JP7537094B2 - Polyester Film - Google Patents

Description

The present invention relates to polyester films.

In recent years, there has been a demand for heat-shrinkable films that have the heat resistance to prevent deformation such as shrinkage at low temperatures of around 90°C for printing and coating special inks, coating agents, etc., and that have the characteristic of shrinking significantly at high temperatures of around 150°C, which is the subsequent shrinkage process. For example, in applications such as packaging applications centered on bottle containers for tea and soft drinks, decorative applications in which the shrinkage of a film is used to impart a sophisticated design to components with complex shapes, and optical release films for forming optical layers, there is an increasing need to achieve both a low heat shrinkage rate at low temperatures and a high heat shrinkage rate at high temperatures. As heat shrinkage films, there are uniaxially stretched films as described in Patent Document 1 that shrink in a specific direction, and films that shrink only in a specific direction by stretching in the horizontal direction and then sequentially biaxially stretching in the vertical direction (Patent Document 1). In addition, it is preferable that the shrink label is a cylindrical shrink label that is attached to a container such as a PET bottle and used, and then the label is easily separated from the bottle at the consumer stage for bottle recycling. Known methods for making labels easier to tear include, for example, using a film made of a material with good tearability, reducing the thickness of the film, and perforating the label (Patent Document 2). Furthermore, research has been conducted on shrink labels that are less likely to cause interlayer peeling during shrink processing and have excellent drop resistance and tearability (Patent Document 3).

International Publication No. 2017/022742 JP 2009-178887 A JP 2015-179133 A

When the transverse-longitudinal sequentially biaxially stretched film described in Patent Document 1 is used as a shrinkable film that requires the above-mentioned low-temperature heat resistance and high-temperature shrinkage properties, the film shrinks significantly at 150°C, but the Young's modulus is too high, making it difficult to separate the label after it is made into a shrink label, etc., which is an issue. In addition, while label separation and the like are taken into consideration for the films described in Patent Documents 2 and 3, the heat shrink tension is small due to the laminated structure including an olefin layer, and uniform heat shrinkage cannot be achieved.

Therefore, the objective of the present invention is to provide a polyester film that solves the above problems.

In order to solve the above problems, the present invention uses a polyester film having a 150°C heat shrinkage rate in the main shrinkage direction of 25% to 45%, a 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction of -6% to 10%, a 90°C heat shrinkage rate in the main shrinkage direction of 14% or less, and a 90°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction of 5% or less, a heat shrinkage tension peak value in the main shrinkage direction converted to 38 μm of 0.7 N/4 mm or more and 3.0 N/4 mm or less, and a Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction of 200 MPa or less.

The polyester film of the present invention has special thermal and mechanical properties, such as a 150°C heat shrinkage rate of 25% to 45% in the main shrinkage direction, a 150°C heat shrinkage rate of -6% to 10%, a 90°C heat shrinkage rate of 14% or less in the main shrinkage direction, and a 90°C heat shrinkage rate of 5% or less in the direction perpendicular to the main shrinkage direction, a heat shrinkage tension peak value of 0.7 N/4 mm or more and 3.0 N/4 mm or less in the main shrinkage direction in terms of 38 μm, and a Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction of 200 MPa or less. This allows the film to shrink uniformly even when it is shrunk with a printed or coated layer provided, and the Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction of 200 MPa or less, making it suitable for use in packaging, molding and decoration, and optical applications.

The polyester film of the present invention will be described in detail below.
The polyester film of the present invention has a 150°C heat shrinkage rate in the main shrinkage direction determined by the measurement method described below of 25% or more and 45% or less. By setting the 150°C heat shrinkage rate in the main shrinkage direction to 25% or more and 45% or less, excellent shrinkage properties can be exhibited when used for packaging applications, molding decoration applications, optical applications, etc. As in the present invention, the method of setting the 150°C heat shrinkage rate in the main shrinkage direction to 25% or more is not particularly limited, but for example, a method of stretching in the main shrinkage direction can be mentioned. For example, if it is intended to shrink at 25% or more at 150°C, it is necessary to stretch at least 1.25 times, and it is preferable to give an orientation to such an extent that the refractive index in the main shrinkage direction is 1.57 to 1.64. However, if the refractive index in the main shrinkage direction is oriented to exceed 1.64, it may be difficult to set the 150°C heat shrinkage rate in the main shrinkage direction to 25% or more and the 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction to 10% or less. In the present invention, the main shrinkage direction refers to the direction in which the heat shrinkage at 150°C is highest when any one direction of the film is set as 0° and the heat shrinkage is measured in directions from that direction at 5° intervals up to 180°.

The polyester film of the present invention has a 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction, which is determined by the measurement method described below, of -6% or more and 10% or less. This range can suppress the occurrence of wrinkles in the heat shrinkage process. Normally, a biaxially stretched film such as a shrink film will shrink significantly in both the longitudinal and width directions. There is no particular limitation on the method for making the heat shrinkage rate in the main shrinkage direction 25% or more and 45% or less, and the 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction -6% or more and 10% or less, as in the polyester film of the present invention, but in the process of producing the polyester film, it is preferable to include a stretching step (I) in which the stretching ratio in the direction perpendicular to the main shrinkage direction is 3.0 times or more, followed by a stretching step (II) in which the stretching ratio in the direction that is the main shrinkage direction is more than 1.1 times and 2.2 times or less. In order to improve uniformity of thickness unevenness, etc., a stretching step in the main shrinkage direction may be included before the stretching in the direction perpendicular to the main shrinkage direction in the stretching step (I). However, in order to make the 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction 10% or less, the stretching ratio in the stretching step is preferably more than 1.0 times and 2.8 times or less, and most preferably 1.6 times or less. In addition, the heat shrinkage rate and heat shrinkage tension can be further controlled by providing a heat treatment step (I) between the stretching step (I) and the stretching step (II) or by providing a heat treatment step (II) after the stretching step (II).

In the present invention, at least in the stretching step (I), the stretching ratio in the direction perpendicular to the main shrinkage direction is set to 3.0 times or more to stretch so as to be oriented and crystallized in the stretching direction, and then in the stretching step (II), the film is stretched in the direction perpendicular to the stretching direction in the stretching direction (main shrinkage direction) by more than 1.0 times and less than 2.2 times, thereby suppressing the orientation crystallization of the resin constituting the film and obtaining a film that is greatly thermally shrunk in the stretching direction (main shrinkage direction) of the stretching step (II). In other words, it is presumed that the amorphous component, which is considered to be a shrinkage component, can be selectively distorted in the stretching direction of the stretching step (II) by slightly stretching in the direction perpendicular to the stretching direction of the stretching step (I) in a state where the film is greatly oriented and crystallized in the stretching step (I). Therefore, in order to control the 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction to -6% or more and 10% or less, it is important to use a crystalline resin as the resin composition to the extent that it can be oriented and crystallized. The degree of oriented crystallization can be understood using the refractive index and the plane orientation coefficient as indicators. In the present invention, the plane orientation coefficient is preferably greater than 0.06, and the plane orientation coefficient is preferably 0.14 or less in terms of distorting the amorphous component without crystallizing it.

The polyester film of the present invention has a 90°C heat shrinkage rate of 14% or less in the main shrinkage direction. In the present invention, it is required to suppress shrinkage deformation in the drying process after printing and applying various functional layers, so the shrinkage rate in the main shrinkage direction at 90°C must be 14% or less. If the 90°C heat shrinkage rate in the main shrinkage direction exceeds 14%, the film will shrink and deform in the drying process after printing and applying various functional layers, and flatness will deteriorate. On the other hand, if the 90°C heat shrinkage rate in the main shrinkage direction is greater than -2% (i.e., thermal expansion of -3% or -4%), the transportability of the film may deteriorate and productivity may deteriorate, so the 90°C heat shrinkage rate in the main shrinkage direction is preferably greater than -2%. There is no particular limit to the method for making the heat shrinkage rate in the main shrinkage direction 14% or less at 90°C. For example, a method in which the refractive index in the main shrinkage direction is 1.57 to 1.64 and the plane orientation coefficient is 0.08 to 0.14 is a preferred method.

The polyester film of the present invention has a 90°C heat shrinkage rate of 5% or less in the direction perpendicular to the main shrinkage direction. This range makes it possible to suppress wrinkles during thermal lamination in packaging applications, for example. The rate is preferably -2% or more and 2% or less. The 90°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction can be controlled by the stretching conditions described below.

The polyester film of the present invention has a heat shrinkage tension peak value in the main shrinkage direction in 38 μm conversion of 0.7 N/4 mm or more and 3.0 N/4 mm or less. More preferably, it is 1.0 N/4 mm or more and 3.0 N/4 mm or less, and even more preferably, it is 2.0 N/4 mm or more and 3.0 N/4 mm or less. By being in this range, the printed layer and the coating layer can be uniformly shrunk. The method of controlling the heat shrinkage tension peak value in the main shrinkage direction in 38 μm conversion to this range is not particularly limited, but it can be achieved, for example, by stretching a homopolyester at a predetermined magnification in the main shrinkage direction. However, in order to make the Young's modulus at 140 ° C. in the direction perpendicular to the main shrinkage direction 200 MPa or less and the heat shrinkage tension peak value in 38 μm conversion of 0.7 N/4 mm or more and 3.0 N/4 mm or less, the key is to use a polyester having the following copolymerization components. As a method for making the Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction 200 MPa, for example, a method of introducing a copolymerization component in a total amount of 6 mol% or more relative to the diol component or glycol component, using polyethylene terephthalate as the main component, is known. However, in such a method, the introduction of the copolymerization component reduces the crystallinity, which in turn reduces the film orientation, and therefore the heat shrinkage tension decreases with the conventional method. For this reason, in the present invention, in order to make the Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction 200 MPa or less while making the heat shrinkage tension peak value 0.7 N/4 mm or more and 3.0 N/4 mm or less, it is important to introduce a copolymerization component, and then perform the heat treatment step (I) after the stretching in the above-mentioned stretching step (I), and to perform the heat treatment at a temperature HS-1 [°C] in the heat treatment step (I) that is 80°C or more and 110°C or less lower than the melting point Tm1 [°C] of the resin constituting the film ((ΔmS [°C] = Tm1 [°C] - HS-1 [°C] -), 80 ≦ ΔmS [°C] ≦ 110). For example, in the case of polyethylene terephthalate, HS-1 [°C] is preferably 140°C or more and 170°C or less. Next, by performing stretching in the main shrinkage direction in the stretching step (II), the Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction can be set to 200 MPa or less, while the heat shrinkage tension peak value in the main shrinkage direction can be set to 0.7 N/4 mm or more and 3.0 N/4 mm or less. If a highly crystalline resin such as a homopolymer is used and HS-1 is set so that ΔmS is 80°C to 110°C, crystallization of the film progresses and film breakage may occur. For this reason, it is important to reduce the crystallinity of the resin by introducing a copolymerization component or the like. If the polyester resin constituting the film has two or more melting points, the lowest melting point is designated as Tm1.

The polyester film of the present invention preferably has a heat shrinkage tension rise start temperature of 70°C or higher, as determined by the measurement method described below. This can suppress wrinkles caused by shrinkage of the original roll during printing and the drying process of the coating layer, and can suppress deterioration of flatness. In order to achieve a heat shrinkage tension rise start temperature of 70°C or higher, it is preferable to use a resin with a glass transition temperature of 75°C or higher and set ΔmS to 80°C or higher and 110°C or higher.

The polyester film of the present invention has a Young's modulus of 200 MPa or less at 140°C in a direction perpendicular to the main shrinkage direction. When the polyester film of the present invention is used for applications in which a coating layer is provided by printing or coating and then punching is performed, various secondary processing is possible if the Young's modulus at 140°C in a direction perpendicular to the main shrinkage direction is within this range. There is no particular limitation on the method for making the Young's modulus at 140°C 200 MPa or less, but preferred methods include introducing a copolymerization component into the polyester resin constituting the film or mixing a soft resin component such as an elastomer.

The glycol or derivative thereof that gives the polyester used in the present invention is preferably 80 mol% or more of ethylene glycol, but other components include aliphatic dihydroxy compounds such as 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,4:3,6-dianhydro-D-glucitol (generic name: isosorbide) and neopentyl glycol, polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol, polypropylene glycol and polytetramethylene glycol, alicyclic dihydroxy compounds such as 1,4-cyclohexanedimethanol and spiroglycol, aromatic dihydroxy compounds such as bisphenol A and bisphenol S, and derivatives thereof.

The dicarboxylic acid or its derivative that gives the polyester used in the present invention is preferably 80 mol % or more of terephthalic acid, but other components include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, and diphenoxyethanedicarboxylic acid, aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, and oxycarboxylic acids such as paraoxybenzoic acid, as well as derivatives thereof. Examples of dicarboxylic acid derivatives include esters such as dimethyl terephthalate, diethyl terephthalate, 2-hydroxyethyl methyl terephthalate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl adipate, diethyl maleate, and dimethyl dimerate.

The polyester film of the present invention preferably has an MOR value (the ratio of the maximum and minimum values of the transmitted microwave intensity measured by a microwave transmission molecular orientation meter) of 1.55 or more and 1.90 or less. By setting the MOR value within this range, the heat shrinkage rate in the direction perpendicular to the main shrinkage direction can be suppressed, and the heat shrinkage characteristics can be biased toward the main shrinkage direction, making it suitable for use in packaging applications such as shrink labels. There is no particular limitation on the method for setting the MOR value within this range, but a preferred control method is to set the birefringence, which is the absolute value obtained by subtracting the refractive index in the main shrinkage direction of the film from the refractive index in the direction perpendicular to the main shrinkage direction, to 0.05 or more, and then set ΔmS to 80°C or more and 90°C or less.

The polyester film of the present invention is preferably a laminated film of two or more layers, consisting of at least a resin layer A mainly composed of a resin A and a resin layer B mainly composed of a resin B. In the present invention, a film-forming method in which ΔmS is set to 80°C or more and 110°C or less is preferable in order to increase the thermal shrinkage tension, but the film is very brittle and frequently breaks, resulting in poor film-forming stability. However, it is preferable to provide a resin layer B as a protective layer, since it suppresses film breakage and improves film-forming stability. The laminated structure may be a two-layer structure of A/B, a three-layer structure of A/B/A, a five-layer structure of A/B/A/B/A, or a multi-layer structure of more than five layers. In addition, an A/B/C laminated structure in which a resin layer C is further provided may be used as long as it does not impair the effects of the present invention.

In the polyester film of the present invention, when the film is a laminated film of resin layer A and resin layer B, it is preferable that the melting endothermic peak temperature TmA of resin A is 10°C or more lower than the melting endothermic peak temperature TmB of resin B. In the present invention, in order to make the heat shrinkage tension fall within the range of the present invention, it is preferable to set the value ΔmS obtained by subtracting the set temperature HS-1 [°C] of the heat treatment step (I) from the lowest melting point Tm1 [°C] of the resins constituting the film (in a polyester film consisting of resin A and resin B, if the melting point Tm of resin A is lower than that of resin B, Tm1 [°C] = TmA [°C]) to 80°C or more and 110°C or less, and the main purpose is to relax the orientation imparted by the stretching in the stretching step (I). In other words, it is desired to relax the orientation of only resin layer A as much as possible in the heat treatment step (I), but if HS-1 is set to a condition close to the melting point TmA of resin A, the film formation stability will be reduced. For this reason, the film-forming stability is improved by providing resin B, which has a higher melting point than resin A. From the viewpoint of film-forming stability, it is preferable that the melting endothermic peak temperature TmA of resin A is lower than the melting endothermic peak temperature TmB of resin B by 15°C or more, and more preferably by 20°C or more.

In the present invention, from the viewpoint of making the Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction 200 MPa or less, the crystalline melting energy ΔHmA of the resin A is preferably 25 J/g or less, and from the viewpoint of suppressing film breakage in the heat treatment step (I), the crystalline melting energy ΔHmA is preferably 10 J/g or more. There is no particular limitation on the method of controlling ΔHmA from 10 J/g to 25 J/g or less, but one preferred method is, for example, introducing a copolymerization component into the polyester resin. From the viewpoint of simultaneously suppressing film breakage in the heat treatment step (I) and achieving a Young's modulus of 200 MPa or less in the direction perpendicular to the main shrinkage direction, a preferred method is to use 80 mol% or more of terephthalic acid and ethylene glycol as the dicarboxylic acid component and diol component of the polyester resin (resin A) constituting the resin layer A, respectively, and to use 2,6-naphthalenedicarboxylic acid, isophthalic acid, 1,4-cyclohexanedimethanol, neopentyl glycol, spiroglycol, and 1,4:3,6-dianhydro-D-glucitol (generic name: isosorbide) as the copolymerization components.

The polyester film of the present invention may have a surface layer on at least one side thereof that exhibits one or more functions selected from the group consisting of hard coat properties, self-repair properties, anti-glare properties, anti-reflection properties, low reflectivity properties, ultraviolet shielding properties, and antistatic properties, for the purpose of smoothing the surface when minute scratches are caused on the film surface by biaxial stretching. A softer surface is preferable from the viewpoint of conformity with shrinkage of the original film.

Next, it is preferable that the polyester film of the present invention is biaxially oriented from the viewpoint of heat shrinkage properties. A preferred manufacturing method for obtaining such a biaxially oriented polyester film is described below using a sequential biaxial stretching method as an example. The present invention is not to be interpreted as being limited to this example.

As a method for producing the polyester film of the present invention, a method for producing a film that undergoes at least a melt extrusion step and a casting step to form a sheet, followed by a stretching step (I), a heat treatment step (I), a stretching step (II), and a heat treatment step (II) in that order is preferred. In the stretching step (I), sequential biaxial stretching may be performed, and it is preferred that the stretching ratio in the film production direction (film flow direction: MD direction (longitudinal direction)) is 1.1 times or more and 2.2 times or less, and the stretching ratio in the direction perpendicular to the film production direction (film width direction: TD direction (transverse direction)) is 3.0 times or more and 6.0 times or less. In the stretching step (I), the stretching ratio in the film production direction (film flow direction: MD direction (longitudinal direction)) is preferably 2.0 times or less from the viewpoint of suppressing the heat shrinkage rate in the direction perpendicular to the main shrinkage direction, and is most preferably 1.5 times or less. In addition, from the viewpoint of making the thermal shrinkage rate in the main shrinkage direction 25% or more, it is preferable that the cumulative stretching ratio in the film formation direction in the stretching steps (I) and (II) is 2.0 times or less, and the stretching ratio in the film formation direction in the stretching step (II) is higher than the stretching ratio in the film formation direction in the stretching step (I). In addition, from the viewpoint of controlling the thermal shrinkage tension peak value to 0.7 N/4 mm or more and 3.0 N/4 mm or less, it is preferable that the temperature (HS-1) in the heat treatment step (I) is -80 ° C or more and -110 ° C or less with respect to the melting point Tm1 of the resin constituting the film. In addition, in the film manufacturing method having a heat treatment step (II) after the stretching step (II), it is preferable that the heat treatment temperature (HS-2) is equal to or higher than the glass transition temperature of the resin and equal to or lower than the glass transition temperature + 40 ° C. In addition, when there are multiple glass transition temperatures, the temperature with the higher glass transition temperature is used as the reference.

The thickness of the polyester film of the present invention is not particularly limited as long as the purpose can be achieved, and it may be about 3 μm to 300 μm, which is the thickness generally used for biaxially stretched films. The thickness may be varied as necessary.

The polyester film of the present invention may be further reinforced with a backing material. Examples of the backing material include biaxially oriented polyester film and biaxially oriented polypropylene.

The polyester film of the present invention is preferably used for packaging applications because it has a low heat shrinkage rate in the low temperature range and uniform heat shrinkage in the high temperature range. It has heat resistance that does not shrink during the drying process after coating of various functional layers such as a printing layer, a weather-resistant layer, an adhesive layer, an adhesive layer, a deposition layer, etc., and is therefore compatible with water-based solvent coating agents. Furthermore, it exhibits high heat shrinkage when heated at high temperatures, so it is excellent in terms of attachment to containers such as bottles, and since the Young's modulus in the direction perpendicular to the main shrinkage direction is 200 MPa or less, labels can be easily removed, and it is preferably used for various packaging applications, mainly for labels.

The polyester film of the present invention can also be used favorably for decorative purposes. It has heat resistance that does not shrink during the drying process after coating with various functional layers such as a printing layer, weather-resistant layer, adhesive layer, bonding layer, deposition layer, scratch-resistant layer, fingerprint-resistant layer, etc., and is compatible with water-based solvent coating agents, so it has excellent heat resistance during the drying process after coating with various functional layers and shows high heat shrinkage when heated at high temperatures, making it applicable to highly designed decoration of components with complex shapes.

(Methods for measuring characteristics and evaluating effects)
The methods for measuring the characteristics and evaluating the effects of the present invention are as follows.

(1) Composition of polyester A polyester film is dissolved in hexafluoroisopropanol (HFIP), and the content of each monomer residue component and by-product diethylene glycol can be quantified using ¹ H-NMR and ¹³ C-NMR. In the case of a laminated film, each layer of the film is scraped off according to the laminate thickness, and the components constituting each layer alone can be collected and evaluated. For the film of the present invention, the composition was calculated by calculation from the mixing ratio at the time of film production.

(2) Film thickness and thickness of each layer The film was embedded in epoxy resin, and the cross section of the film was cut out with a microtome. The cross section was observed at a magnification of 5000 times with a transmission electron microscope (TEM H7100 manufactured by Hitachi, Ltd.) to determine the film thickness and, in the case of a laminated structure, the thickness of each layer.

(3) Film Main Shrinkage Direction An arbitrary direction of the film was set as 0°, and samples were cut out in a size of 150 mm x 10 mm width in directions from 0° to 180° at 5° intervals from there, and marks were made on both ends at intervals of 100 mm (L0). A 3 g weight was hung from the sample, and the sample was placed in a hot air oven heated to 150° C. for 30 minutes for heat treatment. The gauge length (L1) after heat treatment was measured, and the heat shrinkage rate was calculated from the change in the gauge length before and after heating using the following formula.
Heat shrinkage rate (%) = 100 x (L0 - L1) / L0
The measurement was carried out five times in each direction, and the direction with the highest thermal shrinkage rate was determined as the main shrinkage direction.

(4) 90°C heat shrinkage and 150°C heat shrinkage The film was cut into a size of 150 mm long x 10 mm wide so that the length direction was the main shrinkage direction and the direction perpendicular to the main shrinkage direction. Marks were made at both ends at intervals of 100 mm (L0), and the film was placed in a hot air oven heated to the measurement temperature for 30 minutes with a 3g weight hanging therefrom, and heat treatment was performed at a specified temperature. The distance between the gauge marks (L1) after heat treatment was measured, and the heat shrinkage was calculated from the change in the distance between the gauge marks before and after heating using the following formula. Five samples were measured in each direction, and the average value was used for evaluation.
Heat shrinkage rate (%) = 100 x (L0 - L1) / L0
(5) Peak heat shrinkage tension, heat shrinkage tension rise temperature A film that had been left for 24 hours at a temperature of 23°C and a relative humidity of 65% was measured using a TMA/SS6000 (manufactured by Seiko Instruments Inc.) with an initial length of 20 mm and a width of 4 mm from 23°C to 170°C at a heating rate of 5°C/min, and the peak heat shrinkage tension [mN] was read from the obtained heat shrinkage force curve. For films with a film thickness other than 38 μm, the peak value read was divided by the film thickness [μm] of the measured sample, and the result was multiplied by 38 to obtain the peak heat shrinkage tension in the main shrinkage direction in terms of 38 μm. The heat shrinkage tension rise temperature was the temperature at which the value obtained by subtracting the initial load from the load in the measurement result exceeded 10 mN for the first time.

(6) Glass transition temperature, melting endothermic peak temperature (melting point) Tm, crystal melting energy ΔHm
In accordance with JIS 7122 (1987), a differential scanning calorimeter (EXSTAR DSC6220 manufactured by Seiko Instruments) was used to heat 3 mg of resin in a nitrogen atmosphere from 30° C. to 300° C. at a rate of 20° C./min. The resin was then held at 300° C. for 5 minutes, and then cooled to 30° C. at a rate of 40° C./min. The resin was then held at 30° C. for 5 minutes, and then heated from 30° C. to 300° C. at a rate of 20° C./min. The glass transition temperature obtained during this temperature rise was calculated by the following formula (i).
Glass transition temperature=(extrapolated glass transition onset temperature+extrapolated glass transition end temperature)/2 (i)
Here, the extrapolated glass transition onset temperature is the temperature at the intersection of a straight line extending the low-temperature side baseline to the high-temperature side and a tangent drawn at a point where the gradient of the curve of the stepwise change part of the glass transition is maximum. The extrapolated glass transition end temperature is the temperature at the intersection of a straight line extending the high-temperature side baseline to the low-temperature side and a tangent drawn at a point where the gradient of the curve of the stepwise change part of the glass transition is maximum. The peak top temperature of the melting endothermic peak is the melting endothermic peak temperature (melting point). The crystal melting energy ΔHm was calculated in accordance with JIS 7122 (1987) from the area of the maximum endothermic peak obtained from the DSC curve of the obtained differential scanning calorimetry chart (endothermic and exothermic curve) using disk session "SSC/5200" for data analysis.

(7) Young's modulus at 140°C The film was cut into a rectangular sample of 150 mm long x 10 mm wide so that the direction perpendicular to the direction in which the thermal shrinkage rate was the highest in the measurement of (3) was the length direction. Using a tensile tester (Tensilon UCT-100 manufactured by Orientec), a tensile test was performed in the direction perpendicular to the direction in which the thermal shrinkage rate was the highest in the measurement of (3) of the film, with an initial tensile chuck distance of 50 mm and a tensile speed of 300 mm/min. The measurement was performed by setting the film sample in a thermostatic chamber previously set at 140°C, and performing a tensile test after preheating for 90 seconds to measure the Young's modulus. The measurement was performed five times each, and the average value was used.

(8) Refractive index, Δn birefringence, plane orientation coefficient fn
The refractive indexes in the main shrinkage direction, the direction perpendicular to the main shrinkage direction, and the thickness direction (nMD, nTD, and nZD, respectively) of the film determined in (3) were determined using an Abbe refractometer at 25° C., with sodium D line (wavelength 589 nm) as a light source and methylene iodide as a mounting liquid. The birefringence and plane orientation coefficient (fn) were calculated from the determined refractive indexes according to the following formulas.
Birefringence = |nMD-nTD|
Plane orientation coefficient fn=(nMD+nTD)/2-nZD
(9) MOR Value A sample was cut into a size of 10 cm x 10 cm, and the maximum and minimum values of the transmitted microwave intensity were determined using a transmitted microwave transmission type molecular orientation analyzer MOA-2001 manufactured by KS Systems Co., Ltd. (now Oji Scientific Instruments Co., Ltd.). The MOR value was calculated as the ratio of the maximum value to the minimum value (maximum value/minimum value).

(10) Suitability for Packaging Applications (I) Dry Heat Resistance Screen printing was performed on the film surface. Printing was performed using ink U-PET (517) and screen SX270T manufactured by Mino Group Co., Ltd. under conditions of a squeegee speed of 300 mm/sec and a squeegee angle of 45°, and then the film was dried for 5 minutes in a hot air oven at 90°C to obtain a printed layer laminated film. The appearance of the obtained printed layer laminated film was evaluated according to the following criteria.
A: No wrinkles were observed even after drying, and the appearance was good.
B: Some wrinkles were observed after drying, but the appearance was good.
C: Wrinkles were observed after drying, but were at a level that did not cause any practical problems.
D: Wrinkles were observed after drying, and the product was not of a practical level.
A, B and C are passing levels.

(II) Heat shrinkability For the printed layer laminated film prepared in (I), both ends of the film were bonded by heat-cutting and sealing to prepare a cylindrical label. The label was placed on a PET bottle, and the bottle was passed through a tunnel oven at 150°C for 3 seconds to attach the label to the bottle. The shrinkage and appearance were evaluated according to the following criteria.
A: No wrinkles, distortions, or insufficient shrinkage occurred, and the appearance was excellent in design.
B: At least one of wrinkles, distortion, and insufficient shrinkage was observed, but the appearance was excellent in design.
C: At least one of wrinkles, distortion, and insufficient shrinkage was observed, but there was no problem in practical use.
D: At least one of wrinkles, distortion, and insufficient shrinkage was observed, and the product was not of a practical level.
A, B and C are the passing levels.

(III) Label removability For the printed laminate film attached to the bottles prepared in (II), 100 bottles and their labels were separated using a PET bottle label separator "PLS-2000" manufactured by Yamamoto Seisakusho Co., Ltd. The amount of label remaining on the bottle after the removal work was completed was Lx, and the amount of label before removal was _L0 . The value calculated from the following formula was taken as the remaining rate, and the label removability was evaluated according to the following criteria.

Lx/L ₀ × 100 = Survival rate (%)
A: The remaining rate was less than 10%.

B: The remaining rate was 10% or more.

A is the passing level.

(11) Uniform Shrinkage A film cut to A4 size was shrunk in an oven at 150° C., and the orientation angle was measured at any 9 points in the film plane, and the film was judged according to the following criteria.

A: The variation in the orientation angle was ±1°.
B: The variation in the orientation angle was ±2°.

C: The variation in the orientation angle was ±3°.
D: The variation in the orientation angle was ±5°.

A, B and C are the passing levels.

The orientation angle is measured using the following procedure.

An automatic birefringence device KOBRA-21ADH manufactured by Oji Scientific Instruments was used. Samples were cut to 4 cm in the longitudinal direction and 5 cm in the transverse direction, and the maximum refractive index azimuth in the film plane was measured at a wavelength of 590 nm in high retardation measurement mode, which was used to determine the orientation direction. For films strongly stretched in the longitudinal or transverse direction, the strongly stretched (higher magnification) direction was defined as 0°, and the angle between the strongly stretched axial direction and the orientation direction was defined as the orientation angle.

(Production of polyester)
The polyester resin used for film formation was prepared as follows.

(Polyester A)
A polyethylene terephthalate resin (intrinsic viscosity 0.65) containing 100 mol % of terephthalic acid as the dicarboxylic acid component and 100 mol % of ethylene glycol as the glycol component.

(Polyester B)
A polyester resin (intrinsic viscosity 0.75) containing, as dicarboxylic acid components, 94 mol % of a terephthalic acid component and 6 mol % of an isophthalic acid component, and as a glycol component, 100 mol % of an ethylene glycol component.

(Polyester C)
A polyester resin (intrinsic viscosity 0.75) containing, as dicarboxylic acid components, 92 mol % of a terephthalic acid component and 8 mol % of an isophthalic acid component, and as a glycol component, 100 mol % of an ethylene glycol component.

(Polyester D)
A polyester resin (intrinsic viscosity 0.75) containing, as dicarboxylic acid components, 88 mol % of a terephthalic acid component and 12 mol % of an isophthalic acid component, and as a glycol component, 100 mol % of an ethylene glycol component.

(Polyester E)
A polyester resin (intrinsic viscosity 0.75) containing, as dicarboxylic acid components, 86 mol % of a terephthalic acid component and 14 mol % of an isophthalic acid component, and as a glycol component, 100 mol % of an ethylene glycol component.

(Polyester F)
A polyester resin (intrinsic viscosity 0.75) containing, as dicarboxylic acid components, 82 mol % of a terephthalic acid component and 18 mol % of an isophthalic acid component, and as a glycol component, 100 mol % of an ethylene glycol component.

(Polyester G)
A polyester resin (intrinsic viscosity 0.75) containing, as the dicarboxylic acid component, 91 mol % of a terephthalic acid component and 9 mol % of a 2,6-naphthalenedicarboxylic acid component, and as the glycol component, 100 mol % of an ethylene glycol component.

(Polyester H)
A polyester resin (intrinsic viscosity 0.75) containing, as the dicarboxylic acid component, 88 mol % of a terephthalic acid component, 12 mol % of a 2,6-naphthalenedicarboxylic acid component, and, as the glycol component, 100 mol % of an ethylene glycol component.

(Polyester I)
A polyester resin (intrinsic viscosity 0.75) containing 100 mol % of terephthalic acid as the dicarboxylic acid component, 88 mol % of ethylene glycol as the glycol component, and 12 mol % of 1,4-cyclohexanedimethanol as the glycol component.

(Polyester J)
A polyester resin (intrinsic viscosity 0.75) containing 100 mol % of terephthalic acid as the dicarboxylic acid component, 85 mol % of ethylene glycol as the glycol component, and 15 mol % of 1,4-cyclohexanedimethanol as the glycol component.

(Polyester K)
A polyester resin (intrinsic viscosity 0.75) containing 100 mol % of terephthalic acid as the dicarboxylic acid component, 82 mol % of ethylene glycol as the glycol component, and 18 mol % of 1,4-cyclohexanedimethanol as the glycol component.

(Polyester L)
A polyester resin (intrinsic viscosity 0.75) containing 100 mol % of a terephthalic acid component as a dicarboxylic acid component, and 92 mol % of an ethylene glycol component and 8 mol % of a spiroglycol component as glycol components.

(Polyester M)
A polyester resin (intrinsic viscosity 0.75) containing 100 mol % of a terephthalic acid component as a dicarboxylic acid component, 88 mol % of an ethylene glycol component as a glycol component, and 12 mol % of a spiroglycol component as a glycol component.

(Polyester N)
A polyester resin (intrinsic viscosity 0.75) containing 100 mol % of a terephthalic acid component as a dicarboxylic acid component, 84 mol % of an ethylene glycol component as a glycol component, and 16 mol % of a spiro glycol component as a glycol component.

(Polyester O)
A polyester resin (intrinsic viscosity 0.75) containing, as the dicarboxylic acid component, 100 mol % of a terephthalic acid component, and, as the glycol components, 88 mol % of an ethylene glycol component and 12 mol % of a neopentyl glycol component.

(Polyester P)
A polyester resin (intrinsic viscosity 0.75) containing, as the dicarboxylic acid component, 100 mol % of a terephthalic acid component, and, as the glycol components, 82 mol % of an ethylene glycol component and 18 mol % of a neopentyl glycol component.

(Polyester Q)
A polyester resin (intrinsic viscosity 0.75) containing, as the dicarboxylic acid component, 100 mol % of a terephthalic acid component, as the glycol component, 42 mol % of an ethylene glycol component, 15 mol % of an isosorbide component, and 43 mol % of a 1,4-cyclohexanedimethanol component.

(Particle Master Manufacturing)
(Particle Master A)
Polyethylene terephthalate particle master (intrinsic viscosity 0.63) containing aggregated silica having a number average particle size of 1.2 μm in a particle concentration of 5% by mass in polyester A.

(Examples 1 to 52, Comparative Examples 1 to 4)
The compositions of the polyester and particle master resin used are as shown in the table, and the raw materials were fed to a vented co-rotating twin screw extruder with an oxygen concentration of 0.2% by volume, melted at an extruder cylinder temperature of 270°C, and discharged from a T-die in the form of a sheet onto a cooling drum whose temperature was controlled at 25°C, with a short tube temperature of 275°C and a die temperature of 280°C. At this time, electrostatic was applied using a wire electrode with a diameter of 0.1 mm, and the sheet was brought into close contact with the cooling drum to obtain an unstretched sheet. 1st longitudinal stretching, 1st transverse stretching, heat treatment, 2nd longitudinal stretching, 2nd transverse stretching, and heat treatment were performed in this order to obtain a polyester film with the stretch ratio, stretching temperature, and heat treatment temperature shown in the table, respectively.

All of the examples had a 150°C heat shrinkage rate in the main shrinkage direction of 25% to 45%, a 150°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction of -6% to 10%, a 90°C heat shrinkage rate in the main shrinkage direction of 14% or less, and a 90°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction of 5% or less, a heat shrink tension peak value in the MD direction converted to 38 μm of 0.7 N/4 mm or more and 3.0 N/4 mm or less, and a Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction of 200 MPa or less, excellent drying suitability after coating of various functional layers, and suitable shrinkability and secondary processability for subsequent large shrinkage at 150°C.

On the other hand, in Comparative Example 1, the ratio of the first longitudinal stretch was 3.3 times, so the shrinkage component was biased and distorted in the first transverse stretch, and the final 150°C heat shrinkage rate in the longitudinal direction of the film was less than 15%.

In addition, in Comparative Example 2, ΔmS exceeded 110° C., and therefore the peak value of the thermal shrinkage tension was outside the range of the present invention.
In Comparative Example 3, since the crystal melting energy exceeded 25 J/g, it was not possible to obtain a film by crystallization even though TM1-HS-1 was within the range of 80° C. to 110° C.
In Comparative Example 4, since homopolyester was used, the Young's modulus in the direction perpendicular to the main shrinkage direction exceeded 200 MPa, and the label removability was poor.

The polyester film of the present invention has special heat shrinkage properties that suppress shrinkage at low temperatures of around 90°C and shrink significantly at high temperatures of around 150°C with high shrinkage stress, and also has the characteristic of a low Young's modulus in the direction perpendicular to the main shrinkage direction. This makes it possible to dry the film after coating various functional layers while suppressing shrinkage deformation at around 90°C, and then allows it to be used in packaging applications that require secondary processing after large shrinkage at around 150°C, various release applications, process films, etc.

Claims (5)

1. A polyester film having a 150°C heat shrinkage rate in the main shrinkage direction of 25% or more and 45% or less, a 150°C heat shrinkage rate in a direction perpendicular to the main shrinkage direction of -6% or more and 10% or less, a 90°C heat shrinkage rate in the main shrinkage direction of 14% or less, and a 90°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction of 5% or less, a heat shrinkage tension peak value in the main shrinkage direction converted into a 38 μm value of 0.7 N/4 mm or more and 3.0 N/4 mm or less, and a Young's modulus at 140°C in the direction perpendicular to the main shrinkage direction of 200 MPa or less, containing 80 mol % or more of an ethylene glycol component in glycol components and 80 mol % or more of a terephthalic acid component in dicarboxylic acid components, and further containing a copolymer component . The polyester film according to claim 1, having an MOR value (the ratio of the maximum to minimum transmitted microwave intensity measured by a microwave transmission molecular orientation meter) of 1.55 to 1.90. The polyester film according to claim 1 or 2, wherein the heat shrink tension rise start temperature is 70°C or higher. A polyester film according to any one of claims 1 to 3, which is a laminated film of two or more layers having at least a resin layer A whose main component is resin A and a resin layer B whose main component is resin B, and in which the melting endothermic peak temperature TmA of the resin A is at least 10°C lower than the melting endothermic peak temperature TmB of the resin B. The polyester film according to claim 4, wherein the crystalline melting energy ΔHmA of the resin A is 10 J/g or more and 25 J/g or less.