These researchers attempted to visualize and characterize the three-dimensional bulk structure of paper and board using non-intrusive techniques. Previously, they used laser scanning confocal microscopy (LSCM). This technique, although it is capable of nonintrusively visualizing paper samples, is limited by the depth or thickness that can be viewed. Further work using oil immersion lenses and LSCM showed that the quality of the images and interference from non-focal planes could be minimized. However, the researchers were still limited to a thickness of approximately 50 µm.

Explorations were performed with the use of x-ray micro-computed tomography (x-ray CT) to visualize the structure of porous materials. This technique is far superior to other techniques, and the researchers were able to view the entire three-dimensional structure of thick (-300 µm) samples. These three-dimensional images were binarized (black and white) using methodologies developed by the group. This binarized image was then analyzed for pore structure characterization using supercomputing resources. Structural parameters of interest include pore size distribution, average pore diameter, porosity distribution and average porosity, tortuosity, available transfer surface area, and fiber-fiber bonded area. Also, numerical deconvolution of the three-dimensional images to sharpen the image, as well as to obtain better characterizations of the structure, will also be conducted using Supercomputing Institute resources.

Initial estimates of the structure characteristics indicate an immense potential for using this approach. Recent work using x-ray microtomography on paper samples of varying structure indicate that the technique is capable of visualizing the structure. The team’s methods to characterize the internal pore structure revealed interesting information on the anisotropic nature of the internal structure. For example, measurements of tortuosity in the inplane and transverse direction are very different with the transverse direction being more tortuous. This, along with other structural measurements, can explain the differences in transport properties in the in-plane and transverse directions. Efforts are underway in using higher resolution x-ray technique to get the finer details of the structure.