Modified epoxy coatings on mild steel: Tribology and surface energy
Graphical abstract
Introduction
Epoxies are noted for their versatility, high resistance to chemicals, outstanding adhesion to a variety of substrates, toughness, high electrical resistance, durability at high and low temperatures, low shrinkage upon cure, flexibility, and the ease with which they can be poured or cast without forming bubbles [1], [2], [3], [4], [5]. These properties make them eligible for use in various applications such as protective coatings (for appliance, automotive primers, pipes) [6], encapsulation of electrical and electronic devices, adhesives, bonding materials for dental uses, replacement of welding and riveting in aircraft and automobiles, composites materials in space industry, printed circuitry, pressure vessels and pipes, and construction uses such as flooring, paving, and airport runway repair [1], [7].
One of the ways of improving the performance of polymers is by introducing a metallic dispersed phase. Metallic materials have useful properties and characteristics that are crucial for many applications; among them high electric conductivity, paramagnetism, high thermal conductivity as well as good mechanical properties. Combination of polymers with metals results in materials with electrical and magnetic properties comparable to neat metals and with a significant improvement in thermal properties of polymers. Also, the processability is the same as for neat polymers – a significant advantage for speed of production and processing costs.
Al particles added to poly(ethylene oxide) affect electric conductivity as reported by Muszynska and her colleagues [8]. Mamunya et al. used Cu and Ni powders as fillers in an epoxy resin and in poly(vinyl chloride) and studied the concentration dependence of electric and thermal conductivity [9]. Kim and coworkers added a soft Al + Fe + Si magnetic powder to a polymeric matrix to produce magnetic films to be used for shielding of electromagnetic waves [10]. Metal powders have been distributed in low density polyethylene (LDPE) without agglomeration [11], resulting in improvement of tribological properties [12], [13]. Brito and Sanchez [14] used Zn, Cu, and Al as fillers in thermoset polymer systems composed by epoxy and amino resins at several concentrations up to 30 wt.%; they studied mechanical properties as well as thermal decomposition. They observed that the temperature of decomposition decreases when metal is added to epoxy + amine resins, this for all metals and all epoxy:amine ratios.
Modification of epoxies for achieving a wide range of properties including the above mentioned ones can be effective. Fluoropolymer modified epoxy provides better scratch resistance, low surface tension, better overall tribological properties and hydrophobicity [15], [16], [17], [18]. Kumar, Alagar and Mohan developed siliconized epoxy interpenetrating coatings over mild steel wherein Zn powder was used for achieving good corrosion resistance [19]. Bazyliak, Bratychak, and one of us proposed the use of oligomers containing peroxy groups as epoxy modifiers in order to improve adhesion [20], [21], [22]. Polymer composites including carbon nanotubes (CNTs) are used for a variety of applications [23]; epoxies as corrosion protection coatings on steels have been reported [24]. Epoxy powder coatings modified using nano-CaCO3, resulting in tensile and corrosion resistant properties were described by Yu and coworkers [25]. There has also been work reported on blends of thermoplastics like polysulfones and poly(methyl methacrylate) with epoxies for achieving toughness [26], [27]. Kumar, Balakrishnan, Alagar and Denchev used silicone and phosphorus for modifying an epoxy to achieve anticorrosion, antifouling and flame retardant properties [28]. Fibers can also be used to alter the properties of the material [29].
In this work, modification of pure epoxy had been carried using a fluoropolymer and metal powders to achieve an advantageous combination of tribological and hydrophobic properties.
Section snippets
Materials
Epon™ Resin 828, an undiluted clear difunctional bisphenol-A/epichlorohydrin derived liquid epoxy resin, diglycidyl ether of bisphenol-A (DGEBA), from Hexion Speciality Chemical Inc., was used as the base epoxy. The glass transition temperature Tg of the epoxy is 92 °C [16]. The epoxide equivalent weight of the epoxy is 185–192 g/eq. Its chemical formula is:
Fluorinated poly(aryl ether ketone) (FPEK) fluoropolymer was used as one of the modifiers due to its low surface energy nature which could
Composition
Since the coatings were prepared in laboratory atmosphere, there was a chance of formation of a new secondary phase within the metal powders like a metal oxide phase which would affect the tribological properties of the final coating. Hence it was pertinent to analyze the composition of the metal powders to determine the existing phases. Diffraction patterns of Ni, Al, and Ag powders did not show traces of any secondary phases but the diffraction pattern of Zn powder shows some peaks related to
Acknowledgments
A partial financial support has been provided by the Robert A. Welch Foundation, Houston (Grant # B-1203). Detailed comments of a referee are appreciated.
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