Solvent-Free, High Castor Oil Content-Based Pressure-Sensitive Adhesive: Synthesis and Performance

With the growing demand for sustainable and biodegradable adhesives, castor oil has gained attention as a promising bio-based raw material due to its hydroxyl functionality, low toxicity, and environmental benefits. In this study, a solvent-free pressure-sensitive adhesive (CO-PSA) with a high castor oil content (55 wt%) was synthesized via controlled functionalization of castor oil (trifunctional to difunctional) and polymerization with isophorone diisocyanate (IPDI). FTIR confirmed the target chemical structure of CO-PSA by comparing the structures of the starting material, intermediates, and final product. The optimal CO-PSA formulation (CO-PSA-3) exhibited outstanding adhesive performance: initial tack for an 11# steel ball, 180° peel adhesion strength of 12.26 N/25 mm, and static shear holding power of 25 h. Wettability tests confirmed good surface interaction (water contact angle < 80° for all formulations), ensuring effective substrate bonding. Thermal analysis (DSC/TGA) indicated good thermal stability (initial thermal degradation temperature: 266°C) and flexibility at low temperatures (glass transition range: −70°C to −32°C). Degradation studies showed that CO-PSA undergoes complete alkaline hydrolysis within 2 h, forming nanoscale degradation products (~90 nm in diameter). The high–castor-oil (55 wt%), solvent-free, and degradable CO-PSA formulation developed in this study can serve as an environmentally friendly alternative to petroleum-derived adhesives, advancing sustainable adhesive technologies for the packaging industry and related sectors.