Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use


Composition: When papermakers use the word "pitch" they are talking about tacky materials that usually contain resins from the wood. Other materials are likely to be present as well. Wood chips from different types of trees are likely to contain from about 1 to 5% of wood pitch. Usually a significant part of this pitch consists of fatty acids, and, in the case of softwood chips, resin acids. Either of these acids is easily converted to semi-soluble soap form as the pH is raised to the neutral range. A major portion of the pitch is likely to consist of non-ionic triglyceride esters of fatty acids; treatment with strong alkali, as in the case of kraft pulping and bleaching, is expected to hydrolyze the ester bonds, forming acid salts and glycerin. Wood pitch also contains non-saponifyable, oily substances such as beta-sitosterol. Materials from other sources tend to associate with pitch in the same agglomerates, deposits, and spots in the paper product. Susbstances that tend to associate with wood pitch include latex (from repulping of coated broke), components of defoamers (especially ethylene-bis-stearamid), and talc (often used as a pitch-control agent).

Function: Most of the resin contained within a paper sheet causes no difficulty either to the producer or to the user of the paper. Deposition of even a fraction of a percent of the pitch onto papermaking equipment can force an operation to close down for frequent, intensive cleaning ("boilouts") or take expensive measures to control the pitch.

Strategies for Use: Pitch problems can be very complex due to the many different hydrophobic materials that can associate with each other in a paper machine furnish. For this reason, it is highly recommended to do a chemical analysis. This may make it possible to determine the most likely root causes of the problem. One of the most general strategies for avoiding pitch problems is to keep the tacky materials bound to the fibers. This is one of the ideas behind a common practice of adding alum during or immediately after mechanical pulping. The alum complexes with any soaps of resin acids of fatty acids, and the presence of these complexes can serve to bind the multi-component droplets of pitch to the fiber surfaces. Addition of highly cationic polymers such as polyethyleneimine (PEI) to the process can have a similar effect. The relative turbidity of filtrate obtained from different points in the process can be used as an indication of the probable effectiveness of such treatments with cationic materials. It is very common also to add talc or another detackifying agent to coat tacky particles and minimize their tendency to form large agglomerates, deposits, or spots in the product. An effective retention aid system can help limit the filling of press felts with pitch. Spraying the forming fabric with a very dilute solution of highly cationic polymer can create a hydrophilic layer that resists deposition of oleophilic materials. Wet-press felts can rinsed either continuously of periodically with solutions that may include nonionic surfactants, alkali, and solvents.

Cautions: Safe procedures must be followed when collecting samples of pitch deposits. Equipment must be locked out and the air quality checked by qualified people before entering enclosed areas such as headboxes.

Pitch deposition mechanism with alum, pH   Ilustration of how alum may either cause or prevent pitch aggomeration and deposits. The idea is to treat the pitch before it is released from the fiber.


