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|Last update : June 18, 2003|
|2 - Specificity of recycled fibers morphology|
C. Voillot (EFPG)
Recycled pulps are characterized by a mixture of several components, including fibers from different vegetals (softwood, hardwood, principally) which have been treated by several chemical or mechanical processes to prepare the initial pulp, by a mixture of fillers, chemical additives, and then by a specific recycling treatment which includes more or less new chemical treatment if deinking is performed. The effect of recycling with or without deinking are different depending on the kind of pulp (chemical or high yield pulp) and many mechanical, optical and structural sheet properties are affected. The simple effect of drying on a never dried pulp fiber is well known. The main morphological modifications which appeared during recycling are inside the cell wall. Other important modification occurred on the surface of the fiber conducting to bonding strength changes but will not be presented here.
To analyze the modification inside the fiber wall, several techniques have been described, starting from the definition of the Fiber Saturation Point which introduces the internal swelling of the wall, and the Water Retention Value easily performed on the pulp, value which is commonly used to define the hornification of fiber wall due to drying and re-slurrying. Different methods enable to access to the porosity and pores distribution in the cell wall, like the Solute Exclusion Technique which uses different dimensions of macromolecules to enter the pores of the fiber wall. The Mercury Porosimetry, and more recently the Thermoporosimetry based on DSC measurement are also used to describe the pores size distribution of the pulp fibers. All these techniques show that important modifications occurred inside the distribution of the pores during recycling but they are largely dependant on the kind of the pulp (principally chemical pulps versus high yield pulps).
The evolution of the cell wall morphology in terms of lumen diameter, cell wall thickness and collapsity of the wall is also of great importance to characterize recycling effect. For that purpose, new techniques based on specific sample preparation for electron microscopy and image analysis, are of great interest to improve accuracy of the measurement in comparison to optical microscopy based devices.