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Pele Oy
Modern Papermaking
Contents Page
Paper and Board Grades 3
Paper Composition 11
Papermaking Processes 23
Surface Sizing and Coating 69
How to Influence on Process and Paper Quality 83
New Papermaking Developments 92
Thank You for Your Attention 109
2
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PAPER AND BOARD GRADES
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4
European unofficial paper grade classification
Printing and writing papers
Mechanical printing papers
Woodfree printing and writing papers
Paperboards
Cartonboards
Containerboards
Special boards
Tissue
Hygiene products
Other tissue products
Air-laid paper
Specialty papers
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5
Printing paper grades
News
MF
Spesial.
SC-A+ SC-A
SC-B SC-C
MFC
LWC
FCO
HWC
MWC
WF
Unctd
WF
Coated
Relative
Value
Relative Quality
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6
Uncoated
woodfree
Coated
surface
Coated & ca-
lendered
European classification of P&W paper grades
UncoatedWoodfrees
CoatedWoodfrees
Woodfree Printingand Writing Papers
UncoatedMechanicals
CoatedMechanicals
MechanicalPrinting Papers
Next level classification
according to pigment coating
(surface quality)
Pulping Method
(Brightness)
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7
European mechanical paper grades
Newsprint
TD, Bulky etc.
SC-papers
RG and offset
Uncoated
Mechanicals
LWC FCO MFC
Single Coated
MWC HWC
2-3 coatings
Coated
Mechanicals
Mechanical
Paper Grades
Mechanical paper grades include mainly
mechanical pulp (SGW, TMP, CTMP etc.) or
deinked pulp from mechanical recovered
papers.
Amount of bleached softwood kraft pulp
(BSKP) is 0-50 % depending on paper grade.
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8
European woodfree paper grades
Office Papers
Cut Size
like A4, A3
Printing Papers
Folio Sheets
and Rolls
Uncoated
Woodfree
Single Coated
Gloss/Matt
Folio or Rolls
Multi Coated
Gloss/Matt
Folio or Rolls
Coated
Woodfree
Woodfree Papers
in Sheets and Rolls
Woodfree paper grades are made mainly
from chemical hardwood pulp. Some
BSKP must be added to coated grades.
Coated grades can include 5-20%
hardwood BCTMP.
Deinked pulp made from woodfree grades
can be added especially to office papers .
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9
Classification of coated grades
Coated one side
C1S
Single Coated
Rolls Sheets
Gloss Finish Matt Finish
Double Coated Triple Coated
Coated two sides
C2S
Coated
Woodfree
Coated
Mechanical
Coated
Board
Coated Grades
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10
Paper grades and printing methods
Printing
Method
Paper Grade
CSWO HSWO Sheet Fed
Offset
Roto-
gravure
Flexo Elektrogr.
& Inkjet
Newsprint xxx x
MF Specialties xxx xx x x x
SC xx xxx
MFC xxx x x
FCO xxx
LWC xxx xx x
MWC, HWC xxx x x
WFC xx xxx xx
WFU xx x xx xxx
xxx = most common usage, xx = common usage, x = some usage
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PAPER COMPOSITION
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Fibers and paper properties
Chemical pulp can be bleached up to brightness 90 %. Bright mechanical pulps have brightness 75-85 %.
Mechanical pulps give opacity, bulk and stiffness to the paper. Hardwood chemical pulp and softwood mechanical pulp can be used up to 100 % of paper furnish.
Softwood chemical pulp and hardwood mechanical pulp are normally additional pulps to give special properties to printing papers and are not normally utilized without other pulps.
More BCTMP from hardwoods is used for woodfree papers and boards. Some lignin from BCTMP will be dissolved in alkaline papermaking conditions. Dissolved lignin and extractives increase anionic trash and make the control of wet end chemistry more complex.
DIP, mechanical pulps and BCTMP have lower brightness than chemical pulp. Carbonates are best pigments to improve brightness as filler and in coating.
12
Hardwood,
Short fibers
Softwood,
Long fibers
Chemical
Pulp,
Flexible
Mechanical
Pulp, Stiff
Fiber/Pulp
Type
Wet and dry
strength
Stiffness,
opacity
Formation,
brightness
Printability,
runnability
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Hardwood vs. softwood chemical pulp
Short hardwood fibers will be more available than long softwood fibers.
Hardwood kraft gives smoothness, bulk and optical properties. This means that printability of final product is good.
Average length of hardwood pulp fibers is slightly less than one millimeter.
Refined softwood fiber is about 2 mm long. Longer fibers give better strength for coating, finishing and printing purposes.
Filler pigments decrease paper strength at the wet end of paper machine but also in surface sizing and coating where water moistens base paper.
The trend is to increase hardwood and filler and to decrease softwood. However, where softwood is integrated it can be used more together with less expensive filler.
Hardwood Chemical Pulp (Birch)
Softwood Chemical Pulp (Pine)
13
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Fiber combinations in European white papers
Hardwood
100 %
News
SC
White
Kraft
Uncoated
Woodfree
LWC
Opacity
Bulk
Brightness
Coated
Woodfree
Softwood
100 %
Strength Formation
14
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Thin Eucalyptus fiber
with thick fiber wall
Vessel cell of Eucalyptus
Plantation hardwood pulps
Thin and quite long fibers of Eucalyptus having thick fiber wall can be developed by
refining without loss in bulk and tear strength. However, short and thick vessels cells
must be handled to prevent picking problems. There are several usable species of
eucalyptus, which have different properties for papermaking.
Eucalyptus is well suited for all kind of paper and board grades. Acacia is the other
competitive fiber but has thinner fiber walls and is not as good for grades requiring high
bulk and stiffness.
15
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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.
16
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.
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17
Recovered paper usage
Container
Board
Special Office Papers Mixed to Office
Papers Deinked fibers
Hygienic Products Mixed to Tissue
Papers
News, SC, LWC Printing
Papers Deinked fibers
Corrugating
Medium
OCC,
Kraft Paper
Testliner
Board
Office
Waste
ONP
OMG
Mixed
Waste
Recycled fibers
Recycled fibers
Deinked fibers
Cartonboards White Lined
Chipboard
ONP = Old Newspapers OMG = Old Magazines
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18
Uncoated paper raw materials
Material Mech. % WF % Comment
Fibers 60 - 100 70 - 100 Wood or non-wood fibers
Fillers 40 - 0 30 - 0 Mineral or synthetic pigments
Surface sizes - 0 - 5 Starch, CMC, PVA, synthetic size,
optical brighteners etc.
Functional
chemicals 0 - 1 0 - 2
Internal sizes, dyes etc.
(effect on paper properties)
Performance
chemicals for
process
<1 <1 Retention aids, defoamers, biocides etc.
(effect on process performance)
Water 5 - 10 4 - 7 To be in balance with ambient air
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19
Long and short fibers in paper
Most papers contain long fibers (BSKP) to give runnability and short fibers (BHKP or mechanical pulp) to give printability or other end use properties.
Uncoated WF
Newsprint
Kraft Papers (Bleached
or Unbleached)
LWC Magazine
SC Magazine
Coated WF
Long fibers,
BSKP
Short fibers,
BHKP or
Mechanical
pulp
0 % 100 %
0 % 100 %
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20
Conventional LWC base paper raw materials
Chemical pulp 30 - 50%
Bleached softwood kraft, hardwood is not used
Mechanical pulp 70 - 50%
Stone groundwood (SGW), pressure groundwood (PGW),
thermomechanical pulp (TMP) or chemithermomechanical
pulp (CTMP, BCTMP)
Broke
10 - 30% of the primary fiber furnish
Uncoated and coated broke (separately dosed)
Filler pigments
Normally 4 -10 % of base paper (25 -100 % of this amount
returned back as coated broke)
Kaolin clay, talc, calcium carbonate, titanium dioxide.
Functional Chemicals
Cationic starch, slight hydrophobic sizing, dyes
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21
Effect of long fiber addition on paper properties
Positive
Wet and dry runnability Improve
Strength properties Increase (also tear)
Folding endurance Increases
Negative
Printability Decreases
Formation Less uniform
Smoothness Decreases
Porosity Increases
Ink holdout Lower
Bulk and stiffness Decrease
Dimensional stability Decreases
Energy consumption Increases
Costs Increase
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22
Sizing alternatives
Internal size is pumped to the pipe before headbox.
Surface size is added with size press (film sizer today)
Type of Size Internal
Sizing
Surface
Sizing
Dry strength
improvement
(starch, CMC etc.)
WF papers,
mechanical printing
papers, paperboards
WF papers,
WFC not always,
paperboards
Wet strength
improvement
(resins)
Tissue,
packaging papers,
specialties
Can be added
to surface size
Hydrophobic
sizes (water repellent)
WF papers, paperboards
(coated WF not always)
Can be added
to surface size
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PAPERMAKING PROCESSES
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Cardboard recycling process
24
www.millenniumrecycling.com/process/
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25
Pulper
Screw
press
Disperging Post flotation
Thickening 2
Pulp
storage
Thickening 1
Slot
screens
Pulp
storage Cleaners
Flotation Cleaners Hole
screens
Consistencies
= Small = Average = Very high = High
Conventional deinking process
The filtrate from thickening 1 and 2 is flotated and reused in the process again.
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26
Papermaking process
Slushing Refining Forming Pressing Drying
Precalender Coating Finishing Converting
Steam
Coating colour
Pulp bales
Fresh water
...or pulp
Additives
Calender
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27
Papermaking principle (= water removal)
Headbox & wire
Formation
Orientation
Strength
Smoothness
Two-sidedness
>99% 50% 3% 20% 8% 7% 80%
Drying
Strength
Smoothness
Two-sidedness
CD profiles
Indicative
water content
Wet Pressing
Porosity, bulk
Strength
Smoothness
Two-sidedness
Surface Sizing/Coating
Porosity, ink abs.
Surface strength
Smoothness
Brightness, gloss
Two-sidedness
Calendering
Porosity, ink abs.
Smoothness, gloss
Brightness, opacity
Bulk, stiffness
Two-sidedness
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28
Simplified stock preparation in papermaking
Source: Valmet
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Paper machine white water system
The objective of the white water system is to reduce water consumption and to minimize
fiber losses by recirculating water.
29
The amount of suspension
per ton of dry material in
different positions:
Pulp Mix Thick stock
fiber
recovery Fresh
water
Forming
Excess water for
reuse or to effluent
Long circulation
Short
circulation
Additives
White
water
tank
Wire
pit White
water
tower
Dilutions at
web breaks
HB
Stock
prep
Position Consistency
%
m3 / ton
of dry mat.
Stock 4.0 25
To Headbox 0.5 200
After wire 20.0 5
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Conventional approach flow
Source: Knowpap
Old - holes
New - slots
30 Screening
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31
Attention! – 182 m long machine will appear!
Wet end of copy paper machine
Picture: Voith Paper
Wire Section Press section
Headbox & Former
Formation
Orientation
Strength
Smoothness
Two-sidedness
Wet Pressing
Porosity
Bulk
Strength
Smoothness
Two-sidedness
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32
Predrying and surface sizing
Surface sizing or
coating Predrying
cylinders
Drying
Moisture (MD, CD)
Two-sidedness
Curl
Sizing/Coating
Porosity
Ink absorption
Strength
Smoothness, Gloss
Brightness, opacity
Two-sidedness Picture: Voith Paper
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Afterdrying, calendering and reeling
Reeling Calendering
Afterdrying
Calendering
Caliper and porosity
Ink absorption
Smoothness & gloss
Brightness & opacity
Two-sidedness
Bulk and stiffness Picture: Voith Paper
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34
Forming
Drying
Drying Pressing
Coating Reeling Winding Calendering
Surface
sizing
Coated woodfree papermaking line
About 10 m wide and 10 mm thick stock flows from the headbox to the wire. The final
paper caliper is less than 0.1 mm.
About 50% of the paper volume is air.
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35
Formers and speed
Picture: Valmet Paper
Hybrid formers are suitable for non-wood and specialty papers where speed must be slow due to the very difficult dewatering.
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36
Crescent Former for tissue paper
Wire speed is about 20% higher than reeler speed due to the shortening in creping.
Release chemicals can be sprayed on the dryer surface to help creping.
Picture: Voith
Headbox
Yankee dryer
+ hot air hood
Pope reel Press Gap former
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ATMOS tissue technology
According to Voith the big advantage of this technology
is that for premium tissue production it consumes 35%
less energy than TAD and the investment costs are
much lower. While through-air drying uses only air
pressure, ATMOS uses also vacuum.
Depending on application, it also enables fiber savings
and the use of 100% recovered paper furnish.
37
Pictures: Voith
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Retention of fibers, fillers and fines
Fibers are long compared to wire
fabric openings. Retention of long
fibers is good against the wire, but
fillers and fiber fines are smaller
than wire openings.
Mechanical retention of fillers and
fiber fines is possible when the fiber
mat is thick enough with smaller
voids between fibers than in wire
openings.
Common practice is to flocculate
fine material to larger aggregates.
However, this can flocculate also
fibers and impair paper formation.
38
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39
Principle of paper formation
Originally there is over100 times as much water as fibers. Low concentration is needed to be able to avoid flocculation and to control basis weight (thickness).
Suction or pressure against the fabric is needed for dewatering.
Fourdrinier wire is pressing a pattern called wire mark to the paper. This causes two-sidedness.
Twin wire sections are used to avoid two-sidedness and to get easier dewatering with high speed.
Solids content after wire is 18-22 %.
Wire section removes about 98% of the total water. However, very expensive equipment and most of the energy are needed for press- and dryer sections.
To get the final dryness dewatering by pressing and by evaporation is needed after wire section.
Wire fabric
Filtered
web
Free
fibers
in water
Removed water
Picture: Knowpap
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40
Filtration in gap former
Picture: Knowpap
Wire
Wire
Two separate fiber mats are formed
on the wires.
Middle part of the paper web has
lower fines content and lower
bonding strength.
Water removal capacity is more than
double compared to Fourdrinier.
Both surfaces have very little dusting
and linting material (fiber fines and
fillers). This kind of paper is very
suitable for offset printing. In
addition, it is possible to use more
filler without linting.
Fiber orientation is similar on both
surfaces. Curling tendency is very
low.
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41
Paper machine clothing
Press felt Wire fabric
Dryer fabric
Batt fiber needled
to form fine surface
Laminate fabric
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42
Pressing of wet web
There are 1...4 nips in the press section. Earlier nips had only one felt (picture). Today
double felted nips are increasingly used. Solids content after press section is 45 - 55%.
Web will be rough but compacted against the felt side and smooth but open on the roll side.
Paper is bulkier if less wet pressing and more drying is used. This, however, increases
steam consumption.
Felt
Web
Picture: Knowpap
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Wet pressing theory
Wet pressing has a strong effect on the properties of paper. The press geometry, rolls
and their covers, felts and linear pressure combinations must be selected to conform to
the running speed and the paper grade to be produced.
Picture: Valmet
43
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Dryness and porosity with shoe and roll presses
KnowPap 4.0 (2002)
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Press draw and porosity
A high press draw is not only question of runnability but also paper quality is lower when low porosity is needed.
Porosity measurement is also a good tool for evaluating what is a too high press draw
200
400
600
800
0 1 2 3 4 5
Poro
sity,
Ben
dts
en
, m
l/m
in
Press Draw, % Picture modified from: Valmet
45
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46
Effect of press nip on paper
Felt and roll patterns are copied to the paper surface (felt is rough and roll is smooth). Paper web close to the felt is compressed due to the lower water pressure but higher
mechanical pressure. Paper becomes dense but rough on the felt side.
Picture: M.A.MacGregor
Roll side
Felt side
Smooth and open
Rough but dense
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47
One-sided felt and water removal – rough
and compacted felt side surface.
Two-sided felt and water
removal – symmetric web, both
surfaces rough and compacted.
Effect of felt on paper surface
nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
nnnnnnnnnnnnnnnnnnnnnnnnnnmnnnnnnn
nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
Rigid plate like press roll
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48
Impulse in pressing and calendering
Paper is viscoelastic. This means that not only the pressure, but also the time under the pressure has effect in pressing and calendering.
Total effect of pressure forces is related to the sum of pressure impulse in all nips. If speed is doubled it would require double linear load or double number of nips. Shoe
press and belt calender are very effective.
Impulse = pressure x time
Pressure = linear load / nip length
Time = nip length / speed
Impulse = linear load / speed time
Impulse
= area
Impulse = pressure x time = speed
Σ linear load
Nip
pressure
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49
Water content of the web
After wire section there is about 80% water in the web, even if more than 97% of the original water is removed. Removal of the final 2% is very expensive in the press and drying sections.
After press section solids content (and water content) is about 50%.
Picture: Knowpap
Pick-up
felt
H2O
50%
H2O
80%
Press section of a slow machine:
open draw after 2nd nip
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50
Basic concept for woodfree coated and uncoated: two shoe presses with
transferbelt. This gives good runnability and CD profiles, but more two-
sidedness than double-felted last press.
Modern press section
Better web run
through press
No rewetting
after 2nd nip
Quick start-up
with new fabrics
Picture: Voith Paper
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Paperboard machine press sections
On the right press section of a
cartonboard machine has a
separate smoothing press after
double felted shoe press.
Kraftliner machine can have last
press double felted because
smoothness requirements are
not critical (picture below).
51 Pictures: Voith
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Typical cartonboard machine
Cartonboard machines can have higher speeds when there are more wires. Drainage of
each wire is similar to papermaking drainage of grammage less than 100 gsm.
Development of double shoe presses with totally supported web run increases web
dryness to dryers 4-5 %-unit. Increased dryness allows 20 % higher speed, when drying
capacity is limited or 20 % lower energy consumption with same speed.
Higher dryness means that web is stronger when transferred to dryers and there are less
web breaks and sticking to dryer surfaces.
The paper machine in the picture below is Bohui PM1 cartonboard machine in China
supplied by Voith. Smoothing press after double felted shoe presses is without felt.
52
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53
Principle of drying
In dryer section about one ton water must
be evaporated per one ton of final
product.
For paper drying and water evaporation,
heat must be transferred to the wet web.
This is normally done by steam heated
cylinder dryers (30 - 60 pieces).
Evaporated water must be transferred
from the paper machine hood and fresh
dry air blown back. Heat from the
exhaust air is returned back to the
process.
Paper moisture before coating or surface
sizing is 2 - 5%. Final paper moisture is
about double (4 - 10%) mainly depending
on the mineral content and paper grade.
Picture: Knowpap
Exhaust air
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54
Hydrogen bond formation
Hydroxyl
group
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55
Inter-fiber hydrogen bond formation 1
Initial weak bonds via
several water molecule
layers in the beginning
of dryer section.
H O
H O
H O
O H
fiber wall surface
H O H
O H
O H
H O H
H O H
H O H
H O H
H O H
H
O
H O H
H O H
H O H
fiber wall surface
Smook’s Handbook, 1982, adapted
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56
Inter-fiber hydrogen bond formation 2
Stronger bonds via
monolayer of water
O H H O
H
O H
O H
H O H
H O H
H
O
H O
H O
H O
fiber wall surface
fiber wall surface
Smook’s Handbook, 1982, adapted
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57
Smook’s Handbook, 1982, adapted
H O
H O
O H
O H
O H
H
O
H O
fiber wall surface
fiber wall surface
Inter-fiber hydrogen bond formation 3
Direct hydrogen
bonding between
fibers
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58
Remoistening of paper and fiber swelling
O H
O H O H
H
O fiber wall surface
H O
H O
H O
H O H H O
H
H O H
H O H
H O H
H O H
H O H
H O H
H O H
fiber wall surface
H O H
Water molecule
This is why paper
can be recycled!
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Shrinkage profile of conventional paper machine
Edges compared to center have:
• higher weight
• higher caliper
• higher roughness
• higher porosity
• lower dimension stability
• long and slack web to the rolls
59
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PAPER STRUCTURE
Picture: Prof Claire Davies
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61
Structure of paper
Paper structure is porous and there is lot of air between fibers and inside the fiber lumens.
Softwood chemical pulp fibers are mainly collapsed in dry paper sheet (picture).
Paper structure is layered. Main part of fiber area is bonded to the other fibers.
Paper thickness (caliper) is from 40 to 120 µm.
Original thickness of softwood fibers is about 30 µm and hardwood fibers about 20 µm.
There are 5 to 20 fiber layers in a printing paper sheet.
Fibers must be collapsed or broken down to thinner particles to be able to make a smooth and even paper sheet.
Paper structure is oriented,
porous and layered
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62
Breaking lengths of various materials
Breaking length km
Single softwood fiber 100-150
Pine Wood 20-25
Printing papers 2-6
Softwood kraft paper 8-10
Steel 4-5
Aluminum 3-4
Graphite 35-40
Breaking length is the theoretical length of a material strip where it breaks
due to its own weight.
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63
Moisture sorption isotherms for paper
Paper is hygroscopic and in balance with the air temperature and humidity.
Moisture content (m) also depends on the direction of the change (hysteresis).
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64
Evenness of paper
Formation Flocculation (long fibers)
Basis Weight Variation Machine direction (MD)
Cross machine direction (CD)
Residual variation (all directions)
From lot to lot
Two-sidedness Smoothness, gloss
Absorption, density
Color, brightness
Curl, orientation
Orientation Fibers more in MD
Orientation angle to MD ± 0 - 5º
Tensile strength ratio MD/CD = 2...4
MD
CD
Bad formation
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65
MD and CD properties of paper
Compared to cross-machine direction
paper in machine direction:
has more fiber orientation
has higher gloss
is stiffer
has higher tensile strength
has lower tear strength
has lower elongation
has better dimensional stability i.e.
shrinking in drying is bigger in CMD Fibers are more in machine
direction. The upper sheet in
the picture is stiffer (MD =
longer side of copy paper).
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66
Curl directions in sheet moistening
Fibers swell and shrink more in the direction of thickness and paper in the cross machine direction (due to fiber orientation).
MD/CD tensile ratio for roll paper can be 3 - 4 but for sheeted paper it should be 2 - 3 to reduce curl and to improve CD stiffness.
Wire side - more oriented in MD
(not valid if gap former paper)
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Components of grammage variation
Systematic variation in MD and CMD are mathematically separated and the rest of
the variation is called random or residual variation.
MD variation reflects pressure pulsations, CMD variation control of slice and
residual variation stability of the process and headbox.
67
Random or
Residual
Variation
Cross Machine
Direction
Variation
Machine
Direction
Variation
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68
Basic printing paper properties
Importance depends on final usage
information, packaging or hygienic
General properties
basis weight, moisture, caliper
Strength properties
tensile, tear, burst, folding
surface, bond, dusting
Optical properties
brightness, opacity, color
Surface properties
roughness, gloss
Absorption properties
water, oil, ink
Structural properties
formation, orientation, two-sidedness, curl
density/bulk, stiffness
porosity, air permeability
Picture: Knowpap
Bulk / Density
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SURFACE SIZING AND COATING
69
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Main paper coating principles
70
Picture: Katarina Dimic-Misic
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71
Film sizer with air turn
www.mhibeloit.com
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72
Main phases in conventional pigment coating
Drying of wet coating
color with IR, hot air
and drying cylinders
Application of
coating color
Leveling of
coating color
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Main coating methods
Blade coating produces smooth surface but uneven coating. Curtain coating produces
even coating layer but rough surface.
73 Picture: Voith Paper
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Film coating layout
Typical coating processes for LWC
Blade coating layout
Picture: Valmet
74
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Coating section of a cartonboard machine
There can be several coating stations in a coated paperboard machine (2-5 pcs).
The picture below shows a coating sequence top-top-back-top.
75
Picture: Voith
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76
Effects of coating on paper
Coating fills the cavities and covers the base paper surface
increasing smoothness.
Ink absorption decreases.
Surface strength increases and dusting decreases.
Gloss increases, with the objective often being the increase
of print gloss.
Opacity increases, and hopefully also brightness.
Mechanical strength of paper decreases, when coated and
uncoated papers are compared at the same basis weight.
Stiffness decreases when papers are compared at the
same basis weight.
Triple Coated
Uncoated
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77
10,000 X
Surface of Coated Paper
Fine kaolin clay
Ground Calcium Carbonate
Pictures: SMI
Precipitated Calcium Carbonate
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Fillers and coatings in paper
Mineral pigments can be added as a filler before headbox or to the surface as a coating with binders.
Paper Grades
Filler
Pigment
%
Surface size
per side
g/m2
Coating
per side
g/m2
Wo
od
co
nta
inin
g
Newsprint, TMP/GW
Newsprint, DIP
0 - 5
5 - 15
0
0 - 1.5
0
0
Unctd Mechanical, TD, Bulky
SC
5 - 15
15 - 35
0
0
0 - 5
0
Ctd Mechanical, LWC
MWC, HWC
5 - 15
8 - 18
0
0 - 2
5 - 15
20 - 40
Wo
od
free
Uncoated Woodfree, Copy
Printing
15 - 30
10 - 25
1 - 2
1 - 2
0
0 - 5
Coated Woodfree, standard
Premium Art
10 - 15
12 - 18
0 - 2
0 - 2
10 - 15
20 - 35
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Structure of coated paper
Coating thickness is relatively smaller than grammage of coating. Density of coating layer is about double (2000 kg/m3) compared to the base paper density (1000 kg/m3).
Picture:
R. Klein, U. Schulze
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Effect of calendering
It is difficult to make matt but smooth paper which would be ideal for reading.
Glossy Paper Gloss 50-80
PPS <1
Silk or semimatt Gloss 20-40
PPS 1-2
Matt Gloss 10-20
PPS >2
Pictures: Jouni Marttila
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Important properties of coated paper
Good CD profiles (basis weight, caliper, moisture, gloss, roughness, porosity, roll hardness)
Free of faults and holes (for coating), no impurities
Low fiber roughening potential (web offset grades)
High strength (MD tensile, CD tear, internal bond)
Good smoothness and minimum two-sidedness
Good formation (no mottling)
High compressibility (especially for rotogravure)
Optimal porosity and pore distribution
No blistering in heat set offset oven (high temperature)
No cracking (when folding) of higher weights
High brightness and opacity (low grammages)
Good CD stiffness, no curl (web offset grades and sheeted grades)
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Offset paper runnability vs. paper properties
Pressroom runnability Low amount of breaks
Low blistering tendency (heat set)
Low fiber roughening
Good folding
Good register control
Small amount of debris on blanket
Paper properties
Profiles (moisture, basis weight, caliper, orientation etc.)
Tear- and tensile strength
Mechanical faults
Linting and dusting
Blistering resistance (heat set)
Ply-bond
Porosity
Moisture
Number of shives
Stiffness
Density, stretch
dampening
unit
ink unit
plate
blanket
backing
cylinder
Paper with dust
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How to Influence on Papermaking
Process and Paper Quality
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84
Effect of chemical pulp refining on paper
Positive effects Wet web strength
Tensile, surface etc. strengths
Better formation
Coating coverage
Porosity and ink demand
Smoothness and gloss
Negative effects Water removal and solids content
Bulk and stiffness
Compressibility
Opacity and brightness
Drying shrinkage dimension stability
Tear strength
Energy consumption
Pictures: E.Gruber
Internal fibrillation External fibrillation Fiber bonding
+ =
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Woodfree paper process adjustments
Command Variables
Process Parameters BHKP %
Refining
Filler %
Grammage
Drainage -- --- + ---
Retention - ± -- +++
Formation ++ + +++ --
Wet strength -- ++ --- ±
Dry paper runnability -- + -- +
Specific energy cons. + -- +++ ±
+ = positive effect, - = negative effect
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Effect of command variables on paper properties
Paper Properties BHKP %
Refining
Filler %
Grammage
Optical properties ++ -- +++ +
Tear strength - + - - - +++
Other strength properties - ++ --- +++
Better bulk ± -- ± ±
Better smoothness + ++ +++ +
Dimension stability + -- ++ +
Lower porosity + ++ ± +++
Better printability + + +++ +
Total costs/ton + -- +++ ---
+ = positive effect, - = negative effect
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Effects of selected parameters on paper properties
Increasing the right
variables have the effects of
arrows in paper properties
Lo
ng
fibers
(BS
KP
)
Refin
ing
We
t en
d s
tarc
h
Fille
r
Mo
re o
rien
tatio
n
in M
D
We
t pre
ss
ing
Fin
al m
ois
ture
Cale
nd
erin
g
Press dryness
Initial wet web strength, MD
Tear strength, CD
Tensile strength, MD
Dimension stability, CD
Internal bond strength
Smoothness, gloss MD
Porosity
Stiffness, CD
Opacity
Brightness
Costs
Red =
negative
Green =
positive
MD =
Machine
Direction
CD =
Cross direction
= main
reason to
increase
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Advantages of good runnability
More
- filler
- coating
Higher
- speed
- efficiency
Good
Runnability
Decreased
- basis weight
- long fibre content
Less
- energy
- water
Less
- chemicals
- wires/felts
More
- short fibres
- mech. pulp/DIP
Quality
- better
- more even
Lower
- labor cost
- supplies cost
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Improved
runnability
Lower raw
material costs
Longer
wire life
Increased
machine speed
Less steam &
energy/ton
Lower
furnish
cost
Cleaner
system
Steam & el
used only once
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
chemistry
Advantages of low break frequency
Product
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
Lower losses of
fillers & chemicals Higher
press solids
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Example of paper quality control system
Picture: Metso Automation
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Source: Valmet
Consumption values for papermaking
News-
print LWC
Fine
paper
Opti
Concept
News
Opti
Concept
LWC
Electricity
kWh/t of paper 470-570 550-700 500-650 530-630 600-750
Drying steam,
t/t of paper 1,7 - 1,8 1,7 - 1,8 1,8 - 1,9 1,1 - 1,3 1,1 - 1,3
Drying gas,
kg/t of paper 0 0,08 - 0,1 0 25 - 27 30 - 40
Fresh water
m3/t of paper
total for the mill
10 - 15 10 - 15 13 - 18 8 - 13 8 -13
Shower water
m3/t of paper
total for the PM
3 - 5 3 - 5 3 - 5 3 - 5 3 - 5
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New Papermaking Developments
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Basic process principle is old
Combined forming/pressing or pressing/drying ?
Do we need water in forming and coating – dry processes?
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Flocculation, retention and drainage
The basic problem of papermaking is that it is difficult to get good formation,
drainage and retention at the same time.
Flocculated: • Bad formation
• Low strength
• Low opacity
• High porosity
• Good drainage
• Good fiber retention
Good formation • High strength
• Good opacity
• Low porosity
• Slow drainage
• Low fiber retention
Better quality
Picture: E.Gruber
Pele Oy
Headbox dilution in papermaking
The biggest problem of industrial papermaking is fiber flocculation. Flocculation tendency
is the basic reason that there must be 100-1000 times water dilution in the headbox.
The other reason to the high dilution is cross-direction profile control. Final paper is less
than 0.1 mm thick and it should have thickness accuracy about ±1%. This is ±0.001 mm
or ±1 µm. With a 100-fold dilution this accuracy requirement is already ±0.1 mm which is
easier to achieve for a 10 m wide web.
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Fibers occupy a sphere
in the headbox
Stiff fibers form flocs
with friction forces
Flocculation results in
bad paper formation
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Current headbox technology
The principal construction of current hydraulic headbox technology is quite expensive due
to the very large area of highly finished surface.
For operator this kind of headbox is also demanding. Principally there is only one
optimum very narrow flow window for papermaker and outside this window the turbulence
is too high or too low.
The optimum jet-to-wire speed ratio is different for best formation and best fiber
orientation in most cases.
The most demanding flow range might be the filler ply in a multilayer board.
96 Picture: Voith
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High consistency forming
The main improvement of papermaking should be reduction of water usage of the
internal circulations. Conventional solution to this is high consistency forming (1-3%).
In addition to the flocculation tendency the CD accuracy is demanding in high
consistency forming.
It is easy to calculate what would be the slice opening for different headbox
consistencies. When grammage is low slice opening is just some millimeters, which is
demanding for CD accuracy. The calculation below is for 100% retention. In practice
lower retention increases slice opening.
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Slice O
pen
ing
, m
m
Grammage, gsm
0,00
5,00
10,00
15,00
20,00
25,00
30,00
35,00
0 50 100 150 200 250 300 350
HB cons. 1 %
HB cons. 3%
HB cons. 2 %
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High consistency forming and paper quality
The fibers from high consistency forming
are randomly oriented in all directions
rather than in the plane of the web
making this forming process unsuitable
for formation of printing papers.
The random grain orientation is believed
to be due to collision during drainage of
the densely packed fibers. In addition,
the formed web has high bulk, high
porosity, grainy formation, increased z-
direction strength (out of the plane of the
web) and reduced in-plane strength.
While this web is suitable for some
board grades it is not suitable for thin
publication papers.
This old picture on the right shows what
is the difference between filtering
(normal paper) and thickening (high
consistency paper).
98
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High consistency forming headbox
There is a very interesting patent idea (WO 2013024205 A1) of Matti Luukkanen on High
Consistency (HC) forming (2-5%). This could be very suitable for pulp drying machines and
several board grades, especially for the filler ply of three-layer board.
The picture below shows how rotating drum (red) inside a curved chamber produces
turbulence, pressure and flow to the water removal gap between a solid apron and a
moving wire on a dewatering box.
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Consistency Total mass
% tons/dry ton
0,5 200
1 100
2 50
3 33
4 25
5 20
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Foam forming to solve flocculation problems
New possibilities with increased headbox consistency by foam forming:
New paper properties by using special long fibers with good formation
High bulk products with good strength by combination with nanofibrillated
cellulose for insulation materials, filters and tissue products
High bulk with good z-strength for e.g. middle ply of cartonboard
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Bulk [cm3/g] Picture: VTT
Pele Oy
Pilot foam forming machine at VTT Finland 2013
Foam forming gives possibilities to save water, energy and material in papermaking.
101
Picture: VTT
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Microfibrillated cellulose
Microfibrillated cellulose is a potential but still today expensive development for papermaking.
MFC can be made by grinding or refining fibers to small pieces called microfibrils.
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Principle of Valmet water layering technology
With multilayer headbox it is possible to get separation of fiber layers and prevention of
flocculation through the layers by using a water layer between the two fiber layers.
It is possible to put different chemicals and filler between the fiber layers with water
layering. One example is cationic starch.
103
Picture: Valmet
Pele Oy
Multilayer headbox with water layering technology
Valmet had a presentation in PaperCon2015. The following conclusion
is from this presentation:
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Alternatives for containerboard machines
Board making is developed closer to papermaking and relative speed and production
development has been faster than in papermaking. Today there are also gapformers
in board machines.
105
Picture: Valmet
Pele Oy
Wet end rebuild of testliner machine.
This Valmet example is interesting how to get four layers of conventional two-layer
testliner machine by using layering headbox.
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Multigrade cartonboard machine
There are five headboxes and two shoe presses with totally four felts. Five coating stations allow different kind of products.
First calender has hot roll on top side and second calender on bottom side. It is possible to make symmetrical graphical board.
Picture: Valmet
Pele Oy
Impingement drying possibilities
High-speed machines require good dryness after press section to get runnability.
One possibility to save bulk or add filler content is to use impingement drying in
the beginning of dryer section.
108 Picture: Valmet
Pele Oy
THANK YOU FOR YOUR ATTENTION
109
