Opportunities in Wet-End Chemistry: Feature Essay, Posted Nov. 2003
"Does Pitch Have Your Attention?"
Martin A. Hubbe
Dept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh, NC 27695-8005
Citation (public domain): http://www4.ncsu.edu/~hubbe/new
A recent article described "pitch seasons," which can occur almost with the regularity of baseball seasons . There has been something mysterious about pitch, with its unpredictable assaults on the productivity and product quality of paper mills, especially those using mechanical pulps. Fortunately there continues to be progress in our understanding of pitch. Such progress is described in a recent, outstanding textbook by Back and Allen [2 ].
Why pH and water hardness, etc., are important. The behavior of pitch-like materials in the wet-end of a paper machine depends on such factors as pH and the calcium or magnesium ion concentrations. The reason for this sensitivity is that different chemical forms of pitch can exist under different aqueous conditions. At very low pH, say below about 3.5, it is reasonable to expect that the resin acids and fatty acid molecules within a dispersed pitch sample will be mainly in their associated, uncharged state. The uncharged state of the particles, in addition to their water-hating nature, gives rise to danger that they will form deposits, if the pH were indeed to be reduced to very low values. If one were then to isolate the acidic pitch and add sodium hydroxide to increase the pH above about 5.5, then most the acid groups would become saponified. That means that they would be sodium salts. Soaps, as such, don't cause tackiness problems. But just as when you do laundry, soap ions can be precipitated by water hardness ions, forming a tacky scum. Cratin and Murray [3 ] described how it is possible to use pH and ionic concentrations to predict the interfacial tension of water in the presence of pitch dispersions, and also the surface energy of pitch films deposited from such mixtures.
Pitched Battle. By the time you know that you have entered "pitch season," it is a pretty sure bet that your main supplier of papermaking chemicals will know it too. There will be a variety of steps that can be taken. Talc addition ought to be high on your list of options. As noted by Gill , you often can lessen the impact of pitch in your system by attaching it to solid surfaces early in the system, before it has had a chance to agglomerate into particles big enough to act like "dirt" in your sheet. The oil-loving surface of talc will attract pitch-like substances. As long as you add enough talc, the overall tackiness of the system is kept low enough that the rate of pitch deposition onto equipment is reduced, so talc is often called a detackifier. A range of organic chemical products have been developed that also act as detackifiers . A related approach is to use a surfactant to help disperse the pitch by making its surface more water-loving. An article by Borchardt  showed that best results were obtained when a surfactant was selected so that its cloud point, the point where it is just ready to become insoluble, equals the process temperature. A third approach is to use cationic fixatives to bind small (hopefully) pitch particles onto fiber surfaces so that they are retained efficiently at the forming section and not allowed to accumulate in the whitewater system . Such additives as alum, polyaluminum chloride (PAC) and polyamines can be useful for this reason. Recently there have been important advances in fighting pitch with enzymes [8-10]. One idea is to convert triglcyceride fats - a particularly difficult to deal with component of pitch - into fatty acids, which can be more easily controlled by the other methods already mentioned.
Are we having fun yet? How do you know that you are having a pitch problem? If you have to ask, well, maybe you are having a good day. Any maybe your system is already benefiting from some of the proactive things that you can do with the help of God (including the weather, to season the wood chips) and your chemical supplier.