Proceeding Articles

The thermodynamic behaviour (that is, the simultaneous mechanical and thermal behaviour) of paper and other sheet-like materials will be reported. It will be shown that the thermodynamic analysis will reveal much more of the deformation mechanism than the mechanical analysis alone.

Results obtained show that the initial straining of paper is controlled by energy elastic forces and in accordance with Kelvin’s thermoelastic equation. The plastic region straining of paper is controlled by irreversible intrafibre deformation. Simultaneously, some interfibre bond breakage does occur, but this breakage is partial and it is not a prerequisite of plastic deformation of paper.

Paper-like products or paper substitutes made largely from synthetic polymers are classified as synthetic paper, as recently proposed from Japan. (¹) Several different types have been developed and the most important are in the following three groups –

1 . Spunbonded sheets.
2. Paper-like polymer films (including foamed sheets).
3. Synthetic pulp products.

In addition, there are various combinations of synthetic polymers with pulp fibres that have been developed, tested and used as paper products for example, addition of synthetic fibres to pulp fibres, impregnation of pulp fibres with synthetic polymers, lamination of paper and board with synthetic polymers and graft copolymerisation of synthetic polymers to pulp fibres. Some of these processes and products are well known and conventional (such as lamination) and some are experimental only (such as grafting) they will therefore not be further described here. One type of synthetic pulp called fibrids was developed early as a thermoplastic binder in paper. (²,³) In principle, fibrid technology is related to synthetic pulp production (group 3).

There are several reviews and books published on synthetic papers for example, Battista, (⁴) Wolpert, (⁵) Johnson, (⁶) Lunk & Strange, (⁷) Inagaki, (⁸) Kossoff, (⁹) and others. (¹⁰,¹¹)

The mutual interaction between cellulose and plastics has led in recent years to more and more complicated and improved products capable of capturing an extensive share in the paper, board and packaging industry. In most finishing processes, whether they are concerned with the substance of the paper or its surface properties, the maintenance of the morphological structure of cellulose is of decisive importance. This is because the features associated with this structure, especially those of sheet formation and in contrast to synthetic finishing materials, have an enormous influence upon the combined technological properties of the finished product. Indeed, various cellulose derivatives and modified celluloses are known that would achieve the properties obtained when synthetics are added for finishing purposes, but they have the disadvantage in most cases that the fibre structure of cellulose is lost on modification. Therefore, it is not surprising that most composite paper and cellulose products, apart from a few products used mainly in the textile industry, represent an aggregate of the fibre and finishing material.

As this symposium is held in Cambridge, it seems appropriate to speak on semantics, particularly on the terminology of synthetic and plastics papers. Confusion about it prevails in both the paper and plastics industries. It is necessary to emphasis that the synthetic polymers and conventional paper and board industries in some cases compete and in other cases co-operate closely in order to achieve products with required functional properties by making them from materials of both industries, properties that cannot be achieved in products made from materials supplied by one industry only. The consumption of polyethylene for coating paper and board is increasing.

Polymers have been incorporated into paper by (a) solution impregnation, (b) latex impregnation and (c) latex beater addition.

Their effects on the strength properties of the sheet were measured by conventional tests and interpreted in terms of the separate effects of the polymers on bonds and fibres and the distribution of polymer in the sheet. It is noted, however, that the introduction of polymer resulted in time-dependent behaviour that deserves special consideration. Stress relaxation properties were therefore examined and analysed for a variety of polymers and conditions.

A procedure is described in which experimental data relating to relaxation and creep are conveniently obtained and can provide the basis for evaluation and analysis.

The surface properties of hardwood papers are determined largely by the nature of the fibres and vessel elements (henceforth referred to as vessels) and the possibilities for interfibre and fibre to vessel hydrogen bonding. Experiments with Eucalyptus species and tropical hardwoods covering a wide range of wood densities have shown that surface smoothness as well as bulk mechanical properties depend on the lateral conformability of the fibres and that the response to beating, in terms of surface smoothness, is more marked for pulps with fibres of high Runkel ratio. Effective removal of vessels from pulps was accomplished on a laboratory scale by a method based on that used by Jacquelin for flocculation studies. Vessel removal resulted in a drastic reduction in the vessel IGT pick number. A comparison between the picking tendency of two eucalypt pulps with similar external fibre and vessel dimensions, similar vessel concentrations, but with greatly different fibre lumen diameters, indicated that the bonding strength between fibres and vessels is an important factor in picking, as well as vessel size and concentration. Laterally conformable fibres from low density woods can provide the necessary fibre-to-vessel bonding. Beating has a very pronounced effect in reducing pick number and the question arises whether breaking up of the vessels or improved bonding is mainly responsible. The wider use of hardwood resources for fine papers, particularly for offset printing papers, depends on the surface properties that can be attained.

Dr Page presented an interesting film with an informative commentary upon it, which without the film is not of value in itself and so cannot usefully be reproduced, but the authors have provided the following synopsis. 

A unique apparatus has been built and techniques developed that permit a fresh approach to the study of fibre properties. The apparatus is a stress/strain recorder for testing single fibres under axial tensile load. It is fitted with a high power polarising scopial image and the trace of the stress/strain curve can be recorded on cine film, in synchronism, so that the visual appearance of the fibre and its state of stress and strain at any time can be displayed. The detailed physical events that coincide with initiation of yield and failure are dramatically revealed.

The presentation consisted of a series of cine sequences that demonstrate phenomena that relate to the tensile properties of fibres, including –

1. The effect of fibril angle on fibre strength and stretch-to-break.
2. The initiation of rupture by defects.
3. The tensile behaviour of microcompressed fibres.
4. The phenomena that occur when a fibre dries under tensile stress.
5.  The tensile behaviour of wet fibres.

The important roles of sheet roughness and porosity and of fibre morphology on the way liquids spread on paper are revealed by static and dynamic scanning electron microscopy. Some preliminary results are described that demonstrate the potential value of the technique.

An approach to the basic processes of ink absorption into paper coatings is made by considering the way in which oil is absorbed into kaolin-based layers (50 microns thick), formed on flexible polyester substrates. Oil layers are applied to these layers by use of an IGT printability tester . The change in gloss level with time after application of the oil film is used as a method for determining the time taken for an oil film of known thickness to be absorbed.

The capillary penetration equation of Washburn is used as a basis to explore the relationship between the physical properties of the oils and the structure of the coatings.

The basic structure of one coating is also determined by the method of mercury porosimetry and is related to the oil absorption rate by use of the equation of Millington & Quirk for the permeability of the layer.