JPH0651582B2 - Method of chemically strengthening float glass - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a soda-lime float glass, which is used for various moldings such as thin and large-area construction, glass for vehicles, fixtures, etc. The present invention relates to a method for chemically strengthening float glass used in the field of materials.

[Conventional technology]

Float glass has excellent surface smoothness, flatness, thickness uniformity, etc., compared to so-called ordinary flat glass, so it is widely used not only in the fields of construction, vehicles, etc. but also in the field of electronic materials, such as liquid crystal and plasma displays. is there.

Furthermore, as a recent trend, thin glass with a thickness of 4 mm or less has been favored, and the thinner the thickness, the higher the strength is desired.

It is well known that the chemical strengthening method by alkali ion substitution is applied to effectively strengthen thin glass, but when the chemical strengthening method is used as it is for float glass, the glass warps (for example, a thickness of 1 mm). At 0.45 to 1.
(25 mm / 300 mmφ) flatness was impaired, and in particular the flatness required for optical disk substrates and the like (for example, 0.2 mm or less / 300 mmφ at a thickness of 1 mm) could not be obtained.

Warpage is caused by molten metal during glass float molding, usually
It is speculated that this is due to the effect of Sn entering the contact glass surface, but no epoch-making countermeasure against this warp has been found. For example, the Sn leaching surface of glass is ground and polished, and then alkali ion substitution treatment is carried out, but this method not only complicates the process but also easily causes cracks and surface defects, which makes the cost expensive. There was a problem.

According to JP-A-47-39320, the effect is obtained by first contacting the entire surface of the glass with an alkali salt having a desired K / Na ratio and then further contacting it with an alkali salt having a higher K / Na ratio in the alkali ion substitution. Although it is disclosed that the strength of the float glass is expressed, it is intended to suppress the warpage that occurs during the alkali ion substitution due to the difference between the Sn contact surface and the non-contact surface of the float glass which is the object of the present invention. Is completely different.

[Problems to be solved by the invention]

As described above, in the prior art, the process is complicated and costly even if polishing is performed to suppress the warpage that occurs when the float glass is subjected to ion substitution, or a two-step treatment with a salt containing potassium is performed. However, there is a problem that the warp cannot be effectively suppressed.

The present invention provides a novel method for suppressing the occurrence of warpage when strengthening float glass by alkali ion substitution.

[Means for solving problems]

In the present invention, the contact surface with molten metal during float forming of soda lime glass is coated with molten salt containing potassium.
It is characterized in that heat treatment is carried out at 380 ° C. to 650 ° C. for 10 minutes to 70 hours, and then the entire surface of the glass is brought into contact with a molten salt containing potassium to perform heat treatment to strengthen it.

At the time of float molding, molten glass is molten metal (usually S
n) extends along the surface to form a smooth surface (hereinafter referred to as the bottom surface), while its upper surface contacts the atmosphere to form a smooth free surface (hereinafter referred to as the top surface).

On the bottom surface in contact with Sn, Sn penetrates into the glass to a depth of 10 to 20 μm. It is presumed that other components such as soda ions in the glass are relatively reduced due to the infiltration of Sn into the bottom surface.

By the way, it is clear that when the float glass is strengthened by alkali ion substitution as it is, a warp occurs in which the top surface side expands and the top surface side becomes convex,
This is because the alkali ion substitution on the bottom surface is suppressed by the infiltration of Sn into the bottom surface as described above, while the alkali ion substitution on the top surface side is not suppressed, and the substituted alkali ion radius is large. It is speculated that this may be one of the causes.

The present invention has been made in view of this point, first by coating a portion of the soda ions on the bottom surface of the float glass with potassium ions having a large ionic radius to replace the potassium-containing molten salt, by heat treatment When forming a warp that makes the bottom surface convex, and then replacing soda ions on the entire surface of the glass with ions having a larger ionic radius, potassium ions in practice, and strengthening, the top surface side, which should inevitably occur, becomes convex The warp is offset by the warp in which the bottom surface side is convex, and a chemically strengthened glass having no warp is manufactured.

The method of coating the bottom surface of the float glass with a molten salt containing potassium and heat treatment is not specified, but for example, the glass is immersed in a bath of potassium nitrate, sulfate, phosphate or a mixed salt thereof and immediately After pulling up the glass and cooling and solidifying the potassium salt coating the glass, the potassium salt coating the top surface is washed away with water, and after drying, the glass is placed in an electric furnace heated to a predetermined temperature and heated for a predetermined time. To do. The temperature, the time is the range in which the alkali ion substitution on the bottom surface can be performed efficiently and without excess or deficiency, and the glass is not deformed, that is, 380 ° C to 650 ° C.
10 minutes to 70 hours is suitable. That is, the temperature is 380 ℃
If less than, the potassium ion substitution on the bottom surface of the glass is insufficient, therefore there is almost no warp suppressing effect,
If it exceeds 650 ° C, the glass tends to be deformed or clouded and the smoothness is impaired. The treatment time is related to the above temperature, and if it is less than 10 minutes, the temperature needs to be 650 ° C. or higher, but on the other hand, it is not preferable because it involves the aforementioned defects. Further, if it is longer than 70 hours, the substitution of potassium ions on the bottom surface proceeds too much, and the convex warpage becomes remarkable on the bottom surface side, which cannot be offset even in the subsequent chemical strengthening treatment, which is economically disadvantageous. After that, the glass is taken out of the furnace, cooled, washed with water and dried.

The glass obtained has a convex bottom surface side of -0.03 mm to 0.45 mm / 300 mmφ.

After the above treatment, the entire surface of the post-float glass is subjected to ion substitution with potassium to form a surface compression layer on the entire surface of the glass.

This treatment employs a conventionally known method, for example, by immersing the glass in a potassium nitrate bath at about 500 ° C. for 30 minutes to several hours, a potassium substitution layer of about several tens to several tens μm is formed. After this operation, the glass is cooled and washed. Thus, warp is suppressed and chemically strengthened glass can be obtained.

〔Example〕

 Specific examples of implementation will be described below.

Float glass having a size of 300 mmφ and a thickness of 1 mm was prepared, and 5 sheets were provided for each of the following examples. 400
The glass was immersed in a potassium nitrate bath at 0 ° C. and immediately pulled up and allowed to cool. After the glass-coated potassium nitrate solidified, the potassium nitrate on the top surface was removed by a water spray and dried.

Next, the glass was heat-treated in the electric furnace under the following conditions.

Example 1; 610 ° C., 40 minutes Example 2; 520 ° C., 6 hours Example 3; 400 ° C., 16 hours Reference Example 1; 360 ° C., 44 hours After that, the glass was taken out, cooled, washed with water, dried and partially removed. The amount of warpage of the glass was measured.

Further, each glass was immersed in a potassium nitrate bath at 490 ° C. for 150 minutes, taken out, cooled, and washed with water.

For the obtained glass, a shape measuring instrument (DEKTAK manufactured by SLOAN)
The amount of warpage was measured by II) and the surface compressive stress was measured by a stress measuring device. The relationship between the processing conditions and the amount of warpage is shown in Table 1. In the table, Reference Example 1 is out of the suitable treatment temperature condition range, and Reference Example 2 is a case where only chemical strengthening is applied,
Reference Example 3 also shows the case of a green sheet glass that is not subjected to any treatment.

As can be seen from Table 1, according to the present invention, the warp amount is 0.20 mm /
It is less than 300mmφ and is similar to float green sheet glass. The surface compressive stress is in the range of 2600 to 3300 Kg / cm ² , which is equivalent to that of the conventional chemical strengthening method.

[Advantages of the Invention] As described above, the present invention suppresses the warpage that occurs when the conventional float glass is simply chemically strengthened, and the surface compressive stress is also equivalent to that of the conventional method, and it is used in electronic materials fields such as displays and disk substrates. It has a remarkable effect that it can be widely used.

Claims (1)

[Claims] 1. A surface of a soda-lime glass that comes into contact with molten metal during float forming is coated with a molten salt containing potassium, heat-treated at 380 ° C. to 650 ° C. for 10 minutes to 70 hours, and then the entire glass surface contains potassium. A method for chemically strengthening a float glass, which comprises heat-treating by contacting with molten salt to strengthen the glass.