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Sirris Smart Coating workshop - Easy-to-clean and Self cleaning Coatings - 19 May 2011 - State of the art - Heidi Van Den Rul, Sirris
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Sirris Smart Coating workshop - Easy-to-clean and Self cleaning Coatings - 19 May 2011 - State of the art - Heidi Van Den Rul, Sirris

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The desire for self-cleaning or easy-to-clean surfaces has been identified in recent years as an important research topic to enhance the competitiveness of companies. The approaches to obtain such a …

The desire for self-cleaning or easy-to-clean surfaces has been identified in recent years as an important research topic to enhance the competitiveness of companies. The approaches to obtain such a surface are diverse, depending on the specific requirements of the industrial sectors involved. Two classical approaches for dirt repelling surfaces are (i) hydrophobic or superhydrophobic surfaces and (ii) superhydrophilic, photocatalytic surfaces. A brief summary of the principles of these kind of coatings will be given in this presentation, focusing on present state-of-the-art preparation and testing methods. Examples selected from the scientific literature, a patent study on superhydrophobic coatings and commercial products will be presented to identify advantages and problems of present self-cleaning coatings.

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  • 1. State of the art of easy-to-clean and self-cleaningcoatings
    Dr. Heidi Van den Rul
    Sirris Smart CoatingApplication Lab
  • 2. Overview
    Approachesforeasy-to-cleancoatings:
    Hydrophobic
    Superhydrophobic
    Photocatalytic
    Superhydrophobiccoatings:
    What?
    Scientific and patent preparationmethods
    Testing of (commercial) coatings
    Conclusions
    Photocatalyticcoatings:
    Principles and preparation
    Testing of photocatalyticproperties
    Conclusions
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    2
  • 3. Liquid wettability of a flat surface
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    3
    low wettability
    high wettability
    contact angle
    Θ0°
    0<Θ< 90°
    90<Θ<180°
    Θ180°
    Tangent angle of the liquid-vapor interface measured at the threephase contact point
    hydrophilic
    hydrophobic
    water
    oil
    oleophilic
    oleophobic
    Drop at equilibrium: contact anglecanbemeasuredbybalancing the interfacialforces: γSL + γLV cos θ = γSV
    Young equation: cos θ = (γSV - γSL ) / γLV
    Sourcefigures: Shirtcliffe et al., Adv. Coll. Interf. Sci (2009) - naturesraincoats.com
  • 4. Hydrophobic, easy-to-clean coatings
    Beading up of water, low dirtuptake
    Examples:
    Contact angleon flat hydrophobicsurface: max. 115-120°
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    4
    silicone
    PTFE
    sol-gelwithhydrophobic building blocks
    fluorine containing sol-gel coating with low surface free energy.
    Source: inm-gmbh.de
  • 5. Superhydrophobic, self-cleaning coatingsWhat?
    Inspiredby lotus leave:
    Water dropletsball up (contact angle 160°) and rolloff the surface (slippery) of manyplants
    Rolling dropletsgather and transport dust: “self-cleaning”
    Lotus leave has double scaleroughness
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    5
    SEM picture showinghierarchicalroughness of Lotus leave:
    • microbumps (papillae)
    • 6. nanostructure (epicuticularwaxes)
    Nelumbonucifera “Lotus”
  • 7. Superhydrophobic, self-cleaningcoatingsModels explainingbehavior of dropletsonrough surfaces
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    6
    Dropletsmaintains contact withentireroughsurface
    Droplets have no complete contact with the roughsurface at all points
    roughness
    roughness
    Requirementsfor a superhydrophobiccoating:
    • Water contact angle > 150°
    • 8. Water contact anglehysteresis (or sliding angle) < 10°
    Sourcefigures: naturesraincoats.com
  • 9. Superhydrophobic, self-cleaningcoatingsContact anglehysteresis and sliding angle
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    7
    Measures of howgood a droplet can move on a surface:
    Contact anglehysteresis = advancing – receding contact angle
    the lower the hysteresis, the easier the droplet slides
    Wenzel state: high hysteresis
    Cassie-Baxter state: low hysteresis
    Sliding angle = smallestsurfacetiltingangle at which the dropletsrollsoff
    Dependsonsize of droplets
    Dependsonhysteresis
    Sensitive to vibration
    A droplet of liquid on a tilted surface has an advancing contact angle at the front and a receding contact angle at the rear edge
  • 10. Superhydrophobic, self-cleaningcoatings
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    8
    Requirementsfor a superhydrophobiccoating:
    • Water contact angle > 150°
    • 11. Water contact anglehysteresis (or sliding angle) < 10°
  • Superhydrophobic, self-cleaningcoatingsPreparationstrategies
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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    low surfaceenergy + high surfaceroughness
  • 12. Superhydrophobic, self-cleaningcoatingsPreparationmethods
    Lithography
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    10
    Oner et al., Langmuir 16 (2000) 7777
    C.H. Choi, UCLA
  • 13. Superhydrophobic, self-cleaningcoatingsPreparationmethods
    Etching
    Wet chemicaletching of metals
    Plasma etching of polymers
    Laser etching of inorganicmaterials
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    11
    Etched steel
    Etchedcopperalloy
    Etched Cu (0.5 wt% oxalic acid 5-7 days)
    Etched Cu in aq. Solution 2M NaOH + 0.1M K2S2O8 60’
    All etched surfaces are treatedwith a hydrophobic agent afterwards
    Guo et al., J. Coll. Interf. Sci. 353 (2011) 335
  • 14. Superhydrophobic, self-cleaningcoatingsPreparationmethods
    Crystal growth, e.g. hydrothermal
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    12
    Nanolamellatestructures of CaTiO3on Ti
    ZnOcrystals
    ZnOnanowire film
    Spiral Co3O4 nanorodarraysonglass
    All hydrothermallygrownstructures are treatedwith a hydrophobic agent afterwards
    Wu et al., Mat. Lett. 65 (2011) 477
    Wu et al., Mat. Lett. 64 (2010) 1251
    Guo et al., J. Coll. Interf. Sci. 353 (2011) 335
  • 15. Superhydrophobic, self-cleaningcoatingsPreparationmethods
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    13
    Polyelectrolyemultilayercoating made bylayer-by-layermethod
    Porous Cu film byelectrochemicaldeposition
    Porouspolymermembraneobtainedbyphaseseparationof a multicomponent mixture
    SuperhydrophobicPECVD-formedcoatingfrom C6F6
    Coatingformedbytemplating
    Guo et al., J. Coll. Interf. Sci. 353 (2011) 335 – Xue et al., Sci. Techn. Adv. Mater. 11 (2010) 033002 – Crick et al., Chem. Eur. J. 16 (2010) 3568 - Shirtcliffe et al., Adv. Coll. Interf. Sci (2009)
  • 16. Superhydrophobic, self-cleaningcoatingsPreparationmethods
    Depositionfrom “particles”
    (hydrophobized) silica, (mixed with) other metal oxides, (carbon nanotubes)
    Micron ornanoparticlesor micron + nanoparticles or nanoparticles bond to micronparticles
    With/without binder
    Particlesformed in situbysol-gelmethod
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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    Sol-gel route to roughsurface
    Dualsize “raspberry” silicaparticles
    Coatingformedfromsol-gel precursor + silicananoparticles
  • 17. Superhydrophobic, self-cleaningcoatingsPreparationmethods
    Afterrougheningoften a low energycoatingneeds to bedeposited to obtain a superhydrophobiccoating:
    Fluoroalkylsilanes
    Alkyl molecules, e.g. stearic acid
    Non-fluorinatedpolymers
    Alkylsilanes
  • 18. Superhydrophobic, self-cleaningcoatingsPreparationmethods: whichone is relevant?
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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  • 19. Superhydrophobic, self-cleaningcoatingsPreparationmethods: results of a patent study
    20-5-2011
    17
  • 20. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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    Superhydrophobic, self-cleaningcoatingsA commercial coating
    • Superhydrophobiccoating “lotusleafcoatings” (USA)
    “Basedonamorphoussilica and a customengineeredpolymer”
    “Abrasion resistance testing show only a reduction in contact angle by 10 to 20% in worst cases”
    • Test samples onglass
    Testedby Sirris, Smart CoatingApplication Lab
    within CO project – multifunctionalcoatingswithnano and hybridmaterials
  • 21. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be
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    Superhydrophobic, self-cleaningcoatingsA commercial coating: testing
    Water contact angle
    Water sliding angle
    Water contact angle and sliding angleafterabrasionwithcrocktest
    Taber linear abrasion with crock adapter kit, cotton cloth
    Dataphysics contact angle measuring instrument with tilting table
    Tested by Sirris, Smart Coating Application Lab
    within CO project – multifunctional coatings with nano and hybrid materials
  • 22. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    20
    Superhydrophobic, self-cleaningcoatingsA commercial coating: test
    • Contact angle (beforeabrasion) > 150°
    • 23. Sliding angle (beforeabrasion) with 5 µl droplet: 30°
    • 24. Best coating of 5 tested
    • 25. But: strong decrease of contact angle and increase of sliding angle after abrasion: the surface is very abrasion sensitive
    • 26. Applicableonly in “abrasionlimited” environment
    Abrasion with taber linear abraser with crock adapter kit, weight 350 g, cotton cloth
    1 cycle = 2 movementson sample
    Testedby Sirris, Smart CoatingApplication Lab
    within CO project – multifunctionalcoatingswithnano and hybridmaterials
  • 27. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    21
    Superhydrophobic, self-cleaning coatingsresearch @ Smart Coating Application Lab Sirris
    Solvent
    Adhesive sol
    Nanoparticles SiO2
    Micron particles SiO2
    Nanoparticles Al2O3
    Research doneby Sirris, Smart CoatingApplication Lab
    within CO project – multifunctionalcoatingswithnano and hybridmaterials
  • 28. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    22
    Superhydrophobic, self-cleaning coatingsresearch @ Smart Coating Application Lab Sirris
    Coatingfromnanoparticles in solvent
    silica
    Abrasionresistancebetteronmicrostructuredglass
    Research done by Sirris, Smart Coating Application Lab
    within CO project – multifunctional coatings with nano and hybrid materials
  • 29. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    23
    Superhydrophobic, self-cleaning coatingsresearch @ Smart Coating Application Lab Sirris
    Coatingfromnanoparticles + adhesive sol
    onsandblastedglass
    Silica + alumina
    nanoparticles
    Superhydrophobicsurface
    Abrasionresistancebetterwithadhesive sol and/ormicrostructuredglass
    Research done by Sirris, Smart Coating Application Lab
    within CO project – multifunctional coatings with nano and hybrid materials
  • 30. Superhydrophobic, self-cleaning coatingsConclusions
    Superhydrophobiccoatings: veryappealingwithmuchpromiseforself-cleaning and otherapplications (e.g.anti-icing)
    Manypreparationmethods are reported in literature
    Relatively few industrialapplications have resultedfrom the Lotus effect up to now
    Reason: abrasionproblems
    Solutions are available at R&D stage
    Other issues:
    Transparency of a roughsurface
    A superhydrophobicsurface is generallynot (super)oleophobic
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    24
  • 31. Photocatalytic, self-cleaningcoatings
    Photocatalytic:
    Organic, oxidizable and microbialcontaminants are degradedbylighton a suitablecatalyst
    Superhydrophilic:
    water droplets have a very low contact angle – nodropletsbut a water film is formedon a superhydrophilicsurface
    Self-cleaning:
    water wets the surfacecompletely and water film takesalong the degradeddirt
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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  • 32. Photocatalytic, self-cleaningcoatingsPhotocatalysis
    Titanium dioxide:
    Amorphous, anatase, rutile, brookite
    Anatase and rutile are photocatalyticallyactive (rutileloweractivity)
    Band gap 3.2 ev = 380 nm (UV)
    Anatase most commonlyusedphotocatalyst
    cheap, non-toxic, easy to produce, chemically and biologically inert
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    26
    Withphotonenergy > bandgap
    semiconductor
  • 33. Photocatalytic, self-cleaning coatings
    Variousways to manufacturetitaniaphotocatalyticcoatings:
    CVD, sol-gel, precipitation, hydro/solvothermalsynthesis
    “paintlike” layers: stabledispersions of titania in binders
    Stabledispersions of titania are requiredwithadditivessuitable to incorporate in paint
    Binder must beresistant to photoactiveattackby the reactiveradicals
    The particleson the paintsurface must bereadilyaccessible
    TiO2photocatalytic surfaces are commerciallyavailable and have been used in variousapplications (Japan, Europe)
    water and air purification
    self-cleaningglass, concrete products, coatings
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    27
  • 34. Decoloration of dye
    Methylene blue (standard), Methyl orange, Rhodamine …
    In solution
    As stain
    Photo-oxidation of organic film
    Stearic acid (standardforthin films), Palmitic acid
    Degradation of gas
    Ethanol, Propanol
    Measurement of rate of hydroxyl radical generation
    by using specific fluorescent probes
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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    Photocatalytic, self-cleaning coatingsestablised methods to evaluate photocatalytic effect
  • 35. Photocatalytic, self-cleaning coatingsdegradation of methylene blue test
    ISO standard test 10678:2010
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    29
    • 2 identicalcoatingshaving the sameactivesurface
    • 36. In methylene blue solution 10-5M
    • 37. One is stored in the dark – one is exposed to a defineddose of UV light
    • 38. The concentration of methylene blue is measured at specificintervalsduringirradiation
    • 39. Sample 1 shows a decrease in MB concentrationdue to adsorption
    • 40. Sample 2 shows a decrease in MB concentrationdue to adsorption and photocatalyticdegradation
    • 41. Difference = measurefor the photocatalyticactivity of the coating
  • 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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    Photocatalytic, self-cleaning coatingsdegradation of methylene blue test
    commercial coating
    0 2 4 6 h
    Decrease of absorbance/concentration of methylene blue afterUV-irradiation in presence of coating
  • 42. 20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
    31
    Photocatalytic, self-cleaning coatingsdegradation of methylene blue test
    P25 coating
    MB 10-5M 2 h 4 h 6h
    Decrease of absorbance/concentration of methylene blue afterUV-irradiation in presence of coating
  • 43. Photocatalyticcoatings = mature product field
    Titania is UV activated
    Inside room use?
    Doping of TiO2 to have a photocatalytic effect in VIS
    Maintaining of the photocatalytic effect?
    Theoretically TiO2maintainsitsactivity
    But: deactivation of photocatalystbyenvironmental factors
    e.g. volatilesilicon-containingcompounds (fromsealants, cleaningagents, shampoos, printinginksadditives, …) can cover the activesurface
    Inorganiccontaminantscannotberemovedphotocatalytically
    Onorganicsubstrates the reactiveradicalscanalsoattack the substrate and anintermediatelayer is required
    20-5-2011
    © Sirris | www.sirris.be | info@sirris.be |
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    Photocatalytic, self-cleaningcoatingsConclusions
  • 44. 20-5-2011
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    33
    State of the art of easy-to-clean and self-cleaningcoatings
    Thanks to
    IWT forfinancial support
    Joey Bosmans for the experimentalwork
    youforyourattention
    Questions ?
    heidi.vandenrul@sirris.be