JP2008020747A - Light guide plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light guide plate which has a small retardation and can reduce a loss of light on the surface. <P>SOLUTION: The light guide plate 5 is molded by using a resin composition comprising acrylic resin (a) and aliphatic polyester resin (b), whereby retardation can be made small based on the effect of positive and negative birefringent polymers. Therein, by using the aliphatic polyester resin (b) which has high transparency and a low refractive index and is well compatible with the acrylic resin (a), the refractive index can be suppressed low and the loss of the light emitted from a cold cathode tube 1 on the surface of the light guide plate 5 can be minimized. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light guide plate used for a backlight of a liquid crystal display.

  With the recent expansion of the display market, the use of transparent resin materials is increasing as a backlight for liquid crystal displays. The backlight light guide plate refers to a unit that propagates and diffuses light incident from a certain direction and emits light on the liquid crystal unit surface side. As a backlight method, there are a direct type method in which a light guide plate is sandwiched between a light source and a liquid crystal unit, and an edge light method in which a light source is arranged on an edge of the light guide. In recent years, there has been a growing demand for thinness, and the edge light system has become mainstream. In the edge light method, the light transmission distance in the light guide is longer than the direct type, so that the light used in the light guide plate has a high light transmittance to prevent light loss in the light guide plate. Is essential. In addition, the design of the light guide plate is indispensable to emit light incident from the edge to the entire side surface of the liquid crystal unit of the light guide plate.

  As a method for uniformly emitting light incident from the edge of the light guide plate to the entire surface of the light guide plate, a method of performing dot gradation printing on one side (for example, refer to Patent Document 1), and forming unevenness to diffuse light on one side There are a method (for example, see Patent Document 2), a method of attaching a film with gradation, and the like. In general, an injection molding method is used for forming a thin and precise light guide plate and unevenness for diffusing light. In that case, the thinner the residual strain at the time of molding, the greater the retardation, which is an optical characteristic generated from the strain. When the retardation increases, the polarization characteristics of the light emitted from the light guide body are uneven. Since the liquid crystal display device uses the polarization characteristics of light to turn on and off the passage of light, the polarization characteristics of light emitted from the light guide that is a component of the surface light source are as uniform as possible. The polarization characteristics are good. In addition, residual stress remains as a cause of retardation.

As a technique for reducing them, attempts have been made to increase the fluidity of transparent thermoplastic resins. However, in order to increase fluidity, the molecular weight of the resin must be lowered, resulting in a decrease in mechanical strength, which causes a problem in processing or in practical use.
Further, the light loss of the light guide plate occurs on the incident surface from the light source of the light guide plate. This occurs due to surface reflection due to a difference in refractive index between the light guide and air. In order to reduce this, a light guide plate made of a material having a refractive index as small as possible is required.

At present, an acrylic resin, which is a transparent resin material mainly used for a light guide plate, has a large refractive index in a direction perpendicular to the stretched direction due to the characteristics of the resin. Therefore, the acrylic resin molded product has a problem that a certain amount of retardation occurs. Molding here is not limited to injection molding, but also applies to extrusion molding, cast molding, and the like.
Some transparent materials have a higher refractive index in the direction of stretching, such as polycarbonate resin. However, ordinary transparent resins have problems of poor compatibility with acrylic resins, haze values, and light transmittance.
JP-A-6-194526 JP-A-61-127506

  An object of the present invention is to provide a light guide plate that has low birefringence and suppresses the occurrence of retardation as much as possible and has a small absolute value of refractive index.

The inventors of the present invention have made extensive studies in order to further reduce the retardation generated in the formation of the light guide plate. As a result, a light guide plate having a low birefringence could be obtained by molding the light guide plate from a resin composition comprising an acrylic resin and an aliphatic polyester resin. Furthermore, the material can reduce the refractive index while maintaining the transparency of the acrylic resin alone, and can reduce the light loss on the light incident surface.
That is, the present invention is as follows.
1. Acrylic resin (a) more than 50 parts by mass and 99.9 parts by mass or less, aliphatic polyester resin (b) consisting of a resin composition consisting of 0.1 parts by mass and less than 50 parts by mass, and refraction by 633 nm laser light A light guide plate having a rate of 1.456 or more and 1.491 or less.

  According to the present invention, it is possible to obtain a light guide plate having not only transparency but also a small retardation and a small absolute value of refractive index.

Hereinafter, the present invention will be specifically described.
The acrylic resin (a) in the present invention is a methacrylic acid ester such as cyclohexyl methacrylate, t-butyl cyclohexyl methacrylate, methyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, isopropyl acrylate, acrylic acid 2 -A polymer obtained by polymerizing one or more monomers selected from acrylic acid esters such as ethylhexyl.

  Among them, a homopolymer of methyl methacrylate or a copolymer with other monomers is preferable from the viewpoint of heat resistance. Examples of monomers copolymerizable with methyl methacrylate include other alkyl methallylic esters, alkyl acrylates, aromatic vinyl compounds such as styrene, vinyl toluene and α-methyl styrene, acrylonitrile, methacrylonitrile, etc. Vinyl cyanides, maleimides such as N-phenylmaleimide and N-cyclohexylmaleimide, unsaturated carboxylic acid anhydrides such as maleic anhydride, and unsaturated acids such as acrylic acid, methacrylic acid and maleic acid.

  Among these monomers that can be copolymerized with methyl methacrylate, alkyl acrylates are particularly excellent in heat decomposability, and methacrylic resins obtained by copolymerizing acrylates are fluid during molding. Is preferable. In the case of copolymerizing acrylic acid esters with methyl methacrylate, the amount of alkyl acrylates used is preferably 0.1% by mass or more from the viewpoint of thermal decomposition resistance, and 15% by mass from the viewpoint of heat resistance. The following is preferable. The content is more preferably 0.2% by mass or more and 14% by mass or less, and particularly preferably 1% by mass or more and 12% by mass or less. Among these acrylate esters, methyl acrylate and ethyl acrylate are particularly most preferable in terms of the above improvement effect even if they are copolymerized with a small amount of methyl methacrylate. The said monomer which can be copolymerized with methyl methacrylate can also be used 1 type or in combination of 2 or more types.

  The acrylic resin (a) preferably has a weight average molecular weight of 50,000 to 200,000. The weight average molecular weight is preferably 50,000 or more from the viewpoint of the strength of the molded product, and preferably 200,000 or less from the viewpoint of molding processability and fluidity. A more preferable range is 70,000 to 150,000. In the present invention, isotactic polymethacrylate and syndiotactic polymethacrylate can be used simultaneously.

  As a method for producing the acrylic resin (a), for example, generally used polymerization methods such as cast polymerization, bulk polymerization, suspension polymerization, solution polymerization, emulsion polymerization, and anionic polymerization can be used. In this case, it is preferable to avoid the introduction of minute foreign substances as much as possible. From this viewpoint, bulk polymerization or solution polymerization without using a suspending agent or an emulsifier is desirable. When solution polymerization is performed, a solution prepared by dissolving a mixture of monomers in an aromatic hydrocarbon solvent such as toluene or ethylbenzene can be used. In the case of polymerization by bulk polymerization, the polymerization can be started by irradiation with free radicals generated by heating or ionizing radiation as is usually done.

  As the initiator used in the polymerization reaction, any initiator generally used in radical polymerization can be used. For example, azo compounds such as azobisisobutylnitrile, benzoyl peroxide, lauroyl peroxide, t-butyl peroxide When an organic peroxide such as -2-ethylhexanoate is used and polymerization is carried out particularly at a high temperature of 90 ° C. or higher, solution polymerization is common, so the 10-hour half-life temperature is Preferred are peroxides and azobis initiators that are 80 ° C. or higher and are soluble in the organic solvent used. Specifically, 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, cyclohexane Peroxide, 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, 1,1-azobis (1-cyclohexanecarbo Tolyl) and 2- (carbamoylazo) isobutyronitrile. These initiators are used in the range of 0.005 to 5% by mass.

  As the molecular weight regulator used as necessary in the polymerization reaction, any one used in general radical polymerization is used. For example, mercaptan compounds such as butyl mercaptan, octyl mercaptan, dodecyl mercaptan, 2-ethylhexyl thioglycolate are used. Particularly preferred. These molecular weight regulators are added in a concentration range such that the degree of polymerization is controlled within the above range.

  Examples of the aliphatic polyester resin (b) of the present invention include polymers having aliphatic hydroxycarboxylic acid as the main constituent, polymers having aliphatic polyvalent carboxylic acid and aliphatic polyhydric alcohol as the main constituents, and the like. Can be mentioned. Specifically, polymers having aliphatic hydroxycarboxylic acid as a main constituent include polyglycolic acid, polylactic acid, poly-3-hydroxybutyric acid, poly-4-hydroxybutyric acid, poly-4-hydroxyvaleric acid, poly-3- Hydroxyhexanoic acid, polycaprolactone, etc. are mentioned. Polymers mainly composed of aliphatic polycarboxylic acid and aliphatic polyhydric alcohol include polyethylene adipate, polyethylene succinate, polybutylene adipate, and polybutylene succinate. Is mentioned. These aliphatic polyesters can be used alone or in combination of two or more. Among these aliphatic polyesters, a polymer containing hydroxycarboxylic acid as a main constituent is preferable, and a polylactic acid resin is particularly preferably used. These (b) components can use 1 or more types.

  The polylactic acid-based resin is a polymer containing L-lactic acid and / or D-lactic acid as a main constituent component, but within a range not impairing the object of the present invention, other copolymer components 0.1 to 30 other than lactic acid. It may contain mass%. Examples of such other copolymer component units include polyvalent carboxylic acids, polyhydric alcohols, hydroxycarboxylic acids, lactones, and the like. Specifically, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid , Azelaic acid, sebacic acid, dodecanedioic acid, fumaric acid, cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 5-sodium sulfoisophthalic acid, 5-tetrabutylphosphonium sulfoisophthalic acid Polyvalent carboxylic acids such as ethylene glycol, propylene glycol, butanediol, heptanediol, hexanediol, octanediol, nonanediol, decanediol, 1,4-cyclohexanedimethanol, neopentylglycol, glycerin, trimethylol The Bread, pentaerythritol, bisphenol A, aromatic polyhydric alcohol obtained by addition reaction of bisphenol with ethylene oxide, polyhydric alcohols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, glycolic acid, Hydroxycarboxylic acids such as 3-hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid, 6-hydroxycaproic acid, hydroxybenzoic acid, glycolide, ε-caprolactone glycolide, ε-caprolactone, β-propiolactone, δ- Lactones such as butyrolactone, β- or γ-butyrolactone, pivalolactone, and δ-valerolactone can be used. These copolymer components can be used alone or in combination of two or more.

  The polylactic acid resin is a polymer mainly composed of lactic acid, that is, L-lactic acid or D-lactic acid. In the polylactic acid-based resin, the constituent molar ratio of the L-lactic acid unit and the D-lactic acid unit is preferably 100% for both the L-form and the D-form, and either the L-form or the D-form is preferably 85% or more. More preferably, one is 90% or more, and more preferably one is a 94% or more polymer. In the present invention, poly-L lactic acid mainly composed of L-lactic acid and poly-D lactic acid mainly composed of D-lactic acid can be used simultaneously.

The polylactic acid-based resin may be copolymerized with lactic acid derivative monomers other than L-form or D-form or other components copolymerizable with lactide. Examples of such components include dicarboxylic acids, polyhydric alcohols, hydroxycarboxylic acids. Examples include acids and lactones. The polylactic acid resin can be polymerized by a known polymerization method such as direct dehydration condensation or ring-opening polymerization of lactide. If necessary, the molecular weight can be increased by using a binder such as polyisocyanate.
The preferred weight average molecular weight range of the polylactic acid-based resin is preferably a weight average molecular weight of 30,000 or more from the viewpoint of mechanical properties, and preferably 1,000,000 or less from the viewpoint of processability. More preferably, it is 50,000-500,000, Most preferably, it is 100,000-280,000.
As a method for producing the aliphatic polyester resin (b), a known polymerization method can be used. Particularly, for a polylactic acid resin, a direct polymerization method from lactic acid, a ring-opening polymerization method via lactide, or the like is employed. be able to.

  In the present invention, the ratio of the acrylic resin (a) in the resin composition comprising the acrylic resin (a) and the aliphatic polyester resin (b) is that of the acrylic resin (a) and the aliphatic polyester resin (b). More than 50 parts by mass and less than 99.9 parts by mass, preferably more than 60 parts by mass and less than 95 parts by mass, more than 65 parts by mass and less than 90 parts by mass with respect to 100 parts by mass in total. More preferably.

The refractive index can be set according to the ratio of acrylic resin (a) and aliphatic polyester resin (b), and it is 1.456 or more and 1,491 or less. However, by reducing the retardation, the balance with the light reflection loss on the surface 1.470 or more and 1.491 or less are preferable, and 1.480 or more and 1.488 or less are especially preferable.
The retardation in the present invention is represented by the following formula.
Re = (Nx−Ny) d (unit: nm)
Nx: Maximum refractive index in the plane of the light guide plate Ny: Minimum refractive index in the plane of the light guide plate d: Thickness of the light guide plate

  The method for producing the resin composition for the light guide plate of the present invention is not particularly limited, and a known method can be adopted. For example, a method of mixing a glycerin higher fatty acid monoester in a monomer mixture in a desired ratio in advance with a mixer or the like at the time of production of an acrylic copolymer, and mixing and pelletizing using an extruder can be exemplified. . Moreover, it is also possible to add a ultraviolet absorber, a light-diffusion material, dye, a pigment, etc. as needed in the range which does not impair the effect of this invention in the case of a mixing | blending.

The light guide plate of the present invention can be used as a thin backlight light guide plate of a liquid crystal display.
In general, a light guide plate is produced by (1) injection molding method (2) melt extrusion molding, press molding, or cast molding. In the injection molding method, as a shape of the light guide plate, in addition to a basic rectangular parallelepiped, a method using a shaping mold and a method in which a large surface is uneven are performed. In the method of processing a plate-shaped molded body obtained by melt extrusion molding, press molding, or cast molding, a light guide plate is obtained by cutting and polishing the obtained sheet or film-shaped molded body to a predetermined size. It is done. The light guide plate obtained by both methods can control light rays by applying geometrical optically designed unevenness, dot gradation printing, or attaching a dot gradation film.

  The light guide plate according to the present invention can be produced by any of the methods described above. When the light guide plate of the present invention is used in an edge light type backlight system, a method of producing by an injection molding method is preferable from the viewpoint of mass productivity.

The present invention will be described based on examples.
<Measurement item>
(1) Composition analysis of copolymer The dichloromethane solution of the copolymer was cast on a platinum board, dried, and pyrolyzed at 450 ° C. in a nitrogen stream injection and pyrolysis furnace (Shimadzu PYR-2A). The monomer component produced by depolymerization was immediately introduced into a gas chromatography column and analyzed. The results were obtained by quantitative calculation using the polymer analysis result of known composition obtained by bulk polymerization as a standard.
(2) Reduced viscosity 0.15 g of the sample was dissolved in 50 ml of chloroform and measured at 25 ° C. with an Ostwald viscometer.
(3) Total light transmittance
Measurement was performed according to JIS K7361-1, using a haze meter NDH2000 (manufactured by Nippon Denshoku Industries Co., Ltd.).
(4) Retardation measurement Otsuka Electronics phase difference measuring machine RETS-100 was used, and the measurement was carried out using the rotating analyzer method.
The retardation at the center of the light guide plate in FIG. 1 was measured with a spot diameter of Φ5 mm.
The measurement wavelength was 550 nm.
(5) Refractive index measurement
After the measurement sample was dried at 70 ° C. for 3 hours, the refractive index was measured with a 633 nm laser beam using a Metricon prism coupler.

<Raw material>
(A) A monomer mixture consisting of 68.6 parts by weight of methyl methacrylate raw material, 1.4 parts by weight of methyl acrylate, and 30 parts by weight of ethylbenzene was added to 1,1-di-t-butylperoxy- 50 ppm of 3,3,5-trimethylcyclohexane and 1500 ppm of n-octyl mercabtan were added and mixed uniformly. This solution was continuously supplied to a sealed pressure-resistant reactor having an internal volume of 10 liters, polymerized with stirring at 130 ° C. and an average residence time of 2 hours, and then continuously sent to a storage tank connected to the reactor. The volatile matter was removed by the above and further transferred to an extruder to obtain a pellet-shaped acrylic resin. The water content in the resin was adjusted with a hot air drier at 80 ° C. for 12 hours. The reduced viscosity of this acrylic copolymer was 53 ml / g. The composition of the acrylic copolymer was analyzed using a pyrolysis gas chromatography method, and the result of MMA / MA = 98.0 / 2.0 (mass ratio) was obtained.

(B) Raw material of polylactic acid resin Using NatureWorks 4040D manufactured by Cargill Dow Co., Ltd., dried at 60 ° C. for about 1 hour with a hopper dryer, and then dried at 60 ° C. for 24 hours with a vacuum dryer to remove trace impurities.

[Examples 1 and 2 and Comparative Examples 1 and 2]
The above acrylic resin pellets and polylactic acid resin pellets were mixed at a weight ratio shown in Table 1.
It was molded by an injection molding machine IS-100EN manufactured by Toshiba Machine, and the light guide plate of FIG. 1 was obtained.
Molding was performed at a cylinder temperature of 250 ° C and a mold temperature of 80 ° C with a maximum injection stress of 80Mpa.

[Comparative Example 3]
PC-110, which is a polycarbonate resin manufactured by Asahi Kasei Chemicals Corporation, was molded using an injection molding machine manufactured by Toshiba Machine to obtain the light guide plate shown in FIG. Molding was performed at a cylinder temperature of 290 ° C and a mold temperature of 100 ° C with a maximum injection stress of 100Mpa.
In order to function as a light guide plate for a backlight of a liquid crystal display, a configuration as shown in FIG. 2 is adopted. With the light guide plate of the present invention, the reflection loss of light from the cold cathode tube on the surface of the light guide plate can be reduced as compared with a conventionally used light guide such as an acrylic resin alone or a polycarbonate resin.

  Since the light guide plate of the present invention has a much lower retardation than that currently used, it is possible to suppress a change in polarization characteristics of light passing therethrough. In addition, since the refractive index is low, light loss at the light incident surface can be reduced. The light guide plate of the present invention can be used for a backlight unit in a liquid crystal display device.

Example, front view of light guide plate used for evaluation Backlight configuration using light guide plate

Claims (1)

  Acrylic resin (a) more than 50 parts by mass and 99.9 parts by mass or less, aliphatic polyester resin (b) consisting of a resin composition consisting of 0.1 parts by mass and less than 50 parts by mass, and refraction by 633 nm laser light A light guide plate having a rate of 1.456 or more and 1.491 or less.

Images