JPH09208235A - Production of silica glass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the cracking of dry gel in a low temp. region in a firing process and enhance the yield of a product by limiting polyvinyl acetate used for preparing a sol when silica glass is produced by a sol-gel method. SOLUTION: A sol prepd. by hydrolyzing a condensation polymer of a silicon alkoxide with a base catalyst in the presence of polyvinyl acetate and a mixed solvent consisting of methanol and a high b.p. solvent is made gel and this gel is dried and vitrified by firing. The polyvinyl acetate has >=1×10<-4> mol.cm<3> /g second virial. coefft. measured with a light scattering photometer.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing silica glass by a sol-gel method.

[0002]

2. Description of the Related Art The sol-gel method is 1) compared with the conventional method for producing synthetic quartz glass by the vapor phase method.
Low energy cost due to low synthesis temperature 2)
Large and highly homogeneous glass can be obtained without high temperature forming and annealing of glass, which is indispensable for obtaining large and highly homogeneous glass by the vapor phase method. 3) Inject sol into a container and gel Since the shape of the gel can be controlled by the shape of the container in the process, there are advantages such as the cost of processing the glass and reduction, and research is being conducted.

As a process for producing large quartz glass by the sol-gel method, an organic polymer such as polyvinyl acetate is added, a solvent containing a high boiling point solvent such as dimethylformamide and tetrahydrofurfuryl alcohol is used, and a silicon alkoxide of A method has been proposed in which a condensation polymer is hydrolyzed using a base catalyst to prepare a sol, which is gelled, dried to obtain a dry gel, and the silica gel is fired to obtain quartz glass. According to the above method, large-sized quartz glass can be manufactured relatively easily.

[0004]

However, using polyvinyl acetate as an organic polymer and a mixed solvent of a high boiling point solvent such as methanol and dimethylformamide or tetrahydrofurfuryl alcohol as a solvent, a large dry gel is obtained. Then, when quartz glass is produced by firing, cracks occur at a low temperature (200 ° C. or less) in the firing process, resulting in a problem that the product yield is remarkably reduced.

The present invention hydrolyzes a polycondensation product of a silicon alkoxide using a base catalyst in the presence of a mixed solvent of polyvinyl acetate and methanol and a high boiling point solvent to give a sol, which is gelled and dried to give a dried gel. Then, in the method for producing silica glass by the sol-gel method in which the silica glass is then fired, a method for preventing the occurrence of cracks in the low temperature region of the firing process of the dry gel and improving the product yield is provided.

[0006]

The present invention is carried out in the presence of polyvinyl acetate and a mixed solvent of methanol and a high boiling point solvent,
Hydrolysis of a polycondensation product of silicon alkoxide using a base catalyst causes the sol to gel, is dried to give a dry gel, and is then fired to give a glass. The second virial coefficient measured by a scattering photometer is 1 × 10 ^-4 mo
A method for producing silica glass, characterized by using polyvinyl acetate of 1 · cm3 / g2 or more.

That is, in the present invention, as polyvinyl acetate, the second virial coefficient of the methanol solution thereof measured by a light scattering photometer is 1 × 10 ⁻⁴ mol · cm 3 / g 2
It was decided to use the above-mentioned polyvinyl acetate. The second virial coefficient measured using a light scattering photometer at a temperature of 25 ° C. using a methanol solution of polyvinyl acetate has a value of 1 ×
When it is less than 10 ⁻⁴ mol · cm 3 / g 2, cracks are likely to occur when the produced dry gel is vitrified into a glass. In the present invention, the device used for measuring the second virial coefficient is DLS-700 manufactured by Otsuka Electronics Co., Ltd. However, it goes without saying that the second virial coefficient specified in the present invention is not limited to the measuring device. , It is possible to obtain equivalent performance or a similar value, but the measurement may be performed by using another device.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As the solvent, a mixed solvent of methyl alcohol and a high boiling point solvent such as dimethylformamide and tetrahydrofurfuryl alcohol can be used. As the high boiling point solvent, cellosolves such as dimethylacetamide, methyl cellosolve and ethyl cellosolve may be used.

As polycondensation products of silicon alkoxides, polycondensation products such as silicon methoxide, silicon ethoxide and silicon propoxide can be used, but in view of raw material cost and easiness of hydrolysis, polycondensation products of silicon methoxide can be used. The use of polymers is preferred.

As the base catalyst used for the hydrolysis of the condensation product of silicon alkoxide, ammonium hydroxide,
Tetramethylammonium hydroxide, tetraethylammonium hydroxide, n-propylammonium hydroxide, choline and the like can be used, with choline providing suitable results.

[0011]

The cause of cracking in the low temperature region during the firing process of the dry gel and the effect of the present invention on it, which are intended to be solved by the present invention, are considered as follows. The second virial coefficient is known to represent the strength of the interaction between the solvent and the solute. That is, the fact that the second virial coefficient of polyvinyl acetate in the measured methanol solution is small means that the interaction between methanol and polyvinyl acetate is small, and conversely, the polyvinyl acetate molecules in the solution are Is strong, which means that in such a polyvinyl acetate, molecules tend to aggregate in a methanol solution. From this, when polyvinyl acetate having a small second virial coefficient is added to a mixed solution for sol preparation containing methanol, polyvinyl acetate molecules are agglomerated in the solution, and the coexisting liquid phase (condensation of silicon alkoxide) occurs. It is considered that the incorporation of the polymer, dimethylformamide, tetrahydrofurfuryl alcohol, etc.) into the aggregate occurs. Furthermore, when a base catalyst is added to the above-mentioned mixed solution for hydrolysis of a polycondensation product of silicon alkoxide, polyvinyl acetate is easily saponified by the base catalyst to become polyvinyl alcohol, and polyvinyl alcohol is an alcohol present in a large amount in the system. Therefore, the agglomeration of the polymer as described above is further promoted, and the condensation of the polymer containing a large amount of a polycondensation product of silicon alkoxide, dimethylformamide, and tetrahydrofurfuryl alcohol in the system is further promoted. Many collections will occur. When a sol is prepared using a solution containing such polymer aggregates, and gelled and dried, the liquid phase taken inside the polymer aggregates evaporates slowly, so dimethylformamide and It is possible to obtain a dry gel containing a polymer aggregate having a high content of tetrahiddlefurfuryl alcohol. When such a dried gel is fired, dimethylformamide and tetrahydrofurfuryl alcohol inside the polymer aggregate are rapidly released during the heating process, or the aggregate is formed due to the softening of the polymer and the increase in gas pressure inside the aggregate. It is believed that the gel expands and, as a result, the gel cracks. Regarding the cause of cracking as described above, the gel phase of the sol, the liquid phase in the gel was removed by centrifugation, and the specific surface area of the gel obtained by drying this and the dimethylformamide contained therein and tetrahydrofurfuryl alcohol were included. When the amount is analyzed, when polyvinyl acetate having a second virial coefficient smaller than the value specified in the present invention is used, the value of the specific surface area becomes remarkably small,
It is also believed to be supported by the experimental results that the amounts of dimethylformamide and tetrahydrofurfuryl alcohol are high. When polyvinyl acetate having a large second virial coefficient as defined in the present invention is used, the specific surface area of the gel obtained by drying the liquid phase in the gel is increased, while the content of dimethylformamide and tetrahydrofurfuryl alcohol is increased. It is considered that the formation of polymer aggregates that cause the above-described cracking was suppressed because the value of γ becomes smaller.

[0012]

【Example】

Example 1 100 parts by weight of a polycondensation product of silicon methoxide, 50 parts by weight of methanol, 35 parts by weight of dimethylformamide, 27 parts by weight of tetrahydrofurfuryl alcohol, and a second amount measured by the method described in the detailed description of the present invention. Polyvinyl acetate methanol solution with virial coefficient of 2 × 10 ^-4 mol · cm3 / g2 (solid concentration 50%, degree of polymerization: 150
0) 10 parts by weight was added and stirred for 30 minutes, and 1 part by weight of choline methanol solution (1M) was added thereto. Then water 39
A weight part was dropped to prepare a sol. The sol obtained is 60
Pour into a cm-square Teflon-coated stainless steel container, place a lid with polyvinyl chloride film and aluminum foil on the opening with the polyvinyl chloride side facing the gel side, and place it in the container and let stand at room temperature to gel the sol. Turned into Then, the container was transferred to a dryer and heated at 50 ° C. for 30 hours, then the temperature was returned to room temperature, the aluminum foil on the container lid was removed, and then the temperature was raised from 30 ° C. to 120 ° C. in order to dry. went. The obtained dry gel is heated in air from room temperature to 10
20 ℃ / h to 0 ℃, 2 ℃ from 100 ℃ to 200 ℃
When the temperature was raised at a rate of / h and the presence or absence of cracks was examined, no cracks were observed.

Example 2 Polyvinyl acetate methanol solution having a second virial coefficient of 2.5 × 10 ⁻⁴ mol · cm 3 / g 2 measured by the method described in the detailed description of the present invention (solid content 50%, polymerization 15 degrees
00) was used to prepare a dry gel in the same manner as in Example 1, and the presence or absence of cracks in the firing process up to 200 ° C. was examined. No cracks were observed.

Comparative Example Polyvinyl acetate methanol solution having a second virial coefficient of 0.8 × 10 ⁻⁴ mol · cm 3 / g 2 measured by the method described in the detailed description of the present invention (solid content 50%, degree of polymerization) 15
No. 00) was used to prepare a dry gel in the same manner as in Example 1, and the presence or absence of cracks in the firing process up to 200 ° C. was examined. As a result, cracks were observed.

[0015]

According to the method for producing quartz glass by the sol-gel method of the present invention, the dry yield of gel can be dramatically improved and the product cost can be reduced.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroki Terasaki 48 Wadai, Tsukuba City, Ibaraki Prefecture Hitachi Chemical Co., Ltd. Tsukuba Development Laboratory

Claims (1)

[Claims] 1. A polycondensation product of a silicon alkoxide is hydrolyzed using a base catalyst in the presence of a mixed solvent of polyvinyl acetate and methanol and a high-boiling-point solvent to gelate a sol, and is dried to give a dry gel. Then, in the method for producing silica glass that is fired into glass, the second method is to measure a solution of polyvinyl acetate in methanol with a light scattering photometer.
A method for producing silica glass, characterized in that polyvinyl acetate having a virial coefficient of 1 × 10 ⁻⁴ mol · cm 3 / g 2 or more is used.