Method of producing an alkali-free glass

Details Method of producing an alkali-free glass Method of producing an alkali-free glass

2001-07-17

2003-04-15

Colaianni, Michael (Department: 1731)

Glass manufacturing

Processes

Fining or homogenizing molten glass

C065S134100, C501S055000, C501S056000, C501S069000, C501S070000

Reexamination Certificate

active

06546753

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to an alkali-free glass and, in particular, to an alkali-free glass for use as a light transparent glass substrate for a liquid crystal display and the like as well as a method of producing the same.
Conventionally, an alkali-free glass has been used as the light transparent glass substrate for the liquid crystal display and the like. The alkali-free glass for use in such a display is required to be free from bubbles such as blisters and seeds resulting in a display defect, in addition to various characteristics such as heat resistance and chemical resistance.
To meet such a demand, various kinds of alkali-free glasses have been proposed. U.S. Pat. No. 4,994,415 discloses an SiO
2
—Al
2
O
3
—B
2
O
3
—CaO—BaO as one of the alkali-free glasses.
In order to obtain a glass without bubbles, it is important to select a fining agent capable of generating a fining gas both in a relatively low-temperature range in which batch decomposition and degassing is started and in a relatively high-temperature range in which fining and homogenization of a glass are caused. Specifically, the fining agent serves to expel a gas generating during the batch decomposition and degassing of a glass material, and to enlarge very small bubbles remaining in the glass melt during fining and homogenization processes, whereby enlarged bubbles float up in the glass melt and leave the glass melt.
In the meanwhile, the alkali-free glass for use as a glass substrate for a liquid crystal display is high in viscosity of the glass melt so that a melting process is carried out at a high temperature in comparison with a glass containing an alkali component. In the alkali-free glass of the type, the batch decomposition and degassing generally occurs at 1200-1300° C. while the fining and the homogenization are performed at a high temperature of 1400° C. or more. Under the circumstances, the fining agent is required to produce the fining gas in a wide temperature range (on the order of 1200-1600° C.). Presently, As
2
O
3
is widely used as the fining agent.
However, As
2
O
3
is highly toxic and may possibly cause environmental pollution during a manufacturing process of the glass and during disposal of a waste glass. In this reason, the use of As
2
O
3
is being limited.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an alkali-free glass which does not use As
2
O
3
as a fining agent and which is free from bubbles resulting in a display defect, and to provide a method of producing the same.
As a result of various experiments, the present inventors have found the fact that above-mentioned object is achieved by using a combination of Sb
2
O
3
and at least one of SnO
2
and chloride, as a fining agent instead of As
2
O
3
.
According to the present invention, there is provided an alkali-free glass essentially consisting of basic elements of 40-70% SiO
2
, 6-25% Al
2
O
3
, 5-20% B
2
O
3
, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO, and a fining agent of a combination of 0.05-3% Sb
2
O
3
and at least one of 0.05-2% SnO
2
and 0.005-1% Cl
2
, on the base of the weight percent.
According to the present invention, there is also provided a method of producing an alkali-free glass having a basic composition essentially consisting by weight of 40-70% SiO
2
, 6-25% Al
2
O
3
, 5-20% B
2
O
3
, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO, by preparing a glass batch having the basic composition free from alkali metal oxide, melting the glass batch, and forming the glass melt, which is characterized by adding a combination of 0.05-3 wt % Sb
2
O
3
and at least one of 0.05-2 wt % SnO
2
and 0.01-2 wt % chloride in terms of Cl
2
as a fining agent into the glass batch.
Sb
2
O
3
and SnO
2
used in the present invention generate a large amount of fining gas (oxygen gas) as a result of chemical reaction following variation in valence of Sb ions and Sn ions. Specifically, Sb
2
O
3
(trivalent) is at first changed to Sb
2
O
5
(pentavalent) in a low-temperature range on the order of several hundred degree in ° C. and then returns to Sb
2
O
3
(trivalent) around 1200-1300° C. At this time, a large amount of fining gas is released. Likewise, when SnO
2
(quadrivalent) is changed to SnO (bivalent) at 1400° C. or more, a large amount of fining gas is released. On the other hand, chloride is decomposed and volatilized in a temperature range not lower than 1200° C. to generate the fining gas (for example, chroline gas). Particularly, decomposition and volatilization are very active at a high-temperature range not lower than 1400° C. to generate a large amount of fining gas.
Therefore, the use of a combination of Sb
2
O
3
and at least one of SnO
2
and chloride as the fining agent provides a high fining effect at a wide temperature range from a comparatively low temperature for the batch decomposition and degassing to a high temperature for fining and homogenizing. Thus, it is possible to provide the alkali-free glass without bubbles resulting in a display defect.
DESCRIPTION OF PREFERRED EMBODIMENTS
Next, description will be made about a method of producing of an alkali-free glass according to the present invention.
At first, a raw glass mixture or a glass batch is prepared so as to obtain a glass having the above-mentioned composition. Description will hereafter be made about the content of each component in the composition and the reason why the content is so defined.
SiO
2
is a component serving as a network of the glass. The content of SiO
2
is 40-70%, preferably, 45-65%. When the content is less than 40%, chemical resistance is degraded and a strain point of the glass is lowered so that heat resistance is degraded. When the content is more than 70%, high-temperature viscosity is increased so that the meltability is deteriorated and that devitrified substances of cristobalite readily precipitate.
Al
2
O
3
is a component to improve the heat resistance and devitrification resistance of the glass. The content of Al
2
O
3
is 6-25%, preferably, 10-20%. When the content of Al
2
O
3
is less than 6%, the devitrification temperature remarkably rises so that the devitrification is readily caused to occur in the glass. When the content is more than 25%, acid resistance, more particularly, buffered-hydrofluoric-acid resistance is degraded so that the cloudness is readily caused to occur on the surface of a glass substrate.
B
2
O
3
is a component serving as a flux to lower the viscosity and to facilitate melting of the glass. The content of B
2
O
3
is 5-20%, preferably, 6-15%. When the content of B
2
O
3
is less than 5%, the effect as the flux is insufficient. When the content is more than 20%, the hydrochloric acid resistance is degraded and the strain point is lowered so that the heat resistance is degraded.
MgO is a component to decreasing the high-temperature viscosity without lowering the strain point so as to facilitate melting of the glass. The content of MgO is 0-10%, preferably, 0-7%. When the content of MgO is more than 10%, the buffered-hydrofluoric-acid resistance of the glass is seriously degraded.
CaO has a function similar to MgO. The content of CaO is 0-15%, preferably, 0-10%. When the content of CaO is more than 15%, the buffered-hydrofluoric-acid resistance of the glass is seriously degraded.
BaO is a component to improve the chemical resistance and the devitrification resistance of the glass. The content of BaO is 0-30%, preferably, 0-20%. When the content of BaO is more than 30%, the strain point is lowered so that the heat resistance is degraded.
SrO has an effect similar to BaO. The content of SrO is 0-10%, preferably, 0-7%. The content of SrO more than 10% is unfavorable because the devitrification is increased.
ZnO is a component to improve the buffered-hydrofluoric-acid resistance and the devitrification resistance. The content is 0-10%, preferably, 0-7%. When the content of ZnO is more than 10%, the glass tends to be devitrified and the strain point is lowered so that the heat resistance can not be assured.
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