This document discusses differences between hardwood and softwood fibers used for papermaking. Hardwood fibers are shorter and thinner than softwood fibers. Softwoods contain mostly fibers (tracheids) which form the fiber network in paper, while hardwoods also contain vessel and ray cells which can cause issues. Fiber length, diameter, cell wall thickness and wood density impact paper properties like strength, bulk, and formation. The roles of different pulps, fillers and additives in achieving desired paper qualities are also reviewed. Optimal mechanical pulps for printing papers contain fibrillated fibers, fines, and few shives.

1. Pele Oy
Fiber and Pulp Properties for
Papermaking
Pekka Komulainen
Pekka.Komulainen@clarinet.fi
20 August, 2015

2. Pele Oy
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
2

3. Pele Oy
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%).
3
Wood properties Unit
Picea
abies
Pinus
sylvestris
Pinus
radiata
Populus
tremula
Betula
pendula
Eucalyptus
globulus
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

4. Pele Oy
Example of softwood fiber basket
4

5. Pele Oy
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
 Mechanical pulps
 Good runnability and end use properties of mechanical grades
 Formation, printability
 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
5

6. Pele Oy
Roles of different raw materials
 Fillers and coating pigments
 Improve the end use printability properties of paper
 Decrease costs, carbonate widely available
 Save forests
 Additives
 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
6

7. Pele Oy
Wood, fiber and paper properties
7
FIBER PROPERTIES
IN PULPS
Fiber
length
Cell wall thickness
of fibers
Fiber
coarseness
Hemicellulose
content
Nr of Fibers
per unit weight
Bulk, Stiffness,
Wet strength
Light scattering
coefficient (opacity)
Formation, Porosity,
Orientation
Bonding, Density,
Dimension stability
Fiber
Stiffness
Dry strength
Wet Strength
RAW MATERIAL
PROPERTIES
PAPER
PROPERTIES

8. Pele Oy
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,
and converting
8
Paper Grades Short fibers for printability
Long fibers for
runnability
Mechanical grades GW, PGW, TMP, BCTMP, DIP
Long fiber softwood
(BSKP)Woodfree grades BHKP, DIP
Non-wood grades
Several non-woods (bagasse,
wheat straw etc.)
Bamboo, kenaf etc.

9. Pele Oy
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
sheet.
 Tensile index of all pulps improve when pulp
is made to lower freeness.
9

10. Pele Oy
Effects of refining on fibers
 Internal and external fibrillations as well as creation of fines are the main
positive effects of refining.
10
Removal of primary
fiber wall and S1 layer
More fiber hairiness
(external fibrillation)
Delamination and
swelling of fibers
(internal fibrillation)
Creation of fines
Fiber cutting and shortening
Dissolving of material
Straightening of fibers

11. Pele Oy
Final bonding in paper
 Picture on the right describes
bonded fibers after refining and
drying.
 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.
11
Picture:
Hubbe

12. Pele Oy
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.
12
Fiber Lumen
More internal fibrillation,
collapsed – good bonding but
low bulk
More external fibrillation,
not collapsed – good bulk and
bonding

13. Pele Oy
13
Effect of chemical pulp refining on paper
Positive effects
 Wet web strength 
 Fiber bonding and strength 
 Better formation
 Coating coverage 
 Porosity and ink demand 
 Smoothness and gloss 
Negative effects
 Water removal and solids content 
 Bulk and stiffness 
 Paper compressibility 
 Opacity and brightness 
 Drying shrinkage  dimension stability 
 Tear strength 
 Energy consumption 
 CAPEX and maintenance costs 
Pics: E.Gruber
Internal fibrillation External fibrillation Fiber bonding
+ =

14. Pele Oy
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.
14
Main Wood Properties
Average dry density
Tracheid, ray cell and
vessel shares
Fibers after Pulping
Cell wall thickness
Tracheid length
Coarseness
Fibril angle
Fibers to Paper Machine
Ratio fines to fibers (crill)
External fibrillation
Cell wall collapse, density
Pitch and stickies
Wood for Pulping
Cellulose,
hemicellulose
and lignin content
Extractives content

15. Pele Oy
Pulping
process
External
pressurePulp
drying
Pulp
refining
Pulp
yield
Lateral
force
Surface
tension
Sheet
density
Surface
properties
Optical
properties
Strength
properties
Lateral
conform-
ability
Wood
vessels
Wood
density
Degree of
collapse
Bonded
area
Wood
extractives
Fiber
diameter
Wall/dia
ratio
Wall
thickness
Forces during papermaking:Fiber processing:
Wood, fiber and paper properties
Wood
properties:
Paper
properties:
Wood
chemistry
15

16. Pele Oy
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
holdout.
 Spruce fibers have lot of thin-walled early wood
fibers, which form more dense, smooth and strong
paper.
 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.
16
Norway spruce
Pinus radiata

17. Pele Oy
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.
17
Good fiber
for printing paper
Suitable fiber for tissue, copy paper and
cartonboard middle ply

18. Pele Oy
Hardwood chemical pulp
18
 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

19. Pele Oy
Primary and secondary fines of hardwood
 Primary fines of hardwood pulp (ray and vessel cells) cause picking, linting and
reduce bonding.
 Secondary fines formed in refining is mostly thin fibrils and enhance bonding.
19
Picture: Martin A. Hubbe
Primary fines Secondary fines

20. Pele Oy
20
Effect of softwood kraft on paper properties
Positive
 Strength properties Increase (also surface, tear and wet strength)
 Folding endurance Increases
Negative
 Formation Less uniform
 Smoothness Decreases
 Porosity Increases
 Ink holdout Lower
 Bulk and stiffness Decrease
 Dimensional stability Decreases
 Energy consumption Increases
 Costs Increase
Picture: Canfor

21. Pele Oy
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
bonding.
 Good runnability is always more
important than we could imagine.
21
No bonding but very rigid structure!

22. Pele Oy
Dryer section runnability
 There should be high residual tension after
first wet strain to avoid web slackness and
breaks.
 Low tension after first dryers due to
relaxation may lead to slackening of the
wet web.
 This causes wrinkling, bagging, fluttering
and weaving of the web which can lead to
web breaks.
 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.
22

23. Pele Oy
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.
23
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%
dryness).

24. Pele Oy
24
Advantages of low break frequency
Improved
runnability
Lower raw
material costs
Longer
wire life
Increased
machine
speed
Less steam &
energy/ton
Lower
furnish
cost
Higher
press solids
Less starch
etc. needed
Better CD-
profiles
Less shade &
caliper variation
Stronger
paper
Better bulk
& stiffness
Better
printability
Constant
filler content
Productivity
Cost
Efficiency
Easy wet end
chemistryProduct
Quality
Low Break
Frequency
Less effluent and
fresh water/ton
Better and less variable
raw materials
Less Dry
Broke
Stable and better
paper quality
More Net
Tons
Lower Chemical
Consumption
Decreased use of
chemicalsCleaner
system
Steam & el
used only
once

25. Pele Oy
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.
Picture: Knowpap
Long TMP fibers with thick cell
wall are normally not very well
collapsed or fibrillated. Internal
bond of paper tends to be low.
25

26. Pele Oy
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
26

27. Pele Oy
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
tensile strength.
Picture: KARI LUUKKO AND HANNU
PAULAPURO
TAPPI JOURNAL FEBRUARY 1999
27

28. Pele Oy
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.
Picture: PFI
28

29. Pele Oy
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
reject refining)
 Such fiber properties will give publication papers
with improved print quality, formation and
runnability.
29

30. Pele Oy
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.,
1999 International
Mechanical Pulping Conference
Picea Abies
30

31. Pele Oy
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 Grammage
µm g/m
2
1 3
2 6
3 9
4 12
5 15
6 18
T
Wall density ~ 1500 kg/m3
31

32. Pele Oy
Example: pulp characteristics for containerboards
32
 There is no common criteria for pulp
quality. It depends on end products
to be manufactured.
 Linerboard
 Compression strength, burst,
stiffness and porosity are most
important
 For white-top grades printing
properties are important
 Corrugating medium (fluting)
 Compression strength is critical

33. Pele Oy
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
of paper.
 Valmet has also introduced its own online strength measurement, which is based on
external fibrillation or hairiness of fibers.
33

34. Pele Oy
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.
34

35. Pele Oy
35
Fibers are never identical