| Thermally modified wood is widely applied as industrial raw material in outdoor conditions such as landscape and decorative architecture due to its improved dimensional stability,decay resistance and attractive color.However,Wood undergoes degradation in outdoor application due to the influence of numerous exterior conditions,among which ultraviolet light and water are regarded as the most important.A series of changes have taken place in the process of thennal modification,which may cause changes in anti-weathering properties of wood and mechanism of photic action.The mechanism has not yet been elucidated.This study investigated the photic action mechanism of lignin and extractives in thermally modified Scots pine(Pinus sylvestris L.)by using electron spin resonance(ESR)and gas chromatography-mass spectrometer(GC-MS).Three approaches are carried out to improve the anti-weathering property of thermally modified wood by controlling ultraviolet light and water simultaneously.The color stability,glossiness,water repellency and abrasion performance of treated wood were determined.Several methods,such as Fourier transform infrared spectroscopy(FTIR),Ultraviolet-visible spectroscopy(UV-Vis),Scanning electron microscope with energy dispersive X-ray analysis(SEM-EDXA),X-ray diffractometer(XRD)and X-ray photoelectron spectroscopy(XPS)were used to investigate the modification mechanism.The main results of this study are listed below.(1)The polar extractives with low molecular weight that generated during thermal modification could release the photodegradation of thermally modified wood.The concentration of fatty acids decreases from 40.94%to 24.89%significantly while phenolics become more abundant from 7.84%to 54.46%with prolonged exposure.The reduction or even elimination of the fatty acids is presumed to be a physical process,namely,a discharge from wood during exposure to radiation and elevated temperature.Increased amount of phenolics is a result of the photodegradation of wood components.Phenolics play the role as a "barrier" against further photodegradation of thermally modified wood(2)After post-treatment with TiO2 sol and paraffin emulsion,modifiers were adhered onto wood cell wall and the anti-weathering performance of thermally modified wood improves.The results showed that TiO2 was not resistant to water but addition of paraffin emulsion improved the adherence of TiO2 on wood surface.Paraffin emulsion improved the surface hydrophobicity of thermally modified wood and prevented degradation products from leaching out,making wood cells retaining a most complete woodenstructure(3)The stability of color and glossiness of TiO2/PDMS(polydimethylsiloxane)treated thermally modified wood was better than the control group.The use of PDMS improved the hydrophobicity and wear resistance of wood surface,which was not influenced by ultraviolet light But in this coating experiment,the distribution of TiO2 in PDMS is heterogeneous.Agglomeration of nanoparticles also occurs.These phenomena may waken the shielding effect of TiO2.(4)By TiO2/PDMS modification,rutile TiO2 nanoparticles with a paeticle size of 1 um were successfully deposited on the surface on thermally modified wood.The deposition of TiO2 was proved to make thermally treated wood surface UV-resistant by strongly scattering,reflecting and absorbing light,but was nondurable due to the poor leaching resistance.The successive coating by PDMS significantly improved the leaching resistance of TiO2 nanoparticles due to the superior hydrophobicity of PDMS.Chemical boding between modifiers and hydroxyl groups of thermally modified wood makes anti-weathering property of TiO2/PDMS treated wood improved. |
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