FAST FADING PHOTOCHROMIC GLASS

Abstract: This invention is directed to transparent photochromic glas­ ses that can darken to a comfort range, i. e., between about 35-60%, or to a dark range, i.e., below 35%, depending upon glass composition, and will exhibit a faded transmittance after five minutes' removal from actinic radiation higher than 70%. The base compositions of the glasses consist essentially, in weight percent, of about SiO₂ 46 -60 MgO 0-3.5 B₂O₃ 16 -28 CaO 0-6 Al₂O₃ 4 -11 SrO 0-6 ZrO₂ 2 - 6 BaO 0-6 Al₂O₃ + ZrO₂ 6 -13 MgO + CaO + SrO + BaO 0-7 Li₂O 2 - 5 P₂O₅ 0-5 Na₂O 0 - 4 TiO₂ 0-3 K₂O 2.5- 9 For a comfort glass the photochromic elements consist essentially, as analyzed, of Ag 0.13-0.18 Br 0.08 -0.14 Cl 0.16-0.34 CuO 0.001-0.004 For a dark glass the photochromic elements consist essen­ tially, as analyzed, of Ag 0.15-0.3 Br 0.07 -0.14 Cl 0.2 -0.45 CuO 0.004-0.016

Description: Field of the Invention The present invention is related to the preparation of transparent photochromic glasses suitable for the production of ophthalmic and non-prescription lenses which demonstrate the characteristic of very rapid fading. In the present context it will be understood that rapidly fading glasses are glasses which exhibit a luminous transmittance at ambient temperature greater than about 70% five minutes after having been removed from a source of actinic radiation. The state of the art of photochromic glasses, as far as known to Applicant, is described in U.S. Patent 3,208,860, which is the basic photochromic patent, and by the subsequent U.S. Patents 3,795,523; 3,833,511; 3,998,647; 4,000,019; 4,018,965; 4,102,693; 4,130,437; 4,190,451; 4,358,542; 4,407,966; 4,550,087. In US-A-4,407,966 very fast fading photochromic glasses are described and it is stated that the improved photochromic behaviour of such glasses is derived from the discovery that the inclusion of about 0.1-0.15 % by weight of PbO in glasses containing higher Li₂O contents can restore and may even improve upon the darkening faculty of the glass, while permitting the more rapid fading rate imparted by the higher Li₂O content to be maintained. As was explained in U.S. patent No. 4,550, 087, whereas, in general, it was previously desired to develop photochromic glasses exhibiting very low transmittance in the darkened state, recently a great interest has arisen in photochromic glasses capable of reaching a "comfort" range of darkening; i.e., having transmissions between about 40-60%. In both of the two types of glasses, however, the users of lenses prepared therefrom have demanded rapid fading. We have discovered a narrow range of base glass compositions which, by means of careful proportioning of the "photochromic elements", i.e., the concentrations of Ag, Cl, Br, and CuO, can provide two levels of darkening accompanied by very rapid fading. These base glass compositions consist essentially, expressed in terms of weight percent on the oxide basis, of: with the preferred base glasses consisting essentially of: In the framework of the present invention, a "comfort" lens is understood as comprising a lens prepared from a glass which, in a thickness of 2 mm, exhibits the following optical and photochromic properties: (a) a clear (untinted) luminous transmittance (T o ) greater than 88%; (b) a luminous transmittance in the darkened state, i.e., after 15 minutes' exposure to actinic radiation (T D15 ) over the temperature range of 0°-25°C, higher than 35%, preferably above 40%, but lower than 60%; (c) a luminous transmittance in the darkened state, i.e., after 15 minutes' exposure to actinic radiation (T D15 ) at 40°C, no higher than 63%; and (d) a fading rate at room temperature (20°-25°C) such that, after five minutes' removal from the actinic radiation (T F5 ), the glass exhibits a luminous transmittance of at least 75% and, preferably, more than 80%. Preferably, the glasses also demonstrate the two following properties: (A) a difference in the luminous transmittance in the darkened state over the temperature interval of 0°-25°C of less than eight percentage units of transmittance and, preferably, less than six; and (B) a difference in the luminous transmittance in the darkened state over the temperature interval of 25°-40°C of less than 18 percentage units of transmittance and, preferably, less than 16. The last two parameters reflect the independence in photochromic behavior exhibited by the inventive glasses with respect to temperature. To obtain these optical and photochromic properties requires the introduction of the photochromic elements in the following proportions, measured in terms of weight percent as analyzed in the glass: with the preferred concentrations being: The development of haze has been a serious problem in the preparation of photochromic glass lenses darkening to the comfort range. It has been postulated that the mechanism on which the development of haze rests involves a necessary low content of silver which, coupled with a poor rate of nucleation, leads to the growth of large silver halide crystals which cause scattering of light. To avoid this problem, the following relationships must be satisfied: Ag≧0.13% Br:(Cl+Br)≧0.24, preferably ≧0.30 Furthermore, the desired photochromic properties necessitate: 0.25≦αµρ¨Ag:(Cl+Br)<0.60, preferably ≦αµρ¨0.50 In the framework of the present invention, a "dark" lens comprises a lens of a glass which, in a thickness of 2 mm, exhibits the following optical and photochromic properties: (a) a luminous transmittance in the clear state (T o ) greater than 88%; (b) a luminous transmittance in the darkened state, i.e., after 15 minutes' exposure to actinic radiation (T D15 ) over the temperature range of 0°-25°C, lower than 35%, preferably below 30%; (c) a luminous transmittance in the darkened state, i.e., after 15 minutes' exposure to actinic radiation (T D15 ) at 40°C, lower than 58%, preferably below 55%; and (d) a fading rate at room temperature (20°-25°C) such that, after five minutes' removal from actinic radiation (T F5 ), the glass exhibits a luminous transmittance greater than about 70%. Obtaining these optical and photochromic properties requires the presence of the photochromic elements in the proportions below, as analyzed in the glass in weight percent: with the preferred concentrations being: The limits of the ranges specified above for the constituents of the compositions of the base glass are also crucial for obtaining glasses exhibiting good melting and forming behaviors, and possessing the chemical and physical properties required of glasses utilized in optical and ophthalmic applications, as well as the desired photochromic behavior. For example: Al₂O₃ inhibits phase separation and, in like manner to ZrO₂, improves the chemical durability of the glass. However, either of these oxides reduces the stability of the glass with respect to devitrification. Consequently, the following relationship must be respected: 6≦αµρ¨ Al₂O₃+ZrO₂≦αµρ¨13 or 6≦αµρ¨Al₂O₃+ZrO₂≦αµρ¨15, when the glass contains P₂O₅ Because B₂O₃ can exert an adverse effect upon the chemical durability of the glass, the maximum will not exceed 28% and will preferably be lower than 25%. The refractive index (n D ) can be corrected to 1.523 to render the glass useful for ophthalmic applications through adjustment of the ZrO₂ and TiO₂ contents. However, TiO₂ can have three harmful effects on the glass: (1) it confers an undesirable yellow tint; (2) because it absorbs ultraviolet radiation, it decreases the sensitivity of the glass to actinic radiation; and (3) it renders the glass more susceptible to phase separation. Adjustment of refractive index can also be effected through additions of BaO, SrO, CaO, and MgO. Nevertheless, MgO tends to raise the liquidus temperature, and so will preferably be essentially absent from the composition. CaO appears to be more effective in maintaining the desired rapid fading character of the glass, so that its use will be preferred to that of BaO and SrO. In all cases the total MgO+CaO+SrO+BaO should not exceed 7%, with the preferred value comprising between 1 and 4%. P₂O₅ appears to be advantageous in improving the fading rate of the glasses, but it adversely affects the chemical durability of the glass and raises its liquidus temperature. Consequently, a maximum of 5% should not be exceeded. Furthermore, P₂O₅ appears to increase the tendency of the glass to phase separate and, as a result of that fact, should be excluded or limited to proportions lower than 1% where the total of CaO+MgO equals at least 2%. To obtain the desired photochromic behavior and notably great fading rates, careful control of the proportion of alkali metal oxide is indispensable. To illustrate, the cationic ratio Li₂O:total alkali metal content (R₂O) will desirably be greater than 0.4, but less than 0.8, preferably between 0.45-0.75. Furthermore, Na₂O contents greater than 4% appear to adversely affect the fading rate of the glasses. On the other hand, Na₂O appears to increase the mechanical strength which can be obtained through chemical strengthening of the glass. Consequently, the preferred glasses will contain at least 0.4% Na₂O. In like manner, to insure high mechanical strength in the glass through chemical strengthening, the preferred compositions will contain Li₂O in amounts greater than 2%. Finally, up to about 1% total of classic glass colorants such as CoO, Er₂O₃, MnO, and NiO can be incorporated to tint glass. Up to about 1% SnO₂ or up to 4 parts/million (ppm) Pd can be added to provide a brown coloration in the darkened state. When necessary, especially in photochromic glass compositions containing a great proportion of alkali metal and/or alkaline earth metal oxides, up to 1% by weight As₂O₃ and/or Sb₂O₃ can be introduced in order to increase the ratio Cu + :Cu +2 in the matrix. Description of Preferred Embodiments Table I below presents a certain number of glass compositions, expressed in terms of parts by weight on the oxide basis, illustrating the parameters of the invention. Because the total of the individual constituents is equal to or very close to 100, the values indicated can be considered, for all practical purposes, as representing weight percent. Inasmuch as it is not known with which cation(s) the halides are combined and the proportions of them are very small, they are simply reported in the form of chlorine and bromine, in conformity with the usual practice of glass analysis. Finally, since silver is present in a very small amount, it is simply reported in the form of the metal. The actual ingredients of the glass forming batch can comprise any materials, either oxides or other compounds, which, when melted together, are converted into the desired oxide in the proper proportions. The halides are generally added in the form of alkali metal halides. The cationic ratio Li₂O:R₂O is also indicated (L:R). The ingredients of the glass forming batch were combined, carefully mixed together in a ball mill to promote the obtaining of a homogeneous molten mass, and then progressively transferred into a platinum crucible to be heated through the Joule effect. The glass forming batch is melted for three hours at about 1400°C. After forming, the glass is annealed at about 450°C. Analyses of the photochromic elements have indicated losses of these during melting of about 20-25% for Cl, 45-50% for Br, 8-10% for Ag, and 0% for CuO. The glass samples presented above were introduced into an electrically-heated furnace (sample dimensions 4 cm x 4 cm x 0.4 cm) and exposed to the times in minutes and temperatures in °C indicated in Tables II-IX to develop photochromism in them. The samples were then removed from the furnace and thereafter ground and polished to a thickness of about 2 mm. (In general, temperatures between about 600°-675°C have been determined to be satisfactory.) Tables II-IX also report measurements of the photochromic behavior (T o , T D15 , T F5 ) exhibited by the polished samples, utilizing the solar simulator apparatus whose principle is described in U.S. Patent No. 4,190,451. The color of the glasses in the clear and darkened state is defined by the trichromatic coordinates [respectively (x o , y o ), (x D , y D )] determined through the trichromatic colorimetric system of the C.I.E. of 1931 utilizing Illuminant C as the light source. This colorimetric system and light source are explained by A.C. Hardy in the Handbook of Colorimetry , Technology Press M.I.T., Cambridge, Massachusetts (1936). The color in the darkened glass (X D , y D ), is determined after an exposure of 20 minutes at 25°C under a source of ultraviolet light ("Black light-blue lamp"). Finally, Tables VII-IX indicate the weight ratios Ag:(Cl+Br) and Br:(Cl+Br). In the tables below: T o designates the luminous transmittance of a glass in the clear (undarkened) state; T D15 (0) designates the luminous transmittance of a glass in the darkened state after 15 minutes' exposure to the solar simulator source of actinic radiation at 0°C; T D15 (25) designates the luminous transmittance of a glass in the darkened state after 15 minutes' exposure to the solar simulator source of actinic radiation at 25°C; T D15 (40) designates the luminous transmittance of a glass in the darkened state after 15 minutes' exposure to the solar simulator source of actinic radiation at 40°C; ▵TD(0-25) designates the difference in luminous transmittance of a glass in the darkened state over the temperature range of 0°-25°C; ▵TD(25-40) designates the difference in luminous transmittance of a glass in the darkened state over the temperature range of 25°-40°C; and ▵T F5 (25) designates the faded luminous transmittance of a glass after five minutes' removal from the solar simulator source of actinic radiation at 25°C. Examples 1-8 are representative of dark photochromic glass compositions which contain no alkaline earth oxides. Examples 9-22 are representative of dark photochromic glass compositions which contain alkaline earth oxides. Examples 23-29 are representative of dark photochromic glass compositions which contain P₂O₅. Example 28 illustrated the incompatibility that exists between MgO and P₂O₅. The substantial reduction in T o caused by an excess of TiO₂ can be observed in Example 29. Examples 30-35 are representative of comfort photochromic glasses having different base compositions. It should be noted that Examples 16 and 35 have similar base compositions but different levels of Ag, Cl, Br, and CuO. A like situation is present with respect to Examples 17 and 33. Examples 36-40 are representative of comfort photochromic glasses which have the same base composition as Example 33, but contain varying amounts of Ag, Cl, and Br. Table VII below illustrates the effect of the Ag content and the ratio Br:(Cl+Br) on the development of haze in the glass during the heat treatment process. As defined here, haze corresponds to the intensity of diffused light exhibited by the polished samples as measured optically. The level of haze is expressed in an arbitrary scale of units; an acceptable value of haze on that scale being less than 40. Examples 41-48 have the base composition of Example 33. A comparison of Examples 44, 47, and 48 with Examples 45 and 46 demonstrates the effect which the ratio Br:(Cl+Br) has upon the development of haze. Examples 41-43 contain insufficient Ag. Table VIII below illustrates the effect of varying Ag contents on photochromic properties. Examples 49-53 have the base composition of Example 33 except for modifications in the levels of the photochromic elements and the addition of Pd, which imparts a brown coloration to the glasses in the darkened state. Both the ratio Ag:(Cl+Br) and the ratio Br:(Cl+Br) are provided. Table IX below demonstrates the effect varying amounts of Cl and CuO can have upon the photochromic behavior of the glass. Examples 54-60 have the base composition of Example 33, but with differing amounts of the photochromic elements and the addition of Pd which confers a brown tint to the glass in the darkened state. Again, the ratios Ag:(Cl+Br) and Br:(Cl+Br) are provided. Examples 5, 16, 38, 40, 57, and 58 are illustrative of our most preferred embodiments of the inventive glasses, not only because of their photochromic properties, but also in view of their general physical and chemical properties. The photochromic elements are present in such miniscule amounts that their effect upon the general physical and chemical properties of a glass is essentially negligible. Inasmuch as the base compositions of the five glasses are quite similar, Table X records average values of several properties measured on those glasses, utilizing techniques conventional in the glass art.

Claims: A transparent, comfort photochromic glass which, in bodies of 2mm cross section, will exhibit the following optical properties: (a) a clear (undarkened) luminous transmittance higher than 88%; (b) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation over the temperature range of 0°-25°C higher than 35%, but less than 60%; (c) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation at a temperature of 40°C no higher than 63%; and (d) a faded luminous transmittance at 20°-25°C after five minutes' removal from actinic radiation of at least 75%; said glass having a base composition consisting expressed in terms of weight percent on the oxide basis, of and containing photochromic elements, as analyzed in weight percent, of said photochromic elements satisfying the ratios Br:(Cl+Br)≧0.24 and 0.25≦αµρ¨Ag:(Cl+Br)≦αµρ¨0.60 and where CaO + MgO equals at least 2 weight percent, P₂O₅ is less than 1 weight percent; and in which Al₂O₃ + ZrO₂ equals 6-15 weight percent when the glass contains P₂O₅. A photochromic glass according to claim 1 wherein said base composition consists of A photochromic glass according to claim 1 containing photochromic elements, as an analyzed in weight percent, of said photochromic elements satisfying the ratios Br:(Cl+Br)≧0.30 and 0.25≦αµρ¨Ag:(Cl+Br)≦αµρ¨0.50 A photochromic glass according to claim 1 also containing up to 4 ppm Pd. A transparent, dark photochromic glass which, in bodies of 2mm cross section, will exhibit the following optical properties: (a) a clear (undarkened) luminous transmittance higher than 88%; (b) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation over the temperature range of 0°-25°C lower than 35%; (c) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation at a temperature of 40°C lower than 58%; and (d) a faded luminous transmittance at 20°-25°C after five minutes' removal from actinic radiation of greater than 70%; said glass having a base composition consisting expressed in terms of weight percent on the oxide basis, of and containing photochromic elements, as analyzed in weight percent, of and where CaO + MgO equals at least 2 weight percent, P₂O₅ in less than 1 weight percent; and in which Al₂O₃ + ZrO equals 6-15 weight percent when the glass contains P₂O₅. A photochromic glass according to claim 5 wherein said base composition consists of A photochromic glass according to claim 5 containing photochromic elements, as analyzed in weight percent, of A photochromic glass according to claim 5 also containing up to 4ppm Pd.

Inventors: MAZEAU, JEAN-PIERRE, PRASSAS, MICHEL
Applicants: CORNING GLASS WORKS
Application year: 1986
Main classification: C03C4
IPC codes: C03C4, C03C3