The Function of Lubricants in Pigment Coating Ilkka Herrala Laboratory of Paper Technology HUT 25/04/11 Ilkka Herrala / AW...
The Goals of the Thesis  <ul><li>to describe and illustrate the  function of lubricants  in pigment coating . </li></ul><u...
Topics of Discussion <ul><li>E ffects of Lubricants </li></ul><ul><li>Classification of Lubricants </li></ul><ul><li>Poten...
Effects of Lubricants   <ul><li>Lubrication of the wet coating and improvement of flow characteristics (rheology). </li></...
… Effects of Lubricants <ul><li>Reduction or elimination of whiskering and scratches. </li></ul><ul><li>Reduced or elimina...
… Effects of Lubricants <ul><li>Elimination or reduction of pigment dusting, blocking, scaling and linting during finishin...
Classification of Lubricants <ul><li>Aqueous dispersions of  metallic soaps  (soluble / insoluble) </li></ul><ul><li>Wax e...
… Classification of Lubricants <ul><li>Triglycerides (fats) and other fatty acid esters </li></ul><ul><li>Polyglycols and ...
Potential Lubricants <ul><li>should be  attracted  either chemically or physically  to the surface  and thus decrease the ...
Principals of Tribology <ul><li>The word tribology is derived from the Greek word  tribos  meaning rubbing.   </li></ul><u...
Types of Friction <ul><li>Boundary friction </li></ul><ul><li>Hydrodynamic friction </li></ul><ul><li>Elastohydro-dynamic ...
Problems Affected by Paper Friction <ul><li>Roll winding and rewind problems </li></ul><ul><li>Crepe wrinkles within the r...
Dusting and Linting of Coated Paper <ul><li>Insufficient binder  in the coating or at the surface of the dry coating.  </l...
Principals of Rheology <ul><li>Rheology describes the deformation of a body under the influence of stresses.  </li></ul><u...
Viscoelasticity <ul><li>The majority of fluids of technical importance lie somewhere between liquids and solids in their r...
Viscoelasticity Affecting the Quality of Coating  <ul><li>dominant in  high-speed  blade coating </li></ul><ul><li>formula...
New Hypotheses of Lubricant Function <ul><li>Lubricants may  turn the balance  of a viscoelastic coating color  from the e...
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The Function of Lubricants in Pigment Coating

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The purpose of this Master’s thesis is to describe and illustrate the function of lubricants in the pigment coating of paper. The original goal was to show that lubricants might contribute to up-speeding of the pigment coating process. As this appeared to be a very complicated task to prove in reality and required test equipment, which was not available at that time in the Paper Laboratory of Helsinki University of Technology, the experimental part of the thesis was left out and a more comprehensive and versatile literature study of the subject was realized instead. I hope that the advanced level of information served by this literature study could inspire researches interested in this subject to further analyse the function of pigment coating lubricants, and based on this information develop new lubricant products for the pigment coating market.

Pigment coating lubrication can be divided into two categories: lubrication in wet state coating and lubrication in dry (or semi-dry) coating. Lubrication in wet state coating involves hydrodynamic lubrication and boundary lubrication, and occurs when moving pigment surfaces are completely or partially separated by a water layer. The lubricants have now influence on the viscoelastic character of the wet coating color by altering the manner in which various ingredients of a coating color interact with one another. Lubrication in dry coating involves both boundary lubrication, where paper surface and process machinery contact, and elastohydrodynamic lubrication - in connection with supercalendering - where hard roll surfaces are in contact with elastomeric surfaces of coated paper and filled calender rolls. In both cases, absorbed films of lubricants reduce friction between the respective surfaces. Lubricants then influence the manner in which the pigment particles and other materials are aligned with each other after the drying and supercalendering steps. This results in better runnability of the machines and improved optical properties of the finished sheet.

In the synthesis of this Master’s thesis some guidelines are given for evaluating different substances for their lubricating performance in pigment coating: An effective lubricant should be attracted either chemically or physically to the surface and thus decrease the real area of contact, and therefore the adhesion, of the two rubbing surfaces. It should have strong cohesion energy between the molecules. The optimal chain length must be found to ensure insolubility together with low viscosity. Organic Sulfur-compounds could be added to a lubricant in order to ensure lubrication performance under high temperatures and pressures. Finally, novel mechanisms are suggested for lubricant performance, especially in blade coating: Lubricants may turn the balance of a viscoelastic coating color from the elastic side to a slightly viscous side and thus enable the flow of a coating color still in very high coating solids. Together with higher coating solids, higher coating speeds may also be achieved, which was indeed the initial interest of this study. Secondly, there seems to be a link between blade bleeding/extrusion and the slip boundary within the color. One possible mechanism for lubricant function may well be their ability to reduce friction and increase slip at the blade boundary, thus preventing bleeding and dry stalagmite formation at the blade tip from occurring.

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The Function of Lubricants in Pigment Coating

  1. 1. The Function of Lubricants in Pigment Coating Ilkka Herrala Laboratory of Paper Technology HUT 25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  2. 2. The Goals of the Thesis <ul><li>to describe and illustrate the function of lubricants in pigment coating . </li></ul><ul><li>to show that lubricants may contribute to up-speeding of the pigment coating process . </li></ul><ul><li>based on this information develop new lubricant products for the pigment coating market . </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  3. 3. Topics of Discussion <ul><li>E ffects of Lubricants </li></ul><ul><li>Classification of Lubricants </li></ul><ul><li>Potential Lubricants </li></ul><ul><li>Principals of Tribology </li></ul><ul><li>Principals of Rheology </li></ul><ul><li>Lubricants and Viscoelasticity </li></ul><ul><li>Lubricants and Blade Runnability </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  4. 4. Effects of Lubricants <ul><li>Lubrication of the wet coating and improvement of flow characteristics (rheology). </li></ul><ul><li>Better application and metering of the wet coating color. </li></ul><ul><li>Improved leveling. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  5. 5. … Effects of Lubricants <ul><li>Reduction or elimination of whiskering and scratches. </li></ul><ul><li>Reduced or eliminated film split patterning. </li></ul><ul><li>Increased coating smoothness. </li></ul><ul><li>Improvement of finish (gloss). </li></ul><ul><li>Increased resistance to cracking and peeling when paper is folded. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  6. 6. … Effects of Lubricants <ul><li>Elimination or reduction of pigment dusting, blocking, scaling and linting during finishing, slitting, sheeting, and printing operations. </li></ul><ul><li>Easier calendering making possible an acceptable machine calender finish for some grades. </li></ul><ul><li>Enhancement of printability. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  7. 7. Classification of Lubricants <ul><li>Aqueous dispersions of metallic soaps (soluble / insoluble) </li></ul><ul><li>Wax emulsions (polyethylenes) </li></ul><ul><li>High-solids, lubricant blends containing calcium stearate </li></ul><ul><li>Diglycerides (lecithin blends) </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  8. 8. … Classification of Lubricants <ul><li>Triglycerides (fats) and other fatty acid esters </li></ul><ul><li>Polyglycols and polyglycol ethers/esters (e.g. PEG) </li></ul><ul><li>Sulfated and/or sulfonated oils </li></ul><ul><li>Fatty amines and fatty amides (synthetic waxes) </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  9. 9. Potential Lubricants <ul><li>should be attracted either chemically or physically to the surface and thus decrease the real area of contact, and therefore the adhesion, of the two rubbing surfaces (acid > amine groups) . </li></ul><ul><li>strong cohesion energy between the molecules (CH 2 - groups > CF 2 - groups). </li></ul><ul><li>optimal chain length of the fatty acid chain (low viscosity, water insolubility). </li></ul><ul><li>optimal level of the melting point . </li></ul><ul><li>org. Cl- and S-compounds as additives . </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  10. 10. Principals of Tribology <ul><li>The word tribology is derived from the Greek word tribos meaning rubbing. </li></ul><ul><li>It is the science and technology of interacting surfaces in motion and of related practices and subjects. </li></ul><ul><li>Tribology deals with friction and wear. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  11. 11. Types of Friction <ul><li>Boundary friction </li></ul><ul><li>Hydrodynamic friction </li></ul><ul><li>Elastohydro-dynamic friction </li></ul><ul><li>pigment-pigment </li></ul><ul><li>pigment-lubricant-pigment </li></ul><ul><li>dry coating-elastomeric SC rolls </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  12. 12. Problems Affected by Paper Friction <ul><li>Roll winding and rewind problems </li></ul><ul><li>Crepe wrinkles within the roll </li></ul><ul><li>Roll telescoping during transit </li></ul><ul><li>Stack and pallet instability </li></ul><ul><li>Problems with web tracking and print registration </li></ul><ul><li> No feed or multipe feed during sheet feeding </li></ul><ul><li> Registration errors in die cutting or converting </li></ul><ul><li> Corrugator’s runnability problems of several kind </li></ul><ul><li> Friction buildup and dusting during converting operations, especially at SC and printing </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  13. 13. Dusting and Linting of Coated Paper <ul><li>Insufficient binder in the coating or at the surface of the dry coating. </li></ul><ul><li>Excessive penetration of binder into raw stock. </li></ul><ul><li>Improper coating penetration, insufficient screening, or failure to optimize dispersant requirements . </li></ul><ul><li>Overdrying the sheet (moisture content). </li></ul><ul><li>Slight variations in paper machine or coater speed, coating profile, or drying rate causing nonuniformaties in paper. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  14. 14. Principals of Rheology <ul><li>Rheology describes the deformation of a body under the influence of stresses. </li></ul><ul><li>Rheology plays an important role in coating color processability , coater runnability and coated paper properties . </li></ul><ul><li>It still remains unclear which rheological parameters of paper coatings correlate with coater runnability and coating structure . </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  15. 15. Viscoelasticity <ul><li>The majority of fluids of technical importance lie somewhere between liquids and solids in their rheological behavior. </li></ul><ul><li>They are in varying extent both elastic and viscous and are therefore called viscoelastic materials. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  16. 16. Viscoelasticity Affecting the Quality of Coating <ul><li>dominant in high-speed blade coating </li></ul><ul><li>formulations with high solids content </li></ul><ul><li>where relatively large blade loadings are required to obtain a target coat weight </li></ul><ul><li>depends to a great extent on the coating formulation used (type and amount of pigment and soluble additives) </li></ul><ul><li>wet bleeding and scratches </li></ul><ul><li>stalagmites at the blade (back side) </li></ul><ul><li>change in the spreading response of coatings during application </li></ul><ul><li>may promote coverage of the substrate </li></ul><ul><li>leveling-out of blade-induced streaks </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab
  17. 17. New Hypotheses of Lubricant Function <ul><li>Lubricants may turn the balance of a viscoelastic coating color from the elastic side to a slightly viscous side and thus enable the flow of a coating color still in very high coating solids. Together with higher coating solids, higher coating speeds may also be achieved. </li></ul><ul><li>They may have ability to reduce friction and increase slip at the blade boundary thus preventing bleeding and dry stalagmite formation from occurring. </li></ul>25/04/11 Ilkka Herrala / AWL Scandinavia Ab

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