Forschungsinstitut für Pigmente und Lacke, Stuttgart, FPL

Assignment: Investigate the compatibility of selected top coats with primer 900226/32

Client: Zipperling Kessler & Co(GmbH & Co).

Authors: Dr. T. Schauer, Dipl. Chem. A. Joos, E. Praschak

Stuttgart, 24.07.1996

1. Questions to be investigated and object of study

Several factors are important when using organic coatings to protect steel against corrosion. The properties of the primer such as adhesion, porosity and the direct inhibition of corrosion reactions play the decisive role here. Such factors as inter-layer adhesion, porosity and hardness of the top coat must also be regarded as important parameters.

The aim of this study was to investigate the compatibility with the polyaniline primer of top coats based on various binders. Properties to be measured were pendulum hardness, dry and wet adhesion. Selected top coats were also to be subjected to the salt spray test and outdoor weathering conditions.

2. Experimental design

2.1 Substrate

Body panels (150x80x1.5 mm) from Mercedes-Benz were used as substrate. The panels were blasted with îElektrokorund NK 1 No. 100” (Messrs. Würth) and then degreased with a mixture of acetone, ethylacetate and xylene (1:1:1).

2.2 Coatings

The polyaniline primer 900226/32 was applied using pneumatic spray units. The thickness of the coating after drying averaged 27.7 ± 2.5 µm.

The top coats were sprayed on after the primer had dried (24 h). The formulations were used as such, except that thinner was added to improve sprayability. Table 1 shows the formulations of the top coats and the thicknesses of the layers. A total of six two-layer and two three-layer systems were produced.

2.3 Conditioning

To ensure complete drying and curing of the coatings, the coated panels were stored in a climate chamber at a constant temperature of 23°C and 55% relative humidity. The state of the coatings was assessed by measuring their pendulum hardness by the König method (DIN 53 157, 1/87).

2.4 Measuring dry and wet adhesion

To measure dry adhesion, stamps (diam. 7mm) were fixed to the coatings with a two-component epoxy adhesive and subsequently removed with the aid of a puller (Messrs. Instron). The pull-off forces were registered with a measuring sensor and averaged from five independent readings.

Table 1

Formulations and layer thicknesses of top coats tested on the polyaniline primer

top coat

Name

 Formulation/ Parts by wt.

Layer thickness in µm

2-comp. epoxy top coat

'top coat 1'

ERCD 7001 100

Hardener ERX 17

Thinner 12.5

154 ± 7.3
2-comp. epoxy top coat

'top coat 2'

Component A 88

Component B 12

Thinner EG 60

173 ± 7.9
2-comp. polyurethane top coat

'top coat 3'

Base 100

Additive 459 25

Thinner 12.5

123 ± 5.3
2-comp. polyurethane top coat

'top coat 4'

Component A 90

Component B 10

Thinner EG 4

187 ± 7.5
2-comp. acrylic top coat

'top coat 5'

Base 100

Hardener 2330/00 15

Thinner 11224 5

131 ± 16
1-comp. acrylic top coat

'top coat 6'

Paint 100

Thinner 5

144 ± 9.6

Intermediate layer - 'top coat 1'

top coat - 'top coat 3'

'top coat 1'/'top coat 3'

 

50*

197 ± 8.3**

Intermediate layer - 'top coat 1'

top coat - 'top coat 4'

'top coat 1'/'top coat 4'

 

50*

234 ± 15.6**

*Intermediate layer thickness, **Total layer thickness

To measure wet adhesion the test pieces were stored in distilled water. At certain times the test pieces were removed and measured in the same way as for dry adhesion. Since the adhesion of the top coat falls off with storage in water, it was possible for this measurement to fix the stamps to the coating with a commercially available superglue.

2.5 Salt spray test in accordance with DIN 50021-SS

Three two-layer systems ('top coat 1', 'top coat 5' and 'top coat 6') were subjected to the salt spray test. Two panels for each system were scratched with a scratching tool (Sikkens). The back of the test pieces was covered with an adhesive film and the edges were also treated with protective varnish. The prepared panels were placed in a spray chamber (Messrs. Erichsen) and continuously sprayed with NaCl solution (50g NaCl to 1l water, pH 7). During the test the chamber was heated to 35°C.

At certain times the panels were removed and visually assessed for under-film corrosion (DIN 53 167), blistering (DIN 53 209) and degree of rusting (DIN 53 210).

2.6 Outdoor weathering

Two panels each of the same top-coat systems as for the salt spray test were exposed to the local climate and other environmental influences at Hook of Holland on 03.05.96. The coatings were scratched, and the panels were covered on the back with an adhesive film and the edges also treated with a protective varnish.

Outdoor weathering is scheduled to last at least two years. A first inspection of the panels will take place in October 1996 and the results will be notified to the client.

3. Test results

3.1 König pendulum hardness

The pendulum hardness measurements are summarised in Fig. 1 for two-layer systems and in Fig. 3 for three-layer systems.

It will be seen from Fig. 1 that the pendulum hardness of the coating systems investigated is heavily dependent on the top coat used. It is possible to rank the top coats in order of declining pendulum hardness:

'top coat 1' > 'top coat 4' > 'top coat 6' > 'top coat 5' > 'top coat 3' > 'top coat 2'

Looking at the individual binders, it is not possible to establish a general correlation between the type of binder and the pendulum hardness; one epoxy top coat ('top coat 1'), for example, displays the highest pendulum hardness figures, whereas another epoxy top coat ('top coat 2') is characterised by the lowest pendulum hardness values.

It can be seen that both the changes in pendulum hardness over time and the final figures are different for each top coat used and must be studied separately in each case.

3.2 Dry and wet adhesion

The results of the dry and wet adhesion measurements are summarised in Figs. 3 to 6. As far as dry adhesion is concerned (at the time of measurement 0:), the coating systems investigated can be ranked as follows:

'top coat 3' > 'top coat 5' > 'top coat 4' > 'top coat 1'/'top coat 4' > 'top coat 2' > 'top coat 1'/'top coat 3' > 'top coat 1'>'top coat 6'.


Fig. 1 König pendulum hardness for the two-layer systems investigated



Fig. 2 König pendulum hardness for the three-layer systems investigated

Exposing the coating systems to stress by immersion in water and tracing the changes in adhesion yields additional information about the resistance of the systems investigated, and it is also possible to say something about the compatibility of the polyaniline primer with the individual top coats.

Fig. 3 shows the pull-off figures for 'top coat 3' and 'top coat 1'/'top coat 3' coating systems.


Fig. 3 Dry and wet adhesion of 'top coat 3' and 'top coat 1'/'top coat 3' coating systems;
KD = cohesive failure in top coat, KG = cohesive failure in primer,
ZB = interlayer failure in top coat system

In the case of the 'top coat 3' top coat on the polyaniline primer the cohesive failures could only be detected in the first phase of the study. As time went on there was evidence of an increasing tendency for the fracture site to shift into the primer. In the final phase of the study as much as 50% of the fracture was observed in the primer, but there was no adhesive failure. The results indicate generally good compatibility of the 'top coat 3'/polyaniline primer system. The same system, but with the addition of an intermediate layer of 'top coat 1', also displayed no evidence of adhesive failure; in the course of the measurements, however, there were signs of a weakening of the inter-layer adhesion between the 'top coat 1' layer and the 'top coat 3' top coat.

Fig. 4 summarises the results of the dry and wet adhesion measurements for 'top coat 4' and 'top coat 1'/'top coat 4' coating systems. These systems were found to exhibit even better compatibility between the polyaniline primer and the 'top coat 4' top coat: no adhesive failures were detected, and the detachment occurred primarily in the top coat zone. The adhesion between 'top coat 1' and 'top coat 4' was also found to be good.


Fig. 4 Dry and wet adhesion of 'top coat 4' and 'top coat 1'/'top coat 4' coating systems;
KD = cohesion failure in top coat, KG = cohesion failure in primer

The pull-off strength measurements for the 'top coat 1' and 'top coat 2' coating systems are shown in Fig. 5. Both systems were found to display good compatibility: there were no adhesive failures, and the fracture site was mainly in the top coat zone.

Fig. 6 shows the results of the adhesion measurements for the 'top coat 5' and 'top coat 6' systems. In the case of 'top coat 5', despite the high dry adhesion value, there was found to be a marked reduction in the wet adhesion of the polyaniline primer. In the final phase of the study it was almost entirely adhesive failure that was recorded, though the absolute figures for pull-off strength were still above the limits of full delamination.

Relatively low pull-off strength values were found from the start for the 'top coat 6' system, mostly with cohesive failure in the top coat. As time went on the fracture site increasingly shifted into the primer phase. The low pull-off strength values show that the polyaniline primer and the 'top coat 6' are not compatible and that the weak point in this system is in the interfacial zone between the two layers. Water diffuses into this region, resulting in premature cohesive failure. Accelerated blistering can be expected in this system.


Fig. 5 Dry and wet adhesion of 'top coat 1' and 'top coat 2' coating systems;
KD = cohesive failure in top coat, KG = cohesive failure in primer


Fig. 6 Dry and wet adhesion of 'top coat 5' and 'top coat 6' systems;
KD = cohesive failure in top coat, KG = cohesive failure in primer,
AB = adhesive failure

3.3 Salt spray test

The salt spray test was performed on scratched panels for three coating systems - 'top coat 1', 'top coat 5' and 'top coat 6'. Under-film corrosion, blistering and degree of rusting were assessed. The results of this test are summarised in Table 2.

Table 2

Results of salt spray test in accordance with DIN 50021-SS

Duration

'top coat 1'

'top coat 5'

'top coat 6'

h

U*

B**

R***

U

B

R

U

B

R

24

k****

k

k

k

k

k

k

k

k

72

k

k

k

k

k

k

k

m2/g4

k

96

k

k

k

k

k

k

k

m2/g4

k

168

k

k

k

k

k

k

k

m2/g5

k

241

k

k

k

k

k

k

k

m3/g5

k

337

k

k

k

k

m2/g5

k

k

m3/g5

k

505

k

k

k

k

m2/g5

k

k

m3/g5

k

607

k

k

k

k

m2/g5

k

k

m4/g5

k

770

k

k

k

k

m2/g5

k

k

m4/g5

k

1035

k

k

k

k

m2/g5

k

k

m4/g5

k

*U - under-film corrosion (DIN 53 167), **B - blistering (DIN 53 209),
***R - degree of rusting (DIN 53 210), k**** - none

The test showed that all three coating systems provide highly efficient corrosion protection and complete suppression of under-film corrosion over a period of more than 1000 h; this must be regarded as an indication of the good corrosion control properties of the polyaniline primer. In the case of 'top coat 6', however, there was a marked tendency to form blisters, and this made itself felt after only 72 h. The same applied to 'top coat 5' top coat, though here the blistering was not observed until after 337 h. The results of the salt spray test indicate a general need for individual investigation of the compatibility of the top coat with the polyaniline primer.

4. Conclusions

  1. The measurements of the adhesion of coating systems with the polyaniline primer showed that as a rule this primer adheres to the steel surface. In most of the coating systems investigated a cohesive failure took place and the bond between the polyaniline primer and the substrate remained intact. No corrosion phenomena occurred during the continuous exposure to stress by immersion in water.
  2. The polyaniline primer displayed differences in compatibility with the various top coats; on the basis of the tests performed the compatibility rankings are as follows:

    'top coat 1'>'top coat 2'='top coat 4'>'top coat 3'>'top coat 5'>'top coat 6'

    The results indicate that every coating system with the polyaniline primer should be tested for compatibility between the primer and the individual top coat.

  3. The results of the salt spray tests confirm that increased susceptibility of the coating to water can be expected where compatibility between the primer and the top coat is poor. This was the case with 'top coat 5' and 'top coat 6', and manifested itself in the poor inter-layer adhesion and/or reduced adhesion to the substrate, and in the tendency of the coatings to form blisters. In practice this can result in premature failure of the coating.
  4. No corrosion of the substrates was detected for any of the coating systems tested, either in the salt spray test or in the wet adhesion tests. This points to the good corrosion control properties of the polyaniline primer.
  5. The pendulum hardness measurements provide information about the course of complete drying and curing of the coatings over time, though the absolute pendulum hardness values do not correlate so well with the wet adhesion or corrosion resistance of the coatings.

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