By aiming at tailoring the bonding strength of a thermosetting lignin-containing phenol-formaldehyde (LPF) wood adhesive, different fractions of an industrial hardwood alkaline lignin have been prepared through sequential solvent fractionation (i-PrOH, EtOH, and MeOH). Those fractions were comprehensively characterized by GPC, GC, Py/GC−MS, and NMR techniques. Lignin fractions with low molar mass and narrow dispersity, including the i-PrOH-soluble and EtOH-soluble ones, were of high purity and had more reactive sites for LPF adhesive synthesis and better accessibility due to lower degree of condensation than the high molar mass ones. Some recalcitrance of integrating high molar mass fractions covalently into the PF adhesive was observed, which was also true in the case of lignin phenolation. The tailored bonding strength of the LPF adhesive, tested by gluing wood pieces, provided strong evidence for molecular structure−performance correlation; the i-PrOH-PF had the lowest activation energy, the highest curing enthalpy, and the strongest bonding strength of 2.16 MPa. This study demonstrates a clear structure−property-application relationship of technical hardwood lignin in the LPF adhesive field, which might pave the way for a more effective bulk valorization.

Two series of well-defined lignin fractions derived from birch and spruce alkaline lignin (AL) by sequential solvent fractionation (i-PrOH-EtOH-MeOH) were engaged in a structure–property-application relationship study. The bacterial-derived alkaliphilic laccase (MetZyme) extensively catalyzed the oxidation and polymerization of AL fractions in an aqueous alkaline solution (pH 10). Lignin fractions with low molar mass reached a higher polymerization degree due to more phenolic-OH groups serving as reactive sites of oxidation and better lignin-laccase accessibility arose from a lower lignin condensation degree than the high molar mass ones. In comparison, AL fractions from spruce were found to be less reactive toward the laccase-catalyzed polymerization than those from birch, which was attributed to the much pronounced aryl-vinyl moieties’ oxidation. Furthermore, in situ polymerization of birch AL fractions using microfibrillated cellulose as a structural template was conducted in an aqueous medium and a dispersion of nanocellulose with its fiber network evenly coated by aligned lignin nanoparticles was obtained. The present study not only provides fundamental insights on the laccase-assisted oxidation and polymerization of lignin but also presents a new perspective for valorizing lignin in biobased fiber products through green processing of solvent fractionation and enzymatic treatment.

The potential of thirteen aliphatic, aromatic, thiuram and heterocyclic substituted organic disulfide derivatives of the general formula R-S-S-R' as a new group of halogen-free flame retardants (FR) for polypropylene films have been investigated. According to DIN 4102-1 standard ignitibility test, for the first time it has been demonstrated that many of the disulfides alone can effectively provide flame retardancy and self-extinguishing properties to polypropylene (PP) films at already very low concentrations of 0.5 wt%. In an effort to elucidate the mechanism of the thermal decomposition of disulfide derivatives the fragmentation patterns of the evolved gases from a thermogravimetric analyzer (TGA) have been analyzed by simultaneous mass spectrometry (MS) and Fourier transform infrared spectrometry (FTIR). The main decomposition products initiated by homolytic scission of the S-S bond and/or scission of the C-S bond were identified as thiols, aliphatic and aromatic hydrocarbons, isothiocyanates (depending on the disulfide structures) with further evolution of elemental sulfur and sulfur dioxide at temperatures of above 300 o C and 450 o C, respectively. Based on this preliminary study, we have shown that disulfides represented by e.g. diphenyl disulfide (1), 5,5'-dithiobis(2-nitrobenzoic acid) (2), bis(1-phenyl-1H-tetrazol-5yl)-disulfide (4), 2-bisbenzothiazole-2,2′-disulfide (6) and N,N-dithiobis-(phtalimide) (10) constitute a new halogen-free family of additives for flame retarding of polypropylene.

Three unsaturated compounds bearing respectively phosphate, aryl bromide and sulfenamide moieties were used as flame retardants (FR) for flax fabrics. Due to the presence of carbon-carbon double bonds, radiationgrafting was considered to covalently bond these FR onto fiber structure. Grafting efficiency and location of FR molecules were investigated by weight measurements and SEM-EDX observations. Flammability and especially self-extinguishment were assessed by thermogravimetric analysis, pyrolysis-combustion flow calorimetry and a non-standardized fire test already used in previous studies. All FRs were able to diffuse into elementary fiber bulk. Nevertheless only the phosphonated monomer (noted FR-P) was significantly grafted onto flax. Self-extinguishment was obtained for fabrics containing at least around 0.5 wt% of phosphorus. On the contrary the FR content of flax fibers after radiation-grafting procedure and washing was negligible for FR-S and FR-Br, evidencing that these molecules have not been grafted upon irradiation. Moreover, the combination of these molecules prevents the radiation-grafting of other molecules which showed good grafting rate when used alone.

No abstract