Porosidad Del Papel Y El Arte De Hacer Papel 11234w

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Paper Porosity and the Art of Papermaking

Pekka Komulainen [email protected] 24 August, 2015

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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

Some uses of porosity measurement results to process optimization

 Pulps and pulping processes

 Headbox consistency

 Filler types and amount

 Slice and jet controls

 Broke types and amount

 Dewatering arrangement

 Wet end chemistry

 Wet pressing

 Internal sizing

 Sheet release and draws

 Refining

 Drying type

 Fines retention

 Surface sizing

 Air content

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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

Low porosity correlates with: Wet Pressing Good formation

Root cause:

Low press to dryer draw No pinholes

Final benefit of low porosity: Good runnability

No print through Rotogravure printability

Low air content

Low oil absorption

Low silicone pick-up

High internal bond

Limit: Set-off in CSWO

Good bonding

Offset printability

Dense sheet

Good refining

Good strength

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Total process control with porosity

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 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.

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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.

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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

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Increased basis weight

Higher opacity

Decreased Porosity

Less pinholes

Increased moisture Decreased bulk

Lower caliper

Lower ink absorption

Lower stiffness

Less fluting in offset

Better print gloss

Lower tear strength

Lower ink demand

More fiber bonding

Better smoothness Less printthrough

Better formation

Better ply bond

Higher strength Less dusting

Green

= positive

Red

= negative

Lower light scattering Lower opacity and brightness

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Process parameters increasing porosity

Pulsating drainage

Poor wet end chemistry

Reduced refining

Less reject in screening

Higher air content

Less wet end starch

Lower retention

Lower fiber fines content

Less platey fillers

More flocculation

Carbonate fillers

Lower fiber bonding

Coarser fiber type

More pin holes

Less surface size

Higher press draw

Decreased wet pressing

Reduced calendering

Less broke

Higher Porosity

More shrinkage at web edges Softer winding

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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.

Fillers

Fibers

Drainage

Chemicals

Retention

Wet pressing

Refining

Air content

Press draw

Calendering

Screening

Flocculation

Web shrinkage

Winding

Porosity measurement Basis weight Bulk Pin holes Tensile

Caliper

Formation

Scott Bond

Moisture

Opacity

Roughness

Stiffness Print through Print gloss

Ink demand

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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.

MD var.

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Thank You for Your Attention

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