MULTI-LAYER RESEALABLE TAMPER-EVIDENT FILM FOR PACKAGING

Abstract

A multilayer film comprising, in this order: (a) a biaxially oriented polyethylene terephthalate layer, (b) a pressure sensitive adhesive layer, (c) an elastic polyurethane dispersion, and (d) a biaxially oriented polyethylene terephthalate layer.

Layer (d) preferably has the structure A:B:A:C; where A and B preferably comprise crystalline PET and antiblocking particles and C is preferably an amorphous, heat-sealable copolyester.

Claims

1. A multilayer film comprising, in this order: (e) a biaxially oriented polyethylene terephthalate layer, (f) a pressure sensitive adhesive layer, (g) an elastic polyurethane layer, and (h) a biaxially oriented polyethylene terephthalate layer.

2. The film of claim 1, wherein (d) has the structure A:B:A:C; A and B comprise crystalline PET and antiblocking particles and C is an amorphous, heat-sealable copolyester.

3. The film of claim 2, wherein the amorphous, heat-sealable copolyester is an ethylene terephthalate copolymer.

4. The film of claim 3, wherein C is a copolymer of (i) dimethyl terephthalate or terephthalic acid with one or more of azelaic acid, dimethyl azelate, dimethyl sebacate, sebacic acid, isophthalic acid, or 5-sodiumsulfoisophthalic acid, or (ii) of dimethyl terephthalate or terephthalic acid with ethylene glycol, diethylene glycol and/or cyclohexanedimethyl glycol.

5. The film of claim 2, wherein the antiblocking particles are silica, calcium carbonate, glass beads, kaolin, or a mixture of at least two thereof.

6. The film of claim 1, wherein (d) has been biaxially stretched TD/MD 300-400%, at 226-238° C.

7. The film of claim 2, wherein layers (A) and (B) together have a thickness of 2-100 μm, and layer (C) has a thickness of 0.1-10 μm, and layer (d) has a thickness of 2-110 μm.

8. The film of claim 7, wherein layer (d) has a thickness of 8 μm.

9. The film of claim 1, wherein layer (b) comprises at least one styrene block copolymer of at least one styrene monomer and isoprene, butadiene, butylene or a mixture thereof.

10. The film of claim 9, wherein layer (b) comprises a styrene block copolymer of at least one styrene monomer and isoprene forming an SIS block copolymer.

11. The film of claim 9, wherein layer (b) comprises 40-85% of styrene block copolymer and 10-35% of a compatible tackifying resin having a softening temperature of 5-150° C.

12. The film according to claim 11, wherein the pressure sensitive adhesive has a viscosity, at a temperature of at east 130° C., lying within a range located above the power curve η=22 000×(dy/dt+200).sup.−0.82 wherein dy/dt comprises a shear rate between 100 and 1,000 s.sup.−1, a tense strength at a pull rate of 1 ms.sup.−1 lying within the range located below the polynomial curve y=−2.82×10.sup.−16 x.sup.6+5.92×10.sup.−13 x.sup.5−4.97×10.sup.−10 x.sup.4+2.15×10.sup.−7 x.sup.3−4.99×10.sup.−5 x.sup.2+6.26×10.sup.−3 x+4.71×10.sup.−2, wherein y comprises an ordinate representing the stress expressed in MPa and x comprises an abscissa representing the deformation expressed in %; and an elastic modulus G′<5×10.sup.5 Pa at −20 to +40° C.

13. The film according to claim 1, further comprising between layer (a) and layer (b), a layer (a′) of an additional copolyester.

14. The film according to claim 2, wherein layer (C) comprises an anti-fog agent.

15. A container having a resealable lid, wherein said lid is a film according to claim 1.

16. A process for the preparation of a film according to claim 1, comprising coextrusion of layer (d), heating to above glass transition temperature, biaxially stretching the layer, gravure coating of the layer (c) polyurethane dispersion on layer (d), and extrusion laminating layer (d) to layer (a) with layer (b) PSA as the adhesive.

17. The process according to claim 16, wherein layer (d) is biaxially stretched at 238° C.

18. The process according to claim 16, further comprising (i) a corona pretreatment of layer (a) prior to supply of layer (a) to an extrusion lamination apparatus, (ii) a bump corona treatment where a corona treater is present on the production line and effects treatment prior to application of adhesive and lamination, or a combination of (i) and (ii).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 shows the lower viscosity limit of the PSA in terms of viscosity (Pas) versus shear rate (1/s) for an embodiment of the invention.

[0036] FIG. 2 shows the upper limit of tensile strength of the PSA for a pull rate of 1 m/s in terms of stress versus percent deformation for an embodiment of the invention.

[0037] FIG. 3 is a representation of a closed container having the film of the invention as a sealing member.

[0038] FIG. 4 shows a container having the film of the invention as a sealing member, after opening.

[0039] FIG. 5 shows an extrusion lamination process used to prepare the films.

[0040] The multilayer film of the invention can be produced by coextrusion of the layer (d) BOPET, separate coextrusion of layer (a) BOPET, gravure coating of the layer (c) polyurethane dispersion on layer (d) BOPET, and extrusion laminating layer (d) BOPET to layer (a) BOPET with layer (b) PSA as the adhesive. In extrusion lamination, as shown in FIG. 5, an adhesive (54) (in this case, PSA layer (b), the styrene block copolymer), is extruded onto a moving substrate (in this case, BOPET layers (d) and (a), represented by (51) and (52)) through a flat die (53). The layer (c) polyurethane dispersion can be coated in-line with the extrusion lamination, upstream of the flat die (53), or can be coated off-line so that it is already on BOPET layer (d) when fed to the extrusion lamination process (as depicted in FIG. 5). At least the side of the layer (a) BOPET that will contact the (b) PSA can be corona treated in-line with the extrusion lamination, upstream of the flat die (53), to increase the adhesion of the (b) PSA to the layer (a) BOPET. The polymer melt exits the die typically at a high temperature, typically 150 to 330° C., preferably in an embodiment 150 to 190° C. After exiting the die the polymer melt is oxidized when it comes into contact with air over a distance referred to as the air gap. This distance can be optimized for each resin, with a typical range being 5 to 10 inches. Increasing the air gap may improve adhesion through longer oxidation time; however, too high of an air gap will result in lower adhesion from excessive cooling of the polymer. When the melt exits the die the melt film is pulled down into a nip (55) between two rollers (56) and (57), the pressure roll and the chill roll, respectively, situated below the die. The substrates, moving at a velocity which is higher than that of the melt film, draw the film to the required thickness. The pressure between the two rolls forces the film onto the substrates. Further, the film is cooled and solidified by the low temperature of the chill roll, typically at around 50 to 85° F. The draw-down ratio, which is one of the characteristic parameters of the extrusion coating process, is the ratio of the die gap to the thickness of the polymer film on the substrate. A typical draw-down ratio is 20-60. The laminated film is then conveyed through various additional rollers, represented by (58) and (59) and collected on a final roll (60). A representative extrusion coating process is given, for instance, in Crystalline Olefin Polymers, Part II, by R.A.V. Raff and K. W. Doak (Interscience Publishers, 1964), pages 478 to 484, or Plastics Processing Data Handbook, by Dominick V. Rosato (Chapman & Hall, 1997), pages 273 to 277.

[0041] The films of the invention provide the advantage of being capable of forming a resealable lidding for a container, which lidding can comprise the entire surface closure of the container, without the need for die-cutting. Moreover, the presence of the internal polyurethane layer (c), rather than functioning as a haptic or “soft touch” coating intended to modify the surface feel of the film, provides a tamper-evident feature, as when the lid is opened for the first time, the film transitions from clear to cloudy. Layer (c) may further include a dye to enhance this effect.

[0042] A container in accordance with the invention is represented by FIGS. 3 and 4. FIG. 3 shows the sealed container before opening (and after resealing). In FIG. 3, the multilayer film of the invention is represented by (30), made up of the printable BOPET (31), which is adhered by PSA (32) to the elastic polyurethane layer (33), adjacent to the heat sealable BOPET (34) which is heat-sealed to the lip (36) of container (35). In FIG. 4, the container has been opened by peeling back the lid which is represented by (40), and printable BOPET (41), PSA (42), polyurethane (43) and BOPET (44). The portion of the film heat-sealed to the lip (46) of the container (45) contains the remaining portion of the lid which stays adhered thereto, tamper-evident polyurethane (431), and BOPET (441).

ASPECTS OF THE INVENTION

[0043] 1. A multilayer film comprising, in this order: [0044] (a) a biaxially oriented polyethylene terephthalate layer, [0045] (b) a pressure sensitive adhesive layer, [0046] (c) an elastic polyurethane layer, and [0047] (d) a biaxially oriented polyethylene terephthalate layer. [0048] 2. The film of aspect 1, wherein (d) has the structure A:B:A:C; A and B comprise crystalline PET and antiblocking particles and C is an amorphous, heat-sealable copolyester. [0049] 3. The film of aspect 2, wherein the amorphous, heat-sealable copolyester is an ethylene terephthalate copolymer. [0050] 4. The film of aspects 2 or 3, wherein C is a copolymer of (i) dimethyl terephthalate or terephthalic acid with one or more of azelaic acid, dimethyl azelate, dimethyl sebacate, sebacic acid, isophthalic acid, or 5-sodiumsulfoisophthalic acid, or (ii) of dimethyl terephthalate or terephthalic acid with ethylene glycol, diethylene glycol and/or cyclohexanedimethyl glycol. [0051] 5. The film of any of aspects 2-4, wherein the antiblocking particles are silica, calcium carbonate, glass beads, kaolin, or a mixture of at least two thereof. [0052] 6. The film of any of aspects 1-5, wherein (d) has been biaxially stretched TD/MD 300-400%, at 226-238° C. [0053] 7. The film of any of aspects 2-6, wherein layers (A) and (B) together have a thickness of 2-100 urn, and layer (C) has a thickness of 0.1-10 μm, and layer (d) has a thickness of 2-110 μm. [0054] 8. The film of any of aspects 1-7, wherein layer (d) has a thickness of 8 urn. [0055] 9. The film of any of aspects 1-8, wherein layer (b) comprises at least one styrene block copolymer of at least one styrene monomer and isoprene, butadiene, butylene or a mixture thereof. [0056] 10. The film of aspect 9, wherein layer (b) comprises a styrene block copolymer of at least one styrene monomer and isoprene forming an SIS block copolymer. [0057] 11. The film of any of aspects 1-10, wherein layer (b) comprises 40-85% of styrene block copolymer and 10-35% of a compatible tackifying resin having a softening temperature of 5-150° C. [0058] 12. The film according to any of aspects 1-11, wherein the pressure sensitive adhesive has a viscosity, at a temperature of at least 130° C., lying within a range located above the power curve η=22 000×(dy/dt+200).sup.−0.82 wherein dy/dt comprises a shear rate between 100 and 1,000 s.sup.−1, a tensile strength at a poll rate of 1 ms.sup.−1 lying within the range located below the polynomial curve y=−2.82×10.sup.−16 x.sup.5+5.92×10.sup.−13 x.sup.5−4.97×10.sup.−10 x.sup.4+2.15×10.sup.−7 x.sup.3−4.99×10.sup.−5 x.sup.2+6.26×10.sup.−3 x+4.71×10.sup.−2, wherein y comprises an ordinate representing the stress expressed in MPa and x comprises an abscissa representing the deformation expressed in %; and an elastic modulus G′<5×10.sup.5 Pa at −20 to +40° C. [0059] 13. The film according to any of aspects 1-12, further comprising between layer (a) and layer (b), a layer (a′) of an additional copolyester. [0060] 14. The film according to any of aspects 2-13, wherein layer (C) comprises an anti-fog agent. [0061] 15. A container having a resealable lid, wherein said lid is a film according to any of aspects 1-14. [0062] 16. A process for the preparation of a film according to any of aspects 1-14, comprising coextrusion of layer (d), heating to above glass transition temperature, biaxially stretching the layer, gravure coating of the layer (c) polyurethane dispersion on layer (d), and extrusion laminating layer (d) to layer (a) with layer (b) PSA as the adhesive. [0063] 17. The process according to aspect 16, wherein layer (d) is biaxially stretched at 238° C. [0064] 18. The process according to aspect 16 or 17, further comprising (i) a corona pretreatment of layer (a) prior to supply of layer (a) to an extrusion lamination apparatus, (ii) a bump corona treatment where a corona treater is present on the production line and effects treatment prior to application of adhesive and lamination, or a combination of (i) and (ii).

[0065] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0066] The entire disclosures of all applications, patents and publications, cited herein are incorporated by reference herein.

[0067] Unless explicitly noted otherwise, all percentage data for mixtures denote percent by weight, and relate to the corresponding mixture as a whole, comprising all solid or liquid components, without solvents. Furthermore, unless explicitly noted otherwise, all temperatures are indicated in in degrees Celsius (° C.). The following examples are intended to explain the invention without limiting it.

EXAMPLES

[0068] A series of film samples were created to test the suitability of the resulting film having this structure

##STR00001##

for a resealable lidding film for packaging containers.

Top Film Layer

[0069] Film A: 92 gauge (23.3 μm) multilayer biaxially oriented PET film with a Corona treated surface on one side (“Corona Side”) and a coextruded copolyester on the opposite side (“COEX Side”).

Middle Layer

[0070] PSA: Extrudable pressure sensitive adhesive.

Bottom Film Layer

[0071] Film B: 50 gauge (12.7 μm) multilayer biaxially oriented PET film with a coextruded copolyester heat sealable layer that includes a slip package on one side (“Heat Seal Side with Slip”) and a Corona treated surface on the opposite side (“Corona Side”).

[0072] Film C: 52 gauge (13.2 μm) multilayer biaxially oriented PET film with a haptic (“soft touch”) matte coating on one side (“Soft Touch Side”) and a coextruded copolyester heat sealable layer on the opposite side (“Coex Side”).

[0073] Film D: 48 gauge (12.2 μm) multilayer biaxially oriented PET film with a coextruded matte surface on one side (“Matte Side”) and a Corona treated surface on the opposite side (“Corona Side”).

[0074] Film E: 48 gauge (12.2 μm) multilayer biaxially oriented PET film that has been chemically treated on one side (“Chemically Treaded Side”) and a coextruded copolyester on the opposite side (“Coex Side”).

[0075] Film F: 48 gauge (12.2 μm) multilayer biaxially oriented PET film with a very smooth, high coefficient of friction surface on one side (“Smooth Side”) and a coextruded copolyester on the opposite side (“Coex Side”).

[0076] Film G: 53 gauge (13.5 μm) multilayer biaxially oriented PET film with a coextruded copolyester heat sealable layer on one side (“Heat Seal Side”) and a soft touch (haptic) matte coating on one side (“Soft Touch Side”).

[0077] Film H: 37 gauge (9.4 μm) multilayer biaxially oriented PET film with a coextruded copolyester heat sealable layer on one side (“Heat Seal Side”) and a Corona treated surface on the opposite side (“Corona Side”) with multidirectional ‘easy tear’ property.

[0078] Film I: 72 gauge (18.3 μm) multilayer biaxially oriented PET film that has a dead-fold property with a Corona treated surface on one side (“Corona Side”) and an untreated plain PET surface on the opposite side (“Plain Side”).

[0079] Film J: 56 gauge (14.2 μm) multilayer biaxially oriented PET film with multidirectional ‘easy-tear’ property as described in U.S. Pat. No. 7,943,230.

Samples

[0080] Film A was used for the Top Film Layer for all Samples, with the Corona Side of Film A oriented towards the Middle Layer at Interface 1 for the first eight (8) Samples and the COEX Side oriented towards the Middle Layer at Interface 1 for the next seventeen (17) Samples.

[0081] The same PSA was used for the Middle Layer, but with different thicknesses.

[0082] Films B-J were used for the Bottom Film Layer with different sides oriented towards the Middle Layer at Interface 2.

[0083] For each Sample, the Film for the Top Film Layer and the Film for the Bottom Film Layer were fed to a nip into which the PSA for the Middle Layer was extruded as illustrated in FIG. 5. Extrusion was at a temperature of 170° C., over an air gap to the nip of 7 inches, with the chill roll at 70° F.

[0084] Table I summarizes the combinations of films, film orientations, and middle layer thicknesses that were used to create the Samples.

TABLE-US-00001 TABLE I SUMMARY OF SAMPLES Side of Top Bottom Side of Bottom Layer @ Middle Layer Layer Layer @ Sample Interface 1 Thickness (μm) Film Interface 2 1 Corona 6.8 D Corona 2 Corona 6.8 D Matte 3 Corona 13.5 C Soft Touch 4 Corona 10.2 C Soft Touch 5 Corona 13.5 B Heat Seal with Slip 6 Corona 10.2 B Heat Seal with Slip 7 Corona 10.2 F Smooth 8 Corona 10.2 E Chemically Treated 9 COEX 10.2 E Chemically Treated 10 COEX 10.2 E Coex 11 COEX 10.2 D Corona 12 COEX 13.5 C Soft Touch 13 COEX 10.2 C Soft Touch 14 COEX 10.2 B Heat Seal 15 COEX 10.4 G Soft Touch 16 COEX 14.6 G Soft Touch 17 COEX 7.3 G Soft Touch 18 COEX 33.1 G Soft Touch 19 COEX 25.4 G Soft Touch 20 COEX 19.0 G Soft Touch 21 COEX 19.0 I Corona 22 COEX 19.0 J Plain PET 23 COEX 25.4 I Corona 24 COEX 15.6 H Corona 25 COEX 13.1 H Corona

[0085] Samples 1-14 were evaluated as to suitability for a resealable film (on container) application, as determined by Peel Strength, Haze, and Clarity. The tamper evident feature (changing from clear to cloudy when peeled and resealed for the first time) is provided by Samples 3, 4, 12 and 13 (those with the Soft Touch Coating at Interface 2). The Soft Touch Coating listed in Table 1 is a coating of layer (c), namely from an elastic polyurethane dispersion.

[0086] Samples 15-20 show the effect of the amount of PSA used in the middle layer on Peel Strength, Haze and Clarity.

[0087] Samples 21 and 23 show the effect of Film l's dead-fold property due to lack of biaxial orientation (only partially oriented to make it ‘twistable’) on the tear-ability of the bottom film layer.

[0088] Sample 22 shows the effect of Film J's tear-ability on the structure's performance as it is desirable for the bottom film layer to break cleanly upon the first opening.

[0089] Samples 24 and 25 show the effect of BOPET layer (d) as the bottom film layer, but without the haptic (soft touch) coating of layer (c).

Test Method for Peel Strength

[0090] Samples that did not have an outer heat sealable surface (samples 1-14) were coated (about 2 gsm coat weight) on the bottom film layer outer surface with a copolyester heat sealable resin dissolved in 1,3 dioxolane solvent and dried in an oven for 5 minutes. [0091] 1″ wide strips were cut from the film samples and from 0.5 mm APET tray stock. [0092] The heat sealable side of the film samples were heat sealed to the APET tray pieces at 200° C. with 1.5 second dwell time and 40 psi applied pressure using a Labthink Param Classic 513 Gradient Heat Sealer. [0093] The Peel Strength was measured using an MTS Insight 1 tensile device. The film was held in the upper jaw and the tray piece was held in the lower jaw. The sample was peeled at 50 mm/min at 180°. [0094] After peeling, the film was removed from jaws and resealed by pressing the film back onto the tray piece with the tester's thumb 4 times. [0095] Then the sample was re-peeled using the same method. [0096] This was repeated for 4 peels.

[0097] The Peel Strength testing results are summarized in Table II below.

TABLE-US-00002 TABLE II SUMMARY OF PEEL STRENGTH TESTING 1.sup.st Peel 2.sup.nd Peel 3.sup.rd Peel 4.sup.th Peel Strength Strength Strength Strength Sample (gf/in) (gf/in) (gf/in) (gf/in) 1 504 0 0 0 2 562 432 378 207 3 1384 590 430 323 4 1466 999 810 269 5 1356 444 390 409 6 748 249 246 209 7 708 502 315 286 8 785 393 351 302 9 1495 713 523 506 10 1433 724 472 387 11 659 0 0 0 12 1449 628 480 434 13 1347 1253 668 681 14 751 369 413 283 15 1288 517 375 350 16 1558 550 372 332 17 1137 267 173 169 18 1816 675 566 428 19 1560 623 565 402 20 1446 757 546 380 21 1557 697 592 523 22 906 0 0 0 23 1683 708 629 564 24 1371 683 544 544 25 1049 485 401 37

[0098] It is desirable for the 1.sup.st Peel Strength to be greater than 1140 gf/in for adhesive failure and for the 2.sup.nd Peel Strength to be greater than 285 gf/in for adhesive failure.

[0099] The Samples were also tested for Haze and Clarity. Haze was tested using a BYK Gardner haze-gard plus following ASTM method D 1003. The same device was used to measure Clarity. The Haze and Clarity testing results are summarized in Table III below.

TABLE-US-00003 TABLE III SUMMARY OF HAZE & CLARITY TESTING Sample Haze(%) Clarity (%) 1 41.6 68.4 2 45.3 73.7 3 Not Tested Not Tested 4 14.6 85.4 5 Not Tested Not Tested 6 9.5 88.6 7 8.3 86.2 8 6.9 87.0 9 7.9 89.9 10 6.2 91.4 11 41.4 69.4 12 Not Tested Not Tested 13 10.6 90.3 14 9.9 90.7 15 10.2 83.0 16 9.1 83.7 17 13.8 81.6 18 9.0 83.9 19 8.8 84.2 20 8.7 83.8 21 6.9 90.0 22 5.9 90.3 23 6.9 90.1 24 Not Tested Not Tested 25 Not Tested Not Tested
It is desirable to have a Haze value of ≤13.0% and a Clarity value of ≥88.2%