Weathering resistance (UV-shielding) improvement of a polyurethane automotive clear-coating applying metal-organic framework (MOF) modified GO nano-flakes (GO-ZIF-7)

Abstract

In this research, ZIF-7 particles were synthesized over graphene oxide (GO) nanosheets and characterized through various techniques. Then, the influence of the fabricated GO-ZIF-7 nano-hybrids on the weathering resistance and physical-mechanical performance of the PU coatings were investigated. The weathering resistance of PU/GO-ZIF-7, PU/GO, and control PU was examined with the aid of QUV accelerated weathering conditions via several methods such as recording gloss retention and contact angle changes, FTIR, colorimetric, FE-SEM, and AFM analysis. For the PU/GO-ZIF-7 nanocomposite coating exposed to accelerated weathering condition, the changes in carbonyl group relate peak and urethane linkage were less than the changes of these parameters for the control PU sample. The change in the color component (Δb*) decreased to 56.9% of PU/GO-ZIF-7 compared to control PU (70.9%). The change of root mean square (RMS) roughness value of PU/GO-ZIF-7 was less than that of control PU (from 9.44 to 8.04 nm vs. from 14.89 to 8.02 nm). As a result, after a long weathering exposure (UV-A), the fewer changes in aesthetic properties, chemical structure, and topology of the PU/GO-ZIF-7 coating led to less surface degradation and better weathering stability performance. The GO-ZIF-7 nano-hybrids revealed that they are capable of improving the photo-stability of PU coatings by the mixed mechanisms of absorbing UV rays and free radical scavengers. The physical-mechanical characteristics of the nanocomposites were analyzed using dynamic mechanical thermal analysis (DMTA) and tensile tests. The addition of GO-ZIF-7 to PU led to an above 70% and 85% increase of elongation at break and toughness concerning the control PU, respectively.