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THE EFFECT OF SANDSTONE, LIMESTONE, MARBLE AND SODIUM CHLORIDE ON THE POLYMERIZATION OF MTMOS SOLUTIONS

GOINS, ELIZABETH S., WHEELER, GEORGE SEGAN

The Metropolitan Museum of Art, 1000 Fifth Avenue, New York, New York 10028

GRIFFITHS, D., PRICE, CA

Institute of Archaeology, 31-34 Gordon Square, London WC1 H OPY

SUMMARY The effect of different powdered substrates on the hydrolysis, condensation and gelation of water -

MTMOS - ethanol solutions are evaluated by Fourier Transform Infrared Spectroscopy. The hydrolysis rates of MTMOS solutions containing sandstone, weathered sandstone, marble, limestone, calcium carbonate or sodium chloride are compared to a control sample by the use of Principal

Component Analysis (PCA). The condensation reaction of the solutions is evaluated by both FTIR and the time to gelation (T9e1) .

Limestone and marble powders were found to significantly slow the hydrolysis reaction but increase the gelation rate. These trends are consistent with a slight increase in pH of alkoxysilane solutions of pH 5-7. The gels formed from the solutions were of two basic types: 1.) those of the control which were sticky and viscous or 2.) those similar to the base catalyzed solutions (rubbery and weak).

1. INTRODUCTION

The success of alkoxysilane consolidants on carbonate rocks is a topic of some debate. Proprietary alkoxysilane based consolidants, such as the Wacker-Chemie products Wacker H and OH (known as Conservare H and OH in the U.S.A.}, were developed for use on sandstone (as well as bricks and cement). These products were eventually tested and used on marble and limestone (Moncrieff 1977, Moncrieff and Hempel 1977) with reportedly good results (Larson 1984, Bradley 1985). Later work has shown that the carbonate samples consolidated with alkoxysilane solutions do not have as great an increase in mechanical strength as the alkoxysilane consolidated sandstone samples Wheeler et al. 1992, Goins et al. 1995). The discrepencies between the mechanical strength results have generally been attributed to a lack of bonding between the alkoxysilane derived consolidant and the carbonate surface. However, calcite has been shown to influence the condensation reaction of MTMOS (Danehey et al. 1992). A.P.

Laurie, in 1926, noted that, "Limestone and calcareous sandstones are generally sufficiently alkaline in character to render the silicic ester alkaline and so to make the precipitate soft and useless ... " A review of the sol-gel literature (Brinker and Scherer 1990) explains that the major variables in the alkoxysilane polymerization process (the hydrolysis and condensation sequence) are:

1. the type of alkoxysilane and its functionality. 2. the catalyst type (OH- or H+) and concentration (i.e. the pH).

3. the water I alkoxide ratio (r). 4. the reaction temperature. s. whether the system is open or closed (allowing for evaporation and the addition of atmospheric

moisture into the reaction.

These variables will control the polymerization through the growth and aggregation of the sol/silicate

species throughout the transition from the sol to the gel state.

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Figure 1: Sol cluster types formed under acid or base conditions. Under acidic conditions, polymeric type clusters form. Basic conditions, on the other band, form particlute clusters. Intermediate conditions result in clusters that are somewhere in between the two extreme structures depicted here.

Acid catalyzed Hydrolysis is rapid producing many silanol groups

-Si -OH -Si-OH

-Si-OH -Si-OH

These groups then condense slowly by a cluster-cluster mechanism

--Si -0-Si -OH /-Si-OH

-Si-OH

The resulting gel structure is polymeric, made up out of weakly branched sol clusters

Base catalyzed A slow, step wise hydrolysis

R --Si -OH --Si -OH

--Si -OR --Si -OR

The condensation is fast and begins during hydrolysis by a monomer-cluster growth

mechanism

R --Si -0 --Si -OH

~ --Si -OR -Si-OH

The sol clusters formed are highly branched "particles" which gel by aggregation

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The type and concentration of the catalyst will modify the rate of the hydrolysis and condensation reactions by controlling the reaction mechanism (figure 1). Acid conditions produce rapid hydrolysis that produces a sudden increase of silanol groups (Si-OH). The silanol groups slowly polymerize by cluster-cluster growth mechanism into slightly branched clusters which then entangle to form cross linked gels. This type of gel, or xerogel, is called polymeric.

Solutions with an alkaline pH follow a different mechanism. The base catalyzed hydrolysis is much slower than the acid catalyzed system thus the resulting condensation reaction is much faster than the hydrolysis reaction ( figure 1). The silanol monomers polymerize or condense quickly by a monomer cluster growth mechanism and produce highly branched structures. The highly branched clusters eventually aggregate to form a network and are called colloidal or particulate gels.

The presence of weak acids and bases in neutral or near neutral solutions (pH = 5-7) will result in a structure som·ewhere in between the extremes of the acid or base catalyzed solutions. Boonstra and Baken (1990) tested a number of acid catalysts and found that while strong acids such as HCI increased the hydrolysis rate weak acids (e.g. oxalic, malonic, phosphoric) had larger hydrolysis times (i.e. a slower hydrolysis rate). These weak acids do not exhibit acid catalyst behavior but instead act in a manner more like a mild, base catalyst (towards neutral pH).

In order to study the interaction between an alkoxysilane (MTMOS) and carbonate rocks, FTIR was employed to compare the relative rates of hydrolysis between several different ~ystems. Data

I regarding the condensation reaction is determined by measuring the length of time to gelation (lge1) where liie1 = 1/condensation rate. The rates of the sol to gel transition, as measured by the time to gelation, are different from one system to another but generally follow the sequence:

t9e1 (acidic) > !iie1 (neutral) > t9e1 (basic)

2. EXPERIMENTAL

All stone samples were ground mechanically and sieved through 500µm mesh and then allowed to equilibrate in a 55 ± 5% R.H. chamber for 4 - 5 months. 2.00g ± 0.01 of each powdered sample type were weighed out (table 1). The stone was then poured into polypropylene bottles.

Table 1: pH of 212 MTMOS Solutions (molar ratio of water, MTMOS and ethanol)

System pH Concentration of added solids

Marble 5-6 2g/60ml

1Marble 4-5 2g/60ml Limestone 5-6 2g/60ml

1Limestone 4-5 2g/60ml

CaC03 7-8 2g/60ml

Control 4-5 ------Weathered 4-5 2g/60ml Sandstone

NaCl 4-5 2.8 x 10·2 M (100mg/60ml)

Sandstone 4-5 2g/60ml

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2:1 :2 and 4:1 :3 molar ratios of water to MTMOS to ethanol were made up as homogeneous solutions and added to the powdered samples listed in table 1 along with the initial pH of the 212 - sample mixtures. The solutions were left to react in continuous contact with the powders as closed systems for 1056 hours (approximately 44 days), after which, the containers were opened to allow for evaporation of alcohol and water. The 1 Marb and 1 Limestone systems were prepared somewhat differently, the limestone and marble powders were filtered off after one hour's contact with the

solution.

The alkoxysilane solutions were syringed at timed intervals from their containers onto an attenuated total reflectance (ATR) trough with a ZnSe (45°) crystal. The ATR accessory (Perkin- Elmer) mounts inside the FT-IR sample compartment which is constantly purged with water free nitrogen gas. The

FT-IR used throughout is a Perkin-Elmer System 2000. The spectra were collected as a ratio of 32 sample scans to 32 background scans at a resolution of 4 cm-1. The principal compents of the infrared spectral series of each system were determined by the Perkin-Elmer Quant + program.

3. RESULTS

The infrared spectrum of the initial solutions is clearly dominated by the characteristic MTMOS and ethanol fingerprints in the 1300-650 cm-1 region (figure 2) . Peaks involving the Si-O-CH3 (1192 cm-

1,

839 cm-1, 793 cm-1 and 737 cm-1) and ethanol groups (1049 cm-1 , 880 cm-1) may be unambiguously identified. The C-0 stretches of MTMOS and ethanol in the 1100-1000 cm-1 region remain obscured and later complicated by the formation of methanol (C-0 stretch 1030 cm-1) and siloxane bonds. Figure 2 shows the control solution before (the initial spectrum) and after hydrolysis. The main changes are the disapearance of the bands associated with the MTMOS monomer: Si-O-CH3 rocking (1193 cm-\ Si-0-C symetrical stretch (839 cm-\ the Si-0-C anti-symmetrical stretch (793 cm-1) and the peak at 737 cm-1 which has been tentatively assigned to an Si-0 stretching vibration. As these peaks disappear, new bands are formed. In particular, the peak at 923 cm-1 which corresponds to the production of silanol groups (Si-OH stretch) . A low frequency shoulder on the C-0 stretch of ethanol (1048 cm-1) corresponds to the formation of methanol which has a characteristic C-0 stretch at about 1030 cm-1.

Table 2: Infrared Frequencies for MTMOS

Region (cm-1) Assignment Comments

737 -Si - 0 asymetrical stretch MT MOS 760 - 770 -Si - 0 - Si symmetrical Siloxane

stretch

790 -Si - 0 - C asymetrical MTMOS stretch

840 -Si - 0 - C symmetrical MTMOS stretch

880 CH3 or CH2 deformation Ethanol 920 -Si - OH stretch Silano!

1020-1030 -C-0 symmetrical stretch Methanol 1027 -Si - 0 - Si stretch Siloxane

1000 -1100 - C - 0 symmetrical Ethanol stretch

- C 0 asymmetric stretch MTMOS -Si - 0 -Si

antisymmetrical stretch Siloxane 1190 -Si - 0 - CH3 rocking MTMOS 1270 -Si - CH3 deformation MT MOS

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All of the systems exhibited this sequence of change, the disappearance of the MTMOS monomer along with the simultaneous appearance and subsequent growth of the silanol region (920 cm·\ The principal component anlaysis (PCA) of the spectra carried out for each system can be used to determine the main variations within the polymerization reaction. The first principal components factor (PC1) of all the 212 systems studied were the same (figure 2). The systems are clearly dominated by the formation of the Si-OH and increase of the C-OH peak of methanol (923 cm-1 and 1025-1030 cm·1, respectively) at the beginning of the reaction . All of the 212 MTMOS systems have the same mean spectra and identical factor spectra for first principal component thus the PC1 score profiles may be directly compared (figure 4,5 and 6). Each standard number of the scores profile corresponds to one spectrum of the timed sequence. The spectra are in order, so that the first spectra is that of 1 minute into the reaction, standard number 2 is the spectra from 10 minutes and so on. The plots of PC1 (figure 4,5 and 6) for the control, sandstone and limestone spectral series show the kinetic differences in the hydrolysis reaction between the systems. The onset of hydrolysis is slower in the sandstone - MTMOS solution than that of the control. However, the sandstone solution is similar to the control in that the hydrolysis completes quickly once begun. The beginning and ending hydrolysis times, marked by the appearance (beginning) and the time to maximum intensity (ending) of the silanol peak at 920 cm·1, are shown in table 3.

Figure 2: 212 MTMOS - water -ethanol hydrolysis. The initial solution is at the front of the sequence. As the reaction proceeds, the Si-O-CH3 (1190, 839 793 740 cm-

1) peaks of MTMOS

decrease and silanol (923 cm-1) and methanol (1030 cm-1) are formed. Si-0-Si formation also begins during hydrolysis as indicated by the peak at 776 cm-1 and the large increase in the 1030-1020 cm·

1

· region.

1.

0

1200

Figure 3: PClfactor

149. i1

93-12

PC 1

:!5.51

-32.05

l!t .65 4eao.11

1100

793

776

1000 900 800 700 Wavenumber (cm-1)

N.48.81 1968.18 12111.n 698.80

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Figure 4: Plot of PCl scores for 212 molar ratio of water, MTMOS, ethanol control solution.

B.42 56

D . I!~ ..., W, t

A.11 _

IUMI _.

1234

-8.BB - - - - i'&;~~iti"Jl.ifitJAiiliaJliainf9ililffi i-j -~-

T

1e 19 37

Figure 5: Plot of PCl scores for 212 molar ratio of water, MTMOS, ethanol solution in contact with sandstone.

O.i!4

PC I

0.1?

0.1!1

-11. 10

i.t 212Snl Sc

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Table 3: Hydrolysis times for the 212 and 413 MTMOS systems (molar ratios of water, MTMOS and ethanol). The three columns on the left represent the results for the 212 system, the three columns on the right list the results for the 413 solutions. The alkoxysilane solutions were mixed with different powdered materials that are listed in the column at the far left. The time taken to reach the start of the hydrolysis reaction is listed in the column headed with the word "begin." The end of the hydrolysis reaction is listed in the column headed with the word "end." The 413 solutions contain a higher water content than the 212 solution. The increased amount of water has served to speed up the onset of the hydrolysis reaction in the 413 solutions.

212 413 BEGIN END BEGIN END (hours)

LIMESTONE 7.7 112 2 160

1LIMESTONE 6 35 3 45

MARBLE 7 64 5 136 1MARBLE 3 12 1.75 3.5

NaCl2 8 19 2 3 SANDSTONE 5.5 30 2.5 12

WEATHERED 0.75 10.5 1.5 2 SANDSTONE

NaOH2 4 10 0.75 2

CaC02 4 29 5 11

CONTROL 6 12 1.5 3.5

There is some variation within the beginning of the hydrolysis but the main difference is in the duration of the hydrolysis. The fastest 212 hydrolysis reactions are those of the control, sodium hydroxide, weathered sandstone and 1 marble sequences, closely followed by the sodium chloride sample. The hydrolysis rates of the calcium carbonate, 1 limestone and sandstone samples are slowed considerably. The lowest hydrolysis rates are those of the marble and limestone solutions - the limestone hydrolysis rate is the slowest of all being almost double that of the marble.

The 413 systems show the effect of increasing the water to silicon ratio (r). The overall hydrolysis times are decreased in all 413 systems except the limestone, 1 limestone and the marble solutions which have a dramatic increase in the hydrolysis times. The 1 hour contact marble (1 Marb) solution is hardly any different from the control but the marble solution is strongly affected by the increase in water. Also, the 1 hour contact limestone, more soluble than the marble, shows a definite increase in

the completion of the hydrolysis from 35 to 45 hours.

Condensation

As the polymerization proceeds, the evaporation of the solvents becomes the driving force of the reaction (Matos et al. 1992). FTIR spectra of the four major systems, 212 MTMOS control, limestone, sandstone and marble, taken the day before gelation are shown in figure 7. There is very little ethanol left at this stage, as evidenced by the small peak at 880 cm·

1. Also, the silanol

peak at 920 cm·1 is present in all systems. The largest differences among these spectra are in the siloxane bond regions, 1025 cm·1 and 770 cm·1. Clearly, there are more siloxane bonds present in the sandstone and control sols. This difference is even greater in the 413 (figure 8). Figure 9 shows the marble, control, calcium carbonate and sodium hydroxide spectra. The siloxane regions are much greater in the control spectrum. The marble (and limestone) spectra resemble more closely the spectra of MTMOS sols of a higher pH - calcium carbonate and sodium hydroxide.

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Figure 7: 212 MTMOS spectra the day before gelatioo

B c ~ 0 Ill .c

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Table 4 shows the time to gelation for all systems studied (except for sodium hydroxide which gels

within an hour). The beginning of Tgei is the time taken to the initial increase in viscosity. The end of Tge1 is the time taken until a solid soft gel was formed.The marble and limestone in both the 212 and 413 systems, have the shortest gel times. The solutions with high salt I electrolyte content, that is, the weathered sandstone and sodium chloride samples, have the longest times to gelation in both systems.

The main points of interest in this study are that the limestone and marble samples have consistent, short times to gelation and that the 413 system has longer overall gel times. However, the 1 marble and 1 limestone behave differently as their time to gelation significantly decreases in the 413 system.

Table 4: Time to gelation

212 T(gel) days 413 T(gel) days

begin end begin end

WEATHERED 54 62 WEATHERED 63 70 SANDSTONE SANDSTONE SANDSTONE 53 61 SANDSTONE 63 69 LIMESTONE 54 58 LIMESTONE 57 58

MARBLE 53 58 MARBLE 57.5 58 CONTROL 53 60 CONTROL 58 63

CaC03 58 61 CaC03 59 62 1LIMESTONE 55 61 1LIMESTONE 57 59

1MARBLE 60 63 1MARBLE 57 60 NaCl 58 62 NaCl 59 69

The final piece of information to be gathered is the description of the xerogels themselves. Table 5 lists the descriptions for all gels studied. Essentially, two kinds of gels were formed. The limestones

and marbles formed gels that were lumpy and did not adhere to a needle when probed - they were solid but lacked cohesive and adhesive strength. These gels were similar to the calcium carbonate and sodium hydroxide gels. The gels formed by the control, sandstone and weathered sandstone

were more like viscous solutions that, when probed, adhered tenaciously to the probe, forming long strings when pulled. The 1 hour contact marble and limestone as well as the sodium chloride, formed gels that were somewhere in between the two gel types but more like that of the control.

4. DISCUSSION AND INTERPRETATION

The PCA results clearly point to a major difference between the reaction series involving an increase

in the hydrolysis times for the carbonate samples. The measured hydrolysis times for the marble, limestone and the 1 limestone are the most affected. The kinetics and mechanisms of the hydrolysis and condensation reactions are known to determine the structure of the final gel (Boonstra and Baken 1990). Therefore, the gels formed in contact with the carbonate stones must be somewhat different

from those in contact with sandstone.

Both the 212 and the 413 reactions follow the same relative hydrolysis trends. The highest hydrolysis

rates are found in the control , weathered sandstone, 1 marble and sodium hydroxide systems. Sodium chloride, calcium carbonate and sandstone make up a middle category of moderately decreased

rates. The marble, limestone and 1 limestone systems have the lowest hydrolysis rates. Within these general trends there are some exceptions. The sodium chloride system, for example has a great increase in the hydrolysis rate in the 413 solution. Possibly the dilution minimizes the effect that the sodium chloride ions have of stabilizing and isolating (by electrostatic repulsion) the sol

species.

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Increasing the water content in the 413 system decreased the hydrolysis times as expected except for

the limestone, 1 limestone and marble samples. The dramatic increase in their (limestone, 1 limestone and marble) hydrolysis times suggests that the solubility of the surface, and a slight rise in the pH, are

responsible for the effect. The condensation was shown by the FTIR spectra to begin during the hydrolysis reaction. However, how the condensation mechanism is influenced at the closest level, that of the nearest neighbor and

subsequent group size, can not be determined. The FTIR spectra show differences in the siloxane

regions (1025 cm-1 and 770 cm-1) as well as silanol variation (920 cm-1) but this information will vary

greatly depending on: the stage of the reaction (i.e. overall time) to the gel time; the type of system

(i.e. catalyst, salt content etc.) will affect the structure and the length of time from the increase in viscosity to gelation and solubility - some solvents or pH ranges (over 7) will promote

depolymerization processes. In short, the relationship between the sol-cluster structures and their IR

absorptions is unknown. The time to gelation in the 212 systems also indicates that the systems have different sol structures.

The limestone and marble samples are the first to gel. This is consistent with base or weak acid catalyzed hydrolysis at near neutral pH - the pH point of minimum time to gelation. The calcium carbonate has a greater time to gelation due to its pH of slightly greater than 7. The sodium chloride

and weathered sandstone with their slow time to gelation are very consistent - we expect them to stabilize the sol clusters and slow down gelation (Brinker and Scherer 1990, lier 1979). The gels and xerogels derived from the different systems show conclusively that two different sol

structures developed. The limestone and marble systems produced gels that were weak that, when pierced, gave lumpy surfaces with very little adhesive ability or cohesive strength. The control and sandstone systems gave gels that slowly increased in viscosity until the xerogel was formed . This gel

type was sticky and elastic.

Table 5: Xerogel descriptions. Listed below are descriptions of the soft gels and dried gels (xerogels) formed in contact with both the 212 and 413 MTMOS solutions. Two basic types of gels were formed.

Soft, weak rubbery gels were formed in contact with limestone and marble powders. Solutions in contact with the powdered sandstone were like that of the control - a sticky, viscous gel.

Powdered solid Soft Gel Gel at 90 Days Xerogel (all samples brittle)

Control 212 Very sticky, formed long Very hard and brittle Slightly hazy, non-tendrils when probe was strained gel removed

413 Same as control 212 No data same as control Weathered 212 Same as control still soft gel same as control Sandstone

413 Same as control No data same as control Sandstone 212 Same as control still soft _g_el same as control

413 Same as control No data same as control NaOH 212 Clumpy I particulate gel. No data Very hazy I opaque.

Large amount of pore Extremely solution expelled. White I strained/cracked, opaque. Non - coherent separated into large

CIUn:!QS 413 Same as 212 NaOH No data Mostly powder. Some

xerogel present as small o_e_~ue chunks.

CaC03 212 Rubbery, once pierced it No data Slightly hazy. crumbles easily Strained with cracks

running over the surface

413 same as 21 2CaC0 3 same as 212 CaC03 NaCl 212 Rubbery not as weak as No data Opaque, strained

carbonate _g_els 413 Sticky. Similar to control No data slightly hazy,

but not as stri~ non -

strained

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Marble 212 Very weak gels - almost no Cloudy. Hard rubber Slightly hazy, strained cohesive strength. Broken - not brittle or tough. surfaces "clumpy". Whole chunks could

be removed with ease

413 same as 212 Marble No data Cloudy some straining. Orange peel surface texture. Delaminating from marble l(!Yer

Marble 1 Hour 212 Rubbery. Slightly clumpy No data Very slightly hazy, Contact but tougher than 212 non - strained

Marble 413 similar to control gel No data similar to 212 Marble

1 Hour Limestone 212 same as 212 Marble same as 212 Marble Slightly hazy, slightly

strained. 413 same as 212 Marble No data Cloudy, strained,

orange peel surface texture

Limestone 1 Hour 212 similar to control gel No data Very slightly hazy, Contact very slightly strained

413 same as 212 Marble No data 0...E_~ue, strained.

5. CONCLUSIONS

The kinetic study of two MTMOS systems, a 2:1 :2 and a 4:1 :3.5 molar ratio of water to MTMOS to ethanol, via Fourier Transform Infrared Spectroscopy (FTIR) and Principal Component Analysis (PCA), show clear differences between the hydrolysis rates in the carbonate rocks (marble and limestone) and the siliceous rocks (sandstone and weathered sandstone). The limestone and marble samples slowed the hydrolysis rates of the MTMOS solutions. The FTIR analysis of the sols before gelation show differences in the amount of siloxane bond formation thus indicating differences between the sol clusters of the different solutions (carbonate versus siliceous versus the control). The sols formed in contact with the sandstone samples were

similar to those formed in the control system. The condensation reaction was indirectly studied by determining the time to gelation (T9e1). The alkoxysilane systems in contact with the carbonates rocks gelled faster than the other samples. The xerogels formed were also substantially weaker than and different in character to those formed in contact with the sandstones or the control solution. The gels formed in contact with the carbonate

rocks were consistent with those formed by a slight increase in the pH of the solution as reported in the literature (Brinker and Scherer 1990) or as noted by Laurie (1926) for TEOS (ethyl silicate). Clearly, the limestone and marble substrates affect the hydrolysis and subsequent condensation of the MTMOS systems studied and the structures of the xerogels that are ultimately formed. The carbonate rocks raise the pH to around 6, as opposed to the control at pH-5. The degree of branching of the sol clusters is increased leading to a more particulate gel. However, the pH is not raised high enough by the limestones for the hydrolysis and condensation reactions to be that of a base catalyzed system. The results indicate a structure intermediate between that of the control and a base catalyzed

(NaOH) sol/gel.

ACKNOWLEDGEMENT I would like to thank all those involved in this project at the Institute of Archaeology, UCL, London. This paper is based on work completed there as part of the requirements towards the Doctorate of

Philosophy, University College London, completed September 1995.

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