J Mater Sci (2010) 45:2504–2509
DOI 10.1007/s10853-010-4222-0

Fire-resistant nonwovens of EVOH and PET treatedwith amorphous sodium polyborate
Isao Tsuyumoto • Yuichiro Miura • Yukari Hori

Received: 4 November 2009 / Accepted: 9 January 2010 / Published online: 26 January 2010
Ó Springer Science+Business Media, LLC 2010

Abstract Nonwovens made of poly(ethylene terephthalate)(PET) and ethylene–vinyl alcohol copolymer (EVOH) arefound to become highly fire resistant by treating withamorphous sodium polyborate (SPB). The fire-resistantnonwovens with 10-mm thickness show a gradual temperature increase up to 127 °C and no hole, when their back sides
are fired with a gas burner for 12 min. The analysis using theEPMA and the DTA/TG indicates that the SPB foam promotes the formation of char (carbon) layer, and that the charlayer together with the SPB foam insulates inside from theoxygen and the heat.

Introduction
Fire retarding treatment of polymer materials has attracted
much attention from the viewpoint of disaster prevention,
because polymer materials are widely used in our daily life
and their heat release occurs rapidly when they are ignited.
Various compounds such as aluminum hydroxide, magnesium hydroxide, antimony oxides, zinc borate, bromides,and chlorides have been used for fire retardants of polymer
materials, whereas sodium borate has hardly been applied
to synthetic polymer materials [1]. Sodium borate has been
mainly used for wood materials, because it has pronounced
fire retarding effect on cellulose [2]. Polymers containinghydroxyl group (–OH) are considered to show pronounced
fire retarding effect by sodium borate due to the interaction
between hydroxyl group and borate, and we found for theI. Tsuyumoto (&)  Y. Miura  Y. Hori
Department of Applied Chemistry, College of Bioscienceand Chemistry, Kanazawa Institute of Technology,Ishikawa, Japane-mail: tsuyu@neptune.kanazawa-it.ac.jp

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first time that an amorphous sodium polyborate (SPB) has
also pronounced fire retarding effect on ethylene–vinylalcohol copolymer (EVOH). Although a number of reports
have appeared on fire retardancy of ethylene–vinyl acetate
copolymer (EVA) containing such as ammonium polyphosphate, aluminum trihydrate, melamine, and silicate [3–5], there have been few reports on the fire retardancy ofEVOH [6].
The amorphous SPB is prepared by adjusting Na/B ratio
to 0.22 (0.22Na2OB2O3nH2O), and its high solubility towater (up to 5.4 mol/kg of boron at 20 °C) is very appropriate for impregnation treatment of wood, fibers, clothes,
and woven because the dissolved polyborate is efficiently
introduced into the materials inside [7]. The amorphousSPB has a film-forming property, and it yields a solid foam
(0.1–0.3 g/cm3) like polystyrene by evaporating the solution to dryness [8, 9]. This property of the amorphous SPB
greatly contributes to the fire retarding effect because thefoam layer insulates the materials from the heat conduction
and the oxygen diffusion and enhances a char layer formation. The char layer generally consists of an amorphous
carbon and also insulates the underlying layer from theflame [10]. The thermal stability of the char layer isenhanced by the existence of the polyborate. The synergy
effect of the polyborate foam layer and the resultant charlayer is the main reason why the amorphous SPB has
greater fire retarding effect than the other borates. In ourprevious study, we have developed fire-resistant solid
wood and laminated wood satisfying the criterion that thetotal amount of heat release at a heat flux of 50 kW/m2 for
20 min is below 8 MJ/m2 through a process of pressureimpregnation of the amorphous SPB solution [11]. The
amorphous SPB was found to have pronounced fire
retarding effect also on some kinds of adhesives containinghydroxyl groups such as resorcinol formaldehyde (RF),J Mater Sci (2010) 45:2504–2509

polyvinyl alcohol (PVOH), and ethylene–vinyl alcohol
copolymer (EVOH), and to have various potential applications in fields of synthetic polymer materials. In thepresent study, we focus on fire retarding treatment of
EVOH and its fire retarding mechanism. We present here
new fire-resistant nonwovens consisting of the core/cladfiber whose core and clad are polyethylene terephthalate(PET) and EVOH, respectively, treated with SPB.

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remaining over the specimens were brushed off after thesuction. The amorphous SPB (Na/B = 0.22), borax, andboric acid were individually used in this process for comparison. The mixture of borax and boric acid was also
examined by sucking borax after boric acid. The amorphous SPB was used after closely grinding in a mortar witha pestle, because the SPB solution yields a solid foam byevaporation.
Measurement

Experimental
Materials
The nonwovens consisting of core/clad fibers, in which thecore of PET 10 lm in diameter is concentrically surrounded by the clad of EVOH, were used for investigation.
The fibers were fusion bonded at a cross-point of EVOH,
and the total diameter of the fibers was 20 lm. The ethylene content in the EVOH was 44 mol%. The size of thespecimen was 100 mm 9 100 mm 9 10 mm, and thedensity was 0.15 g/cm3.
The SPB solution containing 5.4 mol/kg of boron wasprepared by dissolving 200 g of boric acid (H3BO3)
and 250 g of sodium tetraborate decahydrate (borax,Na2B4O5(OH)48H2O) into 1000 g of water, according toour previous reports [8]. The Na/B molar ratio and thedensity of the solution were 0.22 and 1.14 g/cm3 at 20 °C,respectively. The dissolution was carried out above 60 °C
to accelerate the preparation, and the solution was used
after cooling down to room temperature. In this study, thesolution was used to penetrate the SPB component into thenonwovens. The solid amorphous SPB obtained by evaporating the solutions to dryness was also used for comparison in a different treatment process.
Fire retarding treatment
Both wet and dry processes were investigated for fire
retarding treatment. In the wet process, the nonwovens
were immersed in the SPB solution containing 5.4 mol/kgof boron at ordinary temperatures and pressures for 30 s to
5 min, sandwiched between dry clothes to drain off thesolution, and dried at 115 °C for 24 h. The amount ofincorporation was evaluated by weight percent gain (WPG)of the nonwovens weighed after drying. For comparison,
the conventional borax solution containing 4.8 mol/kg ofboron was prepared by dissolving 558 g of borax into
1000 g of water and used for the immersion at 95 °C,
because the solubility of borax is extremely low at low
temperature. In the dry process, the fire retarding powders
were placed evenly on upside surface of nonwovens andsucked from underside with a suction device. The powders

Fire retarding effect was evaluated by two kinds of tests: amuffle furnace test and a burner combustion test. In themuffle furnace test, the specimens were heated at 750 °C
for 20 min in air, and the effect was evaluated from theweight loss (WL). In the burner combustion test, thespecimens were placed perpendicularly and heated by abutane gas burner whose flame was directed horizontally.
The distance between the specimen and the top of theburner, the whole length of the flame, and the length of thereducing flame were adjusted to 12, 10, and 3 cm,respectively. The holing time of the specimen and thetemperature change at its back side were monitored toevaluate the effect. In the burner combustion test, thespecimens through the dry processes were heated from theupside surface, where the fire retarding powders wereplaced.
The thermal conductivities of the SPB-treated anduntreated nonwovens were measured by the heat flow
method according to ASTM-C518, ISO-8301, and JIS A
1412 (Eko Instruments HC-071H). The thermogravimetry
and differential thermal analysis (TG/DTA, ShimadzuDTG-60) were performed for the SPB-treated anduntreated nonwovens in air at a heating rate of 20 °C/min.
The surface morphology and elemental distribution in theSPB-treated nonwovens before and after heating in air
were investigated by using the scanning electron microscope (SEM) and the electron probe micro analyzer(EPMA, JEOL JXA-8200).

Results and discussion
The WL in the muffle furnace test and the holing time inthe burner combustion test along with the WPG and thetreatment procedures are summarized in Table 1. The
temperature changes at the back side are shown in Fig. 1.
The specimen treated with the SPB through the wet process
showed no hole even by heating for over 720 s, and thetemperature at its back side was increased very graduallyfrom room temperature to 120 °C for 11 min. The backside remained touchable by hand for several minutes eventhough the opposite side was heavily heated by the gas123

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Table 1 The WL in the muffle furnace test and the holing time in theburner combustion test along with the WPG for the specimens
through different treatment processes
Muffle furnace
test

Burner combustion
test

WPG(%)

WL(%)

WPG
(%)

Holing time(s)Wet SPB

94

6095[720Wet borax

204

51

188

150

Dry SPB

47

74

2050

Dry borax

3794

2070

Dry borax ? boric
acid

4591

20160

Untreated

0100

020Fig. 1 Temperature changes at the back sides of the samples from thedifferent treatment processes when their adverse sides were heated bya gas burnerburner. The specimen treated with borax through the wetprocess showed a hole in 150 s, though the WPG was188%. Figure 1 shows a sudden change of the slope of thetemperature change around 150 s, indicating the holing ofthe specimen. Taking into account the holing time, theback side temperature change and WPG, it can be said that
the SPB solution containing 5.4 mol/kg of boron at 20 °C
is advantageous over the conventional borax solution
containing 4.8 mol/kg of boron at 95 °C. The WPG of thedry process specimens were 19–20%, much smaller thanthose of the wet processes. The dry process has a disadvantage in introducing large amount of fire retarding agent
probably because of the clogging in the nonwovens. The
specimen treated with both of boric acid and borax throughthe dry process showed a holing time of 160 s, though theWPG was as small as 20%. The specimens through the dryprocesses showed rather high fire resistance despite theirsmall WPG, because the fire retarding powders were partially aggregated near the surface and the fire spreads fromthe surface. The mixture of boric acid and borax showed123high fire resistance probably because its Na/B molar ratio
was close to that of the SPB. However, the fire resistance of
the SPB powders through the dry process was not so greatcompared to the mixture of boric acid and borax, because
they were not sufficiently introduced inside the specimens
due to adsorption of moisture and agglomeration in ambient atmosphere. Our results indicate that it is essential tointroduce the amorphous SPB homogeneously inside inorder to make the nonwovens highly fire resistant, and thatthe immersion into the SPB solution was the most appropriate treatment process for this. Thus, we analyzed andcharacterized the nonwovens through the wet process.
The thermal conductivities of the SPB-treated anduntreated nonwovens were 0.049 and 0.041 W m-1 K-1,respectively. The thermal conductivity was slightly
increased because the SPB phase partly contributes thethermal conduction. The thermal conductivities of theconventional plastic foam insulator are in the range of
0.020–0.052 W m-1 K-1. The low rate of temperature
increase in Fig. 1 is due to the low thermal conductivity as
well as the high fire resistance. The low thermal conductivity is a favorable factor for fire-resistant materials
because it takes long time for the whole part of the material
to reach the degradation temperature.
The TG and DTA curves for the SPB-treated anduntreated nonwovens are shown in Fig. 2. The untreated
nonwoven showed the weight loss of 100% around 600 °C,
whereas the SPB-treated nonwoven showed the weight lossof 45% around 1000 °C. The weight loss of the SPB-treated
nonwoven started around 250 °C, which was lower than theuntreated one, and it was not completed at 600 °C, at which
the untreated nonwoven vanished. The degradation mechanism was drastically changed by the SPB treatment. The
endothermic peak of the untreated nonwoven around
390 °C indicates a gasification due to the degradation of thepolymer, and various kinds of flammable gas are formed

Fig. 2 TG and DTA curves of the SPB-treated nonwoven through thewet process (black line) in comparison with the untreated nonwoven(gray line)

J Mater Sci (2010) 45:2504–2509

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Fig. 3 SEM images andelement distribution maps
(B, Na, C, and O) for theSPB-treated nonwoven ofEVOH/PET (a), the char formedby heating the SPB-treated
nonwoven of EVOH/PET in air(b), and the SPB-treated
nonwoven of only PET (c)

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above this temperature. The endothermic peak simultaneously indicates that 390 °C was lower than their ignitiontemperatures. Two large exothermic peaks appeared around
450 and 550 °C, indicating that the flammable gases ignited
above their ignition temperatures, and the degradation of
the polymer was further enhanced as a chain reaction. Thisis one of the ordinary thermal degradation processes oforganic polymer materials. In contrast, the SPB-treated
nonwoven showed an exothermic peak around 350 °C due
to the carbonization involving the char formation. The small
exothermic peak around 600 °C is probably due to thecombustion of a portion of carbon in the char. In the SPBtreated nonwoven, the carbonization occurs instead of thegasification because the SPB foam insulates inside from theoxygen and the heat, and the formed char layer also insulate
inside from the oxygen and the heat. As a result, the totalheat release up to 1000 °C in air was drastically reduced bythe SPB treatment. The endothermic peaks at 170, 250, and810 °C depict the melting points of EVOH, PET, and SPB,respectively.
Figure 3 shows the SEM images and the elemental
distribution maps of the SPB-treated nonwoven ofEVOH/PET used in this study (a), the formed char (b),and the SPB-treated nonwoven of PET (c) for comparison. Figure 4 shows the magnified SEM image of theformed char. The elemental distribution maps shows that
B and Na were homogeneously distributed on the fiber
surfaces of the SPB-treated nonwoven of EVOH/PET.
Such homogeneous distribution of B and Na were notobserved for the SPB-treated nonwoven of PET. The SPB
was homogeneously adhered to the fiber surfaces ofEVOH, whereas the SPB was segregated and not efficiently adhered on the fiber surfaces of PET. This good
adherence of SPB to EVOH is related to the pronounced
fire retarding effect on EVOH. In Fig. 4, minute foam

Fig. 4 Magnified SEM image of the char formed by heating the SPBtreated nonwoven of EVOH/PET in air123

J Mater Sci (2010) 45:2504–2509

with the pore sizes of 100 nm scale is observed on thesurface of the underlying char. This indicates that thechar formation was promoted by the insulation effect of
the foam. It is interesting to note that boron and carbon
are observed at the same spot in the element distribution
map of the char, though the concentrations are not uniformly distributed. This may indicate that boron (borate)and carbon are mixed at nanometer scale in the char or
the minute foam and this composition greatly contributes
the fire resistance. Thermal stability of the char isenhanced by the coexistence of an inorganic component
in addition to carbon. Our results give the idea to prepare
new fire-resistant composites from the char obtained byheating the mixture of polymer and SPB. This may leadto new preparation techniques of high-value-added
materials such as carbon fibers, boron fibers, siliconcarbide fibers.

Conclusions
Fire-resistant nonwovens consisting of core (PET)/clad(EVOH) fibers were successfully prepared by immersing
the nonwovens in the SPB solutions. The nonwoven with
10-mm thickness and 94% WPG endured the flame of abutane gas burner for more than 12 min, and its backsidetemperature was 127 °C at 12 min. Similar fire-resistantnonwovens were also prepared through a dry process bysucking from underside of the nonwoven on which borax
and boric acid were placed, though the fire-resistantproperties were disadvantageous over the wet process. Itwas demonstrated in the TG/DTA curves that the SPBtreated nonwoven shows the carbonization instead of thegasification in the range of 300–400 °C because of theSPB foam. The SPB foam and the formed char layer
together insulate inside from the oxygen and the heat.
This mechanism is also supported by the element distribution maps showing the composition of boron(borate) and carbon mixed at nanometer scale. The good
adherence of the SPB to EVOH surface also contributes
the pronounced fire retarding effect. The SPB-treated
nonwovens are expected to be alternative materials forheat insulators such as asbestos, glass fibers, rock wools,
because they are light and firm as well as fire resistant.
Furthermore, the precise investigation of the char formed
from the mixture of polymer and the SPB may lead tothe preparation of new high-value-added materials suchas carbon fibers, boron fibers, silicon carbide fibers.
Acknowledgements The authors are grateful to Messers Yasuro
Araida, and Masato Iizumi of Kuraray Kuraflex Co., Ltd. for valuable
discussions and helpful suggestions.

J Mater Sci (2010) 45:2504–2509

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