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(May 2011)
 
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Laboratory of Pulp and Paper Science and Graphic Arts

III - Paper Physics

III - 2 - X-Ray microtomography applied to fibrous materials

Pierre Dumont, Jean-Francis Bloch, Stéphane Dufreney

X-Ray synchrotron microtomography is an imaging technique allowing the 3D characterization of the microstructure of materials. This technique, used for a decade in our group, is currently at the heart of a Long Term Project (LTP) carried out at ESRF (European Synchrotron Radiation Facility), entitled Heterogeneous Fibrous Materials. This project gathers together few French and International laboratories and is coordinated by LGP2. Its main aim consists in providing a precise description of the microstructure of the materials at a fine scale, that is to say, at a micrometric scale which corresponds to the scale of fibers. Hence, quantitative descriptors of the microstructure have been developed. Moreover, in situ tests are also carried out in order to highlight the influence of different effects such as thermal, mechanical or hygroscopic modifications on the studied microstructure. Finally, getting the physical properties from the collected information, constitutes also a major objective of this project. The studied materials are papers, boards, wires, non-wovens, composites constituted of a polymeric matrix, bio-composites and other more exotic fibrous materials.

Techniques of analysis of fibrous microstructures

Acquisition modes of images obtained from microtomography

The resolution of the images has to be adapted to the characteristic dimensions of the fibers used in the different studied materials in order to describe adequately the studied physical problem. Hence, the obtained images have adapted pixel size, spread from 0.7 to 5 μm. However, the pixel size is not the only parameter to be adjusted. The imaging mode is also fundamental for the image quality. For most of the tested samples (papers and boards), the classical technique in microtomography, that is to say, the absorption mode, is successfully used [Figure 1]. This technique was used for example to study the influence of the process parameters on the microstructure of such materials (PhD of S. Rolland du Roscoat, M. Decain and M. Peralba). In such cases, the duration of a scan was typically of the order of 10 minutes.

Exemples de microtomographies synchrotron à rayons X en mode d’absorption
Figure 1 - Exemples de microtomographies synchrotron à rayons X en mode d’absorption (ESRF, ligne ID19):
(a)microtomographies synchrotron  papier impression-écriture (thèse de S. Rolland du Roscoat),
(b) papier modèle (thèse de C. Marulier).

In the particular case of composite materials, constituted of various components having a weak contrast of X-ray absorption, other techniques of microtomography, as the phase contrast or holotomography, are used. They are specific from synchrotron microtomography. They allow the access for example to the microstructure of composites with polymeric matrix filled with mineral particles and reinforced with roving of glass fibers (example in Figure 2), and to reveal the microstructure of composites based on a polymeric matrix reinforced by natural fibres on wood or annual plants.

composite SMC : matrice polymère polyester chargée en carbonate 
   de calcium + fibres de verre
Figure 2 - Exemples de micrographies 2D obtenues à partir de la microtomographie (ESRF, ligne ID 19)
d’un composite SMC formé par l’assemblage d’une matrice polymère polyester chargée
en carbonate de calcium et mèches de fibres de verre :
(a) image obtenue en mode d’absorption
(b) même image en mode de contraste de phase
(Le et al., Composites : Part A, 2008).

Other materials involve a high contrast of sizes between their different constituents. It is the case for example of filters whose fibers have a diameter of roughly 40 μm. They have to stop fillers which are mineral particles whose size varies from 0.1 μm to 2 μm. In order to observe both constituents, a multiresolution tomographic technique was used. This specific technique was also used in the case of filled papers.

Finally, fibre-reinforced and porous organo-mineral mortars are interesting solutions for the thermal insulation of buildings from their outside walls. Within this context, we have studied the properties of one of these mortars in the fresh state (microstructure, rheology, hydration). To obtain relevant information on the mortar microstructure just after its processing, the Fast Tomography technique had to be used. In this particular case, the duration of a scan had to be decreased from 10 to 1 minute in order to follow the kinetics of the evolution of the structure.

Procedure of image analysis

Obtaining 3D images is nowadays not a final aim. One of the main difficulties is the determination of relevant and discrete descriptors of individual fibres: e.g. labelling of fibres, shape and position of each fibre and fibre-fibre contacts. To obtain such descriptors, image analysis tools are now being developed and have been recently successfully used to analyse 3D microstructures of fibre-reinforced composites. A first step was reached during the PhD of S. Rolland du Roscoat [Figure 3].

processus de segmentation d’une microtomographie d’un échantillon de papier
Figure 3 - Illustration du processus de segmentation d’une microtomographie d’un échantillon de papier
contenant des charges (thèse de S. Rolland du Roscoat) : (a) illustration sur des coupes 2D,
(b) image du volume reconstruit après segmentation où l’on distingue la phase poreuse,
la phase fibreuse (en blanc) et les charges minérales (en rouge).

The segmented images were treated in order to obtain structural parameters [Figure 4]. Few PhD thesis are dedicated to the improvement of these techniques (M. Peralba, O. Guiraud and H. Boulbir). The main aim is to obtain a technique allowing the skeletonization of the fibrous structure and to analyze individual fibers as well as their contact [Figure 5].

Profil de porosité d’une toile de machine à papier   processus de squelettisation du réseau fibreux d’un composite
Figure 4 - Profil de porosité d’une toile de machine à papier calculée
à partir d’une microtomographie obtenue à l’ESRF sur la ligne ID19
(thèse de M. Peralba).
  Figure 5 - Illustration du processus de squelettisation du réseau fibreux d’un composite modèle
à matrice polymère renforcée par des mèches de fibres de verre. Microtomographies obtenues
à l’ESRF sur la ligne ID19 (thèse d’O. Guiraud).

Development of in situ environments

During this LTP, different experimental setups dedicated to mechanical tests were developed in order to carry out in situ tests during the scans. These setups are related to the desired resolution of the images and also to the dimensions of the tested samples. Compression device dedicated to mean resolution (2 to 7.5 μm) – The first setup was developed in order to compress samples whose maximum diameter was around ten millimeters, controlling the force applied on the sample (force up to 500 N) and introducing, eventually, air at controlled temperature and humidity.

Compression and traction setup for high resolution images (0.7 to 2 μm)

In order to test samples whose diameter are smaller (1.4 μm to 5 μm), a miniaturized device dedicated to both tensile and compression tests was designed [Figure 6]. For the tensile tests, due the the poor rigidity of the samples and to avoid to damage them during their manipulation, special jaws were developed. Force sensors (5 N to 50 N) and a piezoelectric engine to control the displacement of the upper jaw complete this setup.

Dispositif de traction-compression à haute résolution
Figure 6 - Dispositif de traction-compression à haute résolution :
(a) installé à l’ESRF sur la ligne ID 19,
(b) détail des mors de traction dans lesquels une éprouvette est insérée (largeur de 1,2 mm)

Microtomography involving humidity control, eventually coupled with mechanical environments

Microtomography during controlled humidities

Materials made of natural fibers are very sensitive to modifications of humidity. A setup allowing the humidification of the surrounding air during the scan was developed at ESRF. This setup was used to analyze the hygroexpansion of numerous samples: printing paper, boards, mats of flax.

Microtomography and in situ mechanical tests

In order to better understand the deformation mechanisms of fibrous network, tensile tests were carried out, using the previously described setup, on various papers, corrugated boards, and on flax fibers, at a resolution of 0.7 μm. For the studied materials, other mechanical solicitations than tensile, have to be considered during its fabrication or its end use. For example, papers are compressed, in the thickness direction during printing or converting. Compression test were carried out at high resolution (0.7 μm) on blotters or corrugated boards. In this case, variable humidity conditions were also considered. In the frame of the study of fibre-reinforced polymer composites, compression tests were carried out in situ involving fast tomography. This technique allows following at a unique scale the deformation and the movement of fibers in the model composite ([Figure 7] – PhD Thesis P. Latil, cooperation with 3S-R)

Evolution de la contrainte normale de compression exercée sur un échantillon de mèche modèle de matériau composite en fonction de la fraction volumique de fibres
Figure 7 - Graphique donnant l’évolution de la contrainte normale de compression exercée
sur un échantillon de mèche modèle de matériau composite en fonction de la fraction
volumique de fibres. Cette fraction de fibres a été estimée grâce aux microtomographies
(ESRF, ligne ID 19) figurant autour du graphique.

Advanced numerical analysis of reconstructed volumes

Digital Image Correlation techniques

In parallel to the determination of quantitative microstructural descriptors, there is a need for the description of kinematical fields of samples, which are deformed in situ during scanning. Here, the Digital Image Correlation technique was applied for the analysis of the hygroexpansion of flax fibre mats, used as reinforcement in composite materials and of folding boards (PhD thesis J. Viguie). This technique gave access to an original description of the heteregeneous displacement and strain fields that were induced by moisture changes [Figure 8]. The variations of these fields might also be correlated to the heterogeneous microstructure of these fibrous materials.

Visualisation 3D d’un carton plat
Figure 8 - (a) Visualisation 3D d’un carton plat à une humidité relative de 20% (à droite) et coupe (ey;ez) selon son épaisseur (à gauche).
(b) Cartes des composantes du champ de déformation du carton plat évaluées à partir des déplacements mesurés par corrélation d’images
dans le plan (ey;ez) pour une variation d’humidité relative de 20% à 50% (thèse de J. Viguié).

Numerical estimation of mesoscopic mechanical and physical properties from scanned volumes

The scanned volumes can complementary be used to determine the physical properties using specific upscaling theoretical and numerical methods. Indeed, flow permeability, thermal, electrical conductivities or mechanical properties of the tested materials were estimated ( PhD thesis M. Decain). This was, for example, performed for the determination of the permeation properties of papermachine wires in the cases of linear and inertial flow regimes. The numerical code is the commercial Geodict software based on the Finite Volume Method (PhD thesis M. Peralba). This technique was also used for composite materials constituted of carbon fibers.

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