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DR/FT INVESTIGATION OF SILYLATED NATURAL STONE -MOLECULAR SURFACE INFORMATION AND MACROSCOPIC FEATURES

BRUCHERTSEIFER, CHR.; BRUGGERHOFF, S.

Zollem-lnstitut beim Deutschen Bergbaumuseum/DMT, Herner Str. 45, 44787 Bochum (FRG)

GROBE, J.

Anorganisch-Chemisches lnstitut, Westfalische Wilhelms-Universitat, Wilhelm-Klemm Str. 10, 48149 Monster (FRG)

GOTZE, H.J.

Lehrstuhl flir Analytische Chemie, Ruhr-Universitat, Universitatsstr. 150, 44780 Bochum (FRG)

SUMMARY

The use of Diffuse Reflectance FT-IR spectrometry (DRIFn in the hydrophobic treatment of natural stones with commercial organosilicon protective agents is described. The results obtained from surface investigation of silylated powders of Schleeriether sandstone (SRS) and Obernkirchener sandstone (OKS) can be correlated to the hygric behaviour of the materials, stored and exposed for eight years. Due to differences in the pore spectrum and chemical composition of the two substrates DRIFT investigation of treated powders suggest highly different interaction with organosilicon compounds resulting in deviating effectiveness and long-term performance of water repellent treatments.

1 INTRODUCTION

Decay of building materials has become a problem of increasing economical extent all over the civilized world. In case of natural stone damage leads to a significant loss of cultural heritage which is documented by numerous case studies dealing with the conservation of historical building substance. But even modern and wide spread building materials such as concrete are affected by environmental influences to an extent held for impossible by now [1]. The various typical and well known damage forms especially of weathered sandstone (crusts, shells) which arise from complex destruction processes due to chemical, physical and biological impact would be impossible without contact of the stone material with water. Since the anthropogeneous weathering components (acid gases, ozone, soot etc.) have increased exponentially and been added to the natural (water, sun, wind etc.) during the last hundred years [2] the key role of water in the destruction of stone has become more and more fatal.

2 WATER REPELLENTS FOR BUILDING MATERIALS AND SURFACE SENSITIVE MEASURING TECHNIQUES

In order to keep water away from the stone facade organosilicon compounds like silanes, siloxanes and silicones offer the possibility to create a hydrophobic polysiloxane network covalently fixed to the building material. Since silyl esters derived from alkyl-trialkoxysilanes X-(CH2)nSi(OR)3 consist of an "inorganic" part (Si(OR)3-group) allowing reaction with the stone surface as well as polycondensation after hydrolysis and an organic, hydrophobic part (alkyl group) repelling water molecules it is obvious that they are preferably applied for hydrophobization in building protection [3]. Strengthening agents which work as an artificial binder in the grain texture of stone are based on silicic acid ester, mostly Tetraethoxysilane (TEOS). Treatment with organosilicon water repellents preserves water vapour

diffusion inside the building material.

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However, with respect to the effectiveness and long-term performance of protective siloxane layers

fundamental and decisive aspects such as

i) molecular functioning of a given polysiloxane structure ii) concentration/depth profile of the agent in the treated material iii) durability and degradation behaviour of the coating

are complex and important fields demanding extensive research . The use of suitable surface sensitive measuring techniques is neccessary to control the structure and efficiency of the protective layer. Information about the kind of interaction with the mineral surface, the degree of polycondensation and the structure of a protective layer is available by the use of several modern spectroscopic measuring techniques. IR and MAS-NMR (Magic Angle Spinning-Nuclear Magnetic Resonance) spectroscopy, for example, allow examination of the bulk properties of siloxane surface layers. Details concerning structur of uppermost molecular layers are obtained from static SIMS (Secondary Ion Mass Spectrometry). Furthermore, the highly surface sensitive technique DRIFT enables to study the interface polymer - stone, thus giving detail about interaction of the coating with

the substrate [4].

3 EXPERIMENTAL SETUP

3.1 Diffuse Reflectance FT-IR spectrometry (DRIFT)

The Diffuse reflectance infrared Fourier Transform technique (DRIFT) has turned out as a very powerful tool for analyzing polymeric surfaces [5]. It is specifically designed to study powdered samples, includes nondestructive sample preparation and is known for its high sensitivity. Many materials (powders, rough surface solids) exhibit diffuse reflection (i.e. incident light is scattered in all directions as opposed to specular (mirror-like) reflection where the angle of incidence equals the angle of reflection. Only the diffusely reflected radiation gives information about the absorption behaviour of the sample. In general, DRIFT spectra are complex and depend strongly upon the conditions under which they are obtained. They can exhibit absorbance as well as reflectance features caused by contributions from transmission, internal and specular reflectance components. Moreover, DRIFT spectra are influenced by sample preparation, particle size, sample concentration etc. influencing quantification of spectra. In order to allow semiquantitative states obtained from Diffuse reflectance spectra the following steps have to be carried out very carefully before recording a DRIFT spectrum: i) In order to minimize or eliminate the contributions of specular reflection due to large particle faces in the collected and detected light the sample must be grinded very extensively thus influencing the packing of the sample. Reliable spectra can be obtained if particle sizes less than 1 O µm are present; ii) Specular reflection produces inverted bands ("reststrahlen bands") in the diffuse reflectance spectrum which are strong for highly absorbing materials such as powdered samples. Dilution with a nonabsorbing standard such as KBr (ca. 1 :10) secures a transmission-like spectrum of the sample due to deeper penetration of the incident beam.

The obtained reflectance spectra are converted into Kubeika-Munk spectra in which linearity between signal intensity and concentration is given allowing quantitative states.

In order to characterize the surface treatment of natural stones with organosilicon protective agents by DRIFT finely powdered stone samples were taken for investigation offering the possibility to load

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the substrate with protective silyl ester to a maximum. In this way reliable spectra giving information about surface characteristics of the coating are easily obtained.

3.2 Examination of the hygric behaviour of treated and weathered natural stones

In 1986 an outstanding field exposure campaign with organosilicon protective agents applied to different natural stones was initiated by the Zollem-lnstitut in cooperation with the German Chemical Industry. Freshly cut stone prisms (300x300x150 mm) were treated with eleven different commercial products. After application (complete soaking for 1 min.) and drying under controlled conditions the material was exposed on metal racks at different locations in Germany representing typical climatic and weathering situations. One set of samples of each variety of stone as well as untreated material was stored in the lab for comparative investigation (reference material).

In the scope of a large investigation program concerning structure and effectiveness/ weathering stability of organosilicon coating selected samples were investigated after eight years of exposure [6]. In order to look for the long-term effect of organosilicon coating under laboratory conditions (reference samples, REF) and under weathering conditions (exposed samples from Dortmund (DO) and Duisburg (DU)) the hygric behaviour of the material was investigated by a) complete long-term water storage; b) subsequent drying profiling of the prisms. Additionally, penetration depths of the agents on the stones were determined optically by the wetting technique. As an exemplary comparative investigation of two systems with different mineral bonding and chemical composition, Schleeriether (SRS, clayey) and Obemkirchener sandstone (OKS, quartzitic) were selected. Initially, the exposed material chosen for hygric tests was sealed a second time at one of the two faces where already sampling had taken place after two years of exposure. The sealed face was chosen as the standing side of a prism for water absorption measurements by the diving method. Drying and careful climatization up to weight constance followed. Due to the all-side water contact water uptake investigation by complete storage of the stones should yield an average value for the remaining hydrophobic effectiveness of the whole treated surface.

Besides water absorption behaviour weathering of stone is further influenced by the drying properties of a given substrate. If loss of humidity in impregnated stone material is accelerated or delayed depends on both pore radii distribution and mineral composition, especially on the content of clay minerals (sheet silicates) which tend to store water molecules, thus extending drying periods which

causes freeze-thaw stress.

4 RESULTS AND DISCUSSION

SRS is a clayey bonded type of sandstone which consists of ca. 60 Vol.% quartz. The material contains different clay minerals, among them kaolinit, montmorillonit and illit. Thus, SRS is susceptible to hygric swelling and frost sprinkling (freeze-thaw-stress), therefore representing a relatively weak material. On the contrary, OKS is a very pure quartzitic sandstone (87 Vol.%) , known as a durable material, containing varying amounts of kaolinit. Further data of the two substrates are

given in table 1.

Table 1. Petrophysical data of SRS and OKS

SRS OKS

Porosity ivol. o/o]_ 17.8 18.3

Pore radii maximum [µm] 3.7 2.2

S_e_ecific surface (m2{91 7.9 0.9

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Stone slabs of the two varieties of stone were smashed into small pieces prior to a grinding process in

a vibrating pot mill (2 min.) with a boron carbide inset. The stone powder was stored at 18°C and 65% relative humidity. Portions of Sg were treated with a threefold amount of the organosilicon protective

agents relevant in the field exposure study. After application and evaporation of the solvents a polycondensation reaction time of four weeks (18°C/65% r.h.) followed. The treated material was diluted with KBr as nonabsorbing standard (1: 10) and the mixture again grinded in a vibrating mill (2

min) prior to recording of the DRIFT spectra with a Perkin-Elmer 1760x FT-IR spectrometer using a Graseby Specac Minidiff DRIFT unit PIN 4500. After the incident light hits the sample surface the DRIFT unit enables highly efficient focusing of the light scattered by the surface using an ellipsoid

and two plane mirrors.

Hygric examinations of both exposed and lab-stored material were carried out by diving of the prismatic stones (SRS: 192h, OKS: 768h) keeping the water stand at 5 cm above the samples. Drying profiling of the material was performed subsequently over 300h. Additionally, optical detennination of penetration profiles of the applied protective agents were carried out by the wetting technique after closing the hygric investigations. Wedge-shaped samples were cut from the environmentally most affected area of the exposed material. Corresponding samples were taken from the reference material. The hydrophobic border zone is distinguished from the untreated inner material by wetting the cut side with water. Usually, the water repellent surface appears lighter than the hydrophilic. Penetration depths were measured with an accuracy of 0,5 mm.

4.1 Investigation of lsobutylsilane treated material

Surface OH groups (Si-OH, Al-OH, Ca-OH) of mineral components are found as centers of covalent bonding with organosilicon compounds in general. DRIFT spectra of both silanized 1 SRS and 2 OKS (Fig.1) exhibit peaks in the OH stretch region (3300-3750 cm-1) due to surface silanol groups (Si-OH) of the clay minerals. In case of kaolinit containing OKS, assignment of signals from the isolated (3704 cm-1) and bridged silanol groups (below 3650 cm-1) is known from earlier investigations [7]. The polycondensate formed from lsobutyltrimethoxysilane (IBTMO), (CH3)2CHSi(OCH3)3, is best characterized by strong bands in the CH stretch region of the DRIFT spectra (isobutyl group: 2955 cm-1 , methoxy group: 2870 cm-1). The substrates were treated with the following silane based agents:

A 100 % , B 40% with catalyst,

C 20% with 20% Tetraethoxysilane (TEOS) and catalyst, (solvent: Ethanol).

A general feature when comparing DRIFT spectra shown above is a much higher signal-to-noise ratio (SNR) in case of silanized OKS which is caused by a higher loaded surface. Thus, a significantly higher relation of signal intensity (CH stretch to OH stretch) for the benefit of the OKS surface is observed. Obviously, due to its high content of quartz as bonding mineral for silane, OKS is covered by a thicker polysiloxane layer compared to SRS. Independent on the material the highest relation of signal intensity is not observed for A pure IBTMO but for the diluted systems B and c . considering the DRIFT spectra of OKS, it is striking that in case of the diluted systems B and c signal intensity of the surface OH groups is reduced significantly compared to A suggesting more covalent interaction between silane and mineral surface.

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3500 2500 3500 2500

wavenumber [cm 'J wavenumber [cm 'J

lA 2 A

)!)

§ )!)

§ § :E

§ w ~

:E

.8 ~

" .8

:>I " :>I

~ - ~ 4000 3500 300l 2500 4000 3500 3000 2500

'lv.lvenumba- [cn1' ) W3Vcnumber [ cm·1)

1B 2B

3500 3000 2500 4000 3500 3000 2500

wavenumber [cm '] wavenumber [cm ')

lC 2C

Fig. 1: DRIFT spectra of the IBTMO based agents A,B and C applied to 1 SRS and 2 OKS

DRIFT results raise the question in how far surface information obtained from the spectra correspond to macroscopic properties of the agents applied to the two substrates. The lower covering of the material surface when using A pure IBTMO indicated by DRIFT is strengthened by earlier studies which reveal very unsatisfying water repellent properties of some types of sandstones treated with pure silane, among them Sander sandstone, a material very similar to SRS, as well as OKS [7] (8]. A further IBTMO based product was used in the field exposure program containing IBTMO 20% with hydrolyzed TEOS 20%. Therefore it was also taken into consideration for macroscopic tests. Fig. 2 gives an overview on the comparative investigation of the diluted agents applied to SRS showing the course in water uptake 1 (expressed as relative value to the maximum water absorption of the untreated material) and desorption 2 (expressed as the relative water content in the sample) versus time. Drying behaviour of the untreated material is also shown in the figure. Considering the hygric

behaviour, the investigated agents are labelled by small letters:

a 40% with catalyst, b 20% with 20% hydrolyzed TEOS, c 20% with 20% TEOS and catalyst.

O~-r-~-.-.--.-~r-r-.-~.--..--,-,------,-,---,--.

0 20 40 00 00 100 120 140 160 100 I [h)

la 100~~~~~~~~~~~~~---,

00

[ 60

i 40 j 20 ___________ .. _....-·-·--

--~- ... ···--0 i.--

lb

le

0 20 40 00 00 100 120 140 100 100 t [h)

o 20 ~ oo oo m m ~ s ® t(b)

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o -l-~-,--~~.--~-.~~.--~-.-~___,

0

2a

2b

100 150 t[h)

I ~~ed REF -~ .• ?.~ .. 1

250

? ~~~~~~~~~~~~~~---,

::.0-6 ~. •' 5 ., l ,, ~4 ·'\. ~3 ~--.. _ 8 ' --... ___ _ _ ~ 2 !'--- ... - - - ..:-::::::-------------~ 1 --------

2c

o +-~-,--~~..--~-.-~~.---~..-~--1

0 100 1ro I (h)

200

I ~ed~~- .~~.1 2&> 300

Fig. 2: Hygric properties (1 water uptake, 2 desorption) of SRS treated with the IBTMO based agents a, band c

In contrast to earlier reports about the effect of pure IBTMO water repellent properties of the lab-stored reference material treated with diluted IBTMO impregnation systems (REF, solid curve) are very satisfying after eight years. Maximum values in water uptake lie between 20-35%. The very high effectiveness in hydrophobization is further based on the drying profiling of treated SRS because drying of untreated SRS is generally delayed relative to other sandstones due to the high content of clay minerals. In all cases the water uptake of the treated material lies distinctly below that of the untreated prism. If, in addition, the hygric properties of the exposed material (DO: dashed, DU: dotted curve) is inspected significant deviations - relative to REF - in remaining effectivity of the three systems are observed: Though maximum water contents of the exposed material treated with system a are higher compared to REF drying behaviour also reveals a still remaining impregnating effect. With respect to the very similarly composed systems b and c, however, drastic differences in the impregnating efficiency of the two agents are registrated. Surprisingly, system b exhibits an excellent efficiency and durability whereas water repellent properties of the stones treated with c have decreased dramatically. In fact, optical determination of penetration depths of the polycondensates formed from the two IBTMO containing systems gives evidence of a more than twice as thicker layer

=7 ....._ ____ ....... .,,.... 1229

(2 mm) in case of b securing depth impregnation whereas c is degraded progressively due to weathering resulting in dramatical decrease of impregnating efficiency compared to REF.

Considering the hygric properties of OKS treated with the corresponding agents it is found that in

accordance with the results obtained from DRIFT measurements efficiency and long-term performance of the treatments is greater due to a higher loaded surface and stronger interaction of the polysiloxane with the quartzitic surface. As a logical consequence, optical penetration depths of the silane based agents lie between 4-5 mm which is more than twice as deeper relative to SRS. Fig. 4 illustrates as example the hygric behaviour of OKS prisms treated with impregnation system c.

80

...... ~.-.:..: o~

0 100 200 300

1

.. . .. .

400 t [h)

000 000 700

2

Fig. 3: Hygric properties (1 water uptake, 2 desorption) of OKS treated with the IBTMO based agent c

As indicated by the low water uptake of both reference and exposed material over the four-fold longer time of water storage compared to SRS efficiency and stability of the treatment is impressive. In case of the drying properties it is obvious that untreated OKS losses humidity distinctly faster than SRS resulting in similar remaining water contents after a shorter drying period (ca. 75-100h) of both treated and untreated stones. On the opposite, low water uptakes of treated SRS lead to enormous differences in drying behaviour comparing exposed SRS treated with b (low water uptake) and c (high water uptake) as well as untreated stones (long-term moisture contents, Fig. 2).

4.2 Investigation of methyl- and methyl/octylsilicone treated material

Silicones or polysiloxanes are higher condensed silyl esters which exhibit - compared to silanes and siloxanes - water repellent properties after a shorter polycondensation period on the stone surface. Two exemplary different types of silicones were selected for investigation:

A methylsilicone, B methyl/octylsilicone mixture,

(in organic solvents).

Fig. 4 shows the DRIFT spectra of A methylsilicone and B methyl/octylsilicone applied to 1 SRS and 2 OKS in the 1500-1100 cm-1 region in which Si-CH3 and further Si-C deformation modes due to the

polycondensate normally are observed.

llllll \"'

IA

_;! §

§ >: ~ ~ "

llOO \

~ -------- 1231 Considering a, maximum water uptake of both REF and exposed material does not exceed 25%. A completely different result is obtained in case of b which reveals a minimized hydrophobic effect for the treated material. On the opposite, system B forms a highly efficient polycondensation product when applied to OKS as indicated by the hygric properties (Fig. 6). Maximum water uptake of both REF and exposed samples lies below 32%.

100 200 300 400 500 600 700 t [b]

Fig. 6: Hygric behaviour (here only water uptake shown) of OKS treated with silicone b

In order to give an interpretation of the effects it is neccessary to examine penetration depths. Actually, an important result is obtained by the wetting technique: While in case of A a value of 1,5-2 mm (OKS: 4 mm) of an optically distinct hydrophobic layer is measured the corresponding layer formed from B shows similar depth but appears optically extremely weak (OKS: 4 mm, optically clear). Obviously, the ability of B to interact with the stone material is distinctly lower compared to A leading to progressive evaporation before polycondensation reaction.

5 CONCLUSIONS

The results reported here show that surface investigation by Diffuse Reflectance enables to elucidate properties of polysiloxane coating on different stone materials. As an important quintessence, semiquantitative states obtained from the spectra can be - approximately - correlated to macroscopic properties of silylated material. In how far the observed trends may be generally accepted is to be proved by systematic investigation of larger sample numbers strictly controlling the conditions under which DRIFT spectra are obtained. Generally, but in special with view on the different impregnation results of similar composed agents applied to SRS, it is obvious that the use of further spectroscopic techniques is neccessary in order to obtain more detailed molecular surface information.

REFERENCES

(1) Umwelt, Umweltschaden und Schutzmaf:\nahmen in der Denkmalpflege, 5, 1990, pp. 230 (2) E. M. Winkler, Stone: Properties, Durability in Man's environment, Springer, Wien, New York 1973 (3) K. M. Rodder, Die Grundlagen der Hydrophobierung mineralischer Baustoffe, Sonderdruck aus

Fassadenschutz and Bausanierung, expert-Verlag, 1983, pp 246 (4) J. Grobe, K. Stoppek-Langner, W . MOiier-Warmuth, S. Thomas, A Benninghoven, B. Hagenhoff,

Grundlagenforschung im Dienst des Bautenschutzes, Nachr. Chem. Tech. Lab. 41 , 12, pp. 1236 (5) S. R. Culler, M. T. McKenzie, L. J. Fina, H. Ishida, J. L. Koenig, Fourier Transform diffuse reflectance

Infrared study of polymer films and coatings: A method for studying polymer surfaces, Appl. Spectrosc. 38

(6), 1984, 791 (6) Chr. Bruchertseifer, S. Bruggerhoff, J. Grobe, K. Stoppek-Langner, Proceedings of the first International

Symposium of "Surface treatment of building materials with water repellent agents", Delft, the Netherlands,

1995, pp. 27.1-27.11 (7) K. Stoppek-Langner, thesis, Westfalische Wilhelms-Universitat Munster, 1991 (8) M. Wessels, diploma thesis, Westfalische Wilhelms-Universitat Munster, 1993 [9] c. J. Bellamy, Infrared Spectra of Complex Molecules, Chapman and Hull, London 1975