GB2579687A

GB2579687A - Composition and method

Info

Publication number: GB2579687A
Application number: GB1906902.0A
Authority: GB
Inventors: Guhl Dieter; Dobrowolska Anna
Original assignee: Keeling and Walker Ltd; Inovink Ltd
Current assignee: Keeling and Walker Ltd; Inovink Ltd
Priority date: 2019-05-16
Filing date: 2019-05-16
Publication date: 2020-07-01
Anticipated expiration: 2039-05-16
Also published as: GB2579687B; GB201906902D0

Abstract

An additive for inkjet printing compositions comprises a near infrared absorbing material having a surface which has been modified by at least two different surface modifying agent. A method of preparing the additive comprises contacting particles of a near infrared absorbing material with first and second surface modifying agents. Use of the additive in an inkjet printing composition for a high speed sintering process is also claimed. The inkjet printing compositions may be used in the formation of three-dimensional structures. The near infrared absorbing material may be selected from doped tin oxides, doped indium oxides, alkali metal tungsten oxides, rare earth hexaborides, alkali and/or transition metal (poly)phosphates, alkali or alkaline earth copper silicates, and transparent inorganic conductive oxides, especially antimony tin oxide, indium tin oxide, caesium tungsten oxide, and copper hydroxyphosphate. The first modifying agent may be an organosilane or organophosphorus compound and the second modifying agent may be a polymer comprising ester groups.

Description

Composition and Method 3D printing is a rapidly developing area for rapid prototyping applications. Within this technology sintering of polymer based powders is an emerging area. Structures are generated by laser radiation to selectively sinter polymer particles together to form individual parts or by illumination of sensitive materials.

An alternative route is the use of infrared (IR) lamps to fuse an entire layer of polymer particles. Details of this process are described for example by Ellis, A., Noble, C., Hartley, L., Lestrange, C., Hopkinson, N., & Majewski, C. (2014). Materials for high speed sintering. Journal of Materials Research, 29(17), 2080-2085. doi:10.1557/jmr.2014.156. In order to generate a pattern on the layer of polymer particles several options are available.

One option uses ink-jet printing of an infrared absorbing ink onto the polymer particle layer.

The structure is printed onto the particles. Afterwards the IR lamp irradiates the area in such a way that only those polymer particles which have absorbed the infrared absorbing ink are fused. Particles which have not been covered with the IR absorbing ink will not sinter and can be removed later. The build-up of a defined structure is achieved by repeated cycles of powder layer, ink jet printing the structure and subsequent fusing by IR radiation.

This process has been described as "High Speed Sintering". Currently the process uses an inkjet ink which contains carbon black as an infrared absorbing agent. Carbon black is a very well known infrared absorbing material. However a disadvantage of using carbon black at the loadings currently required means that only black or dark-grey coloured sintered parts can be achieved. It is not possible to provide colourless nor clean other coloured pieces.

Typical inkjet inks for high speed sintering processes are described in WO 2017/188966 and WO 2017/188965. These documents describe the composition and requirements for an ink being used in an inkjet printer for submitting structural information onto a polymer bed.

Although the possibility of using other NIR absorbers is mentioned only data for carbon black and carbon black based formulations is provided.

WO 201 7/1 88 961 describes the use of a photoluminescent inkjet ink comprising photoluminescent materials and a fusing agent comprising carbon black, near infrared absorbing pigments, tungsten or molybdenum bronze or metal nanoparticles.

WO 2017/180164 discloses the combination of a conductive fusing ink comprising metal particles and a second fusing ink comprising carbon black or other near infrared absorbing pigments including tungsten and molybdenum bronzes or metal nanoparticles used in structuring polymeric powder beds for generation of 3D structures.

These ink compositions of the prior art appear to be limited to aqueous systems.

It would be desirable to provide an inkjet printing composition, which contains a low coloured, highly efficient NIR absorber to allow the sintering of colourless or light coloured parts, and which could be used in solvent based as well as aqueous systems.

Although mentioned as theoretical possibility, the current state of the art does not provide any examples of non carbon-black based inkjet inks formulated in the same way as carbon black inks and which fulfil the requirements of the 3D rapid sintering process.

WO 2007/044106 discloses an ink comprising an IR absorptive pigment having low visible colour and is based on a tin oxide containing a transition metal or a transition metal oxide. A UV curable matrix carrier is also described. This document refers in particular to the use of antimony tin oxide having a particle size of 1 to 20 pm, and the examples show that a concentration of 50% antimony tin oxide must be included in the ink in order to achieve a sufficient result. The type of antimony doped tin oxide described in this document is coarse, coloured and nontransparent.

US2018/0016460 describes a solvent based dispersion comprising infrared absorbing tungsten oxide particles in petroleum based solvents as IR absorbing materials for acrylic resins. The dispersion includes a fatty acid dispersant which is soluble in the petroleum solvent. High absorption in the infrared range is claimed, but no data is provided and a printing process is not described.

Inkjet printing inks are well known and widely established. Typical compositions are described for example in the book "Inkjet based Micromanufacturing" by Brand, Fedder, Hierold et al, Wiley-VCH, 2012. They exist in numerous colours and modifications, but they cannot be used for the high speed sintering process, because usually they have only weak or no absorption in the near infrared light area, which is typically defined as the wavelength range between 700 to 2500 nm.

The basic principle of the high speed sintering process is based on the effect that the ink transfers a NIR absorbing agent onto a polymeric powder bead, which is afterwards irradiated by infrared light. The NIR absorbing agent convert the light into heat, which is used to melt the polymer particles. As a result the polymer powder particles fuse or sinter only in those areas, which had previously been printed with the NIR absorbing ink.

Building up a 3 D polymer structure by this process means that the NIR sintering agent is incorporated into the polymeric structure. In order to minimise the influence of the NIR absorbing material on the polymeric part as much as possible highly effective absorbers are required. These provide a maximum radiation to heat conversion at a minimum concentration.

Additionally the colour of the NIR absorber should be as neutral as possible to enable the system to print transparent, white or coloured structures. Preferably the NIR absorbing material should have nanoscale particle size to allow a good distribution of the material on the polymeric powders and additionally to allow the fabrication of transparent parts.

In order to transfer the structure onto the powder bed, it is necessary to incorporate the IR absorbers into an ink. This requires the IR absorber materials to be converted either into a dispersion or into a product which can be easily redispersed in an ink vehicle.

Incorporating NIR absorbing powders into a solvent based dispersion is a challenge. For example inorganic materials are hydrophilic substances and are not compatible with organic solvents, especially those of non-polar nature. Inorganic NIR absorbing materials provided as dry powders are usually highly agglomerated particles, which need high sheer forces for deagglomeration.

It is known in the art that nanoparticle dispersions in organic solvents can be achieved by wet milling of particles in the presence of dispersing aids.

For example US 2018/0016460 describes the dispersion of IR absorbing materials in non-polar 25 solvents.

However incorporating this type of dispersion in an inkjet carrier ink suitable for a 3D rapid sintering process results in an unstable ink. Dispersions made by the prior art methods decompose at higher temperatures and are not storage stable over prolonged periods of time.

It would be highly beneficial to provide a formulation which can be added to regular inkjet inks to provide additional functionality to convert the regular ink into a high speed sintering ink. It would also be beneficial to be able to provide transparent or coloured 3D pieces.

It is an aim of the present invention to provide compositions useful in high speed sintering processes which overcome at least one disadvantage of the prior art.

According to a first aspect of the present invention there is provided the use of a surface modified near infrared absorbing material as an additive in an inkjet printing composition for a high speed sintering process; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents.

According to a second aspect of the present invention there is provided an additive composition for an inkjet printing composition comprising a near infrared absorbing material; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents.

According to a third aspect of the present invention there is provided an inkjet printing composition comprising a surface modified near infrared absorbing material; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents.

According to a fourth aspect of the present invention there is provided a method of preparing an additive composition for an inkjet printing composition, the method comprising: (a) providing particles of a near infrared absorbing material; (b) contacting said particles of near infrared absorbing material with a first surface modifying material; and (c) contacting said particles of near infrared absorbing material with a second surface modifying material.

According to a fifth aspect of the present invention there is provided a method of preparing an inkjet printing composition, the method comprising: (i) providing an additive composition by the steps of (a) providing particles of a near infrared absorbing material; (b) contacting said particles of near infrared absorbing material with a first surface modifying agent; and (c) contacting said particles of near infrared absorbing material with a second surface modifying agent; and (ii) admixing said additive composition with an base inkjet printing composition.

According to a sixth aspect of the present invention there is provided a method of preparing a three dimensional structure, the method comprising the steps of: (1) applying a polymer precursor layer to a substrate; (2) inkjet printing a composition comprising particles of surface modified near infrared absorbing material; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents; and (3) applying infrared radiation to the substrate.

According to a seventh aspect of the present invention there is provided a near infrared absorbing material which has been contacted with a first surface modifying agent and a second surface modifying agent.

Preferred features of the first, second, third, fourth, fifth, sixth and seventh aspects will now be described. Any feature may apply to any aspect as appropriate.

The present invention involves the use of a surface modified near infrared absorbing material whose surface has been modified by at least two different surface modifying agents.

By near infrared (or NIR) absorbing material we mean to refer to materials that absorb infrared radiation having a wavelength in the range 700 to 2500 nm.

The emission spectrum of commercially available IR lamps is shown in figure 1. Maximum output is at around 1000 to 1100 nm. NIR absorbers that have an absorption maximum in that range typically provide the highest possible heat conversion rate. Such materials are therefore highly advantageous.

Preferably the near infrared absorbing material is an inorganic NIR absorbing material.

Inorganic NIR absorbing materials are favoured due to their broad absorption range. Any suitable NIR absorbing material can be used. Such materials will be known to those skilled in the art.

Preferably the near infrared absorbing material is selected from one or more of: doped tin oxides; doped indium oxides; -alkali metal tungsten oxides; -rare earth hexaborides; -alkali and/or transition metal (poly)phosphates; -alkali and alkaline earth copper silicates; and -transparent inorganic conductive oxides.

In some embodiments the NIR absorbing material is a doped tin oxide.

Suitably in the doped tin oxide less than 50 mol% of the tin atoms are substituted with a dopant.

Preferably at least 0.1 mol% of the tin atoms are substituted with a dopant, preferably at least 0.5 mol%, more preferably at least 1 mol%.

Suitably up to 40 mol% of the tin atoms are substituted with a dopant, preferably up to 30 mol%, more preferably up to 25 mol%.

The level of dopant in the tin oxide will depend on a number of factors including the nature of the dopant(s) and the concentration at which the tin oxide is to be used.

In preferred embodiments the dopant is present in a concentration of from 2 to 20 mol%, preferably 3 to 15 mol%.

The tin oxide may be doped with one or more elements selected from antimony, tungsten, phosphorus, copper, niobium, manganese, fluorine, nickel, vanadium, zinc, bismuth, indium, aluminum, europium, rare earth metals, iodine, chloride and nitrogen.

The tin oxide is suitably doped with one or more elements selected from antimony, tungsten, phosphorus, copper, niobium, manganese, fluorine and nickel.

Preferably the tin oxide is doped with one or more elements selected from antimony, fluorine tungsten and phosphorous.

Most preferably the tin oxide is doped with antimony.

In some preferred embodiments the tin oxide is doped only with antimony. In such embodiments the antimony is present in an amount of from 0.5 to 20 mol%, preferably 2 to 18%, more preferably 5 to 15 mol%.

In some embodiments the tin oxide is doped with antimony and tungsten. In such embodiments antimony is preferably present in an amount of from 6 to 10, preferably 7 to 9, for example about 8 mol% and tungsten is present in an amount of from 1 to 5, preferably 2 to 4, for example about 3 mol%.

In some embodiments the tin oxide is doped with antimony and phosphorus. In such embodiments phosphorous is preferably present in an amount of from 1 to 5, preferably from 2 to 4, for example about 3 mol%, and antimony is present in an amount of from 5 to 10, preferably 7 to 9, for example about 8 mol%.

Antinomy doped tin oxide comprising 5 to 15 mol% antinomy is especially preferred.

In some embodiments the NIR absorbing materials comprises a doped indium oxide. Such materials are based on indium oxide in which some of the indium atoms have been replaced by one or more dopant elements.

Suitable dopant elements include tin, zinc, aluminium, gallium and mixtures thereof.

Preferably at least 0.1 mol% of the indium atoms are substituted with a dopant, preferably at least 0.5 mol%, more preferably at least 1 mol%.

Suitably up to 40 mol% of the indium atoms are substituted with a dopant, preferably up to 30 mol%, more preferably up to 25 mol%.

In some preferred embodiments the indium oxide is doped with tin. In such embodiments the indium is suitably present in an amount of from 1 to 20, mol%, preferably 5 to 15 mol%.

In one embodiment the doped indium oxide comprises 90% indium oxide and 10% tin oxide.

One preferred doped indium oxide is reduced indium tin oxide. This material is obtained by preparing tin doped indium tin oxide and then reacting with a reducing agent. This reduces the oxygen content in the lattice. A gaseous reducing agent (for example hydrogen or carbon monoxide) or a liquid reducing agent (for example oxygenated solvents, citric acid) may be used. The synthesis of such compounds is within the competence of a person skilled in the art.

The oxygen content of the lattice may be reduced by 0.001 to 5 wt%. The reduction process may not only form oxygen deficiencies in the crystal lattice, but also form nanoparticles and nanoalloys of tin and indium providing advantageous properties. The preparation of reduced indium tin oxide compounds suitable for use herein is described by Kim et al (Materials Chemistry and Physics 86, 2004, 210-221) and Guenther of al (Journal of Applied Physics, 104, 034501 (2008)).

Indium tin oxides comprising 85 to 95 mol% indium and 15 to 5 mol% tin are especially preferred.

A further preferred doped indium oxide is indium zinc oxide. This material is obtained by preparing zinc doped indium zinc oxide by solid state synthesis and then optionally reacting with a reducing agent. The synthesis of such compounds is within the competence of a person skilled in the art. In some embodiments the indium zinc oxide may comprise one or more further dopant elements, for example gallium or aluminium.

Indium zinc oxides comprising 80 to 98 mol% indium and 20 to 2 mol% zinc are preferred. In one embodiment the doped indium zinc oxide contains 87% indium oxide and 13% zinc oxide.

In some embodiments the indium zinc oxides may comprise 97 to 98 mol% indium and 2 to 3 mol% zinc.

In some embodiments the NIR absorbing material comprises an alkali metal tungsten oxide. Cesium tungsten oxide is especially preferred.

In some embodiments the NIR absorbing material may comprise a modified caesium tungsten oxide. Preferred materials of this type are described, for example, in GB1815402.1.

Suitable NIR absorbing materials include those of formula M'2M2bWcO^(P(0)nRm)e wherein each of M1 and M2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group. Preferably M1 is caesium and M2 is selected from the group consisting of alkali metals, zinc and tin. More preferably M1 is caesium; M2 is selected from sodium, potassium, tin or zinc; a is 0.22 to 0.4; b is 0.01 to 0.2; c is 1; d is 2.7 to 3; e is 0.05 to 0.25; n is 2; m is 1 and R is an unsubstituted alkyl or aryl group having 6 to 20 carbon atoms.

In some embodiments the NIR absorbing material comprises a rare earth hexaboride. Lanthanum hexaboride is especially preferred.

In some embodiments NIR absorbing material comprises an alkali metal and/or transition metal (poly)phosphate. Suitable compounds of this type include is copper hydroxy phosphate, zinc copper hydroxy phosphate, iron copper phosphate, tin copper phosphate, copper phosphate, zinc copper phosphate and manganese copper phosphate.

In some embodiments NIR absorbing material comprises an alkali and/or alkaline earth copper silicates. One suitable compound of this type is copper magnesium silicate.

In some embodiments NIR absorbing material comprises a transparent conducting oxide, which shows high transparency in the visible region but high absorption in the IR region. Suitable materials of this type include aluminium zinc oxide and zinc indium tin oxides.

Preferred NIR absorbing materials are those which offer lowest possible coloration at highest possible IR absorption.

The NIR absorbing materials may be used alone or in any combination. The combinations and relative proportions of components may be suitably adjusted. This may depend on the absorption of the IR lamp with which the material is to be used.

Especially preferred NIR absorbing materials for use herein include antimony doped tin oxides, partially reduced indium tin oxides, cesium tungsten oxides and magnesium copper silicates.

The NIR absorbing materials absorb NIR light and convert it to heat.

As is known to those skilled in the art, different infrared absorbing materials have varying absorption ranges. Some NIR absorbing materials absorb NIR light and convert it to heat absorption in the range above 1200 nm, whereas cesium tungsten oxide has a maximum absorption in the range of 900 to 1000 nm. Copper Phosphate based materials show absorption in the range of 800 to 1200 nm.

The present invention may involve the use of one or a combination of any of these NIR absorbing materials. The combination may be selected to provide a maximum absorbance corresponding to the IR emissions spectrum of an IR lamp in the 3D printing machine used in the invention. Depending on the lamp's emission profile various combinations may be necessary to achieve an optimal absorption and conversion into heat. Selection of suitable materials and adjustment of the relative ratios of the components is within the competence of the person skilled in the art.

The NIR absorbing materials used in the present invention are surface modified. The surfaces of the near infrared materials used in the invention have been modified by contact with at least two surface modifying agents. A surface modifying agent suitably binds to the surface of the particle and preferably affects one or more properties of the particle. Suitably the surface modifying agent changes the way the particle interacts with other components of a composition.

Suitably the near infrared absorbing materials of the present invention have been contacted with a first surface modifying agent and a second surface modifying agent. The first surface modifying agent is different to the second surface modifying agent. The first surface modifying agent and a second surface modifying agent may be contacted with the surface of the particle of NIR absorbing material either simultaneously or sequentially.

Suitably substantially all of the surface of the particle of NIR absorbing material is coated with at least one surface modifying agent.

The first surface modifying agent may cover some or part of the near infrared absorbing material.

The second surface modifying agent may cover some or part of the near infrared absorbing material.

Preferably the first surface modifying agent covers substantially all of the surface of a particle of the near infrared absorbing material. Preferably the second surface modifying agent forms a coating over substantially all of the surface of the near infrared absorbing material. Suitably the first surface modifying agent forms an inner coating layer directly onto the surface of the NR absorbing material.

Suitably the second surface modifying agent forms a second coating layer on top of the first surface modifying agent.

Because the second surface modifying agent is suitably applied over the first surface modifying agent, the second surface modifying agent may not come into direct contact with the surface of the near infrared absorbing material. However it is intended that such embodiments clearly fall within the scope of the present invention.

In some embodiments the particles of the near infrared absorbing material may be contacted by more than two surface modifying materials.

Suitably the first and second surface modifying agents are selected from polymers, organosilanes, organophosphates, organophosphonates, titanates and coupling agents.

Suitable polymers which may be used as surface modifying agents include polyvinylpyrrolidone, polyacrylates, polyvinylbenzylsulfonates polyvinylacetate, polyethyleneimines, polyvinylsulfonates, polyoxazolidone and polyvinylalcohols.

Preferably the first surface modifying material is selected from organophosphorous compounds and organosilanes. Organophosphorous compounds include organophosphates, organophosphate esters and organophosphonates. Especially preferred compounds of this type are those which include n-alkyl and/or aryl substituents.

Suitable organosilanes are compounds having the formula: R2 wherein each of R1, R2, R3 and R4 is independently OH, an optionally substituted alkyl or aryl group or a group of formula OSi(OR)3 wherein each R is independently an optionally substituted alkyl or aryl group. In some embodiments R1 is an alkyl or aryl group, for example phenyl and each of R2, R3 and R4 is OSi(OR)3 wherein each R is methyl, ethyl or isopropyl.

In some especially preferred embodiments R1 is C81-117 or C181137 and each of R2, R3 and R4 OMe.

Suitable organophosphates are compounds having the formula: wherein each R5, R6 and R' is independently OH, an optionally substituted alkyl or aryl group or ORa wherein each R3 is independently an optionally substituted alkyl or aryl group.

In some embodiments R5, R6 and R7 may be selected from polyethylene glycol or polypropylene glycol residues.

In some preferred embodiments each of R5, R5 and R' is selected from OH, phenyl, octyl, and ORa wherein R3 is octyl or benzyl.

In one especially preferred embodiment, R5 is OH and each of R6 and IR7 is ORa wherein Ra is octyl.

Especially preferred organophosphorus compounds include octylphosphonic acid, methylphosphinic acid, stearylphosphonic acid, trioctylphosphate ester, tributylphosphate ester, phenylphosphonic acid, benzylphosponic acid, trimethylbenzylphosponic acid, methylen(bisphosphonic acid); trioctylphosphite and butylphosphite.

Suitable coupling agents which may be used as the first surface modifying agent are described in US2018/0208803 and include metal coupling agents including an amino group, for example a silane coupling agent including an amino group, a titanate coupling agent including an amino group or an aluminate coupling agent including an amino group.

Preferably the first surface modifying agent is selected from organosilanes, organophosphates and organophosphonates.

Preferably the first surface modifying agent is an organophosphate or organophosphonate.

The second surface modifying agent is suitably a surface active agent having dispersing properties. The second surface modifying material may be a polymeric or non-polymeric material. Preferably the second surface modifying agent is a polymeric material.

Preferably the second surface modifying agent is a polymeric material comprising one or more oxygenated functional groups.

Preferably the second surface modifying agent is a polyester or a derivatised polyester.

Suitably the second surface modifying agent is a polymer comprising ester functional groups. Suitably the second surface modifying agent is a polymeric material comprising ester functional groups and amino functional groups.

Suitably the second surface modifying agent is a polyacrylic or methacrylic acid ester or a copolymer of acrylic, methacrylic acid esters, wherein the ester groups are formed from alcohols selected form polyethylene glycols, polypropylene glycols, fatty acid alcohols or fatty acid amines or phosphates.

One especially preferred compound suitable for use as the second surface modifying material is sold under the trade mark Tego Dispers 1010.

The surface modified near infrared absorbing material of the second aspect of the present invention is suitably prepared according to the method of the fourth aspect.

Step (a) involves providing particles of a near infrared absorbing material.

Preferably the particles of the NIR absorbing material provided in step (a) of the method of the fourth and fifth aspects of the present invention are nanoparticles.

Preferably the NIR absorbing material provided in step (a) of the method of the fourth and fifth aspects of the present invention (the unmodified NIR absorbing material) has a volume-based median particle diameter (D50 where D50 is defined as the diameter where 50% of the total volume of particles has a diameter less than D50; also referred to herein as median particle diameter) of less than 500nm, suitably less than 250 nm, preferably less than 200 nm, more preferably less than 150 nm, preferably less than 100 nm. In some embodiments the NIR absorbing materials was an average D50 particle size of less than 80 nm, for example less than 60 nm or less than 50 nm.

The particle diameters referred to herein are volume-based particle diameters, where the diameter equals the diameter of a sphere having the same volume as a given particle, and may be determined by using commercially available laser diffraction equipment in the recommended manner. It will be understood that the particles do not have to be spherical in shape and that the diameter is merely an equivalent diameter. It will also be understood that the apparatus may measure area-based particle diameter with the volume-based particle diameter derived using standard proprietary calculation methods.

For comparison with particle size measurement by laser diffraction, it may be assumed that the particles have a constant density independent of diameter, so that volume-based and weight-based diameters correspond to each other.

Nanoparticles of the NIR absorbing materials having a particle size within the preferred range may be provided by wet milling. Such techniques will be known to the person skilled in the art.

In step (a) the particles of the NIR absorbing material may be provided in an aqueous phase.

They may be provided at the desired size, for example less than 100 nm, by a known technique, for example wet milling.

In some embodiments step (a) preferably involves providing particles of a near infrared absorbing material by wet milling the material in an aqueous phase to give particles having a D50 size of 40 to 60 nm, for example about 50 nm.

In some embodiments step (a) preferably involves providing particles of a near infrared absorbing material out of a hydrothermal process separated by centrifugationfrom the mother liquor and redispersed in water or solvent to give particles having a D50 size of less than 100 nm.

Step (b) involves contacting said particles of infrared absorbing materials with a first surface modifying agent.

Step (c) involves contacting said particles of infrared absorbing materials with a second surface modifying agent.

Steps (b) and (c) may be carried out sequentially or simultaneously.

Preferably step (b) is carried out before step (c).

Step (b) suitably involves contacting particles of a near infrared absorbing material with a first surface modifying agent.

This can be carried out by any suitable means. Such means will be known to the person skilled in the art and include for example the method described in US2018/0155561 or an analogous method.

Suitably step (b) may involve dispersing the infrared absorbing material in a suitable solvent or water to provide a homogeneous mixture. The first surface modifying agent is suitably added to the reactor under mixing. Where needed the temperature may be increased to facilitate the reaction of the IR material with the first surface modifying agent.

In especially preferred embodiments when dispersed in water the infrared absorbing material has a zeta potential of less than 35 my, preferably less than -50 mV.

Step (c) is preferably carried out after step (b) and thus preferably involves contacting a near infrared absorbing material which has been modified with the first surface modifying agent with a second surface modifying agent.

Step (c) may involve addition of the second surface modifying agent to the dispersion of the particles modified by the first modifying agent. Alternatively there may be a filtration, centrifugation or any other solid/liquid separation step between step (b) and step (c). In such embodiments step (c) may involve adding particles modified by the first modifying agent to a solution of the second surface modifiying agent in either the same or a different solvent to that used in step (b).

Thus in some embodiments the surface modified near infrared absorbing material of the present invention may be obtained by contacting a near infrared absorbing material which has already been modified by contact with a first surface modifying agent with a second surface modifying agent.

In some embodiments step (b) is carried out under aqueous conditions. In such embodiments the method of the fourth aspect may involve a drying step or a solvent exchange step between step (b) and step (c).

Step (c) may be carried out in the presence of a solvent. Suitably this solvent is retained and forms part of the additive composition.

The solvent is suitably selected to provide a high compatibility with typical inkjet ink solvents. Ideally the additive composition is miscible with the base inkjet printing composition at any ratio. The solvent suitably has a high enough boiling point to avoid any evaporation during the circulation of the ink through the inkjet printing head and the ink reservoir in the printer.

However the boiling point also needs to be adapted to the sintering temperature of the polymer during the rapid sintering process. Preferably the solvent evaporates at the sintering temperatures.

The solvent should not dissolve, swell or interact in any other manner with the polymer used for the sintering process.

Aliphatic non-polar hydrocarbons are preferred solvents for use in the additive composition. Suitable solvents are selected from heptane, octane, isooctane, isodecane and polyisoprene.

The second surface modifying agent preferably does not influence the viscosity of the additive composition or the formulated inkjet printing composition. Suitably the second surface modifying agent is non foaming and compatible with other ingredients of the base inkjet printing composition in which the additive composition is to be used.

The amount of the second surface modifying agent used depends on the intended concentration of the surface modified NIR absorbing material in the additive composition and the particle size.

The weight ratio of the material obtained after step (b) to second surface modifying agent is suitably from 10: 0.1 to 10: 5, preferably from 10: 0.8 to 10: 2.

Suitably the nanoparticles of the NIR absorbing material provided by the method of the fourth aspect with a monolayer of a first surface modifying agent and a monolayer of a second surface modifying agent such that the particles of surface modified NIR absorbing material are also of the nanoparticle scale.

Preferably the surface modified NIR absorbing material of the present invention has a volume-based median particle diameter (D50 where D50 is defined as the diameter where 50% of the total volume of particles has a diameter less than D50; also referred to herein as median particle diameter) of less than 500nm, suitably less than 250 nm, preferably less than 200 nm, more preferably less than 150 nm, preferably less than 100 nm.

The first aspect of the present invention relates to the use of a surface modified NIR absorbing material as an additive for an inkjet printing composition.

In a second aspect an additive composition for an inkjet printing composition is provided.

Preferably the additive composition comprises a solvent. This is suitably provided in step (c) of the method of the fourth aspect and is as previously defined herein.

A third aspect of the present invention provides an inkjet printing composition comprising a surface modified NIR absorbing material. By inkjet printing composition we mean to refer to any composition which is suitable for application to a substrate by inkjet printing.

Compositions useful in inkjet printing compositions are generally known in the art. Typically ink compositions used in inkjet printers comprise a pigment to provide colouring to the printed image. In the field of the present invention printing is carried out to apply the NIR absorbing materials. Thus the inclusion of a pigment is optional.

In some embodiments the inkjet printing composition of the present invention comprises pigment. It may comprise more than one pigment.

In some embodiments the inkjet printing composition of the present invention does not comprise a pigment.

When present, the pigment(s) is suitably included in an amount of from 0.001 to 30 wt%, preferably 1 to 8 wt%.

Inkjet printing compositions must match a number of requirements to be used on the different types of inkjet printing heads. The ink formulation must meet specifications for physical as well as for chemical properties. Typical physical properties are viscosity, density or surface tension. The particle size of any solid component is limited by the nozzle diameter of the printhead and by the droplet size, which is jetted onto the substrate. The droplet size is around 1 pm, which suggests that any solid pigment has a D90 significantly below 1 pm. With regard to the chemical properties the ink must match specific requirements for solvent mixtures and monomers. Materials must be non corrosive to avoid any damage of the printhead. Such restrictions will be well known to the skilled person.

It is also desirable that the additive composition of the second aspect of the present invention also complies with these requirements. In addition the additive composition must be stable upon dilution. Suitably dilution of the additive composition of the second aspect by admixture with a base inkjet printing composition does not lead to reagglomeration of the inorganic particles, which could cause a blockage of the printhead nozzles.

It is also desirable that the additive composition of the second aspect and the inkjet printing composition of the third aspect of the present invention have long term heat stability and must survive heating / cooling cycles without decomposition or flocculation of any solids suspended/dispersed therein. This heat stability is particularly important because the inkjet printing composition may be circulated through the printing head. The printing head is exposed to heat from the IR curing lamp as well as from the powder bed, which is operated at elevated temperatures. Operational temperatures above 60°C -80°C are possible. The inkjet printing composition needs to be stable when exposed to elevated temperatures, but also at lower temperatures during times of inactivity or during storage in feeding tanks for the printing system.

Suitably the additive composition of the second aspect and the inkjet printing composition of the aspect are stable to temperatures of up to 60 °C. Preferably they are stable at temperatures of up to 80°C, for example at temperatures of up to 90 °C or up to 100 °C.

Suitably the additive composition of the second aspect and the inkjet printing composition of the aspect are stable at temperatures of less than 30 °C. Preferably they are stable at temperatures of less than 20°C, for example at temperatures of less than 10 °C, less than 5 °C or less than 0 °C.

Suitably the additive composition of the second aspect and the inkjet printing composition of the aspect are stable at temperatures of between 10 and 60 °C. Preferably they are stable at temperatures between 0 and 80°C.

Suitably the additive composition of the second aspect and the inkjet printing composition of the aspect are storage stable. Preferably they are stable on storage at temperatures of between 0 and 40 °C for at least 1 month, preferably at least 6 months, suitably at least 12 months.

Suitably the additive composition of the second aspect and the inkjet printing composition of the aspect are stable to variations of temperature. For example they are preferably stable upon repeated exposure (i.e. 3 or more times, suitably 5 or more times, for example 10 or more times) to temperatures of up to 60 °C. Preferably they are stable upon repeated exposure (i.e. 3 or more times, suitably 5 or more times, for example 10 or more times) to temperatures of up to 80°C, for example at temperatures of up to 90 °C or up to 100 °C.

By stable we mean that the compositions do not degrade. Suitably they do not change colour or viscosity. Suitably they do not separate or produce sediments or precipitation.

The inkjet printing composition of the third aspect is preferably obtained by admixing an additive composition of the second aspect, suitably according to the method of the fifth aspect.

The method of the fifth aspect suitably involves steps of: (i) providing an additive composition by the steps of (a) providing particles of a near infrared absorbing material; (b) contacting said particles of near infrared absorbing material with a first surface modifying agent; and (c) contacting said particles of near infrared absorbing material with a second surface modifying agent; and (ii) admixing said additive composition with an base inkjet printing composition.

Step (i) preferably involves providing an additive composition according to the method of the fourth aspect. Preferred features of steps (a), (b) and (c) of step (i) are suitably as defined in relation to the fourth aspect.

Step (ii) involves admixing said additive composition with a base inkjet printing composition.

Step (ii) suitably involves mixing of the additive composition with a base inkjet printing composition in a specific ratio to form the final inkjet printing composition suitable for high speed sintering process.

Advantageously in the present invention the particles of the surface modified NIR absorbing material (suitably nanoparticles thereof) have at least two protective layers on their surface which provides stability against reagglomeration and allows easy dispersion into a wide variety of base inkjet printing compositions. It has been found that the surface modified NIR absorbing materials of the present invention can be easily dispersed in a wide range of solvents of differing polarities by simple mechanical stirring.

Without the surface modification it would be necessary to generate the nanoparticles by bead milling directly in each solvent mixture separately. The inventive procedure generates nanoparticles which can be dispersed by simple stirring in any suitable inkjet vehicle without further effort or adaption.

The use of two surface modifying agents suitably leads to the formation of a double layer on the surface of the NIR absorbing material. This ensures greatest compatibility of the additive composition with the base inkjet printing composition. Depending on the formulation of the additive composition and the base inkjet printing composition, a single or a mixture of different second modifying surface agents can be used. These are suitably selected to ensure full miscibility without flocculation of collapse of the nanoscale dispersion.

Figure 3 shows a schematic drawing of the surface modification and the interaction to form a double or multilayer on the pigment surface. It is believed that the formation of this layer stabilises diluted dispersions.

The additive composition of the present invention is suitably stable upon dilution.

The surface modified NIR absorbing material is suitably present in the additive composition in an amount of from 2 to 40 wt%. The concentration used may be adjusted depending on the IR lamp used and the particular absorption characteristics of the material.

The amount of solvent in the additive composition is preferably between 30 and 95 wt%, preferably between 65 and 90 wt%, especially preferred between 75 and 85 wt%.

In some embodiments the additive composition comprises a low molecular weight stabilizer.

Preferred stabilizers are aminoalcohols, for example polyoxyethylenamines.

The stabilizer is suitably present in the additive composition in an amount of between 0.1 to 5 wt% of the formulation, preferably between 1 to 2 wt%.

The third aspect of the present invention relates to an inkjet printing composition. In preferred embodiments the inkjet printing composition is prepared by admixture of the additive composition and a base inkjet printing composition.

The inkjet printing composition suitably comprises at least one solvent. It suitably comprises the solvent from the additive composition. It preferably comprises a further solvent. The further solvent may be the same or different.

Suitably the inkjet printing composition comprises a further solvent provided by the base inkjet printing composition. Preferably this solvent is selected from esters, ketones, acetates and polyglycols.

Preferably the inkjet printing composition comprises a plasticiser. In some embodiments the additive composition may comprise a plasticiser.

The plasticiser is suitably included to improve the droplet forming properties of the inkjet printing composition. It may be provided in the additive composition. Preferably it is provided in the base inkjet printing composition. Suitable plasticisers include benzoates, phthalates, trimellitates, adipates, citrates and mixtures thereof Plasticizers are suitably present in the inkjet printing composition in an amount of less than 10 wt% of the formulation, preferably between 5 to 8 wt%.

The preparation of the additive composition may be carried out via mixing the constituents in typical mixing reactors. The mixing can be done by internal stirring or by recirculation of the reactor content with a pump until a homogeneous mixture has been obtained. In some embodiments a solvent, plasticizer and the second surface modifying agent may be homogenized in the reactor and add then the NIR absorbing material which has been already contacted with the first surface modifying agent is admixed under intensive stirring until a homogeneous fluid is obtained. Then the stabilizer can be added at the required level. The mixing of the formulation is continued until a stable dispersion has been obtained. The mixing time can vary between 1 and 10 hours per 1000 litre batch.

The inkjet printing composition should spread in a well defined manner on the surface of the polymer powder particle to deposit the IR absorbing particle, but should not alter the polymer surface.

A possible alternative procedure is to formulate a concentrate comprising the surface modified NIR absorbing material which has been contacted with both the first and second surface modifying agents first and then dilute this to the required final concentration. In this case only 30% -40% of the solvent, the full amount of plasticizer and dispersing aids are homogenized, then the surface modified NIR absorbing material powder is added. The formulation is mixed until being of homogeneous appearance. Typically the viscosity is higher than in the end product and increments of solvents may be added to adjust the viscosity to maintain proper stirring. After a certain time the rest of the solvent is added to achieve the final concentration.

The dispersion may then be filtered over a cartridge filter to remove any oversized or not well dispersed particles.

The versatility of the additive composition of the present invention has been demonstrated by testing in inkjet printing compositions. The additive composition was mixed with different base inkjet printing compositions at different ratios to generate different concentrations of NIR absorbing materials. The resultant inkjet printing compositions were then subjected to the usual test methods to qualify inkjet inks, especially assessment of stability under varying temperatures, the reagglomeration level and the NIR properties.

Figure 4 shows the NIR absorbing properties of the diluted functional fluids. The compositions of the invention performed well in all those tests as the following examples show.

The invention will be further described with reference to the following non-limiting examples and the accompanying figures in which: Figure 1 shows the emission spectrum of commercially available IR lamps; Figure 2 shows the typical powder reflectance spectrum of partially reduced indium tin oxide; Figure 3 shows a schematic drawing of the surface modification and the interaction with the dispersing aid; and Figure 4 shows the NIR absorbing properties of inkjet printing compositions of the present invention, in which: 1 is the IR absorption of the base inkjet printing composition without any additive; 2 is the IR absorption of an inkjet with comprising an additive composition of the invention diluted 1:50 in the base inkjet printing composition and 3 is the IR absorption of an inkjet printing composition of the invention with an additive composition diluted 1:10 in the base inkjet printing composition. In this embodiment the infrared absorbing material is antimony doped tin oxide, the first surface modifying agent is octylphosphonic acid and the second surface modifying agent is Tego Dispers 1010 (RTM).

Preparation examples

Surface modified near infrared absorbing materials of the present invention can be prepared as follows: Example A -Application of the first surface modifying agent 500 gr of Infrared Absorbing material are added in to a 5 I beaker filled with 4 I Deionized water. The pH value is adjusted to 7 -8 by addition of either Ammonia solution or Nitric Acid. Under stirring a solution of the 20 gr Octylsilanetrimethylester and 40 gr Butylphosphonic Acid in Ethanol is slowly added. The mixture is stirred with a high speed mixer for 1 hour and the pH adjusted to pH 7 with ammonia again. The dispersion is separated from the liquid, transferred into a drier and dried for 18 hours under a temperature gradient reaching 110°C at maximum.

Example B -Application of the second surface modifying agent A product obtained according to the method of example A, or a commercially available compound which is a near infrared absorbing material that has been treated with a first surface modifying agent was treated as follows: 2L of aliphatic solvent, Exxsol D140, are added into a 5L beaker. Under stirring 80 gr of the second surface modifying agent is added and homogenized.

400g of the product of example 1/commercially available equivalent was added in portions to establish a homogeneous dispersion. The dispersion is circulated through a bead mill for several hours, until the particle size is below 500 nm. Afterwards the dispersion is diluted with Exxsol D140 to a concentration of 7.5% solids. The dispersion is filtered through a 10 pm filter cartridge to remove any oversized particles.

The following materials were prepared: Example NIR absorbing First surface Second surface material modifying agent modifying agent 1 Antimony doped tin oxide Octylphosphonic Acid Polyacrylate, ammonium salt (inventive) 2 Fluorine doped tin None Tego Dispers 1010 (non-inventive) oxide 3 Indium tin oxide Octyltrimethoxy silane EPL50 (inventive) 4 Indium tin oxide None RKLA (non-inventive) Cesium tungsten butylphosphate RKLA (inventive) oxide 6 Cesium tungsten None Tego Dispers 1010 (non-inventive) oxide 7 Copper Methylphosphonic Acid Tego Dispers 1010 (inventive) hydroxyphosphate 8 Cesium tungsten Phenylphosphonic Acid Tego Dispers 1010 (inventive) oxide and Antimony doped tin oxide 9 Cesium oxide tungsten Octylphosphonic Acid Octyltrimethoxy silane mixture / Tego Dispers 1010 Application Testing: Aliquots of the dispersions are taken and mixed with a base inkjet ink to obtain the NIR absorbing ink. The inkjet ink vehicle was obtained from Nazdar Ink Technologies. The final inks are subject to stability tests at different temperatures. The results are summarized below: Example Mixing ratio with Stability after 24 h Stability after 3 Stability after 3 Ink Vehicle at Room month at room month at 50°C Temperature Temperature 1 (inventive) 1: 10 Stable Stable Stable 2 (non-inventive) 1: 10 Agglomerated Precipitation Precipitation 3 (inventive) 1: 10 Stable Stable stable 4 (non-inventive) 1: 10 Stable Precipitation Precipitation (inventive) 1: 10 Stable Stable Stable 6 (non-inventive) 1: 10 Stable Precipitation Precipitation 7 (inventive) 1: 10 Stable Stable stable 8 (inventive) 1: 10 Stable Stable stable 9 (inventive) 1: 10 Stable Stable stable 1(inventive) 1: 50 Stable Stable Stable The resultant inks were printed on polyamide powders in a 3D inkjet printing machine and gave light coloured well cured and sintered parts.

Claims

Claims 1. The use of a surface modified near infrared absorbing material as an additive in an inkjet printing composition for a high speed sintering process; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents.
2. An additive composition for an inkjet printing composition comprising a near infrared absorbing material; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents.
3. An inkjet printing composition comprising a surface modified near infrared absorbing material; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents.
4. A method of preparing an additive composition for an inkjet printing composition, the method comprising: (a) providing particles of a near infrared absorbing material; (b) contacting said particles of near infrared absorbing material with a first surface modifying material; and (c) contacting said particles of near infrared absorbing material with a second surface modifying material.
5. A method of preparing an inkjet printing composition, the method comprising: (iii) providing an additive composition by the steps of (d) providing particles of a near infrared absorbing material; (e) contacting said particles of near infrared absorbing material with a first surface modifying agent; and (f) contacting said particles of near infrared absorbing material with a second surface modifying agent; and (iv) admixing said additive composition with an base inkjet printing composition.
6. A method of preparing a three dimensional structure, the method comprising the steps of: (1) applying a polymer precursor layer to a substrate; (2) inkjet printing a composition comprising particles of surface modified near infrared absorbing material; wherein the surface of the near infrared absorbing material has been modified by contact with at least two different surface modifying agents; and (3) applying infrared radiation to the substrate.
7. A near infrared absorbing material which has been contacted with a first surface modifying agent and a second surface modifying agent.
8. A use, composition, method or material as claimed in any preceding claim wherein the near infrared absorbing material is selected from one or more of doped tin oxides; doped indium oxides; -alkali metal tungsten oxides; -rare earth hexaborides; alkali and/or transition metal (poly)phosphates; -alkali and alkaline earth copper silicates; and -transparent inorganic conductive oxides.
9. A use, composition, method or material as claimed in claim 8 wherein the near infrared absorbing material is selected from doped tin oxides, doped indium oxides, alkali metal tungsten oxides and alkali metal and/or transition metal (poly)phosphates.
10. A use, composition, method or material as claimed in claim 8 wherein the near infrared absorbing material is selected from antimony tin oxide, indium tin oxide, cesium tungsten oxide and copper hydroxyphosphate.
11. A use, composition, method or material as claimed in any preceding claim wherein the first surface modifying agent covers substantially all of the surface of a particle of the near infrared absorbing material and the second surface modifying agent forms a coating over substantially all of the surface of the near infrared absorbing material.
12. A use, composition, method or material as claimed in claim 11 wherein the first surface modifying agent forms an inner coating layer directly onto the surface of the NR absorbing material and the second surface modifying agent forms a second coating layer on top of the first surface modifying agent.
13. A use, composition, method or material as claimed in any preceding claim wherein the first surface modifying material is selected from organophosphorous compounds and organosilanes.
14. A use, composition, method or material as claimed in any preceding claim wherein the second surface modifying agent is a polymer comprising ester functional groups
15. A use, composition, method or material substantially as hereinbefore described with reference to the examples.