Flame-retardant and tough poly(lactic acid) with well-preserved mechanical strength via reactive blending with bio-plasticizer and phosphorus derivative

/10E/3D achieves a vertical burning (UL-94) V-0 classification and a limiting oxygen index (LOI) of 27.5 %, indicative of satisfactory flame retardancy. Compared with PLA, PLA/10E/3D maintains high thermal stability and shows significantly enhanced UV-protecting and soil degradation properties. Therefore, this work delivers a green and scalable reactive processing method to create flame-retardant, tough yet strong bioplastics with improved soil decomposition and UV resistance, which contributes to sustainable development.


Introduction
The ubiquitous applications of non-degradable petroleum plastics have brought about the depletion of oil resources and increasingly severe environmental problems.In response to sustainable development strategies, the development of biodegradable bioplastics have gained more and more attention in recent years, such as poly(butylene adipateco-terephthalate) (PBAT) [1,2], polybutylene succinate (PBS) [3], and PLA [4,5].Among them, PLA has realized practical applications because of its high mechanical strength, biocompatibility, and wide raw material sources.However, the poor toughness and intrinsic flammability of PLA have obviously limited their high-tech applications [6].
Considering green development, the common strategy to increase the toughness of PLA is to blend it with bio-based flexible polymers or plasticizers.For instance, Ding et al. [7] introduced 5 wt% of polycaprolactone (PCL) into PLA to fabricate a tough PLA composite.Their results showed that the notch impact strength of the resulting PLA composite can be up to 251 kJ/m 2 .Recently, Liu et al. [8] reported toughened PLA (PLA/PBAT/DCP/PEG600DA) composites by blending PLA with dicumyl peroxide (DCP), poly(ethylene glycol) 600 diacrylate (PEG600DA) and PBAT.The impact strength and elongation at break of PLA/PBAT/DCP/PEG600DA reached 44.2 kJ/m 2 and 290 %, respectively.Although these PLA systems feature superior toughness and high biomass content, they usually suffer from low tensile strength and poor flame retardancy, which reduces their practical values.Introducing phosphorus derivatives into PLA has been considered a promising approach to address the flammability issue of PLA due to their low smoke and toxicity, environmental friendliness, and high flame-retardant efficiency [9][10][11][12][13][14].For example, aluminum hypophosphite (ALHP), ammonium polyphosphate (APP), DOPO, etc., have been utilized in the fire-resistant PLA, but as-prepared PLA systems showed unsatisfied mechanical performances because the phosphorus-derived flame retardants will catalyze the decomposition of PLA in the melt-blending process.Therefore, efforts should be intensified to the creation of high-performance PLA combining superior toughness and flame retardancy.
Recently, some toughened and flame-retardant PLA systems have been reported, but most of them involve complex synthesis processes and require large amounts of organic solvents and expensive petroleumbased raw materials, which violates ecologically sustainable development [15][16][17][18][19][20][21][22].Therefore, we chose the low-cost, bio-based epoxy soybean oil (ESO) as a toughening agent [23][24][25][26][27][28], and the commercial phosphorus-derived DOPO as a flame retardant to develop toughened and flame retardant PLA bioplastics by simple reactive blending in this work.Since PLA contains the terminal -COOH group, ESO contains the epoxy group, and DOPO possesses the P-H group, they can undergo in-situ grafting reaction during melt blending to ensure the homogeneous dispersion of ESO and DOPO in the PLA matrix.As-fabricated PLA/10E/3D with 10.0 wt% of ESO and 3.0 wt% of DOPO exhibits superior toughness, with impact strength and elongation at break of 6.7 kJ/m 2 and 106.3 %.Additionally, it achieves enhanced flame retardancy with a UL-94 V-0 rating, with an LOI of 27.5 %.Because of the covalent linkages between PLA, ESO, and DOPO, PLA/10E/3D displays comparable thermal stability to PLA.The introduction of DOPO and ESO brings about the enhanced UV-shielding and soil-decomposition performances of PLA/10E/3D.Hence, this work provides a feasible and eco-friendly strategy for creating high-performance, flame-retardant, tough PLA bioplastics with enhanced soil degradation and UV protection, and well-preserved thermal stability and mechanical strength, which can find wide applications in modern industries.

Fabrication of PLA composites
Firstly, the PLA granules were dried in a vacuum at 80 • C for 12 h to remove moisture.The dried PLA granules were added to the torque rheometer, followed by ESO, and then continuously mixed for 10 min at 200 • C and 70 rpm.DOPO was then introduced, and blended for 10 min.The resultant solid products were heat-compressed at 200 • C under a pressure of 14 MPa into different shapes according to the testing specimen size requirements.The sample is named PLA/xE/xD, where E, D and x represents ESO, DOPO, and their mass ratio, respectively.The fabrication procedures of PLA, PLA with 10.0 wt% ESO (PLA/10E), and PLA with 3 wt% DOPO (PLA/3D) were identical to that of PLA/xE/xD.The detailed formulations are tabulated in Table 1.

Characterization
The differential scanning calorimetry (DSC) was carried out by a DSC Q600 apparatus (TA Instruments, USA) at a ramp rate of 10 • C/min from 30 to 250 • C. Fourier transform infrared (FTIR) spectrometry was carried out using a Nicolet 6700 Fourier transform spectrometer (Thermo Fisher, USA) with potassium bromide pellets.Liquid nuclear magnetic resonance (NMR) spectroscopy was recorded on Bruker Advance III MHz spectrometer (Bruker Corporation, Switzerland) at room temperature using CDCl 3 as solvent containing 0.03 % v/v tetramethylsilane (TMS).According to ASTM D638, the tensile stress-strain plots were investigated on an INSTRON 5966 universal test machine (USA) at a crosshead speed of 5 mm/min.To ensure data reliability, the reported data were the average of at least five specimens.For the notched Izod impact test, it was undertaken on a LIANXIANG impact testing machine (China) based on GB/T 1843-2008.The scanning electron microscopy (SEM) observation was undertaken at 15 kV on an EOL JSW-5510LV instrument (Japan), which was coupled with an energy dispersion Xray (EDX) spectrometer.The specimens need to be gold-plated before SEM observation.The thermogravimetric analysis (TGA) was carried out on a NETZSCH STA449F3 facility (Germany).During this test, 5.0-10.0mg of the powdered specimen was heated from 30 • C to 800 • C at a constant rate of 20 • C/min under N 2 condition.The optical performances of PLA films with the size of 50 mm × 50 mm × 0.2 mm were studied using a Lambda35 UV spectrometer (PerkinElmer, USA).The UV-protection factor (UPF) of the film was specifically calculated according to GB/T 18830-2009.The soil degradation behaviors of PLA and PLA/10E/3D were investigated by burying them in a 7.0 cm deep soil with a green plant and watering every three days (water volume: mL).The gel permeation chromatography (GPC) and SEM observation were conducted on original specimens and samples after 30 and 75 days of burial.The GPC measurement was conducted via a Waters 1525 GPC facility (USA) with an elution agent of CHCl 3 and a flow rate of 1.0 mL/ min.The limiting oxygen index was recorded on a Jiangning JF-3 equipment (China), and the sample was 100 mm × 6.5 mm × 3.0 mm in size.The vertical combustion test was performed on a Jiangning CFZ-3 facility (China), and the specimen size was 120 mm × 13 mm × 3.2 mm.The FTT cone calorimeter (UK) was applied to investigate the burning beheviours of PLA specimens (size: 100 mm × 100 mm × 3.0 mm) at a flux of 35 kW/m 3 .Raman spectroscopy was undertaken on a DXR laser Raman spectrometer (Thermo Fisher, USA) under 532 nm laser excitation.X-ray photoelectron spectroscopy (XPS) was undertaken by an ESCALAB XI + apparatus (Thermo Fisher, USA) with Al Ka radiation source.Thermogravimetric-infrared spectroscopy (TG-IR) was undertaken by a TGA2 equipment (METTLER TOLEDO, Switzerland) linked to a TENSOR II facility (BRUKER, Germany), and the testing condition was the same as that of the above TGA test.

Interphase reaction
As shown in Fig. 1a, during melt blending, PLA with terminal carboxyl group firstly reacted with ESO containing epoxy group to generate an intermediate (PLA/E), and then the residual epoxy group in the PLA/E intermediate reacted with the P-H group in DOPO to form the final PLA/xE/xD bioplastic.The torque variation of PLA and PLA/10E/ 3D during melt-blending is displayed in Fig. 1b.The sudden increase in the PLA torque curve was due to the addition of the PLA pellets, and then the torque gradually decreased and tended to equilibrium, indicating that the complete melting of the PLA particles.In the PLA/10E/3D torque variation curve, the addition of ESO and PLA also led to the increased torque.After that, the torque decreased significantly due to the system viscosity reduction.Then, the grafting reaction between PLA and ESO led to a sudden increase in torque.After the addition of DOPO, the torque fluctuated slightly, which was probably because of the ring-

Table 1
The formulations of different PLA samples.

Mechanical performances
The mechanical properties of PLA samples were characterized in detail, as shown in Fig. 2a-e and Table S1.The tensile strength and elongation at break of PLA are 67.9MPa and 3.9 % (see Fig. 2a-c and Table S1), respectively, showing high mechanical strength but poor toughness.Grafting with ESO and DOPO significantly enhances the toughness of PLA/xE/xD composites and maintains high strength (see Fig. 2d).Among PLA/xE/xD composites, the PLA/10E/3D sample shows the highest elongation at break reaching 106.3 %, which is increased by 26.3 times relative to that of PLA.In addition, the tensile strength of the PLA/10E/3D sample is 52.8 MPa.Notably, both tensile strength and elongation at break of the PLA/10E/3D sample are much higher than those of PLA/10E and PLA/3D.This may be because ESO as a bioplasticizer enhances the toughness, and DOPO as a rigid flame retardant improves the robustness.The enhanced toughness of PLA/xE/xD can also be confirmed by the increased impact strength, as shown in Fig. 2e.Specifically, the impact strength of PLA/15E/3D is 8.1 kJ/m 2 , which is 102.5 % higher than 4.0 kJ/m 2 of PLA.To further evaluate the toughness of our PLA/10E/3D (passing a UL-94 V-0 rating, see 'Flame retardancy' section), its elongation at break and impact strength are compared with those of previous flame-retardant PLA with a UL-94 V-0 rating in Fig. 2f and Table S2 [6,12,15,29,[31][32][33][34][35][36][37][38].Obviously, our PLA/10E/3D exhibits much higher elongation at break and impact strength than previously reported flame-retardant PLA systems.Considering the superior toughness and simple reactive blending process, our PLA/10E/3D outperforms its counterparts.
The toughening mechanism of PLA/xE/xD was investigated by SEM and EDX, with the results shown in Fig. 2g-l and Table S3.The crosssection of PLA obtained from the notched Izod impact test is smooth with many regular stripes.However, both PLA/7.5E/3D and PLA/10E/ 3D exhibit wrinkled fracture surfaces, with obvious shear bands and microcracks.The formation of such a wrinkled cross-section under the action of external forces contributes to energy dissipation, bringing about enhanced toughness.Meanwhile, the elemental mapping photos of PLA/10E/3D in Fig. 2j-l confirm that the C, O, and P elements evenly distribute within the fracture surface.Thus, the in-situ grafting reaction between PLA, ESO, and DOPO enables the PLA/10E/3D bioplastic to form a homogeneous polymer system, which is critical for toughness enhancement.When subjected to external forces, the ESO part within PLA/10E/3D will induce plastic deformation of the matrix, manifested as the generation of shear bands and microcracks, ultimately consuming the fracture energy and thus improving toughness [39][40][41].

Thermal stability
The thermal stability of PLA samples under nitrogen atmosphere was evaluated by TGA.The thermogravimetric (TG) and derivative thermogravimetric (DTG) diagrams are shown in Fig. 3a and b.All DTG diagrams show only one peak, indicating that the PLA samples undergo a single thermal decomposition stage.As shown in Table 2, the temperature at 5 % mass loss (T 5% ) of PLA is 314.2 • C, while those of PLA/ xE/xD samples are also around 314.0 • C, indicating that covalently grafting with ESO and DOPO does not damage the thermal stability of the PLA matrix.The heat-resistance index (T HRI ) values of PLA/xE/xD are also close to that of PLA, further indicative of their comparable thermal stability.Generally, the phosphorus-containing flame retardants will catalyze the decomposition of the PLA matrix during the melt-blending process, resulting in poor thermal stability [42,43].In this work, the DOPO content is low, and ESO promotes the uniform dispersion of DOPO in the PLA matrix and produces grafting reactions, thus counteracting the negative effects of DOPO, so effectively maintaining thermal stability.In addition, the temperature at maximum weight loss rate (T max ) and char yield at 800 • C (Y c ) are both increased for PLA/x-E/xD samples compared to PLA, demonstrating that introducing ESO and DOPO allows PLA to achieve enhanced high-temperature stability and char-formation performances.Hence, the PLA/xE/xD samples maintain their thermal stability and promote carbonization capabilities compared to PLA.

UV-shielding properties
The UV resistance of PLA films (thickness: ~0.2 mm) was investigated by UV spectrophotometer, with the UV plots presented in Fig. 3c.PLA shows high transmittance in the UV-B (280-320 nm) and UV-A (320-400 nm) ranges [44], indicating poor UV resistance.By contrast, the PLA/xE/xD bioplastic exhibits improved UV-shielding properties, in particular the transmittance of PLA/10E/3D is reduced to approximately 0 % in the UV-B range.At lower wavelengths (220-320 nm), PLA/xE/xD samples exhibit similar UV resistance due to their close DOPO content.As the wavelength increases (320-400 nm), the UV light is more easily transmitted through the PLA/xE/xD samples.Due to the low content of ESO, there is a small amount of DOPO in PLA/7.5E/3Dsamples does not reactive with ESO and thus tends to aggregate, which   [45].Therefore, the PLA/xE/xD system with enhanced UV resistance is expected to be widely used in outdoor scenes.

Soil degradability
Due to its biodegradability and environmental friendliness, PLA has become the most commercialized bioplastic [46].Unfortunately, the soil degradability of PLA is relatively slow [47,48].According to previous work, the addition of some plasticizers can increase the degradation rate of PLA in soil [49].To investigate the effects of ESO and DOPO on the soil degradation of PLA, soil burial experiments were conducted on both PLA and PLA/10E/3D specimens, and their structure and molecular weight variations after different time of burial were studied by GPC and SEM techniques (see Fig. 3e-l and Table S4).As shown in Fig. 3e and f, both weight average molecular weight (M w ) and number average molecular weight (M n ) of PLA and PLA/10E/3D are decreased with increasing burial time.Notably, the M w and M n of PLA/10E/3D are much lower than those of PLA on the 75th day, indicative of the enhanced soil degradation.Obviously, the introduction of ESO promotes the degradation of the PLA matrix when it is buried in soil.The enhanced soil degradation of PLA/10E/3D can also be confirmed by the SEM images in Fig. 3g-l.As presented in Fig. 3g-i, PLA maintains its smooth surface even after being buried for 75 days.However, the PLA/10E/3D surface becomes rough, and some cracks can be detected within the surface after being buried for 75 days (see Fig. 3j-l).Such a result further demonstrates the improved soil degradation of PLA/10E/3D relative to PLA.Hence, reactive blending with ESO and DOPO can also enhance the biodegradation of PLA, which promotes sustainable development.

Flame retardancy
The LOI values and UL-94 classifications of PLA samples are listed in Fig. 4a and Table 3. PLA is a highly flammable material, reflected in its low LOI of 19.5 % and severe dripping and violent combustion in UL-94 testing.With the addition of 3 wt% DOPO, the PLA/3D sample achieves the UL-94 V-0 rating, and its LOI is increased to 26.0 %, indicative of the high flame-retardant efficiency of DOPO.However, the PLA/3D sample suffers from poor mechanical properties (See Table S1) because of the catalytic degradation effect of DOPO, which significantly restricts its practical application.Introducing ESO cannot enhance the flame retardancy of PLA, and thus the LOI of the PLA/10E sample is only 19 %, and it cannot achieve any rating in the UL-94 test.When simultaneously incorporating ESO and DOPO, all PLA/xE/xD samples pass the UL-94 V-0 rating, and their LOI values range from 26.2% to 29.4 %, which are higher than those of PLA, PLA/3D, and PLA/10E samples.The P-H group of DOPO in PLA/3D will catalyze the decomposition of PLA, which affects the LOI value.In PLA/7.5E/3D,ESO consumes the P-H group of DOPO by ring-opening reaction, which avoids the PLA matrix degradation caused by the introduction of DOPO.Hence, the LOI of PLA/7.5E/3D is higher than that of PLA/3D.When ESO content is gradually increased, there will be some unreacted ESO molecules remained in the PLA matrix.ESO, as a flammable chemical, will lower the LOI of PLA/E/D.In conclusion, LOI decreases gradually with increasing ESO content, and the PLA/15E/3D has the lowest LOI value.Thus, reactive blending with ESO and DOPO not only effectively enhances the toughness of PLA, but also endows it with improved flame retardancy.
The cone calorimetry is the most representative test method to evaluate the combustion behaviors of materials in a real fire environment, and the corresponding results of PLA and PLA/xE/xD samples are shown in Table S5 and Fig. 4b and c.The time to ignition (TTI) values of three PLA/xE/xD samples are about 60 s, which are comparable to that of PLA, which is consistent with the TGA result.All PLA/xE/xD samples show reduced peak heat release rate (PHRR) and total heat release  The results demonstrate that the gas-phase combustion degree of PLA/10E/3D is lower than that of PLA, which is mainly due to the quenching radical effect of DOPO.The char yield at 500 s (CY) of PLA/10E/3D is 5 times higher than that of PLA, indicative of its condensed-phase flame retardancy.Obviously, PLA/10E/3D feature significantly enhanced flame retardancy due to its reduced gas-phase burning degree and enhanced char-forming ability.

Char residue analysis
The condensed-phase flame-retardant mechanism of PLA/xE/xD was first studied by the Raman technique, with the spectra shown in Fig. 5a.In the spectra, the D peak represents the disordered carbon structure, and the G one represents the ordered carbon structure, and the area ratio of D peak to G peak (I D /I G ) is inversely proportional to the char graphitization degree [30,50] The elemental contents and bonding states of PLA and PLA/10E/3D chars were studied by XPS in Fig. 5b-f and S3 and Table S6.The C and O elements can be found in the char of PLA (see Fig. 5b and S3), and the C, O and P elements can be detected in the residue of PLA/10E/3D (see Fig. 5c).In the C1s spectrum of PLA/10E/3D char, the peaks at 287.5, 285.9 and 284.3 eV are attributed to the C --O, C-O-P, and C-H/C structures, respectively (see Fig. 5d).In the O1s XPS spectrum of PLA/ 10E/3D char, the peaks at 533.8, 533.1 and 531.1 eV are assigned to the C-OH, C-O/P and C --O/P bonds, respectively (see Fig. 5e).In the P2p XPS spectrum of PLA/10E/3D char, the peaks at 134.1 and 133.8 eV belong to the P-C and P --O bonds, respectively [36,54].In addition, according to the XPS results of the PLA/10E/3D char, the phosphorus content is 2.04 wt% (see Table S6), which also indicates that DOPO plays an important role in the char formation in the condensed phase.These results confirm that the pyrolysis products of DOPO participate in the char-forming reaction of the PLA matrix in the combustion procedure, thus improving the flame retardancy of PLA/10E/3D.

TG-IR analysis
The TG-IR test of PLA and PLA/10E/3D was conducted in a nitrogen atmosphere, with the spectra shown in Fig. 6a-f.The gaseous products of PLA and PLA/10E/3D samples are identical in the TG-IR tests (see Fig. 6a-e), including water (3576 cm − 1 ), hydrocarbons (2732 and 1370 cm − 1 ), CO 2 (2356 cm − 1 ), CO (2170 cm − 1 ), carbonyl compounds (1762 cm − 1 ), fragments with C-O group (1238 cm − 1 ), and aliphatic ester (1100 cm − 1 ) [55][56][57].Compared with PLA, PLA/10E/3D shows lower intensities of these absorption peaks (see Fig. 6c and f), indicating that DOPO suppresses the thermal decomposition of PLA.Generally, the phosphorus-containing fragments of DOPO capture the active radicals, thus reducing the degree of gas-phase combustion [52].Due to the low phosphorus content, it is hard to detect the phosphorus-based fragments during the TG-IR test of PLA/10E/3D.However, according to the AEHC results, DOPO effectively suppresses the gas-phase combustion degree of PLA composite.In summary, the P-containing decomposition products of DOPO catalyze the PLA composite to form a dense and foamed char with a high degree of graphitization on its surface during combustion, which can inhibit heat transfer and protect the underlying substrate.Meanwhile, the phosphorus-based radicals derived from DOPO trap the active radicals generated by the degradation of the PLA composite, thereby inhibiting combustion in the gas phase.Therefore, the outstanding flame retardancy of PLA/xE/xD is mainly due to the bi-phase action of DOPO.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
et al. opening reaction between DOPO and ESO.Thus, the torque variation of PLA/10E/3D confirms its interphase reaction during melt-blending.According to previous work, ESO can react with PLA at about 200 • C [29].The DSC curves of PLA, ESO + DOPO (mass ratio = 1:1), PLA + DOPO (mass ratio = 1:1), and PLA + ESO (mass ratio = 1:1) are shown in Fig. 1c and S1.Both PLA and PLA + DOPO only show endothermic melting peaks, indicating that there is no reaction between PLA and DOPO.In the DSC plot of PLA + DOPO, the first endothermic peak at about 120 • C is assigned to the melting of DOPO, and the second one at ~150 • C belongs to the fusion of PLA.Due to the catalytic decomposition effect of DOPO, mixing with it causes PLA to melt prematurely.PLA + ESO shows an exothermic peak in its DSC diagram, of which the onset temperature is approximately 193 • C, verifying that PLA can react with ESO during melt-blending (see Fig. S1).A similar phenomenon can be found in the DSC diagram of ESO + DOPO (see Fig.S1).Thus, PLA, ESO, and DOPO undergo the in-situ grafting exothermic reaction during the melt-blending process.The FTIR spectra of PLA, ESO, DOPO, PLA/10E, and PLA/10E/3D are presented in Fig.1d.The characteristic absorption peak at 2440 cm − 1 in the FTIR spectrum of DOPO belongs to the P-H bond, and that at 822 cm − 1 of ESO is corresponded to the epoxy group[30].Obviously, the absorption peak of the P-H bond completely disappears in the FTIR spectrum of PLA/10E/3D.For both PLA/10E and PLA/10E/3D, the peak intensity of the epoxy group is significantly reduced compared with ESO.In addition, the 1 H NMR spectra of PLA, ESO, PLA/E and PLA/E/D are shown in Fig.S2.The H-3, H-4, H-6, and H-8 peaks of ESO appear in the 1 H NMR of PLA/E, and both newly formed H-7′ and H-10 peaks also appear in the 1 H NMR of PLA/E, indicating the ring-opening reaction of ESO and PLA.All these results further confirm that both ESO and DOPO covalently link to the PLA chains during melt-blending by the reactions between the terminal carboxyl group of PLA, epoxy group of ESO and P-H group of DOPO.

Fig. 2 .
Fig. 2. (a) Tensile stress vs. strain plots of PLA and its composites, (b) tensile strength, and (c) elongation at break of PLA samples, (d) digital image of PLA samples after tensile failure, (e) impact strength of PLA samples, (f) elongation at break and impact strength of PLA/10E/3D and previous flame-retardant PLA with an UL-94 V-0 classification, SEM photographs of cross-sections for (g) PLA, (h) PLA/7.5E/3D, and (i) PLA/10E/3D after the notched Izod impact tests, and elemental mapping images of cross-section for PLA/10E/3D, (j) carbon, (k) oxygen, and (l) phosphorus.

Fig. 3 .
Fig. 3. (a) TG and (b) DTG plots of PLA samples in N 2 condition, (c) the UV-vis transmittance curves, and (d) ultraviolet protection factor (UPF) values of PLA samples, (e) M n and (f) M w of PLA and PLA/10E/3D samples after being buried in soil for different periods of time, and the SEM images of (g-i) PLA and (j-l) PLA/ 10E/3D samples after being buried in soil for different periods of time.
. The I D /I G value of the PLA char is as high as 5.77, while the PLA/xE/xD chars show much lower I D /I G values, which are approximately 3.0.The unreacted small-molecule ESOs are more susceptible to thermal decomposition, leading to a monotonic increase in the I D /I G of PLA/xE/xD with increasing ESO, which is consistent with the results of LOI testing.The results show that the P element in DOPO catalyzes the formation of a dense char layer.The char layers can not only prevent the diffusion of pyrolyzed volatiles but also isolate the thermal and oxygen exchange in the combustion process [51-53].

Fig. 6 .
Fig. 6. 3D IR spectra of (a) PLA and (b) PLA/10E/3D decomposition products, (c) IR spectra of PLA and PLA/10E/3D decomposition products at T max , IR spectra of (d) PLA and (e) PLA/10E/3D decomposition products at different temperatures, and the absorption vs. time curves of the peak at 1238 cm − 1 of PLA and PLA/10E/ 3D samples.

Table 2
TGA data of PLA and PLA composites.theUVresistance at 320-400 nm.By further increasing the ESO content, DOPO can be fully reacted, resulting in enhanced and similar UV resistance properties for PLA/10E/3D and PLA/15E/3D.The ultraviolet protection factor (UPF) values of PLA samples are listed in Fig.3d, in which the UPF value of PLA/10E/3D is up to 139.3 and ~60.0 times higher than 2.3 of PLA.It further confirms the enhanced UV resistance of PLA/10E/3D relative to PLA.The UV-blocking ability of biphenyl group from DOPO is responsible for the enhanced UV resistance of PLA/xE/xD samples Z.Zhang et al.affects

Table 3
LOI and UL-94 results of PLA samples.THR) compared with those of PLA.The PHRR and THR of PLA are 426.7 kW/m 2 and 69.1 MJ/m 2 , while those of PLA/10E/3D are 380.2kW/m 2 and 61.3 MJ/m 2 , with 10.9 % and 11.3 % reductions.Moreover, the PLA/10E/3D sample displays lower average effective combustion heat (AEHC), and higher average carbon monoxide yield (ACOY) than PLA.The average carbon dioxide yield (ACO 2 Y) of PLA/10E/3D is lower than that of PLA. (