Volume change and viscoelastic properties of UV-curable adhesives for precise positioning during curing process and their formulation

Optical components are often adjusted to maintain their alignment along the optical axis. They are fixed at their optimal positions particularly using ultraviolet (UV) ray-curable adhesives. To achieve a micron-level positional accuracy, it is necessary to understand adhesive shrinkage while curing and the resulting deformation of the adhesive joint. However, it is challenging to perform a quantitative evaluation of the cure shrinkage and mechanical properties of adhesives given the process complexity and strong dependence on curing conditions. Therefore, this study clarified the effects of various curing conditions on the curing extent of an adhesive under UV curing for positioning and heat curing as a post-curing process. The cure shrinkage, coefficient of thermal expansion, and viscoelastic properties of the adhesive were also measured. Each material property could be defined as a function of the curing extent to predict the changes in the properties under different curing conditions. The adhesive curing process was divided into three stages: adhesive gelation under UV irradiation, fast reaction stage under heating, and slow reaction stage under heating. The adhesive state changed from viscous to viscoelastic to elastic as it cured, and this process proceeded in the aforementioned three stages. Three different formulae were fitted corresponding to each stage, with good agreement with the experimental measurements.