Double-layer polyester film and laminated metal sheet

Abstract

A double-layer polyester film is of a structure having upper and lower layers. Each of the upper and lower layers comprises a copolyester and an additional resin, and is a uniform mixture of the copolyester and the additional resin. The copolyester comprises SiO2 having a mass fraction of 800-2000 ppm added by means of in-situ polymerization. The copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol. The polyester film has thinner thickness, good thermal adhesion with a metal sheet, excellent resistance to deep drawing and complex deformation processing, and excellent corrosion resistance, and can be widely used in metal packaging industry.

Description

Double-layer polyester film and laminated metal sheet TECHNICAL FIELD

[0001] The present application relates to the field of laminated metal sheets for metal packaging, in particular to a double-layer polyester film and laminated metal sheet. BACKGROUND

[0002] In the metal can industry, the traditional method is to process and form cans by coating the surface of the metal sheet, wherein the solvent-based paint used in the coating process contains bisphenol A type non-food safety substances, and the coating process causes environmental problems and a large amount of energy consumption due to the volatilization of a large amount of solvents. In recent years, the industry has been committed to finding qualified new materials to overcome the above-mentioned problems.

[0003] Laminated iron is a flexible polyester film and a metal sheet directly bonded together by hot melting, which can meet the needs of downstream users in can forming, differentiated printing, filling of various contents, high-temperature sterilization of acid, alkali and salt, and long shelf life.

[0004] With the tightening of environmental protection policies and the increasing attention of people to food safety, laminated iron as an environmentally friendly and high-performance new material has attracted more and more attention in the can industry. In recent years, some enterprises have put their laminated iron products into the market. To meet the differentiated needs of different purposes, the raw materials and film layer structure of the polyester film need to be optimized and designed to meet the different requirements of each process downstream.

[0005] Patent application No. CN1839036 discloses a laminated metal sheet and a drawn can used therein. The patent uses a double-layer film and a metal sheet to cover and combine, which is easy to cause defects such as delamination and falling off during can forming. The ethylene-based ionomer used as an intermediate adhesive layer has poor performance during high-temperature sterilization.

[0006] Patent application No. CN 102432984 A discloses a cast polyester film and a metal sheet and metal can used therein. The patent uses a single-layer polyester film and a metal sheet to cover and combine, which is easy to cause the prepared metal can to have poor performance during high-temperature sterilization of acid, alkali and salt due to insufficient barrier resistance.

[0007] Patent No. CN 106142782 B discloses a polyester film for laminated iron hot sealing and a preparation method thereof. The patent features a three-layer polyester film and a metal sheet to cover and combine. The performance of the film is limited by the high crystallization of the lower layer, which leads to insufficient processing and forming performance, and is not suitable for deep drawing and complex deformation purposes.

[0008] The patent application CN 102463725 A discloses a kind of polyester film and its preparation method, the polyester film of this patent is three layers (ABA) structure used in electronic label field, its characteristics are that two surface layers are same resin, it is difficult to meet the performance requirements such as adhesion with metal plate, contact with mold to meet forming processing, contact with the contents in tank, etc., it is easy to lead to performance compromise difficulty and compromise, it is not suitable for being used in canning industry.

[0009] In the prior art, the single-layer film meeting the deep drawing processing requirement of the coated metal plate lacks high-temperature sterilization performance for filling acid, alkali and salt, the multi-layer film meeting the high-temperature sterilization requirement for filling acid, alkali and salt cannot meet the deep drawing processing requirement; the traditional double-layer film is prone to interlayer peeling during deep processing and forming. The traditional film adopts a silicon chip master batch method to add an opening agent, which leads to too many types of resins in the film, and is not suitable for deep drawing processing.

[0010] SUMMARY

[0011] The purpose of the present application is to provide a double-layer polyester film and a coated metal plate, which can meet the performance requirements of a metal container for food and beverage packaging with deep drawing and complex deformation requirements.

[0012] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions.

[0013] According to one aspect of the present application, a double-layer polyester film is provided, which is divided into an upper layer and a lower layer, the upper layer comprises a copolymerized polyester and other resins, and the lower layer comprises a copolymerized polyester and optional other resins, the copolymerized polyester comprises SiO2 added by in-situ polymerization in a mass fraction of 800-2000 ppm. Preferably, the mass fraction of SiO2 in the copolymerized polyester is 1000-1500 ppm.

[0014] According to one aspect of the present application, a double-layer polyester film is provided, which is divided into an upper layer and a lower layer, each of the upper and lower layers comprises a copolymerized polyester and other resins, which are a uniform mixture of the copolymerized polyester and the other resins, the copolymerized polyester comprises SiO2 added by in-situ polymerization in a mass fraction of 1200 ppm.

[0015] In the double-layer polyester film according to one aspect of the present application, the copolymerized polyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexane dimethanol and neopentyl glycol.

[0016] In the double-layer polyester film according to one aspect of the present application, the melting point of the copolymerized polyester is between 200-240℃. Preferably, the melting point of the copolymerized polyester is between 210-230℃.

[0017] In the double-layered polyester film according to one aspect of the present application, the intrinsic viscosity of the copolyester is 0.68-0.72 dL / g.

[0018] In the double-layered polyester film according to one aspect of the present application, the intrinsic viscosity after solid-phase viscosity increase is 0.75-0.78 dL / g.

[0019] In the double-layered polyester film according to one aspect of the present application, the other resin is selected from one of a homopolyester resin, a titanium white masterbatch-containing polyester resin, and a mixture of a homopolyester resin and a golden polyester masterbatch resin.

[0020] In the double-layered polyester film according to one aspect of the present application, the homopolyester resin is selected from PET, PBT, and PPT. Preferably, the melting point of the homopolyester resin is between 250 and 270°C, preferably between 258 and 265°C.

[0021] In the double-layered polyester film according to one aspect of the present application, the double-layered polyester film is manufactured by co-extrusion and biaxial stretching at a temperature of 250-280°C.

[0022] In the double-layered polyester film according to one aspect of the present application, the mass ratio of the other resin in the upper layer is not more than 40%, and the mass ratio of the other resin in the lower layer is not more than 10%.

[0023] In the double-layered polyester film according to one aspect of the present application, the mass ratio of the other resin in the upper layer is in the range of 10-35% in some embodiments. Preferably, the ratio of the other resin in the upper layer is not more than 30%. In some embodiments, the mass ratio of the other resin in the lower layer is more than 0% to ≤10%. In some embodiments, the lower layer does not contain the other resin.

[0024] In the double-layered polyester film according to one aspect of the present application, the copolyester contained in the upper layer and the lower layer has the same melting point.

[0025] In the method for preparing the double-layered polyester film for use in the packaging of medium- and high-end food and beverages according to one aspect of the present application, the thickness of the upper layer is 8-25 μm, and the thickness of the lower layer is 3-6 μm.

[0026] Preferably, the thickness of the lower layer is 4-5 μm.

[0027] In the method for preparing the double-layered polyester film for use in the packaging of medium- and high-end food and beverages according to one aspect of the present application, the melting point of the copolyester is between 200 and 240°C.

[0028] Preferably, the melting point of the copolyester is between 210 and 230°C.

[0029] According to another aspect of the present application, there is provided a coated metal sheet comprising a metal substrate and a double-layered polyester film as described above coated on the surface of the metal substrate.

[0030] According to another aspect of the present application, there is provided a coated metal sheet comprising a metal substrate and a double-layered polyester film as described above coated on the surface of the metal substrate.

[0031] According to another aspect of the present application, there is provided a coated metal sheet comprising a metal substrate and a double-layered polyester film as described above coated on the surface of the metal substrate. 2 According to another aspect of the present application, there is provided a coated metal sheet comprising a metal substrate and a double-layered polyester film as described above coated on the surface of the metal substrate.

[0032] According to another aspect of the present application, there is provided a coated metal sheet comprising a metal substrate and a double-layered polyester film as described above coated on the surface of the metal substrate.

[0033] Compared with the prior art, the present application has the following beneficial effects: the double-layered polyester film of the present application has excellent thermal adhesion to metal sheets, and is particularly suitable for deep drawing and various complex deformation processes without delamination; and the double-layered polyester film of the present application has better corrosion resistance than single-layered films.

[0034] The SiO2 is added to the polymer by in-situ polymerization, which improves the crystallization of the polyester film as a whole and uniformly; and the high-melting-point resin is avoided by improving the conventional method of adding SiO2 in the form of a master batch. The above two points significantly improve the comprehensive performance of the polyester film, and the complex processing resistance and corrosion resistance of the coated iron are significantly improved. DETAILED DESCRIPTION

[0035] The present application provides a double-layered polyester film, which is divided into two layers, the upper layer comprising copolymerized polyester and other resins, and the lower layer comprising copolymerized polyester and optionally other resins. Generally, the mass ratio of other resins in the upper layer film is not more than 40%, such as not more than 30% or not more than 20%. In some embodiments, the mass ratio of other resins in the upper layer film is in the range of 10-30%. The mass ratio of other resins in the lower layer film is not more than 10%, such as 0-10%. In some embodiments, the mass ratio of other resins in the lower layer film is greater than 0% to ≤10%, such as 2-10%. The thickness of the upper layer is generally 8-25 μm, and the thickness of the lower layer is generally 3-6 μm. Preferably, the thickness of the lower layer is 4-5 μm.

[0036] The copolymer polyester used to form the double-layered polyester film of the present application is preferably a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol and neopentyl glycol. Preferably, the copolymer polyester contains 800 to 2,000 parts by mass of SiO2 added in-situ during polymerization. Preferably, the parts by mass of SiO2 in the copolymer polyester is 1,000 to 1,500 ppm, more preferably 1,200 ppm. As used herein, "in-situ addition" means that the SiO2 is mixed with the monomers (i.e., terephthalic acid, ethylene glycol, isophthalic acid, 1,4-cyclohexane dimethanol and neopentyl glycol) used to synthesize the copolymer polyester, and then polymerized to produce the copolymer polyester of the present application. The copolymer polyester of the present application can be produced using conventional methods for producing PET polyester. Preferably, the copolymer polyester used in the present application has a melting point of 200 to 240°C, more preferably 210 to 230°C. Preferably, the copolymer polyester of the present application has an intrinsic viscosity of 0.68 to 0.72 dL / g. Preferably, the copolymer polyester of the present application has an intrinsic viscosity of 0.75 to 0.78 dL / g after solid state polymerization.

[0037] In the present application, the intrinsic viscosity is measured using a technique commonly used in the art.

[0038] As used herein, "other resins" means resins other than the copolymer polyester described herein, including but not limited to homopolymer polyester resins, titanium dioxide-containing masterbatch polyester resins, golden polyester masterbatch resins, and any mixture thereof.

[0039] The homopolymer polyester resins suitable for use in the present application include but are not limited to PET, PBT and PPT. Preferably, the homopolymer polyester resins have a melting point of 250 to 270°C, preferably 258 to 265°C.

[0040] The titanium dioxide-containing masterbatch polyester resins suitable for use in the present application contain TiO2 in an amount of 40 to 80%, preferably 55 to 65% by mass. Preferably, the titanium dioxide-containing masterbatch polyester resins have a melting point of 250 to 270°C, preferably 255 to 265°C.

[0041] The golden polyester masterbatch resins suitable for use in the present application preferably have a melting point of 250 to 270°C, more preferably 258 to 265°C.

[0042] Preferably, when a mixture of two other resins is used, such as when a mixture of a homopolymer polyester and a golden polyester masterbatch resin is used as the other resins, the mass ratio of the two other resins can be in the range of 1:3 to 3:1.

[0043] Preferably, when the lower layer contains an other resin, the other resin is preferably a homopolymer resin.

[0044] The double-layered polyester film of the present application can be manufactured by co-extrusion and biaxial stretching at a temperature of 250-280°C.

[0045] The double-layered polyester film of the present application can be applied to high-end food and beverage packaging.

[0046] The present application also provides a coated metal sheet comprising a metal substrate and a double-layered polyester film as described in any of the embodiments herein coated on the surface of the metal substrate. The double-layered polyester film of the present application can be heat-bonded to the surface of the metal substrate at a pressure of 2-10 Kg and a temperature of 180-260°C. Typically, the metal substrate has a thickness of 0.10-0.50 mm. Suitable metal substrates for use in the present application can be selected from the group consisting of chromium-plated steel sheet, tin-plated steel sheet, low-tin steel sheet, zinc-plated steel sheet, cold-rolled steel sheet and stainless steel sheet.

[0047] The present application also provides a metal container for high-end food and beverage packaging, which is made of the coated metal sheet of any of the embodiments of the present application.

[0048] The objects, features and advantages of the present application will become more apparent from the detailed description of the non-limiting embodiments thereof, which is sufficient to enable those skilled in the art to make and use the present application, as set out below in the specific embodiments.

[0049] Example 1

[0050] A method for producing a polyester film, which is manufactured by biaxial stretching at a temperature of 250-280°C, comprises a copolymerized polyester and other resins, the copolymerized polyester is PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexane dimethanol and neopentyl glycol, and has a melting point of 200°C. The polyester film is manufactured by double-layered co-extrusion, and comprises two layers, the upper layer has a thickness of 8-25 μm, and the lower layer has a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins, wherein the upper layer contains 70% by mass of the copolymerized polyester with a melting point of 200°C and 30% by mass of homopolymer polyester with a melting point of 262°C; and the lower layer contains 95% by mass of the copolymerized polyester with a melting point of 200°C and 5% by mass of homopolymer polyester with a melting point of 262°C. The copolymerized polyester contains 1200 ppm of SiO2, which is added by in-situ polymerization.

[0051] Preparation of coated iron: the prepared biaxially stretched polyester film is heat-bonded to the surface of a 0.19 mm chromium-plated steel sheet at a pressure of 2-10 Kg and a temperature of 180-260°C to obtain coated iron.

[0052] Example 2

[0053] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 240°C. The polyester film is manufactured by a double layer co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 80% by mass of the copolymerized polyester having a melting point of 240°C and 20% by mass of a homopolymer polyester resin having a melting point of 262°C; and the lower layer film contains 90% by mass of the copolymerized polyester having a melting point of 240°C and 10% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0054] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0055] Example 3

[0056] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C.

[0057] The polyester film comprises a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 230°C. The polyester film is manufactured by a double layer co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 70% by mass of the copolymerized polyester having a melting point of 230°C and 30% by mass of other resins, wherein the other resins comprise 1 / 3 by mass of a homopolymer polyester having a melting point of 262°C and 2 / 3 by mass of a golden polyester master batch resin having a melting point of 262°C; and the lower layer film contains 98% by mass of the copolymerized polyester having a melting point of 230°C and 2% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0058] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0059] Example 4

[0060] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 230°C. The polyester film is manufactured by a co-extrusion method, and the film comprises upper and lower layers, the upper layer having a thickness of 8-25 μm and the lower layer having a thickness of 3-6 μm. The upper layer is a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 90% by mass of the copolymerized polyester having a melting point of 230°C and 10% by mass of a homopolymer polyester having a melting point of 262°C; and the lower layer film contains 100% by mass of the copolymerized polyester having a melting point of 230°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0061] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0062] Example 5

[0063] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 230°C. The polyester film is manufactured by a co-extrusion method, and the film comprises upper, middle, and lower layers, the total thickness of the upper and middle layers being 8-25 μm and the lower layer having a thickness of 3-6 μm. The upper and middle layers are mixtures of the copolymerized polyester and the other resins uniformly mixed, and the resins are identical, containing 90% by mass of the copolymerized polyester having a melting point of 230°C and 10% by mass of a homopolymer polyester having a melting point of 262°C, and are essentially a single layer structure; and the lower layer film contains 100% by mass of the copolymerized polyester having a melting point of 230°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0064] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0065] Example 6

[0066] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 230°C. The polyester film is manufactured by a two-layer co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 70% by mass of the copolymerized polyester having a melting point of 230°C and 30% by mass of a homopolymer polyester having a melting point of 262°C; and the lower layer film contains 90% by mass of the copolymerized polyester having a melting point of 230°C and 10% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0067] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0068] Example 7

[0069] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 210°C. The polyester film is manufactured by a co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 70% by mass of the copolymerized polyester having a melting point of 210°C and 30% by mass of a homopolymer polyester having a melting point of 262°C; and the lower layer film contains 92% by mass of the copolymerized polyester having a melting point of 210°C and 8% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 800 ppm of SiO2 added by in-situ polymerization.

[0070] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0071] Example 8

[0072] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 210°C. The polyester film is manufactured by a double layer co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 68% by mass of the copolymerized polyester having a melting point of 210°C and 32% by mass of a homopolymer polyester having a melting point of 262°C; and the lower layer film contains 90% by mass of the copolymerized polyester having a melting point of 210°C and 10% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 2000 ppm of SiO2 added by in-situ polymerization.

[0073] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0074] Example 9

[0075] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 210°C. The polyester film is manufactured by a double layer co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 65% by mass of the copolymerized polyester having a melting point of 210°C and 35% by mass of a homopolymer polyester having a melting point of 262°C; and the lower layer film contains 90% by mass of the copolymerized polyester having a melting point of 210°C and 10% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0076] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chrome-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0077] Example 10

[0078] A method for producing a polyester film manufactured by a biaxial stretching method at a manufacturing temperature of 250-280°C, the polyester film comprising a copolymerized polyester and other resins, the copolymerized polyester being a PET polyester copolymerized with isophthalic acid, 1,4-cyclohexane dimethanol, and neopentyl glycol, and having a melting point of 230°C. The polyester film is manufactured by a double layer co-extrusion method, the film comprising upper and lower layers, the upper layer having a thickness of 8-25 μm, and the lower layer having a thickness of 3-6 μm. Both the upper and lower layers are a mixture of the copolymerized polyester and the other resins uniformly mixed, wherein the upper layer film contains 65% by mass of the copolymerized polyester having a melting point of 230°C and 35% by mass of a homopolymer polyester having a melting point of 262°C; and the lower layer film contains 90% by mass of the copolymerized polyester having a melting point of 230°C and 10% by mass of a homopolymer polyester resin having a melting point of 262°C. The copolymerized polyester contains 1200 ppm of SiO2 added by in-situ polymerization.

[0079] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0080] Comparative Example 1

[0081] A single layer polyester film is prepared from a modified PET resin having a melting point of 210°C by a casting method.

[0082] Preparation of a coated iron: the prepared single layer polyester film prepared by a casting method is hot-bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0083] Comparative Example 2

[0084] A three layer biaxially stretched polyester film, the upper layer resin being a 3 μm PET resin, the middle layer being a 14 μm resin of a blend of a 265°C PET and a 210°C PET (blend ratio of 7:3), and the lower layer being a 3 μm modified PET resin having a melting point of 210°C.

[0085] Preparation of a coated iron: the prepared biaxially stretched polyester film is hot-bonded to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare a coated iron.

[0086] Comparative Example 3

[0087] A three layer biaxially stretched polyester film of ABA structure, the upper layer resin being a modified PET resin having a melting point of 210°C, the middle layer being a PET resin having a melting point of 265°C, and the lower layer being a modified PET resin having a melting point of 210°C, the three layers having a thickness ratio of 1:8:1.

[0088] Preparation of the coated iron: The prepared biaxially-stretched polyester film was hot-attached to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare the coated iron.

[0089] Comparative Example 4

[0090] A two-layer biaxially-stretched polyester film prepared using a vinyl ionomer as an intermediate adhesive layer.

[0091] Preparation of the coated iron: The prepared biaxially-stretched polyester film was hot-attached to the surface of a 0.19 mm chromium-plated steel plate at a pressure of 2-10 Kg and a temperature of 180-260°C to prepare the coated iron.

[0092] Test Example

[0093] The coated metal sheets obtained in Examples 1-10 and Comparative Examples 1-4 were subjected to draw and redraw (DRD) processing according to the processing conditions shown below, and were formed into can bodies through three press forming processes. The prepared 20 μm functional film was present on both the inside and outside of the can.

[0094] Forming processing conditions (draw and redraw method)

[0095] 1. Blanking diameter: 172 mm.

[0096] 2. First pass processing conditions

[0097] Punch diameter: 114.5 mm;

[0098] Die gap: 0.36 mm;

[0099] Blank holder force: 4000 kg;

[0100] Mold assembly temperature before forming: 55°C.

[0101] 3. Second pass processing conditions

[0102] Punch diameter: 88 mm;

[0103] Die gap: 0.4 mm;

[0104] Blank holder force: 3000 kg;

[0105] Mold assembly temperature before forming: 55°C.

[0106] 4. Third pass processing conditions

[0107] Punch diameter: 65.3 mm;

[0108] Die gap: 0.43 mm;

[0109] Blank holder force: 2000 kg;

[0110] Temperature of the mold assembly before forming: 55°C.

[0111] Necking and flanging was performed after forming using the conventional method for can making.

[0112] The cans produced by the above method were evaluated using the following method. The results are shown in Table 1.

[0113] (1) Process adhesion of the resin film

[0114] The resin film layer adhered to the surface of the steel sheet during the process of the draw and stretch can produced under the above forming conditions was evaluated by the naked eye. The state in which peeling did not occur until the final process was excellent.

[0115] (2) Acid resistance: The acid resistance of the coated iron draw can (can type 691) was evaluated to represent the corrosion resistance. The coated can was filled with 20 g / L citric acid solution, capped, and cooked at 121°C for 60 minutes. After cooling, the sample was removed and the acid spots on the surface were observed. The acid resistance of the coated iron was evaluated based on the results.

[0116] (3) Sulfur resistance: The sulfur resistance of the coated iron draw can (can type 691) was evaluated to represent the corrosion resistance. The coated can was filled with 1% Na2S solution, capped, and cooked at 121°C for 60 minutes. After cooling, the sample was removed and the sulfur spots on the surface were observed. The sulfur resistance of the coated iron was evaluated based on the results.

[0117] (4) Flavor retention: The coated iron draw can (can type 691) was filled with Qingdao Snow Beer (manufactured by Qingdao Beer Company), capped according to the conventional method, and stored in the can at 37°C for three weeks. The can was opened after three weeks of storage, and a panel of 100 people evaluated the flavor retention of the contents. The results were excellent if more than 90 people thought that the flavor of the contents before and after storage was the same, good if more than 60 people thought that the flavor of the contents before and after storage was the same, and poor if less than 60 people thought that the flavor of the contents before and after storage was the same.

[0118] Table 1

[0119]

[0120] Note: In Table 1, ╳ indicates poor; Δ indicates fair; o indicates good; and indicates very good; and - indicates that the condition was not evaluated.

[0121] In the above-mentioned embodiments, the copolymerized polyester and other resins are blended to prepare a film with a two-layer structure, and the film forming properties of the cast film forming method and the biaxial stretching film forming method are good, and a polyester film with a relatively small thickness can be prepared. The polyester film has many advantages such as excellent hot adhesion to a metal plate, excellent deep drawing and complex deformation processing resistance, excellent corrosion resistance, and can be widely used in the metal packaging industry.

[0122] By adding SiO2 in the copolymerized polyester through in-situ synthesis, the intervention of various polyester resins (melting point greater than 245℃) is avoided, and the basic requirement of the opening agent is met, and the comprehensive performance of the film is obviously improved.

[0123] Finally, it should be pointed out that although the present application has been described with reference to the current specific embodiments, those skilled in the art should recognize that the above embodiments are only used to illustrate the present application, and are not used as a limitation on the present application, and various equivalent changes or replacements can be made without departing from the concept of the present application, therefore, any changes or modifications to the above embodiments within the scope of the spirit of the present application will fall within the scope of the claims of the present application.

Claims

1. A double-layer polyester film, characterized in that, The bilayer polyester film consists of two layers, an upper layer comprising a copolyester and other resins, and a lower layer comprising a copolyester and optional other resins. The copolyester comprises SiO2 added in 800-2000 ppm by mass through in-situ polymerization.

2. The double-layer polyester film as described in claim 1, characterized in that: The double-layer polyester film is divided into upper and lower layers. Each of the upper and lower layers includes a copolyester and other resins, forming a homogeneous mixture of the copolyester and the other resins. The copolyester includes 1200 ppm of SiO2 added by in-situ polymerization.

3. The double-layer polyester film as described in claim 1, characterized in that: The copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanediol and neopentyl glycol.

4. The double-layer polyester film as described in claim 1, characterized in that: The other resins are selected from homopolymer polyester resins, titanium dioxide masterbatch polyester resins, gold polyester masterbatch resins, and any mixtures thereof.

5. The double-layer polyester film as described in claim 1, characterized in that: The proportion of other resins in the upper film is no more than 40%, and the proportion of other resins in the lower film is no more than 10%.

6. The double-layer polyester film as described in claim 1, characterized in that: The proportion of other resins in the upper film is no more than 30%.

7. The double-layer polyester film as described in claim 1, characterized in that: The upper layer has a thickness of 8-25 μm, and the lower layer has a thickness of 3-6 μm.

8. The double-layer polyester film as described in claim 7, characterized in that: The thickness of the lower layer is 4-5 μm.

9. The double-layer polyester film as described in claim 1, characterized in that: The melting point of the copolyester is between 200-240°C.

10. The double-layer polyester film as described in claim 9, characterized in that: The melting point of the copolyester is between 210-230°C.

11. The method for manufacturing the double-layer polyester film according to claim 1, characterized in that: The double-layer polyester film is prepared by co-extrusion and biaxial stretching at a temperature of 250-280°C.

12. A coated metal plate, characterized in that: It includes a metal substrate and a double-layer polyester film as described in any one of claims 1-10 covering the surface of the metal substrate.

13. The coated metal plate as described in claim 12, characterized in that: The metal substrate is 0.10 to 0.50 mm thick and is selected from chrome-plated steel plate, tin-plated steel plate, low-tin steel plate, galvanized steel plate, cold-rolled steel plate and stainless steel plate.

14. The method for manufacturing the coated metal plate according to claim 12, characterized in that: The method includes directly laminating a polyester film onto the heated surface of the metal substrate using a hot-melt lamination process.

15. A metal container for mid-to-high-end food and beverage packaging, characterized in that, The metal container is made of a coated metal sheet as described in any one of claims 12-13.