WO1995005936A1

WO1995005936A1 - Runnability of packaging film

Info

Publication number: WO1995005936A1
Application number: PCT/CA1994/000443
Authority: WO
Inventors: Vladimir Ronald Braun; Anthony Chow
Original assignee: Dupont Canada Inc.
Priority date: 1993-08-23
Filing date: 1994-08-16
Publication date: 1995-03-02
Also published as: GB9317473D0

Abstract

A method for the formation of a tube from thermoplastic packaging film on a tube-forming apparatus is disclosed. The method comprises feeding the thermoplastic film to tube-forming apparatus and heat sealing the thermoplastic film into the form of a tube using a longitudinal heat seal. The film used in the method is formed by extrusion of molten synthetic thermoplastic polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100 °C and then cooling to a temperature of less than 40 °C. The amount of tension applied to the film is such that the thickness of the film remains substantially unchanged. The formation of the film and the film are also disclosed. The method results in improvements in the runnability of the film, especially on form, fill and seal apparatus in the packaging of liquids, as shown by the number of leaking pouches.

Description

RUNNABILITY OF PACKAGING FILM

The present invention relates to the runnability of thermoplastic packaging film that has been formed in a blown film process, during the heat sealing of such film using a tube forming apparatus. In particular, the invention relates to runnability of polyethylene film, during the heat sealing steps, on a form, fill and seal apparatus in the packaging of flowable materials, especially liquids. The present invention also relates to a method for the formation of thermoplastic packaging film.

For many years, milk has been packaged in pouches made on vertical form, fill and seal apparatus. Such pouches have been sold to household consumers. More recently, pouches have been used to package other comestibles e.g. mayonnaise, caramel, scrambled eggs, tomato ketchup, salad dressing, preserves and the like. Pouches containing such comestibles are usually sold to "institutional" buyers e.g. restaurants.

Vertical form, fill and seal apparatus is known. Using one such apparatus, synthetic thermoplastic film in the form of a continuous flat web is unwound from a roll and formed into a continuous tube, using a forming section and heat sealing along the longitudinal edges of the film, to form a so-called lap seal or a so-called fin seal. The tube thus formed is pulled vertically downwards to a filling station. The tube is then collapsed across a transverse cross-section of the tube, below the filling station; a transverse heat-seal is made by a sealing device, thus making an air-tight seal across the tube. The material to be packaged e.g. milk, enters the tube continuously or intermittently. The tube is then moved in a downward direction by a predetermined distance, usually under the influence of the weight of the material in the tube. A further transverse heat seal is formed. The heat sealing procedure is such that the pouch containing the packaged material that is thus

SUBSTITUTE SHEET formed is severed from the tube, thereby providing a pillow shaped pouch of packaged material. Thus, the sealing device has sealed the top of the filled pouch, sealed the bottom of the next-to-be formed pouch and severed the tube, all in one operation. A vertical form, fill and seal apparatus of the type described above is available under the trademark PREPAC.

Apparatus of the type described above is disclosed in published PCT application No. WO 92/14601 of David C. Riley, published on September 03, 1992.

In the use of such apparatus, it is very important that a leak-free lap or fin seal be formed. If such a seal is not formed, the contents of the package will leak from the package, causing loss of product and the need to clean the form, fill and seal apparatus, which is commercially unacceptable.

One of the problems associated with the formation of a lap or fin seal in a process of the type described above is the runnablity or tracking of the film being used. If the web of film does not track consistently but rather wanders from side to side even by a small amount, there is a tendency for the lap or fin seal to not be formed in a consistent manner and hence a tendency for the formation of leakers in the resultant pouches. While this problem may be more acute in the formation of the lap or fin seal in a form, fill and seal process than in other processes, film runnability is an important parameter in processes involving the use of continuous webs of film. A method for the improvement of the runnability or tracking of packaging film has now been found.

Accordingly, the present invention provides a method for the formation of a tube from thermoplastic packaging film on a tube-forming apparatus, comprising: (a) feeding the thermoplastic film to tube-forming apparatus; and

SUBSTITUTE SHEET (b) heat sealing the thermoplastic film into the form of a tube using a longitudinal heat seal, said film having been formed by extrusion of molten synthetic thermoplastic polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

In a preferred embodiment of the present invention, the thermoplastic packaging film is a linear low density polyethylene having a density in the range of 0.89 to 0.93 g/cm³, a melt index in the range of 0.1 to 10 dg/min. and a thickness in the range of 10 to 260 microns. The linear low density polyethylene may be in the form of blends of such polyethylene with low density high pressure polyethylene or ethylene/vinyl acetate copolymers, especially in which the linear low density polyethylene comprises at least 75% of the blend.

In another embodiment, the longitudinal heat seal is a lap seal or a fin seal.

In a further embodiment, the longitudinal heat seal is formed during a process for the packaging of a flowable material.

In a still further embodiment, the process is an aseptic packaging process.

The present invention also provides a method for the formation of thermoplastic packaging film comprising the steps of extruding molten thermoplastic polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension

SUBSTITUTE SHEET being such that the thickness of the film remains substantially unchanged.

The present invention also provides thermoplastic packaging film formed by the steps of extruding molten thermoplastic polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

The present invention additionally provides a process for the manufacture of an article from a polyethylene film using film-converting apparatus, in which a continuous sheet of polyethylene film is fed to the apparatus, the improvement comprising using continuous sheet of polyethylene film that has been formed by the steps of extruding molten polyethylene through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

In the drawing, FIG. 1 illustrates results obtained according to the invention and in a control run.

In the use of a vertical form, fill and seal apparatus, a continuous flat web film is unwound from a roll i.e. the film is not in the form a tube. The continuous web of film is formed into a tubular form using a film folding "horn" such that the longitudinal edges of the film overlap. It is important that the longitudinal edges of the film do overlap, because failure to do so will result in the formation of pouches that leak. The film then passes to a heat-sealing station for formation of the longitudinal seal. A heat-

SUBSTITUTE SHEET seal backing plate may be provided to assist in formation of this longitudinal seal. It is preferable to substantially equalize the amount of the film that protrudes on the two sides of the longitudinal seal that is formed; the amount of such protruding or excess film is usually measured in millimetres. So-called film diverters are known for this purpose.

The tubular film that has been formed is then fed towards the section of the apparatus in which the material to be packaged flows into the tube and the horizontal heat seals are formed. Spreader fingers may be used to open up the cross-section of the tubular film. The tubular film passes between jaws for the formation of the horizontal seal and for the separation of a filled pouch from the pouch that has yet to be filled. As discussed above, form, fill and seal apparatus is known. When the film fed to the apparatus is film that has been made in a blown film process, the film has a tendency to wander instead of tracking in a uniform manner. This creates a problem of runnability of the film through the apparatus, which manifests itself in the form of leakers.

In the conventional methods for the manufacture of film using a blown film process, molten polymer is extruded through an annular die, thereby forming a tube of molten polymer. Gas, usually inert gas, is fed under a low to moderate pressure into the tube of molten polymer thereby causing the tube to expand; the pressure inside the tube is controlled so that the film that is ultimately produced has a predetermined thickness. As the molten and now expanded tube of polymer proceeds away from the annular die, the polymer cools and solidifies, thereby forming a tube of the film of the thermoplastic polymer. Various techniques known in the art are normally used to randomize the positions of so-called hard bands during the extrusion process. The tube is then collapsed and slit, and usually wound into a roll.

SUBSTITUTE SHEET The film may be treated with an electric discharge prior to or even subsequent to the winding operation in order to render the film receptive to printing inks.

According to the present invention, the film subsequent to the expansion and collapsing in the process of manufacture, is further heated. This heating may be done in-line prior to winding the film onto a roll, and may even be done prior to slitting when the film is still in the form of a continuous collapsed tube. Alternatively, the film may be unwound and passed through a heating station. Furthermore the roll of film may itself be placed in suitable heat treating apparatus. It is preferred that the film be heat treated in-line in a continuous process. It is further preferred that the heat treatment be carried out after slitting the edges from the collapsed tube to form film.

The film is heated to a temperature in the range of 50-l00°C. In a preferred embodiment, the temperature is in range of 60-90°C, especially in the range of 65-80°C. The heat treatment is carried out with the film under tension, but the tension is such that the film thickness remains unchanged or essentially so. Thus, the heat treatment process is distinct from a film orientation process, in which the film is oriented and consequently the film thickness is reduced.

After the heat treatment, the film is cooled to a temperature of less than 40°C, and especially to a temperature of 35°C or less. The higher temperatures may be beneficial for release of stress in the film. The polymer is preferably a linear low density polyethylene, including the polyethylene manufactured using so-called metallocene catalysts. Such polyethylene preferably has a density in the range of 0.89 to 0.93 cm³, and particularly in the range of 0.91 to 0.92 g/cm³. The polyethylene preferably has a melt index in the range of 0.1 to 10 dg/ in. , and especially in the range of 0.5 to 2 dg/min. The thickness of the polyethylene film may be

SUBSTITUTE SHEET varied over the range normally used for packaging films, especially in the range of 10 to 260 microns and particularly in the range of 15 to 150 microns.

The linear low density polyethylene may be in the form of blends with low density high pressure polyethylene and/or ethylene/vinyl acetate copolymers, as is known in the art of manufacture of films from linear low density polyethylene. In preferred embodiments, the linear low density polyethylene comprises at least 75% of the blend.

While the present invention has been particularly described herein with reference to form, fill and seal apparatus and the packaging of flowable materials in which the formation of the heat seal is particularly critical, the present invention may be used in other end uses. For example, the invention may be used in processes for laminating one film, particularly polyethylene, to another film, in the printing of polyethylene film, in bag making process as well as pouch making processes.

The present invention is particularly useful in aseptic packaging processes, particularly aseptic packaging processes utilizing form, fill and seal apparatus. The regulatory requirements for aseptic packaging processes are very rigid and the formation of leakers results in significant down-time of the apparatus before it is in acceptable condition for resumption of aseptic packaging.

The film used in the process may be in the form of a monolayer film, as particularly described above, or it may be a co-extruded film. The coextruded film will usually contain polymers other than polyethylene, including coextrudable adhesives, ethylene/vinyl alcohol (EVOH) copolymers and/or polyamides. The present invention is illustrated by the following examples.

SUBSTITUTE SHEET Example 1 - A 127 μm thick film was manufactured from a blend of linear low density polyethylene and high pressure polyethylene using a conventional blown film process. The blend was formed from 85 parts by weight of an ethylene/octene copolymer having a density of 0.919 g/cm³ and a melt index of 0.73 dg/min with 15 parts by weight of a high pressure polyethylene having a density of 0.918 g/cm³ and a melt index of 8.4 dg/min. The film was cooled and rolled up in the form of a mill roll of un-slit film. One mill roll of the film was heat treated by passing the film through a system of six heated rollers, which were driven at different speeds in order to maintain the film under tension. Details of the speed of the film on each roller and the film temperatures on each of the heated rollers were as follows:

Roll No . Spee (m/min.) Roll Temperature (°C)

1 29 . 7 70 2 2 2 299..77 65

3 30 . 8 65

4 28 . 9 70

5 29 . 0 25

6 33 . 9 unheated

The film stretched 8% in the machine direction but the gauge profile remained unchanged. Rolls of film having a width of 383 mm were slit from the heat treated mill roll and from a control roll that had not been subjected to the six rollers.

Film from the slit control roll of film was fed to a PREPAC AS-2 pouch-forming apparatus; such apparatus is intended for aseptic packaging of flowable material. The apparatus has a peroxide treatment bath, which was at a temperature of 50°C. The apparatus further uses sterile air, which had a temperature of 45°C. Water-filled pouches having a length of 26 cm and containing 1400 g of water were manufactured. The process was allowed to operate for a period of 15 minutes, at which time a steady process had been obtained. 80 consecutive pouches

SUBSTITUTE SHEET were then collected and numbered in sequence. The offset between the two film edges opposite the vertical fold at the longitudinal seal was measured. It was found that the maximum offset was 18 mm and the minimum was minus 7 mm, representing a total shift in the film as measured by the offset of 25 mm.

The procedure was repeated with the film that had been processed according to the invention. It was found that the offset had a maximum of minus 4 mm and a minimum of minus 10 mm, for a spread of 6 mm.

The results show that the side-to-side wander of the film fed to the apparatus was substantially reduced when the film had been treated according to the present invention i.e. the film had wandered 6 mm in the case of the present invention and 25 mm for the control. This means that the risk of losing the vertical seal in the apparatus was similarly reduced with film according to the invention.

The results obtained are shown in Fig. 1, which shows the sequential positioning of the edges of the film during the running of the process.

SUBSTITUTE SHEET

Claims

CLAIMS :

1. A method for the formation of a tube from thermoplastic packaging film on a tube-forming apparatus, comprising: (a) feeding the thermoplastic film to tube-forming apparatus; and

(b) heat sealing the thermoplastic film into the form of a tube using a longitudinal heat seal, said film having been formed by extrusion of molten synthetic thermoplastic polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

2. The method of Claim 1 in which the longitudinal heat seal is a lap seal or a fin seal.

3. The method of Claim 2 in which the longitudinal heat seal is formed during a process for the packaging of a flowable material.

4. The method of Claim 3 in which the process is an aseptic packaging process.

5. The method of any one of Claims 1-4 in which, in step (a) , the film is heated to a temperature in the range of 60-90°C.

6. The method of any one of Claims 1-4 in which, in step (a) , the film is heated to a temperature in the range of 65-80°C.

SUBSTITUTE SHEET

7. The method of any one of Claims 1-6 in which the film is cooled to 35°C or less.

8. A method for the formation of thermoplastic packaging film comprising the steps of extruding molten thermoplastic, polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50- 100°C and then cooling to a temperature of less than

40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

9. The method of any one of Claims 1-8 in which the thermoplastic packaging film is a linear low density polyethylene having a density in the range of 0.89 to 0.93 g/cm³, a melt index in the range of 0.1 to 10 dg/min. and a thickness in the range of 10 to 260 microns.

10. The method of any one of Claims 1-9 in which the linear low density polyethylene has a density in the range of 0.91 to 0.92 g/cm³, a melt index in the range of 0.5 to 2 dg/min. and a thickness in the range of 15 to 150 microns.

11. The method of Claim 10 in which the polyethylene has been manufactured using a metallocene catalyst.

12. The method of any one of Claims 1-11 in which the polyethylene is in the form of a blend with at least one of low density high pressure polyethylene and ethylene/vinyl acetate copolymer.

13. The method of any one of Claims 1-12 in which the film is heat treated in-line in a continuous process.

SUBSTITUTE SHEET

14. The method of any one of Claims 1-13 in which the heat treatment be carried out after slitting edges from a collapsed tube of the film.

15. Thermoplastic packaging film formed by the steps of extruding molten thermoplastic polymer through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature of 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

16. The film of Claim 15 in which the thermoplastic packaging film is a linear low density polyethylene having a density in the range of 0.89 to 0.93 g/cm³, a melt index in the range of 0.1 to 10 dg/min. and a thickness in the range of 10 to 260 microns.

17. The film of Claim 16 in which the linear low density polyethylene has a density in the range of 0.91 to 0.92 g/cm³, a melt index in the range of 0.5 to 2 dg/min. and a thickness in the range of 15 to 150 microns.

18. The film of Claim 16 in which the polyethylene has been manufactured using a metallocene catalyst.

19. The film of any one of Claims 15-18 in which the polyethylene is in the form of a blend with at least one of low density high pressure polyethylene and ethylene/vinyl acetate copolymer.

20. A process for the manufacture of an article from a polyethylene film using film-converting apparatus, in which a continuous sheet of polyethylene film is fed

SUBSTITUTE SHEET to the apparatus, the improvement comprising using continuous sheet of polyethylene film that has been formed by the steps of extruding molten polyethylene through an annular die to form a tube of said molten polymer, expanding the molten tubular film so-formed and then cooling said film, subsequently heating the film under tension to a temperature to 50-100°C and then cooling to a temperature of less than 40°C, the amount of tension being such that the thickness of the film remains substantially unchanged.

SUBSTITUTE SHEET