The novel application of trifluoromethyl group on flame retardancy properties modification of Schiff Base modified PET copolyester
A
Xinxing
Zhang
1
Luxiang
Ma
2✉
Emailmaluxiang@cdut.edu.cn
1
A
Kunshan Duke University
215316
Kunshan
Jiangsu
China
2
Chengdu University of Technology
610041
Chengdu
Sichuan
China
Xinxing Zhang1, Luxiang Ma2*
1. Kunshan Duke University, Kunshan, Jiangsu, 215316, China
2. Chengdu University of Technology, Chengdu, Sichuan, 610041,China
Corresponding author, Luxiang Ma, maluxiang@cdut.edu.cn
A
Abstract
A novel Schiff Base as a flame-retardant monomer (FSB, 5-((3,5-bis(trifluoromethyl) benzylidene) amino) isophthalic acid) was designed and made to synthesize FSBnPETs (PET copolyester) with flame retardancy and droplet resistance properties by copolymerization, where the functional group of -CF3 here used to modify the Schiff Base flame retardant monomer to further adjust the high cyclizing temperature of its modified PET copolyesters. Results revealed that -CF3 enhanced the temperature of the cyclizing reaction of Schiff Base in Schiff Base modified PET copolyesters from the scope of 250–348 °C to that of 350–398 °C by the influence of its electronic effect on passivating the Schiff Base cyclizing reaction, which further broadened its processing temperature window with better machinability. FSB increased the glass transition temperature (Tg), and reduced the melting temperature (Tm) and the crystallizability of FSBnPETs. FSBnPETs still possessed flame retardancy and droplet resistance properties, FSB10PET (7.4 mol% of FSB) owned V-0 level of UL-94 and 31% value of LOI without dripping phenomenon. The bond of -C-F had not broken down and the fluorine element (F) has not dispersed into the air phase to make an obvious influence on the flame retardancy property of PET modified by Schiff Base, and the FSBnPETs still obey the mechanism of the condense flame retardancy. Additionally, in the new preparing process, the dicarboxylic acid of FSB could speed up the condensation polymerization from the BHET precursors, which could save 50% of the preparation time, showing an exciting economic advantage. The newly synthesized functional group of -CF3 modified Schiff Base could be an ideal selection to modify the Schiff Base modified PET.
Keywords:
Schiff Base
Trifluoromethyl
copolymerization, Poly (ethylene terephthalate)
Flame retardancy
1. Introduction
Polymers are very important chemical materials in the application of industry, one of the earliest developed polymer materials is Poly (ethylene terephthalate), PET, which possessed good spinnability, mechanical properties and thermal stability1–4. Like most polymers, PET is flammable and must be modified in the aspect of its flame retardancy before its application. Hight effective flame retardancy is always a hot point in the field of the PET polymer researches5. In all research directions, the intrinsic flame retardancy by copolymerization was studied tensely due to its distinguish advantages of durable flame retardancy, good miscibility and good molecular structure designability except that it brings deteriorated dripping properties6–8. In recent studies, for the solvation of the problems of flame retardancy and droplet resistance problems, high temperature cyclizing method was invented to develop to meet this new need, in which different types of flame retardant monomer cyclizing in high temperature in their modified PET by copolymerization to form cyclic compounds to promote the property of flame retardancy 6,9,10. wu11 firstly introduced Schiff Base (BA) flame retardant monomer to make BAnPETs copolyester to improve the properties of the flame retardancy and droplet resistance for PET polymer, which showed excellent property, the new synthetic flame retardancy PET copolyesters was also machinable as PET, except that its cyclizing temperature was very low which made it cannot be processed in high temperature resulting from the high reacting activity of flame retardant monomer of Schiff Base, this will restrict its application. The methods need to be found to solve this problem to make progress in this field.
The Schiff Base (BA) cyclizing reaction of BAnPETs copolyesters was a cycloaddition reaction by the imine (-C꞊N-) of several Schiff Base unit12. To low down the electronic density of -C꞊N- of Schiff Base monomer could deactivate its cyclizing reaction to enhance the cyclizing temperature of its modified PET copolyester. Electron absorption group could have such an ideal effect on Schiff Base monomers. Halogen was a kind of electron absorption group13, which was a kind of flame retardant element at the same time14. It could be an ideal modification group for the BA Schiff base flame retardant monomer. In the elements of Halogen (F, Cl, Br, I), F has the best effect of electronic absorption effect. The group of -CF3, which has a high F content, strong electron absorption, high lipophilicity and stable C-F bond could be a preferred choice15. The introduction group of -CF3 into organic compounds can significantly change the acidity, polarity, chemical stability and metabolic stability of the compounds16. Therefore, compounds containing -CF3 are widely used in medicine, pesticides and functional molecular materials17. Based on its strong electron absorption property, it might reduce the electronic density of -C꞊N- in the flame retardant monomer of Schiff Base to passive its high cyclizing reaction, and then to improve the cyclizing temperature of Schiff Base modified PET copolyesters which will further enhance its processability.
Furthermore, economic factors are always considered as key conditions in the process of the modification of polymers. Flame retardant monomer should have dicarboxylic acid or dihydroxy structure in the method of the synthesis of PET polymer by esterification. Former studies of making process of PET polymer showed that reaction monomer with dicarboxylic acid could speed up the PET polycondensation process for saving time 18,19. Considering economic factors, a flame retardant monomer with the dicarboxylic acid for speeding up the polycondensation reaction of PET copolyester could also better the making process of PET flame retardancy copolyester to save time and energy in its application as an additional advantage.
Hence, in this work, a new flame retardancy and monomer of Schiff Base modified by -CF3 with dicarboxylic acid was synthesized to enhance the processability and save preparing time of Schiff Bases modified PET by copolymerization.
2. Experimental
2.1 Materials
1,4-dicarboxybenzene (TPA, AR), ethanol (AR), ethylene glycol (EG, AR), antimony trioxide (Sb2O3, 99.99%), Acetic acid (C2H4O2, 99.9%) and was supplied by Aladdin Co., Ltd. (Shanghai, China). 5-((3,5-bis(trifluoromethyl) benzylidene) amino) isophthalic acid and 5-amino-isophthalic acid were obtained from Macklin Co., Ltd. (Shanghai, China).
2.2 Preparation of 5-((3,5-bis(trifluoromethyl) benzylidene) amino) isophthalic acid (FSB)
FSB was prepared through reactants with aldehyde group (-CHO) and the amino (-NH2), and the group of -CF3 was attached to the reactant with the group of -CHO. Scheme 1 showed the synthetic route below. Keeping at 85 °C in a three-port reaction bottle with the N2 protection, 5-aminoisophthalic acid (A) was completely solved in ethanol solvent, in which 5-((3,5-bis(trifluoromethyl) benzylidene) amino) isophthalic acid (B) (A:B = 1mol:1.2 mol) was added, then the reaction was started and lasted for 6 h. After the reaction was finished the precipitation was filtrated and cleaned with ethanol, which was dried for 8h at 80 °C in the vacuum drying oven.
Yield: 74%. 1H NMR: 8.63 (Ar-H, 2H), 8.40 (Ar-H, 1H), 8.33(Ar-H, 1H), 8.11 (Ar-H, 2H), 9.02 (-CH = N-, 1H) and 13.42 (-COOH, 2H), displayed in Fig. 1.
Scheme 1
Preparation route of
5-((3,5-bis(trifluoromethyl)benzylidene)amino)isophthalic acid (FSB)
Fig. 1
FSB monomer 1HNMR
2.3 BHET (Bis(2-hydroxyethyl) terephthalate) preparation
The synthesis of BHET was prepared by 1,4-dicarboxybenzene (PTA) and ethylene glycol (EG), which was displayed in Scheme 2. TPA: EG (1mol:1.3 mol) were mixed in a 5 L autoclave to react at 245°C for 2 h.
1H NMR: 11.64(-COOH), 8.22–8.30 (Ar-H), 4.80–4.94 (-CH2-CH2- ), 4.73(-OH), shown in Fig. 2.
Scheme 2
Preparation route of BHET
Fig. 2
BHET 1HNMR
2.4 Synthesis of PET copolyesters (FSBnPET)
FSBnPET (“n” represents the mole percentage related to TPA) was made by the method of esterification and the preparation process was displayed in Scheme 3. The preparation method of FSB10PET was described in detailly below. BHET (BHET:TPA = 1mol:1mol), FSB (FSB:TPA = 1mol:10 mol) and Sb2O3 (Sb2O3:TPA = 4.1×10− 4 mol:1mol) were added into a three-port reaction bottle, then the mixture was heat to arrive at 240°C under the protection of N2 and vacuum to react for 60 min to obtain FSB10PET. The other FSBnPETs were synthesized at the same condition.
1H NMR: 10.12 (-N = CH-), 8.67–8.77 (Ar-H), 8.51 (Ar-H), 8.30 (Ar-H), 8.22 (Ar-H), 8.06 (Ar-H), 4.89 (-CH2-O-), 4.76 (-CH2-O-), shown in Fig. 3.
Scheme 3
Preparation route of FSBnPETs
Fig. 3
FSB10PET 1HNMR
2.5 Test methods
The intrinsic viscosities of copolymers were characterized by an ubbelohde viscometer at 25°C, 1,1,2,2-tetrachloroethane /phenol solution (1:1, mass ratio) was used as the solvent.
The 1H NMR was characterized by Bruker AV II 300 MHz NMR, FSB Schiff Base and polymers were solved by DMSO-d6 and CF3COOD respectively, and etramethylsilane (TMS) was used as the reference.
The thermal properties were tested by DSC (the differential scanning calorimetry, TA, Q2000). Firstly, the temperature of PET, FSB5PET, FSB10PET, FSB15PET and were enhanced to 230°C, 250°C, 260°C and 280°C to keep 3 min, respectively, and then the temperature was reduced to 40°C and enhanced to 280°C under the temperature change rate of 10°C min− 1 and the N2 protection gas speed of 50 mL min− 1.
Thermal stabilities of copolyesters were tested by TGA (Thermogravimetric analysis, NETZSCH, 209 F1) under N2, samples experienced temperature changes from 40°C to 700°C (10°C min− 1).
The cyclizing behavior of Schiff Base modified copolyesters was characterized by SETARAM LABSYS EVO TGA/STA-EGA with Ar protection gas speed of 15 mL min− 1, scanning temperature from 40°C to 550°C with the temperature change rate of 10°C min− 1.
Flame retardancy of samples was characterized by Limiting oxygen index (LOI) and Underwriter Laboratory 94 vertical burning (UL-94). JF-3 apparatus was selected for LOI and the sample dimension was 120×6.5×3.2 mm3. GZF-5 instrument was selected for UL-94, the sample dimension was 120 × 13 × 3.2 mm3, and both were based on ASTM D 2863-97 standard.
3. Results and discussion
3.1 Reactivity of FSB in the copolymerization of FSBnPETs
The synthesis conditions of FSBnPET were selected according to the former Schiff Base without the group of -CF3 modified PET (BAnPET)11. According to our analysis, FSB was reacted with the BHET precursor could promote the reaction rate of PET copolyesters, the reaction condition and process were stated in Experimental part shown in scheme 2, when the reaction processed for 60 min, the synthetic PET copolyester climbed the mixing rod in the reaction three-necked and round-bottomed flask, then the reaction was stopped to test its intrinsic viscosity. For BA10PET, it took 120 min for the polycondensation and its intrinsic viscosity was 0.78 dLg− 111, but for FSB10PET, its reaction time was reduced to 60 min and its intrinsic viscosity was 0.97 dLg− 1, which showed obvious advantages in the making process for time-saving. According to the principle of polycondensation, the reasons for FSB accelerating the polymerizing reaction was that the proton acid originating from -COOH of FSB could accelerate the polycondensation of PET, that -COOH of FSB could react with the end groups (-OH) of polyesters to promote the generation of the macromolecule of FSBnPETs, that H2O as a product of the BHET and -COOH of FSB was easier to get out of the reaction system than ethylene glycol (EG) which was the product of the BHET precursor, and that -COOH of FSB can capture the free EG to push the polymerizing reaction equilibrium to move in the direction of the generation of FSBnPET copolyesters18,20,21.
3.2 Thermal properties of FSBnPETs
The incorporation of FSB will change the composition of the molecule chains of PET copolyesters after it experienced the modification by copolymerization. The thermal properties of the synthetic copolyesters were tested by DSC, which were shown in Table 1 and Fig. 4. The glass transition temperature (Tg) of FSBnPETs showed an increasing trend, they were PET (78 °C), FSB5PET (82 °C), FSB10PET (84 °C) and FSB15PET (88°C), respectively, caused by the enhancement of the steric inhibition and intermolecular forces of the molecule chain movement of FSBnPETs, and Tm (the melting point) dropped gradually, they were 242 °C, 221 °C, 207 °C for PET, FSB5PET and FSB10PET, respectively, caused by the changes of the structural properties of molecular chains of PET copolyesters after the incorporation of the FSB22, The increased content of FSB reduced ∆Hm (enthalpy of fusion), ∆Hc (enthalpy of crystallization) and Tc (The crystal temperature) of FSBnPETs, when it reached 15%, Tm, ∆Hm and ∆Hc did not shown in the DSC figures in FSB15PET, resulting from the changes of the molecular structural properties of PET copolyesters after the incorporation of FSB, which influenced the crystallizability of the synthetic copolyesters22.
Fig. 4
DSC melting curves (a) and crystallization (b) of PET and FSBnPETs
|
Samples
|
FSB content (mol%)
|
[η]
(dLg− 1)
|
Tg (°C)
|
Tm (°C)
|
∆Hm (°C)
|
Tc (°C)
|
∆Hc (°C)
|
||
|---|---|---|---|---|---|---|---|---|---|
|
Theoretical
|
Actuala
|
||||||||
|
PET
|
0
|
0
|
0.87
|
78
|
242
|
30.6
|
195
|
43.3
|
|
|
FSB5PET
|
4.8
|
3.2
|
0.83
|
82
|
221
|
26.1
|
172
|
25.8
|
|
|
FSB10PET
|
9.1
|
7.4
|
0.97
|
84
|
207
|
22.5
|
|||
|
FSB15PET
|
13.0
|
11.3
|
0.92
|
88
|
-
|
-
|
-
|
||
| a Actual value was obtained from 1HNMR results | |||||||||
3.3 Thermostability of FSBnPETs
The thermostability of PET copolyesters is essential when they experiencing high temperature23. TG was utilized to test these changes on thermostability, shown in Table 2 and Fig. 5. The results demonstrated that PET and FSBnPET experienced the same mass loss process. T5% gradually dropped, which were 401°C, 398 °C, 393 °C, and 389 °C to PET, FSB5PET, FSB10PET and FSB15PET, respectively, caused by the changes of the chemical molecular structure of PET copolyesters by the modification of FSB. Tmax had few changes, which were 435 °C, 431 °C, 434 °C and 433 °C for PET, FSB5PET, FSB10PET and FSB15PET, respectively. Compared to 13.7% of PET at 700 °C, the carbon residue of FSBnPET were 18.4%, 20.7% and 24.0% for FSB5PET, FSB10PET and FSB15PET, respectively, displaying a increasing trend attribute to that the cyclizing reaction of FSB in FSBnPETs promoted the generation of the carbon residue when it experienced high temperature, the carbon residue of FSBnPETs increases with the improvement of FSB content, which were higher than the Shift base modified PET without -CF3 in the former studied work11, resulting from that the group of -CF3 group can low down the electronic clouds density of benzene ring conjugated structure by its electronic effect, which can effectively protect the C-C bond from decomposition, leading to the improvement of thermal stability of FSB and further enhance the carbon residue of PET copolyesters (FSBnPET)24, that the fluorine atom has the characteristics of low polarizability, small van der Waals radius and electronegativity, and the building energy of bond of the C-F was large, which made the flame retardant monomer FSB difficult to decompose to enhance the carbon residue of its modified PET copolyesters25.
Fig. 5
TGA curves of PET and FSBnPETs under nitrogen
|
Samples T5% Tmax CR (wt%)
|
||||
|---|---|---|---|---|
|
PET
|
401
|
435
|
13.7%
|
|
|
FSB5PET
|
396
|
431
|
18.4%
|
|
|
FSB10PET
|
393
|
434
|
20.7%
|
|
|
FSB15PET
|
389
|
433
|
24.0%
|
|
RC: Carbon Residue.
T5%: The degradation temperature that the weight loss 5% took place.
Tmax: The degradation temperature that the highest mass loss rate took place.
3.1 The high-temperature cyclizing behavior of FSBnPETs
According to the designed method, the introduction of the group of -CF3 should have a positive impact on the high temperature cyclizing reaction of FSBnPETs to enhance its machinability. The cyclizing reaction of FSBnPETs owns a heat changing process which will happen when it experienced high-temperature, TG-DSC technique was used to detect these changes6–8, with the result curves displayed in Fig. 6. Between the range of the temperature of 350–398 °C, The DSC curves demonstrated that an obvious new exothermic peak appeared, caused by the cyclizing reaction of FSB in FSBnPETs, this was obviously different from the testing results of pure PET. Compared to the studied results, its cyclizing temperature range was 250–348 °C for BAnPETs copolyester modified by Schiff Base which did not modify by the group of -CF311, which is lower than the that of FSBnPETs.
FSB Schiff base was a kind of an aromatic one, in which all the elements except the element of fluorine were coplanar to form a conjugate system, which was shown as the yellow shadow route of the atomic stereoscopic structure of FSB displayed in Fig. 7, where π electrons can move to the adjacent aligned orbitals at the conjugated atomic system, and the named detail conjugated forms were also identified shown in Fig. 8. The cyclizing reaction of Schiff Base reacted at high temperature to generate a six-membered cyclic chemical compound in Schiff base modified PET11. The Schiff Base cyclizing behavior could be inferred to the pericyclic reaction in its modified PET, where a small electronic density of -C = N- in Schiff Base led to a reduced reactivity. The electronic effect of -CF3, which could be classified detailly into conjugate effect and induction effect, could reduce electron density of -C = N- of FSB, which led to a reduced reactivity and enhanced the cyclizing temperature of its modified FSBnPETs further 12.aa
The functional theory (DFT) calculation was processed by the ORCA calculation software, the detailed conditions of calculation were b3lyp and 6-311G. The bond order calculation of the structure of -C = N- was carried out and the detailed data was displayed in Fig. 9, which was 1.732 in BAnPETs copolyesters, compared to 1.747 for FSBnPETs, it was reduced caused by the powerful electronic effect of the introduction of the group of -CF3 to modify the originating Schiff base BA, indicating that higher bond energy of -C = N- in FSBnPETs needs a higher temperature to provide enough energy for its decomposition according to the molecular orbital theory26. These two reasons may majorly lead to the cyclizing temperature increase of the new synthetic FSBnPETs copolyesters to further improve their processability.
Fig. 6
TG-DSC characterization related to PET and FSB10PET
Fig. 7
Schematic diagram of atomic conjugation plane of FSB Schiff base
Fig. 8
The reduced intermediate reaction route related to the high temperature cyclizing reaction of FSBnPETs
Fig. 9
The DFT calculation of the structure of -C = N in BAnPET (1) [11] and FSBnPETs (2).
3.4 Flame Retardant Properties and Mechanism of FSBnPETs
Schiff Base has been used as an effective flame retardancy and droplet resistance agent for the property modification of PET polymer 11. UL-94 and LOI were used to test the droplet resistance and flame retardancy properties of FSBnPETs, and the obtained data were displayed in Table 3. PET dripped seriously due to its line structure in molecular chain6,27. UL-94 testing results of FSB5PET with the FSB of 3.2 mol% was V-2 level, showing a limited modification of its dripping property, and that of FSB10PET with FSB of 7.4 mol% arrived at V-0 level without dripping. The results of LOI for PET, FSB5PET, FSB10PET and FSB15PET were 22.0%, 29.0%, 31% and 32%, respectively, indicating that the new synthesized FSB Schiff Base with -CF3 still has a good droplet resistance and flame retardancy function for the property modification of PET.
The flame retardancy mechanism related to Schiff Base modified PET (BAnPET) complied with the condense phase flame retardancy mechanism, which was caused by the formation of the dense carbon layer originating from the high temperature cyclizing reaction of BA Schiff Base, and the droplet resistance properties originating from that Schiff Base cyclized in high temperature to generate cyclic compound which could increase the molten viscosity of its modified copolyesters to further prevent the droplet of the molten state of copolyesters11. Here -CF3 as a kind of molecular modification was introduced to the BA Schiff Base, and Fluorine (F) was one of the halogen flame retardants and it had the possibility of achieving the gas phase flame retardancy mechanism when it was decomposed from the modified polymer (FSBnPETs) into its gas phase. To test whether the element of fluorine decomposed to change the flame retardancy mechanism when the modified polymer experienced a high temperature process, TG-IR was utilized to characterize the gaseous decomposing products of FSBnPET and PET, and their corresponding FTIR spectra of PET and FSB10PET were displayed in Fig. 10 (a) and Fig. 10 (b). As the result showed, the FTIR spectra of gaseous decomposing products of FSB10PET displayed the same product peaks as PET did, the detailed information was shown here. 1039 cm− 1 and 1407 cm− 1 (= C = CH2), 1760 cm− 1 and 2740 cm− 1 (RCHO), 1755 cm− 1 and 3580 cm− 1 (RCOOH), -CH3 (1353 cm− 1), 1088 cm− 1 and 1148 cm− 1 (C-O-C), 21057cm− 1 and 2177cm− 1(CO), 899cm− 1 and 1180 cm− 1 (-C-O) and 725 cm− 1 and 2356cm− 1 (CO2)28–32,. The results demonstrated that -CF3 in FSB did not thermally be decomposed, resulting from that the bond of -C-F was one of the strongest chemical bonds and it owned a very high dissociation energy, which made it difficult to decompose. Therefore, the group of -CF3 did not change the mechanism of flame retardancy in Schiff Base modified PET copolyesters, FSB still performed the mechanism of the condense phase flame retardancy in FSBnPETs copolyesters.
|
Sample
|
LOI (%)
|
UL94
|
||
|---|---|---|---|---|
|
Rating
|
Dripping
|
|||
|
PET
|
22
|
NR
|
Serious
|
|
|
FSB5PET
|
29
|
V-2
|
Modified
|
|
|
FSB10PET
|
31
|
V-0
|
NO
|
|
|
FSB15PET
|
32
|
V-0
|
NO
|
|
Fig. 10
FTIR of the products of gas phase decomposition for PET (a) and FSB10PET (b)
4. Conclusions
In this work, 5-((3,5-bis(trifluoromethyl) benzylidene) amino) isophthalic acid (FSB), a flame retardancy monomer of Schiff Base modified by the group of -CF3, was synthesized to prepare FSBnPETs copolyesters to ameliorate the flame retardancy and droplet resistance properties of PET polymer. The research demonstrated that -CF3 can low down the reaction activity of the cyclizing reaction of Schiff Base in FSBnPETs copolyesters to increase the cyclizing temperature, which was improved to the temperature range of 350–398 °C, compared to 250–348 °C for the BA Schiff Base without -CF3 modified PET (BAnPET), indicating that FSBnPETs has a better machinability with a broadened processing temperature window. Additionally, FSB Schiff Base could produce an effect of accelerating the PET polycondensation reaction to reduce 50% time for the making process of FSBnPETs by the acid catalysis effect originating from its dicarboxylic acid structure. The LOI and UL-94 of FSB10PETs (7.4 mol % of FSB) could reach 31% and V-0 level without dripping phenomenon, respectively, and the group of -CF3 did not break down to affect the flame retardant mechanism of PET modified by Schiff Base. These results demonstrated that the newly synthesized monomer has a good modification effect on the flame retardancy and droplet resistance properties of PET polymer. As the experiments and data exhibited, the electronic effect of the functional group could produce a positive effect to modify the properties of polymers, especially for the ones which have chemical reaction of molecular chain because of the changes of environment, and the dicarboxylic acid of flame retardancy monomer to speed up the polycondensation of PET polymer could be an additional advantage when polymers were modified by copolymerization.
A
A
Author Contributions:
Xinxing zhang, Conceptualization Formal analysis, writing-original draft, writing-review and editing. Surpervisor; Luxiang Ma, Methodology and supervisor. All authors have read and agreed to the published version of this manuscript.
A
Funding:
This research was funded by General Project of Hainan Provincial Colleges and Universities
Acknowledgments:
The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for the DSC analysis.
A
Conflicts of Interest:The authors declare no conflict of interest.
Additional Files
References
1.
Ding L et al (2019) Modification of poly(ethylene terephthalate) by copolymerization of plant-derived α-truxillic acid with excellent ultraviolet shielding and mechanical properties. Chem Eng J 374:1317–1325
2.
Kang MJ, Yu HJ, Jegal J, Kim HS, Cha HG (2020) Depolymerization of PET into terephthalic acid in neutral media catalyzed by the ZSM-5 acidic catalyst. Chem Eng J 398:125655
3.
Yu WG, Zhang XY, Gao XF, Liu HH, Zhang XX (2020) Fabrication of high-strength PET fibers modified with graphene oxide of varying lateral size. J Mater Sci 55:8940–8953
4.
Zhang X, Zhao SC, Mohamed MG, Kuo SAW, Xin Z (2020) Crystallization behaviors of poly(ethylene terephthalate) (PET) with monosilane isobutyl-polyhedral oligomeric silsesquioxanes (POSS). J Mater Sci 55:14642–14655
5.
Jing X-K et al (2013) Thermal Transition Behavior, Thermal Stability, and Flame Retardancy of Low-Melting-Temperature Copolyester: Comonomer Effect. Ind Eng Chem Res 52:4539–4546
6.
Ge X-G et al (2008) Phosphorus-containing telechelic polyester-based ionomer: Facile synthesis and antidripping effects. J Polym Sci Part A: Polym Chem 46:2994–3006
7.
Guo D-M et al (2015) A new approach to improving flame retardancy, smoke suppression and anti-dripping of PET: Via arylene-ether units rearrangement reactions at high temperature. Polymer 77:21–31
8.
Wu Z-Z et al (2018) Effect of biphenyl biimide structure on the thermal stability, flame retardancy and pyrolysis behavior of PET. Polym Degrad Stab 155:162–172
9.
Jing X-K et al (2013) The high-temperature self-crosslinking contribution of azobenzene groups to the flame retardance and anti-dripping of copolyesters. J Mater Chem A 1:9264
10.
Zhao H-B, Chen L, Yang J-C, Ge X-G, Wang Y-Z (2012) A novel flame-retardant-free copolyester: cross-linking towards self extinguishing and non-dripping. J Mater Chem 22:19849
11.
Wu J-N et al (2018) New application for aromatic Schiff base: High efficient flame-retardant and anti-dripping action for polyesters. Chem Eng J 336:622–632
12.
Lei ZQ, Xie P, Rong MZ, Zhang MQ (2015) Catalyst-free dynamic exchange of aromatic Schiff base bonds and its application to self-healing and remolding of crosslinked polymers. J Mater Chem A 3:19662–19668
13.
Lee CY et al (2020) Electronic (donating or withdrawing) effects of ortho-phenolic substituents in dendritic antioxidants. Tetrahedron Lett 61:5
14.
Zhao JC et al (2020) Pyrolysis of a perfluoro copolymer and its contribution to hydrogen fluoride (HF). Polym Degrad Stab 173:8
15.
Alonso C, Martinez de Marigorta E, Rubiales G, Palacios F (2015) Carbon trifluoromethylation reactions of hydrocarbon derivatives and heteroarenes. Chem Rev 115:1847–1935
16.
Egami H, Sodeoka M (2014) Trifluoromethylation of alkenes with concomitant introduction of additional functional groups. Angew Chem Int Ed Engl 53:8294–8308
17.
Lu M et al (2018) Synthesis of oxindoles through trifluoromethylation of N-aryl acrylamides by photoredox catalysis. Org Biomol Chem 16:6564–6568
18.
Chegolya AS, Shevchenko VV, Mikhailov GD (1979) Formation of polyethylene terephthalate in the presence of dicarboxylic-acids. J Polym Sci Pol Chem 17:889–904
19.
Shevchenko VV, Mikhailov GD, Chegolya AS, Polyakova TA (1978) - Polycondensation of polyethylene terephthalate in the presence of dicarboxylic acid. – 9, – 483
20.
Ravindranath K, Mashelkar RA (1984) Finishing stages of pet synthesis - a comprehensive model. AlChE J 30:415–422
21.
Otton J et al (1988) Investigation of the formation of poly(ethylene-terephthalate) with model molecules - kinetics and mechanisms of the catalytic esterification and alcoholysis reactions.2. Catalysis by metallic derivatives (monofunctional reactants). J Polym Sci Pol Chem 26:2199–2224
22.
Dong X, Chen L, Duan R-T, Wang Y-Z (2016) Phenylmaleimide-containing PET-based copolyester: cross-linking from 2π + π cycloaddition toward flame retardance and anti-dripping. Polym Chem 7:2698–2708
23.
Lyon RE (1998) Pyrolysis kinetics of char forming polymers. Polym Degrad Stab 61:201–210
24.
Mei Q et al (2019) Sulfate and hydroxyl radicals-initiated degradation reaction on phenolic contaminants in the aqueous phase: Mechanisms, kinetics and toxicity assessment. Chem Eng J 373:668–676
25.
Kooistra FB et al (2018) FEVE based coating systems: Protecting Steel Bridges for over 30 years. 419, 012007
26.
Weinberg WH (1973) Bond-energy bond-order (bebo) model of chemisorption. J Vac Sci Technol 10:89–94
27.
Li L-J et al (2013) Phosphorus-Containing Poly(ethylene terephthalate): Solid-State Polymerization and Its Sequential Distribution. Ind Eng Chem Res 52:5326–5333
28.
Edge M, Wiles R, Allen NS, McDonald WA, Mortlock SV (1996) Characterisation of the species responsible for yellowing in melt degraded aromatic polyesters.1. Yellowing of poly(ethylene terephthalate). Polym Degrad Stab 53:141–151
29.
Holland BJ, Hay JN (2002) The thermal degradation of PET and analogous polyesters measured by thermal analysis-fourier transform infrared spectroscopy. Polymer 43:1835–1847
30.
Holland BJ, Hay JN (2002) Analysis of comonomer content and cyclic oligomers of poly(ethylene terephthalate). Polymer 43:1797–1804
31.
Deng Y, Zhao C-S, Wang Y-Z (2008) Effects of phosphorus-containing thermotropic liquid crystal copolyester on pyrolysis of PET and its flame retardant mechanism. Polym Degrad Stab 93:2066–2070
32.
Yang W, Song L, Hu Y, Lu H, Yuen RKK (2011) Enhancement of fire retardancy performance of glass-fibre reinforced poly(ethylene terephthalate) composites with the incorporation of aluminum hypophosphite and melamine cyanurate. Compos Part B: Eng 42:1057–1065