Advanced tannin based wood preservatives

ABSTRACT

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 leaching (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). Unfortunately, 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.

Key words: Tannin, Caprolactam, Wood preservative, Durability

1 INTRODUCTION

In the previous centuries, universal solutions for preserving wood were used. From the very beginning, solutions based on oils and waxes were recognized for most of the applications until creosote first, and heavy metal based formulations such as chromate-copper-arsenate (CCA) after, were dominating the market warranting very long service life (Richardson 1993, Ruddick 2011). More recently, restrictions on the use of creosote and heavy-metal based wood preservatives have focused public and government attention on technological developments in the wood protection area, specifically on the availability of more natural alternatives (Evans 2003, Schultz et al. 2007, 98/8/EC, 2011/71/EU). Thus the availability and use of environmentally friendly wood preservatives is nowadays strictly required.

Nowadays, several research groups are working with the objective of finding natural solutions for protecting wood and a very interesting overview on this topic has been recently published by Singh and Singh (2012). In addition of these researches also other groups have developed interesting wood preservatives.

In the very wide research field which proposes to protect wood with natural substances, a considerable work is carried by the researcher that considers tannins as the ideal solution. Following the original work of Laks (Laks et al. 1988) other research groups have decided to follow a similar investigation line (Yamaguchi and Okuda, 1998; Taylor...