WO2004000851A2

WO2004000851A2 - Hydrolysis of silanes and surface treatment with the hydrolysis product

Info

Publication number: WO2004000851A2
Application number: PCT/GB2003/002648
Authority: WO
Inventors: Anthony Vernon Cunliffe; David Arthur Tod; Simon Andrew Torry
Original assignee: Qinetiq Limited
Priority date: 2002-06-25
Filing date: 2003-06-20
Publication date: 2003-12-31
Also published as: AU2003242841A1; WO2004000851A3; AU2003242841A8; GB0214592D0; GB2390092A

Abstract

A process is provided for silane hydrolysis having a good degree of control of reaction rate and presenting mild conditions. The process comprises reacting together a silane having at least one hydrolysable group with water in the presence of a catalyst comprising a rare earth metal salt with a non-nucleophilic ligand. The ligand is selected from trifluoromethanesulfonate, perchlorate, oxalate, acetate and other alkanoate having a chain length of from 2 to 10 carbon atoms, hexafluoroacetylacetonate and acetylacetonate, with triflate as the preferred ligand. The rare earth metal is preferably a lanthanide, especially lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, erbium, thulium or ytterbium. The hydrolysis product may be used in a subsequent surface treatments of materials having a superficial oxide or hydroxide layer or the hydrolysis may alternatively be carried out directly in the presence of the surface to be treated. The process is also useful in the production of silicon-based gels such as solvogels and alcogels and for the formation of silane crosslinked polymers.

Description

Hydrolysis of Silanes

The present invention relates to processes for the hydrolysis of silanes under mild conditions and condensation of the product to form a coating on a surface to be treated.

The acid and base hydrolysis of silanes is a well known reaction and proceeds via the SN₂ reaction mechanism, displacing an atom or functionality attached to the silicon atom to produce the silanol. This reaction is repeated in turn for each of the hydrolysable groups attached to the silicon atom, furnishing the respective silanediol, silanetriol and silanetetraol corresponding to the number of hydrolysable groups which are available. Such reactions are generally carried out in the presence of an effective acid catalyst and in a largely aqueous solution.

The treatment of metal surfaces with hydrolysed silanes in this manner is a well known replacement for Cr⁺⁶ passivation coating, which has known detrimental toxicological and environmental effects. Commonly the silane used is an alkoxy silane and a surface treatment involving hydrolysis of such silanes is well known for aluminium.

The hydrolysis and condensation of alkoxy silanes are also important reactions in a number of other industrial processes such as the formation of hard thin films on ophthalmic lenses, production of cross linked polyalkanes and the formation of new hybrid sol-gel materials.

Another area which utilises alkoxysilanes is in the manufacture of aerogels, xerogels and alcogels. These are transparent materials which, upon drying, have the appearance of glass, with insulation properties better than mineral wool, and which are more heat resistant than aluminium. They can be used in a variety of ways. The key step in their synthesis is the hydrolysis of tetra(alkoxy)silanes, which is generally carried out under acidic or basic conditions. The silanes in this technology field are commonly referred to as orthosilicates; preferably the alkoxy group will either be methoxy (TMOS) or ethoxy (TEOS).

Cross linked polyalkanes are useful polymers, as the production of a crosslink between adjacent polymers (much akin to the vulcanisation of rubber), allows the structural properties of the polymer to be controlled. For example, polyethylene does not crosslink within the polymer matrix and the strength and stiffness is only controlled by naturally occurring entanglement of the polymer molecules. Crosslinking can however be achieved by the introduction of a vinylalkoxysilane into the polymer during processing, which is later hydrolysed to form the silanol, which then condenses with another silanol or alkoxysilane moiety on a neighbouring polymer to form a cross link. Typically, in carrying out this process, a catalyst is used to increase the rate of hydrolysis, such as a tin reagent or more recently one one of the Ambicat™-Masterbatch catalysts, which are based on sulphonic acids.

A further application of the surface treatment process based on silane hydrolysis is that which is used to improve adhesive bondings, for example for rocket motor components. The use of γ-glycidoxypropyltrimethoxysilane (GPS) in the pre-treatment of metal surfaces, prior to bonding with epoxy resins, improves the durability of an adhesive bond to the metal surface. However, the metal involved here is often steel and acid catalysed hydrolysis of silanes in high concentrations of water is unsuitable for the surface treatment of steel, as exposure to acidified water can cause surface corrosion. The possibility of diluting the silane/water solutions by the addition of an organic solvent has been tried but found not to be feasible because it slows the rate of the hydrolysis to unacceptable levels.

The use of acidic or basic solutions is also not desirable or possible in conjunction with certain silanes having substituents which are labile under such conditions (eg. isocyanate functionalities) and this requirement consequently limits the range of silanes which can be used in silane hydrolysis treatment processes. Lewis acids are known to promote silane hydrolysis but they generally decompose in water to form acidic solutions and so too would be considered undesirable and can be regarded in general as being of the same character as the previously discussed acidic solutions. Their use merely confirms that hydrolysis depends on the presence of an effective acid catalyst and a largely aqueous solution.

The use of organometallic catalysts for the hydrolysis of silanes is known, including organotin and organotitanate compounds and Al(acac)₃ (aluminium (III) acetylacetate). However, the organotin and organotitanate catalysts are insoluble in water and their catalytic activity is dependent upon the initial hydrolysis of the catalyst to form an activated hydrolysed catalyst. Also they suffer from the disadvantage of readily catalysing the oligomerisation of silanes. Reaction times when using the Al(acac)₃ catalyst can be excessively long, compared to acid catalysed systems and thus, although hydrolysis can be carried out under mild conditions with this catalyst, the extended^' reaction times makes the use of this method unsuited to industrial applications.

In most surface pre-treatment processes based on the hydrolysis of a silane, typically a 1% by mass solution of an alkoxy silane is hydrolysed by reaction in high concentrations of water and at pHs between 3 and 4.5 and used to treat a metal surface. Acids and bases catalyse the hydrolysis of the alkoxy silane to a product such as the corresponding silanetriol which is then applied to the metal surface, with the result that the silanetriol undergoes condensation reactions with the metal oxide on the surface. Low concentrations of the alkoxy silane can help to minimise competing polymerisation (oligomerisation) reactions of the hydrolysis product.

Mechanistic studies reveal that a complex co-ordination of silane species to the metal surface takes place. Under typical conditions, 2 to 5 nm or 1 to 10 monolayers of silane coating are regarded as a desirable thickness of coating for the purpose of improving adhesive bonding. It is an object of the present invention to provide an improved method of silane hydrolysis, to the corresponding silanol, silanediol, silanetriol or silantetraol by catalytic means, avoiding the use of acidic or basic solutions to initiate hydrolysis. By this means interference with functionalities associated with the silane which are acid or base labile can be minimised, for example the epoxide ring in GPS.

It is a further object to provide surface treatment processes for materials which possess oxide or hydroxide layers on their surfaces, particularly for metals, which processes render treated surfaces which are of a quality which is at least as good as is obtained with prior art methods but which use milder treatment conditions. Accordingly, it is a further object of this invention to provide an improved process for silane surface treatment such as will rapidly catalyse the hydrolysis of silanes under mild conditions in the presence of a minimum amount of water, so as to avoid promoting the surface corrosion of metals, such as steel or magnesium, which are susceptible to attack in aqueous solutions under conditions of high or low pH.

The applicant has now found that silane hydrolysis can be effectively catalysed in a substantially neutral solution comprised mainly of an organic solvent by the use of a rare earth metal salt with a non-nucleophilic ligand.

According to a first aspect of the present invention therefore, there is provided a process for the hydrolysis of a silane having at least one hydrolysable group which comprises contacting the silane with water in the presence of a catalyst comprising a rare earth metal salt with a non-nucleophilic ligand. Preferably the ligand is selected from the group comprising trifluoromethanesulfonate, perchlorate, oxalate, acetate and other alkanoate having a chain length of from 2 to 10 carbon atoms, hexafluoroacetylacetonate and acetylacetonate, but is most preferably a rare earth trifluoromethanesulfonate ("triflate") or perchlorate . Preferably, the rare earth metal will be a lanthanide, particularly lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, erbium, thulium, and ytterbium. Most particularly, the catalyst will be europium (III) triflate, samarium (III) triflate, erbium (III) triflate, cerium (IV) triflate, europium (III) perchlorate or ytterbium (III) triflate.

Where the silane is susceptible to attack promoted by the presence of large excesses of water, the process of the invention may be carried out in an organic solvent with only a minor proportion of water present, typically not more than 4% by mass. To achieve sufficient concentrations of the silane reagent and the catalyst in solution for an efficient hydrolysis process, the organic solvent will generally be a polar solvent, such as methanol, ethanol, isopropanol, acetone, acetonitrile etc.

The rate of silane hydrolysis using the rare earth metal salt catalysts can be modified by the appropriate choice of metal cation, ratio of water to solvent, the solvent polarity and the reactivity of the silane. As the type of solvent and the amount of water present may be settled for any given process by other factors, the main determinant of the rate of hydrolysis is likely to be the choice of appropriate rare earth metal cation. For some applications, it may be desirable that the hydrolysis should proceed relatively slowly, for example when surfaces are being treated and it is desired to avoid or minimise the occurrence of oligomerisation (oligomerisation being a slow process relative to hydrolysis). However, for other applications it will be desirable to achieve rapid hydrolysis of the silane, either because this will assist in achieving oligomerisation, such as in the formation of solgels, aerogels, xerogels and alcogels, or for ease of application to surfaces which are being treated e.g. when it is necessary to spray the reacting material directly onto the surface to be treated. The ready ability to control the rate of the hydrolysis reaction according to the specific requirement may be appreciated to be a particular advantage of the present process over prior art hydrolysis methods.

The catalyst may, if desired, be recovered from the reaction mixture. This may be achieved by a chromatographic method, cation exchange such as a zeolite, or clay, or by conversion to an insoluble salt. In most cases, however, it is likely that it will be cheaper and simpler not to recover the catalyst.

In an alternative arrangement the catalyst, rather than being in solution in the reaction mixture, may be an integral part of a heterogeneous or supported phase, such as to allow a silane which is to be hydrolysed to be brought into intimate contact with the catalyst on the supporting medium to allow hydrolysis to take place. The support for the catalyst may comprise part of a fixed reaction column, such that fluids containing the silane may be passed across the surface of the supported catalyst to achieve the desired hydrolysis or alternatively the support medium and catalyst may be added to a fluid containing a silane to be hydrolysed and be recovered at a later stage. Such a medium may, for example, comprise suitably chemically functionalised glass or resin beads, large bio molecules such as proteins, enzymes or nucleic acid oligomers or inorganic lattices, such as zeolites, clays etc, or indeed any medium which is capable of retaining the catalyst and preferably provides a large surface area.

It may further be the case that the use of a supporting medium as described can be employed to enable a number of different catalysts to be used at the same time so as to afford multiple reactions. Such catalysts may comprise different rare earth salts to give different rates of hydrolysis and crosslinking or to afford a different functional group interconversion.

The silanes to which the process of the invention may be applied are any silane which possesses at least one substituent group that is capable of being hydrolysed, such as an alkoxy group, O-aryl group, a halide, carboxylate, thiol, silthian, mercaptosilane, nitrile, cyanate, peroxy, arnine, silazane, diamino, perchlorate, phosphate, borate, titanate, aluminate, etc. In the case where the hydrolysable group is an alkoxy group, the alkoxy side chain is preferably short from Ci to Cio inclusive, so as not to create steric hindrance effects in the hydrolysis step. Preferably the alkoxy group is a methoxy or ethoxy group. In one method of use of the process of the invention, a hydrolysed silane is generated by the process and is then used to provide a surface treatment for metals, metal alloys and non-metal complexes possessing a superficial oxide or hydroxide layer. Particularly suitable materials for such surface treatment include metals and alloys such as aluminium, aluminium alloys, steels, chromium and its alloys, titanium, magnesium alloys, copper, tin and brass, oxides such as alumina and also non-metallics such as glass, silica, clays and talc.

In an alternative aspect of the process of the invention, hydrolysis of the silane is effected in-situ of the surface to be treated. According to this aspect, a process for treatment of a surface is provided which comprises exposing said surface to a silane having at least one substituent group which is capable of being hydrolysed, in the presence of water and a catalyst comprising a rare earth metal salt with a non-nucleophilic ligand. This surface treatment process is particularly suited to the treatment of steels in view of the mild conditions that may be used.

It will be appreciated that in the case of this latter process, the hydrolysis reaction may be allowed to start before the reactive solution is applied to the surface i.e the reactive solution may conveniently be prepared away from the surface but then applied immediately to it such that reaction continues in the presence of the surface. Thus a reactive solution can be made up freshly from all of the constituent reagents or the solution can be pre-packaged as a two or three part solution mix, keeping the catalyst and water separate from the silane. To apply the solution to the surface a spray which will draw up the two or three part solutions and mix them together during the spraying process is used. The silane, eg GPS, will rapidly hydrolyse to the silanol, diol, and triol and these products will react directly with the surface. -The rapid hydrolysis method would be suited to _ surfaces that cannot be easily located in a dipping tank, such as fixed structures, coverage of large areas, fixed inclined or near vertical surfaces. To achieve the desired surface treatment effect in these cases requires a higher rate of hydrolysis of the silane, which can be achieved by the selection of either ytterbium (III) trifluoromethanesulfonate or erbium (III) trifluoromethanesulfonate as the catalyst.

The silanes used for surface treatment in conjunction with a process of the present invention are usually chosen from a range of specialised silanes, such as the commonly used γ-glycidoxypropyltrimethoxy silane (GPS) which has three hydrolysable alkoxy functional groups and thus, on hydrolysis, forms an activated silanetriol species which then reacts to form a plurality of covalent bonds with the oxide surface. Other silanes of this type which may be used include vinyltrimethoxysilane, chloropropyltrimethoxy silane, 3-(triethoxysilyl) propyl amine, 3-(triethoxysilyl)ethylene diamine, 3-(trimethoxysilyl)propyl methacrylate, 3-(trimethoxysilyl)propyl acrylate and 3-(triethoxysilyl)propyl thiol.

All processes for using hydrolysed silanes for surface treatment will typically involve a preliminary preparation of the surface, usually a metal or alloy surface, to remove impurities on the surface, eg. by processes of degreasing and grit blasting. After this the active solution is then applied to the surface by pressure spraying or by using a material applicator such as a brush or roller or by dipping the surface into a tank which is filled with the active solution. For these methods of application, it is desired that the hydrolysis of the silane to the silaneol, diol and triol be relatively slow, typically 30 to 45 minutes. For this purpose europium(III) trifluoromethanesulfonate and samarium(III) trifluoromethanesulfonate catalysts are preferred since their use gives rise to only a limited degree of oligomerisation and they will provide similar reaction times to those obtained using normal acidic conditions. Where a dipping tank is used the process may be operated as either a batch or continuous process with the reagents being replaced at the same rate that they are used up.

The silane (such as typically GPS), is present in an amount of from 0.01% to

5% by mass, (wherein the percentage is defined as a percentage of the total mass of the formulation), preferably 0.1% to 2% by mass, in the reactive solution in order to minimise the occurrence of oligomerisation. The solvent system typically comprises water and an organic solvent, such as methanol, ethanol, acetone, acetonitrile or isopropanol, wherein the organic solvent is present in an amount of from 0%-99% by mass of the total solvent. The preferred organic solvent system is an alcohol and water mix, with the alcohol ranging from 0% to 99.93% by mass of the total solvent, preferably from 75% to 99% by mass, and water ranging from 0.05%) to 99.98%o by mass, preferably from 2% to 10% by mass of the solvent system. The catalyst is typically present in an amount from 0.01% to 2% by mass of the formulation, preferably in the range from 0.12% to 0.5%, to form the reactive solution. The hydrolysis reaction is generally carried out at standard atmospheric pressure and in a temperature range of from 10°C to 40°C, most preferably at room temperature.

After the completion of surface treatment, the treated material is either allowed to dry at room temperature or is heated in an oven at an elevated temperature such as from 15 to 110°C, particularly around 80°C. Once dried, the material is ready for any further treatment which is to be applied, such as the application of paints or adhesives.

A further use of the process of the present invention is as a method of preparing silicon based gels, which are the precursors to aerogels, xerogels and alcogels. This method comprises the hydrolysis and oligomerisation or polycondensation of a lower alkyl alkoxysilane (usually TMOS or TEOS) with a rare earth metal catalyst in a water and alcoholic solvent. The effective formation of gels from silanes is best carried out at high concentrations of silane, with only minimal quantities of water and solvent required, as each mole of hydrolysed silane will produce 4 molar equivalents of solvent. Commonly other additives, such as metal hydroxides and inert fillers, are added to the solution to create desirable properties in the gel product. A significant advantage of using the rare earth metal salt is that it prevents the solvent being expelled from the formed gel and hence results in the formation of a stable alcogel. Alternatively the alcogel can be dried out to form either an aerogel or a xerogel.

According to a further aspect of the present invention there is provided a method of producing crosslinked poly^'siloxanes and polyhydrocarbons using a rare earth metal salt catalyst.

The hydrocarbon to be cross-linked may include^' alkyl, alkenyl and/or alkynyl groups and further may include per- and poly-halonated derivitives, such as fluorinated and chlorinated polymers, more preferably cross linked polyethylene moieties. The production of crosslinkable polyhydrocarbons and halocarbons is afforded by reacting the polymer with an alkoxysilane such as vinyltrimethoxysilane to form a polymer-trimethoxysilane pendant group; typical methodologies used to incorporate the vinyltrimethoxysilane are the Monosil® or Sioplas® techniques

(Dow Chemicals). The trimethoxy end group is hydrolysed and crosslinked in-situ by means of a rare earth metal salt catalyst. Currently this polymerisation reaction is afforded on a commercial scale using relatively inexpensive dibutyl tin dilaurate.

However the use of tin and its salts is undesirable due to environmental concerns and the low cost of these reagents is expected to be increasingly off-set by the introduction of more stringent procedures for its manufacture and use.

Crosslinked polysiloxanes are obtained by reacting at least one OH terminated silicone polymer with a tetralkoxylsilane to afford the linking means, in the presence of a rare earth metal salt which will cause hydrolysis of the alkoxysilane thus enabling the two groups to undergo a condensation reaction, so affording the crosslinked polysiloxane.

The mild pH conditions that are possible when using the rare earth metal salts for the crosslinking reactions are advantageous and will avoid many unwanted side reactions of other acid or base labile functional groups. In the situation where the silanol product of hydrolysis undergoes a condensation oligomerisation to form a 3D lattice, it is expected that a range of silicon bridging centres will be formed. Furthermore, it will be apparent to a person skilled in the art that careful control of the solvent to silane ratio, the rate of hydrolysis and the rate of oligomerisation will affect the respective amounts of the different silicon bridging centres and hence will allow the properties of the lattice to be adjusted as desired. To produce the desired product any of the lanthanide triflate catalysts can be used to achieve the desired reaction rate, but preferably the rate of hydrolysis will be relatively fast, as previously mentioned.

The invention will now be further described with reference to the following examples thereof. The rate of hydrolysis of silane in each example was followed using nuclear magnetic resonance (NMR) spectroscopy. Unless specified otherwise, deuterated solvents were used during the experiments; it should be noted that the use of protonated water and solvents might cause a slight decrease in reaction rate processes due to the isotope effect. However, when comparisons are being made this effect is sufficiently small to be ignored.

Example 1: (Comparative)

Hydrolysis of GPS in water is catalysed by addition of acid or base, otherwise the rate of reaction is too slow to warrant use in an industrial process (see Table 1). Addition of ethanol or other solvent rapidly decreases the rate of hydrolysis to unacceptable levels. The addition of acetic acid to high ethanol to water ratio solutions, does not increase the rate of silane hydrolysis.

Table 1

Example 2: Effect of Water to Ethanol ratio on Yb(OTf)₃ catalysed system

Deuterated water (d2-water), deuterated ethanol (d6-ethanol) and ytterbium trifluoromethanesulfonate were mixed together according to the mass ratios in Table

2. A known quantity of γ-glycidoxypropyltrimethoxy silane was added to the solution and the reaction was followed using proton nuclear magnetic resonance

(NMR) spectroscopy.

The time taken to 83% conversion was between 4 minutes and 174 minutes.

There is a clear trend that for a given catalyst, with a fixed concentration of silane, for example 0.10 mol ratio, as the water percentage increases the reaction time increases. However, at near 100% water content, the reaction times decrease.

Further experiments were carried out using 0.20 mol ratio, effectively doubling the silane concentration, and the same effects were observed. The apparent decrease in reaction time at near 100% water content implies that a different hydrolysis mechanism could be occurring.

Table 2

Example 3: Effect of solvent on Yb(OTf)₃ catalysed system

Deuterated water (d2-water), deuterated solvent and ytterbium trifluoromethanesulfonate were mixed together according to the mass ratios in Table 3. A known quantity of γ-glycidoxypropyltrimethoxysilane was added to the solution and the reaction was followed using proton NMR spectroscopy.

The time taken to 83% conversion was between 23 minutes and 182752 minutes. Although the reaction was shown to take place in THF and dioxane, for industrial purposes, high concentrations of THF and dioxane are impracticable. It should be noted that the reaction proceeds faster in a more polar solvent and the best results are in protic solvents such as ethanol and methanol.

Table 3

Example 4: Effect of silane on Yb(OTf)₃ catalysed system

Deuterated water (d2-water), deuterated methanol and ytterbium trifluoromethanesulfonate were mixed together according to the mass ratios in Table 4. A^'1 known quantity of silane was added to the solution and the reaction was followed using proton NMR spectroscopy.

The time to 83% conversion was between 6 minutes and 425 minutes. All the silanes underwent hydrolysis; however, vinyl trimethoxysilane and 3- trimethoxysilylpropylacrylate reacted rapidly wit±in 15 minutes. The 3- aminopropyltriethoxysilane rapidly crosslinked within an hour to form a gel like material. The kinetics of this reaction could not be followed by proton NMR spectroscopy. This example shows that the non hydrolysable group attached to the silicon atom has a great effect on the reactivity of the silane. However, for surface coating solutions it will generally be desired that the side chain should have certain specific properties and for this reason the coatings industry tends to be directed towards the use of particular silanes.

Table 4

Example 5: Effect of Lanthanide Series Catalyst on Hydrolysis.

Deuterated water (d2-water), deuterated ethanol and a series of lanthanide catalysts were mixed together according to the mass ratios in Table 5. A known quantity of γ-glycidoxypropyltrimethoxysilane was added to the solution and the reaction was followed using proton NMR spectroscopy. The time to 83% conversion was between 25 minutes and 272 minutes. All the lanthanide (III) (trifluoromethanesulfonate)₃ catalysts were active in hydrolysing γ-glycidoxypropyltrimethoxysilane.

Table 5

La=lanthanum, Pr=praeseodymium, Nd=neodymium, Sm=samarium, Eu=europium, Ga=gadolinium, Dy=dysprosium, Er=erbium, Tm=thulium, Yb=ytterbium TFMS=trifluromethylsulfonate

Example 6: Effect of Lanthanide Series Catalyst on Oligomerisation

Deuterated water (d2-water), deuterated ethanol (d6-ethanol) and catalyst were mixed together according to the mass ratios in Table 6. A known quantity of γ- glycidoxypropyltrimethoxysilane was added to the solution and the oligomerisation reaction was followed using silicon-29 NMR spectroscopy (measured by the rate of loss of monomeric species).

Europium(III) and samarium(HI) trifluoromethanesulfonate ^ were the least effective oligomerisation catalysts and are the preferred catalysts where it is desired to avoid or to minimise oligomerisation, as, for example, in metal surface treatment processes. Ytterbium and erbium (III) tend to promote oligomerisation and hence these catalysts are more suitable for use in crosslinked gel formation processes, such as the generation of solgels, aerogels, xerogels and alcogels.

Silicon-29 NMR measurements were performed at higher concentrations of silane and lower concentrations of water. The rate of oligomerisation at 1% mass of the silane (measured by proton NMR) is lower than the rate for 10%> mass of silane when catalysed by Eu (III) trifluoromethanesulfonate (Table 6). This may also be the case for oligomerisation reactions catalysed by other metals in the lanthanide series.

Table 6

Example 7: Effect of Europium Salt and Temperature on Silane Hydrolysis

Unless otherwise specified, deuterated water (d2-water), deuterated ethanol (d6-ethanol) and catalyst were mixed together according to the mass ratios in Table 7. A known quantity of γ-glycidoxypropyltrimethoxy silane was added to the solution and the hydrolysis reaction was followed using proton NMR spectroscopy (measured by the rate of formation of methanol).

The time to 83% conversion typically took between 11 minutes and more than 2500 minutes. Europium perchlorate was the best catalyst for silane hydrolysis. The activation energy for GPS hydrolysis was calculated to be 57 k per mole. Using the same mol ratio of reactants and solvents, there is found to be little difference between the hydrolysis rates in protic solvents (sample EuTFMS (1), Table 7) and in deuterated solvents.

Table 7

# solvent was protonated ethanol and water (ie. non-deuterated)

Example 8 Effect of La Series Catalyst on metal Surface Condensation

Deuterated water (d2-water), deuterated ethanol (d6-ethanol), europium (III) trifluoromethanesulfonate)₃ (catalyst) and GPS were mixed together according to the mass ratio of 30.1:695:1:7.9 respectively. The solution was hydrolysed at 25°C for one hour. The hydrolysed silane solution was dripped onto degreased, grit- blasted stainless steel and the surface was air dried to remove volatile solvent leaving a thin coating (0.1 to 10 microns) of monomeric hydrolysed silane. The condensation reaction was followed by diffuse reflectance infrared spectroscopy at temperatures in the range of 30°C to 90°C.

For comparative purposes, a condensation reaction according to a current state of the art silane treatment system was carried out. The hydrolysis of GPS in water and ethanol (mass ratio of 1:2:18) was catalysed by the addition of acid (pH of 3.7). The solution ^'was hydrolysed for one hour at 25°C before applying onto degreased, grit-blasted stainless steel. The residual solvent was evaporated from the surface by air drying. The condensation reaction was followed by diffuse reflectance infrared spectroscopy at temperatures in the range of 30°C to 90°C.

The results are shown in Table 8. The disappearance of the hydroxyl band in the infrared spectroscopy (band centred around 3375 cm^"1) gave a measure at which

the monomeric hydroxyl silane condensed to form a highly crosslinked siloxane film. The kinetic curves are complex and are not first, second or simple n order rate processes. A measure of the reaction rate, in Table 8, is given by the peak ratio of the OH band to CH band (centered at 2800cm^"1) at a time interval of 50 minutes. At equivalent temperatures, the condensation reaction has progressed to a greater extent for the europium (III) trifluoromethanesulphonate catalysed system relative to the acid catalysed system.

Near the end of the condensation reaction the OH content forms an equilibrium, as there appears to be no further change with time. This suggests that the condensation process is limited either by diffusion processes or by the chemistry of the structure (eg. steric hindrance). However, at equivalent temperatures, the europium (III) trifluoromethane sulphonate catalysed system contains less OH than the acid catalysed system suggesting that the lanthanide salt catalyses the condensation process to a greater extent than the acid catalysed system.

A comparison of the rare earth metal catalysed reaction process with that of the current state of the art methodology therefore indicates that the lanthanide catalysts not only increases the rate at which the silane condenses to a highly crosslinked film but also it increases the extent of the reaction. Table 8

Example 9: Effect of La Series Catalyst on Aerogel Formation

Hydroperchloric acid, water, ethylene glycol, tetraethoxysilane (TEOS) and a lanthanide catalyst was mixed in the mass ratios as illustrated in Table 9. The solution was vigorously stirred at 45°C until the solution was clear. Diisopropylamme solution was stirred into the mixture which was then cast into a glass container. The gel was aged at 35°C for 18 hours.

Although not very well understood, a measure of the crosslink density/effectiveness is considered to be given by the amount by which the pores of a gel collapse during ageing. In poorly crosslinked gels, the sample shrinks, expelling solvent. This was often the case for gels formed by acid/base catalysed reactions. However, as demonstrated in Example 8, lanthanide series salts catalyse condensation processes to a greater extent than the conventional acid/base catalysts. In the case of aerogels containing lanthanide series salts, there was no observable shrinkage during ageing (Table 9). Table 9

Claims

1. A process for the hydrolysis of a silane having at least one hydrolysable group which comprises contacting the silane with water in the presence of a catalyst comprising a rare earth metal salt with a non-nucleophilic ligand.

2. A process for effecting treatment of the surface of a material having a superficial oxide or hydroxide layer, which comprises applying the product of the process of claim 1 to said surface.

3. A process for effecting treatment of the surface of a material having a superficial oxide or hydroxide layer, which comprises exposing the surface to a silane having at least one substituent group which is capable of being hydrolysed, in the presence of water and a catalyst comprising a rare earth metal salt with a non-nucleophilic ligand.

4. A process as claimed in any of claims 1 to 3 wherein the non-nucleophilic ligand is selected from the group comprising trifluoromethanesulfonate, perchlorate, oxalate, acetate and other alkanoate having a chain length of from 2 to 10 carbon . atoms, hexafluoroacetylacetonate and acetylacetonate.

5. A process as claimed in claim 4 wherein the ligand is trifluoromethanesulfonate or perchlorate.

6. A process as claimed in any of claims 1 to 5 wherein the rare earth metal is a lanthanide.

7. A process as claimed in claim 6 wherein the rare earth metal is lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, erbium, thulium or ytterbium.

8. A process as claimed in claim 7 wherein the catalyst comprises samarium (III) trifluoromethanesulfonate (triflate), europium (III) trifluoromethanesulfonate (triflate), erbium(III) trifluoromethanesulfonate (triflate), europium(III) perchlorate, ytterbium (III) trifluoromethanesulfonate (triflate) or cerium (IV) trifluoromethanesulfonate (triflate).

9. A process as claimed in any one of the preceding claims wherein the catalyst is selected to produce a desired rate of silane hydrolysis.

10. A process as claimed in any one of the preceding , claims wherein the hydrolysable group on the silane comprises an alkoxy group, O-aryl group, halide, carboxylate, thiol, silthian, mercaptosilane, nitrile, cyanate, peroxy, amine, silazane, diamino, perchlorate, phosphate, borate, titanate or aluminate.

11. A process as claimed in claim 10 wherein the hydrolysable group on the silane comprises at least one alkoxy group.

12. A process as claimed in claim 11 wherein the silane has from 2 to 4 alkoxy groups having the same or different lower alkyl substituents.

13. A process as claimed in claim 12 wherein the silane comprises vinyltrimethoxysilane, chloropropyltrimethoxy silane, 3-(triethoxysilyl) propyl amine, 3-(triethoxysilyl)ethylene diamine, 3-(trimethoxysilyl)propyl methacrylate, 3-(trimethoxysilyl)propyl acrylate, 3-(triethoxysilyl)propyl thiol, tetraethoxysilane(TEOS), tetramethoxysilane(TMOS) or γ-glycidoxypropyl- trimethoxysilane (GPS).

14. A process as claimed in claim 13 wherein the silane is γ-glycidoxypropyl- trimethoxysilane (GPS).

15. A process as claimed in any of claims 1 to 14 wherein the catalyst is typically present in an amount of from 0.01% to 2% by mass of the reaction mixture.

16. A process as claimed in any of claims 1 to 15 which is carried out in aqueous solution.

17. A process as claimed in any of claims 1 to 15, which is carried out in a solvent comprised of water and at least one organic solvent.

18. A process as claimed in claim 17 wherein the at least one organic solvent is methanol, ethanol, acetone, acetonitrile or isopropanol.

19. A process as claimed in claim 17 or claim 18 wherein the organic solvent is present in an amount of from 0%> to 99% by mass of the solvent.

20. A process as claimed 19 wherein the organic solvent is present in an amount of from 75% to 99% by mass of the solvent.

21. A process as claimed in claim 2 or claim 3 or in any of claims 4 to 20 when dependent thereon, wherein the material to be treated is a metal or metal alloy.

22. A process as claimed in claim 2 or claim 3 or in any of claims 4 to 20 when dependent thereon, wherein the material to be treated is a non-metallic.

23. A process as claimed in claim 21 wherein the metal or metal alloy is selected from the group comprising aluminium, aluminium alloys, steels, chromium, chromium alloys, titanium, magnesium alloys, copper, tin and brass.

24. A process as claimed in any of claim 23 wherein the surface to be treated is mild steel.

25. A process as claimed in claim 22 wherein the non-metallic is selected from the group comprising glass, silica, clays and talc.

26. A process as clahned in claim 2 or claim 3 or in any of claims 4 to 25 when dependent thereon, wherein the silane is present in an amount of from 0.01% to 5% by mass of the reaction mixture.

27. A process as claimed in claim 26 wherein the silane is present in an amount of from 0.1 % to 2% by mass of the reaction mixture.

28. A process as claimed in claim 26 wherein the silane is present in an amount of 0.01%ι to 5% by mass, the catalyst in an amount of from 0.01% to 2% by mass, together with a solvent comprising an alcohol in an amount of from 0 to 99% by mass of the solvent and water in an amount of from 1% to 99%> by mass of the solvent.

29. A process as claimed in claim 28 wherein the silane is present in an amount of 0.1% to 2% by mass, the catalyst in an amount of from 0.1% to 0.5% by mass, together with a solvent comprising an alcohol in an amount of from 75 to 99% by mass of the solvent and water in an amount of from 2% to 10% by mass of the solvent.

30. A process as claimed in any of claims 26 to 29 wherein the silane is GPS.

31. A process as claimed in any one of the preceding claims wherein the process is carried out at a temperature of from 15 to 40°C.

32. A process as claimed in claim 31 wherein the process is carried out at ambient temperature.

33. A process as clahned in claim 1 or in any of claims 4 to 32 when dependent thereon, carried out under conditions such that a silicon-based gel is obtained.

34. A process as claimed in claim 33 wherein the conditions include a high ratio of silane to solvent.

35. A process as claimed in claim 33 or claim 34 wherein the catalyst and silane to solvent ratio are selected to promote formation of a silicon-based gel.

36. A process as claimed in claim 34 or claim 35 wherein the silane is present in an amount of 75 to 90% by mass, with the alcohol in an amount of from 6 to 22% by mass, water in an amount of from 2% to 10% by mass and the catalyst in an amount of from 0.01% to 2% by mass.

37. A process claimed in any of claims 33 to 36 wherein the silane is tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS).

38. A process as claimed in any of claims 33 to 37 wherein the catalyst is ytterbium (III) trifluoromethanesulfonate or erbium (III) trifluoromethanesulfonate.

39. A process as claimed in any one of the preceding claims wherein the catalyst comprises an integral part of a support phase, said phase being such as to allow a silane which is to be hydrolysed to be brought into intimate contact with the catalyst to allow hydrolysis to take place.

40. A process as claimed in claim 39 wherein the support phase for the catalyst comprises part of a fixed reaction column, such that fluids containing the silane may be passed across the surface of the supported catalyst.

41. A process as claimed in claim 39 wherein the support phase comprises a recoverable medium to which the catalyst is attached.

42. A process as claimed in claim 41 wherein the recoverable medium takes the form of beads, large bio molecules or an inorganic lattice.

43. A process as claimed in claim 42 wherein the bio molecule is a protein, enzyme or nucleic acid oligomer.

44. A process as claimed in claim 42 wherein the inorganic lattice is a clay or zeolite.

45. A process as claimed in claim 1 or in any of claims 4 to 32 when dependent thereon, such that a silane crosslinked polysiloxane is produced.

46. A process as claimed in claim 1 or in any of claims 4 to 32 when dependent thereon, wherein a hydrocarbon is present such that a silane crosslinked polyhydrocarbon is produced.

47. A process as claimed in claim 42, wherein the hydrocarbon may include alkyl, alkenyl, and alkynyl polymers.

48. A process as claimed in claim 43, wherein the hydrocarbon may include polyhalo or perhalo substitued polymers, preferably fluorinated and chlorinated polymers.

49. A process as herein described and with particular reference to the examples.

50. A surface having a superficial oxide or hydroxide layer when treated according to any one of claims 2 to 49.