the base of the housing 19. Since water from the bath 12 tends to be carried on the surface of the sleeve 3 after it leaves the bath 12, an air knife 18 is provided to remove any water carry-round, which could prevent the web 20 from adhering to the sleeve 3. The resultant oriented film can then be subjected to various treatments, for example further washing, bleaching, drying and coating, before being wound up into a roll. Additionally the film can be treated with reactive chemical treatments, e.g. cross linking agents, dyes and/or softeners. The film may include materials typically incorporated into film, e.g. antiblocks, colorants/fillers etc.. Sequentially stretched non-derivatised cellulose film produced on the above apparatus (Samples 2 and 3) was tested for tensile strength and for comparison with the tensile properties of film which has to be drawn in the direction of extrusion only. The samples were cut from the film and tested on an Instron Tensile testing machine at 20° and 65% relative humidity, at an extrusion rate of 200 mm/minute. Sample MD ratio TD ratio MD Tensile TD Tensile Strength Strength Kg/mm 2 Kg/mm 2 1 6.1 2.6 18.7 15.8 2 3.7 2.8 10.9 10.1 3 4.0 0 20.4 4.5 TD = transverse draw MD = machine direction draw Thus it can be seen that a transverse draw ratio of about 2.5:1 produces at least a doubling of the transverse strength in the cellulose film. With reference to Figure 4, there is shown a drum 41 in which the elastomeric sleeve 43 may be supported on at least the internal surface of its lower portion by shaped inserts 44 which may reduce the volume of pressurised air within the drum. Furthermore, the two ends 42 of the drum could be rotationally linked through conical surfaces 46 on the two inserts 44, instead of using the separate joint 6 as shown in Figure 1. With reference to Figure 5, a pair of stationary radial arms 57 and a support roller 58 are housed within the drum 51. The support roller 58 is free to rotate about an axle extending between the two radial arms 57 to internally support the upper part of the sleeve 53. The arms 57 are mounted on non-rotary shafts 56 that pass through rotatable hollow shafts 54,55 forming parts of the ends 52.

Claims: Claims 1. Biaxially oriented cellulosic film, the orientation having been imparted by the stretching, preferably sequential stretching, in different directions of a web of a solution of cellulose in a solvent before removing solvent from the web to form the film. 2. Film according to claim l, wherein the film has substantially balanced properties in the two directions of orientation. 3. Film according to either of the preceding claims, wherein the film is a non-derivatised cellulose film. 4. A process for the production of axially oriented cellulose film, the process comprising extruding or casting a web of a solution of a cellulosic material in a solvent therefor, onto an extensible surface to which it adheres and stretching the web in at least one direction by stretching the extensible surface in at least one said direction. 5. A process as claimed in claim 4, wherein the web is formed by extrusion through a slot die, and is sequentially stretched firstly in the direction of extrusion, and thereafter transverse to the direction of extrusion, and solvent is removed from the web to produce a film of biaxially oriented cellulose. 6. A process according to claim 5, wherein stretching of the web in the transverse direction is effected with the web being in contact with the extensible surface. 7. A process according to claim 6, wherein the extensible surface is formed on an elastomeric sleeve. 8. A process according to claim 5 or 6, wherein stretching of the web in the direction of web extrusion is effected by moving the extensible surface relative to the slot die at a faster rate than the cellulose solution is extruded from the said die. 9. A process according to any of claims 4 to 8, wherein the solution is stretched in the direction of extrusion by a factor of from 1.5:1 to 25:1. 10. A process according to any of claims 4 to 9, wherein the solution is stretched in a direction substantially perpendicular to the direction of extrusion by a factor from 1.5:1 to 8:1. 11. A process according to any of claims 4 to 9, wherein the material is cellulose dissolved in a tertiary-amine oxide solvent . 12. A process according to claim 11, wherein the tertiary-amine oxide is N-methylmorpholine-N-oxide. 13. A process according to any of claims 4 to 12, wherein the solvent is removed using water or some other non- solvent or a dilute solution of the solvent in water or non- solvent . 14. Apparatus for the production of oriented film, the apparatus including an expandable surface onto which a web of polymer solution can be extruded or cast, the surface being expandable to stretch the web adhered thereto in at least one of said biaxial directions. 15. Apparatus as claimed in claim 14, in which the expandable surface is a surface on an elastomeric means . 16. Apparatus according to claim 15, wherein the elastomeric means is in the form of a cylindrical sleeve. 17. Apparatus according to claim 16, wherein the cylindrical sleeve has substantially rigid ends inclined relative to each other and stretching of the sleeve is effected by rotation of said rigid ends with each. 18. Apparatus according to claim 17, wherein the angle between said two ends is adjustable to enable the degree of transverse stretch to be adjusted. 19. Apparatus as claimed in any one of claims 14 to 18, further including an extrusion means for extruding a substantially flat web onto the expandable surface, the surface being movable away from the extrusion means at a faster rate than the solution is extruded, and being expandable in a direction perpendicular to the direction of extrusion. 20. Apparatus according to any of claims 14 to 19, further including a means for removal of solvent from the web and which comprises a bath for water or a dilute solution of the solvent in water. 21. Apparatus according to any of claims 14 to 20, including means for drying the movable and expandable surface before the web makes contact therewith.

Inventors: ASKEW GREGORY JAMES, HAWLEY KEITH NIGEL WILKINSON, LAW STEPHEN JOHN, STREET HEATHER, ASKEW, GREGORY, JAMES, HAWLEY, KEITH, NIGEL, WILKINSON, LAW, STEPHEN, JOHN, STREET, HEATHER
Applicants: ASKEW GREGORY JAMES, COURTAULDS FIBRES HOLDINGS LTD, HAWLEY KEITH NIGEL WILKINSON, LAW STEPHEN JOHN, STREET HEATHER, ASKEW, GREGORY, JAMES, COURTAULDS FIBRES (HOLDINGS) LIMITED, HAWLEY, KEITH, NIGEL, WILKINSON, LAW, STEPHEN, JOHN, STREET, HEATHER
Application year: 1996
Main classification: B29C55
IPC codes: B29C41, B29C55, B29D7, B29K1, B29L7