1977 Volume 1

The porous nature of paper can be regarded as consisting of three components, namely, an external void system, an internal void system and discontinuities. The external void system can be associated with the concept of paper roughness and is totally open in one direction. The internal void system can be regarded as that which can be observed by simple optical microscopy (e.g. paper sections cut at an oblique angle). The discontinuities can be defined as all those phenomena that deprive the voids between fibres of the characteristics which would justify regarding them as capillaries with smooth surfaces. These would include the structural elements of the fibre surface, the fines in paper, the fibrils and the fibril bundles protruding into the interfibre voids, the pits in the fibre surface and pores in the cell walls.

The interactions between the components of the model system proposed above have been investigated using thickness, light transmission and water absorption measurements.

*Shortened version of the original article, prepared by R. W. Hoyland, UMIST, Manchester.

Fibres from a bleached kraft pulp grafted with polystyrene by means of radiation initiation were studied. The purpose of grafting was to incorporate a polymer with a defined rheological behaviour, thereby changing the composition of the cell wall. The grafts had a polystyrene content ranging from 6 to 35 per cent by weight.

In order to decrease the crystallinity of the cellulose, the fibres were treated with aqueous zinc solutions of 65 and 70 per cent concentration. In addition to decrystallisation effects these treatments caused chemical degradation and dissolution of the cellulose.

Mechanical spectroscopy of the grafted fibres revealed an intense interaction between the polystyrene and the carbohydrate macromolecules. Apparently the treatment with zinc chloride causes degradation and structural breakdown of the grafted fibre which results in a less intense interaction between the cellulose and polystyrene phases.

The results of creep measurements in water and toluene indicate that the polystyrene can control the properties of the matrix while the amorphous cellulose still governs the stress transfer between the reinforcing microfibrils.

The mechanism of the wet tensile performance of paper is described in terms of the hydrogen bond theory of paper. Distinctions are made between the action of wet strength agents and the stiffening action of crosslinks. Chemical crosslinks improve the wet stiffness of paper by reducing the moisture sensitivity of the cellulose network to the swelling action of water. Below the fibre saturation point, the effect of water in reducing the tensile modulus of crosslinked paper is quantitatively the same as in uncrosslinked paper. Wet stiffening arises only from the reduction in the fibre saturation point that the crosslinks create. The role of crosslinks as load-carrying elements is not important in wet stiffening. Rather, crosslinks function as swelling restraints to the network, so that a larger fraction of the pre-existing hydrogen bonds function to retain a larger fraction of the paper’s dry tensile modulus. In this respect, even crosslinked paper can be considered a hydrogen-bond-dominated solid.

The modes of action of the important commercial wet strength additives for paper are explained in terms of the physics and chemistry of the interfibre bonding areas therein. The theoretical principles expounded are then used to provide the experimentally-verified prediction that the naturally-occurring biodegradable aminopolysaccharide, chitosan, can function effectively to mitigate the adverse effect of water in papers.

The various modes in which ordinary and modified cellulose fibres and surface modified cellulose microfibrils can interact with water are reviewed. The hydrogel state of fibres can be extruded and dried to form a continuous oriented strand: ‘one dimensional paper’. A colloidal suspension of surface modified cellulose microfibrils can be converted into a paracrystalline gel which exhibits birefringent domains and responds to extrusion through a cylindrical orifice to yield a continuous fibre whose density and x-ray diagram are characteristic of cellulose: ‘reconstituted native cellulose’. The mechanical properties of these two novel materials are reported.

The mechanism of wet strength development in ‘one-dimensional paper’ which had been encapsulated with nascent polyethylene was studied. The polyethylene was confined to the exterior of the substrate and while it does not prevent fibre wetting and swelling the rate of wetting is considerably slowed. As a result, disruption of interfibre bonds is incomplete.

Ladies and Gentlemen. The panel discussion format is a new venture for this series of meetings. The subject is also new to us.

We have on the platform a number of people who are expert in this field of recycling. Each has prepared a substantial contribution, which has been preprinted and which will be presented in the usual way. The panelists cover a wide range of experience between them and the subjects of their more formal contributions were chosen in some cases, rather to obtain a good coverage of the subject, than because they were the first choices of their authors. Today’s session is very much a team effort.

From a review of the literature and our own work, it appears that the problem requiring most urgent attention is the nature of secondary fibres themselves. It is shown that the essential difference between secondary and virgin fibres is their bondability and that this depends on the papermaking process by which the secondary fibres were generated. Mention is made of some treatments which go some way towards restoring the strength of sheets containing recycled fibre.

Selected results are presented of new detailed experiments on progressively recycling virgin pulp in the form of handsheets and machine made paper. Laboratory beaten pulp has been made into handsheets, re-disintegrated and remade (1) over four recycles with rosin/alumsizing and touch up beating to maintain freeness, and (2) over three recycles without sizing or further beating. The same beaten pulp was made into paper on a pilot paper machine and remade over three cycles.

The work has shown clearly that, in the experiments, pilot machine recycling caused much smaller changes of sheet properties than is the case for handsheet recycling. However, the trends of change as recycling progresses are similar and it is likely that the same basic mechanisms are responsible. Differences of stock preparation and differences of formation make distinct contributions to the differences between handsheet and machine recycling. Changes in the chemical composition of the fibres occur during recycling and these were found to be more pronounced in the second handsheet recycling experiment than in the pilot machine trial. Loss of bonding strength is clearly related to a reduction of wet plasticity of the fibres. Machine recycling results support the handsheet data in suggesting a strong possibility that changes in fibre surface condition play an important part in loss of bonding strength, particularly at the first remaking.

In the paper submitted to the Symposium, attention was drawn to the different results obtained by recycling handsheet paper and Fourdrinier machine-made paper. A method has been sought to give some indication of whether this is due to differences in the fibres themselves.

The use of recycled pulps in the paper industry is increasing in importance. This is a result, not only of the growing deficit of fibrous raw materials in many regions of the world, but also of the mounting concern to protect the environment from the effects of waste paper garbage and its products of combustion.