Opportunities in Wet-End Chemistry: Feature Essay, Posted Nov. 2003
"Using Enzymes in Papermaking"
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
According to Jorling, the forest products industry "can be the most environmentally sound enterprise" (Tappi J. 83, 11: 32, 2000). Paper's main ingredient, wood fibers, is naturally renewable, even "organic" in the best sense of the word. Our manufacturing processes are increasingly efficient, and the levels of pollutants have been cut sharply. Much progress to reduce environmental impacts of pulping and papermaking has involved two main strategies - closing up cycles and improving end-of-pipe treatments. The kraft pulping process, invented in 1884, when combined with the 1930's development of the Tomlinson recovery furnace, has represented a high-water mark in recovery of chemicals and efficient use of energy. Water usage per ton of paper product has been cut by about half since 1975. With respect to end-of-pipe issues, an article by Thompson et al. documents recent achievements made by the industry in treating its aqueous wastes (Bioresource Technol. 77, 3: 275, 2001).
Hey, wait a minute, isn't this essay supposed to be about enzymes? The subject of enzymes comes into the picture because, after having said all of those nice things about the paper industry, I failed to mention an emerging trend that often involves enzymes. The new trend is called "green chemistry." The idea is to replace toxic materials in your industrial process with less hazardous materials. This includes the solvents, energy, and byproducts of each reaction, even before the ingredients enter your own factory - a whole "life-cycle" perspective (see, for instance, Pajula and Kärnä, "Life Cycle Scenarios of Paper," Proc. The First EcoPaperTech, Helsinki, 1995, p. 191).
Enzymes are proteins that catalyze chemical reactions in living organisms. Usually an enzyme speeds up just one reaction, e.g. breaking down cellulose chains in the case of a "cellulase." Natural enzymes usually are found as mixtures, since the organism that produces them has a wide range of needs to stimulate or repress different chemical reaction paths. In addition to isolation of new enzymes, corporate teams are developing purer and more concentrated enzyme products suitable for industrial uses. Whatever the use, enzymes usually require strict attention to pH and temperature, and all of them require many minutes or hours to do their work.
In various papermaking applications enzymes have the potential to replace hazardous chemicals. Take, for instance, chlorine. Chlorine is a very effective disinfectant. But some byproducts of chlorination reactions with aromatic materials in wood pulp are among the most toxic materials known (see, for instance, McKague and Carlberg in Pulp Bleaching, Dence and Reeve, Eds., TAPPI PRESS, Sec. 8, Ch 1, p. 751). By contrast, enzymes are formed through biological processes, and most of the byproducts can be rendered inactive by changes as simple as heating or changing the pH.
According to Baker, a typical kraft pulp ought to receive 80 to 150 kW-hours of refining energy per ton of dry fiber to develop inter-fiber bonding ability (Tappi J. 78, 2: 147, 1995). This is another situation where enzymes can help reduce an environmental impact. No source of electrical energy is entirely eco-friendly - whether it involves sulfur emissions from burning or coal, radiation during mining of uranium, or loss of certain ecosystems when hydroelectric dams are put into place. Studies have shown that cellulase enzymes can reduce the energy required for refining (see, for instance Moran, Pulp & Paper Mag., Sept. 1996). On the other hand, too much enzyme treatment can damage fibers (see Eriksson et al., in Enzyme Applications in Fiber Processing, ACS Symp. Ser. 687, 1998, Ch. 4, p. 41).
A related strategy is to add cellulase enzymes to papermaking furnish after the refining. The object then is to remove very fine, fibrillar material from the suspension and from fiber surfaces so that water can drain more easily. In principle, better dewatering means that less water has to be evaporated in the dryer section of a paper machine - a major area of energy consumption. More often, however, the faster dewatering rates mean increased machine speeds and higher production rates (see King et al., Tappi J. 81, 7: 56, 1998).
How do natural organisms break down fats? "Enzymes" is the correct answer. Bad outbreaks of wood pitch deposits in paper mills tend to be associated with high levels of triglyceride fats from the wood. In such cases it has been shown that addition of a lipase (enzyme to break down fats) can reduce the tackiness (see Fischer and Messner, Tappi J. 75, 2: 130, 1992).
Xerographic toner materials tend to resist to biological degradation. Doesn't this imply that enzymes ought to be useless of deinking of wastepaper that contains this kind of printing? Surprisingly, that is not the case. Based on results reported by Prasad and coworkers (Progress in Paper Recycling 1, 3: 21, 1992) certain enzymes can loosen the attachment between toner particles and fibers, making it possible to float out more of the ink and achieve higher brightness.
Enzymes are a versatile option because
nature contains so many of them. We've only scratched the surface in finding
industrial uses. Issues of low concentrations, inadequate purities, unfavorable
pH or temperature sensitivities, and the slowness of reactions will continue
to pose challenges to potential users of enzymatic approaches in papermaking.
But ultimately time is on the side of their increasing use. That is because
enzymes offer a way to further reduce environmental impacts of pulping and papermaking,
beyond what can be done with traditional approaches.