Development of bio-based polyurethane composite adhesives derived from castor oil and epoxidized soybean oil polyols: Influence of fumed silica and chain extender on strength, stability and wettability

Polyurethane (PU) adhesives are widely used in various industries due to their strong bonding and flexibility. This study focused on the preparation of a bio-based PU adhesive derived from castor oil, as well as bio-adhesives formulated using epoxidized soybean oil (ESO) modified with acetic acid and acrylic acid. The study investigated the effect of polyol type, prepolymer/polyol ratios, and silica fillers on lap shear strength. Lap shear strength analysis indicated that ESO modified with acrylic acid exhibited the highest lap shear strength of 3.74 ± 0.55 MPa, followed by ESO with acetic acid (3.31 ± 0.28 MPa) and castor oil (3.12 ± 0.21 MPa). The study also showed that optimizing the prepolymer/polyol ratio significantly influences lap shear strength, with a prepolymer/polyol ratio of 2 yielding the highest shear strength. Furthermore, the addition of fumed silica up to 1 wt% improved mechanical strength by enhancing hydrogen bonding, but excessive filler led to void formation and weaker adhesion. Additionally, water contact angle measurements revealed that the incorporation of fumed silica enhanced the wettability of the bio-adhesive. Thermal analysis confirmed stability up to 280°C, suggesting that these bio-adhesives can serve as a sustainable and customizable alternative. • Preparation of bio-based PU adhesive derived from vegetable oils. • Epoxidized soybean oil modified with acrylic acid exhibited the highest lap shear strength. • Prepolymer/polyol ratio of 2 yielding the highest shear strength. • Improvement of lap shear strength by the addition of fumed silica up to 1 wt%.