Making hemp paper
Hemp Pulp and Paper Production
Gertjan van Roekel jr.
ATO-DLO Agrotechnology, P.O.box 17, 6700 AA Wageningen, The Netherlands
Van Roekel, G J, 1994. Hemp pulp and paper production. Journal of the International Hemp Association 1: 12-14.
A brief review of the history of paper making reveals the important role of hemp in the development of the industry. The technical aspects of classical hemp pulping and paper making and the present status of the hemp pulp and paper industry are discussed. It is shown that for new applications of hemp as a paper making fibre source, new pulp technology is required. This article is the first in a series about hemp pulping and paper making.
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.
Explained – How Hemp Paper is Produced
Paper produced entirely or mostly from the fibers of the hemp plant is known as hemp paper. Hemp or industrial hemp is the non-narcotic kin of drug cannabis or marijuana. Hemp belongs to the same plant species Cannabis Sativa L as marijuana.
It’s difficult to understand how hemp paper is produced unless you understand the history of it because the method has evolved over the decades.
This botanical relationship had led to hemp being outlawed in much of the world in the 20th century. Thankfully, many countries have now corrected this human blunder. Hemp has been decriminalized since it does not possess any psychoactive properties.
Tetrahydrocannabinol (THC), the psychoactive element is present in high concentration in drug cannabis: 7.5 to 10 percent or higher. This gives marijuana its capacity to cause the so-called high. THC presence is limited to 0.3 percent or less in hemp, which means this plant has no psychotropic effects.
Hemp Paper Antiquities
Historians inform us that the world’s earliest paper was made by the Chinese from hemp fibers nearly 2,200 years ago around 150 BCE. It spread from China to the Middle East and then to the rest of the world. All paper used in the world was hemp paper till 1883.
The first printed Bible, known as the Gutenberg Bible or the 42-line Bible, used hemp paper. Mark Twain’s novels also got printed on hemp paper. It was hemp paper that Thomas Paine used to print the leaflets that spread the idea of American independence.
The first two drafts of the American Declaration of Independence were written on hemp paper. The first version of the US Constitution also got drafted on hemp paper. The current usage of hemp paper, however, is restricted mostly to specialty paper.
Ancient Chinese paper
How Hemp Paper Is Made Today?
Fibers sourced from hemp stalks are among the strongest plant fibers in tensile strength. Hemp stalks yield two types of fibers: long bast fibers and short hemp hurds or pulp fibers. Longer bast fibers come from the outer layer of the hemp stem while hemp hurds refers to the woody inner core.
It is possible to use both types of fibers for the production of hemp paper. Paper made from the long bast fibers is crumbly, thin, and coarse. Paper made from hemp hurds or pulp is thicker and softer. The production process of paper from hemp pulp is also easier.
Hemp paper like traditional paper needs the pulping of the fibers into a slurry. This is easier to do using hemp hurds. The first step is to separate the pulp from other plant matter. The next step is to soak the refined pulp in clean water and pound it to get a pulp slurry.
In industrial papermaking, some additives are used for the slurry. The next step is to remove the excess water from the slurry. The paper machine spreads the slurry on a mobile continuous screen for the water to be drained by gravity or vacuum.
Then the wet paper goes through pressing to be dried. The final outcome is a roll of paper. Cutting the rolls into different paper sizes using mechanized cutters is often a separate enterprise that paper mills do not undertake.
One of the major differences between handmade paper and industrialized paper constitutes in the edges. The machine-cut paper has smooth edges. Wooden frames called deckles are used to cut the paper into different sizes in handmade paper. That leaves the edges slightly uneven. Known as deckle-edges, they indicate that the paper is handmade.
Producing handmade hemp paper
Benefits of Hemp Paper
Knowing how hemp paper is made isn’t enough. You need to know the benefits.
Hemp remained the primary source of papermaking for 2000 years until wood pulp substituted it in the 20th century. The renewed interest in using hemp for papermaking is a result of a growing consciousness about several environmental hazards of using tree paper.
Hemp is nature’s solution to all the problems associated with the use of wood paper. Hemp has considerably higher renewability than trees as a source for paper. The quantity of paper generated from one acre of hemp is equal to what four to 10 acres of trees can produce over a period of 20 years.
The reason behind this productivity is the higher cellulose content in hemp stalks. Cellulose is the main ingredient of paper. Hemp stalks contain up to 85 percent cellulose in comparison to about 30 percent cellulose in wood.
The use of chemicals to remove the non-cellulose contents is significantly higher in the case of wood as nearly 70 percent of the wood is non-cellulose. Also, trees need 20 to 80 years to mature. Hemp, in contrast, is ready for use in four months.
Hemp paper is also significantly more durable than paper produced from wood pulp. It does not yellow and cracks as easily as wood paper. Ancient remnants of hemp paper testify to its durability. Hemp paper is a natural substitute for the special acid paper used for conserving important documents.
Hemp Paper and the Environment
If Mother Earth had a choice, she would choose hemp paper over wood paper. Hemp paper is far more environmentally-friendly than tree paper. Deforestation is a case in point. The world lost 502,000 square miles or 1.3 million square kilometers of forest cover between 1990 and 2016.
The National Geographic published this data quoting a World Bank report. The effects of deforestation are grim and far-reaching. It is one of the primary causes of global warming and climate change. To destroy a forest is also to release the carbon dioxide sequestered there into the atmosphere.
In addition, fewer trees imply a reduced capacity for carbon dioxide absorption. Forests are also critical for the natural water cycle to be maintained. Further, there is the loss of natural habitats for a wide range of flora and fauna. That endangers their existence and threatens the planet’s natural biodiversity.
Using hemp instead of trees for papermaking is one eco-friendly response to the problem of deforestation. In addition to reducing the need to fell trees, hemp also regenerates the soil.
Paper companies planting eucalyptus after felling all the trees in an area does not compare with hemp.
Another environment-friendly aspect of hemp paper is that it needs no bleaching. The production of hemp paper thus eliminates the chances of contaminating water with dioxin or chlorine, as paper mills do. The chemicals used in separating hemp fibers from the lignin are far less toxic.
The only plant better suited for papermaking than hemp is kenaf. However, kenaf does not grow as fast as hemp and does not produce as much fiber as hemp does. Hemp has the potential to meet all our paper needs, but kenaf does not.
How Hemp Paper is Made: Present and Future
Contemporary use of hemp paper is mostly limited to the production of specialty paper such as cigarette paper and cosmetic tissue paper. Only a few companies in Europe and North America have experimented with producing writing paper from hemp.
One of the reasons behind the resistance to using hemp as a source for writing paper could be the costs involved in changing the machinery. About 40 to 60 percent of retooling is necessary to switch from tree paper to hemp paper.
Gmund, a large papermill in Germany is increasingly producing large batches of hemp paper varieties, including writing paper. Other paper mills around the world need to undertake the equipment change to produce more hemp paper unless we want to be left with no more trees to fell.
Jaspreet Singh is the Co-Founder and COO of Hemp Foundation. He is passionate about adventures tours, trekking, and long bike rides.There was a time when hemp paper was used instead of tree based paper. Read about the production of hemp paper and why it is considered need of the hour. ]]>