High-performance polyimide coatings for mild steel: the impact of benzophenone, biphenyl, and bisphenol-a core structures

Abstract

Engineered polyimide coatings are widely recognized for their high thermal stability, chemical resistance, mechanical strength, and electrical insulation. In this work, three polyamic acid (PAA) systems (PAA-1, PAA-2, and PAA-3) were synthesized using two diamines and varying dianhydrides [benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA), 4,4′-Biphthalic Anhydride (BPDA) or 4,4′-bisphenol A diphthalic anhydride (BPADA)] to investigate the influence of benzophenone, biphenyl, and bisphenol-A cores on polyimide coating performance. The polyamic acid (PAA) solutions were prepared under a nitrogen atmosphere and subsequently imidized at 250 °C to obtain polyimide coatings (PI-1, PI-2, and PI-3) on mild steel substrates. This study provides insights into how structural variations in the polyimide backbone affect adhesion, thermal stability, and anticorrosion behaviour. Solid-state ¹³C NMR, XPS, FTIR, and XRD confirmed structural integrity, imidization, and molecular packing, while SEM and CLSM revealed uniform surface morphology. PI-3 exhibited superior adhesion (3.03 MPa) due to Fe–O–C bond formation and demonstrated excellent corrosion resistance in salt spray testing (ASTM B117), with <5% red rust after 168 h. PI-2 (biphenyl core) showed the highest thermal stability and storage modulus (1915 MPa), whereas PI-3 (bisphenol-A core) achieved the highest loss modulus (297.46 MPa), tan δ (0.177), and crosslinking density (0.343 mol m⁻³). Enhanced damping capability and chain mobility of PI-3 contribute to crack resistance and long-term anticorrosion performance, highlighting its potential for advanced protective applications.