Precipitated silicas, a process for their preparation and their use in vulcanizable rubber mixtures

A process for producing a precipitated silica having a CTAB surface area, in accordance with ASTM D 3765-92, of 200 to 400 m.sup.2 /g, a DBP index, in accordance with ASTM D 2414, between 230 and 380 ml/100 g as powder and 180-250 g/100 g as granulate, a silanol group density, V.sub.2 -NaOH consumption, of 20 to 30 ml and the following macropore size distribution which is typical of the surface area range involved, determined by means of Hg porosimetry (DIN 66 133) for specific pore size intervals (incremental mode of application):

• ______________________________________ CTAB surface CTAB surface CTAB surface area range:
• area range:
• area range:
• Pore size 200-250 250-300 300-400 interval in m.sup.2 /g m.sup.2 /g m.sup.2 /g nm Hg consumption in ml/g of silica ______________________________________ 10-20 0.27-0.49 0.35-0.50 0.32-0.42 20-30 0.22-0.32 0.15-0.30 0.17-0.22 30-40 0.15-0.21 0.12-0.17 0.12-0.15 40-50 0.11-0.16 0.09-0.12 0.08-0.11 50-60 0.08-0.12 0.06-0.10 0.06-0.09 ______________________________________
• and produced by reacting sulphuric acid with an alkali metal silicate solution,
• which process comprises:
• acidifying water at a temperature from 30.degree. to 90.degree. C. to a pH of 5 to 5.9 by adding small amounts of sulphuric acid;
• introducing simultaneously said alkali metal silicate solution and sulphuric acid in to said acidified water to precipitate silica, the pH of the solution being kept constant at 5 to 5.9 and the reaction solution being agitated during the entire process;
• suspending the introduction of said alkali metal silicate and sulphuric acid when it is detected that the viscosity of the reaction solution increases to halt the precipitation of silica;
• after an interval of 30 to 120 minutes, resuming the introduction of alkali metal silicate and sulphuric acid into the reaction solution until the concentration of silica reaches from 40 to 60 g/liter, thereby producing a silica suspension.