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Paper Porosity and the Art of
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
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.
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
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.
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 fiber fines
Less anionic trash (less hydrophobic material)
More wet end starch and other dry strength
Less fillers or more platelike fillers (talc,
Less air in stock
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
SRE = Specific Refining Energy
Pe = Effective Power, Pt = Total Power, Po = Idle Power
m = Fibre flow (Bone Dry)
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.
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
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.
Porosity control on paper machine
Porosity decreases with:
Higher basis weight
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
Total process control with porosity
High internal bond
Low press to dryer draw
Low air content
Good refining Good bonding
Low oil absorption
No print through
Low silicone pick-up
Limit: Set-off in CSWO
Root cause: Final benefit of
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.
Total process control with porosity
More fiber bonding
Pele Oy Process parameters increasing porosity
Less surface size
Less reject in
Poor wet end
at web edges
More pin holes
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).
Process optimization and paper quality
Fast online porosity measurement is the best link between process performance and total
Stable porosity is a good guarantee of well working process and consistent paper quality.
Scott Bond Roughness
Print throughPin holes Opacity
Print gloss Ink demand
Basis weight Moisture
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.
Picture: ACA Systems
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
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.
Leakage air increases porosity
when speed is over 10 m/s
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
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
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
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.
Some suppliers of online porosity sensors
MCS Porosity Measurement System
In addition, Honeywell, MCS Engineering and ACA Systems supply scanning
Thank You for your attention
Picture: Pekka Komulainen