JP5604823B2 - Cavity-containing heat-shrinkable polyester film - Google Patents

Description

  The present invention relates to a void-containing heat-shrinkable polyester film, and more particularly, to a void-containing heat-shrinkable polyester film suitable for label applications.

  In recent years, stretched films made of polyvinyl chloride resin, polystyrene resin, polyester resin, etc. (so-called so-called, Heat-shrinkable film) has been widely used. Among such heat-shrinkable films, the polyvinyl chloride film has problems such as low heat resistance, generation of hydrogen chloride gas during incineration, and dioxin generation. In addition, polystyrene film has poor solvent resistance and must use ink with a special composition during printing, and must be incinerated at high temperatures. There is a problem of doing. Therefore, polyester-based heat-shrinkable films that have high heat resistance, are easy to incinerate, and have excellent solvent resistance are widely used as shrink labels. The usage tends to increase with the increase.

  In addition, as the heat-shrinkable film, a film that is largely shrunk in the width direction is generally used from the viewpoint of handling during label production. Therefore, the conventional heat-shrinkable polyester film has been produced by stretching at a high magnification in the width direction in order to develop a sufficient shrinkage force in the width direction during heating.

  However, the conventional heat-shrinkable polyester film has a specific gravity heavier than 1, and the PET container and the label could not be separated with water when the label was peeled off from the PET container.

Therefore, a cavity-containing heat-shrinkable polyester film has been studied to reduce the apparent density of the heat-shrinkable polyester film to less than 1 g / cm ³ . There has been proposed a method (Patent Document 1) in which an incompatible thermoplastic resin is mixed into a main raw material of a heat-shrinkable polyester film.

Japanese Patent Application Laid-Open No. 2002-36312

According to the method of mixing an incompatible thermoplastic resin into the main raw material of the heat-shrinkable polyester film described in Patent Document 1, the cavity is formed by mixing an incompatible thermoplastic resin into the main raw material. A heat-shrinkable polyester film can be produced. It can also be estimated that the apparent density can be less than 1 g / cm ³ by increasing the ratio of the incompatible thermoplastic resin.

However, when the film thickness is reduced, it is easy to achieve an apparent density of less than 1 when the production speed is low, such as in an experimental machine. However, with a high-speed production machine, the shear rate at the die outlet when making an unstretched film The draft ratio is increased, and the size (height) of the dispersion diameter of the unstretched film forming the cavity is decreased (hereinafter referred to as flat). Due to the flatness, the height of the cavity of the film after stretching is lowered, and it is difficult to reach an apparent density of 1 g / cm ³ or less. In particular, the cavity near the surface layer in the film height direction is flattened to impair the effect of reducing the apparent density.

  The object of the present invention is to solve the above-mentioned problems of the conventional void-containing heat-shrinkable polyester film, have a light-cutting property, a preferable white appearance, an apparent density smaller than water, and the size of the cavity in the film thickness direction. It is an object of the present invention to provide a void-containing heat-shrinkable polyester-based film having a small variation.

  As a result of intensive studies to solve the above problems, the present inventors have finally completed the present invention. That is, the present invention has the following configuration.

1. A multilayer heat-shrinkable polyester film comprising at least two layers, wherein at least one layer is mixed with polyester in a thermoplastic resin incompatible with the polyester, and is stretched at least uniaxially to contain a cavity; A thermoplastic resin incompatible with the polyester has a melt flow rate (MFR) of 20 g / min viscosity and a flexural modulus of 2000 MPa or more, and a multilayer heat-shrinkable polyester film has the following (1 The void-containing heat-shrinkable polyester film characterized by satisfying the requirements (1) to (3). (1) The hot water thermal shrinkage rate in the width direction when treated in warm water at 95 ° C. for 10 seconds is 50% or more and 80% or less (2) The void-containing heat-shrinkable polyester film is at an arbitrary position (3) The height of the cavity in the cross section of the cavity-containing heat-shrinkable polyester film cut in the width direction at an arbitrary position. The ratio of 3 or less is 2. The void-containing heat-shrinkable polyester film as described in the above item 1, wherein the apparent density is 0.99 g / cm ³ or less.
3. The void-containing heat-shrinkable polyester film as described in the above 1 or 2, wherein the solvent adhesive strength is 2 N / 15 mm width or more and 10 N / 15 mm width or less.
4). Thickness is 30 micrometers or more and 50 micrometers or less, The cavity containing heat-shrinkable polyester film in any one of said 1st-3rd characterized by the above-mentioned.
5. The cavity according to any one of the first to fourth aspects, which is produced under the conditions that the shear rate of the polymer flow in the die when obtaining an unstretched film is 80 sec ^-1 or more and the draft ratio is 18 or more. Contains heat-shrinkable polyester film.
6). The void-containing heat-shrinkable polyester system according to any one of the first to fifth aspects, wherein the thermoplastic resin incompatible with polyester is cyclic polyolefin or a combination of cyclic polyolefin and crystalline polystyrene. the film.
7). Any one of the first to sixth aspects, wherein the B layer having a cavity is an intermediate layer, and the A layer / B layer / A layer structure in which the A layer having no cavity is provided on both surface layers. Void-containing heat-shrinkable polyester film.

The void-containing heat-shrinkable polyester film of the present invention is highly shrinkable in the width direction, which is the main shrinkage direction, and has good shrinkage finishing properties when processed into a label. In addition, even in production conditions where the shear rate and draft ratio of the polymer flow in the die are large when obtaining an unstretched film, the ratio of the height of individual cavities in the film thickness direction of the film after stretching (maximum height of cavities ÷ The minimum height of the cavity) is small, and the apparent density can be 0.99 g / cm ³ or less. Usually, it is known that when the dispersion diameter of the unstretched film is high, the cavity after stretching becomes large and the specific gravity can be lowered. However, if the shear rate of the polymer flow in the die during melting is large, the dispersion diameter at the end in the thickness direction of the void-containing layer constituting the unstretched film (the portion close to the boundary with the adjacent layer) becomes small, and the draft When the ratio is high, the dispersion diameter of the entire void-containing layer constituting the unstretched film becomes small. As a result, the apparent density is unlikely to decrease. In the present invention, paying attention to the viscosity and hardness of the resin incompatible with PET that creates a dispersion diameter so that the dispersion diameter of the unstretched film after melting does not become small, the shear rate and draft ratio of the polymer flow in the die are large. Even under production conditions, the cavities are larger and the apparent density can be reduced. Thereby, after processing into the label of a PET container, it is easy to water-separate a PET container and a label. Therefore, the white heat-shrinkable polyester film of the present invention can be suitably used as a label for a container such as a bottle. When used as a label, the white heat-shrinkable polyester film can be very efficiently used in a container such as a bottle within a short time. It can be mounted, and when it is heat-shrinked after mounting, it can exhibit a good finish with very little wrinkles and insufficient shrinkage, and water separation between the PET container and the label is easy.

  The void-containing heat-shrinkable polyester film of the present invention is excellent in aesthetics, has light-cutting properties without printing or processing, and has excellent aesthetics even when printed.

  Further, the void-containing heat-shrinkable polyester film of the present invention has a high adhesive force when the front and back surfaces (or the same surface) are bonded together with a solvent. Therefore, it can be suitably used for various coated labels including labels such as PET bottles.

Schematic diagram of one cavity to indicate the measurement location of cavity height measurement

Embodiments of the present invention will be specifically described below.
The heat-shrinkable polyester film can be obtained as follows. A polyester composed of a dicarboxylic acid component and a polyvalent glycol component is melt-extruded from an extruder and cooled with a conductive cooling roll (such as a casting roll) to form a film (unstretched film).

  In the extrusion, the copolyester is extruded alone, or a plurality of polyesters (copolyester, homopolyester, etc.) are mixed and extruded. That is, the film may contain a second alcohol component different from the base unit (crystalline unit such as polyethylene terephthalate) and the polyhydric glycol component (ethylene glycol component) constituting the base unit. Good. In the present invention, the content of the acid component and the diol component is the content of the entire polyester constituting the film with respect to the acid component and the diol component when two or more kinds of polyesters are mixed and used. It does not matter whether transesterification has taken place after mixing. In the present invention, terephthalic acid is preferred as the main acid component constituting the base unit, and ethylene glycol is preferred as the main diol component constituting the base unit.

  Any of the above polyesters can be produced by polymerization according to a conventional method. For example, the polyester can be obtained by using a direct esterification method in which a dicarboxylic acid and a diol are directly reacted, or a transesterification method in which a dicarboxylic acid dimethyl ester is reacted with a diol. The polymerization may be performed by either a batch method or a continuous method.

  When a polyester film containing the second alcohol component is stretched, a heat-shrinkable polyester film can be obtained.

  As the second alcohol component, a diol component and a trivalent or higher alcohol component can be used. The diol component includes ethylene glycol, 1,3-propanediol, 1,4-butanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,6-hexanediol, 3-methyl- Alkylene glycols such as 1,5-pentanediol, 2-methyl-1,5-pentanediol, 1,9-nonanediol, 1,10-decanediol; cyclic alcohols such as 1,4-cyclohexanedimethanol; diethylene glycol; Examples include ether glycols such as ethylene glycol, polypropylene glycol, polyoxytetramethylene glycol, alkylene oxide adducts of bisphenol compounds or derivatives thereof; and dimer diols. Trivalent or higher alcohols include trimethylolpropane, glycerin, pentaerythritol and the like.

  Further, the total of at least one monomer component that can be an amorphous component in 100 mol% of the polyhydric alcohol component in all the polyester resins is preferably 14 mol% or more, and more preferably 16 mol% or more. Particularly preferred is 18 mol% or more. Examples of the monomer that can be an amorphous component include neopentyl glycol and 1,4-cyclohexanediol.

  Further, in addition to a heat-shrinkable polyester film having particularly excellent shrinkage finish, in order to improve the shrink finish while having a high heat shrinkage rate, in 100 mol% of the polyhydric alcohol component in the total polyester resin, The amount of neopentyl glycol or 1,4-cyclohexanedimethanol component is preferably 14 mol% or more, more preferably 16 mol% or more, and particularly preferably 18 mol% or more. The upper limit of the component is not particularly limited, but if the amount of the component is too large, the thermal shrinkage rate may become excessively high or the rupture resistance of the film may be deteriorated. It is preferable that it is 35 mol% or less, and it is especially preferable that it is 30 mol% or less.

  In order to improve the shrink finish, the polyester elastomer content in all polyester resins constituting the film is preferably 3% by mass or more. Here, the polyester elastomer is a polyester block copolymer comprising, for example, a high melting crystalline polyester segment (Tm 200 ° C. or higher) and a low melting point soft polymer segment (Tm 80 ° C. or lower) having a molecular weight of 400 or more, preferably 400 to 800. And polyester elastomers using polylactones such as poly-ε-caprolactone in the low melting point soft polymer segment. Moreover, it is possible to adjust the shrinkage | contraction rate of the direction orthogonal to the main shrinkage | contraction direction appropriately by making a polyester elastomer into the said range, and combining with the preferable manufacturing method and conditions mentioned later.

  In common, an aliphatic linear diol having 8 or more carbon atoms (eg, octanediol) or a trihydric or higher polyhydric alcohol (eg, trimethylolpropane, trimethylolethane, glycerin, diglycerin, etc.) It is preferable not to contain a large amount. In the heat-shrinkable polyester film obtained by using polyester containing these diols or polyhydric alcohols, it is difficult to achieve a necessary shrinkage rate, which is not preferable.

  Moreover, it is preferable not to contain diethylene glycol, triethylene glycol, and polyethylene glycol as much as possible. In particular, diethylene glycol is likely to be present because it is a by-product component during polyester polymerization. However, in the polyester used in the present invention, the content of diethylene glycol is preferably less than 4 mol%.

In order to achieve the total light transmittance specific to the film of the present invention and impart light-cutting properties to the film, for example, in the film, particles such as inorganic particles and organic particles are added to the film mass.
It is suitable to contain 0.1-20 mass%, Preferably it is 1-15 mass%. When the content of the particles is less than 0.1% by mass, it is difficult to obtain, for example, sufficient light-cutting property, which is not preferable. On the other hand, if it exceeds 20% by mass, for example, the film strength is lowered, and film formation tends to be difficult.

The particles may be added before polyester polymerization, but are usually added after polyester polymerization. Examples of inorganic particles to be added include kaolin, clay, and calcium carbonate.
, Known inert particles such as silicon oxide, calcium tephrate, aluminum oxide, titanium oxide, calcium phosphate, and carbon black, high melting point organic compounds insoluble in the melt film formation of polyester resin, cross-linked polymers, and metal compounds used in polyester synthesis It may be an internal particle formed inside the polymer during the production of the polyester by a catalyst such as an alkali metal compound or an alkaline earth metal compound. Of these, titanium oxide particles are preferable from the viewpoint of providing the necessary light-cutting properties.

  The average particle size of the particles contained in the film is in the range of 0.001 to 3.5 μm. Here, the average particle diameter of the particles is measured by a Coulter counter method. The average particle size of the particles is preferably 0.001 μm to 3.5 μm, more preferably 0.005 μm to 3.0 μm. If the average particle size of the particles is less than 0.001 μm, for example, it will be difficult to obtain necessary light-cutting properties. When the average particle diameter of the particles exceeds 3.5 μm, for example, the film surface is inferior in smoothness and problems such as printing omission are likely to occur. The average particle diameter of the anatase form is generally 2.0 μm or less, and the average particle diameter of the rutile form is 2.0 μm or more. In order to conceal visible light, a particle size of 2.0 to 3.0 μm is most efficient, and rutile titanium oxide generally has higher concealability than anatase form.

  Titanium oxide particles are classified into anatase and rutile crystal forms. Both are used for kneading plastics. The anatase type is likely to cause yellowing and resin deterioration due to direct sunlight, etc. When used outdoors, the titanium oxide surface is subjected to special treatment (alumina, silica, organic substances, etc.) or the rutile type is selected. There are many.

In the present invention, in order to obtain an apparent density of 0.99 g / cm ³ or less, for example, it is preferable to contain cavities inside the film. For example, a foam material or the like may be mixed and extruded, but a preferred method is to obtain a cavity by mixing an incompatible thermoplastic resin in polyester and stretching it in at least one axial direction. Specific examples of resins incompatible with polyester include polystyrene resins, polyolefin resins, polyacrylic resins, polycarbonate resins, polysulfone resins, and cellulose resins. In order to produce a film having a large cavity under production conditions where the shear rate and draft ratio of the polymer flow in the die are large, it is necessary to increase the dispersion diameter of the resin incompatible with the polyester in the unstretched film. For this purpose, an incompatible resin having a high viscosity and a high strength in a resin incompatible with polyester is preferable. The present inventors have experimented, viscosity melt flow rate (MFR) of at 20 g / mi n hereinafter are preferred, it has been found that more than 2000MPa is preferred flexural modulus hardness. For example, the cyclic polyolefin used in the examples of this patent is a preferred resin that satisfies the conditions of viscosity and hardness. Crystalline polystyrene satisfies the hardness, but is not necessarily low in viscosity. However, crystalline polystyrene has good compatibility with the cyclic polyolefin, and is preferably used in combination with the cyclic polyolefin because the viscosity increases. However, polyolefin resins such as polypropylene are not preferred because of their high viscosity but insufficient hardness. Thus, it is particularly preferable to satisfy both the viscosity and the hardness.

The content of the resin incompatible with the polyester is preferably in the range of 10.0 to 25.0 mass% in terms of film. If the incompatible resin is less than 10.0% by mass, for example, the amount of formation of cavities inside the film is reduced, and the effect of lowering the apparent density tends to be insufficient, which is not preferable. If the incompatible resin exceeds 25% by mass, for example, kneading in the extrusion process tends to be uneven and it is difficult to obtain a stable film, which is not preferable.

  Polystyrene resin refers to a thermoplastic resin containing a polystyrene structure as a basic component. In addition to homopolymers such as atactic polystyrene, syndiotactic polystyrene, and isotactic polystyrene, other components are grafted or block copolymerized. Modified resins such as high-impact polystyrene resins and modified polyphenylene ether resins, and also thermoplastic resins having compatibility with these polystyrene resins, such as polyphenylene ether, are included.

  In preparing the polymer mixture obtained by mixing the polyester and the incompatible resin, for example, the chips of each resin may be mixed and melt-kneaded in an extruder and extruded, or both resins may be preliminarily mixed by a kneader. The kneaded product may be further melt extruded from an extruder. Moreover, a polystyrene resin may be added in the polyester polymerization step, and a chip obtained by stirring and dispersing may be melt-extruded.

  The cyclic polyolefin resin used as a resin incompatible with polyester in the present invention preferably has norbornene and tetracyclododecane units as cycloolefin units. The copolymer unit preferably has an acyclic olefin monomer unit, particularly preferably an ethylene unit. Particularly preferred cycloolefin copolymers are norbornene-ethylene copolymer and tetracyclododecane-ethylene copolymer. Among them, a cyclic polyolefin resin containing 5 to 80% by weight, preferably 10 to 60% by weight of ethylene units is particularly preferable.

  The cyclic polyolefin resin usually has a glass transition temperature of −20 to 400 ° C., but the cyclic polyolefin resin suitably used in the present invention is preferably 100 to 230 ° C., more preferably 130 to 200 ° C. When the glass transition temperature is less than 100 ° C., Tg may be lower than the temperature at the time of unstretched film stretching, and foaming is difficult at the time of film stretching, which is not preferable. On the other hand, if Tg is higher than 230 ° C., it is difficult to uniformly mix the polymer in the extruder and it becomes difficult to extrude the polymer mixture, such as non-uniform film properties.

  The film in the present invention is preferably provided with an A layer substantially free of cavities in addition to the B layer containing many cavities therein. In order to achieve this configuration, different raw materials are charged into different extruders A and B, melted, bonded together in the molten state before the T-die or inside the die, and cooled with a conductive cooling roll (such as a casting roll). To form a film (unstretched film).

  The stretching is preferably uniaxial stretching, but may be biaxial stretching that is stretched at a lower magnification in a direction different from the direction of the uniaxial stretching (film width direction). In the film of the present invention, it is particularly preferable that a B layer containing a large number of cavities is used as an intermediate layer, and A layers having no cavities are provided on both surface layers. At this time, it is preferable not to contain incompatible resin in the A layer as a raw material. By doing so, there is no cavity in the A layer, and the film can maintain the strength after printing. In addition, since there are no cavities, the film does not become weak and has excellent wearability. In addition, since the shrinkage rate is reduced by the formation of the cavity, a high thermal shrinkage rate can be provided by providing a layer without the cavity.

  Furthermore, by using the B layer as an intermediate layer and having an A layer / B layer / A layer structure, undesirable curling can be prevented, and the film surface tends to be smooth, thereby improving printability. . In addition, when a polystyrene resin is added as a resin incompatible with polyester, smoke is generated during melt extrusion, and the process is likely to be fouled and the operability is deteriorated. Will be eliminated and stable production for a long time will be possible.

  Furthermore, additives such as a stabilizer, a colorant, an antioxidant, an antifoaming agent, an antistatic agent, and an ultraviolet absorber may be contained in a desired layer as necessary. Moreover, you may add a fluorescent whitening agent in order to improve the whiteness of a film.

[Intrinsic viscosity]
The heat-shrinkable polyester film of the present invention preferably has an intrinsic viscosity of 0.60 dl / g or more. If the intrinsic viscosity of the heat-shrinkable film is too small, the molecular weight of the polyester that composes the film will be low, which will reduce the durability of the shrinkage stress during heat shrinkage, resulting in defects such as shrinkage whitening and shrinkage spots. It becomes easy, and it becomes inferior to shrink finish and appearance. Moreover, if the molecular weight of polyester falls, the mechanical strength and tear resistance of a film will be reduced.

  The intrinsic viscosity is preferably 0.60 dl / g or more, more preferably 0.63 dl / g or more.

  As a method for increasing the intrinsic viscosity of the film, for example, (1) a method in which a high molecular weight polyester is used as the polyester that is the raw material of the film (for example, the intrinsic viscosity is 0.63 dl / g or more, preferably 0.68 dl / g More preferably, a method of using a polyester of 0.70 dl / g or more), (2) a method of suppressing thermal decomposition and hydrolysis when forming a film by extruding the polyester (for example, preliminarily using a polyester raw material) (3) A method of using a hydrolysis-resistant polyester as the polyester (for example, having an acid value of 25 eq / ton or less) (Method of using polyester), (4) Antioxidant to polyester (for example, 0.01 to 1 quality) A scheme of% of) contain the like.

  As the polymerization catalyst, various conventional catalysts can be used. For example, titanium-based catalyst, antimony-based catalyst, germanium-based catalyst, tin-based catalyst, cobalt-based catalyst, manganese-based catalyst, etc., preferably titanium-based catalyst (titanium tetrabutoxide) Etc.), antimony catalysts (such as antimony trioxide), germanium catalysts (such as germanium dioxide), cobalt catalysts (such as cobalt acetate), and the like.

  Further, the void-containing heat-shrinkable polyester film of the present invention can be subjected to corona treatment, coating treatment, flame treatment, etc. in order to improve the adhesion of the film surface.

The void-containing heat-shrinkable polyester film of the present invention is a film calculated by the following formula (1) from the length before and after shrinkage when treated for 10 seconds in warm water at 80 ° C. under no load. It is preferable that the thermal contraction rate in the longitudinal direction (ie, hot water thermal contraction rate at 80 ° C.) is −2% or more and 4% or less.
Thermal shrinkage rate = {(length before shrinkage−length after shrinkage) / length before shrinkage} × 100 (%)
... Formula (1)

When the hot-water heat shrinkage in the longitudinal direction at 80 ° C. (the direction perpendicular to the main shrinkage direction) is less than −2% (that is, when it exceeds 2% by heat treatment), it is good for use as a bottle label. On the other hand, if the hot water thermal contraction rate in the longitudinal direction at 80 ° C. exceeds 4%, when used as a label, the shrinkage tends to occur during the thermal contraction. It is not preferable. Therefore, the hot water heat shrinkage in the longitudinal direction at 80 ° C. is preferably −2% or more and 4% or less, more preferably −1% or more and 3% or less, and more preferably 0% or more and 2% or less. Note that the measurement temperature of 80 ° C. is adopted as the process for attaching the label to the container, for example, the temperature corresponding to the actual label temperature when passing through the shrink tunnel by steam. A temperature of 80 ° C. is adopted in order to confirm that the above-mentioned problems are unlikely to occur.

  In addition, the void-containing heat-shrinkable polyester film of the present invention has a film width calculated from the length before and after shrinkage according to the above equation 1 when treated in warm water at 95 ° C. for 10 seconds without load. The heat shrinkage rate in the direction (main shrinkage direction) (that is, hot water heat shrinkage rate of 95 ° C.) is preferably 50% or more and 80% or less.

  If the hot water thermal contraction rate in the width direction at 95 ° C. is less than 50%, the shrinkage amount is small, so that wrinkles and tarmi occur on the label after thermal contraction, which is not preferable. However, if the hot water thermal contraction rate in the width direction at 95 ° C. exceeds 80%, it is not preferable because when used as a label, the shrinkage tends to occur during thermal contraction, or so-called “jumping” occurs. Therefore, the hot water heat shrinkage in the width direction at 95 ° C. is preferably 50% or more and 80% or less, more preferably 52% or more and 78% or less, and more preferably 55% or more and 75% or less. Note that the measurement temperature of 95 ° C is adopted because the shrinkage potential in the width direction, which is the main shrinkage direction in which the film can be obtained at maximum, is a major concern of customers, and 95 ° C close to the boiling water temperature to represent it. Is adopted.

In the void-containing heat-shrinkable polyester film of the present invention, the height ratio of individual cavities in a cross section in the width direction at an arbitrary position perpendicular to the film surface is preferably 3 or less. A cross-section perpendicular to the film surface (film width direction) is magnified 2000 times with a scanning electron microscope (manufactured by Hitachi, Ltd., model S-510), photographed, and the cavity traced on the tracing film and painted. The image processing was performed with an image analyzer. The image analysis device used was an image scanner (GT-8000) manufactured by Seiko Epson Corporation, and the obtained image was imported into software (Adobe Photoshop TM 2.5J) of a personal computer (manufactured by Macintosh). Image analysis was performed with the software (Ultimage TM / 242.1.1). At this time, those detected at 0.2 μm or less were excluded so as not to accidentally make the lubricant in the raw material hollow. The height of each cavity was determined from a cross-sectional view showing the shape of the cavity thus obtained. The ratio between the average value of the cavity height and the cavity height (the ratio of the highest cavity to the lowest cavity) was used from the following equation.
Average value of cavity height = total height of individual cavities ÷ number of cavities
... Formula (2)
Cavity Height Ratio = Cavity Height of Cavity with Maximum Height / Cavity Height of Cavity with Minimum Height
... Formula (3)

If the average value of the cavity height is less than 2.3 μm, it is difficult to reduce the apparent density. On the other hand, when the thickness is 6 μm or more, there may be a problem in stability of film forming properties such as fracture, which is not preferable. The average value of the cavity height is preferably 2.3 μm or more and 6 μm or less, more preferably 2.5 μm or more and 5.5 μm or less, and even more preferably 2.8 μm or more and 5 μm or less.

  If the cavity height ratio is greater than 3, the size of the cavities is not uniform, making it difficult to reduce the apparent density. Although it is preferably close to 1, it is not usually 1 at all, and the cavity height ratio may be 1.1 or more. The cavity height ratio is preferably 3 or less, more preferably 2.8 or less, and even more preferably 2.6 or less. The lower limit may be 1.1 or more, or 1.2 or more.

The void-containing heat-shrinkable polyester film of the present invention preferably has an apparent density of 0.99 g / cm ³ or less. Four films were cut into 5.0 cm squares and used as samples. Four samples were stacked, and the total thickness was measured at 10 points using a micrometer with four significant figures, and the total value of the stacked thickness was determined. The average value was divided by 4 and rounded to 3 significant figures to obtain an average thickness per sheet (t: μm). The mass (w: g) of the four samples was measured using an automatic upper pan balance with four significant digits, and the apparent density was determined from the following equation. The apparent density was rounded to 3 significant figures.
Apparent density (g / cm ³ ) = w / (5.0 × 5.0 × t × 10 ⁻⁴ × 4)
= W × 100 / t
... Formula (4)

In the present invention, the apparent density of the film is preferably 0.99 g / cm ³ or less, more preferably 0.95 g / cm ³ or less, and still more preferably 0.90 g / cm ³ or less. The light and light apparent density is a great advantage in mass production, and the PET container and label can be easily separated from each other with water when the PET container is labeled. However, if the apparent density is too small, the strength of the film itself is impaired. Therefore, the apparent density is preferably 0.7 or more, and more preferably 0.75 or more.

  In the void-containing heat-shrinkable polyester film of the present invention, the solvent adhesive strength is preferably 2 N / 15 mm width or more. If the solvent adhesive strength is less than 2 N / 15 mm width, it is not preferable because the label is easily peeled off after the heat shrinkage. The solvent adhesive strength is more preferably 3N / 15 mm width or more, and particularly preferably 4N / 15 mm width or more. Although the solvent adhesive strength is preferably high, the solvent adhesive strength is considered to be about 10 (N / 15 mm) at present as the upper limit in view of the performance of the film forming apparatus. Also, if the solvent adhesive strength is too high, when two films are solvent-bonded to form a label, a situation where the film is unnecessarily bonded to the film is likely to occur, and the productivity of the label may be reduced. Therefore, it may be 8.5 (N / 15 mm) or less, and even if it is 7 (N / 15 mm) or less, it does not matter at all.

  The thickness of the void-containing heat-shrinkable polyester film of the present invention is not particularly limited, but it is preferably 30 μm or more and 50 μm or less, more preferably 35 μm or more and 45 μm or less as the heat-shrinkable film for labels. In addition, when the void-containing heat-shrinkable polyester film of the present invention has a laminated structure, the thickness of each layer is not particularly limited, but is preferably 2 μm or more.

In order to satisfy the above characteristics, the film of the present invention preferably has a layer structure of A / B, A / B / A , or A / B / C. The thickness ratio of the A layer and the B layer is preferably B / A = 2/1 or more, more preferably 4/1 or more, and further preferably 6/1 or more. If it is less than 1/1, it is difficult to achieve both the aesthetics of printability and the reduction of specific gravity. A / B / A is preferable because it suppresses undesirable curling after the shrinkage treatment.

When the C layer is provided, the content of the cavities is arbitrary, but it is possible to add particles for controlling slippage between the bottle and the film during shrinkage.

In the present invention, the total light transmittance is 40% or less, preferably 35% or less, more preferably 30% or less, and still more preferably 20% or less. If it exceeds 40%, the contents may be seen through or the printed matter may be difficult to see, which may be inferior in appearance. The total light transmittance is preferably close to 0%, but may be 5% or more, or 10% or more. Even if it is 15% or more, the light-cutting effect is clear.

  In the present invention, the whiteness is 70 or more, preferably 75 or more, more preferably 80 or more. If it is less than 70, the contents may be seen through or the printed matter may be difficult to see, and the appearance may be inferior. The whiteness is preferably close to 100, but may be 98 or less, or 95 or less.

  The film obtained by this invention can be made into a tube shape, and a film edge part can be joined. In that case, 1,3-dioxolane or a mixed solution with an organic solvent compatible with 1,3-dioxolane or a solvent or a swelling agent having a solubility parameter in the range of 8.0 to 13.8 is applied. It is preferable to bond by drying at a temperature of 70 ° C. or lower to obtain a tubular body before drying. Examples of the solubility parameter include those described in a solvent handbook (edited by the Japan Adhesive Association, published by Nikkan Kogyo Shimbun). The joint portion in the tube may have a width as narrow as possible from 50 mm or more and, of course, is appropriately determined according to the size of the container. A width of 5 mm is standard. Further, the joining portion may be joined in a single line shape, or may be formed by forming a plurality of linear joints over two or more. Since these joints do not damage the film base material, they retain the properties of the polyester polymer as they are and have protective properties such as impact resistance and bottle breakage resistance. It is good without a decrease in the degree of orientation due to shrinkage and no embrittlement phenomenon due to the subsequent heat treatment.

  The attachments using this tube include containers, bottles (including plastic bottles), and can sticks (pipes, sticks, wood, various sticks), preferably attached to bottles mainly composed of polyethylene terephthalate. This facilitates the collection and is effective for separating the specific gravity of the bottle and the label with water when the polyethylene terephthalate bottle is reused.

  Next, although a specific example is demonstrated about the manufacturing method of the cavity containing heat-shrinkable polyester film of this invention, it is not limited to this manufacturing method.

  The polyester raw material used in the present invention is dried using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer, melted at a temperature of 200 to 300 ° C., and extruded into a film. In extruding, any existing method such as a T-die method or a tubular method may be employed. After extrusion, it is cooled rapidly to obtain an unstretched film.

  The extrusion temperature is preferably in the range of 250 ° C to 290 ° C. When the extrusion temperature is lower than 250 ° C., for example, a load is excessively applied and normal extrusion becomes difficult. When the extrusion temperature exceeds 290 ° C, for example, the polyester resin is likely to be thermally deteriorated in the extruder, resulting in problems such as a decrease in mechanical strength of the resulting film and excessive shrinkage in the longitudinal shrinkage resulting in a decrease in shrink finish. Arise.

Further, the shear rate at the die outlet when obtaining an unstretched film is 80 (sec ⁻¹ ) or more,
Even when the draft ratio is 18 or more, it is preferable that the average value of the heights of the cavities is 2.3 μm or more and the ratio of the heights of the cavities is 3 or less. A smaller shear rate and draft ratio is advantageous for reducing the apparent density. However, if the shear rate is low, the amount of resin extruded becomes small and the productivity deteriorates. In addition, when the draft ratio is small, the production speed becomes slow and the productivity becomes worse. Moreover, when it is going to make the speed | rate in the process of making the unstretched film for obtaining stretched film thickness 30 micrometers-50 micrometers into 50 (m / min) or more, a draft ratio will inevitably become high.

Further, the shear rate and draft ratio at the die outlet were obtained from the following formulas (5) and (6).
Shear rate
γ = 6Q / (W × H ² ) (5)
γ: Shear rate (sec ^-1 )
Q: Discharge rate of raw material from the extruder (cm ³ / sec)
W: Width of die outlet opening (cm)
H: Lip gap of the die (cm)
Draft ratio
Draft ratio = V _f / V _l Formula (6)
V _f : Resin die Lip outlet speed (cm / sec)
V _l ; cooling drum take-up speed (cm / sec)

In the range of the above-mentioned large shear rate and large draft ratio, simply mixing an incompatible resin with no particular ingenuity with polyester will flatten the cavity, and the height of the cavity in the film thickness direction will be low. The apparent density could not be lowered sufficiently. Therefore, the present inventors can set the average value of the cavity height in the film thickness direction to 2.3 μm or more and the cavity height ratio to 3 or less even with a large shear rate and a large draft ratio by the following means. Incompatible resins were examined. In order to produce a film having a large cavity under production conditions where the shear rate and draft ratio of the polymer flow in the die are large, it is necessary to increase the dispersion diameter of the incompatible resin in the unstretched film. For this purpose, among resins incompatible with polyester, incompatible resins having high viscosity and high strength (high flexural modulus) are preferable. As a result of experiments by the present inventors, the viscosity was 20 in MFR.
It is preferably at most g / min, more preferably at most 15 g / min, and even more preferably at most 10 g / min. The lower the MFR, the better. However, 0.1 is actually the lower limit. The hardness is preferably 2000 MPa or more, more preferably 2400 MPa or more, and further preferably 2800 MPa or more in terms of flexural modulus. The higher the flexural modulus, the better. However, if it is too high, rupture is likely to occur at the time of stretching and the productivity is deteriorated. Therefore, 6000 MPa can be said to be the upper limit. Of course, for example, it is preferable to contain the incompatible resin as described above in the B layer having a layer structure such as A layer / B layer / A layer.

  Next, when the unstretched film is stretched, first, preheating is performed. The temperature of preheating shall be in the range of Tg + 10 ° C. to Tg + 30 ° C. of the unstretched film. Next, stretching is performed. The stretch ratio is 3.4 times or more and 5.0 times or less, preferably 3.6 times or more and 4.5 times or less with respect to the unstretched film. The stretching temperature is a predetermined temperature in the range of Tg-5 ° C to Tg + 15 ° C. The lower the preheating temperature and the stretching temperature, the higher the stress during stretching, so that the cavity can be enlarged and the apparent density can be reduced. The same applies to the draw ratio. The higher the draw ratio, the higher the stress during drawing, and the larger the cavity and the smaller the apparent density. However, if the stress at the time of stretching is too high, breakage occurs and productivity is deteriorated. Therefore, the above range is optimal for satisfying both conditions for reducing the apparent density and productivity.

  Next, it is preferable to heat-fix the film. The heat setting temperature is in the range of Tg + 10 ° C. to Tg + 20 ° C. Moreover, you may heat-set in the state made to tension | tensile in the extending | stretching direction of a film. In that case, the tension rate is desirably 6% or less. On the other hand, when the heat setting temperature is higher than Tg + 20 ° C., the shrinkage rate in the film width direction is reduced, and the voids are crushed and the apparent density is increased.

  From the viewpoint of controlling the heat shrinkage stress of the film, it is preferable that the number of stretching steps is large. However, if the number of steps is too large, it is difficult to design a stretching facility in industrial production. It is desirable to have 4 or less steps.

  The package obtained using the void-containing heat-shrinkable polyester film of the present invention is formed by covering at least part of the outer periphery with a label provided with perforations based on the void-containing heat-shrinkable polyester film. Examples of packaging objects include plastic bottles for beverages, various bottles, cans, plastic containers such as confectionery and lunch boxes, paper boxes, etc. Hereinafter, these are collectively called packaging objects). Normally, when these objects to be packaged are coated with a heat-shrinkable polyester film-based label containing heat-shrinkable material, the label is heat-shrinked by about 2 to 15% for packaging. Adhere to the body. In addition, printing may be given to the label coat | covered with a packaging target object, and it does not need to be printed.

  The label can be made by applying an organic solvent slightly inside from the edge of one side of the rectangular film, and immediately rolling the film and bonding the edges together to form a label, or roll Apply the organic solvent slightly inside from the edge of one side of the film wound up in the shape of a film, immediately roll up the film, overlap the edges and adhere, cut the tube to make a label . As the organic solvent for adhesion, cyclic ethers such as 1,3-dioxolane or tetrahydrofuran are preferable. In addition, aromatic hydrocarbons such as benzene, toluene, xylene and trimethylbenzene, halogenated hydrocarbons such as methylene chloride and chloroform, phenols such as phenol, and mixtures thereof can be used.

Next, the present invention will be specifically described by way of examples and comparative examples. However, the present invention is not limited to the modes of the examples, and may be appropriately selected without departing from the spirit of the present invention. It is possible to change. Hereinafter, Example 3 will be read as Reference Example 3.

  The evaluation method used in the present invention is as follows.

[Melt flow rate (MFR) of incompatible resin]
Measured according to American Society for Testing Materials Standard ASTM-D1238.

[Flexural modulus]
Measurements were made according to American Society for Testing Materials Standard ASTM-D790.

[Thermal shrinkage (hot water thermal shrinkage)]
The film is cut into a 10 cm × 10 cm square, heat-shrinked by treatment in warm water at a predetermined temperature ± 0.5 ° C. for 10 seconds under no load condition, and then measured in the vertical and horizontal dimensions of the film. In accordance with the following formula (1), the thermal shrinkage rate was determined. The direction in which the heat shrinkage rate is large was taken as the main shrinkage direction.
Thermal shrinkage rate = {(length before shrinkage−length after shrinkage) / length before shrinkage} × 100 (%)
... Formula (1)

[Average value of cavity height, ratio of cavity height]
A cross-section perpendicular to the film surface (film width direction) is magnified 2000 times with a scanning electron microscope (manufactured by Hitachi, Ltd., model S-510), photographed, and the cavity traced on the tracing film and painted. The image processing was performed with an image analyzer. The image analysis device used was an image scanner (GT-8000) manufactured by Seiko Epson Corporation, and the obtained image was imported into software (Adobe Photoshop TM 2.5J) of a personal computer (manufactured by Macintosh). Image analysis was performed with the software (Ultimage TM / 242.1.1). At this time, those detected at 0.2 μm or less were excluded so as not to accidentally make the lubricant in the raw material hollow. The height of each cavity was determined from a cross-sectional view showing the shape of the cavity thus obtained. The ratio between the average value of the cavity height and the cavity height (the ratio of the highest cavity to the lowest cavity) was used from the following equation.
Average value of cavity height = total height of individual cavities ÷ number of cavities
... Formula (2)
Cavity Height Ratio = Cavity Height of Cavity with Maximum Height / Cavity Height of Cavity with Minimum Height
... Formula (3)

[Apparent density of film]
Four films were cut into 5.0 cm squares and used as samples. Four samples were stacked, and the total thickness was measured at 10 points using a micrometer with four significant figures, and the total value of the stacked thickness was determined. The average value was divided by 4 and rounded to 3 significant figures to obtain an average thickness per sheet (t: μm). The mass (w: g) of the four samples was measured using an automatic upper pan balance with four significant digits, and the apparent density was determined from the following equation. The apparent density was rounded to two decimal places.
Apparent density (g / cm ³ ) = w / (5.0 × 5.0 × t × 10 ⁻⁴ × 4)
= W × 100 / t
... Formula (4)

[Whiteness]
The degree of whiteness was measured according to JIS-L1015-1981 method using Z-1001DP manufactured by Nippon Denshoku Industries Co., Ltd.

[Total light transmittance]
The total light transmittance was calculated | required by Nippon Denshoku Industries Co., Ltd. product NDH-1001DP.

[Solvent adhesive strength]
Sealing was performed by applying 1,3-dioxolane to the stretched film and bonding the two together. Thereafter, the seal portion is cut to a width of 15 mm in a direction orthogonal to the main shrinkage direction of the film (hereinafter referred to as the orthogonal direction), and set in a universal tension tester STM-50 manufactured by Baldwin Co., Ltd. A 180 ° peel test was performed at 200 mm / min. And the tensile strength at that time was made into solvent adhesive strength.

[Tg (glass transition point)]
Using DSC (model: DSC220) manufactured by Seiko Denshi Kogyo Co., Ltd., 10 mg of an unstretched film was heated from −40 ° C. to 120 ° C. at a heating rate of 20 ° C./min. From the obtained endothermic curve Asked. A tangent line was drawn before and after the inflection point of the endothermic curve, and the intersection was defined as Tg (glass transition point).

[Shrinkage distortion at the label]
A cylindrical label (a label with the main shrinkage direction of the heat-shrinkable film as the circumferential direction) was prepared by adhering both ends to the heat-shrinkable film with dioxolane. After that, using a Fuji Astec Inc steam tunnel (model: SH-1500-L), a 500 ml PET bottle with a transit time of 2.5 seconds and a zone temperature of 80 ° C. (body diameter 62 mm, minimum neck diameter 25 mm) The label was attached by heat shrinking. At the time of mounting, the neck portion was adjusted so that a portion with a diameter of 40 mm was one end of the label. As an evaluation of the finishing property after shrinkage, the strain in the 360 degree direction on the attached label was measured using a gauge, and the maximum value of the strain was obtained. At that time, the standard was as follows.
○: Maximum strain of less than 2 mm ×: Maximum strain of 2 mm or more

[Label adhesion]
The label was attached under the same conditions as those described above for measuring shrinkage finish. Then, when the attached label and the PET bottle were lightly twisted, it was evaluated as “◯” if the label did not move, or “x” if it slipped through or the label and the bottle were displaced.

  Moreover, the polyester used for the Example and the comparative example is as follows.

The polyester used in the examples is as follows.
Polyester a: Polyethylene terephthalate (Intrinsic viscosity (IV): 0.75 dl / g)
Polyester b: neopentyl glycol 30 mol% and ethylene glycol 70 mol%
Polyester composed of terephthalic acid (IV: 0.75 dl / g)
Polyester c: A polyester comprising 50% by weight of polyester a and 50% by weight of titanium oxide
Steal material (IV: 0.51 dl / g)
Polyester d: Polybutylene terephthalate (IV: 1.24 dl / g)
Raw material e: Cyclic polyolefin resin (manufactured by Polyplastics Co., Ltd.
Product name: TOPAS (registered trademark) 6017
Raw material f: crystalline polystyrene resin (made by Nippon Polystyrene Co., Ltd.)
Product name: G797N))
Raw material g: Crystalline polyolefin resin (Product name: EG8, manufactured by Sumitomo Chemical Co., Ltd.)
Raw material h: crystalline polyolefin resin (product name: DX820, manufactured by Mitsui Chemicals, Inc.)
Raw material i: Cyclic polyolefin resin (manufactured by Polyplastics Co., Ltd.
Product name: TOPAS (registered trademark) 6013

[Example 1]
As the A layer, a polyester obtained by mixing 6% by mass of polyester a, 84% by mass of polyester b, and 10% by mass of polyester d. As the B layer, 45% by mass of polyester b, 15% by mass of polyester c, and 10% by mass of polyester d. % And 30% by mass of the raw material e were melted at 280 ° C., and co-extruded from a T die so that the layer thickness ratio was A layer / B layer / A layer = 20/60/20, and quenched with chill rolls. Thus, an unstretched multilayer film having a thickness of 120 μm was obtained. The unstretched multilayer film had a Tg of 65 ° C. The shear rate at the exit of the T die was 110, and the draft ratio was 21.

  The unstretched film was preheated until the film temperature reached 80 ° C., and then stretched in the transverse direction with a tenter. The stretching was performed at 70 ° C. and 4.0 times. Next, the film was heat-set at 82 ° C. while maintaining the film width at the end of stretching to obtain a film.

  Table 4 shows the evaluation results of the obtained film. A film having good cavity height and apparent density and good shrinkage finish was obtained.

[Example 2]
In Example 1, the layer B was 45% by weight of polyester b, 15% by weight of polyester c, 10% by weight of polyester d, 10% by weight of raw material e, 20% by weight of raw material f, and mixed with Example 1 A heat-shrinkable film was continuously produced by the same method. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. According to Example 1, a good film was obtained.

[Example 3]
In Example 1, the B layer was changed to a formulation in which 45% by mass of polyester b, 15% by mass of polyester c, 10% by mass of polyester d, and 30% by mass of raw material f were mixed. A heat-shrinkable film was continuously produced in the same manner as in Example 1 except that the shear rate at the T-die outlet was 70 sec ⁻¹ and the draft ratio was 15. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. In this example in which only the raw material f (polystyrene resin) is added to the B layer, if the shear rate is reduced and the draft ratio is adjusted to be small, the productivity is lowered, but a good film can be obtained according to Example 1. I was able to.

[Example 4]
In Example 1, the B layer was changed to a formulation in which 45% by weight of polyester b, 15% by weight of polyester c, 10% by weight of polyester d, and 30% by weight of raw material i were mixed. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. A preferred void-containing heat-shrinkable polyester film could be obtained according to Example 1.

[Example 5]
In Example 1, the B layer was changed to a formulation in which 55% by mass of polyester b, 15% by mass of polyester c, 10% by mass of polyester d, and 20% by mass of raw material e were mixed. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. A preferred void-containing heat-shrinkable polyester film could be obtained according to Example 1.

[Example 6]
A heat-shrinkable film was continuously produced in the same manner as in Example 1 except that the shear rate at the T-die outlet in Example 1 was 70 sec ⁻¹ and the draft ratio was 15. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. Although it was slightly inferior to Example 1 in productivity, a very good film could be obtained.

[Example 7]
A heat-shrinkable film was continuously produced in the same manner as in Example 1 except that the shear rate at the T-die outlet in Example 1 was 160 sec ⁻¹ and the draft ratio was 28. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. It was particularly preferable in terms of productivity, and a satisfactory film could be obtained.

[Comparative Example 1]
A heat-shrinkable film was continuously produced in the same manner as in Example 3 except that the shear rate at the exit of the T die was 110 and the draft ratio was 21 in Example 3. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. The cavity height was small, the cavity height ratio was large, and the apparent density was high. When only the raw material f (polystyrene resin) was added to the B layer, when the shear rate was increased and the draft ratio was increased, a film having favorable characteristics could not be obtained.

[Comparative Example 2]
In Example 1, the heat-shrinkable film was prepared in the same manner as in Example 1 except that layer B was 45% by weight of polyester b, 15% by weight of polyester c, 10% by weight of polyester d, and 30% by weight of polyester g. Was produced continuously. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. The height of the cavity was small and the apparent density was high.

[Comparative Example 3]
In Example 1, the heat-shrinkable film was formed in the same manner as in Example 1 except that layer B was 45% by weight of polyester b, 15% by weight of polyester c, 10% by weight of polyester d, and 30% by weight of polyester h. Was produced continuously. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. The height of the cavity was small and the apparent density was high.

[Comparative Example 4]
In Example 1, the heat setting temperature after weft stretching was changed to 110 ° C. And the characteristic of the obtained film was evaluated by the same method as Example 1. The evaluation results are shown in Table 4. The heat shrinkage property in the width direction was insufficient, and the label adhesion and the shrinkage distortion at the label were not satisfactory.

  The void-containing heat-shrinkable polyester film of the present invention has excellent shrinkage properties and light-cutting properties, has a large cavity even at a high shear rate and draft ratio, and has a small apparent density. Can be suitably used.

Claims (7)

A multilayer heat-shrinkable polyester film comprising at least two layers, wherein at least one layer is mixed with polyester in a thermoplastic resin incompatible with the polyester, and is stretched at least uniaxially to contain a cavity; A thermoplastic resin incompatible with the polyester has a melt flow rate (MFR) of 20 g / min viscosity and a flexural modulus of 2000 MPa or more, and a multilayer heat-shrinkable polyester film has the following (1 The void-containing heat-shrinkable polyester film characterized by satisfying the requirements (1) to (3).
(1) The hot water thermal shrinkage rate in the width direction when treated in warm water at 95 ° C. for 10 seconds is 50% or more and 80% or less (2) The void-containing heat-shrinkable polyester film is at an arbitrary position (3) The height of the cavity in the cross section of the cavity-containing heat-shrinkable polyester film cut in the width direction at an arbitrary position. Ratio {maximum height (Hmax) ÷ minimum height (Hmin)} is 3 or less The void-containing heat-shrinkable polyester film according to claim 1, wherein the apparent density is 0.99 g / cm ³ or less.   The void-containing heat-shrinkable polyester film according to claim 1 or 2, wherein the solvent adhesive strength is 2N / 15mm width or more and 10N / 15mm width or less.   The void-containing heat-shrinkable polyester film according to claim 1, wherein the thickness is 30 μm or more and 50 μm or less. The void-containing composition according to any one of claims 1 to 4, which is produced under the conditions that the shear rate of the polymer flow in the die when obtaining an unstretched film is 80 sec ^-1 or more and the draft ratio is 18 or more. Heat-shrinkable polyester film. The void-containing heat-shrinkable polyester film according to any one of claims 1 to 5, wherein the thermoplastic resin incompatible with polyester is cyclic polyolefin or a combination of cyclic polyolefin and crystalline polystyrene. . 7. The structure according to claim 1, wherein the structure is an A layer / B layer / A layer in which a B layer having a cavity is an intermediate layer and an A layer having no cavity is provided on both surface layers. Cavity-containing heat-shrinkable polyester film.

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