Quantitative characterization of flocculation in UV-curable liquid inks by viscosity-temperature measurements

The ongoing need for UV-curable liquid inks with increased color strength has driven ink manufacturers into a never-ending pursuit for highly pigmented ink systems. One of the major difficulties quickly encountered is the rapid drop in flow of ink as pigment load increases, generating numerous problems at all stages of ink lifetime, from manufacturing to final printing. Therefore, a need exists for a reliable method capable of quantitatively characterize flow (or lack thereof) in liquid inks. Study of viscosity-temperature curve provides an accurate way of measuring the lack of flow in this type of systems. Viscosity-temperature behavior of highly pigmented inks can be divided in two regions: At the lower temperatures, ink behaves typically like polymer melt. The Litt model, developed for the latter systems is applied and a universal relationship between the factors A and TC of the model is found. At higher temperatures, viscosity substantially deviates from the model. Actual increase of viscosity with temperature is commonly observed in this region. A critical temperature (CT) can be defined, above which dramatic change in ink flow properties is observed. The term "puffy" is often used to describe the aspect of such dispersion. The created structure can be destroyed by moderate shearing at temperature below CT, fully restoring the original ink properties. These results are interpreted through pigment flocculation, leading to the creation of a physical network of associated pigments. Likewise physical gels, flocculated inks can be characterized by their lack of free flow. Examples of the use of the defined Critical Flocculation Temperature to identify manufacturing and printing problems are presented.