Organic Coatings for Corrosion Control - American Chemical Society

Chapter 8

Study of the Water Barrier Properties of Paints After Natural and Accelerated Photooxidative Degradation

Downloaded by CORNELL UNIV on August 2, 2016 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch008

E. Deflorian, L. Fedrizzi, and P. L. Bonora

Department of Materials Engineering, University of Trento, Trento, Italy The evaluation of the corrosion protection properties of organic coatings is often obtained by the exposure of coated samples to natural environment (for example, marine or industrial atmosphere) and by accelerated weathering tests (salt spray fog, UV irradiation, cycles, etc.). At the end of the test, however, the visual observation of the tested samples, comparing the coating appearance with defined standards, is currently the only checking system. Such procedures generally take a long time and the results are influenced by operator judgment; therefore, they are not completely objective. For this reason, the use of quantitative analysis, for example, EIS (electrochemical impedance spectroscopy) or FTIR can provide information on physical and chemical properties of the coating and on the corrosion process, allowing an early evaluation of the degradation phenomena. In this work the effects of natural UV degradation after exposure for several years in the atmosphere are compared with the consequences of artificial weathering in an UV chamber (ASTM G53) using electrochemical, infrared and calorimetric techniques. The comparison shows the remarkable differences in the two degradation mechanisms giving information on future developments for new testing procedures. The possibility of a reliable life time prediction for an organic coating, in particular concerning the maintenance of the corrosion protection properties, is extremely important for the paint industry. The reduction of the testing time for a new product is proportional to the availability of reliable and accurate testing procedures. For this reason, many different accelerated tests are generally employed for simulating the actual field environment (/). At the same time, however, there are many criticisms about the realistic effectiveness of accelerated degradation (2). Moreover, the evaluation techniques usually involve visual observation of the samples, so the accuracy, and hence, reproducibility are limited. 92

©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


93 The study of the deterioration of organic coatings in the presence of the combined action of UV radiation and water is demanded by the wide use of paints in external environments that are exposed to both solar radiation and humidity. This paper deals with the degradation of both chemical and barrier properties of samples irradiated with artificial and solar UV sources in order to compare the effects of accelerated testing on the degradation mechanisms of organic coatings. The UV-polymer interaction can induce microstructural and chemical changes due to photo-oxidative processes, which, in the presence of water, reduce the corrosion protection properties of paint. Photo-oxidative phenomena may cause important changes in the paint, not only aesthetic (change of color or gloss), but also functional (bonds breakdown and consequent loss of elasticity, reduction of the barrier properties etc.) (3). A simple approach to the study of corrosion protection properties of UVtreated organic coatings has been that of separating in time the two phenomena. First, having the painted products undergo different UV radiation times. Then, immersing them in an aqueous solution and studying the absorption phenomena. The photoxidation in organic coatings causes permanent modification in the polymeric chains through mechanisms of initiation, propagation and termination (4). Under the effect of the energy related to the ultra-violet (UV) irradiation, the polymer can produce free radicals by chemical bond breakdown (initiation). These free radicals can react either with oxygen, producing unstable peroxides, or with other polymeric chains (chain propagation). The final chemical products are obtained from the reaction offreeradical recombination (generally ketones or alcohols). The chemical bond breakdown of the polymeric chains with the formation of oxidized compounds causes the production of small molecules, which are not adherent to the coating and are easily removed in an aggressive environment. After this modification, the polymeric coating becomes more brittle (therefore, some cracks can be produced, e.g. by thermal cycles) and possibly thinner. In the case of pigmented coatings, the loss of polymeric mass leads to a superficial pigment concentration higher than the critical level. This causes a further enbrittlement (5-6). The chemical bond modification can be usefully studied by means of infrared spectroscopy in reflectance (FTIR). Measuring the change in intensity (reflectance) and position of the peaks (wavenumber), it is possible to characterize the relative changes in chemical bonds (7). In order to study the reduction protection properties of an organic coating (without active pigments) as a consequence of the weathering, it is important to know the barrier properties of the paint with regard to water and ions. The evaluation of the water absorption process in an organic coating immersed in a solution is a very important phenomenon. Normally, it preludes paint-metal interface loss of adhesion. Also, it is a presupposition for the electrochemical reaction and the reduction of the resistance to the passage of ions through the coating itself. This process can be studied with accuracy by means of electrochemical impedance spectroscopy (EIS) (8-9). With the same technique, it is also possible to quantify the barrier properties of the coating regarding aggressive ions. The higher the ionic resistance of the paint, the higher the corrosion protection performance. In fact, the presence of defects, as well as incorrect surface preparation, can decrease the ionic resistance and increase the corrosion rate (10-11). The ionic resistance is the effective mechanism in the corrosion protection properties of barrier paints. Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

94


Materials and Experimental Procedure The studied system consists of sand blasted mild steel (Sa 2 1/2) coated with an epoxy polyamide primer (10 μπι) followed by an alky die topcoat cured with isocyanates (150 μιη). The product comes from industrial production. Two different classes of weathered samples are considered: the first group contains samples exposed to an industrial atmosphere (Turin, Italy) for 2, 10, 14 and 19 years. The second group consists of just prepared specimens artificially weathered in a UV chamber. The samples were irradiated with UV radiation according to the ASTM G53 standards. This fixes the UV source spectrum (270-290 nm, peak 313 nm), in addition to the radiation geometry. After some preliminary measurements, four different radiation times were chosen: 24, 48, 96 and 400 hours (the reference sample was not irradiated). The studied samples and the weathering procedure are reported in Table 1. For each of these materials (4 test samples for each radiation time) an electrochemical impedance spectroscopy (EIS) characterization was carried out in order to assess the phenomena of water absorption and underpaint corrosion. The measuring cell consisted of a work electrode (coated metal) of about 30 cm of area immersed in a 3.5% solution, a platinum counter-electrode, and a reference electrode (saturated potassium sulphate SSE, +642mV vs SHE). A potentiostat EG&G PAR 273 and a Solartron 1255 frequency response analyzer were used for the electrochemical measurements. The frequency range for the measurements ranges from 10,000 Hz to 0.001 Hz with a signal amplitude of 20 mV. The samples were also characterized by using differential scanning calorimetry (DSC) for measuring the glass transition temperature (Tg). The temperature range was -60, +100°C and the equipment used a Mettler TC 10A. In order to identify the chemical changes in the polymer structure due to the UV interaction, some FTIR (Nicolet DXL in reflectance) measurements were carried out. 2

Results and Discussion The effects of artificial and solar UV radiation were compared using infrared spectroscopy (FTIR) to verify if differences in the chemical structure can be found before and after UV treatment. Also, if it is possible to chemically distinguish the effects of the two different types of UV degradation. Comparing the new samples, some differences (both in shape and intensity) of peaks are evident in all the UV treated samples (Figure 1). The most important peaks are reported in Table 2 with the relevant wave numbers. These changes are mainly related to the degradation of the functional group biuret (part of the esamethilendiisocyanate), and only marginally, to the urethane group. The comparison between the FTIR measurements obtained on samples exposed to the natural atmosphere and the samples after artificial UV weathering show that these two types of degradation are substantially chemically equivalent. In both cases it is possible to note a sharp reduction of the peak intensity for the wave number related to the biuret group. However, there are a few important differences concerning the mechanisms of chemical degradation. With attention to Figure 1 and Table 2, it is possible to observe a different effect of the weathering procedure on the urethane group. In Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

95

Table 1 : Materials and treatments


SYMBOL new UV48 UV96 UV400 S2 SIO S14 S19

2000

1800

TREATMENT 48 hours ASTM G53 96 hours ASTM G53 400 hours ASTM G53 2 years of solar exposure 10 years of solar exposure 14 years of solar exposure 19 years of solar exposure

1600

1400

1200

1000

1

wave number (cm" ) Figure 1 : FTIR spectrum for new samples (new), samples exposed in the Turin atmosphere for 2 years (S2) and samples treated with UV radiation for 48 hours (UV48).


96 particular, the laboratory-accelerated test induces a higher breakdown of the urethane group than the solar irradiation. This difference is particularly clear considering the stretching of the C-0 bond in the urethane group at 1250 cm' : only the artificial irradiation induces a significant intensity reduction of this peak, while the change in the peak intensity for the naturally weathered samples is very small. This is also true for long term exposure. The main difference distinguishing the natural and artificial UV-treated samples is the thickness of degradation. In Figure 2, the spectra of new samples are compared with the spectra of two weathered samples obtained 40-50 μιη under the surface (the external layer was previously removed). It is possible to note that the chemical degradation of artificially UV treated samples (UV48) involves only the surface layer of the coating, having the spectrum similar to the one obtained on untreated samples. This behavior is also confirmed for very long (400 hours) time of exposure. The case of naturally exposed samples (even after only two years) in which the chemical modification (biuret degradation) is maintained along the whole thickness is different. The reasons for such differences can be found in different aspects. First, it is important to remember that artificial UV weathering, unlike solar exposure, causes the production of a significant amount of ozone, changing the oxidative behavior of the atmosphere. Moreover, the higher UV concentration in the accelerated radiation chamber induces the formation of many free radicals in the external layer of paint. The photo-oxidative degradation can be divided into different principal steps: initiation, i.e. the molecular excitation forming free radicals; propagation, i.e. the interaction between the free radical and other molecules; and termination, i.e. the reaction between twofreeradicals. Under artificially accelerated UV conditions the superficial free radical concentration is very high. Comparing the UV radiation energy described in ASTM G53 and the values of UV solar radiation energy experimentally determined in Turin, the first value is more than two orders of magnitude higher than the second. The competition in the propagation and termination reactions is a function of the free radical concentration. In fact, the propagation rate is proportional, as a first approximation, to the free radical concentration (assuming constant molecular polymer concentration). On the contrary, the termination rate is proportional to the square of the free radical concentration and, therefore, it is more efficient than the propagation reaction in increasing the free radical concentration. These considerations can explain the reason for the thinner modified paint layer for accelerated UV treated samples because, in this case, the termination reaction reduces the interaction layer maintaining the presence of free radicals in the upper layers of the coating. This conclusion is also confirmed by the glass transition temperature (Tg) measurements, which show that all the artificial UV treated samples, have the same Tg as the new ones (about 30°C). This proves that the chemical and microstructural modification of the paint involves only a small superficial part of the mass of the coating. On the contrary, the samples exposed to the Turin atmosphere show a Tg that is higher (35-38°C) by increasing the time of exposure, in agreement with a degradation mechanism acting in most of the thickness of the coating. Further, to determine the chemical modification due to the natural and artificial UV radiation, the influence of weathering on the actual corrosion protection properties of the studied samples was studied. To this purpose the samples were


1


97

Table 2: Wavenumber, chemical bonds and position in the polymer structure of the principal peaks measured in the FTIR spectra. W E V E N U M B E R (CM- )

CHEMICAL BONDS

POSITION O F T H E B O N D IN T H E POLYMER STRUCTURE

1720 1680 1635 1520 1375 1460 1250 1210 1120 1070

C=0 stretching C = 0 stretching C = 0 stretching N - H bend C - H bend O - H , C - H bend C - 0 stretching C - H , C - C stretching C - 0 stretching C - C stretching

urethane ester biuret urethane and biuret


1

urethane ester ester

100-,

CD Ο c ro ο ω

40-

Ο)

20-

UV48 r S2 r - new 0 2000

1800 ' 1600 ' 1400 " 1200 ' 1000 '

800

-1

wave number (cm ) Figure 2: FTIR spectrum for new samples (new), samples exposed in the Turin atmosphere for 2 years (S2r) and samples treated with UV radiation for 48 hours (UV48r) both after 50 μιη of thickness removal.



98 characterized by using electrochemical impedance spectroscopy (EIS). By analyzing the experimental results of this technique it is possible to quantify the barrier properties of the coating and the electrochemical corrosion process occurring on the metal surface. The typical Nyquist impedance plot for natural and artificial weathered samples after some days of immersion in the sodium chloride solution is shown in Figure 3. The shape of the diagram seems to indicate the presence of a single depressed loop related to a single time constant. With a more accurate analysis of the electrochemical data, by fitting the results with specific software, it is possible to define a more complex behavior with more time constants. However, the goal of this study is the coating degradation. Therefore, we have isolated (in the high frequency range) the electrical and dielectrical behavior due exclusively to the organic coating, neglecting in this step of the research, the corrosion process, which is very limited due to the quality of the coating. For frequencies higher than 1 Hz, the equivalent electrical circuit modeling the experimental results is shown in Figure 4 and it consists of a Constant Phase Element (CPE) Qc related to the non-ideal capacitive behavior of the coating in parallel with the ionic resistance through the paint Rp. The electrolyte resistance can be neglected. The first parameter, the coating capacitance Qc, can be expressed by the following formula: Qc = ε ε A/d 0

(1)

where A is the testing area, d the coating thickness, ε the dielectric constant of the medium and ε thefreespace permitivity. If the results are measured per unit of area and the coating thickness is constant, the Qc trend and the ε trend are equivalent. Actually the coating thickness can change slightly (+/-10%) from sample to sample and for this reason we have preferred to represent the result in the form of ε instead of Qc in order to minimize the differences due to thickness variations. The change of Qc during the immersion time is mainly due to water uptake phenomena inducing an increase of the apparent dielectric constant of the coating. The discussion of ε is, therefore, interesting in order to define the change in the water barrier properties of the coating, which can cause electrochemical process on the metal substrate or loss of adhesion at the metal-polymer interface. The value of Rp is a function of the ion diffusion through any kind of defect in the coating. The chance for a barrier organic coating (without active pigments) to protect against corrosion is mainly due to the aptitude to reduce the ion diffusion in the coating; this is necessary in order to equilibrate and to support the corrosion reaction. From this consideration the importance of the Rp evaluation to understand the protection properties of the coating is evident. The following discussion is based on these two parameters (Qc and Rp). In Figure 5 the dielectric constant of both the naturally weathered samples and of the new one versus the time of immersion in NaCl solution is shown in logarithmic scale. During the first 30 minutes of immersion the samples reach an equilibrium with the aqueous environment showing a behavior similar for all the materials with small and insignificant differences. The ε values start to increase after the first hours of immersion and they reach a plateau after about 4-5 days. After the initial period the samples showing the higher increase of ε (and therefore the higher water uptake) in comparison with the new material are the ones with 10 years of natural weathering; 0


99 1.5x10%


g

9

l.OxlO -

2

z (n*cm ) real

Figure 3: Typical Nyquist plot for natural and artificial weathered samples after some days of immersion in the sodium chlorides solution.

Qc

Rp 1

1

w—

Figure 4: Equivalent electrical circuit modeling the organic coating contribution to the total impedance.

Ï0

100

VO'OO10000

time (minutes) Figure 5: Dielectric constant versus the immersion time trend for the new samples and the samples exposed in the Turin atmosphere for 2 years (S2), 10 years (S10) and 19 years (SI9). Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


100 the highest being the sample with 2 years of natural weathering. In a previous work (72), it was found that for polyester paints the UV radiation can increase the water uptake process by increasing both the quantity of water (the saturation coefficient) and the kinetics of uptake (the diffusion coefficient). This behavior was explained by considering the microstructural modification in paints due to photo-oxidation and, because of the hydrophilic action of both thefreeradicals and the products of photooxidation. This result is confirmed by the data of Figure 5, but with a more complex mechanism. There are two competitive phenomena: 1 ) the formation of both free radicals and hydrophilic products of photo oxidation which increase the water uptake; 2) the increase of the Tg (more evident for long term exposure, as previously discussed) causing a reduction of the water uptake which induce on its turn a more rigid polymeric structure. The first aspect is dominant for samples exposed for 2 years in the Turin atmosphere; on the contrary the second mechanism (the reduction of water uptake rate due to the increase of the Tg) is important for samples exposed for 19 years. The result is that for long time UV exposition the water uptake seems to decrease (the values of ε are lower after 10 years than after 2 years and after 19 years the values are lower than for the new material). The UV radiation can induce, for long term exposure, a change in the Tg, as previously discussed. The Tg increases (at values higher than the room and testing temperature) and reduces the water uptake rate. The trend in figure 5 is in agreement with the Tg modification. The results of both the new and the for artificially weathered samples are presented in Figure 6. The behavior is almost the same all the tested materials showing a very similar water uptake process. The absence of differences in the water uptake process is only apparent in contrast with the chemical modifications measured with the infrared characterization. The chemical changes for the studied materials are localized in the upper layers and therefore they do not induce modification in the water barrier properties of the whole coating. The first conclusion arising from Figures 5 and 6 is that for our samples the UV artificial weathering does not seem to be equivalent to natural weathering. This is shown by the limited effects of artificial UV exposure on water barrier properties. As previously discussed, the parameter describing the paint barrier properties to ions is the pore resistance Rp. The results obtained on both naturally weathered and new samples are reported in Figure 7. The initial value of Rp is very high (about 10 Ω-cm ) showing the good corrosion protection properties of this kind of alkydic resin; this is true also after about 20 years of natural exposure. In Figure 7 the starting value of Rp is the first measurable value obtained with the equipment used (it is not possible to measure impedance higher than 10 Ω-cm ). Due to the interaction of the organic coating with the electrolyte, the value of Rp decreases under the effect of ionic diffusion through the paint. During the first week of immersion, all the samples, the new and weathered ones, are characterized by the same Rp trend. In the following weeks, the ionic mobility increases (Rp decreases), for the natural weathered materials, while for the new sample it remains almost constant. This behavior proves the very good protection properties of the alkydic organic coating under investigation because only after several days of immersion in the aggressive solution is it possible to note a difference in the ion resistance which 10

2

10

2


101


25

Ί

ί

'ί'ο

i ' ô o i o ' o o ' ' ' Ύόοοο' time (minutes)

Figure 6: Dielectric constant versus the immersion time trend for the new samples and the samples treated with UV radiation for 48 hours (UV48) and 96 hours (UV96).

10 10

Ε ο

Ε

10

ΰ

CL

10

ε

1000

10000

100000

time (minutes) Figure 7: Pore resistance versus the immersion time trend for the new samples and the samples exposed in the Turin atmosphere for 2 years (S2), 10 years (S10) and 19 years (SI9).


102 8

2

remains not lower than 5·10 Ω·αη (also for the naturally weathered samples), According to other authors (6), the UV radiation can produce some microcracks in the coating, inducing the presence of small defects which increase ionic conductivity. This is the case with our samples. The water barrier properties are, therefore, mainly a function of the polymer structure, while the ionic resistance is a function of microscopic defects produced by the UV treatments. We can conclude that the corrosion protection properties of the organic coating are only slightly degraded by 20 years of exposure in the Turin atmosphere, because the chemical and microstructural modification do not induce the formation of any macro-defect but only a limited reduction of ionic barrier properties. The Rp values obtained by electrochemical impedance data on artificially weathered samples are shown in Figure 8. The initial values are in the order of 10 Ω-cm , similar to the same values of the new samples. Also for the UV treated samples there is a decrease in the pore resistance reaching a plateau value after about 7 days of immersion in the NaCl solution. This trend is similar to the behavior of the new samples but the plateau values are different: lower for the weathered samples (about 1.2 10 Ω-cm ) and higher for the new ones (about 2 10 Ω-cm ). It is important to point out that during the 40 days of testing the naturally degraded samples never actually reach a stationary value of Rp. Accelerated artificial UV weathering induces a ion barrier degradation in the studied organic coatings. This effect is lower than in the case of natural UV degradation. Moreover, by increasing the time of artificial UV treatment till 400 hours the Rp trend is approximately the same as for 48 or 96 hours of treatment. The lower, but measurable, effect of the artificial weathering can be explained (also for this parameter) considering the limited penetration of the UV degradation in the coating thickness for the artificial UV treated materials and, therefore, also the reduced penetration of microdefects.


10

2

9

2

9

2

Conclusions •

By using infrared, thermal and electrochemical techniques it is possible to characterize the organic coating degradation due to photo-oxidation, comparing the effects of different weathering procedures (natural and accelerated) concerning the microstructral, chemical and corrosion protection properties. This approach could be useful in general for understanding the degradation mechanisms and to explain the possible differences in natural and artificial weathering of paints, in order to improve the efficiency of the accelerated degradation test procedures of organic coatings.

•

The chemical bonds in the studied alkyd resin are modified by both artificial and solar UV interaction, as shown by the FTIR characterization, mainly for the degradation of the biuret group. In the case of artificially weathered samples there is also a minor urethane group degradation. However, the chemical modifications are more or less the same for solar or artificial UV treatments.

•

There is a noticeable difference between artificially and naturally weathered samples considering the thickness of degradation. The naturally radiated samples are modified in the whole coating thickness, while in the artificially UV treated samples only the superficial layers are photo-oxidized. This difference is due both Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


103

Figure 8: Pore resistance versus the immersion time trend for the new samples and the samples treated with UV radiation for 48 hours (UV48) and 96 hours (UV96).


104


to the different ozone concentration near the coating surface in the two different cases and to the competition of the propagation and termination reactions of free radicals. For a high intensity of UV radiation (the accelerated test) the high free radical concentration favors the termination reaction, concentrating the chemical modifications in the external layer of the organic coating. •

Because of the differing thickness of degradation, the water barrier properties after the two different UV treatments (natural and artificial) are different. The accelerated UV test does not change the relevance of the water uptake process occurring in new samples. On the contrary, the solar exposure increases both kinetics and amount of the water uptake. There is an inversion of this trend for long term solar exposure, which cause an increase of the glass transition temperature Tg. The more rigid structure causes a reduction of the water uptake kinetics. In conclusion, the accelerated UV test does not correspond to the natural degradation as it does for the water barrier properties.

•

The ion barrier properties are reduced by all the UV treatments. However, because of the high protection properties of the studied alkyd resin, it is not possible to distinguish clearly the trend of the ionic resistance as a function of either the solar or the artificial UV exposure time. The only evident result is the minor effect on ion barrier properties of the artificial UV radiation in comparison to the natural photo-oxidation. From the ionic resistance point of view (which corresponds for barrier coatings to the corrosion protection properties) the accelerated UV test corresponds only partially to the natural degradation.

Acknowledgments The authors thank the Imper S.p.A. in Turin and in particular Dr. Belletti for the samples preparation. The authors acknowledge also Prof. Campostrini of University of Trento and Dr. Fezia of the PPG Italy for the useful discussion about the FTIR results interpretation and Dr. Volcan for the assistance in all the experimental data acquisition and interpretation. Literature Cited 1. Appelman, B.R. Journal of Coating Technology 1980, 62, p. 57. 2. Kendig,M.;Scully, J. Corrosion 1990, 46, p.22. 3. Clough, R.L.; Shalaby, S.W. In Radiation Effects on Polymers ACS Symposium Series, 1991. 4. Pappas, S.P. Prog.Org.Coat 1989 17, p. 107. 5. Armstrong, R.D.; Jenkins, A.T.A.; Johnson, B.W. Advanced in Corrosion Protection by Organic Coatings II, The Electrochemical Society, 1994, p.24. 6. Armstrong, R.D.; Jenkins, A.T.A.; Johnson, B.W. Corrosion Sci. 1995. 37, p. 1615. 7. Bauer, B. Prog. Org. Coat. 1993, 23, p. 105. 8. Monetta, T.; Bellucci, F.; Nicodemo, L.; .Nicolais, L. Prog. Org. Coat. 1993, 21, p. 353. 9. Deflorian, F.; Miskovic-Stankovic, V.B.; Bonora, P.L.; Fedrizzi, L. Corrosion 1994, 50, p.446. Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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10. van Westing, E.P.M.; Ferrari, G.M.; de Wit, J.H.W. Corrosion Sci. 1994, 36, p, 957. 11. Deflorian, F.; Fedrizzi, L.; Locaspi, Α.; Bonora, P.L. Electrochim. Acta, 1993, 38, p. 1945. 12. Deflorian, F; Fedrizzi, L.; Bonora, P.L. Corrosion Sci. 1996, 38, p. 1697.


Organic Coatings for Corrosion Control - American Chemical Society

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