Synthesis and thermal properties of spiro phosphorus compounds

Article abstract of DOI:10.1007/s10973-010-0714-z

Intumescent materials, 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro- [5,5]-undecane-3,9-dioxide and 3,9-dichloro-2,4,8,10-tetraoxa-3,9- diphosphaspiro-[5,5]-undecane having the capacity to produce dehydrating agent, blowing agent, and undergo carbonization during burning have been synthesized. The thermal behavior of the synthesized materials was investigated using differential thermal analysis, thermal volatilization analysis, programmed vacuum pyrolysis-mass spectrometry, flash pyrolysis-mass spectrometry and off-line pyrolysis-gas chromatography-mass spectrometry. The materials show exothermic degradation after 250 °C. Monitoring the release of hydrogen chloride and water, the blowing agents for the production of carbon foam, clearly indicated the superior performance of the pentavalent phosphorus compound over the trivalent phosphorus compound. The major gaseous degradation products released during pyrolysis showed the presence of sufficient quantities of several alkyl-substituted benzenes and fused aromatics. Suitable degradation mechanism has been proposed and discussed to explain the formation of various organics during thermal degradation.

Full text of DOI:10.1007/s10973-010-0714-z

J Therm Anal Calorim (2010) 101:281–287
DOI 10.1007/s10973-010-0714-z
Synthesis and thermal properties of spiro phosphorus compounds
•
Chinnaswamy Thangavel Vijayakumar Nagarajan David Mathan
•
•
Vijayakumar Sarasvathy Thangamani Rajkumar
•
•
Arunachalam Thamaraichelvan Durairaj Ponraju
Received: 8 July 2009 / Accepted: 9 February 2010 / Published online: 17 March 2010
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract Intumescent materials, 3,9-dichloro-2,4,8,10-
tetraoxa-3,9-diphosphaspiro-[5,5]-undecane-3,9-dioxide
and 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro-[5,5]-
undecane having the capacity to produce dehydrating
agent, blowing agent, and undergo carbonization during
burning have been synthesized. The thermal behavior of
the synthesized materials was investigated using differen-
tial thermal analysis, thermal volatilization analysis, pro-
grammed vacuum pyrolysis–mass spectrometry, flash
pyrolysis–mass spectrometry and off-line pyrolysis–gas
chromatography–mass spectrometry. The materials show
exothermic degradation after 250 °C. Monitoring the
release of hydrogen chloride and water, the blowing agents
for the production of carbon foam, clearly indicated the
superior performance of the pentavalent phosphorus com-
pound over the trivalent phosphorus compound. The major
gaseous degradation products released during pyrolysis
showed the presence of sufficient quantities of several
alkyl-substituted benzenes and fused aromatics. Suitable
degradation mechanism has been proposed and discussed
to explain the formation of various organics during thermal
degradation.
Keywords Spiro phosphorus compounds Á
Intumescence Á Thermal analysis Á Flash pyrolysis–
mass spectrometry Á Off-line pyrolysis–GC–MS
Introduction
In the modern world, we the people live with flammable
products such as chemicals, textile, upholstery, computers,
mobile phones, etc. Flammability depends on the chemical
composition of the materials which generate huge amount
of heat, subsequently release toxic corrosive gases on
combustion/pyrolysis [1]. Hence, it is necessary to develop
thermally stable products by modifying the chemical
structure or to make the material thermally stable by
compounding it with flame retardants [2]. Usually, the
incorporation of flame retardant additives or the develop-
ment of flame retardant coating for flammable materials is
a cost effective and easiest method for improved thermal
stability.
Flame retardant is a chemical substance, which inhibits
or retards physically or chemically the combustion process
by releasing non-flammable products or altering the deg-
radation path and allows more time for people to escape
from the fire hazard [3, 4]. Halogenated flame retardants
remove reactive H and OH radicals, which are produced
during combustion. According to environment concerns the
use of halogenated flame retardant systems are restricted
[5–7]. Inorganic metal hydroxides such as aluminum
hydroxide, magnesium hydroxide, etc., are used to retard
the flammability, for effective achievement relatively high
loadings are used and hence it alters the existing property
[8]. Compounds having phosphorus can be effectively used
to impart flame retardancy. It traps the radicals formed in
C. T. Vijayakumar (&) Á N. D. Mathan Á V. Sarasvathy Á
T. Rajkumar
Department of Polymer Technology, Kamaraj
College of Engineering and Technology, S.P.G.C. Nagar,
K. Vellakulam Post, Virudhunagar 625701, India
e-mail: ctvijay22@yahoo.com
A. Thamaraichelvan
Post graduate and Research Department of Chemistry,
Thiagarajar College (Autonomous), Madurai 625009, India
D. Ponraju
Radiological Safety Division, Department of Atomic Energy,
IGCAR, Kalpakkam 603102, India
123

282
C. T. Vijayakumar et al.
the combustion process by releasing the P2, PO, PO2, and
HPO radicals and induces the formation of carbon foam
[5, 9]. Nitrogen containing flame retardants are the envi-
ronment friendly flame retardant, which on heating, relea-
ses non-flammable free nitrogen, ammonia, and weak HCN
which dilute the flammable gases. Nitrogen compounds
itself not act as flame retardant, commonly nitrogen com-
pounds are used as synergist [3, 10]. Boron (borax, boric
acid, and borates) and silicon (silicones, organosilanes,
silsesquioxanes, and silicates) containing compounds are
used to impart thermal stability to the flammable materials.
Both boron and silicon containing compounds form their
oxides during pyrolysis, which prevents the access of
atmospheric air to the substrate and there by diminishing
the formation of combustible gases [11–13]. Intumescent
means mixture of components on heating various chemical
reactions taking place between them and produce multi-
cellular voluminous char, which insulates and protects the
underlying materials from heat, oxygen, and fuel essential
for combustion of the materials [10, 14, 15]. Intumescent
flame retardant system can be used in several materials
such as polymers, metallic materials, textiles, wood, etc.
[16, 17]. Intumescent activity is maximized by using car-
bon rich compounds along with phosphorus compounds as
acid sources which catalyze the carbonization and spumific
agents like urea, urea–formaldehyde resin, dicyandiamide,
melamine, polyamides, etc. On heating, carbonifics, poly-
hydric compounds loss water and give char in the presence
of acid sources; simultaneously nitrogen containing blow-
ing/spumific agents release non-flammable gases which
help to swell char [12]. The intumescent materials can act
as more than one way in imparting flame retardancy
because all the materials used are having more than one
functional group [18].
materials exists, a detailed study of the thermal properties
is essential. In this work, both PSPDPC and TSPDPC were
synthesized and their thermal properties are investigated in
detail and presented.
Experimental
Materials
The synthetic routes of 3,9-dichloro-2,4,8,10-tetraoxa-3,
9-diphosphaspiro-[5,5]-undecane-3,9-dioxide (PSPDPC) and
3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro-[5,5]-
undecane (TSPDPC) are shown in Scheme 1.
Synthesis of 3,9-dichloro-2,4,8,10-tetraoxa-3,9-
diphosphaspiro-[5,5]-undecane-3,9-dioxide
Exactly 2 mol of phosphoryl chloride was added drop wise
for 2 h under nitrogen atmosphere to 1 mol of pentae-
rythritol kept at 90 °C in a round bottom flask fitted with
water condenser. The reaction mixture was refluxed at
100 °C for 20 h. The resultant hard cake was crushed and
refluxed with excess phosphoryl chloride for an additional
2 h. The excess phosphoryl chloride was removed by
decantation followed by distillation under vacuum. The
obtained product was washed with carbon tetrachloride
followed by ethanol and finally with ether. The resultant
white colored PSPDPC was dried under vacuum [21, 22].
Synthesis of 3,9-dichloro-2,4,8,10-tetraoxa-3,9-
diphosphaspiro-[5,5]-undecane
The mixture of pentaerythritol and phosphorus trichloride
in the mole ratio of 1:2 was refluxed in sufficient quantity
of dichloro methane. Refluxing was continued for 1 h after
Phosphorus-based intumescent materials are degrading
at relatively low temperature, due to their weak bonds;
hence it can act as flame retardant by modifying the
decomposition temperature of the substance. During the
decomposition process they form carbon layer which
insulates the transfer of mass, oxygen, and heat during fire
[19, 20]. Phosphorus-based intumescent flame retardant
system can be used either by incorporating as additive to
the materials to be flame retarded or incorporating into the
polymer main chain covalently as one of the monomer or
precursor. Additive types are predominantly used since
reactive flame retardants are expensive due to their special
manufacturing technique [2, 3, 11].
Cl
O
O
O
O
O
O
2 POCl3
–4 HCl
P
P
Cl
PSPDPC
OH
OH
HO
HO
Spiro phosphorous compounds, 3,9-dichloro-2,4,8,
10-tetraoxa-3,9-diphospha-spiro-[5,5]-undecane-3,9-diox-
ide (PSPDPC) and 3,9-dichloro-2,4,8,10-tetraoxa-3,
9-diphosphaspiro-[5,5]-undecane (TSPDPC) are potent
intumescent flame retardant materials. Since the conversion
of these compounds to several structurally diverse
O
O
O
O
2 PCl3
–4 HCl
P
Cl
P
Cl
TSPDPC
Scheme 1 Synthetic routes of PSPDPC and TSPDPC
123

Synthesis and thermal properties of spiro phosphorus compounds
283
100
80
60
40
20
0
the last crystal of pentaerythritol had disappeared. Finally,
the solvent was removed by vacuum distillation. The
colorless crystalline solid obtained was TSPDPC [23].
PSPDPC
TSPDPC
Analysis
Differential thermal analysis (DTA) of the synthesized
materials was performed in nitrogen atmosphere at a
heating rate of 10 °C min-1 from ambient to 450 °C using
Mettler TA 2000 model DTA instrument. The glass appa-
ratus used for thermal volatilization analysis (TVA) was
described elsewhere [24]. TVA analysis was carried out
with 50 mg of the powdered sample in high vacuum. TVA
curves were recorded by heating the material linearly from
ambient to 450 °C at a rate of 10 °C min-1. The total ion
current (TIC) of the volatile products formed during the
programmed heating (10 °C min-1) of the material in a
Perkin Elmer Sigma 3B oven was recorded using a Balzers
Quadrupole Mass Spectrometer QMG 511.
100
200
300
400
Temperature/°C
Fig. 1 DTA traces of PSPDPC and TSPDPC (heating rate:
10 °C min-1
)
complexity of the degradation that is proceeding at this
temperature region and definitely the degradation is not a
single step process.
Flash pyrolysis of the materials at different temperatures
(100–400 °C in 50 °C steps) were carried out for 5 s by
inserting the samples, deposited on a platinum ribbon,
directly into the mass spectrometer (QMG 511). The off-
line pyrolysis of the materials at 400 °C for 10 min in
nitrogen atmosphere was done by using an all glass appa-
ratus described in detail by Vijayakumar [25]. The volatile
low molecular weight products formed during degradation
were collected over a frozen matrix of tetrahydrofuran [26,
27]. Separation and identification of the products were
performed using Hewlett Packard Gas Chromatograph (HP
5890) containing a 12 m column HP-1 (cross-linked
methyl silicone gum) coupled to a Hewlett Packard Mass
Selective Detector (HP 5970). The data analysis was per-
formed using Hewlett Packard Work Station, HP 59970.
Thermal volatilization analysis
Pressure sensitive TVA traces of PSPDPC and TSPDPC
recorded from 100 to 450 °C under vacuum condition are
shown in Fig. 2.
In the TVA trace of PSPDPC, the gradual increase of
pressure is noted from 150 up to 200 °C, where as for
TSPDPC the pressure increase is seen after 180 °C and
leveled around 230 °C. This pressure increase may be
attributed to the softening of the material followed by
degassing of the material and/or to the release of the
absorbed water molecules from the material. The possi-
bility of condensation of the material in high vacuum is
80
Results and discussion
PSPDPC
TSPDPC
Differential thermal analysis
60
DTA curves of PSPDPC and TSPDPC were recorded in
nitrogen atmosphere are illustrated in Fig. 1. The broad
endotherm of PSPDPC at around 240 °C is responsible for
the phase transformation such as melting of the compound
and the broad exotherm having the maximum at 340 °C is
attributed to the release of degradation products. However,
TSPDPC shows a broad exotherm at 327 °C. No endo-
therm is observed for this material. From Fig. 1, it is clear
that the spiro compound TSPDPC starts to degrade at a
lower temperature than the compound PSPDPC. PSPDPC
shows intumescence behavior with melting, but the com-
pound TSPDPC undergoes intumescence without melting.
The unsymmetrical nature of the DTA curve indicates the
40
20
0
100
200
300
400
Temperature/°C
Fig. 2 TVA traces of PSPDPC and TSPDPC (heating rate:
10 °C min-1
)
123

284
C. T. Vijayakumar et al.
80
60
40
20
0
327 °C, whereas TSPDPC shows a considerable increase in
the ion current value at around 305 °C. Both the TVA and
TIC measurements clearly indicate that the material
TSPDPC is showing sudden release of degradation prod-
ucts at a lower temperature than the compound PSPDPC.
From Fig. 3, one can safely state that the total amount of
volatiles released from the compound PSPDPC is com-
paratively higher than from the compound TSPDPC.
PSPDPC
TSPDPC
Evolution profile of H2O and HCl: flash pyrolysis–mass
spectrometry
The normalized mass spectra recorded for the products
obtained by the flash pyrolysis of PSPDPC and TSPDPC at
100
200
300
400
Temperature/°C
Table 1 Flash pyrolysis studies: possible structures for the various
m/z values
Fig. 3 Programmed pyrolysis–total ion current measurements for
PSPDPC and TSPDPC (heating rate: 10 °C min-1
)
Possible structure of the ion
m/z value
.+
18
H2O
also another factor for this pressure change. After 300 °C,
pressure increases gradually and reaches maximum at
317 °C for PSPDPC and 305 °C for TSPDPC. Above
325 °C in PSPDPC and 340 °C in TSPDPC, no change in
pressure was observed. The sudden increase in pressure
after 300 °C within a short time span indicates eruptive
release of gases from both the materials and this may be an
important factor which contributes to the intumescent
behavior of these materials.
.+
26
27
28
29
31
35
36
37
38
39
41
42
43
51
53
54
69
70
81
82
91
105
124
C2H2
.+
C2H3
.+
.+
+
CO , C2H4
+
CHO , C2H5
+
CH2OH
+
Cl
.+
HCl
Total ion current measurement during pyrolysis
+
Cl
The results of programmed pyrolysis of the materials fol-
lowed by the monitoring of the TIC in a mass spectrometer
are shown in Fig. 3. Here, also sudden increase of TIC was
noted above 300 °C for both the materials investigated.
PSPDPC shows the highest ion current value at around
.+
HCl
+
C3H3
+
C3H5
.+
C3H6
+
C3H7
100
80
60
40
20
0
100
80
60
40
20
0
TSPDPC
PSPDPC
+
C4H3
+
C4H5
.+
C4H6
+
C4H5O
.+
C4H6O
.+
C5H5O
.+
C5H6O
+
C4H8Cl
0
20 40 60 80 100 120 140
0
20 40 60 80 100 120 140
+
m/z
m/z
C6H5CO
.+
.+
Fig. 4 Mass spectra of volatiles produced from PSPDPC and
TSPDPC by flash pyrolyzing at 500 °C for 5 s
C8H12O , C7H8O2
123

Synthesis and thermal properties of spiro phosphorus compounds
285
1000
800
600
400
200
0
200
160
120
80
500 °C for 5 s is shown in Fig. 4. Compound PSPDPC
showed intense m/z = 18 and 36 indicating the formation
of water and hydrogen chloride, respectively. The material
TSPDPC showed the presence of intense m/z = 36 (HCl)
peak but the peak responsible for water (m/z = 18) is not
very prominent as is noted for the compound PSPDPC. In
Table 1, the m/z values and the probable structure for the
ions are presented.
PSPDPC
TSPDPC
The evolution profile of the major degradation products
(water and hydrogen chloride) was monitored by flash
pyrolyzing the materials (PSPDPC and TSPDPC) for 5 s at
a specified temperature and then recording the mass spec-
trum from m/z = 10 to 140. Intensity of the m/z = 18
and 36 are found out and recorded against the pyrolysis
temperature (Figs. 5, 6).
40
0
450
Sufficient quantities of gases must be released from the
material under consideration so that it can act as an intu-
mescent material. The carbonaceous foam formation
during degradation is essential for an intumescent material.
From Figs. 5 and 6, it is clear that both the materials
investigated are able to produce large quantities of water
and hydrogen chloride after 300 °C. The amount of gases
released from PSPDPC is considerably higher than from
TSPDPC, as evidenced by the intensity values. All the
above investigations strongly support the fact that the two
spiro phosphorus compounds are having sufficient intu-
mescent activity and the activity is noted above 300 °C.
50
150
250
350
Temperature/°C
Fig. 6 Evolution profile of HCl obtained by flash pyrolyzing the
spiro compounds at different temperatures for 5 s
10
7
20
23
2000
PSPDPC
2
3
14
18
17
8
21
22
12
16
15
11
26
25
27
0
TSPDPC
1
4
29
31
2000
Off-line pyrolysis–GC–MS studies
9
6
30
19
24
28
5
13
0
The total ion chromatograms for the volatile products
obtained by degrading PSPDPC and TSPDPC isothermally
0
5
10
15
20
Time/min
Fig. 7 Off-line pyrolysis–GC–MS studies: total ion chromatogram
for the volatiles obtained by isothermally (400 °C) degrading
PSPDPC and TSPDPC for 10 min
1000
800
600
400
200
0
25
20
15
10
5
PSPDPC
TSPDPC
for a specified period (400 °C for 10 min) are shown in
Fig. 7. The compounds identified on the basis of the
fragmentation pattern noted in their mass spectra are shown
in Table 2.
From Table 2, it is obvious that apart from hydrogen
chloride, several aliphatic and alicyclic chloro compounds
are identified in the volatile products. In spite of benzene
and toluene, various isomeric aromatic hydrocarbons are
also noted among the degradation products. It is interesting
to note spiro compounds containing five carbons atoms on
degradation liberate different isomeric aromatic hydrocar-
bons. From this it is clear that compound or compounds
capable of undergoing isomerization, cyclization, and
polymerization are produced during the degradation of the
spiro compounds. The formation of spiro-[2,2]-pent-1,
0
450
50
150
250
350
Temperature/°C
Fig. 5 Evolution profile of H2O obtained by flash pyrolyzing the
spiro compounds at different temperatures for 5 s
123

286
C. T. Vijayakumar et al.
degrading matrix and induces several ionic rearrangements
and condensations leading to the formation of several
isomeric alkyl-substituted aromatic hydrocarbons.
Table 2 Isothermal pyrolysis of PSPDPC and TSPDPC: products
identified in the volatile fraction
S. no. Retention Name of the products
time/min
Compared to various aromatic compounds, the number
of organophosphorus compounds registered among the
volatile degradation products is very minimum (Table 2).
One possible explanation for this particular observation is
that, the phosphoric and poly phosphoric acids formed
during the thermal degradation may react with carboniza-
tion source and form thermally stable P–C and P–O–C
bonded char (phospho-carbonaceous char) [22]. Similar
behavior was noted by Camino et al. [29] in their detailed
study of the thermal degradation of 3,9-dihydroxy-
2,4,8,10-tetraoxa-3,9-diphosphaspiro-[5,5]-undecane-3,9-
dioxide and proposed the mechanism of formation of
phospho-carbonaceous structure through carbocation route.
The formed phospho-carbonaceous char serves as an
excellent heat resistant barrier.
1
2
0.58
0.66
1.45
1.56
1.60
1.64
2.42
2.84
4.91
5.67
HCl
C2H5Cl—ethyl chloride
3
C3 or C4 compound containing chloro group
C6H6—benzene
4
5
C5H11Cl—2-chloro-2-methyl butane
C3 or C4 compound containing chloro group
C5 compound containing CH3 group
C6H5CH3—toluene
6
7
8
9
C4H5Cl—chloromethylcyclopropene or isomer
10
C8H10—Isomer I (dimethyl benzene isomer/
ethyl benzene)
11
12
6.01
6.02
Unknown
C8H10—Isomer II (dimethylbenzene isomer/
ethyl benzene)
13
14
15
6.69
7.83
8.28
C8H10—Isomer III (dimethylbenzene isomer/
ethyl benzene)
C5H6Cl2—1,1-dichloro-2-ethenylcyclopropane
or isomer
Conclusions
C5H6Cl2—1,1-dichloro-2-ethenylcyclopropane
or isomer
From the foregoing findings one can state that, PSPDPC
shows intumescence at relatively lower temperature with
melting as compared to TSPDPC. The complexity of the
degradation of both PSPDPC and TSPDPC is clearly seen
in the DTA curve and the degradation is multistep process.
The sudden increase in pressure in the TVA traces of the
two spiro compounds indicate the eruptive release of gas-
eous products during thermal degradation and this phe-
nomenon is responsible for the intumescence behavior and
also for the production of voluminous foamed char. Com-
pared to TSPDPC, PSPDPC generates more amounts of
volatiles at a faster rate as evidenced by TIC measure-
ments. The evolution profile of water and hydrogen chlo-
ride from PSPDPC and TSPDPC by flash pyrolysis showed
PSPDPC is able to liberate these compounds comparatively
in high quantities than TSPDPC. Off-line pyrolysis studies
indicate the structure of PSPDPC facilitates the formation
of sterically much constrained spiro pentadiene, which is
responsible for the formation of several isomeric alkylated
aromatic hydrocarbons. The major amount of phosphorus
in both PSPDPC and TSPDPC are involved in the forma-
tion of phospho-carbonaceous char via phosphoric and poly
phosphoric acids formed during the thermal degradation.
16
17
8.47
8.64
Compound containing cyclopropene unit
C9H12—Isomer I (ethylmethyl benzene,
trimethyl benzene)
18
19
20
21
22
23
24
8.68
8.83
9.02
9.27
9.36
9.82
9.95
C9H12—Isomer II (ethylmethyl benzene/
trimethyl benzene)
C9H12—Isomer III (ethylmethylbenzene/
trimethyl benzene)
C9H12—Isomer IV (ethylmethylbenzene/
trimethyl benzene)
C9H12—Isomer V(ethylmethylbenzene/
trimethyl benzene)
C9H12—Isomer VI (ethylmethylbenzene/
trimethyl benzene)
C9H12—Isomer VII (ethylmethylbenzene/
trimethyl benzene)
C9H12—Isomer-VIII (ethylmethylbenzene/
trimethyl benzene)
25
26
27
28
29
12.20
13.70
14.60
16.00
18.80
Compound containing indane nucleus
m/z = 271, 272, 273
m/z = 182
m/z = 345, 346, 347
m/z = 133 (C4H6O3P)
3-diene from PSPDPC is detailed by Vijayakumar et al.
[28]. The formation of such a sterically much constrained
compound is restricted in TSPDPC due to the no avail-
ability of the oxygen in the phosphorus atom. Hence, the
formation of several di- or tri- alkylated benzene is
restricted from the compound TSPDPC. The formed
hydrogen chloride acts as a strong proton donor in the
Acknowledgements This work was financially supported by Indira
Gandhi Centre for Atomic Research (IGCAR) Kalpakkam, India,
under the project no. IGCAR/SG/RSD/RI/2007/KCE&T_1. The
authors would like to thank the Principal and Managing Board of
Kamaraj College of Engineering and Technology, K. Vellakulam Post
625701 and Thiagarajar College of Arts and Science (Autonomous),
Madurai 625009, India for providing facilities to carry out this project
successfully.
123

Synthesis and thermal properties of spiro phosphorus compounds
287
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