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 SiO.sub.2 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.

Claims

1. A double-layer polyester film, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein the upper layer comprises a copolyester and an additional resin, and the lower layer comprises a copolyester and optionally an additional resin, wherein the copolyester comprises 800-2000 ppm by mass of SiO.sub.2 added by in-situ polymerization.

2. The double-layer polyester film according to claim 1, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein each of the upper layer and the lower layer comprises a copolyester and an additional resin, wherein the copolyester and the additional resin form a homogeneous mixture, wherein the copolyester comprises 1200 ppm by mass of SiO.sub.2 added by in-situ polymerization.

3. The double-layer polyester film according to claim 1, wherein the copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol.

4. The double-layer polyester film according to claim 1, wherein the additional resin is selected from the group consisting of a homopolyester resin, a polyester resin comprising a titanium dioxide master batch, a golden polyester master batch resin and any mixtures thereof.

5. The double-layer polyester film according to claim 1, wherein the upper layer film comprises the additional resin in a proportion of not more than 40%, and the lower layer film comprises the additional resin in a proportion of not more than 10%.

6. The double-layer polyester film according to claim 1, wherein the upper layer film comprises the additional resin in a proportion of not more than 30%.

7. The double-layer polyester film according to claim 1, wherein 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 according to claim 7, wherein the lower layer has a thickness of 4-5 μm.

9. The double-layer polyester film according to claim 1, wherein the copolyester has a melting point of 200-240° C.

10. The double-layer polyester film according to claim 9, wherein the copolyester has a melting point of 210-230° C.

11. A method of manufacturing the double-layer polyester film according to claim 1, wherein the double-layer polyester film is made by co-extrusion and biaxial stretching at a temperature of 250-280° C.

12. A film-laminated metal plate comprising a metal substrate and the double-layer polyester film according to claim 1 laminated on a surface of the metal substrate.

13. The film-laminated metal plate according to claim 12, wherein the metal substrate has a thickness of 0.10-0.50 mm, and the metal substrate is selected from the group consisting of a chromium-plated steel plate, a tin-plated steel plate, a low-tin steel plate, a galvanized steel plate, a cold-rolled steel plate and a stainless steel plate.

14. A method of manufacturing the film-laminated metal plate according to claim 12, wherein the method comprises direct lamination of the polyester film on a heated surface of the metal substrate by hot melt lamination.

15. A metal container for medium-end to high-end food or beverage packaging, wherein the metal container is made of the film-laminated metal plate according to claim 12.

16. The film-laminated metal plate according to claim 12, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein each of the upper layer and the lower layer comprises a copolyester and an additional resin, wherein the copolyester and the additional resin form a homogeneous mixture, wherein the copolyester comprises 1200 ppm by mass of SiO.sub.2 added by in-situ polymerization.

17. The film-laminated metal plate according to claim 16, wherein: the copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol; and/or the additional resin is selected from the group consisting of a homopolyester resin, a polyester resin comprising a titanium dioxide master batch, a golden polyester master batch resin and any mixtures thereof; and/or the upper layer film comprises the additional resin in a proportion of not more than 40%, and the lower layer film comprises the additional resin in a proportion of not more than 10%; and/or the upper layer has a thickness of 8-25 μm, and the lower layer has a thickness of 3-6 μm; and/or the copolyester has a melting point of 200-240° C.

18. The metal container for medium-end to high-end food or beverage packaging according to claim 15, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein each of the upper layer and the lower layer comprises a copolyester and an additional resin, wherein the copolyester and the additional resin form a homogeneous mixture, wherein the copolyester comprises 1200 ppm by mass of SiO.sub.2 added by in-situ polymerization.

19. The metal container for medium-end to high-end food or beverage packaging according to claim 18, wherein: the copolyester is a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol; and/or the additional resin is selected from the group consisting of a homopolyester resin, a polyester resin comprising a titanium dioxide master batch, a golden polyester master batch resin and any mixtures thereof; and/or the upper layer film comprises the additional resin in a proportion of not more than 40%, and the lower layer film comprises the additional resin in a proportion of not more than 10%; and/or the upper layer has a thickness of 8-25 μm, and the lower layer has a thickness of 3-6 μm; and/or the copolyester has a melting point of 200-240° C.

20. The metal container for medium-end to high-end food or beverage packaging according to claim 15, wherein the metal substrate of the film-laminated metal plate has a thickness of 0.10-0.50 mm, and the metal substrate is selected from the group consisting of a chromium-plated steel plate, a tin-plated steel plate, a low-tin steel plate, a galvanized steel plate, a cold-rolled steel plate and a stainless steel plate.

Description

DETAILED DESCRIPTION

[0035] The present disclosure provides a double-layer polyester film, wherein the double-layer polyester film comprises two layers including an upper layer and a lower layer, wherein the upper layer comprises a copolyester and an additional resin, and the lower layer comprises a copolyester and optionally an additional resin. Generally, the mass proportion of additional resin in the upper layer film is not more than 40%, for example, not more than 30%, or not more than 20%. In some embodiments, the mass proportion of the additional resin in the upper layer film is 10-30%. The mass proportion of additional resin in the lower layer film is not more than 10%, e.g., 0-10%. In some embodiments, the mass proportion of the additional resin in the lower layer film is greater than 0% to <10%, for example, 2-10%. The upper layer generally has a thickness of 8-25 μm, and the lower layer generally has a thickness of 3-6 μm. Preferably, the lower layer has a thickness of 4-5 μm.

[0036] The copolyester used to form the double-layer polyester film of the present disclosure is preferably a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol. Preferably, the copolyester comprises 800-2000 parts by mass of SiO.sub.2 added by in-situ polymerization. Preferably, SiO.sub.2 in the copolyester has a content of 1000-1500 ppm by mass, preferably 1200 ppm by mass. The term “added by in-situ polymerization” or the like as used herein refers to mixing SiO.sub.2 with the monomers for synthesizing the copolyester (i.e. terephthalic acid, ethylene glycol, isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl dimethanol), and then polymerizing to produce the copolyester according to the present disclosure. A conventional process for preparing PET polyester may be used to prepare the copolyester of the present disclosure. Preferably, the copolyester suitable for the present disclosure has a melting point of 200-240° C., preferably 210-230° C. Preferably, the copolyester of the present disclosure has an intrinsic viscosity of 0.68-0.72 dL/g. Preferably, the copolyester of the present disclosure has an intrinsic viscosity of 0.75-0.78 dL/g after solid phase tackification.

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

[0038] As used herein, “additional resin” refers to a resin other than the copolyester described herein, including but not limited to a homopolyester resin, a polyester resin comprising a titanium dioxide master batch, a golden polyester master batch resin and any mixtures thereof.

[0039] Homopolyesters suitable for the present disclosure include but are not limited to PET, PBT and PPT. Preferably, the homopolyester resin has a melting point of 250-270° C., preferably 258-265° C.

[0040] In the polyester resin comprising a titanium dioxide master batch suitable for the present disclosure, the mass percentage of TiO.sub.2 may be 40-80%, preferably 55-65%. Preferably, the polyester resin comprising a titanium dioxide master batch has a melting point of 250-270° C., preferably 255-265° C.

[0041] The golden polyester master batch resin preferably has a melting point of 250-270° C., preferably 258-265° C.

[0042] Preferably, when a mixture of two additional resins is used, such as a mixture of a homopolyester and a golden polyester master batch resin used as the additional resin, the mass ratio may be in the range of 1:3 to 3:1.

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

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

[0045] The double-layer polyester film of the present disclosure can be used for medium-end to high-end food or beverage packaging.

[0046] There is further provided a film-laminated metal plate according to the present disclosure, wherein the film-laminated metal plate comprises a metal substrate and the double-layer polyester film described in any of the embodiments herein and laminated on a surface of the metal substrate. The double-layer polyester film of the present disclosure can be thermally bonded to the surface of the metal substrate at a pressure of 2-10 kg and a temperature of 180-260° C. Generally, the metal substrate has a thickness of 0.10-0.50 mm. The metal substrate suitable for the present disclosure may be selected from the group consisting of a chromium-plated steel plate, a tin-plated steel plate, a low-tin steel plate, a galvanized steel plate, a cold-rolled steel plate, and a stainless steel plate.

[0047] There is still further provided a metal container for medium-end to high-end food or beverage packaging according to the present disclosure, wherein the metal container is made of the film-laminated metal plate described in any of the embodiments in the present disclosure.

[0048] In the following detailed description, the objectives, features, and advantages of the present disclosure will become clearer and more apparent with reference to the non-limiting examples, and the content is sufficient to enable those skilled in the art to understand and implement the present disclosure.

EXAMPLE 1

[0049] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 200° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 200° C. and 30% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 95% by mass of the copolyester having a melting point of 200° C. and 5% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0050] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 2

[0051] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 240° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 80% by mass of the copolyester having a melting point of 240° C. and 20% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 240° C. and 10% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0052] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 3

[0053] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C.

[0054] The polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 230° C. and 30% by mass of an additional resin, wherein the additional resin comprised ⅓ by mass of a homopolyester having a melting point of 262° C. and ⅔ by mass of a golden polyester master batch resin having a melting point of 262° C.; and the lower layer film comprised 98% by mass of the copolyester having a melting point of 230° C. and 2% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0055] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 4

[0056] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230° C. The polyester film was manufactured by a co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper layer comprised a mixture of the copolyester and the additional resin which were mixed homogeneously, wherein the upper layer film comprised 90% by mass of the copolyester having a melting point of 230° C. and 10% by mass of a homopolyester having a melting point of 262° C.; wherein the lower layer film comprised 100% by mass of the copolyester having a melting point of 230° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0057] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 5

[0058] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230° C. The polyester film was manufactured by co-extrusion of an upper layer, an intermediate layer and a lower layer, wherein the total thickness of the upper layer and the intermediate layer was 8-25 μm, and the thickness of the lower layer was 3-6 μm. The upper layer and the intermediate layer each were a mixture of the copolyester and the additional resin which were mixed homogeneously. These two layers comprised completely the same resins, i.e., 90% by mass of the copolyester having a melting point of 230° C. and 10% by mass of a homopolyester having a melting point of 262° C. These two layers were indeed a monolayer structure. The lower layer film comprised 100% by mass of the copolyester having a melting point of 230° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0059] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 6

[0060] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 230° C. and 30% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 230° C. and 10% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0061] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 7

[0062] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 210° C. The polyester film was manufactured by a co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 70% by mass of the copolyester having a melting point of 210° C. and 30% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 92% by mass of the copolyester having a melting point of 210° C. and 8% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 800 ppm SiO.sub.2 added by in-situ polymerization.

[0063] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 8

[0064] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 210° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 68% by mass of the copolyester having a melting point of 210° C. and 32% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 210° C. and 10% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 2000 ppm SiO.sub.2 added by in-situ polymerization.

[0065] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 9

[0066] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 210° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 65% by mass of the copolyester having a melting point of 210° C. and 35% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 210° C. and 10% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0067] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

EXAMPLE 10

[0068] In the method of producing a polyester film, the polyester film was manufactured by a biaxial stretching process at a manufacturing temperature of 250-280° C., wherein the polyester film comprised a copolyester and an additional resin, wherein the copolyester was a PET polyester modified by copolymerization of isophthalic acid, 1,4-cyclohexanedimethanol and neopentyl glycol, and had a melting point of 230° C. The polyester film was manufactured by a double-layer co-extrusion process, and the film comprised two layers including an upper layer and a lower layer, wherein the upper layer had a thickness of 8-25 μm, and the lower layer had a thickness of 3-6 μm. The upper and lower layers each were a mixture of the copolyester and the additional resin that were mixed homogeneously, wherein the upper layer film comprised 65% by mass of the copolyester having a melting point of 230° C. and 35% by mass of a homopolyester having a melting point of 262° C.; and the lower layer film comprised 90% by mass of the copolyester having a melting point of 230° C. and 10% by mass of the homopolyester resin having a melting point of 262° C. The copolyester comprised 1200 ppm SiO.sub.2 added by in-situ polymerization.

[0069] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

COMPRATIVE EXAMPLE 1

[0070] A monolayer polyester film was made from a modified PET resin having a melting point of 210° C. by using a casting process.

[0071] Preparation of film-laminated steel: the monolayer polyester film prepared using the casting process was thermally 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 obtain the film-laminated steel.

COMPARATIVE EXAMPLE 2

[0072] In a three-layer biaxially stretched polyester film, the upper layer resin was a 3μm PET resin, the intermediate layer comprised a 14μm blended resin of 265° C. PET and 210° C. PET (having a blending ratio of 7:3), and the lower layer comprised a 3μm modified PET resin having a melting point of 210° C.

[0073] Preparation of film-laminated steel: the prepared biaxially stretched polyester film was thermally 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 obtain the film-laminated steel, wherein the lower layer was thermally laminated on the steel plate.

COMPARATIVE EXAMPLE 3

[0074] In a three-layer biaxially stretched polyester film having an ABA structure, the upper layer resin was a modified PET resin having a melting point of 210° C., the intermediate layer comprised a PET resin having a melting point of 265° C., and the lower layer comprised a modified PET resin having a melting point of 210° C. The thickness ratio of the three layers was 1:8:1.

[0075] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

COMPARATIVE EXAMPLE 4

[0076] A double-layer biaxially stretched polyester film was prepared using a vinyl ionomer as an intermediate bonding layer.

[0077] Preparation of film-laminated steel: the polyester film prepared by the biaxial stretching process was thermally 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 obtain the film-laminated steel.

TEST EXAMPLE

[0078] The film-laminated metal plates obtained in Examples 1-10 and Comparative Examples 1-4 were processed using the Draw and Redraw process (DRD) under the following processing conditions. They were formed into can bodies by punching three times. The 20 μm functional film prepared was located on both the inner and outer sides of the cans at the same time.

[0079] Processing Conditions (Draw and Redraw Process)

[0080] 1.Blank diameter: 172 mm.

[0081] 2.First-pass processing conditions

[0082] Punch diameter: 114.5 mm;

[0083] Die clearance: 0.36 mm;

[0084] Blank holder force: 4000 kg;

[0085] Mold assembly temperature before molding: 55° C.

[0086] 3.Second-pass processing conditions

[0087] Punch diameter: 88 mm;

[0088] Die clearance: 0.4 mm;

[0089] Blank holder force: 3000 kg;

[0090] Mold assembly temperature before molding: 55° C.

[0091] 4.Third-pass processing conditions

[0092] Punch diameter: 65.3 mm;

[0093] Die clearance: 0.43 mm;

[0094] Blank holder force: 2000 kg;

[0095] Mold assembly temperature before molding: 55° C.

[0096] After molding, conventional processes in can making were used for necking and flanging.

[0097] The cans prepared by the above methods were evaluated by the methods described below. The results are shown in Table 1.

[0098] (1) Adhesion of Reson Film in Processing

[0099] The resin film layer laminated on the steel plate surface was visually evaluated to see whether it was peeled off or not during the processing steps of the DRD can prepared under the abovementioned forming and processing conditions. The result where no peeling occurred till the final step is excellent.

[0100] (2) Acid resistance performance: After the film-laminated steel was punched into a can (can size 691), acid resistance performance evaluation was performed to represent corrosion resistance performance evaluation. The film-laminated can was filled with a 20 g/L citric acid solution. After the can was capped, the solution was boiled at 121° C. for 60 min. After cooling, the sample was taken out, and spots corroded by the acid on the surface of the sample were observed to evaluate the acid resistance performance of the film-laminated steel.

[0101] (3) Sulfur resistance performance: After the film-laminated steel was punched into a can (can size 691), sulfur resistance performance evaluation was performed to represent corrosion resistance performance evaluation. The film-laminated can was filled with a 1% Na.sub.2S solution. After the can was capped, the solution was boiled at 121° C. for 60 min. After cooling, the sample was taken out, and sulfide spots on the surface of the sample were observed to evaluate the sulfur resistance performance of the film-laminated steel.

[0102] (4) Flavor retention: After the film-laminated steel was punched into a can (can size 691), the can was filled with Tsingtao Snow Beer (manufactured by Tsingtao Brewery Company). After the filled can was sealed by using a conventional method, it was stored at 37° C. for three weeks. The can was opened after the three weeks of storage, and an evaluation panel of 100 people inspected the taste retention of the content. The taste retention is excellent if 90 or more people considered that there was no difference in the taste of the content before and after the storage, good if 60 or more people considered that there was no difference, and poor if less than 60 people considered that there was no difference.

TABLE-US-00001 TABLE 1 Adhesion in Acid Sulfur Flavor Test Item Processing Resistance Resistance Retention Example 1 ⊚ ⊚ ⊚ Excellent Example 2 ⊚ ⊚ ⊚ Excellent Example 3 ⊚ ⊚ ⊚ Excellent Example 4 ⊚ ⊚ ◯ Good Example 5 ⊚ ⊚ ◯ Good Example 6 ⊚ ⊚ ⊚ Excellent Example 7 ⊚ ⊚ ⊚ Excellent Example 8 ⊚ ⊚ ⊚ Excellent Example 9 ◯ ⊚ ⊚ Excellent Example 10 ◯ ⊚ ⊚ Excellent Comparative ⊚ ◯ Δ Δ Example 1 Comparative X — — — Example 2 Comparative X — — — Example 3 Comparative — — — — Example 4 Note: in Table 1, X means poor; Δ means mediocre; ◯ means good; ⊚ means very good; — means unable to be evaluated.

[0103] In the above Examples, the copolyester and an additional resin were blended and then formed into films having a double-layer structure including an upper layer and a lower layer. The formability of the films made by both the casting process and the biaxial stretching process is good, and the polyester films having a small thickness can be prepared. The polyester films have many advantages, such as excellent performance in thermal bonding to a metal plate, excellent endurance in deep drawing processing and complex deformation processing, and excellent corrosion resistance, widely useful in the metal packaging industry.

[0104] By adding SiO.sub.2 to the copolyester in the in-situ synthesis, no conventional silicon-containing polyester chips are used in the polyester films, so involvement of a variety of polyester resins (having a melting point of greater than 245° C.) is avoided. As a result, not only the basic requirements of an anti-blocking agent are satisfied, but also the overall performance of the films can be improved significantly.

[0105] Finally, it should be pointed out that although the present disclosure has been described with reference to the current specific examples, those skilled in the art should appreciate that the above examples are only used to illustrate the present disclosure, and are not used to limit the present disclosure. Various equivalent changes or substitutions can be made without departing from the concept of the present disclosure. Therefore, without departing from the essential spirit of the present disclosure, all changes and variations of the abovementioned examples will fall in the scope of the claims in the present disclosure.