WO2016137459A1

WO2016137459A1 - Core/shell particles comprising a titanium dioxide core

Info

Publication number: WO2016137459A1
Application number: PCT/US2015/017506
Authority: WO
Inventors: Jelena LASIO
Original assignee: The Chemours Company Tt, Llc
Priority date: 2015-02-25
Filing date: 2015-02-25
Publication date: 2016-09-01

Abstract

The disclosure provides a process for generating air voids around particles comprising: attaching guest particles onto host particles through covalent or non-covalent bonding to form a host-guest complex; treating the host-guest complex with a layer of a shell material selected from inorganic oxides or inert polymers to form a shell; and removing the guest particles. These particles are useful as hiding or opacifying agents in coating and molding compositions, plastic products and laminates.

Description

TITLE

CORE/SHELL PARTICLES COMPRISING A TITANIUM DIOXIDE CORE

BACKGROUND OF THE DISCLOSURE

The present disclosure relates to the formation of a core/shell particle, consisting of metal oxide core, such as T1O2, and a mechanically-robust shell that contains air voids. More particularly, the disclosure relates to the core/shell particle where the core is titanium dioxide, T1O2.

In coating compositions, T1O2 is used most commonly as the white pigment, due to its exceptional hiding power ability. The hiding power of the T1O2 pigment arises from its high refractive index, in particular the difference in refractive indices of the pigment itself and the average of the film that contains it. Consequently, in order to improve hiding power performance, the difference in refractive indices between the white pigment and average of the film that contains it should be maximized. Further, in order to maximize pigment performance, agglomeration of pigment particles in the film should be avoided. Therefore, a need exists to improve upon hiding power performance of T1O2 films.

SUMMARY OF THE DISCLOSURE

In a first aspect, the disclosure describes the construction of a hybrid particle, whose core is T1O2, and whose shell is made of a robust material containing air voids. This is done through attachment of guest particles onto the host, a pigment particle, and then coating of the host-guest complex with a mechanically-robust shell material. Finally, the guest particles are removed, generating a core/shell material with air voids in the shell.

In the first aspect, the disclosure provides a process for generating air voids around particles comprising:

(a) attaching guest particles onto host particles through covalent or non-covalent bonding to form a host-guest complex; (b) treating the host-guest complex with a layer of a shell material selected from inorganic oxides, such as silica or alumina, or inert polymers; and

(c) removing the guest particles. Typically, the guest particles are polystyrene or another non-recyclable polymer and the guest particles are removed by calcining. In one embodiment, the guest particles are typically depolymerizable and are removed by thermal or chemical treatment. Examples include PMMA which are removed by thermal treatment, and polyesters or polyamides that are removed by chemical treatment. In another embodiment, the guest particles are calcium carbonate and are removed by treatment with an acid.

By titanium dioxide particles we mean pigmentary sized titanium dioxide particles of average size which may or may not have organic or inorganic surface treatments. More typically the titanium dioxide particles have a particle size of about 100 to about 900nm in size, more typically between about 150 and about 600nm, and still more typically between about 180 and about 270nm.

By "attached" we mean the guest particle(s) are chemically bound to the ΤΊΟ2 particle such that they at least partially coat the surface of the ΤΊΟ2 particle. Guest particles are usually smaller than the host particle, usually between about 20 and about 150nm in size, more typically between about 30 and about 120nm, and still more typically between about 40 and about 1 10nm. Guest particles can be made of polymer material, such as polystyrene, PMMA, or other polymers, or metal carbonates such as CaCO3.

By host-guest complex we mean the agglomerate of a core particle, usually ΤΊΟ2, in the center, surrounded by smaller guest particles attached to it. The host-guest complex should have strong enough attachment or binding between the two types of particles to withstand the conditions of shell deposition, so that guest particles can serve as place holders for air voids. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic description of a process for making core/shell particles with air voids.

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 "TiO2 particle", "the T1O2 particle", or "a T1O2 particle" also includes a plurality of T1O2 particles.

In order to improve hiding power of T1O2 films, the disclosure provides a complex particle that provides benefits of better spacing of T1O2 particles while increasing the refractive index difference between film average and pigment itself. This is achieved with a shell around pigment particles that contain air voids, so that the voids lower the refractive index of the film in which they are embedded, and agglomeration of pigment particles is prevented by the robust shell around the pigment.

The process for generating air voids around particles comprising:

(a) attaching guest particles onto host particles through covalent or non-covalent bonding to form a host-guest complex;

(b) treating the host-guest complex with a layer of a shell material selected from inorganic oxides, such as silica or alumina, or inert polymers; and

(c) removing the guest particles. By "attaching" or "binding" we mean the guest particle(s) are attracted to the ΤΊΟ2 particle such that they at least partially coat the surface of the ΤΊΟ2 particle thereby separating it from adjacent ΤΊΟ2 particles in a crowded system. This attachment can be through chemical bonds (chemically bound) and additionally through supplementary attractive physical forces (physically bound). Chemical bonds are preferred. A physical bond tends to be weaker, less robust, longer distance, and less selective than a chemical bond. A physical bond is a bond mediated by physical forces, specifically electrostatic (Coulomb) and dispersion (Van der Waals) interactions. Physical bonds include ionic bonds, dipole-dipole interactions, hydrogen bonds, ion-dipole interactions, ion-induced dipole interactions, dipole-induced dipole interactions, and dispersion (Van der Waals) interactions. Examples of physical bonds include the interaction between metal cations and halide anions in salt (ionic) and the interaction of methanol with chloroform (dipole-dipole). The interaction between the polar/charged surface of a T1O2 pigment and the uncharged polymeric particle in US 7288146 is an example of ion-induced dipole class of physical bonds.

Typically stronger and shorter range, chemical bonds result in the sharing of an electron pair between reactive partners. Chemical bonds include non-polar covalent, polar covalent, and coordinate covalent or dative bonds. Non-polar and polar covalent bonds involve the sharing of electrons between atoms where one electron is contributed from each atom. Non-polar covalent bonds describe those bonds where there is limited difference in electronegativity between the bound atoms (e.g. C-C), whereas polar covalent bonds describe those bonds where the difference in electronegativity is substantial (e.g, C-O). Coordinate covalent bonds describe bonds where both electrons are contributed from a single atom

Functionalization of either the T1O2 pigment or guest particle allows for attachment of the particles via bonds formed through condensation with the available hydroxylated surface. Examples of functional groups useful for this mode of attachment include but are not limited to phosphonic acids, sulfonic acids, boronic acids and their corresponding salts. One example of chemical bonding is the incorporation of phosphate groups on the surface of the guest particles so that they may, under appropriate conditions, interact with alumina species if present on the surface of an inorganically surface-treated ΤΊΟ2 particle. A further example of a polar covalent bond useful for this mode of particle attachment is the Si-O bond formed by the condensation of silanes with hydroxylated surfaces. Another example of chemical bonding is the polar-covalent bond formed during the ring-opening of an epoxide with reactants including but not limited to thiols, amines, carboxylic acids, and alcohols. Placement of a separate reactant from the pair on the ΤΊΟ2 and the guest particle results in attachment of the two particles post-reaction. Further examples of polar-covalent bonds useful for attachment of ΤΊΟ2 to guest particles include but are not limited to amides, esters, urethanes, ureas, ethers and those bonds resulting from thiol-ene and azide-alkyne reactions. An example of this attachment scheme that proceeds through formation of a non-polar covalent bond would be functionalization with a diene and a dienophile to facilitate a Diels-Alder reaction, forming a cyclic C-C based linker. Linker between the host and guest particle can be a small molecule, or polymeric.

Alternatively, the host-guest complex can be achieved through

mechanochemical methods, when host and guest particles are exposed to high shear conditions, in which, under very high mechanical forces, a chemical bond is formed between host and guest particles. As described above, once the host-guest complex is formed, it is treated with a material that will generate a robust shell on the surface of the complex. The material can be inorganic, such as silica or alumina, or a polymer. Guest particles and the shell material should have different chemical or physical properties, in order to allow selective removal of guest particles after deposition of shell material onto the host-guest complex. For example, if the guest particles are polymeric, it is preferable that the shell is made of an inorganic material, or a polymeric material with orthogonal reactivity, to provide the possibility of selective guest removal. Organic guest particles can be calcined or thermally depolymerized in the presence of inorganic shell, or reacted with a material with which the shell material doesn't react. For example, if the guest particles are made of CaCO3 it would be preferable that the shell is made from a material resistant to acid, as it would be used to remove guest particles. The host/guest complex should be stable to the conditions of the shell deposition. In the case of silica or alumina deposition, that would include resistance in basic conditions and elevated temperatures. In the case of a polymeric shell, that would include resistance to elevated temperatures and/or organic solvents. To generate the shell with air pockets around T1O2 core, the guest particles should be removed in a process that doesn't damage the shell or the core material. In case of organic guest particles, they can be removed through calcination, or thermal depolymerization (in case when the guest particles are made out of PMMA), or in case of CaCO3 guest particles, through acid treatment. The shell should be of sufficient porosity to allow gas escape in calcination or depolymerization processes, or acid intrusion in the case of CaCO3 dissolution.

Host Particles

By host particles we mean pigmentary sized inorganic particles such as titanium dioxide particles of average size which may or may not have organic or inorganic surface treatments. The particles described herein are between about a 50 to about 900nm in size, more typically between about 150 and about 600nm, and still more typically between about 180 and about 270nm.

It is expected that any inorganic particle may be useful in this

disclosure. By inorganic particle it is meant an inorganic particulate material that becomes uniformly dispersed in a composition and imparts color and opacity to the product. Some examples of inorganic powders include but are not limited to ZnS, ΤΊΟ2, CaCO3, BaSO4, ZnO, M0S2, silica, talc and clay.

In particular, titanium dioxide is an especially useful particle in the processes and products of this disclosure. Titanium dioxide (ΤΊΟ2) particle useful in the present disclosure may be in the rutile or anatase crystalline form. It is commonly made by either a chloride process or a sulfate process. In the chloride process, T1CI4 is oxidized to ΤΊΟ2 particles. In the sulfate process, sulfuric acid and ore containing titanium are dissolved, and the resulting solution goes through a series of steps to yield ΤΊΟ2. Both the sulfate and chloride processes are described in greater detail in "The Pigment Handbook", Vol. 1 , 2nd Ed., John Wiley & Sons, NY (1988), the teachings of which are incorporated herein by reference. The particle may be a pigment or nanoparticle. By "pigment" it is meant that the titanium dioxide particles have an average size of less than 1 micron.

The titanium dioxide particle may be substantially pure titanium dioxide or may contain other metal oxides, such as silica, alumina, zirconia. Other metal oxides may become incorporated into the particles, for example, by co-oxidizing or co-precipitating titanium compounds with other metal compounds. If co-oxidized or co-precipitated about 20 wt% of the metal oxide, more typically, 0.5 to 5 wt%, most typically about 0.5 to about 1 .5 wt% may be present, based on the total particle weight.

The titanium dioxide particle may also bear one or more metal oxide surface treatments. These treatments may be applied using techniques known by those skilled in the art. Examples of metal oxide treatments include silica, alumina, zirconia among others. Such treatments may be present in an amount of about 0.1 to about 10 wt%, based on the total weight of the particle, preferably about 0.5 to about 3 wt%.

The inorganic particle may be silanized by treating with at least one silane, or a mixture of at least one silane and at least one polysiloxane. Guest Particle:

By guest particle we mean a particle that is typically smaller than the host particle, and with an ability to form a bond with a host particle. Guest particles are usually smaller than the host particle, usually between 20 and 150nm in size, more typically between about 30 and 120nm, and still more typically between 40 and 1 10nm.

The guest particle is prepared using typically an organic monomer which is polymerized to generate the guest particle. Some monomers for the guest particle include styrene, methyl methacrylate, a-methylstyrene, lactic acid, or formaldehyde, more typically methyl methacrylate, lactic acid, or a-methylstyrene, and still more typically methyl methacrylate or a- methylstyrene . 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, or other Nylons), etc. Further, two monomers can be copolymerized, with one having a functional group that is suitable for attachment to the host particle. The functional groups in the comonomer could be chosen from, but are not limited to siloxanes, amines, azides, carboxylic acids, epoxides, alkenes, alkynes, boronic, sulfonic acids, quarternary amines, etc. Alternatively, guest particles can be functionalized post-synthesis, to install functional groups on their surface through surface treatment with functional linkers, which can be small molecules or polymers or both. The monomers are present in the amount of about 1 to about 60wt%, more typically about 2 to about 50wt%, still more typically about 5 to about 40wt%, based on the total weight of the components used in the preparation of the guest particle. Typically, the particle size of the guest is tunable, and the particle size distribution of the guest particles achieved is narrow, which is advantageous. For example, preparation of the recyclable guest particle by emulsion polymerization is achieved by emulsification of the water-insoluble monomer or a monomer mixture in water, and polymerized using radical or photopolymerization conditions. Radical initiators such as potassium- or ammonium persulfate, and 2,2-azobis(2-methylpropionamidine)

hydrochloride (AIBA) can be used, more typically AIBA. 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, it might be

advantageous to use copolymers in order to introduce charge on the surface of the particle, like for example vinyltimethylammonium chloride benzene, 2-(methacryloxy)ethyltrimethylammonium chloride, etc. In cases where silica is deposited onto the host-guest complex surface, it might be beneficial to use a copolymer with a silyl group, to promote the silica deposition on the particle surface like for instance 3-

(trimethoxysilyl)propylmethacrylate, or other silyl-containing monomers. In some cases, it might be preferable to use comonomers that can crosslink two growing polymer chains, thereby strengthening the guest particle- some of those materials include divinylbenzene or ethylene glycol dimethacrylate. In order to perform the polymerization, the reaction temperature is kept between about 0 and about 100°C, more typically about 15 to about 90°C, still more typically about 25°C to about 70°C.

Guest particles may also comprise metal carbonates such as CaCO3.

By aqueous monomer dispersion we mean water or a mixture of water and surfactant, initiator, defoaming agent, or a suitable buffer in cases where pH needs to be kept in a particular range.

Outer Shell:

The host/guest particle is then coated with a shell material. To generate a silica treatment comprising a coating, layer 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(dinnethyl)octylsilane, or 3-aminopropyl-(diethoxy)nnethylsilane, or siloxanes having general formula RSi(OR)3, Ri R2Si(OR)2, or R1 R2R3S1OR, wherein 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 host/guest particles in dilute ethanol/water solution of ammonia is treated with the solvent based silica precursor, which results in silica deposition on the host/guest particles, generating host/guest/shell particles.

Alternatively, the silica shell can be deposited through a water-based shell deposition process. The water-based silica precursor is sodium silicate, potassium silicate or pre-formed silicic acid; more typically sodium silicate or potassium silicate; still more typically sodium silicate.

The concentration of water-based silica precursor is about 0.005 wt% to about 20 wt%, more typically about 0.005 wt% to about 15 wt%, based on the total weight of the dispersion. Typically, the suspension of host- guest complex particles in water is treated with water-based silica precursor, which results in silica deposition onto the host-guest complex generating host/guest shell particles.

The pH is maintained at about 2 to about 10, more typically about 5 to about 9, to form a silica layer on the host-guest complex 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 comprising a coating, layer or shell on the host-guest complex. The reaction is kept at temperatures between 25-100°C, more typically between 40 and 90°C, still more typically between 50 and 80°C. The host/guest/shell particles are removed from the aqueous solution by centrifugation or filtration, more typically by centrifugation.

Depending on the nature of the guest particle, the recyclable guest particle can be recycled either through thermal depolymerization, or acid- or base hydrolysis. Typically, guest 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 about 290 to about 325°C, to generate hollow particles as well as core monomer. For example, particles comprising

TiO2/poly(methylmethacrylate)/silica can be heated above around about 300°C to generate methyl methacrylate monomer and TiO2/silica particles comprising a void. Further, TiO2/poly(a-methylstyrene)/silica can be heated to about above 60°C to generate TiO2/silica particles comprising a void 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 TiO2/polyacetal/silica with acid should generate TiO2/silica particles comprising voids as well as aldehyde monomer that can be recycled in guest particle synthesis. Similarly, polyesters or polyamides from host/guest/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).

These depolymerization methods allow for formation of TiO2/silica particles comprising a void, as well as, being non-destructive toward guest monomers, allowing for guest material recycling. Applications:

These inorganic host/shell particles having a void are useful as hiding or opacifying agents in coating and molding compositions, plastic products and laminates. EXAMPLES

Example 1 . Guest particle synthesis

To a 3L round bottom flask, equipped with a reflux condenser, thermometer, nitrogen inlet, and magnetic stirring, was added water (720 ml_) and acetone (480 ml_), and methyl methacrylate (31 .95g, 319 mmol), and the mixture was degassed with nitrogen for 1 h. The reaction flask was then heated to 68 °C, and allowed to equilibrate for 10 minutes at that temperature. The initiator (Wako V-44, 1 .86g, 6.0 mmol) was added next, and the reaction was left stirring for 15 minutes. After that time, 3- (trimethoxysilyl)propyl methacrylate (3.99ml_, 16.80mol) was added via syringe, and then the reaction mixture was stirred for 45 more minutes, after which it was allowed to cool to room temperature. The reaction mixture was then filtered through a cotton plug to remove any precipitated solids, and the particle size of the sample was measured as 58nm.

Example 2. Host-Guest Complex formation, followed by silica coating.

The guest sample described in example 1 is mixed with T1O2 in a ratio that allows for at least partial coverage of the surface of T1O2, and the mixture is allowed to stir at room temperature overnight, at a pH that promotes attaching or binding. The pH is then adjusted to 8, and sodium silicate is added slowly, over a period of several hours, simultaneously with addition of HCI, in order to maintain pH8. After the addition, the mixture is left stirring overnight, then centrifuged and washed to isolate the coated host-guest complex.

Example 3. Core/Shell particle with air voids in the shell

The sample described in Example 2 is heated in a high temperature furnace to 500°C for 6h, and then allowed to cool to room temperature. The resulting solid contains TiO2/silica core/shell particles with air voids in the silica shell.

Claims

CLAIMS What is claimed is:

1 . A process for generating air voids around particles comprising:

(b) treating the host-guest complex with a layer of a shell material selected from inorganic oxides or inert polymers to form a shell; and

(c) removing the guest particles.

2. The process of Claim 1 wherein the host particles are inorganic

particles selected from the group consisting of ZnS, T1O2, CaCO3, BaSO4, ZnO, M0S2, silica, talc and clay.

3. The process of Claim 2 wherein the host particles are titanium

dioxide (T1O2).

4. The process of Claim 3 wherein the titanium dioxide (T1O2) is surface treated with metal oxides.

5. The process of Claim 2 wherein the host particles are 100 to about 900nm in size.

6. The process of Claim 1 wherein the guest particles are organic

polymers prepared from polymers selected from the group consisting of styrene, methyl methacrylate, a-methylstyrene, lactic acid and formaldehyde.

7. The process of Claim 1 wherein the guest particles are polystyrene or polyolefin and the guest particles are removed by calcining.

8. The process of Claim 1 wherein the guest particles are

depolymerizable and are removed by thermal or chemical treatment.

9. The process of claim 3 wherein the guest particles are PMMA and are removed by thermal treatment.

10. The process of claim 3 wherein the guest particles are polyester, polyamides, or polyacetals and are removed by chemical treatment.

1 1 . The process of Clainn 1 wherein the guest particles are calcium carbonate and are removed by treating with an acid.

12. The process of Claim 1 wherein the guest particles are about 20 and about 150nm in size.

13. The process of Claim 1 wherein the attaching is by chemical bonding.

14. The process of Claim 1 wherein the attaching is via functional groups selected from phosphonic acid, sulfonic acid, boronic acid and salts thereof.

15. The process of Claim 1 wherein the shell material is inorganic or a polymer.

16. The process of Claim 1 wherein the shell is prepared using a solvent- based silica precursor.

17. The process of Claim 16 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(dimethyl)octylsilane, or 3-aminopropyl- (diethoxy)methylsilane, or siloxanes having general formula

RSi(OR)3, Ri R2Si(OR)2, or R1 R2R3S1OR, wherein R, Ri , R2, and R3, are any of a variety of alkyl or aryl groups.

18. The process of Claim 17 wherein the solvent-based silica precursor is tetraethyl orthosilicate (TEOS) or tetrapropyl orthosilicate (TPOS).

19. The process of Claim 1 wherein the shell is prepared using a water- based silica precursor selected from the group consisting of sodium silicate, potassium silicate, preformed silicic acid, and mixtures thereof.

20. The process of Claim 13 wherein the shell is silica or alumina.