JPS6345084B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing a polarizing film having good polarization properties.

[Conventional technology]

The demand for polarizing film as an important material for liquid crystal displays is increasing, and there is a strong demand for not only polarizing performance but also durability such as moisture resistance and heat resistance.

By the way, polarizing films made of polyvinyl alcohol (PVA) and iodine are used because they have good polarizing properties, but this PVA-iodine polarizing film has some moisture resistance and heat resistance. Due to this problem, other durable thermoplastic-dye polarizing films are being developed.

However, the polarizing film made of this thermoplastic resin-dye does not have sufficient polarizing performance, and is currently not used practically.

On the other hand, as a method for manufacturing polarizing film, a method is adopted in which a base polymer is uniaxially oriented, and a dichroic substance (=polarizing element) such as iodine or dye is arranged in a uniaxial direction. As a method for orienting in the direction, stretching is performed between a low speed roll and a high speed roll. A so-called roll stretching method is generally employed. Among them, so-called long-section stretching, in which the distance between low-speed rolls and high-speed rolls is increased, weakens the tensile strength in the transverse direction due to longitudinal stretching.
It is known that this method is preferable for producing polarizing films because it improves the degree of orientation of the base polymer in the stretching direction (Japanese Patent Application Laid-open No. 40874/1983).

However, when stretching a wide film, the distance between the rolls must be considerably long, and the film must be gradually stretched over the entire stretching distance while being heated in that space. It's not easy. Even if it were possible, it would tend to be unstable in terms of quality, and especially if a thermoplastic resin that tends to crystallize is used as the base material, there is also the problem of vertical tearing.

[Problem that the invention seeks to solve]

An object of the present invention is to produce a polarizing film that does not tear during stretching even when a thermoplastic resin that easily crystallizes is used as a base material, and that is stable and has excellent polarizing performance without the need for long stretching. The purpose is to provide a method.

[Means for solving problems]

In order to achieve the above object, the present inventors have made extensive studies and found that by using curved rolls as low-speed rolls, it is possible to alleviate the tensile force in the transverse direction associated with longitudinal stretching, and that, surprisingly, a short stretch of stretching can achieve sufficient orientation. The inventors have finally arrived at the present invention, which is a method for producing a good polarizing film.

That is, the present invention provides a polarizing film characterized in that when a polarizing film made of a thermoplastic resin and a dichroic substance is manufactured by a roll stretching method, a low speed roll having a curved cross-sectional shape is used as a pair of stretching rolls. This is a film manufacturing method.

The thermoplastic resin used in the present invention is a thermoplastic resin that can be formed into a transparent film and can be stretched. Examples include homo- or copolymers of polyester, polyamide, polyolefin, polyether, polysulfone, polyvinyl, and the like. From the viewpoint of durability such as moisture resistance and heat resistance, polyester resins and polyamide resins are preferable.

The photodichroic substance used in the present invention is a substance that differs in light absorption depending on the orientation of its molecular structure, and examples of known substances include iodine, dyes, and pigments. Particularly preferred are organic pigments such as dyes and pigments, which are selected in consideration of affinity with the thermoplastic resin used.

The amount of photo dichroism used is determined by the degree of coloring in the polarizing film, but is usually 0.001 to 10% by weight, preferably 0.01 to 3% by weight based on the thermoplastic resin.
Weight % is appropriate.

First, in the present invention, an unstretched film made of a thermoplastic resin alone or containing a photodichroic substance is produced. In the case of a thermoplastic resin alone, a dichroic substance is adsorbed onto the surface of an unstretched film or a stretched film. In any case,
The unstretched film is subjected to the next roll stretching.

This unstretched film is produced by a melt extrusion method or a solution casting method with or without adding a dichroic substance to the thermoplastic resin. At this time, it is preferable to suppress crystallization of the thermoplastic resin as much as possible in order to achieve uniform stretching.

Next, in the present invention, the unstretched film is passed through a roll stretching machine using curved rolls as low speed rolls to form a stretched film.

It is preferable that the curved roll used here has a smooth cross-sectional shape as shown in FIGS. When the unstretched film curves along the roll surface, its apparent width L becomes the original film width L.
(see FIG. 3), and when immediately thereafter stretched and taken off by a take-up roll, a uniaxially stretched film preferably having a width of 1 is obtained.

These L and l can be easily measured by marking approximately the center of the film, and the ratio of the difference [(L-l)/L] can be determined as the curvature.

The larger the curvature at this time, the better, but ideally the maximum value should satisfy the following formula between it and the target stretching ratio R.

(L-l)/L=(1-1/√) Of course, if we allow some substantial shrinkage of the film in the width direction between the curved roll and the high-speed roll that is the take-up roll, the curvature ratio will be It does not matter if it is smaller than the above calculation formula. In particular, when considering the flatness of the film in the width direction, it is better to make it slightly smaller.

The curved roll can be made of a core roll usually made of steel, the surface of which is plated with metal such as nickel or chrome, or coated with synthetic resin, rubber, etc., and the surface is usually heated to a temperature higher than the temperature at which it can be stretched. Preferably, it is heated. As the heating means, a method of passing a heating medium through the inside of the roll, or a method of heating from the outside by infrared irradiation or the like can be adopted.

In the present invention, it is preferable to provide a plurality of stretching roll groups each consisting of a pair of the above-mentioned curved roll and a high-speed take-up roll to perform multistage stretching. In particular, when it is necessary to increase the stretching ratio or when using a wide unstretched film, when the curvature ratio of the above-mentioned curving roll cannot be made so large that it approaches (1-1/√) with the one-stage method, It is better to adopt a multi-stage stretching method. In this case, as long as only the last pair of take-up rolls is a flat roll, the intermediate take-up rolls may also be curved rolls.

An example of an embodiment using the one-stage stretching method of the present invention will be explained with reference to FIG. The unstretched film 2 fed from a feed roll 3 consisting of a pair of pinch rolls passes through flat preheat rolls 4 and 4', and then
The film is stretched by a high-speed take-up roll 5 via a curved roll 1 to form a stretched film 2'. Thereafter, heat treatment can be performed as necessary to improve thermal dimensional stability.

At this time, the stretching ratio is as follows:
It is determined by the ratio (Vs/Vp) of the surface running speed (Vp) of the take-up roll 5 and the surface running speed (Vs) of the take-up roll 5, and can be appropriately selected depending on the base resin used, the stretching temperature, etc., but preferably It is 3 times or more, more preferably 4 to 10 times. The stretching ratio in the multi-stage stretching method is the product of the stretching ratios in each stage (total ratio).

〔Effect of the invention〕

According to the method of the present invention, even if a wide unstretched film is used, the orientation of the base polymer can be improved without unduly increasing the distance between the rolls, and as a result, a polarizing film with excellent polarization performance can be made into a relatively compact film. Continuous production is possible using a roll longitudinal stretching machine, so there is a great cost advantage. In addition, the obtained polarizing film not only has excellent polarization properties but also good optical uniformity, and if a resin with excellent moisture resistance and heat resistance is used as the base material, durability is especially required. It can be suitably used in the field of liquid crystal displays.

〔Example〕

The present invention will be further explained below with reference to Examples.

In addition, the light transmittance in the examples is the light transmittance (To) of one polarizing film measured by a self-recording spectrophotometer, and the degree of polarization is the light transmittance (To) of one polarizing film measured with a self-recording spectrophotometer. Alternatively, the light transmittance measured vertically (T ₁₁
Alternatively, T⊥), this is the value calculated using the following formula.

However, all values are measured at the maximum absorbance wavelength (λmax).

Example 1 20 g of anthraquinone disperse dye “Miketon Polyeser BIue TGSF” (manufactured by Mitsui Toatsu Chemical Co., Ltd.) was added to 10 kg of polyethylene terephthalate resin pellets (IV0.71) and mixed uniformly, then an extruder (40 mmφ) was added. Melt extrusion from T-die (290
℃) and quenched to form an unstretched film (300 μm width) with a thickness of 150 μm.

This film is passed through a curved heated roll (maximum diameter 400 mm width) heated to 70°C with a curvature ratio of 0.4 in the roll longitudinal stretching machine shown in Fig. The stretched film passed through the stretching rolls was then heat-treated under tension with a heat setting roll (190°C) to obtain a polarizing film (180 mm width) with a thickness of about 70 μm.

When the optical properties of this polyester polarizing film were measured, the transmittance (To) was 40% and the degree of polarization (V) was 89%, with almost no variation in the width direction.

Comparative Example 1 When the optical properties of a polarizing film obtained by performing longitudinal stretching (4 times) in the same manner as in Example except that the curved roll was replaced with a normal flat roll were measured, To = 46%, V = 52 It was %.

Example 2 10kg of nylon-6 resin pellets (IV0.6) were mixed with Miketon Polyester, an azo dichroic dye.
After adding and mixing 10 g of "Orange 3GSF" (manufactured by Mitsui Toatsu Chemical Co., Ltd.), an unstretched film (width 300 mm) with a thickness of about 100 μm was formed using a T-die extruder.

This film was fed to the roll stretching machine shown in Figure 4, which has curved rolls (curvature ratio 0.3) heated to 110°C, stretched 3.5 times in the longitudinal direction, and then subjected to tension heat treatment with heat setting rolls (180°C). about 40μ
A polarizing film (width 210 mm) with a thickness of m was obtained.

The optical properties of this polyamide polarizing film are
To=38%, V=81%.

Comparative Example 2 The optical properties of a polarizing film obtained by stretching and heat treatment in the same manner as in Example 2 except that the curved roll was replaced with a flat roll were To = 41% and V = 56%.
It was hot.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views showing the shape of a specific example of a curved roll according to the present invention, FIG. 3 is a conceptual diagram showing a change in the width of the film in a curved heating roll, and FIG. It is a process diagram showing one embodiment of the process.

Claims (1)

[Claims] 1. When producing a polarizing film made of a thermoplastic resin and a dichroic substance by a roll stretching method,
A method for producing a polarizing film, characterized in that a low-speed roll of a pair of stretching rolls has a curved cross-sectional shape. 2. The method for producing a polarizing film according to claim 1, wherein the low-speed roll has a smooth convex or concave cross-sectional shape over its entire width. 3. The method for producing a polarizing film according to claim 1 or 2, wherein the thermoplastic resin is selected from the group consisting of polyester resins and polyamide resins. 4. The method for producing a polarizing film according to any one of claims 1 to 3, wherein the dichroic substance is an organic dye.