Benefits of online porosity measurement 24 August 2015

1. Pele Oy Paper Porosity and the Art of Papermaking Pekka Komulainen 24 August, 2015 2. Pele Oy Possibilities to utilize porosity…
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  • 1. Pele Oy Paper Porosity and the Art of Papermaking Pekka Komulainen 24 August, 2015
  • 2. Pele Oy 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. 2
  • 3. Pele Oy 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 3 Some uses of porosity measurement results to process optimization
  • 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. 4
  • 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 5
  • 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 6 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 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. 7
  • 8. Pele Oy 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 8
  • 9. Pele Oy 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. 9
  • 10. Pele Oy Total process control with porosity 10 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:
  • 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 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. 11
  • 12. Pele Oy Total process control with porosity 12 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 13 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 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. 14
  • 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. 15 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 lab.  Machine direction variation seems to be in this case about ± 15 ml/min. This could be corrected by using online porosity measurement. 16 MD var.
  • 17. Pele Oy 17 Thank You for Your Attention
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