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Scanning Electron Microscopy Measurements of the Surface Roughness of Paper Sujit Banerjee,* Rallming Yang, and Charles E. Courchene Institute of Paper Science and Technology, School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 500 Tenth Street NW, Atlanta, Georgia 30332-0620
Terrance E. Conners Department of Forestry, UniVersity of Kentucky, Lexington, Kentucky 40546-0073
A new approach to the measurement of x-y uniformity of the surface of a paper sheet is described. Scanning electron micrographs are taken of both the top and bottom surfaces of a paper sheet and image-analyzed. The images were converted to grayscale, and the standard deviation of the pixel brightness was called the “SEM roughness index” and calculated for each surface. Both commercial newsprint sheets and handsheets made with kraft, TMP, and recycled fibers were examined. Debonders and cationic polymers were added to some sheets. The addition of debonders increases the index on the top side of the sheet but decreases it on the bottom. This is caused by the movement of fines from the top to the bottom side. The addition of cationic polymers increases the SEM roughness index by increasing the degree of microfloc formation. Samples taken across a reel from a commercial paper machine tend to show a mirror image relationship between the top and bottom surfaces. The SEM roughness index is able to detect subtle changes in sheet structure caused by differences in the mode of addition of polymers used for retaining fines in the sheet. Introduction Many measures of uniformity can be applied to a sheet of paper. For example, optical and radiographic means can be used to determine the density variation of the sheet.1-5 Surface smoothness (and roughness) is another index of uniformity that is conventionally measured by a contact method involving a stylus, for example. We reasoned that it should be possible to obtain quantitative information relating to surface uniformity with scanning electron microscopy (SEM), which has been valued for its apparent 3-D views of surfaces. Topography is well-known to affect localized brightness values in scanning electron microscope images.6 In a sheet of paper, fiber edges are highlighted by the production of secondary electrons that are gathered by the electron detector. Similarly, recesses in the paper surface appear as darker wells from which few secondary electrons escape. Hence, very flat surfaces lead to images with uniform brightness levels, whereas rougher surfaces will have a greater amount of brightness variation. With this understanding, we recognized that analysis of the brightness values of the individual pixels in SEM micrographs could represent a new type of noncontact surface uniformity indicator when samples and data acquisition conditions are comparable. This Article describes the SEM technique and relates changes in grayscale uniformity to the surface structure of paper. Because papermaking is a complex process involving the interaction of fiber structure with dewatering physics and chemical modifiers, we examined the surface effects of debonding agents and retention aids in papermaking operations. Debonders are used to increase the bulk softness of papers (especially tissue) by decreasing the sheet stiffness and tensile strength, while retention aids are used to increase the fraction of fines remaining in the sheet after drainage during papermaking. Papers examined * To whom correspondence should be addressed. E-mail: sb@ gatech.edu.
included laboratory-prepared handsheets made from commercial kraft and recycled fiber furnishes, and commercial newsprint samples. Materials and Methods Paper Samples. Bleached softwood kraft pulp (from GeorgiaPacific, Palatka, FL) was disintegrated for 15 000 revolutions in a British disintegrator. The freeness was 515 mL of CSF. Cationic polyacrylamides (c-PAM) PL 2320 and PL 2510F from Eka Chemicals were added after the handsheet mold was halffull of pulp. Handsheets were made in a British handsheet mold equipped with a 100-mesh wire. For the debonder work, handsheets were made with mixtures of thermomechanical pulp (TMP) obtained from Augusta Newsprint, Augusta, GA (410 mL of CSF) and the bleached softwood pulp identified above. The pulps were mixed in different ratios and disintegrated using a British disintegrator for 5000 revolutions and diluted to a solids level of 0.3%. Tetraethyl ammonium bromide debonder was added at 0.5 and 1 wt % (dry basis) and mixed for about 1 h at 49 °C. All handsheets were made according to Tappi T205.7 For the recycle study, the bleached softwood kraft pulp was recycled three times. Analysis Method. Scanning electron microscopy was conducted with a Hitachi S-800 instrument operated at 12 keV. The angle of the scanning beam with respect to the sample surface was 90°. Paper samples were mounted on a stub, coated with gold deposited in a sputter coater, and photographed at either 25× or 100× magnification. Ten locations were randomly selected for image acquisition from different areas of each sample; that is, 10 images were acquired per stub. Each image for every sample was photographed under identical conditions and working distance (18 mm) to permit comparisons. To produce SEM images with the appearance of realistic topography, an electron beam traverses the surface of the vacuumcontained sample in a raster pattern. The impinged surface points in turn emit secondary electrons, and these secondary electrons are collected by a fixed in-column detector. Because of this
10.1021/ie900029v CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
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Figure 1. An SEM micrograph of a hardwood kraft handsheet and the associated histogram of pixel brightness levels. 0 ) black, 255 ) white.
Figure 2. Images of the top and bottom surfaces of a 1:1 kraft:TMP mixture with debonder.
spatial arrangement, the angle between the electron beam and locations on the sample surface will vary slightly; as an analogy, consider looking at the surface of the earth from a tethered balloon, where the viewing angle is a function of the height of the balloon and the size of the area being surveyed. For our analyses, these angles are very small because SEM samples are only a few millimeters across and because the working distance is less than 0.75 cm. SEM micrographs were digitized to 8-bit grayscale images (black ) 0, white ) 255) using Image J software from the National Institutes of Health.8 The pixel brightness data for each image were plotted as 256-level histograms, where the x-axis represents the 0-255 grayscale levels and the y-axis represents the pixel frequency. As an example, Figure 1 shows an SEM image of a hardwood kraft handsheet and its associated histogram. Areas with pinholes or other noncharacteristic “problems” were not included in our analyses because they were clearly outside the normal range of material variation. The histograms from all 10 images were combined and averaged to minimize noise and the influence of random variations due to minor sheet nonuniformity at the microscopic level. The uncertainty determined from several triplicate measurements was less than 1.5%. Results and Discussion The location of the histogram peak (at a grayscale value of 95 in Figure 1) was considered to be the dividing line between the darker and lighter pixels. The lighter areas of the image tend to represent fiber mass, whereas the darker regions relate more to void space; consequently, the position of the peak can change with changes in the brightness of the image. The peak position did not appear to be a sensitive indicator for surface smoothness or lack thereof. We noted that most of the histograms appeared to be symmetrical or only slightly skewed. Hence, we calculated the standard deviation of the mean of the
pixel brightness values and evaluated these standard deviations as a measure of surface uniformity. (Skewness and kurtosis measurements may be appropriate for some materials.) For this application, we named the standard deviation the SEM roughness index. The uncertainty in the index was less than 4% for all of the analyses conducted in this study. Micrographs taken at 25× and 100× provided similar results. To determine whether specimen orientation affected the histograms, a commercial newsprint sample was photographed with its machine direction aligned with the electron detector in the SEM, and also with its machine direction aligned at 90° to the detector. Ten micrographs were taken at each orientation, and histograms were prepared as for the other samples. No differences in the SEM roughness index were found between the histograms taken at these perpendicular orientations, which confirms that specimen orientation does not influence this measurement technique. Effect of Additives: Debonder and Retention Aids. Sheets were prepared from mixtures of kraft and TMP pulp with and without tetraethyl ammonium bromide, a typical debonding agent.9 Micrographs of the top and bottom surfaces are illustrated in Figure 2, where the high fines content of the bottom surface is apparent. SEM roughness values obtained from the top and bottom surfaces of these sheets are shown in Figure 3. Note that the addition of debonder increases the SEM roughness for the top surface but decreases it for the bottom. The probable reason is the loss of fines from the top surface, which would increase the average pore size and thereby decrease uniformity. The corresponding increase in fines on the bottom surface would decrease the SEM roughness. Hence, changes in the SEM roughness index match the visuals in Figure 2 quite well. The change is smaller on the top side because not all of the fines removed from the top surface are deposited on the bottom; some of the fines drain right through. The SEM roughness index
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Figure 3. Effect of debonder on the SEM roughness index for kraft and TMP mixtures. The left and right panels are for the top and bottom surfaces, respectively. Figure 4. SEM roughness index for PM1. Table 1. Effect of c-PAM on the SEM Roughness Index PL 2320
PL 2510F
kg/tonne
top
bottom
top
bottom
0 0.32 0.45 0.68
29 33 39 39
33 41 41
29 33 38 40
34 37 40
Table 2. Effect of Recycling on the SEM Roughness Index times recycled
SEM roughness index Unrefined 675 mL of CSF
0 1 2 3
64 62 61 61
Figure 5. SEM roughness index for PM2.
Refined 250 mL of CSF 0 1 2 3
61 53 53 54
essentially allows one to quantify the effects visible in Figure 2 and provides a rationale. Values for the SEM roughness index for kraft pulp handsheets with and without c-PAMs are reported in Table 1. These polymers act as retention aids; that is, they attract and retain fines to the fiber mat during drainage of the sheet. They also promote fiber flocculation, which reduces uniformity.10,11 As a result, the SEM roughness index rises upon c-PAM addition. Results for the two polymers are similar as shown in Table 1; changes in the top and bottom surfaces are also quite similar. Once again, the changes in SEM roughness can be easily rationalized from standard retention behavior. Effect of Recycling. SEM roughness index values after each step are provided in Table 2. No significant change in the index is seen upon recycling the unrefined fiber. For the highly refined fiber, the SEM roughness drops after the first recycle and remains relatively constant thereafter. Recycling changes the properties of fiber in many ways;12-14 for example, fiber length and flexibility are affected, and it is difficult to ascribe the changes in the roughness index to any of these. However, a recent study proposed that refining occurs in three discrete stages.15 Refining bleached softwood fiber down to 360 mL of CSF removes the primary cell wall and S1 layer while the S2 layer begins to swell. Next, internal delamination occurs within the S2 layer between CSF 360 and 220 mL as confirmed by scanning electron microscopy. The onset of delamination is sudden; dramatic changes in fiber structure occur at about CSF 360 mL. Finally, fiber destruction occurs below 220 mL of CSF. It is possible that fiber destruction occurs over the first recycle.
This would substantially change the nature of the furnish and affect the surface properties of the resulting sheet. Samples from a Newsprint Mill. A reel sample from paper machine 1 (PM1) of a commercial newsprint mill was divided into 16 equal segments across the reel. Images were taken (at 100×), and the resulting SEM roughness index values for the 16 locations across the reel are plotted in Figure 4. Note that the top and bottom profiles are roughly mirror images of each other, which means that any nonuniformity on the bottom surface is offset by that on the top, and vice versa. SEM roughness index values of the top and bottom surfaces were averaged, and the results are provided in the top panel of Figure 4. The average variability is clearly lower than that of either surface because of the cancelation of uniformity variations. This mirror image relationship is likely caused by drainage effects as in the case of the debonder experiments discussed earlier. Some component of the furnish, unknown at present, must be moving from the top to the bottom surface. The source of the drainage effect is not known, but it could relate to nonuniformity in the headbox. Analogous SEM roughness index measurements taken from a second paper machine (PM2) on a different day are illustrated in Figure 5. Here, the variability is much smaller, and there is no obvious mirror image relationship. Yet, the average variability (top panel) is much lower than that in either of the lower panels, confirming that some cancelation of variability occurred. An opportunity to evaluate the practical application of the SEM approach was provided during a mill trial where the mode of addition of retention aids to the stock stream was optimized. As before, reel samples were taken from 16 equal segments, and the SEM roughness index profiles across the reel before and after the optimization are illustrated in Figure 6. There is a clear decrease in variability for the top side as well as for the averaged top and bottom profiles. The optimized mixing was
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Literature Cited
Figure 6. Effect of retention aid optimization on the SEM roughness index.
expected to provide a more uniform concentration of the retention aid across the headbox. While this was not picked up by conventional techniques, the difference was detected by the SEM roughness approach. Conclusions We have developed a new measure of sheet uniformity that is able to detect small changes in x-y uniformity. It detects changes induced by additives such as retention aids and debonders. The approach can measure relatively subtle changes across a reel of commercially manufactured paper. Because the measurement must be performed in a laboratory, the technique is unlikely to find use as a real-time quality control tool. However, it does identify relatively subtle contributions to x-y uniformity and can be useful in analyzing the outcome of process changes, such as that discussed for the retention aid optimization. Acknowledgment This study was funded by a consortium consisting of Stora Enso, Eka Chemicals, Abitibi-Bowater, and the State of Georgia through its TIP3 program. We thank Uma Udayasankar for taking some of the SEM images.
(1) Cresson, T. M.; Tomimasu, H.; Luner, P. Characterization of paper formation, Part 1: sensing paper formation. Tappi J. 1990, 73, 153–159. (2) Keller, D. S.; Pawlak, J. J. β-Radiographic imaging of paper formation using storage phosphor screens. J. Pulp Paper Sci. 2001, 27, 117–123. (3) Paavola, A. Image analysis technology opens a new perspective on paper formation. Proceedings. 61st Appita Annual Conference and Exhibition, Gold Coast, Australia, 23-27, 2006. (4) Tomimasu, H.; Kim, D.; Luner, P.; Minsoo, S.; Esworthy, C. Comparison of four paper imaging techniques: radiography, electrography, light transmission, and soft X-radiography. Tappi J. 1991, 74, 165–176. (5) Wahjudi, U.; Duffy, G. G.; Kibblewhite, R. P. An evaluation of three formation testers using radiata pine and spruce kraft pulps. Appita J. 1998, 51, 423–427. (6) De Silveira, G.; Forsberg, P.; Conners, T. E. Scanning electron microscopy: A tool for the analysis of wood pulp fibers and paper. In Surface Analysis of Paper; Conners, T. E., Banerjee, S., Eds.; CRC Press: Boca Raton, FL, 1995. (7) TAPPI Forming Handsheets for Physical Tests of Pulp, Test Method T 205 sp-06; Tappi Press: Atlanta, GA, 2000. (8) Rasband, W. S. Image J; National Institutes of Health: Bethesda, MD, 2004; http://rsb.info.nih.gov/ij/. (9) Sun, T.; Lindsay, J. D. Reactive compounds to fibrous webs. U.S. patent 6,322,665, 2001. (10) Hartley, W. H.; Banerjee, S. Imaging c-PAM-induced flocculation of paper fibers. J. Colloid Interface Sci. 2008, 320, 159–162. (11) Huber, P.; Pierre, C.; Bermond, C.; Carre, B. Comparing the fiber flocculation behavior of several wet-end retention systems. Tappi J. 2004, 3, 19–24. (12) Hubbe, M. A.; Venditti, R. A.; Rojas, O. J. What happens to cellulosic fibers during papermaking and recycling? A review. Bioresources 2007, 2, 739–788. (13) Tze, W. T.; Gardner, D. J. Swelling of recycled wood pulp fibers: effect on hydroxyl availability and surface chemistry. Wood Fiber Sci. 2001, 33, 364–376. (14) Wistara, N.; Young, R. A. Properties and treatments of pulps from recycled paper. Part I. Physical and chemical properties of pulps. Cellulose 1999, 6, 291–324. (15) Walsh, F. L.; Banerjee, S. Characterization of thin water layers in pulp by tritium exchange. Part 2: Effect of refining on water absorption. Holzforschung 2007, 61, 120–124.
ReceiVed for reView January 7, 2009 ReVised manuscript receiVed February 28, 2009 Accepted March 2, 2009 IE900029V
