Surface energy considerations for offset printing of coated paper and paperboard

Offset printing of coated paper involves the complex interactions of ink with a surface that is charac-terized by three major properties: roughness, porosity, and related pore network structure and surface chemistry (related to surface free energy [SFE]). The effects of porosity and roughness are relatively well understood and are documented in the literature, whereas the influence of surface chemistry is much less studied and therefore the focus of this paper. The key results shown include:i) Coating porosity has a negligible effect on SFE determination by contact angle using two fluids.ii) The chemistry of the latex polymer in the coating formulation dominates the influence on SFE compared to pigment, with any surface energy differences present in the pigment being almost completely masked by latex.iii) Wetting agent and corona treatment can impact water absorption rate and surface spreading of water, resulting in small differences in printability. Increasing the concentration of the surfactant on a coated surface indi-cates switching orientation of the surfactant molecules, giving a "step wise" printing result.When looking to improve offset printability by selection of different pigments, the variation in SFE is less important than variation in either surface roughness or porosity.Application: This paper studies the impact of sheet surface energy on offset printing and attempts to rank its importance against surface porosity and topography. Although the surface energy was shown to be less crucial than porosity for impacting offset printing, it still has the potential to be important for other aspects of converting, such as glueing, and a discussion on how to measure and control surface energy is useful.