Pitch Problems

A deposit will be considered to be pitch-related if the key binding agent is derived from wood. The most common ingredients of wood pitch are resin acids (from softwood), fatty acids (from all kinds of wood), triglyceride fats, and various unsaponifiable materials such as beta-sitosterol. (The word unsaponifiable means that the material cannot be converted into a soap, even at high pH, as in the cases of carboxylic acids and esters.) The tacky properties of pitch can change greatly, depending on whether it has become air-oxidized, polymerized, when the temperature changes, or when it is mixed with other materials.

One of the most effective wet-end additives to combat pitch problems is talc. Talc is a platy, oil-loving mineral. Well designed equipment with good agitation is needed to disperse dry talc well enough so that it can perform its job efficiently. Though talc fillers having large particle sizes (e.g. 5-10 micrometers) are used as a filler in many parts of the world, such as the far east and Finland, it is recommended to use finely divided, high-surface-area talc products for control of pitch. The key is to add enough talc so that the overall tackiness of the surfaces in the system is reduced. A variety of pitch control additives, including products related to polyvinyl alcohol, have effects that are similar in nature to talc and effective in certain furnishes.

The pitchy nature of wood can be highly dependent on the season, the freshness of the wood chips, and the kind of pulping treatment. The situation can be tricky, since the highest tackiness usually is associated with an intermediate condition between liquid-like nature and solid-like nature. These characteristics are affected by temperature, the presence of other materials such as oils and resins, and by pH. The hardness ions, calcium and especially magnesium, often are associated with high levels of tackiness. Polymerization of wood pitch can shift the glass transition temperature of the material, so the maximum in tackiness is also shifted to a higher temperature.

Another approach used by some manufacturers of products that contain mechanical pulps is to add alum very early in the process. The purpose apparently is to keep the pitch-like material associated with the fibers, rather than allow it to float freely in the process water.

Use of an efficient retention aid program often can help to minimize pitch problems by keeping tacky materials associated with the long fibers. Freely floating pitch particles have the potential to self-agglomerate in the white water system. Also they will tend to fill the wet-press felts when they are squeezed out of the paper web, except if they are well bound to fibers with retention aid polymers.

Some further information can be found in two essays titled "Does Pitch Have Your Attention?", and "Pitch and Stickies, A Chemist's View".

References

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).

Anon., "A Primer on Pitch Problems," Tappi 62 (4): 20 (1979).

Dreisbach, D. D., and Michalopoulos, D. L., "Understanding the Behavior of Pitch in Pulp and Paper Mills," Tappi J. 7 (9): 19 (1989).

Gill, R. I. S., "Chemical Control of Deposits - Scopes and Limitations," Paper Technol. 37 (6): 23 (1996).

Kowalski, A., Bouchard, D., Allen, L., Larin, Y., and Vadas, O., "Pitch Expert. A Problem-Solving System for Kraft Mills," AI Mag. Fall 1993, 81.

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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