Weathering of coil-coatings : UV radiation and thermal effects

The effect of heat and of QUV ageing on coil coatings was tested by electrochemical impedance, and the results compared with surface analysis of the polymers by FTIR and XPS. It was shown that UV radiation is more relevant than heat to chemical degradation. A different correlation between water permeation and chemical degradation was observed depending on the coating thickness: for the thinner coatings, the higher UV degradation has corresponded to increased water absorption, whereas in the thicker coating, the bulk effect of heat was more relevant to water permeation.


INTRODUCTION
Coil coatings are widely used for many applications, namely in architecture.Weathering, however, is a major problem in outdoor exposure.Weathering results from the joint action of ultraviolet (UV) radiation, heat, oxygen, humidity and chemical species.These agents induce chemical and physical changes in the polymer, which may vary depending on the nature and intensity of the aggressive agent, but which all result in degradation.Industrially, this degradation is usually assessed by the changes in the optical properties, namely gloss and colour.Although it has been demonstrated that the water barrier properties of a polyester coating decreased with the exposure to UV radiation^ , not much attention is paid in the literature to the loss of corrosion protection provided by the coating to the substrate.In this respect, electrochemical impedance is a powerful technique.It can be used to determine the rate of delamination, the electrical properties of the coating and also to estimate the water uptake from a solution, a parameter of relevance since the permeation of water is the first step in the process of aqueous corrosion of the substrate.Estimation of water uptake is frequently made using the Brasher-Kingsbury equation, which gives the volume fraction of water, (| ) as: In this correlation, C^ and CQ represent the capacitance of the coating at an instant t and extrapolated to t = 0, respectively, and 8^^ is the dielectric constant of bulk water (taken as -80 at ambient temperature).The coating capacitance can be determined from the imaginary impedance at high frequency: In a previous study it was concluded that the Brasher-Kingsbury equation gives higher values of the water content than those estimated from gravimetry, but that qualitatively the information obtained is quite reasonable, since the relative order of water absorption obtained with different formulations was confirmed by gravimetry^ \ In this work the degradation of coil coatings was assessed by electrochemical impedance, in order to determine the effect of elevated temperature and of UV radiation combined with water.The results are interpreted based upon surface analysis (FTIR and XPS).

Materials
The study was made using three commercial coil coatings: polyvinilidene fluorine (PVDF), an architectural polyester (outdoor polyester) and a PVC based coating (PVC plastisol).The coatings were applied on a substrate of (hot-dip) galvanized steal (20 |J.m zinc layer), primed with an acrylic primed (~5 ¡im).All the coatings had red pigment, and the total coating thickness (primer + topcoat) was as follows: PVDF: 20 !im.Polyester: 25 [im and PVC: 200 )Lim.
For the surface analysis, model coatings (grey pigment) were prepared with the following proportions: Coatings were tested as-produced (after curing) and also after accelerated ageing by two different processes: a) Continuous heating at 125 °C during 2 weeks b) QUV ageing.This was made in a standard QUV-weatherometer by a cyclic treatment consisting of 4 h UV-A at 70 °C, alternating with 4 h of condensation at 50 °C.The total time was 3000 h of ageing.

Electrochemical measurements
Electrochemical measurements were made with the sample held vertically against an o-ring, leaving an exposed area of 3.14 cm , during continuous immersion in 3 % NaCl.The measurements were made using a 1255 Solartron frequency response analyser and a 1256 Solartron electrochemical interface.A sine wave of 30 mV (rms) was applied across the cell, in a 3-electrode arrangement.The capacitance measurements were made at a constant frequency of 50 kHz.

FTIR
Spectrum acquisition was made using micro-ATR (Attenuated Total Reflectance).This is an accessory of the FTIR microscope by which an attenuated reflectance spectrum can be acquired from a very small area (area of the micro-ATR crystal, diameter about 100 |lm).Since the information depth of this technique is ~1 |J.m, it is used to acquire chemical information from the topmost surface layer of the coating and not from the bulk.The measurements were repeated a few times to obtain a representative spectrum from each sample.The instrument used was a Perkin Elmer System 2000 Spectrometer with Autoimage microscopy and ATR Accessory.

X-Ray photoelectron spectroscopy (XPS)
XPS was used with the aim of characterising the outermost atomic layers of the polymers.The surface is excited by x-ray radiation, causing ionisation of the atoms, and analysis of the kinetic energy of the emitted electrons leads to the determination of their binding energy.The technique has excellent energy resolution, allowing chemical analysis.The tests were made using a 310 F Microlab (VG Scientific) equipped with a concentric hemispherical analyser, a differentially pumped ion gun and a nonmonochromated Mg mode (Ka = 1253.6eV).Spectra were obtained in the Constant Analyser Energy mode (CAE = 30 eV).

Electrochemical impedance
Under the conditions of the tests, no signs of corrosion were detected on any of the systems.The capacitance values are significantly different for the various coatings, mainly as a result of the different thickness, since the capacitance is inversely proportional to the thickness d {EQ: permitivity of vacuum): The coating capacitance rises significantly in the first 10 h of immersion, becoming nearly stable after that period.This corresponds to absorption of water and ions from solution, which leads to an increase of the dielectric constant of the coating.
The values of capacitance for the "dry" coating, Co, can be obtained by extrapolation of the first part of the curve to t = 0. Once the capacitance CQ is known, the volume fraction of water can be estimated from the Brasher-Kingsbury equation (Fig. 1).
For PVC plastisol a fraction of 10 % of water in the coating was estimated, a value that is approximately twice that of the polyester and five Weathering of coil-coatings: uv radiation and thermal effects AS.GÁSTELA, A.M. SIMÔES, G. DAVIES AND M.G.S. FERREIRA times that of PVDE Also a difference in the shape of the curve is observed, with an inflection after a linear portion of the curve, followed by an acceleration of the permeation processes.This difference can be attributed to the closed porosity of PVC plastisor \ This coating has internal gas bubbles that form in the cure stage, and which lead to closed pores in the bulk of the polymer.Once this water reaches theses pores, they tend to absorb quickly, accelerating the process.
Impedance tests were then performed on samples of the same batch, but aged by one of the methods described above (Figs. 2 to 4)-In the PVDF and the polyester, the absorption curve remained practically unaffected by the heat treatment, whereas the QUV treatment enhanced the water permeation.Surprisingly, however, the results with PVC were opposite: no effect of QUV t^^^(s^''=)     ageing, but an enhancement of water permeation by the treatment at 125 °C.

Surface analysis
Both FTIR and XPS were made using the model samples.The infrared spectra of these coatings are complex, and total identification was not possible.

PVDF
The heat treatment did not introduce any relevant changes in the spectrum, with the peaks practically in the same positions and with the same intensity.With the QUV, only a very slight degradation was detected, consisting of the formation of weak peaks at 1681 and 1616 cm~\ and a rise of intensity in the region 3000-3300 cm~^ (formation of OH groups).Finally, the peak at 1725-1730 cm~\ assigned to the C=0, has become smaller after the QUV test, revealing some degradation of the PMMA.
In the FTIR spectra (Fig. 5) several peaks were identifiedt^l the V,(CH2), v[(0)CH3] and v[(C)CH3] of PMMA at -2954 cm"^ and the v(C=0) at 1725cm-it^^ also of the PMMA.For the PVDF molecule, the characteristic peaks at 1401, 1182 and 1065 cm"^ were also observed.

Polyester
The peaks at 1545 cm~^ and 813 cm"^ correspond to the out-of-plane and in-plane vibration of the triazine bond in the melamine molecule, respectively (Fig. 6   weaker and broadened both by UV and by heat, whereas the 813 cm~^ peak remains unchanged.With the QUV, however, both peaks disappear.This can be interpreted as a breakdown of the bonds in the substituents of the triazine ring and is consistent with the literature^ andvj^ where it is stated that the out-of-plane vibrations is more sensitive to small degradations, whereas the 813 cm~^ peak is affected only by severe degradation.The metoxi group, at 911--913 cm" , was also affected, having disappeared with both the ageing treatments.This is consistent with the formation of a broader band at -3300 cm~\ that corresponds to the region of the O-H and N-H peaks.
According to the literature^ ^^^ the degradation of polyester -melamine system may lead to the formation of amines, alcohols, formats and formamides, as a consequence of the breakdown of the ether and N-C bonds of the cross-linking.
The atomic ratios at the surface of the polyester coating estimated by XPS analysis (Table I) support the FTIR since they suggest some loss of nitrogen at the surface.

PVC
Several characteristic peaks are observed in the reference samples (Fig. 7).From the DOP, the v(C=0) at 1721 cm~^ ^^^ the in-plane vibrations of the ring at 1599 and 1579 cm~^ and the ô(CH3) at 1328 cm"^ were identified^^l For the PVC molecule, only the ô(CH3) at 1425 cm~^ and the stretching v(CH) at 2970 cm-^ were identified.The C-Cl vibrations appear bellow 800 cm" , where identification was not possible.
The 125 °C aged coating revealed a spectrum very similar to the unaged one, except for slight reduction in the 1425 cm" peak and an increased intensity of the 3280-3400 cm" peaks, suggesting an increase of OH bonds^^ "^^^ ^l The QUV ageing induced relevant degradation, with an increase of the peaks at 1550-1700 cm"^ including one peak at 1619 cm"^.This region has  been assigned to C=C double bonds.A significant reduction of the 1425 cm" peak was detected, as well as a general increase of the absorption bellow 1250 cm".Also relevant is the extinction of peaks, namely at 873 cm" , revealing the disappearance of chemical bonds.Elemental quantification of the XPS spectra allowed estimation of atomic ratios (Table II).The results suggest surface oxidation induced by UV radiation, but not heat.Loss of chlorine occurred at the surface with both ageing treatments, although it was more pronounced with the 125 °C treatment.
Deconvolution of the C^g peak was made accounting for 4 peaks.Following the decreasing order of the binding energy, the peaks are: Ci corresponding to C=0, C2 to C-Cl in the PVC molecule, C3 to CH2 in PVC plus the C-O in the phthalate and finally C4 corresponding to the aromatic and aliphatic carbon in the phthalate (plasticizer).The C2/C3 ratio, together with the percentage of C=0 bond, was selected as reflecting the most relevant chemical changes in the polymer (Table III).The decrease of the C2/C3 ratio reveals de-chlorination.This phenomenon was observed in both ageing treatments, but with higher intensity after 125 °C treatment.The C=0 content was significantly increased under the UV radiation, confirming oxidation of the polymer at the surface.

DISCUSSION
The surface analysis has shown that for all the coatings, the QUV ageing has produced higher degradation than the heating to 125 °C.
Of the 3 coatings tested, PVDF was clearly the more resistant to degradation.Nevertheless, some degradation of the co-polymer (PMMA) occurred under de QUV ageing.In this treatment, UV radiation plays a major role, inducing breakdown of chemical bonds with the formation of light compounds, and loss of elements at the surface, whereas the water washes the surface, leaching some of these light compounds away from the surface.
A direct correlation between chemical degradation and water absorption was thus observed in PVDF and polyester coatings.In fact, the more intense degradation corresponded to the QUV ageing, and it was accompanied by a rise in permeability.
The PVC coating revealed a different behaviour.Although the spectroscopic analysis revealed a more severe degradation in the QUV, the rise in water absorption was detected in the samples aged by temperature.The mechanism of ageing explains this difference.As observed before^^*^% the effects of UV radiation are felt only in a thin layer at the outermost part of the film.With the PVDF and polyester, which are very thin, the effect of UV radiation is felt across a thickness that corresponds to a significant portion of the film.Since PVC plastisol is a thick coating, the surface effect of UV radiation affects only a minor fraction of the film.This coating was, however, the most sensitive of the three to degradation, and the only one degraded by heat.The effect of heat, although less effective in terms of chemical degradation, affects the bulk of the polymer.
From the point of view of corrosion performance, the good performance of the PVC plastisol is greatly dictated by its large thickness.

CONCLUSIONS
The joint use of electrochemical impedance to assess the loss of water barrier properties and of spectroscopic surface analysis to determine the degradation mechanisms has proved to be an interesting way of studying the ageing processes of organic coatings.
From the two ageing treatments tested -QUV and heating at 125 °C -QUV was the one that produced a higher chemical degradation.For PVDF and polyester, which are thin coatings, this higher degradation had led to an increase of permeability.
For thick coatings, such as PVC plastisol, although the effect of UV radiation was severe, it was felt only at the surface, and consequently did not affect the water permeability of the film.The effect of heat was in this case more significant in terms of water absorption, because it is felt across the entire thickness of the coating.(Akzo Nobel) with whom we had many useful discussions.

Figure 2 .
Figure 2. Effect of ageing tests on the water absorption of commercial PVDF.

Figure 4 .
Figure 4. Effect of ageing tests on the water absorption of commercial PVC.

Table 1 .
Atomic ratios in Polyester

Table II .
Atomic ratios in PVC plastisolTabla II.Rozones atómicos en el plastisol PVC

Table Hi .
Carbon distribution in PVC plastisolToblo III.Distribución de carbón el el plastisol PVC