Solvent-Free Transformation of EVA Into Pressure-Sensitive Adhesives (PSAs) by Mixing Nonylated Diphenylamine: Improved Tack, Adhesion Strength, and Longevity

Pressure-sensitive adhesives (PSAs) bond surfaces with minimal pressure, eliminating the need for heat, water, or solvents. Ethylene vinyl acetate (EVA) is a well-established polymer in hot melt adhesives; however, research on EVA-based PSAs is limited. This study introduces nonylated diphenylamine (NDPA), a liquid derivative of diphenylamine, as a novel tackifier and antioxidant for EVA-based PSAs. By simply mixing NDPA with EVA at elevated temperatures, an EVA-based PSA is successfully formulated without the use of toxic solvents. The molecular structure of NDPA, characterized by alkyl chains and secondary amines, allows it to interact effectively with both the polyethylene (PE) and vinyl acetate moieties in EVA. These interactions significantly enhance the viscoelastic properties and adhesive strength of the formulations. Notably, the addition of NDPA increases lap shear strength from 98.67 kPa for pure EVA to 516.67 kPa for the EVA-NDPA formulation containing 40 wt.% NDPA. Furthermore, a 50 wt.% inclusion of NDPA achieves the highest peel strength of 6.5 N mm⁻¹, surpassing that of commercial PSA tape. Additionally, NDPA contributes to preventing oxidative degradation, significantly enhancing the longevity of adhesive properties and positioning it as a promising additive for improved EVA-based PSA formulations.