POLARIZING FILM

Abstract: A polarizing film which comprises a film base having thereon an anthraquinone dye of general formula (I): wherein R¹ and R² each represents a hydrogen atom, an alkyl group, an alkoxy group, an aralkyloxy group, an aryl group, a cyclohexyl group, a 4-alkylcyclohexyl group, a carboxylic acid ester group, a carboxylic acid amide group, a halogen atom, ; X represents an oxygen atom, a sulfur atom or an amino group.

Description: The present invention relates to a polarizing film containing an anthraquinone dye. Polyvinyl alcohol (PA) based films having iodine or a dichroic dye fixed thereto are well known as polarizing films. Such conventional polarizing films have good pola­ rizing performances but their resistance to heat and mois­ ture is not so good. In order to solve this problem, lamination with protective films such as those based on cellulose acetate has been proposed and polarizing films manufactured by this method have been commercialized. But depending on the field of use, even such laminated pola­ rizing films sometimes suffer from moisture attack. Polarizing films that employ non-PVA hydrophobic polymers as their base have been reviewed and it has been proposed to form a polyene structure by removing hydrogen halides from vinyl halide based polymers such as PVC and PVDC. However, for several reasons such as low heat resis­ tance, insufficient stability to light and oxygen, and very small latitude in the choice of colors, these polarizing films have not yet gained much popularity. JP-B-3944/74 and JP-A-45153/79 show polyamide based polarizing films. Polarizing films employing this polyamide-dye combination have higher heat and moisture resistance and mechanical strength than those based on the PVA-iodine combination, the PVA-dichroic dye combination, or polyenes, but their pola­ rizing performance is poor as compared with the last-men­ tioned three types of polarizing films. Under these circumstances, the present inventors conducted intensive studies in order to develop a novel polarizing film that not only has high polarizing perfor­ mance but also possesses good use characteristics such as resistance to heat, moisture and weather, and transparency. As a result, the present inventors found that this object can be attained by employing an organic resin polymer as a film base and by incorporating in this polymer a dye that is highly soluble in it and which has high dichroism. The present invention has been accomplished on the basis of this finding. The present invention provides a polarizing film that comprises a film base having thereon an anthraquinone dye of the general formula (I) : wherein R¹ and R² each represents a hydrogen atom, an alkyl group, an alkoxy group, an aralkyloxy group, an aryl group, a cyclohexyl group, a 4-alkylcyclohexyl group, a carboxylic acid ester group, a carboxylic acid amide group, a halogen atom, ; X is an oxygen atom, a sulfur atom or an amino group. The structure of the anthraquinone compound having the formula (I) is hereunder described in detail. The alkyl or alkoxy group denoted by R¹ and R² which may be the same or different typically has 1 to 18 carbon atoms, with the range of 1 to 9 carbon atoms being prefer­ red. The aralkyloxy group denoted by R¹ and R² is prefera­ bly a benzyloxy group, which may be substituted by an alkyl or alkoxy group having 1 to 18, preferably 1 to 9, carbon atoms. The aryl group denoted by R¹ and R² is preferably (wherein R³ is a hydrogen atom or the same alkyl group as denoted by R¹; R⁴ is a hydrogen atom or the same alkyl or alkoxy group as denoted by R¹). The 4-alkylcyclohexyl group denoted by R¹ and R² is preferably a trans-4-alkylcyclohexyl group that is substituted by a C₁-C₉ alkyl group. Examples of the carboxylic acid ester group denoted by R¹ and R² (R⁵ is the same alkyl group as denoted by R¹). Examples of the carboxylic acid amide group denoted by R¹ and R² include Examples of the halo­ gen atom denoted by R¹ and R² include F, Cl and Br atoms. Examples of the organic resin that is used as the film base in the present invention include: polyesters, polycarbonates, polyether sulfones, polyimides, polyamides, vinyl halide polymers, vinylidene halide polymers, polyvinyl alcohols, ethylene-vinyl acetate copolymers, cellulosics, polyvinyl butyrals and liquid crystalline polymers. Illustrative liquid crystalline polymers are copoly­ esters of polyethylene terephthalate-parahydroxybenzoic acid. Among these highly heat- and moisture-resistant polymers, polyesters, polyamides, polyimides, and copoly­ esters liquid crystalline polymers of polyethylene tereph­ thalateparahydroxybenzoic acid are preferably used. The polarizing film of the present invention is produced by incorporating 0.01 to 10 wt%, preferably 0.05 to 5 wt%, of the anthraquinone dye in the film bases mentioned above. Depending on the need, two or more of the anthra­ quinone compounds specified above may be used in admixture. The polarizing film of the present invention may also con­ tain other dichroic dyes, non-dichroic dyes, or any suitable additives such as UV absorbers and antioxidants. The polarizing film of the present invention is typically produced by the following procedure: a composi­ tion comprising an organic resin serving as the film base, an anthraquinone compound and any other necessary component is melted to a homogeneous state and shaped into a film or sheet form; then, the film or sheet is stretched uniaxially by a stretch ratio of from 3 to 12 times at a temperature of from 20 to 200°C and further heat-treated at 100 to 250°C for a period of from one second to 30 minutes so as to make a film with a thickness typically ranging from 30 to 200 µm. If necessary, the drawing operation may be conducted in two directions, for example, a longitudinal direction and a traversal direction at right angles. The liquid crystalline copolyester of polyethylene terephthalate-parahyroxybenzoic acid is a polyester resin that is formed by copolymerizing a polyethylene terephthal­ ate component (A) and a parahydroxybenzoic acid component (B) with the (A) to (B) molar ratio being adjusted to be within the range of 40:60 to 5:95. This resin forms a "thermotropic" liquid crystal which, in a molten state, ex­ hibits anisotropy which observed under a polarizing micro­ scope employing crossed Nicol prisms. In orer to make a polarizing film from this liquid crystalline polyester resin, it is first blended with a dichroic acid and the mixture is melted and formed into a film through a die, typically rectangular or circular in cross section, under appropriate shear and draft conditions. Higher shear rates are desirably employed. Although satis­ factory film orientation and transparency can be attained under the commonly employed film-forming conditions (shear rate: several ten to several hundred sec̅¹), even better results are attainable by forming a film with a shear rate of 1,000 sec̅¹ or above. In order to provide transparency, the thickness of the film to be produced is usually set at a value of not larger than 100 µm. The polarizing film thus produced may be immediately used in the form of a film or sheet. If desired and depend­ ing on the specific object of use, the polarizing film may be subjected to a suitable treatment prior to use, such as formation of a protective layer by lamination with a suita­ ble polymer such as a triacetate, acrylic or urethane poly­ mer, or formation of a transparent film of an electroconduc­ tive material, such as an indium-tin oxide, on the surface of the polarizing film by evaporation, sputtering or coat­ ing. The coefficient of dye orientation (Fdye) of the anthraquinone compound which is to be used as a dichroic dye in the present invention can be calculated by the following equation. Fdye = (D-1)/(D+2) (1) where D is the dichroic absorpt ratio of the dichroic dye containing film and is given by the following formula: D = log(Io/I II )/log(Io/I I ) (2) wherein Io is the transmittance of an undyed film prepared by the same drawing and treating conditions; I I is the transmittance of this film for the case where the plane of polarization of the incident ray is perpendicular to the axis of drawing; and I II is the transmittance of the film for the case where the plane of polarization is parallel to the axis of drawing. Therefore, the value of Fdye indicates the degree of orientation of the dichroic dye and the larger this value, the higher the polarizing performance of the polarizing film. The following examples are provided for the purpose of further illustrating the present invention but it should be understood that various modifications of these examples are possible to the extent that they will not depart from the scope and spirit of the invention. In the examples, parts are shown based on a weight. EXAMPLE 1 A mixture of 100 parts of a polyethylene terephthal­ ate oligomer (ηinh: 0.10 dl/g), 288 parts of parahydroxyben­ zoic acid and 0.13 part of stannous acetate was stirred at 220°C for 1 hour under a nitrogen stream. After the mixture was cooled to 140°C, 266 parts of acetic anhydride was added, followed by stirring for an additional 1.5 hours. With acetic acid being distilled off, the mixture was heated to 275°C and 0.23 part of zinc acetate dihydrate was added. Thereafter, the pressure in the reactor was gradually decreased to 0.3 Torr and the mixture was stirred for 6 hours to complete the polymerization. The resulting polymer was composed of a polyethylene terephthalate and a parahydroxybenzoic acid component in a molar ratio of 20:80 and displayed anisotropy in a molten state when observed under a polarizing microscope equipped with a heat stage. This liquid crystalline polyester resin was shaped into a film with a 30-mm extruding machine through a T-die (die width, 120 mm; lip clearance, 0.2 mm). The other film-­ forming conditions were as follows: temperature, 275°C; shear rate, 2,900 sec̅¹; and draft ratio, 15. The film extrudate was cooled by passage through a cooling water tank with an air gap of 40 mm. Using a Henschel mixer, 1 kg of the polyester resin was blended with 5 g of a dye having the structure noted below: The blend was shaped by passing through a T-die as in the manner described above to make a red colored polarizing film having a thickness of 10 µm. This film had a maximum absorption at a wavelength of 517 nm and its Fdye value was 0.84. EXAMPLE 2 One kilogram of a polyethylene terephthalate resin and 1 kg of an anthraquinone dye as used in Example 1 were mixed in a molten state at 280°C. The blend was shaped into a film having a brilliant red color. This film was stretch­ ed uniaxially at 90°C by a stretch ratio of 5 times on a T.M. Long's stretcher so as to make a red polarizing film with a thickness of 100 µm. This film had a maximum absorp­ tion at a wavelength of 519 nm and its Fdye value was 0.72. EXAMPLE 3 A polarizing film was prepared as in Example 1 except that the anthraquinone dye of the following formula was used: The polarizing film produced had a maximum absorp­ tion at a wavelength of 516 nm and its Fdye value was 0.69. EXAMPLE 4 Additional polarizing films were prepared as in Example 1 using the anthraquinone dyes indentified in Table 1, which also lists the maximum absorption wavelengths and Fdye values of the produced polarizing films. The polarizing film of the present invention not only has good polarizing performance but also possesses superior characteristics such as resistance to heat, mois­ ture and weather, and transparency. Because of these features, the film may effectively used in a practical operation and has a very high commercial value.

Claims: 1. A polarizing film which comprises a film base having thereon an anthraquinone dye of the general formula (I) : wherein R¹ and R² each represents a hydrogen atom, an alkyl group, an alkoxy group, an aralkyloxy group, an aryl group, a cyclohexyl group, a 4-alkylcyclohexyl group, a carboxylic acid ester group, a carboxylic acid amide group, a halogen atom, ; X represents an oxygen atom, a sulfur atom or an amino group. 2. A polarizing film according to claim 1, wherein the anthraquinone dye has the general formula (I) in which R¹ and R² each represents a hydrogen atom, a C₁-C₉ alkyl group, a C₁-C₉ alkoxyl group, a benzyloxy group, a benzyloxy group substituted by a C₁-C₉ alkyl group or a C₁-C₉ alkoxy group, (wherein R³ represents a hydrogen group or a C₁-C₉ alkyl group; R⁴ represents a hydrogen atom, a C₁-C₉ alkyl group, or a C₁-C₉ alkoxyl group, a trans-4-alkylcyclohexyl group substituted by (wherein R⁵ represents a C₁-C₉ alkyl group), chlorine atom or a bromine atom. 3. A polarizing film according to claim 1, wherein the anthraquinone dye is 4. A polarizing film according to claim 1, wherein the anthraquinone dye is 5. A polarizing film according to claim 1, wherein the highly heat- and moisture-resistant film base consists of a poly­ ester, polyamide, polyimide, or a liquid crystalline co­ polyester of polyethylene terephthalate-parahydroxybenzoic acid.

Inventors: MIURA, KONOE, OKUMURA, SEIGO, OZAWA, TETSUO, URASHIMA, HIDETOSHI
Applicants: MITSUBISHI KASEI CORPORATION
Application year: 1987
Main classification: C09B1
IPC codes: G02B1, G02B5, C09B1