Comparison of NMR Cryoporometry, Mercury Intrusion Porosimetry, and DSC Thermoporosimetry in Characterizing Pore Size Distributions of Compressed Finely Ground Calcium Carbonate Structures Patrick A. C. Gane, † Cathy J. Ridgway,* ,† Esa Lehtinen, ‡ Rustem Valiullin, § Istvan Furo ´ , § Joachim Schoelkopf, † Hannu Paulapuro, ‡ and John Daicic | Omya Development AG, CH 4665 Oftringen, Switzerland, Helsinki University of Technology, Box 6300, FI-02015 Hut, Finland, KTH Royal Institute of Technology, Teknikringen 30/36, SE-10044 Stockholm, Sweden, and YKI Institute for Surface Chemistry, Box 5607, SE-11486 Stockholm, Sweden This work investigates for the first time how mercury intrusion porosimetry (MIP), NMR-based cryoporometry, and DSC-based thermoporosimetry compare in revealing the porous character- istics of ground calcium carbonate structures compacted over a range of pressures. The comparison is made using the same source samples throughout. MIP, a much-used method in the characterization of porous structures, has the drawback that the high pressure needed to intrude the mercury may either distort the skeletal porous structure of the sample, especially when compressible materials such as cellulose or binders/latex are present, or lead to a reduction in the measured number of large pores due to the shielding by smaller pores. These effects have previously been addressed using bulk modulus corrections and by modeling the structure permeability to account for the potential shielding. Cryoporometry gives detailed information about the pore size distribution of an imbibition saturated structure. Thermoporosimetry is a relatively new candidate in this field, and it yields both pore size distribution and pore volume. Currently it is somewhat limited in the pore size range detectable, but it is relevant to pigmented coatings. Its potential is identified for capturing the pores involved in the progress of imbibition before saturation is reached. Introduction The topic of porous media characterization covers a vast area of research activity. Important porous media parameters include surface free energy and geometry/ morphology of the interconnecting pore network. The structural properties that characterize the behavior of a porous substrate, with respect to liquid absorption and permeability, can be broken down using a pore and connecting throat concept primarily into the following: porosity, pore size and size distribution, throat size distribution, connectivity, tortuosity, permeability, and pore/throat shape (including aspect ratio). While porosity is directly measurable by an adequate porosimetry method, and permeability can be experi- mentally quantifiable, pore and throat size distribu- tions, connectivity, and, to a certain extent, tortuosity need a more sophisticated description. In particular, to describe throat size distribution, connectivity, and tor- tuosity requires to date adoption of a simulator program, which constructs a model of discrete pores and throats providing a given connectivity derived from porosimetric data. 1-4 Furthermore, the geometry of the pores and throats is best visualized by using an adequate method of microscopy, among which electron microscopy is widely used, which does go some way toward determin- ing connectivity and tortuosity, at least semiquantita- tively. Quantifying connectivity and pore arrangements in space are important issues that still need evaluation when a pore network simulator is involved, as, ideally, it is necessary to reduce the number of ill-defined fitting parameters. Baldwin et al., 5 using a magnetic resonance imaging technique (MRI) and image analysis, charac- terized binary pictures in terms of pore morphology and fractal dimensions. Due to the limited resolution of MRI, this analysis is confined to >10 μm pores. Direct experimental three-dimensional (3D) digitized imaging is in its infancy. For example, microtomography has, as yet, limited resolution (>∼8 μm), 6 just outside the region of micrometers and below the resolution needed for paper coating structures. This, however, is a highly interesting new method, which would allow for an excellent 3D visualization of porous structures and holds much potential for the future. This paper focuses on methods especially relevant to paper and paper coating structures in particular, to evaluate the pore size distribution. The first method we apply is probably the most accepted means to investi- gate a porous structure and adopts intrusion/extrusion porosimetry, where the use of mercury as the nonwet- ting test liquid is the most widespread technique. 7-10 Mercury is an ideal liquid, because it shows a very high contact angle to most solid substrates and, therefore, the Laplace equation can be employed to relate an applied pressure with a relevant pore diameter being intruded by the nonwetting liquid mercury. The main drawbacks of the method are the effects induced by the high pressure applied and the shielding of large pores * To whom correspondence should be addressed. E-mail: cathy.ridgway@omya.com. † Omya Development AG. ‡ Helsinki University of Technology. § KTH Royal Institute of Technology. | YKI Institute for Surface Chemistry. 7920 Ind. Eng. Chem. Res. 2004, 43, 7920-7927 10.1021/ie049448p CCC: $27.50 © 2004 American Chemical Society Published on Web 10/28/2004