Sirris Smart Coating workshop - Easy-to-clean and Self cleaning Coatings - 19 May 2011 - State of...
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Transcript of Sirris Smart Coating workshop - Easy-to-clean and Self cleaning Coatings - 19 May 2011 - State of...
the collective centre of the belgian technology industy
State of the art of easy-to-clean and self-cleaning coatingsDr. Heidi Van den RulSirris Smart Coating Application Lab
Overview
• Approaches for easy-to-clean coatings:• Hydrophobic• Superhydrophobic• Photocatalytic
• Superhydrophobic coatings:• What?• Scientific and patent preparation methods• Testing of (commercial) coatings• Conclusions
• Photocatalytic coatings:• Principles and preparation• Testing of photocatalytic properties• Conclusions
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Liquid wettability of a flat surface
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Θ0° 0<Θ< 90° 90<Θ<180° Θ180°
hydrophilic hydrophobic
high wettability low wettability
contact angle
water
oil oleophilic oleophobic
Drop at equilibrium: contact angle can be measured by balancing the interfacial forces: γSL + γLV cos θ = γSV
Young equation: cos θ = (γSV - γSL ) / γLV Source figures: Shirtcliffe et al., Adv. Coll. Interf. Sci (2009) - naturesraincoats.com
Tangent angle of the liquid-vapor interface measured at the three phase contact point
Hydrophobic, easy-to-clean coatings
• Beading up of water, low dirt uptake• Examples:
• Contact angle on flat hydrophobic surface: max. 115-120°
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PTFEsilicone
sol-gel with hydrophobic building blocks
fluorine containing sol-gel coating with low surface free energy.
Source: inm-gmbh.de
Superhydrophobic, self-cleaning coatingsWhat?• Inspired by lotus leave:
• Water droplets ball up (contact angle 160°) and roll off the surface (slippery) of many plants
• Rolling droplets gather and transport dust: “self-cleaning”• Lotus leave has double scale roughness
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Nelumbo nucifera “Lotus”
SEM picture showing hierarchical roughness of Lotus leave: • microbumps (papillae)• nanostructure (epicuticular waxes)
Superhydrophobic, self-cleaning coatingsModels explaining behavior of droplets on rough surfaces
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Source figures: naturesraincoats.com
Droplets maintains contact with entire rough surface
Requirements for a superhydrophobic coating:• Water contact angle > 150°• Water contact angle hysteresis (or sliding angle) <
10°
Droplets have no complete contact with the rough surface at
all points
roughness roughness
Superhydrophobic, self-cleaning coatings Contact angle hysteresis and sliding angle
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• Measures of how good a droplet can move on a surface:• Contact angle hysteresis = advancing – receding contact angle
• the lower the hysteresis, the easier the droplet slides• Wenzel state: high hysteresis• Cassie-Baxter state: low hysteresis
• Sliding angle = smallest surface tilting angle at which the droplets rolls off
• Depends on size of droplets• Depends on hysteresis• 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
Superhydrophobic, self-cleaning coatings
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Requirements for a superhydrophobic coating:• Water contact angle > 150°• Water contact angle hysteresis (or sliding angle) <
10°
Superhydrophobic, self-cleaning coatingsPreparation strategies
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low surface energy + high surface roughness
Superhydrophobic, self-cleaning coatingsPreparation methods• Lithography
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C.H. Choi, UCLA
Oner et al., Langmuir 16 (2000) 7777
Superhydrophobic, self-cleaning coatingsPreparation methods• Etching
• Wet chemical etching of metals• Plasma etching of polymers• Laser etching of inorganic materials
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(a) Etched steel(b) Etched copper alloy(c) Etched Cu (0.5 wt% oxalic acid 5-7
days)(d) Etched Cu in aq. Solution 2M NaOH
+ 0.1M K2S2O8 60’All etched surfaces are treated with a hydrophobic agent afterwards
Guo et al., J. Coll. Interf. Sci. 353 (2011) 335
Superhydrophobic, self-cleaning coatingsPreparation methods• Crystal growth, e.g. hydrothermal
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Wu et al., Mat. Lett. 65 (2011) 477
ZnO crystals
ZnO nanowire film
Wu et al., Mat. Lett. 64 (2010) 1251
Nanolamellate structures of CaTiO3 on Ti
Spiral Co3O4 nanorod arrays on glass
Guo et al., J. Coll. Interf. Sci. 353 (2011) 335
All hydrothermally grown structures are treated
with a hydrophobic agent afterwards
Superhydrophobic, self-cleaning coatingsPreparation methods
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Porous Cu film by electrochemical deposition
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)
Polyelectrolye multilayer coating made by layer-by-layer method
Coating formed by templating Porous polymer membrane obtained by phase separation of a multicomponent
mixture
Superhydrophobic PECVD-formed coating from C6F6
Superhydrophobic, self-cleaning coatingsPreparation methods• Deposition from “particles”
• (hydrophobized) silica, (mixed with) other metal oxides, (carbon nanotubes)
• Micron or nanoparticles or micron + nanoparticles or nanoparticles bond to micronparticles
• With/without binder• Particles formed in situ by sol-gel method
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Coating formed from sol-gel precursor + silica nanoparticles
Dual size “raspberry” silica particles
Sol-gel route to rough surface
Superhydrophobic, self-cleaning coatingsPreparation methods• After roughening often a low energy coating needs to be
deposited to obtain a superhydrophobic coating:• Fluoroalkylsilanes
• Alkyl molecules, e.g. stearic acid• Non-fluorinated polymers• Alkyl silanes
Superhydrophobic, self-cleaning coatingsPreparation methods: which one is relevant?
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simple upscalable
short time on all substrates
extra functions
lithography - - + -+ -
Wet chemical etching
+ + - - -
Plasma etching + + + - +
Hydrothermal growth
+ + - + -
Electrochemical deposition
- -+ + - -
Layer-by-layer + + - + +
Templating -+ - - - -+
Phase separation + -+ + + -+
CVD - -+ + -+ +
Sol-gel + + -+ + +
Particle deposition
+ + + + +
Superhydrophobic, self-cleaning coatingsPreparation methods: results of a patent study
08-04-2023 17
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• Superhydrophobic coating “lotusleaf coatings” (USA)“Based on amorphous silica and a custom engineered polymer”“Abrasion resistance testing show only a reduction in contact angle by 10 to 20% in worst cases”
• Test samples on glass
Superhydrophobic, self-cleaning coatingsA commercial coating
Tested by Sirris, Smart Coating Application Lab within CO project – multifunctional coatings with nano and hybrid materials
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• Water contact angle• Water sliding angle• Water contact angle and sliding angle after abrasion with crocktest
Taber linear abrasion with crock adapter kit, cotton cloth
Tested by Sirris, Smart Coating Application Lab within CO project – multifunctional coatings with nano and hybrid materials
Dataphysics contact angle measuring instrument with tilting table
Superhydrophobic, self-cleaning coatingsA commercial coating: testing
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• Contact angle (before abrasion) > 150°
• Sliding angle (before abrasion) with 5 µl droplet: 30°
• Best coating of 5 tested
• But: strong decrease of contact angle and increase of sliding angle after abrasion: the surface is very abrasion sensitive
• Applicable only in “abrasion limited” environment
1 cycle = 2 movements on sample
Abrasion with taber linear abraser with crock adapter kit, weight 350 g, cotton cloth
0 5 10 15 20 25 30 35 400
20406080
100120140160180200
179
10786
67 62
LotusLeaf standard coating
water contact angle
water sliding angle
number cycli crock test
angl
e (°)
Tested by Sirris, Smart Coating Application Lab within CO project – multifunctional coatings with nano and hybrid materials
Superhydrophobic, self-cleaning coatingsA commercial coating: test
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Nanoparticles SiO2
Micron particles SiO2
Nanoparticles Al2O3
SolventAdhesive
sol
Research done by Sirris, Smart Coating Application Lab within CO project – multifunctional coatings with nano and hybrid materials
Superhydrophobic, self-cleaning coatingsresearch @ Smart Coating Application Lab Sirris
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Abrasion resistance better on microstructured glass
silica
Coating from nanoparticles in solvent
Research done by Sirris, Smart Coating Application Lab within CO project – multifunctional coatings with nano and hybrid materials
Superhydrophobic, self-cleaning coatingsresearch @ Smart Coating Application Lab Sirris
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Superhydrophobic surface
Silica + alumina
Abrasion resistance better with adhesive sol and/or microstructured glass
nanoparticles
Coating from nanoparticles + adhesive sol
on sandblasted glass
Research done by Sirris, Smart Coating Application Lab within CO project – multifunctional coatings with nano and hybrid materials
Superhydrophobic, self-cleaning coatingsresearch @ Smart Coating Application Lab Sirris
Superhydrophobic, self-cleaning coatingsConclusions• Superhydrophobic coatings: very appealing with much
promise for self-cleaning and other applications (e.g.anti-icing)
• Many preparation methods are reported in literature• Relatively few industrial applications have resulted from the
Lotus effect up to now• Reason: abrasion problems • Solutions are available at R&D stage
• Other issues:• Transparency of a rough surface• A superhydrophobic surface is generally not (super)oleophobic
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Photocatalytic, self-cleaning coatings
• Photocatalytic: Organic, oxidizable and microbial contaminants are degraded by light on a suitable catalyst
• Superhydrophilic: water droplets have a very low contact angle – no droplets but a water film is formed on a superhydrophilic surface
• Self-cleaning: water wets the surface completely and water film takes along the degraded dirt
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Photocatalytic, self-cleaning coatingsPhotocatalysis
Titanium dioxide:• Amorphous, anatase, rutile, brookite• Anatase and rutile are photocatalytically active (rutile lower activity)• Band gap 3.2 ev = 380 nm (UV)• Anatase most commonly used photocatalyst • cheap, non-toxic, easy to produce, chemically and biologically inert
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semiconductor
With photon energy > bandgap
Photocatalytic, self-cleaning coatings
• Various ways to manufacture titania photocatalytic coatings:• CVD, sol-gel, precipitation, hydro/solvothermal synthesis• “paint like” layers: stable dispersions of titania in binders
• Stable dispersions of titania are required with additives suitable to incorporate in paint
• Binder must be resistant to photoactive attack by the reactive radicals
• The particles on the paint surface must be readily accessible
• TiO2 photocatalytic surfaces are commercially available and have been used in various applications (Japan, Europe)• water and air purification• self-cleaning glass, concrete products, coatings
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• Decoloration of dye• Methylene blue (standard), Methyl orange, Rhodamine …• In solution• As stain
• Photo-oxidation of organic film• Stearic acid (standard for thin films), Palmitic acid
• Degradation of gas• Ethanol, Propanol
• Measurement of rate of hydroxyl radical generation• by using specific fluorescent probes
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Photocatalytic, self-cleaning coatingsestablised methods to evaluate photocatalytic effect
Photocatalytic, self-cleaning coatingsdegradation of methylene blue test
• ISO standard test 10678:2010
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• 2 identical coatings having the same active surface• In methylene blue solution 10-5M• One is stored in the dark – one is exposed to a defined dose of UV light• The concentration of methylene blue is measured at specific intervals during irradiation• Sample 1 shows a decrease in MB concentration due to adsorption• Sample 2 shows a decrease in MB concentration due to adsorption and photocatalytic
degradation• Difference = measure for the photocatalytic activity of the coating
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Photocatalytic, self-cleaning coatingsdegradation of methylene blue test
450 475 500 525 550 575 600 625 650 675 700 725 750 775 800 825 850-0.5
0
0.5
1
1.5
2
0 h UV irradiation
2 h
4 h
6 h
wavelength (nm)
Abso
rban
ce
0 2 4 6 h
Decrease of absorbance/concentration of methylene blue after UV-irradiation in presence of coating
commercial coating
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Photocatalytic, self-cleaning coatingsdegradation of methylene blue test
450 475 500 525 550 575 600 625 650 675 700 725 750 775 800 825 850-0.5
0
0.5
1
1.5
2
0 h2 h4 h6 h
Wavelength (nm)
abso
rban
ce
Decrease of absorbance/concentration of methylene blue after UV-irradiation in presence of coating
MB 10-5M 2 h 4 h 6h
P25 coating
• Photocatalytic coatings = 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 TiO2 maintains its activity
• But: deactivation of photocatalyst by environmental factors • e.g. volatile silicon-containing compounds (from sealants,
cleaning agents, shampoos, printing inks additives, …) can cover the active surface
• Inorganic contaminants can not be removed photocatalytically
• On organic substrates the reactive radicals can also attack the substrate and an intermediate layer is required
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Photocatalytic, self-cleaning coatingsConclusions
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State of the art of easy-to-clean and self-cleaning coatings
Thanks to
IWT for financial supportJoey Bosmans for the experimental work
you for your attention
Questions [email protected]