Platinum Corrosion Reducing Premelted Oxide Compositions for Lead Containing Solder Glasses

The present invention relates to premelted oxide composition such as a premelted oxide mixture of PbO and B.sub.2 O.sub.3 used as a major material in the raw bath for making lead containing solder glasses, resulting in batch mixtures that are Pb.sub.3 O.sub.4 free or have reduced Pb.sub.3 O.sub.4 content.

It is desirable that the use of Pb.sub.3 O.sub.4 or PbO oxides in solder glass raw batch materials be reduced or eliminated, to thereby reduce any possible health hazard as well as to reduce costs and to take advantage of the unexpected reduction in platinum corrosion and the increased life of a platinum melter resulting from use of premelted materials as batch constituents.

It is an object of the present invention to provide an improved solder glass that is Pb.sub.3 O.sub.4 free or has a reduced Pb.sub.3 O.sub.4 content in the raw batch by using a premelted oxide mixture such as PbO and B.sub.2 O.sub.3, or PbO and SiO.sub.2, or a mixture thereof; so the glass can be made from raw batch ingredients without using Pb.sub.3 O.sub.4 or PbO in oxide form.

It is an object of the present invention to provide an improvement in a lead containing solder glass containing PbO and/or B.sub.2 O.sub.3 and SiO.sub.2, the raw batch materials including an effective platinum corrosion reducing amount of a premelted oxide mixture of PbO and at least one of the oxides B.sub.2 O.sub.3 and SiO.sub.2.

It is an object of the present invention to provide a method of making a solder glass that is Pb.sub.3 O.sub.4 free or has a reduced Pb.sub.3 O.sub.4 content in the raw batch, the glass composition containing PbO and containing at least one of the oxides SiO.sub.2 and B.sub.2 O.sub.3, the composition optionally containing one or more of the oxides ZnO, BaO, Al.sub.2 O.sub.3 and Bi.sub.2 O.sub.3, the improved method comprising the steps of (A) providing raw batch ingredients to provide the above solder glass compositions; and (B) premixing as a major portion of the raw batch ingredients a premelted oxide mixture of PbO and B.sub.2 O.sub.3 or a premelted mixture of PbO and SiO.sub.2, or a mixture thereof, the premelted oxide composition being above the line ABCD of the ternary diagram of the Figure, the premelted oxide mixture being used in an effective platinum corrosion reducing amount.

These and other objects will be apparent from the description that follows, the appended claims, and the drawings, in which:

The FIGURE is a pseudo-ternary phase diagram of the premelted oxide mixtures including PbO and/or B.sub.2 O.sub.3 and SiO.sub.2 and mixtures thereof.

The present invention provides a raw batch for making a lead glass composition containing PbO and containing at least one of the oxides SiO.sub.2 and B.sub.2 O.sub.3, the composition optionally containing ZnO, BaO, Al.sub.2 O.sub.3 and Bi.sub.2 O.sub.3, the improvement comprising using as a major constituent of the batch an effective platinum corrosion reducing amount of a premelted oxide mixture of PbO and B.sub.2 O.sub.3 or a premelted oxide mixture of PbO and SiO.sub.2 or a mixture of both the premelted mixtures, the premelted oxide composition being above the line ABCD of the pseudo-ternary phase diagram of lead borate solder glass premelted oxide mixtures shown in the FIGURE.

The present invention also provides a method of making a solder glass composition that is Pb.sub.3 O.sub.4 free or has a reduced Pb.sub.3 O.sub.4 content in the raw batch, the glass containing PbO and containing at least one of the oxides SiO.sub.2 and B.sub.2 O.sub.3, the composition optionally containing one or more of the oxides ZnO, BaO, Al.sub.2 O.sub.3 and Bi.sub.2 O.sub.3, the method comprising the steps of (A) providing raw batch ingredients to provide the above solder glass compositions; and (B) premixing with the raw batch ingredients an effective platinum corrosion reducing amount of a premelted oxide mixture of PbO and B.sub.2 O.sub.3 or a premelted mixture of PbO and SiO.sub.2, or a mixture thereof, the premelted oxide composition being above the line ABCD of the ternary diagram of the FIGURE.

Due to the extremely corrosive attack of these lead borate and lead-zinc-borate glasses on refractory, it is necessary to melt them in cold-bottom and cold-walled refractory melters, or in platinum or platinum-rhodium melter bushings or crucibles.

The inventive concept of using premelted ingredients dramatically lowers the rate of attack of platinum (during the melting process) if platinum group metals are used for melting. This can be measured by measuring platinum content of the melted glass. Using premelted ingredients, the platinum content ranges from about 4 or 5 to 15 parts per million, compared to 20 to 35 parts per million for conventional oxide batch.

The present invention provides superior glass quality, the glass being made using a premelted oxide batch having less tendency to crystallize from internal or surface nucleation. This enables nucleating additives to be used in after-processes (such as grinding and blending) with more predictable results.

Use of premelted lead silicate such as 92% PbO.8SiO.sub.2 allows simpler and more complete melting and combination of the high-melting SiO.sub.2 ingredient in these glasses.

The present invention provides unexpected results. The product made with premelted batch has superior DTA (differential thermal analysis) properties, which indicate it is less subject to random internal or surface nucleation, in which nucleation is a major problem in preparing these crystallizable solder lead containing glasses using conventional oxide batch materials.

The present invention provides an improvement preferably in a lead borate glass made in a platinum melter, the glass comprising the following ingredients in approximate weight percent:

the improvement comprising supplying all or part of the PbO, B.sub.2 O.sub.3 and SiO.sub.2 contents in a premelted mixture of oxides including a mixture of PbO and B.sub.2 O.sub.3 and optionally a mixture of PbO and SiO.sub.2 or a mixture of PbO, B.sub.2 O.sub.3 and SiO.sub.2, the balance of the oxides being supplied from raw batch material oxides or carbonates, the premelted mixture of oxides being used in a platinum corrosion reducing amount.

The present invention advantageously provides a lead containing solder glass made with a reduced amount of Pb.sub.3 O.sub.4 or PbO or without using any Pb.sub.3 O.sub.4 by using premelted oxides to reduce costs, health hazards and surprisingly reduce deterioration in platinum furnaces used to melt the raw batch materials. The invention unexpectedly substantially reduces the corrosion of platinum by the batch during melting and firing.

Generally, the premelted oxide (PbO and B.sub.2 O.sub.3 or SiO.sub.2 or mixtures of PbO, B.sub.2 O.sub.3 and SiO.sub.2) has a composition above the line ABCD in the pseudo-ternary diagram of the FIGURE and preferably a composition above the line EF in the diagram.

As seen in the FIGURE, the lefthand corner of the triangle forming the ternary diagram represents a composition of about 85 wt.% PbO and 15 wt.% SiO.sub.2. The righthand corner represents about 92 wt.% PbO and 8 wt.% SiO.sub.2. The apex represents a premelted oxide composition mixture of about 87.5% PbO and 12.5% B.sub.2 O.sub.3. Good results have been obtained with a premelted oxide composition of about 86.25% PbO, 6.25% B.sub.2 O.sub.3 and 7.5% SiO.sub.2 seen at point H.

The points E and F have the premelted oxide composition of about 86.75 wt.% PbO, 8.75% B.sub.2 O.sub.3, and 4.5% SiO.sub.2 at point E and about 88.85 wt.% PbO, 8.75% B.sub.2 O.sub.3 and 2.4% SiO.sub.2 at F.

The point A has a premelted oxide composition of about 85.75% PbO, 3.75% B.sub.2 O.sub.3 and 10.5% SiO.sub.2 while the point D has a composition of about 90.65 wt.% PbO, 3.75% B.sub.2 O.sub.3 and 5.6% SiO.sub.2. As indicated in the diagram, the base line represents no B.sub.2 O.sub.3 while the line BC represents approximately 2.5% B.sub.2 O.sub.3. The top of the triangle of the diagram represents lead borate eutectic of a composition of about 87.5 wt.% PbO and .12.5% B.sub.2 O.sub.3 with no SiO.sub.2.

The premelted oxide has an average particle size of larger than 100 mesh or usually between 4 mesh and 30 mesh. The premelted oxide particles are much coarser than the red lead or Pb.sub.3 O.sub.4 normal batch particle. This particle has an average size generally between 200 and about 325 mesh.

As known in the art, the other raw batch materials that supply the B.sub.2 O.sub.3, SiO.sub.2, etc., are well known such as boric acid, boric anhydride, silica sand, zinc oxide, aluminum oxide, bismuth oxide, and barium carbonate.

The more premelted oxide particles added to the conventional raw batch particles, the more the corrosion of the platinum from the platinum furnace is reduced. The corrosion of the platinum can be reduced from a normal amount of about 20 to 35 ppm using all Pb.sub.3 O.sub.4, to as low as about 4 or 5 to 15 ppm by replacing most of the Pb.sub.3 O.sub.4.

Lead containing glasses were made from raw batch materials including the premelted oxides, the mixing, melting and fining done conventionally.

All the Pb.sub.3 O.sub.4 was replaced by the premelted oxide mixture(s) as follows:

The PbO-B.sub.2 O.sub.3 mixtures or the PbO-SiO.sub.2 mixtures shown in parenthesis are the premelted oxide mixture compositions having an average particle size of about 4 to 30 mesh.

As noted in the examples, substantially all the Pb.sub.3 O.sub.4 was eliminated from the raw batch and excellent results obtained. Benefits are obtained by replacing 25%, 50% or 75% of the Pb.sub.3 O.sub.4, The more Pb.sub.3 O.sub.4 replaced, the better the platinum corrosion reducing results.

The following table shows the range of premelted oxide mixtures and the range of oxides in the glass:

The raw batch materials are mixed and melted conventionally as known in the art. Lead containing glasses are shown, for example, in U.S. Pat. Nos. 3,947,279 (Carl J. Hudecek, assigned to Owens-Illinois, Inc.) and 3,973,975 (J. Francel, et al., assigned to Owens-Illinois, Inc.) which are incorporated by reference.

The premelted mixtures of PbO and B.sub.2 O.sub.3, or PbO and SiO.sub.2 are made by, for example, heating mixtures of litharge and silica, or litharge and boric acid, in a conventional cold-walled, surface heated refractory melter. The melted mixture is then water-fritted and dried to produce the premelted mixtures used as major constituents in the batch of the invention.

This step of preparing the premelted oxide mixtures of PbO and B.sub.2 O.sub.3, or PbO and SiO.sub.2, is much less critical than preparation of the final product melt of lead-zinc-borate crystallizable solder glass, or other lead compositions optionally containing ZnO, BaO, Al.sub.2 O.sub.3, and Bi.sub.2 O.sub.3.

The step of preparing premelted oxide mixtures of PbO and B.sub.2 O.sub.3, or PbO and SiO.sub.2, can employ lower cost raw materials such as litharge or boric acid, and can be done in a conventional non-platinum (refractory) furnace, because of the lower-melting eutectic nature of these mixtures, and their inherent non-crystallizing stability.

The following glass composition was made using premelted oxide mixtures and using regular raw batch materials:

Approximately 1000 pounds of the above glass was melted using lead borate (premelt) tribasic lead silicate and lead monosilicate all from Hammond Lead Co., and zinc oxide/barium carbonate regular raw materials. Only 16.6 percent of the batch was not premelted material.

This run, in a new platinum melter, was immediately followed by a 1000-lb. "control" run using regular non-premelted raw batch. Samples were collected each hour, lab ground and tested for properties and platinum content.

The results were as follows:

1. Platinum content of the glass was as follows:

The above results show much lower platinum with the premelted batch.

2. Glass Properties of the premelted batch:

The glass had excellent properties for a solder glass.