A study of changes in wet-fiber flexibility and surface condition of recycled fibers

It has been generally recognized that the deterioration of recycled pulp is mainly caused by the loss of total bonding strength of recycled pulp while the loss of intrinsic fiber strength is only minor. The loss of total bonding strength may be attributed to one or both of the two fundamental properties of fibers: wet-fiber flexibility and surface condition. Through studies on a series of hemlock pulps with different chemical composition, the changes in these two properties were determined and the main factor which causes the strength loss of recycled pulp was identified. It is found that the dominant factor causing the strength loss of recycled pulp is wet-fiber flexibility. This conclusion is supported by the strength-density relation of recycled pulp and by the changes in pulp Water Retention Value (WRV) on recycling. The other fundamental property of fibers, specific fiber bonding strength, is studied through the application of Page's equation of tensile strength and is found to remain largely unchanged during recycling. The validity of this application is also addressed.