Fire-Safe Polyesters Enabled by End-Group Capturing Chemistry

Article abstract of DOI:10.1002/anie.201900356

Upon heating, polyesters decompose to small molecules and release flammable volatiles and toxic gases, primarily through chain scission of their ester linkages, and therefore exhibit poor fire-safety properties, thus restricting their applications. Report

Full text of DOI:10.1002/anie.201900356

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Accepted Article
Title: Fire–Safe Polyesters Enabled by End–Group Capturing
Chemistry
Authors: Bo-Wen Liu, Li Chen, De-Ming Guo, Xiao-Feng Liu, Yu-Fei
Lei, Xiao-Min Ding, and Yu-Zhong Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201900356
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10.1002/anie.201900356
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Fire–Safe Polyesters Enabled by End–Group Capturing Chemistry
Bo-Wen Liu, Li Chen*, De-Ming Guo, Xiao-Feng Liu, Yu-Fei Lei, Xiao-Min Ding, Yu-Zhong Wang**
Abstract: Polyesters would decompose to small molecules to release
flammable volatiles and toxic gases upon heating, primarily through
chain scission on their ester linkages, therefore exhibiting poor fire
safety that highly restrict their applications. Herein, aiming to elevate
the fire safety of polyesters, we report an end-group-capturing effect
of (bis)oxazoline groups generated from the thermal rearrangement
of N-(2-hydroxyphenyl)phthalimide (HPI) moiety, which was
incorporated into the polyester chain via copolymerization. These
copolyesters, as a result, exhibited high efficiency in retarding the
decomposition speed by capturing the decomposed products,
particularly for the carbonyl-terminated fragments, consequently
increasing the fire safety regarding self-extinguishing, anti-dripping,
and inhibiting heat release and smoke production of the polyesters.
The successful application of this method in both semi-aromatic and
aliphatic polyesters provided promising perspectives to designing
versatile fire-safe polymers.
networks. Among the rest, copolymerizing phosphorus-containing
monomers receives ever-increasing attention due to their high
efficacy, low toxicity and acceptable economic efficiency.
Architecturally, typical polymers with unique structures can also
achieve intrinsic fire safety in the absence of conventional flame-
retardant elements such as phosphorus, silicon, chlorine and
bromine. Through different thermal reactions, these structures
can preclude the decomposition, promote aromatization,
carbonization and further char formation, thus increase the fire
safety. For example, deoxybenzoin-based polymers release
water at the beginning of pyrolysis process, then convert to
diphenylacetylene structures that further promote aromatization
and carbonization at higher temperature[6]. Another example is a
series of bisphenol-1,2,3-triazole (BPT) based polymers including
both thermoplastic and thermosetting ones, which undergo loss
of N2 at high temperature, then go through thermal rearrangement
reaction to phenylindoles and char formation toward fire safety[7].
Analogously, phenylethynyl containing polymers aromatize and
carbonize through specific crosslinking reaction at a high
temperature[8], which can also play a positive role on fire safety.
Undoubtedly, the most crucial element for preparing fire-safe
polymers is to comprehensively understand the decomposition
mechanism during combustion. However, the fire-safe strategies
for every specific polymer still need to be customized because
different polymers undergo different decomposition processes. A
universally applicable method remains unavailable. Polyester is a
category of polymers that contain the ester functional group in the
main chain. Upon heating, polyesters undergo chain scissions at
their ester linkages[12]. This process can be further self-catalyzed
by the pyrolysis products, i.e., the carboxyl- and hydroxyl-
terminated fragments. Recently, Mathias and co-workers[9]
Polymeric materials are a critical class of materials central to
applications from advanced carbon-fiber composites and
functional organics, to electronic and electrical devices, synthetic
fibers and packaging[1]. However, despite the tremendous
convenience they bring to our daily life, serious problems also
exist. Poor fire safety for instance, is the main drawback of most
polymeric materials that severely threatens human lives and
possessions[2]. Different methods have been proposed to
enhance the fire safety of the polymeric materials[3], among which
additive-type flame retardants have been mostly adopted in
practice[4]. Although these flame retardants are low cost, highly
effective and easy to be processed, imperfections like migration
and blooming of the small-molecular additives, deterioration of the
materials’ inherent properties, and potential persistence, bio-
accumulation and toxicity (PBT) for the entire eco-system are still
the urgent issues to be addressed[5]. Therefore, intrinsically fire-
safe polymers, whose characteristic comes from their chemical
design instead of using flame-retardant additives, are highly
desired.
reported
the
thermal
rearrangement
of
N-(2-
hydroxyphenyl)phthalimide (HPI) to benzoxazole with the release
of carbon dioxide, as shown in Scheme 1a[10]. The products
generated from the thermal rearrangement, such as the non-
flammable carbon dioxide and water vapor, as well as the
thermally stable (single or bis)benzoxazole structures, shall all
have positive contributions to fire safety of polymers, particularly
polyesters. In particular, the oxazoline-containing structures that
have long been used as chain extender for polyesters[11] (Scheme
1b), possess a great potential to end-group-capturing effect for
typical polyesters while being applied with fire or heat. Given all
that, we propose a new fire-safe strategy based on the above-
mentioned end-group-capturing effect. Through capturing the
carboxyl-terminated fragments with the benzoxazole structures,
the degradation of polyesters is inhibited, the solid-state residues
are increased, and the release of combustible volatiles at high
temperature is suppressed. Consequently, the fire safety of the
polyesters is improved.
Inspired by molecular design scenario of copolymerization,
diverse intrinsically fire-safe polymers have been designed and
prepared by chemically incorporating different characteristic
structures into their backbones, side chains, or crosslinking
[*]
B. W. Liu, L. Chen, D. M. Guo, X. F. Liu, Y. F. Lei, X. M. Ding, Y. Z.
Wang
The Collaborative Innovation Center for Eco-Friendly and Fire-safety
Polymeric Materials, National Engineering Laboratory of Eco-
Friendly Polymeric Materials (Sichuan), State Key Laboratory of
Polymer Materials Engineering, College of Chemistry, Sichuan
University, Chengdu 610064, China
[*]
E-mail: l.chen.scu@gmail.com; yzwang@scu.edu.cn
Fax: +86-28-85410755; Tel: +86-28-85410755
Herein, we used poly(ethylene terephthalate) (PET), the
most commonly used thermoplastic polyester with the biggest
market share for synthetic fibres in the world[13], as typical
representative to demonstrate the proposed strategy, and
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explored a novel PET-based copolyester with intrinsic fire safety
without using traditional flame-retardant element. With 20 mol%
HPI (relative to dimethyl terephthalate (DMT)) incorporated, the
resultant PET-co-HPI20 exhibited excellent fire safety with great
self-extinguishing and anti-dripping performance, along with ultra-
low heat release and smoke production. Detailed pyrolysis and
decomposition analyses showed that the release of carbonyl-
terminated fragments was inhibited as expected, which further
confirmed the end-group-capturing effect. Further investigation
displayed a great applicability of this method in aliphatic
polyesters like poly(butylene succinate) copolyester (PBS), where
the resultant copolyester PBS-co-HPI20 also exhibited great fire
safety. This suggests that our proposed strategy is a versatile
method for fire-safe polyesters and other polymers.
Scheme 1. Chemistry behind the hydroxyl-containing phthalimide (HPI)
structure. (a) Thermal rearrangement of HPI. (b) Potential chain extending effect
on PET.
Scheme 2. Synthetic procedures of phthalimide monomers (a) and their PET copolyesters (b). TMA = 1,2,4-benzenetricarboxylic anhydride, NMP = 1-methyl-2-
pyrrolidinone, DMT = dimethyl terephthalate, EG = ethylene glycol, 3-HPIM = N-(2-hydroxy-5-methoxycarbonylphenyl)-4-methoxycarbonyl phthalimide, 3-MPIM =
N-(2-methyl-5-methoxycarbonylphenyl)-4-methoxycarbonyl phthalimide, 3-PIM = N-(3-methoxycarbonylphenyl)-4-methoxycarbonyl phthalimide.
HPI-containing monomer (3-HPI) was firstly synthesized by
using classic imidization method[14]. It was then copolymerized
with DMT and EG by melt polycondensation to obtain the HPI-
containing copolyesters named as PET-co-HPIx (Scheme 2),
where x represented the relative molar content of HPI to DMT.
Two reference copolyesters containing phthalimide with different
ortho-substituent groups, PET-co-MPIx and PET-co-PIx, were
also prepared by replacing the hydroxyl group of HPI with methyl
of MPI and hydrogen atom of PI, respectively. Since these PET
copolyesters do not dissolve in conventional solvent for GPC test,
detailed molecular weight data could not be obtained; however,
all these copolyesters exhibited an intrinsic viscosity higher than
0.45 dL/g. Thermogravimetric analysis (TGA) (Figure S13 and
Table S4) revealed that PET-co-HPI had a slightly decreased
extinguished immediately alongside the first ignition, and within 3
s after the second ignition without any dripping (Figure 1a),
exhibiting excellent efficiency of HPI on suppression of both flame
spread and dripping phenomenon. We also noted that PET-co-
HPI20 generated compact char layers (Figure S14) that acted as
an efficient physical barrier to prevent heat/oxygen transfer from
the outside and release of combustible volatiles from the inside.
The limiting oxygen index (LOI, defined as the minimum
concentration of oxygen that support combustion of a polymer)
was further used to evaluate the fire safety of the copolyester. The
results (Figure 1g) showed that the LOI value of PET-co-HPI
copolyesters increased gradually with the HPI content, and
reached a high value of 33% for PET-co-HPI20.
Considering the combustion environment in real fire is quite
different from that of a small bench-scaled tests like UL-94 and
LOI, we conducted cone calorimetry, an oxygen consumption
technique providing a burning scenario much similar to real fire,
to further evaluate the fire safety of the obtained copolyesters, as
shown in Figure 1 and Table 1. The peak heat release rate (pHRR)
of PET-co-HPI copolyesters, as one of the most important
parameters to evaluate fire safety, decreased gradually with the
increase of HPI content. In particular, PET-co-HPI20 had the
lowest pHRR value of 276 kWm-2, which was 62% lower than that
of pure PET (734 kW m-2). Contrastively, although the
incorporation of MPI and PI led to an initial decrease of pHRR for
PET-co-MPI5 and PET-co-PI5, incorporating more monomer did
not further lower the pHRR (Table S6), representing a limited
effect on inhibiting the heat release. The total heat release (THR)
of these copolyesters showed similar trends as pHRR, where
initial decomposition temperature (T5%
,
defined as the
temperature where 5% weight loss occurred), which was caused
by the release of CO2 during thermal rearrangement; while the
temperature at the maximum weight loss rate (Tmax) and the
residue at 700 °C increased because of the formation of the more
thermally stable structure after thermal rearrangement.
Pure PET, owing to its linear structure and low melt viscosity
and strength, generated continuous melt drips during combustion,
barely self-extinguished after ignition (Figure 1a). Thus, we
measured the self-extinguishing and anti-dripping performance of
the obtained copolyesters by UL-94 vertical burning test. The
reference samples displayed an improved but limited anti-dripping
performance. In detail, PET-co-PI20 and PET-co-MPI20 both
dripped once or twice and only achieved a UL-94 V-2 rating
(Figure 1a, Table S5). While on the contrary, PET-co-HPI20 self-
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PET-co-HPI20 showed a 17% reduction while PET-co-MPI20 and
PET-co-PI20 didn’t (Figure 1c).
Figure 1. Fire-safety performance of PET, PET-co-PI20, PET-co-MPI20 and PET-co-HPI20. Combustion process during UL-94 test at different time (a). Heat release
rate (HRR) (b) and total heat release (THR) curves (c) obtained from cone calorimetry, embedded SEM images denoted the burning residue of PET (in black dashed
line box) and PET-co-HPI20 (in pink dashed line box). Heat release rate (HRR) curve obtained from PCFC test (d). (e-f) Smoke production rate (SPR) and carbon
monoxide production rate (COP) obtained from cone calorimetry. (g) Limiting oxygen index (LOI) value.
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Smoke is more dangerous than fire itself in threatening human
lives, and is regarded as the deadliest parameter of materials’ fire
safety. Compared to pure PET, the peak smoke production rate
(pSPR) of PET-co-HPI20 (Figure 1e) decreased sharply by 55%,
while the decrease of PET-co-MPI20 and PET-co-PI20 was about
30%. More importantly, carbon monoxide, as the most common
lethal gas in fire, its production rate (COP) also decreased sharply
for PET-co-HPI (Figure 1f), where the peak COP value of PET-
.
co-HPI20 was only 25% of that of pure PET. Scanning electron
microscopy (SEM) observation (Figure 1b and Figure S15) of the
burning residues after cone calorimetry revealed that PET-co-
HPI20 remained highly compact and consecutive char layer,
where pores barely existed in a relative large scope (300 µm ×
300 µm). This residual char acted as a strong barrier for the
transfer of heat and release of combustible volatiles.
Figure 2. Pyrolysis and decomposition process of PET and PET-co-HPI20. Three-dimensional TG-IR spectra of (a) PET and (b) PET-co-HPI20. (c) Absorbance
intensity change of CO2 and carbonyl group as a function of time obtained from TG-IR. (d). Characteristic chromatographic peak of bisbenzoxazole derivate detected
from Py-GC/MS. (e). Thermal rearrangement process of PET-co-HPI.
Pyrolysis combustion flow calorimetry (PCFC), a non-flaming
method for evaluating the combustibility of milligram samples,
revealed similar trends as cone calorimetry did. In particular, the
pHRR from PCFC (Figure 1d) of PET-co-HPI20 decreased
drastically compared to pure PET, while the decrease of PET-co-
MPI20 and PET-co-PI20 was moderate. Additionally, based on the
Van Krevelene method[15], we investigated the influence of
substituent groups on the heat release capacity (HRC) by
calculating the theoretical HRC (THRC) of these copolyesters
(Table 1, Table S7 and Scheme S1). Assuming that no interaction
existed among the chemical units of the copolyesters, the THRC
of PET-co-HPI20, PET-co-MPI20 and PET-co-PI20 were calculated
to be 322 Jg-1K-1, 338 Jg-1K-1 and 289 Jg-1K-1, which were 50.5%
and 7.0% higher but 9.1% lower than the experimental results,
respectively. Such a large difference between the calculated and
experimental HRC of PET-co-HPI20 confirmed the occurrence of
some interactions (i.e., thermal rearrangement and further
reactions) during its pyrolysis. Such interactions inhibited the heat
release of PET-co-HPI and contributed to the exothermal peak at
350 °C of the HRR curve (Figure 1d).
To understand the detailed pyrolysis process of these PET
copolyesters, we analyzed the GC chromatograms of their
pyrolytic volatile products by pyrolysis-gas chromatography/mass
spectrometry (Py-GC/MS) at 500 °C. Specifically, these products
were mainly divided into two parts, in which the PET part marked
in red and the characteristic part marked in blue (Figure S16, S17,
S18, S19 and Table S8). On the one hand, the results showed
that the reference samples PET-co-MPI20 and PET-co-PI20
pyrolyzed by the conventional chain scission, leading to MPI and
PI-containing fragments respectively (Figure S17 and S18),
meaning no interactions occurred among these chemical units
during pyrolysis. On the other hand, PET-co-HPI20 generated
benzoxazole-containing fragments in addition to HPI-containing
fragments (Figure S19), indicating the occurrence of the thermal
rearrangement from HPI to benzoxazole, as illustrated in Scheme
1a. Furthermore, from the GC chromatogram of PET-co-HPI20 at
600 °C, a small but abnormal chromatographic peak showed up
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at around 791 s (Figure 2d, S20 and S21), which was ascribed
the bisbenzoxazole-containing group. This suggested the
occurrence of the second interaction between HPI and the
thermally rearranged intermediate as illustrated in Figure 2e.
beginning but, in contrast, the relevant speed decreased
gradually as time passed by, and finally became slower than PET.
As a consequence, the copolyester generated more residue than
PET did. Furthermore, the complex viscosity of PET-co-HPI20
showing in the dynamic rheology test on heating mode drastically
increased with the elevating temperature (Figure S23), which
contributed positively to the fire-safe performance, and should act
synergistically with the end-group capturing effect. Taken
together, the end-group-capturing effect was confirmed, and the
detailed two-step decomposition process should be as follows. At
first, when PET-co-HPI copolyesters were heated above 350 °C,
the 1st step thermal rearrangement of HPI to benzoxazole
occurred, then the resulting intermediate and the carboxyl
terminals of PET-co-HPI copolyesters participated in the 2nd
thermal rearrangement to form the bisbenzoxazole-containing
group (Figure 2e), thus leading to the end-group-capturing effect.
The mechanical properties of the fire-safe copolyesters was
further investigated. Compared to pure PET, tensile strength of
PET-co-HPI decreased gradually from 57.9 MPa of PET to 40.4
MPa of PET-co-HPI20; and the tensile modulus showed a similar
trend (Table S2). This was mainly due the destroyed chain
regularity and deteriorated crystallinity of the copolyesters. While
the DMA results exhibited higher storage modulus (G’) of PET-co-
HPI than that of pure PET (Figure S11), which was caused by the
increased chain rigidity and intermolecular interactions.
Table 1. Fire safety parameters of PET and the copolyesters.
Sample
pHRR[a]
[kWm-2]
pSPR[b]
[m2s-1]
pCOP[c]
[10-2 gs-1]
HRC[d]
[Jg-1K-1]
THRC[e]
[Jg-1K-1]
PET
734
430
376
276
0.17
0.12
0.11
0.08
1.29
0.99
0.81
0.53
347
318
316
214
347
289
338
322
PET-co-PI20
PET-co-MPI20
PET-co-HPI20
[a] Peak heat release rate (pHRR) obtained from cone calorimetry. [b] Peak
smoke production rate obtained from cone calorimetry. [c] Peak carbon
monoxide production rate obtained from cone calorimetry. [d] Heat release
capacity (HRC) obtained from PCFC. [e] Theoretical heat release capacity
(THRC) calculated by Van Krevelene method
The end-group-capturing effect derived from the oxazoline-
containing structures, as mentioned before, was further
investigated by thermogravimetric analysis-infrared spectrometry
(TG-IR). As shown in the 3D TG-IR spectra (Figure 2a and 2b),
the pyrolysis products of both PET and PET-co-HPI20 were mainly
consisted of aliphatic ether (1020-1180 cm-1), hydrocarbon (1280-
1470 cm-1), RCOH (1761 and 2742 cm-1), RCOOH (1761 and
3580 cm-1), carbon monoxide (2111 and 2181 cm-1) and carbon
dioxide (2356 and 671cm-1). Regarding the absorbance intensity
of CO2 as a standard, which was the main evolved gas of PET
during decomposition, PET-co-HPI20 generated much less
carboxyl and hydroxyl-terminated fragments than PET did, which
was further confirmed from the intensity change of CO2 and
carbonyl absorbance as a function of time (Figure 2c), indicating
the carboxyl- and hydroxyl-terminated chains were effectively
captured for PET-co-HPI20. We also noted a drastic decrease of
the absorbance intensity of the highly combustible volatiles such
as hydrocarbons, indicating an efficient suppression on fuel
supply for combustion.
Moreover, as summarized in the PCFC (Figure 1d) and TGA
results (Figure S13), and also as reported by Mathias and co-
workers[9], the thermal rearrangement of HPI to benzoxazole
occurred at an elevated temperature above 350 °C, meaning that
the decomposition process of the PET-co-HPI copolyester may
be greatly changed when the copolyester was heated above
350 °C. To confirm such hypothesis, isothermal TGA test was
therein employed. As illustrated in Figure S22, when PET and
PET-co-HPI20 were both isothermally heated for 1 h under
nitrogen at 350 °C, the temperature where conventional chemical
bond fracture dominated the decomposition but the thermal
rearrangement just began, the decomposition (weight loss) speed
of PET-co-HPI20 was faster but the final residue was lower than
that of PET, which was owing to the insufficient thermal
rearrangement reactivity and the initial release of CO2 at the
beginning of thermal rearrangement. When the samples were
heated at a higher temperature, i.e., 380 °C, the temperature
where the thermal rearrangement took charge, the decomposition
speed of PET-co-HPI20 was still faster than that of PET at the
As displayed in the above results, this end-group-capturing
effect showed great applicability in semi-aromatic polyester PET.
Thus, to evaluate the versatile applicability of this method in other
polymers, such as aliphatic polyesters, we prepared the HPI-
containing poly(butylene succinate) copolyesters shorted as PBS-
co-HPI, and investigated their fire safety. With 20 mol% (relative
to dimethyl succinate) of HPI incorporated, PBS-co-HPI20 easily
passed the UL94 V-0 rating without dripping (Figure S24),
representing a great applicability of this method in different
polymeric materials.
In summary, we reported a versatile end-group-capturing
method as an effective alternative for the design and preparation
of diverse fire-safe polyesters based on the thermal
rearrangement of HPI. We used PET copolyester as
a
representative and investigated the detailed fire-safety mode-of-
action. Our work revealed great applicability of this method in both
semi-aromatic and aliphatic polyesters for fire-safety purpose. Its
high applicability makes the method a universal fire-safety state-
of-art for polyesters, and provides a new strategy for fire-safe
polymers.
Acknowledgements
Financial supports by the National Natural Science Foundation of
China (21634006, 51822304 and 51721091) and the Sichuan
Province Youth Science and Technology Innovation Team
(2017TD0006) are sincerely acknowledged.
Keywords: fire safety • thermal rearrangement • end-group
capture • polyester
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10.1002/anie.201900356
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
End-group
capture:
End-group
Bo-Wen Liu, Li Chen*, De-Ming Guo,
Xiao-Feng Liu, Yu-Fei Lei, Xiao-Min
Ding, Yu-Zhong Wang**
capture of benzoxazole from thermal
rearrangement of hydroxyl-containing
phthalimide group toward fire safety for
polyesters is reported. This novel
strategy exhibited great applicability
and high efficiency in both semi-
aromatic and aliphatic polyesters,
providing new thinking on designing
fire-safe polymers.
Page No. – Page No.
Fire–Safe Polyesters Enabled by
End–Group Capturing Chemistry
Layout 2:
COMMUNICATION
Bo-Wen Liu, Li Chen*, De-Ming Guo,
Xiao-Feng Liu, Yu-Fei Lei, Xiao-Min
Ding, Yu-Zhong Wang**
Page No. – Page No.
Fire–Safe Polyesters Enabled by
End–Group Capturing Chemistry
End-group capture: End-group capture of benzoxazole from thermal rearrangement of hydroxyl-containing phthalimide group
toward fire safety for polyesters is reported. This novel strategy exhibited great applicability and high efficiency in both semi-aromatic
and aliphatic polyesters, providing new thinking on designing fire-safe polymers.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.