3-D Characterization of the Structure of Paper and Paperboard and Their Application to Optimize Drying and Water Removal Processes and End-Use Applications

3 Introduction and Background 5 Methods 6 Results and Discussion 10 Tortuosity 18 Pore-Bond Network Represenations 22 Comparison between Commercial and Laboratory Sheets 28 Permeability 31 Effect of papermaking processes on 3D structure 33 Comparison of high and low resolution tomography 37 Surface Structure Characterization 42 Method to predict transport properties using actual 3D structures 49 First Passage Time (FPT) Method 54 Description of Simulation Procedure 55 Transport Properties-Results and Discussion 59 Pore Diffusivity – Summary 68 Simultaneous Diffusion in Paper 69 Introduction and Background 69 Method 71 Results and Discussion 76 Conclusions and Path forward 94 References 98 2 Project Objective: Develop an accurate, direct, non-invasive method to visualize the complex pore structure of paper and board. Characterize the three dimensional bulk structure of paper and paper board using sophisticated image analysis techniques. Develop relationships between pore structure characteristics and transport properties namely, water vapor diffusivity and liquid permeability. ABSTRACT The three dimensional structure of paper materials plays a critical role in the paper manufacturing process especially via its impact on the transport properties for fluids. Dewatering of the wet web, pressing and drying will benefit from knowledge of the relationships between the web structure and its transport coefficients. Among transport, moisture diffusion in paper is central to the understand ing and optimal design of paper products for their performance in different environmental conditions. Our recent research of moisture sorption in paper has indicated that diffusion of water vapor through the pore space is an important mechanism for transport [Ramaswamy, 2001, Huang, 2002]. The effect of the three dimensional structure of the paper sheet on the diffusion of moisture is significant. The structure of the pore space within a paper sheet is imaged in serial sections using x-ray micro computed tomography. The three dimensional structure is reconstructed from these sections using digital image processing techniques. The structure is then analyzed by measuring traditional descriptors for the pore space such as specific surface area and porosity. In addition, morphometric and quantitative stereological techniques are used to characterize the structure. Techniques of mathematical morphology [Serra, 1986] used include erosion, dilation, closing, opening and binarization with subsequent skeletonization. 3 A sequence of microtomographs was imaged at approximately 2 mm intervals and the three-dimensional pore-fiber structure was reconstructed. The pore size distributions for both in-plane as well as transverse pores were measured. Significant differences in the in-plane (XY) and the transverse directions in …