Fundamentals of strength loss in recycled paper

Considerable work has been devoted to the upgrading of recycled chemical (low yield) pulp fibers during the past decade. There is also disagreement on the effectiveness of an upgrading process regardless, whether of chemical or mechanical origin. One serious problem which restricts sustainable progress in the field of fine-paper recycling is the lack of knowledge of the mechanism by which recycling affects the texture and arrangement of the cell wall which ultimately causes inferior properties of the recycled fibers. The deteriorative effect of recycling on fine-paper manifested itself on the loss in potential bonding of recycled fibers. The loss in potential bonding of the recycled fibers translated into hornification (i.e., loss in fiber wet-flexibility) and/or surface deactivation by recycling. The susceptibility of the fibers for hornification rather than surface deactivation during recycling is substantiated with different techniques. It is concluded that the hornification is responsible for inferior properties of recycled fibers. More importantly, observations in the present work suggest that refining/beating does not develop any new surface area. The effect of refining is restricted to a reduction in the rigidity of the lamellae by mechanical fatigue and subsequently, increased swelling and plasticization of the fiber wall. Thus, drying of never-dried fibers (unbeaten or beaten) from water pulls the lamellae toward each other by surface tension forces and binds the lamellae rich in surface by crystallization forces. These forces lead to an increase in the crystallization of the cell wall provided that the condition required for crystallization, is met by the molecular orientation in the cell wall. When these fibers are re-wetted again, the delamination does not reverse completely, and the lamellae remain partially closed. This results in increased rigidity of unraveled lamellae and restricts the internal surfaces of fibers to access by water. The concomitant result is restricted swelling and thus, loss in wet-plasticity of the fibers on recycling. Most of this change takes place in the first cycle. Repeated recycling deteriorates further the wet-plasticity of the fibers. Based on these findings a model is proposed which explains the mechanism by which hornification develops in the fiber wall during recycling. The proposed model also provides new information on the effects of fiber beating or refining.Forestry, Faculty ofGraduat