阅读说明：本技术 背光单元和液晶显示装置 (Backlight unit and liquid crystal display device ) 是由 波川启土 松野有希 于 2018-11-08 设计创作，主要内容包括：背光单元(1)包括导光片(21)和设在导光片(21)的表面的光扩散片(23)。将根据JIS K 7125在温度23℃、湿度50％的条件下测量出的导光片(21)与光扩散片(23)之间的静摩擦系数设为Y<Sub>1</Sub>,且将根据JIS K 7197测量出的导光片(21)的线膨胀系数与光扩散片(23)的线膨胀系数之差设为X时,下式(1)的关系成立。(1)：Y<Sub>1</Sub>≤1.00×10<Sup>－4</Sup>X<Sup>2</Sup>-3.43×10<Sup>－2</Sup>X+2.35。(The backlight unit (1) comprises a light guide sheet (21) and a light diffusion sheet (23) provided on the surface of the light guide sheet (21). The static friction coefficient between the light guide sheet (21) and the light diffusion sheet (23) measured at a temperature of 23 ℃ and a humidity of 50% according to JIS K7125 is Y 1 And the relationship of the following formula (1) is established when the difference between the linear expansion coefficient of the light guide sheet (21) and the linear expansion coefficient of the light diffusion sheet (23) measured according to JIS K7197 is X. (1): y is 1 ≤1.00×10 －4 X 2 ‑3.43×10 －2 X+2.35。)

1. A backlight unit comprising an optical member and a light diffusion sheet provided on a surface of the optical member, characterized in that:

the coefficient of static friction between the optical member and the light diffusion sheet measured at a temperature of 23 ℃ and a humidity of 50% according to JIS K7125 was Y₁，

And X is the difference between the linear expansion coefficient of the optical member and the linear expansion coefficient of the light diffusion sheet measured according to JIS K7197, the following formula (1) is satisfied,

[ equation 1]

Y₁≤1.00×10^－4X²-3.43×10^－2X+2.35 (1)。

2. A backlight unit comprising an optical member and a light diffusion sheet provided on a surface of the optical member, characterized in that:

the coefficient of static friction between the optical member and the light diffusion sheet measured at 65 ℃ and 95% humidity according to JIS K7125 was Y₂，

And X is the difference between the linear expansion coefficient of the optical member and the linear expansion coefficient of the light diffusion sheet measured according to JIS K7197, the following formula (2) is satisfied,

[ formula 2]

Y₂≤9.00×10^－5X²-3.51×10^－2X+3.12 (2)。

3. The backlight unit according to claim 1 or 2, wherein:

the optical component is a light guide sheet.

4. A liquid crystal display device, characterized in that:

comprises a liquid crystal display panel, a polarizing plate and a backlight unit,

the polarizing plate is arranged on the surface of the liquid crystal display panel,

the backlight unit according to any one of claims 1 to 3 arranged opposite to the polarizing plate.

Technical Field

The present invention relates to a backlight unit in a liquid crystal display device and a liquid crystal display device.

Background

In recent years, flat panel displays such as liquid crystal displays and plasma displays have been widely used as display devices because of space saving and high definition. Among them, the liquid crystal display is drawing attention because it is more power-saving and clearer, and research and development of the liquid crystal display are being carried out continuously.

The liquid crystal display device is, for example, a non-emissive display device having a liquid crystal display panel composed of a Thin Film Transistor (TFT) substrate and a Color Filter (CF) substrate arranged to face each other, and a backlight provided on the back side of the liquid crystal display panel. In the CF substrate, a colored layer colored in red, green, or blue, for example, is provided on each sub-pixel constituting a pixel.

A backlight unit of an edge light type (edge light type) or a direct type is attached to the lower surface side of the liquid crystal layer as a backlight. Such an edge-light type backlight unit is disclosed, for example, by including a light source, a light guide plate having a square plate shape and arranged so that an end portion thereof extends along the light source, a light diffusion plate provided on a surface side of the light guide plate and having a light diffusion function mainly, and a prism sheet provided on a surface side of the light diffusion plate and having a refraction function of refracting toward a normal direction side (for example, refer to patent document 1).

Patent document 1: japanese laid-open patent publication No. 2016-95380

Disclosure of Invention

Technical problems to be solved by the invention

Here, in the conventional backlight unit, there are the following problems: since the light diffusion sheet and the light guide sheet have different amounts of expansion and contraction, the light diffusion sheet having a small amount of expansion and contraction bends (wrinkles) due to friction between the light diffusion sheet and the light guide sheet caused by expansion and contraction of the light guide sheet having a large amount of expansion and contraction.

More specifically, the base layer of the light diffusion sheet is mainly composed of polyethylene terephthalate (PET) resin, and the light guide sheet is mainly composed of Polycarbonate (PC) resin or acrylic resin having a higher linear expansion coefficient than polyethylene terephthalate resin. Therefore, there are the following problems: when the light diffusion sheet and the light guide sheet are heated and cooled, a difference in the amount of expansion and contraction occurs between the light diffusion sheet and the light guide sheet, and the light diffusion sheet is bent (wrinkled) due to expansion and contraction of the light guide sheet having a large amount of expansion and contraction.

The present invention has been made to solve the above problems, and an object of the present invention is to: provided is a backlight unit capable of suppressing the occurrence of warping in a light diffusion sheet.

Technical solution for solving technical problem

In order to achieve the above object, a backlight unit of the present invention includes an optical member and a light diffusion sheet provided on a surface of the optical member, the backlight unit characterized in that: the coefficient of static friction between the optical member and the light diffusion sheet measured at 23 ℃ and 50% humidity according to JIS K7125 was Y₁And the relationship of the following formula (1) is established, assuming that X represents the difference between the linear expansion coefficient of the optical member measured according to JIS K7197 and the linear expansion coefficient of the light diffusion sheet.

[ equation 1]

Y₁≤1.00×10^－4X²-3.43×10^－2X+2.35 (1)

Further, another backlight unit of the present invention includes an optical member and a light diffusion sheet provided on a surface of the optical member, the backlight unit being characterized in that: the coefficient of static friction between the optical member and the light diffusion sheet measured at 65 ℃ and 95% humidity according to JIS K7125 was Y₂And X represents the difference between the linear expansion coefficient of the optical member and the linear expansion coefficient of the light diffusion sheet measured according to JIS K7197,the relationship of the following expression (2) holds.

[ formula 2]

Y₂≤9.00×10^－5X²-3.51×10^－2X+3.12 (2)

Effects of the invention

According to the present invention, it is possible to suppress the light diffusion sheet from being bent due to expansion and contraction of an optical member, such as a light guide sheet, disposed to face the light diffusion sheet.

Drawings

Fig. 1 is a cross-sectional view of a liquid crystal display device according to an embodiment of the present invention.

Fig. 2 is a sectional view for explaining a backlight unit in the liquid crystal display device according to the embodiment of the present invention.

FIG. 3 is a graph showing the relationship between the coefficient of static friction and the difference in linear expansion coefficient under normal temperature conditions (temperature 23 ℃ C., humidity 50%) and the occurrence of warpage.

Fig. 4 is a graph for explaining the relationship between the difference between the coefficient of static friction and the coefficient of linear expansion under high temperature conditions (temperature 65 ℃ c., humidity 95%) and the occurrence of warpage.

Detailed Description

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 is a sectional view of a liquid crystal display device according to an embodiment of the present invention, and fig. 2 is a sectional view for explaining a backlight unit in the liquid crystal display device according to the embodiment of the present invention.

As shown in fig. 1, the liquid crystal display device 1 includes a liquid crystal display panel 2, a first polarizing plate 3 (hereinafter, sometimes simply referred to as "polarizing plate 3"), a second polarizing plate 4 (hereinafter, sometimes simply referred to as "polarizing plate 4"), and a backlight unit 5. Among them, a plurality of pixels are arranged in a matrix on the liquid crystal display panel 2, the first polarizing plate 3 is attached to the front surface side (upper surface side in fig. 1, viewer side of the liquid crystal display device 1) of the liquid crystal display panel 2, the second polarizing plate 4 is attached to the back surface side (back surface side, lower surface side in fig. 1, opposite side to viewer side of the liquid crystal display device 1) of the liquid crystal display panel 2, and the backlight unit 5 is provided on the back surface side of the liquid crystal display panel 2.

As shown in fig. 1, the liquid crystal display panel 2 includes a TFT substrate 6 as a first substrate, a CF substrate 7 as a second substrate, a liquid crystal layer 8, and a seal member (not shown), the CF substrate 7 and the TFT substrate 6 are arranged to face each other, the liquid crystal layer 8 is provided between the TFT substrate 6 and the CF substrate 7, and the seal member is formed in a frame shape so as to bond the TFT substrate 6 and the CF substrate 7 to each other and seal the liquid crystal layer 8 between the TFT substrate 6 and the CF substrate 7.

The TFT substrate 6 and the CF substrate 7 are each formed in a rectangular plate shape. The liquid crystal display device 1 has a plurality of photo spacers (not shown) for limiting the thickness of the liquid crystal layer 8 (i.e., a cell gap).

The TFT substrate 6 includes, for example, an insulating substrate, a plurality of gate lines, a plurality of source lines, a plurality of TFTs, a protective film, a plurality of pixel electrodes, and an alignment film (all not shown). The insulating substrate is a glass substrate, a plastic substrate, or the like, a plurality of gate lines are provided on the insulating substrate so as to extend parallel to one another, a plurality of source lines are provided so as to extend parallel to one another in a direction orthogonal to the respective gate lines, a plurality of TFTs are provided at each portion where the respective gate lines intersect the respective source lines, that is, on the respective sub-pixels Pr, Pg, and Pb, a protective film is provided so as to cover the respective TFTs, a plurality of pixel electrodes are provided in a matrix on the protective film and are connected to the respective TFTs, respectively, and an alignment film is provided so as to cover the respective pixel electrodes.

The CF substrate 7 includes an insulating substrate such as a glass substrate or a plastic substrate, a common electrode provided on the insulating substrate, and an alignment film (not shown) provided so as to cover the common electrode.

The liquid crystal layer 8 is made of a nematic liquid crystal material having electro-optical characteristics, or the like.

< backlight Unit >

As shown in fig. 2, the backlight unit 5 is an edge-light type backlight unit. The backlight unit 5 includes a light guide sheet 21, a light source 22, a light diffusion sheet 23, and a prism sheet 24, the light guide sheet 21 guides light incident from an end surface to a front surface side, the light source 22 irradiates the end surface of the light guide sheet 21 with light, the light diffusion sheet 23 is superimposed on the front surface side of the light guide sheet 21, and the prism sheet 24 is provided on the front surface side of the light diffusion sheet 23. The backlight unit 5 further includes a reflection sheet 26, and the reflection sheet 26 is provided on the reverse side of the light guide sheet 21.

The light diffusion sheet 23 has a function of diffusing light rays incident from the back surface side and condensing (condensing and diffusing) the light rays toward the normal direction side, and the prism sheet 24 has a function of refracting light rays incident from the back surface side toward the normal direction side. The reflection sheet 26 has a function of reflecting the light emitted from the back surface side of the light guide sheet 21 toward the front surface side and then incident again on the light guide sheet 21.

< prism sheet >

The prism sheet 24 is disposed opposite to the polarizing plate 4. The prism sheet 24 is formed of a transparent synthetic resin as a main component, particularly a colorless and transparent synthetic resin, because it is required to transmit light. The prism sheet 24 includes a base material layer 35 and a projection array including a plurality of convex prism sections 36 laminated on the surface of the base material layer 35. The convex prism portions 36 are laminated in stripes on the surface of the base material layer 35. The convex prism portion 36 is a triangular prism having a back surface contacting the surface of the base material layer 35.

The lower limit of the thickness of the prism sheet 24 (height from the back surface of the base material layer 35 to the apexes of the ridge prism portions 36) is preferably 35 μm, and more preferably 50 μm. On the other hand, the upper limit of the thickness of the prism sheet 24 is preferably 200 μm, and more preferably 180 μm.

The lower limit of the center-to-center pitch P (see fig. 2) of the ridge prism portions 36 of the prism sheet 24 is preferably 12 μm, and more preferably 20 μm. On the other hand, the upper limit of the center-to-center pitch P of the ribbed prism portions 36 of the prism sheet 24 is preferably 100 μm, and more preferably 60 μm.

The apex angle of the convex prism portion 36 is preferably 85 ° to 95 °. The lower limit of the refractive index of the ribbed prism portion 36 is preferably 1.5, and more preferably 1.55. On the other hand, the upper limit of the refractive index of the ribbed prism portion 36 is preferably 1.7.

The prism sheet 24 may be formed of two prism sheets, or two prism sheets may be used by laminating one prism sheet.

< light diffusion sheet >

The light diffusion sheet 23 is provided on the surface of the light guide sheet 21, and includes a base material layer 37, a light diffusion layer 38 provided on the front surface side of the base material layer 37, and an anti-sticking layer 39 provided on the reverse surface side of the base material layer 37.

The base layer 37 of the light diffusion sheet 23 is formed mainly of a transparent synthetic resin, particularly a colorless and transparent synthetic resin, because it needs to transmit light. The main component of the base layer 37 is not particularly limited, and examples thereof include polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, weather-resistant vinyl chloride, and polyimide.

The light diffusion layer 38 of the light diffusion sheet 23 has a light diffusion material and a binder thereof. The light diffusing material is a particle having a property of diffusing light, and is roughly classified into an inorganic filler and an organic filler. Examples of the inorganic filler include silica, aluminum hydroxide, alumina, zinc oxide, barium sulfide, magnesium silicate, and a mixture thereof. Specific examples of the organic filler include acrylic resin, acrylonitrile resin, polyurethane, polyvinyl chloride, polystyrene, polyamide, polyacrylonitrile, and the like.

The shape of the light diffusing material is not particularly limited, and examples thereof include spherical, cubic, needle-like, rod-like, spindle-like, plate-like, scaly, and fibrous, and among them, spherical beads having excellent light diffusibility are preferable.

The anti-adhesion layer 39 is formed by dispersing resin beads 40 into a resin matrix. The resin beads 40 are arranged on the opposite side of the base material layer 37 in a dispersed manner. Since the resin beads 40 are arranged loosely, the adhesion-preventing layer 39 has a plurality of convex portions formed by the resin beads 40 and a flat portion where the resin beads 40 are not present. The anti-sticking layer 39 is in contact with the light guide sheet 21 provided on the reverse surface side in a state where the plurality of convex portions are scattered, not in contact with the entire reverse surface, thereby preventing sticking and suppressing unevenness in luminance of the liquid crystal display device 1.

The resin forming the anti-sticking layer 39 is not particularly limited, and examples thereof include acrylic urethane (acrylic urethane) based resin, acrylic resin, acrylonitrile resin, polyurethane, polyvinyl chloride, polystyrene, polyamide, polyacrylonitrile, and the like.

Among them, from the viewpoint of preventing generation of the bright point defect, it is preferable to use an acrylic urethane resin to impart flexibility to the anti-sticking layer 39. The bright spot defect is caused by damage to the surface of light guide sheet 21 due to friction between light diffusion sheet 23 and light guide sheet 21. Here, the acrylic urethane resin refers to an acrylic resin having an acrylic skeleton and a urethane skeleton, and for example, a resin obtained by crosslinking an isocyanate resin and an acrylic polyol resin can be used.

< light guide plate >

The light guide sheet 21 is a sheet-like optical member that transmits light emitted from the light source 22 to the inside and emits the light from the surface. The light guide sheet 21 may be formed in a substantially wedge shape in cross section or may be formed in a substantially flat plate shape.

Since light guide sheet 21 needs to transmit light, it is formed mainly of a transparent resin, particularly a colorless and transparent resin. The main component of light guide sheet 21 is not particularly limited, and examples thereof include a polycarbonate resin excellent in transparency and strength, and a synthetic resin such as an acrylic resin excellent in transparency and scratch resistance. Among these, the main component of the light guide sheet 21 is preferably a polycarbonate resin. Since the polycarbonate resin has excellent transparency and a high refractive index, total reflection easily occurs at the interface between light guide sheet 21 and an air layer (a layer formed in the gap between light guide sheet 21 and light diffusion sheet 23 provided on the front surface side of light guide sheet 21 and a layer formed in the gap between light guide sheet 21 and reflection sheet 26 provided on the back surface side of light guide sheet 21), and light can be efficiently propagated. Further, since the polycarbonate resin has heat resistance, it is difficult for the light source 22 to generate heat and thus to deteriorate.

< light source >

Light source 22 is disposed such that the irradiation surface faces (or abuts) the end surface of light guide sheet 21. various light sources can be used as light source 22, for example, light emitting diode (L ED).

< reflective plate >

Examples of the reflection sheet 26 include a white sheet in which a filler is dispersed in a base resin such as polyester, and a mirror sheet in which a metal such as aluminum or silver is deposited on the surface of a film made of polyester to improve the specular reflection.

Here, in the conventional backlight unit, as described above, there are the following problems: the light diffusion sheet having a small amount of expansion and contraction bends due to the expansion and contraction of the light guide sheet having a large amount of expansion and contraction.

The inventors of the present invention paid attention to this point, and found a condition for suppressing the light diffusion sheet 23 from being bent by reducing the friction between the light diffusion sheet 23 and the light guide sheet 21 and suppressing the influence of expansion and contraction of the light guide sheet 21 having a large expansion and contraction amount.

More specifically, the backlight unit 5 of the present invention is characterized in that: the coefficient of static friction between the light guide sheet 21 and the light diffusion sheet 23 is Y₁And the linear expansion coefficient α of the light guide sheet 21₁Linear expansion coefficient α with the light diffusion sheet 23₂The difference (i.e., α)₁－α₂) When X is defined, the relationship of the following expression (3) is established.

[ formula 3]

Y₁≤1.00×10^－4X²-3.43×10^－2X+2.35 (3)

The "static friction coefficient Y" of the formula (3)₁"is a value measured according to JIS K7125 under the conditions of a temperature of 23 ℃ and a humidity of 50%. The "linear expansion coefficient" is a value measured according to JIS K7197.

Further, the static friction coefficient Y between the light guide sheet 21 and the light diffusion sheet 23 is reduced₁And the coefficient of static friction Y is determined₁By setting the relationship of the guaranteed expression (3) within the range, even if the amount of expansion and contraction of the light guide sheet 21 and the amount of expansion and contraction of the light diffusion sheet 23 are greatly different (that is, the linear expansion coefficient α of the light guide sheet 21)₁Linear expansion coefficient α with the light diffusion sheet 23₂A large difference in the difference), the light diffusion sheet 23 can be suppressed from being bent.

Further, the inventors of the present invention also paid attention to the fact that the backlight unit 5 is exposed to high-temperature and high-humidity conditions for a long time when the liquid crystal display device 1 is used for a long time, and found conditions for suppressing the occurrence of warping of the light diffusion sheet 23 under high-temperature and high-humidity conditions (temperature: 65 ℃ C., humidity: 95%).

More specifically, the backlight unit 5 of the present invention is characterized in that: the coefficient of static friction between the light guide sheet 21 and the light diffusion sheet 23 is Y₂And the linear expansion coefficient α of the light guide sheet 21₁Linear expansion coefficient α with the light diffusion sheet 23₂The difference (i.e., α)₁－α₂) When X is assumed, the relationship of the following expression (4) is established.

[ formula 4]

Y₂≤9.00×10^－5X²-3.51×10^－2X+3.12 (4)

The "static friction coefficient Y" of the formula (4)₂"is a value measured according to JIS K7125 under the conditions of a temperature of 65 ℃ and a humidity of 95%.

Further, the static friction coefficient Y between the light guide sheet 21 and the light diffusion sheet 23 is reduced₂And the coefficient of static friction Y is determined₂By setting the relationship of the guaranteed expression (4) within the range, even under high temperature and high humidity conditions (temperature: 65 ℃ C., humidity: 95%), the difference between the amount of expansion and contraction of the light guide sheet 21 and the amount of expansion and contraction of the light diffusion sheet 23 is large, and the light diffusion sheet 23 can be prevented from being bent.

The static friction coefficient Y can be reduced by increasing the hardness of the resin forming the anti-sticking layer 39 in contact with the light guide sheet 21₁、Y₂。

More specifically, since a load due to the weight of the light diffusion sheet 23, the prism sheet 24, and the liquid crystal display device 1 acts on the contact portion between the convex portion of the light diffusion sheet 23 and the light guide sheet 21, if the hardness of the resin forming the anti-sticking layer 39 is low, the convex portion of the light diffusion sheet 23 is deformed, and the contact area between the convex portion of the light diffusion sheet 23 and the light guide sheet 21 is increased. As a result, friction between light diffusion sheet 23 and light guide sheet 21 increases. In view of the above, the present embodiment employs the following configuration: by increasing the hardness of the resin forming the anti-sticking layer 39 in contact with the light guide sheet 21, deformation of the convex portion of the light diffusion sheet 23 is suppressed, thereby preventing the occurrence of a crack in the light diffusion sheetWhile reducing the coefficient of static friction Y₁、Y₂。

The "hardness" referred to herein is a hardness measured in accordance with ISO 14577-1 (indentation hardness test), and can be measured by a microhardness tester (product name: DUH-W201, manufactured by SIMADZU Co.).

In addition, in anti-sticking layer 39, the average radius of the plurality of projections formed by resin beads 40 is reduced, and the contact area between anti-sticking layer 39 and light guide sheet 21 is reduced, so that the static friction coefficient Y can be reduced₁、Y₂。

The "average radius of the convex portions" referred to herein means an average value of the radii of the convex portions obtained by arbitrarily extracting 10 convex portions.

In the embodiment, the light guide sheet 21 formed of polycarbonate, acrylic resin, or the like is described as an optical member disposed to face the light diffusion sheet 23, but any optical member may be used as long as the relationship of the above-described equations (3) and (4) is ensured and the light diffusion sheet 23 can be prevented from being bent.

Examples of the material include a glass sheet, a metal sheet such as an aluminum sheet, and a resin sheet. The resin sheet is made of polyethylene terephthalate resin, polyimide resin, polyvinyl chloride (PVC) resin, acrylonitrile-butadiene-styrene (ABS) resin, Polytetrafluoroethylene (PTFE) resin, polypropylene (PP) resin, or the like.

In the present embodiment, the anti-adhesion layer 39 in which the resin beads 40 are dispersed in the resin matrix is described as an example, but the following configuration may be adopted: in the case where the resin beads 40 are not used, the anti-adhesion layer 39 is given a shape or the base material layer 37 is given a shape. The shape in this case is not particularly limited, and examples thereof include spherical, cubic, needle-like, rod-like, spindle-like, plate-like, scaly, and fibrous shapes. In such a configuration, the relationship between the above equations (3) and (4) is established, and thus the light diffusion sheet 23 can be prevented from being bent.

The invention is illustrated below with reference to examples. The present invention is not limited to the above-described embodiments, and modifications and variations can be made to the following embodiments in accordance with the gist of the present invention, and should not be excluded from the scope of the present invention.

(example 1)

< measurement of Linear expansion coefficient >

A light diffusion sheet formed of an acrylic urethane resin (surface hardness: 0.67mN) and including an anti-sticking layer having protrusions on the surface and an optical sheet (optical member) formed of glass were prepared, and the linear expansion coefficients of the light diffusion sheet and the optical sheet were measured according to TMA (Thermo mechanical analyzer: JIS K7197).

More specifically, first, a sample having a length of 10mm × 5mm in both the mechanical axis (length) direction of the sheet (hereinafter referred to as "MD direction") and the direction orthogonal thereto (hereinafter referred to as "TD direction") was prepared, and then, the load applied to the probe (detection bar) measuring the change in length was adjusted to 4[ kPa ] using a linear expansion coefficient measuring device (manufactured by Hitachi High-Tech Science Corporation, trade name: TMA7100)]And raising the temperature from room temperature to 70 ℃ at a rate of 5 ℃/min in the atmosphere, and calculating a linear expansion coefficient α of each of the optical sheet and the light diffusion sheet of the sample from 30 ℃ to 70 ℃₁、α₂。

It should be noted that the linear expansion coefficient α₁、α₂Calculated according to the following formula (5).

[ formula 5]

Coefficient of linear expansion α₁(or α)₂)＝(1/L₀)×(L₇₀-L₃₀)/(70-30) (5)

Here, L₀Showing the length of the sample (sheet) [ mm]，L₃₀Shows the amount of change in length [ μm ] at 30 DEG C]，L₇₀Shows the amount of change in length [ μm ] at 70 deg.C]。

Then, the linear expansion coefficient α is calculated₁And α₂The difference X (i.e., α)₁－α₂). The results are shown in table 1.

< measurement of coefficient of static Friction >

Then, the anti-adhesion layer of the light diffusion sheet was brought into contact with the optical sheet at a temperature of 23 ℃ and a humidity of 50%Under the conditions, the static friction coefficient Y between the optical sheet and the light diffusion sheet was measured according to the friction coefficient test method (JIS K7125)₁。

More specifically, first, an optical sheet (63mm × 63mm) was placed on a light diffusion sheet (200mm × 150mm), and a weight (63mm × 63mm) of 200g was placed on the optical sheet, and then, a tensile tester (A) was used&Manufactured by D inc, trade name: RGT-1210) of the thickness of the optical sheet, the static friction coefficient Y at that time was measured by stretching the optical sheet at a speed of 100 mm/min₁。

Also, the static friction coefficient Y between the optical sheet and the light diffusion sheet was measured according to the friction coefficient test method (JISK 7125) under the conditions of a temperature of 65 ℃ and a humidity of 95%₂. The results are shown in table 1.

< evaluation of bending >

Next, the light diffusion sheet was evaluated for the presence of warping. More specifically, first, a 5-inch light diffusion sheet was placed on a 5-inch optical sheet and laminated, and prism sheets of the same size (composed of a prism sheet having a thickness of 75 μm and a prism sheet having a thickness of 100 μm, both prism sheets being laminated so that ridge prism portions thereof are orthogonal to each other) were placed on the light diffusion sheet. Then, the mixture was left to stand at 65 ℃ and 95% humidity for 72 hours.

Subsequently, the sheet was returned to room temperature (temperature 23 ℃ C., humidity 50%) and taken out in this state, and after two hours, the presence or absence of warping of the light diffusion sheet was confirmed. The presence or absence of the occurrence of the bending was confirmed by placing a light diffusion sheet alone on a smooth glass plate and visually observing the sheet from the front side under a fluorescent lamp. The results are shown in table 1.

(examples 2 to 13, comparative examples 1 to 11)

The linear expansion coefficient and the static friction coefficient were measured and evaluated for bending in the same manner as in example 1, except that at least one of the hardness of the resin forming the anti-sticking layer, the average radius of the convex portions, and the material of the optical sheet was changed as shown in table 1. The results are shown in table 1.

[ TABLE 1]

< correlation between coefficient of static friction and coefficient of linear expansion >

The coefficient of static friction between the optical sheet and the light diffusion sheet shown in Table 1 was determined as Y₁(temperature 23 ℃ C., humidity 50%) and coefficient of linear expansion α of optical sheet₁And linear expansion coefficient α of light diffusion sheet₂The relationship between the difference X is plotted as a graph against the coefficient of static friction Y₁The relationship between the linear expansion coefficient and the difference X and the occurrence of the bending were investigated. The above results are shown in fig. 3.

Similarly, the coefficient of static friction between the optical sheet and the light diffusion sheet shown in Table 1 was determined as Y₂(temperature 65 ℃ C., humidity 95%) and coefficient of linear expansion α of optical sheet₁And linear expansion coefficient α of light diffusion sheet₂The relationship between the difference X is plotted as a graph against the coefficient of static friction Y₂The relationship between the linear expansion coefficient and the difference X and the occurrence of the bending were investigated. The above results are shown in fig. 4.

As shown in Table 1 and FIG. 3, it is understood that the coefficient of static friction Y is measured at normal temperature (23 ℃ C., 50% humidity)₁When the difference X between the linear expansion coefficients satisfies the relationship of the above expression (3) (i.e., in the cases of examples 1 to 13), the light diffusion sheet is suppressed from being bent.

As shown in Table 1 and FIG. 4, it can be seen that the coefficient of static friction Y is obtained under high temperature conditions (temperature 65 ℃ C., humidity 95%)₂When the difference X between the linear expansion coefficients satisfies the relationship of the above expression (4) (that is, in the case of examples 1 to 13), the light diffusion sheet is suppressed from being bent.

Industrial applicability-

In summary, the present invention is particularly useful for a backlight unit used in a liquid crystal display device.

-description of symbols-

1 liquid crystal display device

2 liquid crystal display panel

3 first polarizing plate

4 second polarizing plate

5 backlight unit

6 TFT substrate

7 CF substrate

8 liquid crystal layer

21 light guide plate (optical component)

22 light source

23 light diffusion sheet

24 prism sheet

26 reflective sheet

35 base material layer

36 ribbed prism part

37 base material layer

38 light diffusion layer

39 anti-adhesion layer

40 resin beads