JPH0834817A - Production of polyvinyl butyral - Google Patents

Abstract

PURPOSE:To produce a microparticulate polyvinyl butyral excellent in transparency and solubility from a vinyl acetate monomer in one step. CONSTITUTION:The production process is one for producing polyvinyl butyral comprising hydrolyzing (saponifying) polyvinyl acetate obtained by radical- polymerizing vinyl acetate to synthesize polyvinyl alcohol and condensing the polyvinyl alcohol with butyl aldehyde, which process comprises adding 0.5-30wt.%, based on the entire weight of the reaction solution, 2-30wt.% concentration aqueous solution of polyvinyl butyral of a degree of acetalization of 5-25mol% to the reactor during the condensation.

Description

Detailed Description of the Invention

The present invention relates to a method for producing polyvinyl butyral. More specifically, the present invention relates to a method for synthesizing fine particle polyvinyl butyral having excellent transparency and solubility from a vinyl acetate monomer in one reaction tank at once.

[0002]

BACKGROUND OF THE INVENTION Polyvinyl butyral is used in a wide variety of applications such as safety glass interlayer films, adhesives, paints and various binders. Many manufacturing methods have been proposed for this polyvinyl butyral. The industrial production method of polyvinyl butyral is as follows. In the first step, polyvinyl acetate is radically polymerized to produce polyvinyl acetate, and the obtained polyvinyl acetate is hydrolyzed (saponified) in an alkaline solution to give polyvinyl alcohol. Is separated, purified, and dried to synthesize polyvinyl alcohol. Next, in the second step, polyvinyl alcohol is dissolved, an acid catalyst and butyraldehyde are added thereto, and this is condensed to produce polyvinyl butyral.

As a method for acetalizing polyvinyl alcohol in the second step, a precipitation method, a dissolution method and a homogenization method have been conventionally known. Among them, the precipitation method can obtain powdery polyvinyl butyral from an aqueous solution in a single step, so that it is more industrial than other methods that require precipitation and separation of polyvinyl butyral in the solution during the reaction and purification. Have significant advantages. However, the process is still complicated and it is not effective in terms of process and cost.

As an attempt to improve the above-mentioned problems, for example, in “Vinyl-based synthetic resin” [published by Japan Society of Polymer Chemistry, page 98, 1949], Mr. Osugi's polyvinyl acetate alcohol was used. Introducing a method of simultaneously adding sulfuric acid and butyraldehyde to a solution and performing butyralization at the same time as deacetic acid, but in addition, there are problems such as this reaction taking a long time and the resulting polyvinyl butyral being colored. It has been pointed out. In addition, USP. 2,105,2
08 (1939), as a catalyst, sulfuric acid, hydrochloric acid, nitric acid,
A method of depositing an acetalized product such as polyvinyl butyral from a water emulsion reaction solution using a mineral acid such as phosphoric acid or a strong organic acid such as toluenesulfonic acid is described.

In a study on the synthesis of polyvinyl formal (I-II) [Osaka Kogyo Kenkyusho, 1949], Yamada et al. Used acetic acid as a solvent, suspended polyvinyl alcohol in it and added aldehyde and hydrochloric acid. A method of completing the acetalization and then mixing with water at 10 ° C. or lower to separate polyvinyl formal is proposed, but this method requires a step of recovering acetic acid.

In "Application of Vinyl Emulsion" [published by Kobunshi Kagaku Kogyokai, page 101, 1956], Motoyama et al. Described a method of saponifying polyvinyl acetate with hydrochloric acid and then acetalizing it. Says. However, the method of synthesizing the above polyvinyl butyral and the like in one step involves re-precipitation with some solvent in any method and involves a step of recovering the polymer, so that uniform particles cannot be obtained. I have.

[0007]

DISCLOSURE OF THE INVENTION The present invention overcomes the above-mentioned conventional drawbacks, and an object of the present invention is to produce fine particle polyvinyl butyral which is excellent in transparency and solubility all at once from vinyl acetate monomer. To provide a method.

[0008]

According to the present invention, polyvinyl acetate obtained by radically polymerizing vinyl acetate is hydrolyzed (saponified) with an acid to synthesize polyvinyl alcohol, and then butyraldehyde is condensed. A method for producing polyvinyl butyral, wherein at the time of condensation, an aqueous polyvinyl butyral solution having a concentration of 2 to 30% by weight and an acetalization degree of 5 to 25 mol% is added to the reaction tank in an amount of 0.5 to 30% by weight based on the total amount of the reaction solution. %, The method for producing fine particle polyvinyl butyral is characterized by its addition.

The radical polymerization of vinyl acetate used in the present invention may be any of bulk polymerization, solution polymerization, emulsion polymerization and granular polymerization. Of the above-mentioned polymerization methods, solution polymerization is preferably used because a relatively high degree of polymerization can be obtained, the heat of polymerization can be easily removed, and the viscosity of the reaction system can be kept low.

Examples of the solvent used in the above solution polymerization of the present invention include methanol, ethanol, benzol, methyl acetate, ethyl acetate, acetic acid and acetone. The solvent used for these polymerizations is preferable because it can be used as it is in the saponification step.

The above-mentioned saponification methods include acid saponification and alkali saponification. Acid saponification can reduce the ash content in the obtained polyvinyl butyral product as compared with alkali saponification, especially after saponification, acetalization reaction. Is carried out subsequently, it is industrially advantageous in that the acid catalyst can be used as it is in the subsequent acetalization step. Examples of the acid catalyst include hydrochloric acid, sulfuric acid, phosphoric acid, paratoluenesulfonic acid and the like. Of these acids, hydrochloric acid is preferably used because the resulting polyvinyl butyral product is less colored than when other acids are used.

The polyvinyl alcohol used in the present invention is polymerized by the above method, and the degree of polymerization is appropriately determined depending on the intended use of the polyvinyl butyral to be obtained, and is usually about 500 to 8,000. The saponification degree is preferably about 75 mol% at the end of acid saponification. This is because the saponification proceeds even after the start of the next acetalization reaction, and in addition to this, the saponification degree of the final product is preferably 90 mol% or more.

Then, water is added to the reaction tank as a reaction solvent in the polyvinyl alcohol, and the mixture is mixed to adjust the concentration. Here, the amount of water is preferably added so that the ratio of the solvent used for saponification is 40% by weight or less based on the total amount of the reaction liquid. When the solvent ratio exceeds 40% by weight with respect to the total amount of the reaction liquid, it is not possible to obtain polyvinyl butyral as fine particles having a uniform diameter by acetalization reaction.

The degree of acetalization used in the present invention is 5 to 2
Although 5 mol% of polyvinyl butyral (hereinafter, abbreviated as low-acetalized polyvinyl butyral) is prepared separately, the degree of polymerization of polyvinyl alcohol used for it is not particularly limited, but usually the average polymerization Those having a degree of 500 to 8,000 are preferably used.
The saponification degree is preferably about 85 mol%.

The above polyvinyl alcohol is acetalized with butyraldehyde (low acetalized polyvinyl butyral) and is used as a nucleating agent in the present invention. When the degree of acetalization is less than 5 mol%, it acts on the particle formation of polyvinyl butyral. If the degree of acetalization exceeds 25 mol%, it will precipitate and separate by itself when added to the reaction solution, and will not be effective as a nucleating agent.

The above low-acetalized polyvinyl butyral is added to the reaction tank as an aqueous solution, the concentration of which is
2 to 30 depending on the use of the obtained polyvinyl butyral
The amount is 0.5 to 30% by weight based on the total weight of the reaction solution. When the concentration of the low-acetalized polyvinyl butyral is less than 2% by weight, there is no effect as a nucleating agent, and when it exceeds 30% by weight, the concentration of the reaction solution is high and the reaction system is uniformly dispersed. And does not give favorable results.

Further, if the amount of the low-acetalized polyvinyl butyral added is less than 0.5% by weight based on the total amount of the reaction solution, it has no effect as a nucleating agent, and if added in excess of 30% by weight. The effect is constant, and it is uneconomical to add more than that.

The time for adding the low-acetalized polyvinyl butyral is most preferably immediately after adding water as a solvent to polyvinyl alcohol. The temperature of the reaction solution at the time of adding the low acetalized polyvinyl butyral is 5
It is preferably -50 ° C. If this temperature is lower than 5 ° C, there is a problem that polyvinyl alcohol microgels are generated and a non-uniform polymer is generated in a solvent-insoluble state, and if it exceeds 50 ° C, fine and uniform particles are not formed. .

[0019]

The method for producing polyvinyl butyral of the present invention is
Degree of acetalization of 2 to 30% by weight is 5 to 25 mol%
The polyvinyl butyral aqueous solution is added to the reaction solution in an amount of 0.5 to 30% by weight with respect to the total amount of the reaction solution, whereby the low acetalized polyvinyl butyral is uniformly dispersed in the reaction solution, and the polyvinyl butyral produced is It acts as a nucleating agent and can be made fine and homogeneous particles can be precipitated from the reaction system.

In the method for producing polyvinyl butyral of the present invention, the pH of the reaction solution is limited to the acidic side such as hydrolysis (saponification) with an acid. By combining with each element, polyvinyl butyral can be continuously produced in the same reaction tank from polyvinyl acetate into fine particles at once and as homogeneous particles, and the obtained polyvinyl butyral does not contain impurities, and safety glass can be obtained. When used as an intermediate film of No. 3, it shows no coloration and shows excellent performance in transparency without image distortion. Further, when it is used in paints and adhesives, it exhibits extremely excellent solubility.

In the method for producing polyvinyl butyral of the present invention, when vinyl acetate is radically polymerized in a solvent, a relatively high degree of polymerization can be obtained, the heat of polymerization can be easily removed, and the viscosity of the reaction system can be kept low. By combining with each element of the present invention, it is possible to produce polyvinyl butyral as fine particles, and as homogeneous particles, the obtained polyvinyl butyral does not contain impurities, and when used as an intermediate film of safety glass, coloring is also obtained. In addition, it shows excellent performance in transparency without causing image distortion. Further, when it is used in paints and adhesives, it exhibits extremely excellent solubility.

[0022]

EXAMPLES Examples of the present invention will be specifically described below, but the present invention is not limited thereto. In the examples, the methods of measuring the degree of polymerization, the degree of saponification, the degree of butyral and the evaluation of the particle diameter were measured and evaluated by the methods shown below. 1. Degree of Polymerization and Degree of Saponification The degree of polymerization and the degree of saponification were measured according to JIS K6726.

2. Method of measuring degree of butyralization The degree of butyralization was measured by a nuclear magnetic resonance (NMR) method. At the time of measurement, the proton nuclear magnetic resonance spectrum was evaluated as follows. First, a 1 wt% deuterated DMSO solution of polyvinyl butyral was prepared, a small amount of tetramethylsilane was added as a standard substance (0 ppm), and measurement was carried out at a measurement temperature of 25 ° C. It is calculated from the integrated value of the absorption peak of. However, CH ₃ of the butyral group of the side chain is 0.8 to 0.
It belongs to the chemical shift of 9 ppm and its integrated intensity (I
In A), CH ₂ of the butyral group of the main chain and the side chain is 1.
It was assigned to a chemical shift of 0 to 1.9 ppm, and its integrated intensity (IB) was read from each chart and calculated by the following formula. Butyralization degree (mol%) = [2 × IA / (3 × IB-
4 x IA)] x 100

3. Evaluation of particle size The particle size was evaluated by the weight ratio of particles passing through each size mesh.

Example 1 50 parts by weight of vinyl acetate monomer, 283 parts by weight of methanol, and 0.1 part by weight of benzoyl peroxide as a catalyst were placed in a reaction tank, and a polymerization reaction was carried out at 78 ° C. for 8 hours. Then, 30 parts by weight of 35% by weight hydrochloric acid was added thereto, and saponification reaction was carried out at 80 ° C. for 10 hours to obtain a methanol solution of polyvinyl alcohol. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1).

Separately, 100 parts by weight of a 10% by weight aqueous solution of polyvinyl alcohol having a polymerization degree of 1,500 and a saponification degree of 99.4 mol% was cooled to 10 ° C., and 2 parts by weight of butyraldehyde was added thereto to form an acetal. The reaction was carried out to prepare low-acetalized polyvinyl butyral having an acetalization degree of 20 mol% (solution 2). The above solution 2 was added dropwise to the solution 1 and stirred for 30 minutes. Then, 20 parts by weight of butyraldehyde was added dropwise thereto and stirred for 2 hours.
The temperature was raised to 0 ° C. and the reaction was continued for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain white polyvinyl butyral powder.
The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

(Example 2) In the same manner as in Example 1, a methanol solution of polyvinyl alcohol was obtained. Next, 660 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 3). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
4 mol% polyvinyl alcohol 5 wt% aqueous solution 200
One part by weight was cooled to 10 ° C., and 2 parts by weight of butyraldehyde was added thereto to carry out an acetalization reaction to prepare a low acetalized polyvinyl butyral having an acetalization degree of 20 mol% (solution 4). The solution 4 was added dropwise to the solution 3 and stirred for 30 minutes. Then add butyraldehyde 2
After stirring for 2 hours while dropping 0 part by weight, the temperature was raised to 50 ° C. and the reaction was further performed for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain white polyvinyl butyral powder. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

Example 3 In the same manner as in Example 1, a methanol solution of polyvinyl alcohol was obtained. Next, 1,132 parts by weight of water was added to this solution and cooled to 10 ° C (solution 5). Separately, 50 parts by weight of a 20% by weight aqueous solution of polyvinyl alcohol having a polymerization degree of 1,500 and a saponification degree of 99.4 mol% were cooled to 10 ° C., and 2 parts by weight of butyraldehyde was added to the mixture to carry out an acetalization reaction. A low acetalized polyvinyl butyral having an acetalization degree of 20 mol% was prepared (solution 6). The solution 6 was added dropwise to the solution 5 and stirred for 30 minutes. Then, 20 parts by weight of butyraldehyde was added dropwise thereto and stirred for 2 hours.
The temperature was raised to 0 ° C. and the reaction was continued for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain white polyvinyl butyral powder. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.
It was shown to.

Example 4 A methanol solution of polyvinyl alcohol was obtained in the same manner as in Example 1. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
4 mol% polyvinyl alcohol 20 wt% aqueous solution 10
0 parts by weight was cooled to 10 ° C., and butyraldehyde 2 was added to this.
An acetalization reaction was performed by adding parts by weight to prepare a low-acetalization polyvinyl butyral having an acetalization degree of 5 mol% (solution 7). The above solution 7 was added dropwise to the solution 1 and stirred for 30 minutes. Then add butyraldehyde 2
After stirring for 2 hours while dropping 2 parts by weight, the temperature was raised to 50 ° C. and the reaction was further performed for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain white polyvinyl butyral powder. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

Example 5 A methanol solution of polyvinyl alcohol was obtained in the same manner as in Example 1. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
4 mol% polyvinyl alcohol 10 wt% aqueous solution 10
0 parts by weight was cooled to 10 ° C., and 2.5 parts by weight of butyraldehyde was added thereto to carry out an acetalization reaction to prepare a low acetalization polyvinyl butyral having an acetalization degree of 25 mol% (solution 8). The solution 8 was added dropwise to the solution 1 and stirred for 30 minutes. Then, 20 parts by weight of butyraldehyde was added dropwise thereto and stirred for 2 hours.
The temperature was raised to 0 ° C. and the reaction was continued for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain white polyvinyl butyral powder.
The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

Example 6 A methanol solution of polyvinyl alcohol was obtained in the same manner as in Example 1. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
60.5 parts by weight of a 4 mol% 0.83 wt% aqueous solution of polyvinyl alcohol was cooled to 10 ° C., and 0.1 part by weight of butyraldehyde was added to the mixture to carry out an acetalization reaction to obtain a low acetalization degree of 20 mol%. An acetalized polyvinyl butyral was prepared (solution 9). The above solution 9 was added dropwise to the solution 1 and stirred for 30 minutes. Then, 20 parts by weight of butyraldehyde was added dropwise thereto, and the mixture was stirred for 2 hours, then heated to 50 ° C. and further reacted for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain white polyvinyl butyral powder. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

(Comparative Example 1) In the same manner as in Example 1, a methanol solution of polyvinyl alcohol was obtained. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. 20 parts by weight of butyraldehyde was added dropwise thereto, and the mixture was stirred for 2 hours, heated to 50 ° C., and further reacted for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain polyvinyl butyral. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

Comparative Example 2 A methanol solution of polyvinyl alcohol was obtained in the same manner as in Example 1. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
4 mol% polyvinyl alcohol 10 wt% aqueous solution 10
0 part by weight was cooled to 10 ° C., and 0.2 part by weight of butyraldehyde was added thereto to carry out an acetalization reaction to prepare polyvinyl butyral having an acetalization degree of 2 mol% (solution 10). While dropping the above solution 10 into the solution 1, 3
Stirred for 0 minutes. Then add butyraldehyde 20
After stirring for 2 hours while adding parts by weight, the temperature was raised to 50 ° C. and the reaction was continued for another 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain polyvinyl butyral. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

Comparative Example 3 A methanol solution of polyvinyl alcohol was obtained in the same manner as in Example 1. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
4 mol% polyvinyl alcohol 10 wt% aqueous solution 10
0 parts by weight was cooled to 10 ° C., and 3.1 parts by weight of butyraldehyde was added thereto to carry out an acetalization reaction to prepare polyvinyl butyral having an acetalization degree of 30 mol% (solution 11). While dropping the above solution 11 into the solution 1, 3
Stirred for 0 minutes. Then add butyraldehyde 20
After stirring for 2 hours while adding parts by weight, the temperature was raised to 50 ° C. and the reaction was continued for another 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain polyvinyl butyral. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

Comparative Example 4 A methanol solution of polyvinyl alcohol was obtained in the same manner as in Example 1. Next, 425 parts by weight of water was added to this solution and cooled to 10 ° C. (Solution 1). Apart from this, the polymerization degree is 1,500 and the saponification degree is 99.
4 mol% polyvinyl alcohol 1% by weight aqueous solution 10.
1 part by weight was cooled to 10 ° C., and 0.02 parts by weight of butyraldehyde was added thereto to carry out an acetalization reaction to prepare polyvinyl butyral having a degree of acetalization of 2 mol% (solution 12). 30 while adding the above solution 2 to the solution 1
Stir for minutes. Then, 20 parts by weight of butyraldehyde was added dropwise thereto, and the mixture was stirred for 2 hours, then heated to 50 ° C. and further reacted for 2 hours. The reaction solution was cooled to room temperature, polyvinyl butyral was separated, neutralized with sodium bicarbonate and washed with water to obtain polyvinyl butyral. The degree of polymerization, the degree of saponification and the degree of acetalization of the obtained polyvinyl butyral are shown in Table 1, and the shapes of reaction products such as particle size are shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

INDUSTRIAL APPLICABILITY The method for producing polyvinyl butyral according to the present invention is a polyvinyl butyral aqueous solution having a concentration of 2 to 30% by weight and an acetalization degree of 5 to 25 mol% and a polyvinyl butyral aqueous solution of 0.5 to the total amount of the reaction solution. When added to the reaction solution in an amount of ˜30% by weight, the low-acetalized polyvinyl butyral is uniformly dispersed in the reaction solution and acts as a nucleating agent for the produced polyvinyl butyral to make it fine and homogeneous. It can be precipitated as particles from the reaction system.

Further, in the method for producing polyvinyl butyral of the present invention, the pH of the reaction solution is limited to the acidic side such as hydrolysis (saponification) with an acid, and the like. By combining with each element, polyvinyl butyral can be continuously produced in the same reaction tank from polyvinyl acetate into fine particles at once and as homogeneous particles, and the obtained polyvinyl butyral does not contain impurities, and safety glass can be obtained. When used as an intermediate film of No. 3, it shows no coloration and shows excellent performance in transparency without image distortion. Further, when it is used in paints and adhesives, it exhibits extremely excellent solubility.

According to the method for producing polyvinyl butyral of the present invention, when vinyl acetate is radically polymerized in a solvent, a relatively high degree of polymerization can be obtained, the heat of polymerization can be easily removed, and the viscosity of the reaction system can be kept low. By combining with each element of the present invention, it is possible to produce polyvinyl butyral as fine particles, and as homogeneous particles, the obtained polyvinyl butyral does not contain impurities, and when used as an intermediate film of safety glass, coloring is also obtained. In addition, it shows excellent performance in transparency without causing image distortion. Further, when it is used in paints and adhesives, it exhibits extremely excellent solubility.

Claims (1)

[Claims] 1. A method for producing polyvinyl butyral by subjecting polyvinyl acetate obtained by radical polymerization of vinyl acetate to hydrolysis (saponification) with an acid to synthesize polyvinyl alcohol and further condensing butyraldehyde. At the time of condensation, an aqueous polyvinyl butyral solution having a concentration of 2 to 30 wt% and an acetalization degree of 5 to 25 mol% is added to the reaction tank in an amount of 0.5 to 30 wt% with respect to the total amount of the reaction liquid. Method for producing fine particle polyvinyl butyral.