Weathering, salt spray corrosion and mar resistance mechanism of clay (nano-platelet) reinforced polyurethane nanocomposite coatings

Abstract

Performance properties like weathering and corrosion resistance are critical for automotive top coats. Using two types of clays Cloisite 30B(C30B) and Cloisite 20 A(C20 A), a 2 K polyurethane (PU) pack coating formulation was modified at nanoscale. These coatings show better resistance to salt spray (500 h) and accelerated weathering conditions (1000 h). When observed under atomic force microscopy (AFM) nanoscale morphology of these nanocomposite coatings show globular (C30B) and rod-like (C20 A) hard segment formation of size˜200 nm, which compared to pristine PU coatings are more uniformly formed and evenly distributed across the microscale surface (surface texturing). Visual, spectroscopic & chemical analysis confirmed better performance of nanocomposite coatings under ultraviolet/humidity and salt cycles. Dry film thickness & gloss; dft1000 (G[1000]) was reduced by 25% (32%) for pure PU, 14.03% (16.7%) for PU-3C30B and 20% (22.7%) for PU-3C20 A respectively. The overall colour difference in CIELab values ΔE* was 9.794, 7.623 and 8.450 for PU, PU-3C30B and PU-3C20 A. Salt spray (500 h) rusted about 35% area of mild steel (ms panels) for PU, while for PU-3C30B and PU-3C20 A it was only 5% and 10% respectively. Fourier transform infrared (FTIR) shows better retention of functional groups in aged and corroded nanocomposite coatings versus pristine coatings. This proves that two dimensional clay particles when duly exfoliated interact with hard segments and play a significant role in resisting weathering forces, salt spray corrosion and even mar.