Poly(lactic acid) (PLA) is one kind of thermoplastic biodegradable polymeric material, which is sourced from starch-rich plants such as cassava, corn, and so on. PLA has many advantages, including rich raw materials, biodegradability, excellent biocompatibility, superior processing and mechanical properties [1–3]. Given this, PLA has been extensively applied in biomedical, packaging materials and other related fields. However, similar to the conventional polyesters, PLA is easy to ignite in the air and accompanied by serious dripping phenomenon, which seriously restricts the application of PLA in the fields with high flame-retardant requirements such as electronic packaging and automobile industry [4,5]. Hence, the attempt to enhance the anti-flammability of PLA has become the focus of both academia and industry.

At present, physical blending flame retardant modification is the most commonly used method for flame-retardant modification of PLA, which refers to the processing of flame retardant PLA materials by adding flame retardants to PLA matrix through melt blending. This strategy possessed the advantages of low cost, convenient operation and easy commercialization. The commonly used additive flame retardants is mainly divided into two categories which are organic and inorganic anti-flammable agents. The organic anti-flammable agents for PLA include phosphorus-based chemicals [6–8], phosphorus/nitrogen-based chemicals [9,10], intumescent flame-retardant systems [11–13], etc., among many organic flame retardants, the phosphorus-containing compounds have become the most popular flame-retardants for PLA owing to their merits of low smoke and environmental friendliness. So far, diverse phosphorus-based flame-retardant additives including 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived chemicals [14–19], hypophosphorous acid-based derivatives [6], poly(ethylene diglycol phenylphosphinate) [20], poly(1, 2-propanediol 2-carboxyethyl phenyl phosphinate) [21], poly(butylene phenyl phosphate) [22], hyperbranched polyphosphate ester [23], and poly(phenylphosphoryl phenylenediamine) [24], have been synthesized for improving the anti-flammability of PLA. However, in order to achieve satisfactory flame retardant effect, it is necessary to add a high content (usually beyond 15%) of phosphorus-based flame-retardant additives to PLA.

Except for the organic flame-retardants, various inorganic flame-retardants such as carbon-based materials [25–28], metal organic frameworks [29,30], silicates [31–34], black phosphorus [35], graphitic carbon nitride [36], molybdenum disulfide [37,38], have been utilized to enhance the anti-flammability of PLA. Compared to organic flame-retardants, inorganic anti-flammable agents are more efficient in decreasing the heat release and smoke emission at a relatively low dosage. However, the use of inorganic flame-retardants alone generally makes PLA difficult to reach UL-94 V-0 classification. Thus, inorganic flame-retardants are mostly used together with organic flame-retardants to create a flame-retardant synergism.

Based on the review of the anti-flammable strategies above, highly efficient anti-flammable technology for PLA remains a challenging task. Recently, the application of bio-based flame retardants for PLA is becoming a new-arising trend [39–42]. As a sucrose-derived diol, isosorbide has attracted extensive attention because of its high stiffness and thermal stability [43]. Isosorbide has been reported to fabricate the flame-retardants for epoxy [44,45] and poly(butylene succinate) [46]. Recently, Wang and collaborators designed a isosorbide-derived poly(phosphoester) as flame-retardant for PLA [47]. PLA containing 20 wt% of isosorbide-derived poly(phosphoester) achieved a LOI of 25.5% and passed UL-94 V-0 classification. To develop isosorbide derivatives with higher flame-retardant efficacy, an isosorbide-derived polyphosphonate was produced via condensation between 2-carboxyethyl phenylphosphinic acid and isosorbide in this study. The effect of the isosorbide-derived polyphosphonate dosage on the thermal and flame-retardant behaviors was investigated. Finally, the flame-retardant mechanism of this isosorbide-derived polyphosphonate was speculated based on the analysis of the pyrolysis products of PLA composites.

The chemical structure of PICPP was characterized using the FTIR instrument. As shown in Fig. 1, CPA exhibits an intense band centered at the 3419 cm⁻¹ that is ascribed to the -OH groups of carboxylic and phosphorus acid, and the absorption peaks at 2925 and 2872 cm⁻¹ are assigned to the asymmetric and symmetric stretching vibration of -CH₂- groups, respectively. The absorption peaks of isosorbide at 2937 and 2875 cm⁻¹ are related to the symmetric and asymmetric stretching vibration of -CH₂- groups, respectively. The broad absorption peak at 3397 cm⁻¹ is assigned to the -OH group, and the absorption peak at 1077 cm⁻¹ originated from the C-O-C group in the pentacyclic structure. Compared to the monomers, the stretching vibration of the hydroxyl groups almost disappears in PICPP attributed to the occurrence of esterification between CPA and isosorbide. Additionally, the absorption band appearing at 1221 cm⁻¹ belongs to the formation of C-O-C_isosorbide.

Fig. 2 depicts the proton and ³¹P-NMR spectra of PICPP. As shown in Fig. 2a, the multiple signals at the chemical shift range from 3.5 to 5.2 ppm (marked 1–6) are ascribed to the protons belonging to the isosorbide cycle. The multiple signals ranging from 7.4 to 7.8 ppm are allocated to the aromatic protons. Additionally, the signals at 2.6 and 2.2 ppm are assigned to the protons in the -CH₂-P(O)- (marked 7) and -CH₂-C(O)- (marked 8) groups, respectively. An intense signal at 43.1 ppm is observed in the ³¹P-NMR spectrum of PICPP (Fig. 2b), implying only one chemical environment for phosphorus atoms. These data correspond well with the anticipated molecular structure, manifesting the successful synthesis of PICPP.

Polymers’ molecular weight is a key factor to affect their performances. The influence of PICPP on the molecular weight of PLA composites was characterized using GPC. Table 1 lists the GPC data of PLA and PLA/PICPP. With the increase of the addition content of PICPP, both the number-average molecular weight (M_n) and weight-average molecular weight (M_w) were declined gradually. Polydispersity index is calculated by the ratio of M_w/M_n, which is generally to assess the molecular weight distribution. It can be seen that the polydispersity index was also declined as the addition content of PICPP increased. PLA was apt to decompose during the processing procedure because of the catalysis of hydroxyl terminated PICPP. That is why the decreased molecular weight and the increased polydispersity index after incorporating PICPP into PLA.

The effect of PICPP on the thermo-mechanical performances of PLA was determined using DMA measurement. Fig. 3a illustrates that the storage modulus (E’) decreased with the increment of temperature in the whole studied scale. The E’ at 30°C of the pristine PLA was 2,179 MPa, whereas the addition of PICPP decreased the E’ at 30°C to 1,956, 1,716 and 1,551 MPa for PLA/PICPP-5, PLA/PICPP-10 and PLA/PICPP-15, respectively. The reduced storage modulus of the PLA/PICPP composites can be ascribed to the declined molecular weight of PLA catalyzed by the presence of PICPP. Additionally, all the samples exhibited a sharply decreased E’ stage corresponding to the transition stage from the glassy state to the rubbery state of PLA. The glass transition temperature (T_g) is identified by the temperature at the maximum tanδ value (Fig. 3b). The T_g of the pristine PLA was 73°C. When the addition content of PICPP was low, the T_g of PLA/PICPP-5 remained unchanged. With the increase of the addition content of PICPP, the T_g of PLA/PICPP-10 and PLA/PICPP-15 was slightly decreased to approximately 69°C. Moreover, the broaden long tail of tanδ peak could due to the fact that G" decreases slowly as temp increases, which implies the chain entanglements are enhanced, supporting the better compatibility.

Typical stress-strain curves of PLA and its composites are shown in Fig. 4, corresponding data were summarized in Table 2. It can be observed that the incorporation of PICPP into PLA matrix showed a negative effect on the mechanical properties of PLA composites. The tensile modulus, tensile strength and elongation at break of pure PLA were 1.31 GPa, 66.45 MPa and 7.03%. Adding 5 wt% PICPP into PLA matrix, the tensile strength of PLA/PICPP-5 slightly declined to 59.79 MPa while the elongation at break of PLA/PICPP-5 increased to 7.81%, and PLA/PICPP-5 presented comparative tensile modulus as that of neat PLA. However, further increasing the addition of PICPP, the tensile strength and elongation at break of PLA/PICPP-10 and PLA/PICPP-15 were obviously decreased, which was attributed to that the presence of PICPP declined the molecular weight of PLA, hence worsening the mechanical properties of PLA composites.

The effect of PICPP on thermal decomposition of PLA was analyzed using TGA. Fig. 5 shows the TGA and differential TGA (DTG) thermograms of PLA and PLA/PICPP under air and nitrogen atmospheres, and Table 3 summarizes the relating data. In Figs. 5a and 5b, PLA thermally degraded in a single stage between 324°C (T_5%, the temperature at 5% weight loss,) and 380°C, and the weight loss was 97.5%. The temperature at the maximum decomposition rate (T_m) of PLA was 363°C as observed in the DTG curve. At 800°C, only 0.4% residue was left. After adding PICPP, PLA/PICPP thermally degraded in one stage. With the increase of the addition content of PICPP, the T_5% of PLA/PICPP was decreased gradually, owing to the earlier decomposition of phosphorus-containing additives [48,49]. The presence of PICPP almost had no change in the residue yield of PLA. Although PICPP had certain charring ability, the formed char was unstable and decomposed at high temperature. Owing to the generation of the intermediate char, the weight loss rate was slowed down during the thermal decomposition process, as demonstrated by the DTG curves. Under nitrogen atmosphere, PLA also behaved a thermal decomposition stage between 333°C (T_5%) and 390°C and yield 1.0% residue (Figs. 5c and 5d). The T_5% and T_m of PLA were slightly higher than those under air, because of the absence of oxidation by oxygen. Similarly, the T_5% of PLA/PICPP was decreased gradually as the addition content of PICPP grew up. From DTG curves, it can be found that the incorporation of PICPP reduced the weight loss rate of PLA during the thermal decomposition process, which was ascribed to the generation of a carbonaceous layer. However, the char was thermally unstable so that cannot resist the high temperature. Thus, the residue yield of PLA/PICPP was very slightly improved by the presence of PICPP.

The LOI and UL-94 tests are employed to assess the anti-flammable behaviors of PLA and PLA/PICPP, and corresponding results are summarized in Table 4. The LOI value of pure PLA was 20.0% which was relatively low, and it completely burned out in the UL-94 test corresponding to no classification. With increasing the addition amount of PICPP, the LOI of PLA/PICPP composites was increased gradually. Specifically, the LOI value of PLA/PICPP-5, PLA/PICPP-10 and PLA/PICPP-15 was 26.0%, 30.0% and 31.0%, respectively. Additionally, PLA/PICPP-5 behaved UL-94 V-1 classification, and PLA/PICPP-10 and PLA/PICPP-15 achieved UL-94 V-0 classification, manifesting that PICPP was an efficient flame retardant additive for PLA. Since the dispersion behavior of flame retardants or the phase separation morphology has an impact on the flame retardant efficiency [50,51], such the high flame retardant effect could be ascribed to the good dispersion state of the PICPP in PLA matrix.

MCC is a widespread apparatus to obtain the heat release rate (HRR) information of polymers just using milligram samples [52,53]. Fig. 6 depicts the HRR vs. temperature curves of PLA and PLA/PICPP, and Table 4 summarizes the relating data. The HRR curve of the pristine PLA showed an intense peak, and its peak HRR (PHRR) value was 690 W/g, implying that PLA was a highly flammable material. With the content of PICPP was increased, the PHRR of PLA/PICPP was decreased gradually. Specifically, the PHRR values of PLA/PICPP-5, PLA/PICPP-10 and PLA/PICPP-15 were reduced to 564, 537 and 463 W/g, respectively. The PLA/PICPP-15 exhibited the lowest PHRR value, 33% lower than the pristine PLA. Additionally, the width of the HRR curves of PLA/PICPP composites became broaden with increasing the content of PICPP. This was consistent with the widen molecular weight distribution of PLA caused by the catalysis of PICPP. The THR value showed a similar changing trend as the PHRR. As increasing the content of PICPP, the THR values of PLA/PICPP-5, PLA/PICPP-10 and PLA/PICPP-15 were declined to 22.1, 21.9 and 20.6 kJ/g, respectively. By contrast to the pristine PLA, the temperature to PHRR (T_P) of PLA/PICPP exhibited a quite slight change, whose changing trend was in good agreement with the T_max observed in DTG curves.

The evolved products of PLA and PLA/PICPP-15 in the vapor phase were detected by TG-FTIR technique. As shown in Figs. 7a and 7b, it was apparent that PLA/PICPP-15 exhibited a similar 3D FTIR spectra as PLA. The typical evolved products appeared at 2,930–2,600, 1,870–1,650, 1,530–1,290, and 1,190–1,000 cm⁻¹ for both PLA and PLA/PICPP-15. Fig. 7c further provides the FTIR spectra of PLA and PLA/PICPP-15 at the maximum weight loss rate. The typical evolved products were identified by the characteristic signals: aliphatic hydrocarbons (stretching vibrations at 2,930–2,800 cm⁻¹; bending vibrations at 1,480–1,300 cm⁻¹), aldehydes (2,800–2,600 cm⁻¹), ketones and carboxylic acids (1,870–1,650 cm⁻¹) and esters (1,190–1,000 cm⁻¹) [11,23,54]. During the thermal decomposition process, the macromolecular chains of PLA broken into several typical small molecules including aliphatic hydrocarbons, aldehydes, ketones and carboxylic acids and esters, which accorded well with the previous reports [11,23]. Among these typical pyrolysis products, carbonyls (aldehydes, ketones and carboxylic acids, etc.) displayed the highest absorbance intensity, implying that they are the majority in the pyrolysis products. Additionally, the absorbance intensity of the peaks at 1,870–1,650 cm⁻¹ of PLA/PICPP-15 was lower than that of PLA, meaning the declined production of carbonyls in the decomposition process. Since carbonyls are flammable pyrolysis products, the declined carbonyl-moieties production may account for the declined heat release parameters in MCC measurement.

Based on the discussion above, the flame retardant mechanism of PICPP can be attributed to the combined gas and condensed phase flame retardant actions. The former plays a flame retardant effect by generating phosphorus-containing free radicals while the latter by forming pyrophosphoric acid or metaphosphoric acid compounds. According to the previous reports [55], the char yield decreased and the emission of phosphorus-containing volatiles increased as the oxidation state of phosphorus decreased. Because the PICPP has a low oxidation state of phosphorus, the flame retardant mode of action of PICPP mainly manifested in the gas phase. The gaseous phase flame retardant action is mainly depicted as follows: Once the PLA composites are exposed to fire sources, PICPP could decompose to generate phosphorus-containing free radicals, which can quench the hydrocarbon free radicals from the decomposed matrix and thus terminate the combustion chain reaction in gas phase. Additionally, some phosphorus-containing compounds derived from the thermal decomposition of PICPP remain in the condensed phase, resulting in the slight promotion of char yield as observed in TGA. Predominant gas phase mode of action in combination with minor carbonization has been reported to be an efficient strategy for flame retardancy enhancement [56].

In this study, an isosorbide-derived polyphosphonate (PICPP) was synthesized and utilized as an anti-flammable agent for PLA. FTIR and NMR spectral data demonstrated the successful synthesis of PICPP. Owing to the catalysis effect of hydroxyl-terminated PICPP on the decomposition of PLA during the processing, the molecular weight of PLA was declined after adding PICPP. The incorporation of PICPP leaded to a reduction in the storage modulus of PLA as the molecular weight was declined. TGA results manifested that PICPP reduced the onset thermal degradation temperature and the weight loss rate of PLA composites during the thermal decomposition process. PLA containing 10 wt% of PICPP achieved a high LOI value of 30.0% and passed the UL-94 V-0 classification. Additionally, the PHRR and THR of PLA containing 15 wt% of PICPP were declined by 33% and 16%, respectively, compared with those of PLA. TGA-FTIR results clarified that PLA/PICPP produced the less typical pyrolysis products especially flammable carbonyls than PLA, which was responsible for the declined PHRR and THR values of PLA/PICPP. This isosorbide-derived polyphosphonate provides a renewable and efficient solution for flame-retardant PLA.