12 mai 2023
Invisible but functionalUV-protecting coatings
Sirris Smart Coating Application Lab
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Weathering of materials
Solar radiation (UV)Moisture (dew, humidity, rain)He
atPollutants (ozone, acid rain)
Salt water
Global plot of UV levels in July—dark brown being the highest (NASA/Goddard Space Flight Center, Scientific Visualization Studio)
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Damage by UV radiation
Loss of mechanical properties and gloss, discoloration of
plastics
Discoloration of dyes
roughening, colour changing, cracking
of wood
Obelisk of Thutmosis I
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Absorption of UV-radiation by polymer or impurity Polystyrenes, aromatic polyesters, aromatic polyurethanes, …
absorb UV light Acrylates, aliphatic polyesters,… cannot absorb light, but
technical polymers contain impurities that can absorb uv-light chromophore = functional group that absorbs the energy Molecule becomes excited (in a higher energy state)
UV-photons have enough energy to break chemical bonds of organic compounds, resulting in the formation of free radicals
Effect of UV radiation on polymers
Photooxidative degradation
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Stabilisation against UV light
By mixing in the material/plastic By coating
Liquid light stabilizers in solvent- and waterborne coatings Solid light stabilizers with melting point close to extrusion temperature for powder coatings
UV absorbers / screeners
(compete for the) absorption of UV-light and avoid radical formation
Quenchers Destroy the excited state of chromophores, accept the energy of the chromophore and release it as harmless heat or radiation
Radical scavengers Destruction of already formed radicals
Peroxide decomposers
Destruction of peroxide radicals
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Stabilizers: UV absorbers
Organic UV absorber molecules: 2-(2-hydroxyphenyl)-benzotriazoles (BTZ) hydroxyphenyl-s-triazines (HPT) 2-hydroxy-benzophenones (BP) oxalanilides
Requirements for good uv protection: Absorb at the wavelength where the polymer has a
maximum sensitivity Triazoles and triazines give the broadest protection
Don’t absorb > 400 nm – undesired colour Filter efficiency ~ Lambert Beer law
Concentration, film thickness, extinction coefficient Absorb better and faster than substrate dissipate absorbed energy quickly repeatedly run this cycle
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Stabilizers: UV absorbers
Why are UV absorbers photostable? ESIPT mechanism: rapid and efficient dissipation of absorbed energy
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Stabilizers: UV screeners Inorganic oxide particles TiO2, ZnO, CeO2
Mechanism: Bandgap absorption Scattering of light
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Stabilizers: UV screeners Inorganic oxide particles TiO2, ZnO, CeO2
Mechanism: Bandgap absorption Scattering of light
Transparency of coating (micron – nanoparticles)
Distribution of stabilizer must be optimized
Good compatibility with other components of formulation
Avoid agglomeration of oxide particles Suppress photocatalytic activity
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Stabilizers: Radical scavengers Interrupt the chain reaction of polymer
degradation 2 different classes:
Cyclic: hindered amine light stabilizers (HALS) Non-cyclic: phenolic antioxidants
2,2,6,6-tetramethyl-piperidine
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UV-protecting coatings
Combination of UV stabilizers in clearcoat: 1,5-2% UV absorber:
color retention 1% HALS radical
scavenger: gloss retention, prevention from cracking, improve life time of UV absorber
Reproduced from “light stabilizers”, Thomas Bolle, from “Additives for coatings”, J. Bieleman
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UV-protecting coatings
Combination of UV stabilizers in clearcoat: 1,5-2% UV absorber:
color retention 1% HALS radical
scavenger: gloss retention, prevention from cracking, improve life time of UV absorber
Reproduced from “light stabilizers”, Thomas Bolle, from “Additives for coatings”, J. Bieleman
ΔE after 54 months exposure in Florida of clearcoats with different UV-absorbers over a basecoat with
different pigmentations
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Applications
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Evaluation methods
Assess the stability of coatings towards weatherability: Outdoor exposure
Florida (tropical climate) worldwide standard Arizona (dry desert climate) Australia (high UV doses) Jacksonville Florida (tropical + air pollution)
Artificially accelerated weathering To drastically reduce evaluation times Xenon QUV Empirical correlation to outdoor exposure:
1 hr Q.U.V. ~ 17 hrs natural radiation1000 hrs = 1.5 m QUV ~ 2 yrs natural radiation8:1 – 25:1
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Testing of coating durability of a printed PVC canvasChallenge: acquire insights in the durability of coated PVC canvas
Fading of color? Cracking of canvas due to longterm
outdoor exposure? Testing of two kinds of coating
Contribution Sirris QUV weathering tests (ISO 4892-3)
1000 hrs Evaluation of the coating compared to
uncoated canvas• Visual evaluation• Colorimetry (L,a,b-determination)• Gloss measurements• Bending tests (cracking of canvas?)
Results QUV on uncoated canvas shows
degradation of the color and gloss Coated canvas shows low degradation No cracking on both coated and
uncoated canvas
2 mai 2023© sirris | www.sirris.be | [email protected] |
+32 498 91 93 14
Heidi Van den Rul, program manager coatings