pH—Viscosity Stability of Water-Based Flexography Inks with Bio-binder: Full Acrylic Replacement

The replacement of petroleum‑derived acrylic binders with sustainable bio‑based alternatives is of increasing interest in water‑based flexographic inks; however, successful adoption requires preservation of storage stability and printed color fidelity. In this study, a bio-binder based on soy protein was evaluated as a partial and full replacement of a conventional acrylic emulsion binder in cyan water‑based flexography ink formulations for linerboard application. The soy replacement of 0–100 wt% in the let‑down ink vehicle were formulated and assessed for storage stability via pH and viscosity measurement over 60 days using a Zahn #2 cup efflux time. The physical properties ink, such as optical ink density and colorimetric performance (CIE L*a*b* and delta E 2000 (ΔE₀₀) color difference) were determined. All formulations maintained stable alkaline pH values (≈ 8.7–9.2) and consistent viscosity within the flexography printing operating window (≈ 25–28 s), indicating good chemical compatibility and resistance to viscosity shift. Optical ink density showed only minor, non‑disruptive changes across formulations. Colorimetric analysis revealed no statistically significant differences in L* or a* values, while modest formulation‑dependent shifts were observed in the b* coordinate at intermediate soy levels. Importantly, ΔE₀₀ values were below 2, a common perceptibility threshold, indicating negligible visual color differences relative to the acrylic control. The results demonstrated that soy protein bio-binder can replace acrylic binder across the full substitution range without compromising storage stability, press‑relevant viscosity, or printed color appearance. This work supports soy protein bio-binder as a sustainable alternative for water‑based flexography inks, enabling reduced reliance on petroleum‑based polymers and associated Volatile Organic Compounds (VOC) emissions while maintaining performance required for packaging applications.