The dry-line method in bast fibre production

This thesis introduces a new harvesting and processing method for bast fibre plants
such as flax fibre, linseed flax (Linum usitatissimum L.) and hemp fibre (Cannabis
sativa L). The new combination of the autumn harvesting of linseeds and the spring
harvesting of fibres is called the dry-line method®. Originally spring harvesting, on
which the dry-line method is based, was introduced for harvesting reed canary grass
(RCG) (Phalaris arundinacea).

Finnish weather conditions are favourable for ideally weathering the plants for the dryline
method of harvesting. Investigations show that Finnish weather conditions cause a
succession of autumn moisture, winter freezing and spring drying of the bast fibre plants.
These beneficial Nordic growing conditions offered by nature can be economically
exploited.

The existing traditional harvesting and processing methods for bast fibre plants normally
present two major problems: high moisture content in the harvested plants and the
problem of detaching the bast fibres from the stems. These problems joined with the
traditional processing technology used are not responding at the moment to the demands
introduced by new industrial products and uses.

Economically the most important aspects of the new dry-line method are the autumn
harvesting of the linseed from the bast fibre plants and the low moisture content of the
fibre yield in the spring. The autumn conditions are normally dry enough for the harvesting
of mature seeds from the tops of the bast fibre plants. For a farmer this guarantees
their income in the form of the linseed yield.

In Finnish latitudes (60–63°N) harvesting is possible for both fibre and linseed flax.
The advantage of using hemp fibre is based on its photoperiodic properties; hemp fibre
does not produce mature seeds in Finland, instead the photoperiodic properties of hemp
fibre in the long daylight conditions results in a high biomass yield.

In the dry-line method the linseed and hemp stems have been harvested in the spring
after the moist autumn and frosty winter. The relative humidity (RH) of the air is low
during the harvesting period, between March and May. This produces an important
economical advantage, the equilibrium moisture content (EMC) of the harvested material
is low (normally 10% wet basis) the material therefore needs no extra indoor drying.
In autumn harvesting, the drying of the biomass is normally a crucial factor because
the industry desires bast fibre material with moisture content between 10–20%
wet basis. In existing harvesting methods it is not possible to detach the bast fibres
before drying the plants.

The winter freezing process on the bast fibre plants results in the easy detachment of the
bast fibres. It is now possible to produce new industrial raw materials using existing
light farm machinery integrated with simple milling and separation equipment. The
new harvesting method enables the use of these materials in applications such as pulp,
moulded fibre packages, composites and oil adsorption products.

On the microscopic level the structure of the fibre’s surface becomes more open when
harvested in the spring. The autumn moisture and repeated winter freezing opens the
fibre bundles and pore structures of the bast fibre plants. During the springtime the
standing plants are dried under low relative humidity air conditions. This produces
much larger surfaces in the bast fibre plant material when compared to the material that
is harvested and processed using the traditional methods.

Using the dry-line method (as expected) results in a decrease in the traditional textile
fibre quality parameters that is in the fibre bundle strength, while the bast fibres are
highly retted. However the detachment of the bast fibres can be carried out easily. These
properties can be utilized in bast fibre harvesting, processing and manufacturing. The
dry-line method is more economical due to the lower drying costs, easy detachment of
bast fibres and higher bast fibre yields during processing when compared to the traditional
methods of harvesting and processing.

The properties of the autumn and spring-harvested plant materials were compared in
this thesis. Some key investigations into potential new dry-line based product lines such
as: adsorption materials, pulp products, and composites were carried out. The focus of
the research presented in this thesis has been new economical production lines and
products for bast fibre plants.