WO2023084203A1

WO2023084203A1 - Borosilicate powder

Info

Publication number: WO2023084203A1
Application number: PCT/GB2022/052829
Authority: WO
Inventors: Karikath Sukumar Varma; Fiona BLACK; Ruksana UDDIN
Original assignee: Pilkington Group Limited
Priority date: 2021-11-10
Filing date: 2022-11-09
Publication date: 2023-05-19
Also published as: EP4430006A1

Abstract

There is provided a method of manufacturing a powder comprising borosilicate particles, the method comprising the steps of: mixing a solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide to form a sol; aging the sol to form a gel; and drying the gel to form particles directly, and/or drying the gel to form an aggregate of particles and grinding the aggregate of particles, and also, a method of manufacturing functionalised borosilicate particles, comprising the steps of: providing a powder comprising borosilicate particles formed from a dried gel; and reacting the borosilicate particles with a functional molecule and/or nanoparticle, and also a powder comprising borosilicate particles, wherein the particles comprise a functional molecule and/or nanoparticle.

Description

BOROSILICATE POWDER

The present invention relates to a powder, in particular a powder comprising borosilicate particles. The invention also relates to a method of manufacturing said powder and uses of same.

Powders, especially ceramic or glass powders, see a wide variety of uses as an industrial material, for example they may be used as glazes, frits, fillers, colourants, functional substrates, and solders in industries including construction, healthcare, dentistry, cosmetics, and electronics. Powders comprising borosilicate particles are of particular benefit, as they have higher chemical durability, lower coefficient of thermal expansion, and higher thermal resistance than alternative compositions, such as soda-lime silica glass. Ceramic powders and glass powders generally comprise a matrix of covalently bonded atoms, commonly incorporating oxygen bridges between metal or semi-metal atoms such as silicon.

Prior methods of providing powders include pulverising waste glass. For example, US 20210188694 Al discloses a process for preparing glass powder product including a step of crushing and milling waste glass.

However, a powder produced from waste glass will have the same composition as the original bulk glass, leading to powder products which are not optimised for their intended use.

In some cases, glass compositions may be specifically produced for the manufacture of glass powders. For example, EP 3437623 Bl discloses a dental glass powder that contains zinc, silicon, and fluorine and does not substantially contain aluminium. Again, a bulk glass is produced and pulverised to provide a powder. However, it is often not economical to modify the composition of a bulk glass to optimise the waste stream composition that leads to the glass powder.

Furthermore, bulk glass compositions often incorporate fluxes to improve melting and flowing behaviour necessary to enable the common manufacturing methods of bulk glass products, such as the float process. However, these fluxes may be undesirable in the powder product. As such, certain compositions of powders may be practically unobtainable from a waste glass pulverisation process. In addition, pulverisation processes are often lengthy and introduce impurities into the resultant powder, due to abrasion of the pulverisation apparatus. For example, alumina ball mills may be used, which introduces alumina particles into the powder.

Prior methods of producing powders directly, without pulverisation of glass melts, have been investigated. For example, US 3762936 A discloses a method of manufacturing borosilicate powder including the steps of intimately contacting colloidally subdivided amorphous silica and a source of boric oxide and heating the mixture to a temperature from 500 to 800 °C to form powdered borosilicate glass.

However, previous routes to powders comprising borosilicate particles have required high temperatures either to form a bulk glass, or to enable the borosilicate matrix forming reaction to proceed. High temperatures, such as those employed in conventional glass making processes, severely limit the components that can be introduced into borosilicate particles. Similarly, glass melts are often highly caustic. This again limits the components that can be introduced into the borosilicate particles.

A prior low temperature method of providing a ceramic powder comprising borosilicate ceramic particles is disclosed in US 20160376419 Al. However, this requires a pH in the range of 2 to 5, which may be incompatible with some components. A further method of providing borosilicate powders is disclosed in US 20140075995 Al, which uses exhaust gas from a glass furnace. However, such a method may not be feasible in situations where it is desired to produce a borosilicate powder without the presence of a glass furnace. A method of providing a nano glass powder is disclosed in US 20120138215 Al.

Borosilicate ceramics may be produced by sol-gel processes. For example, Beckett et al describe in J Sol-Gel Sci Techn (2006) 39:95-101 the formation of borosilicate glasses from silicon alkoxides and metaborate esters in dry non-aqueous solvents, Parashar et al describe in Nat Nano, (2008) 3: 589-594, borosilicate nanoparticles prepared by exothermic phase separation, and WO 98/42627 Al describes a method of producing a sol-gel.

It is desirable to overcome the aforementioned problems to provide a method of manufacturing a powder comprising borosilicate particles that does not require milling and allows a wider range of components to be introduced into the borosilicate particles. As such, according to a first aspect of the present invention there is provided a method of manufacturing a powder comprising borosilicate particles comprising the steps of: mixing a solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide to form a sol; aging the sol to form a gel; and drying the gel to form particles directly, and/or drying the gel to form an aggregate of particles and grinding the aggregate of particles.

Thus, powders comprising borosilicate particles may be provided wherein the matrix of the borosilicate particle has a very low content of, or is essentially free of, alkali and alkaline earth metals. The glasses therefore have very low coefficients of thermal expansion which lead to high thermal shock resistance, and also possess good electrical, weathering, chemical and heat resistance properties. As such, preferably the borosilicate particles comprise less than 5 weight % alkali metal and less than 5 weight % alkaline earth metal.

The step of drying the gel has been found to produce the desired particles either as a powder, or as an aggregate of particles that does not require milling, but may be ground to produce a powder. The skilled person will appreciate that the sol-gel reaction might under some conditions proceed to solidification, whereupon a glassy solid is produced. This glassy solid may then be milled to produce a powder. However, as discussed above, extensive milling processes may introduce impurities and constitute additional steps which may be undesirable considering the efficient processing of powders. Therefore, solidification is prevented. The inventors have found that solidification may be preferably prevented either by adding solvent to the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide or alternatively by halting the solgel reaction prior to solidification by cooling.

An aggregate of particles may be distinguished from a glassy solid, because a network of loosely bound particles is formed rather than a single solid mass, preferably the particles are less than 500 pm in diameter. The aggregate may be easily ground or the particles separated without recourse to milling apparatus.

As such, preferably the solution comprises a solvent and/or the method further comprises a step of cooling the gel to a temperature less than 10 °C following the step of aging the sol to form a gel. Preferably the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a solvent. Preferably, the solvent is an aprotic solvent. Preferably, the solvent is selected from acetone, tetra hydrofuran (THF) and dimethylsulfoxide (DMSO), most preferably tetra hydrofuran (THF). Preferably, the solvent does not comprise water. The inventors have discovered that the sol-gel reaction between a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide may proceed in non-aqueous, and even water free conditions.

Preferably the gel is cooled to less than 5 °C, more preferably less than 2 °C. The inventors have found that cooling preferably halts the sol-gel reaction, which if allowed to continue may impact on or even prevent powder formation. The gel may then be dried to produce either a powder spontaneously, or powder aggregates that may be ground to a lesser extent than what is required for a glassy solid.

The matrix of the borosilicate particles may be optimised for the use of the powder by the selection of sol components, in particular the selection of trialkoxyboroxine and orthosilicate and/or silicon alkoxide.

Preferably, the trialkoxyboroxine is of the formula:

wherein R¹, R² and R³, which may be the same or different, each represent a substituted or unsubstituted alkyl group. Preferably the alkyl group is a lower alkyl containing from 1 to 5 carbon atoms. The alkyl group may be substituted with aryl groups or other organic functional groups.

The skilled person will appreciate that the trialkoxyboroxine is preferably miscible or soluble in the solution.

Preferably, R¹, R² and R³ are each methyl such as trimethoxyboroxine (TMB). Alternatively, R¹, R² and R³ are each ethyl such that the trialkoxyboroxine is triethoxyboroxine (TEB). Alternatively, any one or more of R¹, R² and R³ may be substituted with an electron withdrawing group. In particular, any one or more of R¹, R² and R³ may be substituted with a carbonyl or halo- group. As such, in some embodiments the trialkoxyboroxine is selected from the group comprising: tri(2-chloroethoxy)boroxine, tri(2,2-dichloroethoxy)boroxine, tri(2,2,2- trichloroethoxy)boroxine, tri(3-chloro-l-ethoxy)boroxine, tri(l,3-dichloro-2-propoxy)boroxine, tri(4-chloro-l-butoxy)boroxine, tri(3-trifluoromethylbenzyloxy)boroxine, tri(2- fluorobenzyloxy)boroxine, tri(3-fluorobenzyloxy)boroxine, tri(4-fluorobenzyloxy)boroxine, tri(pentafluorobenzyloxy)boroxine, tri(2,2,3,3-tetrafluorobenzyloxy)boroxine, tri(lH,lH- pentafluoropropoxy)boroxine, tri(lH,lH,5H-octafluoropentoxy)boroxine, and tri(lH,lH- heptafluorobutoxy)boroxine. Suitable trialkoxyboroxine compounds may be prepared according to the procedures disclosed in EP 0970161 Al.

Preferably, the silicon centre of the orthosilicate or silicon alkoxide is capable of co-ordination bonding to an oxygen group of the trialkoxyboroxine, and the ligand on the trialkoxyboroxine must be attracted to the silicon centre of the orthosilicate or silicon alkoxide.

Preferably, the orthosilicate is a tetrasubstituted orthosilicate. Preferably the orthosilicate is tetraethylorthosilicate (TEOS).

Preferably, the silicon alkoxide is of the formula A*p[Si(OR')_Q]sBT where:

A* is an alkyl group; Si is silicon; R' is an substituted or unsubstituted alkyl, alkenyl, allyl, alkynyl, aryl, or aryloxy group; B is an electron-withd rawing group; P is 0, 1, or 2; Q is 1, 2, 3, or 4; S is from 1 to 20 and T is the difference between the valency of Si and the sum of P and Q.

Preferably, A* is a lower alkyl group containing 1 to 10 carbon atoms. Preferably, R' is a lower alkyl group containing 1 to 10 carbon atoms.

Preferably, Q is at least 2 and the R' groups are the same as one another.

Preferably, Q is at least 2 and each R' is a methyl group. Alternatively, Q is at least 2 and each R' is an ethyl group. For example, the silicon alkoxide may be selected from the group comprising: silicon tetraacetate, methyltriethoxysilane (MTES), dimethyldiethoxysilane (DMDES), and phenyl triethoxysilane (PhTES).

In a particularly preferred embodiment of the present invention, the trialkoxyboroxine is trimethoxyboroxine (TMB) and the orthosilicate and/or silicon alkoxide is tetraethylorthosilicate (TEOS). The expected decomposition products of TMB and TEOS are B2O3 and SiCh, respectively, when solvents and organic groups are removed through heating in air or under vacuum. The expected relative ratio of the two oxides is calculated assuming that all boron and silicon atoms present are incorporated into the borosilicate network such that 1 mole of TMB gives 3/2 moles of B2O3 and 1 mole of TEOS gives 1 mole of SiO2.

However, not all boron and silicon atoms are necessarily incorporated into the resultant matrix depending on reaction conditions, as volatile boron compounds may be evolved.

The B₂O3:SiO2 composition of powders can be tuned by varying the ratio of the starting materials. As such in some embodiments the ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide is from 0.05: 1 to 7: 1, preferably from 0.1: 1 to 2: 1, and more preferably from 0.1: 1 to 1: 1. In particularly preferred embodiments, the ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide is preferably from 0.2: 1 to 0.8: 1, even more preferably from 0.3: 1 to 0.6: 1.

Preferably, the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a catalyst. Preferably the catalyst is an acid, preferably trifluoroacetic acid. Preferably the molar amount of trifluoroacetic acid used is at least two orders of magnitude less than the molar amount of the lesser of boroxine, orthosilicate or silicon alkoxide. An acid catalyst may be of particular benefit when the step of mixing a trialkoxyboroxine with an orthosilicate and/or silicon alkoxide to form a sol is in the presence of a polar solvent, in particular acetone.

The matrix of the borosilicate particles may be optimised for the use of the powder by the selection of further additional sol components. As such, preferably the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a metal alkoxide. Preferably, the metal alkoxide is of the formula

A*P[M(OR')Q]SB_T where:

A* is an alkyl group; M is a metal capable of increasing its coordination number; R' is an alkyl, alkenyl, allyl, alkynyl, aryl, or aryloxy group; B is an electron-withdrawing group; P is 0, 1, or 2; Q is 1, 2, 3, or 4; S is from 1 to 20 and T is the difference between the valency of M and the sum of P and Q.

Preferably, Q is at least 2 and the R' groups are the same as one another.

Preferably, Q is at least 2 and each R' is a methyl group. Alternatively, Q is at least 2 and each R' is an ethyl group.

In some embodiments the metal alkoxide is preferably a tetraalkoxy titanium compound, such as titanium(IV) ethoxide, titanium(IV) methoxide, titanium(IV) isopropoxide, a titanium(IV) butoxide, preferably titanium(IV) n-butoxide. The incorporation of titanium in the matrix is thought to improve its mechanical and chemical resistance.

In some embodiments, the metal alkoxide is preferably a tetraalkoxy zirconium compound, such as zirconium(IV) ethoxide, zirconium(IV) methoxide, zirconium(IV) isopropoxide, a zirconium(IV) butoxide, preferably zirconium(IV) n-butoxide. The incorporation of zirconium in the matrix is thought to improve its mechanical and chemical resistance.

In some embodiments, the metal alkoxide is preferably a trialkoxy aluminium compound, such as aluminium ethoxide, aluminium methoxide, aluminium isopropoxide, aluminium butoxide. The incorporation of aluminium in the matrix is thought to improve its mechanical and chemical resistance.

A cooling step may be employed to allow the production of sol on a large scale, which may then be cooled and stored without gelation continuing to solidification for an order of months. The sol, or a portion thereof, may then be brought to room temperature as desired to allow the reaction to proceed to form a gel. The inventors have discovered that it is desirable to control the rate of gelation, so that the sol-gel reaction may be halted accurately, and consistency maintained over the production of the powders and the powders themselves. Factors that influence the rate of gelation include the temperature of the sol, the mixing of the sol, and sol components.

Preferably, as defined herein, a gel is formed wherein at least 30% of the constituent starting materials form a coherent network. Preferably, in the gel formed at least 50% of the constituent starting materials form a coherent network. The presence of a coherent network may be ascertained by methods known to the skilled person, for example, by a scanning vibrating needle curometer, assessment of transparency change, drying forms to a solid, infrared spectrometry or centrifuging etc.

Preferably, during the step of aging the sol to form a gel, the sol is maintained at a temperature of from 0 to 100 °C, more preferably from 5 to 50 °C, yet more preferably from 10 to 30 °C, even more preferably from 15 to 25 °C. In some cases, the temperature may be controlled by heating. However, in some cases controlling the temperature of the aging step may comprise cooling, which may prevent thermal runaway when using components which accelerate the gelation process.

Preferably, the thickness of the gel prior to the step of drying is greater than 0.1 cm, more preferably greater than 1 cm, but preferably less than 10 cm. A thicker gel film may take too long to adequately dry, while a thinner gel film may not form particles spontaneously, or may be difficult to grind to produce particles.

When temperatures lower than 200 °C are employed during the drying step, the drying step may be carried out under vacuum, for example using conventional rotary evaporation equipment, to allow the formation of powders and/or powder aggregates.

The inventors have discovered that preferably low temperatures and gentle reaction conditions allow the production of powders comprising borosilicate particles which comprise functional molecules and/or nanoparticles that were previously unobtainable. In addition, the sol-gel reaction may preferably proceed in substantially "water-free" conditions, allowing borosilicate particles comprising water-sensitive functional molecules and/or nanoparticles to be produced. Therefore, preferably the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises one or more functional molecules and/or nanoparticles.

In particular, the functional molecules and/or nanoparticles may comprise a chromophore, a fluorophore, an antimicrobial agent, bonding moieties, catalysts, organic dyes, or a combination of these. For example, a copper nanoparticle, a silver nanoparticle or Rhodamine 6G dye.

In some embodiments, the functional molecules and/or nanoparticles may be temperaturesensitive. Therefore, preferably the sol and/or gel is not heated to a temperature greater than 300 °C, more preferably the sol and/or gel is not heated to a temperature greater than 200 °C, even more preferably the sol and/or gel is not heated to a temperature greater than 150 °C, yet more preferably the sol and/or gel is not heated to a temperature greater than 100 °C. Higher temperatures are undesirable and may cause functional molecules and/or nanoparticles incorporated in the sol to be degraded. It is of particular benefit that when the functional molecules and/or nanoparticles are temperature-sensitive, the drying step is performed under vacuum.

In some embodiments, the functional molecules and/or nanoparticles may be water-sensitive. Therefore, preferably the solution and/or sol and/or gel are water free. As used herein the term 'water free' means that water is excluded such that the solution and/or sol and/or gel comprise less than 1 weight percent water.

In some embodiments, the functional molecules and/or nanoparticles may be acid-sensitive. Therefore, preferably the solution and/or sol and/or gel have a pH of 6.5 or greater.

In some embodiments, the functional molecules and/or nanoparticles may be base-sensitive. Therefore, preferably the solution and/or sol and/or gel have a pH of 7.5 or lower.

According to a second aspect of the present invention, there is provided a method of manufacturing functionalised borosilicate particles, comprising the steps of: providing a powder comprising borosilicate particles formed from a dried gel; and reacting the borosilicate particles with one or more functional molecule and/or nanoparticle. That is, the inventors have discovered that unreacted sol-gel forming moieties on the borosilicate particles may be reacted with functional molecules and nanoparticles to form covalent bonds with such functional molecules and nanoparticles.

Preferably, the step of reacting the borosilicate particles with a functional molecule and/or nanoparticle takes place in a solvent, preferably an aprotic solvent.

As discussed above, the inventors have discovered that low temperatures and less caustic reaction conditions when compared to glass melts previously used to produce borosilicate particles, preferably allows the production of powders comprising borosilicate particles which comprise functional molecules, moieties or nanoparticles.

Therefore, according to a third aspect of the present invention, there is provided a powder comprising borosilicate particles, wherein the particles comprise one or more functional molecule and/or nanoparticle.

The terms functional molecule and/or nanoparticle does not preclude that same are covalently bound with the borosilicate particles.

Optional features of the first and second aspects of the present invention may also be applied to the third aspect of the present invention in any combination, and vice-versa.

Without wishing to be bound by theory, it is thought that during the sol-gel process functional molecules and/or nanoparticles may be incorporated into the borosilicate particles, either by physical inclusion within pores and/or voids within the particles, or by chemical bonding between the functional molecule and/or nanoparticle and moieties on the surface of the matrix.

Alternatively, following preparation of the borosilicate particles, reactive groups on the surface of the particle may bond with functional molecules and/or nanoparticles.

Preferably, the one or more functional molecules and/or nanoparticles is/are encapsulated in the borosilicate particles. A functional molecule and/or nanoparticle that is encapsulated in the borosilicate particles is preferably combined with the borosilicate particles in such a way that it cannot be removed without at least partial destruction of the particles or denaturing of the functional molecule and/or nanoparticle. For example, the functional molecule and/or nanoparticle may be: physically included within the pores and/or voids such that it cannot escape; and/or chemically bonded to moieties on the surface of the matrix such that they are held within pores and/or voids within the particles.

In particular, the functional molecule and/or nanoparticle may comprise a chromophore, a fluorophore, an antimicrobial agent, bonding moieties, catalysts, or a combination of these.

In some embodiments, the borosilicate particles comprise a matrix wherein the molar ratio of B₂O3:SiO2 measured by decomposition is from 1:99 to 99: 1. In particularly preferred embodiments, the borosilicate particles comprise a matrix wherein the molar ratio of BzC^SiCh measured by decomposition is from 5:95 to 20:80, more preferably from 10:90 to 15:85. Borosilicate particles comprising a matrix where the molar ratio of BzC^SiCh measured by decomposition is within this range are particularly beneficial in terms of chemical and mechanical resistance.

The powders may be used in a variety of contexts. Therefore, according to a fourth embodiment of the present invention there is provided the use of a powder produced by the method of the first embodiment, or produced by the method of the second embodiment, or according to the third embodiment, in a glaze, solder, coating, pressed solid, cosmetic, paint, medicament or electronic device.

The invention will now be described by way of example only and with reference to the accompanying figures, in which:

Figure 1 depicts the expected and measured B2O3 content of example powders in mol% plotted against the TMB content of the starting solution in mol%;

Figure 2 depicts infrared spectra of an example powder and a comparative example powder;

Figure 3 depicts the expected and measured B2O3 content of example powders in mol% plotted against the TMB content of the starting solution in mol%;

Figure 4 depicts infrared spectra of example powders; Figure 5 depicts infrared spectra of an example powder heated to different temperatures between 180 °C and 600 °C;

Figure 6 depicts the thermogravimetric analysis of example G;

Figure 7 depicts the thermogravimetric analysis of example powders produced using varying solvents;

Figure 8 depicts the infrared spectra of example powders produced using varying solvents;

Figure 9 depicts SEM images of borosilicate particles;

Figure 10 depicts SEM images of pores upon borosilicate particles;

Figure 11 depicts the infrared spectra of an example powder, Rhodamine 6G, and an example powder comprising Rhodamine 6G; and

Figure 12 depicts a chart of water content of powders measured by Karl Fischer analysis.

The general procedure for preparing example powders was as follows: a solution of trimethoxyboroxine (TMB) and tetraethylorthosilicate (TEOS) was stirred for three weeks in sealed oven-dried glassware under air, followed by drying in a crucible to 200 °C in air to remove solvents and volatile components, giving a powder in the form of white flakes. In some cases, transparent flakes were formed. Where solvent was used, the amount of solvent was equivalent to half the total moles of the two starting materials. Where the solvent was acetone 0.5 mol% of trifluoroacetic acid was added to the solution as catalyst. Tetraethylorthosilicate (TEOS) was used as received from Sigma Aldrich. Trimethoxyboroxine (TMB) was used as received from Fisher Scientific, TCI or Sigma Aldrich, with a purity of 95% or above. Solvents were purchased from Rathburn Chemicals.

A first series of example powders Al to A9 was prepared to investigate the effect of the starting material ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide upon the matrix composition of the resultant particles.

Example powders Al to A9 were prepared according to the general procedure by stirring with acetone solvent at room temperature for 3 weeks, and the relative amounts of TMB and TEOS starting materials were varied as in Table 1. For each example powder Al to A9 the expected decomposition ratio of B₂O3:SiO2 was calculated, and the actual decomposition measured using Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES), assuming all boron was present as B2O3 and all silicon was present as SiO2, as described in Table 1. A comparative example, a ground sample of Borofloat 33 available from Schott, was included for reference.

Table 1 - Examples of powders and their expected decomposition and measured decomposition ratios.

A clear solution was observed upon stirring for the maximum reaction time of 3 weeks, except for solutions with higher boron contents (solutions A7-A9) which developed a white/yellow milky appearance after a few weeks.

Figure 1 depicts the results of Table 1 in graphical form, wherein the expected and measured B2O3 content of the resultant powder in mol% is plotted against the TMB content of the starting solution in mol%. The graph shows a generally linear trend in the amount of B2O3 in the final powder relative to SiCh as the mol% proportion of TMB added to the starting solution is increased above 40%. At lower TMB:TEOS ratios the points deviate from the trend. However, in all cases the B2O3 content is lower than the expected decomposition ratio. This difference is thought to be due to the loss of volatile borate species formed in early stages of the reaction.

These results show that the B₂O3:SiO2 ratio of the composition of powders comprising borosilicate particles can be tuned by varying the ratio of the starting materials. As such, in some embodiments the ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide is preferably from 0.05: 1 to 7: 1, more preferably from 0.1: 1 to 2: 1, and even more preferably from 0.1: 1 to 1: 1. Notably, the powder of example A4 was most compositionally similar to the comparative example. Therefore, in particularly preferred embodiments, the ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide is preferably from 0.2: 1 to 0.8: 1, even more preferably from 0.3: 1 to 0.6: 1.

The comparative example (CE) is a borosilicate powder known to demonstrate beneficial properties, including chemical resistance, but has the drawbacks of being produced by a high- temperature melting process. In addition, when submitted to melting point analysis, this comparative example powder undergoes a phase change at 915 to 920 °C, and is fully melted at 980 to 990 °C. However, an example powder according to the invention produced via a water- free reaction under gentle reaction conditions by using a ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide of 0.44: 1 did not undergo a phase change or melt even when exposed to temperatures in excess of 1400 °C. It is thought that the borosilicate powder of the comparative example comprises fluxing elements, which are essential to the making process but which are also undesirable in some applications of the end use of a powder.

Figure 2 depicts the infrared spectra for example A4, TMB:TEOS ratio 0.44: 1, against comparative example 1, Borofloat 33. Notably, as shown in Figure 4, the powder of example A4 gave the most similar infrared spectra to that of Borofloat 33. This is consistent with ICP-OES analysis and suggests that both the composition and vibrational structure of the powder prepared via an organic liquid route are similar to that of reference borosilicate glass.

Herein, it has been shown that a compositionally and vibrationally comparable powder to a borosilicate powder of the comparative example may be produced via water-free and gentle reaction conditions by using a ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide of 0.44: 1, and that such a powder may have a very high melting point and temperature stability. Therefore, in particularly preferred embodiments, the ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide is preferably from 0.2: 1 to 0.8: 1, even more preferably from 0.3: 1 to 0.6: 1.

A second series of example powders was prepared using THF as a solvent, and the relative amounts of TMB and TEOS starting materials were varied. As shown in Figure 3, the measured mol°/o of B2O3 in the resultant powder was below the theoretical amount for lower TMB mol% solutions, but the relationship became more linear with higher TMB mol% solutions.

A third series of example powders was prepared using a neat solution of TMB and TEOS, that is, without including a solvent, and the relative amounts of TMB and TEOS starting materials were varied. As shown in Figure 3, neat solutions provided a higher amount of B2O3 in the powder at low TMB mol% solutions compared to solutions with solvents, but still lower than the theoretical amount.

The structure of the example powders was further investigated by FTIR spectroscopy. Figure 4 depicts the infrared spectra for example powders formed from the following ratios of TMB:TEOS from top to bottom: 0.06: 1, 0.16: 1, 0.28: 1, 0.44: 1, 0.67: 1, 1: 1, 1.56: 1, 2.33: 1, 6.14: 1. Each example powder in Figure 3 was formed by stirring for 3 weeks in THF, followed by drying in a crucible to 200 °C, then heating in a crucible to 550 °C.

In general, bands relating to boron groups increase and bands relating to silicon groups decrease with the TMB:TEOS ratio, which is consistent with ICP-OES analysis which showed an increase in B2O3:SiO2 ratio as the amount of TMB in the starting solution increased. The broad band at ca. 1300 cm ¹ (related to BO3 and BO4 vibrations), sharp band at 1190 cm ¹ (attributed to B-OH⁴'¹⁴) and new band at ca. 720 cm ¹ (B-O-B vibrations) grow as the TMB:TEOS ratio increases. The band relating to the Si-0 vibration at 1050 cm ¹ decreases. The bands at ca. 915 cm ¹ and 670 cm ¹ relating to the Si-O-B bend and stretch, grows in intensity up to 1.56: 1 TMB:TEOS, then decreases as the amount of TMB increases further. The band at 3200 cm ¹ is tentatively assigned to overlapping Si-OH and B-OH vibrations, but may also be due to absorbed water molecules which would suggest an increase in the hygroscopic nature of the powders as the amount of B2O3 increases.

An example powder was prepared by stirring a neat solution of TMB and TEOS with TMB:TEOS ratio of 0.44: 1 for 2 weeks then curing at 180 °C, and then portions of the powder were dried at a range of different temperatures between 200 °C and 550 °C. The resultant cured powders were submitted to CHN elemental analysis using a Vario MICROcube Analyser, the results are depicted in Table 2. Table 2

As demonstrated by the elemental analysis results depicted in Table 2, curing at 200 °C results in a powder which comprises 18.6 atomic % carbon and 4.13 atomic % hydrogen. It is thought that these atoms are present in unreacted TMB and/or TEOS moieties. These moieties may be within the particles, or on the surface. Unreacted surface moieties may be available to be further functionalised, leading to functionalised particles. Such functionalisation is not possible with conventional and/or powders, such as that of the comparative example.

Curing at 350 °C results in a powder with greatly reduced carbon and hydrogen content. It is thought that any unreacted moieties are denatured by this temperature.

Curing at 550 °C in air results in a powder with slightly increased carbon content, and reduced hydrogen content, while curing at 550 °C in argon results in a powder with reduced carbon and hydrogen content, compared to curing at 350 °C. It is thought that the particle is absorbing carbon dioxide and water from the atmosphere when cured at this temperature.

An example powder G was prepared by stirring a neat solution of TMB and TEOS with TMB:TEOS ratio of 0.44: 1 for 2 weeks. Portions of the example powder G were heated to different temperatures between 180 °C and 600 °C and then analysed by FTIR, the resultant spectra are depicted in Figure 5. The spectra indicate that changes in the powder occur when heating between 180 °C and 350 °C, but there are no significant structural changes when heating to temperatures greater than 350 °C up to 600 °C. In addition, a portion of the example powder G was submitted to thermogravimetric analysis as depicted in Figure 6. As can be seen from Figure 6 there is a large loss of mass between 100 and 200 °C, thought to be the evaporation of unreacted starting materials and volatile trialkyl borate species, which was confirmed by evolved gas analysis. Thereafter, the powder remains stable until approximately 350 °C, where another loss thought to be due to denaturing of unreacted surface moieties occurs. As such, borosilicate particles produced via the present methods may be functionalised by reaction with surface moieties which are present after the preparation of the particles. For example, they may be functionalised in order that they comprise functional molecules and/or nanoparticles. Alternatively, borosilicate particles produced via the present methods may be immobilised upon substrates by binding with such surface moieties.

Further thermogravi metric analysis was conducted on powders produced using neat solution, or acetone or THF solvent with TMB:TEOS ratio of 0.44: 1. These powders were cured to 200 °C prior to thermogravimetric analysis, which is depicted in Figure 8. These powders each show similar mass loss steps, associated with the loss of unreacted surface moieties, but the end weight percentage is highest for THF, then acetone, then neat solution. As such, it is thought that the use of a solvent, especially THF, results in a more developed matrix that has less unreacted moieties when compared to a neat solution, while still providing particles and/or an aggregate of particles upon drying.

To investigate the effect that solvent has upon the resultant powders, powders produced using neat solution, or acetone or THF solvent were heated to 550 °C, allowed to cool, and analysed by ICP-OES, the results are depicted in Table 3.

Table 3

Analysis by ICP-OES indicates that the boron to silicon ratio is not greatly affected by the solvent selection. These powders were submitted to FTIR analysis, and Figure 8 depicts the infrared spectra of these powders, in which it can be seen that there is not a major difference in the spectra between a neat solution and acetone solvent, while THF solvent produces a slightly different spectra. The weaker peak at 1050 cm ¹ is thought to be due to a reduction in Si-0 bonding, potentially due to more homogenous particle formation.

Table 4

Table 4 provides particle size information for powders produced according to the present method with different ratios of B₂O3:SiO2. The powders were produced by grinding aggregate dried gels of sols which were produced by stirring TMB and TEOS neat for 6 days, and sizes gathered from SEM images by measuring the diameter of the particle. SEM images are included herein as Figures 9 and 10, wherein Figure 9 shows the particles whole at lower magnification, and Figure 10 shows the structure of the pores upon the borosilicate particles at higher magnification.

It can be seen that for all compositions a wide range of particle sizes can be provided. Preferably the powder comprises borosilicate particles with a particle size from 18 to 500 pm, preferably from 100 to 200 pm. In some cases the powder may be processed to produce a powder with a particle size distribution such that 90% or more of particles have a particle size from 100 to 200 pm. Such processing may include size filtration.

The size of pores, where present, is unaffected by heating up to 550 °C. Preferably, the powder comprises particles with pores. Preferably, the pores have a size of from 200 to 400 nm. It can be seen that an increase in B2O3 in the particle matrix leads to greater incidence of pores. Therefore, preferably the particles comprise a BzC SiCfc ratio of at least 60:40, preferably 70:30, and further comprise pores, preferably pores at least 200 nm in size. Such particles may be beneficially combined with functional molecules and/or nanoparticles.

Figure 12 depicts the water content of powders with varying BzC^SiCh ratio. These powders were produced from neat solutions of TMB and TEOS stirred for 6 days, dried then heated to 550 °C in air. The resultant powders were left in air and the water content measured over time by Karl Fischer analysis. It can be seen that the powders are hydroscopic, with the greatest % water increase for 60:40 BzC^SiCh ratio.

A series of example powders was produced which comprise boron, silicon and titanium by stirring a solutions of TMB, TEOS and titanium isopropoxide for 1 day, varying the ratios of the starting materials. The titanium isopropoxide was seen to accelerate the gelling process. The resultant gels were heated to 300 °C to remove unreacted starting materials then submitted to ICP-OES analysis. The compositions of the resultant powders are depicted in Table 5.

Table 5

As shown in Table 5, it is possible to repeatedly produce particles comprising boron, silicon and titanium using the presently disclosed method. Preferably, the mol % of titanium based on oxides is from 5 to 25, more preferably from 10 to 20, even more preferably from 12 to 17.

A series of example powders comprising boron, silicon and aluminium was produced by stirring a solutions of TMB, TEOS and aluminium butoxide for 1 day, with varying ratios of starting materials. The aluminium butoxide was seen to accelerate the gelling process. The resultant gels were heated at 200 °C to remove unreacted starting materials then submitted to ICP-OES analysis. The compositions of the resultant powders are depicted in Table 6. Table 6

As shown in Table 6, it is possible to create produce particles comprising boron, silicon and aluminium by the present methods. Preferably, the mol % of aluminium as AI2O3 from 5 to 20, more preferably from 10 to 18, even more preferably from 12 to 15.

A series of example powders comprising boron, silicon and zirconium was produced by stirring a solutions of TMB, TEOS and Zirconium(IV) isopropoxide for 1 day, with varying ratios of starting materials. The resultant gels were heated to remove unreacted starting materials, samples El, E3, E5 were heated at 300 °C, while samples E2, E4 and E6 were heated at 550 °C. The powders were submitted to ICP-OES analysis to provide the compositions depicted in Table 7. All zirconium borosilicate powder examples produced XRD diffractograms with no measurable reflections, and appear to be amorphous and/or have low crystallinity.

Table 7

As shown in Table 7, it is possible to create produce particles comprising boron, silicon and zirconium by the present methods. Preferably, the mol % of Zirconium as ZrCh is from 5 to 25, more preferably from 10 to 20.

Example powders were made which included functional molecules and/or nanoparticles. Dye containing particles were prepared as follows.

An example powder was made by stirring TMB with TEOS in a solution of THF including organic dye Rhodamine 6G, and then heating the resultant gel to 200 °C to form a solid, then milling the solid to provide a borosilicate powder comprising Rhodamine 6G dye. The powder was rinsed to remove free dye, and then submitted to FTIR analysis. The infrared spectra is depicted in Figure 11, wherein the top spectrum is that of a borosilicate powder without dye, the middle spectrum is the infrared spectrum of Rhodamine 6G dye alone, and the bottom spectrum is the infrared spectrum of the borosilicate powder comprising Rhodamine 6G dye. Hatched rectangles are used to show matching peaks between the spectra. As such, it can be seen from Figure 10 that the Rhodamine 6G dye was successfully incorporated into the borosilicate particles. This result was confirmed visually, as the particles were visibly pink.

An example powder was made by stirring TMB with TEOS in a solution of IPA including organic dye PV19, and then heating the resultant gel to 200 °C to form a solid, then milling the solid to provide borosilicate powder comprising PV19 dye. The powder was rinsed to remove free dye. The PV19 dye was successfully incorporated into the borosilicate particles. This result was confirmed visually, as the particles were visibly purple. The colour remained even when the powder was heated to up to 300 °C.

As such, the conditions of the borosilicate forming reaction are sufficiently mild that organic functional molecules such as dyes may be incorporated in the particles.

An example powder was made by stirring TMB with TEOS in a solution of IPA including organicmetallic dye Blue 15:3, and then heating the resultant gel to 200 °C to form a borosilicate powder comprising Blue 15:3 dye. The powder was rinsed to remove free dye. The Blue 15:3 dye was successfully incorporated into the borosilicate particles. This result was confirmed visually, as the particles were visibly blue. This colour remained even when the particles were heated to 300 °C.

An example powder was made by stirring TMB with TEOS in a solution of IPA including inorganic dye Blue 36, Cobalt Chromite Blue-Green Spinel, and then heating the resultant gel to 200 °C to form a borosilicate powder comprising Blue 36 dye. The powder was rinsed to remove free dye. The Blue 36 dye was successfully incorporated into the borosilicate particles. This result was confirmed visually, as the particles were visibly blue. This colour remained even when the particles were heated above 300 °C, and even when heated to 650 °C.

An example powder was made by stirring TMB with TEOS in a solution of IPA including inorganic dye PG 26, Cobalt Chromite Green Spinel, and then heating the resultant gel to 200 °C to form a borosilicate powder comprising PG 26. The powder was rinsed to remove free dye. The Blue 36 dye was successfully incorporated into the borosilicate particles. This result was confirmed visually, as the particles were visibly green. This colour remained even when the particles were heated above 300 °C, and even when heated to 650 °C.

Claims

23 Claims

1. A method of manufacturing a powder comprising borosilicate particles, the method comprising the steps of: mixing a solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide to form a sol; aging the sol to form a gel; and drying the gel to form particles directly, and/or drying the gel to form an aggregate of particles and grinding the aggregate of particles.

2. A method according to claim 1, wherein the solution comprises a solvent and/or the method further comprises a step of cooling the gel to a temperature less than 10 °C following the step of aging the sol to form a gel.

3. A method according to claim 2, wherein the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a solvent, preferably the solvent is selected from acetone, tetra hydrofuran (THF) and dimethylsulfoxide (DMS), most preferably tetrahydrofuran (THF).

4. A method according to claim 2 or claim 3, further comprising a step of cooling the gel following the step of aging the sol to form a gel, wherein the gel is cooled to a temperature less than 10 °C, preferably the gel is cooled to a temperature less than 5 °C, more preferably the gel is cooled to a temperature less than 2 °C.

5. A method according to any preceding claim, wherein the trialkoxyboroxine is of the formula:

wherein R¹, R² and R³, which may be the same or different, each represent a substituted or unsubstituted alkyl group.

6. A method according to claim 5, wherein R¹, R² and R³ are the same and wherein: each is methyl such that the trialkoxyboroxine is trimethoxyboroxine (TMB); or each is ethyl such that the trialkoxyboroxine is triethoxyboroxine (TEB).

7. A method according to any preceding claim, wherein trialkoxyboroxine is selected from the group comprising: tri(2-chloroethoxy)boroxine, tri(2,2-dichloroethoxy)boroxine, tri(2,2,2- trichloroethoxy)boroxine, tri(3-chloro-l-ethoxy)boroxine, tri(l,3-dichloro-2-propoxy)boroxine, tri(4-chloro-l-butoxy)boroxine, tri(3-trifluoromethylbenzyloxy)boroxine, tri(2- fluorobenzyloxy)boroxine, tri(3-fluorobenzyloxy)boroxine, tri(4-fluorobenzyloxy)boroxine, tri(pentafluorobenzyloxy)boroxine, tri(2,2,3,3-tetrafluorobenzyloxy)boroxine, tri(lH,lH- pentafluoropropoxy)boroxine, tri(lH,lH,5H-octafluoropentoxy)boroxine, and tri(lH,lH- heptafluorobutoxy)boroxine.

8. A method according to any preceding claim wherein the orthosilicate is a tetrasubstituted orthosilicate, preferably tetraethylorthosilicate (TEOS).

9. A method according to any preceding claim, wherein the silicon alkoxide is of the formula

A*p[Si(OR')_Q]sBT where:

A* is an alkyl group; Si is silicon; R' is an alkyl, alkenyl, allyl, alkynyl, aryl, or aryloxy group; B is an electron-withdrawing group; P is 0, 1, or 2; Q is 1, 2, 3, or 4; S is from 1 to 20 and T is the difference between the valency of Si and the sum of P and Q.

10. A method according to claim 9, wherein:

A* is a lower alkyl group containing 1 to 10 carbon atoms; and/or R' is a lower alkyl group containing 1 to 10 carbon atoms; and/or Q is at least 2 and the R' groups are the same as one another, preferably wherein each R' is a methyl group or wherein each R' is an ethyl group.

11. A method according to any preceding claim, wherein the silicon alkoxide is selected from the group comprising: silicon tetraacetate, methyltriethoxysilane (MTES), dimethyldiethoxysilane (DMDES), and phenyl triethoxysilane (PhTES).

12. A method according to any preceding claim, wherein the ratio of trialkoxyboroxine to orthosilicate and/or silicon alkoxide in the solution is from 0.05:1 to 7:1, preferably from 0.1:1 to 2:1, more preferably from 0.1: 1 to 1:1, even more preferably from 0.2: 1 to 0.8:1, yet even more preferably from 0.3:1 to 0.6:1.

13. A method according to any preceding claim, wherein the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a catalyst, preferably the catalyst is an acid.

14. A method according to claim 13, wherein the catalyst is trifluoroacetic acid, preferably the molar amount of trifluoroacetic acid in the solution is at least two orders of magnitude less than the molar amount of the lesser of trialkoxyboroxine, orthosilicate or silicon alkoxide.

15. A method according to any preceding claim, wherein the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a metal alkoxide, preferably a metal alkoxide of the formula

A*P[M(OR')Q]SB_T where:

16. A method according to claim 15, wherein: A* is a lower alkyl group containing 1 to 10 carbon atoms; and/or R' is a lower alkyl group containing 1 to 10 carbon atoms; and/or Q is at least 2 and the R' groups are the same as one another, preferably wherein each R' is a methyl group or wherein each R' is an ethyl group.

17. A method according to claim 15 or 16, wherein the metal alkoxide is: a tetraalkoxy titanium compound, a tetralkoxy zirconium compound or a trialkoxy aluminium compound, preferably selected from the group comprising titanium(IV) ethoxide, titanium(IV) methoxide, titanium(IV) isopropoxide, titanium(IV) butoxide, zirconium(IV) ethoxide, zirconium(IV) methoxide, zirconium(IV) isopropoxide, zirconium(IV) butoxide, aluminium ethoxide, aluminium methoxide, aluminium isopropoxide, and aluminium butoxide. 26

18. A method according to any preceding claim, wherein during the step of aging the sol to form a gel the sol is maintained at a temperature from 0 to 100 °C, more preferably from 5 to 50 °C, yet more preferably from 10 to 30 °C, even more preferably from 15 to 25 °C.

19. A method according to any preceding claim, wherein the solution comprising a trialkoxyboroxine and an orthosilicate and/or silicon alkoxide further comprises a functional molecule and/or nanoparticle.

20. A method according to any preceding claim, wherein the step of drying the gel is carried out under a vacuum.

21. A method of manufacturing functionalised borosilicate particles, comprising the steps of: providing a powder comprising borosilicate particles formed from a dried gel; and reacting the borosilicate particles with one or more functional molecule and/or nanoparticle.

22. A powder comprising borosilicate particles, wherein the particles comprise one or more functional molecule and/or nanoparticle.

23. A powder according to claim 22, wherein the functional molecule and/or nanoparticle is encapsulated in the borosilicate particles.

24. A powder according to claim 22 or 23, wherein the borosilicate particles comprise a matrix wherein the molar ratio of BzC SiCh of the matrix measured by decomposition is from 5:95 to 20:80, more preferably from 10:90 to 15:85.

25. Use of a powder comprising borosilicate particles manufactured by the method of any of claims 1 to 21, or according to any of claims 22 to 24, in a glaze, solder, coating, pressed solid, cosmetic, paint, medicament or electronic device.