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Thermal degradation characteristics of rigid polyurethane foam and
the volatile products analysis with TG-FTIR-MS
Lingling Jiao, Huahua Xiao, Qingsong Wang, Jinhua Sun
*
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, PR China
article info
Article history:
Received 17 July 2013
Received in revised form
29 September 2013
Accepted 30 September 2013
Available online 10 October 2013
Keywords:
Rigid polyurethane foam
Thermal degradation
Volatile products
TG-DSC
In situ FTIR
TG-FTIR-MS
abstract
Thermal degradation characteristics of rigid polyurethane (PUR) foam in both air and nitrogen gaseous
environments were studied using thermogravimetry and differential scanning calorimetry (TG-DSC)
hyphenated techniques. And in situ Fourier Transform Infrared (FTIR) was employed to investigate the
characteristic functional groups of the decomposition residues at different temperatures. It is found that
the thermal degradation of PUR material in air and N
2
present a three-stage and a two-stage process,
respectively. And the degradation reaction rate of PUR in air is accelerated significantly due to the
presence of oxygen. The thermal degradation mechanism of PUR under non-oxidizing gaseous envi-
ronment was evaluated using a TGA instrument coupled with Fourier Transform Infrared and mass
spectrometer (TG-FTIR-MS). HCFC-141b served as blowing agent is detected at the initial stage. The
urethane bond groups of PUR start to break up into isocyanates segments and polyols segments from
about 200
C. With an increase of temperature, the polyols decompose into some kinds of aliphatic ether
alcohol. In the temperature range of 350e500
C, the dominant volatile products are primary amines,
secondary amines, vinyl ethers and CO
2
.
Ó2013 Elsevier Ltd. All rights reserved.
1. Introduction
Polyurethane (PU) is a kind of high polymer with a basic re-
petitive unit urethane bond (NHCOO). It can be produced from the
polymerization of isocyanates, polyols and some kinds of additives.
The products of PU materials are widely used all over the world,
such as coatings, adhesives and elastomers. Rigid polyurethane
foam (PUR) is one of the most important PU products, and in
particular it is usually utilized to produce insulation material for
building facade. However, the PUR materials are highly flammable
and the flame spread quite fast in case of fire. With the increasing
applications of PUR as insulation materials, more and more build-
ing fires were caused, resulting in great economic and life loss. It is
generally accepted that the thermal degradation of materials is the
initial step of solid combustion processes and the pyrolysis prod-
ucts that contain gaseous fuel support the combustion [1]. Thus, it
is of great importance to study the thermal degradation charac-
teristics and volatile products of PUR insulation materials.
Many studies have been conducted on the thermal degradation
mechanism of different kinds of polyurethane materials [2e5].
With different types of isocyanates, polyols or additives, PU can
have different molecular structures and properties. And the ther-
mal degradation behavior depends on the structure of the PUR
material. Chattopadhyay et al. [6] gave a detailed review on the
thermal stability and flame-retardant mechanism of various widely
used polyurethane materials. Information about the processes
occurring during the thermal stress, factors influencing the thermal
stability, and methods to improve the thermal stability and flam-
mability of PUs were presented. Kulesza et al. [7] investigated the
thermal degradation of PUR blown with pentane by using TGeMS,
TGeFTIR and Py/GCeMS. Four temperature ranges of gaseous
products evolution were detected under inert atmosphere and
pentane volatilized at the initial stage. It was suggested that ther-
mal degradation of PUs based on polyester polyols might be
occurred by increased bond strain of polymer chain due to tem-
perature increasing and the sequence of thermal degradation in
polymer chain is from hard segment to soft segment [8].
A large majority of the previous works focused on the influence
of flame retardants on the thermal degradation behavior [9e13].
The flame-retardant mechanism of PU foams and efficiency of
different flame-retardant additives were investigated. However,
few literatures can be found on the degradation mechanism of PUR
material itself when it is heated, especially for building facade fires.
In our earlier work [14], we investigated the kinetic parameters and
the decomposition products of three typical insulation materials for
building insulation system applications. In this study, we attempt to
*Corresponding author. Tel.: þ86 551 360 6425; fax: þ86 551 360 1669.
E-mail address: sunjh@ustc.edu.cn (J. Sun).
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
0141-3910/$ esee front matter Ó2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymdegradstab.2013.09.032
Polymer Degradation and Stability 98 (2013) 2687e2696
obtain more details with respect to the degradation mechanism of
PUR insulation material. The PUR employed in this study had no fire
retardant (FR), and the formula used was typical for building
insulation systems in the materials market. Thermal degradation
characteristics of PUR foam are studied by means of thermog-
ravimetry and differential scanning calorimetry (TG-DSC) hy-
phenated techniques. In order to investigate the characteristic
functional groups of the decomposition residues, in situ Fourier
Transform Infrared (FTIR) is employed. And the volatile products in
helium are studied using a TGA instrument coupled with FTIR
spectroscopy and mass spectrometer (TG-FTIR-MS). We combined
the analysis results of decomposition residues and evolved prod-
ucts in the non-oxidizing gas environment to obtain more infor-
mation on the thermal degradation mechanism.
2. Experimental
2.1. Materials
The materials were produced by Shanghai Saikun rubber and
plastic products co., LTD in China. And the formula (composed of
polyether polyols, isocyanates, catalyst, blowing agent and some
other additives) used in this work was typical for building insu-
lation systems in the materials market. Rigid polyurethane foams
were grinded to powder in mortar for all the experiments.
2.2. TG-DSC hyphenated technique
The TG-DSC experiments were carried out on a SDT Q600 from
TA Instruments in air and nitrogen, respectively. The sample
weights about 4 mg, heated up to 700
C at a heating rate of 10
C/
min, and the flow rate of the both gases was 100 mL/min.
2.3. In situ FTIR tests
Changes in the functional groups of decomposition residues at
different temperatures were characterized by in situ FTIR spec-
troscopy (Nicolet 8700 from Thermo Electron Corporation). The
pressed-disk technique was utilized with KBr and CaF
2
in air and
nitrogen, respectively. Therefore, one of the spectral ranges was
from 4000 to 400 cm
1
while the other was 4000e1000 cm
1
.
2.4. TG-FTIR-MS experiment
On-line testing of gaseous products obtained during the thermal
degradation was performed using a Perkin Elmer Pyris 1 TGA
coupled with Frontier FTIR spectroscopy spectrophotometer and a
Clarus SQ 8T mass spectrometer (TG-FTIR-MS). The sample was
heated from room temperature up to 800
C at a heating rate of
10
C/min in helium, with a gas flow rate of 75 mL/min. The con-
nections for gas transportation between the apparatuses were set
at 190
C to allow the decomposition products in a gaseous state.
3. Results and discussion
3.1. Thermal degradation behavior
Fig. 1 shows the TG and DSC curves of rigid PU foams in (a) ni-
trogen and (b) air at a heating rate of 10
C/min. It is obvious that
the thermal degradation of rigid polyurethane foam (PUR) in ni-
trogen and air show a two-stage and three-stage process, as pre-
sented in Fig. 1(a) and (b), respectively. Table 1 gives the parameters
obtained from TG and DSC results during the thermal degradation
of PUR. In both air and nitrogen, small amount of weight loss (about
3%) can be observed between 110 and 190
C, as listed in Table 1.It
indicates that the evaporation of water and some small molecules
products predominate in this stage and most of the chemical bonds
have not begun to break up (which will be confirmed later).
(a)
(b)
Fig. 1. TG and DSC curves of rigid PU foams in (a) nitrogen and (b) air atmospheres at
10 C/min.
Table 1
Parameters obtained from TG and DSC results of the thermal degradation of PUR.
Atmosphere The temperature range of each stage (
C) Weight loss (%) T
max
(from DSC curve) Residue (%) Heat (J/g)
N
2
110e190 3 340
C20156.42 (absorption)
220e600 76.5
Air 110e190 2.7 314
C, 530
C3þ7201.52 (release)
210e397 46
397e670 48.3
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e26962688
The biggest difference between TG curves in the 2 atm is at the
main degradation stage. And in air, where overlaps exist in the two
exothermic peaks of DSC curve, the total quantity of heat reaches
up to 7201.52 J/g, as shown in Table 1. It is conceivable that the large
amount of heat release can give rise to fire risk. Nevertheless, only a
small endothermic peak is shown in nitrogen, about 156.42 J/g, and
corresponds to the main weight loss stage. The different residues
amounts show that the extent of PUR degradation in air is finished
more completely, as oxygen exists.
3.2. In situ Fourier Transform Infrared spectroscopy
Polyurethane is a kind of high polymer with a basic repetitive
unit urethane bond (eNHCOOe), and can be produced from the
polymerization of isocyanates and polyols. The FTIR spectroscopy of
rigid PU foam in air at room temperature (RT) is shown in Fig. 2. The
FTIR result under nitrogen condition is similar (spectral range from
4000 to 1000 cm
1
).
For isocyanate segments, the symmetric and asymmetric
stretching vibrations of NeH correspond to the broad absorption
bands near 3322.9 cm
1
and 3389 cm
1
, while the medium-strong
peak at 1526.4 cm
1
confirms the in-plane bending vibration of Ne
H. The sharp absorption peaks at around 1726.3 cm
1
and
1221.9 cm
1
are typical for the stretching vibration of esters C]O
and asymmetric stretching vibration of CeO (from NeCOeO),
respectively. And the weak peak near 916.8 cm
1
is due to the Ne
COeO symmetric stretching vibration. The 1604.4 cm
1
absorption
peak is caused by the vibration of C]C in benzene ring and the
several weak peaks near 767e916.8 cm
1
belong to out-of-plane
bending vibration of CeH in multisubstituted benzene ring. For
soft segment polyether polyols, the three peak groups near
2979.5 cm
1
, 2929 cm
1
and 2873 cm
1
are caused by the
stretching vibration of CeH in methyl, methylene and methylene.
And eCH
3
shows the symmetric bending vibration at 1380 cm
1
.
Ether bond CeOeC stretch results in the broad and strong peak at
1082.7 cm
1
.
Fig. 3 illustrates the FTIR results of PUR foam at different tem-
peratures at a heating rate of 10
C/min in (a) air gaseous and (b)
nitrogen gaseous atmospheres. With the increase of temperature,
the evaporation of water can be seen obviously as the absorption
bands near 3500 cm
1
in Fig. 3(a) and (b) is weakened rapidly
under the two conditions of air and N
2
. According to Silverstein
et al. [15], isocyanates show a cumulated double bond stretch in the
2280-2000 cm
1
region. There is an obviously band at 2280 cm
1
at
room temperature in Fig. 3, which shows that there is still some
unreacted isocyanates monomers in the PUR material. And the
absorption band fades away before 200
C under both air and ni-
trogen environments. It means that the remaining monomers
contribute to the small amounts of weight loss during the initial
stage in Fig. 1. However, this absorption peak reappears and
strengthens with temperature increasing from 200
C to 280
Cin
air and 320
CinN
2
. The decomposition mechanism occurs via one
of the three reactions or a combination of them in Scheme 1, which
are similar to those in literatures [5e7]:
Fig. 2. FTIR of PU in air gaseous environment at room temperature.
(a)
(b)
Fig. 3. FTIR of rigid PU foam at different temperature at 10 C/min in (a) air gaseous
and (b) nitrogen gaseous environments.
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e2696 2689
It indicates that the degradation of PUR foams in this circum-
stance starts with the break of urethane bonds according to reac-
tion (1) in Scheme 1. Rupture of hard segments takes place at a
relatively low temperature and great amounts of isocyanates are
formed in the decomposition residues at the early stage. At the
meantime, peaks at 916.8 cm
1
, 1604 cm
1
and 3389 cm
1
weaken
obviously in this degradation process. However, within the tem-
perature range of 280e300
C for N
2
and 320e350
C for air, the
absorption bond 2280 cm
1
almost disappears. This implies that
the isocyanate monomers completely evaporate from PUR in a
narrow range of about 20e30
C. From 260
C in air and 320
Cin
nitrogen, the weakening of the strengths of absorption bands cor-
responding to polyol segments indicates the degradation of polyols.
At 300
C in air, the NeH peak has been very weak and the hard
segments have broken down completely. Furthermore, peak groups
of ether bonds and CeH stretching vibrations from methyl and
methylene fully disappear after 340
C. At temperature above
360
C, each peak in the spectra with very low absorbance suggests
that the material decomposition is finished. Compared to that in
nitrogen, the degradation rate of PUR material during the process in
air is accelerated significantly due to the presence of oxygen. The
initial reaction temperature of the whole reaction in N
2
is
approximately 50
C higher than that in air.
3.3. TG-FTIR-MS
Fig. 4 gives the TG and DTG curves of PU foam degradation. Two
obviously degradation stages are distinguished following the TG
and DTG results, as shown in Fig. 4, and the maximum weight loss
rate takes place at around 350
C. The result of mass spectrometry
experiment reveals that the whole thermal degradation process
involves a three-step evolution of volatile products. And only one
peak is observed from the FTIR Spectroscopy. The two prominent
peaks of total ion chromatogram (TIC) and Gram-Schmidt recon-
struction are in the same range of 300e420
C, corresponding to
the main weight loss stage of TG curve.
Fig. 5 displays the volatile products detected by FTIR at the
initial degradation stage. The broad range of absorptions between
1006.5 cm
1
and 1153.6 cm
1
can be assigned to the stretching
vibration of CeF, while 751.9 cm
1
responds to CeCl. The above-
mentioned absorption peaks reach their maximum values at
160.9
CinFig. 5. And 1,1-dichloro-1-fluoroethane, namely HCFC-
141b, is a kind of physical blowing agent for rigid polyurethane
foam, especially in insulation materials application for building
facade. Fig. 6 shows the mass spectrum of the volatile products of
PU foam at 167.33
C and Fig. 7 presents the evolution of HCFC-
141b. As presented in Figs. 6 and 7, signals from m/z 83, 81, 63
and 61 (CH
3
CCl
2
F) are observed between 110 and 220
C and the
intensities reach to the maximum values approximately at 167
C.
O
C
OR'HNRNCOR R' OH
+
(1)
H
N
RC
O
O
CR
CH
2
R''
R'
RNH
2
CC
R
R' R''
H
CO
2
++
(2)
CO
2
+
RNHR'
O
C
OR'HNR
(3)
Scheme 1. Basic reactions for thermal degradation of polyurethanes.
Fig. 4. TG and DTG curves of PU foam. Total ion chromatogram (TIC) and Gram-
Schmidt reconstruction of the volatile products.
Fig. 5. The FTIR spectra of volatile products during the initial degradation stage.
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e26962690
This suggests that the blowing agent 1,1-dichloro-1-fluoroethane
(CH
3
CCl
2
F) is released as the dominant products at the first
degradation stage.
The strongest absorption of all the evolving products at 360
Cis
shown in Fig. 8, and the FTIR spectra of the evolved gaseous
products obtained at different temperatures are presented in Fig. 9.
The presence of characteristic absorption band at 1100 cm
1
is due
to the stretching vibration of CeOeC bond from ethers with high
polarity. It is the strongest absorption peak in the spectrogram of
Fig. 8 and the peak strengthened rapidly from 325
C. The
Fig. 6. Mass spectra of the volatile products of PU foam at 167.33 C.
Fig. 7. Evolution of HCFC-141b (m/z 81, 83, 61, 63).
Fig. 8. The FTIR spectra of PU gaseous products at 360 C.
Fig. 9. FTIR spectra of evolved gaseous products obtained at different temperatures.
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e2696 2691
absorption peak near 1627 cm
1
confirms the existence of vinyl
ethers. Furthermore, the symmetrical and asymmetrical absorption
bands of CeOeC in aryl alkyl ethers account for the peaks at
1042 cm
1
and 1269 cm
1
. It is quite clear that the absorption peaks
at 2977 cm
1
, 2936 cm
1
and 2887 cm
1
are caused by the
stretching vibrations of CH
3
,CH
2
and CeH respectively. And these
absorption groups appear earlier (from about 250
C), which in-
dicates the formation of decomposition products from polyether
polyol segments. Thus, the peaks near 931 cm
1
,1376cm
1
and
1450 cm
1
also indicate the formation of tert-butyl groups eCe
(CH
3
)
3
. Medium peak at 3700e3550 cm
1
is attributed to the
presence of compounds with OH group(s) among the decomposi-
tion products. The peak exists in a wide temperature range 320e
570
C, as shown in Fig. 9. Meanwhile, the absorption bands near
1514 cm
1
in the same temperature region is caused by the bending
vibration of NeH in aromatic secondary amine. The formation of
CO
2
is confirmed by the two peaks at 2358 cm
1
and 2312 cm
1
,
while the peaks near 1724 cm
1
and 1750 cm
1
suggest the pres-
ence of carbonyl (group) C]O in esters. It can be seen in Fig. 9 that
the carbon dioxide shows two obvious evolving stages in the
temperature regions of 250e410
C and 410e670
C.
Several kinds of small molecule products, such as CO
2
(m/z
44), HCN (m/z 27), CH
3
OCH
3
(m/z 46 in the first stage) and NO
2
(m/z 46, 30 at a higher temperature) are detected by mass
spectrometer, and the evolutions of these products with respect
to temperature are shown in Fig. 10(a) and (b). Similar to the FTIR
results mentioned above, signal m/z 44 reveals that the release of
CO
2
shows a two-stage process in the ranges between 220 and
450
C and 550e650
C respectively. Nevertheless, the first step
in Fig. 10(b) displays an overlap of two peaks. It is worth noting
that small amounts of extremely toxic substance HCN is released
near 365
C, while the maximum weight loss rate takes place at
350
C.
(a)
(b)
Fig. 10. Mass spectra of small molecule products during the thermal degradation.
Fig. 11. Mass spectra of the volatile products of PUR at 230.33 C.
Fig. 12. Evolution of m/z 205, 57, 220, 55 and 206.
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e26962692
The mass spectra of volatile products and evolution of m/z
signals at different temperature regions are given in Figs. 11e16 .
Signal at m/z 106 is the base peak for methyl anilines com-
pounds, and m/z 59 is for all kinds of alcohols, where even vinyl
ethers are detected. With temperature increase, the identified
evolved products from MS experiments and the corresponding
absorption bands according to FTIR results are summarized in
Table 2.
Based on the in suit FTIR experiment analysis above in N
2
, the
thermal degradation mechanism of PUR material under non-
oxidizing gaseous environment can be evaluated. The detailed
Fig. 13. Mass spectra of the volatile products of PUR at 375.83 C.
Fig. 14. Evolution of m/z 59, 57, 58, 85, 55, 103 and 87.
Fig. 15. Mass spectra of the volatile products of PUR at 485.88 C.
Fig. 16. Evolution of m/z 106, 107, 93, 91, 65, 66, 77 and 78.
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e2696 2693
Table 2
Identification of evolving products during the degradation of PUR.
Figures of MS results T
p
(
C) Volatile products Corresponding bands (cm
1
)
Figs. 6 and 7165 CH
3
CCl
2
F 751.9
1006.5e1153.6
Figs. 11 and 12 230 931
1376
1450
Figs. 13 and 14 370 1100
2977
2936
2887
1627
Figs. 15 and 16 485 1514
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e26962694
information on the degradation process is presented in Scheme 2.
Firstly, the blowing agent HCFC-141b and some other small mole-
cules, such as H
2
O and CH
3
OCH
3
, volatilize at the initial stage (from
100 t o 200
C). In addition, some unreacted isocyanate monomers
that remained in the materials contribute to the weight loss. Sec-
ondly, the urethane bond groups of PUR break up into isocyanates
and polyols according to reaction (1) in Scheme 1. As shown in the
results of in situ FTIR, the isocyanates segments formed from
decomposition cannot volatilize and are trapped in the residues
zone. Then, the polyols segments start to decompose into some
kinds of aliphatic ether alcohols as temperature increases. And the
products become more complex with temperature increasing, even
epoxy compound is formed as a result of interactions among
evolved products. From about 350
C, reaction (2) and (3) of
Scheme 1 dominates the process and the materials decompose to
primary amines, secondary amines, vinyl ethers and CO
2
in Scheme
2. Moreover, nitrides HCN and NO
2
are formed in the temperature
range of 300e400
C. At temperature above 500
C, the material
residues continue to decompose into volatile products. Thus the
aliphatic alcohol with branched chains, benzene alkyls and small
molecule CO
2
are generated as well. When the temperature rises to
600
C, the whole thermal degradation process is almost finished.
4. Conclusions
(1) Thermal degradation process of rigid polyurethane foam
(PUR) shows a two-stage and a three-stage in nitrogen and
air, respectively. The total quantity of heat releases in air
reaches up to 7201.52 J/g and only a small endothermic peak
of about 156.42 J/g is shown in nitrogen.
(2) Compared to that in nitrogen, the degradation rate of PUR
material during the whole process in air is accelerated
significantly due to the presence of oxidizing gases. The
initial reaction temperature of thermal degradation in N
2
is
approximately 50
C higher than that in air.
(3) On the basis of in suit FTIR and TG-FTIR-MS experiment
analysis, the mechanism of PUR thermal degradation under
non-oxidizing gaseous environment has been evaluated.
HCFC-141b served as the blowing agent is released at the
initial stage. The urethane bond groups of PUR break up into
CH
3
NH
2
NH
HO OOH
OO
OH
HO OO
OOO
H
NC
O
O
H
2
CO
2
H
2
CO
2
C
O
H
N
H
N
HN
step 1
CH
3
CCl
2
F (HCFC-141b)
H
3
C
O
CH
3
H
2
O
small molecules
products
isocyanates
not reacted
blowing agent
(100-200 °C)
+
step 2
O
H
2
CO
2
H
2
COH
2
+
H
N
NCO
step 3
step 4
++
CO
2
polyether polyols
olefin
ethers
+
++
(200-350 °C)
(350-600 °C)
(start from 200 °C)
H
N
N
H
Scheme 2. Thermal degradation mechanism of rigid polyurethane (PUR) foams under non-oxidizing gaseous environment.
L. Jiao et al. / Polymer Degradation and Stability 98 (2013) 2687e2696 2695
isocyanates and polyols from about 200
C. At the meantime,
the polyols segments decompose to some kinds of aliphatic
ether alcohols and the products become more complex as the
evolved products interact with each other. In the range of
350e500
C, primary amines, secondary amines, vinyl ethers
and CO
2
become the dominant products of PUR. At temper-
ature above 500
C, the aliphatic alcohol with branched
chains and benzene alkyls are generated.
Acknowledgments
This study was supported by National Basic Research Program of
China (973 Program, No. 2012CB719702) and the Research Fund for
the Doctoral Program of Higher Education (No. 20113402110023).
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