WO1992017531A1

WO1992017531A1 - Polyester film coated with metal adhesion promoting copolyester

Info

Publication number: WO1992017531A1
Application number: PCT/US1992/002139
Authority: WO
Inventors: Richard Paul Pankratz
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1991-03-29
Filing date: 1992-03-25
Publication date: 1992-10-15

Abstract

An oriented polyester film coated on one or both sides with a copolyester derived from the condensation of isophthalic acid, terephthalic acid, an aliphatic dicarboxylic acid and a copolymerizable alkylene glycol and having a glass transition temperature (Tg) of at least 30 °C. The coated film provides excellent adhesion to vapor deposited metal coatings.

Description

TITLE

POLYESTER FILM COATED WITH METAL ADHESION PROMOTING COPOLYESTER

.

BACKGROUND OF THE INVENTION

The present invention relates to an oriented polyester film coated on one or both sides with a copolyester primer coating for improving the adhesion of the polyester film to metals applied by vapor

deposition.

Metallic coated polyester films have utility in various packaging and solar control applications. The commercial acceptance of such metal coated films, however, depends on obtaining good adhesion between the metallic layer and the polyester film.

One approach for improving the adhesion of the vapor deposited metal layer to the polyester film is disclosed in U.S. 4,476,189 issued to Posey et al. on October 9, 1984. U.S. 4,476,189 discloses a polyester film coated with a water-dispersible copolyester primer composition comprising terephthalic acid, an aliphatic dicarboxylic acid, and an aromatic sulfonated monomer with an alkylene glycol. Such primer compositions containing an aromatic sulfonated monomer provide inferior wet adhesion properties when used to bond polyester film to a vapor deposited metal. Moreover, such primer coated polyester film compositions are not recyclable for use in food contact applications because the sulfonated monomer is not FDA approved.

Accordingly, it is an object of this invention to provide an oriented polyester film having enhanced adhesion to a vapor deposited metal for use in packaging applications.

Another object of this invention is to provide a copolyester coating composition for biaxially oriented polyethylene terephthalate film which renders the film more adhesive to vapor deposited metals.

SUMMARY OF THE INVENTION The present invention relates to an oriented polyester film coated on one or both sides with a continuous copolyester coating consisting essentially of the condensation product of a diacid component selected from the group consisting of isophthalic acid,

terephthalic acid and an aliphatic dicarboxylic acid of the formula HOOC (CH₂)_nCOOH wherein n is 2 to 11, the total amount of the diacid component being 50 mole %; and a copolymerizable aliphatic or cycloaliphatic alkylene glycol component having from 2 to 11 carbon atoms, the total amount of the glycol component being 50 mole %; wherein said copolyester has a glass transition temperature (Tg) of at least 30°C but not higher than a temperature which is 100°C lower than the melting point of the copolyester, said film having improved adhesion to a further metal coating applied by vapor deposition.

DETAILED DESCRIPTION OF THE INVENTION

The polyester films for use in the invention include polyethylene terephthalate (PET), polyethylene-2,6-naphthalate and poly-1,4-cyclohexylenedimethylene terephthalate. Of these PET is most preferred.

Copolyesters predominantly containing ethylene

terephthalate units can also be used, which copolyesters may contain a minor proportion, preferable not larger than 5 mole %, of dicarboxylic acid components other than terephthalic acid, such as isophthalic acid, sebacic acid and adipic acid, and which may also contain a minor proportion, preferably not larger than 5 mole %, of diol components other than ethylene glycol, such as butanediol, diethylene glycol, triethylene glycol and 1,4-cyclohexanedimethanol. Such polyester films are made by procedures that are well-known in the art.

For example, the polyester resin may be melted and extruded as an amorphous sheet onto a polished revolving casting drum to form a cast sheet of polymer. Then, the cast sheet may be axially stretched in one direction, either in the direction of extrusion (longitudinal direction) or perpendicular to the direction of

extrusion (transverse direction) in the case of

monoaxially oriented film and in two directions in the case of biaxially oriented film, i.e., the film is stretched in both the longitudinal and transverse directions either consecutively or simultaneously. The first stretching step of the cast sheet may be in either of these two orthogonal directions. The amount of stretching, necessary to impart strength and toughness to the film, can range from 3.0 to 5.0 times the

original cast sheet dimensions in one or both

directions. Preferably, the amount of stretching ranges from about 3.2 to 4.2 times the original cast sheet dimensions. The stretching operations are carried out at temperatures ranging from about the second order transition temperature to below the temperature at which the polymer softens and melts.

If necessary, the film can be heat treated, after stretching, for a period of time sufficient to

crystallize the polyester film. Crystallization imparts stability and good tensile properties to the film. When polyethylene terephthalate is heat treated, it is subjected to a temperature ranging from 190°C to 240°C and, more preferably, a temperature ranging from 215°C to 235°C.

The coating copolyesters of the invention are produced by polycondensing a diacid component selected from the group consisting of terephthalic acid,

isophthalic acid and an aliphatic dicarboxylic acid having the formula HOOC (CH₂)_nCOOH wherein n is 2 to 11 with a glycol component selected from the group

consisting of aliphatic and cycloaliphatic alkylene glycols having from 2 to 11 carbon atoms. The total amount of diacid used should be substantially equivalent to the total amount of glycol used on a molar basis, i.e., 50 mole % diacid to 50 mole % glycol.

Preferred copolyesters contain from 0 to 45 mole % isophthalic acid, from 40 to 85 mole % terephthalic acid, from 0 to 30 mole % of an aliphatic dicarboxylic acid, from 30 to 100 mole % of ethylene glycol and from 0 to 70 mole % of an aliphatic or cycloaliphatic

alkylene glycol.

Representative aliphatic dicarboxylic acids used in the invention copolyesters include adipic, succinic, malonic, maleic, fumaric, azelaic, glutaric, sebacic, suberic, itaconic and dodecanedioic acids. Azelaic acid is preferred.

Examples of aliphatic or cycloaliphatic alkyleneglycols used in the invention copolyesters include ethylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-cyclohexanediol and 1,10-decanediol. Ethylene glycol is preferred. It has unexpectedly been found that when relative molar proportions of terephthalic acid, isophthalic acid, aliphatic dicarboxylic acid and alkylene glycol components are selected within limits that provide a copolyester glass transition temperature (Tg) of at least 30°C, a copolyester primed polyester film having excellent adhesion to a vapor deposited metal can be obtained. As the Tg increases above 30°C the adhesion to vapor deposited metal is dramatically improved;

whereas with copolyester Tg's below 30°C the adhesion to vapor deposited metal is markedly decreased.

Copolyesters having the specified diacid and glycol components and glass transition temperatures ranging from 30°C up to 100°C below the melting point of the copolyester when used in making the coated primer film of the invention provide excellent adhesion to vapor deposited metal.

Preferred coating copolyesters of the invention include a copolyester derived from 85 mole %

terephthalic acid, 15 mole % isophthalic acid, 58 mole % ethylene glycol and 42 mole % neopentyl glycol having a Tg of 74°C; a copolyester derived from 80 mole %

terephthalic acid, 20 mole % isophthalic acid and 100 mole % ethylene glycol having a Tg of 70°C; a

copolyester derived from 80 mole % terephthalic acid, 20 mole % azelaic acid, 35 mole % ethylene glycol and 65 mole % neopentyl glycol having a Tg of 66°C; a

copolyester derived from 41 mole % terephthalic acid, 41 mole % isophthalic acid, 18 mole % azelaic acid and 100 mole % ethylene glycol having a Tg of 39°C; and a copolyester derived from 40 mole % terephthalic acid, 40 mole % isophthalic acid, 10 mole % adipic acid, 10 mole % azelaic acid and 100 mole % ethylene glycol having a Tg of 30°C. In the above numerical values the acid mole percent is on the basis of total diacid while the glycol mole percent is on the basis of total glycol.

The copolyester coating of this invention may be applied in-line as a solvent solution or an aqueous dispersion at any stage during polyester film

manufacture. The preparation of such coating solutions or dispersions is well-known in the art, for example, as disclosed in U.S. 4,888,381 and U.S. 3,054,703. The coating may be applied prior to stretching at a point after casting of the amorphous sheet and before uniaxial or biaxial stretching, or subsequent to the uniaxial stretching but prior to biaxial stretching. The heat applied to the coated film during the stretching stages is usually sufficient to evaporate the volatiles and dry the coating. The coating may also be applied off-line subsequent to biaxial stretching of the film, which necessitates a separate drying step.

The solvent or aqueous coating composition may be applied to the polyester film by any of the well-known techniques used in polyester film manufacture, e.g., by roller coating, spray coating, slot coating, immersion coating or, preferably, gravure roller coating.

Moreover, the polyester film surface can be subjected to a corona discharge treatment prior to coating which permits the coating to more easily wet the surface, thereby improving the adhesion of the coating to the polyester film surface.

The solvent or aqueous coating composition may be applied to one or both sides of the polyester film, or it may be applied to one side and a different coating applied to the opposite side.

In some cases, from 5 to 10% of an isocyanate crosslinking agent may be added to the coating

composition to harden the coating, thereby increasing the Tg and further improving adhesion to the vapor deposited metal. The coating composition may also include antioxidants, dyes, pigments, lubricants, antiblocking agents and/or slip agents, e.g., talc, metal oxides, calcium carbonate, silica, titanium dioxide, silicates, aluminates and aluminosilicates.

The coating composition may be applied to the polyester film to produce a final dry coating thickness ranging from 1 × 10^-6 to 5 × 10^-4 inch, which corresponds to a dry coating weight ranging from 0.03 to 15 g/m².

Polyester film which has been coated with the copolyester coating composition of this invention has excellent utility as a base for the production of metallized polyester films. Such films are prepared by forming a metallic coating on a surface of the

copolyester coated film by directing a stream of metal vapor or atoms onto the surface of the film by a vacuum deposition procedure. This is effected by heating the metal in a high vacuum, preferably in the range from about 10^-3 to 10^-5 torr, to a temperature around its melting point such that the vapor pressure of the metal exceeds about 10^-2 torr or by subjecting the metal to a stream of bombarding ions whereby the metal is removed by mass transfer "sputtering". The metal is

subsequently vaporized or sputtered, emitting metal vapors or atoms in all directions which impinge on the film surface, condense and form a thin metallic coating on the film. Applicable metals include zinc, nickel, silver, copper, gold, indium, tin, stainless steel, chromium, titanium and, most preferably, aluminum, as well as the oxides of these metals. The thickness of the applied metal coating depends on the final use of the metallized film. Thicknesses of aluminum for use in packaging applications range from about 100 to 2000 angstroms.

The advantageous properties of this invention can be observed by reference to the following examples which illustrate, but do not limit, the invention. All parts and percentages are by weight unless otherwise

indicated.

Examples 1 to 5 (Comparative Examples 1C to 5C)

A series of copolyesters having the compositions shown in Table 1 were applied as 0.5% solutions in methylene chloride to one surface of a 3 mil biaxially oriented polyethylene terephthalate film. The

crosslinked samples also contained 5% or 10% by weight of an isocyanate crosslinking agent (based on polyester solids). The copolyester coated films were dried at 50°C for one minute and aged for 24 hours at ambient temperature before metallizing. The coating weights were 0.1 gm/m².

Metal adhesion was evaluated by placing samples of the coated films in a laboratory vacuum coater. The vacuum chamber was evacuated to less than 2 × 10^-5 torr pressure and approximately 800 angstroms of aluminum was evaporated onto the coated film surface from a tungsten filament.

The samples were further evaluated by. heat sealing a polyester film having a heat-sealable copolyester on one surface (Melinex® 850 purchased from Imperial

Chemicals Corporation) to the metallized PET at 120°C and 20 psi for 5 seconds. Peel strengths (180°C) were used to quantitatively measure adhesion after the laminated samples had been aged for 24 hours. Adhesion results are given in Table 1 together with the % of aluminum removed from the samples.

As can be readily seen from Table 1, PET film coated with the invention copolyesters of Examples 1 to 5 specifically formulated to have Tg's above 30°C exhibit significant improvement in metal adhesion as compared with uncoated PET film (the control) and with the copolyesters of Comparative Examples 1C to 5C which have Tg's under 30°C and are outside of the invention.

TABLE IA

VAPOR DEPOSITED ALUMINUM ADHESION TO COPOLYESTER PRIMED FILM

Copolyester Composition¹

(Mole %)

Example Glycol Diacid

No. 5% 10%

Tg

°c EG 4T NPG T I AD AZ

Control 80 100 - - 100 - - -

(unprimed

PET)

1 74 58 - 42 85 15 - -

2 70 100 - - 80 20 - -

3 66 35 - 65 80 - - 20

4 39 100 - - 41 41 - 18

5 30 100 - - 40 40 10 10

1C 19 57 - 43 33 33 - 34

2C 10 96 4 - 50 20 - 30

3C -10 100 - - 33 17 - 50

4C -15 100 - - 55 - - 45

5C -20 75 25 - 55 45 - -

TABLE IB

VAPOR DEPOSITED ALUMINUM ADHESION TO COPOLYESTER PRIMED FILM

Aluminum Adhesion

Example gm/in (% A1 removed)

No.

Uncross- Cross- Cross- linked linker² linker²

Control 110 (100) - - - - - -

(unprimed

PET)

1 325(0) 445(0) 360(0)

2 270(0) 315(0) 310(0)

3 335(0) 360(0) 280(0)

4 295(80) 310(80) 300(0)

5 235(80) 235(80) 260(50)

1C 27(90) 42(90) 40(80)

2C 50(90) 75(60) 97(80)

3C 52(90) 77(90) 115(90)

4C 20(90) 100(50) 70(90)

5C 27(90) 17(80) 17(70)

¹ED - Ethylene Glycol T - Terephthalic Acid

4T - 1,4-Butanediol I - Isophthalic Acid

NPG - Neopentyl Glycol AD - Adipic Acid

AZ - Azealic Acid

²Isocyanate crosslinking agent (weight %). Example 6 (Comparative Example 6C)

The copolyester of Example 3 was coated on PET film, the coated film was metallized as previously described and then both wet and dry adhesion were compared (Example 6C) to a commercially available copolyester (Hostaphan® 2600 purchased from American Hoechst Company) containing from 5 to 10% of a

sulfonated isophthalic acid in the polymer.

The metal adhesion was evaluated by heat sealing an electrically discharge treated ethyl methacrylate film (Type 899 purchased from the Dow Chemical Company) to the metallized PET films at 120°C and 50 psi for 15 seconds. In the wet adhesion test, the laminates were first submerged in water for 1 minute, removed and then the peel strengths were measured.

Dry and wet adhesion results are given in Table 2 together with the % of aluminum removed from the

samples. Table 2

VAPOR DEPOSITED ALUMINUM ADHESION

Example Aluminum Adhesion (gm/in) No. (% A1 removed)

Dry Wet

6 925 (0) 626 (10)

6C 990 (0) 112 (95)

The results show that the copolyester of the invention has much better wet adhesion compared to the commercial copolyester.

Claims

What is claimed is :

1. An oriented polyester film coated on one or both sides with a continuous copolyester coating consisting essentially of the condensation product of a diacid component selected from the group consisting of isophthalic acid, terephthalic acid and an aliphatic dicarboxylic acid of the formula HOOC(CH₂)_nCOOH wherein n is 2 to 11, the total amount of the diacid component being 50 mole %; and a copolymerizable aliphatic or cycloaliphatic alkylene glycol component having from 2 to 11 carbon atoms, the total amount of the glycol component being 50 mole %; wherein said copolyester has a glass transition temperature (Tg) of at least 30°C but not higher than a temperature which is 100°C lower than the melting point of the copolyester, said film having improved adhesion to a metal coating applied by vapor deposition.

2. The film of claim 1 wherein the copolyester consists essentially of the condensation product of on the basis of diacid 85 mole % terephthalic acid, 15 mole % isophthalic acid, and on the basis of glycol, 58 mole % ethylene glycol and 42 mole % neopentyl glycol.

3. The film of claim 1 wherein the copolyester consists essentially of the condensation product of on the basis of diacid 80 mole % terephthalic acid, 20 mole % isophthalic acid and on the basis of glycol 100 mole % ethylene glycol.

4. The film of claim 1 wherein the copolyester consists essentially of the condensation product of on the basis of diacid 80 mole % terephthalic acid, 20 mole % azelaic acid, and on the basis of glycol 35 mole % ethylene glycol and 65 mole % neopentyl glycol.

5. The film of claim 1 wherein the copolyester consists essentially of the condensation product of on the basis of diacid 41 mole % terephthalic acid, 41 mole % isophthalic acid, 18 mole % azelaic acid and on the basis of glycol 100 mole % ethylene glycol.

6. The film of claim 1 wherein the copolyester consists essentially of the condensation product of on the basis of diacid 40 mole % terephthalic acid, 40 mole % isophthalic acid, 10 mole % adipic acid, 10 mole % azelaic acid and on the basis of glycol 100 mole % ethylene glycol.

7. A metallized polyester film comprising an oriented polyester base film coated on one or both sides with (a) a continuous copolyester coating consisting essentially of the condensation product of a diacid component selected from the group consisting of

isophthalic acid, terephthalic acid and an aliphatic dicarboxylic acid of the formula HOOCH(CH₂)_nCOOH wherein n is 2 to 11, the total amount of the diacid component being 50 mole %; and a copolymerizable aliphatic or cycloaliphatic alkylene glycol component having from 2 to 11 carbon atoms, the total amount of the glycol component being 50 mole %; wherein said copolyester has a glass transition temperature (Tg) of at least 30°C but not higher than a temperature which is 100°C lower than the melting point of the copolyester and (b) a vapor deposited metal coating on the copolyester coating.

8. The film of claims 1 or 7 wherein the

polyester base film is polyethylene terephthalate.

9. The metallized film of claim 7 wherein the polyester base film is polyethylene-2,6-naphthalate.

10. The metallized film of claim 7 wherein the metal is zinc, nickel, silver, copper, gold, indium, tin, stainless steel, chromium, titanium and aluminum.