RU2520938C1 - Polarising film and method of obtaining it - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: polarising film consists of oriented molecules of a block copolymer of polyvinyl alcohol and polyvinylene, obtained by acid-catalysed thermal dehydration of oriented molecules of polyvinyl alcohol, and additionally contains phosphotungstic acid. A method of production consists in formation of a film of polyvinyl alcohol from a water solution of polyvinyl alcohol and an acid catalyst of thermal dehydration of the said polymer, uniaxial stretching and annealing the said film. As the acid catalyst used is phosphotungstic acid in an amount of 10-30% relative to the polyvinyl alcohol weight. The film is stretched 4-7-fold relative to its initial length and annealed at a temperature of 120-140°C for 1-15 min.

EFFECT: similar optic properties throughout the whole area, increased resistance to moisture and heat, ensuring simplicity, efficiency and environmental friendliness of the film production.

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Description

The invention relates to polarization films for the visible spectral region based on polyvinyl alcohol (PVA) and methods for their preparation. Such films are important components of various optoelectronic devices, including primarily liquid crystal flat displays.

Currently known PVA-based polarization films are divided into three groups depending on the component polarizing light that is part of them: 1) films containing polyiodide complexes; 2) films containing dichroic dyes; 3) films in which PVA chains contain polyvinylylene units. Films are uniaxially elongated. A uniaxial drawing creates a predominant molecular orientation of the polarizing components in the direction of the drawing axis, as a result of which the film acquires polarizing properties.

The subject of the invention is a polarization film based on PVA, the polymer chains of which contain polyvinylylene units. The preparation of such films is based on the acid-catalyzed thermal dehydration of PVA, in which part of the polyvinyl alcohol units of the polymer chain are chemically converted into polyvinyl and, as a result, a block copolymer of polyvinyl alcohol and polyvinyl is formed

$$\cdots -{\left(C{H}_2-CHOH\right)}_n-\cdots \to \cdots -{\left(C{H}_2-CHOH\right)}_{n-k}-{\left(CH=CH\right)}_k-\cdots ,$$

where - (CH ₂ -CHOH) _nk - and - (CH = CH) _k - are, respectively, polyvinyl alcohol and polyvinylylene units of said block copolymer. Uniaxially oriented polyvinylylene units are characterized by strong optical dichroism in the visible region of the spectrum — they transmit light polarized parallel to the orientation axis and strongly absorb light with perpendicular polarization. Therefore, a film consisting of oriented molecules of a block copolymer of PVA and polyvinylylene has polarizing properties.

A polarization film consisting of oriented molecules of the PVA block copolymer and polyvinylylene obtained by acid-catalyzed thermal dehydration of oriented PVA molecules is described in the patent [1].

In the methods proposed in [1] for the preparation of this film for catalysis of thermal dehydration of PVA, strong mineral acids are used: HC1, HI, HBr, H ₂ SO ₄ . In one method, the acid is introduced into a non-oriented PVA film by immersing it in an aqueous solution of acid in methanol or in a mixture of methanol-water. Then the film is subjected to dehydration by annealing it at 150-175 ° C with simultaneous uniaxial drawing. In another method, the PVA film is first oriented (stretched) in one direction, then acid is introduced into it, similarly to the first method, after which the film is annealed at 150-175 ° C. During annealing, the acid contained in the film catalyzes the thermal dehydration of PVA and is simultaneously removed from the film by evaporation.

There are known modifications of the methods for producing a polarizing film described in the patent [1], characterized in that strong mineral acid (BUT) is introduced into the film, keeping it not in an acid solution, but in concentrated (fuming) acid vapor [2-4]. Additionally, these methods include operations to improve the thermal and moisture resistance of the films, as well as improve their polarization characteristics.

A disadvantage of the known methods [1-4] is the use of large quantities of highly corrosive and hazardous human exposure to strong mineral acids. This imposes increased requirements for corrosion protection of equipment and production safety. Another disadvantage of the methods [1-4] is that the dynamics of acid evaporation and, accordingly, its concentration in different parts of the film may vary due to temperature fluctuations and convective motion of acid vapor in contact with the film. As a result, the formation of polyvinylylene units in the structure of PVA during its dehydration occurs non-uniformly, which leads to the appearance of stripes and spotting defects in the staining of films in polarized light and makes them unsuitable for particularly precise optical applications.

Technical solutions are known [5, 6], which make it possible to obtain a polarizing film by partial thermal dehydration of PVA catalyzed by hydrochloric acid without the use of large amounts of acid, and thus give uniform formation of polyvinylylene structures over the film area. In the method [5], a layer of a 1-3-normal HCl solution in the amount of ~ 10 ^-4 ml / cm ² is applied to the carrier film of polyethylene terephthalate or cellulose triacetate and at the same time the carrier film is combined on the applied layer with a uniaxially stretched PVA film, after which the resulting " sandwich "incubated at 115-160 ° C for 3-10 minutes Method [6] is similar to method [5], except that instead of a layer of an aqueous HCl solution, a thin layer of a composition containing a substance is deposited on the surface of the carrier film, which decomposes at elevated temperatures to form HCl. The disadvantage of these methods is their complexity associated with the use of additional special equipment for applying a uniform thin layer of a substance or mixture of substances on one film and combining it on this layer with another film. Another disadvantage of these methods is the increase in material costs due to the use of auxiliary polymer films and reagents.

Closest to the claimed invention is a polarizing film and the method for its preparation described in the patent [1] (prototype).

The objective of the present invention is a polarizing film, the method of obtaining which is simple and economical, does not require corrosion protection of equipment and increased safety measures for production, provides uniform formation of polyvinyl structures and, accordingly, the same optical properties over the entire film area.

The problem is solved in that the polarization film, consisting of oriented molecules of the PVA block copolymer and polyvinylylene obtained by acid-catalyzed thermal dehydration of oriented PVA molecules, additionally contains tungsten-phosphoric acid (PVA).

The concentration of PVA in the film is 10-30%.

A method of obtaining the inventive polarizing film consists in forming a PVA film from a combined aqueous solution of PVA and an acid catalyst for the thermal dehydration of this polymer, uniaxial drawing and subsequent annealing of the film. FVK is used as an acid catalyst, which is added to the PVA solution in an amount of 10-30% relative to the polymer mass, the film is pulled 4-7 times relative to its original length and annealed at a temperature of 120-140 ° C for 1-15 minutes.

We found that PVA (chemical formula H3PW12O40) is an effective catalyst for thermal dehydration of PVA, which allows one to obtain the required degree of dehydration of PVA films at relatively low annealing temperatures (120-140 ° С) even at relatively low (up to 10%) content of PVA in the film . It is important that PVA, in contrast to strong liquid mineral acids (HCl, HI, HBr, H ₂ SO ₄ ), is a non-volatile, environmentally friendly and safe acid catalyst [7]. Due to this, the receipt of the inventive polarizing film does not require anticorrosive protection of equipment and increased safety measures for production. We also found that PVA-PVA films obtained from a joint aqueous solution of PVA and PVA are nanocomposite: PVA is uniformly distributed in the PVA matrix in the form of solid particles with a size of less than 50 nm. Due to the uniform distribution of PVA particles, the PVA dehydration reaction catalyzed by it proceeds uniformly throughout the film, as a result of which the film has the same polarization properties over its entire area. In addition, since the particle size of the CVF is about 10 nm, there is no light scattering on these particles and, accordingly, their presence in the polarization film does not affect its optical transparency.

As will be seen from the examples of implementation of the invention below, films having high polarization properties, heat and moisture resistance, can be obtained by the claimed method with a content of FVC of 10-30% relative to the mass of PVA, uniaxial stretching of the film 4-7 times relative to its initial long, annealing temperature of the film 120-140 ° C and annealing duration of 1-15 minutes

Example 1

A predetermined amount of an aqueous solution of PVA (brand Mowiol 28-99) and PVA with a PVA content of 30% relative to PVA is poured onto the bottom of the Petri dish from modified polystyrene and dried at room temperature for 24 hours. The formed film with a thickness of 50 μm is removed from the substrate, fixed in a manual device for uniaxial drawing of polymer films and stretched 4 times at a given temperature. The film thickness after drawing is approximately 25 μm. The oriented film, which is still fixed in the hood, is annealed in an oven at 120 ° C for 8 minutes. The film thus obtained is practically colorless when viewed in light polarized parallel to the axis of the film draw, and has a gray-brown color in light polarized perpendicularly, which indicates the presence of polarizing properties of the obtained film. The color of the film is uniform over its entire area, there are no defects of “striping” and “spotting” of the coloring.

Two more polarizing films are made in the same way, except that one film is annealed for 12 min at 120 ° C, and the other for 2 min at 140 ° C.

The transmission (T) of the obtained polarization films in the wavelength range of 400-700 nm with parallel (T _|| ) and perpendicular (T _⊥ ) polarization of light relative to the axis of the film extraction was measured on a UV-Vis-NIR Sagu 500 spectrophotometer. After that, the degree of polarization ( k) light film was determined by the formula

k (%) = (T _|| -T _⊥ ) / (T _|| + T _⊥ ) × 100

For perpendicularly polarized light, the spectra of the obtained polarization films have a wide intense absorption band in the region of 400-600 nm with a maximum of about 465 nm. Values T _|| and k at a wavelength of 465 nm for the obtained polarization films are shown in Table 1. The data presented show that the optical characteristics of an elongated film change 4 times from 60 to 26% in T _|| and from 54 to 92% by K with increasing duration and temperature of annealing.

Example 2

Two PVA films containing 30% PVA are cast from an aqueous solution, subjected to uniaxial drawing and annealed, as described in Example 1. In this case, the films are drawn out 7 times and one film is annealed for 12 minutes at 120 ° C, and the other film for 2 minutes at 140 ° C. The optical characteristics of the obtained polarization films, measured as described in example 1, are shown in table 1.

A comparison of the data given in Table 1 for Example 2 with the data for Example 1 shows that for films with the same PVA content, an increase in the degree of stretching of the films at a constant duration and temperature of their subsequent annealing allows one to obtain polarized films with increased polarizing ability without reducing their transparency for parallel polarized light . Example 3

Two films of PVA containing PVA in an amount of 20% by weight of PVA are cast from an aqueous solution, subjected to uniaxial drawing 4 times and annealed, as described in example 1. In this case, one film is annealed for 12 min at 120 ° C, and the other film 3 min at 140 ° C. The optical characteristics of the obtained polarization films are shown in table 1. Comparison of the data in table 1 for example 3 with the data for example 1 shows that a decrease in the concentration of PVA from 30% to 20% upon receipt of the polarizing films by the claimed method allows to increase their polarizing ability and at the same time increase their transparency for parallel polarized light.

Example 4

Three PVA films containing PVA in an amount of 15% by weight of PVA are cast from an aqueous solution, subjected to uniaxial extraction 4 times and subsequent annealing, similar to that described in example 1. In this case, one film is annealed for 17 min at 120 ° C, and the other two films - 3 and 4 min at 140 ° C. The optical characteristics of the obtained polarization films are shown in table 1. Comparison of the data in table 1 for example 4 with the data for examples 1 and 3 shows that for films containing 15% PVA, the values of T _|| and k, comparable with those for films containing 20% or 30% FVC, can be obtained due to a slight increase in the duration of annealing of the films at 140 ° C.

Example 5

Two PVA films containing PVA in an amount of 10% by weight of PVA are cast from an aqueous solution, subjected to uniaxial extraction 4 times and subsequent annealing, similar to that described in example 1. Annealing is carried out at 140 ° C for 10 and 15 min for the first and second film respectively. The optical characteristics of the obtained polarization films are shown in table 1. Comparison of the data in table 1 for example 5 with the data for example 4 shows that a decrease in the concentration of PVA from 15% to 10% can significantly increase the transparency of the films for parallel polarized light, while maintaining them high polarizing ability, but this requires a significant increase in the duration of annealing of films.

Table 1. Content Power Temperature Duration T _|| % k,% FVK,% hoods annealing, ° C annealing, min Example 1 thirty four 120 8 35.3 85,2 thirty four 120 12 30,2 91.5 thirty four 140 2 25.9 92.2 Example 2 thirty 7 120 12 39.5 92.8 thirty 7 140 2 25.6 94.7 Example 3 twenty four 120 12 40.5 90.6 twenty four 140 3 35.0 95.1 Example 4 fifteen four 120 17 57.7 72.9 fifteen four 140 3 53.3 84.5 fifteen four 140 four 28.6 97.0 Example 5 10 four 140 10 48.9 92.8 10 four 140 fifteen 33.7 97.8

Example 6

The polarization film obtained in example 3, with optical characteristics T _|| = 35.0% and k = 95.1% was tested for heat resistance by keeping it at a temperature cabinet at 80 ° C for 50 hours. Measurements of the optical characteristics of the film after these tests yielded T _|| = 35.2% and k = 94.8%, which practically coincide with the values of these characteristics before testing, which indicates a high heat resistance of the obtained polarization film.

Example 7

A polarization film obtained similarly to that described in example 2, with optical characteristics T _|| = 39.0% and k = 91.4% was tested for moisture resistance by aging at a temperature of 35 ° C and a relative humidity of 100% for 16 hours. Measurements of the optical characteristics of the film after these tests yielded T _|| = 49.5% and k = 89.6%. After that, the film was additionally kept at a temperature of 20-25 ° C and a relative humidity of 90% for 140 hours. The optical characteristics of the film were T _|| = 54.6% and k = 85.4%. Thus, the test results show that the obtained polarizing film has good moisture resistance.

Information sources

1. US patent No. 2173304. (prototype)

2. US patent No. 2674159.

3. US patent No. 5666223.

4. US Patent No. 6814899 B2.

5. US patent No. 59973834.

6. US patent No. 7087194 B2.

7. Misono M., Ono I., Koyano G., Aoshima A. Heteropolyacids. Versatile green catalysts usable in a variety of reaction media. Pure appl. Chem. V. 72. No. 7, P. 1305-1311. 2000.

Claims (3)

1. A polarizing film consisting of oriented molecules of the block copolymer of polyvinyl alcohol and polyvinylylene obtained by acid-catalyzed thermal dehydration of oriented molecules of polyvinyl alcohol, characterized in that it further comprises tungsten phosphoric acid. 2. The polarizing film according to claim 1, characterized in that the concentration of phosphoric-tungsten acid in it is 10-30%. 3. A method of producing a polarizing film, which consists in forming a film of polyvinyl alcohol from an aqueous solution of polyvinyl alcohol and an acid catalyst for the thermal dehydration of this polymer, uniaxial drawing and subsequent annealing of this film, characterized in that phosphoric-tungsten acid is used as the acid catalyst, which is taken in an amount of 10-30% relative to the mass of polyvinyl alcohol, the film is pulled 4-7 times relative to its original length and annealed at a temperature of 120-140 ° C for 1-1 5 minutes.