JP7056322B2 - Heat shrinkable polyester film - Google Patents

Description

The present invention relates to a heat shrinkable polyester film suitable for heat shrinkable label applications.

In recent years, in addition to applications such as label packaging, cap seals, and integrated packaging that protect glass bottles or plastic bottles and display products, polyvinyl chloride resin and polystyrene are also used for banding that binds containers such as lunch boxes. A stretched film made of a based resin or a polyester based resin (so-called heat-shrinkable film) is used. Among such heat-shrinkable films, polyvinyl chloride-based films have low heat resistance and generate hydrogen chloride gas during incineration; they have problems such as causing dioxin. In addition, polystyrene film is inferior in solvent resistance, it is necessary to use ink with a special composition at the time of printing, and it is necessary to incinerate it at a high temperature, and a large amount of black smoke is generated with an offensive odor at the time of incineration. There is a problem of doing. On the other hand, polyester-based heat-shrinkable films are widely used as heat-shrinkable labels because they have high heat resistance, are easy to incinerate, and have excellent solvent resistance, and are widely used as heat-shrinkable labels. As the amount increases, the amount used tends to increase more and more.

As a conventional heat-shrinkable polyester film, for example, as described in Patent Document 1, it is common to use an amorphous polyester raw material (amorphous raw material). This is because it is thought that amorphous molecules are involved in the development of shrinkage rate. For example, in the examples of Patent Document 1, it is shown that the shrinkage rate tends to increase as the content of the monomer as an amorphous component increases. However, the heat-shrinkable polyester film using an amorphous raw material has a problem of low heat resistance and poor thickness unevenness.

By the way, usually, when a heat-shrinkable film is used as a bottle label, the edges of the film are fixed to each other with a solvent, an adhesive or the like to form a ring-shaped (tube-shaped) label, which is then put on the bottle and shrunk. The method is adopted. By setting the shrinkage direction to the width direction, shrinkage labels can be continuously produced, which is efficient.

Therefore, Patent Document 2 discloses a heat-shrinkable film that heat-shrinks in the horizontal (width) direction and hardly heat-shrinks in the vertical (longitudinal) direction, and a method for producing the same. In the examples of Patent Document 2, polyethylene terephthalate is used as a raw material, and a film having a wrinkled treatment in the vertical direction is stretched horizontally and uniaxially to produce a film exhibiting desired heat shrinkage characteristics.

Further, the applicant of the present application has sufficient heat shrinkage characteristics in the main shrinkage direction, which is the longitudinal direction (mechanical direction), even if it does not contain a large amount of monomer components that can be amorphous components, and is orthogonal to the main shrinkage direction. Patent Document 3 discloses a heat-shrinkable polyester-based film having a low heat-shrinkability in the width direction (vertical direction) and a small thickness unevenness in the longitudinal direction. In the above patent document, a polyester-based unstretched film containing ethylene terephthalate as a main component and containing 0 mol% or more and 5 mol% or less of a monomer component which can be an amorphous component among all polyester resin components is used for lateral stretching. After that, the film is produced by a biaxial stretching method of longitudinal stretching. For example, as a stretching method A, a simultaneous biaxial stretching machine shown in FIG. 1 is used at a temperature of film Tg or more (Tg + 40 ° C.) or less. After stretching (transverse stretching) in the width direction at a magnification of 5 times or more and 6 times or less, the distance between the clips is widened at a temperature of film Tg or more (Tg + 40 ° C.) to 1.5 times or more to 2.5 times or less. A method of relaxing in the width direction by narrowing the width of the tenter by 5% or more and 30% or less after the transverse stretching while stretching in the longitudinal direction (longitudinal stretching) at a magnification is described.

Further, since all the heat-shrinkable films disclosed in Patent Documents 1 to 3 have high light transmittance, they do not have a light-shielding property against the contents. When packaging contents with an expiration date such as foods and beverages with a heat-shrinkable film, there is also a problem that the contents are deteriorated by external light. Therefore, there is also a desire to reduce the transmission of light and extend the shelf life of the contents.

Japanese Unexamined Patent Publication No. 8-27260 Japanese Unexamined Patent Publication No. 5-169535 International Publication No. 2015/118968

Yuji Kumatani, "Fiber Structure Formation Mechanism and Development of High Performance Fibers", Journal of The Society of Fiber Science and Technology (Fibers and Industry), Vol. 63, No. 12 (2007) A. Mahendrasingam et al “Effect of draw ratio and temperature on the strain induced crystallization of poly (ethylene terephthalate) at fast draw rates” Polym. 40 5556 (1999)

As described above, the film of Patent Document 2 is disclosed as a heat-shrinkable film that shrinks significantly in the width direction. However, in the examples of Patent Document 2, the heat shrinkage rate at 95 ° C. in the width direction is 12.5% at the maximum, and it cannot be said that the shrinkage rate level required for the current heat shrinkage film is satisfied. ..

On the other hand, the heat-shrinkable polyester film described in Patent Document 3 has a main shrinkage direction in the longitudinal direction. Therefore, in the film whose main shrinkage direction is the width direction, a film having a high heat shrinkage rate in the width direction and a small thickness unevenness has not yet been disclosed. Even if the lateral → longitudinal stretching method described in Patent Document 3 is simply changed from longitudinal to transverse stretching, the non-shrinkage direction (longitudinal direction) cannot be relaxed and the shrinkage rate in the longitudinal direction becomes high, which is desired. No film is available. Further, even if the stretching is changed from vertical to horizontal, the thermal shrinkage stress in the width direction may increase.

The present invention has been made in view of the above circumstances, and an object thereof is a heat-shrinkable polyester-based film that does not substantially contain an amorphous component, has a high heat-shrinkability in the width direction, and has a light-shielding property. It is an object of the present invention to provide a heat-shrinkable polyester-based film having a small thickness unevenness and a method for producing the same.

The present inventors have conducted studies to solve the above problems. As a result, in stretching the unstretched film obtained by melting and extruding the raw material resin having low light transmittance, the unstretched film was preheated at a temperature (T1) of (Tg + 40 ° C.) or more (Tg + 70 ° C.) or less, and the preheated film was (T1). It is transversely stretched at a temperature (T2) of Tg + 5 ° C. or higher (Tg + 40 ° C.) or lower, and the stretched film is further stretched at a temperature (T3) of (Tg-10 ° C.) or higher (Tg + 15 ° C.) or lower (however, T1). > T2> T3) By adopting the transverse stretching method, it is possible to control the shrinkage rate in the longitudinal direction and the width direction, and it is possible to achieve both a high heat shrinkage rate in the width direction and a reduction in thickness unevenness, and completed the present invention.

The configuration of the present invention is as follows.
1. 1. A heat-shrinkable polyester-based film containing 90 mol% or more of an ethylene terephthalate unit in 100 mol% of all ester units.
A heat-shrinkable polyester-based film characterized by satisfying the following requirements (1) to (5).
(1) Heat shrinkage in the width direction when shrunk in hot water at 90 ° C for 10 seconds is 50% or more and 75% or less (2) Heat shrinkage in the longitudinal direction when shrunk in hot water at 90 ° C for 10 seconds Rate is -6% or more and 14% or less (3) Heat shrinkage rate in the longitudinal direction is -6% or more and 6% or less when shrunk in hot water at 70 ° C for 10 seconds (4) Thickness spots in the width direction are 1% More than 20% or less (5) Total light transmittance is more than 15% and less than 40% 2. The heat-shrinkable polyester-based film according to 1 above, which further satisfies the following requirement (6).
(6) The maximum heat shrinkage stress in the width direction when shrinking for 30 seconds in hot air at 90 ° C. is 4 MPa or more and 13 MPa.

According to the present invention, it is a heat-shrinkable polyester-based film that does not substantially contain an amorphous component, has a high heat-shrinkability in the width direction, which is the main shrinkage direction, has a light-shielding property, and has thickness unevenness. It was possible to provide a small heat-shrinkable polyester-based film.

1. 1. Polyester raw material used for heat-shrinkable polyester-based film The polyester raw material used for the heat-shrinkable polyester-based film of the present invention has 90 mol% or more of ethylene terephthalate units in 100 mol% of all ester units. It is preferably 95 mol% or more, and most preferably 100 mol%. The ethylene terephthalate unit contains ethylene glycol and terephthalic acid as main constituents. By using ethylene terephthalate, excellent heat resistance and transparency can be obtained as a heat-shrinkable polyester film.

The polyester raw material used in the present invention may contain an amorphous component (amorphous alcohol component and an amorphous acid component), but the ratio of the amorphous alcohol component to 100 mol% of the total alcohol component and the total acid component. The total with the ratio of the amorphous acid component in 100 mol% is suppressed to 0 mol% or more and 5 mol% or less. In the present invention, as described above, it is preferable that the polyester is composed of only an ethylene terephthalate unit, but it is not positively copolymerized, and a constituent unit consisting of terephthalic acid and diethylene glycol is used as a by-product in the ethylene elephthalate unit. May exist. The smaller the content of the amorphous component, the better, and most preferably 0 mol%.

Examples of the monomer of the amorphous acid component (carboxylic acid component) include isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid and the like.
Examples of the monomer of the amorphous alcohol component (diol component) include neopentyl glycol, 1,4-cyclohexanedimethanol, diethylene glycol, 2,2-diethyl1,3-propanediol, and 2-n-butyl-2-. Examples thereof include ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, and hexanediol.

As the polyester raw material used in the present invention, 1,4-butanediol, which is a diol component other than ethylene glycol, may be used as a component other than the above-mentioned ethylene terephthalate and amorphous component. Although 1,4-butanediol lowers the melting point of the polyester film and is useful as a low Tg component, it is preferable that 1,4-butanediol is not contained as much as possible from the viewpoint of the present invention. The content of 1,4-butanediol in the total alcohol component and the total acid component is preferably 10 mol% or less, more preferably 5 mol% or less, and most preferably 0 mol%.

In the present invention, in order to reduce the light transmittance of the film, a method of blending a coloring additive (pigment) in the film, a method of containing fine cavities inside the film, and white printing (so-called white solid) on the entire surface of the film. It is possible to adopt a method of printing) alone or in combination of two or more. The following describes a method of adding a coloring additive and containing a cavity, which is a preferable mode.

As the coloring additive, a white coloring additive is preferable, and any kind of inorganic fine particles and organic fine particles can be selected. Examples of the inorganic fine particles include silica, alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate and the like. Of these, titanium dioxide, calcium carbonate, kaolin, and barium sulfate are preferable, and titanium dioxide is more preferable. Examples of the organic fine particles include acrylic resin particles, melamine resin particles, silicone resin particles, crosslinked polystyrene particles and the like. The average particle size of the fine particles is preferably in the range of about 0.05 to 3.0 μm when measured with a Coulter counter.

The method of blending the fine particles in the polyester raw material is not particularly limited, and for example, it can be added at any stage of producing a polyester resin, but polycondensation is performed at the esterification stage or after the transesterification reaction is completed. It is preferable to add it as a slurry dispersed in ethylene glycol or the like at a stage before the start of the reaction to proceed with the polycondensation reaction. Further, a method of blending a slurry of fine particles dispersed in ethylene glycol or water using a kneaded extruder with a vent and a polyester resin raw material; or a method of blending the dried fine particles and a polyester resin raw material using a kneaded extruder. It may be done by a method of blending with and the like.
The above-mentioned colored matter is preferably added to the film in an amount of 3% by mass or more and 20% by mass (in the range of% or less. When the amount of the colored matter added is 5% by mass or less, the light transmittance required for the film is achieved. It is not preferable because it becomes difficult to do so. The larger the amount of the coloring additive added, the lower the light transmittance, but if it is contained in an amount of 20% by mass or more, the film tends to be brittle and stretched in the film forming process. It is not preferable that the amount of the coloring additive added is 4% by mass or more and 19% by mass or less, and more preferably 5% by mass or more and 18% by mass or less.

As a method of containing cavities, for example, a foaming material or the like may be mixed and extruded, but as a preferable method, a thermoplastic resin incompatible with the polyester is mixed with the polyester as a raw material, and at least one axis is used. By stretching in the direction, a cavity is obtained. The thermoplastic resin incompatible with polyester is arbitrary, and is not particularly limited as long as it is incompatible with polyester. Specific examples thereof include polystyrene-based resins, polyolefin-based resins, polyacrylic-based 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 preferable because of the formability of cavities.

The polystyrene-based resin in the present invention refers to a thermoplastic resin containing a polystyrene structure as a basic component, and grafts or blocks other components in addition to homopolymers such as atactic polystyrene, syndiotactic polystyrene, and isotactic polystyrene. It contains a copolymerized modified resin such as an impact resistant polystyrene resin and a modified polyphenylene ether resin, and further contains a mixture of a thermoplastic resin having compatibility with these polystyrene-based resins such as polyphenylene ether.

The polymethylpentene-based resin in the present invention is a polymer having a unit derived from 4-methylpentene-1 in an amount of 80 mol% or more, preferably 90 mol% or more, and other components include ethylene unit and propylene unit. , Butene-1 unit, 3-methylbutene-1, etc. are exemplified.

The polypropylene-based resin in the present invention includes homopolymers such as isotactic polypropylene and syndiotactic polypropylene, as well as modified resins obtained by grafting or block copolymerizing other components. The thermoplastic resin incompatible with the polyester of the present invention exists in the polyester in a form dispersed in various shapes such as a spherical shape, an elliptical spherical shape, or a thread shape.

Mixtures of thermoplastic resins that are incompatible with polyester may contain various additives, such as waxes, antioxidants, antistatic agents, crystal nucleating agents, thickeners, heat stabilizers, and coloring agents, as needed. Pigments, color inhibitors, UV absorbers and the like can be added. Further, it is preferable to add the above-mentioned coloring additives in combination as fine particles as a lubricant for improving the workability (slipperiness) of the film and as a concealing aid for reducing the total light transmittance.

As a method of blending the above particles into a mixture of a thermoplastic resin incompatible with polyester, for example, it can be added at any stage of producing the polyester resin, but at the stage of esterification or after the completion of the transesterification reaction. , It is preferable to add it as a slurry dispersed in ethylene glycol or the like at a stage before the start of the polycondensation reaction to proceed with the polycondensation reaction. Further, a method of blending a slurry of particles dispersed in ethylene glycol or water using a kneaded extruder with a vent and a polyester resin raw material, or a method of blending dried particles and a polyester resin raw material using a kneaded extruder. It is also preferable to carry out by a method of blending.

The ratio of the incompatible resin is preferably 0% by mass or more and 20% by mass or less with respect to the film. 0% by mass indicates that it does not contain an incompatible resin. In this case, it is preferable to develop the concealing property by adding fine particles as a coloring additive that lowers the total light transmittance described above. Further, if the incompatible resin is contained in an amount of more than 20% by mass, the cavity content increases and the physical strength of the film decreases, which is not preferable. More preferably, it is 5% by mass or more and 15% by mass or less.

The intrinsic viscosity of the polyester raw material is preferably in the range of 0.50 to 0.75 dl / g. If the intrinsic viscosity is lower than 0.50 dl / g, the tear resistance improving effect is lowered, while if it is larger than 0.75 dl / g, the increase in filtration pressure is large, which makes high-precision filtration difficult. The intrinsic viscosity is more preferably 0.52 dl / g or more and 0.73 dl / g or less.
The heat-shrinkable polyester-based film of the present invention can also be subjected to corona treatment, coating treatment, flame treatment, or the like in order to improve the printability and adhesiveness of the film surface.

2. 2. Characteristics of the heat-shrinkable polyester-based film of the present invention The film of the present invention satisfies the above requirements (1) to (5). When a heat-shrinkable film is used for a label of a bottle, the most contributor to the shrinkage finish of the label is specified in (1) at 90 ° C. in the width direction and (2) and (3) in the longitudinal direction. The temperature is 70 ° C. and 90 ° C., and it is technically more difficult to control the shrinkage rate in the temperature zone than in other temperature zones. According to the present invention, the film has a very high heat shrinkage rate in the width direction, which is the main shrinkage direction defined in (1), and a low heat shrinkage rate in the longitudinal direction in (2) and (3). It is very useful in that it is possible to provide a heat-shrinkable polyester-based film having a small thickness unevenness.

2.1 Heat shrinkage rate in the width direction As specified in (1) above, the heat-shrinkable polyester film of the present invention is in the width direction (main shrinkage direction) when immersed in hot water at 90 ° C. for 10 seconds. The shrinkage rate is 50% or more and 75% or less. Here, the "width direction" is a direction orthogonal to the longitudinal direction (machine direction; MD), and is also called a transverse direction (TD). If the heat shrinkage rate in the width direction at 90 ° C. is less than 50%, the film shrinks insufficiently when the container or the like is coated and shrunk, and the film does not adhere to the container cleanly, resulting in poor appearance, which is not preferable. On the other hand, if the heat shrinkage rate in the width direction at 90 ° C. exceeds 75%, the shrinkage rate becomes extremely high when the container or the like is coated and shrunk, and the film is distorted, which is not preferable. The heat shrinkage rate in the width direction at 90 ° C. is preferably 55% or more and 70% or less, and more preferably 60% or more and 65% or less.

2.2. Thermal shrinkage in the longitudinal direction The heat-shrinkable polyester film of the present invention has thermal shrinkage in the longitudinal direction (mechanical direction, MD) when immersed in hot water at 90 ° C. for 10 seconds, as specified in (2) above. The rate is -6% or more and 14% or less. If the heat shrinkage rate in the longitudinal direction at 90 ° C. is less than -6%, it is not preferable because when the container or the like is coated and shrunk, it is easily stretched and wrinkled easily, and a good shrinkage appearance cannot be obtained. On the other hand, if the heat shrinkage rate in the longitudinal direction at 90 ° C. exceeds 14%, distortion and sink marks are likely to occur after shrinkage, which is not preferable. The heat shrinkage rate in the longitudinal direction at 90 ° C. is preferably -4% or more and 12% or less, and more preferably -2% or more and 10% or less.

Further, as specified in (3) above, the heat-shrinkable polyester film of the present invention has a heat-shrinkage rate of -6% in the longitudinal direction (mechanical direction, MD) when immersed in hot water at 70 ° C. for 10 seconds. More than 6% or less. If the heat shrinkage rate in the longitudinal direction at 70 ° C. is less than -6%, it is not preferable because when the container or the like is coated and shrunk, it is likely to be stretched too much and wrinkle easily, and a good shrinkage appearance cannot be obtained. On the other hand, if the heat shrinkage rate in the longitudinal direction at 70 ° C. exceeds 6%, distortion and sink marks are likely to occur after shrinkage, which is not preferable. The heat shrinkage rate in the longitudinal direction at 70 ° C. is preferably -4% or more and 4% or less, and more preferably -2% or more and 2% or less.

2.3. Thickness unevenness in the width direction As defined in (4) above, the heat-shrinkable polyester film of the present invention has a thickness unevenness of 1% or more and 20% or less when the measured length is 1 m in the width direction. If the thickness unevenness in the width direction exceeds 20%, not only appearance defects such as end face misalignment and wrinkles occur when the film is wound as a roll, but also printing defects are likely to occur when printing the film, which is preferable. not. The thickness unevenness in the longitudinal direction is preferably 19% or less, more preferably 18% or less. The smaller the thickness unevenness in the width direction is, the more preferable it is, but considering the performance of the film forming apparatus and the like, it is considered that the limit is about 1%.

2.4. Maximum heat shrinkage stress in the width direction As specified in (5) above, the heat shrinkable polyester film of the present invention has a maximum heat shrinkage stress of 4 MPa or more in the width direction when shrunk in hot air at 90 ° C. for 30 seconds. It is preferably 13 MPa. When the maximum heat shrinkage stress at 90 ° C. in the width direction exceeds 13 MPa during heat shrinkage, it is not preferable because the object to be packaged such as a container is easily deformed. On the other hand, the lower the maximum heat shrinkage stress in the width direction is, the less the deformation of the object to be packaged is, which is preferable. However, at the current state of the art, 4 MPa is the lower limit.

2.5. Total light transmittance The total light transmittance of the heat-shrinkable polyester film of the present invention must be 15% or more and 40% or less. If the total light transmittance is 40% or more, the hiding property of the film is inferior, so that when it is used as a packaging material, the packaging object is exposed to ultraviolet rays, which accelerates the deterioration of the packaging object. It is better if the total light transmittance of the film is less than 15%, but in the present invention, 15% is the limit, so 15% is set as the lower limit.

2.6. Other Characteristics The thickness of the heat-shrinkable polyester film according to the present invention is preferably 5 μm or more and 200 μm or less, considering that it is used for label applications of bottles and banding films used for bundling purposes such as lunch boxes. More preferably, it is 20 μm or more and 100 μm. If the thickness exceeds 200 μm, the weight per area of the film simply increases, which is not economical. On the other hand, if the thickness is less than 5 μm, the film becomes extremely thin, which makes it difficult to handle (poor handling) in a process such as making a tubular label. In addition, it may be difficult to keep the total light transmittance of the film within the range of (5) above.

3. 3. Method for Producing Heat-Shrinkable Polyester Film According to the Present Invention The heat-shrinkable polyester-based film of the present invention is transversely stretched under the following conditions using an unstretched film obtained by melt-extruding the polyester raw material with an extruder. Can be manufactured by Specifically, the preheated film is preheated at a temperature (T1) of (Tg + 40 ° C.) or higher (Tg + 70 ° C.) or lower, and the preheated film is transversely stretched at a temperature (T2) of (Tg + 5 ° C.) or higher (Tg + 40 ° C.) or lower. The laterally stretched film is further stretched at a temperature (T3) of (Tg-10 ° C.) or higher (Tg + 15 ° C.) or lower. Here, the T1, T2, and T3 satisfy the relationship of T1>T2> T3. If necessary, after the second transverse stretching at T3, heat treatment may be performed at a temperature of (Tg-30 ° C.) or more and Tg or less. The polyester can be obtained by polycondensing the above-mentioned suitable dicarboxylic acid component and diol component by a known method. Usually, two or more kinds of chip-shaped polyester are mixed and used as a raw material for a film.

Hereinafter, each step will be described in detail.

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

Then, an unstretched film can be obtained by quenching the sheet-shaped molten resin after extrusion. As a method for rapidly cooling the molten resin, a method of casting the molten resin from a base onto a rotary drum and quenching and solidifying the molten resin to obtain a substantially unoriented resin sheet is preferably used.

The heat-shrinkable polyester-based film of the present invention can be obtained by stretching the obtained unstretched film in the width (lateral) direction by the method described in detail below.

3.2. Lateral Stretching The stretching method for obtaining the heat-shrinkable polyester-based film of the present invention is different from the molecular structure by citing the conventional film-forming method of the heat-shrinkable polyester-based film and Non-Patent Documents 1 and 2. Will be explained in detail while considering.

The details of the molecular structure that governs the shrinkage behavior of the film have not been clarified yet, but it is roughly considered that the oriented amorphous molecules are involved in the shrinkage characteristics. Therefore, the heat-shrinkable polyester-based film is usually produced by using an amorphous raw material and stretching it in a direction to be shrunk (main shrinkage direction, usually width direction). A heat-shrinkable polyester film using a conventional amorphous raw material generally has a stretching ratio (final stretching) of about 3.5 to 5.5 times at a temperature from the glass transition temperature (Tg) to Tg + 30 ° C. It is manufactured by stretching at (magnification). It is considered that the amorphous molecules are oriented by these stretching conditions and the film has a shrinkage rate. The lower the stretching temperature or the higher the stretching ratio, the higher the shrinkage rate (that is, the amorphous molecules are oriented). It will be easier).

On the other hand, when the monomer component (amorphous raw material) that can be an amorphous component as in the present invention is 0 mol% or more and 5 mol% or less and substantially does not contain an amorphous raw material, the temperature is the same as above, that is, When stretched from Tg at a temperature of Tg + 30 ° C., the film shrinks when stretched at a magnification of 2 to 2.5 times, but the same magnification as above, that is, stretching of about 3.5 to 5.5 times is applied. On the contrary, the shrinkage rate of the film decreases. For example, Comparative Examples 1 and 3 in Table 1 described later are all 3.6 at a temperature of 85 to 90 ° C. (Comparative Example 1) or 80 ° C. (Comparative Example 3) using a polyester raw material having Tg = 75 ° C. This is an example in which a film was produced by laterally stretching the film by a factor of 2 (Comparative Example 1) or 2.4 times (Comparative Example 3). In Comparative Example 3 in which the draw ratio is as low as 2.4 times, the shrinkage ratio in the width direction is as high as 55.3%, but in Comparative Example 1 in which the draw ratio is as high as 3.6 times, the shrinkage ratio in the width direction is 26.8%. It dropped significantly.

It is considered that the reason for this is that the molecules oriented by stretching crystallize (aligned crystallization) and inhibit the shrinkage of the film (that is, the shrinkage of the amorphous molecule). For example, FIG. 4 of Non-Patent Document 1 shows the relationship between stress (horizontal axis) and birefringence (vertical axis) in uniaxial stretching of polyethylene terephthalate fiber, and the state of change in molecular orientation can be read from this figure. be able to. That is, in the region where the stretching ratio DR is up to about 2 times, the stress and the birefringence have a linear relationship, and when the stretching is stopped, the stress is relaxed and the birefringence decreases. The decrease in birefringence indicates relaxation of the molecular chain, and when replaced with a film, it is considered to indicate shrinkage of the film (expression of shrinkage rate). On the other hand, when the draw ratio DR exceeds 2 times, it becomes difficult to establish a linear relationship between stress and birefringence, and even if stretching is stopped, no decrease in birefringence can be seen. It is considered that this phenomenon indicates a decrease in the shrinkage rate due to orientation crystallization. From this, it is considered that the shrinkage ratio can be developed in the film under the condition that the orientation crystallization due to stretching does not occur even when the amorphous raw material is not substantially contained as in the present invention. ..

Here, the stretching condition in which the orientation does not crystallize even when stretched is shown in, for example, the photograph of FIG. 2 of Non-Patent Document 2. Here, a crystal peak was observed at low temperature stretching at 85 ° C (= Tg + 10 ° C), whereas no crystal peak was observed at high temperature stretching at about 130 ° C (= Tg + 55 ° C), and the molecule was completely oriented and crystallized. It is shown not to. It is believed that this is because in high temperature stretching, the relaxation of molecules that occurs during stretching is faster than the orientation rate. As a matter of fact, when the present inventors carried out high-temperature stretching at a constant temperature of 130 ° C. on the film production line, the shrinkage rate did not develop because the molecules were not oriented according to the above mechanism. Furthermore, it was found that not only the shrinkage rate was not observed, but also the thickness unevenness was poor (increased) because the stress during stretching did not increase.

Therefore, the present inventors do not perform the entire stretching process as high-temperature stretching (constant) as described above, but instead use a step of hardly orienting the molecules by high-temperature stretching and a step of positively orienting the molecules by low-temperature stretching. By dividing the invention, it was found that the shrinkage rate can be expressed by orienting only amorphous molecules while suppressing the decrease in shrinkage rate due to orientation crystallization, and the thickness unevenness can be suppressed to a low level, and the present invention is completed. I arrived. Specifically, as described in detail below, only amorphous molecules are put into the film by a method of preheating at temperature T1, laterally stretching at temperature T2, and then transversely stretching at temperature T3 (T1> T2> T3). It was found that the temperature can be controlled in the longitudinal direction and the thermal shrinkage in the width direction, and that the thickness unevenness in the width direction can be reduced.

Hereinafter, each step will be described in sequence.

First, the preheating zone is preheated at a temperature T1 of (Tg + 40 ° C.) or higher (Tg + 70 ° C.) or lower. If the preheating temperature T1 is less than (Tg + 40 ° C.), the molecules tend to be oriented and crystallized during the transverse stretching in T2 in the next step, and the heat shrinkage in the film width direction tends to fall below the lower limit of 50% (described later). See Comparative Example 1 in Table 1). Further, when the preheating temperature T1 is (Tg + 40 ° C. or less), the stress applied in the vertical direction due to the neck-in generated by the lateral stretching increases, and the heat shrinkage rate in the longitudinal direction tends to exceed the upper limit of 6%, which is not preferable. On the other hand, if the preheating temperature T1 exceeds (Tg + 70 ° C.), the thickness unevenness in the width direction worsens and easily exceeds the upper limit of 20%, which is not preferable. The preheating temperature T1 is more preferably (Tg + 45 ° C.) or higher (Tg + 65 ° C.) or lower, and further preferably (Tg + 50 ° C.) or higher (Tg + 60 ° C.) or lower.

Specifically, it is preferable to control the passage time of the preheating zone to 2 seconds or more and 10 seconds or less so that the preheating temperature is T1. If the passing time of the preheating zone is less than 2 seconds, the lateral stretching at T2 in the next step is started before the film reaches the preheating temperature T1. Therefore, the same problem as when the preheating temperature T1 is less than (Tg + 40 ° C.) occurs. The longer the passing time of the preheating zone, the easier it is for the film temperature to reach the preheating temperature T1, which is preferable. However, if the passing time is too long, the temperature of the preheating zone is set to exceed the cold crystallization temperature. It is not preferable because the crystallization of the unstretched film is excessively promoted. Further, the longer the passage time of the preheating zone, the larger the production equipment, which is not preferable. A passing time of the preheating zone of 10 seconds is sufficient.

Next, the film preheated at the temperature T1 is transversely stretched at a temperature T2 of (Tg + 5 ° C.) or higher (Tg + 40 ° C.) (sometimes referred to as first transverse stretching). In the first transverse stretching, it is necessary to suppress the molecular orientation due to stretching as described above, and the temperature T2 in the first transverse stretching is set lower than the preheating temperature T1 to be (Tg + 5 ° C) or higher (Tg + 40 ° C) or lower. Control. If the temperature T2 in the first transverse stretching is less than (Tg + 5 ° C.), the same problem as in the preheating occurs, and not only the heat shrinkage in the film width direction tends to fall below the lower limit of 50%, but also in the longitudinal direction. It is not preferable because the heat shrinkage rate tends to exceed the upper limit of 6%. On the other hand, if the temperature T2 in the first transverse stretching exceeds (Tg + 40 ° C.), the thickness unevenness in the width direction tends to exceed the upper limit of 20%, which is not preferable. The temperature T2 in the first transverse stretching is more preferably (Tg + 10 ° C.) or higher (Tg + 35 ° C.) or lower, and further preferably (Tg + 15 ° C.) or higher (Tg + 30 ° C.) or lower.

Further, the draw ratio in the first transverse stretching is preferably 1.5 times or more and 2.5 times or less. When the draw ratio in the first transverse stretching is less than 1.5 times, the effect of suppressing the orientation crystallization becomes small, the shrinkage ratio in the film width direction tends to be less than 50%, and the shrinkage ratio in the longitudinal direction is 6. It is not preferable because it easily exceeds%. On the other hand, if the stretching ratio in the first transverse stretching exceeds 5 times, the thickness unevenness in the width direction tends to exceed 20%, which is not preferable. The draw ratio in the first transverse stretching is more preferably 1.6 times or more and 2.4 times or less, and further preferably 1.7 times or more and 2.3 times or less.

The laterally stretched film is further stretched at a temperature T3 of (Tg-10 ° C.) or higher (Tg + 15 ° C.) or lower (sometimes referred to as second transverse stretching). As described above, in the first transverse stretching, the molecule is stretched at a high temperature so that the molecular orientation is suppressed, but in the subsequent second transverse stretching, on the contrary, it is necessary to positively cause the molecular orientation by stretching. Therefore, in the present invention, the product is stretched at a low temperature so that T2> T3, and specifically, the temperature T3 in the second transverse stretching is (Tg-10 ° C.) or higher (Tg + 15 ° C. or lower). When the temperature T3 in the second transverse stretching is lower than (Tg-10 ° C.), the stress applied in the longitudinal direction due to the neck-in generated in the transverse stretching increases, and the heat shrinkage in the longitudinal direction tends to exceed the upper limit of 6%. It is not preferable because it will end up. On the other hand, when the temperature T3 in the second transverse stretching exceeds (Tg + 15 ° C.), the molecular orientation becomes small and the heat shrinkage in the width direction tends to fall below the lower limit of 50% (comparative example in Table 1 described later). See 5). The temperature T3 in the second transverse stretching is more preferably (Tg-7 ° C.) or higher (Tg + 12 ° C.) or lower, and further preferably (Tg-4 ° C.) or higher (Tg + 9 ° C.) or lower.

Further, the draw ratio in the second transverse stretching is preferably 1.5 times or more and 2.5 times or less. If the stretching ratio in the second transverse stretching is less than 1.5 times, the thickness unevenness in the width direction is deteriorated. On the other hand, when the draw ratio exceeds 2.5 times in the second transverse stretching, not only the shrinkage rate in the width direction tends to decrease, but also the shrinkage stress in the width direction increases. The draw ratio in the second transverse stretching is more preferably 1.6 times or more and 2.4 times or less, and further preferably 1.7 times or more and 2.3 times or less.

The final stretching ratio (the product of the stretching ratio in the first transverse stretching and the stretching ratio in the second transverse stretching) is preferably 3 times or more and 5.5 times or less. If the final draw ratio is less than 3 times, not only the shrinkage rate in the width direction tends to decrease, but also the thickness unevenness in the width direction worsens. On the other hand, when the final draw ratio exceeds 5.5 times, breakage is likely to occur during stretching in the width direction. The final draw ratio is more preferably 3.1 times or more and 5.4 times or less, and further preferably 3.2 times or more and 5.3 times or less.

In the present invention, the preheating temperature T1, the temperature T2 in the first stretching, and the temperature T3 in the second stretching satisfy the relationship of T1> T2> T3. A desired film can be obtained by laterally stretching so that the individual T1, T2, and T3 satisfy the above range while satisfying this relationship.

3.3. Heat treatment The film subjected to the transverse stretching as described above may be heat-treated in a state where both ends in the width direction are gripped by clips in the tenter, if necessary. Here, the heat treatment means heat treatment at a temperature of (Tg-20 ° C.) or more and Tg or less for 1 second or more and 9 seconds or less. Such heat treatment is preferably used because it can suppress a decrease in heat shrinkage and improve dimensional stability after storage over time. If the heat treatment temperature is lower than (Tg-20 ° C.), the above effect of the heat treatment is not effectively exhibited. On the other hand, when the heat treatment temperature is higher than Tg, the heat shrinkage rate in the width direction tends to fall below the lower limit of 50%.
The temperature during the heat treatment is preferably not less than the temperature T3 in the second stretching.

Based on the above heat treatment regulations, all of Examples 1 to 7 and Comparative Examples 1 to 3, 5, 7 (Tg = 75 to 77 ° C.) described later except Example 3 have a heating temperature after transverse stretching. Is 50 ° C. and does not satisfy the above temperature range, so it is not regarded as an example of performing the heat treatment in the present invention. On the other hand, in Example 3, since the heating temperature after lateral stretching was 70 ° C., it is regarded as an example of performing the heat treatment in the present invention.

The longer the heat treatment time, the easier it is to exert the effect, but if it is too long, the equipment becomes huge, so it is preferable to control it to 1 second or more and 9 seconds or less. More preferably, it is 5 seconds or more and 8 seconds or less.

Further, in the heat treatment step, relaxation in the width direction can be performed by reducing the distance between the gripping clips in the tenter. This makes it possible to suppress dimensional changes and deterioration of heat shrinkage characteristics after storage over time.

Next, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the aspects of these Examples and may be modified without departing from the spirit of the present invention. It is possible.

The following characteristics were evaluated for each polyester film shown in Table 2 described later.

[Heat shrinkage rate (hot water heat shrinkage rate)]
A polyester film is cut into a square of 10 cm × 10 cm, immersed in warm water at a predetermined temperature [(90 ° C. or 70 ° C.) ± 0.5 ° C.] for 10 seconds under no load, and then heat-shrinked, and then 25 ° C. The film was immersed in water at ± 0.5 ° C. for 10 seconds, pulled out of the water, the vertical and horizontal dimensions of the film were measured, and the heat shrinkage ratio of each was determined according to the following formula 1. The direction in which the heat shrinkage rate is large was defined as the main shrinkage direction (width direction).
Heat shrinkage rate (%) = {(length before shrinkage-length after shrinkage) / length before shrinkage} × 100 Equation 1

[Thickness spot in the width direction]
A wide strip-shaped film sample with a length of 40 mm in the longitudinal direction of the film and a dimension of 1.2 m in the width of the film is sampled from the film roll, and the measurement speed is 5 m / using a continuous contact thickness gauge manufactured by Micron Measuring Instruments Co., Ltd. The thickness was continuously measured in min along the width direction of the film sample (measurement length was 1 m). The maximum thickness at the time of measurement was Tmax., The minimum thickness was Tmin., And the average thickness was Tave., And the thickness unevenness in the width direction of the film was calculated according to the following formula 2.
Thickness spot (%) = {(Tmax.-Tmin.)/Tave.} × 100 Equation 2

[Maximum heat shrinkage stress]
A sample having a length of 200 mm in the main shrinkage direction (width direction) and a width of 20 mm in the width (longitudinal direction) is cut out from a polyester film, and a strength elongation measuring machine with a heating furnace (Tensilon, a registered trademark of Orientec) is used. It was measured. The heating furnace was preheated to 90 ° C., and the distance between the chucks was set to 100 mm. After temporarily stopping the blowing of the heating furnace and opening the door of the heating furnace and attaching the above sample to the chuck, the door of the heating furnace was immediately closed and the blowing of the heating was restarted. The heat shrinkage stress in the width direction when shrinking in hot air at 90 ° C. for 30 seconds was measured, and the maximum value thereof was taken as the maximum heat shrinkage stress (MPa).

[Total light transmittance]
The measurement was performed using a haze meter "500A" (manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7136. The measurement was performed twice, and the average value was calculated.

[Tg (glass transition point)]
Tg was determined according to JIS-K7121-1987 using a differential scanning calorimeter (model: DSC220) manufactured by Seiko Electronics Inc. Specifically, 10 mg of the unstretched film was heated from −40 ° C. to 120 ° C. at a heating rate of 10 ° C./min, and the endothermic curve was measured. Tangent lines were drawn before and after the inflection point of the obtained endothermic curve, and the intersection was defined as the glass transition point (Tg; ° C.).

[Evaluation of shrinkage finish]
The edges of the polyester film were welded with an impulse sealer (manufactured by Fuji Impulse) to obtain a cylindrical label with the width direction as the circumferential direction. This label was placed on a commercially available PET bottle (with contents; "Oi Ocha" manufactured by Ito En Co., Ltd.) and heat-shrinked by passing it through steam adjusted to 85 ° C. (tunnel passage time: 30 seconds). The shrinkage finish of the label was visually evaluated on a 5-point scale according to the following criteria. The defects described below mean jumping up, wrinkles, insufficient shrinkage, folds at the end of the label, shrinkage whitening, and the like.
5: Best finish (no defects)
4: Good finish (with one defect)
3: There are 2 defects 2: There are 3 to 5 defects 1: There are many defects (6 or more)

<Synthesis of polyester raw materials>
Synthetic polyester raw material A In a stainless steel autoclave equipped with a stirrer, thermometer and partial recirculation cooler, 100 mol% dimethyl terephthalate (DMT) as a dicarboxylic acid component and ethylene glycol (EG) 100 as a polyhydric alcohol component. Ethylene glycol was charged in a molar ratio of 2.2 times that of dimethyl terephthalate, and zinc acetate was used as a transesterification catalyst in an amount of 0.05 mol% with respect to the acid component, and antimony trioxide was used as a polycondensation catalyst. 0.225 mol% was added to the acid component, and the transesterification reaction was carried out while distilling off the produced methanol to the outside of the system. Then, a polycondensation reaction was carried out at 280 ° C. under a reduced pressure of 26.7 Pa to obtain a polyester raw material A having an intrinsic viscosity of 0.58 dl / g. For the intrinsic viscosity, 0.2 g of polyester is dissolved in 50 mL of a mixed solvent of phenol / 1,1,2,2-tetrachloroethane (60/40, weight ratio), and the intrinsic viscosity is 30 ° C. using an Ostwald viscometer. (Dl / g) was measured. This polyester raw material A is polyethylene terephthalate. The composition of the monomer component of the polyester raw material A is shown in Table 1. In Table 1, the content of each monomer component in 100 mol% of the total acid component is in the column of "acid component", and the content of each monomer component in 100 mol% of the total polyhydric alcohol component is in the column of "polyhydric alcohol component". It shows the content of the monomer component.

Synthesis of Polyester Raw Materials B to F As shown in Table 1, polyester raw materials B to F having different monomer components were obtained by the same method as the polyester raw materials A. The polyester raw material B was produced by adding SiO ₂ (Silicia 266 manufactured by Fuji Silysia Chemical Ltd.; average particle size 1.5 μm) as a lubricant at a ratio of 7,000 ppm to polyester. The polyester raw material F was produced by adding TiO ₂ (TA-300 manufactured by Fuji Titanium, average particle size 0.4 μm) as a coloring additive so as to be 50% by mass with respect to the polyester. Each polyester raw material was appropriately formed into chips. In Table 1, TPA is terephthalic acid, BD is 1,4-butanediol, NPG is neopentyl glycol, CHDM is 1,4-cyclohexanedimethanol, and DEG is diethylene glycol as a by-product. The intrinsic viscosities of each polyester raw material are B: 0.58 dl / g, C: 0.72 dl / g, D: 0.80 dl / g, E: 1.20 dl / g, F: 0.58 dl / g, respectively. there were.

Polystyrene In addition to the above polyester raw materials, polystyrene was used as a cavity-developing agent. As the polystyrene, G797N manufactured by Japan Polystyrene was used.

Using the polyester raw materials A to F and the polystyrene raw materials, various polyester films shown in Table 2 were obtained.

[Example 1]
Polyesters A, B and F were mixed at a mass ratio of 85: 5: 10 and charged into an extruder. This mixed resin was melted at 280 ° C., extruded from a T-die, wound around a rotating metal roll cooled to a surface temperature of 30 ° C., and rapidly cooled to obtain an unstretched film having a thickness of about 150 μm. The Tg of the unstretched film was 75 ° C.
The obtained unstretched film was guided to a transverse stretching machine (tenter) and preheated at 140 ° C. for 5 seconds. The preheated film was continuously led to the first half zone of lateral stretching and laterally stretched at 105 ° C. until it became 2.1 times. Subsequently, in the latter half zone of lateral stretching, lateral stretching was performed at 82 ° C. until the temperature increased 1.8 times. The final lateral stretch ratio was 3.8 times. Finally, after heat-treating at 50 ° C. for 3 seconds in the heat treatment zone, the film is cooled, both edges are cut and removed, and the film is wound into a roll with a width of 500 mm to form a transversely stretched film having a thickness of 40 μm over a predetermined length. The film was continuously produced to obtain the film of Example 1.

[Examples 2 to 7, Comparative Examples 1 to 7]
In Example 1, the films of Examples 2 to 7 and Comparative Examples 1 to 7 were produced in the same manner as in Example 1 except that the conditions for lateral stretching were changed as shown in Table 2.

The characteristics of each film thus obtained were evaluated by the above method. These results are also shown in Table 2.

The heat-shrinkable films of Examples 1 to 7 satisfying the requirements of the present invention have a heat-shrinkability in the width direction even though polyethylene terephthalate having a very small proportion of amorphous components of 1 mol% is used. It was high, the thickness unevenness in the width direction was reduced, the heat shrinkage rate in the longitudinal direction was suppressed to a low level, and the shrinkage finish when coated on the label was also good (evaluation 4 or 5). Further, by containing TiO2 and polystyrene in the polyester raw material, the total light transmittance is suppressed to a low level, and the light-shielding property of the contents is also excellent.

In contrast to these examples, in Comparative Example 1, since the temperature T1 at the time of preheating was as low as 92 ° C., the heat shrinkage rate at 90 ° C. in the width direction was as low as 20.4%, and the shrinkage finish of the label was remarkably high. Decreased (evaluation 1).

In Comparative Example 2, the temperature T1 at the time of preheating was controlled to the high temperature specified in the present invention, but the temperature T2 at the time of the first transverse stretching was as low as 72 ° C., which was the same temperature as the temperature T3 at the time of the second transverse stretching. The heat shrinkage rate at 70 ° C. in the longitudinal direction was as high as 8.7%, and the shrinkage finish of the label was lowered (evaluation 2).

In Comparative Example 3, the temperature T1 at the time of preheating, the temperature T2 at the time of the first stretching, and the temperature T3 at the time of the second transverse stretching were all the same as 80 ° C., and the final stretching ratio was as low as 2.4 times, so that the width was wide. The thickness spot in the direction increased to 28.4%. Furthermore, since the raw material did not contain a substance that exhibits light-shielding properties, the total light transmittance was as high as 87.3%.

Since Comparative Example 4 used a polyester film containing a large amount of amorphous components, the thickness unevenness in the width direction was extremely large at 22.0%. Further, as in Comparative Example 3, since the raw material did not contain a substance exhibiting light-shielding property, the total light transmittance was as high as 86.7%.

In Comparative Example 5, the temperature T1 at the time of preheating and the temperature T2 at the time of the first transverse stretching were controlled within the ranges specified in the present invention, but the temperature T3 at the time of the second transverse stretching was as high as 100 ° C., and T2 = T3. This is a processed example. As a result, the heat shrinkage rate at 90 ° C. in the width direction was as low as 18.5%, and not only the shrinkage finish of the label was significantly reduced (evaluation 1), but also the thickness unevenness in the width direction was increased by 23.1%. became.

Since Comparative Example 6 used a polyester film containing a large amount of amorphous components, the thickness unevenness in the width direction was extremely large at 25.4%.

In Comparative Example 7, the temperature T1 at the time of preheating, the temperature T2 at the time of the first stretching, and the temperature T3 at the time of the second transverse stretching were all controlled within the ranges specified in the present invention, so that the shrinkage finish was good (evaluation 5). However, since the raw material did not contain a substance that exhibits light-shielding properties, the total light transmittance was as high as 87.1%.

Since the heat-shrinkable polyester film of the present invention has the above-mentioned characteristics, it can be suitably used for labeling of bottles, bundling applications such as lunch boxes, and particularly applications requiring light-shielding properties.

Claims (2)

A heat-shrinkable polyester film containing 90 mol% or more of an ethylene terephthalate unit in 100 mol% of all ester units.
A heat-shrinkable polyester-based film characterized by satisfying the following requirements (1) to (5).
(1) Heat shrinkage in the width direction when shrunk in hot water at 90 ° C for 10 seconds is 50% or more and 75% or less (2) Heat shrinkage in the longitudinal direction when shrunk in hot water at 90 ° C for 10 seconds Rate is -6% or more and 14% or less (3) Heat shrinkage rate in the longitudinal direction is -6% or more and 6% or less when shrunk in hot water at 70 ° C for 10 seconds (4) Thickness spots in the width direction are 1% 20% or more (5) Total light transmittance is 15% or more and 40% or less The heat-shrinkable polyester-based film according to claim 1, which further satisfies the following requirement (6).
(6) The maximum heat shrinkage stress in the width direction when shrinking in hot air at 90 ° C. for 30 seconds is 4 MPa or more and 13 MPa.