PROCESS FOR PREPARING SILICA/POLYMER HYBRID HOLLOW NANOSPHERES THROUGH INTERFACIAL POLYMERIZATION IN INVERSE MINIEMULSIONS

5 BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

The present disclosure relates to a process for making silica/polymer nanospheres, more particularly to a preparation process for making silica/polymer nanospheres utilizing a mini-emulsion preparation 10 process utilizing solvent based silica precursors; and use of the

silica/polymer nanospheres in coating compositions.

Description of the Related Art

Nanospheres are submicroscopic colloidal systems composed of a

15 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 from micro or miniemulsions via

polymerization reaction at the interface of the droplets, the so-called interfacial polymerization reaction. Interfacial polymerization occurs at the

20 interface of two immiscible phases, for example, oil and water, and a thin shell is formed. In the formation of the shell, the monomers are in either oil or water phase to participate in the reaction. Typically, for the

preparation of core-shell nanocapsules via interfacial polymerization, an microemulsion or miniemulsion is first prepared, either water in oil or oil in

25 water, wherein in the former nanocapsules with an aqueous core

suspended in oil are formed and in the latter nanocapsules with an oily core suspended in water are formed. Existing process for the preparation of hybrid silica/polymeric particles either require a multi-step

polymerization/condensation process to form the layer-by-layer hollow

30 nanospheres, or produce particles that are too large.

A need exists for a hollow nanosphere composed of a single silica/polymeric hybrid shell, wherein the silica is derived from a solvent based silica precursor. It is also needed that the process can be prepared through a one step process and provide superior performance for opacity enhancement in architectural and industrial coatings.

SUMMARY OF THE DISCLOSURE

In a first aspect, the disclosure provides a process for preparing a silica/polymeric hybrid hollow nanosphere comprising:

(a) providing a mixture comprising water, at least one non- reactive solvent; at least one acrylic or styrenic monomer; at least one solvent based silica precursor or a polymerizable silane or combinations thereof; an initiator; and at least one surfactant;

(b) shearing the components of the mixture from (a) with high shear energy at an energy density of at least 10 ^Λ 6 J/m ^A 3 to form a mini-emulsion; and

(c) heating to at least about 50 °C, more typically about 50 ° to about 90 °C; and still more typically about 60 ° to about 80 °C to form, in one step, silica/polymeric hybrid hollow nanospheres.

By non- reactive solvent we mean that the solvent does not substantially react, more typically does not react, with any of the other components added to the reaction.

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

"silica/polymeric hybrid hollow nanosphere", "the silica/polymeric hybrid hollow nanosphere", or "a silica/polymeric hybrid hollow nanosphere" also includes a plurality of silica/polymeric hybrid hollow nanospheres.

The disclosure relates to a process for preparing a silica/polymeric hybrid hollow nanosphere, typically a substantially non-porous

silica/polymeric hybrid hollow nanosphere. By substantially non-porous it is meant that the surface area and porosity of the silica/polymeric shell, typically walls, has to be tuned. Whether the silica/polymeric shell is adequate can be determined by comparing the surface area of the particles with surface area of a smooth sphere, typically a polymer shell, of the same diameter. Typically, we consider the shell substantially non- porous if its surface area does not surpass about 130% of the surface area of a smooth sphere of the same dimensions, i.e., it is about 30% or less higher than the 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. Controlling the ratio between silica precursor and monomers will lead to more or less porous silica/polymeric layers, which can lead to control of the porosity and surface area of the particles.

These silica/polymeric hybrid hollow nanospheres are useful as hiding or opacifying agents in coating and molding compositions. They are also useful as drug delivery systems in the pharmaceutical and medical industries; in food, personal care and cosmetics; and agriculture. These nanospheres have a particle size of about 5 nm to about 400 nm, more typically about 50 nm to about 300 nm, and still more typically about 100 nm to about 250 nm.

The silica/polymeric hybrid hollow nanospheres are prepared by a process comprising:

(a) providing a mixture comprising water, at least one non- reactive solvent; at least one acrylic or styrenic monomer; at least one solvent based silica precursor or a polymerizable silane or combinations thereof; an initiator; and at least one surfactant;

(b) shearing the components of the mixture from (a) with high shear energy at an energy density of at least 10 ^Λ 6 J/m ^A 3 to form a mini-emulsion; and

(c) heating to at least about 50 °C, more typically about 50 ° to about 90 °C; and still more typically about 60 ° to about 80

°C to form, in one step, silica/polymeric hybrid hollow nanospheres.

The non-reactive solvent may be an alkane, a hydrocarbon oil, aromatic hydrocarbon or halogenated hydrocarbon liquid, more typically alkane or hydrocarbon oil. The at least one acrylic or styrenic monomer may be methyl methacrylate, methyl acrylate, n-butyl methacrylate, t-butyl methacrylate, t-butyl acrylate, ethyl glycol dimechacrylate, styrene or divinylbenzene; more typically methyl methacrylate or styrene. The solvent based silica precursor is tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS) tertrapropyl orthosilicate (TPOS), tetrabutyl orthosilicate (TBOS), tetrahexyl orthosilicate, diethoxydimethylsilane, ethoxytrimethylsilane, methoxytrimethylsilane, Trimethoxy(octyl)silane, Triethoxy(octyl)silane, Methoxy(dimethyl)octylsilane, or 3-Aminopropyl- (diethoxy)methylsilane; more typically tetraethyl orthosilicate (TEOS) or tertrapropyl orthosilicate (TPOS)

Some suitable initiators include azo compounds such as 2,2'- azobisisobutyronitrile (AIBN) or 2,2'-azobis(2-methylpropionamide) dihydrochloride (AIBA); metal persulfate such as potassium persulfate (KPS) or sodium persulfate; more typically AIBN or KPS.

Polymerizable silanes such as allyltriethoxysilane,

allyltrimethoxysilane, diethoxy(methyl)vinylsilane,

dimethoxymethylvinylsilane, triethoxyvinylsilane, trimethoxy(7-octen-1 - yl)silane, 3-(trimethoxysilyl)propyl acrylate, 3-(trimethoxysilyl)propyl methacrylate, or vinyltrimethoxysilane are useful in this disclosure, more typically 3-(trimethoxysilyl)propyl acrylate or 3-(trimethoxysilyl)propyl methacrylate. At least one surfactant is part of the mixture in step (a).

Some suitable surfactants include cetyltrimethylammonium bromide

(CTAB), lauryltrimethylammonium bromide, dodecyltrimethylammonium bromide, octyltrimethylammonium bromide, sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), dioctylsulfosuccinate , nonionic surfactants such as alkylphenol polyoxyethylene,

polyoxyethylene glycol alkyl ethers, polyoxypropylene glycol alkyl ethers, octylphenol ethoxylates or poloxamers, more typically SDS, SDBS or CTAB. Some useful commercially available surfactants series include Triton X ^® manufactured by The Dow Chemical Company, Brij ^®

manufactured by Croda International PLC, or Pluoronic ^® manufactured by BASF.

The mixture in step (a) may be prepared in any glass container or stainless steel reaction vessel.

The mixture of the above components is then sheared at an energy density of at least 10 ^Λ 6 J/m ^A 3, more typically about 10 ^Λ 7 J/m ^A 3 to about 5 ^* 10 ^Λ 8 J/m ^A 3, to form a mini-emulsion. Some useful means for shearing include an ultrasonic disruptor, high speed blender, high pressure homogenizer, high shear disperser, membrane homogenizer or colloid mill, more typically an ultrasonic disruptor, high speed blender, or a high pressure homogenizer. Typically shearing occurs for a period of about 5 to about 120 minutes depending on amount of emulsion needed to be prepared and desired emulsion size range, more typically about 30 minutes to about 60 minutes. Typically, shearing is accomplished at room temperature. Optionally, a defoamer may be needed to avoid foaming during emulsifying. Some suitable defoamers include BASF Foamaster®, Dow Corning® 71 and 74 Antifoams.

The mini-emulsion formed in step (b) is then heated to at least about 50 °C, more typically about 50 °C to about 90 °C; and still more typically about 60 °C to about 80 °C to form, in one step, using a sol gel reaction and polymerization, a silica/polymeric hybrid hollow nanosphere. Heating may be accomplished using hot plate, heating mantle or any other heating method. Applications:

These silica/polymeric hybrid hollow nanospheres are useful as hiding or opacifying agents in coating and molding compositions. They are also useful as drug delivery systems in the pharmaceutical and medical industries; in food, personal care and cosmetics; and agriculture.

EXAMPLES

Glossary:

AIBN 2,2'-azobisisobutyronitrile

TEOS tetraethyl orthosilicate

TBOS tetrabutyl orthosilicate

TMSPM 3-(Trimethoxysilyl)propyl methacrylate

CTAB cetyltrimethylammonium bromide

SDS sodium dodecyl sulfate

Example 1 :.

An oily mixture which contained 5.0 g of hexadecane, 36.8 g of octane, 28.8 g of methyl methacrylate, 3.6 g of ethylene glycol

dimethacrylate, 3.6 g of styrene, 0.798 g of AIBN, 5.0 g of TEOS and 1 .0 g of TMSPM was first prepared, and added to a water solution which contains 420.0 g of water, 0.9 g of CTAB and 0.5 g of defoamer

(Foamaster® 1 1 1 , BASF). Miniemulsification was achieved by shearing the mixture for 30 minutes with a high speed blender at 9500 rpm. After forming a stable miniemulsion, the polymerization was started by heating to 70 °C for at least 16 hours. The structure of the resulting particles was analyzed using transmission electron microscopy and shown in Figure 1 . The average particle size of the resulting hollow particles determined by dynamic light scattering is 237.9 nm with a polydispersity of 0.462.

Example 2:.

An oily mixture which contained 5.0 g of hexadecane, 36.8 g of octane, 28.8 g of methyl methacrylate, 3.6 g of ethylene glycol

dimethacrylate, 3.6 g of styrene, 0.798 g of AIBN and 6.0 g of TMSPM was first prepared, and added to a water solution which contains 420.0 g of water, 0.9 g of CTAB and 0.5 g of defoamer (Foamaster® 1 1 1 , BASF). Miniemulsification was achieved by shearing the mixture for 30 minutes with a high speed blender at 9500 rpm. After forming a stable

miniemulsion, the polymerization was started by heating to 70 °C for at least 16 hours. The structure of the resulting particles was analyzed using transmission electron microscopy and shown in Figure 2. The average particle size of the resulting hollow particles determined by dynamic light scattering is 191 .7 nm with a polydispersity of 0.174.

Example 3:

An oily mixture which contained 5.0 g of hexadecane, 36.8 g of octane, 28.8 g of methyl methacrylate, 3.6 g of ethylene glycol

dimethacrylate, 3.6 g of styrene, 0.798 g of AIBN and 12.0 g of TMSPM was first prepared, and added to a water solution which contains 420.0 g of water, 0.9 g of CTAB and 0.5 g of defoamer (Foamaster® 1 1 1 , BASF). Miniemulsification was achieved by shearing the mixture for 30 minutes with a high speed blender at 9500 rpm. After forming a stable

miniemulsion, the polymerization was started by heating to 70 °C for at least 16 hours. The structure of the resulting particles was analyzed using transmission electron microscopy and shown in Figure 3. The average particle size of the resulting hollow particles determined by dynamic light scattering is 183.6 nm with a polydispersity of 0.140.

Example 4: An oily mixture which contained 7.0 g of hexadecane, 5.4 g of methyl methacrylate, 0.6 g of ethylene glycol dimethacrylate, 0.133 g of AIBN, 1 .0 g of TEOS and 0.5 g of TMSPM was first prepared, and added to a water solution which contains 70.0 g of water, 0.07 g of SDS.

Miniemulsification was achieved by ultrasonicating the mixture for 60 s with a Branson sonifier W150 at 100% amplitude and then stopped for 30 s; this cycle was repeated 10 times. To avoid polymerization due to heating, the mixture was cooled in an ice-bath during homogenization. After forming a stable miniemulsion, the polymerization was started by heating to 70 °C for at least 16 hours. The structure of the resulting particles was analyzed using transmission electron microscopy and shown in Figure 4. The average particle size of the resulting hollow particles determined by dynamic light scattering is 171 .4 nm with a polydispersity of 0.239.

Example 5:

An oily mixture which contained 7.0 g of hexadecane, 5.4 g of methyl methacrylate, 0.6 g of ethylene glycol dimethacrylate, 0.133 g of AIBN, 1 .0 g of TBOS and 0.5 g of TMSPM was first prepared, and added to a water solution which contains 70.0 g of water, 0.07 g of SDS.

Miniemulsification was achieved by ultrasonicating the mixture for 60 s with a Branson sonifier W150 at 100% amplitude and then stopped for 30 s; this cycle was repeated 10 times. To avoid polymerization due to heating, the mixture was cooled in an ice-bath during homogenization. After forming a stable miniemulsion, the polymerization was started by heating to 70 °C for at least 16 hours. The structure of the resulting particles was analyzed using transmission electron microscopy and shown in Figure 5. The average particle size of the resulting hollow particles determined by dynamic light scattering is 134.0 nm with a polydispersity of 0.108.

Example 6. Hiding power performance of selected example in coating formulations Hollow silica particles prepared in Example 3 above were tested in an acrylic latex paint formulation. Two formulations were prepared (Table 1 ), one without any hollow silica (control), and one with 5 wt% of materials from Example 3. Thin coating films were made from the two formulations, and they were compared for hiding power (Scoat), using standard protocols of Kubelka-Munk theory of reflectance (Table 2). It is evident that addition of hollow silica particle provides films with superior hiding power. The hollow particles described above are thus seen as good additives for hiding power improvement.

Table 1 . Connposition of paint formulations with and without hollow silica particle.

^* PVC=pigment volume concentration.