Customer reference: Plasma pre-treatment in car body construction

Introduction

The PiezoBrush PZ3 was used by a German sports car manufacturer as part of a series of tests on the efficiency of various surface treatments. The aim was to improve the adhesion of a contaminated surface. The PiezoBrush PZ3 was chosen for this purpose due to its handy design and ease of use. The short set-up time of the process is also a decisive factor. This enables acute, particulate surface activation even in areas that are difficult to access. In the experiment, anodized aluminium test specimens, some of which were oiled with grease, were treated. These were then bonded and subjected to a tensile shear test.

Specimen production

During the test, various aluminum sheets were provided with different degrees of oiling. The test specimens were then treated using the PiezoBrush PZ3. The Nearfield module was used for this. The parameters were set to 100 % power with a 15-second timer. In an alternating movement, the 25 x 12.5 mm processing surface was treated for 15 seconds in staggered paths at right angles to the direction of loading. A distance of approx. 2 mm from the joining part was maintained as constantly as possible. The sheets were then glued together promptly. The dimensions of the test specimens corresponded to those of DIN 1465 and the adhesive thickness was 0.2 mm.

Results

The treatment method was validated using two procedures. On the one hand, test inks were used to verify surface activation before bonding and a contact angle measurement was carried out using distilled water. On the other hand, bonded test specimens were subjected to the tensile shear test. An epoxy adhesive was used as the adhesive.

For the test with the test inks, the test specimens were treated longitudinally on one half (left in the picture) and then the test inks were applied. These clearly show the increase in surface energy. Figure 2 shows the results with a surface tension of the test ink of 40 mN/m.

Furthermore, when treating heavily oiled surfaces, it was noticed that the high temperatures lead to changes in the grease on the surface. Figure 3 shows a slightly altered structure in the marked area. This suggests that heating the grease above its typical processing temperature causes the grease to resinify.

The evaluation of the tensile shear tests and contact angle measurements was carried out in comparison to an untreated surface with the same degree of oiling.

Diagram 1 shows the results of the contact angle measurement.

The plasma pre-treatment lowers the contact angle of the water droplet. This indicates improved surface activation. However, a regressive progression of the contact angle over time can also be seen. This is due to residual surface contamination. The impurities are dissolved in the water droplet and reduce its surface energy.

The results of the tensile shear tests are shown in Diagram 2.

The tensile shear strengths show that plasma pre-treatment increases the adhesive strength up to a degree of oiling of 1. The plasma process is capable of fine cleaning up to this degree of oiling and simultaneously activates the surface. At higher degrees of oiling, the contamination layer prevents surface activation, which is why there is no increase in tensile shear strength. Instead, the tensile shear strength is reduced. This could be due to the fact that the high temperatures of the plasma resinify the oil layer. As a result, the adhesive is less able to incorporate the impurities into its polymer lattice and therefore achieves lower strengths.

Summary

Plasma treatment with the PiezoBrush PZ3 has great potential for the bonding preparation of slightly contaminated surfaces. By activating the aluminum surface, the tensile shear strength could be increased from 25.3 MPa to 26.9 MPa. The PiezoBrush PZ3 is particularly suitable for the pre-treatment of hard-to-reach adhesive seams due to its handy design and good surface activation properties. Further use for the partial surface pretreatment of selected components is to be evaluated in further steps with a focus on the parts for which the device can be used with economic advantages.