Fracionamento de lignina proveniente do processo kraft para obtenção de frações com diferentes massas molares e aplicação em materiais funcionais: espumas de carbono.

Abstract

This thesis has three main objectives: (i) To describe an efficient fractionation method for Kraft lignin (KL) to extract large amounts of low molecular weight lignin, which indicates the presence of a higher content of phenolic hydroxyl groups; (ii) Select KL fractions as a partial substitute for phenol in the synthesis of phenolic resins and correlate their structure-property; (iii) To describe an efficient route for the synthesis of carbon foams (CF) derived from KL fractions by thermal decomposition of a polyurethane (PU) sacrificial mold. The KL from Eucalyptus urograndis wood was refined by the ethyl acetate (EtOAc) fractionation process. The lignin-phenol-formaldehyde (LPF) resins were formulated replacing 25 and 50% of the phenol by fractionated lignin. The lignin used were KL, fraction of KL insoluble in EtOAc (LFIns) and fraction of KL soluble in EtOAc (LFSol). LFSol and LFIns fractions were analyzed for composition, functional group content and thermal properties. The structure-property correlation of the CF produced by the partial replacement of phenol by KL, and its fractions (LFSol and LFIns) was also analyzed. For this, the CF produced were characterized by thermal analysis, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) and electrochemical measurements. Gel Permeation Chromatography (GPC) results indicated that lower molecular weight lignin fractions (LFSol) showed greater solubility in EtOAc. The total hydroxyl group content (by 31P NMR) of the LFSol fraction increased to 4.73 mmol.g-1 after fractionation. The phenolic resin based on LFSol-derived monomers showed adhesion resistance, glass transition temperature and thermal stability with values close to commercial phenolic resins. The better homogeneity of the LPF resin produced from LFSol, with 25% phenol replacement, resulted in adhesion properties similar to the control resin (2.58±0.13 and 2.38±0.23 MPa for the samples control and LFSol, respectively). On the other hand, FLSol can directly replace 15% of the commercial FF resin (corresponding to >30% phenol replacement) showing similar adhesive performance (4.71±0.22 and 4.56±0.43 MPa for the control and LFSol samples, respectively) compared to the commercial control and significantly superior to LFIns (3.38±0.20 MPa) and KL (3.59±0.23 MPa). The results indicated that the foam synthesized with LFSol had very similar properties to the control resin with 100% phenol. The potential of LFSol for application as an electrochemical sensor was also demonstrated, with good selectivity for the detection of hydroquinone in water.