Tissue360 - Spring/Summer 2015 - (Page 16)

feature sheet structure Process effect on tissue Properties Analysis compares TAD, UCTAD, ADT, eTAD, ATMOS, NTT processes BRUCE W. JANDA Conventional pressed tissue processes are increasingly being supplanted by "structured tissue processes" for increased product performance. Through Air Drying (TAD) is the best known of these structured tissue processes. TAD has experienced a renaissance over the past decade. More than 30 percent of USA tissue capacity is now TAD.1 More recently, concern about energy costs, difficult retrofitting, and pressure drop due to high ash content recycled furnishes has spurred development of alternative processes to achieve structured tissue." About 50 percent of new U.S. tissue capacity (recently installed) employed some form of structured process.2 TAD and the alternative structured processes tend to share some key characteristics in both the equipment (hardware) and processes (software) for structuring tissue. This remains an area of secrecy with little product data published. However, publicly available patent information can be used to explore the similarities and differences between the TAD process and the known alternatives. TISSUE PROPERTY TRADEOFFS, OPERATING CURVES Specific tissue process efficacy in creating desirable product performance can be compared using operating curves that describe inherent physical tradeoffs based on the tissue structure. Softness versus strength. The bestknown operating curve is the tradeoff between softness and strength. Softness measurement is the subject of many papers and controversy. However, the inverse and relatively linear relationship to tissue strength has been demonstrated with both panel and mechanical softness measures. Tissue strength is frequently represented by the geometric mean average of the MD (machine direction) and CD (cross direction) tensile break strength. Most tissue is creped, which reduces MD tensile strength while increasing texture, free fiber ends, and bulk. Creping also increases MD stretch, resulting in reduced elastic modulus (stiffness). However, experience has shown that the curve can only be shifted with a major change in creping adhesion or blade Figure 1. Effects of improved formation, fiber, creping on strength versus softness. 16 Tissue360Âș Spring/Summer 2015 angle. Figure 1 illustrates the relative effects of improved formation, fiber, or creping on the strength versus softness relationship based on experience. Tissue strength/softness optimization is well known and can shift the curve only so far. TAD or structured tissue moves the curve 30 - 50 percent versus conventional dry crepe by creating a network of dense and resulting stronger areas in an otherwise un-compacted web (illustrated in Figure 2). Figure 3, taken from the patent literature, shows a side view of both the un-densified pockets and the surrounding densified network that characterize TAD or structured tissue. Figure 3, taken from the patent literature, shows a side view of both the undensified pockets and the surrounding densified network that characterize TAD or structured tissue. Structuring tissue requires the wet web to be differentially pressed or compacted with a pattern. This creates areas of high strength that surround uncompressed islands. Rush crepe from the forming to molding process tends to further create structure and Figure 2. Structured tissue can move the strength/softness curve 30 - 50 percent versus conventional dry crepe. www.tappi.org http://www.tappi.org

Table of Contents for the Digital Edition of Tissue360 - Spring/Summer 2015

A Century of Tissue: Career Experts Explore Past, Future
Sheet Structure Process Effect on Tissue Properties
Rapid Growth Puts GapCon on Global Tissue Stage
New Tissue Technologies Showcase

Tissue360 - Spring/Summer 2015