WO2006105600A1

WO2006105600A1 - Silicone coated metal oxide particles

Info

Publication number: WO2006105600A1
Application number: PCT/AU2006/000454
Authority: WO
Inventors: Geoff James Trotter; Takuya Tsuzuki
Original assignee: Advanced Nanotechnology Limited
Priority date: 2005-04-06
Filing date: 2006-04-06
Publication date: 2006-10-12

Abstract

A process for applying a silicone coating to metal oxide particles is described. The process comprises the steps of: preparing a mixture of an aqueous suspension of metal oxide particles and an aqueous solution of water-soluble organosilicon monomers; and, adding a pH-lowering agent to the mixture so as to initiate polymerisation of the water-soluble organosilicon monomers and form silicone coated metal oxide particles. The process does not require the use of an organic solvent.

Description

"Silicone coated metal oxide particles"

Field of the Invention The present invention relates to a process for applying a silicone coating to metal oxide particles.

The present invention relates particularly though not exclusively to silicone coated metal oxide particles that are re-dispersible in polar and non-polar liquids.

Background of the invention

Metal oxide particles have a long history of use in a variety of applications, including as pigments in paints and inks, additives in rubbers, cosmetics and pharmaceutical products, and as an ultraviolet (UV) blocking agent in sunscreens and protective coatings.

The surface of metal oxide particles is by nature hydrophilic, which makes them non-wettable with organic solvents, oils and plastics that are frequently used as carrier media in the aforementioned applications. Furthermore, some metal oxides such as titanium dioxide and zinc oxide have a high photoactivity, which may result in undesired effects caused by reactions between the metal oxides and other components in the products. It is therefore necessary for metal oxides to be surface treated to produce desirable end properties for many applications.

Metal oxide particles are commonly used as a UV blocking agent in sunscreens, paint systems and plastics, due to their ability to absorb and scatter UV radiation. UV radiation, with wavelengths in the range of 290 nm to 400 nm, is known to be damaging to the human skin, as exposure can cause sunburn, wrinkles and skin cancer in the long term. Protection from UV radiation may be achieved using inorganic (physical) shielding agents, such as zinc oxide or titanium dioxide, or by organic (chemical) shielding agents, such as avobenzene and paraamino benzoic acid (PABA) .

Most organic shielding agents do not provide protection over the full UV spectrum, are prone to photo-degradation and may be irritating to the skin. However, they are traditionally- more cosmetically appealing than inorganic sunscreens, due to the ability to provide adequate coverage without resulting in significant whitening when applied to the skin. In the past, inorganic sunscreens have consisted of micron sized particles, and when a sufficient quantity of the sunscreen is applied to provide the correct level of protection, an opaque, white finish remains on the skin. The micron-sized particles in the sunscreen scatter visible light, which leads to this whitening effect.

In more recent times, nano-sized particles, which have diameters of less than 100 nm, have been used as a means to reduce the whiteness and give a more cosmetically acceptable transparent film on the skin, whilst still providing the required UV protection. As a result, many formulators have begun using zinc oxide nanoparticles in sunscreen formulations.

However, some inherent downfalls arise in the use of nano- sized powders in formulations. Such powders have with a high surface reactivity, which leads to increased interactions between neighbouring particles. Over time this can result in the flocculation of particles in the formulation. In addition, the hydrophilic nature of the metal oxide powder surface leads to a low affinity with oily carrier media, making it extremely difficult to achieve a stable suspension. A generally accepted method to overcome such problems has been to surface coat the metal oxide particles to render then hydrophobic. The surface coating acts to decrease the surface activity of the particles and to increase their affinity with oily carrier media, leading to a more stable dispersion and a product with a longer shelf life. In addition, the surface coating can also reduce the photoactivity of the particles and increase their feel in the formulation, leading to a product with a greater cosmetic appeal.

In paint, rubber and plastic industries, the reduction of photoactivity by surface coating is also beneficial for preventing the degradation of polymers around the incorporated inorganic particles from occurring by the photoactivity, resulting in improved product life time. The hydrophobic modification of particle surfaces is also essential for paint, rubber and plastic industries in order for the inorganic particles to be dispersed stably in the polymer systems.

Organosilicon compounds such as silicone and polysiloxane have been used to surface treat metal oxide powders in an attempt to overcome this problem. Silicones are polymers that have a regular repeating backbone of -Si-O- and contain side groups of varying functionality. Most notably, organosilicon compounds containing methyl side group (dimethyl polysiloxanes) , methyl and hydrogen side groups (methyl hydrogen polysiloxanes) , and alkyl groups (alkyl polysiloxanes) have been utilized. Additionally, the side groups of the silicone may be selected to match the chosen carrier media to enable a greater affinity between the surface treated powders and the carrier media. Several prior art techniques have been developed for coating metal oxide fine particles with silicone or organosilicon. They can be classified into the following four categories;

Δ) use of organosilicon polymers (i,e, silicones) in the presence of organic solvents;

B) use of organosilicon monomers or oligomers in the presence of organic solvents;

C) use of organosilicon monomers or oligomers in a vapour phase; and,

D) use of organosilicon polymers (i,e, silicones) directly on dry powders .

Examples of prior art techniques that belongs to the category A) are described in U.S. patent application number 2003/0161805 and 2004/0156809, Japanese patent 2005289834, and US patents 6,482,519, and 6,440561. The techniques in this category have problems in that the molecules of organosilicon polymers are generally too large to effectively coat nanometre-size particles. In addition, the techniques require the use of organic solvent, which increases production cost and present a health hazard. Furthermore, the coating methods are not compatible with many of the common techniques used to produce metal oxide fine particles, such as mechanochemical synthesis as described in International patent application number PCT/AU99/000368 or PCT/ΔU96/000539 (the contents of which are incorporated herein by reference) . The techniques in category A are also not compatible with liquid phase synthesis, where the particles are dispersed in water prior to the commencement of the coating procedure.

Examples of prior art techniques that belong to category B) are disclosed in U.S. patent 6,132,743, Japanese patent 11/193,354, and in US patent applications 2005/0227859 and 2005/0154124. Although use of organosilicon having lower molecular weight may be used for coating nano-scale particles, these methods still require the use of organic solvents, which causes the aforementioned problems.

An example of a prior art technique that belongs to category C is the method described in US patent 4,882,225. In that patent, the organosilicon vapour comes into contact with a dry powder surface at temperatures of around 100⁰C, where polymerization takes place on the powder surface. This process results in particles with a hydrophobic coating and a reduced photo-catalytic activity. However, low production rates can be a problem, as the system must be left for several hours for polymerization to reach completion. In addition, high temperature organic vapours can present a health hazard, and the need for dry powder means that the particles must be agglomeration free for a continuous coating to be produced. It is quite difficult to produce an unagglomerated dry powder without first applying some form of surface coating.

Examples of prior art techniques that belong to category D are found in U.S. patent 5,756,788 and 6,265126, and US patent application numbers 2005/0255057, 2003/0027896 and 2004/0197359. US patent application number 2005/0255057 discloses a method of coating zinc oxide and titanium dioxide nanoparticles with a siloxane star-graft copolymer, by direct mixing of dry powders of zinc oxide and titanium dioxide into the surface treatment agent. There is a requirement that the zinc oxide and titanium dioxide particle surface be conditioned prior to coating, which comprises removing materials sorbed to the surface or adding dopants to the surface or a combination thereof. The conditioning process of US patent application number 2005/0255057 may be accompanied by vacuum treatment, plasma treatment, gas or fo—"

fluid flushing. Reactive by-products and residues are removed prior to the surface coating. Such a process leads to high production cost. Furthermore, the need for dry powder means that the particles must be agglomeration free for a continuous coating to be produced on each particle.

All of the above techniques produce silicone coated metal oxide particles that are hydrophobic, which means the particles are not miscible with organic matrices having strong polarity, thereby limiting the degrees of freedom of formulation.

In order to overcome the aforementioned problems associated with those prior art techniques, there was a push to develop less expensive and more effective methods to apply surface coating on nanoparticles . One such method is to utilize nanoparticle dispersion in water. Several prior art techniques have been developed for coating metal oxide fine particles with organosilicon in water.

US patent 6,120,596 describes a method of applying a halosilane or organohalosilane coating on pigment particles in water. This method is used to modify the surface properties of organic pigment particles from hydrophobic to hydrophilic. The method cannot be used to produce a hydrophobic surface on the metal oxide particles. US patent application 2004/0091440 discloses a method of coating inorganic powder with polyethylene-modified silicone in water. This process is only capable of producing a hydrophilic surface is produced by this method.

US patent application 2004/0047887 teaches a method of producing silicone-treated inorganic powder using silicone compounds having a Si-H group. The US patent application specifically describes the use of tetramethylcyclotetrasiloxane, dimethylpolysiloxane and octamethylcyclotetrasiloxane. Although it is stated in US patent application 2004/0047887 that the silicone compounds may be brought into contact with the inorganic powder in a solution in a solvent including water and other organic solvents, the specific silicone compounds referenced in US patent application 2004/0047887 are known to be water- insoluble and only soluble in organic solvents having low polarity.

US Patent 5,128,204 describes magnetizable microspheres comprising a polysilsesquioxane network and, distributed within the network, a magnetizable filler chemically bonded to polysilsesquioxane units. The magnetizable microspheres are prepared by dispersing an aqueous suspension of a magnetizable filler, not coated with a dispersing agent, in a solvent, dissolving an alkoxysilane or an alkoxysiloxane in the organic phase, polycondensating to a polysilsesquioxane, removing water, separating, and optionally redispersing the microspheres in water. Hence, this method still requires the use of an organic solvent.

The present invention has been developed to provide an alternative process for applying an organosilicon coating in water to alter the surface properties of metal oxide powders so as to render such particles compatible with carier media having a wide range of polarity, without the need to use organic solvents.

It will be clearly understood that, although prior art use and publications are referred to herein, this reference does not constitute an admission that any of these form a part of the common general knowledge in the art in Australia or in any other country. In the statement of invention and description of the invention which follow, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Summary of the invention

It has been discovered by the Inventors that a highly- effective silicone coating can be applied onto metal oxide particles in water without the need to use of organic solvents or external heat to coat the particles. This is achieved using water-soluble siliconate monomers to form polymethylsilsesquioxane on the surface of the particles.

It has previously been widely understood in the relevant art, that polymethylsilsesquioxane can only be used to form hydrophobic coatings so as to act as a water repellent

(Berthiaume, M., "Silicones in Cosmetics", Principles of

Polymer Science and Technology in Cosmetics and Personal

Care, Marcel Dekker Inc., 1999, pages 308 - 310). However, the Inventors have found that, when metal oxide fine or nano- sized particles are coated with polymethylsilsesquioxane in water using the processes of the present invention, the resulting particles are re-dispersible in either polar liquids, such as water, or non-polar liquids, such as many organic solvents. The coating is also effective in reducing the photoactivity of metal oxides. This remarkably increases the degrees of freedom of formulation.

Thus, according to a first aspect of the present invention there is provided a process for applying a silicone coating to metal oxide particles, the process comprising the steps of: preparing a mixture of an aqueous suspension of metal oxide particles and an aqueous solution of water-soluble organosilicon monomers; and, adding a pH-lowering agent to the mixture so as to initiate polymerisation of the water-soluble organosilicon monomers and form silicone coated metal oxide particles.

Without wishing to be bound by theory, it is understood that the monomers in the aqueous solution of organosilicon monomers remain freely soluble in highly alkaline aqueous solutions (at pH values greater than 12) . When the pH value is reduced, methyl siliconate forms silanetriol, and slowly condenses to form oligomers and siloxane polymers. It is understood that the -Si-OH groups of the hydrolyzed silane initially hydrogen bond with -OH groups on the metal oxide particle surface. As the reaction proceeds, water is lost and a covalent bond is formed. The reaction of hydrolyzed silane with surface -OH ultimately results in the condensation of siloxane polymer which encapsulates the metal oxide particles thereby coating them. Advantageously, the aqueous dispersion and the aqueous The aqueous dispersion may be in water only or in a mixture of water and a water miscible organic solvent.

Preferably, the mixture is homogenised by agitating the mixture to provide a more even distribution of the silicone coating and to discourage agglomeration of the metal oxide particles.

In one embodiment, the step of adding a pH-lowering agent to the mixture is conducted gradually as a sudden change in the pH can cause precipitation of silicone not on the surface of the metal oxide particles. The process may further comprise the step of drying the coated metal oxide particles. Suitable methods for drying include but are not limited to spray-drying or freeze drying. Advantageously, the step of drying the coated particles may be conducted at a temperature less than 400 ⁰C, and more advantageously less than 150 °C.

Advantageously the dried coated metal oxide particles may be re-dispersed in a polar or a non-polar carrier medium to form a suspension. This may be achieved using one or more of the following methods either singly or in combination: ultrasonication, high shear mixing, beads milling, pearl milling, roll milling, mechanical stirring, colloid milling, use of a vibratory stirrer, or use of a conventional paint mixer.

The step of re-dispersing the dried metal oxide particles may include the step of adding one or more dispersants to improve stability of the suspension.

The aqueous suspension of metal oxide particles may comprise particle concentrations of 0.1% to 60 % by weight or from 1 % to 40 % by weight.

The concentration of water-soluble organosilicon monomers in the aqueous solution may be in the range of 0.1 to 20 mg per square metre, 0.1-10 mg per square metre or 0.1 to 5 mg per square metre of the total surface area of the metal oxide particles in the aqueous suspension.

In one embodiment, the mixture of the aqueous suspension of metal oxide particles and the aqueous suspension of water- soluble organosilicon monomers has a pH value of greater than 12 before the step of adding a pH-lowering agent. One of the reasons for keeping the pH high is to mitigate the risk of certain metal oxide particles dissolving or decomposing in the aqueous suspension. Another reason for maintaining a high pH is to disperse the metal oxide particles using electrostatic repulsion force when the isoelectric point of the metal oxide is found at lower pH.

The step of adding a pH-lowering agent may lower the pH of the mixture to a pH less than 12, or less than 10, or less than pH 8 to initiate polymerization of siliconate or silanetriol.

Various pH-lowering agents may be used provided only that the pH-lowering agent does not itself react with the metal oxide particles and does not cause sedimentation of the particles by itself in the pH range of 2 - 13. The pH-lowering agent may be selected from the group comprising acids such as hydrochloric acid, sulphuric acid, nitric acid, or organic acids .

The pH-lowering agent may equally be any other water-soluble substance which lowers the pH in water below 12, or 10 or 8. The pH-lowering agent may added in the form of a precursor compound that forms sodium carbonate, sodium bicarbonate, hydrochloric acid, sulphuric acid or nitric acid. To this end, the pH may be lowered by bubbling carbon dioxide through the mixture .

The process may be conducted at a temperature below 80⁰C or below 60 ⁰C or below 4O⁰C.

In one embodiment, the water-soluble organosilicon monomers are preferably an alkali metal hydrocarbon siliconate in which the hydrocarbon radical is an alkyl radical having up to 5 carbon atoms or a phenyl radical, preferably potassium methyl siliconate and/or sodium methyl siliconate. These compounds have the advantage of being readily available as well as readily soluble in water.

Advantageously, the process further comprises the step of washing to remove by-products of polymerisation and the pH- controlling agent. The by-products are expected to be potassium or sodium compounds depending on which type of salt is used in preparing the siliconate. For example, when hydrochloric acid is used as a pH-controlling agent to reduce pH, potassium methylsilsiloxane forms potassium chloride byproduct. Sodium methylsilsiloxane forms sodium nitride byproduct when nitric acid is used as a pH-controlling agent to reduce pH. Suitable methods for the removal of the pH- controlling agent and other by-products of polymerization include but are not limited to filtration washing or centrifugation washing.

According to a second and third aspect of the present invention there is provided a suspension comprising metal oxide particles coated with silicone according to the first aspect of the present invention, the coated metal oxide particles being dispersed in a polar or a non-polar carrier medium.

Brief Description of the Drawings

In order to facilitate a more detailed understanding of the nature of the invention, preferred embodiments will now be described in detail, by way of examples, which are not to be construed as limiting the invention in any way, to be read in conjunction with the accompanying drawings, in which:

Figure 1 illustrates UVVis specular transmittance of aqueous suspensions of polymethylsilsesquioxane-coated zinc oxide compared with an aqueous suspension of uncoated zinc oxide; Figure 2 illustrates UVVis transmittance of a suspension of polymethylsilsesquioxane-coated zinc oxide in isostearyl neopentanoate;

Figure 3 is a transmission electron microscopy image of zinc oxide particles coated with polymethylsilsesquioxane dispersed in caprylic/capric triglyceride;

Figure 4 illustrates a volume-weighted particle size distribution of zinc oxide particles coated with polymethylsilsesquioxane and dispersed in caprylic/capric triglyceride compared with uncoated zinc oxide particles; Figure 5 shows UVVis transmittance of a suspension of polymethylsilsesquioxane-coated zinc oxide in decamethylcyclopentasiloxane; and,

Figure 6 shows EPR signals of coated and uncoated zinc oxide aqueous suspensions, mixed with an aqueous solution of 5, 5-dimethyl-l-pyrroline N-oxide, upon exposure to UV light (300 nm in wavelength) .

Detailed Description of the Preferred Embodiments

Before the preferred embodiments of the present methods are described, it is understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs .

Throughout this specification, the term "fine particles" is used to refer to sub-micron sized particles having an average particle size not greater than one micrometer. The term "nano-sized particles" or "nanoparticles" refers to particles having an average size not greater than 200 nanometers unless otherwise specified. The present invention is equally applicable to both fine and nano-sized particles. It is to be clearly understood that the present invention is equally applicable to the surface modification of any of a variety of metal oxide particles, including but not limited to zinc oxide, titanium oxide, iron oxide, cerium oxide, zirconium oxide, and aluminium oxide. The processes of the present invention may be used to apply a silicone coating to coated or uncoated metal oxide particles, and mixtures thereof, for example, coated zinc oxide with uncoated iron oxide particles.

Throughout this specification, the term "silicone" is to be understood as polymer or oligomer forms of organosilicon compounds, which consist of polymers made up of silicon-oxygen and silicon-carbon bonds and exclude any monomers. The structure of the resulting polymer is not limited to one- dimensional chain structure but can be a two-dimensional or multi-dimensional network.

To facilitate a better understanding of the processes of the present invention, the following non-limiting examples are provided using zinc oxide as a representative metal oxide being coated with polymethylsilsesquioxane as a representative silicone coating in water. It is to be understood that whilst the present invention removes the need to use organic solvents when coating metal oxide particles with silicone, making it possible to use only water to form the aqueous solution and the aqueous suspension, the silicone coating could also be achieved if a water-miscible organic solvent is added to either the aqueous solution or the aqueous suspension.

It is expected that a person skilled in the art may devise other methods for the addition or removal of the pH-lowering agent and other by-products of polymerization without departing from the inventive concept of the present invention. Furthermore, it is expected that a person skilled in the art may devise other methods for re-dispersion of the coated dry powder into various liquids without departing from the inventive concept of the present invention. All such variations are considered to be within the scope of the present invention for which the following examples are for illustrative purpose only.

Example Ij Coating zinc oxide particles with polymethylsilsesquioxane

An aqueous suspension of zinc oxide particles with a particle concentration of 7 wt% was obtained. The zinc oxide particles had a mean diameter of 32.7 nm with a BET specific surface area of 32.3 m²/g. The pH of the aqueous suspension of zinc oxide was in the order of 9-10.

A potassium methyl siliconate aqueous solution comprising 20% of potassium oxide and 34% of siliconate ions was also obtained. Such a solution is commercially available (for example, Wacker Chemical product name BS-I 6) .

A mixture was then formed by adding 1.4 grams of the potassium methyl siliconate aqueous solution to 100 ml of the aqueous suspension of zinc oxide particles so that the concentration of potassium methyl siliconate aqueous solution was 20 wt% relative to zinc oxide. The pH of the mixture so formed was 12.5, with no obvious sign of dissolution of zinc oxide .

The mixture was homogenised using ultrasonication for 1 min and stirring for 10 min.

Thereafter, whilst stirring, polymerisation was initiated by slowly adding dilute hydrochloric acid until the pH of the mixture was lowered to 7.5. This process took approximately 5 min. During polymerisation, methyl siliconate forms silanetriol, and slowly condenses to form oligomers and siloxane polymers. Without wishing to be bound by theory, it is understood that the -Si-OH groups of the hydrolyzed silane initially hydrogen bond with -OH groups on the metal oxide particle surface. As the reaction proceeds, water is lost and a covalent bond is formed. The reaction of hydrolyzed silane with surface -OH ultimately results in the condensation of siloxane polymer which encapsulates and thereby coats the metal oxide particles. As a by-product of the acid-base reaction, an aqueous solution of potassium chloride was formed.

The mixture was then washed with deionised water while settling the coated zinc oxide particles with repeated centrifugation until the salinity of the supernatant became less than 50 ppm. The product was then dried in a tray at 120 °C for 16 hours to produce a dry white powder of zinc oxide particles coated with polymethylsilsesquioxane.

Example 2: Dispersion of metal oxide particles coated with polymethylsilsesquioxane into water.

A dry white powder of zinc oxide particles coated with polymethylsilsesquioxane was obtained in accordance with Example 1. The powder was ground using a mortar and a pestle. An aqueous dispersion of 0.01 wt% zinc oxide was made and ultrasonicated using an ultrasonic probe for 1 min. No dispersants or surfactants were added.

UVVis transmittance is a measure of the dispersibility of powders in liquid. At a fixed powder concentration and optical path length, a higher transmittance indicates better dispersibility and a lesser degree of particle agglomeration. Figure 1 shows the UVVis spectra of an 0.01 wt% aqueous dispersion of the silicone coated zinc oxide particles of Example 1 at an optical path length of 10 nm. For comparison purposes, a suspension of uncoated zinc oxide particles was formed by adding an ammonium salt of polyacrylic acid (for example, Dispex-N40, CIBA Chemical) of 10 wt% relative to zinc oxide and ultrasonicating the dispersion for 15 minutes.

Although the dispersion of the polymethylsilsesquioxane coated zinc oxide had a shorter ultrasonication time and there was no dispersant added to it, it is evident from

Figure 1 that the dispersion so produced has a UVVis spectrum that is essentially the same or superior to that of an suspension of uncoated zinc oxide dispersed in water with a dispersant present.

Normally, once particles are dried, severe aggregation between particles occurs that makes re-dispersion in liquids almost impossible. However, the polymethylsilsesquioxane coating on the zinc oxide particles allowed re-dispersion of the coated zinc oxide particles to the same low degree of agglomeration as was present before coating and drying, and also increased the ease of dispersion in water.

Example 3; Dispersion of metal oxide particles coated with polymethylsilsesquioxane into an organic liquid carrier medium

A dry white powder of zinc oxide particles coated with polymethylsilsesquioxane was obtained in accordance with

Example 1. The powder was ground using a mortar and a pestle. A formulation was produced using a bead mill, by dispersing the powder in a cosmetically acceptable oil, in this example, isostearyl neopentanoate, with the addition of appropriate dispersants, in this example, a polyhydroxystearic acid (Solsperse 3000, Lubrizol) at 10 wt% relative to dry powder. Isostearyl neopentanoate is a fatty acid ester with polarity between alcohols and hydrocarbons, and is an example of a medium-polarity liquid. The powder concentration of the formulation was 40 wt% .

UVVis spectroscopy was carried out using a quartz cell with an optical path length of 20 micron and a Varian Cary 300Bio UVVis spectrophotometer equipped with an integrating sphere. Figure 2 shows the total, diffuse and specular transmittance spectra for this formulation. High specular transmittance and low diffuse transmittance values indicate (a) a low degree of agglomeration in the particle suspension and (b) good miscibility of particles in isostearyl neopentanoate.

Example 4: Dispersion of metal oxide particles thus coated with polymethylsilsesquioxane into an organic liquid carrier medium

A dry white powder of zinc oxide particles coated with polymethylsilsesquioxane was obtained in the same manner as in Example 1. The powder was ground using a mortar and a pestle. Δ formulation was prepared by dispersing dry powder in a cosmetically acceptable oil, in this example, caprylic/capric triglyceride, with the addition of appropriate dispersants (Solsperse 3000, Lubrizol, at 10 wt% relative to dry powder) . Caprylic/capric triglyceride is another fatty acid ester with polarity between alcohols and hydrocarbons, and is an example of a medium-polarity liquid. The powder concentration of the formulation was 60 wt%.

Figure 3 shows a transmission electron microscopy image

("micrograph") of zinc oxide particles coated with polymethylsilsesquioxane and dispersed in caprylic/capric triglyceride. It is evident in the micrograph of Figure 3, that the silicone coated particles are well dispersed with a low degree of agglomeration.

Figure 4 shows the volume-weighted particle size distribution of zinc oxide particles coated with polymethylsilsesquioxane and dispersed in caprylic/capric triglyceride. The size distribution was measured by photo-correlation spectroscopy, using the particle suspension diluted to 0.005 wt% with caprylic/capric triglyceride. For comparison, the size distribution of the aqueous suspension of zinc oxide particles prior to coating is also shown in Figure 4. The volume-weighted mean particle diameter of the coated particles (3β.6nm) was nearly the same as that of the zinc oxide particles before coating (32.7 nm) .

It is commonly known that metal oxide particles tend to aggregate strongly once they are dried, making the particles almost impossible to be re-dispersed. However, Figures 3 and 4 demonstrate that the silicone coating applied in this embodiment of the process of the present invention enabled re-dispersion of the coated zinc oxide particles to a degree that is almost identical to the original dispersion state, even after drying of the powder.

Example 5: Dispersion of metal oxide particles thus coated with polymethylsilsesquioxane into organic liquids.

A dry white powder of zinc oxide particles coated with polymethylsilsesquioxane was obtained in accordance with Example 1. The powder was ground using a mortar and a pestle. A formulation was prepared by dispersing the dry powder in decamethylcyclopentasiloxane with appropriate dispersants, in this example, a carboxyl-acid-rαodified organosilicon dispersant, at 15 wt% relative to dry powder. Decamethylcyclopentasiloxane is an example of a very low polarity liquid. The powder concentration of the formulation was 40 wt%.

UVVis spectroscopy was carried out using a quartz cell with an optical path length of 20 micron and a Varian Cary 300Bio UVVis spectrophotometer equipped with an integrating sphere. Figure 5 shows the total, diffuse and specular transmittance spectra for the formulation of this example. High specular transmittance and low diffuse transmittance values indicate (a) a low degree of agglomeration in the particle suspension and (b) good irascibility of particles in decamethylcyclopentasiloxane .

Example 6: Photocatalytic activity of metal oxide particles coated with polymethylsilsesquioxane

A dry white powder of zinc oxide particles coated with polymethylsilsesquioxane was obtained in the same manner as in Example 1. The dry powder was dispersed in filtered deionised water at 0.02 wt% using an ultrasonic probe for 15 min. A 25 mM spin trap stock solution of 5, 5-dimethyl-l- pyrroline N-oxide (DMPO), specifically Sigma-Aldrich, 97%, was prepared in filtered deionised water. DMPO is a particularly effective spin trap for hydroxyl (OH) radicals. A formulation was prepared by adding 1 mL of the spin trap stock solution to 100 mL of the aqueous suspension.

A Bruker ESP-300 Electron Paramagnetic Resonance (EPR) spectrometer, equipped with a TE 102 EPR cavity and operated at about 9.73 GHz, was employed to record the time evolution of DMPO-OH during irradiation of the formulations. Irradiation was achieved using a UV light source and wavelength selection was made using a monochromater . The samples were examined in a quartz EPR flat call with 0.3 mm inner thickness. A frequency sweep over the range of 60 GHz, from 3440 to 3500 GHz under a constant flow of -0.6 mL/min using a peristaltic pump was carried out. The UV light was initially blocked, the sample suspension then introduced into the cell using a peristaltic pump, the flow stopped and the sample was then irradiated with monochromatic UV light at a wavelength of 300 nm. For comparison, an uncoated zinc oxide particle suspension of the same mean particle size and particle concentration was made and the EPR measurements carried out in the same manner.

Figure 6 shows the comparison of the broad frequency sweep Electron Paramagnetic Resonance spectra for the samples of uncoated and polymethylsilsesquioxane-coated zinc oxide. It is evident from Figure 6 that the polymethylsilsesquioxane- coated zinc oxide gave 4-6 times lower signal intensity than uncoated zinc oxide, indicating that the polymethylsilsesquioxane-coating suppressed the photocatalytic activity of zinc oxide particles.

Example 7: Use of metal oxide particles coated with polymethylsilsesquioxane in a sunscreen formulation

A dry white powder of zinc oxide particles coated with polymethylsilsesquioxane was obtained in accordance with Example 1. The dry powder was re-dispersed in C12-15 alkyl benzoate (Finsolv-TN, Finetex) to 50 % by weight. A sunscreen formulation was made out of the particle suspension in C12-15 alkyl benzoate, using the ingredients listed below in Table 1. The sunscreen formulation contained 19% by weight of coated zinc oxide particles. Table 1. Sunscreen formulation for the suspension of coated zinc oxide particles in C12-C15 alkyl benzoate.

Ingredients : %w/w

Water 34.95

Propylene glycol 3

Sodium chloride 2

Keltrol F 0.15

Coated zinc oxide suspension 19

C12-15 alkyl benzoate 19

Miglyol 812 14

Polyethylene 617 2.5

Arlacel P135 3

Monomuls 90-018 1

Performalene 400 1

Liquid Germall Plus 0.4

A water phase was prepared by dissolving sodium chloride and propylene glycol in water, and then dispersing Keltrol in the water phase by adding it slowly whilst stirring at 80 - 85°C. An oil phase was prepared by heating the suspension of coated zinc oxide in C12-15 alkyl benzoate along with Performalene 400, polyethylene 617, Arlacel P135, Miglyol 812 and Monomuls 90-018, to 90 - 95°C for 5 min, until melted. The water phase was then added to the oil phase to form a mixture. The mixture was stirred with a high shear mixer, and then cooled down to 40 - 45⁰C. Liquid Germall Plus was then mixed in, and cooled down to room temperature. Table 2 lists the performance values of the sunscreen. The specular extinction coefficient α, is determined using the formula:

Transmittance % = 100* exp (-α.C.L.)

Where, C is the concentration [wt%] and L is the optical path length [mm] . O Q

Table 2: Sunscreen performance values for 19 wt% coated zinc oxide particles.

Now that the preferred embodiments and illustrative examples of the present invention have been described in detail, the present invention has a number of advantages over the prior art, including the following:

(a) the process is organic solvent free which makes the process both more economical and safer for the environment;

(b) the coated metal oxide particles have excellent compatibility with liquids having a wide range of polarity; (c) the coated metal oxide particles exhibit a low degree of agglomeration on drying allowing them to be readily re-dispersed; and

(d) coating the metal oxide particles reduces the photoactivity of the metal oxide fine and nano-sized particles.

Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic inventive concepts. For example, once metal oxide particles are coated with polymethylsilsesquioxane using the examples described above, they are of particular value in many applications where nanocomposites are formed in organic substances having a variety of polarity values. Such applications include sunscreen compositions, cosmetic formulations, ultraviolet- screening paints, ultraviolet-screening plastics, and polymer nanocoinposites . All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

Claims

Claims defining the Invention:

1. A process for applying a silicone coating to metal oxide particles, the process comprising the steps of: preparing a mixture of an aqueous suspension of metal oxide particles and an aqueous solution of water-soluble organosilicon monomers; and, adding a pH-lowering agent to the mixture so as to initiate polymerisation of the water-soluble organosilicon monomers and form silicone coated metal oxide particles.

2. The process of claim 1 wherein the mixture is homogenised by agitating the mixture.

3. The process of claim 1 or 2 wherein the step of adding a pH-lowering agent to the mixture is gradual.

4. The process of any one of claims 1 to 3 further comprising the step of drying the silicone coated metal oxide particles.

5. The process of claim 4 wherein the step of drying the silicone coated particles is conducted at a temperature less than 400 ⁰C.

6. The process of claim 4 wherein the step of drying the silicone coated particles may be conducted at a temperature less than 150⁰C.

7. The process of any one of claims 4 to 6 wherein the dried silicone coated metal oxide particles are dispersed to form a suspension in a polar carrier medium.

8. The process of any one of claims 4 to 6 wherein the dried silicone coated metal oxide particles are dispersed to form a suspension in a non-polar carrier medium.

9. The process of claim 7 or 8 wherein the step of dispersing the dried metal oxide powder to form a suspension includes the step of adding one or more dispersants .

10. The process of any one of claims 1 to 9 wherein the aqueous suspension of metal oxide particles has a metal oxide particle concentration of 0.1% to 60 % by weight of the aqueous suspension.

11. The process of any one of claims 1 to 9 wherein the aqueous suspension of metal oxide particles has a metal oxide particle concentration of 1 % to 40 % by weight of the aqueous suspension.

12. The process of any one of claims 1 to 11 wherein the concentration of water-soluble organosilicon monomers in the aqueous solution is in the range of 0.1 to 20 mg per square metre of total surface area of the metal oxide particles in the aqueous suspension.

13. The process of any one of claims 1 to 11 wherein the concentration of water-soluble organosilicon monomers in the aqueous solution is in the range of 0.1 to 10 mg per square metre of total surface area of the metal oxide particles in the aqueous suspension.

14. The process of any one of claims 1 to 11 wherein the concentration of water-soluble organosilicon monomers in the aqueous solution is in the range of 0.1 to 5 mg per square metre of total surface area of the metal oxide particles in the aqueous suspension.

15. The process of any one of claims 1 to 14 wherein the mixture has a pH value of greater than 12 before the step of adding a pH-lowering agent.

16. The process of claim 15 wherein the pH-lowering agent lowers the pH of the mixture below 12 to initiate polymerisation.

17. The process of claim 15 wherein the pH-lowering agent lowers the pH of the mixture below 10 to initiate polymerisation.

18. The process of claim 15 wherein the pH-lowering agent lowers the pH of the mixture below 8 to initiate polymerisation.

19. The process of any one of claims 1 to 18 wherein the pH- lowering agent is an organic acid.

20. The process of any one of claims 1 to 18 wherein the pH- lowering agent is hydrochloric acid, sulphuric acid, or nitric acid, alone or in combination.

21. The process of any one of claims 1 to 18 wherein the pH- lowering agent is a precursor compound that forms sodium carbonate, sodium bicarbonate, hydrochloric acid, sulphuric acid or nitric acid when added to the mixture.

22. The process of any one of claims 1 to 18 wherein the pH- lowering agent is carbon dioxide that is bubbled through the mixture .

23. The process of any one of claims 1 to 22 wherein the steps of preparing the mixture and adding the pH-lowering agent are conducted at a temperature below 8O⁰C.

24. The process of any one of claims 1 to 22 wherein the steps of preparing the mixture and adding the pH-lowering agent are conducted at a temperature below 60⁰C.

25. The process of any one of claims 1 to 22 wherein the steps of preparing the mixture and adding the pH-lowering agent are conducted at a temperature below 40⁰C.

26. The process of any one of the claims 1 to 25 wherein the water-soluble organosilicon monomers are an alkali metal hydrocarbon siliconate in which the hydrocarbon radical is an alkyl radical having up to 5 carbon atoms or a phenyl radical .

27. The process of claim 26 wherein the water-soluble organosilicon monomers are one or both of potassium methyl siliconate and sodium methyl siliconate.

28. The process of any one of claims 1 to 27 further comprising the step of washing the coated metal oxide particles to remove by-products of polymerisation.

29. The process of any one of claims 1 to 28 wherein the metal oxide is zinc oxide.

30. The process for applying a silicone coating to metal oxide particles substantially as herein described with reference to the accompanying examples. ^■

31. A suspension comprising metal oxide particles coated with silicone according to the method of any one of claims 1 to 29 dispersed in a polar carrier medium.

32. A suspension comprising metal oxide particles coated with silicone according to the method of any one of claims 1 to 29 dispersed in a non-polar carrier medium.

33. A suspension substantially as herein described with reference to the accompanying examples .