Fiber and Pulp Properties for
20 August, 2015
Hardwood vs. softwood cells
Hardwood fibers are about third of the softwood fiber length (1 vs. 3 mm) and 2/3 of
softwood fiber thickness (20/30 μm). In addition, hardwood includes lot of vessel and
ray cells, which can cause so called vessel picking and linting in offset printing.
Picture: Prof. Wimmer
Wood and fiber properties
The big difference between softwoods and hardwoods is amount of real fibers (tracheids).
Only tracheids can form fiber network and help papermaking.
Biggest problem with nonwood fibers is low share of real fibers (commonly less than 50%).
Wood properties Unit
Amount of wood volume
Fibers (tracheids) % 95 93 89 61 65 49
Vessels % 0 0 0 26 25 21
Ray cells etc. % 5 7 11 13 10 30
Average fiber dimensions
Length mm 3,3 3,1 3,3 1.0 1.1 1.0
Diameter µm 31 30 40 20 21 16
Cell wall thickness µm 3,1 3,0 7,0 3,2 3,8 3,8
2 x cell wall/width % 20 20 35 32 36 48
Wood Density (dried) kg/m3
405 550 515 450 640 820
Roles of different papermaking pulps
Softwood chemical pulps
Wet and dry runnability for papermaking, finishing and converting
Ensuring strength and stiffness for packaging materials
Hardwood chemical pulps
Good end use properties of woodfree printing papers and tissues
Good formation, brightness, opacity and printability
Decrease the costs of the fiber furnish
Good runnability and end use properties of mechanical grades
Better yield and lower costs of the fiber furnish
Bulk and stiffness, especially for filler ply of multilayer products
Recycled and nonwood pulps
Decrease fiber furnish costs, can be more environmentally friendly
Enlarge the raw material base for papermaking
Roles of different raw materials
Fillers and coating pigments
Improve the end use printability properties of paper
Decrease costs, carbonate widely available
Improve the papermaking process (performance chemicals)
Improve the end use properties of the paper (functional chemicals)
The desired paper properties can be obtained by
Selecting the proper furnish components
Adjusting the fiber furnish composition
Adjusting the properties of the different fiber furnish components used
Properly controlling the total papermaking and finishing processes
Wood, fiber and paper properties
Cell wall thickness
Nr of Fibers
per unit weight
Pulps and paper grades
Actual fiber furnishes may vary largely and can be quite different especially
in small unintegrated paper mills
Very often the price of fiber seems to be more important than the
performance of fiber in the product; within each end-product the quality
and the price of end-products may vary largely
It is important to understand how each furnish component contributes the
quality of the product and the performance in the paper machine, finishing,
Paper Grades Short fibers for printability
Long fibers for
Mechanical grades GW, PGW, TMP, BCTMP, DIP
Long fiber softwood
(BSKP)Woodfree grades BHKP, DIP
Several non-woods (bagasse,
wheat straw etc.)
Bamboo, kenaf etc.
Tensile strength of different pulps
DIP has normally better tensile strength
than TMP even if it contains filler.
Standard newsprint contains 50-100 % DIP.
TMP fibers of Pinus radiata are coarse and
tend to form bulky, low tensile and porous
Tensile index of all pulps improve when pulp
is made to lower freeness.
Effects of refining on fibers
Internal and external fibrillations as well as creation of fines are the main
positive effects of refining.
Removal of primary
fiber wall and S1 layer
More fiber hairiness
swelling of fibers
Creation of fines
Fiber cutting and shortening
Dissolving of material
Straightening of fibers
Final bonding in paper
Picture on the right describes
bonded fibers after refining and
External fibrils and fines from refining
are an important part of bonding.
Secondary fines (fibrillar) has the
most positive effects.
Collapsed lumen in the ribbon-like
fibers increase bonded area.
Crimped section at fiber crossings
have effect on the rheological
behavior of paper.
Paper density and strength
How to get bonding without density increase? Dry strength chemicals, surface
sizing and micro-fibrilled cellulose are some possibilities.
Gentle refining or low Specific Edge Load (SEL) gives good bulk and bonding
at the same time. Low SEL for never dried hardwood is < 0.5 J/m.
More internal fibrillation,
collapsed – good bonding but
More external fibrillation,
not collapsed – good bulk and
Effect of chemical pulp refining on paper
Wet web strength
Fiber bonding and strength
Porosity and ink demand
Smoothness and gloss
Water removal and solids content
Bulk and stiffness
Opacity and brightness
Drying shrinkage dimension stability
CAPEX and maintenance costs
Internal fibrillation External fibrillation Fiber bonding
Wood, fiber and pulp properties for papermaking
It is important to understand the effect of wood properties on paper quality.
Wood density is a simple measure and well suited to predict paper strength.
Fiber length is not the only characteristics correlating with paper strength.
Latest studies show that even more important are external fibrillation and crill content
after stock preparation. Fibers should be easily refined to save energy.
Main Wood Properties
Average dry density
Tracheid, ray cell and
Fibers after Pulping
Cell wall thickness
Fibers to Paper Machine
Ratio fines to fibers (crill)
Cell wall collapse, density
Pitch and stickies
Wood for Pulping
and lignin content
Forces during papermaking:Fiber processing:
Wood, fiber and paper properties
Fiber collapse and paper properties
Differences in relative cell wall thickness have
effect on paper properties.
An example here is Pinus radiata compared to
Norway spruce. TMP fibers of Pinus radiata are
coarse and tend to form bulky, low tensile and
porous sheet with reduced coating and ink
Spruce fibers have lot of thin-walled early wood
fibers, which form more dense, smooth and strong
Dense paper, however, can have lower light
scattering and stiffness.
From hardwoods eucalyptus is more thick-walled
while birch and acacia are more thin-walled.
Fiber collapse and flexibility
It is important to increase flexibility and collapsibility of thick-walled fibers.
The main means to improve collapsibility is refining.
When using stratified headbox it is possible to fractionate fibers and put coarse
fraction to the middle layer.
for printing paper
Suitable fiber for tissue, copy paper and
cartonboard middle ply
Hardwood chemical pulp
For woodfree paper grades mainly hardwood kraft is used. Thick-walled
eucalyptus can be better than thin-walled or birch.
On the left side paper is bulky and thicker giving better stiffness, which is
important e.g. for copy paper.
Thick-walled fibers are better in cartonboard middle ply because
smoothness is not as important there as bulk and stiffness.
Picture: Celso Foelkel
Primary and secondary fines of hardwood
Primary fines of hardwood pulp (ray and vessel cells) cause picking, linting and
Secondary fines formed in refining is mostly thin fibrils and enhance bonding.
Picture: Martin A. Hubbe
Primary fines Secondary fines
Effect of softwood kraft on paper properties
Strength properties Increase (also surface, tear and wet strength)
Folding endurance Increases
Formation Less uniform
Ink holdout Lower
Bulk and stiffness Decrease
Dimensional stability Decreases
Energy consumption Increases
Fiber type and wet end runnability
Printing papers with mechanical pulp
have better runnability on paper machine
than woodfree grades.
The main reason is that mechanical pulp
fibers are stiffer in wet condition.
The good tensile stiffness of the wet fiber
network is mainly due to elastic pressure
and friction forces between fibers.
It is easy to make a model from four
sticks and note the rigid structure without
Good runnability is always more
important than we could imagine.
No bonding but very rigid structure!
Dryer section runnability
There should be high residual tension after
first wet strain to avoid web slackness and
Low tension after first dryers due to
relaxation may lead to slackening of the
This causes wrinkling, bagging, fluttering
and weaving of the web which can lead to
In modern single felted dryer sections, the
problematic areas of paper with low
tension level are mainly found in
converging and diverging gaps between
the dryer cylinders and the fabric.
Fiber fines and residual tension
The studies of Dr. Retulainen suggest that the residual tension is more closely related to the
tensile stiffness than to the tensile strength. Therefore also the factors, such as the fiber
stiffness, straightness and the activity of the network to bear load, affect the residual tension.
It is interesting to note that stiff TMP fibers blended with only 10 % kraft fines form stronger
wet sheet than kraft fibers and kraft fines.
On the left the effect of fiber and
fines type on the residual
tension (tension at 1% strain
after 0.475 s relaxation) of wet
sheet (compared at 55%
Advantages of low break frequency
Less steam &
Less shade &
Easy wet end
Less effluent and
Better and less variable
Stable and better
Decreased use of
Steam & el
TMP fibers and bonding
TMP fibres are normally not very well fibrillated. Fibrillation is needed to get bonding.
Sometimes there is still latency left to the paper machine. Latency (curling of fibers)
reduces strength and bonding and increases porosity.
Fibrillation can be increased by using fresh/moist wood, alkaline pH, high amount of
reject refining and also with post-refining (however may cut fibers too much).
Freeness as such is a measure of fines content, but not a good measure of fibrillation.
Fibrillation should be checked from microscope fiber pictures.
Long TMP fibers with thick cell
wall are normally not very well
collapsed or fibrillated. Internal
bond of paper tends to be low.
TMP fibrils and flakes
A good quality TMP for printing paper includes fibrils to give bonding and strength as
well as flakes (mainly unbonded material) to improve light scattering and paper opacity.
For cartonboard middle ply flakes are not needed.
Picture: KARI LUUKKO AND HANNU PAULAPURO
TAPPI JOURNAL FEBRUARY 1999
Effect of TMP fines to paper properties
It is important to note that fibrils increase tensile strength without reducing light scattering
and flakes increase light scattering without decreasing tensile strength.
A good TMP includes both flakes and fibrils and thus increases both light scattering and
Picture: KARI LUUKKO AND HANNU
TAPPI JOURNAL FEBRUARY 1999
Fiber wall thickness vs. roughness & opacity
Fiber wall thickness determines very much paper smoothness and opacity.
Increased fiber splitting without fiber cutting (lower freeness) can improve
the situation but not totally.
Optimal mechanical pulp
Optimal mechanical pulp for improved
publication papers should have:
Thin fiber walls (raw material selection)
Fiber fines and large degree of fiber splitting (thin
slot screening and reject refining)
Fibrillated fiber surfaces (reject refining)
Low amount of shives, and especially latewood
shives (reject refining and thin slot screening)
Reasonable fiber length (thin slot screening and
Such fiber properties will give publication papers
with improved print quality, formation and
Fiber Wall Thickness of Norway Spruce
Average fiber wall thickness of Norway spruce TMP is almost 2 µm but there
are some fibers with wall thickness of 3-5 µm.
Mechanical pulps made from mature Pinus radiata can have fiber wall thickness
of about 6 µm, which is three times the Norway spruce value.
Reme, P. A., Kure, K.-A.,
Gregersen, O. W., Helle, T.,
Mechanical Pulping Conference
Importance of fiber wall thickness
It is important to have several fiber layers in a thin paper to get good formation,
smoothness, opacity and gloss. This correlates with thin fiber wall.
Fiber wall of Norway spruce is about 2 µm while Radiata pine has 4-6 µm. In a 40 gsm
paper there can be only 3-4 fibers of Pinus radiata in the total paper thickness.
Area = Perimeter x Wall Thickness, A=P*T
Fiber volume = Area x Length, V=A*L=P*T*L
Coarseness = Fiber weight/Length, C=W/L
C = Volume*Density/Length, C=V*ρ/L=P*T*L* ρ/L= P*T*ρ
Fiber Grammage (g/m2) = Coarseness/fiber Width = P*T*ρ/P*2 = 2*T*ρ = 3*T (µm)
~ P/2 Fiber Fiber
Wall density ~ 1500 kg/m3
Example: pulp characteristics for containerboards
There is no common criteria for pulp
quality. It depends on end products
to be manufactured.
Compression strength, burst,
stiffness and porosity are most
For white-top grades printing
properties are important
Corrugating medium (fluting)
Compression strength is critical
What makes strong kraftliner
It is quite common understanding that fiber length and fiber collapse are the most
important fiber properties having effect on important linerboard properties. This
means that kraft pulp must be strongly refined to get lumen collapse and higher
bonded area. But there is also a third variable what Innventia in Sweden has studied.
PulpEye has recently introduced its CrillEye online crill measurement. Crill is finely
divided cellulosic material - finer than external fibrillation - that is liberated during
refining process. The crill particles are about a hundred times thinner than the fibers.
In spite of the fact that only about one per cent of the weight of fibers and other
particles in the furnish is crill, it can correspond to as much as fifty per cent of the
total free surface area. This shows the importance of crill for the strength properties
Valmet has also introduced its own online strength measurement, which is based on
external fibrillation or hairiness of fibers.
Crill and tensile strength
Research studies at Innventia have shown that crill is the single variable
having the strongest connection to paper strength. Laboratory results in
the figure below show a strong correlation to paper tensile strength index.