invisible but functional - uv-protecting coatings

12 mai 2023

Invisible but functionalUV-protecting coatings

Sirris Smart Coating Application Lab

© sirris | www.sirris.be | [email protected] |

2 mai 2023 2© sirris | www.sirris.be | [email protected] |

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)


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

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


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


Stabilizers: UV absorbers

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 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 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


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


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


Applications

https://kenallennews.files.wordpress.com/2013/03/img_1451.jpg


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


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

[email protected]

Heidi Van den Rul, program manager coatings

invisible but functional - uv-protecting coatings

Technology