JP4916105B2 - Method for producing biaxially stretched polyester laminated film - Google Patents

Description

The present invention relates to a biaxially stretched polyester laminated film having excellent oxygen barrier properties and heat resistance, and biodegradability suitable for obtaining a packaging film.

In order to facilitate the disposal of plastic films, biodegradable films have attracted attention, and various films have been developed. The biodegradable film is subject to hydrolysis and biodegradation in soil and water, gradually breaking down and decomposing the film, and finally changing to a harmless degradation product by the action of microorganisms. As such a film, a film formed from an aromatic polyester resin, an aliphatic polyester resin such as polylactic acid or polybutylene succinate, polyvinyl alcohol, cellulose acetate, starch or the like is known.
Among these biodegradable films, several films made of sulfonate group-containing aromatic polyesters have been proposed as films excellent in transparency, heat resistance and the like (for example, Patent Document 1, Patent Document 2 and Patent Document). However, such a film is excellent in heat resistance, but has a limitation in use as a packaging film because it is inferior in heat sealability and difficult to make an automatic bag.
On the other hand, on one side of a base material layer made of a biaxially stretched film made of a polylactic acid polymer,
A biodegradable laminated film in which a heat seal layer made of a composition of an aromatic copolymer polyester or an aliphatic / aromatic copolymer polyester and a polylactic acid polymer is laminated has been proposed (for example, see Patent Document 4). . However, a film obtained from the composition of the aliphatic / aromatic copolymer polyester and the polylactic acid polymer is inferior in transparency, and its use may be limited when used for a packaging film.
In addition, since a film made of an aliphatic polyester resin is inferior in gas barrier properties, a polyglycolic acid film layer biaxially stretched on a thermoplastic resin film such as polylactic acid is laminated as one method for improving oxygen barrier properties. A gas barrier composite film has been proposed (for example, Patent Document 5).

Japanese Patent Publication No. 5-507109 (Claims) Japanese translation of PCT publication No. 6-505040 (Claims) JP 2000-114912 A (Claims) JP 2002-273845 A (Claims) Japanese Patent Laid-Open No. 10-80990 (Claims)

An object of the present invention is to develop a biaxially stretched polyester laminated film suitable for a packaging material having excellent gas barrier properties and heat resistance and biodegradability.

In the present invention, an acid component comprising 10 to 0.2 mol% of an aromatic dicarboxylic acid component and 10 to 0.2 mol% of an aromatic dicarboxylic acid component and 10 to 0.2 mol% of an aromatic dicarboxylic acid component and at least one surface of a polyglycolic acid film is provided. cyclic dihydric de proxy compound laminated sheet sulfonate group-containing aromatic polyester film formed of a component is laminated by co-extrusion molding biaxially oriented, biaxially oriented polyester further characterized in that heat set it A method for producing a laminated film is provided.

The biaxially stretched polyester laminate film of the present invention is a laminate of a biaxially stretched sulfonate group-containing aromatic polyester film and a biaxially stretched polyglycolic acid film, which is excellent in heat resistance and gas barrier properties, as well as transparency and biodegradation. It also has sex.

Hereinafter, the requirements constituting the present invention will be described.
Polyglycolic acid (A)
The polyglycolic acid (A) according to the present invention is a polymer (aliphatic polyester) obtained by polymerizing glycolic acid or a derivative thereof, and has the following formula (1):
_{(-O-CH 2 -CO-) ··········} (1)
The polymer having a repeating unit represented by the formula (1) is generally a repeating unit represented by the formula (1) 6
A polymer containing 0% by weight or more, more preferably 70% by weight or more, and still more preferably 80% by weight or more.
The molecular weight (melt viscosity) of the polyglycolic acid (A) according to the present invention is not particularly limited as long as it has a film-forming ability, but is usually a temperature of (Tm + 20 ° C.) (that is, a temperature corresponding to a normal melt processing temperature). ) And a melt viscosity η * measured at a shear rate of 100 / sec is 50
It is in the range of 0 to 100,000 Pa · s, more preferably 700 to 50,000 Pa · s, and still more preferably 800 to 20,000 Pa · s. A polymer having a melt viscosity η * of less than 500 Pa · s may be drawn down when melt-molded into a film, or a film melt-extruded from a T-die may be deformed during cooling and difficult to melt. Or, the toughness of the obtained film may be insufficient. Melt viscosity η * is 100,000 Pa
-Polymers exceeding s require high temperatures for melt processing, and polyglycolic acid may cause thermal degradation during processing.
The polyglycolic acid (A) according to the present invention is a crystalline polymer and usually has a melting point of 150.
More preferably, it is in the range of 180-225 ° C, more preferably 210-225 ° C. The heat of fusion (ΔHm) is usually 20 J / g or more, more preferably 30 to 75.
J / g or more, more preferably in the range of 40 to 75 J / g. A polymer having a low melting point or ΔHm may have insufficient gas barrier properties, heat resistance, mechanical strength, and the like.

The polyglycolic acid (A) according to the present invention includes, for example, ethylene oxalate (that is, 1,4-dioxane-2,3-dione), lactide, and lactones (for example, β-propiolactone) as a copolymerization component. , Β-butyrolactone, pivalolactone, γ-butyrolactone,
cyclic compounds such as δ-valerolactone, β-methyl-δ-valerolactone, ε-caprolactone, trimethylene carbonate and 1,3-dioxane; lactic acid, 3-hydroxypropanoic acid, 3-hydroxybutanoic acid, 4- Hydroxycarboxylic acids such as hydroxybutanoic acid and 6-hydroxycaproic acid or alkyl esters thereof; aliphatic or alicyclic diols such as ethylene glycol, trimethylene glycol, 1,4-butanediol, 1,4-cyclohexanediol; 1 type, or 2 or more types, such as aliphatic dicarboxylic acid, such as an acid, adipic acid, sebacic acid, or its alkylester, may be included.
By including such a copolymer component as polyglycolic acid, the melting point of the polyglycolic acid homopolymer can be lowered. Since the molding temperature can be lowered by lowering the melting point of polyglycolic acid, thermal decomposition during molding can be reduced. Moreover, the crystallization speed of polyglycolic acid can be controlled by copolymerization to improve extrusion processability and stretch processability.

The polyglycolic acid (A) according to the present invention can be produced by various known methods. Specific polymerization methods are described, for example, in JP-A-10-80990 and JP-A-11-116666.

Sulfonate group-containing aromatic polyester (B)
The sulfonate group-containing aromatic polyester (B) according to the present invention has an aromatic dicarboxylic acid component (b1) of 90 to 99.8 mol%, preferably 92 to 99.5 mol%, and a sulfonate group-containing aromatic dicarboxylic acid component ( b2) 10 to 0.2 mol%, preferably 8 to 0.5 mol%
And an aliphatic or alicyclic dihydroxy compound component (c), preferably an ethylene glycol component (c1) of 50 to 99 mol%, more preferably 60 to 98 mol%, and a polyoxyalkylene glycol component (c2) 50 To 1 mol%, more preferably 40 to 2
It is a polyester comprising an aliphatic or alicyclic dihydroxy compound component (c) consisting of mol%. The number of moles of the acid component comprising the aromatic dicarboxylic acid component (b1) and the sulfonate group-containing aromatic dicarboxylic acid component (b2) is aliphatic or alicyclic dihydroxy compound component (c).
Is substantially equal to the number of moles.
The sulfonate group-containing aromatic polyester (B) according to the present invention further contains a small amount of a component derived from the aliphatic dicarboxylic acid or aliphatic hydroxycarboxylic acid component (b3), preferably in the range of 30 mol% or less. Also good. When aliphatic dicarboxylic acid (b3) is included, aromatic dicarboxylic acid component (b1), sulfonate group-containing aromatic dicarboxylic acid component (b2)
And the number of moles of the acid component composed of the aliphatic dicarboxylic acid and the number of moles of the aliphatic or alicyclic dihydroxy compound component (c).
The weight average molecular weight of the sulfonate group-containing aromatic polyester (B) according to the present invention is preferably in the range of 10,000 to 500,000. The melt flow rate is
Based on ASTM D-1238, the value measured under a load of 2160 g at 220 ° C. is 0.1.
It is preferable that it is -100 (g / 10min). When the molecular weight and the melt flow rate are within the above ranges, the melt viscosity suitable for extrusion molding is exhibited, and sufficient mechanical strength as a laminated film substrate layer is obtained.

Aromatic dicarboxylic acid component (b1)
The aromatic dicarboxylic acid component (b1), which is a component constituting the sulfonate group-containing aromatic polyester (B) according to the present invention, is a compound in the same category as the aromatic dicarboxylic acid component (b). As the aromatic dicarboxylic acid component (b1), the aromatic dicarboxylic acid component (b)
As well as terephthalic acid or dimethyl terephthalate or mixtures thereof are particularly preferred.
Aromatic dicarboxylic acid component in acid component of sulfonate group-containing aromatic polyester (B) (
b1) is in the range of 90 to 99.8 mol%, preferably 92 to 99.5 mol%. Within this range, the greater the amount of the aromatic dicarboxylic acid component (b1), the higher the rigidity of the resulting polyester. .

Sulfonate group-containing aromatic dicarboxylic acid component (b2)
The sulfonate group-containing aromatic dicarboxylic acid component (b2), which is a component constituting the sulfonate group-containing aromatic polyester (B) according to the present invention, specifically includes, for example, aromatic dicarboxylic acids such as phthalic acid and isophthalic acid. A sulfonic acid metal base (—SO ₃ M) is bonded to the benzene ring as a substituent. Examples of the metal (M) include alkali metals such as lithium, sodium, and potassium, or alkaline earth metals such as magnesium and calcium. Preferable examples include a metal salt of 5-sulfo-isophthalic acid, a metal salt of 4-sulfo-isophthalic acid, and a metal salt of 4-sulfo-phthalic acid. This component is added for the purpose of imparting hydrolyzability and biodegradability to the aromatic polyester copolymer. In particular, 5-sulfo-isophthalic acid sodium salt is preferable because of its superior effect. The aromatic dicarboxylic acid may be an alkyl ester, and can be used, for example, in the form of dimethyl-5-sulfoisophthalic acid sodium salt.
The sulfonate group-containing aromatic dicarboxylic acid component (e) in the acid component of the sulfonate group-containing aromatic polyester (B) is in the range of 10 to 0.2 mol%, preferably 8 to 0.5 mol%. The greater the amount of the sulfonate group-containing aromatic dicarboxylic acid component (b2), the higher the hydrolyzability and biodegradability. However, if an excessive amount is used, the resulting polyester may become water-soluble.

Aliphatic or alicyclic dihydroxy compound component (c)
The aliphatic or alicyclic dihydroxy compound component (c), which is a component constituting the sulfonate group-containing aromatic polyester (B) according to the present invention, is not particularly limited, but is usually an aliphatic dihydroxy compound component, If it is a branched or linear dihydroxy compound having 2 to 12 carbon atoms, preferably 4 to 6 carbon atoms, or an alicyclic dihydroxy compound component, a cyclic compound having 5 to 10 carbon atoms is obtained. Can be mentioned.
Specific examples of the aliphatic or alicyclic dihydroxy compound component (c) include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, and 1,4-butane. Diol, 1,5-pentanediol, 2,4-dimethyl-
2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-
1,3-propanediol, 2,2,4-trimethyl-1,6-hexanediol, especially ethylene glycol, 1,3-propanediol, 1,4-butanediol and 2
, 2-dimethyl-1,3-propanediol (neopentyl glycol); cyclopentanediol, 1,4-cyclohexanediol, 1,2-cyclohexanedimethanol,
1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol and 2,2
, 4,4-tetramethyl-1,3-cyclobutanediol and diethylene glycol,
Examples thereof include polyoxyalkylene glycols such as triethylene glycol and polyoxyethylene glycol, and polytetrahydrofuran, and particularly include diethylene glycol, triethylene glycol and polyoxyethylene glycol or a mixture thereof or a compound having a different number of ether units. It is done. The aliphatic or cycloaliphatic dihydroxy compound component can also be a mixture of different aliphatic or cycloaliphatic dihydroxy compounds.
Among these aliphatic or alicyclic dihydroxy compound components (c), the polyoxyalkylene glycol component (c2) has an effect of appropriately adjusting the mechanical properties, hydrolyzability or biodegradability of the resulting polyester (B). Therefore, the ethylene glycol component (c1) is 50 to 99 mol%, more preferably 60 to 98 mol%, and the polyoxyalkylene glycol component (c2), preferably the diethylene glycol component is 50 to 1 mol%, Is preferably a combination of an aliphatic or alicyclic dihydroxy compound component (c) comprising 40 to 2 mol%.
If the amount of the polyoxyalkylene glycol component (c2) is less than 1 mol%, the hydrolyzability or biodegradability may be insufficient, and if it exceeds 50 mol%, the mechanical properties of the film may be adversely affected. .

Aliphatic dicarboxylic acid or aliphatic hydroxycarboxylic acid component (b3)
The aliphatic dicarboxylic acid or aliphatic hydroxycarboxylic acid component (b3) that may be contained in the component constituting the sulfonate group-containing aromatic polyester (B) according to the present invention is specifically azelaic acid or succinic acid. Examples thereof include aliphatic dicarboxylic acids such as adipic acid, sebacic acid and glutaric acid, and aliphatic hydroxycarboxylic acids such as glycolic acid, hydroxyacetic acid, lactic acid and caprolactone. The aliphatic dicarboxylic acid or aliphatic hydroxycarboxylic acid (b3) lowers the glass transition temperature of the sulfonate group-containing aromatic polyester (B), preferably lower than 70 ° C. For the purpose of improving the decomposability, it is added as one of the copolymerization components.

The production method of the sulfonate group-containing aromatic polyester (B) according to the present invention is known prior to the filing of the present application. For example, JP-A-5-507109, JP-A-6-505040, JP-A-6 -505513 and the like.

Biaxially stretched polyester laminate film The biaxially stretched polyester laminate film of the present invention comprises 90 to 99.8 mol% of the aromatic dicarboxylic acid component and a sulfonate group-containing fragrance on at least one side of the biaxially stretched film made of polyglycolic acid. A biaxially stretched film composed of an sulfonic acid group-containing aromatic polyester composed of an acid component composed of 10 to 0.2 mol% of an aliphatic dicarboxylic acid component and an aliphatic or alicyclic dihydroxy compound component is laminated.
The thickness of the biaxially stretched polyester multilayer film of the present invention is appropriately determined according to the use and is not particularly limited. Usually, the biaxially stretched polyglycolic acid film is 2 to 20 μm.
m, preferably 5 to 15 μm, and the biaxially stretched sulfonate group-containing aromatic polyester film is in the range of 5 to 15 μm, preferably 10 to 30 μm.

The biaxially stretched polyester multilayer film of the present invention may be subjected to surface treatment such as corona treatment or flame treatment on one or both sides as necessary. In addition, the biaxially stretched polyester multilayer film of the present invention may further comprise a random copolymer of high-pressure low-density polyethylene, linear low-density polyethylene, crystalline or low-crystalline ethylene, and an α-olefin having 3 to 10 carbon atoms, depending on the application. A polymer or a random copolymer of propylene and ethylene or an α-olefin having 4 or more carbon atoms, a polybutene, an ethylene / vinyl acetate copolymer, an aliphatic polyester, or a low melting point polymer alone or a laminate thereof. May be. Furthermore, in order to improve the gas barrier properties, an ethylene / vinyl alcohol copolymer, polyamide, polyester, vinylidene chloride polymer, etc. may be laminated by extrusion coating, film lamination,
A metal or an oxide thereof, silica, or the like may be deposited. Of course, in order to increase the adhesion to other substances, the surface of the biaxially stretched multilayer film may be anchored with an adhesive such as imine or urethane, or maleic anhydride-modified polyolefin may be laminated.

The biaxially stretched polyester laminated film of the present invention is produced by biaxially stretching a laminated sheet obtained by coextrusion molding of the polyglycolic acid and sulfonate group-containing aromatic polyester film .
Biaxially oriented polyester laminated film, Ju - blur method, may adopt any of the tenter method, a tenter method is preferred. Biaxial stretching may be simultaneous or sequential. The stretching conditions vary depending on the thermal history of the (unstretched) multilayer sheet, but are performed, for example, at a stretching temperature of 40 to 90 ° C. and a stretching ratio of 1.5 to 6.0 times. After stretching, the obtained biaxially stretched film is heat-treated (heat set). The temperature of the heat treatment is carried out in a temperature range from the crystallization temperature of the polyester to the melting point. If the heat treatment temperature is high, the thermal stability is good, and conversely if the temperature is low, the thermal stability is poor. In other words, it is possible to change the heat shrinkage rate by changing the heat treatment temperature, and the temperature condition can be selected according to the application.
The biaxially stretched polyester laminated film of the present invention is a laminate sheet obtained by co-extrusion molding of the polyglycolic acid and the sulfonate group-containing aromatic polyester, using a stretching roll , 55 to 70 ° C, preferably 55 to 70 ° C. The film is stretched at least 1.5 times, preferably 2 to 4 times in the machine direction at a temperature of 65 ° C., and then at least 1.2 in the transverse direction at a temperature of 60 to 90 ° C., preferably 70 to 80 ° C., using a tenter. It can be produced by stretching 5 times, preferably 2 to 10 times.
When the laminate sheet obtained by coextrusion molding is stretched in the longitudinal direction at a stretching temperature of less than 55 ° C., the multilayer sheet is not stretched uniformly, and the polyglycolic acid layer is cracked when stretched in the transverse direction. There is a possibility that a biaxially stretched polyester laminated film cannot be obtained. The reason is that the laminated sheet can be uniformly stretched even if the stretching temperature in the longitudinal direction exceeds 70 ° C., but the crystallization of polyglycolic acid proceeds when the laminated film stretched beyond 70 degrees is stretched in the transverse direction. This is because the crystal cannot be stretched by lateral stretching, the polyglycolic acid layer is broken, and uniform lateral stretching cannot be performed.
Moreover, when extending | stretching the lamination sheet obtained by coextrusion shaping | molding to a horizontal direction, you may heat the extending | stretching point of a lamination sheet. In order to heat the stretching point, auxiliary heating means such as resistance heating, hot air heating, induction heating, infrared heating or the like can be used. As these auxiliary heating means, infrared rays, particularly far infrared rays, are most preferable because they can efficiently and instantaneously heat a polymer compound such as sulfonate group-containing aromatic polyester and polyglycolic acid according to the present invention.
If the longitudinal stretching temperature of the laminated sheet is less than 60 ° C., the film may be broken or the sheet may be detached from the tenter chuck, and stretching may not be performed. On the other hand, when the temperature exceeds 90 ° C., crystallization of the sulfonate group-containing aromatic polyester and polyglycolic acid proceeds and uniform lateral stretching may not be performed.
<Example>

EXAMPLES Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples unless it exceeds the gist.

The physical property values and the like in Examples and Comparative Examples were obtained by the following evaluation methods.
<Evaluation method>
(1) Tensile properties:
Strip specimens (length: 150 mm, width: 15 mm) in the MD direction and TD direction were taken from the biaxially stretched polyester laminated film, and a tensile tester (Tensilon Universal Tester RTC- manufactured by Orientec Co., Ltd.) with a distance between chucks of 100 mm. 1225) to conduct a tensile test,
The strength (stress) (MPa), elongation (%), and Young's modulus (MPa) at the breaking point were determined.
Elongation (%) was defined as a change in the distance between chucks.
(2) Oxygen permeability:
Using OX-TRAN 2/20 manufactured by Mocon, temperature 20 according to JIS K 7126
The measurement was performed under the conditions of ° C. and humidity 80% RH.
(3) Heat shrinkage rate:
A sample having a length of 120 mm and a width of 15 mm was cut out from the biaxially stretched polyester laminated film, and marked lines were drawn at intervals of 100 mm. Next, the laminated film is subjected to a predetermined temperature (8
After being left in an oven set at 0 ° C., 100 ° C., and 120 ° C. for 15 minutes, it was taken out and left at room temperature for 15 minutes or longer, and the length between marked lines (L: mm) was measured. [(100-L) /
The value of 100] × 100 (%) was defined as the heat shrinkage rate (%).
(4) Melting characteristics:
Based on JIS K7121 and K7122, it calculated | required on condition of the following using DSC (differential scanning calorimeter).
About 5 mg of a sample is precisely weighed and packed in an aluminum pan. As DSC, Q100 manufactured by TA Instruments Co., Ltd. is used, and 250 ° C. at a rate of 20 ° C. to 10 ° C./min in a nitrogen atmosphere of 50 ml / min.
The temperature is raised to 50 ° C. and once melted, then maintained at 250 ° C. for 10 minutes and 20 ° C. at a rate of 10 ° C./min
The solution was cooled down to crystallization, maintained at 10 ° C. for 5 minutes, and then again at a rate of 10 ° C./min.
The temperature was raised to 0 ° C. to obtain a thermal melting curve, and the heat of fusion and melting point (endothermic peak temperature) of the sample were determined from the obtained thermal melting curve.

The polymers used in Examples and Comparative Examples are shown below.
(1) Polyglycolic acid (PGA):
Melt viscosity η *: 1,000 to 1,500 Pa · sec, melting point: 220 ° C., heat of fusion: 6
4 J / g.
(2) Sulfonate group-containing aromatic polyester (B) (SAE):
45 mol% terephthalic acid, 37 mol% ethylene glycol, 9 mol% diethylene glycol, 1 mol% sodium 5-sulfo-isophthalate, 8 mol% hydroxyacetic acid, density:
1.35 g / cm ³ , melting point (Tm): 200 ° C., MFR (220 ° C., 2160 g load):
15 g / 10 minutes.

Example 1
<Manufacture of laminated sheets>
Using a 40mmφ three-type three-layer single-screw extruder equipped with a T-die at the tip, SAE / PGA /
SAE was extruded at a thickness ratio of 70/70/70 / and quenched with a casting roll at 30 ° C. to obtain a laminated sheet having a thickness of 210 μm and a three-layer structure.
The extrusion temperatures of PLA and PGA were 230 ° C. and 270 ° C., respectively.
<Manufacture of biaxially stretched film>
The surface of the obtained laminated sheet is made into a pantograph-type batch biaxial stretching device (manufactured by Bruckner)
KARO-IV type) was preheated with hot air at 70 ° C. for 5 seconds, and then stretched 3.0 times (simultaneous biaxial stretching) in the longitudinal and transverse directions at a speed of 8 m / min. Further, after stretching, the film was heat-set in an atmosphere of 120 ° C. for 10 seconds, and then the stretched film was immediately cooled with a fan to obtain a biaxially stretched laminated film having a three-layer structure having a thickness of 18 μm.
Table 1 shows the physical properties of the obtained biaxially stretched polyester laminated film.

Comparative Example 1
Example 1 except that when the laminated sheet obtained in Example 1 is biaxially stretched, it is preheated at 100 ° C.
However, when the biaxial stretching was performed, the laminated sheet was cracked and biaxial stretching could not be performed.

Reference example 1
<Manufacture of sheets>
Using a 40 mmφ single screw extruder equipped with a T-die at the tip, SAE was extruded and quenched with a casting roll at 30 ° C. to obtain a sheet having a thickness of 210 μm.
The extrusion temperature of SAE was 230 ° C.
<Manufacture of biaxially stretched film>
The obtained sheet was guided to a roll heated to 60 ° C. and roll-stretched three times in the longitudinal direction. An infrared heater (manufactured by WC Heraeus: 2400W / 200V) between the rolls
) Was used to heat the stretching point. This longitudinally stretched sheet was stretched 3.5 times at 80 ° C. in the transverse direction by a tenter-type lateral stretching apparatus, heat-fixed at 160 ° C. for 3 seconds, and wound to obtain a biaxially stretched film.
Table 1 shows the physical properties of the obtained biaxially stretched film.

The biaxially stretched polyester laminated film of the present invention is excellent in transparency from the visibility and aesthetics of the contents required when used for packaging foods, pharmaceuticals, etc., and prevents oxidation of the contents. Excellent gas barrier properties.
Moreover, since the biaxially stretched polyester laminated film of the present invention has excellent biodegradability, its shape can be easily disintegrated after use and can contribute to the protection of the natural environment.
Furthermore, since it has a high melting point as a biodegradable resin, it is also effective as a barrier film having excellent heat resistance.

Claims (1)

On at least one surface of the polyglycolic acid film, the acid component an aromatic dicarboxylic acid component 90 to 99.8 mol% and from 10 to 0.2 mole% sulfonate group-containing aromatic dicarboxylic acid component and an aliphatic or cycloaliphatic dihydric de A biaxially stretched polyester resin obtained by stretching a laminated sheet obtained by coextrusion molding of a sulfonate group-containing aromatic polyester film comprising a roxy compound component in the longitudinal and transverse directions at a stretching temperature of 40 to 90 ° C. A method for producing a biaxially stretched polyester laminated film, wherein the film is heat-treated (heat set) in a temperature range from the crystallization temperature to the melting point of the aromatic polyester.