Note: Descriptions are shown in the official language in which they were submitted.
~0~876
This invention relates to photochromic glasses,i.e. to glass compositions which darken on exposure to
actinic radiation and fade back to their original, normally
colourless, state when no longer exposed.
In our British Patent Specification No.1367903,
we have described and claimed a range of photochromic
glasses compris mg at least -17% by weight P205 as one
of the glass ~orming components, with silver halide
crystals dispersed throughout the glass, the total silver
content of the glass being at least 0.05% by weight Ag.
The specific glasses disclosed in that Specification are
alumino-phosphate glasses comprising not more than 40% by
weight SiO2 and between 9% and 34% by weight Al203 as
further glass forming components, and at least 10% by
weight R20, where R=K, Na or Li. They can also contain
up to 19% by wle~ght B203, though most of the glasses
disclosed contain no more than 3 to 7% B203.
Glasses falling within the claims of British Patent
1367903 are now used in the manufacture of ophthalmic
lenses for both sunglasses and prescription spectacles.
These alumino-phosphate glasses, like the photochromic
borosilicate glasses also available in the market, while
exhibiting desirable photochromic properties, have
relatively slow responses to exposure and removal of actinic
radiation, i.e. slow darkening and fading rates. It is
desirable, particularly for ophthalmic purposes, to have
glasses with faster responses, particularly a faster fading
rate. A rapid fading rate is desirable to aid in adjustment
to a sudden decrease in available light, such as when a
wearer of spectacles with lenses of photochromic glass
enters a dimly-lit room.
~ ' ' .
928~6
An object of the present invention is to provide a range of
photochromic glasses having impxoved properties and, in
particular, glasses which provide an improved combination of
photochromic effect (measured as the induced op-tical dcnsity or
change in light transmission when irradiated with actinic
radiation) and speed of response to irradiation or removal of
radiation.
The invention in one aspect comprehends a photochromic
alumino-phosphate glass having silver halide crystals dispersed
throughout the glass. The glass consists essentially of, in
weight percentages: SiO2 : 8.5 to 25%, A12O3 : 13 to 36.5%,
P2O5 : 7.5 to 33.5%, B2O3 : 7 to 28%, R2O : 7 to 20-5%~ R O :
0 to 21%, TiO2 : 0 to 6%, ZrO2 : 0 to 10%, PbO : 0 to 8%, where
R2O represents at least one of Na2O, K2O and Li2o, the maximum
content of Li2O being 5%, and R'O represents at least one of
MgO, CaO, SrO and BaO, within the following individual limits:
MgO : 0 to 4%, CaO : 0 to 6.5%, SrO : 0 to 10~, BaO : 0 to 21%,
the amount of Sio2 is not less than 16% when the B2O3 content is
less than 8%; said photochromic glass further comprising, as
photochromic components, in weight percentages expressed as
quantities over and above the 100% total of all the non-photo~
chromic components of the glass:
silver, expressed as Ag2O not less than 0.05%
Cl ~- Br 0.20 to 2.0%
Cl 0 to 1.0%
Br 0.08~ to 1.0%
These glasses have been found to have a good combination of
induced optical density on irradiation with actinic light and
rapid darkening on irradiation and rapid fading when irradiation
ceases. It will be understood that, as a general rule, the
darkening and fading times are longer when the induced optical
density is greater.
In these glasses, it is possible for A12O3, B2O3 or P2O5 to
876
be the largest consituent. The preferred range of glasses for
ophthalmic purposes is that in which the largest consti-tutent is
Al2O3 which is present in an amount of not less than 22 weight %,
the content of each of P2O5 and B2O3 is always less than the
content of A12O3 and is such that P2O5 does not exceed 25.5
weight % and B2O3 does not exceed 24.5 weight %. Glasses within
this preferred range can be formulated to have a fast response to
irradiation or the removal of irradiation, coupled with physical
properties which make them suitable for manufacture on a commercial
scale and for use as ophthalmic lenses. For example, the liquidus
temperature and viscosity of the molten glass can be chosen to suit
conventional forming processes, while the hardness of the glass is
appropriate for conventional grinding and polishing processes. The
refractive index can be adjusted to the standard value of 1.523
which is conventional for ophthalmic use, and the glass can have a
good chemical resistance or durability.
In general, it is only practicable to operate with
contents of both Al2O3 and SiO2 at the upper ends of the ranges set
out above in cases where a high viscosity is required at the liquid-
us temperature, which itself is not too high, for e~mple wherethe glass is to be formed into sheet glass.
Another range of glasses within the scope of the pre-
sent invention is that wherein the largest constitutent is B2O3
which is present in an amount of not less than 25 weight %, while
the content of each of Al2O3 and P2O5 is always less than the
content of B2O3 and is slch that Al2O3 does not exceed 20 weight
% and P2O5 does not exceed 20 weight ~.
A further range of glasses according to the present
invention is that wherein the largest constituent is P2O5 which
is present in an amount of not less than 21.5 weight %, while the
content of each of Al2O3 and B2O3 is always less than the content
of P2O5 and is such that A12O3 does not exceed 26 weight % and
B2O3 does not exceed 17.5 weight %.
-- 4
-` ``` -`` 109~87~ -
If the liquidus temperature is made relatively
low, e.g. as a result of the use of a relatively large
amount of B203 and a relatively small amount of SiO2, it is
important to keep a watch that the durability of the
glass (e.g. as tested by absence of attack in acid and
alkali solutions) is still acceptable. The degree of
durability which is acceptable will of course vary
according to the proposed use of the glass. Thus a
glass which has insufficient durability for ophthalmic
use but good photochromlc properties may be of value
for use in instruments or other uses where it is not
exposed to attack. ~ ;
When the B203 level approaches the lower limit,
i.e. is less than 8%, it lS necessary that the SiO2 con-
tent is at least 16% in order to ensure both the desired
fast response!land adequate durability for ophthalmic
purposes. With B203 below 7%, the P205 content must be
kept below 17%.
R20 may be constituted solely by K20, or by a
combination of two or more of K20, Li20 and Na20, or by
Na20 alone. Where R20 is Na20 alone, it should preferably
not exceed 14% by weight, to avoid possible problems in
glass forming and durability.
In the case of glasses intended for ophthalmic
use, it is advantageous for the glasses to be capable of
being toughened by the conventional ion exchange process,
in which larger metal ions are exchanged for smaller metal
ions in a surface layer of the glass to produce a compres-
sive stress therein. The ion exchange is effected by immer-
~0 sing the glass in a molten salt bath.Generally potassium ions
,d~ ' . . .
- : ~
109~876
are exchanged for sodium and/or lithium ions in a
bath of molten KN03, or sodium ions are exchanged for
lithium ions in a molten NaN03 bath. Thus where the
glass is to be chemically toughened in this way it is
preferred that the R20 component should include Na20
and/or Li20. We prefer to use a mixture of alkali
metal oxides, with K20 always present, and to keep each
of Na20 and Li20 below 5%. The depth of penetration
of the exchanged ions, and the compressive stress
produced, can be varied by varying the temperature
of the molten salt bath. In general, the greater the
penetration, the lower the compressive stress and vice
versa, so an advantageous compromise must be found by
experiment.
As indicated above, the silver content of the
; glass, expressed as Ag20, is not less than 0.05% by weight,
because with lower amounts of Ag20 it can be difficult
to achieve adequate darkening. Preferably the Ag20
is not less than 0.06%.
The~glass may further comprise from 1 to 21% by
weight R'O, where R'O re~resents at least one of MgO, BaO,
SrO and CaO, within the following individual limits:
MgO O to 4%
CaO O to 6.5%
SrO O to 10%
BaO O to 21%
For ophthalmic use, it is convenient for the
glass to have a refractive index (nD), measured for light
of the wavelength of the sodium D line, which is as close
as possible to the standard figure of 1.52~. To adjust
-6-
. ~- -
`~ ` lO9Z876
the refractive index to this figure, additions of proportions
of TiG2, ZrO2 and/or PbO can be of value, though care is
needed to avoid problems arising from too large a quantity
of one or more of these components. The amount of TiO2
used should not exceed 6% by weight, in order to avoid dangers
of crystallisation and unwanted colouration of the glass, the
normal preferred limit being 3% by weight. ZrO2 should not
exceed 10 weight % in order to avoid unacceptable increases
in liquidus temperature, the normal preferred limit being
10 7 weight %. PbO can be incorporated in quantities up to
8% by weight. Small quantities of other additives, such
as HfO2 (up to 3%) and ZnO (up to 6%) may be incorporated
for the same purpose. Tinting agents may also be added in
known manner, to provide a fixed tint in addition to the
variable photochromic colourir~.
; As is known, the photochromic effect is produced
by the silver hali~e crystals referred to above. Minor
amounts of copper oxides assist the development of the photo-
chromic effect, and larger amounts may be used to provide a
fixed tinting effect in addition. The preferred amounts of
the photochromic components, namely the silver (expressed
as Ag20), the co~per oxide and the halides (Cl and Br),
.. . .. _ .
_ . .
~ ' ~
- 1~92876
which are expressed in accordance with the normal
convention as quantities over and above the 100% total
of all other components of the glass, are as follows:
Ag20 0.06 to 0.60%
CuO 0.005 to 1.0%
- Cl+Br 0.20 to 2.0%
Cl 0 to 1.0%
Br 0.08 to 1.0%
In most cases, the photochromic effect can be
enhanced by heat treatment of the glass, the appropriate
heat treatment schedule being primarily determined by the
viscosity-temperature relationship of the particular glass.
In general, the heat treatment temperature lies between
the strain point and the softening point of the glass,
the heat treatment time required being several hours at
the lower temperature but only a few minutes at the higher
temperature. At the higher temperature, however, deforma-
tion and clouding of the glass may occur, so it is preferred
for convenience to use a ~temperature 20 to 100C above
the annealing point and a heat treatment time of 10 to
60 minutes.
The schedule may be imposed on the glass directly
after forming or the glass may be annealed and cooled to
room temperature before heat treatment. The cooling rate
to which the glass is subjected after heat treatment is
sometimes found to have an effect on the photochromic pro-
perties of the final product. This cannot be stated as a
general rule, however, and must be determined by experi-
mentation on individual glasses.
The temperature/time schedule imposed on a glassis
also determined by the concentrations of photochromic agents
-8-
,
.
~09Z876
in the gless and the photochromic property requirements of
the final product. In general, the higher the levels of
the components contributing to-the photochromism the
shorter will be the heat treatment schedule, and in some
cases, the photochromism may develop during cooling from
the melt or annealing of the glass. Excessively long
heat treatments are generally to be avoided because they
may lead to some clouding of the glass.
Specific embodiments of the invention will now be
described in more detail by way of example, and with
reference to the following Table I, which sets out
examples of glass compositions in accordance with the
invention, showing their compositions on the oxide basis
and the photochromic effect achie~-ed in terms of the
induced optical density (ODd) and the time ln seconds taken
to fade to a condition of half the total induced optical
density (~ OD FT), measured with standard samples of glass
2 mm thick, in standard simulated solar conditions at
air mass 2 (see Parry Moon, J. Franklin Inst., ~ (1940),
pages 583-617). The induced optical density is the differ-
ence between the optical density of the glass in the fully
darkened state and the optical density in the fully faded
state, the optical density being defined in the conventional
manner as log10 Ii , where Ii is the intensity of the incid-
ent light and It is the intensity of the transmitted light.
The induced optical density is thus a real measure of the
photochromic effect and is in fact directly proportional
to the number of photochromically activated silver atoms in
a given volume of the glass. The time required to fade from
the fully darkened condition to a condition of half the in-
duced optical density (2 OD FT) is thus an effective measure
` -` 109~876
for comparing fading times of glasses having different
values of light transmission in the bleached or faded
state and is comparable with the half-fading times
referred to in our earlier Specification No. 1367903.
Table I also lists the temperature (HTC) at
which each of the glasses was heat treated. A standard
heat treatment time of 20 minutes was used in each case,
for comparative purposes only.
Finally Table I lists the refractive index nD
of most of the glasses.
-10-
` `` `-`- 109:2876
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109~876
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-15-
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---` ``- 109;2~37
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--24--
-`- 1092~76
The following Table II lists a series of photochromic
glass compositions according to the invention which can
be chemically toughened by ion exchange as mentioned above with
the compressive stress in pounds per square inch and depth
of penetration in microns achieved when the ion exchange
is carried out by immersion for 16 hours in a molten KN03
bath at 470C, as well as the photochromic properties of
the toughened glasses. In the case of glasses 174, 175,
and 178, the exchange is of potassium ions for sodium
10 ions. In glass 176, potassium ions are exchanged for
sodium and lithium ions. In glasses 177 and 179,
potassium ions are exchanged for lithium ions. It can
be seen that the chemical toughening process does not
affect the photochromic properties, e.g. by comparing the
properties of glass 174 with the very similar glass 71
in Table I.
- 25 -
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-26 -
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`-` lO~Z876
The compositions listed in the Tables can be made
up in the following manner. The batch is melted under
oxidising or neutral conditions'-at a temperature in the
range 1200 -to 1600C, and after cooling is annealed at
a temperature between 450 and 650C. A final heat
treatment may subsequently be effected at between 20
and 100C above the'annealing point for a period of 10
to 60 minutes. The optimum heat treatment temperature
range for a particular glass may be determined by a
10 gradient furnace technique. In some cases, it may be
necessary to support the glass~during heat treatment to
avoid sagging.
The batches can be made up from conventional glass-
making raw materials, such as carbonates, meta- or ortho
phosphates, nitrates and oxides. The silver and halide
components may be added to the batches in the form of
finely-ground si~ver salts and sodium or potassium halides,
respectively.
Precautions are requi~d during melting to minimise
20 volatilisation losses of batch components. Up to 60%
by weight of the halide components and 30% by weight of
the silver may be lost in this way and the necessary
allowances are required during batch preparation.
The glasses disclosed above have a useful combination
of photochromic effect, measured as induced optical density,
with speed of response to exposure to, or removal of,
actinic radiation. Although in some glasses it will be
seen that the induced optical density is not high, the
speed of response in those glasses is particularly rapid.
30 The glasses can be used for ophthalmic purposes and for other
àpplications where temporary protection from actinic radiation
such as sunlight is required with a return to normal trans-
mission ~hen the actinic radiation is absent. Theymay thus be
used for glazing in buildings or vehicles in some circumstances.
- - 27 -
.
109f~876
The production of photochromic properties in a
glass is associated with the formation of silver halide t
crystals in the glass matrix in a form in which they
are sensitive to actinic radiation. Hence the glass t
maker is not only faced with the problem of choosing L
a glass composition which can be melted and formed satis-
factorily in a particular commercial process, but also the
problem of achieving this in a glass in which silver halide ~-
crystals will be produced in radiation-sensitive form, so
10 as to give the glass satisfactory photochromic properties. ..
Many suggestions have been made to explain the behaviour e
of the silver halide crystals in the glass matrix, and
British Patent Specification 1,428,880 even suggests that
in some circumstances and with certain phosphate glass
compositions, the silver halide may be present in the
glass matrix in non-crystalline segregation phases.
In view of th!e!large number of components it is possible
to incorporate in a glass composition, it is in practice
impossible to investigate fully all the permutations and :
20 combinations of even a selected area of glass compositions
such as is defined in a patent application for a simple
g-lass composition not involving the behaviour of further
additives. The problem is increased in the case of com-
positions where a further physical effect is produced
by the addition of other additives, such as those of the
present invention. We have made a large number of glasses
in the course of our investigation of the composition area
claimed in this application. In the examples selected
from this work and listed in the Tables above to illustra-te
30 our invention, we have in particular illustrated the wide
- 28 -
.
.,~ ,
,
"` 10~287~
variation in composition which is possible within the
defined area in terms of the major glass forming components
A1203, B203 and P205. We have iilustrated how, with
this wide variation, glasses can be obtained with a
good combination of induced optical density on irradiation
with actinic light coupled with rapid darkening on
irradiation and rapid fading when irradiation ceases.
As indicated above, we prefer to operate with A1203
as the major component. Examples are included to
illustrate this for varying relationships of B203 to
P205, i.e. from B203 greater than P205 to B203 equivalent
to P205, and on to where P205 is greater than B203. We
have also provided examples to indicate that it is
feasible to make suitable glasses with either B203
or P205 as the major component. The examples further
illustrate the!possible variations within these ranges,
i e B203~PAl2o3~ P205 and B203~ P2 5 2 3
2 5 A1237 B20~ and P205 7B203~A12o3.
The level of SiO2 in the composition has little
or no effect`on the photochromic properties of the glass
. but does enable one to adjust the forming properties of
the glass, and can, for example, be important in
~ achieving a glass which can be easily toughened by chemical
; means. Hence the adjustment of silica level to accommodate
changes in the other major components (A1203, P205, B203)
is a matter of applying the ordinary skill of the glass-
maker, and the knowledge of the known effects on a glass
` composition of such changes.
Examples are provided in Table I to exemplify the
limits of the permissible ranges for the major components,
.
29
' .
`` 10~87t;
but in addition examples of glasses ln which the major
components are not at the limits of ranges are included
to help to guide the practical glass maker to those areas
where the most useful glasses can be obtained and to indicate
that a large number of glasses exist and have been tested
to identify and prove the valuable compositional area which
is the basis of thls invention. The Examples are in no way
intended to establish discrete areas within our broad dis-
closure in which the advantages of our invention are obtained
10 but to demonstrate that glass-compositions may be selected
over the total area with-a particular preference for
selecting glasses in which A1203 is the major component.
The selection of a suitable base ~lass composition must also
be accompanied by aelection of appropriate quantities of
the photochromic additives, Ag20, CuO, Cl a~ Br. The
possibility of varying the quantities of these additives
in the same base !c~mposition is demonstrated in, e.~. Examples
43 to 49. Other variations in this co~position are shown
in Examples 7, 8, 9, 57 and 58. It will be seen that, in
20 general, with an increase in the level of Ag20 there is an
v increase in induced optical density. It is there~ore
importa~ in selecting a suitable base glass composition
also to experiment with and adjust the level of photochromic
additives to give a desired induced optical density in any
particular glass.
As mentioned above, a final heat treatment may be
i e~fected, and there may be with some compositions a need
to investigate the effect of changes in both the time and
temperature of the heat treatment to cause the separation
30 of silver halide crystals in the glass matrix so as to
achieve an optimum performance from the particular glass.
This can be conveniently done using a sample rod of the glass
.
`` 10~2~37~
` cast in a gradient furnace. Examples showing a variation
.
- in heat treatment temperature wlth some variation in
~- photochromic additives while maintainlng almost the same
base glass composition include Examples 12, 50 to 56, 59 to
61, and 72 to 74.
Further adjustments may be needed in the level of-
- - photochromic_additives and the conditions for heat treatment
if a composition is further adjusted by composition changes
to give a desired`refractlve index such as 1.523. The
adjustment of a glass tb the standard ophthalmic refractive
index of 1.523 + O.D01 can be seen to be feasible with
the glasses of the present invention. The majority of our
Examples in Table I where the index is or has been corrected
to 1.523 + 0.001 are in the area~where Al203 is the major
component in the composition, as this is the area where
the combination of properties achieved has been found
most advantageous ~or commercial scale production of
ophthalmic glasses, but it will be seen that Example 173
also has such a refractive index in a glass composition
in which B203 is the major component.
, .
.~ " . .
,
:,'
~ -31-
