9,10-Dihydro-9-oxa-10-phosphaphenanthrene10-oxide (DOPO) was purchased from Huawei Ruike Chemical Co., Ltd. (Beijing, China). PLA was purchased from Wuxi Diaisheng Epoxy Co., Ltd. (Wuxi, China). Epoxy chain extender (ADR-5481) was purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).

Imine-containing compounds and P-PPD-Ph were synthesized according to literature procedures (Sun and Yao, 2011). PLA, chain extender, and P-PPD-Ph were dried for 4 h under vacuum at 80°C before use. PLA, epoxy chain extender (4 wt‰), and P-PPD-Ph (0, 5, 10, and 15 wt%) were mixed uniformly. Then, the mixture was extruded by using a twin-screw extruder (CTE 35, Coperion Keya Machinery Manufacturing Co., Ltd., China) at a temperature of 180–200°C and a screw speed of 300 rpm. The extruded pellets were then molded into samples for testing using an injection molding machine (CJ80MZ 2N CII, Zhende Plastic Machinery Factory, China) at 180–200°C.

Thermal analysis (TG) was conducted in a Q50 thermal gravimetric analyzer (TA, United States) at a heating rate of 10°C/min and under nitrogen conditions. Approximately, 5 mg of the sample was weighed and placed in an aluminum ceramic with a gas flow of 60 ml/min.

The UL-94 test (SH5300, Guangzhou Xinhe Electronic Equipment Co., Ltd., China) was conducted with sample sizes of 130*10*3.2 mm³ in accordance with the ASTM UL 94-2006.

The LOI measurements were performed on an oxygen index flammability gauge (HC-2C) according to ASTM D 2863-97 standard with a sample dimension of 100 *6.5 *3.2 mm³.

The TG curves under N₂ of PLA and PLA-conjugated flame-retardant composites containing P-PPD-Ph are presented in Figure 1B, and the corresponding data are listed in Table 1A. Relative to PLA/materials without the addition of flame retardant, the addition of flame-retardant P-PPD-Ph reduced the initial decomposition temperature (T−_5%) of PLA-conjugated flame-retardant composites. This may be due to the decomposition temperature of P-PPD-Ph before PLA, and the decomposition of P-PPD-Ph changed the decomposition behavior and induced the PLA-conjugated flame-retardant composites to decompose in advance. Additionally, the residual carbon of PLA-conjugated flame- retardant composites at 700°C increased with the addition of the P-PPD-Ph conjugated flame retardant. This is due to the degradation of phosphorus-containing groups generating heat-resistant residues, which lead to inhibit the decomposition of PLA-conjugated flame-retardant composites (Battig et al., 20121), reduce the maximum thermal decomposition rate, and promote the formation of carbon layers, thereby increasing the char yield of PLA-conjugated flame-retardant composites. Also, when the addition of P-PPD-Ph conjugated flame retardant was 15 wt%, the char residue of the PLA-conjugated flame-retardant composites increased by 4.2% under 700°C. This shows that P-PPD-Ph conjugated flame retardant creates a condensed-phase flame-retardant effect on PLA-conjugated flame-retardant composites.

Figure 1C shows the FTIR spectra of composites at different temperatures in the TG test under N₂ conditions. In the PLA/P-PPD-Ph-15% initial decomposition stage (around 340°C), the absorption peak of P-O near 935 cm⁻¹ appeared in the FTIR spectrum, and 1,599 cm⁻¹ was the absorption peak of P-Ph, and the absorption peaks of CO₂ (2,355 cm⁻¹) and CO (2,177 cm⁻¹ and 2,100 cm⁻¹) appeared. The stretching vibration peak of carbonyl was near 1760 cm⁻¹, and the absorption peak of C–H on the aliphatic chain was at 3,006 cm⁻¹. Also, the absorption peak of hydrocarbons and carbon–oxygen species was 2,638–2,908 cm⁻¹, and the absorption peak of -OH was 3,583 cm⁻¹. With the increase of temperature to the maximum decomposition temperature (about 390°C), the thermal decomposition products of PLA/P-PPD-Ph-conjugated flame-retardant composites were detected by FTIR, and the absorption peak intensity was the strongest and new absorption peaks were detected. 752cm⁻¹ may be the absorption peak of P-C, 1195cm⁻¹ may be the absorption peak of P=O, and 935, 1,047, and 1195cm⁻¹ were the absorption peaks of aromatic phosphorus-containing complexes, which was consistent with the related literature (Yu et al., 2020; Qin et al., 2021). It showed that the thermal decomposition of the PLA conjugated flame-retardant composites may release aromatic esters and aromatic phosphorus-containing compounds. These substances were beneficial to improve the flame retardancy of the conjugated flame-retardant composites and played a gas-phase flame-retardant effect. However, as the thermal decomposition temperature increased to 500°C, the peak intensity of FTIR significantly decreased, and the absorption peaks at 3,006 cm⁻¹ (C–H) and 2,638–2,908 cm⁻¹ (hydrocarbon and carbon–oxygen species, respectively) disappeared. When the temperature reached 600°C (final decomposition stage), the absorption peaks of P=O at 1195cm⁻¹ and 3583cm⁻¹ (-OH) disappeared, which corroborated that the thermal decomposition products of the conjugated flame-retardant composites may have PO₂· and PO·, which can capture H· and OH·, so P-PPD-Ph played a gas-phase flame-retardant role in PLA-conjugated flame-retardant composites.

The effects of the P-PPD-Ph conjugated flame retardant on the flame retardant performance of PLA-conjugated flame-retardant composites were investigated by the vertical burning test (UL-94) and limiting oxygen index test (LOI), and the results are shown in Table 1B. As shown in Table 1B, when the content of P-PPD-Ph conjugated flame retardant is 5 wt%, the PLA-conjugated flame-retardant composites can pass the UL-94 V-0 level. As shown in Figure 1D, the melting dripping occurred in PLA and also in the PLA-conjugated flame-retardant composites. In PLA materials without the addition of flame retardant, splines were dropped in the form of wire-drawing after combustion, while after the P-PPD-Ph conjugated flame retardant was added, the molten droplets after combustion were dropped in the form of water drops. Moreover, PLA without the addition had droplets with fire, and the absorbent cotton was easily ignited, whereas the PLA conjugated flame retardant composites with 5% P-PPD-Ph added had droplets more rapidly without fire, and the absorbent cotton was well reserved. The results show that P-PPD-Ph can decompose the matrix in advance, thus producing molten droplets that take away the heat of the burning matrix (Xi et al., 2016), which is the same as the explanation on TG-FTIR of composites; the quenching and dilution effects of P-PPD-Ph can extinguish the fire on droplets during dripping, thereby the cotton will not be ignited. Also, the LOI value of PLA conjugated flame retardant composites increased from 22.5 to 34.0%, it achieved a good flame-retardant effect, and it can be seen that the prepared P-PPD-Ph conjugated flame retardant had higher flame-retardant efficiency.