AQUEOUS SLURRY COMPRISING INORGANIC OXYGEN-CONTAINING PARTICULATES

The invention relates to aqueous slurries containing inorganic oxygen- containing particulates.

Such aqueous slurries are known in the art. The aqueous slurries of the art, such as those presented in EP-A-0992456 and EP-A-1130630, generally have a limited stability upon storage, especially with pH fluctuations, and the particulates tend to agglomerate to larger particles and eventually settle and form sediment. WO 2008/043703 improves on this technology, but the particles thereof are only stable at low pH and over a small range and the particles are not compatible with most coating resins and other matrices. In order to prevent sedimentation and/or agglomeration, often surfactants are added in considerable amounts, such as described in WO 2004/000916 and WO 2003/084871. Such surfactant-containing slurries generally cannot be suitably used in various applications due to the presence of the surfactant. In polishing applications, for instance, the surfactant will cover the particle surface, causing a reduction in the polishing performance and generally contaminating the surface to be polished. Also in coating formulations the surfactants can be prohibitive. Further it is observed that many of the particle slurries of the prior art have too large an average particle size, due to which they cannot be used widely.

It is an object of the present invention to provide novel storage stable aqueous slurries comprising inorganic oxygen-containing particulates with an average particle size of 165 nm or less.

This objective is achieved with an aqueous slurry comprising modified inorganic oxygen-containing particulates having an average particle size of 165 nm or less, wherein said inorganic oxygen-containing particulates have been modified with a silane coupling agent having a functional group selected from the group

consisting of RiO-CO-, (R2)2N-, R3O-, and R ₄ S-, wherein Ri, R ₂ , R3, and R ₄ are independently chosen from H and a hydrocarbon comprising from 1 to 10 carbon atoms and optionally comprising a heteroatom, with the proviso that the inorganic oxygen-containing particulate is not silicon dioxide.

The degree of surface modification, expressed as the wt% silane based on the total weight of modified inorganic particulate material, is preferably less than 10 wt%, more preferably less than 5 wt%, even more preferably less than 3 wt%. The degree of surface modification is preferably more than 0.001 wt%, more preferably more than 0.01 wt%, and most preferably more than 0.02 wt%. It will be clear to those skilled in the art that a too hydrophobic surface will result in an unstable product, as it will phase separate from the aqueous phase, whereas a too low modification will not result in a less stable slurry. For applications in hydrophobic materials such as organic coatings the material should possess a certain level of surface modification to obtain a good dispersion in the coating.

The aqueous slurry of the present invention was found to be stable over a prolonged storage time and over a wide pH range. The particulates show considerably reduced or no agglomeration upon storage compared to products of the prior art.

It is noted that the term aqueous slurry or aqueous dispersion is used to define that more than 50, more preferably more than 70, and most preferably more than 90 wt% of the liquid phase of the slurry / dispersion consists of water, up to a maximum of 90, preferably up to 95, most preferably up to 100 wt% of water.

The slurry may contain only water as suspending medium or it may contain water and one or more other solvents. The other solvents are preferably organic solvents which are compatible, i.e. miscible, with water. Examples of such solvents include ketones, alcohols such as ethanol and iso-propanol; alkoxylated alcohols such as propylene glycol mono methyl ether and propylene glycol mono ethyl ether; and glycols such as ethylene glycol, propylene glycols

and polypropylene glycols and polyethylene glycols. Preferably, the aqueous slurry of the invention comprises only water as suspending medium.

The aqueous slurries of the invention have an improved pH stability, meaning that the particulates do not agglomerate over a much wider range of pH values than conventional slurries, which are only stable when the pH is between 2 and

4, rarely up to pH=5. The improved effect for slurries according to the invention is particularly observed for particulates which are modified with the silane having the defined additional functional group. Particulates modified with a silane not containing the defined functional group tend to agglomerate in an aqueous medium.

The particulates of the invention may be used in various applications, e.g. coatings, polymers, resins, because the surface modification was found to enhance the compatibility of the particulate material with the host material, so enabling a good dispersion of the particulate material. In such applications the scratch resistance, corrosion resistance and/or UV stability may be positively affected.

The aqueous slurry of the invention comprises inorganic oxygen-containing particulates. The inorganic oxygen-containing particulate material can be any particulate material known to the man skilled in the art capable of forming a suspension as used in the process of the present invention, i.e. where at least part of the solid particulate material is dispersed in the aqueous medium. It is envisioned that the inorganic particulate material of the present invention may already be modified, e.g. it may contain organic constituents or be encapsulated in a second inorganic material, before the particulate material is modified according to the processes of the invention. The inorganic oxygen-containing particulate material generally is selected from oxides, hydroxides, clays, calcium compounds, zeolites, and talc. In one embodiment it is preferred that the particulate is a metal oxide.

Examples of suitable oxides and hydroxides are alumina, aluminium trihydrate, titanium dioxide, zinc oxide, iron oxide, zirconium oxide, cerium oxide, antimony oxide, bismuth oxide, cobalt oxide, dysprosium oxide, erbium oxide, europium oxide, indium oxide, indium hydroxide, indium tin oxide, magnesium oxide, neodymium oxide, nickel oxide, samarium oxide, terbium oxide, tin oxide, tungsten oxide, and yttrium oxide.

Clays typically are cationic or anionic clays. Examples of cationic clays are the smectites, such as montmorillonite. Examples of anionic clays are layered double hydroxides (LDHs) such as hydrotalcite and hydrotalcite-like LDHs. Examples of calcium compounds are calcium carbonate and calcium phosphate.

The present invention also encompasses oxygen-containing particulates containing two or more of the aforementioned inorganic particulate materials. An example of such a compound is a silica coated with cerium(IV) oxide or vice versa.

The present invention further encompasses physical mixtures of two or more of the aforementioned inorganic particulate materials.

Typically, the inorganic oxygen-containing particulate materials of the invention have a number average particle diameter, as determined using a dynamic light scattering method of between 1 and 165 nm. It is envisaged that the particle size distribution of the inorganic oxygen-containing particulate material according to the invention may be bimodal or polymodal.

The amount of particulates in the aqueous slurry generally is at least 0.01 wt%, preferably at least 0.5 wt%, more preferably at least 2 wt%, and most preferably at least 5 wt%, and generally at most 60 wt%, preferably at most 50 wt%, and most preferably at most 40 wt%, based on the total weight of the aqueous slurry.

The slurries of the invention are storage stable. The term storage stable is meant to define a dispersion where:

1 ) the particle aggregation in the dispersion is limited such that the average particle size increases by a factor of less than 4, more preferably less than 3, and most preferably less than 2 during the storage time;

2) the degree of settling, as determined by storing 100 to 200 ml of a dispersion at a solids concentration of 5 wt% for the storage time and evaluating the amount of material that has settled to the bottom, is such that at most 20 wt% more preferably at most 10 wt%, even more preferably at most 5 wt%, and most preferably at most 2 wt% of the dispersed material has settled to the bottom. The storage time in which the dispersion is stable is preferably at least 1 week, more preferably at least 1 month, and most preferably at least 3 months.

In a preferred embodiment the slurries are storage stable over a pH range of from 2, preferably from 2.5, more preferably from 3, up to at least 7, preferably at least 8, and more preferably at least 9. This ensures they can be used in almost any application.

In one embodiment of the present invention, the aqueous slurry comprises inorganic oxygen-containing particulates containing cerium(IV); preferably, the inorganic oxygen-containing particulate contains cerium(IV) oxide.

The inorganic oxygen-containing particulates, in particular the cerium(IV) oxide- containing particulates, may be prepared using any preparation method known in the art. Such methods include precipitation with an oxidizing after-treatment, pyrolytic synthesis, reactive precipitation, and electrochemical synthesis. A preferred method for preparing the particulates of the invention is using reactive precipitation, as this method is simple, easily carried out, requires little energy, is easily controlled, and does not require (re)suspending of solid particulates, whereas e.g. cehum(IV) oxide obtained via pyrolytic synthesis and CeO2 particles obtained after a calcination treatment at a temperature above 400 ⁰ C, which are typically more crystalline and typically have a lower amount of surface-active groups, do require such a redispersion step. This may lead to aqueous slurries with a considerable amount of larger agglomerates, which is

considered undesirable in certain applications such as surface polishing. In addition, particulates obtained using precipitation processes generally have a larger amount of active surface groups compared to particulates obtained using pyrolytic synthesis. This is particularly advantageous when the particulate is to be modified with a silane coupling agent, as the modification is much easier and may even require less of the coupling agent.

The average particle size of the inorganic oxygen-containing particulates of the invention, in particular the cehum(IV) oxide-containing particulates, in accordance with the invention as determined using the dynamic light scattering method, e.g. using a Zetasizer ZS of Malvern Instruments, is below 165 nm. In the context of the present application the terms "particle size" and "average particle size" refer to the intensity average particle size. Preferably, the average particle size of the particulates is below 150 nm, more preferably below 145 nm, more preferably still below 125 nm, even more preferably below 105 nm, and most preferably below 95 nm.

The pH of the aqueous, not yet modified, slurry of the invention generally ranges between 2 and 4. Preferably, the pH of the slurry is at least 2.1 , more preferably at least 2.2, and most preferably at least 2.5, and preferably at most 3.9, more preferably at most 3.7, and most preferably at most 3.5. The pH can be adjusted by adding a base or acid to the aqueous slurry or by using an acidic or alkaline ion exchange resin or by washing out ions via a membrane washing step. The use of ion exchange resins or membrane washing is generally preferred, as no additional salts are introduced to the slurry requiring further steps to reduce the conductivity. The membrane washing process is commonly known in the art for the separation of components and more specifically for the removal of salts from water streams. Reference is made to the Kirk-Othmer Encyclopedia of Chemical Technology Vol. 26, pages 51 -102 ("Water Desalination"), 2007, Wiley and Sons. Suitably, the dispersion is circulated over a membrane with a pump and some pressure is applied due to which the water and the ions are pressed through the membrane. To maintain the required

solids content of the dispersion, water is added to the dispersion. The pore size of the membrane has to be chosen such that the ions are washed out but the particles are retained in the system. Suitably, the membrane is supported, e.g. on a ceramic support. This way the dispersion can be washed to give a low conductivity.

The conductivity of the unmodified slurry generally is below 2 mS/cm; preferably, the conductivity is below 1.5 mS/cm, and most preferably below 1.0 mS/cm, and generally the conductivity is at least 0.01 mS/cm, preferably at least 0.02 mS/cm, and most preferably at least 0.05 mS/cm.

Generally, the conductivity of aqueous slurries made via precipitation is well above 2 mS/cm and should thus be reduced to a value which is in accordance with the invention. The conductivity of the slurry can be reduced using any method known in the art. Suitable examples of such methods are the use of ion exchange resins and/or ion selective membranes. Also methods are contemplated that may alter the pH and the conductivity at the same time. An example of such a method is the use of an acidic or alkaline ion exchange resin or the use of a membrane washing step.

The invention further pertains to an aqueous slurry comprising inorganic oxygen-containing particulates, in particular cehum(IV) oxide-containing particulates, which have been modified with a silane coupling agent having a functional group selected from the group consisting of RiO-CO-, (R2)2N-, R3O-, and R ₄ S-, wherein Ri, R ₂ , R3, and R ₄ are independently chosen from H and a hydrocarbon comprising from 1 to 24, preferably up to 20, more preferably up to 18, and most preferably up to 10 carbon atoms and optionally comprising a heteroatom. It is noted that also mixtures of these silanes can be used.

A combination of two or more of coupling agents can be used in the process of the invention. The coupling agents may be contacted with the inorganic oxygen- containing particulate material as a mixture or separately. The ratio of the

coupling agents may vary as desired. It is also envisaged to change the ratio of the coupling agents in time while they are added to the particulate material. The silane coupling agent is typically selected from the group consisting of silanes, disilanes, oligomers of silane, and silsesquioxanes. Preferred coupling agents are silanes.

The silanes suitable for use in the process of the invention are silanes according to formulae I-VI:

IV V Vl wherein each one of Ri, R2, R3, R ₄ is independently selected from hydrogen or hydrocarbon having between 1 and 20 carbon atoms, of which at least one of Ri, R2, R3, and R ₄ comprises one or more functional groups selected from the group consisting of R1O-CO-, (R2)2N-, R3O-, and R ₄ S-, wherein R ₁ , R ₂ , R3, and R ₄ are independently chosen from H and a hydrocarbon comprising from 1 to 24, preferably up to 20, more preferably up to 18, and most preferably up to 10 carbon atoms and optionally comprising a heteroatom. In one embodiment of the invention, the functional group-containing substituent is connected directly to silicon (Si-R), and thus not connected to the silicon through the ether (Si-OR). If a silane according to any one of the formulae IV to Vl is used in the process of the invention, the ions, and in particular Cl ^" , are preferably removed from the

mixture, for example by using ion-exchange techniques. In order to avoid an additional ion removal step, the silanes according to any one of the formulae I- III are preferred.

Examples of silanes according to the invention are hydrolyzed 3-glycidyloxy- propyl thmethoxysilane (Dynasylan ^® GLYMO), hydrolyzed glycidyloxypropyl triethoxysilane (Dynasylan ^® GLYEO), and hydrolyzed anhydride-functional silanes, e.g., the hydrolysis product of Geniosil GF 20. Further examples of silanes are reaction products of silanes which introduce the desired functionality on the silane, e.g., reaction products of epoxysilanes with amines, e.g. bis(2- hydroxyethyl)-3-amino-2-hydroxypropyl-triethoxysilane, and reaction products of aminosilanes with anhydrides introducing acid group on the particulate material. It is further envisaged that the silane used in the process of the invention is a mixture of two or more silanes according to any one of the formulae I-VI.

In one embodiment of the process of the invention, the inorganic oxygen- containing particulate material which has been modified with at least one of the above coupling agents is subsequently modified with a further coupling agent or with a compound capable of reacting with the coupling agent attached to the particulate material. For example, the modified particulate material containing a glycidyl group is treated with diethanol amine, which renders a modified oxygen- containing particulate material that is more hydrophilic and consequently more compatible with the aqueous medium.

The present invention also relates to a process for preparing the aqueous slurry according to the invention comprising the steps of:

(a) precipitating the inorganic oxygen-containing particulates having an average particle size of below 165 nm by reactive precipitation of a salt of a metal having a lower valence than the same metal in the inorganic oxygen-containing particulate and a base; and

(b) modifying the particulate with a silane coupling agent having a functional group selected from the group consisting of RiO-CO-, (R ₂ ) ₂ N-, R3O-, and R ₄ S-, wherein Ri, R ₂ , R3, and R ₄ are independently chosen from H and a hydrocarbon comprising from 1 to 24, preferably up to 20, more preferably up to 18, and most preferably up to 10 carbon atoms and optionally comprising a heteroatom. It is noted that also mixtures of these silanes can be used.

The invention further pertains to a process for preparing the aqueous slurry according to the invention comprising the steps of:

(a) forming, preferably by precipitation, an aqueous composition comprising inorganic oxygen-containing particulates having an average particle size of below 165 nm, preferably by reactive precipitation of a metal salt and a base; and (b) optionally adjusting the pH of the aqueous slurry to a value between 2 and 4, preferably keeping the conductivity of the aqueous slurry to a value below 2 mS/cm; and

(c) modifying the particulates with a silane coupling agent having a functional group selected from the group consisting of R1O-CO-, (R ₂ ) ₂ N-, R3O-, and R ₄ S-, wherein Ri, R ₂ , R ₃ , and R ₄ are independently chosen from H and a hydrocarbon comprising from 1 to 24, preferably up to 20, more preferably up to 18, and most preferably up to 10 carbon atoms and optionally comprising a heteroatom. It is noted that also mixtures of these silanes can be used.

The term "reactive precipitation" refers to a precipitation process where a salt of a metal having a different valence from that of the metal in the final product is used. For example, for the preparation of CeO ₂ particles use is made of an aqueous solution of a cehum(lll) salt, e.g. cehum(lll) nitrate, to which a base, such as ammonium hydroxide or potassium hydroxide, is added so as to precipitate cerium(lll) hydroxide, which is subsequently oxidized by oxygen, optionally in the presence of a base, so as to oxidize cehum(lll) to form

cerium(IV) hydroxide, after which the cerium(IV) hydroxide is dehydrated to form CeO2. The obtained particles generally differ from particulates obtained with other processes. The amount of reactive groups on the surface, the density of the particles, and their stability as a function of the pH may differ.

The adjustment of step (b) can be performed by any method described above. A preferred method is the use of ion exchange resins, such as alkaline or acidic ion exchange resins, or by washing using a membrane step. With the latter method the pH and the conductivity can be adjusted simultaneously.

The modification of step (c) can be conducted using any method known in the art. If so desired, the modification step can be conducted during step a) and/or b), meaning that the modification is taking place during the step wherein the particles are formed, during the step wherein the pH is adjusted, or during both of these steps.

In a particular embodiment of the invention, step c) is conducted at the same time as step a) wherein the inorganic oxygen-containing particulates are formed in aqueous medium. It was observed that by combining these two steps a composition is obtained with a very low average particle size. In some applications this can be beneficial. To certain extent, this embodiment allows control over the particle size of the particulate that is formed, by adjusting the degree of modification. Typically, a higher modification will result in a smaller particle size and vice versa.

The aqueous slurry of the invention can be used in polishing of substrates, such as those used in the chip making industry. The slurry can further be used in coating compositions, in particular in water borne coating compositions for increasing the scratch resistance of the coating, for example. It may also be used for various purposes, such as UV protection, in organic polymers such as styrenics, acryliates, and polyolefins, particularly polypropylene and polyethylene, or in UV protection for coatings (e.g. Rhodigard ex Rhodia). The

aqueous slurries of the invention, and in particular the particulates contained therein, can further be suitably used in catalyst preparation, as fuel additive, and as anti-corrosion agent.

The present invention will be illustrated in the following Examples.

EXAMPLES

Comparative Example A 8.7 g of Ce(NO3)3 • 6 H ₂ O were dissolved in 102 g demineralized water at room temperature in a glass beaker (Concentration of Ce = 0.2 M). The solution was heated to 75°C using a heating plate. Then 5 g of 25 wt% ammonia solution were added within 30 seconds while vigorously mixing at 9,500 rpm using an Ultraturrax. After approximately 7 minutes the mixing was continued with a magnetic stirrer at a much lower rotation speed for approximately one hour.

The obtained slurry (Comparative Example A) was white. The aqueous slurry had a pH of 3.7 and a conductivity of 18.52 mS/cm.

The average particle size of the particulates as measured using a Malvern Instruments Zetasizer Nano was 78 nm. In a stability test more than 20 wt% of the white solid particulates settled to the bottom within a week.

Silane modified nano-ceria particles

An aqueous CeO ₂ slurry was produced according to the same procedure as given in Example A. The slurry was washed over a CEPAration ceramic membrane from Hydroflux with a pore size of 20nm until the conductivity was below 0.4 mS/cm. It contained 4.8 wt% ceria with an average particle size of 122 nm (as measured using a Zetasizer ZS of Malvern Instruments), a pH of 4.2, and a conductivity of 0.34 mS/cm at 20°C. This slurry was only stable at a pH between 2 and about 4.

Example 1 : Modification of nano-ceria with acid-functional silane Into a 250 ml 3-necked round-bottomed flask equipped with a mechanical stirrer and a thermometer were weighed 0.890 g (4.96 mmoles) of aminopropyl trimethoxysilane and 78.65 g of methanol. Next, 0.510 g (5.10 mmoles) of succinic anhydride was added.

After 75 minutes 25.70 g of the ceria dispersion were added and the temperature was raised to 65°C. After 8 hours the reaction was stopped. The pH of the system was brought to 11 with a 0.1 M potassium hydroxide solution. The resulting dispersion had an average particle size of 165 nm and was stable at a pH of 2, 4, 6, 10, and 11.

Example 2: Modification of nano-ceria with a qlvcidyl-functional silane

Into a 100 ml 3-necked round-bottomed flask equipped with a mechanical stirrer and a thermometer were weighed 4.27 g of a solution of 0.346 g (1.46 mmoles) of glycidyloxypropyl trimethoxysilane in 15.33 g of methanol.

Next, 45.87 g of the ceria dispersion were added and temperature was raised to 65°C. After 16 hours the reaction was stopped. Under the applied conditions, epoxy-silanes will hydrolyze to a diol. The resulting dispersion had an average particle size of 124 nm and was stable at a pH of 2, 4, 6, 10, and 11.

Example 3: Modification of nano-ceria with a bishvdroxy-t-amine-functional silane

Into a 100 ml 3-necked round-bottomed flask equipped with a mechanical stirrer and a thermometer were weighed 11.19 g (0.106 mole) of diethanolamine and 23.85 g (0.101 mole) of glycidyloxypropyl trimethoxysilane. The reaction mixture was heated on an oil-bath at 100°C for 2.5 hours in order to complete the reaction.

Next 0.352 mg of the diethanolamine-glycidyloxypropyl trimethoxysilane reaction product was dissolved in 30.30 g of methanol and 25.52 g of the ceria dispersion were added and the whole was allowed to react for 16 hours at 65 ⁰ C.

The resulting dispersion had an average particle size of 124 nm and was stable over the whole pH at a pH of 2, 4, 6, and 10.

Comparative example B: octyltrimethoxysilane-modified ceria Into a 2,500 ml 3-necked round-bottomed flask equipped with a mechanical stirrer and a thermometer were weighed 100.00 g of the aqueous CeO2 dispersion, containing 4.80 g of ceria and 0.19 g (7.0 mmoles) of octylthethoxysilane. The reaction mixture was allowed to react for 6 hours at 65 ⁰ C. Some deposits were formed on the stirrer. The slurry was stable at pH 3.5 (as for the non-surface-modified ceria). However, increasing the pH resulted in a coarsening of the slurry and above pH 5 particles quickly agglomerated and settled.

Example 4 Chlorosilane in-situ modification via addition in metal salt solution

In this example step c) is performed at the same time as step a) of the process of the invention. 56 g of Ce(NOs)3 ^■ 6 H ₂ O was dissolved in 344 g of demineralized water to which a specified amount of tri-methyl chlorosilane was added. Separately, 21.6 g of potassium hydroxide were dissolved in 378.4 g of water. Both solutions were pre-heated to 40 - 45°C. The base was manually added in one shot to the cerium solution while it was vigorously stirred in a jacketed reactor. After the addition was performed, the reaction mixture was heated to a reaction temperature of 85 - 90 ⁰ C, which took 20 - 25 minutes to reach. The system was then reacted further until the colour changed from yellow/brown via purple to white. The resulting slurry had a pH of about 4.5.

The dispersion was washed over a CEPAration ceramic membrane from Hydroflux with a pore size of 4 nm, until the conductivity was below 1 mS/cm.

After washing, the solids content of the mixture was measured via IR drying at 140 ⁰ C.

The theoretical amount of solid material that should remain in the system if all particles were to stay in the mixture after washing, can be calculated to be about 3 wt%. Based on this figure, the amount of material that was removed from the dispersion after washing was estimated. This value represents the weight fraction of particles that have a size < 4 nm.

The table shows that, on average, the more silane is added, the more material having a pore size of 4 nm is washed out via the membrane. This indicates that with more silane used, a higher fraction of particles having a particle size smaller than 4 nm is formed. These results further indicate that the in-situ addition of a silane can be used to control the particle size.

Example 5

Chlorosilane in-situ modification via base addition

56 g of Ce(NO3)3 ^■ 6 H ₂ O were dissolved in 344 g of demineralized water. Separately, 21.6 g of potassium hydroxide were dissolved in 378.4 g of water to which 0.494 g of th-methyl chlorosilane was added. Both solutions were preheated to 40 - 45°C. The base with silane was added manually to the cerium solution while it was vigorously stirred in a jacketed reactor. After the addition was performed, the reaction mixture was heated to a reaction temperature of 85 - 90 ⁰ C, which took 20 - 25 minutes to reach. The system was then reacted further until the colour changed from yellow/brown via purple to white. The resulting dispersion had a pH of 5.

The slurry was subsequently washed over a CEPAration ceramic membrane from Hydroflux with a pore size of 4 nm until the conductivity was 0.30 mS/cm. After washing, the solids content of the mixture was measured via IR drying at 140 ⁰ C.

The residual amount of material retained in the dispersion was measured to be 0.405 wt%. This means that about 87 wt% of particles have a size < 4 nm. This further shows that the silane can suitably be dosed into the dispersion via the base solution.