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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.
ARTICLE IN PRESS
www.elsevier.com/locate/ijadhadh
0143-7496/$ - see front matter Crown Copyrightr 2005 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijadhadh.2005.03.009
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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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.
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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
1.0% silane 5.0% silane
8.0% silane 12.0% silane
1.5% silane 2.0% 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
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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.
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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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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.
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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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GIC
(kJm-2)
GB only
Process time lag (days)
Fig. 10. Wedge fracture energy as a function of inprocess time
delay.
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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.
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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.
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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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