Volume 2 Extended Abstract 2
Submitted 26th August 1999
Evaluation of Amorphous Zirconia Films as adhesion promoters for organic coatings
F. J. Rodriguez#
, L. Fedrizzi*, R. Di Maggio, S. Rossi , G. Coco, P. Bonora
Dipartimento di Ingegneria dei Materiali, Università degli Studi di Trento
Via Mesiano, 77, 38050 Trento, Italy
*
Dipartimento di ICMMPM, Università di Roma I "La Sapienza"Via Eudossiana, Roma, Italy
#
Depto. Ingenieria Metalurgica, Facultad de Quimica, U.N.A.M., 04510 Mexico D.F., Mexico
Keywords: Organic coatings, Zirconia film, Adhesion, Corrosion.
Introduction
The most important properties of a protective coating are the adhesion to the substrate and the anticorrosive protection [1]. The last one should be obtained both as a barrier to aggressive chemical species or by using anticorrosive pigments like chromate or phosphate. Adhesion to the substrate has been object of some studies because it is supposed that if the protective system fail on adherence the total protection falls down [1, 2].
Chemical pre-treatments for improving adhesion properties of organic coatings on steel have been used for a long, and chromate and phosphate pre-treatments showed to be quite useful. However, by law, many countries have forbidden the use of toxic pre-treatments (chromate-like), in spite of their good anticorrosive properties. Because of that, industry is deeply concerned about the development of a chemical process able to substitute chromate pre-treatments.
The use of phosphate has shown good results on galvanised steel. So, it improves the adhesion of the paint film and the anticorrosive protection. But phosphates are not the best option in ecological terms. On steel it has been employed iron phosphate, zinc phosphate and tricationic one [3].
Other chromate-free pre-treatments have been developed and tested. The choice of the chemical conversion treatment must be based on several criteria: economical, ecological, technical, etc. Some of them have a chromate-like behaviour, but are not economically attractive. The application of zirconium oxide as a pre-treatment on steel appears a promising compromise. On the other hand, ZrO2-CeO2 films, put down on stainless steel by sol-gel techniques [4], resulted protective against dry and wet corrosion [5].
Sol-gel is a process to obtain films of metallic oxides in very mild conditions, but the reproducibility of thickness and quality of them depends on several parameters affecting the precursors solution. In this study it was evaluated if the use of two different complexing reagents and the control of the hydrolysis allows the formation of ZrO2 films suitable as pretreatments promoting adhesion of organic coatings.
Experimental
Low carbon steel was used as substrate. On 7.5cmx10cm degreased samples were deposited amorphous zirconia by dipping. After the deposition, the film underwent thermal treatments in oven at temperature between 150-250°C, in order to remove the most organic load. Repeating the deposition run, the thickness could be increased. The hydrolysis and therefore the formation of the film was made by reacting in controlled humidity chamber (70% RH) after each dipping, except in series 7 and 8 where this step was made using boiling water.
In Table I are reported the concentration of precursor, chelating agent, number of layers, temperature and time of the drying treatment.
Table I.
|
Series |
Alkoxide Molarity
(g/mole) |
N° layers |
Chelating agent |
Temperature (°C) |
Time (min) |
Comment |
|
1 |
0.3 |
3 |
Acetic acid |
250 |
30 |
--- |
|
2 |
0.3 |
3 |
Acetic acid |
250 |
10 |
--- |
|
3 |
0.3 |
3 |
Acetic acid |
150 |
30 |
--- |
|
4 |
0.3 |
5 |
Acetic acid |
150 |
30 |
--- |
|
5 |
0.6 |
3 |
Acetic acid |
150 |
30 |
--- |
|
6 |
0.3 |
3 |
Acetylacetone |
250 |
30 |
--- |
|
7 |
0.3 |
3 |
Acetylacetone |
250 |
30 |
Treated in boiling water |
|
8 |
0.3 |
3 |
Acetylacetone |
150 |
30 |
Treated in boiling water |
|
9 |
|
Degreased steel |
|
10 |
|
Tricationic phosphate |
|
11 |
|
Iron phosphate |
On comparison, the thickness of zirconia films on glasses deposited in the same conditions was measured by using a profilometer Dektak 3. The value was around 0.60 m
m, but for series 4 and 5 it increased up to about 1.0 m
m.
The series 9, 10 and 11 were used as blanks. Degreased steel was coated in order to get a blank with poor adhesion. Tricationic phosphate is a common chemical treatment that improves adhesion of the coating to the substrate as well as corrosion resistance. Iron phosphate has been studied because of its use on the industry due to the low cost.
A multilayer polyester paint coating formed by a primer and a topcoat was applied in all cases. The average dry film thickness (DFT) was about 30m
m. These kind of paints are applied in domestic electrical appliances [3].
Some samples were assessed by using cross-scratched coated samples on Salt Fog Chamber (ASTM B117) evaluating the damage. In order to assess the adhesion promoted by the pre-treatment, quantitative measurements using a pull-off test (Sebastian IV) and measurements of the swelling of the film using N-methyl pyrrolidone [6] , were made.
For the FTIR analysis we have used a Spectrometer BioRad FTS 165, coupled with a BioRad UMA 250 device for the analysis of some samples.
Results and discussion
The results obtained in Salt Fog Chamber test are shown in Table II.
The detachment near the scratch was measured by gently tearing off the disbonded area with a knife. Also blisters were observed in some cases. The disbondment was not uniform along the cross scratch, so the data reported in the table are average values. In Table II also the maximum and the minimum were shown, when the range was too wide.
These results evidenciated a prolonged thermal treatment improves the properties of the zirconia film, because of the evolution of the most of organic component favouring the formation of a ceramic coating on steel. Anyway, a few organic residuals were present in the film even after that treatment [7].
Comparing series 1 and 3, an analogous conclusion can be drawn. The thicker the film, the worse the performance, so that in order to eliminate the most amount of organic residuals and enhance the adhesion, the time or the temperature of the thermal treatment have to be increased.
Table II.
|
Series |
Detachment after 7 days (mm) |
Detachment after 21 days (mm) |
Comments |
|
1 |
0.3 |
0.7 (0.6 to 1) |
No blistering |
|
2 |
0.4 |
1.2 (1 to 1.3) |
No blistering |
|
3 |
0.5 (0.3 a 0.8) |
2 (1.8 to 2.5) |
Small blistering near the scratch |
|
4 |
0.8 (0.5 a 1) |
3.5 (3 to 4) |
Total detachment after 21 days. There were large blisters all over the samples. |
|
5 |
0.4 |
0.6 (0.5 to 1) |
It was difficult to detach |
|
6 |
1.5 (1 a 2) |
--- |
After 7 days, all the paint was detached |
|
7 |
0.6 (0.2 a 1) |
1.8 (1 to 2) |
Different detachment depending on the position in the cross scratch. Blistering |
|
8 |
1.1 (0.5 a 1.8) |
--- |
Disbonded area is brittle. Blistering |
|
9 |
6.5 |
--- |
Blistering all over the sample |
|
10 |
--- |
0.1 (0.1 to 0.2) |
It was difficult to detach |
|
11 |
0.4 |
2.5 (2 to 3) |
There was blistering mainly near the cross-scratch |
The use of a more concentrated solution of alkoxide seems to be positive for a good performance of the zirconia film. It is remarkable that the series 5 resulted the best, along with series 1, in promoting adhesion. In fact the detachment of the film after 7 days is not very different with series 3, made under the same conditions but with different concentration of alkoxide. Moreover the series 5 resulted even better than the other at longer time of analysis.
The microstructure of samples obtained from solution containing the acid appeared more suitable to adhere to the organic coating than the other ones. In fact the stronger chelating action of the acetylacetone with respect the acid and its very difficult hydrolysis could account for the worst behaviour of those samples.
On the other hand, there was no surprise about the results with degreased and coated steel, showing the larger detachment of all tested. It is clear that the use of a pre-treatment is needed, but it is also clear that there is a big difference between the tested ones. The use of iron phosphate as a pre-treatment shows good results in short times of exposure (7 days), compared with the zirconia film in series 1, 5 and 2, but it is not a good option in long services evaluation because its behaviour is one among the worst. Since now, tricationic phosphate looks the best option .
With a few precautions, a ranking of the adhesion behaviour in Salt Spray Fog of our samples could be done. These results brought about the further study was restricted to samples of series 1 and 5.
Swelling with N-methyl pyrrolidone (NMP)
Van Ooij et al [6] have proposed the measure of the adhesion of the paint coating on the base of amounts of its swelling in N-methyl pyrrolidone (NMP). In fact the swelling produces stress, which could detach the paint from the substrate. This test of adhesion of a film to the metal depends on the thickness of the film, the nature of the coating, and especially the pre-treatment on substrate surface. The results of this test are expressed as time of total detachment of the organic film in NMP. This parameter is named NMPRT (N-methyl pyrrolidone retention time). The test is based on the hypothesis that NMP acts on the organic coating, disregarding the substrate and pretreatment.
Actually the presence of organic residuals in the zirconia films of this study, even if negligible, could invalidate the assumption, so that the detachment could be due both to the stresses induced by swelling and a chemical-physical interaction between NMP and the zirconia films. In fact the results of NMP test on our samples didn’t match perfectly with those obtained from the Salt Fog Chamber and showed before. As an example the series 5, which were the best-ranked on Salt Fog Chamber, showed a NMPRT only of 35 seconds. The poor adhesion of the primer on the zirconia films could account for the small NMPRT values recorded.
On the contrary, there was no total detachment of the coating on the samples pre-treated with phosphate, but after 2 hours we could appreciate the formation of blisters reaching a 4M and 7M (according to ASTM D 714), for the iron and tricationic phosphates, respectively.
Anyway the findings showed that the zirconia films, even if are not as good as tricationic phosphate, are better than the iron phosphate.
In order to explain these results, FTIR analyses were recorded on the zirconia film after the detachment. With respect the FTIR spectrum on zirconia pretreatment after deposition, we can appreciate some modifications. In particular the presence of a signal due to the C=O bond asserts the retainement of organic substances, pyrrolidone or a polyester residue (the assignment couldn’t be made doubtless). Some studies about the effect of the NMP on the zirconia thin film will be addressed.
Pull-off test
The tests carried out with a pull-off device (Sebastian IV) have not been useful in this research, because of the type of failure. The failure mode was decohesive in the topcoat, and we did not see the primer. As a result we just can say that the mechanical features of the coating are poor.
Conclusions
On the base of these results, it is easy to infer that thin amorphous zirconia films are better pre-treatment than a commercial iron phosphate, even if it is not so good as the tricationic one. Also, the thermal treatment on forming the zirconia film is important and a very strict control in the use of temperature and time of treatment is a condition in order to obtain the best performance of the ZrO2 film.
References
1. H. Leidheiser, Jr., W. Funke, J.Oil Colour Chem. Assoc. 68, 121 (1985).
2. F. Deflorian, L. Fedrizzi, J. Adhesion Sci. Technol. 13 (5), 629-645 (1999).
3. Werther Neri, Introduzione alla Verniciatura delle Superfici Metalliche, 3a. Edizione, Ed. Tecniche Nuove, Milano, 1990.
4. M. Guglielmi, J. Sol-Gel Sci. Technol. 8, 443-449 (1997).
5. R. Di Maggio, S. Rossi, L. Fedrizzi, P. Scardi, Surface & Coatings Technol. 89, 292-298 (1997).
6. W.J.Van Ooij, R.A.Edwards, A.Sabata, J.Zappia, J.Adhesion Sci. Technol. 7 (8), 897-917 (1993).
7. R. DI Maggio, R. Campostrini, G. Guella, Chem. Mater., 10 (12) 3839-3847 (1998).
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