1989 Volume 3

NA

The effect of industrial beating on tensile strength properties has been compared with laboratory beating and the differences observed have been explained in terms of fibre properties.

Tensile strength development is less pronounced for industrial beating than for laboratory PFI-mill beating. Beating in the laboratory Escher-Wyss conical refiner was found to develop strength to an level intermediate between those of the other two.

The fibre characteristic causing these differences was found to be the ability of the fibres to transmit load in thesheet. Fibre deformations introduced during pulping and bleaching are to a large degree removed by PFI-mill beating. Industrial beating shows very little effect in this respect.Tensile strength development could be explained wholly by changes in fibre swelling (water retention value) and in the ability of the fibres to transmit load. The latter property was evaluated by zero-span measurements on rewetted sheets.

In this overview forming covers all processes from the dilution of thick stock into a mix, using recirculated white water in the short circulation, to the dewatering in the wire section.

Grammage non uniformity in the paper web is to a predominant degree generated by the forming process, and especially the small scale variations summarized in the term formation. The term mass formation is recommended when only grammage is considered and not the optical impressions there of. The forming process also generates the main part of the large scale variations, that is the MD-,CD-and residual grammage variance.

Mass formation has traditionally been evaluated using beta radiography, combined with micro densitometry or image analysis. A new technique involving the direct recording of electron beam transmission is underdevelopment, with promises off aster processing, perhaps even on-line, and high geometrical resolution. Characterization techniques based on the co-occurrence matrix, applicable to image analysis, can be a useful complement to the traditional power spectra techniques.

It has recently been conclusively demonstrated that in flowing fibre suspensions, flocs are kept together by the bending forces of interlocked fibres. To study the dynamic behaviour off lowing fibre suspensions, modern video techniques, high speed movie pictures and image analysis are applied.

To improve grammage uniformity, the mix should be fed to the headbox directly after the dilution of thick stock with white water. No processes like screening or cleaning, from which uncontrolled reject fibre flows are drawn should be allowed in the short circulation. Further the consistency of each material component in the recirculated white water should be controlled, if the content of the different components in the sheet produced is to be held constant.

First pass retention is shown to be a badly defined retention value, and should only be used in comparisons for one paper machine when no changes of the material content in the long circulation occurs.

In the headbox, the tapered manifold is the commonly used means for achieving the coarse distribution across the machine width. From the cross distributor, a tube package can lead either to a stilling chamber, or directly into the out let nozzle. The nozzle has to be fed using maximum open area to avoid flow instability, and the nozzle contraction must be large enough to reduce velocity streaks and relative turbulence to an acceptable level. Mathematical models are now being applied to the calculation of water flow patterns in headboxes.

Local slice lip adjustments, especially on headboxes without let nozzles of low convergence angle, can cause considerable sideways flow on a Fourdrinier wire, and this will have a large effect on the CD grammage variations as well as on the final sheet anisotropy profiles.

There are two basic headbox designs for stratified forming. In one of these, thin,pointed vanes separate the different furnishes. In the other, thicker separation walls generate air wedges, which may separate the furnishes up to the actual starting point for dewatering. In the first case, layer mixing can start already at the headbox, while in the latter four new surfaces between air and mix jets are created, all four being potential sources of disturbance generation.

High consistency headboxes have been developed, with which paper is formed according to an extrusion process. To reduce mix flocculation, various channel shapes are used inside the headboxes.

Sheet build-up generally takes place according to a filtration process, which has an inherent self healing effect. Therefore, the mass formation of a laboratory sheet is better than that of a random sheet. For a machine-made sheet, the comparatively high mix consistency causes floc generation, which may result in a worse large scale mass formation than that of the random sheet.

When evaluating the mechanical and optical characteristics of a machine made paper sample, its properties relative to those of a laboratory sheet from the same furnish may be expressed as the Forming Efficiency.

The Kozeny-Carman equation describing flow through porous beds can not be used to predict filtration dewatering rates during web forming. This is because of the gradual compression of the web by the dewatering forces, the closing of some pores, turbulent flow situations etc. Dewatering capacities must so far be predicted using empirical equations, and parameters evaluated on the basis of dewatering experiments.

The development of forming wires has led to multi-layer designs where both the paper side and the wear side can be optimized simultaneously.

Pressure pulses in hydraulic headboxes are detrimental to Fourdrinier dewatering, since attenuation due to standing wave generation on the wire can create large MD grammage variations.

In Fourdrinier dewatering, several new dewatering elements have been developed, allowing a better control of the activity in the mix on the wire, and thus also of the mass formation of the web formed.

In conventional twin-wire forming,the dewatering pressure is generated by wire tension according to one of two basic principles: roll dewatering with constant or blade dewatering with pulsating dewatering pressure. A combination of these two principles may result in an improved combination of mass formation and retention. Recently a new method for the generation of dewatering pressure has been demonstrated, in which the pressure along the forming zone can be controlled freely, since it is generated by application of local forces and thus not by wire tension.

Multiply products manufactured through simultaneous forming are now used for low grammage products. The problem is to achieve acceptable layer purity as well as layer mass formation. Controlled pressure pulse dewatering could provide the means to reach optimum dewatering conditions.

The influence of forming conditions on product properties is a vast area within which two subjects are discussed: the interrelationship between mass formation and paper strength and finally fibre orientation anisotropy.

The properties of paper are defined on two sampling scales, depending on whether the micro- or the macroscopic physical characteristics of the sheet are of interest. Two models are presented for the structure of homogenized paper, based either on the orientational distribution of the fibres or on that of the interfaces of pores and fibres . Orientational anisotropy arises on the scale of the fibre aggregates through shearing between flocs. Accurate tests were performed of the floc anisotropy, which influences the properties of paper on the size scale of a few mm2.

This contribution is based on the work of Eerd Palokangas, a special student at the Institute of Paper Science and Technology. His investigation was concerned with the effects of formation on hard and soft nip calendering.

Four levels of formation were produced namely poor, medium, good and excellent for a 45g/m’ newsprint furnish. The first three levels were obtained by varying the formation consistency on a pilot plant former, while the sheets with excellent formation were produced on a Formette Dynamique. The same method of wet pressing and drying was used for each set of sheets, however important differences between the web formed sheets and Formette Dynamique sheets were later discovered.

The level of formation or type of calendering did not appear to significantly effect sheet roughness at various levels of sheet densification, as measured by Parker Print Surf. Losses in elastic and strength properties were found, and were higher for hard nip calendering. Interestingly, strength losses were higher for sheets having excellent formation when compared with those having poor formation.

Investigation of this unusual result revealed that the poorly formed sheets produced on the web former, did not have the same initial level of internal stresses (drying stresses) as the sheets with excellent formation. This was determined by rewetting the poorly formed sheets, and drying them under full restraint (as was done with the sheets having excellent formation) . The elastic constants were then measured, and found to be in close agreement with those measured on sheets having excellent formation.

From this limited study we conclude that: the higher the initial level of internal stresses, the greater the reduction of strength properties will be when the sheet is subjected to calendering. Therefore, attention has to be paid to the initial level of internal stress, when strength properties change as a result of calendering are being investigated.

Below are appended some general comments on SESSION 8 “CONTROLLABILITY”, and some implications for the development of control systems, with particular reference to the phenomenon of chaos.

There are many interesting aspects and the examples of process modelling and identification in the papers of this session. The complexity of the papermaking process and hence the requirements of an effective control system are very clearly illustrated. There is a vast useful literature on control theory and applications.

The total process performance and its profitability must determine the role of the control system. The subject can be presented as a series of facets developed in an orderly manner. First there are some basic theoretical definitions such as controllability (in its specialised technical meaning), observability, sampled data systems, instability and chaos. After that, there is the process performance which needs to be considered: as a design or static system; as a dynamic operating system; and quality assurance. It is also important to encompass mill wide performance.

NA

NA

Dr. D.A. Page mentioned in his review of the beating of chemical pulps: “If a unified simple theoretical treatment can be made from a complex phenomenon, it will then have the following three advantages:

• professors can teach it
• students can learn it,
• engineers can practice it”.

Against this statement I feel the Symposium has been successful. The visionary evaluation of the opportunities and challenges of the future papermaking by Mr. R.C. Williams started the Symposium with high spirits. His call for increased cooperation between paper companies in supporting the development of new technology warrants full attention.