Invisible but functional - uv-protecting coatings

Invisible but functional
UV-protecting coatings
Sirris Smart Coating Application Lab
27/10/2016 1© sirris | www.sirris.be | info@sirris.be |

Weathering of materials
Solar radiation (UV)
Moisture (dew, humidity, rain)
Heat
Pollutants (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)

What is UV radiation/light?

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

 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

Stabilisation against UV light
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 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
© sirris | www.sirris.be | info@sirris.be |
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

Stabilizers: UV absorbers
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 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 𝐸 = 𝜀. 𝑐. 𝑑
 Concentration, film thickness, extinction coefficient
 Absorb better and faster than substrate
 dissipate absorbed energy quickly
 repeatedly run this cycle

Stabilizers: UV absorbers
27/10/2016 8
 Why are UV absorbers photostable?
 ESIPT mechanism: rapid and efficient dissipation of absorbed energy

Stabilizers: UV screeners
 Inorganic oxide particles TiO2, ZnO, CeO2
 Mechanism:
 Bandgap absorption
 Scattering of light

Stabilizers: UV screeners
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 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

Stabilizers: Radical scavengers
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 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

27/10/2016 12
 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

27/10/2016 13
 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

Applications

Evaluation methods
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 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 radiation
1000 hrs = 1.5 m QUV ~ 2 yrs natural radiation
8:1 – 25:1

Testing of coating durability of a printed PVC canvas
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Challenge: 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

27/10/2016© sirris | www.sirris.be | info@sirris.be |
+32 498 91 93 14
heidi.vandenrul@sirris.be
Heidi Van den Rul, program manager coatings

Invisible but functional - uv-protecting coatings

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