ENDURECEDORES DE RESINAS EPÓXI BASEADOS EM FRAÇÕES DE LIGNINA KRAFT AMINADAS: INFLUÊNCIA DA MASSA MOLAR NAS PROPRIEDADES TERMOMECÂNICAS E ADESÃO PRÁTICA
The use of aminated lignins as hardeners for epoxy resins has been growing in popularity recently, although the high chemical heterogeneity and polydispersity continue to pose challenges for the development of thermosetting materials based on technical lignins (TL). Enhanced control over the chemical structure and molecular weights (Mw) of TL can be achieved through fractionation processes. Fractions of TL with more homogeneous structures and narrower molecular weight distributions are desired for epoxy hardener applications, although most studies in the literature employ non-fractionated TL for this purpose. To contribute to this scenario, this study aimed to evaluate the influence of Mw and chemical structure of aminated Kraft lignin fractions (AKLF) on the thermomechanical properties of a DGEBA epoxy resin, with AKLF used as hardeners. Technical Kraft lignin (KL) was fractionated using a sequential solvent fractionation (SSF) protocol. Gel permeation chromatography coupled with multi-angle light scattering (GPC-MALS) was used to assess the molecular weights of KL and its fractions (KLF), which also had their chemical structures evaluated through phosphorus nuclear magnetic resonance spectroscopy (31P NMR) and two-dimensional heteronuclear single quantum coherence spectroscopy (2D NMR/HSQC). The lowest molecular weight fraction (F2), the highest molecular weight fraction (F3), and KL were modified via Mannich reaction using two aliphatic diamines: ethane-1,2-diamine (EDA) and butane-1,4-diamine (BDA). The amination of KLF was confirmed through X-ray photoelectron spectroscopy (XPS) and HSQC. The influence of the molecular weights of the diamines used in the amination of the fractions on the thermomechanical properties of the resin was also secondarily evaluated. XPS and HSQC results suggest that AKLF based on F2 are more reactive with EDA, while AKLF based on F3 demonstrated greater reactivity with BDA. This pattern may be attributed to the fact that F2 and F3 possess, respectively, the lowest and highest number of aliphatic hydroxyl groups, as confirmed by 31P NMR. Diamines with longer carbon chains tend to interact more favorably with the fraction exhibiting higher molecular weight and a greater number of aliphatic hydroxyl groups. Finally, an increase in thermal stability and glass transition temperatures (Tg) was observed with the increase in the molecular weights of KLF.