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 Films of these copolyesters are mentioned in the unpublished German Patent Application P 4224161.8, which is incorporated herein by reference.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) -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 hydroxycarboxyiic-acid-derived units, e.g. such derived from p-hydroxyben- zoic acid.

To produce the film, the polymer melt is extruded through a die onto a chill roll where it solidifies, is then biaxially oriented, heat set, optionally post treated and wound on a roll. 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 pentafiuorophenol and hexafluoroisopropanol at a concentration of 0.2 g/dl and a temperature of 25^βC) of the PENBB polymer after extrusion be > 0.5 di/g and preferably > 0.55 dl/g. 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 refractome- ter, optical bench or compensators. For a multilayer film known methods for coextrusion, in-line or off-line coating can be used. The solidified film as ex¬ truded 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, and preferably between 5 to 20.

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

The heat setting takes place at a temperature between the cold cry- stallization temperature and the melt temperature of the copolymer composi¬ tion.

In some cases a surface treatment such as corona, plasma or flame treatment should be employed before winding the film on a roll.

Prior to coating the PENBB film surface with the acrylic coating compo- sition, the film may be surface treated in a conventional manner by exposure to, e.g. an electric corona, plasma or flame treatment. Electric corona disch¬ arge is a conventional surface treatment which is commonly performed on a film to enhance its surface qualities, especially its adhesive and printing properties. Electric corona discharge methods and apparatus are described in U.S. Patent Nos. 3,057,792 and 4,239,973. If the surface treatment follo¬ wed by the coating occurs before stretch orientation, heating the film before stretch orientation will drive off the water. For biaxially oriented film, prefera- bly the corona treatment followed by the acrylic polymer coating may occur during the in-line manufacturing process, either before stretch orientation, or between the machine draw and transverse draw of biaxial stretch orientation, or after stretch orientation. If the surface treatment and coating steps occur after stretch orientation, it is necessary to completely dry the film before winding. If the surface treatment and coating occur before orientation, or between draw during orientation, the later orientation steps which require the film to be heated would drive off the excess water from the acrylic polymer coating. Preferably, the corona treatment and subsequent acrylic polymer coating occur between draws during the stretch orientation step.

The mono- or multilayer biaxially oriented PENBB film is coated on the corona, plasma or flame treated surface preferably with an aqueous disper¬ sion of the acrylic coating composition described below. The coating compo¬ sition may conveniently be applied as an aqueous dispersion or emulsion using any of the well known coating techniques. For example, the film may be coated by roller coating, spray coating, gravure coating, or slot coating. The heat applied to the film during the subsequent pre-heating, stretching, and heat setting stages is generally sufficient to evaporate the water and crosslink the acrylic coating, if a crosslinkable monomer comprises a portion of the acrylic coating.

The coated, biaxially oriented PENBB film may then be heat treated for a period of time necessary to crystallize the film. Crystallization imparts the improved dimensional stability and excellent tensile properties to the PENBB film. The so coated, crystallized and biaxially oriented PENBB film is then wound onto a roll.

The above description describes an aqueous acrylic coating applied in¬ line (during manufacture). While this is the preferred process for the biaxially oriented copolyester film, organic solvent-based acrylic coatings as well as off-line coating (after manufacture of the film is complete) is a further aspect of the invention.

The acrylic copolymers used as primer layers according to this inven¬ tion consist essentially of at least about 50 percent by weight of one or more polymerized acrylic and/or methacrylic monomers. Although not necessary, the acrylic copolymer may contain from about 1 to 15 percent by weight of a copolymerizable comonomer, which in its copolymerized state is then capa¬ ble of intermolecular crosslinking by the application of heat with or without the addition of a separate resinous crosslinking agent.

The acrylic component of the primer copolymers is preferably present at a level of from about 50 to 99 percent by weight and preferably comprises an ester of acrylic acid or methacrylic acid, especially an alkyl ester wherein the alkyl group contains up to 10 carbon atoms such as methyl, ethyl, n- propyl, isopropyl, n-butyl, isobutyl, t-butyl, hexyl, heptyl and n-octyl. Acrylic copolymers derived from a lower alkyl (C, to C₄) acrylate, especially ethyl acrylate, together with a lower alkyl methacrylate provide particularly effecti¬ ve adhesion between the PENBB film and superimposed reprographic or matte layers. Primer copolymers comprising an alkyl acrylate, e.g. ethyl acrylate or butyl acrylate, together with an alkyl methacrylate, e.g. methyl methacrylate, desirably in a total proportion in the range of about 70 to 95 weight percent, are particularly preferred. The acrylate comonomer of such acrylic/methacry- lic combinations is preferably present in a proportion in the range of 15 to 65 mole percent and the methacrylate comonomer preferably in a proportion exceeding that of the acrylate comonomer generally by an amount in the range of 5 to 20 mole percent. The methacrylate is preferably present in a proportion in the range of 35 to 85 mole percent.

Examples of suitable crosslinking comonomers include N-methγlol acrylamide, N-methylol methacrylamide and their corresponding ethers; epoxide materials such as glycidyl acrylate, glycidyl methacrylate and allyl- glycidyl ether; carboxyl containing monomers such as crotonic acid, itaconic acid or acrylic acid; anhydrides such as maleic anhydride or itaconic anhydri¬ de; hydroxy containing monomers such as allyl alcohol and hydroxy ethyl or propyl acrylate or methacrylate; amides such as acrylamide, methacrylamide or maleamide; and isocyanates such as vinylisocyanate or allylisocyanate.

Preferred comonomers from the above are N-methylolacrylamide and N-methylolmethacrylamide, mainly because copolymer chains containing one of these monomers are capable of condensing with one another with the ap¬ plication of heat to form the desired inter-molecular crosslinking. In the case of copolymers containing the other functional monomers, it is necessary to form blends of two or more copolymers containing different functional como- nomers to achieve the desired crosslinking, e.g. blending an acrylic/crotonic acid copolymer with an acrylic copolymer containing isocyanate, epoxide or N-methylol functionality capable of reacting with acidic functional groups.

Specific additional combinations of such blended acrylic copolymers include copolymers containing monomers having epoxide functionality combi- ned with copolymers containing monomers having amine, acid anhydride, carboxyl, hydroxyl, or N-methylol functionality; copolymers containing mono¬ mers having N-methylol or N-methylol ether functionality combined with copolymers containing monomers having isocyanate functionality combined with copolymers containing monomers having carboxyl or hydroxyl functiona- lity; and so forth. Preferably, the functional monomers present in the mixed copolymer systems are present in approximately equimolar amounts.

The acrylic copolymers also may contain up to about 49 percent by weight of one or more halogen-free non-acrylic monoethylenically unsaturated monomers interpolymerized therewith. Suitable comonomers include dialkyl maleates such as dioctyl maleate, di-isooctyl maleate and dibutyl maleate; vinyl esters of versatic acid; vinyl acetate, styrene, acrylonitrile and similar materials.

The preferred crosslinking mixed copolymer compositions for the purposes of this invention are approximately 50/50 blends of an ethyl acryla- te/methyl methacrylate/crotonic acid copolymer with an ethyl acrylate/methyl methacrylate/glycidylacrylate copolymer, blends of an ethyl acrylate/methyl methacrylate/methacrylamide copolymer with an ethylacrylate/methyl metha- crylate/N-methylolacrylamide copolymer; and compositions based on copoly¬ mers of ethyl acrylate/methyl methacrylate/N-methylolacrylamide such as copolymers containing from about 50 to 99 percent by weight of acrylic and/or methacrylic monomers, 0 to 49 percent by weight of the mono-ethyle- nically unsaturated monomer and about 1 to 15 percent by weight N-methy- lolacrylamide. More preferred as copolymers containing from about 70 to 90 percent by weight of acrylic and/or methacrylic monomers, about 0 to 25 percent by weight of the mono-ethylenically unsaturated monomer, and from about 5 to 10 percent by weight N-methyoiacrylamide. As pointed out above, the polymeric acrylic primer composition does not require the presence of an external crosslinking agent such as a melamine or urea/formaldehyde condensate.

The primer coating is applied to the base mono- or multilayer PENBB film preferably as an aqueous dispersion or latex and at a solids concentration within the range of about 0.5 to 15 percent, preferably about 3 to 10 percent by weight. The preferred solids level is such as to yield a final dry coating thickness within the range of about 1 nm to 300 nm, which translates into a solids level on a dry weight basis of from 1 mg/m² to 300 mg/m². The preferred thickness range of the dried PENBB copolyester primer is from 15 to 50 nm, with 25 nm being the target thickness. The thickness of the base mono- or multilayer film to which the coating is applied may range from about 1 to about 250 μm.

The coating may be applied to one or both sides of the mono- or multilayer PENBB film, or it may be applied to one side only with optionally a different coating applied to the opposite side. Additives known in the .art may 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. Normally, it is desirable to in¬ clude a surfactant to enhance the wettability of the aqueous coating to the base PENBB film.

PENBB film primed with the above acrylic composition has excellent utility as a film base for the production of photosensitive reprographic films. 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 diazionium 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 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. These matte coatings may be based on an organic 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 solvents may also include those listed above. Particulate materials include finely divided (less than 10 μm particle size) clays or silica. 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 and U.S. Patent No. 3,624,021.

Yet another application of the primed film of this invention is as a packaging or label material. The primed biaxially oriented PENBB films de- monstrate good adhesion to organic solvent based printing inks and markedly improved UV resistance, dimensional stability and stiffness (tensile strength) when compared with unprimed films, especially unprimed PET films. These inks may comprise organic solvent dispersions or solutions of pigments and/or dyes in combination with acrylic resins or other resins and thickening agents.

Accordingly, the excellent UV resistance, dimensional stability, stiff¬ ness (tensile strength) and adhesive qualities of PENBB film primed with the crosslinked acrylic copolymer layers of this invention to organic solvent based coatings applied thereto renders such film of more universal utility to the manufacturer of finished reprographic, graphic and packaging products.

Reprographic adhesion (Repro) is evaluated using a lacquer comprising (in parts by weight) 9 parts cellulose acetate butyrate of "20 second" grade dissolved in a mixture of 88 parts ethylene glycol monomethyl ether (methyl cellosolve) and 3 parts methyl ethyl ketone with Rhodamine B dye added for color (3 parts of a 1 percent Rhodamine B dye in n-butanol). The lacquer is applied to the coated surface of the film by means of a wire-wound bar, Meyer rod No. 70, and cured in an oven for 5 minutes at 60^β C. The coating is scored with a cross-hatched pattern by means of scalpel. A strip of adhe¬ sive tape (Scotch tape 610) is adhered to the cross-hatched area, rubbed with a fingernail to insure intimate contact and then pulled rapidly from the film. The amount of lacquer remaining in the cross-hatched area is expressed as a percentage of the amount remaining, i.e. no lacquer removed = 100 percent adhesion, all lacquer removed = 0 percent adhesion with intermedia¬ te adhesion values being assessed in terms of the proportion of lacquer area remaining adhered to the film. The test is done twice on two sheets (total of

4 areas tested) and the adhesion value reported as that of the test area showing the most failure.

Repro test results demonstrating less than 95 percent adhesion are not generally considered acceptable, whereas test results approaching or equaling 100 percent adhesion are the target standard.

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 strength 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 example is illustrative of the invention.

EXAMPLE A latex comprising 4.5 percent by weight solids of a copolymer of 60 weight percent of methyl methacrylate, 35 weight percent ethyl acrylate, and

5 weight percent of N-methyloiacrylamide, and a surfactant are applied as a primer coating to a PENBB film which is made of 289 parts by weight of dimethyl 2,6-naphthalene dicarboxylate, 322 parts by weight of dimethyl 4,4'-bibenzoate, 368 parts by weight of ethylene glycol. Granules of this polymer, having a melting point of 281 °C are melted in a single screw ex¬ truder at temperatures of 280 to 320°C and extruded through a sheet die onto a cooling roll, temperature controlled at 30°C. A 120 μm thick film is obtained which is clear and transparent. Its density is 1.31 g/cm³. This pre- film is then sequentially biaxially oriented. The longitudinally drawn film is corona treated by a corona discharge apparatus and thereinafter coated with the latex described 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 transverse direction. The biaxially drawn film is heat set at a temperature of 230°C. These heat treatments result in the crosslinking of the primer coating.

The thus obtained primer coated PENBB film shows the following characteristics compared to a similarly coated PET film.

TABLE 1

As is evident from the above table, stiffness (tensile strength, tensile modulus), elongation at break, shrinkage, and UV resistance are improved as compared to similarly coated PET films. The film produced according to the above example is tested for reprographic adhesion as set forth above and provides greater than 95 percent adhesion as compared with a control uncoa- ted polyester film which exhibited 0 percent adhesion.

Claims

THAT WHICH IS CLAIMED IS:

1. Primer coated, biaxially oriented self-supporting, mono- or multilayer copolyester film, wherein the copolyester is PENBB and wherein the primer coating contains an acrylic polymer.

2. Primer coated biaxially oriented copolyester film according to Claim 1, wherein the primer coating consists essentially of a copolymer of at least 50 percent by weight of polymerized acrylic and/or methacrylic monomers, from 1 to 15 percent by weight of a copolymerized functional group containing comonomer which in the copolymerized state is capable of inter-molecular crosslinking by the application of heat.

3. Primer coated biaxially oriented copolyester film according to Claim 1 or 2, wherein the primer coating contains from 0 to 49 percent by weight of one or more halogen-free mono-ethylenically unsaturated copolymerized monomers.

4. Claim 2 or 3, wherein said functional group present in said functional group containing monomer is selected from the group consisting of amine, amide, acid anhydride, N-methylol, carboxyl, hydroxyl, epoxy and isocyanate.

5. Primer coated biaxially oriented copolyester film according to Claim 2, 3 or 4, wherein said functional group containing comonomer is selected from the group consisting of N-methylolacrylamide and N-methylolmethacrylamide.

6. Primer coated biaxially oriented copolyester film according to any one or more of the preceding claims, wherein the coating is applied on one surfa¬ ce of the film.

7. Primer coated biaxially oriented copolyester film according to Claims 1- 5, wherein the coating is applied on both surfaces of the film.

8. Primer coated biaxially oriented copolyester film according to any one or more of the preceding claims, wherein the film is a monolayer film.

9. Primer coated biaxially oriented copolyester film according to any one or more of the preceding claims with a birefringeance < 0,2.

10. Primer coated 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.

1 1. Use of the primer coated biaxially oriented copolyester film according s to claim 1 as a packaging film.

12. Use of the primer coated biaxially oriented copolyester film according o claim 1 as a reprographic film.