Note: Descriptions are shown in the official language in which they were submitted.
10~15S8
Multifocal ophthalmic lenses are manufactured by sealing
one or more small segments or buttons of a high refractive index
glass into a recess in a major lens blank molded from a crown glass
in a manner well-known to the ophthalmic glassmaking art. Hence,
in conventional practice, the major lens blank is made from a
spectacle crown glass having a refractive index of 1.523 and the
segment or button glass has a higher refractive index, commonly in
the range of 1.57-1.71, the selection thereof being dependent upon
the degree of visual correction demanded in the finished multi-
focal lens.
A recent development in the ophthalmic lens field hasinvolved the introduction of photochromic lenses. Photochromic
glasses, or phototropic glasses as such have been variously called,
are described in United States Patent No. 3,208,860, the basic
patent in the field. As is explained therein, photochromic glasses
darken under the influence of actinic radiation, normally ultra-
violet radiation, and will return to the clear state when the
actinic radiation is removed. Such glasses, index-corrected to
1~523, have been, and are presently being, marketed as ophthalmic
lenses by Corning Glass Works under the regis~ered trade marks
PHOTOGRAY ~ and PHOTOSUN ~
Up to the present time, these glasses have been mar-
keted as "single vision" lenses, i.e., as a single lens having a
refractive index of 1.523. Hence, there has been a need for a
higher refractive index glass which could be sealed to the major
lens blank of photochromic glass to yield a multifocal lens. Such
a glass would exhibit a softening point between about 630-675 C.,
a strain point between about 450-500C., and a coefficient of
thermal expansion (25-300C.) between about 45-60 x 10 7/C. to
insure satisfactory fusing to the photochromic major lens blank
while developing a low stress level seal therebetween, as well as
- possessing the complicated matrix of physical characteristics
demanded of an ophthalmic glass.
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~041S58
Lead flint glasses were originally devised to serve as
higher index segments for multifocal lenses. Examples of such
glasses are illustrated in United States Patent No. 2,528,634.
Barium flint glasses have also been developed for that purpose and
examples of those can be found in United States Patents Nos. ~ - -
2,523,264, 2,523,265, and 2,523,266. The compositions disclosed
in those patents have been proven very useful for the ophthalmic
glass industry. Nevertheless, such have not been suitable for
sealing to photochromic glasses of the type mentioned above mar- ~ -
10 keted by Corning Glass Works of Corning, New York, under desig- -
nations of Corning 8097 and 8098, since the softening temperatures
thereof are too high and/or the coefficients of thermal expansion
are too high.
Therefore, the primary objective of the instant inven-
tion is to provide glasses having a refractive index between about
1.58-1.71, a softening point of about 630-675C., a strain point
of about 450-500C., and a coefficient of expansion of about
45-60 x 10 7/C. thereby rendering them particuIarly suitable as
segments for ophthalmic, multifocal photochromic spectacles.
2Q This objective can be achieved with glasses consisting
essentially, by weight on the oxide basis, of about 3-8% A12O3, -
3-7~ B2O3, 30-60% PbO, and 30-50% SiO2. These glasses are not
photochromic in themselves. However, inasmuch as the button
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comprises but a small part of the composite multifocal lens, it
does not materially detract from the photochromic character sup- `--
plied by the major lens blanks when sealed thereto.
In one aspect of this invention there is provided an '; ~ -~
ophthalmic glass exhibiting a refractive index between about 1.58-
- 1.71, a softening point between about 630-675C., a strain point
3Q between about 450-500C., a coefficient of thermal expansion
(25-30QC.~ between about 45-60 x 10 7~oc., and no surface im-
perfection with a weight loss of no more than about 0.03 mg/cm2
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2 -
104~5S8
after immersion into a 10% by weight aqueous solution of HCl for
ten minutes at 25C., consisting essentially, by weight on the
oxide basis, of about 3-7% B2O3, 3-8% A12O3, 30-60% PbO, and
30-55% SiO2.
Although operable glasses can be produced employing the ~ -
simple quaternary system, various metal oxides are preferably in-
cluded to aid in melting and forming the glass, improving the
chemical durability, altering the refractive index, etc. For
example, minor additions of La2O3 can steepen the viscosity curve
of the glass and increase the acid durability thereof, while re-
taining or even raising the refractive index. These additions
permit the B2O3 content to be increased to result in an overall
softening of the glass at the temperature where it is sealed to
the photochrormic major, while maintaining an adequately high
strain point and a sufficiently low coefficient of thermal expan-
sion to secure a low stress level in the seal. In general, the
total of La2O3 will be held below about 6%. BaO in amounts up
to about 10% can be substituted for PbO. Such substitutions will
maintain the high refractive index and beneficially increase the
dispersion of the glass. Ho~ever, BaO hardens the glass so an
excess must be avoided to insure satisfactory fusion of the seg-
ment to the photochromic major. Upon to about 3% total of Na2O
and~or Li2O can be useful in optimizing sealing characteristics
although some caution must attend their use inasmuch as their
presence tends to raise the coefficient of thermal expansion of
the glass. TiO2 behaves similarly to La2O3 with respect to the
viscostty of the glass and the improvement in durability. Never-
theless, more than about 3% TiO2 significantly limits the trans-
mission of ultraviolet radiation through the segment which, in ~c
turn, inhibits the photochromic reaction in the major.
In summary, useful additions to the quaternary baseglass include: up to 6% La2O3, up to 10% BaO, up to 3% Li2O
- 3 -
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1041558
and/or Na2O, and up to 3~ TiO2. In no event, however, will the
sum of all constituents outside of the base quaternary exceed
about 10~.
A careful balance is required to be maintained between
the A12O3 and B2O3 contents. At least 3% A12O3 is demanded to
attain a liquidus below 1100 C., but the presence of A12O3 adverse-
ly hardens the glass at fusing temperatures. That effect can be
offset through additions of B2O3.
United States Patents Nos. 2,393,448 and 3,493,405 dis-
10 close glass compositions similar to those operable in the instant ~ -
invention. The former patent describes glasses having very low
power factors, viz., less than 0.07%, temperature coefficients less
than 100 parts per million/C., and high dielectric constants ren-
dering them especially suitable for electrical insulating compo-
nents. The latter patent discusses glasses suitable for encapsu-
lating semiconductor devices.
The glasses of the former are alkali-free, and consist
essentially, by weight, of 5-15% A12O3, 5-65% B2O3, 20-40~ PbO,
and 10-50% SiO2. Such broad ranges do, indeed, overlap the -
strictly-defined limits of the present invention. However, there
is no suggestion of utilizing such glasses as high refractive index ; ~ -
opthalmic segments suitable for multifocal photochromic lenses and -
no working example recited therein even closely approaches the
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compositions demanded in the instant invention.
The glasses of the latter patent are also alkali-free
and consist essentially, by weight, of 4-15% A12O3, 0-15% B2O3
45-60~ PbO, 25-40% SiO2, and 2-8% ZnO. Here, again, there is no
disclosure of the utility of those glasses as high rèfractive
index segments for opthalmic multifocal photochromic lenses and,
furthermore, ZnO is cited as a required component. As is shown
infra, the inclusion of ZnO in the glasses of the present inven- '~
tion so deleteriously affects the chemical durability thereof as
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10415S8
to preclude their utility as ophthalmic ~uttons.
Finally, United States Patent No. 3,801,336 is stated to
disclose glass compositions specifically designed to perform as
segments for ophthalmic photochromic glass lenses. The composi-
tions described therein contained about 18-30% B2O3, 20-32~ SiO2,
31-5% PbO~ 9-6~ A12O3, 2-2% BaO, 0-3~ TiO2, 0-5% La2O3, and 4.8%
ZnO. Such glasses are obviously outside the A12O3 and B2O3 ranges
of the present invention and, furthermore, contain ZnO, an unwanted
component in the instant glasses.
10Table I reports a group of glasses, expressed in parts
by weight on the oxide basis, which are operable in the present
invention. The batch ingredients therefor can consist of any
materials, either the oxides or other compounds, which, upon being
melted together, will be converted to the desired oxide in the
proper proportion. The batch ingredients were carefully blended
together and then run into platinum crucibles. The crucibles were
placed into a furnace operating at 1400-1500C. and the batch
melted for four hours. The melts were stirred for one hour to
remove cords, seeds, and other inhomogeneities. The melts were
2Q poured into steel molds to yield square glass slabs about 6" x 6"
x 1/2" and these slabs immediately transferred to an annealer
~operating at about 550C.
; The table also records the standard viscosity-temperature
measurements of softening point (Soft Pt.), annealing point (Ann.
Pt.) and strain point (Str. Pt.~, each in C., the coefficient-of
thermal expansion over the range 25-300C. (Exp.), density in
g/cc, refractive index ~nD), liquidus temperature (Liq.) in C.,
the power factor (P. F.) determined at one megacycle at room tem-
-perature, the dielectric constant (D. C.) determinated at one mega-
cycle at room temperature, and the temperature coefficient ~T. C.)
over the range of room temperature to 100 C. in P.P.M./ C. Each
of these values was determined utilizing techniques
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10415S8
conventional in the glass art. A measure of the chemical durabil-
ity in terms of weight loss in mg/cm (Wt. Loss) and in appearance
is also tabulated.
The chemical durability of the instant glasses was eval-
uated utilizing a test method standard in the ophthalmic industry -
denominated as the AO test. The procedure contemplates five
general steps. First, the surface area of a glass sample (nor-
mally polished) is measured. Second, the sample is weighed.
Third, the sample is immersed into a 10~ by weight a~ueous solution
of HCl for 10 minutes at 25C. Fourth~ the sample is withdrawn
from the HCl solution, washed, and dried. Fifth, the sample is
reweighed and the differencein weight calculated as "weight loss"
in mg/cm2 of sample surface. Any haze, iridescence, or other sur-
face phenomenon is observed visually. 2
A weight loss of no more than about 0.03 mg/cm with no
surface imperfection noted visually is conventionally deemed ,~ -~
required to successfully pass that test. Hence, under Appearance,
N. C. designates no change and Irid. indicates iridescence observed. -~
Arsenic oxide and/or antimony oxide constituted the -
fining agent (F. A.), for the recited glasses. Other fining
agents known to the glassmaking art could be substituted therefor.
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Table II reports glass compositions, melted and formed
in like manner to those set forth in Table I, but which are close
but outside the ranges prescribed in the instant invention. A com-
parison of the resultant properties clearly illustrates the criti-
cality of the compositional parameters which must be observed to
achieve the desired physical properties. Hence, even very minor
modifications can alter the viscosity relationship, the chemical
durability, the liquidus, etc. of the glass.
TABLE II
21 22 23
SiO2 41.0 32.6 34-3 25.0
B2O3 3.8 5.0 0.4 16.6
A123 2.8 6.9 4.4 15.2
PbO 52.4 53.0 60.9 43.3
Li2O - 0.2
ZnO - 2.0
F. A.~ 0.3 0.3 0.3
Soft. Pt. 671 630 679 650
Ann. Pt. 495 478 518 510
20 Str. Pt. 458 444 483 478
Exp. 45.5 60.3 57.4 47.9
Density - 4.039 4.392
nD 1.64 1.67 1.706 1.635
Liq. 1270
Wt. Loss 0.01 0.03 0.02 0.29
Appear. N. C. Irid. N. C. Irid.
As can be observed, glasses 1-19 exhibit the necessary
physical properties for ophthalmic elements. Such glasses also
demonstrate high power factors and temperature coefficients.
30 Example 11 having a nD of 1.5882, Example 12 having a nD f
1.653, and Example 16 having a nD of 1.700 provide three index
steps which are especially useful as segments in multifocal ophthal-
.....
11
1041SS8
mic lenses. Their melting and forming behavior, coupled with their
physical properties, including chemical durability, recommend them
for preferred compositions. Seal stress upon fusion to Corning ~:
8097 photochromic glass is essentially nil. s~
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