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.
Hemp pulp and paper production:
Paper from hemp woody core
Birgitte de Groot
G. v. Prinstererstraat 87, 6702 CP Wageningen, The Netherlands
During my studies I spent various traineeships at paper mills abroad, in one of which hemp and flax bast fibres were used. At the university I made paper out of both woody core and bast fibres. I learnt that the chemical pulping of hemp bast fibres primarily for fibre softening and separation (due to the low lignin content) is not difficult. Most effort was required in the mechanical procedures of beating, refining, and extrusion to cut and develop the long fibre.
Hemp woody was a different matter. I shall explain how hemp woody core can be processed into pulp for paper production. After elucidating the background of the rediscovery of hemp in The Netherlands, discussing trends in pulp production and how hemp woody core may fit in the market, I will briefly discuss my research work and conclude with arguments for scaling up recommendations.
Rediscovery of fibre hemp
Fibre hemp has been an important ingredient of paper since its invention in China, almost 1900 years ago (about 100 AD). This situation remained until the invention of printing presses and the mechanization of paper machines required more feed stock than the rags of hemp and flax could offer. This led about 100 years ago to the use of trees and more aggressive chemicals for pulp (cellulose) and paper production. Fibre hemp almost vanished in The Netherlands, only used for so-called ‘wind-breaks’, protecting vulnerable crops against damage.
In the early 1980s, a group of young farmers in the ‘Veenkolonien’, in North-east Netherlands, together with students of Wageningen Agricultural University, investigated possibilities for a so-called fourth crop. In their region about 50% of the land is used for potato growing (for starch production), about 25% for wheat and about 25% for sugar beets. They were looking for a new crop to broaden their rotation scheme, which would lessen the chemical needs against, for instance, potato diseases. This has led to the rediscovery of fibre hemp in The Netherlands, with potential for both farmers and the pulp and paper industry.
Trends in pulp production
Traditionally, spruce and pine are used for chemical softwood pulp to produce strong thick paper. Chemically treated hardwood pulp was introduced when smoother and thinner printable paper was required. Printing and writing grade papers in The Netherlands consist of about equal amounts of hardwood and softwood fibres.
The pulp and paper industry is always looking for ways to reduce costs. One way is to use faster growing trees, like poplar and Eucalyptus. Another way is to recycle paper, as is increasingly practiced in The Netherlands. Both ways require upgrading: producing better paper from less valuable feed stock. It requires adaptation of machinery and finding ways to handle shorter fibres.
One way to do this is chemically. Chemicals are used for fibre separation and delignification, after which only a mild refining is needed before final pulp production. Much more difficult is mechanical pulping of hardwood fibres: mechanical forces are used for fibre separation and fibrillation. In general, this treatment shortens the fibres, which is acceptable for softwood fibres (which are about 3 mm long), but is detrimental for hardwood fibres of 1 mm and less. This explains why at the moment no thermo-mechanical processes are used for hardwood fibres. However, technology is improving all the time, and chemi-thermomechanical processes are used commercially for hardwood pulp, an ingredient in Light Weight Coated papers.
Traditionally used as filler pulps, hardwood pulps have become important paper constituents (fine papers may contain 70-90% hardwood). To exploit hardwoods to their maxilnum potential, refining processes must be gentle, and must avoid cutting through new refiner designs and medium consistency refining. The trend for modern fine paper making is to develop short fibres rather than to cut long fibres and then develop them. Hardwood can yield a well formed strong sheet, to a point where softwood and hardwood pulps give nearly identical properties (Baker 1995).
|Table 1. Botanical classification of fibre hemp among other renewable fibre sources|
|Subdivision||Class||Family||Fibre crops / wood species|
|Gymnospermae||Coniferae||Piceae||Picea abies (spruce)
Pinus sylvestris (pine)
|Betula verrucosa (birch)
Fagus sylvatica (beech)
Populus tremuloides (poplar)
Cannabis sativa (hemp)
Boehmeria nivea (ramie)
Linum usitatissimum (flax)
|Monocotyledoneae||Gramineae||Triticum vulgare (wheat)|
Market potential for hemp woody core pulp
Botanically, as well as chemically, (Tables 1 and 2), hemp woody core is comparable with hardwood. I wish to stress this, because paper makers commonly consider that all annual crops are alike and must be treated as straw, requiring special effluent treatment due to high silica content. Dicotyledons like hemp do not have the high silica content in the ash, in contrast with monocotyledons like straw or other grasses. As for hemp woody core, bear in mind that hemp is an annual crop, so the wood components are younger than the average tree-fibres used in paper making. Like other plants offering long and strong fibrous material (jute, kenaf, flax) hemp has traditionally been generally regarded as material for textiles. The long bast fibres were used either for sailing and fishing gear, or for strong thin cellulose rich paper. The shorter fibres in the woody core were discarded, or at best used only as fuel.
|Table 2. Chemical composition of some fibre crops and wood species (on unextracted dry wood basis)|
|Picea abies |
Given the reality of improved pulping and refining processes it would be mistaken to ignore hemp woody core, two thirds of the stem, as paper feed stock. When Eucalyptus was introduced paper makers were unimpressed, the fibre length of 0.8 mm being 20% shorter than what they thought necessary. The thought that Eucalyptus pulp was doomed to fail, but now Eucalyptus has become a commercially important pulp, may well also come true for hemp woody core, if it is handled with the same care.
Table 3 summarizes the potential of both hemp bast and hemp woody core fibres. Of course the long cellulose-rich bast fibres are of great value, but the specialty paper market is small, thus it is necessary to also examine other possibilities, particularly textiles and glass-fibre replacement. As is shown, the market potential for hardwood type fibres exists, as they provide smoothness and printability in paper and board. Care should be taken to develop a process for hemp woody core so that it will fit into extant hardwood pulping processes.
Having explained the rationale of employing hemp woody core as a serious paper source, now I will explain why alkaline pulping was studied. The aim of the hemp programme was to design and develop environmentally safe and economically feasible pulping processes. The importance of alkaline pulping is illustrated in the Table 4.
|Table 4. Pulp production, apparent consumption and net imports (in tons) in Europe in 1991 (CEPAC annual statistics)|
|total wood pulp
for EC 12
|total wood pulp||22.3
|total pulp in
|chemical pulp in
Chemical pulp, cellulose, is of great importance in European paper production. Eighty-seven percent of this pulp is produced with the kraft process, using sodium hydroxide and sodium sulfide to produce unbleached pulp, which is bleached with chlorine and chlorine dioxide. Anticipating modern environmental demands in our densely populated country, possibilities of employing less polluting processes (sulfur, chlorine free) were investigated. Only alkaline pulping with sodium hydroxide is a potential pulping process for hemp woody core, and a basis for alkaline-oxygen and alkaline peroxide processes.
During my graduate programme at the University I found that strong and smooth paper can be produced from hemp woody core, as treated with sodium hydroxide. It seemed sensible to study further the kinetics and mechanisms of alkaline pulping of hemp woody core, to support the development and optimization of alkaline pulp production for printing and writing grades. I modeled degradation kinetics in relation to literature data on lignin and carbohydrate degradation.
What happens with hemp woody core when sodium hydroxide is applied? Sodium hydroxide promotes swelling and expansion of hemicellulose. This suppresses cross-fibre fragmentation during mechanical treatment and promotes fibrillation and formation of interfibre bonding during paper making, resulting in better mechanical properties of the paper.
Further treatment with sodium hydroxide, at temperatures around 170o C, promotes delignification (removal and degradation of lignin). This facilitates disintegration of wood in fibrous components and eliminates the colouring substances.
In the literature three consecutive reaction stages are distinguished; I modeled lignin, cellulose and hemicellulose degradation and removal with an integral calculation method, regarding those three consecutive reaction stages as the result of simultaneous reactions, depending on reaction time, temperature, sodium hydroxide concentration.
For my experiments I used a 80 ml flow-through reactor, which was designed at the university, in which woody core chips can be heated up from room temperature to reaction temperature within 6 minutes.
The concentration of NaOH appeared to be of major importance. During impregnation and heating periods when 40-50% of the material is removed, lower NaOH concentrations resulted in higher pulp yields, as less hemicellulose is removed during impregnation.
As elaboration of the modeling procedure and reaction kinetics is beyond the scope of this article, I refer to our articles in Holzforschung. The results, equations for lignin, cellulose and xylan degradation, can be used for optimization. Different pulping conditions optimize pulp production, depending on process costs and pulp prices.
The calculated reaction constant for the main delignification reaction of hemp woody core is about 1.5 times higher than for poplar wood, implying that residence time of hemp in continuous pulping reactors can be shorter compared to hardwood pulping.
The easier delignification may be due to the presence of less condensed polymer structures, as the fibres are younger than the average hardwood fibre. This may be investigated in studies of the relation of anatomy and chemistry to hemp processing.
Recommendations for pilot plant studies
For the following reasons, given Dutch conditions, we consider the more cautious choice between chemical or chemi-thermomechanical pulping of hemp, to be chemi-thermomechanical pulping.
1. The minimal scale for feasible chemical pulping of hemp woody core is relatively high, due to expensive chemical recovery. For the future: Research is underway in Finland, Sweden and Russia into alkaline pulping of annual crops and hardwood without sulfur, determining simplified and cheaper recovery methods.
2. Pilot plant studies for chemi-thermomechanical pulping can be started with one process unit of 10,000 ton pulp per year for extrusion pulping (bast fibres) and one of 25,000 ton pulp per year for chemi-thermomechanical pulping, using refiners (woody core fibres), which can be built up into a full size mill of several units.
3. More units would make the mill more flexible, giving lower risks of failure. The mill can also use other raw materials, like poplar wood, with little adaptation of the equipment.
4. Chemi-thermomechanical pulping of hemp woody core would render up 75-80% of the material to be used for paper, while chemical processes would yield only 40-50% of the material as pulp, the residue to be purified and burned as waste material.
5. The trend towards upgrading is likely to continue. It is likely that in future more use will be made of chemi-thermomechanical pulp, replacing chemical pulp.
Future for hemp woody core
The test papers I produced from alkaline hemp woody core pulp, both bleached and unbleached, show impressive smoothness and strength characteristics. The data can be used as reference values, to which chemi-thermomechanical hemp woody core pulp should be developed, using the right refiner plate configuration and consistency. As hardwood chemi-thermomechanical pulp has been increasingly used for printing and writing grades, in place of hardwood cellulose, it appears that hemp woody core is the most sensible alternative to the employment of hardwoods.
This paper is based on a lecture presented at Bioresource Hemp Symposium, Frankfurt, 2-5 March 1995.