Novel sulfonate-containing halogen-free flame-retardants: Effect of ternary and quaternary sulfonates centered on adamantane on the properties of polycarbonate composites

Article abstract of DOI:10.1039/c7ra06504c

In order to find a more environmentally friendly flame retardant, we have designed a novel halogen-free flame-retardant (FR) system consisting of ternary and quaternary sulfonates centered on adamantane. They are named 1,3,5-tri(phenyl-4-sodium sulfonate)adamantane (AS₃) and 1,3,5,7-tetrakis(phenyl-4-sodium sulfonate)adamantane (AS₄), respectively. Both kinds of FRs were synthesized and compounded with polycarbonate (PC) to study their effects on the properties of PC composites. The results show that the new FR system can improve PC flame retardancy efficiently, and has mechanical strength advantages as well. The PC composites with only 0.1 wt% AS₃ or 0.08 wt% AS₄ can pass vertical burning tests (UL-94) V-0 level, with increasing the value of limiting oxygen index (LOI) to 31.2% or 32.3%. Moreover, they can maintain ideal mechanical properties compared to neat PC simultaneously. Finally, the flame retardant mechanism of this system was verified via thermal analysis, morphology and chemical structure analysis of the char residues.

Full text of DOI:10.1039/c7ra06504c

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Novel sulfonate-containing halogen-free flame-
retardants: effect of ternary and quaternary
sulfonates centered on adamantane on the
properties of polycarbonate composites†
Cite this: RSC Adv., 2017, 7, 39270
*
Jian Wei Guo, Jia Xing Xian and Shu Qin Fu
Dong Yu Zhu,
In order to find a more environmentally friendly flame retardant, we have designed a novel halogen-free
flame-retardant (FR) system consisting of ternary and quaternary sulfonates centered on adamantane.
They are named 1,3,5-tri(phenyl-4-sodium sulfonate)adamantane (AS3) and 1,3,5,7-tetrakis(phenyl-4-
sodium sulfonate)adamantane (AS4), respectively. Both kinds of FRs were synthesized and compounded
with polycarbonate (PC) to study their effects on the properties of PC composites. The results show that
the new FR system can improve PC flame retardancy efficiently, and has mechanical strength advantages
as well. The PC composites with only 0.1 wt% AS3 or 0.08 wt% AS4 can pass vertical burning tests (UL-
94) V-0 level, with increasing the value of limiting oxygen index (LOI) to 31.2% or 32.3%. Moreover, they
can maintain ideal mechanical properties compared to neat PC simultaneously. Finally, the flame
retardant mechanism of this system was verified via thermal analysis, morphology and chemical structure
analysis of the char residues.
Received 12th June 2017
Accepted 3rd July 2017
DOI: 10.1039/c7ra06504c
rsc.li/rsc-advances
attracted the interests of more and more researchers from both
the academic and industrial elds.9–15 However, the general
1 Introduction
non-halogenated ame retardants (FR) such as phosphorus-,
silicon-, and nitrogen-containing FRs are usually not efficient
enough to enable PC to pass V-0 rating in the UL 94 test until
being incorporated with a relatively large amount.6,16–20 For
example, among the above halogen-free FRs, phosphorus-
containing compounds like triphenyl phosphate (TPP) and
resorcinol bis(diphenyl phosphate) (RDP) are the most widely
used FRs for PC and their blends, but generally, to enable PC
passing UL94 V-0 test, the required additive amount should be
as much as 5–15 wt%.21–25 As we known, the additive type ame
retardants are generally incorporated into polymeric materials
by physical means, a large amount of addition may bring in
a variety of problems, such as poor transparency, leaching, and
a reduction in mechanical properties.
According to the study,26–28 organosulfonates such as the
commercially available potassium diphenylsulfone sulfonate
(KSS) are highly effective FRs for PC. In general, very low addi-
tions (no less than 1 wt%) of certain metal salt sulfonates can
provide a self-extinguishing performance to polycarbonate.
However, the halogen-free ame retarding PC composites
incorporated with current sulfonates FRs like KSS still have
drawbacks. For example, higher ame retardancy is limited and
the mechanical properties suffer a loss, which can be attributed
to the poor interfacial adhesion between FR and the polymer
matrix. Therefore, to design a series of novel efficient sulfonate
FRs and to study the effect of their structure on the properties of
With the development of technology, polymer materials are
used with an ever increasing magnitude instead of metal or
ceramics in all kinds of areas. However, re hazards associated
with the use of these polymeric materials are usually present
and can cause heavy loss of life and property.1–3 Therefore, it is
vital to reduce the combustibility of the polymers by using ame
retardants in the development and application of new mate-
rials.4 Polycarbonate (PC) is one of the fastest growing engi-
neering plastics containing excellent physical properties
including transparency, toughness, high heat distortion
temperature, and benecial mechanical properties.5 Generally,
PC shows a V-2 rating in the UL-94 test and the oxygen index is
about 25%, but it is still difficult to meet the needs of more
stringent ame retardant performance in many practical
applications.6–8
For the sake of safety, all kinds of ame retardants have been
developed and used for PC. The halogen-containing ame-
retardants that have high ame-resistant efficiency, however,
have been gradually forbidden due to the generated toxicity and
environmental problems. Because of this, the development of
environmentally friendly halogen-free ame retardants has
School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, China. E-mail: guojw@gdut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra06504c
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PC composites is of great signicance for promoting the a magnetic stirrer. Aer being stirred for 10 min at 55 ꢁC, 0.2 g
development of this eld. aluminium chloride (1.9 mmol) was added slowly and the
In this paper, ternary and quaternary sulfonate centered on reaction solution was heated under vigorous reux for another
adamantane were designed as 1,3,5-tri(phenyl-4-sodium sulfo- 10 minutes. Finally, the system was then poured into an ice
nate)adamantane (AS3) and 1,3,5,7-tetrakis(phenyl-4-sodium water and ether mixture, the resultant undissolved substance
sulfonate)adamantane (AS4), respectively. Owing to the various was ltrated and extracted by chloroform, followed by recrys-
advantages of adamantane such as having stereo-regularity, tallization from benzene to get the nal product of 1,3,5-
multiple substitutability, excellent lipophilicity, good thermal triphenyladamantane.
and oxidative stabilities and high rigidity, the novel sulfonate
The procedure for preparing 1,3,5,7-tetraphenyladmantane
ame retardant with high sulfur content is expected to be more is very similar to that for 1,3,5-triphenyladamantane, except for
efficient for ame retardancy and less negative to mechanical using more catalyst of aluminium chloride (0.6 g) and reacting
property. In this study, these two kinds of FRs were rst for a longer time of 60 min in the same case as shown above. In
synthesized and characterized, then the ammability of PC addition, aer the reaction completed, the product is insoluble,
composites lled with the novel FR were investigated using so it should be obtained as the lter residue not the ltrate aer
a cone calorimeter, as well as the LOI and UL-94 tests. The effect soxhlet extraction from chloroform. Finally, 1,3,5,7-tetraphe-
of the two kinds of FR content on mechanical properties and nyladmantane was gained aer vacuum dried as white solid.
thermal properties of PC/ASn (n ¼ 3, 4) was also systematically
The reaction approaches for both 1,3,5-tri(phenyl-4-
studied. Finally, the ame retarding mechanism was deduced sulfochloride)adamantane and 1,3,5,7-tetrakis(phenyl-4-
from the thermal degradation and the residual char sulfochloride)adamantane are the same. Take 1,3,5-tri(phenyl-
morphology and chemical structure. Using this system, PC 4-sulfochloride)adamantane for example, a typical procedure
composites with only 0.08 wt% AS4 or 0.1 wt% AS3 can pass V- is as follows: 1.35 g 1,3,5-triphenyladamantane (3.72 mmol),
0 and have LOI value higher than 30, and can maintain ideal 20 mL dry dichloromethane was added to a 100 mL three-neck
mechanical properties as well. In addition, it is found that the ask equipped with a reux condenser, a calcium chloride
higher the sulfonate content in FR molecular structure, the drying tube, a dropping funnel and a magnetic stirrer in an ice
better for ame retardant PC.
bath, and then to the reaction system was dropwise added
2.5 mL chlorosulfonic acid and 5 mL dichloromethane within
10 min. The temperature was slowly raised to 35 C and kept
ꢁ
2 Experimental
2.1 Materials
stirring for 75 min. Aer cooling to room temperature, the
organic layer was separated by centrifuge, and rotary evaporated
to get a brown oil which was washed by water and then ltrated.
Finally, 2.35 g (95.7%) solid product of 1,3,5-tri(phenyl-4-
sulfochloride)adamantane was obtained aer vacuum dried at
Polycarbonate (PC, grade: 301-10) was supplied by Dow Chem-
ical Company (American) with a melt index (GB 368283) of 20 g/
10 min and a density of 0.918 g cmꢀ3. PC was dried in the oven
at 120 ꢁC for 8 h prior to blending. Adamantane (99%) was
purchased from Zhang jia gang Xikai Chemical Co., Ltd., China.
Bromine (AR, 99.5%), Aluminium chloride (AR, 99%) and tert-
butyl bromide (AR, 99%) were purchased from Aladdin. Chlor-
osulfonic acid (AR) was supplied by Xiya Reagent Co., Ltd.,
China. The potassium diphenylsulfone sulfonate (KSS) used
here as a reference ame retardant was offered by Guangdong
Ever Sun Corp., China. Benzene (AR) and dichloromethane (AR)
purchased from Guangzhou Chemical Reagent Factory, were
distilled in anhydrous calcium chloride before used. Other
chemicals were all used without further purication.
ꢁ
45 C for 12 h.
The procedure for the nal step is the same for both AS3 and
AS4. To take AS3 for example, a typical method is as follows:
5.0 g 1,3,5-tri(phenyl-4-sulfochloride)adamantane and 300 mL
deionized water was added to a 500 mL ask with a reux
condenser and a magnetic stirrer. The mixture was heated at
100 ꢁC for 24 h to give a clear solution which was distilled under
reduced pressure, and the residue was dried by vacuum oven at
45 ꢁC for 12 h. Next, the resultant solid was dissolved in 50 mL
of water and was neutralized by 10 wt% NaOH solution. Aer
the water was thoroughly removed, the residue was bleached
and vacuum dried to afford 5.0 g (98.4%) the product of AS3 as
a white salt.
2.2 Synthesis of ternary and quaternary sulfonate FR AS3 and
AS4
2.3 Preparation of ame retardant PC composites
The ASn (n ¼ 3, 4) was synthesized through the reactions of
Friedel-cras arylation from 1-bromoadamantane,29 followed by PC and the ame retardants AS3 and AS4 were rst dried over-
substitution reaction, hydrolysis reaction and neutralization night at 120 ꢁC before compounding. The PC composites lled
reaction according to the above reaction route as shown in with 0.06–0.16 wt% AS3 or AS4 FR were prepared in a HL-200
Scheme 1.
internal mixer (Jilin University Science and Education Instru-
A typical procedure for synthesis of 1,3,5-triphenylada- ment factory, China) under 50 rpm for 8 min at the temperature
mantane is as follows: 5.0 g 1-bromoadamantane (23 mmol), of 245 ꢁC, which were denoted as PC/AS3 (0.06%), PC/AS3
6.5 g tert-butyl bromide (50 mmol), and 175 mL benzene was (0.08%), PC/AS3 (0.1%), PC/AS3 (0.12%), and PC/AS3 (0.16%),
introduced into a 500 mL three-neck ask equipped with respectively. PC/AS4 were labeled in the same manner. The
a reux condenser, a calcium chloride drying tube and compounds were then broken into pieces and dried in a vacuum
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Scheme 1 Synthesis route and structures of flame retardants ASn (n ¼ 3, 4).
oven at 90 C over 5 h. Aerwards, the composite pieces were the heat release rate (PHRR, kW mꢀ2), specic extinction area
injection molded or compression molded into standard sizes (SEA), the peak value of smoke production rate (SPR), and CO
for different tests. Unlled PC and the PC composites lled with production rate (COP).
ꢁ
ame retardant KSS of a suitable amount were also manufac-
tured under the same conditions for comparison.
2.4.3 Mechanical tests. The tensile and exural tests were
performed using a PHM-5 universal tester according to an ISO
844 and ASTM D790 standard procedure, respectively. Each
batch included ve specimens to yield averaged values.
2.4 Characterization and measurements
2.4.1 Spectroscopic analysis. The Fourier transform
infrared (FTIR) spectra were recorded with KBr on a Nicolet
6700 FTIR spectrometer from 400 to 4000 cmꢀ1. The hydrogen
nuclear magnetic resonance (1H-NMR) spectra were recorded
on a Bruker AVANCE III HD400-MHz superconducting NMR
spectrometer.
2.4.2 Flammability tests. Limiting oxygen index (LOI)
measurements were carried out using a HP6115 type oxygen
index test apparatus (Foshan, China) following an ASTM D2863-
2008 standard procedure, and the dimensions of all samples
were 80 mm ꢂ 10 mm ꢂ 4 mm. The LOI values were calculated
based on the results of ve tests.
2.5 Thermogravimetric analysis and thermal degradation
kinetics
Thermogravimetric analysis (TGA) and thermal degradation
kinetics were carried out with a NetzschSTA409 PC thermoa-
nalyzer instrument (Netzsch, Germany). For the thermal anal-
ysis of FRs, each sample was heated from 50 to 800 ꢁC at the rate
of 20 ꢁC minꢀ1 under nitrogen atmosphere. To test the thermal
degradation kinetics, each sample was heateꢀd1from 50 to 800 ꢁC
ꢁ
at various rates of 5, 10, 15 and 20 C min under air atmo-
sphere. The Flynn–Wall–Ozawa method was used to analyze the
thermal degradation kinetics parameters.
The vertical burning tests (UL-94) were conducted on a ZRS-2
vertical horizontal burning apparatus (NEARBYMRO, Germany)
according to ASTM D3801 standard, the dimensions of all
samples were 125 mm ꢂ 13 mm ꢂ 3 mm. In the test, ve
sample bars suspended vertically over surgical cotton were
ignited using a 50 W methane gas burner.
2.6 Scanning electron microscopy (SEM)
The SEM observation was performed on a Hitachi S-3400N
scanning electron microscope (Hitachi, Japan) to investigate
the morphology of the char layer of samples aer the LOI tests.
The cone calorimetry test was carried out by using a FTT
0007 cone calorimeter (Fire Testing Technology, UK) in accor-
dance with the ISO 5660 standard procedures. Each specimen,
3 Results and discussion
3.1 Chemical structure characterization of FRs
with a dimension of 100 mm ꢂ 100 mm ꢂ 3 mm, was wrapped The ame retardant AS3 and AS4 were synthesized via the
in aluminum foil and exposed horizontally to an external heat reaction route presented in Scheme 1. The 1H NMR, IR spectra
ux of 35 kW mꢀ2. The recorded parameters included the time and elementary analysis data for each intermediate product
1
to ignition (TTI), heat release rate (HRR, kW mꢀ2), peak value of were provided in the ESI (Fig. S1–S7†). Fig. 1(a) shows the H
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1
As shown in the right side of Fig. 1(a) which is the H NMR
spectrum of AS4, we can see the chemical shis of H protons at
the peak appearing at 2.06 (s, 12H) correspond to the H protons
(a) in CH2 of tetra-substituted adamantyl, and the peaks at
7.515–7.551 (m, 16H) are assigned to H protons (b + c) in ¼ C–H
of benzene ring.
To conrm the successful synthesis of AS3 and AS4, FT-IR
spectra were also recorded and presented in Fig. 1(b). AS3
shows almost the same absorption peaks with AS4. The peaks at
3022 cmꢀ1, 1600 cmꢀ1 and 1495 cmꢀ1 should be attributed to
the stretching vibrations of ¼ C–H and C]C in benzene rings,
respectively, and the characteristic absorption peak of para-
disubstituted benzene ring is observed at 825 cmꢀ1. The peaks
corresponding to stretching vibration and the deformative
vibration of C–H and–CH2 in adamantane appear at 2923, 1446,
712 cmꢀ1. The characteristic absorption of S]O which belongs
to the sulfonate group is observed at 1180 and 1041 cmꢀ1. In
addition, it can be also shown that a broad absorption peak
appears at 3444 cmꢀ1, which should correspond to the signal of
absorbed water. Conclusively, the 1H NMR and the FTIR spectra
conrm that AS3 and AS4 have been synthesized successfully.
3.2 Thermal analysis of FRs
The basic thermal properties of FRs are vital for their applica-
tion in polymers.
Fig. 2 shows the thermal stability of the ame retardant AS3
and AS4 by thermogravimetric analysis under nitrogen atmo-
sphere. Both AS3 and AS4 display the similar TG and DTG curves,
but AS4 has a little higher degradation temperature. As shown in
Fig. 2, about 10 wt% weight loss between 100 and 300 ꢁC results
from the evaporation of (i) absorbed moisture and (ii) crystal
water. According to the weight loss percent of the corresponding
part on the curves, the maximum weight loss between 480 ꢁC and
ꢁ
550 C should correspond to the released SO2 from the decom-
position of the sulfonate groups, and the last stage of weight loss
between 630 ꢁC and 800 ꢁC should be attributed to the pyrolysis
of –ONa from benzene rings. The residues should be 1,3,5-tri-
phenyladamantane or 1,3,5,7-tetraphenyladamantane and
Fig. 1 (a) 1H NMR spectra of AS3 and AS4 with DMSO-d6 as solvent, (b)
FTIR spectra of AS3 and AS4.
NMR spectra of AS3 and AS4 with DMSO-d6 as a solvent, and
Fig. 1(b) the IR spectra.
The 1H NMR spectrum of AS3 is shown in Fig. 1(a), the multi-
peaks at 1.932–2.070 ppm (m, 12H) correspond to the protons
(b + c) of –CH2 in adamantyl. The peak at 2.447 ppm (s, 1H) is
attributed to the (a) proton of –CH in adamantyl, and the peaks
during 7.444–7.564 ppm (m, 12H) are due to (d + e + f + g)
protons in the benzene ring.
Fig. 2 TG and DTG curves of AS3 and AS4 recorded in N2.
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Table 1 Cone calorimetric data with a heat flux of 35 kW mꢀ2
sodium ions. In general, proper decomposition temperature is
very important for additive-type ame retardants to work for
matrix materials. The maximum decomposition peak tempera-
ture which corresponds to the degradation of sulfonate is
Sample
PC
PC/AS3 (0.10%)
PC/AS4 (0.08%)
TTI/s
102
305.9
61.0
680.5
0.133
0.148
72
243.2
53.3
510.3
0.081
0.103
86
247.8
59.4
573.3
0.079
0.089
ꢁ
527.7 C for AS3 and 543.3 ꢁC for AS4, that is much higher than
pHRR (kW mꢀ2
)
THR (MJ mꢀ2
)
)
the thermo-processing temperature of PC composites and
SEA (m3 kgꢀ1
pkSPR (m3 sꢀ1
Mean COY (kg kgꢀ1
matchable with the degradation temperature of PC (Tpeak
¼
)
ꢁ
534.2 C) (see Fig. 7). It demonstrates that the novel FR system
of AS3 and AS4 can well meet the demand for ame retardants of
PC in regards to thermal property.
)
work. The samples of PC/AS3 (0.1%), PC/AS4 (0.08%) and PC are
tested by cone calorimeter and some of the important results are
summarized in Table 1. The heat release rate (HRR), in particular
the peak of HRR (pHRR) value, is proved to be the most useful
parameter to evaluate re safety. Fig. 4 shows the HRR plots for
pure PC, PC/AS3 (0.1%), and PC/AS4 (0.08%) composites.
It can be seen from Table 1 and Fig. 4 that neat PC shows
a very sharp HRR curve at the time range of 100–400 s with
a maximum of 305.9 kW mꢀ2 at 125 s. In comparison, the HRR
curves of PC/AS3 (0.1%) and PC/AS4 (0.08%) both become atter,
indicating the lower intensity of a re. The peak HRR (PHRR)
values decrease to 243.2 kW mꢀ2 and 247.8 kW mꢀ2 for PC/AS3
(0.1%) and PC/AS4 (0.08%), respectively. The total heat release
(THR) value for neat PC is 61.0 MJ mꢀ2, and it also decreases to
53.3 MJ mꢀ2 and 59.4 MJ mꢀ2 for PC/AS3 (0.1%) and PC/AS4
(0.08%), respectively. These results demonstrate that both AS3
and AS4 can effectively reduce the risk of re hazard.
Smoke and toxic gas release performances are other very
important parameters to determine ame-retardant material in
re safety judgment.31 As seen in Table 1, the SEA data which
measures the total generated smoke quantity decreases from
680.5 m3 kgꢀ1 for PC to 510.3 m3 kgꢀ1 and 573.3 m3 kgꢀ1 for
PC/AS3 (0.1%) and PC/AS4 (0.08%), respectively. And the pkSPR
is also decreased from 0.133 m3 sꢀ1 to 0.081 m3 sꢀ1 and
0.079 m3 sꢀ1, respectively. The lower value of SEA and pkSPR
indicates that AS3 and AS4 can both strongly cut down the
3.3 Flammability of PC composites
The ammability of PC/ASn (n ¼ 3, 4) composites was system-
atically evaluated using LOI, UL-94 and cone calorimetry
methods. The results of LOI and UL-94 tests for PC/ASn with
different loadings and PC/KSS composites as the control
samples are displayed in the above Fig. 3. As shown in Fig. 3, the
LOI value of neat PC is 25.8%. Aer being added with the
synthesized new FRs, we can see that both AS3 and AS4
improved the ammability of PC composites efficiently.
According to the JISK 7201 industrial standard, with LOI >30%,
PC/AS3 with AS3 content between 0.1–0.16 wt% and PC/AS4 with
AS4 content 0.08–0.16 wt% within the range studied, can all be
assigned to the rst class ame retardant materials that are
extremely hard to ignite. As to the UL-94 tests, it also can be
clearly indicated from Fig. 3 that the UL-94 rating for neat PC is
V-2. As to the ame retardant PC composites, only 0.1 wt% AS3
or even 0.08 wt% AS4 can enhance PC materials to pass V-
0 rating. So the optimal FR addition amount can be 0.1 wt%
for AS3 and 0.08 wt% for AS4. In contrast, the control samples,
which are added with KSS as FR instead, under the same FR
content, can only reach V-2 with the LOI value of 27.1% for PC/
KSS (0.08 wt%) and 28.6% for PC/KSS (0.1 wt%), respectively.
Thus the above results from both LOI and UL-94 manifest that
our synthesized AS3 or AS4 indeed works well in ame-retarding
of PC composites, and AS4 which has more sulfonate content in
molecular structure is shown more efficient than AS3.
Cone calorimetry, as one of the most important bench-scale
methods for evaluating the ame retarding performance of
materials in a real re case,30 was further investigated in this
Fig. 3 The LOI values and UL-94 results of flame retardant PC
composites PC/AS3 and PC/AS4 with different FR contents. For the
control sample of PC/KSS (0.08%) is 27.1%, V-2, and PC/KSS (0.1%) Fig. 4 Heat release rate curves of PC, PC/AS3 (0.1%) and PC/AS4
28.6%, V-2.
(0.08%) composites.
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Fig. 5 CO release rate curves of PC, PC/AS3 (0.1%) and PC/AS4 (0.08%)
composites.
smoke emission of PC composites. Fig. 5 shows the variation of
CO production during the combustion process. The mean CO
yield of PC is 0.148 kg kgꢀ1. With addition of ASn (n ¼ 3, 4) FR,
the mean CO yield decreased to 0.103 kg kgꢀ1 and 0.089 kg kgꢀ1
for PC/AS3 (0.1%) and PC/AS4 (0.08%), respectively. Moreover,
the rst peak CO production is signicantly decreased from
0.0077 kg kgꢀ1 for neat PC to 0.0043 kg kgꢀ1 and 0.0044 kg kgꢀ1
,
respectively. This means that the AS3 or AS4 ame retardant
system can efficiently suppress poisonous gas emission so as to
increase the likelihood of survival at the beginning of a re.
It is worthwhile to mention that the TTI of pure PC is 102 s,
while it is decreased to 72 s and 86 s with addition of 0.1 wt%
AS3 and 0.08 wt% AS4, respectively. The unexpected decrease in
TTI of PC/ASn composites indicates that AS3 and AS4 may retard
the combustion of PC matrix through accelerating the thermal
decomposition of PC and quickly promoting the formation of
char layers at relatively lower temperature.
Fig. 6 (a) Effect of the FR content on the mechanical strengths of PC/
AS3 and PC/AS4 composites; (b) histogram of the mechanical strengths
of PC, PC/AS3 (0.1%), PC/AS4 (0.08%) and the control samples of PC/
KSS (0.1%) and PC/KSS (0.08%).
From the LOI test, UL-94 test and cone calorimeter results
which are discussed above, it is clear that the ame retardancy
of PC is signicantly enhanced by incorporation of AS3 or
AS4 FRs.
exural strength increases rst and then decreases sharply. At
the optimal addition of each FR for ame retardancy, the ex-
ural strength of PC/AS3 (0.1%) is 84.87 MPa, slightly reduced by
1.8% compared to that of pure PC of 86.47 MPa, and PC/AS4
(0.08%) is 88.75 MPa, increased by 2.6% compared to that of PC.
In contrast, the exural strengths are only 76.88 MPa and
78.88 MPa for the control samples of PC/KSS (0.1%) and PC/KSS
(0.08%), respectively, which is decreased by 11% than PC.
In conclusion, the novel FR system we proposed in this study
can effectively alleviate the deterioration of FR to the mechan-
ical strength of polymeric matrix, and can even improve it to
a certain extent in some cases. The advantages of AS3 and AS4 FR
over the current existing control FR for PC could be attributed to
the low incorporated content and the high stiffness and lipid
solubility of adamantly that make AS3 and AS4 of better
compatibility with PC.
3.4 Mechanical properties
As an additive-type ame retardant, the effect of its incorpora-
tion on the mechanical properties of matrix material is a matter
of concern. Fig. 6 depicts the tensile and exural properties of
PC and its composites. As shown in Fig. 6(a), the tensile
strength of virgin PC is 60.55 MPa. With increasing of AS3
content, the tensile strength of PC/AS3 composites increases
rst and then levels off aer a slight decrease. As to PC/AS4, the
tensile strength keeps gradually increasing and then levels off
with additional AS4 content. With the addition of FR at the
optimal content for ame retardancy, the tensile strength rises
to 61.58 MPa and 61.88 MPa for PC/AS3 (0.1%) and PC/AS4
(0.08%), respectively. While the control samples of PC/KSS
(0.1%) and PC/KSS (0.08%) have the tensile strength of
59.88 MPa and 60.1 MPa, respectively, which are both lower
than that of neat PC. As to the exural test, we see that the
3.5 Flame retardant mechanism
3.5.1 Thermal decomposition properties of PC composites.
exural strength of PC/AS3 is slightly decreasing and then In order to clearly understand the ame retardant mechanism
increasing with the rise of AS3 content, while for PC/AS4, the of PC/AS3 and PC/AS4, the thermal stability of the ame
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retardant PC composites is investigated by TGA. Moreover, the decomposition of PC and promote its char-forming so as to
Flynn–Wall–Ozawa method is used to analyze their thermal result in the decline of the degradation temperature as well as
degradation kinetics.
TTI. Therefore, the decreased thermal stability is essential
Histogram of average activation energy of PC, PC/AS3 (0.1%), rather than a drawback of the multi-sulfonate FR system.
and PC/AS4 (0.08%) with the degradation conversion rate from
Furthermore, to study the degradation behavior of the PC
0 to 70% calculated by the Flynn–Wall–Ozawa method.
composites, the Flynn–Wall–Ozawa method is used to analyze
The thermal degradation and stability of polymers have their thermal degradation kinetics. According to this method,
substantial connection with their ame retardancy. Therefore, the degradation activation energy can be determined directly
TG analysis was used to investigate the thermal degradation from the log b versus 1/T plot as follows.32
properties of PC and its composites. Fig. 7(a) shows the TG and
AE
R
E
log f ðaÞ ¼ log
ꢀ log b ꢀ 2:315 ꢀ 0:4567
DTG curves under air for the PC, PC/AS3 (0.1%), and PC/AS4
(0.08%) composites, respectively. Compared to PC, it is
observed that both the TG and DTG curves of PC/AS3 (0.1%) and
PC/AS4 (0.08%) shi forward to lower temperature successively.
The corresponding data such as onset decomposition temper-
ature (T5%), the temperature at maximum weight loss rate
(Tpeak) are list in the insert table of Fig. 7(a). It is found that the
T5%, Tpeak of PC are 485.2 ꢁC and 534.2 ꢁC, respectively.
However, the T5%, Tpeak are 468.2 ꢁC, 528.8 ꢁC, and 470.4 ꢁC,
521.9 ꢁC for PC/AS3 (0.1%) and PC/AS4 (0.08%) respectively. This
result is in accordance with the variation of TTI listed in Table 1
that the TTI of the PC/AS3 and PC/AS4 composites are shorter
than that of virgin PC. It can be deduced that the sulfonate
groups in AS3 or AS4 ame retardants accelerate the
RT
where A is the pre-exponential factor, R is the gas constant, E is
the activation energy, b is heating rate, T is the Kelvin temper-
ature, and f(a) is the integrated form of the conversion depen-
dence function. The TG curves for PC, PC/AS3 (0.1%), PC/AS4
(0.08%) composites recorded under various heating ow rates
of 5, 10, 15 and 20 ꢁC minꢀ1 under air atmosphere are shown in
Fig. S8–S10.† The activation energy data of the three samples
calculated by the Flynn–Wall–Ozawa method are listed in
Tables S1–S3,† and the average thermal degradation activation
energies within the conversion rate from 0 to 70% are shown in
Fig. 7(b). It can be seen clearly from Fig. 7(b) that the average
activation energy of PC is 176.9 kJ molꢀ1. When AS3 or AS4 is
added, the average activation energy decreases to 160.7 kJ molꢀ1
and 153.5 kJ molꢀ1 for PC/AS3 (0.1%) and PC/AS4 (0.08%),
respectively. Comparatively, the average activation energy of PC/
AS4 (0.08%) is lower than that of PC/AS3 (0.1%), which
demonstrates stronger accelerating decomposition. This result
further manifests that both AS3 and AS4 can accelerate PC
decomposition. AS4, with more sulfonate content in its molec-
ular structure is relatively stronger, which is consistent with
TGA results as shown in Fig. 7(a). This could be due to the SO2
generated from sulfonate degradation which shows high catal-
ysis ability for PC decomposition.
3.5.2 Morphology and structure analysis of char residue. In
order to further understand the ame retarding mechanism of
PC/ASn system, the morphology and chemical structure of
residual char obtained from the sample aer LOI test was
investigated by SEM and FT-IR, respectively. As can be observed
in Fig. 8, the residue char surface of PC is smooth but with
macro-pores and crevices, which cannot provide a good barrier
for the polymer. In comparison, the char layers of both PC/AS3
and PC/AS4 samples are continuous and expansionary with
a multitude of expanded spheres. This intumescent char layer
provides a good ame shield to stop the propagation of heat and
mass to the underneath layer, and delays the volatilization of
combustible gases, it also retards the penetration of oxygen, and
the feedback of heat. Consequently, such a useful char layer as
the ame barrier can make the corresponding PC composites
achieve excellent re retardation.
Fig. 9 shows the FT-IR spectra of virgin PC and the residual
char of PC/AS3 (0.1%) and PC/AS4 (0.08%), respectively. We can
see that the FT-IR spectra for residual char of PC/AS3 and PC/AS4
are almost the same, but are of many differences compared to
that of the virgin PC. The absorption peak in 3439 cmꢀ1 and
1400 cmꢀ1 represents the stretching and plane bending
Fig. 7 (a) TG and DTG curves of PC, PC/AS3 (0.1%), PC/AS4 (0.08%)
composites, (b).
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Fig. 8 SEM photos of the residue char for (a) PC, (b) PC/AS3 (0.1%) and (c) PC/AS4 (0.08%) composites.
the LOI value from 25.8% to 31.2% and 32.3% for PC/AS3 (0.1%)
and PC/AS4 (0.08%), respectively. Moreover, the results from
cone calorimetry show that HRR, THR and COY of both PC/AS3
and PC/AS4 are reduced compared to the neat PC. In addition,
they can maintain good mechanical strength, especially for
PC/AS4 which displays increased mechanical strength in most
cases. Finally, we obtain the ame retarding mechanism of this
new PC/ASn system is that AS3 and AS4 have a high catalytic
effect to accelerate PC thermal degradation and protective char
formation. This novel sulfonate ame retardant system provide
a new green solution for decreasing the deterioration of the
mechanical performance and enhancing the ame retardancy
simultaneously.
Acknowledgements
Fig. 9 FT-IR spectra of virgin PC, and the residual char of PC/AS3
(0.1%) and PC/AS4 (0.08%).
The authors would like to thank Sun Yat-sen University for
plastic processing equipments, Kingfa Sci. and Tech. Co., Ltd.
for supporting cone calorimetry tests. In addition, we thank
James Wilk who is
a English teacher in University of
vibration of –OH groups, respectively. The characteristic peaks
in 1630 cmꢀ1 and 1597 cmꢀ1 are attributed to the –C]C– in the
benzene ring. The above information in FT-IR spectra of residue
char for PC/ASn (n ¼ 3, 4) composites indicate that some
aromatic alcohol compounds are produced in the residual char
of PC/AS3 or PC/AS4 aer combustion.
From the FT-IR results in combination with the above char
morphology and TG analysis of PC and its composites, we can
obtain the ame retarding mechanism as follows: the ame
retardant AS3 or AS4 rst decomposes in case of re and
produces SO2 that serves as a kind of catalyst to accelerate the
decomposition of PC. Subsequently, some compounds con-
taining aromatic alcohol are then generated and further cross-
linked automatically to promote intumescent char formation,
so that the unburned PC is protected with high ame
retardancy.
Massachusetts-Amherst for helping us to check the English
writing of the revised manuscript. Funding: This work was
supported by the National Natural Science Foundation of China
[grant numbers 21476051, 21506038 and 21676058], the Nature
Science Foundation of Guangdong Province [grant number
2014A030310307], and the Science and Technology Project of
Guangdong Province [grant numbers 2013B010403028 and
2016A050502057].
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