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August 14, 2000
Volume 78, Number 33
CENEAR 78 33 p.42
ISSN 0009-2347


Mairin Brennan

Paper has a long history. Its name is derived from papyrus, a sedge that flourishes along the banks of the Nile River. In ancient times, the Egyptians extracted the fibrous interior of this plant to fashion a kind of writing material.

The current method of making paper is straightforward: Plant fibers stick together when the fibers are pulped, spread on a screen, and dried. The Chinese are credited with giving birth to the process in A.D. 105, when an official of the Imperial Court created a sheet of paper from mulberry and other plant tissue fibers, old rags, and hemp waste. Centuries elapsed before the discovery reached Central Asia in the mid-700s. By the end of that century, paper was being produced in Baghdad, during the golden age of Islamic culture. Later on, papermaking was introduced to Europe, and by the 14th century, paper mills existed in Spain, Italy, France, and Germany; cotton and linen rags provided the fiber feedstock. Sheets of paper were made one at a time, by hand.

The advent of mechanized papermaking in the early 19th century increased productivity and upped the demand for rags. But rags were in short supply because demand for paper had been surging since the mid-15th century, when the printing press debuted. Fortuitously, by the time the paper machine was invented, timber was being explored as a substitute for rags. In 1840, a mechanical approach to pulping wood was developed in Germany. In 1866, chemical pulping with sodium sulfite was developed by U.S. chemist Benjamin Tilghman. And about 1880, "kraft" pulping, which uses sodium sulfide and caustic soda, was invented by German chemist Carl F. Dahl. Timber became the dominant feedstock for paper mills.

Wood, cotton, and other plants contain cellulose, the long, linear glucose polymer from which paper is made. In wood, the cellulose is encased in a matrix of lignin, a phenylpropanoid polymer that shields it from harm. Reinforcing lignin are polysaccharides with backbones of various sugar units. To extricate cellulose from these barrier polymers, the tree bark is removed, the wood is chipped, and the chips are pulped. The pulp is then poured on a wire screen, where water drains and the fibers mat. The mat is passed through a series of rollers that dry, press, and "iron" it and add various finishes. Writing papers, for example, are impregnated with a water-resistant substance to prevent ink from spreading.

Mechanical pulping produces pulp in high yield. But the process is energy intensive and removes little lignin, which hurts paper quality. Thus that approach primarily is used in making newsprint, tissue, and other low-quality paper. Kraft pulping, the major chemical pulping process used today, yields less pulp, but it removes enough lignin to produce paper strong enough to make grocery bags (kraft is German for strong). The residual lignin is what colors the bags brown, and it must be bleached out to achieve high-quality white paper. Traditionally, the lignin is oxidized with chlorine and, more recently, chlorine dioxide processes that have caused environmental controversies for the paper industry.

Not surprisingly, environmentally friendly bleaching processes are being pursued, including ozone-based systems. And at least one enzyme-based process developed in the late 1980s in Finland has been used commercially. The process uses xylanase to make lignin more vulnerable to oxidation by attacking the polysaccharides that protect it. Other enzyme-based bleaching approaches currently are being explored, including one at the University of Georgia (UGA), Athens.

Inorganic bleaching catalysts also are in the works. For example, polyoxometalates, such as SiV2W10O40, that oxidize lignin to carbon dioxide and water in a closed-loop cycle have been developed by a team of researchers at the Forest Products Laboratory (FPL, a unit of the Department of Agriculture's Forest Service), Madison, Wis., and Emory University, Atlanta. The technology currently is ready for large-scale pilot-plant evaluation.

An enzyme-based deinking technology that removes ink from newsprint and magazines for reprocessing is already in use. Developed at UGA in the early 1990s, the cellulase-based approach is energy efficient. Mechanical deinking, in contrast, literally "beats" the ink out of the paper. Marketed by Enzyme Deinking Technologies, Norcross, Ga., the new technology has been adopted by various companies in the U.S. and abroad, according to Chief Operating Officer Howard S. Kaplan.

On another front, biopulping is being explored as a way to both reduce energy consumption and improve paper strength in mechanical pulping. Roughly a decade ago, a white rot fungus that chews through lignin was discovered by members of a biopulping consortium that included FPL, pulp and paper companies, and other groups. The fungus was enlisted to predigest wood chips so as to provide a softer substrate. One U.S. company has confirmed the feasibility of the approach on a commercial scale, and others--in the U.S. and abroad--have expressed interest in evaluating it, notes Masood Akhtar, chief executive officer of Biopulping International, Madison, Wis.

Last year, 96.9 million metric tons of paper and paperboard were manufactured in the U.S., according to the American Forest & Paper Association (AF&PA), Washington, D.C. In the process, 99 million cords of wood were consumed (1 cord = 128 cu ft) as well as 37.2 million tons of recycled paper. So much timber is used to make paper (and other wood-based products) that forests must be managed in a way that conserves the ecosystem. To promote such stewardship, AF&PA and its member firms adopted guidelines known as the Sustainable Forestry Initiative ( in 1995.

Rags are still used to make durable, very high quality paper. U.S. paper currency, for example, is made from a mix of cotton recovered from rags and cotton gin trash (75%) and flax waste (25%), notes Peter Hopkins, a spokesman for Crane Co., Dalton, Mass., which has supplied the U.S. Bureau of Engraving & Printing with currency paper since 1879. The long cellulosic fibers in cotton and flax produce paper that stands up to rough handling. And that's why dollar bills, like blue jeans, survive laundry washings so well.


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