Physicochemical Characterization of Papermaking Fibres

The purpose of this paper is to present as complete as possible, a picture of our present knowledge about papermaking fibres and their physico-chemical characteristics. The properties of the papermaking fibres are, in most cases, significantly influenced by the morphology of the wood fibres, but, from one and the same kind of wood fibres, paper can be produced with widely different properties as a result of different pulping and papermaking processes. Dissolution of material from the fibre wall and the middle lamella, structural changes of the polymeric material, and mechanical work on the fibrous material (defibration, refining, and to a certain extent undesired mechanical damage to the fibres in the pulp mill machinery) combine to produce the fibre properties required in the papermaking process. Starting from the structure of the wood, a survey is given of the pattern of dissolution of different important pulping processes and the resulting bulk composition of the pulps. Characterisation of papermaking properties should include effects of both bulk and surface of the fibres and for that reason they are both discussed. The bulk composition has been studied for many years and we have a fairly good knowledge of the main features, although there is still a need for more detailed knowledge. The properties of the fibre surface are less known, but they have been the subject of several recent studies. They are therefore dealt with in more detail, particularly the problem of making reliable and relevant measurements. Dissolution of lignin and other components in the pulping process is also important for the chemical composition of the surface. Mechanical removal of the remaining middle lamella and the outer layers of the fibre wall (the primary wall and SI of the secondary wall) substantially change the surface composition and create fines with a large surface area, which may interact with the wet-end chemicals in the paper mill and decrease the over-all effects of these chemicals. Removal of the outer layers will also change the fibre properties as a whole since, for instance, the SI layer restricts outward swelling of the main part of the secondary wall, S2, and preserves fibre rigidity. Swelling of the fibres influences there fining behaviour of a pulp. For lignin-containing pulps, swelling facilitates refining. For bleached pulps with a very low content of residual lignin, the effect of swelling on refining is rather difficult to as will be briefly discussed.