Compositions: coating or plastic – Coating or plastic compositions – Alkali metal silicate containing
Patent
1990-03-13
1992-09-29
Bell, Mark L.
Compositions: coating or plastic
Coating or plastic compositions
Alkali metal silicate containing
106 383, 21914523, C04B 2826, B23K 3522
Patent
active
051511215
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to lithium silicate binder solutions which are of value in the manufacture, as examples, of welding rods and sand moulds.
BACKGROUND OF THE INVENTION
Silicates are well characterised in the literature, for example `The Chemistry of Silica` by R. K. Iler (published by Wiley Interscience 1979), and they have a number of applications as binders Examples of these applications are in sand moulding for metal casting, protective coatings, sealants, adhesives and as welding rod binders.
In the latter application the silicate binder is required to provide mechanical strength to the formulation around the metal core and have a satisfactory moisture resistance.
Additionally, the effect of binder composition on the evolution of chromium fumes during welding has been described by Kimura et al in a paper presented to the American Welding Society in April 1979 and published in Welding Research Supplement July 1979 The chromium fumes can arise from both the substrate and rod and Kimura demonstrated that reducing the sodium and potassium level in the water soluble binder component leads to a reduction in evolution of chromium fumes while the presence of lithium did not lead to chromium evolution. Thus there is an advantage in using only lithium silicate as the rod binder.
A general background description of welding can be found in Kirk Othner (pub Wiley & Son 1970) vol 22 pages 241 to 252. Although the term welding `rod` is used herein it will be understood that the welding electrode used may have a shape other than the geometric rod.
GENERAL DESCRIPTION OF THE INVENTION
The invention provides a lithium silicate capable of acting as the sole silicate binder in welding rod compositions, sand moulds and the like and having a SiO.sub.2 :Li.sub.2 O weight ratio from about 7.5 to about 9:1. This silicate also provides sand moulds for casting having a particularly good strength. This lithium silicate is preferably obtained as solutions with the properties:
______________________________________ Specific gravity @ 20.degree. C.
1.19 to 1.21
Lithium oxide 2.4 to 2.6% w/w
Silica 20.1 to 21.0% w/w
Weight ratio SiO.sub.2 :Li.sub.2 O
7.5 to 9:1
Solids content 22% to 26% wt
______________________________________
Preferably the solids content is from about 18%, preferably from about 22%, to about 26% by weight Above 26% the solution becomes too viscous to use and below 18% the solution does not act as an efficient binder.
The invention extends to a method of preparing lithium silicate as defined wherein silica hydrogel is mixed with lithium hydroxide solution, preferably under conditions of shear, at a temperature not above about 35.degree. C. The lump silica hydrogel used in the process is obtained from an acid/silicate reaction, for example as disclosed in GB 1215928 (Unilever), and may contain from about 5% to about 80% by weight silica, more usually about 20% to about 50%
The applicants have found lithium silicate of the defined weight ratio provides welding rods of desired strength and water resistance without the requirement to have another soluble silicate present which would give rise to chromium fumes. The use of the lithium silicate of the invention as the sole silicate binder also reduces the total fume evolved during welding.
LITERATURE
The use of a mixture of lithium, potassium and sodium silicates as a welding rod binder solution is described in GB 1532847 (van Bearle) Preparation of a silicates mixture by dissolving silica hydrogel in silicate solution and then admixing lithium hydroxide is provided in U.S. Pat. No. 3,522,066 (Lithium Corp). The applications for lithium silicates have been generally described in Electrical Manufacture September 1964. GB 1183120 (Henkel) describes preparation of lithium silicates by mixing silicic acid and lithium hydroxide solution at a temperature above 80.degree. C. GB 1581182 (Kobe Steel) is directed to the problem of chromium fumes in welding but teaches the use of specified oxides in colloidal form in the binde
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Bell Mark L.
Jones Deborah
Unilever Patent Holdings B.V.
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