Chemically toughened glasses

Details Chemically toughened glasses Chemically toughened glasses

2000-09-14

2003-02-11

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S064000, C501S070000, C501S071000, C428S410000

Reexamination Certificate

active

06518211

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to glasses which are capable of being chemically toughened. In particular the present invention relates to glasses which can be chemically toughened and which are primarily, but not essentially, intended for use in aeronautical and automotive vehicles.
In the chemical toughening, of glass, the surface of the glass is compressed by the substitution of alkali ions in the surface layers of the glass by heavier, larger ions. This is customarily effected in an ion-exchange bath containing one or more salts of the heavier ions. By so doing, the breaking strength of the glass is increased, thereby permitting the glass to withstand static stresses, such as those experienced in aircraft cockpits or cabins, and more dynamic stresses, such as those encountered if the aircraft strikes a flock of birds.
2. Description of the Related Art
Chemically strengthenable glasses are well known. Many of these contain significant quantities of lithium. In, for example, U.S. Pat. No. 4,156,755, there is described and claimed such a glass. However, lithium has the disadvantage of increasing the density of the glass and, in many modern applications for chemically toughened glass, this is not acceptable.
German Patent Specification No 19616633C discloses a wide range of glass compositions, some of which overlap the ranges of the present invention. However, the manner in which they are produced is not revealed. These glasses are used for making display panels and security glazing. However, such glasses essentially contain fluorine and cannot, therefore, be made by the float process. Similarly, Russian Patent Specification No 1146288A also discloses glass compositions which overlap some of the ranges of the composition of the present invention. However, these are not made by the float process and, as it is well known by those skilled in the art, there is a very large difference between float (or flat) glass and container glass of the type described in such Patent Specification.
An alternative method of chemically toughening glass if the glass contains sodium ions is to ion-exchange these for potassium ions. Such a method is disclosed in, for example, International (PCT) Patent Application No. WO 94/108910. Such glass has the added advantage that it has a low density of approximately 2.46 in comparison with conventional float glass which has a density of approximately 2.50. Although such patent alleges that no boron need be present in the glass, it is clear that the glass would be more difficult to melt if boron was absent. In fact, this is borne out by the sole example in the patent which discloses a composition containing nearly 3.5% B
2
O
3
. Boron oxide lowers the viscosity of the glass. This makes the glass easier to melt and, in theory, easier to refine. Moreover, the combination of, in the context of these glasses, high amounts of both boron and potassium allows the low density to be achieved. However, the use of boron is disadvantageous in that it attacks the silica crowns conventionally used in furnaces.
The use of high quantities of potassium also has drawbacks. In particular, large amounts of potassium cause the production of high viscosity foams early in the melting process. These are very slow to collapse and often lead to silica faults in the finished glass which makes it unacceptable from a commercial viewpoint.
It is desirable if the glasses produced have relatively high strain points so that the ion-exchange can be effected at higher temperatures and the desired level of chemical toughening can be achieved in an economically acceptable time. It is known that the strain point can be raised by increasing the quantities of alumina or zirconia in the glasses. However, these materials are extremely refractory and are difficult to melt in a conventional float furnace within an acceptable time. Alkali metal oxides, such as those of lithium, sodium and potassium help to digest alumina and zirconia but have an adverse effect on the strain point and prevent a high surface compressive stress being achieved during the ion-exchange.
Alkaline earth metal oxides have also been utilised in making glasses which are chemically toughened by ion-exchange. However, these also have drawbacks associated therewith. Zinc oxide is not compatible with the float process, due to the ease with which it is reduced to zinc metal, thereby producing an unacceptable bloom on the glass. Calcium oxide interferes with the sodium/potassium ion-exchange and leads to poor penetration whilst magnesium oxide, particularly in the presence of alumina, normally raises the liquidus temperature of the glass to an unacceptably high level. It will be understood that glasses being manufactured on a float plant should have a positive working range, that is to say, a positive difference between the temperature at which the glass has a viscosity of 10,000 poise and the liquidus, also known as the crystallisation, temperature.
OBJECT OF THE INVENTION
The present invention therefore seeks to provide boron-free glasses having a positive working range, which can be readily melted to float glass standards with respect to the inclusion of bubble and solids and which can be chemically strengthened over a period of less than 100 hours to exhibit a surface stress of at least 400 MPa with a depth of ion penetration greater than 200 microns.
In a subsidiary aspect, the present invention also seeks to provide a glass having a low density. In particular, a low density, in the context of the present invention, is less than 2.48 g/cm
3
, preferably less than 2.46 g/cm
3
. This is particularly true if the glass is intended for use in aeronautical applications. Throughout this specification, the amounts of components are given in weight percent unless specifically stated otherwise.
SUMMARY OF THE INVENTION
According to the present invention, there is therefore provided a boron-free float glass composition having a positive working range comprising:
SiO
2
58% to
70% (by weight)
Al
2
O
3
5% to
15%
Na
2
O
12% to
18%
K
2
O
0.1% to
5%
MgO
4% to
10%
CaO
0% to
1%
with the provisos that the sum of the Al
2
O
3
and MgO exceeds 13%, that the sum of the amounts of Al
2
O
3
plus MgO divided by the amount of K
2
O exceeds 3 and that the sum of the Na
2
O plus K
2
O plus MgO exceeds 22%.
We have surprisingly found that the amount of Al
2
O
3
is critical. If the amount of Al
2
O
3
is less than 5%, insufficient stress can be created when the glass is toughened by ion-exchange but if it is greater than 15%, the glass becomes extremely difficult to melt and causes liquidus problems.
MgO has been found to be a highly desirable component of the glasses of the present invention. It assists in lowering the melting temperature whilst simultaneously not affecting the strain point of the glass. Furthermore, it helps to increase the surface stress of the glass during the ion-exchange process.
Both MgO and Al
2
O
3
help to achieve the high surface compressive stress required if the glass is to be used in aeronautical applications. However, when both are present in comparatively high amounts, as in the glasses of the present invention, they can have an adverse effect on the liquidus temperature of the glass.
K
2
O poses many problems when melting glass in a float tank. For example, during the melting process, it causes foaming which breaks up into a scum and eventually appears in the finished glass as an inclusion fault. Nevertheless, in the context of the present invention, it is essential to assist in the diffusion of additional potassium ions from the ion-exchange bath so as to achieve sufficiently deep penetration at a reasonable rate. We have surprisingly found that if the amounts of Al
2
O
3,
MgO and K
2
O are as outlined above, the above-mentioned problems do not arise or are at least minimised.
CaO is often used to lower the melting point of glasses. However, its presence in glasses of the present invention lead to low ion penetration during the ion exchange. It is, therefore, not specifica

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