Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-03-24
2002-08-20
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S070000, C065S030130, C065S030140
Reexamination Certificate
active
06436859
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a glass composition which is capable of being pressed under a softened condition by heating and being chemically strengthened by alkali ion exchange. Such glass composition is suitable for obtaining glass articles such as optical glass of lenses and compact substrate glass. The present invention is more particularly relates to a substrate glass for information recording media that can be suitably used as a magnetic recording medium and so forth. In fact, this substrate glass is prepared by pressing and then chemical strengthening.
Substrate glass for information recording media, for example, is required to be able to be easily melted at a temperature comparable or lower than soda lime silica glass, have a softening temperature lower than soda lime silica glass, be able to be pressed easily and allow the formation of subtle surfaces that follow the press mold, be resistant to breakage as a result of having a thermal expansion coefficient similar to the press die, and have a high degree of so-called chemical strengthening by alkali ion exchange.
Furthermore, although it was common in the past to search for a method for grinding press formed glass to obtain surface smoothness, grinding is not preferable since it requires considerable time and trouble and has high costs. Although there have also been examples advocating the formation of a smooth surface by float process, although the float process is suitable for large-volume production of relatively large size glass such as window glass, it is not suited to manufacturing small, precise substrates such as substrates for information recording media.
Although the specification of U.S. Pat. No. 4,156,755 discloses a SiO
2
—Al
2
O
3
—Li
2
O—Na
2
O glass additionally containing ZrO
2
that is strengthened by ion exchange, this glass has an extremely high content of Al
2
O
3
and ZrO
2
, has high melting and forming temperatures and has problems with melting ease and formability (particularly with respect to press forming).
Japanese Patent Laid-Open Publication No. 10-1329 discloses a glass composition for chemical strengthening and chemically strengthened glass article comprising SiO
2
—Al
2
O
3
—Li
2
O—Na
2
O—CaO, does not contain ZrO
2
and is suitable for float process. In this publication, there is no mention of press forming, and both the temperature at which the glass exhibits a viscosity of 10
2
poises and the temperature at which the glass exhibits a viscosity of 10
4
poises are high. In addition, due to the absence of ZrO
2
, which is known to be a component for providing a glass with excellent water resistance and weather resistance, this glass is believed to be difficult to satisfy water resistance and weather resistance.
Japanese Patent Laid-Open Publication No. 5-32431 discloses a glass, which is to be subjected to chemical strengthening, comprising SiO
2
, Al
2
O
3
, Li
2
O, Na
2
O and ZrO
2
, and a chemically strengthened glass produced thereby. In this publication, there is no descriptions of the temperature at which the glass exhibits a viscosity of 10
2
poises, the temperature at which the glass exhibits a viscosity of 10
4
poises, the softening point and the like of this glass which is to be subjected to chemical strengthening. In addition, although the ZrO
2
component is contained at a relatively high range, this tends to make the glass melting temperature higher and forming, and particularly press forming, difficult.
Japanese Patent Laid-Open Publication No. 10-158028 discloses a glass substrate for magnetic discs, comprising SiO
2
, Al
2
O
3
, Na
2
O, K
2
O, RO and ZrO
2
, where RO is MgO, CaO, SrO or BaO, that is resistant to scratching and suitable for float process. Press forming of this glass is not mentioned in this publication, and the temperature at which the glass exhibits a viscosity of 10
2
poises and the temperature at which the glass exhibits a viscosity of 10
4
poises are both high. In addition, although the K
2
O content is relatively high in comparison with the Na
2
O content, in consideration of the fact that the sodium component in the glass is ion-exchanged by molten potassium salt, efficient ion exchange is thought to be difficult.
Japanese Patent Laid-Open Publication No. 11-199267 discloses a glass substrate for magnetic discs, consisting essentially of SiO
2
, Al
2
O
3
, Li
2
O, Na
2
O, K
2
O, MgO, CaO, ZrO
2
and TiO
2
.
Namely, there have been cases in the prior art having problems with formability including melting ease and press forming, inadequate weather resistance, moisture resistance and water resistance, or being unable to be chemically strengthened by ion exchange.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a lithium-sodium-aluminosilicate glass composition that is capable of providing an ion exchange strengthened glass article through it pressing and then alkali ion exchange.
It is another object of the present invention to provide such ion exchange strengthened glass article.
It is a further object of the present invention to provide a process for producing such ion exchange strengthened glass article.
According to the present invention, there is provided a lithium-sodium-aluminosilicate glass composition comprising a softening point of 700° C. or lower; a first temperature at which the glass composition exhibits a viscosity of 10
2
poises, the first temperature being 1450° C. or lower; a second temperature at which the glass exhibits a viscosity of 10
4
poises, the second temperature being 1000° C. or lower; and a mean thermal expansion coefficient between 30° C. and a glass transition temperature of the glass composition, the coefficient being within a range of from 90×10
−7
/° C. to 130×10
−7
/° C.
According to the present invention, there is provided an ion exchange strengthened glass article prepared from the glass composition by a process. This process includes (a) providing the glass composition; (b) pressing the glass composition at the softening point, thereby to obtain a precursor of the glass article; and (c) bringing the precursor into contact with a molten salt comprising a potassium salt, thereby conducting an alkali ion exchange strengthening.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The glass composition of the invention can be melted easily, can be easily formed by pressing, allows the formation of subtle surfaces that closely follow the press mold, is resistant to breakage by closely approximating the thermal expansion coefficient of the press mold, and can be chemically strengthened at high degree by alkali ion exchange, and more particularly, provides a substrate glass for information recording media that can be preferably used as a magnetic recording medium and so forth.
A pressed glass composition (i.e., a precursor of an ion exchange strengthened glass article) of the present invention is produced in two stages by first melting, clarifying and homogenizing the glass raw materials, producing a preliminary formed glass having a shape that approximates the target shape by, for example, casting the resulting molten glass, and then re-heating this preliminary formed glass and supplying this to press forming. Alternatively, the pressed glass composition of the present invention can be produced in a continuous operation by first producing the above-mentioned preliminary formed glass and while still heated, supplying it to press forming after slightly adjusting the temperature.
When glass exhibits a viscosity of 10
2
poises during its melting, the glass becomes extremely rich in fluidity This is advantageous for melting and homogenizing of the glass. Thus, the temperature at which glass exhibits a viscosity of 10
2
poises is also referred to as the melting temperature. Common soda lime silica glasses are considered to melt easily, and the temperature at which these glasses exhibit a viscosity of 10
2
poises is within the range of 1400-1450° C. Similarly, the temperature at which the glass composition according to the present i
Aratani Shin-ichi
Muramoto Tadashi
Central Glass Company Limited
Crowell & Moring LLP
Group Karl
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