JP2012212000A - Light reflection plate and backlight unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light reflection plate capable of responding to recent displays which are made to be thinner and made of thermoplastic polyester foam capable of achieving high luminance.SOLUTION: A light reflection plate for a backlight unit is made of polyester foam, and includes fine air bubbles having an average bubble diameter equal to or less than 50 μm therein, and its reflection surface side has arithmetic average surface roughness equal to or less than 1.2 μm. Preferably, the light reflection plate is obtained by cutting a fine air bubble layer of a base material including the fine air bubble layer and skin layers on both sides thereof at a cutting surface perpendicular to the thickness direction. Furthermore, preferably, a luminance difference ΔL (≡Ls-Lo) which is a difference between luminance Ls of a surface of the light reflection plate on the cutting surface side and luminance Lo of a surface of the light reflection plate on the opposite side of the cutting surface is equal to or greater than 10 cd/m.

Description

  The present invention relates to a light reflecting plate capable of obtaining high luminance, particularly a light reflecting plate used in a backlight unit such as a liquid crystal television, and a backlight unit using the same.

  Until now, direct-type backlights have been employed in large-screen liquid crystal display devices such as liquid crystal televisions. However, in recent years, liquid crystal televisions have been made thinner, and the backlights employed in liquid crystal televisions have shifted to edge light systems that can be designed to be thin.

  In the edge light type backlight unit, a backlight unit having a structure as shown in FIG. 5 is generally used in order to obtain a function as a surface light source. The liquid crystal backlight unit 11 shown in FIG. 5 is a two-edge type backlight in which the light sources are arranged on the upper and lower sides of the frame among the edge light methods, and includes a light source 13 and a light guide plate 15. A light reflecting plate 1 is provided on the back surface. As the light source 13, CCFL (cold cathode fluorescent lamp) has been mainstream until now, but the transition to LED (light emitting diode) is progressing. Light 17 emitted from the light source 13 enters the light guide plate 15 and propagates through the light guide plate 15 while repeating total reflection. At this time, a part of the light is diffused and emitted from the surface of the light guide plate 15 to the outside due to a light scattering mechanism called a pattern formed inside or on the surface of the light guide plate 15. That is, the light guide plate 15 has a function of emitting light emitted from the side surface to the entire surface of the light guide plate 15, but does not contribute to the brightness of the display with respect to light emitted to the surface opposite to the display side. Therefore, the light reflection plate 1 plays a role of reflecting such light and directing it toward the display side. As one of the light reflectors, a light reflector made of a thermoplastic polyester foam having fine bubbles inside, a thickness of 200 μm or more and a specific gravity of 0.7 or less is known (Patent Document 1).

Japanese Patent No. 2925745

  It is widely known that a conventional light reflecting plate exhibits excellent performance as a reflecting plate for a liquid crystal television because it has a higher reflectance than other reflecting plates due to its fine bubble structure. However, when the sheet thickness is reduced in accordance with the recent needs for thinning of liquid crystal televisions, there is a problem in that the reflectance decreases, and as a result, sufficient luminance cannot be obtained on the display.

  The present invention has been made in view of such problems, and a light reflecting plate made of a thermoplastic polyester foam that can cope with the thinning of displays in recent years and that can obtain high luminance, and the same are used. An object is to provide a backlight unit.

  A first invention is a light reflecting plate for a backlight unit, which is made of a polyester foam and has fine bubbles with an average bubble diameter of 50 μm or less inside, and has an arithmetic average surface roughness of 1 on the reflecting surface side. A light reflector having a thickness of 2 μm or less.

  According to the first invention, since it has fine bubbles inside and the surface roughness on the reflecting surface side is 1.2 μm or less, high brightness can be obtained as compared with the conventional light reflecting plate having the same thickness. Can do. In other words, by setting the surface roughness of the skin layer on the reflective surface side to 1.2 μm or less, the specular component of the reflected light can be increased as compared with the conventional case, and as a result, high luminance can be obtained.

A second invention includes the light reflecting plate according to the first invention, a light guide plate provided on the light reflecting plate, and a light source provided on a side of the light guide plate. It is a backlight unit.
According to the second invention, it is possible to obtain a backlight unit having a high brightness while being thin.

  According to a third aspect of the present invention, there is provided a step of holding a resin sheet made of a polyester resin in a pressurized inert gas atmosphere so that the resin sheet contains an inert gas, and the resin sheet containing the inert gas. And a step of foaming by heating under pressure, having a plurality of bubbles having an average bubble diameter of 50 μm or less inside, and having an arithmetic average surface roughness of 1.2 μm or less on the reflecting surface side It is a manufacturing method of the light reflection board which consists of foams.

ADVANTAGE OF THE INVENTION According to this invention, the light reflection board which consists of a thermoplastic polyester foam which can respond to the thinning of a display in recent years, and can obtain high brightness, and a backlight unit using the same can be provided.
That is, since the light reflecting plate according to the present invention has a high reflectance despite the same sheet thickness as the conventional thermoplastic polyester foam having fine bubbles, it is used as a reflecting plate for the backlight unit. , The brightness can be improved.

The cross-sectional schematic diagram of the light reflection board which concerns on this invention. The cross-sectional schematic diagram of the other example of the light reflection board which concerns on this invention. The figure which shows the manufacturing process of the light reflection board which concerns on this invention. The plot of the arithmetic mean roughness and luminance difference (DELTA) L of the light reflection board which concerns on an Example and a comparative example. 2A is a schematic plan view of an edge light type backlight unit, and FIG. 2B is a schematic diagram of an A-A ′ cross section in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a light reflecting plate 1 according to the present invention. The light reflecting plate 1 is a sheet-like thermoplastic polyester foam having a large number of fine bubbles 3 therein. As the resin used for the light reflecting plate 1, polyethylene terephthalate is particularly preferable. The arithmetic mean roughness Ra (JIS B0601-1994) on the reflecting surface side of the light reflecting plate 1 is 1.2 μm or less.

  Further, as shown in FIG. 1, the skin layer 5b is often provided on the opposite side of the reflection surface. However, if the reflection surface of the light reflection plate 1 is provided, the skin layer 5b may or may not be provided. The skin layer 5b is a layer having a high density on the surface of a member having a thickness of about 10 to 30 μm, which is generated when a polyester foam is molded and foamed.

The light reflecting plate 1 is a resin base material such as a foam, and is formed in a sheet shape, a film shape, a plate shape, or the like. More specifically, a thermoplastic resin sheet having fine bubbles or pores having an average cell diameter of 100 nm or more and 50 μm or less inside can be suitably used. When the average cell diameter exceeds 50 μm, the surface becomes rough and the mechanical strength of the resin sheet becomes insufficient. Moreover, the average bubble diameter is generally 10 μm or less, and may be 1 μm or less.
The average cell diameter can be determined according to ASTM D3576-77. That is, an SEM photograph of a cross section of the thermoplastic resin foam was taken, a straight line was drawn in the horizontal direction and the vertical direction on the SEM photograph, and the length t of the bubble strings crossed by the straight line was averaged. Assuming that the magnification of the photograph is M, the average bubble diameter d can be obtained by substituting it into the following equation.
d = t / (0.616 × M)

  As shown in FIG. 3, the light reflector 1 has a fine bubble layer 9, and a substrate 7 having a skin layer 5 b on both sides is cut by a cut surface 10 orthogonal to the thickness direction. It is preferable to obtain. By cutting one base material 7, a thin light reflection plate 1 can be obtained, and two light reflection plates can be obtained from one base material, which is excellent in mass productivity.

Further, a luminance difference which is a difference between the luminance Ls of the sliced surface on the cut surface 10 side of the light reflecting plate 1 and the luminance Lo of the surface opposite to the cut surface 10 (on the skin layer 5b side when there is a skin layer 5b). ΔL (≡Ls−Lo) is preferably 10 cd / m ² or more. When the single substrate 7 is used without being cut, the surface of the substrate 7 before cutting (the surface on the skin layer 5b side when the skin layer 5b is present) is used as the reflecting surface. That is, if the luminance difference is positive, it means that the luminance of the slice surface is increased compared to the surface different from the surface on the slice side, that is, the surface of the base material 7 before cutting that occurs during foaming / molding. . On the other hand, when the luminance difference is negative, it means that the luminance of the slice surface is lower than the surface of the base material 7 before cutting. In particular, when the luminance difference is 10 cd / m ² or more, an increase in luminance can be experienced with the naked eye. Incidentally, the luminance difference is usually at 100 cd / cm ² or less, are often at 50 cd / cm ² or less.

  The thickness of the light reflection plate 1 is preferably 100 μm or more. If the thickness of the light reflecting plate 1 is less than 100 μm, the mechanical strength is weak and wave wrinkles are likely to occur, which is not preferable. Further, the thickness of the light reflection plate 1 is desirably 1 mm or less. If the thickness of the light reflecting plate 1 exceeds 1 mm, the amount of material used increases, the cost increases, and the demand for thinning currently required in the market cannot be met.

  Further, the specific gravity of the light reflecting plate 1 is preferably 0.7 or less. If the specific gravity is large, the number density of bubbles is not sufficient, and the effect of fine bubbles is reduced.

  Further, in order to ensure the performance of the light reflecting plate, the total reflectance of the light reflecting plate is desirably 90% or more. In addition, the light reflectance as used in this specification means the relative light reflectance with respect to the reflecting plate of a reference. Therefore, when the light reflectance is the same as that of the reference reflector, the light reflectance is 100%. When the light reflectance is higher than that of the reference reflector, the light reflectance exceeds 100%. When the light reflectance is lower than that of the plate, the light reflectance is less than 100%. In the present invention, a white plate in which fine powder of aluminum oxide is hardened is used as a reference.

  In addition, when the light reflection board 1 is a polyester foam, it is desirable that a crystallinity degree is 30% or more. If the degree of crystallinity is less than 30%, it is not preferable because it is soft and easily wrinkles.

  In this invention, you may add 0.1-10 weight part of polyester-type elastomer with respect to 100 weight part of thermoplastic polyester resins. When the addition amount of the polyester elastomer is less than 0.1 parts by weight, the cell diameter of the obtained foam tends to be large, and the dispersion tends to be non-uniform. When the addition amount of the polyester elastomer exceeds 10 parts by weight, it is disadvantageous in terms of cost. The addition amount is more preferably 1 to 5 parts by weight, still more preferably 1 to 3 parts by weight.

  Further, a bead coat can be applied to the surface of the light reflecting plate 1 on the reflecting surface side. If the beads are uniformly applied, the gap with the light guide plate can be kept constant, which is more effective in preventing uneven brightness of the liquid crystal panel. As the beads, for example, hard glass, polymethyl methacrylate (PMMA), or soft polybutyl methacrylate (PBMA) can be used.

  In addition, you may have the skin layer 5a in the reflective surface side like the light reflection board 1a shown in FIG. This skin layer 5a is a dense, flat, thermoplastic resin layer having a thickness of more than 0 μm, in which fine bubbles 3 are hardly distributed and bubbles are not exposed on the surface. That is, the skin layer 5a is a layer that smoothes the surface so that fine bubbles are not exposed on the surface even if the thickness is small. The thickness of the skin layer 5a is preferably 7 μm or less. By reducing the thickness of the skin layer to 7 μm or less, light absorption by the skin layer can be suppressed, so that the luminance can be further improved.

  In addition, additives such as an antioxidant, an ultraviolet ray inhibitor, a lubricant, a pigment, and a reinforcing agent can be appropriately added to the resin used for the sheet or film.

[Method for producing thermoplastic polyester foam sheet]
Next, a method for manufacturing the light reflecting plate 1 will be described. First, a roll is formed by overlapping and winding a thermoplastic polyester sheet and a separator. The separator used here may be any one as long as it has a void through which an inert gas or an organic solvent used as needed freely enters and exits, and the permeation of the inert gas into itself can be ignored. . As the separator, a resin nonwoven fabric or a metal net is particularly suitable. The resin nonwoven fabric is preferably made of polyolefin resin or nylon resin. Moreover, even if it is a nonwoven fabric which consists of a polyester-type resin, if the fiber is extended | stretched and it is difficult for the inert gas to penetrate | permeate, it can be used conveniently. The metal net is generally called a wire cloth, and is preferably a weaving method such as plain weave, twill weave, plain tatami weave, twill tatami weave and the like in which vertical lines and horizontal lines are knitted in a perpendicular direction. Iron, copper, aluminum, titanium, or an alloy thereof can be used as the material, but stainless steel is more preferable in consideration of price and life. On the other hand, the thermoplastic polyester sheet is preferably unstretched. This is because when the thermoplastic polyester sheet is stretched, the gas does not sufficiently permeate into the sheet, and the intended foamed sheet cannot be obtained.

(Organic solvent containing process)
When an organic solvent is contained in the sheet, the crystallinity of the thermoplastic polyester sheet can be increased to 30% or more. As a result, the rigidity of the sheet is increased and it is difficult for the trace of the separator to remain on the surface of the sheet, and the permeation time of the inert gas can be shortened. Depending on the type of separator, the trace of the separator may not remain on the surface of the sheet, so that the treatment containing the organic solvent is not necessarily required. However, from the viewpoint of shortening the gas permeation time, it is preferable to carry out a treatment containing an organic solvent.

  Organic solvents used to increase the crystallinity of the resin sheet include benzene, toluene, methyl ethyl ketone, ethyl formate, acetone, acetic acid, dioxane, m-cresol, aniline, acrylonitrile, dimethyl phthalate, nitroethane, nitromethane, benzyl alcohol Etc. Of these, acetone is more preferable from the viewpoints of handleability and economy.

  As a method for containing the organic solvent in the roll, a method of immersing the roll in the organic solvent or a method of holding the roll in the vapor of the organic solvent is used. The latter method is superior to the former method in that an organic solvent may be used in a small amount and the additive added to the thermoplastic polyester sheet is hardly eluted.

  In the method of immersing the roll in the organic solvent, it is sufficient that the treatment time is several hours to several tens of hours at room temperature, and the content of the organic solvent in the resin does not increase so much even if the treatment is performed for a longer time. The treatment time when the roll is held in the vapor of the organic solvent to contain the organic solvent varies depending on the type of resin to be foamed and the thickness of the sheet. For example, when a roll made of polyethylene terephthalate is put into a closed container saturated with acetone vapor at room temperature and atmospheric pressure and acetone is infiltrated, if the sheet thickness is 0.6 mm, the processing time is 24 hours or more, 0 .9 mm is sufficient if it is 48 hours or longer. By such treatment, the content of acetone in the resin becomes 4 to 5% by weight.

If the roll is preliminarily containing an organic solvent to increase the crystallization degree of the sheet, it is not necessary to increase the crystallization degree by containing a large amount of inert gas. Can be shortened. For example, when carbon dioxide is permeated into a polyethylene terephthalate sheet at 60 kg / cm ² , if the sheet thickness is 0.6 mm, the permeation time is 1 hour or longer, and if it is 0.9 mm, 2 hours or longer is sufficient.

  Moreover, when a roll-shaped sheet | seat is immersed in the organic solvent, you may rewind a sheet | seat with the separator which prepared the sheet separately before the next process, and may produce a roll newly.

(Inert gas impregnation process)
Next, the roll formed as described above is placed in a high-pressure vessel and held in a pressurized inert gas atmosphere so that the thermoplastic polyester sheet contains an inert gas serving as a foaming agent. Examples of the inert gas include helium, nitrogen, carbon dioxide, and argon. Among these, carbon dioxide is preferable because it can be contained in a large amount in the thermoplastic polyester. Penetration pressure of the inert gas is preferably set to 30~70kg / cm ^2, further 50 kg / cm ² or more. The infiltration time of the inert gas is 1 hour or more, and more preferably the gas is infiltrated until it becomes saturated. The gas permeation time and gas permeation amount until saturation is reached vary depending on the type of resin to be foamed, the type of inert gas, the permeation pressure, and the thickness of the sheet. For example, when carbon dioxide is permeated into a polyethylene terephthalate sheet at 60 kg / cm ² , the permeation time is preferably 24 hours or longer if the sheet thickness is 0.6 mm, and 96 hours or longer if 0.9 mm. Under such conditions, the content of carbon dioxide in the resin is 6 to 7% by weight.

(Foaming process)
Further, the roll is taken out from the high-pressure vessel, and only the thermoplastic polyester sheet containing the inert gas is foamed by removing the separator. Under the present circumstances, after taking out from a high pressure container, the bulk specific gravity of the foam obtained can be adjusted by adjusting the time until it makes it foam.

  Specifically, the longer this time, the larger the bulk specific gravity. The heating temperature at the time of foaming is set to be not lower than the melting point and not lower than the glass transition point of the resin. Examples of the heating means include a hot air circulation type foaming furnace, an oil bath, a molten salt bath, and the like, but it is preferable to use a hot air circulation type foaming furnace from the viewpoint of handleability. The foaming conditions in the hot-air circulating foaming furnace are set to a linear velocity such that the foaming temperature is about 240 ° C. and the foaming time is 1 to 5 minutes. Thereafter, the foamed sheet coming out of the furnace is wound up on a thermoforming roll whose temperature is adjusted to 150 ° C. or higher, and cooled to obtain a thermoplastic polyester foam sheet.

(Cutting process)
FIG. 3 is a diagram for explaining a light reflecting plate cutting step. The obtained thermoplastic polyester foam sheet (base material 7) has a fine bubble layer 9, and has a skin layer 5b on both sides. The obtained base material 7 is cut | disconnected by the cut surface 10 orthogonal to a thickness direction, for example in the half position of the thickness direction. By cutting one base material 7, a thin light reflecting plate can be obtained, and two light reflecting plates can be obtained from one base material, which is excellent in mass productivity.

  As a cutting method on the cut surface 10, the base material 1a may be sliced, and may be a band knife method or a canna method. For example, in the band knife method, a surface layer having an arithmetic average roughness of the cut surface of 1.2 μm or less is formed by appropriately adjusting the rotation speed of the band knife and the feed speed of the substrate 1a. For example, it is performed at a knife rotation speed of 1.0 to 1.5 m / s and a line speed of 10 to 15 m / min. In the band knife method, a band knife with a belt-like blade is used as a ring, and the polyester foam sheet is sent to the rotated band knife for cutting. In the band knife method, the cut portion is rubbed with a rotated band knife, so that the skin layer 5a is easily formed.

  Even in the half-cutting by the Kanna method, a surface layer having an arithmetic average roughness of 1.2 μm or less is formed on the cut surface by appropriately adjusting the linear velocity. For example, the linear velocity is 15 to 30 m / min. Thus, the light reflecting plate 1 is formed. Note that the Kanna method is a method in which the blade is pressed against the cut surface of the foam sheet and cut, and for example, the foam sheet is sent to the fixed blade and the cut surface is cut. In the Kanna method, the cut portion tends to leave the state at the time of cutting, so that the skin layer 5a is difficult to be formed and bubbles are easily exposed.

(Effect of the present invention)
According to the present invention, by cutting a single substrate with a fine envelope layer to obtain a surface having a predetermined roughness or less, the thickness of the light reflector is reduced, the mass productivity is improved, and the luminance on the reflective surface side is increased. Can be improved at the same time.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such examples. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the technical idea disclosed in the present application, and these are naturally within the technical scope of the present invention. Understood.

  After adding and kneading 2 parts by weight of a polyester elastomer (Mitsubishi Chemical Corporation, Primalloy (registered trademark) B1942N) to 100 parts by weight of polyethylene terephthalate (Nihon Unipet Co., Ltd., RT-553C), 0.48 mm The sheet was formed into a sheet of thickness × 540 mm width × 355 m length. This resin sheet and an olefin-based non-woven fabric separator were overlapped and rolled into a roll shape so that there was no portion where the surfaces of the resin sheets contacted each other.

  Thereafter, the roll was put in a pressure vessel, pressurized to 5.2 MPa with carbon dioxide, and carbon dioxide was permeated into the resin sheet. The carbon dioxide gas permeation time into the resin sheet was 35 hours. Next, the roll was taken out from the pressure vessel, and only the resin sheet was continuously supplied to a hot air circulation type foaming furnace set at 220 ° C. while removing the separator, and foamed. The obtained foam was uniformly foamed, and the average cell diameter was as very fine as 0.9 μm. The thickness of the foam was 900 μm, and the total reflectance of the foam sheet was 99.9%. Moreover, the thickness of the skin layer formed on both surfaces of the foam sheet was 20 μm.

  The obtained foam sheet was cut in half with a band knife type or canna type slicer to obtain a light reflecting plate. Light reflectors with different surface roughness were obtained by changing the blade type, knife rotation speed, line feed speed, and the like.

[Evaluation method]
(Surface roughness)
Using a surface roughness meter (SJ-201, manufactured by Mitutoyo Corporation), the arithmetic average roughness Ra of the surface on the reflecting surface side of the obtained light reflecting plate was measured. In addition, the measurement was implemented with respect to the direction orthogonal to MD of a light reflection board, and the cutoff value was set to 0.8 mm. In addition, the surface roughness of the surface having the skin layer formed at the time of molding and foaming of the reference reflector was Ra = 1.23 μm.

(Luminance evaluation)
The front luminance when the obtained light reflection plate was applied to the reflection plate of the backlight unit was measured using a surface luminance meter (CA-2000, manufactured by Konica Minolta Co., Ltd.). The light reflecting plate is installed so that its MD coincides with the longitudinal direction of the backlight unit. The luminance Ls when the side sliced by the slicer is the reflective surface side and the luminance Lo when the opposite surface to the slice side is the reflective surface side are measured, and the difference ΔL≡Ls−Lo is obtained. The evaluation value of luminance was used. The luminance evaluation results are shown in Table 1 and FIG.

According to Table 1 and FIG. 4, it can be seen that the luminance difference increases as the arithmetic average roughness on the reflecting surface side decreases. In particular, in examples where the arithmetic average roughness is 1.2 μm or less, the luminance difference is greater than 10 cd / m ^2, and it can be seen that the luminance of the backlight unit using the light reflector according to the examples is high. Further, in each example, there was no skin layer on the reflective surface side, or the thickness was 7 μm or less even if there was a skin layer.

DESCRIPTION OF SYMBOLS 1 ......... Light reflecting plate 3 ......... Bubble 5 ......... Skin layer 7 ......... Base material 9 ......... Fine bubble layer 10 ......... Cut surface 11 ......... Backlight unit 13 ......... Light source 15 ……… Light guide plate 17, 19 ……… Light

Claims (8)

A light reflector for a backlight unit,
Made of polyester foam,
It has fine bubbles with an average bubble diameter of 50 μm or less inside,
A light reflecting plate having an arithmetic average surface roughness on the reflecting surface side of 1.2 μm or less.   2. The light reflecting plate according to claim 1, wherein the light reflecting plate is obtained by cutting a substrate having a fine bubble layer at a cut surface perpendicular to the thickness direction. The luminance difference ΔL (≡Ls−Lo), which is the difference between the luminance Ls of the surface on the cut surface side of the light reflecting plate and the luminance Lo of the surface opposite to the cut surface, is 10 cd / m ² or more. The light reflecting plate according to claim 2, wherein Having a skin layer on the reflective surface side of the light reflector;
The light reflecting plate according to claim 1, wherein the skin layer has a thickness of 7 μm or less.   The light reflecting plate according to claim 1, wherein the light reflecting plate is made of a polyester foam having a thickness of 100 μm or more.   A light reflecting plate according to any one of claims 1 to 5, a light guide plate provided on the light reflecting plate, and a light source provided on a side of the light guide plate. Backlight unit to be used. Holding a resin sheet made of a polyester resin in a pressurized inert gas atmosphere and containing the inert gas in the resin sheet; and
Heating and foaming the resin sheet containing the inert gas, and
It has a plurality of bubbles with an average bubble diameter of 50 μm or less inside,
A method for producing a light reflector made of a polyester foam, wherein the arithmetic average surface roughness on the reflective surface side is 1.2 μm or less.   Furthermore, the manufacturing method of the light reflection board of Claim 7 provided with the process cut | disconnected by the cut surface orthogonal to the thickness direction after the said foaming process.

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