Thermal characteristics of novel NaH2PO4/NaHSO 4 flame retardant system for polyurethane foams

Article abstract of DOI:10.1007/s10973-005-7294-3

Thermal behaviour of NaH₂PO₄/NaHSO₄ flame retardant system for polyurethane (PU) rigid foams was investigated by simultaneous TG/DTA under dynamic conditions. It has been found that the most probable mode of action of 5:3

Full text of DOI:10.1007/s10973-005-7294-3

Journal of Thermal Analysis and Calorimetry, Vol. 86 (2006) 2, 475–478
THERMAL CHARACTERISTICS OF NOVEL NaH PO /NaHSO FLAME
2
4
4
RETARDANT SYSTEM FOR POLYURETHANE FOAMS
1
K. Kulesza , K. Pielichowski and Z. Kowalski
*
2
3
1
Institute of Heavy Organic Synthesis ‘Blachownia’, ul. Energetyków 9, 47-225 KÄdzierzyn-Koüle, Poland
2
Department of Chemistry and Technology of Polymers, Cracow University of Technology, ul.Warszawska 24
1-155 Kraków, Poland
Institute of Inorganic Chemistry and Technology, Cracow University of Technology, ul.Warszawska 24, 31-155 Kraków, Poland
3
3
Thermal behaviour of NaH
neous TG/DTA under dynamic conditions. It has been found that the most probable mode of action of 5:3 (mass/mass)
NaH PO /NaHSO system, which proved to be most efficient in PU flame retardation (in comparison with other NaH PO /NaHSO
2 4 4
PO /NaHSO flame retardant system for polyurethane (PU) rigid foams was investigated by simulta-
2
4
4
2
4
4
compositions), is based on formation of char-promoting phosphoric acids and on thermal stabilisation of PU macrochains by an ex-
cess of sodium dihydrogenphosphate.
Keywords: flame retardation, polyurethane, sodium dihydrogenphosphate, sodium hydrogensulfate, thermal decomposition
Introduction
by dilution and cooling effects, as well as by inducing
cross-linking reactions [9]. Additionally, we have re-
ported that sodium dihydrogenphosphate and sodium
hydrogensulfate show synergistic effect when used
jointly [10]; it was supposed that they react with
phosphoric acids formation [11] that are known to be
active both in gas and condensed phase during poly-
mer combustion [2,12–15]. Recently action of other
acid forming inorganic FR agents has been also de-
scribed; results of TG-MS study of (NH ) SO and
Polyurethanes (PU) are a class of polymers with
broad range of properties and applications. Some of
the most important polyurethane-based materials are
rigid PU foams that are commonly used for building
engineering applications and for thermal insulation in
domestic and commercial refrigeration, as well as fur-
niture components and decorative panelling [1]. Such
a broad spectrum of applications has led to concerns
about the flammability which needs to be thoroughly
re-addressed after the replacement of chlorofluoro-
carbons with other blowing agents that have lower en-
vironmental impact but may support the fire propaga-
tion [2]. Generally, phosphoric acid haloalkyl esters
are commonly used as PU flame retardants [3], but
there is tendency to withdraw halogen-containing
systems due to formation (in their presence) of toxic
decomposition products during fire [4]. Effective
nonhalogen flame retardants are dimethyl-
methylphosphonate and other phosphorus-containing
organic compounds [2]; besides, ammonium
polyphosphate (APP), red phosphorus (RP) and ex-
pandable graphite (EG) are applied as flame retar-
dants of PU foams [3, 5, 6] – e.g. the mixture of APP
or RP with EG was found to be very effective [7, 8],
but its black colour is disadvantageous for a number
of applications. Apart from ammonium salts of phos-
phoric acid, attention was also paid to other inorganic
phosphates - our previous results have shown that so-
dium dihydrogenphosphate flame retards PU foams
4
2
4
(NH ) HPO influence on the pyrolysis of pinus nee-
4
2
4
dles were published [16]. On the other hand, thermal
decomposition of various PU materials is presently
intensively studied, as described in [17–19]. The in-
fluence of NaH PO /NaHSO flame retardant (FR)
2
4
4
system on thermal decomposition of ethoxylated
bisphenol A-based polyetherurethanes blown with
pentane was recently described [20, 21]. DRIFTS,
TG-MS, TG-FTIR and GC-MS data indicate that
mixture of sodium dihydrogenphosphate and sodium
hydrogensulfate causes change of degradation mech-
anism of PU at the start of degradation (180–250°C) –
more thermally stable groups are formed by urethane
bond decomposition reaction leading to formation of
secondary amines and CO , while base foam (without
2
addition of the FR system) degrades with first order
amines, vinyl bonds and CO formation [20, 21]. Ad-
2
ditionally, the FR system catalyses cross-linking reac-
tions those lead to intensive char formation.
Thus, the aim of this work was to explain the
mechanism of interaction of NaH PO and NaHSO by
2
4
4
*
Author for correspondence: kulesza@icso.com.pl
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2006 Akadémiai Kiadó, Budapest

KULESZA et al.
thermal analysis methods in order to apply the knowl-
edge gained during systematic thermal studies for even
more effective flame retardation of rigid PU foams.
Experimental
Materials
Sodium dihydrogenphosphate (NaH PO ) and disod-
2
4
ium pyrophosphate (Na H P O ), which is a by-prod-
2
2
2
7
uct of NaH PO thermal degradation, were products
2
4
of Chemical Works ‘Alwernia S.A.’, Alwernia, Po-
land, and sodium hydrogensulfate (NaHSO ) was ob-
tained from POCh, Gliwice, Poland.
4
Novel flame retardant system formulation
NaH PO /NaHSO flame retardant system was ob-
2
4
4
tained by grinding of 5 mass parts of NaH PO with 3
2
4
parts of NaHSO in porcelain mortar. Burning tests of
4
PU foam modified with 20% of NaH PO /NaHSO
2
4
4
Fig. 1 Results of thermal analysis of sodium dihydrogenphos-
phate. Heating rate 5 K min
–1
have shown synergistic effect of these compounds;
the highest efficiency was obtained at 5:3
(
mass/mass) ratio of components [11].
Techniques
Thermal analyses were carried out using simultaneous
thermogravimetric/differential thermal analyser TA In-
–1
struments 2960 at a heating rate of 5 K min under air
atmosphere. Samples’ mass was ca. 20 mg. FTIR spec-
tra were obtained for 10 mg samples in 300 mg KBr disk
using Unicam’s spectrometer Mattson 3000.
Results and discussion
A comparison of the endothermal peaks in DTA curve
of sodium dihydrogenphosphate decomposition
(
Fig. 1) with the corresponding peaks in DTA profile
of disodium pyrophosphate (Fig. 2) shows that the lat-
ter is an intermediate product, formed during sodium
dihydrogenphosphate thermal degradation.
Dehydration of sodium dihydrogenphosphate
into pyrophosphate form should stoichiometrically
proceed with 7.5% of sample mass decrease – in our
experiment we have observed only 6.84% mass loss.
Besides, 13.84% of the total mass loss was found,
while stoichiometric sodium dihydrogenphosphate
full dehydration requires 15%. We suppose that the
difference arises from other phosphates presence in
an industrial-grade sample. Conclusion of disodium
pyrophosphate formation agrees with recent
Vlase et al. results [22]. Thermal decomposition of
the second component of flame retardant system ana-
Fig. 2 Results of thermal analysis of disodium pyrophosphate.
Heating rate 5 K min
–1
lysed in the course of this work (sodium
hydrogensulfate) is displayed in Fig. 3.
In the first stage, 3.04% of hygroscopic water
was lost, followed by melting and dehydration lead-
ing to the formation of solid Na . Dehydration of
anhydrous NaHSO into Na S O should cause 7.5%
2 2 7
S
O
2
2
7
4
of mass loss; in Fig. 3 can be seen that in this degrada-
476
J. Therm. Anal. Cal., 86, 2006

2 4 4
NaH PO /NaHSO FLAME RETARDANT SYSTEM
Fig. 3 Results of thermal analysis of sodium hydrogensulfate.
–
Heating rate 5 K min
1
Fig. 4 Thermal degradation of NaH
(
2 4 4
PO /NaHSO 5:3
–1
mass/mass) mixture. Heating rate 5 K min
tion step the sample lost 9.91% of mass – such a dif-
ference is caused by the presence of monohydrate of
sodium hydrogensulfate in the investigated sample. In
the next step, sodium pyrosulfate melts and degrades
tion at 130°C, followed by two dehydration steps giv-
ing overall reaction (3)
5
NaH PO + 3NaHSO ®
2 4 4
with SO evolution.
3
®
Na H P O + 3Na SO + 4H O
2
(3)
After analysis of the individual components, ther-
mal degradation of 5:3 (mass/mass) NaH PO /NaHSO
4
mixture was studied and results are shown in Fig. 4.
2
5
5
16
2
4
2
4
If flame retardant contains 1.83% of hygroscopic
water then reaction (3) yields 7.36% of water, while
two dehydration steps at 177 and 239°C produce
.55% of water (Fig. 4) – the small difference is due
to the influence of next dehydration step and presence
of small amount of NaHSO ·H O. FTIR spectra do
not show differences between thermally untreated
sample of flame retardant and sample heated up to
40°C (Fig. 5). It suggests that endothermic effect at
30°C is not an effect of chemical reaction, but is re-
sulted from melting of a new, previously formed com-
ponent in FR system. Phosphoric acid formation
would be rather connected with dehydration step.
Summarizing the results obtained, the most
probable idea assumes melting effect at 130°C and,
during the following dehydration step, phosphoric
acid formation, which acts as charring agent during
Hygroscopic water evolution (1.83%) is fol-
lowed by an endothermic effect at 130.4°C (not con-
nected with any mass loss) which is could be a reper-
cussion of reaction (1); such endothermic effect does
not appear during thermal analysis of mixture’s com-
ponents. Next degradation steps are also other than it
could be concluded from the thermal degradation
route of separated components of mixture – the reac-
tion of sodium dihydrogenphosphate with sodium
hydrogensulfate can proceed as presented in reactions
7
4
2
1
1
(
1) and (2).
NaH
NaH PO + H PO ® NaH P O + H O­ (2)
PO
+ NaHSO
® H
PO
+ Na
SO
(1)
2
4
4
3
4
2
4
2
4
3
4
3
2
7
2
Maximum efficiency of NaH PO /NaHSO mix-
2
4
4
burning, while an excess of NaH PO increases PU
2 4
ture as a flame retardant (when only first reaction oc-
curs) should be observed at the mass proportion of
thermal stability – it agrees with the our previous re-
sults (NaH PO4 improves alkyloxylate-based PU
2
1
:1, while, when first and second reaction are running
foams thermal stability and influences the degrada-
tion mechanism [10] – it can be assumed that
NaH PO presence stabilizes PU macrochains) and
both, mixture ought to be the most active at 2:1 mass
ratio. Correlation of the highest activity at 5:3 mass
ratio with the thermal analysis result suggests the
2
4
with the present results – the flame retardant system
under investigation loses water by the way which is
more similar to phosphoric acids than to a salt.
presence of Na H P O16 as intermediate active com-
2 5 5
ponent, which is formed by no-water producing reac-
J. Therm. Anal. Cal., 86, 2006
477

KULESZA et al.
References
1
2
O. Gunter, Polyurethane Handbook, Hanser, Munich, 1994.
A. F. Grand and C. A. Wilkie (Eds.), Fire Retardancy of
Polymeric Materials, Marcel Dekker, New York, 2000.
J. Green, J. Fire Sci., 14 (1996) 353.
3
4
5
J. Murphy, Reinf. Plastics, 45 (2001) 42.
M. Modesti, F. Simioni and P. Albertini, Cell. Polym.,
13 (1994) 113.
6
7
R. William and H. Bell, US patent 2168706 A, 25/06/1986.
S. Duquesne, R. Delobel, M. Le Bras and G. Camino,
Polym. Degr. Stab., 77 (2002) 333.
8
9
M. Modesti and A. Lorenzetti, Polym. Degr. Stab.,
78 (2002) 167.
K. Pielichowski, K. Kulesza and E. M. Pearce, J. Polym.
Eng., 22 (2002) 196.
1
1
0 K. Pielichowski, K. Kulesza and E. M. Pearce, J. Appl.
Polym. Sci., 88 (2003) 2319.
1 K. Pielichowski and K. Kulesza, Thermal stability and
Fig. 5 Comparison of FTIR spectra of NaH
2
PO
4 4
/NaHSO 5:3
(mass/mass) mixture 1 – under ambient conditions and
– after conditioning for 2 h at 140°C
th
flame retardancy of rigid polyurethane foams, The 14
2
Annual BCC Conference on Flame Retardancy – Recent
Advances in Flame Retardancy of Polymeric Materials,
Stamford, CT, June 2–4, 2003.
1
1
2 G. Camino, L. Costa and M. P. Luda di Cortemiglia,
Polym. Degr. Stab., 33 (1991) 131.
3 J. Green, J. Fire Sci., 10 (1992) 470.
Conclusions
Sodium dihydrogenphosphate normally decomposes
with sodium pyrophosphate formation, but in the pres-
ence of sodium hydrogensulfate it melts before the de-
composition occurs – it is manifested by an endothermal
effect at ca. 130°C. Sodium dihydrogenphosphate and
sodium hydrogensulfate give synergistic flame retarda-
tion effect – the most probable mode of action is based
on char-promoting phosphoric acids formation and ther-
mal stabilization of PU units by an excess of sodium
dihydrogenphosphate.
14 E. D. Weil, Handbook of Organophosphorus Chemistry,
Ed. R. Engel, Marcel Dekker, New York, 1992.
1
1
5 S. Lu and I. Hamerton, Prog. Polym. Sci., 27 (2002) 1661.
6 A. Pappa, K. Mikedi, N. Tzamtis and M. Statheropoulos,
J. Therm. Anal. Cal., 84 (2006) 655.
1
1
1
7 K. Pielichowski, D. SÓotwiÕska and J. Pielichowski, J.
Therm. Anal. Cal., 63 (2001) 317.
8 E. F. S. Vieira, A. R. Cestari, S. F. Zawadzki and
S. M. Rocha, J. Therm. Anal. Cal., 75 (2004) 501.
9 W. W. SuÓkowski, S. Mistarz, T. Borecki, M. MoczyÕski,
A. Danch, J. Borek, M. Maci·üek and A. SuÓkowska,
J. Therm. Anal. Cal., 84 (2006) 91.
2
2
2
0 K. Kulesza and K. Pielichowski, J. Anal. Appl. Pyrolysis,
76 (2006) 249.
Acknowledgements
1 K. Kulesza, K. Pielichowski and K. German, J. Anal.
Appl. Pyrolysis, 76 (2006) 243.
2 T. Vlase, G. Vlase and N. Doca, J. Therm. Anal. Cal.,
Auhors are grateful to Dr. Zbigniew Wzorek (Cracow Univer-
sity of Technology) for his help with TG/DTA measurements.
80 (2005) 207.
Received: August 21, 2005
Accepted: June 27, 2006
DOI: 10.1007/s10973-005-7294-3
478
J. Therm. Anal. Cal., 86, 2006