Potential of Papermaking Fibers
22 October 2015
This presentation aims to review the effects of stock treatment,
drying and recycling on fiber properties.
It is common knowledge that the pulp made in the mill process is
inferior to the laboratory-made reference pulp.
Another big difference is in never-dried pulp compared to dried
Third interesting thing related to the previous changes is the
difference between virgin pulp compared to recycled pulp.
The purpose of this review is to present some information related
to all these three topics.
My LinkedIn profile can be found by following this link:
Wood is a complex natural composite built up of fibers that are glued together by lignin.
Fibers consist of fibrils that are held together by lignin and hemicellulose.
Fibrils are built up of bundles of microfibrils.
Volumetric composition of hardwood and softwood
Fibers 90 - 95 %
Ray cells 5 - 10 %
Fibers 27 - 76 %
Vessels 7 - 55 %
Ray cells 5 - 25 %
Wood cell types
Fibers/tracheids are best for papermaking. Other cell types cause mainly problems.
This is one reason that softwood fibers are best and nonwood fibers worst fibers.
Breaking lengths of some materials
Breaking length is a very good measure of material
strength because it takes the density of the material
into account. The following list of breaking lengths
Graphite 37 km
Eastern white pine wood 23
Paper from bleached softwood 8-10
We can notice that wood strength is more than
double compared to softwood paper strength which
is about double to steel strength.
There is good potential to improve paper strength
closer to wood strength.
Laboratory pulp compared to mill pulp
Tear strength with same tensile strength in lab pulp is always better than in mill
pulp. What are the basic reasons to this difference?
How could we improve mill pulps?
Laboratory vs. industrial digester
It is well known that a laboratory digester (left picture) produces stronger fibers than an
industrial digester (right picture).
Chemically and mechanically the treatment of fibers is different in laboratory vs. mill.
Picture: YokogawaPicture: Thwing-Albert
Fiber length of laboratory pulp compared to mill pulp
Measured fiber length of mill
pulp is shorter than with
Induced damage points, fiber
deformations and porosity
changes occurred during
processing are taught to be
responsible for most of the
observed strength loss when
industrial and laboratory pulps
from softwood are compared.
Fiber kappa of mill vs. laboratory pulps
Laboratory cooking can produce much more homogenous delignification than
Lab-cookedvs. commercial pulp
0 10 20 30 40 50 60 70 80 90
COV = 0.54
COV = 0.34
Picture: TAPPI Journal Dec 2002
Pulping and fiber wall strength
Fiber wall is like a brick wall. In the wood the
strong cellulose groups are surrounded with
lignin and hemicellulose.
During cooking the wall looses most of lignin
There will be lot of porosity in the structure and
it is vulnerable to mechanical forces in the
At the end of cooking more than 50 % of the
wall volume is dissolved.
Fiber length can be original, but coarseness is
only half and strength of individual fiber is much
less than in the beginning.
Lower kappa – more lignin
has been eaten from the fiber wall
Capillary structure of a delignified
fiber (McIntosh 1950)
Defects and failure zones in fibers
Examples of non-homogeneous zones in fibers include curl, kink, dislocation,
microcompression and twist.
In mill operation, fiber damages can be induced accidentally or intentionally, by shearing
at high consistency. Some pulps are highly susceptible to damages, others are more
The most important damage might be curl which is not necessarily stable. It is readily
removed from some pulps but not from others. Curl can be stabilized by certain
treatments, notably by heat treatment at high consistency.
Curl and other damages are often disregarded because they cannot be easily measured.
Yet in practice their effects often dominate the properties of pulp suspensions, wet webs
and dry sheets. Ignoring these effects has led to costly surprises, both in research and
Fiber curl and paper properties
Curled fibers decrease tensile strength but increase strain, tear strength, bulk and
porosity. Very small amount of curled fibers can have big influence.
Some papers can benefit of this e.g. sack kraft, absorbent papers and tissue papers.
Cartonboard middle layer can also benefit of higher bulk and better board stiffness.
Straight fibers of commercial
softwood bisulphite pulp of
Highly curled fibers of
bleached softwood kraft pulp
Moderately curly fibers of
commercial dried bleached
softwood kraft pulp
Pictures: D. H . Page, R. S. Seth et al.
Fiber curl and kinks made with kitchen mixer
Spruce kraft pulp fibers were beaten with PFI beater 2000 revs and deformed in Hobart
kitchen mixer for 0, 15 and 45 minutes. The aim was to introduce fiber deformations
without changing fiber swelling.
Homogenization was carried out at room temperature 25 °C and consistency of 9%.
Olli Joutsimo, KCL Finland
Tensile strength deterioration after cooking
Immediately after cooking fibers have
good tensile strength, but later in the
process tensile strength is deteriorated
already before refining.
The reason could be mechanical forces
due to blowing, pumping, mixing etc.
The picture shows how mixing in the
Hobart mixer increases fiber curl and
the number of kinks thus decreasing
tensile strength. However, paper strain
is increased at the same time.
This seems to be quite similar behavior
as latency of mechanical pulp.
Olli Joutsimo, KCL Finland
Improvement of tear strength after cooking
When tensile strength is decreased due
to forces in mechanical mixing, tear
strength is increased at the same time.
Olli Joutsimo, KCL Finland
Reduction of TMP fiber curl
In the figure below is tensile index versus fiber curl before and after LC-refining and
tested after no, cold and hot disintegration. The handsheets were made without
This shows how latency removal and low consistency refining can improve tensile
strength and reduce fiber curl.
864 Nordic Pulp and Paper Research Journal Vol 27 no.5/2012
Bleached pulp process
There are too many high turbulence shearing forces to fibers in the pulp mill.
Should we use completely different pumping and mixing technology.
At least pumping against valves should be avoided. May be volumetric pumps
could be better.
Latency removal of mechanical pulp
Latency of mechanical pulp has been known for a long time. This experience should
also be used in chemical pulp treatment.
Latency removal involves removal of fiber curling, which occurs at high-consistency
treatment, by means of mixing the pulp a certain period of time at a lower
consistency (2 - 4 %) and a temperature of 70 – 80 °C.
Typical process in a softwood stock preparation
To save pulp strength there should be different low turbulence mixing and pumping
principles than today. Minimum amount of turbulence and low consistencies.
No valves, use of speed controlled pumps, minimal stock mixing.
Two refiners in series or newest refiner designs would give more even result (fiber
straightening) and lower SEL.
Why not only one mixer
instead of two chests?
Are these really needed?
Low specific edge load
Low consistency pulper
Online measurement of single fiber properties
Measurements of single fiber properties are new tools to develop and control
papermaking process and paper quality.
Fibrillation effects on fibers
Target of a good refining is to get external fibrillation (hairy surface and fibril fines)
and internal fibrillation (delamination of fiber layers).
Unrefined softwood Refined hairy softwood
Dimas Dwi Prasetyo Nugroho (2012)
Adam A. Brancato
Delaminated hairy fiber
Bonding of refined fibres
External fibrillation can make very long bonds
compared to fiber thickness.
There are more bonds in fiber crossings.
Good bonding requires fiber flexibility and lumen
collapse to get more intimate contact.
Fiber fines and higher surface tension enhance
bonding especially at fiber crossings.
Curled fibers decrease relative bonded area by
geometrically preventing bonding. Low
consistency refining straightens fibers.
Electrostatic environment in refining can have a
large effect on internal fibrillation and refining
Electrostatic environment in refining
Water retention value (WRV) is a good measure how much fibers are delaminated
The picture below shows how much better refining and papermaking can be in
sodium environment compared to acidic environment.
Bäckströn and Hammar, Counterions & refining, 2010
Tensile index and fiber fines content
It is quite common opinion that fiber length is most important to get high tensile
strength but also fines content is very important.
Small amounts of fiber fines have big effect on tensile strength of chemical pulp.
Fines in the picture means material through mesh № 200 (75 μm).
Picture below is simplified and redrawn from the Thesis of Yana Zaytseva (2010).
So called crill is much finer than this fines.
0 2 4 6 8 10
Fines content, %
Crill and tensile strength
Crill consists of thin fibrils that are partially or completely loosened from the fibers. Crill
fibril thickness (250 nm) is about one hundred of fiber thickness.
Despite the fact that crill represents only approx. 1% by weight of the particles in a
suspension, it may contribute to as much as 50% of the free surface.
Research studies at Innventia have shown that crill is the single variable having the
strongest connection to paper strength. Lab results in the figure below show a strong
correlation to paper tensile strength.
High definition image analysis vs. tensile strength
Valmet has introduced a new pulp analyzer based on microscopic online testing.
The picture below shows how sensitive the measurement is. Small changes in external
fibrillation percentage have large influence on tensile strength.
Wet paper vs. wet soil structure
When the solids content of paper is 20 - 50% the
behavior of the suspension is very similar as soil
suspensions, which are very well studied. The picture
on the right and the following text is from soil behavior:
“Due to its surface tension, water molecules in the
interparticle voids bond the soil grains at their interface
with the air that is present in the voids and where
The smaller the grain size, the greater the bonding or
apparent cohesion. For example, suction effects on
uniformly graded gravel would be negligible while the
effects on well-graded gravel could be significant.
Even a small amount of fines in sand can result in
measurable cohesion. In the context of reinforced
walls and slopes”.
This explains very well why small amount of fiber fines
have great effect on paper strength. This also explains
that fines will be transferred to the contact points with
water where it will be most effective for bonding.
Drying stresses and tensile strength
Machine made papers always have higher tensile strength in the machine direction.
How much higher depends on fiber orientation (normal range of tensile ratio 2-4).
In addition, sheet stress under drying has great effect on tensile strength.
Kärenlampi, P., Niskanen, K., Fapet
Structural changes in fiber and paper properties
The effects on the table are my subjetive opinions and valid only for chemical pulp.
Mechanical pulp fines is less bonding compared to chemical pulp fines.
Tensile Internal Light Smooth-
strength bond scattering ness
+++ + – ++ –
+ = positive change – = negative change
++ +++ –– + ––
++ +++ –– + ––
++ + – – +
Nonwood fibers and linting in offset printing
Nonwoods and some special hardwoods have very much
vessels and fine fiber material including parenchyma,
epidermal and ray cells.
Several studies have shown that most of the linting
material in uncoated paper is parenchyma and ray cells.
These cells are so small and light that they follow accept
in screening and cleaning. Refining has practically no
effect on small cells.
Old paper machines have Fourdrinier wire at least in the
beginning of the wire section. This means that fines and
dusting material is more on the top side of the paper.
In addition, press section often has last felt on the bottom
position. This means that there will be more bonds
between fibers on the bottom side.
Old wire and press section concepts lead to high linting
tendency on the top side. If now filler content is high, it
also means that the filler concentrates to the top side and
prevents fiber bonding even more.
Lint from offset blanket
Nonwood fiber problems
Nonwood fibers normally have about 50 % of the area other cells than fibers, such
parenchyma, epidermal and vessel cells. These cells are not good for papermaking.
Nonwood pulp has high fines content and fibers can have thin cell walls. These together
lead to dewatering and runnability problems as well as to low paper stiffness.
bulk & stiffness
For woodfree papers it is common to refine softwood and hardwood separately (left figure).
Good practice is to refine first softwood, then blend hardwood to softwood and refine them
together (right figure).
Sequential refining seems to give better tensile strength with same energy consumption.
VAIL MANFREDI: 2006 Pan Pacific Conference Advance in Pulp and Paper Sciences & Technologies
Optimal SW/HW refining
This sequential refining is more flexible than common refining, gives better result than
completely separate refining and requires less equipment and energy than completely
Original refiners vs. modern refiners
Old refining method was so called stamper, where fibers were beaten with big
hammers. Fiber flocks got during long time treatment of pressure forces but very little
shear forces. This saved fiber length.
Modern refiners such as Valmet Optifiner Pro have high capacity and lower energy
Should the refiners create even more pressure forces and less shear forces?
Hornification in pulp drying
Internal fibrillation can be described as the breakage of the crosslinks between microfibrils
during beating. It reduces the effective moment of inertia of the cell wall, thus increasing
wet fiber flexibility, conformability, and collapsibility. As a result, internal fibrillation mainly
enhances inter-fiber bonding and improves paper tensile strength.
A schematic illustrating possible change in pore structure resulted from beating previously
dried pulps. It shows that even though the pore volume of previously dried pulps can be
recovered by beating, the permanent changes to pore structure have occurred. That is,
some pores, which are closed in drying, are not reopened by normal levels of beating.
In dried pulp
Internal Fibrillation in Never-dried and Once-dried Chemical Pulps
XINSHU WANG, THAD C. MALONEY AND HANNU PAULAPURO
Helsinki University of Technology, Espoo, Finland
Inter-fiber hydrogen bonding and debonding
After pulping and bleaching never-
dried pulp fibrils in fiber wall are
better separated than in a dried and
After pulp drying, slushing and
refining there are still some
irreversible hydrogen bonds inside
the fiber walls.
This means that the fiber is stiffer
and not as prone to collapse as
never dried pulp. Inter-fiber bonding
of dried pulp is not as good as with
Fibrillation and hornification in drying
Simplified presentation of external and internal fibrillation and hornification in drying.
These explain the differences between never-dried and dried pulps.
Integrated paper and board mills can benefit compared to mills using bale pulp.
Pictures: Hubbe et al. (2007)
Good bonding Less bonding
Never-dried fiber versus recycled
Never-dried fiber has more free fibrils and OH-groups for bonding to other fibers than
Especially recycled fiber shows hornification i.e. less free OH-groups and fibrils.
Recycled pulp also has more frayed fibrils and fiber curl.
Juan Cecchini 2015
Tensile strength and paper density
Never-dried pulp is stronger and requires
less refining energy for a certain tensile
strength and freeness.
It is well known that paper density and
tensile strength correlate when kraft pulp
For almost all papers bulk and tensile
strength are both desired properties.
It is interesting to know, if this
combination is better with never-dried
The curve on the right shows that even if
the never-dried pulp is stronger, the
combination of strength and bulk is same
with dried and never-dried pulps i.e. the
points follow a linear relation.
Picture: Xinshu Wang (2006)
Pele Oy Pore volume of never-dried and once-dried
Even though the pore volume of
previously dried pulps can be recovered
by beating (i.e., the fibers can be
reswollen), some pores are not reopened
by normal levels of beating. In other
words, beating does not completely
The likely explanation is that strong
irreversible hydrogen bonding is formed
between microfibrils in drying which is not
broken when the fiber is beaten.
Thus, beating mainly disrupts and loosens
macrofibrils (aggregated microfibrils),
creating large-sized pores in the cell wall.
Internal Fibrillation in Never-dried and Once-dried Chemical Pulps
XINSHU WANG, THAD C. MALONEY AND HANNU PAULAPURO
Helsinki University of Technology,Espoo, Finland
hw-nd = hardwood never-dried
hw-od = hardwood once-dried
Tensile strength of bamboo pulp
Tensile strength of never-dried bamboo pulp is better than after first drying.
Heijnesson-Hulten et al. (2013)
Xinshu Wang’s dissertation
Drying of pulps greatly reduces pulp swelling,
enhancing dewatering but impairing tensile
strength. Dried pulps offer a far better
combination of dewatering and tensile strength
than never-dried pulps. One possible reason is
that some small hard-to-dewater pores in the
fiber wall are irreversibly closed by drying, which
enables better dewatering.
However, pulp drying is energy-consuming.
Pressing pulps to high dryness may provide an
economical way to improve dewatering, while
maintaining paper strength properties. Pressing
hornifies pulps, which promotes dewatering but
impairs tensile strength to a certain extent.
On the other hand, pressing causes fibers to
flatten, with the flattened fibers providing more
surface contact for bonding, thus increasing
density and tensile strength. Never-dried pulps
which were pressed before refining were found
to give both improved dewatering and better
Xinshu Wang’s Dissertation (cont.)
The refining results support the earlier view
that internal fibrillation is largely produced by
a cyclic compressive action. It is suggested
that fibers need to be turned over in refining
and compressed from different directions in
order to disrupt their internal structure and
cause internal fibrillation.
Compression also facilitates fiber
straightening, but does not promote external
fibrillation and fines generation.
At the same swelling level, more straightened
pulps give higher tensile strength, and pulps
with less fines and external fibrillation enable
better dewatering. Hence, to achieve an
optimum combination of dewatering and
tensile strength, chemical pulp refining
should aim at increasing internal fibrillation,
straightening fibers, and keeping the amount
of fines and external fibrils at a low level.
If pulps are refined so that moisture content
after press section is same, once-dried pulp
requires 100, pressed never-dried pulp about
40 and never dried pulp about 25 kWh/t.
Beaten fibers of kraft pulp and hornification
The fibers in the picture are freeze-
dried to keep the fibrils visible and
out of the fiber surface.
When fibers are dried there will be
irreversible bonds between fibrils
and fiber surface. This is one part
of hornification and cause reduced
Effect of pulp drying on papermaking process and paper quality
Lower wet and
can be used
shrinkage Less curl
Free of micro-
How many times fibers are recycled?
It is a very common misunderstanding that recycled fibers are recycled 5 to 10
times. The picture below shows that only one out of every eight fibers is recycled
more than two times, even at a 70% closed-loop recovery rate.
Effect of recycling on CTMP
In this trial Eastern spruce CTMP was used.
It can be seen that there is more bonding with recycling.
Increased bonding causes higher strength and density, but lower tear and scattering
R.C. HOWARD and W. BICHARD
Effect of recycling on chemical pulp
In recycling bonding potential of bleached chemical pulp fibers decrease due to
hornification. This means that tensile and burst strengths can decrease up to 15%.
Tear strength and scattering coefficient increase as usual when bonding decreases.
R.C. HOWARD and W. BICHARD
Fiber lines in pulp mills and stock preparation lines in paper mills should be more
designed towards gentle fiber treatment and saving of fiber strength. New types of
pumps and tank agitating systems are needed.
Main refiners should be developed to create more normal forces and less shear
forces. Probably all fibers do not need refining at all. Refining only softwood
chemical pulp could be enough in many cases.
Fines content is important for bonding. It might be easier to get enough fines by
refining only a small part of pulp to a very low freeness.
Another possibility could be to use a small part such a pulp which is easy to refine
and make fines. Several nonwood pulps and some hardwoods would have this
kind of potential.
However, it should be remembered that best fines is so called secondary fines,
which is fibril material from S2 layer of the fiber.
Recovered paper and board is still today very good material for paper and board
industry. Correct treatment of recycled fibers is the key to get good products from
100% recycled material.