*3.3. DSC*

The reaction energy was measured on soda lignin, polymer and blends of lignin and polymer to assess the thermal degradation. Enthalpy measurements were similar for PLA/40%Lignin (27 J/g) compared with neat PLA (31 J/g). At higher a temperature of 376 ◦C (PLA) the enthalpy of 628 J/g was similar with PLA/20%Lignin and higher than PLA/40%Lignin (362 ◦C, 409 J/g), see Figure 2. No significant variations in Tm (temperature maximum degradation) were found with the addition of lignin to PLA matrix. The temperature of maximum degradation occurred between 360–400 ◦C for all biocomposite samples. Decomposition of aromatic rings is expected above 500 ◦C [36]. The glass

transition temperature (Tg) for soda lignin in our study was found to be at 109 ◦C. Additionally, the Tg value can be correlated with the molecular weight of lignin [51]. This value is in accordance with the values reported in the literature. The Tg for neat PLA is found to be at 71 ◦C in this study. The glass transition temperature of the PLA/Lignin biocomposites showed significant shift of the Tg from 71 ◦C towards lower temperature, Tg of 59 ◦C for PLA/Lignin 20%. This can be explained by different molecular factors such as interchain hydrogen bonding, crosslinking density, rigid phenyl groups, and molecular mass [52].

**Figure 2.** Differential scanning calorimetry (DSC) thermographs of Lignin, PLA, and PLA/lignin biocomposites.

## *3.4. Mechanical Properties*

The filaments (PLA, PLA/20%Lignin, and PLA/40%Lignin) were used to 3D print dogbones for further characterization (Figure 3). The stress−strain curves of the biocomposites with unmodified lignin showed that the mechanical strength decreased when the lignin content increased (Figure 4), which is in accordance with previous studies [21,22]. This is most probably due to a low fusing between the printing layers, especially in biocomposites with 40 wt % of lignin. However, when the printing temperature was increased to 215 ◦C, the biocomposite revealed a relative increase of the mechanical properties. This was presumptively due to an improved adhesion of the 3D printed layers. The strength−strain curves also show that lignin led to a more fragile biocomposite that resists less deformation before rupture. Consequently, the elastic modulus decreased by 25%–32%, compared to the neat PLA sample. The addition of low content of lignin in PLA biocomposites presented an improvement of the ductility. However, lignin content above 10% has caused a decrease in the plasticity of biocomposites, either for acetylated or unmodified lignin [53].

**Figure 3.** 3D printed dogbones for mechanical testing.

**Figure 4.** Stress−strain curves for the different biocomposites

#### *3.5. Scanning Electron Microscopy (SEM)*

The SEM images (Figure 5) of the fractured surface of the biocomposites provided an insight into the bonding interaction between lignin and PLA. The analysis of the PLA/40% Lignin sample revealed a 3D printed structure where the printed threads are clearly visible, thus confirming a poor inter-layer adhesion and a corresponding low mechanical performance (Table 1). In order to find a suitable printing temperature of the lignin-containing biocomposite and thus increase the inter-layer bonding, two additional temperatures were tested during the 3D printing process (215 ◦C and 230 ◦C). The results revealed that a suitable printing temperature for the biocomposite filaments developed in this study was 215 ◦C (Table 1 and Figure 4). The SEM pictures showed the improvement of the bonding of the printed layers, which favored the mechanical properties of the samples. However, increasing the printing temperature to 230 ◦C led to a decrease of the tensile strength and modulus. This was potentially caused by the degradation of the carbohydrates in the lignin fraction, which usually occurs over 220 ◦C, becoming volatile gases and presumptively creating microstructures within the polymeric matrix. Our results are in accordance with literature results of Thakur et al. [20] and Watkins et al. [50].

**Figure 5.** Scanning electron microscopy (SEM) analysis of the fracture surface of tensile tested dogbones.

#### *3.6. FT-IR Spectra and XRD Analysis*

FT-IR spectroscopy was applied to assess the functional groups of PLA/Lignin biocomposites at different temperatures. Generally, the curves of PLA/lignin showed similar bands with increased emissivity compared to neat PLA (Figure 6). Neat PLA showed peaks around 2995 cm<sup>−</sup><sup>1</sup> and 2930 cm<sup>−</sup>1, which are associated to the asymmetric and symmetric stretching vibration of CH3 group. The intense peak at 1749 cm<sup>−</sup><sup>1</sup> is attributed to the C=O stretching vibration [54]. The peak at 1450 cm<sup>−</sup><sup>1</sup> corresponds to CH3 anti-symmetric bending vibration. The peaks at 1385 cm<sup>−</sup>1, 1360 cm<sup>−</sup>1, 1316 cm<sup>−</sup>1, and 1300 cm<sup>−</sup><sup>1</sup> are associated to the deformation, symmetric, and bending mode of the CH group, respectively. The peaks at 1182 cm<sup>−</sup>1, 1084 cm<sup>−</sup>1, and 1038 cm<sup>−</sup><sup>1</sup> are attributed to C-O-C stretching vibrations [55]. This peak is obviously higher in the PLA/40%Lignin curve, which indicates that addition of lignin led to a higher content of hydroxy groups. In addition, the biocomposites containing lignin showed a small peak at 1510 cm<sup>−</sup><sup>1</sup> due to the C=C groups of the aromatic rings of lignin.

**Figure 6.** FT-IR spectra of PLA and PLA/lignin biocomposites.

X-ray analysis revealed a change in crystallinity as the lignin was included in the formulation (Figure 7). PLA exhibits a broad peak at 2θ degrees = 10◦–25◦ associated with the semicrystalline nature of PLA. The appearance of peaks at 2θ = 32◦ and 34.5◦ in lignin-containing biocomposites indicated further crystallization of PLA, due to the action of lignin as nucleating agen<sup>t</sup> [56].

**Figure 7.** X-ray diffraction analysis (XRD) patterns of PLA and PLA/Lignin biocomposites.

## *3.7. Antioxidant Properties*

Antioxidant capacity of the biocomposites was measured and expressed as radical scavenging activity (RSA). PLA shows a low antioxidant (activity only associated with the surface interaction with the radical ABTS) (Figure 8). However, the lignin-containing biocomposites show a significantly higher radical scavenging activity (~50%) due to the antioxidant activity of lignin. Although, the biocomposites containing 40% lignin have a slightly higher RSA compared to the PLA/20% Lignin, the differences are not significant. The reaction of ABTS with the biocomposite material occurs mostly on the surface of the specimens, which may explain this behavior. The antioxidant activity of the lignin has been reported previously in several studies [45,57,58], including the blending of low amounts of kraft lignin (0.5 wt %–3 wt %) in PLA for potential biomedical applications [44]. Materials with high antioxidant activity are highly demanded for application in food packaging and biomedicine. A wide variety of antioxidant compounds are described in literature (resveratrol, curcumin, ascorbic acid, carotenoids, etc.), however, these compounds are generally expensive. For this reason, the use of lignin in biocomposites is proposed as a low-cost option to produce materials with high antioxidant capacity.

**Figure 8.** Antioxidant activity of PLA and biocomposites.

Keep in mind that lignin is a natural antioxidant, and lignin has been proposed to stabilize a given material against photo- and thermo-oxidation [14–17,23]. The antioxidant property of soda lignin has been confirmed in this study where the printed materials containing lignin has a significant larger antioxidant capability compared to PLA (Figure 7). This suggests that the biocomposites developed in this study are also suitable for additional materials, e.g., food packaging applications.

The suitability of the PLA/lignin biocomposite filament for 3D printing was also tested, by printing a smartphone protective case (Figure 9). The printing process revealed a good performance of the lignin-containing filament, and a functional protective case was effectively 3D printed. PLA/Lignin filaments are a plausible option for lignin utilization with potential in, e.g., rapid prototyping and consumer products [59]. It is worth to mention that the typical smell from some lignins was not detected during the extrusion of the filaments or during the printing process, which is an additional advantage of using soda lignin in PLA biocomposite materials.

**Figure 9.** 3D printing of a smartphone protective case with PLA/lignin biocomposite filament.
