| Résumé : |
The sensitivity of automotive coatings particularly coatings for plastics, to scratching, has been a growing concern among automakers. Scratching may result from such predelivery events as polishing of minor defects embedded in the paint, or post-delivery events such as car wash ristles, dirt embedded under a cloth utilized in polishing the car, tree branches, and the like. Warranty cannot separate out which event is the more prevalent (e.g., predelivery or post-delivery to the customer) form of damage on plastics. Data available on coatings for metal, however, does suggest thot isocyanate-based crosslinked systems perform more poorly than their melamine-based crosslinked counterparts when exposed to in-plant (predelivery) handling. This work attempts to correlate the scratch resistance behavior of hydroxyl-functional acrylic or polyester-isocyanate-functionalized carbamate-melamine-, functionalized silane-melamine-, and hydroxyl-functional acrylic or polyester-melamine-crosslinked systems, both in their green state (right out of the oven, less than one week post-cure time) and in their infancy in the field (simulated 250 kJ Xenon arc Weather-ometer), to surface attributes such as toughness, hardness, and elasticity. Known scratch methodologies, namely the crockmeter and Ford five-finger laboratory test methods, in addition to a newly described compressive shear loading device (herein termed Scratcho), are used to compare the clearcoat systems as applied over one-component hydroxyl functional acrylic or polyester melamine crosslinked black basecoats. It is shown that the scratch testing methodology utilized to assess the scratch resistance of the coating system dictates results, and that Scratcho compares very well with crockmeter results. Functional carbamate-melamine crosslinked one-component coatings and functionalized silane-melamine crosslinked one-component coatings appear to outperform selected two-component coatings, which in turn outperform one-component hydroxyl functional acrylic or polyester melamine crosslinked coatings. Material attributes such as surface hardness, toughness (as measured through the method of essential work), and the ability to recover from an applied load are most important in the ability of the coating to resist damage. |
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Scratch resistance behavior of automotive plastic coatings in JOURNAL OF COATINGS TECHNOLOGY (JCT), Vol. 74, N° 925 (02/2002)
