WO2015189624A2

WO2015189624A2 - Functionalised material

Info

Publication number: WO2015189624A2
Application number: PCT/GB2015/051723
Authority: WO
Inventors: Suelen Barg; Esther García-Tuñon BLANCA; Eduardo Saiz GUTIERREZ
Original assignee: Imperial Innovations Limited
Priority date: 2014-06-11
Filing date: 2015-06-11
Publication date: 2015-12-17
Also published as: GB201410439D0; WO2015189624A3; GB2527098A

Abstract

The present invention relates to an ink composition comprising a functionalised material, a pH regulating agent, and an aqueous solvent, wherein the functionalised material comprises inorganic particles functionalised with a pH responsive polymer.

Description

Functionalised Material

TECHNICAL FIELD

The present invention relates to an ink composition comprising functionalised inorganic particles, a method of preparing them, methods of printing an article with the ink composition and a functionalised material comprising inorganic particles.

BACKGROUND

The field of 3D printing and additive manufacturing for the printing of ceramic products is underdeveloped when compared to the printing of polymer or metal products. Printing inks for 3D objects often require complicated formulations with multiple additives, for example dispersants, binders, that are specifically formulated for each material to be printed. The inks also require the fine control of pH, and/or mixtures of volatile solvents. There is therefore a need for simplified inks which have the required resolution and structural stability for printing 3D articles made, for example, from ceramic materials.

SUM MARY OF THE INVENTION

It has been determined that an ink composition comprising inorganic particles functionalised with a pH responsive polymer, provides an ink which has the required resolution and structural stability for the printing of 3D articles.

Accordingly, in a first aspect, the present invention provides an ink composition comprising a functionalised material, a pH regulating agent, and an aqueous solvent, wherein the functionalised material comprises inorganic particles functionalised with a pH responsive polymer.

In a second aspect, the present invention relates to a method of preparing an ink

composition comprising the steps of

(a) preparing a functionalised material according to the first aspect by mixing an aqueous suspension of inorganic particles with an aqueous solution of a pH responsive polymer;

(b) adding a pH regulating agent. In a third aspect, the present invention provides a use of a composition according to the first aspect, as an ink for 3D printing.

In a fourth aspect, the present invention relates to a method of printing an inorganic article comprising the steps of

(a) printing an ink composition to form an article;

(b) drying the printed article; and optionally

(c) curing the printed article.

In a fifth aspect, the present invention provides an article obtainable by printing a

composition according to the first aspect.

In a sixth aspect, the present invention provides a functionalised material comprising inorganic particles functionalised with a pH responsive polymer.

In a seventh aspect, the present invention provides a method of preparing the functionalised material of the sixth aspect, comprising the steps of mixing an aqueous suspension of inorganic particles with an aqueous suspension of a pH responsive branched polymer, at a pH of at least 7.

In an eighth aspect, the present invention provides an aqueous suspension comprising the functionalised material as described herein.

BRIEF DESCRIPTION OF THE FIGURES

In the context of the following figures, detailed discussion and examples, an exemplary pH- responsive polymer may be referred to as a pH-responsive branched copolymer surfactant (BCS).

Figure 1 shows (a) the adsorption behaviour of BCS molecules onto positively charged Al₂0₃ surfaces and (b) the Langmuir isotherm fit of the adsorption data.

Figure 2 shows the interactions between BCS and SiC.

Figure 3 shows the kinetics of the self-assembly process for Al₂0₃ functionalised with BCS.

Figure 4 shows the rheology studies of the Al₂0₃ inks. DETAILED DESCRIPTION The following definitions pertain to chemical structures, molecular segments and substituents:

The term "alkyl" as used herein refers to a branched or unbranched saturated hydrocarbon group which may contain from 1 to 20 or 1 to 12 carbon atoms. Preferably, a lower alkyl group contains from 1 to 6, preferably 1 to 4 carbon atoms. Methyl, ethyl and propyl groups are especially preferred. "Substituted alkyl" refers to alkyl substituted with one or more substituent groups. Typical substituent groups include, for example, halogen atoms, nitro, cyano, hydroxyl, cycloalkyi, alkyl, alkenyl, haloalkyl, alkoxy, haloalkoxy, amino, alkylamino, dialkylamino, formyl, alkoxycarbonyl, carboxyl, alkanoyl, alkylthio, alkylsulfinyl, alkylsulfonyl, alkylsulfonato, arylsulfinyl, arylsulfonyl, arylsulfonato, phosphinyl, phosphonyl, carbamoyl, amido, alkylamido, aryl, aralkyl and quaternary ammonium groups, such as betaine groups. Of these substituent groups, halogen atoms, cyano, hydroxyl, alkyl, haloalkyl, alkoxy, haloalkoxy, amino, carboxyl, amido and quaternary ammonium groups, such as betaine groups, are particularly preferred. When any of the foregoing substituents represents or contains an alkyl or alkenyl substituent group, this may be linear or branched and may contain up to 12, preferably up to 6, and especially up to 4, carbon atoms. A cycloalkyi group may contain from 3 to 8, preferably from 3 to 6, carbon atoms. An aryl group or moiety may contain from 6 to 10 carbon atoms, phenyl groups being especially preferred. A halogen atom may be a fluorine, chlorine, bromine or iodine atom and any group which contains a halo moiety, such as a haloalkyl group, may thus contain any one or more of these halogen atoms.

Terms such as "(meth) acrylic acid" embrace both methacrylic acid and acrylic acid.

Analogous terms should be construed similarly.

Terms such as "alk/aryl" embrace alkyl, alkaryl, aralkyl (e.g. benzyl) and aryl groups and moieties.

Molar percentages are based on the total monofunctional monomer content.

Molecular weights of monomers and polymers are expressed as weight average molecular weights, except where otherwise specified.

An ink composition according to the present invention comprises a functionalised material, a pH regulating agent and an aqueous solvent, wherein the functionalised material comprises inorganic particles functionalised with a pH responsive polymer. The functionalised material comprises inorganic particles with pH responsive polymer bound thereto. Binding may be covalent, adsorption of hydrophobic groups to the inorganic particles, electrostatic interactions or a combination thereof. Accordingly, the functionalised material comprises covalent bonding between the inorganic particles and the pH responsive polymer.

Inorganic material according to the present invention includes, but is not limited to, Al₂0₃, SiC, Si0₂, Ti0₂, mullite (AI₂0₃2Si0₂), iron oxides (FeO, Fe₂0₃, magnetite (Fe₃0₄)) and solid- oxide perovskite structures for fuel cells cathodes e.g.

(where Ln=La, Sm, Nd, Gd, Dy),

(LSCF) for example

Lao 8Sr_{0 2}Feo 8Co_{0 2}, and Lao 6Sr_{0 4}Feo 8Co_{0 2}, and Ln 1-x A x M 12-y Mn y O 32-δ , where Ln=La, Nd, Pr,

A= Ca, Sr and M=transition metal) .

Preferably, the inorganic particles are selected from the group of Al₂0₃ and SiC, Si0₂, Ti0₂, AI₂0₃2Si0₂, solid oxides with perovskites structure, iron oxides and magnetite.

Preferably, when the composition comprises Al₂0₃ particles, the composition may comprise up to 45 vol/v% Al₂0₃. Preferably, the composition may comprise between 10-50 vol/v%, 20-45 vol/v%, 30-40 vol/v% Al₂0₃.

Preferably, when the composition comprises SiC, the composition may comprise up to 35 vol/v% SiC. Preferably, the composition may comprise between 10-40 vol/v%, 20-35 vol/v% SiC.

The inorganic particles may be functionalised with a pH responsive polymer.

Preferably, the functionalised material may comprise covalent binding between the inorganic particles and the pH responsive polymer.

A pH responsive polymer is characterized by a reversible response to an external stimuli, i.e. pH. A pH responsive polymer is a polymer whose solubility in a solvent (usually in aqueous solvent) changes dependent upon solution pH. Accordingly, a solution or suspension of a pH responsive polymer will have different rheological properties at different pH values.

The following definitions of the pH responsive polymer apply to all aspects and embodiments of the invention described herein.

The pH responsive polymer may be a copolymer.

The pH responsive polymer may be a branched copolymer. The pH responsive polymer may comprise at least one residue of a chain transfer agent.

Preferably, the polymer further comprises poly(ethylene glycol) and methacrylic acid residues.

Preferably, the polymer further comprises ethylene glycol dimethacrylate residues.

Preferably, wherein the inorganic particle is Al₂0₃, the Al₂0₃ to polymer ratio is between 15: 1 and 75: 1 (by weight).

Preferably, wherein the inorganic particle is SiC, the SiC to polymer ratio is between 15: 1 and 63: 1 (by weight).

A chain transfer agent (CTA) is a molecule which may reduce the molecular weight of the polymer during a free-radical polymerisation via a chain transfer mechanism and prevents polymer gelation. The CTA may be a molecule comprising a thiol group and can be either monofunctional or multifunctional. The CTA may be a hydrophobic monomer. Examples of hydrophobic CTAs include, but are not limited to linear and branched alkyl (e.g. C₂-Ci₈alkyl) and aryl thiols (e.g. mono- or di-thiols) such as dodecanethiol (DDT), octadecyl mercaptan, 2-methyl-l-butanethiol and 1 ,9- nonanedithiol. The residue of the chain transfer agent may comprise 0.05 to 30 mole %, of the copolymer (based on the number of moles of monofunctional monomer).

The polymer may be, for example, as described in US 2012/0059069, US 201 1/0313054, EP 2102256, US 201 1/0172314, which are hereby incorporated by reference.

The pH responsive branched copolymer may comprise

(a) at least one ethyleneically monounsaturated monomer;

(b) at least one ethyleneically polyunsaturated monomer;

(c) at least one residue of a chain transfer agent;

(d) at least two chains formed from (a) being covalently linked, other than at its ends, by a bridge at residue (b); and wherein

(i) at least one of (a) to (c) comprises a hydrophilic residue;

(ii) at least one of (a) to (c) comprises a hydrophobic residue; (iii) at least one of (a) to (c) comprises a moiety capable of forming a non- covalent bond with at least one of (a) to (c).

In the present invention, ethyleneically monounsaturated monomer may be referred to as "monofunctional monomer". The ethyleneically polyunsaturated monomer may be referred to as "multifunctional monomer".

The pH responsive branched copolymer may comprise the general formula (I)

in which

E and E' each independently represent a residue of a chain transfer agent or an initiator;

G and J each independently represent a residue of a monofunctional monomer having one polymerisable double bond per molecule;

L is a residue of a multifunctional monomer having at least two polymerisable double bonds per molecule; each R, independently, represents a hydrogen atom or an optionally substituted alkyl group;

X and X' each independently represent a terminal group derived from a termination reaction; g, j and I represent the molar ratio of each residue normalised so that g + j = 100, wherein g and j each independently represent 0 to 100, and I is≥ 0.05; and m and n are each independently≥ 1 ; at least one of E, E', G, J and L is a hydrophilic residue; and at least one of E, E', G, J and L is a hydrophobic residue. g may be 0-10, preferably 1 to 10, more preferably 4-6. j may be 90-100, preferably 94-96. I may be 10. Preferably, G may be a poly(ethyleneglycol) residue. J may be a methacrylic acid residue. E and E' may be a residue of dodecanethiol.

Preferably, the polymer has a composition according to formula (II)

(II) wherein G' is poly(ethylene glycol), J' is carboxyl, E and E' each independently represent a residue of a chain transfer agent or an initiator, X and X' each independently represent a terminal group derived from a termination reaction, L is ethylene glycol diester, g, j and I represent the molar ratio of each residue normalised so that g + j = 100, wherein g and j each independently represent 0 to 100, and I is≥ 0.05; and m and n are each independently ≥ 1 .

The monofunctional monomer may comprise any carbon-carbon unsaturated compound which can be polymerised by an addition polymerisation mechanism, for example vinyl and allyl compounds. The monofunctional monomer may be hydrophilic, hydrophobic, amphiphilic, anionic, cationic, neutral or zwitterionic in nature. The monofunctional monomer may be selected from but not limited to monomers such as vinyl acids, vinyl acid esters, vinyl aryl compounds, vinyl acid anhydrides, vinyl amides, vinyl ethers, vinyl amines, vinyl aryl amines, vinyl nitriles, vinyl ketones, and derivatives of the aforementioned compounds as well as corresponding allyl variants thereof. Other suitable monofunctional monomers include hydroxyl-containing monomers and monomers which can be post-reacted to form hydroxyl groups, acid-containing or acid-functional monomers, zwitterionic monomers and quaternised amino monomers. Oligomeric, polymeric and di- or multi-functionalised monomers may also be used, especially oligomeric or polymeric (meth)acrylic acid esters such as mono(alk/aryl) (meth)acrylic acid esters of polyalkyleneglycol or

polydimethylsiloxane or any other mono-vinyl or allyl adduct of a low molecular weight oligomer. Mixtures of more than one monomer may also be used to give statistical, graft, gradient or alternating copolymers. Thus, G and J each independently represent a residue of a monofunctional monomer as described above.

Vinyl acids and derivatives thereof include (meth)acrylic acid, fumaric acid, maleic acid, itaconic acid and acid halides thereof such as (meth)acryloyl chloride. Vinyl acid esters and derivatives thereof include C C₂o alkyl(meth)acrylates (linear and branched) such as methyl(meth)acrylate, stearyl(meth)acrylate and 2-ethyl hexyl(meth) acrylate

aryl(meth)acrylates such as benzyl(meth)acrylate, tri(alkyloxy)silylalkyl(meth)acrytates such as trimethoxysilylpropyl(meth)acrylate and activated esters of (meth)acrylic acid such as N- hydroxysuccinamido(meth)acrylate. Vinyl aryl compounds and derivatives thereof include: styrene, acetoxystyrene, styrene sulfonic acid, vinyl pyridine, vinylbenzyl chloride and vinyl benzoic acid. Vinyl acid anhydrides and derivatives thereof include maleic anhydride. Vinyl amides and derivatives thereof include (meth)acrylamide, N-(2- hydroxypropyl)methacrylamide, N-vinyl pyrrolidone, N-vinyl formamide,

(meth)acrylamidopropyl trimethyl ammonium chloride, [3-((meth)acrylamido)propyl]dimethyl ammonium chloride, 3-[N-(3-(meth)acrylamidopropyl)-N,N-dimethyl]aminopropane sulfonate, methyl (meth)acrylamidoglycolate methyl ether and N-isopropyl(meth)acrylamide. Vinyl ethers and derivatives thereof include methyl vinyl ether. Vinyl amines and derivatives thereof include: dimethylaminoethyl(meth)acrylate, diethylaminoethyl(meth)acrylate, diisopropylaminoethyl(meth)acrylate, mono-t-butylaminoethyl(meth)acrylate,

morpholinoethyl(meth)acrylate and monomers which can be post-reacted to form amine groups, such as vinyl formamide. Vinyl aryl amines and derivatives thereof include: vinyl aniline, vinyl pyridine, N-vinyl carbazole and vinyl imidazole. Vinyl nitriles and derivatives thereof include (meth)acrylonitrile. Vinyl ketones and derivatives thereof include acreolin.

Hydroxyl-containing monomers include: vinyl hydroxyl monomers such as hydroxyethyl (meth)acrylate, hydroxy propyl (meth)acrylate, glycerol mono(meth)acrylate and sugar mono(meth)acrylates such as glucose mono(meth)acrylate. Monomers which can be post- reacted to form hydroxyl groups include vinyl acetate, acetoxystyrene and glycidyl

(meth)acrylate. Acid-containing or acid functional monomers include: (meth)acrylic acid, styrene sulfonic acid, vinyl phosphonic acid, vinyl benzoic acid, maleic acid, fumaric acid, itaconic acid, 2-(meth)acrylamido 2-ethyl propanesulfonic acid, mono-2- ((meth)acryloyloxy)ethyl succinate and ammonium sulfatoethyl (meth)acrylate. Zwitterionic monomers include (meth)acryloyl oxyethylphosphoryl choline and betaines, such as [2- ((meth)acryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide. Quaternised amino monomers include (meth)acryloyloxyethyltri-(alklaryl)ammonium halides such as

(meth)acryloyloxyethyltrimethyl ammonium chloride.

Oligomeric and polymeric monomers include oligomeric and polymeric (meth)acrylic acid esters such as mono(alk/aryl)oxypolyalkyleneglycol(meth)acrylates and

mono(alklaryl)oxypolydimethyl-siloxane(meth)acrylates. These esters include: monomethoxy oligo(ethyleneglycol) mono(meth)acrylate, monomethoxyoligo(propyleneglycol)

mono(meth)acrylate, monohydroxy oligo(ethyleneglycol) mono(meth)acrylate, monohydroxy oligo(propyleneglycol) mono(meth)acrylate, monomethoxy poly(ethyleneglycol)

mono(meth)acrylate, monomethoxy poly(propyleneglycol) mono(meth)acrylate,

monohydroxy poly(ethyleneglycol) mono(meth)acrylate and monohydroxy

poly(propyleneglycol) mono(meth)acrylate. Further examples include: vinyl or allyl esters, amides or ethers of pre-formed oligomers or polymers formed via ring-opening

polymerisation such as oligo(caprolactam), oligo(caprolactone), poly(caprolactam) or poly(caprolactone), or oligomers or polymers formed via a living polymerisation technique such as poly(1 ,4-butadiene).

The corresponding allyl monomers to those listed above can also be used where

appropriate.

Examples of monofunctional monomers are: amide-containing monomers such as

(meth)acrylamide, N-(2-hydroxypropyl)methacrylamide, N,N'-dimethyl(meth)acrylamide, N and/or N'-di(alkyl or aryl) (meth)acrylamide, N-vinyl pyrrolidone, [3-((meth)acrylamido)propyl) trimethyl ammonium chloride, 3-(dimethylamino)propyl(meth)acrylamide, 3[N-(3- (meth)acrylamidopropyl)-N,N-dimethyl]aminopropane sulfonate,

methyl(meth)acrylamidoglycolate methyl ether and N-isopropyl(meth)acrylamide;

(Meth)acrylic acid and derivatives thereof such as (meth)acrylic acid, (meth)acryloyl chloride (or any halide), (alkyl/aryl)(meth)acrylate, functionalised oligomeric or polymeric monomers such as monomethoxy oligo(ethyleneglycol) mono(meth)acrylate, monomethoxy

oligo(propyleneglycol) mono(meth)acrylate, monohydroxy oligo(ethyleneglycol)

mono(meth)acrylate, monohydroxy oligo(propyleneglycol) mono(meth)acrylate.

monomethoxy poly(ethyleneglycol) mono(meth)acrylate, monomethoxy poly(propyleneglycol) mono(meth)acrylate, monohydroxy poly(ethyleneglycol) mono(meth)acrylate, monohydroxy poly(propyleneglycol) mono(meth)acrylate. glycerol mono(meth)acrylate and sugar mono(meth)acrylates such as glucose mono(meth)acrylate; vinyl amines such as aminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, diisopropylaminoethyl (meth)acrylate, mono-tbutylamino (meth)acrylate, morpholinoethyl(meth)acrylate, vinyl aryl amines such as vinyl aniline, vinyl pyridine, N-vinyl carbazole, vinyl imidazole, and monomers which can be post-reacted to form amine groups, such as vinyl formamide; vinyl aryl monomers such as styrene, vinyl benzyl chloride, vinyl toluene, cx-methyl styrene, styrene sulfonic acid and vinyl benzoic acid; vinyl hydroxyl monomers such as hydroxyethyl (meth)acrylate, hydroxy propyl (meth)acrylate, glycerol mono(meth)acrylate or monomers which can be post-functionalised into hydroxyl groups such as vinyl acetate, acetoxy styrene and glycidylmeth)acrylate; acid-containing monomers such as (meth)acrylic acid, styrene sulfonic acid, vinyl phosphonic acid, vinyl benzoic acid, maleic acid, fumaric acid, itaconic acid, 2- (meth)acrylamido 2-ethylpropanesulfonic acid and mono-2-((meth)acryloyloxy)ethyl succinate or acid anhydrides such as maleic anhydride; zwitterionic monomers such as (meth)acryloyl oxyethylphosphoryl choline and betainecontaining monomers, such as [2- ((meth)acryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide; quaternised amino monomers such as (meth)acryloyloxyethyltrimethyl ammonium chloride.

The corresponding allyl monomer, where applicable, can also be used in each case.

Functional monomers, that is monomers with reactive pendant groups which can be post or pre-modified with another moiety following polymerisation can also be used such as glycidyl (meth)acrylate, tri(alkoxy)silylalkyl (meth)acrylates such as

trimethoxysilylpropyl(meth)acrylate, (meth)acryloyl chloride, maleic anhydride, hydroxyalkyl (meth)acrylates, (meth)acrylic acid, vinylbenzyl chloride, activated esters of (meth)acrylic acid such as N-hydroxysuccinamido m(meth)acrylate and acetoxystyrene.

trimethoxysilylpropyl(meth)acrylate, (meth)acryloyl chloride, maleic anhydride, hydroxyalkyl (meth)acrylates, (meth)acrylic acid, vinylbenzyl chloride, activated esters of (meth)acrylic acid such as N-hydroxysuccinamido m(meth)acrylate and acetoxystyrene. When the monofunctional monomer is providing the necessary hydrophilicity in the copolymer, it is preferred that the monofunctional monomer is a residue of a hydrophilic monofunctional monomer, preferably having a molecular weight of at least 1000 Daltons.

Macromonomers (monomers having a molecular weight of at least 1000 Daltons) are generally formed by linking a polymerisable moiety, such as a vinyl or allyl group, to a preformed monofunctional polymer via a suitable linking unit such as an ester, an amide or an ether. Examples of suitable polymers include mono functional poly (alkylene oxides) such as monomethoxy [poly (ethyleneglycol) ] or monomethoxy [poly (propyleneglycol) ] , silicones such as poly (dimethylsiloxane) s, polymers formed by ring-opening polymerisation such as poly (caprolactone) or poly (caprolactam) or mono-functional polymers formed via living polymerisation such as poly (1 , 4-butadiene) .

Preferred macromonomers include monomethoxy [poly (ethyleneglycol) ] mono

(methacrylate) , monomethoxy [poly (propyleneglycol) ] mono (methacrylate) and mono (meth) acryloxypropyl-terminated poly (dimethylsiloxane) .

When the monofunctional monomer is providing the necessary hydrophilicity in the copolymer, it is preferred that G and/or J is a residue of a hydrophilic monofunctional monomer, preferably having a molecular weight of at least 1000 Daltons.

Hydrophilic monofunctional monomers contain hydrogen bonding and/or permanent or transient charges. Hydrophilic monofunctional monomers include (meth) acryloyi chloride, N- hydroxysuccinamido (meth) acrylate, styrene sulfonic acid, maleic anhydride, (meth) acrylamide, N- (2- hydroxypropyl) methacrylamide, N-vinyl pyrrolidinone, N- vinyl formamide, quaternised amino monomers such as (meth) acrylamidopropyl trimethyl ammonium chloride, [3- ( (meth) acrylamido) propyl] trimethyl ammonium chloride and (meth) acryloyloxyethyltrimethyl ammonium chloride, 3- [N- (3- (meth) acrylamidopropyl) -N, N- dimethyl] aminopropane sulfonate, methyl (meth) acrylamidoglycolate methyl ether, glycerol mono (meth) acrylate, monomethoxy and monohydroxy oligo (ethylene oxide) (meth) acrylate, sugar mono (meth) acrylates such as glucose mono (meth) acrylate, (meth) acrylic acid, styrene sulfonic acid, vinyl phosphonic acid, fumaric acid, itaconic acid, 2- (meth) acrylamido 2- ethyl propanesulfonic acid, mono-2- ( (meth) acryloyloxy) ethyl succinate, ammonium sulfatoethyl (meth) acrylate, (meth) acryloyi oxyethylphosphoryl choline and betaine- containing monomers such as [2- ( (meth) acryloyloxy) ethyl] dimethyl- (3- sulfopropyl) ammonium hydroxide. Hydrophilic macromonomers may also be used and include monomethoxy and monohydroxy poly (ethylene oxide) (meth) acrylate and other hydrophilic polymers with terminal functional groups which can be post-functionalised with a polymerisable moiety such as (meth) acrylate, (meth) acrylamide or styrenic groups.

Hydrophobic monofunctional monomers include C|_-2o alkyl (meth) acrylates (linear and branched and (meth) acrylamides, such as methyl (meth) acrylate and stearyl (meth) acrylate, aryl (meth) acrylates such as benzyl (meth) acrylate, tri (alkyloxy) silylalkyl (meth) acrylates such as trimethoxysilylpropyl (meth) acrylate, styrene, acetoxystyrene, vinylbenzyl chloride, methyl vinyl ether, vinyl formamide, (meth) acrylonitrile, acreolin, 1- and 2- hydroxy propyl (meth) acrylate, vinyl acetate, glycidyl (meth) acrylate and maleic acid. Hydrophobic macromonomers may also be used and include monomethoxy and monohydroxy poly (butylene oxide) (meth) acrylate and other hydrophobic polymers with terminal functional groups which can be post- functionalised with a polymerisable moiety such as (meth) acrylate, (meth) acrylamide or styrenic groups.

Hydrophilic monofunctional monomers include: (meth)acryloyl chloride, N- hydroxysuccinamido (meth)acrylate, styrene sulfonic acid, maleic anhydride,

(meth)acrylamide, N-(2-hydroxypropyl)methacrylamide, N-vinyl pyrrolidinone, N-vinyl formamide, quaternised amino monomers such as (meth)acrylamidopropyl trimethyl ammonium chloride, [3-((meth)acrylamido)propyl]trimethyl ammonium chloride and

(meth)acryloyloxyethyltrimethyl ammonium chloride, 3-[N-(3-(meth)acrylamidopropyl)-N,N- dimethyljaminopropane sulfonate, methyl (meth)acrylamidoglycolate methyl ether, glycerol mono(meth)acrylate, monomethoxy and monohydroxy oligo(ethylene oxide) (meth)acrylate, sugar mono(meth)acrylates such as glucose mono(meth)acrylate, (meth)acrylic acid, vinyl phosphoriic acid, fumaric acid, itaconic acid, 2-(meth)acrylamido 2-ethyl propanesulfonic acid, mono-2-((meth)acryloyloxy)ethyl succinate, ammonium sulfatoethyl (meth)acrylate, (meth)acryloyl oxyethylphosphoryl choline and betaine-containing monomers such as [2- ((meth)acryloyloxy)ethylj dimethyl-(3-sulfopropyl)ammonium hydroxide. Hydrophilic macromonomers may also be used and include monomethoxy and monohydroxy

poly(ethylene oxide) (meth)acrylate and other hydrophilic polymers with terminal functional groups which can be post-functionalised with a polymerisable moiety such as (meth)acrylate, (meth)acrylamide or styrenic groups.

Hydrophobic monofunctional monomers include C C₂o alkyl(meth)acrylates (linear and branched and (meth)acrylamides, such as methyl(meth)acrylate and stearyl(meth)acrylate, aryl(meth)acrylates such as benzyl(meth)acrylate, tri(alkyloxy)silylalkyl(meth)acrylates such as tri-methoxysilylpropyl(meth)acrylate, styrene, acetoxystyrene, vinylbenzyl chloride, methyl vinyl ether, vinyl formamide, (meth)acrylonitrile, acreolin, 1- and 2-hydroxy propyl(meth)acrylate, vinyl acetate, and glycidyl(meth)acrylate. Hydrophobic macromonomers may also be used and include monomethoxy and

monohydroxypoly(butylene oxide) (meth)acrylate and other hydrophobic polymers with terminal functional groups which can be post-functionalised with a polymerisable moiety such as (meth)acrylate, (meth)acrylamide or styrenic groups.

Responsive monofunctional monomers include (meth)acrylic acid, 2- and 4-vinyl pyridine, vinyl benzoic acid, N-isopropyl(meth)acrylamide, tertiary amine (meth)acrylates and

(meth)acrylamides such as 2-(dimethyl)aminoethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, diisopropylaminoethyl (meth)acrylate, mono-t-butylaminoethyl (meth)acrylate and N-morpholinoethyl (meth)acrylate, vinyl aniline, vinyl pyridine, N-vinyl carbazole, vinyl imidazole, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, maleic acid, fumaric acid, itaconic acid and vinyl benzoic acid. Responsive macromonomers may also be used and include monomethoxy and monohydroxy poly(propylene oxide) (meth)acrylate and other responsive polymers with terminal functional groups which can be post-functionalised with a polymerisable moiety such as (meth)acrylate, (meth)acrylamicie or styrenic groups.

The multifunctional monomer or brancher may comprise a molecule containing at least two vinyl groups which may be polymerised via addition polymerisation. The molecule may be hydrophilic, hydrophobic, amphiphilic, neutral, cationic, zwitterionic, oligomeric or polymeric. Such molecules are often known as cross-linking agents in the art and may be prepared by reacting any di- or multifunctional molecule with a suitably reactive monomer. Examples include di- or multivinyl esters, di- or multivinyl amides, di- or multivinyl aryl compounds, di- or multivinyl alk/aryl ethers. Typically, in the case of oligomeric or polymeric di- or multifunctional branching agents, a linking reaction is used to attach a polymerisable moiety to a di- or multifunctional oligomer or polymer. The brancher may itself have more than one branching point, such as T-shaped divinylic oligomers or polymers. In some cases, more than one multifunctional monomer may be used. When the multifunctional monomer is providing the necessary hydrophilicity in the copolymer, it is preferred that the multifunctional monomer has a molecular weight of at least 1000 Daltons.

The corresponding allyl monomers to those listed above can also be used where

appropriate.

Preferred multifunctional monomers include but are not limited to divinyl aryl monomers such as divinyl benzene; (meth)acrylate diesters such as ethylene glycol di(meth)acrylate, propyleneglycol di(meth)acrylate and 1 ,3-butylenedi(meth)acrylate; polyalkylene oxide di(meth)acrylates such as tetraethyleneglycol di(meth)acrylate, poly(ethyleneglycol) di(meth)acrylate and poly(propyleneglycol) di(meth)acrylate; divinyl (meth)acrylamides such as methylene bisacrylamide; silicone-containing divinyl esters or amides such as

(meth)acryloxypropyl-terminated poly(dimethylsiloxane); divinyl ethers such as

poly(ethyleneglycol)divinyl ether; and tetra- or tri-(meth)acrylate esters such as

pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate or glucose di- to penta(meth)acrylate. Further examples include vinyl or ally esters, amides or ethers of preformed oligomers or polymers formed via ring-opening polymerisation such as

oligo(caprolactam), oligo(caprolactone), poly(caprolactam) or poly(caprolactone), or oligomers or polymers formed via a living polymerisation technique such as oligo- or poly(1 ,4-butadiene). Macrocrosslinkers or macrobranchers (multifunctional monomers having a molecular weight of at least 1000 Daltons) are generally formed by linking a polymerisable moiety, such as a vinyl or aryl group, to a pre-formed multifunctional polymer via a suitable linking unit such as an ester, an amide or an ether. Examples of suitable polymers include di-functional poly(alkylene oxides) such as poly(ethyleneglycol) or poly(propyleneglycol), silicones such as poly(dimethylsiloxane)s, polymers formed by ring- opening polymerisation such as poly(caprolactone) or poly(caprolactam) or polyfunctional polymers formed via living polymerisation such as poly(1 ,4-butadiene).

Macrocrosslinkers or macrobranchers (multifunctional monomers having a molecular weight of at least 1000 Daltons) are generally formed by linking a polymerisable moiety, such as a vinyl or aryl group, to a pre-formed multifunctional polymer via a suitable linking unit such as an ester, an amide or an ether. Examples of suitable polymers include di-functional poly (alkylene oxides) such as poly (ethyleneglycol) or poly (propyleneglycol) , silicones such as poly (dimethylsiloxane) s, polymers formed by ring- opening polymerisation such as poly (caprolactone) or poly (caprolactam) or poly-functional polymers formed via living polymerisation such as poly (1 , 4-butadiene) .

Preferred macrobranchers include poly (ethyleneglycol) di (meth) acrylate, poly

(propyleneglycol) di (meth) acrylate, methacryloxypropyl-terminated poly (dimethylsiloxane), poly (caprolactone) di (meth) acrylate and poly (caprolactam) di (meth) acrylamide .

Hydrophilic branchers contain hydrogen bonding and/or permanent or transient charges. Hydrophilic branchers include methylene bisacrylamide, glycerol di (meth) acrylate, glucose di- and tri (meth) acrylate, oligo (caprolactam) and oligo (caprolactone) . Multi end- functionalised hydrophilic polymers may also be functionalised using a suitable polymerisable moiety such as a (meth) acrylate, (meth) acrylamide or styrenic group.

Hydrophobic branchers include divinyl benzene, (meth) acrylate esters such as

ethyleneglycol di (meth) acrylate, propyleneglycol di (meth) acrylate and 1 ,3- butylene di (meth) acrylate, oligo (ethylene glycol) di (meth) acrylates such as tetraethylene glycol di (meth) acrylate, tetra- or tri- (meth) acrylate esters such as pentaerthyritol tetra (meth) acrylate, trimethylolpropane tri (meth) acrylateand glucose penta (meth) acrylate . Multi end- functionalised hydrophobic polymers may also be functionalised using a suitable

polymerisable moiety such as a (meth) acrylate, (meth) acrylamide or styrenic group.

Branchers include: methylene bisacrylamide, glycerol di(meth)acrylate, glucose di- and tri(meth)acrylate, oligo(caprolactam) and oligo(caprolactone). Multi end-functionalised hydrophilic polymers may also be functionalised using a suitable polymerisable moiety such as a (meth)acrylate, (meth)acrylamide or styrenic group.

Further branchers include: divinyl benzene, (meth)acrylate esters such as ethyleneglycol di(meth)acrylate, propyleneglycol di(meth)acrylate and 1 ,3-butylene di(meth)acrylate, oligo(ethylene glycol) di(meth)acrylates such as tetraethylene glycol di(meth)acrylate, tetra- or tri-(meth)acrylate esters such as pentaerthyritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate and glucose penta(meth)acrylate. Multi end-functionalised hydrophobic polymers may also be functionalised using a suitable polymerisable moiety such as a (meth)acrylate, (meth)acrylamide or styrenic group.

Branched polymers are polymers comprising at least two non-linear linked polymer chains, i.e. with a structure comprising a main chain and at least one further polymeric chain branching off the main chain.

Branched polymers of the invention are polymer molecules which are engineered to have a finite size. Branched polymers are soluble addition polymers and include statistical, graft, gradient and alternating branched polymers. Cross-linked polymers, unlike branched polymers of the invention, generally grow while monomers are available and can be arbitrarily large. Cross-linked polymers generally form gels.

Branched polymers according to the present invention are obtainable by an addition polymerization process. An addition polymerisation process may be a conventional free- radical polymerisation method. A branched copolymer may be prepared using a free-radical polymerisation method by polymerising a monofunctional monomer with a multifunctional monomer in the presence of a chain transfer agent and a free radical initiator. Preferably, the molar ratio of the multifunctional monomer to the monofunctional monomer is between 0.0005 and 1 .

Multifunctional responsive polymers may also be functionalised using a suitable

polymerisable moiety such as a (meth)acrylate, (meth)acrylamide or styrenic group such as polypropylene oxide) di(meth)acrylate.

Thus, L is a residue of a multifunctional monomer as described above. The copolymer must contain a multifunctional monomer. In other words, I is≥ 0.05 in formula (I). It is preferably 0.05 to 50, more preferably 0.05 to 40, particularly 0.05 to 30 and especially 0.05 to 15.

It is preferred that R and R' in formula (I) each independently represent a hydrogen atom or an optionally substituted lower alkyl group (a Ci_-4 alkyl group).

X and X' each independently represent a terminal group derived from a termination reaction. During conventional radical polymerisation, some inherent and unavoidable termination reactions occur. Common termination reactions between free-radicals are typically bimolecular combination and disproportionation reactions which vary depending on the monomer structure and result in the annihilation of two radicals. Disproportionation reactions are thought to be the most common, especially for the polymerisation of (meth) acrylates, and involve two dead primary chains, one with a hydrogen terminus (X or X = H) and the other with a carbon-carbon double bond (X or X = -C=CH₂). When the termination reaction is a chain transfer reaction, X or X is typically an easily abstractable atom, commonly hydrogen. Thus, for instance, when the chain transfer agent is a thiol, X and/or X can be a hydrogen atom.

As will be apparent from formula (I), m + I equals the number of polymerisable groups in L and n is the total number of repeat units in the copolymer. Preferably, m is 1 to 6, more preferably 1 to 4.

A preferred branched pH responsive copolymer may comprise at least two ethyleneically monounsaturated monomers, wherein one of the ethyleneically monounsaturated monomers is (meth)acrylic acid or a (meth)acrylic acid derivative, and wherein one of the ethyleneically monounsaturated monomers is a poly(ethyleneglycol)(meth)acrylate or a

poly(ethyleneglycol) derivative. A preferred branched pH responsive copolymer according to the invention may be a branched copolymer comprising residues of the ethyleneically monounsaturated monomer methacrylic acid (MAA) and polyethylene glycol methacrylate (PEGMA), the ethyleneically polyunsaturated monomer ethyleneglycol di methacrylate (EGDMA), and the chain transfer agent dodecanethiol (DDT). pH responsive copolymers may be described using the following nomenclature:

(monofunctional monomer G)_g (monofunctional monomer J)_j (multifunctional monomer L)i (chain transfer agent)_d wherein the values in subscript are the molar ratios of each constituent normalised to give the monofunctional monomer value as 100, that is, g + j = 100. The degree of branching is denoted by I. The subscript d refers to the molar ratio of the chain transfer agent.

In one embodiment, the monofunctional monomer G may be PEGMA, the monofunctional monomer J may be MAA, the multifunctional monomer L may be EGDMA, and the chain transfer agent may be DDT.

In a preferred embodiment, the polymer may be

PEGMAg/MAA_rEGDMA|-DDT_d wherein g may be 4-6, j may be 94-96, I may be 10 and d may be 10. In a more preferred embodiment, the polymer may be PEGMA5/MAA95-EGDMA10-DDT10.

Preferably, the polymer has the formula MFM₁₀o-EDGMA₁₀-DDT₁₀, wherein MFM is a monofunctional monomer and is nominally set to a molar value of 100, and EGDMA is present in molar equivalent of 10 relative to the total MFM. Preferabyl, the MFM may be methacrylic acid, poly(ethylene glycol) methacrylate, or mixtures thereof.

The initiator is a free-radical initiator and can be any molecule known to initiate free-radical polymerisation such as azo-containing molecules, persulfates, redox initiators, peroxides, benzyl ketones. These may be activated via thermal, photolytic or chemical means.

Examples of these include but are not limited to 2, 2' -azobisisobutyronitrile (AIBN) , azobis (4-cyanovaleric acid) , benzoyl peroxide, cumylperoxide, 1-hydroxycyclohexyl phenyl ketone, hydrogenperoxide/ascorbic acid. Iniferters such as benzyl- N, N-diethyldithiocarbamate can also be used. In some cases, more than one initiator may be used. The initiator may be a macroinitiator having a molecular weight of at least 1000 Daltons. In this case, the macroinitiator may be hydrophilic or hydrophobic. Preferably, the residue of the initiator in a free-radical polymerisation comprises 0 to 5% w/w, preferably 0.01 to 5% w/w and especially 0.01 to 3% w/w, of the copolymer based on the total weight of the monomers.

The use of a chain transfer agent and an initiator is preferred. However, some molecules can perform both functions.

The non-covalent bond in the branched polymer may be a hydrogen bond. The non- covalent bond may be formed by Van der Waal forces, by ionic interactions, or by pi-pi interactions.

In a preferred embodiment, the copolymers comprise a functionality that can hydrogen-bond with each other in response to external stimulus. This hydrogen-bonding requires the presence of hydrogen-bonding donor and acceptor groups of the branched copolymer. An example of a branched copolymer capable of hydrogen bonding comprises ethyleneglycol and meth(acrylic) acid residues.

The copolymer may be responsive to a change in external stimulus. Preferably, the stimulus is a change in pH. The change in pH allows a change in the properties, for example the rheological properties, of the polymer in an aqueous solution. For example, in their neutral form, the polymers are dehydrated, compact and non-hydrophobic. In their ionic form, they are hydrated, swollen and hydrophilic. The change in pH allows the ionisation or

deionisation of the moieties on the polymer by protonating or deprotonating them. For example, carboxylic moieties may be deprotonated at high pH, leading to an increased electrostatic interaction between the branches. When the polymer is covalently bound to a particle, this allows an electrostatic interaction at high pH to occur between the positively charged particle surface.

When the polymer has structure PEGMA5/MAA₉5-EGDMA1₀-DDT1₀, at pH 9.68, the degree of protonation (a) is 0. No MAA residues in the BCS are protonated at pH 9.68 or above. There is no aggregation of the BCS. At pH 6.46, the degree of protonation (a) is 0.5, indicating that 50% of the MAA residues in the BCS are protonated at pH 6.46. At pH 3.88, the degree of protonation (a) is 1 , indicating that 100% of the MAA residues in the BCS are protonated at pH 3.88 or below, which means full aggregation of the BCS.

The aqueous solvent may be water or a mixed solvent system comprising water and one or more solvents (which are preferably miscible with water). Preferably, the solvent is water. An aqueous solvent enables the full formation of hydrogen bonds to obtain a change in the rheological properties of the functionalised material. Small amounts of organic solvent, for example methanol or ethanol, may be used.

The pH regulating agent is any substance which when dissolved in aqueous solution alters the pH of the solution. It may be an acid or a base or a compound which dissolves in aqueous solution to form an acid or base. Preferably, the pH regulating agent is an acid or a compound which dissolves to form an acid. In a preferred embodiment, the pH regulating agent may be a sugar acid or a compound which dissolves in aqueous solution to form a sugar acid. The ink composition may comprise 0.1 to 10 wt/v%, preferably 0.2 to 5 wt/v% pH regulating agent.

A more preferred embodiment comprises glucono-5-lactone as a pH regulating agent. The glucono-5-lactone dissolves to gluconic acid and then the gluconic acid drops the pH. This leads to a homogenous drop of pH. Preferably, the ink composition comprises between 0.5 to 10 wt/v% of glucono-5-lactone. Preferably, the ink composition comprises 1 , 2, 3, 4, 5 wt/v% glucono-5-lactone.

Preferably, the inorganic particle is Al₂0₃ and the composition comprises 2 wt/v% glucono-δ- lactone.

Preferably the inorganic particle is SiC and the composition comprises 2 to 5 wt/v% glucono- δ-lactone.

Preferably, the pH of the composition is less than the pKa of the ionisable groups in the polymer.

The ink compositions may be prepared by mixing an aqueous suspension of a functionalised inorganic particle with an aqueous solution of BCS. The pH of the resulting mixture may be adjusted to at least 7 and the mixture is stirred for at least 10 minutes and up to 4 hours. In one embodiment, the pH is adjusted to at least 8. In one embodiment, the mixture may be stirred for at least 30 minutes. The mixture may be stirred for up to 60 minutes.

Octanol may be added to the BCS/functionalised inorganic particle/pH regulating agent suspension. This eliminates bubbles in the suspension and provides a homogenous paste.

To further eliminate bubbles, the BCS/functionalised inorganic particle/pH regulating agent suspension may be conditioned using a planetary mixer. In order to print self-supporting 3D structures, an ink must have well controlled viscoelastic properties that enable a stable flow through the deposition nozzle and then "set" immediately retaining the shape. The inks must have a shear thinning behaviour (viscosity decreases with shear stress) so they can be easily injected through a small nozzle. At the same time, they should exhibit a solid-like (G'>G") behaviour, and high values of viscoelastic properties (storage modulus, G').

As the pH of the composition decreases upon the addition of a pH regulating agent, the functionalised material forms a weak gel-like network. Network formation is established where the storage (G') and loss (G") moduli are equal. As the pH is further decreased, more links are established until the hydrogen bonds create a strong network.

Preferably, the pH of the composition is less than 6.

The storage modulus is measured by oscillation measurement, using a parallel plate with 40 mm diameter. The oscillation measurement is performed with a solvent trap cover to prevent solvent evaporation. The oscillation settings are: amplitude sweep at 0.1 Hz, under strains from 0.0001 up to 1000.

Preferably, the storage moduli G' is at least 10kPa at strains below 1 % (measured at 0.1 Hz with 40mm parallel plate), preferably at least 50 kPa, 100 kPa, 250 kPa, 500 kPa, 1000kPa.

For Al₂0₃ particles, absorption of BCS occurs via 3 mechanisms: 1) the interactions of the hydrophobic chain ends (DDT) on the surfaces; 2) the electrostatic interaction between the carboxylic anions in the MAA residues (COO^") with the positively charged particle surfaces; and 3) the establishment of chemical covalent bonding between the carboxylic residues and the metal oxides on the surface of the particles.

SiC has negatively charged surfaces at pH>2. The attachments may take place through the interactions of the hydrophobic ends of the polymer and the surface of the particle as well as the establishment of chemical covalent bonding between the carboxylic residues and the metal oxides on the surface of the particles. The negatively charged surfaces will repel the negative carboxylic groups in BCS promoting a change in conformation of the BCS layer on the surface of the particle. These repulsions are likely to lead to the formation of loops and comb-like structures around the particles which has an effect on the rheological behaviour of SiC suspensions. In another embodiment, the invention provides an ink composition precursor which may comprise the formulation of the ink composition as defined in any embodiment of the first aspect of the invention absent the pH regulating agent.

There is also provided a method of preparing an ink composition which may comprise the steps of preparing a functionalised material according to the first aspect of the invention by mixing an aqueous suspension of the inorganic particle with an aqueous solution of a pH responsive polymer; and adding a pH regulating agent.

Preferably, octanol is added after the first step.

Preferably, the pH of the composition is less than 5.

The invention provides a method for printing an article comprising the steps of printing the ink composition to form an article; drying the printed article; and optionally curing the printed article. The article may be a ceramic article.

The method may further comprise the step of preparing an ink composition according to the first aspect.

Rheological properties of an ink composition according to the invention enable its use for the production of three dimensional articles. The ink composition is preferably for use in 3D printing and is of particular use in extrusion filament printing. The ink composition may also be used for coating, injection moulding and tape casting.

Preferably, the printing may be extrusion filament printing. In extrusion filament printing, an ink composition is printed by extrusion of a filament of the ink composition through a printing nozzle. Printing of thin filaments is possible and the printing nozzle may have a diameter of, for example, 100-1000 μηι, preferably 100-500 μηι (giving a corresponding printed filament diameter). Filaments can be built up in layers to provide a 3D printed article. The ink may be used for continuous extrusion.

The printing method may be a 3 dimensional printing method, to produce a 3 dimensional printed article.

Filaments can be built up in layers to provide a 3D printed article. The ink may be used for continuous extrusion.

The drying may be according to methods known in the art. Drying may be in a convective oven with or without humidity control. Drying may also take place in a microwave oven. The drying step may comprise lyophilising the printed article. The drying step may comprise lyophilising the printed article. Freeze-drying, or lyophilising helps avoid shrinkage and allows the formation of 3D objects with smooth surfaces while preserving fine printing features down to the low micrometre range.

The curing may comprise the step of heating the ceramic article. The heating temperature may be between 200-2400 °C. Preferably, the heating may be at a temperature of between 900-1900°C. The heating may be under a reducing atmosphere.

Al₂0₃ articles may be cured by sintering without pressure in air at a temperature between 1400 and 1800 °C. Drying time may be up to 24 hours.

SiC articles may be cured by sintering under vacuum and reducing atmosphere at a temperature between 1700 and 2400 °C. Drying time may be up to 24 hours.

The present invention provides a functionalised material comprising an inorganic particle functionalised with a pH responsive polymer.

Preferably, the functionalised material comprises covalent binding between the inorganic material pH responsive polymer.

Preferably, the polymer is as defined in the first aspect of the invention.

Preferably the inorganic particle is Al₂0₃, preferably the inorganic particle is SiC.

The present invention provides a method of preparing the functionalised material of the sixth aspect, comprising the steps of mixing an aqueous suspension of an inorganic particle with an aqueous suspension of a branched polymer, at a pH of at least 7, preferably at least 8, or at least 9.

Preferably, the inorganic particle suspension comprises 10-50 vol% inorganic particles in an aqueous solution.

The present invention provides an aqueous suspension comprising the functionalised material as described herein.

The present invention will now be explained in more detail by reference to the following non- limiting examples:

Examples

The pH responsive polymer, with a ratio of ethylene glycol(EG) to methacrylic acid residues (MAA) is 1.1 : 1 , was synthesised as follows: A mixture of PEGMA (6.732 g, 6 mM, 5 molar equivalents), MA (10.000 g, 1 16 mM, 95 molar equivalents), ethyelene glycol dimethacrylate (EGDMA, 2.42 g, 12 mM, 10 molar

equivalents), and 1-dodecanethiol (DDT, 2.472 g, 12 mM, 10 molar equivalents) was degassed. Ethanol (200 ml) was degassed separately and added to the monomer mixture. After heating to 70 °C, the polymerization was initiated by addition of AIBN (200 mg) and was left stirring for 48 h. Ethanol was then removed by distillation and the polymer was washed with cold diethyl ether and dried.

A pH responsive polymer (BCS) stock solution with a concentration of 8 wt/v% was prepared in distilled water at pH 12.

The inorganic particles were functionalised by mixing a solution of pH responsive BCS (1-2 wt/v%) at pH 8 with an aqueous suspension of inorganic particles. The functionalization of Al₂0₃ particles yielded stable suspensions with up to 43 vol% aluminium oxide. The functionalization of SiC particles yielded stable suspensions with up to 35 vol% silicon carbide. The inorganic particles may be conditioned by adding up to 2.5 wt% octanol to eliminate bubbles.

For AI2O₃ particles, the adsorption behaviour was studied by measuring the sulphur concentration (directly correlated with BCS concentrations) in the supernatants after centrifugation by inductive coupled plasma atomic emission spectroscopy (ICP-AES).

Figure 1 (a) shows the adsorption data of BCS molecules onto positively charged Al₂0₃ surfaces obtained by measuring the sulphur concentrations of supernatants. The amount of molecules increases with the initial concentration of BCS (c₀). The adsorption isotherm shows that the amount of BCS attached to the surface increases with the initial BCS concentrations of the liquid. The experimental data confirm the BCS attachment to the ceramic surfaces. The closest fit to a theoretical model appears to be the Langmuir model (see Figure 1 (b)). Figure 1 (b) shows the Langmuir isotherm fit of the adsorption data (R² = 0.94237).

The BCS attachment to the SiC surfaces was evaluated using BCS molecules functionalised with rhodamine. The results show that the BCS molecules interact with the SiC surfaces. The results prove that the BCS molecules interact with the SiC surfaces (see Figure 2). The amount of BCS molecules attached to the surface depends on the intitial (C₀) and equilibrium (c_ep) concentrations. At BCS concentrations of up to 3 wt/v% the amount of molecule adsorbed on the surface increases. This behaviour can be described by a

Langmuir isotherm curve when plotted against the BCS concentrations in equilibrium (c_eq). However, higher initial BCS concentrations (4-5 wt/v%) lead to a drop in BCS adsorption and do not follow a theoretical model.

Figure 2 shows the BCS/SiC interactions. Figure 2(a) shows the attachment related to initial concentration. Initially the adsorption increases with initial BCS concentrations in the liquid, however above 3 wt/v%, the amount of adsorbed molecules gradually decreases with higher BCS concentrations. The region highlighted in orange was used to determine the adsorption isotherms. Figure 2(b) shows the adsorption data vs. the initial concentration for the region highlighted in orange in (a). The red dashed line shows the max adsorption values. Figure 2(c) shows the Langmuir adsorption isotherm of BCS molecules onto SiC surfaces (R²= 0.9739). From this fitting we obtained the maximum amount that could be adsorbed (rmax = 0.1071 μηιοΙ that corresponds to 30 BCS mg per g of SiC).

Figure 3 shows the kinetic of the self-assembly process, showing the change on the viscoelastic properties G', and G" (storage and loss modulus respectively) for a 43vol% Al₂0₃ with BCS (1wt/v%) and 1 wt/v% G5L. Initially, the suspension has a liquid like behaviour (G">G'), and G', G" values around 1 GPa. The activation of the hydrogen bonds increases the magnitude of these properties up to values close to 1000Pa, 30 minutes after lowering the pH.

The ink compositions comprising functionalised inorganic particles were prepared by mixing a suspension of the functionalised inorganic particles (1 or 2 wt/v%) at pH 8 with glucono-δ- lactone (0.5 to 2 wt/v% related to the suspension volume) while gently stirring under vacuum.

The suspensions are subjected to ball milling for 24 hours.

Afterwards, the suspensions are conditioned by adding 2.5wt% of 1-octanol under continuous stirring and light vacuum to eliminate the bubbles. The ceramic/BCS ratio is 75/1 for the optimized Al₂0₃ ink (with 2 w/v G5L to trigger self-assembly) and around 58/1 for the SiC (with amounts between 2 and 5w/v G5L). The pH trigger is added under continuous stirring and left at least for 2 hours in order to assemble the ceramic particles into a 3D network, thus leading to very stable ceramic "gels". Before printing, the ceramic ink is conditioned by using a planetary mixer (Thinky mixer ARE 250) for approx. 15 mins at 2000 rpm in mixing mode and another 15 mins at 2000 rpm in de-foaming mode. This step eliminates bubbles and provides a very homogenous paste, what is critical to provide a continuous flow during the printing process. Figure 4 shows the rheology of the Al₂0₃ ink compositions. Figure 4(a) shows the viscoelastic properties of the composition absent of pH-regulating agent compared with fresh ink (60 minutes after addition of the pH-regulating agent) and with the soft and optimum inks. Figure 4(b) shows that fresh ink with values of G' below 2kPa are not adequate for 3D printing.

It is possible to print inks with different textures; for example, "soft" inks can flow through smaller nozzles better than the "optimum" ink (Figure 4). The "optimum" ink has the texture of a very thick cream that completely retains the shape of the nozzle and provides strong filaments during the printing process. Both inks have the same flow behaviour (shear thinning) but differ on the values of the viscoelastic properties. The "soft" ink has higher G', and G" values (around 20kPa) than the original suspension (below 10Pa), but much lower than the "optimum" ink (around 200kPa). There are also differences on the LVR (liner viscosity region); strong inks have a smaller LVR at lower strains (Figure 4).

Suspensions of magnetite and perovskites structure materials functionalised with BCS as described above have been prepared in an analogous process as set out above. Addition of glucono-5-lactone affected the rheology in an analogous manner to the example above.

While preferred embodiments of the present invention have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.

The work leading to this invention has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n° 301909.

Claims

1. An ink composition comprising a functionalised material, a pH regulating agent, and an aqueous solvent, wherein the functionalised material comprises inorganic particles functionalised with a pH responsive polymer.

2. The ink composition according to claim 1 , wherein the functionalised material comprises covalent binding between the inorganic particles and the pH responsive polymer.

3. The ink composition according to any one of claims 1 or 2, wherein the inorganic particles comprise Al₂0₃, SiC, Si0₂, Ti0₂, AI₂0₃2Si0₂, solid oxides with perovskites structure, iron oxides or magnetite.

4. The ink composition according to claim 3, wherein the composition comprises up to 43 vol% Al₂0₃.

5. The ink composition according to claim 3, wherein the composition comprises up to 35 vol% SiC.

6. The ink composition according to any one of claims 1-5, wherein the polymer is a copolymer.

7. The ink composition according to any preceding claim, wherein the pH responsive copolymer comprises

(a) at least one ethyleneically monounsaturated monomer;

(b) at least one ethyleneically polyunsaturated monomer;

(c) at least one residue of a chain transfer agent;

(i) at least one of (a) to (c) comprises a hydrophilic residue;

(ii) at least one of (a) to (c) comprises a hydrophobic residue;

(iii) at least one of (a) to (c) comprises a moiety capable of forming a non- covalent bond with at least one of (a) to (c).

8. The ink composition according to any of claims 1 to 7, wherein the pH responsive copolymer may comprise the general formula (I)

in which

X and X' each independently represent a terminal group derived from a termination reaction; g, j and I represent the molar ratio of each residue normalised so that g + j = 100, wherein g and j each independently represent 0 to 100, and I is≥ 0.05; and m and n are each independently≥ 1 ; at least one of E, E', G, J and L is a hydrophilic residue; and at least one of E, E', G, J and L is a hydrophobic residue.

The ink composition according to any one of claims 1-6, wherein the pH responsive er is a branched pH responsive polymer comprising at least one residue of a chain transfer agent, optionally wherein the polymer further comprises poly(ethylene glycol) and methacrylic acid residues, optionally wherein the polymer further comprises ethylene glycol dimethacrylate residues.

10. The ink composition according to any one of claims 6-9, wherein the polymer has the formula PEGMAg/MAA_rEGDMA|-DDT₁₀, wherein g + j = 100 and EGDMA is 10 molar equivalents relative to PEGMA/MAA.

11. The ink composition according to claim 10, wherein g is 4-6, j is 94-96, 1 is 10 and d is 10.

12. The ink composition according to claim 6, wherein the polymer has a composition according to formula (II)

(II) wherein G' is poly(ethylene glycol), J' is carboxyl, E and E' each independently represent a residue of a chain transfer agent or an initiator, X and X' each independently represent a terminal group derived from a termination reaction, g, j and I represent the molar ratio of each residue normalised so that g + j = 100, wherein g and j each independently represent 0 to 100, and I is≥ 0.05; and m and n are each independently≥ 1 .

13. The ink composition according to any one of claims 1-4, or 6-12, wherein the Al₂0₃ to polymer ratio is between 15: 1 and 75: 1.

14. The method according to any one of claims 1-3, or 5-12, wherein the SiC to polymer ratio is between 15: 1 and 63: 1.

15. The ink composition according to any preceding claim, wherein the storage moduli G' is at least 10kPa at strains below 1 % (measured at 0.1 Hz with 40mm parallel plate).

16. A method of preparing an ink composition comprising the steps of

(a) preparing a functionalised material according to any one of claims 1- 13 by mixing an aqueous suspension of the inorganic particle with an aqueous solution of a pH responsive polymer;

(b) adding a pH regulating agent.

17. The method according to claim 16, wherein octanol is added after step (a).

18. The ink composition according to any one of claims 1-15, or the method according to claim 16-17, wherein the pH regulating agent is an acid, base, or substance which dissolves in an aqueous solution to form an acid or base.

19. The ink composition or the method according to claim 18, wherein the pH regulating agent is glucono-5-lactone.

20. The ink composition or the method according to any of claims 18-19, wherein the composition comprises between 0.5 to 10 wt/v% of glucono-5-lactone.

21. The ink composition or the method according to claim 20, wherein the inorganic particle is Al₂0₃ and the composition comprises 2 wt/v% glucono-5-lactone.

22. The ink composition or the method according to claim 20, wherein the inorganic particle is SiC and the composition comprises 2 to 5 wt/v% glucono-5-lactone.

23. The ink composition according to any one of claims 1-15, or 18-22 or the method according to any one of claims 16-20, wherein the pH of the composition is less than the pKa of the ionisable groups in the polymer.

24. The ink composition according to any one of claims 1-15, or 18-22, or the method according to any one of claims 16-20, wherein the pH of the composition is less than 5.

25. Use of a composition according to any one of claims 1-15, or 18-22, as an ink for 3D printing.

26. A method of printing an inorganic article comprising the steps of

(a) preparing an ink composition according to any one of claims 1-15, or 18-22 to form an article;

(b) drying the printed article; and optionally (c) curing the printed article.

27. The method according to claim 26, wherein the method further comprises the step of preparing an ink composition according to any one of claims 1-15, or 18-22, prior to step (a).

28. The method according to any one of claims 26-27, wherein the drying comprises lyophilising the printed article.

29. The method according to any one of claims 26-28, wherein the inorganic article comprises Al₂0₃, and wherein the curing step comprises sintering without pressure in air at a temperature between 1400 and 1800°C, optionally for up to 24 hours.

30. The method according to any one of claims 26-28, wherein the inorganic article comprises SiC, and wherein the curing step comprises sintering under vacuum and reducing atmosphere at a temperature between 1700 and 2400°C, optionally for up to 24 hours.

31. The method according to any of claims 26-30, wherein the printing is extrusion filament printing.

32. The method according to any one of claims 26-31 , wherein the printing is a three- dimensional printing method, to produce a three dimensional printed article.

33. An article obtainable by printing a composition according to any one of claims 1-15, 18-22 or as produced by a method of any one of claims 26-31.

34. A functionalised material comprising an inorganic particle functionalised with a pH responsive polymer.

35. The functionalised material according to claim 34, wherein the inorganic particle is covalently functionalised with a pH responsive polymer.

36. The functionalised material according to any one of claims 34-35, wherein the inorganic particles Al₂0₃, SiC, Si0₂, Ti0₂, AI₂0₃2Si0₂, ceramic solid-oxides with perovskite structure, iron oxides or magnetite.

37. The functionalised material according to claim 36, wherein the inorganic particle is Al₂0₃.

38. The functional material according to claim 36, wherein the inorganic particle is SiC.

39. The method according to any one of claims 36-37, wherein the Al₂0₃ to polymer ratio is between 15: 1 and 75:1.

40. The method according to any one of claims 36 or 38, wherein the SiC to polymer ratio is between 15: 1 and 63: 1.

41. The functionalised material according to any one of claims 34-40, wherein the polymer is as defined in claims 6-15.

42. A method of preparing the functionalised material of any one of claims 34-40, comprising the steps of mixing an aqueous suspension of an inorganic particle with an aqueous suspension of a pH responsive polymer, at a pH of at least 7.

43. The method according to claim 42, wherein the inorganic particle suspension comprises 0.5-5 wt% inorganic particles in an aqueous solution.

44. The method according to any one of claims 42-43, wherein the Al₂0₃ to polymer ratio is between 15/1 and 75/1.

45. The method according to any one of claims 42-43, wherein the SiC to polymer ratio is between 15/1 and 63/1.

46. An aqueous suspension comprising the functionalised material according to any one of claims 34-40.

47. A composition, method, use or material as substantially described herein with reference to or as illustrated in one or more of the examples or accompanying figures.