WO2016050277A1

WO2016050277A1 - Process for the preparation of functionalized zinc oxide nanoparticulate powders

Info

Publication number: WO2016050277A1
Application number: PCT/EP2014/070913
Authority: WO
Inventors: Dino BRUS; Franc ŠVEGL; Zorica Crnjak Orel
Original assignee: Kemijski inštitut
Priority date: 2014-09-30
Filing date: 2014-09-30
Publication date: 2016-04-07

Abstract

The present invention relates to a process for the preparation of surface-modified zinc oxide particles comprising the steps of (i) providing a suspension of Zn(OH)₂ particles in an alcoholic medium; (ii) reacting said suspension of Zn(OH)₂ particles with an Si-containing surface modifier, whereby a mixture of surface-modified ZnO particles and surface-modified Zn(OH)₂ particles suspended in an alcoholic medium is obtained; (iii) drying said mixture of (ii), wherein surface-modified Zn(OH)₂ particles are converted to surface-modified ZnO particles during said drying; thereby obtaining dried surface-modified ZnO particles.

Description

PROCESS FOR THE PREPARATION OF FUNCTIONALIZED ZINC OXIDE

NANOPARTICULATE POWDERS

Field of the invention

The present invention relates to a process for the preparation of surface functionalized zinc oxide (ZnO) nanoparticles in the form of solid powder which can readily be re-dispersed to obtain stable suspensions in water and/or organic media for a desired application. Highly pure, crystalline ZnO nanoparticles with spherical shape and narrow particle size distribution of e.g. 40 to 80 nm, coated with layer of silica or or gano functional silica can be prepared quickly and at large scale and low cost using inexpensive materials via a stable low temperature process. Furthermore, the functionalized ZnO powder can be easily dispersed in water and organic media and applied e.g. in cosmetic sunscreen preparations, as stabilizer in plastics, in paints, in coatings and as antimicrobial active ingredient.

Background of the invention The preparation of zinc oxide nanoparticles by dry and wet methods, is well known. The burning of zinc is a classical dry method known in the art. Aggregated particles with a broad size distribution are produced, which need additional grinding and addition of surface-active agents to prepare stable dispersions in the sub -micrometer range.

The finely divided zinc oxide nanoparticles are readily produced by wet-chemical routes, such as hydrothermal synthesis and precipitation processes. The prior art shows plethora of wet chemistry synthetic processes for the preparation of zinc oxide nanoparticles with various morphologies and size distributions. However, not many are appropriate for cost efficient industrial large scale production. The hydrothermal synthesis takes place in an expensive high pressure batch reactor at elevated temperatures. Usually costly precursor chemicals are used and the addition of dispersant is required to control the size of ZnO particles. Although the synthesis proceeds at relatively low temperature (300°C and below) a long time is required until reaction is completed. Precipitation from zinc salts by bases in aqueous media is another well-known technique. It is usually more cost efficient and generally produces zinc hydroxide and/or carbonate and/or some solid complex precipitates with anions which need purification by washing, separation of liquid from solid phase and conversion of solid precipitate by thermal treatment to zinc oxide. The thermal treatment of washed and dried powder at higher temperatures has an adverse effect on morphology of nanoparticles due to sintering processes. Often micrometer sized aggregates are formed which can only be broken down, usually incompletely to the primary particles by grinding.

The precipitation processes are normally conducted in batch reactors. However it is difficult to obtain good homogeneity in batch processes, in particular if the size of the reactor is large, e.g., after scale-up. As a result, the precipitate obtained has non-uniform particle size and morphology.

Precipitation processes in tubular reactors are generally known. WO 98/02237 describes the process for the production of copper oxalate powders by precipitation from a liquid reaction mixture passing along a tubular reactor (Segmented Flow Tubular Reactor - SFTR). The precipitation reaction is effected by sub-dividing the reaction mixture into a plurality of discrete volumes or segments which are passed, preferably under plug flow conditions along a tubular reactor separated by discrete volumes of a separating fluid which is immiscible with the reaction mixture. Within each volume of the reaction mixture, the conditions for the precipitation reaction are substantially identical so that a uniform product is obtained from each volume of the reaction mixture. This type of tubular reactors has shown excellent performances in liquid-liquid reactions as precipitation of different types of nanoparticles. When compared with batch reactors, SFRT shows many advantages. It can be operated in a continuous mode and ensure controlled hydrodynamics, residence times, and reaction conditions.

As described by Aimable et. al. (2010, Processing and Application of Ceramics 4(3): 107— 114), the preparation of nonagglomerated re-dispersible nanoparticles of ZnO (particle size < 100 nm) was successfully accomplished by using SFTR. The ZnO nanoparticles were prepared via an alkaline precipitation from zinc nitrate and sodium hydroxide aqueous solutions at 90°C in the presence of poly( acrylic acid) (PAA Mw 2000). After precipitation reaction, the suspended powder was washed with ultra pure water, before being filtered and dried for 24 hours at 70°C.

Nanoparticles of zinc oxide are found in many applications. Zinc oxide (ZnO) protects against UV radiation and has been used for sunscreening for decades. When compared with organic UV absorbers, inorganic UV absorbers such as ZnO, exhibit various advantages, e.g., thermal stability, better resistance to photo degradation and better migration stability. Zinc oxide with particle sizes below 100 nm is particularly suitable for use as UV absorber. It is desirable that nanoparticles exhibit the highest possible transparency in the visible wavelength region and the highest possible absorption in the region of near ultraviolet light (UV-A region, ca. 320 to 400 nm wavelength).

Common applications of nanoparticulate zinc oxide as UV absorber are in cosmetic sunscreen compositions, formulation of paints and coatings, in transparent organic- inorganic hybrid materials, as a protection of UV- sensitive organic pigments, as stabilizer in plastics and as antimicrobial active ingredient. In most of these applications zinc oxide is incorporated into a polymer matrix. The property of the zinc oxide particles to increase the rate of photocatalytic degradation of organic polymers surrounding them is often disadvantageous.

The common applications of nanoparticulate zinc oxide as UV absorber and in other applications need further surface treatment or functionalization of the zinc oxide nanoparticles and preparation of stable dispersions. The shelf life (pot life) of these dispersions is thereby increased.

As shown in prior art, the surface modification of zinc oxide particles, for example, with amorphous layers comprising silicon oxides or aluminum oxides can protect organic polymer matrix from photocatalytic degradation without considerable influence on UV- absorbing properties of zinc oxide particles.

DEI 9907704 describes the production of nanoscale zinc oxide particles in a batch from zinc acetate via an alkaline precipitation. WO 90/06874 describes UV-absorbing chemically inert compositions in form of an aqueous slurry comprising particles consisting of ZnO with surface coating of Si0₂ and A1₂0₃.

US 20030172845 describes a process for the preparation of nano-zinc oxide dispersions stabilized by hydroxyl group-containing inorganic polymers. The inorganic polymers described can be obtained by hydrolysis and condensation of monomeric and/or oligomeric alkoxysilanes or organoalkoxysilanes (sol-gel process). As described in this document, the preparation of stabilized nano-zinc oxide dispersions is a three-step process. The preparation of a dispersion of the zinc oxide nanoparticles in a halogen-containing medium is accomplished according to procedure in e.g. DE-A-19907704 (step 1), the dispersion is then added to the hydroxyl group-containing inorganic polymer (step 2), then the halogen- containing constituents are removed by distillation (step 3).

US 2012/0097068 describes a process for the preparation of surface modified zinc oxide nanoparticles with reduced photocatalytic activity, which have Si-O-alkyl groups and are soluble in organic solvents. The ZnO nanoparticles are dissolved in a solvent, in the presence of ammonia or amines with a tetraalkyl orthosilicate and optionally with an organosilane.

EP 1167462 describes metal oxide powders coated with a tight silica coating of 0.1 to 100 nm used in an ultraviolet-screening cosmetic formulation. The metal oxide particles with a silica coating are furthermore also treated with a hydrophobicizing agent. The silica coating is formed with the help of tetraalkoxysilanes in aqueous solution. The hydrophobicizing agents used are alkylalkoxysilanes.

US 20030104198 describes metal oxide particles such as zinc oxide and titanium dioxide having a metal oxide core coated with silicon dioxide. The coating is accomplished by dispersing metal oxide particles, adding base, preferably ammonia, and at least one compound of the type X_nSi(OR)4-_n. The reaction product is separated and optionally washed with water and dried.

H. Wang, et al. (2002, Chemistry Letters, 630-631) describe ZnO nanoparticles which are coated with silica with the help of a two-stage procedure. First, a mixture of a tetraethoxysilane, ethanol and aqueous ammonia solution is prepared. ZnO nanoparticles are then added to this solution. The ZnO particles provided with silica coatings of about 20 nm exhibit reduced photocatalytic activity.

In the aforementioned prior art, processes for obtaining surface modified zinc oxide particles include at least two separate processes.

The first process is the preparation of nanoparticulate ZnO, usually in the form of aqueous and/or organic solvent suspension.

A second process is the surface modification of the ZnO particles, accomplished by dispersing zinc oxide powder in appropriate media and adding/reacting appropriate precursors for coating formation.

A key factor for industrial production is to obtain surface-coated ZnO nano powder. The cost of transporting and storing suspensions even at 30% weight solids fraction is significantly higher than transporting a dried powder. The key for further applications is the ability to re-disperse the powder after drying in water and/or organic media to obtain stable suspensions. An object of the present invention was therefore to provide an efficient continuous process for production of surface modified zinc oxide nanoparticles in the form of nanoparticulate solid powder which powder can be readily re-dispersed to obtain stable aqueous and/or alcohol suspensions for desired application.

The present invention combines the two processes of ZnO nanoparticles preparation and surface modification to a single efficient large-scale industrial process.

This object is achieved by reacting Zn(OH)₂ particles in suspension with a Si-containing surface modifier, preferably at elevated temperatures, and drying (preferably at elevated temperatures), to obtain a surface-modified (i.e., functionalized) nanoparticulate powder of ZnO particles coated with a Si-containing coating (e.g., silica and/or thin layer of silica containing organosilanes and/or organofunctional groups at outer surface) which prevents particle agglomeration, reduces photocatalytic activity and enables re-dispersibility of nano powder in aqueous and/or organic media. Brief summary of the invention

One aspect of the invention relates to a process for the preparation of surface-modified zinc oxide (ZnO) particles comprising the steps:

(i) providing a suspension of Zn(OH)₂ particles in an alcoholic medium; (ii) reacting said suspension of Zn(OH)₂ particles with an Si-containing surface modifier, whereby a mixture of surface-modified ZnO particles and surface-modified Zn(OH)₂ particles suspended in an alcoholic medium is obtained;

(iii) drying said mixture of (ii), wherein at least part of (or all of) said surface- modified Zn(OH)₂ particles of said mixture are converted to surface-modified ZnO particles during said drying; thereby obtaining dried surface-modified ZnO particles.

The drying in step (iii) preferably takes place at elevated temperatures. More preferably, the drying step comprises increasing the temperature of the mixture of (ii) by at least 10°C, or at least 20°C, or at least 30°C, or at least 40°C, or at least 50°C, or at least 70°C, or at least 90°C above a temperature (e.g., a reaction temperature) at step (ii). The drying step (iii) may comprise increasing the temperature of the mixture if (ii) to a temperature of at least 30°C, or at least 40°C, or at least 50°C, or at least 70°C, or at least 90°C, or at least 110°C, or at least 130°C, or at least 140°C, or at least 150°C, or at least 160°C.

In one embodiment, the drying step (iii) comprises spray-drying said mixture of surface- modified ZnO particles and surface-modified Zn(OH)₂ particles in a spray dryer. Preferably said spray-drying is effected in a stream of drying gas having an initial temperature of from 90°C to 160°C, more preferably an initial temperature of from 100°C to 140°C. Preferably, said drying gas is cooled from said initial temperature to a final temperature of from 20°C to 80°C, more preferably of from 50°C to 70°C, during the spray-drying process.

In preferred embodiments of the invention, at least 50%, or at least 60%, or at least 70%>, or at least 80%>, or at least 90%>, or at least 95%>, or at least 97%>, or at least 99%>, or at least 99.9% of the surface-modified Zn(OH)₂ particles of the mixture are converted into surface- modified ZnO particles during the drying step.

Preferably, step (ii) comprises a step (ii. l) of mixing said Zn(OH)₂ particles in suspension and said Si-containing surface modifier, most preferably in a high shear mixer. Step (ii) may further include a step (ii.2) of incubating said Zn(OH)₂ particles in suspension and said Si-containing surface modifier. The incubation may take place at a temperature of from 10 to 30°C, e.g., room temperature, e.g., for a period of from 5 to 60 min, preferably of from 10 to 20 minutes.

The alcoholic medium preferably comprises an alcohol selected from the group consisting of ethanol, methanol, propanol, ethylene glycol, diethylene glycol, and polyol; preferably ethanol, methanol, propanol or ethylene glycol.

In accordance with the invention, the alcoholic medium preferably contains less than 10%, or less than 5%, or less than 2%, or less than 1% by weight of water (based on the total weight of the alcoholic medium). The alcoholic may also be free of water. The Si-containing surface modifier is preferably selected from the group consisting of tetraalkyl-ortosilicate, organosilane, organofunctional silane, polyfunctional alkoxysilane and silanols.

In a preferred embodiment, the Si-containing surface modifier has the general formula:

wherein RO is independently methoxy, ethoxy, acetoxy, or any hydrolysable group; preferably methoxy, ethoxy, acetoxy; and

X is Ci-C₂o alkyl radical, aryl, phenyl or an organofunctional group.

X may hence be selected from C1-C15 alkyl,

alkyl, C1-C4 alkyl and phenyl. For example, X may be selected from methyl, ethyl, propyl, i-butyl, n-octyl. X, however, can also be an organofunctional group selected from amino, methacryloxy, epoxy, benzylamino, vinyl, vinyl-benzyl-amino, chloropropyl, melamin, ureido, mercapto, disulfido and tetrasulfido. In another preferred embodiment, the Si-containing surface modifier has the general formula:

wherein: Y, Y' are each selected from hydrogen, substituted or unsubstituted Ci-C2o-alkyl radical and C₆-aryl radical (preferably Ci-Cs-alkyl radical or C₆-aryl radical); R is selected from Ci-C8-alkyl radical and phenyl radical (preferably Ci-C4-alkyl radical or phenyl radical); a, b and c are independently 0, 1, 2, 3 or 4; and the sum of (a + b + c) = 4.

In another preferred embodiment, the Si-containing surface modifier has the general formula: [(R¹0)_a(Y¹)₃-aSi]₁-(CH₂)_k-X-(CH₂)₁-[SiY²3-b(OR²)_b]_J wherein: R¹, R² are independently of each other Ci-Cs-alkyl or C₆-aryl radical; Y¹, Y² are independently each other substituted or unsubstituted Ci-C₂o-alkyl, or substituted or unsubstituted C₆-aryl radical; a, b are independently of one another 1, 2 or 3; i and j are integers, wherein (i + j) is greater than or equal to 2; k and 1 are independently of one another integers from 0 to 10 (inclusive); and X is a bridging unit to which (i + j) alkoxysilyl groups [(R¹0)_aY¹ ₃__aSi] or [SiY² ₃__b(OR²)_b] are bonded via a chemical bond. X, within the context of the present application, is preferably selected from -(CH₂)_n-, -NH-, - NH-CH₂-CH₂-NH-, -NH-CO-NH-, -N(CH₃)-, -CHR-, -Ar-, -CH₂-Ar-CH₂-, -0-, -S-, -S-S-, -(CF₂)_n-, and -CH=CH-, wherein Ar stands for aryl. In particularly preferred embodiments, the Si-containing surface modifier is selected from the group consisting of: CH₃-Si(OCH₃)₃, C₂H₅-Si(OCH₃)₃, phenyl-Si(OCH₃)₃, CH₃- Si(OC₂H₅)₃, C₂H₅-Si(OC₂H₅)₃, phenyl-Si(OC₂H₅)₃, n-butyl-Si(OC₂H₅)₃, n-butyl- Si(OCH₃)₃, i-butyl-Si(OC₂H₅)₃, i-butyl-Si(OCH₃)₃, i-propyl-Si(OC₂H₅)₃, i-propyl- Si(OCH₃)₃, i-octyl-Si(OC₂H₅)₃, i-octyl-Si(OCH₃)₃, (CH₃)₂Si(OCH₃)₂, (C₂H₅)₂Si(OCH₃)₂, (CH₃)(phenyl)Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (C₂H₅)₂Si(OC₂H₅)₂,

(CH₃)(phenyl)Si(OC₂H₅)₂, (n-butyl)₂Si(OC₂H₅)₂, (n-butyl)₂Si(OCH₃)₂, (i- butyl)₂Si(OC₂H₅)₂, (i-butyl)₂Si(OCH₃)₂, (i-propyl)₂Si(OC₂H₅)₂, (i-propyl)₂Si(OCH₃)₂, (CH₃)HSi(OCH₃)₂, CH₃HSi(OC₂H₅)₂, (CH₃)₂Si(0-phenyl)₂, (C₂H₅)₂Si(0-phenyl)₂, n- octadecyl- Si(OC₂H₅)₃, n-octadecyl- Si(OCH₃)₃, n-hexadecyl- Si(OC₂H₅)₃, and n- hexadecyl- Si(OCH₃)₃.

In other particularly preferred embodiments, the Si-containing surface modifier is selected from the group consisting of: (CH₃)₃Si(OCH₃), (C₂H₅)₃Si(OCH₃), (CH₃)₂(phenyl)Si(OCH₃), (CH₃)₃Si(OC₂H₅), (C₂H₅)₃Si(OC₂H₅), (CH₃)₂(phenyl)Si(OC₂H₅), (n-butyl)2Si(OC₂H₅)2, (n-butyl)₂Si(OCH₃)₂, (i-propyl)₂Si(OC₂H₅)2, (i-propyl)₂Si(OCH₃)₂, (CH₃)HSi(OCH₃)₂, CH₃HSi(OC₂H₅)₂, (CH₃)₂Si(0-phenyl)₂, (C₂H₅)₂Si(0-phenyl)2, (CH₃)(phenyl)₂Si(OCH₃), (CH₃)(phenyl)₂Si(OC₂H₅), (CH₃)₃Si(0-phenyl), (C₂H₅)₃Si(0- phenyl), (phenyl)₃Si(OCH₃), (phenyl)₃Si(OC₂H₅), (i-propyl)(CH₃)₂Si(OC₂H₅), (i- propyl)(CH₃)₂Si(OCH₃), (i-propyl)₂(CH₃)Si(OC₂H₅), (i-propyl)₂(CH₃)Si(OCH₃), (i- propyl)₃Si(OC₂H₅), (i-propyl)₃Si(OCH₃)₃, (n-butyl)(CH₃)₂Si(OC₂H₅)₂, (n- butyl)(CH₃)₂Si(OCH₃), (n-butyl)₂(CH₃)Si(OC₂H₅), (n-butyl)₂(CH₃)Si(OCH₃), (n- butyl)₃Si(OC₂H₅), and (n-butyl)₃Si(OCH₃).

The Si-containing surface modifier may also be selected from the group consisting of: vinyltrichlorosilane, vinyltris( -methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, γ-acetoxypropyltrimethoxysilane, γ-cyanopropyltrimethoxysilane, isocyanatopropyltrimethoxysilane, y-(methacryloyloxypropyl)trimethoxysilane, β-(3 ,4 epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidyloxypropyltrimethoxysilane, γ- glycidoxypropylmethyldiethoxysilane, N- (aminoethyl)y-aminopropyltrimethoxysilane, N- (aminoethyl)y-aminopropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane, N- phenyl-y-aminopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, and γ- chloropropyltrimethoxysilane, dimethylvinylmethoxysilane and dimethy lviny lethoxysilane .

Alternatively, or additionally, the Si-containing surface modifier is selected from: bis(triethoxysilyl)ethane, l,8-bis(triethoxysilyl)octane, bis(triethoxysilyl)ethane, 1,2- bis(trimethoxysilyl)decane, bis(trimethoxysilylethyl)benzene, and bis(triethoxysilyl)ethylene, bis(y-(triethoxysilyl)propyl)disulfide, bis(y-

(triethoxysilyl)propyl)amine, N,N -bis(y-(trimethoxysilyl)propyl)ethylenediamine and 1,3- bis(glcydoxypropyl)tetramehyldisiloxane. Preferably, the Si-containing surface modifier is added at a molar ratio (i.e., Si-containing surface modifier to Zn(OH)₂) of 0.01 to 0.5.

According to a preferred embodiment of the inventive process, step (i) of providing a suspension of Zn(OH)₂ particles in an alcoholic medium comprises (or consists of) the following the steps:

(i.1) providing a first aqueous solution comprising at least one zinc salt;

(1.2) providing a second aqueous solution comprising at least one base;

(1.3) mixing said first and second aqueous solutions, whereby Zn(OH)₂ is formed and Zn(OH)₂ particles precipitate to form an aqueous suspension; (i.4) removing at least part of the liquid phase (e.g. 50%, or 60%, or 70%>, or

80%)) of said aqueous suspension thereby obtaining concentrated Zn(OH)₂ particles;

(i.5) re-dispersing said concentrated Zn(OH)₂ particles in an alcoholic medium, thereby obtaining Zn(OH)₂ particles suspended in an alcoholic medium.

Steps (i.4) and (i.5) may be repeated 1, 2, 3 or multiple times. The zinc salt used according to the invention is preferably selected from the group consisting of zinc nitrate, zinc chloride and zinc acetate.

The base used according to the invention is preferably selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), rubidium hydroxide (RbOH), caesium hydroxide (CsOH) and ammonia; preferably from sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonia.

The aqueous solution in step (i. l) preferably comprises less than 10%>, or less than 5%>, or less than 2%, or less than 1%, or less than 0.1% (most preferably less than 5%) by weight of alcohol (based on the total weight of the liquid).

In preferred embodiments, the mixing in step (i.3) is conducted in a segmented flow tubular reactor (SFTR). Said mixing step (i.3) preferably occurs at a temperature within the range of from 10°C to 90°C, preferably of from 30°C to 70°C, or from 40°C to 60°C.

The separation step (i.4) preferably comprises filtration, decantation or centrifugation.

Preferably, the re-dispersion in step (i.5) occurs in a high shear mixer. Detailed description of the invention

The present invention relates to processes for the preparation of surface modified zinc oxide particles, preferably nanoparticles, in the form of solid powder which can be readily re-dispersed to obtain stable suspensions in water and/or organic media for any desired application. The surface-modified zinc oxide particles and suspensions prepared according to the invention find application as stabilizers in plastics, paints, in coatings and as antimicrobial active ingredient. Such products also form part of the present invention.

In one embodiment of the invention, the expression "particle" is to be understood as meaning primary particle, i.e., excluding agglomerated particles.

The term "nanoparticles" is used to refer to particles having an average diameter of below 100 nm (number average diameter), as determined by means of transmission electron microscopy (TEM) or scanning electron microscopy (SEM). Alternatively, the expression "nanoparticles" shall be understood to be particles having a volume median diameter D(v, 0.5) of 100 nm. The volume median diameter D(v, 0.5) is the diameter where 50% of the distribution is above and 50% is below that diameter. In one embodiment, the particle size is determined according to DIN 53206. The diameter, in the context of the present invention, is taken to be the longest linear dimension through the particle in 3 dimensional space.

"Zink oxide" shall mean ZnO. "Zink hydroxy de" shall mean Zn(OH)₂. Collective terms specified for various substituents have the following meaning: Ci-C2o-alkyl: straight-chain or branched hydrocarbon radicals having up to 20 carbon atoms, for example Ci-Cio-alkyl or Cn-C₂₀-alkyl, preferably Ci-Cio-alkyl, for example Ci- C₃-alkyl, such as methyl, ethyl, propyl, isopropyl, or C4-C6-alkyl, n-butyl, sec-butyl, tert- butyl, 1 , 1-dimethylethyl, pentyl, 2-methylbutyl, 1 , 1-dimethylpropyl, 1 ,2-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl, hexyl, 2-methylpentyl, 3-methylpentyl, 1 , 1- dimethylbutyl, 1 ,2-dimethylbutyl, 1 ,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3- dimethylbutyl, 3,3-dimethylbutyl, 2-ethylbutyl, 1 , 1 ,2-trimethylpropyl, 1 ,2,2- trimethylpropyl, 1 -ethyl- 1-methylpropyl, l-ethyl-2-methylpropyl, or Cy-Cio-alkyl, such as heptyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl, 1 , 1 ,3,3-tetramethylbutyl, nonyl or decyl, and isomers thereof.

Aryl: a mono- to trinuclear aromatic ring system comprising 6 to 14 carbon ring members, e.g. phenyl, naphthyl or anthracenyl, preferably a mono- to binuclear, particularly preferably a mononuclear, aromatic ring system.

Ci-C2o-Alkoxy is a straight-chain or branched alkyl group having 1 to 20 carbon atoms (as specified above) which are attached via an oxygen atom (-0-), for example Ci-Cio-alkoxy or Cn-C2o-alkoxy, preferably Ci-Cio-alkyloxy, particularly preferably Ci-C₃-alkoxy, such as, for example, methoxy, ethoxy, propoxy.

The term "high shear mixer" is well known in the art. Any general-purpose high-shear mixer may be used. A high shear mixer useful for the present invention is IKA T-25 digital Ultra-Turrax from IKA-Werke, Staufen, Germany.

The present invention provides an industrially relevant continuous process for production of surface modified ZnO nanoparticles in the form of nanoparticulate solid powder. The functionalized ZnO nano powder is readily re-dispersible to obtain stable aqueous and/or alcohol suspensions for desired application. The process according to the invention is defined by the appended claims. It thus comprises the following steps:

(iii) drying said mixture of (ii), wherein surface-modified Zn(OH)₂ particles of said mixture are converted to surface-modified ZnO particles during said drying; thereby obtaining dried surface-modified ZnO particles.

The aqueous medium preferably contains less than 5% (or less than 3%, or less than 1%, or less than 0.1%) by weight of other liquid constituents, such as alcohol. Preferably, the aqueous medium includes at least 95%, or at least 97%, or at least 99%, or at least 99.9% by weight of water (excluding suspended solids).

The alcoholic medium preferably contains less than 5% (or less than 3%, or less than 1%, or less than 0.1 %) by weight of other liquid constituents, such as water. Preferably, the alcoholic medium includes at least 95%, or at least 97%, or at least 99%, or at least 99.9% by weight of alcohol (excluding suspended solids). The surface modified ZnO particles of the present invention preferably have a volume median diameter D(v, 0.5) of less than 100 nm.

According to the invention, step (i) of providing a suspension of Zn(OH)₂ particles in an alcoholic medium may include the steps of

(a) preparing aqueous solutions of at least one zinc salt (solution 1) and of at least one preferably strong base (solution 2), and

(b) mixing (and optionally incubating) solutions 1 and 2.

Preferably, the mixing in step (b) takes place in a tubular reactor, such as a Segmented Flow Tubular Reactor (SFTR). During the mixing, nanoparticulate Zn(OH)₂ particles are formed and precipitate out of the solution to form an aqueous suspension. Subsequently, the aqueous medium may be at least partially removed and the particles may then be re- suspended in an alcoholic medium. A high shear mixer is preferably used for re- suspension. An alcohol colloid suspension is preferably formed. The precursor solution (solution 1) has preferably a Zn ion concentration of 100 to 500 mM, more preferably of 200 to 400 mM. The concentration is adequate to prevent generation of other phases in preparation for ZnO nanoparticles with uniform size and shape. The Zn salt, according to the invention, may be zinc nitrate, zinc chloride and zinc acetate. The precursor solution is prepared by adding and dissolving an appropriate amount of Zn salt in deionized water under agitation. The deionized water is preferably heated to 60°C to 80°C to increase the speed of dissolution, if necessary.

The aqueous base solution (solution 2) to be used according to the invention may in general be prepared by any substances which are able to produce a pH of from about 8 to about 13, preferably of from about 9 to about 12.5, in aqueous solution depending on their concentration. Preference is given to using alkali metal hydroxides, such as sodium or potassium hydroxide, alkaline earth metal hydroxides, such as calcium hydroxide or ammonia. Particular preference is given to using sodium hydroxide, potassium hydroxide and ammonia. The strong base solution is prepared by dissolving an appropriate amount of strong base as an alkali metal hydroxide (e.g NaOH) or dilute concentrated aqueous ammonia solution to appropriate concentration.

The concentration of the strong base is preferably chosen so that a hydroxyl ion concentration in solution 2 is in the range from 100 to 2000 mM, more preferably from 400 to 800 mM. The hydroxyl ion concentration in solution 2 is chosen depending on the concentration and the valence of the zinc ions. For example, in the case of a solution 1 with a concentration of zinc ions of 200 mM, solution 2 has preferably a hydroxyl ion concentration of 400 mM.

Preferably the mixing (and/or incubation) step (b) is carried out continuously. The mixing of solutions 1 and 2 results in precipitation of Zn(OH)₂. The mixing may occur continuously in a segmented flow tubular reactor (SFTR). The basic design and principle of operation of SFRT are well known and described e.g. in US6458335. The SFTR may be composed of three distinct parts: (1) a mixer which ensures that the reactant solutions are efficiently mixed, (2) a segmenter, which alternately supplies individual volumes or segments of the reaction mixture and segments of a "separating" fluid (gas or liquid), which is substantially immiscible with the reaction mixture, and (3) a tubular reactor, through which segments of reaction mixture and separating fluid are transported in an alternating fashion. The tubular reactor is preferably placed in a thermostatic bath.

Conveniently, solution 1 and solution 2 may be stored in two vessels made of e.g. of stainless steel and equipped with a spear reaching almost to the bottom of the keg. The volume of the vessel may be 50 liters. The vessel may be equipped with self-closing valve consisting of a spring loaded valve mechanism connected to the top of the spear. The self- closing valve seals the contents in the vessel from the outside environment and allows compressed gas in to push the solution out. The pressurized gas according to the invention may in general be any inert gas or gas mixture, preferably nitrogen or argon. The self- closing valve is opened by a coupling fitting which has two valves that control the flow of solution out of and compressed gas into the keg. As the solution is dispensed from the vessel, more pressurized gas is forced in to maintaining the pressure on the solution and constant flow rate at exit. The preferred pressures of pressurized nitrogen in the vessels with Zn nitrate precursor solution 1 and base solution 2 are 1.5 bar to 2.0 bar. The solutions 1 and 2 from step (a) may be dispensed from the vessels at constant flow into the micro-mixer where the initial supersaturation is created. The micromixer is in Y- configuration or T-configuration and any design described in US6458335 can be applied. Particularly preferred is T-micro-mixer with Hartridge-Roughton configuration where the feed flows are displaced from a direct collision geometry to create an internal vortex in the mixing chamber. The initial contacting of reagents and the mixing time in the mixer are important parameters, which will influence the nucleation rate. The Hartridge-Roughton mixing device appears to be the most efficient with a constant of micro-mixing time down to 1.2 ms. Two dimensions for the Hartridge-Roughton mixing device are used: 2 mm internal diameter for inlet tubes and 4 mm for outlet tube. The reacting mixture is then segmented in a segmenter by nitrogen gas. The segmenter comprises of concentrically located tubes with defined annual space between them. The inner tube with inner diameter in the range from 0.2 to 2 mm, depending on the desired flow rate is of a shorter length than outer tube which may have an inner diameter of 2 to 5 times of the inner tube. In the present embodiment of the invention the inner diameter of inner tube of 1 mm and outer tube of 5 mm is preferred. The mixing chamber is defined between the downstream end of inner tube and a segmenting region in the form of a restriction element which is located within outer tube and has a central bore of diameter between 2 to 4 mm depending on the desired flow rate. The preferred diameter of the bore is 2 mm. The distance between the downstream end of the inner tube and the upstream end of the restriction element is in the range 0.5 to 5 mm, preferably 5 mm. The distance may be adjusted by virtue of inner tube being located in position by a screw thread arrangement.

The outer tube is provided with an arm, the outlet tube of micromixer delivering the reactant mixture. As such, the reactant mixture enters, and fills the annular space between inner and outer tube of segmenter. The pulsed stream of nitrogen gas (~ 2 -5 s) is supplied by means of an electro pneumatic control valve system. Small suspension volumes (~ 0.3 - 2 cm³) are thus created, producing micro-droplets entering the tubular reactor. The continuous plug flow comprised of alternate, discrete volumes of reactant mixture and separating nitrogen gas is passed to tubular reactor. These small volumes ensure a high homogeneity inside each droplet, all circulating through the tube with an identical residence time and heat exchange. The residence time is determined by the flow speed, the tube length, and reaction kinetics. According to a particularly preferred embodiment of the present invention the residence time of reaction mixture is of from 10 to 30 min (e.g., around 15 min) for tube diameter 2 mm and tube length 5 m. The tubular reactor, segmenter and mixer may be of any material which is not degraded and which remains unaltered in contact with the reactants and separating fluid or separating gas. Further criteria for the choice of material are its ability to be wetted by the reactants and separating fluid and non-adherence to the precipitated particles. Examples of suitable materials include plastics (e.g. PMMA, PTFE, PE, PO, and the like), glasses (e.g. chemistry glass, borosilicate glass, vitreous silica and the like) and metals (e.g. stainless steel, aluminum, titanium or alloys thereof, and the like). In the embodiment of this invention the preferred material for tubular reactor is PTFE.

The system can easily be scaled up. For example a multiple-tube configuration with 1000 parallel tubes was tested. The efficiency of precipitation step in SFRT was shown to be between 70 and 80%. Zn(OH)₂ particles precipitate during or after the mixing step. In order to separate the nanoparticulate Zn(OH)₂ precipitate from the aqueous medium, a separation step (c) may optionally be performed. The separation step (c) may be performed by, for example, decantation, filtration or centrifugation. If required, the aqueous dispersion can be concentrated prior to isolating the precipitated particles by means of a membrane process such as nano-, ultra-, micro- or crossflow filtration and, if appropriate, can be at least partially freed from undesired water-soluble constituents, for example alkali metal salts, such as sodium chloride or sodium nitrate. The losses in separation step are due to thickening and washing in the range between 10 - 20 %. It has proven to be advantageous to carry out the separation step (c) at temperature in the range from 10°C to 50°C, e.g., at room temperature. It may thus be required to cool the aqueous suspension obtained in step (b) to such lower temperature.

It is favorable to thicken the precipitate, which enables fast removal of supernatant. Particularly good thickening of the material and thus also particularly complete removal of the precipitate from the by-products of the precipitation is achieved by centrifugation. In one embodiment, the precipitate is first washed with deionized water and then with an alcoholic medium.

The process may further include a step (d) of dispersing the washed precipitate in an alcoholic medium. Re-dispersion is preferably effected in a high speed shear mixer, e.g., at 5000 to 15000 rpm, e.g. at 10000 rpm, for 5 to 15 minutes, e.g. 10 minutes. Alternatively, an ultrasonic dispenser may be used. A stable, milky alcohol colloid suspension may then be obtained.

The alcoholic medium according to the invention may, e.g., comprise ethanol (CH₃CH₂OH), methanol (CH₃OH), propanol, or butanol. Ethylene glycol or other polyols may also be used. According to a preferred embodiment of the present invention, the alcoholic medium is ethanol. Industrial grade ethanol may be used.

According to the invention, in step (ii) of reacting said suspension of Zn(OH)₂ particles with an Si-containing surface modifier comprises addition of tetraalkyl-ortosilicate or at least one organo functional alkoxysilane to the alcoholic suspension, preferably during intensive mixing, e.g., using high speed shear mixer at 5000 to 15000 rotations/min, particularly preferably at 10000 rotations/min, for 5 to 10 minutes, preferably 10 minutes, or by using ultrasonic dispenser or similar dispersing equipment.

The tetraalkyl orthosilicate added is preferably tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, or tetrabutyl orthosilicate. Tetraethyl orthosilicate is very preferably used.

The content of tetraalkyl orthosilicate in the process according to the invention can vary within a very wide range, for example depending on the reactivity of the silicate or the desired coating thickness or density. As a rule, from 0.01 to 1.0 molar equivalents of tetraalkyl orthosilicate, based on Zn(OH)₂, are used. Preference is given to from 0.05 to 0.5 molar equivalents of tetraalkyl orthosilicate, in particular from 0.1 to 0.2 molar equivalents of tetraalkyl orthosilicate, based on the Zn(OH)₂.

In step (ii), the Si- containing surface modifier may be an organo alkoxysilane of the formula (I): (Y)_a(Y^')_bSi(OR)_c (I) where Y, Y' are each hydrogen, an optionally substituted Ci-C₂o-alkyl or C₆-aryl radical; R is a Ci-C8-alkyl radical or a phenyl radical; a, b and c, independently of one another, may be 0, 1, 2, 3 or 4 and (a+b+c)=4.

Preferably Y, Y' are each hydrogen, Ci-Cs-alkyl or C₆-aryl radical; R is a Ci-C4-alkyl radical or a phenyl radical; a, b and c, independently of one another, may be 0, 1, 2 or 3 and (a+b+c)=4.

Particularly preferred Si-containing surface modifiers are the following organo alkoxysilanes and organo functional alkoxysilanes:

CH₃-Si(OCH₃)₃, C₂H₅-Si(OCH₃)₃, phenyl-Si(OCH₃)₃, CH₃-Si(OC₂H₅)₃, C₂H₅-Si(OC₂H₅)₃, phenyl-Si(OC₂H₅)₃, n-butyl-Si(OC₂H₅)₃, n-butyl-Si(OCH₃)₃, i-butyl-Si(OC₂H₅)₃, i-butyl- Si(OCH₃)₃, i-propyl-Si(OC₂H₅)₃, i-propyl-Si(OCH₃)₃, i-octyl-Si(OC₂H₅)₃, i-octyl- Si(OCH₃)₃, (CH₃)₂Si(OCH₃)₂, (C₂H₅)₂Si(OCH₃)₂, (CH₃)(phenyl)Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (C₂H₅)₂Si(OC₂H₅)₂, (CH₃)(phenyl)Si(OC₂H₅)₂, (n-butyl)₂Si(OC₂H₅)₂, (n-butyl)₂Si(OCH₃)₂, (i-butyl)₂Si(OC₂H₅)₂, (i-butyl)₂Si(OCH₃)₂, (i-propyl)₂Si(OC₂H₅)₂, (i- propyl)₂Si(OCH₃)₂, (CH₃)HSi(OCH₃)₂, CH₃HSi(OC₂H₅)₂, (CH₃)₂Si(0-phenyl)₂, (C₂H₅)₂Si(0-phenyl)₂, n-octadecyl- Si(OC₂H₅)₃, n-octadecyl- Si(OCH₃)₃, n-hexadecyl- Si(OC₂H₅)₃, n-hexadecyl- Si(OCH₃)₃, and/or (CH₃)₃Si(OCH₃), (C₂H₅)₃Si(OCH₃), (CH₃)₂(phenyl)Si(OCH₃), (CH₃)₃Si(OC₂H₅), (C₂H₅)₃Si(OC₂H₅), (CH₃)₂(phenyl)Si(OC₂H₅), (n-butyl)₂Si(OC₂H₅)₂, (n-butyl)₂Si(OCH₃)₂, (i-propyl)₂Si(OC₂H₅)₂, (i-propyl)₂Si(OCH₃)₂, (CH₃)HSi(OCH₃)₂, CH₃HSi(OC₂H₅)₂, (CH₃)₂Si(0-phenyl)₂, (C₂H₅)₂Si(0-phenyl)₂, (CH₃)(phenyl)₂Si(OCH₃),

(CH₃)(phenyl)₂Si(OC₂H₅), (CH₃)₃Si(0-phenyl), (C₂H₅)₃Si(0-phenyl), (phenyl)₃Si(OCH₃), (phenyl)₃Si(OC₂H₅), (i-propyl)(CH₃)₂Si(OC₂H₅), (i-propyl)(CH₃)₂Si(OCH₃), (i- propyl)₂(CH₃)Si(OC₂H₅), (i-propyl)₂(CH₃)Si(OCH₃), (i-propyl)₃Si(OC₂H₅), (i- propyl)₃Si(OCH₃)₃, (n-butyl)(CH₃)₂Si(OC₂H₅)₂, (n-butyl)(CH₃)₂Si(OCH₃), (n- butyl)₂(CH₃)Si(OC₂H₅), (n-butyl)₂(CH₃)Si(OCH₃), (n-butyl)₃Si(OC₂H₅), (n- butyl)₃Si(OCH₃). Other preferred Si-containing surface modifiers are organofunctional alkoxysilanes, such as vinyltrichlorosilane, vinyltris( -methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, γ-acetoxypropyltrimethoxysilane, γ-cyanopropyltrimethoxysilane, isocyanatopropyltrimethoxysilane, y-(methacryloyloxypropyl)trimethoxysilane, β-(3 ,4 epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidyloxypropyltrimethoxysilane, γ- glycidoxypropylmethyldiethoxysilane, N- (aminoethyl)y-aminopropyltrimethoxysilane, N- (aminoethyl)y-aminopropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane, N- phenyl-y-aminopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, and γ- chloropropyltrimethoxysilane, dimethylvinylmethoxysilane and dimethylvinylethoxysilane. Alkoxysilanes having a perfluorinated or partial-fluorinated alkyl group may also be used.

Other preferred Si-containing surface modifiers are polyfunctional alkoxysilanes of the formula (II)

[(R¹0)a(Y¹)₃__aSi]₁-(CH₂)_k-X-(CH₂)₁-[SiY² ₃__b(OR²)b]_J (II) wherein R¹, R², independently of one another, are each Ci-Cs-alkyl or C₆-aryl radical; Y¹, Y², independently of one another, are each an optionally substituted Ci-C₂o-alkyl or C₆-aryl radical; a, b, independently of one another, are each 1, 2 or 3; i and j are each an integer wherein (i+j) is greater than or equal to 2; k and 1, independently of one another, are each an integer from 0 to 10; and X is a bridging structural unit. X is preferably selected from - (CH₂)„-, -NH-, -NH-CH₂-CH₂-NH-, -NH-CO-NH-, -N(CH₃)-, -CHR-, -Ar-, -CH₂-Ar-CH₂- , -0-, -S-, -S-S-, -(CF₂)_n-, and -CH=CH-.

Particularly preferred Si-containing surface modifiers are the following non- functional and functional dipodal silanes: bis(triethoxysilyl)ethane, l,8-bis(triethoxysilyl)octane, bis(triethoxysilyl)ethane, l,2-bis(trimethoxysilyl)decane, bis(trimethoxysilylethyl)benzene, bis(triethoxysilyl)ethylene, bis(y-(triethoxysilyl)propyl)disulfide, bis(y-

(triethoxysilyl)propyl)amine, N,N -bis(y-(trimethoxysilyl)propyl)ethylenediamine and 1,3- bis(glcy doxypropy l)tetramethy ldisilo xane .

The Si-containing surface modifier can also be a mixture comprising at least two of tetraalkoxysilanes, organo silanes, organofunctional silanes, and silane based inorganic polymers. These can be obtained, for example, by hydrolysis and condensation of various organo alkoxy silanes. Preference is given here to mixtures of organosilanes of the formula YSi(OR)₃ with tetraalkoxysilanes of the formula Si(OR)₄, and mixtures of organosilanes of the formula Y₂Si(OR)₂ with tetraalkoxysilanes of the formula Si(OR)₄, where Y and R have the meanings given in formula (I). The addition of the organosilane can take place before, during or after the addition of the tetraalkyl orthosilicate. Preferably, the tetraalkyl ortho silicate is added first and then the organosilane.

Hydroxyl group-containing inorganic polymers containing the elements silicon and aluminium can also be used. These precondensed oligomeric species can be obtained in an appropriate manner, by, for example, hydrolyzing and condensing aluminium alkoxides with alkoxy and/or organoalkoxysilanes. The preparation of such hetero condensates is well known, for example, from "Sol-Gel Science", J. Brinker, 1990.

The content of organosilanes and/or organofunctional silanes in the process step (ii) can vary within a wide range, for example depending on the reactivity of the silane or the desired thickness or density of the surface coating/modification. As a rule, from 0.01 to 0.5 molar equivalents of organosilane, based on the Zn(OH)₂, are used. Preferably, from 0.02 to 0.3 molar equivalents of organosilane, more preferably from 0.05 to 0.2 molar equivalents of organosilane, based on the amount of Zn(OH)₂ are used.

In a preferred embodiment the inorganic precursor monomers and/or oligomers are only hydro lyzed to a degree where the hydroxyl groups quantitatively bind to the surface of Zn(OH)₂ particles and the condensed inorganic polymer based layer is obtained which has the appropriate density and thickness. In order that the hydrolysis and condensation does not continue too far, eventually resulting in gellation, only from 0.1 to 1.0 mol of water, preferably from 0.3 to 0.5 mol of water, is added per mole of hydro lysable group (e.g. Si- OR). The reaction takes place at a content of preferably less than 5% by weight of water, based on the total amount of solvent and water.

Step (ii) may include the addition of a catalyst. The catalysts may be used optionally for the hydrolysis and condensation of precursor monomers and oligomers. The catalyst may be acids, bases, metal complexes and organometallic compounds. Preference is given to use bases like ammonia alkali metal (Li, Na, K) and alkaline earth metal (Mg, Ca, Ba) hydroxides or strong acids like hydrochloric acid, nitric acid and weak acids like formic acid, acetic acid and p-toluenesulphonic acid. In a preferred embodiment of the invention, ammonia and/or hydrochloric acid is added to a final concentration of 0.1 M to 1.5 M in the alcoholic medium. After the surface modification reaction has taken place in process step (ii) the surface modified zinc oxide-hydroxide nanoparticles are separated from alcoholic medium in a drying step. The drying may be spray drying. The spray-drying may take place in conventional spray-drying equipment, e.g., using a supersonic nozzle and Teflon filter bag for collection of nanoparticles in a stream of confluent hot air. The gas inlet temperature (initial temperature) is preferably 90°C to 160°C, or 100°C to 140°C, e.g., 120°C. The exit temperature of the drying gas (final temperature) is preferably 20°C to 80°C, or 50°C to 70°C, e.g., 60°C.

During the drying step (iii), conversion of zinc hydroxide particles to crystalline (wurtzite type) ZnO nanoparticels takes place. Furthermore, condensation reactions of surface modifiers on the surface of the nanoparticles may proceed; preferably completely. Spray-drying is particularly advantageous, because it enables direct continuous preparation of surface functionalized ZnO nanoparticlulate powder, which preserves the nanoscale properties of the particles, thus greatly improving nanoparticle usability.

The efficiency of the process is normally above 50 %. The efficiency of precipitation step in SFRT may be between 70 and 80%, the losses due to thickening and washing are in the may range between 10 and 20 % and the losses in drying step at spray drier may be between 20 to 30 %. The efficiency in drying step can be improved by installing the electrostatic filter at the end of the drying line.

According to one embodiment of the present invention, the surface-modified nanoparticulate powder contains particles with a diameter of from 40 to 80 nm (D(v, 0.5)). Preferably greater than 80% by number (more preferably greater than 90% by number) of the particles are within that range. This is particularly advantageous since good re- dispersibility is ensured within this size distribution. This size range is also advantageous since, for example following re-dispersion of such surface-modified zinc oxide nanoparticles, the resulting suspensions are transparent and thus do not affect the coloring when added to cosmetic formulations. Moreover, this also gives rise to the possibility of use in transparent films.

The nanoparticulate particles according to the invention provide a high light transmittance in the region of visible light and for a low light transmittance in the region of near ultraviolet light (UV-A). Preferably, the ratio of the logarithm of the percentage transmission (T) at a wavelength of 360 nm and the logarithm of the percentage transmission at a wavelength of 450 nm [In T(360 nm)/ln T(450nm)] is at least 15, particularly preferably at least 20.

The preparation process of the modified ZnO nanoparticles in powder form according to the invention permits a very efficient and controlled hydrophobization or hydrophilization of the particles surface. The modified ZnO particles according to the invention are present, for example, as constituents of liquid formulations or of powders.

The continuous process for preparation of surface-modified nanoparticulate ZnO powder can easily be scaled up to industrial production scale. The production of surface coated ZnO nano powder ready for transport is much more cost effective as producing and storing suspensions.

The surface-modified nanoparticulate powder of ZnO is readily re-dispersible in a liquid medium (polar or nonpolar solvents, depending on the modification of the particle surface) and forms stable suspensions. This is particularly advantageous since, as a result of this, uniform incorporation, for example into plastics or films, is possible.

The present invention provides surface-modified ZnO nanoparticulate particles appropriate for use as UV protectants in cosmetic sunscreen preparations, as stabilizer in plastics and as antimicrobial active ingredient. The nanoparticles according to the invention are in particular appropriate for UV protection in polymers. In this application, the particles either protect the polymers themselves against degradation by UV radiation, or the polymer composition comprising the nanoparticles is in turn employed in the form of a protective film or applied as a coating on the surface, for example of wood, plastics, fibers or glass. According to a further preferred embodiment of the present invention, the surface-modified nanoparticulate powder of ZnO is re-dispersible in water, where it forms stable suspensions. This is particularly advantageous since this opens up the possibility of using the material according to the invention for example in cosmetic formulations.

The present invention shall now be explained in terms of the following non-limiting Examples.

Examples

Example 1 - Continuous preparation of nanoparticulate zinc oxide powder without surface modification (not claimed)

Aqueous solutions of zinc salt, solution 1, and strong base, solution 2, were prepared. Solution 1 comprised 59,50 g of zinc nitrate (Ζη(ΝΟ)₃*6Η₂0, reagent grade 98%, Sigma Aldrich) per liter and had a zinc ion concentration of 0.2 mo 1/1. Solution 2 comprised 16 g of sodium hydroxide per liter and thus had a hydroxyl ion concentration of 0.4 mol/1. The 30 liters of solution 1 and 30 liters of solution 2 were placed in two keg vessels equipped with self-closing valve. The vessels were tapped by coupling fitting equipped with valve for control of the flow of solution out of the keg into the partitioning chamber of segmented flow tubular reactor (SFTR) and a valve to control the pressure of compressed nitrogen on the solution (around 2 atm) to dispense the solution out of the vessel. A scale- out configuration of SFTR consisting of 1000 tubes running in parallel was used. Solutions 1 and 2 were continuously metered into the partitioning chambers of SFTR via separate feed tubes, in each case at a metering rate of 0.50 1/min. The solutions 1 and 2 were partitioned in the partitioning cambers equipped with 1000 exit lines each, internal diameter 2 mm, flow rate 0,5 ml/min. The metered portions of solution 1 and 2 coming out of single exit lines are mixed in micromixer, segmented in a segmenter and passed as discrete segments of suspension (volume of approx. 0.1 - 0.5 cm³) separated by nitrogen gas bubbles of similar volume along a tubular reactor having a length of 5 m and internal diameter of 2 mm. The residence time of the segments in the reactor was 15 minutes and the reaction was conducted at ambient temperature (~ 23 °C). A white suspension coming out of 1000 tubes of SFTR was collected with funnel into a plastic vessel. The freshly produced suspension was thickened by centrifuge at 10000 rpm. The supernatant was decanted and the pasty cake was re-dispersed in deionized water. The process of thickening by centrifuging and decantation of supernatant was repeated. The pasty cake was then re- dispersed in ethanol (industrial grade) to obtain milky stable suspension in alcoholic medium.

The ethanol suspension was dried in a spray-dryer to obtain unmodified white ZnO nanoparticulate powder. The solid colloidal milky particles were separated from alcohol by spray drying on conventional spray drier equipped with supersonic nozzle and Teflon filter bag for collection of nanoparticles in a stream of confluent hot air with input temperature 120°C and exit temperature 60°C. The total mass of nanoparticulate ZnO powder obtained was 220 g.

The resulting powder had, in the UV-VIS spectrum, the absorption band at about 350-360 nm characteristic of zinc oxide and in IR spectrum, the absorption band between 600 and 400 cm^"1 characteristic for lattice vibrations of ZnO. The X-ray diffraction of the powder showed exclusively the diffraction reflections of hexagonal wurtzite zinc oxide. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reviled that the resulting powder had particle size distribution between 40 and 80 nm with an average particle size of about 60 nm. The dynamic light scattering (DLS) measurements reviled soft agglomeration into agglomerates with average size of 240 nm.

As prepared ZnO nanoparticulate powder was readily re-dispersed in alcohol and water. Example 2- Continuous preparation of surface modified nanoparticulate zinc oxide powder - ZnO nanoparticles coated with silica coating.

Aqueous solutions of zinc salt, solution 1, and strong base, solution 2, were prepared.

Solution 1 comprised 59,50 g of zinc nitrate (Zn(NO)3*6H₂0, reagent grade 98%, Sigma Aldrich) per liter and had a zinc ion concentration of 0.2 mol/1. Solution 2 comprised 16 g of sodium hydroxide per liter and thus had a hydroxyl ion concentration of 0.4 mol/1.

The precipitation reaction was carried out on single line segmented flow tubular reactor (SFTR).

1000 ml of solution 1 and 1000 ml of solution 2 were continuously metered into SFTR using Ismatec Reglo Digital IP-30, four channel peristaltic pump via two separate feed tubes, in each case at a metering rate of 0.5 ml/min. The metered portions of solution 1 and 2 are mixed in micromixer, segmented in a segmenter and passed as discrete segments of suspension (volume of approx. 0.1 - 0.5 cm³) separated by nitrogen gas bubbles of similar volume along a tubular reactor having a length of 5 m and internal diameter of 2 mm. The residence time of the segments in the reactor was 15 minutes and the reaction was conducted at ambient temperature (~ 23°C). A white suspension coming out of SFTR was collected into a plastic vessel. The freshly produced suspension was thickened by centrifuge (10 000 rpm). The supernatant was decanted and the pasty cake was re- dispersed in deionized water. The process of thickening by centrifuging and decantation of supernate was repeated. The pasty cake was then re-dispersed in ethanol (2000 ml - industrial grade) to obtain milky stable suspension. With stirring, 3 g of TEOS (0.2 molar equivalents of tetraalkyl orthosilicate, based on the zinc hydroxide solid precipitate content) were introduced into ethanol suspension. The homogenization of suspension was achieved by intensive mixing at 15000 rotations/min, for 5 minutes, using ultra high shear mixer (IKA T-25 digital ULTRA-TURRAX). The homogenized suspension of the precipitate containing TEOS was spray dried on conventional spray drier equipped with supersonic nozzle and Teflon filter bag for collection of nanoparticles in a stream of confluent hot air with input temperature 120°C and exit temperature 60°C. The total mass of silica coated nanoparticulate ZnO powder was 6 g.

The resulting powder exhibited in the UV-VIS spectrum the absorption band at about 350- 360 nm characteristic of zinc oxide and in IR spectrum, the absorption band between 600 and 400 cm^"1 characteristic for lattice vibrations of ZnO and absorption bands between 900 cm^"1 and 1200 cm^"1, corresponding to Si-O-Si stretching vibrations. No other bands were present in the IR spectra demonstrating that the powder was dry, and the conversion of zinc hydroxide to zinc oxide as well as the polycondensation reactions of hydro lyzed TEOS monomers and oligomers on the surface of the ZnO particles were complete.

The X-ray diffraction of the powder showed exclusively the diffraction reflections of hexagonal wutzite zinc oxide. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reviled that the resulting powder had particle size distribution between 50 and 80 nm with an average particle size of about 70 nm. The dynamic light scattering (DLS) measurements revealed slight agglomeration into agglomerates with average size of 251 nm.

The silica coated ZnO nanoparticulate powder was readily re-dispersed in alcohol and water to form stable suspensions.

Example 3 - Continuous preparation of surface modified nanoparticulate zinc oxide powder - ZnO nanoparticles coated with acrylate functionalized silica coating.

Aqueous solutions of zinc salt, solution 1, and strong base, solution 2, were prepared as described in Example 2. The precipitation reaction was carried out on single line segmented flow tubular reactor (SFTR) as described in Example 2.

The thickening and washing of precipitate was accomplished as in Example 2. The surface modifier in the form of y-(methacryloyloxypropyl)trimethoxysilane (MEMO), the amount of 5 g (0.2 molar equivalents of silane monomer precursor, based on the zinc hydroxide solid precipitate content) was introduced into fresh ethanol suspension under intensive stirring. The homogenization of suspension was achieved by further intensive mixing at 15000 rotations/min, for 5 minutes, using ultra high shear mixer (IKA T-25 digital ULTRA-TURRAX).

The homogenized suspension of precipitate containing methacryloxy functionalized silane was spray dried on conventional spray drier equipped with supersonic nozzle and Teflon filter bag for collection of nanoparticles in a stream of confluent hot air with input temperature 120°C and exit temperature 60°C. The total mass of silica coated nanoparticulate ZnO powder was 7 g.

The resulting powder exhibited in the UV-VIS spectrum the absorption band at about 350- 360 nm characteristic of zinc oxide and in IR spectrum, the absorption band between 600 and 400 cm^"1 characteristic for lattice vibrations of ZnO and absorption bands between 900 cm^"1 and 1200 cm^"1, corresponding to Si-O-Si stretching vibrations. The absorption bands at 1719^"1 and 1637 cm^" correspond to stretching vibrational modes of C=0 and C=C bonds and the absorption bands around 2900 cm^"1 typical of the stretching vibrational modes of the ≡CH, =CH₂, and -C¾ groups of MEMO. On the other hand, vibrational modes around 3430 cm^"1 and 1616 cm^"1 corresponding to -OH groups and to H₂0, indicate that the material absorbs some moisture.

The X-ray diffraction of the powder showed exclusively the diffraction reflections of hexagonal wutzite zinc oxide. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reviled that the resulting powder had particle size distribution between 50 and 100 nm with an average particle size of about 80 nm. The dynamic light scattering (DLS) measurements reviled soft agglomeration into agglomerates with average size of 145 nm.

The methacryloxy functionalized silica coated ZnO nanoparticulate powder was readily re- dispersed in alcohol and water to form stable suspensions. Example 4 - Continuous preparation of surface modified nanoparticulate zinc oxide powder - ZnO nanoparticles coated with amine functionalized silica coating.

The thickening and washing of precipitate was accomplished as in Example 2.

The surface modifier in the form of γ-aminopropyltriethoxysilane (APTES), the amount of 4.4 g (0.2 molar equivalents of silane monomer precursor, based on the zinc hydroxide solid precipitate content) was introduced into fresh ethanol suspension under intensive stirring. The homogenization of suspension was achieved by further intensive mixing at 15000 rotations/min, for 5 minutes, using ultra high shear mixer (IKA T-25 digital ULTRA-TURRAX).

The homogenized suspension of precipitate containing amino functionalized silane was spray dried on a conventional spray drier equipped with supersonic nozzle and Teflon filter bag for collection of nanoparticles in a stream of confluent hot air with input temperature 120°C and exit temperature 60°C. The total mass of silica coated nanoparticulate ZnO powder was 6 g.

The resulting powder exhibited in the UV-VIS spectrum the absorption band at about 350- 360 nm characteristic of zinc oxide and in FT-IR spectrum, the absorption band between 600 and 400 cm^"1 characteristic for lattice vibrations of ZnO and absorption bands between 900 cm^"1 and 1200 cm^"1, corresponding to Si-O-Si stretching vibrations. The FT-IR spectrum of the APTES-functionalized sample showed the CH₂ asymmetric and symmetric stretching modes are observed at 2930 and 2865 cm^"1, respectively, confirming the presence of the propyl chain of the APTES molecule. Also present is the CH₃ asymmetric mode at 2975 cm^"1 suggesting that some ethoxy groups have not been completely hydro lyzed and, consequently, that the APTES molecules are not fully bonded either with the ZnO surface or/and with a neighboring APTES molecule (horizontal polymerization). The remaining spectral features include an absorbance feature at 1575 cm^"1, corresponding to the N¾ scissor vibration and confirming the presence of the N¾ terminal group of APTES molecules. In addition to the N¾ scissor mode, another feature at 1615 cm^"1, corresponding to the asymmetric -NH₃ ⁺ deformation mode, is present. The weak feature at 1490 cm^"1 is also assigned to the symmetric -NH₃ ⁺ deformation mode. The presence of these modes suggests that when the samples are exposed to air after drying, water molecules are weakly bonded to the N¾ groups, thereby allowing for the protonation of the amine. The broad feature in the 2500-3500 cm^"1 (on which the CHx stretching modes are superposed) is evidence that water molecules undergo hydrogen bonding, most likely with the amino groups of the coating. The X-ray diffraction of the powder showed exclusively the diffraction reflections of hexagonal wutzite zinc oxide. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reviled that the resulting powder had particle size distribution between 70 and 100 nm with an average particle size of about 90 nm. The dynamic light scattering (DLS) measurements reviled soft agglomeration into agglomerates with average size of 130 nm.

As prepared amino functionalized silica coated ZnO nanoparticulate powder was readily re-dispersed in alcohol and water to form stable suspensions.

Example 5 - Continuous preparation of surface modified nanoparticulate zinc oxide powder - ZnO nanoparticles coated with epoxy functionalized silica coating. Aqueous solutions of zinc salt, solution 1, and strong base, solution 2, were prepared as described in Example 2.

The precipitation reaction was carried out on single line segmented flow tubular reactor (SFTR) as described in Example 2.

The thickening and washing of precipitate was accomplished as in Example 2. The surface modifier in the form of γ-glycidyloxypropyltrimethoxysilane (GLYMO), the amount of 4.7 g (0.2 molar equivalents of silane monomer precursor, based on the zinc hydroxide solid precipitate content) was introduced into fresh ethanol suspension under intensive stirring. The homogenization of suspension was achieved by further intensive mixing at 15000 rotations/min, for 5 minutes, using ultra high shear mixer (IKA T-25 digital ULTRA-TURRAX) .

The homogenized suspension of precipitate containing epoxy functionalized silane was spray dried on conventional spray drier equipped with supersonic nozzle and Teflon filter bag for collection of nanoparticles in a stream of confluent hot air with input temperature 120°C and exit temperature 60°C. The total mass of silica coated nanoparticulate ZnO powder was 6 g.

The resulting powder exhibited in the UV-VIS spectrum the absorption band at about 350- 360 nm characteristic of zinc oxide and in FT-IR spectrum, the absorption band between 600 and 400 cm^"1 characteristic for lattice vibrations of ZnO and absorption bands between 900 cm^"1 and 1200 cm^"1, corresponding to Si-O-Si stretching vibrations. The presence of epoxy bands at 916 and 1250 cm^"1 in FTIR spectra suggest that epoxy group is preserved. The broad feature in the 2500-3700 cm^"1 (on which the CH stretching modes are superposed) is evidence that water molecules undergo hydrogen bonding on the surface of the coating.

The X-ray diffraction of the powder showed exclusively the diffraction reflections of hexagonal wutzite zinc oxide. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reviled that the resulting powder had particle size distribution between 50 and 80 nm with an average particle size of about 70 nm. The dynamic light scattering (DLS) measurements reviled soft agglomeration into agglomerates with average size of 140 nm.

As prepared epoxy functionalized silica coated ZnO nanoparticulate powder was readily re-dispersed in alcohol and water to form stable suspensions.

Claims

1. A process for the preparation of surface-modified zinc oxide particles comprising the steps:

(iii) drying said mixture of (ii), wherein surface-modified Zn(OH)₂ particles are converted to surface-modified ZnO particles during said drying; thereby obtaining dried surface-modified ZnO particles.

2. The process of claim 1, wherein said drying step (iii) includes increasing the temperature of said mixture of (ii), preferably by at least 10°C above a temperature present at step (ii).

3. The process of claim 1 or 2, wherein said drying step (iii) comprises drying said mixture of (ii) in a spray dryer using a stream of drying gas having an initial temperature of from 90°C to 160°C.

4. The process of claim 3, wherein said drying gas is cooled to a final temperature of from 20°C to 80°C during spray-drying.

5. The process of any one of the preceding claims, wherein at least 50% by weight of said Zn(OH)₂ particles present said mixture of (ii) are converted to ZnO particles during said spray-drying step (iii).

6. The process of any one of the preceding claims, wherein step (ii) comprises a step (ii. l) of mixing said Zn(OH)₂ particles in said alcoholic medium and said Si- containing surface modifier in a high shear mixer.

7. The process of any one of the preceding claims, wherein said alcoholic medium comprises an alcohol selected from the group consisting of ethanol, methanol, propanol, ethylene glycol, diethylene glycol, and polyol.

8. The process of any one of the preceding claims, wherein said alcoholic medium comprises less than 5%(wt) of water.

9. The process of any one of the preceding claims, wherein said Si-containing surface modifier is selected from the group consisting of tetraalkyl-ortosilicate, organosilane, organofunctional silane, polyfunctional alkoxysilane and silanols.

10. The process of claim 9, wherein the Si-containing surface modifier has the general formula:

wherein RO is methoxy, ethoxy, acetoxy, or any hydrolysable group; and X is Ci-C₂o alkyl, aryl, phenyl or an organofunctional group.

1 1. The process of claim 10, wherein X is an alkyl selected from methyl, ethyl, propyl, i-butyl, n-octyl.

12. The process of claim 10, wherein X is an organofunctional group selected from amino, methacryloxy, epoxy, benzylamino, vinyl, vinyl-benzyl-amino, chloropropyl, melamin, ureido, mercapto, disulfido and tetrasulfido.

13. The process of claim 9, wherein the Si-containing surface modifier has the general formula:

(Y)_a(Y^')_bSi(OR)_c wherein:

Y, Y' are each selected from hydrogen, substituted or unsubstituted Ci-C₂o-alkyl radical and C₆-aryl radical;

R is selected from Ci-Cs-alkyl radical and phenyl radical; a, b and c are independently 0, 1 , 2, 3 or 4; and the sum (a + b + c) = 4.

14. The process of claim 9, wherein the Si-containing surface modifier has the general formula:

[(R¹0)_a(Y¹)₃-aSi]₁-(CH₂)_k-X-(CH₂)₁-[SiY²3-b(OR²)_b]_J wherein: R¹, R² are independently of each other Ci-Cs-alkyl or C₆-aryl radical;

Y¹, Y² are independently each other substituted or unsubstituted Ci-C₂o-alkyl, or substituted or unsubstituted C₆-aryl radical; a, b are independently of one another 1, 2 or 3; i and j are integers, wherein (i + j) is greater than or equal to 2; k and 1 are independently of one another an integer from 0 to 10 (inclusive); and

X is a bridging unit.

15. The process of claim 9, wherein the Si-containing surface modifier is selected from the group consisting of: CH₃-Si(OCH₃)₃, C₂H₅-Si(OCH₃)₃, phenyl- Si(OCH₃)₃, CH₃-Si(OC₂H₅)₃, C₂H₅-Si(OC₂H₅)₃, phenyl-Si(OC₂H₅)₃, n-butyl-Si(OC₂H₅)₃, n-butyl-Si(OCH₃)₃, i-butyl-Si(OC₂H₅)₃, i-butyl-Si(OCH₃)₃, i-propyl-Si(OC₂H₅)₃, i-propyl- Si(OCH₃)₃, i-octyl-Si(OC₂H₅)₃, i-octyl-Si(OCH₃)₃, (CH₃)₂Si(OCH₃)₂, (C₂H₅)₂Si(OCH₃)₂, (CH₃)(phenyl)Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (C₂H₅)₂Si(OC₂H₅)₂,

16. The process of claim 9, wherein the Si-containing surface modifier is selected from the group consisting of: (CH₃)₃Si(OCH₃), (C₂H₅)₃Si(OCH₃), (CH₃)₂(phenyl)Si(OCH₃), (CH₃)₃Si(OC₂H₅), (C₂H₅)₃Si(OC₂H₅), (CH₃)₂(phenyl)Si(OC₂H₅), (n-butyl)₂Si(OC₂H₅)₂, (n-butyl)₂Si(OCH₃)₂, (i-propyl)₂Si(OC₂H₅)₂, (i-propyl)₂Si(OCH₃)₂, (CH₃)HSi(OCH₃)₂, CH₃HSi(OC₂H₅)₂, (CH₃)₂Si(0-phenyl)₂, (C₂H₅)₂Si(0-phenyl)₂, (CH₃)(phenyl)₂Si(OCH₃), (CH₃)(phenyl)₂Si(OC₂H₅), (CH₃)₃Si(0-phenyl), (C₂H₅)₃Si(0- phenyl), (phenyl)₃Si(OCH₃), (phenyl)₃Si(OC₂H₅), (i-propyl)(CH₃)₂Si(OC₂H₅), (i- propyl)(CH₃)₂Si(OCH₃), (i-propyl)₂(CH₃)Si(OC₂H₅), (i-propyl)₂(CH₃)Si(OCH₃), (i- propyl)₃Si(OC₂H₅), (i-propyl)₃Si(OCH₃)₃, (n-butyl)(CH₃)₂Si(OC₂H₅)₂, (n- butyl)(CH₃)₂Si(OCH₃), (n-butyl)₂(CH₃)Si(OC₂H₅), (n-butyl)₂(CH₃)Si(OCH₃), (n- butyl)₃Si(OC₂H₅), and (n-butyl)₃Si(OCH₃).

17. The process of claim 9, wherein the Si-containing surface modifier is selected from the group consisting of: vinyltrichlorosilane, vinyltris(P- methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, γ- acetoxypropyltrimethoxysilane, γ-cyanopropyltrimethoxysilane, isocyanatopropyltrimethoxysilane, y-(methacryloyloxypropyl)trimethoxysilane, β-(3 ,4 epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidyloxypropyltrimethoxysilane, γ- glycidoxypropylmethyldiethoxysilane, N- (aminoethyl)y-aminopropyltrimethoxysilane, N- (aminoethyl)y-aminopropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane, N- phenyl-y-aminopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, and γ- chloropropyltrimethoxysilane, dimethylvinylmethoxysilane and dimethy lviny lethoxysilane .

18. The process of claim 9, wherein the Si-containing surface modifier is selected from: bis(triethoxysilyl)ethane, l,8-bis(triethoxysilyl)octane, bis(triethoxysilyl)ethane, 1 ,2-bis(trimethoxysilyl)decane, bis(trimethoxysilylethyl)benzene, and bis(triethoxysilyl)ethylene, bis(y-(triethoxysilyl)propyl)disulfide, bis(y- (triethoxysilyl)propyl)amine, N,N -bis(y-(trimethoxysilyl)propyl)ethylenediamine and 1,3- bis(glcydoxypropyl)tetramehyldisiloxane.

19. The process of any one of the preceding claims, wherein said Si-containing surface modifier is added at a molar ratio (Si-containing surface modifier to Zn(OH)₂) of from 0.01 to 0.5.

20. The process of any one of the preceding claims, wherein step (i) of providing a suspension of Zn(OH)₂ particles comprises:

(i.1) providing a first aqueous solution comprising at least one zinc salt;

(i.2) providing a second aqueous solution comprising at least one base; (1.3) mixing said first and second aqueous solutions, whereby Zn(OH)₂ is formed and Zn(OH)₂ particles precipitate and form an aqueous suspension;

(1.4) removing at least part of the liquid phase of said aqueous suspension thereby obtaining concentrated Zn(OH)₂ particles; (i.5) re-dispersing said concentrated Zn(OH)₂ particles in an alcoholic medium, thereby obtaining Zn(OH)₂ particles suspended in an alcoholic medium.

21. The process of claim 20, wherein said zinc salt is selected from the group consisting of zinc nitrate, zinc chloride and zinc acetate.

22. The process of any one of claims 20-21, wherein said base is selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), rubidium hydroxide (RbOH), caesium hydroxide (CsOH) and ammonia.

23. The process of any one of claims 20-22, wherein said mixing in step (i.3) is in a segmented flow tubular reactor (SFTR).

24. The process of any one of claims 20-23, wherein said mixing step (i.3) occurs at a temperature within the range of from 10°C to 90°C.

25. The process of any one of claims 20-24, wherein said separation step (i.4) comprises filtration, decantation or centrifugation.

26. The process of any one of claims 20-25, wherein said re-dispersion in step ( i.5 ) occurs in a high shear mixer.