PAPER



OUTLINE
Reading
CHAPTER 16 IN THE TEXT, 389 ff.

Introduction
People have written on many different kinds of plant materials. The Egyptians used papyrus (Cyperus papyrifera, Cyperaceae). They hammered out the stems flat and glued them together by the edges. Rice paper from the Orient is still made by pounding sheets cut from the pith of the rice paper plant (Fatsia papyrifera, Araliaceae). Both the Mayans and Polynesians developed a method of making paper like products by pounding the bark of fig species (amatl or amate, Ficus spp.) or the paper mulberry (Broussonetia papyrifera) (both Moraceae), respectively.
In southeastern Asia, people wrote with a stylus on bamboo sections. After a period of time, the writing "appeared" on the bamboo surface. None of these substances were true paper.
Paper is made from plant fibers that have been separated from one another and then matted together into a thin sheet. The Chinese produced a product of this type about 100 A.D. They also used paper mulberry bark for this purpose. Other fibers such as flax, hemp and even rags were also used at times. The fibers, floating in liquid, were allowed to settle in a thin film on a screen. The screen was shaken in order to interlock the fibers.
Upon drying, the sheet of paper was peeled from the screen. Papermaking was spread across Asia and to Europe by the Arabs. Paper reached Spain about 1000 years after it was developed in China.
Most papers were made there from linen, cotton or hemp. Paper reached most of Europe about 700 A.D. Gutenberg - about 1450, was one of several of this period who developed printing with movable type. However, this early printing was not on paper.
Because of the hand labor involved, paper was expensive and not readily available. However, a papermaking machine was developed in the late 1700's and that lowered the cost of paper. The increased production that resulted caused a shortage in the raw materials, at that point mostly rags.
Today, most of the world's paper is made from woods of various types. About 40% of the world's wood use is to make paper. However, esparto grass, kenaf, bamboo, and sugarcane bagasse are also used.

Composition of wood
Wood is about 50% cellulose and 30-40% lignins. The fibers and xylem tracheids in the wood are almost pure cellulose.

The mechanical process
In this process, the wood pulp is ground and the fibers still are mixed with pectins and lignin. The fibers are short and weak. The resultant paper is not of good quality and yellows quickly. Today newspapers, catalogs, paper towels, cardboard, cheap magazines, building boards, etc. are made by this process. None of these is expected to last indefinitely.
The yield of paper compared to the amount of wood used is relatively high, often about 90%. About one fourth of all paper is made by the mechanical process.
Made from spruce, fir, some pine and hemlock. These trees have light colored woods and long fibers.

The soda process
In 1851, a process in which wood was treated with sodium hydroxide (NaOH) was devised which dissolved away the lignins from the wood fibers. This process produces a relatively weak paper. Blotters are usually made by this process. Yields are about 48%. A number of hardwoods are usually used.
Usually uses hardwoods. Aspen, cottonwood, basswood, beech, birch, maple and gum.

The sulfite process
This process was developed in 1857. Solutions of sulfites and sulfur dioxide (sulfurous acid) are used to dissolve the lignins. The fiber has good strength properties and can be bleached with chlorine and calcium hypochlorite. Unfortunately, paper made by the sulfite process has a high acid content and gradually the paper becomes brittle and disingegrates.
The acid doesn't come from the digestion itself, but from acid added later in the processing. Most books printed since 1850 have been printed on this kind of paper. Yields are less than 50%. Mostly softwoods that are low in resin. Spruce, fir, hemlock, tamarack, some pines, but also birch and aspen.
Used for books, bond paper, wrapping paper, tissue, rayon, and to mix with other pulps.

The sulfate or Kraft process
In this case, the wood is treated with sodium hydroxide and sodium sulfide. This process was developed about 1885. Just about any kind of wood can be used. Normally the pulp is not bleached. Yields still less than 50%. Conifer woods are commonly employed. The xylem tracheids of these trees are longer than the xylem vessels of hardwoods (2.0-4.0 to 0.5-1.5 mm).
Tall oil is a by product of pulping. This material contains resins and fatty acids.

Paper manufacture
Once the fibers have been prepared they are made into a slurry which is fed onto a large screen (a Fourdrinier screen). Resins, gums, starches, or rosin can be added as sizing agents or talc or clay can be added as fillers at this stage. Alum is also added to bind the sizes to the fibers.
Dyes may also be added. A vacuum under the screen sucks excess water out. The screen moves back and forth and tangles the fibers. See the diagrams on pg. 391 and 392.
The large screen is continuous and rotates. The screen passes between rollers which press out part of the water and help form the surface of the paper. On another portion of the screen, the paper is heated to drive off excess water. The paper may also be surface coated.
Although paper made by the mechanical process has poor stability, much newsprint is made in this manner. Most of the rest of the world's paper is made by the sulfite and sulfate processes.

Sources of paper
Most paper in the U.S. is made in the Southeast. Most of our newsprint is imported from Canada. In recent years, attempts have been made to grow row cropped crops suitable for paper manufacture. Among these are kenaf (Hibiscus cannabinus, Malvaceae) and sunn hemp (Crotalaria juncea, Fabaceae). Both are already used in some Third World countries.


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Revised April 2005




© David S. Seigler, Integrative Biology 363, Plants and Their Uses, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.