J Mater Sci (2011) 46:5371–5377
DOI 10.1007/s10853-011-5475-y

Highly flame retardant coating consisting of starch
and amorphous sodium polyborate
Isao Tsuyumoto • Yuichiro Miura • Masato Nirei •Shota Ikurumi • Takenori Kumagai

Received: 17 February 2011 / Accepted: 14 March 2011 / Published online: 25 March 2011
Ó Springer Science+Business Media, LLC 2011

Abstract Mixture of starch and amorphous sodium
polyborate (SPB) is found to show high flame retardancy,
when its aqueous solution is deposited and dried on organic
polymer materials such as polyethylene terephthalate(PET) nonwoven, rigid polyurethane (RPU) foam, andpolypropylene (PP) nonwoven. The PET nonwoven(10 mm thickness) and the RPU foam (10 mm thickness)coated with the mixture endure the premixed flame ofbutane gas burner with length of 100 mm for more than
12 min. The PP nonwoven (0.7 mm thickness) endures thenonpremixed flame with length of 65 mm in the 45 degrees
Meckel burner test for more than 2 min. The backsidetemperatures in the both tests remain below 130 °C. The
thermal analyses and the SEM observation indicate themechanism that the SPB foam promotes the carbonizationof starch and that the carbonized layer together with the
SPB foam insulate inside from oxygen and heat.

Introduction
Flame-retardant treatment of organic polymer materials isessential from the viewpoint of disaster prevention, because
their heat release occurs rapidly after ignition. A wide
variety of flame retardants have been developed and widelyused in the organic polymer materials for buildings, household appliances, and automobiles. There are, however,environmental concerns in the use of halogen or phosphorus
based flame retardants, because the flame retardants

I. Tsuyumoto (&)  Y. Miura  M. Nirei  S. Ikurumi 
T. Kumagai
Department of Applied Chemistry, College of Bioscience andChemistry, Kanazawa Institute of Technology, Ishikawa, Japane-mail: tsuyu@neptune.kanazawa-it.ac.jp

themselves are toxic in most cases and toxic products may bereleased in fire, disposal, and normal use [1]. Therefore,
halogen-free and phosphorus-free flame retardants with low
environmental pollution risk are particularly needed. New
flame retardants such as expandable graphites [2, 3], silicones [4] have been developed and applied to organic
polymer materials. Intumescent polymers and their flame
retardance have been also intensively investigated [5].
Sodium borate has high flame retardant effect on cellulose-based materials such as wood, paper, and cotton [6]. Itssolubility to water has been found to rise up to 5.24 mol/kg(as to boron) at 20 °C by adjusting Na/B ratio to 0.22, whilethat of the conventional borax (sodium tetraborate octahydrate, Na2B4O5(OH)4 8H2O) is one order of magnitude
lower at room temperature [7–9]. The dry residue of thesolution is amorphous sodium polyborate (SPB), and its
film-forming property as well as the high concentration
contributes the flame retardant effect to base materials. We
have found that SPB has high flame retardant effect also on
EVOH (ethylene–vinyl alcohol copolymer) and polyurethane, and we have successfully prepared a flame retardant
polyethylene terephthalate (PET) nonwoven with EVOH
adhesive and a flame retardant rigid polyurethane (RPU)
foam by impregnation of the SPB solution [10, 11].
In flame retardant treatment, coating process would be
advantageous over the impregnation process in that theequipment is simple and post-treatment is possible on
ready-made building materials and commodities. The pure
SPB solution is not suitable for the coating process, because
the SPB powder precipitates nonuniformly on the basematerial and its adhesiveness to the base material is low. A
binder or primer is essential to provide flame retardance bycoating with the SPB. As mentioned above, sodium borate
has high flame retardant effect on cellulose-based materials,
and we considered that starch is appropriate for the binder

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of the SPB. The SPB is supposed to have high flame
retardant effect on starch, because the carbonized layer
formation due to the dehydration of cellulose contributes
the flame retardancy and this should apply to the othergeneral carbohydrates such as starch. The synergy effect offlame retardancy due to the carbonized layer formation is
expected together with the binding effect of starch.
In the present study, we found a significant flame
retardant effect of the mixture of starch and the SPB, andsuccessfully developed highly flame retardant RPU foam,
PET/EVOH nonwoven, and polypropylene (PP) nonwoven
by simply coating with the mixture. We present their flame
retardant performance and discuss the mechanism of thesynergy effect.

J Mater Sci (2011) 46:5371–5377

was deposited on the base materials and dried at 115 °C toevaluate the flame retardant performance. The mixture
placed on the base material was manually spread using aspatula to form a homogeneous layer. The adhesive amountof the mixture was mostly determined by its viscosity
because the mixture was spread as widely as possible.
Transparent smooth films were formed on the base materials, and the appearance was the same as that of starchglue. The adhesive amount of the flame retardant wasevaluated by weight percent gain (WPG) weighed after the
drying. As for the mixture composition, the concentrationof starch was changed in the range 1–5 wt% by fixing theconcentration of SPB at 22.9 wt%, because the saturated
concentration of SPB at room temperature is 22.9 wt%.
More dilute solutions were also used to investigate theeffect of WPG.

Experimental
Measurement
Base materials
Three types of base materials were tested: PET/EVOH
nonwoven, RPU foam, and PP nonwoven. The PET/EVOH
nonwovens consisted of core/clad fibers of PET/EVOH.
The core of the fiber was made of PET with diameter of
10 lm, and it was concentrically surrounded by the clad of
EVOH. The fibers were fusion bonded at a cross pointof EVOH, and the total diameter of the fibers was 20 lm.
The thickness of the specimen was 10 mm, and the density
was 0.15 g/cm3. The RPU foams were made from polymeric methylene diphenyl diisocyanate (PMDI) and polyether polyol. The thickness was 10 mm, and the density
was 0.056 g/cm3. The PP nonwoven (0.7 mm thickness,
83.5 g/m2) was made of the PP fiber by the melt blown
method. All the samples were 20 cm 9 20 cm in size.
Flame retardant
The SPB solution 22.9 wt%, containing 5.4 mol/kg ofboron, was prepared by dissolving 20 g of boric acid
(H3BO3) and 25 g of sodium tetraborate decahydrate(borax, Na2B4O5(OH)48H2O) into 100 g of water,according to our previous reports [7]. The Na/B molar ratio
and the density 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 wasused after cooling down to room temperature. More concentrated solutions than 22.9 wt% can be prepared at elevated temperatures by dissolving more boric acid andborax, e.g., 34.9 wt% at 40 °C, 42.4 wt% at 50 °C, and57.7 wt% at 80 °C.
A given amount of starch powder (potato starch forfood) was mixed with the SPB solution around over 90 °C
to dissolve and to increase viscosity. The mixed solution

123

Flame retardant effect was evaluated by two types of burner
combustion tests: a perpendicular burner test and the 45
degrees Meckel burner method. In the perpendicular burner
test, the samples were placed perpendicularly and heated bya premixed butane gas burner whose flame was directedhorizontally. The distance between the sample and the top of
the burner, the whole length of the flame, and the length of
the reducing flame were adjusted to 120, 100, and 30 mm,
respectively. In the 45 degrees Meckel burner method, thesamples were arranged at 45° angles to the horizontal planeand heated by the Meckel burner whose nonpremixed flame
was 65 mm in length and directed perpendicularly. In bothtests, the holing time of the sample and the temperaturechange at its backside were monitored. The PET nonwoven
and the RPU foam were tested by the perpendicular burner
test, whereas the PP nonwoven was tested by the 45 degrees
Meckel burner method. The heating capability of the premixed flame with length of 100 mm in the perpendicularburner test was greater than that of the nonpremixed flamewith length of 65 mm in the 45 degrees Meckel burner
method, and thus the former test was appropriate for flame
retardant materials with large thickness.
The thermal properties in air were investigated by thedifferential thermal analysis and thermogravimetry (DTATG, Shimadzu DTG-60) at a heating rate of 20 °C/min.
The surface morphology was investigated by using thescanning electron microscopes (SEM, JEOL JSM-5610).

Results and discussion
The concentrations of starch and SPB in the deposited
mixture, starch to SPB weight ratio, the WPG, holing timeand backside temperature for the PET/EVOH nonwoven in

J Mater Sci (2011) 46:5371–5377

5373

the burner combustion test are shown in Table 1. Compared among the mixture compositions with 22.9 wt% of
SPB, the flame retardant effect was significantly increased
by mixing starch with SPB. The surfaces of the specimens
were only carbonized, and did neither ignite nor burn. Thisis the first observation of a significant synergy effect fromthe combined use of starch and SPB. In particular, the PET/EVOH nonwoven treated with the mixed solution with
3.0 wt% of starch and 22.9 wt% of SPB endured the premixed flame of butane gas burner with length of 100 mm
for more than 12 min. The mixture composition with
3.0 wt% of starch and 22.9 wt% of SPB was found to show
higher flame retardant effect than those with 1.0 or
5.0 wt% of starch. The flame retardancy was closely
dependent on the WPG, and thus changing the WPG is themost effective measure to control the flame retardant performance. The level of flame retardancy is controllable byadjusting the concentration, the viscosity and the WPG.
The flame retardant performance of the RPU foamcoated with SPB and starch mixture are shown in Table 2.
The flame retardant effect coated with SPB alone wassmall, and such specimens showed holes within 48 s. In
contrast, the specimens coated with the mixture of starchand SPB endured the 100 mm flame of premixed gas
burner for more than 12 min. The surface of the specimens
was only carbonized, and did neither ignite nor burn,similarly to the treated PET/EVOH nonwoven. The flame

retardant effect by coating with the mixture of SPB andstarch stood comparison with that by the vacuum or pressure impregnation of SPB solution, which we have reported
previously on the RPU foam [10]. The coating process wasadvantageous over the impregnation process in that theequipment was simple and the treatment and drying processes were speedy. The backside temperatures were generally lower than those of the treated PET/EVOH
nonwoven, because the thermal conductivity of the RPU
foam, 0.022 W m-1 K-1, is lower than that of the PET/EVOH nonwoven, 0.041 W m-1 K-1.
The holing time and backside temperatures of PP nonwovens in the 45 degrees Meckel burner test is shown in
Table 3, and their appearance after the test are shown in
Fig. 1. The untreated PP nonwoven was easily ignited andthe whole area was consumed. The PP nonwoven coatedwith SPB alone was ignited in 10 s, and some part of thesample, 128 cm2 in 400 cm2, was slowly consumed by
combustion in 67 s. The PP nonwoven coated with the
mixture of starch and SPB endured the flame of Meckelburner for more than 2 min. The surface was only carbonized and did not show ignition nor melting. It was firstfound that the SPB and starch mixture was also highly
effective for PP. This may suggest that the flame retardant
effect of the mixture is irrelevant to the kind of basematerials, and thus there are potential applications to allkinds of combustible materials.

Table 1 Holing time and backside temperature of the PET/EVOH nonwoven coated with the SPB and starch mixture in the burner combustion
test

Premixed burnerwith 100 mm flame

Meckel burnerwith 65 mm flame

StarchSPB

Starch/WPG

Holing time

Backside temperature (°C)

(wt%)

(wt%)

SPB ratio

(%)

(s)

30 s

1 min

3 min

6 min

9 min

12 min

0

0

0

0

10

–

–

–

–

–

–

0

22.9

0

18.0

30

42.6

–

–

–

–

–

0

22.9

0

37.5

50

24.1

41.2

–

–

–

–

0
1.0

22.9
22.9

0
0.044

79.2
42.5

90
700

24.1
23.5

27.7
28.6

–
98.8

–
133.9

–
153.0

–
–

3.0

22.9

0.134

48.4

No hole

23.8

30.4

52.5

95.0

115.1

129.6

5.0

22.9

0.226

52.3

110

24.0

27.7

–

–

–

–

0.4

21.1

0.019

34.3

410

23.9

26.0

46.3

95.5

–

–

0.8

19.5

0.041

33.2

170

25.0

31.1

–

–

–

–

1.1

17.7

0.062

18.8

150

24.9

30.7

–

–

–

–

1.1

17.7

0.062

21.8

210

25.0

27.5

63.4

–

–

–

2.3

12.2

0.189

22.2

150

24.9

41.3

–

–

–

3.2

8.3

0.386

8.6

20

–

–

–

–

–

–

4.5

2.3

1.96

9.9

20

–

–

–

–

–

–

1.1

17.7

0.062

4.0

No hole

29.2

37.9

100.2

127.2

131.5

131.9

2.1

12.2

0.172

10.6

No hole

25.5

33.3

53.6

68.3

111.0

111.0

The concentrations of starch and SPB in the mixed solution are shown. The samples were coated manually using a spatula and thus the WPG wasdependent on the viscosity of the mixed solution

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J Mater Sci (2011) 46:5371–5377

Table 2 Holing time and backside temperature of the rigid polyurethane foam coated with the SPB and starch mixture in the burner combustion
test

Premixed burnerwith 100 mm flame

Potato starch

StarchSPB

Starch/WPG

Holing time

Backside temperature (°C)

(wt%)

(wt%)

SPB ratio

(%)

(s)

30 s

1 min

3 min

6 min

9 min

12 min

0

0

0

0

13

–

–

–

–

–

–

0

22.9

0

8.9

21

–

–

–

–

–

–

0
3.0

34.9
22.9

0
0.131

23.8
51.1

48
No hole

22.8
24.8

–
27.8

–
61.5

–
75.2

–
88.2

–
89.0

3.0

34.9

0.086

105.5

No hole

21.3

22.1

43.4

65.6

80.1

93.3

3.0

47.4

0.063

184.1

No hole

23.5

25.3

38.9

46.0

51.6

60.3

The concentrations of starch and SPB in the mixed solution are shown. The samples were coated manually using a spatula and thus the WPG wasdependent on the viscosity of the mixed solution

The DTA/TG curves of the starch powder alone and of
the dry residue of the mixed solution containing 1.0%
starch and 22.9% SPB are shown in Fig. 2. The pure starchshowed thermal decomposition and the combustion of thedecomposition product in the range 300–400 °C. The
endothermic and exothermic peaks in the DTA curve are
due to the decomposition and the combustion, respectively,
and they are superimposed in the temperature range
300–400 °C. The broad exothermic peak around 540 °C is
due to the combustion of carbonized residue. In contrast,
the dry residue of the mixed solution showed an endothermic peak around 250 °C and exothermic peaks at 596
and 662 °C. The endothermic peak is due to the dehydration of starch involving carbonized layer formation, whichis enhanced by SPB, and the exothermic peaks are due tothe combustion of the carbonized layer. The endothermic
peak at 815 °C is the melting point of SPB. SPB suppresses
the combustion involving thermal decomposition and promotes the carbonization. Such behavior is closely related to
the flame retardant mechanism of the SPB and starch
mixture.
Figure 3 shows the DTA/TG curves of the untreated
PET/EVOH nonwoven and that coated with the mixture of
starch and SPB. The untreated PET/EVOH nonwoven
showed an endothermic peak in the range 330–400 °C due
to thermal decomposition, and two broad exothermic peaksin the range 400–600 °C due to the combustion of thethermal decomposition product. As for the PET/EVOH

nonwoven coated with the mixture of starch and SPB, theexothermic peaks in the range 400–600 °C were suppressed and a small exothermic peak appeared around
615 °C. This indicates that the thermal decomposition andits combustion were suppressed and carbonization proceeded instead. The exothermic peak around 615 °C is dueto the combustion of carbonized layer. Figure 4 shows the
DTA/TG curves of the untreated RPU foam and that coatedwith the mixture of starch and SPB. The untreated RPUfoam showed four broad exothermic peaks in the range
200–600 °C due to the combustion of thermal decomposition product. As for the RPU foam coated with the
mixture of starch and SPB, such exothermic peaks in therange 200–600 °C were suppressed, and broad exothermic
peaks in the range 650–800 °C appeared instead. It isconsidered that carbonization was enhanced in the range
200–600 °C due to the suppression of the thermal
decomposition, and the combustion of carbonized layer
gradually occurred in the range 650–800 °C. Figure 5
shows the DTA/TG curves of the untreated PP nonwoven
and that coated with the mixture of starch and SPB. The
untreated PP nonwoven showed monotonous weightdecrease and heat emission in the range 220–400 °C due tothermal decomposition and the combustion of the decomposition product. By the treatment with the mixture, heatemission in the range 220–400 °C was suppressed, andexothermic peaks appeared at 600 and 670 °C. Similarly tothe case with the treated RPU foam, the carbonization was

Table 3 Holing time and backside temperature of the PP nonwoven coated with the SPB and starch mixture in the burner combustion test

Meckel burnerwith 65 mm flame

Potato starch

StarchSPB

Starch/WPG

Holing time

Backside temperature (°C)

(wt%)

(wt%)

SPB ratio

(%)

(s)

10 s

20 s

30 s

60 s

90 s

120 s

0

0

0

0

10

184.7

–

–

–

–

–

0

42.4

0

287

10

119.2

–

–

–

–

–

3.0

42.4

0.07

311

[120

49.0

55.6

58.1

62.6

77.9

84.9

The concentrations of starch and SPB in the mixed solution are shown. The samples were coated manually using a spatula and thus the WPG wasdependent on the viscosity of the mixed solution123J Mater Sci (2011) 46:5371–5377

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Fig. 1 The appearance of the PP nonwovens coated with the SPB only (a) and coated with the mixture of the SPB and starch (b), after the 45
degrees Meckel burner test

Fig. 2 The DTA/TG curves of the starch powder alone (gray line)
and of the dry residue of the mixed solution containing 1.0% starchand 22.9% SPB (black line)

enhanced in the range 220–400 °C and the combustion of
the carbonized layer occurred at 600 and 670 °C.
Figure 6 shows the cross-sectional SEM image of the
RPU foam coated with the mixed solution containing
22.9 wt% of SPB and 3.0 wt% of starch (WPG 56.7%)
after enduring the 12 min-heating by the premixed flamewith length of 100 mm. The upper half shows the SPB
foam formed by heating and the lower half shows the
carbonized cells of the RPU foam. Because the SPB has thefilm-forming property, the SPB transformed to the SPB
foam by incorporating the water vapor emitted by thethermal decomposition of starch. The surface cellular filmof the SPB foam insulated both starch and base materials
from oxygen, and the carbonization of both starch and basematerials was enhanced. The RPU foam transformed to thatwith the carbonized cells as observed in the lower half ofFig. 6. The carbonized layer originated from starch isconsidered to exist between the SPB foam and theFig. 3 The DTA/TG curves of the untreated PET/EVOH nonwoven(gray line) and that coated with the mixture of starch and SPB (blackline)

Fig. 4 The DTA/TG curves of the untreated RPU foam (gray line)
and that coated with the mixture of starch and SPB (black line)

carbonized RPU foam. Both the SPB foam and the carbonized materials insulate inside from oxygen and heat,and contribute the flame retardant effect. Figure 7a and bshows the cross-sectional SEM images of the PP nonwoven

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Fig. 5 The DTA/TG curves of the untreated PP nonwoven (gray line)
and that coated with the mixture of starch and SPB (black line)

coated with the mixed solution containing 42.4 wt% of
SPB and 3.0 wt% of starch (WPG 311%) after enduring
2 min-heating by the Meckel burner. The images weretaken from the backside of the sample. The layer appearing
on the extreme left in Fig. 7a is the SPB foam, and theaccumulated layers appearing on its right are carbonized
PP nonwoven. The carbonization of the PP nonwoven wasenhanced by the SPB foam. In Fig. 7b, microspheres of
SPB are observed on the left hand and a dense carbonizedlayer originated from starch is observed on the right hand.
Similarly to the case with the RPU foam, the SPB foamlayer and the carbonized layers originated from both starchand PP nonwoven are generated and accumulated on thesurface, and both contribute the flame retardant effect byinsulating inside from oxygen and heat.
In the present study, we first found a significant synergy
effect of the combined use of starch and SPB. The combined use of SPB and starch was inspired by the experimental fact that SPB has great flame retardant effect on
cellulose [6]. The mechanism of the synergy effect of the
combined use is deduced from our present experimental

Fig. 7 The cross-sectional SEM images of the PP nonwoven coatedwith the mixed solution of SPB and starch after enduring 2 minheating by the Meckel burner

results and the flame retardant mechanism of SPB on cellulose. Starch mixed with SPB decomposes to carbon andwater vapor by heating, because SPB insulates starch fromoxygen. The emission of water vapor enhances the formation of SPB foam and simultaneously dilutes thedecomposition product gas. The SPB foam enhances the
carbonization of base materials and starch by insulation ofoxygen. Both the SPB foam and the dense carbonized layer
insulate inside the base materials from oxygen gas and heat
conduction. The mechanism suggests that the mixture of
SPB and starch is categorized into a kind of intumescent
flame retardant. Furthermore, starch also acts as a binder of
SPB to improve adhesiveness to the base materials. The
improvement of the adhesiveness also contributes the significant flame retarding effect.

Conclusions

Fig. 6 The cross-sectional SEM image of the RPU foam coated withthe mixture of the SPB and starch after enduring the 12 min-heatingby the premixed flame123

Mixture of SPB and starch was first found to show significant flame retardant effect when deposited on organic
polymer materials. Three kinds of base materials were
investigated using the mixture: the PET/EVOH nonwoven,

J Mater Sci (2011) 46:5371–5377

the RPU foam, and the PP nonwoven. The treated PET/EVOH nonwoven and the treated RPU foam (both 10 mmthickness) endured intensive heating by premixed gas
burner for 12 min, while the treated PP nonwoven (0.7 mm
thickness) endured heating by Meckel burner for 2 min. It
is noteworthy that the present flame retardant coating was
also effective for PP, and this suggests that the mixture cangive the flame retardant effect to all kinds of combustible
materials. In the SEM images of the heated samples, SPB
foam was formed on the surface, and dense carbonizedlayer was formed beneath the SPB foam. Both the SPB
foam and the carbonized layer insulate the samples insidefrom oxygen gas and heat conduction. The mixture of SPB
and starch can be categorized into a kind of intumescent
flame retardant, because starch enhances the formation ofboth the SPB foam and the carbonized layer simultaneously by its dehydration. Only one significant problem atthis moment is that the coating of the mixture of starch and
SPB is weak in water, though starch contributes the
improvement of adhesiveness to the base materials as abinder of SPB. We are trying to improve water resistanceusing the other polysaccharides. Nevertheless, it is important to note that a high flame retardancy can be given bysimply coating with the conventional starch glue containing SPB as a post-processing technique. There are a wide
variety of potential applications for this technique such as
decorative materials in buildings, interior decoration of

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automobiles, housing of machines, household appliances,and furniture. We believe the present flame retardant isadvantageous over the conventional ones in costs, convenience, and environmental load.

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