NZ227361A - Multi-layer plastics packaging film having at least one ethylene vinyl alcohol copolymer layer and at least one polyolefin-based layer - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £27361 NEW ZEALAND PATENTS ACT 1953 No.: 227361 Date: 16 December 1988 227 3 61 '•w' COMPLETE SPECIFICATION "Improvements in or Relating to Packaging Films" WE TRANSPAK INDUSTRIES LIMITED, a company duly incorporated under the laws of New Zealand of 23-25 Porana Road, Takapuna, Auckland, New Zealand hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: 227361 This invention relates to plastics packaging films.

It is an object of the invention to provide a plastics packaging film which will at least provide the public with a useful choice.

Accordingly, in one aspect, the invention consists in a multi-layer mono-axially oriented plastics packaging film containing at least one layer of ethylene vinyl alcohol copolymer and at least one other layer containing a polyolefin based plastics material wherein the ethylene vinyl alcohol copolymer is oriented to at least four times draw ratio in a single direction.

In a further aspect the invention consists in a method of forming a multi-layer plastics packaging film by combining at least one layer of ethylene vinyl alcohol copolymer and at least one other layer of a polyolefin based plastics material and then subjecting the multi-layer film to stretching in the longitudinal direction to at least four times draw ratio while restraining the film against substantial contraction in the transverse direction- In still a further aspect the invention consists in a method of forming a multi-layer plastics packaging film by combining at least one layer of ethylene vinyl alcohol copyolymer and at least one other layer of a polyolefin based plastics material; subjecting the multi-layer film to stretching in the longitudinal direction to substantially three times draw ratio while restraining the film against substantial contraction in the transverse direction; laminating the oriented film with at least one substantially non-oriented film containing a polyolefin based plastics material; and then subjecting the laminate to orientation in the same direction as the oriented film has already been oriented.

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

Various preferred forms of the invention will now be described.

This invention provides a multilayer plastics packaging film which comprises layers of ethylene vinyl alcohol copolymer (EVOH) and a polyolefin based plastics material, preferably linear low density polyethylene (LLDPE). The layers are preferably co-extruded or laminated into a tube or sheet and are held in combination by an adhesive promoting polymer. The resulting tube or sheet is subsequently subjected to mono-axial orientation in the longitudinal direction to a draw ratio of at least four times and, in ideal circumstances,up to eight times. Stretching typically occurs at a temperature lying in the range 80-130°C.

Alternatively, the extruded film containing EVOH is uniaxially oriented to a draw ratio of about three, laminated to further film layers and the resulting laminate then oriented in 227 3 6 1 the same direction to a draw ratio of about two.

More preferably the multilayer film contains at least one barrier layer A consisting of an ethylene vinyl alcohol copolymer containing 20 to 60 mole-% ethylene, and which is at least 90 mole-% saponified, or containing the EVOH in a mixture. The melt flow index of the EVOH is in the range of 1 to 20 g/10 min. (measured at 210°C and 2160g), and its density is in the range of 1.10 to 1.25 g/cm^. Preferably, the EVOH contains 29 to 48 mole-% ethylene, with a melt flow index range of 3 to 16 g/10 min. (measured at 210°C and 2160g), and density in the range of 1.12 to 1.21 q/cm.3.

Immediately adjacent to the surface of the barrier layer A is an adhesion promoting layer B, preferably consisting of a modified polyolefin or containing a modified polyolefin in a mixture. The adhesive modified polyolefin is preferably a homopolymer or copolymer of polyethylene or polypropylene grafted with a carboxylic acid or its anhydride. A preferred composition contains a maleic anhydride grafted polymer of polyethylene or polypropylene, with a density in the range of 0.88 to 0.95 g/cm^, and melt index of 0.1 to 10 g/10 min. (at 190°C, 2160g).

Immediately adjacent to layer B is a polyolefin based layer C, which is preferably heat sealable, and which preferably consists of a linear low density polyethylene (LLDPE) or a linear low density polyethylene in a mixture. The LLDPE is preferably a copolymer containing ethylene and one or more of butene, hexene, or octene, with a melt index range of 0.1 to 30 227361 g/10 min. (at 190°C, 2160g) and density in the range of 0.88 to 0.96 g/cm^. One such preferable composition contains a copolymer of ethylene and octene, with a melt index of 0.5 to 2 g/10 min. (at 190°C, 2160g) and density of 0.920 to 0.940 g/cm^.

The multilayer film can be formed by simultaneously melting and co-extruding the separate material layers through a flat or circular film forming die, then cooling and solidifying the melt to form a multilayer film of thickness from 20 to 1000 microns. Such processes are known as flat film co-extrusion or blown film co-extrusion. Alternatively, one or more of the film layers may be formed separately, and then combined in a film lamination process such as extrusion lamination or co-extrusion lamination. The solidified multilayer film, as manufactured from any of the above processes, may then be heated to a temperature of 80 to 130°C and stretched in the longitudinal direction to a draw ratio of 3 to 10 times. It may then be heat set, and cooled to retain the mono-axial orientation.

Alternatively a multilayer film containing the EVOH layer may be mono-axially oriented as previously described, to a draw ratio of about three, laminated to one or more layers of a polyolefin, preferably LLDPE or LDPE, containing film, and the resulting laminate then further oriented, in the longitudinal direction, to a draw ratio of about 2 to 2.5.

There are many different possible configurations of film structures which can be manufactured from the above process. The film produced from the first co-extrusion or lamination step may be in tubular or flat sheet form, and may contain anywhere from 3 to 10 layers. A preferred structure is a five layer film, with a configuration of C-B-A-B-C, typical layer thickness ratios being 8-1-2-1-8. Other preferred configurations are A-B-X-C, C-B-A-C, or A-B-C, where layer X represents a polyolefin based layer which is different from C.

The film is preferably subjected to mono-axial orientation on a form of mono-axial stretching machine which supports the film on spaced rollers and achieves the orientation of the film between particular leading and trailing rollers the leading roller rotating at a higher speed than the trailing roller. In other words, the orientation is achieved due to the variation in speed between adjacent rollers. We believe that advantages are derived by effecting the orientation over a very short length of film and we have obtained extremely good results by ensuring that the orientation is effected by rollers, spaced at approximately 8 mm apart, and preferably not more than 10 mm apart, and having different peripheral speeds.

The roller surfaces are heated to a surface temperature in the order of 80-130°C. Further, the frictional interaction between the film and the roller surfaces prevents substantial width reduction of the film as it is oriented. Some width redirection does occur although it is minimal. The upper limit of width reduction is in the order of 10 per cent and typically we have experienced width reduction of the order of 5-7 per cent. 227 3 6 1 Testing of the oriented film in the transverse direction would indicate that while the film is supported in a transverse direction during orientation in the machine direction, no discernible orientation of the film occurs in the transverse direction.

The mono-axial stretching or orientation step may be carried out in line with the initial forming operation, or as a separate out of line process. In either case, film can be fed to the orientation stage in continuous form as either a single sheet, a collapsed tube, or as multiple separate sheets and/or tubes. By choosing different polymers and processing conditions, separate sheets and/or tubes may or may not be thermally laminated while being mono-axially oriented. The processing configuration may also be adjusted such that separate sheets are oriented to different draw ratios with a single processing step. In a preferred configuration, a single sheet or collapsed tube of film with structure C-B-A-B-C or C-B-A-B-X is passed through a set of rollers heated at 90 to 130°C, and stretched at a draw ratio of 5 to 7 times. The resulting uniaxially oriented film is cooled to ambient temperature, and collected in roll form. EXAMPLES Example I: A five layer film (I-a), of configuration C-B-A-B-C, was formed by blown film co-extrusion to an average thickness of 200 microns. Its outside layers (C) consisted of an octene copolymer linear low density polyethylene with a density of 0.930 g/cm^ and melt index of 1.00 g/10 min. (190°C, 2160g)- The adhesive layers (B) consisted of a polymer based on maleic anhydride grafted polyethylene of density 0.91 g/cm^ and melt index of 1.80 g/lOmin. (190°C, 2160g). Layer (A) consisted of an ethylene vinyl alcohol copolymer, with an ethylene content of 38 mole-%, density of 1.17 g/cm^ and melt flow index of 8 g/lOmin. (210°C, 2160g). The average layer thickness ratios were 8-1-2-1-8. The film (I-a) was fed continuously, in single sheet form, through a series of rollers heated to temperatures of 90 to 130°C. The film was stretched approximately 6.5 times in the longitudinal direction, cooled to ambient temperature, and the resulting film (I-b) collected in roll form. The average thickness of (I-b) was 31 microns, and (I-b) was observed to be much improved in optical, stiffness, and tensile properties over (I-a). Table 1 lists some comparitive physical properties of (I-a) and (I-b). From this table, it is seen that the 6.5 times uniaxially oriented (I-b) is significantly improved in surface gloss, tensile strength in the machine direction, stiffness, impermeability to the passage of oxygen. Further, the oriented film showed surprisingly little sensitivity to the effects of moisture when compared to the unorientated film. It should also be noted that oxygen transmission is also used as a measurement of film permeability to other gases, such as carbon dioxide and nitrogen.

Example II: The co-extruded film (I-a) from Example I was fed continously in the form of a collapsed tube through the heated rollers at 90 to 130°C. The inside surface of the tube became heat sealed to 227361 itself, making a thermal laminate. Samples of this laminate were manufactured with stretch ratios in the longitudinal direction of 1:1 to 7:1. Physical property measurements for the samples, labelled (Il-a) to (Il-d), are listed in Table 2. The data again show the beneficial effects of uniaxial orientation on optical, tensile, stiffness, and barrier properties.

Example III: The flex crack resistance (or pinhole resistance) of uniaxially oriented samples from Examples I and II was measured, and compared to that of unoriented samples. In Table 3, sample Ill-a is an unoriented five layer co-extrusion of 90 microns total thickness. Its structure is very similar to the co-extruded base film from Examples I and II, except that the surface layers contain a mixture of 75% octene LLDPE (0.920 g/cm3, 1.00 g/10 min at 190°C, 2160g), and 25% LDPE (0.920 g/cm3, 1.00 g/10 min at 190°C, 2l60g); the melt index and density of the adhesive layers are 3.50 g/10 min (190°C, 2160g.) and 0.91 g/cc; the EVOH barrier layer contains the same 38 mole-% ethylene and 1.17 density, with the melt flow index now 3.2 g/10 min at 190°C, 2160g. Sample Ill-b is the 6.5 times oriented film described in Example 1, and Sample III-c is the 7 times oriented film described in Example 2. The film samples were conditioned to equilibrium (at 23°C, 75% R.H.), Gelbo flex tested to known degrees, and measured for oxygen transmission rate after flexing. The results show the oxygen barrier of the oriented films to be more resistent to flexural failure than the unoriented film. 227 36 1 Example IV: Example II was repeated, this time using an EVOH barrier layer with 42 mole-% ethylene, density of 1.12 g/cm^, and melt flow index of 15 g/10 min at 210°C, 2160g. Table IV lists the physical property measurements of film samples with draw ratios 1:1 to 7:1 (Samples IV-a to IV-d). Tensile, stiffness, and optical properties of these films were found to be very similar to those measured in Example II. However, the higher ethylene content of the EVOH barrier layer used in this example results in higher oxygen transmission rates. These permeability rates are decreased with orientation, but not to the same extent as the lower ethylene content EVOH used in the previous examples.

We have found that films manufactured by the method described above are heat sealable, and show improved gas impermeability, flex crack resistance, stiffness, tensile strength, optical qualities, and reduced water sensitivity of the EVOH. The films produced by the method may be used to package both food and non-food products requiring moisture and oxygen barrier packaging.

Flexible co-extruded films containing EVOH have been produced in the past and are well known for their advantageous gas barrier properties. However, these films generally suffer from poor flex crack (or pinholing) resistance, and susceptibility of their gas barrier to moisture. It is known that the molecular orientation which occurs with film stretching improves the gas barrier and reduces the water sensitivity of EVOH films. It is also known that uniaxial orientation or 227 361 stretching of the linear low density polyethylene film improves its stiffness, tensile, and optical properties. However, conventional wisdom would suggest that EVOH films cannot be oriented to draw ratios of greater than about three. We have found that by combining EVOH layers with suitable carrier layers we can orient EVOH to a substantially greater degree and, in doing so, we can achieve physical properties which are quite surprising as is indicated in the attached tables. 2 2 7 3 6 1 TABLE 1 EXAMPLE I - FILM PROPERTY COMPARISON PROPERTY FILM SAMPLE NUMBER I-a I-b THICKNESS (microns) 200 31 ORIENTATION DRAW RATIO 1:1 6.5:1 SURFACE GLOSS (%) 64 153 ULTIMATE TENSILE STRENGTH (MPa) Machine Direction 18 258 Tranverse Direction 21 18 STIFFNESS, 2% SECANT (MPa) Machine Direction 342 659 Tranverse Direction 272 420 OXYGEN TRANSMISSIONS:1 (cc 25um/(m2 24hr atm)) AT 25° C, 0% R.H. 7.6 2.2 AT 25° C, 75% R.H. 9.1 3.2 AT 25° C, 93% R.H. 23 4.1 1 Oxygen transmissions are reported per 25 um total thickness of film. Film samples were conditioned at the indicated temperature and humidity conditions for at least 2 weeks before measuring. Measurements were performed with 100% oxygen atmosphere, with a Mocon 100 Oxtran device. 2 2 7 3 6 1 EXAMPLE II - FILM PROPERTY COMPARISON PROPERTY FILM SAMPLE NUMBER Il-a ii-b II-c Il-d ORIENTATION DRAW RATIO 1:1 4:1 6:1 7:1 THICKNESS (microns) 400 100 67 57 SURFACE GLOSS (%) 64 - 112 ULTIMATE TENSILE STR. (MPa) Machine Direction 18 91 190 265 Tranverse Direction 21 32 24 20 STIFFNESS, 2% SECANT (MPa) Machine Direction 342 588 915 1080 Tranverse Direction 272 516 662 641 OXYGEN TRANSMISSIONS:1 (cc 25um/(m2 24hr atm)) AT 25° C, 0% R.H. 7.6 - - 2.8 AT 25° C, 75% R.H. 9.1 4.8 3.0 3.0 AT 25° C, 93% R.H. 23 - 3.2 1 Measurements are reported per Film samples were conditioned at humidity conditions for at least Measurements were performed with MOCON 100 Oxtran device. um thickness of film, the indicated temperature and 2 weeks before measuring. 100% oxygen atmosphere, with a a - * * TABLE 3 227 < EXAMPLE III - FLEX CRACK RESISTANCE EFFECT OF FILM FLEXING ON OXYGEN BARRIER PROPERTIES SAMPLE Ill-a SAMPLE Ill-b SAMPLE III-C ORIENTATION DRAW RATIO 1:1 6.5 : 1 7:1 THICKNESS (microns) 90 31 57 NUMBER OF GELBO FLEX CYCLES1 OXYGEN TRANSMISSION2 (cc/m2 24hr atm) 0 0.9 0.4 0.2 90 0.3 1.7 0.2 270 •k*k 0.7 0.3 450 ** 0.8 0.6 675 ** 2.5 900 ** ★ *■ * * >900 ** ** ** o 1 Samples were conditioned to 23° C, 75%RH before flexing on Gelbo flex testing device, ASTM F392-74 2 Measurement of Oxygen transmissions at ambient conditions using "Detector film method" (DIE ANG. MAKRO CHEMIE, 88, 1980, Q; 209-221) ** Denotes Oxygen transmission greater than 2500 (cc/m2 24hr atm), which is the upper test limit, and represents essentially complete loss of barrier.

TABLE 4 EXAMPLE IV - FILM PROPERTY COMPARISON FILM SAMPLE NUMBER PROPERTY IV-a IV-b IV-c IV-d ORIENTATION DRAW RATIO 1:1 4:1 6:1 7:1 THICKNESS (microns) 404 102 66 59 OPTICAL HAZE (%) .8 3.2 2.1 2.3 ULTIMATE TENSILE STR.(MPA) Machine Direction Tranverse Direction 18 15 117 19 205 19 271 22 STIFFNESS, 2% SECANT (MPa) Machine Direction 346 600 1098 1151 Tranverse Direction 388 529 796 586 OXYGEN TRANSMISSION:1 (cc 25um/(m2 24hr atm)) AT 25° C, 75% R.H. 14.5 12.2 10.0 7.8 1 Measurements are reported per 25 um total thickness of film. Film samples were conditioned at the indicated temperature and humidity condition for at least 2 weeks before measuring. Measurements were performed with 100% oxygen atmosphere, with a Mocon 100 Oxtran device. 227 3 6 1

Claims (16)

WHAT WE CLAIM IS:

1. A multi-layer plastics packaging film containing at least one layer of ethylene vinyl alcohol copolymer and at least one other layer containing a polyolefin based plastics material, wherein the ethylene vinyl alcohol copolymer is oriented to at least four times draw ratio in a single direction.

2. A film as claimed in Claim 1 wherein the layer(s) of ethylene vinyl alcohol copolymer comprise the minor constituent of said film.

3. A film as claimed in Claim 1 or Claim 2 wherein said film is formed by co-extruding said layers.

4. A film as claimed in any one of Claims 1 to 3 wherein said film is stretched in the longitudinal direction to up to eight times draw ratio.

5. A film as claimed in claim 4 wherein said film is stretched in the longitudinal direction to approximately seven times draw ratio.

6. A film as claimed in any one of the preceding claims wherein said polyolefin based plastics material comprises linear low density polyethylene.

7. A film as claimed in Claim 6 comprising outer layers of linear low density polyethylene with a single layer of ethylene vinyl alcohol copolymer therebetween, each of said layers being separated by an adhesive promoting polymer.

8. A method of forming a multi-layer plastics packaging film by combining at least one layer of ethylene vinyl alcohol copolymer and at least one other layer of a polyolefin based plastics 227361 material and then subjecting the multi-layer film to stretching in the longitudinal direction to a least four times draw ratio while restraining the film against substantial contraction in the transverse direction.

9. A method as claimed in Claim 7, said method comprising co-extruding the layers while disposing an adhesive promoting polymer therebetween.

10. A method as claimed in Claim 8 or Claim 9 including the step of mono-axially orientating said 151m to a draw ratio of up to eight

11. A method as claimed in Claim 10 including the step of mono-axially orienting said film to a draw ratio of substantially seven.

12. A method of forming a multi-layer plastics packaging film by combining at least one layer of ethylene vinyl alcohol copolymer and at least one other layer of polyolefin based plastics material; subjecting the multi-layer film to stretching in the longitudinal direction to substantially three times draw ratio while restraining the film against substantial contraction in the transverse direction; laminating the oriented film with at least one substantially non-oriented film containing a polyolefin based plastics material; and then subjecting the laminate to orientation in the same direction as the oriented film has already been oriented.

13. A method as claimed in Claim 12 wherein the laminate is oriented to a draw ratio of at least two.

14. A multi-layer mono-axially oriented plastics packaging film as claimed in Claim 1 and substantially as hereinbefore described with particular reference to any of the examples.

15. A method of forming a multi-layer plastics packaging film as claimed in Claim 8 or Claim 12 and substantially as herein described.

16. A package when formed from the film as claimed in any one of Claims 1 to 7 "M"1 o'X ^ naim 14 ft* ^ DATED THIS DAY OFJ^2- frt $ A. J. e*3K & .SON f< 1*15 J -17- 5® AGENTS FOR THE APPLICANT,