Preparation and Mechanism of Toughened and Flame-Retardant Bio-Based Polylactic Acid Composites

As a biodegradable thermoplastic, polylactic acid (PLA) shows great potential to replace petroleum-based plastics. Nevertheless, the flammability and brittleness of PLA seriously limits its use in emerging applications. This work is focused on simultaneously improving the flame-retardancy and toughness of PLA at a low additive load via a simple strategy. The PLA/MKF/NTPA biocomposites were prepared by incorporating alkali-treated, lightweight, renewable kapok fiber (MKF) and high-efficiency, phosphorus-nitrogenous flame retardant (NTPA) into the PLA matrix based on the extrusion–injection molding method. When the additive loads of MKF and NTPA were 0.5 and 3.0 wt%, respectively, the PLA/MKF/NTPA biocomposites (PLA3.0) achieved a rating of UL-94 V-0 with an LOI value of 28.3%, and its impact strength (4.43 kJ·m−2) was improved by 18.8% compared to that of pure PLA. Moreover, the cone calorimetry results confirmed a 9.7% reduction in the average effective heat of combustion (av-EHC) and a 0.5-fold increase in the flame retardancy index (FRI) compared to the neat PLA. NTPA not only exerted a gas-phase flame-retardant role, but also a condensed-phase barrier effect during the combustion process of the PLA/MKF/NTPA biocomposites. Moreover, MKF acted as an energy absorber to enhance the toughness of the PLA/MKF/NTPA biocomposites. This work provides a simple way to prepare PLA biocomposites with excellent flame-retardancy and toughness at a low additive load, which is of great importance for expanding the application range of PLA biocomposites.


Introduction
As global industrialization continues, people are faced with the problems of fossil resource depletion and the resulting environmental pollution [1]. Consequently, replacing traditional petroleum-based materials with renewable bio-based materials has become a hot topic of current research. Among them, PLA, a thermoplastic material with good biodegradability, biocompatibility, and processability, is an important product in the renewable chemistry industry [2,3]. Specifically, PLA is produced by polymerizing lactic acid or lactide from the fermentation of crops [4][5][6], which has shown great potential for application in emerging fields, such as the auto industry, in electronic devices, and so on [3,7]. However, the flammability and poor toughness of PLA have limited its application in emerging fields [3,8].

Preparation of PLA/MKF/NTPA Biocomposites
The original KF (150-200 μm) were pretreated with 0.8 wt% NaOH solution to remove waxes from their surfaces and referred to as modified KF (MKF) after washing and drying. Then, the PLA and MKF were dried (70 °C, 6 h) before use. Subsequently, the PLA, MKF, and NTPA were uniformly blended in a mixer. The mixed sample was granulated with a twin-screw extruder ; the temperature profiles of the twin-screw extruder were 165, 170, 175, 178, 180, and 180 °C during the melt compounding. After the extrusion operation, the composites were cut into pellets. Then, the final test specimens were molded with an HTF86X1 hydraulic injection molding machine (Zhejiang Haitian Equipment Company, China) at the temperature parameters of 165, 170, 175, 178, and 180 °C, and the formulations are given in Table 1.

Characterization
The tensile and flexural strengths of the composites were measured with a Reger computer-controlled RGT-20A mechanical apparatus (Shenzhen, China) at rates of 5 and 2 mm·min −1 following the ASTM D638 and D790 standards, respectively. The notched impact strength was performed with an XJC-5 impact tester (Hebei, China) per the ASTM D256 standard.
The limiting oxygen index (LOI) tests were carried out with a JF-3 oxygen index instrument (Jiangning, China) based on the ASTM D2863 standard. The size of the specimens was 130 mm × 6.5 mm × 3.2 mm. The UL-94 tests for the specimens (130 mm × 13 Scheme 1. The chemical structure of NTPA.

Preparation of PLA/MKF/NTPA Biocomposites
The original KF (150-200 µm) were pretreated with 0.8 wt% NaOH solution to remove waxes from their surfaces and referred to as modified KF (MKF) after washing and drying. Then, the PLA and MKF were dried (70 • C, 6 h) before use. Subsequently, the PLA, MKF, and NTPA were uniformly blended in a mixer. The mixed sample was granulated with a twin-screw extruder (SLJ-20); the temperature profiles of the twin-screw extruder were 165, 170, 175, 178, 180, and 180 • C during the melt compounding. After the extrusion operation, the composites were cut into pellets. Then, the final test specimens were molded with an HTF86X1 hydraulic injection molding machine (Zhejiang Haitian Equipment Company, China) at the temperature parameters of 165, 170, 175, 178, and 180 • C, and the formulations are given in Table 1.

Characterization
The tensile and flexural strengths of the composites were measured with a Reger computer-controlled RGT-20A mechanical apparatus (Shenzhen, China) at rates of 5 and 2 mm·min −1 following the ASTM D638 and D790 standards, respectively. The notched impact strength was performed with an XJC-5 impact tester (Hebei, China) per the ASTM D256 standard.
The limiting oxygen index (LOI) tests were carried out with a JF-3 oxygen index instrument (Jiangning, China) based on the ASTM D2863 standard. The size of the specimens was 130 mm × 6.5 mm × 3.2 mm. The UL-94 tests for the specimens (130 mm × 13 mm × 3.2 mm) were carried out on a CZF-2-type instrument (Jiangning, China) based on the ASTM D3801-1996 standard. A cone calorimetry instrument (West Sussex, UK) was used to evaluate the combustion performance of samples following the ISO 5660 standards. The samples with a size of 100 mm × 100 mm × 3 mm were measured at the heat flux of 35 kW·m −2 .
The microscopic morphology characteristics of the specimens were observed with a scanning electron microscopy (SEM) instrument (JSM7500F microscope, Japan).
X-ray photoelectron spectroscopy (XPS) was performed using an ultra-high vacuum system and a Kα hemispherical electron analyzer (Thermofisher Scientific Company, Waltham, MA, USA). The corresponding C, N, O, and P elements were analyzed.
Thermogravimetric coupled with Fourier transform infrared (TG-IR) analysis of the samples was performed with the TGA Q5000 IR thermogravimetric analyzer interfaced with the Nicolet 6700 FTIR spectrophotometer. The 5.0 mg sample was heated under a nitrogen atmosphere from 50 to 800 • C with a flow rate of 20 mL·min −1 .  Figure 1. When the MKF was introduced, the tensile and flexural strength of PLA/MKF biocomposites decreased, whereas their ductility and impact strength increased significantly. When the additive load of MKF increased to 0.5 wt%, the maximum impact strength (5.54 kJ·m −2 ) of the PLA/MKF biocomposites was obtained, which was an increase of 48.5% over neat PLA. Similar results were also reported by Qian et al. [35] and Wang et al. [36]. These researchers observed that the incorporation of fiber-like materials gave rise to an improvement in toughness at the sacrifice of the tensile strength of PLA composites. In comparison with bamboo cellulose nanowhiskers (BCNW) [35], only 0.5 wt% MKF significantly improved the toughness of the PLA/MKF biocomposites, indicating that the lightweight hollow MKF has a clear advantage in giving PLA superior toughness at a low additive load. 100 mm × 100 mm × 3 mm were measured at the heat flux of 35 kW·m −2 .

Results and Discussion
The microscopic morphology characteristics of the specimens were observed scanning electron microscopy (SEM) instrument (JSM7500F microscope, Japan).
X-ray photoelectron spectroscopy (XPS) was performed using an ultra-high v system and a Kα hemispherical electron analyzer (Thermofisher Scientific Compan tham, MA, USA). The corresponding C, N, O, and P elements were analyzed.
Thermogravimetric coupled with Fourier transform infrared (TG-IR) analysi samples was performed with the TGA Q5000 IR thermogravimetric analyzer inte with the Nicolet 6700 FTIR spectrophotometer. The 5.0 mg sample was heated u nitrogen atmosphere from 50 to 800 °C with a flow rate of 20 mL·min −1 .

Mechanical Property Analysis of PLA/MKF Biocomposites
To identify the toughening role of MKF, the mechanical properties of PLA/MK composites were characterized. The tensile strength (60.35 MPa), flexural strength MPa), elongation at break (7.67%), and notched Izod impact strength (3.73 kJ·m −2 ) PLA are depicted in Figure 1. When the MKF was introduced, the tensile and f strength of PLA/MKF biocomposites decreased, whereas their ductility and strength increased significantly. When the additive load of MKF increased to 0.5 w maximum impact strength (5.54 kJ·m −2 ) of the PLA/MKF biocomposites was ob which was an increase of 48.5% over neat PLA. Similar results were also reported b et al. [35] and Wang et al. [36]. These researchers observed that the incorporation o like materials gave rise to an improvement in toughness at the sacrifice of the strength of PLA composites. In comparison with bamboo cellulose nanowhiskers (B [35], only 0.5 wt% MKF significantly improved the toughness of the PLA/MKF bioc sites, indicating that the lightweight hollow MKF has a clear advantage in givin superior toughness at a low additive load.

Fire Safety Analysis of PLA/MKF/NTPA Biocomposites
From previous studies, it was determined that 0.5 wt% MKF imparted ex toughness to PLA composites, thus identifying 0.5 wt% as the optimum additive lo a consequence, the PLA/MKF/NTPA biocomposites were prepared by introducing to improve their flame-retardant performance.
The flame-retardant effect of NTPA on the PLA/MKF/NTPA biocomposites w vestigated, and the related data are illustrated in Table 1. Neat PLA was highly infl ble and burned with a large number of molten drops, which was attributed to th degradation of molecular chains. When NTPA was added in the amount of 3 w PLA3.0 biocomposite passed the UL-94 V-0 rating with an LOI value of 28.3%. Al

Fire Safety Analysis of PLA/MKF/NTPA Biocomposites
From previous studies, it was determined that 0.5 wt% MKF imparted excellent toughness to PLA composites, thus identifying 0.5 wt% as the optimum additive load. As a consequence, the PLA/MKF/NTPA biocomposites were prepared by introducing NTPA to improve their flame-retardant performance.
The flame-retardant effect of NTPA on the PLA/MKF/NTPA biocomposites was investigated, and the related data are illustrated in Table 1. Neat PLA was highly inflammable and burned with a large number of molten drops, which was attributed to the rapid degradation of molecular chains. When NTPA was added in the amount of 3 wt%, the PLA3.0 biocomposite passed the UL-94 V-0 rating with an LOI value of 28.3%. Although NTPA could not completely inhibit the melt dripping of the PLA/MKF/NTPA biocomposites, Polymers 2023, 15, 300 5 of 13 the droplets from the PLA/MKF/NTPA biocomposites did not ignite the absorbent cotton in the UL-94 test because of the excellent gas-phase flame-retardant effect of NTPA, thus giving the PLA/MKF/NTPA biocomposites good fire safety.
The fire safety of the PLA/MKF/NTPA biocomposites was further investigated by simulating the combustion behavior of the composites under fire disaster conditions via cone calorimetry tests [13]. The curves of the heat release rate (HRR) and total heat release (THR) of the biocomposites are depicted in Figure 2. The related data are also presented in Table 2. With the incorporation of NTPA, the time to ignition (TTI) of biocomposites increased from 51 s for PLA0 to 74 s for PLA3.5, indicating that NTPA inhibited the ignition of PLA biocomposites. In Table 2, the peak HRR (pHRR) of PLA0 was 657 kW·m −2 , which was lower than that of neat PLA (766 kW·m −2 ) in our previous work [15]. This was attributed to the production of a thin char layer by MKF during the combustion of PLA0, which acted as a barrier in the condensed phase. When the flame broke through the barrier layer, the HRR increased rapidly again. Thus, the THR of PLA0 was higher, rising to 120.4 MJ·m −2 . When NTPA was incorporated, the pHRR of the PLA/MKF/NTPA composites slightly increased. This phenomenon was attributed to the decomposition of NTPA that then promoted the degradation of the bio-based matrix. However, the THR and the average effective heat of combustion (av-EHC) of PLA3.5 were reduced by 13.0% and 12.3%, respectively, compared with those of PLA0. Meanwhile, the total smoke release (TSR) of the PLA biocomposites increased from 4.96 m −2 ·m −2 for PLA0 to 268.02 m −2 ·m −2 for PLA3.5, which corresponds with the increasing NTPA content. This indicated that the efficient flame-retardant effect of NTPA on PLA biocomposites was mainly due to the gas-phase flame-retardant effect [50]. In addition, Vahabi et al. reported a general dimensionless criterion FRI (Flame Retardant Index, Equation (1)) that was used to evaluate the flame-retardancy of thermoplastic materials [51]. (1) simulating the combustion behavior of the composites under fire disaster conditions via cone calorimetry tests [13]. The curves of the heat release rate (HRR) and total heat release (THR) of the biocomposites are depicted in Figure 2. The related data are also presented in Table 2. With the incorporation of NTPA, the time to ignition (TTI) of biocomposites increased from 51 s for PLA0 to 74 s for PLA3.5, indicating that NTPA inhibited the ignition of PLA biocomposites. In Table 2, the peak HRR (pHRR) of PLA0 was 657 kW·m −2 , which was lower than that of neat PLA (766 kW·m −2 ) in our previous work [15]. This was attributed to the production of a thin char layer by MKF during the combustion of PLA0, which acted as a barrier in the condensed phase. When the flame broke through the barrier layer, the HRR increased rapidly again. Thus, the THR of PLA0 was higher, rising to 120.4 MJ·m −2 . When NTPA was incorporated, the pHRR of the PLA/MKF/NTPA composites slightly increased. This phenomenon was attributed to the decomposition of NTPA that then promoted the degradation of the bio-based matrix. However, the THR and the average effective heat of combustion (av-EHC) of PLA3.5 were reduced by 13.0% and 12.3%, respectively, compared with those of PLA0. Meanwhile, the total smoke release (TSR) of the PLA biocomposites increased from 4.96 m −2 ·m −2 for PLA0 to 268.02 m −2 ·m −2 for PLA3.5, which corresponds with the increasing NTPA content. This indicated that the efficient flame-retardant effect of NTPA on PLA biocomposites was mainly due to the gas-phase flame-retardant effect [50]. In addition, Vahabi et al. reported a general dimensionless criterion FRI (Flame Retardant Index, Equation (1)) that was used to evaluate the flame-retardancy of thermoplastic materials [51].
As shown in Table 2, the FRI of the composites improved with the addition of NTPA, and the FRI of PLA3.5 was 1.55 times more than that of PLA0. The results indicated that PLA/MKF obtained better flame retardancy with the introduction of NTPA.    As shown in Table 2, the FRI of the composites improved with the addition of NTPA, and the FRI of PLA3.5 was 1.55 times more than that of PLA0. The results indicated that PLA/MKF obtained better flame retardancy with the introduction of NTPA.

Thermal Stability of PLA/MKF/NTPA Biocomposites
The TG and DTG curves of the PLA/MKF/NTPA biocomposites are presented in Figure 3 under N 2 . The corresponding data are summarized in Table 3. As with PLA0, there was only one decomposition process for all PLA composites. However, the T initial (the 5 wt% degradation temperature) reduced from 326.5 • C for PLA0 to 316.6 and 293.4 • C for PLA2.5 and PLA3.0, respectively, due to the catalytic degradation effect of NTPA. Meanwhile, the T max (maximum degradation temperature) of the PLA3.0 biocomposites shifted forward with increasing NTPA level, and their R max (weight loss rate at T max ) lowered from 31.8%·min −1 for PLA0 to 24.1%·min −1 . This demonstrated that NTPA efficiently lowered the thermal decomposition rate of the PLA/MKF/NTPA biocomposites. In addition, the char residue at 800 • C of the PLA3.0 biocomposites increased from 0.48% for PLA0 to 1.44%. These results demonstrated that the introduction of NTPA was beneficial for PLA/MKF to obtain flame-retardancy. wt% degradation temperature) reduced from 326.5 °C for PLA0 to 316.6 and 293.4 °C PLA2.5 and PLA3.0, respectively, due to the catalytic degradation effect of NTPA. M while, the Tmax (maximum degradation temperature) of the PLA3.0 biocomposites sh forward with increasing NTPA level, and their Rmax (weight loss rate at Tmax) lowered f 31.8%·min −1 for PLA0 to 24.1%·min −1 . This demonstrated that NTPA efficiently low the thermal decomposition rate of the PLA/MKF/NTPA biocomposites. In addition char residue at 800 °C of the PLA3.0 biocomposites increased from 0.48% for PLA 1.44%. These results demonstrated that the introduction of NTPA was beneficia PLA/MKF to obtain flame-retardancy.

Flame-Retardant Mechanism of PLA/NTPA/MKF Biocomposites
According to the combustion mode of different flame-retardant materials, the fla retardant mechanism is classified into condensed and gas-phase mechanisms [23]. condensed-phase flame-retardant mechanism of the PLA/MKF/NTPA biocomposites analyzed by examining photos, SEM images, and the elemental composition of char dues after the CCT. The relevant images and data are shown in Figure 4. PLA0 bu completely without any residual char (Figure 4a). When NTPA was introduced, the re ual char of the PLA/MKF/NTPA biocomposites increased and gradually formed a ne complete char layer (Figure 4b-d). The microscopic morphology of the PLA/MKF/N biocomposites' char residue was continuous, and the pores and cracks on the surfa the residue gradually decreased with increasing NTPA content, which indicated the lytic carbonization effect of NTPA on the biocomposites. Specifically, compared PLA2.5 (Figure 4e), the cracks and pores of residual char for PLA3.0 (Figure 4f) di peared gradually, and a compact and denser char formed, which effectively prevented

Flame-Retardant Mechanism of PLA/NTPA/MKF Biocomposites
According to the combustion mode of different flame-retardant materials, the flameretardant mechanism is classified into condensed and gas-phase mechanisms [23]. The condensed-phase flame-retardant mechanism of the PLA/MKF/NTPA biocomposites was analyzed by examining photos, SEM images, and the elemental composition of char residues after the CCT. The relevant images and data are shown in Figure 4. PLA0 burned completely without any residual char (Figure 4a). When NTPA was introduced, the residual char of the PLA/MKF/NTPA biocomposites increased and gradually formed a nearly complete char layer (Figure 4b-d). The microscopic morphology of the PLA/MKF/NTPA biocomposites' char residue was continuous, and the pores and cracks on the surface of the residue gradually decreased with increasing NTPA content, which indicated the catalytic carbonization effect of NTPA on the biocomposites. Specifically, compared with PLA2.5 (Figure 4e), the cracks and pores of residual char for PLA3.0 ( Figure 4f) disappeared gradually, and a compact and denser char formed, which effectively prevented the internal and external mass and heat during the combustion of the PLA/MKF/NTPA biocomposites. As a result, PLA 3.0 obtained a satisfactory UL-94 V-0 rating. In Figure 4h, XPS analysis indicated that the elemental contents of C, N, O, and P in the residual char of PLA3.0 were 77.13, 6.10, 15.50, and 1.27 wt%, respectively. The N and P elements in the char residue further supported the catalytic charring effect of NTPA [15].  The gas-phase flame-retardant mechanism of the PLA/MKF/NTPA composites was studied with TG-IR analysis, and the relevant data are shown in Figure 5. The 3D TG-FTIR spectra (Figure 5a-c) of the PLA/MKF/NTPA biocomposites suggested that the IR absorption intensity of the biocomposites diminished with the improvement of NTPA content, which indicated that NTPA reduced the total amount of gas-phase thermal decomposition products of the PLA/MKF/NTPA composites. The FTIR spectra at Tmax of the biocomposites were compared in Figure 5d. With the introduction of NTPA, the intensity of the absorption peaks of alkanes (C-H), carbonyls (C=O), and ethers (C-O-C) [52,53] in the pyrolysis products of PLA/MKF/NTPA biocomposites decreased significantly, and the absorption peaks of aromatic compounds and PO· appeared at 1586, 1483, and 1056 cm −1 [15]. The interesting results indicated that NTPA changed the pyrolysis path of the composites and that the benzene-and phosphorus-containing radicals generated by NTPA decomposition acted as quenchers in the gas phase.
The flame-retardant effect of NTPA on the composites was found in the condensed phase as well as in the gas phase. The flame-retardant effect of NTPA in the PLA/MKF/NTPA biocomposites was dominated by the gas-phase flame-retardant mechanism and supplemented by the condensed-phase flame-retardant mechanism. The gas-phase flame-retardant mechanism of the PLA/MKF/NTPA composites was studied with TG-IR analysis, and the relevant data are shown in Figure 5. The 3D TG-FTIR spectra (Figure 5a-c) of the PLA/MKF/NTPA biocomposites suggested that the IR absorption intensity of the biocomposites diminished with the improvement of NTPA content, which indicated that NTPA reduced the total amount of gas-phase thermal decomposition products of the PLA/MKF/NTPA composites. The FTIR spectra at T max of the biocomposites were compared in Figure 5d. With the introduction of NTPA, the intensity of the absorption peaks of alkanes (C-H), carbonyls (C=O), and ethers (C-O-C) [52,53] in the pyrolysis products of PLA/MKF/NTPA biocomposites decreased significantly, and the absorption peaks of aromatic compounds and PO· appeared at 1586, 1483, and 1056 cm −1 [15]. The interesting results indicated that NTPA changed the pyrolysis path of the composites and that the benzene-and phosphorus-containing radicals generated by NTPA decomposition acted as quenchers in the gas phase.
The flame-retardant effect of NTPA on the composites was found in the condensed phase as well as in the gas phase. The flame-retardant effect of NTPA in the PLA/MKF/NTPA biocomposites was dominated by the gas-phase flame-retardant mechanism and supplemented by the condensed-phase flame-retardant mechanism.

Mechanical Properties and Toughening Mechanism of PLA/MKF/NTPA Biocomposites
The effect of NTPA on the mechanical properties of PLA/MKF/NTPA was evaluat and the corresponding data is shown in Figure 6. The tensile strength, flexural streng and notched Izod impact strength of the PLA/MKF/NTPA biocomposites exhibited a creasing trend with increasing NTPA levels. This was because the rigid NTPA molecu presented a certain plasticizing effect in the PLA/MKF/NTPA biocomposites, thus res ing in a slight reduction of mechanical properties. However, compared to neat PLA (3 kJ·m −2 , Figure 1), the impact strength of PLA2.5 (4.86 kJ·m −2 ) and PLA3.0 (4.43 kJ·m −2 ) s increased by 30.3% and 18.8%, respectively. Specifically, PLA3.0 achieved a UL-94 rating and showed momentous application potential in flame-retardant and toughen PLA biocomposites.
The fracture surface morphology of the PLA/MKF/NTPA biocomposites was char terized by SEM, as shown in Figure 6d-f. The fracture surface of the PLA/MKF/NT biocomposites became significantly rougher compared to that of neat PLA [15]. Furth more, a fiber-like structure was observed on the fracture surfaces of the biocomposi which was attributed to the fracture of the filled MKF. These fracture surface morpho gies indicated that NTPA and MKF were homogeneously dispersed and compatible in PLA matrix, and that there was no obvious agglomeration of NTPA and MKF. The cor sponding toughening mechanism is elucidated in Figure 6c. During the impact proc MKF acted as an energy absorber to dissipate stress and enhance the impact strength the PLA/MKF/NTPA biocomposites.

Mechanical Properties and Toughening Mechanism of PLA/MKF/NTPA Biocomposites
The effect of NTPA on the mechanical properties of PLA/MKF/NTPA was evaluated, and the corresponding data is shown in Figure 6. The tensile strength, flexural strength, and notched Izod impact strength of the PLA/MKF/NTPA biocomposites exhibited a decreasing trend with increasing NTPA levels. This was because the rigid NTPA molecules presented a certain plasticizing effect in the PLA/MKF/NTPA biocomposites, thus resulting in a slight reduction of mechanical properties. However, compared to neat PLA (3.73 kJ·m −2 , Figure 1), the impact strength of PLA2.5 (4.86 kJ·m −2 ) and PLA3.0 (4.43 kJ·m −2 ) still increased by 30.3% and 18.8%, respectively. Specifically, PLA3.0 achieved a UL-94 V-0 rating and showed momentous application potential in flame-retardant and toughened PLA biocomposites.
The fracture surface morphology of the PLA/MKF/NTPA biocomposites was characterized by SEM, as shown in Figure 6d-f. The fracture surface of the PLA/MKF/NTPA biocomposites became significantly rougher compared to that of neat PLA [15]. Furthermore, a fiber-like structure was observed on the fracture surfaces of the biocomposites, which was attributed to the fracture of the filled MKF. These fracture surface morphologies indicated that NTPA and MKF were homogeneously dispersed and compatible in the PLA matrix, and that there was no obvious agglomeration of NTPA and MKF. The corresponding toughening mechanism is elucidated in Figure 6c. During the impact process, MKF acted as an energy absorber to dissipate stress and enhance the impact strength of the PLA/MKF/NTPA biocomposites. To clarify the flame-retardancy and toughening levels of the prepared PLA/MKF/NTPA biocomposites, filler loadings and impact strength increments were used to compare with previously reported PLA composites containing biofiber materials. The comparison results are shown in Table 4 and Figure 7. It should be noted that the impact strength increments were derived by comparison with neat PLA. In general, low additive loading means low cost, whereas high loading often causes deterioration of tensile properties due to poor dispersion. Compared with previously reported PLA composites [8,37,45,46,54], the PLA/MKF/NTPA biocomposites exhibited excellent flame-retardancy and toughness at low additive loads (only 3.5 wt%), and they thus showed potential applications in large-scale production.  To clarify the flame-retardancy and toughening levels of the prepared PLA/MKF/NTPA biocomposites, filler loadings and impact strength increments were used to compare with previously reported PLA composites containing biofiber materials. The comparison results are shown in Table 4 and Figure 7. It should be noted that the impact strength increments were derived by comparison with neat PLA. In general, low additive loading means low cost, whereas high loading often causes deterioration of tensile properties due to poor dispersion. Compared with previously reported PLA composites [8,37,45,46,54], the PLA/MKF/NTPA biocomposites exhibited excellent flame-retardancy and toughness at low additive loads (only 3.5 wt%), and they thus showed potential applications in large-scale production.

Conclusions
In this work, the flame-retardant and toughened PLA biocomposites were prep successfully by introducing low additive loads of bio-based MKF and NTPA. Only wt% additive load of NTPA enabled the PLA/MKF/NTPA biocomposites to pass the 94 V-0 rating, and the LOI value of the PLA/MKF/NTPA biocomposites increased 19.2% to 28.3%. In addition, the av-EHC of the PLA3.0 biocomposites declined by 9 and their char residue increased significantly. The excellent fire safety of the PLA bio posites was attributed to the gas-phase quenching effect of the phosphorus-and benz containing radicals of NTPA as well as the barrier effect of char residues induce NTPA. In addition, the homogenously dispersed lightweight MKF enabled the im strength of the PLA/MKF/NTPA biocomposites to obtain an 18.8% increase over neat This work presented a simple, low-cost way for the preparation of toughened and fl retardant PLA biocomposites at low additive loads, and it has great potential for emer fields.

Conclusions
In this work, the flame-retardant and toughened PLA biocomposites were prepared successfully by introducing low additive loads of bio-based MKF and NTPA. Only a 3.0 wt% additive load of NTPA enabled the PLA/MKF/NTPA biocomposites to pass the UL-94 V-0 rating, and the LOI value of the PLA/MKF/NTPA biocomposites increased from 19.2% to 28.3%. In addition, the av-EHC of the PLA3.0 biocomposites declined by 9.8%, and their char residue increased significantly. The excellent fire safety of the PLA biocomposites was attributed to the gas-phase quenching effect of the phosphorus-and benzene-containing radicals of NTPA as well as the barrier effect of char residues induced by NTPA. In addition, the homogenously dispersed lightweight MKF enabled the impact strength of the PLA/MKF/NTPA biocomposites to obtain an 18.8% increase over neat PLA. This work presented a simple, low-cost way for the preparation of toughened and flame-retardant PLA biocomposites at low additive loads, and it has great potential for emerging fields.
Author Contributions: Conceptualization, methodology, K.X. and L.L.; investigation, data analysis, K.X., C.Y. and C.D.; formal analysis, K.X., Y.X. and B.L.; resources, L.L.; writing-original draft preparation, K.X.; writing-review and editing, Y.X. and L.L.; supervision, project administration, funding acquisition, Y.X., B.L. and L.L. All authors have read and agreed to the published version of the manuscript. Institutional Review Board Statement: Not applicable.

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