Special Issue
of adhäsion KLEBEN & DICHTEN 3/2004
Photo: Volkswagen AG
Nano-modified
ambient temperature curing
epoxy adhesives
Figure 1: Car assembly line
Ambient temperature curing
structural adhesives have to
meet ever increasing demands.
By modifying epoxy adhesives
with reactive liquid rubbers and
nanoparticles they can be raised
to new levels of performance.
Stephan Sprenger, Christian Eger,
Anthony Kinloch, Jin H. Lee,
Ambrose Taylor, Dave Egan
he use of room temperature curing
adhesives is of great interest in
many areas, including automotive
T
applications. In quite a few future applications (figure 1) the two component adhesive
has to possess the same performance as the
heat curing adhesives currently used, regardless of the other advantages the room
temperature system might have to offer.
This is a challenge to adhesive formulators,
as well as to adhesive raw material manufacturers. Most crash-resistant structural
epoxy adhesives currently in use are modified
with reactive liquid polymers (HYCAR RLP)
/1, 2/.
Investigations of heat curing one component epoxy adhesives have shown that the
performance of rubber-modified adhesives
can be increased significantly by the addition of a small amount of filler particles
with an average particle size of approximately 20 nm /3/.
Initial tests with room temperature curing
systems confirmed the existence of a synergy between "soft" µm-sized rubber domains and "hard" nm-sized filler particles
for two part epoxy adhesives /4/. The results
of our recent extensive research work will
be presented in this paper.
Table 1: Formulations of two-component model epoxy adhesives
2C series containing approx. 18 % Hycar ATBN
Reactive liquid rubbers
in two part adhesives
Butadiene acrylonitrile copolymers with
amino end groups (Hycar ATBN) are commonly used in the hardener component of
two part epoxy adhesives. The desired property profile of the hardener is achieved by
further blending with polyamines, cycloaliphatic polyamines, polyamidoamines,
imidazoles, accelerators etc.. Although, the
partial immiscibility of the reactive liquid
rubber with some amines and the higher
viscosity have to be recognised in the formulation.
During the cure, a partial phase separation
occurs and µm-sized rubber domains are
formed (usually 1 - 5 µm). The rubber molecules in the domains are crosslinked to the
epoxy resin matrix by the amine endgroups, thus improving the toughness
of the resin dramatically. A fraction of the
rubber is not participating in the domain
formation; but these long flexible polymer
chains are crosslinked into the resin matrix.
As a result of the lower network density, the
adhesive elongation is increased significantly; the modulus and glass transition
temperature are lowered.
It is of interest to note that no domains are
formed when using these reactive liquid
polymers with methacrylate end-groups
(Hycar VTBN) in acrylic adhesives. The
reason might be different solubility parameters and compatibilities.
Nano particles
in epoxy applications
Nanoparticles are of great interest as raw
materials for adhesives; having several
Table 2: Formulations of two-component model epoxy adhesives
2Ca series containing approx. 12.5 % Hycar ATBN
Table 3: Formulations of two-component model epoxy adhesives
2Cb series containing approx. 9.5 % Hycar ATBN
advantages compared to conventional
micron-sized fillers: at the same loading level
the number of particles (and their surface
area) is orders of magnitude higher. As a
consequence the interactions with the resin
matrix are much more intensive. Even at
high loading levels (40 %) the viscosity is
not increased much because there is no
formation of agglomerates.
Nano-modified resins are transparent due
to the small particle size; this is especially
important for coatings applications. The
particles can penetrate close meshed fabrics
and are therefore very suitable for the reinforcement of composites, especially if injection methods are used for manufacturing.
Recently masterbatches of surface-modified
SiO2-nanoparticles in epoxies and acrylates
have become available in technical quantities
/5/: Nanopox‚ and Nanocryl‚ respectively.
In industrial production, spherical particles
with a particle size of 20 nm and a very
narrow particle size distribution are formed
using a sol gel process /6/. The surface
modification prevents agglomeration and
enables crosslinking into the resin matrix
during cure.
The use of these nanoparticles in epoxy
resin formulations gives significant improvements in several properties: high
toughness and high modulus (unlike the
"classic" toughening using elastomers
which lowers the modulus). The coefficient
of thermal expansion can be improved.
Thermal and chemical stabilities are
unchanged and can be improved at higher
loading levels. Viscosity and rheology of the
resin is changed only slightly. By adding
more than 20 % of nanoparticles, scratch
resistance and wear resistance can be increased significantly. Compression strength
can be improved as well.
Figure 2: Mode I fracture energy versus addition level of nano-SiO2
Figure 3: Lap shear strength on untreated aluminium versus addition level
of nano-SiO2
Synergy between reactive
liquid rubber
and nanoparticles
In an earlier investigation /3/ of heat curing epoxy adhesives it was found that
adhesives modified with nanoparticles
only could not compete with a toughened,
rubber-modified adhesive.
Very surprisingly a synergy between the two
different modifications was discovered /3, 7/:
the performance of a very tough rubbermodified one part epoxy adhesive can be
improved significantly by adding small
amounts of nano-SiO2.
The next step is to investigate the combination of "soft" µm-sized rubber domains
Figure 4: Lap shear strength on oil-treated EG steel versus addition level
of nano-SiO2
and "hard" nm-sized filler particles in room
temperature curing epoxy adhesives. It has
to be taken into account that in such
systems besides rubber domains and silica
particles there are long, flexible elastomer
molecules (of the reactive liquid rubber)
which are linked within the matrix. This is
different compared to heat curing one part
epoxy systems.
The formulations of the model adhesives
investigated are given in tables 1 - 3. All the
adhesive formulations are toughened by
adding an amine functional butadiene acrylonitrile copolymer (Hycar ATBN) to the
polyamidoamine hardener. In the 2C series
33 % of the polyamidoamine hardener is
replaced by the reactive liquid rubber; in the
2Ca series this is 25 % and in the 2Cb
series 20 %. The Nanopox was added to
the resin component in various concentrations.
The NBR-content of the formulations varies
slightly within a series due to stoichiometry.
All adhesive joints were cured for 24 hours
at room temperature, followed by a 2 hour
postcure at 60 °C; regardless of the substrate
used or the test performed. Note that
90 - 95 % of the lap shear strength of the
postcured samples can be achieved by simply curing for one week at room temperature. Hence, postcuring at elevated temperatures is therefore not necessarily a "must".
The glass transition temperatures, determined by DMTA, were found to be around
70 – 80 °C for all formulations. The addition of nanoparticles has no apparent influence on Tg.
The adhesive fracture energy, determined
using the tapered double-cantilever beam
test (BS 7991) was investigated first. The
substrate material was a chromic-acid
etched automotive aluminium alloy. The
results are shown in figure 2. There is a
remarkable increase in toughness for low
addition levels of nanoparticles. When lower elastomer levels are used, in series 2Ca
and 2Cb, the maximum toughness moves
towards lower nano addition levels, and the
loss in performance with higher levels of
nanoparticles is more drastic.
Lap shear tests were performed according
to DIN 55-283 using untreated automotive
aluminium alloy (grade 6016) and Anticorit
RP 4107 S oil pretreated electrogalvanized
steel. (0.8 mm thick). The results are given
in figures 3 and 4.
With untreated aluminium substrates,
significant improvements in lap shear
strength are found when surface modified
Figure 5: Roller peel strength on chromic acid etched aluminium versus addition
level of nano-SiO2
Figure 6: Wedge impact performance on degreased steel versus addition level
of nano-SiO2
Figure 7: Lap shear strength after VDA ageing versus addition level of nano-SiO2
nanoparticles are added. Again the performance goes through a maximum at low
addition levels. However, in contrast to the
fracture energy results, the maximum shifts
to higher addition levels of nanoparticles
with lower rubber contents. Chromic acid
etched aluminium substrates show similar
behaviour.
The lap shear test results with oil-treated
steel as substrate are less evident. Series
2Cb shows the typical curve with significantly improved lap shear strengths and a
maximum at low concentrations of nanosilica. With 12.5 % and 18 % reactive liquid
rubber (series 2Ca and 2C) there seems to
be no improvement.
For further investigations the model adhesive formulations of series 2C were chosen.
Figure 5 shows the results of the roller peel
tests according to ASTM D 3167 using
a chromic acid etched aluminium alloy
(type 2024 T3).
Tests were performed at room temperature
and at -40 °C. At low temperatures a more
distinct maximum and a higher absolute
value are found, together with a slight shift
of the maximum towards lower levels of
nanoparticles. Compared to the toughened
rubber-modified adhesive without nanoSiO2 the performance is improved by more
than 100 %.
For the wedge impact tests according to ISO
11343, performed at 2 m/s, degreased steel
(grade ST 05, 0.8 mm thick) was used as
the substrate. Cohesive failure was found
with all test specimens. The test results are
given in figure 6. Again the typical picture:
an extraordinary improvement in toughness
is found at low levels of Nanopox in the
formulation. Further trials at low temperatures are under way.
Lap shear strength test specimens using the
untreated automotive alloy (grade 6016)
were subjected to 10 cycles of ageing according to VDA 621-415. Each cycle consists
of 1 day of salt spray, followed by 4 days of
condensed water climate changes, followed
by two days of room temperature storage.
The unaged test specimens were stored for
70 days at ambient temperature.
After ageing the typical picture regarding
the relationship between lap shear strength
and nano filler particle content is still more
or less the same. The performance level is
still above the automotive industry requirements regarding hem flange adhesives for
hang-on parts.
When evaluating the lap shear strength
after ageing it should be noted that model
adhesive formulations have been used. By
adding corrosion inhibitors, e. g. zinc phosphates, and other stabilizers the loss in
strength after ageing should be reduced
significantly.
The results of further investigations regarding low temperature performance (e. g.
wedge impact) and cataplasma ageing
should be available soon.
Having conclusively shown the existence of
a synergy between reactive liquid rubbers
(Hycar RLP) and surface-modified SiO2nanoparticles (Nanopox), still the question
regarding the mechanism responsible is not
answered yet.
In other studies it was confirmed that
nanoparticles are extremely effective at
reinforcing resins due to their huge surface
area causing very intensive interactions with
the resin matrix.
The molecular network of the room temperature curing systems investigated is relatively wide meshed, due to the crosslinked
elastomer molecules which do not participate in the phase separation and rubber
domain formation. The polyamidoamine
molecules of the hardener exhibit a certain
molecular chain length as well. Evidently
the two separate mechanisms of reinforcement by filler particles and toughening
by elastomer domains complement each
other. The clarification of the mechanism
of this "µm - nm"-synergy is the subject of
intensive research.
The results of this investigation clearly show
the potential of room temperature curing
epoxy adhesives. There is a huge interest
not only for automotive applications, but also for adhesive bonding of composites, e. g.
wind turbine blades, and aerospace applications.
Summary
Toughening using reactive liquid rubbers is
used in many structural adhesives applications and gives an excellent price/performance ratio. By the synergistic combination
of "classic" rubber toughening and the addition of surface-modified SiO2-nanoparticles,
room temperature curing epoxy adhesives
can be raised to new levels of performance.
These new high-performance raw materials open up exciting new applications for
two part epoxy adhesives.
π
Authors & Literature
Prof. Dr. A. J. Kinloch, J.H. Lee, Dr. A. C. Taylor: Imperial College London, Mechanical
Engineering Dept., Exhibition Road, London SW7 2BX, U.K., www.me.ic.ac.uk/AACgroup/
D. Egan: Noveon Inc., 9911 Brecksville Rd, Cleveland, OH 44141, U.S.A.,
www.noveoninc.com
Dr. C. Eger, Dr. S. Sprenger: hanse chemie AG, Charlottenburger Str. 9,
D-21502 Geesthacht, Germany, www.hanse-chemie.com
stephan.sprenger@hanse-chemie.com
Acknowledgements:
The authors wish to thank DaimlerChrysler AG Sindelfingen (Germany) PWT/VWL
for performing the VDA ageing tests.
/1/ N.N.: "Review: Two-Component Structural Adhesives for Metals, Plastics and other
Substrates", Technical Information BFGoodrich Company (1992)
/2/ Kinloch, A.J.: "Adhesives in Engineering"; Proc. Inst. Mech. Eng.; 48th T. Hawskley
Mem. Lect., London, 11.12.96 (1996)
/3/ Sprenger, S.; Eger, C.; Kinloch, A.; Taylor, A.; Lee, J.; Egan, D.: "Nanoadhesives:
Toughness and high strength", Adhäsion, Kleben & Dichten, 03/2003, pages 24 – 30
(2003)
/4/ Kinloch, A.J., Lee, J.H., Taylor, A.C., Sprenger S., Eger, C., Egan, D.: "Toughening
Structural Adhesives via Nano- and Micro-Phase Inclusions", J. Adhesion 79,
Number 8 – 9, pages 867 – 873 (2003)
/5/ Product profile Nanopox®; hanse chemie AG (2002)
/6/ Patent application WO 02/08 37 76 A1 (2002)
/7/ Patent pending