Proceeding Articles

The studies upon which this contribution is based were made to investigate the structure and properties of high stretch papers. Webs produced by compaction and by creping, the two main commercial processes, were examined by light microscopy and physical testing.

The micro photography shows a variety of web configurations found in crepe papers, including examples of wave formations, internal delamination and two sidedness. The characteristic fibre orientation and densification are illustrated by photo micrographs of webs taken before and after the compacting process.

The mechanical behaviour of high stretch papers is illustrated by typical stress/strain curves and a discussion of their behaviour during the process of straining.

After a review of the development of extensible papers, a description of the double-roll compacting process and its variables is given. Its principal feature is the venturi section formed in the nip between a rubber and a steel roll, between which the paper web passes in a semi-dry state. On running the rubber roll more slowly than the steel roll, the web will shrink in the machine-direction. Experiments on a pilot machine showed an increase in the compacting effect with increasing nip pressure and speed difference, though with certain limitations. When considering nip width and peripheral speed difference as primary variables, however, linear relationships with the paper properties were found. The nip width will vary with the nip pressure and rubber thickness and hardness.

The mechanism of double-roll compacting is considered to involve tangential forces, which move the rubber towards the back side of the nip, where it contracts, thereby shrinking the web. The structure of the resulting extensible paper was examined by photo micrographs of surface and cross-sections, by measuring the thickness changes on stretching and by load elongation measurements. The fibres appear curved after the compacting operation. This will result in the breaking of bonds when stretching the paper and in an ultimate breaking load lower than for flat kraft. The total rupture energy, however, is considerably higher.

An apparent increase in the rubber roll diameter on increasing nip pressure was observed. This will cause a decrease in the mean speed difference at the nip. At a limited set speed difference, the rubber roll was found to change from being driven to be driving on increasing the nip pressure. In an appendix, the nip width and the slip have been treated theoretically as well as experimentally.

The high frequency shear mechanical behaviour of cellulose pulp/ watersystems during theprocess of drying from 3 percent solids to total dryness has been non-destructively and continuously monitored by the technique of ultrasonic impedometry. Unusual fibre/water interactions have been detected at both extremes of the concentration range studied. These interactions are given interpretation in molecular terms.

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Following a review of the effects of machine- and cross-direction forces in the web during drying on the stress/strain properties of the finished paper,the nature of the axial forces in the individual fibres during tension drying is discussed in the light of the theory of the structure. The effects on the mechanical properties and structure of individual holocellulose pulp fibres of tension drying have been carefully investigated. An unusual extensional behaviour was observed at the onset of drying. It was found that tension during drying promoted substantial increments in tensile strength, Young’s modulus and crystallite orientation; generally, the spring wood fibres under went larger changes than the summer wood ultimate elongation was reduced and crystallinity remained unchanged.

Partial removal of the hemicelluloses resulted in large decrements in tensile strength and Young’s modulus, a phenomenon not attributable to degradation of the fibre or to such side effects as swelling; the levels of these mechanical properties were reduced to those of ordinary pulp and cotton fibres. The relative enhancement of tensile strength and Young’s modulus in the extracted fibres caused by tension drying was much greater than that observed in the holocellulose pulp fibres, the latter property rising almost to that of the holocellulose fibres dried under load as the drying load was increased. The crystallinity of the extracted fibres (as determined by the method of half-width of a diffraction peak) was higher than that of the original holocellulose pulp, suggesting enhanced cellulose/cellulose bonding within the fibre, which, in turn, seems to account for the tension drying behaviour.

Theory and experimental data relating to the possible effects of tension drying on the zero-span tensile strength of machine-made paper are presented. It is indicated that more work needs to be done in this area and, more generally, on the effect of tension drying of individual fibres on all the mechanical properties of paper.

The goal of this study was to gain a greater understanding of the changes in the mechanical properties of spring wood fibres during the consolidation of the web. It was found that the elastic modulus of freely dried fibres E is essentially tripled by any axial load applied during drying. Their tensile strength t is increased and their stretch of is decreased in proportion to the loads applied during drying up a twofold change. The elastic modulus measured at 90°to the fibre axis E1, was found to be about 0.1E. These fibre properties are of paramount importance to the stress/strain properties of paper, which are described quantitatively in terms of the fibre properties and sheet structure in our second contribution to this symposium.

Changes in the axial dimension of chemical pulp fibres were studied, using various combinations of drying and wetting. It was found that the commonly experienced elongation, brought about as a result of wetting, was exchanged for a shrinkage when the applied load before wetting was considerably lower than that applied before the preceding drying. Dimensional stabilisation was found for certain combinations of drying and wetting loads.

In addition, fibre stiffness was studied. During drying, the stiffness increased sharply within a dry solids content range of 15-35 percent. Upon further drying, the stiffness of the latewood fibres did not change, whereas that of the earlywood fibres was decreased. The former effect is most likely associated with an increased modulus of elasticity, whereas the latter is probably a result of changes in the fibre cross-section involving collapse.

The importance of fibre flexibility to the consolidation of the paper web and to sheet formation has long been recognised and a number of methods have been developed to measure fibre stiffness.(1 -4) The fundamental resistance of fibres to flexural deformations is given by the flexural rigidity EI, the product of modulus of elasticity and moment of inertia. The methods of Seborg & Simmonds (1) and Forgacs et al.(2) do not yield flexural rigidity values, that of Nethercut (3) requires fibres of 7-8 mm length and is therefore unsuitable for wood fibres; only the method of Samuelsson(4) has been successful in providing a quantitative measure of flexural rigidity. Duncker et al. (in the paper presented at this symposium) have used the Samuelsson method in studying the effect of drying on the flexural rigidity of fibres. It is the purpose of this contribution to describe a new method for measuring the flexural rigidity of fibres and to present some data on the effect of drying on flexural rigidity.

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The thermal softening of isolated samples of lignin, hemicellulose and cellulose has been investigated by observation of the thermally induced collapse of a column of powder under constant gravitational load. Softening temperatures of lignins ranged from 127-193°C. Birch xylan and pine glucomannan softened at 167° and 181’C, respectively. Sorption of water by lignin and hemicellulose caused pronounced decrease of the softening temperature-in some cases,to as low as 54°C. Softening points of both dry and moist lignins or hemicelluloses have been shown to correlate with the temperature at which the sample develops adhesive properties. The softening and adhesive behaviour has been explained in terms of the concept of the glass transition for amorphous polymers. Sorbed water is considered to act as a low molecular weight diluent in plasticising the polymer chains and lowering the glass transition temperature.

Celluloses were found to soften at temperatures greater than 230°C. In contrast to lignin and hemicellulose, sorption of water by the cellulose had negligible effect on the softening temperature. This difference was probably due to the crystalline nature of cellulose and indicated that water did not plasticise individual cellulose chains at the molecular level.