**1. Introduction**

The impact of worldwide production and consumption of petroleum-based plastics on economic and ecological sustainability has attracted widespread attention; these crises can be alleviated by developing bio-based plastics [1–3]. As an important environmentally friendly bio-based polymer, polylactic acid (PLA) can be obtained from corn, wheat, sugar beet, etc.; it completely degrades to carbon dioxide and water in a composting environment [4–6].

Recently, PLA materials have been commercialized in the fields of packaging materials, textile materials and medical materials because of their multiple advantages over traditional petro-polymers, including excellent mechanical properties, transparency, biocompatibility, biodegradability and processability [7–10]. However, the limiting oxygen index (LOI) of PLA is only 19%, which makes it easy to burn; a large number of molten droplets are formed during the burning process, which significantly restricts its commercial applications on a large scale [11,12].

Various flame-retardant additives have been incorporated into PLA matrices to solve this problem [13–18]. Polyphosphates, such as ammonium polyphosphate (APP), melamine polyphosphate, tris (hydroxymethyl)-aminomethane polyphosphate and isosorbide-based polyphosphonate, were introduced to PLA as halogen-free flame-retardants to increase its flame retardancy [19–22]; APP was often used in intumescent flame-retardants (IFR). As an eco-friendly flame-retardant, IFRs exhibit excellent flame retardancy on PLA due to their high effectiveness, low smoke production, low toxicity and low corrosiveness [23,24].

**Citation:** Zhang, Q.; Liu, H.; Guan, J.; Yang, X.; Luo, B. Synergistic Flame Retardancy of Phosphatized Sesbania Gum/Ammonium Polyphosphate on Polylactic Acid. *Molecules* **2022**, *27*, 4748. https://doi.org/10.3390/ molecules27154748

 Academic Editors: Marek Brzezi ´nski and Małgorzata Ba´sko

Received: 11 June 2022 Accepted: 22 July 2022 Published: 25 July 2022

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However, the char formation of pentaerythritol that usually acts as a carbon source in traditional IFRs is poor; a large amount of IFR needs to be added to achieve a vertical burning test (UL-94) V-0 rating, which leads to a significant decrease in the mechanical properties of PLA materials. Therefore, finding a carbon source with high flame-retardant efficiency is the key to improving the flame-retardant efficiency of IFRs in PLA.

With the development of bio-based materials and the increasing demand for ecologically sustainable development, research on bio-based flame-retardants has attracted the attention of many researchers. Bio-based polysaccharides such as starch, cellulose, lignin, cyclodextrin and chitosan have been used as "green" carbon sources in the flame-retardant modification of PLA [25–29]. Sesbania gum (SG) is a natural polysaccharide found in the seeds of sesbania, a native plant of China. It is a galactomannan; its main structure consists of one galactose on every other mannose unit. SG is an attractive biopolymer because of its abundance, low cost, biodegradability and potential application in the production of biodegradable films [30]. The large number of hydroxyl groups on the surface of these polysaccharides resulting in obvious phase separation from a hydrophobic matrix (such as PLA), resulting in poor interfacial compatibility between them and PLA. DOPO is commonly used as a reactive flame-retardant with high thermal stability, good antioxidant property, excellent flame-retardant ability and environmental compatibility. The reactive P–H bond in its structure may interact with unsaturated double bonds. Endic anhydride (EA), bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride, also has high reactivity, containing both unsaturated double bonds and anhydride bonds. The P–H bond of DOPO can undergo an addition reaction with the double bond of EA; the resulting product is further covalently bonded to the side chain of SG to reduce the polarity of SG and increase its compatibility with PLA. In the present work, SG was modified with DOPO and EA to prepare phosphorylated sesbania gum (DESG). Ammonium polyphosphate (APP), an acid source and gas source in IFRs, was combined with DESG and then added to PLA, resulting in a PLA-based composite material with flame retardancy. The synergistic flame retardancy of DESG and APP on PLA was investigated, thermal properties and morphologies of the resulting composites were characterized and the flame-retardant mechanism of DESG and APP in PLA was proposed.
