Weatherability and Leach Resistance of Wood Impregnated with Nano-Zinc Oxide
© The Author(s) 2010
Received: 12 April 2010
Accepted: 1 June 2010
Published: 15 June 2010
Southern pine specimens vacuum-treated with nano-zinc oxide (nano-ZnO) dispersions were evaluated for leach resistance and UV protection. Virtually, no leaching occurred in any of the nano-ZnO–treated specimens in a laboratory leach test, even at the highest retention of 13 kg/m3. However, specimens treated with high concentrations of nano-ZnO showed 58–65% chemical depletion after 12 months of outdoor exposure. Protection from UV damage after 12 months exposure is visibly obvious on both exposed and unexposed surfaces compared to untreated controls. Graying was markedly diminished, although checking occurred in all specimens. Nano-zinc oxide treatment at a concentration of 2.5% or greater provided substantial resistance to water absorption following 12 months of outdoor exposure compared to untreated and unweathered southern pine. We conclude that nano-zinc oxide can be utilized in new wood preservative formulations to impart resistance to leaching, water absorption and UV damage of wood.
Zinc oxide has a long history as a UV stabilizer and preservative component in coatings. Recent reports on nano-zinc oxide have primarily examined zinc oxide nanoparticles with respect to exterior coatings to improve photostability [1, 2], as a component of UV coatings for nanocomposites [3, 4], or modeling UV permeability of nano-ZnO-filled coatings . Relatively few reports exist on impregnating wood with nanometals or the changes that nanometals impart on wood durability [6, 7]. Impregnating wood with nano-ZnO offers the advantage over nano-coatings of long-term protection from photo-degradation and biological deterioration, particularly if the nanotreatment is leach resistant and bioactive.
Pyrolytic preparations of nanometals, such as zinc or copper, affect several elemental characteristics such as size, charge and dispersion properties in such a way that may improve their properties for wood preservation compared to similar soluble preparations . Nanometals created by pyrolysis demonstrate precisely controlled particle size in the 1–100-nanometer range that may improve penetrability of the chemical into wood relative to nanometals prepared by milling. Nanometer-size particles of metals have increased surface area when evenly dispersed in a layer. If the particle size was smaller than the diameter of the wood window pit (< 10,000 nm) or the opening of the bordered pit (400–600 nm), complete penetration and uniform distribution would be expected . Nanoparticles demonstrate high dispersion stability, but in concentrated form, they are subject to Van der Waals forces. Surfactants are typically added to increase dispersion stability thereby enabling liquid dispersion of higher concentrations of nanometals. Matsunaga et al.  conducted a study on microdistribution of a micronized copper wood preservative (10–700 nm). They observed numerous particle deposits of copper in ray tracheids and pit lumens within the wood. These deposits created a different microdistribution pattern in wood treated with the micronized copper than was observed in wood treated with other copper-based preservatives. Fixation of micronized copper is believed to occur primarily through deposition in pit chambers and on tertiary cell wall layers rather than via chemical reaction .
In a preliminary study, Clausen et al.  evaluated the feasibility of nano-zinc oxide as a wood preservative. In laboratory tests, they reported that nano-zinc oxide-treated wood inhibited some decay fungi, but not those that are otherwise zinc-tolerant (e.g. Postia placenta). Nano-zinc oxide inhibited termite feeding and caused moderate termite mortality and inhibition. Nano-zinc-treated wood was leach resistant, but weathering results were too preliminary to draw conclusions. Here, we report on weathering characteristics of southern pine vacuum-treated with nano-zinc oxide after 12 months of exposure.
Materials and Methods
Nano-zinc oxide (Nanophase Technologies Corporation, Romeoville, IL, USA) was provided as an aqueous dispersion containing 50% 30 nm zinc oxide particles with a dispersant. Zinc sulfate was obtained from Mallinckrodt Chemicals, St. Louis, MO, USA.
Treatment retention pre- and post-weathering
End grain checking following weathering
Ave. checks (#)
Leaching procedures were similar to AWPA E11-06 standard method  and are described in detail Kartal et al. . Briefly, five specimens (19 × 19 × 19 mm) per treatment were placed into 500-mL beakers, submerged in 100 mL of DI water and subjected to a vacuum to impregnate the specimens with the leaching solution. Samples were mildly agitated for 14 days, and leachates were collected after 6 h, and 1, 2, 4, 6, 8, 10, 12 and 14 days. Leachates were analyzed for zinc with ICP-AES and expressed as ppm zinc for the average leach rate of the 5 blocks per treatment concentration.
Water absorption of unweathered southern pine and weathered specimens treated with nano-ZnO
Nano-zinc oxide-impregnated SP (1.6 kg/m3 retention) demonstrated leach resistance in laboratory tests and following outdoor exposure for 12 months. Nano-zinc oxide also protected wood from water absorption and UV degradation for at least 12 months under conditions of natural weathering. The surface properties of 30-nm zinc oxide may have inhibited zinc-sensitive microbes, i.e. fungi and bacteria, which may be partially responsible for erosion, graying and deterioration of wood surfaces in outdoors exposure. These beneficial properties of nano-zinc oxide suggest that it should be considered as one component in the development of new multi-component wood preservatives.
The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.
The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright.
We thank Dan Foster, Chemist, for conducting the ICP-AES analyses.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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