Evaluation of the changes in paper structure by the laboratory wet pressing conditions Yong Joo Sung * , D. Steven Keller 1 ESPRI, Department of Paper Science and Engineering, State University of New York, Syracuse, NY, USA Received 19 July 2007; accepted 9 January 2008 Abstract The effects of the laboratory wet pressing conditions on the structural parameters of paper were investigated systematically. Especially, the local variation of apparent density was evaluated by applying the twin laser profilometer (TLP) and the storage phosphor b-radiographic imaging system. The results showed that the densification by wet pressing was distributed across the entire sample area. The relationship between the wet pressing densification and the dryness (water removal by the wet pressing) was found to be different depending on the felt fabric, which indicated the higher dryness without increased densification could be possible by choosing proper fabric condition. The effects of the fabric type on the surface structure were also investigated. # 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Keywords: Wet pressing; Densification; Local structure; Dryness; Fabric; Surface structure 1. Introduction Even though the distribution of fiber that forms the basic structure of paper occurs in the forming process, the post- forming processes, including wet pressing, drying and calendering, also affect the paper structure. The densification by wet pressing can especially have major effects on paper structure. For example, the supple wet web subjected to the wet pressing process is likely to be deformed and compressed between pressing fabrics or the pressing roll. It is, therefore, an evidence that each post-forming process such as the wet pressing should be expected to produce totally different paper structures that may further be affected by the operating conditions. Although the effects of wet pressing on the paper structure appear straightforward [1–3], the actual changes in the relationship between grammage and thickness especially, in scale of 1 mm or less have had little attention in the literature. Furthermore, most of researches have been focused on the one or two structural properties, for example, local mass variation, formation, rather than overall consideration of the inhomoge- nity, especially thickness and apparent density, because of the difficulties in the measurement of local thickness. In this study, the systematic approach for evaluating the changes in paper structures by the laboratory wet pressing was presented. Two different commercial felt fabrics were used for this study. The changes in paper structure including the surface structure depending on the pressure of wet pressing were investigated. 2. Experimental 2.1. Sample preparation The handsheet samples of 100 g/m 2 were formed and then couched according to TAPPI standard method T205 sp-02. The standard research pulps available from the National Institute of Standards & Technology (NIST) were used for this work [4]. The softwood pulp has a nominal coarseness of 17.6 mg/100 m and a mean fiber length (length weighted) of 2.85 mm. The hardwood pulp has a nominal coarseness of 9.5 mg/100 m and a mean fiber length of 0.65 mm. Both were beaten using a laboratory valley beater according to TAPPI standard method T200 sp-01. The final Canadian Standard Freeness (CSF) of each pulp was 521 ml for the hardwood and 634 ml for the www.elsevier.com/locate/jiec Available online at www.sciencedirect.com Journal of Industrial and Engineering Chemistry 14 (2008) 328–332 * Corresponding author. Current address: K&G Corporation, 302 Shinseong- Dong, Yuseong-Gu, Deajeon 305-805, Republic of Korea. E-mail address: yosung17@gmail.com (Y.J. Sung). 1 Current address: Department of Paper and Chemical Engineering, Egr. Building, Room 64G, Miami University, Oxford, OH 45056, United States. 1226-086X/$ – see front matter # 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.01.005