Proceeding Articles

A theoretical model relating the print density and mottle of solid prints to the optical properties of the paper, the ink, the amount of ink and the unevenness of the ink distribution in the print has been developed.

The model differs from previous models in that the effects of speckle are included and in that the effects of the variables mentioned above can be clearly distinguished. The model is compatible with the assumption that stochastic processes govern the ink distribution and that the intensity of these processes may be related to the structure of the paper. Only a brief review of some results is given.

The means of defining formation in terms of its subjective appearance was explored by processing the transmitted light signal from various paper samples on an analogue computer using equations related to physiological attributes. Some psychological aspects of the interpretation of images are discussed and the relevant aspects of both of these is described. It is demonstrated how this approach was implemented in an instrumental technique to quantify hitherto subjective assessments of quality.

The statistical significance of the aspects of microformation and macroformation is explained and practical analysis is achieved by dissecting an area into strips. The value of each strip is derived from the content of the microformation, these values are then analysed for mean and standard deviation to assign numbers for both the microformation and macroformation. The instrument outputs a weighted value of the sum of the two figures on a continuous sampling basis.

The instrument has been used in the laboratory for sample testing,in particular cigarette tissue, also on a papermachine making tracing paper, where a continuous record is produced.

The textural properties discussed include surface softness and bulk softness in tissues and towelling, handle in printing and writing papers and drape in non-wovens. These properties are discussed in terms of individual measurable factors and complexities are shown. Inadequacies of objective test methods are described and the necessity for subjective judgment noted. A brief account is given of psycho-physics in the field of tactile perception and examples are given of its basis and application in subjective testing. Tactile perception is shown to provide a high degree of accuracy and precision.

The results of psycho-physical scaling experiments have shown that a property such as softness can easily be scaled like other subjective variables such as loudness and brightness. They have also shown that bulk softness and surface softness should be treated separately.

In search for physical measurements correlating with the subjective properties, one has to proceed along slightly different pathways. For bulk softness, a known physical property-bending stiffness (or, more correctly, inverse bending stiffness)- has been chosen as a good substitute for subjective bulk softness.

An instrument using a ‘synthetic fingertip’ has been developed for measuring surface softness. The basic principles of the system, as well as recent experimental results are reported.

Most attempts to quantify features of paper involve the use or creation of scientific instruments. This contribution claims attention to an entirely different approach in which human judgments can be organised and structured to provide viable quantification that aligns with practical assessment in production and in the market place. Three of the many applications are given as examples.

Cellulose has the environmental advantage that it is bio-degradable and that paper made from it is readily recycled by repulping. It is perhaps surprising that cellulose keeps as well as it does and that some books keep for centuries. In spite of much research work, it is impossible to predict the permanence of individual samples really precisely from artificial ageing tests, though the factors to affect ageing are fairly clear. The quality of the furnish has always been known to be important. Acidity has been studied for a long time and the permanence of existing books can be improved, at a cost, by neutralisation. Storage temperature is important, but has been largely ignored, although it could be corrected in many existing libraries.

The thermal stability of papermaking pulps under conditions of accelerated ageing was examined. Degradation was found to be extremely complex and influenced both by the technical processes of papermaking and by the experimental methods used. At moderate temperatures as used for accelerated ageing, the folding endurance test was a sensitive measure of degradation. Methods of reducing the variability of the test were considered and a new technique capable of handling widely different results was developed and evaluated.

Several papermaking pulps were artificially aged and the resultant loss of strength attributed to the interaction between a gradual fibre strength reduction and a rapid increase in interfibre bonding. Excessive initial interfibre bonding decreased handsheet stability. Softwood sulphite and kraft pulps were more stable than a birch kraft pulp, but the behaviour of sulphite pulps varied greatly. The presence of lignin or of high hemicellulose content was not of itself sufficient to cause rapid ageing.

The yellow colouration that occurs when a cellulose material is exposed to heat and light is considered to be an indication of its degradation.⁽¹⁻³⁾ By using diffuse reflection spectra techniques, we have been able to determine the absorption bands that appear when cellulose is heated in darkness. We have also been able to demonstrate the reversibility of the colouration effect, as this disappears if the cellulose material is exposed to daylight at ambient or low temperature.

Photorestoration can be used for obtaining a higher degree of whiteness than in the initial state; in addition, the colouration effect returns if the unprocessed cellulose initially subjected to light action is placed away from light (post irradiation effect) and it is more intense if the temperature is increased.

The bleaching of cellulose materials by light has been remarked by several research workers, ⁽⁴⁻⁷⁾ but, to our knowledge, the photochromic nature of these colour reactions was not demonstrated following these observation. During the course of our investigations, we have noticed that this phenomenon occurs for various cellulose materials whether it be lignified cellulose such as newsprint or cellulose in the most pure form we have been able to procure.

Probable causes for the deterioration of the physical properties of paper as a result of repeated handling have been determined. A flexing test has been devised that evaluates the relative durability of paper.

Investigations with the scanning electron microscope indicate that the fibrillar component of paper deteriorates during flexing and is probably responsible for the decline in stiffness as well as the modulus of paper. Because of the ever increasing use of automatic document handling equipment, stiffness retention with handling is essential, since limp documents are difficult, if not impossible, to process automatically. Unfortunately, stiffness declines rapidly as paper is flexed or handled.

The rates of deterioration for all other physical properties of paper during flexing are independent of each other and vary from one paper to another. The rate at which certain properties deteriorate is independent of the quality of the paper. No correlation has been found between any paper property and durability.

The influence of humidity and temperature on the change in elastic properties of single fibres and the corresponding papers are shown to be similar.

Furthermore, expansional changes due to variation in the atmospheric conditions are shown to be related to the elastic properties of the sheet . For thermal expansion in paper, the product of coefficient of linear expansion and the elastic properties is approximately constant in all directions. A similar relationship is also followed in hygroexpansion. In the experiments reported, a slightly higher value in the machine direction was obtained for the product of the dimensional changes and the elastic modulus.

It is also noted that the geometric means of the dimensional instability and elastic properties in machine and cross directions coincide with the same properties of the isotropic sheet.

Hygrodimensional changes for single fibres were measured by means of following the rotational twist of the fibres caused by sorption. It is deemed from these experiments that the fibre swelling is proportional to the moisture regain.

Finally, it is shown that the viscoelastic properties of cellulose material change in an irregular manner during changes of the moisture regain. These effects cause considerable reduction in the creep to failure time (lifetime) of paper in compression or tension under transient atmospheric conditions.