U.S. Pat. No. 4,479,819 describes the preparation of glass articles exhibiting excellent polarization in the infrared region of the radiation spectrum. These polarizing articles are prepared from glasses containing particles of silver halide dispersed therein. The silver halide is selected from the group consisting of AgCl, AgBr, and Agl.
The method disclosed comprises four basic steps:
(1) a batch for a glass containing silver and at least one halide selected from the group consisting of chloride, bromide and iodide is melted and the melt shaped into a glass body of a desired configuration.
(2) that glass body is subjected to a heat treatment at a temperature at least above the strain point of the glass, but not in excess of 50xc2x0 C. above the softening point of the glass, for a period of time sufficient to cause the generation of silver halide particles therein, the particles being selected from the group consisting of AgCl, AgBr, and Agl, and ranging in size between about 200-5000 xc3x85; thereafter
(3) the glass body is elongated under stress at a temperature above the annealing point of the glass, but below the temperature at which the glass demonstrates a viscosity of about 107 MPa (108 poises), such that the silver halide particles are elongated to an aspect ratio of at least 5:1 and aligned in the direction of the stress; and then
(4) the elongated glass body is exposed to a reducing atmosphere at a temperature above about 250xc2x0 C., but no higher than about 25xc2x0 C. above the annealing point of the glass, for a period of time sufficient to develop a reduced surface layer on the glass article having a thickness of at least 10 microns (≈0.0004xe2x80x3) and, preferably, about 50 microns (≈0.002xe2x80x3), wherein at least a portion of the elongated silver halide particles is reduced to elemental silver particles having aspect ratios greater than 2:1 and being deposited in and/or upon the elongated particles.
The principal objective of the invention disclosed in that patent is to produce glass articles displaying excellent polarizing properties over the infrared portion of the radiation spectrum, preferably within the region of 700-3000 nm (7000-30,000 xc3x85), but also at longer wavelengths, e.g., 3 to 5 microns.
The dichroic ratio is defined as the ratio existing between the absorption of radiation parallel to the direction of elongation and the absorption of radiation perpendicular to the direction of elongation. The sharper (taller and narrower) the peaks, the higher the dichroic ratio. Sharp peaks occur with the presence of relatively small particles.
Nevertheless, the particles must not be too small. With particles smaller than about 100 xc3x85, the mean-free path limitations to the conduction electrons cause the peak to broaden. Moreover, small particles demand very high elongation stresses to develop the necessary aspect ratio. The likelihood of glass body breakage during a stretching-type elongation process is directly proportional to the surface area of the body under stress. There is, then, a very practical limitation as to the level of stress that can be applied to a glass sheet or other body of significant bulk. In general, a stress level of several MPa (a few thousand psi) has been deemed to comprise a practical limit.
A commercial glass was developed based on the teachings of the -819 patent. However, since initial demand was not large, an available glass, used for ophthalmic lenses, was employed.
That glass required a refractive index of at least 1.523. To achieve that value, substantial amounts of both ZrO2 and TiO2 were incorporated in the glass composition. The glass selected for commercial purposes, was a mixed alkali aluminoborosilicate containing about 5% ZrO2 and about 2% TiO2. The latter was necessary to provide the refractive index required for ophthalmic purposes.
This glass provided excellent properties, but encountered forming problems. These problems arose because there were two different devitrification, that is, crystallization, phases associated with the glass. These phases were a silver halide (AgX) phase and a rutile (TiO2) phase.
These two phases had different liquidus temperatures. Each of these different phases can lead to problems either during the forming process, or during subsequent heat treatments. These problems could be coped with in molding ophthalmic blanks due to the nature of the ophthalmic lens blank molding process. Such blanks are relatively small, can be formed quickly, cooled and mechanically reshaped, if necessary.
The situation is very different in producing polarizers. Here, the glass is cast as bars, or drawn in relatively thick sheets. Bars are cast in a standard thickness. However, in the interest of economy, the width of the bar is as great as possible, while avoiding devitrification. Thus, it would be desirable to form bars up to at least 30.5 cms (one foot) in width.
The AgX phase is influenced by the amount of AgX present in the glass composition. However, for polarization purposes, the AgX content is dictated by the desired polarization behavior. The AgX content may range up to about 0.20 percent by weight, but the AgX liquidus tends to increase accordingly. Thus, the AgX liquidus may range up to 1020xc2x0 C., but is preferably maintained below 1000xc2x0 C., that is xe2x89xa6995xc2x0 C.
The rutile phase liquidus is the temperature at which rutile (TiO2) crystals start forming. In the present glass, this temperature is greater than 1040xc2x0 C. This greatly limits the ability to process wide bars and sheets, without encountering crystals forming, that is, without the glass devitrifying. The AgX liquidus temperature is essentially fixed. Therefore, it would be desirable to achieve a rutile phase liquidus temperature near, or preferably somewhat below, the value for the AgX phase, that is xe2x89xa6995xc2x0 C.
The ideal condition would be to eliminate the TiO2 content entirely. However, that is not feasible for a couple of reasons. TiO2 is more effective as a refractive index control than ZrO2, while having a minimal effect on photochromic properties, that is particle size. While larger amounts of ZrO2 might be employed, this is undesirable since, as is well known, there is a deleterious effect on the liquidus and the glass becomes difficult to melt.
Further, the 1.523 value of refractive index is not required for a polarizer. Nevertheless, it is desirable to maintain a somewhat lower value, particularly to match an anti-reflection coating, if such coating is applied. Accordingly, a refractive index value of 1.500-1.520 is highly preferred in a polarizing glass.
Perhaps the most important reason stems from the need to switch production back and forth between an ophthalmic glass and a polarizing glass in a single glass melting unit. It was found that the switch to an ophthalmic glass, containing slightly over 2% TiO2, could be made much more rapidly if the TiO2 were not completely removed from the composition for polarizer production. Thus, a considerably longer time was required to obtain stable photochromic properties when the original glass was titania-free. However, with some titania in the original polarizer glass, the properties required in the ophthalmic glass could be stabilized in a much shorter time. Economically, of course, this is highly significant.
It is then a purpose of the present invention to provide an improved glass polarizer of the type disclosed in U.S. Pat. No. 4,479,819.
It is a further purpose to provide a glass polarizer having excellent polarizing characteristics in the infrared portion of the spectrum due to elongated, silver chloride, bromide and/or iodide particles in the glass.
It is another purpose to provide a glass polarizer as disclosed in U.S. Pat. No. 4,479,819, but having AgX and rutile liquidus temperatures not over 1020xc2x0 C., but preferably about 995xc2x0 C.
It is a still further purpose to provide a glass polarizer having its polarization characteristics imparted by elongated, silver chloride, bromide and/or iodide particles in the glass, and having a rutile liquidus value not higher than the AgX liquidus value of the glass, and preferably equal to or slightly lower.