ORIGINAL PAPER Application of electrochemical impedance spectroscopy in characterization of structural changes of printing plates Sanja Mahovic Poljacek & Dubravko Risovic & Tomislav Cigula & Miroslav Gojo Received: 30 March 2011 /Revised: 20 June 2011 /Accepted: 26 June 2011 # Springer-Verlag 2011 Abstract The porous structure of the aluminium oxide surface of lithographic printing plate (PP) has a most significant influence on the quality of the imprints. This study presents the results of application of electrochemical imped- ance spectroscopy (EIS) in characterization of PPs' porous structures and their changes during chemical processing. Two common PP types—thermal and conventional—were investigated. The influence of the processing solution’ s working age on topographical changes of PP surface and associated change in the impedance spectra are investigated and discussed. The equivalent electrical circuit models reproducing the observed EIS spectra are proposed. Based on these models two mechanisms of surface’ s topography changes responsible for degradation of PP performance due to the processing are identified and discussed. Keywords Printing plate . Aluminium oxide . Anodic film . Pore sealing . Porous structure . EIS Introduction Aluminium surface suitable for use as a lithographic printing plate (PP) consists of two different areas: ink- receptive image areas, which carry a photosensitive coating, and fountain solution-retaining non-image areas (aluminium oxide film). In order to improve the fountain solution adhesion on the aluminium oxide film and to enhance the adhesion of the photosensitive coating during the printing process [1, 2], the aluminium foil needs to be roughened by electrochemical processes of graining and anodic oxidation [3–6]. These processes ensure the formation of specific anodic layer on the aluminium foil which consists of a thin nonporous compact oxide layer (barrier layer) and an outer extremely porous oxide layer. Size and quality of the grained microstructure, i.e., of the porous layer will influence the printing performance and durability of the PPs [7, 8]. Consequently, the porous structure of the lithographic PP has a most significant influence on the quality of the imprints because the adsorption of the fountain solution on the nonprinting elements prevents the adsorption of printing ink on those areas during reproduction process. The photoactive coating, after exposure in the PP making process, becomes soluble in the alkaline solution. Alkaline solution (pH ≈ 13) mainly based on sodium or potassium hydroxides is used to remove this coating from nonprinting parts of the PPs, supposedly without signifi- cantly affecting the structure of the aluminium oxide layer. Nevertheless, the amorphous structure of the aluminium oxide can be affected by such chemical processing [9], inducing changes that impair the quality of the PP. The extent of these undesirable effects presumably also depends on the working age of the processing solution. The immersion of the aluminium oxide porous layer into the alkaline solution as required by processing technology could change the topography of the porous layer and consequently result in degradation of the PP performance. This kind of structural changes influencing the functional properties of the PPs could occur as a consequence of (pore) sealing process [9]. Hence, as the adsorption, i.e., the technological performance of a PP, depends on the porosity of aluminium oxide structure, characterization of this S. Mahovic Poljacek (*) : T. Cigula : M. Gojo Faculty of Graphic Arts, University of Zagreb, Getaldiceva 2, HR-10000 Zagreb, Croatia e-mail: smahovic@grf.hr D. Risovic Molecular Physics Laboratory, Ruder Boskovic Institute, POB 180, HR-10002 Zagreb, Croatia J Solid State Electrochem DOI 10.1007/s10008-011-1490-9