On the compression properties of paper - implications for calendering

Abstract: The objective of this work was to study and describe thedeformation of paper in the thickness direction in a calendernip that in some cases was heated. The temperature distributionin the thickness direction was evaluated using modelexperiments. From these measurements, the thermal diffusivityand the apparent thermal conductivity were calculated assumingthe paper to be a semi-infinite plate. It was found that theapparent thermal conductivity increased with increasingdistance from the heated surface, indicating that the heattransfer coefficient may have influence on the temperaturedistribution in the paper.The deformation of the paper in the thickness direction whensubjected to a haversine pressure pulse could be predictedusing material properties evaluated from creep-recovery studiesand a generalisation of Boltzmann's superposition principle.The influence of temperature on the material properties wasdescribed using an Arrhenius expression. The temperaturedistribution in the thickness direction during calendering wasobtained using finite element simulations. The results of thesesimulations were in fair agreement with the experimentalresults. At high roll temperatures (up to 200 ºC), thebest agreement between the results of the viscoelasticdescription and the permanent deformation after calendering wasobtained when the thermal softening of paper was modelled usinga series coupling of two Arrhenius-expressions.The density profile in the thickness direction aftercalendering was evaluated using image analysis ofcross-sections. The uncalendered paper exhibited no differencein density between the floc and non-floc areas. It thereforeseems likely that the density variations noted aftercalendering were created in the nip. A temperature gradientcalendering had, in a sense, a rather limited effect on thedensity gradient in the thickness direction of the paper.Keywords:Calendering, compression, creep, density,finite element analysis, heat transfer coefficient, mechanicalproperties, temperature, thermal conductivity, viscoelasticity,z-direction