Proceeding Articles

Tensile test failure lines across 15 mm wide strips of uncalendared newsprint were found to pass largely through areas of below average grammage. The breakline in commercially calendered newsprint passed largely through areas of above average grammage. At intermediate degrees of calendering both higher and lower than average grammages were involved . Tensile tests of 0-5 mm wide calendered newsprint samples also showed an inverse relationship between strength and grammage. The number of sulphite fibres and their angles to the machine-direction of the paper (the straining direction) were also shown to influence tensile strength.

The layered structure of paper is a necessary consequence of its method of manufacture by deposition of fibres from a low concentration suspension in water or air. It results in an extreme anisotropy of the in-plane and out-of-plane properties. Reported and estimated values of compressibility, strength, permeability of fluids and light transmission, in the two directions, are compared, although such comparisons are often only speculative in the absence of direct measurements of the properties of non-layered papers.

The total result of the layered arrangement of fibres is a material that is relatively dense and smooth, stiff, strong in tension, but liable to crease and delaminate. Such a combination of properties determines its performance in the four main areas of application-

1. For printing, it offers a compact, smooth surface, but poor processing strength
and opacity.
2. For packaging, good puncture resistance and easy creasability of board, but
poor folding endurance.
3. For hygienic and disposable products, good processing strength, but poor
softness and absorbency.
4. For barrier and filter media, good strength, but a dense packing.

Attempts are discussed that are aimed at overcoming these limitations of the layered structure.

Other sheet materials, such as felt and leather, possess a non-layered structure that strongly affects their performance. The possibilities are discussed of extending the areas of application of paper and of paper-like materials through making its structure non-layered.

MR RADVAN mentioned (in the section on page 145 of attempts to produce a three-dimensional paper structure) several different approaches to the attempts at orienting the fibres in the Z-direction with the claim of increasing the bulk. As he also mentioned, this is of the greatest interest in the production of wet-laid non-wovens.

In addition to the method of upending some of the fibres, especially the short fibres and enmeshing them vertically in the web, a further method has been discovered. ⁽¹⁾ This uses crimped synthetic short-cut fibres in such a way that only a section of a fibre is located in the Z-direction and it is not necessary therefore to upend a complete fibre. Furthermore, this does not involve any after-treatment.

A recently introduced theory of light reflectance based on a paper sheet behaving in its reaction to light as a stack of spaced parallel layers is described briefly. Utilising the theory, the contributions of cell wall thickness, lumen size, degree of lumen collapse, absorption coefficient of cell wall material and extent of interfibre bonding to the light-scattering ability of a sheet are discussed in a quantitative way. The conclusions are illustrated by examples of paper sheets in which there is clear evidence that their particular optical properties may be traced to one or more of the above parameters. Thus, for example, it is shown that to make sheets of high opacity while sacrificing neither brightness nor strength, fully bleached fibres with thin cell walls and narrow lumens are the most desirable.

The papermaking bond between cellulose surfaces can be increased markedly by pretreatment of the surfaces in a corona plasma in oxygen or air. The effect is probably due to oxidative degradation of the molecules of cellulose near the surface. Similar increase in bonding can be achieved by treatment with ozone gas. This is the basis of the Paprizone process, in which paper made from mechanical pulp is brightened and strengthened by pretreatment with hydrogen peroxide and ozone.

When two surfaces of a thermoplastic such as polythene are pressed together at elevated temperature, autohesion can take place. Pretreatment of the surfaces in a corona plasma lowers the temperature at which autohesion occurs. This effect is probably not caused by surface oxidation, since it can be produced by both oxidising and non-oxidising plasmas. Its origin is uncertain, but a possible explanation is that the discharge causes electret formation in the polymer sheet, which promotes autohesion by facilitating interdiffusion of the polymer molecules on contiguous surfaces. Autohesion of thermoplastics will be most important in processes such as pressing and calendering of sheets containing the new synthetic fibres.

Preliminary results have indicated that effects similar to those described above may be obtained by treating cellulose and polythene surfaces in a microwave plasma. The advantage of using microwave energy is that the plasma can be made to fill the entire volume of the treatment chamber, rather than being restricted to a short spark as in the case of a corona discharge.

The thermally induced bond between cellulose and thermoplastic polymers is increased also by pretreatment of the surface in a corona plasma. The effect is usually most marked when both surfaces are treated, but treatment of the polymer alone will produce a good bond. For wood/polymer adhesion, it is possible to produce a bond equivalent to that obtained by use of a conventional plywood adhesive bd pressing a thin sheet of corona-treated polythene between two sheets of wooy veneer.

Hardboards can be made by hot pressing an air-dry mixture of wood fibre with a finely powdered polymer. Treatment of the polymer powder in a corona plasma,before mixing and pressing, produces a substantial improvement in both the strength and water resistance of the finished board. It is possible to make a board of pressure-refined aspen fibres and powdered polythene, which is as strong as conventional hardboards, but which shows about one third of their dimensional instability on soaking or boiling in water.

The object of this survey is to review recent work on the mechanical properties of paper with particular reference to the role of fundamental parameters and to attempt an assessment of the current position. Precisely which parameters are to be distinguished as fundamental is of course still a major field for research, so it is hoped that a spirit of reasonableness will be perceived in the interpretations that are offered below.

As a preliminary to the survey, some physical and chemical properties of cellulose are collected together with associated properties of fibres and certain structural features of paper. In its end usage, the mechanical attributes of paper are influenced markedly by environmental factors such as temperature and the presence of moisture, but it still remains a problem to separate these effects in basic terms. Rheologically, paper has a long memory of its past history, but, its structural heterogeneity makes this memory appear erratic to the experimenter and it often yields counterexamples to embarrass theoretical developments. Nevertheless, there has accumulated a large body of data on the mechanical behaviour of paper and there has been considerable success in fitting this into a working patchwork of respectable theory that has served well in stimulating experimental discoveries, thereby providing for a continued regenerative development, which is the subject of this article.

The early work borrowed and developed rheological models from the textile industry and the subsequent avalanche of data needed to evaluate empirical parameters exposed an intricate interdependency of effects. This stimulated a retraction into the apparently safer realm of molecular theory by employing classical physical chemistry to exploit the accumulating data on hydrogen bonds. The reaction to the enthusiastic development of molecular models must have been dismay in many scientific papermakers, for valuable as they clearly were (and still are) these models said nothing about fibres, which manifestly distinguish paper from other materials. Accordingly, general attention was turned to models that appeared more faithful in a structural sense and might support a theory of fracture, which phenomenon of course was well known to be an eager accompaniment of any mechanical treatment. There emerged two principal developments, statistical geometry and linear network theories. Firstly, the former provided a clinically statistical treatment of fracture in open networks. Various refinements of the linear network theories were successful for well-behaved synthetic fibrous networks. Inevitably, they were not able to overcome the obstacle that physical properties of natural fibres depend on the structure and treatment of the sheet. So fitting factors of dubious heritage had to be used to fix the initial slope of predicted stress/strain curves and to represent fracture by a subsequent decay of that slope. Meanwhile, statistical geometry gave rise to a kind of statistical elasticity that acknowledged the fibrous structure and successfully derived from it a covariance with deformation of the network during straining. The same obstacle prevented absolute predictions (as had faced the linear theories) and fracture was not considered. On the other hand, two independent approaches to fracture that seemed to represent the mechanism in a phenomenological way depended on energy rather than geometrical considerations, but experiments show that-at least for thin sheets-the fracture process is governed by geometrical properties of the network.

As for the future of research on mechanical properties of paper, it is the view of the author that treatment of paper as a heterogeneous continuum will prevail while linear network theories will be recognised as being with little value for predicting the behaviour of real paper. There is clearly a rich field of research for organic and physical chemists both at the level of fibre-to-fibre bonds and in the treatment of bulk properties to answer the many questions that remain, about the role of hemicellulose and migration of water, about the interdependence of rheological- and thermal effects and about the extent to which the molecular structure and disposition of fibrils can influence bonding and hygroexpansivity, to name but a few.

Byway of introduction, the present state of knowledge is reviewed on the deformations of paper under uniaxial tension in the three principal directions- elongation, lateral contraction and change in thickness. Their measurement is briefly described in order to show the relationship between the experimental situation and the definition of the terms by which the deformations are characterised.

Following this, the results of tensile tests with straining cycles are presented. These show, apart from the elongation, the pattern of the cross-sectional changes and their reversible and irreversible components. Using laboratory handsheets, the effects studied were of manufacturing variables such as the grammage, the degree of refining and calendering, also of the furnish and of loading.

These investigations were supplemented by measurements on machine-made papers. It was found that the relationship between plastic elongation and total elongation is almost the same for all papers. There is also a strong similarity between the patterns of lateral contraction of the various papers, for which Poisson ratios ᵛₓᵥ = 0.16 – 0.34 and ᵛₓᵥ = 0.04 – 0.10 were found. By contrast, the change in thickness varies considerably from paper to paper and shows negative as well as positive Poisson ratios.

The comparison with results obtained on other sheet-like materials such as packaging foils and various printing substrates demonstrates the peculiarities of the deformation behaviour of paper.

Concluding the article, an attempt is made t o arrive at a general formulation of the load/deformation behaviour of paper and it is here that the significance of the investigated fundamental properties of paper for its conversion and use become apparent.

Use of paper for structural purposes has prompted research toward better utilisation of materials, determining potential stiffness and procedures to attain such stiffness.

Stiffness is dependent on thickness and a new definition of thickness is proposed that facilitates the quantification of paper stiffness with traditional engineering concepts without introducing errors caused by surface roughness.

Perhaps equally important as thickness in achieving potential stiffness is specific gravity, because the modulus of elasticity will usually vary as the cube of specific gravity if densification occurs while paper is wet.

The most efficient means for control of stiffness is restraint during drying, with a potential for threefold increases in modulus of elasticity. Even larger increases in stiffness in the order of five or sevenfold are awaiting a better means of controlling fibre orientation.

A new method is being developed for quantifying interfibre bonding that will be independent of sheet grammage and additives.

Moisture is seen as the greatest obstacle to making high performance structural fibre products because of the inability of these products to maintain reasonable levels of stiffness when wet, even when treated with synthetic resins . In spite of this problem, the potential high stiffness and strength-to-weight ratios that are available in wood fibre makes the creation of new structural products from pulp fibres inevitable.

Web failure in the papermaking and printing operations is considered as an in-plane tearing mechanism and two aspects of this-in-plane edge tear and in-plane started tear-are discussed.

A technique is proposed for the measurement of in-plane edge tearing strength and this is used to assess disc slitter performance and the potential of new approaches to slitting. It is shown that water jet slitting offers a possible alternative to conventional disc slitting, yielding edges with higher and more uniform edge tearing strength than those obtained from conventional disc slitting. The technique is used also to evaluate the influence of various edge defects on edge tearing strength. It is found that, whereas shives have no significant effect on the edge tearing strength of newsprint evaluated in this investigation, short edge cuts cause significant reductions in edge tearing strength. The strain concentration about web defects is examined using laser holographic interferometry and the observations are in qualitative agreement with those obtained with the edge tear tester.

Whereas the edge tearing strength is a measure of the force required to initiate a tear in the edge of a paper web, the in-plane started tearing strength is an energy measure of the flaw-carrying ability of the web. A pendulum tester, designed to evaluate this parameter, is used to determine how the degree of bleaching, the extent of beating and the drying history of chemical pulp in conjunction with the drainage characteristics of groundwood affect the in-plane started tear characteristics of paper. The relationship between Elmendorf and in-plane started tear is also discussed.

For a body containing a crack, the Griffith energy balance criterion for crack growth is –

G = – (∂U/∂A)

where G is the strain energy release rate for a fixed length l of the specimen, U is the elastic energy stored in the body and A is the area of the fractured surface.