Sustainable Design of Zwitterionic Polyurethane Acrylate Emulsions for Pressure-Sensitive Adhesive Packaging

Pressure-Sensitive Adhesives (PSAs) are widely used in flexible packaging due to their rapid adhesion, excellent transparency, and mechanical flexibility. However, traditional solvent-based PSAs pose environmental hazards; although waterborne alternatives are more eco-friendly, they often suffer from limited adhesion, poor wetting on low-energy surfaces, low mechanical strength, and inadequate microbial resistance without additional additives. To overcome these challenges, two hybrid polyurethane acrylate (PUA) systems were synthesized: a conventional PUA (prepolymer-01) and a zwitterionic-based PUA (ZPUA, prepolymer-02). From these prepolymers, five PSA emulsions were formulated: one from acrylic monomers (PSA-01), one from PUA (PSA-02), and three from ZPUA (PSA-03, PSA-04, PSA-05). The incorporation of zwitterionic segments into the PUA backbone was confirmed by FTIR spectroscopy. Thermogravimetric analysis (TGA) demonstrated the superior thermal stability of ZPUA over its non-zwitterionic counterpart. ZPUA-based emulsions exhibited excellent wettability on low-energy surfaces, a key advantage for packaging applications. Moreover, tensile testing showed that zwitterionic PSA films (PSA-03 to PSA-05) outperformed their non-zwitterionic counterparts (PSA-01, PSA-02) in both tensile strength and elongation at break. Improved adhesion was also observed on difficult-to-bond surfaces, confirming the benefit of zwitterion integration. Importantly, the ZPUA emulsions displayed intrinsic antimicrobial activity, potentially eliminating the need for formalin as a preservative.