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Paint. Paint. Paint. Paint. Paint. Paint. a liquid with a colour?. This lecture. Function and structure of paint Rheology of paint Drying of a paint layer Paint brush. Product. function. properties. structure. composition. processing for structure. chemistry.

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  1. Paint Paint Paint Paint Paint Paint a liquid with a colour?

  2. This lecture • Function and structure of paint • Rheology of paint • Drying of a paint layer • Paint brush

  3. Product function properties structure composition processing for structure chemistry the role of physics (transport phenomena) in product engineering

  4. Paint coating, decorating, protecting physics impervious to water (vapour) coloured - colour fast - adhering - flexible - strong layered structure, pigment dispersed in a polymer production of binder, pigment and solvent pigment dispersion in binder and solvent chemistry brushing, spraying, drying

  5. Structure of Paint before drying pigment binder solvent after drying or impermeable porous depends on the ratio of the volumes of pigment and binder

  6. Composition of Paint volume fractions paint film pigment 0.05..0.7 binder 0.95..0.2 solvent 0.2..0.6 - air 0.0..0.2 other <0.05 thickeners, wetting agents, drying agents, biocides...

  7. ‘Pigment Volume Concentration’ serious blistering little critical PVC rusting none metal protection wall paint

  8. Colouring and Fillers pigment: colouring and fillers colouring: discrete particles (not dissolved) fine precipitates expensive inorganic: TiO2, carbon black organic: any number of colours fillers: talcum, clay, CaCO3…, natural or milled cheap give thickness and ‘schuurbaarheid’

  9. Pigment Size cumulative distribution Small particles: not visible Large particles: expensive colouring coarse particles filler wavelength of light

  10. C C C C C C C C C C C C C C O O Binder conjugated double bond final form: a polymer linear polymers: cellulose lacquer remain soluble in a solvent peroxide radical cross-linked polymers insoluble after formation two-component binders C C C C C C C mostly industrial O cross linking oxygen linked binders ‘drying’ O the most common ones C C C C C C C

  11. Solvent gives a low viscosity during application requirements lower alkanes (heptane), aromatics (xylenes), ketones (acetone) and alcohols (butanol) low viscosity drying time miscibility with binder aqueous (latex) paints are becoming important: water sticky polymer pigment

  12. When are Liquids Miscible? (1) when they do not differ too much in polarity heptane toluene ethanol water heptane no mixing toluene ethanol water no mixing increasing polarity

  13. + + + + + = = = = = + + + + + When are Liquids Miscible? (2) O C C C C C O H H H C C C C - + = C + - + + no charge separation separate charges increasing polarity demixing lowers the energy

  14. Solubility Parameter 6 10 20 heptane ethanol water toluene alkyd resins common binders acrylates miscibility when solubility parameters match small molecules mix more easily cross linked polymers are insoluble

  15. Paint: Which Viscosity? no settling of pigment not too much force required surface should level there should be no dripping

  16. Settling (1) pigment now after one year settling of a single particle velocity allowed about one molecule diameter per second lower limit of viscosity velocity gradient

  17. Levelling (1) much wider than a hair effect of surface tension + + high low pressure high flow

  18. Levelling (2) + +

  19. ‘shock’ Dripping no shock when

  20. Force Required this force should not be too large

  21. no settling smoothing no dripping small force paint must be non-Newtonian

  22. Rheology of Paint (1) newton bingham casson

  23. Thixotropy before stirring after stirring

  24. Adjusting the Viscosity 2 1 in this region 1 2 here

  25. Viscosity of Liquid Mixtures (high shear adjustment) empirical parameter for liquids of similar structure mass fraction of binder

  26. Viscosity of Suspensions not accurate!

  27. Gel Formation (low shear adjustment) polymer networks between the particles break down at high shear reform quickly (thixotropy) usually via a separate gel forming polymer, but can also be caused by the binder

  28. Drying -- the First Stage just after painting: much solvent in film vapour pressure of solvent mass transfer by diffusion and free convection heat transfer relation available: from this, the mass transfer relation

  29. Properties density difference for volatile solvents plays some role diffusivity molar volumes of component as liquid air:

  30. Vaporisation of Solvent binder + pigment vaporisation of solvent diffusion through the polymer

  31. Diffusion through Polymer (Stage 2) diffusivity depends strongly on concentration skin formation

  32. Drying regimes • Evaporation and free convection • order: hours • Diffusion through film • order: weeks • Polymerization of binder • order: months to years

  33. Paint brushes

  34. Paint Brush with Frank Winkel sable hairpencil ordinarybrush whitewashbrush 25 mm 35 mm 50 mm hair thickness

  35. Brushing Parameters painting rateu brush thicknessT hair diameter d velocity betweenhairs, v volume fraction of paint, e paint heightH paint thicknessh W

  36. u T d e v H h W Energy Changes contributions to the change of the system Gibbs energy: increasing interfacial energy brush / air (or paint / air-in-brush) 1 decreasing gravitational energy of paint in brush 2 frictional losses of paint (between the paint hairs) 3 decreasing interfacial energy air / surface due to paint 4

  37. Slow Painting, Small Brush d increasing interfacial energy brush / air (or paint / air-in-brush) 1 hairs e interfacial area per volume of paint decreasing interfacial energy air / surface due to paint 4 = 1 4 maximum paint thickness is a fraction of the hair thickness

  38. Splotching a brush will lose its paint spontaneously when: decreasing gravitational energy of paint in brush increasing interfacial energy brush / air (or paint / air-in-brush) > 2 1 or when so when the brush is: filled too far has too thick hairs

  39. Painting Quickly (1) increasing interfacial energy brush / air 1 frictional losses of paint (between the brush hairs) 3 decreasing interfacial energy air / surface due to paint 4 paint thickness determined by + = 1 3 4 consider brush as parallel slits with same surface / volume: v losses per dispersed volume of paint: s proportional to the painting rate

  40. Painting Quickly (2) paint thickness 1 expected: painting rate 0 0 2 4

  41. 40 usual values 20 0 0 0.2 0.4 Measurements ‘Histor Hoogglans’ paint on a toilet door laid down flat

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