Improvement of UV resistance of wood surfaces by using ZnO nanoparticles

doi:10.1016/j.polymdegradstab.2012.01.013

Polymer Degradation and Stability

Volume 97, Issue 4, April 2012, Pages 592-596

https://doi.org/10.1016/j.polymdegradstab.2012.01.013 Get rights and content

Abstract

Nanoparticles of some of the inorganic metal oxides are known to absorb UV radiation. In this study, the UV resistance of rubberwood (Hevea brasiliensis) coated with zinc oxide (ZnO) nanoparticles dispersed in maleic anhydride modified polypropylene (MAPP) was evaluated. The photostability of nano ZnO blended polyurethane (PU) exterior clear coating was also evaluated. The rubberwood specimens treated/coated with nanoparticle blended MAPP and PU coatings were exposed to xenon light source in a weather-o-meter. The colour and chemical changes on exposed wood surfaces were analyzed. The coatings without nanoparticles exhibited photo-yellowing and degradation of wood surfaces due to UV light exposure. Dispersion of ZnO nanoparticles in MAPP and PU coatings restricted the colour changes and photodegradation of wood polymers.

Introduction

Natural wood exposed to solar radiation undergoes rapid colour change and breakdown of wood polymers in the surface layer [1], [2], [3]. The combined effect of solar radiation and other environmental agents like water and temperature, described as weathering, severely affects the life of timber under exterior applications. The colour stability of natural wood to light exposure is also an important issue from aesthetic point of view. The phenomenon of photodegradation of wood and the different ways of its stabilization has been studied extensively [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Surface photo-stabilization significantly extends the service life of coated wooden products placed in outdoor situations [13].

To ensure long life, wood substrates are usually coated with various decorative and protective finishes such as paints, transparent finishes and varnishes. Transparent coating systems that allow the natural features of wood (i.e., grain, colour, texture) have short life span under outdoor exposure. Because of UV light transparency of the clear coats and extreme sensitivity of wood component, particularly lignin, to UV light degradation, deterioration occurs underneath clear finishes, which results in coating failure. Therefore, stability of wood substrate against moisture along with photodegradation is important.

A common failure mechanism for organic coatings is photodegradation due to UV exposure. In order to minimize such failures, coatings are formulated with transparent UV-absorbing molecules. A good UV absorber must be colourless and should be able to transform the absorbed UV energy into less harmful energy before reaching the substrate. UV absorber must also be photo-stable by itself. Organic phenolic molecules, such as benzophenone and its derivatives have been extensively tried and used as an organic UV absorber. However, due to their relatively low molecular weight, these additives can migrate out of applied coatings, either to the coating surface or into the substrate [14], [15], and therefore, have limitations in providing the long-term protection.

Inorganic UV-absorbing materials mainly based on metal oxide films or particles can be used to absorb or scatter light. Organic–inorganic hybrid materials consisting of metal nano-oxides and organic coatings have received significant interest in the recent past. Nano-composites formed by dispersion of inorganic nanoparticles in a polymer matrix improve abrasion, scratch resistance, heat resistance, toughness and stiffness [16]. Moreover, the transparency of nano-composite is not affected by incorporating nanoparticles in the polymer matrix.

Nano objects of some of the metal oxides e.g., zinc oxide (ZnO) and titanium dioxide (TiO₂) is known to absorb UV radiation. Nanoparticles of ZnO have the ability to offer UV protection to coatings and underlying substrates while also being transparent in the visible spectrum. The potentiality of nanoparticles has resulted in significant technological advancement in methods for the production of nanoparticles for numerous applications. However, the difficulty of dispersing nanoparticles and integrating them into polymer systems has limited the commercialization of nano-particle enabled products [17].

Performance of coatings incorporated with nano metal oxide has been studied recently [18], [19], [20]. The nanoparticles have been found to have superior resistance to UV degradation compared to organic HALS at appropriate loading levels [18]. Incorporation of zinc oxide (ZnO) into polypropylene significantly reduced photodegradation and surface cracks [19]. ZnO-based UV nano-composites for wood coatings are found to have increased weather fastness and improved optical properties [21]. Impregnation of wood with zinc oxide nanoparticles has been found to improve weathering resistance of wood [22]. Application of pigment modified nanosols has been found to improve weathering stability of thermally modified wood [23].

In this study, ZnO nanoparticles were incorporated in maleic anhydride graft polypropylene (MAPP) and polyurethane (PU) based clear coating and exposed to UV light to assess the effectiveness of nano objects for protection of wood surfaces against UV degradation. MAPP was selected as coating material because acid anhydrides are reactive with wood –OH groups. Maleic anhydride content of the polymer can react with hydroxyl groups associated with wood constituents. Polypropylene content present in MAPP may make wood surfaces hydrophobic. Moreover, MAPP forms a transparent coating and therefore effect of dispersion of nanoparticles can be easily studied. Commercially available polyurethane (PU) based exterior clear coatings was also blended with ZnO nanoparticles and wood surfaces coated with ZnO blended PU were exposed to UV light source in a weather-o-meter and its photostability was assessed.

Section snippets

Materials

Specimens of rubberwood (Hevea brasiliensis) of size 100 × 45 × 4 mm (radial × tangential × longitudinal) were prepared from defect free wood. Maleic anhydride modified polypropylene (MAPP) (fusa bond-61305, DUPONT) and zinc oxide nanoparticles (<100 nm, Sigma Aldrich) were used in the study.

Treatment of wood surfaces with ZnO nanoparticles blended MAPP

MAPP was blended with 7.5% of zinc oxide nanoparticles. The mixture of zinc oxide nanoparticles and molten MAPP was blended using a Torque Rheometer (Thermo Scientific, HAAKE Polylab QC). 36 g of MAPP was

Colour changes

The effect of light exposure on surfaces of untreated and MAPP treated rubberwood is shown in Fig. 1a and b, respectively.

The colour of the untreated specimens darkened upon irradiation. The rapid colour change in untreated wood surface due to exposure is indicated by decreasing value of lightness (L*) and increase in the chromaticity parameters a* and b* (Fig. 2a and b). The decreasing values of L* indicates that the wood surface becomes darker after light exposure (ΔL* became −7.15 after

Conclusions

The performance of wood can be enhanced by incorporation of nanoparticles in the coatings. Untreated and MAPP treated rubberwood exhibited significant degradation of wood polymers due to UV light irradiation which resulted in colour changes on wood surfaces. FTIR spectra showed degradation of wood polymer lignin and also breakdown of MAPP coating. Dispersion of ZnO nanoparticles in MAPP restricted the colour changes and photodegradation of wood polymers. Incorporation of ZnO nanoparticles in PU

Acknowledgments

The authors wish to thank the Director, Institute of Wood Science and Technology, Bangalore for his support and encouragement.

References (24)

P.D. Evans et al.
Polym Degrad Stab
(2002)
C.A.S. Hill et al.
Polym Degrad Stab
(2001)
K.K. Pandey
Polym Degrad Stab
(2005)
H. Zhao et al.
Polym
(2006)
U. Muller et al.
J Photochem Photobio
(2003)
W.C. Feist et al.
D.N.S. Hon
R.S. Williams
H. Derbyshire et al.
Holz als Roh- und Werkstoff
(1981)
R.S. Williams
J Appl Polym Sci
(1983)

P.D. Evans et al.

Wood Sci Technol

(1992)

P.D. Evans et al.

J Wood Chem Technol

(2010)

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Improvement of UV resistance of wood surfaces by using ZnO nanoparticles

Abstract

Introduction

Section snippets

Materials

Treatment of wood surfaces with ZnO nanoparticles blended MAPP

Colour changes

Conclusions

Acknowledgments

Polym Degrad Stab

Polym Degrad Stab

Polym Degrad Stab

Polym

J Photochem Photobio

Holz als Roh- und Werkstoff

J Appl Polym Sci

Wood Sci Technol

J Wood Chem Technol