Update of older presentation

1. Pele Oy
Paper Porosity and the Art of
Papermaking
Pekka Komulainen
Pekka.Komulainen@clarinet.fi
February, 2018

2. Pele Oy
Porosity and air permeability
 Porosity is measured by air flow through paper web
over standard pressure difference.
 All papers have voids between fibers. Scientific porosity
means void volume compared to total volume. That is
different than air permeability.
 Air permeability is called porosity in the following
presentation.
 Continuous porosity measurement tells what has been
made in papermaking and predicts what will happen in
printing or converting.
 Online porosity measurement is different compared to
laboratory measurement. The main difference is that
paper sheet is not moving in the laboratory and the web
speed of online measurement can vary very much.
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www.mcsengineering.com

3. Pele Oy
What can be optimized with porosity measurement
 Pulps and pulping processes
 Filler types and filler amount
 Broke types and amount
 Chemicals and wet end chemistry
 Fiber refining and swelling
 Fines and filler retention
 Air content and foaming
 Headbox consistency and formation
 Slice and jet controls
 Wire table dewatering
 Wet pressing and calendering
 Sheet release and draws
 Drying type, free or restrained
 Surface sizing and coating
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4. Pele Oy
Porosity for total process control
 Fibers , refining, fines, fillers and wet end chemistry have effect on drainage, flocculation
and retention. All these effect on porosity, which is a good measure of paper quality.
 Formation is a measure of small scale variations. Porosity depends on these variations as
well as the level of paper quality.
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5. Pele Oy Porosity control before headbox
Porosity decreases with:
 Less softwood kraft pulp
 More fine mechanical or hardwood pulp
 More coated broke or DIP (normally)
 More refining
 More fiber fines
 Less anionic trash (less hydrophobic material)
 More wet end starch and other dry strength
chemicals
 Less fillers or more platelike fillers (talc,
delaminated clay)
 Less air in stock
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6. Pele Oy
Refining and porosity
 Conventional control of refining is to control Specific Refining Energy
 SRE control of refiners cannot decrease pulp quality variations
 With online porosity measurement this can be corrected
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SRE = Specific Refining Energy
Pe = Effective Power, Pt = Total Power, Po = Idle Power
m = Fibre flow (Bone Dry)

7. Pele Oy
Online porosity control and refining
 In series refining fibers are stiffer in the first refiner. If the load is too
high, fiber cutting and fines formation will increase. This decreases
freeness and drainage on the wire.
 In the second (and third) refiner fibers are more flexible and specific
edge load can be higher without fiber cutting and fines formation.
However, internal fibrillation advances and more and more flexible
fibers compensate the lower binding power of fiber cutting.
 Same porosity can be achieved with higher CSF when using increasing
loading order. Total energy can be saved, drainage on the wire is
increased and lower headbox consistency can be used to get better
formation even if the fibers are longer and paper strength is better.
 Loading shares between refiners is only one example how online
porosity can be utilised to optimise refining. This would not be possible
by using freeness measurement, which mostly reflects fines content.
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8. Pele Oy
Press draw and porosity
 A high press draw is not only question of lower runnability but also paper quality. Higher
press draw means higher porosity and lower strength. Most of the papers require low
porosity but high bulk and strength. Porosity measurement is a good tool to control
press draw.
 Wet pressing itself always reduces porosity and increases strength. Wet strain after nip
has a very negative effect, increasing porosity and decreasing strength. High press
draw also increases paper roughness and decreases printability.
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Picture: Valmet

9. Pele Oy
Porosity control on paper machine
Porosity decreases with:
 Higher basis weight
 Better formation
 Good retention
 Slow and gentle water removal
 More wet pressing
 Less press draw
 Lower drying shrinkage
 More surface size
 More film forming size (PVA, CMC)
 Less surface size penetration
 Pigmented surface size
 More calendering
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www.mcsengineering.com

10. Pele Oy
Total process control with porosity
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High internal bond
Sheet compaction,
Low press to dryer draw
Wet Pressing
Good formation
Low air content
Good refining Good bonding
Good strength
Low oil absorption
No pinholes
Good runnability
No print through
Dense sheet
Rotogravure printability
Offset printability
Low silicone pick-up
Limit: Set-off in CSWO
Low porosity
correlates with:
Root cause: Final benefit of
low porosity:

11. Pele Oy
Total process control with porosity
 Constant porosity – Consistent quality
 Papermakers know that final porosity of cigarette, filter and
sack papers is very important but also printability and
runnability properties of printing papers correlate well with
final paper porosity.
 Most on-line instruments measure and control only one paper
property. In addition to final paper porosity, which is an
important property for some paper grades, online porosity
measurement is a universal analyse of paper quality for most
paper grades due to the fact that almost all process
parameters have a clear effect on porosity.
 On the other hand, porosity correlates well with almost all
physical paper properties. If porosity is constant, paper
process performs well and paper quality is consistent.
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Paper
making
Paper
porosity
Paper
quality

12. Pele Oy
Total process control with porosity
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Decreased
bulk
Better
formation
Lower
caliper
Lower
stiffness
Better
ply bond
Better
smoothness
Less print-
through
Less
pinholes
Higher
opacity
Better
print gloss
Lower ink
absorption
Increased
basis weight
Increased
moisture
Decreased
Porosity
More fiber bonding
Higher
strength
Lower tear
strength
Lower light
scattering
Lower opacity
and brightness
Less
dusting
Lower ink
demand
Less fluting
in offset
Green
Red
= positive
= negative

13. Pele Oy Process parameters increasing porosity
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Higher
Porosity
Lower
retention
Less broke
Softer winding
Decreased
wet pressing
Coarser fiber
type
Less surface size
Carbonate
fillers
Less platey
fillers
Less wet
end starch
Less reject in
screening
Reduced
refining
Poor wet end
chemistry
Pulsating
drainage
More shrinkage
at web edges
Reduced
calendering
Higher press
draw
More
flocculation
Higher air
content
More pin holes
Lower fiber
bonding
Lower fiber
fines content

14. Pele Oy
Total process control with online porosity
 Porosity is an overall measure of process performance. Porosity
measurement is like freeness in stock preparation. When porosity is
stable process is performing well.
 Porosity measurement is an excellent analyse before surface sizing
and coating. Measurement not only shows the history of the paper
web but also predicts how good coverage of size or coating can be
achieved in the following process.
 Film coating and cases where the positive effects should be made
with a minimum coat weight are well suited.
 When porosity is low, size or coating penetration is reduced and the
effect of coating on the paper surface is better. This is important
when basis weight is low or coating colour cost is high.
 Especially when low brightness base paper is coated to get white
surface low porosity is important (coated kraftliner).
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15. Pele Oy
Process optimization and paper quality
 Fast online porosity measurement is the best link between process performance and total
paper quality.
 Stable porosity is a good guarantee of well working process and consistent paper quality.
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Tensile
Bulk
Formation
Caliper Stiffness
Scott Bond Roughness
Print throughPin holes Opacity
Print gloss Ink demand
Basis weight Moisture
Drainage
Fillers
Wet pressing
Retention
Fibers
Press draw
Air content
Chemicals
Flocculation
Refining Screening
Web shrinkage
Calendering Winding
Porosity measurement

16. Pele Oy
MD and CD variation in porosity
 CD variation of porosity is very stable. It can easily be measured in laboratory.
 Machine direction variation seems to be in this case about ± 15 ml/min. This could
be corrected by using online porosity measurement.
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MD var.
Picture: ACA Systems

17. Pele Oy
Calibration with laboratory measurement
 Paper is moving in online measurement and
there is always some leakage air included in the
measurement. This air depends on two
parameters: machine speed and paper
roughness.
 Most paper grades are so sensitive that for
runnability reasons it is not possible to increase
the tightening suction high enough to totally
eliminate the leakage air.
 This means that the calibration curve with
laboratory measurement is not totally linear even
if the paper quality is same.
 Normally there are several basis weights of each
paper grade and these are run with different
speeds. With lower basis weight and higher
speed the share of leakage air is higher. The
relation to speed is not even. Normally surface
air film starts to build when speed is > 10 m/s.
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Leakage air increases porosity
when speed is over 10 m/s
Paper web
Measured air
Air
through

18. Pele Oy
Challenges of online porosity compared to lab
 Different paper grades normally have
different roughness. Higher roughness
means that there is more leakage air and the
calibration curve of different paper grades is
slightly different. This causes problems
especially in high speed multigrade paper
production.
 The other thing is dwell time over the
measurement orifice. Dwell time depends on
machine speed. It is well known that dwell
time has effect on the air flow.
 With higher dwell time in the laboratory paper
structure will compress more and porosity is
lower compared to online measurement.
 Last but not least, air temperature and
humidity are not constant in online
measurement.
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Online web speed 10 m/s and
0.01 m orifice mean that
measurement time is one ms.
Paper is viscous and not
compressed – porosity is higher.
Laboratory measurement time is
several seconds – paper is
slightly compressed and porosity
is lower.

19. Pele Oy
Other challenges
 To avoid leakage it is very important to install
the porosity measurement head completely
aligned with paper web. Each supplier can
give instructions for the installation.
 Normally suppliers can give guarantees for
the workability of the measurement.
However, recycled fibers with stickies and
dust can be so difficult that the contacting
head is not working continuously.
 In any case, maintenance of the equipment
and changing of filters must be done so often
that the system is working as planned.
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www.mcsengineering.com

20. Pele Oy
Some suppliers of online porosity sensors
 ABB Porolog
http://new.abb.com/pulp-paper/abb-in-pulp-and-paper/products/lorentzen-wettre-
products
 MCS Porosity Measurement System
www.mcsengineering.com
 Microperm Smart
http://www.avatronsystems.fi/m.php
 Permi
http://www.aca.fi/permi/
 In addition, Honeywell, MCS Engineering and ACA Systems supply scanning
porosity systems.
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21. Pele Oy
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Thank You for your attention
Picture: Pekka Komulainen