E. Masson

Antioxidant characteristics of hydrolysable and polyflavonoid tannins : An ESR kinetics study

As part of an investigation of the role of tannin as antioxidants, the radical formation and radical decay reactions of some polyflavonoid and hydrolysable tannins has been followed by electron spin resonance (ESR) techniques. Comparative kinetics were determined for both light-induced radicals and by radical transfer from a less stable chemical species for the tannin alone and when the tannin is in a methanol solution. The five parameters which appear to have a bearing on the very complex pattern of the rates of tannin radical formation and radical decay were found to be (1) the extent of the colloidal state of the tannin in solution, (2) the stereochemical structure at the interflavonoid units linkage, (3) the ease of heterocyclic pyran ring opening, (4) the relative numbers of A- and B-rings hydroxy groups, and (5) solvation effects when the tannin is in solution. It is the combination of these five factors that appears to determine the behavior as an antioxidant of a particular tannin under a set of application conditions.

Comparative kinetics of induced radical autocondensation of polyflavonoid tannins. I. Modified and nonmodified tannins

Comparative kinetics of the radical autocondensation induced by SiO2 of a polyflavonoid tannin, namely quebracho tannin a mostly profisetinidin/prorobinetinidin tannin, in its natural extract state, sulfited, carbohydrate free, and in its adhesive intermediate form were carried out by electron spin resonance (ESR). The results obtained not only confirmed the existence of strong radical mechanisms of tannin autocondensation in the presence of dissolved SiO2, but also pointed out new effects of interest in such a reaction. The SiO2 induced autocondensation proceeds at a faster rate, and radical surge and concentration decay appear to be more marked and more rapid the more colloidal the tannin solution is. This appears to indicate that a colloid-induced intramicellar radical mechanism of the reaction is at work. As a consequence the intensity and rate of the radicals surge and decay decreases passing from more colloidal to less colloidal tannin solutions. Thus, the decay rate decreases passing from natural tannin to adhesive intermediate, to carbohydrate-free tannin, to almost disappear for sulfited tannin. The colloidal state of the solution appears to depend mostly, but not only, on the presence of the polymeric carbohydrates in the extract.

Comparative kinetics of the induced radical autocondensation of polyflavonoid tannins. III. Micellar reactions vs. cellulose surface catalysis

Cellulose surface catalysis of the induced autocondensation of tannins has been found to occur also for the radical mechanism of the reaction. Equally effective in accelerating pyran ring opening of polyflavonoids by radical mechanisms appears to be the presence of any substance leading to micellar structures in the tannin solution. Thus, soaps and synthetic and natural polymeric colloids have been found to accelerate the reaction. The micellar effect is distinct and different from the surface catalysis of the process induced by cellulose. Amorphous and crystalline cellulose appear to differ somewhat as regards the extent of catalytic activation. As for ionic mechanisms, also for catalyzed radical mechanisms, procyanidins invert their favorite reaction of cleavage of the interflavonoid bond to favor, instead, pyran ring opening. This inversion of the favorite reaction is induced both by the presence of cellulose and by the level of the micellar state of the reaction.

Formaldehyde and VOCs emissions from bio-particleboards

Formaldehyde and other volatile organic compounds (VOCs) emissions were determined from particleboards manufactured with natural adhesives. Thus, mimosa tannin with hexamine solution (T) and a blending of glyoxalated wheat straw lignin solution mixing with mimosa tannin and hexamine (TL) in 50/50 proportion were used as resins. These ones were compared with a traditional synthetic urea formaldehyde (UF) adhesive. Formaldehyde and other VOCs were determined and quantified with high performance liquid chromatography (HPLC) and gas chromatography–mass spectrometry (GC–MS) methods. Formaldehyde, acetaldehyde, acetone/acroleine and hexanaldehyde were the main compounds detected by HPLC. Toluene, ethylbenzene, xylene and following terpenes: α-pinene, β-pinene, 3-carene and limonene were determined by GC–MS. Results were obtained according to the EN-16000. After three days, emission results were the following: (a) formaldehyde UF: 240 > T:66 > TL:34 μg/m2 h; (b) acetaldehyde TL:46 > T:40 > UF:29 μg/m2 h; (c) acetone/acroleine TL:53.9 > UF:39.5 > 29.3 μg/m2 h; (d) hexanaldehyde T:14.6 > TL:13.7 > UF:11.7 μg/m2 h; (e) toluene UF:19.4 > TL and T: 9.4 μg/m2 h; (f) ethylbenzene UF:17.4 > TL and T: 7.6 μg/m2 h; (g) m-xylene UF 69.11 > TL and T 26.6 μg/m2 h; (h) o-xylene UF:16.0 > TL and T: 5.6 μg/m2 h; and (i) terpenes were emitted with lower rate than all the other compounds. In general, higher emissions have been noticed in natural adhesives than in the UF. Thus, the main terpenes found were α-pinene: UF:8.5 < TL:14.8 < T:19.7; β-pinene: UF:1.8 < TL:2.6 < T:3.6 and 3-carene UF:3.6 < TL and T: 4.9. Concerning limonene, no difference was observed whether the used adhesive was natural or not.

Emission Gases in Linear Vibration Welding of Wood

Analysis of the gases, vapour and degradation volatiles emitted as smoke from the welding interface during linear vibration welding of beech and oak woods has shown that the compounds in such a smoke are water vapour, CO2, and degradation compounds from wood polymeric carbohydrates as well as from amorphous lignin. For the two hardwoods tested the main carbohydrates contributing to the volatile compounds are xylan hemicelluloses. Numerous compounds, in very small proportions, derived from the degradation and rearrangement reactions of lignin have also been identified. The proportion of CO2 emitted is very low, and neither CO nor methane are emitted due to the relatively low temperature reached. Experiments in the temperature range of linear wood welding but at prolonged time have shown that the main component of the smoke produced during welding appears to be water vapour. There is no emission of gases or degradation volatiles after welding has ended.