Mechanical Performance of Calcium Phosphate-Reinforced Epoxy/Episulfide Polymeric Adhesives for Structural Metal Bonding

In this study, P0, P1, and P2 polymer-based adhesive systems were systematically investigated to evaluate the effects of polymer composition and Ca₃(PO₄)₂ filler content on their adhesion performance. Single-lap shear tests conducted according to ASTM D1002 revealed that both polymer architecture and filler loading play critical roles in determining the shear strength and overall mechanical behavior of the adhesive joints. Among the Ca₃(PO₄)₂-free systems, the copolymer-based P2 adhesive exhibited the highest shear strength (0.47 MPa), compared to P1 (0.20 MPa) and P0 (0.19 MPa), demonstrating the beneficial effect of copolymerization on load-bearing capacity and interfacial adhesion. For Ca₃(PO₄)₂-reinforced adhesive systems, filler incorporation significantly enhanced joint performance up to an optimum content. In particular, the addition of 1 wt% Ca₃(PO₄)₂ to the P2 system yielded the highest shear strength (0.87 MPa). However, further increases in filler loading led to reduced performance, with shear strength values decreasing to 0.47 MPa at 2 wt% and 0.19 MPa at 4 wt%. This decline at elevated filler contents is attributed to particle agglomeration and weakened interfacial interactions, which limit efficient stress transfer within the adhesive matrix. Overall, the findings demonstrate that controlled Ca₃(PO₄)₂ incorporation combined with appropriate copolymer design provides an effective strategy for tailoring the adhesion properties of polymer-based adhesive systems. The developed formulations show strong potential for high-performance structural bonding applications requiring enhanced strength and reliability.