Hyperbranched alkyd/magnetite-silica nanocomposite as a coating material

Highlights

• A conformal series of castor oil based hyperbranched Alkyd/Fe₃O₄@SiO₂ nanocomposites was developed as a coating material.

• Structure-property relationship was studied by incorporating different nanofiller concentrations in the alkyd matrix.

• Shape and size control of nano-Fe₃O₄@SiO₂ is crucial to achieve well-dispersed nanospheres with good filler properties.

• Well-dispersed nanoparticles (especially 0.5%) in alkyd matrix exerted improved mechanical and anticorrosive properties.

Abstract

Engineering innovative nanomaterials with low volatile organic content (VOC) has awarded great interest to control air pollutant emissions. We designed a highly branched alkyd matrix suitable for surface coating from castor oil via polyesterification. A simple A₂ + B₃ (di- and tri-functional monomers) methodology was used to prepare the hyperbranched polyester from natural multifunctional monomers. Magnetite-coated silica (Fe₃O₄@SiO₂) particles with 60–70 nm average diameter were prepared by in situ method that binds magnetite nanoparticles to silica nanospheres. The magnetite size and attaching efficiency were controlled by the concentration of chemicals and reflux duration. The nanocomposite coating was prepared by solution casting. The structure-property relationship was studied for different concentrations of nanofiller in the alkyd matrix. The surface and anticorrosive properties were studied via contact angle and salt spray tests. Mechanical performance and thermal stability were assessed by various methods. The highest improvement was achieved with nanofiller insertion up to 0.5% Fe₃O₄@SiO₂ nanospheres.

Introduction

Worldwide concern about environmental pollution and climate change has directed modern research toward green chemistry [1]. Polymeric alkyds are widely used for metal coating for their low cost, high performance, ease of application, bio-degradability and superior adhesion [2], [3]. They are also better in color retention, durability, and anticorrosive performance [4]. One advantage of hyperbranched alkyd over other dendritic polymers is the possibility of single-step preparation from commercially available monomers [5], [6]. The A₂ + B₃ approach, i.e. ester linkage formation between a dibasic acid and trifunctional alcohol, is a simple technique for preparing hyperbranched polyesters [7].

Alkyd resin paint is a polyester-based material derived from vegetable oil containing polyunsaturated fatty-acids [8], [9]. Recently, vegetable oils have received great interests because of their renewability and sustainability [10]. Castor oil is a cheap raw material for the production of eco-friendly resins and one of the few naturally occurring glycerides as the fatty acid portion is 90% [11]. It is a non-drying oil but can be converted into a drying oil by dehydration [12]. Dehydration of castor oil is usually performed by catalytic processes at 250 °C under inert gas [11]. Coatings based on dehydrated castor oil (DCO) fatty acid can reach full cure via oxidative crosslinking [11], [12], [13]. Alkyd paints based on DCO fatty acid have advantages of non-yellowing, high durability, anticorrosion, and weather resistance [14]. Incorporation of inorganic nanofiller in alkyd resin can enhance its performances [15], [16], [17]. A great interest was given to bifunctional nanomaterials that possess dual structure and exhibit extra-ordinary characteristics [18]. Magnetite nanospheres with large surface area afford active sites suitable for different applications [19]. Using Stöber silica as a support for assembling magnetite nanoparticles (NPs) onto its surface can protect these nanomaterials from degradation and improves their biocompatibility [20].

Fe₃O₄@SiO₂ nanofiller structure combines the properties of two phases with varied chemical composition and crystal structure [21]. High anticorrosive properties were achieved by insertion of Fe₃O₄@SiO₂ in epoxy composite coatings [22]. The environmental impact of painting industry is mostly associated with the volatile organic content (VOC) level [23]. Highly branched alkyd nanocomposites provide newly developed materials with double merits: (i) get rid of VOC problems; (ii) property enhancement [24].

This study introduces castor oil-based hyperbranched alkyd/Fe₃O₄@SiO₂ nanocomposites as a cost effective and green material with enhanced coating characteristics. An in-situ large scale synthesis method that efficiently attaches magnetite NPs to silica nanospheres is reported in this work. To the best of our knowledge, this is the first study to prepare and evaluate the tailored alkyd/Fe₃O₄@SiO₂ nanocomposites. This research highlights the significance of the extent of dispersion of Fe₃O₄@SiO₂ nanofillers in determining the improvement in the mechanical, thermal and anticorrosive properties of the nanocomposites.