GB941367A - Improvements in or relating to coating impervious granular material - Google Patents

Abstract

A process for the preparation of resin-coated particles of impervious granular material suitable for use in the production of shell moulds, for example sand or silicon carbide, comprises applying an aqueous alkaline solution of a liquid, water-soluble initial condensation product, produced by the condensation of a phenolic compound with less than an equimolar amount of an aldehyde at a temperature not exceeding 75 DEG C. in the presence of an alkaline condensation catalyst, to a mass of the impervious particles to coat the surfaces thereof, adding an acid to the wetted mass in an amount to lower the pH of the said aqueous solution to 2 or below, and converting said initial condensation product to a Novolak resin in situ on said particles by heating at a temperature not exceeding 75 DEG C. whilst agitating the mass to dry it. A hardening agent, e.g. hexamethylene tetramine, may be incorporated with the resincoated particles during the drying stage. The phenolic compound may be phenol, a cresol, a xylenol, or mixed cresols and xylenols. The aldehyde may be formaldehyde, furfuraldehyde, acetaldehyde or propionaldehyde. The molar ratio of phenol to aldehyde may be from 1 : 0.6 to 1 : 0.85. Alkaline condensation catalystinclude NaOH, KOH, Ca(OH)2, Ba(OH)2, Sr(OH)2, the corresponding carbonates, and NH4OH, whilst the acid used to lower the pH may be hydrochloric, sulphuric or phosphoric.ALSO:A process for the production of resin-coated particles of impervious granular material suitable for use in the production of shell moulds, for example sand, silicon carbide or glass, comprises applying an aqueous alkaline solution of a liquid, water-soluble initial condensation product, produced by the condensation of a phenolic compound with less than an equimolar amount of an aldehyde at a temperature not exceeding 75 DEG C. in the presence of an alkaline condensation catalyst, to a mass of the impervious particles to coat the surfaces thereof, adding an acid to the wetted mass in an amount to lower the pH of the said aqueous solution to 2 or below, and converting said initial condensation product to a novolak resin in situ on said particles by heating at a temperature not exceeding 75 DEG C. whilst agitating the mass to dry it. A hardening agent, e.g. hexamethylene tetramine, may be incorporated with the resin-coated particles during the drying stage. The phenolic compound may be phenol, a cresol, a xylenol, or mixed cresols and xylenols. The aldehyde may be formaldehyde, froformaldehyde, acetaldehyde or propionaldehyde. The molar ratio of phenol to aldehyde may be 1:0,6 to 1:0,85. Alkaline condensation catalyst include NaOH, KOH, Ca(OH)2, Ba(OH)2, Sr(OH)2, the corresponding carbonates, and NH4OH, whilst the acid used to lower the pH may be hydrochloric, sulphuric or phosphoric.