EIS study on failure process of two polyurethane composite coatings

https://doi.org/10.1016/j.porgcoat.2010.04.017Get rights and content

Abstract

The performance of acrylic polyurethane composite coating and aliphatic urethane composite coating in 3.5% NaCl solution under ultra-violet radiation was studied with methods of EIS, SEM and FTIR. For aliphatic polyurethane coating, the coating resistance decreased and the coating capacitance and porosity increased more quickly than acrylic polyurethane coating. The acrylic polyurethane composite coatings showed better performance than aliphatic polyurethane composite coatings. IR spectra showed that under UV irradiation conditions, the failure mechanism of the two polyurethane coatings was the transformation of sec-amide to primary amides. The fractured bonds in acrylic polyurethane were mainly C–O bonds, while in aliphatic polyurethane they were mainly C–N bonds. The lower protection property of aliphatic polyurethane coating may be mainly attributed to the C–N bonds which are more liable to be broken.

Introduction

Modified polyurethane paints are widely used as topcoats due to their good anti-corrosion properties and weather resistance. Particularly, acrylic polyurethane and aliphatic polyurethane coatings are the most widely used types. The aliphatic polyurethane is based on aliphatic isocyanate, which reacts with acrylic ester containing-hydroxyl, and has strong adhesion, good chemical resistance and aging resistance [1], [2], [3], [4]. But aliphatic polyurethane is easy to become yellow because of the decomposition of urethane bonds. Therefore acrylic polyurethane topcoat is widely used in offshore oil platforms, bridges, ships and automobile industries. Owing to its rapid non-destructive characteristics, electrochemical impedance spectroscopy (EIS) technology has become one of the most important methods in study of the coating failure process [5], [6], [7], [8]. Some authors have done EIS studies on acrylic polyurethane topcoats. Armstrong et al. [9], Zhang et al. [10] and González-García et al. [11] studied the variations of coating porosity resistance, coating capacitance, polarization resistance and double layer capacitance of acrylic polyurethane coating before and after UV aging process. Irigoyen et al. [12], Xu et al. [13], Wang et al. [14] and Hu et al. [15] analyzed the aging behavior of polyurethane coating, and suggested that the fractured bonds were mainly N–C and C–O bonds during the photo-degradation process of polyurethane topcoat. Yang et al. [16] and Marchebois et al. [17] studied the relationship between penetrative diffusion of the electrolyte solution in the coating and corrosion of metal under the coating. However, studies on failure mechanism of aliphatic polyurethane coatings are much less. In this paper, the failure mechanisms of two kinds of polyurethane coating systems, acrylic polyurethane topcoat/micaceous ferric oxide epoxy intermediate/zinc-rich epoxy primer and aliphatic polyurethane topcoat/micaceous ferric oxide epoxy intermediate/zinc-rich epoxy primer, in 3.5% NaCl solution under UV aging were studied mainly using EIS and FTIR, and the difference in the protective properties of the two coating systems was discussed.

Section snippets

Experimental materials

The coatings used include zinc-rich epoxy primer (two-component), micaceous ferric oxide epoxy intermediate, aliphatic polyurethane topcoat and acrylic polyurethane topcoat. The main compositions of the coatings are as follows.

Type 801 zinc-rich epoxy coating: 4.15% epoxy resin, 86.75% zinc powers (8–12 μm) and 2.25% polyamide resin (produced by Beijing HCBM New Material Co., Ltd., China).

Type 701-2 epoxy middle coating with mica iron: 43.1% epoxy resin, 6.68% chlorinated paraffin, 7.84% iron

EIS characteristics of the coatings

Fig. 1 is the EIS plots of polyurethane composite coatings in 3.5% NaCl solution under UV aging. At the beginning, the Nyquist diagrams show very large slopes which are almost vertical to the horizontal axis (Fig. 1a and b), while the Bode diagrams show straight lines with the slope of −1 (Fig. 1c and d). The coating impedances for both polyurethane composite coatings are larger than 1010 cm2). The results show that the coatings at this time were good shielding layers with large impedances

Conclusions

  • 1.

    Under the combined effect of immersion in 3.5% NaCl solution and UV, for aliphatic polyurethane coating, the coating resistance decreased and the coating capacitance and porosity increased more quickly than acrylic polyurethane coating. The acrylic polyurethane composite coatings showed better performance than aliphatic polyurethane composite coatings. Under the testing condition, for aliphatic polyurethane composite coating the coating resistance decreased below 106 Ω cm2 after 28 days, while

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