Condensed tannin reactivity inhibition by internal rearrangements: Detection by CP-MAS 13C NMR

Abstract

Tannin extracts were prepared from barks of maritime pine (Pinus maritimus), aleppo pine (Pinushalepensis), Norway spruce (Picea abies) and from a mixture of barks of different pine species (Pinusspp., namely 20% black pine (Pinus thunbergii)+ 30% Scots pine (Pinus sylvestris) + 20% maritime pine(Pinus maritimus) + 10% Aleppo Pine (Pinus halepensis) + 5% Douglas fir (Pseudotsuga menziesii) +15% Norway spruce (Picea abies)) at a paper mill. The 13C-NMR spectra were carried out from thesetannins. Blending systems adhesives were prepared with 5% paraformaldehyde to measure the reactivityby gel time, and to determinate the mechanical resistance by thermomechanical analysis (TMA) andthe internal bond strength (IB) according to the European Norm EN-312. The CP MAS 13C NMR bandsat 163 ppm and the increase number of smaller bands between 20 and 65 ppm clearly indicate that theextraction of procyanidin-type tannins has been incorrect yielded high relative proportions of catechinicacid structures and diarylpropanol-catechinin acid dimer caused by internal flavonoid rearrangement, andcausing the blockage of the C6 and C8 reactive sites, rendering unusable the tannin for wood adhesiveapplications. Thus, the gel times are very long except for the maritime pine tannin with a value of 51(s). The TMA analysis gave very low MOE values for almost all the tannin extract samples, except formaritime pine. This situation was confirmed by the IB test. Only maritime pine tannin presented a goodmechanical resistance with a value of 0.6 (MPa). Only this sample fulfils the standard requested.