1. Andreas Lutz
F
or the last ten years or so, spe-
cially flexibilised one-component
epoxy structural adhesives have
been used alongside conventional join-
ing methods in vehicle body shells. As
well as offering a significant increase in
stiffness, these adhesives also produce
highly durable bonds and improve the
impact strength of the body under dy-
namic loads. These structural adhesives
are growing in importance because of
the increased use of modern high-
strength steels and special alloys in ve-
hicle bodies. In particular in the case of
high-strength steels, there is a signifi-
cant improvement in the durability of
joints as a result of the use of adhesives
when compared with conventional join-
ing processes.
The problem
When repairing bodywork, it is impor-
tant to return it to its original state. Un-
til now, two-component epoxy structural
adhesives have been used. Although
these adhesives can be cured at room
temperature, they do not have the ideal
properties for this type of application.
For example, in comparison with one-
component structural adhesives, they
produce bonds with less mechanical
strength and, in particular, lower levels
of overall dynamic strength. As a result,
it is not possible to restore the bodywork
components to their full, original
strength.
The solution
In order to meet these requirements for
repairs, a crash-resistant two-compo-
nent structural adhesive has been devel-
oped, with mechanical properties very
similar to those of original equipment
one-component systems.
Crash-resistant two-component adhesives for repairs
As good as new
The use of adhesives for bonding high-strength steels is preferable
to other conventional joining methods because of the increased durability
of the joints. Two-component epoxy adhesives have in the past been used
for repairs, but they were not able to restore the bodywork components
to their full, original strength.
Figure 1: Impact peel strength of the new two-component repair adhesive at 23 °C
and -40 °C in comparison with a conventional product when used on medium-
strength steels and aluminium (Z: hot-dip galvanised steel, Ze: electrogalvanised
steel, AC 120: aluminium AA 6016, pretreated with Alodine 2040)
Impactpeelstrength[N/mm]
40
30
20
10
0
New two-component repair adhesive Conventional repair product
Z,23 °C
Z,– 20 °C
ZE,23 °C
Z,– 20 °C
ZE,23 °C
AC120,23 °C
Z,23 °C
AC120,–20 °C
AC120,23 °C
299-51637/0609

2. 10
The new adhesive was developed on
the basis of conventional two-component
structural adhesives, by making specific
modifications to both the A and the B
component using special polymers,
epoxy resins and adhesion promoters.
The special feature of these new resin
components is the ability to ensure the
efficient plasticisation of the adhesive in
its cured state. The result is a considera-
ble increase in the mechanical properties
of the bond when compared with existing
products, particularly under dynamic or
static peel load. The addition of the spe-
cial polymers has no negative effects on
key adhesive properties, such as the
modulus or glass transition temperature.
Mechanical strength
Table 1 shows the E-modulus and flexu-
ral strength of the newly developed
crash-resistant repair adhesive in com-
parison to previous two-component re-
pair adhesives and current crash-resist-
ant one-component structural adhesives.
Two striking features are the considera-
ble increase in flexural strength, which
indicates an improvement in the flexibi-
lisation of the adhesive, and a compara-
tively high modulus, which allows for a
stiffer bond similar to that produced by
a one-component bodywork adhesive.
The increased flexibility of the new
two-component repair adhesive is re-
flected, among other things, in its high
dynamic strength when compared with
current repair systems. The figures are
even comparable with those of crash-re-
sistant one-component structural adhe-
sives. Figure 1 shows the impact peel
strength of the adhesive bonds at differ-
ent temperatures on different types of
steel and on aluminium.
The quasi-static tensile shear
strengths and T-peel strengths are also
high and comparable with those of one-
component structural adhesives. Table 2
shows a summary of these figures. The
T-peel strengths of the new two-compo-
nent adhesive are higher than those of a
conventional product and similar to those
of a one-component structural bodywork
adhesive. The quasi-static tensile shear
strengths of the products are all similar.
Summary
A two-component repair adhesive has
successfully been developed which:
1. Has mechanical strengths similar to
those of crash-resistant one-compo-
nent structural adhesives
2. Offers significantly increased dy-
namic impact peel strength and static
T-peel strength compared with other
two-component adhesives.
3. Produces bonds with good levels of
mechanical strength on high-strength
steels.
4. Also gives positive results in conven-
tional corrosion tests.
These properties make the new two-
component adhesive ideally suited to
vehicle bodywork repairs, as it allows all
the properties of the body components
to be returned almost to their original
state. ¢
New crash-
resistant two-
component struc-
tural adhesive
Conventional
two-compo-
nent structural
adhesive
Conventional
crash-resistant
one-component
structural adhesive
E-modulus MPa 1100 1200 1400
Flexural strength % 25 10 15
New crash-
resistant two-
component struc-
tural adhesive
Conventional
two-compo-
nent structur-
al adhesive
Conventional
crash-resistant
one-component
structural adhesive
Tensile shear strength
[MPa]
Hot-dip galvanised steel 31 28.6 32.8
Electrogalvanised steel 21.9 21.7 22.6
AA 6016, pretreated
with Alodine 2040
21.2 22.2 23.1
T-peel strength [N/mm]
Hot-dip galvanised steel 9.2 6.4 10.2
Electrogalvanised steel 10.1 7.2 10.8
AA 6016, pretreated
with Alodine 2040
8 6 9
Author
Dr. Andreas Lutz (ALutz@dow.com) is
head of product development in the
Metal Bonding Europe division of Dow
Europe GmbH, Automotive, Horgen
(Switzerland).
Table 1: E-modulus and flexural strength of the new repair adhesive compared with
conventional one-component and two-component structural adhesives.
Table 2: Tensile shear and T-peel strength on steel surfaces. The new two-compo-
nent repair adhesive compared with a conventional product and a crash-resistant
one-component structural adhesive.
299-51637/0609