VIEWS: 6 PAGES: 18 POSTED ON: 8/19/2011
DENSITY-RELATED PROPERTIES IN 2-LAYER STRATIFICATION. Abstract. This article describes results of 2-layer stratification of through air dried, creped and uncreped (flat) sheets on a slow speed (40 fpm) pilot paper machine, using two different, "strong and weak" model systems, one consisting of refined and unrefined NSWK the other NSWK and Eucalyptus. The study dealt with the effects of BW, composition, and layer orientation on creped and uncreped (i.e. flat) strength, density, bulk, and water absorptive capacity. Surprisingly, stratification showed no benefit in improving density- related properties; stratified sheets had the same density, bulk, and water absorptive capacity as homogeneous sheets of similar strength and composition. However, compared to the homogeneous controls, stratification altered the strength of the creped web. The direction of the change depended on the orientation of the layers; the drier side layer lost 30% more, while the air side 30% less than the unstratified homogeneous control. These findings would most likely apply to the more popular 3-layer stratification. Furthermore, they imply that stratification would not improve any other density-related tissue product properties, like, for instance, sheet flexibility and bulk softness. However, the findings do not contradict the well-known overall softness and economic benefits of 3-layer stratification. The article also shows how to compare products of different Basis Weight using corrected Breaking Length and corrected Bulk/BW, since these properties are not BW-independent. Introduction. In the last 2 decades stratification has become widespread as a technique for making premium grade tissue. The use of three different layers has provided considerable economic benefit by allowing cheaper furnish to be placed in the middle layer, and also afforded a means for increasing the product softness, or at least surface smoothness, through the use of superior furnish components in the outer layers. These benefits, while seldom documented systematically in the Literature, are intuitively obvious. It is, however, by no means clear how stratification affects other important properties, like strength, cushioniness, flexibility, bulk, and water absorptive capacity. The present work as undertaken to quantify the effect of stratification on strength, bulk, water absorptive capacity, and density, at a wide range of Basis Weights and at different layer orientations. To reduce the number of variables the study was restricted to 2-layer stratification using two different, "strong and weak" model systems, one consisting of refined and unrefined NSWK the other NSWK and Eucalyptus. The study was carried out on a slow speed (40 fpm), through air dried (TAD) pilot paper machine. Since the TAD method allowed the easy removal of the sheet before creping, both creped and uncreped (flat) sheets could be studied separately. The present work utilizes several seldom-used testing methods and calculating procedures. Instead of apparent density, based on product thickness, it uses true density, determined by the oil saturation method. Since oil only penetrates into the basic portion of the sheet, it is not influenced by the pattern imparted by creping. This method, aside from being more accurate, allows comparison between creped and uncreped sheets. The second novel method relies on the use of "-corrected Breaking Length" to compare strength at different BWs. Regular BL tacitly assumes that the TS-BW relationship is not only linear but the two variables are directly proportional to each other. Its calculation simply divides TS by BW. Since, as has been found by several researchers (xx) and is very clear from this work that there is a considerable BW-intercept (dubbed the -value), ignoring it in the usual BL calculating procedure leads to considerable error when comparing products of different BWs. The concept of -corrected BL takes this fact into consideration and requires that the -value be subtracted from the appropriate BW before BL is calculated. Similarly, Bulk does not directly proportional to BW, something implicitly assumed when Bulk/BW is used to compare products at different BWs. Bulk/BW in fact is highly dependent on BW. The BW-intercept (dubbed the -value) for the Bulk-BW relationship is extremely high. The concept of -corrected Bulk/BW takes this fact into consideration and requires that the -value be subtracted from the appropriate BW before Bulk/BW is calculated. These calculating procedures allow comparison of properties for the wide BW range and layer composition in the study. While the results may not be immediately comparable to other works, the extra accuracy of these procedures is a compensation. Experimental Procedure. The runs have been carried out on a 14 in wide pilot paper machine in a trough air dried mode at 50 fpm, employing 2 layer stratification and a model furnish of "strong" and "weak" components. Two different systems have been studied, a refined and unrefined Irving pulp (as the strong and weak components, respectively), and an Irving-Santa Fe Eucalyptus system. The experimental conditions are shown in Tables 1-4. Results and Discussion. Tensile Strength-Basis Weight for homogeneous sheets. As mentioned above, the study was carried out on 2 different model furnish systems, both modeling mixtures of a strong and a weak furnish. The first such system consisted of a refined and unrefined Irving pulp, to stand for the strong and weak components. The second system utilized a refined Irving pulp for the strong, and an unrefined Santa Fe Eucalyptus as the weak component. The test results for the first system are given in Tables 1 and 2, for the second in Tables 3 and 4. Since the findings for both systems were surprisingly similar, I will be discussing exclusively the first system. Uncreped sheets. Strength-BW data for the 3 different homogeneous sheets, 100% strong, 100% weak, and 50-50 mixture are shown in Figure 1. As can be seen, for all 3 compositions tensile strength vary linearly with Basis Weight. The relation, however, is not direct proportionality; there is a considerable x-intercept, in accordance with several other researchers who noticed that (1,2)…The relationship has the form: Equ.1 GMT=a*(BW-) Where "a" is the slope, a is the intercept. I shall be referring to this intercept as the "" value. It probably represent the minimum BW required to form a coherent load-bearing layer. It is especially important for tissue weight sheets for which failure to take it into account can lead to considerable error when using the BW-normalized strength, or Breaking Length. The concept assumes that the slope of the relationship is constant for all points, and the line goes through the origin. In this article, I will use the real slope, or the "-corrected Breaking Length", which is obtained by subtracting the -value for each composition from Basis Weight. Hence: Equ. 2 BL =(GMT)/(BW-) As can be seen, "a" from equation 1 is the BL. This value now is, of course, BW-independent and can be used to characterize the furnish strength. While the most commonly used unit for BL is meters, in this article, for ease of handling and comparison, I will use mixed units, in which GMT is in oz/in and BW in lbs/ream. The conversion to meter is easy; multiplying by 658.42 will yield meters. I am sure the wisdom of my usage will become obvious. In the rest of the article BL will mean BL in mixed units (mu). As can be seen, there are 3 distinct lines for the 3 compositions. All cut the axis at approximately 5 lbs/ream. Since all 3 compositions contain the same fiber, one would expect the same -value. By and large this is so, the regression values are 5.19, 4.25, and 5.08 for the strong, weak, and 50-50 mixture, respectively. The corresponding BL's (the actual slopes) are BLs = 4.99 mu, BLw 1.77 mu, and BLm 3.47 mu (where the subscipts "s", "w" and "m" refer to the strong, weak, and mixed compositions, respectively. The ratio of BLs between the strong and weak furnish, (BLs)/ (BLw) is 4.99/1.77, or 2.81. It should be pointed out that the weight average law of mixing applies quite well, the calculated BL for the 50-50 mixture is 3.38 mu. This compares quite well with the actual number of 3.47 mu. Creped sheets. Figure 2 shows the same relationship for the creped sheets. Once more, the 3 distinctly different lines are obtained for the 3 compositions. The -values are 3.43, 5.03, and 5.68. These should be closer to each other and maybe slightly higher than the corresponding uncreped values, but the data is not adequate to yield greater accuracy. Therefore I have used these values to calculate the BLs. They are 2.45 mu, 0.44 mu, and 1.36 mu. For the mixture, the weight average law predicts a BL of 1.44 vs. the actual value of 1.36, a fairly good agreement. Interestingly, strength breakdown due to creping has changed the ratio of BLs between the strong and weak furnish from 2.81 to 5.57. In other words, strength loss in creping was higher in the weak component. Before leaving this section, I would like to point out the difference between the conventional, uncorrected BL and -corrected BL, using the conditions for the mixed composition. These only vary in BW, hence they should have the same BL. Conventional BL values are 594, 695, and 742 for the 3 BWs. The corresponding BL values are 878, 912, and 890. Obviously, the latter are far more constant for all BWs. Tensile Strength-Basis Weight for stratified sheets. 50-50 strong-weak mixtures. In the original series of runs, the 50-50 furnish mix was run in at 3 different BWs in 2 different layer orientation, strong layer on top (strong-weak, designated as "s-w") and weak layer on top (weak strong, designated as "w-s"). The data for these is given in Table 1, (Run Numbers 10, 14, 17, 18, 22, and 25). When TS-BW was plotted, somewhat surprisingly there was a considerable difference between the 2 orientations, the s-w line was well above of the w-s line; I had expected that the 2 orientation would be similar to each other, and to the 50-50 homogeneous sheets. Owing to the experimental design, the 2 orientations were run on different days. Despite all efforts, the strength of the 2 furnishes changed from one day to the next, casting some doubt as to the validity of these findings. To be absolutely certain, the 2 series were rerun on the same day at 4 different BWs (Runs 25-32), and tested in duplicates. The results for these are shown in Figures 3 and 4 for the uncreped and creped series. Figure 3 shows the tensile strength-BW relationship for the uncreped sheets. The data points fall on two distinct lines for the 2 orientation. This time without a doubt the s-w line was above that of the w-s line. The BL for the former (calculated using weighted average of the -values) was 3.92, the latter 3.42, a 15% difference in favor of the s-w orientation. Even more surprising, this difference became more magnified for the creped sheets, as shown in Figure 4. The two orientations fall on 2 distinct lines. BLws was 1.92 mu, while BLsw was 1.2 mu, a difference of 58% While not shown in a graph form, the curves for the 2 series were quite similar to those found for the original, suspect ones (Run Numbers 10, 14, 17, 18, 22, and 25). Obviously the difference is real. I have found no immediately obvious explanation for the difference between the uncreped points. For now accepting it as it is, there is still the extra widening of the gap during creping to account for. Clearly, the weight average law cannot explain the difference, since the overall composition for the two orientations is the same. One way to explain the difference is to assume that the layer against the drier side loses strength differently than the air side. If the magnitude of the different "extra strength loss" (ESL) for the 2 cases were known, the weight average law would still apply. The following is a description of such an approach. Let's assume that the layers against the drier side breaks down to a strength level which is different than the strength of the pure components (i.e. BLs and BLw). Using subscripts "d" and "a" for air-and-drier side, we can set up the following equations: Equ. 2: BLsd =BLs *ESLd Equ. 3: BLwd =BLw *ESLd Equ. 3: BLsa =BLs *ESLa Equ. 4: BLwa =BLw *ESLa The terms BLsd ,BLwd ,BLsa ,BLwa are the "true" strengths of the different layers, and can now be used to calculate total strength, suing the weight average law: Equ 5: BLsw = 0.5*BLsa + 0.5*BLwd Equ 6: BLws = 0.5*BLwa + 0.5*BLsd Solving the above equations we obtain the following final results: ESLd = 0.725, and ESLa = 1.435. This means that when a layer is against the drier side, its strength will be 27.5% less (i.e. 100-72.5%) than that of the 100% pure component creped homogeneously, while any air side layer will be 43.5% higher than that of of the 100% pure component creped homogeneously. To show that these are the correct values, let us use real numbers: BLswcalc = 0.5*2.44*1.44+0.50.44*0.725 , which yields BLswcalc=1.91, which is identical to the value obtained for BLsw. Similarly, BLwscalc = 0.5*2.44*0.725+0.50.44*1.435, which yields BLwscalc=1.2 , which is identical to the value obtained for BLsw. The above treatment, somewhat simplistically assumed that the 2 different layers break down uniformly within each layer, hence only one number for the drier side and one for the air side. In practice, there is most likely a gradient of breakdown. Furthermore, it ignored the 15% difference, already existing between the uncreped sheets. Still, the success of these to explain the discrepancy is heartening. It should also be clear that the values for ESLd and cannot be entirely ESLw arbitrary. To see this, let us try to apply the above values to a homogenous case, assuming that they can be thought of as consisting of 2 layers, one air side and one drier side. Using such a procedure for BLs BLscalc = 0.5*1.99*0.725+ 0.5*1.99*1.435 we obtain BLscalc = 2.174, compared with the actual value of 1.99 for BLs. The error is about 8 %. Had I used the pair of values 0.7 and 1.3 for ESLd and ESLa there would have been perfect agreement between the calculated and real values for the homogeneous case. Thus it can be concluded that for the particular paper machine used the drier side layer will lose about 30% more than that of the 100% pure component creped homogeneously, while any air side layer will be 30 % higher than that of the 100% pure component creped homogeneously. Undoubtedly, these values will be different for different paper machine, but the direction of the differences will be the same. One further point emerges from the above discussion. Since there is a difference between drier and air side strength, actually probably a gradient, this means that for a homogeneous sheet, and probably for most stratified as well, there is also a density gradient. And since the drier side layer is most likely weaker, its density will also be lower. Strength-density relationships. We have seen that there can be a change in the tensile strength-BW relationship between homogeneous and stratified sheets of the same composition. While this is of considerable theoretical interest, an even more important question is whether stratification can alter the important strength-related tissue properties. Since bulk, water absorptive capacity, and some components of softness (bulk softness, possibly flexibility, but not surface softness) all depend inversely on density, it is instructive to compare the BL-density relationship of homogeneous and stratified sheets. Figures 5 and 6 shows such relationships respectively for uncreped and creped sheets for all compositions. As may be seen in both Figures, there is an excellent correlation between BL and density; all points fit the regression lines extremely well. Furthermore, all points, homogeneous and stratified, are so close to the regression line that the only conclusion that can be made is that stratification does not affect the BL-density relationship, for a given strength, both homogeneous and stratified sheets have the same density. Interestingly, and somewhat surprisingly, as can be seen by comparing Figure 5 and 6, the creped and uncreped lines are fairly similar, at the same density they yield about the strength for the uncreped and creped sheets. This means that creping does not affect the basic sheets structure. These same two conclusion may be drawn from the unit water absorptive capacity (UWA)-strength relationship, which will be discussed in the next section. It should be pointed out that, while the strength-density is generally a linear relationship when strength is varied by a constant method, refining in this instance, it is by no means necessary that the same relationship should hold for different compositions, nor even when strength is changed by breakdown during creping. The fact that it is the case for refined and unrefined pulp merely means that by changing the ratio of strong and weak components, we vary strength as well. It is not clear why creping breakdown should also be along the line. Water absorptive capacity-strength relationships. As mentioned above, unit water absorptive capacity (UWA) depends inversely on density, hence directly on Breaking Length. Figure 7 shows the UWA-BL relationship for both uncreped and creped sheets. As has been the case for the density there is an excellent correlation between BL and UWA; all points fit the regression lines extremely well. Furthermore, all points, homogeneous and stratified, are so close to the regression line that the only conclusion that can be made is that stratification does not affect the BL- UWA relationship. As for the strength-density relationships, at the same strength water absorptive capacity is quite similar for the creped and uncreped sheets; in fact there is a slight advantage for the uncreped sheets. This shows that creping does not affect the underlying sheet structure by de-densification, though it obviously imparts shaped bulk and stretch. Bulk-strength relationships. It may be recalled that the tensile strength-BW relationship was described by a linear relationship as given by equation 1 as GMT = a*(BW-), which means that GMT is directly proportional to BW. Actually, a somewhat similar relationship exists for TWA-BW relationship. TWA is not directly proportional to BW, there is a y-intercept. However its value is so small that it could be neglected for the purpose of this work. One can, however not neglect a similar intercept for the Bulk-BW relationship. The relationship is shown in Figure 8. While the relationship is linear, it shows an extremely pronounced y-intercept, Bulk0. Its value is 135, 160.3, and 137 TMI Bulk units respectively for strong, weak, and 50-50 mixture. This intercept means that for the strong furnish, the Bulk at zero BW would be 135. Conventional Bulk/BW calculation, however, implicitly assumes that the intercept is zero, hence when used to compare sheets of different BWs it is in error. This can be seen, for instance, by looking at the strong series. The only difference among the 3 conditions is BW. The density of these points is quite constant. The conventional Bulk/BW values for the 3 conditions are 16.96, 11.5 and 10.07. Clearly, we cannot use conventional Bulk/BW values for comparisons across BWs. To see how correct comparison may be obtained, we can use the same thought process as for BL. The actual Bulk-BW relationship has the form: Equ. 3 Bulk-c = b*BW, or Equ. 4 Bulk=b*(BW+), where "b" is the slope and is the BW intercept. Bulk is now directly proportional to (BW+). We can call Bulk/(BW+) as -corrected Bulk/BW, or (Bulk/BW). Clearly, Bulk/(BW+) is now independent of BW. To see this, let's calculate Bulk/BW again for the 3 strong points, using the appropriate -value of -31.62 (the BW-intercept in Figure 8. We obtain 4.82, 4.21, and 4.28, fairly constant values. This not surprising, since the Bulk-intercept at zero BW may be thought of as the contribution of shaped Bulk to total bulk. Subtracting this amount from actual Bulk figures yields basic, weight-bearing, water-absorbing Bulk, and the corresponding Bulk/BW is counterpart of density. If density is constant, so should be Bulk/BW. In fact, while not shown in graphical form, there is an excellent correlation between corrected Bulk/BW and density. In the rest of the discussion Bulk/BW will mean (BW+). Plotting it against BL (-corrected) we can compare homogeneous and stratified products for all BW range. This is shown in Figure 9 for creped strong, weak, 50-50 homogeneous, and 50-50 strong-weak and weak-strong stratified sheets. For the stratified sheets the average of all 3 homogeneous -values for used in the calculations. As can be seen, stratified and homogeneous sheets fall on the same line, showing that stratification has no benefit in Bulk generation. Conclusions. The present study, using 2-layer stratification and the simplest possible furnish system of the same refined and unrefined furnish has shown no advantage for stratification for water absorptive capacity, bulk, or density. There is no particular reason to believe that using the far more widespread 3-layer stratification, or different type of furnish components would alter these conclusions. However, the findings do not contradict the well-known overall softness and economic benefits of 3-layer stratification. Figure 1 200 150 100 50 0 0 5 10 15 20 25 30 35 40 dat a points fo r strong regression line for st rong dat a points fo r weak regression line for weak dat a points fo r 50-50 mixture regression line for 50-50 mixt ure Figure 2 100 80 60 Tensile Strength 40 20 0 0 5 10 15 20 25 30 35 40 Basis Weight data points for strong regression line for strong data points for weak regression line for weak data points for 50-50 mixture regression line for 50-50 mixture Figure 3 140 120 100 Tensile Strength 80 60 40 20 0 0 5 10 15 20 25 30 35 40 Basis Weight data points for strong-weak regression line for strong-weak data points for weak-strong regression line for weak-strong Figure 4 60 50 40 Tensile Strength 30 20 10 0 0 5 10 15 20 25 30 35 40 Basis Weight dat a point s for strong-weak regression line for st rong-weak dat a point s for weak-st rong regression line for weak-st rong Figure 5 3500 3000 2500 Breaking Length, m 2000 1500 1000 500 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 Density, g/cc data points for strong (homogeneous) data points for weak (homogeneous) data points for 50-50 mixture (homogeneous) all data points regression line for all points Figure 6 6000 5000 4000 Breaking Length, m 3000 2000 1000 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 Density, g/cc dat a point s for strong (h omogeneous) dat a point s for weak (ho mogeneous) dat a point s for 50-50 mixt ure (ho mogeneous) all dat a point s regression line for all point s Figure 7 8 7 6 5 UWA g/g 4 3 2 1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 BL, m dat a point s for crep ed sheets regression line for creped sheet s dat a point s for uncrep ed sheets regression line for uncreped sheet s Figure 8 Bulk vs. BW 600 600 31.69 500 400 300 Bulk 200 100 0 0 100 200 40 30 20 10 0 10 20 30 40 BW St ron g Regression Line 50-50 Mix Regression lin e Weak Regression Line Figure 9 10 9 8 7 6 Bulk/BW, TMI units 5 4 3 2 1 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Breaking Length, m Strong 50-50-Mix Weak Regression Line 50-50 strong-weak 50-50 weak-strong Calculations. Homogenous sheets, using "strong" component as an example: (BL)s =(GMT)/(BW-s), [(Bulk/BW)]s =(GMT)/(BW-s), Stratified sheets of any composition, using "s/w" as an example: (BL) s/w =(GMT)/(BW-[s* fs, + w* fw]), Stratified sheets for 50-50 composition only: [(Bulk/BW)] s/w =(Bulk)/{BW-[(s+ m +m)/3]}