Paradox effects of flake carbonyl iron on the photodegradation behaviors of epoxy-based wave-absorbing coatings: Photo-catalytic and UV blocking

Highlights

• Paradox effects of FCI on the photodegradation behaviors of epoxy coatings

• The addition of FCI accelerated the photo-oxidation on the surface of EP coatings

• The degradation depth of EP/FCI coating was decreased due to UV blocking effects of FCI

Abstract

Polymer-based electromagnetic wave-absorbing (EWA) coatings that can effectively dissipate electromagnetic radiation are in high demand. While researchers on the development of lighter and more efficient EWA coatings have been carried out extensively, there is limited information on their durability and the underlying degradation mechanisms. In this work, epoxy (EP)/flake carbonyl iron powder (FCI) coatings were selected as the model EWA coating. UV aging experiments of specimens were conducted up to about 240 h to study the effects of additional FCI on the photo-oxidative behaviors of coatings. The evolutions in the surface topography, chemical structure, hydrophilicity of aged coatings were systematically studied by Scanning electron microscopy (SEM), Laser scanning confocal microscopy (LSCM), Fourier transform infrared spectroscopy (FTIR), and static contact angle measurement, and further combined with micro-FTIR to investigate the degradation diffusion behavior from the exposed surface to the bulk of coatings. The results showed that the addition of FCI significantly accelerated the photo-oxidation of EP coatings, which was attributed to the corrosion of FCI during aging and the concomitant photo-catalytic degradation effects of its corrosion products. Moreover, the cross-sectional analysis revealed a non-uniform distribution of EP degradation species across the thickness direction, while it is interesting to find that the degradation depth of EP/FCI coating was lowered compared with that of pure EP coating, which could be due to the UV blocking effect of FCI. This work clearly revealed the underlying mechanisms for the distinct dependence of surface and cross-sectional degradation of coatings on the incorporation of FCI, which has laid crucial groundwork for durability evaluation and performance optimization of EWA coatings.

Introduction

Polymer-based electromagnetic wave-absorbing (EWA) coatings, have found significant applications in the field of industry, construction, commercial and military [1], [2], with advantages of flexibility, easy processing, light weight, etc. The absorption of electromagnetic wave usually is to add electromagnetic wave absorbers in the polymeric coatings. According to the microwave loss mechanism, traditional absorbers can be divided into resistance absorbers (such as carbon derivatives, conductive polymers, etc.), dielectric absorbers (such as silicon nitride and iron nitride, etc.), and magnetic dielectric absorbers (such as ferrite, carbonyl iron, etc.) [3]. Recently, novel absorbers with low density, high absorption, wide absorbance frequency range [1], [4], [5] have also attracted significant interest and have been designed through methods of nanomaterials hybridization [6], surface or doping modification [7], and three-dimensional structural design [8], [9]. For instance, Gu et al. [10] designed porous two-dimensional Fe₃N alloy/carbon nanosheet composites. The combination of magnetic/dielectric materials and the introduction of porous structure led to excellent impedance matching performance, with the minimum reflection loss (RL) values reaching striking −65.6 dB with a thickness of 2.05 mm. Zhang et al. [8] successfully prepared the aerogel with the peculiar three-dimensional network structure and porous structure using cellulose, chitosan, and polyaniline. The enhanced conductivity loss ability and the space charge polarization together boosted its electromagnetic absorption properties. Although excellent efforts have been done for developing novel EWA coatings with high absorption, wide absorbance frequency range, and thin thickness [1], [4], their durability is still a big concern considering that they are usually going to be subjected to the harsh environment, such as marine, plateau, etc., with intensive irradiation, high temperature or salt mist.

The electromagnetic absorbers, such as ferrite, carbonyl iron, and polycrystalline iron fibers are often susceptible to oxidize or corrode when exposed to the atmosphere environment [7], [11], [12], which would significantly impair their wave absorption ability. Great endeavours have been made to improve the chemical and oxidation stability of those absorbers through inert protective coatings [7], [13], [14], such as SiO₂ [13], aluminum [14], and carbon [15]. For example, Zhou et al. [14] found that the electromagnetic absorption bandwidth values (RL < −10 dB) of FCI decreased from 3.9 GHz to 0 GHz after the high-temperature oxidation; however, the antioxidant property of FCI can be significantly improved with a thin layer of the aluminum coating on its surface, and its absorption bandwidth stayed around 2.1 GHz after the oxidation. Zhang et al. [15] applied a carbon coating on the surface of FeSiAl powders using the CCVD method, and the carbon coating improved the conductivity and dielectric loss without affecting the magnetic loss of the powders, while also improving the corrosion resistance of the powders. Whereas, the reliability of polymer-based EWA coatings depends not only on the properties of electromagnetic wave absorbers themselves but also largely on the polymeric part. The polymer matrix is an integral part of EWA coatings, necessary to glue the wave absorbers together and protect them from environmental stresses experienced in their service environment. Polymer matrix is also vulnerable to environmental attack, which could lead to the chain scission [16], [17], cross-linking [18], cracking [19] or blistering [20], etc., and finally resulted in its function loss. Peng et al. [21] studied the ozone degradation behaviors of carbonyl iron/epoxy coatings, found that the addition of carbonyl iron can suppress the oxidation process on the coating surface induced by ozone. For polymers, there are two main methods to prevent their aging degradation, blocking external aging factors and removing unstable intermediates. For example, adding UV stabilizers or fillers can effectively shield the photooxidation of polymer by ultraviolet light [22], while the addition of free radical capture agents can terminate the free radical chain-breaking reaction of polymer and prevent the degradation of polymer [23]. Unfortunately, investigations on the degradation behaviors of polymer part of wave-absorbing coatings, especially the interaction between the corrosion of absorbers and degradation of polymer matrix are still scarce. A thorough investigation on such relationships would help evaluate the reliability of coating products and assist the design of new EWA coatings.

Hence, this paper will introduce our work on the reliability evaluation of polymer-based EWA coatings. The system was based on an epoxy (EP)/flake carbonyl iron (FCI) wave-absorbing coating. Flake carbonyl iron was used as it is one of the most widely studied magnetic absorbents due to its large saturation magnetization, high Curie temperature [24], [25]. As to the polymer matrix, epoxy was adopted considering its extensive applications with the advantages of high thermal stability, chemical resistance, and excellent adhesive properties. UV aging experiments were carried out as UV light is the most intense aging factor to initiate the degradation of polymers. As the first part of this research, the present work will be focused on curing kinetics and photodegradation mechanisms of EP/FCI wave-absorbing coating, emphasizing the influence of the addition of FCI and its corrosion on the degradation and failure of EP. Surface and cross-sectional changes of coatings were investigated to reveal the underlying degradation mechanisms. This work will provide a scientific basis for a better understanding of the degradation and failure of EWA coatings.