A multi-functional coating based on acrylic copolymer modified with PDMS through copolymerization

Highlights

• Multi-functional acrylic-polydimethylsiloxane (PDMS) copolymers were synthesized for application in harsh industrial environments.

• Acrylic-PDMS copolymer films exhibited high hydrophobicity, low water absorption ratio and excellent dirt pickup resistance.

• The enhanced anti-corrosion performance of the copolymers was verified by EIS and salt spray tests.

• An approach to developing multi-functional copolymers through molecular design is illustrated.

Abstract

The chemical modification of acrylic copolymer was performed through copolymerization of vinyl-terminated polydimethylsiloxane (PDMS) and acrylic monomers by solution polymerization in order to synthesize acrylic-PDMS copolymers that could possess multi-functional properties required for application in harsh industrial environments. The synthesis of acrylic-PDMS copolymers was designed to possess combined advantages of the PDMS and acrylic copolymer. The acrylic-PDMS copolymers formed dispersion particles to be stable in water and the hydrophilic groups of the acrylic copolymer was distributed outside of the particles which is intended to stabilize the copolymer particles in water, and this could make it possible to prepare water-based coatings. The acrylic-PDMS showed more flexible copolymer chains, allowing the surface of acrylic-PDMS copolymer films to form with minimum defects, compared to the acrylic copolymer film, and with significantly decreased surface energy. The copolymer coatings exhibited high hydrophobicity with contact angles of the copolymer films increased to more than 90°. The high contact angle and low surface energy contributed to the low water absorption ratio and low dirt-pick performance of the copolymer films. The enhanced anti-corrosion performance of the copolymers was verified by electrochemical impedance spectra (EIS) and salt spray tests. The incorporation of PDMS segments in the acrylic-PDMS copolymers has been shown to be an effective approach to developing multi-functional coating with high chemical, corrosion and thermal resistances.

Introduction

Polydimethylsiloxane (PDMS) is frequently used in coating preparation and modification due to its desirable properties, including high chain flexibility, high hydrophobicity, low surface energy and surface tension, low glass transition temperature, good chemical resistance and high thermal stability [[1], [2], [3]]. However, the relatively high cost and high hydrophobicity of PDMS have prevented its wide application in waterborne coatings [4]. Acrylic copolymers are a low cost material that possesses a variety of properties including good film-forming, high gloss and high transparency [[5], [6], [7], [8]], permitting its wide applications in adhesives and additives, and especially in waterborne coatings [[9], [10], [11], [12], [13]]. However, the relatively lower chemical, corrosion and thermal resistances of acrylic copolymers have limited its application in harsh industry environmental conditions. If a coating could be designed and synthesized by modifying acrylic copolymer with vinyl-terminated PDMS, it could be a coating with multi-functional properties required for application in harsh environmental conditions such as in the petrochemical plants. In the petrochemical industry, sour water gas plants often have indispensable equipment for treating acid sewage. The operation conditions of acid water tank are relatively harsh, with the main mediums in acid sewage being various acids, phenol and oils and the operating temperature being about 65–70 °C [14]. Under such conditions, the acid water tank protection would need coatings that possess multi-performance including chemical resistance, anti-corrosion, thermal stability and easy-cleaning. Epoxy resin-based coatings are commonly used in petrochemical plants [15]. The cost of it is high and this is a restriction for the wide application. To prepare multi-functional coatings with multiple performance, modifying acrylic copolymer with other polymers is the most efficient method [16,17]. Previous studies have shown that acrylic polymers modified with PDMS usually exhibit superior performance to those of their single components. For instance, Li et al. prepared acrylic-PDMS composite latex by emulsion polymerization [18]. The thermal stability of the composite latex enhanced from 325 °C to 362 °C. The contact angle and corrosion resistance of the dried acrylic-PDMS composite latex increased with the content of PDMS. In addition, the water absorption ratio decreased linearly with the increased PDMS component, which reduced from 15.5 % to 5.11 %. However, the corrosion resistance performance of the acrylic-PDMS copolymers was seldom investigated in these studies.

The modification of acrylic copolymers can be classified into physical and chemical modification according to the force type between acrylic copolymers and PDMS [19]. Physical blending is the simplest and most common method to modify acrylic copolymers. For instance, latex blends of acrylic copolymer and PDMS were prepared to improve the dirt pickup resistance [20]. However, it was found that when the PDMS content reached to about 30 %, the latex blends cannot form a uniform film. In most cases, the PDMS is not easy to uniformly distribute in the matrix by physical blending when the content increases [21,22]. In addition, the compatibility between PDMS and acrylic copolymers is really low [23,24]. The repeat unit of PDMS polymer chain is –Si–O–, and the side chains of PDMS are −CH₃ groups with no active groups of PDMS to react with acrylic or curing agents [25]. This means there is no covalent bond between the two polymer systems. In this situation, PDMS may be removed easily from the film surface and the properties of PDMS will be lost at the same time. In contrast, chemical modification allows covalent bonds be formed between acrylic copolymers and PDMS. At the presence of covalent bonds, mixing at the molecular level can be achieved to combine corresponding superior properties of two copolymers into a co-polymer system. The commonly used method of introducing PDMS units to acrylic copolymer chain is emulsion polymerization [[26], [27], [28], [29]]. The emulsion polymerization can be divided into conventional emulsion polymerization, mini emulsion polymerization, seeded emulsion polymerization and other synthetic polymerization techniques [30,31]. The PDMS does not have reactive group to react with acrylic monomers. To incorporating PDMS units into acrylic copolymer chain, the PDMS needs to be modified with silane to introduce vinyl groups. For instance, the PDMS was modified with γ-methacryloxy propyl trimethoxyl silane to obtain reactivity, and then copolymerized with acrylic monomers to prepare acrylic-PDMS composite latex by emulsion polymerization [18]. In the polymerization, surfactants are one of the main components [32]. However, one obvious disadvantage of this method is the presence of surfactants that would be residual in polymer system. For waterborne coatings, the remained surfactants could certainly cut down the water resistance [33]. In addition, most of the surfactants are with low molecular weight and therefore with high ability to move [34], consequently the surfactants could simply move between the coating and substrate. This is a vital factor contributing to the decrease in the adhesion of coatings on substrate. Moreover, the surfactant remaining in polymers may have harmful effect on environment. Considering these factors, surfactant-free polymerization should be developed in copolymerization of vinyl-terminated PDMS and acrylic copolymers. In this study, these issues are addressed by acrylic-PDMS copolymers being synthesized by free radical polymerization without the assistance of surfactants.