Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Patent
1992-05-08
1994-11-29
Chaudhuri, Olik
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
2523132, 2523156, 502233, C01B 3312
Patent
active
053688333
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to new silica sols, to a process suitable for the production of the sols and to the use of the new sols in the production of paper. More particularly the invention relates to new silica sols which have particles with a high specific surface area and which have a fairly high content of so called microgel. The new sols are particularly suitable for use in the production of paper as additives in combination with cationic polymers.
Silica sols, which term is used herein for silica hydrosols, are aqueous systems with very small silica particles which can be used in several fields of application, among other things dependent on the particle size. At the production of paper silica based sols with anionic particles have gained an increased utilization during the last few years. The silica sols are here used as additives to the stock in combination with cationic polymers, primarily to increase retention and dewatering at papermaking. In the European patent 41056 is for example disclosed the use of colloidal silica sols in combination with cationic starch in the production of paper. The PCT applications WO 86/00100 and WO 86/05826 disclose combinations of silica sols, with particles where at least the surface groups contain aluminum, and cationic natural polymers and cationic polyacrylamides, respectively. It is generally stated that the silica particles have a specific surface area within the range of from 50 to 1000 m.sup.2 /g. The commercially used sols at production of paper are of the type which have colloidal particles with a particle size usually from about 4 to about 7 nm, i.e. a specific surface area of from about 700 to about 300 m.sup.2 /g and above all sols with particles having a specific surface area of about 500 m.sup.2 /g have been used commercially. It has generally been considered that sols with colloidal particles of the above given size have given the best results and they have also been preferred with regard to their stability. Quite generally it has been desired silica sols which are to be used commercially that they are as monodisperse as possible, i.e. that they have as narrow particle size distribution as possible. Consequently work at the production of the sols has aimed at avoiding formation of microgel.
The present invention relates to new silica sols which are characteristic in that they have a comparatively low S-value and in that the sol particles have a high specific surface area. It has been found that these sols which contain anionic particles are useful at the production of paper and similar products and that they hereby, in combination with cationic polymers, give very good improvement of retention and dewatering. In addition to silica sols as defined in the appended claims the present invention also relates to a process for the production of silica sols and to the use of the sols, as defined in the appended claims.
As mentioned above, the present silica sols are characteristic in that they have a high specific surface area, within the range of from 750 to 1000 m.sup.2 /g. The given specific surface area has been measured by means of titration with NaOH according to the method described by Sears in Analytical Chemistry 28 (1956): 12, 1981-1983. The specific surface area is suitable within the range of from 750 to 950 m.sup.2 /g and preferably within the range of from 800 to 950 m.sup.2 /g. Another characteristic property of the present silica sols is their S-value, which in contrast to the S-value for known commercial sols used in paper production, is comparatively low. The S-value of the present sols is within the range of from 8 to 45 per cent, suitably from 10 to 35 per cent and preferably from 10 to 30. The given S-value has been measured and calculated as described by Iler, R. K. & Dalton, R. L. i J. Phys. Chem. 60(1956), 955-957. The S-value can be said to be a measure of the degree of aggregate of microgel formation and a lower S-value indicates a higher microgel content and can be seen as a measure of the amount of SiO.sub.2, in per cent by we
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Johansson Hans E.
Larsson Bo V.
Chaudhuri Olik
Eka Nobel AB
Horton Ken
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