31
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).
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) |
Angiospermae | Dicotyledonae | Betulaceae Fagaceae Saliceae Cannabinaceae Urticaceae Linaceae |
Betula verrucosa (birch) Fagus sylvatica (beech) Populus tremuloides (poplar) Cannabis sativa (hemp) Boehmeria nivea (ramie) Linum usitatissimum (flax) |
Monocotyledoneae | Gramineae | Triticum vulgare (wheat) Phylostachys puberula |
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) | |||||
glucan | xylan | mannan | lignin | ||
Picea abies (spruce)1 Pinus sylvestris (pine)1 |
4 4.3 4 |
7.6 7.2 |
10 3 7.6 |
2 8.6 2 7.8 |
|
Betula verrucosa (birch)1 Cannabis sativa (hemp), woody core bast fibres |
3 7.5 3 7.7 6.7 |
2 4.6 1 6.7 1.5 |
0.5 1.2 1.9 |
1 9.5 2 2.1 4.0 |
|
Triticum vulgare (wheat)2 |
3 0.4 12.2 |
1 0.4 9.9 |
|||
1Rydholm 1965, 2Nordkvist 1989 |
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.
Table 3. Market potential of fibre hemp as paper furnish (prices at 1 March 1995) | |||||
BAST |
% |
COMPARABLE PULP/CELLULOSE |
VALUE US$/T |
PURPOSE | POTENTIAL (NL) T/YEAR |
hempfibre |
53 | cott. linters cellulose abaca cellulose |
2000 4000 |
specialty papers |
3,000 |
softwood CTMP softwood cellulose |
715 825 |
strength in testliner, LWC, sanitary |
60,000 |
||
woody core |
56 | hardwood CTMP (aspen) |
735 | stiffness and printability, in board, and coated grades |
130,000 |
hardwood cellulose birch/eucalypt mixed hardwoods |
815 765 |
printing/ writing |
130,000 |
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.
Table 4. Pulp production, apparent consumption and net imports (in tons) in Europe in 1991 (CEPAC annual statistics) | |||
production | apparent consumption |
net import |
|
total wood pulp for EC 12 |
9.4 106 |
18.3 106 |
8.9 106 |
total wood pulp | 22.3 106 |
18.5 106 |
-3.8 106 |
total pulp in W. Europe |
31.7 106 |
36.8 106 |
5.1 106 |
chemical pulp in W. Europe |
19.0 106 (87% kraft) |
22.10 106 |
3.1 106 |
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