Special Issue

Polymer Materials Technology

Development of the New Halogen-Free Flame Retardant System
by Nanoencapsulation
Jae-Kyung Kim*, Hirofumi Kawanaka*

A large amount of flame retardant is required for high flame retardance of polymer materials, but their physical properties are remarkably lowered. Meanwhile,a demand for the halogen-free flame retardant that does not allow the generation of toxic gases
during combustion has been strongly required. But, so far, the halogen-free flame retardant having a performance as high as the halogen flame retardant is not available. Here we propose thenanoencapsulation technology of the halogen-free flame retardant as one promising method toachieve a high flame retardant effect with a small amount of a conventional flame retardant. We
succeeded to nanoencapsulate the halogen-free flame retardant in a layered compound byusing a co-milling method as a nanoencapsulation technology. We found out that the nanocapsules are co-mixed, with the exfoliated state and the intercalated state in the resin. The results
show that the prepared nanocapsule has a good flame retardant effect in spite of a small
amount of the flame retardant in the nanocapsule. Also, the synergy effect for the highly flame
retardance has been investigated by combining the commercially available flame retardant with
the nanocapsule. Then, the cross-linked foam which is highly combustible was applied to confirm the synergy effect, and the material was classified as UL94 V-0.

ABSTRACTINTRODUCTION
1.1. INTRODUCTION
Recently, polymer materials are widely used in a variety of
applications, such as building materials, vehicles etc. But
there was a problem of reduction in physical properties,
because a large amount of the flame retardant is required
to obtain flame retardance of the polymer materials.
Meanwhile, a demand for the halogen-free flame retardant that does not allow the generation of toxic gases
during combustion has been strongly required. But, sofar, the halogen-free flame retardant having a performance as high as the halogen flame retardant is not supplied in spite of the development and the commercialization of many halogen-free flame retardants by various
manufacturers. Then, the development of a halogen-freeflame retardant, that makes possible to obtain a highflame retardant effect with a small amount of flame retardant, was desired. Therefore, we proposed the nanoencapsulation of the flame retardant as one promising method.
1.1 Development Situation of the Nanocomposite
material
As shown in Figure 1, the conventional nanocomposite
intercalation reaction occurs in a solution because theammonium salt used in the reaction can be soluble inwater and alcohols. Especially, a bulky and large molecular weight ammonium salt, such as the quaternary ammonium salt, is frequently used. The quaternary ammonium
salt can intercalate between the layered compound, andthe interlayer distance of the layer compound can be
expanded. Due to the expanded layered compound, it iseasy to exfoliate in the resin by the mixing process.
Moreover, by inserting the flame retardant between thelayers as the second stage of the intercalation process,the nanocomposite can have the flame retardant property.
By mixing the layered compound, which is obtainedwith this method, into the resin, the layered compound
can be exfoliated, and uniformly dispersed (nano-dispersion) in the resin, then the uniform nanocomposite can beproduced. Moreover, as described above, the flame retardant intercalated between the layers can be dispersedwith the exfoliated layered compound in the resin.

* Research & Development Div.
Polymer Materials R&D Laboratories

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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation

Ion exchange reaction
In solution

In organic solvent

in a resin

Intercalationin the first step

Intercalationin the second step

Dispersed

Layeredcompound・2 step intercalation
・Use of combustible material
・Drying of organic solvent

Quaternary ammonium salt
(Combustible)

Dispersion ofcompletely exfoliated layer

Figure 1 Conventional nanocomposite technology.

However, by applying this method to the flame retardant, a problem is created. Most of the final purpose of
the conventional nanocomposite material is the complete
layer exfoliation in the resin. In other words, full exfoliationof the layered compound in the mixing process results inthe exposure of flame retardant, and its degradation iseasily caused by the shear stress, the processing temperature and the contact with other agents. Therefore, theflame retardant effect can not be exerted.
1.2 Advantages of the Technology
As shown in Figure 2, it is possible to directly insert theflame retardant in the layered compound without using
the ammonium salt for expanding the layered compound.
And the inserted flame retardant, instead of the quaternary ammonium salt which has no flame retardance,
expands into the interlayer distance and the nanocapsuleis easily exfoliated in the resin by mixing as well as theflame retardant being nano-dispersed into the resin withthe exfoliated layer. Therefore the amount of the flame
retardant could be dramatically decreased by the nanoencapsulation. In addition, many flame retardants werehardly soluble to solvents, and the insertion was difficult
by the conventional solution methods, but this technology
is effective to the intercalation reaction regardless of theInterlayer
distance ＋
Layered compound

hardly soluble compounds. Also, this technology can beapplied not only to the flame retardant but also to themany other additives which are hardly soluble into solutions. Furthermore, this technology has the following
advantages:
・As the solution method which is needed to a large
amount of solvent, the drying process is not necessary, and a great deal of energy is not consumed.
・There is no aggregation problem in drying and in
granulation.
・The low cost manufacturing is possible as the process is simple.
In addition, the nanocapsule prepared by the co-milling
method can be dispersed with both the exfoliated andintercalated states. So, the intercalated additives can beprotected from the shear stress, the thermal history andthe influence of other additives in mixing with the resin.
Thus, the flame retardant effect can be improved.
Accordingly, this technology is called nanoencapsulation
technology, distinguishing it from the conventional nanocomposite method, because the nanocapsule is dispersed with both the exfoliated and intercalated states in
the resin.

Insertion reaction

Flame retardantInterlayer
distance
Layered compound(nanoencapusulation)

Mechanical reaction

Dispersed in the resin

Rotation

Ball milling process
Layeredcompound

Resin

Ball
Intercalation reaction by co-milling

Co-existing system of exfoliated and intercalated flame retardantFigure 2 Nanoencapsulation of flame retardant prepared by the co-milling process.

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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation

2.1 Preparation of the Nanocapsule
The intercalation reaction to obtain the nanocapsule wascarried out in a ball-milling equipment in the desired ratioof the layered compound (swelling mica) and the halogen
free flame retardant (polyphosphate ester) under therequired reaction conditions.
2.2 Dispersion of Nanocapsules in the Resin
The prepared nanocapsule were dry-blended with anengineered plastic resin at each concentration of (5, 20,
30%) and dispersed by a twin-screw extruder under therequired condition which has been preliminarily adjusted.
Thus a resin in which the nanocapsule was dispersed
was prepared. The sample, which had the same composition of the nanocapsule, but was not nanoencapsulated,
was prepared to confirm the validity of the nanocapsuleon the flame retardance.
2.3 Evaluation of the Nanocapsules
Qualitative analysis to investigate the intercalation of theflame retardant in the nanocapsule was measured by the
X-ray diffraction. Also, the thermal properties and thequantitative analysis of the flame retardant in the nanocapsule were carried out by Thermogravimetry-Differential
Thermal Analysis (TG-DTA) method. Furthermore, theflame retardance of the nanocapsule was confirmed bymeasuring the self-extinguishing time in vertical combustion and Oxygen Index test.

QUALITATIVEANALYSISANALYSIS AND
AND QUANTI3.3. QUALITATIVEQUANTITATIVEANALYSISTATIVEANALYSIS
OF THE NANOCAPOF THE NANOCAPSULES
SULES
After the intercalation reaction by the co-milling method,
the phenomenon, in which the powder was firmly adhering on the inner wall surface of the ball-mill as shown in
Figure 3, was observed. This is the phenomenon where
the powder is activated by the intercalation reaction andis easily adhering on the metal surface. It is consideredthat the reaction is properly progressing.

Figure 3 In the ball-mill after co-milling.

The X-ray diffraction analysis results of the layered compound and the obtained nanocapsule are shown in
Figure 4.

70000

Intensity［－］

EXPERIMENTS
2 2 EXPERIMENTS2θ= 9.22°

60000

Interlayer distanceis expanded

50000
40000
30000

2θ= 7.10°

20000
10000
0

4

5

6

7

8

9

10

11

12

2θ［degree］
Figure 4 X-ray diffraction analyses.
(blue line: layered compound, red line: nanocapsule)

The X-ray diffraction intensity curve for the layered compound shown in Figure 4 (blue line) has a peak at around2θ=9°. According to the reference1), this peak is derived
from the non-insertion portion of the layered compound.
The lattice plane spacing d, is approximately 0.92 mm by
the Brrag’s equation below.

nλ= 2 d sinθd : Lattice plane spacingθ : Angle between the lattice plane and X-ray
λ : X-ray wave lengthn : Integer
Diffracted X-ray

Incident X-ray

Wave length λIncident angle θθLattice spacing dd sin θ2θFigure 5 Bragg’s refraction.

On the other hand, a newly peak appeared at 2θ=7°
for the nanocapsule as shown in Figure 4 (red line). The
lattice plane spacing by the peak of 2θ=7° is calculated
as 1.26 nm, and we confirmed that the layer spacing is
expanded by the intercalation reaction. In other words, it
can be deduced that the polyphosphate ester is intercalated between the layers of the layered compound. And,the amount of the halogen-free flame retardant in the
nanocapsule was measured by using the Thermogravimetry
analysis (TG analysis). TG analysis results for the layered
compound, the halogen free retardant and the obtained
nanocapsule are shown in Figure 6. In the case of the layered compound in Figure 6 (①), no weight reduction isobserved in the range from room temperature to 600℃.
And, in the case of the polyphosphate ester in Figure 6 (②),
the weight is sharply decreased at 300℃ and the decomposition occurs. In addition, the Thermogravimetry curve
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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation

M (meq/100 g) = 105 × { A (mg)−B (mg) } / C(TG curve) of the nanocapsule is shown in Figure 6 (③).
As shown in Figure 6, the decomposition temperature of
the nanocapsule is 300℃ to 600℃ or higher, and shiftedto the higher temperature more than 150℃ compared tothe decomposition temperature 300 to 420℃ of the polyphosphate ester. In addition, the weight is gradually
reduced. It means that the heat resistance was enhanced
and the sustainable release was possible by trapping thepolyphosphate ester in the nanocapsule. From the above
results, we concluded that the nanocapsule was successfully obtained. From the TG-DTA curve, the amount of theflame retardant (meq/100 g) in the layered compound per
100 g of the layered compound is calculated as approximately 8 meq/100 g from the following equation.

160

M : Intercalation amount (meq/100 g)
A : TG at decomposition starting point (mg)
B : TG at decomposition finishing point (mg)
C : Molecular weight of the flame retardant
In the above equation, “meq” means milliequivalent. 1
equivalent weight is a unit indicated as a number ofcharges, 1 gram equivalent is equivalent to a quotient (g)obtained by dividing the molecular weight by the valenceof the molecule.

140

①

120

Decomposition temperature 300℃－ 420℃

100

100
80

No weight reduction

60

80
60

40

40

20

20

0

②

120

Weight［%］140

Weight［%］B (mg)

0

100

200

300

400

500

0

600

0

100

200

Temperature［℃］
Weight
［%］

500

600

700

③ Decomposition temperature rises to 300－600℃ or more
Fouling

90

80
0

400

in
ent ceem
rov stan
Imp t resi
hea

100

300

Temperature［℃］

Intercalated material
100

200

300

400

500

600

Temperature［℃］

Figure 6 Thermogravimetry curves (TG curves)( ① Layered compound, ② Polyphosphate ester, ③ Nanocapsule).

The degree of the exfoliation dispersion in the mixing process of the nanocapsule material and of the non-nanocapsule material, described in 2.2, was evaluated by the
X-Ray diffraction method. The evaluation result is shownin Figure 7. In comparison with the X-Ray diffraction intensity curve shown in Figure 4 (red line), the peak positionof the nanocapsule in the resin is located at 2θ=4.6°,
lower than that (2θ=7° ) of the nanocapsule. Also, theintensity of the peak is lower and broader. It is clarified
that the nanocapsules are partially exfoliated in the resinby shear stress of the extruder and some parts of thenanocapsules are dispersed while maintaining the intercalated state. On the other hand, in the case of the nonnanocapsule material, the peak of the 2θ＝9° does notshift to the lower angle side, and this implies that the dispersion into the resin occurs without exfoliation.

35000

Intensity［－］

4.4. DISPERSIONDISPERSIONOF
OF THETHE NANOCAPSULES
NANOCAPSULESININTHE
THERESIN
RESIN

30000
25000
20000
15000

③
②
①

10000
5000
0

0

2

4

6

8

10

12

14

2θ［degree］
Figure 7 X-ray diffraction intensity ( ① neat resin, ② nonnanocapsule material, ③ nanocapsule material).

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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation

THEFLAME
FLAMERETARDANCERETARDANCE OF
5.5. THE
OF
THENANOCAPSULE
NANOCAPSULE
THE
The flame retardance of the nanocapsule and the nonnanocapsule was evaluated with the self-extinguishingtime after 10 s of ignition vertically as well as the oxygen
index. The concentration of the nanocapsule and the nonnanocapsule in the resin is 5 wt %. Also, neat resin is evaluated with above tests as the standard data. At first, theresults of self-extinguishing time are shown in Table 1.

Table 1

The results of the self extinguishing time with neat resin,
non-nanocapsule material, and nanocapsule material.

① Neat resin

N＝1 N＝2 N＝3 N＝4 N＝5

Self-extinguishing time(s)*NG② Non-nanoencapsulatedmaterial

N＝1 N＝2 N＝3 N＝4 N＝5

Self-extinguishing time(s)

42

*NG

*NG

*NG

72

*NG

*NG

*NG

51

③ Nanoencapsulated material N ＝ 1 N ＝ 2 N ＝ 3 N ＝ 4 N ＝ 5
Self-extinguishing time(s)

22

22

22

40

36

46 sec78 sec120 sec

Figure 9 Deformation of the char layer by combustion time.

* NG：Complete combustion

As shown in Table1, the neat resin was completely
combusted after ignition vertically during 10 s. But, thenanocapsule and the non-nanocapsule material immediately generated a char (a char layer formed by the combustion) after the ignition and it was observed that the
combustion of the material was delayed. However, theself-extinguishing time for the non-nanocapsule material
was dispersed and 30 s longer than for the nanocapsulematerial. Next, the oxygen index of the neat resin, nonnanocapsule material, nanocapsule material was carriedout. The results are shown in Figure 8.

23.5

Oxygen Index［－］the nanocapsule material shows the higher oxygen index
than the non-nanocapsule material, the char layer after
the combustion and the combustion behavior wereobserved.
Figure 9 shows the deformation of the char layer withcombustion time.
These samples were taken from the oxygen index testof the materials with the same concentration of the nonnanocapsule in the resin.

23
22.5
22
21.5

Figure 9 shows that the char layer is deformed according to the progress of the combustion. It means that thechar layer of the non-nanocapsule material was deformed
and collapsed by the combustion, and the resin exposedby the collapsed char layer was re-ignited. It is speculated that it is difficult for the char layer formed by the nonnanocapsule to suppress the diffusion of the flammablegas and the oxygen which cause the combustion.
Moreover, due to the low strength of the char layer, it iseasier to collapse. And then the mixing of the flammableand the oxygen in the resin exposed by the collapsed
char layer occurs and the combustion was continuing.
Based on this speculation, the char layers of the nanocapsule material and of the non-nanocapsule material
after the combustion were compared to confirm the flame
retardant effect of the nanocapsule material. Figure 10
shows the surfaces of the char layers after the combustion, and Figure 11 shows the cross sections of the charlayers.

21
20.5
20

0

5

10

15

20

25

30

35

Concentration［%］Non-nanocapsule material

Nanocapsule material

Figure 8 The effect of the concentration of the non-nanocapsulematerial and the nanocapsule material with respect tothe oxygen index. (▲：the non-nanocapsule material,■：the nanocapsule material).

As shown in Figure 8, the oxygen index is increased
with the increase of the concentration of the nanocapsuleand the non-nanocapsule in the resin. Compared, theoxygen index of the nanocapsule material is higher thanthat of the non-nanocapsule material. To understand why

Figure 10 The surface of the char layers after combustion.

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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation

As shown in Figure 10, the surface of the nanocapsulematerial has a roughness structure more than that of the
non-nanocapsule material. As shown in the TG curve of
the nanocapsule in Figure 6, the weight reduction is slowly and the curve shifts to the high temperature side. From
this fact, it is speculated that the flame retardant, which
was well-dispersed with the exfoliation of the nanocapsule in the resin, forms the uniform char layer at the initialstage of the combustion. After that, the secondary charlayer is formed by the decomposition of the flame retardant intercalated in the nanocapsule existing on the surface and in the inside of the material. As the result, thedense char layer with the roughness structure on the surface of the material is formed.

Nanocapsule material

Non-nanocapsule material

Figure 11 The cross section of the char layer after combustion.

As shown in Figure 11, in the case of the non-nanocapsule material, a large cavity was formed in the char layer.
On the other hand, the nanocapsule material does nothave a large cavity but has the homogeneous char layer.
Although the char layer is formed by the combustion of
the non-nanocapsule material, it is difficult to form thehomogeneous char layer due to the poor dispersion of
the flame retardant in the resin. And then, the dense of
the char layer could be lowered and the combustion in
the char layer by mixing the flammable gas and the oxygen occurs from the parts in which the char layer is notformed. So, the large cavity in the char layer could beformed.
Due to the large cavity in the char layer, the char layer
collapses easily as shown in Figure 9, and then the combustion from the exposed resin continues.
On the other hand, as described in 1.2 section, thenanocapsule material could have the uniform char layer
because the flame retardant had been nano-dispersed in
the resin. Then the large cavity shown in the non-nanocapsule material was not formed.
From the above description, we conclude that the following speculation can be carried out. The non-nanocapsule material can not form the dense char layer becauseof the poor dispersion of the flame retardant in the resin,but the nanocapsule material can form the dense charlayer by the nano-dispersed flame retardant and the large
cavity in the char layer is not created. Moreover, someflame retardants nano-dispersed in the resin still maintainthe intercalated state, and the secondary char layer isalso formed by their decomposition. The results canimprove the strength of the char layer. Therefore, the diffusion of the flammable gas and the oxygen can be suppressed by the dense and strong char layer, and then the
combustion can be prevented. This is why the oxygen
index of the nanocapsule material is increased.

FLAMERETARDANT
RETARDANT MECHANISMMECHANISM OF
6.6. FLAME
OF
THENANOCAPSULE
NANOCAPSULE
THE
The flame retardant system by nanoencapsulation and its
effectiveness has been described above. By the way, due
to the higher demand for the products which correspondto high standard of the flame retardance like UL94 V-0,
the further improvement of the flame retardance could benecessary. Accordingly, a possible method to improve theflame retardance with the basic concept of the flame
retardant mechanism is investigated.
Figure 12 shows the basic concept of the flame retardance. The combustion is as follows. The melting and thethermal decomposition of the organic material are causedby the heat radiation and the thermal diffusion in thematerial by the heat source. The thermal decomposedmaterials (mainly flammable gas) diffused from the insideand outside of the materials mix with the ambient oxygen
and the combustion occurs. In short, the combustion iscaused by the following chain reaction: heating →decomposition→ diffusion → ignition.
A flame retardant mechanism at each step (the following ① to ⑥) shown in Figure 12 is effective to inhibit the
combustion chain. As shown in Table 2, in general, theflame retardant mechanism is categorized into the gas
phase reaction and the solid phase reaction.

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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation①Cooling by an endothermic dehydration reaction withheating.
②Increasing an amount of the solid residue and reducing an amount of a flammable gas.
③Inhibition of the flammable gas diffusion.
④Dilution of the oxygen and the flammable gas concentration by generating a nonflammable gas in thethermal decomposition.
⑤Blocking the diffusion of the oxygen by forming the
inflammable protective layer.
⑥Suppressing the radical chain reaction by interrupting free radicals generated in the combustion process.

Heating①
Melting②
Thermal decomposition

Generation of flammable gas③④

Oxygen⑥⑤

Combustion

Figure 12 The basic concept of the flame retardance.2)

Based on the basic concept shown in Figure 12, theflame retardance by the nanocapsule corresponds to ③
and ⑤. That is, the flame retardance of the nanocapsulemostly corresponds to the solid phase reaction because
the homogeneous formation of the char layer by well-dispersed nanocapsule improves the gas barrier, and thenthe diffusion of the oxygen and the flammable gas can besuppressed. In the case of the gas phase in the combustion process, the combustion generally occurs with mixing the flammable gas and the oxygen. So, we consider
that the flame retardance of the nanocapsule could be
improved by adding the flame retardant which plays arole in the gas phase reaction. Then, the flame retardantsystem (generation of an inflammable gas in the thermal
decomposition) shown in Figure 12-④, which dilutes theconcentration of the flammable gas and the oxygen, wasapplied. For the above purpose, the flame retardance ofthe nanocapsule is investigated by adding the nitrogencompound corresponding to the gas phase flame retardant system as shown in Table 2-(4).

THECOMPOSITIONSCOMPOSITIONS OF CROSS7.7. THE
CROSSLINKEDFOAMFOAMANDAND EVALUATIONEVALUATION OF
LINKED
OF
FLAMERETARDANCERETARDANCE
FLAME
For the issue described in the previous section, the combination of the nanocapsule and a nitrogen compound (acommercial flame retardant) was applied to the polyolefin
foam which is highly combustible. And, the flame retardance for the compound was evaluated by the oxygen
index and the UL94 vertical flame retardant test. The
results show in Table 3 and Figure 13.
Table 3

Table 2

The flame retardant mechanism of the flame retardantsystem.3)

Flameretardantmechanism

Representative example
1）A radical trapping effect by the Halogen
compounds and the Phosphorus compounds.

Gas phase

The comparison of the flame retardance between
specimens mixed with the commercially available flame
retardants.
Item

Non-nanocapsule

Nanocapsule

Oxygen index

30.6

31.8

UL94 Vertical flame
retardant test

Not applicableV- 02）A synergy effect by Halogen compounds+ Antimony trioxide (Zinc borate, ZnS etc.).
3）An endothermic dehydration reaction by the
Hydrate metal compounds.
4）The inflammable gas generation and the oxygen
dilution by Nitrogen compounds.
1) A char formation by the Phosphorus compounds.
2) A thermal insulating inorganic layer formation by
the Hydrate metal compounds + the Siliconcompounds.

Solid phase3) A thermal insulating inorganic layer formation by
the Hydrate metal compounds +the Zinc borate.
4) A foamed char formation by Intumescing system(Ammonium polyphosphate (APP) + Nitrogencompounds).
5) Nanocomposite + Silicon oxide by conventional
flame retardant system + Char formation

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Development of the New Halogen-Free Flame Retardant System by Nanoencapsulation

Non-nanocapsule

Nanocapsule

The nanoencapsulation of the halogen-free flame retardant was successfully obtained by the co-milling method.
And from the results of the X-ray diffraction, the nanocapsule was well-dispersed with both exfoliated and intercalated states in the resin. The prepared nanocapsule has agood flame retardant effect in spite of a small amount of
the flame retardant in the nanocapsule, and it is obviousthat the flame retardance can be improved by theincrease of the amount of the flame retardant in the nanocapsule. Furthermore, the flame retardant cross-linked
foam which was assigned to class UL94 V-0 could beobtained by using the nanocapsule and the commercial
flame retardant. It is because of the combination of theflame retardants induced the synergy effect on the flame
retardance.

Clamp

Combustion

Extinguishing

UL 94 not applicable

CONCLUSION
8.8. CONCLUSION

UL94 V-0 level

Figure 13 The comparison between the UL94 combustion test
results.

As shown in Table 3, the specimens prepared by thecompositions of the non-nanocapsule were not applicable in the UL94 vertical flame retardant test because thespecimens were nearly burned down to the clamp after
the ignition. On the other hand, the specimens preparedby the compositions of the nanocapsule were extinguished immediately in the first ignition, and were extinguished in the second ignition as well, thus it was
assigned to class UL94 V-0. From the results of the flame
retardant test, the nanocapsule has a good performancefor its flame retardance in comparison to the non-nanocapsule. And, as shown in Table 2, the combination of thetwo types of flame retardant system (the solid phase reaction induced by the nanocapsule and the gas phase reaction induced by the commercial flame retardant) indicatedthe synergy effect. So, the flame retardance of the foam
could be improved up to the high flame retardant level.

REFERENCES
1) Haruo Shiramizu: “Clay Mineralogy, Chapter 6 Clay Mineral
Detailed Exposition” Asakura Publishing Co., Ltd. (in Japanese)
2) Masashi Nakamura et al. “Halogen and Antimony Free Flame
Retardant for Semiconductor Sealing Material” Matsushita ElectricWorks technical report, 2001 (in Japanese)
3) Hitoshi Nishizawa “Direction of Future Research of Flame
Retardant Technology” Topics Report 2005, Technical committee offlame retardant materials (in Japanese)

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