Elongated-shaped silica sol and method for preparing the same

A method for preparing a stable alkaline aqueous silica sol having an SiO.sub.2 concentration of from 1 to 6% by weight, in which the colloidal particles of said silica have a particle size of 40 to 300 millimicrons as measured by dynamic light-scattering, and have an elongated shape having a uniform thickness of 5 to 20 millimicrons and an elongation of 5 to 30 times the thickness in only one plane, comprising the following steps (a), (b) and (c):

• (a) mixing at a temperature of 2.degree. to 50.degree. C. an aqueous solution containing at least one compound selected from the group consisting of water-soluble calcium salt and magnesium salt with an aqueous colloidal liquid of an active silicic acid containing from 1 to 6% by weight of SiO.sub.2 but not containing colloidal silica particles having a size larger than 3 millimicrons and having a pH value higher than 4 and up to 5 in an amount of from 1500 to 8500 ppm as a weight ratio of CaO or MgO or a mixture of said CaO and MgO to SiO.sub.2 of the said active silicic acid;
• (b) mixing at 2.degree. to 50.degree. C. an aqueous solution of at least one compound selected from the group consisting of alkali metal hydroxides, water-soluble organic bases, water-soluble silicates of alkali metal hydroxide and water-soluble silicates of water-soluble organic base with the aqueous solution obtained in step (a) in a molar ration of SiO.sub.2 /M.sub.2 O from 20 to 200, where SiO.sub.2 indicates the total silica content derived from said active silicic acid and from said silicate, and M indicates an alkali metal atom of said alkali metal hydroxide or an organic base molecule of said organic base;
• and
• (c) heating the mixture obtained in step (b) at 60.degree. to 250.degree. C. for 0.5 to 40 hours.

A method for preparing a stable alkaline aqueous silica sol having an SiO.sub.2 concentration of from 1 to 30% by weight, in which the colloidal particles of said silica have a particle size of 40 to 300 millimicrons as measured by dynamic light-scattering, and have an elongated shape having a uniform thickness of 5 to 20 millimicrons and an elongation of 5 to 30 times the thickness in only one plane, comprising the following steps (a), (b), (c) and (d):

• (a) mixing at a temperature of 2.degree. to 50.degree. C. an aqueous solution containing at least one compound selected from the group consisting of water-soluble calcium salt and magnesium salt with an aqueous colloidal liquid of an active silicic acid containing from 1 to 6% by weight of SiO.sub.2 but not containing colloidal particles having a size larger than 3 millimicrons and having a pH value of from 2 to 4 in an amount of from 1500 to 8500 ppm as a weight ratio of CaO or MgO or a mixture thereof with respect to SiO.sub.2 of the said active silicic acid.
• (b) mixing at 2.degree. to 50.degree. C. an aqueous solution of at least one compound selected from the group consisting of alkali metal hydroxides, water-soluble organic bases, water-soluble silicates of alkali metal hydroxide and water-soluble silicates of water-soluble organic base with the aqueous solution obtained in step (a) in a molar ratio of SiO.sub.2 /M.sub.2 O form 20 to 200, where SiO.sub.2 indicates the total silica content derived form said active silicic acid and the silica content of said water soluble silicate, and M indicates an alkali metal atom of said alkali metal hydroxide or an organic base molecule of said organic base;
• and
• (c) heating the mixture obtained in step (b) at 60.degree. to 150.degree. C. for 0.5 to 40 hours and then
• (d) removing anions and water from said silica sol prepared in step (c) to provide an anion concentration of 0.1% by weight or less and an SiO.sub.2 concentration of from 1 to 30% by weight.

A method for preparing a stable alkaline aqueous silica sol having an SiO.sub.2 concentration of from 0.5 to 25% by weight, in which the colloidal particles of said silica have a particle size of 40 to 500 millimicrons as measured by dynamic light-scattering, and have an elongated shape having a uniform thickness of 5 to 40 millimicrons and an elongation of 5 to 30 times the thickness in only one plane, comprising the following steps (a'), (b') and (c'):

• (a') mixing at a temperature of 2.degree. to 50.degree. C. an aqueous solution containing at least one compound selected from the group consisting of water-soluble calcium salt and magnesium salt with an aqueous acidic silica sol containing colloidal silica particles having an average particle diameter of from 3 to 30 millimicrons, said acidic silica sol having from 0.5 to 25% of SiO.sub.2 by weight and having a pH value of from 1 to 5, in an amount of from 0.15 to 1.00% by weight of CaO, MgO or a mixture of said CaO and MgO to SiO.sub.2 of said acidic silica sol;
• (b') mixing at 2.degree. to 50.degree. C. an aqueous solution of at least one compound selected form the group consisting of alkali metal hydroxides, water-soluble organic bases, water-soluble silicates of alkali metal hydroxide and water-soluble silicates of organic base with said liquid as obtained in step (a') in a molar ratio of SiO.sub.2 /M.sub.2 O from 20 to 300, where SiO.sub.2 indicates the total silica content derived from said acidic silica sol and from said silicate, and M indicates an alkali metal atom of the said alkali metal hydroxide or an organic base molecule of said organic base;
• and
• (c') heating the mixture obtained in step (b') at 60.degree. to 300.degree. C. for 0.5 to 40 hours to form said colloidal silica particles in said heated mixture having an elongated shape and having a thickness larger than the particle size of said colloidal silica particles of said step (a').

A method for preparing a stable alkaline aqueous silica sol having an SiO.sub.2 concentration of from 1 to 40% by weight, in which the colloidal particles of said silica have a particle size of 40 to 500 millimicrons as measured by dynamic light-scattering, and have an elongated shape having a uniform thickness of 5 to 40 millimicrons and an elongation of 5 to 30 times the thickness in only one plane, comprising the following steps (a'), (b'), (c') and (d'):

• (a') mixing at a temperature of 2.degree. to 50.degree. C. an aqueous solution containing at least one compound selected from the group consisting of water-soluble calcium salt and magnesium salt with an aqueous acidic silica sol containing colloidal silica particles having an average particle diameter of from 3 to 30 millimicrons, said acidic silica sol having from 0.5 to 25% of SiO.sub.2 by weight and having a pH value of from 1 to 5, in an amount of from 0.15 to 1.00% by weight of CaO, MgO or a mixture of said CaO and MgO to SiO.sub.2 of said acidic silica sol;
• (b') mixing at 2.degree. to 50.degree. C. an aqueous solution of at least one compound selected from the group consisting of alkali metal hydroxides, water-soluble organic bases, water-soluble silicates of alkali metal hydroxide and water-soluble silicates of organic base with said liquid as obtained in step (a') in a molar ratio of SiO.sub.2 /M.sub.2 O from 20 to 300, where SiO.sub.2 indicates the total silica content derived from said acidic silica sol and from said silicate, and M indicates an alkali metal atom of the said alkali metal hydroxide or an organic base molecule of said organic base;
• and
• (c') heating the mixture obtained in step (b') at 60.degree. to 300.degree. C. for 0.5 to 40 hours to form said colloidal silica particles in said heated mixture having an elongated shape and having a thickness larger than the particle size of said colloidal silica particles of said step (a'), and then
• (d') removing anions and water from said silica sol prepared in step (c') such that the anion concentration is 0.1% by weight or less and the SiO.sub.2 concentration of said silica sol is from 1 to 40% by weight.

A method for preparing a stable alkaline aqueous silica sol having an SiO.sub.2 concentration of from 1 to 30% by weight, in which the colloidal particles of said silica have a particle size of 40 to 300 millimicrons as measured by dynamic light-scattering, and have an elongated shape having a uniform thickness of 5 to 20 millimicrons and an elongation of 5 to 30 times the thickness in only one plane, comprising the follow steps (a), (b), (c), and (d):

• (a) mixing an aqueous solution of at least one compound selected from the groups consisting of water-soluble salt of calcium or magnesium with an aqueous colloidal liquid of an active silicic acid containing from 1 to 6% by weight of SiO.sub.2 and not containing colloidal silica particles having a size larger than 3 millimicrons and having a pH value higher than 4 and up to 5 in an amount of from 1500 to 8500 ppm as a weight ratio of CaO or MgO or a mixture thereof with respect to SiO.sub.2 of the said active silicic acid at a temperature of 2.degree. to 50.degree. C.;
• (b) mixing an aqueous solution of at least one compound selected from the group consisting of alkali metal hydroxides, water-soluble organic bases, water-soluble silicates of alkali metal hydroxide and water-soluble silicates of water-soluble organic base with the liquid obtained in step (a) in a molar ratio of SiO.sub.2 /M.sub.2 O from 20 to 200, where SiO.sub.2 indicates the total silica content derived from said active silicic acid and the silica content of said water-soluble silicate, and M indicates an alkali metal atom of said alkali metal hydroxide or an organic base molecule of said water-soluble organic base at 2.degree. to 50.degree. C.;
• (c) heating the liquid obtained in step (b) at 60.degree. to 250.degree. C. for 0.5 to 40 hours, whereby to form a stable sol of colloidal silica;
• and then
• (d) removing anions and water from the sol obtained in step (c) to provide an anion concentration of 0.1% by weight or less and an SiO.sub.z concentration of from 1 to 30% by weight in the sol.