ème Congrès annuel, Section technique, ACPP th Annual Meeting, Technical Section, CPPA. Paper strength - paper formation relations

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1 ème Congrès annuel, Section technique, ACPP th Annual Meeting, Technical Section, CPPA Paper strength - paper formation relations Jean-Philippe Bernié and W.J.Murray Douglas Department of Chemical Engineering Pulp & Paper Research Centre McGill University, Montréal ABSTRACT The relationship between some physical properties of commercial newsprint, g/m 2, and paper formation was determined. A new method in formation measurement was used which provides the nonuniformity of formation over a range of scale of formation.5-45mm. MD and strength is influenced strongly by formation nonuniformity over just a narrow band of scale of formation, 1 to 25mm. Between 2/3 and 3/4 of the variability in tear strength is determined by formation nonuniformity in this 1-25mm range. RÉSUMÉ Nous avons étudié la relation entre des propriétés physiques et la formation de papier journal de grammage g/m 2. Nous utilisons une nouvelle méthode de mesure de la formation qui nous permet de mesurer l intensité de la non-uniformité de la formation en fonction de l échelle de formation, de.5 à 45mm. La corrélation entre la force de déchirure et la formation est de l ordre de R 2 variant entre.63 et.78 pour une échelle de formation de 1 à 25mm. Introduction Various properties of paper which determine its value are dependent in part on the quality of formation. The extent of nonuniformity of formation can be determined by time consuming and relatively expensive radiographic techniques. In common practice, light transmission image analysis provides a fast, convenient and acceptable alternative to radiography. The coefficient of variation of local grey level is accepted as a satisfactory approximation of the function of local grammage, the definition of formation number. A number of commercial test instruments provide an index of formation proportional to a grey level based formation number. A shortcoming of such techniques is that, while giving a measure of how much nonuniformity of formation there is, they provide no information as to how this nonuniformity is distributed across the sheet. This shortcoming applied also to an improvement in formation determination we reported earlier [1]. Papermakers have long used words such as cloudy, grainy, mottled to recognize quantitatively the importance of the nature of the distribution of formation nonuniformity. Sheets may have the identical formation number yet display a clearly different distribution of formation nonuniformity, thus being of quite different formation. Determination of paper nonuniformity Using in the present study a 1x1mm field of view, we provide a quantitative measure of the distribution of formation nonuniformity [2], evaluated over a range of scale of formation about.5 to 45mm. Presentation of the intensity of formation nonuniformity as a function of scale of formation implies a line, termed the Paper Formation Line. For the research reported here the formation nonuniformity was determined at eight values of scale of formation:.5, 2, 4.8, 1, 12, 2.5, 35 and 45mm. Thus for each sheet the Paper Formation Line was determined for the formation nonuniformity at these eight values of scale of formation. In this way it is possible to determine over what range of scale of formation a particular paper property is being influenced by the quality of sheet formation.

2 Application to paper strength A major Canadian paper company provided us with samples from 6 different production runs of newsprint, grammage g/m 2, along with determinations of, MD tear, MD tensile and MD stretch of this paper. In order to have a statistically representative sample, 1 images of each sheet were taken. For each image we determined the nonuniformity of formation as a function of scale of formation over the range.5 to 45mm, i.e. the Paper Formation Line. The results for the 1 images of each sheet were then combined to produce the Paper Formation Line representative of that sheet. For every paper property we then determined the correlation with our measurement of formation nonuniformity, evaluated at each of the eight values of scale of formation noted above. There was no significant correlation for MD tensile and MD stretch. For, the results are given on Fig.1. It is evident from the R2 values that the -paper formation relationship is very much stronger over an intermediate range of scale of formation than at smaller or larger scales. This effect is seen explicitly in the plot of the R2 correlation values from Fig. 1 as a function of scale of formation, presented as Fig. 2; the equivalent results for MD tear appear as Fig. 3. Discussion of results From Fig. 2 and 3 it is apparent that there is a narrow plateau region over which the R 2 correlation values are remarkably high, in the range This plateau region occurs in a well defined region of scale of formation, between about 1 to 25mm. Thus the quality of formation over the regions of scale of formation.5 to 1mm and 25 to 45mm have little effect on MD and strength. As factors other than formation also affect tear strength, it is impressive to obtain a measure of formation which indicates that from 2/3 to 3/4 of the variability in tear strength is caused by formation. It would be expected that in relations between paper properties and formation quality, the range of scale of formation that is important would vary with both the type of paper and the particular paper property. This expectation has now been demonstrated. Thus for the tear strength of newsprint it is the quality of formation in the 1-25mm range of scale of formation that is of prime importance. For the case of the print quality of uncoated offset paper, it is by contrast the quality of formation in the 4-8mm range of scale of formation that we found recently to be determinant [2]. Moreover, by identifying the scale of formation range that is important, the print quality - formation correlations obtained were about twice as high as those obtained when formation was determined with several commercial test instruments. Thus the widely held view in the industry that the dependence of print quality on paper formation is higher than that indicated by current formation test instruments has been shown to be correct. In fact, the dependence of print quality on formation is about twice as important as previous formation measurements had indicated. SUMMARY As curent paper formation test instruments provide no information concerning how formation nonuniformity is distributed across a sheet, it has not been possible to examine quantitatively the relation between the distribution of formation nonuniformity and various paper properties. The recent appearance of this capability [2] permits determination of the dependence on the distribution of formation nonuniformity for specific properties of specific grades of paper. The present study establishes that it is the quality of formation in the 1-25mm range of scale of formation which influences significantly the MD and strength of g/m 2 newsprint. Over the 1-25mm range of scale of formation, the R 2 correlation between tear strength and formation nonuniformity is notably high, in the range , indicating that 2/3 to 3/4 of the variability in tear strength comes from this component of formation. By contrast we reported recently [2] that it is the nonuniformity of formation in the 4-8mm range of scale of formation which exerts a major influence on the offset print quality of uncoated fine paper. With no overlap in the range of scale of formation influencing these different properties

3 of different grades of paper, 4-8mm in one case, 1-25mm in the other, it becomes apparent that it is only with the determination of formation nonuniformity as a function of scale of formation that the effects of formation on specific properties of specific grades of paper can be determined. Moreover, because the correlations of paper properties and formation quality are exceptionally strong when determined for the specific range of scale of formation concerned, this procedure becomes highly advantageous for use in a control loop for adjustment of papermaking parameters and forming section operation to optimize end-use properties such as printability and strength properties. These properties are now shown to be considerably more dependent on formation nonuniformity than had previously been thought based on current formation test instruments. REFERENCES [1] Bernié, J.-Ph. and Douglas, W.J.M., Local grammage distribution and formation of paper by light transmission image analysis, Tappi Journal, 79 (1): 193 (1996) [2] Bernié, J.-Ph. and Douglas, W.J.M., Exploration of the print quality-paper formation relation, 1997 Tappi Process and Product Quality Conference: 73 (1997) ACKNOWLEDGEMENTS The present work was supported by the Network of Centres of Excellence (N.C.E.) program of the Canadian government. (Paper presented at the 84th Annual Meeting of the Technical Section, CPPA, Montréal, Jan. 98) (Article présenté au 84ème Congrès Annuel de la Section Technique, ACPP, Montréal, Janvier 98)

4 .8 Coefficient of correlation Scale of formation, mm Fig. 2: /scale of formation correlation.8 Coefficient of correlation Scale of formation, mm Fig. 3: MD tear/scale of formation correlation

5 21 31 y = x r 2 = y = 53.92x r 2 = Scale of formation:.5mm Scale of formation: 2mm Scale of formation: 4.8mm 8 y = x r 2 =.49 y = x r 2 = Scale of formation: 1mm Scale of formation: 12mm y = 7.446x r 2 = y = 12.2x r 2 = Scale of formation: 2mm 25 Scale of formation: 35mm Figure 1: Effect of scale of formation on the formation nonuniformity - relation