Polym. Bull. (2018) 75:4967–4976
https://doi.org/10.1007/s00289-018-2311-4
ORIGINAL PAPER

High flame retardancy of amorphous sodium silicate
on poly(ethylene-co-vinyl acetate) (EVA)
Isao Tsuyumoto1

Received: 11 July 2017 / Revised: 23 January 2018 / Accepted: 12 March 2018 /Published online: 15 March 2018
Ó Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract Amorphous sodium silicate (ASL) is found to show high flame retardancy
on poly(ethylene-co-vinyl acetate) (EVA), in contrast to crystalline sodium disilicate, Na2Si2O5, with similar composition. The EVA polymer (ethylene content:
67 wt%) loaded with 33 wt% or more of ASL shows self-extinguishing properties
and meets the UL94V-0 rating, while the EVA polymer loaded with 33 wt% of thecrystalline Na2Si2O5 does not even meet the UL94 HB rating. The flame retardant
effect of ASL is much higher than the conventional halogen-free additives such as
magnesium hydroxide. The TG measurements of the flame retardant EVA samples
and the scanning electron microscope observations of the combustion residues
suggest that the remarkable flame retardancy is due to the film-forming properties of
ASL.
Keywords Flame retardancy  EVA  Silicate

Introduction
Flame retardant treatments are essential for polymer materials to prevent firedisasters, because their heat release after ignition increases rapidly. A wide variety
of flame retardants have been developed and used in the polymer materials ofhousehold appliances, buildings, automobiles, trains, and cables. Halogen- or
phosphorus-based flame retardants, however, cause environmental concerns,
because they are toxic in most cases and toxic products may be released in fire,& Isao Tsuyumoto
tsuyu@neptune.kanazawa-it.ac.jp
1

Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute ofTechnology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan

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disposal, and ordinary use [1]. Magnesium hydroxide and aluminium hydroxide
have been widely used as halogen- and phosphorus-free additives in polyolefin
polymers, e.g., poly(ethylene-co-vinyl acetate) (EVA) for manufacturing cables, butsuch inorganic hydroxides need to be added in large quantities by 50 wt% or moreto achieve the practical level of flame retardancy. Boehmite, AlO(OH) andhydrotalcite, Mg4Al2(CO3)(OH)123H2O have been reported as alternative halogenand phosphorus-free flame retardant additives for EVA [2], and zinc borates havebeen reported to be synergistic agents in flame-retardant aluminium hydroxide- ormagnesium hydroxide-filled EVA [3]. However, such additives are needed in large
quantities as ever to achieve the practical level of flame retardancy. New halogenand phosphorus-free flame retardants are still highly desirable, while other new
halogen- and phosphorus-free flame retardants have been also reported so far forvarious polymer materials [4], e.g., expandable graphite [5], organically modified
montmorillonite [6], triazinic polyol [7].
Our research group has reported the remarkable flame-retardant effects ofamorphous sodium borate on polymer materials such as nonwovens of ethylene–
vinyl alcohol copolymer (EVOH) and poly(ethylene terephthalate) (PET) [8], rigid
polyurethane foam (RPU) [9, 10], laminated wood [11]. Amorphous sodium borate
was introduced inside the materials by impregnation of a concentrated sodium
polyborate solution. The dry residue of the solution was an amorphous phase of
sodium borate, and the amorphous phase was found to show higher flame-retardant
effects than crystalline phases such as sodium tetraborate decahydrate (borax), boricacid. It is generally known that borate shows similar behavior as silicate, and silicate
is far more abundant than borate on the earth. The production cost andenvironmental load of the flame retardants will be reduced by developing silicatebased flame retardants of the similar performance.
Sodium silicate solution (water glass) has been widely used as a flame retardant
and an adhesive of cellulose-based materials such as paper, cardboard, plywood, andcotton fabrics [12–15]. Silica and silicone resin have also been used for flameretardant additives of polymer materials [16, 17]. On the other hand, sodium silicate
powder has not been applied to the flame-retardant additives of the polymer
materials, and there have been no reports on the flame retardant effects of sodium
silicate powder when melt-mixed with the polymer materials. Considering the factthat some kinds of polymer materials show flame retardancy by coating with sodium
silicate-based solution, sodium silicate powder is expected to give flame retardancy
to the polymer materials by melt-mixing. Sodium silicate powder is advantageousover the other additives, because it contains no hazardous elements and is madefrom abundant natural resources.
This study focused on the potential application of sodium silicate powder to theflame retardants of EVA. The aim is to investigate the relationship between the
flame retardancy and the character of sodium silicate.

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Experimental
Materials
Amorphous sodium silicate (ASL) was prepared by drying a sodium silicate solution(17.7 wt% Na2O, 36.9 wt% SiO2, 45.4 wt% H2O, Na/Si molar ratio 0.93, density
1.70 g/cm3, Fuji Kagaku, Osaka, Japan) at 200 °C for 24 h. Crystalline sodium
disilicate, Na2Si2O5, was prepared by drying the same solution at 200 °C for
14 days or more. Each sodium silicate sample after drying was crushed into fine
powder using mortar and pestle.
EVA used in the present study was a commercially available extrusion-grade(EV170, Du Pont-Mitsui Polychemicals, Tokyo, Japan) with ethylene content of
67 wt%, melting point of 62 °C, and melt flow rate (MFR) of 1 g/10 min (at 190 °C
and a load of 2.16 kg). The EVA pellet was melt-mixed with the sodium silicate
powder using a counter-rotating twin-screw mixer (Musashino Kikai, Tokyo, Japan)
at 80 °C and 60 rpm for 15 min. The formulations of the EVA–ASL mixtures werein the range of 91 wt% EVA–9 wt% ASL to 50 wt% EVA–50 wt% ASL. For
comparison, the mixtures of 67 wt% EVA–33 wt% crystalline Na2Si2O5 and 67
wt% EVA–33 wt% Mg(OH)2 were also prepared. Sample pieces in size of
125 mm 9 13 mm 9 3 mm were fabricated by hot-press molding at 80 °C and
10 MPa for 5 min.
Measurement
The crystal structures of the sodium silicates were identified by the powder X-ray
diffraction (XRD) measurements (XD-D1, Shimadzu, Kyoto, Japan) using graphite
monochromatized Cu Ka radiation (k = 0.154059 nm) at 30 kV and 30 mA. The
X-ray profiles were collected between 2° and 120° with a step interval of 0.02° and acount time of 1 s/step. The differential thermal analysis and thermogravimetry(DTA/TG, DTG-60, Shimadzu, Kyoto, Japan) was performed for the sodium
silicates and the flame retardant EVA in air at a heating rate of 20 °C/min. The
sodium silicates were measured after drying at 200 °C for specified time andcrushing into fine powder using mortar and pestle. As for the flame retardant EVA,
4–6 mg of the sample was cut from the whole specimen and used for themeasurement. Flame retardant effect was evaluated according to the American
National Standard UL 94 (ASTM D635-14, D3801-10) using rectangular samples of
EVA in size of 125 mm 3 13 mm 3 3 mm [18]. The surface morphology of thesamples after the combustion tests was investigated using a scanning electron
microscope (SEM, JSM-5610, JEOL, Tokyo, Japan) with an accelerating voltage of
10 kV.

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Fig. 1 XRD patterns of the sodium silicates obtained by drying the sodium silicate solutions at 200 °C
for a 24 h and b 14 days

Results and discussion
Figure 1 shows the XRD patterns of sodium silicates obtained by drying the sodium
silicate solution at 200 °C for 24 h and 14 days. Only one broad peak was observedin Fig. 1a, and this indicates that drying the sodium silicate solution for 24 h yielded
an amorphous sodium silicate. In Fig. 1b, the sharp peaks emerged by extending thedrying time to 14 days, and the peaks were assigned as b-Na2Si2O5 (sodium
disilicate, natrosilite) reported in the literature (PDF4 00-029-1261). The crystalline
phase of b-Na2Si2O5 was found to form by drying the sodium silicate solution for
14 days. Since the Na/Si molar ratio of the sodium silicate solution is 0.93, thecrystalline phase of b-Na2Si2O5 should form with some fraction of the other phase(Na/Si \ 0.93). Because all the sharp peaks in Fig. 1b are assigned as b-Na2Si2O5,
some fraction of amorphous phase is considered to remain with crystalline bNa2Si2O5. These two samples were fine white powders showing no difference intheir appearance and texture. The TG curve of the amorphous sodium silicate
showed a weight decrease by 5.8% from 200 to 860 °C and a melting point at
855 °C, while that of the crystalline Na2Si2O5 showed the same melting point at
855 °C and a smaller weight change, 1.9%, as shown in Fig. 2. This indicated thatthe amorphous sodium silicate contained 5.8 wt% of hydrated water and the
crystalline Na2Si2O5 contained less hydrated water. The DTA/TG curves for the two
samples did not show much difference except for the amount of water release. The
type of the product can be controlled by changing the drying time, and this is inagreement with the reported phase diagram of Na2O–SiO2–H2O [19]. Additionally,
to our knowledge, all commercially available sodium silicate powders were
crystalline phases.
Table 1 summarizes the combustion properties of the EVA samples melt-mixed
sodium silicates in the UL 94 test. A remarkable difference in flame retardancy on
EVA was observed between the amorphous sodium silicate and the crystalline one.
The sample of 67 wt% EVA–33 wt% crystalline Na2Si2O5 and the virgin EVA

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Fig. 2 DTA/TG curves for the sample powders of ASL and crystalline Na2Si2O5 in air
Table 1 Combustion properties of the EVA samples melt-mixed with the sodium silicates in the UL94
test
Additive

Loading (wt%)

UL94

Horizontal burning rate (mm/min)

Dripping

None

0

–

102

Yes

ASL

9

–

75

No

ASL

17

–

65

No

ASL

23

–

63

No

ASL

29

HB

35

No

ASL

33

V-0

–

No

ASL

38

V-0

–

No

ASL

41

V-0

–

No

ASL

44

V-0

–

No

ASL

47

V-0

–

No

ASL

50

V-0

–

No

Na2Si2O5

33

–

79

Yes

Mg(OH)2

33

–

225

Yes

showed dripping and burning rates greater than 40 mm/min in the horizontal
burning (UL 94 HB) test, which did not satisfy the UL 94 HB requirements. In
contrast, dripping was not observed for the EVA samples containing 9 wt% or moreof ASL, and notably the EVA samples containing 33% or more of ASL achieved the
UL 94V-0 standard. Self-extinguishing properties were observed for the samples
containing 33% or more of ASL in both the UL 94 HB and V-0 tests, while thesample containing 29% of ASL did not show the self-extinguishing properties andmet the UL 94 HB standard. The EVA samples containing 33% or more of ASL
burned with a small flame with length of 1–2 cm before the self-extinguishing,while the flame of that with the crystalline Na2Si2O5 was over 10 cm in length.

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Additionally, 67 wt% EVA–33 wt% Mg(OH)2 sample showed dripping and did notmeet the UL 94 HB nor V-0 standards. It can be said that ASL has much greater
flame retardancy on EVA compared with the conventional Mg(OH)2.
Figure 3 shows the appearance of the combustion residues after the UL 94 HB
tests. Black char remained with white powder on it, and the white powder covering
the black char spread with increasing the content of ASL. It is considered that thewhite powder covering the black char brought about the self-extinguishingphenomena of the samples containing ASL by 33% or more. The content ofhydrated water in ASL was as small as 5.8%, and this is much smaller than that ofconstitution water in Mg(OH)2, 30.9%, which is calculated from the theoreticalweight decrease when Mg(OH)2 decomposes into MgO and H2O. Thus, the great
flame retardancy is neither due to the cooling effect nor gas dilution effect ofhydrated water because such effects of hydrated water in ASL should be muchsmaller than that of Mg(OH)2. As for the EVA–ASL mixtures, gasification burning
is considered to be suppressed by other mechanism. Figure 4 shows the SEM
images of the carbonized regions of the sample residues. The white powder on thesurface of the black char of the EVA–ASL samples was the flakes and foam of ASL
as shown in Fig. 4a, and these protected the char from oxygen and heat. Such flakes
and foam were observed a little for the crystalline Na2Si2O5, and the foam was
isolated and did not link together, as shown in Fig. 4b. It is important to establish
the network of foamed structure by linking together throughout the overall specimen
like percolation for the flame retardancy, and this depends on the fraction of the
ASL. A little foam observed for the crystalline Na2Si2O5 is considered to be derived
from the remaining fraction of the amorphous phase. The remarkable difference inthe flame retardancy between the EVA samples containing 29 and 33% of ASL in
Table 1 and Fig. 3 presumably depend on whether or not the network of the foamed
structure is formed throughout the overall specimen. The observed self-extinguishing properties are due to this shielding effect of the network of the flakes and foam,

Fig. 3 Appearance of the combustion residues after the UL 94 HB test. The flame was applied to theright ends of the samples. Their ASL contents (wt%) are shown together

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Fig. 4 SEM images of the carbonized regions of the sample residues. Each EVA sample was melt-mixed
with 33 wt% of a ASL and b crystalline Na2Si2O5

and this is the first observation of an intumescent-like flame retardant effect of theamorphous sodium silicates.

Fig. 5 TG curves of the formulations EVA–ASL and EVA–crystalline Na2Si2O5 in comparison with thevirgin EVA. a 50 wt% ASL, b 33 wt% crystalline Na2Si2O5, c 33 wt% ASL, d 9 wt% ASL, and e virgin
EVA

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Figure 5 shows the TG curves of the EVA samples containing ASL (9, 33, and50 wt%) and crystalline Na2Si2O5 (33 wt%) along with the virgin EVA. The
thermal degradation of EVA has been reported to proceed in three stages. The firststage (250–350 °C) can be assigned to the evolution of acetic acid due to thedecomposition of the vinyl acetate groups [3, 20]. The second stage (425–500 °C)
corresponds to the degradation of the polyethylene chains [3, 20, 21]. The third
stage (550–600 °C) may be assigned to the oxidation of carbonaceous residue [3].
The thermal degradation behaviors of our samples corresponded with those reported
in the literature. The first-stage degradation of the flame-retardant samples started atlower temperature than the virgin sample and ceased at almost the same temperatureas the virgin sample. In this stage, the evolution of combustible degradation
products such as acetic acid does not enhance the combustibility because thetemperature is below their ignition points. The gas emission below the ignition
points at early stage may be favorable to the flame retardancy. The second-stage
degradation of the flame retardant samples occurred at a little higher temperature
region than the virgin sample, and the temperature region was gradually increased
with increasing flame-retardant additives. In this stage, the flame retardant additives
are considered to affect the mechanism of gasification and carbonization, i.e.,
suppress gasification burning and enhance carbonization. In the third stage, weightdecrease due to the oxidation of carbonaceous residue ceased faster compared withthe virgin sample. This suggests the formation of incombustible char from theflame-retardant samples instead of combustible carbonaceous substance. It isnoteworthy that the EVA samples containing 33 wt% ASL and 33 wt% crystalline
Na2Si2O5 showed almost the same TG curves, though their flame retardancies wereremarkably different. The slight deviation between the two curves is due to thehydrated water content of ASL. The correspondence of the two TG curves suggeststhat the remarkable flame retardancy is due to not only the changes in the thermal
degradation behaviors but also changes in the mechanical properties of EVA matrix
brought by the amorphous network of ASL such as melt viscosity increase and the
suppression of dripping.
It is considered that the remarkable flame-retardant effect as well as the dripping
suppression is due to film-forming properties of the amorphous phase. The
amorphous sodium silicate, a highly viscous glass, constructs a network in thepolymer matrix and transforms into glass film when heated. The glass film, swollen
by decomposition gas, subsequently changes into the glass foam. The stiffness ofthe glass network suppresses the dripping, and the glass film (flakes) and foamshield the polymer matrix from oxygen and heat. Carbonized layers made frompolymer matrix also protect inside from oxygen and heat. This flame retardant
mechanism is similar to those of the intumescent flame retardants, though the
expansibility is apparently lower. Generally, such effects are not expected forpulverized crystalline phases, since they only keep existing as powders in polymer
matrix. Our results give a new design guideline of flame retardants that amorphous
phases are advantageous over the crystalline phases with similar compositions.

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Conclusions
A remarkable flame retardant effect of ASL on EVA was found. Self-extinguishing
was observed for EVA samples melt-mixed with ASL by 33 wt% or more, anddripping was thoroughly suppressed for the EVA samples melt-mixed by 9 wt% or
more. The amorphous sodium silicate is considered to transform into glass film andfoam on sample surfaces when heated, and these as well as carbonized layers protect
polymer matrix inside from heat and oxygen. Dripping is also suppressed by thestiffness of glass network in the polymer matrix. Our findings are solely due to using
amorphous sodium silicate instead of crystalline ones, and this suggests that new
properties can be expected even for well-known conventional flame-retardant
additives by controlling their crystallinity and structures.
Acknowledgements The author is grateful to Messrs. Ryo Kuwata, Kohei Matsumoto, and Fumiki Gotofor their assistance in the experiments in this work. This work was supported by a Grant-in-Aid for
Scientific Research (C), KAKENHI, no. 16K05946, from the Japan Society for the Promotion of Science(JSPS).

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