Molecular mechanisms involved in creep phenomena of paper

Abstract

The phenomenon of mechanosorptive creep (i.e., the increasing creep occurring in some hygroscopic materials subjected to moisture cycling) was studied for paper from a molecular point of view. Paper was tested in creep at different loading levels in a constant high humidity of 90% relative humidity (RH) and in a cyclic climate between 30 and 90% RH. Throughout the creep tests, spectra from the mid‐ and near‐IR, as well as dynamic mechanical data, were recorded to determine molecular changes occurring with time. In tensile stress scans the instantaneous, dynamic elastic modulus was found to increase. It is suggested that this increase was due to orientation of the cellulose molecules, which was detected as changes in the mid‐IR spectra at 1160 cm−1 assigned to the C1OC4 stretching. During creep in constant and cyclic humidity, the modulus was found to increase with time, more so for the cyclic humidity. Changes in the mid‐IR spectra at 1184 and 1030 cm−1, which is assigned to CH2, CH, and CO, may indicate sliding between the cellulose chains. The near‐IR measurements mainly showed differences in the moisture content. In stress scans the moisture content increased with increasing tensile load. In creep at constant 90% RH, the moisture content was also found to increase in a manner similar to the stress scan. In the cyclic humidity with a conditioning time of 70 min at 90% RH the moisture content decreased successively with increasing numbers of cycles. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1590–1595, 2001

A critical discussion of the physics of wood–water interactions

“…Hence, the equilibrium moisture state will change during a sorption process also after the step change when the ambient vapour pressure is held constant. The interdependence between moisture state and stress state is also seen when external loads are applied (Olsson and Salmén 2001).…”

“…In wood cell walls, the restraining effect of cellulose microfibrils on the swelling amorphous polymers, hemicelluloses and lignin, might cause significant stress gradients without large gradients in bound water concentration being present. Time-dependent behaviour of wood cell walls has been ascribed to shearing in the material directions (Balashov et al 1957;Lotfy et al 1972;Olsson and Salmén 2001;Engelund and Svensson 2011). The rate of sorption should therefore be related to the rate of relaxation of shear swelling stresses in wood cell walls.…”

Modelling time-dependent mechanical behaviour of softwood using deformation kinetics

“…The mechanism behind the TDMB has been ascribed to sliding between the MFs (Balashov et al 1957;Lotfy et al 1972;Olsson and Salmén 2001). This sliding over time is controlled by flow processes within the material, where the internal hydrogen bonds between the constituents are broken, moved and reformed in new configurations (van der Put 1989;Hanhijärvi 1995;Bonfield et al 1996).…”

Mechano-sorptive creep under compressive loading – A micromechanical model

“…The parameter E c has the dimensional unit Pa and contains the effects of the modulus of elasticity for the linear chain molecular and the molecular network. A decelerating creep rate is captured by this creep law, which will give a creep response resembling those of paper and fibres (Brezinski, 1956;Haslach, 1994;Sedlachek, 1995;Olsson and Salmén, 2001). Written in a slightly different form, this kind of creep law has been used to model creep of single fibres (Sedlachek, 1995).…”