Effect of alkyl groups in organic part of polyoxo-metalates based ionic liquids on properties of flame retardant polypropylene

Article abstract of DOI:10.1016/j.tca.2016.03.020

The structure-property relationship of a series of polyoxo-metalates based ionic liquids (PILs) modified polypropylene (PP)/Intumescent flame retardant (IFR) composites were studied. The results showed that the chemical structure of PILs has a great effect on properties of PP composites. The flame retardancy of PP/IFR/PILs composites with a shorter alkyl group are better than that with a longer alkyl group. The PP/IFR/PIL composite with 0.5 wt% PIL with 1-butyl-3-methyl imidazole and 14.5% IFR gets the UL-94 V-0, but achieving the same UL-94 grade needs no less than 19.5 wt% IFR and 0.5 wt% PIL with 1-octyl-3-methyl imidazole. In addition, PILs modified by different groups have great effects on the mechanical properties, the PP/IFR/PIL composites containing 1-dodecyl-3-methyl imidazole obtain the best notch impact strength. The results show that the flame retardant and mechanical properties of PP/IFR/PILs composites can be tailored by changing the cation structure of PIL.

Full text of DOI:10.1016/j.tca.2016.03.020

Thermochimica Acta 631 (2016) 51–58
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Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Effect of alkyl groups in organic part of polyoxo-metalates based ionic
liquids on properties of flame retardant polypropylene
Shengjiao Chen, Chengle Wang, Juan Li∗
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang
315201, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure-property relationship of a series of polyoxo-metalates based ionic liquids (PILs) modified
polypropylene (PP)/Intumescent flame retardant (IFR) composites were studied. The results showed that
the chemical structure of PILs has a great effect on properties of PP composites. The flame retardancy
of PP/IFR/PILs composites with a shorter alkyl group are better than that with a longer alkyl group. The
PP/IFR/PIL composite with 0.5 wt% PIL with 1-butyl-3-methyl imidazole and 14.5% IFR gets the UL-94
V-0, but achieving the same UL-94 grade needs no less than 19.5 wt% IFR and 0.5 wt% PIL with 1-octyl-
3-methyl imidazole. In addition, PILs modified by different groups have great effects on the mechanical
properties, the PP/IFR/PIL composites containing 1-dodecyl-3-methyl imidazole obtain the best notch
impact strength. The results show that the flame retardant and mechanical properties of PP/IFR/PILs
composites can be tailored by changing the cation structure of PIL.
Received 31 December 2015
Received in revised form 8 March 2016
Accepted 11 March 2016
Available online 15 March 2016
Keywords:
Polyoxo-metalates based ionic liquids
Chemical structure
Intumescent flame retardant
Synergistic
Polypropylene
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
the efficiency and cost performance of IFR in PP is an important
research topic.
Polypropylene (PP) is one of the most popular resins among all
plastics due to its excellent properties and high cost performance.
However, the flammability of PP limits its applications in many
fields. So it is necessary to improve the flame retardant properties of
PP. A lot of work have been done in the past several decades to over-
come this drawback of PP and great progresses have been achieved
[1–10]. Usually the flammability of PP is modified by adding flame
retardants into PP or grafting flame retardant elements onto PP. The
used flame retardants include halogen, phosphorus, silicon etc.
Intumescent flame retardant (IFR) is one of the most suitable
halogen-free candidates for PP because of matched thermal and
chemical performance between them. IFR is usually a compound
including acid source, carbon source and blowing agent. It shows
lots of excellent advantages and environmentally friendly charac-
teristics. However, its flame retardant efficiency is not high enough
because only covered by a char layer with good quality and high
quantity, can good flame retardancy be obtained. Generally the
loading of commercial IFRs is needed more than 25 wt% to achieve
the UL-94 V-0 as shown in many literatures [11–15]. And the addi-
tion of IFRs will deteriorate the mechanical properties of polymers.
Besides, the cost of materials will be increased also. So, increasing
Designing novel flame retardants or introducing cata-
lysts/synergists into the traditional IFRs systems are usual
methods to increase the flame retardant efficiency of IFRs [16–22].
Among all methods, adopting catalysts/synergists is more con-
venient and most of them are inorganic materials. Though these
catalysts/synergists have good effects on flame retardancy, they
are difficult to be dispersed uniformly into the resin because
of their low content and poor compatibility. Organic-inorganic
hybrid materials show better potential as synergist than single
inorganic materials due to they are composed of inorganic and
organic parts. Polyoxometalate based ionic liquid (PIL) is a kind
of organic-inorganic hybrid material and can behave as efficient
catalyst in many reactions, also in flame retardant fields in our
previous work [23,24]. It is found that good synergistic effect
exists in PP/IFR/PIL system. The anion of PIL affects the flame
retardant properties of PP greatly. However, the effects of organic
structure of cation in PILs on the flame retardant performance of
PP composites are still confused. What is even more important is
that the cation structure may modify the compatibility between
PILs, IFR and PP and influence the mechanical performance of PP
composites. Therefore, it is a feasible method to balance the flame
retardant and mechanical performance of PP/IFR/PILs composites
by changing chemical structure of PILs.
This paper focuses on the effect of organic structure of cation
in PILs on the flame retardant and mechanical properties of
∗
Corresponding author.
E-mail address: lijuan@nimte.ac.cn (J. Li).
http://dx.doi.org/10.1016/j.tca.2016.03.020
0040-6031/© 2016 Elsevier B.V. All rights reserved.

52
S. Chen et al. / Thermochimica Acta 631 (2016) 51–58
C4H9
80
H3C N
C4H9
N
H3C
H3C
N
+ C4H9Cl
N
Cl-
24 h
1
2
C4H9
3-
25
PMo12O40
3HCl
+
3
H PMo12O40
+
H C
3
N
3
N
N
N
3
Cl-
Scheme 1. Preparation road of PIL4.
PP/IFR composites. PILs with different chemical structure were
prepared based on the reactions between 12-phosphomolbdic
acid (PMoA) and 1-butyl-3-methyl imidazolium ([C4]) chloride, 1-
octyl-3-methyl imidazolium ([C8]) chloride, 1-dodecyl-3-methyl
imidazolium ([C12]) chloride and 1-octadecyl-3-methyl imida-
zolium ([C18]) chloride, respectively. The synergistic effects of
these catalysts on the thermal, flame retardant and mechanical
properties of PP composites were investigated by using UL-94
vertical burning tests, limiting oxygen index (LOI), thermogravi-
metric analyzer (TGA) and notched impact strength tests. Scanning
electron microscopy (SEM) was adopted to observe the morphol-
ogy of char residues after combustion and crack surface of PP
composites.
3.1. Characterization
LOI values were obtained by a 5801 digital oxygen index ana-
lyzer (Kunshan YangYi test Instrument Co., Ltd.) according to
ASTM D2863-97. The specimens used for the test were 100.0
mm × 6.5 mm × 3.2 mm in dimension.
UL-94 vertical tests were performed on an AG5100B vertical
burning tester (Zhuhai Angui Testing Equipment Company, China)
according to ASTM D3801. The specimens used for the test were
100.0 mm × 10.0 mm × 3.2 mm in dimension.
Thermal gravimetric analysis (TGA) experiments were per-
formed on a Mettler Toledo TGA/DSC1 Analyzer. About 5 mg
specimens were heated from 25 ◦C to 800 ◦C at a heating rate of
10 ◦C/min under nitrogen (N2)(50 ml/min).
The SEM morphology of char residue obtained after LOI test
was observed by S4800 (Hitachi Corp., Japan). All the samples were
coated with a thin layer of conductive gold before examination.
Notched impact strength measurement was carried out with a
machine XJ-50Z provided by Chengde Dahua testing machine Co.
Ltd. according to ISO180:2000.
2. Experimental
2.1. Materials
Ammonium polyphosphate (APP) (n > 1500) was supplied by
Presafer (Qingyuan, China) Phosphor Chemical Company Limited.
Pentaerythritol (PER) was purchased from Aladdin Industrial Inc.
(Shanghai, China). PP (F401) was obtained from Yangzi Oil Co.,
with a melt index of 2.0 g/min (230 ◦C/2.16 kg). 1-methyl imidazole
(MIm) was obtained from Sinopharm Chemical Reagent Co., Ltd.
PMoA (Chemical formula: H3PMo12O40·nH2O) and 1-butyl chlo-
ride, 1-octyl chloride, 1-dodecyl chloride, 1-octadecanoyl chloride,
of analytical purity were purchased from Aladdin Reagent Chemical
Factory. All reagents were used without further purification.
4. Results and discussion
4.1. Flame retardant properties
The flame retardant performance of PP/IFR/PILs composites are
shown in Table 1.The pure PP has a LOI value of 17.0 and is not
classified (NC) in the UL-94 test. When 20 wt% IFR is added, the
LOI value increases to 25.7, but is failed in the UL-94 test. All the
samples with 0.5 wt% PILs and 19.5 wt% IFR show increased flame
retardancy. The PP composites containing PIL4, PIL8 and PIL12 can
be classified the UL-94 V-0 without melt dripping and their LOI
values are 30.2, 31.3 and 30.0, respectively. However, the PP com-
posites containing PIL18 only passes the UL-94 V-2, and obtains a
LOI value 25.2. In addition, the composites with total addition of
15 wt% IFR/PIL4 have a LOI value 28.0 and pass the UL-94 V-0 test.
While the PP composites with PILs containing a longer alkyl chain
fail in the UL-94 test at the same formulation. The results show
that the structure of organic cations in PILs has great effects on the
flame retardancy of PP/IFR/PILs composites. The composites con-
taining shorter alkyl chains achieve better flame retardancy than
that of longer alkyl chains. It is needed to be noted that the PP11
containing PIL12 can be classified the UL-94 V-0, while that for PP8
is NC.
The effect of PILs content on flame retardancy of PP compos-
ites is also researched as shown in Table 2. Though the LOI values
change slightly for the four kinds of PP composites, it has a trend
of increasing firstly and then decreasing with the content of PILs
increasing. However, all PP composites do not pass the UL-94 V-
0 tests except PP6. The PIL18 containing the longest alkyl chain
possesses the poorest flame retardancy. The PILs may catalyze the
charring reaction of PP/IFR during combustion. More or less of PILs
do not help to improve the flame retardant properties. Only a suit-
able dose can increase the flame retardant efficiency. Therefore,
only the PP6 achieves the UL-94 V-0. The data suggest that organic
structure of PIL plays a great role in the flame retardant properties
2.2. Preparation of PILs
1-butyl 3-methyl imidazolium chloride ([C4]Cl) was synthe-
sized by stirring1:1 molar ratio of imidazolium and 1-butyl chloride
at 80 ◦C for 24 h in nitrogen atmosphere. After recrystallization
from ethyl acetate, the intermediate product was obtained. Then
0.47 g of [C4]Cl and 2.88 g of PMoA were dissolved in deionized
water, respectively, and mixed under constant stirring for 12 h.
Then a white precipitate was formed. The product was filtered and
washed for several times with deionized water until chloride-free
(AgNO3 aqueous test). Finally, the obtained [C4]PMo (PIL4) was
dried overnight at 80 ◦C in oven. The preparation road of PIL4 is
shown in Scheme 1, the other PILs are similar to PIL4. In addition,
[C8]PMo, [C12]PMo, [C18]PMo are marked as PIL8, PIL12, PIL18,
respectively. Their HNMR is shown in Fig. S4 and Table S4.
3. Preparation of PP composites
PP composites were prepared by using a Brabender mixer at
200 ◦C with roller speed 50 rpm for 10 min. The ratio of APP and
PER mixture was 3:1 (wt/wt). After mixing, the samples were hot-
pressed at about 200 ◦C under 10 MPa for 3 min into sheets in the
dimensions of 100.0 mm × 100.0 mm × 3.2 mm and then cut into
suitable sample bars for LOI and UL-94 testing.

S. Chen et al. / Thermochimica Acta 631 (2016) 51–58
53
Table 1
Flame reatardant properties of different PP/IFR/PILs composites.
Samples
PP (wt%)
IFR (wt%)
Catalyst (wt%)
LOI (vol%)
UL-94
Melt Dripping
t1 + t2 (s)
PP0
PP1
PP2
PP3
PP4
PP5
PP6
PP7
100
80
82
85
80
82
85
80
82
85
80
82
85
80
82
85
0
0
0
0
0
17.0
25.7
24.6
23.7
30.2
29.4
28.0
31.3
30.1
27.8
30.0
28.8
26.8
25.2
23.0
21.9
NC
NC
NC
NC
V-0
V-0
V-0
V-0
NC
Y
Y
Y
Y
/+
20.0
18.0
15.0
19.5
17.5
14.5
19.5
17.5
14.5
19.5
17.5
14.5
19.5
17.5
14.5
0+/
0+/
0+/
0 + 0
0 + 0
0 + 2
0 + 1
0+/
0+/
0 + 1
0 + 1
0+/
1 + 7
/+
0.5 PIL4
0.5 PIL4
0.5PIL4
0.5 PIL8
0.5 PIL8
0.5 PIL8
0.5 PIL12
0.5 PIL12
0.5PIL12
0.5 PIL18
0.5 PIL18
0.5 PIL18
N/N
N/N
N/N
N/N
N/Y
N/Y
N/N
N/N
N/Y
Y/Y
Y
PP8
PP9
NC
PP10
PP11
PP12
PP13
PP14
PP15
V-0
V-0
NC
V-2
NC
NC
Y
/+
Note: IFR is APP/PER = 3:1 (wt%/wt%).
Table 2
Flame retardant properties of different PP/IFR/PILs composites.
Samples
PP (wt%)
IFR (wt%)
Catalyst (wt%)
LOI (vol%)
UL-94
Melt Dripping
t1 + t2 (s)
PP0
PP3
100
85
85
85
85
85
85
85
85
85
85
85
85
85
0
0
0
17.0
23.7
27.3
28.0
27.4
26.0
27.8
26.7
26.4
26.8
24.5
20.0
21.9
20.7
NC
NC
NC
V-0
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
Y
Y
Y
N/N
Y
Y
Y
Y
Y
Y
Y
Y
Y
/+
15.0
14.75
14.5
14.0
14.75
14.5
14.0
14.75
14.5
14.0
14.75
14.5
14.0
0+/
0+/
0 + 2
0+/
0+/
0+/
0+/
0+/
0+/
/+
PP16
PP6
PP17
PP18
PP9
PP19
PP20
PP12
PP21
PP22
PP15
PP23
0.25 PIL4
0.5 PIL4
1.0 PIL4
0.25 PIL8
0.5 PIL8
1.0 PIL8
0.25 PIL12
0.5 PIL12
1.0 PIL12
0.25 PIL18
0.5 PIL18
1.0 PIL18
/+
/+
/+
Y
of PP/IFR composites. In addition, because 15 wt% IFR/PIL4 is the
critical value for PP composites to be classified in the UL-94 test, it
is sensitive to the content change.
samples containing 15 wt% IFR, only PP6 is remained, all the other
five samples are burned up. In addition, the samples containing
total 20 wt% IFR are also researched, the results are showed in
Fig. 2g–l. It is found that PP4, PP7 and PP10 keep good shape after
testing so they can obtain the UL-94 V0. While PP13 is out of shape
and forms a little char residue, it only achieves the UL-94 V2. Both
the LOI and UL-94 results suggest that the organic cation tailors the
flame retardant performance of PP composites. It is worth men-
tioning that the color of PP changes evidently because of PILs. This
will limit its application in industry, so it need to be improved in
the future.
Figs. 1 and 2 show the char residues of PP and PP composites after
LOI and UL-94 tests, respectively. Pure PP burns with melt-dripping,
and little char residue is held on the surface of PP0 (Fig. 1a). Intu-
mescent char residues are observed on the surface of PP3, however
they cannot be swelled enough due to the insufficiency of formed
char. The char residues for PP6 show largest expanding volume
and best flame retardant properties, because the char layer pro-
vides a good barrier to shield the combustible gas and heat flow
from transferring into the resin. Of course its char quality is good
enough meanwhile. The char residues for PP9 and PP12 look expan-
sive, but their quantity and quality are not good enough to act
as effective protective layer for underlying resin when the total
content of IFR is only 15 wt%. So the two composites do not get
good results in LOI and UL-94 tests. There is only a little char cov-
ered on PP15, its flame retardant properties are poor inescapably.
Fig. 2 is the photos of char residues after UL-94 tests. For all the
4.2. Thermal degradation behaviors
Fig. 3 is the TGA and DTG results of different PILs and IFR. The
TGA of IFR shows three stages. APP and PER reacts in the first
stage (T < 280 ◦C) and forms esters mixtures. In a second step, the
blowing agent decomposes to yield gaseous products (i.e., evolved
ammonia from the decomposition of APP) which result the char to
swell (280 ◦C ≤ T ≤ 350 ◦C). The intumescent material is not ther-
mal stable and decomposes at higher temperatures and then loses
its foamed character at about 430 ◦C [25]. The major weight loss
of PILs is at 350–550 ◦C, which is primarily the decomposition of
organic moiety. All of the PILs show good thermal stability, for
example, 1 wt% degradation temperature (T1wt%) of PIL12 achieves
the lowest value 251 ◦C among all PILs. The melting point of PP is
164–170 ◦C usually, so PILs can meet its processing requirement.
Most of organic parts of PILs decompose before 430 ◦C, the longer
of the alkyl chains, the more the weight loss is. The weight loss
for PIL is PIL4 < PIL8 < PIL12 < PIL18, the opposite order is obtained
Fig. 1. Photos of the char residues after LOI test for PP composites (a–f) PP0, PP3,
PP6, PP9, PP12, and PP15, (g–l) PP0, PP1, PP4, PP7, PP10, and PP13.

54
S. Chen et al. / Thermochimica Acta 631 (2016) 51–58
Fig. 2. Photos of the char residues after UL-94 test for PP composites (a–f) PP0, PP3, PP6, PP9, PP12, and PP15, (g–l) PP0, PP1, PP4, PP7, PP10, and PP13.
Fig. 3. (a) TGA and b) DTG curves of different PILs and IFR under N2 atmosphere.
Table 3
Exp. and cal. TGA data of IFR and catalysts under N2 atmosphere.
Samples
T 1wt% (◦C)
T 5wt% (◦C)
T 10wt% (◦C)
T 50wt% (◦C)
Residues at 800 ◦C (wt%)
ꢀwt%
IFR
PIL4
172
293
172
174
296
172
174
251
172
182
282
172
185
217
344
218
209
351
218
214
335
218
219
331
218
222
253
409
256
249
395
256
253
365
256
256
357
256
259
512
–
515
513
–
24.9
63.7
25.2
35.4
61.4
25.9
39.0
54.2
25.7
34.3
43.2
25.3
36.6
IFR/PIL4-Cal
IFR/PIL4-Exp
PIL8
IFR/PIL8-Cal
IFR/PIL8-Exp
PIL12
IFR/PIL12-Cal
IFR/PIL12-Exp
PIL18
IFR/PIL18-Cal
IFR/PIL18-Exp
10.2
13.1
8.6
514
517
514
522
–
513
515
11.3
for main peak temperature in DTG. The alkyl volatile released by
PILs will be fuel for combustion. These fuels offset the effect of IFR
before the degradation of PP resulting in poor flame retardancy for
PP composites.
Fig. 4 and Table 3 show TGA curves and related data of PILs and
IFR under N2. The temperatures at 5 wt%, 10 wt%, 50 wt% are noted
as T5wt%, T10wt%, T50wt%, respectively. The calculated (Cal.) curves are
the results calculated from the curves of IFR and PILs based on their
weight percentage in the blend, and the experimental (Exp.) curves
are the TGA results of the IFR/PILs blend.
The exp. T1wt% is higher than the cal. one suggesting PILs increase
the thermal stability of IFR at lower temperatures, and effect of
PILs with long alkyl group is more evident than short alkyl group.
Compared to IFR, the thermal degradation speed for the IFR/PILs
becomes slower above 500 ◦C. It illuminates that the PILs mod-
ify the thermal stability of char resulting in more char residues at
higher temperatures. Especially, the char residue for the IFR/PIL8 at
800 ◦C is 39.0 wt% while the cal. value is only 25.9 wt%. The exp. char
residues at 800 ◦C are 10.2 wt%, 8.6 wt% and 11.3 wt% for IFR/PIL4,
IFR/PIL12 and IFR/PIL18, respectively, which are also higher than
that of the cal. one. It is interesting that the quantity of char residues
is not corresponding to good flame retardancy for alkyl group.
In PP/IFR/PILs composites, the IFR and PILs are dispersed in PP
resin, so their thermal degradation is different from single IFR/PILs.
Fig. 5 shows TGA of PP/IFR/PILs composites, the related data are
listed in Table 4. For all samples, the thermal degradation at lower
temperature is accelerated by addition of PILs or IFR. However, all
the exp. curves shift to higher temperature compared to their cal.
curves suggesting a better thermal stability at higher temperature.
The exp. T1wt% values of PP6, PP9, PP12 and PP15 are 286 ◦C, 207 ◦C,
212 ◦C and 175 ◦C, in which only PP6 is 63 ◦C higher than that of the
cal. one, the others are all lower than that of the cal. one. In addition,
the char residues at 800 ◦C for PP6 and PP12 are not only higher than
the cal. values but also higher than that of PP/IFR (PP3) composites.
However, for PP9 and PP15 composites, the exp. and cal. curves
almost coincide with each other at the temperature range from 500
to 800 ◦C. The results reveal that best synergistic effect exists among
IFR, PIL4 and PP, and that for PIL12 is poorer than PIL4 but better
than PIL8. PIL18 obtains the poorest synergistic effect. The PIL8 and
PIL18 improve char residues of IFR in Fig. 4, but show poor syn-

S. Chen et al. / Thermochimica Acta 631 (2016) 51–58
55
100
100
80
a
b
80
60
60
PIL8
PIL4
40
20
IFR
40
IFR
IFR/PIL8-Cal
IFR/PIL8-Exp
IFR/PIL4-Cal
IFR/PIL4-Exp
20
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Temperature
(
)
Temperature
(
)
100
80
60
40
20
100
80
c
d
60
PIL18
PIL12
IFR
IFR
40
IFR/PIL18-Cal
IFR/PIL18-Exp
IFR/PIL12-Cal
IFR/PIL12-Exp
20
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Temperature
(
)
Temperature
(
)
Fig. 4. Cal. and Exp. TGA curves of (a) PIL4, (b) PIL8, (c) PIL12 and (d) PIL18 under N2.
100
80
60
40
20
0
100
a
b
80
60
PP
PP
40
20
0
IFR/PIL4
PP6-Cal
PP6-Exp
IFR/PIL8
PP9-Cal
PP9-Exp
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Temperature
(
)
Temperature
(
)
100
80
60
40
20
0
100
80
60
40
20
0
c
d
PP
PP
IFR/PIL18
PP15-Cal
PP15-Exp
IFR/PIL12
PP12-Cal
PP12-Exp
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Temperature
(
)
Temperature
(
)
Fig. 5. Cal. and Exp. TGA curves of (a) PP6, (b) PP9, (c) PP12 and (d) PP15 under N2 atmosphere.
Table 4
Cal. and exp. TGA data of PP/IFR/PILs composites under N2 atmosphere.
Samples
T1wt%
(
◦C)
T5wt%
(
◦C)
T10w%
(
◦C)
T50w%
(
◦C)
Residues at 800 ◦C (wt%)
PP
374
230
225
223
286
226
207
232
212
235
175
406
374
383
377
398
375
379
377
390
376
380
420
411
416
412
426
411
429
412
436
411
432
446
447
459
447
459
447
469
448
469
447
468
0.0
3.7
5.6
5.3
7.5
5.9
5.8
5.2
7.1
5.5
5.6
PP3-Cal
PP3-Exp
PP6-Cal
PP6-Exp
PP9-Cal
PP9-Exp
PP12-Cal
PP12-Exp
PP15-Cal
PP15-Exp

56
S. Chen et al. / Thermochimica Acta 631 (2016) 51–58
Fig. 6. SEM photographs of char residue of (a) PP3, (b) PP6, (c) PP9, (d) PP12 and (e) PP15.
ergistic effects on the charring behavior of PP/IFR/PILs composites.
This may be the major factor that determines the flame retardant
performance of PP/IFR/PILs composites.
8
6
4
2
0
4.3. Morphology analysis of char residue
Fig. 6 shows the SEM photos of char residues for PP and its com-
posites after LOI tests. Whether PP3, PP9, PP12 or PP15, there are
holes in the char layer as noted by arrows. The holes in PP3 are
larger than other samples, while those in PP15 are smaller and more
than other samples. For PP3, the char is not continuous because of
the lack of IFR. However for PP15, the longer alkyl chains provide
more fuels for combustion, and more gas will escape from the char
residue, small and crowded holes are thus formed. The char of PP6
is smooth, few holes are observed which means good char quality.
It is well known that a perfect char layer should have continuous,
compact surface, and porous interior. It is because that PP6 has
more perfect char layer that PP12, it obtains better flame retardant
performance than PP12.
PP0
PP2
PP5
Samples
PP8
PP11
PP14
Fig. 7. Notch impact strengths of PP composites.
4.4. Mechanical properties
Fig. 7 gives the notch impact strength of different PP compos-
ites. It is found that IFR causes the decrease of impact strength for
PP composites. While PILs increase impact strength of PP/IFR/PILs

S. Chen et al. / Thermochimica Acta 631 (2016) 51–58
57
PILs and PP. However, with the alkyl chains lengthening, the notch
impact strength increase firstly and then decrease after the carbon
number is more than 12. PP has a side methyl group, while the
long alkyl chains just like polyethylene. When the carbon number
is larger than 12, the compatibility between PILs and PP become
poor again. Of course, this may be connected with secondary crys-
tallization behaviors of alkyl which will be discussed in the future.
In a word, the organic structure of PILs can tailor the flame retar-
dancy and mechanical properties of PP composites meanwhile. It
provides a good method to get a balance of over-all properties.
5. Conclusions
Relationship between chemical structure of PILs and flame
retardancy and mechanical performance of PP/IFR/PILs compos-
ites were studied in this paper. The results showed that the flame
retardant properties of PP/IFR composites are improved by intro-
ducing PILs, however with the alkyl group lengthening, their flame
retardancy decreases again. For example, the PP/IFR/PIL composite
with 15% IFR/PIL4 passes the UL-94 V-0 test without melt-dripping,
while achieving the same grade needs more than 20 wt% IFR/PIL8.
The flame retardancy of PP/IFR/PIL18 even becomes poorer than
PP/IFR. Only the catalyst promotes the formation of perfect intu-
mescent charring layer, can good flame retardant properties be
obtained. In addition, PILs modified by different organic groups
have great effects on the mechanical properties. The addition of
IFR deteriorates the mechanical properties of PP. The notch impact
strength of PP composites with 18 wt% IFR is only 68% of that for
PP. While all the PP/IFR/PIL composites show better notch impact
strength than PP/IFR, especially PP/IFR/PIL12 obtains the best notch
impact strength as well as PP. The results are based on three factors:
Firstly, the groups in PIL tailor their thermal degradation and mod-
ify the synergism with IFR; Secondly, degradation of longer alkyl
chains in PIL provides fuel for combustion; Thirdly, the longer alkyl
chainsin PILincreasethecompatibilitybetweenPILs, IFRandPPand
modify mechanical and flame retardant performance. Therefore,
the matched chemical structure of PILs and suitable formulation of
PP/IFR/PILs bring good balance between physical and mechanical
properties for PP.
Fig. 8. SEM images of fracture morphology for PP composites.
composites evidently though their content is only 0.5 wt%. With
the alkyl chains lengthening, the notch impact strength increase
firstly and then decrease after the carbon number of alkyl group is
more than 12. The PP composites including PIL12 obtain the best
impact strength. The results shows the organic structure has great
influences on the mechanical properties of PP/IFR/PILs composites.
Though they are all alkyl group, the compatibility between them
and PP is different which may be related with their length and
condition.
The fracture surface of PP composites after impact testing is
observed by using SEM as shown in Fig. 8. The fracture surface
of PP shows some flaws and wrinkles suggesting toughness of PP.
After introducing IFR, some large holes and cracks are found, and
the interface between PP and IFR is evident meaning poor com-
patibility between them. Introducing 0.5 wt% PIL4 into PP/IFR, the
fracture morphology of PP5 is changed. Large holes are decreased
and wrinkles are observed which can absorb energy. This phe-
nomenon shows PIL4 modify the compatibility to a certain degree
though it is not perfect. For PP8, PP11 and PP14, few large holes are
observed, but there are small holes exist in PP8 and PP 14, few holes
appear in PP11. This indicates that PIL12 has the best compatibility
in PP/IFR composites, and this contributes to good impact strength.
Based on the results of flame retardancy, thermal behaviors,
mechanical properties and morphology analysis, it is concluded
that the organic structure of PILs not only tailors their flame
retardancy but also influences the mechanical properties of PP com-
posites. On the one hand, PILs tailors the flame retardancy of PP
composites by changing the content of PMo in PILs and modifying
the activity of PILs. The synergistic effects of PILs are related to the
content of PMo indicating PMo is the effective component. With
the alkyl chains lengthening, the content of PMo in the same PILs
is decreased, so the synergistic effect is weakened. Therefore, the
flame retardancy of PP13 is as poor as PP1. Meanwhile, the organic
cations containing alkyl chains will degrade from 300 ◦C to 500 ◦C,
the released alkyl residue will become fuel for combustion. Though
the content of PILs is low, it is a key factor to offset the effect of IFR
resulting in the decrease of flame retardancy for PP composites. On
the other hand, the alkyl chains modify the compatibility between
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
This work is financially supported by the National Natural Sci-
ence Foundation of China (Nos. 21274159, 51473178), and Program
for Ningbo Innovative Research Team (Grant 2015B11005) and
Ningbo Natural Science Foundation (No. 2013A610138).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tca.2016.03.020.
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