Note: Descriptions are shown in the official language in which they were submitted.
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 ~
2 . . .:
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~ ~ 2~77~1~
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 ' ', ':
.. . . . . , I ~ , .. . . .
~ 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 . .
g ':
~. 2 0 7 7 ~ 1 ~
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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2~7~8~
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);
12
.. . . .
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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~
,~. ' .,' ,
2Q7~
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
14
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,
r~.
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
16
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;
17
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f~ ~ ' ' .
2~77X~
(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
21
,,,
, :
,. ............................................................... .
........ ",
~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
.
22
~ - 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
23
.
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- , , . ,, , , ., , , :
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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
24
. ." , , . , . . . , :, ;. . . , ,. , , ~, : .
- 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.
,
,l~ .
- - :
7~8~
'":
EXAMPLB 1
....
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
,~
27
.,~ ,
~''' ' '''~'
~ 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
28
,
.,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 %.
.
- 29
/
, , . , .. , . ,........... , .. , : ,: ,.: ., : .. .. :
,, , ,.,, . , , ~ :, . : : . . . ,,: ;: . : " .... .. ..
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 .::
.! .
.~, ~.'
r~
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 %. ' ~;
, .....
~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 %. ~:
~`
;~ - .
' ,
~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 `
9' . :
`;,, ,
. ' ,` ,,,
. :^` 2~7~
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 %.
BXAMPLE 10
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 %.
1. :,
' 32 ~ :
, . . . .
,. ~ ',
.
:, , - ; . - . ~ . .,: :. , ,,, ,. : ,: ~ , ,
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
33
,, :
~: ' . , , ' ! ' . ,' ,, ' ,
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.
' ..~ .: '~,
,'' " : ..
3~ .-
;, ".:,~'
~77~
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
,
., . . . ~ . ..
.; , , , . , . : . ~ ; ~ :
:, ' ; .................... ; '' ,, , ,: : : ,
. ,. . : :
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
, ''~''~ .
6 ;':,
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~ 2~77~
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).
37
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39
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