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THE CONSOLIDATION OF NATURAL STONE WITH A STONE STRENGTHENER ON THE BASIS OF POL Y-SILICIC-ACID-ETHYLESTER

KOBLISCHEK, PETER J. INTERACRYL GmbH, D-00459 Frankfurt/Main, Germany

SUMMARY Only a prehydrolized silicic acid ethyl ester can be applied safely and gives better results in consolidation. The MOTEMA®-Stone Strengthers are ready-to-use products designed for the restoration of natural stone, stuc~o and frescos which are damaged by weathering influences. In addition other building materials such as bncks or terracotta can be consolidated. During application, the material's capillary system absorbs the product and will transport it within deeper zones. The Poly-Silicic-Acid-Ethyl-Ester reacts with the water absorbed at the capillary walls and silicic acid gel is formed. In the past years, this technology was been used on many objects in Europe, including the works in the Forum Romanum in Rome, the reconstruction of the Greek Theatre, part of the temple of Apollo Licio in Metaponto in the south of Italy, the southportal of the cathedral in Worms and this year on buildings in Petra. In 1978 we did not pay attention to the safety-relevant data of the monomer tetra ethyl silicat (silicic acid ethyl ester), because we did not intend to use a monomer ester as a stone strengthener. On the market "solvent free" stone strengthener are offered as very environmentally beneficial. This means a solvent free, catalized silicic tetra ethyl ester. We have examined the possibility of the combination of the polyester within the monomer ester as a solvent, in order to be able to offer also a "solvent free" stone strengthener. To do this, we had to consider safety-related data. We determined that the MAC of tetra ethyl silicate is only 20 ppm, but that it has no odour befor 85 ppm. This means that one can inhale a toxic amount before noticing any odour.

ZUSAMMENFASSUNG Nur ein vorhydrolysierter Kieselsaureester ist tor den Verarbeiter nicht gesundheits-schadlich und einfach sowie sicher in der Anwendung. Die MOTEMA®-Steinfestiger werden gebrauchsfertig geliefert und sind geeignet fur die Festigung von Natursteinen, vorzugsweise Sandsteinen, Stuck, Putzen, Fresken und Terrakotta, wenn diese durch Bewitterung ihre Festigkeit im Gefuge verloren haben. Bei der Anwendung wird das Produkt von den Kapillaren und Poren aufgenommen und bildet dort mit dem dort vorhandenen Wasser die neue Verkittungssubstanz Kieselgel = Siliziumdioxid · Wasser. Seit 1978 wird diese Methode der Festigung an vielen namhaften Objekten in Europa angewandt. Hierzu zahlen u.a. das Forum Romanum, das wiederaufgebaute griechische Theater und der Tempel des Apollo Licio in Metaponto, das Sudportal des Domes in Worms und Gebaude in Petra. Im Jahr 1978 hatten wir den sicherheitsrelevanten Kenndaten des monomeren Tetraethylsilikats (Kieselsauretetraethylester) keine Beachtung geschenkt, denn wir wollten den monomeren Ester als Steinfestiger nicht einsetzen. Auf dem Markt werden jetzt "losemittelfreie" Steinfestiger als besonders umweltfreundlich angeboten. Es handelt sich hier um losemittelfreien, katalysierten Kieselsaure­tetraethylester. Wir untersuchten die Moglichkeit der Kombination des Polykiesel-saureesters in dem monomeren Ester als Losungsmittel, um so ebenfalls einen "losungsmittelfreien" Steinfestiger anbieten zu konnen. Hierfur muBten wir uns sicherheitstechnischen Kenndaten ansehen. Dabei stellten wir fest, daB der MAK-Wert von Tetraethylsilikat nur 20 ppm betragt, aber erst ab 85 ppm riechbar ist. Das bedeutet, der Verarbeiter atmet schon eine toxische Menge ein, bevor er dies riecht.

1 INTRODUCTION

The reason for weathering of natural stone and in particular of sand stone is the loss of the grain-grain­linkage. The reduction of adhesion can penetrate deeply on the inside of the stone. The purpose of strengthening is to restore the bond between separate grains. Today organic silicic aci~ ester~ are almo~t exclusively used. Through hydrolysis, alcohol is deco~posed and absor~ed as the bin~er S102-gel. This gel should reach all weathered areas, penetrating uniformly and deeply into the material.

2 SELECTION OF PRODUCTS

Earlier products such as baryta water, fluates and water-gla~ did mor~ har~ than ~ood to.the na~ural stone and are of no importance today. Also the reconstructive satura~on. with reaction resin ~olutions according to DIN 16945 must be judged very sceptically. The application of polymer solution, e.g. polymethylmethacrylate, is esp~~ially suitable _for selected n~tural stones. However, those products, also in combination with hydrophobizing agents, will not be examined here. [1]

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Silicic acid ester are esters of ortho silicic acid with the following structural formula:

Both esters react in the same way:

O·R

I R·O-Si-O· R

I O·R

Si

R

H,O

=silicon,

= CH3- (methyl) or = C2H5- (ethyl) .

Si(OR)4 + 4 H20 -> Si02 • aq + 4 ROH

R-OH = methanol: CHpH, or= ethanol: C2H50H

The monomer methyl ester is considerably more reactive than ethyl ester. Because it is highly toxic -causing serious necrosis of cornea, with danger of blindness - this methyl ester cannot be purchased on the free market an consequently cannot be applied. This leaves only ethyl ester.

This silicic acid ethyl ester (SAE) has a molecular weight of 208 and a boiling point of 163°C and hydrolizes slowly in water, which leads to the problem of evaporation. Much of the applied compound would evaporate before it could penetrate deeply into the weathered stone. For this reason this monomer ester is catalized generally using dibutyltinlaurat. The hydrolysis of the ester then proceeds more quickly. This reaction is strongly influenced by temperature, moisture on and in the object and atmospheric humidity. Under disadvantageous conditions, there is a danger of glazing on the surface.

The tensile strength cracks with catalized customary silicic acid ethyl ester products depending on depth of penetration - as described by Ettl/Schuh (2) - are explainable by the different reactions due to temperature and humidity within the stone.

Profile of strengthening of an unconsolidated (broken line) and a consolidated (beam) reed sand stone of the same origin.

3 ALTERNATIVES

The monomer silicic acid esters were al.ready on the market for more than 1 o years in 1978, when our customers asked for a slowly hydrolysing stone strengthener Chemistry prescri'b · · · es an increase m

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pressure and temperature as a means of accelerating chemical reactions. Both are out of question for stone consolidation. '!"'polymerization at room temperature proceeds substantially quicker. if the first phase takes place in advance in a separate reaction before remixing prepolymerisat with the monomers. This reaction is accomplished in the workshop by increased temperature. We examined whether the Trommsdorf-effect is also applicable by silicic acid ethyl ester after pre-hydrolysis. Corresponding references were found in the bibliography. [3]

In test series, the most favourable relation between viscosity and reactivity were determined. We found an oligomer consisting of an average of 4 molecular components to be especially suitable. The structural formula can be presumed as follows:

H5C,0 OC2H5 OC2H5 - I I I C2H50 - Si- O - Si- 0 - Si-OC H

I I I 2 5

H5C20 0 OC2H5

I H5C,0 - Si-OC2H5

- I OC2H5

This oligomer has an average molecular weight of appr. 650 and a Si02 -content of 40 to 50 % w/w. The viscosity rises from 1 mPa·s of the monomer ethyl ester to 5 mPa·s, which is still acceptable. The increased viscosity can be reduced by adding a solvent. This is necessary anyway, because a stone strengthener with an Si02-content of 40 to 50 % w/w should not be applied. How can the secretion "in situ", i.e. in the depth of the stone, be controlled? On every crystal surface in a stone and particularly on quartz there exists a stratum of water molecules. This so-called Helmholtz-Stratum has a strength of ca. 3 molecule layers.

Helmholtz-Stratum with anions, cations and water-dipoles [EAGLAND]

Region A

I i.h.p. o.h.p. shear

plane

8 ' Ao;on 6 ' C•t;oo 8 : Water dipole

There is a zone inside the stone which contains water and which is required_ for the hydrolys_is of the poly silicic ·d t The enrichment of water can be additionally supported by using a hygroscopic solvent, such as

:~an~~- ~thanol has a boiling point of 78°C, can be combined with silicic acid (ethyl) ester products as well as with water in every proportion and is physiologically harmless.

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The most important data of the ready-to-use PSAE-product inethanol:

Form: Colour: Odour: Active content: Specific gravity: Viscosity: Flash point: Hazard classification:

liquid colourless alcoholic, aromatic 28% Si02

0,94/20°C 2 mPa·s/23° C (Brookfield) 12°C

3 (flammable liquids)

The slower hydrolysis in comparison with the catalized silicic acid ester was accepted intentionally, because it has a decisive advantage: during the hydrolysis of ester a relatively large amount of ethanol has to escape from the system. The consequence is a strong shrinkage and high tensile strength which are caused in the gel. During the hydrolysis of a silicic acid ester, silanole are developed as an intermediate stage.

I I -Si-OR + H-0-H <-> -Si-OH + ROH (1)

I I

I I I I -Si-OH + RO-Si- <-> -Si-0-Si- +ROH (2)

I I I I

I I I I -Si-OH + HO-Si- <-> -Si-0-Si- +ROH (3)

I I I I R-Si(OH), + 2 H20 -> Si02 + 4 ROH (4)

This becomes apparent through a passing hydrophobical stage on the consolidated stone. If the gel exists as a silanole for a longer time and if during this stage a large amount of ethanol can be separately evaporated, the shrinkage of the gel is substantionally diminished after the complete hydrolysis and dehydration. Tension is reduced and the consolidations are more uniform. This is shown in the following diagram. The none-pre­condensed, high catalized product shows a rough increase of the consolidation towards the surface. The pre­condensed product on the other hand has a well-balanced consolidation profile with a smooth inward course.

10

8

2

O L---'---'--'-----'----'---'-----'~'---'--'--'---'----'-~

0 20 40 Depth (mml

Regensburger Grunsandstein

PSAE SAE

60

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Foto 1 . Pure silicic acid gel with parts of not finilized hydrolysis.

Foto 2: MOTEMA®- PKSE-Steinfestiger 28 on Sander-Sandstone.

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Foto 3: MOTEMA®-PKSE-Steinfestiger 28 solid and glutinousfree structure on

Sander-Sandstone .

4 SAFETY RELEVANT DATA

In 1978 we did not pay attention to the safety-relevant data of the monomer tetra ethyl silicat (silicic acid tetra ethyl ester). because we did not intend to use a monomer ester as a stone strengthener.

On the market "solvent free" stone strengtheners are offered as very environmentally beneficial. This means a solvent free, catalized silicic acid tetra ethyl ester. We have examined the possibility of the combination of a polyester within the monomer ester as a solvent, in order to be able to offer also a solvent free stone strengthener. To do this, we had to consider safety-related data. We determined that the MAC of tetra ethyl silicate is only 20 ppm, but that it has no odour before 85 ppm. This means that one can inhale a toxic amount before noticing any odour. (6) The following table compares the most important safety-relevant data of silicic acid ethyl- (SAE) and poly silicid acid ethyl esters (PSAE) . ·

Mol.-weight: Boiling point: CAS-Nr. EG-Nr. Danger symbol: Possible dangers:

Exposition limit: (MAC) Smellability: Steam pressure: Minimum level for odour: Toxic reactions:

-skin

- eyes - inhalation

SAE

208 163°C 78-10-4 014-005-00-0 Xn, =hazardous flammable, hazardous by inhalation, irritates the eyes, respiratory organs and skin 20 ppm = 170 mg/m3

85 ppm < 2 hPa 85 ppm

moderately irritating not sensitized irritating noxious

PSAE

::: 650 > 200°c 68412-37-3

none no particular dangers known

none

350 ppm (Ethanol) 2,4 hPa 350 ppm (ethanol)

not irritating

slightly irritating none

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5 CONCLUSION

In our opinion, only a stone strengthener based on a poly silicic acid ester can satisfy the requirements of natural stone an working hygiene.

REFERENCES

(1) KOBLISCHEK, P. J.: Polymere in der Denkmalsanierung, Proc . .IV. ICPIC, TH-Darmstadt, S. 441-446, 19.-21 .09.1984.

(2) ETTL, H.; SCHUH, H.: Konservierende Festigung von Sandsteinen mit Kieselsaureethylester, Bautenschutz + Bausanierung 12 (1989) 35-38.

[3] CHVATAL, T.: Vergleichende Untersuchung der wichtigsten modernen Steinkonservierungsmit-tel. Steinmetz und Bildhauer (197 4) H.2, 96 - 101 , H.3, 154 - 161

[4] WENDLER, E.: Gesteinsfestigung mit marktilblichen und modifizierten Kiesels~ureesterprodukten , Proc. 2. Fachtagung Natursteinsanierung Bern ' 95, INTERACRYL AG, (1995) .

[5] KOBLISCHEK, P. J .: Protection of Surfaces of Natural Stone and Concrete through Polymers. Proc. 1. International Symposium on Surface Treatment of Building Materials with Water Repellent Agents, pp. 2:1-12, Delft, (1995).

The REM-fotos with kind permission of FMPA-Stuttgart. diploma thesis from Mr. Michael Bottner.