Controlled Undercure in Radiation-Curable Pressure-Sensitive Adhesives: The Conversion Window, Residual Unsaturation as a Specification, and Migration from a Permanently Soft Layer

A pressure-sensitive adhesive is not qualified by driving its photocure as far as possible. It must remain sufficiently compliant and dissipative to wet a counter-surface and develop tack while retaining enough cohesive network strength to resist shear, edge lifting, transfer and internal failure. In a radiation-curable PSA, cure therefore creates a bounded material-state problem rather than a simple maximisation problem. The useful state depends on formulation, coating weight, irradiation history, network formation and the measurement used to describe that state. Direct UV-PSA studies show that tack, peel adhesion and cohesive/shear performance can move in different directions as crosslinking advances. Chemical conversion, gel fraction, residual carbon-carbon double bonds and residual monomer are likewise non-equivalent measurements. ER-190 defines the evidence required to specify and requalify a deliberately bounded undercured PSA state without turning one published dose, conversion value, residual-unsaturation level or residual-monomer concentration into a universal target.