Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-04-07
2001-09-04
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S076000, C501S078000, C501S079000, C501S900000, C501S903000, C501S065000, C501S065000, C501S065000, C501S065000
Reexamination Certificate
active
06284687
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to high refractive index inorganic glasses; said index being more specifically between 1.70 and 1.78. Said glasses have particularly interesting uses as optical or ophthalmic glasses and are most particularly appropriate for manufacturing segments which enter into the making of multifocal corrective lenses. Said multifocal corrective lenses which incorporate at least one segment in a glass of the invention make up an integral part of the present invention.
BACKGROUND OF THE INVENTION
A multifocal lens is constituted of a principal lens in glass and of one or more segments in glass which are fused onto it. These segments have a higher refractive index than that of the principal lens; this enables obtaining areas of higher corrective power. Said segment(s) is (are) normally soldered onto the principal lens during a heat treatment bringing the whole of the glasses to a temperature close to the softening point of the glass constituting the principal lens. Consequently, the segment glasses must have characteristics of viscosity close to those of the glass of the principal lens. It must also have close thermal expansion characteristics, so that the resulting soldering has a very low strain level.
The objective of the present invention was to obtain glasses of refractive index between 1.70 and 1.78, fusible particularly to an ophthalmic glass manufactured by the Applicant Company (Corning S.A.). of index 1.6. of Abbe number 41 and which has the following physical characteristics:
thermal expansion coefficient: 100×10
−7
K
−1
(between 25 and 300° C.),
softening point: 610° C.
strain point: 460° C.,
annealing point: 490° C.
Said glass of the Applicant Company bears the Code-name 8008. It was described in U.S. Pat. No. 4,540,672.
It has been determined experimentally that, in order to be fusible to said glass Code-named 8008, the glasses of the invention must have a thermal expansion coefficient between about 90 and 110×10
−7
K
−1
and a softening point between about 570 and 620° C.
Furthermore, it was desired that said glasses of the invention have:
a low tendency to devitrify and a high liquidus viscosity (≧10 Pa.s); which facilitates their forming and limits their costs of production; and
as much as possible, a good chemical durability.
The technical problem to be solved within the context of the present invention has therefore been that of providing glasses having the specifications below:
high refractive index: 1.70≦n≦1.78;
thermal expansion coefficient: between about 90 and 110×10
−7
K
−1
(between 25 and 300° C.);
softening point: between about 570 and 620° C.
low tendency to devitrify and high liquidus viscosity (≧10 Pa.s).
Within the context of an advantageous variant of the invention, it was sought that said glasses, having the specifications above, have furthermore a good chemical durability.
PRIOR ART
According to the prior art, numerous glasses having a refractive index higher than 1.7 and which are able to be used as segment glass are already known. On the other hand, to the knowledge of the Applicant, there exists few, even none of them which have the specifications indicated above.
In the Application FR 97 05357 of Apr. 30, 1997, the Applicant has itself described glasses which have the refractive index, the thermal expansion coefficient and the softening point sought after here. Said glasses have the composition by weight below:
SiO
2
27-36
B
2
O
3
0-9
Li
2
O
1-5
Na
2
O
1-7
K
2
O
0-7
CaO
0-7
BaO
4-13
ZnO
0-8
La
2
O
3
4-15
PbO
4-27
TiO
2
7-18
ZrO
2
0-9
Nb
2
O
5
0-8
with: Li
2
O + Na
2
O + K
2
O
6-15
SiO
2
+ TiO
2
+ ZrO
2
42-55
PbO + TiO
2
+ ZrO
2
+ Nb
2
O
5
29-40.
They do not however have optimized devitrification characteristics. In fact, amongst said glasses, the most efficient from this point of view only have a liquidus viscosity of greater than 3 Pa.s which can attain (for the preferred compounds) a maximum of 7 Pa.s.
As indicated above, the Applicant has sought to obtain glasses having a liquidus viscosity of at least 10 Pa.s; the forming, of such classes being largely facilitated.
The Applicant has, in an entirely surprising way, obtained glasses having the specifications above (thus having notably a liquidus viscosity equal to or greater than 10 Pa.s) by modifying a little the composition of those described in the Application FR-A-2 508 433. The glasses described in said Application have the composition by weight below:
SiO
2
28-<33
Li
2
O
0-1
Na
2
O
2-9
K
2
O
0-4
PbO
>28-34
BaO
0-<22
La
2
O
3
0-20
TiO
2
0-9
ZnO
0-4
ZrO
2
2-6.5
Nb
2
O
5
0-10
with: Li
2
O + Na
2
O + K
2
O
4-10.5
BaO + La
2
O
3
14-<22
TiO
2
+ Nb
2
O
5
>4-12
TiO
2
+ Nb
2
O
5
+ ZrO
2
6-17
They could not, a priori, lead obviously to the invention insofar as they have a softening point notably higher than that sought after: 640-700° C. instead of 570-620° C. (see the specifications above).
Furthermore, it is hereby stressed on the fact that there does not exist, to the knowledge of the person skilled in the art, any general rules for increasing assuredly the liquidus viscosity of an inorganic glass.
SUMMARY OF THE INVENTION
The glasses of the invention have the properties required for the specifications as given above. In other words, in order to have the specifications, the Applicant proposes a solution (glass compositions) which is entirely original.
The Applicant proposes novel glasses having the following composition of oxides, expressed in percentages by weight:
SiO
2
29-35
B
2
O
3
0.5-5
Li
2
O
0.5-3
Na
2
O
4-8
K
2
O
0-2
PbO
28-40
BaO
4-14
La
2
O
3
4-12
TiO
2
3-8
ZnO
0-8
ZrO
2
0-4
Nb
2
O
5
0-5
with: Li
2
O + Na
2
O + K
2
O
5.5-9.
REFERENCES:
patent: 4568651 (1986-02-01), Sagara
patent: 5162826 (1992-11-01), Morgan et al.
patent: 5204292 (1993-04-01), Michimata
patent: 5744409 (1998-04-01), Hashimoto et al.
Comte Marie M. J.
Marques Paulo
Corning S.A.
Group Karl
Peterson Milton M.
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