JP6014415B2 - Scattering polarizer and liquid crystal display device comprising the same - Google Patents

Description

  The present invention relates to a scattering polarizer that can be used as a brightness enhancement film and the like, and more specifically, has scattering anisotropy that transmits light in a specific polarization direction and reflects light in another polarization direction. The present invention relates to a scattering polarizer.

  Conventionally, as a configuration example of a liquid crystal display (LCD), as shown in FIG. 1, a glass substrate, a polarizing film, a brightness enhancement film, a diffusion film, a light guide plate and a light source, and a reflection are provided on the back side (back side) of the liquid crystal cell. A configuration in which sheets or the like are sequentially laminated has been adopted.

In such a configuration, the polarizing film transmits only light of a specific polarization direction (P wave, linearly polarized light) and supplies it to the liquid crystal cell, and absorbs light of other polarization directions (S wave). If only the polarizing film is used, the amount of light supplied to the liquid crystal cell is reduced and the image becomes dark.
Therefore, by arranging a brightness enhancement film on the light source side of the polarizing film as in the above configuration, the amount of light in the polarization direction transmitted by the polarizing film is increased, and the amount of light that can be supplied to the liquid crystal cell is increased. Brightening is done.

As this kind of brightness enhancement film, a film using a scattering polarizer is known.
When a scattering polarizer is used as a brightness enhancement film, if the polarization direction of light passing through the brightness enhancement film matches the polarization direction of light passing through the polarization film, light in the polarization direction absorbed by the polarization film can be obtained. The light is reflected on the light source side by the brightness enhancement film on the near side, and while the reflection and scattering are repeated between the brightness enhancement film and the reflection sheet, the polarization direction of the light changes and passes through the polarization film. As a result, the luminance of the image can be improved.

  As such a scattering-type polarizer, for example, as disclosed in Patent Documents 1 to 3, a polarizer obtained by laminating and stretching two kinds of materials is known. These polarizers use the reflection characteristics at the laminated interface to impart polarization reflection characteristics and to precisely control the thickness of each layer, thereby exhibiting high luminance enhancement characteristics. However, such a multilayer structure type polarizer requires uniform multilayer lamination and precise control of the thickness of each layer, so that there is a concern that the manufacturing process becomes complicated and productivity is remarkably lowered.

Conventionally, there has been known a scattering polarizer obtained by uniaxially stretching a polymer blend having a phase separation structure composed of two types of polymers having different birefringence.
Such a scattering type polarizer has scattering anisotropy in which the degree of scattering of polarized light is different between the stretching direction and the perpendicular direction, and therefore selectively transmits light in a specific polarization direction and light in other polarization directions. Can be selectively reflected or scattered. For example, Patent Documents 4 and 5 include a continuous phase and a dispersed phase, and the refractive index difference between the continuous phase and the dispersed phase is greater than 0.05 along the first axis and orthogonal to the first axis. A polarizer is disclosed that is less than 0.05 along the second axis and has a diffuse reflectance of at least 30% for polarized light, and a method for manufacturing the same.
Furthermore, Patent Document 6 includes a dispersed phase mainly composed of a polyethylene naphthalate resin and a continuous phase mainly composed of an acrylic resin and rubber, and is perpendicular to the orientation direction of the dispersed phase on the optical film surface. An optical film having a parallel axis and a refractive index difference between a dispersed phase and a continuous phase of greater than 0.05 and less than 0.1 is disclosed.
These polarizers are characterized in that they exhibit polarization characteristics by utilizing scattering reflection at the interface between the continuous phase and the dispersed phase. Compared with the multilayer structure type polarizer described above, Since control is relatively easy, it has a feature of high productivity.

JP 09-506984 A Japanese Translation of National Publication No. 09-506985 JP-T 09-507308 Special Table 2000-506989 JP 2000-506990 A JP 2011-197299 A

  However, the polarizers disclosed in Patent Documents 4 to 6 have insufficient polarization transmission characteristics even if they are formed in a range that determines the difference in refractive index between the continuous phase and the dispersed phase. Furthermore, since the compatibility between the continuous phase and the dispersed phase is low, a relatively large dispersed phase is formed, and the dispersed phase is uniaxially stretched. There is a problem that tearing occurs.

  Conventionally, in order to develop high brightness enhancement performance, as disclosed in Patent Document 5, it has been considered that high diffuse reflection characteristics with respect to polarized light are necessary. The inventors have come up with the idea that it is necessary to sufficiently enhance the characteristics in order to develop high brightness enhancement performance.

  That is, an object of the present invention is to provide a scattering polarizer that has high mechanical strength and high polarization transmission characteristics and is particularly suitable as a brightness enhancement film.

  The present inventors have found that it is important to control the morphology of the dispersed phase as a factor that improves the mechanical strength of the polarizer and sufficiently enhances the transmission characteristics with respect to polarized light. It was discovered that the factor that controls the viscosity was derived from a specific diol residue in the polyester resin, and the present invention was completed.

  That is, the present invention is a scattering polarizer comprising a uniaxially oriented film containing at least two thermoplastic resins and having a sea-island structure with a continuous phase (I) and a dispersed phase (II), At least one of the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II) is a polyester resin having a diol residue derived from isosorbide. It is a scattering polarizer characterized by being.

  In the present invention, the thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II) are both polyester resins, and the thermoplastic resin It is preferable that (A) and the thermoplastic resin (B) are different types of polyester resins.

  The present invention also relates to a thermoplastic resin (A) that forms the continuous phase (I) and a thermoplastic resin (B) that forms the dispersed phase (II), one of which is a diol residue derived from isosorbide. It is preferable that the other is a polyester-based resin, and the other is a polyethylene naphthalate-based resin.

  The scattering type polarizer proposed by the present invention is suitable as a scattering type polarizer capable of improving luminance because it has low anisotropy in mechanical strength such as cracks and tears and has high polarization transmission performance. Can be used. Moreover, the liquid crystal display device provided with the scattering type polarizer of this invention can be provided.

It is sectional drawing which showed an example of the general structure of a liquid crystal display (LCD).

  Hereinafter, a scattering polarizer (referred to as “the present polarizer”) as an example of an embodiment of the present invention will be described.

<This polarizer>
The present polarizer is a scattering polarizer comprising a uniaxially oriented film containing at least two kinds of thermoplastic resins and having a sea-island structure with a continuous phase (I) and a dispersed phase (II).
At this time, the thermoplastic resin forming the continuous phase (I) is referred to as a thermoplastic resin (A), and the thermoplastic resin forming the dispersed phase (II) is referred to as a thermoplastic resin (B). That is, the continuous phase (I) and the dispersed phase (II) are mainly composed of different thermoplastic resins.

(Thermoplastic resin used in this polarizer)
The thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II) are not particularly limited as long as they do not deviate from the scope defined in the present invention. However, it is preferable that the intrinsic birefringence of the thermoplastic resin (A) and the thermoplastic resin (B) are both positive.
For example, thermoplastic resins having a negative intrinsic birefringence, such as polystyrene resins and acrylic resins, can also be used in the present invention. However, since these have bulky molecular structures in the side chains of the molecular chains, they are mechanical. Anxiety remains in strength. Therefore, it is preferable to use a thermoplastic resin having a positive intrinsic birefringence from the viewpoint of preventing a decrease in mechanical strength when a polarizer is formed. However, this does not prevent the use of a thermoplastic resin having excellent mechanical strength and a negative intrinsic birefringence.

The absolute value of the difference between the average refractive index of the thermoplastic resin (A) forming the continuous phase (I) and the average refractive index of the thermoplastic resin (B) forming the dispersed phase (II) is 0. A value larger than 05 is preferred.
When the intrinsic birefringence of the thermoplastic composition (A) and the thermoplastic resin (B) are both positive, the absolute value of the average refractive index difference between the (A) and the (B) is 0.05. When the orientation is below, it is considered that a difference in refractive index in the axis (S axis) parallel to the orientation direction of the continuous phase (I) and the disperse phase (II) is unlikely to occur.

Moreover, it is preferable that the magnitude relationship of the average refractive index of said (A) and said (B) and the magnitude relationship of the birefringence of said (A) and said (B) are equal. That is, if the magnitude relationship of the average refractive index between (A) and (B) is (A)> (B), the magnitude relationship between the birefringence of (A) and (B) is It is preferable that (A)> (B). If the magnitude relationship does not match, it is considered that a difference in refractive index between the axis (S axis) parallel to the orientation direction of the continuous phase (I) and the dispersed phase (II) is unlikely to occur.
However, if the polarizer oriented in the uniaxial direction belongs to the range defined by the present invention, the absolute value of the average refractive index difference between (A) and (B), the average refractive index and birefringence This is not the case with the matching of the magnitude relationship.

Moreover, it is preferable that at least one of the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II) is a crystalline thermoplastic resin. . If it is a crystalline thermoplastic resin, the polymer chains are easily oriented when oriented in a uniaxial direction, and the refractive index of the continuous phase (I) and the dispersed phase (II) with respect to the axis (S axis) parallel to the orientation direction. This is preferable because the difference is easily increased and the polarization reflection characteristics on the S axis are easily improved.
The crystalline thermoplastic resin generally refers to a thermoplastic resin that has a crystal melting peak temperature (melting point), and more specifically, in differential scanning calorimetry (DSC) performed in accordance with JIS K7121. Thermoplastic resins whose melting point is observed include those in a so-called semi-crystalline state. Conversely, a thermoplastic resin whose melting point is not observed in DSC is referred to as “amorphous”.

  Such a crystalline thermoplastic resin is not particularly limited. For example, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, polyethylene succinate , Polyester resins such as polybutylene succinate, polylactic acid, poly-ε-caprolactam, polyethylene resins such as high-density polyethylene, low-density polyethylene, and linear polyethylene, ethylene-vinyl acetate copolymer, ethylene- ( (Meth) acrylic acid copolymer, ethylene- (meth) acrylic acid ester copolymer, ethylene-vinyl alcohol copolymer, ethylene-vinyl chloride copolymer, ethylene-vinyl acetate-carbon monoxide copolymer, ethylene- Vinyl acetate -Ethylene copolymers such as vinyl chloride copolymer, ethylene-α olefin copolymer, polypropylene resin, polybutene resin, polyamide resin, polyoxymethylene resin, polymethylpentene resin, polyvinyl alcohol resin Fluorine resins such as polytetrafluoroethylene and polyvinylidene fluoride, cellulose resins, polyetheretherketone, polyetherketone, polyphenylene sulfide, engineering plastics such as polyparaphenylene terephthalamide, and super engineering plastics it can. Of these, polyester resins are preferable, and crystalline aromatic polyester resins are more preferable.

  In general, polyester-based resins often have a positive intrinsic birefringence, and among them aromatic polyester-based resins have a high birefringence, so that the continuous phase (I) with respect to the axis parallel to the orientation direction (S-axis) This is preferable because the refractive index difference of the dispersed phase (II) can be easily increased and the polarization reflection characteristics on the S axis can be easily improved.

In this polarizer, one of the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II) is crystalline or In the case of a semicrystalline thermoplastic resin, the other thermoplastic resin is preferably an amorphous thermoplastic resin.
As will be described later, this polarizer is preferably subjected to heat treatment from the viewpoint of dimensional stability after being oriented in a uniaxial direction. During the heat treatment, the crystalline thermoplastic resin is easily crystallized in the orientation direction. The film is heat-set with a large anisotropy of refractive index with respect to a parallel axis (S axis) and an axis perpendicular to the orientation direction and parallel to the film surface (P axis). On the other hand, the amorphous thermoplastic resin tends to be relaxed in orientation during heat treatment, and as a result, the refractive index difference between the continuous phase (I) and the dispersed phase (II) with respect to the S axis is increased. Since it tends to make it easy, it is preferable.

  The amorphous thermoplastic resin is not particularly limited, and examples thereof include polyester resins, polycarbonate resins, polyolefin resins, and polyphenylene ether. Among these, amorphous polyester resins and polycarbonate resins are preferable.

(Polyester resin having a diol residue derived from isosorbide)
In the present polarizer, the thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II) have been described above. Furthermore, it is important that at least one of the thermoplastic resin (A) and the thermoplastic resin (B) is a polyester resin having a diol residue derived from isosorbide.
By having a diol residue derived from isosorbide, the heat resistance of the polyester resin is improved, so that the deflection and dimensional stability of the polarizer due to heat can be improved.
Further, the polyester resin having a diol residue derived from isosorbide and the thermoplastic resin (A) for improving the compatibility of the other thermoplastic resin and forming the continuous phase (I) and the dispersed phase (II) The formation of a fine sea-island structure of the thermoplastic resin (B) that forms the film is achieved.

  Polyester resins having diol residues derived from isosorbide include aromatic polyester resins and aliphatic polyester resins derived from dicarboxylic acid residues and diol residues, polyester carbonates having carbonate bonds, etc. Is also included as a concept. In addition to the above, copolymers obtained by various combinations of various dicarboxylic acid residues and diol residues and polymer blends are also included.

  Examples of preferable dicarboxylic acid residues for constituting a polyester resin having a diol residue derived from isosorbide include terephthalic acid, isophthalic acid, 2-chloroterephthalic acid, 2,5-dichloroterephthalic acid, 2- Methyl terephthalic acid, 4,4-stilbene dicarboxylic acid, 4,4-biphenyl dicarboxylic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, bisbenzoic acid, bis (p-carboxyphenyl) Aromatic dicarboxylic acids such as methane, anthracene dicarboxylic acid, 4,4-diphenyl ether dicarboxylic acid, 4,4-diphenoxyethane dicarboxylic acid, 5-Na sulfoisophthalic acid, ethylene-bis-p-benzoic acid, succinic acid, adipine Acid, sebacic acid, azelaic acid, dodecanedioic acid, , 3-cyclohexane dicarboxylic acid, residues derived from aliphatic dicarboxylic acids or their ester derivatives, such as 1,4-cyclohexane dicarboxylic acid. These dicarboxylic acid residues may be used alone or in combination of two or more.

  Furthermore, as a preferable mixture of diol residues for constituting a polyester resin having a diol residue derived from isosorbide, for example, an ethylene glycol residue as a first residue and 1,4- Selected from the group consisting of butanediol, neopentyl glycol, isosorbide, diethylene glycol, polytetramethylene glycol, and 1,4-cyclohexanedimethanol, spiroglycol, 2,2,4,4-tetramethylcyclobutane-1,3-diol It is important to include isosorbide by using residues derived from at least one selected from the above. Preferred diol residues constituting a polyester resin having a diol residue derived from isosorbide include an ethylene glycol residue as the first residue, a 1,4-cyclohexanedimethanol residue as the second residue and an isosorbide residue. The thing using group is mentioned.

  Furthermore, the thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II) are both polyester resins, and the thermoplastic resin (A) and It is preferable that the thermoplastic resin (B) is a different type of polyester resin.

As described above, in general, a polyester-based resin often has a positive intrinsic birefringence, and has a high birefringence. It is easy to increase the refractive index difference of (II).
In addition, both the thermoplastic resin (A) and the thermoplastic resin (B) are polyester resins, and the thermoplastic resin (A) and the thermoplastic resin (B) are different types of polyester resins. In this case, since the compatibility between (A) and (B) is high, the continuous phase (I) and the dispersed phase (II) are easy to form and have a fine sea-island structure.
Among these, it has been found that polyester resins having a diol residue derived from isosorbide show good compatibility with other polyester resins.
Therefore, in an island structure elongated in the orientation direction of the dispersed phase (II) generated when oriented in the uniaxial direction, the dispersion in the axis (P-axis) direction perpendicular to the orientation direction of the dispersed phase (II) and parallel to the film surface The diameter tends to be in a range equal to or less than the wavelength of visible light, and the effect of facilitating the improvement in luminance in the present polarizer is brought about.

  The thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II) are both polyester resins, and the thermoplastic resin (A) and When the thermoplastic resin (B) is a different type of polyester resin, one of the thermoplastic resin (A) and the thermoplastic resin (B) is a crystalline polyester resin, particularly a crystalline fragrance. It is preferable that the other is a polyester-based resin and the other is an amorphous polyester-based resin.

  Polyester resins that can be preferably used in the present polarizer include those derived from dicarboxylic acid residues and diol residues, those derived from hydroxycarboxylic acids such as lactic acid, those derived from ε-caprolactam, etc. There is no particular limitation. Specifically, for example, aromatic polyester resins such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, polyethylene succinate, polybutylene succinate. In addition to aliphatic polyester resins such as polylactic acid and poly-ε-caprolactam, polyester carbonate having a carbonate bond is also included as a concept. In addition to the above, copolymers obtained by various combinations of various dicarboxylic acid residues and diol residues and polymer blends are also included.

  Examples of preferred dicarboxylic acid residues include terephthalic acid, isophthalic acid, 2-chloroterephthalic acid, 2,5-dichloroterephthalic acid, 2-methylterephthalic acid, 4,4-stilbene dicarboxylic acid, 4,4-biphenyldicarboxylic acid Acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, bisbenzoic acid, bis (p-carboxyphenyl) methane, anthracene dicarboxylic acid, 4,4-diphenyl ether dicarboxylic acid, 4,4- Aromatic dicarboxylic acids such as diphenoxyethanedicarboxylic acid, 5-Na sulfoisophthalic acid, ethylene-bis-p-benzoic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, dodecanedioic acid, 1,3-cyclohexanedicarboxylic acid Fats such as acids and 1,4-cyclohexanedicarboxylic acid Residues derived from dicarboxylic acids or their ester derivatives. These dicarboxylic acid residues may be used alone or in combination of two or more.

  Preferred mixtures of diol residues include, for example, ethylene glycol residue as the first residue, 1,4-butanediol, neopentyl glycol, isosorbide, diethylene glycol, polytetramethylene glycol, and 1,4 as the second residue. -What used the residue induced | guided | derived from at least 1 sort (s) chosen from the group which consists of cyclohexanedimethanol, spiroglycol, 2,2,4,4-tetramethylcyclobutane-1,3-diol is mentioned. Preferably, an ethylene glycol residue is used as the first residue, and an isosorbide residue or a 1,4-cyclohexanedimethanol residue is used as the second residue.

  Among them, in the thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II), one of them is a polyester resin having a diol residue derived from isosorbide It is preferable that the other is a polyethylene naphthalate resin because it has a high average refractive index and a high birefringence.

  When a polyethylene naphthalate resin is used, the weight average molecular weight is preferably 30,000 or more, the intrinsic viscosity is preferably 0.5 dl / g or more, and the glass transition temperature is preferably in the range of 70 ° C to 120 ° C, The melting point is preferably in the range of 240 ° C to 270 ° C.

  Moreover, when using polyethylene naphthalate type-resin, it is preferable to use what has YI value in the range of -10-10, especially in the range of 3-3. When the polyethylene naphthalate resin is made of a mixture, the YI value of each resin is preferably in the range of −10 to 10. If the YI value is in the range of −10 to 10, for example, by incorporating it into a liquid crystal display or the like as a brightness enhancement film, it is possible to further improve the image clarity, and to further increase the brightness improvement rate. it can.

A commercial item can also be used as a polyethylene naphthalate-type resin.
For example, Teonex TN8065S (polyethylene naphthalate homopolymer, manufactured by Teijin Chemicals Ltd., intrinsic viscosity 0.71 dl / g), Teonex TN8065SC (polyethylene naphthalate homopolymer, manufactured by Teijin Chemicals Ltd., intrinsic viscosity 0.55 dl) / G), Teonex TN8756C (polyethylene naphthalate and polyethylene terephthalate copolymer, manufactured by Teijin Chemicals Ltd., intrinsic viscosity 0.65 dl / g) and the like can be mentioned as preferred examples.

(Composition of this polarizer)
The mixing mass ratio of the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II) constituting the polarizer is (A) / (B ) = 10% by mass / 90% by mass to 90% by mass / 10% by mass. Such a mixed mass ratio is preferable because the dispersed phase does not decrease too much, and scattering at the interface between the continuous phase and the dispersed phase is reduced, and there is no fear that the polarization reflection characteristic is deteriorated.
The present polarizer may contain other thermoplastic resin as long as it contains at least one of the thermoplastic resin (A) and the thermoplastic resin (B). Two or more thermoplastic resins corresponding to the plastic resin (B) may be included.

(Other ingredients)
For the purpose of improving the dispersibility of the dispersed phase (II), an additive such as a compatibilizing agent (C) may be added to the polarizer as necessary.

  The compatibilizing agent (C) can be selected from conventional compatibilizing agents depending on the type of continuous phase and dispersed phase. For example, the compatibilizing agent (C) has a polycarbonate resin, an ester resin, a resin having an epoxy group, or an oxazoline ring. Examples thereof include a block copolymer or a graft copolymer comprising at least one resin selected from a resin and a resin having an azlactone group, and at least one resin selected from a styrene resin, polyphenylene oxide, and polyamide. Among these, from the viewpoint of improving dispersibility, a resin having an epoxy group or an oxazoline group is particularly preferable, and an epoxy-modified one is particularly preferable.

  The blending ratio in the case of adding the compatibilizer (C) is 0.1 to 20 parts by mass, preferably 0 with respect to 100 parts by mass in total of the thermoplastic resin (A) and the thermoplastic resin (B). 2 to 15 parts by mass, particularly 0.2 to 10 parts by mass, more preferably 1 to 10 parts by mass.

  Further, as additives other than the compatibilizing agent (C), various additives such as antioxidants, heat stabilizers, light stabilizers, hydrolysis inhibitors, impact modifiers, etc., do not impair the characteristics of the present invention. Can be added.

(Refractive index difference)
In this polarizer, the absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) is larger than 0.05 on an axis parallel to the orientation direction (S axis). In addition, it is preferably larger than 0.05 in an axis (P axis) perpendicular to the orientation direction and parallel to the film surface.

  Since the absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) is larger than 0.05 in the axis parallel to the orientation direction (S axis), the S A high scattering reflection characteristic is exhibited with respect to the polarized light in the axis, and the luminance improving ability of the obtained scattering polarizer can be improved. The absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) is more preferably greater than 0.1 and further greater than 0.15 on the S axis. preferable.

The absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) is 0.05 on an axis (P axis) perpendicular to the orientation direction and parallel to the film surface. By being larger, it is possible to develop high transmission characteristics with respect to the polarized light on the P axis.
On the other hand, the absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) is preferably smaller than 0.1. By being smaller than 0.1, the scattering at the interface between the continuous phase (I) and the dispersed phase (II) is suppressed when the dispersed diameter in the P-axis direction of the dispersed phase (II) described later is in the preferred range in the present invention It is preferable because the transmission characteristic on the P-axis can be expressed.

As a means for making the absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) larger than 0.05 in the axis parallel to the orientation direction (S axis), The absolute value of the difference in average refractive index between the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II) is larger than 0.05. A method of selecting the thermoplastic resin (A) and the thermoplastic resin (B) and adjusting them to a preferred range, a free width uniaxial stretching, a constant width uniaxial stretching, a tensile stretching method, an inter-roll stretching method, a roll Orientation of the thermoplastic resin (A) that forms the continuous phase (I) and / or the thermoplastic resin (B) that forms the dispersed phase (II) by stretching such as rolling, and the difference in birefringence A method of adjusting to a preferred range using the above, or the continuous phase ( To the thermoplastic resin (A) and / or the thermoplastic resin (B) forming the dispersed phase (II), adding other compatible thermoplastic resin or refractive index adjuster, The method of adjusting to a preferable range etc. are mentioned.
The absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II) is 0.05 on an axis (P axis) perpendicular to the orientation direction and parallel to the film surface. As a means for increasing, the absolute value of the difference in average refractive index between the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II) is A method of selecting the thermoplastic resin (A) and the thermoplastic resin (B) so as to be larger than 0.05 and adjusting them to a preferable range, a free width uniaxial stretching, a constant width uniaxial stretching, a tensile stretching method Orienting the thermoplastic resin (A) that forms the continuous phase (I) and / or the thermoplastic resin (B) that forms the dispersed phase (II) by stretching such as a roll-to-roll stretching method or a roll rolling method. And adjusting to a preferred range using the difference in birefringence Alternatively, the thermoplastic resin (A) that forms the continuous phase (I) and / or the thermoplastic resin (B) that forms the dispersed phase (II) and other thermoplastic resins or refractive indexes that are compatible with each other. The method etc. which add a regulator etc. and adjust to a preferable range are mentioned.

(Dispersed diameter in the P-axis direction of the dispersed phase (II))
In this polarizer, the dispersion phase (II) preferably has a dispersion diameter in the P-axis direction of 10 nm or more and 200 nm or less. The dispersion diameter is measured by the method described later.

Since the present polarizer is oriented in a uniaxial direction, the dispersed phase (II) is a flat ellipsoid or a fiber. If the dispersion diameter in the P-axis direction of the disperse phase (II) is 10 nm or more and 200 nm or less, the dispersion phase (II) is sufficiently smaller than the wavelength order of light, and therefore an axis perpendicular to the orientation direction and parallel to the film surface (P axis) Even if the refractive index difference between the continuous phase (I) and the dispersed phase (II) is large, sufficient transmission characteristics can be exhibited.
In this polarizer, the dispersion diameter in the P-axis direction of the dispersed phase (II) is 10 nm or more and 200 nm or less in order to develop higher polarization transmission characteristics with respect to the polarization on the P-axis. Is preferred. Among these, the dispersion diameter in the P-axis direction of the dispersed phase (II) is preferably 10 nm or more and 100 nm or less, because higher transmission characteristics are expressed with respect to the polarized light on the P-axis.

  On the other hand, when the dispersion diameter in the P-axis direction of the disperse phase (II) exceeds 200 nm, scattering at the interface between the continuous phase (I) and the disperse phase (II) tends to occur, and the polarization transmission characteristics are deteriorated. In addition, anisotropy of mechanical properties of the scattering polarizer is likely to occur, and defects such as cracking and tearing are likely to occur. On the other hand, when the thickness is less than 10 nm, the continuous phase (I) and the dispersed phase (II) can be regarded as being compatible with each other, and it is difficult to develop the polarization characteristics.

  As means for setting the dispersion diameter in the P-axis direction of the dispersed phase (II) to 10 nm or more and 200 nm or less, the thermoplastic resin (A) that forms the continuous phase (I) and the dispersed phase (II) are formed. A method of sufficiently kneading the thermoplastic resin (B) to be incompatible with a twin screw extruder or the like, and a thermoplastic resin (B) that forms the dispersed phase (II) by uniaxial stretching When forming a film in a sufficiently extending method or T-die casting method, the thermoplastic resin (B) forming the dispersed phase (II) is sufficiently extended by increasing the take-up speed (cast roll speed). Method, a method of sufficiently stretching the thermoplastic resin (B) forming the dispersed phase (II) by increasing the take-up speed when forming a film by an inflation method, and a fiber diameter of 10 nm or more and 200 nm or less Fibers, a method of impregnating the like in the thermoplastic resin forming the continuous phase (I) (A).

(Polarization transmittance)
The present polarizer preferably has an average polarization transmittance of 80% or more at a measurement wavelength of 400 nm to 700 nm on the P axis. Conventionally, it was considered that a high diffuse reflection characteristic with respect to polarized light is necessary to develop a high brightness enhancement capability. However, although the reason is not clear, the present inventors have a transmission characteristic with respect to the polarized light on the P axis. It has been found that making a sufficient increase contributes greatly to improving the luminance improvement capability.

  The average polarization transmittance is more preferably 82% or more, and particularly preferably 84% or more. In order to set the average polarized light transmittance to 80% or more, the absolute value of the difference between the refractive index of the continuous phase (I) and the refractive index of the dispersed phase (II), or a diol residue derived from isosorbide By controlling the size of the dispersed phase (II) according to the kneading conditions of the polyester-based resin and other thermoplastic resin, and reducing the dispersed diameter in the P-axis direction of the dispersed phase (II), Achievable.

(Mechanical strength)
Further, the polarizer preferably has a tear strength in the P-axis direction measured in accordance with JIS K7128-3 of 600 N / cm or more. More specifically, a test piece is prepared according to JIS K7128-3, and evaluated by the tear strength measured by the right-angled tearing method at a temperature of 23 ° C. and a test speed of 200 mm / min.
When the tear strength is 600 N / cm or more, this polarizer has excellent mechanical strength when used as a brightness enhancement film, and causes problems such as breakage during handling when incorporated in a liquid crystal display device. The possibility is very small.

As described in the prior art, the conventional polarizer has low compatibility between the continuous phase and the dispersed phase, so a relatively large dispersed phase is formed, and the dispersed phase is uniaxially stretched. Anisotropy also occurs, and when a load is applied in the P-axis direction, there is a problem that cracks and tears easily occur along the S-axis.
In contrast, the present polarizer surprisingly has a mechanical strength that could not be achieved in the past. The tear strength in the P-axis direction measured in accordance with JIS K7128-3 is preferably 600 N / cm or more, more preferably 800 N / cm or more, and further preferably 900 N / cm or more, It is especially preferable that it is 1000 N / cm or more.

  As a means for setting the tear strength in the P-axis direction measured in accordance with JIS K7128-3 to 600 N / cm or more, at least two types of thermoplastic resins constituting the polarizer have a positive intrinsic birefringence. It is possible to select a product or to select a product with good compatibility. In addition, by selecting a polyester-based resin having a diol residue derived from isosorbide in at least one thermoplastic resin, the size of the dispersed phase (II) during kneading with another thermoplastic resin Can be controlled. Furthermore, in the case where a crystalline thermoplastic resin is used for the dispersed phase (II), the dispersion diameter is finely oriented in the P-axis direction and then sufficiently crystallized. Can also be achieved.

(Film forming method)
As a method for producing the present polarizer, first, a mixed resin composition containing at least two kinds of thermoplastic resins may be melted to form a sheet. At this time, the method for forming the film is not particularly limited, and examples thereof include a T-die casting method, a calendar method, and an inflation method. Among these, the T die casting method is preferable from the viewpoints of film formation stability and production efficiency.
When adopting the T-die casting method, for example, at least two kinds of thermoplastic resins are dried, supplied to an extruder, and heated to a temperature equal to or higher than the melting point of the resin to be melted. Then, the cast composition may be formed by extruding the melted composition from the slit-shaped discharge port of the T die and firmly solidifying it on a cooling roll.
Although the extrusion temperature of the sheet depends on the flow characteristics of each resin, when a polyethylene naphthalate resin is used, it is preferably about 270 ° C. to 340 ° C., more preferably 280 ° C. to 320 ° C. If the extrusion temperature is 270 ° C. or higher, the sheet can be formed sufficiently for the molten resin to flow. On the other hand, if it is 340 ° C. or lower, the sheet characteristics are less likely to deteriorate due to thermal decomposition of the resin.
Further, the ratio of the discharge amount Q (kg / h) to the screw rotation speed N (rpm); Q / N (kg / h / rpm) is preferably in the range of 0.16 to 1.00, 0.18 More preferably, it is in the range of ˜0.90.
Thus, the compatibility of the mixed resin composition can be controlled by adjusting the extrusion temperature and the discharge amount Q to suitable ranges.

(Orientation method)
The polarizer is made of a film oriented in a uniaxial direction as described above. The orientation direction may be either the film take-up (flow) direction (MD) or the direction perpendicular to MD (TD). In order to more effectively express the properties of this polarizer, the MD is used. Orientation is preferred. That is, in this polarizer, it is preferable that the axis parallel to the alignment direction (S axis) is MD, and the axis perpendicular to the alignment direction and parallel to the film surface (P axis) is TD.
In addition, the orientation method is not particularly limited. For example, as described above, a sheet formed by the T die casting method is uniaxially stretched to MD or TD, or when the film is formed by the T die casting method, the sheet is taken up. Examples thereof include a method of drafting MD by increasing the speed (cast roll speed) and a method of drafting MD by increasing the take-up speed when forming a film by the inflation method.
Especially, the method of carrying out the uniaxial stretching of the sheet | seat formed by the T die-casting method to MD or TD as mentioned above is preferable from a viewpoint of film forming stability or production efficiency improvement.

(Stretching method)
Here, “uniaxial stretching” means stretching that is positively performed in only one direction. For example, in the process of film formation, heat treatment, winding, etc., the direction is naturally different from the one direction. It is intended to include the case of being stretched. Objectively, this is a case where the stretching ratio in one direction is two times or more of the stretching ratio in the direction orthogonal thereto.

By uniaxially stretching in this manner, the dispersed phase can be arranged and fixed in a substantially constant direction in the continuous phase, and the anisotropic scattering function can be exhibited.
By uniaxial stretching, the refractive index difference between the continuous phase (I) and the dispersed phase (II) is increased in the stretching direction, and the dispersed phase (II) is elongated in the stretching direction. Are included within the preferred scope of the present invention. Therefore, the polarized light in the direction in which the refractive index difference with respect to the S axis increases is scattered, and the dispersion diameter is sufficiently small with respect to the wavelength order of light for the P axis, so that the polarized light is transmitted. A scattering type polarizing element can be produced.

The stretching method may be any of free-width uniaxial stretching and constant-width uniaxial stretching, and any of a stretching method, an inter-roll stretching method, a roll rolling method, and other methods may be employed.
The stretching temperature is preferably a temperature within the range of the glass transition temperature (Tg) of the resin to (Tg + 50 ° C.). When the stretching temperature is within this range, stretching can be performed stably without breaking during stretching.
The draw ratio is not particularly limited, but for example, it is preferably 2 times or more for MD or TD, preferably 3 to 9 times for MD or TD, particularly 4 to 7 times for MD or TD. Among them, in order to reduce the dispersion diameter in the P-axis direction of the dispersed phase (II) and adjust it to the preferred range of the present invention by extending the dispersed phase (II) when formed with an unstretched sheet formed into a film. In particular, it is particularly preferable to stretch in the MD.

The stretched sheet is preferably heat-treated to impart heat resistance and dimensional stability.
For example, when a polyethylene naphthalate resin is used, the heat treatment temperature is preferably 180 to 230 ° C, and more preferably 180 to 200 ° C. The treatment time required for the heat treatment is preferably 1 second to 5 minutes.

(Thickness)
The thickness of the polarizer is not particularly limited. For example, when used for a brightness enhancement film, the thickness is preferably 50 μm to 250 μm, particularly preferably 100 μm to 200 μm. Generally, when the thickness of the scattering polarizer is increased, the number of scattering increases, so that the polarization reflection property in the axis parallel to the alignment direction (S axis) is improved. There is a tendency that the polarization transmission characteristic in an axis (P axis) perpendicular to the direction and parallel to the film surface is lowered.
However, as in the present invention, by setting the dispersion diameter in the P-axis direction of the dispersed phase (II) to 10 nm or more and 200 nm or less, the polarization reflection characteristic on the S-axis is improved with respect to the increase in thickness, and The effect of suppressing the decrease in the polarization transmission characteristic on the P-axis is brought about.

<Explanation of terms>
In the present invention, the form of the scattering polarizer is not particularly limited, and includes a plate form, a sheet form, a film form, and other forms.

  In general, "film" refers to a thin flat product that is extremely small compared to its length and width and whose maximum thickness is arbitrarily limited, and is usually supplied in the form of a roll (Japan) Industrial standard JISK6900), and in general, “sheet” refers to a product that is thin by definition in JIS and generally has a thickness that is small instead of length and width. However, since the boundary between the sheet and the film is not clear and it is not necessary to distinguish the two in terms of the present invention, in the present invention, even when the term “film” is used, the term “sheet” is included and the term “sheet” is used. In some cases, “film” is included.

In addition, the expression “main component” in the present specification includes the meaning of allowing other components to be contained within a range that does not hinder the function of the main component unless otherwise specified.
At this time, although the content ratio of the main component is not specified, the main component (when two or more components are main components, the total amount thereof) is 50% by mass or more, preferably 70% in the composition. It occupies at least 90% by mass, particularly preferably at least 90% by mass (including 100%).

In the present invention, when expressed as “X to Y” (X and Y are arbitrary numbers), “X is preferably greater than X” and “preferably Y”, with the meaning of “X to Y” unless otherwise specified. It means “smaller”.
Further, in the present invention, when expressed as “X or more” (X is an arbitrary number), it means “preferably larger than X” unless otherwise specified, and “Y or less” (Y is an arbitrary number). ) Includes the meaning of “preferably smaller than Y” unless otherwise specified.

The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to these examples, and various applications are possible without departing from the technical idea of the present invention.
Here, the take-up (flow) direction of the sheet or film when the sheet or film is manufactured is indicated by MD, and the orthogonal direction thereof is indicated by TD.

<Measurement and evaluation method>
First, measurement methods and evaluation methods for various physical property values of samples obtained in Examples and Comparative Examples will be described.

代表的なポリマーの固有複屈折率の推定値を下記に記す。
・ポリエチレンナフタレート：０．３３
・ポリエチレンテレフタレート：０．２３
・ポリ１，４−シクロへキシレンジメチレンテレフタレート：０．１７
・ポリメタクリル酸メチル：−０．００４３ (1) Average Refractive Index / Intrinsic Birefringence The average refractive index is a raw material used in Examples and Comparative Examples according to JIS K7124, using an Abbe refractometer manufactured by Atago, using sodium D line (589 nm) as a light source. The average refractive index of was measured.
The intrinsic birefringence is a refractive index value representing the polarizability per monomer, which is a structural unit of the molecular chain, and can be estimated by the Lorentz-Lorentz equation represented by the following equation (1).

The estimated value of the intrinsic birefringence of a typical polymer is shown below.
・ Polyethylene naphthalate: 0.33
-Polyethylene terephthalate: 0.23
Poly 1,4-cyclohexylene dimethylene terephthalate: 0.17
Polymethyl methacrylate: -0.0043

(2) Evaluation method of polarized light transmittance The polarized light transmittance was measured by attaching an integrating sphere to a spectrophotometer (manufactured by Hitachi, Ltd .: U-4000). A polarizing film is attached to the incident light side of a light source having a wavelength of 300 nm to 800 nm, an absorbing polarizing film is inserted on the light source side, and the light source is only linearly polarized light vertically polarized. A polarizer was inserted, and the polarization transmittance of the scattering polarizer was measured with respect to an axis (P axis) perpendicular to the stretching direction and parallel to the film surface (P axis) and a parallel axis (S axis) to the stretching direction.
The evaluation criteria for the P-axis were evaluated as “◯” when the average polarization transmittance from 400 nm to 700 nm was 80% or more, and “X” when the average was less than 80%.

(3) Dispersion diameter of disperse phase (II) in the P-axis direction The cross section of the film obtained with a transmission electron microscope (TEM) was observed and judged according to the following criteria.
◯: The dispersion diameter in the P-axis direction of the resin forming the dispersed phase (II) is 10 nm or more and 200 nm or less.
X: The dispersion diameter in the P-axis direction of the resin forming the dispersed phase (II) is less than 10 nm or greater than 200 nm.

(4) Luminance evaluation In a backlight unit (“plus one VGA” manufactured by Century, 8 inch, model number: LCD-8000V), a reflection sheet, a light guide plate with an LED light source, a diffusion sheet, a prism sheet (2 sheets), a sample ( The brightness enhancement film) and the polarizing film were sequentially laminated and fixed, and the central brightness of the screen at a distance of about 50 cm was measured with a brightness meter (Minolta, model: LS-100).
The luminance (reference) when the sample sheet was not incorporated was measured, and the ratio to the luminance was calculated as the luminance improvement rate (see the following formula (2)). The larger this value, the higher the brightness.
Formula (2): Brightness improvement rate = (luminance when the sample sheet is assembled / luminance before incorporation of the sample sheet)

(5) Tear strength A test piece was prepared according to JIS K7128-3 (1998), and the P-axis direction and S-axis direction were measured at a temperature of 23 ° C. and a test speed of 200 mm / min. The tear strength was measured and judged according to the following criteria.
A: Tear strength is 1000 N / cm or more.
○: Tear strength is 600 N / cm or more and less than 1000 N / cm.
X: Tear strength is less than 600 N / cm.

<Examples 1 and 2>
Polyethylene naphthalate resin (average refractive index: 1.646, intrinsic birefringence: positive, Tg: 118 ° C., Tm: 261 ° C., intrinsic viscosity 0.71 dl / g, hereinafter referred to as A-1) and amorphous Polyester resin (average refractive index: 1.568, intrinsic birefringence: positive, Tg: 109 ° C., dicarboxylic acid unit: terephthalic acid 100 wt%, diol unit: ethylene glycol 30 wt%, 1,4-cyclohexanedimethanol 45 wt% , Isosorbide 25%, hereinafter referred to as B-1) at a mass mixing ratio shown in Table 1 and after sufficient mixing, while being fed with a constant mass feeder, at 290 ° C. with a φ40 mm twin screw extruder Extrusion kneading (ratio of discharge amount Q (kg / h) and screw rotation speed N (rpm); Q / N = 0.26 (kg / h / rpm)), cooling and solidifying to a thickness of 600 μm To prepare a sheet. The obtained sheet was cut out, tilted by 90 °, passed through a tenter heated to a preheating temperature of 145 ° C., a stretching temperature of 130 ° C., and a heat treatment temperature of 180 ° C. (heat treatment time of 32 seconds), and stretched 6 times to MD. Thus, a sheet-like polarizer was obtained. The evaluation results of the obtained polarizer are shown in Table 1.

<Example 3>
Polyethylene naphthalate resin (A-1) and amorphous polyester resin (average refractive index: 1.566, intrinsic birefringence: positive, Tg: 97 ° C., dicarboxylic acid unit: terephthalic acid 100 wt%, diol unit: ethylene The same procedure as in Examples 1 and 2 except that 37% by weight of glycol, 50% by weight of 1,4-cyclohexanedimethanol, 13% of isosorbide, hereinafter referred to as B-2) was blended at a weight mixing ratio shown in Table 1. Thus, a sheet-like polarizer was obtained. The evaluation results of the obtained polarizer are shown in Table 1.

<Comparative Examples 1 and 2>
Polyethylene naphthalate resin (A-1) and amorphous polyester resin (average refractive index: 1.551, intrinsic birefringence: positive, Tg: 110 ° C., dicarboxylic acid unit: terephthalic acid 100 wt%, diol unit: 1 , 4-cyclohexanedimethanol 79 wt%, 2,2,4,4-tetramethylcyclobutane-1,3-diol 21 wt%, intrinsic viscosity 0.77 dl / g, hereinafter referred to as B-3) in Table 1. A sheet-like polarizer was obtained by the same method as in Examples 1 and 2 except that the blending ratio was as shown. The evaluation results of the obtained polarizer are shown in Table 1.

<Comparative Examples 3 and 4>
Polyethylene naphthalate resin (A-1) and amorphous polyester resin (average refractive index: 1.552, intrinsic birefringence: positive, Tg: 117 ° C., dicarboxylic acid unit: terephthalic acid 100 wt%, diol unit: 1 , 4-cyclohexanedimethanol 65 wt%, 2,2,4,4-tetramethylcyclobutane-1,3-diol 35 wt%, intrinsic viscosity 0.69 dl / g, hereinafter referred to as B-4) in Table 1. A sheet-like polarizer was obtained by the same method as in Examples 1 and 2 except that the blending ratio was as shown. The evaluation results of the obtained polarizer are shown in Table 1.

<Comparative Example 5>
Polyethylene naphthalate resin (A-1) and amorphous polyester resin (average refractive index: 1.539, intrinsic birefringence: positive, Tg: 109 ° C., dicarboxylic acid unit: terephthalic acid 100 wt%, diol unit: ethylene Except for blending 52 wt% glycol, 5 wt% diethylene glycol, 43 wt% spiroglycol, intrinsic viscosity 0.72 dl / g, hereinafter referred to as B-5) in the weight mixing ratio shown in Table 1, Examples 1 and 2 A sheet-like polarizer was obtained by the same method. The evaluation results of the obtained polarizer are shown in Table 1.

  As is clear from the above results, it can be seen that the scattering polarizer of the present invention has a high mechanical strength and a high polarization transmission property, and is a scattering polarizer particularly suitable as a brightness enhancement film. On the other hand, the sheet of the comparative example was insufficient in both mechanical strength and polarization transmission characteristics.

  That is, the scattering polarizer of the present invention can exhibit excellent handling properties and brightness enhancement characteristics when incorporated in a liquid crystal display device as a brightness enhancement film.

Claims (4)

  A scattering polarizer comprising a uniaxially oriented film containing at least two thermoplastic resins and having a sea-island structure of a continuous phase (I) and a dispersed phase (II), wherein the continuous phase (I At least one of the thermoplastic resin (A) forming the dispersed phase (II) and the thermoplastic resin (B) forming the dispersed phase (II) is a polyester resin having a diol residue derived from isosorbide. A scattering polarizer.   The thermoplastic resin (A) that forms the continuous phase (I) and the thermoplastic resin (B) that forms the dispersed phase (II) are both polyester resins, and the thermoplastic resin (A) and the thermoplastic resin. The scattering polarizer according to claim 1, wherein the resin (B) is a different type of polyester resin.   In the thermoplastic resin (A) forming the continuous phase (I) and the thermoplastic resin (B) forming the dispersed phase (II), one is a polyester resin having a diol residue derived from isosorbide The scattering polarizer according to claim 1, wherein the other is a polyethylene naphthalate resin.   A liquid crystal display device comprising the scattering polarizer according to claim 1.

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