Silicon Chemistry 1: 397–402, 2002.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Solvent effect on TEOS film formation in the sandstone consolidationprocess ∗

Ramon Zarraga 1, Dolores E. Alvarez-Gasca 2 & Jorge Cervantes 1, ∗∗1Facultad de Quımica, Universidad de Guanajuato, Guanajuato, Gto., 36050, Mexico,2Facultad de Arquitectura, Universidad de Guanajuato, Guanajuato, Gto., 36000, Mexico,∗∗Author for correspondence (e-mail: jauregi@quijote.ugto.mx)

(Received 11 February 2003; accepted 7 August 2003)

Key words: consolidants, film formation, sandstone, solvent effect, TEOS

Abstract

Alkoxysilanes, low-viscosity monomers capable of polymerizing into the porous network of stone by a sol-gelprocess, are widely used as consolidants in the restoration of stone monuments. However, since consolidation is anon-reversible application capable of causing serious harmful side effects to the original material, stone consoli-dation is almost always considered a very risky intervention. Alkoxysilanes are insoluble in water, so co-solventedsystems are very often used, but even knowing that the solvent is a determinant parameter for sol-gel reactions,there is still a lack of information regarding how it can influence the stone-alkoxysilane affinity. For two differenttetraethoxysilane-acidic co-solvented systems we are reporting both the morphological characteristics showed bygels formed in situ and the affinity reached with the stone in the sandstone consolidation process. Aqueous solutionsof ethanol and methyl-ethyl-ketone (MEK) were the solvents compared. SEM and 29Si Solid State NMR were usedto determine the alkoxysilane performance on these silicic-based materials. It was found that using MEK solutionsresulted in appreciable sandstone-alkoxysilane interaction, forming a more homogeneous film. On the other hand,ethanol does not promote alkoxysilane-sandstone compatibility. A brittle film is obtained when ethanol is used.

Introduction

In recent years, an apparent acceleration in the rate ofstone decay and the growing worldwide interest in pre-serving historic buildings are promoting a significantincrease in the number of studies on restoration. Usingalkoxysilane-based products as stone consolidants topreserve decayed quartz-bearing rocks, like sandstonein historical buildings, has become a common prac-tice in the last decades [1–3]. The market impact isindicated by the fact that 50% of non-funcionalizedsilicon compounds are directed towards architecturalcoatings and mineral consolidation [4]. Alkoxysilanes,such as tetraethoxysilane (TEOS), are applied as lowviscosity monomers or dimers in solutions that may in-clude water, ethanol, other organic solvents (generally

∗ This paper is dedicated to Professor Kohei Tamao, 2002Kipping Award winner.

MEK and acetone) and some organometallic catalysts.In a recent overview of current research on stonepreservation, consolidation is considered as an ac-tive conservation process “. . . where stone is severelyweakened by decay, some form of consolidation maybe necessary to restore some strength. Ideally, onemight hope to make the stone at least as strong as itwas originally, so it might resist further decay. . . ” [5].The drawback is that contradictory reports regardingthe performance of alkoxysilane consolidants prolif-erate in the literature. For some conservationists theyperform well, for some others they perform poorlyon substrates with equivalent mineral composition [6].Actually we can say that consolidation processes arenot entirely mastered. And to make things more com-plex, harmful side effects are very frequent in con-solidation interventions, suggesting that there is stilla long road ahead until full mastery of the process isreached [7, 8].

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Scheme 1.

The main reason behind this lack of consistencyin results seems to be basically related to the level ofchemical affinity achieved by the alkoxysilane-stone,as well as the characteristics in the resulting film[9–12]. Sol-gel chemistry is directly involved in theconsolidation process from the moment the alkoxysil-ane solution is in contact with the stone substrateuntil the polymeric film is deposited and dried. In allcases it has been established by Brinker and Schererthat the chemical processes behind gel formation aregreatly influenced by the presence of, among others,the solvent and/or co-solvent added [13].

The effect of the solvent on the consolidation pro-cess has been largely neglected. Diverse formulationswith different kinds of solvents have been added toTEOS in order to decrease its concentration and vis-cosity, making the initial solution able to penetratedeeply into the substrate [5]. In order to start thepolymerization, water is required (both in acid andorganometallic catalyzed reactions), but the process isconsiderably more efficient if a co-solvent is addedto achieve immediate miscibility. Popular commer-cial consolidants (Wacker OH and Tegovakon V) arewater-free. However, considering that the purpose hereis to maximize chemical interactions with the surfacesilanols in the stone, water is needed as has been sug-gested by some authors [14, 15] in order to obtainreactive pre-hydrolyzed species. On the other hand,hydrolysis and condensation reactions are complicatedby solvent interactions, particularly in their affecton the orientation of the silanol molecules. It hasbeen suggested [16] that intramolecular condensation(cyclization) is promoted in acidic ethanol solutions,a prejudicial situation for the homogeneous gelationnecessary to achieve an effective consolidation pro-cess. Also, if an alcohol is used, it does not merelyact as a solvent. Alcohol, because it is a reactionproduct, when added to the alkoxysilane/water systemmay cause the reverse of reactions 2 and 3 shown inScheme 1 (esterification and alcoholysis) to occur. Theextent to which this occurs depends upon the amountof the alcohol added and the pH of the system [13].

Application of IR spectroscopy to study alkoxy-silane-silanol reactions is limited by the strong back-ground absorption bands of bulk siloxane structures[14]. However, we have shown recently that it is pos-sible to determine the sandstone decay degree by SolidState 29Si NMR methods. We have also suggested thatthe value of consolidation obtained is consistent withsome particular modifications in the Q2–Q3 region inthe 29Si NMR spectrum, which can be attributed tothe chemical compatibility achieved between the al-koxysilane species and the surface silanols on the rock[11].

In the present work we use 29Si NMR methodsand SEM to investigate how the TEOS consolidationprocess is influenced by the use of different kinds ofsolvents in aqueous-acidic systems, in this particularcase ethanol and methyl-ethyl-ketone.

Experimental

Procedure

Different 5 × 5 × 3 cm (35.0 ±1 g) samples ofsound (BET surface area = 5.3 m2 g−1) and par-tially decayed (6.2 m2 g−1) pink quarry stone, themost common sandstone employed in Mexico’s cent-ral region, were collected from Jaral Church duringrecent renovations. Mineralogical analysis and XRDconfirmed that both samples belonged to the samebed, showing also that the composition of the soundstone is mainly albite and quartz, with a small quantityof hipersterne and hematite. Partially decayed stonesshow a lower level of albite and diverse levels of ka-olinite [11, 17]. This is evidence that sodium feldspar(albite) has been hydrolyzed to clay materials in theprocess known as kaolinization [18].

Data for each sample were obtained after its col-lection, and again after they all were sprayed andpermitted to react for 3 weeks in black plastic bagswith 10 mL of two different TEOS solutions at pH = 3(hydrochloric acid was used). The consolidant solu-tions used were (% vol): (1) 65% TEOS, 25% EtOHand 10% water; (2) 65% TEOS, 25% MEK and 10%water. We named them TEOS EtOH and TEOS MEK,respectively.

For the compatibility assessment, blank experi-ments were carried out mixing powders of the un-treated samples with both the different xerogels ob-tained aside from TEOS MEK and TEOS EtOH solu-tions, in the same proportion as we used in theconsolidation part.

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Chemical reagents

Tetraethoxysilane (TEOS), 99+% was obtained fromethanolysis of SiCl4 in a pilot plant facility [19].Reagent grade solvents used were hydrochloric acid(HCl), ethanol (EtOH) and methyl-ethyl-ketone(MEK) and were purchased from Aldrich. All reagentswere used as received without any further purification.

Scanning Electron Microscopy (SEM)

A palladium-gold alloy was vacuum evaporated ontothe dried samples. They were then investigated usinga LAICA S-420σ Scanning Electron Microscope.

Nuclear Magnetic Resonance (NMR)

29Si MAS NMR spectra were recorded on a VarianUnity Plus 300 MHz spectrometer operating at 59.58MHz. A 7-mm diameter silicon nitride rotor with kel-F caps was used. The rotor spin rate was 4 KHz,with a delay time of 8 s, and 1600 transients wereaccumulated.

Results and discussion

Information on chemical affinity and gel morphologywas determined using two experimental methods, 29SiMAS NMR spectroscopy and Scanning Electron Mi-croscopy (SEM). The results are discussed below.

29Si MAS Nuclear Magnetic Resonance (NMR)

29Si MAS NMR spectra from the sound stone andthe partially decayed sandstone are shown in Fig-ure 1. Interpretation of 29Si NMR spectra is basedon data given in the literature [11, 20, 21]. The bigand narrow quartz signal around –100 ppm persists inboth samples, but the broad feldspar signal between–83 and –94 ppm is lower in the decayed sample,indicating some kaolinization has occurred. Partiallydecayed stone mixed with xerogels proceeding fromTEOS MEK and TEOS EtOH solutions exhibits aslight increase in the Q3 region (around –93 ppm),as shown in Figure 2. Evidently this change is due tothe abundance of xerogels in the Q3 species, which ismore intense in TEOS EtOH, as can be seen in Fig-ure 3. Small rings (cyclic trimers), formed appreciablyonly in the presence of ethanol, can also be observedin Figure 3(a) by the narrow peak at –87.7 ppm.

Figure 1. 29Si NMR spectra of untreated samples. (a) The partiallydecayed sandstone; (b) the sound sandstone.

Figure 2. 29Si NMR spectra of partially decayed sandstone mixedwith powders of xerogels using (a) TEOS with EtOH, and (b) TEOSwith MEK.

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Figure 3. 29Si NMR spectra of xerogels using (a) TEOS with EtOH,and (b) TEOS with MEK.

The same stone, consolidated with both (a) TEOSEtOH and (b) TEOS MEK, respectively, is illustratedin Figure 4. It can be concluded from the higher sig-nal increase observed in both the Q2 and Q3 regions(around –87 and –93 ppm) that a superior chem-ical interaction was achieved with TEOS dissolvedin methyl-ethyl-ketone. The higher polarity of eth-anol makes this solvent avid for the available silanolgroups in the stone. That is, the substrate, polymerand solvent form a competitive acid-base triangle. Be-sides, the significant tendency of alkoxysilanes to formrings instead of condensing directly with the stone inacidic ethanol solutions results in an even higher lackof chemical affinity.

Scanning Electron Microscopy (SEM)

Figure 5 shows a typical decayed sandstone, with highporosity, some feldspar crystals and some kaoliniticmaterial product from the weathering processes. InFigure 6 a characteristic film obtained in situ fromTEOS EtOH solution can be observed. It looks brittleand thick. On the other hand, a much more continu-ous and thin film formed from TEOS MEK solutionis shown in Figure 7. The reason for this remarkable

Figure 4. 29Si NMR spectra of partially decayed sandstone consol-idated using (a) TEOS with MEK, and (b) TEOS with EtOH.

behavior seems to confirm the effect above mentionedof the higher polarity shown by ethanol. The cycliza-tion rate constants are raised for the oligomers recentlyformed in the solution, as has been suggested [16]and observed by 29Si NMR techniques in Figure 3(a),producing a bulky and less homogeneous coating.

Conclusions

The sandstone consolidation process with alkoxysil-anes is decidedly influenced by the kind of co-solventused, including water, in the solutions.

MEK allows appreciable chemical interactionsbetween the sandstone and the alkoxysilanes. Thisaffinity was observed with NMR techniques throughthe signal increase observed in both the Q2 and Q3

regions. The way the hydrolyzed species attach tospecific silanols in the stone is still unclear.

Lack of chemical affinity and bad quality coatingsare obtained using ethanol, a very polar solvent. This

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Figure 5. Partially decayed sandstone. Bar = 10 µm.

Figure 6. Partially decayed sandstone consolidated using TEOSwith EtOH. Bar = 10 µm.

is probably due to the disablement of the superficialsilanols and the tendency to form rings shown by al-koxysilanes in this kind of chemical environments. Weare currently investigating different ways to increasethe number of available silanols on the stone surfacein order to achieve a better chemical affinity.

Acknowledgements

The authors wish to thank CONACYT and SIGHO-CONACYT (Mexico), grant 19990204002, for finan-cial support.

Figure 7. Partially decayed sandstone consolidated using TEOSwith MEK. Bar = 10 µm.

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