UV induced microcellular foaming—A new approach towards the production of 3D structures in offset printing techniques

The generation of microcellular foams via a photochemical initiation mechanism is a new approach aimed at the one step production of three dimensional structures in offset printing techniques. The photochemical foaming involves the excitation of a selected photoacid generator with ultra-violet (UV) light to release Brønsted acids. Carbon dioxide, formed upon the reaction of the Brønsted acid with calcium carbonate particles, is used as blowing agent. In order to achieve efficient proton formation and consequently generate a sufficient amount of blowing agent, photosensitizers are employed. Long wavelength absorbing anthracene derivates are added to capture a higher fraction of the light source, a conventional mercury arc lamp, to sensitize the photolysis of the photoacid generator. The microcellular foaming was performed with a commercially available UV curable offset ink formulation containing triacrylate oligomers. To ensure that the gas bubbles are trapped in the cured resin a balance has to be found between the curing speed of the ink and the foaming speed. In the present study crucial process parameters including photoacid generator level, choice of photosensitizer, light intensity and concentration of calcium carbonate particles are evaluated and their influence on the foam properties are discussed. Both the cell morphology and the expansion of the film thickness are characterized with optical microscopy and mechanical methods. The results clearly show that the UV assisted foaming leads to the formation of dense and uniform microcells by applying optimized process parameters. Moreover, the relative thickness of the ink layer can be raised up to 90% which makes the one-step production of three dimensional structures (e.g. reliefs) at short reaction times and ambient temperatures feasible.