1989 Volume 2

Lucas Washburn theory has been used for many gears to model the penetration of liquids into paper where the rate of penetration is a function of the balance between surface tension forces and viscous drag. Interfacial contact angle is assumed to be constant and the pore morphology is reduced to an’ equivalent cylindrical pore radius. In reality, the pore morphology in paper is extremely complex. The interactions of liquids with paper are largely determine by local variation in Young-Laplace equilibria. Thus, the rate-determining factors for penetration of liquids in paper may be the distribution of divergence and convergence in pore wall geometry and the presence of discontinuities. In this paper, the penetration of liquids into paper coatings is examined as a function of pore morphology, pigment shape, and packing order. Sorption phenomena such as the “wetting delay” observed in uncoated but sized papers is explained in terms of the difference in morphology of surface and bulk pores. Fractal assembly rules are applied to morphological subunits in pigment coatings such that the rate of liquid penetration in a continuous coating could be predicted.

Many printing papers are pigment coated in order to obtain a paper surface with superior printing properties. To achieve this it may be assumed that a coating layer of uniform thickness or mass is extremely important.

A soft X-ray technique has been developed in order to study the coating mass distribution on coated papers. In order to demonstrate the significance of this technique a wood-free and a wood-containing base paper were coated in a pilot coater. On the wood-containing base paper, blade coating with a flooded nip unit was compared with coating in a short dwell time (SDTA) unit. In the former unit, two blades of the same thickness but with different tip areas were used. The result indicated that the most uniform coating layer was obtained when the coating was performed in the flooded nip coater and with a blade of low tip area. This is interpreted as being due to a high specific load on the base paper when it passes under the blade tip, which smoothes out the irregularities in the base paper surface so that the coating film flowing out under the blade tip and the corresponding coating layer are of uniform thickness or mass.

On the fine paper the effect of the strategy of drying the coating layer has been investigated. It was found that the coating layer is redistributed during its consolidation process, which is interpreted as being due to movements in the base paper, as a result of the water pick up from the wet coating layer which swells its fibers and break bonds between them.

The coated fine paper was printed in a sheet-fed offset press and the mottle in the print was evaluated. The correlation between the print mottle and the coating mass distribution was very good.

A new approach to characterize the roughness of paper in contact with ink under compression in the printing nip is proposed. The printing roughness is calculated from the parameters of the ink coverage function contained in the ink transfer equations. The approach assumes an identity between the ink transfer coverage function and the pore shape function of the surface pores. Although the printing roughness correlates well with standard roughness/porosity tests, different regression lines result from different printing conditions . The printing roughness was found to be inversely linearly related to the logarithm of the printing pressure with the slope of the regression line representing a measure of the compressibility of the paper surface. The compressibility is independent of the printing pressure but, for rough papers, is a function of the nip dwell time.

Recent developments in industrial practice are briefly reviewed and then improvements of our understanding of calendering are reviewed under the headings: the assessment of surface properties, the compressibility of paper, the effects of calendering variables on paper properties (for hard (iron) roll calendering, soft roll calendering and temperature or moisture gradient calendering, and the effect of calendering on paper strength) and rolling contact phenomena. The Concluding Comments list the implications of these industrial and scientific developments for future technical development and research into the calendering processes.

The last 8-9 years have seen further expansions and rationalisations within the paper industry so it is not surprising to read regular reports of new finishing lines on new or modernised machines. Particularly noticeable, however, have been the numerous installations of new supercalenders for LWC grades and the development of competing uncoated filled mechanical printing grades, also supercalendered. Also, there is a trend for makers of standard machine calendered newsprint to produce higher quality mechanical printing grades which require a supercalender finish.

These developments have increased the already considerable interest in the possibility of reducing the costs of supercalendering compared in detail by Muller & Schmidt with machine calendering and with on-line soft roll calendering, using an 8 roll machine (1) . Although the analysis needs to be updated in the light of experience with modern 2 x 2 roll tandem soft calenders, not then in service, the relative importance of running costs, capital investment and production losses through down time are carefully compared. Supercalendering of newsprint produced at 1000 m/min was estimated to cost DM16/ton compared with about DM4 for conventional machine calendering, before down time losses adjust them to DM25 and 27 respectively. (At that time, the on-line alternative was apparently far more expensive) . The higher selling price of the supercalendered paper was not allowed for.

Since 1980, there has been a rapid application of on-line soft roll calendering, in place of the conventional machine calendering with hard (iron) rolls. Excluding the gloss calenders developed about 1962, there are probably nearly two hundred installations, which, it may be said, give a supercalender type finish to papers which were usually not finished that way, or which were lightly supercalendered. Obviously, there has been considerable interest in seeing how far this on-line process can go to produce conventional supercalender grades.

Before reviewing the scientific developments in these operations, it is useful to review briefly the practical improvements which have been made since about the time of the review by Peel, Kerekes and Baumgarten (2) .

In bleached, white top and coated packaging paper and board, paper gloss is a desirable property, whereas in printing papers print gloss is most desirable. One section of this paper deals with gloss development of uncoated paper and board and another section deals with coated paper. A final section addresses print gloss.

The enhanced effect of hot calendering on paper gloss and print gloss, as compared to “air leak” smoothness, is illustrated. This enhancement is due to a thermal softening con centrated at the outermost layer of the paper in contact with the highly polished hot steel roll acting preferentially on the topographical irregularities up to a few microns.

Brushing of coated papers produces some densification of the outermost coating layer, thereby filling the surface voids. Brushing improves both paper gloss and print gloss, as well as the gloss uniformity with no significant effect on the ‘fair leak” smoothness.

A brief review of pertinent works in the field of contact mechanics is made, with emphasis placed on works related to the calendering process, starting with Hertz, assumptions and ending with non-Hertzian effects, including thin-layer covered rolls, sliding, sticking, “micro-slip”, and friction effects.

Emphasis is placed on the effect that changing Poisson’s ratio has on the behavior of stress and strain in the nip and how this is related to soft-nip calendering parameters (the relative rotational velocities of soft-covered and mating rolls, for example).

High resolution color graphics of the state of strain in the nip zone are presented, as modelled by finite-element analysis of the contact problem in soft-nip calendering.

The modelling of the calendering process has been largely empirical, resulting in “creep” equations which relate the finished paper properties to calender parameters. Such modelling has the utility of optimizing calendering configurations for the attainment of a desired paper finish. This approach demonstrates the limitations of machine calendering and other alternatives to reach higher levels of surface finish are suggested.

This study endeavours to establish an understanding of the physical basis for the form of the calendering creep equation. A simple physical model of calendering has been developed which allows at least the qualitative prediction of various calendering effects. The physical model of paper compression is based on the non-Hookean behaviour of paper under compression which is known to arise from the statistical distribution of the number of fibers in a paper web. Elastic constants associated with the exponential stress-strain relation for paper determine the dependencies of caliper reduction on moisture, temperature and fiber processing. A simple viscoelastic model suggests that the dependencies on moisture and temperature may not be as autonomous as they appear in the usual forms of the creep equation. This observation is corroborated by experimental data obtained on pilot laboratory calenders.

This paper discusses the controllability of paper making. We have chosen to take a broad view on the subject . With such an approach, there are four major factors that can contribute to the over-all controllability, viz. : the process with its actuators, the measurement possibilities (sensors), the information technology and, not the least, the humans involved in the paper making operation.

In the introduction to this paper, we try to put controllability in perspective . In order to make our following discussion less abstract, we then describe three cases related to controll ability in which STFI has been involved. The first deals with CD control ; the second with closed-loop quality control; the third with wet-end/retention control . Following these cases, we then discuss the four above-mentioned components that contribute to controllability and we conclude by indicating where we believe the major development needs and opportunities lie, viz. :

– we still lack essential controllability due to inadequate process design .

– new measurement possibilities are essential in order to improve the observability and thereby controllability.

– the human organization has a key role in controllability . Full automation is utopia. The operators have an increasingly difficult task. Their motivation is important .

– knowledge-based systems is an interesting area. Such systems can be of great value, but much development work is still needed .

– it is important to increase the fundamental knowledge of paper making and to establish the knowledge in such a way that it can be used for better control .

The concepts and methodology of system identification and adaptive control in papermaking an presented and discussed. In particular the crucial problems of dead time compensation and proper interpretation of scanning gauge measurements are reviewed. Two new approaches developed recently in our laboratory to overcome these problems are presented.

The key points in extending the monovariable adaptive control schemes to the multivariable case are discussed. Successful applications of selftuning control to machine and cross-directional control of moisture and basis weight are outlined. The difficulties associated with multivariable adaptive control are illustrated by an example of colour control on a paper machine.

Finally, novel approaches to adaptive control such as genetic algorithms, neural networks, and multimodel techniques are presented. The promise of these recent advances in adaptive control is illustrated by an example of automatic compensation for species changes in kraft pulping.

Mechanisms causing instability and change are commonplace in nature. Similar phenomena exacerbate the tasks of devising, building and maintaining systems for efficiently manufacturing paper to a tight specification and high standard of uniformity.

In the case of a machine calender stack, CD control can impart stability to an otherwise unstable system. Instability of the headbox or approach flow system can however seriously affect the MD control of basis weight. Some types of instability lie outside the reach of control, and must if possible be eliminated through improved equipment design. For this reason a good understanding of instability is necessary.

This paper considers some specific examples of unstable behaviour, including eddy and vortex formation, waves and other amplification mechanisms on Fourdrinier wires, corrugation growth, uneven wear, self-excited vibration, and thermal deformation affecting calender stacks.