JP6163869B2 - Light diffusion sheet - Google Patents

Description

  The present invention relates to a light diffusing sheet for scattering light incident from the outside and emitting it to the outside.

  As is well known, in lighting fixtures such as LED lighting fixtures, various display devices such as laser display devices or liquid crystal displays, and other various optical devices, a light diffusion sheet for diffusing light from a light source is used. Arrangement is made in the path or on the light exit side to the outside.

  For example, in an LED lighting apparatus, in order to reduce glare peculiar to an LED light source, a light diffusion sheet is adhered to a light-transmitting cover such as glass or resin. In laser display devices, laser light from a laser light source is incident on the end of a light guide plate, and characters and figures are optically displayed outside the light guide plate. In order to scatter light in the light guide plate, a light diffusion sheet is provided on the end face of the light guide plate on which the laser light is incident, and only when laser light is used. First, a light diffusion sheet is disposed on the surface (display surface side) of the light guide plate in various optical display devices.

  As this type of light diffusion sheet, recently, a large number of transparent fine particles called beads are formed on the surface (usually one side) of a base sheet (film) made of a soft resin such as PET (polyethylene terephthalate). A material in which a body (generally microspheres) is dispersed and a diffusion layer fixed with a transparent binder resin such as an acrylic resin is formed is widely used.

  By the way, when such a light diffusion sheet is actually used, the back surface (the surface opposite to the side on which the light diffusion layer is formed) is often disposed so as to be in contact with some optical member. For example, in a liquid crystal display device or the like, a light diffusion sheet may be disposed on the surface side (image display surface side) of a light guide plate for guiding light from a light source so as to be in contact with the light guide plate. In such a case, if the back surface of the light diffusing sheet is partially attached to the surface of the counterpart material, for example, the light guide plate, the brightness of the display image becomes uneven. Therefore, for example, as shown in Patent Document 1, Patent Document 2, etc., a layer (backcoat layer) for preventing sticking is formed on the surface (back surface) opposite to the light diffusion layer in the light diffusion sheet. It has been performed conventionally.

  Such sticking of the light diffusion sheet to the opposite material on the back side is sometimes referred to as a sticking phenomenon. Therefore, also in this specification, the sticking to the counterpart material on the back side as described above is referred to as a sticking phenomenon, and the back coat layer for preventing the sticking phenomenon is referred to as a back sticking preventing layer.

  As the back surface sticking prevention layer, a structure in which a large number of transparent fine particles as a sticking preventing material are generally dispersed in a transparent binder resin such as an acrylic resin is applied. However, unlike the light diffusion sheet on the front side, the back surface sticking prevention layer is desired not to scatter, reflect or absorb light as much as possible. Therefore, as the fine particles of the back surface sticking prevention layer, those having a particle size much smaller than the fine particles of the light diffusion layer are used, and the dispersion density and blending ratio (ratio to the binder resin) are also light diffusion. Usually much less than the layer. Furthermore, the thickness of the back surface sticking prevention layer should just have the minimum thickness which can hold | maintain the blocking prevention material (microparticle) of a small particle size. Therefore, in general, the thickness of the back surface sticking prevention layer is usually much thinner than the thickness of the light diffusion layer. For example, the thickness of the light diffusion layer is usually about 3 μm to 15 μm, whereas the thickness of the back surface adhesion preventing layer is often about 2 μm to 7 μm.

  In manufacturing such a light diffusion sheet having a three-layer structure, a coating liquid for forming each layer, that is, a light diffusion layer, is formed on each surface of a base sheet (film) made of PET (polyethylene terephthalate) or the like. Apply each coating solution prepared by adding each binder resin (for example, acrylic resin) and solvent (for example, toluene) to the anti-adhesive material (microparticle) for the microparticles or the back surface anti-adhesion layer, and dry. In general, the solvent is blown off, and the light diffusion layer and the back surface sticking prevention layer are respectively formed on the front side surface and the back side surface of the base sheet.

Japanese Patent No. 4209703 JP 2002-99220 A

It has been found that the light diffusion sheet having a three-layer structure having the back surface sticking prevention layer as described above has the following problems.
That is, the phenomenon that the single plate-like light diffusion sheet is warped, particularly the phenomenon that the light diffusion layer side is curled so as to be a concave curved surface (this phenomenon is hereinafter referred to as a curling phenomenon) is likely to occur. I found out.
If the curling phenomenon occurs in this way, there arises a problem that it becomes difficult to align or stack the single-plate light diffusion sheets. Also, when assembling various lighting fixtures and display devices that use light diffusion sheets, it may interfere with the handling of the light diffusion sheet, may not be correctly installed in the place where it should be installed, and the other material (eg light guide plate) There is a problem that the gaps between them are not properly aligned and the gaps between them are not uniform. Therefore, the occurrence of the curling phenomenon has become an obstacle to the automation of the process for actually applying the light diffusion sheet to various final products such as lighting equipment and display devices, and has become one of the causes of defective products.
Conventionally, however, the problem of such a curl phenomenon has not been sufficiently recognized, and no specific countermeasure has been taken.

  The present invention has been made against the background of the above circumstances, so that the curling phenomenon that the single-plate light diffusion sheet is warped does not occur, and there is a problem in handling the light diffusion sheet and applying it to the final product. An object is to provide a light diffusing sheet that does not occur.

  In order to solve the above-mentioned problems, the present inventors have conducted extensive experiments and studies on measures for preventing the occurrence of a curl phenomenon in a light diffusion sheet having a three-layer structure having a back surface sticking prevention layer. The binder resin of the diffusion layer is left uncrosslinked, while the binder resin of the back side anti-sticking layer on the back side is crosslinked to effectively prevent the light diffusion sheet from warping even when it is a single plate. As a result, the present invention has been made.

Specifically, the light diffusion sheet of the basic aspect (first aspect) of the present invention is:
A transparent base sheet having flexibility;
A light diffusion layer laminated on the first surface of the base sheet;
It has a back surface sticking prevention layer laminated on the second surface opposite to the first surface in the base sheet with a thickness smaller than the thickness of the light diffusion layer,
The light diffusion layer includes a large number of transparent first microparticles arranged in a dispersed manner, and a first binder for fixing and holding the first microparticles on the first surface of the base sheet. Prepared,
The back surface sticking prevention layer includes a large number of transparent second fine particles dispersedly arranged, and a second binder for fixing and holding the second fine particles on the second surface of the base sheet. With
Further, the first binder is made of an uncrosslinked resin, and the second binder is made of a crosslinked resin ,
The second fine particles have an average particle size smaller than the average particle size of the first fine particles, and the dispersion density of the second fine particles in the back surface anti-sticking layer is the light. It is smaller than the dispersion density of the first microparticles in the diffusion layer .

The light diffusion sheet according to the second aspect of the present invention is the light diffusion sheet according to the first aspect,
The light diffusion sheet has a thickness of 3 to 15 μm and a thickness of the anti-sticking back layer of 2 to 7 μm.

The light diffusing sheet according to the third aspect of the present invention is the light diffusing sheet according to any one of the first and second aspects.
In the light diffusion sheet, the first binder is an uncrosslinked acrylic resin, and the second binder is a crosslinked acrylic resin.

Furthermore, the light diffusion sheet according to the fourth aspect of the present invention is the light diffusion sheet according to any one of the first to third aspects,
The light diffusion sheet has an average particle size of 1 to 30 μm of the first fine particles and an average particle size of 5 to 12 μm of the second fine particles.

  The light diffusion sheet of the present invention is a light diffusion sheet formed by forming a back surface sticking prevention layer on the surface opposite to the light diffusion layer in the base material sheet, while the binder of the back surface sticking prevention layer is crosslinked, The binder of the light diffusing layer is left uncrosslinked, so that the curling phenomenon that the single-plate light diffusing sheet is warped can be prevented or suppressed. It is possible to effectively prevent troubles in handling the light diffusing sheet and applying it to the final product due to warping.

It is a schematic diagram which expands and shows the one part cross section of the light-diffusion sheet of one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail.

[Configuration of Light Diffusion Sheet of Embodiment]
In FIG. 1, the light-diffusion sheet of embodiment of this invention is shown.
In FIG. 1, a light diffusion layer 3 is formed on one surface (first surface) 1 </ b> A of a base material sheet 1 made of a flexible transparent resin having flexibility, and further on the light diffusion layer 3 in the base material sheet 1. On the other hand, the back surface sticking prevention layer 11 is formed on the opposite surface (second surface) 1B, and the light diffusion sheet 5 having a three-layer structure as a whole is formed.

The light diffusing layer 3 is for scattering light emitted from the base sheet 1 side and radiating the light to the outside. The light diffusing layer 3 is used for fixing and holding a large number of transparent microparticles (first microparticles) 7 arranged in a dispersed manner and the large number of microparticles 7 on one surface 1A of the base sheet 1. Transparent binder (first binder) 9.
Further, the back surface adhesion preventing layer 11 prevents the occurrence of a phenomenon (sticking phenomenon) that partially adheres to the opposing material facing the back surface of the light diffusion sheet 5, for example, the surface of the light guide plate, At the same time, it is a layer that transmits light from a counterpart material such as a light guide plate to the base sheet 1 side. The back surface sticking prevention layer 11 is formed of a thin layer in which a large number of transparent fine particles 13 are dispersed in a binder (second binder) 15 as a sticking prevention material. The thickness T2 of the back surface sticking prevention layer 11 is set to be smaller than the thickness T1 of the light diffusion layer 3.
The resin of the binder (second binder) 15 of the back surface sticking prevention layer 11 is crosslinked, while the resin of the binder (first binder) 9 of the light diffusion sheet 5 is not crosslinked. It has become.

  In the present specification, the term “transparent” means having light transmittance that does not hinder the use as a light diffusing sheet, and depending on the use and usage of the light diffusing sheet, it is colored and transparent. It is also permissible. The light transmittance of the entire light diffusion sheet is preferably 70% or more, more preferably 80% or more, and still more preferably 90% or more in terms of transmittance of visible light (380 nm to 800 nm).

[Operation / Effect of Embodiment]
While the resin of the first binder of the light diffusion layer is in a non-crosslinked state (uncrosslinked state), the second binder of the back surface sticking prevention layer is in a cross-linked state (crosslinked state), thereby forming a single plate It is possible to prevent the occurrence of a phenomenon (curl phenomenon) in which the light diffusion sheet is warped, or at least to suppress the degree of warping. The reason is considered as follows.

  In the production of the light diffusion sheet having a three-layer structure, the coating liquid for forming each layer is applied to each surface of the base sheet (film) and dried as described above, so that the solvent in the coating liquid is removed. In general, the light diffusion layer and the back surface adhesion preventing layer are formed on the front side surface and the back side surface of the base sheet, respectively. In the process of drying the coating solution, each coating layer shrinks, but each layer is constrained by a base sheet (film) in its surface direction (direction along the surface), so each layer after drying has The tensile stress in the plane direction remains. That is, both the light diffusion layer after coating and drying and the back surface sticking prevention layer have a residual tensile stress in the surface direction.

  By the way, the back surface sticking prevention layer of the light diffusion sheet having a three-layer structure is usually thinner than the light diffusion layer on the front surface side. In particular, in practice, the thickness of the back surface adhesion preventing layer is 2 to 7 μm, which is much smaller than the thickness of the light diffusion layer (usually 3 to 15 μm). For this reason, conventionally, the residual tensile stress in the surface direction accumulated in the light diffusion layer on the front side is much larger than the residual tensile stress in the surface direction accumulated in the back surface anti-sticking layer on the back side.

  For example, when the light diffusion sheet wound in a coil state is cut or cut into a single plate shape, the light diffusion sheet tends to be deformed in a direction to release the residual stress. However, if the residual tensile stress in the surface direction of the light diffusion layer on the front side is much larger than the residual tensile stress in the surface direction of the back surface anti-sticking layer on the back side, the light diffusion sheet is in the surface direction of the light diffusion layer on the front side. The residual tensile stress is released in a direction (compression direction), that is, in a direction in which the front side is concavely curved. In addition, since the back side anti-sticking layer on the back side is small in thickness, the tensile strength in the surface direction is much smaller than the tensile strength of the light diffusion layer on the front side, so that the front side is deformed to be concavely curved. Therefore, the resistance of the back surface sticking prevention layer is also small. In combination with these, it is considered that the light diffusing sheet has a phenomenon of curling so that the front side is concavely curved, that is, a curling phenomenon.

  However, in this embodiment, the binder (first binder) of the light diffusion layer is not cross-linked, and therefore its strength (strength in the plane direction) is relatively small. On the other hand, since the binder (second binder) of the back surface adhesion preventing layer is cross-linked, the strength (surface direction strength) of the back surface adhesion preventing layer is larger than that of the light diffusion layer on the front side. Therefore, for example, when the residual tensile stress of each layer is released by cutting or cutting out from the coil state, the light diffusion sheet is warped so that the front side is concavely curved due to the large residual tensile stress of the light diffusion layer on the front side. On the other hand, the back surface sticking prevention layer has a large resistance, and as a result, the occurrence of the curling phenomenon can be prevented or at least suppressed.

[Preferred Conditions / Materials of Embodiment]
The base sheet supports the light diffusing layer and transmits light at the same time. The base sheet only needs to have a degree of flexibility that allows the base sheet to be along various surfaces. The material is not particularly limited, but usually PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PP (polypropylene), PC (polycarbonate), polyester, polyamide or the like is used. The thickness of the base sheet is not particularly limited, but is preferably 12 to 250 μm.

The microparticles (first microparticles) of the light diffusion layer are the main body for diffusing light incident on the light diffusion layer from the base sheet side, and the shape of the particles is In general, a sphere (such as a true sphere or an elliptical sphere close thereto) is preferable, but the shape is not limited thereto, and a particle shape used for a general light diffusion sheet can be arbitrarily applied. As the material of the first fine particles, as will be described later, a resin having a glass transition temperature (Tg) of 90 ° C. or higher, a resin having no glass transition temperature, or internally crosslinked, and less than 90 ° C. It is desirable to use a resin that is not denatured, further glass or the like, and a polystyrene resin is usually used. Further, there is no glass transition temperature, and internally cross-linked polystyrene may be used. Further, the average particle diameter of the first fine particles is not particularly limited, and may be 1 μm to 30 μm. In addition, it is not necessary to use a thing with a uniform particle size as a 1st microparticle, The particle size may vary, and it is used by mixing several types of microparticles which have different average particle diameters It doesn't matter.
In the present specification, the average particle diameter means a cumulative median diameter obtained by measuring a particle dispersion with a laser diffraction particle size distribution analyzer. That is, when the cumulative curve is obtained by setting the total volume of the sample particle group as 100%, the particle diameter is the point at which the cumulative curve becomes 50%.

  The binder (first binder) of the light diffusion layer is left uncrosslinked. When the first binder is crosslinked, the strength of the light diffusion layer is increased, which may promote the curling phenomenon. Therefore, the first binder is left uncrosslinked.

Furthermore, it is preferable to use a resin having a glass transition temperature (Tg) of 90 ° C. or higher and 140 ° C. or lower as the binder (first binder) resin of the light diffusion layer.
When the glass transition temperature of the first binder is less than 90 ° C., when the light diffusion sheet is wound in a coil shape, the first binder is softened particularly between the inner and outer layers of the coil in a high temperature atmosphere or a high temperature and high humidity atmosphere. Light diffusion sheets tend to stick to each other. If the glass transition temperature is 90 ° C. or higher, sufficient hardness can be maintained at the normal winding temperature, so that the light diffusion sheets can be bonded between the inner and outer layers of the coil even in a wound state where tension is applied. It is possible to effectively prevent sticking.

On the other hand, the higher the glass transition temperature of the binder of the light diffusion layer (first binder), the greater the effect of preventing sticking in a high temperature atmosphere or a high temperature and high humidity atmosphere, but when the glass transition temperature exceeds 140 ° C, Another problem may arise. That is, when the glass transition temperature of the binder of the light diffusion layer is extremely high, the binder becomes too hard at a normal atmospheric temperature (for example, room temperature) or a low temperature atmosphere. Therefore, when the light diffusing sheets overlap each other, such as when wound around a coil, scratching is likely to occur, and further, the curling phenomenon may be promoted. That is, if the glass transition temperature of the first binder is high (especially if the glass transition temperature exceeds 140 ° C.), even if the first binder remains uncrosslinked, internal stress is generated and the curling phenomenon is promoted. There is a risk that.
Therefore, the glass transition temperature of the binder (first binder) of the light diffusion layer is preferably 90 to 140 ° C.

  The specific resin type of the binder (first binder) of the light diffusion layer is preferably a transparent resin whose glass transition temperature can be adjusted to 90 to 140 ° C., and has an adhesiveness to the fine particles. The resin is not particularly limited as long as it is a good resin. For example, polyester resin, acrylic resin, polyvinyl chloride resin, ethylene-vinyl acetate copolymer resin, vinyl chloride-vinyl acetate copolymer resin, polyvinyl acetal resin, epoxy resin Melamine resin, phenol resin, and copolymer resins thereof can be used. Here, in terms of transparency and refractive index, it is particularly preferable to use an acrylic resin as the first binder.

On the other hand, as the material for the microparticles (first microparticles) of the light diffusion layer, a resin having a glass transition temperature of 90 ° C. or higher, a resin having no internal glass transition temperature, glass, or the like may be used. desirable. The reason is as follows.
That is, the higher the glass transition temperature of the first fine particles, the greater the effect of preventing the sticking phenomenon as described above in a high temperature atmosphere or a high temperature and high humidity atmosphere. In the case of using a thermoplastic resin such as polystyrene, if the glass transition temperature exceeds 140 ° C., another problem may occur. In other words, when the glass transition temperature of the microparticles made of thermoplastic resin is remarkably high, the microparticles in a normal atmosphere temperature (for example, room temperature) or a low temperature atmosphere become too hard, and therefore when the microparticles are wound around a coil, If the diffusion sheets overlap, the light diffusion sheets may be scratched.
Accordingly, the glass transition temperature of the first microparticles is generally desirably 90 ° C. or higher, and particularly when a thermoplastic resin such as polystyrene is used as the microparticles, the glass transition temperature is 90 to 140 ° C. It is desirable.

  In addition to satisfying the above-mentioned conditions for the glass transition temperature, the fine particles are not modified (deformed) at a temperature lower than 90 ° C., that is, softened and plastic deformation becomes larger than at room temperature, It is desirable to use a material that does not become brittle, or whose transparency is not lowered or yellowed.

  As a specific material for the first fine particles, a transparent material that can be adjusted to a glass transition temperature in the range of 90 ° C. or higher and 140 ° C. or lower, or a material that does not have a glass transition temperature, Any material may be used as long as the material has good shape retention as a sphere without softening or embrittlement at less than ° C, does not cause a decrease in transparency or yellowing, and can easily produce minute spheres. Although not used, for example, polystyrene resin, polymethylmethacrylate resin, nylon and the like can be used, and further, a glass material such as quartz glass can be used. Here, in terms of transparency and high refractive index, it is particularly preferable to use polystyrene. The glass transition temperature of general polystyrene is about 100 ° C. Further, there is no glass transition temperature, and internally cross-linked polystyrene may be used.

  The total thickness of the light diffusion layer is preferably 3 to 15 μm. If it is less than 3 μm, a part where particles are not formed on the base material is produced in the process of drying the paint, and the light diffusion performance is lowered. Be encouraged.

  Further, the blending ratio of the first fine particles and the first binder in the light diffusion layer is usually preferably 5: 1 to 0.5: 1 in terms of mass ratio. The reason is that when the above-mentioned mass ratio is larger than 5, the binder is small and particles are easily lost, and when it is smaller than 0.5, the light diffusion performance is deteriorated.

  The back surface sticking prevention layer is a thin layer in which a large number of transparent fine particles (second fine particles) are dispersed in a binder (second binder) resin as a sticking prevention material.

The resin of the binder (second binder) of the back surface sticking prevention layer is crosslinked. When the second binder is crosslinked, the strength of the back surface sticking prevention layer is increased, and in particular, the tensile strength in the surface direction (direction along the surface) is increased. As a result, when the residual tensile stress of the light diffusing layer is released, the resistance of the back surface sticking prevention layer against the warp of the light diffusing sheet accompanying the release of the residual tensile stress of the light diffusing layer is increased, and the curling phenomenon can be prevented or suppressed. It becomes. Therefore, the second binder is not particularly limited as long as it can be crosslinked, and as the resin, the glass transition temperature (Tg) is the same as the binder of the light diffusion layer (first binder). ) Is preferably 90 ° C. or higher and 140 ° C. or lower.
When the glass transition temperature of the second binder is less than 90 ° C., the sticking (sticking phenomenon) to the counterpart material such as the light guide plate as described above is likely to occur, and particularly in a high temperature atmosphere or a high temperature and high humidity atmosphere, There is a possibility that the binder becomes soft and a sticking phenomenon easily occurs.

On the other hand, the higher the glass transition temperature of the binder (second binder) of the anti-back-sticking layer, the greater the effect of preventing the occurrence of blocking phenomenon. For example, the back-sticking at the time of cutting out from a coil or during normal storage As the hardness of the prevention layer is increased, the effect of preventing the occurrence of the curling phenomenon is further increased. However, when the glass transition temperature exceeds 140 ° C., another problem may occur. That is, when the glass transition temperature of the second binder is extremely high, the binder becomes too hard at a normal atmospheric temperature (for example, room temperature) or a low temperature atmosphere. Therefore, it becomes easy to scratch each other when the light diffusion sheets overlap each other, such as when wound around a coil.
Therefore, the glass transition temperature of the binder (second binder) of the back surface adhesion preventing layer is also preferably 90 to 140 ° C.

  The specific resin type of the binder (second binder) for the back surface adhesion preventing layer is a transparent resin that can be cross-linked and desirably can adjust the glass transition temperature to 90 to 140 ° C. The resin is not particularly limited as long as the resin has good adhesion to the microparticles. For example, a polyester resin, an acrylic resin, a polyvinyl chloride resin, an ethylene-like resin, as in the case of the light diffusion layer binder (first binder). Vinyl acetate copolymer resin, vinyl chloride-vinyl acetate copolymer resin, polyvinyl acetal resin, epoxy resin, melamine resin, phenol resin, and copolymer resins thereof can be used. Here, in terms of transparency and refractive index, it is preferable to use an acrylic resin as the second binder.

In order to crosslink the resin of the binder (second binder) in the back surface sticking prevention layer, the acid value (equivalent of the crosslinking group) of the binder resin is preferably 0.5 to 10 mgKOH / g. When the acid value is less than 0.5 mg KOH / g, it is difficult to sufficiently increase the strength of the back-sticking preventing layer due to a decrease in cross-linking points, and the effect of preventing the curling phenomenon may be insufficient. . On the other hand, if the acid value exceeds 10 mgKOH / g, an unreacted functional group may remain and the back surface sticking prevention layer may be colored. In addition, it is necessary to add a crosslinking agent to the binder (second binder) resin in the back surface sticking prevention layer. Examples of the crosslinking agent include isocyanate-based, epoxy-based, oxazoline-based, and carbodiimide-based resins. Can be used, preferably those based on isocyanate.
Specific isocyanate-based crosslinking agents include TDI, MDI, HDI, IPDI, XDI, H6XDI, NDI, and NBDI, as shown in Table 1 below. HDI, IPDI, and XDI types are preferred.

In addition, since the fine particles (second fine particles) in the back surface sticking prevention layer usually have a small blending ratio, the glass transition temperature has little influence on the sticking phenomenon and the curling phenomenon, and therefore Although the glass transition temperature is arbitrary, it is preferably 90 ° C. or higher and 140 ° C. or lower as in the case of the light diffusion layer microparticles (second microparticles), or there is no internal cross-linking type glass transition temperature. It is appropriate to use one.
Specifically, for example, polystyrene resin, polymethyl methacrylate (PMMA), or the like can be used.

Here, the back surface sticking prevention layer does not need to have a light diffusion function as in the case of the light diffusion layer, but rather has high light transmittance without diffusing or reflecting light. Is desired. Therefore dispersion density of fine globules (second fine globules) in the back surface sticking prevention layer, you remarkably smaller than the dispersion density of the fine particle material in the light diffusion layer (first minute particle bodies). The blending ratio of the second microparticles to the second binder in the back surface sticking prevention layer is also based on the blending ratio of the microparticles (first microparticles) to the binder (first binder) in the light diffusion layer. However, it is desirable to make it extremely small. Furthermore, it is desired that the thickness of the back surface sticking prevention layer is much smaller than the thickness of the light diffusion layer. The average particle diameter of the second fine particle material of the back sticking prevention layer shall be the much smaller average particle diameter than the average particle diameter of the first fine particle material of the light diffusing layer.

Specifically, the dispersion density of the second fine particles in the back surface sticking prevention layer is preferably 50 to 300 particles / mm ² . If the dispersion density of the second microparticles is less than 50 particles / mm ² , a sufficient anti-sticking effect may not be obtained, whereas if it exceeds 300 particles / mm ² , the light transmittance decreases. There is a fear. Here, the dispersion density means the average number of fine particles per unit area on the surface along the surface of the back surface sticking prevention layer.

  In addition, the blending ratio of the second fine particles and the second binder in the back surface sticking prevention layer is usually preferably in the range of about 0.2: 100 to 5: 100 in terms of mass ratio. If the blending ratio of the second microparticles is too small, the effect of preventing sticking cannot be obtained. On the other hand, if the blending ratio of the second microparticles is too large, the light transmittance of the back surface sticking prevention layer is lowered. There is a risk of it.

  The thickness of the back surface sticking prevention layer is desirably 2 to 7 μm. If the thickness is less than 2 μm, particles may be lost. On the other hand, if the thickness exceeds 7 μm, the light transmittance of the back surface sticking prevention layer may be reduced.

  The average particle size of the second fine particles of the back surface sticking prevention layer is preferably 5 to 12 μm. If the average particle size of the second fine particles is less than 5 μm, the sticking prevention effect may not be sufficient. On the other hand, if the average particle size of the second fine particles exceeds 12 μm, the particles may be lost.

  In addition, about a back surface sticking prevention layer, in order to exhibit a sticking prevention function, a part of 2nd microparticle is protruded from a 2nd binder, and the unevenness | corrugation is formed in the surface of a back surface sticking prevention layer deep.

〔Production method〕
The method for producing the light diffusion sheet (three-layer structure) of the embodiment as described above is not particularly limited, but it is usually desirable to apply the following method.

  That is, a coating liquid for the light diffusion layer is prepared by previously mixing the first fine particles for constituting the light diffusion layer, the first binder and a solvent (for example, toluene, methyl ethyl ketone), etc. A coating liquid for the back surface adhesion preventing layer is prepared by mixing the second fine particles for constituting the adhesion preventing layer, the second binder resin, a crosslinking agent and a solvent. And after applying and drying one of the coating liquid for the light diffusing layer and the coating liquid for the back surface adhesion preventing layer on one side of the film to be the base sheet, the other side What is necessary is just to apply | coat a coating liquid on the surface on the opposite side of a film, and to dry. Here, the crosslinking of the resin of the second binder in the back surface sticking prevention layer may be advanced by appropriately managing the temperature and time at the time of coating and drying. Alternatively, the light diffusing sheet after the coating step can be stored for several hours to several days in a high-temperature environment (for example, 40 ° C.), so that the crosslinking of the binder resin can be advanced.

  Examples of the present invention are shown below together with comparative examples.

[Example 1]
This example is a light diffusing sheet of the embodiment shown in FIG. 1, that is, a light having a three-layer structure in which a light diffusing layer is formed on the front side of a base sheet and a back surface sticking preventing layer is formed on the opposite side. In the production of the diffusion sheet, the binder resin of the light diffusion layer is uncrosslinked, and the back surface sticking prevention layer is an crosslinked example.
The following coating liquid A1 for forming a light diffusion layer not containing a crosslinking agent is applied to one side of a transparent polyethylene terephthalate film (“A4300” manufactured by Toyobo Co., Ltd., thickness: 100 μm) as a base sheet after drying. The bar coater was applied so that the work amount was 8.0 g / m ² .
The following coating liquid B1 containing HDI-based isocyanate as a crosslinking agent is used as a coating liquid for forming a back surface adhesion preventing layer on the surface opposite to the surface coated with the coating liquid A1 in the polyethylene terephthalate film. Then, it was applied by a bar coater so that the coating amount after drying was 4.6 g / m ² to obtain a light diffusion sheet. In addition, the binder of the light-diffusion layer after drying remains uncrosslinked, and the binder of the back surface sticking prevention layer after drying is crosslinked.
Here, the thickness of the light diffusion layer after drying (uncrosslinked) is 6.5 μm, and the thickness of the back surface adhesion preventing layer (crosslinked) after drying is 4.0 μm. Moreover, the average particle diameter of the whole polystyrene particle (1st microparticles | fine-particles) in the light-diffusion layer after drying is 8.3 micrometers. On the other hand, the dispersion density of the PMMA particles (second microparticles) in the back surface sticking prevention layer (crosslinked) after drying is about 120 particles / mm ² . In addition, the thickness of each layer enlarged the sheet | seat cross section with the microscope, measured 10 points | pieces (thickness of the part in which particle | grains do not exist) of the binder part, and made it the average value.
(Coating fluid A1)
Acrylic resin A (glass transition temperature Tg = 105 ° C.) 8.0 parts by mass crosslinked polystyrene particles (Techpolymer SBX-6 manufactured by Sekisui Plastics Co., Ltd .; average particle size 6.4 μm, no glass transition temperature)・ ・ 13.2 mass parts cross-linked polystyrene particles (Techpolymer SBX-12, manufactured by Sekisui Plastics Co., Ltd .; average particle size 11.7 μm, no glass transition temperature) 9.6 mass parts cross-linked polystyrene particles (sekisui) Techpolymer SBX-17 manufactured by Seisaku Kogyo Co., Ltd .; average particle size 16.1 μm, no glass transition temperature) 1.2 mass parts toluene 63.0 mass parts (coating liquid B1)
Acrylic resin A (glass transition temperature Tg = 105 ° C.) 19.6 parts by mass PMMA particles (Techpolymer SSX-108 manufactured by Sekisui Plastics Co., Ltd .; average particle size 8.0 μm, no glass transition temperature).・ 0.2 parts by mass HDI-based isocyanate (Coronate HL made by Nippon Polyurethane) ... 0.2 parts by mass Toluene ... 80.0 parts by mass

[Example 2]
A light diffusing sheet was obtained in the same manner as in Example 1 except that in the coating liquid A1 of Example 1, acrylic resin B (glass transition temperature Tg = 95 ° C.) was used instead of acrylic resin A.
In Example 2 as well, the binder of the light diffusion layer after drying remains uncrosslinked, and the binder of the back surface sticking prevention layer after drying is crosslinked.
Moreover, the thickness of the light diffusion layer after drying (uncrosslinked), the thickness of the back surface sticking prevention layer after crosslinking (crosslinked), the average particle diameter of the entire polystyrene particles (first microparticles) in the light diffusion layer, and The dispersion density of the PMMA particles (second fine particles) in the back surface sticking prevention layer is the same as in Example 1.

Example 3
In the coating liquid A1 of Example 1, it replaced with the acrylic resin A, and except having used the acrylic resin C (glass transition temperature Tg = 135 degreeC), it carried out similarly to Example 1, and obtained the light-diffusion sheet.
In Example 3 as well, the binder of the light diffusion layer after drying remains uncrosslinked, and the binder of the back surface adhesion preventing layer after drying is crosslinked.
Moreover, the thickness of the light diffusion layer after drying (uncrosslinked), the thickness of the back surface sticking prevention layer after crosslinking (crosslinked), the average particle diameter of the entire polystyrene particles (first microparticles) in the light diffusion layer, and The dispersion density of the PMMA particles (second fine particles) in the back surface sticking prevention layer is the same as in Example 1.

Example 4
A light diffusing sheet was obtained in the same manner as in Example 1 except that the acrylic resin D (glass transition temperature Tg = 85 ° C.) was used in place of the acrylic resin A in the coating liquid B1 of Example 1.
In Example 4 as well, the binder of the light diffusion layer after drying remains uncrosslinked, and the binder of the back surface sticking prevention layer after drying is crosslinked.
Moreover, the thickness of the light diffusion layer after drying (uncrosslinked), the thickness of the back surface sticking prevention layer after crosslinking (crosslinked), the average particle diameter of the entire polystyrene particles (first microparticles) in the light diffusion layer, and The dispersion density of the PMMA particles (second fine particles) in the back surface sticking prevention layer is the same as in Example 1.

Example 5
A light diffusing sheet was obtained in the same manner as in Example 1 except that the acrylic resin E (glass transition temperature Tg = 145 ° C.) was used in place of the acrylic resin A in the coating liquid B1 of Example 1.
In Example 5 as well, the binder of the light diffusion layer after drying remains uncrosslinked, and the binder of the back surface adhesion preventing layer after drying is crosslinked.
Moreover, the thickness of the light diffusion layer after drying (uncrosslinked), the thickness of the back surface sticking prevention layer after crosslinking (crosslinked), the average particle diameter of the entire polystyrene particles (first microparticles) in the light diffusion layer, and The dispersion density of the PMMA particles (second fine particles) in the back surface sticking prevention layer is the same as in Example 1.

[Comparative Example 1]
A light diffusing sheet was obtained in the same manner as in Example 1 except that the following coating liquid B2 not containing a crosslinking agent was used in place of the coating liquid B1 in Example 1.
In this example, the binder of the light diffusion layer after drying and the binder of the back surface adhesion preventing layer remain uncrosslinked.
In addition, the thickness of the light diffusion layer (uncrosslinked) after drying, the thickness of the back surface sticking prevention layer (uncrosslinked) after drying, the average particle diameter of the entire polystyrene particles (first microparticles) in the light diffusion layer, and The dispersion density of PMMA particles (second microparticles) in the back surface sticking prevention layer is the same as in Example 1.
(Coating fluid B2)
Acrylic resin A (glass transition temperature Tg = 105 ° C.) 19.8 parts by mass PMMA particles (Techpolymer SSX-108 manufactured by Sekisui Plastics Co., Ltd .; average particle size 8.0 μm, no glass transition temperature).・ 0.2 parts by mass Toluene ... 80.0 parts by mass

[Comparative Example 2]
A light diffusing sheet was obtained in the same manner as in Example 1 except that instead of the coating liquid A1 in Example 1, the following coating liquid A2 containing HDI isocyanate as a crosslinking agent was used.
In this example, the binder of the light diffusion layer after drying and the binder of the back surface adhesion preventing layer are both crosslinked.
Moreover, the thickness of the light diffusion layer after drying (crosslinked), the thickness of the back surface sticking prevention layer after crosslinking (crosslinked), the average particle diameter of the entire polystyrene particles (first microparticles) in the light diffusion layer, and The dispersion density of the PMMA particles (second fine particles) in the back surface sticking prevention layer is the same as in Example 1.
(Coating fluid A2)
Acrylic resin A (glass transition temperature Tg = 105 ° C.) 7.8 mass parts cross-linked polystyrene particles (Sekisui Plastics Co., Ltd. Techpolymer SBX-6; average particle size 6.4 μm, no glass transition temperature) ··· 13.2 parts by mass crosslinked polystyrene particles (Techpolymer SBX-12, manufactured by Sekisui Plastics Co., Ltd .; average particle size average particle size 11.7 µm, no glass transition temperature) ··· 9.6 parts by mass crosslinked polystyrene particles (Techpolymer SBX-17 manufactured by Sekisui Plastics Co., Ltd .; average particle size 16.1 μm, no glass transition temperature) 1.2 parts by mass HDI-based isocyanate (Coronate HL manufactured by Nippon Polyurethane) 0.2 Part by mass Toluene 68.0 parts by mass

[Comparative Example 3]
In Example 1, the coating liquid B2 containing no crosslinking agent was used instead of the coating liquid B1, and the coating liquid B2 was coated with a coating amount of the back surface adhesion preventing layer after drying of 8.0 g / except that was applied by a bar coater so that the m ^2, thereby obtaining the light diffusion sheet in the same manner as in example 1.

  The light diffusion sheets obtained in Examples 1 to 5 and Comparative Examples 1 and 2 were evaluated for sticking as follows, and the light diffusion sheets were evaluated for the curl phenomenon. The evaluation results are shown in Table 2.

[Sticking evaluation]
Assuming that the light diffusion sheet is used as a panel light member, an adhesion test with an acrylic plate was performed.
The light diffusion sheet and the acrylic plate are stacked so that the back surface sticking prevention layer of the light diffusion sheet obtained in Examples 1 to 6 and Comparative Examples 1 to 3 is in contact with the acrylic plate (Acrylite, manufactured by Mitsubishi Rayon). A load of ² kg / cm ² was applied to the sample and allowed to stand at 60 ° C. and 90% RH for 2 weeks, and then the light diffusion sheet was confirmed to adhere to the acrylic plate. Evaluation is as follows visually.
Adhesion between the acrylic plate and the sheet is not seen. : ○
The adhesion between the acrylic plate and the sheet is partially seen. : △
Adhesion between the acrylic plate and the sheet can be seen almost entirely. : ×

[Curl evaluation]
The light diffusion sheet was cut into a 10 cm square, the height of the four corners of the sheet was measured with a metal ruler, and the average value was obtained. In addition, the numerical value of curl was defined as a positive value at the four corners of the sheet when the light diffusing layer side was bent, and conversely, a negative value was set at the four corners of the sheet when the back sticking prevention layer side was bent.

  As is clear from Table 2, it was confirmed that the light diffusing sheets of Examples 1 to 5 of the present invention did not cause curl or even slightly caused curl. Moreover, it was confirmed that sticking hardly occurs in the light diffusion sheets of Examples 1 to 5 of the present invention.

On the other hand, it was confirmed that in the light diffusion sheet of Comparative Example 1 in which the binder resin of the back surface sticking prevention layer was not crosslinked, the curl was large and sticking was remarkably generated.
Further, in the light diffusing sheet of Comparative Example 2 in which the binder resin of the light diffusing layer was cross-linked, the curl was greatly generated in the reverse direction (the direction in which the light diffusing layer becomes a convex curved surface).

  In Comparative Example 3 in which the binder resin of the back surface sticking prevention layer was not cross-linked and the thickness of the back sticking prevention layer was increased by increasing the coating amount of the back surface sticking prevention layer, no curling occurred. , Sticking has occurred remarkably.

  The preferred embodiments and examples of the present invention have been described above, but the present invention is not limited to these embodiments and examples. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Base material sheet, 3 ... Light diffusion layer, 5 ... Light diffusion sheet, 7 ... 1st microparticle, 9 ... 1st binder, 11 ... Back surface sticking Anti-sticking layer, 13 ... second fine particles, 15 ... second binder.

Claims (4)

A transparent base sheet having flexibility;
A light diffusion layer laminated on the first surface of the base sheet;
It has a back surface sticking prevention layer laminated on the second surface opposite to the first surface in the base sheet with a thickness smaller than the thickness of the light diffusion layer,
The light diffusion layer includes a large number of transparent first microparticles arranged in a dispersed manner, and a first binder for fixing and holding the first microparticles on the first surface of the base sheet. Prepared,
The back surface sticking prevention layer includes a large number of transparent second fine particles dispersedly arranged, and a second binder for fixing and holding the second fine particles on the second surface of the base sheet. With
Further, the first binder is made of an uncrosslinked resin, and the second binder is made of a crosslinked resin,
The second fine particles have an average particle size smaller than the average particle size of the first fine particles, and the dispersion density of the second fine particles in the back surface anti-sticking layer is the light. A light diffusion sheet, wherein the light diffusion sheet is smaller than a dispersion density of the first microparticles in the diffusion layer.   The light diffusion sheet according to claim 1, wherein the light diffusion layer has a thickness of 3 to 15 μm, and the back surface sticking prevention layer has a thickness of 2 to 7 μm.   The light diffusion sheet according to any one of claims 1 and 2, wherein the first binder is an uncrosslinked acrylic resin, and the second binder is a crosslinked acrylic resin. 4. The average particle size of the first microparticles is 8.3 to 30 μm, and the average particle size of the second microparticles is 5 to 12 μm. The light diffusing sheet described in 1.

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