Coating composition for the foundry industry, containing particulate, amorphous silicon dioxide and acid

Abstract

A coating composition is described, for use in the foundry, in particular comprising particulate, amorphous silicon dioxide (SiO2) and an aqueous phase having a pH of at most 5, and also coated, waterglass-bound foundry molding elements, especially coated, waterglass-bound foundry molds and foundry cores, which each comprise a coating composition of the invention. Further described is the use of a coating composition of the invention for producing a coating on a waterglass-bound foundry molding element and a method for producing a waterglass-bound foundry molding element (mold or core) coated with a water-containing refractory coating. Likewise specified is a kit whose contents include a coating composition of the invention.

1 . A coated, waterglass-bound mold or coated, waterglass-bound core for use in the foundry, comprising a coating composition comprising
 (a) an aqueous phase having a pH of at most 5,   (b) particulate, amorphous silicon dioxide, and   (c) one or more further refractories.   
     
     
         2 . The coated mold or coated core as claimed in  claim 1 , where the waterglass-bound mold or the waterglass-bound core comprises particulate, amorphous silicon dioxide. 
     
     
         3 . The coated mold or coated core as claimed in  claim 2 , where the primary particles of the particulate, amorphous silicon dioxide of constituent (b) (i) are spherical and/or (ii) possess a D90<10 μm, determined by laser diffraction,
 where the primary particles of the particulate, amorphous silicon dioxide of constituent (b) (i) are spherical and possess a sphericity of 0.9 or more, determined by evaluation of two-dimensional microscope images. 
 
     
     
         4 . The coated mold or coated core as claimed in  claim 1 , where the constituent (c) comprises one or more substances selected from the group consisting of quartz, aluminum oxide, zirconium dioxide, aluminum silicates, phyllosilicates, zirconium silicates, olivine, talc, mica, graphite, coke, feldspar, diatomite, kaolins, calcined kaolins, metakaolinite, iron oxide, and bauxite,
 and/or   where the constituent (a) comprises one or more acids, having a pKa<5, which are selected from the group consisting of inorganic and organic acids,
 where the organic acids are selected from the group consisting of mono-, di-, and tricarboxylic acids, 
   and/or
 where the inorganic acids are selected from the group consisting of hydrochloric acid, nitric acid, and phosphoric acid, 
   and/or   comprising in or as constituent (b) a particulate, amorphous silicon dioxide which as a secondary constituent comprises (i) zirconium dioxide, (ii) carbon and/or (iii) a Lewis acid   and/or   where the aqueous phase (a) has a pH of at most 4.   
     
     
         5 . The coated mold or coated core as claimed in  claim 1 , comprising one or more or all of the following constituents:
 one or more biocides,   one or more wetting agents,   one or more rheological additives, and   one or more binders.   
     
     
         6 . The coated mold or coated core as claimed in  claim 1 , where the ratio of the total mass of inorganic and organic acids in the aqueous phase (a) in relation to the total mass of the coating composition is in the range from 0.1 to 10%,
 and/or   where the ratio of the water to the total mass of the aqueous phase of constituent (a) is greater than 50%,   and/or   where the coating composition possesses a solids content of less than 80 wt %, based on the total mass of the coating composition,   and/or   where the coating composition possesses a fraction of particulate, amorphous silicon dioxide of constituent (b) in the range from 1 to 30 wt %, based on the total mass of the coating composition   and/or   where the coating composition possesses a total fraction of particulate, amorphous silicon dioxide of constituent (b) and of further refractories of constituent (c) in the range from 25 wt % to 80 wt %, based on the total mass of the coating composition.   
     
     
         7 . The coated mold or coated core as claimed in  claim 1 , comprising one or more binders, in a total amount of not more than 2 wt %, based on the total mass of the coating composition. 
     
     
         8 . The coated mold or coated core as claimed in  claim 1  for use in the casting of a metal melt having a temperature >900° C. 
     
     
         9 . The coated mold or coated core as claimed in  claim 4 , wherein the organic acids are selected from the group consisting of mono-, di-, and tricarboxylic acids, which are solid at 25° C. and 1013 mbar. 
     
     
         10 . The coated mold or coated core as claimed in  claim 9 , wherein the organic acids are selected from the group consisting of citric acid and oxalic acid. 
     
     
         11 . The coated mold or coated core as claimed in  claim 5 , wherein the one or more binders comprises polyvinyl alcohol. 
     
     
         12 . The coated mold or coated core as claimed in  claim 7 , comprising one or more binders, in a total amount in the range from 0.05 to 0.80 wt %, based on the total mass of the coating composition. 
     
     
         13 . The coated mold or coated core as claimed in  claim 6 , where the ratio of the total mass of inorganic and organic acids in the aqueous phase (a) in relation to the total mass of the coating composition is in the range from 1 to 5%. 
     
     
         14 . A kit for producing the coated, waterglass-bound mold or coated, waterglass-bound core of  claim 1 , including in separate components
 (U) a coating composition comprising
 (a) an aqueous phase having a pH of at most 5, 
 (b) particulate, amorphous silicon dioxide, and 
 (c) one or more further refractories, and 
   (V) a binder comprising waterglass.   
     
     
         15 . A process for producing a waterglass-bound mold coated with a water-containing refractory coating or a waterglass-bound core coated with a water-containing refractory coating, for use in the foundry, comprising the following steps:
 (1) providing or producing a coating composition comprising
 (a) an aqueous phase having a pH of at most 5, 
 (b) particulate, amorphous silicon dioxide, and 
 (c) one or more further refractories, 
   (2) providing or producing an uncoated, waterglass-bound mold or an uncoated, waterglass-bound core, and   (3) applying the provided or produced coating composition from step (1) to the provided or produced mold or the provided or produced core.   
     
     
         16 . A coated mold or coated core, producible by a process as claimed in  claim 15 . 
     
     
         17 . The process as claimed in  claim 15 , where the provided or produced uncoated mold or the provided or produced uncoated core comprises particulate, amorphous silicon dioxide,
 and/or   where the uncoated, waterglass-bound mold or the uncoated, waterglass-bound core is produced in step (2) by curing a provided or produced molding material mixture
 by gassing with carbon dioxide, 
 by admixing esters or phosphates 
   or
 by gassing with hot air in a heated tool. 
   
     
     
         18 . The process as claimed in  claim 15 ,
 where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place at a temperature of the provided or produced mold or provided or produced core of >50° C.,   and/or   where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place by an application process selected from the group consisting of spraying, dipping, flow coating, and spreading.   
     
     
         19 . The process as claimed in  claim 18 , where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place at a temperature >70° C. and <100° C. 
     
     
         20 . The process as claimed in  claim 18 , where the applying to the provided or produced uncoated mold or the provided or produced uncoated core takes place by dipping.