Organosilanes Adhesion

8/20/2019 Organosilanes Adhesion

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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.

238 11 Organosilanes: Adhesion Promoters and Primers



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

Organosilanes: Adhesion Promoters and Primers 11 239





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




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




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.




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




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




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




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




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




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




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




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




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




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




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




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




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




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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Organosilanes Adhesion