Optical Glass With Refractive Indexes of 1.60-1.69 and Abbe Numbers of at Least 54 With Particularly Low Density and Particularly Good Chemical Stability

1. Technical Field of the Invention

This invention relates to a new optical glass which is distinguished from known glasses of the same optical type in that it has a significantly lower density and improved chemical stability.

2. Background Art

Whereas known crown glasses of this optical type are molten in a glass system of SiO.sub.2 --B.sub.2 O.sub.3 --BaO (with very high BaO percentages of up to 50 percent by weight), the glasses according to the present invention comprise the essential components SiO.sub.2 --Ba.sub.2 O.sub.3 --CaO--La.sub.2 O.sub.3.

Accordingly, it is a general object of the present invention to provide an optical quality crown glass combining the properties of a high refractive index, a high Abbe number, a low density, and good chemical stability.

Another object of the present invention is to provide such glass compositions which contain less than 12 percent by weight BaO.

A further object of the present invention is to provide glass compositions having a refractive index of 1.635-1.645, an Abbe number of at least 59.8, and a density not greater than 3.0 g/cm.sup.3.

An additional object of the present invention is to provide glass compositions having a refractive index of 1.670-1.685, an Abbe number of at least 54, and a density not greater than 3.4 g/cm.sup.3.

A more particular object of the present invention is to provide lens blanks and polished spectacle lenses made from the novel glass compositions herein.

Upon study of the specification and appended claims, further objects, features and advantages of the present invention will become more fully apparent to those skilled in the art to which this invention pertains.

Briefly, the above and other objects, features and advantages of the present invention are attained in one aspect thereof by providing a glass composition comprising 5-30 percent by weight, preferably 6-19 percent by weight, SiO.sub.2 and 20-55 percent by weight, preferably 29-53 percent by weight B.sub.2 O.sub.3. In order to guarantee refractive indexes higher than 1.60, preferably 1.62-1.68 or even higher, and simultaneously a low density of less than 3.5 g/cm.sup.3, preferably 2.9-3.3 g/cm.sup.3, CaO is required in an amount of 13-40 percent by weight, preferably 20-29 percent by weight. La.sub.2 O.sub.3 is added in amounts of 3-25 percent by weight, preferably 11-25 percent by weight, in order to improve the chemical stability of the glass, particularly its acid resistance.

To this basic glass system, one or more additional components may be added as required: 1-7 percent by weight ZrO.sub.2 significantly raises the refractive index and resistance to acids, although the Abbe number drops. 1-10 percent by weight ZnO, 1 to 12 percent by weight BaO, 1-10 percent by weight SrO, and 1-9 percent by weight MgO are optional components, as well as 1-9 percent by weight Al.sub.2 O.sub.3 and 1-2 percent by weight TiO.sub.2. As TiO.sub.2 lowers the Abbe number very strongly, even in small amounts, its addition must be made judiciously.

In the following table, two known glasses (LaK21 and LaKN12, seen SCHOTT Catalogue No. 311d, 1980) are compared with two glasses according to the present invention; .rho. is density in g/cm.sup.3. SR is resistance to acids as defined in the above cited SCHOTT Catalog, the contents of which are incorporated by reference herein. Briefly, this chemical or acid resistance is determined by: Removal of a 0,1.mu. layer by acid (pH=4,6; acetate standard buffer solution) at 25.degree. C. in time units (seconds). Example: For LaK N12 removal of a 0,1.mu. layer takes place in 20 seconds.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. In the following Examples, the temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight.

The glasses according to the invention are produced according to techniques known in the art, e.g. in the following manner: The components (oxides, carbonates and nitrates) are weighed out according to the desired formula. Refining agents, e.g. As.sub.2 O.sub.3 or Sb.sub.2 O.sub.3, are added in quantities of 0.1 to 1 percent by weight. This preparation is well mixed and melted down in a ceramic tank or in a platinum crucible at 1250.degree.-1350.degree. C., refined and homogenized, and than allowed to cool while being stirred down to a casting temperature of 1000.degree.-1080.degree. C. The melt is then poured into iron moulds or further mechanically processed, e.g. by press moulding. By way of example, the table headed "Sample Melts" lists seven typical examples of melt compositions and their properties.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operaing conditions of this invention for those specifically used in the examples. From the foregoing description, one skilled in the art to which this invention pertains can easily ascertain the essential characteristics thereof and, without departing from the spirit and scope of the present invention, can make various changes and modifications to adapt it to various usages and conditions.

As can be seen from the present specification and examples, the present invention is industrially useful in providing a new type of optical crown glass suitable for high quality applications such as in the manufacture of eyeglass lenses.