High refractive index glasses, multifocal corrective lenses...

Details High refractive index glasses, multifocal corrective lenses... High refractive index glasses, multifocal corrective lenses...

2000-06-02

2001-03-06

Epps, Georgia (Department: 2873)

Optics: eye examining, vision testing and correcting

Spectacles and eyeglasses

Ophthalmic lenses or blanks

C351S172000, C501S076000, C501S079000, C501S903000

Reexamination Certificate

active

06196684

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 several 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 dilation 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 dilation 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 that, in order to be fusible to said glass Code-named 8008, the glasses of the invention must have a thermal dilation coefficient between about 90 and 110×10
−7
K
−1
and a softening point between about 570 and 620° C.
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 dilation coefficient: between 90 and 110×10
−7
K
−1
(between 25 and 300° C.);
softening point: between about 570 and 620° C.
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 of them which have the specifications indicated above; i.e. which combine a refractive index higher than 1.7 and properties of dilation and viscosity such as stated above or close to those stated above. Thus, the Applicant has picked out in said prior art:
U.S. Pat. No. 4,568,651 (HOYA) describing glasses having the following composition of oxides expressed in percentages by weight:
SiO
2
14-24
B
2
O
3
 6-14
Al
2
O
3
0-4
SiO
2
+ B
2
O
3
+ Al
2
O
3
26-34
MgO
 0-10
CaO
 0-15
BaO
 2-24
SrO
 0-17
ZnO
 0-12
BaO + SrO + CaO + ZnO
15-30
PbO
38-50
ZrO
2
 0-3.
These glasses have an index between 1.74 and 1.78, a thermal dilation between 82 and 95×10
−7
K
−1
, and a softening point between 560 and 640° C.
These properties are close to those which were sought after within the context of the invention. However, the composition of the glasses described in said U.S. patent is completely different from that of the glasses of the present invention. These prior art glasses have a very high PbO content and are devoid of alkalis and TiO
2
;
the Japanese patent JP-A-61 146730 (NIPPON KOGAKU KK) describing glasses having the following composition of oxides, expressed in molar percentages:
SiO
2
 6-30
B
2
O
3
15-44
Li
2
O
 7-18
Na
2
O + K
2
O
0-3
MgO + CaO + SrO + BaO + ZnO
 0-35
La
2
O
3
 3-10
ZrO
2
3-5
Nb
2
O
5
 3-8.
These glasses have an index of 1.7 to 1.78, a dilation of 98 to 114×10
−7
K
−1
and a softening point of 548 and 588° C. However, here again, the composition range is very different to that which is claimed here; these glasses contain neither TiO
2
, nor PbO.
The U.S. Pat. No. 4,824,809 (SCHOTT) and the three Japanese patents JP-A-8 26765, JP-A-8 26766 and JP-A-8 25957 (NIKON CORP.) may also be mentioned which describe, respectively:
for the US patent, glasses having a refractive index of 1.73 to 1.78. The dilation and viscosity of said glasses are not specified. These glasses have contents in PbO and, in TiO
2
and in alkalis, respectively much higher and much lower than those of the glasses of the invention;
for the JP patents, glasses having refractive indices and softening points close to those of the glasses of the invention. The thermal dilation coefficients of said glasses are not specified. The composition ranges of said glasses are very different to that claimed here: the glasses in particular contain neither any PbO, nor any TiO
2
.
In order to have the specifications such as stated above, the Applicant therefore proposes an entirely original solution (glass compositions).
SUMMARY OF THE INVENTION
The Applicant proposes novel glasses having the following composition of oxides, expressed in percentages by weight:
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.
The glasses of the invention are obtained essentially from oxides identified above, taken in the amount specified above. It is certainly not excluded that other oxides intervene (vide infra) but, in any case, said other oxides do not intervene in significant amounts (always ≦5% , generally ≦3% by weight, even ≦1% by weight) and do not have a significant influence upon the properties sought after. They only intervene for adjustments.
DESCRIPTION OF THE INVENTION
The principal constituents of the glasses of the invention are SiO
2
, PbO, TiO
2
and La
2
O
3
. Said glasses also contain Li
2
O, Na
2
O and BaO. As regards the optional components (B
2
O
3
, K
2
O, CaO, ZnO, ZrO
2
and Nb
2
O
5
), it is specified here, to all useful ends, that the minimal intervening amount from which they exert a significant effect is generally in the order of 0.5%. Thus, the glasses of the invention cannot contain optional constituents or, if they do contain them, it is generally in a minimal amount of 0.5% (% by weight).
According to an advantageous variant of the first object of the invention, the glasses have the following composition of oxides expressed in percentages by weight:
SiO
2
28-35
B
2
O
3
2-9
Li
2
O
2-5
Na
2
O
2-6
K
2
O
0-5
BaO
 5-13
ZnO
0-7
La
2
O
3
 4-14
PbO
10-27
TiO
2
 9-15
ZrO
2
0-4
(obviously with, as indicated above:
Li
2
O + Na
2
O + K
2
O
 6-15
SiO
2
+ TiO
2
+ ZrO
2
42-55
PbO + TiO
2
+ ZrO
2
29-40
(no Nb
2
O
5
))
The high indices of the glasses of the invention are obtained by maintaining the sum of the TiO
2
, PbO, ZrO
2
and Nb
2
O
5
contents between 29 and 40%. The elements used preferably for obtaining this index are TiO
2
and PbO. This is why the minimal content of these elements must be 7 and 4% respectively. ZrO
2
, in addition to its role upon the index, enables improving the chemical durability. The high cost of the starting materials which are sources of Nb
2
O
5
renders this oxide less interesting than the previous ones. Too high a TiO
2
, ZrO
2
or Nb
2
O
5
content causes a devitrification at low viscosity , while too high a PbO content is unfavorable for the density and the dispersion.
SiO
2
, TiO
2
and ZrO
2
are the elements which most increase the softening point. It has been observed that, in order to obt

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