By Yong Zhang and Bridgette Hendrix, Huntsman Corporation, Advanced Materials Division
Two-component (2K) epoxy coatings have long been used for industrial protective applications due to their excellent adhesion to a wide range of substrates, superior chemical resistance, and good mechanical properties.1 Today, those in the industry who are looking for innovations that can boost coating performance heavily prioritize high performance and productivity in particular. Their main goal is to extend coating life service time and use the fewest coating layers possible. Additives, or performance boosters, are commonly used for this purpose in a formulation. One example of an additive is tougheners, which are used in many high-performance industrial protective coatings to improve their toughness, flexibility, adhesion, and long-term performance.
Phenoxy resins are unique polymer-based additives that have become available to formulators. Phenoxy resins are the product of bisphenol A and epichlorohydrin, with the epoxide ring opened. They are a tough and ductile thermoplastic material with high cohesive strength and good impact resistance. The backbone ether linkages and pendant hydroxyl groups promote wetting and bonding to polar substrates and fillers. The typical phenoxy resin structure is shown in Figure 1.
The structure of the phenoxy resin is a polyhydroxyether with terminal alpha-glycol groups. Weight-average molecular weights range from approximately 25,000 to above 60,000, with n ranging from 30 to above 60. This long chain linear polyhydroxylether structure allows for excellent adhesion, impact and abrasion resistance, flexibility, and chemical resistance in coating applications.
Phenoxy resins are soluble in a variety of materials, including ketones, glycol ethers, and glycol ether esters. This article discusses the use of a phenoxy solvent solution, in a solventborne zinc-rich primer formulation. A performance comparison with a commercially available zinc-rich primer will also be discussed, specifically comparing adhesion, dry time, hardness, impact resistance, and salt spray corrosion resistance.
Raw materials and testing panel preparation
A commercial, two-component (made up of part A and part B), solventborne zinc-rich primer was chosen in this study as the commercial control, referred to as Control. A solventborne phenoxy solution, referred to as PKA (the phenoxy resin additive), was added to part B (the amine curing agent side) as an additive. The physical properties of the Control and PKA are described in Tables 1 and 2, respectively.
The two-component coatings were prepared by mixing part A and part B in an overhead mixer on the 100- to 300-gram scale. By adding different amounts of PKA, which was based upon the total formulation weight, to part B, several phenoxy-modified coating formulations were obtained, and those three are referred to as 1%, 2%, and 4%.
Testing panels were prepared by drawing down the two-component coatings over xylene degreased steel panels, generally using a 10-mil gap 3-inch drawdown bar. The panels were typically allowed to cure for 7 days at 23 ˚C and 50% relative humidity before testing, unless otherwise noted. The dry film thickness of each testing panel was about 3–5 mil.
Coatings were drawn down onto glass substrates with a wet film thickness of 75 µm and set on a B.K. Linear Drying Time Recorder. The dry-to-touch time was visually assessed after dragging a needle through the coating over the course of 24 hours, according to ASTM D5895.
Continue reading in the May-June digital issue of CoatingsTech.