Rheological Properties of UV-curable Flexographic Ink (II) Influence of Interfacial Interactions in Relation to Rheological Variables of Ink and its Printability

Abstract

The rheological properties of UV flexographic (flexo) inks were investigated. UV flexo suspensions of carbon black in liquid medium containing various binding materials, such as acylate pre-polymers, di-/multi-functional monomers, and diluents, were used as sample inks. The inks were characterized on a rheometer in terms of steady and dynamic behaviors. The rheological properties of polymer-pigment systems are a function of some interfacial parameters that characterize the degree of "affinity" of pigment surfaces to the polymer medium. The addition of pigments into liquid medium introduces polymer-pigment junctions besides the polymer-polymer junctions due to entanglement. These newly created junctions arise from the sorption of pigments to the polymer chains. In this study, we found that the frequency dependence of storage modulus G' at a certain strain amplitude is proportional to the number of acryloyl groups in the multi-functional monomer (TMPTA<PETTA<<DPHA). The chemical difference due to chain length (TMPTA<GPTA) is another factor affecting the rheological properties of UV flexo ink. In addition, we surmise that a transient network structure is formed by mechanical interactions that are induced by the addition of high concentrations of small solid particles such as carbon black to a polymer or a multi-phase solution. In such a system, the particle chains form the network structure, although the formation of polymer bridges by adsorbing to adjacent particles is another possibility.