Silica-based sols

Abstract

A process for the production of an aqueous sol containing silica-based particles which comprises (a) acidifying an aqueous silicate solution to a pH of from 1 to 4 to form an acid sol; (b) alkalizing the acid sol at an SiO 2 content within the range of from 4.5 to 8% by weight; (c) allowing particle growth of the alkalized sol for at least 10 minutes; or heat-treating the alkalized sol at a temperature of a least 30° C.; (d) alkalizing the obtained sol to a pH of at least 10.0; and (e) optionally concentrating the sol obtained according to (b), (c) or (d) to provide an aqueous sol containing silica-based particles and having a specific surface area of at least 90 m 2 /g aqueous sol; as well as an aqueous sol containing silica-based particles obtainable by the process. The invention also relates to an aqueous sol containing silica-based particles which sol has a specific surface area of at least 115 m 2 /g aqueous sol and an S-value within the range of from 10 to 45% or contains silica-based particles having a specific surface area of at least 550 and less than 1000 m 2 /g SiO 2 . The invention further relates to the use of the aqueous sol containing silica-based particles as a drainage and retention aid in the production of paper as well as a process for the production of paper from an aqueous suspension containing cellulosic fibers, and optional filler, in which silica-based particles and at least one charged organic polymer are added to the cellulosic suspension.

1 . Process for the production of an aqueous sol containing silica-based particles which comprises: 
 (a) acidifying an aqueous silicate solution to a pH of from 1 to 4 to form an acid sol;    (b) alkalising the acid sol at an SiO 2  content within the range of from 4.5 to 8% by weight to a pH of at least 7;    (c) allowing particle growth of the alkalised sol for at least 10 minutes;    (d) alkalising the obtained sol to a pH of at least 10.0; and    (e) optionally concentrating the sol obtained according to (b), (c) or (d) to provide an aqueous sol containing silica-based particles and having a specific surface area of at least 90 m 2 /g aqueous sol.    
     
     
         2 . Process for the production of an aqueous sol containing silica-based particles which comprises: 
 (a) acidifying an aqueous silicate solution to a pH of from 1 to 4 to form an acid sol;    (b) alkalising the acid sol at an SiO 2  content within the range of from 4.5 to 8% by weight;    (c) heat-treating the alkalised sol at a temperature of at least 30° C.;    (d) alkalising the heat-treated sol to a pH of at least 10.0; and    (e) optionally concentrating the sol obtained according to (b), (c) or (d) to provide an aqueous sol containing silica-based particles and having a specific surface area of at least 90 m 2 /g aqueous sol.    
     
     
         3 . Process according to  claim 1  or  2 , characterised in that it comprises (e) concentrating the sol obtained according to (c) or (d) to provide a sol having a specific surface area of at least 95 m 2 /g aqueous sol.  
     
     
         4 . Process according to  claim 1 ,  2  or  3 , characterised in that the alkalisation according to (b) and (d) is carried out by means of an aqueous silicate solution.  
     
     
         5 . Process according to any of  claims 1  to  4 , characterised in that the particle growth and heat-treatment according to (c) is carried out at a temperature within the range of from 35 to 95° C.  
     
     
         6 . Process according to any of  claims 1  to  5 , characterised in that the particle growth and heat-treatment according to (c) is carried out for 20 to 240 minutes.  
     
     
         7 . Process according to any of  claims 1  to  6 , characterised in that the alkalisation according to (d) produces a sol having a molar ratio of SiO 2  to M 2 O, where M is alkali metal or ammonium, within the range of from 15:1 to 30:1 and a pH of at least 10.6.  
     
     
         8 . Process according to any of  claims 1  to  7 , characterised in that it further comprises addition of an aluminium-containing compound and/or a boron-containing compound.  
     
     
         9 . Process according to any of  claims 1  to  8 , characterised in that the silica-based particles obtained have a specific surface area of at least 550 m 2 /g SiO 2 .  
     
     
         10 . Aqueous sol containing silica-based particles obtainable by a process according to any of  claims 1  to  9 .  
     
     
         11 . Aqueous sol containing silica-based particles, characterised in that it has a specific surface area of at least 115 m 2 /g aqueous sol and the silica-based particles have a specific surface area of at least 550 and less than 1000 m 2 /g SiO 2 .  
     
     
         12 . Aqueous sol containing silica-based particles, characterised in that it has a specific surface area of at least 115 m 2 /g aqueous sol and an S-value within the range of from 10 to 45%.  
     
     
         13 . Aqueous sol according to  claim 11  or  12 , characterised in that it has a molar ratio of SiO 2  to M 2 O, where M is alkali metal or ammonium, within the range of from 15:1 to 40:1.  
     
     
         14 . Aqueous sol according to  claim 12  or  13 , characterised in that the silica-based particles have a specific surface area of at least 550 m 2 /g SiO 2 .  
     
     
         15 . Aqueous sol according to any of  claims 11  to  14 , characterised in that it has an S-value within the range of from 25 to 35%.  
     
     
         16 . Aqueous sol according to any of  claims 11  to  15 , characterised in that it has a silica content of at least 10% by weight.  
     
     
         17 . Use of an aqueous sol containing silica-based particles according to any of  claims 10  to  16  or produced by a process according to any of  claims 1  to  9  as a drainage and retention aid in the production of paper.  
     
     
         18 . Process for the production of paper from an aqueous suspension containing cellulosic fibres, and optional fillers, which comprises adding to the suspension silica-based particles and at least one charged organic polymer, forming and draining the suspension on a wire, characterised in that the silica-based particles are obtained by a process according to any of  claims 1  to  9  or present in an aqueous sol according to any of  claims 10  to  16 .  
     
     
         19 . Process for the production of paper which comprises: 
 (a) providing an aqueous suspension containing cellulosic fibres, and optional fillers;    (b) providing an aqueous sol containing silica-based particles, the sol having a specific surface area of at least 90 m 2 /g aqueous sol and the silica-based particles having a specific surface area of less than 1000 m 2 /g SiO 2 ;    (c) providing at least one charged organic polymer;    (d) adding the charged organic polymer and the silica-based particles to the suspension;    (e) forming and draining the obtained suspension on a wire.    
     
     
         20 . Process for the production of paper which comprises: 
 (a) providing an aqueous suspension containing cellulosic fibres, and optional fillers;    (b) providing an aqueous sol containing silica-based particles having a specific surface area of at least 90 m 2 /g aqueous sol and an S-value within the range of from 10 to 45%;    (c) providing at least one charged organic polymer;    (d) adding the charged organic polymer and the silica-based particles to the suspension;    (e) forming and draining the obtained suspension on a wire.    
     
     
         21 . Process according to  claim 19  or  20 , characterised in that the sol has a specific surface area in the range of from 95 to 150 m 2 /g aqueous sol.  
     
     
         22 . Process according to  claim 19 ,  20  or  21 , characterised in that the silica-based particles have a specific surface area of at least 550 m 2 /g SiO 2 .  
     
     
         23 . Process according to any of  claims 19  to  22 , characterised in that the charged organic polymer is cationic starch or cationic polyacrylamide.  
     
     
         24 . Process according to any of  claims 19  to  23 , characterised in that the aqueous sol is diluted to a silica content of from 0.05 to 5% by weight before adding the silica-based particles to the suspension.  
     
     
         25 . Process according to any of  claims 19  to  23 , characterised in that the silica-based particles are added to the suspension in an amount of from 0.005 to 0.5% by weight, calculated as SiO 2  and based on dry cellulosic fibres and optional fillers.