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        Last update : June 18, 2003
 
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            1 - Morphological characterization of pulp components. State of the art and new techniques            
C. Voillot (EFPG)

Logo EFPGRaw materials for the manufacture of paper and board are at a very large extent coming from vegetal pulps. All cellulosic cells can be used to make a paper sheet, but the resulting product will have properties depending widely on the morphology of the cells used. From non woody components which are characterized by a very large variety of cells morphology, to the common softwood and hardwood species, and taking into account the process used to get the individual cell, there are a lot of possibilities to make a paper sheet. Due to the variety of uses of paper and board (printing, packaging, sanitary, filtering,..) the properties measured on a sheet become more and more numerous and generally they are classified in "mechanical properties", "textural properties", "optical properties" or "thermal properties". Among these properties, the mechanical ones are of great importance for the runnability and productivity of the paper machine. For optical properties, it is well known that the fillers used in the sheet are also of great importance and particularly their morphology.

The morphology of the pulp fibers can be described with a large variety of measurements, basically geometrical measurement for the description of the shape of the fibers taking into account the distribution of these values. Considering the general shape of fibers, the length, width, wall thickness, lumen diameter, are the more commonly measured values, including statistical index needed to characterize the distribution of these values.

Since several years, new electronic devices based on image analysis of very diluted pulp suspensions have been developed to measure these principal characteristics, with increase in accuracy and speed. These devices use specific software to calculate the general index, and it is of interest to precise the way these values are calculated, particularly the "weighted value" of the length (weighted in length, mass, projected area).

The main difficulty is the large ratio "length to width" of the majority of pulp fibers, which need high pixel number to keep a good accuracy in the measurement of width and if possible wall thickness.

Relation between morphology and mass is obtained with the "coarseness" value which is also measured with these new electronic devices. Coarseness is a useful index to evaluate the number of fibers per gram which is a convenient index to classify the different pulps, and is of great interest to compare mechanical or structural properties of sheet on this basis.

Other morphology indexes like curl, vessels amount (for hardwood pulps), can be measured with these electronic devices, but are not yet very much exploited. Fibril orientation and fiber flexibility are some particular values which are also related to morphology, but which measurement remains difficult with a great dispersion of the value, and which are more in relation with the internal cell wall properties.

 

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