Scaling behavior in the scratching of automotive clearcoats

Abstract

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.

An Overview of the Scratch Resistance of Automotive Coatings: Exterior Clearcoats and Polycarbonate Hardcoats

“…In addition, the initial fracture scratch resistance also increases. However, an increase in nano-scratch resistance does not always lead to an increase in large scale scratch resistance, as has been demonstrated via nano-scratch testing [5].…”

“…Unfortunately, each type of scratch has a unique damage mechanism, so improving a clearcoat's resistance to one type of scratch may not improve its resistance to the other type. Considerable effort has recently been put into formulating clearcoats with a noticeable improvement in scratch resistance [4][5][6][7][8][9][10]. Unfortunately, it is unclear how good is "good enough" when comparing laboratory scratch data to customer perception.…”

“…Details about the macro-scratch testing apparatus and clearcoats tested were previously published [5]. Of note is the shallow depth of penetration and the small shoulders that remained after deformation by the scratch tip.…”

“…Less differentiation was observed between the clearcoats' mar resistance (deformation @ 5 mN). The crosslink density of the UV curable clearcoats is significantly higher (5-8 × 10 −3 mol/cc) than that of the thermally cured clearcoats (~2 × 10 −3 mol/cc) [5,11]. While the nano-scratch behavior is improved by an increase in crosslink density, the increase comes at the price of long-term cracking resistance [11], which implies the flaw sensitivity of the coating has been raised to an untenable level.…”

“…Morphological changes that occur during weathering may reduce the effectiveness of this approach [17]. Also, it has been shown that forces during scratch events can reach upwards of 35 N [5], a thin nanoparticle/resin layer will likely not be able to survive these loads, resulting in deep scratches of similar morphology to those seen in standard thermoset systems.…”