CN114834027A

CN114834027A - Die for extrusion molding of polyvinyl alcohol film, optical polyvinyl alcohol film, polarizing film and preparation method thereof

Info

Publication number: CN114834027A
Application number: CN202210288227.5A
Authority: CN
Inventors: 李承龙; 古文正; 张飞雄; 徐勇彪; 吕新坤; 姜维; 王宏星
Original assignee: Chongqing Spectrum New Material Technology Co ltd
Current assignee: Chongqing Spectrum New Material Technology Co ltd
Priority date: 2022-03-22
Filing date: 2022-03-22
Publication date: 2022-08-02
Anticipated expiration: 2042-03-22
Also published as: CN114834027B

Abstract

The invention relates to the technical field of polymer films, and discloses a die for extrusion molding of a polyvinyl alcohol film, an optical polyvinyl alcohol film, a polarizing film and a preparation method thereof, wherein the width a of an upper die opening and the width b of a lower die opening of the die are respectively 10-3000 mu m; the surface roughness of the upper die opening platform and/or the lower die opening platform of the die meets the following requirements: ra is more than or equal to 0.002 mu m and less than or equal to 0.5 mu m, Rz is more than or equal to 0.02 mu m and less than or equal to 1 mu m; where Ra is the arithmetic mean deviation of the profile and Rz is the maximum height of the profile. The polyvinyl alcohol film prepared by the mold can keep better flatness at a micro scale, so that the tensile property and the dyeing property of the film are improved, and the polyvinyl alcohol film can eliminate micro fine lines, have thinner stripe width, unobvious spots and less foreign matters under the condition of high-light-intensity backlight. The polarizing film produced by dyeing and stretching the polyvinyl alcohol film has fewer mottled defects, higher transmittance and higher degree of polarization.

Description

Die for extrusion molding of polyvinyl alcohol film, optical polyvinyl alcohol film, polarizing film and preparation method thereof

Technical Field

The invention relates to the technical field of polymer films, in particular to a preparation method of a mold for a polyvinyl alcohol film, an optical polyvinyl alcohol film and a polarizing film.

Background

A polarizer, which is an important component of a Liquid Crystal Display (LCD), is mainly used to control optical rotation of light passing through a liquid crystal, and is generally made of a polarizing film formed by oriented adsorption of iodine or dichroic dye of a dichroic dye to a polyvinyl alcohol-based resin film. With the thinning of LCD products, the polarizer is also developed to be thinner and larger in size. Therefore, thinning of a polyvinyl alcohol (PVA) film for a polarizer is a problem to be solved first. However, the PVA film prepared by the prior art becomes thinner, and then a plurality of production problems occur, for example, after the thickness of the PVA film is reduced, high-magnification stretching is difficult to realize in the back-end dyeing and stretching process, and the phenomena of tearing and breaking easily occur, so that the thin PVA polarizing film product has weak stripes and dyeing defects, and the problems of low transmittance and low polarization degree of the PVA polarizing film are caused.

Disclosure of Invention

The invention aims to solve the problems that after the thickness of a PVA film is reduced, high-rate stretching is difficult to realize in the back-stage dyeing and stretching process, tearing and breaking are easy to occur, and finally the transmittance of a polarizing film is low and the polarization degree is low in the prior art, and provides a mould for a polyvinyl alcohol film, an optical polyvinyl alcohol film and a preparation method of the polarizing film.

In order to achieve the above object, a first aspect of the present invention provides a die for extrusion molding of a polyvinyl alcohol-based film, the die having an upper die width a and a lower die width b each of 10 to 3000 μm.

The second aspect of the invention provides a method for preparing an optical-grade polyvinyl alcohol film, which comprises the following steps:

and extruding the polyvinyl alcohol slurry through the die, and drying to form the optical-grade polyvinyl alcohol film.

In a third aspect, the invention provides an optical-grade polyvinyl alcohol film, wherein the thickness of the optical-grade polyvinyl alcohol film is 10-75 μm; preferably 20-60 μm;

preferably, the optical-grade polyvinyl alcohol film has the number of the micro-fine grains being less than or equal to 4 and the number of the specific foreign matters being less than or equal to 4 in each square meter.

The micro-texture observed in the present invention is typically a faint fine stripe running in the MD direction of the film, typically greater than 5cm in length, and running parallel in the MD direction. The micro-fine lines are caused by the difference between the thickness of the stripe region and the thickness of the periphery, and the difference in thickness is usually less than 1 μm.

The specific foreign matter observed in the invention refers to polyvinyl alcohol gel foreign matter, which usually appears in a strip shape, a folded shape, a point shape or a block shape, and the outline of the foreign matter is easy to observe because the optical transmittance of the gel foreign matter is different from that of a PVA wet film. The foreign matter that this application indicates needs to make the differentiation with the foreign matter that the process problems of thick liquids such as insufficient dissolution or insufficient filtration leads to in the thick liquids, and the process problems of above-mentioned thick liquids can be solved through optimizing polyvinyl alcohol thick liquids dissolving process, extrusion process, especially promotion coarse filtration, fine filtration level.

The fourth aspect of the present invention provides a method for preparing a polarizing film, which comprises the following steps:

and dyeing and stretching the optical polyvinyl alcohol film to obtain the polarizing film, wherein the polarization degree of the polarizing film is more than or equal to 99.9%, and the transmittance of the polarizing film is more than or equal to 43%.

The fifth aspect of the invention provides a polarizing film, wherein the polarization degree of the polarizing film is greater than or equal to 99.9%, and the transmittance of the polarizing film is greater than or equal to 43%.

Through the technical scheme, the polyvinyl alcohol film prepared by the mold can keep better flatness at a micro scale, so that the tensile property and dyeing property of the film are improved, micro fine lines of the polyvinyl alcohol film can be eliminated under the condition of high-light-intensity backlight, and foreign matters are fewer. The polarizing film produced by dyeing and stretching the polyvinyl alcohol film has fewer mottled defects, higher transmittance and higher degree of polarization.

Drawings

FIG. 1 is a schematic diagram showing the results of the mold for a polyvinyl alcohol film of the present invention.

Description of the reference numerals

1. Go up die orifice 2, die orifice 3, go up die orifice platform

4. Lower die opening platform

Detailed Description

The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.

The invention provides a mould for extrusion molding of a polyvinyl alcohol film, which comprises an upper template and a lower template which are arranged in parallel, wherein a space formed between the two templates is a flow channel of a polyvinyl alcohol melt obtained by extrusion melting, mixing and plasticizing of polyvinyl alcohol; two templates are used for preliminarily shaping the polyvinyl alcohol melt. In the structure of the upper template and the lower template, a platform is formed at the position where the polyvinyl alcohol melt flows out of the mold, namely the tail end position of the flow channel of the upper template and the tail end position of the lower template, and the platform is respectively and correspondingly formed into an upper die orifice and a lower die orifice of the mold. As shown in fig. 1, the width a of the upper die orifice 1 and the width b of the lower die orifice 2 of the die are respectively 10-3000 μm, preferably, the width of the upper die orifice 1 and the width of the lower die orifice 2 of the die are respectively 80-1000 μm; a. b is less than 10 mu m, the die has high processing difficulty and the die durability tends to be poor; a. when b is larger than 3000 μm, defects such as fine streaks, and gel foreign matter are present on the film surface of the raw film formed from the polyvinyl alcohol slurry, and the stability of the quality of the raw film of the polyvinyl alcohol extruded from the die tends to be poor. The opening of the die orifice of the die can be adjusted, the adjusting mode is not limited, and the mode that the die orifice elastic sheet is extruded by a bolt commonly used in the industry is preferably adopted for adjustment.

Furthermore, the included angle e of the upper die orifice and the included angle f of the lower die orifice of the die are respectively 10-75 degrees, and the included angle of the die orifice refers to the included angle formed by the bevel edge of the die orifice and the perpendicular bisector. e. The durability of the mold is a problem when the angle f is less than 10 °; e. when the angle of f is more than 75 ℃, the die tends to have fine streaks. e. f may be equal or unequal, preferably equal. The radius of an upper die opening fillet x and the radius of a lower die opening fillet y of the die are respectively 0-2mm, the fillet is a fillet generated by arc surface transition formed by the intersection line of an adjacent die opening platform surface and a die flow channel surface, and the radius of a transition circle corresponding to the fillet is the radius of the fillet and represents the size of the fillet; when the radius of the fillet x and the radius of the fillet y are both larger than 2mm, the quality of the polyvinyl alcohol base film extruded from the die tends to deteriorate, and uniform film formation tends to be impossible.

Further, the surface roughness of the upper die opening platform 3 and/or the lower die opening platform 4 of the die meets the following requirements: 0.002 μm Ra 0.5 μm, 0.02 μm Rz 1 μm, preferably 0.002 μm Ra 0.075 μm, 0.02 μm Rz 0.2 μm; wherein Ra is the arithmetic mean deviation of the profile, Rz is the maximum height of the profile, and if Ra is less than 0.002 μm, Rz is less than 0.02 μm, the processing difficulty is high, and the durability tends to be poor; when Ra is more than 0.5 μm and Rz is more than 1 μm, there is a tendency that fine streaks, defects such as gel foreign matter and the like are present on the film surface of the green film formed from the polyvinyl alcohol slurry, and the quality stability of the green film of the polyvinyl alcohol extruded from the die is deteriorated. The surface roughness Ra of the surfaces of the upper die platform 3 and the lower die platform 4 may be equal or unequal, preferably equal. The surface roughness Rz of the surfaces of the upper die land 3 and the lower die land 4 may be equal or unequal, preferably equal. Ra and Rz can be evaluated by a known test method as a known concept. As shown in fig. 1, upper and lower die platforms 3 and 4 include horizontal platform portions and rounded corners. The die is a clothes hanger type die, a fishtail type die, a T-shaped die (a support tube type die) or a distribution screw die. The polyvinyl alcohol sizing agent can be uniformly dispersed in the width direction through the die, and the discharging speed of the polyvinyl alcohol sizing agent from the die is basically consistent in the width direction.

The structure and the shape of the mould, particularly the mould opening part of the mould, have great influence on the film forming process of the polyvinyl alcohol slurry. The polyvinyl alcohol aqueous slurry is mostly formed into a film by a wet method, compared with a melting method, a polyvinyl alcohol blank film formed by the wet method contains a large amount of solvent water, so that the problem of local drying and unbalanced drying of the polyvinyl alcohol blank film is more easily caused by unreasonable structure of a die orifice of a traditional casting die in the film forming process. According to the invention, the widths of the upper die opening and the lower die opening of the die, the surface roughness of the platforms of the upper die opening and the lower die opening of the die and the r angle of the die opening of the die are controlled to a specific level, so that a polyvinyl alcohol blank film has no drying imbalance after the polyvinyl alcohol slurry is extruded from a slit of the die, and the uneven deformation of the film surface caused by the die opening of the die is reduced, thereby reducing the defects of the optical polyvinyl alcohol film such as micro-fine lines, foreign matters and the like.

Further, the die orifice surface of the die is hardened by chromium plating, and from the viewpoint of durability, the die orifice surface is hardened, preferably by chromium plating and then finely ground to a target level.

The second aspect of the present invention provides a method for preparing an optical-grade polyvinyl alcohol film, based on a mold for extrusion molding of a polyvinyl alcohol film, comprising the steps of:

extruding the polyvinyl alcohol slurry through the die, and drying to form the optical-grade polyvinyl alcohol film; the temperature of the die is 50-99 ℃, and the drying temperature is 50-99 ℃; the thickness of the polyvinyl alcohol film is 10-75 μm; preferably, the thickness of the polyvinyl alcohol-based film is 20 to 60 μm, and more preferably, the thickness of the polyvinyl alcohol-based film is 30 to 50 μm.

The invention adopts the method that the polyvinyl alcohol slurry is extruded to the surface of the drying roller or the mirror surface steel belt for drying, in order to ensure that the extrusion rate of the PVA slurry from the die is consistent, the extrusion rate of the slurry to the surface of the drying roller or the mirror surface steel belt is uniform and flat, and the extrusion rate of the slurry in the width direction of the die orifice is consistent.

The third aspect of the invention provides an optical polyvinyl alcohol film prepared by the preparation method of the optical polyvinyl alcohol film, wherein the thickness of the optical polyvinyl alcohol film is 10-75 μm; preferably 20-60 μm;

The fourth aspect of the present invention provides a method for preparing a polarizing film, which uses the above method for preparing an optical polyvinyl alcohol film, and comprises the following steps:

and dyeing and stretching the optical polyvinyl alcohol film to obtain the polarizing film.

The fifth aspect of the invention provides a polarizing film prepared by the preparation method of the polarizing film, wherein the polarization degree of the polarizing film is not less than 99.9%, and the transmittance of the polarizing film is not less than 43%.

According to a particularly preferred embodiment of the present invention, the above-mentioned mold for extrusion molding of a polyvinyl alcohol-based film is used for the production of an optical-grade polyvinyl alcohol film, wherein the upper die width a and the lower die width b of the mold are respectively 80 to 1000 μm, and the surface roughness of the upper die land and/or the lower die land of the mold satisfies: ra is more than or equal to 0.002 mu m and less than or equal to 0.075 mu m, Rz is more than or equal to 0.02 mu m and less than or equal to 0.2 mu m, and the thickness of the polyvinyl alcohol film layer is 30-50 mu m.

The drying temperature of the drying roller or the steel belt is preferably 50-99 ℃; the drying time is preferably such that the moisture content is from 8% to 40%, preferably from 17% to 25%; after drying, subjecting the preformed optical-grade polyvinyl alcohol film to multi-roller drying and drying in an oven, humidifying, further controlling the humidity of the preformed optical-grade polyvinyl alcohol film to be 2% -5%, cutting edges to ensure that the cutting edges of the film are regular and have no warpage, and rolling to obtain the film; the thickness uniformity of the optical grade polyvinyl alcohol film is preferably controlled within + -1 μm.

The present invention is not particularly limited in terms of the source of all raw materials, and may be commercially available.

The raw material used in the invention comprises polyvinyl alcohol, the polymerization degree of the polyvinyl alcohol is preferably 1000-; the degree of alcoholysis of the polyvinyl alcohol is preferably greater than or equal to 99%, more preferably from 99.9% to 99.99%.

In the invention, the used raw materials also comprise the polyvinyl alcohol and a solvent; the mass ratio of the polyvinyl alcohol to the solvent is preferably 1: (0.6-9), more preferably 1: (1-7), and more preferably 1: (1.5-5), most preferably 1: (2-4); the solvent is preferably one or more of dimethyl sulfoxide, glycerol or water, and is more preferably water. The source of the solvent is not particularly limited in the present invention, and a commercially available product of the solvent may be used, or the solvent may be prepared by itself according to a technical scheme for preparing the solvent, which is well known to those skilled in the art.

In the invention, the raw materials also comprise a plasticizer; the mass ratio of the polyvinyl alcohol to the plasticizer is preferably 1: (0.05-0.3), particularly preferably 1: (0.08-0.13); the plasticizer is preferably one or more of glycerol, diglycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol or trihydroxymethyl propane, more preferably one or more of glycerol, triethylene glycol, tetraethylene glycol or trihydroxymethyl propane, and most preferably glycerol and/or triethylene glycol; the source of the plasticizer is not particularly limited in the present invention, and a commercially available product of the plasticizer may be used, or the plasticizer may be prepared by itself according to a technical scheme for preparing the plasticizer well known to those skilled in the art.

In the invention, the raw material also comprises a surfactant. The mass ratio of the polyvinyl alcohol to the surfactant is preferably 1: (0.0001-0.03), more preferably 1: (0.0001-0.003), and more preferably 1: (0.0001-0.001), most preferably 1: 0.0005. the surfactant is preferably an anionic surfactant and/or a nonionic surfactant, more preferably one or more of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, polyoxyethylene octylphenol ether, polyoxyethylene nonylphenol ether or lauric acid diethanolamide, and more preferably sodium dodecyl sulfate and/or lauric acid diethanolamide; the source of the surfactant is not particularly limited in the present invention, and commercially available products of the surfactant can be used, or the surfactant can be prepared by itself according to a method well known to those skilled in the art for preparing the surfactant.

In the present invention, the raw material further includes a crosslinking substance. The form of the crosslink is not limited, and examples thereof include a chemical bond, a coordinate bond, and an intermolecular interaction. The crosslinking substance is preferably a substance containing a group capable of forming a bond or an interaction with a hydroxyl group. The crosslinking substance preferably contains a number of groups that can bond or interact with each other of 2 or more. The molar ratio of the active group to the hydroxyl group of the polyvinyl alcohol is preferably 1 (20-2000), more preferably 1 (100-300). The substance causing chemical crosslinking is preferably: maleic anhydride, phthaloyl chloride, phthalic anhydride, succinic anhydride, phthalic acid, epichlorohydrin, formaldehyde, glutaraldehyde, boric acid, potassium permanganate and the like. The substance causing complexation or intermolecular interaction is preferably an iron compound, a barium compound, a boron-containing compound, a manganese compound, a cobalt compound, or a copper compound. Among the above crosslinking substances, glutaraldehyde is preferable.

The optical-grade polyvinyl alcohol film prepared by the preferred embodiment of the invention can keep better flatness in a micro scale, so that the tensile property and dyeing property of the film are improved. In the process of manufacturing the polyvinyl alcohol film, particularly in the process of uniformly dispersing polyvinyl alcohol slurry to the whole casting surface direction (TD direction of the film) through the die and extruding the polyvinyl alcohol slurry to the surface of a drying roller or a steel belt, the design of the die orifice of the die can eliminate tiny fine lines and reduce foreign matters of the polyvinyl alcohol film under the condition of backlight with high light intensity. The polarizing film prepared by dyeing and stretching the optical-grade polyvinyl alcohol film has fewer color spot defects.

The micro-fine lines and specific foreign matters observed in the invention mean that a film product with the length of 50cm and the width of 50cm is cut along the length direction (MD direction) of the film, and the film is irradiated by a 6500k white light source under a dark room environment, wherein the length of the light source is consistent with the width of the product. A white back plate (length, width, 3.5m) was introduced, and the light source, the film, and the white back plate were arranged in parallel in this order, with a distance of 50cm between the light source and the film, and a distance of 275cm between the film and the white back plate. In the case of strong white light, fine lines, specific foreign matters, etc. existing in the polyvinyl alcohol film are amplified on the white back sheet, and the quality of the film is evaluated by counting the above defects. Wherein, the fine lines are classified as follows: a: the number of the fine lines is less than or equal to 1; b: the number of the fine grains is more than 1 and less than or equal to 2; c: the number of the fine grains is more than 2; the specific foreign matter amount is classified as follows: a: the foreign matter is less than or equal to 1; b: foreign matter is more than 1 and less than or equal to 3; c: 3 < foreign matter.

The mottling of the polarizing film observed in the present invention means that the polarizing film produced from the polyvinyl alcohol film of the present invention was placed on a surface light source (backlight) in a dark room with an observation polarizer (a polarizer having a transmittance of about 43%) placed thereon in such a manner that a cross nicol prism was present with respect to the observation polarizer, and then the polarizing film was irradiated with light (at a luminance of 15000cd) from the backlight through the observation polarizer, visually observed at a position 1m directly above the polarizing film, and the mottling evaluation of the polarizing film was evaluated based on the following criteria: a: no stain was observed; b: stains can be observed, but not significantly, at a level that is practically unproblematic; c: a level of mottling that is problematic in practice can be observed.

The thickness test method of the polyvinyl alcohol film refers to HG/T4185-2011; the transmittance and degree of polarization of the polarizing film were tested by GB/T31379-2015.

The method for manufacturing the polarizing film for evaluating the color spots adopts a wet process. A rectangular test piece having a length of 12cm × 20cm in the width direction was sampled from the width direction center of the PVA film obtained as described above, both ends of the test piece in the length direction were fixed to a stretching jig so that the dimension of the stretched portion reached 10cm × 20cm in the width direction, the test piece was immersed in water at a temperature of 30 ℃ for 30 seconds, uniaxially stretched in the length direction at a stretching speed of 20 cm/min to 1.5 times the original length, then immersed in an aqueous solution of iodine/potassium iodide at a temperature of 30 ℃ containing iodine at a concentration of 0.03 mass% and potassium iodide at a concentration of 3 mass% for 60 seconds, and uniaxially stretched in the length direction at a stretching speed of 20 cm/min to 3 times the original length; subsequently, the polarizing film was immersed in an aqueous boric acid/potassium iodide solution containing boric acid at a concentration of 4 mass% and potassium iodide at a concentration of about 5 mass% at a temperature of 60 ℃, uniaxially stretched at a stretching speed of 20 cm/min (4 th stage stretching) in the longitudinal direction to 5.8 times the original length, then immersed in an aqueous potassium iodide solution containing potassium iodide at a concentration of 3 mass% for 10 seconds to perform fixing treatment, and then dried in a dryer at 60 ℃ for 4 minutes to obtain the polarizing film.

The present invention will be described in detail below by way of examples.

Example 1

Preparing polyvinyl alcohol slurry: according to the mass ratio of polyvinyl alcohol, glycerol, sodium dodecyl sulfate and water being 10: 1: 0.03: 20. Wherein the polymerization degree of the polyvinyl alcohol is 2300, and the alcoholysis degree is 99.99%. And dissolving and homogenizing the slurry in a reaction kettle, coarsely filtering the slurry, feeding the coarse filtered slurry into an extruder, and finely filtering the coarse filtered slurry, and feeding the fine filtered slurry into a die.

The die parameters of the die are as follows: the mould is a clothes hanger type mould; the width a of the upper die opening and the width b of the lower die opening of the die meet the following requirements: a, b, 100 μm; the surface roughness of going up die orifice platform and die orifice platform satisfies respectively: ra 0.005 μm, Rz 0.05 μm; the included angle e of the upper die opening and the included angle f of the lower die opening are both 45 degrees; the radius of the upper die opening fillet x and the radius of the lower die opening fillet y are respectively 0.02 mm.

And (3) after the polyvinyl alcohol slurry is molded by a mold, drying the polyvinyl alcohol slurry on a drying roller at the drying temperature of 93 ℃, and obtaining a preformed film after the moisture content of the film reaches 20%. The preformed film may be passed through a multi-roll dryer, oven, further controlling the film to 3.5%. And drying to obtain the optical polyvinyl alcohol film, wherein the thickness of the optical polyvinyl alcohol film is 30 μm, and the thickness uniformity is controlled within +/-0.5 μm.

The optical-grade polyvinyl alcohol film is subjected to defect detection, and the detection result is shown in table 1, wherein the film has fine lines: a; foreign matter: a; color spots: A.

the results of the color unevenness, the degree of polarization and the transmittance of the polarizing film obtained from the optical-grade polyvinyl alcohol film are shown in table 1.

Example 2

The die parameters of the die are as follows: the mould is a clothes hanger type mould; the width a of the upper die opening and the width b of the lower die opening of the die meet the following requirements: a is 10 μm; the surface roughness of going up die orifice platform and die orifice platform satisfies respectively: ra 0.002 μm, Rz 0.02 μm; the included angle e of the upper die opening and the included angle f of the lower die opening are both 75 degrees; the radius of the upper die opening fillet x and the radius of the lower die opening fillet y are respectively 0 mm.

Example 3

The process of example 2 was followed except that the die parameters of the die were as follows:

the die parameters of the die are as follows: the mould is a clothes hanger type mould; the width a of the upper die opening and the width b of the lower die opening of the die meet the following requirements: a ═ b ═ 3000 μm; the surface roughness of going up die orifice platform and die orifice platform satisfies respectively: ra 0.5 μm, Rz 1 μm; the included angle e of the upper die opening and the included angle f of the lower die opening are both 10 degrees; the radius of the upper die opening fillet x and the radius of the lower die opening fillet y are respectively 2 mm.

The obtained optical-grade polyvinyl alcohol film has defect detection results shown in table 1, and the film has fine lines: a; foreign matter: a; color spots: A.

Comparative example 1

The method of example 2 is followed except that the upper die width a and the lower die width b of the die satisfy: and a is 4000 μm.

The obtained film was subjected to defect detection, and the film had fine lines as shown in table 1: c; foreign matter: c; color spots: C.

the results of the color unevenness, the degree of polarization and the transmittance of the polarizing film obtained from the film are shown in table 1.

Comparative example 2

The method of example 2 was followed except that the surface roughness of the upper and lower die platforms each satisfied: ra 0.6 μm, Rz 1.2 μm.

The obtained film was subjected to defect detection, and the film had fine lines as shown in table 1: b; foreign matter: a; color spots: B.

Comparative example 3

The process of example 2 was followed except that the included angle of the upper die e and the included angle of the lower die f were both 80 °.

The obtained film was subjected to defect detection, and the film had fine lines as shown in table 1: c; foreign matter: c; color spots: B.

Comparative example 4

The method of example 2 was followed except that the radius of each of the upper die fillet x and the lower die fillet y was 2.5 mm.

The obtained film was subjected to defect detection, and the film had fine lines as shown in table 1: a; foreign matter: b; color spots: C.

The performance test results of the optical polyvinyl alcohol film and the polarizer in the examples and the comparative examples are shown in Table 1.

TABLE 1

As can be seen from the results of the examples, the comparative examples and table 1, compared with the comparative examples of the optical polyvinyl alcohol film and the polarizing film prepared by the conventional mold, the optical polyvinyl alcohol film prepared by the mold of the invention has the advantages that the tiny fine lines are eliminated, the stripe width is thinner, the specks are not obvious, and the foreign matters are less in the case of the backlight with high light intensity; the polarizing film produced by dyeing and stretching the polyvinyl alcohol film has fewer mottled defects, higher transmittance and higher degree of polarization.

The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including combinations of various technical features in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.

Claims

1. The die for extrusion molding of the polyvinyl alcohol film is characterized in that the width a of an upper die opening and the width b of a lower die opening of the die are respectively 10-3000 mu m.

2. The mold of claim 1, wherein the included angle of the upper die orifice angle e and the included angle of the lower die orifice angle f of the mold are respectively 10 ° to 75 °.

3. The die of claim 1, wherein the respective radii of the upper die fillet x and the lower die fillet y of the die are 0-2 mm.

4. The mold according to claim 1, wherein the surface roughness of the upper and/or lower die land of the mold is such that: ra is more than or equal to 0.002 mu m and less than or equal to 0.5 mu m, Rz is more than or equal to 0.02 mu m and less than or equal to 1 mu m; where Ra is the arithmetic mean deviation of the profile and Rz is the maximum height of the profile.

5. The mold according to any one of claims 1 to 4, wherein the upper and lower die widths a and b of the mold are respectively 80 to 1000 μm.

6. The mold of claim 5, wherein the die surface of the mold is hardened by chrome plating.

7. A preparation method of an optical-grade polyvinyl alcohol film is characterized by comprising the following steps:

extruding the polyvinyl alcohol slurry through the die of any one of claims 1-6, and drying to form the optical-grade polyvinyl alcohol film.

8. The method according to claim 7, wherein the temperature of the mold is 50 to 99 ℃ and the drying temperature is 50 to 99 ℃.

9. An optical grade polyvinyl alcohol film obtained by the production method according to claim 7 or 8, wherein the optical grade polyvinyl alcohol film has a thickness of 10 to 75 μm; preferably 20-60 μm;

10. A method for producing a polarizing film, characterized by comprising the steps of:

the optical grade polyvinyl alcohol film of claim 9 is dyed and stretched to obtain the polarizing film.

11. A polarizing film obtained by the production method according to claim 10, wherein the polarizing film has a degree of polarization of 99.9% or more and a transmittance of 43% or more.