A selection of five different automotive clearcoat varnish samples were obtained from Cytec Industries Inc., a company which supplies crosslinking additives to automotive coating manufacturers. These samples were selected to give a wide range of mar resistance, i.e., the ability of the coating to resist damage caused by light abrasion. The difference between mar and scratch resistance is that mar is related only to the relatively fine surface scratches which spoil the appearance of an automotive coating. The NST is therefore ideally suited to simulating such scratch damage in a controlled way, for example, damage caused in service due to car washing, polishing, grit impact, or other environmental conditions. A quantitative description of these effects is usually given in terms of gloss reduction, increase in haze or shift in grey level.
Haze measurements can also reveal the anisotropy of the scratch direction.
The five measured samples are summarized in Table 1 with their known mar resistance ratings. Five progressive normal load scratches were performed on each sample and average values taken for the two critical loads, L c 1 and L c 2. The diamond stylus used had a tip radius of 10m m. 1 summarizes the evolution of penetration depth for a scratch made on each of the five samples and it is evident that the coatings exhibiting good mar resistance have a greater depth but a much smoother depth profile. The less mar-resistant coatings have an increasingly rough profile after the applied load has exceeded the first critical point, L c 1.