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

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

08-04-2023© Sirris | www.sirris.be | [email protected] | 2

Liquid wettability of a flat surface


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°


PTFEsilicone

sol-gel with hydrophobic building blocks

fluorine containing sol-gel coating with low surface free energy.

Source: inm-gmbh.de

http://upload.wikimedia.org/wikipedia/commons/8/8c/Dimethylpolysiloxan.png

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


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


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


• 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


Requirements for a superhydrophobic coating:• Water contact angle > 150°• Water contact angle hysteresis (or sliding angle) <

10°

Superhydrophobic, self-cleaning coatingsPreparation strategies


low surface energy + high surface roughness

Superhydrophobic, self-cleaning coatingsPreparation methods• Lithography


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


(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


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


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


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?


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


• 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

08-04-2023© Sirris | www.sirris.be | [email protected] 19

• 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


Dataphysics contact angle measuring instrument with tilting table

Superhydrophobic, self-cleaning coatingsA commercial coating: testing


• 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 (°)


Superhydrophobic, self-cleaning coatingsA commercial coating: test


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


Abrasion resistance better on microstructured glass

silica

Coating from nanoparticles in solvent




Superhydrophobic surface

Silica + alumina

Abrasion resistance better with adhesive sol and/or microstructured glass

nanoparticles

Coating from nanoparticles + adhesive sol

on sandblasted glass



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


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


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


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


• 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


Photocatalytic, self-cleaning coatingsestablised methods to evaluate photocatalytic effect

Photocatalytic, self-cleaning coatingsdegradation of methylene blue test

• ISO standard test 10678:2010


• 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



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



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


Photocatalytic, self-cleaning coatingsConclusions


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]

Technology

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