EP0673313A1

EP0673313A1 - Copolyester primed biaxially oriented copolyester film

Info

Publication number: EP0673313A1
Application number: EP93901393A
Authority: EP
Inventors: Cynthia Bennett; E-Won Choe; John Anthony Flint; Bodo Kuhmann
Original assignee: Hoechst AG; Hoechst Celanese Corp
Current assignee: Hoechst AG; CNA Holdings LLC
Priority date: 1992-12-09
Filing date: 1992-12-09
Publication date: 1995-09-27
Also published as: JPH08504379A; EP0673313A4; WO1994013479A1

Abstract

The invention deals with a biaxially oriented self-supporting mono- or multilayer copolyester film having a continuous copolyester primer coating on one or both sides thereof, wherein the copolyester film is a PENBB film and wherein said copolyester primer coating contains the polyester condensation product of the following monomers or their polyester forming equivalents: (a) a suitable aromatic dicarboxylic acid; (b) an aliphatic dicarboxylic acid of the formula HOOC-(CH2)n-COOH wherein n ranges from 1 to 11 and/or (c) a sulfomonomer containing an alkali metal sulfonate group and; (d) stoichiometric quantities of a copolymerizable aliphatic or cycloaliphatic alkylene glycol having from 2 to 11 carbon atoms. PENBB is a copolyester containing units of formula (I).

Description

COPOLYESTER PRIMED BIAXIALLY ORIENTED COPOLYESTER FILM

The present invention relates to biaxially oriented PENBB film coated on one or both sides with a primer coating which renders the PENBB film surface receptive to additional coating or printing from aqueous or organic solvent based systems.

Background Of The Invention

Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), is widely used as a base for drafting film, photographic film and reprographic film, as well as for packaging and labeling applications.

PET films, however, need to be improved especially in their modulus, heat stability, dimensional stability, hydrolysis resistance, water pickup and UV stability. Moreover PET film needs to have a more hydrophilic surface in order to be more receptive to coating or printing. In most applications where the film is to serve as a base or support for other coatings, it must be coated on one or both sides with a primer coating which adheres to the film and is receptive as well to other coatings applied to it. For example, U.S. Patent Nos. 2,627,088 and 2,698,240 teach a primer coating for PET film comprising a terpolymer composition of vinylidene chloride, acrylic ester and itaconic acid. This primer layer is said to have excellent adhesion to the polyester surface and to water or alcohol based photographic gelatin layers subsequently coated thereon. U.S. Patent No. 3,447,947 teaches a drafting film comprising a PET film base coated with a primer layer which may comprise, inter alia, an organic solvent-soluble mixed isophthalic polyester or polyester-amide or polyester oxazoline. Such layers are said to provide good adhesion to both the polyester base film and to hydrophobic film forming drafting layers ap- plied from organic solvent and containing finely divided toothing agents such as silica, as well as to photographic silver halide emulsion layers and light sensitive diazo layers. However, the application of such coatings by the film manufacturer is not particularly attractive because of the requirement that they be applied as solutions or dispersions in organic solvent. Health and safety factors dictate against the in-plant utilization of organic solvents in coating processes undertaken during film manufacture. Other primer layers include the thermoset acrylic or methacrylic layers taught in U.S. Patent No. 3,819,773 which can be applied to the PET film in the plant from aqueous medium. However, these primer layers suffer the dis¬ advantage that they do not provide commercially adequate adhesion for aqueous based coatings such as aqueous printing inks or aqueous matte drafting layers applied directly to them.

Compositions containing certain water dispersible .or water soluble copolyesters and copolyester amides have been developed which are taught to be useful as adhesives for various substrates, including metals, papers and polyester film, or as binders or sizing agents for synthetic fibers. Such mate- rials are disclosed in U.S. Patent Nos. 3,563,942 and 3,779,993. Aqueous dispersions of similar materials are also disclosed in U.S. Patent No. 4,340,519, said materials comprising copolyester compositions containing the polyester reaction product of an aromatic dicarboxylic acid, an aliphatic dicarboxylic acid, an aromatic dicarboxylic acid containing a metal sulfonate group and polyol components, said composition further containing a water soluble organic compound having a boiling point of 60 ^* C to 200 ^* C. Such copolyesters are preferably a blend of crystalline and non-crystalline copolye¬ sters. These materials are disclosed to have utility as hot melt adhesives, coatings and binders for plastic fibers or films, including polyester plastics. More recently, water soluble or dispersible copolyesters of a type similar to those described in U.S. Patent Nos. 3,563,942 and 3,779,993 above have been disclosed as primer layers for PET film to improve the adhesion to the film of photosensitive layers. For example, U.S. Patent Nos. 4,252,885 and 4,304,851 teach a priming layer applied to PET film as an aqueous dispersion comprising a polyester having free-functional acid groups derived from the condensation of one or more glycols with one or more tri- functional organic acids. Such primers are disclosed to enhance the adhesion of photographic layers to PET film. European Patent Publication No. 0029620 teaches a PET priming layer useful for gelatin based photosensitive coatings, which primer is based on an aqueous dispersion of the condensa¬ tion product of ethylene glycol and a mixture of terephthalic acid, isophthalic acid and a salt of sulfoisophthalic acid. U.S. Patent No. 4,181 ,528 teaches that the adhesion of hydrophiiic photographic emulsions to PET film is enhan¬ ced by the application to the film of a sub-coating comprising an aqueous dispersion of gelatin, a water soluble polyester similar to those discussed above and a polyfunctional aziridine crosslinking agent. While these and other priming layers or sublayers are effective in enhancing the adhesive qualities of PET film for certain applications, there are limitations. For example, whereas many thermoplastic or thermoset acrylic or methacrylic primers may provide suitable adhesion to photographic or printing lacquer compositions containing organic solvents, such primers do not provide commercially satisfactory adhesion to many aqueous based coatings or printing inks without the application of a second priming or subbing layer over the first primer. Conversely, many of the water dispersible polyester primers referred to above may provide satisfactory adhesion directly to aqueous or alcohol based systems, but will not provide commercially satisfactory adhesion directly to organic solvent based lacquers and printing inks.

Moreover, the support layer still needs to be improved in stiffness (tensile modulus), tensile strength, heat stability, dimensional stability, water pickup and UV stability. The development of a coated film whose surface would be receptive to both aqueous and organic solvent coatings for printing and whose support layer shows improved mechanical properties as well as lower moisture absorption and higher UV resistance would be of great signifi¬ cance to the film industry.

Accordingly, it is an object of this invention to provide a biaxially oriented self-supporting film material receptive to aqueous and organic sol¬ vent based coatings and improved mechanical stability, lower moisture absorption and higher UV resistance. The invention, therefore, relates to biaxially oriented self-supporting mono- or multilayer copolyester film having a continuous copolyester primer coating on one or both sides thereof, whe¬ rein the copolyester film is a PENBB film and wherein said copolyester primer coating contains the polyester condensation product of the following mono- mers or their polyester forming equivalents:

(a) a suitable aromatic dicarboxylic acid;

(b) an aliphatic dicarboxylic acid of the formula HOOC-(CH₂)_n-COOH wherein n ranges from 1 to 1 1 and/or

(c) a sulfomonomer containing an alkali metal sulfonate group and (d) stoichiometric quantitites of a copolymerizable aliphatic or cy- cloaliphatic alkylene glycol having from 2 to 1 1 carbon atoms. Most of these coatings are disclosed for PET film in U.S. Patent No. 4,476,189.

U.S. Patent No. 3,008,934 discloses copolyesters containing as acid derived units 4,4'-bibenzoate and a host of other dicarboxyiates including

2,6-naphthalic dicarboxylate. it also discloses oriented fibers and films prepa¬ red from these copolyesters, however, biaxially oriented PENBB films are not disclosed or envisioned. In particular, those films with improved stiffness (tensile modulus) and tensile strength in both MD and TD as well as thermo- stability, UV stability, hydrophobicity, dimensional stability and impermeability toward gases in comparison to PET film are not disclosed in U.S. Patent No. 3,008,934.

The copolyester priming layers may be applied to the PENBB film at any suitable stage during manufacture of the film, i.e., before or during the stretching operations. The resultant primed PENBB film is found to provide excellent adhesion to many aqueous or organic solvent based coatings subse¬ quently applied thereto and also to metallic coatings subsequently applied thereto by metallizing techniques and exhibits improved mechanical stability, lower moisture absorption and higher UV resistance. Detailed Description Of The Invention

The biaxially oriented copolyester film base for the purpose of this invention is made from PENBB.

PENBB as mentioned hereinbefore is a copolyester containing as acid- derived unit at least 5 mole percent of a radical of the formula

(bibenzoate, BBJ

In the case that more than 10 mole percent of terephthalic acid derived radicals are present in the copolymer, the content of bibenzoate derived units is at least 25 mole percent. Films of these copolyesters are mentioned in the unpublished German Patent Application P 4224161.8, which is incorporated herein by reference. Preferably PENBB is a copolyester wherein at least 80 mole percent of the acid derived units (NBB) consist of bibenzoate (20 to 80 mole percent, preferably 40 to 60 mole percent) and naphthalate (80 to 20 mole percent, preferably 60 to 40 mole percent). The remaining 20 or less mole percent may consist of other acid derived units, which e.g. affect the melting point or the crystallization kinetics. Preferably at least 80 mole percent of the diol-derived units consist of -O(CH₂)₂-O-units. The remaining 20 or less mole percent consist of other diol-derived units, which e.g. may also affect the melting point or the crystallization kinetics. It may also be desirable to replace minor amounts of the acid- and/or diol-derived units with hydroxycarboxylic-acid-derived units, e.g. such derived from p-hydroxyben- zoic acid.

In order to achieve the desired mechanical properties in the biaxially oriented PENBB film it is recommended that the IV value (inherent viscosity, as measured in a 1 : 1 weight-ratio mixture of pentafluorophenol and hexa- fluoroisopropanol at a concentration of 0.2 g/dl and a temperature of 25 ^* C) of the PENBB polymer after extrusion be > 0.5 dl/g and preferably > 0.55 di/g. The copolyester is obtained by polycondensation of the corresponding diacid or lower dialkyi diester and the corresponding diol. Both components should preferably be employed in equimolar ratios. It may, however, be advantageous to employ one of the components - especially the diol -- in excess, for instance in order to influence the reaction kinetics or to serve as a solvent. The polycondensation is carried out according to known processes used, e.g. in the production of polyethylene terephthalate (PET). Usually about 100 mole percent of the dicarboxylic acid or dialkyldicarboxylate or a corresponding mixture of two or more of these acids and/or esters are mixed with > 100 mole percent of the corresponding diol(s). This mixture is then heated to about 200 " C, preferably in the presence of a transesterification catalyst, until sufficient lower alkyl alcohol has been removed from the mixture via distillation. This reaction yields an oligomer or a low molecular weight polyester, which is subsequently subjected to polycondensation, preferably in the presence of a stabilizer and/or catalyst. Useful stabilizers and catalysts can be polyphosphates, triorganyl phosphates, antimony trioxi- de or tetraalkoxy titanate(IV) or mixtures of triphenylphosphate and antimony trioxide. A preferred process for the production of such copolyesters is described in U.S. Patent Application Serial No. 07/735,553 which is incorpo- rated herein by reference. A further increase in molecular weight can be achieved by solid phase polycondensation at a temperature just below the melting point under vacuum, or a stream of dry air or inert gas.

To produce the film, the polymer melt is extruded through a die onto a chill roll where it solidifies and is then biaxially oriented, heat set, optionally post treated and wound on a roll. For a multilayer film known methods for coextrusion, in-line or off-line coating can be used. The solidified film as extruded on the chill roll should be obtained in an essentially amorphous state. To achieve this, the melt film must be pinned to the chill roll by a known method such as electrostatic pinning or vacuum, air knife or the like. The biaxial orientation of the film is achieved by stretching the film at elevated temperature in the machine (MD) and transverse direction (TD). This stretching can be either simultaneous or sequential. In the case of sequential stretching the first stretching step can be in either MD or TD, followed by stretching in the other direction. The orientation in MD can also be achieved in several steps, either one after another prior to stretching in TD, or before and after the TD stretching. Preferred temperatures for stretching lie bet- ween the glass transition temperature and about 30 ^* C above the cold cry¬ stallization temperature of the PENBB copolymer composition in use (both temperatures can easily be measured on amorphous films by DSC). The total stretch ratios (Λ) in MD and TD lie between 1 : 2 and 1 : 10, preferably between 1 : 2.5 and 1 : 5. The product of the total stretch ratios should be between 1 to 30 preferably between 5 to 20. Biaxial drawing is performed such that the birefringeance is < 0.2, preferably < 0.1 to ensure adequately isotropic properties. Birefringeance as mentioned herein is the absolute value of the difference between the maximum and minimum refractive indices in the plane of the film, as measured on common instruments such as Abbe refractometer, optical bench or compensators.

In order to optimize properties, relaxation steps can be included in the orientation and heat setting processes.

The heat setting takes place at a temperature between the cold crystal¬ lization temperature and the melt temperature of the copolymer composition. In some cases a surface treatment such as corona, plasma or flame treatment should be employed before winding the film on a roll.

The copolyester primer coating of this invention, either in form of an aqueous dispersion or an organic solvent based solution/dispersion may be applied in-line at one of the two following stages during the PENBB film manufacture: in the pre-draw stage at the point between the casting of the amorphous sheet and the first stretch such as disclosed, for example in British Patent No. 1 ,41 1 ,564 or in the interdraw stage subsequent to the uniaxial drawing but prior to biaxial drawing such as disclosed in U.S. Patent No. 4,214,035. Normally, the heat applied to the film during the stretching or final conditioning stages is sufficient to evaporate the water or otherwise dry the primer coating. In one preferred embodiment, the primer coating is applied after the PENBB film is uniaxiaily stretched, that is, after the film is stretched in one direction, but before the film is stretched in the orthogonal direction. In another preferred embodiment, the PENBB film is first stretched in the longi- tudinai direction prior to coating. In this preferred embodiment, after longitu¬ dinal stretching, the film is coated by any of the well known techniques employed in the art. For example, coating may be effected by roller coating, spray coating, slot coating or immersion coating. In a preferred embodiment, the PENBB film is coated by means of gravure roller coating. Also, the uniaxiaily drawn film is preferably subjected to a surface treatment such as corona discharge, plasma or flame treatment as is known in the art. This treatment decreases the hydrophobic character of the PENBB film surface, which permits the water-based coating to more easily wet the surface and thus improve the adhesion of the coating to the surface. As indicated above, the copolyesters of this invention suitable as primer coatings for PENBB films are produced by polycondensing (a) a suit¬ able aromatic dicarboxylic acid, (b) an aliphatic dicarboxylic acid of the formula HOOC-(CH₂)_n-COOH, wherein n ranges from 1 to 1 1 , and/or (c) a sulfo onomer containing an alkali metal suifonate group, and (d) stoichiome- trie quantities of a copolymerizable aliphatic or cycloaliphatic alkylene glycol having from 2 to 1 1 carbon atoms.

Suitable aromatic dicarboxylic acids are those having 1 , 2, 3 or 4 benzene rings which are either fused or attached to one another by single bonds. These aromatic dicarboxylic acids may be substituted with 1 or up to 4 substituents, such as COOH, OH, carboxylic acid esters having 1 to 4 carbon atoms, amides, amines, etc. Suitable radicals are e.g. -OH, -COOR {R = H, C^C^alkyl), -NR-R², -NR¹-C(O)-R² and -C(O)-NR¹R² (R\R² being identi¬ cal or different and representing H,

Preferably suitable aromatic dicarboxylic acids are terephthalic acid, isophthalic acid, 4,4'-bibenzoic acid, 2,6-naphthoic dicarboxylic acid and trimelitic acid. Examples of dicarboxylic acids suitable as component (b) of the co¬ polyesters include malonic, adipic, azelaic, glutaric, sebacic, suberic, succinic, brassyiic acids and mixtures thereof, or their polyester forming equivalents.

The term "polyester forming equivalent" as used herein means reac- tants having groups capable of undergoing condensation reactions to form polyester linkages, which groups include carboxylic groups as well as lower alkyl esters thereof, e.g. dimethyl terephthalate, diethyl terephthalate, and many other corresponding esters, halides or salts.

Examples of glycols suitable as component (d) include ethylene glycol; 1 ,5-pentanediol; 1 ,6-hexanediol; neopentyl glycol; 1 ,10-decanediol; cycloh- exane dimethanol; and similar materials. Preferably the glycol should be aliphatic or cycloaliphatic, with no ether linkages in the molecular chain.

Ethylene glycol is the much preferred glycol.

In order to render the copolyester water dispersible or even water soluble, it is preferred to either add a sulfomonomer (c) containing a metal sulfonate group or even substitute component (b) entirely with such a sulfo¬ monomer. Examples of preferred sulfomonomers are those containing a metal sulfonate group attached to a dicarboxylic aromatic nucleus (compo¬ nent c) which materials are represented by the general formula

S0₃M

wherein:

M is an alkali metal cation;

Z is a trivalent aromatic radical; and X and Y are carboxyl groups or polyester forming equivalents.

Such monomers are disclosed in U.S. Patent Nos. 3,563,942 and

3,779,993, which are incorporated herein by reference. Species of such monomers include sodium sulfoterephthalic acid; sodium 5-sulfoisophthalic acid; sodium sulfophthalic acid; 5-(p-sodiosulfophenoxy)-isophthalic acid; 5- (sulfopropoxy) isophthalic acic^* sodium salt; and like materials as well as their polyester forming equivalents, such as the dimethyl esters. Preferably M is Na\ Li⁺ or K⁺ .

In order to achieve good results in a primed PENBB film offering satis¬ factory adhesion to both aqueous and solvent based coatings and to metal layers, it has been found that component (a) (the aromatic dicarboxylic acid) should be present in the solid part of the primer composition within the about 60 to 98 mole percent range. If component (b) (the aliphatic dicarboxylic acid) is present and component (c) (the sulfomonomer) is not present, the amount of component (b) ranges from about 2 to 40 mole percent. If compo- nent (c) is present and component (b) is not present in the primer composi¬ tion, the amount of component (c) ranges from about 2 to 40 mole percent. If both components (b) and (c) are present in the primer composition, the sum of both components range from about 2 to 40 mole percent. In order to selectively improve metal adhesion, it has been found that component (a) should preferably be present in an amount of from about 65 to 98 mole percent, component (b) should be present in an amount of from about 0 to 30 mole percent, and component (c) should be present in an amount of from about 2 to 20 mole percent.

Especially in order to improve the adhesion of printing, it has been found that component (a) should preferably be present in an amount of from about 60 to 75 mole percent, component (b) should be present in an amount of from about 15 to 25 mole percent, and component (c) should be present in an amount of from about 6 to 15 mole percent.

In any event, the added mole percentages of all components (a), (b) and (c) is 100 mole percent. The diol component (d) is added in at least stoichiometric amounts, preferably in excess of about 5 to 1 5 mole percent.

In the synthesis of the copolyester primer coatings of this invention, it is most preferred to employ all of the starting diacid monomers in their lower dialkyi ester form, particularly in the dimethyl ester form This allows more consistent control of the formation in-situ of dialkylene glycols, e.g. diethyle- ne glycol when the glycol reactant is ethylene glycol. It has been discovered that the presence of significant amounts of such materials in the copolyester composition may detract from the good adhesive properties of the primer layers. For the same reason, the sulfomonomer is preferably employed as the alkali metal salt rather than as the free sulfonic acid, which, however, is not excluded from the scope of this invention.

The copolyester coatings suitable for the purposes of this invention may be further characterized as having an acid number of less than 10, preferably from about 0 to about 3, a number average molecular weight of less than about 50,000 and an RV (relative viscosity measured as a 1 percent solution in dichloroacetic acid at 25 ^* C, using an Ubbelohde capillary viscome- ter) within the range of about 300 to 700, more preferably in the range of about 350 to 650.

As indicated above, the primer coating of the present invention is applied to the base PENBB film as an aqueous dispersion and at a solids concentration within the range of about 0.5 to 15 wt.-%, preferably about 3 to 10 wt.-%. The preferred solids level is such as to yield a final dry coating thickness within the range of about 1 to 300 nm, which translates into a solids level on a weight basis of from 1 to 300 mg/m². The preferred thick¬ ness range of the dried copolyester primer is from 1 5 to 50 nm, with 25 nm being the target thickness.

The coating may be applied to one or both sides of the PENBB film, or it may be applied to one side and a different coating such as a thermosetting acrylic or methacrylic coating applied to the opposite side, such as taught in U.S. Patent No. 4,214,035. In some cases, it may be desirable to include a hardening agent in the copolyester coating formulations, e.g. from about 1-20 percent by weight of a melamine or urea/formaldehyde condensation product, to further modify the properties of the primer coating. Other additives known in the art may also be present in the coating formulation such as antistatic agents, wetting agents, surfactants, pH regulating agents, anti-oxidants, dyes, pigments, slip agents such as colloidal silica, and the like.

The copolyester primer coatings of this invention exhibit excellent heat stability and accordingly any scrap primed film made during production can be mixed with fresh polyester, re-melted and re-fed to the film forming extruder to produce oriented film. Such PENBB film produced containing up to about 70 percent by weight of coated scrap reclaim exhibits good quality, color and appearance with very little if any perceptible degradation of properties due to the presence of the coating impurity. Thus, the primed PENBB film of this invention offers a distinct commercial advantage to the film manufacturer over many other primed films, such as films primed with vinylidene chloride containing polymers as disclosed in U.S. Patent Nos. 2,627,088 and 2,698,- 240, which tend to degrade and discolor when reclaimed as set forth above. Biaxially oriented PENBB film primed with the copolyester composition of this invention has excellent utility as a film base for the production of photosensitive reprographic films, due to its excellent dimensional stability. Such films are prepared by forming a coating on a surface of the primed PENBB film of a photosensitive composition comprising an organic solvent solution of a resinous binder containing or impregnated with a light sensitive diazonium compound, and drying said coating. Resinous binders suitable for this purpose include cellulose acetate butyrate, cellulose acetate, cellulose acetate propionate as well as vinyl polymers such as polyvinyl acetate. Suitable solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone, ethylene glycol monomethyl ether and mixtures thereof. These reprographic coatings and their method of application and use are well known in the art.

Similarly the primed biaxially oriented PENBB film of this invention forms an excellent substrate for the application of matte coatings which render the film suitable for use as a drawing or drafting material, due to its excellent dimensional stability. These matte coatings may be based on either an aqueous or organic solvent composition comprising a resinous binder and a finely divided particulate material which serves as a "toothing agent". The resinous binder may include the resinous materials referred to above as well as acrylic or methacrylic resins. The organic solvent may also include those listed above. Particulate materials include finely divided (less than 10 μπ\ particle size) clays or silica. Water dispersible or water soluble binders such as polyvinyi alcohol or hydrolyzed polyvinyl acetate may also be employed. Other ingredients such as thickeners or dispersing agents may also be present in such matte formulations. Similar matte formulations are disclosed for example in British Patent No. 1 ,072,122. Yet another application of the primed biaxially oriented PENBB film of this invention is as a packaging or label material. The primed films are heat sealable, hydrolysis resistant and demonstrate improved adhesion to aqueous and organic solvent based printing inks when compared with unprimed PET film. These inks may comprise aqueous/alcohol dispersions or solutions of pigments and/or dyes in combination with acrylic resins and thickening agents, or organic solvent dispersions or solutions of such dyes or pigments in combination with resinous materials. Surprisingly, it has been found that the biaxially oriented PENBB film itself provides a much better gas barrier toward water vapor, CO₂ or O₂ than known PET films. In yet another embodiment of the invention, the primer coated biaxially oriented PENBB film has been found excellent in use as a base for the produc¬ tion of metallized PENBB films, due to reduced water absorption, stiffness and improved mechanical strength. Such films are prepared by well known prior art techniques such as by forming a coating on a surface of the coated biaxially oriented PENBB film of a metal by directing a stream of metal vapor or atoms onto the surface of the film by a vacuum deposition technique. This is effected by heating the metal in a high vacuum, preferably in the range of about 10^'3 to about 10^"5 Torr, to temperatures above its melting point such that the vapor pressure of the metal exceeds about 10^'2 Torr or it is effected by subjecting the metal to a stream of bombarding ions whereby the metal is removed by mass transfer, known as "sputtering" . When these conditions are achieved, the metal is vaporized or sputtered, emitting metal vapor or atoms in all directions. These vapor or atoms impinge on the film surface, condense and thereby form a thin metallic coating on the film Metals applicable to this process are zinc, nickel, silver, copper, gold, indium, tin, iron, chromium, titanium and, most preferably, aluminum, and include also the oxides of such metals. The thickness of the applied metal coating is a matter of preference depending upon the final use of the metallized film Thicknesses for aluminum in packaging applications range from about 300 to 600 A, while thicknesses in solar applications are generally less than 100 A. A major application of the metal coated biaxially oriented PENBB film of this invention is as a packaging or label material. The side of the film not coated with the metal may be coated with a heat sealable material such as is disclosed in British Patent No. 1 ,249,015. Here, too, the improved gas barrier properties of the PENBB film itself are advantageous.

Accordingly, the excellent mechanical properties, along with the UV stability and improved gas barrier combined with the adhesive qualities of biaxially oriented PENBB film primed with a copolyester layer of this invention to both aqueous based and organic solvent based coatings applied thereto render such film of more universal utility to the manufacturer of finished reprographic, graphic, and packaging products. The mechanical properties are measured in a tensile testing machine made by Zwick (Ulm, Germany) on 15 mm wide strips of film. The initial distance between the chucks is 100 mm and the crosshead speed is 100 mm/min for the strength and elongation determination and 10 mm/min for the tensile modulus determination. The UV resistance is tested by measuring the retention of tensile elongation after exposure to UV light in a "Suntest" apparatus manufactured by Heraeus (Hanau, Germany) for 14 days.

The following examples are illustrative of the invention.

EXAMPLE 1

A water dispersible copolyester containing on the acid side approxima¬ tely 73 mole percent terephthalic acid, 20 mole percent adipic acid and 7 mole percent of the sodium salt of 5-sulfoisophthalic acid, and on the glycol side 100 mole percent ethylene glycol is prepared by the following procedure. A 2-liter, stainless steel reaction flask, equipped with an anchor stirrer, a thermocouple to measure the temperature of the vessel contents, a 46 cm Claisen/Vigreaux distillation column with a condenser and a receiver flask, an inlet port, and a heating mantle, is preheated to 190 ^β C, swept with nitrogen, and charged with 907.3 g of dimethyl terephthalate, 223.0 g of dimethyl adipate, 132.7 g of dimethyl-5-sulfoisophthalate-sodium salt and 794.5 g of ethylene glycol. A buffer (Na₂CO₃.10H₂O-3.577 g) and a transesterification catalyst (Mn(OAc)₂.4H₂O-0.563 g) are added. The mixture is stirred and heated while methanol distilled from the flask. During the distillation, the vessel temperature is gradually raised to 250 ^° C. When the weight of distilla¬ te is the same as the theoretical methanol yield, an ethylene glycol solution containing 0.188 g of phosphorous acid is added. The distillation column is replaced with a gooseneck vapor takeoff with a receiver. Ethylene carbonate (20 g) is added (neat) to the reaction mixture, and vigorous off-gassing (CO₂) starts immediately. The CO₂ evolution subsides after about 10 minutes. A 240 Torr vacuum is applied, and the polycondensation catalyst (0.563 g Sb₂O₃ in an ethylene glycol slurry) is added. The mixture is stirred under 240 Torr vacuum for 10 minutes, after which the pressure is reduced from 240

Torr to 20 Torr in 10 mm Torr/minute increments. With the system at 20 Torr vacuum, the vessel temperature is raised from 250 ^β C to 290" C at a rate of 2^° C/minute. When the vessel temperature reaches 290 ^°C, the stirrer speed is reduced, and the pressure is lowered to 0.1 Torr or less. At that point, a stirrer motor ammeter reading is obtained. The viscosity of the polymer is controlled by allowing the polycondensation to proceed to fixed values for the change in stirrer motor current (ΔA) of 5 Amperes. After the desired molecular weight is attained, nitrogen is used to pressurize the vessel and to force the molten polymer out of the bottom plug of the vessel into an ice water quench bath.

An aqueous dispersion of the copolyester prepared as described above is made by adding 60 g of the copolyester in granular form to one liter of water maintained at about 90 ° C in a two liter stainless steel beaker under conditions of vigorous stirring. After the copolyester is completely dispersed, it is cooled to room temperature, filtered, after which 1 .12 g of a 50 percent solids aqueous dispersion of colloidal silica (50 % solids) is added under mixing conditions. This dispersion is then applied as a primer to PENBB film by the follo¬ wing procedure:

PENBB granules made out of 289 parts by weight of dimethyl 2,6- naphthalene dicarboxylate, 322 parts by weight of dimethyl 4,4'-bibenzoate and 368 parts by weight of ethylene glycol are melted in a single screw extruder at temperatures of 280° to 320 ^*C and extruded through a sheet dye onto a cooling roll, temperature controlled at 30 ^* C. A 120 μm thick film is obtained which is clear and transparent. This cast sheet is longitudinally stretched and is then corona treated by a corona discharge apparatus and thereafter coated with the copolyester dispersion prepared above by reverse gravure coating.

The corona treated, longitudinally drawn, coated film is dried at a temperature of about 100 " C. Thereafter the film is stretched in the trans¬ verse direction to produce a biaxially drawn filrm The biaxially drawn film is heat set.

EXAMPLE 2 A water dispersible copolyester containing on the acid side approxima¬ tely 90 mole percent isophthalic acid and 10 mole percent of the sodium salt of 5-sulfoisophthalic acid, and on the glycol side 100 mole percent ethylene glycol is prepared by the following procedure.

A 2-liter, stainless steel reaction flask, equipped with an anchor stirrer, a thermocouple to measure the temperature of the vessel contents, an 46 cm Claisen/Vigreaux distillation column with a condenser and a receiver flask, an inlet port, and a heating mantle is preheated to 190^* C, swept with nitrogen, and charged with 1065.6 g of dimethyl isophthalate, 180.6 g of dimethyl-5- sulfoisophthalate-sodium salt and 756.9 g of ethylene glycol. A buffer (Na₂CO₃.10H₂O-3.577 g) and a transesterification catalyst (Mn(OAc)₂.4H₂O- 0.563 g) are added. The mixture is stirred and heated while methanol distil- led from the flask. During the distillation, the vessel temperature is gradually raised to 250 ^* C. When the weight of distillate is the same as the theoretical methanol yield, an ethylene glycol solution containing 0.188 g of phospho- rous acid is added. The distillation column is replaced with a gooseneck vapor takeoff with a receiver. Ethylene carbonate (20 g) is added (neat) to the reaction mixture, and vigorous off-gassing (CO₂) starts immediately. The CO₂ evolution subsides after about 10 minutes. A 240 Torr vacuum is ap- plied, and the polycondensation catalyst (0.563 g Sb₂O₃ in an ethylene glycol slurry) is added. The mixture is stirred under 240 Torr vacuum for 10 minu¬ tes, after which the pressure is reduced from 240 Torr to 20 Torr in 10 Torr/Minute increments. With the system at 20 Torr vacuum, the vessel temperature is raised from 250 ^° C to 290 ' C at a rate of 2" C/minute. When the vessel temperature reaches 290 " C, the stirrer speed is reduced, and the pressure is lowered to 0.1 Torr or less. At that point, a stirrer motor amme¬ ter reading is obtained. The viscosity of the polymer is controlled by allowing the polycondensation to proceed to fixed values for the change in stirrer motor current (ΔA) of 2.3 Amperes. After the desired molecular weight is attained, nitrogen is used to pressurize the vessel and to force the molten polymer out of the bottom plug of the vessel into an ice water quench bath. An aqueous dispersion of the copolyester prepared in accordance with this example is made by adding 60 g of the copolyester described above in granular form to one liter of water maintained at about 90^" C in a two liter stainless steel vessel under conditions of vigorous agitation. After the co¬ polyester is completely dispersed, it is cooled to room temperature and filtered, after which 1 1.2 g of a 50 percent solids aqueous dispersion of colloidal silica (50 wt.-% solids) is added under mixing conditions.

This dispersion is applied to the same biaxially oriented PENBB film as in Example 1 . Compared to known primer coated PET films, the primer coated PENBB films according to this invention exhibited commercially satis¬ factory adhesion to typical reprographic, graphic arts and packaging coatings. The improved mechanical properties, heat stability, dimensional stability, water absorption, UV stability and gas barrier compared to similarly coated biaxially oriented PET films can be seen from Table 1 . TABLE 1

Claims

THAT WHAT IS CLAIMED IS:

1 . A biaxially oriented self-supporting mono- or multilayer copolyester film having a continuous copolyester primer coating on one or both sides thereof, wherein the copolyester film is a PENBB film and wherein said copolyester primer coating contains the polyester condensation product of the following monomers or their polyester forming equivalents:

(a) a suitable aromatic dicarboxylic acid;

(c) a sulfomonomer containing an alkali metal sulfonate group and;

(d) stoichiometric quantities of a copolymerizable aliphatic or cy- cloaliphatic alkylene glycol having from 2 to 1 1 carbon atoms.

2. A biaxially oriented copolyester film according to Claim 1 , wherein the primer coating is applied in the form of an aqueous dispersion or an organic solvent solution.

3. A biaxially oriented copolyester film according to Claim 1 or 2, wherein the primer coating composition is applied in-line either before uniaxial drawing or after uniaxial drawing, but prior to the second draw step.

4. A biaxially oriented copolyester film according to Claim 3, wherein the primer coating composition is applied after uniaxial drawing.

5. A biaxially oriented copolyester film according to any one or more of the preceding claims, wherein the coating is effected by roller coating, spray coating, slot coating or immersion coating.

6. A biaxially oriented copolyester film according to any one or more of the preceding claims, wherein prior to coating the film is subjected to a corona discharge, flame or plasma treatment.

7. A biaxially oriented copolyester film according to any one of the prece¬ ding claims, wherein the aromatic dicarboxylic acid is one having 1 , 2, 3 or 4 fused or attached benzene rings which benzene rings may be substituted with one or more identical or different radicals selected from the group consisting of:

-OH, -COOR (R = H, C₁-C₄-alkyl), -NR¹R², -NR¹-C(O)-R² and -C(O)- NR¹R² (R¹,R² being identical or different and representing H, C C₄- alkyl).

8. A biaxially oriented copolyester film according to Claim 7, wherein the aromatic dicarboxylic acid is selected from the group consisting of: terephthalic acid, isophthalic acid, 4,4'-bibenzoic acid, 2,6-naphthoic dicar¬ boxylic acid and trimelitic acid.

9. A biaxially oriented copolyester film according to any one or more of the preceding claims, wherein the aliphatic dicarboxylic acid is selected from one or more members of the group consisting of: malonic, adipic, azelaic, glutaric, sebacic, suberic, succinic, brassγlic acid and their polyester forming equivalents.

10. A biaxially oriented copolyester film according to any one or more of the preceding claims, wherein the alkylene glycol is selected from the group consisting of: ethylene glycol, 1 ,5-pentanediol, 1 ,6-hexanediol, neopentyl glycol, 1 ,10- decanediol and cyclohexane dimethanol.

1 1 . A biaxially oriented copolyester film according to Claim 10, wherein the alkylene glycol is ethylene glycol.

12. A biaxially oriented copolyester film according to any one or more of the preceding claims, wherein the sulfomonomer is represented by the formu¬ la X - Z - Y

I

SO₃M

wherein

M is an alkali metal ion,

Z is a trivalent aromatic radical, and

X and Y are carboxyl groups or polyester forming equivalents.

13. A biaxially oriented copolyester film according to Claim 12, wherein the

sulfomonomer is selected from the group consisting of sodium sulfotereph- thalic acid, sodium 5-sulfoisophthalic acid, sodium sulfophthalic acid, 5-(p- sodiosulfophenoxyHsophthaiic acid, 5-(sulfopropoxy) isophthalic acid sodium

salt.

14. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the aromatic dicarboxylic acid is present in an

amount of from 60 to 98 mole percent and wherein the aliphatic dicarboxylic

acid is present in an amount of from 2 to 40 mole percent (in the case that

no sulfomonomer is present) and the sulfomonomer is present in an amount

of from 2 to 40 mole percent (in the case that the aliphatic dicarboxylic acid

is not present) and wherein the added amount of the aliphatic dicarboxylic

acid and the sulfomonomer ranges from 2 to 40 percent (in the case that

both aliphatic dicarboxylic acid and the sulfomonomer are present).

1 5. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the primer coating is applied as an aqueous

suspension at a solid concentration within the range of 0.5 to 1 5 weight

percent.

16. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the copolyester film is coated with the same or

different primer coatings on each side.

17. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the copolyester film contains up to 70 weight

percent of coated scrap reclaim, which coated scrap reclaim originates from

a film according to Claim 1 .

18. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the copolyester film is metallized.

19. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the film is a monolayer film.

20. A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the birefringeance of the PENBB layer is < 0.2.

21 . A biaxially oriented copolyester film according to any one or more of

the preceding claims, wherein the PENBB has an IV of 0.5 dl/g.

22. Use of a biaxially oriented copolyester film according to Claim 1 for the

production of photosensitive reprographic film.

23. Use of a biaxially oriented copolyester film according to Claim 1 for the

application of matte coating.

24. Use of a biaxially oriented copolyester film according to Claim 1 as dra¬

wing or drafting material.

25. Use of a biaxially oriented copolyester film according to Claim 1 as

packaging or label material.

26. Use of a biaxially oriented copolyester film according to Claim 1 as me-

tallizable film.

27. Use of a biaxially oriented copolyester film according to Claim 1 in the

production of polarizers.