|
| Lignin | Some key lignin characteristics |
|
| Kraft lignin (KL) | (i) Containing ash of 1–2% and suitable for low-value-added applications [25] |
| (ii) Having a high concentration of condensed structures with strong ether bonds and phenolic hydroxyl groups [35, 36] |
| (iii) Containing the organic matter such as polysaccharides and a few extractive fractions [37] |
| (iv) Molecular weight variability from low to the high molecular weight of 3,000–188,000 g·mol−1 [38] |
| (v) Solubility is limited but can be increased by sulfonation [39] |
| (vi) KL or modified KL can be used as dispersing agents, additives, and thermoset polymers mainly in low molecular weight [39] |
| (vii) KL can produce thermoset polymers when it is condensed with formaldehyde and cross-linked with phenol [39] |
| (viii) KL with higher molecular weight can be used as a thermoplastic mixture or copolymer [38] |
|
| Lignosulfonate (LS) | (i) High ash content of 4–8% and a high molecular weight than KL [40] |
| (ii) The high molecular weight of 1,000 to 50,000 Da [40] than KL with a polydispersity index (PI) of 4.2–8 [33, 41] |
| (iii) Many impurities (approximately 30% by weight), such as ash or carbohydrates [41] |
| (iv) Required to modify for high compatibility with other polymeric materials [42] |
| (v) Dissolve in a variety of aqueous solutions but not in organic solvents [40] |
| (vi) Good water solubility [40] |
| (vii) Having a variety of functional groups (carboxylate, phenolic, as well as sulfur-containing groups) provide exceptional colloidal properties, dispersants, additives (plasticizers), surfactants, and flocculants [43] |
|
| Soda lignin (SL) | (i) Sulfur-free and has a higher purity than KL and LS [44] |
| (ii) Suitable for a range of high products (resins, chemical reactants, composites, and antioxidant agents) and natural feed supplements for monogastric animals [45–47] |
| (iii) Commonly derived from soda or soda-anthraquinone pulping of agricultural wastes or perennial crops [44–46] |
| (iv) Soda lignin contains lower phenolic and aliphatic hydroxyl groups [38] |
|
| Organosolv lignin (OL) | (i) OL has high reactive groups, no sulfur, high purity, and the highest reactivity and is nontoxic [36, 48] |
| (ii) High-quality lignin for higher value-added applications such as chemicals [49] |
| (iii) High solubility in an organic solvent but insoluble in water [50] |
| (iv) The possible application in the adhesive in wood panel production, a monomer in a biopolymer, antioxidant, and pesticide [51] |
| (v) Low content of residual carbohydrates [52] |
|
