Liquid retention in porous coatings: effect of pore geometry and material interactions on drying efficiency

摘要

Drying of prints is a major energy-consuming step in the production of printed media, especially in heatset web offset and emerging high speed inkjet applications - the challenge being greater for oil-based ink systems. To determine the major influencing factors in respect to coating structure and chemical formulation, the dynamic drying rates for mineral oil saturated ground calcium carbonate (GCC) and modified calcium carbonate (MCC) coatings are examined as a function of temperature and for various levels of binder dosage, at first using styrene-acrylate latex with minimal oil interaction and then adopting styrene butadiene types showing diffusive absorption properties to oil. Two internal stages of drying can were identified for both structures: a rising drying rate period and a falling rate period as a function of temperature. In the rising rate period, the effect of film flow, being a major internal mass transport mechanism, was identified to be of particular relevance for the case of GCC based structures. In the falling rate period, the absorbent/absorbate interactions become more dominant in determining the liquid retention. This was observed by an extended thermogravimetry peak of a limited drying rate. In case of noninteracting latex this may have purely structural causes due to the decreased permeability, i.e. increased diffusion resistance, with increasing binder content. However, in the case of the more interacting styrene butadiene latex types, the resulting structural changes due to swelling of latex, observed in form of a higher level of oil uptake into the latex itself, affect the permeability to an extent that leads to a limited drying rate over an extended temperature range, even at low binder concentrations. Furthermore, structural changes due to disruption of the pigment packing, and the resulting increase in liquid permeability, have an effect on the liquid retention, provided the general level of the permeability poses a limiting factor as for the GCC structures.