Hélène Fulcrand

The aim of this work was an investigation of the ability of gallic (GA) and ellagic (EA) acids, which are phenolic compounds encountered in various plants, to act as flame retardants (FRs) for epoxy resins. In order to improve their fireproofing properties, GA and EA were treated with boric acid (to obtain gallic acid derivatives (GAD) and ellagic acid derivatives (EAD)) to introduce borate ester moieties. Thermogravimetric analysis (TGA) highlighted the good charring ability of GA and EA, which was enhanced by boration. The grafting of borate groups was also shown to increase the thermal stability of GA and EA that goes up respectively from 269 to 528 °C and from 496 to 628 °C. The phenolic-based components were then incorporated into an epoxy resin formulated from diglycidyl ether of bisphenol A (DGEBA) and isophorone diamine (IPDA) (72, 18, and 10 wt.% of DGEBA, IPDA, and GA or EA, respectively). According to differential scanning calorimetry (DSC), the glass transition temperature (T_g) of the thermosets was decreased. Its values ranged from 137 up to 108 °C after adding the phenolic-based components. A cone calorimeter was used to evaluate the burning behavior of the formulated thermosets. A significant reduction of the peak of heat release rate (pHRR) for combustion was detected. Indeed, with 10 wt.% of GA and EA, pHRR was reduced by 12 and 44%, respectively, compared to that for neat epoxy resin. GAD and EAD also induced the decrease of pHRR values by 65 and 33%, respectively. In addition, a barrier effect was observed for the resin containing GAD. These results show the important influence of the biobased phenolic compounds and their boron derivatives on the fire behavior of a partially biobased epoxy resin.

Rationale: The analysis of tannins is currently usually done by liquid chromatography after the chemical depolymerization of tannins. However, one of the main limitations of this method is the difficulty in specifically detecting each constitutive tannin monomer in a complex matrix, as numerous compounds co‐elute with the monomers, thereby compromising the analysis. Methods: The depolymerization of tannins under acidic conditions and in the presence of thioglycolic acid releases the various constitutive monomers, either as terminal units or as extension units. The tannin subunits were then quantified using extracted‐ion chromatogram (EIC) mass spectrometry, which required the determination of the ionization efficiency of the monomers. Thus, we used AgBF4‐assisted coupling for the hemisynthesis of the noncommercial epicatechin gallate dimer. Results: The EIC showed that the derivatives of the extension units were twice as ionized as the terminal units. Unlike the UV chromatograms, this new EIC‐based method is more specific and accurate because it is not impacted by the co‐elution phenomenon. This result, when added to the linearity obtained on a large range of the calibration curves, allowed for quantification of tannin subunits from EIC in complex mixtures with less pretreatment of the samples. Conclusions: Due to its specificity and sensitivity, this EIC‐based method represents a significant step toward improving the quantification of the tannin composition of samples. The results of this study should allow the oxidation markers to be quantified more accurately and taken into account, thereby providing a better indication of the actual tannin composition.

An improved analytical depolymerization method for characterizing condensed tannins was developed with menthofuran (3,6-dimethyl-4,5,6,7-tetrahydro-1-benzofuran) as the nucleophilic trapping reagent. Herein, menthofuran was compared with routinely used nucleophiles, phloroglucinol and 2-mercaptoethanol. At 30°C and in the presence of 0.1 M HCl, menthofuran displayed the outstanding ability to enable the fast and full depolymerization of procyanidin B2 using only a 1:1 molar ratio of both reactants. In the same conditions, phloroglucinol and 2 mercaptoethanol led to a reaction equilibrium with significantly lower conversion yields. Application to commercial tannin extracts showed that a menthofuran to extract weight ratio of 1 gave the same yields of procyanidin constitutive units as 10-fold higher mol. eq. phloroglucinol and 100 fold 2-mercaptoethanol. Finally, guidelines for implementing the menthofuran depolymerization method are proposed to assess the tannin content and composition of extracts as well as of plant materials without prior extraction.