Alkali-Resistant Glass Compositions

1. Field of the Invention

This invention relates to alkali-resistant glass compositions and particularly, though not exclusively, to such compositions which can be formed into glass fibres.

2. Description of the Prior Art

It is known to include a proportion of zirconia (ZrO.sub.2) in glass compositions for enhancing their resistance to alkalis. It is also known that the inclusion of substantial proportions of boron oxide (B.sub.2 O.sub.3) and/or alkali metal oxides (R.sub.2 O) has a deleterious effect on the alkali resistance of the glass, although both these constituents have other favourable properties which have hitherto been considered to make their inclusion desirable. For example, both B.sub.2 O.sub.3 and R.sub.2 O act as fluxes to aid melting and thus help to overcome the tendency of ZrO.sub.2 to make melting difficult, and they can also improve the characteristics of the glass for drawing glass fibres.

It is an object of the present invention to provide glass compositions with a particularly high alkali resistance.

According to the present invention, the major components of the glass composition comprise, in weight percentages:-

The total of SiO.sub.2 + ZrO.sub.2 + RO being not less than 94% by weight of the glass, where RO represents at least one divalent oxide of the group consisting of CaO, MgO, SrO, BaO and ZnO, the amount of said divalent oxide or oxides lying within the ranges, in weight percentages: CaO 20 to 45%; MgO 0 to 10%; SrO 0 to 8%; BaO 0 to 10% and ZnO 0 to 5%.

The balance of the composition may consist of at least one of the following constituents: TiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, Fe.sub.2 O.sub.3, F.sub.2 and R.sub.2 O, where R.sub.2 O represents K.sub.2 O, Na.sub.2 O, or Li.sub.2 O, the amount of any one of the said constituents not exceeding 5% by weight of the composition. Preferably the amount of R.sub.2 O does not exceed 3% by weight of the composition.

The glass compositions according to the invention thus contain relatively large proportions of ZrO.sub.2, while being free from, or containing only low proportions of, B.sub.2 O.sub.3 and R.sub.2 O. In spite of the absence or low level of these fluxing agents, it has proved possible to melt the glass compositions quite readily.

When subjected to standard tests for chemical durability in aqueous and in alkaline environments, such glass compositions have shown excellent results. It has also proved possible to form them into glass wool fibres, e.g. by high temperature blown type processes.

Specific embodiments of glass compositions in accordance with the invention will now be described by way of example.

The following Table 1 lists 18 glass compositions consisting of ZrO.sub.2, SiO.sub.2 and CaO, illustrating the use of four different values for ZrO.sub.2 (18, 14, 10 and 8 weight %) with varying proportions of SiO.sub.2 between 45 and 65 weight % and correspondingly inversely varying proportions of CaO between 45 and 21 weight %.

Table 2 lists 24 further glass compositions, each of which is identified by a number C/07 followed by a three-figure number. For most of the glasses in Table 2, the composition is given both in mole percentages and in weight percentages and the liquidus temperature and the results of standard chemical durability tests are given. In the latter tests, 10 gm samples of the glass are ground to a powder and treated with 50 c.c. of the specified reagent (water, N/10 NaOH and N.NaOH respectively). The weights of the component oxides extracted (in milligrams) are listed in Table 2.

The liquidus temperatures of glasses C/07/208 to 216 inclusive were not precisely measured, but it was ascertained that they all fell within the range from 1400.degree. C. to 1500.degree. C. The chemical durability of glasses C/07/211 and 213 was not measured because these glasses are very similar to glass 212, differing only in the content of ZrO.sub.2 with consequential adjustment to the SiO.sub.2 content, and they can consequently be confidently predicted to have similar chemical durability, 211 being slightly less good due to its lower ZrO.sub.2 and 213 being slightly better due to its higher ZrO.sub.2 content.

When using the maximum permissible amount of SiO.sub.2 (65 weight %) a proportion of up to 5 weight % Na.sub.2 O may be included, as in glasses C/07/208 and 209, to improve the drawing characteristics of the glass and thereby facilitate the formation of fibres. These glasses also contain the minimum permissible amount of RO (20 weight%) in the form of CaO. In general, the amount of RO increases as the amount of SiO.sub.2 is reduced. As shown by the foregoing Examples, CaO may vary between 20 and 45%, and up to 10% of the RO may consist of MgO, as in glass C/07/214. Up to 10% of the RO can be BaO, up to 8% of the RO can be SrO, and up to 5% of the RO can be ZnO, if desired. With MgO or SrO present, a slight lowering of the liquidus temperature can be achieved, which is beneficial for formation of glass fibres. A small amount of TiO.sub.2 can also be included, as in glass C/07/179, to produce a similar lowering of the liquidus temperature, but TiO.sub.2 also tends to reduce the alkali resistance so it can only be used to a limited extent, i.e. up to 5 weight %. Al.sub.2 O.sub.3 produces similar effects.

B.sub.2 O.sub.3 or F.sub.2 could also be included in amounts of up to 5 weight % to assist melting. Fe.sub.2 O.sub.3 may be present in the customary small amounts (up to 0.5 weight %) which result from the normal impurities in raw materials.