JP5230998B2 - Polyester film - Google Patents

Description

  The present invention relates to a polyester film, and more particularly to a white polyester film that can be suitably used as a reflective film.

  In order to obtain a reflective film with high reflective performance, it is known to use a white polyester film containing a large amount of barium sulfate particles in polyester. For example, in Japanese Patent Laid-Open No. 2000-37835, 10 to 80% by weight of barium sulfate particles are contained in a polyester film, and in Japanese Patent Laid-Open No. 2005-125700, 10 to 25% by weight of barium sulfate particles are contained in a polyester film. Yes.

  By the way, in the process of manufacturing a polyester film, when the polyester composition is melt-extruded, a filtration filter is provided to remove foreign substances. If the polyester composition contains barium sulfate particles in such a high concentration, the filtration filter may be clogged during melt extrusion to form a film, and a sudden increase in filtration pressure may occur.

  This increase in filtration pressure is noticeable when the dispersibility of the particles is poor. When the filtration pressure rises, the discharge pressure at the time of melt extrusion fluctuates, or in a severe case, the discharge itself cannot be performed. If a filter with a large opening is used as a filtration filter in order to avoid an increase in the filtration pressure, a large aggregate is formed in the film, or in severe cases, the film is broken.

JP 2000-37835 A JP 2005-125700 A JP 2004-50479 A

  An object of the present invention is to solve such a problem of the prior art, and while having a layer containing barium sulfate particles, there is no increase in filtration pressure in the film forming process, and it is difficult to break, and stably forms a film. It aims at providing the polyester film which can be manufactured.

That is, the present invention comprises a composition of barium sulfate particles and polyester, the content of the barium sulfate particles in the composition is 31 to 60% by weight, and the composition is obtained by the ion coupling plasma method. It is a polyester film characterized in that the content of zinc element is 150 to 1000 ppm.

  According to the present invention, in order to obtain a high reflectance, a layer containing a large amount of barium sulfate particles is provided. A polyester film can be provided.

Hereinafter, the present invention will be described in detail.
[Barium sulfate particles]
The average particle diameter of the barium sulfate particles in the present invention is preferably 0.3 to 3.0 μm, more preferably 0.4 to 2.5 μm, and particularly preferably 0.5 to 2.0 μm. By using barium sulfate particles having an average particle diameter in this range, a film having high glossiness is not formed with coarse protrusions on the film, and without causing clogging of a filter used for filtration during melt extrusion. Can be produced.
The barium sulfate particles may be either plate-like or spherical. The barium sulfate particles may be subjected to a surface treatment to improve dispersibility.

  The content of barium sulfate particles in the polyester composition constituting the polyester film is preferably 31 to 60% by weight, more preferably 35 to 55% by weight, and particularly preferably 40 to 50% by weight. By containing in this range, it is possible to obtain a film that has a sufficiently high reflectance and is not easily torn.

  The particle size distribution of the barium sulfate particles is preferably 5.0 μm or less, more preferably 4.8 μm or less, and particularly preferably 4.6 μm or less as a 90% volume particle size (D90) integrated from the small particle size side. By using barium sulfate particles having this particle size distribution, there are few coarse particles, filter clogging does not occur, and a polyester film can be stably formed.

[polyester]
As the polyester in the present invention, a polyester comprising a dicarboxylic acid component and a diol component is used. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, adipic acid, and sebacic acid. Examples of the diol include ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and 1,6-hexanediol. Of these polyesters, polyethylene terephthalate is preferred. As the polyester, a copolyester is preferably used.

  When the copolymer polyester is used, the ratio of the copolymer component is preferably 3 to 15 mol%, more preferably 5 to 14 mol%, and still more preferably 7 to 13 mol% per the total dicarboxylic acid component. By using a copolymer polyester containing a copolymer component in this range, even when a layer containing a large amount of barium sulfate particles, for example, 40% by weight or more is contained, the polyester film is stably produced without breaking. Can be membrane.

  When polyethylene terephthalate is used as the polyester, for example, isophthalic acid or 2,6-naphthalenedicarboxylic acid can be used as the copolymer component. Copolyethylene terephthalate obtained by copolymerizing isophthalic acid or 2,6-naphthalenedicarboxylic acid in the range of 1 to 20 mol% is a preferred polyester in the present invention.

[Zinc element content]
It is important that the composition of barium sulfate and polyester constituting the polyester film of the present invention has a zinc element content of 150 to 1000 ppm. If the zinc element content is less than 150 ppm, the barium sulfate particles tend to aggregate and easily become foreign matter. On the other hand, if the zinc element content exceeds 1000 ppm, the polyester deteriorates. A polyester film made of a composition containing zinc element in the range of 150 to 1000 ppm is less likely to aggregate the barium sulfate particles, and can stabilize the barium sulfate particles in the molten polyester. The reason is not clear, but it is presumed that the zinc particles with some charge are gathered on the surface of the barium sulfate particles, and the barium sulfate particles are stabilized by the repulsive force (electrostatic repulsion effect) due to the charges. .

  In order to make zinc element in the range of 150 to 1000 ppm, zinc element may be added to the composition of polyester and barium sulfate particles, polyester or barium sulfate particles. The zinc element can be added, for example, in the production process of barium sulfate particles, the master chip production process containing barium sulfate particles in high concentration in the polyester, or the melt-kneading process in the film production process.

  The elemental zinc contained in the polyester composition is preferably derived from metallic zinc. For this reason, it is preferable to add zinc element as metallic zinc powder. Of the metal zinc powder, scaly zinc powder is preferable from the viewpoint of further improving the dispersibility of the barium sulfate particles.

[Film thickness]
The thickness of the polyester film of the present invention is, for example, 25 to 350 μm, preferably 40 to 300 μm, and particularly preferably 50 to 250 μm. When the thickness is within this range, a film having a high reflectance can be obtained. On the other hand, no increase in reflectivity can be expected even if the film is made thicker than 350 μm.
The polyester film of the present invention may be composed of a single layer or may be a laminated film composed of two layers, three layers or more layers.

[Reflectance]
In the polyester film of the present invention, the reflectance of at least one surface thereof is preferably 96% or more, more preferably 97% or more, further preferably 98% or more, as an average reflectance having a wavelength of 400 to 700 nm. If it is less than 96%, it is not preferred because sufficient screen brightness cannot be obtained when used as a reflector in a backlight unit of a liquid crystal display device.

[Filtration polyester filtration]
In the present invention, a nonwoven fabric type filter having an average opening of 10 to 100 μm, preferably an average opening of 20 to 50 μm, made of a stainless steel fine wire having a wire diameter of 15 μm or less is used as a filter for filtering molten polyester when forming a polyester film. It is preferred to use and filter the molten polyester. By performing this filtration, it is possible to obtain a polyester film that can be suitably used as a reflective film for a backlight unit of a liquid crystal display device with few coarse foreign matters.

[Production method]
Hereinafter, as an example of a method for producing the polyester film of the present invention, a method for producing a two-layer laminated film of A layer / B layer will be described as an example.
A laminated unstretched sheet is produced by a simultaneous multilayer extrusion method using a feed block from a polymer melted from a die. That is, the polymer melt for forming the A layer and the polymer melt for forming the B layer are laminated to form, for example, an A layer / B layer, using a feed block, and are spread on a die and extruded. . At this time, the polymer laminated by the feed block maintains the laminated form.

  The unstretched sheet extruded from the die is cooled and solidified by a casting drum to form an unstretched film. This unstretched film is heated by roll heating, infrared heating or the like, and stretched in the longitudinal direction to obtain a longitudinally stretched film. This stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. The stretching temperature is preferably not less than the glass transition point (Tg) of the polyester, more preferably not less than Tg and not more than 70 ° C. from Tg. The stretching ratio is preferably 2.2 to 4.0 times, more preferably 2.3 to 3.9 times in both the longitudinal direction and the direction orthogonal to the longitudinal direction (hereinafter referred to as the transverse direction). By setting it as the draw ratio of this range, a film with few thickness spots can be produced, without breaking during film formation.

  Subsequently, the film after longitudinal stretching is subjected to lateral stretching, heat setting, and thermal relaxation in order to form a biaxially oriented film. These processes are performed while the film is running. The transverse stretching process starts from a temperature higher than the glass transition point (Tg) of the polyester. And it is performed while raising the temperature to (5 to 70) ° C. higher than Tg. Although the temperature rise in the transverse stretching process may be continuous or stepwise (sequential), the temperature is usually raised sequentially. For example, the transverse stretching zone of the tenter is divided into a plurality along the film running direction, and the temperature is raised by flowing a heating medium having a predetermined temperature for each zone. The transverse stretching ratio is preferably 2.5 to 4.5 times, more preferably 2.8 to 3.9 times, although it depends on the required characteristics of this application. By setting the draw ratio within this range, a film with less thickness unevenness can be produced without breaking during film formation.

  The film after transverse stretching is preferably heat treated at a constant width or a width reduction of 10% or less at a temperature (Tm-20 to 100) while holding both ends to reduce the thermal shrinkage. When the temperature is higher than this, the flatness of the film is deteriorated, and the thickness unevenness becomes large, which is not preferable. On the other hand, if the heat treatment temperature is lower than (Tm-80) ° C., the thermal shrinkage rate may increase. Moreover, in order to adjust the thermal shrinkage in the region of (Tm-20 to 100) ° C. or lower in the process of returning the film temperature to room temperature after heat setting, both ends of the film being gripped are cut off, and the take-up speed in the film vertical direction Can be adjusted and relaxed in the vertical direction. As a means for relaxing, the speed of the roll group on the tenter exit side is adjusted. As the rate of relaxation, the speed of the roll group is reduced with respect to the film line speed of the tenter, preferably 0.1 to 1.5%, more preferably 0.2 to 1.2%, particularly preferably 0.3. The film is relaxed by performing a speed reduction of ˜1.0% (this value is referred to as “relaxation rate”), and the longitudinal heat shrinkage rate is adjusted by controlling the relaxation rate. Further, the width of the film in the horizontal direction can be reduced in the process until both ends are cut off, so that a desired heat shrinkage rate can be obtained.

Hereinafter, the present invention will be described in detail by way of examples. Each characteristic value was measured by the following method.
(1) Film thickness A film sample was measured for 10-point thickness with an electric micrometer (K-402B manufactured by Anritsu), and the average value was taken as the thickness of the film.

(2) Thickness of each layer A sample was cut into a triangle, fixed in an embedded capsule, and then embedded in an epoxy resin. Then, after embedding the sample with a microtome (ULTRACUT-S) into a thin film section having a thickness of 50 nm in parallel with the microtome, the specimen was observed and photographed with a transmission electron microscope at an acceleration voltage of 100 kv. The thickness of each layer was measured and the average thickness was determined.

(3) Average reflectance An integrating sphere was attached to a spectrophotometer (Shimadzu UV-3101PC), and the reflectance when BaSO ₄ white plate was 100% was measured over 400 to 700 nm. The reflectance was read out and the average value within this range was obtained. In addition, when the structure of the film had a reflectance of one surface higher than the reflectance of the other surface, the measurement was performed from the surface having a high reflectance.

(4) Stretchability The film was stretched 2.5 to 3.4 times in the longitudinal direction and 3.5 to 3.7 times in the transverse direction to form a film, and it was observed whether it could be stably formed. Evaluation was made according to the following criteria.
○: A film can be stably formed for 1 hour or more.
X: Cutting occurs within 1 hour, and the film cannot be stably formed.

(5) Glass transition point (Tg), melting point (Tm)
Using a differential scanning calorimeter (TA Instruments 2100 DSC), the measurement was performed at a heating rate of 20 m / min.

(6) Zinc element content When the film is a single layer film, it is a sample for measurement as it is, and when the film is a laminated film composed of a plurality of layers, it is peeled off layer by layer and measured. did. A certain amount of sample was baked in an oven at 600 ° C. for 3 hours, and the remaining 8 g was dispersed in 40 ml of 0.5N HCl solution, stirred for 30 minutes and then centrifuged to obtain ICP (ion coupling plasma method). The amount of zinc element was measured.

(7) Particle size distribution of barium sulfate particles The particle size distribution was measured using a laser scattering particle size distribution analyzer SALD-7000 manufactured by Shimadzu Corporation. Before the measurement, the dispersion in ethylene glycol was weighed so that the barium sulfate particle powder was equivalent to a 5% by weight slurry concentration, stirred for 10 minutes with a mixer (for example, National MXV253 type cooking mixer), and cooled to room temperature. Then, it supplied to the flow cell system supply apparatus. Then, in the supply device, ultrasonic treatment was performed for 30 seconds for defoaming (the strength of the ultrasonic treatment was 60% from the position showing the MAX value of the knob of the ultrasonic treatment device), and then the measurement was performed. . And 90% volume particle size (D90) was calculated | required from the particle size distribution measurement result.

(8) Average particle diameter of barium sulfate particles The measurement was performed using a laser scattering particle size distribution analyzer SALD-7000 manufactured by Shimadzu Corporation in the same manner as D90. From the particle size distribution measurement results, the value of 50% volume particle size (D50) was defined as the average particle size.

(9) Evaluation of foreign matter On the highly reflective surface of the film, using a halogen lamp in a dark room, observe a film sample of A3 size, and color other than white with a diameter of 0.3 mm or more As a result, the number of foreign matters was measured visually. In the case of the elongated one, the average of the major axis and the minor axis was taken as the diameter.

(10) Filter pressure When the resin containing barium sulfate particles is converted by discharge amount (kg) / filter area (m ² ) with a nonwoven fabric type filter having an opening of 30 μm, 100 kg when filtered at 120 kg / m ² The filter pressure difference (ΔP: unit is MPa) increased from the initial pressure at the time of filtration was evaluated.

[Example 1]
132 parts by weight of dimethyl terephthalate, 18 parts by weight of dimethyl isophthalate (12 mol% based on the acid component of the polyester), 98 parts by weight of ethylene glycol, 1.0 part by weight of diethylene glycol, 0.05 part by weight of manganese acetate, 0 parts of lithium acetate .012 parts by weight were charged into a rectifying column and a flask equipped with a distillation condenser, and heated to 150 to 235 ° C. with stirring to distill methanol to conduct a transesterification reaction. After the methanol was distilled off, 0.03 part by weight of trimethyl phosphate and 0.04 part by weight of germanium dioxide were added, and the reaction product was transferred to the reactor. Subsequently, while stirring, the pressure in the reactor was gradually reduced to 0.5 mmHg and the temperature was raised to 290 ° C. to carry out a polycondensation reaction. Using this polyester resin for layers A and B, a master batch of barium sulfate was prepared, and metal zinc powder was added at the time of preparing the master chip to adjust the addition amount shown in Table 1.

  Using these raw materials, supply them to two extruders each heated to 285 ° C, and use a two-layer feed block device in which layer A polymer and layer B polymer are A / B in layer A and layer B Then, the sheet was formed into a sheet shape from a die while maintaining the laminated state. Further, an unstretched film obtained by cooling and solidifying the sheet with a cooling drum having a surface temperature of 25 ° C. was heated at the described temperature, stretched in the longitudinal direction (longitudinal direction), and cooled by a roll group at 25 ° C. Subsequently, while holding both ends of the longitudinally stretched film with clips, the film was guided to a tenter and stretched in a direction perpendicular to the longitudinal direction (lateral direction) in an atmosphere heated to 125 ° C. Thereafter, heat setting was performed in the tenter at the temperature shown in Table 2, and longitudinal relaxation was performed and lateral width was placed under the conditions shown in Table 2, followed by cooling to room temperature to obtain a biaxially stretched film. Table 2 shows the physical properties of the obtained film.

  In the table, TPA means terephthalic acid, IPA means isophthalic acid, and NDC means 2,6-naphthalenedicarboxylic acid.

[Examples 2 to 8]
A film was produced under the film forming conditions shown in Table 2 by changing the blending amount shown in Table 1 and the dicarboxylic acid component of the polyester. In Examples 1 to 4, a two-layer film is used using a two-layer feed block, in Examples 5 and 6, a single-layer film is used without using a feed block, and in Examples 7 and 8, a three-layer feed block is used. A three-layer film was prepared.

[Comparative Examples 1-6]
Film formation was performed under the conditions shown in Tables 1 and 2. In Comparative Examples 1, 2, and 5, the stretchability was poor, and it was difficult to continuously form films. In Comparative Examples 3 and 4, there were many foreign matters, and in Comparative Example 6, the dispersibility of barium sulfate was poor, and a predetermined amount of voids did not occur in the film.

  Since the polyester film of the liquid crystal display device of the present invention has a high reflectance and has few foreign substances, it can be suitably used as a reflector of a backlight unit of the liquid crystal display device. In addition, bases for paper substitutes, such as cards, labels, stickers, home delivery slips, video printer image paper, ink jet image paper, barcode printer image paper, posters, maps, dust-free paper, thermal transfer paper, offset printing paper, etc. It can be used as a material, and can also be used as a base material for display sheets, white boards, telephone cards, IC cards, and back sheets of solar cells.

Claims (4)

The composition of barium sulfate particles and polyester, the content of barium sulfate particles in the composition is 31 to 60% by weight, and the content of zinc element in the composition determined by the ion coupling plasma method Is a polyester film, characterized by being 150-1000 ppm.   The polyester film according to claim 1, wherein the barium sulfate particles have an average particle size of 0.3 to 3 μm.   The polyester film according to claim 1, wherein all zinc elements are derived from metallic zinc. In producing the polyester film according to claim 1, a composition comprising 31 to 60% by weight of barium sulfate particles and polyester in the presence of 150 to 1000 ppm of zinc as a content determined by an ion coupling plasma method. A method for producing a polyester film, comprising a step of melt-kneading.