FIELD OF THE INVENTION
The present invention relates to a single or multilayer, coated biaxially oriented polyester film which can withstand high temperature and water conditions under pressure without loss of optical density and barrier properties.

BACKGROUND OF THE INVENTION
It is common in the polyester film industry to produce specialty high oxygen and moisture barrier films for use as protective overwrap materials on foods and other oxygen or moisture sensitive products. This can be done with a variety of technologies including, but not limited to coextrusion of barrier polymers such as poly(ethylene vinyl alcohol) “EVOH”, poly (ethylene naphthalate) “PEN” and polyamides, or by subsequent coating of the film using barrier polymers such as EVOH or poly(vinylidene chloride) PvDC.
Another common approach for creating a high moisture and oxygen barrier is to coat the surface of the film with a layer of metal, such as aluminum or others like silicon oxide (SiOx), aluminum oxide (AlOx) etc. typically using vacuum deposition. These layers provide a nearly impenetrable and therefore excellent moisture and oxygen barrier as well as an economical cost for the applications. This is the most common way to produce metal-coated polyester film for creating moisture and oxygen barrier, which is opaque in nature. SiOx and AlOx deposition provides transparent barrier properties.
These Polyester based barrier films are used as part of packaging material for storage of food stuff and enhancing the shelf life of the food. Application like ready to use packaging film require packaging film to have functions such as boiling water resistance and heat resistance in order to prevent the packaging from dissolving away during thermal sterilization and prevent the contents from suffering a deterioration in flavor due to the dissolution. Such film are well known, and are used for packaging of food like vegetables, meat, fruit, and other, dried food. Such packaging material store food for a sufficiently long shelf life at room temperature or in a refrigerator or a freezer. So there is a need when such packaging material is placed in boiling water, or microwave for food preparation, it should withstand the cooking conditions and gas barrier properties.
EP1479513 talks about gas barrier film where barrier layer is SiOx, which is protected by anchor coat layer film comprising a mixture containing an oxazoline group-containing resin.
The present invention has been made in view of the above-mentioned problems faced by conventional films, and provides a polyester film superior in physical and mechanical properties and suitable for application to fields associated with boiling and pressurized treatment, which require tenacity, bending resistance, bag breakage resistance on dropping, impact resistance and the like.

OBJECT OF THE INVENTION
The present invention provides an improved coated biaxial oriented polyester film, which forms a good bond to metals, such as aluminum. It also withstands heat sterilization conditions efficiently and provides a coating over the metalized film, to withstand the harsh boiling and pressure conditions.

SUMMARY OF THE INVENTION
In an aspect, the present invention provides a coated biaxial oriented polyester film wherein coating consists of acrylic coating, polyurethane coating and a cross linker.
In another aspect, the present invention provides a process for preparation of a coated biaxial oriented polyester film comprising the steps of:
a) metallizing polyester film using a metal; and
b) coating metallized film using a coating composition, wherein the coating composition consists of acrylic coating, polyurethane coating and a cross linker.
DETAILED DISCRIPTION OF THE INVENTION
The present invention provides an improved polyester film, which is especially suitable for packaging snack foods and other consumer goods.
In an aspect, the present invention provides a coated biaxial oriented polyester film wherein coating consists of acrylic coating, polyurethane coating and a cross linker.
In another aspect, the present invention provides a process for preparation of a coated biaxial oriented polyester film comprising the steps of:
a) metallizing polyester film using a metal; and
b) coating metallized film using a coating composition, wherein the coating composition consists of acrylic coating, polyurethane coating and a cross linker.
The acrylic coating is a water based coating having acrylate copolymers. Acrylic coating contain hydrophobic component and hydrophilic component. The hydrophobic component can be selected from different acrylates and hydrophilic component be acrylic or methacrylic acid. The hydrophobic component selected is acrylate and are selected from monomer containing different acrylate components. Acrylate components are selected from methyl methacrylate, ethyl acrylate, butyl acrylate, and other functional acrylate components like acetoacetoxy ethyl methacrylate (AAEM), hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxybutyl(meth)acrylate. Acid components be acrylic acid, methacrylic acid and alike.
The co-polymer selected is having glass transition temperature (Tg) above 0ºC. More preferably, Tg should be 0ºC to 50ºC. Most preferably, Tg of co-polymer is above 20ºC to 50ºC. The process of preparation of different acrylic coating resin is mentioned in US2016/0319169, US5194550 and US6465591. US2016/0319169 talks about preparing acrylic emulsion for adhesive application; US5194550 talks about preparing acrylate emulsion using styrene for adhesive application and US6465591 talks about preparation of acrylic coating for paper film and rubber.
The polyurethane dispersion coating is a water based coating and can be aromatic or aliphatic urethane. The current polyurethane tried is aliphatic urethane. Aliphatic polyurethane is prepared by using a polydiol and Isocyanates. Polydiol can be polytetramethyleneglycol or polyhexamethylenegycol or a mixture thereof. Polydiols are linear polyether glycols with hydroxyl groups at both ends. Polyols can have molecular weight ranging from 650 to 4000. Based on molecular weight it can be liquid to solid in nature.
The Polyol considered is polytetramethyleneglycol (PTMG) having molecular weight ranging from 250 to 1500. The Polyol considered is polytetramethyleneglycol (PTMG) having molecular weight ranging from 250 to 1500. Isocyanates which are used in making water dispersible prepolymer, are made by using commonly used isocyanates such as hexamethylene diisocyanate (HDI), methylene dicyclohexyl diisocyanate or hydrogenated MDI (HMDI) isophorone diisocyanate (IPDI), isophorone diisocyanate, cyclohexane diisocyanate, cyclohexylmethane diisocyanate, phenylene diisocyanate, toluenediisocyanate, napthalenediisocyanate, dimethylol propionic acid (DMPA) is widely used for synthesizing water dispersible PUDs (polyurethane dispersions). It has two hydroxyl group and one acid group. Hydroxyl group of DMPA react with isocyanates to form PUDs.
Aminosulphonic acid are used for preparing PUDs and proved valuable as chain extending agents. Amino sulphonic acid is used for increasing the hydrophobicity of PUDs. Amino sulphonic acid that can be used are 1-propanesulfonic acid, 2- [3- [(2-aminoethyl) amino] propionamido] -2 -methyl-, monosodium salt; monosodium salt of ethylenediamino-2-ethanesulfonic acid; monosodium salt of ethylenediamino-3-propanesulfonic acid.
Chain lengthening agent used is hydrazine based, having molecular weight in range of 32 to 400. The hydrazine is used as hydrazine, hydrazine hydrate or aqueous hydrazine.
The process of preparation of PUD is mentioned in U.S. Patent numbers US4870129, US5594065 and US4797427. US4870129 is about polyurethane for adhesive application, US5594065 is about PU and acrylic hybrid for water based PUD, US4870129 is about use of diamines as chain extenders in PUD.
The cross linker selected can be polyaziridine based cross linker, carbodimide based crosslinker, melemiane and/or formaldehyde based cross linker. The addition of cross linker to acrylic or polyurethane produce a marked improvement in water and chemical resistance even at elevated temperature. The cross linker is available with Cross linker CX-100 from DSM, Xama 7 from Polyaziridine Global, etc; carbodimide crosslinker are available with Nisshinbo Chemicals Inc, etc, Melamine formaldehyde based crosslinker as Cymel 303, Cymel 327, Cymel 325 etc from Cytec.
The present invention relates to the coating composition that bear high temperature and pressure conditions without affecting the critical properties of the film. The coating is performed over the metallized polyester film and properties like barrier optical density, are monitored.
In an embodiment, coating can be performed over the metallized polyester film, where metallized polyester film can be obtained by spurting metal over the plain polyester film or over the coated polyester film.
In the embodiment polyester film can be corona treated polyester film or coated polyester film. The coating applied can be acrylic, or co-polyester coating which are commonly used over the polyester film for metallization.
In the present embodiment, polyester film is biaxial oriented polyester, where polyester film is oriented in both machine and transverse direction to produce biaxial oriented polyester film.
The polyester film is prepared by process comprising the steps of:
a) allowing dicarboxylic acid and alkylene glycol to react along with additives to obtain molten polyester, having Tg between 55°C to 80°C; and
b) extruding the said molten polyester through a die and quenching the same on chilled rollers to obtain transparent polyester film.
Dicarboxylic acid is selected from terephthalic acid or dimethyl terephthalate either used alone or in combination with two or more components which may be 2,6 naphthalene dicarboxylic acid, isophthalic acid, and phthalic acid, aliphatic dicarboxylic acid like adipic acid or ester of same. Alkylene glycol is ethylene glycol either used singly or in combination with two or more component which comprises of diethylene glycol, propylene glycol, butanediol, hexanediol, neopentyl glycol, trans-1,4 cyclohexanediol and cis-1,4-cyclohexanediol.
The dicarboxylic acid and alkylene glycol are mixed in the ratio of 1:0.3 to 1:0.5 along with the additives like antiblocking agents, antioxidants, stabilizers and other suitable compounds at high temperature to produce polyester chips which is then melted in extruder and oriented to produce polyester film. The orientation is usually done above Tg of the polyester.
The polyester film can be multilayer or single layer polyester film. Single layer polyester film is prepared by extrusion of polyester film containing suitable anti block for providing the sooth run ability of the film. Multilayer film is prepared by co-extrusion of two or more polyester resin layers together to produce polyester film. Multilayer polyester film has more than one layer structure preferably three layers where two layers are outer layers and third layer is base layer. Both outer layers are on the opposite sides of base layer. The outer layer of the film can have different or same composition. Base layer is generally consists of polyester resin along with small amount of silica. One or both outer layer have polyester along with anti-block for smooth run ability of film. The antilock can be inorganic or organic in nature. Inorganic anti-block generally used is silica and widely available by the name of Sylobloc or like. Variety of Sylobloc that are available commercial are Sylobloc 42, Sylobloc 44, Sylobloc 46, Sylobloc 35 etc. The use of organic and inorganic anti-block to obtain better run ability of film is well known in the art.
Polyester film so formed has thickness in range of 6 micron to 150 micron, preferably from 6 micron to 100 micron and more preferably from 6 micron to 50 micron.
The metallization can be done over coated polyester film or plain polyester film. The treated polyester film is generally biaxial oriented polyester film which is coated with water dispersible co-polyester resin. The common water dispersible co-polyester resin readily available are AQ55 from Eastman, WB 630 and WB 730 from Sensho Chemicals Japan.
These co-polyester resins are dissolved in water at 70-90ºC in presence of small quantity of isopropyl alcohol and /or Ethylene glycol tertiary butyl ether and / or ammonia. The solid content of dissolved co-polyester resin is approx. 10wt%. 0.5-5wt% of crosslinker is added in solution to crosslink the copolyester resin. Crosslinker generally taken is melamine based crosslinker and available as Cymel 303, Cymel 327 from Cytec. This co-polyester coating containing cross-linker is applied inline during BOPET film production before the transverse direction orientation process to produce coated biaxial oriented polyester film which is well known in the art.
The metallization performed over the coated or plain polyester film. Metallization can be opaque or transparent in nature. Opaque metallization performed by deposition aluminum metal over the surface and transparent metallization is performed by deposition AlOx or SiOx over the surface. These metallization is used to improve the water and oxygen barrier properties of the polyester film. Higher barrier properties shows that food packed using barrier film will have high shelf life. The opaque metallization can be monitored by Optical density method where higher the optical density means higher the metal coverage over the film and better the barrier properties. The optical density of opaque polyester film is generally kept in the range of 0.5 to 5.0.
The barrier layer of the film is made up of aluminum or AlOx and it can be opaque and transparent.
In an embodiment, the present invention provides a barrier coated polyester film where barrier layer is applied over plain polyester film.
In another embodiment, barrier layer is applied over co-polyester coated polyester film and where co-polyester coating is applied inline during polyester film production.
The coating composition of the present invention contains acrylic coating composed of hydrophobic and hydrophilic components where hydrophobic component is acrylate and hydrophilic component is acrylic acid or methacrylic acid. It also contains polyurethane coating where polyurethane coating is aliphatic in nature.
The coating composition of the present invention has thickness in the range of 0.05 to 5gsm (gram/meter2).
The coating so performed over the barrier layer coated polyester film. The coating is performed in the range of 0.05gsm to 5.0gsm. The coating is performed by gravure based offline coating method. Higher the coating thickness better will be property retention after high temperature boiling test.
The polyester film of the present invention is single or multi layered, where a water based coating is applied over the metal barrier layer, to protect the metal and barrier properties of the metal barrier layer. The metal barrier layer can be aluminum layer or silicon oxide layer or aluminum oxide layer. Top coating is over the metal barrier layer where the coating is formed by crosslinking of acrylic coating, and polyurethane coating. The crosslinking of acrylic and polyurethane is performed by using polyaziridine based crosslinking agents. The coating composition should be such that coating can withstand high temperature in presence of moisture under pressure without effecting the barrier metal layer and barrier properties of the film. The film should also withstand the gelbo test condition without generation of pinholes after subjecting film to high temperature in presence of moisture under pressure conditions. The conditions that coating should withstand are high temperature of 121ºC in presence of water for 30 minutes in a closed autoclave under pressure of 0.24Mpa.
Test performed:
High Temperature boiling test – The metallized coated film is subjected to a high temperature test under a closed system in presence of water. The coated metallized film is subjected to temperature of 121ºC for 30 minutes. The sample is then tested for property change.
Gelbo test – The metallized coated polyester film is subjected to
Optical Density – The metallized coated film is observed for optical density check as per ASTM.
OTR – Oxygen vapour transmission was checked as per ASTM 3985
Pin Holes –Average number of pin holes was checked per 100 square meter. This test is performed by spreading any colored ink over the test sample surface. Test sample is kept over a white cloth that has capability to retain ink. Number of ink marks over the cloth per 100 square meter of test samples are checked.
Printability – Printability was checked using Rexta from Toyo Ink. Test sample is printed using this ink and checked of removal of ink using scotch tape no. 610 by hand pull method. If Ink transfers from test sample to scotch tape by hand pull method, shows failure of printing on test sample.
Unless stated to the contrary, any of the words “comprising”, “comprises” and includes mean “including without limitation” and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.
The following example is given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLES
Metallized polyester film of 12 micron thickness is coated using a coating composition containing acrylic coating, polyurethane coating and cross-linker.
The acrylic coating contains acrylate copolymers with small amount of acid. The acrylate selected is methyl methacrylate, methacrylate, Ethyl methacrylate and Butyl acrylate. Acid is methacrylic acid. The Acrylic coating has glass transition temperature (Tg) of about 25ºC.
Preparation of acrylic coating
Added polyvinyl alcohol solution (5wt%, 151gms) in water (400gms) and ethoxylated acetylenic surfactant (6.3gms), sodium lauryl ether sulfonate (22gms), and tert butyl hydroxyl peroxide (0.5gms). Added solution of sodium salt of hydroxymethane sulfonic acid (1wt%, 150gms). To this solution added a mixture of containing butyl acrylate (68gms), methacrylate (20gms), 2-hydroxy methyl methacrylate (50gms), acrylic acid (18gms) and methyl methacrylate (35gms). pH was maintained at 7.0-8.0 by adding triethylamine. The reaction was allowed to react at 55-60ºC for 3-4 hours. After reaction was completed, reaction mass was cooled and pH was maintained at 7.5 to 8.0 by adding liquor ammonia.
Preparation of polyurethane coating
The polyurethane coating is an aliphatic polyurethane. Aliphatic polyurethane was prepared by mixing PTMG (52.5gms), dimethylol propionic acid (2gms), isophorone diisocyanate (28.4gms), hydrazine hydrate (9gms) and diamine-2,2-{-2 amino ethyl)amino}ethane sulphonic acid (8gms).
For preparing prepolymer for PUD, PTMG of molecular weight 1000 and dimethylol propionic acid was taken and agitated at 50-70ºC where the mixture is in melted stage. At this temperature isophorone diisocyanate was added slowly with vigorous agitation. Added dibutyltin dilaurate catalyst (0.01gms) at 80-90º C and agitated for 3-4hours till NCO content maintained at 5.5wt%.
Hydrazine hydrate and diamine-2, 2-{-2-amino-ethyl)amino}ethane sulphonic acid were added in water and during high speed stirring prepolymer was added. Now ethyl acetate was removed during distillation. PUD so formed was neutralized by triethylamine.
Crosslinker taken is polyaziridine which is commercially available as CX-100 from DSM.
Example – 1
The metallized polyester film was prepared by metallization of plain polyester film using aluminum based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.15 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 2
The metallized polyester film was prepared by metallization of plain polyester film using AlOx based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.15 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 3
The metallized polyester film was prepared by metallization of co-polyester (WB730 from Sensho Japan) coated polyester film using aluminum based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.15 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 4
The metallized polyester film was prepared by metallization of co-polyester (WB730 from Sensho Japan) coated polyester film using AlOx based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.15 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 5
The metallized polyester film was prepared by metallization of plain polyester film using aluminum based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.20 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 6
The metallized polyester film was prepared by metallization of plain polyester film using AlOx based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.20 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 7
The metallized polyester film was prepared by metallization of co-polyester (WB730 from Sensho Japan) coated polyester film using aluminum based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.20 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.

Example – 8
The metallized polyester film was prepared by metallization of co-polyester (WB730 from Sensho Japan) coated polyester film using AlOx based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.20 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight is 0.25gsm.
Example – 9
The metallized polyester film was prepared by metallization of plain polyester film using aluminum based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.10 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25 gsm.
Example –10
The metallized polyester film was prepared by metallization of plain polyester film using AlOx based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.10 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
Example – 11
The metallized polyester film was prepared by metallization of co-polyester (WB730 from Sensho Japan) coated polyester film using aluminum based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.10 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.

Example – 12
The metallized polyester film was prepared by metallization of co-polyester (WB730 from Sensho Japan) coated polyester film using AlOx based metal and metallized film was coated with coating composition containing acrylic coating, polyurethane coating and cross linker in a ratio of 1:1:0.10 respectively. The water was added as a solvent for keeping solid content of 10wt%. The coating was performed where coating weight was 0.25gsm.
The details of examples are summarized below.
Table 01

Example Treatment Type Barrier Layer Top Coating
Acrylic Polyurethane Cross linker
1 Plain Film Aluminum 1 1 0.15
2 Plain Film AlOx 1 1 0.15
3 Co-Polyester Aluminum 1 1 0.15
4 Co-Polyester AlOx 1 1 0.15
5 Plain Film Aluminum 1 1 0.20
6 Plain Film AlOx 1 1 0.20
7 Co-Polyester Aluminum 1 1 0.20
8 Co-Polyester AlOx 1 1 0.20
9 Plain Film Aluminum 1 1 0.10
10 Plain Film AlOx 1 1 0.10
11 Co-Polyester Aluminum 1 1 0.10
12 Co-Polyester AlOx 1 1 0.10

The samples produced were analyzed for optical density and barrier properties. Results are shown below.

Table 02
Example Optical density Barrier (OTR) Pin Holes Printability
1 2.5 1.2 1-2 No ink Transfer
2 0.05 2.1 1-2 No ink Transfer
3 2.5 0.89 1-2 No ink Transfer
4 0.05 1.9 1-2 No ink Transfer
5 2.5 1.2 1-2 No ink Transfer
6 0.05 2.1 1-2 No ink Transfer
7 2.5 0.89 1-2 No ink Transfer
8 0.05 1.9 1-2 No ink Transfer
9 1.4 1.2 1-2 No ink Transfer
10 0.02 2.1 1-2 No ink Transfer
11 2.5 0.89 1-2 No ink Transfer
12 0.05 1.9 1-2 No ink Transfer

The metallized coated polyester film is subject to high temperature boiling test and checked for changes in property. Results are shown below.
Table 03
Example Optical density Barrier (OTR) Pin Holes Printability
1 2.5 1.4 1-2 No ink transfer
2 0.05 2.4 1-2 No ink transfer
3 2.5 0.91 1-2 No ink transfer
4 0.05 2.1 1-2 No ink transfer
5 2.5 1.3 1-2 No ink transfer
6 0.05 2.5 1-2 No ink transfer
7 2.5 0.93 1-2 No ink transfer
8 0.05 2.0 1-2 No ink transfer
9 1.6 -- -- --
10 0.02 -- -- --
11 1.8 -- -- --
12 0.02 -- -- --

The coated film produced from example 1 to 8 was kept for aging for about 90 days and checked for property change after high temperature boiling test. Results are shown below.
Table 04
Example Optical density Barrier (OTR) Pin Holes Printability
1 2.5 1.4 1-2 No ink transfer
2 0.05 2.4 1-2 No ink transfer
3 2.5 0.91 1-2 No ink transfer
4 0.05 2.1 1-2 No ink transfer
5 1.2 -- -- --
6 0.03 -- -- --
7 1.5 -- -- --
8 0.03 -- -- --

CLAIMS:WE CLAIM:
1. A coated biaxial oriented polyester film wherein coating comprises of acrylic coating, polyurethane coating and a cross linker.
2. A process for preparation of a coated biaxial oriented polyester film comprising the steps of:
a) metallizing polyester film using a metal; and
b) coating metallized film using a coating composition, wherein the coating composition consists of acrylic coating, polyurethane coating and a cross linker.
3. The process as claimed in claim 2, wherein the metal is selected from a group consisting of aluminum, silicon oxide and aluminum oxide.
4. The coated biaxial oriented polyester film as claimed in claim 1 and 2, wherein the acrylic coating consists of hydrophobic components and hydrophilic components.
5. The coated biaxial oriented polyester film as claimed in claim 4, wherein the hydrophobic components are selected from a group consisting of methyl methacrylate, ethyl acrylate, butyl acrylate, acetoacetoxy ethyl methacrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl(meth)acrylate and the hydrophilic components are selected from a group consisting of acrylic acid and methacrylic acid.
6. The coated biaxial oriented polyester film as claimed in claim 4, wherein the hydrophilic components are selected from a group consisting of acrylic acid and methacrylic acid.
7. The coated biaxial oriented polyester film as claimed in claim 1 and 2, wherein the polyurethane coating consists of aliphatic urethane selected from a group consisting of polydiol, polytetramethyleneglycol and polyhexamethylenegycol or a mixture thereof; isocyanates selected from a group consisting of hexamethylene diisocyanate (HDI), methylene dicyclohexyl diisocyanate, hydrogenated MDI (HMDI) isophorone diisocyanate (IPDI), cyclohexane diisocyanate, cyclohexylmethane diisocyanate, phenylene diisocyanate, toluenediisocyanate, napthalenediisocyanate; and dimethylol propionic acid.
8. The coated biaxial oriented polyester film as claimed in claim 1 and 2, wherein the crosslinker is selected from a group consisting of polyaziridine based cross linker, carbodimide based crosslinker, melemiane based cross linker and formaldehyde based cross linker.
9. The process as claimed in claim 8, wherein the crosslinker is a polyaziridine based cross linker.