Hemp Pulp & Paper Productionpublished: January 1st, 1994
History of paper making
The use of fibre hemp (Cannabis sativa L.) for pulp and paper dates back more than 2,000 years. The oldest surviving piece of paper in the world was discovered by archeologists in 1957 in a tomb near Sian in Shensi province, China (Temple 1986). It is about 10 cm square and can be dated precisely between the years 140 and 87 BC. This paper and similar bits of paper surviving from the next century are thick, coarse, and uneven in their texture. They are all made of pounded and disintegrated hemp fibres. Paper historians agree that the earlier Egyptian papyrus sheets should not be referred to as paper, because the fibre strands are woven and not "wet-laid" (Hunter 1957). The Chinese paper-making craftsmanship was transferred to Arabic and North-African countries, and from there to Europe. The first European paper making was reported in the first half of the 16th century (Hunter 1957). Until the early 19th century, the only raw material available for paper making was rags. Rags are worn-out clothes. Since at that time clothing was solely made of hemp and flax (sometimes cotton), almost all paper in history was thus made of hemp and flax fibres. With the industrial revolution, the need for paper began to exceed the available rag supply.
Although hemp was the most traded commodity in the world up to the 1830s (Conrad 1993), the shortage of rags threatened the monopoly for hemp and flax as paper-making fibres. This was the major incentive for inventors and industries to develop new processes to use the world's most abundant (and cheap) source of natural fibres: our forests. Currently, only about 5% of the world's paper is made from annual plants like hemp, flax, cotton, sugarcane bagasse, esparto, wheat straw, reeds, sisal, abaca, banana leaf, ananas and some other more exotic species. The world hemp paper pulp production is now believed to be around 120,000 tons per year (FAO 1991), which is about 0.05 % of the world's annual pulp production volume. Hemp pulps are generally blended with other (wood-) pulps for paper production. There is currently no significant production of 100 % true hemp paper.
Renewed interest in hemp paper
The recent renewed interest in hemp as a paper-making fibre seems to originate from a strong environmental motive. All primary forests in Europe, and most in North America have been destroyed, amongst others for paper production. Now we accuse the nations which still have primary forests of not guarding theirs. In Europe all trees harvested for paper making were intended for that purpose, so there seems to be no valid reason to switch to a non-wood or "tree-free" fibre source. This of course is a little different in the Americas and in Asia and Australia, where primary forests are cleared at a huge environmental cost. In these regions hemp has a number of advantages as an alternative source of paper-making fibre. Hemp does not need pesticides or herbicides, and yields three to four times more usable fibre per hectare per annum than forests. And last but not least: paper recycling was invented to make up for the mistake of cutting down our primary forests. Technically speaking, one doesn't need to recycle hemp paper, because it is a renewable raw material. One disadvantage of using hemp or other annual plants as fibre source is that the present pulping technology has been optimised for tree-fibre pulping, so some adjustments in the pulping processes need to be made when applying this technology to hemp fibres. Before going into technical details, we will first examine the technology of pulp and paper making.
Pulping and paper making
Paper making is essentially the rearranging of elementary fibres from whatever source (a tree, a hemp stalk, an old pair of jeans or even a scoop of algae) into a flat thin sheet. Elementary fibres are the basic building blocks of trees and many plants. The average paper making fibre is about 2 mm long and about 20 micrometers (0.02 mm) thick. All fibres are assembled of chains of cellulose molecules, arranged as a rigid structure. These building blocks are glued together with other biological components (lignins, pectins), which give a certain flexibility and strength to the tissue, so that the tree or plant can bend at high stresses, and doesn't break in a storm, and is able to carry its seeds and fruits.
The following explains what is needed to process a fibre source into paper (Smook 1982): Pulping (from fibre source to pulp):
* cleaning: all non-fibrous components need to be removed from the raw material, and the remaining fibres must be cleaned of dirt, rocks and other contaminants.
* fiberizing: the elementary fibres are taken apart by either chemically removing the glue that holds them together, or mechanically tearing the fibre structure apart. From this step on, the material is referred to as "pulp".
* cutting: especially hemp fibres are too long to give a homogeneous paper sheet, so the fibres have to be cut to the right size.
* classification: the fibres suitable for use in paper are separated from the ones too short, too long, too wide, too thin, too crooked, too dirty and too old. Fibres can be classified by weight (centrifugal and gravitational processes) and size (variou s sieving processes).
* bleaching: optionally, the suitable fibres may be bleached to a higher "whiteness". The whiter a sheet, the better the contrast with the ink. Old-style pulp mills use chlorine compounds with hazardous side-effects. Modern pulp mills use oxygen-based bleaching (compounds like oxygen, ozone and peroxide). Hemp pulp can be bleached with relatively harmless hydrogen peroxide. For some applications bleaching is not required, for instance for packaging paper and board.
* refining: this is a separate process step in which the fibre surfaces are "roughened". The greater surface roughness of a fibre, the better it adheres to other fibres in the paper sheet and the greater the strength of the paper. Papermaking (from pulp to paper):
* Dilution: in order to lay the fibres evenly into a homogeneous sheet, the pulp is diluted with large amounts of water (sometimes up to 200 times as much water as fibre pulp).
* Formation: the fibre-water slurry is poured on a fine mesh wire. Most of the water will fall through the wire, leaving the fibres to settle into a flat sheet.
* Drying: in the next steps, the wet sheet is dried by subsequent pressing and steam heating.
* Sheeting: finally, the formed sheet is cut to the required size. These processes are essentially the same for manual paper making and for modern paper machines, with the difference that the old paper maker put out one handmade sheet per minute, and the state-of-the-art Fourdrinier newsprint paper machine puts out 15,000 square meters a minute: a 10 meter wide sheet at 90 kilometers an hour! Remainders of the hemp pulp industry Although there are thousands of non-wood paper mills in the world, only a few of them use hemp as a fibre source. At present 23 paper mills use hemp fibre, at an estimated world production volume of 120,000 tons per annum. Most of the mills are located in China and India, and produce moderate quality printing and writing paper. Typically, these mills do not really have a fixed source of fibre, but they simply use whatever can be found in the region. About 10 of the mills are located in the western world (US, UK, France, Spain, eastern Europe, Turkey), and these mills produce so-called specialty papers such as: * cigarette paper: even popular American cigarette brands have a 50% hemp cigarette paper and filter.
Some countries still have legislation prescribing the use of hemp in cigarette paper, because other fibres (like spruce) generate hazardous fumes when incinerated (!). * filter paper (for technical and scientific uses) * coffee filters, tea bags * specialty non wovens * insulating papers (for electrical condensators) * greaseproof papers * security papers * various specialty art papers These papers can generally only be produced from special fibres like hemp, flax, cotton and other non-wood fibre sources. The average hemp pulp and paper mill produces around 5000 tons per annum. This should be compared to a "normal" pulp mill for wood fibre, which is never smaller than 250,000 tons per annum. The only reason the remaining mills can still produce at this extremely small size is that there is a very special use for the pulp. This partly explains the high price for a hemp pulp: about US $2500 per ton versus about US $400 for a typical bleached wood pulp. The remaining mills in the western world are unable to cope with western environmental regulations because of their small size and archaic technology. Some mills survive by shipping their waste water to a large wood pulp mill nearby, others have to close down.
There is a clear shift in capacity towards countries that do not as yet take environmental problems very seriously. One reason for the high price of hemp pulp is the inefficient pulping processes used. Another reason is that hemp is harvested once a year (during August) and needs to be stored to feed the mill the whole year through. This storage requires a lot of (mostly manual) handling of the bulky stalk bundles, which accounts for a high raw material cost. Classical pulping technology Most mills predominantly process the long hemp bast fibres, which arrive as bales of cleaned ribbon from preprocessing plants located near the cultivation areas. The bales are opened and fed into a spherical tank, called a digester. Water is added (5 to 10 times the fibre weight), together with the cooking chemicals to remove the "glue" components lignin and pectin from the fibres. Most mills use sodium hydroxide and sulphur cocktails. The fibres are cooked for several hours (sometimes up to eight hours) at elevated temperature and pressure, until all fibres are separated from each other. After cooking, the cooking chemicals and the extracted binding components are separated from the fibres by washing with excess water. This is where most of the polluting waste emerges from the process. Often wastes are discharged as such into the local surface water. The remaining clean fibres are then fed into a Hollander beater, which is best compared to an industrial size bathtub, with a large wheel revolving around a horizontal axis at one point in the tub. The wheel pumps the pulp round and round, and meanwhile cuts the fibres to the right length, and also gives the fibres the required surface roughness for better bonding capacity. This beating goes on for up to twelve hours per batch. Some mills add bleaching chemicals in this beating process, other mills pass the pulp from the beating machines to separate tanks for bleaching.
These separate bleaching treatments often use chlorine compounds, which are also discharged into the environment. The bleached pulp is then ready to be pumped to the paper machine, or can be pressed to a dryness suitable for transportation to a paper mill elsewhere. The processing time of more than twenty hours make this process very expensive, as the costly equipment and handling must be depreciated over a very low throughput. Necessity for new technology New applications for hemp as a paper making raw material require a new pulping technology which must be able to use hemp from wet storage. Some new technologies have been developed, albeit in laboratory or on pilot scale. The next item in this series about hemp pulping and papermaking will discuss these new technologies and their benefits.
References * Abel E.L. 1980. Marihuana, the first twelve thousand years. Plenum press, New York, 289 pp. * Conrad C., 1993. Hemp, lifeline to the future. Creative Xpressions Publishing, Los Angeles, California. * FAO 1991. The outlook for pulp and paper to 1995. Paper products, and industrial update. Food and Agricultural Organisation of the United Nations, Rome. * Hunter, D. 1957. Papermaking, the history and technique of an ancient craft. 2nd Ed. Albert A. Knopf * Smook G.A. 1982. Handbook for pulp & paper technologists. 2nd Ed. Angus Wilde Publications, Vancouver, B.C. * Temple R.K.C, 1986. China, land of discovery and invention. Patrick Stevens Ltd., United Kingdom.