Flame-Retardant Rigid Polyurethane Foams Prepared with
Amorphous Sodium Polyborate
Isao Tsuyumoto, Yasutoshi Onoda, Fumiya Hashizume, Eisuke Kinpara
Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology,
Applied Chemistry, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan
Received 23 November 2010; accepted 27 December 2010
DOI 10.1002/app.34025
Published online 1 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Highly flame-retardant rigid polyurethane(RPU) foams were prepared by vacuum impregnation orpressure impregnation of sodium polyborate (SPB) solution. The RPU foams (10 mm thickness, 134–690% SPB)
endured the flame of a butane gas burner for 167–1144 s,
and the backside temperatures remained below 45 C until
penetration occurred. The heat releases from the RPU
foams with 150 and 400% SPB in air in the cone calorimeter tests were 8.8 and 4.1 MJ/m2, respectively. The

INTRODUCTION
Rigid polyurethane (RPU) foams are widely used as
insulating materials in buildings, household appliances, and automobiles because their thermal conductivities (0.022–0.029 W m1 K1) are much smaller
than those of the other insulating materials, such aspolystyrene foam, glass wool, and rock wool. Theirhigh flammability, however, is undesirable for suchapplications from the viewpoint of disaster prevention, and their fire-retardant treatments are essential
in actual use. A wide variety of fire retardants havebeen investigated by many researchers, and the mostwidely used fire retardants in RPU foams are basedon halogen or phosphorus or a combination of
both.1–7 There are, however, environmental concernswith their use because the fire retardants themselves
are toxic in most cases and toxic products may bereleased not only in normal use but also in fires andin disposal.8 Therefore, fire retardants with low
environmental pollution risk are particularly needed.
Halogen-free and phosphorus-free fire retardants,
such as expandable graphites,9,10 silicones,11–13 aluminum trihydroxide,14 and magnesium hydroxide,15
have been developed, and all of them have been
adapted for polyurethane foams. On the other hand,sodium borate has been conventionally used for
Correspondence to:kanazawa-it.ac.jp).

I.

Tsuyumoto(tsuyu@neptune.

Journal of Applied Polymer Science, Vol. 122, 1707–1711 (2011)

C 2011 Wiley Periodicals, Inc.
V

analyses with thermogravimetry, differential scanningcalorimetry, and scanning electron microscopy indicatedthe flame-retardant mechanism, in which the SPB foam
and the formed char layer together insulated the inside
C 2011 Wiley Periodicals, Inc.
from oxygen gas and heat. V
J Appl Polym Sci 122: 1707–1711, 2011

Key words: flame retardance; polyurethanes; thermal
properties

wood materials because it has pronounced fireretardant effects on cellulose.16 Despite its potential
applications, there has been no report on the fire
retarding of polyurethane foam with sodium borate.
An amorphous sodium polyborate (SPB) preparedby the adjustment of the Na/B ratio to 0.22 has beenfound to be soluble in water up to 5.4 mol/kg of boron at 20 C.17–19 The dissolved polyborate can be
efficiently introduced into the materials’ inside witha high-concentration solution, and thus, it is very
appropriate for the impregnation treatment of wood,fibers, clothes, and wovens. Furthermore, the amorphous SPB has a film-forming property and generates a solid foam by evaporating the solution to dryness.18,19 The foam layer insulates the materials fromheat conduction and oxygen diffusion and enhanceschar-layer formation. The char layer and the foamlayer insulates the underlying layer from the flame.17
Amorphous SPB has a greater fire-retardant effectthan other borates because of the synergy effect ofthe polyborate foam layer and the resultant charlayer. In our previous study, we found a pronounced fire-retardant effect of amorphous SPB on
ethylene–vinyl alcohol copolymer and developed
new fire-resistant nonwovens consisting of the core/clad fiber, whose core and clad were poly(ethylene
terephthalate) and ethylene–vinyl alcohol copolymer,
respectively.20 In this study, we aimed to develop
fire-resistant insulating materials from organic polymer foams. We found a pronounced fire-retardant
effect of amorphous SPB also on RPU foams andfirst developed fire-resistant RPU foams.

1708

TSUYUMOTO ET AL.

EXPERIMENTAL
Materials
RPU foam made from polymeric methylene diphenyl diisocyanate (PMDI) and poly(ether polyol)
was used. The thickness and the density were 10
mm and 0.056 g/cm3, respectively. SPB solutions
containing 5.4 and 9.9 mol/kg boron were preparedby the dissolution of boric acid (H3BO3) and sodium
tetraborate decahydrate [borax, Na2B4O5(OH)48H2O]
into water to adjust the Na/B ratio to 0.22, according
to our previous reports.19 The solutions were prepared around 60 C to accelerate the dissolutions. In
the fire-retardant treatment, a solution of 5.4 mol/kg
was used at room temperature after cooling,whereas one of 9.9 mol/kg was used at 40 C to prevent precipitation.
Fire-retardant treatment
The fire-retardant SPB was introduced into the RPU
foam in wet processes with the SPB aqueous solution. Because the RPU foam shed water and was notpermeable to the SPB solution at ordinary pressure,
we used two types of procedures for the treatments:pressure impregnation and vacuum impregnation.
In the pressure impregnation process, the SPB solutions were impregnated in an autoclave at gaugepressures of 0.2–1.0 MPa for 20 min. In the vacuum
impregnation process, the pressure of a vacuum desiccator with the samples was diminished to gaugepressures of 0.8 to 0.4 MPa (absolute pressure ¼
0.2–0.6 MPa) with a rotary pump, and then, the SPB
solution was introduced into the desiccator. The
samples were immersed in the solution for 10 min.
All of the samples were dried at 115 C in an ovenafter the impregnations. The introduced amount of
SPB was evaluated by weight percentage gain
(WPG) in SPB grams per gram of RPU, which wasweighed after the drying.
Measurement
The fire-retardant effect was evaluated by a burner
combustion test and a cone calorimeter (CCM) test.
In the burner combustion test, the samples wereplaced perpendicularly and heated by a butane gas
burner whose flame was directed horizontally. The
distance between the sample and the top of theburner, the whole length of the flame, and the lengthof the reducing flame were adjusted to 12, 10, and 3
cm, respectively. The penetration time of the sample
and the temperature change at its backside weremonitored. In the CCM test, heat release from asample with a surface area of 99  99 mm2 wasmonitored when it was irradiated with infrared in
air at a heat flux of 50 kW/m2. The amount of heat
Journal of Applied Polymer Science DOI 10.1002/app

TABLE I
Treatment Conditions, WPGs in SPB (g/g of RPU), andPenetration Times of the Samples in the Burner
Combustion Tests
Gauge
Concentration WPG Penetration
Sample pressure (MPa) (mol/L as to B) (%)
time (s)
A
B
C
D
E
F
G
H
I
J

1.0
0.8
0.6
0.4
0.2
0.8
0.6
0.8
0.6
0.4

5.4
5.4
5.4
5.4
5.4
9.9
9.9
5.4
5.4
5.4

255
248
255
232
260
690
526
362
221
134

796
609
461
256
227
1144
671
548
187
167

Gauge pressures were positive for the pressure impregnation and negative for the vacuum impregnation.

release from the sample was calculated from O2
consumption in the exhaust gas and recorded for
20 min. A total amount of heat release below 8 MJ/
m2 at a heat flux of 50 kW/m2 for 20 min was thenecessary criterion for noncombustible materials by
Japanese Building Standards.
Thermogravimetry (TG; Shimadzu DTG-60, Kyoto,
Japan) was performed for the SPB-treated and theuntreated RPU foams in air at a heating rate of20 C/min. Differential scanning calorimetry (DSC;
Shimadzu DSC-60) was also performed for the SPBtreated and the untreated RPU foams in air at aheating rate of 10 C/min. The surface morphology
was investigated with scanning electron microscopy(SEM; Keyence VE-9800, Hitachi S-3000).

RESULTS AND DISCUSSION
The treatment conditions, WPG, and penetration
time in the burner combustion test are summarizedin Table I. In the pressure impregnation, the fire
resistance indicated by the penetration timeincreased with increasing gauge pressure, although
the WPG was almost constant irrelevant to thegauge pressure. This was because the SPB solution
was impregnated into the foam more homogeneously at higher pressures. In general, flame retardants must be permeated homogeneously into thebase materials on the microscopic scale to achieve
high fire resistance because the materials ignite fromtheir easily burnable parts and thermally damage
the other parts. As for vacuum impregnation, theWPG and penetration time increased with increasing
gauge pressure. Unlike the pressure impregnation,
WPG reached 363 and 690% at 0.8 MPa with 5.4
and 9.9 mol/kg solutions, respectively. The temperature change at the backside of the heated sample isshown in Figure 1. The temperatures increased

FLAME-RETARDANT RPU FOAMS

1709

Figure 1 Temperature changes at the backsides of samples A, E, F, and H (see Table I) when their adverse sides
were heated by a gas burner.

Figure 3 DSC curves of the vacuum impregnated RPU
foams with 270 and 568% SPB in comparison with theuntreated RPU foam in air (t ¼ temperature).

gradually with heating until steep increases took
place just before the penetration. The backside temperatures before penetration were all below 60 C,
and this indicated that the SPB-treated RPU foam
acted as a highly effective insulator against flame. In
contrast, the untreated RPU foam showed a steep
temperature increase immediately after heating, andits penetration time was 15 s. It is noteworthy that
the SPB-treated RPU foams of 1 cm thickness withdensities of 0.19 g/cm3 (WPG ¼ 255%) and 0.44 g/
cm3 (WPG ¼ 690%) endured the flame of the butane
gas burner for 796 and 1144 s, respectively. The fireresistant and heat-insulating performance of thisRPU foam reached the level of practical use.

The amount of heat release from the vacuum
impregnated samples of 10 mm thickness in the CCM
test is shown in Figure 2. The total amounts of heatrelease for 20 min were 8.8 and 4.1 MJ/m2 for the
samples with WPGs of 150 and 400%, respectively.
At the initial stage of the irradiation up to 2 min, the
SPB foam layer and the char layer were formed by
the gasification of the RPU. The considerable heatrelease was observed only in this stage, and the moderate heat release was observed after this because of
the two kinds of layers insulated the inside from oxygen gas and heat. The appearance of the samples after the CCM tests was black on the surface and lightbrown on the reverse because of the carbonization.
Because the total amount of heat release below 8 MJ/
m2 is the necessary criterion for noncombustible
materials by the Japanese Building Standards, thiswas the first development of noncombustible organic
polymer-based materials that satisfy the criterion.
Figures 3 and 4 show the DSC and the TG curvesof the vacuum impregnated and untreated RPU

Figure 2 Heat release rate and total heat release from thevacuum impregnated RPU foams with (a) 150% SPB and(b) 400% SPB in the CCM test (t ¼ time).

Figure 4 TG curves of the vacuum impregnated RPU
foams with 270 and 568% SPB in comparison with theuntreated RPU foam in air (t ¼ temperature).
Journal of Applied Polymer Science DOI 10.1002/app

1710

foams, respectively, in air. In the DSC curve, theheat release of the untreated RPU foam started
around 210 C because of the combustion of the gasification product, and the heat release rate wasincreased with increasing temperature.21,22 The TG
curve indicates that this gasification (degradation)
process proceeded up to around 650 C for theuntreated RPU foam. In the DSC curves of the SPBtreated RPU foam, the heat releases were suppressedin comparison with the untreated sample, and interestingly, the sample with 568% SPB showed endothermic decomposition instead of heat release. In
general, the degradation processes occurring atelevated temperature were thermodynamically endothermic, and thus, the endothermic DSC curve indicated that the gasification (degradation) without the
combustion proceeded because of the insulation ofoxygen gas by the SPB foam. This result supports
the flame-retardant mechanism in which the SPB
foam insulated the inside from oxygen gas. As
shown in the TG curve of Figure 4, the thermal degradation of the untreated RPU foam occurred in twostages: 200–400 and 400–620 C. By the vacuum
impregnation of 270% SPB, the degradation temperature regions moderately decreased to 200–370 and
370–570 C, and the total weight loss ratio was
reduced to 53%, although the thermal degradation
curve was similar to that of the untreated RPU
foam. The lowering of the temperature at which thethermal degradation occurred may have been due tothe reduction of the fraction of closed cell space by
the vacuum impregnation. The reduction of the total
weight loss was due to the existence of SPB, whichusually showed only about a 14% weight loss,
mainly by the desorption of hydrated water. Unlike
these, the RPU with 568% SPB started the weightloss around 100 C. Given that the first steep weightloss of about 15% around 150 C was due to the desorption of hydrated water of the SPB, the thermal
degradation of the RPU foam gradually proceeded
above 150 C. The gasification (degradation) proceeded without oxygen because the SPB foam insulated the inside from air. The gasification products
did not ignite because oxygen gas was dilutedaround the products, and the ignition temperaturesof the products formed without oxygen were higher
than their formation temperature. The total weightlosses until 1000 C were 53 and 35% for the samples
with 270 and 568% SPB, respectively, and these values suggest that the base material, RPU foam,
mostly disappeared. However, we found that blackglassy residues remained even after the samples
were heated at 1000 C in air. There is a possibility of
the formation of high-value-added fire-resistantmaterials, such as carbon fiber and boron fiber,
because the pure SPB yielded white sodium borate
by heating at 1000 C.
Journal of Applied Polymer Science DOI 10.1002/app

TSUYUMOTO ET AL.

Figure 5 SEM images of (a) the vacuum impregnated
RPU foam and (b) that after heating by the gas burner.
The sample for part b was carbonized, and its appearance
to the naked eye was black char with white fine powder
on it. The images in parts a and b were taken at accelerating voltages of 15 and 2 kV with and without carbon coating, respectively.

Figure 5 shows the SEM images of the RPU foam
after the vacuum impregnation of SPB [Fig. 5(a)]and that after heating by the gas burner [Fig. 5(b)].
As shown in Figure 5(a), the closed cells in the RPU
foam were partly ruptured by the pressure difference, and the SPB solution was considered to be
impregnated inside through the ruptured cells andprecipitated on the foam wall. The RPU foam structure mostly remained the same after the vacuum
impregnation except for the point that the closed
cells were partly ruptured. The microstructure also
indicated that the stiffness and the thermal insulating properties were not reduced by the vacuum
impregnation. The carbonized part of the SPBtreated RPU foam formed by heating endured theflame of the burner for 3–19 min, as already shownin Table I and Figure 1. The SEM image in Figure5(b) indicates that the enduring carbonized part consisted of overlapped highly viscous dense char.
Interestingly, imbricate, scalelike incrustation wasobserved on the char at the lower left. This was

FLAME-RETARDANT RPU FOAMS

probably a fine precipitate of the SPB because thecarbonized part consisted of black char and finewhite powder on it by naked eye observation. This
supported the mechanism in which the SPB foam
insulated the inside from oxygen and accelerated thecarbonization.

1711

high-value-added materials, such as carbon fibers,boron fibers, and silicon carbide fibers.
The authors are grateful to Toru Nishino, Ikuo Nishiyama,
Tamekazu Ono, and Masahiro Tomijima of Kurabo Co., Ltd.,for supplying samples of RPU foams and for their helpful
suggestions.

References
CONCLUSIONS
Flame-retardant RPU foams were successfully prepared by the vacuum impregnation or pressure
impregnation of SPB solutions. The RPU foam with
10 mm thickness and 690% SPB endured the flameof a butane gas burner for 19 min, and its backsidetemperature was below 45 C for 19 min. The flame
retardance was controllable by changes in theimpregnation conditions and WPG. The total heatreleases for 20 min in the CCM test were 4.1 and 8.8
MJ/m2 for the samples with WPGs of 150 and 400%,
respectively. This was the first organic polymer
material that satisfied the necessary criterion (<8MJ/m2 for 20 min) for noncombustible materials.
DSC and TG analyses indicated that the heatreleases were suppressed because of the insulationof oxygen gas by the SPB foam. In the SEM image,overlapped dense char was observed in the carbonized part of the sample, which endured the flame of
the gas burner. These results support the flame-retardant mechanism in which the SPB foam and theformed char layer together insulated the inside fromoxygen gas and heat. The RPU foam developed inthis study was a flame-retardant heat insulator withlight weight (<0.44 g/cm3). There are various applications in industries, including heat insulators forbuildings, household appliances and automobiles,
gaskets for sealing, and fire-resistant clothes. This
could be an alternative to materials such as asbestos,glass fibers, and rock wools. Further analysis of thechar may lead to new preparation techniques of

1. Singh, H.; Jain, A. K. J Appl Polym Sci 2009, 111, 1115.
2. Singh, H.; Jain, A. K.; Sharma, T. P. J Appl Polym Sci 2008,
109, 2718.
3. Hirschler, M. M. Polym Adv Technol 2008, 19, 521.
4. Wang, J. Q.; Chow, W. K. J Appl Polym Sci 2005, 97, 366.
5. Weil, E. D.; Levchik, S. V. J Fire Sci 2004, 22, 183.
6. Levchik, S. V.; Weil, E. D. Polym Int 2004, 53, 1585.
7. Tanga, Z.; Maroto-Valera, M. M.; Andrésena, J. M.; Millerb, J.
W.; Listemannb, M. L.; McDanielb, P. L.; Moritab, D. K.;
Furlanb, W. R. Polymer 2002, 43, 6471.
8. Purser, D. In Fire Retardancy of Polymeric Materials; Grand,
A. F.; Wilkie, C. A., Eds.; Marcel Dekker: New York, 2000;
p 449.
9. Thirumal, M.; Khastgir, D.; Singha, N. K.; Manjunath, B. S.;
Naik, Y. P. J Appl Polym Sci 2008, 110, 2586.
10. Shi, L.; Li, Z. M.; Xie, B. H.; Wang, J. H.; Tian, C. R.; Yang, M.
B. Polym Int 2006, 55, 862.
11. Berta, M.; Lindsay, C.; Pans, G.; Camino, G. Polym Degrad
Stab 2006, 91, 1179.
12. Matsukawa, K.; Inoue, H. J Polym Sci Part C: Polym Lett 1990,
28, 13.
13. Inoue, H.; Ueda, I.; Nagai, S. J Appl Polym Sci 1988, 30, 2039.
14. Smissaert, L.; VanHoeland, P. J Fire Sci 1986, 4, 192.
15. Ashley, R. J.; Rothon, R. N. Plast Rubber Compos Process
Appl 1991, 15, 19.
16. Lewin, M.; Weil, E. D. In Fire Retardant Materials; Horrocks,
A. R.; Price, D., Eds.; Woodhead: Cambridge, England, 2001;
p 55.
17. Tsuyumoto, I.; Oshio, T. J Wood Chem Tech 2009, 29, 277.
18. Tsuyumoto, I. J Am Ceram Soc 2005, 88, 1628.
19. Tsuyumoto, I.; Oshio, T.; Katayama, K. Inorg Chem Commun
2007, 10, 20.
20. Tsuyumoto, I.; Miura, Y.; Hori, Y. J Mater Sci 2010, 45, 2504.
21. Shufen, L.; Zhi, J.; Kaijun, Y.; Shuqin, Y.; Chow, W. K. Polym
Plast Tech Eng 2006, 45, 95.
22. Thirumal, M.; Singha, K.; Khastgir, D.; Manjunath, B. S.; Naik,Y. P. J Appl Polym Sci 2010, 116, 2260.

Journal of Applied Polymer Science DOI 10.1002/app