Sol-gel composition for producing glassy coatings

Glassy coatings are made by curing in situ a coating of a sol gel of boron triethoxide, water, alcohol, and the alkoxides of: titanium and zirconium. A platey filler such as mica is present. The coating may be applied to teeth as a dental fissure sealant or varnish to protect restorations, or as an inherently coloured cosmetic coating, or as a prophylactic coating.

This invention relates to a sol gel composition for use in producing glassy 
coatings, to a process for producing a glassy coating using the 
composition, to a method For cosmetic colouring of teeth using the process 
and to a method for prophylactic protection of teeth using the process. 
Fissure sealants have been demonstrated as effective in reducing incidence 
of tooth decay and inhibiting decay even after it has started, but have 
not gained universal acceptance in general practice. These fissure 
sealants are understood to have relatively low durability, adhering to the 
tooth with a rather short half-life (5 years). 
Sol-gels would not be considered for dental use, since the curing of sol 
gels is typically undertaken in a slow furnace, which would pose clinical 
difficulties. Thus, U.S. Pat. No. 5,068,208 discloses a method of making 
optical elements of gradient-index glass by a sol-gel route including 
adding together a partially hydrolysed silicon alkoxide, an alkoxide of Ti 
or Zr, and an alkoxide of B, Al or Ge, adding water while agitating, 
moulding the mixture for sufficient time to form a gel, washing the 
moulded gel, acid-leaching the washed gel to remove some metal oxide, 
fixing the gel in aqueous alcohol, drying the gel and sintering. 
Silicon-based sol gels often rely on the presence of sodium alkoxide, and 
it has been suggested that sodium does not enhance the chemical resistance 
or mechanical durability of glass. However, an alternative is taught by 
EP-A-482659, in which a thin glass film is produced on a substrate by a 
sol-gel method, comprising applying to the substrate a hydrolysable 
solution containing a metal alkoxide, water, alcohol and an acid, exposing 
to ammonia and heating to form a thin glas film in situ. The metal 
alkoxide is selected from Si, Ti, Ge, Al and B, of which Si, and Si+Ti, 
are exemplified. 
According to the present invention, a sol gel, xerogel or heat-consolidated 
gel composition comprises: a hydrolysable boron ester (such as boron 
triethoxide or tripropoxide) or boron salt convertible to the oxide by 
thermal or oxidative decomposition (such as boron nitrate or citrate); a 
solvent (alcoholic or non-alcoholic); alkoxide(s) of any one, two or more 
of aluminium and of Gp VB and/or GpIVB metals (preferably zirconium 
together with titanium); and a filler having a mean particle size of up to 
3 .mu.m in one dimension and 5-100 .mu.m in the other two dimensions. Part 
of the boron ester can be substituted by appropriate compounds of other 
non-metallic glass formers. Some of these compositions have been found to 
be usefully stable. Where not deleterious, a GpIA or IIA metal alkoxide 
(such as sodium or calcium) may be present. Some or all of such metal may 
advantageously be present as the fluoride. Preferably the number of metals 
present (not counting B or Si) is at least two, e.g. three or four, of 
which one is preferably Ti. The composition may further comprise water, 
which may however be supplied otherwise, e.g. by exposure to air. The 
metal alkoxide, if there is just one, may be Ti or Zr or Al. Preferred 
combinations of metal alkoxides are: (i) Zr, Ti; (ii) Al, Ti, Na; (iii) 
Ti, Na, Zr; (iv) Al, Ti, Zr; and (v) Al, Ti, Zr, Na. The metals (not 
counting Na) are preferably present in amounts up to one-sixth (by number 
of atoms) of the boron. Solid oxides e.g. fine neodymium oxide powder may 
be dispersed into the sol gel. 
Preferably, the sol gel composition synthesis was characterised by an 
ageing step, during which moisture was admitted to the composition at a 
rate under 1% of the rate in free air. This controls the rate of 
hydrolysis and consequently of `polymerisation` of the molecules of the 
composition without destabilisation, which rate can in principle be 
monitored via an increase in viscosity of the composition, or e.g. by 
infra-red spectroscopy, by differential scanning calorimetry, by 
thermogravimetric analysis, by nuclear magnetic resonance or by electron 
spin resonance. 
The filler may be for example laponite, zeolite, kaolinite or vermiculite, 
or preferably the filler is in the form of flat plates such as talc or 
mica, or a mixture, optionally coated (preferably by chemical vapour 
deposition) with for example titanium dioxide, chromium oxide or ferric 
oxide or a mixture. Such materials are harmless if swallowed in the small 
quantities in which they might spall off. The filler preferably comprises 
up to 30% by weight of the composition, and more particularly preferably 
comprises 20-30 wt % where the sol gel proportion of the composition is a 
sol gel yielding 5 to 15 (e.g. 10) wt % on curing, and the filler 
preferably comprises 5-10 wt % where the composition yields 0.1-1 wt % on 
curing. 
Other coating methods may also be used, alternatively or in addition, such 
as deposition of silane. This can promote adhesion and enhance mechanical 
properties. As silanising agents, compounds containing a glycidoxy organic 
group and a trimethoxysilyl group may be used, such as Dow Corning Z6040 
(trade mark), 
##STR1## 
The mean filler particle size is preferably 0.1 to 2 .mu.m in one 
dimension and 5-100 .mu.m in the other two dimensions. Where the latter 
dimensions are 5-20 .mu.m, and the sol gel preferably yields 5-15 e.g. 10 
wt % on curing, the filler preferably comprises from 20 to 30% by weight 
of the composition (i.e. before curing), and will be suitable for 
single-coating applications, to mask tooth discolourations. Where those 
dimensions are 10-60 .mu.m, the filler preferably comprises 10-20% by 
weight of the composition. In the case of xerogel, which is 90 volume % 
air, a platey filler will improve its mechanical properties such that it 
could be used as an insulating material, or the cavities can be used as 
drug reservoirs, which will slowly release, on a tooth or otherwise. Where 
the sol gel yields 0.1-1 wt % on curing and the filler is correspondingly 
preferably 5-10 wt %, a better performance is often obtained, and the sol 
gel may be applied in multiple coatings to the tooth. 
The filler reduces the incidence of crazing in the cured sol-gel (not only 
in dental applications) by physically reducing the bulk of sol gel needed, 
thus making what there is of it more elastic. It also absorbs incident 
laser energy and re-emits it to the sol gel, accelerating the latter's 
curing. The filler also improves the abrasion resistance of the cured sol 
gel glassy coating. Given that the yield of glass from sol-gels is 
preferably about 5-10% by weight, and can be 0.1 to 1% or even less as 
already indicated, the filler when present will in such cases thus be a 
major component of the product. In the case of mica, which tends to fall 
out of sol-gel suspension quite rapidly, it may be incorporated into the 
sol-gel when or immediately after the latter is made up; as the sol-gel 
`polymerises`, the polymers grow on the mica, improving its suspension and 
bonding, but as a precaution it may be advisable to shake it before use, 
or else the mica may be added to the sol gel at any later stage, e.g. 
immediately before use. On the other hand, a too-perfect suspension is to 
be avoided; as it is, the mica advantageously settles into pits and 
fissures, whither it is drawn by surface tension. 
The hydrolysable boron ester is preferably boron triethoxide or boron 
tripropoxide, or any other boron alkoxide may be used. The molar 
proportion of water:boron may be (3/4 to 3):1, preferably (1 to 2):1, for 
example 11/2:1. To induce successful hydrolysis in the composition, water 
must be added homogeneously to prevent localised excesses leading to 
precipitation. For example, water may be added by stirring the composition 
vigorously under an atmosphere of high humidity, or by allowing the 
composition to stand in a sealed container with a few pinholes to allow 
very slow ingress of atmospheric moisture. Part of the B, as already 
mentioned, can be substituted by appropriate compounds of other 
non-metallic glass formers. Boron nitrates or boron citrate, optionally 
esterified, may be tried. The solvent may comprise hydrophobic materials 
such as partly or wholly halogenated methane, e.g. CCl.sub.4, or 
tetrahydrofuran, or diethylether, or hydrophilic materials such as ketones 
e.g. acetone or alcohols e.g. ethanol optionally containing up to an equal 
volume of propanol (iso or n) preferably from 1/2 to 3/4 volumes (e.g. 60 
ethanol:40 propanol). The proportion of water plus solvent may be such 
that the composition yields 1-10 g boron oxide per 100 g (the filler being 
included in the 100 g). In an alternative sol-gel preparation method, 
solvents (including water) may be absent and an intermediate solid may be 
converted into an applyable liquid sol gel composition by controlled 
exposure to atmospheric moisture. 
A process for producing a glassy coating according to the invention 
comprises applying a sol gel composition e.g. as set forth above, to an 
object to be coated and curing the coating e.g. by flame (very miniature 
flames can be used in the mouth) e.g. butane flame heating, otherwise by 
radiation from the tip of a diathermy needle or preferably by laser, for 
example a CO.sub.2 laser, with an energy input to the object of preferably 
1 to 2000 mJ/mm.sup.2 preferably applied at a rate which does not cause 
overheating leading to cracking or flaking of the film, e.g. a travelling 
spot of e.g. 150 .mu.m diameter illuminating any one point for from e.g. 
1/2 to 100 milliseconds such as 20 to 60 ms, the laser having a power of 
such as under 4W (more preferably up to 1W), preferably for a duration of 
0.1 to 4 (e.g. 1/2 to 3) seconds. This energy input is expected to raise 
the overall tooth temperature by only 1.degree.-2C.degree., excess heat 
being removed by the blood supply to the pulp. A CO.sub.2 laser may be 
tuned to 10.6 .mu.m as is most usual, or may be tuned to or near 9.6 .mu.m 
(e.g. 91/2-10 .mu.m), which is most strongly absorbed by natural tooth, 
and may be of pulsed output. This is useful if it is desired to fuse 
(physically incorporate) the sol gel into the enamel or dentine by 
temporarily or permanently vitrifying these, a procedure which requires 
high laser power outputs and which is expected to make the enamel more 
resistant to caries. The pulse width and frequency can be varied to suit 
the thickness of the film to achieve good consolidation, and, if desired, 
vitrification. An Nd:YAG 1.06 .mu.m laser could be used, but needs a 
chromophore in the sol gel to absorb it. More generally, the radiation 
should be of a wavelength which is totally absorbed and converted to heat 
within the sol gel and first few underlying microns of the tooth or other 
substrate. Apart from the quoted examples of .lambda.=10.6 .mu.m and 1.06 
.mu.m, the mid infra-red range of .mu..apprxeq.2 to 6 .mu.m (e.g. 5 .mu.m) 
may be used. The coating as applied (before curing) may be up to 30 .mu.m 
thick, preferably 1-10 .mu.m e.g. 2-10 .mu.m. The preferred energy input 
of 1 to 2000 mJ/mm.sup.2 thus includes a preferred range where only 
consolidation of the film is required, e.g. 20 to 500 such as 50 to 200 
mJ/mm.sup.2, and a preferred range where vitrification of part of the 
substrate tooth is required, e.g. 500 to 2000 mJ/mm.sup.2. 
A method for cosmetic colouring of a tooth according to the invention 
comprises using the process set forth above, wherein the said object is 
the tooth. The tooth may have been treated with restorative material such 
as glass alkenoate cement, for which the present invention can be regarded 
as providing a protection. The sol-gel may include a pigment. The 
neodymium oxide powder suggested above imparts a remarkably evenly 
distributed blue colour to the glass. Alternatively, preferably the filler 
is so formulated as to appear a tooth-like colour in the applied 
thickness. Alternatively, the tooth is stained cosmetically, and the stain 
retained by the applied coating. As a side-effect, prophylactic benefits 
may be obtained. 
A method for prophylactic protection of a tooth according to the invention 
comprises using the process set forth above, wherein the said object is 
the tooth. The tooth may have been treated with restorative material such 
as glass alkenoate cement, for which the present invention can be regarded 
as providing a varnish. Preferably the filler is so formulated as to 
appear a tooth-like colour in the applied thickness. As a side-effect in 
that case, cosmetic benefits may be obtained. In all these methods, the 
option (explained above) of fusing tile enamel, at least superficially, 
may be adopted. 
Preferably the tooth is cleaned beforehand e.g. mechanically or by 
acid-etching. 
The present invention provides a method whereby drugs may be released 
slowly, comprising allowing a coating produced by xerogel as set forth 
above and charged with the drug to ablate. 
The two forms of product derived from sol-gel, viz glass and xerogel, 
differ in the physical organisation of their polymeric structures: 
(i) Sol-Gel derived glass: A high density polycondensed lattice or network 
with minimal porosity. 
(ii) Sol-Gel derived xerogel: A polymeric structure which is highly porous 
in the 100 nm range and of correspondingly low density, having trapped 
organic residues and being mechanically weak. The formation of a xerogel 
is a direct indication of sufficient hydrolysis to yield a useful glassy 
material. The deposition of a thin film from these sol-gels will depend 
upon dilution factor and nature of the solvents used. It is important to 
note these sol-gels once synthesised will continue to undergo hydrolysis 
and condensation. 
Defect free glassy films are important for adequate tooth protection, and 
require careful attention to two crucial stages in the sol-gel process 
once a continuous liquid coating has been applied: 
(i) Sol liquid-to-gel transition 
(ii) Consolidation of gel to glass. 
Stage (i) needs to be slow which implies controlled rate of solvent loss, 
otherwise the shrinkage of resulting gel is rapid and uneven leading to a 
fractured coating. The gel has to be partially dried and then given even 
surface heat treatment. Stage (ii), viz heat treatment, also needs to be 
carefully controlled, otherwise the film will crack and/or blister. The 
glass coating is vulnerable to cracking during heat treatment where 
shrinkage occurs, as density increases, mainly in the vertical direction 
and not the horizontal. Thin coatings that are less than 1/4 .mu.m 
generally do not suffer from cracking and have better mechanical 
durability. Following this finding, efforts to develop a sol gel glass 
having the same coefficient of thermal expansion as natural tooth were 
discontinued as unnecessary. 
Using liquid spreading techniques likely to be available in ordinary 
clinical practice would however yield unconsolidated coatings on tooth 
surfaces having a thickness of approximately 1-20 e.g. 5-10 .mu.m. 
(Applying a drop from a dropwise dispenser, it spreads across the tooth 
surface spontaneously.) As indicated above, the addition of inert fillers 
such as mica flakes is desirable; it permits thicker yet crack-free 
consolidated glass coatings and improves xerogel coatings. Curing gives 
rise to a consolidated film which is thinner than the unconsolidated 
coating because of loss of solvent and organic constituents. Allowing for 
this, an ideal post-curing (consolidated) film thickness to aim for is 
e.g. 0.1 to 1 .mu.m. Sol-gel derived coatings may be applied to: 
(i) Fissure sealing 
(ii) Sealing marginal gaps arising from old restorations 
(iii) Entire tooth crown surface protection 
(iv) Root canal therapy, e.g. sealing tubules 
(v) Lining freshly prepared cavities (blocking open tubules 
(vi) Protection of cavities freshly restored with filling materials. 
(vii) Replacing the use of porcelain veneers for aesthetically coating 
discoloured enamel surfaces 
(viii) Slow release of fluoride for topical application to tooth 
(ix) Controlled release of drugs for example in the treatment of dentine 
hypersensitivity or periodontal disease, and 
(x) Impregnation of porous structures for mechanical strengthening and 
other purposes e.g. drug release, enamel disorders and dental material 
improvement. 
There are certain preferred ranges of compositions of the sol-gel. 
Considering atoms of B, Na (or equivalent), Al, Zr and Ti (or equivalent), 
boron preferably accounts for at least 40, more preferably at least 50%. 
Sodium is preferably under 50% (on an atomic basis again) such as 1-40%, 
more preferably 5-30%. Aluminium may be 5-15%, and titanium and/or 
zirconium and/or vanadium and/or niobium and/or tantalum 3-15%, more 
preferably 5-10%, and/or not exceeding one-fifth of the boron. Boron is 
desirable as a glass-former, and sodium should be limited as it makes the 
glass less resistant to acid.

The invention will now be described by way of example. 
Boron alkoxides were made as follows. 
1 Boron Triethoxide 
Boron trialkoxide can be synthesised by the dehydration of mixtures of 
alcohol with boric oxide (equation 1) or boric acid (equation 2). The 
reactions are both slow and require long reflux times. 
EQU B.sub.2 O.sub.3 +6ROH.fwdarw.2B(OR).sub.3 +3H.sub.2 O 1 
EQU B(OH).sub.3 +3ROH.fwdarw.B(OR).sub.3 +3H.sub.2 O 2 
Boric acid and ethanol (eq2) were used to synthesise boron triethoxide. The 
mixture was refluxed with continuous stirring. 1:3 mole of boric acid to 
ethanol was used. 
______________________________________ 
Required Used 
______________________________________ 
B(OH).sub.3 (white solid) 
61.83 g (1 mole) 
61.00 g 
Ethanol 138 g (3 moles) 
138.38 g 
______________________________________ 
The above mixture was refluxed for 34 hours and on cooling a white solid 
(boric acid) remained unreacted; a small percentage appeared to 
precipitate out of solution. The reflux temperature was found to be 
80.degree.-81.degree. C. A sample of 10 ml was taken after 24 hours of 
refluxing, which on cooling begins to flocculate. A white semi-gelatinous 
precipitate results, leading then to a suspension. On standing the 
precipitate settles out leaving a separate clear liquid above the white 
precipitate. The clear liquid was decanted and stored for future use. The 
clear liquid, on evaporation, produced a white solid which is thought to 
be the required alkoxide mixed with boron oxide. 
The clear liquid is thought to be composed of a mixture of boron 
triethoxide, ethanol and water, which on access to moisture from the 
atmosphere and loss of solvent leads to gelling and boron oxide formation. 
1000 .mu.l of the above clear liquid has a mass of 0.802 g, and produces 
0.081 g of white solid on evaporation to dryness, that is a yield of 
10.1%. 
2 Boron Tripropoxide 
Conditions: Reflux With Continuous Stirring. 
Excess propanol was used in an attempt to increase the yield of boron 
tripropoxide. (1:6 mole used). 
______________________________________ 
Required Used 
______________________________________ 
B(OH).sub.3 30.91 g (0.5 moles) 
31.8 g 
Boric acid 
Propan-l-ol 180 g (3 moles) 180.7 g 
______________________________________ 
total reflux time 6 hours, temperature approximately 82.degree. C. 
Synthesis stage 1, 3 hours reflux with only 90 g of propanol then further 
90 g of propanol was added and refluxed for a further 3 hours. 
The 1000 .mu.l of supernatant has a mass of 0.7 g (3 hrs reflux with 1:3 
mole reaction) and yields 0.07 g of solid, that is 10%. 
It is again assumed the above supernatant is composed of a mixture of boron 
alkoxide, alcohol and water. 
Synthesis of Boron Based Sol-Gels 
Series I 
The above synthesised boron alkoxide will be used as the precursor for 
boron oxide. 
The following metal alkoxides have been investigated. 
Titanium tetraisoproxide (liq) 
Zirconium tetra sec-butoxide (liq) 80% in butanol 
Boron triethoxide (liq) in ethanol 
Boron tripropoxide (previously made) in propanol 
EXAMPLE 1 
EQU Ethanol+Ti+Zr+B 
The order of addition of the components and their respective amounts were 
as follows: 
______________________________________ 
Ethanol + 2.331 g (3000 .mu.l) 
Ti tetraisopropoxide + 
0.369 g (400 .mu.l) 
Zr tetra sec butoxideq + 
0.278 g 
Boron triethoxide solution (600 ml) 
described above 
______________________________________ 
Ethanol+Ti+Zr produced a clear liquid but the addition of the boron 
triethoxide produced a white precipitate which remains in suspension. The 
product was however of some utility. 
EXAMPLE 2 
EQU Ethanol+Ti+B+Zr 
The order of addition of the components and their repective amounts were as 
follows: 
______________________________________ 
Ethanol + 2000 .mu.l 
Ti tetraisopropoxide + 
0.373 g (400 .mu.l) 
Boron triethoxide solution + 
0.144 g (200 .mu.l): A white 
precipitate appeared 
Zr tetra sec butoxide 
4.255 g: On shaking, the 
precipitate redissolves, a 
phenomenon requiring a certain 
minimum Zr above that in 
Example 1. 
______________________________________ 
A xerogel was then made from this solution. 
1000 .mu.l of the solution was exposed slowly to atmospheric moisture in a 
25 ml glass bottle closed with a perforated (pin holes) cap. 
Observations: 
24 hours Approximately 50% of the liquid had evaporated and the sol-gel 
system remained clear. 
4 days The sol-gel system set to a fractured white gel-like material 
approximately 10-20% of the original volume. Clear flakes had formed on 
the inner side walls of the glass bottle. This is a clear indication of 
the formation of a xerogel from solution adhering to the side walls and 
having evaporated. 
This sol-gel can thus yield clear xerogel material and/or a white 
`xerogel-like material`. 
EXAMPLE 3 
EQU Ethanol+Ti+Zr 
The order of addition of the components and their respective amounts were 
as follows: 
a higher quantity of Ti & B and a smaller quantity of Zr. 
______________________________________ 
Ethanol + 4000 .mu.l 
Ti tetraisopropoxide + 
0.940 g (1000 .mu.l) 
Boron triethoxide in ethanol + 
0.801 g (1000 .mu.l) of the 
solution described at the 
outset of the Examples 
Zr tetra sec butoxide 
2.139 g 
______________________________________ 
The ethanol+Ti+B produces an intense white precipitate suspension. Addition 
of Zr and vigorous shaking for 1 minute leads to total dissolution of the 
white precipitate yielding a clear solution with a yellow hue (due to the 
inherent yellow colour of Zr tetra sec butoxide solution). 
The white precipitate was not identified, but titanium dioxide is ruled out 
(because it is insoluble) and titanium ethoxide is ruled out because it is 
miscible with ethanol. 
A xerogel was then made from this solution. 
1000 .mu.l of the solution was exposed slowly to atmospheric moisture in a 
25 ml glass bottle closed with a perforated (pin holes) cap. 
Observations: 
24 hours later Approximately 50% of the liquid had evaporated and the 
sol-gel had set to a white gel-like material. 
4 days later The white gel-like material dried out and fractured. There was 
also a presence of clear flakes of a xerogel peeling from inner walls of 
the glass container. 
This sol-gel can thus yield clear xerogel and/or a white `xerogel-like 
material`. 
EXAMPLE 4 
From the yellow-hued clear solution of Example 3, 1000 .mu.l was taken. To 
it were added successive aliquots of 100 .mu.l of boron ethoxide solution 
in ethanol (0.08 g boron ethoxide per aliquot). The first four aliquots 
were added without any sign of turbidity (precipitate/suspension 
formation) and the system remained clear. With the fifth and sixth 
aliquots, the system became slightly turbid and opaque with possibly a 
slight increase in viscosity. From the seventh to the tenth aliquot the 
system progressively became turbid and viscous and a white 
precipitate/suspension formed which set to gel within 3-5 minutes of 
adding the final aliquot of boron triethoxide. The system tolerated more 
boron with these successive small additions than when the same amount of 
boron was added all at once. 
EXAMPLE 5 
EQU Ethanol+B+Zr+Ti 
The order of addition of the components and their respective amounts were 
as follows: 
______________________________________ 
Ethanol + 2000 .mu.l 
Boric triethoxide + 1.641 g 
Zr tetra sec butoxide + 1.698 g 
Ti tetraisopropoxide 4.600 g 
______________________________________ 
Ethanol+Boric triethoxide+Zr produces an intense white 
precipitate/suspension. The addition of excess Ti and vigorous shaking 
leads to complete dissolution of the precipitate and the reaction is 
noticeably exothermic. 
From this and Example 4, it appears that both Ti and Zr induce the 
gel/precipitate formation (most likely to be B(--O--)n complex) which in 
the presence of a certain level of Ti or Zr alkoxide leads to complete 
dissolution and a clear stable sol-gel. 
A simple Boron+Zirconium sol-gel via this route does not seem to be 
possible. 
A xerogel was then made from this solution. 
1000 .mu.l of the solution was exposed slowly to atmospheric moisture in a 
25 ml glass bottle closed with a perforated (pin holes) cap. 
Observations: 
24 hrs 50% volume loss, sol-gel remained clear 
4 days Set as previously to a fractured white gel-like material which 
dehydrates to a white fractured solid. There was also the presence of 
clear flakes of xerogel material peeling off the container walls. 
1000 .mu.l yielded 0.301 g of white xerogel-like solid. 
The 0.301 g solid was heated to 1080.degree. C. for 2.5 hours and yielded 
0.137 g of a fractured white ceramic-like material. 
This sol-gel can thus yield clear xerogel and/or a white xerogel-like 
material. 
EXAMPLE 6 
EQU Ethanol+B+Ti+Zr 
The order of addition of the components and their respective amounts were 
as follows: 
______________________________________ 
Ethanol + 2000 .mu.l 
Boron triethoxide + 1.634 g (2000 .mu.l) 
Titanium tetraisopropoxide (pure) + 
(2000 .mu.l) 
Zr tetra sec butoxide 
first addition 2.134 g and 
second addition making 
it up to 4.773 g. 
______________________________________ 
The first addition of Zr did not clear the suspension but the second 
addition of Zr taking the total amount to 4.773 g completely cleared the 
suspension resulting in a clear yellow-hued sol-gel. 
A xerogel was then made from this solution. 
1000 .mu.l of the solution was exposed slowly to atmospheric moisture in a 
25 ml glass bottle closed with a perforated (pin holes) cap. 
Observations 
24 hrs Approximately 50% reduction in volume: the sol-gel remained clear. 
4 days A fractured white solid resulted. Flakes of clear xerogel were also 
present. 
This sol-gel can thus yield clear xerogel and/or a white `xerogel-like 
material`. 
EXAMPLE 7 
EQU Si+Al+Ti+Na+Zr+B 
A previously synthesised stable sol-gel was used which had been given 
limited access to atmospheric moisture for 19 days during synthesis. This 
had the composition: 
Tetraethoxysilicon 99.62 g 
Zr tetra sec butoxide 11.2 g (i.e. 13.79 g of an 80% solution in 
butan-1-ol) 
Al tetra sec butoxide 19.87 g 
Na ethoxide 9.3g 
This yielded a semi-solid, then eventually a clear liquid. 
To 1000 .mu.l of this Si+Al+Ti +Na +Zr sol-gel was added 200 .mu.l of boron 
triethoxide solution. No reaction was observed and the system remained 
clear. 1000 .mu.l of the liquid was exposed slowly to atmospheric moisture 
in a 25 ml glass bottle closed with a perforated (pin holes) cap. 
Observations: 
2 hrs The sol-gel remained clear 
24 hrs The sol-gel remained clear but has become viscous. 
48 hrs The sol-gel remained clear and viscous. 
72 hrs Set to a clear semi-gel/solid state 
96 hrs Set to a clear solid (xerogel state) 
144 hrs Clear solid, i.e. xerogel 
EXAMPLE 8 
EQU Si+Ti+Al+Na+B (NB no zirconium) 
A previously synthesised stable sol-gel was used which, after 4 days' 
exposure, was sealed in a vessel and stored at -10.degree. C. for over a 
year. This sol-gel had the composition: 
Tetra ethoxysilicon (TEOS) 66,15 g 
Ti tetra isopropoxide 26.68 g 
Al tetra sec butoxide 24.42 g 
Na ethoxide 9.06 g 
and had been made in this order of mixing: TEOS+Al +Ti (still a clear 
liquid) then+Na produces a precipitate/gel-like mass. Limited exposure to 
atmospheric moisture led to conversion from solid state to a stable usable 
liquid. 
To 1000 .mu.l of this stable usable liquid was added 600 .mu.l of boron 
triethoxide. 
The system remained clear and appeared to be stable. 
1000 .mu.l of the liquid was exposed slowly to atmospheric moisture in a 25 
ml glass bottle closed with a perforated (pin holes) cap. 
Observations: 
12 hours The sol-gel remained clear 
24 hours The sol-gel remained clear but had become viscous. 
48 hours The sol-gel remained clear and viscous. 
72 hours Set to a clear semi-gel/solid consistency 
96 hours Set to a clear solid (xerogel state) 
144 hours Continued to lose solvent, became fractured. 
EXAMPLE 9 
EQU B+Zr+Ti+Na 
To 2000 .mu.l of the clear solution of Example 6 (light yellow in colour) 
was added 0.167 g sodium ethoxide (yellow powder). 
The sodium ethoxide only dissolved sparingly producing a more strongly 
yellow/orange-coloured solution. A large proportion of the sodium ethoxide 
did not dissolve. Propanol (200 .mu.l) was added to dilute the solution 
and aid dissolution of the sodium ethoxide. There were no adverse 
reactions to propanol, almost all of the sodium ethoxide dissolving to 
yield a strongly yellow/orange liquid. 
Excess sodium ethoxide appeared to continue to dissolve over the next 1-3 
hours. There then appeared to be signs of turbidity in the sol-gel, but 
the turbidity clears in time. 
A xerogel was then made from this solution. 
1000 .mu.l of the solution was exposed slowly to atmospheric moisture in a 
25 ml glass bottle closed with a perforated (pin holes) cap. 
Observations: 
24 hours Remained as a clear liquid 
96 hours Set to a solid gel-like clear material, with also a clear xerogel 
coating on the side walls of the glass bottle. 
EXAMPLE 10 
EQU B+Zr+Ti+Al 
To 1000 .mu.l of the clear solution of Example 6 (yellow coloured liquid) 
was added 0.269 g of aluminium tripropoxide (clear viscous liquid). 
This produced an exothermic reaction on mixing. A semi-gelatinous liquid 
resulted which was semi-translucent. Propanol (3000 .mu.l) was added to 
dilute the system; no adverse reactions were observed. Within 5-10 minutes 
of shaking/mixing, a clear transparent yellow liquid resulted. 
A xerogel was then made from this liquid. 
1000 .mu.l of the liquid was exposed slowly to atmospheric moisture in a 25 
ml glass bottle closed with a perforated (pin holes) cap. 
Observations 
12 hours Remained as clear liquid 
24 hours Set to a white gel-like solid. There was also a presence of clear 
flakes of xerogel. 
96 hours White fracture xerogel-like solid resulted, together with a clear 
xerogel coating on the side walls of the glass bottle. 
EXAMPLE 11 
EQU B+Ti+Al 
______________________________________ 
Boron isopropoxide 4.004 g + 
(5000 .mu.l) 
Titanium isopropoxide 2.864 g + 
(3000 .mu.l) 
Aluminium sec-butoxide 3.121 g 
______________________________________ 
The mixture remained a clear liquid at all stages, suggesting the utility 
of boron isopropoxide as a sol gel component of compositions according to 
the invention, and also suggesting the possibility of a B/Ti sol gel. 
The yield in B was about 13%, and is typically 10-13%.