A facile approach to fabricate bright blue heat-resisting paint with self-cleaning ability based on CoAl₂O₄/kaolin hybrid pigment

Highlights

• Superhydrophobic CoAl₂O₄ hybrid pigment was prepared based on its rough surface.

• Hybrid pigments exhibited high water contact angle and low sliding angle.

• The obtained paint presented good self-cleaning and excellent weathering properties.

• Superhydrophobic heat-resisting paint could be sprayed on various substrates.

Abstract

A bright blue and low-cost heat resisting paint with excellent self-cleaning ability was prepared by adding superhydrophobic CoAl₂O₄/kaolin hybrid pigment (K-HP). Kaolin (K) was served as a suitable matrix for anchoring CoAl₂O₄ nanoparticles to prepare K-HP, and then it was modified with hexadecyltrimethoxysilane to construct the superhydrophobic hybrid pigment based on the rough structure of K-HP. The sprayed coating of the superhydrophobic heat-resisting paints exhibited the good self-cleaning performance with high water contact angle of 164.3 ^o and low sliding angle of 1.2 ^o below 380 °C, as well as the excellent color properties (L*= 41.72, a* = − 3.01, b* = − 62.93) and thermal stability even at 800 °C. Incorporation of K-HP not only improves the thermal stability of the heat-resisting paint, but also effectively protects steel plate substrate from damage during the calcining process. Therefore, it is expected to provide a facile approach to fabricate the chromatic self-cleaning heat-resisting paint with excellent thermal stability.

Introduction

Most of the traditional heat-resistant paints are extensively used in many high temperature industrial applications to prevent the external corrosion, where coating requires to have a good thermal stability at high temperature (about 350–800 °C) (Mayer, 1998, Dhoke et al., 2009, Miyashita et al., 2016, Hsu et al., 2017, Zhou et al., 2017). Actual researches on traditional heat resistant paints have two main issues. First the economies, related to the price of the involved inorganic colored pigments are expensive, which likely leads to the high production cost of the heat-resistant paints. Second they tend to get contaminated by the pollutants as most of them are hydrophilic (Zhou et al., 2016). Therefore, it is urgent to develop a kind of novel heat-resistant paint with low-cost and self-cleaning ability.

Various approaches such as chemical vapor deposition, co-condensation and sol-gel processing have been used for fabrication of hydrophobic materials via combining with the design of the rough structure and the modification of the low surface energy reagents (Zhang et al., 2004, Steele et al., 2010, Huang and Lin, 2014). For example, Li et al. prepared polysiloxane/multiwalled carbon nanotubes nanocomposites with high water contact angles, ultralow sliding angles, excellent stability and good healing capability, and its application properties were investigated as ultrastable, healable and superhydrophobic coatings (Li and Zhang, 2015). Liu et al. prepared a nanotubular coating with both high transparency and healable superhydrophobic self-cleaning properties (Liu et al., 2016). However, most of those methods and materials were mainly used for the fabrication of hydrophobic coating rather than hydrophobic paint. What's more, most of these approaches displayed complicated procedures, low mechanical properties and high costs, so that they were not suitable for large-scale applications. Plainly, there is a need for development of a simple and low-cost approach to fabricate a hydrophobic paint, especially for heat resisting paint with self-cleaning ability.

In general, the paint consists of resins, fillings, solvent, additives and colored pigments, in which the resins and colored pigments play an important role to thermal integrity of heat-resistant paint (Allen et al., 2004, Cho et al., 2004). The coloring pigments not only decorate the surface of the substrate, but also improve their thermal stability, scratch and abrasion resistance of the coatings without affecting their optical, scratch hardness, adhesion and so on (El Saeed et al., 2015). As a typical of inorganic pigments, cobalt aluminate (CoAl₂O₄) is a promising blue colored pigment in the heat-resistant paint due to its unique properties such as high refractive index, good weather resistance and excellent thermal stability (Lu et al., 2013, Soleimani-Gorgania et al., 2015, Yoneda et al., 2016, Álvarez-Docio et al., 2017). However, the price of CoAl₂O₄ is very high among the inorganic pigments due to the scarcity of cobalt ores in the world, while the color and acid resistance are poor owing to the limitation of the traditional solid-phase method, in which the chemical composition and particle size of the resulting pigment are not uniform, and the amounts of impurities are high (Soleimani-Gorgania et al., 2015). In our previous work, it has been confirmed that the coprecipitation method incorporating of clay minerals not only decreased the calcination temperature and the production cost for preparation of cobalt blue pigments, but also prevented from the agglomeration during high temperature crystallization process to improve the color and weather resistance properties (Mu et al., 2015, Zhang et al., 2017).

K is one of the clay materials widely used for a large number of applications (Juettner et al., 2007, Gilkes and Prakongkep, 2016, Mishra et al., 2017), especially in the heat-resistant paint due to the excellent thermal stability and the unique flake-like morphology (Prasad et al., 1991, Murray, 2006, Guo et al., 2016). Therefore, K was employed to fabricate the K-HP after anchoring of CoAl₂O₄ nanoparticles on the surface of K, and then K-HP was modified using hexadecyltrimethoxysilane (HDTES) to obtain superhydrophobic K-HP, and then HDTES modified K-HP was finally included in the heat-resistant paint system to obtain the blue heat resisting paint with self-cleaning property in this study. The effect of the added amounts of HDTES, ammonia saturated ethanol solution, H₂O and hydrolysis reaction time on the superhydrophobic property of hybrid pigments were investigated in detail by fixing the mass of K-HP. The self-cleaning and heat-resistant properties of the as-prepared heat-resistant paint containing superhydrophobic K-HP were also studied after being sprayed onto steel plate.