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Abstract: Being amongst the most versatile molecules available to promote adhesion,
organosilanes nonetheless remain a bit of a conundrum for some as one needs to know their
chemistry thoroughly in order to utilize them in a useful fashion. In the first instance, this work
starts with a short presentation of organosilanes defining this class of chemicals as well asexplaining their genesis followed immediately by a section of this chapter dedicated to their
chemistry which includes a few words of warning. Closely related are the necessary interactions
that such molecules have to develop on materials in order to fulfill their main role as adhesion
promoters, and this is described too but more particularly when considering metallic
substrates and polymers. Reactions in a medium are also covered briefly as silanes are
commonly included in a matrix which precludes a more comprehensive section of silanes
behavior in formulations later on in this document. Is also covered other usages of silanes than
as adhesion promoters together with principles to follow in order to chose a silane for
a particular application. The following section is concerned with the use of silanes as primersparticularly where the user aims to improve adhesion or protect from corrosion. To complete
this work, a small section covers some other organic or nonorganic adhesion promoters.
11.1 Introduction
Although initially the development of organosilanes adhesion promoters was a response to
a technological need, more recent developments in the use of such molecules illustrate the
reflection of recent legislation and need for more ‘‘green’’ materials. This is true in particular
when considering such applications as replacements for chromic acid rinses used in the
aerospace industry or corrosion protection where silanes are used as primers in the purest
sense of the word. Unfortunately or fortunately, what lies at the heart of this class of
molecules is their chemistry. It is both the tenor of their flexibility and complexity, and full
understanding of such matters is of foremost importance. As such, explanations of the
various intrinsic reactions they may undergo together with a review of how such molecules
interact with a variety of materials is essential. The obvious next step from using silanes as
adhesion promoters/primers consists into incorporating the silane within a formulation in
order to interact with a surface while saving a step in the process. For example, an adhesive
formulation may already contain a silane which is destined to diffuse and bond with thesurface of choice. Apart from the obvious economy of time, one can anticipate large savings
of a more real nature. This chapter will therefore also cover reactions of silanes when not in
solution, i.e., for organosilanes incorporated in formulations such as inks or adhesives.
Another major part is concerned with the parameters which influence the deposition of
organosilanes as primers, some of which are often overlooked, as well as with the various uses
of silanes either as adhesion promoters or otherwise including their sometimes controversial
usage as corrosion protection coatings. One part is also dedicated to the manner in which one
should choose one’s silane, although this section is certainly not exhaustive. The ultimate in
the matter remaining a series of tests and experiments for the particular system considered.However, the present author acknowledges that this is not always timely or practical,
especially when one considers the resources required and, hence, some pointers are provided
to perform an adequate selection. Finally, and although this chapter will examine primarily
organosilane adhesion promoters as well as their use as primers, and within formulations,
a short summary of other available adhesion promoters will also be presented at the end of
the chapter.
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11.2 What Silanes are and Where do they Come From
11.2.1 Definition
Organosilanes are hybrid molecules with at least one hydrolysable alkoxysilane. They usually bear
the alkoxysilane functionality at one end and an organic functionality at the other end. The
general formula is R 0-Si(OR)3, where R 0 corresponds to the main chain as well as any desired
functionality, while OR is the hydrolysable alkoxy functionality. However, it is possible to find
bis-silanes with alkoxysilanes at both ends of the molecules and one functionality within the
middle chain. Various combinations of such structures may also be purchased according to the
particular application sought after and/or the type of usage. There are many silanes available, but
the most used bear the following organic functionalities: vinyl (-C=C-, double bond), epoxy,
amino (primary amine as well as others, -N-) and mercapto (-S). They may be applied fromsolution as well as incorporated in formulations. They may be used on their own or in
conjunction with other silanes. They can be deposited on several substrates: glass, metallic, as
well as, more recently, organic. Their applications are vast and include composites, paints,
adhesives, paints, and inks, and they can impart many properties on and to the materials they
are used with, including hydrophobicity, temperature, and abrasion resistance. But mostly, what
organosilanes are most famous for is their usage as primers and adhesion promoters where their
role is to promote and improve adhesion between two dissimilar and/or incompatible materials.
11.2.2 The Origin of Organosilanes
The thought of creating such molecules originated from the will to render glass and an organic
matrix compatible as well as to increase the durability of glass fibers–based composites. Indeed,
although such composites were exhibiting strong initial strength, they would ‘‘fall apart’’ once
introduced into a humid and hot environment. The debonding was attributed to water ingress
and hydrolysis of glass. These composites were also exhibiting problems with stresses across the
interface because of the very different properties of the glass and the matrix in terms of thermal
expansion (Pluedemann 1991). A hybrid molecule with chemistry similar to glass on the one
hand and similar to an organic matrix on the other hand was needed. A few organic reactions later,the first silane was born, itself a composite of the chemistry of glass (alkoxysilane part hydrolyzing
into a silanol) and any chosen matrix (organic functionality born on the molecule in order to
interact with a matrix). This was of course a major contribution to the world of chemistry and
science by Edward D. Pluedemann, probably better known as ‘‘Mr Silane.’’ They were obviously
used on glass first, but their usage subsequently progressed onto the surface of metals and a lot
recently on polymeric materials too (Pluedemann 1991, Smith 1999). The chemistry of those
molecules is at the origin of their being but is also at the very root of their flexibility and the many
domains in which they find valuable applications. Therefore, the following section will cover the
chemistry of organosilanes and what other reactions they can undergo, including how they interact with other media be it with their surfaces or matrices.
11.3 Silanes Chemistry
First and foremost, one should make the distinction between all silanes molecules and the more
specific organosilanes which are used in solutions or formulated systems as adhesion
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an epoxy or a vinyl as in the work by Blum et al. as seen in >Fig. 11.3, where the study was
performed using proton nuclear magnetic resonance (1H-NMR) (Blum et al. 1991):
(A) represents the signal of methoxysilane (reactant) and (B) the resonance of methanol.
The decrease of A versus the increase of B indicates that hydrolysis is taking place. Once theintensity of B is constant, the reaction is complete. This work is also a very good illustration of
the rapidity with which amino-functionalized silanes can hydrolyze. It is very important to
isolate them from water if not in use as they are autocatalyzed for both hydrolysis and
condensation to such an extent that such molecules will even react with the air moisture and
absorb CO2. Such reactions can be controlled better paradoxically if performed in an aqueous
solution as one can monitor the kinetics of reaction using catalysis by setting the pH of the
solution. The catalysis for hydrolysis is usually acidic, although it is possible to use basic
catalysis which is usually preferred for condensation, particularly self-condensation. A small
paragraph on catalysis will be provided at the end of this section.In addition, one should also emphasize one particular aspect of organosilane hydrolysis:
the effect of other solvents and more particularly that of alcohol, understood as the organic
class of molecules of course. It is often advised as largely seen in the literature to proceed to the
hydrolysis of a silane in aqueous solution; often also, one is advised to use an alcohol together
with water as another solvent, and many authors happily mention using a mixture of water and
either methanol or ethanol while waiting for a given time (of say 20–30 min) for hydrolysis.
Si O
Oa
b
c
O
CH3
CH3
H2N
H2N H2N
H2N
H2O
H2N
++
3C2H5OH3H2OSi OH
HO
OH
Si OH
OH
+ Si OH
OH
Si OH
OH
HO +
HO HO OSi
HO
AI
Si OH
HO
OH
HO Al
Al
Al
AlHO
HO
Al
Al
+ Si
OH
O Al
Al
Al
AlHO
Al
Al
Al
Al
Al
AlHO
HO
Al
H3C
H2N
H2NH2N
. Fig. 11.2
(a) Schematic of APS organosilane hydrolysis, (b) schematic of APS organosilane self-
condensation, and (c) APS organosilane condensation with aluminum surface
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This is rather contra-intuitive as this clearly contradicts Le Chatelier principle. Indeed,
hydrolysis of alkoxy functionalities should lead to the production of the corresponding alcohol.
By example, an ethoxy will produce ethanol. Hence, adding the correct alcohol (only correct by
the virtue that it will allow avoidance of alcoholysis, see below) can only slow down or even stop
the hydrolysis reaction. Such problem has been illustrated clearly by some work where the
hydrolysis is shown to clearly being extremely or even on existent when methanol is added toa solution of g-glycidoxy propyl trimethoxysilane (GPS) (Abel et al. 2006). Again, NMR was
used, and it was possible to show that adding methanol in solution was significantly slowing
down the hydrolysis reaction even if only added at a concentration of 10% (v/v) in solution.
However, notwithstanding the problem of volatile organics, one can comprehend that in some
cases, it is preferable to use an organic solvent because it evaporate faster but remains the
problem of controlling hydrolysis. A good answer to the conundrum would be to use
a concentrated aqueous solution at ideal pH (see catalysis section below) and then diluting
in an organic solvent once hydrolysis has occurred in order to obtain a solution in which the
monomer is stable, i.e., with a longer shelf life. This is not valid though for aminosilanes.
11.3.1.2 Condensation
Two types of condensation are possible. One is self-condensation, while the other corresponds
to condensation with a substrate of choice such as a metallic substrate. The latter will be
covered in more detail in another section below. By definition, condensation corresponds to
10
B
A
15 20 25
t (min)
30 355
. Fig. 11.3
Illustration of hydrolysis: A represents the methoxysilane signal and B the methanol resonance in
an extract of a nuclear magnetic resonance spectrum. The decrease of A versus the increase of B indicates that hydrolysis is taking place. Once the intensity of B is constant the reaction is
complete
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the reaction of two hydroxyls (-OH) functionalities, leading usually to the formation of water
and an ether bond; there is a possibility, however, that condensation may occur before
hydrolysis has taken place, but this is not as likely unless catalysis is being used. > Figure 11.2b
illustrates the reaction. Similar to hydrolysis, condensation and particularly condensation asoligomer formation is influenced by the value of the pH of the aqueous silane solution. Both
condensation and hydrolysis can be controlled using catalysis, although, as mentioned before,
amino-functionalized silanes are autocatalyzed with an extremely rapid tendency to polymer-
ize and are also influenced by the amount of silanols present. Condensation is mostly
performed under basic catalysis. One of the interests of the users would be of course to be
able to establish whether a solution of silane has polymerized without resorting to complex and
often expensive tools. One of the recommendations on the silane ‘‘grapevine’’ is to check
whether the solution has gone cloudy. This, however, is not valid if the silane polymerizes clear,
which is indeed the case of g-GPS but not of g-aminopropyl triethoxysilane (APS) or vinylsilanes which polymerize cloudy. One should also be aware that in spite of the opinion of
various authors on the subject, organosilanes will not always go down happily on a surface in
order to form bonds. In the case of oligomeric solutions, this is totally false and can lead to
premature bond failure if used as a primer.
11.3.1.3 Alcoholysis
Alcoholysis is the exchange of the alkoxy borne on the silane with the corresponding alcohol
functionality if alcohol molecules are used within a silane solution. By example, if the silane isan ethoxy and that a solution containing methanol is made, then there is a possibility that the
ethoxy may substitute for a methoxy.
11.3.1.4 Catalysis
Either hydrolysis or condensation may be controlled using acidic or basic catalysis. Essentially,
when examining these two reactions, one would ideally like to be able to freeze them in time in
order to obtain solutions with long shelf life. Although this is only possible by, for example,
cooling down a solution/mixture, it is preferable to use a solution of monomeric-hydrolyzedsilanes as a primer solution and catalysis conveniently allows this. The kinetic constant of the
hydrolysis and/or condensation reaction vary as a function of the pH of the solution. If acidic
catalysis is examined for hydrolysis, then the constant, plotted as log k as a function of pH,
exhibits a ‘‘V-shaped’’ curve with a minimum specific value of pH. This value corresponds to
the slowest kinetics of the reaction, usually around pH 6 for hydrolysis and 10 for condensa-
tion. >Figure 11.4 shows an example of such a shape. The position of the minimum of the
curve will itself depend on the structure of the molecule.
11.3.2 Specific Interactions at the Interface
11.3.2.1 Interactions with a Metallic Substrate
As far as metallic substrates are concerned, the anticipation is that the silane will react with
hydroxyls functionalities present at the surface in a reaction similar to that of self-condensation.
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An example of APS bonding on aluminum is given in > Fig. 11.2c . It is important to point out
that such interaction will be facilitated by a prior hydrolysis, but this is not necessarily
compulsory, as silanes can even hydrolyze within a formulation and subsequently condense.
This, for example, is particularly easy for aminosilanes which are autocatalyzed for both hydro-
lysis and condensation. A silane film is often shown as a ‘‘clean’’ monolayer being deposited on the
surface of choice. This is far from reality, and often the film deposited on a substrate will represent
several layers and have formed cross-linking of its own. This said, it is possible to deposit very thin
films as well as monolayers in order to study the interaction of a possible silane with a substrate.
Such methods include the use of adsorption isotherms, for example, as described by Watts in >Chap. 10 of the present volume, or modelling of a silane layer using adequate softwares. Davis
et al. showed that some silanes form a stable interaction when presented with a model aluminum
surface; depending on the molecule, the interaction can occur via the silanol, the organic
functionality, or forming a bridge while interacting with both sides of the molecule (Davis
1997). Kinloch et al. have also shown that the order, meaning geometry, in which silane will
deposit on a surface is correlated with the length of the chain attached to the silane part, where it
is above a length of 20 carbon atoms. At certain lengths, the molecules will order with a particular
angle to the surface of choice, something not seen for short silane chains or below a length of 10
carbons (Hobbs and Kinloch 1998). Another example where the directionality of the interactioncan be demonstrated is shown in the work of George et al. (George et al. 1996). In order to
understand the interfacial chemistry of certain joints formed in the semiconductor industry and
using angle-resolved x-ray photoelectron spectroscopy, George et al. have studied the interac-
tions of APS with a silicon wafer as well as with Pyralin, a polyimide precursor. Their work shows
that APS absorbs CO2 indeed and can interact on both sides of the molecule with the substrate.
The most interesting feature of the article is the demonstration that under a temperature of
phenyl-bis (2-methoxyethoxy)silanol
Acidic
catalysis Basiccatalysis
0–6
–5
–4
–3
l o g k o b s
–2
–1
0
1
2
2 4 6
pH
8 10 12 14
a
. Fig. 11.4
Illustration of the pH dependence of the kinetics of hydrolysis for phenyl-bis
(2-methoxyethoxysilanol). On the left of the triangle, one can see acidic catalysis, while on the
right, basic catalysis occurs
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125C, a thin APS film (1.5 nm) can ‘‘flip’’ from one side to the other (from –NH2 interacting to
silanol interacting), an interesting fact as most drying and cure processes will involve a step at
temperature usually preferably above a threshold that will rid of most of the water present in
the film. What remains is demonstrating the existence of bond formation. While acid-base typeof bonds can sometimes be determined using x-ray photoelectron spectroscopy, showing the
presence of covalent bond formation can be readily achieved by various methods such as time
of flight secondary ion mass spectrometry (ToF-SIMS) but also with more accessible and
cheaper techniques such as infrared spectroscopy (Abel et al. 2004; 2000). > Figure 11.5 shows
the region of nominal mass 71 in a positive mass spectrum (ToF-SIMS) for a GPS-coated grit-
blasted aluminum where the fragment illustrating a bond formation between aluminum and
×101
8.0
SiOAl+
(1.35 mmu)
Si2CH3+
(3.1 mmu) SiC2H3O+
(1.97 mmu)
C3H3O2+
(–0.79 mmu)
C4H7O+
(2.26 mmu)
C5H11+
(3.4 mmu)
mass resolution (SiOAl+: 8950)
7.0
6.0
5.0
I n t e n s i t y
4.0
3.0
2.0
1.0
70.85 70.90 70.95 71.00 71.05 71.10 71.15mass/u
. Fig. 11.5
High-mass resolution of ToF-SIMS spectrum in the positive mode of grit-blasted aluminum
coated with GPS at nominal mass 71. The most intense peak may be assigned to AlOSi+ and
provides proof of silane bonding to aluminum substrate. The difference in mass between
assignments and recorded masses is provided in mmu
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a silanol is clearly visible, as shown by the fragment AlOSi+. This was initially shown by Gettings
and Kinloch at nominal mass m/z=100u for FeOSi+ easier to demonstrate in spite of the lack of
mass resolution because of its occurrence at an even mass which is unusual for most organic
fragments, as shown in >Fig. 11.6 (Gettings and Kinloch 1977).
11.3.2.2 Interactions with a Polymer or Polymeric Substrate
Although the interaction of organosilanes to metals or more generally surface-bearing hydroxylfunctionalities is usually well known, it is another matter when interactions of the same
molecules to polymer are concerned. If the silane has been selected properly, it is usually
anticipated that the organic functionality borne by the silane will interact with the polymer of
choice in some way either by forming a covalent bond or a Lewis (electronic type of interac-
tion) acid-base type of bond. This is exemplified easily when a silane is incorporated in an
epoxy resin; as most structural adhesives contain an epoxy part and an amine curing agent, one
0
100
1000
Y i e l d ( c o u n t s / s )
10 000
100 000
20 40 60
Polysiloxane
formation
(Atomic mass units)
bonds
– Fe – O – Si
80 100 120
FeAlO2+
FeSiO+
FeO2–
SiO2–
SiO3H–
SiO2H–
SiO3
–
FeO–
FeO+
SiOH+
Fe2+
FeH+
Fe+
Ca+
N2+
Si+
K+
Al+Na+CH
x
+
Cl–
OH–
O–
C2–
H–
. Fig. 11.6
ToF-SIMS spectrum in the positive mode of detection of steel coated with silane, showing
the presence of an ion at nominal mass m/z=100u and indicating the presence of FeOSi+ resulting
from bond formation of steel with silanols
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can see that an aminosilane will also react with the epoxy resin, and that an epoxy silane (such
as g-glycidoxy propyl trimethoxy silane or GPS) will react with the curing agent. Such
a reaction was shown to occur by Rattana et al. using thin film deposition of an amine-
curing agent on the top of an epoxy silane. The reaction mechanism is the same as seen for thecuring of the resin itself (Rattana et al. 2002). Other authors have shown similarly that
a methacrylate-functionalized silane will bond to an unsaturated polyester resin (Dow Corning
2011). Just as for any interactions with metals, any film formed by the silanes on polymeric
surfaces cannot be assimilated to a simple monolayer and, in the case of two organic materials
(polymer/silane), this leads to the hypothesis proposed initially by Pluedemann that a silane
coupling agent can form an interpenetrating network as a possible mechanism of adhesion
(Pluedemann 1991). This provides another mode of interaction between silanes and polymers,
where no chemical reaction is necessary.
11.3.3 Reactions When not in Solution
Silanes can be incorporated in formulations as will be discussed later in this work. However,
and for the sake of clarity, reactions in a bulk material will be briefly covered here. Silanes used
in formulations can be used for different purposes but, most of the time, they are present in
order to aid bonding between the matrix of choice (whether adhesive, ink, or else) and the
chosen substrate. In order to do so, they have to undergo the exact same reactions that a silane
and/or a primer prepared from a solution should, namely hydrolysis and condensation. The
particular difficulty resides in the observation of reactions within a system which may containother sources of silicon and where the silane is introduced at small concentrations of the order
of 1% (w/w). Abel et al. have had some success in observing the reactions undergone, in
particular, by an aminosilane within an epoxy system. Using ToF-SIMS, it was possible to
observe peaks which are indicative of hydrolysis at nominal mass 79 (Si(OH)3+) as well as other
fragments characteristic of condensation on both substrate (bonding) and as self (polymeri-
zation) (mass 71 and 72, for respectively AlOSi+ and Si2O+). In these figures, two
superimposed spectra are shown, and they correspond to two different regions close to the
interface of an adhesive-containing APS deposited on aluminum foil. An example of such
fragments is shown in > Figs. 11.7 , > 11.8 , and > 11.9 , where high-mass resolution spectra areshown for the masses mentioned above, together with other fragments present at the same
nominal mass. The lower spectrum corresponds to the region closer to the substrate for
nominal mass 79 and 72 while AlOSi+ was only observed for this region. For such reactions
to occur, water must be present in some form; the most obvious way in which water may be
found in a system is within the matrix itself or adsorbed on the substrate. Increase of
temperature when curing an adhesive, for example, may be a further incentive for bond
formation, be it covalent bond formation with the susbtrate or self-condensation. When an
aminosilane is used, there is no doubt that the autocatalysis is aiding the reactions to occur as
well as their identification.
11.4 Uses of Organosilanes Other than Adhesion Promoter
It is actually useful to look at the actual application for which organosilanes can be used rather
than a particular field as well as the actual properties of materials provided by silanes. The
major use of silane is obviously in the context of adhesion and very specifically as an adhesion
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promoter; however, this concept can be turned on its head and provide functionalized surfaces
where silanes do not interact or with very small interactions. Most silanes bear alkoxysilanesand another functionality; usually the latter is designed to interact with another material, but it
is possible to prepare silanes bearing an alkyl chain opposite to the alkoxys. This results in
a molecule only capable to interact with van der Waals interactions. Such molecules when
grafted/bonded to a surface of choice may therefore be used as water repellent and contribute
to properties of materials in this manner. Apart from the obvious usage, this may, for example,
provide such properties as thyxotropy to adhesive and paints and also help in increasing
toughness of materials as a ‘‘non-interaction’’ as the interface of two dissimilar materials can
provide a deflection path for a fracture crack and hence increase resulting toughness. Another
application uses the same concept as an adhesion promoter, and this is when a silane is used asa cross-linker. The very same functionalities used to promote adhesion can also be used within
a matrix or formulation and promote cross-linking by forming bonds with neighbouring
functionalities. Silanes can also impart temperature resistance, and to do so, the organic
functionalities that the silane must bear are very specific. A good example of this is a phenyl
functionality, which is not surprising as such chemistry also provides higher temperature
resistance to molecules that bear it. Other applications include mechanical improvement with
x102
x102
C6H7+
C5H3O+
AlO3H4+
Si(OH)3+
35 ppm
M/ D
M>7400
3.0
5.0
I n t e n s i t y
1.0
2.5
3.5
0.5
1.5
I n t e n s i t y
mass / u
78.96 79.00 79.04 79.08
. Fig. 11.7
High-mass resolution at nominal mass 79, showing the presence of Si(OH)3+ and indicating that
the silane incorporated in the formulation has hydrolyzed
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abrasion resistance as well as corrosion protection which will be described in more detail in > Sect. 11.6 as it is invariably connected to adhesion.
11.5 Selection of a Coupling Agent
The general rules when choosing a silane is to work with a molecule which will be compatible
with the system considered. This is why often the chemistry of the silane used will be consideredand will provide a fairly good rule of thumb for selection. Examples include epoxy silane for
epoxy adhesives, although an aminosilane can work as well, and vinyl can be used for rubber.
The compatibility in the chemistry goes further than just exhibiting similar functional groups.
Another way to choose a silane is of course to consider which type of reaction such molecule
may undergo. For example, a vinyl silane will be compatible with a system that reacts through
free radical process/polymerization. In that sense, an epoxy silane such as g-glycidoxy propyl
trimethoxy silane is compatible with an epoxy resin not only on the account of similar
chemistry but because it will undergo the exact same reaction through the epoxy ring than
cure reactions based on the same functionality in the resin/adhesive.
11.6 Organosilanes as Primers
When a material is to be bonded to another, various case scenarios may be considered. One
would, of course, like to improve adhesion between these materials by treating the surface in
some way which can be observed macroscopically by increased wetting. However, other
x102
5.0 AlOSi+
4.0 10 ppm
M/ D M>9500
2.0
I n t e n s i t y
1.0
SiC2H3O+
Al2OH+
71Ga+
C5H11+
C4H7O+
mass / u
70.95 71.00 71.05 71.10
. Fig. 11.8
High-mass resolution at nominal mass 71, showing the presence of AlOSi+ and indicating that the
silane incorporated in the formulation has bonded with the substrate
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considerations include protection of the substrate against corrosion or oxidation, especially
but not only if metallic, as well as increasing the surface area available for bonding. These aims
may be achieved in many ways with a treatment of the surface to bond. Abrasive mechanical,chemical, plasma, flame and coatings treatments are available and produce surfaces with
improved, often combined characteristics (like higher surface energy and increased surface area
by example), and properties which vary in benefit according to whatever technique is used and
which effect is desired as well as how some of these methods are combined (abrasion plus primer).
One method that combines in attaining several of the properties considered in the previous
paragraph is the application of a primer. This is achieved by depositing a relatively thin film of
material, usually in liquid form, on the material to be bonded or coated. The role of the primer
is to act as some kind of barrier (against possible corrosion for example) and/or to provide
functionalities for bonding and hence make the two surfaces to be bonded more compatible.One particular type of molecules which seems to be used with some success in both cases is
organosilanes. The use of silanes in such applications is popular because of its relative
harmlessness compared to for example chromium VI based treatments. They can also be
prepared in aqueous solutions and this helps for applications where volatile organic com-
pounds are either prohibited or reduced considerably as indicated by recent European Direc-
tives making them a good application in an industrial context. As described above these
x102
x102
2.5
C5H
12
1.0
1.5
2.0
I n t e n s i t y
Si2O
4.0
0.5
C3H6NO
2.0
3.0
I n t e n s i t y
mass / u
71.95 72.00 72.05 72.10 72.15
1.0
. Fig. 11.9
High-mass resolution at nominal mass 72, showing the presence of Si2O+ and indicating that the
silane incorporated in the formulation has polymerized
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molecules were designed in order to compatibilize glass and coatings and in particular in order
to increase durability of matrix to glass fibers in composites acting in stopping untimely
hydrolysis of the glass acting therefore both as a barrier and providing ideal chemistry at the
interface. Subsequently it was also found that silanes could be used on metals and more recently on polymers. Their use will be examined in the light of two of the mains aims for primers: as
a barrier and creation of functionalities/compatibilization with other materials. In other words
in two domains where silanes have potentially a great importance: adhesion and corrosion
sciences.
11.6.1 Organosilanes as Adhesion Primers
Many parameters may influence the mechanical properties of a joint formed with a silaneprimer and the interactions formed when depositing the silane. One can roughly divide them
in three categories prior to implementation in the field. Firstly, the solution used initially, then
the application, and finally the curing of the coating and/or adhesive deposited on the top of
the silane layer. If one considers the solution first, the tenor of using a silane primer or coating
properly is to know the chemistry. One point which is never emphasized enough is that one
should advise strongly to use a solution which only contains monomers of the hydrolyzed
silane of choice or failing that a solution that does not contain oligomers of the silane of choice.
Bertelsen and Boerio have shown quite cleverly using both fracture mechanics as well 29Si NMR
and vibrational spectroscopy that poor durability of a joint formed with an epoxy adhesive and
a hydrolyzed epoxy silane could be explained by the oligomerization of the solution used which
is related with time and hence shelf life (Bertelsen and Boerio 2001). > Figure 11.10 shows the
relationship between the amount of polymerization present in the solution and the correlation
with the onset of the increase of the crack length of a test joint. In other words, the higher the
amount of oligomers/polymerization in the solution, the least durable is the joint. Similarly,
other coworkers have shown that the amount of adsorption of g-GPS decreases as a function of
hydrolysis time which is consistent with Bertelsen and Boerio’s studies (Abel et al. 2000). This
also undoubtedly illustrates the importance of the hydrolysis parameters in solutions which
include, to name but a few, hydrolysis time, pH of the solution, concentration of silane-used
solvents (from aqueous only to mixtures of alcohol). Porrit underwent a very exhaustive study of the parameters which can influence durability when a particular silane primer is used in an
epoxy adhesive joint (Porrit 2001). The parameters considered included details concerning the
solution used (as exemplified by the sentence above) but also considered the substrate surface
treatment prior to coating the silane solution, mode of deposition such as dipping or brushing
the solution on, as well as time and temperature of cure for the adhesive. In his work, the
durability tests were performed using the rather convenient and quick Boeing wedge test, and
he was able to eliminate unfavourable conditions and obtain the optimum concentration of
silane, time, and pH of hydrolysis for an aqueous solution, lag time between drying and
bonding, mode of deposition, time and temperature of drying. Parameters considered forthe surface treatment included type of cleaning and, type of surface roughening. This shows
that to obtain the full benefits of silane usage, one must know in detail not only the chemistry
but also the favourable or unfavourable conditions such application may encounter once
deposited on a substrate. This section is going to review some of these parameters.
First and foremost, the cleanliness of the substrate should be of uppermost importance but
is not always necessary, which confuses the silane application issue further. Usually, one should
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deposit a coating or film on a clean surface to avoid any weak boundary effect, but it is not
unusual to see that some adhesives and/or coatings subsequently deposited on a surface can
absorb the contaminants or displace them (usual culprits include and not particularly in
industry only polydimethyl siloxane (PDMS) and materials akin to polytetrafluroethylene
better known under the designation Teflon™ or PTFE). For example, Watts and Pricket haveshown that adhesion to composite samples exhibiting PDMS on their surface can be good and
explained this fact by assuming that the contaminant was being absorbed by the adhesive
(Pricket 2003). Producing a similar final effect, it is assumed that silanes deposited on a surface
can displace contamination. This is probably true of organics but is not the case for salts such as
carbonates, and one should be aware that anything present on the surface prior to bonding may
compromise strength and/or durability of a joint.
0
20
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
30 40 50
Oligomer Concentration (%)
Initial crack length T o t a l c r a c k l e n g t h ( i n )
60 70 80 90
0
10
20
30
40
50
60
70
80
90%SiOH
%Dimer
%Network
2 4 6 8
Hydrolysis time
b
a
% C
o m p o s i t i o n
10 12 14 16 18 20 22
. Fig. 11.10
(a) Relationship between composition and hydrolysis time for a 10% solution of g-GPS inwater. (b) Relationship between the composition in a 10% solution and total crack length of
wedge test specimens prepared from adherends treated with a 1% g-GPS solution
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Another point of importance is the roughness of the substrate. It is usually thought that the
rougher the surface, the better the adhesion or at least the mechanical anchorage of the coating
on a substrate. Actually, what roughening produces in indeed an increase in surface area available
for any coating, but this translates in particular into an increased number of available bonding
sites. This is one of the reasons why phosphoric or chromic acid anodizing are so attractive for
aluminum-based alloys notwithstanding their barrier properties. When Porritt studied the effect
of surface preparation on the durability of joint prepared, the best effect was reported for
a ‘‘boiled’’ aluminum surface where the microstructure obtained resembles that of phosphoric
acid anodizing albeit at a smaller scale. The various parameters that were shown to be of
importance were as follows: pH of the water used to hydrolyze GPS, time of hydrolysis, conditions
of deposition, temperature of drying, and concentration of the solution. >Figure 11.11 shows
the results of the measurements of the fracture energy resulting from the optimized surfacetreatment using g-GPS. This figure actually shows two optimized treatments: one optimized
treatment corresponds to the best parameters as cited above, while the best treatment utilizes
the same parameters and 1 h hydration (‘‘boiling’’) of the aluminum prior to deposition of the
silane primer. It is rather remarkable that this final treatment happens to exhibit even better
durability than the system usually assumed to be the best in terms of aluminum-based alloys
for aerospace, namely chromic acid anodizing and phosphoric acid anodizing.
11.6.2 Organosilane as Corrosion Protection Films
The advantage of a protective coating containing silanes is supposed to be twofold: it can
protect the metal on which the film will be applied, and it is also supposed to interact
chemically with both substrate and covering coating. The treatment usually consists into the
application of a mixture of silanes as it has been shown that a mixture exhibits a better
performance in terms of corrosion protection than a single silane solution (van Ooij et al. 2005).
2.0
2.5 1 hr hydration + silane
Optimised silane pre-treatment
Grit-blasted only
1.5
CAA pre-treatmentPAA pre-treatment
0.0
0.5 F r a c t u r e e n e r g y ( k J m – 2
)
1.0
0 20 40 60 80 100 120 140 160 180
Exposure time (hrs)
. Fig. 11.11
Fracture energy comparison for several treatments, including two optimized silane treatments.
CAA denotes chromic acid anodizing, while PAA means phosphoric acid anodizing
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Much has been done to ascertain the efficacy of such corrosion protection primers as shown
by one of the main protagonist of such methods, Win van Ooij based at the University of
Cincinnati (van Ooij et al. 2005; Zhu and van Ooij 2004).
The mixture is typically a combination of ‘‘monosilanes’’ and ‘‘bis-silanes,’’ where theformula of the former is X 3Si(CH2)nY and the later is X 3Si(CH2)nY(CH2)nSiX 3 (or even X 3Si
(CH2)mSiX 3). X represents the hydrolysable group and Y a functional group such as an amine
or vinyl. The bis-silanes provide a pronounced hydrophobic polysiloxane layer resulting from
SiOSi linkages. Unfortunately, such molecules are usually fairly hydrophobic and make the use
of other solvents than water necessary, which is a clear handicap for the use of such solutions.
Such systems include the use of bis-[triethoxysilyl]ethane (BTSE, (OC2H5)3Si(CH2)2Si)).
However, bis-silanes functionalized with amines are compatible with aqueous solutions and
have allegedly shown the same resistance to corrosion as chromate based systems. Such systems
are tested for corrosion resistance using various methods, mostly electrochemistry based.
11.7 Silanes in Formulations
Silanes can be used in various ways, but when it comes to adhesion, they are either deposited as
a coating or are incorporated within formulations. Those formulations include adhesives,
coatings, and inks, but the list is not exhaustive. The intention is for these molecules to find
their way toward the substrate and help bonding at the interface by rendering the substrate and
the material deposited on it more compatible. This kind of application is important as it helps
reducing time of application and hence saves money but also because silanes are considered as
a substitute to many other not so green materials. Using silanes in such a way can be considered
as a ‘‘built-in’’ (within adhesive or coating for example) primer deposition as it is possible to
show that a silane incorporated within a formulation will diffuse toward the desired interface.
This is why relevant research will be described in this section.
The main problem encountered here is that there is hardly any control of the rate of the
reactions happening within the system and similarly no control of the diffusion be it rate
amount, type, etc. To confuse the issue further, many manufacturers will incorporate several
organosilanes within a system on the off-chance that they have ‘‘missed something’’ without
regard for the possible consequences on the interactions at the interface and durability of such joints. Although not many studies have been performed on the subject due to the intrinsic
difficulty of the research, two were performed at the University of Surrey with two very
different materials. One was concerned with the adhesion of polyamide coatings to steel in
order to protect such items as dishwasher baskets from damage and corrosion; the other with
the adhesion of an epoxy-based adhesive on aluminum to avoid the use of a silane (or other)
primer. In both cases, only one organosilane was incorporated, and the aim was for the
durability and/or protection resistance of the coating to be improved compared to
a formulation without silane. In accordance with the general rules of silane choice, an epoxy
functionalized (GPS) was chosen for the epoxy adhesive or an aminosilane (APS) for either theadhesive or the polyamide material (Guichenuy et al. 2004; 2006a, b). In both cases, it was
found that the addition of a silane is beneficial to durability and/or corrosion resistance. It was
also possible to follow the diffusion of respective silanes using the now well-known method of
ultralow-angle microtomy (ULAM) which allows access, amongst other advantages, to the still
bonded material and hence the intact interface between either epoxy adhesive and aluminum
or polyamide and steel (Hinder et al. 2005). > Figure 11.12 shows an example of reconstructed
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diffusion profile of the concentration of silicon from the interface (distance zero) with the
substrate toward the bulk of materials for an epoxy system. The analysis was performed using
XPS after microtoming samples and calculation of the actual depth covered through ULAM cut
(this is achieved through a simple trigonometry calculation, for more details see (Hinder et al.
2005)). Silicon is obviously used as a marker for the silane, and all precautions have been taken to
eliminate other sources of silicon, including the silica (incidentally functionalized with an alkyl
silane), usually included in the formulation to achieve thixotropy of the adhesive. One can see
that the concentration of silicon is higher toward the substrate for aluminium whereas it has been
shown that it increases also for while it was shown to be higher toward the steel and the air/
polyamide interface in Guichenuy et al.’s work (Guichenuy et al. 2004, 2006a, b). This indicates
that the silane has indeed diffused toward the desired interface but also toward the external
surface of the coating. One can point out that the diffusion seems to occur in a similar way and
achieve similar concentration of silicon at interfaces around 1–2 at.%. In the two examples
provided, only the silanes are identical, while all other materials and conditions are totally different: two different substrates, a thermoplastic versus a thermoset as polymeric matrix
which implies that the factors determinating the diffusion are similar and correspond to the
molecule properties and behavior rather than anything else. This diffusion is most likely to
occur when either material is still in a liquid/soft form with the difference that the temperature
used is not the same, although it does not seem to be reflected in the diffusion.
Several driving forces can be invoked here. In order for the molecule to segregate to the
interface with the substrate, it has to be mobile enough, and the local enrichment has to be
driven by thermodynamic considerations. They have been described in detail by Abel and
Watts as well as by Guichenuy (2006b), but they can be summed up here. The metallic substratecapable of forming bonds with the silane may act as a sink for the silane molecules, solubility
parameters should be considered also to assess the compatibility of matrix to silane to check on
exclusions mechanisms as well as possible of interdiffusion with matrix as shown by Gentle
et al. (1992). Another possible explanation lies into the value of the chemical potential (partial
molar Gibbs free energy) of the silane, within the matrix considered as flow will occur
spontaneously from a region of high chemical potential to a region of low chemical potential.
2.4
1.8
0.6
1.2
[ S i ] ( a t . % )
0 8 16 24 32 400.0
Distance (um)
0%
0.5%
1%
2%
. Fig. 11.12
Schematic indicating diffusion of silane within a formulation. On the left is the interface with
aluminum and, on the right, the bulk of the adhesive
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Several concentrations were tested, and one can find that if such a system is to be used, the
concentration to be used has to be optimized and most likely will go an optimum then exhibit
a detrimental effect on the properties considered. For example, adhesives containing a range of
concentrations of GPS or APS were tested, and it became clear that too much silane withina formulation can exhibit similar properties to a formulation without silane, exemplifying
beautifully the famous saying of ‘‘overegging the pudding.’’
11.8 Non-silane Coupling Agents and/or Adhesion Promoters
Although silane adhesion promoters are probably the most known, used, and versatile, there
are many others. Some are based on titanium, chromium, or zirconium, while others, still
based on silicon chemistry totally differ in their structure from regular organosilanes. This
section will highlight briefly a few examples with their chemistry, mechanisms, and fields of applications.
11.8.1 Metal-Based Coupling Agents: Example of Zirconium andTitanium
Together with titanium, zirconium has a similar chemistry to that of silicon because of the
group they belong to in the periodic table of elements. Zirconium tends to form polymeric
species both in aqueous and solvent-based solution chemistry with a clear tendency to formbonds with oxygen. The zirconium compounds are classified as either anionic cationic or
neutral. If organic species are involved in a system, it is generally admitted that zirconium will
react readily with carboxyl groups rather than oxygenated functions such as an ether com-
pound. They find applications, for example, in the industry of printing inks, lithographic
printing plates for which they also contribute to corrosion resistance, aluminum drinking cans
and pigment coating, as well as dental coupling agent between enamel polymeric filler. Several
mechanisms can be put forward, and they include hydrogen bonding with zirconium as well as
reactions with carbonyl functionalities. Titanium complexes on the other hand are believed to
interact through surface hydroxyl groups. Other adhesion promoters based on other metals arealso available, including one of the earliest known in the field dating from the 1960s and based
on a methacrylate-chrome complex (Volan®).
11.8.2 Other Silicon-Based Coupling Agents
Some other coupling agents more recently highlighted include molecules which are again based
on silane chemistry but do not have the same structure as organosilanes. Such an example is the
use of cyclic azasilanes in particular in the field of microelectronics and optoelectronics. The
interest resides in a usage for a field in which the presence of water is totally undesirable andallows the avoidance of parasite by-products with the surface of nano-objects. Another reason
for which the conventional adhesion promoters cannot be used is the propensity of
organosilanes to self-condense or polymerize, inducing the creation of nanoscale domains
thus potentially creating disturbances in the system or compromising its properties. For such
an application, volatile molecules are needed to be able to use vapor phase deposition, one of
the main deposition methods at the nanoscale. Another factor consists into the poor amount of
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sites functionalized using conventional silanes. If one considers pyrogenic silica, for example,
only half at most will be functionalized which is far from desirable. Arkles et al. ( 2004) have
shown the potential of such molecules and synthesized seven of this type with various
functionalities on both the silicon and nitrogen atoms, including alkoxy functionalities witheither one or two cycles in their structures. Cyclic azasilane molecules have been used to modify
fillers with a showing higher bonding efficiency than a linear analogue (Vendamuthu et al.
2002), achieving a high-density monolayer deposit. The authors also concluded that although
nitrogen had a catalytic action on bonding, further activity is provided by a ring-opening
reaction.
11.9 Conclusions
Many parameters can influence the application of organosilane to whichever system is of interest. The major obstacle in their use is often the lack of knowledge of their mechanism as
well as their chemistry, and although one would like to be able to pick any molecule and get on
with work, this is not truly possible when using silanes. The only true answer is to test the
system, coating, or formulation of interest to ascertain which are the optimum conditions to
use a silane or even a mixture as is often done for corrosion protection for example or for
formulations sometimes on the ‘‘of chance’’ to miss something out!!! However, when chosen
carefully, silanes can provide an exceptional primer treatment even more performing than
accepted though soon to be redundant surface treatments such as chromium VI based rinses.
Testing does not have to always involve complex instrumentation, and often industry will usesimple tests to decide on optimum conditions and concentrations. These molecules can find
applications in so many domains that a little care and dispersing a few myths is well worth the
effort and, hopefully, this work will have helped in achieving this goal.
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