Surface checking of wood is increased by photodegradation caused by ultraviolet and visible light
The aim of this research was to test the hypothesis that exposure to solar radiation increases the checking of wood exposed to the weather, and to examine the causes and spectral dependency of such an effect. Lodgepole pine decking samples were exposed outdoors under filters, which blocked selected regions of the solar spectrum while allowing other weathering factors to act on samples. Surface checking in samples was quantified after 12, 24 and 36 weeks of exposure, and the chemical and micro-structural changes occurring at weathered wood surfaces were examined. Check numbers and dimensions were greater in samples exposed under a filter to the full solar spectrum than in samples exposed under filters that blocked the transmission of UV, visible or infrared radiation. Samples that were shielded from more energetic wavelengths developed less checking and also showed less delignification at the exposed wood surfaces. Checks developed at the margins of rays and propagated at the interface between adjacent tracheids, close to the middle lamella. We conclude that exposure to UV and visible light increases the tendency of wood to check during exterior exposure. Our findings point to a link between changes in cell micro-structure as a result of photodegradation of lignin and the development of visible checks in wood exposed outdoors.
Surface Discoloration Analysis and Lignin Degradation Fragments Identification of UV-Irradiated Moso Bamboo (Phyllostachys pubescens Mazel)
Color changes caused by artificial UV radiation of Moso bamboo (Phyllostachys pubescens Mazel) were recorded as a function of exposure time to obtain the maximum absorption trend by the Kubelka-Munk (K-M) spectra. Lignin photolysis into smaller molecules was evaluated using spectrophotometry and gas chromatography-mass spectrometry (GC-MS) analysis. Results showed that the K-M absorption peak increased in the yellow and red regions (360 to 500 nm) when compared with the untreated sample, which was in accordance with the yellow-red shift by visual observation. The maximum absorption of the K-M spectra from UVB phototreated bamboo was in the UV region. GC-MS analysis showed that benzene carbonyls, organic acid, and esters were the major types of photolysized molecules of bamboo lignin, which were derived from the C-C bonds adjacent to the α-carbonyl. UV irradiation (295 to 400 nm) resulted in the breakdown of carbonyl and unsaturated C-C groups conjugated to aromatic ring at the C., which partly contributed to the quick discoloration at the initial 100 h of UV irradiation.
Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder
Abstract In order to obtain high-strength aluminum joints, ultrasonic soldering of 1070 aluminum was conducted under liquidus temperature of Sn–Zn hypereutectic solder. A device for ultrasonic soldering was assembled, which propagated ultrasonic vibrations in a direction perpendicular to joining surfaces. This device joined 1070-Al using quasi-melting Sn–Zn hypereutectic solder without using any artificial spacers. The strength of the solder joints was evaluated by tensile tests. The optimum joining conditions were determined, and the effects of solder compositions and soldering temperature on the joint strength and the solder layer thickness were examined. In this ultrasonic soldering process, the highest tensile strength was obtained for the solder joints fabricated at 220 °C for the Sn–23Zn and Sn–40Zn solder compositions. The joint strength was equivalent to that of 1070-Al heat treated at 220 °C. The sound joints were obtained at 300 °C using Sn–82Zn solder, the liquid phase volume fraction of which was theoretically only 0.24. The present work also revealed that the thickness of retained solder layer in the joint after ultrasonic soldering could be estimated. Accordingly, ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints.