1981 Volume 2

The relationship between ‘handle’ and the bending properties of paper and paper-board is investigated statistically. The handle of Paperboard was recognised to be explained by bending stiffness. In contrast,  the handle of paper is complicated by bending stiffness alone. The highest correlation coefficient is obtained for paper by applying the formula Sm²/d where Sm is the maximum bending moment at the curvature of 2.5cm⁻¹ and d is the thickness of the paper.

A simple method of observing the liveliness of paper is also proposed.

I would like to draw your attention to the fact that the fibres of which paper is made retain their individuality in the sheet, even in dense papers.

In many experiments, conditions are such that the discontinuous structure of paper has little chance to show. This is why the modulus of elasticity, for example, comes out the same whether it is determined by bending, by a sonic method, or by stretching in a tensile tester of standardized inertia. Such experiments assure us that the laws of continuum physics are consistent and that it is still worth reading and studying them.

The aim of the present investigation is to find out those variables of the paper-making process that cause variation in the light transmission of paper, independently of basis weight variation. Furthermore, is intended to discover under what conditions and to what accuracy the distribution of areal mass (formation) can be characterised by the areal distribution of light transmittance. The study is carried out by measuring values of beta-ray transmission and light transmission at exactly the same points of paper samples using an aperture of 1 mm diameter and analysing the correlation of the results.

At this stage of the study the effects of furnish composition, beating, wet pressing, and calendering on the correlation between mass distribution and distribution of transmittance have been analysed. It can be seen that at least prolonged beating and heavy calendering change the distribution of transmittance in such a way that the optical formation measurement does not give a true picture of the distribution of mass.

The factors which are responsible for the surface reflection of coated paper at specular angles have been explored by means of models of specular reflection and of coating roughness. The model of specular reflection used is based upon the addition of the roughnesses which arise from independent sources, i.e. the roughness caused by the base paper on the one hand, and by the coating pigments on the other. Coating surface roughness is approached theoretically by the incorporation in a model of the influences of particle shape and size, size distribution and the hypothetical statistics of particle position at the surface.

The theoretical predictions have been tested with the aid of experimental data, on specular reflection, Hunter gloss, profilometric roughness, and particle size. The samples studied were different types of coated paper, blade-coated on a pilot scale.

Multidimensional scaling, a statistical technique that permits separation and identification of the principal factors used by people when judging differences and preferences between pairs of test stimuli, has been adapted to the subjective evaluation of print quality. Numerical values of the factors involved in subjective print quality evaluation are used to establish the relationship with corresponding physical print qualities and related paper properties. Information is also generated concerning the preferences and reliability of each judge and the degree to which each judge agrees with other judges in a professional group.

Multidimensional analysis of the subjective evaluation of wire-mark in solid letterpress prints indicates that the degree to which lines appear in the wire-mark pattern is as disturbing as the overall wiremark intensity.

Mottle, show-through, contrast, and paper colour are found to be of importance in the judgement of stereo letterpress print quality; while mottle, liming, show through, and set-off are found to be significant in polymer plate letterpress printing trials. Physical tests are compared for their ability to predict these subjective print quality factors.

A fluid penetration technique is described which enables measurements to be made of penetration rate into coatings, of total pore volume, and of the subsequent permeation rate of fluid through the film. From these measurements, the number of pores and their length and diameter can be calculated assuming a simple model of pore structure. The lengths of pores calculated in this way are shown to be unrealistically large and a more complex model structure is postulated involving large and small pores serially connected. In order to determine pore sizes for this model it is necessary to generate an additional equation and this is achieved by measuring air permeation through the film. Pore sizes determined in this way are comparable in magnitude with those measured by other techniques.

Evidence is offered which suggests that air flow through coatings is molecular rather than viscous and that fluid pressures within the coating pore structure can be negative.

An image analysis technique was developed by means of which the mass distribution of paper was characterised by measurements of the light transmitted through it. To establish the ability of the image analyser to assess the mass distribution accurately, comparisons were made with the established method of beta-radiography. On the basis of these comparisons it was concluded that the small scale mass distribution can be assessed by image analysis of the light transmitted through paper.

The remaining aspects of the study demonstrate applications of the image analysis technique which include: analysing the variation in the mean and variance of the optical densities of different commercial newsprint samples: investigating the relationship between the mass distribution of newsprint and machine type: and comparing this with other methods used to evaluate mass distribution and formation of paper.

The mechanical equilibrium of two fluids separated by a curved interface requires the existence of a hydrostatic pressure, Δp between them; this is given by the Laplace equation:
Δp = -yc

Experimental and analytical techniques are discussed for the application of image analysis to the measurement of fibre length, width, coarseness, and curl. It is shown that automated and manual procedures agree well and the crossed fibre problem can be reduced to insignificance by appropriate sample preparation procedures and the use of a curl factor as a recognition function for crossings. Image analysis is used to characterise the introduction of curl in kraft pulp fibres by high consistency beating, and the removal of curl in the hot disintegration of refiner mechanical pulp.

Much has been done in studies of the light and beta-ray images of paper for formation and Mass Density Distribution respectively. This paper presents the comparison of the images obtained by the two methods using random data analysis techniques, such as filter theory, auto-correlation functions, transfer functions and coherence. Only objective measurements will be used so that visual optical illusions will not confuse the results. Types of paper to be studied will include calendered and uncalendered papers.