Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
Effects of surface pre-treatment on adhesive bonding of VICTREX® PEEK™
1. Introduction
The aim of this study was to find settings for different surface treatments in order to enhance adhesive
bonding of PEEK™ polymer. The list below shows companies involved and surface pre-treatments
employed:
 University of Erlangen-Nuernberg, Elangen, Germany (LSP)
surface roughening, acid etch, excimer laser treatment and preliminary tests
 Fraunhofer Institut Lasertechnik, Aachen, Germany (ILT)
UV-lamp and plasma treatment
 Imperial College, London, UK
corona treatment
 Lipp-Terler, Austria
high temperature PEEK-film extrusion
2. Experimental procedure
All tests were performed at room temperature. A two component epoxy Araldit AV138M and hardener
HV998 from Ciba was used. Although room temperature cure is possible, a 15 minute heat cure at
100°C was preferred. The reference state for each test was a sample which had been acetone wiped
only prior to bonding. This is referred to as the untreated sample.
Standard grade unfilled VICTREX® PEEK™ 450G tensile bars (DIN 53455, 3mm x 10mm cross
section) were moulded at LSP who also manufactured bonding jigs to be used at LSP and ILT.
Samples were cut in half and acetone wiped prior to surface treatment. Bonding occurred in a jig for
proper alignment at an overlap length of 12.5mm to form a single overlap shear test specimen. A
constant pressure during heat cure was applied using bull dog clamps. Tensile bars were tested at a
cross head speed of 10mm/min, 10 replicates for each
set of conditions. LSP was able to compensate the
induced bending moment in a tensile test by allowing an
offset of 3mm between both cross heads. The tensile
tester at ILT could not accommodate for this bending
moment.
Imperial College had shown that corona discharge is
limited to samples thinner than 3mm. Therefor a different
test procedure was required for corona treated samples.
Here, test strips 210mm long and 20mm wide were
prepared from PEEK™ film 0.3mm thick which had
kindly been supplied by Lipp-Terler in Austria. This film is
slightly lower in molecular weight than the 450G grade
used for tensile bars. Any differences in behaviour
regarding adhesive bonding are highly unlikely. T-peel
joints were acetone wiped prior to treatment and were
prepared with a controlled bond thickness of 0.25mm.
Joints were placed between steel plates and clamped for constant pressure during heat cure as
above. Displacement rate in T-peel tests according ISO 11339 was 100mm/min.
3. Results
3.1. General comments
In a single overlap shear test the force required for failure of the bonded sample is measured. Upon
division by surface area a bond strength is evaluated. Depending on failure mode different cross
sections need to be considered:
a) Interfacial failure occurs if crack propagation is along the interface between adhesive and substrate
PEEK™. In this case bond strength is reported as lap shear strength in MPa (N/mm 2) which is the
measured force divided by the area of bonded overlap.
b) Bulk or substrate failure occurs if the adhesive joint is strong enough to withstand applied forces. In
this case the tensile bar breaks outside of the bonded area. Therefore bond strength in MPa is a
normal tensile stress which is calculated by dividing measured force by cross section of fractured
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
tensile bar. Due to the complex state of stress with stress concentrations at end of overlap the
measured tensile strength of a bonded sample is lower than the tensile strength of a simple test bar.
3.2. Surface roughening
Abrasive paper (SiC, #600) and grit blast using glass spheres between 100m and 200m at 5 bar air
pressure for 2 seconds at a distance of 30cm from the substrate were used for plain roughening.
Lap shear strength of 10 replicates for each condition is shown in Figure 1. Surface roughening
increased bond strength from 0.7MPa for the untreated samples nearly 3 times to 2.1MPa. There is no
significant difference with respect to abrasive method used. All samples failed at the interface between
adhesive and substrate PEEK™. Surface roughness increased from 0.12m for untreated samples to
0.8m and 2.6m in case of grit blast and abrasive paper respectively.
3.3. Acid etch
Samples were dipped in chromic sulphuric acid for 2s, 10s, 30s and 60s, rinsed with de-mineralised
water and dried in air prior to bonding. The formulation was as follows:
0.15l H2O + 0.25kg K2Cr2O7 + 1,65l H2SO4.
As opposed to untreated samples lap shear strength increased nearly 8 times as depicted in Figure 2.
There is no significant difference with respect to treatment time. Maximum lap shear strength obtained
was 5.5MPa. Failure mode was interfacial.
Major effects in roughness and wetting behaviour happened within the first two seconds of treatment
time just like changes in bond strength: Acid etch changed surface roughness from 0.12m to 0.18m
after 2s, increasing only slightly to 0.2m after 60s. These changes are small as compared to those
imposed by mechanical abrasion. Surface chemistry changed and therefore wetting was improved as
measured by contact angle . Within 2s  decreased rapidly from 77.4° to 72.8°; further decrease to
70.6° after 60s is minute.
3.4. Treatment with excimer laser
The XeCl-laser supplied a system energy of 2J at a wavelength of 308nm. With the laser optics and
set-up involved energy densities of 0.5, 1.0 and 4.3mJ/mm 2 were selected. Samples were treated with
these 3 energy densities at 1Hz employing 1 and 10 passes across the sample surface. With
exception of the lowest settings (0.5mJ/mm 2, 1pass) all conditions were above ablation threshold. This
means that aside from chemical modifications in the surface layer particles of (degraded) PEEK™ are
thrown out producing ablation products or debris.
Figure 3 shows that bond strength is greatly increased as compared to untreated samples. Bond
strength seems to be independent of energy density, number of passes and whether ablation products
are wiped off or not. Lap shear strengths of up to 5.6MPa were measured. All samples failed at the
interface.
Although settings were above ablation threshold surface roughness was shown unchanged. Changes
in surface chemistry were in the same order of magnitude as in acid etched samples. Again, the most
rapid decrease in contact angle from 77.4° to 72.8° happened upon the least severe treatment where
after no more changes seem to occur. Thus, contact angle correlates to bond strength.
3.5. Treatment with UV-lamps
UV-lamps with a wavelength of 172nm and 222nm were used at ILT for treatment of PEEK™ polymer.
Samples were placed inside a chamber at a distance of 100mm from the radiation source. The
chamber was flushed with either air or argon. Treatment times were 30s and 240s (4 minutes). The
results of this series of experiments are shown in Figure 4 and 5.
In all cases bond strength has been increased. After 30s treatment time lap shear strength has been
increased from 1.2MPa (untreated at ILT) to values between 2MPa and 4.2MPa depending on
wavelength and process gas. Upon increasing treatment time to 4 minutes lap shear strength has
reached values close to 5MPa independent of both wavelength and process gas.
Contact angle of untreated samples was measured to be 76.9° at ILT which is very similar to 77.4° as
measured at LSP. In general contact angle decreases as bond strength increases. The amount of
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
decrease in contact angle is much more severe in UV-lamp treated samples than it is in laser treated
samples. Laser treatment changed  from 77° to 72° while UV treatment changed  from 77° to
values between 30 and 40°. Wetting behaviour improves drastically.
It is important to note that for this type of geometry and surface treatment the failure mode partially
changed from interfacial to bulk fracture. After 4 minutes of treatment in air 2 out of 10 samples failed
within the bulk of PEEK™ polymer. In argon up to 6 out of 10 samples failed within the bulk. This
circumstance indicates that the limiting factor for this combination of geometry/adhesive/treatment is
no longer a weak interface nor the adhesive itself. A reduction of the stress concentration at the end of
overlap could move the locus of failure back into the adhesive joint allowing for further increase of
bond strength.
For a general comparison of results between test houses 10 replicates of UV radiation treated
samples at 222nm in air for 4minutes duration (done at ILT) were also tested at LSP. Bond strength
determined at LSP was 4MPa with 6 bulk failures (ILT: 5MPa, 2 bulk failures). These differences are to
be expected and should not be stressed too much. Both test houses determined bond strengths at the
border line between interfacial and bulk failure. Consequently these results are to be considered
similar.
3.6. Plasma treatment
Plasma treatment occurred in a chamber with air and argon as process gas. The distance between
sample and electrode was 0.8mm and 2.0mm. Samples were exposed to 5 passes and 40 passes.
Figure 6 shows an increase in lap shear strength with increasing number of passes. The figure further
shows that argon as process gas is more effective than air as process gas. There is no clear influence
of gap size on bond strength. However, using only 0.8mm gap resulted in more bulk failures than
using 2.0mm gap under same conditions. The most aggressive conditions were shown to be 0.8mm
gap, 40 passes in Argon where 7 out of 10 samples failed outside the bonded area within the bulk.
The remaining 3 samples showed an average of 5MPa lap shear strength.
Measurement of contact angle showed an instantaneous decrease to approximately 35° which
remained nearly constant for all settings as shown in Figure 7. There is no unique correlation between
bond strength and  since bond strength changes with parameter settings while contact angle does
not.
3.7. Corona treatment
The corona treatment apparatus used consisted of a high frequency power source, transformer, multifin metallic electrode and motor controlled table. Treatment levels were varied by changing power,
table velocity and number of passes. This way distinct values in energy between 0.04J/mm 2 and
2.64J/mm2 were adjusted. The minimum power required for stable discharge was 350W.
Following some initial non-linearity the load for further peel remained approximately constant. These
maximum loads are shown in Figure 8 for different treatment levels. Upon corona treatment peel force
increased from 0.13N for the untreated sample to approximately 2.3N. Peel force was independent of
treatment level. Cracks in untreated joints were running along interface between adhesive and
PEEK™ film. Cracks in corona treated joints were running close to the interface such that mixed mode
failure occurred with the crack displacement jumping back and forth between interface and adhesive.
3.8. Comparison of treatment methods
A comparison of maximum lap shear strength obtained with each surface pre-treatment of tensile bars
is shown in Figure 9. Corona treatment is excluded here since experiments had to be different.
Simple mechanical abrasion allowed for an increase in bond strength to 2.1MPa. Further improvement
of adhesive bonding to values between 5MPa and 5.6MPa resulted from any of the investigated
physio-chemical treatments. In terms of measured bond strength there is no significant difference with
respect to type of treatment. However, if number of bulk failures out of 10 replicates is included into
this consideration then treatment with UV-lamps at 172nm in argon as well as plasma treatment in
argon seem to enhance adhesion the most.
3.9. Changing locus of failure from bulk to interface: geometrical effects
Before the series of experiments listed above started a one component epoxy AV119 from Ciba was
chosen to eliminate possible errors in mixing. The required heat cure for 50minutes at 140°C was no
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
problem since volumetric changes induced by crystallisation resulting in internal stresses would not
occur below Tg of PEEK™ polymer. All 3 test houses started their experimental work with this epoxy.
The results were exceptionally good and yet devastating: lap shear and T-peel tests after acid etch,
corona treatment and exposure to UV-lamps resulted in bulk failure in every case. The conclusion was
that by choosing AV119 as adhesive and the above stated test conditionsno further improvement of
surface modification was necessary. Any increase in bond strength as induced by surface treatment
could not be measured. Since AV119 seemed to be quite compatible with PEEK™ polymer in terms of
mechanical and thermal performance some preliminary tests were required to move locus of failure
back to the interface adhesive-PEEK™.
In a regular tensile test PEEK™ polymer shows cold drawing and a tensile strength in the order of
100MPa. In case of substrate failure tensile bars always snapped at the end of overlap in a brittle
manner at values way below tensile strength of PEEK™ polymer. Therefore the complex state of
stress within the joint must lead to a stress concentration followed by crack initiation at end of overlap.
Due to notch sensitivity of PEEK™ the tensile bar will fracture brittle. Thus, efforts were made to
reduce or withstand stress concentration. Those means which showed the most impact are discussed
below and shown in Figure 10. All testing was done after 30s of acid etch using between 5 and 10
replicates at a pull rate of 10mm/min.
- change in length of overlap
Joints with 5mm, 12.5mm and 30mm overlap were prepared using 3mm PEEK™ 450G. These results
are labelled 1, 2 and 3 in Figure 10. Bond strength is lowest in case of 5mm joints. A slight and
perhaps insignificant maximum of 42MPa is reached at an overlap of 12.5mm. Failure was always
within the substrate. At 12.5mm overlap a force of 1250N was recorded. A simple tensile bar without
stress concentrations would read in the order of 3000N prior to cold drawing. If a joint needs to
withstand higher forces then the stress concentration at ends of overlap must be reduced.
- using an oversized fillet
To reduce stress concentration experiments with different bond lengths were repeated using a thick
fillet of adhesive at ends of overlap. Those results are labelled as 4, 5 and 6. Indeed, bond strength
did increase in all cases. Still, each sample snapped within the bulk of PEEK™ at end of overlap.
Maximum strength was 69MPa corresponding to a force of 2060N which demonstrates the importance
of joint design in terms of reducing stress concentration at end of overlap.
As sudden changes in sample thickness is poor plastic design practice attempts were made to bevel
off ends of samples prior to bonding. Still, 100% bulk failure occurred with strengths between 34MPa
and 42MPa depending on bevel angle. Even with a crack carved into the adhesive failure at 36MPa
occurred within the bulk of PEEK™.
-changing sample thickness
Attempts were made to change state of stress within the joint by increasing sample thickness from
3mm to 4mm at 12.5mm overlap. It was expected that bending moment and elongation within the joint
would change. In this instance samples 2 and 7 of Figure 10 need to be compared which all failed
within the substrate. Increased sample thickness decreased fracture stress.
-changing grade of PEEK™
Another interesting comparison is between PEEK™ 450G and PEEK™ 450GL30, both 4mm thick.
Glass fibre reinforced PEEK™ polymer has increased tensile strength and stiffness. PEEK™ 450G
failed within the bulk at 27MPa. PEEK™ 450GL30 failed at the interface with a lap shear strength of
8.4MPa. These samples are labelled as 7 and 8 in Figure 10. Hence, a compatible solution for this
adhesive bonding project would have been this epoxy AV119 in conjunction with 4mm tensile bars
made of glass fibre reinforced PEEK™ 450GL30 since locus of failure was at the interface between
PEEK™ and the adhesive. Although we expected further improvements of surface treatments to be
measurable we could not be certain just how far we may improve adhesion before locus of failure
would again be within the bulk of PEEK™.
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
3.10.
Summary
Adhesive bonding of PEEK™ 450G has been studied after surface pre-treatment using mechanical
abrasion, acid etch, excimer laser, UV-radiation source, plasma treatment and corona discharge. The
quality of adhesive joints was measured in single lap shear tests of tensile bars with a cross section of
3mmx10mm and an overlap of 12.5mm. Since corona discharge was restricted to thinner samples a
different procedure was necessary. In this case 0.3mm PEEK™ film was used for evaluation of peel
force in T-peel tests. Joints were prepared with a two component epoxy from Ciba (AV138M/HV998).

Mechanical abrasion increased lap shear strength to approximately 2MPa as opposed to
untreated samples. Considering its simplicity mechanical abrasion may be a useful technique for
many applications.

Acid etch, excimer laser, UV-radiation source and plasma treatment increased bond strength to
values between 5MPa and 5.6MPa. The performance of these physio-chemical treatments is
better than simple abrasion. At 5.6MPa lap shear strength a force of 700N is acting on the surface
causing interfacial failure.

UV-radiation and plasma treatment may be viewed as being most beneficial since up to 7 out of
10 replicates failed within the bulk of PEEK™ polymer, the remaining at the interface.

Corona treatment increases peel force nearly 17-fold. A comparison to lap shear strength as
measured in remaining experiments is not possible.

Treatment level does effect strength and number of bulk failures in case of plasma treatment and
UV radiation. No significant effect of treatment level on bond strength has been found in case of
the remaining techniques (abrasion, acid etch, laser, corona).

No more failure within the adhesive joint occurred upon changing adhesive to a 1 component heat
cure epoxy from Ciba (AV119). Treating samples by acid etch, corona and UV-radiation moved
locus of failure 100% into the bulk of PEEK™. Further improvement of bond strength by surface
treatment is not possible for the joint geometry used in these experiments.

Upon studying geometrical effects which impact on the stress concentration at end of overlap we
have come to the conclusion, that bonding of PEEK™ works very well. Depending on joint
geometry forces up to 2000N were acting on the adhesive joint. For comparison: 3000N would be
measured at tensile strength of a single tensile bar without stress concentrations. This part of the
study revealed that joint design has a major impact on forces that may be applied to the joint.

Acid etch is of low cost but requires careful handling. The remaining techniques need expensive
equipment.

It is most important to choose an adhesive which is compatible to boundary conditions of the
application and to make a proper joint design.
The value of these experiments would greatly increase if a few more tests were performed in order to
enhance key properties of PEEK™ such as high temperature and chemical resistance or suitability for
medical applications. Therefore we suggest to use acid etch as surface treatment for its simplicity and
vary the following:

different epoxies to show performance at high temperatures

other types of adhesive for good performance with respect to hydrolysis resistance, USP VI
classified,...

other type of adhesives with respect to easy application such as film adhesives, instant adhesives
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
lap shear strength [MPa]
3
2,5
2
1,5
1
0,5
0
untreated
grit blast
treatment
lengthwise
SiC-paper
untreated
grit blast
0,7
0,3
0,12
1,9
0,3
0,79
lap shear strength [MPa]
standard deviation
roughness [m]
crosswise
SiC-paper
SiC-paper
SiC-paper
lengthways
crosswise
1,8
2,1
0,3
0,3
2,57
lap shear strength [MPa]
Figure 1: lap shear strength of PEEK™ polymer using different abrasive methods
7
6
5
4
3
2
1
0
untreated
treatment time
lap shear strength [MPa]
standard deviation
2s
untreated
0,7
0,3
10s
treatment time
2s
5,1
0,4
30s
10s
5
0,5
60s
30s
5,5
0,1
60s
5,2
0,3
Figure 2: lap shear strength of PEEK™ polymer after different treatment times in chromic sulphuric
acid
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
1 pass
10 passes
25.10.99
10 passes, no ablation
lap shear strength [MPa]
7
6
5
4
3
2
1
0
untreated
energy density [mJ/mm 2]
1 pass
lap shear strength [MPa]
standard deviation
10 passes
lap shear strength [MPa]
standard deviation
10 passes, no ablation
lap shear strength [MPa]
standard deviation
0,5
1
energy density [mJ/mm2]
untreated
0,7
0,3
4,3
0,5
1
4,3
5,6
0,1
5,4
0,2
5,5
0,2
5,5
0,3
5,6
0,3
5,1
0,2
5,5
0,2
Figure 3: lap shear strength of PEEK™ polymer upon treatment with an excimer laser using different
energy densities.
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
lap shear strength [MPa]
7
222nm
6
172nm
5
4
3
2
1
0
untreated
UV lamp
untreated
30s air
30s Ar
240s air
240s Ar
30s air
222nm
strength deviation
1,18
0,07
3,35
0,48
2,2
0,08
4,97
0,26
4,93
0,13
bulk
0
0
0
2
4
30s Ar
240s air
172nm
strength deviation bulk
2,06
4,24
4,76
5,03
0,06
0,25
0,19
0,13
240s Ar
222nm/LSP
strength deviation
0
1
2
6
4,03
0,24
bulk
6
Figure 4: lap shear strength of PEEK™ polymer treated with UV-lamps of different wavelength in air
and argon as process gas
90
222nm
80
172nm
contact angle [°]
70
60
50
40
30
20
10
0
untreated
30s air
UV lamp
untreated
30s air
30s Ar
240s air
240s Ar
angle
76,9
61,8
66,6
37,5
37,8
222nm
deviation
1,5
1,8
1,6
1,1
0,8
30s Ar
bulk
0
0
0
2
4
240s air
240s Ar
172nm
angle deviation bulk
52,4
56,4
28,5
39,9
0,5
1,4
0,5
2,1
0
1
2
6
Figure 5: contact angle of PEEK™ polymer treated with UV-lamps in 2 environments at 2
wavelengths
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
7
lap shear strength [MPa]
0,8mm gap
2,0mm gap
6
5
4
3
2
1
0
untreated
gap size
untreated
5 passes air
5 passes Ar
40 passes air
40 passes Ar
strength
1,18
2,69
3,61
4,17
4,98
5 passes air
0,8mm gap
deviation
0,07
0,13
0,24
0,11
0,19
5 passes Ar 40 passes air 40 passes Ar
bulk
0
0
2
6
7
strength
3,03
3,59
4
4,53
2,0mm gap
deviation
0,27
0,33
0,17
0,16
bulk
0
1
1
5
Figure 6: lap shear strength of PEEK™ polymer after plasma treatment in air and argon at two gaps
90
0,8mm gap
80
2,0mm gap
contact angle [°]
70
60
50
40
30
20
10
0
untreated
gap size
untreated
5 passes air
5 passes Ar
40 passes air
40 passes Ar
angle
76,9
35,4
37,3
32,4
35,6
5 passes air 5 passes Ar 40 passes air 40 passes Ar
0,8mm gap
deviation
1,5
0,6
0,7
0,4
0,7
bulk
0
0
2
6
7
angle
37,6
34,7
37,3
37,8
2,0mm gap
deviation
0,7
1,1
0,7
0,6
bulk
0
1
1
5
Figure 7: contact angle of PEEK™ polymer after plasma treatment in air and argon at two gaps
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
3
peel force [N]
2,5
2
1,5
1
0,5
0
0
0,04
0,12
0,92
energy [J/mm2]
energy [J/mm2]
1,32
2,64
peel force [N]
force
deviation
0,13
0,05
2,3
0,2
2,2
0,3
2
0,2
2,4
0,3
2
0,2
0
0,04
0,12
0,92
1,32
2,64
Figure 8: peel force of corona treated PEEK™ film as determined by T-peel test
lap shear strength [MPa]
7
6
5
4
3
2
1
0
untreated
roughening
acid etch
laser
UV lamp
plasma
surface treatment
surface treatment
bulk failure
 [MPa]
std. dev.
untreated roughening acid etch
0
0
0
0,7
2,1
5,5
0,3
0,3
0,1
laser
0
5,6
UV lamp
6
5,03
plasma
7
4,98
0,1
0,13
0,19
Figure 9: comparison of different surface treatment methods: 6 bulk failure with UV treatment and 7
bulk failure with plasma treatment; remaining failures were interfacial
Victrex Europa GmbH
Dr.-Ing. Horst Sandner
25.10.99
90
bond strength [MPa]
80
70
60
50
40
30
20
10
0
0
Nr.  [MPa] dev.
0
1,5
0,2
1
23,8
5,9
2
41,8
8,9
3
39,6
4,6
4
56
8,2
5
54,2
12,5
6
68,7
17,9
7
26,6
4,2
8
8,4
1,3
1
2
3
overlap length [mm]
5
12,5
30
x
x
x
x
x
x
x
x
x
4
5
6
7
8
fillet
grade
failure
mode
no yes
x
3mm/450G
IF
x
3mm/450G
B
x
3mm/450G
B
x
3mm/450G
B
x
3mm/450G
B
x
3mm/450G
B
x
3mm/450G
B
x
4mm/450G
B
x
4mm/450GL30
IF
force
[N]
188
714
1254
1188
1680
1626
2061
1064
1050
Figure 10: Tests under varying geometrical conditions in order to move locus of failure from bulk to
interface. Samples 0 (untreated) and 8 (4mm 450GL30) failed at the interface, the remaining samples
within the bulk of PEEK™