PAPER AND INK INTERACTIONS Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 274 Prediction of surface structure of coated paper and of ink setting time by infrared spectroscopy Juraj Gigac, Miroslava Kasajová, Marta Maholányiová, Monika Stankovská and Michal Letko KEYWORDS: Coated papers, FTIR, Image analysis, SEM, Ink setting rate, Surface structure SUMMARY: Pigments and latex have an essential influence on coated surface structure and on ink setting. The combination of attenuated total reflection infrared spectroscopy (ATR-FTIR) and photoclinometry based on image analysis, measured at 10° illumination of paper surface in defined directions (MD and CD), allows to predict surface pores area and latex in coating as well as ink setting rate. Information about surface pores area and latex was provided by SEM surface analysis. Ink setting rate was evaluated by the set-off method. Dynamics of liquid penetration was studied by ultrasound method. Infrared spectroscopy and X-ray microanalysis using SEM/EDS method were used to determine kaolin, calcium carbonate and latex contents in coating. ADDRESSES OF THE AUTHORS: Juraj Gigac (gigac@vupc.sk), Pulp and Paper Research Institute, Lamačská cesta 3, 85104, Bratislava, Slovakia Corresponding author: Juraj Gigac Structural differences play an important role in interaction of paper with liquids in the course of production and manufacture processes as well as during utilization. Microtopography, porosity and surface energy are the most discussed properties due to their influence on wetting and penetration dynamics of liquids occurring at surface sizing, coating and printing processes. Structural differences of a solid surface affect also functional properties such as friction coefficient and permeability of fluids (liquids and gases) which are important at manipulation, storage and protection of packaged goods. Non-destructive methods are increasingly used as they allow obtaining information in real time and help to control a number of paper properties (basis weight, humidity, filler content, whiteness, coloring, formation, pinholes and impurities) during manufacture. Development of new contactless methods for paper testing is interesting as these methods are useable for on-line control of product quality. Infrared spectroscopy in near (NIR) and middle (MIR) region appropriately complements information about raw materials and technology used for the production of an unknown type of paper. Infrared spectroscopy in near region (NIR) enables us to obtain information suitable for off-line and on-line control of strength properties of paper (Behnsen 2008; Gigac, Fišerová 2011). The technology of coating weight measurement IQMultiCoat from Metso Automation (Williamson 2006) was applied five years ago, in M- real´s Tako mill (Finland), using a combination of NIR sensor, to identify kaolin as well as cellulose, moisture and latex binders, and MIR sensor to determine the amount of calcium carbonate pigment. Photoclinometry is a promising method for on-line evaluation of paper properties in visible range of electromagnetic radiation (Dahl et al. 2006; Gigac et al. 2012). This method is used in astronomy to measure surface topography of cosmic bodies (Muinonen et al. 1989; Korokhin et al. 2010). One of the first applications in paper industry was the measurement of wet paper web on paper machine wire at optimization of paper formation by changing the adjustment of activity of dewatering elements (Kiviranta, Paulapuro 1992). Ink setting problems may arise in multicolor offset printing of coated papers. Low setting rate increases ink imprint on reverse side of next paper sheet. On the other hand, high setting rate causes low print gloss. Evaluation of coating layer structure (roughness, size of pores and porosity) on ink setting (Xiang et al. 2004) and print gloss (Jeon, Bousfield 2004) showed that pore structure of coating layer is the principal control factor of surface appearance, optical properties and printing characte- ristics. The greatest influence on coating porosity is by the applied pigment particles (type, form and particle size distribution). Moreover, the interaction of ink with coated paper is significantly affected by calendering (Hiorns et al. 1998; Bluvol et al. 2003). Generally, fast ink setting can be achieved by application of fine pigments as well as by steep particle size distribution. Coarser pigment offers bigger but less pores. Finer pigment with flat particle size distribution curve provides higher density of pores and faster ink setting. Besides permeability methods of pore size distribution measurement based on measurement of pressure by extrusion of fluid from saturated porous materials, there are other microscopic techniques such as SEM electron microscopy (Chinga 2002; Chinga, Helle 2003; Ström et al. 2010) and AFM atomic force microscopy on the surface and cross-section of paper (Di Risio, Yan 2006), thermoporometric technique based on differential scanning calorimetry DSC (Lehtinen, Paulapuro 2002) and cryoporometry based on nuclear magnetic resonance NMR (Furó, Daicic 1999). The volume and type of latex binder as well as drying conditions significantly affect pore size and distribution of pore size of coating layer. Higher contents of acrylonitrile groups and higher gel contents of latex increased print gloss (Enamoe et al. 2000). Glass transition temperature Tg of latex polymer binder has great influence on paper gloss and coating porosity. The influence of styrene-butadiene (SB) latex Tg, thickener type (carboxymethyl cellulose CMC and ethyl hydroxyethyl cellulose EHEC), its molecule weight and quantity added on the formation and consolidation of coating were investigated in the study of model coating colors using AFM technique (Kugge 2004). Latex with higher Tg migrates much more easily to the top coating layer. The result is more complete filling of pores and higher gloss. Increased addition of CMC retards binder migration and higher molecule weight of CMC prolongs Brought to you by | Uppsala University Library Authenticated Download Date | 3/20/19 3:11 PM