White heat-shrinkable polyester-based film

Abstract

To provide a white heat-shrinkable polyester-based film and label having a high perforation opening property, and also a good label appearance.SOLUTION: A white heat-shrinkable polyester-based film has ethylene terephthalate as a main constituent component, contains one or more kinds of monomer components that can serve as an amorphous component of 13 mol% or more of the total polyester components, and satisfies the following (1) to (6), (1) a heat shrinkage ratio in a longitudinal direction of 0% or more and 15% or less when processed 10 sec. in hot water of 98°C, (2) a heat shrinkage ratio in a width direction of 50% or more and 80% or less when processed 10 sec. in hot water of 98°C, (3) a trans conformation ratio A1 / A2 being a ratio of an absorbance A1 at 1,340 cm-1 by a polarization ATR-FTIR method to an absorbance A2 at 1,410 cm-1 is 0.3 or more and 0.65 or less in the longitudinal direction, (4) a total light transmissivity of the film is 10% or more and 40% or less, (5) a tear propagation resistance in the longitudinal direction after 20% shrinkage in the width direction in a hot air oven is 1 N / mm or more and 80 N / mm or less, and (6) a distortion index of a molecular orientation angle is 0° or more and 15° or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a white heat-shrinkable polyester film, a white heat-shrinkable polyester label, and a package using the white heat-shrinkable polyester film or label. More specifically, the present invention relates to a white heat-shrinkable polyester film that has light-blocking properties and excellent perforation-opening properties and is suitable for label applications, and a label and package using the white heat-shrinkable polyester film. [Background technology]

[0002] In recent years, stretched films made of polyvinyl chloride resins, polystyrene resins, and polyester resins have been widely used as heat-shrinkable films for applications such as label packaging, cap seals, and integrated packaging that combine the protection of glass bottles, PET bottles, and other products with product labeling. Among these films, polyvinyl chloride films have problems such as poor heat resistance, the generation of chlorine gas when incinerated, and the generation of dioxins. Polystyrene films, on the other hand, have poor solvent resistance, require the use of inks with special compositions when printed, and, unless incinerated at high temperatures, produce large amounts of black smoke accompanied by an unpleasant odor upon incineration. For these reasons, polyester heat-shrinkable films, which have high heat resistance, are easily incinerated, and have excellent solvent resistance, have come into widespread use.

[0003] Furthermore, heat-shrinkable films that shrink significantly in the width direction are generally used for ease of handling during label production, and therefore conventional heat-shrinkable films have been produced by stretching at a high ratio in the width direction to achieve the desired shrinkage ratio when heated.

[0004] In particular, labels attached to beverage containers, such as dairy products, where UV rays can cause deterioration of the contents, require high light-blocking properties. They also need a high heat shrinkage rate so that they can tightly seal the container from the bottom to the opening without any gaps when shrunk, meaning they must be shrunk at high temperatures before being attached.

[0005] For example, Patent Document 1 describes a white heat-shrinkable polyester film that has excellent light blocking properties due to the provision of a layer containing titanium oxide as an intermediate layer, and that has low curling, a low specific gravity, and can be extruded at high speed due to the inclusion of voids.

[0006] However, the white heat-shrinkable polyester film described in Patent Document 1 is hardly stretched in the longitudinal direction perpendicular to the main shrinkage direction, so when it is shrunk to cover a PET bottle as a label, the label cannot be torn along the perforations, and may break during the process of opening (i.e., perforation opening is poor).

[0007] Furthermore, unlike the production of general biaxially stretched films, it is not possible to increase the roll speed during longitudinal stretching to increase the speed of the production line, which results in poor productivity.

[0008] On the other hand, when polyester heat-shrinkable films are stretched in the longitudinal direction to improve the perforation opening properties, the mechanical strength in the longitudinal direction is increased and the perforation opening properties are improved, but shrinkage occurs in the longitudinal direction. Therefore, when the film is shrunk onto a PET bottle and used to cover the bottle as a label at high temperatures, the film shrinks significantly in both the circumferential and perpendicular directions of the bottle, causing distortion in the label and making it look very unattractive.

[0009] Therefore, Patent Document 2 describes a film that has excellent perforation tearability and also suppresses shrinkage in the longitudinal direction, by performing intermediate heat treatment at high temperature after stretching in the longitudinal direction.

[0010] However, the present inventors have studied the method described in Patent Document 2, and have found that when the film is stretched in the longitudinal direction at a high ratio and then heat-treated at a high temperature, a bow-shaped distortion (the so-called bowing phenomenon) occurs due to shrinkage stress acting in the longitudinal direction, and the molecular orientation angle is significantly distorted, particularly near the ends.This causes distortion when the film is shrunk and used as a heat-shrinkable film, damaging the appearance, and deformation of the distorted parts changes the thickness of the intermediate layer containing the white pigment in the deformed parts, resulting in uneven light transmittance and uneven color in the appearance at the distorted parts. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 6519720 [Patent Document 2] Patent No. 5637272 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention has been made in light of the problems of the prior art. That is, an object of the present invention is to provide a white heat-shrinkable polyester film and label that solve the problems of the conventional heat-shrinkable polyester films described above, and that have good perforation tearability when attached as a label and also have good label appearance when attached.

[0013] Another object of the present invention is to provide a white heat-shrinkable polyester film that has light-blocking properties without being subjected to printing or processing and has an excellent appearance. [Means for solving the problem]

[0014] As a result of extensive investigations, the present inventors have found that the above problems can be solved by the following means, and have arrived at the present invention. That is, the present invention comprises the following configurations. 1. A white heat-shrinkable polyester film that contains a polyester resin containing ethylene terephthalate as its main constituent component and that contains 13 mol% or more of one or more monomer components that can become amorphous components in the total polyester resin components, and that satisfies the following requirements (1) to (6). (1) When treated in 98°C hot water for 10 seconds, the hot water shrinkage rate in the longitudinal direction is 0% to 15%. (2) When treated in hot water at 98°C for 10 seconds, the hot water shrinkage rate in the width direction is 50% or more and 80% or less. (3) 1340 cm of heat-shrinkable polyester film measured by polarized ATR-FTIR method -1 Absorbance A1 and 1410cm -1 When the ratio A1 / A2 of the absorbance A1 at the film thickness to the absorbance A2 at the film thickness is taken as the trans conformation ratio, this trans conformation ratio must be 0.3 or more and 0.65 or less in the longitudinal direction, which is the non-shrinking direction of the film. (4) The total light transmittance of the film must be between 10% and 40%. (5) After shrinking the material by 20% in the width direction in a hot air oven, the breaking elongation when tensile tested in the length direction at 1000 mm / min is 70% or more and 300% or less. (6) The distortion index of the molecular orientation angle is 0° or more and 15° or less. 2. A white heat-shrinkable polyester film as described in 1 above, characterized in that the tear propagation strength in the longitudinal direction after shrinking by 20% in the width direction in a hot air oven is 50 N / mm or less, and the ratio of the tear propagation strengths in the longitudinal direction to the width direction (longitudinal direction / width direction) after shrinking by 20% in the width direction in a hot air oven is 1.0 or more and 11.0 or less. 3. The white heat-shrinkable polyester film according to item 1 or 2 above, characterized in that the tensile breaking strength in the longitudinal direction is 60 MPa or more and 200 MPa or less. 4. The white heat-shrinkable polyester film according to any one of the above items 1 to 3, characterized in that the shrinkage stress in the width direction is 2 MPa or more and 18 MPa or less. 5. Apparent specific gravity is 0.9g / cm 3 More than 1.3g / cm3 The white heat-shrinkable polyester film according to any one of the above items 1 to 4, characterized in that: 6. A label using the white heat-shrinkable polyester film according to any one of items 1 to 5 as a base material, and having perforations or a pair of notches formed therein. 7. A package characterized by using the white heat-shrinkable polyester film described in any one of items 1 to 6 as a base material, and covering at least a portion of the outer periphery with a label having perforations or a pair of notches, which is then heat-shrunk. [Effects of the Invention]

[0015] The white heat-shrinkable polyester film of the present invention has high shrinkability in the width direction, which is the main shrinkage direction, and high mechanical strength in the longitudinal direction perpendicular to the width direction. Furthermore, when used as a label, it has good perforation tearability, allowing the film to be neatly torn along the perforations from the start of tearing to the end of tearing. Furthermore, because the heat shrinkage rate in the longitudinal direction, which is perpendicular to the main shrinkage direction, is low and the orientation angle is small, when the film is heat-shrunk and attached to a bottle or the like as a label, it results in a good shrinkage finish with little wrinkles or distortion. The package of the present invention has a good tearability of the coated label, allowing the coated label to be neatly torn along the perforations with a moderate force.

[0016] The white heat-shrinkable polyester film of the present invention is lightweight and has excellent appearance, and has light-blocking properties without being printed or processed, and has excellent appearance even when printed.

[0017] Furthermore, the white heat-shrinkable polyester film of the present invention has extremely high adhesive strength when bonded to both sides (or to the same side) using a solvent, making it suitable for use in various coated labels, including labels for PET bottles. DETAILED DESCRIPTION OF THE INVENTION

[0018] The polyester used in the present invention is one whose main constituent is ethylene terephthalate. That is, it contains 50 mol % or more, preferably 60 mol % or more, of ethylene terephthalate. Other dicarboxylic acid components constituting the polyester of the present invention include aromatic dicarboxylic acids such as isophthalic acid, naphthalenedicarboxylic acid, and orthophthalic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, and decanedicarboxylic acid, and alicyclic dicarboxylic acids.

[0019] When an aliphatic dicarboxylic acid (such as adipic acid, sebacic acid, or decanedicarboxylic acid) is contained, the content is preferably less than 3 mol %. Heat-shrinkable polyester films obtained using polyesters containing 3 mol % or more of these aliphatic dicarboxylic acids are not preferred because they tend to have insufficient film stiffness during high-speed installation.

[0020] It is also preferable not to contain trivalent or higher polycarboxylic acids (for example, trimellitic acid, pyromellitic acid, and anhydrides thereof). Heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids are not preferred because they make it difficult to achieve the required high heat shrinkage rate.

[0021] Examples of the diol component constituting the polyester used in the present invention include aliphatic diols such as ethylene glycol, 1-3 propanediol, 1-4 butanediol, neopentyl glycol, and hexanediol; alicyclic diols such as 1,4-cyclohexanedimethanol; aliphatic ether diols such as diethylene glycol; and aromatic diols such as bisphenol A.

[0022] The polyester used in the white heat-shrinkable polyester film of the present invention is preferably a polyester whose glass transition point (Tg) is adjusted to 60 to 80° C. To adjust the glass transition point, it is preferable to incorporate one or more of cyclic diols such as 1,4-cyclohexanedimethanol, diols having 3 to 6 carbon atoms (for example, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, hexanediol, etc.), and aliphatic ethers such as diethylene glycol.

[0023] Furthermore, the polyester used in the white heat-shrinkable polyester film of the present invention must contain one or more monomer components that can be amorphous components in 100 mol % of polyhydric alcohol components or 100 mol % of polycarboxylic acid components in the entire polyester resin in a total amount of 13 mol % or more, more preferably 16 mol % or more, even more preferably 18 mol % or more, and particularly preferably 20 mol % or more. Examples of monomers that can become amorphous components include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, 1,4-butanediol, and hexanediol and diethylene glycol. Of these, neopentyl glycol, 1,4-cyclohexanedimethanol, and isophthalic acid are preferred. However, if too much of one or more monomer components that can become amorphous components is used, the heat shrinkage properties may be unnecessarily large or the mechanical properties may be insufficient. Therefore, the total content is preferably 40 mol% or less, and more preferably 35 mol% or less.

[0024] The polyester used in the white heat-shrinkable polyester film of the present invention preferably does not contain a diol having 8 or more carbon atoms (e.g., octanediol, etc.) or a polyhydric alcohol having a valence of 3 or more (e.g., trimethylolpropane, trimethylolethane, glycerin, diglycerin, etc.). A white heat-shrinkable polyester film obtained using a polyester containing such a diol or polyhydric alcohol will have difficulty achieving the required high heat shrinkage rate.

[0025] It is also preferable that the polyester used in the white heat-shrinkable polyester film of the present invention contains as little triethylene glycol and polyethylene glycol as possible.

[0026] In addition, various additives, such as waxes, antioxidants, antistatic agents, crystal nucleating agents, viscosity reducers, heat stabilizers, coloring pigments, color inhibitors, and UV absorbers, can be added to the resin forming the white heat-shrinkable polyester film of the present invention as needed. It is preferable to add fine particles as a lubricant to the resin forming the white heat-shrinkable polyester film of the present invention to improve the workability (slipperiness) of the polyethylene terephthalate resin film. Any fine particles can be selected, but examples of inorganic fine particles include silica, alumina, titanium dioxide, calcium carbonate, kaolin, and barium sulfate. Examples of organic fine particles include acrylic resin particles, melamine resin particles, silicone resin particles, and cross-linked polystyrene particles. The average particle size of the fine particles can be appropriately selected as needed within the range of 0.05 to 3.0 μm (as measured with a Coulter counter).

[0027] The particles can be added to the resin that forms the white heat-shrinkable polyester film at any stage during the production of the polyester resin, but it is preferable to add the particles as a slurry dispersed in ethylene glycol or the like during the esterification stage or after the transesterification reaction is completed and before the polycondensation reaction is initiated, and then the polycondensation reaction is allowed to proceed. Alternatively, a vented kneading extruder may be used to blend a slurry of the particles dispersed in ethylene glycol or water with the polyester resin raw material, or a kneading extruder may be used to blend dried particles with the polyester resin raw material.

[0028] In the present invention, in order to adjust the total light transmittance of the film to a specific narrow range and impart light blocking properties to the film, it is suitable to incorporate particles such as inorganic particles or organic particles in the film in an amount of 0.1 to 20 mass %, preferably 1 to 15 mass %, based on the mass of the film. If the particle content is less than 0.1 mass %, it is undesirable, for example, because it becomes difficult to obtain sufficient light blocking properties. On the other hand, if it exceeds 20 mass %, it is undesirable, for example, because the film strength decreases and film formation becomes difficult.

[0029] The particles may be added before polyester polymerization, but are usually added after polyester polymerization. Examples of the inorganic particles to be added include known inert particles such as kaolin, clay, calcium carbonate, silicon oxide, aluminum oxide, titanium oxide, calcium phosphate, and carbon black; high-melting-point organic compounds that are insoluble during melt-forming of polyester resins; and internal particles formed inside polymers during polyester production by metal compound catalysts used in synthesizing crosslinked polymers and polyesters, such as alkali metal compounds and alkaline earth metal compounds. Among these, titanium oxide particles are preferred from the viewpoint of imparting the necessary light-blocking properties.

[0030] 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 size of the particles is measured by the Coulter counter method. The average particle size of the particles is preferably 0.001 μm or more and 3.5 μm or less, and more preferably 0.005 μm or more and 3.0 μm or less. If the average particle size of the particles is less than 0.001 μm, for example, it is likely to be difficult to obtain the required light blocking properties, which is undesirable. If the average particle size of the particles exceeds 3.5 μm, the smoothness of the film surface is poor, which is likely to cause problems such as missing prints, which is undesirable.

[0031] In the present invention, in order to adjust the apparent specific gravity, it is preferable to incorporate fine cavities inside. For example, a foaming material or the like may be mixed and extruded, but a preferred method is to obtain cavities by mixing an incompatible thermoplastic resin into the polyester and stretching it in at least one direction. The thermoplastic resin incompatible with polyester used in the present invention is arbitrary and is not particularly limited as long as it is incompatible with polyester. Specific examples include polystyrene-based resins, polyolefin-based resins, polyacrylic resins, polycarbonate-based resins, polysulfone-based resins, and cellulose-based resins. In particular, polystyrene-based resins or polyolefin-based resins such as polymethylpentene and polypropylene are preferred in terms of the ability to form cavities.

[0032] The polystyrene-based resin refers to a thermoplastic resin containing a polystyrene structure as a basic constituent element, and includes homopolymers such as atactic polystyrene, syndiotactic polystyrene, and isotactic polystyrene, as well as modified resins obtained by grafting or block copolymerizing other components, such as impact-resistant polystyrene resins and modified polyphenylene ether resins, and further mixtures of these polystyrene-based resins with thermoplastic resins that are compatible with them, such as polyphenylene ether.

[0033] Furthermore, the polypropylene resin in the present invention includes homopolymers such as isotactic polypropylene and syndiotactic polypropylene, as well as modified resins obtained by graft or block copolymerization of other components.

[0034] In preparing a polymer mixture obtained by mixing the polyester and an incompatible resin, chips of each resin may be mixed and melt-kneaded in an extruder and then extruded, or both resins may be pre-kneaded in a kneader and then melt-extruded from the extruder. Also, a polystyrene resin may be added in the polyester polymerization step, and chips obtained by stirring and dispersing the resin may be melt-extruded.

[0035] In the film of the present invention, it is preferable to provide a layer B, which has fewer cavities than layer A, on at least one side of layer A, which contains many voids inside. To achieve this structure, it is preferable to feed different raw materials A and B into different extruders, melt them, bond them in the molten state before or inside a T-die, solidify them in close contact with a cooling roll, and then stretch them by the method described below. In this case, it is preferable that the amount of incompatible resin in Layer B, which is the surface layer or one side of the film as a raw material, is less than that in Layer A. This reduces the number of voids in Layer B and the surface roughness, resulting in a film that does not impair the aesthetic appearance of the print. In addition, because there are parts of the film that do not have many voids, the film does not become weak and has excellent wearability.

[0036] Furthermore, the white heat-shrinkable polyester film of the present invention may be subjected to corona treatment, coating treatment, flame treatment, or the like in order to improve the adhesiveness of the film surface.

[0037] When the white heat-shrinkable polyester film of the present invention is treated in 98°C hot water for 10 seconds under no load, the heat shrinkage rate in the longitudinal direction of the film (i.e., the 98°C hot water heat shrinkage rate) calculated from the lengths before and after shrinkage using the following formula (1) must be 0% or more and 15% or less, and the heat shrinkage rate in the transverse direction must be 50% or more and 80% or less. Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) ...Equation (1)

[0038] If the longitudinal hot water heat shrinkage rate at 98°C is less than 0% (i.e., the material expands upon heat treatment), the printed pattern will become distorted, and a good shrink appearance will not be achieved when used as a bottle label, which is undesirable. Conversely, if the longitudinal hot water heat shrinkage rate at 98°C is greater than 15%, when used as a label, bow-shaped distortion will likely occur in the direction perpendicular to the application direction upon heat shrinkage, which is undesirable. Therefore, the longitudinal hot water heat shrinkage rate at 98°C is preferably between 0% and 15%; more preferably between 1% and 10%; and even more preferably between 1% and 8%. The reason for using a measurement temperature of 98°C is that when used as a label for beverages and other containers that require light blocking properties, the label is expected to be shrunk by high-temperature steam in a steam tunnel or the like to cover the container from the bottom to the opening.

[0039] Furthermore, when the white heat-shrinkable polyester film of the present invention is treated in hot water at 98°C for 10 seconds under no load, if the hot water heat shrinkage rate in the width direction of the film calculated from the length before and after shrinkage using the above formula (1) is less than 50%, the shrinkage amount is so small that the label after heat shrinkage will wrinkle or sag, which is undesirable. Conversely, if the hot water heat shrinkage rate in the width direction at 98°C is more than 80%, when used as a label, distortion during heat shrinkage is likely to occur, or so-called "jumping up" will occur, which is undesirable. The lower limit of the hot water heat shrinkage rate in the width direction at 98°C is preferably 55% or more, and particularly preferably 60% or more. The upper limit of the hot water heat shrinkage rate in the width direction at 98°C is particularly preferably 75% or less.

[0040] Furthermore, the white heat-shrinkable polyester film of the present invention preferably has a widthwise shrinkage stress of 2 MPa or more and 18 MPa or less when heated to 90°C. A widthwise shrinkage stress of less than 2 MPa when heated to 90°C is undesirable because the film tends to loosen and wrinkle when used as a bottle label, making it difficult to obtain a good shrinkage appearance. Conversely, a widthwise shrinkage stress of more than 18 MPa when heated to 90°C is undesirable because the film tends to distort during heat shrinkage when used as a label. The lower limit of the widthwise shrinkage stress when heated to 90°C is more preferably 4 MPa or more, even more preferably 5 MPa or more, and particularly preferably 6 MPa or more. The upper limit of the widthwise shrinkage stress when heated to 90°C is more preferably 15 MPa or less, and particularly preferably 13 MPa or less.

[0041] The white heat-shrinkable polyester film of the present invention preferably has a longitudinal tear strength of 1 N / mm or more and 50 N / mm or less after being shrunk by 20% in the width direction in a hot air oven at 90°C, and the ratio of the longitudinal tear strength to the width direction is preferably 1 or more and 11 or less.

[0042] [Method for measuring tear strength] The film is shrunk 20% in the width direction in a hot air oven adjusted to 90°C for 20 seconds, and then sampled into test pieces of the specified size in accordance with JIS-K-7128-2. The tear resistance of the notched test pieces in the longitudinal and width directions of the film is then measured using an Elmendorf tear tester. The tear strength per unit thickness is then calculated using the following formula 2: Tear strength = tear resistance (N) / thickness (mm) Formula 2 Furthermore, the tear strength ratio is calculated using the following formula 3 from the tear strength value after shrinking the sample by 20% in the width direction in the above-described hot air oven at 90°C. Tear strength ratio = longitudinal tear strength / transverse tear strength Formula 3

[0043] If the longitudinal tear strength after shrinking 20% ​​in the width direction in a 90°C hot air oven is less than 1 N / mm, it is undesirable because, when used as a label, it may easily tear due to impact, such as being dropped during transportation. Conversely, if the tear strength exceeds 80 N / mm, the resistance in the initial stage of tearing the label increases, not only does the force required to cut increase, but the film may also tear during cutting (ease of tearing), resulting in poor tearability. The lower limit of the tear strength is preferably 4 N / mm or more, and more preferably 6 N / mm or more. The upper limit of the tear strength is preferably 45 N / mm or less, and even more preferably 40 N / mm or less.

[0044] If the tear strength ratio in the longitudinal direction to the width direction is 11 or more, when used as a label, the film will tear relatively easily in the width direction, and when pulled in the longitudinal direction and cut along the perforation, the tear will tend to flow in the width direction, causing the film to tear along the way, which is undesirable. Conversely, if the tear strength ratio is 1 or less, the film will tear very easily in the longitudinal direction, making it more likely that the label will tear if dropped or otherwise impacted during transportation.

[0045] Furthermore, the white heat-shrinkable polyester film of the present invention preferably has a tensile breaking strength in the longitudinal direction of 60 MPa or more and 200 MPa or less when the tensile breaking strength is determined by the following method.

[0046] [Method for measuring tensile breaking strength] In accordance with JIS-K7127, a rectangular test piece of a specified size is prepared, and the test piece is held at both ends with a universal tensile tester and subjected to a tensile test at a tensile speed of 200 mm / min. The strength (stress) at the time of tensile failure in the longitudinal direction of the film is calculated as the tensile breaking strength.

[0047] A longitudinal tensile breaking strength of less than 60 MPa is undesirable because it weakens the "stiffness" when labeling and attaching to bottles, etc. Conversely, a tensile breaking strength of more than 200 MPa is undesirable because it results in poor cuttability (ease of tearing) in the initial stage of tearing the label. The lower limit of the tensile breaking strength is preferably 65 MPa or more, more preferably 70 MPa or more, and particularly preferably 80 MPa or more.

[0048] The present inventors have investigated the problem of heat-shrinkable films tearing during opening along their perforations and have found that films with poor openability show a significant decrease in elongation after shrinking and attaching them as labels, making them more susceptible to tearing, whereas films with improved openability according to the present invention show almost no decrease in film elongation. Specifically, the white heat-shrinkable polyester film of the present invention must have a longitudinal tensile elongation at break of 70% or more and 300% or less, as measured by the following method: the film is shrunk by 20% in the width direction in a hot air oven adjusted to 90°C, and then a tensile test is conducted at a tension speed of 1000 mm / min.

[0049] [Method for measuring tensile elongation at break after shrinkage] After shrinking the film by 20% in the width direction in a hot air oven adjusted to 90°C, strip-shaped test pieces of a specified size are prepared in accordance with JIS-K7127, and the test pieces are gripped at both ends with a universal tensile tester and subjected to a tensile test at a tensile speed of 1000 mm / min. The elongation (strain) at tensile break in the longitudinal direction of the film is calculated as the tensile break elongation.

[0050] If the tensile breaking elongation in the longitudinal direction after shrinking the film by 20% in the width direction in a hot air oven adjusted to 80°C falls below 70%, it is undesirable because the film will tear easily when opening along the perforation after being attached as a label, resulting in poor cuttability. Conversely, a higher tensile breaking elongation is desirable because it is less likely to tear, but with the method of the present invention, 300% is the technical limit. The lower limit of the tensile breaking elongation is preferably 75% or more, more preferably 80% or more, and particularly preferably 90% or more.

[0051] The lower limit of the thickness of the white heat-shrinkable polyester film of the present invention is preferably 10 μm or more from the viewpoint of obtaining the necessary light blocking properties. On the other hand, the upper limit is not particularly limited, but a thickness exceeding 300 μm is uneconomical and undesirable because the weight increases when used as a label. In addition, when the white heat-shrinkable polyester film of the present invention has a laminated structure, the layer containing particles and voids is preferably at least 5 μm or more in order to obtain light transmittance. The thickness of each of the other layers is not particularly limited, but is preferably 2 μm or more.

[0052] In the present invention, the apparent specific gravity of the film is 1.2 g / cm 3 It is preferably 1.15 g / cm or less, and more preferably 1.15 g / cm 3 More preferably, 1.1 g / cm or less 3 The following is the reason. A low apparent specific gravity and light weight are significant advantages in mass production, and the white heat-shrinkable polyester film of the present invention can achieve a desirable light weight due to the presence of voids inside. In particular, by adopting the machine-transverse stretching method described below, a larger areal stretching ratio can be adopted compared to conventional uniaxially stretched films having voids, and an even lower apparent density can be obtained. However, an apparent specific gravity that is too low will impair the strength of the film itself, so the apparent specific gravity is set to 0.9 g / cm. 3 It is preferable that this is equal to or greater than this.

[0053] In the present invention, the total light transmittance must be 40% or less, preferably 35% or less, more preferably 30% or less, and even more preferably 20% or less. A transmittance of more than 40% is not preferable because it may result in poor appearance, such as the contents being visible through the film or the printed matter being difficult to see. Regarding the lower limit, the lower the total light transmittance, the better from the viewpoint of the ability to conceal the contents. However, an increase in the number of particles increases the specific gravity, and an increase in voids reduces the strength. Therefore, the lower limit of the total light transmittance that can be achieved by the product of the present invention is 10%.

[0054] The heat-shrinkable polyester film of the present invention has a 1340 cm -1 Absorbance A1 and 1410cm -1 The ratio A1 / A2 (hereinafter referred to as absorbance ratio) of A1 to A2 at the time of shrinkage must be 0.5 or more in the main shrinkage direction of the film (hereinafter referred to as width direction), and 0.3 to 0.65 in the direction perpendicular to the main shrinkage direction (hereinafter referred to as length direction).

[0055] The absorbance ratio represents the trans-conformation ratio of the molecular orientation. In developing the present invention, the inventors conducted extensive research into the molecular orientation state that results in a tensile elongation at break of at least 50% at a tension speed of 1000 mm / min after shrinking by 20% in the width direction, and discovered that the trans conformation ratio is related to cuttability by changing the film-forming conditions. That is, the inventors focused on the molecular orientation (trans conformation ratio) in a film biaxially stretched in the longitudinal direction (MD) and the width direction (TD), and investigated the trans conformation ratio in the longitudinal direction to determine what molecular orientation exhibits favorable cuttability and satisfies the required heat shrinkage properties, leading to the present invention.

[0056] That is, the inventors have obtained experimental results showing that changes in the trans conformation ratio and cuttability are related by changing the stretching temperature and the film-forming conditions in the longitudinal direction (described later). The trans conformation is thought to represent the alignment state of molecular chains, and it is believed that a high trans conformation ratio makes it easier for the molecular chains to maintain an aligned state in the longitudinal direction even after heat shrinking in the width direction, which not only makes it easier for tears to propagate along the molecular chains but also maintains a high tensile breaking elongation of the film, improving perforation cuttability.

[0057] The absorbance ratio in the longitudinal direction of the film must be 0.3 to 0.65. If the absorbance ratio in the width direction of the film is less than 0.3, the molecular orientation will be low, resulting in increased resistance when torn and poor perforation cutting properties. The absorbance ratio is more preferably 0.32 or more, and even more preferably 0.35 or more. On the other hand, if the absorbance ratio in the longitudinal direction of the film exceeds 0.65, the molecular orientation will be too high, making the film more susceptible to tearing due to impact and increasing the thermal shrinkage rate in the longitudinal direction. This will make the label more susceptible to wrinkling and distortion after shrinkage. The absorbance ratio in the longitudinal direction is more preferably 0.57 or less, and even more preferably 0.55 or less.

[0058] On the other hand, the absorbance ratio in the film width direction is preferably 0.5 or more. If the absorbance ratio in the film width direction is less than 0.5, the molecular orientation will be low, resulting in a decrease in shrinkage stress, making the film more likely to loosen when worn, and making it difficult to achieve the heat shrinkage rate required for a heat-shrinkable film. The absorbance ratio in the film width direction is more preferably 0.55 or more, and even more preferably 0.6 or more.

[0059] Furthermore, the white heat-shrinkable polyester film of the present invention must have a molecular orientation angle of 15 degrees or less when the film width direction is set to 0 degrees. The upper limit of the molecular orientation angle is more preferably 13 degrees or less, and even more preferably 12 degrees or less. As for the lower limit, the closer the molecular orientation angle is to 0 degrees, the better.

[0060] In the present invention, the longitudinal direction of the film is defined as the X axis, the width direction of the film as the Y axis, and the thickness direction of the film as the Z axis. When viewed on the XY plane of the film, the direction with the greatest degree of molecular orientation is referred to as the molecular orientation axis. The molecular orientation angle refers to the angle at which the molecular orientation axis is misaligned with the longitudinal or width direction of the film. To measure the molecular orientation angle, a rectangular sample is taken from one end of the film to the other end in the width direction. The molecular orientation angle (the angle in the direction of the molecular orientation axis) of each cut-out film sample is measured using a molecular orientation angle measuring device (MOA-6004) manufactured by Oji Scientific Instruments. The width direction of the film is defined as 0 degrees. When the direction of the molecular orientation axis is less than 45 degrees relative to the width direction, the difference from 0 degrees is measured. When the direction of the molecular orientation axis is greater than 45 degrees, the difference from 90 degrees is measured. The molecular orientation angle of each rectangular sample taken at the edge and center of the film is measured using the above method, and the difference in molecular orientation angle between the edge and center is taken as the distortion index of the molecular orientation angle, as expressed by the following equation 4. Molecular orientation angle distortion index = (absolute value of the difference in molecular orientation angle between samples taken from the edge and the center) Equation 4

[0061] The molecular orientation angle is small in the central portion of the film in the width direction, and is usually 15° or less. On the other hand, the difference in molecular orientation is large near the edges of the film, exceeding 15°. This is because when an amorphous polyester film used for heat-shrinkable film is biaxially stretched in the longitudinal and transverse directions, after stretching in the longitudinal direction (longitudinal direction), the film is preheated in order to be stretched in the transverse direction (width direction) using a tenter machine or the like, and strong shrinkage stress acts in the longitudinal direction, which easily distorts the molecular orientation angle into a bow shape (prone to bowing). In the present invention, this problem is addressed using the method described below.

[0062] To satisfy the above characteristics, the film of the present invention may be composed of a single layer. However, a preferred layer configuration is one having at least two or more layers. A particularly preferred layer configuration is a B / A / B layer configuration, with an A layer containing voids and a B layer containing fewer voids than the A layer. The absence of voids in the B layer, which forms the surface layer, improves surface smoothness and reduces the occurrence of missing dots during printing, which is preferable for aesthetic appearance during printing. The thickness ratio of A layer to B layer is preferably A / B = 1 / 1 or greater, more preferably 2 / 1 or greater, and even more preferably 4 / 1 or greater. A ratio of less than 1 / 1 makes it difficult to achieve both aesthetic printability and reduced apparent density. A B / A / B layer configuration is also preferable for suppressing undesirable curling after shrinkage treatment.

[0063] The method for producing the white heat-shrinkable polyester film of the present invention is not particularly limited, but will be described below by taking an example. The white heat-shrinkable polyester film of the present invention can be obtained by melt-extruding a polyester raw material containing ethylene terephthalate as the main constituent component and containing at least 13 mol% in total of one or more monomer components that can become amorphous components in the entire polyester resin component using an extruder to form an unstretched film, and then biaxially stretching and heat-treating the unstretched film using the specified method described below. If necessary, multiple resin composition raw materials can also be co-extruded to obtain a laminated unstretched film.

[0064] When melt-extruding the raw resin, it is preferable to dry the polyester-based raw material using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer. After drying the polyester-based raw material in this way, it is melted at a temperature of 200 to 300°C using an extruder and extruded into a film. For such extrusion, any existing method such as a T-die method or a tubular method can be used.

[0065] When a layer formed by mixing the polyester and an incompatible resin is melt-extruded, it is preferable to control the domain size of the incompatible resin generated in the polyester to 0.1 μm or more and 50 μm or less. A domain size of 0.1 μm is undesirable because the void size generated by stretching becomes too small, resulting in a high apparent specific gravity and the required light transmittance cannot be obtained. A domain size of 50 μm or more is undesirable because the void size becomes too large, resulting in a decrease in tensile breaking elongation and poor perforation tearability. In order to uniformly disperse the incompatible resin and control the domain size, it is preferable to perform melt-extrusion using a kneading extruder having two or more screws.

[0066] In a layer containing a blend of polyester and an incompatible resin, it is preferable to control the domain shape of the incompatible resin in the unstretched film so that it is longer in the longitudinal direction than in the width or thickness direction. This domain shape allows the voids generated after stretching to be longer in the longitudinal direction, facilitating tear propagation along the longitudinal direction and improving perforation tearability. To control the domain shape, the ratio of the cooling roll speed to the linear velocity of the molten resin extruded from the die (draft ratio) is preferably 2 to 20 times, more preferably 3 to 15 times. If the draft ratio is less than 2 times, the incompatible resin domains will not be stretched in the longitudinal direction, making it impossible to obtain a longitudinally elongated shape. If the draft ratio is more than 20 times, necking will be significant, increasing the thickness of the edges, which will make it difficult to achieve uniform longitudinal stretching in the subsequent process and increasing distortion of the orientation axis in the width direction, which is undesirable.

[0067]

[0033] Furthermore, it is preferable to obtain the white heat-shrinkable polyester film of the present invention by stretching the obtained unstretched film in the longitudinal direction under predetermined conditions as described below, quenching the longitudinally stretched film, once subjecting it to heat treatment and relaxation treatment in the longitudinal direction, cooling the heat-treated film under predetermined conditions, stretching it in the width direction under predetermined conditions, and heat-treating it again. A preferred film-forming method for obtaining the white heat-shrinkable polyester film of the present invention will be described in detail below.

[0068] As described above, conventionally, heat-shrinkable polyester films have been produced by stretching an unstretched film only in the direction in which it is desired to shrink (i.e., the main shrinkage direction, usually the width direction). As a result of the inventors' investigation of conventional production methods, they have found that the conventional production methods of heat-shrinkable polyester films have the following problems. Simply stretching the film in the width direction increases the tear strength in the longitudinal direction, as mentioned above, making it difficult to open the perforation when used as a label. In addition, it is difficult to increase the line speed of the film-making equipment. If the method of stretching in the longitudinal direction and then in the width direction is adopted, shrinkage force in the width direction can be generated, but shrinkage force in the longitudinal direction will also be generated at the same time, resulting in a poor finish after shrinking and attaching to the label. If the method of stretching in the longitudinal direction, followed by heat treatment and then stretching in the width direction is adopted, the thermal shrinkage rate in the longitudinal direction can be reduced, but as mentioned above, the molecular orientation angle in the width direction becomes large due to bowing, and when made into a label, distortion is likely to occur after shrinkage and installation.

[0069] Based on the above findings, the present inventors have come to the conclusion that, in order to simultaneously achieve good perforation-opening properties and shrinkage finish, it is necessary to reduce the distortion of the molecular orientation angle caused by longitudinal stretching while orienting the molecules in the film in the longitudinal direction. As a result, by taking the following measures when producing a film by a so-called longitudinal-transverse stretching method in which the film is stretched in the longitudinal direction and then in the width direction, it is possible to achieve a molecular state that is oriented in the longitudinal direction but does not contribute to shrinkage force, thereby obtaining a white heat-shrinkable polyester film that simultaneously achieves good perforation-opening properties and shrinkage finish, and have come up with the present invention.

[0070] [Method for producing heat-shrinkable polyester film of the present invention] The heat-shrinkable polyester film of the present invention is produced by the following procedure. (1) Control of longitudinal stretching conditions (2) Intermediate heat treatment after longitudinal stretching (3) Relaxation in the longitudinal direction (4) Natural cooling between intermediate heat treatment and transverse stretching (heat blocking) (5) Forced cooling of the film after natural cooling (6) Control of transverse stretching conditions (7) Heat treatment after transverse stretching Each of the above means will be explained below in order.

[0071] (1) Control of longitudinal stretching conditions In the production of a film by the longitudinal-transverse stretching method of the present invention, the stretching temperature must be Tg or higher and Tg+30° C. or lower, and the film must be stretched longitudinally at a magnification of 3.3 times or higher and 5.0 times or lower.

[0072] Furthermore, the longitudinal stretching of the present invention is preferably carried out in a multi-stage stretching process of two or more stages, and in particular, it is preferable that the stretching distance of the film between the first-stage stretching rolls is longer than that between the second-stage stretching rolls. The inventors of the present invention have investigated a stretching mode that suppresses the bowing phenomenon, i.e., that simultaneously reduces the molecular orientation angle in the longitudinal direction and improves the trans conformation ratio in the longitudinal direction. They have found that increasing the distance over which the film is stretched between the rolls in the first half of stretching reduces the yield stress and is advantageous for reducing the distortion of the molecular orientation angle in the longitudinal direction, while shortening the distance between the rolls in the second half of stretching improves the stress reached during stretching and is advantageous for improving the trans conformation ratio. When performing two-stage stretching, it is preferable to strike a balance between these factors. In particular, the stretching distance between the rolls in the first half of stretching is preferably 150 mm or more and 500 mm or less, and the stretching distance between the rolls in the second half of stretching is preferably adjusted within a range of 0.4 to 0.8 times the distance between the rolls in the first half of stretching. When two-stage stretching is performed, the stretching ratio in the first stage is preferably 1.2 to 2.0 times, and the stretching ratio in the second stage is preferably 2.2 to 3.5 times.

[0073] When stretching in the longitudinal direction, a large total longitudinal stretching ratio tends to improve the trans-conformation ratio in the longitudinal direction and improve perforation cuttability. However, if the longitudinal stretching ratio is too high, the film will undergo oriented crystallization after longitudinal stretching, making it more likely to break during the transverse stretching process, which is undesirable. Therefore, the upper limit of the total longitudinal stretching ratio is set to 5.0. The longitudinal stretching ratio is more preferably 4.8 times or less, and even more preferably 4.4 times or less. On the other hand, if the longitudinal stretching ratio is too small, the thermal shrinkage in the longitudinal direction will be small, but the trans-conformation ratio in the longitudinal direction will also be small, which will increase tearing in the longitudinal direction and tend to deteriorate perforation cuttability, and the tensile breaking strength will also be low, which is undesirable. The longitudinal stretching ratio is preferably 3.0 times or more, more preferably 3.2 times or more, and even more preferably 3.3 times or more.

[0074] (2) Intermediate heat treatment after longitudinal stretching In order to thermally relax the molecules oriented in the machine direction, heat treatment is performed after machine stretching. At this time, after machine stretching the unstretched film, it is necessary to hold both ends in the width direction with clips in a tenter and then heat treat it at a temperature of Tg+40°C or higher and Tg+120°C or lower for 6.0 seconds or higher and 12.0 seconds or lower (hereinafter referred to as intermediate heat treatment). By performing such intermediate heat treatment, it is possible to reduce the thermal shrinkage in the machine direction caused by machine stretching.

[0075] The temperature of the intermediate heat treatment is preferably Tg+40°C or higher, and more preferably Tg+42°C or higher. Preferably, the intermediate heat treatment temperature is Tg+110°C or lower, more preferably Tg+100°C or lower. If the temperature of the intermediate heat treatment is too high, the molecular chains oriented by longitudinal stretching will change to crystals, making it impossible to obtain a high heat shrinkage rate after transverse stretching. On the other hand, the time of the intermediate heat treatment must be adjusted appropriately depending on the raw material composition within the range of 6.0 seconds to 12.0 seconds. The amount of heat applied to the film is important in the intermediate heat treatment, and if the temperature of the intermediate heat treatment is low, a long intermediate heat treatment time will be required. However, if the intermediate heat treatment time is too long, the equipment will become large, so it is preferable to adjust the temperature and time appropriately.

[0076] (3) Relaxation process in the longitudinal direction To reduce the molecular orientation angle while maintaining the molecules oriented in the longitudinal direction by longitudinal stretching, thermal relaxation is preferred. By incorporating a relaxation step, the residual shrinkage stress in the longitudinal direction of the film after longitudinal stretching can be reduced, suppressing distortion of the molecular orientation angle during intermediate heat treatment or cross-direction stretching, and improving the shrinkage finish. Furthermore, even when relaxed in the longitudinal direction, the transconformation ratio in the longitudinal direction can be maintained by leaving a certain amount of molecular chains oriented in the longitudinal direction. We investigated methods for controlling tear strength and tensile breaking strength. We found that these can be controlled by relaxing the film in the longitudinal direction using either or both of the following methods (i) and (ii).

[0077] (i) A step of heating the longitudinally stretched film at a temperature of Tg or higher and Tg+60°C or lower, and relaxing the film by 10% to 50% in the longitudinal direction for 0.05 to 5 seconds using rolls with different speeds. The heating means may be a temperature-controlled roll, near infrared rays, far infrared rays, a hot air heater, or the like.

[0078] (ii) In the intermediate heat treatment step, the distance between the holding clips in the opposing tenter is reduced to perform relaxation in the longitudinal direction by 10% to 40% for a time of 0.1 seconds to 12 seconds.

[0079] Each step will be explained below.

[0080] (i) Relaxation after longitudinal stretching After longitudinal stretching, the film is heated to a temperature of Tg or higher and Tg+60°C or lower, and is desirably relaxed in the longitudinal direction by 10% to 50% for a time of 0.05 seconds to 5.0 seconds using rolls with different speeds. If the temperature is lower than Tg, the film after longitudinal stretching does not shrink and relaxation cannot be performed, which is not preferred. On the other hand, if the temperature is higher than Tg+60°C, the film crystallizes, resulting in poor transparency and other properties, which is also not preferred. The film temperature during relaxation is more preferably Tg+10°C or higher and Tg+55°C or lower, and more preferably Tg+20°C or higher and Tg+50°C or lower. The bottom is even better.

[0081] The time for relaxing the film in the longitudinal direction after longitudinal stretching is 0.05 seconds or more and 5 seconds or less. If the time is less than 0.05 seconds, the relaxation time will be too short, and the temperature will be lower than Tg. If the temperature is not high enough, it will cause uneven relaxation, which is not desirable. If it is longer, the film can be relaxed at a lower temperature, but the equipment becomes larger. Therefore, it is preferable to adjust the temperature and time appropriately. The relaxation time is preferably It is 0.1 seconds or more and 4.5 seconds or less, and more preferably 0.5 seconds or more and 4 seconds or less.

[0082] Furthermore, if the relaxation rate in the longitudinal direction of the film after longitudinal stretching is less than 10%, the film will not split in the longitudinal direction. If the relaxation rate in the longitudinal direction of the film after longitudinal stretching is greater than 50%, the tear strength in the longitudinal direction increases, which is undesirable because the relaxation rate of the film after longitudinal stretching is unsatisfactory, and the perforation cuttability deteriorates. The relaxation rate of the film after longitudinal stretching is more preferably 15% to 45%, and even more preferably 20% to 40%.

[0083] (ii) Relaxation during intermediate heat treatment In the intermediate heat treatment step, it is desirable to relax the film in the longitudinal direction by 20% to 40% for a time of 0.1 to 12 seconds by shortening the distance between the holding clips in the opposing tenters. A relaxation rate of less than 20% is undesirable because the molecular orientation in the longitudinal direction is not sufficiently relaxed, resulting in a high thermal shrinkage rate in the longitudinal direction. A relaxation rate of more than 40% is undesirable because it increases the tear strength in the longitudinal direction and deteriorates the perforation cuttability. A relaxation rate of 22% or more is more preferable, 38% or less is more preferable, and 36% or less is even more preferable.

[0084] The time for longitudinal relaxation in the intermediate heat treatment step is preferably 0.1 seconds or more and 12 seconds or less. If it is less than 0.1 seconds, the relaxation time will be too short, and unless the temperature is raised above Tg + 40°C, relaxation unevenness will occur, which is not preferable. If the relaxation time is longer than 12 seconds, there will be no problem with the film, but the equipment will become large, so it is preferable to adjust the temperature and time appropriately. The relaxation time is more preferably 0.3 seconds or more and 11 seconds or less, and even more preferably 0.5 seconds or more and 10 seconds or less.

[0085] (4) Natural cooling between intermediate heat treatment and transverse stretching (heat blocking) In the production of films using the longitudinal-transverse stretching method of the present invention, intermediate heat treatment is required after longitudinal stretching. After the longitudinal stretching and intermediate heat treatment, the film must be passed through an intermediate zone where no active heating is performed for a time of 0.5 to 3.0 seconds. That is, an intermediate zone is preferably provided in front of the transverse stretching zone of the tenter for transverse stretching. The film after longitudinal stretching and intermediate heat treatment is introduced into the tenter and passed through this intermediate zone for a predetermined time, after which forced cooling, as described below, is performed and transverse stretching is then performed. Additionally, in the intermediate zone, it is preferable to block the accompanying flow accompanying the running of the film and the hot air from the cooling zone so that when a strip of paper is dropped without the film passing through the intermediate zone, the paper piece hangs almost completely vertically. Note that passing through the intermediate zone for a time shorter than 0.5 seconds is undesirable because the transverse stretching becomes high-temperature stretching and the transverse thermal shrinkage cannot be sufficiently increased. Conversely, a time of 3.0 seconds is sufficient, and setting a time longer than this is undesirable because it wastes equipment. The time required to pass through the intermediate zone is more preferably 0.7 seconds or more, even more preferably 0.9 seconds or more, more preferably 2.8 seconds or less, and even more preferably 2.6 seconds or less.

[0086] (5) Forced cooling of the film after natural cooling In the production of a film by the longitudinal-transverse stretching method of the present invention, it is necessary to actively force-cool the film so that its temperature is Tg or higher and Tg + 40°C or lower, rather than simply stretching the naturally cooled film transversely. By performing such a forced cooling treatment, the molecular chains oriented in the longitudinal direction are fixed, making it possible to obtain a film that has good perforation tearability when used as a label. The temperature of the film after forced cooling is preferably Tg + 2°C or higher, even more preferably Tg + 4°C or higher, more preferably Tg + 35°C or lower, and even more preferably Tg + 30°C or lower.

[0087] When a film is forcibly cooled, if the temperature of the film after forcibly cooling remains above Tg + 40°C, the heat shrinkage rate of the film in the width direction will be low, resulting in insufficient shrinkage when made into a label. However, by controlling the film temperature after forcibly cooling to Tg + 40°C or lower, it is possible to maintain a high heat shrinkage rate in the width direction of the film. Furthermore, if the film temperature after forcibly cooling remains above Tg + 40°C, the stress of the transverse stretching performed after cooling will be small, resulting in low shrinkage stress in the width direction and poor conformability to the bottle. By performing forcible cooling so that the film temperature after cooling is Tg + 40°C or lower, it is possible to maintain a high shrinkage stress in the width direction.

[0088] (6) Control of transverse stretching conditions Transverse stretching must be performed in a tenter with both widthwise edges held by clips at a temperature of Tg + 5°C to Tg + 40°C, at a stretching ratio of 3 to 7. Stretching under these conditions aligns the molecules in the widthwise direction, enabling the film to exhibit high shrinkage force in the widthwise direction, resulting in a film with good perforation tearability when used as a label. A transverse stretching ratio of less than 3 is undesirable because it reduces the thermal shrinkage rate in the widthwise direction and also reduces the stress applied to the film during stretching, resulting in increased thickness unevenness. On the other hand, a transverse stretching ratio of more than 7 is undesirable because it results in excessively large voids in the film, reducing the apparent specific gravity and the mechanical strength of the film due to the voids. The transverse stretching ratio is preferably 3.5 times or more, more preferably 4 times or more, more preferably 6.5 times or less, and even more preferably 6 times or less. The transverse stretching temperature is more preferably Tg+10°C or higher, even more preferably Tg+13°C or higher, more preferably Tg+37°C or lower, and even more preferably Tg+34°C or lower. When stretching in the transverse direction, if the stretching temperature exceeds Tg+40°C, the thermal shrinkage rate in the width direction decreases. However, by controlling the stretching temperature to Tg+40°C or lower, it is possible to increase the shrinkage rate in the width direction. Furthermore, if the stretching temperature exceeds Tg+40°C, the stress during transverse stretching decreases, the shrinkage stress in the width direction decreases, and the film's ability to conform to the bottle deteriorates. By controlling the transverse stretching temperature to Tg+40°C or lower, it is possible to increase the shrinkage stress in the width direction. Furthermore, if the film temperature exceeds Tg+40°C, the stretching stress during transverse stretching decreases, which tends to increase thickness unevenness in the width direction, which is undesirable.

[0089] On the other hand, if the stretching temperature is lower than Tg+5°C, the stress applied to the film during stretching will be too great, making it more likely to break during transverse stretching, and the increase in voids inside the film will reduce the apparent specific gravity of the film and its mechanical strength, which is undesirable.

[0090] (7) Heat treatment after transverse stretching (final heat treatment) After transverse stretching, the film must be finally heat-treated in a tenter at a temperature of Tg or higher but not exceeding Tg + 50°C for 1 to 9 seconds while both widthwise edges are held with clips. Heat-treatment temperatures higher than Tg + 50°C result in a decrease in the heat shrinkage in the width direction, and the heat shrinkage at 98°C becomes less than 50%, which is undesirable. Heat-treatment temperatures lower than Tg cannot adequately relieve the shrinkage stress generated by stretching, resulting in excessively high shrinkage stress, which makes the film prone to wrinkles and distortion during shrinkage. While a longer heat-treatment time is preferable, a longer time would require larger equipment, so a time of 9 seconds or less is preferred. [Example]

[0091] The present invention will be specifically explained below by showing examples, but the present invention is not limited to these examples.

[0092] [Absorbance ratio] The infrared absorption spectrum was measured using an FT-IR spectrometer "FTS 60A / 896" (Varian) with a measurement wavenumber range of 650 to 4000 cm-1 and an accumulation count of 128, using polarized light by the ATR method. The ratio A1 / A2 of the absorbance A1 at 1340 cm-1 and the absorbance A2 at 1410 cm-1 was taken as the absorbance ratio (trans conformation ratio).

[0093] [Heat shrinkage rate (hot water heat shrinkage rate)] The film was cut into a 10 cm x 10 cm square and immersed in warm water of a specified temperature ±0.5°C for 10 seconds without load to allow it to shrink, then immersed in water at 25°C ±0.5°C for 10 seconds and removed from the water, measuring the longitudinal and transverse dimensions of the film, and calculating the thermal shrinkage rate according to the following formula (1). The direction with the largest thermal shrinkage rate was defined as the main shrinkage direction. Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

[0094] [Shrinkage stress] A sample measuring 200 mm in length in the main shrinkage direction and 20 mm in width was cut from the heat-shrinkable film and measured using a Toyo Baldwin Co., Ltd. (now Orientec) heating furnace-equipped strength and elongation tester (Tensilon (registered trademark of Orientec)). The heating furnace was preheated to 90°C, and the distance between chucks was set to 100 mm. The airflow from the heating furnace was stopped temporarily, the furnace door was opened, and the sample was attached to the chuck. The door was then quickly closed and the airflow was resumed. The shrinkage stress was measured for 30 seconds or more, and the shrinkage stress (MPa) after 30 seconds was determined. The maximum value during the measurement was recorded as the shrinkage stress (MPa).

[0095] [Tear strength] The film was cut to 150 mm in the longitudinal direction and 238 mm in the width direction, fixed to a metal frame measuring 190 mm in the width direction and 150 mm or more in the longitudinal direction, and placed in a 90°C hot air oven for 20 seconds to shrink the film by 20% in the width direction, and then sampled as a test piece of the specified size in accordance with JIS-K-7128-2.The tear resistance of the notched test piece in the longitudinal and width directions of the film was then measured using an Elmendorf tear tester manufactured by Toyo Seiki.

[0096] [Tensile breaking strength] A rectangular test piece was prepared, measuring 140 mm in the measurement direction (longitudinal direction of the film) and 20 mm in the direction perpendicular to the measurement direction (width direction of the film). Using a universal tensile testing machine "DSS-100" (Shimadzu Corporation), both ends of the test piece were held with chucks for 20 mm on each side (distance between chucks: 50 mm), and a tensile test was performed at an ambient temperature of 23°C and a tensile speed of 200 mm / min. The strength (stress) at tensile failure was recorded as the tensile failure strength.

[0097] [Breaking elongation after shrinkage] The film was cut to a length of 150 mm longitudinally and 238 mm transversely. The film was then fixed to a metal frame measuring 190 mm transversely and 150 mm longitudinally. The film was then placed in a 90°C hot-air oven for 20 seconds to shrink the film by 20% transversely. Afterwards, rectangular test specimens measuring 100 mm longitudinally and 20 mm transversely were prepared. Tensile tests were performed using a Shimadzu DSS-100 universal tensile testing machine. The specimens were clamped at both ends with 20 mm chucks (30 mm distance between chucks) at an ambient temperature of 23°C and a tensile speed of 1,000 mm / min. The strain (elongation) at break was recorded as the fracture elongation.

[0098] [Apparent specific gravity] The film was cut into a 10.0 cm square to prepare a sample. The total thickness of this sample was measured at 10 different locations using a micrometer to four significant figures, and the average total thickness was calculated. This average value was rounded to three significant figures to obtain the average thickness t (μm) per sheet. The mass w (g) of the sample was measured to four significant figures using an automatic top-pan balance, and the apparent specific gravity was calculated using the following equation 5. The apparent specific gravity was rounded to three significant figures. Apparent specific gravity (g / cm 3 ) =w / (10.0×10.0×t×10 -4 )=w×100 / t...Equation 5

[0099] [Distortion index of molecular orientation angle] Samples measuring 140 mm x 100 mm (longitudinal x transverse) were taken from the opposing left and right edges of the film and at the center of the film in the transverse direction. The molecular orientation angles of these three samples were measured using a molecular orientation angle measuring device (MOA-6004) manufactured by Oji Scientific Instruments Co., Ltd. The absolute value of the difference in molecular orientation angle between each sample taken from the edge and the center of the film was calculated, and the maximum absolute value of the difference between the left and right edges was calculated using the following formula 4, which was used as the distortion index of the molecular orientation angle. Molecular orientation angle distortion index = (absolute value of the difference in molecular orientation angle between samples taken from the edge and the center) Equation 4

[0100] [Total light transmittance] If the label was printed, a cloth was wetted with ethyl acetate and the ink on the label was wiped off with the cloth. The total light transmittance of unprinted labels or labels with the ink removed was measured using a Nippon Denshoku Industries Co., Ltd. NDH-1001DP.

[0101] Label shrinkage distortion Cylindrical labels (labels with the main shrinkage direction of the heat-shrinkable film aligned circumferentially) were fabricated by bonding both ends of a heat-shrinkable film pre-printed with a 10 mm grid pattern using dioxolane. The labels were then placed on 500 ml PET bottles (body diameter 62 mm, minimum neck diameter 25 mm) and passed through a Fuji Astec Inc. steam tunnel (model SH-1500-L) at a zone temperature of 95°C for 2.5 seconds to heat-shrink the labels and attach them to the bottles. The neck was adjusted so that the 40 mm diameter was located at one end of the label. The finish after shrinkage was evaluated by measuring the deviation of the grid pattern from the horizontal plane in a 360° direction on the body of the attached label, and the maximum distortion was calculated. Evaluation was based on the following criteria.

[0102] ◎: Maximum distortion less than 1.0 mm ○: Maximum distortion 1.0 mm or more and less than 2.0 mm ×: Maximum distortion 2.0mm or more

[0103] [Label adhesion] The labels were attached to the PET bottles under the same conditions as those for the shrinkage distortion of the labels described above. The label adhesion was evaluated according to the following criteria. ◎: There is no slack between the attached label and the PET bottle, and when the bottle cap is fixed and the label is twisted, the label does not move. ○: When the bottle cap is fixed and the label is twisted, the label does not move, but there is a little slack between the label and the PET bottle. ×: When the bottle cap is fixed and the label is twisted, the label shifts.

[0104] [Wrinkles on label] The label was attached to a PET bottle under the same conditions as those for the shrinkage distortion of the label described above, and the occurrence of wrinkles was evaluated according to the following criteria. ◎: No wrinkles of 2 mm or larger. ○: The number of wrinkles 2 mm or larger is 1 to 2. ×: Three or more wrinkles of 2 mm or larger.

[0105] [Perforated opening] Labels with perforations perpendicular to the main shrinkage direction were attached to PET bottles under the same conditions as those for the shrinkage strain test described above. The perforations were formed by 1-mm-long holes spaced 1 mm apart, with two perforations running 22 mm wide and 120 mm long along the length of the label. The bottles were then filled with 500 ml of water and refrigerated at 5°C. Immediately after removal from the refrigerator, the perforations on the labels were torn with fingertips. The number of bottles that did not tear cleanly along the perforation or that were torn during tearing and could not be removed from the bottle was counted, and the failure rate (%) of perforation-opening defects was calculated for all 50 bottles. A failure rate of 20% or less was considered acceptable for practical use.

[0106] <Preparation of polyester raw materials> A stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser was charged with 100 mol% dimethyl terephthalate (DMT) as the dibasic acid component and 100 mol% ethylene glycol (EG) as the glycol component, with a molar ratio of 2.2 times that of the methyl ester. A transesterification reaction was carried out using 0.05 mol% (relative to the acid component) zinc acetate as a transesterification catalyst, while the resulting methanol was distilled off. Subsequently, 0.025 mol% (relative to the acid component) antimony trioxide was added as a polycondensation catalyst. Polycondensation was carried out at 280°C under reduced pressure of 26.6 Pa (0.2 Torr), yielding Polyester A with an intrinsic viscosity of 0.70 dL / g. This polyester was polyethylene terephthalate. Furthermore, the polyesters (B, C, D, E, and F) listed in Table 1 were synthesized using the same method as above. When producing Polyester F, SiO2 (Sylysia 266 manufactured by Fuji Silysia Corporation; average particle size 1.5 μm) was added as a lubricant at a ratio of 7,000 ppm to the polyester. Polyester G was produced by adding 50% by weight of titanium dioxide (TA-300 manufactured by Fuji Titanium) to Polyester A and feeding the mixture into a twin-screw extruder. In the table, NPG stands for neopentyl glycol, CHDM stands for 1,4-cyclohexanedimethanol, and DEG stands for diethylene glycol. The intrinsic viscosities were 0.76 dl / g for Polyester B, 0.75 dl / g for Polyester C, 0.73 dl / g for Polyester D, 0.68 dl / g for Polyester E, 0.70 dl / g for Polyester F, and 0.67 dl / g for Polyester G.

[0107] [Table 1]

[0108] [Example 1] The raw material for Layer B was a mixture of the above-mentioned polyesters A, B, D, and F in a weight ratio of 5:60:30:5. The raw material for Layer A was a mixture of polyesters B, D, and G in a weight ratio of 55:25:20, with 6 wt. % polystyrene resin (G797N, manufactured by Nippon Polystyrene) added. The raw materials for Layers A and B were fed into separate twin-screw extruders, mixed, and melted. The resulting mixture was joined in a feed block and melt-extruded through a T-die at 280°C. The resulting mixture was then wrapped around a rotating metal roll cooled to a surface temperature of 30°C at a draft ratio of 5x, resulting in an unstretched film with a thickness of 336 μm and a B / A / B laminate structure (B / A / B = 68 μm / 200 μm / 68 μm). The take-up speed of the unstretched film (the rotational speed of the metal roll) was approximately 20 m / min. The Tg of the unstretched film was 67°C.

[0109] The unstretched film obtained as described above was then introduced into a longitudinal stretching machine having a plurality of rolls arranged in series, and stretched in two stages in the longitudinal direction by utilizing the difference in the rotation speed of the rolls. Specifically, the film was preheated on a heated roll until the film temperature reached 85°C, and then stretched 1.4 times over a stretching distance of 200 mm by utilizing the difference in rotation speed between a low-speed rotating roll set at a surface temperature of 80°C and a medium-speed rotating roll set at a surface temperature of 80°C. The longitudinally stretched film was then stretched 2.9 times over a stretching distance of 120 mm by utilizing the difference in rotation speed between a medium-speed rotating roll set at a surface temperature of 80°C and a high-speed rotating roll set at a surface temperature of 80°C (therefore, the total longitudinal stretching ratio was 4.06 times).

[0110] The film immediately after longitudinal stretching was passed through a heating furnace. The interior of the heating furnace was heated with a hot air heater and the set temperature was 95°C. Utilizing the speed difference between the rolls at the entrance and exit of the heating furnace, the film was relaxed by 30% in the longitudinal direction. The relaxation time was 0.6 seconds.

[0111] As described above, immediately after longitudinal stretching, the film was forcibly cooled at a cooling rate of 40°C / sec using a cooling roll (a high-speed roll positioned immediately after the second longitudinal stretching roll) set to a surface temperature of 30°C. The cooled film was then introduced into a tenter and passed successively through an intermediate heat treatment zone, a first intermediate zone (natural cooling zone), a cooling zone (forced cooling zone), a second intermediate zone, a transverse stretching zone, and a final heat treatment zone. The length of the first intermediate zone in the tenter was set to approximately 40 cm, and shielding plates were installed between the intermediate heat treatment zone and the first intermediate zone, between the first intermediate zone and the cooling zone, between the cooling zone and the second intermediate zone, and between the second intermediate zone and the transverse stretching zone. Furthermore, in the first and second intermediate zones, the hot air from the intermediate heat treatment zone, the cooling air from the cooling zone, and the hot air from the transverse stretching zone were blocked so that when a strip of paper was hung down without the film passing through the zone, the paper would hang down almost completely vertically. In addition, the distance between the film and the shielding plate was adjusted so that most of the accompanying flow of the film was blocked by the shielding plate provided between the intermediate heat treatment zone and the first intermediate zone when the film was being passed through. In addition, the distance between the film and the shielding plate was adjusted so that most of the accompanying flow of the film was blocked by the shielding plate at the boundary between the intermediate heat treatment zone and the first intermediate zone and at the boundary between the cooling zone and the second intermediate zone when the film was being passed through.

[0112] The longitudinally stretched film introduced into the tenter was first heat-treated in an intermediate heat treatment zone at 170°C for 5.0 seconds, and then introduced into the first intermediate zone where it was naturally cooled by passing through the zone (passage time = approximately 1.0 second). The naturally cooled film was then introduced into a cooling zone where it was actively cooled by blowing low-temperature air onto it until its surface temperature reached 100°C. The cooled film was then introduced into a second intermediate zone where it was naturally cooled again by passing through the zone (passage time = approximately 1.0 second). The film that had passed through the second intermediate zone was then introduced into a transverse stretching zone where it was preheated until its surface temperature reached 85°C, and then stretched 4.0 times in the width direction (transverse direction) at 85°C.

[0113] The transversely stretched film was then introduced into the final heat treatment zone, where it was heat treated at 85°C for 5.0 seconds, cooled, and then cut and removed from both edges. The film was then wound into a 500mm wide roll, continuously producing a biaxially stretched film of approximately 30µm in thickness over a predetermined length. The properties of the resulting film and label were evaluated using the methods described above. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0114] Example 2 The polyester raw materials for layer B were a mixture of polyester A, polyester C, polyester D, and polyester F in a weight ratio of 5:60:30:5. The raw materials for layer A were a mixture of polyester C, polyester D, and polyester G in a weight ratio of 55:25:20. The materials were melt extruded in the same manner as in Example 1, except that the raw materials for layer A were a mixture of polyester C, polyester D, and polyester G in a weight ratio of 55:25:20. The materials were then stretched longitudinally in the same manner as in Example 1, resulting in a biaxially stretched film having a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0115] Example 3 The film was melt extruded in the same manner as in Example 1, except that 6% by weight of polypropylene (FO-50F Grand Polymer) was added to the raw material for Layer A instead of 6% by weight of polystyrene resin, and then longitudinally stretched, naturally cooled, forcedly cooled, transversely stretched, and finally heat-treated in the same manner as in Example 1, to obtain a biaxially stretched film with a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0116] Example 4 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Subsequently, natural cooling, forced cooling, transverse stretching, and final heat treatment were carried out in the same manner as in Example 1, except that a 40% relaxation treatment was carried out in the longitudinal direction of the film in a heating furnace at 95°C, resulting in a biaxially stretched film having a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0117] Example 5 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Subsequently, natural cooling, forced cooling, transverse stretching, and final heat treatment were carried out in the same manner as in Example 1, except that a 50% relaxation treatment was carried out in the longitudinal direction of the film in a heating furnace at 95°C, resulting in a biaxially stretched film having a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0118] Example 6 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Subsequently, natural cooling, forced cooling, transverse stretching, and final heat treatment were carried out in the same manner as in Example 1, except that a 20% relaxation treatment in the longitudinal direction of the film was carried out in a heating furnace at 95°C, resulting in a biaxially stretched film having a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0119] Example 7 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Subsequently, natural cooling, forced cooling, transverse stretching, and final heat treatment were carried out in the same manner as in Example 1, except that a 30% longitudinal relaxation treatment was performed during the intermediate heat treatment, resulting in a biaxially stretched film having a width of 500 mm and a thickness of 30 μm. Therefore, the total relaxation rate in the longitudinal direction of the film was 30%. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0120] Example 8 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. The film was then subjected to a 20% relaxation treatment in the longitudinal direction in a 95°C heating furnace, and a 10% relaxation treatment was also performed during the subsequent intermediate heat treatment. The natural cooling, forced cooling, transverse stretching, and final heat treatment were then carried out in the same manner as in Example 1, resulting in a biaxially stretched film with a width of 500 mm and a thickness of 30 μm. Therefore, the total relaxation rate in the longitudinal direction of the film was 30%. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0121] Example 9 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Thereafter, natural cooling, forced cooling, transverse stretching, and final heat treatment were carried out in the same manner as in Example 1, except that the temperature in the intermediate heat treatment was changed to 130°C, and a biaxially stretched film having a width of 500 mm and a thickness of 30 μm was obtained. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0122] Example 10 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Thereafter, natural cooling, forced cooling, transverse stretching, and final heat treatment were carried out in the same manner as in Example 1, except that the temperature in the intermediate heat treatment was changed to 170°C, and a biaxially stretched film having a width of 500 mm and a thickness of 30 μm was obtained. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0123] Example 11 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and longitudinally stretched in the same manner as in Example 1. Thereafter, natural cooling, forced cooling, and transverse stretching were carried out in the same manner as in Example 1, and the final heat treatment temperature was changed to 105°C, resulting in a biaxially stretched film having a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0124] Comparative Example 1 Extrusion was carried out in the same manner as in Example 1 to obtain an unstretched film having a thickness of 165 μm. The unstretched film was then introduced into a tenter and preheated to a surface temperature of 80°C, after which it was stretched 5.5 times in the width direction (transverse direction) at 76°C to obtain a uniaxially stretched film with a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3. Although the shrinkage finish was good, the film was not longitudinally stretched, and therefore tended to tear when cut along the perforations, making it difficult to cut.

[0125] Comparative Example 2 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and stretched in two longitudinal directions by utilizing the difference in roll rotation speed. Specifically, the unstretched film was preheated on a preheated roll until the film temperature reached 85°C, and then stretched 2.6 times by utilizing the difference in rotation speed between a low-speed rotating roll set at a surface temperature of 80°C and a medium-speed rotating roll set at a surface temperature of 80°C. The longitudinally stretched film was then stretched 1.4 times by utilizing the difference in rotation speed between a medium-speed rotating roll set at a surface temperature of 90°C and a high-speed rotating roll set at a surface temperature of 80°C (hence, the total longitudinal stretching ratio was 3.64 times). The subsequent steps, including natural cooling, forced cooling, transverse stretching, and final heat treatment, were carried out in the same manner as in Example 1, except that relaxation after stretching was not performed, the temperature in the intermediate heat treatment was changed to 160°C, and the transverse stretching temperature was changed to 95°C. A biaxially stretched film having a width of 500 mm and a thickness of 30 μm was obtained. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3. Because no relaxation in the longitudinal direction was carried out, the distortion of the molecular orientation angle became large, and there were many distortions and wrinkles when the film was shrunk, making it less practical.

[0126] Comparative Example 3 The polyester raw materials for Layer B were a mixture of Polyester A, Polyester B, Polyester E, and Polyester F in a weight ratio of 5:60:30:5. The raw materials for Layer A were a mixture of Polyester A, Polyester B, and Polyester E in a weight ratio of 10:80:10. The raw materials for Layer A were a mixture of Polyester A, Polyester B, and Polyester E in a weight ratio of 10:80:10. The blend was melt extruded in the same manner as in Comparative Example 2, except that 10% by weight of polystyrene resin (G797N, manufactured by Nippon Polystyrene) and 10% by weight of titanium dioxide (TA-300, manufactured by Fuji Titanium) were added during blending. The blend was then longitudinally stretched in the same manner as in Comparative Example 1, resulting in a biaxially stretched film with a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3. As in Comparative Example 2, no longitudinal relaxation was performed, resulting in significant distortion of the molecular orientation angle. The film exhibited significant distortion and wrinkles upon shrinkage, making it unsuitable for practical use.

[0127] Comparative Example 4 The same polyester raw material as in Example 1 was melt-extruded in the same manner as in Example 1 and stretched in the machine direction in a single stage by utilizing the difference in roll rotation speed. Specifically, the unstretched film was preheated on a preheated roll until the film temperature reached 85°C, and then stretched 4.0 times over a stretching distance of 120 mm by utilizing the difference in rotation speed between a low-speed rotating roll set at a surface temperature of 80°C and a high-speed rotating roll set at a surface temperature of 80°C. Subsequent steps, including natural cooling, forced cooling, transverse stretching, and final heat treatment, were carried out in the same manner as in Example 1, resulting in a biaxially stretched film with a width of 500 mm and a thickness of 30 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3. Although the shrinkage finish was good, the single-stage stretching resulted in significant distortion in the molecular orientation angle, resulting in significant distortion and wrinkles during shrinkage finish, making the film less practical.

[0128] [Table 2A]

[0129] [Table 2B]

[0130] [Table 3A]

[0131] [Table 3B] [Industrial Applicability]

[0132] The white heat-shrinkable polyester film of the present invention has excellent properties such as easy perforation tearability, light blocking properties, and light weight, and can therefore be suitably used for bottle labels.

Claims

1. A white heat-shrinkable polyester film comprising a polyester resin having ethylene terephthalate as a main constituent component and containing 13 mol% or more of one or more monomer components that can become amorphous components in the total polyester resin components, and satisfying the following requirements (1) to (6). (1) The hot water heat shrinkage rate in the longitudinal direction when treated in hot water at 98°C for 10 seconds is 0% or more and 15% or less. (2) The hot water shrinkage rate in the width direction when treated in hot water at 98°C for 10 seconds is 50% or more and 80% or less. (3) 1340 cm of heat-shrinkable polyester film measured by polarized ATR-FTIR method -1 Absorbance A1 and 1410 cm -1 When the ratio A1 / A2 of the absorbance A1 at the time of film shrinkage to the absorbance A2 at the time of film shrinkage is defined as the trans conformation ratio, the trans conformation ratio is 0.3 or more and 0.65 or less in the longitudinal direction, which is the non-shrinking direction of the film. (4) The total light transmittance of the film is between 10% and 40%. (5) The tear propagation strength in the longitudinal direction after shrinking by 20% in the width direction in a hot air oven is 1 N / mm or more and 80 N / mm or less. (6) The distortion index of the molecular orientation angle is 0° or more and 15° or less.

2. 2. The white heat-shrinkable polyester film according to claim 1, wherein the tear propagation strength in the longitudinal direction after shrinking by 20% in the width direction in a hot air oven is 1 N / mm or more and 50 N / mm or less, and the ratio of the tear propagation strengths in the longitudinal direction to the width direction (longitudinal direction / width direction) after shrinking by 20% in the width direction in a hot air oven is 1.0 or more and 11.0 or less.

3. 3. The white heat-shrinkable polyester film according to claim 1, wherein the tensile strength at break in the longitudinal direction is 60 MPa or more and 200 MPa or less.

4. 4. The white heat-shrinkable polyester film according to claim 1, wherein the shrinkage stress in the width direction is 2 MPa or more and 18 MPa or less.

5. Apparent specific gravity is 0.9 g / cm 3 Above, 1.3g / cm 3 5. The white heat-shrinkable polyester film according to claim 1, wherein the film has the following properties:

6. A label using the white heat-shrinkable polyester film according to any one of claims 1 to 5 as a substrate, and having a perforation or a pair of notches formed therein.

7. A packaging body, characterized in that the white heat-shrinkable polyester film according to any one of claims 1 to 6 is used as a base material, and a label having perforations or a pair of notches is covered on at least a part of the outer periphery thereof and heat-shrunk.

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