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Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Chemistry of
Organofunctional Silanes
and Applications in
Construction Industry
Franc Švegl
AMANOVA d.o.o.
Tehnološki park 18, SI-1000 Ljubljana, Slovenia
E-mail: [email protected]
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Table of Contents

Introduction



Chemistry of Organofunctional Silanes – Silicon Esters



Hydrolysis – Condensation
POSS frameworks
Applications in Construction Industry









Silicon
Silicon vs. Carbon Chemistry
Silane coupling agents - general concept
Silane coupling agents - thermoset composites
Silane coupling agents – mineral fillers
Hydrophobicity, hydrophilicity and silanes
Silanes and corrosion
Hybrid materials
Aerogels
Silanes and cement
Conclusions
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction - Silicon
Silicon shows a rich variety of chemical properties and can be found from such bulk commodities as
concrete, clays and ceramics, through more chemically modified systems such as soluble silicates, glasses,
and glazes to recent industries based on silicone polymers and solid-state electronics devices.
Concrete
Ceramics
Clays
Soluble
silicates
Silicon
Solid-state
electronics devices
Glasses
Silicone
polymers
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction – Silicon preparation
Group IV
Preparation:
- reduction of high-purity silica or quartz with carbon in an electric arc furnace (over
1900 °C):
SiO2 + C → Si + CO2
metallurgical grade silicon
at least 98% pure
SiO2 + 2 C → Si + 2 CO
Using this method, silicon carbide (SiC) may form – it can be eliminated by keeping concentration of SiO2
high: 2 SiC + SiO2 → 3 Si + 2 CO.
Crystallization-Purification:
-crystallization by Czochralski method,
-float-zone silicon (FZ-Si),
-dissolution of metallurgical silicon powder in an acid
-zone melting,
polycrystalline silicon
-conversion to a silicon compound that can be more easily purified by distillation and
then conversion of that compound back into pure silicon
Si + 2 Cl2 → SiCl4
SiCl4 + 2 Zn
2 HSiCl3
or
Si + 3 HCl → HSiCl3 + SiCl4 + H2
→
950 °CSi + 2 ZnCl2 (DuPont process – ultra pure silicon)
1150 °C
→
Si + 2 HCl + SiCl4 (Siemens process – high-purity silicon)
3 SiCl4 + Si + 2 H2 → 4 HSiCl3
face-centered cubic structure
4 HSiCl3 → 3 SiCl4 + SiH4
SiH4 → Si + 2 H2
(Renewable Energy Corporation fluidized bed technology using silane)
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction – Silicon as raw material
The direct synthesis of
methylchlorosilanes, known
as the Rochow reaction is the
reaction of methyl chloride
with elemental silicon in the
presence of copper catalyst.
hydrolitic
reaction
reduction
The direct process is the
most widely used basic
technique in the silicone
industry.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction – Silicon vs. Carbon
In their most stable state, silicon and carbon will both conveniently bond to four other atoms.
Silicon-based chemicals exhibit significant physical and chemical differences compared to carbon-based
analogous.
Prominent differences in bond strength:
 silicon does not form double or -bonds as readily as carbon does;
 the -Si-Si- bond (230 kJ mol-1) is weak in comparison to the -C-C- bond (356 kJ mol-1),
 the -Si-O- bond (368 kJ mol-1) is stronger than the corresponding -C-O- σ-bond (336 kJ mol-1).
In general, bonds with electronegative elements are stronger with silicon than with carbon. In particular, the
silicon-fluorine bond is extremely strong (582 kJ mol-1).
Bond strength
Electronegativity
C – C 347 kJ mol-1
Si – H
323 kJ mol-1
C – H 410 kJ mol-1
Si – C
326 kJ mol-1
(by Pauling)
Si
1,90
H
2,20
C
2,55
Si – Cl
398 kJ/mol-1
C – Cl 330 kJ mol-1
Cl 3,16
Si – O
464 kJ mol-1
C – O 360 kJ mol-1
O 3,44
Si – F
582 kJ mol-1
C – F 488 kJ mol-1
F 3,98
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Bond strength
Si – Si 230 kJ mol-1
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction – Silicon vs. Carbon
nucleophile
(F-, RO-, HO-, etc.)
Silicon has a lower electronegativity (1.9) as carbon (2.5) or oxygen
(3,4) and consequently, -C-Si- bonds and –Si-O- bonds are
polarized, rendering the silicon open to attack by nucleophiles (F-,
Cl-, RO-, HO-, …).
Silicon is bigger atom as carbon therefore the bonds between silicon
and other atoms are in general longer than the equivalent bonds
between carbon and the corresponding atoms. The increased bond
lengths between silicon and other atoms in comparison to the
corresponding systems involving carbon enables hard nucleophiles
(in particular F-) to react at sterically hindered silicon centers.
partly
polarized
Bond length
Si – C
1,89 Å
C–C
1,54 Å
In comparison, bonds of silicon to H are relatively weak (i.e.
hydrides like triethylsilane (Et3SiH) are reducing agents since the
Si–H bond is relatively weak (~323 kJ mol-1)). The carbon-silicon
bond is strong enough (~326 kJ mol-1) for trialkyl silyl groups to
survive a wide variety of synthetic transformations, but it is weak
enough to be selectively cleaved when required using mild
conditions.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction - Silicon vs. Carbon Chemistry
While carbon forms an extensive series of compounds with hydrogen, the list of silicon analogs is
limited. Compounds between silicon and hydrogen are called hydrides and can be viewed as analogs of
alkanes. The first silicon hydrides were made in 1857 by Friedrich Wöhler and Heinrich Buff who reacted
Al/Si alloys with aqueous HCl. The compounds prepared were analysed by Charls Friedel and Albert
Landenburg in 1867 shown to be SiH4 and SiHCl3. The first homologue, Si2H6, was prepared by Henri
Moissan and Samuel Smiles in 1902 from the protolysis (HCl) of magnesium silicide (Mg2Si). The thermal
instability and great chemical reactivity of the compounds precluded further advances until Alfred Stock
developed his greaseless vacuum techniques and first began to study them as contaminants of his boron
hydrides in 1916. He proposed the names silanes and boranes by analogy with alkanes.
Silanes are branched or unbranced chains (up to n=8).
All are colorless volatile liquids or gases at room
temperature. They are extremely reactive and
spontaneously ignite or explode in air. There is a steady
increase in the boiling point with increasing molecular
weight and chain length, and trisilane is the first
member of the series that is a liquid at room
temperature. Instability increases with increasing
number of Si-Si bonds, and only gas forms of SiH4
(silane) and Si2H6 (disilane) are stable at room
temperature.
Silane general formula SinH2n+2
chemical
stability
increase
SiH4
silane (g)
H3Si-SiH3
disilane (g)
H
H3Si-Si-SiH3
trisilane (liq)
H
etc.. n=8
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
b.p.
increase
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction - Silicon vs. Carbon Chemistry
A silane that contains at least one carbon-silicon bond (e.g. CH3-Si-) is known as an organosilane or
organofunctional silane.
In general, the organofunctional group will act chemically the same in the organosilicon compound as it
does in the carbon-based compound. For example, the distance of the amine, or other organofunctional
group, from silicon will determine whether the silicon atom affects the chemistry of the organofunctional
group. If the organic spacer group is e.g. a propylene linkage (-CH2CH2CH2-), then the organic reactivity
in the organofunctional silane will be similar to organic analogs in carbon chemistry.
silane
silicon
ester
organosilane
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Introduction - Naming Silicon Compounds
Silane SiH4 is the simplest hydride and provides the basis of nomenclature for all silicon chemistry.
Compounds are named as derivatives of silane with substituents prefixe. Two or more substituents are listed
alphabetically with substituted organic moieties being named first, followed by simple organic fragments.
Alkoxy substituents are named next, followed by acyloxy, halogen and pseudohalogen groups, e.g.
ethylmethylethoxysilane,
C2H5(CH3)SiH(OC2H5),
and
(3-chloropropyl)
methylchlorosilane,
ClCH2CH2CH2SiH(CH3)Cl.
Silanols
Silazanes
Cyclic silanes
Silanes
Chlorosilanes
(silylamines)
SiH4
H3Si-OH
H3SiNHSiH2NHSiH3
silanol
silane
SiCl4
H3Si-SiH3
tetrachlorosilane
disilane
HSiCl3
silane diol
H
trichlorosilane
Ph3Si-ONa
H3Si-Si-SiH3
H
trisilane
etc.. n=8
trisilazane
H2Si-(OH)2
sodium
1,1,3,3-tetramethyldisilazane
triphenylsilanolate
methyldichlorosilane
CH3-SiHCl2
Silicon group as a unit
H3Si-
cyclohexasilane (R = H)
Shorthand notation for the metylsiloxanes
and polymethylsiloxanes
silyl
Siloxanes
H
H3Si-O-SiH3
-Si-
disiloxane
H
Me3Si-O-SiMe3
H3SiO-
siloxanyl
hexamethyldisiloxane
Me3Si-
trimethylsilyl
Me3SiO-
silylene
M
Me
-O-Si-O-
Me
O
-O-Si-OD
O
-O-Si-OO
Me
Me3Si-O-SiMe3
T
MM
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Q
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Chemistry of Organofunctional Silanes
Silicon Esters – alkoxy derivatives of silanes
Silicon esters are silicon compounds that contain an oxygen bridge from silicon to an organic group.
Tetraalkoxysilanes
Substituted trialkoxysilanes
OR group:
R’ group:
methoxy -OCH3
(tetramethoxysilane - TMOS),
ethoxy -OC2H5
(tetraethoxysilane -TEOS),
or acetoxy –OOCCH3
(tetraacethoxysilane)
-H, -Cl, -CH3, -C2H5,
phenyl, or organofunctional
group, such as amino,
methacryloxy, epoxy,
isocyanate, etc.
Applications of tetraalkoxysilanes cover a broad range.
These compounds are classified roughly to whether
the Si-OR bond is expected to remain intact or to be
hydrolyzed in the final application.
Applications in which the Si-OR bond is hydrolyzed
include: binders for foundry-mold sands used in
investment and thin-shell castings, binders for
refractories, resins, coatings, sol-gel glasses, crosslinking agents, and adhesion promoters. Applications in
which Si-OR bond remains intact include heat transfer
and hydraulic fluids.
Substitution of one or more organic groups
which can be either inert or reactive brings to the
compound new functionality.
Methyl- and phenyltrialkoxysilanes are primarily
used in the production of silicone resins and
coatings. Longer chain materials, eg. propyl-,
isobutyl-, and octyltrialkoxysilanes, are used in
hydrophobic coatings, primarily for masonry and
concrete.
Many organosilanes with reactive functional
group are used as coupling agents.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: hydrolysis - esterification
Hydrolysis
Alkoxy silanes react readily with water by reaction called hydrolysis.
The hydrolysis reaction, through the addition of water, replaces alkoxide group (OR) with hydroxyl group
(OH-) which becomes attached to the silicon atom. Depending on the amount of water and catalysts present,
hydrolysis may go to completion, so that all OR groups are replaced by OH, or stop while the silicon is only
partially hydrolyzed.
Esterification
Because water and alkoxides are immiscible, a mutual solvent such as an alcohol is utilized. It should be
emphasized, however, that the addition of solvents and certain reaction conditions may promote reverse
reactions like esterification, in which an alcohol molecule displaces a hydroxyl group to produce an alkoxide
ligand plus water as a by-product.
R’-Si(OR)3
hydrolysis
+ nH2O
R’-Si(OR)(3-n)(OH)n + nROH
esterification
Transesterifiaction
Transesterification is a reaction in which an alcohol displaces an alkoxide group to produce an another
alcohol molecule. It often occurs when alkoxides are hydrolyzed in alcohols containing different alkyl groups
and in multicomponent systems that employ several alkoxides with differing alkoxide substituents.
R’’OH + R’-Si(OR)3 → R’-Si(OR)2OR’’ + ROH
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: hydrolysis mechanisms
Although hydrolysis can occur without addition of an external catalyst, it is most rapid and complete when
they are employed. The hydrolysis reaction can be catalyzed by acids and bases (mineral acids (HCl) and
ammonia are most generally used, however, other catalysts as acetic acid, KOH, amines, KF, and HF can be
used).
The rate and extent of the hydrolysis reaction is most influenced by the strength and concentration of the
acid- or base catalyst. All strong acids and bases behave similarly, whereas weaker acids and bases require
longer reaction times and higher concentration of catalyst to achieve the same extent of reaction.
Acid-Catalyzed
Mechanism
Si – OR + H+
+
Si – O-R
H
SN2 -Si
R’
R’
R’
Under acidic conditions, it is likely that an alkoxide group is protonated in a rapid first step. Electron
density is withdrawn from the silicon atom, making it more electrophilic and thus more susceptible to attack
from water. This results in the formation of a penta-coordinate transition state with significant SN2-type
character (bimolecular nucleophilic substitution). The water molecule attacks from the rear and acquires a
partial positive charge. The positive charge of the protonated alkoxide is correspondingly reduced, making
alcohol better leaving group. The transition state decays by displacement of an alcohol and inversion of the
silicon tetrahedron.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: hydrolysis mechanisms
Under basic conditions the hydroxyl anion attacks the silicon atom. Again, an SN2-type mechanism has
been proposed in which the -OH displaces -OR with inversion of the silicon tetrahedron. The SN2** and SN2*
mechanisms involve a stable 5-coordinated intermediate state, which decays through a second transition
state in which any of the surrounding ligands can acquire a partial negative charge. Hydrolysis occurs only by
displacement of an alkoxide anion, which may be aided by hydrogen bonding of the alkoxide anion with the
solvent. Because the silicon acquires a formal negative charge in the transition state, SN2** or SN2*,
mechanisms are quite sensitive to inductive as well as steric effects.
Base-Catalyzed Mechanism
OH- + R’-Si(OR)3
R’
R’
R’-Si(OR)2(OH) + RO-
R’
Base-catalyzed hydrolysis of silicon alkoxides proceeds much more slowly than acid-catalyzed hydrolysis at
an equivalent catalyst concentration. Basic alkoxide oxygens tend to repel the nucleophile, -OH. However,
once an initial hydrolysis has occurred, following reactions proceed stepwise, with each subsequent alkoxide
group more easily removed from the monomer then the previous one. Therefore, more highly hydrolyzed
silicones are more prone to attack. Although hydrolysis in alkaline environments is slow, it still tends to be
complete and irreversible.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: factors effecting hydrolysis
Steric Effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If
atoms are brought too close together, there is an associated cost in energy due to overlapping electron
clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and
reactivity.
Steric effects occur as:
 hindrance or steric resistance - the size of groups within a molecule prevents chemical reactions that are
observed in related smaller molecules;
 steric shielding –a charged group on a molecule is seemingly weakened or spatially shielded by less
charged (or oppositely charged) atoms, including counterions in solution (Debye shielding);
 steric attraction - molecules have shapes that are optimized for interaction with one another;
 chain crossing - a random coil molecule can't change from one conformation to a closely related shape
by a small displacement;
steric repulsions - different parts of molecular system repeal one another.
Steric factors exert the greatest effect on the hydrolitic
stability of organoxysilanes. Any complication of alkoxy
group retards the hydrolysis of alkoxysialnes, but
hydrolysis rate is lowered the most by branched alkoxy
groups. The retarding effect of the bulkier ethoxide
group is clearly evident for the acid hydrolysis of
tetraalkoxysilanes.
steric
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: factors effecting hydrolysis
Inductive Effects
The inductive effect is a transmission of charge through a chain of atoms in a molecule by electrostatic
induction. The electron cloud in a σ-bond between two unlike atoms like Si and O or C is not uniform and is
slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of
bond polarization, where the more electronegative atom has a slight negative charge (δ-) and the other
atom has a slight positive charge (δ+). If the electronegative atom is then joined to a chain of atoms, the
positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect,
also known as the -I effect. The alkyl groups are less electron-withdrawing than hydrogen and are
considered as electron-releasing. They exert an electron releasing character indicated by the +I.
inductive
Electron withdrawing substituents such as –OH or
–OSi help to stabilize the negative charge on
silicon, causing the hydrolysis rate to increase with
the extent of OH substitution, whereas
electronproviding
substituents
cause
the
hydrolysis rate to decrease.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: factors effecting hydrolysis
Solvent effects
Solvents are added to prevent liquid-liquid phase
separation during the initial stages of the hydrolysis
reaction and to control the concentrations of silicate
and water that influence the hydrolysis and
condensation kinetics.
Solvents may be classified as polar or nonpolar and
as protic (containing removable or labile proton) or
aprotic.
With regard to solvating power the following
characteristics are important:
• polarity,
• dipole moment and
• the availability of protons.
Brinker and Scherer, Sol-Gel SCience, 1990
The polarity largely determines the solvating ability for polar or nonpolar species. More polar solvents (eg.
water, alcohol or formamide) are normally used to solvate polar, silicate species and surfaces. Less polar
solvents such as dioxane or tetrahydrofuran (THF) may be used in alkyl-substituted or incompletely
hydrolyzed systems. Ether alcohols such as methoxyethanol or ethoxyethanol exhibit both polar and
nonpolar character and are useful when a distribution of polar and nonpolar species is present in a solution.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: factors effecting hydrolysis
Solvent effects
The dipole moment, m, of a solvent determines the length over which the charge on the species can be felt
by surrounding species. The lower the dipole moment, the larger this length becomes.
The availability of labile protons determines whether anions or cations are solvated more strongly
through hydrogen bonding. Because hydrolysis is catalyzed either by hydroxyl (pH > 7) or hydronium ions
(pH < 7), solvent molecules that hydrogen bond to hydroxyl ions or hydronium ions reduce the catalytic
activity under basic or acidic conditions, respectively. Therefore, aprotic solvents that do not form hydrogen
bond to hydroxyl ions have the effect of making hydroxyl ions more nucleophilic, whereas protic solvents
make hydronium ions more electrophilic. Hydrogen bonding may also influence the hydrolysis mechanisms.
Hydrogen bonding of the solvent may
facilitate inversion of silicon tetrahedron by
activating poor leaving groups.
The availability of labile protons also influences the extent of the
reverse reactions, reesterification or siloxane bond alcoholysis or
hydrolysis. Aprotic solvents do not participate in reverse reactions
such as reesterification or hydrolysis, because they lack sufficiently
electrophilic protons and are unable to be deprotonated to form
sufficiently strong nucleophiles (eg. OH- or OR-) necessary for
these reactions. Therefore compared to alcohol or water, aprotic
solvents such as a THF or dioxane are considerably more inert.
Reverse reactions
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: condensation
Two partially hydrolyzed molecules can link together in a condensation reaction. Subsequent condensation
reactions, involving the silanol groups (Si-OH), produce siloxane bonds (Si-O-Si) plus the by-products small
molecules of water or alcohol. Under most conditions, condensation commences before hydrolysis is
complete.
alcohol condensation
R’(OR)2Si-OR + HO-Si(OR)2R’
R’(OR)2Si-O-Si(OR)2R’ + ROH
alcoholysis
water condensation
R’(OR)2Si-OH + HO-Si(OR)2R’
R’(OR)2Si-O-Si(OR)2R’ + H2O
hydrolysis
Typical sequence of condensation products is monomer,
dimer, linear trimer, cyclic trimer, cyclic tetramer, and higher
order rings. This sequence of condensation requires both
depolymerization (ring opening) and the availability of
monomers which are in solution equilibrium with the oligomeric
species and/or are generated by depolymerization (reverse
reactions of acoholysis and hydrolysis). Alcoholysis and
hydrolysis of siloxane bonds provide a means for bond
breakage and reformation allowing continual restructuring of
the growing polymers.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: condensation mechanisms
As with hydrolysis, condensation can proceed without catalyst, however, their use in organosiloxanes is
highly helpful. Furthermore, the same type catalysts are employed: generally those compounds which exhibit
acidic or basic characteristics.
Base-catalyzed condensation
The most widely accepted mechanism for the basic-catalyzed condensation reaction involves the attack of
a nucleophilic deprotonated silanol on a neutral silicate species. This reaction pertains above the isoelectric
point of silica (> pH 1,5 – 4,5, depending on the extent of condensation of the silicate species), where
surface silanols may be deprotonated depending on their acidity. The acidity of silanols depends on other
substituents on the silicon atom. When basic OR and OH are replaced with OSi, the reduced electron
density on Si increases the acidity of the protons on the remaining silanos. This favors reactions between
larger, more highly condensed species, which contain acidic silanols, and smaller, less weakly branched
species. The condensation rate is maximized near neutral pH where significant concentrations of both
protonated and deprotonated silanols exist. A minimum rate is observed near the isoelectric point.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: condensation mechanisms
Acid-catalyzed condensation
The acid-catalyzed condensation mechanism involves a
protonated silanol species. Protonation of the silanol
makes the silicon more electrophilic and thus more
susceptible to nucleophilic attack. The most basic
silanol species (silanols contained in monomers or
weakly branched oligomers) are the most likely to be
protonated. Therefore, condensation reactions may
occur preferentially between neutral species and
protonated silanos situated on monomer end groups of
chains. The condensation of alkylsilanetriol involves
protonated silanol.
The proposed reaction mechanisms for both base- and
acid-catalyzed condensation involve penta- or
hexacoordianted transition states or intermediates.
Because of this, the condensation reaction kinetics will
be influenced by both steric and inductive factors.
bimolecular
intermediate
hexacoordinated
silicon atom
bimolecular
intermediate
pentacoordinated
silicon atom
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: factors affecting condensation
Steric, inductive and solvent effects
Steric effects
Substituents attached to silicon that reduce steric crowding in the transition state or intermediate will
enhance the condensation kinetics. Bulky groups attached to silicon or perhaps partial condensation of the
silicon involved in the condensation reaction are expected to retard the condensation process.
Inductive effects
Replacement of more electron-providing OR groups with progressively more electron-withdrawing OH and
OSi groups stabilizes the negative charge on the anionic nucleophile involved in the base-catalyzed
condensation reaction and therefore should enhance the kinetics. Extensive hydrolysis and condensation
should destabilize the positively charged intermediate or transition state involved in the acid-catalyzed
condensation reaction and thus retard the condensation kinetics.
In organosilanes organic substituents influence the acidity of silanols involved in condensation. Electronproviding alkyl groups reduce the acidity of the corresponding silanol. This should shift the isolelectric point
toward higher pH values. Conversely, electron-withdrawing groups (OH or OSi) increase the silanol acidity,
and the condensation rate for oligomeric species occurs at about pH 2.
Solvent effects
Depending on the pH, either protonated or deprotonated silanos are involved in the condensation
mechanism. Because protic solvents hydrogen bond to nucleophilic deprotonated silanos and aprotic
solvents hydrogen bond to electrophilic protonated silanols, protic solvents retard base-catalyzed
condensation and promote acid-catalyzed condensation. Aprotic solvents have the reverse effect.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
TEOS: hydrolysis and condensation to silica
Silicon dioxide never forms directly during
hydrolysis. According to Iler, polymerization of
monomer Si(OH)4 occurs in three stages:
 polymerization of monomer to form particles,
Si(OH)4
 growth of particles,
 linking of particles into chains,
 and then networks that extend throughout the
liquid medium, thickening it into a gel.
The condensation takes place in such a fashion
as to maximize the number of Si-O-Si bonds and
minimize the number of terminal hydroxyl groups
through internal condensation. Thus rings are
quickly formed to which monomers add, creating
three-dimensional particles. These particles
condense to the most compact state leaving OH
groups on the outside.
The three-dimensional particles serve as nuclei. Further growth occurs by an Ostwald ripening mechanism
whereby particles grow in size and decrease in number as highly soluble small particles dissolve and
reprecipitate on larger, less soluble nuclei. Growth stops when the difference in solubility between the
smallest and largest particles becomes only a few ppm. Due to greater solubility, growth continues to larger
sizes at higher temperatures, especially above pH 7.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
TEOS: hydrolysis and condensation to silica
Iler divides the polymerization process into three approximate pH domains < pH 2, pH 2-7, and >pH 7.
pH 2 appears as a boundary, since the point of zero
charge (PZC), where the surface charge is zero, and
the isoelectric point (IEP), where the electrical
mobility of the silica particles is zero, both are in the
range pH 1-3. pH 2 represents a metastability region
where the observed gel times are quite long. Below
pH 2, the polymerization rate is proportional to H+
conc.
pH 7 appears as a boundary because both the silica
solubility and dissolution rates are maximized at or
above pH 7 and because the silica particles are
appreciably ionized above pH 7 so that particle
growth occurs without aggregation or gelation. Since
the gel times decrease steady between pH 2 and pH
6. It is generally assumed that above the IEP the
condensation rate is proportional to OH- conc.
Above pH 7 all the condensed species are more likely to be ionized and therefore mutually repulsive, growth
occurs primarily by the addition of monomers to more highly condensed particles rather than by particle
aggregation. Due to the greater solubility of silica and the greater size-dependence of solubility above pH 7,
growth of the primary particles continues by Ostwald ripening. Particles grow rapidly to a size that depends
mainly on temperature. Higher temperatures produce larger particles due to greater silica solubility. In the
absence of salt, no chaining or aggregation occur, because the particles are mutually repulsive. The addition of
salt reduces the thickness of the double layer at given pH, dramatically reducing the gel times.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: condensed structures
R‘ = CH3, C2H5, etc...
R = -OR‘, aryl, alkyl, -SH, COOH, -SO3H, -NCO, -NR‘‘,
methacryloyl, halogen,
epoxy, styryl, etc...
f=2
(SiO2)n silica
f=4
Rn-Si(OR‘)4-n
n = 1..4
Polyhedral
Oligosilsesquioxanes
(POSS)
[R-SiO1.5]n
6 < n < 12
polyhedral
oligomers
Oligo- and
polysiloxanes
f=3
f=3
f=3
f=2
f=3
[R-SiOm]n
n=∞
m > 1.5
insoluble gels
f=3
[R-SiOm]n
n<∞
m > 1.5
soluble
amorphous
polymers
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: sol-gel processing
The complete hydrolysis of tetratalkoxysilanes under highly controlled conditions usually without the
presence of fillers, is associated with sol-gel technology. Sol-gel is a method for preparing specialty metal
oxide glasses and ceramics by hydrolyzing a chemical precursor or mixture of chemical precursors that pass
sequentially through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in
a continuous liquid phase (gel) state before being dehydrated to a glass or ceramic.
The use of sol-gel processing has
increased dramatically since 1980.
A variety of techniques have been
developed to prepare fibers,
microspheres, thin films, fine nano
powders, and special monoliths.
Applications for this technology
include
protective
coatings,
catalysts, piezoelelctric devices,
waveguides, lenses, high strength
ceramics,
superconductors,
insulating materials, and nuclear
waste encapsulation. The flexibility
of sol-gel technology allows unique
access to multicomponent oxide
systems and low temperature
process regimes.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Silicon Esters: other reactions
The Si-OR bond undergoes a variety of reactions apart from hydrolysis and condensation. The reactivity
of the Si-OR bond is in many cases analogous to the Si-Cl bond, except that the reactions are more
sluggish. These reactions become increasingly more sluggish with greater bulk and steric screening of
the alkoxy group. Interesting reactions of Si-OR group are as follows:
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Polyhedral Oligo- and Heterosilsesquioxanes
Polyhedral oligosilsequioxanes (POSS) are three-dimensional structures derived from the hydrolytic
condensation of trifunctional organosilicon monomers (RSiX3 e.g., RSiCl3 and RSi(OMe)3 ). These reactions
can produce a wide variety of interesting products, ranging from small oligomers and discrete clusters to
complex resins and T-gels. Fully-condensed silsesquioxanes have the idealized empirical formula [RSiO3/2]n,
whereas incompletely-condensed silsesquioxanes possess reactive Si-OH groups which are potentially
capable of forming additional Si-O-Si linkages via elimination of water.
Many hydrolytic condensation reactions
produce synthetically useful quantities of
fully-condensed
POSS
frameworks
containing 6, 8, 10 and/or 12 Si atoms.
T6
T8
T10
Fully-condensed [RSiO3/2]n frameworks
can form in both polar and non-polar
solvents under both acidic and basic
conditions.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
T12
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Polyhedral Oligo- and Heterosilsesquioxanes
A related family of polyhedral Si/O clusters can be prepared via base-catalyzed equilibration of
tetrafunctional silicon monomers, such as Si(OEt)4. These clusters are functionalized silicates (i.e.,
[(SiO2)n(SiO4)m]4m-) rather than silsesquioxanes (i.e., [RSiO3/2]), but they exhibit many similarities to fullycondensed POSS frameworks. For example, base-catalyzed equilibrium of Si(OMe)4, of Si(OEt)4, silicic
acid, or SiO2 can be performed under conditions where the major Si-containing species in solutions is
[Si8O20]8-.
A wide variety of fully-condensed POSS and spherosilicate frameworks can be prepared from commercially
available silane monomers. These easily synthesized frameworks have many potential uses and serve as
the basic starting materials for more elaborate derivatives.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Hydridosilsesquiosanes
Polyhedral
Hydridosilsesquioxane
cages
Polyhedral hydridosilsesquioxanes like H8Si8O12, H10Si10O15 and several
larger [HSiO3/2]n frameworks are of special interest because they are easy to
synthesize in pure form and commercially available. They serve as
precursors to functionalized silsesquioxanes and spherosilicates because of
the ease with which Si-H bonds can be synthetically manipulated.
The oxidation of H8Si8O12 and other hydridosilsesquioxanes can be
accomplished with variety of reagents. Most of these reactions appear to
involve initial attack of nucleophile on a framework Si-H groups. Hydroxide is
an effective catalyst for the oxidation of Si-H to Si-OH by water.
Si8O12H8
Hydrosilylation is one of the most
important and general methods for
forming Si-C bonds, and it has been
used
extensively
to
produce
octafunctional silsesquioxanes from
H8Si8O12 and α-olefins. The reaction is
quite general, but it often produces
mixtures of products resulting from
both α-addition and β-addition to the
olefin. In addition, hydrosilylation
reaction of H8Si8O12 with vinyl silanes
are also complicated by vinyl/H
exchange.
Hydrosilylation
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Incompletely-condensed POSS frameworks
The hydrolytic condensation of RSiX3 produces high yields of compounds that are intermediates along the
way to fully condensed POSS frameworks. These incompletely condensed POSS frameworks have a wide
range of applications.
condensation
+
partial
hydrolysis
(c-C6H11)SiCl3
(c-C6H11)7Si7 O7(OH)3
(c-C6H11)8Si8 O9(OH)2
incompletely
condensed
incompletely
condensed
(c-C6H11)6Si6O9
completely
condensed [RSiO3/2]n
framework
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Incompletely-condensed POSS frameworks and
heterosilsesquioxanes
Heterosilsesquioxanes are formally derived by substitution of a main-group, transition-metal or f-element
atom for one or more Si atoms in a silsesquioxanes. When the heteroatoms are metal atoms, the term
metallsilsesquioxane is often used.
A wide range of heterosilsesquioxanes are available via functionalization of incompletely condensed
silsequioxanes. There are five general methods for attaching heteroatoms to an incompletely condensed
silsesquioxane framework:
(1) reaction of SiOH groups with metal alkyl
complexes;
(2) metathesis of SiOH groups for less acidic
alkoxide or amide ligands;
(3) base-assisted (e.g. Et3N) reaction of SiOH
groups with active metal halides;
(4) reaction
of
Me4Sb
stabilized
silsesquioxanes with metal halides; and
(5) reaction of Tl-stabilzed silsequioxanes with
metal halides or triflates.
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
POSS frameworks as …
Soluble models (for silica, silica supported catalysts, aluminosilicates, etc.):
Because of their high Si/O content and short range structural similarities to known SiO2 polymorph,
silsesquioxanes and metallasilsesquioxanes have been used extensively as models for silica surfaces, silica
supported catalysts, aluminosilicates and monodisperse titanium containing catalysts.
Precursors to hybrid inorganic/organic materials:
Incompletely condensed frameworks have been used as comonomers for new families of silsequioxane
based polymers and as building blocks for network solids. Fully condensed POSS frameworks have been
used for an even broader spectrum of applications, including photocurable resins, liquid crystals,
elektroactive films, and building blocks for catalytically active organometallic gels.
One of the most promising applications for silsesquioxanes is
the use of fully condensed POSS frameworks possessing
one potentially polymerizable pendant group as comonomers
in traditional thermoplastic resins. These comonomers, which
are synthesized via corner capping reactions of trisilanols
can dramatically improve performance characteristics of
many resin systems. Lichtenhan has pioneered the use of
POSS monomers in advanced thermoplastics and developed
an entire chemical tree of monomers suitable for
polymerization and grafting reactions. Both the pool of readily
available POSS monomers and the range of applications for
these monomers are expanding rapidly.
X = H, Cl, OH, OSiMe2H, vinyl, allyl,
ethylnorbornenyl, glycidyl,
p-styryl, (CH2)3O-methacryl,
(CH2)3Cl, (CH2)3CN, (CH2)2CO2Me
R = c-C6H11 or c-C5H9
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013
Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“
Organofunctinal Silane Applications
Adhesives
and Sealants
Precision
casting
Sol-Gel,
Hybrid materials
Aeroges
Spin-On
RTV
CVD
Organofunctional
silanes
Cements and
Ceramics
Water Repellents and
Surface Protection
Glass Frosting
Hydraulic and
Heat-Transfer
Flluids
Paints, Inks and
Coatings
Bonded
Phases
Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013