Wet Web Strength

The ability of the never-dried web of paper to resist breakage can be critical to process efficiency, especially in the case of traditional paper machine systems in which the paper web passes unsupported between the couch, various wet-press felts, and between dryer cans.

Though it is sometimes true that low wet web tensile strength may be correlated with a high incidence of breaks after the couch or within the wet-press section, it is important to keep in mind that the problem is inherently two-dimensional. That is, the likelihood of web breakage is usually a function of both the wet-web tensile strength and the ability of the web to stretch. As more and more water is removed from the paper web, the tensile load to failure rises, but the percent stretch to failure falls. True success of any measure to improve wet-web performance has to show a combination of improvements in tensile load and stretch to failure.

One of the most sure-fire ways to increase wet-web strength is to increase the proportion of long fibers, such as softwood kraft or CTMP fibers. The down-side of this approach is that such fibers will tend to make the paper somewhat more floccy, and a nonuniform sheet is expected to fail at its thin areas when subjected to strength testing. Wet-web strength may show modest improvements due to increased refining. Bent or curled fibers are reputed to contribute greater stretching ability to a wet web, which could possibly be an advantage in some situations (but not desirable in terms of dimensional stability of the paper).

Low or variable wet-web strength is sometimes due to effects of surface-active agents. This makes sense when you consider that fibers in a wet web are mainly held together by surface tension forces, operating through meniscuses. The capillary forces are proportional to the interfacial tension, and that tension tends to be reduced by any surfactants. It is possible that the surfactants also act to lubricate the surfaces of the fibers, allowing them to slide past each other, thus lowering the wet-web tensile strength. This type of problem often can be reduced by identifying the major source or sources of surfactants, decreasing dosages of surfactants where appropriate, or improving washing processes where appropriate.

References:

Seth, R. S., "The Effect of Fiber Length and Coarseness on the Tensile Strength of Wet Webs: A Statistical Geometry Explanation," Tappi J. 78 (3): 99 (1995).

Seth, R. S., Barbe, M. C., Willimans, J. C. R., and Page, D. H., "The Strength of Wet Webs: a New Approach," Tappi 65 (3): 135 (1982).

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (m_hubbe@ncsu.edu). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.


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