Precipitated silica used as reinforcing filler for elastomers

Abstract

The invention relates to a novel process for the preparation of precipitated silica which can be used as a reinforcing filler for elastomers. The invention also relates to novel precipitated silicas in the form of powder, granules or, preferably, substantially spherical beads, these silicas being characterized in that they have a BET specific surface of between 185 and 250 m 2 /g,a CTAB specific surface of between 180 and 240 m 2 /g,and a pore distribution such that the pore volume V2 made up of the pores with a diameter of between 175 and 275 Å represents less than 50% of the pore volume V1 made up of the pores with diameters of less than or equal to 400 Å, a pore volume (V d1 ), made up of the pores with a diameter of less than 1 μm, of greater than 1.65 cm 3 /g, a fineness value (F.V.) of between 70 and 100 Å, and a content of fines (τ f ), after deagglomerability with ultrasound, of at least 50%.

1 . Process for the preparation of precipitated silica, of the type comprising the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by separation and drying of this suspension, characterized in that: 
 the precipitation is carried out in the following way: 
 (i) an initial stock solution is formed containing at least some of the total amount of the silicate used in the reaction and at least one electrolyte, the concentration of silicate (expressed as SiO 2 ) in the said initial stock solution being between 50 and 60 g/l,  
 (ii) the acidifying agent is added to the said stock solution until a pH of between 7 and 8.5 for the reaction medium is obtained,  
 (iii) the acidifying agent is added to the reaction medium along with, where appropriate, simultaneously, the remaining amount of the silicate,  
   the separation comprises a filtration and washing operation using a filter equipped with a means of compacting,    a suspension having a solids content of less than 17% by weight is dried by spraying.    
     
     
         2 . Process according to  claim 1 , characterized in that the said concentration of silicate (expressed as SiO 2 ) in the said stock solution is between 55 and 60 g/l.  
     
     
         3 . Process according to either of claims  1  and  2 , characterized in that the said electrolyte is sodium sulphate, its concentration in the initial stock solution being between 12 and 20 g/l, in particular between 15 and 20 g/l.  
     
     
         4 . Process according to one of  claims 1  to  3 , characterized in that after the simultaneous addition of acidifying agent and of the remaining amount of silicate, an additional amount of acidifying agent is added to the reaction medium, preferably until a pH of between 4 and 6 is obtained in the reaction medium.  
     
     
         5 . Process according to one of  claims 1  to  3 , characterized in that the total amount of the silicate used in the reaction is introduced in step (i) and in that, in step (iii), the acidifying agent is added until a pH of between 4 and 6 is obtained in the reaction medium.  
     
     
         6 . Process according to one of  claims 1  to  5 , characterized in that a suspension with a solids content of between 14.5 and 16.5% by weight is dried by spraying.  
     
     
         7 . Process according to one of  claims 1  to  6 , characterized in that the said drying is carried out using a nozzle atomizer.  
     
     
         8 . Process according to one of  claims 1  to  7 , characterized in that the separation comprises a filtration, a washing and then a compacting operation, using a filter press.  
     
     
         9 . Process according to  claim 8 , characterized in that the said compacting is carried out by introduction of air at a pressure of less than 4.5 bar for 20 to 40 seconds, for example for about 30 seconds.  
     
     
         10 . Pressure according to  claim 9 , characterized in that the said pressure is between 3.8 and 4.3 bar.  
     
     
         11 . Process according to one of  claims 1  to  10 , characterized in that the dried product is subsequently ground and then optionally agglomerated.  
     
     
         12 . Process according to one of  claims 1  to  10 , characterized in that the dried product is then agglomerated.  
     
     
         13 . Precipitated silica, characterized in that it has: 
 a BET specific surface of between 185 and 250 m 2 /g,    a CTAB specific surface of between 180 and 240 m 2 /g,    a pore distribution such that the pore volume V2 made up of the pores with a diameter of between 175 and 275 Å represents less than 50% of the pore volume V1 made up of the pores with diameters of less than or equal to 400 Å,    a pore volume (V d1 ), made up of the pores with a diameter of less than 1 μm, of greater than 1.65 cm 3 /g,    a fineness value (F.V.) of between 70 and 100 Å,    a content of fines (τ f ), after deagglomerability with ultrasound, of at least 50%.    
     
     
         14 . Silica according to  claim 13 , characterized in that it has a pore distribution such that the ratio V2/V1 is not more than 0.45.  
     
     
         15 . Silica according to either of claims  13  and  14 , characterized in that it has a pore volume (V d1 ), made up of the pores with a diameter of less than 1 μm, of at least 1.70 cm 3 /g, preferably between 1.70 and 1.80 cm 3 /g.  
     
     
         16 . Silica according to one of  claims 13  to  15 , characterized in that it has a fineness value (F.V.) of between 80 and 100 Å.  
     
     
         17 . Silica according to one of  claims 13  to  16 , characterized in that it has a median diameter (φ 50 ), after deagglomerability with ultrasound, of less than 8.5 μm, preferably between 5 and 7 μm.  
     
     
         18 . Silica according to one of  claims 13  to  17 , characterized in that it has an ultrasound deagglomerability factor (F D ) of greater than 5.5 ml, preferably greater than 9 ml.  
     
     
         19 . Silica according to one of  claims 13  to  18 , characterized in that it has a pore volume (V3), made up of the pores with a diameter of between 100 and 300 Å, of at least 0.82 cm 3 /g, in particular of at least 0.85 cm 3 /g.  
     
     
         20 . Silica according to one of  claims 13  to  19 , characterized in that it has a total pore volume (TPV) of greater than 3.0 cm 3 /g, for example between 3.1 and 3.4 cm 3 /g.  
     
     
         21 . Silica according to one of  claims 13  to  20 , characterized in that it has a packed filling density (PFD) of greater than 0.28, in particular at least equal to 0.30.  
     
     
         22 . Silica according to one of  claims 13  to  21 , characterized in that it has an oil absorption value DOP of between 230 and 330 ml/100 g, preferably between 250 and 300 ml/100 g.  
     
     
         23 . Silica according to one of  claims 13  to  22 , characterized in that it is in the form of substantially spherical beads with an average size of at least 80 μm.  
     
     
         24 . Silica according to one of  claims 13  to  22 , characterized in that it is in the form of powder with an average size of at least 15 μm.  
     
     
         25 . Silica according to one of  claims 13  to  22 , characterized in that it is in the form of granules at least 1 mm in size.  
     
     
         26 . Use, as a reinforcing filler for elastomers, of a silica obtained by the process according to one of  claims 1  to  12  or of a silica according to one of  claims 13  to  25 .