US 5,149,389 AGrant
Laminated Metal Sheet
Issue Date:1992-09-22
•35 Claims
•2 Drawing Sheets
Abstract
A laminated metal sheet, the metal sheet having adhered on at least one major surface thereof a film of a non-crystalline polyester, and a process for the preparation thereof. The laminated metal sheet is particularly useful for forming into drawn and wall ironed (DWI) cans. The laminate is also useful for forming other non-retorted packaging components.
Metadata
Assignee
- CMB Foodcan plc
Inventors
- Peter J. Heyes
- Nicholas J. Middleton
- Alison M. Brown
Application Information
Application Number:US 5125448
Filing Date:1990-04-18
Priority Date:1987-10-15
Art Unit:111
Classifications
IPC:
B32B 3126
Field of Search:
428220156623;626;659;457;458;483454;456;457272.4;309.9;322;324.4
Patent Drawings (2 sheets)
Description
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described in further detail, by way of example only, with reference to the following Examples, and with reference to the following drawings, in which:
FIGS. 1 and 2 show diagrams of apparatus suitable for performing the process of the present invention;
FIG. 3 shows a section taken through a laminate in accordance with the invention and comprising mono-layered polymer films (A) laminated to a metal strip (M);
FIG. 4 shows a section taken through a laminate similar to that of FIG. 3 but having a composite multi-layered polymer film (A) laminated to a metal strip (M);
FIG. 5 shows a section taken through a laminate similar to that of FIG. 4 but containing an additional film (B) of thermoplastic polymer laminated to the opposite side of the metal strip (M);
FIG. 6 shows a can end formed from a laminate in accordance with the invention; and
FIGS. 7a and 7b respectively show a drawn and wall-ironed can and a draw-redraw can formed from a laminate in accordance with the invention.
EXAMPLES 1 to 13
Polymer/metal/polymer laminates were prepared by a lamination process performed in apparatus as illustrated schematically in FIG. 1 or FIG. 2 of the accompanying drawings. A metal sheet M was pre-heated by infra-red or induction heating to an appropriate temperature T.sub.1 by a heater 1. Temperature T.sub.1 is usually within the range 140.degree. C. and 330.degree. C. Polyester films A and B were fed from feed rolls 2 and 4 and laminated to the opposite sides of the pre-heated metal sheet between lamination rolls 6, 8, typically having a diameter of 100-400 mm. Lamination was generally performed using a nip force of 200-400 N per metre between the lamination rolls.
In the lamination nip, intimate and uniform, wrinkle-free contact between the metal sheet and the polymer films is established. Downstream of the lamination rolls the resultant laminate is re-heated, preferably by use of an induction heater 10 or by infra-red heating, to a laminate temperature T.sub.2 at which the polymer films (A) will interact with and become strongly bound to the metal sheet. Temperature T.sub.2 is usually within the range 240.degree. to 330.degree. C. for polyesters. The metal polymer laminate is held at temperature T.sub.2 for a short period of time, usually no more than 2 seconds, and is then rapidly and uniformly quenched with water to a temperature below the glass transition point of the polyester in the films, for example about 80.degree. C. for PET. Quenching can be performed in any conventional manner, but typically can be performed by passing the laminate through a tank 12 of water as shown in FIG. 1 or by passing the laminate through curtain 14 of quenching water as shown in FIG. 1 and FIG. 2.
In general, the process illustrated in FIG. 1 with the lamination being performed in a vertical mode is preferred. Vertical movement of the metal strip through the lamination stage tends to allow a higher quench rate and gives better and more uniform quenching.
FIG. 1 also shows a schematic diagram of a typical temperature profile which would be found in the process illustrated in the apparatus of FIG. 1.
Thus, laminates were prepared from the materials given in Table I by preheating the metal strip by infra-red or induction heating, passing the metal strip and polymer films into a pair of nip rolls and laminating both major metal surfaces simultaneously with the polymer films. The resultant laminate was reheated by infra-red or induction, held for two seconds and quenched rapidly and uniformly with cold water.
Table II sets out a number of Examples showing the results obtained when preparing such laminates using various metal temperatures (T1) in the pre-lamination stage and various reheating temperatures (T2) in the post-lamination stage.
Temperature T.sub.2 is measured by pyrometer responding to radiation emitted at a wavelength of about 7.9 microns.
The resin of polyester Type I and Type II contains 95 mole percent terephthalic acid and 91 mole percent ethylene glycol.
The laminate formability was assessed by coating coverage after draw and wall ironing the laminate in two stages:
Stage 1: a cup (height 35 mm, diameter 86 mm) was drawn from the laminate, suitably lubricated.
Stage 2: a can body (diameter 65 mm, height 130 mm) was formed by redrawing and wall ironing.
After forming, the cans were rinsed in water and dried. Coating coverage was assessed by immersion in acidified copper sulphate for two minutes and visually inspecting for copper deposits or the "enamel rating" technique using a sodium chloride solution, a voltage of 6.3 V and measuring current in milliamps.
The influence of lamination temperatures on the polyester coating structure and formability of the laminate was assessed by x-ray diffraction. In this technique, the film or laminate is placed in an x-ray diffractometer. Count rates are measured when the flat samples are exposed to a beam of substantially monochromatic x-rays using an appropriate detector. The sample and detector are rotated in line with respect to the beam, maintaining the geometry such that the angle between the sample and beam (.theta.) and beam and detector remain in the ratio 1:2, as in a normal Powder diffraction scan. This data generates information on planes parallel to the sample surface.
In biaxially oriented PET, the (1,0,0) plane gives a high count rate at .theta. = 13.degree. C. but in amorphous PET the peak is absent. The ratio of 0 = 13.degree. peak heights for laminate and film is related to the amount of retained orientation in the laminate. Our results are presented as the ratio of peak heights and the laminated PET coating peak height for .theta. = 13. Laminate material B laminated in accordance with the teaching of GB 2123746 to retain orientation (see Example 4) had poor formability and failed to make cans without metal breaking or severe coating disruption. However, when laminate material B was processed to eliminate orientation and crystallinity, as in Example 3, it had excellent formability and afforded good protection after forming.
Similarly, biaxially oriented coextruded laminate materials C and J also gave good formability if amorphous, and poor formability if they retained orientation in the laminate (compare Example 5 with Example 6 and Example 13 with Example 14).
Cast, unoriented PET or PBT coatings were effective, provided they were laminated to produce an amorphous and not a crystalline condition. A crystalline condition is produced for example by slow cooling from the reheated stage.
PBT materials are considerably more expensive than polyesters closely related to PET (Examples 1 to 8 and 10 to 16).
Examples 11 and 12 show that laminates formed from polypropylene materials of the type described in GB 2003415 exhibited poor formability. Such laminates were found to give metal failure in can forming.
Examples 15 and 16 show the effectiveness of polyester coatings derived from biaxially oriented films of different film crystallinities.
The extent of re-orientation of the polyester coatings was assessed by examining a drawn and wall-ironed can made with the laminate and the conditions of Example 5. The following results were obtained:
The XRD data confirms that the laminated sheet had amorphous coatings and shows that the upper can wall is slightly oriented, more so for the can outside coating.
The lower 2 mm of the can wall external coating were not significantly affected by can forming and remained essentially amorphous.
The external walls of DWI cans formed from the laminates of Examples 1 to 13, 15 and 16 were decorated by a conventional dye sublimation process. The quality of the resultant decoration was found to be excellent, provided the label was not in contact with the lower 2 mm of the can wall, region "d" of FIG. 7a.
Claims
What is claimed is:
1. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a film of a non-crystalline polyester adhered directly thereto, said polyester being formed by polymerisation of a difunctional acid component comprising at least 60 mole percent terephthalic acid and a dihydric alcohol component comprising at least 60 mole percent ethylene glycol, which process comprises adhereing to one or both major surfaces of the metal sheet a monofilm of a polyester which is formed from a difunctional acid component comprising at least 60 mole percent terephthalic acid and a dihydric alcohol component comprising at least 60 mole percent ethylene glycol, said monofilm or monofilms being adhered to the metal sheet by heating the metal sheet to a temperature (T1) above the melting point of the polyester film, the temperature (T1) being such that during lamination of the polyester film or films to the metal sheet, the outer surfaces of the polyester film or films remain below their melting points, laminating the film or films to the metal sheet, reheating by indirect means the laminate to a temperature (T2) above the melting points of the polyester films; and after holding at these elevated temperatures, quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester coating resins, the laminating reheating and quenching conditions being such that the polyester monofilm or monofilms in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
2. A process according to claim 1, wherein the temperature to which the laminate is re-heated after lamination is from 220.degree.-330.degree. C.
3. A process according to claim 1, wherein the indirect means for re-heating the laminate is an induction heating means.
4. A process according to claim 1, wherein the or each polyester film which is applied to the metal sheet is a cast thermoplastic polyester.
5. A process according to claim 4, wherein the cast thermoplastic polyester is polyethylene terephthalate.
6. A process according to claim 1, wherein the or each polyester film which is converted into non-crystalline or amorphous form is a biaxially oriented polyester having a semi-crystalline structure.
7. A process according to claim 6, wherein the biaxially oriented polyester is a biaxially oriented polyethylene terephthalate.
8. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a composite film of a non-crystalline polyester adhered directly thereto, said polyester comprising a copolyester formed by polymerisation of a difunctional acid component comprising at least 60 mole percent terephthalic acid and a dihydric alcohol component comprising at least 60 mole percent ethylene glycol, which process comprises adhering to one or both major surfaces of the metal sheet a composite film or films of said copolyester having a polyester inner layer (A1) and a polyester outer layer (A2), said composite film or films of said copolyester being adhered to the metal sheet by a process which comprises (i) heating the metal sheet to a temperature (T1) above the softening point of the polyester inner layer (A1) but below the melting point of the polyester outer layer (A2), (ii) laminating the film or films to the metal sheet, and (iii) reheating by indirect means the laminate so that the metal sheet reaches a temperature above the melting point of the or each polyester film, (iv) after holding at these elevated temperatures, quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester coating resins, (v) the laminating, reheating, and quenching conditions being such that the polyester film or films in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
9. A process according to claim 8, wherein the temperature to which the laminate is re-heated after lamination is from 220.degree.-330.degree. C.
10. A process according to claim 8, wherein the indirect means for re-heating the laminate is an induction heating means.
11. A process according to claim 8, wherein the or each composite film of said polyester which is applied to the metal sheet is a co-extruded cast composite polyester film.
12. A process according to claim 8, wherein the or each composite film of said polyester which is applied to the metal sheet is a composite co-extruded polyester film comprising prior to the conversion into non-crystalline or amorphous form: (A1) an inner layer of a substantially non-crystalline linear polyester having a softening point below 200.degree. C. and a melting point below 250.degree. C. but above 150.degree. C., and (A2) an outer layer of a biaxially oriented linear polyester having a crystallinity greater than 30%.
13. A process according to claim 12, wherein the linear polyester of inner layer (A1) is an amorphous copolymer of ethylene terephthalate and ethylene isophthalate or a copolymer formed from terephthalic acid and two dihydric alcohols.
14. A process according to claim 12, wherein the outer layer (A2) is biaxially oriented polyethylene terephthalate.
15. A process according to claim 8 wherein said film has an outer layer of coextruded polyethylene terephthalate homopolymer.
16. A process according to claim 8, wherein said film has an outer layer of coextruded polyethylene terephthalate homopolymer with an intrinsic viscosity between 0.6 and 0.7 as measured by gel permeation chromotography.
17. A process according to claim 8, wherein a different coextruded composite film is adhered to each side of said metal sheet and the outer layer of at least one of those films is a polyethylene terephthalate homopolymer.
18. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a film of a non-crystalline copolyester adhered directly thereto, said copolyester consisting essentially of a copolymer having at least about 60 mole percent of ethylene terephthalate and the remainder being less than about 40 mole percent of copolymerized ester units other than ethylene terephthalate, which process comprises adhering to one or both major surfaces of the metal sheet a monofilm or monofilms of said copolyester, said monofilm or monofilms being adhered to the metal sheet by heating the metal sheet to a temperature (T1) above the melting point of the polyester film, the temperature (T1) being such that during lamination of the polyester film or films to the metal sheet, the outer surfaces of the polyester film or films remain below their melting points; laminating the film or films to the metal sheet; reheating by indirect means the laminate to a temperature (T.sub.2) above the melting points of the polyester films; and after holding at these elevated temperatures, quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester film or films, the laminating, reheating and quenching conditions being such that the polyester monofilm or monofilms in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
19. The process according to claim 18, wherein said copolyester consists essentially of at least about 75 mole percent of ethylene terephthalate and the remainder being less than about 25 mole percent of copolymerized ester units other than ethylene terephthalate.
20. The process according to claim 19, wherein said copolyester consists essentially of at least about 80 mole percent of ethylene terephthalate and the remainder being less than about 20 mole percent of copolymerized isophthalate.
21. The process according to claim 18, wherein said copolyester is defined by: ##STR2## in which R.sub.1 is a divalent hydrocarbon group at least 60 mole percent of which consists of p-phenylene groups; R.sub.2 is a divalent hydrocarbon group at least 60 mole percent of which consists of --C.sub.2 H.sub.4 -- groups with A=1 and wherein, for the remaining mole percent of R.sub.2, A is an integer.
22. The process according to claim 21, wherein R.sub.1 is at least 75 mole percent p-phenylene groups; and R.sub.2 is at least 75 mole percent --C.sub.2 H.sub.4 -- groups with A=1.
23. The process according to claim 22, wherein R.sub.2 is at least 80 mole percent p-phenylene groups; and R.sub.2 is at least 80 mole percent --C.sub.2 H.sub.4 -- groups with A=1.
24. The process according to claim 21 wherein said remaining mole percent of R.sub.2 is selected from the group consisting of diethylene glycol and cyclohexane dimethanol.
25. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a film of a non-crystalline copolyester adhered directly thereto, said copolyester consisting essentially of a copolymer having at least about 60 mole percent of polyethylene terephthalate which process comprises adhering to one or both major surfaces of the metal sheet a monofilm or monofilms of said copolyester, said monofilm or monofilms being adhered to the metal sheet by heating the metal sheet to a temperature (T1) above the melting point of the polyester film, the temperature (T1) being such that during lamination of the polyester film or films to the metal sheet, the outer surfaces of the polyester film or films remain below their melting points; laminating the film or films to the metal sheet; reheating by indirect means the laminate to a temperature (T2) above the melting points of the polyester films; and after holding at these elevated temperatures, quenching repidly the polyester coated metal to a temperature below the glass transition points of the polyester film or films, the laminating, reheating, quenching conditions being such that the polyester monofilm or monofilms in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
26. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a film or a non-crystalline copolyester adhered directly thereto, said copolyester consisting essentially of a copolymer having at least about 60 mole percent of polyethylene terephthalate and the remainder being less than about 40 mole percent of modifying agents which process comprises adhereing to one or both major surfaces of the metal sheet a monofilm or monofilms of said copolyester, said monofilm or monofilms being adhered to the metal sheet by heating the metal sheet to a temperature (T1) above the melting point of the polyester film, the temperature (T1) being such that during lamination of the polyester film or films to the metal sheet, the outer surfaces of the polyester film or films remain below their melting points; laminating the film or films to the metal sheet; reheating by indirect means the laminate to a temperature (T2) above the melting points of the polyester films; and after holding at these elevated temperatures, quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester film or films, the laminating, reheating, and quenching conditions being such that the polyester monofilm and monofilms in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
27. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a composite film of a non-crystalline copolyester adhered directly thereto, said copolyester consisting essentially of a copolymer having at least about 60 mole percent of ethylene terephthalate and the remainder being less than about 40 mole percent of copolymerized ester units other than ethylene terephthalate, which process comprises adhering to one or both major surfaces of the metal sheet a composite film or films of said copolyester having a polyester inner layer (A1) and a polyester outer layer (A2), said composite film or films of said copolyester being adhered to the metal sheet by a process which comprises: (i) heating the metal sheet to a temperature (T1) above the softening point of the polyester inner layer (A1) but below the melting point of the polyester outer layer (A2); (ii) laminating the film or films to the metal sheet; (iii) reheating by indirect means the laminate so that the metal sheet reaches a temperature above the melting point of the or each polyester film; and (iv) after holding at these elevated temperatures, quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester coating resins; wherein (v) the laminating, reheating and quenching conditions are such that the polyester film or films in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
28. The process of claim 27, wherein said polyester consists essentially of at least about 75 mole percent of ethylene terephthalate and the remainder being less than about 25 mole percent of copolymerized ester units other than ethylene terephthalate.
29. The process of claim 28, wherein said polyester consists essentially of at least about 80 mole percent of polyethylene terephthalate and less than about 20 mole percent of isophthalate.
30. The process of claim 27, wherein said polyester is defined by: ##STR3## in which R.sub.1 is a divalent hydrocarbon group of at least 60 mole percent of which consists of p-phenylene groups; R.sub.2 is a divalent hydrocarbon group at least 60 mole percent of which consists of --C.sub.2 H.sub.4 -- groups with A=1, and wherein, for the remaining mole percent of R.sub.2, A is an integer.
31. The process of claim 30, wherein R.sub.1 is at least 80 mole percent p-phenylene groups; and R.sub.2 is at least 80 mole percent --C.sub.2 H.sub.4 -- groups with A=1.
32. The process of claim 30, wherein R.sub.1 is at least 20 mole percent p-phenylene groups; and R.sub.2 is at least 20 mole percent --C.sub.2 H.sub.4 -- groups with A=1.
33. The process according to claim 30 wherein said remaining mole percent of R.sub.2 is selected from the group consisting of diethylene glycol and cyclohexane dimethanol.
34. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a composite film of a non-crystalline copolyester adhered directly thereto, said copolyester consisting essentially of a copolymer having at least about 60 mole percent of polyethylene terephthalate, which process comprises adhering to one or both major surfaces of the metal sheet a composite film of said copolyester having a polyester inner layer (A1) and a polyester outer layer (A2), said composite film or films of said copolyester being adhered to the metal sheet by a process which comprises: (i) heating the metal sheet to a temperature (T1) above the softening point of the polyester inner layer (A1) but below the melting point of the polyester outer layer (A2); (ii) laminating the film or films to the metal sheet; (iii) reheating by indirect means the laminate so that the metal sheet reaches a temperature above the melting point of the or each polyester film; and (iv) after holding at these elevated temperatures; quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester coating resins; wherein (v) the laminating, reheating and quenching conditions are such that the polyester film or films in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
35. A process for preparing a laminated metal sheet having on one or both major surfaces thereof a composite film of a non-crystalline copolyester adhered directly thereto, said copolyester consisting essentially of a copolymer having at least about 60 mole percent of polyethylene terephthalate and the remainder being less than about 40 mole percent of modifying agents, which process comprises adhering to one or both major surfaces of the metal sheet a composite film of said polyester having a polyester inner layer (A1) and a polyester outer layer (A2), said composite film or films of said polyester being adhered to the metal sheet by a process which comprises: (i) heating the metal sheet to a temperature (T1) above the softening point of the polyester inner layer (A1) but below the melting point of the polyester outer layer (A2); (ii) laminating the film or films to the metal sheet; (iii) reheating by indirect means the laminate so that the metal sheet reaches a temperature above the melting point of the or each polyester film; and (iv) after holding at these elevated temperatures, quenching rapidly the polyester coated metal to a temperature below the glass transition points of the polyester coating resins; wherein (v) the laminating, reheating and quenching conditions are such that the polyester film or films in the metal/polymer laminate is or are converted into non-crystalline or amorphous form.
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