Optimization of One-Step, Low-VOC, Chromate-Free Novel Primer Coatings Using a Taguchi Method Approach

Abstract

In this paper, we have used a Taguchi method approach for the optimization of 5 different novel room-temperature cured primers. These novel primers were developed for the replacement of the chromate-based Cr(VI) technology currently used for the corrosion protection of aerospace alloys such as AA 2024-T3. However, Cr(VI) has been identified by the Environmental Protection Agency (EPA) as toxic and carcinogenic and a replacement for it is being sought. The 5 systems differ in the type of organofunctional silane used for formulating the primer. The five different organofunctional silanes used were bis-(triethoxysilylpropyl) ethane (BTSE silane), bis-(triethylsilylpropyl) tetrasulfide (bis-sulfur silane), and combinations of the above silanes in the weight ratios 1:2, 2:1 and 1:1 respectively. The components of the room temperature primer were acrylate resin, an epoxy resin, one of the five organofunctional silane system and a crosslinker. An L9 orthogonal array was chosen with the parameters set as the components of the primer. The levels were varied through three different amounts for the resins and the silane or silane combinations. The crosslinker amount was fixed and was varied by using three different types of crosslinker. Various corrosion performance tests were conducted on the samples coated with the primers formulated. Electrochemical Impedance spectroscopy (EIS), 3.5 wt-% NaCI aqueous solution immersion test, DI water contact angle before and after exposure to 3.5 wt-% NaCI aqueous solution, tape adhesion test (ASTM D 3359), MEK double rub test (ASTM D 4752), chemical resistance test (ASTM D 3912) were used as performance evaluation tests. Using the results obtained from these experiments an optimized formulation was obtained for primers formulated with each of the five different primer systems.