Producing higher quality bath and facial tissue at commercial production rates is a challenging proposition
Sam Archer and Gary Furman (Nalco Company)
The tissue maker normally focuses on the creping process, or the addition of different chemistries to enhance system performance. However, it has been known for some time that formation can have a direct impact on finished tissue quality (Schiel, 1984). One type of formation that is often overlooked is the embedded structures within the sheet.
These structures are normally associated with the wet web forming process. By utilizing photo-microscopy and FFT mathematical algorithms, these structures can be identified and their impacts on sheet properties understood and optimized.
EMBEDDED STRUCTURES. Forming a wet fiber web at current production speeds is a very dynamic process. The design of the forming fabric can create a topographical relief and fiber support structure. This surface can result in variable fiber distribution that mirrors the fabric (LaFond 2004). Each fabric, by design, will have a different surface topography and will result in different wet web characteristics.
• Dimensionally uniform webs will result from forming the sheet on a surface that has a high degree of in-plane support. The sheet can be thought of as flat, or planar, with a relatively constant thickness.
• Webs having embedded structures are typically created on forming surfaces with high surface relief. Thickness and basis weight are variable and reflect the fabric design.
Both types of sheet architectures can impact sheet strength and overall softness. Figure 1 schematically illustrates the relationship between wire design and resulting sheet structures. The formed sheets are shown in a simple stylistic fashion.
EMBEDDED STRUCTURES AND LAMINATION TO THE YANKEE. After the sheet is formed, it is transferred to the felt with the embedded structures basically intact.
As can be seen in Figure 2, embedded structures can have a significant effect on pressing and lamination of the sheet to the Yankee. Wet webs that are dimensionally uniform will be uniformly laminated to the Yankee dryer, while webs containing embedded structures will have higher pressing due to decreased land area.
Good lamination of the sheet to the Yankee dryer is critical to successful creping. With dimensionally uniform sheets, coating materials having high wet tack adhesion characteristics are critical. The web is transferred and adhered to the Yankee dryer coating through re-wet and entanglement mechanisms that establish intimate contact between the sheet and the dryer (Furman and Su, 1992).
With webs containing embedded structures, the choice of the Yankee coating system is somewhat problematic. The coating needs to be durable enough to withstand the localized pressure and hydraulic forces in the suction pressure roll nip, but also soft enough to allow close sheet contact and mechanical entanglement. Successful commercialization of any tissue product lies in the ability to modify the properties of the coating to match the process and product design (Furman, 2004).
EMBEDDED STRUCTURES AND THE CREPING PROCESS. Once the sheet is laminated to the Yankee and dried to the desired level, it is creped to produce the desired product properties. Euler’s Equation helps explain what occurs at the tip of the doctor blade. This equation defines the critical force, Pcr, necessary to fold, or buckle, a beam of any material. In tissue making the beam length, L, is the released sheet from the tip of the doctor blade to the first point of remaining contact to the Yankee dryer (Hollmark, 1983). The force is being applied parallel with, but opposite to the travel of the sheet around the Yankee dryer (see Euler’s Equation). If a dimensionally uniform sheet is tightly and uniformly adhered to the Yankee dryer, L is generally shorter and the critical force to fold, Pcr, is large. The energy that is transferred into the sheet begins to mechanically disrupt hydrogen bonds that were formed in the drying process and bulging or elastic deformation can occur (McConnel, 2004).
By capitalizing on this type of transformation it is possible to develop extremely soft, “velvety” tissue products.
If the sheet releases further above the doctor blade tip the critical force Pcr is low and folding predominates. The distance between embedded structures will define L. Sheets with embedded structures can have even crepe bars, limited by the number of CD fabric elements per unit length. The surface feel can be very silky and a few free fiber ends may be present.
The conclusion reached by using Euler’s Equation matches what seems intuitively obvious.
The sheet should and does fold at the weakest point, between the higher basis weight embedded structures.
FFT TECHNOLOGY. Although the existence of embedded structures has been known by the paper maker and scientist for years, only recently, has it been possible to view, study and understand the impact on tissue softness and strength. Reasonably priced digital imaging microscopes and image analysis software (Image-Pro‚ Plus, Media Cybernetics, Silver Spring, MD, USA) containing Fast Fourier Transform, FFT, capabilities have opened the technical door to every tissue maker. To analyze a tissue sample, a 10X-magnified formation image is captured with a digital imaging device (Figure 3B). The image is then analyzed with software utilizing FFT algorithms to find all the repeating units. This will result in a frequency spectrum (Figure 3C). The repeating units are then digitally removed from the spectrum and a new image is created that contains no embedded structures (Figure 3D). Floc and sheet defects other than those caused by the wire can be viewed at this point. A new image is then created by digitally subtracting the image without embedded structures from the original image. The result (Figure 3E) is an image showing the embedded structures that were in the original image.
CASE STUDIES. Following are two examples showing different embedded sheet structures and how the FFT technology can help identify hidden opportunities for improvement.
• Case 1: A tissue maker wanted to improve product softness in order to penetrate a market that had previously been unattainable. The stated desire was to increase the crepe bar count 45% (from 25 to 38 crepe bars per cm), thereby improving overall surface smoothness.
- Image and FFT Analysis: By comparing the images of the crepe bar structure (Figure 3A) and images of the embedded structures (Figure 3E), obtained from an FFT analysis, it was possible to assess the opportunity for improvement. It appeared that the tissue maker had already achieved at least 85% of the crepe bar generation potential. Elimination of crepe structure irregularities could increase crepe bar count by another 10%. It was also likely that more free fiber ends could be generated through high adhesion creping. The result should be a net improvement in perceived surface smoothness.
- Outcome: By trialing a softer high adhesion coating system, the tissue maker was able to improve crepe bar uniformity and achieve the 10% improvement in count. Surface feel was improved as more surface free fiber ends were observed. (Figures 3F and 3G) are 40X images used to document tissue produced during Base and Trial conditions.
• Case 2: The tissue maker owned two tissue machines (Tissue Machine 1 and Tissue Machine 2) producing the same products with the “same” process. It was unclear why TM 1 consistently produced sheets with higher bulk that were slightly less soft.
The tissue maker wanted each machine to be able to produce the same product properties.
- Image and FFT Analysis: Digital formation images of tissue from each machine (Figure 4A and 4B) clearly showed that there were significant differences between the sheets. By applying the FFT technology it was possible to visualize (Figure 4C and 4D) the embedded structures that were integral to the sheet. TM 1 had a very well defined and pronounced embedded structure, while TM 2 had a more uniform and finer structure. The bulk of the creped sheet and lower softness values, from TM 1, was probably due to the magnitude of the embedded structures and their impact on the creping transformation.
- Outcome: Operations personnel at the tissue facility are now considering different fabric designs as an important element of improving the consistency of products produced from the two machines.
SUMMARY:
• Focusing effort on understanding and improving the way the wet web is formed and optimizing the creping process will result in improvements in quality and productivity.
• Image capture and FFT technologies can be utilized as tools to build an understanding of the impacts of different embedded sheet structures on the creping transformation and production of quality tissue. •
References:
Furman, G. S. and Su, W., “ A review of chemical and physical factors influencing Yankee dryer coatings,” Nordic Pulp and Paper J., Vol. 8 no. 1, 217-222, 1993.
Furman, G., Grigoriov, V., Su, W., Kaley, C., “Effects of Modifying Agents on Adhesive film Properties – Findings Toward Improved Yankee Coatings”, Tissue World Americas, Miami Beach, FL, September 21-23, 2004.
Hollmark, H., “Mechanical Properties of Tissue”, In the Handbook of Physical and Mechanical Testing of Paper and Paperboard, Vol. 1, editied by R.E. Mark, pp. 497 – 521, 1983.
LaFond, J., “Tissue Forming Fabric Design and Application”, TAPPI - 2002 Tissue Runnability Short Course, New Orleans, LA, May 21-23, 2002.
McConnel, W., “The Science of Creping”, Tissue World Americas, Miami Beach, FL, September 21-23, 2004.
Schiel, C., “Economic and Technical Aspects in the Production of Lightweight Sanitary Tissue – Between Headbox and Yankee”. Das Papier, Vol. 38, no. 9, pp. 417 – 429, 1984.
Acknowledgements:
The authors would like to thank the following people:
Dr. Ross Gray (Nalco) for developing the FFT tissue application technology.
Jeff Herman (Albany International) for his help in providing useful wire images and his thoughtful review of this work. •