CA2077811A1

CA2077811A1 - Polyester laminated metal sheet

Info

Publication number: CA2077811A1
Application number: CA002077811A
Authority: CA
Inventors: Takaaki Okamura; Atsuo Tanaka; Tsuneo Inui; Akio Miyachi
Original assignee: Toyo Kohan Co Ltd
Current assignee: Toyo Kohan Co Ltd
Priority date: 1992-09-09
Filing date: 1992-09-09
Publication date: 1994-03-10
Also published as: GB2280638A; GB9316390D0; GB2280638B; FR2695865A1; NL9201649A; DE4232251A1

Abstract

ABSTRACT

A polyester resin film laminated metal sheet comprises a biaxially oriented polyester resin film containing about 40 to about 60 weight % of polybutylene terephthalate and about 40 to about 60 weight % of polyethylene terephthalate, and having a glass transition temperature of about 40 to about 65°C and a minimum time for half crystallization at least below about 20 seconds. The biaxially oriented polyester resin film is heat bonded to one or both sides of a metal sheet 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 and thereafter quenching. The polyester resin film laminated metal sheet according to the invention can be used for the outside of can stocks such as the outsides of can ends, can bodies in three piece cans, drawn cans, drawn and redrawn cans and screw caps, all of which are treated with hot steam and hot water in a retort for the sterilization of the packed foods.

Description

77~ 1 ~POLYE8TER I~lIN~TED ~Eq!AL }~ ET

FIE~D OF ~H~ INV~NTION

The present invention is directed to a polyester resin film laminated metal sheet, and to articles manufactured therefrom.

:.
BACRGRO~ID O~F THE~ INVENTION ~ `

Presently, metal sheet stock, for example, such as eleckrotinplate, tin free steel (TFS), and aluminum are widely used for can stock after applying one or more coats of lacquer.
However, the employ of such lacquer coatings have associated drawbac~s including increased energy costs due to extended curing times, and the discharge of solvent during curing which must be disposed, for example, by incineration to prevent environmental pollutlon.
To avoid such problems as mentioned above, the lamination of a thermoplastic resin film on a metal sheet has recently been descrlbed. For example, U.S. Patent NoO 4,517,255 describes a ethod or laminating a crystalline polyester resin film *o a metal sheet heated to a temperature above the melting temperature of the polyester resin ~ilm, and thereafter immediately quenching ~

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the laminate. In this re~erence, the crystalline polyester resin film is said to be sufficiently adhered to the metal sheet by an amorphous non-oriented polyester resin layer which is formed at the interface of the crystalline polyester resin film and the metal sheet as a result of the heating step. However, when a can manufactured by the polyester resin ~ilm laminated metal sheet according to this method is treated by hot steam and water at 100C to 130C in a retort for the sterilization of foods packed therein, innumerable milky spots which deteriorate the commodity value of the packaging are observed on the outside of the can made by the polyester resin film laminated me~al sheet when hot water droplets accumulate in part of the surface of the polyester resin film laminated metal sheet, although the laminated polyester resin film is not peeled off from the metal sheet. It is thought that such milXy change is due ~o the difference of the recrystallization speed of the amorphous non-oriented polyester resin layer in the part contacting hot steam from that in the part contacting hot water. If the surface of the polyester resin film laminat~d metal sheet is uniformly wetted by hot water or hot steam, the milky spots are not be observed, because the entire surface of the polyester resin film laminated metal sheet changes uniformly.
Laid-Open Japanese Patent Application No. Hei 3-212433 describes a method for producing a copolyester resin film laminated metal sheet said to have excellent resistance to such milky chanye by a retort treatment. In particular, this patent ,: :: , . , ,: : :
:, is characteri~ed ~y laminatin~ a copolyester resin film con~isting of J5 to 99 mole % of polyethylene terephthalate and 1 tQ 25 mole % of other polyeste.-c on a l~etal sheet at a kemperature ~elow ~he melting t2~tperature and above the softening temperature o~ the copolyester resin filr~l, and thereafter ra~idly quenchiny ~he lam-.nate in order to decr2ase the amount of the amorphous copol~lester resin layer formed as a result of heating step during iamination, wl.lich causes ~he mil~y change by a ret~rt treatment such as desc~ibed a~ove. In the method according to this r2L-erencP, a small amount of the amorphous non-o~iented copolyester layer is inevitably formed between the surface of the metal sheQt an~ t'ne biaxially criented copolyester resin layer. :
lf the amorphous non-oriented layer is not formed at all, the laminated biaxially oriented copoly2ster resin film will not adhere to the met21 sheet. Thus, if the milky change is eli.minated, th2 laminated copolyester resin film may peel off ~;
during foI~ing cpera~ion, because the amorphous non-oriented co~olyester resin layer is scant and non-uniform.
Further, according to this ^eference, it is very difficult t.o obtain a polyes~-er resin film laminated ~etal sheet which exhi~its gooc'. ad~lesion and forma~lity and does no'~ exhibit a ~il'y chang2 during r~tort tre~.. cment, because ~he polyester resin ;~
softened at a temperature below the melting temperature and above ~he softening temperature of th-e employed polyes~er resin film has hi~h viscosity and 'che surIac2 of the metal sheet is not u/lifonnly wetted ~y the melted copolyester resin.
1, , ~ 207~8~

It is possible experimentally to prevent the milkY ~hange of the laminate having an amorphous non-oriented layer by the methods described in these patents by a use o~ special retort, wherein the laminate is usually in contact with hot steam or water only, or bv a retort treatment at higher temperature.
However, these methods are not economical. Furthermore, this milky change may be also prevented hy reheating the laminate before quenching at a temperature above a ylass transition temperature for a long time, for exampl~ at 160 C for 120 seconds in ~he production process of the polyester resin film laminated metal sheet. However, this reheating method is not suitable ~or the continuous production of the polyester resin film laminated metal sheet at high speed, and is not suitable from the standpoint of economy, because an addition of reheating equipment is necessary.
In U.S. Patent No. 4,614,691, the surface of the polyester resin film laminated metal sheet does not become milky by contact with hot steam and water in a retort because the presence of an amorphous non-oriented polyester resin layer is substantially reduced as a result of lamination at a temperature below the melting temperature of the polyester resin film. However, the coatinq and curing of the specified adhesive precoated to khe polyester resin film which is an absolute requirement in this reference is disadvantageous from the standpoints of the material cost and the treatment of a large volume of solvent discharged during curing the precoated adhesive which causes air pollution.

~ 2~7~

- Japanese Patent Publication No~ Sho 57-23584 describes a metal structure covered with a thermoplastic polyester resin produced by esterification of dicarboxylic acid, in which terephthalic ac~d is at least 45 mole % of dicarboxylic acid with diol, and in which 1,4-butane diol is at least 55 mole % of said diol. The polyester resin has a relative viscosity of 1.2 to 1.8 a tack point of not lower than 130C, and a degree of crystallinity of up to 30%. In this reference a metal substrate is covered with a thermoplastic resin containing above 45 mole %
of polybutylene terephthalate which has an almost amorphous non- ~
oriented structure. Therefore, the surface of the laminate .
according to this reference may become milky during retort treatment, because the laminated amorphous non-oriented layer is non-uniformly recrystallized by retort treatment. Therefore, the ~: -laminate according to this reference can not be used for the applications treated with hot steam and water in a retort after packing fosds.
Additionally, Japanese Patent Publication No. Sho 60-4058 describes a can end produced by a polyester resin laminated metal sheet, which comprises heat bonding a polyester resin on a metal sheet, wherein the polyester resin is produced by esterification o~ dicarboxylic acid in which tereph~halic acid is at least 66 mole % of said dicarboxylic acid with diol, and in which 1,4-butane diol is at least 45 mole % of said diol ~ave with the resulting product having an intrinsic viscosity of ~.7 to 2.8.
According to this reference, the can end can not be treated in a ,, '' ' ,; , ' , :

- 2~81~
retort after packing foods, because the laminated polyester layer is constructed b~ the same method as that in ~apanese Patent Publication No. Sho 57-23~84, and thus becomes milky by retort treatment.
Accordingly, a primary objective of the present invention is to provide a polyester resin film laminated metal sheet which has improved resistance to such milky and discoloration change as discussed above which is observed on the outside of a can after retort treatment for sterilization of the packed foods, and which also has improved adhesion of the laminated polyester resin film to a metal sheet and formability to can ends, can bodies in three piece cans, drawn cans, drawn and redrawn cans and screw caps.
It is another objective of this invention to provide an economical method for the continuous production of such polyester resin film laminated metal sheet at high speedO

8~MMARY OF TH~ INVENT10~

The aforesaid objectives are accomplished in accordance with the present invention, which provides a method for the production of a polyester resin film laminated metal sheet having excellent resistance to milky change and other discolorat,i~n by retort treatment. The present invention comprises heat bonding a biaxially oriented polyester resin film consisting of primarily 7 ' ', ':

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~ 2077~

polybutylene terephthalate and polyethylene terephthalate, or a biaxially oriented polyester resin film wherein a portion of the polyethylene terephthalate is substituted by polyethylene isophthalate, having specified characteristics on one or both sides of the surface of a treated metal sheet having excellent adhesion to the employed polyester resin film, and thereafter qusnching. -~
More particularly, the present inventive method comprises heat bonding a biaxially oriented polyester resin film to one or both sides of a metal sheet having a coating of a single layer of ~i hydrated chromium oxide or a double layer comprised of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide. The biaxially oriented polyester resin film comprises of about 40 to about 60 wt % polybutylene terephthalate and about 40 to about 60 wt % polyethylene terephthalate, and has a glass transition temperature of about 40C to about 65C, and a minimum half crystallization ~ime of at least below about 20 seconds on one or both sides of the metal sheet.
The present invention is mora fully explained in accordance with the following detailed description, including preferred embodiment~.

DE~AI~ED DESCRIPTION OF ~H~ INVENTION

In the present inventive method, a biaxially oriented polyester resin film is heat bonded to one or both sides of a ., , , ., . . ,, :i.' ,: :, i:, : ' , ,' ' ' ' ,,, ' ' ,, ` ' ' .. ':' ; ." `, ' . ' ,' ~77~
metal sheet, which metal surface has been treated to preferably provide excellent adhesion properties for the polyester resin film, and the polyester resin film bonded metal sheet is thereafter quenched. The biaxially oriented polyester resin ~ilm consists primarily of ~olybutylene terephthalate and polyethylene terephthalate, or a biaxially oriented resin film in which a part of the aforesaid polyethylene terephthalate is substituted by polyethylene isophthalate.
In accordance with this invention, several characteristics of the polyester resin bonded metal sheet are important and include the following:
(l) characteristics of the employed polyester resin film;
~ 2) characteristics of the polyester resin film after lamination to the metal sheet, especially the characteristics of the polyester resin layer contacting the surface of the metal sheet;

(3) use of the surface treated metal sheet which preferably has excellent adhesion to the laminated polyester resin film; and (41 selection of laminating conditions in response to the characteristics of the employed polyestsr resin film.
In accordance with the present invention, all of the above-described factors are controlled within their respective :
preferred ranges, to obtain a polyester resin film laminated metal sheet having excellent resistance to the milky change . .
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during retort treatment; eXcellen~ adhesion of the laminatedpolyester resin film to the metal sheet; and an excallent formability to can ends and drawn cans.
The present invention can be ~urther summarized by laminating a biaxially orien~ed polyester resin film wherain the amorphous non-oriented layer formed between a biaxially oriented film and the metal sheet immedia~ely after lamination is recrystallized at a optimum and preferred speed before quenching the laminate.
The polyester resin film laminated metal sheet according to the present invention can be used for the outside of can stocks such as tha outsides of can ends, can bodies in three piece cans, drawn cans, drawn and redrawn cans and screw caps, all of which are treated with hot steam and hot water in a retort for the sterilization of the packed foods.

The Polyester Resi~ Film The polyester resin film laminated metal sheet according to the present invention is produced by heat bonding a biaxially oriented polyester resin film on a metal sheet wherein the polyester resin film consists of about 40 to about 60 weight % of polybutylene terephthalate and about 40 to abou~ 60 weight % of polyethylene terephthalate, and wherein the re~in has a glass transition temperature of about 40 to 65C, and at least b~low 20 .. . . . . . , . - . . . . .

2~77~
seconds of a minimum time for half crystallization to a metal sheetO Further, 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. After bonding of the polyester resin film the polyester resin film bonded metal sheet i5 quenched.
In the process of the present invention, it is very important that at least a portion of the amorphous non-oriented polyester resin layer formed between a biaxially oriented polyester resin film and a metal sheet by heat bonding is recrystalliæed before quenching the resulting laminate. In particular, if a polyester resin film having high crystallization speed is used for the present invention, a substantial portion of the amorphous non-oriented polyester layer formed between the biaxially oriented polyester resin film and the metal sheet is recrystallized before quenching the laminate. As a result, the laminated polyester resin film is peeled off from the metal sheet by severe forming~ If a polyester resin film having low crystallization speed is used, the formed amorphous non-oriented polyester resin layer is substantially not recrys~allized before quenching the laminate, and as a result, the polyester resin film :~
laminated metal sheet having excellent resistance to the milky change by retort treatment which is the objective of the present invention can not be continuously produced at ~igh speed, .
although the adhesion of the laminated polyester resin film to ~
he metal sheet is good. ~ .

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Therefore, it has been found ~ be indispensable in the present invention to use a polye~ter resi~ film having a minimum time for half crystallization o~ at least below about 20 seconds.
It is more preferable to use a polyester resin film having about 0.5 to about 12 seconds of the minimum time for half crystallization from the point of stable and high speed production of the laminate according to the present invention.
The minimum t.ime for half crystallization as used in the invention is defined as the minimum time for half crystallization o~ the employed polyester resin at a range in temperature wherein the employed polyester resin is crystallized. This property can be determined by using an apparatus for the measurement of polymer crystallization speed, for example, the apparatus of Trade name MK-701 made by Kotaki Co., Ltd., and can be calculated ;~
by the following Avrami's equations:

1 - X = Exp (-K.tn) (It - Iq) 1 - X = (Io - Ig) where, X represents the crystallinity of the employed polyester , resin:
K represents a constant for crystallization speed of the employed polyester resin;
n represents avrami index;
t represents time (seconds);

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'I.o represents a tran~paren~ in~ensity o~ depolarized light at the startiny point in the measure~ent, or more particularly Io r~pr2sents a ~alue which can b~ obtalned after 'che melted ~nlyester resi.n is clropped 'nto a si.ii.con oil bath for cr~s-tallization and tnen ~ept for 10 seconds;
It represents a transparent intensity of depolarized light a~Ler t seconds ln tho measureme~t, or more particularly it rep.r2s~nts the v~lue after 10-~t se~.ond; and Ig represents a transpa~ent intensi~y of depolarized light at the end point in the measure;nen~, or Ig represents the value .:
in which I ~ log t CU~2 shows almost a straight line.
Al'chough it is possi~le to use ~olyester resin films having --.
varlous compositions and havin~ all o~LimU~ crystallization speed in acoordance wich tne ~resent invention, the use of a biaxially .
~rien'Lecl polyest~r resin ~ilm comprising of about 40 to about 60 :
weight % poly~utylene terephthalate and about 40 to about 60 :~:
weight % polyethyl2ne terephthalate is prererable from the .;
stanapoint of charactQ.rlstics and economy. A polyester resin Lilm whex~in a part o- polyethylene terephthalate is substituted ;;
hy polyechylene isoph'~hal~te Ol other polymers can clso be used ~.
in the presenk invenlcion. ~owe~.re~, much attention m-lst be paid ko th2 addi~,on of these pol~ners. For i~stance, it is not pxeerable that the amount of ~olyet'nylene isophthalate added in : :
the polyester resin film contai-ning about 40 tq about ~0 weight % ..
O'î polybutylene terep'nthalate i.5 a~ove about 15 weight % of polyeth~ylene terephthalate, because the crystallization speed of 1~

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this polyester re~in film becomes slo~ with the increase in the amount of polyethylene isophthala~e. I~ the amount of polybutylene terephthalate is below about 40 weight % in the employed polyester resin film, the surface of the laminated polyester resin film on the metal sheet may change to milky by retort treatment, because the amorphous non~oriented polyester resin layer formed between the biaxially oriented polyester resin film and the metal sheet is not sufficiently recrystallized before quenching the laminate. Furthermore, the polyester resin film having below about 40 weight % of polybutylene terephthalate may not be suitable for the continuous production of the polyester resin film laminated metal sheet according to the present invention at high speed, because the crystallization speed of this polyester resin film becomes slow and a greater part of amorphous non-oriented layer formed during heat bonding to the metal sheet remains without recrystallization, although the formed amorphous non-oriented layer is recrystallized if the laminate is reheated for a long time or is slowly quenched after lamination to the metal sheet. If the amount of poly~utylene terephthalate is above about 60 weight % in the employed polyester resin film, the surface appearance of the laminated polyester resin film may become noticeably poor by oligomer of the polyester resin film which is isolated on the surface of the polyester resin film in the production process of the polyester re~in film, and each other surface of the coiled laminate may be stuck in the forming process, because the polyester resin film , " ;: ,,. . : , , ~ .; ~. ;

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having above about 60 weight % of polybutylene ~erephthalate has a low glass transition temperature. Furth~rmore, the polyester resin film containing polybutylene terephthalate becomes expensive with the increase in the amount of polybutylene terephthalate, and the industrial production of it becomes difficult. Therefore, in view of the above the use of the polyèster resin film comprising about 40 to about 60 weight % of polybutylene terephthalate and about 40 to about 60 weight ~ of polyethylene terephthalate is preferred in ~he present invention.
~ he glass transition temperature of the employed polyester resin film is also an important factors in the present invention The glass transition temperature of the employed polyester resin film can be measured at a heating rate of 10C/minute, for example, in a differential scanning calorimeter (SS10) made by Seiko Denshi Kogyo Co. In the present invention, it is necessary to use a polyester resin film having a glass transition temperature of about 40C to about 65C. If the polyester resin film having a glass transition temperature of below about 40C is used, each o~her surface of the coiled laminate may become stuck -in the ~orming process. Furthermore, the corrosion resistance of this polyester resin film laminated metal sheet becomes poor, because this polyester resin film becomes poor in the barrierability. In the present invention, the upper limit of the glass transition temperature of the employed pq~yester resin film is automatically decided by the composition of polyester resin film. Namely, the grass transition temperature of the employed ' '', ' ' . . ' . . ' . ' . ' . :' .' . ' ' ' po~ ~ster resin fi]rn is kept ~elow about 65C, becauise the amount of polybutylene-terephthalat~ in the employed polyester resin `L~ m is ~referably restric~e~ to the ran~e of about 40 to ~o weig~t % from the point of t;ne r~slstance to milXy chanqe by retort treatment. Although the pol~ester resin film having a glass transition temperature above about 65C can be produced by a decrease in the amount of polybutylene terephthalate blended in polyethylene t~rephthalate, it becomes poor in the resistance to the milky change by retort treatment.
The mechanical property of the employed polyes~er resin ~ilm is also ~m~iortarlt factor f~om the standpoint of formability of ~he ~Glyester resin film. Specifically, the elongation at breaX
of the poly~ster resin film, which can be determined at the speed of 100 ~m/mir. at 25C in an ordir,ar~ tensile testing machine, should be at leasc a`~ove about 80 ~- If a polyester resln film havin~ helo~J about 30 % of elongation at ~reaX is used for the present inven~ion, r.lany cracks can arise in the laminated film by li~ht for~.in~ to can erds, hecause the formability of s~id film }: ecomes, poor.
The preierable thio~ness of th2 polyester resin film used in the pres2nc inven:_ion is about ~ ro a~out 80 um, and mor~
~r~f~r~bly ~ut 3.0 ta about 30 um I~ th2 'chicXness of the emfrlOyf~d polv2scer resin film is abouc belo~l 5 um, good corrosion rf?sistance~ a,t~r -on~ing may not be obtained and the c~ntinuous lafminaLion of tn2 thin polye~t~r x~sin ~ilm LO the metal sheec ~ay become dlficulc. Ifhe US ~:r polyester resin film having a f, '~f ', .~: . , ' ' .: :

~ 2 0 7 7 ~ ~ ~

thickness above about 80 um becomes economically undesirable for the film to be laminated to the metal sheet, because it is expen~ive as compared with epoxy phenolic lacquer widely used in the can industry.
It is al~o contemplated in this invention that additives such as antioxidant~, stabilizers, pigments, antistatic agents, lubricants and corrosion inhibitors and other known additives and adjuvants in amounts known and desired for various performance characteri6tics can be added during thP manufacturing process of the polyester resin film.
The characteristics of the polyester resin film after lamination to the metal sheet will now be discussed.
In the present invention, it is preferably from the standpoint of the resistance to the milky change by retort treatment, the adhesion to the metal sheet after forming and the corrosion resistance after forming that the characteristics of the polyester resin film after lamination to the metal sheet are controlled by all of the following factors in some optimum, and thus preferred range. These factors include:
(~) relative ratio of refractive index of the laminated polyester resin film which is measured in the length-wi e direction of the inner side contacting with the metal sheet, after peeling off from the metal sheet;
(2) relative ratio of the density in the laminated polyester resin film;

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(3) refractive index in the measured lengthwise, widthwise and oblique direction of the laminated polyester resin film, measured from the outer ~ide; and (4) residual degree of biaxial orientation in the ~.
laminated polyester resin film.
The relative ratio o~ refractive index and the relative ratio o~ the density in the laminated polyester resin film can he de~termined as follows: Samples (a), (b) and (c) can be prepared by the following methods, respectively: :
Sample (a): a polyester resin film laminated metal sheet is immersed into dilute hydrochloric acid solution at 25C.
After dissolution of the metal sheet, the obtained film is rinsed with water ~or 3 hours and then dried in a desiccator in the .
presence of silica gel (drying agent) for 1 day at 30C.
Sample (b): The same polyester resin film laminated metal sheet as prepared in sample (a) is heated in a nitrogen . `.
atmosphere at a temperature of the melting temperature of said film + 30C for 1 minute and them immediately immersed into -~
liquid nitrogen. After that, the only laminated film is obtained by using the same method as in the preparation of sample (a).
Sample (c): The same polyester re~in film laminated metal sheet as prepared in sample (a) is heated for one hour in a nitrogen atmosphere at a temperature where a maximum density is obtained within a range of crystallization temperature of the laminated polyester resin film. After that, the only laminated ~8 '~ , . '; ',; :: .: ' ~ilm is obtained by usin5 ~che same method as in the preparaticn o. sampl~ ~a~
The refractit~e inde~ in the lerisr~thwise direction of sample (a~, (b~ and (c) is measured from t~le inner side contacting with the me~al sheet b'l using a refractometer and the density of sam~les (a), (b) and ~c) is al50 l~easured, respscctiYely.
.~:n th.- present in~ention, sample (b) corresponds to tho stat~ h~rein the fil.-n ha~ an almost a-iQorphous non-oriented ~;
structuro, and sample (c) cor;esponds to the state wherein the ;
film has maximum crystal].i~ation structure. ~-The relative ratio in the refracti~le index of the laminated -polyester rosin film (R) is calculated by the following equation:
R (~) = {(Ra - Rb)/(Rc - Rh)} x 100 where, Ra, Rb and Rc represent the refractive index in the lengthwiso direction of samples (a), (b) and (c) measured by the method describe~ above, res~ectively.
The relative ratio of t:he denslty in the la~inated pclyester resin fllm (D) is calculated ~y the following equation: -D (~ (Da - D~)/(Dc - Db)} ~ 100 whexe, Da~ ~ and Dc represent the density of samples (a), (b) an~ (c).
In s_he pr85en~ in~ention, R is prefera~ly controlled within th2 rarlfJ~ o~ a~out ~2 to a~out 95 ~. If R i5 les.~ than a~out ~2 % ~ the S~faS~e OL the l--~min~ted polyes~er resin film may change to milk~ ~ a rc-tort treatl,lent, althous~h the adhesion of the lalr;inatod polyester resin film to the meSs~al sheet is good, f ,s,' . . .

2~7~

because it is thought that a greater portion of the laminated crystalline biaxially oriented polyester resin film will change to a layer having poor resistance to milky change, in a similar manner as the amorphous layer. On the other hand, if R is more than about 95 %, the adhesion o~ the laminated polyester resin film to the metal sheet may become poor, because a larger portion of the crystalline biaxially oriented layer remains even after the lamination to the metal sheet.
D is also preferably controlled with the range of about 35 to about 90 %. If D is less than about 35 ~, it may become very difficult to prevent the milky change from occurring by a retort treatment, because it is thought that a greater portion of the laminated crystalline biaxially oriented polyester resin film may change to the amorphous non-oriented layer and may be not recrystallized before quenching the laminate. It is also preferable in the present invention the D is not more than about 90%, because many cracks may arise in the laminated polyester resin film by the deterioration in formability of the laminated film, specifically formability by impact forming.
Furthermore, the refractive index in the lengthwise, widthwi~e and obligue directions of the laminated polyester resin film measured from the outside of the film (RI) and the residual degree of biaxial orientation of the laminated polyester resin film (BO) are also important factors to be considered in the pre~ent invention. RI of the laminated polyester resin film is , . . .
, ~ .
- ~ :
. .
,, .. . , ,,~,,.~,:

, , , ' . ' t : :
f , ., ~asured by using a refractometer and BO is determined by the fiollowing procedures:
(1) the X-ray diffracti.on intensity of the polyester ~esin ~ilm before an~ af~er lamination to the metal sheet is ~asured ~ hin 2 range of 2~ - 20 to 30; ~-(21 a ~oint of 2~ = 20 and a point of 2~ = 30 are conn.ected ',~y a strai~ht llne~ and this line is designated as the base line;
(3) a heig~t of the ~eak appearing in 2e = 23 to 29 of the diffraction inrensity curve from the base line is measured; and the heiynts in the polyester resin film before and ~:
after lamination -t~ ~he metal sheet is represented by Ia and Ib, respestively; and (~ a residual de~:cee of biaxial orientation (BOj is r-ep-resented by the following e~lation~
BO (%) = Ib/Ia x 100 :.
Xf the ~.I i.s less than about 1.59, the formability by impact fonni~g ~ay become poor, and if the RI is more than about 1.67, the elonga~ion of ~he lami.nated polyester resin film may become ~oor. In par'cicula-r, it is prefQra~le in the present invention that RI is controllPd to within ~ range of about 1.59 to about 1.67 ~rom ~he standpoint of 'che formability oE the lamlnated fil~ll. On the other h~.nd, il BO is less than a~out 20 %, the Lorma~ili-c~ o~ the laminated ~olyester resin film may ~ecome poor ~,nd t~e surface of ~he laminate~ polyester resin film may change to lr.ilXy by a retort -c.reatment. In the case of BO greater than ,,, , :
,. ............................................................... .
........ ", ~778~

about 85%, the adhesion of the laminated polyester resin film to the metal ~heet may become poor. TherePore, B0 is preferably controlled to a range of about 20 to about 85 %.

. . .

The Metal_Substrate ~ ~-.

Metal sheet useful in this invention can be steel sheet, tin plated steel sheet, nickel plated steel sheet and aluminum sheet.
Further, in accordance with the present invention to provide the desired excellent adhesion properties of the metal sheet to the polyester resin 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 and plated nickel in the metal -sheet is preferably below about 5.6 g/m2 and about 3.0 g/m2, respectively, for reasons of economy. However, if the amounts of plated tin and plated nickel are below about 0.95 g/m2, the effect of plated tin or nickel on such characteristics, for example, as corrosion resistance to ~he packed food, is hardly apparent, despite of the addition of a further plating process.
As mentioned above, it is an important factor in the present invention that the employed metal sheet be 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 .

~ - 2 ~ 7 ~

chromium oxide, in order to obtain an excellent adhesion of the laminated polyester resin film to ~he metal sheet af~er for~,ing to can ends and drawn cans.
- The preferred amount of hydrated chromium oxide as chromium is about 5 to about 25 mg/m2 in the single layer or the double layer. The preferred amount of metallic chromium in the double layer is about lO to about 150 mg/m2~ If the amount of hydrated chromium oxide as chromium is below about ~ mg/m2 or above about -25 mg/m , the adhesion of the laminated polyester r~sin film to the metal sheet may become poor after forming even if the amount of metallic chromium is about 10 to about 150 mg/m2, when the -polyester resin film laminated metal sheet is exposed to hot steam and hot water in a retort. It is preferable that the deposition of metallic chromium improves the adhesion of the laminated polyester resin film to the metal sheet and the corrosion resistance of the obtained laminate. However, the deposition of metallic chromium above about 150 mg/m2 is , unnecessary in the present invention, because the corrosion -resistance is not substantially improved, even if metallic chromium above abou't 150 mg/m2 is deposited.
.
When the polyester resin film laminated metal sheet in accordance with the present invention is used for the can stock wherein high corrosive foods and beverages are packed and treated with hot steam and hot water in a retort, the polyester resin ~ilm, wherein one side of said polyester resin film contacting with the metal sheet is precoated uniformly and thinly with a .

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207~l.a thermosetting resin, such as epoxy-phenolic resin, may be laminated on the surface treated metal she~t or the polyester resin film may be la~inated on the surface treated metal sheet .~ precoated with the thermosetting resin described above. However, such precoating with a thermosetting resin on an employed polyester resin film or the employed metal sheet may be ; expensive.

- Production of the Polyester :
Resin Film Laminated Metal Sheet . ' .
In accordance with the present inventive method the biaxially oriented polyester resin film having the above . described characteristics is continuously heat bonded to a surface treated metal sheet under conditions wherein the temperature of the metal sheet to be laminated by the polyester re~in film, the thickness of the employed metal sheet, the thickness of the employ~d polyester resin film, the surface temperature of the employed laminating roll, and the pressure added to the laminating roll, the time until cooling the laminate after lamination, are controlled within preferred ranges in respon~e to the recited characteristics of the e~ployed polyester resin film.
In particular, it is important and very much preferred in the present inven~ion that the metal sheet to be laminated with . ." , , . , . . . , :, ;. . . , ,. , , ~, : .

- 2~7~

polyester resin film be maintained a~ a temperature above the melting temperature of the employad polyester resin film, and the ~urface temperature of a laminating roll be controlled at a temperature below the melting temperature of the employed polyeitex resin film. If the tempsrature of the metal sheet is below the melting temperature of the employed polyester resin film, the laminated polyester resin film may not be sufficiently adhered to the metal sheet and can be peeled off from the metal sheet by light forming. Furthermore, if the surface temperature of the laminating roll is at a temperature above the melting temperature of the employed polyester resin film, the continuous and stable production of the polyester resin film laminated metal sheet according to the present invention may become very difficult, because the outside of the laminated polyester resin film can be melted by heat transmitted from the laminating roll and adhered to the laminating roll.
Generally, it is also preferred in the production of the :~
polyester resin film laminated metal sheet according to the present invention that a polyest~r resin f1lm having higher ~.
degree of biaxial orientation is laminated on the metal sheet heated to higher te~.sperature. On the other hand, in accordance -.
with the invention a polyes~er resin film having a lower degree of biaxial orientation is laminated to the metal sheet heated to a lower temperature. In the u~ie of a thinner polyester resin film or the use of a thicker metal sheet, it is preferable to :-decrease a surface temperature of metal sheet or laminating roll. .

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Mc.~ely, it is i~nportant i.n '~he prQSent invention t~at the accors which e~fect -che characteristic~ of the laminated polyester resin gilm 2re controll~d ~o ~e wit~ pr~erred o~timum range.
~ he ~ethod for heating the raetal sheet to be laminated with the polyesteL resi-n film is no'~ cri-tical to the 7~resen-t invent~on. '~o~reYer~ i-rom the ~tand;30irlt of t'ne continuous Gnd s'cable ~ro~uc~iorl o~ ~he laminate at high spee~, ccnduction neating by rolls hea'ced hy incluction heating, induction hea'cing arld/or resistance~ hea'~ g whicr. 2re l~sed for reflo~ing electroplat~-~d tin in the produckion pr~cess of electrotinpla'ce are. suitable as ~ ~ne-thod for neGting the metal sheet, because the raetal sheet can be rapidly hea'~ed and the temperature of the heated rnetal shee~ can he easil-y controlled. Furthermore, it i5 also pr~fera~le in the ~Jrese.nt ln-~ention ihat heating wich a roll heatad by hot steam or heatin~ in i?.n eleciric oven can be uso a~
an auxiliary raethod for pr~he-till~J the me~al sheet to b2 1 arlinated .
The present invention is explalned in fu ther detall by th2 following examp.les. It is to be understood, ho~ever, that -these ~xa~n7~1es a-sa ~G' il lustra~ciYQ ~ rposes only a7.~d are n-3L intende~
to limi~ the scope o- th~i lnv~ .,.on or claims and spirit -thereo-~7.
in a-ny ~.~ay.

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A biaxially oriented polyester resin film consisting of 50 ~.
weight % of polybutylene terephthalate and 50 weight % of polyethylene terephthalate having a thickness of 12 um, a minimum time for half crystallization of 7.5 seconds, a glass transition temperature of 49C and an elongation at break of 132 % was laminated by using a pair of laminating rolls wherein the surface temperature was 120C on both sides of a TFS strip. The TFS
strip had a thickness of 0.22 mm and a width of 250 mm, and a coating of metallic chromium of 105 mg/mZ and hydrated chromium :
oxide of 17 mg/m2 as chromium which had been heated to 250C by using a pair of rolls heated by induction heating at a laminating ;
speed of 25 m/min. After 4 seconds, the laminate was quenched in water having a temperature of 35C. In the laminated polyester resin film of the obtained laminate, R was 40 %, D was 61 %, RI
was 1.636 to 1.650 and BO was 62 %.

., .
', .' ; ~X~NPL~ 2 .',', . ~, The samc polyester resin film as in Example 1 was laminated by u~ing a pair of laminating rolls, wherein the surface temperature was 120C on both sides of the same TFS strip as in 1 Example 1, which had been heated to 255C by using a pair of ,~

.,~ , ~''' ' '''~' ~ 2~7~

rolls heated by induction heating at a laminating speed of 60 ~ m/min. After 0.7 seconds~ the laminate was quenched in water having a temperature of 35C. In the laminated polyester resin film of the obtained laminate, R was 2~ %, D was 38 %, RI was l.610 to l.633 and B0 was 21 %.

EX~PL~ 3 The same polyester resin film as in Example l was laminated by using a pair of laminating rolls, wherein the surface temperature was 100C on both sides of the same TFS strip as in Example l, which had been heated to 238C by using an induction heating at a laminating speed of 25 m/min. After 8 seconds, the laminate was quenched in water having a temperature of 35C. In the laminated polyester resin film of the obtained laminate, R
- was 93 ~, D was 88 ~, RI was l.638 to l.655 and B0 was 84 %.

~XA~PLE 4 ,~ ~

A biaxially oriented polyester resin film having the same compo~ition as in Example ~, but also having an elongation at break of 105 % and a higher degree of orientation compared to that in Example l, was laminated on a tin plated steel strip wherein a double layer consisting of a lower layer of 30 mg/m2 of , .,I

;

~ ~ 7 ~

metallic chromium and an upper layer o hydrated chromium oxide of 10 mg/m2 as chromium was formed, and afterwards electroplated with 1.0 g/m2 of tin under the same conditions as in Example 1.
After 6 seconds, the laminate was qu~nched in water having a temperature of 35C. In the laminated polyester resin film of the obtained laminate, R was 66 %, D was 75 %, RI was 1.655 to 1.660 and B0 was 68 ~. -~XANPL~ 5 ..
..

A biaxially oriented polyestsr resin film having the same composition as in Example 1, but having an elongation at break of :
148 % and a lower degree of orientation compared to that in ~xample 1, was laminated by using a pair of laminating rolls :
wherein the surface temperature was 100C on the same TFS strip as in Example 1 which had been heated to 256C by using a pair of ~-rolls heated by induction heating. After 2 seconds, the laminate was quenched in water having a temperature of 35C. In the laminated polyester resin film of the o~tained laminate, R was 28 %, D was 40 %, RI was 1.590 to 1.613 and BO was 37 %.

.

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2~7~

E~AMPL~ 6 A biaxially oriented polyester resin film consisting of 42 %
polybutylene terephthalate and 58 ~ polyethylene terephthalate which had a thickness of 12 um, a minimum time for half crystallization of 20 seconds and a glass transition temperature of 58C was laminated on both sides of the same TFS strip as in Example 1 under the same conditions as in Example 1. After 10 seconds, the laminate was quenched in water having a temperature of 35C. In the laminated polyest~r resin film of the obtained ~ laminate, R was 36 %, D was 67 %, RI was 1.636 to 1.655 and BO
! was 72 ~-' ~ EXAMPL~ 7 .~ A biaxially oriented polyester resin film consisting of 58 ql polybutylene terephthalate and 42 weight % polyethylene `i terephthalate which had a thickness of 12 um, a minimum time for : half crystallization of 2,8 seconds, a glass transition ` temperature of 42C and an elongation at break of 121 % was '"! laminated on the same TFS strip as in Example 1 under the same conditions as in Example 1. After 4 seconds, the laminate was ;~ quenched in water having a temperature of 35C. In the laminated ,. . .
, 30 .::

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2~7~

polyester resin film of the obtained laminate, R was 51 %, D was 63 %, RI was 1.632 to 1.650 and BO was 52 %. ' ~;

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~MPL~ 8 . .

:~ ~he same polyester resin film as in Example 1 was ~ :
laminated by using a pair of laminating rolls wherein the surface ~::
temperature was 125C of TFS strip having a thickness of 0.20 mm, a width of 250 mm, a surface coating of metallic chromiu~ of 75 mg/m2 and hydrated chromium oxide of 13 mg/m2 as chromium which ;'~ had been heated to 255C by using a pair of rolls heated by an induction heating at a laminating speed of 30 m/min. After 8 seconds, the laminate was quenched in water having a temperature of 65C. In the laminatPd polyester resin film of the laminate, R was 35 %, D was 50 %, RI was 1.615 to 1.632 and B0 was 25 %. ~:

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' , ~X~MPLB 9 The same polyester resin film as in ~xample 1 was laminated ~ r ' by using a pair of laminating rolls wherein the surface '~. temperature was 90C on the same TFS strip as in Example 8 which .j 31 `

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had been heated to 242C by using a pair of rolls heated by an induction heating at a laminating speed of 25 m/min. ~fter 6 seconds, the laminate was quenched in water having a temperature of 35C. In the laminated polyester resin film of the obtained lamina~e, R was 79 %, D was 86 %, RI was 1.639 to 1.657 and B0 was 85 %.

A biaxially oriented polyester resin film consisting of 53 weight % polybutylene terephthalate and 47 weight % polyethylene terephthalate which had a thickness of 25 um, a glass transition temperature of 45C, a minimum time of half crystallization of 5.3 seconds and an elongation at break of 123 % was laminated by using a laminating roll wherein the surface temperature was 110C
on an aluminum strip having a thickness of 0.24 mm, a width of 250 mm and a coating of hydrated chromium oxide of 7 mg/m2 as chromium which had been heated to 252C by using a pair of rolls heated by induction heating at a laminating speed of 25 m/min.
After 2 seconds, the laminate was quenched in water having a temperature of 65C. In the laminated polyester resin film of ., : .:. .
the obtained laminate, R was 37 %, D was 62 %, RI was 1.652 to J 1. 665 and 80 was 65 %.

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:, , - ; . - . ~ . .,: :. , ,,, ,. : ,: ~ , , Comparat~ye ~x~mple 1 The same polyes~er resin film as in Example 1 was laminated by using a pair of laminating rolls wherein the surface temperature was 90C on the same TFS as in Example 1 which had been heated to 263C by using pair of rolls heated by induction heating at a laminating speed of 60 m/min. After that, the laminate was immediately ~uenched in wa~er having a temperature of 35C. In the laminated polyester resin film of the obtained laminate, R was 20 %, D was 33 %, RI was 1.615 to 1.641 and BO
was 24 %.

Comparativ2 Bxample 2 The same polyester resin film as in Example 1 was laminated by using a pair of laminating rolls wherein the surface ; temperature was 90C on the same TFS strip. Other laminating conditions were the same as in Example 3. In the laminated polyester resin film of the laminate, R was 97 %, D was 92 %, RI
was 1.639 to 1.657 and BO was 87 %.

comparative Example 3 ~ A biaxially oriented polyester resin film consisting of 63 weight % of polyhutylene terephthalate and 37 weight ~ of polyethylene terephthalate which had a thickness of 12 um, a glass transition temperature of 38C, a minimum time for half ,, :

~: ' . , , ' ! ' . ,' ,, ' , crystallization of 1.7 seconds and a elongation at break of 112 % - .
was laminate~ on the same TFS strip as in Example 1 under the same conditions as in Example 1. In the laminated polyester resin film o~ the obtained laI~inate, R was 62 %, D was 72 %, RI
was 1.636 to 1.658 and B0 was 34~.

.

.

co~arativ~ ~m~le 4 A ~iaxially oriented polyethylene terephthalate film havin~
a thickness oî 12 um, a glass transition temperature of 74C, a minimum ti.me for half crystallization of 42.3 seconds, an elonyation at break of 122 % and the same biaxially orientation ;~
as that in Example 1 was laminated by using a pair of laminatillg rolls wherein the surface temperature was 1~0C on the same TFS ~:~
strip as in Example 1 which had been heated to 287C by using a pair of rolls heated b~ induction heating at laminating speed of ~ .
25 m/min. Arter 10 seconds, the laminate was quenched in water having a temperature of 50C. In the laminated polyethylene .
terephthalate film of the obtained laminate, R was 16 ~, D was 43 %, and RI was 1.642 to 1.656 and BO wa 39 ~
In Examples l to 10 and Comparative E~amples l to 4, 0.1 weight % of a spherical SiO2 having an average diameter o~ 1.5 - .
um was added as a lubricant in the ~roduction process of the .
employed polyester film.
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The characteristics of ~amples ob~ained by Exampl~S 1 to 10 and Comparative Example 1 to 4 were evaluated by the following testing methods, after the measurement of the coating weight on the resultant metal sheet by an X-ray fluorescent method. The results are shown in tables 1 and 2.
The characters in tables 1 and 2 are explained as follows:

(1) Adhesion of the laminated polyester resin film after formina to a drawn cup.

The resultant metal shePt was cut to a circular blank having a diameter of B0 mm by a punch press. The blank was deeply drawn to form a cup at a drawing ratio of 2Ø The adhesion of the laminated polyester resin film to metal sheet was evaluated by the degree of the peeling off of the polyester resin film from the formed cup body and then divided into 5 ranks, wherein 5 was excellent, 4 was good, 3 was fair, 2 was poor and 1 was bad.

.. .

(2) Resistance to milky change by a~retort treatment.

The resultant metal sheet was cut to a size of 50 mm x 100 mm. The cut sample which was placed on stainless sheet having a temperature of 20 or 30C was treated by hot steam having a temperature of 120C for 30 minutes in a retort. After that, the surface appearance o~ sample exposed by hot steam was evaluated with the naked eye and divided into 5 ranks, wherein 5 , ., . . . ~ . ..

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was excellent (no apparent mil~y change), 4 was good, 3 was fair, 2 was poor and 1 was bad (a subs~antial amount of milky change).

A
(3) ~o~nabilitY of the laminated film.
!The resultant metal sheet was cut to a size of 50 mm x 50 ~-. The cui sample was formecl by ~ drop of a stsel r~d having a ~ -~
diameter of the point of 1/2 inches and a weight of 1 kg from the height of 30 cm by using a Du Pont impact test machine. The formability of sample was evaluated by the de~ree of cracks in the laminated film and divided into 5 ranks, wherein 5 was excellent (no cracks apparent), 4 was good, 3 was fair, 2 was poor and 1 was bad (innumerable cracks). ~ -., . - , .

j ~ (4) E mabilitv of the lamina~ed f;lm after heatinq.
, ..
The resultant metal sheet was cut to a size of 50 mm x 50 mm after heating at 205C for 10 minutes. The heated sample was ~,evaluated by the same method as in (3). ;~

'~:

1(53 Corrosion resis~ance aEter forming to a drawn cup.
.; ., .
~The rssultant metal sheet was cut to a circular blank hauing `~a diameter of 60 mm by a punch press. the blank was deeply drawn ;~to ~orm a cup at a drâwing ratio of 1.5. The obtained cup was , ''~''~ .

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filled by 10 ml of 1 % acetic acid solution and then was stored at 37C for 1 month.
The corrosion resistance of sample was evaluated by the degree of black spots observed on the side wall of the cup, and divided into 5 ranks, wherein 5 was excellent (no black spots apparent), 4 was good, 3 was fair, 2 was poor and 1 was bad (innumerable black spots).

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Claims

1. A polyester resin film laminated metal sheet which comprises, a biaxially oriented polyester resin film comprising about 40 to about 60 weight % of polybutylene terephthalate and about 40 to about 60 weight % of polyethylene terephthalate, and having has a glass transition temperature of about 40 to about 65°C and a minimum time for half crystallization at least below about 20 seconds, heat bonded to one or both sides of a metal sheet 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 and thereafter quenching.

2. The laminated metal sheet of claim 1, wherein up to about 15 weight % of said polyethylene terephthalate is substituted by polyethylene isophthalate in said polyester resin film.

3. The laminated metal sheet of claim 1, wherein said polyester the resin film has a minimum time for half crystallization of about 0.5 to about 12 seconds.

4. The laminated metal sheet of claim 1, wherein a relative ratio of the refractive index (R) and a relative ratio of the density (D) which is calculated by the following equations (1) and (2) in the polyester resin film after lamination to said metal sheet is about 22 to about 95 % and about 35 to about 90 %, respectively.

R = ((Ra - Rb)/(Rc - Rb)) x 100 ....(1) D = ((Da - Db)/(Dc - Db)) x 100 ....(2), where Ra and Da represents the refractive index in the lengthwise direction of the polyester resin film measured from the inner side contacting with said metal sheet and the density of the polyester resin film which is peeled off from said polyester resin film laminated metal sheet, respectively, and where Rb and Db represent the refractive index measured by the same way as Ra, and the density of the polyester resin film which is peeled off from said polyester resin film laminated metal sheet after heating in a nitrogen atmosphere at a temperature of the melting temperature of said polyester resin film + 30°C for 1 minute and then immediately quenched into liquid nitrogen, respectively, and where Rc and Dc represent the refractive index measured by the same way as Ra, and the density of the polyester resin film which is peeled off from said polyester resin film laminated metal sheet after heating for one hour in a nitrogen atmosphere at a temperature where the maximum density is obtained within a range of crystallization temperature of the laminated film and then gradually quenched, respectively.

5. The laminated metal sheet of claim 4 wherein RI and BO
in the polyester resin film after lamination to said metal sheet is about 1.59 to about 1.67 and about 20 to about 85 %, respectively, and where RI represents the refractive index in the lengthwise, widthwise and the oblique directions of the laminated polyester resin film measured from the outer side, and where BO represents the residual degree of biaxial orientation of the laminated polyester resin film.

6. The laminated metal sheet of claim 1, wherein said metal sheet is selected from the group consisting of steel sheet, tin plated steel sheet, nickel plated steel sheet and aluminum sheet.

7. The laminated metal sheet of claim 6, wherein said metal sheet is tin plated steel sheet with about 0.05 to about 5.6 g/m2 of tin.

8. The laminated metal sheet of claim 6, wherein said metal sheet is nickel plated steel sheet with about 0.05 to about 3.0 g/m2 of nickel.

9. The laminated metal sheet of claim 1, wherein the amount of hydrated chromium oxide is about 5 to about 25 mg/m2 as chromium in said single layer or said double layer formed on said metal sheet.

10. The laminated metal sheet of claim 1 wherein the amount of metallic chromium is about 10 to about 150 mg/m2 in said double layer formed on said metal sheet.

11. A method for producing said polyester resin film laminated metal sheet according to claim 1, wherein said polyester resin film is laminated on one or both sides of said metal sheet which has been heated to a temperature of the melting temperature of said polyester resin film(Tm) to about Tm + 50°C
by using a laminating roll, and where the surface temperature of the laminating roll is about 50°C to Tm-10°C, and then quenched within about 10 seconds.