JP3036896B2 - Method for producing polyvinyl acetal resin - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyvinyl acetal resin having a reduced melt viscosity and solution viscosity.

[0002]

2. Description of the Related Art Polyvinyl acetal resins such as polyvinyl butyral are widely used as interlayer materials for laminated glass, as a raw material for adhesives and paints.

As such a raw material resin, a polyvinyl butyral resin produced by a precipitation method using an aqueous medium is exclusively used. The reason is that the precipitation method using this aqueous medium is industrially advantageous because there is no need to recover expensive solvents as in the solution method.

However, in the precipitation method, a large amount of catalyst (for example,
Since the acid catalyst is used in an amount of 2 to 4% by weight, it is necessary to make the particles fine (for example, 50 μm or less) due to a problem in purification. In this case, the acetalization reaction had to be performed at a relatively low temperature (0 to 30 ° C.) in order to obtain uniform particles without aggregation of the particles.

[0005] If the reaction is not carried out at a relatively low temperature, scale adheres to the inner wall of the reactor, making it impossible to operate for a long time. The quality may be degraded, and from this point, it is necessary to carry out the acetalization reaction at a relatively low temperature.

However, when the acetalization reaction is performed at a relatively low temperature, gelation of polyvinyl alcohol occurs during the acetalization reaction, and the melt viscosity and the melt viscosity of the obtained resin increase.

When a laminated glass having a large curvature is manufactured using such a resin, it is difficult to follow the curved surface of the glass because of its high melt viscosity. Therefore, air is generated between the glass plate and the interlayer. Remains, and it is not possible to efficiently produce a high-quality laminated glass. In addition, in the case of producing an adhesive or a paint, unless a large amount of solvent is used, the coating workability is deteriorated, and an inexpensive adhesive or paint cannot be produced.

In US Pat. No. 2,720,501, an aqueous polyvinyl alcohol solution and an aldehyde are partially reacted at 50 to 90 ° C. while circulating in a loop reactor, and then stirred in a kettle reactor. 60-100 ° C while
A method for producing a polyvinyl acetal resin by a precipitation method by completely reacting the same with the above method is disclosed.

In this method, the flow rate of the fluid circulating in the loop reactor is 5 feet / second (about 1.5 m / second).
This is set as above. And in the embodiment, 12 feet /
Set to seconds. By performing the reaction under such conditions, aggregation of the obtained resin particles is prevented.

[0010]

However, when the acetalization reaction is carried out at a relatively high temperature (50-100 ° C.) according to this method, aggregation of the resin particles is prevented to some extent depending on the conditions. However, the scale adheres to the inner wall of the reactor due to the stickiness of the resin, making it impossible to operate for a long time, or the scale may fall off from the inner wall and be mixed into the resin, resulting in poor quality of the resin. It turned out that there was a problem that occurred.

The present invention is intended to solve such a problem. It is an object of the present invention to provide a resin having improved solvent solubility, a reduced solution viscosity and a reduced melt viscosity, and a resin particle. It is an object of the present invention to provide a method for producing a polyvinyl acetal resin which can prevent aggregation of the resin and prevent the scale from adhering to the inner wall of the reactor.

[0012]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention relates to a method in which an aqueous solution of polyvinyl alcohol and an aldehyde are partially reacted at 30 to 90 ° C. while circulating in a loop reactor. With stirring in the reactor
In the method for producing a polyvinyl acetal resin by completely reacting at 30 to 90 ° C., the loop-shaped reactor inner wall is formed of glass, and the degree of acetalization is at least 40.
The reaction up to mol% is carried out under the condition that the number of Reynolds nozzles (Re) of the fluid circulating in the loop reactor becomes 5000 or more.

Further, preferably, the surface roughness of the glass on the inner wall of the loop reactor is 0.5 μm or less in Rz value (however,
The value is a 10-point average roughness measured in accordance with ISO R468 using a stylus-type roughness measuring instrument.)

Hereinafter, the present invention will be described in detail with reference to the reactor shown in FIG. FIG. 1 is a schematic explanatory view showing one example of a reaction apparatus used in the present invention. This reaction apparatus includes a polyvinyl alcohol dissolving tank 1, an aldehyde tank 3, a loop reactor 10, and a kettle reactor 2.

The polyvinyl alcohol dissolving tank 1 and the loop reactor 10 are connected by a supply pipe 4, and the aldehyde tank 3 and the loop reactor 10 are connected by a supply pipe 6. The loop reactor 10 and the kettle reactor 2 are connected by a transfer pipe 5. 17 is a withdrawal valve.

The loop reactor 10 is provided with a circulating pump 7, a pressure gauge 8, and a flow meter 9;
6 is provided with supply pumps 14, 16 respectively.
Further, the kettle-shaped reactor 2 is provided with a reflux condenser (not shown) in order to prevent unreacted aldehyde from flowing out of the system.

In such a reaction apparatus, an aqueous solution of polyvinyl alcohol is prepared in the polyvinyl alcohol dissolving tank 1. As the polyvinyl alcohol, polyvinyl alcohol having an average polymerization degree of 200 to 2600 and a saponification degree of 90 to 100% is generally used. And generally 5-12
% By weight. Alternatively, the viscosity, measured with a 4% aqueous solution at 20 ° C., is generally adjusted to be between 4 and 60 centipoise.

An acid catalyst is added to the aqueous solution of polyvinyl alcohol. Acid catalysts include hydrochloric acid, formic acid, phosphoric acid,
Sulfuric acid, paratoluenesulfonic acid and the like are used. For example, in the case of hydrochloric acid, these acid catalysts are generally contained in an amount of 0.05 to 1.0% by weight. Alternatively, the pH of the aqueous solution is generally
Adjusted to be 2.

The aldehyde tank 3 is prepared with an aldehyde or an aqueous solution thereof. As the aldehyde, a single aldehyde or a mixed aldehyde of two or more aliphatic aldehydes having 1 to 6 carbon atoms such as formaldehyde, acetaldehyde, propionaldehyde and butyraldehyde is generally used. Lower aldehydes such as formaldehyde increase the heat resistance of the resin, and higher aldehydes such as butyraldehyde increase the solvent solubility of the resin. In particular,
The use of a mixed aldehyde is preferable because the physical properties can be easily balanced. The aldehyde includes aldehydes such as paraformaldehyde and paraacetaldehyde which can be converted into the above aldehyde.

First, an aqueous solution of polyvinyl alcohol containing an acid catalyst is supplied from a feed pump 4 to a loop reactor 10.
Aldehyde or an aqueous solution thereof is supplied to the loop reactor 10 by the supply pump 6 at a constant rate. The amount of aldehyde relative to the amount of polyvinyl alcohol is generally supplied in excess of 5 to 40% by weight over the theoretical amount. The supply is precisely controlled so as not to fluctuate.

An aqueous solution of polyvinyl alcohol and an aldehyde or an aqueous solution thereof are mixed through the junction of the loop reactor 10, and the mixed fluid is circulated in the loop reactor 10 by the circulation pump 7. At this time, the loop reactor 10
Is formed of glass and the number of Reynolds nozzles (Re) of the fluid circulating in the loop reactor 11 is 5000 or more, preferably
It is necessary to carry out the reaction under such conditions that the amount becomes 7000 to 10,000 or more. The reason is as follows.

Whether or not the scale adheres to the inner wall of the reaction apparatus is determined by the relationship between the adhesive force of the resin to the inner wall of the apparatus and the force for separating the adhered scale from the inner wall of the apparatus. For example, if the peel force of the scale is higher than the adhesive force, the resin does not adhere to the inner wall of the device.

Polyvinyl acetal resins such as polyvinyl butyral have different properties depending on the degree of acetalization. Polyvinyl acetal resin produced by a precipitation method using an aqueous medium is generally in a gelled state in which the formation of particles is incomplete and the degree of acetalization is up to 40 mol%, and is highly sticky. It was found that it easily adhered to the scale.

Therefore, when the inner wall of the loop reactor 10 is formed of various materials such as stainless steel, glass, and synthetic resin, and the reaction liquid having an acetalization degree of up to 40 mol% is examined for adhesion to various materials, It was found that a material having a smaller contact angle with water (that is, a material that is more easily wetted with water) is less likely to adhere. As a result of confirming the adhesion of the reaction solution using glass having a contact angle with water of 7 °, it was found that the reaction solution hardly adhered to the glass. In the present invention, since the reaction solution uses water as a medium, it is presumed that the water layer formed on the glass surface when the glass surface is wetted with water prevents the adhesion of the resin.

When the inner wall of the loop reactor 10 is made of glass as described above, the resin does not easily adhere, but it is insufficient to prevent the adhesion of scale. Therefore, in order to increase the peeling force of the scale due to the particles up to the acetalization degree of 40 mol%, the fluid circulating in the loop reactor 10 is focused on the Reynolds number (Re) of the fluid forming the strong turbulence. When the number of Reynolds nozzles (Re) was changed to various values and the adhesion of the scale was examined, if the number of Reynolds nozzles (Re) of the fluid was 5000 or more, the aggregation of the resin particles was prevented and the scale adhered to the inner wall of the apparatus. Turned out not to be.

Here, the Reynolds number (Re) of the fluid is given by the formula: loop pipe diameter × flow velocity × fluid density / fluid viscosity. Therefore, by appropriately setting these values, the Reynolds number (Re) of the fluid can be made 5000 or more. The flow rate of the fluid can be adjusted by adjusting the circulation pump 33. The average residence time of the fluid circulating in the loop reactor 10 is generally 1 to 10 minutes.

In this case, the surface roughness of the glass forming the inner wall of the loop reactor 10 is 0.5 μm or less in Rz value (however, the Rz value conforms to ISO R468 using a stylus-type roughness measuring instrument. 10 point average roughness measured by the method described above). This is presumed to be due to a decrease in the anchor when the resin adheres. In addition, as a glass, soda-lime glass, potash glass, lead glass, borosilicate glass, quartz glass, enamel, etc. are mentioned.

As described above, the polyvinyl alcohol aqueous solution and the aldehyde are circulated in the loop reactor 10, and the amount corresponding to the supply of the polyvinyl alcohol aqueous solution and the aldehyde is discharged by operating the withdrawing valve 32.
The discharged slurry liquid of the partial reaction is put into a kettle reactor 40. In order to prevent agglomeration of resin particles and adhesion of scale, the degree of acetalization of the resin in the partial reaction put into the kettle reactor 40 should not exceed 40 mol%, so that aggregation of resin particles and adhesion of scale will not occur. Prevention, preferably 55 mol% or more.

Here, the degree of acetalization is determined, for example, by using a reaction product obtained by neutralizing, washing and drying the reaction solution (slurry solution) in a usual manner, and using 10 to 20% by weight of dimethyl sulfoxide (%) of the reaction product. create a DMSO-d ₆₎ solution, can be measured by ¹³ C nuclear magnetic resonance spectroscopy using an isotope ¹³ C as resonance species. When performing the acetalization reaction in the loop reactor 10, the degree of acetalization has a distribution,
The acetalization degree referred to in the present invention is an average value of the acetalization degree having a distribution. When a mixed aldehyde is used, it means the sum of the degrees of acetalization of each aldehyde.

As the pot-shaped reactor 40, an ordinary pot-shaped reactor equipped with a stirring blade is used. The inner wall of the pot-shaped reactor 10 and other piping portions may be made of any material such as stainless steel and a glass lining material, but a glass lining material is preferably used. While rotating the stirring blade and stirring,
The reaction is generally carried out at 30 to 90 ° C. for 2 to 5 hours, followed by aging to complete the reaction, followed by cooling. Thereafter, the powder is neutralized, washed with water, and dried by a conventional method to produce a powdery or granular polyvinyl acetal resin. The degree of acetalization of the polyvinyl acetal resin varies depending on the application, but is generally set to 60 to 75 mol% in total acetalization degree.

[0031]

As described above, using the inside of the loop reactor and the kettle reactor, the inner wall of the loop reactor is formed of glass, and the reaction having an acetalization degree of at least up to 40 mol% is carried out in the loop reactor. When the reaction between polyvinyl alcohol and aldehyde is carried out by a precipitation method using an aqueous medium under the condition that the Reynolds number (Re) of the fluid circulating through the water is 5000 or more, the partial reactant in the loop reactor is strongly disturbed. Due to the high stirring action, even if the reaction temperature is relatively high, such as 30 to 90 ° C., aggregation of the resin particles is prevented.

In addition, the above-mentioned high stirring action and the action of the water layer formed on the glass surface combine to prevent the adhesion of scale to the vessel wall. In particular, when the surface roughness of the glass on the inner wall of the loop reactor is set to 0.5 μm or less in Rz value, the number of anchors on the glass surface is reduced, which is more effective in preventing the adhesion of scale to the vessel wall.

[0033]

EXAMPLES Examples and comparative examples of the present invention will be described below. Example 1 In this example, the reaction apparatus shown in FIG. 1 was used.

The polyvinyl alcohol dissolving tank 1 has a capacity of 10
The aldehyde tank 3 is made of stainless steel with a capacity of 6 liters. Also,
The loop reactor 10 was formed by an inner glass lining pipe having a diameter of 25 mm, and its capacity was set to 10 liters. The pot-shaped reactor 2 is made of a 100-liter capacity inner glass lining material. Each of the above glass linings has a surface roughness of 0.3 μm in Rz value.

First, 62 liters of pure water was put into the polyvinyl alcohol dissolving tank 1, and an average degree of polymerization of 1700 and a degree of saponification of 9 were added thereto.
After charging and dispersing 7,500 g of 9.0 mol% polyvinyl alcohol, the temperature was raised to 90 ° C. to completely dissolve the polyvinyl alcohol, and then cooled and maintained at 60 ° C. Further, 160 g of 35% hydrochloric acid was added as a catalyst. Further, 1700 g of acetaldehyde and 2150 g of butyraldehyde were put into the aldehyde tank 3 and mixed well.

The loop reactor 10 was filled with pure water, heated and maintained at 60 ° C., and circulated by operating the circulation pump 7. In addition, 11 l of pure water and a concentration of 35
Then, 25 g of hydrochloric acid was added thereto and stirred, and the mixture was heated and maintained at 60 ° C.

The polyvinyl alcohol aqueous solution containing the acid catalyst and the mixed aldehyde were supplied into the loop reactor 10. The two are mixed through the junction, and the mixture is circulated in the loop reactor 10 by the circulation pump 7. At this time, the number of Reynolds (Re) of the fluid circulating in the loop reactor 10 is set to 7236 (0.025 m of loop pipe diameter,
Flow rate 4m / sec, density of the fluid 1013 kg / m ^3, viscosity of the fluid
(0.014 kg / m · sec), and the reaction proceeded while preventing coalescence of particles.

An amount corresponding to the supply of the aqueous solution of polyvinyl alcohol and the mixed aldehyde is discharged by adjusting the discharge valve 17, and is put into the kettle reactor 2. The supply pumps 4 and 6 were controlled so that the supply amounts of the aqueous polyvinyl alcohol solution and the mixed aldehyde were completed in 60 minutes. When the slurry liquid was sampled from the outlet of the loop reactor 10 and the degree of acetalization of the resin was measured by a nuclear magnetic resonance spectroscopy, the degree of acetoacetalization was 12.9.
Mol%, butyralization degree was 29.6 mol%.

The reaction and aging were performed at 65 ° C. for 4 hours with stirring while setting the rotation speed of the stirring blade of the kettle reactor 2 to 250 rpm, the peripheral speed of the stirring blade to 2 m / sec, and the usage efficiency to 10 w / liter. ,
Cooled to 40 ° C. Thereafter, the mixture was neutralized, washed with water and dried in a conventional manner to produce a granular polyvinyl butyral resin having a particle size of 5 to 10 μm.

The degree of acetalization of the obtained polyvinyl butyral resin was determined by nuclear magnetic resonance spectroscopy. As a result, the degree of acetoacetalization was 39.5 mol% and the degree of butyralization was 31.0 mol%.

Using a B-type viscometer, the concentration was 5% by weight.
Was measured with a mixed solvent (equivalent mixed solvent of ethanol and toluene). The solution viscosity was 60 centipoise and low. Using a 25 mmΦ membrane filter (pore size 3 μm), 5 cm ³ of the above solution was
When the filtration time was measured under a load of 500 g / cm ² , the filtration time was 5 seconds or less, which was quick.

Example 2 In Example 1, 2200 g of a 35% by weight aqueous solution of formaldehyde and 3080 g of butyraldehyde were used instead of 1700 g of acetaldehyde and 2150 g of butyraldehyde. Other than that, it carried out similarly to Example 1.

In this case, the particle size is 5 to 10 μm. The degree of formalization of the obtained polyvinyl acetal resin is 2
2.9 mol% and the butyral degree was 47.2 mol%. Solution viscosity was low at 65 centipoise. The filtration time of the solution was as short as 5 seconds or less.

Example 3 In Example 1, 2200 g of a 35% by weight aqueous formaldehyde solution was used instead of 1700 g of acetaldehyde and 2150 g of butyraldehyde, and 750 g of acetaldehyde and 1850 g of butyraldehyde were used. Other than that, it carried out similarly to Example 1.

In this case, the particle size is 5 to 10 μm. The degree of formalization of the obtained polyvinyl acetal resin is 2
5.2 mol%, the degree of acetoacetalization was 17.2 mol%, and the degree of butyralization was 27.5 mol%. The solution viscosity was low at 62 centipoise. The filtration time of the solution was as short as 5 seconds or less.

Example 4 An aqueous propionaldehyde solution was prepared in the same manner as in Example 1 except that 1700 g of acetaldehyde and 2150 g of butyraldehyde were used.
2240 g and butyraldehyde 2150 g were used. Other than that, it carried out similarly to Example 1.

In this case, the particle size is 5 to 10 μm. The resulting polyvinyl acetal resin had a propion acetalization degree of 39.4 mol% and a butyralization degree of 30.2 mol%. Solution viscosity was low at 58 centipoise. The filtration time of the solution was as short as 5 seconds or less.

Example 5 In Example 1, the flow rate was changed to 3 m / sec, and the Reynolds number (Re) of the fluid circulating in the loop reactor 10 was 5426.
Was adjusted to Other than that, it carried out similarly to Example 1.

In this case, the particle size is 5 to 10 μm. The degree of acetoacetalization of the obtained polyvinyl acetal resin was 38.9 mol%, and the degree of butyralization was 30.5 mol%. The solution viscosity was low at 64.5 centipoise. The filtration time of the solution was as short as 5 seconds or less.

Comparative Example 1 In Example 1, the flow rate was changed to 1.5 m / sec, and the number of Reynolds (Re) of the fluid circulating in the loop reactor 10 was 27.
Adjusted to 15. Other than that, it carried out similarly to Example 1.

In this case, the particle size is 50 to 150 μm.
The degree of acetoacetalization of the obtained polyvinyl acetal resin was 31.2 mol%, and the degree of butyralization was 30.5 mol%. The solution viscosity was high at 102.4 centipoise.
The filtration time of the solution was as short as 5 seconds or less.

Comparative Example 2 Pure water (3460 g) was placed in a 5-liter separable flask, and 320 g of polyvinyl alcohol having an average degree of polymerization of 1700 and a saponification degree of 99.0 mol% was added thereto to completely dissolve. The aqueous polyvinyl alcohol solution was heated and maintained at 50 ° C., and 244 g of 35% hydrochloric acid was added thereto.

To this, 62 g of paraacetaldehyde and 80 g of butyraldehyde were added to proceed with the reaction. During this operation, the temperature was cooled from 50 ° C to 2 ° C. The reaction time between them is 13
It was time. This was heated to 60 ° C and heated for 2 hours to carry out a reaction and aging, and then cooled to 40 ° C. Then, it was neutralized, washed with water and dried in a usual manner to produce a granular polyvinyl butyral resin.

In this case, the particle size is 1 to 5 μm. The degree of acetoacetalization of the obtained polyvinyl acetal resin was 41.5 mol%, and the degree of butyralization was 28.5 mol%. The solution viscosity was high at 88 centipoise. The filtration time of the solution was as slow as 120 seconds.

Comparative Example 3 In Comparative Example 2, 71 g of a 35% by weight aqueous solution of formaldehyde and 120 g of butyraldehyde were used instead of 62 g of paraacetaldehyde and 80 g of butyraldehyde. The reaction time from 50 ° C. to 2 ° C. was changed to 12 hours.
Other than that, it carried out similarly to the comparative example 2.

In this case, the particle size is 1 to 5 μm. The degree of formalization of the obtained polyvinyl acetal resin is 2
The degree of butyralization was 5.1 mol% and 43.9 mol%. The solution viscosity was high at 92 centipoise. Also, the filtration time of the solution was slow at 360 seconds.

Comparative Example 4 In Comparative Example 2, instead of 62 g of paraacetaldehyde and 80 g of butyraldehyde, 71 g of a 35% by weight aqueous solution of formaldehyde, 25 g of paraacetaldehyde and 80 g of butyraldehyde were used. The reaction time from 50 ° C. to 2 ° C. was changed to 16 hours. Other than that, it carried out similarly to the comparative example 2.

In this case, the particle size is 1 to 5 μm. The degree of formalization of the obtained polyvinyl acetal resin is 2
The degree of acetoacetalization was 7.5 mol%, and the degree of butyralization was 26.6 mol%. The solution viscosity was high at 85 centipoise. The filtration time of the solution is 350
Seconds were slow.

Comparative Example 5 In Comparative Example 2, 62 g of acetaldehyde and 80 g of butyraldehyde were used instead of 62 g of paraacetaldehyde and 80 g of butyraldehyde. Further, 244 g of hydrochloric acid having a concentration of 35% by weight was changed to 57 g of hydrochloric acid having a concentration of 35% by weight. Further, the reaction time from 50 ° C. to 2 ° C. was changed to 24 hours.
Other than that, it carried out similarly to the comparative example 2.

In this case, the particle size is 100 μm or more.
The degree of acetoacetalization of the obtained polyvinyl acetal resin was 27.5 mol%, the degree of acetoacetalization was 18.4 mol%, and the degree of butyralization was 34.4 mol%. The solution viscosity and the filtration time of the solution were measured, but were not dissolved in the mixed solvent (a mixed solvent of an equal amount of ethanol and toluene).

[0061]

As described above, in the method of the present invention, the aqueous polyvinyl alcohol solution and the aldehyde are partially reacted at 30 to 90 ° C. while circulating in the loop reactor, and then stirred in the kettle reactor. In the method for producing a polyvinyl acetal resin by completely reacting at 30 to 90 ° C. while forming a loop-shaped reactor inner wall with glass, a reaction having an acetalization degree of at least up to 40 mol% is performed in a loop-shaped reactor. Reynolds number (Re) of the fluid circulating in
This is carried out under the condition of being not less than 00, whereby the aggregation of the particles of the resulting vinyl acetal resin is prevented.
Also, adhesion of scale to the vessel wall is prevented.

Therefore, the acetalization reaction at a relatively high temperature (30 to 90 ° C.) becomes possible, gelation of polyvinyl alcohol is prevented during the acetalization reaction, and the solution viscosity or the viscosity of the obtained polyvinyl acetal resin is reduced. Melt viscosity can be reduced. In addition, a long-time operation is possible, and a high-quality resin can be manufactured without mixing of scale.

Therefore, the polyvinyl acetal resin obtained by the method of the present invention prevents air from remaining between the glass plate and the interlayer film due to a decrease in melt viscosity even when producing a laminated glass having a large curvature, High quality laminated glass can be manufactured efficiently. Further, it is possible to manufacture an adhesive or a paint having good coating workability and low cost by using a relatively small amount of a solvent. The method of the present invention has the above advantages.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing one example of a reaction apparatus used in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Polyvinyl alcohol dissolution tank 2 Pot-shaped reactor 3 Aldehyde tank 4 Supply pipe 5 Transfer pipe 6 Supply pipe 7 Circulation pump 10 Loop reactor 14 Supply pump 16 Supply pump 17 Extraction valve

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-275310 (JP, A) JP-A-2-123104 (JP, A) JP-A-54-47975 (JP, A) JP-A-1- 318009 (JP, A) JP-A-3-41104 (JP, A) JP-A-60-71617 (JP, A) JP-A-53-86785 (JP, A) JP-A-2-49002 (JP, A) JP-B-49-29305 (JP, B1) U.S. Pat. No. 2,702,501 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08F 8/28 C08F 16/34 C03C 27/12

Claims (2)

(57) [Claims] 1. An aqueous polyvinyl alcohol solution and an aldehyde are partially reacted at 30 to 90 ° C. while circulating in a loop reactor, and then stirred in a kettle reactor for 30 minutes.
In a method for producing a polyvinyl acetal resin by completely reacting at ~ 90 ° C, a loop-shaped reactor inner wall is formed of glass, and a reaction having an acetalization degree of at least up to 40 mol% is performed in the loop-shaped reactor. A method for producing a polyvinyl acetal resin, wherein the method is performed under the condition that the number of Reynolds (Re) of a circulating fluid is 5000 or more. 2. The surface roughness of the glass on the inner wall of the loop reactor is 0.5 μm or less in terms of Rz value (however, the Rz value is a 10-point average measured in accordance with ISO R468 using a stylus-type roughness measuring device). The method for producing a polyvinyl acetal resin according to claim 1, wherein the roughness is roughness).