Optical bonding utilizing aqueous Al2O3–P2O5-lactate sol-gel adhesives

In the field of optical bonding, traditional optical adhesives typically contain toxic organic polymers and solvents, limiting their widespread application. Hence, there is an urgent need for environmentally friendly aqueous adhesives. While there have been efforts to develop green adhesives, few report their performances with optical substrates. The unique requirements for optical adhesives include: optically transparent; the refractive index match of the substrate; minimal stress during and after curing. This presents a challenge to ensure compatibility between green and optical. This study introduces a novel sol-gel adhesive derived from an Al 2 O 3 –P 2 O 5 -lactate system. This adhesive is biocompatible, uses water as the sole solvent. It exhibits exceptional optical clarity, with more than 99 % transparency in the visible light and near-infrared regions. Furthermore, it provides reliable bonding strength , particularly when bonded with sapphire crystal substrates, reaching a maximum of 5.96 MPa. Interfacial chemistry analysis reveals the critical role of acidic components (such as phosphoric and lactate groups) in adhesives accounted for the bonding mechanism. By increasing the molar ratio of phosphoric acid , the bonding strength of adhesive was significantly enhanced across various substrates. This sol-gel adhesive not only adheres to green chemistry principles but also shows promising potential for optical bonding applications.