Fire hazards management for polymeric materials via synergy effects of pyrolysates-fixation and aromatized-charring

Journal Pre-proof Fire hazards management for polymeric materials via synergy effects of pyrolysates-fixation and aromatized-charring Teng Fu (Conceptualization) (Methodology) (Data curation) (Writing - original draft) (Writing - review and editing) (Visualization), De-Ming Guo (Validation), Lin Chen (Validation), Wan-shou Wu (Validation), Xiu-Li Wang (Conceptualization) (Project administration) (Funding acquisition) (Writing - review and editing), Yu-Zhong Wang (Conceptualization) (Project administration) (Funding acquisition) (Writing - review and editing)

PII:

S0304-3894(20)30026-1

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122040

Reference:

HAZMAT 122040

To appear in:

Journal of Hazardous Materials

Received Date:

2 December 2019

Revised Date:

6 January 2020

Accepted Date:

6 January 2020

Please cite this article as: Fu T, Guo D-Ming, Chen L, Wu W-shou, Wang X-Li, Wang Y-Zhong, Fire hazards management for polymeric materials via synergy effects of pyrolysates-fixation and aromatized-charring, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122040

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Fire hazards management for polymeric materials via synergy effects of pyrolysates-fixation and aromatized-charring

Teng Fu, De-Ming Guo, Lin Chen, Wan-shou Wu, Xiu-Li Wang*, Yu-Zhong Wang*

The Collaborative Innovation Center for Eco-Friendly and Fire-Safety Polymeric Materials,

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National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, College of Chemistry, Sichuan University,

*Corresponding

authors.

Tel.

&

Fax:

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Chengdu 610064, China

+86-28-85410755.

E-mail

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[email protected] (Xiu-Li Wang) and [email protected]. (Yu-Zhong Wang)

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address:

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Graphical Abstract

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Highlights

A fire-safe tactic with nitrogen/phosphorus-free molecules is proposed.

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Aryl ether ketone structure decreases the chemical risks of flame retardant issues.

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Fire safety is acchieved by binding aliphatic fragments and aromatized charring.

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ABSTRACT: Previous approaches to suppressing fire hazards are concentrated on brominated flame retardants (BFRs) or phosphorus flame retardants (PFRs). However, their chemical hazards to health and environment have not been able to be ignored currently. It is quite urgent to propose a durable and environmentally-friendly fire-safe strategy, which can eliminate migration, release, and environmental hazards of traditional flame retardants during their use,

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disposal, and recycling. Herein, we design a fire-responsive molecule (FRM) only containing C, H, and O elements based on full-life-cycle consideration and achieve fire safety via the

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synergy of specifically binding aliphatic fragments and aromatized charring in combustion. The resulting polymer shows low fire hazard with excellent self-extinguish, low organic

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volatiles/heat/smoke release, and good comprehensive performance. This polymer can be

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fabricated to fibres with potential applications for textiles, and electronics, etc. Therefore, we achieve a durable fire-safe strategy without flame-retardant chemicals hazard. This approach

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can fundamentally eliminate the potential chemical hazards associated with the introduction of halogen or phosphorus flame retardants and give a new vision about solving the “flame

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retardant chemical hazard issues” that are debated, tracked, and evaluated for several decades.

KEYWORDS: fire-safe polymer, fire hazards, chemical safety, flame retardant, smoke suppression

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1. Introduction Fire hazards are one of the most common disasters in the world that threaten public safety and social development. However, the frequent occurrence of fire in recent years, such as Notre-Dame cathedral fire, Brazil National Museum fire, and London Grenfell Tower fire, makes people aware of the urgency for fire prevention in the ancient and modern architecture. These blaze disasters cause severe casualties, unrecoverable property loss, and permanent

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artifacts disappearance. Therefore, supplies with fire safety property are beginning to be considered more seriously[1-5].

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Hydrocarbon-based polymers, one of the largest and most versatile necessary chemicals, e.g., general polymers, functional materials, natural polymer material, are the most typical

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flammable materials and display large fire load[6]. Physically adding flame suppressant, such

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as brominated flame retardants (BFRs), phosphorus flame retardants (PFRs), and nanoparticles[7-9], is the conventional methods for fire-safe strategy. Through different

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reactions, they can interfere with the chemistry of the flame, promote the flame-extinguishing, or form physical barriers that block heat and mass transfer [3, 10, 11]. However, for physical

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manners, their molecules/particles may migrate and invalid during long-term use, resulting in

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failure of the fire-safe performance, which will bring a severe burden to future fire prevention. Moreover, many halogenated flame retardants are found to be persistent, bioaccumulate and/or toxic (PBT) and banned[12-15]. Some PFRs have been detected in various commercial products, air, and water. They threaten the ecosystems and their possible risks in the organism and environmental impacts cannot be ignored[16-22]. No-halogen and no-phosphorus reactive 4

flame-retardant strategy can avoid the lost efficacy of fire-safety, the migration of flame retardants, bioaccumulation, and water pollution caused by the usage of additive BFRs and PFRs. Given all that, the environmentally friendly fire-safe materials, which can maintain fire-proof effects both now and in the future, is quite urgent. Recently, some thermal response N and/or P elements-containing structure, which can slow heat conduction and reduce flammable gas emissions by carbonization effect, are

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designed and chemically incorporated for fire prevention of polymers. These structures enhance the fire-safe properties through specific chemical char-forming characteristics, such

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as cross-linking, rearrangement reaction, graphitization, end-group capturing, intumescent carbonization[23-29]. However, at the end of the life of these materials, the incorporated

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nitrogen/phosphorous elements make the pyrolysis recycling more complex and decrease the

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quality of the oil product, which brings big problems for their recycling[30]. Todd Emrick and co-workers have developed a series of flame retardants or standalone materials based on

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deoxybenzoin moieties. The deoxybenzoin shows a fireproof function by carbonization at high temperatures [31-33]. If the polymer only contains C, H, and O elements, its full life

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cycle (production, use, recycling) will be greener and environmentally friendly[34-36].

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Considering the life cycle of polymeric materials, the no-halogen, no-phosphorus, and no-nitrogen flame-retardant structure should be designed to eliminate the argument of the use/prohibition of flame retardants. In traditional chemistry reactions, compounds containing aryl ether ketone structures undergo a cleavage reaction at high temperatures. The ether and ester bonds break randomly, 5

resulting in non-flammable carbon dioxide, thermally stable aromatic compound (dibenzofuran, biphenyl diphenoxybenzene, benzophenone, 9H-fluoren-9-one, etc.) as well as graphitization char derived from the fluorenone structure[37]. Besides, the aryl ketone derivatives, like benzophenone, converted to their triplet states, can attract small radicals[38-40], which are fundamental to sustaining combustion reactions, i.e., the aryl ketone probably reduces the concentration of radicals in the flame zone. Given all that, the

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chemical characteristics mentioned above of aryl ether or ketone structure with no-halogen/phosphorus elements shall have positive contributions towards polymer fire-safe.

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Herein, inspired by the molecular chemical reactions, we design a fire-responsive molecule (FRM) and propose a durable fire-safe strategy via the synergy of fragment fixation

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and aromatized promotion during pyrolysis and combustion. The no-halogen and

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no-phosphorus FRM structure, only containing C, H, and O elements, can be chemically incorporated into polymeric materials, avoid the migrate and invalid problem, and show

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environmentally friendly features. The FRM is chemically incorporated into poly(ethylene terephthalate) (PET), showing typical and colossal fire hazard[41], to verify its fire-safe and

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smoke suppression effects. The FRM-based polymer (FRMxPET, synthesis, and

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characterization are shown in SI), without any flame-retardant elements (such as P, Br, N), exhibits excellent fire-proof performance with great self-extinguishing and char-forming ability. The FRM structure can respond to fire and control its hazard by fixating aliphatic segments produced in pyrolysis and transforming to oxygen-doped aromatic rings during combustion. Moreover, the inherent fire-safe FRM20PET is fabricated to fire-safe polymeric 6

fibres and exhibits potential applications for textiles, and electronics, etc. Therefore, we achieve a fire-safe strategy based on full-life-cycle consideration without flame-retardant chemicals hazard.

2. Result and discussion 2.1 Fire hazards assessments

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We first studied the inherent fire-safe performance of polymers. As we all know, limited oxygen index (LOI), which gives the oxygen concentration (vol. %) needed to keep a material

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burning, as an essential index of fire resistance, is widely used to measure the flame retardance of materials. As shown in Fig. 1A, the LOI values of FRM5PET, FRM10PET, and

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FRM20PET increase to 26.2, 28.0, and 32.0, respectively. The FRMxPETs, especially the

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FRM20PET, exhibit excellent flame retardance. From the images after LOI tests (Fig. 1B), the FRM structure can promote the carbonization of polyesters to form char layer in combustion

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and enhance the flame retardance effectively. Further, the UL-94 tests are carried out to investigate the self-extinguishing of the FRMxPETs. As shown in Table S1, the incorporation

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of FRM structure can enhance the self-extinguishing property. The pure PET burns out in the

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UL-94 test at the 2nd ignition. And FRM20PET shows good self-extinguishing. The 1st and 2nd after-flame time of FRM20PET are both 2 s, which is obviously lower than the t1 (10 s) and t2 (15 s) of PET.

Then, the thermal degradation processes and char forming ability of FRMxPET is indicated by thermal gravimetric analysis (TGA) under nitrogen and air atmosphere. The 7

TGA and DTG data are shown in Fig. S1. The char residue (CR) in nitrogen are shown in Fig. 1A. Compared with pure PET (8.6 wt%), the incorporation of FRM can significantly promote the carbonization of copolyester. The char residue at 700 °C (CR700) of FRM5PET, FRM10PET, FRM20PET are 17.1 wt%, 20.3 wt%, and 27.0 wt%, respectively. The TGA analysis in nitrogen illustrates that the FRM structure can enhance the char forming performance. The char residue at 500°C (CR500) or 550 °C (CR550) in the air atmosphere is

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shown in Table S2. Compared with PET (CR500, 15.6 wt% and CR550, 2.6 wt%), the CR500 and CR550 of FRM20PET increase to 25.8 wt% and 19.8 wt%, respectively. This result illustrates

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the FRM structure can also promote the formation of char in the air atmosphere at a relatively high temperature (500-550 °C) and delay thermal-oxidative degradation. Combining the LOI

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and TGA tests, the FRM structure can slow the thermal decomposition down and increase the

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formation of char, which can prevent the heat transfer to the internal of polymeric materials as a protective barrier.

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Further, the theoretical value and experimental LOI and CR are compared to evaluate the fire-safe performance of polyesters. Usually, the char forming performance and LOI of

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polymeric materials, especially the hydrocarbon-based polymers made up of carbon (C),

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hydrogen (H), and oxygen (O), have a great relationship with their molecular-level chemical structure [42-45]. Therefore, the relationship among the char forming performance, LOI, and the chemical structure is elucidated according to the Van Krevelen equations (Formula S1 and S2, Table S3). The applicability of the equations for polymer has been analyzed in SI. The group division diagram and the char-forming tendency (CFT) value of functional groups are 8

shown in Fig. 1C. As shown in Fig. 1D, the CFT of carbonyl, ethylene, and ether groups are 0, which means the aliphatic groups do not participate in the carbonization process. The calculated CR and LOI values, as well as the experimental values, are shown in Fig. 1E, 1F and Table S3. Unexpectedly, the experimental CR value of FRMxPETs is significantly higher than the theoretical CR value, and the experimental LOI value is also obviously more elevated than the theoretical LOI value. This result reflects the existence of the unique intermolecular

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reaction among “terephthalate, ethylene glycol, aryl ether or aryl ketone structure derived from FRM” at the high temperature. And the specific chemical reaction, occurring at high

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temperature and combustion, endows the polymer with a surprising char forming ability and fire-safe performance. These chemical reactions are discussed in depth below.

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MCC is an effective milligram-scale measurement system for evaluating the heat release,

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which serves as a valuable precursor to larger scale flammability tests such as UL-94 and cone calorimetry. The heat release capacity (HRC) and the heat release data are shown in Fig.

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1G and Table S4. The HRC of PET, FRM5PET, FRM10PET, and FRM20PET are 383, 282, 258, and 201 J/g K, respectively. The FRMxPETs, especially the FRM20PET, show low HRC.

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The incorporation of the aryl ether ketone structure can evidently decrease the HRC.

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Generally, the materials with HRC values lower than about 300 J/g K can self-extinguish and show excellent fire-safety performance because the heat release is not high enough to overcome heat losses and so the flame cannot propagate[44, 45]. The MCC results illustrate the FRM20PETs exhibit low heat release behavior.

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The fire-safe property and the combustion behavior are further assessed with oxygen consumption cone calorimetry, which relates to fire developing scenarios. The HRR and THR curves of PET and FRM20PET are shown in Fig. 1H. Other detailed data are shown in Fig. S2 and Table S5. The FRMxPETs display the lower PHRR and THR. Compared with pure PET (734 kW m-2), the PHRR of FRM20PET (405 kW m-2) reduces by 44.8%. The low PHRR illustrates the FRM20PET is less susceptible to cause flame propagation in the fire. The THR

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of FRM20PET is also reduced to 64 MJ m-2 compared with pure PET (70 MJ m-2). The FRM structure can enhance the char residue of the polymer during combustion. Compared with

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pure PET (its char residue is 9.0 wt%), the char yield of FRM20PET is increased to 27.7 wt%. The increase of the char residue means the decrease of flame fuel in the gas phase. Moreover,

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the average effective heat of combustion (av-EHC) of the fuel is also discussed. The av-EHC

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of FRM20PET is 17.6 MJ/kg, which is lower than the av-EHC of PET (19.2 MJ/kg). Further, we explore the effect of FRM structure on smoke release. The FRM structure can reduce the

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smoke release of PET. The total smoke release (TSR) is suppressed. When 20 mol% modified monomers are introduced, the TSR (1230 m2 m-2) of FRM20PET is 29.8% lower than that of

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pure PET (1752 m2 m-2).

Further, we research the smoke suppression effects of FRM structure by intuitive

observation. The observation of the smoke formation process through shielded fire has always been a difficult problem in material combustion analysis. Here, for the first time, we construct a laser lighting system, a high-speed camera with filter filtering the firelight and ambient-light 10

for in situ observation of smoke formation processes during combustion (Actually the combustion in cone calorimeter is more proper for observing the smoke formation. But due to the occlusion of the cone heat, the whole fire field cannot be analyzed. Instead, the combustion in the LOI test is chosen to observe the smoke formation processes of the full fire field. The apparatus diagram is shown in Fig. 2 and S5, the detailed information and are explained in SI). As shown in Movie S1, for pure PET, it melts during the violent combustion

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process with almost no obvious carbonization and produces heavy smokes. As a comparison, we can intuitively find that FRM20PET produces less smoke than PET. Besides, the

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FRM20PET quickly forms a char layer while melting. The FRM structure not only exhibits smoke suppression function but also improves the char forming capacity.

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The above fire hazards assessment results illustrate the FRM is quite a fireproof

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molecule structure, which can play a firefighting role in combustion chemistry.

2.2 Fireproof processes analysis

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The above analysis illustrates the FRM molecule shows a fire-proof function during

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combustion and enhances the fire-safety properties obviously, in which FRM20PET possesses the best fire-safety property. Undoubtedly, the most crucial element for the fire-safe function is the fire-safe chemical and physical effects during combustion, so we further analyze the chemical pyrolysis process of FRM20PET.

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The main pyrolysis process of FRM20PET and the molecular-level chemical reaction of the ether ketone ether structure at high temperature are researched by PY-GC-MS. The total ion chromatogram and corresponding M/z are also shown in Fig. S3. The detailed chromatographic and mass spectrometry data of PET and FRM20PET cleavage products are shown in Table S6 and S7. The cleavage products of FRM20PET are quite different from the cleavage products of PET, comparing the chromatogram peaks and

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their molecular weight. For PET, almost no cleavage products form at 12.5-18.0 min retention time, while FRM20PET shows more peaks with higher molecular weight. Since the column of

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the PY-GC-MS instrument is a non-polar column, which means the higher the retention time is, the higher the molecular weight and the more stable the pyrolysates are. That is,

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FRM20PET forms more stable pyrolysis products.

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For FRM20PET (shown in Fig. 3), containing both the BHET and FRM units, parts of its pyrolysis processes are similar to PET (Fig. S4). The ester bonds, involving a cyclic transition

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state, cleave to form benzoic acid derivatives and vinyl terephthalate derivatives. On the one hand (Path 1a), the derivatives of benzoic acid continue to break, producing a series of

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flammable small molecule radicals (hydrogen, benzene, methyl, ethyl, vinyl, vinyl alcohol

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radicals). Small molecule radicals promote further fragmentation of the polymer molecular segments, and they also react to form flammable compounds. On the other hand (Path 1b), the chemical bonds of vinyl terephthalate (B1) continue to break. The vinyloxy structure is cleaved, and the 4-(ethoxycarbonyl)benzoic acid (B2) is formed. The B1 can also react with hydrogen radicals and form ethyl vinyl terephthalate (B3). Then, after intramolecular bond 12

cleavage, the vinyl benzoate (B4) is produced. The B4 can react with phenyl, vinylidene, carbonyl or hydrogen radicals and forms vinyl [1,1'-biphenyl]-4-carboxylate (B5), vinyl 4-vinylbenzoate (B6), vinyl 4-formylbenzoate (B7) and ethyl benzoate (B8). Then, the ethyl 4-(2-(benzoyloxy)ethyl)benzoate (B9), butane-1,4-diyl dibenzoate (B10) and methyl benzoate (B11) are formed after the intermolecular reaction and intramolecular bond cleavage. The B11 also undergoes intermolecular reactions and form ethane-1,2-diyl dibenzoate (B12) and

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methyl (E)-4-(2-(benzoyloxy)vinyl)benzoate (B13).

The FRM structure exhibits specific pyrolysis processes (Fig. 3), which is the main

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reason that the FRM20PET shows good flame retardance and char forming performance. In the pyrolysis (Path 2), after the aryl ether bond cleave and the intramolecular reactions, the

and

produce

(4-(4-ethoxyphenoxy)phenyl)(phenyl)methanone

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break

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aryl ketone structure transforms to 9H-fluoren-9-one (C1). Meanwhile, the aryl ether bonds (C2)

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(4-(4-methoxyphenoxy)phenyl)(phenyl)methanone (C3). Then, the hydroxyethoxy cleavage intramolecular

cyclization

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and

of

C2

and

C3

occur

and

forms

(8-methyldibenzo[b,d]furan-2-yl)(phenyl)methanone (C4). The C2 and C3 can also bind

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methyl radicals and produce phenyl(9H-xanthen-2-yl)methanone (C5). The aryl ketone

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structure breaks and forms 1-ethoxy-4-phenoxybenzene (C6). After that, the C6 reacts with ethyl radicals and forms 2-ethoxy-9-methyl-9H-xanthene (C7). The proposed pyrolysis processes illustrate that the FRM structure can respond to the high temperature (fire) and transform to oxygen-containing stable aromatic heterocyclic rings and aromatic ketone compounds. These converted-stable aromatic molecules can lower the concentration of the 13

combustible and show a fire-proof effect in the gas phase[46]. Moreover, they are more likely fixated in the condensed phase, participate in the carbonization, and enhance the char forming ability of polymers. Besides, these phosphorus/nitrogen-free pyrolysis products illustrate that FRMxPET polymers do not require dephosphorization and denitrification at the end of the life cycle during their recycling. That is, this approach is friendly-environmentally.

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Moreover, the Raman, laser scanning confocal microscopy (LSM), and SEM tests are used to characterize the compactness and physical morphology of the char layer after

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combustion. The two most important peaks of carbon in the Raman spectrum residue are the D-band absorption peak of 1360 cm-1 and the G-band absorption peak of 1580 cm-1, which

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represents defective graphitized carbon and SP2 hybridized carbon structure, respectively[47].

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The ratio of the peak area to the ID/IG is related to the size of the carbon layer. The larger the ID/IG, the smaller the crystallite size and the denser the carbon layer[48]. As shown in Fig. 4A,

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we performed a fitting calculation through Origin 9.0. The ID/IG of PET, FRM5PET, FRM10PE, and FRM20PET are 2.30, 2.40, 2.56, and 2.69, respectively. Compared with pure

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PET, with the introduction of FRM structure and the increase of its content, the ID/IG of

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FRMxPETs gradually increases, which indicates that the carbon layer formed by FRMxPETs has a smaller crystalline structure and a denser carbon layer. The macroscopic appearance of the residual char layer after cone calorimeter tests at 20

magnification is analyzed by LSM. From the digital images (shown in Fig. S6), with the incorporation and increase of FRM structure, fewer bubble holes are observed, and the char 14

layer becomes denser. The more intuitive three-dimensional-physical morphology of the char layer is reconstructed and assessed in Fig. 4B. The white/red color means the higher place of the char layer, while the blue/green color represents the lower position of the char layer. The more pronounced the color difference in the partial area, the more discontinuous the carbon layer is. And the more holes observed mean that the carbon layer is less compact. Conversely, the carbon layer is more continuous and denser. As shown in the images, PET forms a

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non-continuous and porous char layer structure. Its char layer is easily broken during combustion and cannot prevent the heat and mass transfer. While the FRMxPETs form a more

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continuous and compact char layer, which coincides with the Raman results.

In the high-magnification SEM images (500 X, Fig. 4C), we observe that some

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micro-nano structures are produced in the char surface of FRMxPETs after cone calorimeter

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tests, compared with the smooth char surface of PET. In 10000 magnification SEM images, with the increase of FRM content, FRMxPETs tend to form a regular "fence"-like structure on

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the surface of the carbon. The formation of this unusual structure indicates that unique chemical reactions among the aryl ether ketone structures may take place at high temperatures

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and during combustion. The FT-IR is carried out to confirm the detailed structure of the char

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layer. As shown in Fig. S7 and Table S8, parts type of infrared absorption peaks of the char layer for PET and FRM20PET are basically the same. These peaks assign to the aromatic rings, carbonyl bonds, ether groups. For FRM20PET, it shows specific aryl ether bonds, which illustrate the aryl ether rings remain in the char layer (condensed phase). Moreover, we analyze the char layer by PY-GC-MS and deduce the specific structure of the char layer (Fig. 15

S8 and Table S9). The pyrolysates are aromatic compounds, such as, benzene, phenol, acetophenone, 1,1'-biphenyl) and aryl ether/aryl ketone structures (oxydibenzene, dibenzo[b,d]furan,

1-ethoxy-4-phenoxybenzene,

(8-methyldibenzo[b,d]furan-2-yl)(phenyl)methanone

and

(9-methyl-9H-xanthen-2-yl)(phenyl)methanone. That is, the char layer after combustion is constituted of an aryl ether-doped or aryl ketone-doped aromatization structure. The

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fire-respond conversion of FRM structure to the oxygen-containing stable aromatic heterocyclic rings and aromatic ketone compounds play a physical barrier in the condensed

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phase.

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Based on the above results, we can conclude that the FRM structures can suppress the flame hazards and shows fire-safe function both in gas and condensed phases. As shown in

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Fig. 5, the FRM structure fixates methyl or ethyl segments generated by copolyester and forms oxygen-containing aromatic compounds. These flame-retardant aromatic compounds

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can reduce the concentration of gas fuels and play a firefighting role in the gas phase. At the

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same time, the FRM structure is more inclined to aromatize into the condensed phase, promote carbonization, and prevent heat and mass transfer by forming a compact aryl-ether-doped and aryl-ketone-doped physical barrier. Hence, the inherent fire-safe polyester with no-halogen and no-phosphorus is obtained.

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2.3 Comprehensive performances evaluation Further, we examine the thermal properties, which affect the processability and application temperature range of the polymer. As shown in Fig. 6A and S9, with the incorporation of FRM, the glass transition temperature (Tg) of FRMxPETs increases, which is due to the introduction of the rigid ether ketone ether structure reducing the flexibility of the

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polymer chains. The Tg of copolyesters is all above 80 °C, which is higher than that of PET. The crystallization temperature (Tc), crystallization enthalpy (ΔHc), melting point (Tm), and

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melting enthalpy (ΔHm) of FRMxPETs gradually decrease with the increase of the FRM content. Introducing 10 mol% FRM, almost no crystallization peaks are observed in the

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cooling curve of FRM10PET, and a cold crystallization peak appears in the subsequently

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heating scan. For FRM20PET, both the melting peak and the crystal peak disappear in its DSC curves. However, its Tm still can be determined by melting point meter (about 200 °C). This

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phenomenon illustrates that FRM20PET has a meager crystallization rate and it cannot crystallize during DSC testing. In a word, the above results indicate the introduction of FRM

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structure affects the regularity and symmetry of the polymer chains and decreases the

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crystalline ability. It is worth noting that all polymers still maintain crystallization ability and show the melting point in the range of 200-233 °C. The initial decomposition temperature (T5%, defined as the temperature of 5 wt% weight

loss, in Fig. 6A) of FRM5PET, FRM10PET, FRM20PET in N2 are 391, 392 and 394 °C, respectively. All polymers show excellent thermal stability. The FRM structure has no 17

negative impact on the thermal stability. The tensile property of FRMxPETs is also investigated. Their stress-strain curves and detailed data are shown in Fig. S10 and Table S10. The yield stresses of FRM5PET, FRM10PET, and FRM20PET (56.3, 53.9, and 55.8 MPa, respectively) are almost consistent with PET (57.2 MPa), demonstrating the incorporation of

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the fire-responsive structure has no adverse effect on the stretching yield stresses.

What is more, we study the spinnability of no-halogen and no phosphorous fire-safe

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FRM20PET. The diagram of trial spinning experiment devices is shown in Fig. 6B. FRM20PET can be fabricated to light-yellow fibers (Fig. 6C). As shown in the SEM

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micrographs, the surface of the obtained fibers is smooth, and the fiber diameter is uniform.

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The above results reveal that this fire-safe polymeric material FRM20PET shows the potential

3. Conclusion

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application as textiles, and electronics, etc.

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We provide a novel approach to solve the “flame retardant chemical safety issues” that

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environmentalists, materials scientists, regulators, and flame-retardant industry debate, track, and evaluate for several decades. Based on full-life-cycle consideration, an inherent fire-safety strategy, and a fire-safe material, via introducing a fire-responsive molecule containing aryl ether ketone structure with no flame-retardant elements, is proposed. The FRM structure can endow the polymer with superior fire-safe function, excellent 18

self-extinguish, and low fire hazard. The ether ketone ether structure can fixate methyl or ethyl segments generated by polymers and reduce the concentration of gas fuels. It is more inclined to aromatize into a condensed phase and forms a compact physical barrier. The FRM20PET also shows the excellent comprehensive performance of the material. Moreover, it can be fabricated to fire-safe polymeric fibers and exhibit potential applications for textiles, and electronics, etc. Consequently, we propose a novel strategy towards fire safety without

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flame-retardant chemicals risk and give a better prospect forward fire safety.

Corresponding Author

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* E-mail: [email protected]

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* E-mail: [email protected]

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AUTHOR INFORMATION

Notes

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The authors declare no competing financial interest.

Declaration of interests

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.

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ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grants 21634006, 51827803 and 51721091), and the Sichuan Province Youth Science and Technology Innovation Team (No. 2017TD0006).

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CRediT authorship contribution statement Teng Fu: Conceptualization, Methodology, Data Curation, Writing-Original Draft, Writing Review & Editing, Visualization

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De-Ming Guo: Validation

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Wan-shou Wu: Validation

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Lin Chen: Validation

Review & Editing

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Xiu-Li Wang: Conceptualization, Project administration, Funding acquisition, Writing-

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Yu-Zhong Wang: Conceptualization, Project administration, Funding acquisition, Writing-

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Review & Editing

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[32] R. E. Lyon and T. Emrick Non-halogen fire resistant plastics for aircraft interiors. Polym. Adv. Technol. 19 (2008) 609-619. [33] K. A. Ellzey, T. Ranganathan, J. Zilberman, E. B. Coughlin, R. J. Farris and T. Emrick Deoxybenzoin-based polyarylates as halogen-free fire-resistant polymers. Macromolecules 39 (2006) 3553-3558. [34] A. Rahimi and J. M. García Chemical recycling of waste plastics for new materials production. Nature Reviews Chemistry 1 (2017) 0046. [35] R. Miandad, M. A. Barakat, A. S. Aburiazaiza, M. Rehan and A. S. Nizami Catalytic

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[48] J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt and S. R. P. Silva Raman spectroscopy on

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amorphous carbon films. J. Appl. Phys. 80 (1996) 440-447.

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Figure captions

0

Limited Oxgen Index 10 15 20 25

5

PET

B 30

LOI CR

21.0

8.6

26.2

FRM5PET

17.1 28.0

FRM20PET

27.0 0

5

10

15 20 CR (wt%)

C

25

32.0

30

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Group division diagram

The CFT value of function group

Theoretical CR Theoretical CR 27.0 27.0 Experimental CR Experimental CR

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CFT

17.1 17.1

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1212

0

PET

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383 350

300

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282

258

250

FRM20PET

600 405 400 200

28.3 28.3

F

PET

FRM20PET 1752

1500

1230

1000 500 0

0

H

28.0 28.0 26.2 26.2 25.6 25.6 24.3 24.3

2000

734

0

20 P

ET M

FR

10 P

ET M

5P

FR

FR

M

PE

T

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200

ET

201

HRR (kW m-2)

800

TSR (m2 m-2)

400

E

Theoretical LOI Theoretical LOI 32.0 32.0 Experimental LOI Experimental LOI

2222 21.0 21.0 2020 20.6 20.6

8.6 8.6 11.5 11.5 8.0 8.0

PE F PE T FRRM T M5 P E FR 5 P E T FR M T M10 P FR 10 P ET FR M E T M20 P 20 P E ETT

0

2626 2424

14.6 14.6

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88

0

D

2828

19.3 19.3

1616

4

3232 3030

20.3 20.3

2020

1.25

3434

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2828

CR (wt%) CR (wt%)

Function group

HRC (J g-1 K-1)

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20.3

PE F PE T FRRM T M5 P E F 5 PE T FRRM T M10 P F 10 P E FRRM ETT M20 P 20 P E ETT

FRM10PET

G

Images after LOI tests

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Limited Oxygen Index Limited Oxygen Index

A

100 200 300

Time (s)

0

100 200 300

Time (s)

Fig. 1 (A) The comparison for LOI and CR of PET and FRMxPETs. (B) The digital images of PET and FRMxPETs after LOI tests. (C) The group division diagram of FRMxPETs. (D) The 26

CFT value of functional groups based on Van Krevelen equations. (E) The theoretical and experimental CR of PET and FRMxPETs. (F) The theoretical and empirical LOI of PET and FRMxPETs (G) The HRC data of PET and FRMxPETs in MCC tests. (H) HRR and TSR curves of PET and FRM20PET in developing fire scenarios.

Digital images in LOI test at 28.0 vol % O2 by laser lighting system and high speed camera High speed cameras

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Carbonization effects

Laser source

Firelight filter

5s

10 s

10 s

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5s

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PET 0s

15 s

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0s

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FRM20PET

15 s

20 s

25 s

20 s

25 s

Fig. 2 The smoke formation processes of PET and FRM20PET in LOI tests by shielding flame

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of the laser lighting system and a high-speed camera.

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Thermal pyrolysis way of FRM20PET

Path 1

Path 1b

Path 1a

Divinyl terephthalate

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Derivatives of benzoic acid

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Path 2

Aryl ether bond cleavage

Aryl ketone bond cleavage

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Intramolecular reaction

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Hydroxyethoxy cleavage and

Binding radicals

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intramolecular cyclization

Binding radicals

Fig. 3 Main pyrolysis processes of FRM20PET. The pyrolysis temperature is 500 °C.

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D band

800 1200 1600 2000 Wavenumber (cm-1)

FRM20PET

FRM10PET

G band

ID/IG=2.56

D band

800 1200 1600 2000 Wavenumber (cm-1)

G band

D band

800 1200 1600 2000 Wavenumber (cm-1)

ID/IG=2.69

Intensity (a.u.)

ID/IG=2.40

Intensity (a.u.)

Intensity (a.u.)

FRM5PET

G band

Intensity (a.u.)

PET ID/IG=2.30

A

G band

D band

800 1200 1600 2000 Wavenumber (cm-1)

C

B

PET

500 X FRM20PET

FRM10PET

FRM20PET

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FRM10PET

FRM5PET

FRM5PET

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PET

10000 X

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20 X

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Fig. 4 (A) The Raman spectra of PET, FRM5PET, FRM10PET, and FRM20PET after cone test. (B) The macroscopic appearance of the residual char layer at 20 magnification by LSM

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analysis. (C) The SEM images of the char layer at 500 (up) and 10000 (down) magnification.

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Gas phase

Condensed phase

Flame-retardant gas compounds

Aliphatic fragment fixation Physical Barrier

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Firefighting

Char layer

Carbonization

Pyrolysis and burning

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Fig. 5 Proposed fireproof modes in the condensed phase and gas phase. The FRM structures

(A)

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can manage the flame hazard and shows fire-safe function both in gas and condensed phases.

Melting temperature

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Glass transition temperature

FRM 20PET

88 ° C

84 ° C

FRM 10PET

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82 ° C

FRM 5PET

81 ° C

PET

80

82

84

86

88

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(B) Spinning experiment

Initial decomposition temperature

200 ° C

90 180

394 ° C

225 ° C

392 ° C

233 ° C

391 ° C

250 ° C

200

220

240

Temperature (° C)

(C) FRM20PET fibers

Gottfert

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RG50

Spinneret

Primary fiber

30

395 ° C

260 380

390

400

410

Fig. 6 (A) Thermal performance of PET and FRMxPETs. (B) The diagram of the trial spinning experiment. (C) The processing performance study. Digital and SEM images of

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FRM20PETs fibers are given.

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