Barrier film with enhanced formability and a method for making thereof

Abstract

A film comprises an outer heat sealable layer mainly made of a (co)polyolefin with the Vicat softening temperature not exceeding 130° C., at least one heat resistant layer mainly made of at least one polar (co)polymer selected from the group including predominantly aliphatic (co)polyamides and aromatic (co)polyesters, and at least one core adhesive layer from a material capable of adhering both to (co)polyolefins and to polar (co)polymers. The heat resistant layer comprises not less than 15% of at least one predominantly aliphatic copolyamide with the melting temperature not exceeding 205° C. A method comprises the stages of coextrusion, biaxial stretching, annealing and winding up of the resulting film into a roll.

Claims

1. A coextruded biaxially-oriented barrier film, comprising, (i) at least one outer heat sealable layer, comprising a (co)polyolefin with a Vicat softening temperature not exceeding 130° C., (ii) at least one heat resistant layer, comprising at least one polar (co)polymer selected from the group consisting of (co)polyamides and aromatic (co)polyesters, and (iii) at least one core adhesive layer made of a material capable of adhering to both (co)polyolefins and polar (co)polymers, wherein a. said heat resistant layer, if it consists essentially of (co)polyamides, comprises not less than 15% of at least one copolyamide with a melting temperature not above 205° C.; b. a shrinkage value of the film in each a machine direction and a transverse direction after immersion for 10 seconds in water at a temperature of 90° C. is less than 10%; c. a shrinkage value of the film in at least one of the machine direction and the transverse direction is less than 3% after dwelling for 10 seconds in a drying cabinet preheated to 100° C.; and d. the film is capable of being thermoformed substantially free of ruptures, and the ratio between a mean thickness in unstrained film areas and a mean minimum thickness of the film in a resulting formed product is not more than 4.5, when vacuum thermoforming is carried out automatically by a thermoforming machine at a preset temperature of 100° C., a heating time of 3 s, a forming time of 2.5 s and a vacuum system pressure of 2 mm Hg by means of a forming die with four molding cavities in the form of rectangular parallelepipeds, each with the overall dimensions of 147×143×50 mm and rounding edges with a radius of 20 mm, assembled in a rectangular block with 18 mm gaps between the molding cavities in the machine direction and 15 mm gaps in the transverse direction.

2. The film according to claim 1, wherein said aromatic (co)polyester forming part of the heat resistant layer is selected from the group consisting of polyethylene terephthalate, glycol-modified polyethylene terephthalate, polybutylene terephthalate, crystallizing copolymer of polyethylene terephthalate and polybutylene terephthalate, and any mixture of the foregoing (co)polymers.

3. The film according to claim 1, wherein the adhesive layer material is a product of grafting of maleic anhydride onto polyolefin containing monomeric units of propylene or ethylene.

4. The film according to claim 1, wherein said film additionally has at least one EVOH oxygen barrier layer, comprising a polar copolymer of ethylene and vinyl alcohol, whereby the ethylene content is not higher than 50 mole %.

5. The film according to claim 1, wherein said film additionally has at least one moisture barrier layer comprising an olefin (co)polymer.

6. The film according to claim 1, wherein said film additionally has at least one heat resistant layer comprising aliphatic (co)polyamide, which is a core layer.

7. The film according to claim 1, wherein said film comprises from 3 to 25 layers.

8. The film according to claim 1, wherein said heat sealable layer contains at least one (co)polymer, comprising (i) monomeric units of ethylene or propylene and (ii) monomeric units of at least one of the monomers selected from the group consisting of α-olefin with a number of carbon atoms from 2 to 14, vinyl esters, esters of unsaturated carboxylic acids, unsaturated carboxylic acids and salts thereof, whereby a total quantity of the monomers (ii) does not exceed 40 mole %.

9. The film according to claim 8, wherein said vinyl esters are represented by vinylacetate, and wherein said unsaturated carboxylic acids are selected from the group consisting of acrylic acid and methacrylic acid, whereas their esters contain residues of normal or branched alcohols with a number of carbon atoms from 1 to 6, and said salts contain cations, selected from the group consisting of Li+, Na+, Mg2+, and Zn2+.

10. The film according to claim 1, wherein at least one of said copolyamide of said heat resistance layer having the melting temperature not exceeding 205° C. contains monomeric units of such amino acids as comprises units of monomers selected from the group (i) consisting of γ-aminobutyric acid, δ-aminovaleric acid, ε-aminocaproic acid, .Math.-aminoundecanoic acid, κ-aminolauric acid, tetramethylenediamine, hexamethylenediamine, dodecamethylenediamine, adipic acid, azelaic acid, sebacic acid, dodecanoic acid, and optionally not more than 15% w/w of monomers selected from the group (ii) consisting of isophthalic acid, terephthalic acid, meta-xylylenediamine and para-xylylenediamine.

11. The film according to claim 10, wherein said heat resistant layer additionally contains not more than 85% of at least one (co)polyamide with a melting temperature of above 205° C., containing monomeric units of at least one monomer selected from the group (i) consisting of γ-aminobutyric acid, δ-aminovaleric acid, ε-aminocaproic acid, .Math.-aminoundecanoic acid, κ-aminolauric acid, tetramethylenediamine, hexamethylenediamine, dodecamethylenediamine, adipic acid, azelaic acid, sebacic acid and dodecanoic acid, and optionally not more than 15% w/w of monomers selected from the group (ii) consisting of isophthalic acid, terephthalic acid, meta-xylylenediamine and para-xylylenediamine.

12. The film according to claim 10, wherein said heat resistant layer additionally comprises semiaromatic (co)polyamides selected from the group consisting of PA 6I/6T and PA MXD6.

13. A method of preparation of a biaxially-oriented and annealed barrier film according to claim 1, including the stages of coextrusion, biaxial stretching, annealing and winding up into a roll, wherein i. an area stretch ratio at biaxial stretching is not less than 8, and ii. the stage of annealing is accompanied by relaxation with shrinkage value more than 25% in at least one of the machine direction and the transverse direction, and by surface relaxation with the area shrinkage value more than 40%.

14. The method according to claim 13, wherein said coextrusion is flat-die coextrusion of a flat film, whereas the stages of biaxial stretching, annealing and relaxation are carried out by means of a tenter frame.

15. The method according to claim 13, wherein i. said coextrusion is injection molding of a primary tube through an annular die, ii. said biaxial stretching is achieved by gas-blowing of the primary tube with simultaneous longitudinal drawing of the film by means of two pairs of nip rolls to produce a secondary tube, iii. subsequent annealing and relaxation of the tubular film is carried out at a tube gas pressure lower, than that at production of the secondary tube, and at a controlled reduced tension created by two other pairs of nip rolls used for production of the resulting tube, which is then collapsed into a flattened tubular film, and a ready-to-use film is obtained by cutting of said flat flattened tubular film into a desirable number of prolonged sheets.

Description

DESCRIPTION OF THE FIGURES USED TO DISCLOSE THE INVENTION

(1) FIG. 1 shows a top view of the test forming cavity projected against the film. The arrow shows the machine direction (MD) of the film.

(2) FIG. 2 shows a general view of the product formed from the film (idealized view).

EXAMPLES OF THE INVENTION EMBODIMENTS

(3) The following examples, provided exclusively as illustration of the present invention, without limitation of spirit and scope of claims, disclose the most preferable embodiments of the present invention.

Materials Used in the Examples and their Abbreviations

(4) Polyamides

(5) PA 6—pelletized semicrystallinepolycaprolactam—polyamide 6—Ultramid® B3 grade supplied by BASF.

(6) PA 6/66—pelletized semicrystalline copolymer of caprolactam, adipic acid and hexamethylenediamine—copolyamide 6/66—(PA 66 monomer content about 15%), Ultramid® C3 grade supplied by BASF.

(7) PA 6I/6T—pelletized amorphous copolymer of hexamethylenediamine, terephthalic and isophthalic acids (copolyamide 6I/6T)—Selar® PA grade supplied by DuPont de Nemours.

(8) PA-AB—pelletized masterbatch to prevent adhesion in the film wound into a roll (antiblock additive), for polyamide 6—Polybatch AB PA 3013 grade supplied by A. Schulman.

(9) Polyesters

(10) PET—pelletized semicrystalline polyethylene terephthalate, Lighter C93 grade supplied by Equipolymers GmbH.

(11) PET-AB—pelletized antiblockmasterbatch for PET, Polybatch PT AB 050 G grade supplied by A. Schulman.

(12) Polyolefins

(13) POP1—pelletized polyolefin plastomer (copolymer of ethylene and α-olefin), Queo 0201 grade supplied by Borealis AG.

(14) POP2—pelletized polyolefin plastomer (copolymer of ethylene and α-olefin), Exact® Plastomer 5062 grade supplied by Exxon Mobile.

(15) coPP—pelletized random copolymer of propylene, ethylene and butene, Adsyl 5C30F grade supplied by Lyondell Basell.

(16) IO—pelletized polyolefin ionomer (copolymer of ethylene and acrylic acids, partially zinc-neutralized), Surlyn 1601 grade supplied by DuPont de Nemours.

(17) PE-AB—pelletized antiblock masterbatch for low density linear polyethylene, Polybatch FSU 1010E grade supplied by A. Schulman.

(18) Gas Barrier Materials

(19) EVOH—pelletized copolymer of ethylene and vinyl alcohol with the mole content of ethylene at 32%, Evasin 3251 F grade supplied by Chang Chun Petrochemical Co. Ltd.

(20) Polymer Adhesives

(21) AD1—pelletized maleinized linear low density polyethylene, Modic® M603E grade supplied by Mitsubishi Chemical Europe GmbH.

(22) AD2—pelletized maleinized copolymer of ethylene and an acrylate, Bynel 21E787 grade supplied by DuPont de Nemours.

Example 1. Film with Practically Isotropic Pliability

(23) Nine compositions shown in the corresponding column of Table 1 are loaded into nine extruders with different weight productivities of nine-layer extrusion machine, melted and mixed therein. Then the melts are fed to a nine-layer die, where a nine-layer extrudate is formed, wherein Layer A is the inner layer, and Layer I is the outer layer. Next, it is extruded through an annular die to form the primary tube, which is quenched by cold water down to a temperature of 13-15° C.

(24) The solidified primary tube with the average diameter of about 310 mm and the average thickness of 447 μm is then transferred, by means of a system of active nip rollers, to a tubular infrared heater, where it is heated to the temperature of 80° C., and further subjected to orientation stretching with the MD and TD draw ratios of 2.85 and 3.2, respectively. Subsequent annealing of the produced oriented tubular film in the form of air-inflated tertiary tube is carried out in tubular convection heaters during 3-5 seconds, the temperature of the tertiary tube at the heater outlet being 135-140° C. In the process of annealing, adjustment of the rotational speed of the inlet and the outlet pairs of active nip rollers constraining the tertiary bubble, and maintenance of the bubble diameter at 705 mm provide for 30% dimensional relaxation (shrinkage) of the film in both directions. The parameters of drawing and shrinkage during relaxation step are given in Table 2. After annealing, the collapsed tubular film is passed over the cooling roller to bring the temperature down to 25° C., and is then wound-up into a roll containing about 1,000 m of film. The resulting film has the mean diameter of about 695 mm (flat width about 1,092 mm) and a thickness of about 100 μm.

(25) In the process of the roll rewinding, four parallel blades cut off the flat tubular film so as to detach about 10 mm of the film material from the fold lines and simultaneously cut the film into 6 prolonged sheets in the form of three double strips, each about 357 mm wide. Then each of the resulting sheets is wound up in a separate roll.

(26) After that the film is delivered to meat processing facilities, where it is used as the bottom film for the fabrication of:

(27) a. trays for frankfurter-type sausage packaging, by means of a forming die shown in FIG. 1, with the use of a Multivac R145 thermoformer supplied by Multivac Group (film surface area occupied by the formed products ˜71%) under the conditions described in the foregoing method of determination of the maximum thinning;

(28) b. containers for packaging of whole chicken weighing about 1.2-1.4 kg, by means of a forming die measuring 220×300×120 mm, with the use of a Webomatic APS ML 4600 packaging machine, at transverse intervals of 80 mm (film surface area occupied by the formed products ˜62%).

(29) In both cases the following forming parameters are preset on control panels of the thermoformers: temperature 100° C., preheating time 3 s, forming time 2.5 s and vacuum system pressure 2 mm Hg.

(30) The resulting packages are loaded into a refrigerated truck for delivery to a chain shop located at a distance of 20 km from the meat processing facility. Upon arrival at the place of destination, all packages are unloaded and inspected for damages, and records are made of the percentage of packages with ruptured film.

(31) The results of production tests of this film and of the films from other examples are shown in Table 3.

(32) TABLE-US-00001 TABLE 1 Example 1 and comparative Examples 2 and 7, comparative examples 3, 4*, 6 example 5 Layer α**, % Composition, % α**, % Composition, % A 10 PET (98) + 15 PA 6 (68) + PET-AB (2) PA 6/66 (20) + PA 6I/6T (10) + PA-AB (2) B 5 AD 1 (100) 5 AD 2 (100) C 30 coPP (100) 30 coPP (100) D 5 AD 2 (100) 5 AD 2 (100) E 8 PA 6 (70) + 11 PA 6 (70) + PA 6/66 (20) + PA 6/66 (20) + PA 6I/6T (10) PA 6I/6T (10) [PA 6 (70) + PA 6I/6T (30)] F 4 EVOH (100) 5 AD 2 (100) G 8 PA 6 (70) + 4 EVOH (100) PA 6/66 (20) + PA 6I/6T (10) [PA 6 (70) + PA 6I/6T (30)] H 5 AD 2 (100) 5 AD 2 (100) I 25 POP1 (98) + 20 IO (98) + PE-AB (2) PE-AB (2) *for Comparative Example 4, the composition of polyamide layers E and G is shown in square brackets **αshare of the extruder productivity in the total productivity of the whole unit.

Example 2. Film with a Strong Anisotropy of Pliability

(33) The first stage of production thereof is the extrusion of primary tube with the structure shown in Table 1 for Example 2, the diameter of 362 mm and the thickness of 492 μm.

(34) Here and in the next examples, tubular films are made according to the technology of Example 1 with the end diameter and thickness as in this Example (1), but with the stretching and relaxation shrinkage as shown in Table 2. Accordingly, they have the same diameter (flat tube width) as in Example 1; the tube is cut into the same number of flat sheets as in Example 1, and subjected to the same tests as in Example 1; the test results are shown in Table 3.

Example 3 (Comparative). Film Oriented and Insufficiently Strongly Shrunk During Relaxation

(35) The first stage of production thereof is the extrusion of primary tube with the structure shown in Table 1 for Example 3, the diameter of 248 mm and the thickness of 639 μm. Subsequent operations are carried out as in Example 2.

(36) TABLE-US-00002 TABLE 2 Example Draw ratio Shrinkage during relaxation, % No. MD TD Surface MD TD Surface 1 2.85 3.2 9.12 30 30 51 2 2.85 3.2 9.12 10 40 46 3 c.e. .sup.1 2.85 3.5 9.98 20 20 36 4 c.e. 2.85 3.2 9.12 30 30 51 5 c.e. 2 3.5 7 5 30 33.5 6 c.e. .sup.a no stretching no relaxation 6 c.e. .sup.b no stretching no relaxation 7 4 3.5 14 40 30 58 .sup.1 c.e.—comparative (example) .sup.a, b films differ in the mean thickness - 100 and 150 μm, respectively. .sup.c Surface shrinkage during relaxation a.sub.s in % is calculated as follows α.sub.s = 100 (1 − (1 − α.sub.md/100) × (1 − α.sub.td/100)) = α.sub.md + α.sub.td − α.sub.md × α.sub.td/100, where α.sub.md and α.sub.td is linear shrinkage during relaxation in the machine direction and in the transverse direction, respectively.

Example 4 (Comparative). Film Containing Only High-Melting-Point Polyamides

(37) The first stage of production thereof is the extrusion of the primary tube with the structure shown in Table 1 for Example 4 [the composition of the polyamide layers is shown in square brackets] and the geometric parameter as in Example 1. Subsequent operations are carried out as in Example 1.

Example 5 (Comparative). Poorly Oriented Film

(38) The first stage of production thereof is the extrusion of primary tube is extruded with the structure shown in Example 5, Table 1, with the diameter of 283 mm and thickness of 466 μm. Subsequent operations are carried out as in the Example 2.

Example 6 (Comparative). Unoriented Films. Variants a and b

(39) Two film samples with the structure described in Example 1, the diameter of 695 mm and the thickness of 100 μm (Variant a) and 150 μm (Variant b) are prepared by the well-known method of blown tube coextrusion. The melted tubular extrudate is air-blown immediately after it emerges from the annular die to form a bubble with a diameter of about 700 mm, which is then cooled in a cold air flow and wound-up into a roll. The film thickness is varied by changing the overall capacity of the unit (with preservation of the capacity ratios of the separate extruders).

(40) Subsequent operations with the resulting tubular films are carried out an in Example 2.

Example 7. Flat-Die Extruded Films Subjected to Subsequent Tenter Stretching and Relaxation

(41) Nine compositions shown in Table 1 for Examples 2, 5 and 7, are loaded, melted and mixed in nine extruders operating at different capacities. Then the melts are fed to a nine-layer die, where a nine-layer melt extrudate is formed. Next the material is extruded through a flat die 2,000 mm wide with a gap of 1.1 mm (1,100 μm) to form the melted primary sheet transferred to the cooling shaft at the temperature of 20° C., where the sheet is quenched down to the temperature of 25° C. in its transition from the melted state to the solid state. The thickness and width of the primary sheet is controlled by adjustment of the rotational speed of the first pair of active nip rolls, while maintaining the overall productivity of the extruder constant. Then the primary sheet 1,160-1,161 mm wide and 588 μm thick is reheated with preheated shafts to the temperature of 85° C. and transferred, through a system of rolls, to the tenter frame for simultaneous biaxial stretching and simultaneous biaxial relaxation. The tenter frame is used for the sheet stretching in the MD and TD directions with the draw ratios of 3.5 and 4, respectively. Then the stretched sheet is transferred, together with the clamps, to the heating zone through a system of rolls. There the sheet is heated to a temperature of 145-150° C. and subjected, by mutual convergence of the clamps in two directions, to controlled relaxation and shrinkage in the machine direction and in the transverse direction by 40% and 30%, respectively. After quenching with an air flow, the clamp-deformed edges of the prepared 100 μm thick sheet are trimmed, and the sheet proper is cut into 9 strips each about 357 mm wide, which are then wound-up into a roll.

(42) Further tests of these films are carried out as in Example 1.

(43) TABLE-US-00003 TABLE 3 Shrinkage, %.sup.2 Tray Minimum Percentage Average in water in air at forming thickness of chicken film at 90° C., 100° C., reject of the packages Film of thickness.sup.1, Max. 10 s 10 s rate, %.sup.3 chicken damaged in Example No. mm thinning MD TD MD TD frankfurters chickens tray, μm transportation, %.sup.4 1 100 4.4 1 1 1 1 1 0 35 0 2 100 4 1 2 0 1 1 0 39 0 3 c.e. 100 3.8 15 16 13 14 10 2 41 0 4 c.e..sup.5 100 4.2 1 1.5 0 0 25 8 37 0 5 c.e. 100 4.7 6 0 2 0 38 11 33 15 6 c.e..sup.a 100 5.2 0 0 0 0 0 0 30 48 6 c.e..sup.b 150 4.9 0 0 0 0 0 0 47 1 7 100 4 1 0 0 0 2 0 39 0 .sup.1The thickness is roughly equal to the mean thickness in the unstrained areas .sup.2MD and TD—machine direction and transverse direction of the film .sup.3Percentage of ruptured trays .sup.4Percentage relative to the number of originally intact trays .sup.5Trays and containers from Comparative Example 4 are significantly deformed.

(44) Tables 1, 2 and 3 show that the films made by the invented method can be thermoformed by deep drawing under the severe conditions practically free of ruptures, in contrast to the prior art oriented films. The invented films are not prone to excessive thinning in certain areas, as confirmed by the film maximum thinning values (not more than 4.5) in the formed products (soft trays or containers), and demonstrate a very low shrinkage (less than 3%) at the forming temperature, which provides for preservation of the shape preset at fabrication. The invented film is suitable, despite the reduced original thickness, for fabrication of mechanically strong products, which, notwithstanding the smaller mean thickness of the original film, display a greater mechanical strength than the unoriented films traditionally used for the purpose.