Note: Descriptions are shown in the official language in which they were submitted.
CA 02506289 1997-03-06
76199-48D
BIAXIALLY ORIENTED POLYESTER FILM
FOR LAMINATING METALLIC SHEETS
This is a divisional application of Canadian
Patent Application No. 2,199,334 filed March 6, 1997.
This specification describes two major aspects as
mentioned more in detail later. The subject matter claimed
in the parent application has been restricted to the first
major aspect, whereas the subject matter claimed in this
divisional application is restricted to the second major
aspect. However, it should be borne in mind that the
expression "the present invention" or the like encompasses
both of these aspects.
The present invention relates to a biaxially
oriented polyester film for laminating onto a metallic
sheet. In more detail, it relates to a biaxially oriented
polyester film for laminating onto a metallic sheet, which
provides excellent taste retention of foodstuffs in contact
with the film and is especially suitable for internal
coating of metallic cans.
It has been widely practiced to coat metallic cans
on their inner and outer surfaces of a solution or
dispersion with various thermosetting resins such as epoxy
based resins and phenol based resins dissolved or dispersed
in a solvent, for the purpose of preventing corrosion.
However, these thermosetting resin coating processes present
problems in that it takes a long time for drying and that
the large amount of organic solvent used causes
environmental pollution.
Methods for solving these problems include
laminating a film on the metallic sheet, such as a steel
sheet, aluminum sheet, or any metallic sheet which has been
1
CA 02506289 1997-03-06
76199-48D
surface-treated by plating. When the film-laminated
metallic sheet is ironed or drawn to produce metallic cans,
the film is required to have the following properties:
(1) To be excellent in processability during
lamination onto metallic sheets, before forming.
(2) To be excellent in adhesion to metallic
sheets.
(3) To be excellent in processability, without
causing pinholes or other defects, after forming.
(4) Not to cause polyester film delamination,
cracking or pinholes when any impact acts on the metallic
cans.
(5) Not to adsorb the fragrant components of the
beverage, etc. contained in the cans, and not to cause any
material to be dissolved out of the film for impairing the
flavor of the beverage, etc. contained in the cans
(hereinafter this property is referred to as "taste
property").
To meet these requirements, many proposals have
been made. For example, JP-B-60-052179 discloses a resin
composition consisting of a polyethylene terephthalate based
resin, polybutylene terephthalate resin and ionomer, and
JP-A-02-057339 discloses a copolymerized polyester film
having a specific crystallinity. However, the resin
composition disclosed in JP-B-60-52179 is inferior in heat
resistance, causes adsorption of flavor of beverages, the
ingredient of the resin composition is apt to dissolve into
the beverages, and furthermore, is inferior in long term
storage property. On the other hand, the film disclosed in
JP-A-02-057339 is inferior in taste property after retorting
2
CA 02506289 1997-03-06
76199-48D
and, accordingly, its storage taste property for a long term
or at a high temperature is inferior, and it has been
especially difficult to satisfy excellent taste property and
excellent processability simultaneously.
The present invention addresses the above
mentioned problems and seeks to solve them by providing a
biaxially oriented polyester film, which is excellent in
taste property and has excellent processability suitable for
metallic cans.
SUMMARY OF THE INVENTION
According to a first aspect, the present invention
provides a biaxially oriented polyester film for laminating
onto a metallic sheet, which film comprises a polyester
having a melting point of 246 to 280°C, wherein the film has
an inside average orientation ratio, RinAVE. and an outside
average orientation ratio, Rp"tAVE. as measured by Raman
spectrometry, of 6 or less and 8 or more, respectively.
That means, the film, in the laminate is to have a 1 to 3 um
thick portion (a) which is to lie close to the metallic
sheet and a 1 to 3 um thick portion (b) which is to lie
remote from the metallic sheet, the portions (a) and (b) are
to be disposed at a neck of a can formed from the laminate,
and the portions (a) and (b) have respective average
orientation ratios Ri"AVE and RoutAVE, as measured by Raman
spectrometry, of 6 or less and 8 or more.
According to a second aspect, the present
invention also provides a biaxially oriented polyester film
for laminating onto a metallic sheet, which film comprises a
polyester layer (A) of a polyester having at least 93 moleo
thereof of units derived from at least one of ethylene
terephthalate and ethylene naphthalate and a polyester layer
3
CA 02506289 1997-03-06
76199-48D
(B) of a polyester containing an ionomer and which film has
a face orientation factor of 0.10 to 0.15. This film shows
also an excellent impact resistance.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS OF THE INVENTION
The polyester film according to the first aspect of
the present invention has a melting point of 246 to 280°C,
preferably 250°C to 270°C for improved heat resistance and
long-term storage taste property. To secure good taste
property after heat treatment such as retorting, the amount
of ethylene terephthalate and/or ethylene naphthalate is
preferably 93 moleo or more, more preferably 95 moleo or
more. In particular, metallic cans coated with such films
and filled with beverage can be stored for a long time while
maintaining good taste property. On the other hand, as far
as the taste property is not impaired, another dicarboxylic
acid and/or another glycol can also
3a
CA 02506289 1997-03-06
be copolymerized. The dicarboxylic acids which can be copolymerized include,
for
example, aromatic dicarboxylic acids such as isophthalic acid, naphthalene-
dicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfone-dicarboxylic
acid,
diphenoxyethanedicarboxylic acid, 5-sodiumsulfoisophthalic acid and phthalic
acid,
aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid,
sebacic
acid, dimer acid, malefic acid and fumaric acid, alicyclic dicarboxylic acids
such as
cyclohexynedicarboxylic acid and hydroxycarboxylic acids such as p-
hydroxybenzoic
acid. On the other hand, the glycols which can be copolymerized include, for
example, aliphatic glycols such as 1-4-butanediol, propanediol, pentanediol,
hexanediol and neopentyl glycol, alicyclic glycols such as
cyclohexanedimethanol,
aromatic glycols such as bisphenol A and bisphenol S and diethylene glycol. As
a
matter of course, two or more of these dicarboxylic acids and glycols can also
be used
together.
If a copolymerized polyester is used as the polyester in a film of the present
invention, the component preferably copolymerized with ethylene terephthalate
is 2,6-
naphthalenedicarboxylic acid for improved processability and taste property,
or dimer
acid, 1,4-cyclohexanedimethanol or 1,4-butanediol for improved impact
resistance, or
isophthalic acid for improved processability and adhesion to a metallic sheet
in the
. manufacturing process for producing a can, but the component is not limited
to those
2 0 listed above.
Furthermore, a blend of two or more polyesters can also be used as the
polyester in a film of the present invention. For example, a blend of
polyethylene
terephthalate and polyethylene naphthalate, a blend of polyethylene
terephthalate and
dimer acid-copolymerized polyethylene terephthalate, or a polyester obtained
by
copolymerizing polyethylene terephthalate, cyclohexanedimethanol, terephthalic
acid
4
CA 02506289 1997-03-06
and isophthalic acid, can be used. When a blend is melt-
extruded, ester interchange may occur to promote
copolymerization. However, the extent of the ester
interchange is not limited.
Furthermore, in the polyester film of the present
invention, polyester (C) can be further laminated. Polyester
(C) is preferably a polyester having ethyleneterephthalate
and/or ethylenenaphthalate as the main component. For
controlling the average orientation ratios and improving to
metallic sheet, a polyester (C) having a melting point lower
than the substrate polyester by 2-30°C~ preferably 3-10°C may
be used. Hy laminating polyester (C) with the substrate
polyester, more excellent processability and taste property
can be achieved. Because polyester (C) has good adhesion to
a metallic sheet together with a good processability, it is
preferable to laminate so that the polyester layer (C)
adheres to the metallic sheet. The thickness ratio (the
substrate polyester; polyester (C) is preferably 1=20 to
201, more preferably, 110 to 10=1. However, if the melting
point of polyester (C) is lower than 246°C, the ratio is
preferably 2=1 to 20e1 for improved taste property.
In this invention, when a polyester layer (A) made
of a polyester composition with 93 mol% or more of ethylene
terephthalate and/or ethylene naphthalate as component units
and a polyester layer (H) made of a polyester composition
containing an ionomer are laminated and characterized by
being 0.10 to 0.15 in the face orientation factor, an
excellent taste property as well as excellent impact
5
CA 02506289 1997-03-06
resistance can be achieved.
In the polyester (A) of the present invention, to
secure good taste property after heat treatment such as
retorting, the amount of ethylene terephthalate and/or
ethylene naphthalate is preferably 93 mol% or more, more
preferably 96 mol% or more.because metallic cans coated
internally with films of such polyesters and filled with
beverage can maintains good taste property for a
5a
CA 02506289 1997-03-06
long time. On the other hand, as far as the taste property is not impaired,
another
dicarboxylic acid and/or another glycol can also be copolymerized. The
dicarboxylic
acids which can be copolymerized include, for example, aromatic dicarboxylic
acids
such as isophthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic
acid,
diphenylsulfone-dicarboxylic acid, diphenoxyethanedicarboxylic acid, S-sodium-
sulfoisophthalic acid and phthalic acid, aliphatic dicarboxylic acids such as
oxalic
acid, succinic acid, adipic acid, sebacic acid, dimer acid, malefic acid and
fumaric acid,
alicyclic dicarboxylic acids such as cyclohexynedicarboxylic acid and
hydroxycarboxylic acids such as p-hydroxybenzoic acid. On the other hand, the
p glycols which can be copolymerized include, for example, aliphatic glycols
such as
propanediol, butanediol, pentanediol, hexanediol and neopentyl glycol,
alicyclic
glycols such as cyclohexanedimethanol, aromatic glycols such as bisphenol A
and
bisphenol S and diethylene glycol. Of course, two or more of these
dicarboxylic acids
and glycols can also be used together.
Furthermore, as far~as the effects of the present invention are not impaired,
the
copolymerized polyester can also be copolymerized with a polyfunctional
compound
such as trimellitic acid, trimesic acid or trimethylolpropane.
In the present invention, a blend of two or more of the above polymers can
also
be used.
2 0 The melting point of the polyester constituting the polyester layer (A) in
a film
of the present invention is preferably 246 ° C to 280 ° C, more
preferably 250 ° C to
275 ° C, for improved taste property and heat resistance. If the
melting point is higher
than 280°C, the processability may deteriorate.
The polyester constituting the polyester layer (B) in a film of the present
invention is a polymer of a dicarboxylic acid and a glycol. The dicarboxylic
acids
CA 02506289 1997-03-06
which can be used include, but are not limited to, aromatic dicarboxylic acids
such as
terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic
acid,
diphenylsulfonedicarboxylic acid, diphenyletherdicarboxylic acid,
diphenoxyethane-
dicarboxylic acid, cyclohexanedicarboxylic acid, diphenyldicarboxylic acid,
diphenoxyethanedicarboxylic acid and 5-sodiumsulfoisophthalic acid, aliphatic
dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic
acid, dimer
acid, malefic acid and fumaric acid, alicyclic dicarboxylic acids such as
cyclohexanedicarboxylic acid and hydroxycarboxylic acids such as p-
hydroxybenzoic
acid. On the other hand, the glycols which can be used include, but are not
limited to,
aliphatic glycols such as ethylene glycol, butanediol, propanediol,
pentanediol,
hexanediol and neopentyl glycol, polyols such as polyethylene glycol,
alicyclic
glycols such as cyclohexanedimethanol and aromatic glycols such as bisphenol A
and
bisphenol S. Of course, two or more of these dicarboxylic acids and glycols
can also
be used together.
In the present invention, the method for incorporating into the polyester
constituting the polyester layer (B) an ionomer is not especially limited. For
example,
the ionomer can be added to and mixed with the polyester. Specifically the
polyester
and the ionomer can be, as required, premixed by a blender, mixer, etc., and
rendered
molten and kneaded using an ordinary or vent type single-screw or double-screw
2 o extruder or kneaded in molten state with a crosslinking agent, or when the
polyester
film is produced, the polyester and the ionomer can be mixed and melt-
extruded. As
another method, an ionomer can be added when the polyester is polymerized.
The ionomer content in the polyester (B) is not especially limited, but is
preferably 0.1 to 50 wt%, more preferably 1 to 30 wt%, still more preferably 5
to 30
wt% for improved impact resistance as well as adhesion and heat resistance. If
the
CA 02506289 1997-03-06
ionomer content is not in this range, especially if it is larger than 50 wt%,
heat
resistance may become poor.
The ionomer may be a copolymer of an a-olefin and an ionic salt of an
unsaturated carboxylic acid containing monovalent or divalent metal ions. The
ionomer can be, for example, a copolymer of ethylene and an unsaturated
dicarboxylic
acid such as acrylic acid or methacrylic acid, or a copolymer of ethylene and
an
unsaturated dicarboxylic acid such as malefic acid or itaconic acid, in which
some or
all of the carboxylic groups are neutralized by a monovalent or divalent metal
such as
sodium, potassium, lithium, zinc, magnesium or calcium.
t p In a film of the present invention, a known crosslinking agent can also be
added
to the polyesters constituting the polyester layers (A) and (B). The method of
adding
it is not especially limited. For example, the crosslinking agent can be added
and
mixed with the polyester during polymerization, or a polyester containing the
crosslinking agent can be blended.
In a film of the present invention, to make both the processability and taste
property particularly excellent, and furthermore, to make impact resistance
particularly
excellent, a laminate structure may be adopted since the polyester (B)
containing an
ionomer is excellent in adhesion to the metal and has excellent processability
and
since the polyester layer (A) has excellent taste property. In this case, it
is preferable
20 that the polyester layer (B) is adhered to the metal. Furthermore, the
ratio of the
thickness of the polyester layer (A) to that of the polyester layer (B) is
preferably 1
20 ~ 20 : I, more preferably 1 : 10 --10 : I.
In a film of the present invention, the difference in melting point between
the
polyester constituting the polyester layer (A) and the polyester constituting
the
polyester layer (B) is preferably 30°C or smaller, more preferably
20°C or smaller. If
8
CA 02506289 1997-03-06
it exceeds 30°C, the processability deteriorates.
When the polyesters of the present invention are produced, any known reaction
catalyst and agent for preventing discoloration can be used. The reaction
catalysts
which can be used include, but are not limited to, alkali metal compounds,
alkaline
earth metal compounds, zinc compounds, lead compounds, manganese compounds,
cobalt compounds, aluminum compounds, antimony compounds and titanium
compounds. The agents for preventing discoloration which can be used include,
but
are not limited to, phosphorus compounds. Preferably usually at any optional
stage
before the production of the polyester is completed, an antimony compound,
germanium compound or titanium compound is added as a polymerization catalyst.
As for the method, for example, in the case of a germanium compound, the
germanium compound is added as a powder, or as described in JP-A-54-022234,
the
germanium compound can be dissolved into the glycol used as a monomer of the
polyester. The germanium compounds which can be used include, for example,
germanium dioxide, germanium hydroxide containing crystal water, germanium
alkoxide compounds such as germanium tetramethoxide, germanium tetraethoxide,
germanium tetrabutoxide and germanium ethylene glycoxide, germanium phenoxide
compounds such as germanium phenolate and germanium B-naphthalate, phosphorus
containing germanium compounds such as germanium phosphate and germanium
2 o phosphite and germanium acetate. Above all, germanium dioxide is
preferable. The
antimony compounds which can be used include, but are not limited to, antimony
oxides such as antimony trioxide and antimony acetate. The titanium compounds
which can be used include, but are not limited to, alkyl titanate compounds
such as
tetraethyl titanate and tetrabutyl titanate.
For example, a method in which germanium dioxide is added as the germanium
CA 02506289 1997-03-06
compound in the production of polyethylene terephthalate is described below.
Terephthalic acid and ethylene glycol are interesterified or esterified, and
germanium
dioxide and a phosphorus compound are added. The mixture is in succession
polycondensed at a high temperature under reduced pressure, until a certain
amount of
diethylene glycol present is removed, to obtain a germanium element containing
polymer. Furthermore, preferably the obtained polymer is subjected to solid
phase
polymerization reaction at a temperature lower than the melting point of the
polymer
under reduced pressure or in an inactive gas atmosphere, to decrease the
acetaldehyde
content, for achieving a predetermined intrinsic viscosity and content of
carboxyl end
o groups.
In the polyesters of the present invention, the diethylene glycol content is
preferably 0.01 to 3.5 wt%, and especially in the case of the polyester
constituting the
polyester layer (A), it is more preferably 0.01 to 2.5 wt%, still more
preferably 0.01 to
2.0 wt%, since excellent taste property can be maintained even after many heat
treatments such as the heat treatment in the step of can production and
retorting after
can production. In this case, it is considered that the oxidative
decomposition
resistance at higher than 200°C is improved by the diethylene glycol
component.
Moreover, any known antioxidant can also be added to the polyesters
constituting the
polyester layers (A) and (B) by 0.0001 to 1 wt%. Furthermore, as far as the
properties
2 0 ~'e not impaired, diethylene glycol may be added during polymer
production.
To secure better taste property, the acetaldehyde content of the film is kept
at
preferably 25 ppm or less, more preferably 20 ppm or less. If the acetaldehyde
content
exceeds 25 ppm, the taste property is poor. The method for keeping the
acetaldehyde
content of the film at 25 ppm or less is not especially limited. For example,
to remove
the acetaldehyde produced by thermal decomposition when the polyester is
produced
CA 02506289 1997-03-06
by polycondensation reaction, etc., the polyester can be heat-treated under
reduced
pressure or in an inactive gas atmosphere at a temperature lower than the
melting point
of the polyester. As another preferable method, the polyester is subjected to
solid
phase polymerization under reduced pressure or in an inactive gas atmosphere
at a
temperature higher than 155°C and lower than the melting point. As
another method,
the polymer is melt-extruded using a vent type extruder. As still another
method, the
polymer is melt-extruded at an extrusion temperature higher than the melting
point of
the polymer component highest in melting point and not higher than 30°C
+ that
melting point, preferably higher than that melting point and not higher than
25°C +
that melting point within a short time, preferably within an average melting
time of
less than 1 hour.
In accordance with one aspect of the present invention, the film has an inside
average orientation ratio, RinAVE, and an outside average orientation ratio,
RoutAVE, as
measured by Raman spectrometry, of 6 or less and 8 or more, respectively
R~~nve is preferably 4 or less, more preferably, 3 or less and R°u~ve
is preferably
10 or more, more preferably 11 or more. If RinAVE exceeds 6, the adhesion
between the
film and the metallic sheet becomes insufficient so that the problem of
cracking may
arise. Furthermore, the above properties deteriorate with the lapse of time
especially
after retorting. On the other hand, if Rou~nvE is lower than 8, the flavor
component, etc.
2 0 of the beverage, etc. is adsorbed more, to impair the taste property.
Furthermore, in
addition to adsorption, the components of the beverage, etc. permeate the film
in large
quantities, deteriorating the film. In this invention, as explained later,
each of RinAVE
and RuatAVE means an average orientation intensity ratio of a 1 to 3 ,um thick
portion (a)
which is to lie close to the metallic sheet and a 1 to 3 fem thick portion (b)
which is to
lie remote from the metallic sheet, and which said portions (a) and (b) are to
be
CA 02506289 1997-03-06
disposed at a neck of a can formed from the laminate. Thus, RieAVB and RcatAVE
are
those values measured, by Raman spectrometry, after formation of the can by
laminating at 50 m/min to a TFS steel sheet heated to 170-280°C so as
to have the
polyester layer (B) or polyester layer (C) (or in the case of a mono-layer
film the
casting surface) adhering to the steel sheet, and quickly cooling and drawing
the
laminate by a drawing machine [drawing ratio (maximum thickness/minimum
thickness) = 1.3, drawn in a temperature range within which the laminate is
drawable].
To achieve the above values of RieAVE and R°~wvE, for example, the
face
orientation factor is preferably 0.10 to 0.15, more preferably is 0.10 to
0.14, and
t 0 preferably the film is a laminated Elm having polyester layer containing
ionomer, and
furthermore, the film preferably has a crystalline size x of (100) face of 6nm
or less,
more preferably 5 . 5nm or less, as measured by X-ray diffractometry.
Alternatively, the above values of RinAVE and R°°uve can be
achieved by, for
example, making the difference between the face orientation factor of the
layer on the
metallic sheet side and that on the non-metallic-sheet side preferably 0.001
to 0.05 and
more preferably 0.005 to 0.02 or making the face orientation factor of the
laminated
film of the polymers having different melting temperatures preferably 0.10 to
0.14,
more preferably 0.105 to 0.13. When polyesters having different melting
temperatures
are used, the difference of the melting temperatures is preferably 2 to
30°C. Of
2 0 course, methods for achieving the values of R~nve and RoaWVE are not
limited to the
above.
Moreover, in a film of the present invention, the face orientation factor is
preferably O.lO to 0.15, since the ease with which the Elm is able to laminate
onto the
metallic sheet, its subsequent processability and its high impact resistance
can be
maintained. Especially for higher processability, the face orientation factor
is
12
CA 02506289 1997-03-06
preferably 0.10 to 0.145, more preferably 0.10 to 0.14. If the face
orientation factor is
too high, ability to laminate and processability deteriorate. Thus, the taste
property
after can forming also deteriorates. On the other hand, if the face
orientation factor is
less than 0.10, the film formability deteriorates. The face orientation factor
is obtained
by measuring the surface of the film on the non-metallic-sheet side. The face
orientation factor depends on stretching conditions and the properties of the
polyesters. For example, the stretching ratio (area ratio), although it
depends on the
kind of polyester, is preferably 6 to 12 times, and stretching temperature is
preferably
the glass transition temperature + 10°~C or more and glass transition
temperature +
60°~ or less, more preferably, the glass transition temperature + 25~
or more and
glass transition temperature +50'L or less. The heat treating temperature is
preferably, the melt temperature -10090 to the melt temperature -15°x,
and a heat
treatment time of 1 to 15 seconds is particularly preferable. Too long a heat
treatment
time brings about high crystallinity and too high a face orientation factor.
Too short a
heat treatment time makes it possible to control face orientation factor
within a desired
range; however, dimensional stability of the film is then apt to becomes
impaired.
Furthermore, it is desirable to cool sufficiently after casting and
stretching. Of course,
methods for making polyester film of this invention are not limited to the
above.
When the polyester film consists of at least two or more layers, the
difference
2 o between the face orientation factor of the layer on the metallic sheet
side and that on
the non-metallic-sheet side is preferably 0.001 to 0.05 and more preferably
0.005 to
0.02. This can be achieved, for example, by providing a temperature difference
between both surfaces of the film in the steps of manufacture, such as
casting,
stretching in machine direction, stretching in transverse direction or heat
treatment.
In the present invention, to further improve the taste property, the intrinsic
13
CA 02506289 1997-03-06
viscosity of the polyester is preferably 0.5 dl/g or more, more preferably
0.55 dl/g or
more, still more preferably 0.6 or more. If the intrinsic viscosity is less
than 0.5 dl/g,
the taste property is aggravated undesirably due to dissolved out oligomer.
In the polyester film of the present invention, the mean value of the
elongation
at breakage in each of the machine direction of the film and the transverse
direction is
preferably 130% or more, more preferably 140% or more, still more preferably
150%
or more, for higher processability. This can be achieved by selecting
polyester and
controlling film forming conditions, particularly, stretching temperature and
stretching
ratio, but not limited only thereto. The stretching temperature for machine
direction is
preferably, Tg + 25°~C or more and not so high so that the film tends
to stick to the roll.
The stretching ratios are, for machine direction 2.0 to 3.5 times and for
lateral
direction, 2.0 to 3.5 times.
The thickness irregularity of the polyester film of the present invention is
preferably 30% or less, more preferably 20% or less. If the thickness
irregularity
exceeds 30%, uniform forming is difficult, and pinholes and cracks may occur
after
forming.
In the polyester film of the present invention, the mean value of the heat
shrinkage in each of the machine direction of the film and the transverse
direction at
150 ° C is preferably 0.5 % to 10%, more preferably 1 % to 5 %. If the
heat shrinkage is
2 0 higher than 10%, ability to laminate deteriorates, and hence subsequent
processability
may deteriorate. Methods for achieving the above stated average heat shrinkage
include, of course, controlling conditions for stretching and heat treatment;
however,
for example, it can be said that, to prevent decreasing heat shrinkage, a high
heat
treatment temperature or a long heat treatment time or heat setting with
relaxation is
effective. The heat setting with relaxation may be controlled to provide
relaxation at
14
CA 02506289 1997-03-06
desired ratios for both of the machine direction and the transverse direction,
however,
3-10% is preferable for each direction and it can be one stage or mufti-stage.
In the present invention, the crystalline size x of (100) face of the
biaxially
oriented film is preferably 6 nm or less, more preferably 5.5 nm or less,
still more
preferably 5 nm, especially preferably 4.5 nm or less, as measured by X-ray
diffractometry, for improved ability to laminate and processability. If the
crystalline
size x of (100) face exceeds 6 nm, ability to laminate and processability may
be
insufficient. The crystalline size x of (100) face is obtained by reflected X
ray
diffraction using Scherrer's formula. A crystalline size of (100) face of 6 nm
or less is
determined by the polymers constituting the film, additives, stretching
conditions and
heat treatment conditions, and can be achieved by setting them as desired. For
example, it can be achieved by lowering the heat treatment temperature and
keeping
the heat treatment time within 10 seconds, but these must be set in a range to
satisfy
the properties required for the film.
The thickness of the biaxially oriented film of the present invention is
preferably 3 to 50 ~.cm, more preferably S to 35 ,um, still more preferably 8
to 30 ,um
for improved processability after lamination on the metal, coatability onto
the metal,
impact resistance and taste property.
In the present invention, for improved heat resistance and taste property,
biaxial
2 0 stretching is necessary. The biaxial stretching can be either simultaneous
biaxial
stretching or sequential biaxial stretching.
The method for producing the biaxially oriented film of the present invention
is
not especially limited. For example, the respective polyesters are dried,
supplied to
different conventional extruders, melted, laminated and extruded from a slit
die as a
sheet, brought into close contact with a casting drum by, for example,
electrostatic
CA 02506289 1997-03-06
application, to be cooled and solidified, for obtaining a cast film. The
stretching
method can be either simultaneous biaxial stretching or sequential biaxial
stretching.
The cast sheet is stretched in the machine direction and transverse direction
of the
film, and heat-treated to obtain a film with an intended face orientation
degree. For
improved quality of the film, the use of a tenter is preferable, and
sequential biaxial
stretching to at first stretch in the machine direction and then to stretch in
the
transverse direction, or simultaneous biaxial stretching is desirable. The
stretching
ratio is 1.6 to 4.2 times, preferably 1.7 to 4.0 times in the respective
directions. Either
of the stretching ratios in the machine and transverse directions can be
larger, or the
stretching ratios can be equal. The stretching speed is desirably 1000%/min to
200000%/min. The stretching temperature can be any optional temperature higher
than the glass transition temperature of the polyester and lower than the
glass
transition temperature + 100°C, but preferably 80 to 170°C.
Furthermore, the
biaxially oriented film is heat-treated in an oven or on a heated roll or any
other
conventional method. The heat treatment temperature can be any optional
temperature
of 120 ° C to 250 ° C, but preferably 120 ° C to 245
° C. The heat treatment time is
optional, but preferably 1 to 60 seconds. The heat treatment can be effected
while the
film is relaxed in the machine direction and/or the transverse direction.
Furthermore,
re-stretching can also be effected once or more in each direction, and it can
also be
2 0 followed by heat treatment.
For better handling and processability of the film of the present invention,
it is
preferable that internal and/or external grains, inorganic and/or organic
grains of 0.01
to 10 ~cm in average grain size are present in an amount of 0.01 to 50 wt%. It
is
especially preferable that in a film used on the inner surface of a can,
internal grains,
inorganic grains and/or organic grains of 0.1 to 5 ~cm in average grain size
are present
16
CA 02506289 1997-03-06
in an amount of 0.01 to 3 wt%. For precipitating internal grains, conventional
techniques can be used. For example, techniques proposed in JP-A-48-061556, JP-
A-
51-012860, JP-A-53-041355 and JP-A-54-090397 can be used. Furthermore, other
grains as proposed in JP-A-55-020496 and JP-A-59-204617 can also be used
together.
If grains of more than 10 ,um in average grain size are used, the film is
likely to have
defects. The grains include inorganic grains of, for example, wet process
silica, dry
process silica, colloidal silica, aluminum silicate, titanium oxide, calcium
carbonate,
calcium phosphate, barium sulfate, alumina, mica, kaolin, clay and organic
grains with
styrene, silicone or acrylic acid as a component. Among them, inorganic grains
of wet
process and dry process colloidal silica and alumina and organic grains with
styrene,
silicone, acrylic acid, methacrylic acid, polyester, or divinylbenzene as a
component
can be preferably used. Two or more kinds of these internal grains, inorganic
grains
and/or organic grains can also be used together.
When the film is used on the inner surface of a can, the center line average
roughness Ra is preferably 0.005 to 0.10 ~cm, more preferably 0.01 to 0.05
~cm.
Furthermore, if the ratio Rt/Ra of maximum roughness Rt to center line average
roughness Ra is 4 to 50, preferably 6 to 40, high speed can productivity can
be
enhanced. The center line average roughness Ra of the layer (A) is preferably
0.002 to
0.04 ~crn, more preferably 0.003 to 0.03 ~cm, because the taste property can
be
2 0 improved.
Furthermore, surface treatment of the film by, for example, corona discharge,
can improve the adhesion properties. In this case, the E value is 5 to 50,
preferably 10
to 45.
Moreover, the film of the present invention can be coated in various ways.
The metallic sheet of the present invention is not especially limited, but for
17
CA 02506289 1997-03-06
improved processability, a metallic sheet of, for example, iron or aluminum,
is
preferable. When a metallic sheet of iron is used, it can also be coated on
the surface
with an inorganic oxide layer, for example, a chemical conversion coating
layer
typically formed by chromic acid treatment, phosphate treatment, chromic
acid/phosphate treatment, electrolytic chromic acid treatment, chromate
treatment or
chromium chromate treatment. Especially a chromium hydrate oxide layer of 6.5
to
150 g/m2 as chromium metal is preferable, and furthermore, a malleable metal
plating
layer of, for example, nickel, tin, zinc, aluminum, gun metal or brass can
also be
formed. In the case of tin plating, the plating amount is preferably 0.5 to 15
mg/m2,
and in the case of nickel or aluminum plating, 1.8 to 20 g/m2.
The biaxially oriented polyester film of the present invention can be suitably
used to cover the inside surface of a two-piece metallic can produced by
drawing or
ironing after laminating the film onto a metallic sheet. It can also be
preferably used
to cover the cover of a two-piece can or the body, cover and bottom of a three-
piece
can because it can adhere well to metals and is good in processability.
Examples
Embodiments of the present invention are described in more detail below with
reference to the following Examples. The properties were measured and
evaluated
according to the following methods.
(1) Face orientation factor
The surface of a polyester layer (A) was measured using an Abbe's
refractometer with sodium D line (589 nm in wavelength) as the light source.
From
the refractive indexes in the longitudinal direction, lateral direction and
thickness
direction (Nx, Ny, Nz), the face orientation factor fn = (Nx + Ny)/2 - Nz was
18
CA 02506289 1997-03-06
calculated. Measurements were taken on the surface destined to form the
surface
remote from the metallic sheet.
(2) Melting point
A polyester was dried, rendered molten and quickly cooled, and the melting
point was measured at a heating rate of 10 ° C/min by a differential
scanning
calorimeter (DSC-2 produced by Perkin Elmer).
(3) Elongation (%) at breakage of film
The elongation (%) at breakage was measured using a Tensilon tensile testing
machine, at a tensile speed of 300 mm/min using a sample of 10 mm in width and
100
o mm in length.
(4) Heat shrinkage
Lines were marked on a film sample at a 200 mm interval, and the film was cut
to have a width of 10 mm and suspended in the longitudinal direction. It was
loaded
with 1 g in the longitudinal direction and heated using 190°C air for
20 minutes. The
length between the marked lines was measured, and the shrinkage of the film
was
expressed as a percentage to the original length.
(5) Crystalline size x
The crystalline size x of (100) face was obtained by reflected X ray
diffraction
using Schemer's formula. The measuring X ray wavelength was 0.15418 nm, and
the
2o diffraction of (100) face was observed at a Bragg angle of about
12.7°.
(6) Thickness irregularity
The thicknesses of a film sample were measured at 100 mm intervals for 2 m in
the longitudinal direction, to obtain the mean value Xo, and the maximum value
of the
values obtained by subtracting the mean value Xo from the respective measured
thicknesses was expressed by Xm. The thickness irregularity was expressed by
the
19
CA 02506289 1997-03-06
value of Xm/XO as a percentage. In this case, this measurement was repeated 10
times, and the mean value was adopted as the thickness irregularity.
(7) Intrinsic viscosity of polyester
A polyester was dissolved into ortho-chlorophenol, and the intrinsic viscosity
was measured at 25 ° C.
(8) Inside average orientation ratio, RinAVE, and outside average orientation
ratio, RoutAVE
Measurement was effected using Ramaonor T-64000 (Jobin Yvon), A.r+ laser
as the light source and CCD (Jobin Yvon 1024 x 256) as the detector.
At SO m/min, a film and a 0.24 mm thick TFS steel sheet heated to 170280
° C
were laminated to have the polyester layer (B) or polyester layer (C) (in case
of mono-
layer film, the casting surface) adhering to the steel sheet, and the laminate
was
quickly cooled and drawn by a drawing machine (drawing ratio (maximum
thickness/minimum thickness) = 1.3, drawn in a drawable temperature range).
The
neck portion was cut out and embedded in an epoxy resin, and a section was cut
off in
the longitudinal direction by a microtome. The section was in the longitudinal
direction/thickness direction of the film.
After measurement, the average orientation intensity ratio of the inner 1 to 3
,um portion adjacent to the metallic sheet at the neck of a laminated can
measured by
2 0 Raman spectrometry was expressed as RinAVE, and the average orientation
intensity
ratio in the outer 1 to 3 ,um portion remote from the metallic sheet measured
by
Raman spectrometry, as R°~~,vE. The neck portion was found in the usual
way and the
ratio of the can diameter before and after "neck-in" was 1/0.85
(before/after).
(9) Processability
a. Before heat treatment
CA 02506289 1997-03-06
At SO m/min, a film and a 0.24 mm thick TFS steel sheet heated to
170280°C
were laminated to have the polyester layer (B) or polyester layer (C) (in the
case of a
mono-layer film, the casting surface) adhering to the steel sheet, and the
laminate was
quickly cooled and drawn by a drawing machine (drawing ratio (maximum
thickness/minimum thickness) = 1.3, drawn in a drawable temperature range) to
obtain
a can.
Thus obtained can was filled with 1 % common salt water and allowed to stand
for one day. Then, a voltage of 6 V was applied between an electrode in the
common
salt water and the metallic can, and 10 seconds later, the current value was
read. The
mean value for 10 cans was obtained.
Class A: Less than 0.001 mA
Class B: 0.001 rnA to less than 0.01 mA
Class C: 0.01 mA to less than 0.05 mA
Class D: 0.05 mA or more
b. After heat treatment
The cans obtained in the above were heat treated at 230°C for 10
seconds, and
were formed into neck so that the ratio of the can diameter before and after
neck-in is
1/0.82 (before/after). The cans were retorted at 120°C for 30 min. and
allowed to
stand for one day in 37°C water, filled with 1 % common salt water and
allowed to
2 o stand for one day. Then, a voltage of 6 V was applied between an electrode
in the
common salt water and the metallic can, and 10 seconds later, the current
value was
read. The mean value for 10 cans was obtained.
Class A: Less than 0.1 mA
Class B: 0.1 mA to less than 0.2 mA
Class C: 0.2 mA to less than 0.4 mA
21
CA 02506289 1997-03-06
Class D: 0.4 mA or more
(10) Taste property
The cans (6 cm in diameter and 12 cm in height) formed as above were
pressurized and steamed at 130 ° C for 60 minutes, filled with water,
sealed at 40 ° C,
allowed to stand for one month, and opened. The change of odor was evaluated
by a
sensory test according to the following criterion:
Class A: Odor did not change at all.
Class B: Odor little changed.
Class C: Odor changed a little.
Class D: Odor greatly changed.
(11) Impact resistance
350g of water was filled into a can and sealed. After allowing to stand at 30~
for 72 hours, an impact was applied by dropping the can from 50 cm height onto
concrete ground so that the angle of the bottom of the can relative to the
ground was
45 ° . Next, after removing the contents of the can, its internal
surface was masked
with para~n wax, filled with 1% common salt water, allowed to stand for one
day,
and a voltage of 6 V was applied between an electrode in the common salt water
and
the metallic can, and 5 seconds later, the current value was read. The mean
value for
cans was obtained.
2 0 Class A: Less than 0.3 mA
Class B: 0.3 mA to less than 0.5 mA
Class C: 0.5 mA to less than 1.0 mA
Class D: 1.0 mA or more
(12) As polyesters, the following polyesters were used.
Polyester A: Polyethylene terephthalate (PET) of 0.64 dl/g in intrinsic
viscosity
22
CA 02506289 1997-03-06
and 256°C in melting point
Polyester B: PET of 0.70 dl/g in intrinsic viscosity and 254°C in
melting point
Polyester C: PET of 0.82 dl/g in intrinsic viscosity and 251 °C in
melting point
Polyester D: Isophthalic acid copolymerized polyethylene terephthalate (PET/I:
3 moles of isophthalic acid) of 0.68 dl/g in intrinsic viscosity and
248°C in melting
point
Polyester E: PET/I (6 moles of isophthalic acid) of 0.67 dl/g in intrinsic
viscosity and 240°C in melting point
Polyester F: PET/I (13 moles of isophthalic acid) of 0.67 dl/g in intrinsic
viscosity and 225°C in melting point
Polyester G: Polyester G containing ethylene copolymer ionomer was obtained
by melt-kneading 95 parts by weight of the polyester A and 5 parts by weight
of Zn
ionomer of ethylene-methacrylic acid copolymer of 18 wt% in methacrylic acid
content and 65% in Zn neutralization degree by a double-shaft kneader.
Polyester H: Polyester H containing ethylene copolymer ionomer was obtained
by melt-kneading 80 parts by weight of the polyester B and 20 parts by weight
of Zn
ionomer of ethylene-methacrylic acid copolymer of 20 wt% in methacrylic acid
content and 70% in Zn neutralization degree) by a vent type double-shaft
kneader.
Polyester I: Trimellitic acid copolymerized polyethylene terephthalate (0.7
2 0 mol% of trimellitic acid)
Polyester J: Blend of 90 wt% of polyethylene naphthalate (PEN: 0.73 in
intrinsic viscosity and 266 ° C in melting point) and 10 wt% of
polyethylene
terephthalate (PET: 0.68 in intrinsic viscosity and 254°C in melting
point).
Polyester K: 2,6-naphthalenedicarboxylic acid copolyrnerized polyethylene
terephthalate (5 mol% of 2,6-naphthalenedicarboxylic acid) of 0.68 in
intrinsic
23
CA 02506289 1997-03-06
viscosity and 246.4°C in melting point.
Polyester L: PET/I (4.5 moles of isophthalic acid) of 0.69 dl/g in intrinsic
viscosity and 247°C in melting point
Polyester M: PET/DA (3 moles of dimer acid) of 0.71 dl/g in intrinsic
viscosity
and 249°C in melting point.
Polyester N: PE/CT (6 moles of 1,4-cyclohexanedimethanol) of 0.78 dl/g in
intrinsic viscosity and 246°C in melting point.
Polyester O: PE/CT (33 moles of 1,4-cyclohexanedimethanol) of 0.76 dl/g in
intrinsic viscosity and 194 ° C in melting point.
Polyester P: PET/I (25 moles of isophthalic acid) of 0.67 in intrinsic
viscosity
and 197°C in melting point.
Polyester Q: 2,6-naphthalenedicarboxylic acid copolymerized polyethylene
terephthalate (7.5 mol% of 2,6-naphthalenedicarboxylic acid) of 0.66 in
intrinsic
viscosity and 233.9 ° C in melting point
Example 1
The polyester B destined to form a polyester layer (A) and the polyester G
destined to form a polyester layer (B) were respectively sufficiently dried,
separately
rendered molten according to a conventional method, and co-extruded from
adjacent
dies, and the laminate was quickly cooled to be solidified, to obtain a cast
laminate
2p film. The cast laminate film was stretched to 3.1 times at 112°C in
the machine
direction, stretched to 3.0 times at 115°C in the transverse direction,
and heat-treated
at 200°C at a relaxation of 5% for 5 seconds. The film properties and
can properties
are shown in Tables 2 and 3. Very excellent processability and taste property
could be
obtained.
24
CA 02506289 1997-03-06
Examples 2 to 8
Films as shown in Table 1 were obtained as
described in Example 1, except that the polyesters used, film
production method, etc. were changed. As shown in Tables 2
and 3, excellent film properties and can properties could be
confirmed. In Example 8, the melting point of the polyester
(C) after extrusion was 247°C.
Examples 9 to 14
Films as shown in Tables 4 and 5 were obtained as
described in Example 1, except that the polyesters used, film
production method, etc. were changed. As shown in Tables 5
and 6, excellent film properties and can properties could be
conf i rmed .
Comparative examples 1 to 3
Films as shown in Tables 1 and 2 were obtained as
described in Example 1, except that the polyesters used, film
production method, etc. were changed. As shown in Table 3,
the properties of the films obtained were poor.
Comparative examples 4 to 6
Films as shown in Table 6 were obtained as
described in Example 1, except that the polyesters used, film
production method, etc. were changed. As shown in Table 6,
the properties of the films were poor.
CA 02506289 1997-03-06
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26
CA 02506289 1997-03-06
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CA 02506289 1997-03-06
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CA 02506289 1997-03-06
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CA 02506289 1997-03-06
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34
CA 02506289 1997-03-06
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