GB2276347A

GB2276347A - Double layered thermoplastic resin laminated metal sheet

Info

Publication number: GB2276347A
Application number: GB9306313A
Authority: GB
Inventors: Takaaki Okamura; Kenzo Matsui; Tsuneo Inui
Original assignee: Toyo Kohan Co Ltd
Current assignee: Toyo Kohan Co Ltd
Priority date: 1993-03-26
Filing date: 1993-03-26
Publication date: 1994-09-28
Anticipated expiration: 2013-03-26
Also published as: DE4311481C2; DE4311481A1; GB9306313D0; FR2703298A1; GB2276347B; NL9300575A; FR2703298B1; CA2092939A1

Description

2276347 DOUBLE LAYERED THERMOPLASTIC RESIN LAKINATED META1b SHEET The

present invention relates to a metal sheet laminated with a double layered thermoplastic resin consisting of a polyester resin inner layer and a polycarbonate outer layer which is adhered to said polyester inner layer by using conventional film laminating or extrusion coating techniques.

Metal sheet stock such as electrotinplate, tin free steel (TFS) and aluminum sheet are widely used for can stock after applying one or more coats of lacquer. However, the use of such lacquer coatings has associated drawbacks including increased energy costs due to extended curing times and the discharge of solvent during curing which must be disposed of, forex,mpleby incineration to prevent environmental pollution.

To avoid such problems, the lamination of thermoplastic resin film on a metal sheet has recently been attempted. See, for example, U.S. Patent Nos. 4,517,255, 4,614,691 and U.S. Patent Application Serial Nos. 380,994, 419,217 and Japanese Patent Application No. Hei 2-418199.

U.S. Patent No. 4,517,255 relates to a process for lamination of a crystalline polyester resin film to a metal sheet by heating the sheet to above the melting point of the Polyester resin and thereafter immediately quenching the laminate. The laminate according to this patent has already been investigated for can stock such as drawn cans, can ends and five gallon cans because the laminated crystalline polyester resin film is sufficiently adhered to the metal sheet by an amorphous non-oriented polyester resin layer which is formed at the interface of the crystalline polyester film and the metal sheet as a result of the heating step. However, this laminate cannot be used for applications requiring more severe formability such as a drawn and ironed can and a drawn and stretch formed can, because the formability beccmes poor due to the presence of the crystalline polyester layer on the outside of this laminate.

0Drawn and ironed" refers to multiple drawing and ironing, ironing being a thinning of the metal sheet by reduced clearance between the punch and die- That is, the gap is less than the thickness of the sheet prior to ironing. "Drawn and stretch formed" refers to multiple drawing and stretch forming, wherein any thinning of the can wall occurs through stretching and bending. The gap between the punch and die is not smaller than the thickness of the can wall.

U.S- Patent No. 4,614,691 relates to a process for production of a metal sheet covered with polyester resin film which includes - 2 - laminating a biaxially oriented polyester resin film which has been precoated with a small amount of an epoxy resin containing a curing agent of epoxy resin to a metal sheet which has been heated to a temperature of 2200C to below the melting point of polyester resin film. The laminate according to this patent has been used for some applications such as a shallow drawn can and a can end because this laminate has excellent corrosion resistance after forming compared with that in the laminate according to U.S. Patent No. 4,517,255- However, this laminate cannot be used for applications requiring more severe formability such as a drawn and ironed can and a drawn and stretch formed can because the for-mability of this laminate is insufficient for the continuous production of these drawn cans described above.

In order to improve the formability of these laminates described above, U. S. Patent Application Serial Nos. 380,994 and 419,217 and Japanese Patent Application No. Hei 2-418199 were proposed.

In U.S. Patent Application Serial No. 380,994, a copolyester resin film consisting of 75 to 99 mole % of polyethylene terepthalate and 1 to 25 mole % of other polyester resin having a softening temperature of 170 to 2350C, a raelting temperature of 210 to 250'C' and an elongation at break of 150 to 400% is laminated on a metal sheet which has been heated to the temperature of the melting point of the employed polyester resin film (Tm) SO'C. In U.S. Patent Application Serial No. 419,217, a copolyester resin film having the same composition and the same melting point as those in U.S. Patent Application Serial No. 380,995, and havIng a refractive index in the thickness direction of 1.5000 to 1.5500 and a refractive index in the film's planar dimensions of 1.6000 to 1.6600 is laminated on a metal sheet which has been heated to the temperature of the melting point of the employed polyester resin film (Tm) 500C.

Japanese Patent Application No- Hei 2-418199 relates to a copolyester resin laminated metal sheet for a drawn and stretch formed can which comprises a 0.01 to 0.15 planar orientation coefficient in the outermost layer of the laminated polyester resin and a 0 to 0. 10 planar orientation coefficient in the polyester resin layer contacting the surface of a metal sheet after lamination of polyester resin to the metal sheet.

Presently, the laminates produced under the restricted conditions of these patents are used for a drawn and stretch formed can which is treated with hot steam having a temperature of 120 to 0 C in a retort af ter drinks such as green tea, black tea or oolong tea are packed. These laminates can also be used for drawn and stretch formed cans packed with other foods and drinks, if the cans are produced and packed under normal conditions, i.e., conditions under which dents do not arise during production, packing, or transport. However, some dents may arise in part of the can body by impacts with other cans in the can making process or the transporting process after packing foods and drinks.

The part of the inside of the can body opposite the impact from the outside of the can body is locally corroded, because cracks arise in the laminated polyester resin film.

Therefore, these laminates cannot be used for drawn and stretch formed cans in which severely corrosive foods and drinks are packed, if the cans may be subjected to denting during production, packing, or transport.

In order to solve the problem described above, the polyester resin film laminated metal sheet according to Japanese Patent Application No. Hei 2-418198 has been developed. The laminate according to this patent is able to solve the problem described above. However, this laminate cannot be used for a drawn and stretch formed can in which carbonated beverages having severe corrosivity are packed at temperatures below about SOC, because many cracks arise in the laminated polyester resin film on the inside of the can body by the inpact of cans against each other in the transporting process after packing the beverage at low temperature. The laminate according to this patent does not have good resistance to denting at a low temperetture.

Further-more, in the production of these laminates, it is necessary that the set temperature in laminating the polyester resin film is controlled within a narrow range, because it is otherwise impossible to obtain a laminate having satisfactory characteristics required for a drawn and stretch formed can.

Accordingly, it is a first object of the present invention to provide a thermoplastic resin laminated metal sheet having excellent formability, excellent adhesion of the double layer to a metal sheet, excellent denting resistance, particularly excellent denting resistance at the low temperatures required for drawn and ironed cans and drawn and stretched formed cans in which severely corrosive foods and drinks are packed at a low temperature after severe forming.

Furthermore, it is a second object of the present invention to provide a method for continuous high speed production of the thermoplastic resin laminated metal sheet without severe control of the laminating temperature.

The first objective of the present invention can be accomplished by the formation of a double layered thermoplastic resin consisting of an outer layer of polycarbonate resin and an inner layer of polyester resin on a netal sheet.

Thus, according to one aspect of the present invention there is provided a laminate comprising a layer of hydrated chromium oxide on at least one side of a metal sheet, with a polyester resin inner layer bonded directly to said layer of hydrated chromium oxide, said polyester resin inner layer comprising recurring units the formula 0 0 0 Ii il il (wherein R 2 represents an alkylene group having 2 to 6 carbon atoms and R3 represents an arylene group having 2 to 24 carbon atoms), and bonded directly to said polyester resin inner layer a polycarbonate resin outer layer comprising recurring units having the formula 0 11 (wherein R1 is either an aliphatic hydrocarbon group having 2 to 10 carbon atoms or an aromatic hydrocarbon having 6 to 18 carbon atoms).

The second object of the present invention can be accomplished by continuous high-speed lamination of the double-layered thermoplastic resin described above on a metal sheet which is heated to the melting temperature (Tm) of the polyester resin used to Tm + 150'C in which the inner polyester resin layer contacting the surface of the metal sheet is in a non-crystalline state.

Thus, in another aspect the present invention provides a method for producing a laminate, said method comprising the steps of heating a metal sheet having a layer of hydrated chromium oxide on at least one side to a temperature of Tm to Tm + 1SO'C (wherein Tm is the melting temperature of the polyester resin), bonding directly to said layer of hydrated chromium oxide a polyester resin inner layer comprising recurring units having the formula 0 0 0 11 11 11 (wherein R 2 represents an alkylene group having 2 to 6 carbon atoms and R3 represents an arylene group having 2 to 24 carbon atoms), bonding directly to said polyester resin inner layer a polycarbonate resin outer layer comprising recurring units having the formula 0 11 (wherein R1 is an aliphatic hydrocarbon group having 2 to 10 carbon atoms or an aromatic hydrocarbon having 6 to 18 carbon atoms), and quenching the laminate.

This thermoplastic resin laminated rnetal sheet can be used for some applications in which severe formability, excellent corrosion resistance after forming is required, such as deeply drawn cans formed by multiple drawing, drawn and stretch formed cans, drawn and ironed cans, and can ends wherein a tab f or easy opening is attached. In particular, this thermoplastic resin laminated metal sheet is suitable for a carbonate beverage can in which excellent denting resistance at a low temperature and excellent corrosion resistance are required, even if these cans produced or transported continuously in the can making process and beverage packing process under abnormal conditions.

The polycarbonate resin which f orms an outer layer in the laminate according to the present invention comprises recurring units represented by the following formula (l):

wherein R1 represents an aliphatic hydrocarbon group having 2 to 10 carbon atoms or an aromatic hydrocarbon group having 6 to 18 carbon atoms - The polycarbonate resin is preferably selected froin an aromatic polycarbonate resin such as poly dihydroxy diphenyl 2,2 propane carbonate (Bisphenol A polycarbonate), poly dihydroxy diphenyl methane carbonate, poly dihydroxy diphenyl ethane carbonate, poly dihydroxy diphenyl 2, 2 butane carbonate, poly dihydroxy diphenyl 2, 2 pentane carbonate, poly dihydroxy diphenyl 3, 3 pentane carbonate, poly dihydroxy diphenyl 2, 2 hexane carbonate.

An aliphatic polycarbonate resin can be used for applications in which severe formability is required but excellent heat resistance and excellent retortability are not required, because it does not have excellent heat resistance. For economy, bisphenol A polycarbonate resin is most preferable for the present invention.

Furthermore, the mechanical properties of the polycarbonate resin laminated on the polyester resin inner layer are also an important factor from the standpoint of the fornability and the denting resistance of the laminate obtained. Specifically, the elongation at break of the polycarbonate resin layer laminated on the polyester resin inner layer, which is determined at a speed of 100 mm/min. at 250C in an ordinary tensile testing machine, should be within the range of 70 to 300%, preferably 100 to 200%.

Although the laminated polycarbonate resin layer is better than the laminated polyester resin inner layer in denting resistance, if a polycarbonate resin having below 70% elongation at break is used in the present invention, many cracks also arise in r-he laminated polycarbonate resin layer in addition to the polyester resin inner layer of the laminate after severe forming or denting by impact. That is, the formability and denting resistance of the polycarbonate resin becomes noticeably through a decrease in elongation at break- on the other hand, if a polycarbonate resin having over 300-1 elongation at break is used in the present invention, many film hairs arise in the laminate during severe forming or cutting the laminate, because the laminated polycarbonate resin layer is noticeably elongated.

The polyester resin which forms an inner layer in the laminate according to the present invention comprises recurring units represented by the following general formulw- (2) or (3)- C - (14 -0- R2-- 0 - c. (3) -li wherein R2 represents an alkylene group having 2 to 6 carbon atoms and R3; represents an alkylene group or an arylene group having 2 to 24 carbon atoms.

Furthermore, in the present invention, the use of polyester resin having a 0.3 to 1.8 intrinsic viscosity (IV value) is preferable. If a drawn and stretch formed can produced by using the polycarbonate resin laminated metal sheet in which a polyester resin having an 1V value below 0.3 is used as the inner layer is impacted from the outside of the can, many cracks arise in the laminated polyester resin layer and polycarbonate resin layer, even if this polyester resin layer is uniformly covered with polycarbonate resin outer layer. That is, a polyester resin having an IV value below 0.3 is poor in denting resistance and the cracks in the laminated polyester resin layer are transmitted to the polycarbonate resin outer layer by the impact. on the other hand, a polyester resin having an iv value above 1.8 is not uniformly adhered to the metal sheet, because this polyester resin has high viscosity and the metal sheet is uniformly wetted by the melted polyester resin, even if it is sufficiently melted. Therefore, this polyester resin is not suitable for the inner layer of the laminate according to the present invention.

The IV value described above is calculated by th6 following equation from the viscosity measured in a solution in which 0.3 g of the polyester resin is dissolved in 25 ml of o chlorophenol at 350C.

71 SP [n) + K [n]2C 775P Specificity viscosity En] intrinsic viscosity (IV value) K Constant (0.247) c Volume concentration (g/100 ml) It is preferred to use a thermoplastic polyester resin selected from the group consisting of polyethylene terephthalate, poly(butylene/ethylene terephthalate), poly(ethylene terephthalatel isophthalate), poly (ethylene terephtha late /adipate), poly(ethylene terephthalate/sebacate), poly(butylene terephthalate/isophthalate) and poly(ethylenelbutylene terephthalate/isophthalate). Namely, a polyester resin in which at least one member consisting of polyethylene isophthalate, polybutylene terephthalate, polyethylene adipate, polyethylene sebacate, polybutylene isophthalate is copolymerized with polyethylene terephthalate or blended with polyethylene terephthalate is used in addition to polyethylene terephthalate. In particular, the use of the copolyester resin or the blend polyester resin described above is suitable for some applications in which severe formability is required, compared with the use of a polyester resin - l5 - consisting of only polyethylene terephthalate- Furthermore, it is preferable to use a polyester in which polybutylene terephthalate (PBT) is blended with polyethylene tere-phthalate (PET) at a weight ratio of PBT/PET of 0.1/1 to 1.7/1 for some applications requiring excellent heat resistance, excellent retortability (resistance to milky change of the laminated resin layer by a retort treatment) and severe formability. If the polyester resin having below 0-111 or above 1.7/1 of PBT/PET weight ratio is not used for the laminate in which severe formability such as a drawn and stretch forined can is required, the formability of this polyester in the laminated state on the metal sheet becomes poor- However, the increase in the amount of PBT in the blended polyester resin consisting of PBT and PET has a slightly bad effect on the flavor of foods and drinks. Therefore, if many cracks arise in the laminated polycarbonate resin layer of the laminate according to the present invention which is used for the inside of the can, the flavor of the packed foods and drinks may become slightly bad, although the polycarbonate resin layer contacting directly with the packed foods and drinks does not change the flavor of the packed foods and drinks- In the case described above, it is preferable to use a blended polyester resin having below 1.211 of PBT/PET weight ratio.

Moreover, the retortability of the laminate according to the present invention becomes slightly poor through the decrease in the - 11L weight ratio of PBT/PET- Therefore# in the use of the laminate according to the present invention for the outside of the can which is treated with hot steam in a retort after packing foods and drinks, it is preferable to use the polyester resin having above 0.6/1 PBT/PET weight ratio.

As described above, the PBT/PET weight ratio in the blended polyester resin consisting of PBT and PET is changed depending on the use to which the can is put. The blended polyester resin having 0.611 to 1.211 of PBT/PET weight ratio can be used for all applications in the present invention.

The thicknesses of the polycarbonate resin outer layer and polyester resin inner layer should be selected from the standpoint of the required characteristics and economy. Although the thickness of each resin layer is not especially limited, it is preferable in the present invention that the thickness of the polycarbonate resin outer layer is 3 to 40 gm and the thickness of the polyester resin inner layer is 0-2 to 20 gm. If the thickness of the polyester resin inner layer is below 0.2 gm, the laminated resin layer may be peeled off during severe forming, such as a drawn and stretch formed can. If the thickness of the polyearbonate resin outer layer is below 3 gm, denting resistance of the obtained laminate may become poor. A laminate having a thickness above 40 gm of the polycarbonate resin outer layer and above 20 gm of the polyester resin inner layer is not suitable for - 1,5 - the precoated can stock, because it is not as economical as metal sheet coated with epoxy phenolic lacquer which is generally used in the can making industry.

Metal sheet useful in the present invention can be steel sheet, steel sheet plated with at least tin, nickel and zinc, and aluminun sheet. To provide the desired adhesion properties of the metal sheet to the laminated polyester resin inner layer, the metal sheet is covered with a single layer of hydrated chromium oxide or a double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide- The amount of plated tin, nickel and zinc on the metal sheet is preferably below 2 3-0 g/m ' respectively, for reasons of economy. However, if the 2 amount of plated tin, nickel and zinc is below about 0. 05 g/m respectively, the effect of plated tin, nickel and zinc on the corrosion resistance to the packed foods and drinks is hardly apparent, despite the addition of a further plating process.

As described above, it is an important factor in the present invention that the employed metal sheet is covered with a single layer of hydrated chromium oxide or a double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide in order to obtain good adhesion of the laminated resin layer to the metal sheet after severe forming such as a drawn and stretch formed can and a drawn and ironed can.

^ 1G - The preferred amount of chromium as hydrated chromium oxide is about 3 to about 30 mg/in2 in the single layer or the double layer. The preferred amount of metallic chromium in the double 2 layer is about 10 to 200 MC1/M If the amount of chromium as hydrated chromium oxide is below about 3 mg/m2 or above about 30 M91m2, the adhesion of the laminated resin layer to the metal sheet may become poor after severe forming even if the amount of metallic chromium is about 10 to 200 M9/M 2, particularly if the laminate is exposed to hot steam in a retort. It is preferable that the deposition of metallic chromium improves the adhesion of the laminated resin layer to the metal sheet and the corrosion resistance of the obtained laminate. However, the deposition of metallic chromium above about 200 mg/m2 is unnecessary in the present invention, because the corrosion resistance is not substantially improved, even if metallic chromium above about 200 mg1M 2 is deposited.

The laminate according to the present invention may be produced by the following methods:

(1) A double layered non-oriented, monoaxially oriented or biaxially oriented thermoplastic resin film consisting of a polycarbonate resin outer layer and a polyester resin inner layer is laminated on one or both sides of a metal sheet having a single layer of hydrated chromium oxide or a double layer consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium which has been heated to a temperature of Tm to Tm+150 a C, and then the laminate is gradually or rapidly quenched.

(2) A non-oriented, monoaxially oriented or biaxially oriented polyester resin film or a melted polyester resin is laminated on one or both sides of a metal sheet having a single layer of hydrated chromium oxide or a double layer consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium which has been heated to a temperature of Tm to Tm+1500C, and thereafter a non-oriented, monoaxially oriented or biaxially oriented polycarbonate resin film or a melted polycarbonate resin is laminated on the polyester resin inner layer at a temperature of Tm to Tm+150C. After that, the laminate is gradually or rapidly quenched(3) A coextruded thermoplastic resin consisting of an outer layer of polycarbonate resin and an inner layer of polyester resin is laminated on one or both sides of a metal sheet having a single layer of hydrated chromium oxide or a double layer consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium which has been heated to a temperature of the glass transition temperature of the employed polyester resin (Tg)+300C to Tm+1500C and then the laminate is gradually or rapidly quenched.

(4) One side of a non-oriented, monoaxially oriented or biaxially oriented polycarbonate resin film is laminated with a melted polyester resin by an extrusion laminating method. After that, this double layered film is laminated on one or both sides of a metal sheet having a single layer of hydrated chromium oxide or a double layer consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium which has been heated to a temperature of Tm to Tm+1500C and then the laminate is gradually or rapidly quenched.

In the above methods (1) to (4), Tm represents the melting temperature of the employed polyester resin which exhibits an endothermic peak in an ordinary differential thermal analysis run at a heating rate of 100C/min. In blended polyester resin consisting of PBT/PET, two endothermic peaks are usually observed- In this case, the endothermic peak at higher temperature is used for the melting temperature of the polyester resin.

While all the methods (1) to (4) described above can be used for producing the laminate according to the present invention, from the standpoint of continuous and stable production at high speed and apparatus for such production, it is preferable to produce the laminate by using the film laminating method (1) or (4). In the method for producing the laminate described above. an important factor is that almost all the polyester resin inner layer has a non- - 1,1 - oriented structure in order to obtain an excellent adhesion of the laminated resin layer to the metal sheetWith the use of monoaxially or biaxially oriented double layered resin film, the greater parts of the laminated polyester resin inner layer should be changed to a non-oriented structure from the monoaxially or biaxially oriented structure after lamination to the metal sheet, even if monoavially or biaxially oriented structure remains in the laminated polycarbonate resin outer layer, because the state of the polycarbonate resin outer layer does not affect the characteristics of the laminate. Therefore, the metal sheet laminated with the double layered resin film having a thicker polyester resin inner layer should be heated to comparatively higher temperature than Tm. On the other hand, the metal sheet laminated with the double layered f i lin having a thinner polyester resin inner layer may be heated to slightly higher than Tm_ That is, it is unnecessary to closely control the temperature of the metal sheet to be laminated with the double layered resin film, if the temperature of the metal sheet is maintained in the range of Tm to Tm+1500C. However, if the temperature of the heated metal sheet is below Tra, the laminated double layered resin film does not sufficiently adhere to the metal sheet, because the polyester resin inner layer is not sufficiently melted- On the other hand, the heating of the metal sheet to above Tm+1500C is not preferable, because the polyester - 2 ak ^ resin inner layer may be decomposed and the characteristics of the obtained laminate become poor.

The method for heating the metal sheet is not critical to the present invention. However, from the standpoint of the continuous and stable production of the laminate at high speed, conduction heating by rolls heated by induction heating, induction heating andlor resistance heating which are used for reflowing electroplated tin in the production process of electrotinplate are suitable as a method for heating the metal sheet, because the metal sheet can be rapidly heated and the temperature of the heated metal sheet can be easily controlled.

Furthermore, it is also preferable in the present invention that heating with a roll heated by hot steam or heating in an electric oven can be used as auxiliary methods for preheating the metal sheet to be laminated.

In the method for producing the laminate according to the present invention, whether the laminate is gradually quenched or rapidly quenched is selected by the composition and the state of the employed polyester resin, the characteristics required in the obtained laminate, and the conditions and the method for forming the obtained laminate. For example, in the production Of a laminate in which excellent retortability is required, quenching conditions should be selected in which fine crystals appear by the recrystallization of the non-oriented polyester resin inner - 21 - layer just after lamination- The present invention is explained in further detail by the following illustrative examples.

Exan.p 1 e 1 A biaxially oriented double layered thermoplastic resin film consisting of bisphenol A polycarbonate outer layer and a polyester resin inner layer having PBT/PET weight ratio of 111 (thickness of each resin layer: 12 gin, melting temperature of polyester resin: 252OC) was laminated by using a pair of laminating rolls on both sides Of a TFS strip having a thickness of 0.26mm, a width of 2c;n 2 and a temper of T-5 (metallic chromium: 110 mglm 7 hydrated chromium oxide: 18 mg/m 2 as chromium) which had been heated to 280 OC by using rolls heated by induction heating- After that, the laminate was quenched by water after 3 seconds. In the obtained laminate, the elongation at break of the polycarbonate resin layer was 110>. and an IV value of the polyester resin was 0.72.

2Z - Example 2 A non-oriented double layered thermoplastic resin film consisting of

bisphenol A polycarbonate outer layer and polyester resin inner layer having PBT/PET weight ratio of 0.2/1 (thickness of polycarbonate resin layer: 20 pm, thickness of polyester resin layer: 10 pm, melting temperature of polyester resin: 254OC) was laminated on both sides of TFS having a thickness of 0.26 mm, a width of 250 mm and a temper of T-S (metallic chromium: 60 mg/m2 chromium as hydrated chromium oxide: 25 mg11n2) which had been heated to 2900C by using rolls heated by induction heating, and thereafter the laminate was immediately quenched by water. In the obtained laminate, the elongation at break of the polycarbonate resin layer was 150% and an IV value of the polyester resin was 0-58Example 3

A biaxially oriented double layered thermoplastic resin film consisting of bisphenol A polycarbonate outer layer and polyethylene terephtha late/ po lyethylene isophthalate copolyester resin inner layer produced by a condensation polymerization of 100 mole of ethylene glycol and dicarboxylic acid consisting of 88 mole of terephthalic acid and 12 mole % of isophthalic acid (thickness of polycarbonate resin layer: 15 pm, thickness of copolyester resin layer: 10 gm, melting temperature of copolyester: 228'C) was laminated on both sides of the same TFS strip as in Example 1 under the same conditions as in Example 1. In the obtained laminate, the elongation at break of the polycarbonate resin layer was 210% and the 1V value of the polyester resin was 0.62.

Example 4

A coextruded double layered thermoplastic resin consisting of bisphenol A polycarbonate resin and polyester resin having PBT/PET weight ratio of 0711 (thickness of each resin layer is 4m, melting temperature of polyester resin: 253OC) was laminated on both sides Of a TFS strip having a thickness of 0.26 mn, a width of 2 250 mm and a temper of T-5 (metallic chromium: 150 nglin ', hydrated chromium oxide: 7 mg/m2 as chromium) which had been heated to 2600 C. in the state wherein polyester resin layer contacted with the surface of TFS Strip. After that, the laminate was immediately quenched by water- In the obtained laminate, an elongation at break of polycarbonate resin layer was 113% and an IV value of polyester resin was 0.64.

- 2Lf- - Example 5

A cold rolled steel strip having a thickness of 0.26 mm, a width of 250 mm and a temper of T-5 was electrolytically degreased and then pickled under known conditions. The steel strip, after rinsing with water, was electroplated with 1.5 g/m2 of tin by using a tinplating electrolyte consisting of 80 g/1 of stannous sulfate. 60 g/1 of phenolsultonic acid (65% solution) and 0.06 g/1 of ethoxylated a-naphthol in water under 20 Aldm 2 of cathodic current density at an electrolyte temperature of 4SOCAfter rinsing with water, TFS film consisting of an upper layer of chromium 13 ing/M12 as hydrated chromium oxide and a lower layer of metallic chromiun of 90 Ing/M2 was formed by cathodic treatment on both sides of the tin plated steel strip by using an electrolyte consisting of 50 g/1 of chronic acid and 0.5 g/1 of sulfuric acid in water under 40 A/dn2 of cathodic current density at an electrolyte temperature of so ú) C. The thus treated tin plated steel strip was rinsed with hot water and dried.

A non-oriented double layered thermoplastic resin film consisting of bisphenol A polycarbonate outer layer and polyester resin inner layer having PBT/PET weight ratio of 1.511 (thickness of polycarbonate resin layer: 7 An, thickness of polyester resin layer: 5 pin, melting temperature of polyester resin: 2SO'C) was laminated on both sides of a tin plated steel strip under the same conditions as in Example 1, except that the temperature of the heated tin plated steel strip was 255 in C. In the obtained laminate, t',.,e elongation at break of polycarbonate resin layer was 120% and the IV value of polyester resin was 0.73.

Comparative Example 1 A non-oriented bisphenol A polyearbonate film having a thickness of 25 gm was laminated on both sides of the same TFS strip as in Example 1 which had been heated to 3000C, and then the laminate was immediately quenched. In the obtained laminate, the elongation at break of bisphenol A polycarbonate layer was 103%.

Comparative Example 2 A biaxially oriented polyethylene terephthalatelpolyethylene isophthalate copolyester resin film produced by a condensation polymerization of 100 mole % of ethylene glycol and dicarboxylic acid consisting of 88 mole % of terephthalic acid and 12 mole % of isophthalic: acid (thickness: 25 gm, melting temperature: 228OC) was laminated on both sides of the same TFS as in Example 1 under the same conditions as in Example 1, except that the temperature of the heated TFS strip was 2400C. In the obtained laminate, the IV value of the polyester resin was 0.60.

- 26 Comparative Example 3 The same copolyester resin film as in Comparative Example 2 which was precoated with a resin composition consisting of 80 parts of epoxy resin havinq an epoxy equivalent of 3000 and 20 parts of resol product from paracresol and dried at 1000C (thickness of copolyester resin film: 25 Am, amount of resin composition: 0.8 g1M2 after drying) was laminated on both sides of the same TFS as in Example 1 under the same conditions as in Example 1, in the state in which the side precoated with the resin composition contacted with the surface of the TFS strip, except that the temperature of the heated TFS was 2400C. In the obtained laminate, the IV value of the copolyester resin was 0.58.

The resultant laminate was formed to a drawn and stretch formed can under the following forming conditions:

Forming conditions A- Drawing process Diameter of circular blank 187 mm Drawing ratio: 1.50 B. Redrawing process First redrawing ratio 1.29 Second redrawing ratio 1. 24 Third redrawing ratio 1.20 - 27+ - Curvature radius in a corner of dies used for redrawing process: 0.4 mm Load f or preventing wrinkles: 6000 kg C. Average ratio of thickness of steel sheet in can body to can end: -20%.

The characteristics of the drawn and stretch formed can obtained under the conditions described above were evaluated by the following testing methods. The results are shown in the following Table.

(1) Adhesion of the laminated resin layer after forming:

The adhesion of the laminated resin layer to the metal sheet was evaluated by the degree of peeling off of the resin layer in a cup obtained in each redrawing process by naked eye.

(2) Formability of the laminated resin layer:

The formability of the laminated resin layer was evaluated by a current value between an anode of the metal exposed through cracks of the laminated resin layer on the inside of the obtained drawn and stretch formed can in which 3% of sodium chloride solution was filled and a cathode of stainless steel rod inserted in the can at a constant voltage of 6.3 volts- (3) Denting resistance of the laminated resin film:

Four samples having a width of 30 mm and a height of 120 mm were cut froin the obtained drawn and stretch formed can- A steel rod having a diameter at a point of 112 -inch, a weight of 1 kg was dropped from a height of 40 mm to the position of 10 mm from the can bottom of the sample which corresponds to the outside of the obtained can. After that, the denting resistance of the laminated resin layer was evaluated by an average current value between an anode of metal exposed through cracks of the laminated resin layer in the side corresponding to the inside of the can and a cathode of stainless steel rod contacted through a sponge including 3% sodium chloride solution at a constant voltage of 6.3 volts.

(4) Denting resistance at low temperature The denting resistance at low temperature was evaluated by using the same sample as in (3) under the same conditions as in (3), except the sample was tested immediately after immersion into ice water for 5 minutes.

Table

Comp. C=P. Comp.

Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Ex.1 Ex.2 Ex.3 Tin Base metal sheet TFS TFS TFS 77S plate TFS 7S TFS Outer layer PC PC PC PC PC PC P-PI P-PI C-ompositAon of laminated resin layer Inner layer PB/P PB/P P-PI PB/P PB/P None None Lacq.

(weight ratio) (1/1) (0,2/1), (0 -7:u (1.5/1 Thickness of laminated Outer layer 12 20 15 15 7 25 25 25 resIn layer (P) - - Inner layer 12 10 10 is 5 0 0 0.8 Adhesion to Peel metal sheet after Good Good Good Good Good In 2nd Good Good can forming redraw Characteristics Formability (M) 0 D D 0 0 0.01 D of laminated resin layer Denting resistance (mA) 0 0 0 0 0 - O.DI 0.01 Denting resistance at low temp. (mA) 0 0 0 0 0 - 0,2D D.D5 Remarks: PC represents bisphenol A polycarbonate.

PB/P represents PBT/PET weight ratio in the employed polyester resin. PPI represents a copolyester resin in which 100 mole % of ethylene glycol was polymerized with 86 mole % of terephthalic acid and 12 mole % of isophthalic acid. represents no measurempnt pf current as the laminated resin layer was peeled off.

59375.559

Claims

1. A laminate comprising a layer of hydrated chromium oxide on at least one side of a metal sheet, with a polyester resin inner layer bonded directly to said layer of hydrated chromium oxide, said polyester resin inner layer comprising recurring units the formula 0 0 0 11 11 11 (wherein R2 represents an alkylene group having 2 to 6 carbon atoms and R3 represents an arylene group having 2 to 24 carbon atoms), and bonded directly to said polyester resin inner layer a polycarbonate resin outer layer comprising recurring units having the formula 0 11 (wherein R1 is either an aliphatic hydrocarbon group having 2 to 10 carbon atoms or an aromatic hydrocarbon having 6 to 18 carbon atoms).

2. A laminate as claimed in claim 1 wherein said polycarbonate resin is an aromatic polycarbonate resin having an elongation at break of 70 to 300%.

3. A laminate as claimed in claim 1 or claim 2 wherein said polycarbonate resin is bisphenol A polycarbonate.

4. A laminate as claimed in any one of the preceding claims wherein said polyester resin inner layer has an intrinsic viscosity (IV value) of 0.3 to 1.7.

5. A laminate as claimed in any one of the preceding claims wherein said polyester resin inner layer is a blended polyester resin having PBT/PET weight ratio of 0.1/1 to 1.7J1.

6. A laminate as claimed in any one of the preceding claims wherein said polycarbonate resin outer layer has a thickness of 3 to 40 gm and said polyester resin inner layer has a thickness of 0.2 to 20 gm.

7. A laminate as claimed in claim 1 having a polyester resin inner layer on one side of the metal sheet which is a polyester resin having a PBT/PET weight ratio of 0.1/1 to 1.2/1 and a polyester resin inner layer on the other side of the metal sheet which is a polyester resin having a PBT/PET weight ratio of 0.6/1 to 1.7/1.

8. A laminate as claimed in any one of the preceding claims wherein said metal sheet is steel or steel plated with at least one of tin, nickel and zinc, and aluminium.

9. A laminate as claimed in any one of the preceding claims wherein the amount of chromium is 3 to 30 mg/m2 as hydrated chromium oxide.

10. A laminate as claimed in any one of the preceding claims further comprising a layer of metallic chromium between said metal sheet and said layer of hydrated chromium oxide.

11. A laminate as claimed in claim 10 wherein the amount of metallic chromium is 10 to 200 mg/m2.

12. A method for producing a laminate, said method comprising the steps of heating a metal sheet having a layer of hydrated chromium oxide on at least one side to a temperature of Tm to Tm + 150C (wherein Tm is the melting temperature of the polyester resin), bonding directly to said layer of hydrated chromium oxide a polyester resin inner layer comprising recurring units having the formula 0 0 0 11 11 11 (wherein R 2 represents an alkylene group having 2 to 6 carbon atoms and R3 represents an arylene group having 2 to 24 carbon atoms), bonding directly to said polyester resin inner layer a polycarbonate resin outer layer comprising recurring units having the formula 0 11 -C-RI-O-C- (wherein R1 is an aliphatic hydrocarbon group having 2 to 10 carbon atoms or an aromatic hydrocarbon having 6 to 18 carbon atoms), and quenching the laminate.

13. A method as claimed in claim 12 wherein said polyester resin film is melted when bonded to said metal sheet.

14. A method as claimed in claim 12 or claim 13 wherein said polycarbonate resin film is laminated to the inner film at a temperature of Tm to Tm + 150C.

15. A method as claimed in any one of claims 12 to 14 wherein said polycarbonate resin film is melted when bonded to said inner film.

16. A method as claimed in any one of claims 12 to 15 wherein said metal strip is heated to a temperature of Tg + 30'C to Tm + 1SO'C, wherein Tg is the glass transition temperature of the polyester resin, and said 1 polyester resin inner layer and said polycarbonate resin outer layer are coextruded onto said metal strip.

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