CA2172019A1

CA2172019A1 - Adhesive-free, sterilisation-resistant multilayer film for packaging applications

Info

Publication number: CA2172019A1
Application number: CA002172019A
Authority: CA
Inventors: Andreas S. Gasse; Rudi Klein
Original assignee: Wolff Walsrode AG
Current assignee: Dow Produktions und Vertriebs GmbH and Co oHG
Priority date: 1995-03-21
Filing date: 1996-03-18
Publication date: 1996-09-22
Also published as: ES2141408T3; DE59604147D1; EP0733471A3; EP0733471B1; DE19510136A1; ATE188647T1; DK0733471T3; EP0733471A2

Abstract

The present invention relates to multilayer, thermoformable and sterilisation-resistant film with an A / B / A / B / C
layer structure, wherein - A is a polyamide (PA), - B is a polymeric coupling agent (HV) with a cry-stallite melting point of greater than or equal to 121C and - C is a polyethylene (PE), consisting of a polyethylene homopolymer, of a polyethylene copolymer or of a polyolefin blend prepared from polyethylene, wherein at least one component has a crystallite melting point of greater than or equal to 121C.

Description

Adhesive-free, sterilisation-resistant multilayer film for ~ackaging applications The present invention relates to multilayer films for packaging applications, preferably in the foodstuffs sector. The films are characterised in that they are thermoformable and heat sealable and are distinguished in that they may be produced without adhesives and may be used for applications requiring sterilisation.
Various processes are known for the sterilisation-of films or the pack contents in packaging films. A distinction is drawn between gas sterilisation ~conventionally with ethylene oxide), gamma ray sterilisation and steam sterilisation. These processes are conventionally performed - in autoclaves. Sterilisation is performed at temperatures of above 100C, wherein the material to be sterilised is exposed to this temperature for 30 minutes. The most frequently used temperature is 121C in a counterpressure autoclave. The prior art relating to "films for steam sterilisation" will now be described.

Films which are heat sealable must have at least two layers, wherein one layer acts as the support and one as the sealing layer. While, if the support is aluminium, paper or an oriented film, it may indeed be sterilisation-resistant, it cannot be thermoformable.
Examples of such materials are described in EP 474 587, JP 56082 247, JP 05051867, DE 3623 568 and EP 21 578.
Suitable support materials which are in principle thermoformable are unoriented polymer layers such as polycarbonate (NL 7214460), polystyrene (BE 877 054), polypropylene (CA 1150 461, JP 54032 584), polybutylene terephthalate (JP 06226930) etc..

` ~172(~19 _.

A large proportion of sterilisation-resistant films may be produced using laminating adhesives (DE 3545 768, EP 474 587, JP 03254939, EP 26 954, EP 21 578, DT 2829 871, BE 877 054, JP 51124 180, NL 7214460, EP 50 255, EP 225 164). The disadvantages of such films in comparison with adhesive-free films are greater stiffness and poorer thermoformability.

Polypropylene, polypropylene copolymers or polymer blends containing polypropylene-based polymers are preferably used as the sealing layer material (JP 03254 939, JP 03-126 557, EP 0629227, JP 63270 140, EP 288 972, DE 3545 768, DE 3431 364, DE 3315 652, EP 21548, JP 54032 584, JP 51124 180, NL 7214460). While such materials do indeed ensure sterilisability due to their elevated crystallite melting points, for the same reason they may be heat sealed only at very high sealing temperatures, which is generally undesirable.

Sterilisation resistance to 121C was explicitly mentioned only in the patents EP 0629227, EP 50 255, EP 21578 and NL 7214460. In all cases the sealing layers are polypropylene or polypropylene copolymers. Polyethylenes are also mentioned, but lower sterilisation temperatures are stated (for example JP 56082247: ~ 100C).

To summarise, it may be stated that it is predominantly laminated composite films containing polypropylene sealing layers which are proposed for sterilisation applications.
These have the disadvantage that they have poor thermoformability and that very high sealing temperatures are required for sealing.

The object thus arose of providing a film based on polyamide and polyethylene, preferably for packaging foodstuffs, which is sterilisable (121C, 30 minutes) and 217201~

simultaneously has very good thermoformability. The film should be adhesive-free and should have good interlayer adhesion values both before and after sterilisation. It must be possible to seal such a film at the lowest possible sealing temperature.

Surprisingly, this object could be achieved by a multilayer, thermoformable and sterilisation-resistant film with an A / B / A / B / C layer structure, which film is characterised in that A is a polyamide (PA), B is a polymeric coupling agent (HV) with a crystallite melting point of greater than or equal to 121C and C is a polyethylene (PE), consisting of a polyethylene homopolymer, or of a polyethylene copolymer or of a polyolefin blend prepared from polyethylenes, wherein at least one component has a crystallite melting point of greater than or equal to 121C.

The polyethylene (PE) is here preferably a linear low density polyethylene (LLDPE) or a polyolefin blend prepared from linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). The linear low density polyethylene (LLDPE) is itself a copolymer of ethylene and a C3 to C12 a-olefin with a melt flow index (MFI) of 0.1 to 20 g/min (190C; 2.16 kg), a density of 0.915 to 0.955 g/cm3 and a crystallite melting point of greater than or equal to 121C, the low density polyethylene (LDPE) iS a polyethylene with a melt flow index (MFI) of o.1 to 20 g/min (190C; 2.16 kg) and a density of 0. 915 to 0.935 g/cm3. The ratio of LLDPE to LDPE should be 99:1 to 30:70, preferably 85:15 to 50:50.

The polyamide layer consists of the aliphatic polyamides PA-6, PA-11, PA-l 2, PA-66, PA-6,66, PA-6,8, PA-6,9, PA-6,10, PA-6,11, PA-6, 12, of a copolymer prepared from the monomer units contained therein, of an aromatic or W W 5428 - 4 ~
partially aromatic [polyamide] or of a mixture of the stated polyamides. A mixture of PA-6 and an aromatic polyamide constitutes an important embodiment which is characterised by a ratio of PA-6 to aromatic polyamide of 90:10 to 80:20.

The polymeric coupling agent is an anhydride-modified polyethylene, an acid copolymer of ethylene, an acid-modified ethylene vinyl acetate, an acid-modified ethylene (meth)acrylate, anhydride-modified ethylene (meth)acrylate, an anhydride-modified ethylene vinyl acetate, an acid/acrylate-modified ethylene vinyl acetate or a polymer blend containing at least one of the stated coupling agents. The coupling agent layer (HV) preferably consists of an anhydride-modified polyethylene. Of these, maleic anhydride grafted linear low density polyethylenes (LLDPE) are in turn preferred.

The total thickness of the film is 20 to 500 ~m, preferably 50 to 350 ~m. The layer thicknesses are characterised in that the sum of the thicknesses of the A layers constitutes 20 to 70~ of total film thickness, wherein the ratio of the sum of the thicknesses of the B layer to the sum of the thicknesses of the A layers is 0.1 to 1 and the ratio of the thickness of the C layer to the sum of the thicknesses of the A layers is 0.2 to 3.

The film may in principle additionally contain conventional additives and auxiliary substances. For example, in order to modify the slip properties of the film, at least one layer may contain lubricants and/or antiblocking agents.
Antiblocking agents should here preferably be incorporated in the outer layers of the film, lubricants in the internal layers too.

217201q The film is suitable for printing. At least one layer may be coloured or printed.

The film is particularly suitable for packaging applications, particularly for packaging foodstuffs. The film may here equally well be used both for hot fill products and for heating the contents in the film up to sterilisation temperatures. The film is suitable for packaging meat and sausage products, milk products, fish and smoked products, ready prepared dishes, bread and bakery products and medical or technical devices.-Surprisingly, it is possible by means of the composition ofthe film according to the invention to satisfy the particular requirements of sterilisation resistance combined with good thermoformability. It was not to be expected from the prior art that a sterilisation-resistant, heat sealable multilayer film with good thermoformability could be produced without adhesives.
It was also surprising that, firstly, the interlayer adhesion and sealing strength of films produced according to the invention were higher after sterilisation than those of composite films produced by adhesive lamination and, secondly, were distinctly higher than before sterilisation, which is not possible with prior art adhesive laminated films.

The claimed film is conveniently produced either in a single stage by blown or flat film coextrusion or in multiple stages by coextrusion with extrusion coating. It is cost effective to coextrude a 4-layer support with the structure PA / HV / PA / HV which is then coated in line on the HV side with the sealing layer polymer.

Extrusion coating of LLDPE is in principle characterised bysevere "neck in" due to very narrow molecular weight distributions (MW/Mn = 3 to 3.5i Mw = weight average, Mn =
number average). This may be offset by the preparation according the invention of a polymer blend from LLDPE and LDPE, wherein in such cases the grades selected for the coating should preferably have a wide molecular weight distribution (MW/Mn = 7 to 8).

The extrusion of LLDPE is characterised by comparatively high pressures. This effect is counteracted by the use of relatively high extrusion temperatures. These temperatures should preferably be 260 to 280C during coating.

Conventional prior art machine designs are used, wherein in the case of blown film coextrusion the process is characterised in that the melt is shaped into a tube which is inflated, cooled and, at the other now cool end, collapsed and held closed by pinch rolls and the film is then wound. In the case of flat film coextrusion, so called chill roll units are used having the particular feature of large chill rolls which take off the molten film as it leaves the die.

One feature used to evaluate the invention was thermoformability. In order to determine thermoformability, the previously produced film samples were tested in modern automatic thermoforming machines as are used in the packaging industry (for example Tiromat, Multivac). To this end, the film webs clamped in the machine were heated in sections by a hot plate. Heating may be performed to this end either from the sealing side or also from the opposite side to the sealing side. The films preheated at hot plate temperatures of 100 to 110C were then thermoformed into a tray of edge dimensions 185 x 115 mm. The depth of the tray was varied between 20 and 70 mm.

W W 5428 - 7 ~
The thermoforming result was marked as follows:

1. The thermoforming result is unsatisfactory, an unacceptable number of attempts to form a tray results in tears in the film:
Rating "-"

2. The thermoforming result is good, there is no tearing and shaping of the tray is satisfactory:
Rating "+"

3. The thermoforming result is very good, there is no tearing and shaping of the tray is very good:
Rating "++"
A number of the trays produced were filled with water and sealed with a suitable lidding film at the sealing station of the above-stated automatic thermoforming machines.
Sealing temperatures were 145C. The packages produced in this manner were sterilised at 121C for 30 minutes in a counterpressure autoclave. Interlayer adhesion was then measured at the corners of the tray and compared with the interlayer adhesion at the corners of the tray before sterilisation.
Interlayer adhesion (VH) is tested to DIN 53 357, method B
and the test is intended to provide information as to how strongly the individual films of a composite film adhere together at the weakest point. Interlayer adhesion is taken to be the maximum force necessary in order to delaminate the film to be tested from a test strip of 15 mm in width for a length of 10 mm at a separation angle of 90 degrees.
Interlayer adhesion is stated in N relative to the width of the strip (N/15 mm).

21720~9 Edge-sealed pouches of dimensions 100 x 150 mm filled with water were also produced from the sample films and also sterilised at 121C for 30 minutes in a counterpressure autoclave. The sterilisation result was assessed in the categories "ok" (the edge-sealed pouch withstood sterilisation without damage) and "defective" (the edge-sealed pouch is defective after sterilisation, defects occur in the sealing layer in the seam area).

Comparative examples The following examples are intended to illustrate the subject matter of the invention.

A. Example 1:

Five-layer, adhesive-free film with the structure 2035 / 10 / 35 / 10 / 55 ~m The four-layer PA / HV / PA / HV support of the film was coextruded as a flat film and the LLDPE layer was then extrusion coated in line. The PA used was polyamide 6 of a density of 1140 kg/m3 with a crystallite melting point of 219C and a relative solution viscosity of 3.8 (PA
concentration 1~, temperature 25C, measured in m-cresol), the HV used was a maleic anhydride grafted linear low density polyethylene (LLDPE) of a density of 922 kg/m3 with a crystallite melting point of 125C and a melt flow index (MFI 190/2.16) of 3.1 g/10 min and the PE used was a copolymer of ethylene and octene of a density of 935 kg/m3 with a crystallite melting point of 126C and a melt flow index (MFI 190/2.16) of 4.4 g/10 min.

B. Example 2:

Five-layer, adhesive-free film with the structure 35 / 10 / 35 / 10 / 55 ~m The film was produced in the same manner as described in example 1. The same polyamide (PA) was used as in example 1, the HV used was a maleic anhydride grafted linear low density polyethylene (LLDPE) of a density of 910 kg/m3 with a crystallite melting point of 125C and a melt flow index (MFI 190/2.16) of 4.0 g/10 min and the PE used was a polymer blend prepared from a copolymer of ethylene and octene (LLDPE) of a density of 935 kg/m3 with a crystallite - melting point of 126C and a melt flow index (MFI 190/2.16) of 4.4 g/10 min and a low density polyethylene LDPE of a density of 923 kg/m3 with a crystallite melting point of 111C and a melt flow index (MFI 190/2.16) of 3.8 g/10 min.
The ratio of LLDPE:LDPE was 80:20.

C. Example 3:

Five-layer, adhesive-free film with the structure 35 / 10 / 35 / 10 / 55 ~m The film was produced as described in example 1. The same polyamide (PA) was used as in example 1, the same coupling agent (HV) as in example 2 and the PE used was a polymer blend prepared from the same materials as in example 2. The ratio of LLDPE:LDPE was 50:50.

D. Comparative example 1:

Five-layer, adhesive-free film with the structure 26 / 10 / 26 / 10 / 98 ~m The film was produced as described in example 1. The same polyamide (PA) was used as in example 1 and the same coupling agent (HV) as in example 2. The PE used was an LDPE of a density of 934 kg/m3 with a crystallite melting point of 104C and a melt flow index (MFI 190/2.16) of 3.0 g/10 min. The LLDPE contained 700 ppm of erucamide as lubricant.

- E. Comparative example 2:

Six-layer, adhesive-free film with the structure 25 / 9 / 25 / 9 / 76 / 26 ~m The film was produced in a similar manner to that described in example 1. In this case, the sealing layer was coextrusion coated onto the four-layer support such that the sealing layer consists of two PE layers. The same polyamide (PA) was used as in example 1 and the same coupling agent (HV) as in example 2. PE 5 is an LDPE of a density of 915 kg/m3 with a crystallite melting point of 104C and a melt flow index (MFI 190/2.16) of 8.0 g/10 min and PE 6 is an ethylene/vinyl acetate copolymer ( 5~ VAc) of a density of 925 kg/m3 with a crystallite melting point of 101C and a melt flow index (MFI 190/2.16) of 7.5 g/10 min.

F. Comparative example 3:

Two-layer, adhesive-laminated film with the structure 70 / - / 75 ~m The PA layer and PE layer were produced separately as flat or blown film and subsequently laminated using a two-component polyurethane adhesive system (K). The PA is the same as used in example 1 and the PE is an LLDPE of a density of 935 kg/m3, a crystallite melting point of 125C
and a melt flow index (MFI 190/2.16) o 0. 5 g/10 min.

G. Comparative example 4:

Two-layer, adhesive-laminated film with the structure 70 / - / 75 ~m The PA layer and PE layer were produced separately as flat or blown film and subsequently laminated using a two-component polyurethane adhesive system (K). The PA is the same as described in example 1 and the PE used was a polymer blend prepared from a copolymer of ethylene and butene (LLDPE) of a density of 921 kg/m3 with a crystallite melting point of 117C and a melt flow index (MFI 190/2.16) of 0.8 g/10 min and a low density polyethylene LDPE of a density of 923 kg/m3 with a crystallite melting point of 111C and a melt flow index (MFI 190/2.16) of 2.0 g/10 min.
The ratio of LLDPE:LDPE was 50:50.

Table 1 shows the thermoformability and sterilisation resistance ratings of the stated films A, B, C, D, E, F and G. Thermoformability was determined by thermoforming trays 2~72019 and sterilisation resistance was determined by producing edge-sealed pouches and sterilising them as described above.

Table 1:
Evaluation of film structure, sealing layer, thermoformability and sterilised edge-sealed pouches for examples A to G.

Film structureSealing layer Thermo-Edge-sealed pouch formability Adhesive-free According to After sterilisation the invention (30 min, 1 21 C) A Example 1 yes yes + + ok B Example 2 yes yes + + ok C Example 3 yes yes + + ok D Comparison 1 yes no + + defective E Comparison 2 yes no + + defective F Comparison 3 no yes + ok G Comparison 4 no no + defective It may be seen that the laminated film with a sealing layer not according to the invention (G) neither has good thermoformability nor is it sterilisation-resistant; while the adhesive-free films (D, E) do indeed have very good thermoformability, they are not sterilisation-resistant;
while the laminated film with a sealing layer according to the invention (F) is indeed sterilisation-resistant, it does not have particularly good thermoformability; and sterilisation resistance accompanied by very good thermoformability is achievable only with films produced according to the invention (A, B, C).
Table 2 describes interlayer adhesion before sterilisation [VH(before)] and after sterilisation [VH(after~] and the change [VH(after) - VH(before~]~ The sterilisation-resistant samples A, B, C and F were investigated. Production of the trays (drawing depth 60 mm), sterilisation (30 minutes, 121C) and measurement of interlayer adhesion were performed as described above.

Table 2:
Interlayer adhesion before sterilisation [VH(before)] and after sterilisation [VH(after~] and change [VH(after) - VH(before)], measured at the tray corners of thermoformed deep packages 60 mm in depth. Sterilisation: 30 minutes, 121C.

VH~before~VH(after~VH(a,te,~ ~ VH(before~
[N/15 mm][N/15 mml[N/15 mml AExample 1 2.6 4.8 2.2 BExample 2 3.6 7.2 3.6 CExample 3 2.3 5.8 3.5 DComparison 1not measured not measured not measured EComparison 2not measured not measured not measured FComparison 3 1.3 1.7 0.4 GComparison 4not measured not measured not measured It may clearly be seen that, even before sterilisation, interlayer adhesion at the tray corner of comparison film F
is too low for standard applications (requirement: VH > 2 N/15 mm) and is distinctly lower than for films A, B and C
produced according to the invention. It may moreover be noted that virtually no change in interlayer adhesion occurs after sterilisation for film F, whereas interlayer adhesion values are approximately doubled after sterilisation for films A, B and C.

Claims

1. A multilayer, thermoformable and sterilisation-resistant film with an A / B / A / B / C layer structure, wherein A is a polyamide (PA), B is a polymeric coupling agent (HV) with a crystallite melting point of greater than or equal to 121°C
and C is a polyethylene (PE), comprising a polyethylene homopolymer, a polyethylene copolymer or a polyolefin blend prepared from polyethylene, wherein at least one component has a crystallite melting point of greater than or equal to 121°C.

2. A film according to claim 1, wherein the polyethylene (PE) is a linear low density polyethylene (LLDPE) or a poly-olefin blend prepared from a linear low density polyethylene (LLDPE) and a low density polyethylene (LDPE).

3. A film according to claim 2, wherein the linear low density polyethylene (LLDPE) iS a copolymer of ethylene and a C3 to C12 .alpha.-olefin with a melt flow index (MFI) of 0.1 to 20 g/min (190°C; 2.16 kg), a density of 0.915 to 0.955 g/cm3 and a crystallite melting point of greater than or equal to 121°C.

4. A film according to claim 2, wherein the low density polyethylene (LDPE) has a melt flow index (MFI) of 0.1 to 20 g/min (190°C; 2.16 kg) and a density of 0.915 to 0.935 g/cm3.

5. A film according to claim 2, 3 or 4, wherein the ratio of LLDPE to LDPE is 99:1 to 30:70.

6. A film according to claim 1, wherein the polyamide comprises an aliphatic polyamide PA-6, PA-11, PA-12, PA-66, PA-6,66, PA-6,8, PA-6,9, PA-6,10, PA-6,11 or PA-6,12, a copolymer prepared from the monomer units contained therein, an aromatic or partially aromatic [polyamide] or a mixture of the said polyamides.

7. A film according to claim 2, 3 or 4, wherein the polyamide comprises an aliphatic polyamide PA-6, PA-11, PA-12, PA-66, PA-6,66, PA-6,8, PA-6,9, PA-6,10, PA-6,11 or PA-6,12, a copolymer prepared from the monomer units contained therein, an aromatic or partially aromatic [polyamide] or a mixture of the said polyamides.

8. A film according to any one of claims 1 to 4 or 6, wherein the polymeric coupling agent is an anhydride-modified polyethylene, an acid copolymer of ethylene, an acid-modified ethylene vinyl acetate, an acid-modified ethylene (meth)-acrylate, anhydride-modified ethylene (meth)acrylate, an anhydride-modified ethylene vinyl acetate, an acid/acrylate-modified ethylene vinyl acetate or a polymer blend containing at least one of the coupling agents stated in this claim.

9. A film according to claim 7, wherein the polymeric coupling agent is an anhydride-modified polyethylene, an acid copolymer of ethylene, an acid-modified ethylene vinyl acetate, an acid-modified ethylene (meth)acrylate, anhydride-modified ethylene (meth)acrylate, an anhydride-modified ethylene vinyl acetate, an acid/acrylate-modified ethylene vinyl acetate or a polymer blend containing at least one of the coupling agents stated in this claim.

10. A film according to any one of claims 1 to 4, 6 or 9, wherein the coupling agent is an anhydride-modified polyethylene.

11. A film according to any one of claims 1 to 4, 6 or 9, wherein the coupling agent (HV) is a maleic anhydride grafted linear low density polyethylene (LLDPE).

12. A film according to any one of claims 1 to 4, 6 or 9, wherein the total thickness of the film is 20 to 500 µm.

13. A film according to any one of claims 1 to 4, 6 or 9, wherein the sum of the thicknesses of the A layers constitutes 20 to 70% of total film thickness, wherein the ratio of the sum of the thicknesses of the B layer to the sum of the thicknesses of the A layers is 0.1 to 1 and the ratio of the thickness of the C layer to the sum of the thicknesses of the A layers is 0.2 to 3.

14. A process for preparing a film according to any one of claims 1 to 4, 6 or 9, wherein the film is prepared in a single stage by coextrusion as a blown or flat film or in a multistage process comprising extrusion and extrusion coating.

15. A process for preparing a film according to claim 14, wherein the sequence of layers A / B / A / B is coextruded and layer C is then coated in line.

16. A use of a film according to any one of claims 1 to 4, 6 or 9, as a packaging film.

17. A use of a film according to claim 16, as a packaging film for foodstuffs.