Proceeding Articles

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.

In general, only qualitative information is available concerning the mainly negative effect of the various processing stages of papermaking, after-processing and usage, on the recyclability and properties of wastepaper as a raw material for further paper making. However, it is quite unknown to what extent such negative influence is exerted by the various processing stages as, for instance, stock preparation, wet pressing, drying, calendering and supercalendering, printing, corrugating, and so on.

Esterification of pulp fibres have been performed in pyridine and toluene. The influence on swelling and the mechanical properties of sheets are shown to be different at the same level of esterification obtained by the two methods. This is interpreted to be due to the topological position of the acetyl group in the cell wall. Thus, esterification in toluene tends to allocate the esterification to the surface while in pyridine the interior of the cell wall is made accessible. At low degree of esterification in pyridine an increase in swelling is obtained. Thus, the hydrophobic groups may be seen as structure-breakers primarily by keeping the carbohydrate chains apart which otherwise would tend to form water-inaccessible regions, due to extensive hydrogen bonding. This maximum in swelling is maintained even in the dry state. Thus, the equilibrium moisture content is higher in a fibre moderately esterified in pyridine than a nontreated fibre. The introduction of acetyl groups is also shown to have an influence on the reswelling of the fibre after drying. The loss in swelling after drying is considerably less after drying for a moderately esterified fibre than for a nontreated fibre. Consequently the retention of the strength of the sheet has been improved. Essentially moderately esterified fibres which have been once dried yield a sheet with a strength equal to never dried virgin fibres.

The experiments with the fibres esterified in toluene did not yield the same effect indicating that the presence of hydrophoric groups inside the cell wall is necessary in order to maintain the swelling properties of the fibres. The results indicate new ways of permanenting the strength potential of recycled fibres and have illustrated the fundamental mechanism leading to the decrease of strength potential due to the drying of virgin fibres.

Thispaper is intended to summarise and follow up the papers presented at the Spring Conference of the B.P. and B.M.A. Technical Section in London in 1973 and the Eucepa Symposium in Bratislava in 1976.

In 1968 the Fibre Institute of the Netherlands Organisation for Applied
Scientific Research (TNO) initiated, in close co-operation with the Research
Association of the Netherlands Paper Industry, a programme to promote
the use of waste paper. This programme, which is still in progress, covers –

1 . An overall comparison of mills which use clean and/or mixed waste paper.
2. Pulping, deflaking, cleaning, dispersing, de-inking and bleaching.
3. Treatment of sludges.
4. An analysis of raw materials losses.

The studies are based mainly on comparisons of recycling equipments installed in Dutch paper and board mills. Special trials were made using the pilot plants of paper machine manufacturers.