JP7392770B2 - Hollow-containing polyester resin film - Google Patents

Description

The present invention relates to a cavity-containing polyester film that is easy to mass produce, has a low oligomer content, can suppress oligomer precipitation, and has excellent hiding properties and whiteness.

Synthetic paper, which is a paper substitute mainly composed of synthetic resin, has superior water resistance, moisture absorption dimensional stability, and surface stability compared to natural paper, and is used as a material for labels, stickers, posters, recording paper, and packaging materials. It is used for many things such as. It also has excellent reflectance and whiteness, so it is used in reflective plates for displays and lighting equipment. Polyethylene resins, polypropylene resins, polyester resins, etc. are used as the main raw materials for synthetic paper, but polyester resins, typified by polyethylene terephthalate, are particularly popular due to their excellent mechanical and thermal properties. Since then, it has been used in a wide range of applications.

Generally speaking, methods for obtaining films with functions similar to those of paper include a method of incorporating a large amount of fine cavities inside the film, and a method of surface treatment such as sandblasting, chemical etching, or matting treatment on a flat film. Examples include a method of roughening the surface by performing . Among these, the former method of containing a large amount of fine cavities inside the film not only provides paper-like hiding properties and whiteness, but also reduces the cost per area because the film itself can be made lighter. It is widely used due to its advantages such as being able to hold the image in place, providing appropriate flexibility and cushioning properties, and providing excellent image clarity during printing.

Generally speaking, the method of creating fine cavities inside a film is to mix an incompatible thermoplastic resin (hereinafter referred to as an incompatible resin) into a polyester resin, and then An example of this method is to obtain a sheet in which a resin is dispersed and stretch it in at least one axial direction to cause interfacial peeling between the polyester resin and the incompatible resin to form cavities. Examples of incompatible resins for forming cavities with polyester resins include polyolefin resins such as polyethylene resins, polypropylene resins, and polymethylpentene resins (for example, see Patent Documents 1 to 3), and polystyrene. based resins (for example, Patent Documents 4, 5
) is preferably used.

Polyester resin generally contains oligomers, so when used for long periods as labels, stickers, and reflectors, the oligomers contained in polyester resin films may precipitate on the film surface. There is a concern that the quality of the film base material surface may change. For example, the adhesion of the adhesive layer applied to the film surface may deteriorate, and the surface reflectance and brightness may decrease. There is also a concern that process contamination may occur in post-processes such as annealing and resin coating. As a method of suppressing the precipitation of oligomers from a hollow-containing white polyester resin film, there is an example of laminating a resin coating film on the film base material, but this does not necessarily reduce the amount of oligomers contained in the film base material. Therefore, the effect could not be said to be sufficient (see, for example, Patent Document 6).

Japanese Unexamined Patent Publication No. 49-34755 Japanese Unexamined Patent Publication No. 2-284929 Japanese Unexamined Patent Publication No. 2-180933 Special Publication No. 54-29550 Japanese Patent Application Publication No. 11-116716 JP2007-298963A

The present invention aims to solve the problems of the conventional microvoided polyester resin films as described above. The purpose of this invention is to provide a hollow polyester film with excellent whiteness.

That is, the present invention has the following configuration.
1. A cavity-containing polyester film in which a layer (B layer) made of a polyester resin containing inorganic particles is laminated on both sides of a layer (A layer) containing cavities inside, the layer A being made of a polyester resin and a polyester resin. Consists of a composition containing a thermoplastic resin that is incompatible with the resin, and contains 25% by mass or more and 90% by mass or less of polyester resin recycled from PET bottles based on all the polyester resins constituting the polyester film. A cavity-containing polyester film, wherein the content of ester structural units derived from an isophthalic acid component is 0.2 mol% or more and 2.0 mol% or less with respect to all ester structural units constituting the polyester film.
2. The cavity-containing polyester film according to the above item 1, wherein the content of the cyclic trimer oligomer in the polyester film is 0.60% by mass or less.
3. The cavity-containing polyester film according to the first or second aspect, wherein the polyester resin recycled from PET bottles has an intrinsic viscosity of 0.60 dl/g or more and 0.75 dl/g or less.
4. The first method is characterized in that the thermoplastic resin incompatible with the polyester resin in layer A is at least one resin selected from polystyrene resins, polymethylpentene resins, and polypropylene resins. The cavity-containing polyester film according to any one of -3.
5. The cavity-containing polyester film according to any one of the first to fourth aspects, wherein the inorganic particles in layer B are at least one selected from titanium oxide, calcium carbonate, barium sulfate, and silica.
6. The cavity-containing polyester film according to any one of the above items 1 to 5, which has an optical density of 0.55 or more (converted to a thickness of 50 μm) and a color tone b value of 4 or less (converted to a thickness of 50 μm).

According to the present invention, it is possible to provide a cavity-containing polyester film that can suppress oligomer precipitation while maintaining mass productivity, has excellent hiding properties and whiteness, and can be suitably used for labels, stickers, reflectors, etc. Even after long-term use as described above, there is little change in surface quality such as tackiness and brightness.

The present invention will be explained in detail below.
In the cavity-containing polyester film of the present invention, the polyester resin that is the main component of layer A and layer B is a polymer synthesized from dicarboxylic acid or its ester-forming derivative and diol or its ester-forming derivative. Typical examples of such polyester resins include polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, and polyethylene-2,6-naphthalate. Polyethylene terephthalate is preferred from the viewpoint of mechanical properties, heat resistance, and cost. preferable.

Further, other components may be copolymerized with these polyester resins as long as the purpose of the present invention is not impaired. Specifically, the copolymerization component includes dicarboxylic acid components such as isophthalic acid, naphthalene dicarboxylic acid, 4,4-diphenyldicarboxylic acid, adipic acid, sebacic acid, and ester-forming derivatives thereof. Further, diol components include diethylene glycol, hexamethylene glycol, neopentyl glycol, and cyclohexanedimethanol. Also included are polyoxyalkylene glycols such as polyethylene glycol and polypropylene glycol. The amount of copolymerization is preferably within 10 mol%, more preferably within 5 mol%, per repeating unit.

In the method for producing the polyester resin of the present invention, first, using the aforementioned dicarboxylic acid or its ester-forming derivative and diol or its ester-forming derivative as main starting materials, an esterification or transesterification reaction is carried out according to a conventional method. Examples include a method of manufacturing by further performing a polycondensation reaction at high temperature and reduced pressure.

The intrinsic viscosity of the polyester resin pellets of the present invention is preferably in the range of 0.50 to 0.9 dl/g, more preferably in the range of 0.55 to 0.85 dl/g, from the viewpoint of film formability and recyclability. range.

It is preferable that the polyester resin in the present invention contains a polyester resin recycled from PET bottles. The crystallinity of the polyester used in PET bottles is controlled to improve bottle moldability and appearance, and as a result, the crystallinity of the polyester resin is 0.5 mol% based on the total ester constituent units in the polyester resin. Those containing 10.0 mol% or less of an isophthalic acid component and an ester structural unit derived from any diol component represented by ethylene glycol or diethylene glycol may be used. Further, polyesters whose intrinsic viscosity is increased by further performing solid phase polymerization after liquid phase polymerization are sometimes used. Polyester resin pellets recycled from PET bottles are usually made by washing, pulverizing, and heating and melting the PET bottles to re-pelletize them, but they are also made by solid-phase polymerization to increase the intrinsic viscosity. I don't mind. The intrinsic viscosity of the polyester resin recycled from PET bottles is preferably in the range of 0.60 to 0.75 dl/g. When the intrinsic viscosity is 0.60 dl/g or more, the resulting film becomes difficult to break, making it easy to stably operate film production, which is preferable. On the other hand, it is preferable that the intrinsic viscosity is 0.75 dl/g or less, since the filtration pressure of the molten fluid will not increase too much and it will be easy to stably operate the film production. Generally, when a polyethylene terephthalate resin is subjected to solid phase polymerization, the amount of oligomers contained in the resin, especially PET cyclic trimer, which is the largest in the content, is smaller than that obtained when polyethylene terephthalate resin is subjected to liquid phase polymerization. The upper limit of the cyclic trimer oligomer contained in the recycled polyester resin made from PET bottles is preferably 0.7% by mass, more preferably 0.5% by mass, and even more preferably 0.4% by mass. be.

The lower limit of the content of the polyester resin recycled from PET bottles in the hollow-containing polyester film is preferably 25% by mass, more preferably 30% by mass, and even more preferably 50% by mass. When the content is 25% by mass or more, the amount of oligomers contained in the hollow polyester film decreases, and precipitation of oligomers can be suppressed, which is preferable. Furthermore, in terms of utilization of recycled resin, a high content rate is preferable in terms of contributing to reducing environmental load. The upper limit of the content of polyester resin recycled from PET bottles is preferably 90% by mass, more preferably 85% by mass.

The upper limit of the amount of oligomers contained in the cavity-containing polyester film of the present invention, especially the ester cyclic trimer that is the largest in content, is preferably 0.60% by mass, more preferably 0.55% by mass, More preferably, it is 0.40% by mass. It is preferable that the content of the ester cyclic trimer is 0.60% by mass or less because it facilitates suppressing oligomer precipitation from the film.

The lower limit of the content of the ester structural units derived from the isophthalic acid component in all the ester structural units constituting the polyester resin contained in the cavity-containing polyester film of the present invention is preferably 0.2 mol%, and more preferably is 0.5 mol%, more preferably 0.9 mol%. The upper limit of the content of ester structural units derived from the isophthalic acid component in all ester structural units constituting the polyester resin contained in the film is preferably 2.0 mol%, more preferably 1.8 mol%. , more preferably 1.5 mol%. If it is 2.0 mol % or less, crystallinity is less likely to decrease and heat shrinkage rate can be lowered, which is preferable. In addition, the ester structural unit derived from an isophthalic acid component means an ester structural unit consisting of an isophthalic acid component as a dicarboxylic acid component and any diol component represented by ethylene glycol, diethylene glycol, and the like.

Next, the thermoplastic resin that is incompatible with the polyester resin used in the present invention will be explained. Thermoplastic resins that are incompatible with the polyester resin used in the present invention include those that are uniformly mixed into the polyester resin in a dispersed state and peel off at the interface with the base resin during stretching, causing cavity formation. Any resin may be used, but preferred examples include polystyrene resin, polyolefin resin, cyclic polyolefin resin, polyacrylic resin, polycarbonate resin, polysulfone resin, and cellulose resin. Examples include. These can be used alone, or two or more types can be used in combination if necessary, or they can be copolymerized to impart a suitable affinity with the polyester. Among these, polyolefin resins such as polystyrene resins, polymethylpentene resins, and polypropylene resins can be preferably used.

The preferred blending amount of the above-mentioned incompatible resin varies depending on the amount of cavities required for the final film, stretching conditions, etc., but it is usually 3% by mass or more based on the total amount of the resin composition. It is selected from a range of less than 40% by mass, more preferably from 5 to 30% by mass. If the amount is 3% by mass or more, cavities can be reliably formed in the stretching process, and lightness, flexibility, drawability, writability, etc. can be easily obtained, which is preferable. On the other hand, when the content is less than 40% by mass, not only good stretchability is maintained, but also heat resistance, strength, and stiffness are maintained, which is preferable.

In addition, within a range that does not impair the purpose of the present invention, these polyester resins or optionally used polypropylene resins may contain small amounts of other polymers, antioxidants, heat stabilizers, matting agents, pigments, It may also contain ultraviolet absorbers, optical brighteners, plasticizers, or other additives. In particular, in order to suppress oxidative deterioration of the polypropylene resin, it is preferable to include an antioxidant or a heat stabilizer. The types of antioxidants and heat stabilizers are not particularly limited, but include, for example, hindered phenol type, phosphorus type, hindered amine type, etc., and these may be used alone or in combination. The content is preferably in the range of 1 to 50,000 ppm.

In the present invention, the void-containing polyester film may contain inorganic particles in the polyester resin or in the incompatible resin, if necessary, in order to improve concealment properties and whiteness. Examples of the inorganic particles include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, titanium oxide, and zinc sulfide. Barium and silica are preferred. Further, these inorganic particles may be used alone or in combination of two or more types. These particles can be incorporated into the film by adding them in advance to the polyester resin or to the incompatible resin.

In the present invention, the method of mixing inorganic particles with the polyester resin or the incompatible resin is not particularly limited, and after dry blending the polyester resin and the incompatible resin, the polyester resin and the incompatible resin are directly fed into a film forming machine. Examples include a method in which a polyester resin and an incompatible resin are dry blended, and then melt-kneaded using various general kneading machines to form a masterbatch.

Next, a method for forming a hollow polyester film according to the present invention will be described, but the method is not particularly limited thereto. For example, after drying a mixture consisting of the above-mentioned composition in a conventional manner, it is melt-extruded into a sheet form from a T-shaped nozzle, brought into close contact with a casting drum by an electrostatic application method, and cooled and solidified to obtain an unstretched film. . Next, the unstretched film is stretched and oriented. In the following, the most commonly used sequential biaxial stretching method, particularly a method in which the unstretched film is longitudinally stretched in the longitudinal direction and then transversely stretched in the width direction, will be described. Let's explain with an example. First, in the longitudinal stretching step, the film is heated and stretched by 2.5 to 5.0 times between two or multiple rolls having different circumferential speeds. The heating means at this time may be a method using a heating roll or a method using a non-contact heating medium, or a combination of these may be used, but the temperature of the film may be set between (Tg - 10°C) and (Tg + 50°C). It is preferable to set it as a range. Next, the uniaxially stretched film is introduced into a tenter and stretched 2.5 to 5 times in the width direction at a temperature of (Tg - 10°C) to Tm - 10°C or less to obtain a biaxially stretched film. However, Tg is the glass transition temperature of the polyester resin, and Tm is the melting point of the polyester. Further, it is preferable that the film obtained above is subjected to heat treatment if necessary, and the treatment temperature is preferably in the range of (Tm-60°C) to Tm.

The cavity-containing polyester film of the present invention has a layer structure consisting of a composition that is incompatible with a polyester resin, and has a polyester film containing inorganic particles on both sides of a layer (A layer) containing cavities inside. It is necessary to form a laminated structure in which layers (B layer) made of resin are laminated. If the A layer containing the incompatible resin is exposed to the surface layer, some of the exposed incompatible resin dispersed particles will cause process contamination such as roll stains, which is not preferable.

The ratio of the sum of the thicknesses of layer B laminated on both sides of layer A is preferably in the range of 1 to 40% with respect to the total thickness of the film, from the viewpoint of cavity development and suppression of exposure of incompatible resin. , more preferably 5 to 30%. It is preferable that the sum of the thicknesses of the B layers is 1% or more, since exposure of the incompatible resin in the A layer to the film surface can be suppressed. On the other hand, it is preferable that the sum of the thicknesses of the B layers is 40% or less, since it becomes easy to form cavities in the A layer to obtain sufficient lightness and cushioning properties.

In the present invention, examples of inorganic particles contained in layer B include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, titanium oxide, zinc sulfide, etc. , titanium oxide, calcium carbonate, barium sulfate, and silica are preferred, and titanium oxide is particularly preferred. Further, these inorganic particles may be used alone or in combination of two or more types. These particles can be incorporated into the film by adding them to the polyester resin in advance.

The upper limit of the average particle diameter of the inorganic particles contained in layer B is preferably 5.0 μm, more preferably 3.0 μm, particularly from the viewpoint of printing quality when providing a printing layer etc. in post-processing. Preferably it is 2.5 μm. Further, the lower limit of the average particle diameter of the inorganic particles is preferably 0.1 μm, particularly preferably 0.2 μm, from the viewpoint of slipperiness and concealment properties in the film manufacturing process and post-processing process. The method for measuring the average particle diameter of inorganic particles is to observe the inorganic particles in the cross section of the film with a transmission electron microscope or scanning electron microscope, observe 20 non-agglomerated inorganic particles, and calculate the average value. This can be done by a method of adjusting the average particle size. The shape of the inorganic particles is not particularly limited as long as the object of the present invention is met, and spherical particles and irregularly shaped non-spherical inorganic particles can be used. The particle diameter of irregularly shaped particles can be calculated as a circular equivalent diameter. The equivalent circle diameter is a value obtained by dividing the area of the observed inorganic particle by π, calculating the square root, and doubling the square root.

The content of inorganic particles in layer B is preferably from 5 to 40% by mass, more preferably from 7 to 30% by mass. It is preferable that the content is 5% by mass or more because it can improve hiding properties and whiteness. On the other hand, when the content is 40% by mass or less, not only good film formability is maintained, but also the mechanical strength of the film is maintained. The content of the inorganic particles in the B layer is preferably 1 to 30% by mass, more preferably 2 to 20% by mass. It is preferable that the content is abnormal by 1% by mass because it is easy to improve concealing properties and whiteness. On the other hand, a content of 30% by mass or less is preferable because not only good film-forming properties are maintained but also the mechanical strength of the film is maintained.

Further, the cavity-containing polyester film of the present invention may be provided with a coating layer on at least one side thereof in order to improve the wettability and adhesion of printing inks and coating agents. The compound constituting the coating layer is preferably a polyester resin, but there are also means for improving the adhesion of ordinary polyester films such as polyurethane resin, polyester urethane resin, acrylic resin, polyether resin, etc. Compounds disclosed as are applicable. Further, a crosslinked structure may be formed in order to improve the adhesion durability of these easily adhesive layers. By containing a crosslinking agent, it becomes possible to further improve the adhesion under high temperature and high humidity conditions. Specific crosslinking agents include urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, carbodiimide-based, and the like. Further, in order to promote the crosslinking reaction, a catalyst or the like may be used as appropriate.

The coating layer can also contain lubricant particles in order to impart slipperiness, matteness, ink absorbency, etc. to the surface. Particles may be inorganic particles or organic particles, and are not particularly limited, but include (1) silica, kaolinite, talc, light calcium carbonate, heavy calcium carbonate, zeolite, alumina, Barium sulfate, carbon black, zinc oxide, zinc sulfate, zinc carbonate, zirconium oxide, titanium dioxide, aluminum silicate, diatomaceous earth, calcium silicate, aluminum hydroxide, calcium carbonate, magnesium carbonate, calcium phosphate, magnesium hydroxide, barium sulfate, etc. Inorganic particles, (2) Acrylic or methacrylic, vinyl chloride, vinyl acetate, nylon, styrene/acrylic, styrene/butadiene, polystyrene/acrylic, polystyrene/isoprene, polystyrene/isoprene, methyl methacrylate / Organic particles such as butyl methacrylate, melamine, polycarbonate, urea, epoxy, urethane, phenol, diallyl phthalate, polyester, etc. are used, but in order to give the coating layer appropriate slipperiness. , silica is particularly preferably used.

As a method for providing the coating layer, commonly used methods such as a gravure coating method, a kiss coating method, a dip method, a spray coating method, a curtain coating method, an air knife coating method, a blade coating method, and a reverse roll coating method can be applied. The coating step can be carried out by any method such as coating before stretching the film, coating after longitudinal stretching, or coating on the surface of the film after stretching.

In the cavity-containing polyester film thus obtained, it is possible to use a self-regenerating raw material for the A layer, which is made of scrap film from edges or breakage troubles generated during the film forming process. The content of the self-regenerating raw material is preferably 5 to 60% by mass based on the total amount of each composition in layer A from the viewpoint of raw material cost reduction, whiteness, and film formability. Further, although a self-regenerating raw material may be contained in layer B, it is preferable not to contain it from the viewpoint of deterioration of whiteness and exposure of the incompatible resin in the self-regenerating raw material.

The void-containing polyester film in the present invention preferably has an apparent density of 0.8 to 1.3 g/cm3, more preferably 0.90 to 1.2 g/cm3. It is preferable that the apparent density is 0.8 g/cm 3 or more, since there are not too many cavities and ease of handling is obtained during post-processing such as printing and during use. If it is 1.3 g/cm3 or less, sufficient lightness and cushioning properties can be obtained, which is preferable. Incidentally, the apparent density is a value obtained by a measuring method described in the evaluation method described below.

The cavity-containing polyester film in the present invention preferably has an optical density (OD value) of 0.55 or more, more preferably 0.6 or more. It is preferable that the OD value is 0.55 or more, since sufficient hiding properties can be obtained, and when used for labels, etc., the clarity of the image when printed can be obtained. The upper limit of the OD value is preferably 1.5. A value of 1.5 or less is preferable in terms of whiteness and cost. Incidentally, the OD value is a value obtained in terms of a thickness of 50 μm obtained by the measurement method described in the evaluation method described below.

The cavity-containing polyester film in the present invention preferably has a color tone b value of 4.0 or less, more preferably 3.0 or less. If the b value is 4.0 or less, the whiteness will be at a satisfactory level, and when used as a label, clarity during printing will be obtained, which is preferable. The lower limit of the color tone b value is preferably −5.0. It is preferable that the b value is -5.0 or more, since the film does not have an excessively strong blue tint, and resolution can be satisfied in a well-balanced manner when used as a printing base material.

The thickness of the cavity-containing polyester film of the present invention is arbitrary, but preferably 20 to 300 μm.

The cavity-containing polyester film obtained in this way has excellent mass productivity, has a low oligomer content and can suppress oligomer precipitation, is lightweight and has excellent cushioning properties, has good hiding properties and whiteness, and has excellent labelling. It is suitably used as a base material for cards, packaging materials, reflective materials, etc.

The present invention will be specifically described below with reference to Examples. Note that the present invention is not limited to the embodiments described below. In addition, each evaluation item in Examples and Comparative Examples was measured by the following method.

(1) Intrinsic viscosity [η]
It was dissolved in a mixed solvent of phenol/tetrachloroethane = 60/40 (mass ratio) and measured at 30°C using an Ostwald viscometer. Note that the measurement was performed three times, and the average value was determined.

(2) Amount of Isophthalic Acid The content of ester structural units derived from terephthalic acid and isophthalic acid contained in the raw polyester and the polyester constituting the film was calculated by the following method. After dissolving the sample in 0.1 ml of a mixed solution of deuterated chloroform and trifluoroacetic acid (volume ratio 1/1), 0.5 ml of trichloroethylene was added and dissolved at 135°C to prepare a sample solution. 200'' (manufactured by Varian), proton NMR was measured. The peak intensity of a predetermined proton was calculated, and the content (mol %) of ester structural units derived from terephthalic acid and ester structural units derived from isophthalic acid in 100 mol % of ester structural units was calculated.

(3) Content of cyclic trimer in polyester film The content of cyclic trimer in polyester film was measured by the following method. 100 mg of the pulverized film sample was accurately weighed, dissolved in 3 ml of hexafluoroisopropanol/chloroform mixture (volume ratio = 2/3), and further diluted by adding 20 ml of chloroform. 10 ml of methanol was added to this to precipitate the polymer, which was then filtered. The filtrate was evaporated to dryness and made up to volume with 10 ml of dimethylformamide. The cyclic trimer was then quantified using the high performance liquid chromatography method described below.

(Measurement condition)
Equipment: L-7000 (manufactured by Hitachi) Column: μ-Bondasphere C18 5μ 100 angstrom 3.9 mm x 15 cm (manufactured by Waters)
Solvent: Eluent A: 2% acetic acid/water (v/v)
Eluent B: Acetonitrile Gradient B%: 10 → 100% (0 → 55 minutes)
Flow rate: 0.8ml/min Temperature: 30℃
Detector: UV-258nm

(4) Apparent Density Cut out four pieces of film into 5.0 cm squares, stack the four pieces together, use a micrometer to measure the total thickness at 10 points using four significant digits, and stack the four pieces together. The average value of the thickness was determined. This average value was divided by 4 and rounded to three significant digits to obtain the average thickness per sheet (t: μm). The mass (w:g) of the same four samples was measured to four significant figures using an automatic balance, and the apparent density was determined from the following formula. Note that the apparent density was rounded to three significant digits.
Apparent density (g/cm3) = w/(5.0 x 5.0 x t x 10-4 x 4)

(5) Optical density (OD value)
It was measured using a transmission densitometer "Model Ihac-T5" manufactured by Ihara Electronics Co., Ltd., and converted to a film thickness of 50 μm. Note that the higher the optical density value, the greater the hiding power.

(6) Color tone b value The color tone b value was measured according to JIS-8722 using a Nippon Denshoku Co., Ltd. color difference meter (ZE6000), and was converted to a film thickness of 50 μm. It was determined that the smaller the b value, the higher the degree of whiteness and the weaker the yellow tinge.

(7) Oligomer adhesion evaluation The film was hot pressed with a stainless steel plate at 160°C, and the degree of staining of the stainless steel plate before and after pressing was visually determined. The pressing pressure was 4.9×10 6 N/m 2 (5 kgf/cm 2 ), and the pressing was carried out 30 times for 30 seconds each time, and the evaluation was determined as follows. For films coated with easily adhesive resin, stains on the surface of the coated layer were evaluated.
◎: No dirt was visible on the stainless steel plate.
○: A little dirt was visible on the stainless steel plate, but it was at a level that poses no problem for practical use.
×: White stains were clearly visible on the stainless steel plate.

[Preparation of polyethylene terephthalate resin (I)]
When the temperature of the esterification reactor was raised to 200°C, 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were charged, and while stirring, 0.017 parts by mass of antimony trioxide was added as a catalyst. 0.064 parts by mass of magnesium acetate tetrahydrate and 0.16 parts by mass of triethylamine were charged. Next, the pressure and temperature were increased to carry out a pressure esterification reaction under the conditions of a gauge pressure of 0.34 MPa and 240° C., and then the esterification reactor was returned to normal pressure, and 0.014 parts by mass of phosphoric acid was added. Furthermore, the temperature was raised to 260° C. over 15 minutes, and 0.012 parts by mass of trimethyl phosphate was added. After 15 minutes, dispersion treatment was performed using a high-pressure dispersion machine, and an aqueous solution of sodium tripolyphosphate was added to the silica particles to contain 0.1% by mass of sodium atoms, and 35% of the coarse particles were cut by centrifugation treatment. An ethylene glycol slurry of silica particles having an average particle diameter of 2.5 μm that had been filtered through a metal filter with an opening of 5 μm was added as a particle content of 0.2 parts by mass. After 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reactor, and a polycondensation reaction was performed at 280° C. under reduced pressure.
After the polycondensation reaction is complete, it is filtered using a Naslon filter with a 95% cut diameter of 5 μm, extruded into a strand from a nozzle, and cooled and solidified using cooling water that has been previously filtered (pore size: 1 μm or less). , cut into pellets. The intrinsic viscosity of the obtained polyethylene terephthalate resin was 0.62 dl/g, and the PET cyclic trimer oligomer content was 0.69% by mass. (Hereafter abbreviated as PET resin (I).)

[Preparation of polyethylene terephthalate resin (II)]
In the above-mentioned production of PET resin (I) having an intrinsic viscosity of 0.62 dl/g, a polyethylene terephthalate resin (II) having an intrinsic viscosity of 0.62 dl/g and containing no silica particles was obtained. (Hereinafter, it will be abbreviated as PET resin (II).) The PET cyclic trimer oligomer content was 0.69% by mass.

[Preparation of titanium oxide-containing master pellet (M1)]
A mixture of 50% by mass of PET resin (II) with an intrinsic viscosity of 0.62 dl/g and 50% by mass of anatase titanium dioxide with an average particle size of 0.3 μm (electron microscopy) was placed in a vented twin-screw extruder. The mixture was supplied and kneaded to obtain titanium oxide-containing master pellets (M1). The PET cyclic trimer oligomer content was 0.70% by mass.

[Preparation of cavity forming agent]
60% by mass of polymethylpentene (PMP) resin with a melt viscosity of 1,300 poise, which is a thermoplastic resin incompatible with polyester resin, 20% by mass of polypropylene (PP) resin with a melt viscosity of 2,000 poise, and molten 20% by mass of a polystyrene (PS) resin having a viscosity of 3,900 poise was mixed into pellets, fed into a vented twin-screw extruder temperature-controlled at 285°C, and kneaded to obtain a cavity forming agent.

[Preparation of polyester resin (III) recycled from PET bottles]
After removing foreign substances such as remaining beverages and labels from plastic beverage bottles, the flakes obtained by crushing are melted in an extruder, and the filter is sequentially changed to a finer aperture size to remove even finer foreign substances twice. was filtered, and thirdly filtered through a filter with the smallest opening size of 50 μm, extruded into a strand from a nozzle, cooled and solidified using cooling water that had been previously filtered (pore diameter: 1 μm or less). , and cut into pellets to obtain polyester resin (III). The ratio of ester constitutional units in the obtained polyester resin (III) was terephthalic acid-derived ester constitutional units/isophthalic acid-derived ester constitutional units = 98.6/1.4 (mol%), and the intrinsic viscosity of the resin was It was 0.65 dl/g, and the PET cyclic trimer oligomer content was 0.38% by mass.

[Preparation of copolymerized polyester resin aqueous dispersion (a)]
95 parts by mass of dimethyl terephthalate, 95 parts by mass of dimethyl isophthalate, 35 parts by mass of ethylene glycol, 145 parts by mass of neopentyl glycol, 0.1 part by mass of zinc acetate and 0.1 part by mass of antimony trioxide were charged into a reaction vessel, and the mixture was heated at 180°C. The transesterification reaction was carried out over a period of 3 hours. Next, 6.0 parts by mass of 5-sodium sulfoisophthalic acid was added and the esterification reaction was carried out at 240°C for 1 hour, and then at 250°C under reduced pressure (10 to 0.2 mmHg) for 2 hours. A polycondensation reaction was carried out to obtain a copolyester resin having a number average molecular weight of 19,500 and a softening point of 60°C. 300 parts by mass of the obtained copolymerized polyester resin and 140 parts by mass of butyl cellosolve were stirred at 160°C for 3 hours to obtain a viscous melt. 560 parts by mass of water was gradually added to this melt, and the mixture was stirred for 1 hour. Thereafter, a uniform pale white copolyester resin aqueous dispersion (a) having a solid content concentration of 30% was obtained.

[Preparation of polyurethane resin aqueous solution (b)]
100 parts by mass of polyester diol (OHV: 2000eq/ton) consisting of adipic acid, 1,6-hexanediol, neopentyl glycol (molar ratio: 4/2/3) and 41.4 parts by mass of xylylene diisocyanate. After mixing and reacting at 80 to 90°C for 1 hour under a nitrogen stream, it was cooled to 60°C, 70 parts by mass of tetrahydrofuran was added and dissolved, and a urethane prepolymer solution (NCO/OH ratio: 2.2, free Isocyanate group: 3.30% by mass) was obtained. Subsequently, the urethane prepolymer solution was heated to 40° C., and then 45.5 parts by mass of a 20% by mass aqueous sodium bisulfite solution was added and reacted at 40 to 50° C. for 30 minutes with vigorous stirring. After confirming the disappearance of the free isocyanate group content (solid content), the self-crosslinking polyurethane resin aqueous solution (b) containing isocyanate groups was diluted with emulsified water and blocked with sodium bisulfite having a solid content of 20% by mass. I got it.

[Preparation of coating liquid (c) for forming easily adhesive resin layer]
5.3 parts by mass of 30% by mass aqueous dispersion of copolymerized polyester resin (a), 14.6 parts by mass of self-crosslinking polyurethane resin aqueous solution (b), 42.6 parts by mass of water, and isopropyl alcohol. 32.5 parts by mass of were mixed. Furthermore, a 10% by mass aqueous solution (0.35 parts by mass) of a fluorine-based surfactant (polyoxyethylene-2-perfluorohexylethyl ether) and isostearylamide methylammonium ethosulfate salt were dissolved in ethylene glycol monobutyl ether. 1.6 parts by mass of a 17.5% by mass solution was added. Next, the pH of the above mixture was adjusted to 6.2 with a 5% by mass aqueous sodium bicarbonate solution, and the mixture was microfiltered through a felt-type polypropylene filter with a filtration particle size (initial filtration efficiency: 95%) of 10 μm, and the coating liquid ( c) was prepared.

(Example 1)
[Manufacture of unstretched film]
47% by mass of the PET resin (II), 40% by mass of polyester resin (III), 5% by mass of titanium oxide-containing master pellets (M1), and 8% by mass of a cavity forming agent were mixed and vacuum-dried to form a cavity-containing polyester. This was used as the raw material for layer A. On the other hand, 70% by mass of the PET resin (I) and 30% by mass of titanium oxide-containing master pellets (M1) were mixed into pellets, vacuum dried, and used as a raw material for the inorganic particle-containing polyester B layer. These raw materials were supplied to separate extruders, melted at 285°C, and layered so that the cavity-containing polyester A layer and the inorganic particle-containing polyester B layer were in the order of B/A/B, with a thickness ratio of 10/80. /10, and extruded from a T-die onto a cooling drum adjusted to 30°C to produce an unstretched film having a two-layer, three-layer structure.

[Cavity-containing polyester film production]
The obtained unstretched film was uniformly heated to 70° C. using a heating roll, and longitudinally stretched 3.4 times between two pairs of nip rolls having different circumferential speeds. At this time, as an auxiliary heating device for the film, an infrared heater (rated at 20 W/cm) equipped with a gold reflective film in the middle of the nip roll was installed facing both sides of the film (at a distance of 1 cm from the film surface) and heated. The uniaxially stretched film thus obtained was introduced into a tenter, heated to 140°C, stretched horizontally to 4.0 times, fixed in width, heat treated at 235°C, and further stretched in the width direction at 210°C. By relaxing it by 3%, a hollow polyester film with a thickness of 50 μm was obtained. Table 1 shows the results of the apparent density, OD value, and color tone b value.

(Example 2)
Except that the raw materials for the cavity-containing polyester A layer were changed to 27% by mass of PET resin (II), 60% by mass of polyester resin (III), 5% by mass of titanium oxide-containing master pellet (M1), and 8% by mass of cavity forming agent. A cavity-containing polyester film was obtained in the same manner as in Example 1. The results are shown in Table 1.

(Example 3)
Cavities were formed in the same manner as in Example 1, except that the raw materials for the cavity-containing polyester A layer were 87% by mass of polyester resin (III), 5% by mass of titanium oxide-containing master pellets (M1), and 8% by mass of the cavity-forming agent. A polyester film was obtained. The results are shown in Table 1.

(Example 4)
The raw materials for the cavity-containing polyester A layer are 87% by mass of polyester resin (III), 5% by mass of titanium oxide-containing master pellets (M1), and 8% by mass of the cavity forming agent, and the raw materials for the cavity-containing polyester B layer are PET resin (I). ) 15% by mass, polyester resin (III) 55% by mass, and titanium oxide-containing master pellet (M1) 30% by mass, but a hollow polyester film was obtained in the same manner as in Example 1. The results are shown in Table 1.

(Example 5)
In Example 3, the coating liquid (c) for forming an easily adhesive resin layer was applied to the casting drum contact surface of a uniaxially stretched film longitudinally stretched 3.4 times by a reverse kiss coating method to reduce the resin solid content after drying. The coating was applied to a thickness of 0.3 μm. The uniaxially stretched film having the coating layer is guided into a tenter while drying, heated to 140°C, horizontally stretched 4.0 times, width fixed, heat treated at 235°C, and further stretched 3 times in the width direction at 210°C. % relaxation, a hollow polyester film having a thickness of 50 μm was obtained. The results are shown in Table 1.

(Example 6)
In Example 4, the coating liquid (c) for forming an easily adhesive resin layer was applied to the casting drum contact surface side of a uniaxially stretched film longitudinally stretched 3.4 times by a reverse kiss coating method to reduce the resin solid content after drying. The coating was applied to a thickness of 0.3 μm. The uniaxially stretched film having the coating layer is guided into a tenter while drying, heated to 140°C, horizontally stretched 4.0 times, width fixed, heat treated at 235°C, and further stretched 3 times in the width direction at 210°C. % relaxation, a hollow polyester film having a thickness of 50 μm was obtained. The results are shown in Table 1.

(Comparative example 1)
Cavities were formed in the same manner as in Example 1, except that the raw materials for the cavity-containing polyester A layer were changed to 87% by mass of PET resin (II), 5% by mass of titanium oxide-containing master pellets (M1), and 8% by mass of the cavity forming agent. A containing polyester film was obtained. The results are shown in Table 1.

(Comparative example 2)
Except that the raw materials for the cavity-containing polyester A layer were changed to 77% by mass of PET resin (II), 10% by mass of polyester resin (III), 5% by mass of titanium oxide-containing master pellet (M1), and 8% by mass of cavity forming agent. A cavity-containing polyester film was obtained in the same manner as in Example 1. The results are shown in Table 1.

(Comparative example 3)
In Comparative Example 1, the above-mentioned coating liquid (c) for forming an easily adhesive resin layer was applied to the casting drum contact surface side of a uniaxially stretched film longitudinally stretched 3.4 times by a reverse kiss coating method to reduce the resin solid content after drying. The coating was applied to a thickness of 0.3 μm. The uniaxially stretched film having the coating layer is guided into a tenter while drying, heated to 140°C, horizontally stretched 4.0 times, width fixed, heat treated at 235°C, and further stretched 3 times in the width direction at 210°C. % relaxation, a hollow polyester film having a thickness of 50 μm was obtained. The results are shown in Table 1.

(Comparative example 4)
The raw materials for the cavity-containing polyester A layer are 84% by mass of PET resin (II), 5% by mass of titanium oxide-containing master pellets (M1), 10% by mass of polypropylene (PP) resin with a melt viscosity of 2,000 poise, and PEG as a dispersant. A cavity-containing polyester film was obtained in the same manner as in Example 1, except that 1% by mass of (molecular weight 4000) was added. The results are shown in Table 1. In Comparative Example 4, since a dispersant was added, the dispersed particle diameter of the polypropylene resin was reduced, the apparent density was increased, the hiding property was decreased, and the color b value increased.

According to the polyester film of the present invention, it is excellent in mass production, has a low oligomer content, can suppress oligomer precipitation, and has excellent hiding properties and whiteness, and can be used for labels, stickers, reflective plates, etc., and can be used for a long time. It is possible to provide a cavity-containing polyester film that exhibits little change in surface quality such as tackiness and brightness even when used as a polyester film.

Claims (9)

A cavity-containing polyester film in which a polyester layer (layer B) made of a polyester resin containing inorganic particles is laminated on both sides of a polyester layer (layer A) containing cavities inside, wherein the layer A is made of polyester. Consisting of a composition containing a resin and a thermoplastic resin that is incompatible with the polyester resin,
Contains a polyester resin containing ester structural units derived from the isophthalic acid component with respect to all the polyester resins constituting the polyester film; A cavity-containing polyester film having a content of from 0.2 mol% to 2.0 mol%. The cavity-containing polyester film according to claim 1, further comprising a coating layer on at least one side of the film for improving wettability and adhesion. The void-containing polyester film according to claim 1, which has an apparent density of 0.8 to 1.3 g/cm ³ . The cavity-containing polyester film according to claim 1, wherein the content of inorganic particles in layer B is 5 to 40% by mass. The cavity-containing polyester film according to claim 2, wherein the coating layer is made of polyester resin and contains lubricant particles. The hollow polyester film according to claim 1, which is used for labels, stickers, cards, or packaging materials. The thermoplastic resin that is incompatible with the polyester resin in layer A is at least one resin selected from polystyrene resin, polymethylpentene resin, and polypropylene resin,
The cavity-containing polyester film according to claim 1, wherein the inorganic particles in layer B are at least one selected from titanium oxide, calcium carbonate, barium sulfate, and silica. The cavity-containing polyester film according to claim 1, which has an optical density of 0.55 or more (based on a thickness of 50 μm) and a color tone b value of 4 or less (based on a thickness of 50 μm). The cavity-containing polyester film according to any one of claims 2 to 8 , wherein the content of the cyclic trimer oligomer in the polyester film is 0.60% by mass or less.