1973 Volume 2

Probable causes for the deterioration of the physical properties of paper as a result of repeated handling have been determined. A flexing test has been devised that evaluates the relative durability of paper.

Investigations with the scanning electron microscope indicate that the fibrillar component of paper deteriorates during flexing and is probably responsible for the decline in stiffness as well as the modulus of paper. Because of the ever increasing use of automatic document handling equipment, stiffness retention with handling is essential, since limp documents are difficult, if not impossible, to process automatically. Unfortunately, stiffness declines rapidly as paper is flexed or handled.

The rates of deterioration for all other physical properties of paper during flexing are independent of each other and vary from one paper to another. The rate at which certain properties deteriorate is independent of the quality of the paper. No correlation has been found between any paper property and durability.

The influence of humidity and temperature on the change in elastic properties of single fibres and the corresponding papers are shown to be similar.

Furthermore, expansional changes due to variation in the atmospheric conditions are shown to be related to the elastic properties of the sheet . For thermal expansion in paper, the product of coefficient of linear expansion and the elastic properties is approximately constant in all directions. A similar relationship is also followed in hygroexpansion. In the experiments reported, a slightly higher value in the machine direction was obtained for the product of the dimensional changes and the elastic modulus.

It is also noted that the geometric means of the dimensional instability and elastic properties in machine and cross directions coincide with the same properties of the isotropic sheet.

Hygrodimensional changes for single fibres were measured by means of following the rotational twist of the fibres caused by sorption. It is deemed from these experiments that the fibre swelling is proportional to the moisture regain.

Finally, it is shown that the viscoelastic properties of cellulose material change in an irregular manner during changes of the moisture regain. These effects cause considerable reduction in the creep to failure time (lifetime) of paper in compression or tension under transient atmospheric conditions.

In my introduction to session 1, I made the point that most of the material presented in this session could be seen almost as a continuation of the corresponding part of the Oxford symposium in 1961. This continuity, which was very noticeable in other sessions also, is one of the assets and attractions of these symposia. There are people (and I have met several) who think that such a continuity signifies lack of ideas, a state of stagnation. They argue that we must be ‘with it’, that there must be something new, even topical, every time. I do not agree with this at all. On the contrary, if we let ourselves be dominated by the notion that there must be a frantic search for a new ‘in’ subject every time we plan a symposium, we are in danger of becoming organisers of yet another set of technical gatherings, useful, most likely, but not necessarily bearing the stamp of fundamental research.

The mechanical properties of paper was a main topic both at the 1961 Oxford meeting and the 1965 Cambridge meeting and, as a better understanding of paper’s strength properties will always be of great interest, it will certainly be included as a separate session in the programme of future symposia.

Professor Rånby opened the meeting with a concentrated review of the synthetic paper field. It was a reasonably well referenced review with a very useful classification scheme. Perhaps it is still premature to select synthetic pulp as the most promising approach to synthetic paper, but it is the chairman’s privilege to speculate in this way. The pollution problem, mentioned by the speaker, is mainly due to the pulp mills and is a problem chiefly in the Scandinavian countries. In other countries, paper and pulp mill pollution is less serious compared with sewage effluent. Also, the shortage in petroleum may well be ‘across the board’ and any cuts made could also affect the polymer industry. All in all, the review was a very valuable contribution and should stimulate considerable interest in the field. The subject of synthetic paper is wide open for fundamental research; from such research, much can also be learned about conventional paper and papermaking.

It became obvious at session 4 with its subject of barrier and absorption properties, the requirements for paper as a substrate, that we touched only the tip of an iceberg top. Very little is known about coating, although it has been carried out for 2,000 years.

In my introduction to the session on printability, I spoke of the complexity of this field. It is a subject that is difficult to come to grips with and, believe me, I find it difficult to organise an appraisal of where we are and what we should be doing in the future!

The emphasis in this session was on the largely subjective nature of the optical and tactile properties of paper. Remembering the theme of the conference, I regret that I cannot list a series of experimental discoveries which can be immediately used to improve the end product. I can list a number of impressions that I have received and that, I believe, will help manufacturers in their attempts to do so.

The reaction between paper and its environment is by definition the ecology of paper. This word was once understood only by biological scientists and scholars of Greek, but it is now on everybody’s lips, so session 7 should have proved immensely popular. Of course, we have largely confined our considerations to the response of paper to its environment or to imposed conditions of temperature, humidity and mechanical or chemical stress; we have not been too concerned today with the way in which the total environment, as it involves man, is affected by paper and by the technological processes that constitute papermaking-or more broadly, the pulp and paper industry. It is as well to note, however, the connection between paper’s response to and its effect on the environment, if only to ensure, as the sledgehammers of change swing ever faster, that we do not lose sight of the relevance of our discussions to the current pressing problems on the interface between the paper industry and the rest of society. Nor, we hope, will the relevance be lost on those responsible for sponsoring our attendance at the next symposium.