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Hemp pulp and paper production:
Paper from hemp woody core

Birgitte de Groot
G. v. Prinstererstraat 87, 6702 CP Wageningen, The Netherlands

Introduction
        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).

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.

        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.

Alkaline pulping
        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.

        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.

Systematic approach
        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.

Results
        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.

References

• Baker, C.F. 1995.  Good practice for refining the types of fibre found in modern paper furnishes.  Tappi Journal, 78(2): 147-153.
• De Groot, B., J.E.G. van Dam and K. van 't Riet 1995.  Alkaline pulping of hemp woody core: kinetic modeling of lignin, xylan and cellulose degradation.   Holzforschung, in press.
• De Groot, B., J.E.G. van Dam, R.P. van der Zwan and K. van 't Riet 1994.   Simplified kinetic modeling of alkaline delignification of hemp woody core.   Holzforschung, 48(3): 207-214.
• De Groot, B., D.J. van Zuilichem, R.P. van der Zwan 1988.  The use of non-wood fibres in The Netherlands in: Proceedings of the 1988 International Non-Wood Fibre Pulping and Papermaking Conference, Beijing, China: 216-222.
• Nordkvist, E., H. Graham, P. Aman 1989.  Soluble lignin complexes isolated from wheat straw (Triticum arvense) and red clover (Trifolium pratense).  J. Sci. Food Agric. 48(3): 311-321.
• Rydholm, S.A. 1965.  Pulping processes.  Interscience Publishing, New York.