U.S. patent application number 09/769259 was filed with the patent office on 2001-08-16 for heat-shrinkable polyester films.
Invention is credited to Hayakawa, Satoshi, Morishige, Chikao, Ohashi, Hideto, Sato, Maki, Tabota, Norimi.
Application Number | 20010014729 09/769259 |
Document ID | / |
Family ID | 26584386 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014729 |
Kind Code |
A1 |
Hayakawa, Satoshi ; et
al. |
August 16, 2001 |
Heat-shrinkable polyester films
Abstract
Heat-shrinkable polyester films suitable for label use, wherein
the value of tan .delta. for dynamic viscoelasticity in a main
shrinkage direction of the film is 0.15 or higher at 65.degree. C.
and takes a maximum of 0.40 or higher at a temperature of
65.degree. C. to 100.degree. C. both inclusive, and the heat
shrinkability in the main shrinkage direction of the film after
treatment in hot water at 80.degree. C. for 10 seconds is 30% or
higher, have excellent shrinkage characteristics over a wide range
of temperature extending from low temperatures to high
temperatures, particularly in the low temperature range, which may
cause only rare occurrence of shrinkage spots, wrinkles, strains,
longitudinal sinking, and other defects during heat shrinkage, and
which may further have excellent break resistance.
Inventors: |
Hayakawa, Satoshi; (Aichi,
JP) ; Ohashi, Hideto; (Aichi, JP) ; Tabota,
Norimi; (Aichi, JP) ; Sato, Maki; (Aichi,
JP) ; Morishige, Chikao; (Aichi, JP) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
2000 PENNSYLVANIA AVE, NW
SUITE 5500
WASHINGTON
DC
20006-1888
US
|
Family ID: |
26584386 |
Appl. No.: |
09/769259 |
Filed: |
January 26, 2001 |
Current U.S.
Class: |
528/272 ;
528/271; 528/295.3 |
Current CPC
Class: |
Y10T 428/1345 20150115;
Y10T 428/26 20150115; Y10T 428/254 20150115; Y10T 428/1328
20150115; C08J 2367/02 20130101; Y10T 428/1334 20150115; C08J 5/18
20130101 |
Class at
Publication: |
528/272 ;
528/271; 528/295.3 |
International
Class: |
C08G 063/16; C08G
063/00; C08G 063/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 2000 |
JP |
020195/2000 |
Feb 2, 2000 |
JP |
025101/2000 |
Claims
1. A heat-shrinkable polyester film, wherein the value of tan
.delta. for dynamic viscoelasticity in a main shrinkage direction
of the film is 0.15 or higher at 65.degree. C. and takes a maximum
of 0.40 or higher at a temperature of 65.degree. C. to 100.degree.
C. both inclusive, and the heat shrinkability in the main shrinkage
direction of the film after treatment in hot water at 80.degree. C.
for 10 seconds is 30% or higher.
2. The heat-shrinkable polyester film according to claim 1, wherein
the value of tan .delta. for dynamic viscoelasticity in the main
shrinkage direction of the film is 0.05 or higher at 60.degree.
C.
3. The heat-shrinkable polyester film according to claim 1, wherein
the value of tan .delta. for dynamic viscoelasticity in the main
shrinkage direction of the film takes a maximum of 0.40 or higher
at a temperature of 65.degree. C. inclusive to 80.degree. C.
exclusive.
4. The heat-shrinkable polyester film according to claim 1, wherein
the rate of initial break in a direction perpendicular to the main
shrinkage direction of the film is 0%.
5. The heat-shrinkable polyester film according to claim 1, wherein
the heat shrinkability in a direction perpendicular to the main
shrinkage direction of the film after treatment in hot water at
80.degree. C. for 10 seconds is 10% or lower.
6. The heat-shrinkable polyester film according to claim 1, wherein
the film is made of a polyester comprising as a constituent monomer
at least one of dimer acid and dimer diol.
Description
FILED OF INVENTION
[0001] The present invention relates to heat-shrinkable polyester
films, and more particularly, to heat-shrinkable polyester films
suitable for label use, which may cause only rare occurrence of
shrinkage spots, wrinkles, strains, longitudinal sinking, and other
defects during heat shrinkage, and which may further have excellent
break resistance.
BACKGROUND OF THE INVENTION
[0002] In the past, heat-shrinkable films have been widely used for
various applications, such as shrink-wrap films, shrinkable labels,
and cap seals, by utilization of their property of causing
shrinkage by heating. In particular, heat-shrinkable stretched
films made of vinyl chloride resins, polystyrene resins, polyester
resins, or other resins have been used as labels on various
vessels, such as polyethylene terephthalate (PET) vessels,
polyethylene (PE) vessels, and glass vessels.
[0003] However, vinyl chloride resins have serious problems
including low heat resistance and evolution of hydrogen chloride
gas in their incineration. In addition, when heat-shrinkable films
of vinyl chloride resins are used as shrinkable labels on PET and
other vessels, the labels should be separated from the vessels in
the process of recycling the vessels.
[0004] In contrast, films of polystyrene resins or polyester resins
cause no evolution of harmful substances such as hydrogen chloride
gas in their incineration, and therefore, these films have been
expected to take the place of vinyl chloride resin films as
shrinkable labels on vessels.
[0005] However, polystyrene resin films, although they exhibit good
shrinkage finish in appearance after shrunk, have poor solvent
resistance, so that they require the use of special ink in their
printing. They also have serious problems in their disposal, e.g.,
they require incineration at high temperatures, in which case they
may cause evolution of black smoke and bad smell in large
quantities.
[0006] As materials that can solve the above problems, polyester
resin films have been extremely expected to serve, and there has
been a steady increase in their amounts for use. The conventional
heat-shrinkable polyester films as described above, however, cannot
have satisfactory heat-shrinkage characteristics. More
particularly, they easily cause the occurrence of shrinkage spots
or wrinkles during heat shrinkage, and they further have some
serious problems, when used for covering the bodies of vessels such
as PET bottles, PE bottles, and glass bottles, and then shrunk,
including distortion of letters or patterns after the shrinkage,
which have been previously printed on the films before the
shrinkage, and further including insufficient adhesion of the films
to the vessels. In addition, they have poor shrinkability as
compared with heat-shrinkable polystyrene films, so that they
should be shrunk at higher temperatures to attain the desired
degree of shrinkage, which further causes serious problems
including the deformation of bottles and the occurrence of
whitening.
[0007] In general, when heat-shrinkable films are used for covering
vessels and then shrunk on a large scale for industrial production,
there has been a method in which the films formed into labels,
tubes, bags, or other shapes are fitted on the vessels, and then
allowed to pass, while being carried on a belt conveyor, through a
shrinkage tunnel of such a type that the films are heat shrunk by
steam blowing (ie., steam tunnel) or a shrinkage tunnel of such a
type that the films are heat shrunk by hot air blowing (ie., hot
air tunnel). The efficiency of heat transmission in the steam
tunnel is higher than that in the hot air tunnel, and therefore,
the use of a steam tunnel can result in heat shrinkage with higher
uniformity to give good shrinkage finish as compared with the use
of a hot air tunnel. However, the conventional heat-shrinkable
polyester films as described above are inferior in shrinkage finish
after shrunk through a steam tunnel to heat-shrinkable vinyl
chloride resin films and heat-shrinkable polystyrene resin films.
On the other hand, the irregularity of internal film temperature
easily occurs during heat shrinkage through a hot air tunnel. As a
result, in particular, there easily occur shrinkage spots,
wrinkles, strains, and other defects. For this reason, the
conventional heat-shrinkable polyester films as described above are
also inferior in shrinkage finish after shrunk through a hot air
tunnel to heat-shrinkable vinyl chloride resin films and
heat-shrinkable polystyrene resin films.
SUMMARY OF THE INVENTION
[0008] Under these circumstances, the present inventors have
extensively studied to provide heat-shrinkable polyester films
suitable for label use, which have excellent shrinkage
characteristics over a wide range of temperature extending from low
temperatures to high temperatures, particularly in a low
temperature range, which may cause only rare occurrence of
shrinkage spots, wrinkles, strains, longitudinal sinking, and other
defects during heat shrinkage, and which may further have excellent
break resistance. As a result, they have found that such
heat-shrinkable polyester films can be obtained by the control of
dynamic viscoelasticity and heat shrinkability after treatment in
hot water.
[0009] Thus the present invention provides heat-shrinkable
polyester films wherein the value of tan 6 for dynamic
viscoelasticity in a main shrinkage direction of the film is 0.15
or higher at 65.degree. C. and takes a maximum of 0.40 or higher at
a temperature of 65.degree. C. to 100.degree. C. both inclusive
(unless otherwise indicated, the range of numerical values referred
to herein includes those at both upper and lower limits), and the
heat shrinkability in the main shrinkage direction of the film
after treatment in hot water at 80.degree. C. for 10 seconds is 30%
or higher.
DETAILED DESCRIPTION OF THE INVENTION
[0010] For the heat-shrinkable polyester film of the present
invention, the value of tan .delta. for dynamic viscoelasticity in
the main shrinkage direction of the film should be 0.15 or higher,
preferably 0.20 or higher, at 65.degree. C.
[0011] As used herein, the value of tan 6 refers to a value defined
by tan .delta.=G'/G" where G' and G" are storage modulus and loss
modulus, respectively, which can be determined by applying sine
stress to a sample and measuring the delay of sine strain as the
response of the sample.
[0012] In the process of industrial production where
heat-shrinkable films formed into labels, tubes, or other shapes
are fitted on vessels and then heat shrunk through a shrinkage
tunnel, the temperature on the surface of the vessels in contact
with the heat-shrinkable films, although it may vary with the type
of process or vessel used, is generally kept at a temperature of
85.degree. C. or lower. The value of tan 6 for dynamic
viscoelasticity in the main shrinkage direction of the film at such
low temperatures is a factor determining the occurrence of
shrinkage spots, wrinkles, strains, and other defects during heat
shrinkage. In particular, if the value of tan 6 for dynamic
viscoelasticity in the main shrinkage direction of the film is 0.15
or higher at 65.degree. C., the process of industrial production
where heat-shrinkable films are heat shrunk through a shrinkage
tunnel involves only rare occurrence of shrinkage spots, wrinkles,
strains, and other defects.
[0013] The value of tan .delta. for dynamic viscoelasticity in the
main shrinkage direction of the film may preferably be 0.05 or
higher, more preferably 0.10 or higher, at 60.degree. C., in which
case the film has particularly excellent shrinkage characteristics
at low temperatures and exhibits particularly good shrinkage
finish.
[0014] In addition, the value of tan .delta. for dynamic
viscoelasticity in the main shrinkage direction of the film should
take a maximum at a temperature of 65.degree. C. to 100.degree. C.
both inclusive. If the value of tan .delta. for dynamic
viscoelasticity in the main shrinkage direction of the film takes a
maximum at a temperature lower than 65.degree. C., the film has
deteriorated break resistance at room temperature and comes to
easily cause a change in physical properties with the lapse of
time. For example, during storage at room temperature for a long
time, shrinkability at low temperatures of 70.degree. C. or lower
is decreased, which causes a problem that shrinkage finish may
become poor. If the value of tan .delta. for dynamic
viscoelasticity in the main shrinkage direction of the film takes a
maximum at a temperature of higher than 100.degree. C., shrinkage
finish also becomes poor in a low temperature range. The value of
tan .delta. for dynamic viscoelasticity in the main shrinkage
direction of the film may preferably take a maximum at a
temperature of 65.degree. C. inclusive to 80.degree. C. exclusive,
in which case the film has particularly excellent shrinkage
characteristics at low temperatures and exhibits particularly good
shrinkage finish.
[0015] The value of tan .delta. for dynamic viscoelasticity in the
main shrinkage direction of the film should take a maximum of 0.4
or higher. If the maximum value of tan .delta. is lower than 0.4,
the polyester in the film has too high crystallinity to cause
whitening phenomenon by partial crystallization during heat
shrinkage or to make worse or impossible adhesion between two films
with an organic solvent such as tetrahydrofuran, which has been
usually carried out in the process of tubing. To attain more stable
shrinkage finish in appearance and adhesion with an organic
solvent, the value of tan .delta. for dynamic viscoelasticity in
the main shrinkage direction of the film preferably takes a maximum
of 0.6 or higher, more preferably 0.8 or higher.
[0016] The heat shrinkability in the main shrinkage direction of
the film after treatment in hot water at 80.degree. C. for 10
seconds should be 30% or higher. If the heat shrinkability in the
main shrinkage direction of the film after treatment in hot water
at 80.degree. C. for 10 seconds is lower than 30%, shrinkage finish
becomes poor because of insufficient shrinkage. To attain more
stable shrinkage finish in appearance, the heat shrinkability in
the main shrinkage direction of the film after treatment in hot
water at 80.degree. C. for 10 seconds may preferably be 40% or
higher, more preferably 50% or higher.
[0017] For the heat-shrinkable polyester film of the present
invention, the rate of initial break in a direction perpendicular
to the main shrinkage direction of the film may preferably be 0%.
If the rate of initial break is higher than 0%, the film has
deteriorated break resistance. In the shrinkable polyester film,
molecules are oriented along the main shrinkage direction, so that
the deterioration of break resistance easily causes the occurrence
of splitting along the direction of molecular orientation,
resulting in a problem that the film may be broken by tension in
the process of printing or tubing to decrease the efficiency of
process operation.
[0018] For the heat-shrinkable polyester film of the present
invention, the heat shrinkability in a direction perpendicular to
the main shrinkage direction of the film after treatment in hot
water at 80.degree. C. for 10 seconds may preferably be 10% or
lower. If the heat shrinkability in a direction perpendicular to
the main shrinkage direction of the film is higher than 10%,
shrinkage finish becomes poor by shrinkage in a direction
perpendicular to the main shrinkage direction of the film (ie.,
occurrence of longitudinal sinking). To attain more stable
shrinkage finish in appearance, the heat shrinkability in a
direction perpendicular to the main shrinkage direction of the film
after treatment in hot water at 80.degree. C. for 10 seconds may
preferably be 7% or lower, more preferably 5% or lower, and still
more preferably 2% or lower.
[0019] The heat-shrinkable polyester film of the present invention
is not particularly limited to any thickness, but it may preferably
have a thickness of 10 to 200 .mu.m, more preferably 20 to 100
.mu.m, as shrinkable films for label use.
[0020] The heat-shrinkable polyester film of the present invention
is made of at least one polyester composed mainly of dicarboxylic
acid components such as aromatic dicarboxylic acids, aliphatic
dicarboxylic acids, or ester derivatives thereof, and polyhydric
alcohol components. In the dicarboxylic acid components, the
aromatic dicarboxylic acids may include terephthalic acid,
isophthalic acid, naphthalene-1,4- or -2,6-dicarboxylic acid, and
5-sulfo-isophthalic acid sodium salt. The aliphatic dicarboxylic
acids may include dimer acid, glutaric acid, adipic acid, sebacic
acid, azelaic acid, oxalic acid, and succinic acid. The ester
derivatives of these dicarboxylic acids may include dialkyl esters
and diaryl esters. If necessary, oxycarboxylic acids such as
p-oxybenzoic acid, or polycarboxylic acids such as trimellitic
anhydride and pyromellitic anhydride may be used in combination
with the above dicarboxylic acid components. The polyhydric alcohol
components may include alkylene glycols such as ethylene glycol,
diethylene glycol, dimer diol, propylene glycol, triethylene
glycol, 1,4-butanediol, neopentyl glycol,
1,4-cyclohexanedimethanol, 1,6-hexanediol,
3-methyl-1,5-pentanediol, 2-methyl-1,5-pentanediol,
2,2-diethyl-1,3-propanediol, 1,9-nonanediol, and 1,10-decanediol;
ethylene oxide adducts of bisphenol compounds or their derivatives,
trimethylol propane, glycerin, pentaerythritol,
polyoxytetramethylene glycol, and polyethylene glycol. In place of
the polyhydric alcohols, .epsilon.-caprolactone can also be
used.
[0021] The polyester in the heat-shrinkable polyester film of the
present invention may preferably contain as a constituent monomer,
dimer acid as at least one dicarboxylic acid component or dimer
diol as at least one polyhydric alcohol component. The
incorporation of dimer acid and/or dimer diol as a constituent
monomer(s) of the polyester in the heat-shrinkable polyester film
of the present invention makes it possible to increase the value of
tan .delta. for dynamic viscoelasticity at 60.degree. C. in the
main shrinkage direction of the film, while keeping the film having
good break resistance, which leads to easy control of film
characteristics.
[0022] In this case, the heat-shrinkable polyester film of the
present invention may contain either or both of dimer acid and
dimer diol, and the amount of dimer acid or dimer diol contained
may usually be 1 to 20 mol %, preferably 1 to 15 mol %, and more
preferably 1 to 7 mol %, based on the total amount of carboxylic
acid components or polyhydric alcohol components.
[0023] The dimer acid or dimer diol is a mixture of components
containing as the main components those which have the structures
of the formulas: 1
[0024] wherein X is COOH or CH.sub.2OH; 2
[0025] wherein X is COOH or CH.sub.2OH. The ratio of components may
preferably be (I):(II)=10:90 to 90:10. The dimer acid and dimer
diol may preferably be those which have been washed with water for
purification.
[0026] The polyester material used in the present invention may be
a homo-polyester or a mixture of two or more polyesters. The
mixture of two or more polyesters may be a mixed system of
polyethylene terephthalate and at least one copolyester, or a
combination of at least two copolyesters. The copolyester may also
be used in combination with polybutylene terephthalate,
polycyclohexylene dimethylterephthalate, or other homopolyesters.
Mixing of two or more polyesters with different second-order
transition temperatures (Tgs) may also be useful for the present
invention. Specific examples of the polyester are those composed of
terephthalic acid and isophthalic acid as dicarboxylic acid
components and ethylene glycol, dimer diol, and polytetramethylene
glycol with a molecular weight of 500 to 3000 as polyhydric alcohol
components, and these polyesters may be used in a single
copolymerized system or in a mixed system of two or more
copolymers. These polyesters can be produced by, but not limited
to, melt polycondensation according to the ordinary methods. They
may also be produced by any other method of polymerization. For
polycondensation, various catalysts may be used, such as antimony
oxide, germanium oxide, or titanium compounds. The degree of
polymerization for the polyester is not particularly limited, but
they preferably have an intrinsic viscosity of 0.3 to 1.3 dL/g,
more preferably 0.5 to 1.3 dL/g, from the viewpoint of film
production.
[0027] To the polyester used in the present invention, there may be
added for the purpose of preventing coloring and gel formation as
well as improving heat resistance, various kinds of metal salts, or
phosphoric acid or phosphate esters. The metal salts may include
magnesium salts such as magnesium acetate and magnesium chloride,
calcium salts such as calcium acetate and calcium chloride,
manganese salts such as manganese acetate and manganese chloride,
zinc salts such as zinc acetate and zinc chloride, and cobalt salts
such as cobalt acetate and cobalt chloride. The total amount of
metal salts added, except the above catalyst for polycondensation,
may usually be 300 ppm or smaller, as the respective metal ions,
relative to the polyester produced. The phosphate esters may
include trimethyl phosphate and triethyl phosphate. The total
amount of phosphoric acid or phosphate esters added may usually be
200 ppm or smaller, in terms of phosphorous, relative to the
polyester produced.
[0028] If the total amount of metal ions added, except the above
catalyst for polycondensation, is greater than 300 ppm or the total
amount of phosphorous is greater than 200 ppm, relative to the
polyester produced, the resulting polymer causes remarkable
coloring and deterioration in resistance to heat and degradation
with river water.
[0029] In this case, from the viewpoint of resistance to heat and
degradation with river water, the molar atomic ratio of the total
amount of phosphorous to the total amount of metal ions is
preferably in the range of 0.4 to 1.0. If the molar atomic ratio is
smaller than 0.4 or higher than 1.0, the resulting polymer causes
remarkable coloring and formation of coarse particle, which is not
preferred.
[0030] The production of the polyesters used in the present
invention is not particularly limited to any process, but it can be
carried out by any process of production, including the direct
polymerization method in which dicarboxylic acids are directly
reacted with glycols and the resulting oligomers are subject to
polycondensation; and the transesterification method in which
dimethyl esters of dicarboxylic acids and glycols are subjected to
transesterification, followed by polycondensation.
[0031] The above metal ions, or phosphoric acid or phosphate esters
may be added at any step. In general, metal ions may preferably be
added when starting materials are placed in a reaction vessel, ie.,
before transesterification or esterification, and phosphoric acid
or phosphate esters may preferably be added before
polycondensation.
[0032] To the polyesters in the film of the present invention,
there may be added, if necessary, fine particles such as those of
silica, titanium dioxide, kaolin, or calcium carbonate, and there
may also be added various additives including antioxidants,
ultraviolet light absorbers, antistatic agents, coloring agents,
and antimicrobial agents.
[0033] The value of tan 6 for dynamic viscoelasticity at 65.degree.
C. in the main shrinkage direction of the film, the temperature at
which the value of tan .delta. takes a maximum, the maximum value
of tan .delta., and the heat shrinkability in the main shrinkage
direction of the film after treatment in hot water at 80.degree. C.
for 10 seconds can be controlled within the above ranges by the use
of a polyester material(s) in the film as described above or by the
control of the conditions of film production as described below, or
by a combination of both.
[0034] The following will describe a typical process for the
production of the heat-shrinkable polyester film of the present
invention. Polyester materials which can be used in the present
invention are dried using a dryer such as hopper dryer or paddle
dryer, or a vacuum dryer, and melt extruded into a film shape at a
temperature of 200.degree. C. to 300.degree. C. Alternatively,
undried polyester materials are melt extruded into a film shape
under the removal of water in an extruder of the vent type. For
extrusion, any of the conventional methods can be used, such as
T-die method or tubular method. The extrusion and subsequent rapid
cooling give an unstretched film, which is then subjected to the
process of stretching. To attain the objective of the present
invention, the main shrinkage direction of the film may preferably
be taken as the transverse direction (ie., the direction running
along the film surface and perpendicular to the direction of
extrusion) from a practical point of view. Therefore, the following
will describe a typical example of the process of film production
in which the main shrinkage direction of the film is taken as the
transverse direction. However, the process of film production in
which the main shrinkage direction of the film is taken as the
machine direction (i.e., the direction of extrusion) can also be
carried out substantially in the same manner as described below,
except that the direction of stretching is turned 90 degrees around
the line perpendicular to the film surface.
[0035] The process of stretching in which the main shrinkage
direction of the film is taken as the transverse direction may
include uniaxial stretching in the transverse direction with a
tenter. When a film is stretched in the transverse direction with a
tenter, the film should be preheated prior to the step of
stretching so that film temperature falls within the range of
Tg+0.degree. C. to Tg+60.degree. C. where "Tg" as used herein
refers to the second-order transition temperature of a polyester(s)
in the film.
[0036] To attain uniform distribution of thickness in the
heat-shrinkable polyester film of the present invention, the step
of preheating may preferably be carried out by hot air blowing with
a heat transmission coefficient of 0.0013 cal/cm.sup.2
sec.multidot..degree. C. (0.0054
J/cm.sup.2.multidot.sec.multidot.K) or lower.
[0037] The step of stretching should be carried out at a stretch
ratio (or the total stretch ratio, ie., a product of the respective
stretch ratios, for multi-stage stretching) of 2.3 to 7.3,
preferably 3.8 to 5.2, in the transverse direction at a temperature
ranging from Tg+0.degree. C. to Tg+40.degree. C. In the multi-stage
stretching, the temperature of the first-stage stretching is
preferably set lower than the temperature of preheating.
[0038] After the step of stretching or between any two stages in
the multi-stage stretching, heat treatment may preferably be
carried out under 0% to 15% elongation or relaxation at a
temperature of 60.degree. C. to 110.degree. C. If necessary,
additional heat treatment may preferably be carried out at a
temperature of 40.degree. C. to 100.degree. C.
[0039] The process of stretching in which the main shrinkage
direction of the film is taken as the transverse direction may
include biaxial stretching both in the transverse direction and in
the machine direction. The steps of biaxial stretching may be
carried out successively or simultaneously, and if necessary, it
may be followed by additional stretching. In the successive biaxial
stretching, the steps of stretching may be carried out in any
order, e.g., in the machine direction and then in the transverse
direction, or in the transverse direction and then in the machine
direction, or in the machine direction and then in the transverse
direction and then again in the machine direction, or in the
transverse direction and then in the machine direction and then
again in the transverse direction.
[0040] The step of stretching in the machine direction may be
carried out at a stretch ratio of 1.0 to 2.3, preferably 1.1 to
1.8, and more preferably 1.1 to 1.4, at a temperature of
Tg+0.degree. C. to Tg+50.degree. C., preferably Tg+10.degree. C. to
Tg +40.degree. C. The stretching in the machine direction makes
possible improvement in the break resistance of the heat-shrinkable
polyester film. However, if a film is stretched at a stretch ratio
of higher than 2.3 in the machine direction, the heat shrinkability
in the direction perpendicular to the main shrinkage direction of
the film after treatment in hot water at 80.degree. C. for 10
seconds has a tendency to become 10% or higher. Therefore, such
conditions for stretching in the machine direction are not
preferred for the film production according to the present
invention.
[0041] For the prevention of heat evolution in the film during
stretching to reduce the irregularity of internal film temperature,
the step(s) of stretching may preferably be carried out by hot air
blowing with a heat transmission coefficient of 0.0009
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0038
J/cm.sup.2.multidot.sec.multidot.K) or higher, preferably 0.0013 to
0.0020 cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0054 to
0.0084 J/cm.sup.2.multidot.sec.multidot.K).
[0042] For example, a particularly preferred process of stretching
comprises the following steps in this order:
[0043] 1) Preheating so that film temperature falls within the
range of Tg+0.degree. C. to Tg+60.degree. C. by hot air blowing
with a heat transmission coefficient of 0.0013
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0054
J/cm.sup.2.multidot.sec.multidot.K) or lower;
[0044] 2) First-stage stretching at a stretch ratio of 1.8 to 2.3
in the transverse direction at a temperature ranging from the
preheating temperature -30.degree. C. to the preheating temperature
-20.degree. C. with a heat transmission coefficient of 0.0009
cal/cm.sup.2.multidot.sec.- multidot..degree. C. (0.0038
J/cm.sup.2.multidot.sec.multidot.K) or higher, in which low
temperature stretching and a difference in temperature between the
preheating and the first-stage stretching make an increase in the
value of tan .delta. at low temperatures;
[0045] 3) Heat treatment under 3% to 10% relaxation in the
transverse direction at a temperature ranging from the first-stage
stretching temperature +3.degree. C. to the first-stage stretching
temperature +5.degree. C.;
[0046] 4) Second-stage stretching at the total stretch ratio of 3.8
to 4.2 (i.e., a product of the respective stretch ratios in the
first-stage stretching and the second-stage stretching) in the
transverse direction at a temperature ranging from the first-stage
stretching temperature +5.degree. C. to the first-stage stretching
temperature +10.degree. C. with a heat transmission coefficient of
0.0009 cal/cm.sup.2.multidot.sec.- multidot..degree. C. (0.0038
J/cm.sup.2.multidot.sec.multidot.K), in which high temperature
stretching after the heat treatment under relaxation makes a
decrease in shrinkage stress and heat shrinkability in the machine
direction;
[0047] 5) Heat treatment under 3% to 8% elongation in the
transverse direction at a temperature ranging from the first-stage
stretching temperature to the first-stage stretching temperature
-5.degree. C., in which the elongation makes an increase in heat
shrinkability in the transverse direction.
[0048] As described above, the heat-shrinkable polyester film of
the present invention can have the desired shrinkage
characteristics by a combination of the polyester composition of
starting materials in the film prodcution and the process of
stretching employed therein.
[0049] The heat-shrinkable polyester film of the present invention
may have at least one layer, such as an anti-fogging layer, which
may be formed on the surface thereof, if necessary.
EXAMPLES
[0050] The present invention will be further illustrated by some
examples and comparative examples; however, the present invention
is not limited to these examples.
[0051] The following will describe the methods for measuring some
physical properties of films.
[0052] (1) Dynamic Viscoelasticity
[0053] A film was cut into a strip of 4 cm in length along the main
shrinkage direction and 5 mm in width along the direction
perpendicular thereto, and the measurement of dynamic
viscoelasticity was carried out with this sample using a dynamic
viscoelasticity measuring apparatus available from ITK Co., Ltd.
under the conditions that measurement length was 3 cm, displacement
was 0.25%, and frequency was 10 Hz, in which the value of tan
.delta. at a temperature ranging from 60.0.degree. C. to
60.4.degree. C. was taken as the value of tan .delta. at 60.degree.
C. The value of tan .delta. was reported as an average of those
obtained from two samples. (2) Heat Shrinkability
[0054] A film was cut into a square of 10 cm.times.10 cm with two
sides parallel to the main shrinkage direction and to the direction
perpendicular thereto, respectively, and this sample was
heat-shrunk by immersion under no load in hot water at
80.+-.0.5.degree. C. for 10 seconds and then measured for side
lengths in the main shrinkage direction and in the direction
perpendicular thereto, respectively. The heat shrinkability was
determined from the side lengths by the following equation: 1 Heat
Shrinkabilty = Side length before shrinkage - Side length after
shrinkage Side length before shrinkage .times. 100 ( % )
[0055] (3) Main Shrinkage Direction
[0056] A film was measured for heat shrinkability as described
above in (2). The direction of a side corresponding to the larger
value of heat shrinkability was referred to as the main shrinkage
direction.
[0057] (4) Shrinkage Finish
[0058] A film was printed with three inks of glass, gold and white
colors, and cut into a rectangular of 225 mm in width along the
main shrinkage direction and 110 mm in height along the direction
perpendicular thereto. This sample was formed into a cylindrical
label of 110 mm in height and 110 mm in folding diameter (i.e.,
length in the width direction when the label was folded flat) by
attaching one end to the other in the main shrinkage direction (the
width of margins for attachment were 5 mm) with a solvent such as
1,3dioxolane. The label was fitted on a glass bottle (300 mL) and
heat-shrunk by allowing the labeled glass bottle to pass through a
shrinkage tunnel with a hot air at 130.degree. C. (air speed, 10
m/sec) for a passage time of 10 seconds. The shrinkage finish was
determined by visual observation for the number of shrinkage spots
and evaluated at 5 ranks by the following criteria.
[0059] rank 5: best finish (no shrinkage spot)
[0060] rank 4: good finish (1 shrinkage spot)
[0061] rank 3: bad finish (2 shrinkage spots)
[0062] rank 2: worse finish (3-5 shrinkage spots)
[0063] rank 1: worst finish (6 or more shrinkage spots)
[0064] in which ranks 4 and 5 were regarded as acceptable.
[0065] (5) Rate of Initial Break
[0066] A film was cut into a strip of 15 mm in width along the main
shrinkage direction and 100 mm in length along the direction
perpendicular thereto, and this sample was measured for elongation
at break in the main shrinkage direction and in the direction
perpendicular thereto according to JIS-C-2318. The measurement of
elongation at break was carried out for 20 samples (i e., n=20),
and the number (x) of samples exhibiting 5% or smaller elongation
at break was determined. The rate of initial break was calculated
from the values of x and n by the following equation:
Rate of nitial break=(x/n).times.100 (%)
Example 1
[0067] In a stainless steel autoclave equipped with a stirrer, a
thermometer, and a condenser of the partial reflux type were placed
starting materials at a composition of 80 mol % dimethyl
terephthalate and 20 mol % dimethyl isophthalate as dicarboxylic
acid components and 96 mol % ethylene glycol and 3 mol % dimer diol
("HP-1000" available from Toagosei Chemical Industry Co., Ltd.) as
polyhydric alcohol components so that the polyhydric alcohol
components were 2.2 times as high in molar ratio as the
dicarboxylic acid components, and transesterification was carried
out with 0.05 mol % zinc acetate (relative to the acid components)
as a catalyst under the removal of methanol by distillation from
the system. After that, 1 mol % polytetramethylene glycol with a
molecular weight of 650 (relative to the acid components) and 0.025
mol % antimony trioxide (relative to the acid components) as a
catalyst were added to cause a polycondensation. This gave a
copolyester composed of 80 mol % terephthalic acid and 20 mol %
isophthalic acid as dicarboxylic acid components and 96 mol %
ethylene glycol, 3 mol % dimer diol and 1 mol % polytetramethylene
glycol with a molecular weight of 650 as polyhydric alcohol
components. The copolyester thus obtained had an intrinsic
viscosity of 0.7 dl/g.
[0068] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was then subjected to stretching at
a stretch ratio of 1.15 in the machine direction at 80.degree. C.,
pre-heating at 103.degree. C. for 8 seconds with a heat
transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 2.0 in the transverse direction at 75.degree. C.
with a heat transmission coefficient of 0.0015
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0062
J/cm.sup.2.multidot.sec.multidot.K), heat treatment under 6%
relaxation in the transverse direction at 78.degree. C. for 3
seconds, second-stage stretching at the total stretch ratio of 4.0
in the transverse direction at 80.degree. C. with a heat
transmission coefficient of 0.0014
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0060
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under 5%
elongation in the transverse direction at 73.degree. C. for 6
seconds gave a heat-shrinkable polyester film of 43 .mu.m in
thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Example 2
[0069] A copolyester composed of 79 mol % terephthalic acid, 15 mol
% isophthalic acid and 6 mol % dimer acid ("Prepol 1009" available
from Unichema Chemicals, Ltd.) as dicarboxylic acid components and
88 mol % ethylene glycol, 10 mol % neopentyl glycol and 2 mol %
polytetramethylene glycol with a molecular weight of 650 as
polyhydric alcohol components was prepared by the same method of
polymerization as used in Example 1. The copolyester thus obtained
had an intrinsic viscosity of 0.72 dL/g.
[0070] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 190 .mu.m in
thickness. The unstretched film was subjected to stretching at a
stretch ratio of 1.20 at 78.degree. C. in the machine direction,
pre-heating at 105.degree. C. for 8 seconds with a heat
transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 1.8 in the transverse direction at 75.degree. C.
with a heat transmission coefficient of 0.0015
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0062
J/cm.sup.2.multidot.sec.multidot.K), heat treatment under 5%
relaxation at 76.degree. C. for 3 seconds, second-stage stretching
at the total stretch ratio of 4.1 in the transverse direction at
80.degree. C. with a heat transmission coefficient of 0.0014
cal/cm.sup.2.multidot.s- ec.multidot..degree. C. (0.0060
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under 5%
elongation in the transverse direction at 73.degree. C. for 6
seconds. This gave a heat-shrinkable polyester film of 44 .mu.m in
thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Comparative Example 1
[0071] A copolyester composed of 97 mol % terephthalic acid and 3
mol % isophthalic acid as dicarboxylic acid components and 71.5 mol
% ethylene glycol, 28 mol % neopentyl glycol and 0.5 mol %
polytetramethylene glycol with a molecular weight of 650 as
polyhydric alcohol components was prepared by the same method of
polymerization as used in Example 1. The copolyester thus obtained
had an intrinsic viscosity of 0.70 dL/g.
[0072] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 195 .mu.m in
thickness. The unstretched film was subjected to pre-heating at
105.degree. C. for 9 seconds, stretching at a stretch ratio of 4.3
in the transverse direction at 83.degree. C., and heat treatment
under no elongation at 75.degree. C. for 10 seconds. This gave a
heat-shrinkable polyester film of 45 .mu.m in thickness. The main
shrinkage direction of the film was corresponding to the transverse
direction. The physical properties of the film thus obtained are
shown in Table 1.
Comparative Example 2
[0073] A copolyester composed of 92 mol % terephthalic acid and 8
mol % isophthalic acid as dicarboxylic acid components and 77 mol %
ethylene glycol and 23 mol % 1,4-butanediol as polyhydric alcohol
components was prepared by the same method of polymerization as
used in Example 1. The copolyester thus obtained had an intrinsic
viscosity of 0.70 dL/g.
[0074] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to pre-heating at
95.degree. C. for 8 seconds, first-stage stretching at a stretch
ratio of 2.3 in the transverse direction at 80.degree. C.,
second-stage stretching at a stretch ratio of 1.7 in the traverse
direction at 85.degree. C., and heat treatment under no elongation
at 85.degree. C. for 15 seconds. This gave a heat-shrinkable
polyester film of 44 .mu.m in thickness. The main shrinkage
direction of the film was corresponding to the transverse
direction. The physical properties of the film thus obtained are
shown in Table 1.
Comparative Example 3
[0075] A copolyester composed of 62 mol % terephthalic acid and 38
mol % isophthalic acid as dicarboxylic acid components and 78 mol %
ethylene glycol, 21 mol % butanediol and 1 mol % polytetramethylene
glycol with a molecular weight of 650 as polyhydric alcohol
components was prepared by the same method of polymerization as
used in Example 1. The copolyester thus obtained had an intrinsic
viscosity of 0.70 dL/g.
[0076] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to pre-heating at
90.degree. C. for 8 seconds, first-stage stretching at a stretch
ratio of 1.6 in the transverse direction at 80.degree. C.,
second-stage stretching at a stretch ratio of 2.5 in the traverse
direction at 75.degree. C., and heat treatment at 73.degree. C. for
10 seconds. This gave a heat-shrinkable polyester film of 45 .mu.m
in thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Comparative Example 4
[0077] A copolyester composed of 83 mol % terephthalic acid and 17
mol % 2,6-naphthalenedicarboxylic acid as dicarboxylic acid
components and 83 mol % ethylene glycol, 15 mol % butanediol and 2
mol % polytetramethylene glycol with a molecular weight of 650 as
polyhydric alcohol components was prepared by the same method of
polymerization as used in Example 1. The copolyester thus obtained
had an intrinsic viscosity of 0.70 dL/g.
[0078] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to pre-heating at
105.degree. C. for 8 seconds, first-stage stretching at a stretch
ratio of 2.5 in the transverse direction at 85.degree. C.,
second-stage stretching at a stretch ratio of 1.16 in the
transverse direction at 90.degree. C., and heat treatment under no
elongation at 73.degree. C. for 10 seconds. This gave a
heat-shrinkable polyester film of 45 .mu.m in thickness. The main
shrinkage direction of the film was corresponding to the transverse
direction. The physical properties of the film thus obtained are
shown in Table 1.
Example 3
[0079] In a stainless steel autoclave equipped with a stirrer, a
thermometer, and a condenser of the partial reflux type were placed
starting materials at a composition of 28 mol % dimethyl
terephthalate and 72 mol % dimethyl naphthalate as dicarboxylic
acid components and 88 mol % ethylene glycol and 11 mol % dimer
diol ("HP-1000" available from Toagosei Chemical Industry Co.,
Ltd.) as polyhydric alcohol components so that the polyhydric
alcohol components were 2.2 times as high in molar ratio as the
dicarboxylic acid components, and transesterification was carried
out with 0.05 mol % zinc acetate (relative to the acid components)
as a catalyst under the removal of methanol by distillation from
the system. After that, 1 mol % polytetramethylene glycol with a
molecular weight of 650 (relative to the acid components) and 0.025
mol % antimony trioxide (relative to the acid components) as a
catalyst were added to cause a polycondensation. This gave a
copolyester composed of 28 mol % terephthalic acid and 72 mol %
naphthalenedicarboxylic acid as dicarboxylic acid components and 88
mol % ethylene glycol, 11 mol % dimer diol and 1 mol %
polytetramethylene glycol with a molecular weight of 650 as
polyhydric alcohol components. The copolyester thus obtained had an
intrinsic viscosity of 0.69 dl/g.
[0080] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to stretching at a
stretch ratio of 1.15 in the machine direction at 80.degree. C.
with a heat transmission coefficient of 0.0201
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0837
J/cm.sup.2.multidot.sec.multidot.K), pre-heating at 103.degree. C.
for 8 seconds with a heat transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 2.0 in the transverse direction at 75.degree. C.
with a heat transmission coefficient of 0.0015
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0062
J/cm.sup.2.multidot.sec.multidot.K), heat treatment under 6%
relaxation in the transverse direction at 78.degree. C. for 3
seconds, second-stage stretching at the total stretch ratio of 4.0
in the transverse direction at 80.degree. C. with a heat
transmission coefficient of 0.0014
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0060
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under 5%
elongation in the transverse direction at 73.degree. C. for 6
seconds. This gave a heat-shrinkable polyester film of 43 .mu.m in
thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Example 4
[0081] A copolyester composed of 51 mol % terephthalic acid, 5 mol
% isophthalic acid, 35 mol % 2,6-naphthalenedicarboxylic acid and 9
mol % dimer acid ("Prepol 1009" available from Unichema Chemicals,
Ltd.) as dicarboxylic acid components and 89 mol % ethylene glycol,
10 mol % neopentyl glycol and 1 mol % polytetramethylene glycol
with a molecular weight of 650 as polyhydric alcohol components was
prepared by the same method of polymerization as used in Example 3.
The copolyester thus obtained had an intrinsic viscosity of 0.70
dL/g.
[0082] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to stretching at a
stretch ratio of 1.20 in the machine direction at 78.degree. C.
with a heat transmission coefficient of 0.0201
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0837
J/cm.sup.2.multidot.sec.multidot.K), preheating at 105.degree. C.
for 8 seconds with a heat transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 1.8 in the transverse direction at 75.degree. C.
with a heat transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), heat treatment 5% relaxation
in the transverse direction at 78.degree. C., second-stage
stretching at the total stretch ratio of 4.1 in the transverse
direction at 80.degree. C. with a heat transmission coefficient of
0.0015 cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0062
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under 5%
elongation in the transverse direction at 73.degree. C. for 6
seconds. This gave a heat-shrinkable polyester film of 44 .mu.m in
thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Example 5
[0083] A copolyester composed of 30 mol % terephthalic acid and 70
mol % 2,6-naphthalenedicarboxylic acid as dicarboxylic acid
components and 89 mol % ethylene glycol, 10 mol % dimer diol
("HP-1000" available from Toagosei Chemical Industry Co., Ltd.) and
1 mol % polytetramethylene glycol with a molecular weight of 650 as
polyhydric alcohol components was prepared by the same method of
polymerization as used in Example 3. The copolyester thus obtained
had an intrinsic viscosity of 0.70 dL/g.
[0084] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 400 .mu.m in
thickness. The unstretched film was subjected to stretching at a
stretch ratio of 2.3 in the machine direction at 80.degree. C. with
a heat transmission coefficient of 0.0201
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0837
J/cm.sup.2.multidot.sec.multidot.K), preheating at 105.degree. C.
for 8 seconds with a heat transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 2.5 in the transverse direction at 85.degree. C.
with a heat transmission coefficient of 0.0015
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0062
J/cm.sup.2.multidot.sec.multidot.K), heat treatment under 5%
relaxation at 88.degree. C., second-stage stretching at a stretch
ratio of 1.6 in the transverse direction at 90.degree. C. with a
heat transmission coefficient of 0.0016
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0065
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under 5%
elongation in the transverse direction at 75.degree. C. for 10
seconds. This gave a heat-shrinkable polyester film of 43 .mu.m in
thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Comparative Example 5
[0085] A copolyester composed of 100 mol % terephthalic acid as a
dicarboxylic acid component and 68 mol % ethylene glycol, 31 mol %
neopentyl glycol and 1 mol % polytetramethylene glycol with a
molecular weight of 650 as polyhydric alcohol components was
prepared by the same method of polymerization as used in Example 3.
The copolyester thus obtained had an intrinsic viscosity of 0.70
dL/g.
[0086] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 195 .mu.m in
thickness. The unstretched film was subjected to preheating at
110.degree. C. for 8 seconds with a heat transmission coefficient
of 0.0011 cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), stretching at a stretch ratio
of 4.5 in the transverse direction at 83.degree. C. with a heat
transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under no
elongation at 70.degree. C. for 10 seconds. This gave a
heat-shrinkable polyester film of 43 .mu.m in thickness. The main
shrinkage direction of the film was corresponding to the transverse
direction. The physical properties of the film thus obtained are
shown in Table 1.
Comparative Example 6
[0087] A copolyester composed of 8 mol % terephthalic acid and 92
mol % 2,6-naphthalenedicarboxylic acid as dicarboxylic acid
components and 90 mol % ethylene glycol and 10 mol % 1,4-butanediol
as polyhydric alcohol components was prepared by the same method of
polymerization as used in Example 3. The copolyester thus obtained
had an intrinsic viscosity of 0.68 dL/g.
[0088] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to preheating at
105.degree. C. for 8 seconds with a heat transmission coefficient
of 0.0011 cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 2.3 in the transverse direction at 90.degree. C.
with a heat transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), second-stage stretching at a
stretch ratio of 1.7 in the transverse direction at 85.degree. C.
with a heat transmission coefficient of 0.0011
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under 5%
elongation in the transverse direction at 90.degree. C. for 15
seconds. This gave a heat-shrinkable polyester film of 46 .mu.m in
thickness. The main shrinkage direction of the film was
corresponding to the transverse direction. The physical properties
of the film thus obtained are shown in Table 1.
Example 6
[0089] A copolyester composed of 78 mol % terephthalic acid and 22
mol % isophthalic acid as dicarboxylic acid components and 81 mol %
ethylene glycol, 18 mol % butanediol and 1 mol % polytetramethylene
glycol with a molecular weight of 650 as polyhydric alcohol
components was prepared by the same method of polymerization as
used in Example 3. The copolyester thus obtained had an intrinsic
viscosity of 0.67 dL/g.
[0090] This polyester was melt extruded at 280.degree. C. and then
rapidly cooled to give an unstretched film of 180 .mu.m in
thickness. The unstretched film was subjected to preheating at
90.degree. C. for 8 seconds with a heat transmission coefficient of
0.0011 cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0045
J/cm.sup.2.multidot.sec.multidot.K), first-stage stretching at a
stretch ratio of 1.6 in the transverse direction at 80.degree. C.
with a heat transmission coefficient of 0.0015
cal/cm.sup.2.multidot.sec.multidot..de- gree. C. (0.0062
J/cm.sup.2.multidot.sec.multidot.K), second-stage stretching at a
stretch ratio of 2.5 in the transverse direction at 75.degree. C.
with a heat transmission coefficient of 0.0014
cal/cm.sup.2.multidot.sec.multidot..degree. C. (0.0060
J/cm.sup.2.multidot.sec.multidot.K), and heat treatment under no
elongation at 60.degree. C. for 10 seconds. This gave a
heat-shrinkable polyester film of 45 .mu.m in thickness. The main
shrinkage direction of the film was corresponding to the transverse
direction. The physical properties of the film thus obtained are
shown in Table 1.
1 TABLE 1 Heat shrinkability after treatment in hot Tan .delta.
Temperature water at 80.degree. C. value value maximum for maximum
Main Perp. Rate of Shrinkage Tg at 60.degree. C. at 65.degree. C.
value value of tan .delta. direction direction initial break finish
Example 1 59 0.12 0.26 0.90 76 55.0 -1.0 0 5 Example 2 52 0.17 0.40
0.98 72 57.5 0.0 0 5 Comp. Ex. 1 69 0.02 0.06 0.90 85 47.0 -2.0 0 3
Comp. Ex. 2 62 0.01 0.03 0.37 77 27.0 2.0 20 1 Comp. Ex. 3 59 0.38
0.83 0.85 63 58.5 -1.0 100 4 Comp. Ex. 4 72 0.01 0.02 0.86 95 49.5
12.5 0 2 Example 3 62 0.05 0.25 0.88 95 58.5 -1.5 0 5 Example 4 63
0.05 0.19 0.68 87 42.5 1.5 0 4 Comp. Ex. 5 72 0.01 0.05 0.97 89
60.0 -2.0 0 3 Comp. Ex. 6 89 0.00 0.02 0.38 97 29.0 2.0 0 1 Example
5 62 0.05 0.27 0.89 95 59.5 13.5 0 4 Example 6 58 0.10 0.34 0.92 69
59.5 -1.0 70 4
[0091] The heat-shrinkable polyester films of the present invention
exhibit excellent shrinkage finish over a wide range of temperature
extending from low temperatures to high temperatures, particularly
in the low temperature range, which provides very beautiful
appearance with rare occurrence, if any, of shrinkage spots,
wrinkles, strains, and other defects. The heat-shrinkable polyester
films of the present invention may further have excellent break
resistance. Therefore, they can preferably be used for various
applications including shrinkable labels, cap seals, and
shrink-wrap films.
* * * * *