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The impregnation process of Scots pine and beech samples with tannin solutions was investigated. The two materials involved in the process (impregnation solution and wood samples) are studied in depth. Viscosity of mimosa tannin solutions and the anatomical aspect of beech and Scots pine were analysed and correlated. The viscosity of tannin solutions presents a non-newtonian behaviour when its pH level increases, and in the case of addition of hexamine as a hardener, the crosslinking of the flavonoids turns out to be of great importance. During the impregnation of Scots pine (Pinus sylvestris L.) and beech (Fagus sylvatica L.), the liquid and solid uptakes were monitored while taking into consideration the different conditions of the impregnation process. This method allowed to identify the best conditions needed in order to get a successful preservative uptake for each wooden substrate. The penetration mechanism within the wood of both species was revealed with the aid of a microscopic analysis. Scots pine is impregnated through the tracheids in the longitudinal direction and through parenchyma rays in the radial direction, whereas in beech, the penetration occurs almost completely through longitudinal vessels.

The easy leaching of boron from wood preservation formulations has limited the use of this highly active fungicide. The recently discovered adduct with flavonoids allows boron to be retained for longer periods of time within wood and consequently to extend its life. Two different leaching treatments were compared, and the fungal and termite decay were examined. The biological tests showed extremely high resistance of the leached samples against both fungus (Coriolus Versicolor and Coniophora Puteana) and termites (Reticulitermes Santonensis). The retention of 2.5 kg/m³ was determined as the threshold of efficacy of boron in flavonoid-based wood preservative. Furthermore, solid state 13C-NMR analysis of the tannin resin indicated that boron can be covalently fixed to the tannin-hexamine network.

Protecting timber with natural products is one of the most significant challenge in wood technology since decades, but recently also the political institutions show the need for this challenge to be win. Tannin-based wood preservatives were studied for a long time (Laks et al. 1988), but only combining the knowledge in tannin chemistry developed in more recent years (Pichelin et al. 1999) it was possible to produce tannin-based formulations that enabled to fix in the timber structure resisting Jeaching (Thevenon et al. 2009). The first generation of water resistant tannin-based wood preservative presented very good mechanical properties, enhancement of fire resistance and also outstanding biocide properties (Tondi et al. 2012). Unforlunately, the weathering tests underlined the limit of these innovative formulations: On the one hand the dark color of the treated sample brings to higher absorption of UV-light with the consequence of breaking down the tannin polymer and on the other hand the rigid tannin polymer did not resist the continuous dimensional changes of wood (Tondi et al. 2013). With this work we would like to present a second generation of tannin wood preservative in which the effect of polyethylene glycol and caprolactam have been studied. The latter have been added to the tannin-based formulation in order to increase the elasticity of the tannin polymer whether with a blend or with a real copolymer. The results of these tests have shown a considerable improvement of the properties of the sample treated with tannin-caprolactam solution especially in terms of resistance against water and biological attacks. Some improvements have been registered also for the dimensional stability and for the weathering even if these enhancements do not solve the problem completely yet.