Graduate Thesis Or Dissertation
 

Evaluation of various chemical treatments to prevent the abiotic deterioration of southern pine surfaces through outdoor screening trials

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  • Rapid discoloration and checking of wooden surfaces when exposed outdoors has caused wood to fall out of favor as an outdoor building material, especially in applications where appearance is important. The surface appearance of wood products exposed outdoors can be improved through the application of film forming coatings or by reducing exposure to solar radiation and moisture. A shift towards environmentally friendly building materials has renewed interest in the outdoor application of wood products, where both biotic and abiotic durability is required, often in the absence of large overhangs or maintenance-intensive film forming coatings. The deterioration processes of wood surfaces have been extensively investigated. It is generally accepted that the loss of lignin and hemicellulose from the surface due to free radical reactions initiated by UV light absorption and moisture leads to discoloration and erosion. In addition, it is thought that these chemical reactions also weaken the wood surface, resulting in checking. Further, a large body of research has been assembled investigating different treatments, primarily film forming, that prevent the effects of weathering. Yet, only chromic acid treatments have emerged as an economical means for reducing the effects of outdoor exposure without obscuring the natural figure of the wood. However, due to the toxic nature of these compounds, these treatments have never been commercialized. In this body of work, two outdoor screening weathering studies were conducted to investigate the interactions between the wood surface and four classes of promising surface protection compounds. Representative water repellents, pigments, organic UV inhibitors, and carbon based wood preservatives were selected for the initial study. Samples were pressure impregnated with the candidate systems and subjected to exterior exposure at two sites. Exposure sites were located in the high desert of Oregon and on the leeward side of the island of Hawaii. These locations represent the extremes experienced by most wood based building materials. The Oregon site has low rainfall but high UV exposure while the Hawaiian site has both high rainfall and high UV. Samples were removed from the Oregon site at predetermined radiation intervals over a one year period, while the samples exposed in Hawaii were removed after six months of weathering. Following exposure, the samples were evaluated for discoloration, checking and changes in the chemical composition of the surface. The results from this study indicated that none of the selected treatments completely prevented the effects of weathering on the wood surface; yet some fared better than others in at least some of the categories analyzed. The study also indicated that the mode of deterioration differed significantly between the two exposure locations and that chemical degradation of the surface did not necessarily correlate with checking or discoloration. Red and yellow iron oxide impregnated loblolly pine (Pinus taeda) samples experienced less discoloration and reduced lignin loss during the first study at both exposure locations, suggesting that iron oxides may alter the interaction between UV light and the wood polymers. In a follow up, study loblolly pine samples were impregnated with iron oxide pigments differing in particle size, shape and loading to determine what characteristics of iron oxide influenced surface protection properties. These samples were only exposed at the eastern Oregon location and removed over a three month period at predetermined radiation intervals. The results from this work suggest that crystal shape, size and pigment concentration influenced polymer losses from the wood surface as well as surface discoloration. However, the results suggested that checking was not affected by any of the variables investigated. While these studies suggest that the abiotic deterioration of wood surfaces can be controlled through the application of different transparent and semitransparent treatments, no single compound was completely effective. This implies that combinations of partially effective compounds should be investigated. In addition, the data suggest that, lignin and hemicellulose loss do not substantially affect check formation during the first months of outdoor exposure. Instead, the data suggests that wood / water relations dominate. The different exposure locations used in the first study also highlighted differences in weathering characteristics caused by climate. More research must be conducted to understand the effects of different climates on performance and to use these data to tailor surface protection systems to match exposure conditions.
  • Keywords: Checking and discoloration, Water repellents, Iron oxide pigments, Hawaii, Eastern Oregon, Wood surfaces, Surface protection, Climate, Weathering, Fourier transform infrared spectroscopy (FTIR)
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