Attributing the crosslinking density to water vapor transmission rate of an acrylic-melamine automotive clearcoat
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Attributing the crosslinking density to water vapor transmission rate of an acrylic-melamine automotive clearcoat M. Nasirzadeh, H. Yahyaei , S. M. Lashgari, M. Mohseni
Ó American Coatings Association 2020 Abstract In this study, the effect of crosslinking density of an acrylic-melamine clearcoat on water vapor transmission rate was evaluated. Four types of acrylic resins with different OH contents were used as the main polymeric backbone of the acrylic-melamine clearcoat. Also, three different acrylics to melamine– formaldehyde ratios were used in the coating formulations to obtain various crosslinking densities. It was observed that other film properties such as surface hydrophilicity and molecular homogeneity had a crucial impact on the clearcoat water vapor transmission rate together with the crosslinking density. Surprisingly, results showed that samples with acrylic resin with less OH content and lower melamine–formaldehyde crosslinker had the lowest water vapor transmission rate among other samples. These results were in agreement with the water contact angle and the data deduced from dynamic mechanical thermal analysis (DMTA) results of clearcoats. Keywords Crosslink density, Water vapor transmission rate, Acrylic-melamine clearcoat
Introduction A car body consists of different coating layers to protect it from the environment. These organic coatings are usually composed of up to five layers, namely a thin phosphate conversion coating, an electrodeposited one, a primer surfacer, a metallic basecoat, and finally a clear top coat. Among these, the clearcoat is the top M. Nasirzadeh, H. Yahyaei (&), S. M. Lashgari, M. Mohseni Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, TehranP.O. Box 15875-4413, Iran e-mail: [email protected]
layer that is exposed to degrading environments, including UV irradiation from the sunlight, destructive mechanical factors like carwash and stone chipping, and chemicals such as acid rain. Preventing water vapor permeation through the film has been effective to control the fate of the coating.1 Therefore, this test is crucial to perform and it can be found in the paint testing plan of almost every car manufacturer. The clearcoat, like most of the organic coatings, is composed of a suitable binder that is responsible for film formation and adhesion to the surface. Different resins are used as binders based on their properties and compatibility with other coating components including additives and solvents. Acrylic-melamine coatings are one of the most used clearcoat systems.1 Acrylic resins have excellent resistance to light initiated deterioration, which makes them a suitable component for clearcoat composition.2 The use of melamine–formaldehyde will cause the clearcoat to be cured and make a 3D crosslinked network. In a typical acrylic-melamine coating, a 30 to 70 ratio between melamine to acrylic resin is used.3 In this combination, two types of reactions may cause network formation; one is the condensation reaction between polyols of
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