*3.7. Molecular Weights*

The reactivity and physicochemical property are indicated by critical parameters such as molecular mass. The GPC curve in Figure 7 depicts the molecular weight of numberaverage (Mn), weight-average (Mw), and polydispersity index (PDI, Mw/Mn) of reference and isolated lignin from BL. The obtained molar mass distribution of lignin in this study is in the range of the Kraft lignin in a THF-based system reported by Baumberger et al. [70] (Mn = 200–2000 Da, Mw = 1500–50,000 Da). The number of Mw, Mn, and PDI depends on the isolation method, biomass source, and purification [71]. Figure 8 also shows that the trend of both Mw and Mn is reference lignin > single-step lignin > fractionated-step lignin. Markedly, the trend was similar to AIL content and lignin solubility in an organic solvent. However, the opposite trend is seen against the S/G ratio and total phOH. Reference lignin has a higher molecular mass, AIL content, and solubility in an organic solvent, yet it has a lower S/G ratio, Tg value, and total phOH than isolated lignin. This result contrasts with Gordobil et al. (2018), where a positive correlation between molecular weight, S/G ratio, and total phOH was observed. Different analytical methods may affect the S/G ratio and total phOH, <sup>31</sup>P NMR vs Py-GCMS (S/G ratio) and UV-Vis (total phOH). Higher AIL content resulted in high molecular weight and ASL content due to different governing mechanisms of cleavage bonds and functional groups in lignin. This finding was similar

to the results reported by Stiefel et al. [72], where the insoluble acid is slightly correlated (r<sup>2</sup> = 0.739) with the molecular weight in lignin from different treatments [72].

*Polymers* **2022**, *14*, x FOR PEER REVIEW 16 of 20

**Figure 8.** GPC curve including number-average (Mn), weight-average (Mw), and polydispersity index (PDI, Mw/Mn) of reference lignin, single-step lignin, and fractionated-step lignin. **Figure 8.** GPC curve including number-average (Mn), weight-average (Mw), and polydispersity index (PDI, Mw/Mn) of reference lignin, single-step lignin, and fractionated-step lignin.

*3.8. Future Potential of A. mangium Lignin from BL in Adhesive Applications for Wood-Based Composites*  The structural features of *A. mangium* lignin extracted from BL exhibited a strong link to many alternative ways in its possible applications, according to the findings of this study. The fingerprint result (UATR-FTIR, 1H, and 13C NMR) and elucidation structure by The higher molecular weight of reference lignins was attributed to a higher percentage of the G unit [73]. This finding substantiated that reference lignins are derived from softwood. Higher PDI of fractionated-step lignin indicated wider molecular weight distribution as well as the existence of the impurities that positively correlated to ash content (Table 1) [52].

#### Py-GCMS showed a higher abundance of G-unit which is the most active unit in phenolic resin polymerization. Besides, the high Tg value of isolated lignin is suitable for wood *3.8. Future Potential of A. mangium Lignin from BL in Adhesive Applications for Wood-Based Composites*

adhesive applications. The result was similar to lignin from coconut husk that was examined by Abd Latif et al. [74] as an alternative material for lignin-phenol-glyoxal adhesives. Markedly, high MW indicated higher content of aromatic protons which has a better chance of polymerization. Hence, lignin from the single-step method would be more suitable for wood adhesive applications. Another consideration is the presence of large amounts of phenolic hydroxyls in the isolated lignin structure which means making them reactive to create linkage with aldehyde [18]. **4. Conclusions** This study investigated the chemical and physical properties of lignin derived from pulp mill factory residue (*Acacia mangium*) using diverse techniques. Lignin was successfully isolated through single-step and fractionated-step dilute acid precipitation. Accord-The structural features of *A. mangium* lignin extracted from BL exhibited a strong link to many alternative ways in its possible applications, according to the findings of this study. The fingerprint result (UATR-FTIR, <sup>1</sup>H, and <sup>13</sup>C NMR) and elucidation structure by Py-GCMS showed a higher abundance of G-unit which is the most active unit in phenolic resin polymerization. Besides, the high Tg value of isolated lignin is suitable for wood adhesive applications. The result was similar to lignin from coconut husk that was examined by Abd Latif et al. [74] as an alternative material for lignin-phenol-glyoxal adhesives. Markedly, high MW indicated higher content of aromatic protons which has a better chance of polymerization. Hence, lignin from the single-step method would be more suitable for wood adhesive applications. Another consideration is the presence of large amounts of phenolic hydroxyls in the isolated lignin structure which means making them reactive to create linkage with aldehyde [18].

#### ing to fingerprint analysis by FTIR, 1H, and 13C NMR, unique lignin peaks such as aromatic unit guaiacyl (G) and syringyl (S) were observed. The results were confirmed by the com-**4. Conclusions**

mercial lignin used as a reference. Dilute hydrochloric acid obtained high acid-soluble lignin (ASL) content. In contrast, fractionated-step lignin had lower lignin content than single-step. Still, it had a linear correlation against total phenolic hydroxyl (phOH) content, thermal stability, G-unit, and molecular weight distribution. More condensed G-unit in single-step lignin induced higher molecular weight distribution (Mw and Mn) and Tg value and total phOH. Single-step precipitation obtained the highest lignin yield, ~35.39 %. Comprehensive analysis of technical lignin aided in gathering knowledge of the structure and properties of lignin in suggesting better valorization strategies and enhanced future potential for wider industrial application of lignin as a renewable raw material. This study investigated the chemical and physical properties of lignin derived from pulp mill factory residue (*Acacia mangium*) using diverse techniques. Lignin was successfully isolated through single-step and fractionated-step dilute acid precipitation. According to fingerprint analysis by FTIR, <sup>1</sup>H, and <sup>13</sup>C NMR, unique lignin peaks such as aromatic unit guaiacyl (G) and syringyl (S) were observed. The results were confirmed by the commercial lignin used as a reference. Dilute hydrochloric acid obtained high acid-soluble lignin (ASL) content. In contrast, fractionated-step lignin had lower lignin content than single-step. Still, it had a linear correlation against total phenolic hydroxyl (phOH) content, thermal stability, G-unit, and molecular weight distribution. More condensed G-unit in

single-step lignin induced higher molecular weight distribution (Mw and Mn) and Tg value and total phOH. Single-step precipitation obtained the highest lignin yield, ~35.39 %. Comprehensive analysis of technical lignin aided in gathering knowledge of the structure and properties of lignin in suggesting better valorization strategies and enhanced future potential for wider industrial application of lignin as a renewable raw material.

**Author Contributions:** Conceptualization, N.N.S., E.B.S. and W.F.; methodology, N.N.S., E.B.S., W.S. and W.F.; formal analysis, N.N.S., W.F., A.K., E.W.M. and A.H.I.; investigation, N.N.S., E.B.S., F.F., F.P.S., M.I. and M.A.R.L.; resources, N.N.S. and W.F.; data curation, N.N.S., E.B.S., M.I. and M.A.R.L.; writing—original draft preparation, review and editing, N.N.S., W.F., P.A., V.S. and M.G.; supervision, N.N.S., W.S. and W.F.; project administration, N.N.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by project from Research Organization for Engineering Science, National Research and Innovation Agency (BRIN) Indonesia No. 26/A/DT/2021 "Valorization of Black Liquor from Pulp Mill by Product as Antimicrobial Agent for Textile". This research was also supported by project No. HИC-Б-1145/04.2021 "Development, Properties, and Application of Eco-Friendly Wood-Based Composites" carried out at the University of Forestry, Sofia, Bulgaria. This research was also supported by the Slovak Research and Development Agency under contracts No. APVV-18-0378 and APVV-19-0269.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Authors acknowledge the Research Organization for Engineering Science National Research and Innovation Agency (BRIN) for their funding support of this project. The authors also wish to thank Advanced Characterization Laboratories Cibinong—Integrated Laboratory of Bioproduct, National Research and Innovation Agency through E- Layanan Sains, Badan Riset dan Inovasi Nasional for the facilities, scientific and technical support form.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

