## 2 Materials and Methods
### Characterization Tests  
Fourier transform infrared (FTIR) analysis was performed on the bio-based prepolymer, without and with lignin incorporation. A _Spectrum Two Perkin Elmer_ spectrometer was used, and the spectra were captured from 4000 to 600 cm\({}^{-1}\) and analyzed in _Perkin Elmer Spectrum IR_ software, version 10.7.2. Gel permeation chromatography (GPC) tests were carried out in the prepolymer without and with lignin incorporation to determine their molecular weight average number (M\({}_{\mathrm{n}}\)) and molecular weight average weight (M\({}_{\mathrm{w}}\)). Tetrahydrofuran (THF) was used as the solvent, according to ISO 13885-1, at a concentration of 1 mg/mL. The samples underwent filtration and were then injected into a column set, which consisted of a precolumn (8 \(\times\) 50 mm) PSS SDV and columns (8 \(\times\) 300 mm) PSS SDV analytical linear S, both with a particle size of 3 \(\upmu\)m, and filled with polystyrene crosslinked with divinylbenzene (gel). Prior to elution, the columns were equilibrated at 40 \({}^{\circ}\)C, and then a flowing rate of 1.0 mL/min was used. Dual detectors held at 40 \({}^{\circ}\)C were employed to monitor the eluent: the first was a UV/Visible detector (model JASCO UV-4075) set at 254 nm and the second was a refractive index detector (model JASCO RI-4030). The molecular weights were calculated based on the retention time using a conventional calibration method with narrow polystyrene standards (M\({}_{\mathrm{n}}\) ranging from 266 to 59,300 Da). DMA tests were performed to assess the influence of the aromatic groups of lignin in the thermo-mechanical behavior and in the glass transition temperature (\(T_{\mathrm{g}}\)) of the bio-based VE resin, determined according to ISO 6721-11. The experiments were conducted according to the ASTM E1640 [40] standard in a _Q800 TA Instruments_ dynamic mechanical analyzer. The specimens were tested in a dual cantilever configuration, considering a deformation amplitude of 15 \(\upmu\)m and 10 \(\upmu\)m, respectively, in BVE and BVEL specimens, as well as an oscillatory frequency of 1 Hz. The tests were performed from \(-\)30 \({}^{\circ}\)C to 150 \({}^{\circ}\)C, at a heating rate of 2 \({}^{\circ}\)C/min. For each type of resin, two replicate specimens were tested and, in the case of the BVEL resin, two runs were carried out to assess the effects of exposure to elevated temperature (above the post-curing temperature) regarding the thermomechanical behavior of the material, and a third specimen was tested after post-curefor a longer period at a higher temperature to assess the effects of the additional post-cure. Table 2 lists the specimens tested, indicating the post-curing conditions and the number of DMA runs. Thermogravimetric analyses (TGA) were also conducted according to ASTM E2550-21 [41] standard on a _Hitachi-STA/200_ analyzer. These tests aimed at quantifying the mass variation of the resins as a function of temperature and estimating their decomposition temperatures. The experiments were conducted on two samples for each resin with mass of 15-30 mg, from ambient temperature (20 \({}^{\circ}\)C) up to 600 \({}^{\circ}\)C, at a heating rate of 10 \({}^{\circ}\)C/min, and in both oxidative (synthetic air) and inert (nitrogen) atmospheres (air flow of 100 mL/min). The mechanical behavior of the resins was compared by means of tensile and shear tests conducted according to ASTM D638-14 [42] and ASTM D5379/D5379M [43] standards, respectively, and the shear tests were performed according to the V-notched beam method. For each test type, five replicate specimens of the BVE resin and three of the BVEL (due to the aforementioned difficulties in obtaining cured material for testing) were tested in an _Instron_ universal testing machine (model 5982, with load capacity of 100 kN); the tests were performed under displacement control, at a displacement rate of 1 mm/min.