Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR ADJUSTING THE FIBROUS PROPERTIES OF PULP
The present invention relates to a method according to the preamble of claim 1
for adjust-
ing the fibrous properties of pulp. The invention also relates to a method for
manufacturing
the pulp according to claim 17 and for manufacturing a fibrous product
according to the
preamble of claim 18.
Paper mills use different wood materials in papermaking, depending on the type
of end
products, such as the paper or packaging products that are made of
intermediary products,
such as chemical and/or mechanical oz chemi-mechanical pulps. Generally,
different paper
grades are manufactured from both long and short fibres. Long-fibred raw
material imparts
strength to the paper, short-fibred material in turn imparts smoothness and
printing quali-
ties to the paper. For example, in the manufacture of the long-fibred chemical
pulp portion
of fme grade paper, raw material is used with an average fibre length
(weighted by the
length) of about 2 mm. The average fibre length of the long-fibred portion of
magazine
paper material should preferably be over 2.2 mm. The average fibre length of
the long-
fibred chemical pulp of the raw material used in the manufacture of paper (or
board) that
has top-quality printing and other properties should preferably be 2.3 mm.
Such a raw ma-
terial is called armouring (reinforcement pulp) fibre and it can be used
advantageously, for
example, in the manufacture of top-quality LWC (light weight coated) paper
with a low
basis weight. The printing properties of such paper are good, but not a great
deal of mate-
rial is used. The strength of the paper thus comes from the armouring fibre
and the printing
properties from the mechanical pulp.
in the methods currently used, -the fibre length of waod-is not -adjusted at
all or the adjust-
ment is carried out by classifying the wood, on arrival at a plant, according
to the log di-
ameter or the felling method. When raw material with varying fibre lengths are
mixed in
the right proportions, the mean value of the wood's fibre length can be set at
the correct
level. As the fibre length of the wood from a first thinning is shorter than
that of older
wood, the wood from the first thinning is combined with older wood and,
possibly, with
sapwood. However, the problem arises that the greater the amount of young
growing tim-
ber that is used as raw material, the shorter the fibre. Sawmill chips must be
mixed with the
raw material to increase the mean value of the fibre length in order to
produce raw material
with the desired fibre length for paper manufacturers. Nonetheless, it is
difficult to keep the
fibre length of the wood material at the desired level.
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In connection with this invention, it has been observed that the fibre length
of wood mate
rial obtained from the different parts of wood is different. On the basis of
these observa-
tions, the surprising discovery has been made that the number of annual rings
of a tree has
an impact on the tree's fibre length. When the wood material is classified
into categories
according to the number of annual rings, and wood material is taken from a
certain cate-
gory, a wood material with a homogeneous fibre length is obtained.
Previously, the fibrous properties of wood have been examined for individual
trees or
growing timber only. The patent publication US 2001/0018308 tried to
accomplish a cer-
tain fibre coarseness (less than 22 mg/100 m) and it was discovered that this
goal could be
reached by using treetops and wood from thinning. However, the publication
neither ex-
amined the number of the trees' annual rings by log or groups of logs, nor
divided the
wood material into categories according to the number of annual rings. The
wood was
roughly divided into grown wood and young wood only and it was considered that
the de-
sired fibre coarseness would be reached in this way by taking some young wood
or grown
wood or both in the right proportions. This may be so for trees grown in
certain conditions
and for a certain tree family or species. In order to really obtain wood
material with fibre
dimensions of homogeneous quality, the wood material must be examined much
more ac-
curately. The publication mentioned above does not present a solution to the
problem of
decreasing the variation in the wood material's fibre dimensions.
The international patent publication WO 00/72652 examines standing trees or
the sectional
planes of cut wood by scanning them by means of electromagnetic short-wave of
micro-
wave energy and by making an image of the scanned data. In this way, various
wood prop-
erties such as knots, were discovered, which cannot be observed from the
outside. On the
basis of the observations, the wood material was selected for various
purposes, such as
turning lathes, sawmills, for chemical pulp or paper. The publication also
mentions obser-
vations of annual rings but nothing about observing the number of annual rings
or dividing
the wood material into categories according to the number of annual rings. Nor
does it
mention any improvements in the smoothness of the fibre dimensions of the wood
mate-
rial.
The Japanese patent publication JP A 11-232427, G 06T 1100 describes a method
for de-
fining the number of annual rings, and the patent publication US 5,335,790
describes a
method for classifying the wood according to the brightness and the structure
or the texture
of the surface.
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According to the 'present invention, the fibrous properties of pulp, such as
chemical or pa-
per pulp can be adjusted by using, in the manufacture of chemical or chemi-
mechanical
pulp, a wood material, which is selected on the basis of the number of the
tree's annual
rings.
More specifically, the solution according to the invention is mainly
characterized by what
was stated in the characterising part of claim 1.
The number of the tree's annual rings correlates with its fibre dimensions,
such as the fibre
length and the fibre coarseness. Classifying the wood material into categories
on the basis
of the number of annual rings results in a wood material with homogeneous
fibre dimen-
sions. If the categories are appropriately selected, the variation in
dimensions within a
category is also minor. The definition of the annual rings can be mechanized
and carried
out at any stage between wood harvesting and pulping, if talking about making
chemical
pulp, or the number of annual rings can be defined at any stage between wood
harvesting
and grinding or refining (a (C)TMP refiner), if mechanical or chemi-mechanical
pulp is
made.
The wood material can be classified into categories on the basis of the number
of annual
rings in various ways. By now selecting the raw material from a certain
category, a fibre
with dimensions at the desired level can be obtained. On the other hand,
combining and
mixing the wood material that is divided into the different categories can
result in a fibre
with the desired, preselected fibre dimensions. To obtain the preselected
fibre dimensions,
it may be necessary to divide the annual ring categories in a different way
depending on
the sort of wood in question..
The invention provides considerable advantages. Accordingly, the method
described herein
can be used to improve the quality of wood material and to obtain a wood
material with
homogeneous fibre dimensions for various processes. The method according to
the inven-
tion can be used to adjust the fibrous properties of chemical pulp, mechanical
pulp or
chemi-mechanical pulp, to adjust the smoothness of the properties and to
decrease any
variation in quality. The method can be used to improve the quality of fibre
products. As
the fibrous properties can be affected since the felling of the tree, and not
until after meas-
wring the fibrous properties, the method is also advantageous in terms of
economy and
production.
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In the following, the invention is described more closely with the aid of a
detailed specifi
cation and a few application examples.
Fig. 1 shows the correlation between the annual rings and the fibre lengths
and the fibre
coarseness (with standard length deviations) of the top logs of spruce, which
are obtained
in regeneration cutting, when the material is classified into annual ring
categories;
Fig. 2 shows the correlation between the tops, which are classified according
to the di-
ameter, and the fibre length and the fibre coarseness (with standard length
deviations) of
spruce logs, which are obtained in regeneration cutting;
Fig. 3 shows the correlation between the butt logs, which are classified
according to the
number of annual rings, and the fibre length and the fibre coarseness (with
standard length
deviations) of spruce, which are obtained in regeneration cutting;
Fig. 4 shows the correlation. between the butt logs, which are classified
according to the
diameter, and the fibre length and the fibre coarseness (with standard length
deviations) of
spruce, which are obtained in generation cutting;
Fig. 5 shows the correlation between the top logs, which are classified
according to the
log's age/annual rings, and the fibre length and the fibre coarseness (with
standard length
deviations) of spruce;
Fig. 6 shows the correlation between the top logs, which are classified
according to the
diameter in thinning, and the fibre length and the fibre coarseness (with
standard length
deviations) of spruce;
Fig. 7 shows the correlation between the butt logs, which are classified
according to the
-_- .-... annual rings in thinning, and the-fibre-length.and the fibre
coarseness (with standard-length .
deviations) of spruce;
Fig. 8 shows the correlation between the butt logs, which are classified
according to the
diameter in thinning, and the fibre length and the fibre coarseness (with
standard length
deviations) of spruce;
Fig. 9 shows the correlation between the top logs, which are classified
according to the
annual rings in regeneration cutting, and the fibre length and the fibre
coarseness (with
standard length deviations) of pine;
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Fig. 10 shows the correlation between the top logs, which are classified
according to the
diameter in regeneration cutting, and the fibre.length and the fibre
coarseness (with stan-
dard length deviations) of pine.
Pulp in the present invention refers to chemical pulp, mechanical pulp or
chemi-
5 mechanical pulp. Pulp, the fibre dimensions of which are at the preselected
level, can be
used in the manufacture of paper, board or packaging materials or in any other
processes
that utilize pulp.
According to the present method, the wood material is classified according to
the number
of the log's annual rings. The annual rings of a tree are formed, when the
tree grows peri-
odically, and they can be distinguished from one another. For example, in the
cross-section
of Finnish conifer trees, one can observe the alternating light-coloured
'springwood rings'
of cells/fibres with large cavities and thinner walls and 'summerwood rings'
of cells/fibres
with small cavities and thick walls. In this example, the age of the log in
years is obtained
by counting the number of these concentric cycles. The periodic nature in turn
is a conse-
quence of the variation in the environmental conditions, such as light, heat
and water sup-
ply. The alternation of seasons between a warm summer and a cold winter
induces the
formation of annual rings, as well as, for example, a dry summer and a rainy
winter.
The classification of wood "by log" means that the number of annual rings is
determined
for each tree harvested from a forest,~the cut parts of which are called logs.
When logs are
known to be close to one another in terms of the numbers of annual rings, "by
groups of
logs" correspondingly refers to the definition of the number of annual rings
from a group
of two or more logs. The classification is preferably carried out mechanically
or by model-
ling. The division/sorting __ _. _ _ .according to annual rings .can, be
carried out at any processing
stage of the wood, when making chemical pulp (after felling the tree and
before pulping) or
when making mechanical or chemi-mechanical pulp (after felling the tree and
before
grinding or before the refining process). The number of annual rings can be
determined, for
example, near the chopping machine, after which the logs are classified into
their respec-
tive piles according to the number of annual rings. It is preferable to define
the number of
annual rings as early as at the chopping machine in the forest in connection
with felling the
wood. For example, the chopping machine can have a device at its end, which
reads the
annual rings in the same manner as bar code readers read bar codes. In this
case, the wood
material can be directly classified according to the number of annual rings,
when the tree is
felled, decreasing the need to classify the logs at a later stage. The
classification can pref
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6
erably be carried out mechanically in the plant by counting the number of
annual rings or
by classifying in accordance with the markings that are based on the number of
annual
rings or modelling and obtained by means of the felling machine.
The method according to the present invention can be applied to the
manufacture of
chemical or paper pulp, when using wood that has a periodic growth habit as
raw material.
Trees that have periods of quick and slow growth have such a growth habit, the
periods
resulting from fluctuations in heat, light and/or water supply, as described
above.
The wood material can either be softwood or hardwood. Softwood, for example,
comprises
the spruce and the pine. Hardwood comprises birch, aspen, hybrid aspen,
poplar, beech
tree, red beech, hornbeam, oak, alder, maple, acacia and eucalyptus.
The fibre dimension property refers to the fibre length and the fibre
coarseness, for exam-
ple. The fibre length refers to the arithmetic mean value of the fibre length
distribution, the
mean value of the fibre length distribution weighted by the fibre length or
the mean value
weighted by the weight. Of these, the mean value weighted by the length is
generally used
to provide the best description of the technical potential of fibre.
The fibre coarseness refers to the weight of a fibre sequence of chemical or
paper pulp, for
example, the weight of a meter of the fibre sequence in milligrams. The above
can be
measured using equipment especially developed for this purpose, such as the FS-
200 and
Fibrelab instruments. The devices are based on measuring the fibre dimensions
by optical
measuring methods in a medium that flows in suitable filter troughs. The
results obtained
by means of the devices are device-specific and, therefore, not absolutely
accurate. Good
laboratories have developed in-house standards and calibrations for the
measuring.
In connection with the present invention, we have surprisingly discovered
that, when the
raw material is classified into different categories according to the number
of annual rings,
certain desired fibre lengths and dimensions as well as extraordinary
smoothness levels are
achieved. The number of categories can be 2 to 60; preferably the number of
categories is
2 to 6, typically 3 to 4 or 3 to 5.
According to a preferred embodiment, the classification can comprise the
following cate-
gories, for example: less than 20 annual rings, 21 to 30 annual rings, 31 to
40 annual rings,
over 40 annual rings. If the wood material is classified, for example, into
the following
categories: less than 20 annual rings, 21 to 30 annual rings, 31 to 40 annual
rings, over 40
annual rings, the following correlations are obtained:
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Number of annual rings Fibre length
less than 20 2
21 to 30 2.3
31 to 40 2.4
over 40 2.5
According to another preferred embodiment, the annual rings are classified
into the fol-
lowing categories: less than 10 annual rings, less than 20 annual rings, less
than 30 annual
rings, less than 40 annual rings, less than 50 annual rings, over 50 annual
rings.
By classifying the raw material by log or group of logs into different
categories according
to the number of annual rings, the desired fibre dimension property, such as
the fibre
length, is obtained by selecting the wood material separately from a certain
category or by
combining the raw material obtained from different categories in the right
proportions.
The present method can be used to manufacture mechanical, chemical or chemi-
mechanical pulp from the selected raw material.
For example, if a fibre product is to be made of softwood, the fibre length
(the mean value
weighted by the length) of the product being less than 2.0 mm, typically, a
wood material
with less than 20 annual rings at the butt of the log should be selected. If a
fibre product is
to be made of softwood, its fibre length being in the range of 2Ø..2.3, a
wood material is
selected, which has 21 to 30 annual rings at the butt of the log.
Correspondingly, to obtain
a fibre length in the range of 2.3. ..2.5, a wood material is selected, which
has 31 to 40 an-
nual rings at the butt of the log, and to obtain a fibre length in the range
of 2.5...3.5, a
wood material is selected;-wfierein the number of the log's annual rings
at~~tlie butt of the
log is over 40 annual rings. In the example, the butt is measured but,
alternatively, meas-
uring can be carried out at the log's top end, and the annual ring can be
changed, e.g., with
the aid of a model, so that it corresponds to the number of the butt's annual
rings as a
function of the log's length.
To obtain the preselected fibre dimensions, it may be necessary to divide the
annual ring
categories of different sorts of wood in a different way. The fibre lengths of
birch, for ex-
ample, range from about 0.7 mm to 1.2 mm and the annual ring categories must
be selected
so that they give the desired fibre length for this range.
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The method according to the invention for manufacturing pulp that has
preselected fibre
dimension properties typically comprises the following processing stages:
- wood with a periodic growth habit is selected as the wood material,
- according to its annual rings, the wood material is classified by log or
group of logs
into categories that represent a certain fibre dimension, e.g., the fibre
length and/or
the coarseness,
- the raw material is selected separately from a certain category or by
combining the
raw materials of different categories partly or fully so that the preselected
fibre di-
mensions are achieved, and
- mechanical, chemical or chemi-mechanical pulp is manufactured of the wood
mate-
rial.
Different coniferous or broad-leaved trees can be classified according to the
annual ring
categories of a log or a group of logs, and an improvement in quality can be
achieved in
the adjustment of the fibre dimensions andlor the smoothness tolerance of
chemical, me-
chanical or chemi-mechanical pulp and utilized in improving the quality of the
manufac-
fared products.
A method according to the invention for manufacturing a fibre product that has
preselected
fibre dimension properties, typically comprises the following processing
stages:
- categories of annual rings that represent certain fibre dimensions are
defined for the
wood material by logs or groups of logs and according to the number of annual
rings,
- the wood material is selected from the category of annual rings, which
provides the
preselected fibre dimension properties,
- mechanical, chemical or chemi-mechanical pulp is manufactured of the wood
mate-
rial, and
- the fibre product is made of the pulp.
The following non-limiting examples illustrate the invention:
A total of 442 samples were taken of the sweep of spruce wood (from
regeneration cutting
and thinning) coming to the plant. The tops and butt parts of the trees were
sorted sepa-
rately and their percentage was determined. According to the results, the
volume (and at
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this accuracy also) the weight fraction of the tops from the regeneration
cutting was 54%
and that of the butt parts 15%, and the portion of the tops from thinning was
17% and that
of the butt parts was 14% of the entire sweep of spruce coming to the plant.
This material
was measured and classified according to the log thickness and/or the number
of annual
rings.
Example 1
The trees felled in connection with the regeneration felling of spruce were
classified by top
log, according to the number of annual rings, into the categories of <20
annual rings, 21 to
30 annual rings, 31 to 40 annual rings and >40 annual rings. The results are
shown in Fig.
1.
The portion of wood in the category of <20 annual rings was 12%, the fibre
length ranged
between 1.98 and 2.16 (standard deviation), the average fibre length weighted
by the
length was 2.07 mm. The fibre coarseness in this category ranged between 0.149
and 0.160
mg/m (st.dev.).
The portion of wood in the category of 21 to 30 annual rings was 38%, the
fibre length
ranged between 2.20 and 2.38 (st.dev.), and the average fibre length was 2.29
mm. The
fibre coarseness in this category ranged between 0.163 and 0.172 (st.dev.)
mg/m.
The portion of wood in the category of 31 to 40 annual rings was 25%, the
fibre length
ranged between 2.36 and 2.48 mm (st.dev.), the average fibre length was 2.42
mm. The
fibre coarseness in this category ranged between 0.171 and 0.178 mg/mm
(st.dev.).
The portion of wood in the category of >40 annual rings was 25%, the fibre
length ranged
between 2.46 and 2.59 mm (st.dev.), the average , fibrevlength was 2.52 mm.
The fibre
coarseness in this category ranged between 0.184 and 0.178 mg/mm (st.dev.).
Example 2
The tops of the trees felled in connection with the regeneration cutting of
spruce were clas-
sified according to the diameter into the categories of <80 mm, <100 mm, <120
mm, <140
mm, <160 mm, >160 mm. When the fibre length and the fibre coarseness of the
categories
were measured, it was observed that the fibre lengths and the fibre
coarsnesses were partly
or fully overlapping, as shown in Fig. 2. For example, in the categories of
<80 mm and
<140 mm, the fibre lengths are fully overlapping and, thus, the classification
by diameter
hardly has any significance in practice.
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Example 3
The trees felled in connection with the regeneration cutting of spruce were
classified by
butt log, according to the number of annual rings, into the categories of <30
annual rings,
<40 annual rings and >40 annual rings. The results axe shown in Fig. 3.
5 The portion of wood in the category of <30 annual rings was 10%, the fibre
length ranged
between 2.21 and 2.44 mm (calculated st.dev.), the average fibre length
weighted by the
length was 2.32 mm. The fibre coarseness in this category ranged between 0.163
and 0.175
mglm (st.dev.).
The portion of wood in the category of <40 annual rings was 21%, the fibre
length ranged
10 between 2.40 and 2.56 mm, the average fibre length was 2.48 mm. The fibre
coarseness in
this category ranged between 0.173 and 0.182 mg/m (st.dev.).
The portion of wood in the category of >40 annual rings was 69%, the fibre
length ranged
between 2.46 and 2:68 mm, the average fibre length was 2.57 mm. The fibre
coarseness in
this category ranged between 0.188 and 0.176 mg/m (st.dev.).
Example 4
The wood felled in connection with the regeneration cutting of spruce was
classified by
log, according to the diameter, into the categories of <100 mm, <120 mm and
>120 mm.
When measuring and calculating the fibre length and the fibre coarseness of
the categories,
it was discovered that the fibre lengths and the fibre coarsenesses (st,dev.)
are partly over-
lapping, as shown in Fig. 4.
Example 5
The-top pails ~of the trees felled in connection with the thiiiniiig of spiace
were classified
according to the age or the number of annual rings into the categories of <20
annual rings,
21 to 30 annual rings, 31 to 40 annual rings and >40 annual rings. The results
are shown in
Fig. 5.
The portion of wood in the category of <20 annual rings was 20%, the fibre
length ranged
between 1.99 and 2.19 mm (st.dev.), the average fibre length weighted by the
length was
2.09 mm. The fibre coarseness in this category ranged between 0.150 and 0.162
mg/m
(st.dev.).
The portion of wood in the category of 21 to 30 annual rings was 47%, the
fibre length
ranged between 2.23 and 2.46 mm (st.dev.), the average fibre length weighted
by the
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length was 2.35 mm. The fibre coarseness in this category fanged between 0.164
and 0.177
mg/m (st.dev.).
The portion of wood in the category of 31 to 40 annual rings was 21%, the
fibre length
ranged between 2.36 and 2.50 mm (st.dev.), the average fibre length weighted
by the
length was 2.43 mm. The fibre coarseness in this category ranged between 0.172
and 0.179
mg/m (st.dev.).
The portion of wood in the category of >40 annual rings was 21 %, the fibre
length ranged
between 2.43 and 2.60 mm (st.dev.), the average fibre length weighted by the
length was
2.51 mm. The fibre coarseness in this category ranged between 0.174 and 0.185
mg/m
(st.dev.).
Example 6
The top parts of the trees felled in connection with the thinning of spruce
were classified
according to the diameter into the categories of <100 mm, <140 mm, <120 rnm,
<160 mm
and >160 mm. When measuring the fibre length and the fibre coarseness of the
categories,
it was discovered that the fibre lengths and the fibre coarsenesses (st,dev.)
are partly or
fully overlapping, as shown in Fig. 6.
Example 7
The butt parts of the trees felled in connection with the thinning of spruce
were classified
by log and according to the number of annual rings into the categories of <20
annual rings,
21 to 30 annual rings, 31 to 40 annual rings and >40 annual rings. The results
are shown in
Fig. 7.
The portion of wood in the category of <20 annual rings was 4%, the fibre-
length.ranged
between 1.97 and 2.16 mm (st.dev.), the average fibre length weighted by the
length was
2.06 mm. The fibre coarseness in this category ranged between 0.149 and 0.160
mg/m
(st.dev.).
The portion of wood in the category of <30 annual rings was 27%, the fibre
length ranged
between 2.28 and 2.46 mm (st.dev.), the average fibre length weighted by the
length was
2.37 mm. The fibre coarseness in this category ranged between 0.167 and 0.176
mg/m
(st.dev.).
The portion of wood in the category of <40 annual rings was 33%, the fibre
length ranged
between 2.45 and 2.57 mm (st.dev.), the average fibre length weighted by the
length was
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2.51 mm. The fibre coarseness in this category ranged between 0.176 and 0.183
mg/m
(st.dev.).
The portion of wood in the category of >40 annual rings was 21%, the fibre
length ranged
between 2.55 and 2.64 mm (st.dev.), the average fibre length weighted by the
length was
2.60 mm. The fibre coarseness in this category ranged between 0.182 and 0.188
mg/m
(st.dev.).
Example 8
The logs felled in connection with the thinning of spruce were classified
according to the
diameter into the categories of <100 mm, <120 mm and >120 mm. When measuring
the
fibre length and the fibre coarseness of the categories, it was discovered
that the fibre
lengths and the fibre coarsnesses (st.dev.) are partly or fully overlapping,
as shown in Fig.
8.
Example 9
The top logs of the trees felled in connection with the regeneration cutting
of pine were
classified according to the number of annual rings into the categories of <20
annual rings,
21 to 30 annual rings, 31 to 40 annual rings, 41 to 50 annual rings and >50
annual rings.
The results are shown in Fig. 9.
The portion of wood in the category of <20 annual rings was 2%, the fibre
length ranged
between 1.57 and 1.79 mm (st.dev.), the average fibre length weighted by the
length was
1.68 mm. The fibre coarseness in this category ranged between 0.197 and 0.206
mglm
(st.dev.).
The portion of wood in the category of 21-. to..30 annual rings was 13%, the
fibre length- -
ranged between 1.85 and 2.07 mm (st.dev.), the average fibre length weighted
by the
length was 1.96 mm. The fibre coarseness in this category ranged between 0.207
and 0.214
mg/m (st.dev.).
The portion of wood in the category of 31 to 40 annual rings was 14%, the
fibre length
ranged between 1.99 and 2.16 mm (st.dev.), the average fibre length weighted
by the
length was 2.08 mm. The fibre coarseness in. this category ranged between
0.212 and 0.216
mg/m (st.dev.).
The portion of wood in the category of 41 to 50 annual rings was 24%, the
fibre length
ranged between 2.12 and 2.22 mm (st.dev.), the average fibre length weighted
by the
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length was 2.17 mm. The fibre coarseness in this category ranged between 0.215
and 0.218
mg/m (st.dev.).
The portion of wood in the category of >50 annual rings was 47%, the fibre
length ranged
between 2.19 and 2.28 mm (st.dev.), the average fibre length weighted by the
length was
2.24 mm. The fibre coarseness in this category ranged between 0.217 and 0.221
mg/m
(st.dev.).
Example 10
The tops of the trees felled in connection with the regeneration cutting of
pine were classi-
fied according to the diameter into the categories of <100 mm, <120 mm, <140
mm, <160
mm and >160 mm. When measuring the fibre length and the fibre coarseness of
the catego-
ries, it was discovered that the fibre lengths and the fibre coarsenesses
(st,dev.) are partly
or fully overlapping, as shown in Fig. 10.
