Progress in Organic Coatings 44 (2002) 307–315 The influence of substrate absorbency on coating surface chemistry Hamad Al-Turaif ∗ , Douglas W. Bousfield, Pierre LePoutre Department of Chemical Engineering, Paper Surface Science Program, University of Maine, Orono, ME 04469, USA Received 11 March 2002; accepted 26 April 2002 Abstract The composition of the top surface of a coating layer can influence its functional properties or subsequent processing steps. The effect of the substrate absorbency on the coating surface chemistry is reported. Different coating systems containing a kaolin clay pigment, fine or coarse precipitated calcium carbonates, and a common latex binder were examined. The influence of a soluble polymer added into the coating was characterized. The surface chemistry was measured with attenuated total internal reflectance (ATR) and X-ray photoelectron spectroscopy (XPS). Absorbent substrates generate bulky coatings with high voids and low gloss. Rapid dewatering by the absorbent substrate pulls the small particles, like latex binder, away from the top layers causing a low latex concentration at the surface. On non-absorbent substrates, the addition of the soluble polymer generates coating layers with higher void volume, lower gloss, and lower latex concentrations at the coating surface. However, on absorbent substrates, polymer addition causes coatings with lower void volumes and higher gloss. In this case, the rapid dewatering and mobility of particles is reduced by the polymer, which helps to retain the small particles at the surface. As a result, latex concentration at the surface increases with polymer addition on absorbent substrates. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Substrate absorbency; Soluble polymer; Paper coatings; Surface chemistry 1. Introduction In the multicolor printing of coated publication papers, particularly in offset and ink-jet printing, a problem of mot- tle, or non-uniformity in the print density, is sometimes ob- served. In offset printing, this unevenness has been shown to be related to a non-uniformity in ink setting rate, itself caused by a non-uniformity in the local absorptivity of the coating surface [1]. In ink-jet printing, the problem is related to the uneven spreading and absorption of the ink on the uneven surface [2]. Pore size and pore volume are consid- ered to be important in controlling the coating absorbency. Absorbency decreases with increasing binder content and decreasing void fraction [3]. When coatings are applied to substrates that have non-uniform absorption properties, this in turn may cause non-uniform coating layers. Therefore, it is important to understand the relationship between coating properties and substrate absorbency. The influence of sub- strate absorbency on void volume, gloss, and surface chem- istry is not clear in the literature. ∗ Corresponding author. Present address: Chemical and Materials Engi- neering Department, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia. E-mail address: halturaif@hotmail.com (H. Al-Turaif). In the paint industry, intercoat adhesion and gloss uni- formity can be affected when the substrate is porous and its porosity is uneven, leading to non-uniformity in the sur- face chemistry and absorbency of the paint. The literature is not clear on what components in the paint can compensate for non-uniform substrates or what conditions can lead to non-uniform paint layers. When a water-based coating is applied on an impervi- ous substrate, water leaves by evaporation. Unless certain surface tension driven flows form during evaporation, the coating consolidates into a uniform film whose surface is homogenous in terms of microstructure, chemical compo- sition and absorbency. If the coating or paint is applied on an absorbent substrate, the aqueous phase is removed by capillary movement through the substrate’s capillaries. Of- ten this is rapid compared to evaporation. Consolidation is thought to take place through the formation of a filter-cake. When dewatering is rapid, typically less than a second, the pigment particles may not have time to orient and pack densely. In some cases, large pores may form that allow fines and latex particles to move in the thickness direc- tion. As a result, a gradient in binder content as well as in pigment size distribution has been reported [4–6]. Other flow-induced processes have been observed, leading to seg- regation [7]. When the coat thickness varies locally, due to 0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0300-9440(02)00071-1