Zinc-Containing Phosphate Glasses

Table I records a number of glass compositions, expressed in terms of mole percent on the oxide basis, operable in the present invention. Table IA reports the same compositions, but wherein the values of the individual components have been converted to weight percent. The actual batch ingredients for the glasses can comprise any materials, either the oxides or other compounds, which, upon being melted together, will be converted into the desired oxides in the proper proportions. For example, Li.sub.2 CO.sub.3 is conveniently employed as the source of Li.sub.2 O. Because it is not known with which cation it is combined and because the amount included was relatively quite small, the fluoride content is simply recorded in percent in addition to the base glass composition.

The batch materials were compounded, ballmilled together to aid in securing a homogeneous melt, and then charged into silica crucibles. After placing lids thereon, the crucibles were introduced into a furnace operating at about 1000.degree. C. and maintained at that temperature for about 3 hours. Each melt was poured into a steel mold to produce a rectangular glass slab having dimensions of about 8".times.4".times.0.5" and the glass slab annealed overnight at 300.degree. C.

Rectangular tab-shaped pieces weighing about 30-40 grams were cut from the slabs and heat treated in cups of aluminum foil to temperatures within the range of 350.degree.-450.degree. C., and glass cane was hand drawn from each cup to obtain a close approximation of the working temperature of the glass.

Whereas the above description reflects laboratory melting and forming practice only, it will be appreciated that the recited compositions are capable of being melted in large scale melting units and shaped into desired configurations utilizing forming techniques conventional in the glassmaking art.

Samples having the approximate dimensions of 35.times.25.times.15 mm were cut from each glass slab and then ground and polished for testing the chemical durability thereof. In one test each sample was carefully weighed and then immersed into a bath of boiling water. After a residence time of six hours, the sample was removed from the bath, dried in the ambient environment, and thereafter weighed to determine any loss of weight by the sample. In a second test each sample was carefully weighed and then immersed into a bath operating at 75.degree. C. of a 0.3% by weight aqueous solution of SUPER SOILAX detergent marketed by Economics Laboratories, St. Paul, Minn., the solution exhibiting a pH of about 10. After a dwell period of 24 hours, the sample was removed from the bath, dried in the ambient environment, and reweighed to measure any weight loss. In a third test each sample was carefully weighed and then introduced into an autoclave. The temperature within the autoclave was raised to 125.degree. C. and a steam atmosphere of 33 psi generated. After an exposure of four hours, the sample was removed from the autoclave, dried in the ambient environment, and again weighed to determine any loss in weight. The Tg of the glass was measured utilizing standard differential scanning calorimetry techniques.

Table II reports the percent weight loss in the boiling water test (H.sub.2 O), the percent weight loss in the SUPER SOILAX test (SUPER), the percent weight loss in the steam test (Steam), the Tg of the glass in .degree.C., and the working temperature of the glass in .degree.C. as determined by pulling cane (Pull). In certain instances, the samples exhibited a frosted appearance after exposure to the steam test. That phenomenon is noted by the designation (Fr). One example crystallized (Cryst) during the pull test.

The measures of resistance to attack by water exhibited by Examples 1-37 are very exceptional for phosphate-based glass compositions, and are particularly noteworthy since the glasses demonstrate such low transition and working temperatures. The sharp deterioration in chemical durability occurring in glass compositions ever: slightly outside the ranges prescribed in the inventive glasses is immediately evident with a review of Examples 38-40. Hence, in Example 38 additions of Al.sub.2 O.sub.3 and/or substitutions of Cu.sub.2 O for R.sub.2 O and/or ZnO would be required to improve their durability to the level of that exhibited by Example 3 which is lower in P.sub.2 O.sub.5 content and within the most preferred region of compositions. Example 39 clearly illustrates the need for at least two alkali metal oxides in the glass. In Example 40 the level of P.sub.2 O.sub.5 is too high. In Examples 41-44 the level of ZnO is too low leading to poor glass stability (readily devitrifies). Example 42 indicates that where the level of ZnO is low and the concentration of P.sub.2 O.sub.5 is relatively high, not only does the glass exhibit poor stability, but it also demonstrates poor durability.

The high resistance to moisture attack renders the inventive glasses eminently suitable for a wide range of applications including, for example, food service, electronic devices, optical and ophthalmic lenses, and coatings for optical waveguides. Furthermore, because the viscosity relationships exhibited by the inventive glasses at relatively low working temperatures render them capable of being shaped into articles of complex configurations through extrusion, injection molding, and other techniques conventionally employed in the plastic forming art, the range of products for which the inventive glass-ceramics are useful is greatly broadened.