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

Technical Report · Sun Aug 29 00:00:00 EDT 2004

DOI:https://doi.org/10.2172/883703· OSTI ID:883703

Shri Ramaswamy, University of Minnesota; B.V. Ramarao, State University of New York

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. 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. A sequence of microtomographs was imaged at approximately 2 m 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 pore characteristics are found and may help partly explain the different liquid and vapor transport properties in the in-plane and transverse directions. Results with varying sheet structures compare favorably with conventional mercury intrusion porosimetry data. Interestingly, the transverse pore structure appears to be more open with larger pore size distribution compared to the in plane pore structure. This may help explain the differences in liquid and vapor transport through the in plane and transverse structures during the paper manufacturing process and during end-use application. Comparison of Z-directional structural details of hand sheet and commercially made fine paper samples show a distinct difference in pore size distribution both in the in-plane and transverse direction. Method presented here may provide a useful tool to the papermaker to truly engineer the structure of paper and board tailored to specific end-use applications. The difference in surface structure between the top and bottom sides of the porous material, i.e. "two-sidedness" due to processing and raw material characteristics may lead to differences in end-use performance. The measurements of surface structure characteristics include thickness distribution, surface volume distribution, contact fraction distribution and surface pit distribution. This complements our earlier method to analyze the bulk structure and Z-D structure of porous materials. As one would expect, the surface structure characteristics will be critically dependent on the quality and resolution of the images. This presents a useful tool to characterize and engineer the surface structure of porous materials such as paper and board tailored to specific end-use applications. This will also help troubleshoot problems related to manufacturing and end-use applications. This study attempted to identify the optimal resolution through a comparison between 3D images obtained by monochromatic synchrotron radiation X-CT in phase contrast mode (resolution 1 m) and polychromatic radiation X-CT in absorption mode (res.5 m). 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 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. In addition, pore space diffusion model has been developed to simulate simultaneous diffusion in heterogeneous porous materials such as paper containing cellulose fibers and void spaces. Stochastic dynamic approach along with random walk simulation has been implemented to model simultaneous diffusion in 3D matrix of cellulose fibers and pores. This model is suitable for simulating simultaneous diffusion in porous materials under variety of conditions including low relative humidity conditions where diffusion occurs predominantly through one medium i.e. pore space and high humidity conditions where both mediums (i.e. fiber and pore spaces ) are highly conductive. Both pore as well as effective diffusivity values for paper samples of varying structure were compared with the experimental values and are in fair agreement especially through the thickness direction of samples. In addition to this, intrinsic fiber phase diffusivity has been estimated for the first time using a combination of simulation and experimental data.

• Research Organization: Univ. of Minnesota, Minneapolis, MN (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: FC36-00ID13873
• OSTI ID: 883703
• Report Number(s): ID 13873; TRN: US0703106
• Country of Publication: United States
• Language: English

Related Subjects

43 PARTICLE ACCELERATORS
CELLULOSE
COMPUTERIZED TOMOGRAPHY
DIFFUSION
DRYING
FIBERS
HUMIDITY
HYDRAULICS
IMAGE PROCESSING
MANUFACTURING
MERCURY
PORE STRUCTURE
POROSITY
POROUS MATERIALS
PRESSING
RADIATIONS
RAW MATERIALS
RESOLUTION
SPECIFIC SURFACE AREA
SYNCHROTRON RADIATION
WATER REMOVAL
paper
board
porous materials
3D structure
X-ray tomography
image analysis
structure
characterization
porosity
surface area
transport properties
diffusivity
permeability
experimental
numerical methods
water removal