A Strategy to Achieve the Inherently Flame-retardant PA56 by Copolymerization with DDP

As a bio-based polyamide, polypentamethylene adipate (PA56) is restricted in many fields due to its flammability. In this research, flame-retardant PA56 copolymers (FRPA56s) were designed and successfully synthesized with the introduction of 9,10-dihydro-10-[2,3-di(hydroxy carbonyl)propyl]-10-phosphaphenanthrene-10-oxide (DDP) unit. The thermal properties, thermal degradation and flame behavior of the resultant copolymer were investigated by DSC, TGA, LOI, UL-94 and cone calorimeter tests, respectively. Compared to PA56, the peak value of heat release rate and total heat release of FRPA56 containing 1.2 mol% DDP decrease by 35% and 19%, respectively, indicating the high flame-retardant efficiency of DDP. It is also found that FRPA56 with only 1.0 mol% DDP passes V-0 grade in the UL-94 test while its thermal performance is almost the same as homopolymer PA56. The pyrolysis products in both the condensed and gas phases were further systematically analyzed by SEM, Raman, FTIR, X-ray photoelectron spectroscopy and TGA-GC/MS. The flame-retardant mechanism was also proposed for FRPA56s.


Introduction
Bio-based polypentamethylene adipate (PA56) is a novel industrial bio-based polymer, which is now mainly synthesized by polycondensation of bio-based 1,5-pentanediamine (PDA) and petroleum adipic acid. Compared with the traditional petroleum-based polyamides, bio-based PA56 is environmentally sustainable and has better performances such as higher moisture absorption, easier dyeing, and DDP is becoming a promising reactive flame retardant due to the high thermal stability and excellent flame-retardant properties, which has been successfully incorporated into polyester, polyamide and polyamide elastomer via copolymerization method [17][18][19][20][21][22][23][24][25]. However, although DDP exhibits the potential to improve the flame retardance of polymers, few investigations about its applications in PA56.
In this research, DDP was used as a phosphorus-containing reactive flame retardant to improve the flame retardance of PA56. The inherently flame-retardant PA56 copolymers (FRPA56s) were synthesized by melt polycondensation with PDA, AA and DDP as the monomer. The chemical structures were characterized by FTIR and NMR. The thermal properties were evaluated with DSC. The thermal degradation and flame behavior of FRPA56s were fully investigated by TGA, LOI, UL-94 and cone calorimeter tests. Subsequently, the condensed phase of FRPA56s after the cone calorimeter test was analyzed by SEM, Raman, FTIR and effect on the performance of polymers [12]. Thus, reactive flame retardants have become the trend to improve flame resistance.

Scheme 1
The diagram of flame-retardant PA56 copolymers (FRPA56s) synthetic route in Scheme 1 and the composition of FRPA56s is summarized in Table S1.

Characterization
FTIR spectra were recorded on a Nicolet 8700 spectrometer with a diamond crystal attenuated total reflection (ATR) Smart accessory. Data was collected over 32 scans with a resolution of 4 cm − 1 . Advanced ATR calibration and baseline correction were performed on data using Omnic software. 1 H NMR and 31 P NMR were acquired on Bruker Avance 600 MHz spectrometer. All the chemical shifts were referenced to TMS.
The crystallization and melting behavior of FRPA56s were performed on PerkinElmer DSC 4000 apparatus. The sample (∼5 mg) was sealed in an aluminum pan and high purity nitrogen was used as the purging gas. The sample was firstly melted at 270 ℃ for 3 min to remove the thermal history, then cooled to 30 ℃ at 20 ℃/min (cooling scan). After being held at 30 ℃ for 3 min, it was heated again to 270 ℃ at 20 ℃/min (second heating scan). The peak maxima of the cooling and the second heating scans were taken as crystallization temperature (T c ) and the melt temperature (T m ), respectively, and the area of the peak as the crystallization enthalpies (∆H c ) and the melt enthalpies (∆H m ), respectively.
Thermal degradation behavior was evaluated using Mettler-Toledo TGA2 SF/1100 under nitrogen and air atmosphere. The heating rate was 10 ℃/min from 30 ℃ to 700 ℃ and the purge flow of 50 mL/min. The UL94 vertical burning tests were conducted on CFZ-II burning tester according to ASTM D3801 with specimen dimensions of 130 mm × 13 mm × 3.2 mm. The limiting oxygen index (LOI) values of FRPA56s were estimated on ZR-1 instrument in terms of ASTM D2863 with the specimen dimensions of 115 mm × 10 mm× 4 mm. The cone calorimetric tests were assessed using a VOUCH-6810 cone calorimeter following ASTM E1354. Squared FRPA56s specimens of 100 mm × 100 mm × 3 mm were horizontally irradiated under a heat flux of 50 kW m − 2 .
SEM was carried out on Hitachi Model S-4700 to visualize the morphology of FRPA56s residue char after cone calorimetric tests. Raman spectra of residue char were performed using a Renishaw Via-Reflex Raman spectrometer with a laser of 532 nm. X-ray photoelectron spectroscopy (XPS) of residue char were recorded on a Thermo Fisher Scientific EscaLab 250Xi system to obtain the element composition with an Al Kα radiation at 1486.8 eV.
The pyrolysis products of CEPPA, PA56 and PA56-1.2DDP at 350, 400, 450, 500 and 600 ℃ were obtained by TGA-GC/MS using the Mettler-Toledo TGA2 SF/1100, XPS. The pyrolysis products of FRPA56s in the gas phase were monitored by TGA-GC/MS. Based on these results, the flame-retardant mechanism was proposed for FRPA56s.

Preparation of the PDA-DDP salt
DDP (17.313 g, 0.05 mol) was dissolved in ethanol (200ml) and transferred to a 500 ml three-necked flask equipped with a mechanical stirrer under 60 ℃. After DDP was dissolved completely, stoichiometric PDA (5.109 g, 0.05 mol) was dissolved in ethanol to avoid PDA volatilization, and then slowly dropped into the ethanol solution of DDP. After all the PDA was added completely, the temperature was raised to 75 °C, and the reaction was continued for 3 h under reflux. PDA-DDP salt (white powder, 95% yield) was obtained by filtration, washed with ethanol and dried in a vacuum oven at 60 °C for 12 h.

Synthesis of flame-retardant PA56 copolymers (FRPA56s)
The FRPA56 samples containing 0.5, 0.7, 1.0 and 1.2 mol% DDP (labeled as PA56-0.5DDP, PA56-0.7DDP, PA56-1.0DDP and PA56-1.2DDP, respectively) were synthesized in a 300 ml autoclave (LB300, Shanghai LABE Instrument Co., Ltd, China) which was equipped with a nitrogen inlet and a mechanical stirrer. PDA-AA salt, PDA-DDP salt and deionized water (30 ml) were added to the autoclave according to the feeding molar ratio. The air in the autoclave was replaced with nitrogen three times before the reaction. Then the autoclave was heated to 220 ℃ and maintained for 1 h. The pressure was controlled at 2.0 MPa during this period. Subsequently, the temperature of the autoclave was increased to 270 ℃, whereas the pressure was decreased to normal pressure. Then the reaction was continued for 1 h at 270 ℃ under vacuum (< 300 Pa) and FRPA56 was obtained after cooling. The preparation process of FRPA56s is shown around 3300 and 3085 cm -1 are associated with the stretching vibration of N-H of amide A and amide B, respectively. The peaks around 1642 cm -1 correspond to the amide I bands arising from C = O stretch vibrations, while the peaks at 1548 cm -1 are attributed to the amide II bands resulting from C-N stretch vibrations and N-H bending [26]. The FTIR spectra of FRPA56s are very similar to that of PA56 because of the low DDP content. In addition, in the range of 1740 − 1580 cm -1 (the enlarged drawing in Fig. 1b), the peak at 1702 cm -1 corresponds to the stretching vibration of C = O in DDP, which confirms the incorporation of DDP which was linked to the Agilent 5977 gas chromatographymass spectrometry (GC/MS).

Structure analysis of PA56 and FRPA56s
The inherently FRPA56s were synthesized according to Scheme 1. The FTIR spectra of PA56 and FRPA56s are presented in Fig. 1. In the spectrum of PA56, the bands  that DDP acts as a nucleating agent in the crystallization process of FRPA56s and accelerates the crystallization rate of FRPA56s [28]. Notably, the increase of T c for FRPA56 containing 1.2 mol% DDP is not obvious compared with PA56, which is due to the higher steric hindrance generated by the high content of DDP, restricting the movement of molecular chains in FRPA56s. In addition, The phosphaphenanthrene structure of DDP destroys the symmetry and regularity of molecular chains in FRPA56s, which restricts the arrangement of molecular chains into the lattice, resulting in the formation of thinner lamellae. Therefore, compared with PA56 (255 ℃), the T m of FRPA56s moves slightly towards 251 °C with the DDP content increased to 1.2 mol%. Furthermore, the crystallinity (X c ) of PA56-0.5DDP is almost the same as PA56, as listed in Table 1. As the increase of DDP content, the crystallinity shows a decreasing tendency as the symmetry and regularity of molecular chains are destroyed for FRPA56s. into the PA56 chain by the copolymerization [18]. Besides, the amide I band of FRPA56s shows a blue shift compared to PA56, indicating the strengthened interactions between molecular chains of FRPA56s. Furthermore, two shoulder peaks around 1253 and 1186 cm -1 arise from the gauche conformation of CH 2 -NH in PA56, which indicates the presence of γ crystal form in PA56 and FRPA56s [27].
The 1 H NMR spectra of PA56 and FRPA56s with deuterosulfuric acid as solvent are displayed in Fig. 2a. The signals at 1.7 (a), 2.1 (b) and 3.8 (c) ppm are assigned to the methylene in PDA unit, the peaks at 3.0 (d) and 2.0 (e) ppm are assigned to the methylene in AA unit. The chemical shifts between 7.5 and 8.6 ppm (i) belong to the protons in the phosphaphenanthrene ring of DDP unit. The insert clearly shows the phosphaphenanthrene ring protons peaks of FRPA56s. In addition, as shown in Fig. 2b

Thermal properties and degradation behavior of PA56 and FRPA56s
The crystallization and melting behavior  Table 1. The crystallization temperature (T c ) of FRPA56s decreases after the first increase with the DDP content increasing and is higher than that of PA56, indicating

Flame retardant properties
The results of the UL94 vertical burning and LOI test are shown in Table 3. Although PA56 possesses self-extinguishing during combustion, it only reaches V-2 grade in the UL94 vertical burning test due to the melting droplets produced by PA56 easily igniting the absorbent cotton (Figure S3) Fig. 4. The initial thermal decomposition temperature (T 5% ), maximum decomposition temperature (T max ) and residue char at 700 ℃ are summarized in Table 2. Under the nitrogen atmosphere, PA56 shows a typical one-stage degradation process with the T 5% at 394 °C and T max at 440 °C. All FRPA56s exhibit two primary thermal decomposition processes, indicating that the incorporation of DDP changes the degradation mechanism of PA56.
Notably, as the DDP content increased from 0.5 to 1.2 mol %, the T 5% of FRPA56s decreased from 375 to 361 °C. The phenomenon is ascribed to the breakage of the P-O-C and O-P = O bonds in FRPA56s in advance, which releases flammable small molecules into the gas phase [18]. In addition, FRPA56s release phosphorus-containing radicals to capture H and OH radicals in the gas phase and generate phosphoric acid compounds which accelerate the degradation of FRPA56s at 403-411 °C (T max,1 ). As the temperature further increased to near 452 °C (T max,2 ), the amide fragments of FRPA56s occurs scission. Besides, the residue char of FRPA56s increases to 2.95% with the DDP content increased to 1.2 mol%, revealing that DDP promotes the formation of residue char. Under the air atmosphere, the degradation behavior of FRPA56s is consistent with the nitrogen atmosphere. It includes the primary degradation process of FRPA56s Cone calorimetric tests are performed to assess the dynamic combustion behavior of PA56 and FRPA56s. The relevant combustion parameters to evaluate the fire disaster include the time to ignition (TTI), total heat release (THR), heat release rate (HRR), peak heat release rate (p-HRR), maximum average rate of heat emission (MARHE), total smoke release (TSR), CO production rate (COP), average effective heat of combustion (av-EHC), specific extinction area (SEA) and char residue are provided in Fig. 5; Table 4. As shown in Fig. 5, PA56 burns rapidly after ignition with a TTI of 54 s, the p-HRR value of PA56 arrives at 1091 kW·m − 2 within 131 s (time to p-HRR) and the THR value of PA56 reaches 117 MJ·m − 2 . Compared with PA56, the TTI and time to p-HRR of FRPA56-1.2DDP are shortened to 41 and 59 s, respectively, which is ascribed to the breakage of the P-O-C and O-P = O bonds in advance [32]. The p-HRR value of PA56-0.7DDP and PA56-1.2DDP reduces to 1009 and 713 kW·m − 2 , with 8% and 35% decrements compared with PA56, respectively. THR shows a similar trend as p-HRR and those values of PA56-0.7DDP and PA56-1.2DDP decline to 98 and 95 MJ·m − 2 with the decrement of 16% and 19% compared to PA56, respectively. All results clarify that DDP is a valuable flame retardant as it significantly inhibits the total heat release of FRPA56s [31]. MARHE is defined as the peak of the cumulative heat emission divided by time, which is a good measure of the propensity for fire development under real-scale conditions, playing an important role as a fire safety parameter. As shown in Table 4, the MARHE value of PA56 reaches 466 MJ·m − 2 , while the MARHE values of PA56-0.7DDP and PA56-1.2DDP decrease to 444 and 416 kW·m − 2 , respectively, indicating that the overall fire performance of PA56 has improved. SEA and TSR reflect the smoke release amount during the combustion. Compared with PA56, the SEA and TSR value of PA56-1.2DDP increases to 197 m 2 ·kg − 1 and 1218 m 2 ·m − 2 from 71 m 2 ·kg − 1 and 481 m 2 ·m − 2 ( Fig. 5c; Table 4), respectively, indicating that FRPA56s produce more aromatic volatiles during combustion. Av-EHC reveals the combustible degree of volatiles in the gas phase. As shown in Table 4, compared with PA56 (31 MJ·Kg − 1 ), the av-EHC value of PA56-1.2DDP reduces to 24 MJ·Kg − 1 , revealing that the combustion degree of FRPA56s weakens. The p-COP value rises to 0.014 g/s for PA56-1.2DDP from 0.007 g/s for PA56, which suggests that PA56-1.2DDP burns incompletely compared with PA56. FRPA56s releases PO, PO 2 and phenyl radicals to capture H and OH radicals in the flame zone by quenching effect, leading to incomplete combustion of FRPA56s [33]. Notably, the residual char slightly increased to 0.97% with the DDP content of 1.2 mol%, which is consistent with TGA results.    microscopic surface. PA56 forms porous and fragile char layers with a smooth surface, while the char layers of FRPA56s are compact. In addition, a plentiful of corrugated folds are found on the surface, which serves as a skeleton to strengthen the residual char, interrupting the transfer of heat and oxygen between the flame and unburned part [28]. At higher magnification (10000x), the carbon spheres are observed on the surface of residual char, and their amounts extend with DDP content increases. These microstructures prove that DDP has undergone chemical reactions (dehydration and carbonization) during combustion [15]. The compactness is further quantitatively compared by the integral area ratio I D /I G (at 1372 and 1585 cm − 1 ) in Raman spectra. The larger I D /I G , the smaller crystallite size and the denser the char layers [34]. The I D /I G value increases in the following order: PA56 (2.95) < PA56-0.7DDP (3.97) < PA56-1.2DDP (4.51), indicating that DDP promotes the formation of a denser char layer with a smaller microcrystalline structure, which is consistent with the SEM results. Figure 7a shows the FTIR of the residual char after the cone calorimeter test for PA56 and FRPA56s. Compared with PA56, FRPA56s emerge phosphorus-containing groups peaks at 1216, 1150, 1120, 1092 and 1055 cm − 1 , which is ascribed to phosphorous /hypophosphorous acid derivatives. The peak at 1216 cm − 1 is related to R(OR) (OH)P = O group, the band around 1155 cm − 1 is attributed to R(OH) 2 P=O group, the signal at 1120 cm − 1 derived from the stretching vibrations of P-O-C and two peaks at 1092 and 1055 cm − 1 correspond to the P-OH bond [35]. Those results further reveal that DDP generates the phosphoric acid and hypophosphorous acid derivatives during combustion. The phosphoric acid and hypophosphorous acid derivatives further react (dehydration and carbonization) with polyamide to form the phosphorus-containing dense char layers with polyaromatic structure. The char layers cover the surface Analysis of the condensed phase Figure 6 summarizes the digital photographs, SEM micrographs and Raman spectra of residual char layers after cone calorimetric tests for PA56, PA56-0.7DDP and PA56-1.2DDP. The residual char of PA56 after combustion is negligible while that of FRPA56 increases with DDP content. The SEM images further perform the details of the Subsequently, o-hydroxybiphenyl and fluorene continue to cleave, forming stable gas products containing phenyl groups. Figure 9 depicts the pyrolysis process diagram of DDP. Figure 8b and c show the total ion chromatogram of PA56 and PA56-1.2DDP. The corresponding major gaseous products are presented in Fig. 8e and f, respectively. The α-methyl next to the amide bond and amino group of PA56 is easily scissile, leading to decarboxylation, deamination, alkylation and acylation reaction [37]. The main pyrolysis products of PA56 are cyclopentanone (e1) and pyridine (e2) accompanied by CO 2 and NH 3 [37]. The cyclopentanone and pyridine further capture the molecule radicals (methyl, ethyl and pentyl radicals) forming cyclopentanone derivatives (e3, e4, e5, e7, e8, e10) and nitrogen-containing compounds (e6, e9, e10, e11). Some characteristic pyrolysis products of PA56, such as cyclopentanone (e1/f1), [1,1'-bicyclopentyl]-2-one (e5/f5), pyrrolidine(e6/f6), are also detected in the chromatogram of PA56-1.2DDP, indicating that FRPA56s also releases nonflammable gas (CO 2 and NH 3 ) by decarboxylation and deamination of pentamethylene adipamide structure. In addition, methylbenzene (d1/f2) and o-hydroxybiphenyl (d8/f8) generated from DDP is also found in the chromatogram of PA56-1.2DDP. The result indicates that PA56-1.2DDP releases PO and Ph radicals during the thermal pyrolysis process to capture H and OH and other radicals, interrupting the free radicals chain reaction and exothermic process of PA56-1.2DDP by quenching effect, thereby suppressing the combustion of FRPA56s in the gases phase. Besides, some novel nitrogencontaining pyrolysis substances, such as f9, f10, f11 and f12, are also found in the chromatogram of PA56-1.2DDP. The nonflammable gases (CO 2 , NH 3 and nitrogen-containing compounds (f9, f10, f11 and f12)) formed from the pyrolysis of PA56-1.2DDP dilute the concentration of combustible volatiles, further depressing the combustion degree of PA56. of the matrix, isolating heat and oxygen, preventing further pyrolysis of the PA56 and terminating the combustion of PA56 eventually [13]. Figure 7b presents the XPS survey spectra of PA56, PA56-0.7DDP and PA56-1.2DDP. The C, N and O elements on the surface of the sample are detected, and the P element appears in the residue char of PA56-0.7DDP and PA56-1.2DDP [28,36]. To further analyze the chemical structure of residue char, the corresponding C 1s , O 1s , and P 2p spectra of PA56 and PA56-1.2DDP are depicted in Fig. 7c-e. The C 1s peaks of PA56 and PA56-1.2DDP are spilled into four characteristic bands at 286.4, 285.6, 284.9 and 284.4 eV, which are designated to the C = O, C-N, C-O and C-C, respectively [36]. In the O 1s spectra of PA56, two peaks around 533.6 and 531.0 eV is ascribed to C-O and C = O. The residue of PA56-1.2DDP exhibits an additional peak at 532.4 eV, which corresponds to P-O [28]. For the P 2p spectrum of PA56-1.2DDP, two peaks occur at 134.5 and 133.5 eV are attributed to P-O-C and O-P = O groups of phosphoric /hypophosphorous acid and its derivatives, which coincides with FTIR results, further demonstrating that DDP plays a flame retardant role in the condensed phase [36].

Analysis of the gas phase
The total ion chromatogram of DDP at 350, 400, 450, 500 and 600 °C are shown in Fig. 8a, along with the detailed pyrolysis products in Fig. 8d. As shown in Fig. 8a and d, the main pyrolytic products of DDP are 6-heptenoic acid (d6), o-hydroxybiphenyl (d8), fluorene (d9), 9-methyl-9 h-fluorene (d10), 2-(1-phenylethyl) phenol (d11), 1-phenoxy-2-(2-propen-1-yl) benzene (d12) and triphenylene (d13). O-hydroxybiphenyl and fluorene are formed by the cleavage of the P-C bond in DDP, accompanied by the release of PO, PO 2 and phenyl radicals during the pyrolysis process. Fig. 8 The total ion chromatogram (a-c) and main pyrolysis products (d-e) of DDP, PA56 and PA56-1.2DDP in TGA-GC/ MS tests FRPA56 releases PO, PO 2 and phenyl radicals in the gas phase, which capture H and OH radicals in the flame zone, interrupting the exothermic processes by quenching effect, suppressing the combustion of polymer in the gases phase. In addition, the nonflammable gases dilute the concentrations

Flame-retardant mechanism of FRPA56s
Based on the above-mentioned degradation and flame behaviors analysis, the flame-retardant mechanisms of DDP incorporated FRPA56s are proposed. As shown in Fig. 10,   Fig. 9 Main pyrolysis processes of DDP (a) and FRPA56-1.2DDP (b) of flammable gases and oxygen in the gas phase and take away the heat during combustion. In the condensed phase, FRPA56s form the phosphorus-containing dense char layers with polyaromatic structure during combustion, the dense char layers cover the surface of the matrix, isolating heat and oxygen, preventing further pyrolysis of the PA56 and terminating the combustion of PA56 eventually.

Conclusions
In summary, the inherently flame-retardant PA56 copolymers are successfully prepared with the simple introduction of 9,10-dihydro-10-[2,3-di(hydroxy carbonyl) propyl]-10-phosphaphenanthrene-10-oxide (DDP) unit. V-0 grade is achieved for bio-based PA56 by the incorporation of only 1.0 mol% DDP in the UL94 vertical burning test, which confirms that the DDP copolymerization strategy is effective to improve flame retardancy of bio-based PA56. The pyrolysis products in both condensed and gas phases were analyzed with SEM, Raman, FTIR, XPS and TGA-GC/ MS for FRPA56s. Based on the analysis, the flame-retardant mechanism of FRPA56 was proposed: (i) FRPA56s can release PO, PO 2 and Ph radicals as free radical scavenging agents, inhibiting the combustion through the quenching effect in the gas phases; (ii) the nonflammable gases generated by FRPA56s, such as CO 2 , NH 3 and nitrogen-containing compounds, reduce the concentrations of flammable gases and oxygen in the gas phase and take away the heat during combustion; (iii) FRPA56s form dense char layers during combustion cover the surface of the matrix, isolating heat and oxygen, preventing further pyrolysis of the PA56.