Wood Modified by Inorganic Salts: Mechanism and Properties. I. Weathering Rate, Water Repellency, and Dimensional Stability of Wood Modified with Chromium (III) Nitrate Versus Chromic Acid
Keywords:
Weathering, water repellency, dimensional stability, chromic acid, inorganic salts, chromium nitrate, modified woodAbstract
Chromic acid treatment of wood improves surface characteristics. A simple dip or brush application of 5% aqueous chromic acid to a wood surface stops extractive staining, improves dimensional stability, retards weathering of unfinished wood, and prolongs the life of finishes. A better understanding of how chromic acid effects these improvements may facilitate the development of even better treatments. The improvements may be due to a combination of factors: oxidation of wood by hexavalent chromium compounds, formation of coordination complexes with trivalent chromium, and the presence of insoluble chromium compounds at the surface. Most trivalent chromium compounds do not become fixed in wood (i.e., do not form water-insoluble wood-chemical complexes). However, chromium (III) nitrate treatment of western redcedar (Thuja plicata), southern pine (Pinus sp.), and ponderosa pine (Pinus ponderosa) produced modified wood with properties similar to chromic acid-treated wood. Evaluation of the chromium (III) nitrate- and chromic acid-modified wood by leaching, Xenon arc-accelerated weathering, and swellometer experiments clearly demonstrated similar fixation, erosion, and water-repellent properties.
Based on the results from chromium (III) nitrate-treated wood, fixation through coordination of trivalent chromium with wood hydroxyls and formation of insoluble chromium appear to be the critical factors in improving the performance of the treated wood. Thus, the oxidation of the wood surface by hexavalent chromium appears to be less important and may not be required for improving properties.
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