Rheology of polyflavonoid tannin–formaldehyde reactions before and after gelling. II. Hardener influence and comparison of different tannins

Abstract

Good correspondence of the gel-time values obtained by two different methods, GЈ ϭ GЉ and 1/ 0 30, was observed for different types of natural and modified tannin extracts. The pH presents the predominant effect on both the activation energies and the gel times observed, while the proportion of a paraformaldehyde hardener has a much lesser effect on these parameters. The rate constants of the different phases of the reaction of polycondensation with formaldehyde, both before and after the gel point, were obtained for the six commercial tannin extracts tested. The viscoelastic properties of the different tannins/formaldehyde gels were measured. The gel stiffness S, relaxation coefficient n, and relaxation time were determined and their dependence on the proportion of the formaldehyde hardener, on the temperature, and on the type of tannin was determined. The gel stiffness S appears to be influenced considerably by the proportion of the formaldehyde hardener. Its value decreased as the percentage of the hardener increased: This was due to early network immobilization and the resulting lower level of crosslinking resulting from it. The influence on S of the temperature is not very pronounced. The relaxation coefficient n appears to depend mainly on the reactivity of the tannin used: The faster the reactivity, the higher was the value of n. This appears to be valid exclusively in tannin extracts where the colloidal state is still present, while it is not valid in extracts where the colloidal state was eliminated by, for example, solvent extraction.

Quantitative and qualitative composition of bark polyphenols changes longitudinally with bark maturity in Abies alba Mill.

“…Condensed tannins have a high proportion of hydroxyl groups due to their phenolic constituents, which confer a high degree of reactivity towards compounds such as proteins, polysaccharides and aldehydes. Reactivity of the hydroxyl groups with formaldehyde makes condensed tannins particularly suitable for the efficient manufacture of adhesives, resins and foams (Pizzi and Stephanou 1994;Garnier et al 2002). Identification of bark fractions containing high proportions of condensed tannins and tannins with high degrees of polymerization will facilitate the industrial valorization of bark co-products within the forest products supply chain.…”

Wood production, wood technology, and biotechnological impacts

“…At certain pHs, tannins and formaldehyde transform together into high-molecular branched condensation-products that are poorly hydrolisable (Pizzi 1977, Pizzi & Roux 1978, Roussow et al 1980, Prasetya & Roffael 1991, Garnier et al 2002. Tannins of different sources (from different tree species, different habitats, etc.)…”

“…differ in chemical structure and require therefore different amounts of formaldehyde to start the condensation reactions (Prasetya & Roffael 1991, Dix et al 1998b, Tahir et al 2002, Rosamah 2003, Hashida et al 2006. The most predominant factor is nevertheless the pH that must be alkaline in the range of around pH 10 (Garnier et al 2002).…”

Mechanical potential of eco-OSB produced from durable and nondurable species and natural resins

“…Another environmental concern is the control of volatile and semivolatile compounds derived mainly from adhesives (resins). Natural resins based on lignin (Lei et al 2007;Mansouri et al 2007a) or tannin (Garnier et al 2002;Ballerini et al 2005) are options for environment-friendly products.…”