benefits of online porosity measurement 24 august 2015
TRANSCRIPT
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Possibilities to utilize porosity measurements
Continuous porosity measurement tells what has been made in papermaking and what will be happen in printing or converting.
Air permeability is called porosity in the following presentation.
Porosity is measured by air flow speed 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 porosity or air permeability in this presentation.
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Process control and porosity
Pulps and pulping processes Filler types and amount Broke types and amount Wet end chemistry Internal sizing Refining Fines retention Air content
Headbox consistency Slice and jet controls Dewatering arrangement Wet pressing Sheet release and draws Drying type Surface sizing
Some uses of porosity measurement results to process optimization
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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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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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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)
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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 will advance more and more flexible fibers compensate the lower binding power of fiber cutting.
Same porosity can be achieved with higher CSF when using increased 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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Porosity control on paper machine
Porosity decreases with: Higher grammage 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
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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 a good tool for evaluating what is a too high press draw. Higher press draw increases roughness and decreases strength.
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Total process control with porosity
High internal bond
Low press to dryer drawWet 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:
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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 analyser 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 with almost all physical paper properties. If porosity is constant, paper process performs well and paper quality is consistent.
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Total process control with porosity
Decreasedbulk
Betterformation
Lowercaliper
Lowerstiffness
Betterply bond
Bettersmoothness
Less print-through
Lesspinholes
Higheropacity
Betterprint gloss
Lower inkabsorption
Increasedbasis weight Increased
moisture
DecreasedPorosity
More fiber bonding
Higherstrength
Lower tearstrength
Lower lightscattering
Lower opacityand brightness
Lessdusting
Lower inkdemand
Less flutingin offset
Green
Red
= positive
= negative
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Process parameters increasing porosity
HigherPorosity
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
screeningReduced refining
Poor wet end
chemistry
Pulsating
drainage
More shrinkage
at web edgesReduced
calendering
Higher press draw
More flocculation
Higher air content
More pin holes
Lower fiber
bonding
Lower fiber fines content
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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 analyzer 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, coating colour cost is high or when coating low brightness base paper e.g. kraftliner.
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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.
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
Pele Oy MD and CD variation in porosity
CD variation of porosity is very stable. It can easily be measured in lab. 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.
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Thank You for Your Attention