Breaks of the paper web (Dry-end, Size press, Wet-end)
BREAKS of the paper web can have various causes, many of which have little to do with wet-end additives. For instance, the drives of the paper machine may be unstable or poorly adjusted. Some detective work may be needed to determine why the paper web is breaking. The location at which the web is breaking may be your first clue:
DRY-END BREAKS can result from (a) weak points or holes in the paper, (b) insufficient ability to stretch, relative to the draw applied to the paper, (c) air-handling and fluttering issues, or (d) adhesion of the paper web to tacky surfaces.
Weak points may result from variations in retention or drainage. The ability of the paper web to stretch to some degree without breaking is expected to be a function of the paper's dryness [Seth et al. 1982], so local over-drying may produce a less stretchable ("brittle") area of the paper web; this may happen, for instance, if there is a momentarily low basis weight. Alternatively, a sudden increase in the amount of fines retained in paper is likely to weaken the paper and slow the rate of dewatering. A wetter web may have insufficient tensile strength to make it through parts of the paper machine system.
Holes may be the result of biological slime becoming incorporated into the paper. Slime can be controlled by a biocide program. The paper machine system is especially vulnerable to slime in the headbox area. Since the headbox comes after the pressure screens, any slime that gets entrained into the flow is likely to end up in the paper. Biocide treatments often involve a combination of an oxidizing agent such as chlorine dioxide and one or more antibacterial agent [Edwards, 1996; Hoekstra, 1991].
Excessive adhesion of the web to dryer can surfaces sometimes results from deposition of tacky substances [Douek et al. 1997]. Tacky substances may become transferred from the paper surface and build up on dryer cans. Subsequently, the paper may adhere excessively to the dryer can surface and become torn. Papermakers often address this type of problem with the installation of doctor blades to minimize build-up of anything onto the can surfaces. Wet-end chemical approaches include the use of talc [Allen et al. 1993] and other detackifying agents. A more effective use of retention aids also is likely to keep tacky materials better incorporated into the paper so that it doesn't deposit as much onto dryer cans.
SIZE-PRESS BREAKS can result from holes or weak areas in the sheet, but they also are often related to internal sizing issues. The problem can be especially vexing on paper machines that have traditional "pond"-type size presses (not blade-metered, rod-metered, or gate-roll film-presses). Application of starch to the surface of paper has the potential to temporarily weaken the paper, especially if there is insufficient hydrophobic nature imparted by internal sizing agents. Click on the appropriate links to get information about optimization of internal sizing with rosin, AKD, or ASA wet-end sizes. Rosin sizing problems may be related to dosage of either the rosin or an aluminum compound, pH, or addition points. AKD sizing problems can be due to insufficient dosage, poor retention, or inadequate drying. Since AKD is relatively slow-curing, any wet spots (perhaps due to drops of condensate landing on the paper web) are likely to result in unsized areas. Likewise, streaks of higher moisture may fail to become well sized. Sometimes AKD sizing issues can be minimized by over-drying the paper before the size press, though such practices tend to embrittled the resulting paper. ASA sizing is less susceptible to cure problems because of higher reactivity. However, poor sizing efficiency can result if there is significant decomposition of the size before the paper is dried. Premature decomposition of ASA can be minimized by using the emulsion immediately after its preparation, acidifying the emulsion by adding alum or adipic acid to the starch solution to lower the pH, adding the ASA after the hydrocyclone cleaners, and maintaining high first-pass retention.
Breaks of the wet web of paper after the couch roll or in the wet-press section can have a variety of causes. Many causes are likely to be related to wet-web mechanical properties such as wet-web tensile strength and stretch. Excellent analyses of wet-web strength issues have been published [see references that follow]. In brief, it has been shown that the ability of a wet web to resist breakage is a function of both tensile strength and stretch, and that both of these variables are affected by moisture content. That means that any variations in moisture content after the forming section are likely to cause large variations in wet-web strength properties. Since it is impractical to adjust draws to compensate for very rapid changes in stretch of the wet-web, large variations easily can result in a web break.
A common way to increase wet-web tensile strength is to increase the softwood content of the furnish. For example, some papermakers adopt a practice of increasing softwood and/or reducing filler content when they thread up a fussy paper machine after a break. For sake of completeness, it might also be mentioned that wet-web tensile strength and toughness are favored by high surface tension of the white water. Surfactants and other materials that tend to lubricate the contacts between fibers tend to weaken the wet web.
Other factors that may cause wet-end breaks are closely related to those mentioned in the case of dry-end breaks. Deposition of tacky materials onto press felts and transfer rolls in the wet-press section can result in excessive adhesion of the paper in these areas.
Allen, L. H., Cavanagh, W. A., Holton, J. E., and Williams, G. R., "New Understanding of Talc Addition May Help Improve Control of Pitch," Pulp Paper 67 (13): 89 (1993).
Braitberg, L. D., "Controlling Pitch Accumulations in Paper Mill Systems," Tappi 49 (11): 18A (1966).
Douek, M., Guo, X-Y., and Ing, J., "An Overview of the Chemical Nature of Deposits/Stickies in Mills Using Recycled Fiber," Proc. TAPPI 1997 Recycling Symp., 313 (1997).
Edwards, J. C., "Biocides - Bug Killers that Enhance Pulpmaking and Papermaking Processes," TAPPI J. 79 (7): 71 (1996).
Hoekstra, P. M., "Fundamentals of Slime Control," TAPPI 1991 Chemical Processing Aids Short Course Notes, 55 (1991).
Pikulik, I. I., "Wet-Web Properties and their Effect on Picking and Machine Runnability," Pulp Paper Canada 98 (1): 161 (1997).
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 (firstname.lastname@example.org). 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.