A Practical Formulation Strategy for Spray-Applied Waterborne 2K Wood Coatings: Emulsion Design, Hardener Selection, and Rheology Tuning

Waterborne two-component (2K) coatings are attractive for spray-applied wood finishing because crosslinking can provide durable films while reducing VOC emissions; however, practical use is often limited by short post-mixing workability, viscosity drift after activation, and restricted film-forming feasibility under ambient conditions. This study established a stepwise formulation strategy by sequentially screening emulsion Tg distribution, neutralizer–pH conditions, methacrylic acid (MAA) content, hardener type, and rheology packages. Increasing shell Tg progressively raised minimum film-forming temperature, whereas gel time increased sharply beyond an intermediate range, defining a practical trade-off between ambient film formation and post-mixing workability. Neutralizer identity strongly affected the gel time–pH response, and a practical condition around pH 6.6 was selected for subsequent screening. Increasing MAA reduced particle size but also increased viscosity and, above 3 wt%, caused pronounced foaming after activation. Hardener screening showed that film-forming viability had to be satisfied before viscosity stability could be used for ranking; an HDI/IPDI-based hardener gave the lowest viscosity drift among the film-forming candidates. Final validation showed stable appearance and largely unchanged film properties from 0 to 7 h after mixing, with the first measurable deviations appearing at 8 h.