GB2059943A - Photochromic glasses - Google Patents

Abstract

A fast reacting photochromic alumino- phosphate glass having silver halide crystals dispersed throughout the glass, and a green colouration induced by the presence of a chromium oxide in the finished glass, wherein the finished glass also includes an oxide of antimony and/or an oxide of arsenic, and wherein the fading rate of the glass upon its removal from actinic radiation is such that in the course of fading from its fully darkened condition to its fully faded condition at least 25% of the total possible change in optical density of the glass occurs within 16 seconds. By including an arsenic oxide and/or an antimony oxide in a fast reacting alumino-phosphate photochromic glass composition in addition to a chromium oxide, one can overcome the deterioration in photochromic properties which is observed when a chromium oxide alone without antimony and/or arsenic oxide is added to such a glass composition.

Description

SPECIFICATION Photochromic glasses The present invention relates to photochromic glasses, i.e. to glass compositions which darken on exposure to actinic radiation and fade back to their original faded state when they are no longer exposed to such radiation.

In our British Patent Specification No. 1,367,903, we have described and claimed a range of photochromic glasses comprising at least 17% by weight P205 as one of the glass forming 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 R2O, where R=K, Na or Li. They can also contain up to 19% by weight B2O3, though most of the glasses disclosed contain no more than 3 to 7% B203. Glass compositions of this type which we have made have had a brown colouratin in the darkened state.However, although the glass compositions exemplified in the specification of British Patent No. 1,367,903 exhibit desirable photochromic properties, they have relatively slow responses to exposure and removal of actinic radiation, i.e. they have relatively slow darkening and fading rates.

In our British Patent Specification No. 1515642 we have described and claimed a range of photochromic glasses having faster responses, particularly a faster fading rate, than the glass compositions exemplified in our British Patent Specification No. 1,367,903. The glass compositions of British Patent Specification No.

1515642 are photochromic alumino-phosphate glasses having silver halide crystals dispersed throughout the glass and comprising, as non-photochromic components in weight percentages: SiO2 8.5 to 25% Al203 13to36.5% P205 7.5 to 33.5% B203 7 to 28% R2O 7 to 20.5% where R2O represents one or more of Na2O, K2O and Li2O, the maximum content of Li2O being 5%; the amount of SiO2 is not less than 16% when the B203 content is less than 8%; and, as photochromic components, expressed as weight percentages over and above the 100% total of all the non-photochromic components of the glass: silver, expressed as Ag2O not less than 0.05% Cl+Br 0.20 to 2% When heat treated to develop optimum photochromic properties as discussed in the above-mentioned patent specification No. 1515642, 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. These and other presently available fast response photochromic glasses utilized for ophthalmic and other applications exhibit a grey colouration upon activation with actinic radiation.The colour of these glasses is a function of the composition utilized to produce the photochromic glass, and of the thermal treatment utilized to convert the potentially photochromic glass as formed into a photochromic article exhibiting the desired sensitivity to actinic light.

There is in existence a market for tinted or coloured glasses. The tints or colours are generally produced by the addition of metal oxides which are well known to have particular colouring effects when added either alone or in combination with other oxides having a colouring effect. The addition of such oxides to produce a colouring effect in photochromic glasses has also been suggested and disclosed. Thus, for example, British Patent Specification No. 1,260,416 suggests the use of Mn2Os, NiO, Cur203, Fe2O3 or CoO as colouring agents for a copper-free photochromic glass such as a borosilicate glass. British Patent Specification No. 1,274,780 also describes borosilicate glass compositions which are photochromic and which have definite, adjustable colours in the irradiated state.Such colouration is achieved by adding to a melting batch of borosilicate glass or borate glass containing silver halide at least one element selected from vanadium, chromium, manganese and cobalt, the individual elements being in an electropositive valency state. It is therefore known that borosilicate photochromic glasses can be coloured by the addition of metal oxides. As stated above our main developments in photochromic glasses have not been in borosilicate glasses but in aluminophosphate glasses. In both alumino-phosphate and borosilicate photochromic glasses, a so-called second generation of glasses has recently been developed and placed on the market. This second generation of photochromic glasses refers to such glasses which have a fast response on darkening and fading in comparison with the photochromic glasses which were first made available in the marketplace.In the case of our alumino-phosphate glasses, the second generation alumino-phosphate glasses, the second generation alumino-phosphate glass has been marketed under the trademark "Reactolite Rapide" and has a composition which falls within the scope of the claims of British Patent Specification 1,515,642. Typical fading and darkening rates of this type of fast response alumino-phosphate glass are given in Table II below (see Glass 1).

There is a recognised market need for ophthalamic photochromic glasses with a tint other than the brown or grey colour usually associated with the presence of silver halides as photochromic components in a glass composition. It is clear from the art on the subject of tinting or colouring photochromic glasses that, while problems might be encountered in colouring photochromic glass, e.g. British Patent Specification 1,260,416 is restricted to the colouring of a copper-free photochromic glass, no suggestion has been made that there would be any difference in the effect which the various colouring oxides might have on the photochromic properties. Thus in the past the colouring oxides have all been treated as being equivalent in performance as regards their use for colouring photochromic glasses.

We have now found, in the case of the second generation of alumino-phosphate glasses of the kind disclosed in British Patent Specification No. 1,515,642, that in fact this is not so. It is, we have found, possible to obtain satisfactory grey colours with little or no change in photocromic properties when using cobalt and nickel oxides. This is not the case when an attempt is made to produce a green colour by incorporation of a chromium oxide or oxides alone or associated with other colouring oxides at a level necessary to obtain a green colour. In contrast to the position with other colouring agents, such as a nickel oxide or a cobalt oxide, while the desired colour and transmission in the clear or faded state are obtained as expected, there is an unacceptable deterioration in the photochromic properties of the glass and particularly in the fading rate of the darkened glass.

We have now found that by suitably modifying the base glass composition used in melting fast response alumino-phosphate photochromic glasses, one can prevent the deterioration in photochromic properties which occurs when a chromium compound is included in the batch composition to produce a green colour, or at least minimize such deterioration so that the resultant glass has acceptable fast response photochromic properties.

According to the present invention, there is provided a fast reacting photochromic alumino-phosphate glass having silver halide crystals dispersed throughout the glass, and a green colouration induced by the presence of a chromium oxide in the finished glass, wherein the finished glass also includes an oxide of antimony and/or an oxide of arsenic, and wherein the fading rate of the glass upon its removal from actinic radiation is such that in the course of fading from its fully darkened condition to its fully faded condition at least 25% of the total possible change in optical density of the glass occurs within 16 seconds.

Thus by including an arsenic oxide and/or an antimony oxide in a fast reacting alumino-phosphate photochromic glass composition in addition to a chromium oxide, one can overcome the deterioration in photochromic properties which is observed when a chromium oxide alone without antimony and/or arsenic oxide is added to such a glass composition.

Fast reacting alumino-phosphate glass compositions which can be suitably modified in accordance with the present invention are, for example, disclosed in British Patent Specification No. 1515642.

The green colour of the glass in its faded state will generally differ from the colour of the glass in its darkened state.

In this specification, the term "base glass composition" is used to mean the glass composition without colouring agents and without the additives necessary to produce the photochromic properties of the glass.

As a result, and in accordance with conventional practice in the art, the content of photochromic components and colouring agents is expressed over and above the 100% total of all non-photochromic components.

In general,the colouring agents are added as oxides.

The quantity of colouring agents added to the base glass composition to generate the chromium oxide or oxides, the arsenic oxide or oxides, and the antimony oxide or oxides in the finished glass will depend to a large extent on the transmission in the faded state which it is desired to achieve in the glass, and this in turn is dependant upon the thickness of the article made from the photochromic glass.It is unlikely that a glass with a transmission in the faded state of 40% or less in any thickness will be of any value for general use as a photochromic glass, and we prefer to aim for a transmission in the faded state of the order of 70% at 2 mm thickness and 25"C, with a transmission in the darkened state of the order of 15% to 20% at 2 mm thickness and 25 C. Such transmissions can be achieved with less than 0.45% by weight Cr203 in the finished glass, though generally less than 0.10% by weight Cr2 3 will be sufficient. The total quantity of oxides of antimony and/or oxides of arsenic required in the finished glass in order to avoid any unacceptable change in photochromic properties is of the order of 1% or less, although greater amounts of these materials may be present if desired.

In giving any guidance as to the appropriate levels of compounds to be added to the batch to give Cr203, As203 and/or Sb203 in the finished glass, it must be remembered that it is common practice in the glass industry to prepare glass compositions using continuous or semi-continuous processes in which one generally begins by preparing an uncoloured glass which is then changed to a coloured glass by addition of suitable additives to the batch of components which is about to be melted. Under these conditions, one or more additions of additives can be made so as to produce a glass of desired colour and light transmission.

This can be done by changing the quantity or type of additive until one obtains a glass having the desired properties. Even when a glass having the particular colour and other properties has been obtained, the glass must be monitored for any movement outside set tolerances and, if necessary, appropriate changes must be made to the batch composition so that the properties of the glass made from it are within the set tolerances.

Thus, for example, once the man practiced in the art knows that a Cr203 content of 0.01% (in the glass as analysed) will give a transmission in the faded state of the order of 70% at 2 mm thickness and 25"C in a fast fading alumino-phosphate composition which without Cr203 would have a transmission in the faded state of the order of 90%, and that a Cr2O3 content in the glass of 0.25% will correspondingly give a transmission in the faded state of the order of 40%, it is possible for him to take into account the presence of any other additives which may influence the colour and transmission and to arrive at an appropriate amount of chromium compound to add to the batch to obtain the particular transmission desired for the end product.

Examples are provided in Table I below to illustrate the effectiveness of the presence of Sb203 or As203 in the finished glass in maintaining a satisfactory photochromic response in the glass composition. The examples also illustrate the range of green colourations obtained during our experiments and hence give some further guidance to the man practiced in the art as to satisfactory levels of additives.

If desired, one or more other colouring oxides, such as those of nickel and cobalt, may be added to modify the tone of the green colour of the finished glass.

The following Examples illustrate, but do not limit, the present invention.

A convenient method for comparing the photochromic properties of glass compositions is to measure the percentage of the total possible change in optical density which occurs during standard darkening or fading conditions for a specified period.

For the purposes of this specification, the optical properties of the glass compositions are measured on standard samples of glass 2mm thick at 25"C, in standard simulated solar conditions at air mass 2 (see Parry Moon, J. Franklin Inst., 230 (1940), pages 583-617).

The fully darkened state of the glass compositions of the present invention is defined as the state reached by a standard sample of photochromic glass 2mm thick at 250C after exposure for 23 minutes to the above-mentioned standard simulated solar conditions at air mass 2.

Table I below gives the compositions and optical properties of a number of glasses. Glasses 1 to 5 are for the purpose of comparison, and glasses 6 to 14 are examples of glass compositions which illustrate but do not limit the present invention. The base glass composition in weight % was in all cases as follows: SiO2 17.7 P205 15.1 Li2O 1.4 CaO 2.9 TiO2 0.5 At203 27.6 B203 13.8 K20 10.9 BaO 9.3 ZrO 0.8 Each glass, after having been melted and formed, was heat treated at 659"C for 14 minutes.

In Table I, glass lisa fast reacting photochromic glass with a grey colouration without any colouring additives and its photochromic properties are of the same order as the glass available on the market under the trade name "Reactolite Rapide". Glasses 2 to 5 illustrate the effect of Cr203 on the fading rate of the finished glass while Glasses 6to 14 illustrate the invention and show that by addition of Sb203 Or AS203 the photochromic response as illustrated by the percentage of total possible fading achieved after 16 seconds (F 16) is of the same order as the rate of fading achieved with Glass 1.

TABLE I Glass Number 1 2 3 4 5 Colouring Agents and 5 Photochromic Compo nents(1) Cr203 - 0.041 0.052 0.024 0.024 10 Sb203 As203 CoO - 0.015 0.01 0.0029 - 15 NiO - 0.015 0.009 0.007 Ag 0.34 0.28 0.28 0.28 0.28 20 CuO 0.027 0.03 0.03 0.029 0.03 Cl 0.47 0.26 0.26 0.27 0.27 Br 0.20 0.22 0.21 0.22 0.22 25 % TRANSMISSION IN FADED STATE 89.6 66.0 67.4 79.3 77.3 30 COLOUR (a -1.9 -8.0 -10.2 -5.7 -10.95 CO-ORDINATES FADED GLASS (b 3.1 3.4 6.8 3.1 10.5 35 %TRANSMISSION IN DARKENED STATE 15.5 19.3 21.8 16.3 14.3 40 COLOUR (a 1.8 -2.7 -4.0 -1.4 -4.5 CO-ORDINATES DARKENED GLASS (b -2.6 0.9 3.3 -0.5 0.4 45 F16 35.4 15.4 16.6 18.9 18.0 TABLE I Continued Glass Number 6 7 8 9 10 Colouring Agents and Photochromic Compo nents(1) Cr203 0.010 0.062 0.045 0.063 0.064 Sb203 0.075 0.09 0.1 0.1 0.1 AS203 CoO - 0.0090 0.0117 0.0077 0.0041 NiO - 0.0021 0.025 0.010 0.033 Ag 0.35 0.338 0.334 0.304 0.312 CuO 0.041 0.032 0.033 0.032 0.034 Cl 0.39 0.44 0.41 0.48 0.40 Br 0.13 0.19 0.19 0.20 0.17 %TRANSMISSION IN FADED STATE 77.9 69.2 68.0 69.4 70 COLOUR (a -10.3 -7.9 -6.0 -7.9 -8.4 CO-ORDINATES FADED GLASS (b 7.7 2.9 3.2 3.9 8.7 %TRANSMISSION IN DARKENED STATE 20.7 13.2 13.4 15.2 16.2 COLOUR (a -1.9 -1.3 -0.7 -1.2 -2.0 CO-ORDINATES DARKENED GLASS (b 2.2 -1.6 -1.2 -1.4 1.3 F16 43.3 33.3 34.3 34.3 34.1 TABLE I Continued Glass Number 11 12 13 14 Colouring Agents and Photochromic Components(1) Cr203 0.094 0.094 0.090 0.097 Sb203 0.30 0.40 0.80 As203 - - - 0.80 CoO NiO Ag 0.355 0.36 0.341 0.35 CuO 0.048 0.044 0.048 0.035 Cl 0.46 0.42 0.43 0.44 Br 0.15 0.14 0.14 0.10 %TRANSMISSION IN FADED STATE 72.8 73.3 72.3 70.7 COLOUR (a -10.6 -10.5 -10.6 -10.9 CO-ORDINATES FADED GLASS (b 7.1 7.3 9.5 9.7 %TRANSMISSION IN DARKENED STATE 19.6 20.3 28.6 20.3 COLOUR (a -2.0 -3.7 -3.5 -1.4 CO-ORDINATES DARKENED GLASS (b 1.4 1.3 4.5 1.9 F16 37.3 32.5 41.4 37.4 Key 1: Measured as weight percentages over and above the 100% total of the base glass composition.

We have found that an appropriate system of values for monitoring the colour of the finished glass is the Hunter L,a,b Colour Scale System (see Measurement of Appearance by R.S. Hunter, pages 122 and 123), and the colour of the glasses given in Table i is defined using this system. Under this system a colour is generally defined as a point (a, b) on a plane by reference to two perpendicular axes a and b.

It should be noted that no variation has been made in the base glass composition of the various examples.

This has been done in order to reduce the number of variables and thus provide a satisfactory comparison.

However, although only one base glass composition is exempiified, it is considered that the invention is generally applicable to any fast response alumino- phosphate photo-chromic glass to which it is desired to impart a green colouration without any unacceptable change in photochromic properties due to the presence of a chromium oxide.

Table II below gives the F16, F600, D8 and D60 values of Glasses 1,2 and 7 of Table I. These values are the percentages ofthetotal possible change in optical densitywhich have occurred afterfading fromthefully darkened state for 16 and 600 seconds, and after darkening from the fully faded state for 8 and 60 seconds, respectively.

TABLE -II.

OPTICAL -1 GLASS NO; PROPERTIES 1. 2 7 F 16 35.4 15.4 33.3 F 600 91.4 85.9 83.9 D8 44 44.1 41.3 D 60 75.9 64.7 80.4 It can be seen from Table II that in using a comparison of this nature, the figure which can be used most clearly to indicate the change in photochromic response is the percentage of total possible fading achieved after 16 seconds (F16). We have found in the course of our experimental work that with the alumino-phosphate glasses investigated, there is at least a factor of 2 difference between the glasses which contain chromium oxides without either As203 or Sb203 present, and the glasses which contain a chromium oxide and As2O3 and/or Sb203 as regards the percentage of total possible fading achieved after 16 seconds.

Claims (8)

1. Afast reacting photochromic alumino-phosphate glass having silver halide crystals dispersed throughout the glass, and a green colouration induced by the presence of a chromium oxide in the finished glass, wherein the finished glass also includes an oxide of antimony and/or oxide of arsenic, and wherein the fading rate of the glass upon its removal from actinic radiation is such that in the course of fading from its fully darkened condition to its fully faded condition at least 25% of the total possible change in optical density of the glass occurs within 16 seconds.

2. A photochromic glass according to Claim 1, the glass being an alumino-phosphate glass having silver halide crystals dispersed throughout the glass and comprising, as non-photochromic components in weight percentages: SiO2 8.5 to 25% Awl203 13to36.5% P205 7.5 to 33.5% B203 7 to 28% R2O 7 to 20.5% where R2O represents one or more of Na2O, K2O and Li2O, the maximum content of Li2O being 5%; the amount of SiO2 being not less than 16% when the B203 content is less than 8%; and, as photochromic components, expressed as weight percentages over and above the 100% total of all the non-photochromic components of the glass: silver, expressed as Ag2O not less than 0.05% Cl + Br 0.20 to 2% the finished glass composition further including a chromium oxide, an oxide of antimony and/or an oxide of arsenic.

3. A photochromic glass according to Claim 1 or 2, wherein the amount of chromium oxide in the finished glass is not greater than 0.45% by weight.

4. A photochromic glass according to Claim 3, wherein the amount of chromium oxide in the finished glass is notgreaterthan 0.10% by weight.

5. A photochromic glass according to any one of the preceding Claims, wherein the total amount of oxides of antimony and oxides of arsenic in the finished glass is greater than 1% by weight.

6. A photochromic glass according to any one of the preceding Claims, wherein the glass includes a colouring agent which modifies the tone of the green colour of the finished glass.

7. A photochromic glass according to Claim 6, wherein the colouring agent is an oxide of nickel or an oxide of cobalt.

8. A photochromic glass according to Claim 1 substantially as hereinbefore described in the Examples as any one of Glasses 6 to 14.