JP2002162508A - Light diffusing film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light diffusing film which is superior in light transmittance and light diffusivity, without having to utilize light diffusing effect due to a surface shape. SOLUTION: When parallel rays of visible light are made incident perpendicular to one face of the light diffusing film, the opposite face of the incident part is used as a measurement part and the luminance of rays emergent from the measurement part is measured, when luminance obtained in the perpendicular direction is represented by L0 and luminance obtained in a direction, which is inclined from the perpendicular direction by 30 deg., with the measurement part as the center being represented by L30, the diffusion coefficient αof the light diffusing film defined by he expression α=log10(L30/L0) satisfies α>-1.5 and the surface haze is <=10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light diffusing film suitably used for a backlight of a liquid crystal display, a lighting device, and the like.

[0002]

2. Description of the Related Art In recent years, a large number of displays using liquid crystals have been used as display devices for personal computers, televisions, mobile phones, and the like. These LCD displays are
Since the light source itself is not a light emitter, display can be performed by irradiating light from the back side with a backlight. In addition, the backlight employs a surface light source structure called a sidelight type or a direct type in order to meet the requirement that the entire screen be uniformly illuminated in addition to simply irradiating light. Above all, a sidelight type, that is, a backlight that irradiates light from a side surface to a screen, has been applied to a thin liquid crystal display used for a notebook-type personal computer or the like in which thinning and miniaturization are desired. In general, this type of sidelight type backlight employs a light guide plate system that uses a light guide plate that uniformly propagates and diffuses light to uniformly irradiate the entire liquid crystal display. The light guide plate is engraved with a pattern so that light incident from the side surface is emitted in the vertical direction, and has a non-uniform light distribution due to the pattern. Therefore,
In this type of liquid crystal display, it is necessary to provide a light diffusing film on a light guide plate to make the light uniform so as to obtain high-quality images by increasing in-plane uniformity.

The performance required of such a light diffusing film includes not only high light diffusing property but also extremely high light transmitting property. By increasing the light transmittance, light from the backlight can be used efficiently, so that higher luminance and lower power consumption can be achieved.

Conventionally used light diffusing films include, for example, (1) a transparent thermoplastic resin as described in JP-A-4-275501, which is formed into a sheet,
A diffusion sheet (light diffusive film) obtained by subjecting the surface to physical unevenness, or
JP-A-59108 discloses a light diffusing film obtained by coating a light diffusing layer made of a transparent resin containing fine particles on a transparent substrate film such as a polyester resin.

[0005]

The light diffusing film shown in the above (1) and (2) has a light diffusing effect obtained by a light diffusing layer formed on the surface of the film and coated with irregularities or fine particles. It can be called a diffusion type light diffusion film. These do not contain particles that diffuse light inside the support or the like, and thus the diffusivity depends only on the surface shape. Therefore, for example, when laminating with other base material or the like, if an adhesive is applied to the surface of these light diffusing films, the surface becomes smooth and the diffusivity disappears,
Or, adverse effects such as a decrease in diffusivity due to deviation from the initial optical design. In addition, since the surface has already been processed, it is impossible to perform processing for imparting another function.

[0006] In the field of liquid crystal display members, there has been a demand for higher performance, higher efficiency, thinner and lighter weight. Alignment and adhesion are considered. For this reason, it is difficult for a conventional light-diffusing film utilizing only the light-diffusion effect due to the surface shape to be multifunctional while maintaining a sufficient diffusion property.

Therefore, there is a demand for a light diffusing film which does not utilize the light diffusion effect due to the surface shape but utilizes the light diffusion effect due to the diffusion component contained inside the film. Thus, the present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found a light-diffusing film excellent in light-transmitting and light-diffusing properties without using a light-diffusing effect due to a surface shape. And arrived at the present invention.

An object of the present invention is to provide a light-diffusing film excellent in light-transmitting and light-diffusing properties without utilizing the light-diffusing effect of the surface shape.

[0009]

In the light diffusing film of the present invention, a parallel ray of visible light is incident perpendicularly to the film surface, and the parallel surface of the incident light is emitted from the measurement site with the opposite surface as the measurement site. In the case of measuring the luminance of a light beam, the luminance obtained in the vertical direction is L ₀ , and the luminance obtained in the direction inclined 30 degrees from the vertical direction around the measurement site is L _30.
Where the diffusion coefficient α defined by the following equation (1) is
It is a light-diffusing film characterized by satisfying -1.5 <α and having a surface haze of 10% or less.

[0010] The laminated light diffusing film of the present invention has the following preferred embodiments. (a) A parallel ray of visible light is applied in a direction 3 perpendicular to the film surface.
When the light is emitted at an angle of 0 ° and the surface opposite to the incident region is used as a measurement region and the luminance of the light beam emitted from the measurement region is measured, the luminance obtained in the vertical direction is L ′ ₀ , In addition, assuming that the luminance obtained in a direction inclined by 30 degrees from the vertical direction around the measurement site is L' ₃₀ , the diffusion coefficient β defined by the following equation (2) satisfies β <1.

[0011] (b) a non-light diffusion layer A made of at least a thermoplastic resin,
The light diffusing layer B is composed of at least a thermoplastic resin and a light diffusing component, and has a cross section of A / B / A. (c)
The non-light diffusion layer A is a stretched film made of a crystalline polymer compound. (d) The non-light diffusion layer A and the light diffusion layer B are stacked without interposing an adhesive layer. (e) Total light transmittance is 60% or more.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the light diffusion film of the present invention, a parallel ray of visible light is perpendicularly incident on the film surface, and the brightness of the light beam emitted from the measurement site is defined as a surface opposite to the incident site. In the measurement, when the luminance obtained in the vertical direction is L ₀ , and the luminance obtained in a direction inclined 30 degrees from the vertical direction around the measurement site is L ₃₀ , the diffusion coefficient defined by the following equation 1 α is −1.5 <α
And a surface haze of 10% or less.

[0013] Here, the parallel light of visible light may be a white light source such as a halogen lamp or a metal halide lamp.
A monochromatic light source such as an e-Ne laser may be used, but a parallel light beam is preferable.

As an index of light diffusivity, haze (turbidity) obtained by the following relational expression is generally used. Haze refers to the percentage of the amount of light scattered and transmitted out of the incident light beam while the incident light passes through the sample from the light source.
The total light transmittance is T _t , the diffuse transmittance is T _d , and the linear transmittance is T
When _p, the total light transmittance T _t is represented by the following formula, also _{T t = T d + T p} , the haze H _t is expressed by the following equation.

[0015] A higher value of this haze indicates that the sample diffuses the incident light.

When the light diffusivity is examined in more detail from a practical viewpoint, the point is how much the incident light can be scattered over a wide range. However, since the haze is only a matter of whether or not the haze deviates from the incident light beam, it is considered necessary to discuss the scattering angle of the incident light beam for practical evaluation. For example, 2
When evaluating the types of samples, the haze value is the same,
A phenomenon occurs in which the practical diffusivity differs. this is,
This is due to the fact that the angle of scattering of the incident light is different in each sample.

The scattering coefficient α of the present invention represented by the above formula (1) is an index capable of evaluating the scattering property of incident light rays and thus the practical light diffusing property as a diffusion film.
Is larger than -1.5, practically sufficient diffusivity can be obtained. The scattering coefficient α is more preferably larger than -1.0 from the viewpoint of diffusivity.

Further, the light diffusivity of the present invention is such that a parallel ray of visible light of a film is incident at an angle of 30 degrees with respect to the vertical direction of the film surface, and the opposite surface of the incident portion is set as a measurement portion and emitted from the measurement portion. When performing luminance measurement of a light beam, the luminance obtained in the vertical direction is L ′ ₀ , and the luminance obtained in a direction parallel to the incident light beam and inclined at 30 degrees from the vertical direction around the measurement site is L ′ ₃₀ . In this case, the diffusion coefficient β defined by the following equation 2 preferably satisfies β <1, and more preferably satisfies β <0.75.

[0019] Diffusion coefficient β obtained for a ray incident at an angle of 30 degrees
Is smaller than 1, it is possible to obtain a light diffusing film having uniform light diffusing properties not only with incident light from the front direction but also with no directionality.

The light diffusing film of the present invention preferably has a surface haze of 10% or less. Next, the surface haze will be described.

The haze described above includes those derived from the surface shape and those derived from the internal structure of the substance. The surface haze in the present invention refers to the former. The former surface haze indicates the shape of irregularities on the sample surface and the state of scattered light on the surface due to surface processing. The latter haze is called an internal haze, and indicates a state of scattered light caused by added particles, voids, and the like included in the sample.

The surface haze in the present invention can be determined by preparing a model made of a transparent resin in which the form of the sample surface layer is copied by the method described later, and measuring the haze of the model. With this method, it is possible to measure the diffusion effect obtained only from the influence of the surface morphology.

The light-diffusing film of the present invention has a surface haze of 10% or less. Surface haze is preferably 8% or less,
More preferably, it is 5% or less. 10% surface haze
The film exceeds the surface is not sufficiently smooth, the effect on the diffusivity of the surface shape can not be ignored,
The function may be degraded at the time of surface processing or bonding with another member, which is not preferable. That is, it can be said that the light diffusion film utilizes only the internal diffusion effect because the surface diffusion effect is not used.

In the present invention, in order to satisfy the above conditions, the structure of the light diffusing film is a non-light diffusing layer A made of a thermoplastic resin a, and a light diffusing layer B made of a thermoplastic resin b and a light diffusing component. Then, the configuration of the cross section is preferably A / B / A.

The non-light diffusing layer A is preferably substantially non-light diffusing. Here, “substantially non-light diffusible” means that the internal haze is 0% or more and 10% or less. The internal haze is determined by performing haze measurement in a state where the sample is immersed in a liquid having the same refractive index as the substance of the surface layer of the sample.

The non-light diffusing layer A is made of at least a thermoplastic resin a. As the thermoplastic resin a, for example, polyester resins such as polyethylene terephthalate, polyethylene-2,6-naphthalate, polypropylene terephthalate, polybutylene terephthalate, polycarbonate, polystyrene, polyethylene, polypropylene,
Polyolefin resins such as polymethylpentene, polyamides, polyethers, polyesteramides, polyetheresters, polyvinylchloride, poly (meth) acrylates, copolymers containing these as main components, or mixtures of these resins, etc. And are not particularly limited.

Of these, a crystalline polymer compound is preferably used as the thermoplastic resin a, and further, the non-light diffusion layer A is preferably a stretched film.

As the crystalline polymer compound used as the non-light diffusion layer A, polyester resins such as polyethylene terephthalate, polyethylene-2, 6-naphthalate and the like are preferably used. Further, as the stretched film, a film stretched uniaxially or biaxially can be used, and it is particularly preferable that the film is stretched biaxially.
By laminating such a stretched film, the mechanical properties and heat resistance of the light diffusing film can be improved.

One of the preferable means for controlling the surface haze to 10% or less is to use a stretched film for the non-light diffusion layer A. Since it is substantially non-light diffusing, it does not contain fine particles or the like that precipitate on the surface, so that the surface is smooth and a surface haze of 10% or less can be easily achieved.

Another preferable means is to remove any surface irregularities by laminating an arbitrary thermoplastic resin on the surface, and then press-heating and pressing at a temperature higher than the glass transition temperature of the thermoplastic resin using a flat heating surface. The surface haze can be easily reduced to 10% or less.

The light diffusion layer B of the present invention comprises at least a thermoplastic resin b and a light diffusion component. Thermoplastic resin b
As the resin, the resins mentioned for the thermoplastic resin a can be used. Further, as the light diffusion component, for example, glass,
Inorganic fine particles such as silica, barium sulfate, titanium oxide, magnesium sulfate, magnesium carbonate, and calcium carbonate; or organic cross-linked fine particles made of an acrylic resin, an organic silicone resin, a polystyrene resin, a urea resin, a formaldehyde condensate, a fluororesin, or the like; Polymethyl pentene, polypropylene, polyolefin resins represented by polyethylene, polyethylene terephthalate, polyethylene
Examples of the resin include polyester resin represented by 2,6-naphthalate and the like, resin fine particles (islands having a sea-island structure) made of acrylic resin, bubbles, and the like. Among these, it is preferable to use resin fine particles or bubbles dispersed in the thermoplastic resin b in an island shape as the light diffusing component.

As described above, in the case of the internal diffusion type light diffusing film composed of the thermoplastic resin b and the light diffusing component, by satisfying the above expression (1) or (2), sufficient light diffusion can be obtained. Will show. When the above formula is not satisfied, for example, when placed on the light guide plate of the backlight, it is apparent that the light engraved on the light guide plate has poor light diffusibility such that the dots formed on the light guide plate are seen through.

In order to form an internal diffusion type light diffusing film satisfying the above formula (1) or (2), for example, a thermoplastic resin b and a light diffusing component constituting the light diffusing layer B shown below are used. , The ratio of the light diffusion component in the light diffusion layer B, the thickness of the light diffusion layer B, and the like.

It is important that the difference in the refractive index between the thermoplastic resin b and the light diffusing component is different. By making these have different refractive indices, light refraction occurs at the interface and incident light is scattered. The absolute value of the refractive index difference between the thermoplastic resin b and the light diffusing component is preferably 0.01 or more and 0.4 or less, more preferably 0.05 or more and 0.4 or less, and most preferably 0.1 or more and 0.4 or less. preferable.

The shape of the light diffusing component is preferably spherical. The term “spherical” as used herein refers to a spherical body, and does not necessarily need to be a true sphere, but in order to obtain uniform scattering, a true sphere having no anisotropy in shape is preferable.

The average particle size of the light diffusing component is 1 to 50.
μm is preferable, 1 to 30 μm is more preferable,
20 μm is most preferred. In order to realize high transmittance, which is an important factor in the performance of the light diffusing film, it is important to consider the wavelength of light and the average particle size of the light diffusing particles, and control the size within the above range. It is realized by doing. Further, the average particle size of the light diffusing component is 1
When the diameter is smaller than μm, coloring of the transmitted light may be observed. Therefore, by controlling the transmittance within the above range, it is possible to simultaneously suppress the coloring of the transmitted light.

The average particle size of the light diffusing component can be measured as follows. The diffusion film is observed from the surface direction with a transmission optical microscope, 200 light diffusion particles appearing in the visual field are randomly selected, and the average particle diameter is measured using an image processing device or the like. Even if it is not a perfect circle, after determining the area, it is converted to a perfect circle to determine the particle size.

The volume fraction of the light diffusing component in the light diffusing layer B is preferably 50% or less.
0% is more preferred. If it exceeds 50%, the transmittance of the light-diffusing film may be reduced, or the flow characteristics of the resin may be adversely affected.

The thickness of the light diffusion layer B is preferably from 10 to 500 μm in consideration of the use of the thin film and workability.
0 to 300 μm is more preferable, and 30 to 200 μm is still more preferable. Further, the resin layer on the surface layer is usually 1 to 5
It is selected in the range of 0 μm, preferably 2 to 30 μm, more preferably 3 to 20 μm.

The combination of the above conditions is a preferred embodiment that can be achieved by the present invention, but the above formula (1) or (2)
As an example of a combination range satisfying, for example, the absolute value of the refractive index difference between the thermoplastic resin b and the light-diffusing component is 0.1 or more.
2 or less, the average particle diameter of the light diffusion component is 5 to 15 μm, and the volume fraction of the light diffusion component in the light diffusion layer B is 10 to 30.
%, And the thickness of the light diffusion layer is 50 to 100 μm. Of course, other than the range given as an example, it is possible to satisfy Expression (1) or Expression (2), and in that case, it can be achieved by taking the following measures. Considering the above example as a reference, when the difference in the refractive index between the thermoplastic resin b and the light diffusing component is reduced, the volume fraction of the light diffusing component should be increased or the film thickness of the light diffusing layer B should be increased. Can be achieved. If the difference in refractive index increases,
Conversely, it can be achieved by reducing the volume fraction of the light diffusion component or reducing the thickness of the light diffusion layer B. Similarly, regarding changes in the volume fraction of the light diffusion component and the film thickness of the light diffusion layer B, the other factors (other than the average particle size of the light diffusion component) are inversely increased or decreased with respect to the increase or decrease in the respective values. Can be achieved.

In the present invention, it is preferable that the non-light-diffusing layer A and the light-diffusing layer B are laminated without interposing an adhesive layer. When bonded via an adhesive layer, changes in film performance such as a decrease in transmittance or a change in light diffusivity due to the adhesive layer may occur, and poor bonding may result in poor flatness and in-plane uniformity. This is not preferable because of the possibility of adverse effects.

The method for producing the light diffusing film of the present invention is described below.

As a method for realizing the three-layer lamination of A / B / A of the non-light-diffusing layer A and the light-diffusing layer B, the non-light-diffusing layer A is provided on both sides of the light-diffusing layer B formed in a sheet shape in advance. A method in which a resin is poured and formed, and a transparent layer A previously formed in a sheet shape is laminated on both surface layers of a light diffusion layer B previously formed in a sheet shape via an adhesive layer or by thermocompression bonding. Or a method in which the non-light-diffusing layer A and the light-diffusing layer B are co-extruded and laminated at once. Among these methods, a method in which the non-light diffusion layer A and the light diffusion layer B are co-extruded and laminated at a stretch is most preferably used. By co-extrusion, a film with excellent adhesion can be obtained without interposing an adhesive layer, and since it can be laminated at a stretch by co-extrusion, the process of preparing in advance offline or applying or applying an adhesive layer Since the step of attaching is omitted, high productivity can be realized.

In the present invention, it is preferable to use a stretched film as the non-light diffusing layer A. However, by laminating the non-light diffusing layer A and the light diffusing layer B made of a crystalline polymer compound by co-extrusion, It is possible to laminate the stretched film by stretching and heat treatment as it is after extrusion, and it is possible to obtain a light diffusing film excellent in high light transmittance, high light diffusion, surface flatness and mechanical properties and heat resistance at once in line. It can be said that this is an excellent manufacturing method.

The total light transmittance of the light diffusing film of the present invention is preferably 60% or more, more preferably 70% or more, and most preferably 80% or more. If the total light transmittance is less than 60%, the brightness may be insufficient when the light diffusing film is incorporated into a display,
In order to obtain high luminance, low power consumption cannot be expected.

The total light transmittance and haze can be measured by an integrating sphere light transmittance measuring device (haze meter) in accordance with Japanese Industrial Standard JIS K7105 “Testing method for optical properties of plastics”.

Various additives can be added to the light-diffusing film of the present invention as long as the effects of the present invention are not lost. The layer to which the additive is added may be either a light diffusion layer or a non-light diffusion layer. Examples of additives to be added include, for example, pigments, dyes, fluorescent whitening agents, antioxidants, heat-resistant agents, light-proofing agents, weathering agents, antistatic agents, release agents, compatibilizers, and the like. it can. Further, it is also possible to form an antistatic layer, a hard coat layer and the like on the surface of the light diffusing film of the present invention.

The light diffusing film of the present invention is suitably used for a backlight of a liquid crystal display, a lighting device, and the like.

(Evaluation Method) Haze The haze was measured using a fully automatic direct-reading haze computer HGM-2DP manufactured by Suga Test Instruments Co., Ltd. In order to measure the surface haze, the following molding work was performed.

"Molding of film surface" Condensation reaction type silicone S manufactured by Dow Corning Toray Silicone Co., Ltd.
H9555 and a dedicated curing agent SH9555K in a weight ratio of 9:
1 and degassed while stirring under vacuum for 20 minutes. Subsequently, the mixed silicone rubber is applied on a PET film of 100 μm, and the surface of the sample to be molded is placed thereon, and is pressed down firmly with an ink roller. Heat treatment at 60 ° C. for 30 minutes to cure the silicone rubber. The sample is peeled off, and a photopolymerizable composition having the following composition is poured into a mold.

[Photopolymerizable composition] Acrylic monomer
KAYARAD HX-620 (Nippon Kayaku Co., Ltd.) 5
0 parts, chlorinated polyester oligomer Evecryl EB
-584 (manufactured by Daicel UCB), 50 parts
Photopolymerization initiator Irgacure 184 (Ciba-Geigy Specialty Chemicals) 10 parts, Photopolymerization accelerator
Kayacure EPA (manufactured by Nippon Kayaku Co., Ltd.) 2 parts.

After injecting the photopolymerizable composition, the composition is placed on a 100 μm PET substrate and pressed with an ink roller. Then, it is cured by exposure to 1 J / cm ² . The silicone mold is peeled off to obtain the desired surface model.
By measuring the haze of the obtained model, the surface haze is measured.

The internal haze was measured by immersing the film in a quartz cell into which a standard solution having the same refractive index as the surface resin of each film was injected. The surface haze is 10% or less, and the inner surface is sufficiently smooth so that the surface is sufficiently smooth and the light diffusing property does not decrease even when the surface is adhered, in order to impart another function to the surface and to bond it to another member. A sample having a haze of 50% or more was evaluated as ○, and the others were evaluated as ×.

B. Diffusion coefficient α, β Halogen light source device M manufactured by Moritex Corporation as a light source
Straight light guide M using HF-D100LR
SG4-1100S and the condenser lens MLS-60P were connected to form a parallel light beam having a diameter of about 5 mm. The measurement was performed using a luminance meter LS-110 manufactured by Minolta Co., Ltd. as a luminance meter.

[Measurement of diffusion coefficient α] Irradiation of incident light perpendicular to the film surface (the irradiated part is defined as a measurement point)
Then, the luminance of the emitted light beam is measured on the opposite surface using a luminance meter. L ₀ is measured at a position where the measurement point is perpendicular to the film surface (parallel to the incident light), and L ₃₀ is a position where the measurement point is inclined at 30 degrees with respect to the film surface (3 ° with respect to the incident light).
(0 ° tilted position). L ₀ and L obtained
Calculate α from ₃₀ . ○, −
In the case of 1.5 or less, it was evaluated as ×.

[Measurement of Diffusion Coefficient β] An incident light beam is irradiated on the film surface at an angle of 30 degrees (the irradiated portion is set as a measurement point), and the luminance of the outgoing light beam is measured on the opposite surface using a luminance meter. . L ′ ₀ is measured at a position where the measurement point is perpendicular to the film surface, and L ′ ₃₀ is measured at a position where the measurement point is parallel to the incident light beam (a position inclined by 30 degrees with respect to the film surface). Α is calculated from the obtained L ′ ₀ and L ′ ₃₀ . 1
The case where the size was smaller was rated as ○, and the case where the size was greater than or equal to x.

C. Dot concealing property When the film was placed on the light guide plate of the backlight and the light was turned on, it was visually confirmed whether or not the dot pattern engraved on the light guide plate could be recognized. When the dot shape could not be recognized and the emission was uniform and good, ○ was given. When the outline of the dot was clearly seen, × was given.

D. Lamination characteristics As a model for lamination with other members, the rate of change in haze when laminated with a transparent acrylic plate (1 mm thick) was measured. The haze here indicates the total haze Ht. As a bonding method, here, an acrylic adhesive was applied to a transparent acrylic plate, the film was brought into close contact, and the film was pressed from the film side with an ink roller and bonded. When the rate of change before and after laminating the haze was within 5%, it was evaluated as ○, and when it exceeded 5%, as X. In addition, at the time of bonding, a case where air bubbles easily enter between the film and the acrylic plate, or a case where it was difficult to bond flatly, were evaluated as x.

E. Total light transmittance The total light transmittance was measured at the same time as the haze measurement using a full automatic direct reading haze computer HGM-2DP manufactured by Suga Test Instruments Co., Ltd.

[0060]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not necessarily limited to these.

(Example 1) In a main extruder, an isophthalic acid component was added to polyethylene terephthalate (PET) in an amount of 23 m.
ol% copolymerized polyester resin (melting point 190 ° C,
Glass transition temperature 70 ° C., 80% by weight, 20% by weight of polymethylpentene (melting point: 235 ° C.), and 0.5% by weight of polyethylene glycol were mixed, and a separate extruder was used to feed PET (melting point: 265). ℃)
A three-layer co-extrusion having PET on both surface layers was performed by a predetermined method, and cooled on a mirror-finished cast drum by an electrostatic application method to produce a three-layer laminated sheet. The thus obtained three-layer laminated sheet is stretched three times in the longitudinal direction at 90 ° C., then stretched 3.3 times in the width direction at 95 ° C. through a preheating zone of 90 ° C. with a tenter, and further stretched at 230 ° C. 2
Heat treatment was performed for 0 second to obtain a light diffusing film having a total thickness of 80 μm. The thickness of the surface PET layer of the obtained light diffusing film was 8 μm on one side. The inner layer had a sea-island structure in which a polyester resin obtained by copolymerizing an isophthalic acid component at 23 mol% constituted a sea component and polymethylpentene constituted an island component. The island component was spherical and had an average particle size of about 10 μm. Table 1 shows the results. Surface haze 0.7%, α = −
0.6> −1.5 and β = 0.58 <1. The dot concealing property, that is, the diffusing property was excellent.

Even when the light diffusing film of the present invention is bonded to an acrylic plate, the rate of change in total haze is 1%.
Less, and the lamination characteristics were excellent. As described above, it was easy to make the light diffusing film of the present invention multifunctional by bonding it to another member. A light-diffusing film having high light-diffusing property and high light-transmitting property, which is excellent in practical use, utilizing the internal diffusivity was obtained.

Example 2 A chip obtained by mixing 80% by weight of a polyester resin obtained by copolymerizing 23 mol% of an isophthalic acid component with PET, 20% by weight of polymethylpentene, and 0.5% by weight of polyethylene glycol in Example 1. Was supplied, and a single film consisting of only the internal diffusion layer was extruded without laminating PET on the surface layers on both sides, and cooled on a mirror-finished cast drum by an electrostatic application method to produce a sheet. Since the sheet melted after the stretching and heat treatment steps, it was manufactured as an extruded plate without the stretching step. The film thickness was set to 64 μm, which is the same as that of the internal diffusion layer in Example 1. Thereafter, a polymethyl methacrylate resin previously processed into a sheet shape was thermocompression-bonded to a surface layer on both sides of the extruded plate by a hot press to produce a three-layer laminated sheet. The film thickness of the surface polymethyl methacrylate layer was 8 μm as in Example 1. Table 1 shows the results
Shown in

Even when bonded to an acrylic plate, the rate of change in total haze was less than 1%, and the bonding characteristics were excellent. Thus, the light-diffusing film of the present invention was easily multifunctional by bonding to another member. High light diffusivity that is practically excellent, utilizing the internal diffusivity,
In addition, a light diffusing film having high light transmittance was obtained.

Example 3 85 parts by weight of a polycarbonate resin, silica fine particles 15 having an average particle diameter of 8 μm were placed in an extruder.
A chip mixed with parts by weight is supplied, and a non-added polycarbonate resin is separately supplied to a sub-extruder, and co-extrusion of a molten three-layer having no added polycarbonate on both surface layers by a predetermined method is performed. It was cooled on a mirror-finished cast drum by an application method to produce a three-layer sheet. The thickness of the diffusion layer was 90 μm, and the surface layer was 10 μm on one side. Table 1 shows the results. In addition, even when bonded to an acrylic plate, the rate of change in total haze was less than 1%, indicating excellent bonding characteristics. As described above, the light-diffusing film of the present invention can be easily multifunctional by bonding to another member. A light-diffusing film having high light-diffusing property and high light-transmitting property, which is excellent in practical use, utilizing the internal diffusivity was obtained.

(Comparative Example 1) A chip obtained by mixing 80% by weight of a polyester resin obtained by copolymerizing 17% by mol of isophthalic acid with PET and 20% by weight of polypropylene was supplied to a main extruder. The PET was supplied, co-extrusion of the molten three layers having PET on both surface layers by a predetermined method, and cooling on a mirror-finished cast drum by an electrostatic application method to produce a three-layer laminated sheet. The three-layer laminated sheet thus obtained was stretched three times in the longitudinal direction at 90 ° C., and then stretched 3.3 times in the width direction at 95 ° C. through a preheating zone of 90 ° C. with a tenter.
C. for 20 seconds to obtain a light diffusing film having a total thickness of 50 .mu.m. The thickness of the surface PET layer was 5 μm on one side. Table 1 shows the results.

The surface haze was 1% and the internal haze was 90.
2%. Regardless of the surface morphology, it can be said that the film utilizes the diffusivity inside the film. However, α = −1.61 <−1.5, β = 1.52> 1, and with respect to the dot concealing property, the dots of the backlight are observed through, and the concealing property is poor, that is, the diffusing property is poor. It became a film. Also, even when bonded to an acrylic plate, the rate of change in total haze was less than 1%, and the bonding characteristics were excellent.

Comparative Example 2 A chip obtained by mixing 80% by weight of a polyester resin obtained by copolymerizing 23% by mole of isophthalic acid component with PET, 20% by weight of polymethylpentene, and 0.5% by weight of polyethylene glycol in Example 1. Was supplied, and a single film consisting of only the internal diffusion layer was extruded without laminating PET on the surface layers on both sides, and cooled on a mirror-finished cast drum by an electrostatic application method to produce a sheet. Since the sheet melted after the stretching and heat treatment steps, it was manufactured as an extruded plate without the stretching step. The film thickness was set to 64 μm, which is the same as that of the internal diffusion layer in Example 1. Table 1 shows the results.

Α = −0.72> −1.5, β = 0.2 <
1 and a film excellent in dot hiding properties was obtained, but the surface haze was as high as 32.5%, and the haze change ratio before and after bonding was as high as 6.7%. In addition, it was difficult to perform a smooth bonding without bubbles. The smoothness of the surface layer is insufficient, so that it is difficult to impart other functions to the surface and to adhere and bond with other members.

Comparative Example 3 A coating material for a light diffusing layer having the following composition was applied to one surface of a 100 μm transparent PET substrate so as to have a dry film thickness of 15 μm to obtain a light diffusing film.

[Coating composition for light diffusing layer] 100 parts of polyester resin Byron 200 (manufactured by Toyobo Co., Ltd.), 70 parts of acrylic resin bead polymer MBX-8 (manufactured by Sekisui Plastics Co., Ltd.), 200 parts of toluene, 200 parts of methyl ethyl ketone.

Table 1 shows the results. It was a diffusion film utilizing almost only the surface shape. Lamination was extremely difficult, and air bubbles between the film and the acrylic plate could not be removed. Further, the haze change rate was 35.4%, which was very high. It was difficult to impart other functions to the surface and to adhere and attach to other members.

[0073]

[Table 1]

[0074]

According to the present invention, a light-diffusing film having excellent light-transmitting and light-diffusing properties can be obtained without utilizing the light-diffusing effect of the surface shape.

The light-diffusing film of the present invention has a smooth surface and high light-transmitting and high-light diffusing properties.
It is possible to provide a high-luminance and uniform high-quality image.

Claims (6)

[Claims] 1. A parallel ray of visible light is incident perpendicularly to a film surface, and a surface opposite to the incident portion is set as a measurement portion.
When performing luminance measurement of a light beam emitted from the measurement site, when the luminance obtained in the vertical direction is L ₀ , and the luminance obtained in a direction inclined by 30 degrees from the vertical direction around the measurement site is L ₃₀ , The diffusion coefficient α defined by the following equation (1) satisfies −1.5 <α, and the surface haze is 1
A light diffusing film having a content of 0% or less. 2. A method according to claim 1, wherein parallel light rays of visible light are incident at an angle of 30 degrees with respect to the direction perpendicular to the film surface, and the luminance of light rays emitted from the measurement area is measured using the opposite surface of the incident area as a measurement area. Let L ′ ₀ be the luminance obtained in the vertical direction, and L ′ ₃₀ be the luminance obtained in a direction parallel to the incident light beam and inclined by 30 degrees from the vertical direction with the measurement site as the center. The diffusion coefficient β defined is β
The light diffusing film according to claim 1, wherein <1 is satisfied. 3. A light-diffusing layer A comprising at least a thermoplastic resin and a light-diffusing layer B comprising at least a thermoplastic resin and a light-diffusing component, wherein the cross-sectional structure is A / B / A. 3. The light diffusing film according to 2. 4. The light-diffusing film according to claim 3, wherein the non-light-diffusing layer A is a stretched film made of a crystalline polymer compound. 5. The light diffusing film according to claim 3, wherein the non-light diffusing layer A and the light diffusing layer B are laminated without interposing an adhesive layer. 6. The light diffusing film according to claim 1, wherein the total light transmittance is 60% or more.