Laminated polyester film to be laminated on metal plate

A laminated polyester film comprises: a first layer formed of a first aromatic copolymer having a terminal carboxyl group centration of 10 to 35 equivalent/10.sup.6 g; and a second layer of a molten blend having a terminal carboxyl group concentration of 40 to 80 equivalent/10.sup.6 g, which comprises a second aromatic polyester containing ethylene terephthalate as a main recurring unit and a third aromatic polyester containing tetramethylene terephthalate as a main recurring unit. The laminated polyester film exhibits excellent moldability when a metal plate laminated therewith is deep drawn, and is excellent in impact resistance and flavor retaining property after can making.

TECHNICAL FIELD 
This invention relates to a laminated polyester film to be laminated on a 
metal plate. More specifically, it relates to a laminated polyester film 
to be laminated on a metal plate, which exhibits excellent moldability in 
can-making processing such as drawing of a metal plate laminated with the 
film, and which enables the production of metal cans such as beverage cans 
and food cans which are excellent in heat resistance, resistance to retort 
treatment, flavor retaining property, impact resistance, rustproof 
property and the like. 
PRIOR ART 
Inner and outer surfaces of metal cans generally have coatings for 
protection against corrosion. For simplifying the manufacturing step, 
improving sanitary conditions and preventing environmental pollution, 
there have been recently developed methods of imparting rustproof 
properties to metal cans without using any organic solvent. One of the 
methods is to coat metal cans with a film of a thermoplastic resin. That 
is, studies have been being made of a method in which a plate of 
tin-plated steel, tin-free steel or aluminum is laminated with a film of a 
thermoplastic resin and the resultant laminate is drawn to make cans. 
Attempts have been made to use a polyolefin film or a polyamide film as 
the above film of a thermoplastic resin, but not all of moldability, heat 
resistance, flavor retaining property and impact resistance are satisfied. 
On the other hand, a polyester film, or a polyethylene terephthalate film 
in particular, is drawing attention as one having well-balanced 
properties, and several proposals have been made to use it as a base film 
as follows. 
(A) A metal plate is laminated with a biaxially oriented polyethylene 
terephthalate film through an adhesive layer of a polyester having a low 
melting point, and the resultant laminate is used as a material for making 
cans (see Japanese Laid-open Patent Publication Nos. Sho 56-10,451 and Hei 
1-192,546). 
(B) A metal plate is laminated with a film of an aromatic polyester having 
amorphous nature or very low crystallinity, and the resultant laminate is 
used as a material for making cans (see Japanese Laid-open Patent 
Publication Nos. Hei 1-192,545 and Hei 2-57,339). 
(C) A metal plate is laminated with a heat-set, biaxially oriented 
polyethylene terephthalate film having a low orientation degree, and the 
resultant laminate is used as a material for making cans (see Japanese 
Laid-open Patent Publication No. Sho 64-22,530). 
As for (A), the biaxially oriented polyethylene terephthalate film is 
excellent in heat resistance and flavor retaining property, while it is 
poor in moldability so that it is whitened (causes fine cracks) or broken 
during can-making processing which entails large deformation. 
As for (B), the film used is an amorphous or very low crystalline aromatic 
polyester film and therefore has excellent moldability, while the film is 
poor in flavor retaining property and is liable to embrittle when printing 
is effected on the film, cans are post-treated for retort treatment, or 
cans are stored for a long period of time, and the embrittled film is 
liable to break due to an external impact. 
As for (C), the laminate is to produce an effect in a region between (A) 
and (B), while the film has not yet attained a low orientation degree 
which can be applied to can-making processing. Further, even if the 
laminate is moldable in a region where the degree of deformation is small, 
the film is liable to embrittle when printing is thereafter effected or 
when the can is subjected to retort treatment for sterilizing canned 
contents, and the embrittled film is liable to break due to an external 
impact, as discussed for (B). 
Further, Japanese Laid-open Patent Publication No. Hei 5-339,348 proposes a 
polyester film to be laminated with a metal plate and processed 
thereafter, which is formed from a copolyester having a specific melting 
point, a specific glass transition temperature and a specific 
concentration of a terminal carboxyl group. Japanese Laid-open Patent 
Publication No.Hei 6-39,979/1994 proposes a polyester film to be laminated 
on a metal plate and processed thereafter, which is a laminate of a 
copolyester having a specific melting point and a specific glass 
transition temperature. However, when cans using these films are used, for 
example, as beverage containers, and in particular, when cans are filled 
with contents such as mineral water, the influence of the polyester film 
on taste and flavor differs according to kind of the content. Therefore, a 
further improvement of polyester film to be laminated on a metal plate for 
mold-processing is desired. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide a novel laminated 
polyester film to be laminated on a metal plate. 
Another object of the present invention is to provide a laminated polyester 
film to be laminated on a metal plate, which has an improved property of 
retaining the flavor of its contents and improved impact resistance after 
can making while it retains excellent moldability, heat resistance and 
resistance to retort treatment of a polyester film. 
Other objects and advantages of the present invention will be apparent from 
the following description. 
According to the present invention, the above objects and advantages of the 
present invention can be attained by a laminated polyester film to be 
laminated on a metal plate, which comprises: 
(A) a first layer formed of a first aromatic copolyester (a1) which 
contains terephthalic acid as a main dicarboxylic acid component and 
ethylene glycol as a main glycol component, and which has (a2) a melting 
point in the range of 210.degree. to 245.degree. C., (a3) a glass 
transition temperature of at least 60.degree. C., (a4) a terminal carboxyl 
group concentration in the range of 10 to 35 equivalent/10.sup.6 g, and 
(a5) an intrinsic viscosity in the range of 0.52 to 0.8 dl/g, and 
(B) a second layer formed of a molten blend having a terminal carboxyl 
group concentration in the range of 40 to 80 equivalent/10.sup.6 g, which 
comprises 60 to 99% by weight of a second aromatic polyester (b1) which 
contains ethylene terephthalate as a main recurring unit, and which has 
(b2) a melting point in the range of 210.degree. to 255.degree. C., and 
(b3) an intrinsic viscosity in the range of 0.52 to 0.8 dl/g and 1 to 40% 
by weight of a third aromatic polyester (c1) which contains tetramethylene 
terephthalate as a main recurring unit, and which has (c2) a melting point 
in the range of 180.degree. to 223.degree. C., and (c3) an intrinsic 
viscosity in the range of 0.52 to 1.65 dl/g, and 
(C) which exhibits excellent moldability when a metal plate laminated 
therewith is deep-drawn. 
As described above, the laminated polyester film of the present invention 
comprises the first layer (A) and the second layer (B). 
The first layer is formed of the first aromatic copolyester which contains 
terephthalic acid as a main dicarboxylic acid component and ethylene 
glycol as a main glycol component. 
Illustrative examples of a dicarboxylic acid other than the terephthalic 
acid constituting the first aromatic copolyester include aromatic 
dicarboxylic acids such as isophthalic acid, phthalic acid and naphthalene 
dicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, 
azelaic acid, sebacic acid and decanedicarboxylic acid; alicyclic 
dicarboxylic acids such as cyclohexanedicarboxylic acid; and the like. 
Similarly, illustrative examples of a diol other than ethylene glycol 
constituting the first aromatic copolyester include aliphatic diols 
represented by HO--(CH.sub.2).sub.n --OH (n=3 to 10) such as butane diol 
and hexane diol; branched glycols represented by HO--CH.sub.2 --C(R).sub.2 
--CH.sub.2 --OH (R=alkyl group having 1 to 4 carbon atoms) such as 
neopentyl glycol; diethylene glycols (DEG); triethylene glycols (TEG); 
alicyclic diols such as cyclohexane dimethanol; polyoxyalkylene glycols 
such as polyethylene glycol and polypropylene glycol; and the like. They 
may be used alone or in combination of two or more. 
The first aromatic copolyester preferably contains an ethylene isophthalate 
unit in addition to an ethylene terephthalate unit. 
The first aromatic copolyester has a melting point in the range of 
210.degree. to 245.degree. C. When the melting point is below 210.degree. 
C., heat resistance deteriorates. When the melting point is above 
245.degree. C., moldability is greatly impaired. The melting point of the 
first aromatic copolyester is preferably in the range of 215.degree. to 
235.degree. C. 
The first aromatic copolyester has a glass transition temperature of at 
least 60.degree. C. When the glass transition temperature is below 
60.degree. C., satisfactory flavor retaining property cannot be ensured. 
Since such a copolyester is a copolyester having a high glass transition 
temperature, an isophthalic acid-copolymerized polyethylene terephthalate 
is particularly preferred. 
The glass transition temperature of the first aromatic copolyester is 
preferably at least 70.degree. C. 
Since the melting point and glass transition temperature of the first 
aromatic copolyester depend on kinds and amounts of copolymer components, 
kinds and amounts of the copolymer components are experientially selected 
to satisfy the above values. 
The measurements of the melting point and glass transition temperature of 
the copolyester are carried out by a method for obtaining a melting peak 
and a glass transition temperature peak at a temperature elevation rate of 
20.degree. C./minute, using a Du Pont Instruments 910 DSC. The amount of a 
sample is approximately 20 mg. 
Further, the first aromatic copolyester has a terminal carboxyl group 
concentration of 10 to 35 equivalent/10.sup.6 g. When the concentration is 
above 35 equivalent/10.sup.6 g, the property of retaining the flavor of 
can's liquid contents such as a beverage or food, which come in contact 
with the copolyester inner layer of the can deteriorates significantly. 
This is considered to be caused by strong interaction between the terminal 
carboxyl group and the components of the liquid contents which destroys 
the balance of the components. 
The first aromatic copolyester has a terminal carboxyl group concentration 
preferably in the range of 10 to 30 equivalent/10.sup.6 g, more preferably 
in the range of 10 to 25 equivalent/10.sup.6 g. 
The first aromatic copolyester having a terminal carboxyl group 
concentration in the above range can be produced by an ester exchange 
method (DMT method) or esterification method (Direct Polymerization 
method) for producing copolyesters in which the following special reaction 
conditions are employed. For instance, the first aromatic copolyester can 
be produced by using a metal compound used for the ester exchange method 
or esterification method, such as a compound of Mg, Mn, Zn, Ca, Li, Ti, Ge 
or the like in a smaller amount than usual and setting a polycondensation 
reaction temperature milder (lower) than usual to perform a 
polycondensation reaction. 
The metal compound (catalyst) used in the above polycondensation reaction 
is not particularly limited, but is selected preferably from an antimony 
compound, titanium compound, germanium compound and the like. Among these, 
particularly preferred is a germanium compound from a view point of flavor 
retaining property. 
Preferred examples of the antimony compound include antimony trioxide, 
antimony acetate and the like. Preferred examples of the titanium compound 
include titanium tetrabuthoxide, titanium acetate and the like. Preferred 
examples of the germanium compound include (a) amorphous germanium oxide, 
(b) fine crystalline germanium oxide, (c) a solution prepared by 
dissolving germanium oxide in glycol in the presence of an alkaline metal, 
alkaline earth metal or a compound of any one of these compounds, (d) a 
solution of germanium oxide in water, (e) germanium tetrachloride, and (f) 
a solution of tetraethoxy germanium in glycol. 
The first aromatic copolyester has an intrinsic viscosity in the range of 
0.52 to 0.8 dl/g, preferably 0.54 to 0.75 dl/g, more preferably 0.57 to 
0.71 dl/g. 
The first aromatic copolyester may contain inert fine particles as 
required. The inert fine particles have an average particle diameter 
preferably in the range of 0.05 to 0.6 .mu.m, and may be preferably 
contained in an amount of 0.01 to 1% by weight based on the first aromatic 
copolyester. The inert fine particle is preferably spherical with 
preferred examples thereof including spherical silica, spherical titanium 
oxide, spherical zirconium, and spherical silicone resin particle. 
The second layer (B) constituting the laminated polyester film of the 
present invention is formed of a molten blend of the second aromatic 
polyester and the third aromatic polyester. 
The second aromatic polyester contains ethylene terephthalate as a main 
recurring unit. 
Examples of the aromatic dicarboxylic acids other than terephthalic acid 
and the diol other than ethylene glycol constituting the second aromatic 
polyester are the same as those provided for the first aromatic 
copolyester. 
The second aromatic polyester may contain at least one dicarboxylic acid 
component selected from the group consisting of aromatic dicarboxylic 
acids other than terephthalic acid, aliphatic dicarboxylic acids and 
alicyclic dicarboxylic acids in an amount of 2 to 19 mol % based on the 
total of all dicarboxylic acid components and may also contain at least 
one glycol selected from the group consisting of aliphatic glycols other 
than ethylene glycol and alicyclic glycols in an amount of 2 to 19 mol % 
based on the total of all glycol components. 
The second aromatic polyester has a melting point in the range of 
210.degree. to 255.degree. C. When the melting point is below 210.degree. 
C., heat resistance deteriorates, and when the melting point is above 
225.degree. C., moldability is greatly impaired due to too high 
crystallinity of the polymer. 
The melting point of the second aromatic polyester is preferably in the 
range of 220.degree. to 245.degree. C. 
The second aromatic polyester has an intrinsic viscosity of 0.52 to 0.8 
dl/g, preferably 0.54 to 0.75 dl/g, more preferably 0.57 to 0.71 dl/g. 
The second aromatic polyester has a glass transition temperature preferably 
in the range of at least 60.degree. C., more preferably in the range of at 
least 70.degree. C. 
The third aromatic polyester which is the other component constituting the 
second layer (B), contains tetramethylene terephthalate as a main 
recurring unit. 
Examples of the aromatic dicarboxylic acid other than terephthalic acid 
constituting the third aromatic polyester are the same as those provided 
for the first aromatic copolyester. Examples of the diol other than 
tetramethylene glycol include aliphatic diols represented by 
HO--(CH.sub.2).sub.m --OH (m=2, 3, 5 to 10) such as ethylene glycol and 
hexane diol, branched glycols represented by HO--CH.sub.2 --C(R).sub.2 
--CH.sub.2 --OH (R=alkyl group having 1 to 4 carbon atoms) such as 
neopentyl glycol, diethylene glycols (DEG), triethylene glycols (TEG), 
alicyclic diols such as cyclohexane dimethanol, polyoxyalkylene glycols 
such as polyethylene glycol and polypropylene glycol, and the like. They 
may be used alone or in combination of two or more. 
A preferred example of the third aromatic polyester is polytetramethylene 
terephthalate homopolymer. 
The third aromatic polyester has a melting point in the range of 
180.degree. to 223.degree. C. When the melting point is below 180.degree. 
C., heat resistance deteriorates significantly. 
The melting point of the third aromatic polyester is preferably in the 
range of 195.degree. to 223.degree. C. 
The third aromatic polyester has an intrinsic viscosity in the range of 
0.52 to 1.65 dl/g, preferably 0.54 to 1.55 dl/g, more preferably 0.57 to 
1.50 dl/g. 
The molten blend of the second layer (B) contains 60 to 99% by weight of 
the second aromatic polyester and 1 to 40% by weight of the third aromatic 
polyester based on the total of the second aromatic polyester and the 
third aromatic polyester. 
When the third aromatic polyester is contained in an amount of less than 1% 
by weight and the second aromatic polyester in an amount of more than 99% 
by weight, impact resistance at low temperatures cannot be improved. When 
the third aromatic polyester is contained in an amount of more than 40% by 
weight and the second aromatic polyester in an amount of less than 60% by 
weight, the heat resistance of the film of the second layer (B) decreases 
and its impact resistance is insufficient. 
The difference between the melting point of the second aromatic polyester 
and that of the third aromatic polyester is preferably less than 4.degree. 
C. This greatly facilitates preparation of the molten blend and provides a 
desired molten blend with ease. 
The molten blend preferably contains 90 to 60% by weight of the second 
aromatic polyester and 10 to 40% by weight of the third aromatic 
polyester. 
The molten blend has a terminal carboxyl group concentration in the range 
of 40 to 80 equivalent/10.sup.6 g. When the terminal carboxyl group 
concentration is below 40 equivalent/10.sup.6 g, its adhesion to a metal 
plate is poor. When the terminal carboxyl group concentration is above 80 
equivalent/10.sup.6 g, thermal decomposition of the polyester proceeds 
drastically, and, undesirably, stability of the film-forming process is 
deteriorated and deteriorated foreign matters are included into the film. 
The terminal carboxyl group concentration of the molten blend is preferably 
in the range of 43 to 75 equivalent/10.sup.6 g, more preferably 45 to 70 
equivalent/10.sup.6 g. 
The terminal carboxyl group concentration of the molten blend having a 
terminal carboxyl group concentration in the above range can be attained 
by the terminal carboxyl group concentration of either one of the second 
aromatic polyester and the third aromatic polyester. 
The polyester having a relatively high concentration of a terminal carboxyl 
group can be produced by an ester exchange method (DMT method) or 
esterification method (Direct Polymerization method), which are conducted 
under the following special reaction conditions. For instance, the 
polyester can be produced by (1) a method for obtaining a polyester using 
a metal compound used for ester exchange or esterification, such as a 
compound of Mg, Mn, Zn, Ca, Li, Ti, Ge or the like, in a larger amount 
than usual, (2) a method for promoting a polycondensation reaction while a 
temperature higher than usual and/or a duration longer than usual are 
retained in the latter stage of the polycondensation reaction, and (3) a 
method for copolymerizing a monomer which is liable to undergo thermal 
decomposition, such as an aliphatic carboxylic acid having 4 to 12 carbon 
atoms or an aliphatic glycol. Also, a method for blending a predetermined 
amount of a so-called re-processed polymer having a relatively high 
concentration of terminal carboxyl group in which thermal decomposition 
proceeds relatively well may be adopted. 
Further, a method for reacting an acid anhydride such as phthalic anhydride 
or succinic anhydride with the second aromatic polyester and the third 
aromatic polyester may be adopted. In this case, when an acid anhydride is 
added, for example, to such an extent that the terminal carboxyl group 
concentration exceeds 80 equivalent/10.sup.6 g, thermal stability of the 
second aromatic polyester and the third aromatic polyester is, 
undesirably, greatly lowered due to the unreacted acid anhydride. 
The metal compound (catalyst) used in the above polycondensation reaction 
is not particularly limited, but preferably selected from an antimony 
compound, a titanium compound, a germanium compound and the like. Among 
these, the germanium compound is particularly preferred for the second 
aromatic polyester from a view point of flavor retaining property. 
Preferred antimony compounds include antimony trioxide, antimony acetate 
and the like. Preferred titanium compounds include titanium 
tetrabuthoxide, titanium acetate and the like. Preferred germanium 
compounds include (a) amorphous germanium oxide, (b) fine crystalline 
germanium oxide, (c) a solution prepared by dissolving germanium oxide in 
glycol in the presence of an alkaline metal, alkaline earth metal or a 
compound of any one of these compounds, (d) a solution of germanium oxide 
in water, (e) germanium tetraoxide, (f) a solution of tetraethoxy 
germanium in glycol, and the like. 
At least one of the second aromatic polyester and the third aromatic 
polyester which constitute the second layer (B) preferably contains inert 
fine particles in an amount of 0.03 to 0.5% by weight based on the molten 
blend to improve handling property (winding property) in the film-forming 
process. 
The inert fine particles, that is, a lubricant, may be organic or 
inorganic, while it is preferably inorganic. Examples of the inorganic 
lubricant include silica, alumina, titanium dioxide, calcium carbonate, 
barium sulfate and the like, and examples of the organic lubricant include 
a cross-linked polystyrene particle, silicone resin particle and the like. 
Any of these lubricants have an average particle diameter preferably in 
the range of 0.8 to 2.5 .mu.m. When the average particle diameter is above 
2.5 .mu.m, a pinhole starting from a coarse particle (having a particle 
diameter of 10 .mu.m or more, for example) is generated in that portion of 
the film which is deformed or the film is likely to break in some cases, 
when a metal plate laminated with the film is deep-drawn to make a can. 
In view of the prevention of occurrence of pinholes, the lubricant is 
preferably a monodisperse lubricant having an average particle diameter of 
0.8 to 2.5 .mu.m and a particle diameter ratio (long diameter/short 
diameter) of 1.0 to 1.2. Specific examples of such a lubricant include 
spherical silica, spherical titanium dioxide, spherical zirconium, 
spherical silicone resin particles and the like. 
The above-described first aromatic polyester, second aromatic polyester and 
third aromatic polyester used in the present invention may contain other 
additives such as an oxidant, thermal stabilizer, viscosity modifier, 
plasticizer, adhesion improving agent, nucleating agent, ultraviolet 
absorber, antistatic agent and the like. 
The laminated polyester film of the present invention has a structure of a 
laminate composed of the first layer (A) and the second layer (B). This 
laminate-structured film can be produced by a method in which the first 
aromatic polyester and a molten blend of the second aromatic polyester and 
the third aromatic polyester forming respective layers are separately 
molten, co-extruded and laminate-fused before solidified, and then the 
laminate is biaxially oriented and heat-set, or a method in which each 
polyester for a respective layer is separately molten and extruded to 
prepare films, and then the films are laminate-fused before or after 
stretched. The heat-set temperature can be selected from the range of 
150.degree. to 220.degree. C., preferably 160.degree. to 200.degree. C. 
when the stretched film is heat set. 
In the laminated polyester film of the present invention, the first layer 
(A) has a refractive index in the thickness direction preferably in the 
range of 1.505 to 1.550, more preferably in the range of more than 1.510 
and 1.540 or less. When the refractive index is too low, the film is 
insufficient in moldability, and when the refractive index is too high, 
the film may have a nearly amorphous structure and may be poor in heat 
resistance. 
The laminated polyester film of the present invention has a thickness 
preferably in the range of 6 to 75 .mu.m, more preferably 10 to 75 .mu.m, 
particularly preferably 15 to 50 .mu.m. When the thickness is below 6 
.mu.m, the polyester film is liable to break in processing, and when the 
thickness is above 75 .mu.m, the film has excess in quality which is 
economically disadvantageous. 
The thickness of the second layer (B) (adhesive layer) differs according to 
the surface roughness of the metal plate. In the case of an ordinary 
smooth surface, it is sufficient that the film has a thickness of at least 
2 .mu.m to achieve stable adhesion. Particularly, when importance is 
attached to resistance to retort treatment and rustproof property, the 
film preferably has a thickness of at least 12 .mu.m. Therefore, the ratio 
(TA/TB) of the thickness of the first layer (A), TA, to the thickness of 
the second layer (B), TB, is preferably in the range of 0.02 to 0.67, more 
preferably 0.04 to 0.43, particularly preferably 0.04 to 0.25. 
Specifically, in the case of a 25 .mu.m thick polyester film, the 
thickness of the second layer (B) (adhesive layer) is set to 15 to 24.5 
.mu.m, preferably 17.5 to 24 .mu.m, more preferably 20 to 24 .mu.m. 
As the metal plate to be laminated with the laminated polyester film of the 
present invention, particularly the metal plate for making cans, 
tin-plated steel, tin-free steel and aluminum plates are suitable. 
Lamination of the polyester film onto the metal plate can be conducted by 
the following method. 
The metal plate is heated to a temperature equal to, or higher than a 
melting point of the film, and laminated on the second layer (B) (adhesive 
layer) of the laminated polyester film. The resultant laminate is cooled 
so that the surface portion (thin layer portion) of the film which is in 
contact with the metal plate is brought into an amorphous state and 
intimately bonded to the metal plate. 
Further, in the laminated polyester film of the present invention, the 
first layer (A) is generally in direct contact with the second layer (B), 
but an additional layer may be provided between the first layer (A) and 
the second layer (B) as required. For instance, another thin adhesive 
layer, undercoating layer or electrical discharge-treated layer may be 
provided between the first layer (A) and the second layer (B). An 
additional layer may be laminated on the other side of the second layer 
(B) opposite to the side in contact with the first layer (A) as required. 
When the second layer (B) of the laminated polyester film is formed of the 
second aromatic polyester only, adhesion to the metal plate deteriorates 
greatly and, when the second layer (B) is formed of the third aromatic 
polyester only, the resulting film becomes soft and viscous when it is 
laminated with a metal plate, with the result of poor workability. In 
either case, good laminating property cannot be obtained by using only one 
of the aromatic polyesters. 
Further, when the polyester film is formed of the first layer (A) only, the 
film is insufficient in adhesion and impact resistance, while when the 
polyester film is formed of the second layer (B) only, the film is poor in 
flavor retaining property. Therefore, both cases are inappropriate.

EXAMPLES 
The present invention will be further explained hereinafter with reference 
to the following Examples. 
Examples 1 to 7 and Comparative Examples 1 to 6 
A polyethylene terephthalate (having an intrinsic viscosity of 0.64 dl/g 
and containing 0.3% by weight of titanium dioxide having an average 
particle diameter of 0.3 .mu.m) prepared by copolymerizing a component 
shown in Table 1 in the presence of a polycondensation catalyst shown in 
Table 1 according to an EI method and a molten blend prepared by blending 
the second aromatic polyester and the third aromatic polyester shown in 
Table 1 were individually dried, molten and co-extruded through adjacent 
dies according to conventional methods to laminate and fuse the 
extrudates, and the laminate was solidified by quenching to form an 
unstretched laminated film in which the polyethylene terephthalate formed 
a first layer (A) and the molten blend formed a second layer (B) (adhesive 
layer). 
Then, the above unstretched film was stretched in the longitudinal 
direction at a stretch ratio of 3.0 at 100.degree. C. and then stretched 
in the transverse direction at a stretch ratio of 3.2 by elevating 
temperature from 100.degree. C. to 150.degree. C., and the stretched film 
was heat-set at 200.degree. C. to give a biaxially oriented film. 
The thus obtained film had a thickness of 25 .mu.m. The first layer (A) had 
a thickness of 5 .mu.m and the second layer (B) had a thickness of 20 
.mu.m. 
Comparative Example 1 is a film in which the second layer (B) (adhesive 
layer) is formed of the second aromatic polyester alone, and Comparative 
Example 2 is a film in which the second layer (B) (adhesive layer) is 
formed of the third aromatic polyester alone. 
Comparative Example 7 
A single-layered film which was formed of the first layer (A) of Example 1 
alone was prepared. The thus obtained film had a thickness of 25 .mu.m. 
TABLE 1 
__________________________________________________________________________ 
First layer (A) 
Copolymer component Terminal carboxyl group 
concentration 
Composition 
mol % 
Tm (.degree.C.) 
Tg (.degree.C.) 
Polycondensation catalyst* 
(Equivalent/10.sup.6 
__________________________________________________________________________ 
g) 
Comp. Ex.1 
IA 12 229 73 Ge 25 
Comp. Ex.2 
IA 12 229 73 " 25 
Ex.1 IA 12 229 73 " 25 
Ex.2 IA 12 229 73 " 25 
Ex.3 IA 12 229 73 " 25 
Ex.4 IA 12 229 73 " 25 
Ex.5 AA 9 240 60 " 25 
Ex.6 IA 12 229 73 Sb 25 
Ex.7 IA 12 229 73 " 25 
Comp. Ex.3 
IA 12 229 73 " 40 
Comp. Ex.4 
IA 12 229 73 " 25 
Comp. Ex.5 
IA 3 251 75 " 25 
Comp. Ex.6 
IA 12 229 73 " 25 
Comp. Ex.7 
IA 12 229 73 " 25 
__________________________________________________________________________ 
Second layer (B) 
Terminal 
Second Aromatic polyester Third Aromatic polyester 
carboxyl 
Poly- group 
Copolymer conden- Copolymer concentra- 
Basic component sation Basic 
component tion 
compo- 
Composi- 
mol 
Tm Tg catal- 
Weight 
compo- 
Composi- 
mol 
Tm Weight 
(equivalent/ 
nent 
tion % (.degree.C.) 
(.degree.C.) 
yst* 
(%) nent 
tion % (.degree.C.) 
(%) 10.sup.6 
__________________________________________________________________________ 
g) 
Comp. Ex.1 
PET IA 20 210 
71 Ge 100 -- -- -- -- -- 40 
Comp. Ex.2 
-- -- -- -- -- " -- PBT -- -- 223 
100 50 
Ex.1 PET IA 12 229 
73 " 70 PBT -- -- 223 
30 43 
Ex.2 PET IA 12 229 
73 " 70 PBT IA 5 214 
30 43 
Ex.3 PET SA 9 240 
60 " 70 PBT -- -- 223 
30 43 
Ex.4 PET IA 20 210 
71 " 70 PBT -- -- 223 
30 50 
Ex.5 PET IA 12 229 
73 Sb 70 PBT -- -- 223 
30 43 
Ex.6 PET IA 12 229 
73 Ge 70 PET SA 25 197 
30 43 
Ex.7 PET IA 3 251 
75 " 70 PBT IA 15 198 
30 43 
Comp. Ex.3 
PET IA 12 229 
73 " 70 PBT -- -- 223 
30 43 
Comp. Ex.4 
PET IA 12 229 
73 " 70 PBT -- -- 223 
30 25 
Comp. Ex.5 
PET IA 12 229 
73 " 70 PBT -- -- 223 
30 43 
Comp. Ex.6 
PBT IA 15 198 
32 " 70 PBT -- -- 223 
30 43 
Comp. Ex.7 
-- -- -- -- -- " -- -- -- -- -- -- -- 
__________________________________________________________________________ 
*Ge: Amorphous germanium oxide, Sb: Sb.sub.2 O.sub.3 
In Table 1, IA stands for isophthalic acid, AA adipic acid, SA sebacic 
acid, PET polyethylene terephthalate, and PBT polybutylene terephthalate. 
Further, Tg represents a glass transition temperature and Tm a melting 
point. Terminal carboxyl group concentrations were obtained in accordance 
with A. CONIX method (Makromol. Chem. 26, 226(1958)). 
Each of the fourteen films obtained in Examples 1 to 7 and Comparative 
Examples 1 to 7 was laminated with a 0.25 mm thick tin-free steel plate 
heated at 230.degree. to 260.degree. C. and at the same time fused 
together by pressing them with a roll heated at 90.degree. to 150.degree. 
C. from the opposite side of the metal plate. Then, the resulting 
laminates were cooled with water to obtain one surface- or both surfaces- 
coated steel plates. 
These coated steel plates were evaluated for their laminating property on 
the basis of the following standards. 
(1) Laminating property 
(A) standards for air bubbles and wrinkles 
.largecircle.: No bubbles and wrinkles could be seen. 
.DELTA.: A few bubbles and wrinkles could be seen in a length of 10m. 
X: Many bubbles and wrinkles could be seen. 
(B) standards for heat shrinkage 
.largecircle.: Heat shrinkage percentage was less than 2%. 
.DELTA.: Heat shrinkage percentage was 2% or more and less than 5%. 
X: Heat shrinkage percentage was 5% or more. 
The tin-free steel plates laminated with the above-described polyester 
films were cut into disk-shaped pieces having a diameter of 150 mm, and 
the pieces were deep-drawn at four stages with a drawing die and a punch 
to produce side-seamless containers having a diameter of 55 mm (to be 
abbreviated as "cans" hereinafter). 
The above cans were observed and tested, and evaluated on the basis of the 
following standards. 
(2) Deep-draw processability-1 
.largecircle.: A laminate could be processed without causing any defect on 
a film, and the film showed no opacification or breakage. 
.DELTA.: Whitening of the film was observed at upper portion of the metal 
can. 
X: Breakage was observed in some portions of the film. 
(3) Deep-draw processability-2 
.largecircle.: A laminate was deep-drawn without causing any defect on a 
film, and when the inner film-coated surface was subjected to a rustproof 
test (hereinafter referred to as ERV test) (1% NaCl aqueous solution was 
charged in the can, an electrode was inserted therein, the can body was 
used as an anode, and when a voltage of 6 V was applied, an electric 
current value was measured), the current value was 0.1 mA or less. 
X: A film visually showed no defects, while the measured current value was 
more than 0.1 mA in ERV test. When the part through which the electric 
current was passed was magnified for observation, pinhole-like cracks 
starting at coarse lubricant particles were observed in the sample film. 
(4) Adhesion 
Excellently deep-drawn cans were filled with water to full and subjected to 
a retort treatment for 90 minutes at 120.degree. C. in a steam sterilizer. 
Thereafter, the cans were stored at 50.degree. C. for 3 months. A cross 
was cut on the thus obtained cans to observe the adhesion of the film. 
.largecircle.: The film was firmly adhered to the steel plate and did not 
peel off even when a cross-cut was given. 
.DELTA.: Adhesion slightly deteriorated by a cross-cutting. 
X: The film peeled off by a cross-cutting. 
(5) Rust-proof property 
Excellently deep-drawn cans were filled with an aqueous 5% NaCl solution to 
full and stored at 50.degree. C. for 7 days. Rust generation was visually 
observed for ten of the cans as one group. The results were evaluated as 
follows. 
.largecircle.: Generation of rust was not observed in all of the ten cans. 
.DELTA.: Generation of rust was observed in 1 to 5 of the cans. 
X: Generation of rust was observed in 6 or more of the cans. 
(6) Impact resistance 
Excellently deep-drawn cans were filled with water to full, and cooled to 
10.degree. C. Ten water-filled cans of the same laminate as one group were 
dropped from a height of 30 cm on a polyvinyl chloride tiled floor. Then, 
the cans were subjected to an ERV test. The results were evaluated as 
follows. 
.largecircle.: All of the ten cans showed a current value of 0.1 mA or 
less. 
.DELTA.: 1 to 5 of the cans showed a current value of more than 0.1 mA. 
X: 6 or more of the cans showed a current of more than 0.1 mA, or cracks in 
the film were observed immediately after dropping. 
(7) Resistance to embrittlement under heat 
Excellently deep-drawn cans were heated at 200.degree. C. for 5 minutes and 
thereafter evaluated for impact resistance in the same manner as described 
in (6) above. 
.largecircle.: All of the ten cans showed a current value of 0.1 mA or 
less. 
.DELTA.: 1 to 5 of the cans showed a current value of more than 0.1 mA. 
X: 6 or more of the cans showed a current value of more than 0.1 mA, or 
cracks in the film are observed after heating at 200.degree. C. for 5 
minutes. 
(8) Resistance to retort treatment 
Excellently deep-drawn cans were filled with water to full, subjected to a 
retort treatment at 120.degree. C. for an hour in a steam sterilizer, and 
thereafter stored at 50.degree. C. for 30 days. Then, ten water-filled 
cans of the same laminate as one group were dropped from a height of 50 cm 
on a polyvinyl chloride tiled floor. Then, the cans were subjected to an 
ERV test. 
.largecircle.: All of the ten cans showed a current value of 0.1 mA or 
less. 
.DELTA.: 1 to 5 of the cans showed a current value of more than 0.1 mA. 
X: 6 or more of the cans showed a current of more than 0.1 mA, or cracks in 
the film are observed immediately after dropping. 
(9) Flavor retaining property 
Excellently deep-drawn cans were each filled with 10 bottles of mineral 
water and tightly closed. The cans were stored at 37.degree. C. for 4 
months and opened, and the beverage was sensory-tested for a change in 
taste and flavor. 
.circleincircle.: No change in taste and flavor 
.largecircle.: Slight changes in taste and flavor were found in 1 or 2 of 
the cans. 
.DELTA.: Small changes in taste and flavor were found in 3 to 4 of the 
cans. 
X: Changes in taste and flavor were found in 5 or more of the cans. 
The results of the above nine evaluations are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Deep draw 
Deep draw 
Rust- Resistance 
Flavor 
Laminating process- 
process- proof 
Impact 
to embrit- 
Resistance 
retain- 
property ability - 
ability - 
Adhe- 
pro- 
resis- 
tlement 
to retort 
ing 
(A) 
(B) 
1 2 sion 
perty 
tance 
under heat 
treatment 
property 
__________________________________________________________________________ 
Comp. Ex.1 
.largecircle. 
.largecircle. 
.largecircle. 
X .largecircle. 
.largecircle. 
X X .DELTA. 
.largecircle. 
Comp. Ex.2 
X X -- -- -- -- -- -- -- -- 
Ex.1 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.circleincircle. 
Ex.2 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.circleincircle. 
Ex.3 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.circleincircle. 
Ex.4 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.circleincircle. 
Ex.5 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
Ex.6 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
Ex.7 .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.circleincircle. 
__________________________________________________________________________ 
Deep draw 
Deep draw 
Rust- Flavor 
Laminating process- 
process- proof 
Impact 
Resistance 
Resistance 
retain- 
property ability - 
ability - 
Adhe- 
pro- 
resis- 
to embrit- 
to retort 
ing 
(A) 
(B) 
1 2 sion 
perty 
tance 
tlement 
treatment 
property 
__________________________________________________________________________ 
Comp. Ex.3 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
X 
Comp. Ex.4 
.largecircle. 
.DELTA. 
.largecircle. 
X .DELTA. 
.DELTA. 
.DELTA. 
.largecircle. 
.largecircle. 
.largecircle. 
Comp. Ex.5 
.largecircle. 
.largecircle. 
.DELTA. 
X .largecircle. 
.largecircle. 
.DELTA. 
.largecircle. 
.largecircle. 
.largecircle. 
Comp. Ex.6 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.DELTA. 
.DELTA. 
X .DELTA. 
.largecircle. 
Comp. Ex.7 
.largecircle. 
.largecircle. 
.largecircle. 
X .DELTA. 
.DELTA. 
X .DELTA. 
.DELTA. 
.largecircle. 
__________________________________________________________________________ 
Table 2 clearly shows that the cans using the polyester film of the present 
invention are excellent in laminating property, deep-draw processability, 
resistance to embrittlement under heat, retort resistance, rustproof 
property and impact resistance, and in particular, in flavor retaining 
property and adhesion.