WO1998042627A1

WO1998042627A1 - Sol-gel synthesis of alkali-free borosilicate glass

Info

Publication number: WO1998042627A1
Application number: PCT/GB1998/000837
Authority: WO
Inventors: Karikath Sukumar Varma
Original assignee: Pilkington Plc
Priority date: 1997-03-20
Filing date: 1998-03-19
Publication date: 1998-10-01
Also published as: GB9705764D0; AU6511498A

Abstract

A method of producing metalloborates (including silicoborates) comprising reacting a trisubstituted boroxine with a tetrasubstituted orthosilicate or metal alkoxide in the substantial absence of hydroxyl groups to form a sol-gel so that the rate of reaction may be controlled and that the time taken for the reaction mixture to form a gel is substantially increased. The reaction may be carried out in the presence of a solvent and of a catalyst.

Description

SOL-GEL SYNTHESIS OF ALKALI-FREE BOROSILICATE GLASS

The present invention relates to a method of producing a sol-gel. A sol-gel is basically a reaction product which is initially formed as a liquid but which, as the reaction proceeds, forms a gel and, ultimately a solid. Such substances, in their liquid form, can be used as materials for coating other materials such as glass or, in their solid form, can be formed into monolithic structures. More particularly, the present invention is directed to a process in which the sol-gel is a borosilicate structure.

It is well known to produce borosilicate glass by the "Sol-Gel" method. Thus, according to Tohge N„ Matsuda A. and Minami T., J. Am. Ceram. Soc, C-13-15 (1987), such a method comprises boiling tetraethylorthosilicate (hereinafter referred to as TEOS) which has the formula Si(OEt) under reflux in the presence of ethanol and nitric acid for four hours at 70°C. Triisopropylborate B(O-iPr)₃ is then added and the mixture boiled under reflux for a further four hours at 70°C.

In a modification of this process, as described in Mat Res. Soc. Symp. Vol 32 (1984), D.M. Haaland and C.J. Brinker reacted the TEOS with trimethylborate B(OCH₃)₃ in ethanol in the presence of HC1. Pellegri et al. Jour. Sol-Gel Science and Technology 2, 519-523 (1994) carried out the same reaction bur with nitric acid and also including sodium acetate in the formulation to act as a precursor for Na₂O inclusion. In both of these cases, boiling under reflux at 70°C for four hours is necessary to effect partial hydrolysis of the TEOS. Furthermore, the final reaction necessitated stirring of the reaction mixture for at least 24 hours at room temperature.

In a further known system, as disclosed in Andrianov et al. Inst. Hetero-organic compounds; Acad. Sci. USSR., diacetoxy methyl phenyl silane is reacted with tributyl borate and dibutoxy methyl phenyl silane in the presence of boron triacetate. Such reaction is carried out at 170°C and the reaction time is between 3 and 24 hours. Irwin et _§]_, J. Non-Crystalline Solids 89(1987) 191-205, describes the reaction of TEOS with tris(trimethyl siloxy) boron using both acid, using HC1, and basic, using NH4OH, catalysis. Again, the reaction would only take place by boiling under reflux.

Referring back to Tohge, as referenced above, TEOS is hydrolysed with water in the presence of a mineral acid and ethanol to form silanol , Si(OH)₄. Silanol is an extremely reactive compound and will undergo a condensation reaction with itself to form a siloxane with the elimination of water which causes accelerates the reactions. The siloxane structure will cross-link. The silanol can, as indicated above, be reacted with a trialkyl borate whereby a cross-linked structure containing Si-O-B bonds will be produced. In either of these cases, the reaction product is initially a liquid but, once the polymerisation has continued to a certain degree, that is to say, the molecular weight of the polymer exceeds a certain level, a gel is formed. Ultimately, instead of a gel, a solid is formed. At a late stage in the reaction process, water and die ethanol must be removed.

Instead of permitting the elimination of water in a condensation reaction of two molecules of silanol with one another, the silanol may be reacted with a metal alkoxide such as a tetraalkoxy titanium. If such reaction is carried out, instead of a siloxane lattice having a plurality of Si-O-Si bonds, the lattice will now also include Si-O-Ti and possibly Ti-O-Ti bonds. By carrying out this modification, it is possible to change the properties of the reaction product. The other prior art work referred to above is basically concerned with modifying the starting material. This in turn determines the alcohol which is produced during the silanol-formation step and this determines what product is finally obtained after drying and removal of the alcohol. Alternatively, the mineral acid has been altered or which alcohol is included in the reaction mixture. These latter, particularly the alcohol, alter the rate at which the reactions occur.

As well as siloxane gels, borate gels have also been prepared. These have, hitherto, been primarily derived from alkoxides and alkoxyborates. In alkoxy borates, d e boron is trigonal coplanar with sp² hybridisation. In this configuration, the boron is very electrophilic and is able to undergo a nucleophilic addition reaction to form a tetrahedrally coordinated species in the presence of a strong nucleophile such as an alkoxide. The gel formation takes place by hydrolysis and condensation reactions which involve nucleophilic attack of -OH or - OR groups on trigonal boron with the elimination of alcohol and water by S mechanisms. Since tetrahedrally coordinated boron atoms lack sp³d hybridisation, they are unlikely to undergo nucleophilic reactions. They are, therefore, kinetically stable towards hydrolysis or alcoholysis and no condensation reactions are possible between two tetrahedrally coordinated boron species. The most likely route to the formation of a polyborate network is therefore through the condensation of hydrolyzable trigonal boron monomers and unhydrolyzable tetrahedral boron monomers forming primary units of six-membered rings. Additional hydrolysis of -OR ligands bonded to trigonal boron atoms is required to form extended polymers in which the primary units are linked by B-O-B bonds. However, in the presence of moisture, die six-membered boroxine ring will be hydrolysed to non-cyclic organoborates and boric acid.

Multi-component gels are also known. In particular, borosilicate gels are of interest At present, there are two main types of synthesis of such gels, one being an aqueous system and the other being an alkoxide based system. In d e former, the aim is to co-precipitate different oxides which necessitates the choice of suitable precursors. In the latter, eitiier mixed-alkoxides or metal precursors thereof are hydrolysed or alkoxides are sequentially added to partially hydrolysed precursors. Whichever method is used, water and, usually, an inorganic acid or base are required to hydrolyse the reactants and to provide some control over the sol to gel transformation.

The disadvantages of these known methods are that it has been considered necessary to use fairly strong reaction conditions, such as boiling under reflux, to bring about the formation of Si - O -B bonds. Moreover, conventional wisdom has it that it is necessary to have water, or at least hydroxyl groups, present so as to bring about the hydrolysis reactions. There is, therefore, the likelihood of the sol including unreacted water. If one then attempts to use the sol to provide a coating on glass, such water, either on its own or in conjunction with water vapour in the atmosphere, is likely to attack the boron part of the sol, hydrolysing it to boric acid. In practical terms, this means that the coating is hazy which is totally undesirable.

The present invention ti erefore seeks to provide a method of producing a sol-gel which overcomes the disadvantages of the prior art systems. More particularly, the present invention seeks to provide a method of making a borosilicate sol-gel which overcomes the above-identified disadvantages by reducing the severity of the reaction conditions necessary to produce the Si -O - B bonds and which obviates, or at least minimises the possibility of breakdown of the sol-gel after it has been formed by the action of hydroxyl groups, particularly water.

According to the present invention, there is provided a method of making a borosilicate sol-gel comprising reacting a tri-substituted boroxine of the formula:- OR- , I β

₃0 ^■ e> o€.. y

wherein Ri, R₂ and R₃, which may be the same or different, each represent a substituted or unsubstituted lower alkyl group containing 1 to 5 carbon atoms, alkenyl or alkynyl with a tetrasubstituted orthosilicate or metal alkoxide of the formula :- A_P[M(OR')Q]_SB_T in which A is a lower alkyl group containing from 1 to 10 carbon atoms, M is a metal selected from Groups 3,4,5 and 6b of d e periodic table (including silicon), R' is a lower alkyl group containing 1 to 10 carbon atoms, alkenyl, alkynyl or aryl, B is an electron-withdrawing group, P is 0,1 or 2, Q is 1,2,3 or 4, S is 1 to 20 and T is the difference between the valency of M and the sum of P and Q with the provisos that the metal must be capable of expanding its coordination number, the metal centre must be capable of coordination bonding to an oxygen group of the boroxine and die ligand on the boroxine must be attracted to d e metal of the orthosilicate or metal alkoxide.

Surprisingly, we have found that by carrying out the reaction in this manner, the hydrolysis and condensation reactions occur but at a controlled rate. The reason why this should be so is not fully understood but two possible reasons are that boroxines of d e above type are Lewis acids whilst most B-(OR)₃ compounds (where R is alkyl) are not It is also believed mat the reaction of the boroxine with the orthosilicate involves the breaking of a single bond in the boroxine ring which leads to dimerisation and the breaking of a further ring bond to cause subsequent polymerisation.

It is preferred if R_, R₂ and R₃ are die same as one another and each represents a methyl or ethyl group. In one embodiment of die present invention, if more tiian one OR' groups are present, these may be the same or different are identical to one another. Desirably, however, each represents a methyl or ethyl group. Alternatively or additionally, any one or more of die R' groups may be an alkyl group substituted with an electron withdrawing group. In such a case, die electron witiidrawing group may be a carbonyl group or a halo-group. We have made a range of halogenated boroxines which are useful in the context of the present invention. These halogenated boroxines include tri(2-chloroethoxy)boroxine, tri(2,2- dichloroethoxy)boroxine, tri(2,2,2-trichloroethoxy)boroxine, tri(3-chloro- 1 - propoxy)boroxine, tri( 1 ,3-dichloro-2-propoxy)boroxine, tri(4-chloro- 1 -butoxy)boroxine, tri(3-trifluoromedιylbenzyloxy)boroxine, tri(2-fluorobenzyloxy)boroxine, tri(3- fluorobenzyloxy)boroxine, tri(4-fluorobenzyloxy)boroxine, tri(2,3,4,5,6- pentafluorobenzyloxy)boroxine, tri(2,2,3,3-tetrafluoropropoxy)boroxine, tri(lH,lH- pentafluoropropoxy)boroxine, tri(lH,lH,5H-octafluoropentoxy)boroxine and tri(lH,lH- heptafluorobutoxy)boroxine. In a particularly preferred embodiment of d e present invention, however, the boroxine is trimethylboroxine and the silicate is tetraetiiyl orthosilicate.

Alternatively, Ri, R₂ and/or R₃ may represent an allyl or aryloxy group which may be substituted or unsubstituted. If substituted, it is preferred if the one or more substituents are halo-groups. The method may, if desired be carried out in d e presence of a solvent.

A preferred solvent is tetrahydrofuran. Alternatively, d e solvent may be a polar solvent used in the presence of an acid. In such a case, it is preferred if the solvent is acetone and the acid is trifluoroacetic acid. In tins embodiment, die amount of acid used is extremely small and is of d e magnitude of 10²M if the orthosilicate and the boroxine concentrations are substantially molar.

The process of the present invention is preferably carried out at room temperature. However, there may be occasions when heating is desirable.

It is believed that the process of the present invention leads to the initial formation of both polyborates and polysUicates which co-exist in the same mixture in the form of microgels. Normally, these would tiien react to form die polysilicate gel which would solidify reasonably rapidly. Accordingly, in a preferred aspect of the present invention, d e reaction mixture is cooled which prevents the formation of the borosilicate. We have found that such cooling prevents e mixture from solidifying for a length of time which is of d e order of months. When it is desired to use d e mixture, it is simply necessary to allow d e reaction mixture to return to room temperature.

Alternatively, the reaction may be allowed to proceed to completion whereupon a glassy or ceramic, so-called "monolithic" structure is produced which is then heated to a temperature of below 600°C, preferably below 400°C, so as to form a glass.

One of the chief intended uses of die sol-gel of the present invention is in die field of coating glass. It will be readily appreciated that, because the reaction product does not gel or solidify until d e temperature tiiereof is raised to room temperature, coating glass widi d e reaction product can be effected in a controlled manner and makes it possible to produce higher quality coatings than has hitherto been possible using the sol-gel route.

The invention will be further described, by way of illustration only, with reference to die following non-limitative Examples. Example 1

Trimethoxyboroxine (17.3g) and tetraethylorthosilicate (20.8g) in equimolar concentrations were mixed in completely dry glassware which had been flame-dried and were allowed to react for two hours at room temperature. Trifluoroacetic acid (0.8g : approximately 0.01M) was used as a catalyst. The decomposition products of trimethoxyboroxine (hereinafter referred to as TMB) and tetraethylorthosilicate (hereinafter referred to as TEOS) are assumed to be B₂O₃ and SiO₂ respectively and the equimolar amounts of TMB and TEOS should, in tiieory, produce the decomposition products in the ratio of 60:40 (B₂O₃ : SiO₂). The reactants are then diluted with acetone (220g) which had been dried over molecular sieves.

The reaction product will gel in six to eight hours at room temperature. However, once the gel has formed, it is of no use for coating purposes. Accordingly, die reaction was stopped after four hours by refrigeration. The reaction product is used to coat glass by a dipping process. In general terms, the coating produced can be modified by altering die viscosity of the material and/or by varying the rate at which the glass is introduced into and withdrawn from die reaction product. Conversely, if the viscosity of the reaction product is known, the thickness of the coating produced can be determined. Example 2

The procedure adopted in Example 1 was followed with the exception that the amount of TMB was varied. In this Example, 12g of TMB was utilised witii the aim of producing a decomposition products ratio of 50:50. At room temperature, gelation occurred approximately eight hours after the TMB and TEOS were mixed. Example 3

Example 1 was repeated again with only 8g of TMB being employed with the aim of producing a 40:60 B₂O₃:SiO₂ ratio. The gelation of the product was slowed but nevertheless occurred within 24 hours. Example 4

In this Example, the amount of TMB was reduced still further to 5 g so as to produce a 30:70 B₂O₃:SiO₂ ratio. Gelation of the product was noticeably slower and the reaction mixture remained liquid for a period in excess of 48 hours. Example 5

In an attempt to make a 20:80 B₂O₃:SiO₂ product, Example 1 was repeated yet again witii the amount of TMB reduced still further to 2.6g. Gelation of the reaction product did not occur witiiin a two- week period. The mixture was successfully used to coat glass by the dipping technique despite the non-gelation of the reaction mixture. Such a composition has extremely high stability and a long shelf life. Example 6

A procedure as set out in Example 1 was carried out with die exception that d e reaction mixture, after two hours, was diluted with tetrahydrofuran (THF) (220g) rather than acetone. It proved unnecessary to catalyse the reaction and gelation occurred in under thirty hours. It was found that the gels formed in THF rather than acetone were considerably more transparent. Example 7

The procedure of Example 1 was repeated yet again but this time in the absence of any solvent. The TMB and TEOS were allowed to react at room temperature. A steady increase in viscosity occurred over a period of time and die mixture had solidified over a period of several weeks. Example 8

Equimolar amounts of TMB (17.3g) and titanium (IV) isopropoxide (28.5g) were mixed and underwent an instantaneous reaction. Dilution of the TMB with acetone (220g) prior to the addition of the titanium isopropoxide was necessary to produce a more controlled reaction. Example 9

TMB (17.3g) and TEOS (20.8g) were reacted as described in Example 1. An equimolar amount of titanium (IN) isopropoxide (28.5 g) was then added in an attempt to make a product having a 1:1:1 molar ratio of B₂O₃:SiO₂:TiO₂. However, a violent reaction took place with d e immediate precipitation of a white solid. In an attempt to overcome tins, acetone (220g) was used to dilute die TMB TEOS mixture prior to the addition of die isopropoxide. No catalyst proved necessary and gelation of the reaction product occurred rapidly. Example 10

TMB (17.3g) and TEOS (20.8g) were reacted as described in Example 1. The reaction mixture was diluted with acetone (220g) prior to the addition of titanium (IV) isopropoxide (18.9g). These amount were selected in an attempt to obtain a 3:3:2 ratio of B₂O₃:Siθ2:TiO_.. Gelation was much more controlled than in Example 9 but, nevertheless, die reaction mixture had gelled witiiin three hours. Example 11

Example 10 was repeated but witii the amount of titanium (IV) isopropoxide reduced by half in an attempt to obtain a 3:3:1 molar ratio. A clear solution was obtained which slowly turned into a translucent gel. Example 12

As described in Example 1, equimolar amounts of TMB (17.3g) and TEOS (20.8g) were reacted. The reaction mixture was diluted with acetone (220g) and aluminium tri- secondary butoxide was added thereto. Gelation of the reaction mixture occurred rapidly. The above amounts are designed to give a molar ratio of B₂O₃:SiO₂:Al₂O₃ of 1 : 1 : 1. It was found by reducing die amount of the aluminium compound added, the reaction could be better controlled with the preferred ratio being 3:3:1 -that is to say, by reducing the amount of the aluminium compound to one-tiiird of the amount mentioned in this Example.

It will be readily apparent to ti ose skilled in die art ti at various minor modifications could be made to die present invention without departing from die scope thereof. The sol- gels made by the method of die present invention have a variety of uses. Primarily, however, they will be used for coating glass to provide the glass with, for example, improved fire- retardancy. One method of coating glass with such sol-gels is simply to dip the glass into the sol-gel composition. The coatings thus produced may have a variety of functions. Moreover, additives may be incorporated in the TEOS/TMB reaction to create special properties of the sol-gel and of die coated glass in a manner which is well known per se in die field of sol-gel chemistry. Examples of the types of additives which may be employed include hydrophilic agents, hydrophobic agents, surfactants, dyes, photochromic materials, radiation absorption material, nanoparticles and electroactive materials.

Claims

CLAIMS:-

1. A method of producing a borosdicate sol-gel comprising reacting a boroxine of the formula:-

R₃Q - ^

wherein R╬╣,R₂ and R₃, which may be the same or different, each represent a substituted or unsubstituted lower alkyl group containing from 1 to 5 carbon atoms, alkenyl, alkynyl, aryl or aralkyl with a tetrasubstituted orthosilicate or metal alkoxide of the formula:-

A_P[M(OR')_Q]sB╧ä in which A is a lower alkyl group containing from 1 to 10 carbon atoms; M is a metal selected from Groups 3,4,5 and 6b of the periodic table (including silicon); R' is a lower alkyl group containing from 1 to 10 carbon atoms, alkenyl, alkynyl or aryl; B is an electron- wimdrawing 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 with the provisos that the metal must be capable of increasing its coordination number, the metal centre must be capable of coordination bonding to an oxygen group of the boroxine and the ligand on the boroxine must be attracted to the metal of the orthosilicate or metal alkoxide.

2. A method as claimed in Claim 1 wherein R╬╣,R₂ and R₃ are the same as one another and each represent a metiiyl group.

3. A metiiod as claimed in Claim 1 or 2 wherein Q is at least 2 and the OR' groups are the same as one another.

4. A method as claimed in Claim 1 or 2 wherein Q is at least 2 and die OR' groups are the same as one anotiier.

5. A mediod as claimed in Claim 4 or 5 wherein R' represents a methyl or an ethyl group.

6. A mediod as claimed in Claim 1 wherein die electron withdrawing group is a carbonyl group.

7. A mediod as claimed in Claim 1 wherein the electron withdrawing group is a halo- group.

8. A mediod as claimed in Claim 7 wherein the halo group is a chloro-group or a fluoro- group.

9. A mediod as claimed in Claim 8 wherein the boroxine is selected from the group consisting of 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(2,3,4,5,6- 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.

10. A method as claimed in Claim 1 wherein die boroxine is trimetiioxyboraxine.

11. A method as claimed in Claim 1 wherein the ordiosilicate is tetraethylorthosilicate.

12. A mediod as claimed in any preceding Claim wherein the reaction is effected in the presence of a solvent

13. A method as claimed in Claim 12 wherein the solvent is selected from the group consisting of acetone, tetrahydrofuran and dimethylsulfoxide.

14. A method as claimed in any preceding claim wherein die reaction is carried out in die presence of a catalyst

15. A method as claimed in Claim 14 wherein the catalyst is trifluoroacetic acid.

16. A method as claimed in Claim 15 wherein the molar amount of catalyst used is at least two orders of magnitude less dian the molar amounts of the boroxine and the ordiosilicate or metal alkoxide.

17. A mediod as claimed in Claim 1 wherein R' is selected from allyl or aryloxy.

18. A mediod as claimed in any preceding claim wherein the reaction is effected at an elevated temperature.

19. A mediod as claimed in any preceding claim wherein the reaction is terminated by cooling die reaction mixture.

20. A method of producing a borosilicate sol-gel as claimed in Claim 1 substantially as hereinbefore described with reference to die foregoing Examples .

21. A borosilicate sol-gel whenever produced by a mediod as claimed in any one of claims 1 to 20.

22. A method of coating glass comprising applying a composition as claimed in Claim 21 to a sheet of glass.

23. A method as claimed in Claim 22 wherein die composition is applied to die glass sheet by dipping the sheet into the composition.