COST Action FP1006“Bringing new functions to wood through surface modification”

Boris Forsthuber, Vjekoslav Živković,Gerhard Grüll, Hrvoje Turkulin

Photoprotection of wood against theinfluence of visible light

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300 350 400 450 500 550 600 650 700 7500

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Window glass - easy glazing Window glass - double glazing Window glass - double glazing

+ UV-coating

Tra

nsm

issi

on [%

]

Wavelength [nm]

UV Vis

UV-Vis Transmission throughwindow glass

2(-2‘-hydroxy-phenyl)-benzotriazole class

UV-filtering →

e.g. by UV-absorber:

2(-2‘-hydroxy-phenyl) -s-triazine class

or

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� Xenon arc light exposure of clearcoated pear-tree veneer (behindwindow-glass)

Influence of visible light

0 6 24 48 72 96Xenon arc light exposure [h]

WithUV Absorber

WithoutUV Absorber

→ Influence of visible light!

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Possibilities of wood visible light photostabilization

� Use of micronized pigments- e.g. micronized TiO2 (Rutile)

� Use of semi-transparent coatings- e.g. Iron Oxide Pigments

� Use of photo-antioxidants- e.g. HALS radical scavengers

Visible light Photostabilization

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UV-Vis Spectra

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without UV absorber/screener with UV absorber micronized TiO

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semi-transparent ironoxide

Tra

nsm

issi

on [%

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Wavelength [nm]

UV Vis

Semi-transp.ironoxide

Micron.TiO2

UV-absorber

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Colour stabilization– micronized pigments

Highest Absorbancein the visible region

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� Influence of Pigment concentration

� Influence of Pigment particlesize

Colour stabilization– semi-transparent coatings

Schauwecker et al. (2014)

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� HALS radical scavengers

Denisov-cycle:

Scavenging radicals

Ganna Gryn‘ova et al. (2012)

Non-radicalproducts

UV-Absorber+

Monomeric HALS primer

TiO

2

Monomeric HALS impregnation

BT

Z

HP

T

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� Acid/Base interaction

→ Interaction with acidic compounds

HALS – Possible Interactions

pKb ~ 4-6 pKb ~ 5-7 pKb ~ 8-10

BasicitySchaller et al. (2009)

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HALS can show antagonistic effects with phenolicantioxidants

� Hydrogen Abstraction:

� Coupling reaction:

� Increased formation free radicals by homolytic decomposition ofhydroperoxide

� → Might lead to an increased discolouration/bleaching on dark wood species

HALS – possible Interactions

Lucki et al. (1984)

(Yamashita and Ohkatsu, 2002)

Prieto et al. (2014)

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� Volatilty of HALS α (molecular weight)-1

� Certain Mobility is needed

� Grafting of HALS onto the wood surface

HALS - Volatility

Grelier et al. (1997)

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� Photoprotection of wood against visible light can be done, using � UV-Absorber/Pigments with a certain absorbance in the visible area

� Semi-transparent coatings

� Sterically hindered radical scavengers (e.g. HALS)

� Absorbance in the visible area leads to an alteration of the woods colour in the initial state

� HALS are effective in visible light protection

� Several properties might limit the effectiveness of HALS� Basicity

� Interaction with phenolics

� Volatility

Conclusion

Standort Stetten Standort Arsenal

Boris Forsthuberb.forsthuber@holzforschung.at

Tel. +43/1/798 26 23-20

www.holzforschung.at