Synthesis of novel caged phosphate esters and their flame retardant effect on poly(vinyl chloride) blends

Article abstract of DOI:10.1246/cl.150374

Novel caged phosphate esters were synthesized. Flame-retardant mechanism of poly(vinyl chloride) (PVC) blends plasticized with these synthesized caged phosphate esters was investigated with limiting oxygen index (LOI), thermogravi-metric analysis (TGA), scanning electron microscopy (SEM), and cone calorimeter tests. The results showed that caged phosphate esters improved the thermal stabilities of PVC blends: LOI values of PVC blends increased from 24.2% to 35.9%, peak heat release rate decreased from 379.04 to 258.79 kw m^-2, and total smoke increased from 1877.9 to 3698.1 m² m^-2; hence, the flame retardancy of PVC blends was improved.

Full text of DOI:10.1246/cl.150374

CL-150374
Received: April 21, 2015 | Accepted: June 1, 2015 | Web Released: June 6, 2015
Synthesis of Novel Caged Phosphate Esters and Their Flame Retardant Effect
on Poly(vinyl chloride) Blends
Puyou Jia, Meng Zhang,* Lihong Hu,1,2 Guodong Feng, and Yonghong Zhou*
1
1,2
1
1
1
Institute of Chemical Industry of Forest Products, Chinese Academy of Forest (CAF), Nanjing 210042, P. R. China
2
Institute of New Technology of Forestry, Chinese Academy of Forest (CAF), Beijing 100091, P. R. China
(E-mail: zyh@icifp.cn)
Novel caged phosphate esters were synthesized. Flame-
retardant mechanism of poly(vinyl chloride) (PVC) blends
plasticized with these synthesized caged phosphate esters was
investigated with limiting oxygen index (LOI), thermogravi-
metric analysis (TGA), scanning electron microscopy (SEM),
and cone calorimeter tests. The results showed that caged
phosphate esters improved the thermal stabilities of PVC blends:
LOI values of PVC blends increased from 24.2% to 35.9%, peak
with a mechanical stirrer, condenser pipe, and thermometer. The
mixture was stirred at 140 °C for 6 h to finish the reaction. Then
the dimethylbenzene was evaporated at 160 °C. FT-IR (KBr,
¹
1
cm ): 3391 (O­H, stretching vibration); 2958, 2910 (C­H,
stretching vibration); 1470 (C­H, deformation vibration); 1300
(P=O, stretching vibration); 1021 (P­O­C, stretching vibration);
873 (skeleton vibration of caged bicyclic phosphates); 1741
(C=O, stretching vibration); 1635 (C=C, stretching vibration).
¹
2
31
heat release rate decreased from 379.04 to 258.79 kw m , and
total smoke increased from 1877.9 to 3698.1 m m¹2; hence, the
flame retardancy of PVC blends was improved.
P NMR (DMSO-d6): ¤ ¹7.60 (P=O).
2
24.4 g of PEPA and 20.4 g of acetic anhydride was mixed in
150 mL of dimethylbenzene; the mixture was put in a three-
necked round-bottom flask equipped with a mechanical stirrer,
condenser pipe, and thermometer. The mixture was stirred at
135 °C for 8 h to finish the reaction. Then the dimethylbenzene
Poly(vinyl chloride) (PVC) is one of the more widely used
1
¹1
engineering plastics. It has broad application in many areas
was evaporated at 160 °C. FT-IR (KBr, cm ): 3391 (O­H,
2
such as wire materials, window flames, pipes, and so on. But
stretching vibration); 2958, 2910 (C­H, stretching vibration);
1470 (C­H, deformation vibration); 1300 (P=O, stretching
vibration); 1021 (P­O­C, stretching vibration); 873 (skeleton
plasticized PVC is combustible and releases smoke and poison-
ous gas on burning, because most of the plasticizers burn easily.
The combustibility restricts the application of plasticized PVC
material. Thus, there is an urgent need to improve the flame
retardancy of PVC using flame-retardant plasticizers. Flame-
retardant plasticizers with high plasticizing effect on PVC will
expand the application range of PVC materials. In this paper,
novel caged flame-retardant esters have been synthesized. PVC
blends containing caged phosphate esters (CP) were tested by
limiting oxygen index (LOI), scanning electron microscopy
3
1
vibration of caged bicyclic phosphates). P NMR (DMSO-d6): ¤
¹7.60 (P=O).
24.4 g of PEPA and 29.6 g of acetic anhydride was mixed in
180 mL of dimethylbenzene; the mixture was put in a three-
necked round-bottom flask equipped with a mechanical stirrer,
condenser pipe, and thermometer. The mixture was stirred at
140 °C for 6 h to finish the reaction. Then the dimethylbenzene
¹
1
was evaporated at 160 °C. FT-IR (KBr, cm ): 3391 (O­H,
stretching vibration); 2958, 2910 (C­H, stretching vibration);
1470 (C­H, deformation vibration); 1300 (P=O, stretching
vibration); 1021 (P­O­C, stretching vibration); 873 (skeleton
vibration of caged bicyclic phosphates); 3010 (C=C, stretching
(SEM), thermogravimetric analysis (TGA), cone calorimeter
tests, and universal tensile testing machine. All these inves-
tigations were carried out to further understand the mechanism
of the flame retardancy of PVC blends containing CP.
3
1
Pentaerythritol phosphate (PEPA) was synthesized accord-
ing to reported procedures.3 Figure 1 showed the synthesis
process of CP. 24.4 g of PEPA, 56.85 g of oleic acid, and 0.57 g
vibration). P NMR (DMSO-d6): ¤ ¹7.60 (P=O).
,4
PVC/dioctyl phthalate (DOP)/calcium stearate/zinc stea-
rate (100/40/1/1(g)) of a, PVC/DOP/CP/calcium stearate/zinc
stearate (100/30/10/1/1(g)) of b (CP1), c (CP2), and d (CP3)
were prepared in a Haake Reomix 600p (Haake Instrument
Crop., Germany) with two corotors. The processing time was
2¹
of SiO4 /TiO2 were mixed in 200 mL of dimethylbenzene. The
mixture was put in a three-necked round-bottom flask equipped
5
min at 45 rpm. The mixture temperature was 160 °C. Then the
dumbbell plasticized PVC blends were prepared in a Haake
Minijet Micro injection molding machine (Haake Instrument
Crop., Germany) according GB/T 17037.1-1997 (China).
The LOI values were measured using a JF-3 oxygen index
measuring instrument (Nanjing Lei instrument Co., Ltd., China)
according to Plastics-Determination of burning behavior by
oxygen index (GB/T 2406.1-2008, China). Table 1 showed the
LOI value of PVC blends, PVC plasticized with DOP is a
flammable polymeric material, and its LOI value is only 24.4%.
In combination with 10 g CP1, CP2, and CP3, respectively; the
LOI value reached 32.2%, 30.8%, and 35.9%, respectively. The
results indicated that PVC plasticized with CP exhibited high
efficiency in enhancing flame retardancy.
Figure 1. Synthesis of CP.
1220 | Chem. Lett. 2015, 44, 1220–1222 | doi:10.1246/cl.150374
© 2015 The Chemical Society of Japan

Table 1. Thermal, flame retardant, and mechanical properties of PVC blends
Elongation
at break
Tensile
strength
/MPa
Td
TP1
/°C
TP2
/°C
LOI
/%
Char residue
/%
pHRR
/kw m
TSR
/m m
Samples
¹2
2
¹2
/°C
/%
a
b
c
d
258.6
272.6
279.1
283.7
294.4
302.9
310.1
316.7
461.2
468.8
471.9
471.9
24.4
32.2
30.8
35.9
6.46
9.81
10.93
13.29
379.04
296.23
279.65
258.79
1877.9
3578.6
3247.3
3698.1
452.85
367.71
390.26
416.38
28.14
37.39
35.13
33.86
Figure 3. SEM micrographs of residue’s surface of PVC blends.
decomposition temperature (Td), the maximum weight-loss
temperature rate (TP1 and TP2) of PVC plasticized with CP were
higher than that of DOP, indicating that CP could improve the
thermal stability of PVC blends compared to DOP.
Carbon layers of PVC blends after LOI tests were inves-
tigated with a Hitachi 3400-1 (Hitachi, Japan) scanning electron
microscope (SEM) instrument, operated at 12 kV. Figure 3
shows SEM micrographs of residue’s surface of PVC blends.
Surfaces of residues b, c, and d are significantly different from
that of a. There are many folds and gaps in the surfaces of a,
which cannot prevent oxygen and heat getting into the PVC
matrix; hence, the surface structure of the char residue cannot
improve the thermal stability and flame retardancy of PVC
blends. The surfaces of char residues b, c, and d present a
compact and consolidated carburization zone; no folds can be
found on their surfaces. There are very few gaps in their
surfaces, so that oxygen and heat are efficiently prevented from
getting into PVC matrix. This compact and consolidated
carburization zone structure can improve the thermal stability
and flame retardancy of PVC blends.
Figure 2. TGA and DTG curves of PVC blends.
Thermogravimetric analysis (TGA) was carried out in a
TG209F1 thermal analysis instruments (Netzsch Instrument
¹
1
Corp., Germany) in N atmosphere (50 mL min ) at a heating
2
¹
1
rate of 10 °C min . The samples were measured in a platinum
pan with a mass of about 5 mg and scanned from 40 to 800 °C.
TGA curves of PVC blends are shown in Figure 2; the data
including the degradation temperature (Td), temperature at the
maximum weight-loss temperature rate (TP), and char residue at
6
00 °C are summarized in Table 1. From Figure 1, the degra-
dation courses of all the PVC blends could be divided into two
stages. The first stage degradation at around 90­350 °C is the
fastest and corresponds to the formation and stoichiometric
elimination of HCl. The second stage at above 350 °C is
attributed to crosslinking involving C=C bonds. Thermal
degradation of polyenes involves cyclization and splitting of
Combustion properties were evaluated using a cone calo-
3
rimeter. All samples (100 © 100 © 2 mm ) were exposed to a
FTT200 cone calorimeter (FTT Instrument Crop., UK) under a
¹
2
heat flux of 35 kW m
according to ISO-5660 standard
5
,6
procedures. In order to further investigate the flame retardancy
of PVC blends, the cone calorimeter tests were carried out.
The flame retardant data are showed in Table 1. The peak
heat release rate (pHRR) decreased significantly from 379.04 to
1
,4,5
chains.
DTG curves of PVC blends show two degradation
peaks at around 300 and 460 °C, corresponding to the two faster
thermal degradation stages. Char residues of sample a, b, c, and
d are 6.46%, 9.81%, 10.93%, and 13.29%, indicating that CP
could promote the formation of char residue. From Table 1,
¹
2
258.79 kw m , and the total smoke release (TSR) increased
2
¹2
from 1877.9 to 3698.1 m m . The decrease of HRR value and
Chem. Lett. 2015, 44, 1220–1222 | doi:10.1246/cl.150374
© 2015 The Chemical Society of Japan | 1221

the increase of char residue’s mass because the formation of
compact and consolidated carbon residue layer limited the
emission of volatile thermal degradation products and protected
them from oxygen. The carbon residue layer structure could
maintain the PVC blend thermal stability and improve its flame
retardancy.
This work was supported by National 12th Five-year
Science and Technology Support Plan (Grant No. 2015-
BAD15B08); Jiangsu Province Natural Science Foundation of
China (Grant No. BK20141074).
References
Based on the above results, we can conclude that phos-
phorous-containing components such as metaphosphoric acid
and pyrophosphoric acid and P­O­C, P­OH, P­H components,
generated from the pyrolysis of CP, formed a protective film on
1
M. A. da Silva, M. G. A. Vieira, A. C. G. Maçumoto, M. M.
Beppu, Polym. Test. 2011, 30, 478.
Y. Saeki, T. Emura, Prog. Polym. Sci. 2002, 27, 2055.
X. Yang, N. Ge, L. Hu, H. Gui, Z. Wang, Y. Ding, Polym.
Adv. Technol. 2013, 24, 568.
J. Chen, S. Liu, J. Zhao, Polym. Degrad. Stab. 2011, 96,
1508.
H.-Q. Peng, Q. Zhou, D.-Y. Wang, L. Chen, Y.-Z. Wang,
J. Ind. Eng. Chem. 2008, 14, 589.
L. Ye, Q. Wu, B. Qu, Polym. Degrad. Stab. 2009, 94, 751.
P. Jia, M. Zhang, C. Liu, L. Hu, G. Feng, C. Bo, Y. Zhou,
RSC Adv. 2015, 5, 41169.
L. Zhang, M. Zhang, L. Hu, Y. Zhou, Ind. Crops Prod. 2014,
52, 380.
2
3
the surface of the carbonaceous char layer, and could promote
formation of char residue as dehydrating agent.7
­10
The char
4
5
residue was blown by the evolving gases that led to foaming and
expansion of the char layer. The carbonaceous char layer is
effective in slowing down mass and heat transfer between the
gas and the condensed phases and to protect the underlying
6
7
7
material from further combustion. Plasticizing effect on PVC
could be investigated by the elongation at break and tensile
strength. Tensile data was determined according to GB/T
8
9
1040.1-2006 (China) under ambient conditions, using E43.104
Universal Testing Machine (MTS Instrument Crop., China).
From Table 1, sample a presents higher elongation at break and
lower tensile strength than sample b, c, and d, indicating that
plasticizing effect of CP on PVC is lower than that of DOP.
G. Feng, P. Jia, L. Zhang, L. Hu, M. Zhang, Y. Zhou, Korean
J. Chem. Eng. 2015, 32, 1201.
10 L. Zhang, M. Zhang, Y. Zhou, L. Hu, Polym. Degrad. Stab.
2013, 98, 2784.
1222 | Chem. Lett. 2015, 44, 1220–1222 | doi:10.1246/cl.150374
© 2015 The Chemical Society of Japan