Comparison of 3D Structural Characteristics of High and Low Resolution X-Ray Microtomographic Images of Paper and Board

It is desirable to know the three-dimensional structure of paper to understand how it influences paper characteristics, like mechanical, optical and transport properties. X-ray micro-(computed) tomography (X-μCT) and image analysis allow visualisation and characterisation of the 3D structure of porous materials such as paper and board. To better understand the structure property relationship in paper and board it is essential to identify the necessary optimal resolution for visualization and characterization. This study attempted to identify the optimal resolution through a comparison between 3D images obtained by monochromatic synchrotron radiation X-μCT in phase contrast mode (resolution ≈ 1 μm) and polychromatic radiation X-μCT in absorption mode (res. ≈ 5 μm). It was found that both resolutions have the ability to show the expected trends when comparing different paper samples. The low resolution technique shows fewer details resulting in lower specific surface area, larger pore channels, characterized as hydraulic radii, and lower tortuosities, where differences between samples and principal directions are more difficult to detect. The low resolution images are also noisier. The disadvantages of the high resolution images are high cost and limited availability of hard x-ray beam time as well as the small size of the sample volumes imaged. The results show that the low resolution images can be used for comparative studies, whereas the high resolution images may be better suited for fundamental research on the paper structure and its influence on paper properties, as one gets more accurate physical measurements.