WO2002051630A2

WO2002051630A2 - Multilayer barrier shrink films and process for their manufacture

Info

Publication number: WO2002051630A2
Application number: PCT/IL2001/000856
Authority: WO
Inventors: Israel Sarid; Arthur Bobovitch; Yiftach Nir
Original assignee: Syfan Saad (99) Ltd.
Priority date: 2000-12-25
Filing date: 2001-09-10
Publication date: 2002-07-04
Also published as: IL140543A0; WO2002051630A3; AU2001288035A1

Abstract

Multilayer polymeric structures, particularly heat-shrinkable structures, wherein the polymers of which the layers tat are formed are chosen from the following calsses: Class A: a sealing and humidity-protection layer essentially consisting of ethylene copolymers and ethylene/alpha-olefin copolymers and blends thereof; Class B: a tie layer essentially consisting of maleic anhydride grafted polyethylenes; and, in any order; Class C: barrier layers essentially consisting of ethylene vinyl alcohol copolymers (EVOH), or EVOH containing up to 30% of a polyamide; or Class D: polyamides, such that when Class C and Class D polymers are found on adjacent layers, they cannot be the same.

Description

MULTILAYER BARRIER SHRINK FILMS AND PROCESS FOR

THEIR MANUFACTURE

Field of the Invention

This invention relates to novel shrinkable, multilayer film structures,

comprising specific compatible contiguous layers, particularly useful for

flexible packaging, e.g. of food, and to a method for their manufacture.

Background of the Invention

Heat-shrinkable multilayer film structures, useful particularly for

packaging purposes, are known in the art, and various structures of such

fils and methods for their manufacture have been described.

US Patent No. 4,064,296 describes heat-shrinkable multilayer packaging

films having a layer of a hydrolyzed ethylene-vinyl acetate copolymer,

preferably formed by coextruding a layer of such copolymer between two

other polymeric layers, at least one of which is cross-linkable, and

thereafter orienting the multilayer structure.

US 4,643,943 describes an oriented multilayer film comprising five layers,

a cross-linked core layer, two cross-linked interior layers and two

cross-linked surface layers. US 5,114,795 discloses a multilayered gas barrier film which comprises at

least one layer of polymer having low permeability to moisture and a

polymer blend layer containing a gas barrier polymer, wherein the second

polymer is functionalized for compatibility with the gas barrier polymer so

as to allow the polymer blend layer to adhere to the first polymer layer. A

unique feature of these structures is said to be that the layer, providing

the gas barrier properties, is modified so as to provide good adhesion to a

polyolefin or other moisture barrier layer bonded thereto, without the

necessity of a separate, adhesive tie layer.

US 5,895,694 describes a chlorine-free multilayer film material which

comprises a gas barrier layer comprising a non-chlorine-containing

organic polymer, two tie layers, each contacting one side of said barrier

layer, an inner surface layer, and an outer surface layer.

US 5,993,922 relates to polymeric compositions capable of providing

enhanced cross-linking efficiency, to single and multilayer films having

the said composition contained within at least one layer of said film. It

further relates to a method of treating said film to provide enhanced

cross-linking within selected layers of the film, and to the resulting

cross-linked film product and articles made from said cross-linked film product. The multilayer film described in this reference has at least one

layer containing a polymer cross-linking enhancer (PCE) composition

comprising a copolymer having polymeric units derived from at least one

polyene monomer, at least one C2-C20 olefinic monomer, and further

comprising a polymeric mixture comprising at least one polymer having

polymeric units derived from said polyene monomer, and at least one

polymer having polymeric units derived from at least one C2-C20 olefinic

monomer, each of at least one layer formed with PCE composition being

cross-linked to a greater degree than at least one layer of said film and at

least one layer forming a major surface of the film is sealable.

US 6,051,292 discloses a multilayer packaging film comprising layers

having various degrees of cross-linking when subjected to electron beam

radiation, wherein the outer layer has a high degree of cross-linking and

the inner layer has a low degree of cross-linking.

The prior art has dealt with two difficulties. First of all, in order to

achieve satisfactory barrier properties, an internal EVOH has to be

introduced into the film. EVOH provides high oxygen barrier properties,

however, it is very cumbersome to stretch it in the double-bubble process

and to achieve high shrinkage. The solution is to either blend it with

Nylon, but then the barrier properties are inferior to pure EVOH, or to cross-link the primary extruded tube by electron-beam irradiation. The

later process requires expensive equipment. In addition the barrier

properties of film with EVOH is diminished by humidity.

It is therefore a purpose of this invention to provide a multilayer

structure, particularly a heat-shrinkable film, with high oxygen and

moisture barrier, polymeric structure in which the various layers are

chemically strongly bonded.

It is another purpose of this invention to provide a multilayer barrier

heat-shrinkable polymeric structure, which can be easily oriented for

example in a double-bubble process, without the need for irradiation and

while achieving satisfactory shrinkage properties.

It is still a further purpose of this invention to provide such a structure,

which allows to manufacture film with high barrier properties, equal to

those achieved in a five-layer structure containing pure EVOH as a

barrier layer.

It is yet another purpose of the invention to provide a film structure, in

which the barrier properties are not affected by the film exposure to high

humidity. It is still a further purpose of the present invention to provide a process for

the manufacture of such multilayer polymer films.

Other purposes and advantages of the invention will appear as the

description proceeds.

Summary of the Invention

The present invention provides a multilayer polymeric film, particularly a

heat-shrinkable and gas barrier film, which is made of specific compatible

layers provided between the contiguous layers thereof.

The invention further provides a process for producing the aforesaid

multilayer structures, which comprises co-extruding a plurality of the

several specific film layers. The different layers must be bonded to each

other to form a multilayer structure. The various layers have preferred

chemical natures, which can be classified in four different classes. Said

classes will be designated as Class A, B, C and D, for the sake of brevity.

Class A: a layer comprising polyethylene polymers and copolymers of

higher α-olefines or blend of different such copolymers, preferably — but

not limitatively - with densities from 0.87gr/cm³ to 0.95 gr/cm³, and melting temperatures preferably - but not limitatively - from 70°C to

135°C; or, alternatively, ethylene copolymers, such as EVA, EBA, EAA

and EMA, alternatively, ionomers, copolymers of ethylene and methacrylic

acid, thermally linked to metal ions (e.g., Surlyn Ex. DuPont).

Class B: a tie layer comprises of maleic anhydride grafted polyolefins, such

as ADMER™ NF 478 or NF578 (Ex Mitsui Chemicals), or BYNEL

( Ex Du Pont).

Class C: barrier layers essentially consisting of ethylene vinyl alcohol

copolymers (EVOH), or EVOH containing up to 30% of a polyamide, and

Class D: polyamides, such as N-MXD6 (a unique aliphatic polyamide resin

containing meta-xylene groups, and copolyamides, such as Nylon 6/12

(nylon 6 to nylon 12 ratio 1:1).

When C and D are adjacent, the two layers should be chemically different,

but should be compatible to adhere one to the other.

The invention, in particular preferred embodiments, may provide a three

or more layer structure. Indicating by A, B, C and D respectively any

polymers of the above classes A, B , C and D respectively, a five-layer structure may be symbolized by A/B/C/B/A and a seven-layer structure

may be symbolized as A/B/C/D/C/B/A.

Brief Description of the Drawings

In the drawings:

- Fig. 1 is a schematic illustration of the double-bubble process;

- Fig. 2 is a schematic illustration of a five-layer film; and

- Fig. 3 is a schematic illustration of a seven-layer film.

Detailed Description of Preferred Embodiments

The production of a polymer film begins with the melting of a polymer

composition, followed by its extrusion through a die. This extrudate is

further processed, e.g. by tenter drawing or double-bubble orientation, to

give the polymer film with its desired thickness and properties.

In Fig. 1 the double-bubble technique for making the film is schematically

illustrated. Polymer granules from a feeder (1) are melted in an extruder

(2). The melted polymer is continuously pushed through an annular die (6)

to produce a tubular tape that is inflated by air flowing around a mandrel

to form a first bubble (3) which is externally cooled by a quenching water

flow (4). After passing through a quench bath (5), the first bubble has

already solidified, and the tape is collapsed and transported by the use of nip rollers (9) to an oven or to hot water bath (7), which brings the tape to

its orientation temperature. Air pressure separates the inner surfaces of

the tube and produces the second bubble (8). The circumference of the

bubble is several-fold larger than that of the tube, and the tube is

oriented in the transverse direction (TD) by this stretching. In the

machine direction (MD) orientation is produced by stretching the tube

longitudinally, the rollers at the entrance to the stretching zone, before

the bubble, rotating at a peripheral speed which is several-fold lower than

the exit speed. After cooling, the second bubble is axially cut and rolled

onto spindles (10) for packaging and further processing (racking). An

ultraviolet oven (11) may be added at any location between the tape

solidification point and the second bubble. In this case, the formulation

includes a suitable concentration of an appropriate photo-initiator, and a

cross-linking agent if required.

By using a coextrusion die, more than one layer can be simultaneously

extruded, each layer with the same or different polymer composition to

form a multilayer film. In the case of the double-bubble technique, the die

will be coaxial annular die where the number of passageways is

determined by the number of layers desired in the produced film. When two contiguous layers are compatible, that it to say when they bind

sufficiently well to each other when coextruded in their molten states, the

film may be processed after extrusion in the same way as a single-layer

film.

However, incompatible layers must be bound together using an

intermediate adhesive (or tie) layer.

Fig 2. schematically illustrates a five-layer structure (A B/C/B/A), in which

the outer layers (A) are PE, adjacent to a tie layer (B) and the central

layer is Nylon, N-MXD6 (C).

The formation of a seven-layer structure (A B/C/D/C/B/A), is schematically

shown in Fig. 3, in which the outer layer is PE (A), adjacent to a tie layer

(B) and the three central layers are Nylon, N-MXD6 (C) and EVOH (D).

According to a preferred embodiment of the invention multilayer

polymeric structures are formed, preferably forming a heat shrinkable

film.

According to another preferred embodiment of the invention the film

layers are selected from the group consisting of ethylene copolymers, a tie layer comprising maleic anhydride grafted polyolefins, ethylene vinyl

alcohol copolymers and polyamides.

According to yet another preferred embodiment of the invention the film is

produced using the double-bubble technique.

According to a further preferred embodiment of the invention, the two

external layers comprise polyethylene copolymers.

According to a preferred embodiment of the invention the film production

process comprises coextruding several layers of the selected from the

above polymer groups.

All the above and other characteristics and advantages of the invention

will be further explained through the following illustrative and

non-limitative examples.

Example 1

A five-layer structure, A/B/C/B/A- was produced as follows:

The external "skin" layers A comprised 97% LLDPE, Dowlex 5056 ex

DOW and 3% antiblock masterbatch ( synthetic silica). When not otherwise indicated percentages given in respect of material contents are

by weight.

Layer B was a tie layer, comprised of 100% NF578 ex, Mitsui chemicals.

Layer C, the central layer, was Nylon N-MXD6 6007.

The above structure was formed by extrusion of the different polymers,

which lead to an A/B/C/B/A structure having relative thicknesses of

34/8/16/8/34 %, respectively. The primary tube thickness was 230 microns.

A five-layer extrusion line manufactured by Kuhne was used. The

extrusion temperature was that known for N-MXD6 extrusion, i.e., 250-

270°C. The barrel temperature was: PE and tie 220°C, MXD-6 250°C.

The die Temp was : 250°C.

A film of 25 microns thickness was obtained by orientation of the tube:

Relative thickness ( microns): PE-5.5, Tie-2, N- MXD6 - 10.

Stretching ratio (MDxTD): 3.3x2.7

OTR ( Oxygen Transmission Rate)- 12 cc/m²/day/atm/ @ 23°C,

90% RH( Relative Humidity)

Shrink % @ 95°C ( MD/TD) - 29/27 Example 2

A seven-layer structure was produced forming the following structure:

A/B/C/D/C/B/A, having relative layer thicknesses of 26/4/12/16/12/4/26 %,

respectively. The above structure was obtained by extrusion, using the

same conditions as described in Example 1.

The polymers used were:

Layer A- LLDPE 5056E ( DOW), density 0.92 g/cm³, Layer B- Tie, NF578

(Mitsui), density 0.91 g/cm³, Layer C- N-MXD6 (MGC) , density 1.22g/cm³,

Layer D- EVOH G156B (Kuraray), density 1.12 g/cm³-.

A 250 micron primary tube was oriented to produce a 25 micron thick film.

The relative thickness of each layer was :

6.5, B- 1, C- 3, D- 4 microns.

OTR- 17 cc/m²/day/atm/ @ 25°C and RH 0 and 90%

Shrink % - 90°C MD/TD - 28/30 Example 3

The production of other seven-layer structures is exemplified as follows:

The following structure was extruded A/B/C/D/C/B/A having relative %

thicknesses of 26/4/12/16/12/4/26, respectively.

The same procedure as described in Example 2 was employed, but the

EVOH layer was E-105B ex By EVAL Europe (density: 1.14 gr/cc). The

conditions were:

Barrel Temp. : PE and Tie: 220°C, N-MXD6 250°C, EVOH 230° C

The die Temp: 250°C

The thickness of the layers (microns) was: PE-5.5, Tie-2, MXD-6- 4,

EVOH-2. Total thickness 25 microns.

Stretching ratio (MDxTD) 3x3

OTR- 7.7 cc/m²/day/atm/ @ 23°C, 0 % RH.

Shrink % @ 95°C ( MD/TD) - 35/31

Example 4

A seven-layer film structure. The relative layer thickness % ratio was as

in Example 3. The same conditions as described for Example 3 were

employed, but a different Nylon was used, i.e., F28 Ex EMS. The same results for the film thickness and stretching ratio were obtained

as in Example 3.

OTR- 17.6 cc/m²/day/atm/ @ 23°C, 0% RH

Shrink % @ 95°C (MD/TD) - 31/29

Example 5

The same conditions as in Example 4 were used, except for the tie layer

which was NF498 Ex Mitsui Chemicals, and the Nylon layer which was

N-MXD6 6007.

OTR- 9.7 cc/m²/day/atm/ @ 23°C, 90% RH

Shrink % @ 95°C (MD/TD) - 31/34

Example 6

A seven-layer structure (A/B/C/D/C/B/A), was produced in the same

conditions as in Example 2. The relative layer thicknesses were

21/5/18/12/18/5/21%, respectively. Total thickness 25 microns.

Layer A - LLDPE ( 5056E Ex. DOW)

Layer B - Tie (NF 578 Ex. Mitsui Chemicals)

Layer C - Copolyamide Nylon 6/12, (CF6 Ex. EMS)

Layer D- EVOH (G156B Ex. Kuraray) OTR- 29 cc/m²/day/atm/ @ 25°C, 0% RH

65 cc/m²/day/atm/ @ 25°C, 90% RH

Shrink % @ 90°C (MD/TD)- 20/24

Example 7

A five-layer structure (A/B/C/B/A), was produced in the same conditions as

in Example 3. The relative thicknesses were 37/5/16/5/37%, respectively.

The total film thickness was 25 microns.

Layer A - LLDPE (5056E Ex. DOW)

Layer B - Tie ( NF 578 Ex. Mitsui Chemicals)

Layer C - EVOH and Copolyamide Nylon (E105B Ex. Kuraray +

10% CF6 Ex. EMS)

OTR - 32 cc/m²/day/atm/ @ 25°C, 0% RH

78 cc/m²/day/atm/ @ 25°C, 90% RH

Shrink % 90°C (MD/TD) - 20/24

While embodiments of the invention have been described by way of

illustration, it will be understood that the invention can be carried out by

persons skilled in the art with many modifications, variations and adaptations, without departing from its spirit or exceeding the scope of the

claims.

Claims

1. Multilayer polymeric structures, particularly heat-shrinkable

structures, wherein the polymers of which the layers are formed are

chosen from the group consisting of:

Class A: a sealing and humidity-protection layer essentially

consisting of ethylene copolymers and ethylene/alpha-olefin copolymers

and blends thereof;

Class B: a tie layer essentially consisting of maleic anhydride

grafted polyethylenes; and, in any order:

Class C: barrier layers essentially consisting of ethylene vinyl

alcohol copolymers (EVOH), or EVOH containing up to 30% of a

polyamide, and

Class D: polyamides;

wherein, when Class C and Class D polymers are found on adjacent layers

they cannot be the same.

2. Multilayer structures according to claim 1, comprising five or more

layer structures.

3. A structure according to claim 1, wherein the Class C and/or Class D

polymers comprise Nylon or MXD6.

4. Multilayer structure according to claim 2, comprising two outer

layers of class A polymer and an inner layer of class B polymer.

5. Multilayer structure according to claims 2 or 3, comprising two outer

layers of class A polymers, a central layer of class C polymers, and two

intermediate layers of class B polymers.

6. Multilayer structure according to claims 2 or 3, comprising two outer

layers of polymers of class A, two central layers of polymers of class C, and

one central layer of polymers of class D, and two intermediate layers of

polymers of class B.

7. Process for the manufacture of multilayer polymeric structure,

particularly shrinkable structure for packaging purposes, which comprises

co-extruding a plurality of layers of different polymer compounds selected

from class A, class B, class C and class D compounds.

8. Multilayer structure according to claim 1, wherein one, several or all

layers, excluding Nylon layers, are cross-linked.

9. Multilayer film wherein Nylon or N-MXD6, or their blend are used as

separate layer in contact with the EVOH layer.

10. Multilayer polymeric structure, substantially as described and

illustrated.

11. Process for making a polymeric structure substantially as described

and illustrated.