JP5048954B2 - Reflective sheet - Google Patents

Description

  The present invention relates to a reflective sheet, and more particularly to a reflective sheet suitably used for a backlight light source of a liquid crystal display or a display body. In particular, it can be used for a backlight unit having a thickness of 1 cm or less used for a thin panel using a light source such as an LED or a cold cathode tube such as a mobile phone, a PDA, or a notebook PC.

  In recent years, reflection sheets have been used in various fields, and in particular, many are used as main components of liquid crystal display devices such as mobile phones, personal computers, and televisions. Especially, liquid crystal display devices used for mobile phones. It is important to be able to reduce the thickness, power consumption, and weight. In addition, improvement of the display quality of the liquid crystal display device is also desired. For this purpose, it is necessary to supply a large amount of light to the liquid crystal portion. In order to satisfy the above requirements, it is necessary to increase the amount of light supplied from the light source, and there is a demand for a reflection sheet that has high reflection efficiency, high brightness, and diffuse reflection. In particular, due to the recent trend toward downsizing, the transition from the cathode tube to the LED and further reduction in the number of LEDs used are expected to improve the diffuse reflectance. Moreover, in order to exhibit the wavelength characteristic of LED more efficiently, improvement in the reflectance on the shorter wavelength side than the conventional 550 nm is desired.

  The backlight unit of the liquid crystal display device includes a method in which a light source is placed directly below the liquid crystal unit and a method in which the light source is placed beside a transparent light guide plate. In the former method, the reflector is arranged so as to reflect the light of the lamp below the liquid crystal unit, and in the latter method, the light guide plate beside the light guide plate and below the light guide plate so as to reflect the light. Placed in. All of these reflection sheets are required to have excellent punchability and bending workability in consideration of productivity as well as high reflection efficiency.

  Conventionally, as a material of the reflection sheet, a surface of a metal plate such as aluminum is bonded to a reflection sheet having a metal thin film layer mainly composed of silver, or a metal plate such as aluminum coated with a white pigment. A white polyethylene terephthalate sheet is used as a reflection sheet (see, for example, Patent Documents 1 and 2). In addition to a polyethylene terephthalate sheet, a polyolefin-based reflective sheet has also been reported (see Patent Document 3).

  Furthermore, some reflective sheets comprising a sheet having fine bubbles, a porous sheet containing a specific amount of an inorganic filler, and a laminated sheet thereof have been reported. (For example, refer to Patent Documents 4, 5, and 6) This realizes more excellent light reflectivity by including a large number of reflection layers not only on the surface of the reflection sheet but also inside thereof.

  With the recent thinning of liquid crystal display devices, a method of arranging the light source of the backlight unit beside the light guide plate has been widely used, and further, by reducing the number of light sources, the light source from one side of the backlight unit can be used on the display screen. There has been a growing demand for uniform and even dispersion over the entire surface. As a result, with respect to the reflection sheet, there has been an increasing demand for a diffuse reflection type that is excellent in uniform dispersion from the conventional regular reflection type. The problem is that the difference becomes large.

JP-A-2-13925 JP 59-8882 A Japanese Utility Model Publication No. 57-60119 Japanese Patent Laid-Open No. 7-230004 JP 2002-98811 A Japanese Utility Model Publication No. 2003-136619

  An object of the present invention is to solve the above-mentioned problems and to obtain a reflection sheet that can achieve thinning, diffuse reflection characteristics, high reflectivity at short wavelengths, and maintenance of mechanical characteristics while having high reflection characteristics. is there.

As a result of intensive studies in view of the above prior art, the present inventors have completed the present invention.
That is, the present invention is as described in [1] to [10] below.
[1] A reflection sheet comprising inorganic particles and a thermoplastic resin, having a porosity of 50 to 85%, and a base sheet bonded to both surfaces of a porous sheet having a thickness of 20 to 100 μm. When the vertical direction with respect to one surface of the reflecting sheet is 0 °, (i) the reflection intensity in the 0 ° direction when irradiated with light having a wavelength of 550 nm at an angle of 25 ° is 85% or more ( ii) A reflection sheet having a reflection intensity in the 0 ° direction of 80% or more when irradiated at an angle of 45 °, and (iii) a reflection sheet having a reflection intensity in the 0 ° direction of 75% or more when irradiated at an angle of 75 °. .
[2] The above reflection sheet having a diffuse reflectance of 95% or more when irradiated with light having a wavelength of 430 nm.
[3] The above reflective sheet, wherein the porous sheet contains 50 to 96% of the inorganic particles and 4 to 50% of the thermoplastic resin in a weight percentage based on the total amount of the inorganic particles and the thermoplastic resin.
[4] The reflective sheet as described above, wherein the thermoplastic resin is polyolefin.
[5] The above reflective sheet, wherein the polyolefin is made of polyethylene having an intrinsic viscosity of at least 5 dl / g.
[6] The above reflective sheet, wherein the porous sheet is stretched in at least one axial direction.
[7] The reflective sheet as described above, wherein the base material sheet on at least one surface has a thickness of 4 to 25 μm.
[8] The reflection sheet as described above, wherein the base sheet on at least one surface is made of a thermoplastic resin.
[9] The reflective sheet as described above, wherein the thermoplastic resin is polyester or polyolefin.

  The reflective sheet of the present invention has a waist and is excellent in handling properties (handleability). Surprisingly, even if the base sheet is bonded to both sides of the porous sheet, it has high reflection characteristics according to the angle of the porous sheet itself and hardly reduces the diffuse reflectivity. It has excellent characteristics.

  Therefore, according to the present invention, it is possible to provide a reflective sheet that can be reduced in thickness and retain mechanical properties while having sufficient reflective properties. In particular, it can be suitably used as a reflection sheet incorporated in a small-sized liquid crystal display device such as a mobile phone and a notebook personal computer.

  Furthermore, in the liquid crystal display device, due to the fact that the light source is arranged beside the light guide plate and the liquid crystal display device is made thinner, the conventional cold cathode tube is moving in the direction of adopting the LED, while the LED is wavelength dependent on the relative energy. Have sex. The reflective sheet of the present invention can have a high reflectance in the range of, for example, a wavelength of 430 nm, preferably a wavelength of 400 to 500 nm, more preferably 420 to 480 nm, and even more preferably 430 to 480 nm. It is convenient to use.

Hereinafter, the present invention will be described in detail.
[Porous sheet]
The porous sheet in the present invention is made of a thermoplastic resin containing inorganic particles, and has a space (void) in which a large number of pores are connected. A layered structure can be seen inside. Such a porous sheet has good reflection characteristics by having such voids. The porosity is 50 to 85%, preferably 55 to 80%.

  Examples of the inorganic particles include calcium carbonate, barium sulfate, barium titanate, titanium oxide, talc, clay, silica, and aluminosilicate. It is preferable to apply inorganic particles from the viewpoint of reflection efficiency, cost, and disposal of the sheet. Among them, calcium carbonate, barium sulfate, titanium oxide, silica, and aluminosilicate are particularly preferable.

  The shape of the particles is not particularly mentioned but is preferably a plate shape or a flat shape. For example, when stretched to produce a porous sheet if it is plate-shaped or flat, the surface direction of these particles is arranged relatively in parallel with the layered structure in the sheet, and the effect of assisting light reflection is obtained. It is because it becomes easy to express.

  The inorganic particles preferably have an average particle size of 0.01 to 10 μm. If the average particle size is less than 0.01 μm, the reflection performance is lacking, which is not preferable. On the other hand, if it exceeds 10 μm, tearing occurs during the production of the porous sheet, resulting in a decrease in productivity or an increase in diffuse reflection components, resulting in a reflection characteristic equal to or less than that of a conventional white film, which is not preferable. .

  The porous sheet preferably contains 50 to 96% of the inorganic particles and 4 to 50% of the thermoplastic resin in a weight percentage based on the total amount of the inorganic particles and the thermoplastic resin. The particle content is more preferably 70 to 94%, and even more preferably 80 to 92%. If the content of inorganic particles is higher than 96%, there may be a problem in mechanical strength. On the contrary, when the content of the inorganic particles is less than 50%, the reflection / scattering interface composed of the thermoplastic resin, the air layer, and the inorganic particles is reduced, so that sufficient reflection characteristics cannot be obtained.

As a thermoplastic resin which comprises the said porous sheet, polyolefin resins, such as polyethylene and a polypropylene, can be mentioned as a preferable example.
Preferably, the thermoplastic resin is substantially made of polyethylene having an intrinsic viscosity of at least 5 dl / g. If it is less than 5 dl / g, the strength of the porous sheet may be insufficient.

The porous sheet in the present invention preferably has a specific gravity of 2.0 or less from the viewpoint of weight reduction.
The porous sheet in the present invention has a thickness of 100 μm or less. When the thickness is larger than 100 μm, it hardly contributes to the improvement of reflection characteristics. The lower limit of the thickness is 20 μm. If it is smaller than 20 μm, the reflection characteristics tend to be remarkably lowered.

  The porous sheet of the present invention is preferably oriented by stretching in at least one axial direction. By stretching, a layered porous structure is developed. Such a structure increases the reflection interface, and at the same time, its reflection component expresses not only scattering reflection but also regular reflection component, so it contributes to suppressing the glare due to reflection at the same time as reflecting like a mirror. It is thought that. The preferred thickness is 25 to 75 μm, more preferably 25 to 60 μm.

  Examples of such stretching methods include longitudinal and transverse sequential biaxial stretching, longitudinal and transverse simultaneous biaxial stretching, longitudinal and uniaxial stretching, and lateral uniaxial stretching, and good reflection characteristics are obtained by controlling the directionality and intensity distribution of reflected light. For this purpose, longitudinal and lateral simultaneous biaxial stretching or longitudinal and lateral sequential biaxial stretching is preferable, and longitudinal and lateral sequential biaxial stretching is most preferable in consideration of productivity and cost.

  The porous sheet in the present invention is characterized in that a layered porous structure is formed in the thickness direction of one sheet. The number of layers when observed from the cross-sectional direction is preferably at least 5 layers, more preferably 10 layers or more in the thickness direction of the sheet. If the number of layers is less than 5, the intensity of interface reflection is small, and high reflectance cannot be achieved. Such a layer structure can be observed by SEM, for example.

  Although there is no restriction | limiting in particular as a manufacturing method of the said porous sheet, For example, it can manufacture by extending | stretching substantially gelatinized film forming and the obtained gelled sheet. For example, after dispersing an inorganic dielectric powder as inorganic particles in an appropriate gelling solvent using a milling device or the like, a thermoplastic resin as a binder and the remainder of the appropriate gelling solvent are added, The thermoplastic resin and the solvent are heated to dissolve to form a sol.

  The solubilized composition thus obtained is shaped into a sheet at a temperature equal to or higher than the gelation temperature, and the sheet is rapidly cooled below the gel point to prepare a gelled sheet. This gelled sheet can be produced by stretching uniaxially or biaxially at a temperature equal to or higher than the glass transition point of the thermoplastic resin and then heat-setting. Examples of the gelling solvent include usually decalin, hexane, paraffin, xylene and the like when polyethylene is used as the thermoplastic resin. Two or more of these may be used in combination. In addition, the drying can be controlled, and the stretching can be performed in a state where the solvent remains to some extent.

  If the porous sheet is within the scope of the object of the present invention, a lubricant, a pigment, a dye, an antioxidant, a fluorescent brightener, an antistatic agent, an antibacterial agent, an ultraviolet absorber, and a light stabilizer, if necessary. Additives that impart functionality such as an agent, a heat stabilizer, a light-shielding agent, and a matting agent can be blended.

[Base material sheet]
In this invention, the base material sheet is bonded together on both surfaces of the said porous sheet. By laminating this base material sheet, the mechanical properties that are often insufficient with the porous sheet alone, especially handling performance during assembly such as insertion or installation into the liquid crystal backlight panel by the user, are improved. Can be prevented from being detached.

  Since the base sheet affects the reflection characteristics, it needs to be transparent as long as it does not interfere with the reflection characteristics of the porous sheet, but it is installed on the light incident side (sometimes referred to as the reflective surface). The substrate sheet to be used is preferably highly transparent, and more preferably 2% or less in terms of haze.

  As a material for such a base sheet, a thermoplastic resin film is preferred. Examples of such thermoplastic resins include polyesters such as polyethylene terephthalate and polyolefins such as polypropylene.

  As the thickness of the base material sheet, a thinner one is preferable because the entire reflection sheet can be thinned, but a preferable specific range is 4 to 25 μm. If the thickness is less than 4 μm, problems such as handling of the base sheet may occur. In the case of 25 μm or more, depending on the case, it may not be possible to meet the demand for thinning the reflection sheet.

  Also, the base material sheet on the side where the incident light passes through the porous sheet (non-reflective surface side) is excellent in handling property of the base material sheet itself and the reflective sheet. It is desirable to use a superior one. The base sheet on the non-reflective surface side can be the same as the base sheet on the reflective surface side (thermoplastic resin film). For example, aluminum or steel metal foil can be used. It is. In addition, the same kind and the same thickness of the base material sheet to bond together are more preferable at the point of dimensional stability. Moreover, it is also possible to adjust thickness with the base material sheet of a non-reflective surface side according to a use.

[Reflection sheet]
The reflective sheet of the present invention preferably has a diffuse reflectance at a wavelength of 430 nm of 95% or more. When the reflectance is less than 95%, sufficient brightness cannot be obtained when the reflective sheet is used as a substrate for a backlight reflector of a liquid crystal display. In addition, if the reflectance is 120%, it has sufficient reflection characteristics, and in order to increase the reflectance further, the weight and bulk of the reflection sheet may be affected, so at a wavelength of 430 nm The diffuse reflectance is more preferably 96.0% or more, still more preferably 97.0% or more, and the upper limit is preferably 120%.

The reflective sheet of the present invention is characterized by high reflection intensity even when light having a wavelength of 550 nm is irradiated at a specific angle. In particular,
(I) The reflection intensity when irradiated at an angle of 25 ° is 85% or more,
(Ii) The reflection intensity when irradiated at an angle of 45 ° is 80% or more,
(Iii) The reflection intensity when irradiated at an angle of 75 ° is 75% or more,
It is. Thus, the reflection sheet of the present invention maintains a high reflection intensity with almost no decrease in reflection intensity over a wide range of irradiation angles of at least 25 to 75 ° (preferably 0 to 90 °). In this case, the angle when irradiated perpendicularly to the surface of the reflective sheet is 0 °, and the direction parallel to the surface of the reflective sheet is 90 °.

  In addition, although the one where the reflection intensity when irradiated at the angle of 25 degrees, 45 degrees, and 75 degrees is higher is preferable, the reflective sheet which bonded both surfaces of the porous sheet of Example 1 mentioned later with the film of the thermoplastic resin, for example However, the upper limit of these angles is about 100%, 95% and 90%, respectively, although it depends on the thickness of the film.

  The thickness of the reflective sheet of the present invention is preferably 20 to 100 μm, more preferably 25 to 75 μm. When the thickness is less than 20 μm, it is not preferable because the reflectance is lowered or the handling property is lowered when the sheets are laminated. On the other hand, even if the thickness exceeds 100 μm, the reflection characteristics are not improved. In addition, it cannot meet the demand for thin reflection films in recent years.

[Method for producing reflective sheet]
The reflective sheet of the present invention is manufactured by laminating the base sheet on both surfaces of the porous sheet.
Examples of the bonding method include a method in which an adhesive layer is formed on the surface of the base sheet, and a porous sheet is bonded to the adhesive layer and bonded. Or you may heat-press a base material sheet | seat on a porous sheet. As a method for forming the adhesive layer, a common coating method such as a comma coater method, a gravure method, or the like can be used. This can be selected according to the fluidity of the adhesive. The nip conditions at the time of bonding, such as temperature, pressure, and time, can be adjusted while looking at the type of adhesive and the characteristics of the reflective sheet.

For the formation of the adhesive layer, a general adhesive or double-sided tape can be used. As the adhesive, for example, a modified olefin, polyester, epoxy, urethane, nylon, or acrylic adhesive can be used. Use of an adhesive having excellent transparency is preferable because the reduction in reflection characteristics is small.
The thickness of the adhesive layer is preferably thinner from the viewpoint of reflection characteristics. The thickness is preferably 10 μm or less, more preferably 5 μm or less. If it exceeds 10 μm, the reflection characteristics, thermal dimensional stability, handling properties, etc. are affected, which is not preferable. The lower limit is not particularly limited, but 1 μm is preferable.

Hereinafter, although the specific example of this invention is given and demonstrated, this invention is not limited to this.
In addition, the value in an Example was measured with the following method.
(1) Reflection intensity Reflection with respect to a reference sample at an emission light angle of 25 °, 45 °, 75 ° and 0 ° wavelength of 550 nm with a spectrocolorimeter (trade name “CM-512m3” manufactured by Konica Minolta Sensing). Indicates strength (%). As a reference sample, a white ceramic tile (CM-A64) attached with CM-512m3 was used. In this case, the angle when irradiated (emitted) in the direction perpendicular to the surface of the reflective sheet is 0 °, and the direction parallel to the surface of the reflective sheet is 90 °.
(2) Diffuse reflectance:
An integrating sphere is attached to a spectrophotometer (trade name “UV-3101PC” manufactured by Shimadzu Corporation), and the reflectance is 400 to 800 nm at a measurement light incident (reflection) angle of 5 ° when the BaSO ₄ white plate is 100%. Measured over time. The diffuse reflectance (%) at wavelengths of 430 nm and 550 nm was measured.
(3) Porosity:
It calculated | required by the following formula | equation from the apparent density (rho) and true density (rho) 0 of the porous sheet measured by the well-known method.
Porosity = (ρ0−ρ) / ρ0 × 100 (%)
(4) Sheet thickness:
Measurement was performed with Mitutoyo “Lightmatic” using a probe having a diameter of 4 mmΦ and a flat measurement surface.

[Example 1]
To 19 parts by weight of decalin, 14 parts by weight of paraffin and 6 parts by weight of ultra high molecular weight polyethylene (made by Ticona Co., Ltd.) having an intrinsic viscosity of 15 dl / g (measured in decalin at 135 ° C.) and 2 parts by weight of a dispersant are added and oxidized. 59 parts by weight of titanium (manufactured by Sakai Chemical Co., Ltd.) was dispersed. The dispersion was dissolved at 165 ° C. using a biaxial kneading extruder and extruded at 160 ° C. through a flat film extrusion die. Next, the extruded product was cooled and solidified, and heat-treated at 80 ° C. for 30 minutes. This sheet was stretched 5 times in the MD direction at 120 ° C., then 10 times in the TD direction, and heat-set at 130 ° C. Thereafter, it was washed with methylochrome, dried, and heat-set at 120 ° C. to prepare a porous sheet having a thickness of 75 μm, a porosity of 69%, and an inorganic particle content of 90% by weight.

  For the base sheet, a transparent polyethylene terephthalate (PET) film (“Tetron” HPE film (thickness: 16 μm) made by Teijin DuPont Film) on one side of the porous sheet serving as the reflective surface and transparent PET on the non-reflective surface Using a film (“Tetron” HPE film (thickness 16 μm) made by Teijin DuPont Films), an acrylic resin was applied as an adhesive on one side of each of these PET films to provide an adhesive layer (thickness 2 μm). The above PET film was bonded to a reflection sheet. This reflective sheet had good handleability and was able to produce a backlight with high productivity. The characteristics of the obtained reflection sheet are shown in Table 1 below.

[Example 2]
To 21 parts by weight of decalin, 5 parts by weight of ultrahigh molecular weight polyethylene (made by Ticona Co., Ltd.) having 13 parts by weight of paraffin and an intrinsic viscosity of 15 dl / g (measured at 135 ° C. in decalin) and 2 parts by weight of a dispersant are added and oxidized. 60 parts by weight of titanium (manufactured by Sakai Chemical Co., Ltd.) was dispersed. The dispersion was melted at 170 ° C. using a biaxial kneading extruder and extruded at 155 ° C. through a flat film extrusion die. Next, the extruded product was cooled and solidified, and heat-treated at 80 ° C. for 20 minutes. This sheet was stretched 4.5 times in the MD direction at 125 ° C., then 13 times in the TD direction, and heat-set at 130 ° C. Thereafter, it was washed with methyloic acid, dried, and heat-set at 120 ° C. to prepare a porous sheet having a thickness of 40 μm, a porosity of 72%, and an inorganic particle content of 92% by weight.

  The base sheet is a transparent PET film (“Tetron” GE film (thickness 12 μm) made by Teijin DuPont Film) on one side of the porous sheet that is the reflective surface, and a transparent PET film (Teijin DuPont film on the non-reflective surface) “Tetron” GE film (thickness 12 μm)), an acrylic resin as an adhesive is applied to one side of each of these PET films to provide an adhesive layer (thickness 1 μm), and the PET film is pasted on both sides of the base sheet A laminated reflection sheet was prepared. This reflective sheet had good handleability and was able to produce a backlight with high productivity. The properties of the obtained reflection sheet are summarized in Table 1 below.

[Example 3]
The porous sheet of Example 2 was used. The base sheet is made of a polyolefin film transparent on the reflective surface (“Alphan” OPP film (6 μm thick) manufactured by Oji Paper Co., Ltd.) and a polyolefin film transparent on the non-reflective surface (“Alphan” OPP manufactured by Oji Paper Co., Ltd.) A reflective sheet was prepared in the same manner as in Example 2 except that the film (thickness: 6 μm) was used. The properties of the obtained reflection sheet are summarized in Table 1 below.

[Example 4]
To 20 parts by weight of decalin is added 13 parts by weight of paraffin and 5 parts by weight of ultra high molecular weight polyethylene (made by Ticona Co., Ltd.) having an intrinsic viscosity of 15 dl / g (measured at 135 ° C. in decalin) and 2 parts by weight of a dispersant. 60 parts by weight of barium acid (manufactured by Kyoritsu Co., Ltd.) was dispersed. The dispersion was melted at 170 ° C. using a biaxial kneading extruder and extruded at 155 ° C. through a flat film extrusion die. Next, the extruded product was cooled and solidified, and heat-treated at 80 ° C. for 15 minutes. This sheet was stretched 6.0 times in the MD direction at 120 ° C., then 15 times in the TD direction, and heat-set at 135 ° C. Thereafter, it was washed with methylochrome, dried, and heat-set at 120 ° C. to prepare a porous sheet having a thickness of 20 μm, a porosity of 72%, and an inorganic particle content of 90% by weight.

  The base sheet has a transparent PET film on the reflective surface (“Tetron” HB3 film made by Teijin DuPont Film (thickness 25 μm)), and a transparent PET film on the non-reflective surface (“Tetron” HB3 film made by Teijin DuPont Film (thickness) A reflective sheet was prepared in the same manner as in Example 1 except that 25 μm)) was used. The properties of the obtained reflection sheet are summarized in Table 1 below.

[Reference Example 1]
Table 1 below summarizes the characteristics of a reflective sheet made of a white PET film (“UX” manufactured by Teijin DuPont Film, thickness of 150 μm).

[Reference Example 2]
Table 1 below summarizes the characteristics of the reflection sheet made of a silver reflection sheet (Reiko 60 W, thickness 65 μm).

  The porous sheet of the present invention is excellent in reflection characteristics and handling properties, and can be thinned as a whole, so it can meet the demand for thinning, for example, an LED or a cold cathode tube such as a mobile phone or a PDA. It is useful for a backlight unit having a thickness of 1 cm or less used for a thin panel as a light source.

Claims (8)

It consists of inorganic particles having an average particle size of 0.01 to 10 μm and a thermoplastic resin, has a porosity of 50 to 85%, and has a haze of 2% or less on both surfaces of a porous sheet having a thickness of 20 to 100 μm . When the base sheet is a reflection sheet, and the vertical direction with respect to one surface of the reflection sheet is 0 °,
When the spectrocolorimeter irradiates light with a wavelength of 550 nm at outgoing light angles of 25 °, 45 °, and 75 °, and measures the reflected light of 0 ° received light, and determines the reflection intensity (%) with respect to the reference sample. (I) The reflection intensity in the 0 ° direction when irradiated at an angle of 25 ° is 85% or more, and (ii) the reflection intensity in the 0 ° direction when irradiated at an angle of 45 ° is 80% or more, (Iii) The reflection intensity in the 0 ° direction when irradiated at an angle of 75 ° is 75% or more, and the inorganic particles are contained in an amount of 80 to 92% in a weight percentage based on the total amount of the inorganic particles and the thermoplastic resin. Reflective sheet. An integrating sphere is attached to the spectrophotometer, and the diffuse reflectance is 95% or more when irradiated with light having a wavelength of 430 nm at an incident (reflection) angle of 5 ° when the BaSO ₄ white plate is 100%. Item 6. A reflection sheet according to Item 1. A thermoplastic resin is a polyolefin, the reflection sheet according to any one of claims 1-2. The reflective sheet according to claim 3, wherein polyethylene having an intrinsic viscosity of at least 5 dl / g is used as the polyolefin. Porous sheet is formed by stretching in at least one axial direction, the reflection sheet according to any one of claims 1-4. The reflective sheet according to any one of claims 1 to 5 , wherein the substrate sheet on at least one surface has a thickness of 4 to 25 µm. Reflecting sheet according to any one of claims 1 to 6, the base sheet of at least one surface of a thermoplastic resin. The reflective sheet according to claim 7 , wherein the thermoplastic resin is polyester or polyolefin.