structural analysis by x-ray microtomography of a strained nonwoven papermaker felt
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2002 72: 480Textile Research JournalX. Thibault and J.-F. Bloch
Structural Analysis by X-Ray Microtomography of a Strained Nonwoven Papermaker Felt
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Manuscript received August 6, 2001; accepted November 16. 2001. ,
Structural Analysis by X-Ray Microtomography of a StrainedNonwoven Papermaker Felt
X. THIBAULT AND J.-F. BLOCH
Paper Physics Department, Ecole Franaise de Papeterie et des Industries Graphiques,38402 Saint-Martin dHres, France
Textiles are used in many industrial applications, especially in papermaking. During thepressing of paper, felts are used to recover the water expressed from the wet sheet. Toimprove this operation, felt structures have to be characterized. X-ray microtomographyis a powerful tool for reconstructing the complex organization of such fibrous materials.This method is first described, then examples of measurements are shown and discussed.This nondestructive technique appears to be an excellent tool for investigating woven andnonwoven structures. The fine description of the structure permits the characterization ofstructural parameters and numerical modeling of physical phenomena of this true three-dimensional structure.
Textiles are used in many industries for different ap-
plications. The technique we present here may be used inalmost any case, but we will focus our attention in this
paper on one particular sector: papermaking. In particu-lar, the experimental results will illustrate the ability ofour set-up for felts used in the pressing section.
Paper pulp contains only roughly 5% mass fiber and 95%mass water. Thus the main aim of the whole process is
essentially to remove the water from the wet fibrous pad toobtain a paper sheet. At the beginning of paper making, thepulp is laid on a fabric, and the suspension is concentratedby filtration. The drainage is improved by depressor ele-ments such as vacuum boxes or foils in contact with the
fabric. Next, the sheet supported by the felt is pressed
between rolls. Finally, the wet sheet is dried by heating therolls to remove the excess water and to reach a siccity (ratioof dry and humid masses) of 0.95. The energy necessary toremove the same quantity of water by drying is six times
higher than by pressing. Hence, optimization of the pressingsection impacts the entire paper mill economy.
Different means exist to improve pressing efficiency. Inparticular, one way is to improve sheet consolidation byoptimizing the structure of the felt. Often in pressing stud-ies, the flow resistance due to the felt structure is eitherrelated to physical parameters following an empirical law ornot taken into account at all. In order to justify such laws,experimental work must consider felt flow resistance. Ex-perimentally, when the porosity goes from 0.65 to 0.30
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, the resulting permeability may run from 10-10 to10- 12 2 m2 following an exponential relationship with theporosity. However, recent works tend to show that feltpermeability in modem pressing techniques is not a majorfactor for the efficiency of unit operation and may beneglected . But the structure of the felt remains at theforefront of pressing physics because of its water stockcapacity, the problems of rewetting, or phenomena relatedto the uniformity of pressure application. A 3D reconstruc-tion of felt has never been published to our knowledge.Some works have focused on fabric or scrim reconstruction
[ 13, 10, 8] but never on a whole felt because of the difficultyof accounting for fiber needling.
Characterization of fibrous structures, and felt in partic-ular, has always been within the scope of many scientificworks [1, 2, 7, 9]. For example, confocal laser scanningmicroscopy has been used to obtain a three-dimensionalstructure . Indeed, it is very well known that the pressingfelts structure has a huge influence on its physical proper-ties. Therefore, many experimental techniques such as mi-croscopy have been used to visualize the complex organi-zation of the fibers inside porous networks. However, it hasbeen experimentally very difficult to obtain the whole struc-ture with an acceptable accuracy, that is to say, less than afew microns on a large sample. Furthermore, the sampleshave to be carefully prepared, and the structure may bedamaged.
In this work, we construct a 3D characterization of thefelt structure using a local x-ray microtomography tech-nique. We first briefly describe this tool and then presentsome examples to illustrate our experiments.
The technique used here is called microtomography. Itdiffers from x-ray computed tomography only because theyhave different spatial resolutions. X-ray absorption com-puted tomography is widely used for medical imaging andis accomplished by reconstructing a 2D or 3D image of theobject from attenuation measurements at different angularpositions. This technique is greatly improved by the use ofsynchrotron radiation. Unlike laboratory x-ray sources, syn-chrotron radiation offers the possibility of selecting x-rayswith a small energy bandwidth from the wide, continuous
energy spectrum while retaining a sufficiently intense beam.It allows high spatial resolution images to be generated.Moreover, hardening artefacts, which often occur withpolyenergetic beams in classical tomographic imaging, areattenuated. Synchrotron x-ray computed microtomography,which provides high-resolution 3D images, is well suitedfor studying the porous structure of felt.The fine description of the structure is the main aim of
this study. The computed sample diameter is 6.7 mm,
and the images are recorded with a 6.6 micron pixel size.Another possible technique involves x-ray phase contrasttil, 51. but we did not use this technique here becausetransmission gives excellent results in our experiments.Furthermore, this technique is much faster than phasecontrast. In classical scanners, the x-ray source and thedetector rotate around a fixed sample. This is obviouslynot possible with a synchrotron radiation source. There-fore, the rotation is applied to the sample itself (seeFigure 1 ). The sample to be analyzed is mounted on atranslation/rotation stage, allowing precise alignmentwith the beam. A 2D detector records the beam attenu-ation produced by the sample for different angular posi-tions, as shown in Figure 1. A typical scan involves 900projections of the sample over 180. Different lenses andscintillators may be used to adjust the pixel size to thesample size. Moreover, we have used a monochromatorin the experimental set-up. The detector is of primaryimportance in the set-up, since it determines the spatialresolution of the image. The detector we used is based onthe Frelon CCD (charge couple device) camera developedby the ESRF Detector Group. There are two cco sizes-1024 by 1024 elements and 2048 by 2048 elements. Athin scintillation layer deposited on glass converts x-raysto visible light. Light optics (associated with each cam-era) magnify the image on a screen and project it onto theCCD. The CCD camera is mounted perpendicular to thex-ray beam in order both to protect it and to avoid directinteractions that cause noise in the recorded images.