Reactive-Diluent Selection in Photocurable Formulations

A reactive diluent is not an inert viscosity reducer. It changes the liquid formulation, participates in cure and becomes part of the resulting network or residual-species population. Selection must therefore connect processing viscosity, compatibility, photoreactivity, achievable conversion, shrinkage, network state, volatility and hazard classification. Optimising any one of these dimensions in isolation can displace risk into another: a low-viscosity monomer can increase volatility or alter shrinkage; higher functionality can accelerate gelation yet restrict late conversion; a flexible spacer can reduce shrinkage in one formulation while reducing modulus; and lower nominal hazard cannot be inferred from a structural label alone. This review presents a bounded selection architecture for photocurable formulations. It defines the material identity and specimen state needed for comparison, separates screening properties from formulation-level evidence, and treats viscosity, conversion, shrinkage and cured performance as coupled responses rather than intrinsic rankings. Direct studies show that reactive-diluent structure can change volatility, dilution efficiency, cure response and thermomechanical properties, but their numerical results remain system specific. The central recommendation is a staged comparison in a controlled base formulation, followed by verification under the intended process, post-cure and service state. Selection is a multi-criteria qualification decision, not a search for a universally best diluent.