New method for the measurement of fibres orientation on the paper faces applied to the analysis of hygro-instability in paper sheets

For a given change in the humidity of a paper the hygro-instability of the sheet depends of two main factors:  the sheet structural parameters which reflects its structure: the sheet porosity, the fibres orientation distribution mainly in the sheet plane and the variations of these parameters across the paper sheet thickness.  the water reactivity of the components of the paper sheet which depends mainly of the sheet composition: the cellulosic and the fillers content, the nature of the fibres, softwood or hardwood, their degree of delignification, the beating degree of the pulp, the fines content, the conditions of the sheet drying with a more or less conditions of restraint. In this research initiated at the University of Beira Interior (UBI) with the collaboration of the Ecole Française de Papeterie (EFPG), we have tried to identify these main parameters of the paper hygro-instability. For this purpose we characterise the sheet structural parameters by optical methods developed at the Optical Centre of UBI.