Heat shrinkable film

Abstract

The present invention refers to a multilayer heat shrinkable film characterized by a combination of desirable properties, such as high shrinkage, good optical properties, excellent sealability and puncture resistance. The invention further is directed to a method of producing said film. The invention is further directed to the use of said film or bags and pouches made therefrom for packaging goods as for example food products.

Claims

1. A heat shrinkable, co-extruded film whose puncture resistance is increased by at least 100% by immersion in hot water 90° C. for 4 seconds in a way that the thickness of the film before and after immersion has a maximum difference of 3%, comprising: (i) an outer layer comprising an ionomer with acid content of at least 10% per weight; (ii) an intermediate layer; (iii) an inner layer comprising an ethylene alpha olefin copolymer with density less than 0.905 g/cc; and (iv) an oxygen barrier material layer, wherein shrinkage at 85° C. in each of the machine and transverse directions of the heat shrinkable film is at least 40%.

2. The film of claim 1, wherein the intermediate layer comprises EVA.

3. The film of claim 1, wherein the oxygen barrier material comprises PVDC.

4. The film of claim 1, wherein the oxygen barrier material comprises EVOH.

5. The film of claim 1, wherein the ionomer is neutralized with a sodium cation.

6. The film of claim 5, wherein the neutralization ratio is at least 50%.

7. The film of claim 1, wherein the inner layer comprises a blend of two ethylene alpha olefin copolymers with density less than 0.905 g/cc.

8. The film of claim 1, wherein the inner layer comprises a blend of two ethylene alpha olefin copolymers with densities less than 0.900 g/cc.

9. The film of claim 1, wherein the inner layer comprises less than 50% per weight of EVA copolymer.

10. The film of claim 1, wherein the oxygen barrier material layer is located in the intermediate layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) According to a first aspect, the invention is directed to a heat shrinkable film whose puncture resistance is increased by at least 100% by immersion in hot water 90° C. for 4 seconds in a way that the thickness of the film before and after immersion has a maximum difference of 3%.

(2) In a preferred embodiment, the film according to the invention comprises an outer layer, an intermediate layer and an inner layer, where the outer layer surface of the film comprises an ionomer with acid content of at least 10 weight % and the inner layer comprises an ethylene alpha olefin copolymer with density less than 0.905 g/cc.

(3) According to a preferred version of the invention, the ionomer resin comprises at least 15 weight % of methacrylic or acrylic acid.

(4) According to a further preferred version of the invention the ionomer resin is at least 20% neutralized, more preferably at least 50% neutralized. In a further preferred version the ionomer is neutralized with sodium cation.

(5) According to a further preferred version, the outer layer comprises at least 90% ionomer per weight, more preferably at least 95%.

(6) According to a further preferred version the outer layer comprises less than (and up to) 5 wt % of polybutylene. Above 5 wt % the material becomes difficult to stretch.

(7) According to a further preferred version the outer layer comprises slip and/or antiblock agents. Preferably the slip and antiblock agents are provided as a masterbatch where the host polymer is ionomer or ethylene acrylic acid copolymer or ethylene methacrylic acid copolymer.

(8) In the film according to the present invention, the inner heat sealing layer may comprise a single polyolefin or a blend of different polyolefins. In a preferred version of the structure, the sealing layer comprises at least one homogeneous polyolefin.

(9) In a further preferred version the sealing layer comprises a homogeneous ethylene alpha olefin copolymer with a density of less than 0.905 g/cc.

(10) In a further preferred version the sealing layer comprises a homogeneous alpha olefin copolymer with a density of less than 0.900 g/cc.

(11) In a further version the inner layer comprises a blend of two ethylene alpha olefin copolymers with a density of less than 0.905 g/cc

(12) In another version of the invention the inner layer comprises a blend of two homogeneous ethylene alpha olefin copolymers which have both densities less than 0.900 g/cc.

(13) In a further preferred version of the invention the inner layer does not comprise EVA. The reason is that addition of EVA deteriorates the resistance of the heat seal of the bag during the shrink process. In another version the percentage of the EVA in the inner layer blend is less than 50% per weight, preferably less than 40% per weight.

(14) In a preferred version of the invention, the multilayer film comprises an oxygen barrier material, preferably PVDC, EVOH, polyamide or polyester.

(15) Preferably the oxygen barrier material is located in an intermediate layer of the film.

(16) In general PVDC is preferred as barrier material for the invention due to its insensitiveness under high humidity conditions which are often encountered in meat packing operations. EVOH can be used as well but is more sensitive to humidity

(17) Between the sealing layer and the oxygen barrier layer, a tie layer could be used. Preferred materials for this tie layer may be ethylene ester copolymers, such as ethylene vinyl acetate copolymers, ethylene methyl acrylate copolymers and other materials well known in the art. A preferred version might include maleic anhydrite modified ethylene ester copolymers or maleic anhydrite modified LLDPE. Commercial trademarks are for example BYNEL® from Dupont and ADMER® from Mitsui.

(18) Between the outer and the barrier layer there may be another layer incorporating a tie layer. Preferred materials for this tie layer may be ethylene ester copolymers, such as ethylene vinyl acetate copolymers, ethylene methyl acrylate copolymers and other materials well known in the art. A preferred version might include maleic anhydrite modified ethylene ester copolymers or maleic anhydrite modified LLDPE. Commercial trademarks are for example BYNEL® from Dupont and ADMER® from Mitsui.

(19) A further preferred option for the intermediate layers is a blend of EVA and propylene ethylene copolymer.

(20) A preferred construction of the multilayer film is as follows

(21) Outer layer-intermediate layer-barrier layer-intermediate layer-inner layer Other preferred constructions are as follows.

(22) Outer layer-intermediate layer 1-intermediate layer 2-barrier layer-intermediate layer 3-intermediate layer 4-inner layer

(23) Any of the layers described above may also include additives well known in the art such as slip agents, antiblock, polymer processing aids, antistatic, antifog, acid scavengers, odour scavengers and the like. A person skilled in the art may select the right additives according to any particular needs.

(24) The thickness of the film is preferably in the range 10 to 150 microns, more preferably 20 to 120 microns. The thickness of the outer layer is preferably from 1 to 50 microns, more preferably 3 to 25 microns. The thickness of the inner layer is preferably 5 to 100 microns, more preferably 10 to 60 microns.

(25) In order the material to have a high shrinkability at 85° C., it is needed to be biaxially oriented either by using the double bubble process or the tenter frame process.

(26) Both processes are well known in the art. The double bubble process is especially preferred.

(27) In a further aspect of the invention the film or some layers of the film are irradiated. A preferable method is e-beam or UV radiation or gamma ray. Other methods are also known in the art.

(28) According to a further aspect, the present invention discloses a bag or pouch comprising a film according to the present invention.

(29) In a further aspect, the invention is directed to the use of the films or the bag or pouch of the invention for packaging food. For example, the food item is put inside the bag and the air is removed (vacuumising) with the help of a vacuum device. Subsequently, the open end of the bag is sealed and the vacuum pack is placed in a heat shrinking media such as hot water under a temperature that ensures the shrink of the pack (eg 90° C.). The pack than is ready and is characterized by appealing appearance and long shelf life because of the absence of oxygen.

(30) The present invention is now described by the following Examples:

Examples

(31) A 5 layer film is produced in a double bubble (the double bubble method is described in U.S. Pat. No. 3,456,044) commercial line with the following recipe

(32) Inner (sealing layer), 56% P1+40% P2+4% additives

(33) Adjacent layer 70% E1+30% PP1

(34) Barrier layer PVDC commercial grade

(35) Adjacent layer 70% E2+30% PP1

(36) Outer layer 95% I1+5% ADDITIVES

(37) See table 1

(38) The thickness of the structure is 23/9/5/9/8 starting from the inner and going to the outer layer.

(39) TABLE-US-00001 TABLE 1 Melt Index Density Melting Type Description Manufacturer g/10 min g/cm.sup.3 point ° C. E1 EVA Polimeri 0.3 0.935 93° C. FC45 E2 EVA Polimeri 0.4 0.938 88° C. FD50 PP1 PP copolymer Dow 0.885 I1 Ionomer with DUPONT 0.9 0.94 94° C. acid content 15% P1 AFFINITY DOW 1.6 0.896 94° C. PF1140 P2 TAFMER 4085 MITSUI 3.5 0.885 Less than 70° C.

(40) The inventive material is compared to an already existing product named TRF 55 which does not comprise ionomer in the outer layer.

(41) Tests

(42) 1. Haze measurement. The haze measurement was done according to ASTM D 1003. 2. Gloss measurement. This was done according to BS 2782. 3. Shrinkage measurement done according to ASTM D 2732 at 85° C. 4. Puncture resistance test. Puncture resistance is measured using a MECMESIN instrument comprising a vertical stand VERSA TEST, a load cell AFG 100N, which is mounted on the stand, and a digital height gauge MITUTOYO SDV 60B. A cylindrical shaft ending to a conical indenter is attached to the load cell. The indenter has an angle of 60° and a tip of 0.5 mm in diameter. The film is conditioned at 23° C. and about 50% RH for at least 24 hours prior to the measurement. Square samples of 8.5 cm×8.5 cm are prepared and clamped on a 5 cm diameter circular base. The indenter moves perpendicular to the film and force is measured as the indenter comes into contact with and ruptures the film. The speed of the indenter is set to 25 mm/min. Maximum force needed to puncture the film is recorded. The puncture force of a material is determined by averaging the values obtained for at least 5 samples of the material tested. 5. Puncture resistance test at restrained shrinkage sample.

(43) In order to obtain an area of a film that has undergone restrained shrinkage, we use the following procedure: i) We take a 250×200 bag of the material under question. ii) We measure the thickness of the material at an area about 8 cm below the open end of the bag. iii) We put inside the bag a cylinder, which is 4 cm high and has a diameter of 14 cm. iv) We vacuum the package using a vacuum chamber and seal it about 2 cm from the open end of the bag. v) We place the package obtained in a shrink bath heated at a temperature of 90° C. for 4 seconds. vi) After the shrinkage of the package, we open the bag, take the cylinder out and cut a piece of the material at the same area where we had originally measured thickness. The two thickness measurements should be identical (so average thickness difference should be less than 3%), as the material has not been shrunk at this specific area, since it has undergone restrained shrinkage.

(44) TABLE-US-00002 TABLE 2 PUNCTURE PUNCTURE SHRINKAGE RESISTANCE RESISTANCE (M D/TD) IN UNSHRUNK IN RESTRAINED HAZE GLOSS 85 C. FILM SHRUNK FILM Inventive 9 108 47/52 11 Newtons 27 Newtons material TRF 55 9 110 53/40 15 Newtons 21 Newtons

(45) Therefore one can see that the inventive material is similar to TRF 55 in optics, has much bigger shrinkability in 85° C. conditions and presents an increase in puncture at least 100% after restrained shrinkage in pieces with same thickness (difference of average thicknesses before and after immersion in water is less than 1%).

(46) Thickness of the inventive film before immersion was 55.2 microns and after immersion 55.5 microns whereas the thickness of TRF 55 before immersion was 56 microns while after immersion was 56.3 microns.

(47) It will be understood that various details of the presently disclosed subject matter can be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.