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A REVIEW OF INTERFACIAL ASPECTS IN WOOD COATINGS

A REVIEW OF INTERFACIAL ASPECTS IN WOOD COATINGS. Mari de Meijer Drywood Coatings. TOPICS:. Coating penetration into substrate Wood surface energy and wetting Adhesion Wood surface preparation. PENETRATION OF COATINGS. Techniques for assessment Influence of wood anatomy

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A REVIEW OF INTERFACIAL ASPECTS IN WOOD COATINGS

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  1. A REVIEW OF INTERFACIAL ASPECTS IN WOOD COATINGS Mari de Meijer Drywood Coatings

  2. TOPICS: • Coating penetration into substrate • Wood surface energy and wetting • Adhesion • Wood surface preparation

  3. PENETRATION OF COATINGS • Techniques for assessment • Influence of wood anatomy • Influence coating properties • Relevance to performance

  4. Techniques for assessment Static: • Light and fluorescence microscopy,dyeing the coating or the subtrate • Confocal laser • SEM (+EDAX) Dynamic: • Rate of uptake (volume / droplets) • No dynamic microscopic techniques

  5. Examples softwood

  6. Examples softwood

  7. Examples softwood

  8. Examples hardwood

  9. Schematic overview of possible penetration

  10. Influence coating properties Model capillary flow, static situation Length filled capillary (L), liquid surface tension (g L) cosine of the contact angle (q ) of wetting liquid capillary radius (r) acceleration of gravity g (9,8 m s-2) density of the liquid (rL)

  11. Influence coating properties Model capillary flow, dynamic situation Length filled capillary (L), liquid surface tension (g L) cosine of the contact angle (q ) of wetting liquid capillary radius (r) viscosity paint (h) time (t)

  12. CAPILLARY UPTAKE CELL WALL WOOD CONTENT PAINT INCREASING SOLIDS SELECTIVE UPTAKE WATER OR SOLVENT FLOW COATING COATING

  13. VISCOSITY - SOLIDS

  14. VISCOSITY - SOLIDS WATER SOLUBLE LINSEED OIL

  15. WETTING COATING gcoating< gwood Viscosity can also be limiting the wetting

  16. WETTING COATING

  17. Relevance to performance • Carrier of functional additives like biocides • Improvement of adhesion by providing mechanical anchoring • Improving the exterior durability • Esthetical aspects like clarity of grains (‘anfeuerung’) and pore wetting

  18. Wood surface energy and wetting • Critical surface energy • Polarand dispersecomponents • Lifshitz-van-der-Waals and (Lewis) acid-base components • Young’s equation:s= sl + l cos  • Drop or Wilhelmy plate with various liquids

  19. Wood surface energy and wetting • Theory assumes: thermodynamic equilibrium and a chemically homogeneous solid surface, flat and not influenced by chemical interaction or adsorption of the liquid to the surface ? !!!

  20. Overview of literature data (mJ m-2)

  21. Adhesion / adherence • Impact of the measurement technique • Reduction adhesion by energy stored in the coating because of internal stress • Work expended in deformation during peeling or torsion of the coating • Impact of mechanical anchoring • Influence of moisture in coating or wood • Molecular forces between coating and wood that determine the interfacial adhesion (true adhesion)

  22. Adhesion analysis • X cut of cross-hedge test • dolly pull-off • dolly torques test • peeling in testing machine • atomical level (AFM etc, not on wood)

  23. Peel tape test

  24. Peel tape test

  25. Mechanical anchoring

  26. Mechanical anchoring

  27. Strong impact on adhesion: dry >> vapour > liquid Dry state: too high to measure > 600 J/m2 Moisture & adhesion

  28. stored strain energy work of adhesion work stored in plastic deformation interfacial work of adhesion Moisture & adhesion • Factors influencing the measured adhesion: • WT = cw + Wp - • Interfacial work of adhesion: molecular interaction • Plastic deformation: negligible • Stored strain energy due to internal stress : differential hygroscopic expansion coating and wood

  29. Moisture & adhesion c:coating thickness E: coating elasticity : poisson ratio (0.4) coating: swelling coating wood: swelling wood

  30. Moisture & adhesion c:coating thickness E: coating elasticity : poisson ratio (0.4) coating: swelling coating wood: swelling wood

  31. Calculated – measured adhesion Wacw = gc + g w - g cw Wawet = gCL + gWL - gCW

  32. Calculated – measured adhesion

  33. Adhesion promoting technologies • Pretreatment of the wood by flame-ionisation or plasma- treatment • Incorporation of adhesion promoting monomers in acrylic dispersions • Reducing the wateruptake and / or swelling of the coating by crosslinking of the polymer or reducing the hydrophilicity • Chemical crosslinking between coating and wood

  34. Wood surface preparation • Sanding: reduction of penetration • Rough sawing: increase in coating uptake • Planing: possibility of cell compression

  35. Wood surface preparation • Sanding: reduction of penetration • Rough sawing: increase in coating uptake • Planing: possibility of cell compression Deformed cells Source: SHR Timber Research

  36. Cell compression • Solventborne: expansion during weathering • Waterborne: expansion during coating application Coated with solventborne paint Coated with waterborne alkyd paint Exposed to water Source: SHR Timber Research

  37. CONCLUSIONS • A combination of the anatomical wood structure and flow of the coating determines coating penetration • Differences in penetration of coatings are mainly determined by the increase in viscosity with solid content due to selective uptake of water or solvent in the cell wall • Wetting and surface tension of the coating seem to play a minor role and insufficient wetting is often due to a limitation by viscosity

  38. CONCLUSIONS • Surface energy determinations in terms of polar – dispersive parts or lifshitz vander waals – acid base components has been made for many wood species but are not usefull in understanding the adhesion of coatings • In general the surface energy of wood is equal or higher than the surface energy of a liquid coating which means that wetting is not a limiting factor

  39. CONCLUSIONS • Penetration of coatings into the outer pores of wood certainly contributes to improving the adhesion of a coating, especially under wet conditions. • A very deep penetration will not directly contribute to adhesion but might reduce the differences in dimensional change between coating and wood and reduce stress in the coating • The adhesion of a coating to wood is particularly critical under wet conditions. Waterborne coatings (both acrylic and alkyd based) have a lower wet adhesion than solventborne ones. One reason might be the higher swelling by moisture but other unknown factors seem to play a role too.

  40. CONCLUSIONS • The surface preparation can have a major impact on the coating performance if wood cells are strongly compressed during planing. • The subsequent expansion of the cells can lead to high grain raising or premature cracking of the coating

  41. GAPS IN KNOWLEDGE • The rheology of coatings at increasing solid content or during drying is hardly known but is essential to understand differences in penetrating capacity. • Impact of a penetrating primer on the weathering performance. Seems to be positive, but why? • Reduction of coating adhesion under wet conditions. Improved knowledge in this field is required to understand why adhesion is sometimes insufficient.

  42. Thank you for your long lasting attention!!

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