Abstract
Herein, we synthesized two new phthalimide-functionalized benzoxazine monomers, pPP-BZ and oPP-BZ, through Mannich reactions of 2-(4-hydroxyphenyl)isoindoline-1,3-dione (pPP) and 2-(2-hydroxyphenyl)isoindoline-1,3-dione (oPP), respectively, with p-toluidine and paraformaldehyde. The structures of these two monomers were confirmed using Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopy. We used differential scanning calorimetry, FTIR spectroscopy, and thermogravimetric analysis to study the polymerization behavior and thermal stability of the monomers and their corresponding polybenzoxazines. Poly(pPP-BZ) and poly(oPP-BZ) were formed on mild steel (MS) through spin-coating and subsequent thermal curing polymerization. We used various corrosion testing methods to examine the effect of the curing temperature on the corrosion resistance of the coated MS samples in 3.5 wt.% aqueous solution of NaCl. Among our tested systems, the corrosion rate reached a low of 2.78 µm·Y−1 for the MS coated with poly(pPP-BZ)180 (i.e., the coating that had been cured at 180 °C); this value is much lower than that (4.8 µm·Y−1) reported for a maleimide-based benzoxazine compound (MI-Bz)/33 wt.% ACAT (amine-capped aniline trimer) blend. Thus, the incorporation of the imide functional group into the PBZ coatings is an effective strategy for affording high-performance corrosion resistance.
Highlights
Intra and intermolecular hydrogen bonds among their heterocyclic polymer chains impart polybenzoxazines (PBZs) with many useful properties, including high thermal stability, high glass transition temperatures, high char yields, near-zero shrinkage upon polymerization, low water absorption, low flammability, excellent mechanical and dielectric properties, low surface free energies, and high water contact angles (WCAs) [1,2,3,4,5,6,7,8,9,10]
The ring-closing reaction of oPP-BZ monomer was much easier than pPP-BZ, because of the existence of intermolecular hydrogen bond in oPP, which accelerates the ionization OH group during stages forming of oPP-BZ monomer, this behavior was reported elsewhere [5]
From reactions of pPP and oPP, respectively, with p-toluidine and paraformaldehyde in 1,4-dioxane. Their corresponding polymers obtained after thermal curing had high values of Tg (>212 ◦ C), as determined through differential scanning calorimetry (DSC) analysis
Summary
Intra and intermolecular hydrogen bonds among their heterocyclic polymer chains impart polybenzoxazines (PBZs) with many useful properties, including high thermal stability, high glass transition temperatures, high char yields, near-zero shrinkage upon polymerization, low water absorption, low flammability, excellent mechanical and dielectric properties, low surface free energies, and high water contact angles (WCAs) [1,2,3,4,5,6,7,8,9,10]. Aromatic polyimides are high-performance polymers that display high thermal stability, excellent mechanical properties, and high chemical resistance [21]; Coatings 2020, 10, 1114; doi:10.3390/coatings10111114 www.mdpi.com/journal/coatings. Coatings 2020, 10, 1114 their inert imide rings and strong interchain interactions make them useful in, for example, plastics, films, laminating resins, insulating coatings, and high-temperature structural adhesives [22]. The introduction of a sulfonic acid unit into the benzoxazine backbone has led to corresponding PBZs displaying good thermal stability, low methanol permeability, and excellent acid resistance, making them promising materials for proton exchange membranes in direct methanol fuel cells [23]. Organic coatings are often employed to protect iron and mild steel (MS)
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