USING HEMP FOR MAKING PAPER
USING HEMP FOR MAKING PAPER
Paper can be made from nearly any fibrous plant and indeed, throughout history, it has been. Paper starts out as plant fiber, which is then beaten and/or immersed in chemicals until the fibers are separated and collapsed. In this way, each fiber offers the maximum surface area for bonding to the other fibers. This material is called pulp, and the process is called pulping. In a water suspension, these fibers are then poured over a screen. As the water drains out, the fibers come closer and closer together, forming a tightly bonded mesh which can be remarkably strong. When dry, this material is paper. Thus, the structure of paper consists of a randomly ordered mass of fibers, held together mainly by hydrogen bonding.
Today, paper is made on a long, rolling screen traveling as much as 4,000 feet per second, and measuring perhaps 30 feet wide. The mill that produces this paper typically costs in excess of one billion dollars to build. An estimated 300 million gallons of water are used to produce the newsprint read in the US each day.
Numerous chemicals and processes are involved in breaking down the fiber into pulp and rendering it suitable for paper making. Although the chemical recovery process in modern paper making is fairly efficient, considerable toxicity surrounds almost any mill, mainly from chlorine compounds as well as effluents which change the pH and temperature of runoff waters, harming the biological balance. Because of these hazards, new processes are necessary.
Cellulose and Lignin
The fiber characteristics of each plant vary. All plant fibers contain strands of cellulose called microfibrils, and these are generally grouped into a bundle that forms the fiber. The fibers are oriented roughly parallel to one another like string cheese. These fibers are held together and interpenetrated by lignin, the presence of which reduces the strength and diminishes the optical brightness of paper.
Lignin is a sticky, dark, water-resistant, brittle sort of sap which gives strength to the plant while it is living but causes many difficulties in paper making. Chemically, it is a large, irregular polymer composed mainly of phenylpropane units linked together in three dimensions. In order for paper to be supple, durable and bright, most of the lignin must be removed. Most of the chemicals used in a paper mill are used in the effort to remove lignin.
The Pulping Process
The vast majority of fiber pulped worldwide is pulped by the Kraft process, which is a very sophisticated application of sodium hydroxide (lye) in an enormous pressure cooker. The resulting pulp is then washed and bleached to remove lignin still remaining after the initial cooking. Trees are an excellent source of fiber, and the fibers are of good quality for paper making, especially those of softwood trees. An advantage of using trees instead of annual crops is that trees can be harvested year-round. But of course, the liabilities are tremendous: deforestation and the attendant acceleration of the greenhouse effect; loss of habitat for many species; and pollution and flooding from erosion runoff.
Advantages of Hemp for Paper Making
Hemp is used for paper making in China, Slovenia, the Czech Republic, Germany, Holland, England, France, and other countries. Nonwood paper making technologies are being explored most vigorously in countries which do not have sophisticated industry, and in countries with few forests. However, nonwood fiber accounts for only 10 percent of the paper currently being produced.
The hemp plant, Cannabis sativa L., yields two fibers which are usable for paper production: the outer bark, or bast fiber (1020 percent) and the pithy inner core, or hurd (50 percent) (de Meijer, 1992). The fibers of the outer bark are very long, about 20 mm and make paper that is very strong. For that reason these fibers are superior to wood fibers for most paper making (van der Werf, 1994). Fibers from hemp hurds, however, are far shorter than bast fibers, at about 3 mm yielding a paper that tears easily. This fiber is comparable in many ways to that of hardwoods. Paper can be made from the whole stalk, effecting a compromise between the relative strengths of the two types of fiber. Hemp bast fiber can also be mixed with weaker fibers such as cereal straw, an agricultural waste product that is generally burned in the fields. The paper sold by Living Tree is an example of such a mixture.
Another advantage of hemp fiber for paper making is the low lignin content, which at around seven percent is about one-fourth to one-fifth that of wood. This means that good brightness values can be obtained using environmentally safe peroxide or oxygen bleaching cycles.
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