Method of making laminates having reduced oxygen permeability

Abstract

Laminates of polymeric films and solvent-based polyurethane adhesive formulations for preparing them are provided. The adhesive formulations include a hydroxyl-terminated polyester that forms crystalline polyester domains after reaction with an appropriate polyisocyanate, but prior to completion of cure. The result is an adhesive layer that substantially enhances the oxygen barrier properties of the adhesive and, therefore, of the laminate as a whole, while offering desirable convenience of application even at relatively low temperatures. The laminates may also exhibit desirable retention of barrier properties following flex-cracking.

Claims

1. A method of making a laminate, comprising (a) preparing an adhesive mixture by (i) providing a single species of polyisocyanate (A) as an A-component; (ii) also providing a hydroxyl-terminated polyester (B), formed from a single species of a linear aliphatic diol having terminal hydroxyl groups and from 2 to 10 carbon atoms, and a single species of a linear dicarboxylic acid, the polyester having a number average molecular weight from 300 to 5000, and being solid at 25 C., and having a melting point of 80 C. or below, the hydroxyl-terminated polyester (B) being incorporated as substantially miscible solids in a carrier solvent in an amount of at least 20 percent by weight, based on the weight of (B) and the carrier solvent, to form a B-component, wherein the carrier solvent comprises a non-protonated solvent; (b) either (i) mixing the A-component and the B-component at an NCO/OH ratio from 1 to 2 to form an adhesive mixture (I), or (ii) reacting all or a portion of the A-component and a portion of the B-component at an NCO/OH ratio of from 2 to 8 to form a prepolymer (C) and then mixing the remaining portion of the B-component and any remaining portion of the A-component with the prepolymer (C) to form an adhesive mixture (II) having an NCO/OH ratio from 1 to 2; (c) applying a layer of at least one of the adhesive mixtures (I) and (II) to a first polymeric film; the adhesive mixture (I) or (II) having been prepared closely prior to applying the layer to the first polymeric film; (d) positioning the second polymeric film proximal to the layer and distal to the first polymeric film, such that the layer is between the first and second polymeric films; and (e) allowing the adhesive mixture (I) or (II) to fully react, at a temperature of 50 C. or higher, and then to cure under conditions such that crystalline polyester domains are formed prior to completion of cure; such that a laminate is formed.

2. The method of claim 1 wherein the solids are present in an amount of from 20 percent to 80 percent, based on the weight of the A-component and the B-component combined.

3. The method of claim 1 wherein the hydroxyl-terminated polyester is formed from a C3-C6 diol and a dicarboxylic acid selected from the group consisting of adipic acid, azelaic acid, sebacic acid, and combinations thereof.

4. The method of claim 1 wherein the polyisocyanate is selected from the group consisting of polymeric hexamethylene diiocyanate (HDI trimer isocyanurate), methylene diphenyl diisocyanate, dicyclohexylmethane 4,4-diisocyanate, and toluene diisocyanate.

5. The method of claim 1 wherein the carrier solvent is selected from the group consisting of ethyl acetate, methyl ethylketone, dioxolane, acetone, and combinations thereof.

6. The method of claim 1 wherein an acrylate viscosity modifier is included in the B-component prior to mixing the A-component and the B-component together.

7. The method of claim 1 wherein the conditions of the reaction and cure include a temperature of less than 30 C. and a time of at least 7 days.

8. The method of claim 1 wherein the first polymeric film and the second polymeric film are independently selected from the group consisting of metalized, non-metalized, aluminum oxide-coated and silicon oxide-coated films of low density polyethylene, oriented polypropylene, polyester, polyamide, polyethylene terephthalate, cellophane, and combinations thereof.

9. The method of claim 1 wherein, under ASTM F392/F392M flex-crack testing, the laminate shows a rate of oxygen transmission after flex-crack that is at least 60 percent of the rate before flex-crack.

Description

EXAMPLES 1-2 AND COMPARATIVE EXAMPLES A-F

(1) Solvent-based adhesive formulations representing Examples (Ex.) 1-2 are prepared including the components shown in Table 1. Comparative Examples (CEx.) A-F are also prepared as shown in Table 1. Identifications of components are as follows: BESTER 86 is a crystalline polyester that is a solid at 25 C. and has a melting point <80 C. It is available from Rohm and Haas Italia. MOR-FREE C79 is a hydroxyl-containing component and is a blend of materials including castor oil (an ester but not a polyester), and no crystalline polyester. It is part of a solventless, two-component polyurethane adhesive system available from The Dow Chemical Company. ADCOTE 301A+350A is a solvent-based, two-component polyurethane adhesive system available from The Dow Chemical Company. The hydroxyl-containing component is based on polyethers. MOR-FREE 200C is an HDI based isocyanurate that can be used as part of a solventless or solvent-based, two-component polyurethane adhesive system available from The Dow Chemical Company. MOR-FREE 698A is an MDI based prepolymer containing a non-crystalline polyester. It is part of a solventless, two-component polyurethane adhesive system available from The Dow Chemical Company. MODAFLOW RESIN is a flow modifier containing an acrylic polymer (>99.0%), available from Cytec Industries.

(2) The formulations are prepared by mixing the viscosity modifier into the hydroxyl-containing component, and then mixing the hydroxyl-containing component and the polyisocyanate component in a method known to those skilled in the art as a one-shot polyurethane method. This is carried out at a temperature of 25 C. under ambient pressure, using ULTRA-TURAX equipment for mixing.

(3) TABLE-US-00001 TABLE 1 Component OH NCO Viscosity Solids (% Mix Ratio (%, OH Component Component Modifier by weight) component to Ex. or CEx. (w/w) (w/w) (w/w) in Solvent NCO component) Ex. 1 BESTER 86, MOR-FREE MODAFLOW, 40% in 100:60 95.2% 200C 4.8% ethyl acetate Ex. 2 BESTER 86, MOR-FREE MODAFLOW, 40% in 100:60 95.2% 200C 4.8% ethyl acetate CEx. A BESTER 86, MOR-FREE MODAFLOW, n/a 100:60 95.2% 200C 4.8% CEx. B BESTER 86, MOR-FREE MODAFLOW, n/a 100:60 95.2% 200C 4.8% CEx. C BESTER 86, MOR-FREE MODAFLOW, n/a 100:60 95.2% 200C 4.8% CEx. D C79, 33.3% MOR-FREE n/a n/a 100:50 698A CEx. E C79, 33.3% MOR-FREE n/a n/a 100:50 698A CEx. F ADCOTE ADCOTE n/a 40% in 60:40 301A, 60% 350A, 40% ethyl acetate n/a means not applicable, i.e., not included in that formulation

(4) The combined materials, now termed the adhesive mixture composition, are then quickly fed into the tray of a NORDMECANNICA LABO COMBI 400 laminator for use in laminating together two polymer films. Using a typical doctor blade, the adhesive mixture is applied to the upward-facing surface of a first polymer film, denominated the carrier web; a second polymer film, denominated the secondary web, is then placed on top of the adhesive layer; and the two films are nipped together. The thickness of the adhesive layer is determined, pre-nip and as an average, by taking a presized sample of the laminate and weighing it first with the adhesive on it, and then weighing it again after wiping all of the adhesive off with a solvent-wet cloth. The resulting laminate is rolled onto a reel. Coating weight is in excess of 2 g.sup.2/m; nip roll temperature is greater than or equal to 50 C., most preferably greater or equal to 80 C.; film temperature of the secondary web is greater than or equal to 25 C., preferably greater or equal to 40 C.; and curing temperature is greater than 15 C. but less than 30 C.

(5) Testing is then carried out for bond strength following 7 days cure time, as well as oxygen transmission rate. Testing protocol for OTR corresponds to ASTM D 3985. Results of the testing are then recorded in Table 2.

(6) TABLE-US-00002 TABLE 2 Adhesive Bond Strength, Carrier Secondary Nip T. thickness 7 days ***OTR Web Web [ C.] [m] [N/15 mm] [cm.sup.3O.sub.2/m.sup.2/day] Ex. 1 PETP LDPE 90 3.54 Tear 28.3 Ex. 2 PETP LDPE 60 3.11 Tear 56.3 C. Ex. A Met oPP oPP 90 2.7 m. transf.* 2.32 CEx. B PETP LDPE 90 2.63 2.8 34.0 CEx. C oPP LDPE 90 2.59 d. coex** 73.1 CEx. D PETP LDPE 45 2.60 5.5 93.4 CEx. E oPP LDPE 45 2.45 d. coex 9886.5 CEx. F PETP LDPE 50 3.59 Tear 114.40 *m. transf. = metal transfer **d.coex = delamination of the coextruded film ***OTR = oxygen transmission rate

(7) The results show that the Examples 1 and 2 show significant improvements in OTR when compared to the same web combination under the same or similar nip temperature (CEx. B and CEx. F). The solvent-based inventive adhesive, as shown in Examples 1 and 2, does not require preheating of either the carrier film or of the adhesive prior to application and the resulting laminate shows generally better bond strengths.

Examples 3-6

(8) Flex-crack testing of a variety of samples is carried out under a protocol similar to ASTM F392/F392M. Results are shown in Table 3.

(9) TABLE-US-00003 TABLE 3 OTR before flex- OTR after flex- Example Carrier Secondary cracking cracking No. Web Web [cm.sup.3O.sub.2/m.sup.2/day] [cm.sup.3O.sub.2/m.sup.2/day] Ex. 3 Met oPP oPP 20 Up to 70 Ex. 4 Met oPP PETP 1.25 1.2 Ex. 5 Met oPP oPP 1.8 3 Ex. 6 Met oPP Paper 3.5 12.95