In-Can Stability and Premature Polymerisation in Radiation-Curable Formulations: Inhibitor Mechanisms, Residual Acidity, Thiol Chemistry, Pigment/Filler Effects, Metallic-Ink Instability, Packaging Redox and Thermal Transport Risk
Segurola, Juan
2026-08-27 · Report · Version 1.9
Premature in-can polymerisation is a multivariable failure in which a radical-curable liquid formulation loses its induction period during manufacture, filling, storage or transport. The decisive event is not simply the presence of a polymerisable acrylate, methacrylate, thiol or photoinitiator, but a rate imbalance: radical generation becomes equal to or greater than the combined rates of radical scavenging and termination. Hydroperoxides generated during prior oxidation can remain latent until activated by heat, light, redox-active metals, reducing agents or an unsuitable inhibitor environment. Residual acrylic acid or methacrylic acid can increase corrosion and alter inhibitor or amine speciation, whereas residual 2-hydroxyethyl acrylate (HEA) and 2-hydroxyethyl methacrylate (HEMA) primarily increase the pool of reactive unsaturation and must not be treated as equivalent to acid number. Thiol-containing resins add distinct routes involving thiolate formation, thiyl radicals, disulfides, radical thiol-ene propagation and base-catalysed thiol- Michael addition. Pigments, dyes, graphene, graphite and metallic or mineral fillers can either suppress or promote instability through grade-specific combinations of inhibitor adsorption, surface redox chemistry, extractable metals, residual acids, water, dispersants and photochemical activity. UV metallic inks and coatings are a particularly demanding case: high- area aluminium, copper or copper-alloy flakes, passivation defects, adsorptive loss of soluble inhibitor, photoinitiator/co-initiator compatibility, hydroperoxide burden, heat and shear can converge to shorten induction time. Packaging and thermal history are therefore part of the formulation: qualified HDPE, PP or other thermoplastic containers, or metal containers with continuous internal can coatings and sealed/overcoated welds and seams, reduce direct resin-metal contact. This review compares MEHQ, BHT, aluminium N-nitrosophenylhydroxylamine-type salts and generic copper dithiocarbamates, with explicit separation of established mechanisms from adjacent evidence and proposed pathways. It concludes with a source-attribution test matrix covering clear resins, thiol systems, pigmented formulations, carbon-filled materials, packaging contacts and summer or maritime transport profiles.