GB2146322A - Phototropic glass - Google Patents

Abstract

A phototropic silicate glass which, in 2mm glass thickness at 23 DEG C has a transmission in non-light exposed state of >/= 86%, after 10 minutes light exposure a transmission of < 28% and a brightness recovery half time of 2.5 minutes, consists of SiO2, B2O3, > 6% ZrO2, alkali-metal oxide, alkaline earth oxide and 0-12% other oxides, the molecular ratio of SiO2 to B2O3 being less than 4.0 and the molecular ratio of ZrO2 to alkalioxides being from 0.22-0.37. Typically SiO2 = 45-62%, B2O3 = 15-20%, ZrO2 = 6-8.5%, alkali metal oxide = 4-15%, alkaline earth oxide = 0.2-10%.

Description

SPECIFICATION Phototropic glass Phototropic glasses are mostly used for spectacle lenses and have been described in numerous publications. They possess, measured in 2 mm glass thickness at 23"C in accordance with standardised procedures, certain characteristic phototropic data which, at best, are as follows: Initial transmissivity (AT) in non-light-exposed state 88 % saturation transmission (ST) after 10 minutes light exposure 27 % recovery halftime (RHWZ) at end of exposure 3 min.

recovery after 30 min (R30) after termination of exposure 70 % ; (Although the Lambert-Beersche Law is not equally applicable to phototropic glasses as it is in respect of tinted glasses, for glass thicknesses other than 2 mm, the interrelation depicted in Fig. 1 is valid so that it is possible to refer to a uniform comparison thickness.) The most important properties of phototropic lens glasses are the achievable change in transmission and the recovery speed after darkening. With a constant initial transmissivity in non-light exposed state, the achievable change or variation in transmissivity can be observed by saturation transmission whilst recovery speed can be read from recovery half-time.A simple characteristic index for judging the phototropism of a glass was found in the product of saturation transmission in % and recovery half time in minutes: P = ST x RHWZ.

A graphic illustration of today's known, commercially available phototropic spectacle lens glasses (Fig. 2) shows that the best glasses come to a value of P = slightly more than 50. The curve for P = 50 in Fig. 2 represents the limit according to the current state of the art in phototropism.

The use of the index P as the product of saturation transmission and recovery halftime particularly takes into account the fact that for equal glass thickness and temperature darker glass mostly recover more slowly and lighter glasses mostly recover more rapidly after light exposure. It is desired to obtain glasses with a transmissivity of less than 28% after exposure, irrespective of recovery speed and glasses which recover lightness at a speed no slower than that corresponding to a recovery half time of 2.5 minutes, irrespective of the previously reached saturation transmission.

It is the aim of the invention to provide phototropic glasses whose phototropic properties are characterised by a product P which is less than or equal to 45, preferably 40, and whose saturation transmission (2 mm at 23"C) is either between 1 2 and 20% or between 24 and 27%, using preferably the same basic glass for both ranges of differential transmissivity variations. These two ranges are hatched in Fig. 2.

A further objective of the invention are phototropic glasses which, by virtue of the glass composition, have such technological properties that, with appropriate viscosity-and crystallization values, a continuous production of spectacle lens glasses is effortlessly possible by melting, cutting and pressing using conventional production plant.

In borosilicate glasses, the tendency for phase-separation in the basic glass as influential factor in respect of the later precipitation or separation of the silver-halide containing phase as phototropic carrier agent is generally influenced by the ratio of silicic acid to boric acid. The quotient Six2/8203, which must be calculated on a molecular basis, should be < 4.0 for glasses according to this invention. Thus the objectives of the invention may be realised with the glass compositions according to this invention, particularly such with a high zirconium content.

In glass chemistry zirconium oxide has become known primarily as a seeding agent for the initiation of crystallization (e.g. in vitreous ceramics). Higher zirconium percentages, for example over 3 per cent by weight in a batch, will, according to past experience, result in increased crystallization of such a glass. For this reason, high zirconium-oxide contents are generally undesirable in order to avoid technical problems. However, it was actually found that zirconium oxide contents over 6 wt.% in the compositions according to this invention have no adverse effect on crystallization but that a higher zirconium content has a positive effect on the demixing behaviour of the basic glass.Through this demixing behaviour of the basic glass, a second phase separation process can be influenced which leads to the precipitation of the silver halide phases so that in the final effect the zirconium oxide content affects the quantity of precipitated silver halide phase, the texture of such precipitation and thus the phototropism itself.

In respect of the importance of the phase separation in the basic glass for phototropism, the concentration of the alkali oxides in the compositions according to this invention is important. In particular, the kinetics of the biackening and recovery processes can be influenced by the alkali metal oxide.

It was found that the ratio of zirconium oxide to the sum of the alkali metal oxides represents a good identifying index for the compositions according to this invention, Calculated on a molar basis, the quotient ZrO2/ER20 should be between 0.22 and 0.37.

In the application of phototropic glasses for spectacle lenses, two different groups were found interesting for commercial purposes.

The glass type of one group has a saturation transmissivity between 24 and 27% at 2 mm and 23"C; the other has a saturation transmissivity of 12 to 20% at 2 mm and 23"C.

By means of the present invention it is possible to produce both glass types from the same glass solely by varying the silver-halide concentration. This has great technological advantages because it means that a change over from one type to the other does not require changing the glass in the melting unit (glass tank).

Whilst for a characterisation and identification of the glass compositions, the synthesis composition may well be used in respect of the basic glass as is common practice in glass making because there are no significant losses from the composition, e.g. due to evaporation, during the production of the glass, a different type of characteristic identification must be used in respect of the silver, chlorine and bromine components. For, although nowadays large tank furnaces for phototropic glass operate with less than 10% losses in respect of these three volatile elements, in the experimental crucible vapourisation losses may easily come to over 50%.For this reason, an analytical method must be applied for characteristic identification of the glasses according to this invention in respect of their content of silver, chlorine and bromine The simplest and at the same time accurate method is that of X-ray fluorescence analysis which again operates by composition-appropriate standards which are calibrated titrimetically in accordance with wet-chemical testing methods. All concentration data for silver, chlorine and bromine given in this application have been obtained in this fashion.

The elements silver, chlorine and bromine are determined successively argentometrically by a single titration after a special decomposition process applied to the glass samples in the cold of the production of a special solution composition. With the aid of a titration-recording pen, the potential variation in relation with the addition of silver-ion-containing solution is faultlessly recorded. The analysis readings are obtained from the turning points of successive potential 'jumps' in the course of the precipitation of iodine-bromide- and chlorine-ions into corresponding silver salts. In developing this analysis method, the applicants made sure that this method provides correct results.

It was found that the aims of the present invention may be obtained without problems with the following basic composition (in per cent. by weight): SiO2 45.0 to 62.0 B203 15.0 to 20.0 ZrO2 6.0 to 8.5 alkali metal oxide 4.0 to 15.0 alkaline earth metal oxide 0.2 to 10.0 other oxides 0 to 12.0 Particularly good results are obtained if the basic glass has the following composition (in wt %):: SiO2 52.0 to 60.0 B203 16.0 to 19.5 Li2O 0 to 3.0 Na2O 0 to 8.0 K20 0 to 15.0 CaO 0.2 to 4.0 MgO 0.2 to 2.0 SrO O to 5.0 BaO O to 7.0 ERO 0.4 to 10.0 Al203 0 to 5.0 TiO2 0 to 2.5 PbO O to 1.0 rare earth oxides 0 to 6.0 P205 0 to 2.0 One example of such a composition is hereafter specified in per cent. by weight: SiO2 56.21 B203 18.17 ZrO2 6.28 Li2O 1.77 Na2O 3.55 K20 6.00 CaO 0.48 MgO 0.97 Al203 4.60 TiO2 1.92 Ag2O 0.22 Cl 0.175 Br 0.230 TeO2 0.01 CuO 0.015.

Table I shows 1 9 examples of compositions for glasses according to this invention as well as the properties obtained with these compositions.

The colour of phototropic glasses in light exposed state state depends on the annealing conditions (prehistory of heat treatment) and on doping. Gold, palladium, tellurium and tin oxide are well proven doping substances for producing a brownish colour in light exposed state. The glasses according to this invention can be doped with these materials in order to show a brown coloration when light exposed. Without such doping, their colour in light exposed condition is grey-brown, gray and reddish and reddish grey.

The generation of the silver-halide-containing phase precipitations or separations as phototropic carriers occurs generally (although this should not be taken as a restrictive condition) after glass production in the course of a second independent annealing step or stage.

The phototropism is produced in this annealing stage in the temperature range between 400 and 700"C and times which are inversely proportional to temperature. Within certain limits, saturation transmissivity and recovery half times are also varied in this stage.

For example, from one and the same glass by annealing at low temperature, a faster recovering glass can be produced than would be obtained with higher annealing temperatures.

However, various factors such as clouding tendency colour type in light exposed state, sensitivity to stimulating radiation are affected by such variation in annealing conditions so that it would seem advisable to control the improvement of the kinetics in phototropic glasses with simultaneous high transmission variation at light exposure preferably via the composition and not so much via annealing conditions.

Since in their basic glass composition the glasses according to this invention cover only a narrow viscosity range, they were all tempered at the same annealing temperature. Annealing conditions were 640"C and 1 hour. By varying these conditions, it is possible to vary the phototropic data of the glasses according to the invention.

Example of embodiment A glass according to this invention (example 2 in table) is obtained, for example, by weighing out the following raw materials in the following quantities: Sipur 797.54 g.

anhydrous boric acid 275.85 g.

sodium bromide 5.98 g common salt 9.90 g.

potassium carbonate 132.35 g.

aluminummonohydrate 89.00 g.

rutile 28.83 g.

sodium-silicozirconate 187.91 g.

lithium carbonate 66.21 g.

magnesium carbonate 35.49 g.

calcium carbonate 12.80 g copper oxide 0.226 g.

silver nitrate 10.99 g.

dry-mixing them and gradually feeding into a platinum crucible at 1400"C.

The resulting melt is refined at 1380çC for 45 minutes and then in molten condition around 1000"C passed between two water-cooled rolls. This produces a plate of glass, 2.2 mm thick, which has passed quickly enough through the critical ranges of phototropic precipitation. After cutting up, test samples from this plate are annealed at 635"C for one hour and after cooling ground and polished to 2 mm glass thickness.

The subsequently applied standard phototropic testing process at 23"C shows an intitial transmission in non-light exposed state of 90% and after 10 minutes irradiation with Xenon light a saturation transmission of 26%. The glass recovers at a recovery halftime of 1.5 minutes and has achieved a transmissivity of 80.5% after 30 minutes recovery time.

Wet-chemical analysis of the glass showed the following glass composition in per cent. by weight: SiO2 55.49 B203 18.43 ZrO2 6.66 Li2O 1.80 Na2O 3.60 K20 6.09 MgO 1.00 CaO 0.50 Al203 4.44 TiO2 1.94 CuO 0.013 Ag 0.210 Br 0.143 Cl 0.207

1 2 3 4 5 6 7 8 9 -SS,85 55,49 57,17 56,21 55,49 55,85 56,35 54,G5 56,65 B2O3 18,55 18,43 18,99 18,17 18,43 18,55 18,22 19,90 18181 Zero2 6,70 6,66 7,93 6,28 6,66 6,70 6,29 6,61 6,79 LO 1,81 1,80 1,85 1,77 1,80 1,81 1,78 1,53 1,83 3,62 3,62 3,60 3,70 3,55 3,60 3,62 4,04 3,72 3,7 X2O 5,12 5,09 5,24 6,00 6 ,09 5,12 5,29 5,82 5,19 w tR20 10,55 10,49 10,79 11,32 11,49 10,55 11,11 11,07 11,69 0,50 CaO 0,50 0,51 0,48 0.50 0 S0 0,20 0,49 0,48 0,20 c MgO 1,00 1,00 1,03 0,97 1,00 0,20 0,97 0,99 0,30 Q - - - - - 1,50 - - bud 0,40 - - - - - - - Tr9D 1,M l,so 1,54 1,45 1,50 1,90 1,46 1,47 0,50 ul A12 3 4,46 4,44 - 4,60 4,44 4,46 4,61 4,41 4,53 TiO2 1,95 1,94 1,99 1,92 1,94 1,95 1,92 1,85 1,98 i PO t Fa2O3 '- 1 or 1,54 - - - - - P205 - - - - - - - - analyt. Ag 0,35 0,33 0,35 0,369 0,348 0,33 0,364 0,35 0,33 Ci C1 0,t94 0,209 0,206 0,247 0,207 0,199 0,224 0,201 0,214 Br 0,157 0,130 0,160 0,140 0,155 0,140 0,139 0,165 0,125 TeO2 0,010 0,006 - - 0,010 - - 0,006 CuO 0,015 0,015 0,012 0,015 0,015 0,017 0,015 0,015 0,010 Pd - - 0,0006 - 0,0008 - - - molar t 3,5 3'5 3,5 3,1 3,5 3,5 2,6 3,2 3,5 0,31 > 0,31 0,31 0,36 0,28 0,30 0,31 0,28 0,31 0,32 1,00 1,00 1,00 1,03 1,04 1,04 1,04 1,03 1,00 1,04 1,03 Ag AT () 89 90 89 90 89 88 90 90 89 ST ST t} 19 18 19 13 18 19 16 18 18 aM9Z fninl 2,2 2,4 2,1 2,6 2,1 2,2 2,2 1,9 2,2 | < ! R 30 (r) 80 79 80 81 81 79 81 S1 80 N P a S2XREN2 42 43 40 34 37 41 35 3 4 39

10 11 12 13 14 15 16 17 IS 19 SÒ2 55,14 55,14 55,14 55,97 56,22 57,13 56,35 55,05 56,27 55,65 a203 18,32 18,32 18,32 18,59 18,82 19,39 19,11 18,79 18,89 17,31 Zero2 6,36 6,36 6,36 6,71 7,49 8,46 6,86 6,92 6,72 7,88 Li2O 1,80 1,80 1,80 1,35 1,45 1,85 1,96 1,72 1,92 1,82 1 20 3,59 3,59 3,64 3,98 4,08 3,42 4,02 3,42 4,21 6,06 6,06 6,06 6,06 5,73 5,98 6,16 ,45 5,78 6,12 6,18 11,93 11,45 11,45 10,72 11,41 11,99 10,83 11,52 11,46 1,21 t I C3O 0,50 0,50 0,50 0,50 0,30 0,36 0,30 0,50 0,20 0,20 I1,00 1,00 1,00 1,00 1,01 0,60 0,64 0,40 1,10 1,40 0,80 / SrO - - - - - - - < B^O - - - - - - 0,60 - - 1,50 1,50 1,50 1,50 1,51 0,90 0,90 1,40 1,60 1,60 1,00 Si = A12O3 4,66 4,56 4,66 4,51 3,52 - 3,80 4,22 3,21 4,50 I TiCZ 1,95 1,us 1,95 1,95 1,65 1,75 1,65 1,40 0,75 1,25 - - - - - - - - 0,10 0,20 0,5 \La2P20OJ - - - 0,5 1,00 0,26 CAY 0,26 0,23 0,23 0,23 0 0,27 0,24 0,28 0,29 0,27 0 0,26 b?aLy=;;i 0,26 0,23 0123 0,24 0,23 0,26 0,22 0,27 0,31 0,26 0,27 0,17 0,17 0,165 0,18 0,16 0,17 0,16 0,t9 0,19 0,18 0,17 0 0 0,006 0,006 0,006 0 0 0 0 0 0 - 0,015 0,015 0,015 0,015 0,016 0,012 0,014 0,017 0,015 0,016 Pd 1ar SiO, 3,49 3,49 3,49 3,49 3,46 3,41 3,42 3,40 3,45 3,73 B203 DO1ar Zero2 0,27 0,29 0,29 0,33 0,35 0,36 0,31 0,31 0,30 0,33 c-R2o - ,65 1,71 lr83 1,70 1,61 1,5er m Ag 1,65 1,71 1,83 1,70 1,61 1,5 1,66 1,74 1,63 1,68 AT AU 90 91 90 89 91 90 89 90 88 91 ST ST (*) 24 25 25 28 25 28 26 27 26 28 , REWZ cmin, 1,9 1,5 1,8 1,6 1,5 1,5 1,5 1,4 1,6 1,6 Ia R 30 (z) 78 81 79 81 81 81 80 80 80 81 N P a SIXRHW2 45 38 45 44 38 42 39 39 42 44

Claims (5)

1. A phototropic alkali metal-alkaline earth metal-boron-zirconium-silicate glass which contains, dispersed in the glass, silver-halide containing precipitation products and, where applicable, sensitizers such as CuO as phototropic agents, the essential glass components being SiO2, B203, ZrO2, alkali metal oxide, alkaline earth metal oxide and at most 1 2 per cent by weight of other oxides, wherein for a 2 mm glass thickness at 23"C transmission in non-light-exposed state is greater than 86%, after 10 minutes, light exposure transmission is less than 28%, and half time for recovery is 2.5 minutes, wherein the amount of ZrO2 contained in the glass is greater than or equal to 6.0 % by weight wherein the molecular ratio SiO2:B203 is less than or equal to 4.0, and wherein the molecular ratio Zoo,: the sum of alkali metal oxides is 0.22 to 0.37.

2. Phototropic glass according to claim 1, containing in % by weight: SiO2 45.0 to 62.0 B203 15.0 to 20.0 ZrO2 6.0 to 8.5 alkali metal oxide 4.0 to 15.0 alkaline earth metal oxide 0.2 to 10.0 other oxides 0 to 12.0

3. Phototropic glass according to claim 2, containing in % by wt: SiO2 52.0 to 60.0 B203 16.0 to 19.5 Li2O 0 to 3.0 Na2O 0 to 8.0 K20 0 to 15.0 CaO 0.2 to

4.0 MgO 0.2 to 2.0 SrO O to 5.0 BaO O to 7.0 Sum of RO 0.4 to 10.0 Awl203 0 to 5.0 TiO2 0 to 2.5 PbO O to 1.0 rare earth oxides 0 to 6.0 P20s 0 to 2.0 4. Glass according to claim 3, having a saturation transmission for a 2mm glass thickness at 23"C after standard light exposure of less than or equal to 20%, said glass containing at least 0.28 per cent by weighr of analytically determined Ag, and the weight ratio (Cl + Br): Ag being greater than or equal to 1.54.

5. Glass according to claim 3, having a saturation transmission for a 2 mm glass thickness at 23"C after standard light exposure of 24% to 28%, and containing from 0.15 to 0.33 per cent by weight of analytically determined Ag, and the weight ratio (Cl + Br) : Ag being less than or equal to 1.05.