Study of Weathering Resistance of Different Refinish Coatings

摘要

High weathering resistance is an important factor for automotive coatings. Humidity is one of the most important phenomena causing degradation upon which the appearance of coating, such as gloss and hue will be affected. On the other hands, mechanical damages are often inevitably occurred on automotive bodies during the assembling process, and as a result, repair finishes are frequently required to refinish the failure areas. 2-K PU and acid-catalyzed alkyd/melamine resins are generally the best options for the repair job. The coatings made from these two resins show different weathering resistance in service. This work was an attempt to study the -effect of material and processing parameters of two current repair refinishes on a type of appearance alteration reportedly observed after a while the car was exposed to the environmental atmosphere. Accelerated weathering tests (XENON and humidity), thermal analysis (DSC,DMTA,TGA), FTIR spectroscopy as well as mechanical (pendulum hardness and MEK test) and optical (gloss, spectrophotometer) evaluations were conducted on differently prepared coatings with respect to the coating type, curing temperature, curing time and catalyst, to elucidate the mechanism of the fading phenomena. It was found that the PU system tended to show less alteration compared with the alkyd/melamine one and also that the curing temperatures exceeding 80oC were needed to obtain a highly cross-linked network, leading to lower fading. Moreover, humidity and uv radiation have also been shown to synergistically affect the occurrence of such a phenomenon.