GB2152870A

GB2152870A - Apertured film process

Info

Publication number: GB2152870A
Application number: GB08401395A
Authority: GB
Inventors: Peter Joseph Metcalfe; Andrew James Carter
Original assignee: Smith and Nephew Associated Companies PLC
Current assignee: Smith and Nephew PLC
Priority date: 1984-01-19
Filing date: 1984-01-19
Publication date: 1985-08-14
Also published as: GB8401395D0; GB2152870B

Abstract

A process for the preparation of an elastic apertured film which comprises a blend of 1,2 polybutadiene and an incompatible polymer which forms a discrete particulate phase within a matrix of 1,2 polybutadiene characterised in that the process comprises stretching a film of said blend having thicker and thinner areas until apertures are formed in the thinner areas, films suitable for use in the process, apertured films prepared by the process and articles for contact with the body which comprise such apertured films are described.

Description

SPECIFICATION Apertured film process The invention relates to a process for the preparation of elastic apertured films of 1,2 polybutadiene blends, materials therefor and their use in articles for bodily contact.

British Patent Application No. 2103537A discloses elastic apertured films suitable for use in adhesive medical dressings which comprise a blend of 1,2 polybutadiene and an incompatible polymer which forms a discrete phase with matrix of 1,2 polybutadiene. This patent application further discloses a process of preparing the apertured films which comprises stretching a film of the 1,2 polybutadiene blend and perforating the stretched film to form apertures therein using a conventional perforating method such as a method involving the use of punches and dies, heated needles or protruberances hot gases or electrical spark discharge. Such conventional perforating methods normally involve the use of complex apparatus which often operate at relatively low speeds.The process of preparing the elastic apertured films of the above mentioned patent application thus involves a separate perforating step which can add substantially to the cost of preparing the film.

A process for the preparation of elastic apertured films which comprise a blend of 1,2 polybutadiene nd an incompatible polymer has now been found which avoids the need of using a separate perforating step and which is therefore more economical than the prior art processes disclosed in the above aforementioned patent application.

The present invention provides process for the preparation of an elastic apertured film which comprises a blend of 1,2 polybutadiene and an incompatible polymer which forms a discrete particulate phase within a matrix of 1,2 polybutadiene characterised in that the process comprises stretching a film of said blend having thicker and thinner areas until apertures are formed in the thinner areas.

In the process of the invention the apertures in the 1,2 polybutadiene blend film are formed by stretching of the film. The process of the invention therefore does not comprise a separate conventional perforating step and is therefore more economical than a process of preparing apertured 1,2 polybutadiene blend films which employ such a perforating step.

Apt 1,2 polybutadienes for use in the invention are thermoplastic 1,2 polybutadienes. Favoured thermoplastic 1,2 polybutadienes contain a major amount of syndiotactic 1,2 polybutadiene having a crystallity of 15% to 30% (for example 25% to 30% and have an average molecular weight in excess of 100,000.

Preferred 1,2 polybutadienes for use in this invention are grades RB810, RB820 and RB830 made by Japan Synthetic Rubber Company. The 1,2 polybutadiene may contain other materials such as antioxidants and fillers but preferably does not contain fillers.

The incompatible polymer used in the invention when blended with the 1,2 polybutadiene forms a discrete particulate phase within a matrix of 1,2 polybutadiene.

Suitable incompatible polymers include those derived from the polymerisation of vinyl hydrocarbons for polystyrene and other polyolefins such as polyethylene and polypropylene. An especially suitable incompatible polymer is polystyrene. The polystyrene may be an unmodified grade (homopolymer) or a rubber modified grade (high impact polystyrene) High impact polystyrene is a preferred incompatible polymer.

A particularly preferred polystyrene is a high impact polystyrene (HIPS) is known as Styron 485 (Trade Mark) made by Dow Chemicals Co. Limited and available from R.H. Cole Limited as reference 6MW.

The proportion of 1,2 polybutadiene and incompatible polymer in the blend may vary according to the individual polymers used in the blend.

The blends used in this invention, however, suitably contain 50% to 90% by weight and preferably contain 55% to 85% by weight of 1,2 polybutadiene.

A preferred blend contains 60% by weight of 1,2 polybutadiene (for example RB830) and 40% by weight of high impact polystyrene (for example 6MW). Another preferred blend containing 80% by weight of 1,2 polybutadiene (for example RB810 and 20% by weight of high impact polystyrene (for example 6MW).

The 1,2 polybutadiene and incompatible polymer can be formed into a blend by any suitable conventional method. It is preferred, however, that the blending of the polymers is carried out under hot melt conditions although premixing of the granules of the polymers can be carried out at room temperature.

The 1,2 polybutadieneiincompatible polymer blend used in the process of the invention is in the form of a film. The blending of the polymers therefore may be carried out during the film forming process. It is preferred, however, that the polymers are preblended before being used to form a film.

The 1,2 polybutadiene/incompatible polymer blend film used in the process of the invention has a thicker and thinner areas. Such a film can be stretched to form an elastic apertured film.

Accordingly in another aspect the invention provides an elastic film which comprises a blend of 1,2 polybutadiene and an incompatible polymer which forms a discrete particulate phase within a matrix of 1,2 polybutadiene, which film has thicker and thinner areas and which on stretching is capable of forming an elastic apertured film.

The film of the invention has thicker and thinner areas which will normally and preferably have been formed by embossing. On stretching the embossed film voiding of the film occurs which is then followed by rupture of the thinner areas to form apertures.

The thinner areas of the embossed film therefore will be favourably less than 0.4mm thick, desirably less than 0.25mm thick and preferably Iss than 0.15mm thick. The overall thickness of the embossed film can suitably be 0.75mm to 2mm and preferably 0.1 mm to 1 mm thick.

The embossed pattern of thicker and thinner areas provided on the film will be chosen to ensure that undue propagation of rupture will be prevented. Suitable embossed pattern include those disclosed in British Patent Nos. 914,489, 1,055,963, 1,045,485 and 1,110,051.

Apt embossed films of the invention have on one side thereof a regular pattern of discrete depressions.

Suitable patterns of this type are disclosed in British Patent 1,075,485. Favoured embossed films of the invention have on one side thereof a regular pattern of conical shaped depressions.

Further apt films of the invention have a regular pattern of thicker and thinner areas provided by, on one side of the film a first set of straight parallel ribs and grooves extending in one direction and on the other side of the film, a second set of parallel ribs and grooves which intersect at an angle with the first set. Suitable embossed patterns of this type are disclosed in British Patent 1,075,485. Favoured films of the invention comprise a first set of straight parallel ribs and grooves which extend in the machine direction of the film and a second set of straight parallel ribs which extend in a transverse direction which is perpendicular to that of at the machine direction.

The grooves and ribs of the embossed film can have variety of shapes. Grooves and ribs with a trapezoidal shape have been found to be especially suitable.

The films of the invention can suitably have 2 to 40 and preferably 4 to 20 thinner areas per cm in one direction of the film.

The embossed films of the invention can be convenientallyformed by a melt embossing process in which extruded film of the 1,2 polybutadiene blend is cast as a hot melt between the nip of two rotating embossing rollers which are maintained at a temperature lower than that of the melt temperature of the polymer blend for example 20"C. The pattern on the surface of embossing rollers can be adapted to the desired embossed pattern on the film. One of the embossing rollers therefore can have a plain surface when the film having an embossed pattern on only one surface is required. Typical film melt embossing processes are disclosed in the aforementioned British patents.

The embossed 1,2 polybutadiene blends films of the invention can be stretched until apertures are formed in the thinner areas to form elastic apertured films. The embossed film can be stretched in one direction for example the machine direction of the film or a direction which is transversed to the machine direction or in two directions for example in both machine and transverse directions. The film can suitably stretched in any one direction at stretch ratio of 1.25:1 to 5:1 and preferably at a stretch ratio of 11 to 4:1.

The stretching can be carried out at temperatures which are below that of the melt temperature of the 1,2 polybutadiene used in the blend. Suitable temperatures for stretching the embossed 1,2 polybutadiene blend film are between 10 C and 90"C. The higher stretching temperatures for example stretching temperatures between 50"C to 900C can be used to form of elastic apertured films with open area of apertures greater than 40%; After each stretching the stretched film is allowed to contract.

The elastic apertured films prepared by the process of the invention has a plurality of apertures. The apertures may suitable have an approximately eliptical shape (for a uniaxially stretched film) or an approximately circular shape or square for a biaxially stretched film. Typically the apertures in the film can have a dimension of between 0.1 mm and 2mm. The area of the apertures can suitably be 1%to 80% of the total area of the film.

The thickness of the elastic apertured films formed by the process of the invention will normally be between 0.025 and 0.5mm and preferably 0.05 and 0.4mm.

The elastic apertured films of the invention can suitably have a recoverable elastic strain of at least 50% and preferably at least 100% at 20 C.

In one preferred form the apertured film can be a net having intersecting strands. Favoured nets have longitudinal and transverse intersecting strands and have an area of apertures of greater than 40% of the total area of the net.

The elastic apertured films formed by the process of the invention are conformable and are therefore suitable for use in articles for bodily contact such as medical dressings, bandages and sanitary products.

This is a further aspect the invention provides an article for body contact which comprises an elastic apertured film prepared by the process of the invention.

Elastic apertured films having aperture open area of less than 40% have been found to be suitable for use as a backing layer for an adhesive medical dressing such a first aid dressings.

The adhesive medical dressings are preferably moisture vapour transmitting. Favoured adhesive medical dressings therefore comprise of moisture vapour transmitting adhesive layer which can be continuous or discontinuous. Suitable continuous adhesive layers comprise an acrylate ester copolymer or a polyvinyl ether. Preferred acrylate ester copolymer adhesives are disclosed iri British Patent Application No.

2,081,721A. A favoured acrylate ester copolymer adhesive comprises a copolymer of 47 parts by weight of 2-ethylhexyl acrylate, 47 parts by weight of n-butyl acrylate and 6 parts by weight of acrylic acid.

Elastic apertured films including nets with a high open area of apertures, typically 50% to 80% of the total area of the film, can be used as wound facing layer on an absorbent dressing or as a cover layer on absorbent sanitary product for example a sanitary towel, diaper or incontinence pad.

Example I Extrusion compound preparation An extrusion feedstock was prepared as follows: (i) 80 parts by weight of 1,2 polybutadiene polymer granules JSR RB810 supplied by Japanese Synthetic Rubber Co., were combined with 20 parts by weight of High Impact polystyrene (HIPS) granules (Styron 485, manufactured by the Dow Chemical Co. Ltd., and supplied by R.H. Cole Ltd., reference 6MW), and the resultant mixture tumbled to ensure a sufficiently even dispersion of one type of granule in the other had been obtained for the efficient operation of the next stage of the process.

(ii) The mixture of granules from (i) was loaded into the hopper of a Brabender 19mm extruder. This extruder was equipped with a tube, a polyolefin type screw which had a length to a diameter ratio of 25:1 and a compression ratio of 3:1.

The extrusion was carried out using a die temperature of 1 80"C and a screw speed of 120 rpm. The tube produced was drawn through a water bath maintained at 20"C at a speed sufficient to reduce its diameter to approximately 2mm. After travelling a distance of 1 metre through this bath the tube was passed over a foam pad to remove excess moisture before being cut into 3-5mm lengths using a laboratory model granulator (Accrapak).

The granules were collected from the granulator and then dried for four hours at 90"C in air circulating ovens in trays 2.5mm deep.

Film Extrusion The dried compound prepared above was used to produce an embossed film as follows: The compound was fed into the hopper of a Brabender 19mm extruder equipped as above, with the exception of the die, which in this case was a 150mm wide flat film flexible lip die set to a gap of 0.254mm and maintained at a temperature of 185 C. Using a screw speed of 110 rpm, a melt film was extruded vertically downwards and fed into the nip of a two roller chill casting unit (nip casting speed 1.0 m/min) located directly below the die at a distance of 50mm.

One roller of this chill casting unit had circumferential grooves (one per transverse millimetr) defined between flat topped roller ridges with, in cross section, 45" inwardly tapering sides. The flat tops of the ridges had a width of 0.203mm. The film was taken off over this roller. The other roller had axial grooves (one per circumferential millimetre) having the same geometry and dimensions as those of the other roller.

The film produced was 136mm wide, had an overall thickness of 0.88mm and a weight of 240gsm. It had on one surface longitudinal ribs (one per 1 .Omm measured in the transverse direction), and on the other surface transverse ribs (one per 1.11 mm) measured in the machine direction.

The continuous membranes remaining between the points of intersection of the two sets of ribs were approximately rectangular in shape, having dimensions of 0.20mm x 0.1 5mm, the smaller dimension being parallel to the machine direction and thickness of 0.08mm.

The ribs at their base, i.e. where they formed the boundaries of the membranes, were 0.71 mm wide (longitudinal ribs) and 0.94mm wide (transverse ribs).

Film stretching A sample of the above grooved film, 1 20mm wide, was gripped in the jaws of a laboratory Hounsfield tensometer so as to have a gauge length of 1 00mm and with the longitudinal ribs parallel to the proposed direction of stretching.

The stretching operation was then carried out, at 55"C with the jaws separating at a rate of approximately 50mm per minute. In this manner the grooved film was stretched until a stretch ratio of 2.4:1 had been attained, by which stage the membranes had split and become apertured. The apertured film was then allowed to contract. A sample of the deformed portion was then stretched in a similar manner in the direction which was perpendicular to the above stretch direction, until stretch ratio of 2.2:1 was achieved.

The film was again allowed to contract.

The apertured film produced by the above operations was 0.18mm in overall thickness and had a weight per unit area of 113gsm.

The apertures in this net were almost circular in shape, the slightly larger axis being aligned in the direction which was the machine direction in the unstretched film. The maximum dimensions of the apertures were 0.91 5mm and 0.905mm. The distance between centre points of the apertures was 1.51 mm in the direction which had been machine direction (MD) in the unstretched film and 1.22mm in the transverse direction (TD) perpendicular to this. The apertured film had an open area which was 45% of the total area of the film.

Example 2 The film stretching procedure was the same as in Example 1, with the exception that the stretching was carried out at room temperature. The draw ratios obtained were 2.6:1 in the machine direction and 3.5:1 in the transverse direction.

The dimensions of the apertured film obtained were as follows: Overall thickness = 0.20mm Weight = 122gsm Aperture dimensions Major axis (TD) 0.63mm Minor axis (MD) 0.37mm Distance between centre points 1.45mm (TD) 1.25mm (MD) Open area: 40% of total area of film.

Example 3 The film used in this example was the same as the film in example 1.

The film stretching procedure was the same as in example 1 with the exception that the stretching was carried out at room temperature in the machine direction only. The draw ratio attained was 2.75:1.

The dimensions of the net obtained were as follows: Overall thickness 0.21mm Weight 158gsm Aperture dimensions Major axis (TD) 0.30mm Minor axis (MD) 0.21mm Distance between centre points 1.41mm MD 0.87mm TD Open area: 4% oftotal area ofthefilm.

Example 4 An extrusion compound blend was prepared in the same manner as Example 1 using (i) 80 parts by weight of Polyisobutadiene polymer granules (JSR RB830, supplied by the Japanese synthetic Rubber Co.) which was blended with 20 parts by weight of High Impact Polystyrene granules (Styron 485, manufactured by the Dow Chemical Co. Ltd., and supplied by R.H. Cole, ref. 6MW).

Film extrusion The dried compound prepared above was used to produce an embossed film as follows: The compound was fed into the hopper of a Brabender 19mm extruder equipped as above, with the exception of the die, which in this case was a 150mm wide flat sheet die set to a gap of 0.254mm and maintained at a temperature of 180 C. Using a screw speed of 75rmp, a melt film was extruded longitudinally and fed into the nip of a two roller chill casting unit located 50mm from the die face.

The roller of this chill casting unit was provided with 1 conical projection per 0.725mm in both circumferential and axial roller directions. The height of these projections above the roller surface was 0.21 mm and their diameter at the roller surface was 0.42mm.

The other roller in the chill casting unit was a plain steel roller maintained at a temperature of 40"C. The casting nip speed was 2.3m/min.

This film had a series of depression in one surface, which were approximately conical in nature. Where the depressions interrupted the surface of the film their diameter was 0.31 mm. Their depth was such that the film was not perforated. Their centre to centre separation was 0.76mm measured in the machine direction and 0.71 mm measured in the transverse direction.

Film stretching A sample of the above film, 1 20mm wide, was gripped in the jaws of a laboratory hounsfield tensometer so as to have a gauge length of 70mm with the transverse direction parallel to the proposed direction of stretching.

The stretching operation was then carried out at 57"C with the jaws separating at a rate of 50mm per minute. A stretch ratio of 1.5:1 was obtained after which the film was allowed to contract. During this stretching procedure apertures were formed, associated with the depressions in the film. A sample of the deformed film was then stretched in a similar manner in the direction which was perpendicular to the above stretch direction, until a stretch ratio of 2:1 was achieved. The film was again allowed to contract.

The relaxed film contained apertures which were eliptical in shape, their dimensions being 0.76mm x 0.34mm, with the larger dimension being aligned to the machine direction.

The centre to centre separation of these apertures was 1.2mm in what had been the machine direction during extrusion and 0.78mm in the direction perpedicular to this.

The film thickness was 0.10mm and the film weight per unit area was 50gsm. The apertured film so formed had an open area which was 22% of the total area of the film.

Example 5 An extrusion blend compound was prepared as follows: (i) 60 parts by weight of polyisobutadiene polymer granules (JSR RB830, supplied by the Japanese Synthetic Rubber Co.), were combined with 40 parts by weight of High Impact Polystyrene granules (Styron 485, manufactured by the Dow Chemical Co. Ltd., and supplied by R.H. Cole Ltd., ref. 6MW), and the resultant mixture was tumbled to ensure a sufficiently even dispersion of one type of granule in the other had been obtained for the efficient operation of the next stage of the process.

(ii) The mixture of granules from (i) was loaded into the hopper of a Reifenhauser S60 60mm extruder. This extruder was equipped with a multi rod die which had 12 circular outlet channels, each 3mm in diameter.

The extruder was also equipped with a polyolefin type screw which had a length to diameter ratio of 20:1 and a compression ratio of 3:1.

The extrusion was carried out using a die temperature of 190or and a screw speed of 38 rpm. The filaments produced were drawn through a water bath maintained at 20"C at a speed sufficient to reduce their diameters to approximately 2mm.

After travelling a distance of 1 metre through this bath the filaments were passed through a chamber equipped with a hot air blower, to remove excess moisture before being cut into 3-5mm lengths using a laboratory model granulator (Accrapak).

The granules were collected from the granulator and then dried for four hours at 90"C in air circulating ovens in trays 2.5cm deep.

Film extrusion The dried compound prepare above was used to produce an embossed film as follows: The compound was fed into the hopper of a Reifenhauser S60, 60mm extruder equipped as above, with the exception of the die, which in this case was a 600mm wide flat film flexible lip die set to a gap of 0.254mm and maintained at a temperature of 185"C. Using a screw speed of 34rpm, a melt film was extruded vertically downwards and fed into the nip of a two roller chill casting unit located directly below the die at a distance of 125mm.

One roller of this chill casting unit had circumferential grooves (one per transverse millimetre) defined between flat topped roller ridges with, in cross section, 45" inwardly tapering sides. The flat tops of the ridges had a width of 0.203mm. The film was taken off over this roller. The other roller had with axial grooves (one per circumferential millimetre) having the same geometry and dimensions as those of in the other roller.

Both rollers were maintained at a temperature of 40"C.

At a nip casting speed of 2mlmin the film produced had an overall thickness of 0.69mm.

Film stretching A sample of the above grooved film 100mm wide was gripped in the jaws of a floor model Instron fitted with a temperature control chamber so as to have a gauge length of 50mm with the longitudinal ribs parallel to the proposed direction of stretching.

The stretching operation was then carried out at 90"C with the jaw separating at 50mm/min. In this manner the grooved sheet was stretched until a stretch ratio of 2.5:1 had been attained by which stage the membranes had split and become apertured. The apertured film was then held under tension for 5 minutes before being allowed to contract. A sample of the deformed portion was then stretched in a similar manner in a direction which was perpendicular to the above stretch direction, until a stretch ratio of 3:1 was achieved. The film was left to cool to room temperature under tension. The film was then allowed to contract. The net produced by the above operations was 0.29mm in overall thickness and had a weight per unit area of 85gsm.

The apertures of this net were essentially rectangular in shape, the larger axis being aligned in the direction which was the transverse direction in the unstretched film. The maximum dimensions of the apertures were 1.78mum and 1.43mum. The distance between centre points of the apertures were 1.92mum in the direction which had been the machine direction in the unstretched film and 2.31 mm in the direction perpendicular to this.

The net had an open area which was 56% of the total area of the net.

Example 6 The apertured film of Example 2 was coated with a continuous layer (0.03mm thick) of a moisture vapour transmitting crylate ester copolymer adhesive. The apertured film was coated with adhesive using a transfer method in which a solution of the adhesive was coated onto a release coated paper and dried and the adhesive coated paper was laminated under pressure to the film. The acrylate ester copolymer adhesive used in this example comprised a copolymer of 47 parts by weight of n-butyl acrylate, 47 parts by weight of 2 ethylhexyl acrylate and 6 parts by weight of acrylic acid made according to the method given in British Patent Application No. 2,081,721A.

The adhesive coated elastic apertured film was then used to make first aid dressings. The first aid dressings when applied to volunteers were found to be highly conformable to skin.

Claims

1. A process for the preparation of an elastic apertured film which comprises a blend of 1,2 polybutadiene and an incompatible polymer which forms a discrete particulate phase within a matrix of 1,2 polybutadiene characterised in that the process comprises stretching a film of said blend having thicker and thinner areas until apertures are formed in the thinner areas.

2. A process as claimed in claim 1 in which the incompatible polymer comprises polystyrene.

3. A process as claimed in claim 2 in which the polystyrene is a high impact polystyrene.

4. A process as claimed in any of claims 1 to 3 in which the blend contains 55% to 85% by weight of 1,2 polybutadiene.

5. A process as claimed in any of claims 1 to 4 in which the film is stretched in two directions.

6. A process as claimed in any of claims 1 to 5 in which the film is stretched in any one direction at a stretch ratio of 1.5:1 to 4:1.

7. An elastic film suitable for use in the process of claim 1 which comprises a blend of 1,2 polybutadiene and an incompatible polymer which forms a discrete particulate phase within a matrix of 1,2 polybutadiene, which film has thicker and thinner areas and which on stretching is capable of forming an elastic apertured film.

8. A film as claimed in claim 7 which has been formed by embossing.

9. An embossed film as claimed in claim 8 which has on one side a regular pattern of discrete depressions.

10. An embossed film as claimed in claim 8 which has on one side a first set of straight parallel ribs and grooves and on the other side a second set of straight parallel ribs and grooves which intersect at an angle with the first set.

11. An elastic apertured film prepared by the process of claims 1 to 6.

12. An elastic apertured film as claimed in claim 11 which is in the form of a net.

13. An article for body contact which comprises an elastic apertured film of claims 11 or 12.

14. An article as claimed in claim 13 in which the article is a dressing or an absorbent sanitary product.