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|You are here: Home > Technique > Processes > Scientific report of the LGP2 > Paper physics > Microscopy and material characterization||Update: July 21, 2011|
|Scientific report of the LGP2 (2006-2009)|
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|Researchers of the LGP2
Raphaël Passas, Bertine Khélifi, Cindy Arnoldi
The activities related to the microscopy may be divided into three main axes involving different experimental techniques such as stereo-microscope, scanning microscope or optical microscope coupled with image analysis.
Wood fibers do not have the same morphological properties. Depending on the species, the length, width or cell wall porosity may vary considerably. During pulping, fibers undergo mechanical and/or chemical actions, leading to modifications of the microfibrils at the nanometrical scale. The morphological knowledge at different scales (from nanometer to millimeter) may lead to the optimization of the fibre/process in function of the desired end use. In the context of the PhD of E. Micromatis (collaboration with Tembec S.A., 2007), the optimization of the pulp production has been based on the morphological analyzes obtained from the tool (MorFi) developed in our Laboratory in cooperation with the Centre Technique du Papier (CTP) and Techpap, following the initial work of A. Tessadro (PhD defended in 1995). A second application of such morphological data consists in the simulation of fibrous network aiming in the prediction of paper structures. These works (PhD of C. Barratte (Huyck), 2001, and R. Vincent (Tembec), 2006) have lead to the elaboration of virtual sheets [Figure 1].
|Figure 1 - Image MEB (haut) et simulation (bas) d’une feuille de papier
[Thèse R. Vincent, Grenoble INP-Pagora, TEMBEC]
Papers have always been manufactured using different fibers (bamboo, mulberry, rags, wood, annual plants…). Nowadays, all species combined, they contain more than 50 % of recovered fibers. The analysis of the fibrous composition is based essentially on observations carried out on optical microscopes in order to determine the fibers in the studied material. The identification is based on criteria of botany for different types of cells (tracheid, fiber, vessel, parenchyma…). A complementary analysis on mineral fillers may be carried out in order to determine the nature of the filler and the spatial distribution, both at the surface and in the bulk [Figure 2].
|Figure 2 - Microanalyse X et répartition des atomes dans l’épaisseur du papier
[Plaquette présentation de l’ESEM, R. Passas, Grenoble INP-Pagora]
These results are obtained from the observation involving a scanning electron microscope coupled with X-ray analyses techniques. For deinked pulps, the analyses are more complex as the raw materials come from different origins. Considering the optimization of the deinking process, it is necessary to have a better knowledge of the fibrous suspension in particular at the level of ink particles. One of the main issue is to be able to fractionate the different constituents (ink particles, fillers, fibres) in order to only process the desired flux [Figure 3]. Optical microscopy coupled with image analysis should allow the development of a quantitative method dedicated to the different elements (PhD Thesis of S. Kumar, in collaboration with CTP).
|Figure 3 - Suspension issue de fibres de récupération
(gauche : lumière réfléchie, fond blanc ; droite : fluorescence
[Thèse S. Kumar, Grenoble INP-Pagora, CTP]
All the constitutive elements of a suspension may contribute to the structure of the sheet and hence, will define its structure, optical and mechanical properties. Consequently, the porosity of the surface and the roughness are two main characteristics to be taken into account for ink penetration during printing and for print quality [Figure 4].
|Figure 4 - Surface d’un papier partiellement imprimé
[B. Khélifi, Grenoble INP-Pagora]
The quality of the print may be evaluated visually. Furthermore, the coupling between optical microscopy and image analysis may provide quantitative information on screen ruling, screen angle or registration. The determination of the heterogeneity of the grey levels of the halftone dots may constitute a benefit compared to the classical analysis carried out with a densitometer and offer information on the local modifications of the materials. The sustainable development is also a valid notion for fibrous materials and the new products reinforced with nonwood plants, for example, have to be studied and developed. It is therefore necessary, for the observation of such materials, to use high resolution electron microscopes or scanning probe microscopes with dedicated protocols.
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