The present disclosure generally relates to the field of flexible packaging. More specifically, the present invention relates to a heat-sealable peelable polyester-based lidding which can be heat-sealed on thermoformed rigid containers such as including but are not limited to APET, RPET and polyvinyl chloride (PVC). The present invention also discloses chemical composition of a heat-sealable and peelable layer coated on a BOPET film. BACKGROUND OF THE INVENTION
As modern lifestyle demands more time on a professional front and less time for a personal front, everyone looks forward to ready-prepared meals. The heat sealable and peelable flexible packaging film has a significant role in ready-to-eat food packaging with the easy opening without causing any damage or spillage due to the usage of scissors, knife, manual tearing, and breaking at end users. Conventionally, such films can be produced by coating the layer of sealable polymers dissolved in organic solvents such as Ethyl acetate, IPA or Ethyl alcohol onto the BOPET substrate. In some instances, such films can be made by hot-melt extrusion coating techniques.
Another conventional method includes a multilayer extrusion process in which the base layers are PET-based, and the skin layer (sealable layer) is amorphous polyester-based. The most widely used process is a coating process where a sealable polymer preferably EVA (Ethylene-vinyl acetate) is dissolved in organic solvents such as Ethyl acetate, IPA or ethyl alcohol, followed by coating through an offline gravure coating technique onto the base substrate. However, all the conventional techniques mentioned aforementioned has one or more disadvantages. For example, EVA coating involves the use of solvents that being volatile can easily escape to the environment, thereby causing threats thereto. The solvents recovery is an inefficient and expensive process; the VOC emissions cause a threat to the environment. Moreover, such coatings have medium to low-level performance with APET, and PVC containers. As per some conventional methods, different peelable-sealable films can be coextruded. For example, the peelable lidding film can be prepared using a multistep approach including the base film layer as biaxially oriented Polyester

film, coated with the hot-melt resin/adhesives to form a skin layer that serves as peelable seal film. The commonly used coextruded resins for peelable sealing include ethylene-vinyl acetate (EVA) copolymer resin, a modified polyester copolymer resin, ethylene acrylate resin, etc. However, EVA-based such compositions also have limited performance with respect to peelable properties and film transparency. Vinyl acetate Co-polymer is also not compatible with PET polyester hence it has recycling issues as well.
Therefore, in light of the foregoing discussion, there exists a need for developing safe peelable lidding film with a greener high performance and environmentally, friendly and efficient coating without the use of any solvent.
OBJECTS OF THE INVENTION
The present invention relates to a peelable and heat-sealable polyester-based lidding including a heat-sealable surface layer and polyester-containing core layer. The heat-sealable layer includes 0.8-1.5 gsm dry coating thickness of thermoplastic polyurethane polymer. The heat sealable layer also includes a wax additive, contributing as a peeling agent as well as an anti-blocking agent. Another object of the present invention is to provide a specially formulated aqueous PUD, which is coated on the BOPET film. Such a film provides a heat-sealable layer when thermally sealed as a lid onto APET, and/or PVC thermoformed food containers. Such a lid can be smoothly peelable at the required peel force.
Another objective of the present invention is to provide a coating on a polyester film that has all the constituents approved for direct food contact and having a minimal thickness. The film involves a dry coating thickness of 0.8-1.5 gsm thereon. The film substrate has a thickness in the range of 12 to 50microns.
Yet another objective of the present invention is to provide an aqueous coating that is transparent, stretchable and has anti-fog properties. Such coatings can be applied in-line or offline.
SUMMARY OF THE INVENTION

In one aspect of a present invention, a heat-sealable peelable polyester based film is disclosed. The film includes a biaxially oriented polyester-based base film having a skin layer. The skin layer is coated in-line or offline with an aqueous heat-sealable and peelable coating. The coating includes thermoplastic polyurethane in the range of 40-80 wt%, wax additive in the range of 1-15 wt %, and amine crosslinker in the range of 0.1-2 wt%. The skin layer is amorphous polyester layer. The coated side of the film seals to thermoformed plastic containers.
In another aspect of the present invention, an aqueous heat-sealing and peeling coating for polyester based film is disclosed. The coating includes thermoplastic polyurethane in the range of 40-80 wt%, wax additive in the range of 1-15 wt %, and amine crosslinker in the range of 0.1-2%.
In yet another aspect of the present invention, a method for preparing a heat-sealable and peelable film with a composition is disclosed. The method includes a biaxially oriented polyester-based film having a base layer and a skin layer. The skin layer is coated inline or offline with an aqueous heat-sealable and peelable coating. The method includes diluting polyurethane dispersion (PUD) solids in the range of 20-45wt% in water, followed by adding wax emulsions in the range of 0.1-5 wt% to the diluent PUD. Thereafter, the method involves mixing the above contents until obtaining a homogeneous mixture. Further, the method includes adding the crosslinker in the range of 0.1-2 wt% to the homogeneous mixture, followed by mixing until obtaining a uniform dispersion. Then, the dispersion undergoes filtering, followed by applying the dispersion on a treated polyester film; and drying the film.
STATEMENT OF THE PRESENT INVENTION
Accordingly, the present invention relates to an easy peelable polyester-based lidding which can be heat-sealed on the thermoformed container substrates not limited to APET, and PVC. The invention also discloses the peelable and heat-sealable layer composition. BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the embodiment will be apparent from the following description when read with reference to the accompanying drawings. In the drawings, wherein like reference numerals denote corresponding parts throughout the several views:
Referring to Figure 1, illustrated is a flowchart depicting a method (100) for forming an aqueous heat sealable and peelable polyester film, in accordance with an illustrative embodiment of a present invention;
Referring to Figure 2, illustrated is a result graph depicting the sealing and peeling strengths of the lidding film with containers substrates of APET, and PVC at different sealing temperature, in accordance with the illustrative embodiment of the present invention; and
Referring to Figure 3, illustrated is a result graph depicting sealing and peeling strengths of the lidding film with the container substrates of APET, and PVC in accordance with the illustrative embodiment of the present invention; and Referring to Figures 4A-4E, show mechanism involved in sealing and peeling of the lidding film with the container substrates of APET, and PVC in accordance with the illustrative embodiment of the present invention DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and

"comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
As used herein, the singular forms "a", "an", "the" include plural referents unless the context clearly dictates otherwise. Further, the terms "like", "as such", "for example", "including" are meant to introduce examples which further clarify more general subject matter, and should be contemplated for the persons skilled in the art to understand the subject matter.
The reference in this specification to any prior art publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgement or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavor to which this specification relates.
The term "Flexible packaging" herein relates to any packaging or a part thereof whose shape can be changed readily. For example, bags, pouches, tubes, and so on.
The term "Polyethylene terephthalate (PET)" herein defines a form of polyester, which can be extruded, cast, and stretched bidirectional to form a thin film for packaging for the purposes of packaging foods, beverages, pharmaceutical products like tablets, and so on.
The term "Biaxially-oriented polyethylene terephthalate (BOPET)" herein defines a polyester film that is obtained by stretching the PET film in two perpendicular directions thereof, achieving the BOPET film. The film is well preferred for flexible packaging due to its special properties such as high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties (low gas permeability), and electrical insulation (good insulation performance), good deflection, good hygroscopicity, good optical property, and so on. For packaging purposes, the packaging film must bear properties of good heat resistance, low thermal shrinkage, good machinability, good printability, good transparency, good barrier to oxygen and water vapour, good impact resistance; good puncture resistance. The BOPET film is considered the most preferred

flexible film for packaging food and non-food products. In the case of inline coatings, the BOPET film can be treated with inline corona. In case of offline coatings, an inline primer can be applied on the BOPET film with water dispersible copolyester or polyurethane dispersions or acrylics for high surface energy. In the present invention, the preferred films may include such as but are not limited to amorphous BOPET film (A-BOPET), recycled polyethylene terephthalate (RPET), and so on. In the present invention, the preferred film is a biaxial-oriented film which is chemically treated. In some embodiments, the film is a recycled PET film.
The term "Amorphous BOPET (A-BOPET)"film herein relates to a biaxial-oriented film having a skin layered coextruded amorphous co-polyester on the PET.
The term "Chemically treated BOPET film" herein relates to the BOPET film being treated chemically with chemicals, for example, acrylic, polyurethane, and so on.
The term "Coating" herein relates to a thin layer of a covering applied on a substrate.
The term "Polyurethane dispersion (PUD)" herein relates to a polyurethane polymer resin or acrylic resin which is dissolved in a low volatile solvent, followed by suspension in water. The solvent being volatile may escape from the solution, leaving the PUD on the surface.
The term "Wax emulsion" herein relates to a mixture of at least one wax with water.
The term "Heat sealability" herein relates to making at least two thermoplastic surfaces by application of heat and pressure, sealing thereof. The term "Peelability" herein relates to removal of the polyester film without tearing. Such peelability requires a certain peeling force to peel the film or lid from the container or packaging.
The present embodiment discloses an aqueous coating composition to be applied on a base polyester film. The base film can be a mono-layer or multi-layer film. In some embodiments, the base film can include two or three layers. The base film can be a mono-axially or a biaxially-oriented film. Biaxial orientation may be

sequential or simultaneous by known methods. In addition, the base film can be a polyester-based film. For example, the base film can include polyethylene terephthalate (PET). In some embodiments, the thickness of the base film is 12-50 microns. In some embodiments, the base film is a biaxially-oriented polyethylene terephthalate (BOPET) film. The BOPET film can include such as but are not limited to a normal BOPET, A-BOPET, chemically -treated BOPET film, recycled PET (rPET) film, and/or combinations thereof.
Typical polyester resins used in the base film can include such as, but are not limited to: homopolyesters or copolyesters of polyethylene terephthalate-co-isophthalate copolymer,polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethylene terephthalate-co-naphthalate copolymer, polyethylene-co-cyclohexylene terephthalate, polyether-ester block copolymer, ethylene glycol or terephthalic acid-based polyester homopolymers and copolymers, and combinations thereof.
In some embodiments, the base film is a two-layer film including a first layer on a side of a second layer. In some embodiments, the two-layer base film can be a first layer coextruded with a second layer. In some embodiments, the base film can be a skin layer. The polyester-based layer may have an intrinsic viscosity in the range of 0.5-0.9 dl/g. In some embodiments, the skin layer is a chemically treated layer. In some embodiments, the preferred film is amorphous (A-BOPET) layer. In some embodiments, the preferred film is recycled PET layer. In some other embodiments, the preferred film is a combination of A-BOPET film and the rPET film. In such an embodiment, the film includes rPET in the range of 10-90 wt% in addition to that of the remaining including the A-BOPET film and/or chemically treated film. For example, the film includes 80% of rPET + 20% of A-BOPET.
The present embodiment discloses an aqueous coating to be applied on the BOPET film. The coating can be applied inline or offline on the film. In some embodiments, the polyester film is applied with a primer, followed by offline application of the coating composition. Such primers are polymeric and may include such as but are not limited to poly (ethylenimine) (PEI), ethylene acrylic acid copolymer (EAA), etc. and other existing primers which are known in the art.

The coating is formulated such that a layer thereof has a direct contact with food under various storage conditions such as either frozen, refrigerated or at room temperature or when heating or cooking in a microwave or in a conventional oven. Thickness of the coating may vary between 0.8 gsm and 1.5 gsm, and more preferably between 1.0 and 1.5 gsm. In such an aqueous coating, the heat seal composition is thermoplastic, which softens at sealing temperatures in the range of 120°C-170°C, thereby providing good heat seal properties. Such a coating being water based is environmentally friendly.
In some embodiments, the present invention is applicable for storing various
materials including such as but are not limited to foodstuffs, medical appliances,
pharmaceuticals, and/or combinations thereof, and so on. Examples of the food
stuffs may include such as but are not limited to dry and semi-solid food items
like cereals, coffee, tea, crispy snacks, confectionaries, dairy products, meat, sea¬
foods, pet-foods, fruits, dry-fruits, and so on. Examples of the medical appliances
may include such as but are not limited to syringes, bottle caps, surgical gloves,
cotton, nursery aids, surgical instruments, speculums, umbilical catheters,
thermometers, nebulizers, diagnostic strips, medicine strips, table packaging
wrappers, and so on. Examples of the pharmaceuticals include such as but are not
limited to various medicines, tablets such as paracetamol, and so on.
The composition includes polyurethane dispersion solids; wax emulsion; cross
linker; and water. The polyurethane dispersion (PUD) solids are in the range of
20-45%.The most preferred raw materials used for the preparation of the
polyurethane dispersion are aliphatic polyols which have a molecular weight
range of up to 1000. Some of such suitable polyols include such as but are not
limited to polyether diols, polyester diols, or multi-functional polyols, hydroxyl-
containing epoxides, polybutadienepolyols, polyisobutylenepolyols,
polyhydroxypolyacetals, polyacrylatepolyols, hydroxyl-containing
polycaprolactones and the like, and mixtures thereof. Polyether polyols, polyester polyols, and polycarbonate polyols are preferred. However, to improve the heat seal properties, the polyols which contain three or more hydroxyl groups play an important role. In the embodiment, polyols which have at least three or more hydroxyl groups with molecular weight up to 500 are preferred. Few such

examples namely, glycerol, 1,2, 6-hexanetriol, 1,1-trimethylolpropane, N'-tetrakis(2-hydroxyethyl)-ethylenediamine, etc. The PUD preparation also involves the use of many suitable isocyanates such as butane-l,4-diisocyanate, ethylene diisocyanate, hexane-l,6-diisocyanate, 2,4-diisocyanate, 1,3,5-trimethyl cyclohexane - 2,4-diisocyanate, etc. The dispersion of Polyurethane can be achieved by using the appropriate chain pendant ionic group, non-ionic hydrophilic in the backbone structure of PUD. The usual anionic group incorporated into the polyurethane backbone is a carboxylic acid, sulfonic acid, sulphate, and phosphate group, etc. The most preferred pendant group used is a carboxylic acid group.
The composition also includes wax additive in the range of about 0.1-5%.Examples, the wax additive may include such as but are not limited to Carnauba wax, and so on. The wax additive is configured to provide peelability to the film and also protecting thereof from blocking.
The coating also includes multi-functional crosslinkers. The cross linkers can be in the range of about 0.1-2.0%; and remaining water as a diluent. Examples of the cross linker may include such as but are not limited to azirdine, N-methylol acrylamide, N-methylolmethacrylamide and their corresponding ethers, and so on. The viscosity and total solids of the coatings were adjusted at -30-50 wt% ratio using DI water. In some embodiments, the coating includes anti-fog in the range of0.5-5wt%.
In the embodiment, the coated film of the present invention is heat sealable to APET, and PVC container substrates. The film is made by coating a layer of heat sealable polymer on the BOPET substrate. The aqueous coating is water based, and applied to the polyester film uses water as a diluent instead of an organic solvent. Therefore, the coating composition is safer for manufacturing and is the environment friendly. Further, the coated polyester film is transparent, heat sealable, and peelable.
As shown in Figure 1, a method (100) for forming an aqueous heat sealable and peelable polyester film is disclosed. The method (100) includes a number of steps; sequence thereof may be exemplary for the persons skilled in the art to better understand the present invention. The method (100) involves diluting

polyurethane dispersion (PUD) solids in the range of 20-45wt% in water at step 102. The method (100) involves adding wax emulsions in the range of 0.1-5wt% to the diluent PUD, followed by mixing until obtaining a homogeneous mixture at step 104. The method (100) further involves adding cross linker in the range of 0.1-2wt% to the homogeneous mixture, followed by mixing the above contents until obtaining a uniform dispersion at step 106. Thereafter, the dispersion undergoes filtering at step 108, and coating on a treated polyester film at step 110. Finally, the film undergoes drying at step 112. The film undergoes drying in oven at 170°C for 20 seconds. The film further undergoes testing for heat sealable and peelable properties. Experimental details
So that various embodiments may be more readily understood, reference is made to the following examples which are intended to illustrate the various embodiments, but not limit the scope thereof, a) Preparation of heat-sealable peelable coating
The heat-sealable peelable coating was prepared using constituents enlisted in the following table:

SI No Ingredients Qty (grams)
1 PUD 62
2 Crosslinker 0.1
3 Carnauba wax emulsion 5
4 Antifog agent 0.2
5 Water 32.5
The PUD was charged in a stainless steel container under continuous agitation.
Item 2-4 were added individually under continuous mixing. The resultant coating
emulsion had a solid of approximately 30% and a # 4 Ford cup viscosity of 15
seconds.
Example 2.Coating of a Flexible BOPET film
A BOPET film having a thickness of about 30 microns was corona-treated at a
flux of 0.2 mJ/cm2. The coating was coated using the gravure technique onto the
BOPET film and dried in an oven at about 160°C (320°F) for about 10 seconds.

Results:
The obtained polyester film coated with the aqueous formulation of the present
invention was tested and the following results were achieved:

SI.
No Test parameters Experimen tl Experimen t2 Experimen t3 Experimen t4
1 Coating thickness (dry gsm) -1.5 -1.5 -1.5 -1.5
2 Haze (%) 6.5 6.8 6.2 6.8
3 Water resistance Pass Pass Pass Pass
6 Heat seal strength (Average) (140°C/lsec/70P SI)gF/25 mm APET RPET PVC -650 -550 -1200 -650 -550 -1150 -680 -500 -1100 -680 -540 -1300
Further, as shown in Figure 2, the graph between Heat Sealing Temperature on X-axis and Seal Strength on Y-axis depict the sealing strength for both the types of containers- PVC and APET with respect to the sealing temperature. While Figure 3 depicts the smooth peel performance without any jerks or without any much change of peel force at 140°C, 1 second, and 5 bar pressure. Figures 4A-4E show a mechanism of interaction of the peelable and sealable film with the BOPET film. As shown in Figure 4A, Film A has a first interior layer 402which is the BOPET layer and acts as a Base layer, whilelayer 404 is a second middle layer, which is a primer or amorphous layer. The layer 404 can be coextruded or inline coated on the layer 402. The film A further has an outer layer 406 which is a heat sealant layer. The heat sealant layer is a coated layer which includes polyurethane and wax additive. Layer 408 of film B represents the container substrate which is a heat sealable layer and it can be an APET, RPET, or PVC.
As the film A and film B are brought together into contact under a seal bar with a combination of temperature, pressure, and time, heat seal is formed. At first, the film A and the film B create an interface when brought together where molecular inter diffusion across the interface takes place as shown in Figure 4B. At elevated

temperature and applied pressure, layers404 and406 of the film A melt and get into interdiffusion with each other as shown in Figure 4C. The consequent mixed layers further get into interdiffusion with the layer 408 of the film B and creates a molecular entanglement as shown in Figure 4D.
The layer 406 (sealant polymers) are of low molecular weight, and cannot provide deep entanglement, however it has strong adhesion properties which enhances the seal strength. The amorphous layer of layer 404 diffuses with the layer 408 (e.g. APET), where, heat sealing is due to the stronger adhesion properties and diffusion dynamics of the polymers rather than crystallization. Therefore, the combination of the stronger adhesion properties and interdiffusion dynamics leads to a leak-proof heat seal.
Peel strength is a function of fracture energy or energy to create new surfaces during the peeling process. In this embodiment of the film, when force is applied to the heat seal, such as by the force associated with pulling apart, sealant layer 410 breaks and creates a peeled surface by cohesive failure. Layer 410 from the Figure 4E represents the peeled surface due to cohesive failure. The sealant layer consists of wax additives that do not have any direct role in heat sealable-peelable properties, but it helps to hinder the cold seal behaviour of the sealant layer. As aforementioned hereinabove, the film is applied to lidding of APET and PVC containers which is not limiting. In some embodiments, the present invention also relates to a composition of heat sealable layer including polyurethane dispersion (PUD) coating. Such films can be used, in particular but also not limited to food and food related materials. The aqueous coating to be applied onto the polyester film uses water as a diluent instead of organic solvent, hence safe for packaging. Therefore, the coating composition is safer to the consumers and the environment. Further, the polyester film is transparent (<6% haze), heat sealable and peelable. Despite of the heat sealable capabilities, the film has a peeling force strength of >600gf/25mm on APET containers. Further, such film has a peeling force strength of >1200gf/25mm on PVC containers. Hence, such film is sealable onto APET and PVC container substrates for storing various food items, pharmaceutical, medical items, etc. As the present invention provides coating thickness of 0.8-1.5gsm for the polyester films, thereby saves additional cost of raw materials.

Hence, the present invention provides polyester film and a coating composition for the purposes of flexible packaging to the environment which are green and safe to the environment.
The foregoing descriptions of exemplary embodiments of the present disclosure have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments were chosen and described in order to best explain the principles of the disclosure and its practical application, to thereby enable others skilled in the art to best utilize the disclosure and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions, substitutions of equivalents are contemplated as circumstance may suggest or render expedient but is intended to cover the application or implementation without departing from the spirit or scope of the claims of the present disclosure.

We Claim
1. A heat-sealable peelable polyester based film comprising a biaxially oriented polyester-based base film having a skin layer, the skin layer coated in-line or offline with an aqueous heat-sealable and peelable coating, the coating comprising thermoplastic polyurethane in the range of 40-80 wt%, wax additive in the range of 1-15 wt %, and amine crosslinker in the range of 0.1-2 wt%.
2. The film as claimed in claim 1, wherein the skin layer is amorphous polyester layer.
3. The film as claimed in claim 1, wherein coated side of the film seals to thermoformed plastic containers.
4. The film as claimed in claim 1, wherein thickness of the coating is in the range of0.6-2.0gsm.
5. The film as claimed in claim 1, wherein the coating includes anti-fog additive in the range of 0.5-5wt%.
6. The film as claimed in claim 1 or 2 or 3, wherein core layer of the film comprising recycled PET (rPET) in the range of 0- 90 wt%.
7. An aqueous heat-sealing and peeling coating for polyester based film comprising thermoplastic polyurethane in the range of 40-80 wt%, wax additive in the range of 1-15 wt %, and amine crosslinker in the range of 0.1-2%.
8. A method for preparing a heat-sealable and peelable film with a composition comprising a biaxially oriented polyester-based film having a base layer and a skin layer, the skin layer coated inline or offline with an aqueous heat-sealable and peelable coating comprising thermoplastic polyurethane in the range of 40-80 wt%, wax additive in the range of 1-15 wt %, and amine crosslinker in the range of 0.1-2%., the method comprising:
diluting polyurethane dispersion (PUD) solids in the range of 20-45wt% in
water;
adding wax emulsions in the range of 0.1-5 wt% to the diluent PUD,
followed by mixing until obtaining a homogeneous mixture;

adding the crosslinker in the range of 0.1-2 wt% to the homogeneous
mixture, followed by mixing until obtaining a uniform dispersion;
filtering the dispersion;
applying the dispersion on a treated polyester film; and
drying the film.