Carl-Mikael Tåg

Influence of surface structure on wetting of coated offset papers

Abstract To obtain more knowledge of the properties affecting print quality and adhesion characteristics in the printing process, attention has been directed to the nature of surface energy. The aim was to compare different surface-energy calculation models and to investigate the influence of surface roughness on wetting of coated offset papers. The wetting process was studied by static contact angle measurements using a series of reference liquids. Topographical characterization was carried out using atomic force microscopy. Surface energy components were determined using different calculation models. The determination can be considerably simplified using a mono-monopolar model, which has been proven in previous studies. The surface energy components were derived from both apparent and topography-corrected contact angles. The surface topography had a significant effect on wetting of the samples studied.

Radial Spreading of Inks and Model Liquids on Heterogeneous Polar Surfaces

Novel models are developed and previously published simple models characterizing the spreading of probe liquids and inks on solid surfaces are evaluated with water and ethylene glycol. The aim is to determine the most appropriate models which would be suitable to describe liquid and ink spreading on both chemically and structurally heterogeneous paper surfaces. The surface energy and its components (Lifshitz–van der Waals, acid and base) for the paper samples have previously been characterized at semi-equilibrium conditions. The equilibrium work of adhesion and work of spreading is found to be linearly related to the maximum print tack force of model inks. The dynamic spreading are found to be determined by coupled spreading mechanisms, being related both to the surface structure and surface chemistry.

Dynamic Ink Oil Absorption in a Pigmented Porous Coating Medium Studied by Near-Infrared Diffuse Reflectance Spectroscopy

The rate of absorption (both long and short timescale) of typical heatset offset printing ink oils, namely mineral and linseed oil, has been studied on model ground calcium carbonate coating pigment tablets containing various amounts of either styrene–acrylic or styrene–butadiene binder. The pore structure characteristics of the tablets were determined using mercury intrusion porosimetry. The movement of the oils both on the surface of and within the porous structure of the pigment/binder tablets was studied under the influence of pressure-less capillary flow with subsequent diffusion through the connected void volume of the tablet. The wetting was analyzed by near-infrared (NIR) diffuse reflectance spectroscopy both as a single probe measurement and by hyperspectral imaging. The results showed that the rate of oil filling the structure was strongly dependent on the binder amount in the structure as well as the binder chemistry (oil- or non-absorbing binder), which supports previous findings. The liquid properties, and especially the viscosity of the liquid (oil), influenced the absorption rate. The gradients in absorbance indicated the presence of latex blocking access to some pores and reducing connectivity.

Diffractive optical element-based glossmeter for the on-line measurement of normal reflectance on a printed porous coated paper

Gloss of a product, such as print gloss, is mainly inspected with conventional white light glossmeters both at laboratory or production facilities. However, problems occur in conventional gloss measurement when the inspected surface is vertically moved in the plane of incidence and reflection or when the measurement area is small or curved. For a partial solution to these problems, we have previously introduced diffractive optical element-based glossmeters (DOGs) for the gloss inspection in laboratories and off-line use. We present a new construction of DOG, termed μDOG 1D, for the one-dimensional on-line print gloss measurement, in the form of the reflectance determination normal to the surface. The function of the glossmeter is demonstrated by laboratory tests and on-line measurements at a heat-set web offset printing machine. It is shown that gloss (i.e., normal reflectance) and minute gloss variation of papers and prints can be measured at the printing line using the glossmeter. This glossmeter is expected to be useful in real-time monitoring of the gloss and surface-specific absorption not only in the printing industry but also in inspection of products in other industrial sectors, such as metal finishing, laminating, paper, and construction materials manufacturing.

Dynamic Spreading of Polar Liquids on Offset Papers

The aim of this study was to investigate the spreading of sessile drops of polar probe liquids (water and ethylene glycol) on pigment coated offset papers. Furthermore, existing theoretical models for spreading were applied to evaluate the experimental results. The hydrodynamic model gave a better fit to the results at lower spreading rates, while the molecular-kinetic theory gave a good fit over a larger velocity range. Factors introduced to correct for the exponential dependency on time of the drop base radius and its contact angle can be interpreted as coupled processes. Differences in the spreading between the papers were found to correlate with the acid and base components of the surface energy, rather than with differences in surface roughness.

Acid–base properties of polymer-coated paper

The wetting behavior of a series of polymer-coated papers has been studied. Different ways of determining the acid–base properties of the polymers are presented. The well-known van Oss–Chaudhury–Good (vOCG) bi–bi polar model is compared with more simplified mono–bi polar and mono–mono polar models. The effect of surface roughness on the wetting was also studied with atomic force microscopy. The overall wetting of each probe liquid was evaluated by calculating the work of adhesion to the polymer surfaces. It is shown that ethylene glycol and water may be considered as mono polar liquids, which simplifies the original vOCG-model. It is also shown that in most cases the surface energy values are in the same range when using both the complex bi–bi polar approach and the simpler mono–mono polar approach. The different polymers used are found to be of a predominating basic character.

Impact of fountain solution addition level on tack and optical properties of the print of a mechanically distributed water-in-oil thin film emulsion

The impact of addition and evaporation of an alcohol/surfactant stabilised fountain solution and offset ink thin film emulsion has been studied using a printing ink application device. The ink rheology testing technique (TackOscope), incorporating the possibility to apply the aqueous liquid (print fountain solution) to mix into an oil-based ink to create an emulsion in a twin roll nip, was used firstly in its standard form to provide information relating to the tackiness property during emulsification. The ink used showed a continuous tack increase over time, superposed on this trend, and addition of intermediate amounts of fountain solution were shown to decrease the tack of the emulsion monotonically. After evaporation, the tack finally returns back to its expected undisturbed level, following a fountain solution-free ink tack development, the result being a sigmoidal evolution of tack recovery to this point. Secondly, by introducing a defined non-absorbent substrate in a single pass between the rollers, transfer of the emulsion could be achieved. Expressing the behaviour as a function of the emulsion component to ink ratio, it was, therefore, possible to study the influence on the optical properties of the final thin layer in the form of a print. ADDRESSES OF AUTHORS: Carl-Mikael Tåg (carlmikael.tag@storaenso.com), Stora Enso Support Centre Mönchengladbach, DE-41600, Mönchengladbach, Germany; Pertti Silfsten, Kai-Erik Peiponen (pertti.silfsten@uef.fi, kai.peiponen@uef.fi), Institute of Photonics, University of Eastern Finland, Joensuu Campus, P.O. Box 111, FI-80101 Joensuu, Finland; Patrick A.C. Gane (patrick.gane@omya.com), School of Chemical Technology, Bioproducts and Biosystems, Aalto University, FI-00076 Aalto, Helsinki, Finland; Cathy J. Ridgway (cathy.ridgway@omya.com) Omya International AG, CH-4665 Oftringen, Switzerland Corresponding author: Carl-Mikael Tåg