Derivatization process for producing cationic colloidal silica

Abstract

To provide a process for producing cationic colloidal silica which is surface-modified by at least one aminoalkoxysilane having an amino group in a cationic form.SOLUTION: The production method comprises (i) reacting, under alkaline conditions, negatively charged colloidal silica with at least one aminoalkoxysilane having an amino group in a cationic form at above 20°C to produce surface-modified colloidal silica having a net negative surface charge, and (ii) reversing the surface charge of the net negatively charged surface-modified colloidal silica by contacting the net negatively charged surface-modified colloidal silica with an ion exchange resin to obtain positively charged surface-modified colloidal silica.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a manufacturing method for producing cationic colloidal silica that is surface-modified with at least one aminoalkoxysilane having an amino group in cationic form. [Background technology]

[0002] Silica sols contain small, discrete silica particles suspended in a liquid phase, typically aqueous. The pH of the sol is usually alkaline, and the silica particles tend to have a negative charge, which is balanced by cations in the sol, typically alkali metal cations. The negative charge provides a repulsive force between particles, which helps to avoid particle aggregation and gelation. Due to this advantage of negatively charged silica sols, many different silica modification methods have been proposed to further strengthen and modify the negative charge on silica sols, as described, for example, in U.S. Pat. No. 10,647,887 and U.S. Patent Application Publication No. 2021 / 0380419.

[0003] It is also possible to modify silica particles so that their surface negative charge is reduced or reversed to a positive charge. Such "charge-reversed" sols may be stable at lower pH, which is useful for applications where more alkaline pH values ​​are undesirable.

[0004] One method for producing charge-reversed silica sols is to coat or modify the surface of silica particles with cationic trivalent or tetravalent metal species. One example is described in U.S. Pat. No. 3,007,878, which discloses a method for producing polyvalent metal-oxygen coated colloidal silica particles by adding colloidal silica to a silica sol containing a basic salt solution of a metal with a valence of 3 to 4. The result is silica sol particles with surfaces modified with metal oxides, metal hydroxides, and / or hydrated metal oxides. U.S. Pat. No. 3,139,406 also discloses a method for producing coated silica colloidal particles in which colloidal silica is added to a trivalent acidic metal salt, where anions (e.g., chloride ions) bound to the salt are subsequently removed using a strong base anion exchange resin.

[0005] However, metal modification of the surface of silica sol is not always desirable.Therefore, attempts have also been made to prepare cationic charged silica sol modified with alternative nonmetallic cationic groups, such as nitrogen-containing compounds or sulfur-containing compounds, such as polyethyleneimine-derived silanes or aminoalkylsilanes.The nitrogen-containing groups of these silanes are protonated at pH levels below their respective pKb values, providing cationic charge on the silica surface.They are described, for example, in US Patent Application Publication No. 2022 / 0112401 or US Patent No. 9,803,108.

[0006] However, derivatization with cationic silanes has also been found to suffer from several drawbacks, such as the tendency of cationic colloidal silica to aggregate during the preparation process and / or insufficient surface coverage of the cationic groups on the silica surface. Aggregation leads to changes in the particle size and particle size distribution of colloidal silica dispersions, which is unacceptable for dispersions used in chemical mechanical planarization (CMP) applications. Furthermore, known preparation processes are complex, requiring various process steps and sometimes requiring intermediate cleaning steps, which are wasteful and expensive. Furthermore, currently used derivatization processes use an acidic pH to ensure an overall cationic charge on colloidal silica, which suffers from slow reaction kinetics.

[0007] Therefore, there is a need for an improved derivatization process for producing cationic colloidal silica that preferably overcomes at least one, and more preferably all, of the above-mentioned drawbacks of known processes. Summary of the Invention

[0008] The present invention provides a method for producing cationic colloidal silica surface-modified with at least one aminoalkoxysilane having an amino group in cationic form, comprising: (i) reacting negatively charged colloidal silica with at least one aminoalkoxysilane having an amino group in cationic form at above 20°C under alkaline conditions to produce a surface-modified colloidal silica having a net negative surface charge; (ii) reversing the surface charge of the net negatively charged surface-modified colloidal silica by contacting the net negatively charged surface-modified colloidal silica with an ion exchange resin to obtain a positively charged surface-modified colloidal silica. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows a schematic representation of the method of the present invention according to the subject matter of the examples of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention provides a method for producing cationic colloidal silica surface-modified with at least one aminoalkoxysilane having an amino group in cationic form, the method comprising: (i) reacting colloidal silica with at least one aminoalkoxysilane having an amino group in cationic form under alkaline conditions at above 20°C to produce a surface-modified colloidal silica having a net negative surface charge; (ii) reversing the surface charge of the net negatively charged surface-modified colloidal silica by contacting the net negatively charged surface-modified colloidal silica with an ion exchange resin to obtain a positively charged surface-modified colloidal silica.

[0011] Such a process has surprisingly been found to provide cationic surface-modified silica particles without any change in particle size or particle size distribution before or after modification. The modified silica particles do not aggregate and have a uniform distribution of positive charges on the surface. Furthermore, the manufacturing method of the present invention involves only a very small number of simple process steps.

[0012] Without wishing to be bound by theory, it is assumed that these beneficial effects, particularly the suppression of aggregation, are achieved by carrying out the actual surface modification with aminoalkoxysilane in step (i) under alkaline conditions, thereby ensuring that the overall charge on the silica surface is still negative. Only in the second step is the pH of the silica changed to acidic to obtain a positively charged surface-modified colloidal silica.

[0013] In the first step (i) of the manufacturing method of the present invention, negatively charged colloidal silica is reacted under alkaline conditions with at least one aminoalkoxysilane having an amino group in cationic form above 20°C to produce a surface-modified colloidal silica having a net negative surface charge.

[0014] In the manufacturing method of the present invention, the negatively charged colloidal silica can be any conventional silica (also referred to in this disclosure as silica sol). The starting aqueous silica sol is basic, typically having a pH in the range of 8.0 to 12.0, e.g., 9.0 to 11.0. Other components of such a sol include one or more monovalent cations, typically selected from alkali metal cations, and organic cations, e.g., quaternary ammonium cations, e.g., those of the general formula NR4 + wherein each R is independently H and a cation of C optionally substituted with one or more groups selected from —OH or —NH 1-6 In some embodiments, the monovalent cation is selected from one or more of alkali metal cations, such as lithium, sodium, and potassium, and ammonium ions (NH + ) Typically, potassium is the only or predominant alkali metal. The alkali metal is often derived from a starting soluble silicate solution (e.g., water glass) that can be used to make colloidal silica using conventional processes.

[0015] Examples of suitable alkali metal silicates or water glasses that can be used to make the starting aqueous silica sol include conventional materials such as lithium, sodium, and potassium silicates, preferably sodium silicate.

[0016] The alkali metal content in the starting silica sol is typically in the range of 0.1 to 5.0 wt. % expressed as alkali metal oxide, and in some embodiments, it is 0.2 to 3.0 wt. %.

[0017] The silica concentration in the starting aqueous silica sol is typically in the range of 1 to 40% by weight, for example, 2 to 35% by weight or 3 to 30% by weight.

[0018] The surface area of ​​the colloidal silica particles in the silica sol is not limited and can be calculated from NaOH titration according to the method of Sears (Sears; Anal. Chem., 1956, 28(12), 1981-1983).

[0019] The starting silica sol colloidal silica particles suitably have an average particle diameter of 2 to 150 nm, preferably about 3 to about 100 nm, In a preferred embodiment, the average particle diameter is in the range of 10 to 100 nm.

[0020] Particle size can be calculated from the titration surface area using the method described in "The Chemistry of Silica" by Iler, K. Ralph, page 465, John Wiley & Sons (1979). For silica particles of 2.2 g cm -3 and assuming all particles are of the same size, have a smooth surface area, and are spherical, the particle size can be calculated from Equation 1

[0021]

number

[0022] The density of the starting aqueous silica sol depends at least in part on the silica content and is typically between 1.01 and 1.20 g cm -3 is within the range.

[0023] The dynamic viscosity of the starting aqueous silica sol is typically less than 40 cP, for example less than 30 cP, particularly less than 20 cP. In an embodiment, the dynamic viscosity of the starting aqueous silica sol is less than 10 cP. These viscosities are measured at 20.0°C. The viscosity of the silica sol described in the present disclosure can be measured using a conventional rotational viscometer. One method that can be used is ASTM D4016-14.

[0024] The starting sol is typically an aqueous sol, but may contain other components, such as water-miscible organic solvents, such as one or more lower alcohols (e.g., one or more C1-C4 alkyl alcohols). If other components are present, they typically constitute less than 10% by weight, e.g., less than 5% by weight, of the sol.

[0025] In aqueous systems, colloidal silica particles may be dispersed in the presence of one or more stabilizing cations, which are typically K + , Na + , Li + , NH4 + and the general formula NR4 + The cations are selected from organic cations such as the quaternary ammonium cations of the formula:

[0026] Examples of sols that can be used as the starting aqueous silica sol include silica sols commercially available from Nouryon Chemicals BV under the trade name Levasil™.

[0027] In the process of the present invention, in step (i), negatively charged colloidal silica is reacted with at least one aminoalkoxysilane having an amino group in cationic form under alkaline conditions. Preferably, the reaction is carried out at a pH above 8, more preferably above 9. In some embodiments, the reaction is carried out at a pH below 12, preferably below 11, more preferably below 10. In some embodiments, the reaction is carried out at a pH within the range of 8 to 11, preferably within the range of 8.5 to 10.5, or even more preferably within the range of 9 to 10, for example, within the range of 9 to 9.5.

[0028] The pH may be adjusted using common bases such as potassium hydroxide and ammonium hydroxide, however, a mixture of potassium hydroxide and potassium nitrate has been found to be particularly desirable as such a mixture may allow for better ion exchange during step (ii) of the process of the present invention, resulting in a less acidic and viscous final product obtained in step (ii).

[0029] By maintaining alkaline conditions in step (i) of the process of the present invention, the silanol groups on the silica surface are sufficiently deprotonated, keeping the silica in an anionic form and ensuring resistance to aggregation.

[0030] In the manufacturing method of the present invention, negatively charged colloidal silica is reacted with at least one aminoalkoxysilane having an amino group in cationic form at a temperature above 20° C., for example above 30° C., preferably above 40° C., and even more preferably above 50° C. In some embodiments, the negatively charged colloidal silica is reacted with at least one aminoalkoxysilane having an amino group in cationic form at a temperature between 20 and 100° C., for example between 30 and 100° C., or between 40 and 100° C., preferably between 50 and 90° C., and even more preferably between 50 and 80° C.

[0031] The aminoalkoxysilane may be based on any silane containing an amino group, such as a primary amine group, a secondary amine group, or a tertiary amine group. In some embodiments, the aminoalkoxysilane may be based on an aminoalkylalkoxysilane. In some embodiments, the aminoalkoxysilane is based on an aminoalkylalkoxysilane selected from the group consisting of aminopropyltrialkoxysilanes such as aminopropyltriethoxysilane (APTES) and aminopropyltrimethoxysilane, aminobutyltrialkoxysilanes such as aminobutyltriethoxysilane and aminobutyltrimethoxysilane, aminoethyltrialkoxysilanes such as aminoethyltriethoxysilane and aminoethyltrimethoxysilane, N,N-(diethylaminomethyl)triethoxysilane (DEAMS), N,N-(diethylaminomethyl)trimethoxysilane N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS), N-(2-aminoethyl)-3-aminopropyltriethoxysilane, N-aminoethylaminoethylaminopropyltrimethoxysilane, N-aminoethylaminoethylaminopropyltriethoxysilane, N-aminoethylaminoethylaminopropyltriethoxysilane, and combinations thereof. In certain preferred embodiments, the aminoalkoxysilane may be based on an aminopropyltrialkoxysilane, especially aminopropyltriethoxysilane (APTES).

[0032] In the present invention, where step (i) is carried out under alkaline conditions, the alkaline conditions are adjusted so that the aminoalkoxysilane has an amino group in cationic form. Each specific aminoalkoxysilane has a specific pH at which it is protonated. Those skilled in the art can obtain this specific pH from the literature and adjust the alkaline conditions accordingly according to the specific aminoalkoxysilane selected. For example, the primary amine group of APTES is protonated below pH 10.5, and therefore the aminopropyl group of APTES is cationic below pH 10.5. Therefore, the specific alkaline pH suitable for step (i) of the present invention also depends on the specific aminoalkoxysilane used for silane derivatization.

[0033] The aminoalkoxysilane may be added to the colloidal silica or diluted in a suitable solvent. In other words, in some embodiments, the aminoalkoxysilane may be diluted in a solvent before reacting with the colloidal silica. The solvent may be water or a hydrophilic organic compound, preferably a hydrophilic organic compound. It has been found that such dilution, especially dilution in a hydrophilic organic compound, significantly suppresses the self-condensation of the aminoalkoxysilane and / or the reactivity of the aminoalkoxysilane.

[0034] Hydrolysis of the alkoxy groups is necessary to obtain reactive silanol groups that can subsequently react by condensation with silanol groups on the silica surface to form bonds between the silica and the hydrolyzed aminoalkoxysilane. Addition of a hydrophilic alkoxysilane, such as APTES, to an alkaline aqueous dispersion at alkaline pH results in immediate hydrolysis of the alkoxy groups, after which the hydrolyzed silane species react by condensation with the silica surface.

[0035] In some embodiments, the hydrophilic organic compound may be selected from alcohols, ethers, esters, nitriles such as acetonitrile, or combinations thereof. Preferably, the hydrophilic organic compound is an alcohol. More preferably, the hydrophilic organic compound is selected from methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, decanol, and combinations thereof. It is particularly preferred that the alcohol is ethanol.

[0036] The amount of aminoalkoxysilane added to silica is not particularly limited. However, in order to maintain a net negative surface charge on the surface-modified colloidal silica obtained in step (i) of the production method of the present invention, the surface modification with aminoalkoxysilane must lead to an overall net negatively charged silica. In other words, after step (i), there will be more negatively charged deprotonated silanol groups (i.e., SiO ) on the silica than the positively charged organic groups derived from the aminoalkoxysilane. - For example, 1 nm on silica 2 The total number of silanol groups per unit area is 8 μmol SiOH / m 2 It is known from the literature that the silanol concentration is about 4.9, which corresponds to a pH of 1.26 groups / nm. At pH 9.0, a significant proportion of the silanol groups are ionized, i.e., deprotonated. At this pH, silica has a silanol concentration of 1.26 groups / nm. 2 is estimated to have a negative surface charge of approximately 2.1 μmol of SiO - / m 2 (Searrs, GW, Anal. Chem., 28, (1956, 1981-1983). At a slightly higher pH, about pH 9.3, the surface charge is even higher, about 2.9 μmol SiO - / m 2 (See Iler, RK, The Chemistry of Silica, (1979), p. 356.) Based on this, a person skilled in the art can determine the appropriate maximum amount of aminoalkoxysilane to be added to colloidal silica for surface modification, taking into account the actual pH used.

[0037] In some embodiments, the amount of aminoalkoxysilane added in step (i) to react with the colloidal silica is adjusted to reduce the amount of free SiO 2 on the colloidal silica. - It is selected to be at most 40% of the amount of groups, such as at most 30%, preferably at most 20%, more preferably at most 10%, even more preferably at most 5%.

[0038] In the second step (ii) of the production method of the present invention, the surface charge of the net negatively charged surface-modified colloidal silica obtained in step (i) of the production method of the present invention is reversed by contacting the net negatively charged surface-modified colloidal silica with an ion exchange resin to obtain a positively charged surface-modified colloidal silica.

[0039] In principle, any commercially available strong cationic ion exchange resin may be used. The ion exchange resin is preferably a strong acid cation (SAC) exchange resin containing sulfonic acid groups. SAC exchange resins typically consist of a styrene matrix cross-linked with divinylbenzene functionalized with sulfonic acid groups. Such resins are commercially available, for example, from Lanxess (Germany) under the trade name Lewatit or from DuPont (USA) under the name Amberlite. Ion exchange resins convert metal ions to H + H + It is provided in the form of

[0040] The contact of the net negatively charged surface-modified colloidal silica with the ion exchange resin is usually carried out in a column, where the net negatively charged surface-modified colloidal silica is loaded on top and then passed through a column packed with ion exchange resin. The cation-exchanged silica is collected at the bottom outlet of the column. During contact, deprotonated SiO - The potassium ions on the surface of the net negatively charged surface-modified colloidal silica obtained from step (i) of the production method of the present invention related to the group H + This results in the formation of a neutral silanol group. As a result, the deprotonated SiO - All (or at least most) of the groups are H + The surface charge is controlled by surface derivatization with aminoalkoxysilanes, which contain amino groups in cationic form. Furthermore, the use of cation exchange within the column results in rapid charge reversal of the surface-modified silica particles, which is important for preventing particle aggregation.

[0041] The positively charged surface-modified colloidal silica obtained in step (ii) of the production method of the present invention usually has an acidic pH, preferably less than 7, more preferably less than 5, even more preferably less than 4, for example a pH in the range of 2 to 4 or even 3 to 4. [Example]

[0042] The following non-limiting examples show how to practice the invention.

[0043] Example 1: Preparation of APTES-modified cationic colloidal silica surface Preparation of solutions for controlled alkalinization: 80 g of potassium hydroxide (KOH) was added to 920 g of deionized water and stirred to dissolve the KOH. This resulted in a KOH(aq) solution containing 7 wt% KOH(aq). 1 g of potassium nitrate (KNO3) was added to 74 g of the 7 wt% KOH(aq) solution and stirred to dissolve the KNO3. A total of 75 g of the resulting KOH / KNO3(aq) was ready for use.

[0044] Controlled alkalinization of deionized colloidal silica: 5000 g of 40 nm deionized colloidal silica (available from Nuryon Chemicals BV under the trade name Levasil) was added to a 6 L multi-necked round flask equipped with a baffle to ensure good mixing. The round flask was placed in an electric heating jacket and an electric motor with an adjustable stirring speed was connected. Stirring was initiated at approximately 300 rpm. 75 g of KOH / KNO3 (aqueous) solution was dosed into the flask at a dosing rate of 7-8 mL / min. The product was stirred for 1 h after all the KOH / KNO3 solution had been added. The pH of the resulting mixture was 9.2.

[0045] Preparation of cationic aminopropyltriethoxysilane (APTES) solution in ethanol: 9 g of aminopropyltriethoxysilane (APTES) was added to 81 g of 99.5% ethanol (spectroscopic grade) in a flask. The APTES / ethanol solution was stirred to mix well.

[0046] Heating and adding the APTES / ethanol solution: The alkalized colloidal silica dispersion was stirred at 300 rpm and heated to 60°C. When the temperature of the alkalized colloidal silica reached 60°C, the APTES / ethanol solution was added at a rate of 1.24 mL / min until a total volume of 31.2 mL of the APTES / EtOH solution was added. The resulting mixture was stirred at 60°C for 3 hours and then allowed to cool to room temperature under continuous stirring. The reaction of APTES with the silica surface was confirmed to be relatively rapid and complete.

[0047] Cation exchange of APTES derivatized products: A strong acid cation exchange resin (SAC; sulfonic acid type; IEX capacity 1.8-2.0 equivalents / L) was placed in a 50 mm diameter column. + The APTES-derivatized product was then passed through the column. The cation-exchanged product obtained from the column had a pH of 3.4 and a conductivity of approximately 350 μS / cm. Furthermore, the resulting positively charged surface-modified colloidal silica did not exhibit any aggregation and remained in colloidal form. It was also confirmed that the positively charged groups were evenly distributed over the entire surface of the silica.

[0048] The process of Example 1 is shown schematically in Figure 1. Initially, at high pH, ​​colloidal silica (circles containing Si atoms) is partially negatively charged. This provides electrostatic stabilization and high reactivity toward aminoalkoxysilanes such as APTES. During step (i) of the present process, colloidal silica is reacted with aminoalkoxysilane APTES in ethanol to obtain colloidal silica that transfers some of its silanol groups to the (hydrolyzed) aminoalkoxysilane. The resulting colloidal silica contains positively charged groups derived from the aminoalkoxysilane, but has an overall net anionic surface charge and is therefore amphoteric. The surface-modified colloidal silica with a net negative surface charge is then contacted with a strong acid cation exchange resin (cation IX resin) to convert the anionic deprotonated silanol groups to protonated silanol groups. At acidic pH, the resulting colloidal silica is positively charged and carries a net positive surface charge.

[0049] Example 2: Comparison of surface modification under acidic and alkaline conditions Test A: APTES modification at alkaline pH, 0.2 μmol / m 2 3500 g of 40 nm deionized colloidal silica (available from Nuryon Chemicals BV under the trade name Levasil) was poured into a 6 L multi-neck round flask equipped with a baffle. The round flask was placed in an electric heating jacket. Agitation (impeller) was started at 300 rpm.

[0050] Alkylation: 140 g of a 4 wt % KOH solution (1.56 g) containing dissolved KNO was added at a rate of 14 ml / min using a peristaltic pump. The dosing time was approximately 10 minutes. The pH after this step was 9.3.

[0051] APTES Modification: The alkalized silica sol was stirred and heated to 65°C. 46 ml of a solution consisting of 3.70 g of APTES diluted with 33 g of ethanol (99.5%) was added through a dip pipe at a rate of 2.3 ml / min. The dosing time was 20 minutes. Heating was continued for another 10 minutes. Heating was then stopped and the product was allowed to cool to below 40°C under continuous stirring. The pH after this step was 9.7.

[0052] Cation exchange: The APTES-derivatized product was applied to a column containing approximately 300 ml of a strong acid cation exchange resin (Lewatit MDS 1368). The pH of the recovered product was 3.1. The concentration was 24.3 wt. % SiO2 according to a density of 1.1549 g / ml.

[0053] Test B: APTES modification at pH 2.3, 0. 2 μmol / m2 700 g of 40 nm deionized colloidal silica (available from Nuryon Chemicals BV under the trade name Levasil) was poured into a baffled, multi-necked 1 L round flask. The round flask was placed in an electric heating jacket. Agitation (impeller) was started at 300 rpm.

[0054] Oxidation: The pH was adjusted to 2.3 with 24 g of 1% HNO3.

[0055] APTES Modification: The acidified silica sol was stirred and heated to 60°C. 9.2 ml of a solution consisting of 0.74 g of APTES diluted with 6.7 g of ethanol (99.5%) was added through a dip pipe at a rate of 0.46 ml / min. The dosing time was 20 min. Heating was continued for another 10 min. Heating was then stopped and the product was allowed to cool to room temperature under continuous stirring. The pH after this step was 3.9. The concentration was 24.3 wt% SiO2 according to a density of 1.1549 g / ml.

[0056] The APTES-derivatized products were analyzed to determine particle size distribution (PSD) by dynamic light scattering (DLS, measured in 0.5 wt% SiO2 in 20 mM NaCl solution) and zeta potential (measured in 1.0 wt% SiO2 at pH 3.9 adjusted with HNO3). The results are shown in Table 1.

[0057] [Table 1]

[0058] The results show that under alkaline conditions (within the limits of precision of DLS measurements, i.e., 0.5-1.0 nm z-average), there is no change in particle size distribution after APTES modification of the starting silica material. The z-average of the initial deionized colloidal silica was found to be 43.4 nm (z-average by DLS). When using this method, the particle size was 0.2 μmol APTES / m after APTES surface modification. 2 ), was found to be 43.6 nm. In contrast, EtOH-diluted APTES (0.2 μmol APTES / m 2 Prior to surface modification with HCl, the original deionized colloidal silica was further acidified with nitric acid to a pH of 2.1, and the resulting average particle size (z-average by DLS) was found to be 46.4 nm, an increase of 3 nm (approximately 7%).

[0059] Although the invention has been described with reference to an exemplary embodiment, it will be understood that various modifications are possible within the scope of the invention.

[0060] As used herein, unless expressly indicated otherwise, the word "or" is used to mean an operator that returns a true value when either or both stated conditions are met, as opposed to the operator "exclusive or," which requires that only one of the stated conditions be met. The word "comprising" is used in the sense of "including," not "consisting of." All prior teachings identified above are incorporated herein by reference. Admission of any prior-published document herein should not be construed as an admission or representation that its teachings were general knowledge in Europe or elsewhere as of the date hereof.

Claims

1. 1. A method for producing cationic colloidal silica surface-modified with at least one aminoalkoxysilane having an amino group in cationic form, comprising: (i) reacting negatively charged colloidal silica with at least one aminoalkoxysilane having an amino group in cationic form at above 20°C under alkaline conditions to produce a surface-modified colloidal silica having a net negative surface charge; (ii) reversing the surface charge of the net negatively charged surface-modified colloidal silica by contacting the net negatively charged surface-modified colloidal silica with an ion exchange resin to obtain a positively charged surface-modified colloidal silica.

2. The method of claim 1 , wherein the ion exchange resin is a strong acid cation (SAC) exchange resin containing sulfonic acid groups.

3. 3. The method according to claim 1, wherein in step (i), the reaction of colloidal silica with the at least one aminoalkoxysilane having an amino group in cationic form is carried out at a pH above 8.

4. 4. The process according to claim 1, wherein in step (i), the reaction of the colloidal silica with the at least one aminoalkoxysilane having an amino group in cationic form is carried out at a pH of 8.5 to 10.5, preferably at a pH of 9 to 9.

5.

5. The method of any one of claims 1 to 4, wherein the at least one aminoalkoxysilane is an aminoalkylalkoxysilane.

6. The method of any one of claims 1 to 5, wherein the at least one aminoalkoxysilane is aminopropyltriethoxysilane.

7. 7. The method according to claim 1, wherein the at least one aminoalkoxysilane having an amino group in cationic form is diluted in a hydrophilic organic compound such as an alcohol, an ether, an ester, or a nitrile before reacting with the colloidal silica.

8. The method according to claim 7 , wherein the hydrophilic organic compound is an alcohol.

9. The method according to claim 7 or 8, wherein the hydrophilic organic compound is ethanol.

10. The process according to any one of claims 1 to 9, wherein step (i) is carried out at a temperature of from 40°C to 100°C, preferably from 50°C to 80°C.

11. The amount of at least one aminoalkoxysilane having an amino group in cationic form added in step (i) to react with the colloidal silica is such that the amount of free SiO on the colloidal silica is － The process according to any one of claims 1 to 10, wherein the amount of the groups is selected to be at most 20%, preferably at most 10%.

12. The process according to any one of claims 1 to 11, wherein the positively charged surface-modified colloidal silica obtained in step (ii) has a pH of less than 5, preferably in the range of 3 to 4.

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