Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-08-13
2004-09-21
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S067000
Reexamination Certificate
active
06794323
ABSTRACT:
The invention relates to a borosilicate glass of high chemicals resistance and to its uses.
Fused glass/metal seals which are used in a chemically corrosive environment, for example in the construction of chemical installations or reactors, require glasses which have a very high resistance to both acidic and basic media. Moreover, the thermal expansion of sealing glasses of this type has to be matched to the chemically highly resistant metals or alloys which are used. In this context, it is desirable for the coefficient of linear thermal expansion to be close to or slightly below that of the metal which is to be sealed, so that during cooling of the fused seal, compressive stresses are built up in the glass, these stresses first ensuring a hermetic seal and secondly preventing tensile stresses from building up in the glass, which would promote the occurrence of stress crack corrosion. When using Fe—Ni—Co alloys, e.g. Vacon® 11, with a coefficient of thermal expansion &agr;
20/300
of 5.4×10
−6
/K, or zirconium (&agr;
20/300
=5.9×10
−6
/K) or zirconium alloys, glasses with an expansion coefficient &agr;
20/300
of between >5 and 6.0×10
−6
/K are required as sealing glasses for fused glass/metal seals.
A crucial parameter for characterizing the workability of a glass is the working point V
A
at which the viscosity of the glass is 10
4
dpas. It should be low, since even slight reductions in V
A
lead to a considerable fall in production costs, since the melting temperatures can be reduced. Furthermore, a V
A
which is as low as possible is also advantageous in the production of the fused glass/metal seal, since it is then possible to avoid overheating the parts which are to be fused together, since fusion can occur either at a lower temperature or within a shorter time. Finally, when using glasses with a relatively low V
A
it is possible to prevent the seal being adversely affected and, in the most extreme circumstances, leaking as a result of evaporation and recondensation of glass components. Furthermore, the working interval of a glass, i.e. the temperature difference between the working temperature V
A
and the softening point E
W
, the temperature at which the viscosity of the glass is 10
7.6
dpas, is also of significance. The temperature range within which a glass can be worked is also known as the “length” of the glass.
Applications as primary packaging material for pharmaceuticals, such as ampoules or small bottles, also require glasses which have a very high chemical resistance with respect to acidic and basic media and, in particular, a very high hydrolytic stability. Furthermore, a low coefficient of thermal expansion is advantageous, since this ensures a good thermal stability.
Furthermore, the physical-chemical behavior of the glass during its further processing is of importance, since this has an influence on the properties of the end product and on its possible applications.
If a preform made from borosilicate glass which contains alkali metals, e.g. a tube, is processed further under hot conditions to form containers such as ampoules or small bottles, highly volatile alkali metal borates evaporate. The evaporation products condense in cooler regions, i.e. deposits are formed on the vessels, which have an adverse effect on their hydrolytic stability. Therefore, this phenomenon is disadvantageous in particular for applications of the glass in the pharmaceuticals sector, for example as primary packaging material for pharmaceuticals. The patent literature has already described glasses which have high chemicals resistances but are in need of improvement in particular with regard to their hydrolytic stability and/or have excessively high working points and/or do not have the desired expansion coefficients.
DE 42 30 607 C1 proposes chemically highly resistant borosilicate glasses which can be fused to tungsten. They have expansion coefficients &agr;
20/300
of at most 4.5×10
−6
/K and, according to the examples, working points of ≧1210° C.
The borosilicate glasses described in the publication DE 37 22 130 A1 also have a low expansion of at most 5.0×10
−6
/K.
The glasses described in patent DE 44 30 710 C1 have a relatively high SiO
2
content, namely >75% by weight and >83% by weight of SiO
2
+B
2
O
3
in combination with an SiO
2
/B
2 O
3
ratio of >8, and little Al
2
O
3
, a composition which does make them highly chemically resistant but leads to disadvantageously high working points. These glasses, which in some cases have levels of ZrO
2
(up to 3% by weight) and the ZrO
2
-containing borosilicate glasses described in DD 301 821 A7 likewise have low thermal expansions of at most 5.3×10
−6
/K and 5.2×10
−6
/K and, in particular on account of their ZrO
2
contents, are highly resistant to lyes, but relatively susceptible to crystallization.
The glasses described in DE 198 42 942 A1 and DE 195 36 708 C1, have very high chemicals stabilities, being classified as belonging to hydrolytic, acid and lye class 1. However, the abovementioned drawbacks also apply to these glasses, on account of their high levels of ZrO
2
.
Moreover, in the glasses of the prior art, the problem of the evaporation of alkali metals described above during the hot further processing of preshaped glass bodies will continue to occur.
This problem is neither referred to nor solved in DE 33 10 846 A1, which describes BaO-free laboratory glasses.
It is an object of the invention to find a glass which satisfies high demands both with regard to the chemicals resistance, i.e. belongs to lye class 2 or better, to hydrolytic class 1 and to acid class 1, and on workability and has little evaporation of alkali metals.
This object is achieved by borosilicate glass having a composition, in percent by weight based on oxide content of:
SiO
2
70-77
B
2
O
3
6-<11.5
Al
2
O
3
4-8.5
Li
2
O
0.15-2
Na
2
O
4.5-9.5
K
2
O
0-5
with Li
2
O + Na
2
O + K
2
O
5-11
MgO
0-2
CaO
0-2.5
with MgO + CaO
0-3
ZrO
2
0-<0.5
CeO
2
0-1,
and optionally at least one standard refining agent in an amount sufficient for refining.
The glass according to the invention has an SiO
2
content of 70 to 77% by weight, preferably of 70.5 to 76.5% by weight of SiO
2
. Higher levels would increase the working point excessively and reduce the coefficient of thermal expansion too far. If the SiO
2
content is reduced further, in particular the resistance to acids would deteriorate. An SiO
2
content of <75% by weight is particularly preferred.
The glass contains 6 to <11.5% by weight, preferably 6.5-<11.5% by weight, particularly preferably at most 11% by weight of B
2
O
3
. B
2
O
3
reduces the working temperature and the melting temperature while, at the same time, improving the hydrolytic stability. This is because B
2
O
3
bonds the alkali metal ions which are present in the glass more securely into the glass structure. While lower contents would not reduce the melting point sufficiently far and would lead to an increase in the susceptibility to crystallization, higher contents would have an adverse effect on the acids resistance.
The glass according to the invention contains between 4 and 8.5% by weight, preferably up to 8% by weight, of Al
2
O
3
. Like B
2
O
3
, this component bonds the alkali metal ions more securely into the glass structure and has a positive effect on the resistance to crystallization. At lower contents, the susceptibility to crystallization would rise accordingly and, in particular with high B
2
O
3
contents, there would be an increased evaporation of alkali metals. Excessively high levels would make their presence felt in terms of an increase in the working-and melting points.
For the glasses according to the invention, it is essential for the levels of the individual alkali metal oxides to be within the following limits:
The glasses contain 4-9.5% by weight, preferably 4.5-9% by weight of Na
2
O. They may contain up to 5% by weight of K
2
O and up to 2% by weight, preferably up to 1.5%
Bartsch Reiner
Kunert Christian
Peuchert Ulrich
Bolden Elizabeth A.
Group Karl
Schott Glas
Striker Michael J.
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