Silica-Based Particles, Method of Producing the Same, Paint for Forming Coating Film and Coated

Abstract

To provide silica-based particles with a low refractive index, the hollow and spherical silica-based particles include porous materials or cavities inside an outer shell thereof by growing porous composite oxide particles (primary particles) adjusting a ratio of a quantity of added silica sources versus a quantity of added sources for an inorganic oxide other than silica and then removing the inorganic oxide other than silica. The dispersion liquid of silica-based particles is aged at a range from the room temperature to 300° C., cleaning is performed according to the necessity, and hydrothermal processing is performed at a temperature in the range from 50 to 300° C.

1 . A method of producing silica-based particles comprising the steps (a), (b), (c) and (e): 
 (a) a step in which, when a dispersion liquid of composite oxide particles is prepared by simultaneously adding an aqueous silicate solution and/or an acidic silicic acid solution and an aqueous solution of an alkali-soluble inorganic compound in an alkali aqueous solution or in an alkali aqueous solution with seed particles dispersed therein, if required, the aqueous silicate solution and/or the acidic silicic acid solution and the aqueous solution of alkali-soluble inorganic compound are added so that the molar ratio of MO x /SiO 2  are in a range from 0.01 to 2, herein MO x  denoting an inorganic oxide other than silica and SiO 2  denoting silica to prepare the dispersion liquid of composite oxide particles with an average diameter (D p1 ) in a range from 3 to 300 nm,    (b) a step in which the aqueous silicate solution and/or the acidic silicic acid solution and the aqueous solution of alkali-soluble inorganic compound are added at the molar ratio MO x /SiO 2  smaller than that employed in the step (a) above to prepare a dispersion liquid of the composite oxide particles with the average diameters (D p2 ) of up to 500 nm,    (c) a step in which an acid is added to the dispersion liquid of composite oxide particles and then at least a portion of elements constituting the composite oxide particles other than silicon is removed to prepare a dispersion liquid of silica-based particles, and    (e) a step in which cleaning is performed according to the necessity, and the dispersion liquid of silica-based particles is aged at a range from the room temperature to 300° C.    
     
     
         2 . The method of producing silica-based particles according to  claim 1 , wherein a value of B/A (A: the molar ratio MO x /SiO 2  in step (a), B: the molar ratio MO x /SiO 2  in step (b)) is 0.8 or below.  
     
     
         3 . The method of producing silica-based particles according to  claim 1 , wherein a ratio (D p1 /D p2 ) of the average diameter (D p1 ) of the composite oxide particles versus the average diameter (D p2 ) of the composite oxide particles is in the range from 0.4 to 0.98.  
     
     
         4 . The method of producing silica-based particles according to  claim 1 , wherein the step (b) and/or the step (c) above are performed in the presence of an electrolytic salt at a molar ratio (M E /M S ) of 10 or below in which M E  denotes a mole number of the electrolytic salt and M S  denotes a mole number of SiO 2 .  
     
     
         5 . The method of producing silica-based particles according to  claim 1  further comprising a step (d) performed between the step (c) and the step (e): 
 (d) a step in which an organic silicon compound expressed by the following chemical formula (1) and/or a partially hydrolyzed product thereof, and an alkali aqueous solution, if necessary, are added in the dispersion liquid of silica-based particles obtained in the step (c) above to form a silica-coating layer on the particles:      R n SiX (4-n)   (1)    wherein R denotes a not-substituted or substituted hydrocarbon group having 1 to 10 carbon atoms, an acrylic group, an epoxy group, a methacrylic group, amino group, or a CF 2  group; X denotes an alkoxy group having 1 to 4 carbon atoms, a silanol group, a halogen or hydrogen; and n indicates an integral number in the range from 0 to 3.    
     
     
         6 . The method of producing silica-based particles according to  claim 1  further comprising a step (f): 
 (f) a step in which an organic silicon compound expressed by the following chemical formula (1) and/or a partially hydrolyzed product thereof, and an alkali aqueous solution, if necessary, are added in the dispersion liquid of silica-based particles obtained in the step (e) above to form a silica-coating layer on the particles:      R n SiX (4-n)   (1)    wherein R denotes a not-substituted or substituted hydrocarbon group having 1 to 10 carbon atoms, an acrylic group, an epoxy group, a methacrylic group, amino group, or a CF 2  group; X denotes an alkoxy group having 1 to 4 carbon atoms, a silanol group, a halogen or hydrogen; and n indicates an integral number in the range from 0 to 3.    
     
     
         7 . The method of producing silica-based particles according to  claim 1 , wherein pH of the alkali aqueous solution or of the alkali aqueous solution with seed particles dispersed therein is 10 or more.  
     
     
         8 . The method of producing silica-based particles according to  claim 1  further comprising a step (g) after the step (e) or the step (f): 
 (g) a step in which cleaning is performed according to the necessity, and then hydrothermal processing is performed at a temperature in the range from 50 to 300° C.    
     
     
         9 . The method of producing silica-based particles according to  claim 1  further comprising a step (h) after the step (g): 
 (h) a step in which the organic silicon compound expressed by the following chemical formula (1) and/or a partially hydrolyzed product thereof, and an alkali aqueous solution, if necessary, are added in the dispersion liquid of silica-based particles obtained in the step (g) above to form a silica-coating layer on the particles:      R n SiX (4-n)   (1)    wherein R denotes a not-substituted or substituted hydrocarbon group having 1 to 10 carbon atoms, an acrylic group, an epoxy group, a methacrylic group, amino group, or a CF 2  group; X denotes an alkoxy group having 1 to 4 carbon atoms, a silanol group, a halogen or hydrogen; and n indicates an integral number in the range from 0 to 3.    
     
     
         10 . The method of producing silica-based particles according to  claim 1 , wherein the hydrothermal processing is repeated several times.  
     
     
         11 . The method of producing silica-based particles according to  claim 1 , wherein the inorganic oxide other than silica is alumina.  
     
     
         12 . A method of producing silica-based particles further comprising: 
 a step in which the dispersion liquid of silica-based particles obtained by the method according to  claim 1  is cleaned, dried, and calcinated, if required.    
     
     
         13 . The method of producing silica-based particles according to  claim 1 , wherein an average diameter of the particles is in the range from 5 to 500 nm.  
     
     
         14 . The method of producing silica-based particles according to  claim 1 , wherein a content of an alkali metal oxide in the silica-based particles or the dispersion liquid of the silica-based particles is 5 ppm or below as expressed by a content of M 2 O per silica-based particle (M: an alkali metal element).  
     
     
         15 . The method of producing silica-based particles according to any of  claim 1 , wherein contents of ammonia and/or ammonium ions in the silica-based particles or the dispersion liquid of the silica-based particles is 1500 ppm or below as expressed by a content of NH 3 .  
     
     
         16 . Silica-based particles having a porous material and/or cavities in an outer shell thereof, wherein a ratio (S B /S C ) of a specific surface area (S B ) of the particles measured by the BET method versus a specific surface area (S C ) expressed by the following expression is in the range from 1.1 to 5:  
           S   c (m 2 /g)=6000 /Dp (nm)*ρ 
       Dp denoting an average diameter of the silica-based particles, and ρ denoting a density (g/ml).  
     
     
         17 . The silica-based particles according to  claim 16 , wherein the average diameter is in the range from 5 to 500 nm.  
     
     
         18 . The silica-based particles according to  claim 16 , wherein a thickness of the outer shell is in the range from 5 to 20 nm.  
     
     
         19 . The silica-based particles according to  claim 16 , wherein the refractive index is in the range from 1.15 to 1.38.  
     
     
         20 . A paint for forming coating film comprising: 
 the silica-based particles obtained by the production method according to  claim 1 , or the silica-based particles according to  claim 16;  and    a matrix for forming a coating film.    
     
     
         21 . The paint for forming coating film according to  claim 20  further comprising: 
 oxide-based particles other than the silica-based particles.    
     
     
         22 . A coated substrate having a coating film containing the silica-based particles produced by the production method according to  claim 1  or the silica-based particles according to  claim 16  and a matrix for forming a coating film formed singly or together with other coating film on a surface thereof.