JP2012207153A - White film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a white film excellent in reflectance and hiding properties, particularly, to provide a white film having a high reflectance and high light diffusion properties and suitable as a reflecting member for a surface light source.SOLUTION: The white film is a film having bubbles containing a void nucleating agent within the film. In respective bubbles, when obtaining the ratio (L/R) of the length (L) of the bubble in a surface direction of the film to the length (R) of the bubble in the thickness direction of the film, voids, whose (L/R) is 1.5 to <5, account for ≥50% of the total void area, and the white film has a specific gravity of ≤1.0.

Description

The present invention relates to a white film. More specifically, the present invention relates to a white film having high reflectivity and light diffusibility, suitable as a reflective member for a surface light source.

  In recent years, many displays using liquid crystals have been used as display devices used in personal computers, televisions, mobile phones, and the like. Since these liquid crystal displays themselves are not light emitters, display is possible by installing a surface light source called a backlight from the back side and irradiating light. In addition, the backlight has a structure of a surface light source called a side light type or a direct type in order to uniformly irradiate the entire screen, not just irradiating light. In particular, a side light type backlight using an LED as a light source is applied to a thin liquid crystal display used for a television application, a monitor application, a notebook personal computer or the like for which thinning and energy saving are desired.

  In general, a sidelight type backlight using this LED employs a light guide plate method in which the LED is used as an illumination light source and the entire liquid crystal display is uniformly irradiated from the edge of the light guide plate that uniformly propagates and diffuses light. Yes. In this illumination method, in order to use light more efficiently, a reflection plate is provided on the back surface of the light guide plate to efficiently reflect light diffused from the light guide plate toward the liquid crystal screen.

Reflectors and reflectors used for such surface light sources for liquid crystal screens (collectively referred to as surface light source reflecting members) are required to have a high reflection function as well as a thin film. A film containing fine bubbles inside is used alone, or a film in which these films are bonded to a metal plate, a plastic plate or the like has been used. In particular, when a film containing fine bubbles inside is used, it is widely used because of its excellent brightness improvement effect and uniformity. Such fine bubbles can be obtained by adding a component (void nucleating agent) incompatible with the resin and stretching the resin in one or more directions. Such a film containing fine bubbles inside is disclosed in a patent publication (see Patent Documents 1 and 2). Moreover, although there are usually many types of void nucleating agents, examples in which two or more types of void nucleating agents are used in combination are also disclosed (Patent Documents 3 to 4). Furthermore, a technique for improving the reflectivity and concealability by controlling the dispersion of the ratio of the length in the thickness direction of the void shape to the length in the in-plane direction is also disclosed. (Patent Document 5)
In addition, a film whitened by containing a large number of fine bubbles inside the film has high whiteness because it has high reflectivity, and is therefore preferable for image printing media, particularly image receiving paper for sublimation transfer systems. It is used.

JP-A-6-322153 JP-A-7-118433 JP 2001-225433 A JP 2001-288291 A JP 2004-339403 A

  However, in order to improve the efficiency of the light from the illumination light source in recent years, higher reflectivity and concealment are required for the reflective film. However, in the reflective film using the void nucleating agent, void nuclei are required. Reflective films with a simple increase in the amount of agents and reflective films using two or more types of void nucleating agents have also been proposed, but this has not led to dramatic improvements in light reflectivity and backlight luminance characteristics. There is a need for improved reflectivity and concealment of reflective films.

  Also in the field of image printing media, with higher image quality and higher definition of digital photos, image receiving paper is required to have higher whiteness and improved reflectivity of light reflecting films.

In order to solve the above problems, the white film according to the present invention has the following configuration.
(1) A film having bubbles containing a void nucleating agent inside the film, and for each bubble, the bubble length (L) in the film surface direction and the bubble length (R) in the film thickness direction When the ratio (L / R) was determined, voids having a ratio (L / R) of 1.5 or more and less than 5 accounted for 50% or more of the total void area, and the specific gravity of the white film was 1.0 or less A white film characterized by
(2) The white film according to (1), wherein the void nucleating agent comprises a resin and / or inorganic particles that are incompatible with a main resin component constituting the white film.
(3) The white film of (1) to (2), wherein the main resin component constituting the white film is an aromatic polyester.
(4) The white film of (1) to (3) having a thickness of 30 μm or more and 600 μm or less.

  According to the white film of the present invention, a white film having practically sufficient reflectance and concealability can be provided.

  The white film of this invention contains the main resin component which comprises a white film, and the component (void nucleating agent) which is incompatible with this resin component. By uniformly extruding these mixtures and stretching them in at least one direction, it becomes easy to generate bubbles uniformly in the film.

  As a main resin component which comprises a white film, what does not have absorption in visible region is preferable. In addition, since the white film in the present invention reflects and diffuses light at the gas-solid interface between the bubbles inside the film and the resin, the refractive index of the resin component forming the solid phase is different from the refractive index of the gas phase. Is preferably large. When the refractive index difference is small, reflection at the gas-solid interface does not occur so much, and as a result, a desired light reflection effect cannot be obtained. Since the refractive index of gas and vacuum is substantially 1.0, in order to obtain substantially effective light reflectivity, the refractive index of the resin component is preferably 1.4 or more, more preferably 1 .5 or more. Examples of resins that satisfy such conditions include polyolefins and polyesters. Among them, an aromatic polyester having favorable dimensional stability, mechanical properties, handling properties (handling properties) and a high refractive index can be preferably used.

Further, among aromatic polyesters, polyethylene terephthalate (hereinafter abbreviated as PET), polyethylene-2,6-naphthalene dicarboxylate, polypropylene terephthalate, polybutylene terephthalate, poly-1,4-cyclohexylenedimethylene terephthalate, etc. Since it can be obtained at a low cost and the film-forming property is good, it can be particularly preferably used.

  These polyesters may be homopolymers or copolymers, but are preferably homopolymers. Examples of copolymer components in the case of a copolymer include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, and diol components having 2 to 15 carbon atoms. Examples of these components include isophthalic acid. , Adipic acid, sebacic acid, phthalic acid, sulfonate group-containing isophthalic acid, and ester-forming compounds thereof, diethylene glycol, triethylene glycol, neopentyl glycol, polyalkylene glycol having a molecular weight of 400 to 20,000, and the like. .

  In these polyester resins, various additives within a range not impairing the effects of the present invention, such as fluorescent brightener, heat stabilizer, oxidation stabilizer, ultraviolet absorber, light stabilizer, antistatic agent, dye, A dispersant or the like may be added.

  Next, the void nucleating agent added to form bubbles will be described. The void nucleating agent is not necessarily the same as the main resin component constituting the white film and can be dispersed in the resin component in the form of particles, and examples thereof include inorganic fine particles, organic fine particles, and various thermoplastic resins. .

  The void nucleating agent is preferably one that can form bubbles with itself as a nucleus, such as calcium carbonate, magnesium carbonate, zinc carbonate, titanium oxide (anatase type, rutile type), zinc oxide, barium sulfate, zinc sulfide, basic Inorganic particles such as lead carbonate, mica titanium, antimony oxide, magnesium oxide, calcium phosphate, silica, alumina, mica, talc, polyolefin resin such as polyethylene, polypropylene, polybutene, polymethylpentene, polystyrene resin, polyacrylate resin, polycarbonate resin Thermoplastic resins such as polyacrylonitrile resin, polyphenylene sulfide resin, and fluororesin are preferably used. The void nucleating agent may be a single type, or two or more types of components may be used. When using a thermoplastic resin, it is preferable to use a thermoplastic resin incompatible with the main resin component constituting the white film. The main resin component as used in the field of this invention shows resin which occupies 51% or more in resin which comprises the continuous phase (matrix) in a white film.

  When the main resin component constituting the white film is an aromatic polyester resin, the incompatible thermoplastic resins include olefin resins such as polyethylene, polypropylene, polybutene, and polymethylpentene, styrene resins, and polyacrylate resins. Polycarbonate resin, polyacrylonitrile resin, polyphenylene sulfide resin, fluorine resin and the like are preferably used. These may be homopolymers or copolymers, and may be used in combination of two or more thermoplastic resins. Among these, polyolefins such as polypropylene, polymethylpentene and cycloolefin copolymer having a small critical surface tension are preferable, and polymethylpentene and cycloolefin copolymer are particularly preferably used.

  It is preferable from the point of a reflectance that the said void nucleating agent contains 10 to 45weight% in the whole layer containing the void which comprises a white film. More preferably, it is 25 to 40% by weight. If it is less than 10%, the reflectivity is not preferable because there are few cavities and the reflectivity is low. On the other hand, if it exceeds 45%, not only is the bubble connected and the reflectance is lowered, but also tearing is likely to occur in the film production process, and productivity may be lowered.

  The white film of the present invention is a film having air bubbles inside the film, and for each air bubble, the ratio of the air bubble length (L) in the film surface direction to the air bubble length (R) in the film thickness direction. When (L / R) is calculated, it is necessary that voids having (L / R) of 1.5 or more and less than 5 occupy 50% or more of the entire void area.

  Since the bubbles having the (L / R) of 1.5 or more and less than 5 occupy 50% or more of the entire void area, many non-parallel interfaces with respect to the film surface are formed. And the reflection performance of the whole white film can be improved. When the (L / R) is 5 or more, there are many interfaces parallel to the film surface, the light diffusibility cannot be improved, and the reflection performance of the entire white film cannot be improved. Moreover, when the (L / R) is less than 1.5, the area of the interface between the resin and the gas phase necessary for reflection is small, and a sufficient reflectance cannot be obtained.

  A specific method for measuring the (L / R) and void area of the bubbles in the present invention is shown below. (1) Take a picture of a cross-sectional image obtained in accordance with a method described later (the cross-sectional image may be obtained by an electronic method). (2) A sufficiently transparent film or tracing paper is overlaid on the photograph, and the shape of bubbles in the photograph is traced. However, the thickness of the pen tip used for tracing is 0.5 mm or less. (3) The image of the film or tracing paper traced using a CCD camera or the like was taken into a personal computer, and the cross-sectional area of each bubble was determined using the image analysis software to obtain the void area. Furthermore, (L) and (R) were measured for each bubble present in the film, and then (L / R) was calculated.

  Examples of the method of setting the (L / R) in the above range include a method in which a void nucleating agent is contained in an amount of 25 to 40% by weight, the film is stretched in the width direction after being multistage stretched in the longitudinal direction. .

  In addition, it is not impeded to form a skin layer on one side or both sides of a white film in which fine bubbles are formed by a method such as coextrusion. Furthermore, various particles, a fluorescent brightening agent, and the like may be added to the skin layer. By laminating such a skin layer, mechanical strength, surface smoothness and the like can be imparted to the white film.

  In addition, various coating liquids are applied using a well-known technique in order to impart easy adhesion, antistatic property, ultraviolet light absorption performance, etc. to the surface of the white film as long as the effects of the present invention are not impaired. In order to improve the impact resistance, a hard coat layer or the like may be provided. The coating may be performed at the time of film production (in-line coating), or may be performed on a white film after film production (off-line coating).

  Furthermore, for the purpose of imparting electromagnetic wave shielding properties or bending workability, aluminum, silver, or the like may be bonded or vapor-deposited on one surface of the white film.

  The apparent specific gravity that is a measure of the bubble content of the white film of the present invention needs to be 1.0 or less. More preferably, it is 0.5 or more and 0.8 or less. When the specific gravity is less than 0.5, the mechanical strength as a film may be insufficient, or problems such as easy breakage and poor handleability may occur. On the other hand, if it exceeds 1.0, the bubble content is too low and the reflectance tends to decrease.

  30-600 micrometers is preferable and, as for the thickness of the white film in this invention, 80-450 micrometers is more preferable. When the thickness is less than 30 μm, it becomes difficult to ensure the flatness of the film, and unevenness in brightness tends to occur when used as a surface light source. On the other hand, when it is thicker than 600 μm, when it is used as a white film for a liquid crystal display or the like, the thickness becomes too large to meet the demand for thinning.

  In the white film of the present invention, the light reflectance is preferably 90% or more, more preferably 95% or more, and still more preferably 100% or more from the viewpoint of light reflectivity and backlight luminance characteristics. When the light reflectance is less than 90%, the film is inferior in concealability, and when used as a white film for a liquid crystal display, sufficient brightness may not be obtained. The upper limit of the reflectance is not particularly specified, but is preferably 200% or less from the viewpoint of light reflectance and color.

  Next, although an example is demonstrated about the manufacturing method of the white film of this invention, it is not limited to this example.

  In a composite film forming apparatus having a main extruder and a sub-extruder, a mixture of aromatic polyester resin chips and a void nucleating agent constituting a white film that has been sufficiently vacuum-dried as necessary is heated. Supply to the extruder. Moreover, in order to laminate | stack a skin layer, the chip | tip of the aromatic polyester resin which performed sufficient vacuum drying as needed, an inorganic particle, and a fluorescent whitening agent are supplied to the heated subextruder.

  Here, as a method for obtaining the white film of the present invention, the content of the void nucleating agent is preferably 20% to 45% by weight. By containing such an amount of particles, a large number of bubbles with small (L / R) can be generated.

  In this way, the raw materials are supplied to each extruder and laminated (A / B or A / B / A) so that the polymer of the sub-extruder comes to at least one side of the polymer of the main extruder in the T die composite die. To form a melt-laminated sheet.

  The melt-laminated sheet is tightly cooled and fixed on a cooled drum to produce an unstretched laminated film. At this time, in order to obtain a uniform film, it is desirable to apply static electricity to make it adhere to the drum. Thereafter, if necessary, a desired light reflecting film is obtained through a stretching step, a heat treatment step, and the like.

  The stretching method is not particularly limited, and there are a sequential biaxial stretching method in which stretching in the longitudinal direction and stretching in the width direction are separated and a simultaneous biaxial stretching method in which stretching in the longitudinal direction and stretching in the width direction are performed simultaneously.

  As a method of sequential biaxial stretching, for example, there is a method in which the unstretched laminated film 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. It is common.

  In the stretching process in the longitudinal direction, it is also effective to reduce (L / R) by using a multi-stage stretching method in which stretching is performed in two or more stages. When performing multistage stretching, it is desirable that the stretching temperature in the previous stretching stage is higher than the stretching temperature in the subsequent stretching stage. It is also a desirable stretching method to increase the stretching ratio in the previous stretching step as much as possible. Hereinafter, one of the preferable aspects in the case of performing two-stage stretching will be exemplified. First, with respect to the stretching temperature, the stretching temperature in the first step is set to (Tg + 5 ° C.) to (Tg + 15 ° C.) with respect to the glass transition temperature Tg of the main resin constituting the white film, and the stretching temperature in the second step is (Tg− 5 ° C.) to (Tg + 5 ° C.). Regarding the draw ratio, the draw ratio of the first stage is preferably 1.2 to 2.0 times, and the draw ratio of the second stage is 2.5 to 3.2 times in order to reduce the (L / R). Is preferable.

  After stretching in the longitudinal direction, stretching in the width direction can be subsequently performed. In the sequential biaxial stretching method, both ends of the film are guided to a heated tenter while being held by clips, and can be stretched in a direction perpendicular to the longitudinal direction (lateral direction or width direction).

  On the other hand, as a method of simultaneous biaxial stretching, for example, the both ends of the above-mentioned unstretched laminated film are guided to a heated tenter while being gripped by clips, and stretched in the width direction and simultaneously accelerated the clip traveling speed. Thus, there is a method of simultaneously stretching in the longitudinal direction. 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. At this time, stretching is performed in two stages, and the stretching temperature in the first stage is set to (Tg + 5 ° C.) to (Tg + 30 ° C.) with respect to the glass transition temperature Tg of the main resin constituting the white film. (Tg−5 ° C.) to (Tg + 5 ° C.), the first stage draw ratio is preferably 1.2 to 2.0 times, and the second stage draw ratio is 2.5 to 3.2 times. L / R) is preferable in order to reduce.

  In order to impart planar stability and dimensional stability to the biaxially stretched laminated film thus obtained, heat treatment (heat setting) is subsequently performed in the tenter, and after uniform cooling, cooling to near room temperature and winding up By this, the white film of the present invention can be obtained.

[Measurement and evaluation method of characteristics]
(1) The cross section of the film is cut perpendicular to the thickness direction using (L / R) and void area (i) microtome of bubbles.
(Ii) Using a scanning electron microscope S-2100A type (Hitachi, Ltd.) (hereinafter SEM), the cross section is cut at an appropriate magnification (500 to 200) so that 100 to 200 bubbles are contained in one screen. (10,000 times magnification) and take a picture of the screen to obtain a screen image. In the present invention, the magnification was determined so that in principle 150 could be entered, and the total number of bubbles that were effective as measurement objects contained in the screen was measured. When taking a picture of the screen, an electronic method may be used.
(Iii) From the image obtained by magnifying observation, for each bubble, the bubble length (L) in the film surface direction, the bubble length (R) in the film thickness direction, and the ratio (L / R) ) Here, L and R are not necessarily the length in the surface direction and the length in the thickness direction of the actual bubbles in the film, but are determined by the length in the surface direction of the bubbles and the length in the thickness direction obtained from the image. Shall.
In addition, bubbles having a cross-sectional area (void area) of less than 0.1 μm ² are not measured. Also, bubbles that are not entirely contained in the screen (for example, bubbles that are partially missing at the edge of the screen) are not measured.
(Iv) The sum and ratio of void areas having (L / R) of 1.5 or more and less than 5 with respect to the total void area are obtained.
(V) A ratio was obtained for each of five randomly extracted visual fields, and the average value was defined as a ratio of void areas having (L / R) of 1.5 or more and less than 5.

(2) Specific gravity A white film was cut into a size of 5 cm × 5 cm and measured using an electronic hydrometer SD-120L (Mirage Trading Co., Ltd.) based on JIS K7112 (1980 version). In addition, 5 sheets were prepared for each white film, each was measured, and the average value was used as the specific gravity of the white film.

(3) Reflectance spectrophotometer U-3410 (Hitachi Ltd.) with φ60 integrating sphere 130-0632 (Hitachi Ltd.) (inner surface made of barium sulfate) and 10 ° tilt spacer The light reflectance at 560 nm was determined. In addition, the value which measured and calculated | required the light reflectance from the coating layer side of a white laminated film was made into the reflectance of the said white film. A part number 210-0740 (aluminum oxide) manufactured by Hitachi Instrument Service Co., Ltd. was used as the standard white plate.
[material]
(1) Polyester resin (a)
Using terephthalic acid as the acid component and ethylene glycol as the glycol component, adding to the polyester pellets that can obtain antimony trioxide (polymerization catalyst) to 300 ppm in terms of antimony atoms, performing a polycondensation reaction, limiting viscosity Polyethylene terephthalate pellets (PET) having 0.63 dl / g and carboxyl end group amount of 40 equivalents / ton were obtained. When the heat of crystal fusion was measured using a differential thermal analyzer, it was 4.186 J / g or more, which is a crystalline polyester resin. It was 250 degreeC when melting | fusing point Tm of this resin was measured.

(2) Incompatible resin (b)
A cyclic olefin resin “TOPAS” (manufactured by Polyplastics) having a glass transition temperature of 178 ° C. and a melt volume rate MVR (260 ° C./2.16 kg) of 4.5 ml / 10 mim was used. In addition, when the heat of crystal fusion was measured using the differential thermal analyzer, it was less than 4.186 J / g and was an amorphous resin.

Example 1
The raw material mixture shown in Table 1 was vacuum dried at 180 ° C. for 3 hours, then supplied to the extruder, melt-extruded at 280 ° C., filtered through a 30 μm cut filter, and then introduced into a T-die composite die. did.

  Next, in the T-die composite die, the polyester layer (B) is merged so as to be laminated (B / A / B) on both surface layers of the polyester layer (A), and then co-extruded into a sheet shape to form a melt-laminated sheet. The melt-laminated sheet was closely cooled and solidified by electrostatic application on a drum maintained at a surface temperature of 25 ° C. to obtain an unstretched laminated film. Subsequently, the unstretched laminated film was preheated with a group of rolls heated to a temperature of 100 ° C., then stretched 1.3 times in the longitudinal direction (longitudinal direction), further cooled to 83 ° C., and then 2. After stretching by 8 times, the film was cooled by a roll group having a temperature of 25 ° C. to obtain a uniaxially stretched film. Thereafter, the both ends of the uniaxially stretched film were stretched 3.3 times in the tenter at 110 ° C. in the direction perpendicular to the longitudinal direction (lateral direction) while being held by clips. Subsequently, a heat treatment at 190 ° C. was performed in a heat treatment zone in the tenter, and after 4% transverse treatment at 180 ° C., further 1% relaxation treatment was carried out at 110 ° C. The film was wound up to obtain a film having a thickness of 188 μm.

  The void area of (L / R) 1.5-5 of the obtained film was 68% of the total void area, and the white film excellent in the reflectance was obtained.

[Examples 2-3]
Except for the conditions shown in Table 1, film formation was carried out under the same conditions as in Example 1 to obtain a laminated white film having a thickness of 188 μm. Various properties of the film are shown in Table 1.

[Comparative Examples 1-3]
Except for the conditions shown in Table 1, film formation was carried out under the same conditions as in Example 1 to obtain a laminated white film having a thickness of 188 μm. Various properties of the film are shown in Table 1. There were few voids with a small (L / R), and the reflectance was low compared with Example 1.

  The film of the present invention is suitably used as a reflective member for a surface light source.

Claims (4)

A film having bubbles containing a void nucleating agent inside the film, and for each bubble, a ratio of the bubble length (L) in the film surface direction to the bubble length (R) in the film thickness direction ( L / R), the voids of which (L / R) is 1.5 or more and less than 5 occupy 50% or more of the total void area, and the specific gravity of the white film is 1.0 or less. White film. The white film according to claim 1, wherein the void nucleating agent comprises a resin and / or inorganic particles incompatible with a main resin component constituting the white film. The white film according to claim 1 or 2, wherein a main resin component constituting the white film is an aromatic polyester. The white film according to claim 1, which has a thickness of 30 μm or more and 600 μm or less.