JPH0774283B2 - Cavity-containing polyester film - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, when used as a base material for labels, posters, recording papers, and packaging materials, does not cleave the surface of the base material even after peeling off the adhesive tape from above. Relates to a polyester film containing a large amount of fine voids having excellent strength.

[0002]

2. Description of the Related Art Synthetic paper, which is a paper substitute mainly made of synthetic resin, has a higher water resistance, a higher dimensional stability in moisture absorption, and a higher dimensional stability than natural paper.
It has excellent surface smoothness, printing gloss and clarity, and mechanical strength. In recent years, application development has been promoted by making use of these advantages. Polyethylene, polypropylene, polyester, etc. are used as the main raw materials for synthetic paper. Among them, polyester typified by polyethylene terephthalate is excellent in that it has high heat resistance and is strong and has a wide range of properties. It can be used for various purposes.

As a method for obtaining a film having a function similar to that of paper mainly made of polyester, (1) a method in which a large amount of fine voids are contained in the film or a conventional flat polyester film is used (2-1) ) Sandplast treatment or (2-2) Chemical etching treatment or (2-
3) A method of roughening the surface by a matting treatment (method of laminating a matting agent together with a binder), etc. is disclosed. Among them, the method (1) of containing a large amount of fine voids inside the film allows the film itself to be lightweight and imparts appropriate flexibility, which enables clear printing and transfer. There is an advantage.

As a method for forming fine voids inside the film, conventionally, a polymer which is incompatible with polyester is melt-kneaded with an extruder to obtain a sheet in which the polymer is dispersed in the form of fine particles, and further the sheet is obtained. There is disclosed a method of forming voids around fine particles by stretching. As a polymer that is incompatible with polyester used for generating voids (hereinafter referred to as void developing agent), a polyolefin resin (for example, JP-A-49-
134755) and polystyrene resins (for example, Japanese Patent Publication No. 49-2016, Japanese Patent Publication No. 54-29550).
Gazette) and polyarylate resin (for example, Japanese Patent Publication No.
(28097 gazette). Of these, polystyrene and polypropylene are preferable because they easily form cavities and are inexpensive.

However, the void-containing polyester film thus obtained has a drawback that the surface will be cleaved when the adhesive tape is applied to the surface and then peeled off.
In addition, when the void-containing polyester film is adhesively processed or laminated and used as a label or a packaging material, the base material is easily cleaved. For such defects, it is possible to laminate a layer strong against cleavage on the surface by co-extrusion method, laminating method or coating method, but such a method requires new capital investment and processing steps. .

[0006]

DISCLOSURE OF THE INVENTION According to the present invention, a polyester film containing a large amount of fine cavities and having a strong surface strength is produced in a stretched film production process comprising a single-layer extrusion process to produce a label or a poster. The present invention provides a base material suitable as a base material for recording paper and packaging materials.

[0007]

That is, according to the present invention, a polymer mixture obtained by mixing a polyester mainly composed of ethylene terephthalate repeating units with a thermoplastic resin incompatible with the polyester is oriented at least uniaxially. the air <br/> sinus-containing polyester film of a single layer having a large number of fine voids are produced by, the cavity contained in the surface layer to a depth of 3μm from the surface is at 4 vol% or less, and the entire layer The present invention relates to a void-containing polyester film having an average void content of 8% by volume or more.

In the present invention, the polyester composed mainly of ethylene terephthalate repeating units is
It is a thermoplastic polyester containing preferably 70 mol% or more of ethylene terephthalate repeating units obtained by a polymerization reaction of terephthalic acid and / or a derivative thereof and ethylene glycol. The polyester may be copolymerized with other dicarboxylic acid components, and / or diol components, and / or oxycarboxylic acid components. Further, the polyester may be blended with polyester such as polybutylene terephthalate and / or polyethylene 2,6-naphthalate and / or polycyclohexylene dimethylene terephthalate. The polyester can be produced by melt polymerization by a conventional method, but the polyester is not limited to this and may be a polyester obtained by another polymerization method. The degree of polymerization of the polyester is preferably 0.3 to 1.2 in terms of intrinsic viscosity.

The thermoplastic resin incompatible with the polyester in the present invention means polystyrene resin, polyolefin resin, polyamide resin, polyacrylic resin, cellulose resin, petroleum resin, synthetic rubber, natural rubber, polycarbonate. Examples thereof include resins, polysulfone-based resins, polyarylate-based resins and polyether-based resins. That is, it is a resin that forms a sea-island structure by mixing with the polyester and melt-kneading to cause microphase separation. Preferably,
It is preferable that the polyester is not easily decomposed when melt-extruded with the polyester, and is also difficult to react with the polyester. Further, it is preferable that the thermoplastic resin is dispersed in the polyester with a particle diameter of 0.1 to 50 μm when it is made into a polymer mixture with the polyester. Further, it is preferable that the interface between the polyester and the particles of the thermoplastic resin is formed when the polymer mixture is stretched, and cracks are likely to occur to easily form voids. For example, polystyrene resin is suitable.

The polystyrene resin is a polystyrene homopolymer obtained by polymerizing a styrene monomer or a random copolymer, a block copolymer or a graft copolymer mainly composed of repeating units of styrene obtained by copolymerizing other monomers. is there. Further, blends and polymer alloys obtained by blending such polymers with other polymers are also included. For example, general-purpose amorphous polystyrene, crystalline polystyrene having stereoregularity, impact polystyrene, acrylonitrile-styrene copolymer, acrylonitrile-butadiene-styrene resin and the like can be mentioned.

The polymer mixture obtained by mixing the polyester and the thermoplastic resin of the present invention is obtained, for example, by mixing chips of each resin, melt-kneading in an extruder, and then extruding and solidifying. In addition to this, a method in which both resins are kneaded by a preliminary kneader is further melt-extruded from an extruder and solidified, or a thermoplastic resin is added in the polyester polymerization step, and the chips are obtained by stirring and dispersing. It can also be obtained by a method of melt-extruding and solidifying.
The polymer obtained by solidification is usually in a non-oriented state or a weakly oriented state. The thermoplastic resin is dispersed in the polyester in various shapes such as a spherical shape, an elliptic spherical shape, or a thread shape. The dispersion diameter is spherical and has a diameter of 0.1 to 30 μm.

Further, in order to obtain a void-containing film in the present invention having only a small number of voids in the surface layer, the dispersion state of the particles of the thermoplastic resin has a distribution in the thickness direction, and the surface layer is smaller than the particle diameter of the central layer. The distribution is preferably such that the particle size in the vicinity is smaller. The sheet of the polymer mixture of the polyester resin containing the particles of the thermoplastic resin in which the dispersed state has a distribution in the thickness direction, the viscoelasticity of the polyester and the thermoplastic resin at the time of melting and the extrusion conditions are selected. Obtained by For example, by increasing the shearing stress acting in the slit of the extrusion die to a value higher than usual, normally, the state of dispersion of the thermoplastic resin that is uniform in the thickness direction,
Near the surface layer is one-tenth of the particle size of ordinary thermoplastic resin.
It is possible to obtain a sheet of a polymer mixture in which the following fine particles are present, and particles having a normal particle size are present near the center.

The polymer mixture contains a pigment,
It is also possible to add a colorant, a light fastness agent, a fluorescent agent, an antistatic agent, and the like. The polymer mixture thus obtained is further stretched between rolls having different speeds (roll stretch), stretched by gripping and expanding with a clip (tenter stretch), and stretched by expanding by air pressure (inflation stretch). At least one axis is subjected to orientation treatment by, for example, At this time, peeling occurs at the interface between the dispersed polystyrene resin and the polyester, and many cavities are generated in the polymer mixture. Therefore, the amount of the polystyrene-based resin mixed with the polyester varies depending on the amount of target cavities, but is preferably 1% by weight to 35% by weight with respect to the entire polymer mixture. Below 1% by weight,
There is a limit to increase the amount of voids, and the desired flexibility, lightness, and drawability cannot be obtained. Conversely, 40% by weight
In the above case, the heat resistance and strength of the polyester film are significantly impaired. Further, it becomes difficult to obtain a void-containing film that has high surface strength and is difficult to cleave.

The conditions for orienting the polymer mixture are also closely related to the formation of cavities. For example, in the case of the most commonly used sequential biaxial stretching step, in the case of the sequential biaxial stretching method in which a continuous sheet of the polymer mixture is roll-stretched in the longitudinal direction and then tenter-stretched in the width direction, The stretching temperature is 50 to 140 ° C, and the magnification is 1.2 to 5 times.
The temperature of the tenter stretching is 60 to 150 ° C, and the magnification is 1.2 to
5 times is preferable.

It is also possible to obtain a void-containing film of the present invention in which only the surface layer has a small number of cavities, by making the temperature of stretching have a distribution in the thickness direction and stretching the surface layer at a temperature higher than that of the central layer. The uniaxially oriented void-containing film is superior to the biaxially oriented void-containing film in terms of surface strength and difficulty in cleavage. However, the biaxially oriented void-containing film is preferable in terms of the balance of mechanical strength in the machine direction and the transverse direction and the uniformity of thickness. The cavity-containing film which has been further stretched and oriented has 13
When the heat setting treatment is performed at 0 ° C. or higher, preferably 180 ° C. or higher, dimensional stability at high temperature can be improved. When the heat setting process is performed, the cavities generated by the stretching are generally reduced. It is also possible to reduce the cavities only in the surface layer by raising the temperature of only the surface layer to obtain the void-containing film of the present invention in which only the surface layer has few cavities. The void-containing film oriented only in the uniaxial direction is useful as a shrinkable film or an easily tearable film. The present invention relates to a void-containing polyester film in which voids have been generated by the alignment treatment, and therefore it is necessary to orient at least uniaxially.

In the void-containing polyester film thus obtained, the void content of the voids contained in the surface layer from the surface to the depth of 3 μm is 4% by volume or less, and the average void content of the entire layer is 8% by volume or more. It is necessary to be. If the surface layer from the surface to a depth of 3 μm contains more than 4% by volume, sufficient surface strength and cleavage strength cannot be obtained. Further, even when the surface layer having a porosity of 4% by volume or less is thinner than 3 μm, sufficient surface strength and cleavage strength cannot be obtained. Therefore, in the present invention, the surface layer portion having less cavities than the central portion needs to have a depth of 3 μm or more, and the cavity rate of the cavities contained therein should be 4% by volume or less.

Furthermore, the average porosity of the voids in the entire layer must be 8% by volume or more. If the average void content of the entire layer is less than 8% by volume, the flexibility peculiar to the void-containing film becomes insufficient and clear printing or transfer cannot be performed.
Also, the effect of reducing the weight of the film itself is reduced. The average porosity of the entire layer is preferably 50% by volume or less, and a void-containing film having an average porosity of 50% by volume or more is difficult to manufacture because it frequently breaks in the stretching process, and the formed film has a surface. Is not preferable because the strength and tensile strength are insufficient.

More preferably, by providing a coating layer on at least one side of the void-containing polyester film of the present invention, the wettability and adhesiveness of inks and coating agents are improved. The compound constituting the coating layer,
Polyester resins are preferable, but in addition to these, compounds disclosed as means for improving the adhesiveness of ordinary polyester films such as polyurethane resins, polyester urethane resins, and acrylic resins are applicable.

As a method for providing a coating layer, a gravure coating method, a kiss coating method, a dipping method, a spray coating method, a curtain coating method, an air knife coating method, a blade coating method, a reverse roll coating method or the like which is usually used. Can be applied. As the step of applying, a method of previously applying to the surface of the mixed polymer before the orientation treatment, a method of applying to the surface of the uniaxially oriented cavity-containing film and further orienting it in the orthogonal direction, Any method such as coating on the surface of the finished cavity-containing film is possible.

The thickness of the coating layer varies depending on the desired level, but is usually 0.01 to 10 μm. It is also possible to appropriately add a colorant, a matting agent, an antistatic agent, an ultraviolet absorber, a crosslinking agent, etc. to the coating layer depending on the purpose. The void-containing polyester film thus obtained is superior in the surface strength and the strength against cleavage as compared with the void-containing polyester film using polystyrene or polypropylene which has been conventionally proposed as a void-developing agent. When a polyester film is used as a substrate, a label, a poster, a recording paper, a packaging material, etc., which has a strong surface strength and is difficult to cleave, can be obtained.

[0021]

In the present invention, the polyester mainly composed of ethylene terephthalate repeating units is used in order to satisfy the heat resistance and mechanical strength of the void-containing polyester film. In the present invention, the thermoplastic resin is mixed with the polyester to obtain a polymer mixture, in which fine particles of the polystyrene resin are dispersed in the polyester to form void nuclei generated by the following alignment treatment. To make.

In the present invention, the polymer mixture is oriented at least uniaxially in order to generate a large number of fine voids in the polymer mixture. By making the cavity, the film can be made lighter, the workability is improved, and the price per area is reduced. In addition, the inclusion of the cavities increases flexibility and enables clear printing when printing or transferring. Further, by containing voids, light shielding properties and whiteness can be obtained. Further, many projections derived from the polystyrene resin are formed on the surface of the film, and writing with a pencil or a ballpoint pen is possible.

In the present invention, the reason why the amount of cavities contained in the surface layer 3 μm is 4% by volume or less is to increase the strength of the surface, and the strength of the surface also improves the resistance to cleavage. . On the other hand, the average porosity of the entire layer is set to 8% by volume or more in order to obtain flexibility for clear printing and transfer.

In order to reduce the amount of voids only in the surface layer, it is effective to make the dispersed particles of the thermoplastic resin in the polymer mixture finer near the surface layer than around the center. It can be obtained by selecting the melt viscosity characteristics of the resin and the conditions for melt extrusion using an extruder. The difference in the dispersed particle size of the thermoplastic resin between the vicinity of the surface layer and the center of the polymer mixture seems to be due to the shear stress in the extrusion die, but it is not clear.

It is also possible to suppress the amount of cavities near the surface layer by giving a distribution of the drawing temperature in the thickness direction during drawing. Further, it is possible to reduce only the cavities near the surface layer by treating the film in which the cavities are generated at a high temperature.

[0026]

EXAMPLES Examples and comparative examples of the present invention will be described below. The measurement / evaluation methods used in the present invention are shown below. 1) Intrinsic viscosity of polyester Polyester was dissolved in a mixed solvent of phenol (6 parts by weight) and tetrachloroethane (4 parts by weight) and measured at 30 ° C. 2) Melt flow index of polystyrene resin Measured at 200 ° C. under a load of 5 Kg according to JIS K-7210.

3) Porosity of the surface layer of the cavity-containing film A photograph of the surface of the cross section of the film near the surface layer was taken with a scanning electron microscope, and then the cavity in the region from the surface layer to a depth of 3 μm was traced on the tracing film and painted. Was subjected to image processing by an image analyzer, and the void ratio was obtained by the area ratio, and this value was directly displayed as volume%. -Scanning electron microscope used S-510 scanning electron microscope manufactured by Hitachi Ltd.-Image analysis processor used Luzex IID (manufactured by Nireco Co., Ltd.) 4) The same as the cavity ratio 3) in the central portion of the cavity-containing film. By the method, a region having a thickness of 20 μm near the center was obtained.

5) Apparent Specific Gravity of Film The film is accurately cut into a square of 5.00 cm × 5.00 cm, and its thickness is measured at 50 points to obtain an average thickness tμ.
m, and weigh it to 0.1 mg to wg,
It was calculated by the following formula. Apparent specific gravity (−) = W / 5 × 5 × t × 10000 6) Average porosity of film Calculated by the following formula. Average porosity (volume%) = 100 × (1-true specific volume / apparent specific volume) However, true specific volume = x ₁ / d ₁ + x ₂ / d ₂ + x ₃ / d ₃ + ...
+ X _i / d _i + ... Apparent specific volume = 1 / apparent specific gravity of film In the above formula, x _i represents the weight fraction of the i component, and d _i represents the true specific gravity of the i component. The values of true specific gravity used in the calculations in the examples were polyethylene terephthalate 1.40, anatase type titanium dioxide 3.90, and general-purpose polystyrene resin 1.050.91.

7) Concealment of Cavity-Containing Film According to JIS K6714, Poic integrating sphere type H. T.
The light transmittance of the film was measured using an R meter (manufactured by Nippon Seimitsu Optical Co., Ltd.). The smaller this value, the higher the concealing property.
It was 27% when the commonly used electrostatic copying pulp paper was measured. 8) Surface Strength Using Cellotape (18 mm width, made by Nichiban), the surface strength was evaluated by a Cellotape peeling test. The peeling angle was set in the direction of about 150 ° with the cavity-containing flume kept flat. The area of the peeled film containing voids was differentiated as follows. Class 5 ... The whole was peeled off Class 4 ... Almost peeled off Class 3 ... About half peeled off Class 2 ... Almost no peeled off Class 1 ... No peeled off at all

Example 1 As a raw material, (1) 86% by weight of polyethylene terephthalate resin having an intrinsic viscosity of 0.62 (2) 10% by weight of general-purpose polystyrene having a melt flow index of 1.8 g / 10 minutes
(3) 4% by weight of anatase type titanium dioxide was melt extruded from a T-die at 285 ° C. by a twin screw extruder,
Approximately 600 μm by sticking and solidifying on a cooling roll with electrostatic force
An unstretched sheet of the polymer mixture was obtained. At this time, the gap between the T-die slits was 2.0 mm, and the average flow velocity of the melt of the polymer mixture in that portion was 8.5 m / sec.
Subsequently, the unstretched sheet is rolled at 80 ° C. using a roll stretching machine.
At a temperature of 130 ° C and a lateral draw of 3.5 times at 220 ° C and 3% at 220 ° C.
Heat setting was performed while relaxing. The void ratio of the surface layer of the obtained void-containing film is 1% by volume, the central portion is 18% by volume,
The average porosity of the whole was 16% by volume. The portion with few cavities was present from the surface layer to a depth of about 5 μm.
The void-containing film obtained in this example had excellent surface strength. When the cross section of the unstretched sheet of the polymer mixture of this example was observed with a scanning electron microscope, the average dispersed particle size of polystyrene in the central portion was 4.8 μm, while the dispersed particle size near the surface layer was small. Was 0.6 μm on average.

Example 2 In Example 1, the thickness was reduced to about 1 by increasing the discharge amount.
After obtaining an unstretched sheet of the polymer mixture having a thickness of 200 μm, it was stretched in the same manner and then heat set to obtain a cavity-containing film having a thickness of about 105 μm. The void content of the surface layer of the obtained void-containing film was 1% by volume, the central portion was 18% by volume, and the average void content of the whole was 17% by volume. The portion with few cavities existed from the surface layer to a depth of about 8 μm. The void-containing film obtained in this example had excellent surface strength.

Comparative Example 1 In Example 1, the gap between the T-die slits was set to 4.0.
mm, and the average flow rate of the melt of the polymer mixture between the slits was 4.2 m / sec to obtain an unstretched sheet of the polymer mixture having a thickness of about 600 μm, which was then similarly stretched and then heat set. Then, a film containing voids having a thickness of about 600 μm was obtained.
The void content of the obtained void-containing film was 12% by volume, the central portion was 20% by volume, and the average void content of the whole was 19% by volume. The void-containing film obtained in this comparative example had poor surface strength. When the cross section of the unstretched sheet of the polymer mixture of this comparative example was observed with a scanning electron microscope, the dispersed particle diameter of polystyrene was 5.9 μm on average, and the particles were dispersed substantially uniformly in the thickness direction.

COMPARATIVE EXAMPLE 2 In obtaining an unstretched sheet of the polymer mixture in Comparative Example 1, the unrolled sheet of the polymer mixture having a thickness of about 1200 μm was obtained by making the take-up speed to the cooling roll half of that in Comparative Example 1. After that, a void-containing film having a thickness of about 107 μm was obtained in the same manner. The void content of the obtained void-containing film was 13% by volume, the central portion was 18% by volume, and the average void content of the whole was 18% by volume. The surface strength of the void-containing film obtained in this comparative example was poor.

Example 3 As a raw material (1) 90 parts by weight of polyethylene terephthalate resin having an intrinsic viscosity of 0.62 (2) 10% by weight of general-purpose polystyrene having a melt flow index of 1.8 g / 10 min
Was used in the same manner as in Example 1 to obtain a cavity-containing film having a thickness of about 51 μm. The void content of the obtained void-containing film was 1% by volume, the central portion was 17% by volume, and the average void content of the whole was 15% by volume. The portion with few cavities existed from the surface layer to a depth of about 6 μm. The void-containing film obtained in this example had excellent surface strength.

Example 4 In Example 3, the thickness was reduced to about 1 by increasing the discharge rate.
A cavity-containing film of 03 μm was obtained. The void content of the obtained void-containing film was 1% by volume, the central portion was 16% by volume, and the average void content of the whole was 15% by volume. The portion with few cavities was present from the surface layer to a depth of about 10 μm. The void-containing film obtained in this example had excellent surface strength.

Comparative Example 3 Using the raw materials having the same composition as in Example 3, T
-The same applies after obtaining an unstretched sheet of the polymer mixture having a thickness of about 600 m by setting the distance between the die slits to 4.0 mm and setting the average flow rate of the melt of the polymer mixture between the slits to about 8.7 m / min. To obtain a cavity-containing film having a thickness of about 52 μm. The void content in the surface layer of the obtained void-containing film was 10% by volume, the central portion was 17% by volume, and the average void content of the whole was 16% by volume. The surface strength of the void-containing film obtained in this comparative example was poor.

[0037]

[Table 1]

[0038]

EFFECTS OF THE INVENTION The void-containing polyester film of the present invention has lightness, flexibility and concealing property which are not present in a biaxially stretched polyester film which does not substantially contain voids, and has been conventionally proposed. The surface strength is superior to the void-containing polyester film obtained by using polypropylene or polystyrene as the void-developing agent. Further, since it is not necessary to apply the conventionally proposed method of improving the surface strength by lamination, a void-containing polyester film having excellent surface strength can be obtained even with a single-layer extrusion equipment. Therefore, the void-containing polyester film of the present invention can be used in a wide range of fields such as labels, posters, recording papers, and packaging materials, and it is expected that the obtained products will have durability.

Continuation of front page (72) Inventor Katsuro Kuze 2-1-1 Katata, Otsu, Shiga Prefecture Toyobo Co., Ltd. Research Institute Internal Researcher Hitomi Morita (56) References JP-A 64-18631 (JP, A)

Claims (1)

[Claims] 1. A large number of fine cavities produced by orienting at least uniaxially a polymer mixture obtained by mixing a polyester mainly composed of polyethylene terephthalate repeating units with a thermoplastic resin incompatible with the polyester. in the void-containing polyester film of a single layer having, void content of the cavity included in the surface layer to a depth of 3μm from the surface, is 4% by volume or less, and the average void content of the entire layer, 8 vol% to 50 A void-containing polyester film characterized in that the content thereof is not more than volume%.