*3.1.1 Lignin-based fire retardant*

In recent research literature, it has been found that intumescent flame retardants, such as ammonium polyphosphate (APP), as a non-reactive, inorganic material can be added to polymers as a substitute for halogen flame retardant, being compatible with many polymers and biopolymers [56]. APP is a reaction results of ammonia and phosphoric acid. Therefore, as an additive used for intumescent coating in flame-retardant applications, APP has both function of acid and gas source. When a product containing APP meets fire, APP acts as a flame retardant by a chemical effect in the condensed phase called intumescence. As a result, a carbon foam is formed at the surface of the material which acts as an insulating layer, preventing further decomposition of the material. It has been shown that APP has high content of phosphorus and nitrogen, environmentally friendly, good thermal stability, low smoke, and nontoxicity [11]. These characteristics makes the intumescent flame superior to conventional flame retardants. In recent years, lignin and chitosan has been used as a carbon source in different flame-retardant formulations [12, 13, 57].

Lignin based flame retardants can be prepared by directly physical blending or by chemical modification [58, 59]. A disadvantage of the physical method is

uneven multicomponent mixing, which has a negative impact on flame retardancy. During the chemical modification, the hydroxyl groups present in lignin structure will react with desired functional groups, such as ammonia, phosphoric acid given a lignin with modified structure which is suitable to be used in intumescent flameretardant formulations [60–63]. Zhang et al. [64] showed that lignin modified with urea and combined it with ammonium polyphosphate (APP) was successfully used as a novel intumescent flame retardant (IFR) system to improve the flame retardancy of polylactic acid (PLA). Moreover, Liu et al. [63] showed that novel lignin-based flame retardant was done by chemically grafting nitrogen, phosphorus and copper elements into lignin structure to improve the flame retardance of wood- plastic composites. Lignin nanoparticles can also be used in different intumescent flame-retardant formulations. Collet et al. used for the first-time lignin nanoparticles modified with phosphor in intumescent flame-retardant formulations [65].

Char yield during combustion of a polymer is an important characteristic when the polymer is to be used as a flame retardant or as additive for intumescent coating. We have observed that during thermal decomposition lignin produces high char yield up to 45%, as seen in **Figure 4**. We believe that the high char layer has a positive effect on smoke suppression and therefore er lignin a promising additive in intumescent flame-retardant formulations.

Our hypothesis on formation of a larger and denser charring layer helps in improving smoke suppression is in accordance with literature results of Dai et al. [59]. The mechanism involved here is similar when APP is used, where the hydroxyl groups present in lignin structure reacts with phosphoric acid and ammonia, as illustrated in **Figure 5.**

As such, both phosphor and nitrogen are introduced in lignin structure, having a function of an acid and gas source in intumescent flame-retardant formulations. We strongly believe that the synergic effect of both nitrogen and phosphor incorporated in lignin structure can improve the fire-resistance properties.

**Figure 5.** *Illustration of incorporation of phosphor and nitrogen in lignin structure.*

#### *3.1.2 Antibacterial activity of lignin*

The antimicrobial property of biopolymers has been shown literature to depend on several factors, such as molecular mass, concentration, ability to be fixed into the wood structure and electrical charge [66]. Biopolymers with higher molecular masses have low biocidal activity, compared to high molecular mass biopolymers [67]. Literature studies shown that the antimicrobial activities of lignin can be inhibited by the presence phenolic monomers in lignin [68]. The lignin's antimicrobial activity depends on biomass source, the presence of hydroxyl and methoxy groups, and the extraction methods as follows: softwood organosolv > softwood kraft > grass organosolv due to the effect of acid-soluble lignin content [69]. Lignin as an antimicrobial agent is being used in commodity products like in plastic production [70], textile [71–73], medical materials, pest control, and healthcare products [74]. Lignin's and lignin nanoparticles chemical modification and combination with metals, for example Cu-lignin combination, have been shown to increase antimicrobial activity [75]. Thus, the use lignin as an antibacterial agent is believed to be a high value approach for lignin valorization.
