Non-destructive adhesion control

Transcription

1 Non-destructive adhesion control Impedance spectroscopy can continuously monitor the intercoat adhesion. Andrzej Miszczyk, Thadeus Schauer. In contrast to other test methods, impedance spectroscopy (IS) is capable to measure the intercoat adhesion of, e.g., basecoat/clearcoat systems in situ and non-destructively. Specifically, the loss of interlayer adhesion follows the same trend as the changes in activation energy of ion mobility in the interlayer region, which are measurable via IS. Using conductive microelectrodes between the layers, the method can continuously monitor and quantify adhesion changes under real conditions. The adhesion of coatings to the substrate and the interlayer adhesion between separate coating layers are important parameters that determine the quality, protective properties and long time performance of coatings [1-3]. From a theoretical point of view, adhesion strength originates from the interfacial and intermolecular forces. The measured adhesion strength, defined as the force needed to detach a coating, is usually greater because the applied force is not only used for delamination but also for plastic deformation and other irreversible processes [4]. This is why the evaluation of the real adhesion is a challenge, especially in the case of systems with a high adhesive bond. Also, variable environmental parameters like temperature, humidity and UV radiation influence the protective properties of coatings [5] and in particular the adhesive strength [2, 6]. The known adhesion tests are mostly destructive and do not allow the monitoring of such influences. Thus, there is a need for new alternative methods for a quantitative evaluation of the adhesion of coatings, the early detection of its loss and the monitoring of its changes. Impedance spectroscopy (IS) was chosen for this purpose. For electrochemical impedance spectroscopy (EIS) measurements [7-10] to be carried out, an electrolyte cell is necessary. However, impedance spectroscopy monitoring of interface adhesion and the protective properties of coatings requires no such cell. The method is therefore suitable not only for laboratory testing but also for the monitoring of coatings at atmospheric conditions. This paper highlights the details and capabilities of this new measuring technique. Materials and methods An automotive waterborne base coat and a two-component waterborne polyure-thane clear coat were tested on standard steel test panels (190 mm x 105 mm x 1 mm) at thicknesses of 25 µm and 30 µm, respectively. Two ca. 5 µm thick, 1 cm wide and 8 cm long stripes of an electroconductive ink ("Orgacon E1-P1030", Agfa-Gevaert, Belgium) were applied on the basecoat 1 cm apart from each other and then overcoated with a clearcoat. They were used as sensor electrodes in the impedance measurements (Figure 1). For the temperature variations of the sample, a programmed PID heating device was used. EIS measurements were carried out on samples exposed to a defined relative humidity (rh). These were calibrated with saturated salt solutions of potassium carbonate (43% rh), potassium chloride (84% rh) and potassium sulphate (97% rh) (Figure 1). Impedance data were obtained with a two-electrode system "Solartron 1255" Frequency Response Analyzer and a high impedance buffer "Atlas 9881" over a frequency range from 64 khz down to 1 mhz using a sinusoidal signal with a 60 mv amplitude. A Faraday cage was used for electromagnetic shielding. Humidity changes highly affect the impedance The initial impedance data for specimens, after conditioning them in an exsiccator at 4% rh to reduce the influence of moisture, are typical for dielectric materials (Figure 2). To monitor the influence of humidity, the specimens were then exposed consecutively to atmosphere with 43% rh, 97% rh, 43% rh, and 83% rh, each for two days, at a temperature of 23 C (Figure 3). Impedance spectra were taken at different times of exposure at the different humidity levels (Figure 4). The results for one identical specimen after 24 h of immersion in water are also shown. In Figure 4, a strong dependence of the interlayer impedance on the air humidity can be inferred. The tendency for the impedance at low and high frequency limits to decrease with increasing humidity is also evident. This behaviour can be associated with the water ingress into the coating and the interlayer region, and with a build-up of water conductive paths. The fast full regeneration of the interlayer impedance upon the return from high to low humidity loading is also remarkable. This is evidence for a quasi-reversible transport of water into and out of the coating, driven by the change in outer humidity. Interlayer region tends to accumulate water To compare the interlayer impedance with the integral impedance of the whole coating, measurements using the conventional working electrode/auxiliary electrode geometry were carried out for the same loading conditions. The impedances for this measurement are more than one order of magnitude greater than those for the interlayer region. This indicates that the interlayer region is a weak point of the tested coating system and is more likely to accumulate water and develop conductive paths than the bulk of the coating [11]. The ingress of water vapour into the coating causes a decrease of the impedance modulus as well as the disappearance of the high-frequency time constant and the emergence of a new one at lower frequencies. Such behaviour can be related to the detection of imperfections (electrolytic paths) between coating layers. Under the influence of water as a penetrating agent, the imperfections/weak places are filled and conductively activated. The high-frequency part of the equivalent circuit is defined by the dielectric behaviour of the bulk of the coating (C 1 ) and the interlayer resistance (R 1 ) [12] (Figure 5). The low-frequency part of the equivalent circuit describes additional effects coming from the bulk of the coating and the interlayer under loading with water (C 2, R 2 ) [12]. The ionic mobility in an interlayer region depends on the attractive forces between the layers. In a strong field of high attractive forces between the layers (good adhesion), the ionic mobility will be smaller as in a weak field of attractive forces (poor adhesion), as schematically shown (Figure 6). Pronounced temperature dependence To obtain the temperature dependence of the impedance, measurements were made at temperatures of 30, 35, 40, 45, 50, 55 and 60 C with rh values of 43%, 84%, and 97%. Examples of impedance spectra for the interlayer region, obtained at 40 and 60 C with these humidities are shown in

2 Figures 7 and 8. The strong influence of both humidity and temperature on the impedance of the interlayer region is evident. To assess the influence of the air humidity on the ionic mobility in the interlayer region, the semicircle diameters in Figures 7 a and b were plotted as a function of 1/T, where T is the absolute temperature (Arrhenius plot, Figure 8). The slopes of the curves are directly related to the activation energy of the ionic mobility. Consequently, this activation energy, which is a measure of the ionic mobility in the interlayer region, can be taken as a criterion for the quantification of interlayer adhesion. It has been shown [13] that the same trend exists between the results of blister tests and the activation energy of the ionic mobility, but with the blister test the interlayer adhesion can be evaluated directly. Both data show a similar trend and correlate well: the higher the activation energy, the better the adhesion. Non-destructive, continuous adhesion monitoring Until now it has been impossible to monitor changes in the interlayer adhesion during the atmospheric exposure because of the destructive character of conventional adhesion tests. With this new technique, it is possible to monitor continuously the influence of environmental parameters on the adhesion strength. For instance, Figure 9 shows the changes of the real part of the impedance at 10 khz during a sinusoidal temperature cycling between 30 C and 70 C at rh 40 %. From this plot, the interdependence between the measured real part of the impedance and the temperature of the environment is evident. Using data at a frequency of 10 khz is advantageous over low-frequency data, because the latter generally need an additional fitting step to give the same information. Using the impedance data, it is possible to derive the activation energy during each cycle from the relative Arrhenius plots. The Arrhenius plots for the separate temperature cycles are shown in Figure 10. To make the procedure as simple as possible, the data refer to a maximum and a minimum temperature of 70 C and 30 C, respectively, and to one frequency. Following the changes of R 10kHz in the course of cycling, a very easy quantification and monitoring of the interlayer adhesion of coatings is possible. [13] A. Miszczyk, T. Schauer, Prog. Org. Coat., in press. Results at a glance - Impedance spectroscopy (IS) has been used for the quantitative evaluation and monitoring of the interlayer adhesion in an automotive coating system and for studying the effect of moisture and temperature on the interlayer adhesion. - A new concept of impedance measurements with conductive microelectrodes placed in the interlayer region is introduced. - The transport and accumulation of water and ions in the interlayer region can be evaluated. - A strong sensitivity of the interlayer impedance on the outer air humidity and temperature was observed and the formation of conductive paths was documented. - Interlayer adhesion loss follows the same trend as the changes of the activation energy of the ion mobility in the interlayer region. This can be derived from the interlayer resistance, measured with impedance spectroscopy. - From the Arrhenius plots of the interlayer resistance R the slope of R(1/T) can be determined. This parameter correlates with the activation energy of the ion/water diffusion in the interlayer region and can be regarded as a measure of the interlayer adhesion. - It is possible to record continuously the interlayer adhesion during the exposure time under various changing temperatures and humidities. This means that a simple and quick one-frequency measurement can be carried out for a given temperature. The authors: > Andrzej Miszczyk, Ph.D. is Senior Lecturer at the Department of Electrochemistry, Corrosion and Materials Chemistry, Faculty of Chemistry, Gdansk University of Technology, Poland. He is currently on the leave from the Faculty, conducting research at the Research Institute for Pigments and Coatings Stuttgart, Germany. > Dr. Thadeus Schauer is Department Head at the Research Institute for Pigments and Coatings Stuttgart, Germany. Acknowledge The authors acknowledge the financial support of the research project (AiF-No N) from the German Federal Ministry of Trade and Commerce which provided funds through the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen Otto von Guericke e.v. (AiF). References [1] C.H. Hare, Paint Film Degradation, Mechanisms and Control, Published by SSPC, Pittsburgh, 2002, USA [2] H. Leidheiser, W. Funke, JOCCA, 70, (1987), p. 121 [3] R.A. Dickie, Prog. Org. Coat. 25, (1994), p. 3 [4] M. Piens, H. De Deurwaerder, Prog. Org. Coat., 43, (2001), p. 18 [5] A. Miszczyk, K. Darowicki, Prog. Org. Coat., 46, (2003), 49 [6] D.Y. Perera, Prog. Org. Coat., 28, (1996), 21 [7] F. Deflorian, S. Rossi, P.L. Bonora, J. Coat. Technol., 72, (2000), 81 [8] J.J. Senkevich, J. Mat. Sci., 35, (2000),1359 [9] S. Duval, M. Kedam, M. Sfaira, A.Srhiri, H. Takenouti, J. Electrochem. Soc., 149, (2002), p. B520 [10] J.J. Suay, M.T. Rodriguez, K.A. Razzaq, J.J. Carpio, J.J. Saura, Prog. Org. Coat., 46, (2003), p. 121 [11] A. Miszczyk, T. Schauer, to be published. [12] J. Fleig, J. Maier, Proc. Electroceramics V, vol. 2, University of Aveiro, Portugal, 1996, p. 413.

3 Figure 1: Scheme of a specimen with conductive microelectrodes and the impedance spectroscopy measuring arrangement. Figure 2: Impedance spectrum for the interlayer region of a basecoat/clearcoat system at the beginning of the exposure at 4% rh.

4 Figure 3: Schematic diagram of the humidity changes for the specimen loading.

5 Figure 4: Impedance spectra for the interlayer region of a basecoat/clearcoat system after exposure to different humidities, and after 24 h immersion in water.

6 Figure 5: Equivalent circuits for a two-layer coating with weak places in the interlayer. left: unloaded; right: loaded with water vapour. C1, R1 and C2, R2 are the bulk capacity (C) and resistance (R) for an unloaded and a loaded specimen, respectively. Figure 6: Mass transport in an interlayer region with (a) low attractive forces between surfaces (low adhesion) and (b) high attractive forces (good adhesion).

7 .

8 Figure 8: Log R vs. 1000/T for various relative humidities. R is the intercept resistance at low frequencies. Figure 9: Real part of the impedance at 10 khz ReZ10kHz depending on the temperature cycling for the basecoat/ clear-coat system.

9 Figure 10: Real part of the impedance at 10 khz (R10kHz) as a function of 1/T, for a number of consecutive temperature cycles between 70 C and 30 C, and the corresponding delta R data.