Multilayer heat shrinkable film comprising styrene polymer or a blend of styrene polymers

Abstract

The present invention provides a multilayer heat shrinkable film incorporating an oxygen barrier material and layer containing a styrene polymer or blend of styrene polymers, where the shrinkage of the film in at least one of MD, TD is at least 30% at 90 C. The invention is further directed to a method of manufacturing a bag from said multilayer heat shrinkable film.

Claims

1. A multilayer film, comprising: a) an outer layer comprising styrene-butadiene block copolymer; b) a first tie layer adjacent the outer layer comprising ethylene-vinyl acetate; c) a gas barrier layer adjacent the first tie layer comprising polyvinylidene chloride; d) a second tie layer adjacent the gas barrier layer comprising ethylene vinyl acetate; and e) a sealing layer adjacent the second tie layer selected from the group consisting of metallocene-catalyzed ethylene-alpha olefin copolymers alone or combined with additional ethylene-alpha olefin copolymers and not including ethylene-vinyl acetate, wherein the metallocene-catalyzed ethylene-alpha olefin copolymer has a density less than about 0.905 grams/cm.sup.3 and the additional ethylene-alpha olefin copolymer is not an ethylene-propylene copolymer; f) where the film is heat shrinkable and shrinks at temperatures of about 90 C.

2. The film of claim 1, wherein the outer layer has a thickness that is about 12% of a total film thickness.

3. The film of claim 1, wherein the sealing layer has a thickness between about 0.4 mils and about 2.0 mils and the gas barrier layer has a thickness between 0.1 mils and about 0.5 mils.

4. The film of claim 1, wherein the gas barrier layer has a thickness that is about 11% of a total film thickness.

5. The film of claim 3, wherein the sealing layer has a thickness that is about 46% of a total film thickness.

6. The film of claim 1, wherein the styrene-butadiene block copolymer has a styrene content of more than 50% by weight.

7. The film of claim 1, wherein the first tie layer comprises a blend of ethylene methacrylate copolymer, ethylene vinyl acetate copolymer, and metallocene-catalyzed linear low density polyethylene.

8. The film of claim 1, wherein the second tie layer comprises a blend of ethylene methacrylate copolymer, ethylene vinyl acetate copolymer and metallocene-catalyzed linear low density polyethylene.

9. A bag comprising a sealed multilayer film, wherein the sealed multilayer film comprises: a) an outer layer comprising styrene-butadiene block copolymer; b) a first tie layer adjacent the outer layer comprising ethylene-vinyl acetate; c) a gas barrier layer adjacent the first tie layer comprising polyvinylidene chloride; d) a second tie layer adjacent the gas barrier layer comprising ethylene vinyl acetate; and e) a sealing layer adjacent the second tie layer selected from the group consisting of metallocene-catalyzed ethylene-alpha olefin copolymers alone or combined with additional ethylene-alpha olefin copolymers and not including ethylene-vinyl acetate, wherein the metallocene-catalyzed ethylene-alpha olefin copolymer has a density less than about 0.905 grams/cm.sup.3 and the additional ethylene-alpha olefin copolymer is not an ethylene-propylene copolymer; f) wherein the film is heat shrinkable and shrinks at temperatures of about 90 C.

10. The bag of claim 9, wherein the outer layer has a thickness that is about 12% of a total film thickness.

11. The bag of claim 9, wherein the sealing layer has a thickness between about 0.4 mils and about 2.0 mils and the gas barrier layer has a thickness between 0.1 mils and about 0.5 mils.

12. The bag of claim 11, wherein the gas barrier layer has a thickness that is about 11% of a total film thickness.

13. The bag of claim 11, wherein the sealing layer has a thickness that is about 46% of a total film thickness.

14. The bag of claim 9, wherein the styrene-butadiene block copolymer has a styrene content of more than 50% by weight.

15. The bag of claim 9, wherein the first tie layer comprises a blend of ethylene methacrylate copolymer, ethylene vinyl acetate copolymer, ethylene vinyl acetate copolymer, and metallocene-catalyzed linear low density polyethylene.

16. The bag of claim 9, wherein the second tie comprises a blend of ethylene methacrylate copolymer, ethylene vinyl acetate copolymer and metallocene-catalyzed linear low density polyethylene.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) According to a first and main aspect, the invention provides a multilayer heat shrinkable film comprising at least

(2) a) a layer comprising an oxygen barrier material,

(3) b) a layer comprising a styrene polymer or a blend of styrene polymers, where the shrinkage of the film in at least one of MD, TD (machine direction, transverse direction) is at least or more than 30% at 90 C.

(4) In one preferred embodiment of the invention, the layer of the styrene polymer or blend of styrene polymers is the outer layer of the structure, being exposed to the environment.

(5) In a further embodiment, between the layer of the styrene polymer or blend of styrene polymers and the barrier layer there is a bonding layer which may include ethylene ester copolymers or ethylene alpha olefin copolymers.

(6) Additionally, the multilayer heat shrinkable film of the invention may comprise a heat sealable inner layer. The inner layer may be made of a heat sealable thermoplastic polymer or polymer blend. The heat sealable inner layer may be of any material conventionally used for its heat sealing capability. Heat sealable thermoplastic polymers are recognized by those skilled in the art as being capable of heat sealing to themselves at a variety of time, pressure and temperature conditions. For example, at a given pressure either a relatively high temperature may be applied briefly or a lower temperature may be applied for a longer period of time to obtain similarly suitable seals. It is to be appreciated that persons skilled in the art will be able to select sealing parameters such as temperature, pressure and time of application that depend upon such factors as the type of heat sealing equipment used. The term inner layer generally relates to that side of the multilayer heat shrinkable film, which is not exposed to the environment in a given product form (e.g. bag), but the good to be packaged.

(7) This inner sealing layer may for instance be an alpha olefin copolymer or a blend of alpha olefin copolymers, e.g. a VLDPE, EVA, plastomer or a blend thereof. Alternatively, a styrene polymer or a blend of styrene polymers may also be used as the inner heat sealing layer. In another embodiment, a cycloolefin copolymer or a blend of cycloolefin copolymer and alpha olefin copolymer may be the inner sealing layer.

(8) Suitable polymers and copolymers may include alpha olefins such as members of the polyethylene family such as linear low-density polyethylene (LLDPE); very low-density polyethylene sometimes referred to as ultra low-density polyethylene (VLDPE and ULDPE respectively), plastomers or blends thereof. Still other suitable thermoplastic polymers as the heat sealable inner layer include polypropylene, ethylene-propylene copolymer or an ionomer. It is to be understood however that the thermoplastic polymers mentioned herein are not intended to be an exhaustive list, but merely exemplary.

(9) In a preferred embodiment of the invention, the outer layer (directly exposed to the environment) comprises a styrene copolymer in a blend with a slip agent.

(10) Preferred styrene polymers are polystyrene homopolymer, SBC (styrene butadiene copolymers), SBS copolymers, SIS copolymers, SEBS copolymers, Ethylene Styrene interpolymers and blends thereof. Suitable materials are K RESIN from Chevron Philips Chemical, FINA-CLEAR from Atofina and Styrolux from BASF. A preferred styrene homopolymer is STYRON from Dow. In one preferred embodiment, the styrene content of the material is more than 50% by weight. This increases the stiffness of the material, thus making it more easy to convert.

(11) One functional requirement of the barrier layer may be that together with the other layers, the oxygen transmission rate through the entire multilayer film may be below about 20 cc/l m.sup.2/24 hrs/atm, measured at 23 C, 75% RH. This is to avoid spoilage of certain food products, for example meat enclosed in the multilayer film package due to oxygen passage from the environment through the film wall. This requirement may be satisfied by numerous well-known barrier layer materials. For example, these may include certain of the polyamides (Nylon), hydrolyzed ethylene vinyl acetate copolymer (EVOH) and preferably a vinylidene copolymer. Vinylidene chloride-vinyl chloride (PVDC) is a commonly used copolymer but vinylidene chloride-methyl acrylate copolymer (MA-VDC) may also be used.

(12) The thickness of the heat sealable inner thermoplastic first layer may be between about 0.4 mils and about 2.0 mils. Thinner layers may not perform the described functions while thicker layers may not appreciably improve processability of the film and may reduce total film performance.

(13) The barrier layer thickness may be between about 0.1 mils to about 0.5 mils. Thinner barrier layers may not perform the intended functions and thicker layers do not appreciably improve performance.

(14) Regardless of the structure of the film, one or more film additives may be incorporated, such as slip agents, antiblock agents, colorants, odor inhibitors, oxygen inhibitors etc. The skilled in the art technician can be well aware of these additives.

(15) While the composition of the film may eliminate the need for irradiation of the multilayer film, it is to be understood that the multilayer heat shrinkable film in accordance with the invention may be irradiated to further enhance puncture and temperature resistance.

(16) According to a preferred embodiment, the multi-layer heat shrinkable film of the invention comprises the following components: a) an outer layer of a styrene amorphous copolymer b) an adjacent (bonding) layer of a blend of EMA copolymer, EVA copolymer and metallocene LLDPE c) a barrier core layer comprising PVDC copolymer d) an adjacent (bonding) layer of EVA copolymer or EMA copolymer, EVA copolymer and metallocene LLDPE e) an inner sealing layer comprising EVA, VLDPE, or a plastomer or a blend thereof.

(17) The heat shrinkable multilayer film in accordance with the present invention may be in the form of a tubular article or flat sheets. The multilayer film may be formed into bags useful for the packaging of meats. The multilayer film may be especially useful for those bags manufactured by hot bar or impulse sealing, and after meat is inserted therein, impulse heat sealing may be used to seal the mouth.

(18) One end of the bag may be heat sealed by adhesion between contiguous inner layer surfaces in a direction transverse to the oppositely located sidewalls of the bag. The mouth end of the bag may be impulse heat sealable by fusion bonding between contiguous inner layer surfaces after filling the bag with food.

(19) A bag from the multilayer film may be manufactured such that the inner layer forms the inside portion of the bag while the outer layer forms the outside portion of the bag. Accordingly, the inner layer of the tubular multilayer film may be the heat sealable layer, which is easily sealed by hot bar sealing. Moreover, because of the latter, the mouth of the bag may be more easily sealed by impulse heat sealing.

(20) Thus, according to a second aspect, the invention is directed to a method for manufacturing a bag from a multilayer shrinkable film as defined above, comprising the steps of: a) forming a bag, in which the inner layer of the multilayer heat shrinkable film as defined herein forms the inside portion of the bag while its outer layer forms the outside portion of the bag, b) inserting a good to be packaged into the bag, and c) sealing the bag by hot bar or impulse sealing

(21) Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. All references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the examples are illustrative only and not intended to be limiting.

EXAMPLES

(22) In this series of tests, bags of the same size (400 mm long230 mm wide) and of 60 micron thickness were fabricated from films comprising a heat sealable thermoplastic inner layer, an adjacent layer adhered on one side to the sealable inner layer, an oxygen barrier layer, an adjacent layer and an outer layer. The core layer in most tests was a vinylidene chloride copolymer type oxygen barrier. The film was the biaxially oriented heat shrinkable type, prepared by the double or trapped bubble method as broadly described in the aforementioned Palkhe U.S. Pat. No. 3,456,044. In particular, all layers were simultaneously coextruded.

(23) Certain physical properties of bags were measured by either of the test procedures discussed below.

(24) Shrinkage values were obtained by measuring unrestrained shrink of the stretched film at 90 C. for five seconds. Four test specimens were cut from a given sample of the oriented film to be tested. The specimens were cut to 10 cm in the machine direction by 10 cm. in the transverse direction. Each specimen was completely immersed for 5 seconds in a 90 C. water bath. The distance between the ends of the shrunken specimen was measured. The difference in the measured distance for the shrunken specimen and the original 10 cm was multiplied by ten to obtain the percent of shrinkage for the specimen. The shrinkage for the four specimens was averaged for the MD shrinkage values of the given film sample, and the shrinkage for the four specimens was averaged for the TD shrinkage value.

(25) The tensile modulus compared below has been measured according to ASTM D 882 with an Instron testing device. The optical properties were tested according to ASTM D1003(haze) and BS 2782(gloss).

(26) The bag making testing was done with a use of a commercial bag making machine.

Example 1

(27) A heat shrinkable multilayer film was produced having:

(28) a) an outer layer of a styrene amorphous copolymer b) an adjacent layer of a blend of EMA copolymer, EVA copolymer and metallocene LLDPE c) a barrier core layer d) an adjacent layer of EVA copolymer e) an inner sealing layer
More specifically Outer layer: 98% S1+2% slip masterbatch, 12% of the total structure Adjacent layer 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 19% of the total structure Barrier layer: PVDC copolymer, 11% of the total structure Adjacent layer: 100% E1, 12% of the total structure Inner sealing layer: 20% E1+10% E2+32% P1+30% V1+8% ADDITIVES, 46% of the total structure.

Example 2

(29) A heat shrinkable multilayer film was produced having:

(30) a) an outer layer of a styrene amorphous copolymer b) an adjacent layer of a blend of EMA copolymer, EVA copolymer and metallocene LLDPE c) a barrier core layer d) an adjacent layer of EMA copolymer, EVA copolymer and metallocene LLDPE e) an inner sealing layer
More Specifically Outer layer: 98% S1+2% slip masterbatch, 12% of the total structure Adjacent layer 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 19% of the total structure Barrier layer: PVDC copolymer, 11% of the total structure Adjacent layer: 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 12% of the total structure Inner sealing layer: 20% E1+10% E2+32% P1+30% V2+8% ADDITIVES, 46% of the total structure

Comparative Example 3

(31) As a comparative example, we use a commercial product of FLEXOPACK under the name FMX containing no styrene.

Example 4

(32) Outer layer: 98% S1+2% slip masterbatch, 12% of the total structure Adjacent layer 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 19% of the total structure Barrier layer: PVDC copolymer, 11% of the total structure Adjacent layer: 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 12% of the total structure Inner sealing layer: 20% E1+10% E2+32% P1+30% P2+8% ADDITIVES, 46% of the total structure

Example 5

(33) Outer layer: 98% S1+2% slip masterbatch, 12% of the total structure Adjacent layer 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 19% of the total structure. Barrier layer: PVDC copolymer, 11% of the total structure. Adjacent layer: 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 12% of the total structure. Inner sealing layer: 30% E1+62% P2+8% ADDITIVES, 46% of the total structure.

Example 6

(34) Outer layer: 98% S1+2% slip masterbatch, 12% of the total struct6ure Adjacent layer 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 19% of the total structure Barrier layer: PVDC copolymer, 11% of the total structure Adjacent layer: 15% M1+15% M2+45% E1+20% LL1+5% ADDITIVES, 12% of the total structure Inner sealing layer: 30% E1+42% P2+20% C1+8% ADDITIVES, 46% of the total structure

(35) The films were e-beam radiated with a dose of 4 MRAD.

(36) TABLE-US-00001 TABLE 1 SHRINKAGE COMPARISONS SHRINKAGE SHRINKAGE SHRINKAGE SHRINKAGE SAMPLE 80 C. 85 C. 90 C. 95 C. NOTES EXAMPLE 1 28/28 39/37 50/48 55/46 EXAMPLE 2 30/30 38/35 50/47 55/47 EXAMPLE 3 30/32 38/37 49/45 55/49 EXAMPLE 4 31/31 42/40 50/50 57/52 EXAMPLE 5 27/28 37/37 50/45 55/47 EXAMPLE 6 32/32 44/44 50/50 57/55

(37) According to TABLE 1, there is practically no difference in the comparison of shrinkage between the three different materials.

(38) In the following table the optical property comparisons are explained.

(39) TABLE-US-00002 TABLE 2 HAZE AFTER SAMPLE HAZE GLOSS SHRINKAGE NOTES EXAMPLE 1 8.2 92 30 Excellent clarity after shrink onto product EXAMPLE 2 9 92 36 Excellent clarity after shrink onto product EXAMPLE 3 10.5 81 40 Good clarity after shrink onto product EXAMPLE 4 7.5 97 25 Excellent clarity after shrink onto product EXAMPLE 5 8 95 27 Excellent clarity after shrink onto product EXAMPLE 6 11.5 94 45 Excellent clarity after shrink onto product

(40) Measurement of tensile modulus has been also carried out, according to ASTM D 882.

(41) TABLE-US-00003 TABLE 3 TENSILE NOTES MODULUS DURING BAG SAMPLE MD/TD NOTES MAKING EXAMPLE 1 210/195 stiff Very easy to convert EXAMPLE 2 239/209 stiff Very easy to convert EXAMPLE 3 173/175 soft Convertible only with reduced machine speed. Blocks during bag making. EXAMPLE 4 205/190 Easy to convert EXAMPLE 5 230/225 Very easy to convert EXAMPLE 6 250/250 Very easy to convert

(42) The resins used are explained in table 4

(43) TABLE-US-00004 TABLE 4 Melt Index Melting g/10 Density point Type Description Manufacturer min. g/cm.sup.3 C. E1 EVA Dupont 3135 X 0.35 0.93 95 E2 EVA 1005 VN2 0.40 0.928 102 V1 VLDPE Dow Attane SL 4100 1.00 0.912 122 P1 Plastomer Dow Affinity VP 8770 1.0 0.885 74 PB1 Polybutylene Shell 8640 2 118 LL1 LLDPE Atofina M3410 1 0.934 123 S1 SB DK13 10 1.01 COPOLYMER M1 EMA Atofina LOTRYL 2-3.5 0.95 61 copolymer 29MAO3 M2 EMA Atofina LOTRYL 0.5 0.94 70 copolymer 24MA005 P2 Plastomer Dow Affinity PL 1880 1 0.902 100 C1 Cycloolefin Ticona Topas 8007 1.02 copolymer