Ammonium Polyphosphate Intercalated Yttrium-Doped Layered Double Hydroxides to Enhance the Thermal Stability and Flame Retardancy of Poly(Lactic Acid)

The flammability of the biodegradable plastic PLA limits its application in industrial fields with high flame-retardant requirements. This paper provides a novel strategy for constructing refractory and thermostable PLA composites using layered double hydroxides (LDHs) chemically modified with ammonium polyphosphate (APP). XRD, FT-IR, SEM-EDS, and TEM confirm that the goal of LDHs has been successfully prepared. The thermal stability and combustion behavior of PLA composites were evaluated by the thermogravimetric analysis (TGA) and cone calorimetry tests (CCT). The crystallization behavior and tensile performances were also examined. The results showed that the incorporation of 15wt% MgAlY-APP-LDHs practically makes the PLA composites reach the UL-94 V-0 grade. There were 43% and 20% reduction in the PHRR and THR of PLA/15APP-LDHs respectively due to the catalytic effect of Y elements and barrier effects of LDHs, which was a major performance against fire hazards. Furthermore, the increase in crystallinity and the decrease in mechanical strength of PLA composites are attributed to the nucleation of LDHs. In short, this research introduces the production of multifunctional PLA composites through APP intercalation of LDHs, which are deemed as prospective candidates for the next generation of sustainable plastics products.


Introduction
The global plastics market is expected to grow at a rate of 5% per year. The excessive use of petroleum-based plastics has caused the problem of "white pollution" and has seriously threatened the Earth's ecological environment. To meet the growing demand for plastics, the effective strategy involved the production of biodegradable plastics. PLA is a biodegradable polymer with favorable biocompatibility, which can be obtained from renewable resources. Therefore, PLA is considered a potential substitute for petroleum-based plastics [1][2][3]. In addition, as one of the most promising "green plastics," PLA is widely used in packaging, agricultural, biomedicine, and other fields on account of its excellent mechanical properties, high pellucidity, easy processing, etc. [4][5][6], while the poor flame retardancy of PLA seriously restrains its applications in emerging fields such as electronic devices, automobiles, and building materials [7,8]. To circumvent this problem, adding flame retardants to PLA is an effective strategy to combat fire risk [9][10][11].
To date, halogen-based flame retardants show a negative impact on the environment due to their toxic combustion products, and these side effects have limited its application [12].Phosphorus-containing compounds are efficient flame retardants for polymers. However, it is expected to cause a decrease in mechanical properties [13]. Metal-hydroxides are flame retardants used in various merchant products. The endothermic decomposition of metal-hydroxides can decrease the combustion performance of flammable materials and release water into the vapor phase to dilute the blaze [14]. Their main disadvantages are low flameretardant efficiency and poor compatibility with common polymers. Thus, excessive addition will lead to the deterioration of the mechanical characters of the material.
With the development of nanotechnology, huge opportunities are provided for flame-retardant materials [15][16][17]. Compared with traditional flame retardants, the impressive effect of nanosized additives is to significantly inhibit the HRR of the polymer. As a consequence, more attention has concentrated on finding effective nanofillers to improve different properties of the polymer. The most widely used nanomaterials are carbon nanotubes [18], graphene [19,20], nanooxides [21], and nanoclays [22]. They change the degradation pathway of the polymer and form a highperformance char layer, which will restrict the passage of heat through the underlying material [23][24][25]. Other nanoparticles that show a unique effect on polymers are LDHs. LDHs is considered an emerging layered inorganic nanofiller, LDHs may raise the flame retardancy of polymer material owing to their flexible chemical components and distinct structure.
APP is well known for its nontoxic, environmental protection, and high-performance [40,41]. Although APP has good expansion properties, their flame-retardant effect is not ideal. The thick layer of APP formed during combustion is easily separated from the polymer matrix, so we need to find new ways to balance the flame retardancy and other properties. In previous research, APP and LDHs were simply mixed together physically. Several studies have confirmed that APP intercalating LDHs can further enhance the fire retardancy of resins. For instance, Gao et al. [42] used a solvent mixing method to intercalate APP into LDHs, and compounded with zinc borate (ZB) to prepare PP/APP-LDHs/ZB composites. The results showed that the PHRR of the PP containing 20 wt% APP-LDHs and 2 wt% ZB was significantly reduced by 58%. Liu et al. [43]. embedded APP and dye Acid Red 88 (AR88) together in the LDH interlayer. When the LDHs loading was 25 wt%, the composites' PHRR was remarkably reduced by 63%. In view of this, a new strategy for APP intercalation containing rare earth LDHs is proposed, as shown in Figure 1. Subsequently, LDHs were introduced into the PLA to improve its fire retardancy.

Synthesis of the MgAlY-APP-LDHs.
The MgAlY-APP-LDHs were synthesized via the traditional hydrothermal approach. For MgAlY-APP-LDHs (Mg : Al : Y = 20 : 9 : 1), the preparation steps were as follows: Firstly, disperse APP (50 g) in 100 mL deionized water to make suspension A. Solution B was prepared by dissolving MgCl 2 (20 mM), AlCl 3 (9 mM) and YCl 3 ·6H 2 O (1 mM) in 100 mL deionized water. Subsequently, solution B was added dropwise in suspension A under continuous stirring, and the pH value of the mixture was controlled at around 10 by NaOH (4 M). The mixed solution was transferred to an autoclave and hydrothermally reacted at 100°C for 24 hours. Afterward, the product was washed alternately in ethanol and deionized water to a pH of 7 and finally dried in a vacuum box at 80°C for 12 hours to obtain the MgAlY-APP-LDHs.

Preparation of PLA/MgAlY-APP-LDH composites.
Both PLA and MgAlY-APP-LDHs were vacuum dried at 70°C for 12 h; this step is critical because PLA moisture level affects its properties and can cause its degradation during compounding. The melt blending process used a twinscrew extruder; the rotor operates at a rate of 40 rpm to achieve a stable PLA melt flow. The concentration of MgAlY-APP-LDHs in the PLA composites was specified as 0, 5, 10, 15, 20, or 25 wt%. For the sake of simplicity, PLA/ 5APP-LDHs represent the PLA composites containing 5 wt% MgAlY-APP-LDHs.
where ΔH 0 m was the enthalpy melting of pristine PLA with a value of 93 J/g and φ was the quality score of PLA [44].

Mechanical
Properties. Tensile mechanical testing was conducted using the CTM 6103 testing machine (MTS, China) in the light of GB/T 1040.1-2006 to measure the tensile properties of the composites. The specimen dimensions were 75 × 5 × 2 mm 3 , all tensile tests were carried out with 1 mm/min.

Results and Discussion
3.1. XRD of LDHs. Figure 2 shows the XRD patterns of pure APP and MgAlY-APP-LDHs. An obvious (003) characteristic diffraction peak at 2θ = 9:68°was observed in the singleintercalation APP-LDH pattern; the result indicated that a typical layered structure is formed. According to the formula 2d sin θ = nλ (λ = 0:154056 nm, n = 1), the resultant    [45]. By comparing the XRD patterns of APP and LDHs, it can be found that no characteristic peaks of APP were observed in LDHs, which also confirms that APP molecules were successfully inserted into the interlayer of LDHs. In other words, APP is not simply physically mixed with LDHs.
3.2. FT-IR of LDHs. This part discusses the FTIR characterization of products ( Figure 3). The wide absorption band observed around 3400 cm −1 -3500 cm −1 corresponds to the stretching vibration of the O-H group by water molecules, thus confirming the presence of the hydroxyl groups on LDHs. The peak associated with H-OH bending appears at 1643 cm -1 , which is also due to the interlayer of H 2 O. Furthermore, the bands appear at 1141, 907, and 1440 cm -1 due to the vibration of the P-O and N-H groups, respectively, indicating the existence of phosphate in the LDHs. The characteristic peaks of LDHs are well displayed, and all the spectroscopic results confirmed that APP molecules are not decomposed and successfully entered the interlayer of LDHs. Figure 4(a) displays the surface morphologies of the LDHs. The LDHs are plate-like nanoparticles with a dimension of 100-300 nm; the relatively small LDHs nanoparticles facilitate their sufficient dispersion in the polymers and raise compatibility with polymers. Beyond this, the embedding of APP will not change the shape and structure of LDHs, which further proves that the target LDHs has been prepared successfully. This agreed with the outcomes from previous studies [46]. show that pure APP begins to decompose at around 300°C, and two obvious weight-loss peaks can be observed at 327°C and 583°C. LDHs decompose at a lower temperature than APP; the interlayer moisture will be lost when the tempera-ture is close to 200°C. The temperature continues to rise to about 200~450°C; it will remove the hydroxyl (-OH) on the laminate, and its layered structure will be destroyed simultaneously. When the temperature continues to rise to about 200~450°C, the hydroxyl (-OH) on the laminate will be removed, and the layered structure will be destroyed simultaneously. When the temperature is greater than 450°C, LDHs are completely decomposed into metal oxides and absorb a lot of heat, which reduces the surface temperature of the polymer. Furthermore, the metal oxides will adsorb toxic gases due to their high surface area and large porosity. Meanwhile, the expanded char layer formed by the reaction of metal oxides and polymer degradation products can block the intrusion of heat and achieve the efficient flame retardant. Table 1 lists the T i , T max , and the char residual percentage of PLA and its composites to comprehensively evaluate their relative thermal stability. Compared with PLA, the T i of PLA/5APP-LDHs composites has increased by 7°C, while the T i of the composites loaded with 25 wt% APP-LDHs has been greatly reduced. The reason for this phenomenon may be that as the loading of LDHs increases, it will promote and catalyze the degradation of PLA in the initial stage. Besides, the char layer formed by LDHs slows down the heat release rate of PLA, thereby protecting the PLA matrix from flame degradation. As for T max , all PLA/APP-LDH composites increased; the introduction of 25 wt% LDHs increases the T max from 362.76°C to 368.0°C.However, T HRI will decrease at high filling volume, so it is a challenge to make the polymer achieve high-efficiency flame retardancy with a low addition of LDHs. Additionally, PLA displayed one-step degradation within 307°C~377°C, and the maximum

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flame-retardant requirements. For the designed APP-LDHs, the intercalation effect of APP is beneficial to the interaction between LDHs and PLA matrix, so that LDHs act as a barrier in the condensed phase, which are reflected in the increase in char residue. At the same time, APP generates phosphorus-containing free radicals (PO· and HPO·) in the gas phase, which will interfere with the combustion of PLA and quickly take away the heat of combustion. Therefore, the synergistic effect of these two components in the two stages enables PLA composites to obtain good flame retardancy in the UL-94 tests.

Cone Calorimeter Tests.
CCT is used to assess the fire retardancy of composites [36]. Figures 6(a)-6(d) display the HRR, THR, TSP, and residual mass curves of PLA and its composites. Moreover, several major data such as TTI, PHRR, and THR are shown in Table 2. Table 3 lists pure PLA with a high PHHR value of 457 kW/m 2 , which reflects its poor fire retardancy. Particularly, PLA/15APP-LDHs show a high PHRR reduction value up to 43%. In addition, compared with pure PLA (76 MJ/m 2 ), PLA loaded with 15 wt% APP-LDHs can reduce the THR value to 61 MJ/m 2 . The decrease of THR indicates a great majority of organic structures in the PLA participated in the carbonization  Advances in Polymer Technology process and remained in the condensed phase instead of being converted into "fuel" in the gas phase, which is consistent with the TGA results. Moreover, Figure 7(c) clearly shows that the TSP of PLA/15APP-LDHs composite has a significant decrease compared to pure PLA. The possible reason is that the introduction of LDHs reduces the volatilization of organic matter and inhibits the generation of smoke [51]. As for residual mass, it was found that this parameter was increased with the addition of APP-LDHs. The addition of 15 wt% APP-LDHs can make the char residue of the composite material reach about 18%. In general, the CCT results indicate that APP-LDHs effectively prevent the degradation of PLA material and improve its flame retardancy. The flame-retardant mechanism can be explained from the condensed phase; APP first thermally decomposes to generate polyphosphoric acid and ammonia and then interacts with -OH or hydroxyethyl cyanurate to form phosphate ester [52]. After the phosphate ester is dehydrated, a layer of char foam and viscous molten layer will be formed on the polymer surface to suppress thermal oxygen exchange [53,54]. Meantime, magnesium oxide and aluminum oxide formed after thermal decomposition will accumulate on the surface of PLA to play a role in suppressing the flame.
FRI [55] (flame-retardant index) was a major parameter used to measure the fire retardancy of polymers according to CCT data, which has been calculated by the following formula.

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In summary, FRI < 1 was regarded as the low level of flame retardancy, and 1 < FRI < 10 was regarded as the high-level flame-retardant performance. The FRI value of adding 15 wt% APP-LDHs was 1.63, indicating the flame retardancy of PLA/15APP-LDH composites has the best performance.
3.6.3. Residue Analysis. The analysis of residual char will provide crucial information about the flame-retardant mechanism. Figure 8(a) shows that PLA is almost completely burned out with no residual char. A thin char layer was formed on the surface of PLA by the introduction of LDHs, and the residual mass increased significantly with the increase of APP-LDH content.
Besides, it can be clearly seen from the SEM of the char residue that the pores and cracks of the char layer were significantly reduced after adding LDHs, and the surface morphology was more homogeneous and denser. It may be due to the fact that the catalysis of the Y element helps to form a high-performance carbonized layer [56,57]. The preferred char layer exerts a certain isolation effect during the pyrolysis process of the PLA composites and suppresses the free exchange of flammable volatiles. Significantly, the char layer of PLA/15APP-LDH composite has a denser and more complete microstructure than others (see Figures 9(b)-9(f)), which is more helpful to slowing down combustion and inhibiting the release of heat and smoke [58]. Thus, PLA/15APP-LDHs obtained satisfactory fire resistance.
3.7. DSC of PLA and Its Composites. The DSC test described the crystallization and melting behavior of PLA and PLA composites. Table 4 displays that the T g of pure PLA is 55.50°C. The PLA composites exhibit a single T g indicating excellent thermodynamic compatibility between LDHs and PLA substrate. The T g of PLA composites has increased due to the introduction of LDHs, which means that the temperature range in which PLA is applied has been expanded. The T cc of PLA is 120.20°C, and there is a wide cold crystallization zone at 110~140°C. The T cc of PLA composites is significantly lower than that of PLA. We further confirmed that LDHs as nucleating agents promote the formation of ordered crystal structure of PLA. It can be observed from Figure 10 that the T m of PLA composites obviously moved to the higher temperature. The increase in T m demonstrates that the capability of the PLA molecular segment to move is possibly enhanced thanks to the presence of LDHs. Interestingly, two T m peaks appeared after the introduction of LDHs; the new peaks moved to higher temperature and became stronger with the amount of LDHs increased. It was illustrated that LDHs played a nucleation role and accelerated the transformation of disordered α′ crystal to a more perfect α crystal construction in PLA, causing the increase of the χ c of PLA [59]. Thus, the χ c of PLA/15APP-LDHs was improved from 14.23% of PLA to 37.62%. As we all know, the mechanical properties of semicrystalline polymers are affected by crystallinity [60,61]. It is essential to inspect the influence of LDHs on the mechanical properties of PLA composites, as discussed below.
3.8. Mechanical Properties of PLA and Its Composites. Figure 11 displays the tensile strength of pure PLA that is 44.26 MPa and the elongation at break that is only 21.34%. There is no yielding behavior, implying its typical strong and brittle characteristics. In the initial stage, the addition of LDHs causes the decreased tensile strength and elongation at break. The possible reason is that the LDH nanosheets incline to agglomerate in the PLA matrix, and agglomerates act as stress concentration points leading to brittleness. Although with the continuous increase of LDHs, the mechanical properties of composite materials have rebounded slightly, it is still difficult to maintain the original performance, which is also one of the urgent problems that

Conclusions
In the research, the nanoflame-retardant MgAlY-APP-LDHs were successfully prepared by a simple hydrothermal method and fully characterized. APP-LDHs endowed PLA composites with outstanding fire safety and crystallization performance. PLA/APP-LDH nanocomposites were prepared by melt blending, and their thermal stability, flame retardancy, and mechanical properties were systematically studied. TG results showed that APP-LDHs enhance the T i , T max , and char residue of PLA, which confirmed that APP-LDHs have a positive effect on the thermal stability of PLA composite. In addition, only 15 wt% APP-LDHs improved flame retardancy of PLA and made PLA pass the UL-94 V-0 grade. The PHRR and THR of PLA/15APP-LDH composite dropped by 43% and 20% compared with pure PLA. This was attributed to the cooperative working mechanisms of APP and LDHs in both condensed and gas phases. Furthermore, the nucleation of APP-LDHs increased the crystallinity of PLA while slightly affecting the tensile strength of PLA. This work supplied a new strategy for the design of heat-resistant and fire-retardant PLA nanocomposites, which held great potential in various realistic applications.

Data Availability
The data used to support the findings of this study is included within the article.

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.