Effectiveness of micronizing zinc borate to improve its fungicidal properties

The effectiveness was evaluated for an established wood preservative, zinc borate, prepared in an agitating bead mill to extensively reduce its particle size. The generated micro- or nano-particles of zinc borate were characterized by X-ray powder diffraction, and the particle size distribution was determined to evaluate the effect of milling. Then the fungicidal effectiveness of the zinc borate of both milled and unmilled samples were assayed against brown- and white-rot fungi, on culture medium and on conifer and hardwood as substrates. Treated wood samples were subjected to leaching tests. Scanning electron microscopic images of wood samples were examined to analyze the distribution of zinc borate within the wood. The micronized zinc product kept its crystal structure intact, and it increased the proportion of particles with diameters below 100 nm by 25% when compared to the unmilled product. Malt extract-agar medium supplemented with 2.5% of w/w milled and unmilled zinc borate inhibited fungal growth tested. Both milled and unmilled zinc borate protected the wood when not subjected to leaching. The milled sample of zinc borate improved resistance to leaching, which would allow its application in environments of high moisture content; however, it did not improve the fungicidal action against decay fungi.