WO2015138141A1

WO2015138141A1 - Process for preparing functionalized hollow silica particles with low porosity

Info

Publication number: WO2015138141A1
Application number: PCT/US2015/017799
Authority: WO
Inventors: Jelena LASIO; Anilkumar Raghavanpillai; Francis J. Woerner
Original assignee: E I Du Pont De Nemours And Company
Priority date: 2014-03-11
Filing date: 2015-02-26
Publication date: 2015-09-17

Abstract

The disclosure provides a process for preparing the functionalized hollow silica particles comprising: providing a core-shell silica particle comprising a template core particle and a silica treatment, wherein the core-shell silica particle has an outer surface; and wherein the silica treatment is prepared using a solvent-based silica precursor; creating a functionalized surface on the core-shell silica particle, wherein the functionalized surface is prepared using sulfonic acid, phosphonic esters, carboxylic acid, amines, epoxides, or boronic acids, and removing the template core particle to form a functionalized hollow silica particle. These inorganic hollow particles are useful as hiding or opacifying agents in coating and molding compositions. They are also useful as photonic band gap or thermal insulation materials.

Description

TITLE

PROCESS FOR PREPARING FUNCTIONALIZED HOLLOW SILICA PARTICLES WITH LOW POROSITY

BACKGROUND OF THE DISCLOSURE

The present disclosure relates to a process for making functionalized hollow silica particles. More particularly, the disclosure relates to functionalized silica particles with substantially impervious silica shells.

Nano core/shell particles are submicroscopic colloidal systems composed of a solid or liquid core surrounded by a thin polymer or inorganic shell. This solid or liquid core is removed to form hollow nanospheres. Such core-shell systems may be prepared by deposition of the shell material onto a template particle, wherein the shell material can be either organic, inorganic, or hybrid. The selective removal of the core (template) material without disturbing the shell generates hollow particles. In many applications of hollow nanoparticles, for example in cases where they are used as drug delivery agents, or catalyst support, it is desired for the porosity and surface area of the material to be high, in order to ensure the delivery of the host molecule, or enough surface area for efficient catalysis. In applications in which delivery of air voids is important for the application, such as in photonic band gap materials or coatings, it is desirable that the porosity of the shell is minimized, to ensure the integrity of the central void of the hollow particle. Therefore a need exists for synthetic methods for substantially impervious hollow particles. SUMMARY OF THE DISCLOSURE

The disclosure provides a process for the synthesis of functionalized hollow silica nanospheres, and allows for control of the porosity and surface area of the silica shells, providing access to substantially impervious hollow particles with tunable surface properties. The surface properties of the particles are tuned through grafting with a variety of functionalized alkoxysilanes.

In a first aspect, the process for preparing the functionalized hollow silica particles comprises:

(i) providing a core-shell silica particle comprising a template core particle and a silica treatment, more typically a coating, wherein the core-shell silica particle has an outer surface; and wherein the silica treatment is prepared using a solvent-based silica precursor;

(ii) creating a functionalized surface on the core-shell silica particle, wherein the functionalized surface is prepared using sulfonic acid, phosphonic esters, carboxylic acid, amines, epoxides, boronic acids or quarternary amines, and

(iii) removing the template core particle to form a functionalized hollow silica particle.

The template core particle from the core-shell silica particle may be removed before or after functionalization, more typically before

functionalization. If removed after functionalization, it is important the core removal does not damage the functionalized surface. For example, with a-methyl styrene, core removal is at low temperatures so no harm is done to the functionalized surface if core removal is achieved after

functionalization. Also, acid- or base-labile core materials that can be removed by hydrolysis can be removed before or after functionalization.

More typically the silica treatment is substantially impervious. By 'substantially impervious' we mean the surface area and porosity of the silica shell, typically walls, has to be tuned. Whether the silica shell is adequate can be determined by comparing the surface area of the particles with calculated surface area of a smooth sphere of the same diameter. Typically, we consider the shell substantially impervious if its surface area does not surpass about 130% of the calculated surface area of a smooth sphere of the same dimensions, i.e., it is about 30% or less higher than the surface of the core-shell silica particle compared to the calculated surface area of a smooth sphere of the same diameter, more typically about 125% of the smooth sphere surface area, and still more typically about 120% of the smooth sphere surface area of the same dimensions. Porosity of the particles, measured in pore volume should typically be lower than about 0.16cm³/g, more typically lower than about 0.10cm³/g, still more typically lower than about 0.08cm³/g. Addition of various amounts of the silica precursor will lead to more or less porous silica layers, which can lead to control of the porosity and surface area of the particles. Further, the silica precursor may be added in stages to modulate the porosity of the particles as well as their surface. Lastly, calcination at temperatures higher than 500°C may decrease the porosity and surface area of the particles without increasing the thickness of the wall.

In the first aspect, the process for preparing the core-shell silica particle comprising a template core particle and a silica treatment comprises:

a) providing a template core particle, more typically prepared using emulsion polymerization;

b) coating the template particle with a solvent- based silica precursor such as tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), tetrapropyl orthosilicate (TPOS), tetrabutyl orthosilicate (TBOS), tetrahexyl orthosilicate,

diethoxydimethylsilane, ethoxytrimethylsilane,

methoxytrimethylsilane, trimethoxy(octyl)silane,

triethoxy(octyl)silane, methoxy(dimethyl)octylsilane, or 3- aminopropyl-(diethoxy)methylsilane or siloxanes with general formula RSi(OR)3, Ri R2Si(OR)2, or R1 R2R3S1OR, where R, Ri , R2, and R3, can be any of a variety of alkyl or aryl groups; more typically tetraethyl orthosilicate (TEOS) or tetrapropyl orthosilicate (TPOS); and

c) maintaining the pH at about 2 to about 10 to form a silica treatment, typically a coating, on the template particle. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the process for making hollow silica particles, as described in examples 1 -1 1 .

Figure 2 shows functionalization of hollow silica particles with phosphonate ester through use of diethyl-(2-

(triethoxysilyl)ethyl)phosphonate, as described in Example 12.

Figure 3 shows the scheme for converting the phosphonate ester- functionalized silica from Figure 2, example 12 into phosphonic acid- functionalized silica particles through hydrolysis of phosphonic ester groups, as described in Example 13.

Figure 4 shows functionalization of hollow silica particles with (3- glycydoxypropyl)trimethoxysilane, as described in Examples 14 and 15.

Figure 5 shows the process for further functionalization of the epoxy silyl-funtionalized silica with glycine, to generate carboxyl group- functionalized silica particles through an amine linkage, as described in Example 14.

Figure 6 shows the process for further functionalization of the epoxy silyl-funtionalized silica with thioglycolic acid, to generate carboxyl group- functionalized silica particles through a thioether linkage, as described in Example 15.

Figure 7 shows the process for functionalization of hollow silica particles with (diethoxyphosphoryl)methyl-2- ((triethoxysilyl)ethyl)carbamate, as described in Example 16.

DETAILED DESCRIPTION OF THE DISCLOSURE

In this disclosure "comprising" is to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more features, integers, steps, or components, or groups thereof. Additionally, the term "comprising" is intended to include examples encompassed by the terms "consisting essentially of and "consisting of." Similarly, the term "consisting essentially of is intended to include examples encompassed by the term "consisting of."

In this disclosure, when an amount, concentration, or other value or parameter is given as either a range, typical range, or a list of upper typical values and lower typical values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or typical value and any lower range limit or typical value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the disclosure be limited to the specific values recited when defining a range.

In this disclosure, terms in the singular and the singular forms "a," "an," and "the," for example, include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to "hollow inorganic particle", "the hollow inorganic particle", or "a hollow inorganic particle" also includes a plurality of hollow inorganic particles.

A process for preparing the hollow inorganic particles, typically hollow silica, through intermediacy of template particles, onto which the shell material is deposited, to generate core/shell particles. The process of silica deposition is such that it allows tuning of the surface area and porosity of the silica shells, thereby allowing for synthesis of impervious core/shell particles. The core material is removed to generate hollow particles. The resulting hollow particles are then functionalized with a variety of alkoxysilanes to generate functionalized hollow particles, with tunable porosity and surface area.

The particles described herein are between about a 100 to about 900nm in size, more typically between about 150 and about 600nm, and still more typically between about 180 and about 270nm. The disclosure describes the process for hollow silica particles with tunable porosity and surface area, as well as the functional group on the hollow particles' surface. The process for preparing the functionalized hollow silica particles comprises:

(i) providing a core-shell silica particle comprising a template core particle and a silica treatment, more typically a coating, wherein the core-shell silica particle has an outer surface; and wherein the silica treatment is prepared using a solvent-based silica precursor, whereby the silica treatment is substantially impervious;

(ii) creating a functionalized surface on the core-shell silica particle, wherein the functionalized surface is prepared using sulfonic acid, phosphonic esters, carboxylic acid, amines, epoxides, boronic acids, quarternary amines, etc., and

(iii) removing the template core particle to form a functionalized

hollow silica particle. The template core particle may be removed before or after

functionalization, more typically before functionalization. If removed after functionalization, it is important the core removal does not damage the functionalized surface. For example, with a-methyl styrene core removal is at low temperatures so no harm is done to the functionalized surface if core removal is achieved after functionalization. Also acid- or base-labile core materials that can be removed by hydrolysis can be removed before or after functionalization.

The core-shell silica particle comprising a template core particle and a silica treatment, typically a coating, is prepared by a process comprising: a) providing a template core particle, more typically prepared using emulsion emulsion polymerization;

b) coating the template particle with a solvent- based silica precursor such as tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS) tetrapropyl orthosilicate (TPOS), tetrabutyl orthosilicate (TBOS), tetrahexyl orthosilicate, diethoxydimethylsilane, ethoxytrimethylsilane, methoxytrimethylsilane,

trimethoxy(octyl)silane, triethoxy(octyl)silane,

methoxy(dimethyl)octylsilane, or 3-aminopropyl- (diethoxy)methylsilane or siloxanes with general formula

RSi(OR)3, Ri R2Si(OR)2, or R1 R2R3S1OR, where R, Ri , R2, and R3, can be any of a variety of alkyl or aryl groups; more typically tetraethyl orthosilicate (TEOS) or tetrapropyl orthosilicate (TPOS); and

c) maintaining the pH at about 2 to about 10 to form a silica

treatment on the template particle.

The template particle or core is prepared using typically an organic monomer which is polymerized to generate template particles, or dispersed in water to generate template particles of the appropriate size. Some monomes for the template include styrene, methyl methacrylate, polyacrylic acid, a-methylstyrene, lactic acid, formaldehyde, or

copolymers like Surlyn® (copolymer of ethylene and methacrylic acid) more typically styrene, methyl methacrylate, a-methylstyrene, Surlyn®, and still more typically methyl methacrylate, styrene, or polyacrylic acid. Similarly, a group of two monomers can be chosen for a copolymerization, such as a variety of diacids and dialcohols for polyester polymers (like polyethylene terephthalate, PET), diacids and diamides for various polyamides (like Nylon 6,6, or other Nylons), etc. Typically, the particle size of the template is tunable, and the particle size distribution of the template particles achieved is narrow, which is advantageous. For example, preparation of the template particle or core by emulsion polymerization is achieved by emulsification of the water-insoluble monomer or a mixture of in water, and polymerized using radical polymerization conditions. Radical initiators such as potassium- or ammonium persulfate, and 2,2-azobis(2-methylpropionamidine)

hydrochloride (AIBA) can be used. Surfactant can also typically be used. Some examples of suitable surfactants include sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), poly-(vinylpyrrolidinone) PVP, etc. In some cases, silyl group-containing monomers, such as 3- (trimethoxysilyl)propylmethacrylate can be used, in order to facilitate the silica deposition in the subsequent step. In order to perform the

polymerization, the reaction temperature is kept between about 25 and about 100°C, more typically about 45 to about 90°C, still more typically about 55°C to about 75°C.

Alternately, the template particle or core may be inorganic, for example calcium carbonate, or other inorganic particles onto which silica can be deposited.

The template particle or core is then coated with a shell material to generate a core/shell particle. To generate a silica treatment or shell, at least one solvent-based silica precursor is used. Some examples of solvent-based silica precursors include tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS) tetrapropyl orthosilicate (TPOS), tetrabutyl orthosilicate (TBOS), tetrahexyl orthosilicate,

diethoxydimethylsilane, ethoxytrimethylsilane, methoxytrimethylsilane, trimethoxy(octyl)silane, triethoxy(octyl)silane,

methoxy(dimethyl)octylsilane, or 3-aminopropyl-(diethoxy)methylsilane or siloxanes with general formula RSi(OR)3, Ri R2Si(OR)2, or R1 R2R3S1OR, where R, Ri , R2, and R3, can be any of a variety of alkyl or aryl groups; more typically tetraethyl orthosilicate (TEOS) or tetrapropyl orthosilicate (TPOS). When using organic siloxanes, the reaction is typically done in a dilute ethanol/water ammonia solution, with or without sonication.

Typically, the suspension of template particles in dilute ethanol/water solution of ammonia is treated with the solvent based silica precursor, which results in silica deposition on the recyclable template particles, generating core shell particles. In the case of a water-based silica precursor, such as sodium- or potassium silicate, the template particles are suspended in water, and the silicate agent is added either dropwise, over a period of time, or all at once.

The pH is maintained at about 2 to about 9, more typically about 5 to about 8 to form a silica layer on the recyclable template particle and the reaction times are held between about 1 to about 24 hours, more typically about 1 .5 to about 18 hours, still more typically about 2 to about 12 hours. This results in the deposition of a silica treatment or shell on the recyclable template particle or core. The solids are removed from the aqueous solution by centrifugation or filtration, more typically by centrifugation.

Typically, in order to form impervious silica shells, surface area and porosity of the silica walls have to be tuned. Whether the silica shell is adequate can be determined by comparing the surface area of the particles with calculated surface area of a smooth sphere of the same diameter. Typically, we consider the shell impervious if its surface doesn't surpass about 130% of the calculated surface area of a smooth sphere of the same dimensions, i.e., it is about 30% or less higher than the surface of the core-shell silica particle prior to functionalization, more typically about 125% of the smooth sphere surface area, and still more typically about 120% of the smooth sphere surface area of the same dimensions. Porosity of the particles, measured in pore volume should typically be lower than about 0.16cm³/g, more typically lower than about 0.10cm³/g, still more typically lower than about 0.08cm³/g. Addition of various amounts of the silica precursor will lead to more or less porous silica layers, which can lead to control of the porosity and surface area of the particles. Further, the silica precursor may be added in stages to modulate the porosity of the particles as well as their surface. Lastly, calcination at temperatures higher than 500°C can decrease the porosity and surface area of the particles without increasing the thickness of the wall.

The core may then be removed before or after grafting of a variety of alkoxysilanes onto the surface of the silica particles to form hollow silica particles having a functionalized surface. In one embodiment, removal of the template may be achieved through calcination, namely heating the material to 300-800 °C, more typically 400 C-600 °C, and most typically 450-550 °C. In the case of CaCO3/silica core/shell particles, hollow particles are typically obtained through reaction with acid. If the core is made of recyclable material, it may be recycled either through thermal depolymerization, or acid- or base hydrolysis. In a specific embodiment, core materials made out of poly-(a-methylstyrene), PMMA, various polyamides, as well as styrene are depolymerized at increased temperatures, with the temperatures of depolymerization varying with the polymer used. Some suitable temperature ranges include about 250 to about 450°C, more typically about 275 to about 400°C, still more typically from 290-325°C, to generate hollow particles as well as core monomer. For example, poly(methylmethacrylate)@silica core/shell particles can be heated above around about 300°C to generate methyl methacrylate monomer and hollow silica particles. Further, poly(a- methylstyrene)@silica can be heated to about above 60°C to generate hollow silica particles and a-methylstyrene monomer. Alternatively, acid- or base-labile core materials can be hydrolyzed instead of thermally depolymerized to generate hollow particles with possibility of monomer recycling. Polymers such as Delrin® (polyacetal), poly(lactic acid), as well as other polyesters can be depolymerized through acid hydrolysis. For example, treating polyacetal@silica with acid should generate hollow silica as well as aldehyde monomer that can be recycled in template particle synthesis. Similarly, polyesters or polyamides from core/shell particles can be recycled in the same fashion to generate diacid/dialcohol (diacid/diamine) monomer couples as well as hydroxylic or amino acids as monomers (like in the case of polylactic acid, for example). When the core is calcium carbonate, it is removed by acid treatment, which generates hollow particles. The functionalized surface on the silica particle may be prepared using sulfonic acid, phosphonic esters, carboxylic acids, amines, epoxides, boronic acids, quaternary amines, etc. Grafting of a variety of alkoxysilanes onto the surface of the hollow silica particles provides functionalized hollow silica particles. A large spectrum of functionalities can be introduced onto the silica surface, for example silyl phosphonates, phosphonic acids, amines, alcohols, epoxides, carboxylic acids, thiols, thioethers, carbamates, isocyanates, quarternary ammonium ions, etc. The grafting process includes mixing the grafting agent with silica particles, with or without the solvent, with optional heating of the material, in the temperature range 25-150°C, more typically 60- 130°C, still more typically 80-120°C, with or without the application of vacuum, in order to remove the volatile byproducts, like water or alcohols. In one embodiment of the disclosure, the hollow silica particles were functionalized with (diethoxyphosphoryl)methyl-2- ((triethoxysilyl)ethyl)carbamate, introducing phosphonate functionality on the surface. In another embodiment of the invention, the silica particles were functionalized with diethyl [2-(triethoxysilyl)ethyl]phosphonate to generate phosphonate-functionalized silica particles. Then, in another embodiment, phosphonate ester functionality on the surface of the silica particles was hydrolyzed to generate phosphonic acid-functionalized hollow silica particles. In another embodiment of the disclosure, silica particles were treated with (3-glycidopropyl)trimethoxysilane, to generate epoxy functionality on the silica surface. The epoxy silica was then treated, in one embodiment of the disclosure, with glycine, to introduce carboxylic acid functionality through an amine linkage on the particle. In another, the epoxy silica was treated with thioglycolic acid to introduce the carboxylic functionality through a thioether group.

APPLICATIONS

These inorganic hollow particle dispersions are useful as hiding or opacifying agents in coating and molding compositions. They are also useful as photonic band gap or thermal insulation materials. EXAMPLES

Example 1 . Polystyrene template particle synthesis

To a 2L four-neck round bottom flask, equipped with a mechanical stirrer, thermometer, a reflux condenser, and a nitrogen inlet, was added styrene (18mL, 157.1 mmol), and 600mL of degassed water.

Polyvinylpyrrolidinone, PVP (100mg) solution in 100mL of degassed water was then added. The resulting mixture was stirred at room temperature for 15min. The mixture was degassed by bubbling nitrogen for 20min. To the reaction was then added a degassed solution of 2,2-azobis(2- methylpropionamidine) hydrochloride, AIBA(100mg, 1 .1 mmol) in 100mL water, and the reaction was heated to 70°C overnight. Particle size analysis of the resulting suspension revealed particles with average particle size of 250nm.

Examples 2-1 1 show hollow silica particle synthesis in which porosity and surface area of the particles was systematically decreased to generate impervious particles.

Example 2. Hollow silica particle synthesis

To a 1 L Erlenmeyer flask was added 10OmL of PS suspension, followed by 700ml_ of EtOH, and 20ml_ of aq. NH₄OH. The flask was placed in a sonicating bath, to which was added 1 ml_ of TEOS via a syringe pump, at a 0.1 mL/min rate. The resulting suspension was left sonicating for 2h, and ethanol was removed in vacuo. The resulting slurry was centrifuged, and washed with ethanol twice to yield a white solid. The resulting material was calcined in a tube furnace at 500°C (r.t.-500 at 1 °C/min, then 5h at 500°C. Results are shown in Table 1 . Example 3. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 2ml_ of TEOS were added. Results are shown in Table 1 .

Example 4. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 3ml_ of TEOS were added. Results are shown in Table 1 .

Example 5. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 4ml_ of TEOS were added. Results are shown in Table 1 .

Example 6. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 5ml_ of TEOS were added. Results are shown in Table 1 .

Example 7. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 6ml_ of TEOS were added. Results are shown in Table 1 . Example 8. Hollow silica particle synthesis.

Example 2 was repeated with the following exception: 7ml_ of TEOS were added. Results are shown in Table 1 .

Example 9. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 8ml_ of TEOS were added. Results are shown in Table 1 .

Example 10. Hollow silica particle synthesis

Example 2 was repeated with the following exception: 9ml_ of TEOS were added. Results are shown in Table 1 .

Example 1 1 . Hollow silica particle synthesis

Example 2 was repeated with the following exception

were added. Results are shown in Table 1 .

Table 1

Example 12. Grafting the HSP with phosphonate ester

Solid hollow particles (10g) were dispersed in 300mL dimethyl formamide (DMF). To this suspension was added a diethyl-(2- (triethoxysilyl)ethyl)phosphonate (10ml_, 31 .Ommol ), and the mixture was heated to 120°C overnight. The resulting material was centrifuged to remove the DMF solvent, and washed with ethanol. The presence of grafting groups was measured by TGA and ESCA.

Example 13. Phosphonic acid-functionalized particles

To a 1 L round bottom flask, equipped with an addition funnel and a reflux condenser was added 25.6g of phosphonate ester-functionalized particles (Example 12) and 400mL dichloromethane, and the mixture was kept under nitrogen. To the mixture was added trimethylsilyl bromide (75ml_), dropwise, via an addition funnel. Upon addition, the mixture was heated to reflux for 18h. The mixture was then cooled to room

temperature and the volatiles removed in vacuo. To the residue was then added 150ml_ of methanol, and 50ml_ of dichloromethane, and the mixture was left stirring at room temperature overnight. The silica material was centrifuged to remove from the solvent and excess reagents (9,000 rpm for 20 minutes), and washed with water and methanol. ToF SIMS data confirmed the presence of phosphonic acid functionality, and

disappearance of phosphonic ester functionality.

Example 14. Epoxide-functionalized hollow silica particles, followed by reaction with glycine

To a mixture of 10ml_ of DMF and 500uL of triethylamine (TEA) was added 38.5mg of hollow silica particles (avg size ~250nm), and the mixture was sonicated in an ultrasound (US) bath for ~15min. The mixture became milky white, and 1 ml_ of (3-glycydoxypropyl)trimethoxysilane was added, and the mixture heated to 120°C. After three more hours, the mixture was cooled to r.t., and 24mg glycine in 200uL of water was added to the mixture, which was left stirring overnight. The sample was isolated by centrifuging and washing the solids with ethanol twice, and drying the solids. The material was analyzed by ESCA to confirm the presence of nitrogen atoms on the silica surface. Example 15. Epoxide-functionalized hollow silica particles, followed by reaction with thioglycolic acid

To 10ml_ of DMF was added 36.5mg of hollow silica particles (avg size ~250nm), and the mixture was sonicated in a US bath for ~15min. To the mixture was addedl ml_ of (3-glycydoxypropyl)trimethoxysilane, and the mixture was heated to 120°C overnight. The mixture was cooled to r.t, and, 50ΌμΙ_ of thioglycolic acid was added, and the mixture left stirring for two days. The sample was isolated by centrifuging and washing the solids with ethanol twice, and drying the solids. The material was analyzed by ESCA to confirm the presence of sulfur atoms on the silica surface.

Example 16. Phosphonic ester (carbamate)

Solid hollow particles (1 g) were dispersed in dilute ammonia solution (7wt%, 20ml_), with sonication. The resulting suspension was added to 30ml_ DMF, and the water was removed in vacuo. To this suspension was added (diethoxyphosphoryl)methyl-2-((triethoxysilyl)ethyl)carbamate (1 g, 2.49mmol), and the mixture was heated to 120°C overnight. The resulting material was centrifuged to remove the DMF solvent, and washed with ethanol. The presence of grafting groups was measured by TGA and ESCA.

Example 17. Hiding power performance of selected examples in coatings formulations

Some of the hollow silica particles shown in the examples above were tested in an acrylic latex paint formulation. Five formulations were prepared (Table 2), one without any hollow silica (control), two with

2.5wt% of materials from Examples 12 and 13, respectively, and two with 4.8wt% and 7.2wt% of material from Example 12, respectively. Thin coating films were made from the five formulations, and they were compared for hiding power (Scoat), using standard protocols (Table 3). It is evident that addition of hollow silica particles provides films with superior hiding power. The hollow particles described above are thus seen as good additives for hiding power improvement. Table 2.

Composition of paint formulations with and without hollow silica particles.

Table 3.

Dry film PVC and hiding power data from formulations in Table 2.

*PVC=pigment volume concentration.

Claims

CLAIMS What is claimed is:

1 . A process for preparing the functionalized hollow silica particles comprises:

(i) providing a core-shell silica particle comprising a template core particle and a silica treatment, wherein the core-shell silica particle has an outer surface; and wherein the silica treatment is prepared using a solvent-based silica precursor;

(ii) creating a functionalized surface on the core-shell silica

particle, wherein the functionalized surface is prepared using sulfonic acid, phosphonic esters, carboxylic acid, amines, epoxides, or boronic acids, and

2. The process of Claim 1 wherein the core-shell silica particle

comprising a template core particle and a silica treatment is prepared by a process comprising:

a) providing a template core particle;

b) coating the template particle with a solvent- based silica

precursor; and

c) maintaining the pH at about 2 to about 10 to form a silica treatment on the template particle.

3. The process of Claim 2 wherein the treatment is a coating.

4. The process of Claim 3 wherein the silica treatment is

substantially impervious.

5. The process of Claim 2 wherein the template core particle may be removed before functional ization.

6. The process of Claim 2 wherein the template core particle is prepared using emulsion polymerization.

7. The process of Claim 3 wherein the silica treatment has a surface area that is less than 130% higher than the surface area of a smooth sphere of identical radius.

8. The process of Claim 1 wherein the functionalization of the

surface of the silica is achieved through grafting.

9. The process of Claim 8 wherein the grafting agent is diethyl-(2- (triethoxysilyl)ethyl)phosphonate, whereby a phosphonate ester- functionalized surface is formed .

10. The process of Claim 9 wherein the phosphonate ester- functionalized surface containing particle is hydrolyzed, whereby a phosphonic acid-functionalized hollow silica particle is formed.

1 1 . The process of Claim 8 wherein the grafting agent is (3- glycydoxypropyl)trimethoxysilane , whereby a epoxide- functionalized hollow silica particle is formed.

12. The process of Claim 1 1 wherein the epoxide-functionalized hollow silica particle is reacted with glycine whereby a carboxylic acid-functionalized hollow silica particle with an amine linkage is formed.

13. The process of Claim 1 1 wherein the epoxide-functionalized hollow silica particle is reacted with thioglycolic acid whereby a carboxylic acid-functionalized hollow silica particle with a thioether linkage is formed.

14. The process of Claim 8 wherein the grafting agent is

(diethoxyphosphoryl)-methyl-2-((triethoxysilyl)ethyl)carbamate.

15. The process of claim 1 wherein the solvent based silica

precursor is tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS) tetrapropyl orthosilicate (TPOS), tetrabutyl orthosilicate (TBOS), tetrahexyl orthosilicate,

diethoxydimethylsilane, ethoxytrimethylsilane,

methoxytrimethylsilane, trimethoxy(octyl)silane, triethoxy(octyl)silane, methoxy(dinnethyl)octylsilane, or 3- aminopropyl-(diethoxy)nnethylsilane or siloxanes with general formula RSi(OR)3, Ri R2Si(OR)2, or R1 R2R3S1OR, where R, Ri , R2, and R3, can be any of a variety of alkyl or aryl groups.

16. The process of claim 15 wherein the solvent based silica

precursor is tetraethyl orthosilicate (TEOS) or tertrapropyl orthosilicate (TPOS).

17. The process of claim 1 wherein the template core particle

comprises poly-(methylmethacrylate), poly-(alphamethylstyrene), polyamide or polystyrene.

18. The process of claim 1 wherein the template core particle

comprises polyacetal poly(lactic acid) or polyester.

19. The process of claim 1 wherein the template core particle

comprises calcium carbonate.

20. The process of claim 17 wherein the template core particle

comprising a silica treatment is heated to promote

depolymerization.

21 . The process of claim 16 wherein the template core particle

comprising a silica treatment is treated with acid.

22. The process of claim 18 wherein the template core particle

comprising a silica treatment is treated with acid.

23. The process of claim 1 wherein the particle formed is about

100 nm to about 900 nm in size.

24. The process of claim 1 wherein in pore volume of the silica

treatment is lower than about 0.16cm³/g.

25. The process of claim 1 wherein the pH is maintained at about 8 to about 10.