1973 Volume 1

My lord, ladies and gentlemen, sixteen years ago, we met in Cambridge on the occasion of the first international symposium organised by the Fundamental Research Committee of the Technical Section of the British Paper and Board Maker’s Association. The subject was The Fundamentals of Papermaking Fibres.

How honoured I am to be asked to address this symposium.

Most administrators are agreed that the decision on the ‘right’ amount of  money to spend on research and development is an art rather than a science. Here, I am quoting from the recent report by Lord Rothschild. Whether or not one agrees with his conclusions-and, from the fur and feathers that flew at the time of the publication of his report, it was obvious that some did not this statement is hardly likely to be challenged, as many of us have found to our cost when faced with making or advising on such decisions. In desperation, one can search for the magic formula, but all too often (when found) it turns out to be a paragram compounded in equivocation.

Not only is one faced with the question of how much to spend, but also the interrelated question of what to spend it on and where.

Three terms that refer to the distribution of local grammage in the plane of a sheet of paper are introduced-the intensity, macroscale and microscale of mass distribution. The measurement of these quantities by an optical method and by beta radiography is discussed. Data are obtained by a specially designed microdensitometer and analysed by a frequency analyser. Results are presented as wavelength spectra of mass distribution.

Experimental spectra for handsheets and machine-made sheets are compared with theoretical spectra for sheets composed of fibres having randomly distributed orientations and positions. It is shown both theoretically and experimentally that it is important to use a small measuring area.

A poor mass distribution is demonstrated to have a negative influence on the scattering coefficient and opacity of paper. It is shown how the mass distribution and other paper properties affect the visual appearance of multi-ply board and the lookthrough and print-through of paper. The concept of mass distribution is extended to the distribution of ink on paper. The connections between the mass distribution of ink and the mottle of the prints are discussed.

Finally, the adverse effect of a poor mass distribution on the strength properties of handsheets and machine-made sheets is demonstrated using recent (partly unpublished) experimental data by Cavlin & Rudström.

In any attempt to predict the durability or runnability of paper in its end uses, a means must be sought that is capable of measuring minimum strengths. Through the use of double exposure interference holography, an analysis is made of the strain variation of newsprint webs under critical tensile loading conditions. The coefficient of variation of strain is compared with formation values, which ideally should be measures of mass distribution. The QNSM, MK Systems and STFI formation testing instruments were used for this purpose and it was found that, in the case of the latter instrument, the index of determination of strain variation could reach 0-81. An assessment of the thermal uniformity of paper webs during tensile straining is facilitated by a liquid crystal thermal analysis and it is found that paper exhibits a particularly uniform distribution of temperature suggesting a constant work field model. While the influence of web defects on strength uniformity is briefly examined in this work, reference is made to a more detailed study to be found in another contribution to this symposium.

Concerning Lyne & Hazell’s paper, Formation testing as a means of monitoring strength uniformity, particularly with reference to the use of holographic interferometry, we wish to make the following comments.

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.