Abstract:The scratch resistance of coatings used on two highly visible automotive applications (automotive bodies and window glazings) were examined and reviewed. Types of damage (scratch vs. mar), the impact on customers, and the causes of scratch events were investigated. Different exterior clearcoat technologies, including UV curable and self-healing formulations were reviewed, including results from nano-and macro-scratch tests. Polycarbonate hardcoat glazings were tested vs. annealed glass samples using a Taber abraser, with the resulting damage analyzed using transmitted haze measurements and optical profilometry. A correlation between the damage seen in glass samples (many smooth, shallow mars) and the best hardcoat samples (fewer, deeper scratches) and the haze measurements was discussed. Nano-scratch results showed similar fracture forces, but measurably improved mar resistance for the hardcoats/glass system compared to exterior clearcoats.

Photolatent base crosslinked epoxy thiol coatings were characterized using transmission FTIR spectroscopy, Raman spectroscopy, UV spectroscopy, and Vickers micro-hardness testing. Curing rate constants were calculated for a series of formulations with varying concentrations of photolatent-DBN, ITX (2-isopropyl thioxanthone), and BP (benzophenone), as well as varying curing times. A design of experiments demonstrated that increased UV dose had the greatest impact on increasing the curing rate of the clearcoat system. BP wt% proved to have little impact on the clearcoat curing rate, while PL-DBN and ITX exhibited minimal improvement above 2.5 wt% and 1 wt%, respectively. Vickers micro-hardness results showed that irradiation dose had no impact on the final physical properties of the clearcoat. However, a 24-h ambient post-cure proved to increase the final hardness of the clearcoat under some conditions.

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The scratch resistance of four clearcoat formulations was evaluated using a CSM nano-scratch tester, an AMTEC-Kistler simulated carwash tester, and a laboratory scale macro-scratching tester. Significant differences in the rank-order of all the clearcoats were found when comparing the scratch and mar behavior using macro-scratching, nano-scratching, and AMTEC-Kistler testing. Field vehicles were also examined where the mean and median widths of scratches found on vehicles were 237 and 141 lm, respectively. The range of loads associated with events with the potential to create real-world scratches was found to be significantly higher (5-35 N) than the forces needed to make scratches of the same mean size as those seen in the field (7-10 N). These results indicated that a significant improvement in the scratch resistance of these clearcoats would be needed before customers would notice improvements in their paint finish's scratch resistance.

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