Weathering of wood surfaces: characteristics, mechanisms, and prevention
Photodegradation of southern yellow pine, its mechanisms and means of prevention were studied. Changes in brightness and color were recognized when wood was exposed outdoors or to ultraviolet light. Scanning electron micrographs showed that most of the cell walls on irradiated transverse surface were separated at the middle lamella region. Half-bordered pits and bordered pits on irradiated longitudinal surf aces were found to be readily destroyed by ultraviolet light. It was also noted that progressively granular surfaces formed on the cell walls of latewood in response to irradiation with ultraviolet light. Spectrophotometric studies on the chemical changes of exposed wood, lignin, and cellulose revealed that photochemical reactions primarily took place in lignin, leading to the generation of carbonyl- and carboxylic-containing degradation products of low molecular weight.
Formation of hydroperoxides, one of the mechanisms attributed to the photodegradation of wood, was illustrated from iodometry UV absorption studies. The hydroperoxide concentration at the wood surface increased when wood was irradiated in the presence of singlet oxygen generators. The hydroperoxide concentration decreased when wood was irradiated in the presence of singlet oxygen quenchers. These findings imply the participation of singlet oxygen and the formation of hydroperoxides at the photo-irradiated wood surfaces.
It was found that the photodegradative effects on wood surfaces were mitigated by treating wood surfaces with aqueous solutions of chromic acid or ferric chloride, by treating with penetrating chemical agents such as trial, glycol and 1-octadecanol, or by coating with clear polymeric ultraviolet stabilizers such as homo- and copolymer of 2-hydroxy-4(3-methacryloxy-2-hydroxypropoxy)benzophenone. Experimental results showed that the polymeric ultraviolet stabilizers provided the best protection of the treatments tried, and were themselves stable and resistant to photodegradation. Possible chemistry and mechanisms of protection provided by organic and inorganic chemical agents are discussed.