1973 Volume 1

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.

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.