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.

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

6. nanostructure (epicuticularwaxes)Nelumbonucifera “Lotus”

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

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 | 14 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 | 16

19. Superhydrophobic, self-cleaningcoatingsPreparationmethods: results of a patent study 20-5-2011 17

21. 20-5-2011 © Sirris | www.sirris.be | info@sirris.be 19 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

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” environmentAbrasion 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 | 25

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 | 28 Photocatalytic, self-cleaning coatingsestablised methods to evaluate photocatalytic effect

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

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 | 32 Photocatalytic, self-cleaningcoatingsConclusions

44. 20-5-2011 © Sirris | www.sirris.be | info@sirris.be | 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