stevenson alkoxysilane stone consolidants

7/26/2019 Stevenson Alkoxysilane Stone Consolidants

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ALKOXYSILANE STONE CONSOLIDANTS: THE

EFFECT OF THE STONE SUBSTRATE ON THE

POLYMERIZATION PROCESS

Elizabeth Stevenson Goins

Thesis submitted for the degree of

Doctor of Philosophy

in the Faculty of Science of

niversity College London

March 1995

Department of Materials Science

Institute of Archaeology

University College London

LOI IL)


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bstr ct

Alkoxysilane sol - gel chemistry and the use of a lkoxysilane stone co nsolidants are

reviewed. The consolidated sandstone and limestone samples are subjected to a three -

point bend test to determine the modulus of rupture MOR ). Mechanical testing

procedures and crack formation theories are reviewed. Scanning Electron Microscopy

SEM ) is used to study the fracture surface s and cross sections of gels deposited within

the stone pores. Tetraethoxysilane TEOS), methyltrimethoxysilane MTM OS),

coupling agents amino and glycidoxy functional alkoxysilanes), epoxy and acrylic

resins are tested. The MO R and SEM results indicate differences between the gels

formed in contact with each of the rock types. Fourier Transform Infrared

Spectrosco py FTIR) is used to mon itor the hydrolysis and conden sation reactions of

2:1:2 and 4:1:3.5 molar ratios of water, MTMOS and ethanol solutions in contact with

powdere d marble, limestone, sandstone and weathered sandstone containing soluble

salts). Principal Component Analysis PCA ) is used to identify any major chemical

trends in the FTIIt spectra. The limestone and sandstone are found to slow the

hydrolysis reaction considerably. The time to gelation T

) is determined in order to

comp are condensa tion and gelation rates among the different systems. The resulting

xerogels are emp irically described for each solution. The limestone a nd marble sam ples

decrease the time to gelation and form we ak particulate - type gels.


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3

able of contents

ALKOXYSLLANE STONE CONSOLIDANTS: THE EFFECT OF THE STONE

SUBSTRATE UPON [HE POLYMERIZATION

1 A B S T R A C T

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

ACKNOWLEDGEMENTS

1

PREFACE

2

1 T H E C H E M I S T R Y O F A L K O X Y S I LA N E S

3

1 .1 OV ERJ 7EW

3

1 .2 E F FE C T O F S Q L U J7 O N T Y P E

5

1 2 1 Aqueous solutions

5

1 2 2 Nonaqueous Solutions

9

1 .3 EFF EC T O F O RG A N O F T JN C T I O N A L G R O U P S


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1.3.1 Neutral O rganofunctional Groups

0

1.3.2 Cationic and Anionic Organofunctional Silanes

1

1 4

T H E H Y D RO L Y S IS A N D C O N D EN S AT IO N REAC T IO N S : S Q L FO RI I

Pt A TION

4

1 4 1 Hydrolysis

4

1 4 2 Hydrolysis Mechanisms

8

1 4 3 Condensation

0

1 4 4 Condensation Mechanisms

2

1 5S U A P I M A R Y

4

1 6

R E F E R E NCE S

2 THE S L

To GEL TRANSITION: POLYM ERIZATION AGING AND FILM

F o R M A T I O N

7

2 1 OVERVIEW

7

2 2 AGING PROCESSES

8

2 3 DRYING

0

2 4 STRUCT URAL EVOLUT ION

1

2 5 FILM FORMATION

2

2 5 1

Film D eposition

3

2 5 2

Effect of Solvents Upon Alkoxysilane Film Formation

5

2 5 3

Effect of Stone S ubstrates U pon A lkoxysilane Film F ormation

9

2 6 SILANO L BoND FORMATION

1

2 7 SUMMARY

5

2 8 REFERENCES

7

3 A R E V I E W O F A L K O X Y S I L A N E P O L Y M E R S A S C O N S O L I D A N T S F O R S T O N E 5 9

3 1 O V E R V I E W

3 2

AucoxY sII vvE CONSOUDANTS

1

3.2.1 Overview

1

3.2.2 T etraethoxysilane

2


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6

5 9

S u M M A R Y

46

5.10 REFEREN CES

48

6 EFFECT OF STONE

SUBSTRATES UPON M TMOS PO LYMERIZATION

52

6 1

OV ER V IEW

5

6.1.1 Fourier Transform Infrared Spectroscopy

56

6.1.2 Principal Component Analysis PCA)

59

6.2 E LT PERJMENTA L

6

6.2.1 P reparation of Alkoxysilane Solutions and Stone Sam ples

6

6.2.2 Equipm ent

64

6 .3 R E S U L T S

64

6.3.1 Ban d A ssignments

64

6.3.2 Chemom etric Analysis of Spectral Data by PCA

67

6.3.3FT IR

7

6.3.4 Gelation

84

6.3.5 Xero gel Description

86

6 4 DiscussioN

88

6 .5 CON CL US ION

93

6.6 REFEREN CES

94

7 CoNcLusioNs AND

FUTURE W ORK

96


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ist of figures

Figure 1 1: Overview of the So/ Gel Process__________________________ 13

Figure 1-2. Ov erall R eactions of A lkox y si lane Polym erization_____________ 15

Figure 1-3. Polymerization behavior of silica after 1/er 1979) 17

Figure 1-4: Predominant Course of Condensation

of

S ilane T rio/s in A nhy drous

Solvents

2

Figure1-5: A m inopropy lsilane trio _______ ________ _______ __

3

Figure 1-6: inductive effects of some constituents attached to silicon 28

Figure 1-7: Possible mechanism for acid catalyzed hydrolysis _________________ 29

Figure 1-8: Possible mechanism for base catalyzed hydrolysis_________________ 30

Figure 2-1: A ging

polymeric gels after Brinker and Scherer 1991) 39

Figure 2-2: A ging

particulate gels (after B rinker and S cherer 1991) ____ _____ 4 0

Figure 2-3 . Film deposition (dip coating process)________________________ 4 4

Figure 2-4: Possible liquid sol lramsport m echanism _____________________ ___ 50

Figure 3-1 . A lkox ysilane and silicone resin distribution in L aspra and H ontoria S tone66

Figure 5-1: Diagram of the Three Point Bend Rig

18

Figure 5-2: Increase in M OR of treated lim estone

21

Figure 5-3: increase in M OR

of

treated sandstone

21

Figure 5-4: Increase in M OR

of

treated porous lim estone

22

bigure 5-5: L ffect of changing RI . upon 212 M IM OS -

27

Figure 5-6: Eff ect of changingR.H . upon 212 M T M OS -

28

hgure 5-7: L ff ect of changing R H. upon 212 M IM OS

28

Figure 5-8: Ef fect of changing R H. upon 413 M IM OS -

29

hgure 5-9. L ffect of changing RH . upon 413 M IM OS -

29

Figure 5-10: Effect of changing R H. upon 413 M T M OS

30

hgure 5-11: L ff ect of changing RH . upon W acker H

30

Figure 5-12: Eff ect of changing RH . upon W acker H

30


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Figure 5-13. Effec t of

changingR.H. upon

Wacker ___________________

131

Figure 5 14: Lffect of changing ItH.

upon

Wacker OH _________________ 131

Figure 5-15: Effect o f changingR .H. upon

W acker OH ______________ 131

1igure 5-16: Lf fect of changing RJI. upon W acker OH __________________ 132

Figure 5-17: Fracture surface of sandstone - 413 M 7M OS _________________ 133

Figure 5-18: Cross - section of sandstone and 413 M JM OS _________________ 135

Figure 5-19: Cross - section of sandstone and neat M T M OS _______________ 136

Figure 5-20 : Fracture surface

of

sandstone and W acker H__________________ 137

Figure 5-2 1: Fracture surface of sandstone and W acker Hpre-treated w ith ethanoll37

Figure 5-22: Fracture surface of sandstone and 212 M iM OS pre - treated w ith

ethanol

38

1-igure 5-23: Cross - section of sandstone and 3- A PS _____________________ 139

Figure 5-24: Cross -section of lim estone and W acker H identified by ED S dot

mppngorS 140

Figure 5-25: Possible gel structures w ithin lim estone and sandstone structures_ 145

1igure6 1: PCA matrix definition _________________________________ 160

Figure 6-2: MT M OS - w ater - acetone solution__________________________ 165

Figure6 3: PCi 212 control factor __________________________________ 167

Figure 6-4: Me an Plot of 212 M IM OS control __________________________ 169

Figure6-5: PCI 212 control score profile______________________________ 169

Figure6-6: 212 MT M OS control sequence _____________________________ 170

Figure 6-7: 212 M I M O S

sandstone score profile _________________________ 171

Figure 6-8: 212 MT M OS limestone score profile__________________________ 171

Figure 6-9: 212 MIM OS control sequence _____________________________ 173

Figure 6 10: Hydrolysis times_________

75

Figure 6-11: 212 MIM OS control sequence -

76

Figure 6-12:212 M TM OS sandstone sequence

77


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Figure 6 13: 212 MTMOS Limestone sequence ___________________

78

Figure 6-14: 8:1 Water to MIMOS in acetone_________________ -

79

Figure 6-15: 212 MTMOS control sequence ____________________

8

Figure6-16: MIMOS xerogel _______________________________

8

Figure 6-17: 212 MTMOS systems before gelation ________________

8

Figure 6-18: 413 MJMOS systems before gelation ________________

8

Figure 6-19. 413 MTMOS systems before gekition _______________

83

Figure6-20. 413 MIMOS gel times____________________________

85

Figure 6-21: 212 M1MOS ge/atE on times______________________

85

Figure 6-22. 212 versus 413 ge/ation times_______________________

85

Figure 6-23: Time to gelation of one hour contact marble and limestone

85

Figure 6-24: 212 MIMOS systems at

0.8

9


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1

List of tables

Table4-1. Description

of

T hin S ections_________________________________ 86

Table4-2. Porosity of Stone Samples___________________________________ 88

Table4-3: Soluble Content of

Stone S amples_____________________________ 88

Table 4-4. M ethyl Red A dsorption and Color Change on Powd ered Sam ples_____ 92

Table 4-5: M ethyl R ed A dsorption under Optical M icroscopy ________________ 93

Table 4-6: S om e Phy sical Properties

of

W ater M IM OS and Ethanol __________ 98

T able 4-7: Conso lidants01

Thble 5-1:

fferentMiMOS

Treatments

16

Table 5-2: Limestone MOR results

24

Table 5-3: Sandstone MOR results

25

Table 5 4: Porous Limestone MOR results 126

Table6-1: pH of 212 M IM OS Solutions ______________________________ 163

T able 6-2: Infrared Frequencies for M 7M OS ____________________________ 166

Table 6-3: Gelation lim es of 212 and 413 M IM OS system s _________________ 184

Table

6 :

M 1M OS x erogel description________________________________ 186


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knowledgments

Over the course of my studies so many peop le helped me that I hardly know where to

begin. First, I would like to thank m y supervisor Dr. Da fydd G riffiths for his support

and help throu ghout this thesis. I would also like to thank D r. Clifford Price, Dr. Nancy

Ross, Professor Dave W illiams, Dr. Sue Upton and everyone else at Perkin Elmer UK,

Mr. Salim, Andrew Osborne, D r. Olivier Pages for their support, advice and

encourag ement. I would especially like to thank Dr. Geo rge W heeler at the

Metropo litan Museum o f Art for editing this thesis and helping me to ge t through it. I

would also like to thank Sydney W illiston for helping me to get started and giving me a

break when I needed one.

Thanks to all my friends who helped me h ave such a great time in En gland. Caroline

Veling, Andy Fairbairn, Pete Guest (and Daw n), John Dennison, Natasha Meader,

Gyles lannone, Dave and Lina, Tim Greg ory, Tim Green, Big Steve Strongman , Little

Steve Chaddock and I can t forget Sim, Pat and Nutty

To my family, Mom, Dad, Sarah, Clint and Tuppence - thanks for your love and

patience. Thanks to all my American friends for putting up with me during the writing

up phase I really do have a persona lity (I swear ) - Michael Roberts, Bobby a nd Robs,

Fred and Gail Reuss, Joel and Nancy Leidy, Marcia and Richard Shihadeh, and Matt

Reuss. Finally, I would like to thank Lee Sw ift for his love and support.


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The purpo se of this work was to learn more abo ut the interaction of alkoxysilane

consolidants and rock substrates, particularly those with calcareous co mpo nents. The

conservation literature repo rted inconsistencies in alkoxysilane consolidant performance

on calcareous substrates such as limestone and m arble. In order to clarify much o f the

confusion regarding the alkoxysilane consolidants, a review of alkoxysilane chemistry is

undertaken in the first chapter. The second chapter reviews film and gel form ation from

the sol - gel literature but with an em phasis on concep ts useful to conservation

applications. The third chapter reviews the conservation literature concerning

alkoxysialne stone consolidants.

The m echanical testing of a num ber of different consolidants and coupling agents was

carried out to look for any major trends. This was intended to be a quick m ethod of

narrowing dow n the field for further research. The Fourier Transform Infrared

Spectroscopy FTIR) was intended to be the main thrust of this research.

Methyltrimethoxysilane MTM OS) was chosen for detailed study because it is often

used as a stone con solidant and its infrared spectrum is simpler than that of

Tetraethoxysilane TEOS).


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000 ____

000

000

Uniform Particles

Gelation I

Xerogel F i lm

Heat

Dense Fi lm

Heat

Dense Ceramic

(from Brinker and Scherer 1990)

3

I The Chemistry of lkoxysilanes

1 1 Overview

Brinker and Scherer (1990) define the sol - gel process as the preparation of ceramic

materials by preparation of a sol, gelation of the sol, and remov al of the solvent. The sol

may be p roduced from inorganic or organic precursors (e.g. nitrates or alkoxides) and

may consist of dense oxide particles or polymeric clusters. See fIgure 1-1 for a

schematic representation of the sol-gel process.

Figure 1 1: Overview o

the Sol Gel Process

-100 -

00001

r

0

Sol

Fibers

II

Solvent

Extraction

G el

Evaporat ionof

c

Xerogel

Aerogel


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The precursor of each colloid system

is

a metal or metalloid element that is surrounded

by various ligands - the ligands do no t contain metal/metalloid atoms. In the case

o

alkoxysilane solutions, aqueous system s tend to lead to the form ation of particulate

silica sols in which the dispersed phase is nonpolymeric. Polymeric silica sols are usually

obtained from non aqueous solutions, that is, containing solvents other than or in

addition to water (Brinker and Scherer 1990).

Depending upon the conditions during the reaction (i.e. pH, amount of water, solvent

type, etc.), either a particulate or po lymeric gel is forme d. Particulate gels are held

together by Van der Waal's forces and so are initially weaker than the covalently

bonded polym eric gels.

As the sols condense, oligomers formed under acidic conditions) may form bond s at

random to give network like formations. Sols formed in basic or aqueous solutions,

on the other hand, tend to be particulate. As the solvent evaporates, the so clusters

become more compact and condense farther. Network structures tangle together and

the particulate clusters aggregate into a more com pact structure

nown

as a xerogel.

The gel state may be defined as the point when a molecule reaches macroscopic

proportions and ex tends throughou t the solution. The gel contains a continuous solid

skeleton enclosing a continuous liquid phase. W hen the last bond of this giant molecule

is formed then the gel point is said to have been reached and an increase

n

viscosity is

apparent.

Polymerization is a general term used to refer to the transition from the so to gel state.

The entire process is comprised of several steps: the initial hydrolysis, and the

subsequent condensation, drying and aging processes. Each step is sensitive to

environmental factors and to the make-up of the original solution. Each step is also

influenced by the conditions of the preceding step (for example, the two step processes

where the hydrolysis is acid catalyzed but the pH is raised for the condensation /

gelation). In the case of alkoxysilanes hydrolysis results

in

the formation of silanols Si -

OH ) which then condense to form siloxane bonds. The reactions n

figure 1-2 are

usually given to define the overall process.


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15

Figure 1 2: Ov erall Reactions ofAlkoxysslane Polym erizat ion

1.) Hydrolysis

Si - OR

H

0

i - O H

ROH

Esterification

2.) Alcohol Condensat ion

Si-OR HO-Si

4

=Si-O-SiE+ROH

Alcoholysis

3.) Water Condensation

Si - OH HO - Si

Si

-0- Si

H2O

Hydrolysis

The hydro lysis reaction is greatly affected by the pH of the solution, the type and

concentra tion of catalyst, and the water to si l icctratio r). Acid catalyzed hydrolysis

with low H

O:Si r) ratios leads to sols with we akly branched structure s. At the other

extreme, base catalyzed hydrolysis w ith large H

O:Si ratios yields highly condensed ,

particulate sols. Conditions that range betw een the two extreme s result in interm ediate

structures Brinker, Scherer 1990). The process i s extremely complex and is explained

in greater detail later in this and following chapters.

W ithin this text , aqueous solutions are defined as systems consist ing of water an d

alkoxysilane only; non-aque ous refers to solvent or co-solvent and silane system s i .e.

water and alcohol.

1.2 Effect of Solution Type

1.2.1 Aqueous solutions

Gelation often referred to as polym erization) occurs, in alkoxysilane systems, by the

linking together of si lanol units via cond ensation re actions. The following discussion is


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6

based on the w ork of her (1979), who states that there

s no relation or analogy

between silicic acid polym erized in an aqueous system and condensation-type organic

polymers .

There are two basic types of polym erized structures that will form in aqueous solutions

(no solvent) dependent upon the pH of the system. First, at a pH of less than 4.5

th e

condensation reaction is slow. The viscosity increases with time because there is

essentially n o charge on the silanol groups so flocculation or agg regation occurs which

finally leads to gelation. Therefore, acidic solutions lead to the form ation of network

structure via the slow building of oxygen bridges between the silanol units, l ithe pH is

greater than 7, the sols bear a negative charge thu s increasing stabilization in dilute

solutions although these eventually cluster together to form larger particles (see figure

1-3 for a schematic representation).

11cr (1979) lists the basic steps and conditions as follows:

(a) If the Si(OH ') is formed at a concentration greater than 100 - 200 ppm

s

Si

, and

n

the absence of a solid phase that the silanol might deposit on, then the monom er

polym erizes to dimers and other oligomeric species.

(b) Below p H o f 2 (the isoelectric point of silanols is between 2 -3) the H con centration

4ffects the polymerization rate, above pH 2 the rate is proportional to the hydroxyl ion

concentration.

(c) At the early stages of the polymerization reaction, ring structures are preferentially

formed. This is followed by m onomer addition to the ring structure and linking together

of the cyclic polymers to form larger three dimensional m olecules. The silicic acid has a

strong tendency to maxim ize the siloxane bond formation and minim ize SiOH groups

by condensing internally to the m ost compact state possible with the SiOH groups

oriented towards the water interface. At temperatures of less than 80C, the

condensation may not be com plete and the internal phase, or core, will still contain

SiOH groups.


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ithp

particle size increases

V

three

dimensional

gel networks

U

V

M O N O M E R

DI M ER

CYCLIC

I

PARTICLE

pH

stevenson alkoxysilane stone consolidants

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