JP2008069217A - Polyester film and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensively produceable polyester film having high reflectivity, and hardly causing breakage of the film. <P>SOLUTION: The polyester film having pores obtained by using a resin incompatible with the polyester as nuclei in the inside, contains a thickener in the polyester film. The method for producing the polyester film is also provided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a polyester film having air bubbles having a core made of a resin incompatible with polyester. More specifically, it relates to a polyester film that contains a large number of fine bubbles inside, has excellent reflectivity and concealment properties, has good film forming stability, and is excellent in productivity and cost. The present invention relates to a polyester film that can be suitably used for a reflection sheet for a light device and a lamp reflector, a reflection sheet for a lighting fixture, a reflection sheet for a lighting signboard, a back reflection sheet for a solar cell, and the like.

  In recent years, displays employing liquid crystal panels have been widely used as display devices for mobile tools such as personal computers, televisions, and mobile phones. Since a liquid crystal display itself is not a light emitter, it is generally performed to irradiate light by installing a surface light source called a backlight on the back side. As a light emission source of the backlight, a fluorescent tube or a light emitting diode is often used in order to make use of the characteristics of the liquid crystal display such as thinness, weighing, and low power consumption. These light sources generally have a dot or line shape, and in order to make a surface light source, in many cases, a method of diffusing a light beam emitted from the light source to the liquid crystal display surface side and emitting light is adopted. ing. Further, in order to prevent loss of light flux, the surface facing the exit surface is a light reflection surface inside the surface light source, and by introducing the light flux emitted from the light emitter between the light reflection surface and the exit surface, As much light flux as possible is emitted from one surface, that is, the exit surface. As means for introducing the light beam emitted from the light emitter between the reflection surface and the emission surface, the following two means are generally employed.

  One is that the luminous flux emitted from the light emitting source is once introduced into the flat transparent resin-formed body (light guide plate), and emitted from the luminous body between the light reflecting film and the emission surface. The light beam is introduced. In this case, the light emitted to the reflecting surface side is reflected by the light reflecting film, introduced to the emitting surface side, and reused as the emitted light, whereby the luminance of the emitting surface can be improved. Such a backlight is generally called a side light type or an edge light type because a light emitting source is often arranged at a peripheral position of the screen when the screen is viewed from the front (see Patent Document 1).

  The other is to introduce a light beam emitted from the light emitter between the light reflection film and the emission surface by disposing a light emitting source between the emission surface and the reflection surface. Also, the light emitted to the reflecting surface side is reflected by the light reflecting film, introduced to the emitting surface side, and reused as the emitted light, so that the luminance of the emitting surface can be improved. Such a backlight is often called a direct type because a light emitting source is located directly under the liquid crystal display surface.

In particular, for a thin liquid crystal display used for a notebook personal computer or the like for which a thin and small size is desired, an edge type, that is, a type of backlight that irradiates light from the side of the screen is applied.
In general, this edge type backlight uses a cold cathode tube as an irradiation light source from the edge of the light guide plate, uses a light guide plate that uniformly propagates and diffuses light, and uses a light guide plate method that uniformly illuminates the entire liquid crystal display. It has been adopted. In this illumination method, in order to use light more efficiently, a lamp reflector having an opening on the light emitting side is provided around the cold cathode tube, and light diffused from the light guide plate is efficiently used on the liquid crystal screen side. Therefore, a reflector is provided under the light guide plate. Thus, the loss of light emitted from the cold cathode ray tube can be minimized and used on the liquid crystal screen side, and a function of brightening the liquid crystal screen is provided.

  On the other hand, for large screens such as liquid crystal televisions, the direct backlight type has been adopted because it is not possible to increase the screen brightness with an edge type backlight. In this method, a cold cathode ray tube is provided in parallel at the lower part of the liquid crystal screen, and the cold cathode ray tubes are arranged in parallel on the reflector. The reflection plate is used in a flat shape on the back surface of the cold cathode ray tube, or is used so as to be installed on a portion where the cold cathode ray tube is formed in a semicircular concave shape.

  Lamp reflectors and reflectors used in such surface light sources for liquid crystal screens (collectively referred to as surface light source reflecting members) have excellent brightness improvement effects and uniformity, and therefore contain fine bubbles inside. A film (see, for example, Patent Document 2) is generally used. Among them, a film containing flat bubbles inside by a method such as stretching a resin sheet in which incompatible components are dispersed is widely used as a white reflecting member because it has particularly high whiteness and reflectivity. (See Patent Document 3). Furthermore, there is an example in which the surface gloss of such a white film is adjusted to achieve high functionality (see Patent Document 4).

In recent years, there has been a demand for a film for a reflecting member that has a higher reflectance due to an increase in the size of a screen and an energy saving effect, and at the same time has a low manufacturing cost as the price of the final product decreases. In order to obtain a high reflectivity, it is necessary to increase the number of bubbles contained in the interior and increase the number of light reflecting interfaces.For this reason, if the content of resin incompatible with polyester is increased, Since the dispersibility of the compatible resin is deteriorated, the reflectance is lowered, and there is a significant problem that the production efficiency is lowered due to film breakage in the stretching process, and it is difficult to produce a film for a reflecting member that is inexpensive and has a high reflectance. It is.
JP 63-62104 A JP-A-7-118433 JP-A-6-322153 JP 2000-348450 A

  The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a polyester film that has a high reflectivity and is less prone to film breakage and can be manufactured at low cost.

The present invention uses the following means in order to solve this problem. That is,
It is a polyester film which has air bubbles having a resin incompatible with polyester as a core, and a thickener is contained in the polyester film.

  ADVANTAGE OF THE INVENTION According to this invention, the polyester film which has the high reflectance and has the microbubble which cannot produce a film tear easily during manufacture and can be manufactured cheaply can be obtained.

  In the present invention, it is necessary that the polyester film contains air bubbles having a resin incompatible with the polyester resin as a nucleus. High reflectance can be obtained by light reflection due to the difference in refractive index between the polyester resin and the bubbles. The method for forming the polyester film as described above is not particularly limited. For example, a polyester resin and a resin incompatible with the polyester are kneaded and extruded, and a sheet in which the polyester incompatible resin is finely dispersed in the polyester resin is obtained. An example is a method in which fine flat bubbles are formed inside the film by taking advantage of the fact that after the formation, the sheet is stretched to cause peeling at the interface between the polyester resin and the polyester incompatible resin. When the bubble nucleating agent is an inorganic particle, the reflectance is inferior because bubble formation due to interfacial peeling becomes insufficient.

  Examples of the polyester resin used in the present invention include polyethylene terephthalate (hereinafter abbreviated as PET), polyethylene-2,6-naphthalene dicarboxylate, polypropylene terephthalate, polybutylene terephthalate, and poly-1,4-cyclohexylene diester. Preferred examples include methylene terephthalate. Moreover, you may contain a copolymerization component in these polyester resins. Examples of copolymer components include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, and diol components having 2 to 15 carbon atoms. Examples of these include isophthalic acid, adipic acid, and sebacin. Examples thereof include acid, phthalic acid, sulfonate group-containing isophthalic acid, and ester-forming compounds thereof, diethylene glycol, triethylene glycol, neopentyl glycol, and polyalkylene glycol having a molecular weight of 400 to 20,000.

  The resin incompatible with the polyester in the present invention is a thermoplastic resin other than polyester, and is a thermoplastic resin that is incompatible with the polyester. A resin having a large effect of forming cavities in the film is preferred. More specifically, the incompatible resin refers to a system in which polyester and the incompatible resin are melted by a known method, preferably a differential scanning calorimeter (DSC), dynamic viscoelasticity measurement or the like. When measured, it is a resin in which Tg corresponding to the incompatible resin is observed in addition to the glass transition temperature corresponding to polyester (hereinafter abbreviated as Tg).

  The melting point of such an incompatible resin should be equal to or slightly lower than the melting point of polyester, and higher than the temperature at which the film is heat-set during crystallization to be crystallized (heat treatment temperature). Particularly preferred. From this point, among the incompatible resins, polyolefin resins such as polyethylene, polypropylene, polybutene, and polymethylpentene, polystyrene resins, polyacrylate resins, polycarbonate resins, polyacrylonitrile resins, polyphenylene sulfide resins, and fluorine resins are preferably used. It is done. These incompatible resins may be homopolymers or copolymers, and two or more incompatible resins may be used in combination. Among these, polyolefin resins such as polypropylene and polymethylpentene having a low surface tension are preferable, and polymethylpentene is most preferable. Since polymethylpentene has a relatively large difference in surface tension from polyester and a high melting point, polymethylpentene has a feature that the effect of forming cavities per added amount is large, and is particularly preferable as an incompatible resin.

  In the present invention, it is necessary that the polyester film contains a thickener. In order to obtain high reflectivity and high productivity, it is necessary to contain a large number of fine bubbles in the polyester film. To this end, a resin that is incompatible with polyester, which is the nucleating agent for bubbles, is contained in the polyester resin. It is necessary to disperse in a fine and uniform state. When the thickener is not contained, the dispersibility of the resin incompatible with the polyester is deteriorated, and there is a tendency that the film breaks frequently due to the influence of the viscosity decrease in the melt extrusion process. By adding a small amount of the thickener, it is possible to suppress dispersibility in the molten state and re-aggregation after melting without increasing the production cost.

  In addition, it is an effective method for reducing the manufacturing cost to make a part that does not become a product, such as a film-forming edge, into a melt chip and reuse it in film production, but the productivity is greatly lowered due to a decrease in viscosity. However, by containing a thickener and increasing the viscosity, the raw material can be reused without reducing the productivity, so that a cheaper film can be obtained.

  The thickener in the present invention increases the melt viscosity of the polyester resin, and examples thereof include a resin exhibiting a high melt viscosity and a polymerizing agent.

  Examples of the resin having a high melt viscosity include polyolefin resins or elastomers such as polyethylene, polypropylene, and ethylene-ethyl acrylate copolymers, styrene-ethylene-butadiene block copolymers, styrene-butadiene-styrene block copolymers, and the like. And styrene-based elastomers. These resins having a high melt viscosity are usually added in an amount of 2 to 50 parts by weight, preferably 10 to 20 parts by weight, based on 100 parts by weight of the polyester resin.

  The polymerizing agent is an additive having a functional group having polyester resin reactivity and polymerizing by chemically bonding with the polyester resin. Examples of the functional group that reacts with the polyester resin include an oxazoline group, a carboxyl group, an epoxy group, an isocyanate group, a carbodiimide group, and the like. As a thickener used in the present invention from the viewpoint of reactivity with the polyester resin, an oxazoline is used. It is preferable to use those having a group, an epoxy group, or a carbodiimide group. In particular, when a polyfunctional product is used, polyester molecules can be linked by a crosslinking reaction, and a polyester resin having a higher viscosity can be obtained even at the same temperature. Specific examples include 1,4-phenylene bisoxazoline, 1,3-phenylene bisoxazoline, polyethylene glycol-diglycidyl ether, bisphenol A diglycyl ether, terephthalic acid diglycidyl ether, and the like. When these polymerizing agents are used as thickeners, they are usually added in an amount of 0.1 to 20 parts by weight, preferably 0.1 to 10 parts by weight, based on 100 parts by weight of the polyester resin.

  In the present invention, a production method in which at least a thickener and a polyester resin are kneaded in advance to form a chip is preferable. By kneading the thickener and the polyester resin in advance, fluctuations in viscosity are reduced, so that more stable production is possible. In addition, it is particularly preferable to use a method of adding a thickener when melting or chipping a portion that does not become a product, since the raw material can be reused without reducing productivity.

  In the polyester film of the present invention, various additives, for example, a fluorescent brightener, a crosslinking agent, a heat stabilizer, an antioxidant, an ultraviolet absorber, an organic lubricant, an organic, Inorganic fine particles, fillers, light resistance agents, antistatic agents, nucleating agents, dyes, dispersants, coupling agents and the like may be added. Among these, as the inorganic fine particles, those capable of forming fine flat bubbles with themselves as the core are preferable. For example, calcium carbonate, magnesium carbonate, zinc carbonate, titanium oxide (anatase type, rutile type), zinc oxide, barium sulfate, Zinc sulfide, basic lead carbonate, mica titanium, antimony oxide, magnesium oxide, calcium phosphate, silica, alumina, mica, talc, kaolin and the like can be used. Among these, it is particularly preferable to use calcium carbonate and barium sulfate which have little absorption in the visible light region of 400 to 700 nm. If there is absorption in the visible light range, problems such as a decrease in luminance may occur.

  In the case of organic fine particles, those that do not melt by melt extrusion are preferred, and crosslinked fine particles such as crosslinked styrene and crosslinked acrylic are particularly preferred. The above fine particles may be used alone or in combination of two or more.

  In the present invention, another layer may be laminated on at least one surface of the film layer having air bubbles by a method such as coextrusion. By having such a composite layer structure, surface smoothness and mechanical strength can be imparted to the film, and the production process can be stabilized.

  In this invention, 30-1200 micrometers is preferable and, as for the total thickness of a polyester film, 40-800 micrometers is more preferable. When the thickness is less than 30 μm, it is difficult to ensure the flatness of the film, and unevenness in brightness tends to occur when used as a reflector for a surface light source. On the other hand, when it is thicker than 1200 μm, problems such as extremely low productivity and a thick backlight unit occur.

In the present invention, the apparent density of the polyester film is preferably 0.5 to 1.3 g / cm ³ . When the apparent density is less than 0.5 g / cm ³ , film breakage tends to occur, and when it exceeds 1.3 g / cm ³ , the reflectance tends to be inferior.

  In this invention, it is preferable that the average major axis in the film | membrane of resin incompatible with polyester is 0.3-3.0 micrometers. If the average major axis is less than 0.3 μm or exceeds 3.0 μm, the reflectance tends to decrease, which is not preferable. A method for controlling the average major axis of the resin incompatible with the polyester within the above preferable range is not particularly limited. For example, in addition to the polyester and the incompatible resin, a dispersant may be added. Is a preferred method. Examples of the dispersant exhibiting the above-described effects include olefin polymers or copolymers having polar groups such as carboxyl groups and epoxy groups, and functional groups reactive with polyester, diethylene glycol, polyalkylene glycol, and surfactants. In addition, a thermal adhesive resin or the like can be used. Of course, these may be used alone or in combination of two or more. Among them, a copolymer resin of a polyalkylene glycol, an aliphatic diol component having 2 to 6 carbon atoms, and a polyester resin composed of terephthalic acid is compatible with the polyester resin which is the main constituent unit of the polyester layer (B) and polyolefin. From the viewpoint of improving the dispersibility of the resin, a block copolymer of polyethylene glycol and polybutylene terephthalate is particularly preferable. Such a dispersant may be used as a polyester obtained by copolymerizing a dispersant in a polymerization reaction in advance, or may be used directly as it is.

  The polyester film of the present invention is preferably used as a reflective member of a liquid crystal display, and particularly preferably used as a surface light source member called a backlight. As a surface light source, (A) at least a rectangular light guide plate, a point-like or linear light source, and a light beam emitted from a triggering light source are in one light incident side of the light guide plate or in a relation of opposite sides to each other. A surface light source (edge-type backlight) using a means for introducing light from two incident light sides, (B) a plurality of linear fluorescent tubes arranged in parallel, and a light reflector on the back side of the fluorescent tubes There are two types of surface light sources (direct backlight) having a structure, and the polyester film of the present invention can be suitably used in any of the above.

Next, although an example is demonstrated about the manufacturing method of the light reflection film of this invention, it is not a thing limited to this example. In a film forming apparatus having an extruder, a polyester resin chip that has been sufficiently dried as necessary, and a mixture of a polyester incompatible resin and a thickener are supplied to a heated main extruder. The incompatible resin may be added by using a master chip prepared by uniformly melting and kneading in advance, or may be directly supplied to a kneading extruder. The thickener additive may also be supplied to the kneading and extruding machine directly using a master chip that is uniformly melt kneaded and reactively extruded in advance, or may be supplied directly to the kneading extruder. Is preferred. In order to form a composite layer by coextrusion, raw materials having different compositions may be supplied to a plurality of extruders. The polyester resin mixture thus extruded is cooled and solidified on a cooling drum to obtain an unstretched sheet. The unstretched sheet is guided to a heated roll group, stretched in the longitudinal direction (longitudinal direction, that is, the traveling direction of the film), and then cooled by a cooling roll group. When applying in-line coating, it can also apply | coat using this bar | burr method etc. after this process. Subsequently, both ends of the film stretched in the longitudinal direction are guided to a heated tenter while being held by clips, and stretched in a direction perpendicular to the longitudinal direction (lateral direction or width direction), and subsequently heat-treated in the tenter ( A polyester film can be obtained by performing heat setting), uniformly cooling slowly, and then cooling to near room temperature. In addition, as a stretching method, the both ends of the unstretched sheet are guided to a heated tenter while being gripped by a clip, and stretched in the width direction, and at the same time, the clip traveling speed is accelerated to stretch in the longitudinal direction. There is a way to do it at the same time. This simultaneous biaxial stretching method has an advantage that the film surface does not come into contact with the heated roll, and therefore there is no scratch that becomes an optical defect on the film surface.

The white laminated biaxially stretched film thus obtained can be provided with a functional layer such as a coating layer having ultraviolet absorbing ability by a microgravure plate / kiss coat.
[Measurement and evaluation method of characteristics]
The characteristic value of this invention is calculated | required with the following evaluation method and evaluation criteria.

(1) Apparent density A film is cut into a size of 100 mm × 100 mm and precisely weighed with an electronic balance. The thickness of 8 points is measured for each sample with a calibrated digital micrometer (M-30, manufactured by Sony Precision Technology Co., Ltd.), and the average thickness is calculated for each sample. Is the apparent density.
Apparent density = precise balance weight (g) x 100 / average thickness (µm).

(2) Average major axis of incompatible resin Film samples were frozen on the cooling stage of an electric microtome (ST-201: manufactured by Nippon Microtome Laboratories Co., Ltd.) and observed with a steel knife. The film was cut according to any cutting direction), the cut surface was sputtered with carbon, and a cross-section observation sample was prepared. Subsequently, using a transmission electron microscope HU-12 type (manufactured by Hitachi, Ltd.), the cross section of the film was magnified 3,000 times, and a cross-sectional photograph was taken. Next, the long diameter (incompatible resin size measured on a straight line passing through the center of gravity of the incompatible resin, about 100 incompatible resins observed as the core of the cavity observed from this cross-sectional photograph, The longest part) was measured using a metal ruler, and the average value was taken as the average major axis.

(3) Reflectance (560nm)
Shimadzu UV-Vis spectrophotometer UV-2450 is equipped with an integrating sphere ISR-2200, and relative reflectivity at 560 nm is measured with barium sulfate as standard plate and standard plate as 100% under the following conditions. And what has the reflectance of 98% or more was set as the pass.

<Measurement conditions>
Scanning speed: Medium speed Slit: 5.0nm
Reflection angle: 8 °.

<Standard plate creation method>
34 g of barium sulfate white standard reagent (EASTMAN White Reflectance Standard Cat No.6091) was put into a cylindrical recess having a diameter of 50.8 mm and a depth of 9.5 mm, and compressed using a glass plate, and the compression density was about 2 g / cm. ^Three barium sulfate white standard plates were prepared.

(4) Film-forming stability Evaluation was performed based on the number of occurrences of film tearing. Evaluation was performed in terms of the number of tears per day, and the determination was made according to the following criteria. In addition, (circle) and (triangle | delta) are pass.
○: Good (Nearly torn (less than once / day))
Δ: Slightly inferior (breakage occurs occasionally (1 to 2 times / day))
X: Inferior (Torn frequently (twice / day or more)).

(Example 1)
As a thickener, 20 parts by weight of 1,3-phenylenebisoxazoline (hereinafter abbreviated as PBO) and 80 parts by weight of polyethylene terephthalate (hereinafter abbreviated as PET) are kneaded, reacted and extruded into a chip (hereinafter referred to as a chip). , Abbreviated as compound PET) in advance. Compound PET 2% by weight, PET 87% by weight, polymethylpentene 10% by weight, polybutylene terephthalate-polytetramethylene terephthalate block copolymer (hereinafter referred to as PBT-PTMG) 1% by weight What was adjusted so that it might become was mixed and dried, the raw material was supplied to Extruder A, melt extruded at 280 ° C., filtered through a 30 μm cut filter, and introduced into a T-die composite die. On the other hand, calcium carbonate with an average particle size of 1.2 μm, adjusted to 10% by weight and 90% by weight of PET, is mixed and dried, the raw material is supplied to Extruder B, melt extruded at 280 ° C., and cut by 30 μm. And then introduced into a T-die composite die. Next, melt co-extrusion was performed by a feed block in a T die composite die so as to have a B / A / B configuration, and cooling was performed on a mirror-cast cast drum by an electrostatic application method to prepare a three-layer laminated sheet. . This laminated sheet was stretched 3.3 times in the longitudinal direction at a temperature of 90 ° C., and then stretched 3.3 times in the width direction at 120 ° C. through a preheating zone at 100 ° C. with a tenter. Furthermore, it heat-processed for 10 second at 220 degreeC, and obtained the polyester film by which the heat treatment of extending | stretching 200 micrometers was carried out after cooling.
The thickness constitution of each layer was B / A / B = 14 μm / 172 μm / 14 μm.

  The results are shown in Table 1. The obtained film had high reflectivity and excellent film forming stability.

(Example 2)
The raw material supplied to the extruder A was the same as that of Example 1 except that compound PET was 5% by weight and PET was 84% by weight.

  The results are shown in Table 1. The obtained film had high reflectivity and excellent film forming stability.

(Example 3)
A mixture of 98% by weight of the film-forming edge portion of the film obtained in Example 1 with a crusher and 2% by weight of PBO as a thickener was melted and extruded at 280 ° C. Then, the chip was discharged, cooled with water, and cut to prepare a regenerated chip A. Example 1 except that the obtained recycled chip A 40% by weight, polymethylpentene 6.8% by weight, PET 52.2% by weight, PBT-PTMG 1% by weight was dried and supplied to the extruder A. It carried out like.

  The results are shown in Table 1. The obtained film had high reflectivity and excellent film forming stability. Moreover, it was particularly excellent in terms of manufacturing cost.

(Comparative Example 1)
The composition of the raw material A was carried out in the same manner as in Example 1 except that compound PET was not used and PET was 89% by weight.

  The results are shown in Table 1. The obtained film was inferior in reflectance.

(Comparative Example 2)
The film-forming edge portion of the film obtained in Comparative Example 1 was crushed with a crusher, melted and extruded at 280 ° C., then discharged from the die into a gut shape, cooled with water, and cut to produce a regenerated chip B. Example obtained except that dried mixture B obtained 40% by weight, polymethylpentene 6.8% by weight, PET 52.2% by weight, PBT-PTMG 1% by weight was dried and supplied to Extruder A 1 was carried out.

  The results are shown in Table 1. The obtained film was inferior in reflectance, and at the same time, tearing started and inferior film formation stability.

  The present invention relates to a polyester film containing a large number of fine bubbles inside, excellent in reflectivity and concealing property, having good film forming stability, and excellent in productivity and cost. It can be suitably used for a backlight device and a reflection sheet for a lamp reflector, a reflection sheet for a lighting fixture, a reflection sheet for a lighting signboard, a back reflection sheet for a solar cell, and the like.

Claims (5)

A polyester film having air bubbles having a resin incompatible with polyester inside as a core, wherein the polyester film contains a thickener. The polyester film according to claim 1, wherein the apparent density of the film is 0.5 to 1.0 g / cm ³ , and the average major axis of the resin incompatible with the polyester is 0.3 to ³ µm. The polyester film according to claim 1 or 2, wherein the thickener has an oxazoline group, an epoxy group, or a carbodiimide group. The method for producing a polyester film according to any one of claims 1 to 3, wherein at least a thickener and a polyester resin are previously melted and kneaded to form a chip. The polyester film of Claims 1-3 used as a reflection member of a liquid crystal display.