Polyflavonoid tannins — a main cause of soft-rot failure in CCA-treated timber

Summary

Polyflavonoid tannins are proven to be fast-reacting with CCA solutions and hence, to be strong competitors of the structural wood constituents for fixation of CCA preservatives. The consequence of this effect is that even relatively small amounts of tannin cause severe undertreatment of the structural wood constituents which in turn badly affects the long term durability of CCA treated timber. The effect is compounded by heavy disproportionation between tannins and structural wood constituents of Cu, Cr and As. This leads to the well-known high susceptibility to soft-rot attack in eucalyptus species and in vineyard posts even experienced with some susceptible softwoods. Relationships found by other authors between soft-rot incidence and lignin content in CCA-treated timber are proven here to be only part of the total failure mechanism. The total mechanisms of resistance and failure are due to the balance of distribution of reactions among the various proportions of highly reactive tannins and more abundant but less reactive lignin and carbohydrates present in any wood. As a consequence of the clarification of these mechanisms the liability of different woods to soft-rot attack can then be accurately determined. Solutions to the problem are presented and discussed.