Understanding the Relationship Between Silane Application Conditions

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International Journal of Adhesion & Adhesives 26 (2006) 215

Understanding the relationship between silane application conditions,

bond durability and locus of failure

M.-L. Abela, R.D. Allingtonb, R.P. Digbyc, N. Porrittd, S.J. Shawb,, J.F. Wattsa

aSchool of Engineering, University of Surrey, Guildford, UKbDefence Science and Technology Laboratory, Porton Down, Wiltshire, UK

cAirbus, Filton, Bristol, UKdFuture Systems Technology Division, QinetiQ, Farnborough, Hampshire, UK

Accepted 10 March 2005

Available online 17 May 2005

Abstract

The extent to which an organosilane surface treatment regime can promote durability enhancement of an adhesively bonded

aluminium alloy system has been determined. Results have revealed the range of application and film-conditioning parameters

which contribute to joint durability in a simple Boeing wedge joint. Organosilane solution parameters relating to solvent type,

solution concentration, pH and hydrolysis time have all been shown to influence resultant durability. Interestingly, parameters such

as film drying temperature and in-process time delay (time interval between application of the organosilane to the alloy surface and

subsequent bonding) have little influence on joint performance. The factors responsible for the durability variations observed have

been considered using various surface analytical techniques. Superficially, failure surfaces indicative of interfacial failure between

substrate and adhesive have been observed. More detailed characterisation using both XPS and SIMS has indicated failure processes

associated with a diffusion zone comprising aluminium oxide and the organosilane.

Crown Copyright r 2005 Published by Elsevier Ltd. All rights reserved.

Keywords: Organosilanes; Surface treatments; Moisture resistance; Durability

1. Introduction

The use of adhesive bonding in the manufacture of

load bearing structures can provide numerous benefits in

comparison to more traditional joining techniques such

as mechanical fastening. Such advantages include

improved performance characteristics, resulting largely

from the weight reductions adhesive bonding canprovide, together with significant reductions in both

procurement and life-cycle maintainability costs.

Unfortunately a lack of confidence in the ability of

bonded joints to withstand the range of environmental

and loading conditions typically encountered in many

applications has prevented their widespread use. One

issue of concern has been associated with atmospheric

moisture. Many studies and much experience with

bonded structures have demonstrated the adverse effect

moisture can have on a bonded joint[1].In particular, in

many of these investigations, the precise nature of

environmentally driven failure has been shown to be

associated with the substrateadhesive interphase, thusshowing this region to be the primary zone of weakness.

Thus, for bonded aluminium structures, in order to

promote long-term durability, surface treatment of the

alloy prior to bonding has generally been regarded as a

vital component of the manufacturing process.

Currently much evidence exists indicating that the

strongest and most durable adhesive bonds to many

types of metallic substrate are provided by pre-treating

alloys with a range of aggressive and toxic chemicals.

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0143-7496/$ - see front matter Crown Copyrightr 2005 Published by Elsevier Ltd. All rights reserved.

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Corresponding author. Tel.: +441980614989;

fax: +441980613611.

E-mail address: [email protected] (S.J. Shaw).
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This has been particularly true of aluminium alloys

where extensive use has been made of hexavalent

chromium compounds and strong acids in surface

treatment specifications. However, in recent years there

has been growing pressure from government and

environmental bodies to remove such substances from

manufacturing processes. To this end considerable efforthas been directed toward the application of organosi-

lanes as a potential pre-treatment process[27]. Indeed,

organosilanes have been employed as the basis for

adhesion promoting surface treatments for many years.

In particular, their use in promoting moisture stability in

glass reinforced plastics applications are well known[3].

In addition, they have also been considered and used in

various adhesive bonding applications, both in terms of

initial manufacture and repair. Thus, although con-

siderable experience has been gained in the use of

organosilanes as adhesion promoting substances, most if

not all current applications are based on empirically

determined silane formulations and application proce-

dures. Thus, the most efficient methods of application,

together with the mechanisms by which these materials

enhance (and on some occasions do not enhance) the

strength and durability of bonded joints, has not been

fully understood.

In an attempt to enhance our knowledge of organo-

silanes as the basis of surface treatments for aluminium

alloys, our laboratories have led and participated in an

extensive collaborative research project. Known as the

International Collaborative Programme on Organosi-

lane Adhesion Promoters (ICOSAP)[8], this has had, as

its primary aim, the development of a detailed under-standing of the relationships between silane application

variables, solution and surface chemistry, joint dur-

ability and mechanisms of failure. The results obtained

from much of this investigation are considered and

discussed in this paper.

2. Experimental

2.1. Materials

The organosilane employed throughout this investi-gation has been a proprietary system based on g-

glycidoxypropyltrimethoxysilane (g-GPS). The substrate

material employed was 2024 T3 unclad aluminium alloy.

The adhesive used was a 120 1C curing proprietary film

adhesive based on a toughened epoxy. This system was

selected due to the absence of silane coupling agent in

the formulation.

2.2. Experimental Procedures

All joint durability experiments were conducted using

the Boeing wedge joint in accordance with ASTM D

3762-79. The test arrangement employed is shown in

Fig. 1. Prior to surface treatment and bonding, each

25mm 150 mm adherend was chamfered at one end to

aid the eventual insertion of the wedge for durability

assessment. The substrates were then degreased using

fine grade Scotch Brite abrasive pads with commercial

grade liquid detergent, followed by rinsing in runningtap water whilst subjecting the adherends to further

abrasion with a fresh piece of Scotch Brite. The

adherends were then dried with unpigmented tissue

paper followed by grit blasting with 50 mm alumina grit.

Prior to use, g-GPS was removed from cold storage and

allowed sufficient time to reach ambient temperature.

The main silane solution and application variables

considered in this programme were:

a) solution pH

b) silane concentration

c) the nature of the solventd) hydrolysis time

e) drying temperature

f) time lag between silane application and bonding.

To provide a starting point for the investigation, the

silane pretreatment process initially selected was based

on a 1% silane solution (aqueous) in which pH was

controlled at a value of 5. Prior to application of the

solution to the alloy adherends, a pre-hydrolysis time of

1 h was allowed and, following application, the resultant

silane film was dried at a temperature of 93 1C. Each

process variable was evaluated in turn, optimised andthen fixed so that the next variable could be examined

and optimised.

The pH was controlled by acid (acetic) or alkali

(sodium hydroxide) adjustment, with a pH range of

from 3 to 11 studied. g-GPS concentrations of from 0.1

to 12% vol/vol were examined, with solution hydrolysis

times ranging from 10 min to 48 h. In addition to pure

aqueous solutions, water/methanol solvent combina-

tions were also examined with effort focusing on

methanol concentrations of 10, 50 and 90%.

Following preparation of the silane solution, and the

required period of hydrolysis, the silane solution wasapplied by brush onto the grit-blasted adherend surface,

with brushing continuing for a 10-min period. The

adherends were then placed on edge and tapped on dry

tissue paper so as to remove excess silane solution. The

adherends were then placed horizontally, pre-treated

side up, on aluminium foil and inserted into a fan-

assisted oven for accelerated drying at 93 1C for 1h.

Following drying, the adherends were removed from the

oven and placed on clean aluminium foil and allowed to

cool to below 30 1C at ambient temperature. Although

most of the experiments were conducted with a drying

temperature of 93 1C, a set of experiments were carried

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M.-L. Abel et al. / International Journal of Adhesion & Adhesives 26 (2006) 215 3


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out with drying at 23 1C so as to assess the importance

of this variable on joint durability.

Following surface pre-treatment, adhesive film pre-

viously cut to the required dimensions was sandwiched

between two prepared adherend coupons. The resultant

wedge test specimens were then sandwiched between two

stainless steel plates, with PTFE coated non-porous

glass cloth used as a release ply. All test specimens were

shimmed using aluminium spacers to give a bond linethickness of 0.2 mm and subsequently cured at 1201C

for 60 min under a pressure of 0.21 MPa. Throughout

this work five replicates per experimental condition were

employed.

Following cure, the long edges of the joints were

subjected to a polishing procedure so as to aid

inspection of the bondline during ageing. A wedge was

then carefully driven into the unbonded end of the joint,

with the entire specimen then being placed in a

desiccator at 23 1C for 1 h. The length of the initial

crack introduced into the bondline by insertion of the

wedge was then determined using a travelling micro-scope. Any further advance of the crack was then

assessed as a function of time during exposure of the

joint to 96% RH at 50 1C for 7 days (achieved using a

potassium sulphate saturated salt solution).

Following removal of the joints from the ageing

environment, the specimens were prized apart at the

bondline so as to provide evidence as to the nature of the

failure process responsible for any advancement of the

initial crack. In addition to visual observation of the

failure surfaces, X-ray photoelectron spectroscopy

(XPS) and time of flight secondary ion mass spectro-

metry (ToF-SIMS) were employed with selected joints

to provide a detailed indication of the nature of the

surfaces exposed after the environmentally induced

failure. In the case of XPS, surface analysis was carried

out using a VG Scientific ESCALAB Mk II system

operated in the constant analyser mode, at a pass energy

of 50 eV. An MgKaX-ray source was used and take off

angle was set at 451.

ToF-SIMS analysis was carried out on a VG Scientific

type 23 system equipped with a double stage time offlight analyser and a MIG 300 PB pulsed liquid metal

gallium ion source. Specimens were etched in the pulsed

analyser mode with spectra collected after 0, 1.5, 2.5, 3.5

and 4.5 h pulsed etching.

3. Results and discussion

As previously indicated, the starting point for the

silane solution variable study was an aqueous solution

with 1% silane and pH 5, subjected to a pre-hydrolysis

time of 1 h. Following application of the solution to thealuminium alloy surface, the resultant silane film was

dried at 93 1C for 1h.

Boeing wedge test results obtained from joints

subjected to this pre-treatment procedure are shown in

Fig. 2. Also shown is data relating to both a simple grit-

blast and degrease surface treatment, together with

results obtained from joints previously subjected to acid

anodising surface treatment techniques.

In this figure, and throughout this discussion, fracture

energy values obtained from the wedge tests are

indicated as functions of time in the environment. The

fracture energy values were calculated from knowledge

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3.2

20

25

25

150

20

Unbonded

Adhesive

Fig. 1. Boeing wedge joint.

M.-L. Abel et al. / International Journal of Adhesion & Adhesives 26 (2006) 2154


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These reveal the substantial extent to which solution

pH influences the durability enhancing characteristics of

a 1% g-GPS aqueous solution. Perhaps the first point to

note is that all the silane solutions, irrespective of

solution pH, promoted durability characteristics super-

ior to the base line, grit-blast only specimens. However,

the best performing system was the pH 5.0 solution withpH solutions of 3, 9, and 11 exhibiting similar but

slightly lower GIc values after 72 h joint exposure.

Interestingly, a pH of 7 was shown to promote

environmental resistance significantly inferior to all

other solution pH conditions.

To assess the extent to which solution pH exhibits

similar effects at other silane solution concentrations,

further Boeing wedge experiments were conducted with

silane concentrations ranging from 0.1 to 12%. Fracture

energy, GIc, data obtained from the longest exposure

times employed (approximately 168 h) are shown in

Table 1 for some of the solution concentration/pH

combinations examined. The results reveal extremely

complex trends with, at each of the solution concentra-

tions studied, there existing an optimum solution pH

value, these existing at pH values of 5 for silane

concentrations of 0.1 and 1% and pH 7 for a silane

concentration of 12%, suggesting an increase in

optimum pH with increasing silane concentration.

In attempting to discuss these effects it is necessary to

consider the reactions which are believed to exist within

the aqueous silane solution prior to application to the

alloy surface (Fig. 4).

As indicated, two main reactions occur within the

silane solution. First, in the presence of water the alkoxygroups are sequentially hydrolysed resulting in the

gradual build up of silanol species. Second, the silanols

condense to form oligomers, which gradually increase in

size i.e. converting to dimers/trimers and eventually

larger molecules. In considering these reactions, Erick-

son and Plueddemann have proposed that solution pH

effectively controls both the hydrolysis and condensa-

tion reactions [15]. For example, for organofunctional

trialkoxy silanes in aqueous media, alkoxy silanes

hydrolyse rapidly under mildly acidic conditions to

form monomeric silanols, but condense slowly to

oligomeric variants. However condensation reactions

tend to proceed rapidly at pH values in excess of 7.

These general observations have been confirmed by Xue

et al who observed substantial increases in hydrolysis

rates with aqueous g-GPS solutions on reducing pH

from 6 to 3.5[16]. Tesoro and Wu have reviewed various

aspects of silane solution chemistry and have commen-

ted on the influence of solution pH in relation to bothhydrolysis and condensation reactions [17]. In high-

lighting the importance of these reactions they also

suggested that, for practical adhesion promoting appli-

cations, the effectiveness of the silane was influenced by

the extent of condensation; high levels of condensation

having detrimental effects on adhesion promoting

capability. This would seem logical since the condensa-

tion reactions indicated in Fig. 4 would clearly lead to

the elimination of silanol groups which, if retained,

would promote reaction with surface oxide hydroxyls to

produce a covalent bond. The greater the number of

covalent bonds generated across the interface, the

greater the likelihood of a durable bond. Hence the

apparent necessity for enhancing silane hydrolysis whilst

minimising resultant silanol condensation. Results

obtained from the relatively low silane solution con-

centrations in this study do appear to support this view

with, at silane concentrations of 1%, solution pH values

of 3 and 5 providing the highest GIc values and hence

better durability behaviours. This appears not to be the

case at higher solution concentrations however and the

factors which could be responsible, are discussed in the

next section.

3.2. Influence of silane concentration

Results obtained from Boeing wedge experiments

conducted on joints prepared from silane solution

concentrations ranging from 0.1 to 12%, at a solution

pH of 5, are indicated in Fig. 5.

At this particular pH the results reveal the important

influence of silane concentration on eventual joint

durability performance. Clearly, at this pH, concentra-

tions of 0.5% and 1% provide the highest GIc values

after 168 h exposure and thus the best durability.

Although silane concentrations above and below these

values promote lesser degrees of durability enhance-ment, the results inFig. 5continue to indicate superior

durability enhancement in comparison to base-line data

i.e. joints simply pre-treated via grit-blast/degrease

processes prior to joint formation. Having said this,

the differences between grit-blast/degrease joints and

those prepared from 12% silane solution concentrations

are clearly marginal (at this pH), therefore, indicating

clearly the importance of solution concentration on

eventual joint durability performance.

The extent to which the durability enhancement/silane

concentration relationship varies as a function of

solution pH is indicated in Fig. 6.

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Table 1

The effect of pH on 168h fracture energy for 0.1, 1 and 12% silane

concentrations

Silane pH 168h fracture energy (kJ m2)

0.1% silane 1% silane 12% silane

3 0.142 0.655 0.183

5 0.427 0.708 0.130

7 0.208 0.290 0.569

9 0.210 0.509 0.468

11 0.164 0.541 0.387



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As highlighted above, a pH 5 solution results in an

optimum silane concentration of 1% with a 168-h

exposure GIc value of 0.71 kJm2. Interestingly, two of

the other pH solutions examined, namely 3 and 11, show

similar trends with peaks in durability performance at

1% silane concentrations. The exceptions to this trendare for pH 7 and 9 solutions. With the former a silane

concentration of 5% appears to promote optimum

durability characteristics although it is important to

recognise that problems relating to experimental varia-

bility was encountered with this specific silane concen-

tration. Indeed at all the solution concentration/pH

variations examined, this particular solution (5%, pH 7)

together with pH 5, 1% silane solution concentrations

provided the highest level of durability enhancement

observed. At pH 9, silane concentration appears to have

little influence on extent of durability enhancement at

concentrations of from 1 to 12% (after an initial steep

increase inGIc from 0.1 to 1%). The factors responsible

for these effects will clearly be related to the complex

nature of both solution and interface chemistries and in

particular the manner in which they are influenced by

both silane concentration and solution pH.

In discussing these complex trends it is pertinent tonote that several researchers have considered the issue of

silane solution concentration effects. Sung et al, working

with Al2O3/polyethylene joints modified with g-amino-

propyltriethoxysilane (g-APS), found an interesting

correlation between 1801 peel strength and the concen-

tration of aqueous g-APS solutions [18]. In their work

optimum peel strength was observed at an g-APS

concentration of approximately 2 vol%. Further in-

creases in silane concentration beyond 2% did not

change the peel strength significantly.

Ishida et al have proposed, from FTIR investigations

on g-APS/glass interfaces, a silanetriol-oligomer balance

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R - Si(OR)x

(1 ) HYDROLYSIS R-Si(OR)3

3H2O

3ROH

R-Si(OH)3(2) CONDENSATION

2Si(OH)3

2H2O

SUBSTRATE

OH OH OH

OH OH OH

R R R

HO Si Si OO Si OH

+

Organic group reacts with adhesive Inorganic group reacts with metal

substrates

Fig. 4. Organosilane hydrolysis and condensation reaction mechanisms.



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heavily dependent on silane solution concentration

[19,20]. Indeed, with g-APS, a very low concentration

of 0.15% was observed with concentrations at or below

this value comprising virtually complete monomeric

triol. Increasing oligomer content was observed forsilane concentrations above 0.15%. This would be

expected to have perhaps a significant influence on the

nature of the silane film deposited; low solution

concentrations promoting a more uniform film with

high concentrations promoting defect formation within

the silane interphase. Ishida suggested, as a result, that it

would, therefore, be likely that the mechanical proper-

ties of a resultant composite with a silane above the

transition concentration may not be optimum [20].

Ishida further proposed the possibility that this phe-

nomenon, a so-called onset of association, may not

simply hold true for g-APS and that it may be a general

trend in surface modification with organosilane adhe-

sion promoters. We are not aware of any similar

findings with the organosilane employed in this study

(g-GPS), but it seems reasonable to assume that a

transitionary effect of this type could have influenced

the results in our work.

Kaul et al examined the influence of silane solution

concentration on the durability of Al2O3/polyethylene

joints primed with g-APS [21]. Solution concentration

was shown to exhibit a substantial affect on durability,

with a concentration range of 0.31% providing

optimum performance. Interestingly, concentrationsbeyond 2% were shown to exhibit particularly poor

results, with durabilities roughly comparable to non-

silane treated joints. Similar silane concentration effects

were observed with Al2O3/g-APS/nylon-6 joints with, in

this case, an optimum silane concentration of 0.3%

observed[21]. Indeed, at this specific concentration the

integrity of the silane-modified interfacial region was

enhanced sufficiently to drive failure away from the

normally relatively weaker interfacial zone into the

nylon 6 substrate.

Osterholtz and Pohl have commented on the im-

portance of minimising silanol condensation in aqueoussolutions by maintaining silane concentrations below

1% by weight for what they describe as typical

organofunctional silanes [22]. Kuhbander and Mazza

studied silane solution concentration effects with aqu-

eous solutions of g-GPS and found, via Boeing wedge

tests, that optimum durability performance was pro-

vided by solution concentrations of between 1% and 2%

[13]. As previously mentioned, however, in their work

Kuhbander and Mazza primed all silane treated surfaces

with a chromated-based primer and thus direct compar-

ison with our work is difficult. However, it is of interest

to note that their preferred solution concentrations were

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0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Exposure time (h) to 96 % rh at 50 C

Fractureenergy(kJm

-2)

Base line - no silane pH 5.0 - no silane

0.1% silane 0.5% silane




3.0% silane

Fig. 5. Fracture energy against exposure time for pH 5.0 hydrolysing solution at various silane concentrations.

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12

Silane conc. (%)

GIC(kJm-2)

pH3

pH5

pH7

pH9

pH11

GB only

Fig. 6. Wedge fracture energy as a function of solution concentration

at various pH values.



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similar to the optimum concentrations obtained from

our investigation.

Work by Quinton and Dastoor used SIMS analysis to

build upon earlier work with XPS analysis [23]. In this

study, they were able to demonstrate not only the

variation in silicon containing species on a silane

modified crystalline aluminium oxide surface, but alsovariation in the concentration of Al-O-Si species

associated with the silane layer. SIMS, as with XPS,

indicated a maximum in the concentration of silicon

containing species at a relatively low silane concentra-

tion of 0.75%. Following this low concentration peak,

the concentration of silicon was found to gradually

increase to something approaching an asymptotic

maximum at approximately 12% PTMS concentration.

However the concentration of Al-O-Si species, as

demonstrated by the presence of the mass 71 peak,

although a maximum in concentration was observed at

approximately the same silane concentration, the peak

was followed by a gradual decline in mass 71 intensity

with increasing PTMS concentration. In considering the

factors responsible for this effect Quinton and Dastoor

hypothesised the existence of two distinct surface species

involved in the silane adsorption process i.e. monomeric

silanols and condensed forms of PTMS. At low

concentrations, where the presence of monomeric

silanols would be high, it is reasonable to assume that

a relatively large number of available silanols would

react with surface hydroxyls to produce interfacial

covalent bonds i.e. Al-O-Si bonds. Conversely, at high

PTMS concentrations, for a given combination of

experimental conditions, a significant degree of silanolcondensation would have almost certainly occurred.

Thus the number of silanols available for reaction with

the oxide surface would have been less per unit silicon

concentration.

Thus, the results obtained from our work would

appear to confirm the importance of solution concen-

tration in promoting significant adhesion and durability

enhancement on bonded systems. Indeed, as with our

results, a considerable body of evidence suggests that

silane solution concentrations should be maintained at

fairly low levels; low concentrations being necessary to

minimise the extent to which neighbouring silanetriolmolecules would wish to make contact and undergo

condensation. Although we have no conclusive evidence

to confirm the existence of the onset of association

mechanism, it appears likely that this effect was, partly

at least, responsible for the solution concentration

trends observed in this work. In theory, the GIc v silane

concentration trends highlighted in Fig. 6 would

perhaps be expected to provide some confirmation of

this mechanism. For example, if an onset of association

mechanism is, partly at least, responsible for the silane

concentration effects, variation in solution pH should

have a predictable effect. Specifically, since pH exerts an

influence on the balance between initial silane hydrolysis

and further condensation, then we would expect that

lower pH values would allow the use of higher silane

concentrations since, although silanols would be in close

proximity, the low pH value would retard their desire to

condense. In other words, the peak values ofGIc in the

GIcsilane concentration trends indicated in Fig. 6should move to higher silane concentrations with

decreasing pH. Unfortunately the trends in Fig. 6 are

unable to confirm this hypothesis, an insufficient

amount of data almost certainly contributing to this

fact.

Bearing in mind the optimum solution concentration

observed, for all further activities e.g. studies of solution

age, solvent type, film drying temperature etc, a solution

based upon 1% g-GPS at pH 5 was used so as to

maintain further experimental studies at a manageable

level.

3.3. Nature of the solvent

Although it is widely accepted that organosilanes are

best used in dilute solution, a review of the literature

reveals some ambiguity as to the nature of the solvents

most appropriate for film formation [16,22,2429].

Although aqueous solutions have been studied the

most, many workers have employed solvent combina-

tions, with ethanol or methanol additions to aqueous

solutions being particularly noteworthy. It was, there-

fore, considered useful to examine the nature of the

solvent employed in the silane solutions to assess theeffects on durability performance as measured from

Boeing wedge joints. In this particular case, methanol/

water solvent combinations were investigated ranging

from full aqueous solutions (the primary solvent

employed throughout most of this study) to a 90/10

methanol/water system. Silane concentration was main-

tained at 1% with a pH of 5, with a hydrolysis time of

1 h prior to application of the solution to the alloy

surface. The results obtained are shown in Fig. 7 as

fracture energy, GIc, (as obtained from Boeing wedge

joints after 168 h exposure) plotted against the concen-

tration of methanol in the hydrolysing solution. Asclearly indicated for the solution conditions employed,

the incorporation of methanol into the hydrolysing

solution has a negative effect on GIc and hence joint

durability. Indeed, with a 90/10 methanol/water binary

solvent system, the joint durability characteristics which

result are only marginally superior to those provided by

a simple grit-blast/degrease surface treatment.

Several authors have considered the influence of

solvent modification on solution chemistry and/or

eventual bond performance when employed as an

adhesion promoter. Ishida has proposed that one of

the primary roles of alcohol in aqueous silane solutions

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is to prevent and/or delay the formation of highly

oligomerised siloxane aggregates[30].

Gledhill et al., working with g-GPS on mild steel

substrates, compared an aqueous silane solution with a

95/5 ethanol/water system[25]. With g-GPS maintained

constant at 1% with pH maintained natural, joint

durability was found to be highly dependent on the

nature of the solvent. In this case, the aqueous solutions

were shown to be substantially superior to their ethanol/

water-based counterparts. Again the detailed factorsresponsible for this observation were not discussed,

although it was recognised that silane solution chemistry

was almost certainly responsible for the substantial

differences in performance observed. Thus the results

from this earlier work would seem to support the results

obtained from the current study i.e. that the incorpora-

tion of alcohols into aqueous solutions generally has a

detrimental effect on bond performance, particularly

resistance to hot/wet environments. However, since the

introduction of methanol into the solution will almost

certainly modify the kinetics of both the hydrolysis and

condensation reactions, it is feasible that modificationsto other solution parameters e.g. silane concentration

and hydrolysis time, would have influenced the nature of

the silane adsorbed on to the surface and hence

durability. Further work to investigate this effect would

be of interest.

3.4. Solution hydrolysis time

Several studies have been conducted to consider the

nature of the reactions which occur in an aqueous silane

solution as a function of solution age [20,31]. As

mentioned previously, most of these studies have

demonstrated that two main reactions occur i.e. initial

sequential hydrolysis of the alkoxy groups, followed by

condensation of the resultant hydroxyls. Some effort has

been devoted to assessing the extent to which these two

primary reactions are influenced by various solution

variables [20,31]. Since the nature and extent of these

reactions will be dependent upon time, it was recognisedthat the age of the silane solution prior to application to

the aluminium substrate could be important. Previous

work conducted by Gledhill et al, in which the influence

of solution age with mild steel substrates was studied,

revealed significant effects[25]. Hence solution age prior

to application was considered an important variable in

this study.

To investigate this effect, experiments were focused on

g-GPS aqueous silane solutions having a silane concen-

tration of 1% at a pH of 5. Solutions were hydrolysed

for times between 10 min and 48 h prior to application.

The results obtained from these experiments are shown

inFig. 8.

As indicated, two hydrolysis times, 10 min and 1 h

exhibited the best durability enhancing effects, with all

other hydrolysis times exhibiting lower and similar GIcvalues. Although a 10 min hydrolysis time had approxi-

mately similarGIc values to those obtained after 1 h, the

amount of experimental scatter obtained from the

former was high, leaving considerable doubt as to the

reproducibility of results at this short hydrolysis time.

Thus an optimum hydrolysis time of 1 h was considered

appropriate under the solution conditions employed.

This observation is in agreement with the work of

Gledhill et al who observed optimum hydrolysis times ofbetween 30 and 90 min with g-GPS aqueous solutions

applied to mild steel substrates [25]. Similarly, Khu-

bander and Mazza observed an optimum hydrolysis

time of 1 h with an aqueous 1% g-GPS solution applied

ARTICLE IN PRESS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 20 40 60 80 100

Methanol conc. (%)

GIC(

kJm-2)

GB only

Fig. 7. Plot ofGIc v methanol conc for 168h ageing.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50

GIC

(kJm-

2)

Baseline Data - no silane

Hydrolysis Time (hrs)

treatment

Fig. 8. Wedge fracture energy as a function of solution hydrolysis

time.



10/14

to a primed aluminium alloy substrate [13]. Abel et al

employed NMR techniques to assess the change in the

molecular structure of g-GPS (aqueous solutions) as a

function of time and noted that complete hydrolysis

under the precise solution conditions employed in this

work occurred after 1 h, with only slight condensation of

the silanols produced[32]. A detailed study by Bertelsonand Boerio essentially confirmed these observations with

various concentrations of g-GPS in both aqueous and

solvent/water combinations indicating silanol formation

in less than an hour [31]. Furthermore, using Si-29

NMR spectroscopy, they also observed the onset of

condensation reactions leading to the formation of

dimers and, eventually, network type species. In the case

of a 10% g-GPS aqueous solution at a pH of 5, they

were able to observe the gradual reduction in silanol

concentration after 4 h hydrolysis with a corresponding

increase in dimer and network species concentration

after this time. Indeed after 4 h reaction the dimer

concentration was reported to be approximately 22%

with significant increases in this concentration

observed with increasing time up to 8 h. Of particular

interest, Bertelson and Boerio [31] observed a clear

correlation between solution chemistry and the wedge

test results obtained from the work of Khubander and

Mazza [13] with relatively poor wedge test results

correlating well with the formation of dimer and

network species in the silane solutions. Thus the results

from our work, and the various studies highlighted

above, would suggest that optimum durability enhance-

ment is provided by solutions in which virtually

complete hydrolysis of alkoxy groups has occurred withminimal silanol condensation.

Interestingly, all of the hydrolysis times studied

provided durability characteristics superior to the

base-line non-silane treated joints.

3.5. Influence of film drying temperature

A key parameter likely to influence both the chemical

nature of the silane layer deposited on an alloy surface,

and its subsequent durability enhancing effect, is drying

temperature. To consider this issue in this study, two

silane film drying temperatures were employed i.e. 23and 93 1C. All other aspects of the silane application

process were maintained constant at 1% concentration,

pH 5 with a hydrolysis time of 1 h. The results obtained

from the Boeing wedge experiments conducted are

shown in Fig. 9. As indicated, virtually identical GIc

exposure time trends were obtained, suggesting a

virtually insignificant influence of drying temperature

under the experimental conditions employed. This result

is of particular interest in that recent work, conducted

by Abel et al, has shown that variations in the surface

chemical structure of silane films deposited onto

aluminium alloy do indeed occur with variations in

drying temperature[33]. The factors responsible for this

apparent anomaly are unclear. However, one possibility

is that the elevated temperatures employed during the

adhesive cure cycle, 120 1C, resulted in similar surface

chemical characteristics to the extent that differences in

interfacial chemistry were negligible following the

bonding process. Further work, particularly considering

the influence of film drying temperature with cold-cure

epoxy adhesives, would help to resolve this intriguing

issue. Interestingly, several other research groups have

considered the effects of this important experimental

parameter and a brief overview of some of the research

conducted would be instructive.

A substantial body of work conducted by Baker et al

concluded that optimum durability characteristics with

aluminium alloy substrates pretreated with g-GPSsolutions was provided by a subsequent film drying

temperature of 93 1C [12]. Similar work conducted by

Gledhill et al. on a mild steel alloy strongly suggested

that room temperature (20 1C) drying conditions pro-

moted greater durability enhancing effects than elevated

temperature drying[25]. Sung et al observed, from DSC

experiments, increasing Tg of a bulk silane (g-APS) film

deposited on an Al2O3 substrate with increasing drying

time at 110 1C [18]. They attributed this effect to

increasing film molecular weight and/or crosslinking

with extensive drying. This proposed mechanism was

supported further from observations from SEM-EDXacross an interface between a silane film and a

polyethylene substrate. Results from this analysis

revealed silicon profiles across the interfacial zone with

increasing sharpness the greater the drying time. This

observation was attributed to decreasing interdiffusion

of g-APS into the polyethylene as the silane is

polymerised/crosslinked further on continued drying.

Of particular interest, these authors noted that elevated

temperature drying of the silane film resulted in a

reduction in joint strength together with a change in

locus of failure from cohesive within the polyethylene

substrate to mixed mode close to the interface. These

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0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Fractu

reenergy(kJm-2)

Exposure time (h)

93C dried wedges

23C dried wedges

Fig. 9. Boeing wedge crack growth for silane film drying temperatures

of 23 1C and 93 1C.



11/14

detrimental effects were attributed to a lack of

interdiffusion between the polyethylene and silane due

to increased molecular weight/crosslinking in the latter

caused by the increased drying temperature. Later work

by the same authors considered the influence of silane

film drying on the durability of polyethyleneAl2O3si-

lane joints in a hot/wet environment[21]. Interestingly,drying of the film at elevated temperature (110 1C) was,

at least for moderate drying times, found to enhance

durability. An enhanced inability of water to diffuse

through the silane layer, due to increased crosslinking,

was proposed as the cause of this effect.

Culler et al, exploring reactions at an interface

between an epoxy matrix resin and a g-APS silane film,

observed a direct correlation between the extent of

reaction and the degree of condensation of the g-APS

[34]. In conclusion, they proposed that maximum

reactivity at the interface would be achieved when the

degree of condensation is smallest, clearly implying that,

for optimum performance, the siloxane layer should not

be highly condensed.

Work by Phillips and Hercules has indicated that

heating an adsorbed silane film increases the water

resistance of that film[35]. Cave and Kinloch suggested

that durability enhancements found from the use of

silanes deposited onto metallic surfaces without subse-

quent elevated temperature drying, could have been

associated with the influence of heat cure of the adhesive

on the structure of the silane layer[26].

Nishiyama and co-workers studied the adsorption of

g-MPS on a colloidal silica and observed, via GPC, that

the molecular nature of the adsorbed silane varied withdrying time [36]. In particular both an increased film

molecular weight together with a decrease in the amount

of removable physisorbed silane was observed with

increased drying time.

Kuhbander and Mazza have noted silane manufac-

turers information stating that elevated temperature

drying of silane films is beneficial since heating removes

hydrolysis products and solvents and promotes more

effective silanol condensation [13]. In addition, from

Boeing wedge experiments conducted with silane film

drying temperatures of 60 1C to 107 1C, they observed

optimum durability performance at 931

C. Interestingly,the adhesive employed by Kuhbander and Mazza was

identical to that used in this study, with, in all

probability, the same cure conditions.

Bertelsen and Boerio have proposed that the oligo-

merisation required for good adhesion with silane films

(after deposition) can be obtained through drying of the

film at elevated temperatures[31].

Clearly, the substantial body of research conducted

on this particular experimental variant has not proved

conclusive in determining whether elevated temperature

drying is desirable across a range of silane substrate

combinations.

3.6. Influence of in-process time delay

The time interval between drying of the deposited

silane film and the formation of the bonded joint will be

of considerable practical importance in both initial

manufacture and repair operations. In such circum-

stances it will, of course, be important to know how longthe treated substrate will remain active prior to any

deterioration in its durability enhancing capacity. In

many industrial bonding operations conventional wis-

dom has been that a structure should undergo contin-

uous processing from surface treatment to bonding and

adhesive cure within approximately 24 h. To ascertain

the degree to which applied silane coatings remain

active following application, Boeing wedge experi-

ments were conducted in which the time interval

between silane application and subsequent bonding

were varied between 0 and 17 days. In each case, during

the time delay period, silane treated specimens were

stored in a dust free cabinet at 231C, 50% relative

humidity. The results obtained are indicated inFig. 10.

The trends reveal that, under the specimen storage

conditions employed, little or no reduction in durability

occurs up to approximately 7 days. Although an

increase in the in-process time interval to 17 days

resulted in a notable reduction in durability, the

deterioration in silane surface activity was still not

sufficient to reduce GIc down to values typical of base-

line non-silane treated joints.

The reduction in durability enhancing characteristics

on increasing the in-process time interval from 7 to 17

days may be due to a reduction in the number of activesites on the surface. This could be due to adsorption of

atmospheric contamination or some continuing reactions

(e.g. silanol condensation) within or at the surface of the

silane layer, which could reduce the number of sites

available for reaction/interaction with the adhesive layer.

In spite of the uncertainty relating to the mechanisms

responsible for the above mentioned trends, it is

clear that, provided care is taken to avoid gross

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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 5 10 15 20

GIC

(kJm-2)

GB only

Process time lag (days)

Fig. 10. Wedge fracture energy as a function of inprocess time

delay.



12/14

contamination of the silane treated surface, the time

interval between silane application and subsequent

bonding is not critical up to approximately 7 days. This

observation suggests considerable flexibility in both

manufacture and repair operations. However it is of

course likely that the degree of surface deterioration will

depend on storage conditions. Thus it would be ofinterest to explore the extent to which property variations

occur with respect to this experimental variable.

3.7. Fracture surface characterisation

On completion of each Boeing wedge experiment, the

joints were separated to allow examination of the failure

surfaces. Throughout the study, under all silane treatment

conditions employed, failure within the regions of crack

propagation following ageing in the aggressive environ-

ment, was apparent interfacial. In other words, within

this region of the joint, one failure surface had the visualappearance of aluminium alloy, whereas its counterpart

was ostensibly adhesive. A detailed analysis using XPS

was employed to characterise the failure process occur-

ring within joints previously subjected to the optimised

silane treatment (aqueous 1% concentration, pH5, 1 hr

hydrolysis time, 93 1C drying temperature) and which had

been subjected to an ageing period of one week.

The photograph inFig. 11shows the failure surfaces of

a wedge joint separated after just one week exposure to the

96% rh, 50 1C environment. As indicated, four regions of

the failure surfaces are highlighted. Region 1 corresponds

to the zone which resulted from initial insertion of the

wedge. Region 2 was formed as a result of crack growththrough the adhesive layer in the first hour following

insertion of the wedge (prior to ageing in the hostile

environment). Region 3, the region of primary interest in

this study, resulted from crack growth following insertion

of the joint into the 96% rh, 50 1C environment. Following

the prescribed ageing time in the hostile environment (in

this case one week) joints were removed and split-open to

reveal the failure surfaces. Region 4 corresponds to crack

propagation within the bondline which occurred during

this final separation process.Of primary significance inFig. 11are the differences

in locus of failure between region 3 on the one hand, and

regions 1, 2 and 4 on the other. Region 3 indicates a

zone of virtually complete apparent interfacial failure,

with all other regions demonstrating failure through the

adhesive layer.

Region 3 was subjected to XPS analysis to determine

the precise nature of the failure process. The survey

spectrum given inFig. 12was obtained from adhesive

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Fig. 11. Boeing wedge fracture surface following exposure to test environment for one week.

Fig. 12. XPS survey spectrum from the adhesive failure surface of

one-week aged joint.



13/14

failure surface area.Fig. 13shows the survey spectrum

for the opposing aluminium failure surface. Quantifi-

cation of both surfaces was undertaken and the resultant

data is summarised inTable 2.

The nitrogen and carbon signals recorded from XPS

suggest the presence of adhesive on both sides of the

failure zone, thus indicating a mixed mode of failure at

this exposure time. The presence of the aluminium 2p

signal from the adhesive surface confirms the contribu-

tion of the oxide layer to the overall failure process at

the short exposure condition. Furthermore, silicon wasclearly observed on both adhesive and aluminium

failure surfaces, this clearly indicating the involvement

of the organosilane in the degradation and failure

processes. Based upon this information, a schematic

representation of the degradation and failure events

occurring within the first week of ageing is depicted in

Fig. 14. As indicated, under the ageing conditions

employed, crack growth is seen to proceed through a

composite zone comprising oxide, silane and adhesive.

Interestingly, Arnott et al, working with g-GPS treated

2024-T3 aluminium alloy, observed similar Boeing

wedge failure surfaces to those observed in the current

study [37]. In their case they also observed, from XPS

analysis, the existence of Al, Si, C and O on both sides of

the failed joint i.e. on the apparent adhesive and metallic

surfaces. This would, as with our studies, imply failure

located within an oxide/silane/adhesive diffusion zone.

Such observations clearly provide interesting clues as to

how further improvements in durability could perhaps

result from modifications to both the silane and oxide

layers.

4. Concluding remarks

The application and solution conditions employed

contribute to the overall durability enhancing character-

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Fig. 13. XPS survey spectrum from the aluminium failure surface of

one-week aged joint.

Table 2

XPS quantification for one-week aged wedge joints

Sample ID Elemental concentration, (at%)

O N C Si Al

Standard wedge metal interface 39 2 44 2 8

Standard wedge polymer interface 28 1 67 4 2

Silane layer

Epoxy

Aluminium

oxide

Diffusion layer

containing silane and

aluminium oxideWedge fracture pat96%rh, 50Ch

Fig. 14. Schematic of a sectioned wedge joint showing the apparent failure path for joint aged for one week.



14/14

istics of the organosilane treatment. Specifically, solu-

tion variables such as the nature of the solvent

(methanol versus water), solution pH, silane concentra-

tion and hydrolysis time prior to application, have all

been shown to be important. With the aluminium alloy

employed, an aqueous solution with a silane concentra-

tion of 1% at pH 5 and hydrolysed for 1 h prior toapplication has been shown to produce best results.

Indeed under these conditions durability characteristics

comparable to a chromic acid anodise treatment has

been demonstrated.

Silane film drying temperature has been shown to

have an insignificant effect on resultant joint durability.

The factors responsible for this effect, in particular

conflicting observations from the literature relating to

changes in silane film chemical structure, are unclear.

Surface characterisation techniques based upon mi-

croscopy, XPS and SIMS have indicated degradation

and failure processes associated with both the alumi-

nium oxide and silane layers via a so-called oxide-silane

diffusion zone. Such observations, particularly the

extent to which the oxide layer contributes to the failure

process with the optimised silane treatment, indicates

the regions of the interphase from which further

improvements in joint durability could emerge.

In this study no attempt has been made to investigate

the effects of the various solution parameters on the

stability of the epoxide group on the silane molecule.

Further studies should be carried out to investigate this

potentially important issue.

Acknowledgements

The authors would like to acknowledge the financial

support of the MoD Corporate Research Programme.

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Understanding the Relationship Between Silane Application Conditions