Characterizing the conservation effect of clear coatings on photodegradation of wood
Introduction
Wood is mainly composed of cellulose, hemicelluloses, lignin, and extractives. It is a favorite indoor and outdoor constructions material because of its abundance, attractive appearance and easy processing. However, unprotected wood is susceptible to discoloration and deterioration that reduce its mechanical and physical properties (Hon, 2001, Pastore et al., 2004, Williams, 2005). Many methods, such as surface coating, chemical modification and impregnation with chemicals or plastics have been found to be able to extend service life of wood (Deka and Saikia, 2000, Gindl et al., 2003, Barnes et al., 2005, Chang and Chang, 2006). Among these methods, surface coating is the most common way to protect wood against degradation and to enhance its distinctive appearance (Hayoz et al., 2003, Buckle et al., 2005). Therefore, characteristics of wood coatings to withstand mechanical or chemical abrasion and to tolerate expansion and contraction of wood are critical to prolong their protective functions (Schwalm et al., 1997).
Polyurethane polymers are considered to be high-quality coatings. In the coating industry, two-component solvent-based polyurethane systems have been widely used because desirable properties can be obtained by adjusting makeup components (Lai and Quinn, 1995, Kim and Pail, 1999). Polyurethane coatings provide not only room temperature cure response, but also excellent abrasion resistance, high strength, hardness, flexibility, adhesion to wood materials, great modulus with elongation at break, and good chemical resistance (Fiori, 1997, Kultys and Pikus, 2001). However, wood coated with clear polyurethane coating is still susceptible to photoyellowing (Sigh et al., 2001). Photodiscoloration is unavoidable even if wood is coated with non-yellowing or durable clear coatings, such as aliphatic polyurethane coatings (Chang and Chou, 1999). Yellowing of both clear coating and underlying wood, or either of them may cause the discoloration. In the CIE L∗a∗b∗ system, discoloration of a specimen after light irradiation is described by color difference (ΔE∗). However, color difference contributed by clear coating or underlying wood can not be proportioned separately from a single ΔE∗ value in the coated wood system.
Objectives of this study were to study photostability properties of coated wood systems and to investigate ways to improve polyurethane-coated wood against photodiscoloration. Photodiscoloration of coated wood was simulated by wood specimens covered with different free polyurethane films with/without a commercial light stabilizer (Tinuvin-1130). A light reflection model based on color factors (tristimulus values) X, Y, and Z was established in order to quantify proportion of discoloration caused by clear coating and underlying wood separately. The model was employed to discuss the efficacy of different clear coating treatments. Chemical changes of wood under different films were also characterized after lightfastness test.
Section snippets
Wood specimens and formulation of coatings
China fir (Cunninghamia lanceolata) wood 50 mm × 75 mm × 10 mm in dimension was used in this study.
Commercial aromatic polyurethane (PU) and aliphatic polyurethane (PUA) (Lignal Co., Germany) with a solid content ca. 51% and 27%, respectively, were used in this study. Also used were PU and PUA containing 2% Tinuvin-1130 (hydroxyphenyl benzotriazole light absorber) based on solid content of the clear coatings, which were respectively labeled as PUS and PUAS.
Preparation of polyurethane-coated wood and free films
Wood specimens were coated with PU and PUA
Photodiscoloration of wood coated or covered with polyurethane films
Color difference value (ΔE∗) of wood covered with free PU film (24.96) was similar to that of PU-coated wood (22.24) after 24 days of irradiation. The ΔE∗ values of PUS-coated wood and wood covered with PUS film were almost identical (18.85 and 18.61). Difference in ΔE∗ between PUAS-coated wood (16.93) and wood covered with PUAS film (13.71) was slightly greater but still was visually indistinguishable. Results obtained herein indicated that discoloration of wood covered with free films was
Conclusions
Wood is susceptible to photodiscoloration even it is coated with PU (photoyellowing type polyurethane) or PUA (non-photoyellowing type polyurethane) coatings. In this study, wood specimens covered with different free films were used to simulate clear-coated wood system. A light reflection model was established to quantify discoloration caused by the coating films and underlying wood separately. UV transmittance of PU film decreased while that of PUA film increased after lightfastness test.
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