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clearcoat automotive paint systems by electrochemical properties measurements
Progress in Organic Coatings 54 (2005) 384–389
Evaluation of the weathering performance of basecoat/clearcoat automotive paint systems by electrochemical properties measurements N. Tahmassebi a,b , S. Moradian a,∗ , S.M. Mirabedini c a
Faculty of Polymer and Color Engineering, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran b R&D Center, Irankhodro Co., P.O. Box 11365-1313, Tehran, Iran c Iran Polymer and Petrochemical Institute, P.O. Box 14965-115, Tehran, Iran Received 9 May 2005; received in revised form 4 August 2005; accepted 5 August 2005
Abstract The aim of the present study was to investigate the possibility of evaluating the weathering performance of a basecoat/clearcoat automotive paint system through the determination of its electrochemical properties. To this end, the electrochemical properties of a basecoat/clearcoat automotive paint in a 3.5% solution of NaCl in deionized water were measured at different weathering exposure times. A constant phase element (CPE) was used for describing the electrochemical behavior of the coatings under test. The values of the CPE parameters, i.e. Y0 (the CPE constant) and n (the CPE power) were subsequently correlated to the extent of photo-oxidation (as measured by appearance, surface roughness, FTIR, surface tension and adhesion measurements) of clearcoat at the surface, in the bulk and at the interface between the basecoat and the clearcoat. The result showed that the electrochemical parameters Y0 and n provide ready means for comparing the weathering performances of basecoat/clearcoat automotive paint systems. Increases in the value of Y0 together with decreases in the value of n with increasing weathering exposure times suggest increased possibilities for the onset of cracking in the clearcoat itself in addition to its propagation towards the basecoat. Additionally, sudden variations in the values of Y0 and n are indicative of increases for the clearcoat to peel off. © 2005 Elsevier B.V. All rights reserved. Keywords: Basecoat/clearcoat; Automotive paint; Weathering performance; Electrochemical impedance spectroscopy; Constant phase element
1. Introduction The ability to accurately predict the weathering performance of automotive paint systems in short periods of time is essential for the design and development of new improved coating products. This prediction has become more difficult to do as modern basecoat/clearcoat automotive paint systems have supplanted traditional monocoat systems. The rate of weathering of traditional monocoat systems was proportional to the gloss loss and so was easily measurable at short times of outdoor exposure. In contrast, the basecoat/clearcoat systems usually possess excellent gloss and appearance retention ∗
Corresponding author. Tel.: +98 21 6468243; fax: +98 21 6468243. E-mail address: [email protected] (S. Moradian).
0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.08.004
over such comparative periods of time but fail eventually by catastrophic clearcoat cracking or peeling [1]. Extensive research has been undertaken to find a method for the evaluation of the weathering performance of modern basecoat/clearcoat automotive paint systems. At first, it was assumed that, cracking and peeling occur when a given amount of chemical degradation has taken place in the clearcoat. Therefore, a wide range of techniques were proposed to follow the encountered chemical changes including infrared spectroscopy [2–5], nuclear magnetic resonance spectroscopy [6], electron spin resonance [7], hydroperoxide titration [8] and time of flight secondary ion mass spectroscopy [9]. Weathering tests based on chemical degradation lead to the development of new resin and crosslinker formulations, hindered amine light stabilizers (HALS) and ultraviolet
N. Tahmassebi et al. / Progress in Organic Coatings 54 (2005) 384–389
light absorber (UVA) additives as well as new protocols for accelerated weathering tests [10,11]. However, weathering tests based on the rates of change in chemical compositions still could make no comment on the physical tolerance of the basecoat/clearcoat systems to such changes in the chemical composition. Therefore, Hill et al. proposed a dynamic mechanical analysis (DMA) to probe the network structure and physical repercussions of the undergone changes in the chemical composition as a function of weathering exposure [12]. While DMA is a powerful tool, it can only yield the net results of the chemical degradation in the basecoat/clearcoat network but fails to clarify the path by which these changes have been achieved. Chemical stress relaxation was offered in order to follow the path and net changes of the network and their physical repercussion [13]. Thereafter, it was argued that when a material fails due to the propagation of cracks, one of the most meaningful properties of the material relating to this failure could be the critical fracture energy or the fracture toughness. Therefore, determination of the critical fracture energy was proposed for the assessment of the weathering performance of the basecoat/clearcoat systems [14,15]. Investigating the critical fracture energy showed that even the most brittle basecoat/clearcoat systems would require an applied stress in order to crack. A mismatch in the humidity and thermal expansion coefficients between automotive paint layers and substrate will cause stresses to arise when the humidity and/or temperature of the environment are changed. If these stresses exceed the adhesive and or the cohesive strengths of the basecoat/clearcoat systems, then cracking or peeling will occur [16]. Therefore, workers considered the evaluation of the stress and the fracture energy in order to anticipate longterm weathering performances of basecoat/clearcoat automotive paint systems [17]. However, it must be noted that the evaluation stress and fracture energy in layers of multilayer automotive paint systems are by no means easily attainable mainly due to the difficulties in handling, preparation of samples and the lack of theoretical understanding of fracture mechanics of thin films. Therefore, on the basis of a presupposition of a correlation between stress development and electrochemical properties of coatings, attempts were made in the present paper to investigate the possibility of predicting the weathering performance of basecoat/clearcoat automotive paint systems through the determination of their electrochemical properties [18]. Measurement of electrochemical properties by the aid of electrochemical impedance spectroscopy (EIS) proved to be useful for the elucidation of several aspects of coatings such as adhesion, water permeability (humidity and water immersion resistance) and corrosion resistance [19–21]. On the other hand, investigation of the correlation between stress development and electrochemical properties of coatings shows that differences in the water uptake between the hydrophilic and hydrophobic regions of the coating may
385
cause development of local stresses. These local stresses create and enlarge the pathway passages of water into coatings and reduce barrier properties, i.e. losses in corrosion resistance [18]. Weathering gives rise to non-uniform degradation of clearcoats which themselves contain heterogeneous structures with low and high crosslink density domains [22]. Therefore, weathering may cause development of local stresses in clearcoats, enlarging the pathway passages of water resulting in a reduction in the barrier properties and an increase in water uptake.
2. Experimental 2.1. Investigated paint system The substrate used throughout the experiments was steel panel containing a zinc phosphate layer, which was further furbished by a chrome free treatment. The phosphated substrates were electrocoated at Irankhodro Car Factory by pigmented epoxy paint produced by PPG Industries and were cured at 180 ◦ C for 20 min. A further applied polyester/melamine primer layer provided by a local paint company (Taba Shimi Co.) was cured at 145◦ C for 25 min. A final coat of acrylic/melamine basecoat solvent borne containing aluminum flake and a medium solid solvent borne acrylic/melamine clearcoat produced by another local paint company (Rangafarin Co.) was applied wet on wet and cured at 145 ◦ C for 25 min. Details of the stabilization package are the proprietary of the coating supplier who did not provide information on HALS and UVA content. 2.2. Weathering conditions Accelerated weathering tests were performed in accordance to the Peugeot D27 1389-95 standard by means of an Atlas Xenotest Beta LM weather-o-meter containing a Xenon arc light source with inner and outer quartz/quartz filters and an irradiance of 0.55 W/m2 at 340 nm. Exposure condition included a 102 min time interval for the dry period (relative humidity 50%, dry temperature 54 ◦ C) followed by an 18 min time interval for spraying with deinodized water. 2.3. Photo-oxidation measurements The surface tension of the clearcoat was determined by means of a Kruss G10 Contact Angle Measuring System. Test fluids were water, formamide and diiodomethane. The surface tension has calculated from Ownes–Wendt–Rabel–Kaelble theory [23,24]. A Bruker FTIR Spectrometer model IFS48 was employed to determine the bulk and the interface photo-oxidation of the clearcoat. The clearcoat was scraped off and ground into a fine powder together with KBr, and then the clearcoat/KBr mixture was pressed into pellets for transmission FTIR analysis.
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N. Tahmassebi et al. / Progress in Organic Coatings 54 (2005) 384–389
2.4. Appearance measurements Specular gloss was measured at a 20◦ angle of incidence at different time intervals of exposure by means of a BYKGardner Micro-Tri Gloss® glossmeter and expressed the gloss retention, i.e. the percentage of gloss as a function of weathering time. Waviness of the clearcoat’s surface (orange peel) was measured by means of a BYK-Gardner Wave-Scan Plus 5 orange peel meter and expressed as the shortwaves.
Fig. 1. Equivalent electrical circuit.
2.5. Measurement of physical and mechanical properties Glass transition temperature of clearcoat was measured by means of a PL-DSC by with heating from 10 to 160 ◦ C with heating rate 10 ◦ C/min. Adhesion between the clearcoat and the basecoat was measured by means of an Erichsen’s adhesion Tester Model 525. 2.6. EIS measurements The three-electrode electrochemical cell was obtained by sticking a transparent plastic cylinder on the painted substrate under test and filling it with 3.5% aqueous solution of NaCl. The impedance data were obtained at the open circuit potential using a potentiostat/galvanostat PGSTAT 30 Autolab (Metrohm). The reference electrode was Ag/AgCl, while the counter electrode was a platinum rod. The operating parameters were the frequency range between 0.1 Hz and 50 kHz with eight frequencies within one decade and a 20 mV AC signal. The lower frequencies do not provide more information due to the relative capacitive impedance response of the coatings. The area of exposed working electrode to the electrolyte was 28 cm2 . Before the EIS measurements were started the panels were put in a constant air circulation stove for a period of 48 h at 40 ◦ C in order to lose the water uptake during the weathering test and water absorbed from the atmosphere. Five samples were used for each EIS measurements at different weathering time.
3. Results and discussions The EIS results were analyzed by fitting the data to an equivalent electrical circuit consisting of a CPE (constant phase element, QC ) in parallel with a coating resistance (Rp ) and both in series with the electrolyte resistance (Re ), as is shown in Fig. 1. The electrolyte resistance could be neglected because comparatively it is very small. Coating resistance was difficult to determine correctly due to the high impedance value of coating especially during the early stages of immersion times. Therefore, with this electrical circuit, it is possible to use a CPE for describing the electrochemical behavior of coatings. The CPE is an impedance element with the special property of having its phase angle independent of frequency. It is defined by Eq. (1) where Z is the impedance of the CPE
Fig. 2. The values of Y0 vs. immersion time.
in ohms, ω is the angular frequency in rads−1 , n is the CPE power, α is the constant phase angle of the CPE in rad and Y0 is the CPE constant in ohms−1 . Y0 has the numerical value of the admittance (1/|Z|) at the angular frequency equal to 1 rads−1 . The value of n indicates the extent to which the impedance of an intact paint film deviates from that of an ideal capacitor [25]. ZCPE =
(jω)−n Y0
when
n=
α π/2
(1)
n = 1 an ideal capacitance; n = 0 an ideal resistance; n = 0.5 the Warburg impedance. Figs. 2 and 3 show variation of the values of Y0 (the CPE constant) and n (the CPE power) versus the square root of immersion time in a NaCl solution for weathered and nonweathered coatings. As can be seen, the values of Y0 increase while the values of n decrease with immersion time as well as with the weathering exposure time. Weathering time also
Fig. 3. The values of n vs. immersion time.
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387
Fig. 4. The surface photo-oxidation vs. weathering time.
influenced the initial values of Y0 and n. In addition for exposure times of 558 and 800 h the differences between the values of Y0 and n of the coatings are relatively small while the differences in the values of Y0 and n between the coatings with 800 and 1010 h exposure time are relatively large. As is known, weathering causes photo-oxidation of the surface, the bulk and the interface of the clearcoat leading to the formation of polar groups [26]. On the other hand, the value of Y0 is proportional to the number and the polarizablility of the polar groups and the value of n reflects the interaction between the polar groups within the coating [25]. Therefore, electrochemical behaviors of weathered and non-weathered coatings may be explained through the photo-oxidation of the surface, the bulk and the interface of the clearcoat. Weathering causes an initial rapid increase in polarity of the surface of the clearcoat at the beginning of the exposure time, followed by a more gradual increase as is shown in Fig. 4. In fact, weathering induces an increase in the number and interaction of polar groups and in turn leads to an increase in the value of Y0 and a decrease in the value of n. Surface photo-oxidation also enlarges the surface microstructure and roughness of the clearcoat. As shown in Fig. 5, surface photooxidation and the surface roughness of the clearcoat show the same trend. Surface photo-oxidation gives rise to ablation of the surface of the clearcoat at the beginning of the weathering cycle hence increasing surface roughness. Continued weathering further ablates the top surface of the clearcoat until the surrounding matrix could no longer support the degraded
Fig. 5. The surface roughness vs. weathering time.
Fig. 6. FTIR spectra of bulk photo-oxidation (a) before and (b) after 558 h weathering time.
clearcoat, which falls off, and the depths of the roughness left by the degraded clearcoat are reduced and a new surface of the clearcoat appears. The newly produced surface contains lower polar groups, which in turn reduces the permeation of water into the clearcoat. Simultaneously with these changes in surface of the clearcoat, weathering also gives rise to bulk photo-oxidation of the clearcoat. The infrared spectra of the bulk of clearcoat before and after 558 h of weathering exposure time are shown in Fig. 6. The increase in the intensity of the OH, NH regions (3000–3600 cm−1 ) was attributed to the oxidative formation of alcoholic and carboxylic acid groups as well as the scissions of the acrylic/melamine crosslinks. The scission of the acrylic/melamine crosslinks leads to the formation of melamine methylol groups. These groups could self condense further to form melamine/melamine crosslinks. Evaluating bulk photo-oxidation of clearcoat shows that there are two competing phenomena. Firstly, the formation of hydrophilic groups increases the values of Y0 and then the mobility of polar groups, which in turn decreases the values of n. Secondly, the self-condensation of the melamine methylol groups causes a reduction in the polar groups and giving increases in n and Tg as is shown in Fig. 7. Reduction of polar groups is more evident for long-term weathering exposure times and also gives rise to the formation of a more dense polymeric structure and hence a reduction in the water uptake. At longer exposure times Tg would be increased further producing a more rigid polymeric structure, which may lead to the production of some micro-cracks as a result of loss of the coating’s ability to relax the stress. These micro-cracks favor a further permeation of water causing a sudden large increase in Y0 and a reduction in the value of n. Further weathering also causes interfacial photo-oxidation between the clearcoat and basecoat, which appears as a loss of adhesion as depicted in Fig. 8. Local loss of adhesion at the interface causes further accumulation of water in these regions leading to sudden changes in the values of Y0 and n.
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coatings due to the formation of polar groups. Increasing the number of polarizable groups will increase the dielectric constant and Y0 . But an increase of the intensity of interaction in combination with an increase in the number of polarizable groups will probably not be reflected by an increase in the dielectric constant. This is opposite to the CPE interpretation where it will result in an increase in Y0 and a decrease in n. In fact, the interpretation of the coating impedance using Y0 and n provides more detailed information about the mechanisms involved in the photo-oxidation and its effects on Tg and the coating’s ability to relax stresses.
4. Conclusion
Fig. 7. Tg vs. weathering time.
Generally, water uptake in weathered and non-weathered coatings, gives rise to breakage of intermolecular hydrogen bonds between the polar groups giving increases in the mobility of each polar group. Water uptake also decreases the Tg of the coatings due to plasticizing effects. The net result of these phenomena is an increase in the value of Y0 and a decrease in the value of n. Of course these changes are slowed down due to water saturation of the coating during longer water immersion times. Because the variations obtained in the CPE power (n) were very small and the value was always very close to 1, CPE could also be expressed by Eq. (3): Qc =
εε0 A d
(3)
where A is the exposed area of the working electrode, d the coating thickness, ε the dielectric constant of coating and ε0 is the free-space permittivity. The variation in QC with immersion time can be attributed to water uptake within the coating leading to increases in the dielectric constant of the coating. Therefore, the dielectric constant (ε) could be used as a deterministic parameter in order to follow the variations in the water barrier properties of the coating caused by the degree of photo-oxidation encountered in the accelerated weathering test [27,28]. The photooxidation phenomena increases the dielectric constant of the
Fig. 8. Adhesion vs. interface photo-oxidation of clearcoat.
Evaluation of the electrochemical properties of coatings provides ready means for comparing the weathering performance of basecoat/clearcoat automotive paint systems. Accordingly, initial high increases in the values of Y0 and reductions in the values of n will most probably initiate the onset of cracking in the clearcoat and its growth towards the basecoat. Sudden changes in the values of Y0 and n are indicative of a more increased tendency for the clearcoat to peel off. Additionally, with the present assessing procedure, it is possible to evaluate simultaneously the water uptake, corrosion resistance and weathering performance of the paint system under test.
References [1] D.R. Bauer, Chemical criteria for durable automotive topcoats, J. Coat. Technol. 66 (835) (1994) 57–65. [2] D.R. Bauer, Degradation of organic coatings. I. Hydrolysis of melamine formaldehyde/acrylic copolymer films, J. Appl. Polym. Sci. 27 (1982) 3651–3662. [3] D.J. McEwen, M.H. Verma, R.O. Turner, Accelerated weathering of automotive paints measured by gloss and infrared spectroscopy, J. Coat. Technol. 59 (755) (1987) 123–129. [4] D.R. Bauer, M.C. Peck, R.O. Carter III, Evaluation of accelerated weathering tests for a polyester–urethane coating using photoacoustic infrared spectroscopy, J. Coat. Technol. 59 (755) (1987) 103–109. [5] R.O. Carter III, M.C. Paputa Peck, D.R. Bauer, The characterization of polymer surfaces by photoacoustic fourier transform infrared spectroscopy, Polym. Degrad. Stab. 23 (1989) 121–134. [6] D.R. Bauer, R.A Dickie, J.L. Koenig, Magic angle 13 C NMR of cured and degraded acrylic copolymer/melamine formaldehyde coatings, J. Polym. Sci.: Polym. Phys. 22 (1984) 2009–2020. [7] J.L. Gerlock, D.R. Bauer, L.M. Briggs, R.A. Dickie, A rapid method of predicting coating durability using electron spin resonance, J. Coat. Technol. 57 (722) (1985) 37–45. [8] D.F. Mielewski, D.R. Bauer, J.L. Gerlock, Determination of hydroperoxide concentration in crosslinked polymer coatings containing hindered amine light stabilizer, Polym. Degrad. Stab. 41 (1993) 323–331. [9] J.L. Gerlock, T.J. Prater, S.L. Kaberline, J.E. de Vries, Assessment of photooxidation in multi-layer coating system by time of flight secondary ion mass spectrometry, Polym. Degrad. Stab. 47 (1995) 405–411.
N. Tahmassebi et al. / Progress in Organic Coatings 54 (2005) 384–389 [10] D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C. Paputa Peck, R.O. Carter III, Photo-stabilization and photo-degradation in organic coating containing a hindered amine light stabilizer. Part VII. HALS effectiveness in acrylic melamine coatings having different free radical formation rates, Polym. Degrad. Stab. 36 (1992) 9–15. [11] D.R. Bauer, J.L. Gerlock, R.A. Dickie, Rapid reliable tests of clearcoat weatherability: a proposed protocol, Prog. Org. Coat. 15 (1987) 209–221. [12] L.W. Hill, H.M. Korzeniowski, M. Ojunga-Andrew, R.C. Wilson, Accelerated clearcoat weathering studied by dynamic mechanical analysis, Prog. Org. Coat. 24 (1994) 147–173. [13] M.E. Nichols, J.L. Gerlock, C.A. Smith, Rates of photooxidation induced crosslinking and chain scission in thermoset polymer coatings. I, Polym. Degrad. Stab. 56 (1997) 81–91. [14] M.E. Nichols, C.A. Darr, C.A. Smith, M.D. Thouless, E.R. Fischer, Fracture energy of automotive clearcoats. I. Experimental method and mechanics, Polym. Degrad. Stab. 60 (1998) 291–299. [15] M.E. Nichols, J.L. Gerlock, C.A. Smith, C.A. Darr, The effects of weathering on the mechanical performance of automotive paint systems, Prog. Org. Coat. 35 (1999) 153–159. [16] M.E. Nichols, C.A. Darr, Effect of weathering on the stress distribution and mechanical performance of automotive paint systems, J. Coat. Technol. 70 (885) (1998) 141–149. [17] M.E. Nichols, Anticipating paint cracking: the application of fracture mechanics to the study of paint weathering, J. Coat. Technol. 74 (924) (2002) 39–46. [18] Y. Perera Dan, T. Nguyen, Hygroscopic stress and failure of coating/metal system, Paint, Varnish, Ink and Adhesive Industry, Eurocoat Congress, Italy, 1996.
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[19] A. Amirudin, D. Thierry, Application of electrochemical impedance spectroscopy to study the degradation of polymer coated metals, Prog. Org. Coat. 26 (1995) 1–28. [20] S.M. Mirabedini, G.E. Thompson, S. Moradian, J.D. Scantlebury, Corrosion performance of powder coated aluminum using EIS, Prog. Org. Coat. 46 (2003) 112–120. [21] Z. Ranjbar, S. Moradian, M.R. Mohammedzade Attar, EIS investigation of cathaphoreticaly electrodeposited epoxy coating having different EEWs, Prog. Org. Coat. 51 (2004) 87–90. [22] M.R. Vanlandingham, T. Nguyen, E. Byrd, J.W. Martin, On the use of the atomic force microscope to monitor physical degradation of polymeric coating surfaces, J. Coat. Technol. 73 (923) (2001) 43–50. [23] C.M. Hansen, Measuring surface tension: the droplet method, Eur. Coat. J. 11 (1994) 839–846. [24] M. Osterhold, K. Armbruster, Correlation between surface tension and physical paint properties, Prog. Org. Coat. 33 (1998) 197–201. [25] E.P. Van Westing, G.M. Ferrari, J.H.W. de Wit, The determination of coating performance with impedance measurements. I. Coating polymer properties, Corros. Sci. 34 (9) (1993) 1511–1530. [26] N. Tahmassebi, S. Moradian, Predicting the performances of basecoat/clearcoat automotive paint systems by the use of adhesion, scratch and mar resistance measurements, Polym. Degrad. Stab. 83 (2004) 405–410. [27] N. Tahmassebi, S. Moradian, S.M. Mirabedini, Anticipating the longterm weathering performance of basecoat/clearcoat automotive paint systems by electrochemical impedance measurement, Asia Pacific Coating show, Thailand, 2004. [28] F. Deflorian, L. Fedrizzi, S. Rossi, F. Buratti, P.L. Bonora, Electrochemical characterization of organic coatings for the automotive industry, Prog. Org. Coat. 39 (2000) 9–13.
Evaluation of the weathering performance of basecoat/clearcoat automotive paint systems by electrochemical properties measurements N. Tahmassebi a,b , S. Moradian a,∗ , S.M. Mirabedini c a
Faculty of Polymer and Color Engineering, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran b R&D Center, Irankhodro Co., P.O. Box 11365-1313, Tehran, Iran c Iran Polymer and Petrochemical Institute, P.O. Box 14965-115, Tehran, Iran Received 9 May 2005; received in revised form 4 August 2005; accepted 5 August 2005
Abstract The aim of the present study was to investigate the possibility of evaluating the weathering performance of a basecoat/clearcoat automotive paint system through the determination of its electrochemical properties. To this end, the electrochemical properties of a basecoat/clearcoat automotive paint in a 3.5% solution of NaCl in deionized water were measured at different weathering exposure times. A constant phase element (CPE) was used for describing the electrochemical behavior of the coatings under test. The values of the CPE parameters, i.e. Y0 (the CPE constant) and n (the CPE power) were subsequently correlated to the extent of photo-oxidation (as measured by appearance, surface roughness, FTIR, surface tension and adhesion measurements) of clearcoat at the surface, in the bulk and at the interface between the basecoat and the clearcoat. The result showed that the electrochemical parameters Y0 and n provide ready means for comparing the weathering performances of basecoat/clearcoat automotive paint systems. Increases in the value of Y0 together with decreases in the value of n with increasing weathering exposure times suggest increased possibilities for the onset of cracking in the clearcoat itself in addition to its propagation towards the basecoat. Additionally, sudden variations in the values of Y0 and n are indicative of increases for the clearcoat to peel off. © 2005 Elsevier B.V. All rights reserved. Keywords: Basecoat/clearcoat; Automotive paint; Weathering performance; Electrochemical impedance spectroscopy; Constant phase element
1. Introduction The ability to accurately predict the weathering performance of automotive paint systems in short periods of time is essential for the design and development of new improved coating products. This prediction has become more difficult to do as modern basecoat/clearcoat automotive paint systems have supplanted traditional monocoat systems. The rate of weathering of traditional monocoat systems was proportional to the gloss loss and so was easily measurable at short times of outdoor exposure. In contrast, the basecoat/clearcoat systems usually possess excellent gloss and appearance retention ∗
Corresponding author. Tel.: +98 21 6468243; fax: +98 21 6468243. E-mail address: [email protected] (S. Moradian).
0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.08.004
over such comparative periods of time but fail eventually by catastrophic clearcoat cracking or peeling [1]. Extensive research has been undertaken to find a method for the evaluation of the weathering performance of modern basecoat/clearcoat automotive paint systems. At first, it was assumed that, cracking and peeling occur when a given amount of chemical degradation has taken place in the clearcoat. Therefore, a wide range of techniques were proposed to follow the encountered chemical changes including infrared spectroscopy [2–5], nuclear magnetic resonance spectroscopy [6], electron spin resonance [7], hydroperoxide titration [8] and time of flight secondary ion mass spectroscopy [9]. Weathering tests based on chemical degradation lead to the development of new resin and crosslinker formulations, hindered amine light stabilizers (HALS) and ultraviolet
N. Tahmassebi et al. / Progress in Organic Coatings 54 (2005) 384–389
light absorber (UVA) additives as well as new protocols for accelerated weathering tests [10,11]. However, weathering tests based on the rates of change in chemical compositions still could make no comment on the physical tolerance of the basecoat/clearcoat systems to such changes in the chemical composition. Therefore, Hill et al. proposed a dynamic mechanical analysis (DMA) to probe the network structure and physical repercussions of the undergone changes in the chemical composition as a function of weathering exposure [12]. While DMA is a powerful tool, it can only yield the net results of the chemical degradation in the basecoat/clearcoat network but fails to clarify the path by which these changes have been achieved. Chemical stress relaxation was offered in order to follow the path and net changes of the network and their physical repercussion [13]. Thereafter, it was argued that when a material fails due to the propagation of cracks, one of the most meaningful properties of the material relating to this failure could be the critical fracture energy or the fracture toughness. Therefore, determination of the critical fracture energy was proposed for the assessment of the weathering performance of the basecoat/clearcoat systems [14,15]. Investigating the critical fracture energy showed that even the most brittle basecoat/clearcoat systems would require an applied stress in order to crack. A mismatch in the humidity and thermal expansion coefficients between automotive paint layers and substrate will cause stresses to arise when the humidity and/or temperature of the environment are changed. If these stresses exceed the adhesive and or the cohesive strengths of the basecoat/clearcoat systems, then cracking or peeling will occur [16]. Therefore, workers considered the evaluation of the stress and the fracture energy in order to anticipate longterm weathering performances of basecoat/clearcoat automotive paint systems [17]. However, it must be noted that the evaluation stress and fracture energy in layers of multilayer automotive paint systems are by no means easily attainable mainly due to the difficulties in handling, preparation of samples and the lack of theoretical understanding of fracture mechanics of thin films. Therefore, on the basis of a presupposition of a correlation between stress development and electrochemical properties of coatings, attempts were made in the present paper to investigate the possibility of predicting the weathering performance of basecoat/clearcoat automotive paint systems through the determination of their electrochemical properties [18]. Measurement of electrochemical properties by the aid of electrochemical impedance spectroscopy (EIS) proved to be useful for the elucidation of several aspects of coatings such as adhesion, water permeability (humidity and water immersion resistance) and corrosion resistance [19–21]. On the other hand, investigation of the correlation between stress development and electrochemical properties of coatings shows that differences in the water uptake between the hydrophilic and hydrophobic regions of the coating may
385
cause development of local stresses. These local stresses create and enlarge the pathway passages of water into coatings and reduce barrier properties, i.e. losses in corrosion resistance [18]. Weathering gives rise to non-uniform degradation of clearcoats which themselves contain heterogeneous structures with low and high crosslink density domains [22]. Therefore, weathering may cause development of local stresses in clearcoats, enlarging the pathway passages of water resulting in a reduction in the barrier properties and an increase in water uptake.
2. Experimental 2.1. Investigated paint system The substrate used throughout the experiments was steel panel containing a zinc phosphate layer, which was further furbished by a chrome free treatment. The phosphated substrates were electrocoated at Irankhodro Car Factory by pigmented epoxy paint produced by PPG Industries and were cured at 180 ◦ C for 20 min. A further applied polyester/melamine primer layer provided by a local paint company (Taba Shimi Co.) was cured at 145◦ C for 25 min. A final coat of acrylic/melamine basecoat solvent borne containing aluminum flake and a medium solid solvent borne acrylic/melamine clearcoat produced by another local paint company (Rangafarin Co.) was applied wet on wet and cured at 145 ◦ C for 25 min. Details of the stabilization package are the proprietary of the coating supplier who did not provide information on HALS and UVA content. 2.2. Weathering conditions Accelerated weathering tests were performed in accordance to the Peugeot D27 1389-95 standard by means of an Atlas Xenotest Beta LM weather-o-meter containing a Xenon arc light source with inner and outer quartz/quartz filters and an irradiance of 0.55 W/m2 at 340 nm. Exposure condition included a 102 min time interval for the dry period (relative humidity 50%, dry temperature 54 ◦ C) followed by an 18 min time interval for spraying with deinodized water. 2.3. Photo-oxidation measurements The surface tension of the clearcoat was determined by means of a Kruss G10 Contact Angle Measuring System. Test fluids were water, formamide and diiodomethane. The surface tension has calculated from Ownes–Wendt–Rabel–Kaelble theory [23,24]. A Bruker FTIR Spectrometer model IFS48 was employed to determine the bulk and the interface photo-oxidation of the clearcoat. The clearcoat was scraped off and ground into a fine powder together with KBr, and then the clearcoat/KBr mixture was pressed into pellets for transmission FTIR analysis.
386
N. Tahmassebi et al. / Progress in Organic Coatings 54 (2005) 384–389
2.4. Appearance measurements Specular gloss was measured at a 20◦ angle of incidence at different time intervals of exposure by means of a BYKGardner Micro-Tri Gloss® glossmeter and expressed the gloss retention, i.e. the percentage of gloss as a function of weathering time. Waviness of the clearcoat’s surface (orange peel) was measured by means of a BYK-Gardner Wave-Scan Plus 5 orange peel meter and expressed as the shortwaves.
Fig. 1. Equivalent electrical circuit.
2.5. Measurement of physical and mechanical properties Glass transition temperature of clearcoat was measured by means of a PL-DSC by with heating from 10 to 160 ◦ C with heating rate 10 ◦ C/min. Adhesion between the clearcoat and the basecoat was measured by means of an Erichsen’s adhesion Tester Model 525. 2.6. EIS measurements The three-electrode electrochemical cell was obtained by sticking a transparent plastic cylinder on the painted substrate under test and filling it with 3.5% aqueous solution of NaCl. The impedance data were obtained at the open circuit potential using a potentiostat/galvanostat PGSTAT 30 Autolab (Metrohm). The reference electrode was Ag/AgCl, while the counter electrode was a platinum rod. The operating parameters were the frequency range between 0.1 Hz and 50 kHz with eight frequencies within one decade and a 20 mV AC signal. The lower frequencies do not provide more information due to the relative capacitive impedance response of the coatings. The area of exposed working electrode to the electrolyte was 28 cm2 . Before the EIS measurements were started the panels were put in a constant air circulation stove for a period of 48 h at 40 ◦ C in order to lose the water uptake during the weathering test and water absorbed from the atmosphere. Five samples were used for each EIS measurements at different weathering time.
3. Results and discussions The EIS results were analyzed by fitting the data to an equivalent electrical circuit consisting of a CPE (constant phase element, QC ) in parallel with a coating resistance (Rp ) and both in series with the electrolyte resistance (Re ), as is shown in Fig. 1. The electrolyte resistance could be neglected because comparatively it is very small. Coating resistance was difficult to determine correctly due to the high impedance value of coating especially during the early stages of immersion times. Therefore, with this electrical circuit, it is possible to use a CPE for describing the electrochemical behavior of coatings. The CPE is an impedance element with the special property of having its phase angle independent of frequency. It is defined by Eq. (1) where Z is the impedance of the CPE
Fig. 2. The values of Y0 vs. immersion time.
in ohms, ω is the angular frequency in rads−1 , n is the CPE power, α is the constant phase angle of the CPE in rad and Y0 is the CPE constant in ohms−1 . Y0 has the numerical value of the admittance (1/|Z|) at the angular frequency equal to 1 rads−1 . The value of n indicates the extent to which the impedance of an intact paint film deviates from that of an ideal capacitor [25]. ZCPE =
(jω)−n Y0
when
n=
α π/2
(1)
n = 1 an ideal capacitance; n = 0 an ideal resistance; n = 0.5 the Warburg impedance. Figs. 2 and 3 show variation of the values of Y0 (the CPE constant) and n (the CPE power) versus the square root of immersion time in a NaCl solution for weathered and nonweathered coatings. As can be seen, the values of Y0 increase while the values of n decrease with immersion time as well as with the weathering exposure time. Weathering time also
Fig. 3. The values of n vs. immersion time.
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Fig. 4. The surface photo-oxidation vs. weathering time.
influenced the initial values of Y0 and n. In addition for exposure times of 558 and 800 h the differences between the values of Y0 and n of the coatings are relatively small while the differences in the values of Y0 and n between the coatings with 800 and 1010 h exposure time are relatively large. As is known, weathering causes photo-oxidation of the surface, the bulk and the interface of the clearcoat leading to the formation of polar groups [26]. On the other hand, the value of Y0 is proportional to the number and the polarizablility of the polar groups and the value of n reflects the interaction between the polar groups within the coating [25]. Therefore, electrochemical behaviors of weathered and non-weathered coatings may be explained through the photo-oxidation of the surface, the bulk and the interface of the clearcoat. Weathering causes an initial rapid increase in polarity of the surface of the clearcoat at the beginning of the exposure time, followed by a more gradual increase as is shown in Fig. 4. In fact, weathering induces an increase in the number and interaction of polar groups and in turn leads to an increase in the value of Y0 and a decrease in the value of n. Surface photo-oxidation also enlarges the surface microstructure and roughness of the clearcoat. As shown in Fig. 5, surface photooxidation and the surface roughness of the clearcoat show the same trend. Surface photo-oxidation gives rise to ablation of the surface of the clearcoat at the beginning of the weathering cycle hence increasing surface roughness. Continued weathering further ablates the top surface of the clearcoat until the surrounding matrix could no longer support the degraded
Fig. 5. The surface roughness vs. weathering time.
Fig. 6. FTIR spectra of bulk photo-oxidation (a) before and (b) after 558 h weathering time.
clearcoat, which falls off, and the depths of the roughness left by the degraded clearcoat are reduced and a new surface of the clearcoat appears. The newly produced surface contains lower polar groups, which in turn reduces the permeation of water into the clearcoat. Simultaneously with these changes in surface of the clearcoat, weathering also gives rise to bulk photo-oxidation of the clearcoat. The infrared spectra of the bulk of clearcoat before and after 558 h of weathering exposure time are shown in Fig. 6. The increase in the intensity of the OH, NH regions (3000–3600 cm−1 ) was attributed to the oxidative formation of alcoholic and carboxylic acid groups as well as the scissions of the acrylic/melamine crosslinks. The scission of the acrylic/melamine crosslinks leads to the formation of melamine methylol groups. These groups could self condense further to form melamine/melamine crosslinks. Evaluating bulk photo-oxidation of clearcoat shows that there are two competing phenomena. Firstly, the formation of hydrophilic groups increases the values of Y0 and then the mobility of polar groups, which in turn decreases the values of n. Secondly, the self-condensation of the melamine methylol groups causes a reduction in the polar groups and giving increases in n and Tg as is shown in Fig. 7. Reduction of polar groups is more evident for long-term weathering exposure times and also gives rise to the formation of a more dense polymeric structure and hence a reduction in the water uptake. At longer exposure times Tg would be increased further producing a more rigid polymeric structure, which may lead to the production of some micro-cracks as a result of loss of the coating’s ability to relax the stress. These micro-cracks favor a further permeation of water causing a sudden large increase in Y0 and a reduction in the value of n. Further weathering also causes interfacial photo-oxidation between the clearcoat and basecoat, which appears as a loss of adhesion as depicted in Fig. 8. Local loss of adhesion at the interface causes further accumulation of water in these regions leading to sudden changes in the values of Y0 and n.
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coatings due to the formation of polar groups. Increasing the number of polarizable groups will increase the dielectric constant and Y0 . But an increase of the intensity of interaction in combination with an increase in the number of polarizable groups will probably not be reflected by an increase in the dielectric constant. This is opposite to the CPE interpretation where it will result in an increase in Y0 and a decrease in n. In fact, the interpretation of the coating impedance using Y0 and n provides more detailed information about the mechanisms involved in the photo-oxidation and its effects on Tg and the coating’s ability to relax stresses.
4. Conclusion
Fig. 7. Tg vs. weathering time.
Generally, water uptake in weathered and non-weathered coatings, gives rise to breakage of intermolecular hydrogen bonds between the polar groups giving increases in the mobility of each polar group. Water uptake also decreases the Tg of the coatings due to plasticizing effects. The net result of these phenomena is an increase in the value of Y0 and a decrease in the value of n. Of course these changes are slowed down due to water saturation of the coating during longer water immersion times. Because the variations obtained in the CPE power (n) were very small and the value was always very close to 1, CPE could also be expressed by Eq. (3): Qc =
εε0 A d
(3)
where A is the exposed area of the working electrode, d the coating thickness, ε the dielectric constant of coating and ε0 is the free-space permittivity. The variation in QC with immersion time can be attributed to water uptake within the coating leading to increases in the dielectric constant of the coating. Therefore, the dielectric constant (ε) could be used as a deterministic parameter in order to follow the variations in the water barrier properties of the coating caused by the degree of photo-oxidation encountered in the accelerated weathering test [27,28]. The photooxidation phenomena increases the dielectric constant of the
Fig. 8. Adhesion vs. interface photo-oxidation of clearcoat.
Evaluation of the electrochemical properties of coatings provides ready means for comparing the weathering performance of basecoat/clearcoat automotive paint systems. Accordingly, initial high increases in the values of Y0 and reductions in the values of n will most probably initiate the onset of cracking in the clearcoat and its growth towards the basecoat. Sudden changes in the values of Y0 and n are indicative of a more increased tendency for the clearcoat to peel off. Additionally, with the present assessing procedure, it is possible to evaluate simultaneously the water uptake, corrosion resistance and weathering performance of the paint system under test.
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