*2.5. Characterisation*

The lignin and carbohydrates analyses were performed according to the standard procedures of NREL (NREL/TP–510–42623). The lignin samples were hydrolysed in two steps using sulfuric acid: first step hydrolysis was done with 72 wt % H2SO4, 30 ◦C for 60 min, followed by the second step with 4 wt % H2SO4 at 121 ◦C for 1 h). The resulting supernatant was filtered (0.2uL) and the filtrates were analysed by High Performance Anion Exchange Chromatography (HPAEC, ICS-5000, Dionex, CarboPac PA1 4 × 250 mm column), for carbohydrates. Klasson lignin (acid insoluble lignin) contents were determined gravimetrically, while acid soluble lignin was determined by using TAPPI method (TAPPI UM 250). The absorbance was measured with UV-VIS spectrometry at 205 nm, ε of 110. Carbohydrates were detected on a pulsed amperometry detector (PAD). All carbohydrate contents are reported as anhydrosugars. Purity of lignin samples was calculated from the sum of the ash and sugar results.

#### 2.5.1. Thermo-Gravimetric Analysis (TGA) and Di fferential Scanning Calorimetry (DSC)

TGA was used to determine the thermal stability, decomposition temperature, and char yield for soda lignin, PLA, and di fferent blends of lignin/PLA. The analyses were performed using a Netzsch Jupiter F3 equipment, operating in nitrogen environment. Samples for TGA for each measurement were maintained at 14 ± 5 mg and scans were preformed from 30 ◦C to 800 ◦C with the heating rate of 10 ◦C/min to observe thermal degradation and stability of lignin, PLA, and the corresponding biocomposites. DSC was performed to measure the glass transition temperature (Tg) of soda lignin.

#### 2.5.2. Scanning Electron Microscopy (SEM)

The fracture surface of the dogbones after mechanical testing was sputtered with a layer of gold to make it conductive under the electron beam. Scanning electron microscopy (SEM, Hitachi SU3500, Hitachi High-Technologies Co., Tokyo, Japan) was performed in secondary electron imaging mode using an acceleration voltage of 5 kV and a working distance of 5–6 mm.

## 2.5.3. Mechanical Testing

Test specimens obtained from the 3D printing process were used for tensile testing of the biocomposites.

The 3D printed dogbones were mechanically tested with a Zwick Roell Proline (Zwick GmbH & Co. KG, Ulm, Germany) and a load cell of 2.5 kN. Four specimens of each series were tested. The speed and the grip distance were 20 mm/min and 50 mm, respectively.

#### 2.5.4. Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectra were collected using a spectrometer FTIR-ATR Perkin Elmer Spectrum (Perkin Elmer, Waltham, United States). Two single spectra were collected in the wavelength range from 4000 to 450 cm<sup>−</sup><sup>1</sup> with a resolution of 4 cm<sup>−</sup><sup>1</sup> and a total of 40 scans.

#### 2.5.5. X-ray Di ffraction Analysis (XRD)

X-ray di ffraction analysis of the test specimens was carried out in a Bruker D8 Discover Instrument (Bruker Corporation, Karlsruhe, Germany) with a monochromatic source (CuK α1) over an angular range of 5−50◦ at a scan speed of 1.56◦/min.
