JPH04358817A - Transparent polyester film, sheet and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a polyester film or sheet with high crystallinity and excellent transparency and a method for manufacture thereof by a method wherein a film or a sheet melt-molded by using a specified resin is quenched to manufacture a transparent film or sheet with a low degree of crystallinity, which is then aged and heat-treated. CONSTITUTION:A film or sheet prepd. by melt-molding of a copolyester resin wherein 2-25mol% of the repeating units are ester units consisting of terephthalic acid and an alkylene oxide adduct of bisphenol and remaining repeating units are mainly ester units consisting of terephthalic acid and 1,4-butanediol, is quenched to prepare a film or sheet with a low degree of crystallinity, which is then aged at a temp. of Ta<=Tcc and is heat-treated at a temp. in the range of Tg<=Tb<=Ta-2 and Tb>Ta, wherein Ta is an aging temp. ( deg.C); Tcc is a cold crystallization temp. ( deg.C) of the resin measured by means of JIS K7121 differential scanning calorimetry at a rate of temp. rise of 10 deg.C/min; Tg is a glass transition temp. of the resin measured by the same means; Ta is a m.p. of the resin measured by the same means.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to polyester films and sheets with high crystallinity and excellent transparency, and a method for producing the same.

0002

[Prior art and problems to be solved by the invention] Aromatic polyesters represented by polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) have excellent heat resistance, and a good balance of mechanical strength, gas permeability, etc. Due to its excellent physical properties, it is used as an engineering plastic in a wide range of fields. Among these polyesters, PBT exhibits high crystallinity and can be said to exhibit the above-mentioned excellent properties, but on the other hand, due to its high crystallinity, it cannot be used in applications that require transparency. Ta. In addition, since the crystallization rate of PET is low, it is relatively easy to obtain a transparent film with low crystallinity by rapidly cooling the molten polymer. is insufficient, so it is necessary to proceed with crystallization by annealing. However, when the annealing temperature is raised to increase the crystallization efficiency, it becomes cloudy, and when an attempt is made to increase the crystallization efficiency by adding an additive such as a nucleating agent, transparency is lost due to the additive itself. Furthermore, although it is possible to obtain a highly crystalline polymer by gradually cooling the polymer from the molten state, it is still impossible to obtain a highly transparent product due to the scattering of visible light by the spherulites. An object of the present invention is to solve these problems and provide a polyester film and sheet having excellent performance, and a method for producing the same.

0003

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have made extensive studies and found that an aromatic copolyester into which a specific amount of a specific comonomer unit is introduced is used as a raw material polyester, and It has been discovered that by treating the films and sheets obtained from this under specific conditions, it is possible to provide films and sheets that have high crystallinity and do not whiten or lose transparency even in a heated atmosphere, and have developed the present invention. It has been completed. That is, in the present invention, 2 to 25 mol% of the repeating units consist of ester units of terephthalic acid or its ester-forming derivative and an alkylene oxide adduct of bisphenols and/or an alkylene oxide adduct of dihydroxynaphthalenes, and the remaining A film or sheet with low crystallinity is prepared by rapidly cooling a film or sheet obtained by melt-molding a copolyester resin whose repeating units are mainly composed of ester units of terephthalic acid or its ester-forming derivative and 1,4-butanediol. After aging at a temperature selected to satisfy the following formula (1), the following formula (2) and the following formula (3
) A method for manufacturing a transparent polyester film or sheet, characterized by heat treatment at a temperature within the range of 120°C, and a relative crystallinity of 50% or less obtained by the manufacturing method.
This invention relates to transparent polyester films and sheets that maintain a light transmittance of 75% or more even after heat treatment. Formula (1) Ta≦Tcc
(℃) Formula (2) Tg≦Tb≦Tm-2 (
°C) Formula (3) Tb>Ta
(℃) (However, Ta: Aging temperature (℃) Tb: Heat treatment temperature (℃) Tcc: Cold crystallization temperature of the resin (℃) measured by differential thermal analysis method based on JIS K7121 at a heating rate of 10℃/min Tg: Glass transition temperature of resin measured by differential thermal analysis method based on JIS K7121 at a heating rate of 10°C/min (°C) Tm: JIS K712
A heating rate of 10°C/mi was determined by differential thermal analysis based on
Melting point of the resin measured at n (℃) and derivatives thereof, examples of which include one or more selected from dialkyl esters and diacylated products. Among these, preferred are terephthalic acid and dialkyl esters thereof, and particularly preferred is dimethyl terephthalate. The oxyalkyleneoxy group forming the butylene terephthalate unit of the copolymerized polyester constituting the present invention is introduced by using 1,4-butanediol as a monomer raw material. The present invention is characterized in that, in addition to these, specific amounts of alkylene oxide adducts of bisphenols and/or alkylene oxide adducts of dihydroxynaphthalenes are used as raw material compounds for forming the copolymerized polyester.

Examples of preferable alkylene oxide adducts of bisphenols include the following general formulas (I) to (II).
) are listed.

[0005]

[Chemical formula 1]

In the formula, R is mainly -CH2CH2-, -C
H2CH2OCH2CH2-, -CH(CH3)CH
A group selected from 2-, -CH2CH(CH3)-, which may be the same or different.

[0007]

[Chemical 2]

In the formula, Y1 to Y8 are groups mainly selected from hydrogen, alkyl groups, alkoxy groups, chlorine, and bromine, and each may be the same or different, but hydrogen is particularly preferred. Specific examples of raw material compounds include those obtained by adding alkylene oxide to the bisphenols shown below. Namely, bis(4-hydroxyphenyl)sulfone, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)ketone, bis(4-hydroxyphenyl)
hydroxyphenyl) sulfide, bis(4-hydroxyphenyl) sulfoxide, bis(4-hydroxyphenyl) ether, bis(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)cyclohexane,
Bis(4-hydroxyphenyl)cycloalkane, bis(4-hydroxyphenyl)alkane, 2,2-bis(
3,4'-hydroxyphenyl)propane, 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 4,4'-diphenol, 4,4'-[1,4-phenylenebis(1-methylethylidene) )] Bisphenol, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane, 2,2-bis(3,5-
dibromo-4-hydroxyphenyl)propane, bis(
3,5-dimethyl-4-hydroxyphenyl) sulfone, bis(3,5-dibromo-4-hydroxyphenyl)
Examples include alkylene oxide adducts such as sulfone. Among them, particularly preferred from the viewpoint of polymer preparation are a 2 mole ethylene oxide adduct of bis(4-hydroxyphenyl)sulfone, a 2 mole propylene oxide adduct of bis(4-hydroxyphenyl)sulfone, and
, 2-mol ethylene oxide adduct of 2-bis(4-hydroxyphenyl)propane, 2 mol propylene oxide adduct of 2,2-bis(4-hydroxyphenyl)propane, 2 mol ethylene oxide adduct of bis(4-hydroxyphenyl)sulfoxide adduct, 2 moles of propylene oxide adduct of bis(4-hydroxyphenyl) sulfoxide, 2 moles of ethylene oxide adduct of bis(4-hydroxyphenyl) sulfide, 2 moles of propylene oxide adduct of bis(4-hydroxyphenyl) sulfide, 2 moles of ethylene oxide adduct of bis(4-hydroxyphenyl)ether, 2 moles of propylene oxide adduct of bis(4-hydroxyphenyl)ether, bis(
4-hydroxyphenyl)methane ethylene oxide 2
molar adduct, 2 molar propylene oxide adduct of bis(4-hydroxyphenyl)methane, 2 molar ethylene oxide adduct of 4,4'-diphenol, 2 molar propylene oxide adduct of 4,4'-diphenol, bis (4
-hydroxyphenyl)alkane ethylene oxide 2
molar adduct, 2 molar propylene oxide adduct of bis(4-hydroxyphenyl)alkane, 2 molar ethylene oxide adduct of bis(4-hydroxyphenyl)ketone, 2 molar propylene oxide adduct of bis(4-hydroxyphenyl)ketone etc.

Examples of preferable alkylene oxide adducts of dihydrodinaphthalenes include the following general formula (III
).

0010

[C3]

In the formula, R is mainly -CH2CH2-, -C
H2CH2OCH2CH2-, -CH(CH3)CH
2-, CH2CH(CH3)-, and each group may be the same or different. In the formula, Y1 to Y8
is a group mainly selected from hydrogen, an alkyl group, an alkoxy group, chlorine, and bromine, and each may be the same or different, but hydrogen is particularly preferred. Examples of specific raw material compounds include 2 moles of ethylene oxide adduct of 2,6-dihydroxynaphthalene, 2 moles of propylene oxide adduct of 2,6-dihydroxynaphthalene, 4 moles of ethylene oxide adduct of 2,6-dihydroxynaphthalene, 2,6
- 2 moles of ethylene oxide adduct of dihydroxy-3-bromonaphthalene, 2 moles of ethylene oxide adduct of 2,6-dihydroxy-3-methylnaphthalene, 2,7-
Examples include 2 moles of ethylene oxide adduct of dihydroxynaphthalene, 2 moles of ethylene oxide adduct of 2,7-dihydroxy-3-chloronaphthalene, etc. Among them, 2 moles of ethylene oxide adduct of 2,6-dihydroxynaphthalene is particularly useful for the preparation of polymers. preferred in terms of

[0012] The value of the molar fraction ratio of the alkylene oxide adduct residue of bisphenols or the alkylene oxide adduct residue of dihydroxynaphthalenes to the total structural unit is as follows: It is necessary that each of the oxide adduct residues alone and both together accounts for 2 to 25 mol %, particularly preferably 5 to 20 mol %, based on the total constituent alcohol residue components. If the above value is less than 2 mol%, the crystallization rate is too high, making it difficult to prepare a film or sheet with low crystallinity and excellent transparency, especially a sheet with a wall thickness of 1.0 mm or more. When preparing , it is preferably 5 mol % or more. On the other hand, if it is more than 25 mol %, the physical properties such as mechanical strength and gas permeability, and heat resistance of the film or sheet will decrease.

In addition, in producing the copolyester, small amounts of other components may be used in addition to the above-mentioned essential starting materials depending on the purpose. Examples of other ingredients used here include isophthalic acid, orthophthalic acid,
Aromatic dicarboxylic acids such as diphenic acid and 2,6-naphthalene dicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid and sebacic acid, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, and dialkyl esters or diacyls thereof. compound, ethylene glycol, diethylene glycol, triethylene glycol, 1,3-propanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,10-
Aliphatic diols such as decanediol, alicyclic diols such as 1,4-cyclohexanediol and 1,4-cyclohexanedimethanol, trimethyl or higher functional diols such as trimethyl trimesate, trimellitate, trimethylolpropane, and pentaerythritol. polyfunctional compounds,
Examples include monofunctional compounds such as stearyl alcohol and o-benzoylbenzoic acid, hydroxycarboxylic acid derivatives such as methyl p-hydroxyethoxyphenylcarboxylate, and polyalkylene glycols such as polybutylene glycol. It is possible to use.

These copolyesters can be produced by interfacial polycondensation, melt polymerization, solution polymerization, etc. using conventionally known condensation reactions and transesterification reactions. Further, by using a solid phase polymerization method in which the obtained resin is heat-treated under reduced pressure or in the presence of an inert gas, it is possible to obtain a product with an even higher degree of polymerization.

Methods for forming films and sheets from molten resin include the T-die method and the inflation method, but the T-die method is preferred. In terms of moldability, the intrinsic viscosity of copolyester resin is 0.7.
The above is desirable. Note that the intrinsic viscosity is a value measured in orthochlorophenol at 25°C. Further, in the method of the present invention, after extrusion, it is necessary to rapidly cool the product to once form a low crystallization film or sheet.

Regarding the preferable thickness of the film or sheet, if the film or sheet is thick, only the surface portion becomes transparent by rapid cooling, while the inside becomes slowly cooled and tends to whiten easily. If the thickness of the film or sheet is too thin, the mechanical strength of the film or sheet itself will not be exhibited and the excellent practical effects will be diminished. Therefore, if the preferred thickness range of the film or sheet is specified numerically, it is 0.01 to 2.5 mm, more preferably 0.02 to 1 mm. Alternatively, after producing a film or sheet by rapid cooling, uniaxial or biaxial stretching may be performed to obtain a predetermined thickness.

In the present invention, after the film or sheet is prepared, it is aged at a temperature selected to satisfy the following formula (1). This is a film placed in a heating medium at a predetermined temperature, such as hot water.
This can be done by dipping the sheet, placing it in a dryer at a predetermined temperature, blowing warm air on it, or using radiant heat such as infrared rays. Formula (1) Ta≦Tcc (℃) (However, T
a: Aging temperature Tcc: Resin cold crystallization temperature (°C) measured at a heating rate of 10°C/min by differential thermal analysis method based on JIS K7121) If the aging temperature is higher than Tcc (°C), a sudden Crystallization causes films and sheets to become cloudy, which is undesirable. The time required for aging depends on the content of alkylene oxide adduct residues of bisphenols and/or alkylene oxide adduct residues of dihydroxynaphthalenes in the copolymerized polyester resin, the aging temperature, and the thickness of the film or sheet. It depends on the situation. The higher the content, the lower the aging temperature, or the thicker the film or sheet, the longer the aging time is required. In addition, if the ripening temperature is low, a very long time will be required for ripening, which is unfavorable in terms of productivity, etc., and therefore the ripening temperature is preferably 30° C. or higher. After the film or sheet is aged at a temperature selected to satisfy the above formula (1), it is further processed according to the following formula (2) and the following formula (3).
) is heat treated at a temperature within the range of Equation (2) Tg≦Tb≦Tm-2 (℃) Equation (3) Tb>Ta (
(℃) (However, Tb: Heat treatment temperature (℃) Tg: Glass transition temperature of resin measured at a heating rate of 10℃/min by differential thermal analysis method based on JIS K7121 (℃) Tm: Differential thermal analysis based on JIS K7121 The particularly preferable temperature range is Tg+5
≦Tb≦Tm-10 (°C). If Tb is lower than the glass transition temperature, the time required for heat treatment will be too long, which is undesirable.On the other hand, if it is set to a high temperature, the treatment time can be shortened, but if Tb is too close to the melting point, partial melting may occur due to uneven heat treatment. Yes, it's not good. By performing such treatment, the film or sheet has high crystallinity while maintaining transparency, and thereafter, this transparency and high crystallinity are stably maintained. If the preferable relative crystallinity (CR) of the transparent highly crystalline film or sheet after heat treatment is specified, the CR is 50% or more. That is, if the relative crystallinity of the film or sheet is less than 50%, the advantages based on crystallinity such as gas permeability, heat resistance, and surface hardness will not be exhibited. This is desirable from a practical standpoint, and the present invention has made it possible to provide such films and sheets. Note that the relative crystallinity is a value determined by the DSC measurement method described later.

Furthermore, since the transparency of a film or sheet largely depends on the smoothness of its surface, if high transparency is required, it is desirable that the press plate or cooling roller be as smooth as possible. However, the film
If a heavy feeling is desired for the sheet, the surface of the film or sheet may be made uneven to reduce transparency. Even films and sheets that have undergone these treatments will lose their advantages based on transparency when their light transmittance drops below 75%. Therefore, if we define a preferable range for transparent films and sheets based on their transparency, the light transmittance will be 75%. % or more and which maintains a light transmittance of 75% or more even after heat treatment at 120° C. is practically desirable, and the present invention has made it possible to provide such films and sheets. It should be noted that other thermoplastic resins and inorganic fillers may be supplementarily added to the composition of the present invention depending on the purpose as long as the effects of the present invention are not impaired.

[0019]

Effects of the Invention As described above, the highly crystalline transparent film or sheet made of the specific resin obtained by the present invention has
A transparent film or sheet with a low crystallinity is prepared by rapidly cooling a melt-molded film or sheet, and this is heat-treated after aging to increase the crystallinity without impairing transparency. Has excellent effects. 1) The resin has high heat resistance, maintaining a light transmittance of 75% or more without decreasing transparency even under high temperature conditions of around 120 °C, and is highly crystalline, so it has excellent gas barrier properties and heat shrinkage resistance. Suitable for food packaging for microwave cooking, etc. 2) Since transparency is imparted without impairing mechanical properties, it can also be used for window glass protection films, etc.

[0020]

[Examples] The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited thereto. The conditions for measuring the main characteristic values are as follows. (1) Melting point, cold crystallization temperature, glass transition temperature JIS
It was measured by differential thermal analysis (DSC) based on K7121 at a heating temperature of 10°C/min. (2) Relative crystallinity The film or sheet was cut into a sample for DSC measurement, and the measurement was conducted based on differential thermal analysis. The relative crystallinity is calculated using the following formula. CR=[(ΔHm-|ΔHcc|)/|(ΔHc)HO
MO｜]×100(%) (However, ΔHm; 10℃/mi
Heat of crystal fusion ΔHcc measured by increasing temperature at n; 10°C/mi
The heat of transition (ΔHc) of the cold crystallization peak measured by increasing the temperature at n; To find the original relative crystallinity of the sample, since the crystals melt after progressing,
From the heat of crystal fusion (ΔHm) to the heat of transition at the cold crystallization peak (Δ
The absolute value of Hcc) is subtracted. (3) Cut out the light transmittance film or sheet and JIS K7105
Measurements were made based on. (4) Cut out the oxygen permeability film or sheet and test it according to JIS K7126.
Measurements were made based on. (5) Light transmittance and relative crystallinity after heating After cutting out the film or sheet and placing it in a blow dryer at a temperature of 120 °C for 10 minutes, the light transmittance was determined based on JIS K7105 and the above (2). Relative crystallinity was determined from the conditions.

Production Example 1 (Synthesis of Polyester A) Addition of 229.2 parts by weight of dimethyl terephthalate, 196.8 parts by weight of 1,4-butanediol, and 2 moles of ethylene oxide to 2,2-bis(4-hydroxyphenyl)propane. A reactor equipped with a stirrer and a distillation tube was charged with 56.0 parts by weight of tetrabutyl titanate, and after being sufficiently purged with nitrogen, the temperature was raised to 160 °C under normal pressure.
Stirring was started. Furthermore, the temperature was gradually raised to distill off methanol as a by-product. When the temperature reached 240 °C, the pressure inside the reactor was gradually reduced to 0.1 torr.
Stirring was continued for 3.0 hours at a pressure of 3.0 hours to obtain a copolyester resin with an intrinsic viscosity of 0.79. The introduction rate of the 2 mole ethylene oxide adduct residue of 2,2-bis(4-hydroxyphenyl)propane was determined from 1H-NMR measurement using trifluoroacetic acid-d1 as a solvent. Subsequently, the polyester resin was pelletized and subjected to solid phase polymerization under a nitrogen stream to obtain a highly polymerized polyester having an intrinsic viscosity of 1.24. The properties of the obtained polyester were evaluated as described above. The results are shown in Table 1.

Production Examples 2 to 8 (Synthesis of Polyesters B to H) Instead of the 2 mole ethylene oxide adduct of 2,2-bis(4-hydroxyphenyl)propane, the monomers shown in Table 1 were added in the prescribed amounts. Polymerization was carried out in the same manner as in Production Example 1 except that various copolymerized polyester resins were obtained. The obtained polyester was subjected to solid phase polymerization in the same manner as in Production Example 1, and then its characteristics were evaluated. Table 1 shows the results.
Shown below.

Comparative Production Example 1 (Synthesis of Polyester I) 1
, 4-butanediol and 2,2-bis(4-hydroxyphenyl)propane, melt polymerization and solid phase polymerization were carried out in the same manner as in Production Example 1, except that the added amounts of the 2-mol ethylene oxide adduct of 2,2-bis(4-hydroxyphenyl)propane were changed to the values shown in Table 1. A copolymerized polyester was obtained. The properties of the obtained polyester were evaluated in the same manner as in Production Example 1. The results are shown in Table 1.

Comparative production example 2 (synthesis of polyester J) 1
, 4-butanediol and bis(4-hydroxyphenyl)sulfone, except that the added amount of the 2-mol ethylene oxide adduct was changed to the value shown in Table 1. Melt polymerization and solid phase polymerization were carried out in the same manner as in Production Example 2, and copolymerization was performed. Polyester was obtained. The properties of the obtained polyester were evaluated in the same manner as in Production Example 2. The results are shown in Table 1.

Comparative Production Example 3 (Synthesis of Polyester K) Dimethyl terephthalate and 1,4-butanediol are shown in Table 1.
Polybutylene terephthalate resin (PBT) was obtained by melt polymerization and solid phase polymerization at the monomer ratio shown in . The properties of the obtained polyester were evaluated in the same manner as in Production Example 1. The results are shown in Table 1.

Examples 1 to 8, Comparative Examples 1 to 3 In order to clarify the differences in the properties of films and sheets due to differences in raw material polyester, conditions for rapid cooling from the molten state, thickness of the film and sheet, aging conditions, and heat treatment conditions were determined. Polyesters A to K were evaluated while keeping the value constant. That is, a molten polymer at 240°C was extruded from a T-die die onto a cooling roll at 25°C, and the extrusion speed was adjusted so that the thickness of the film or sheet was 0.10 mm.
The quenching conditions for the sheet, film, and sheet thickness were all the same. These films and sheets were aged in a constant temperature bath at various temperatures and times shown in Table 2, and then heat-treated by placing them in a dryer at 100° C. for 10 minutes. Table 2 shows the evaluation results for the transparency, crystallinity, and gas permeation resistance of these films and sheets.

Examples 9 and 10, Comparative Examples 4 and 5 Films and sheets were produced and evaluated in the same manner as in Example 1, except that the aging temperature was changed. The results are shown in Table 3.

Examples 11 and 12, Comparative Example 6 Films and sheets were produced and evaluated in the same manner as in Example 1, except that the heat treatment temperature was changed. The results are shown in Table 4.

Examples 13 and 14 Films and sheets were produced and evaluated in the same manner as in Example 1, except that the thickness of the films and sheets was changed. The results are shown in Table 5.

[0030]

[Table 1]

*1 Dimethyl terephthalate *2 1
,4-butanediol*3 2-mole ethylene oxide adduct of 2,2-bis(4-hydroxyphenyl)propane*4 2-mole ethylene oxide adduct of bis(4-hydroxyphenyl)sulfone*5 2,2-bis(4 -hydroxyphenyl)propane with 4 moles of ethylene oxide adduct*6 Bis(4-hydroxyphenyl)sulfone with 2 moles of propylene oxide adduct*7 4,4'-[1,4-phenylenebis(1-methylethylidene)]bisphenol 2 mole ethylene oxide adduct of 2,6-dihydroxynaphthalene *9 Not observed.

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

Claims (4)

[Claims] Claim 1: 2 to 25 mol% of the repeating units consist of ester units of terephthalic acid or its ester-forming derivative and an alkylene oxide adduct of bisphenols and/or an alkylene oxide adduct of dihydroxynaphthalenes, and the remaining The repeating unit is mainly terephthalic acid or its ester-forming derivative and 1,4
- Prepare a film or sheet with a low degree of crystallinity by rapidly cooling a film or sheet obtained by melt-molding a copolyester resin consisting of ester units with butanediol, and at a temperature selected so that the following formula (1) is obtained. A method for producing a transparent polyester film or sheet, which comprises further heat-treating at a temperature within the range of the following formula (2) and the following formula (3) after aging. Formula (1) Ta≦Tcc
(℃) Formula (2) Tg≦Tb≦Tm-2 (
°C) Formula (3) Tb>Ta
(℃) (However, Ta: Aging temperature (℃) Tb: Heat treatment temperature (℃) Tcc: Cold crystallization temperature of the resin (℃) measured by differential thermal analysis method based on JIS K7121 at a heating rate of 10℃/min Tg: Glass transition temperature of resin measured by differential thermal analysis method based on JIS K7121 at a heating rate of 10°C/min (°C) Tm: JIS K712
A heating rate of 10°C/mi was determined by differential thermal analysis based on
Melting point of the resin measured at n (℃)) 2. The method for producing a transparent polyester film or sheet according to claim 1, wherein the film or sheet is formed by a T-die method. 3. A transparent polyester film or sheet obtained by the production method according to claim 1 or 2, which has a relative crystallinity of 50% or less and maintains a light transmittance of 75% or more even after heat treatment at 120°C. [Claim 4] The thickness of the film or sheet is 0.01.
The transparent polyester film or sheet according to claim 3, which has a thickness of 2.5 mm.