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Andreas Lutz

For 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 problemWhen 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 solutionIn 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 newThe 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)

Impa

ct p

eel s

tren

gth

[N/m

m]

40

30

20

10

0New two-component repair adhesive Conventional repair product

Z, 2

3 °C

Z, –

20

°C

ZE, 2

3 °C

Z, –

20

°C

ZE, 2

3 °C

AC 1

20, 2

3 °C

Z, 2

3 °C

AC 1

20, –

20

°C

AC 1

20, 2

3 °C

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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 strengthTable 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.

SummaryA 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