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Thermochemical Approach for Screening of Alternative Metal Oxides as a Flame Retardant of Modacrylic Fiber

  • Takeshi Tanaka EMAIL logo , Osamu Terakado and Masahiro Hirasawa
Published/Copyright: April 5, 2016
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High Temperature Materials and Processes
From the journal High Temperature Materials and Processes Volume 36 Issue 3

Abstract

In the view of the exploring novel flame retardants for polymers, modacrylic fibers, which consist of acrylonitrile and vinylidene dichloride, containing metal oxide have been investigated by thermogravimetric–mass spectrometry (TG-MS) analysis. It was found that, among the examined oxides, germanium and antimony oxides formed the corresponding volatile chlorides through the reactions of oxides with hydrogen chloride formed during thermal decomposition of the polymer. The results have been discussed in the framework of thermochemistry. Based on the equilibrium calculation of the polymer–oxide mixture, the predominance diagrams of the M-O-Cl systems (M = Sb and Ge) show that the chlorides are the most stable phases at 573 K, at which temperature the major decomposition of the polymer starts. These results suggest that GeO2 would be a possible candidate of a flame retardant for chlorinated polymers. However, combustion experiments revealed an insufficient performance of the oxide. The inductively coupled plasma with atomic emission spectroscopy (ICP-AES) analysis showed the reactivity of GeO2 for HCl was inferior to that of Sb2O3, and X-ray fluorescence spectrometer (XRF) analysis of the solid thermal decomposition products showed that the evaporation of germanium was less intense than that of the conventional antimony system. This result is presumably due to the smaller rate of the chlorination of GeO2 than that of Sb2O3.

Keywords: modacrylic fiber; flame retardancy; predominance diagram; GeO2

Introduction

Modacrylic fiber, which consists of copolymerized acrylonitrile and vinylidene chloride, containing Sb2O3 is an inherently self-extinguishing flame-retardant fiber that imparts excellent flame retardancy to fabrics in blends with other flammable fibers, such as cotton or polyester fiber [1]. Although antimony oxide has been used for many years as best flame retardant of halogenated polymer, alternative flame-retardant additives are desired due to the environmental concerns.

Many investigators have studied the mechanism of its chemical action as a flame retardant by different approaches and reached an almost common mechanism, in which antimony oxide reacts with halogenated compounds and forms antimony halides that act as a flame inhibitor in gas phase [2–5]. However, it is still unclear whether it acts as free radical scavengers or as diluted gas against oxygen. Furthermore, it is not clarified as well whether there are no alternative candidates which acts as a flame retardant with same behavior as antimony oxide, although various studies have been conducted so far. Zinc compounds (zinc borate, zinc stannate, zinc hydroxyl stannate and its related compounds) give good flame retardancy for poly(vinyl chloride) (PVC), through the enhancement of char formation [6–8]. Molybdenum oxide and related compounds are also char-forming additives that function as smoke suppressants [9–11].

Some hydroxides, alumina trihydrate or magnesium hydroxide, are also used as flame retardants for PVC because of their cost advantage [12]. It is believed that these hydroxides provide flame-retardant effects both in the gas phase and the solid phase [13]. Nano-fillers such as hydrotalcite or some hydroxides give high flame retardancy at low addition levels in PVC [14]. In our recent study, flame retardancy of fabrics consisting of modacrylic fiber containing various suspended metal compounds and cellulosic fiber has been investigated by means of vertical flame test (ISO15025 procedure A) and limiting oxygen index (LOI). It has been found that excellent flame retardancy is achieved by fine-grained MoO3 particles. The after-flame time in vertical flame test and the LOI value are improved with decreasing particle size of MoO3. The flame retardancy of MoO3 of the particle size about 0.1 μm is comparable to that of Sb2O3 [15]. In the study, the mechanism of its flame retardancy has been discussed by X-ray fluorescence (XRF) spectrometer measurement of the fabric specimen after the vertical flame test and thermogravimetric analysis (TGA) of various types of samples. These analytical data indicate that MoO3 works as flame retardant in solid phase.

To clarify the mechanism of flame retardancy mentioned above, a lot of studies have focused on the measurement of thermal decomposition behavior or on the analysis of the residue after thermal treatment. On the contrary to this situation, the thermodynamics of the chlorination reaction between the metal oxides and halogenated compound are not well investigated so far.

In our previous studies, the reactivity of oxide with chlorinated compounds, emitted during thermal degradation of PVC, was discussed in terms of thermodynamics [16, 17]. Although it is difficult to represent the combustion condition with thermodynamic terms because the local partial pressures should change during thermal decomposition of polymers, we believe the thermodynamic approach is effective on discussing the reaction of metal oxide with chlorinated compounds, the reaction that contributes to the flame retardancy for polymer materials.

The objective of our present work is to find out modacrylic fiber containing metal oxides that vaporize as metal chloride under the thermal degradation process by analyzing evolved gases from the specimen and to estimate the mechanism by using thermodynamic consideration. Furthermore, we trace the reason why antimony oxide is superior to other metal oxides as a flame retardant.

Experimental procedures

Materials and preparation of specimens

The modacrylic fiber with metal oxides was prepared as follows. A copolymer consisting of 52 wt.% of acrylonitrile, 46.8 wt.% of vinylidene chloride and 1.2 wt.% of sodium styrenesulfonate was synthesized, and the copolymer was dissolved in acetone. Then, 10 parts by weight of metal oxide was added to 100 parts by weight of the copolymer to prepare a spinning solution. The spinning solution was extruded into an aqueous/acetone solution through a nozzle. The fibers thereby formed were washed with water and dried and thermally treated. Finally, the fiber was crimped and cut into a length of 51 mm.

In the present study, we selected the metal oxides based on the prerequisites as follows: (1) boiling point of the metal chloride is lower than the dominant decomposition temperature of modacrylic polymer (750 K), (2) the metal oxides have ever been studied as flame retardant or smoke suppressant of halogen containing materials and (3) the metal oxides can be applied in terms of commercial use. From the point of views, we have selected SnO2, MoO3 and Bi2O3 as the research subject of the present study. We also evaluated Sb2O3 as reference. Furthermore, we also evaluated GeO2 as an alternative candidate of Sb2O3. (Details are stated in the Section “Results and discussion”). The metal oxides in the present study were fine-grained to submicron particle size by a bead mill and dispersed in fiber in order to make the reaction easier between the metal oxides and hydrogen chloride.

Methods

The analysis of evolved gases (m/z = 0–300) was carried out with a TGA (Rigaku Corp., ThermoPlus) coupled with a mass spectroscope analyzer (Agilent Technologies, 5973inert) (TG-MS). A sample (typical mass of 1 mg) was placed on a platinum pan. The flow rate of the carrier gas consisted with He and O2 (80 and 20 wt.%, respectively) was adjusted to 250 ml/min. Then, the furnace was heated from 293 to 923 K at a heating rate of 10 K/min.

The analysis of the metal elements, which remained in the char residues formed by thermal degradation analysis, was carried out with an inductively coupled plasma with atomic emission spectroscopy (Shimadzu Corp., Agilent Technologies, ICPS-7510) (ICP-AES). The Sb containing samples decomposition was performed by pressurized acidolysis method, after adding sulfuric acid, nitric acid and hydrofluoric acid. The Ge containing sample decomposition was performed by alkaline resolution method, after adding sodium carbonate and potassium carbonate.

Thermochemical calculations were performed with a software package FactSage Ver.6.4 (Thermfact and GTT-Technologies). In the present study, we evaluated the equilibrium oxygen and chloride partial pressures by assuming the simplified sample composition (acrylonitrile: vinylidene chloride: metal oxide = 50:50:10 (wt.%)).

Results and discussion

TG-MS analysis

Figure 1 shows the mass spectra of evolved gases derived from additive-free modacrylic fiber at the heating range of 293–923 K. As seen in the figure, modacrylic fiber has mainly evolved HCl, CO, CO2, H2O and FeCl3･6H2O, the last of which may be caused by stainless column corrosion under the heating condition. Figures 2–5 indicate the temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing Sb2O3, SnO2, MoO3 and Bi2O3, respectively. As for the modacrylic fiber containing Sb2O3, SbCl3, HCl, Cl2 and H2O have been detected at 500–530 K and CO2 and H2O have been detected at 790–850 K, while no metal chloride gases have been detected for other metal oxides. The results support the previous reports [18–20] that Sb2O3 is unique substance which has a potential to act as gas phase flame retardant. Here, we should note that SnO2, MoO3 and Bi2O3 act as flame retardant in the solid phase because we have confirmed the flame retardancy of modacrylic fiber containing these metal oxides by vertical flame spread test and LOI test [15].

Figure 1: The mass spectra of evolved gases derived from additive-free modacrylic fiber at the heating range of 293–923 K.
Figure 1:

The mass spectra of evolved gases derived from additive-free modacrylic fiber at the heating range of 293–923 K.

Figure 2: The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing Sb2O3.
Figure 2:

The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing Sb2O3.

Figure 3: The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing SnO2.
Figure 3:

The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing SnO2.

Figure 4: The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing MoO3.
Figure 4:

The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing MoO3.

Figure 5: The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing Bi2O3.
Figure 5:

The temperature dependence of intensities for mass numbers related to the evolved gases from the sample of modacrylic fiber containing Bi2O3.

Thermodynamics analysis

Thermodynamic aspects of the chlorination reaction between the metal oxides and hydrogen chloride derived from modacrylic polymer have been investigated with the software package FactSage Ver.6.4. Figures 6–9 show the potential stability diagrams of the Sb-Cl-O, Sn-Cl-O, Mo-Cl-O and Bi-Cl-O systems at 573 K. The calculation result of partial pressures for chlorine and oxygen calculated by the method stated in Section “Methods” is displayed as circle in the figures. The values of the partial pressures of hydrogen chloride, water, oxygen and chlorine are also listed at the bottom of each figure. As seen in the figures, Sb2O3 can be chlorinated under the equilibrium condition and potentially stable as SbCl3 gas at 573 K. The result indicates that Sb2O3 can react with hydrogen chloride derived from modacrylic polymer and forms SbCl3 gas under the thermal degradation process. In contrast to this, SnO2 can be chlorinated under the equilibrium condition, and liquid SnCl2 is potentially stable at 573 K. As for MoO3, solid MoO2 is potentially stable. This result supports our previous study for modacrylic fiber containing MoO3, where we observed that MoO3 was reduced to tetravalent oxide at high temperature under inert atmosphere. As for Bi2O3, liquid Bi is potentially stable. From this consideration, it is clear that these three metal oxides are difficult to act as flame retardants in the gas phase.

Figure 6: The phase diagram of the Sb-Cl-O systems at 573 K.
Figure 6:

The phase diagram of the Sb-Cl-O systems at 573 K.

Figure 7: The phase diagram of the Sn-Cl-O systems at 573 K.
Figure 7:

The phase diagram of the Sn-Cl-O systems at 573 K.

Figure 8: The phase diagram of the Mo-Cl-O systems at 573 K.
Figure 8:

The phase diagram of the Mo-Cl-O systems at 573 K.

Figure 9: The phase diagram of the Bi-Cl-O systems at 573 K.
Figure 9:

The phase diagram of the Bi-Cl-O systems at 573 K.

The results evaluated by thermodynamic calculation coincide with TG-MS results in the present study and the knowledge from the previous researches. Therefore, we deduce the method is adequate to decide reactivity of metal oxide with halogenated compounds which contribute to the flame retardancy for polymer materials, and hence, the thermodynamic approach deserves more than a passing notice.

Based on thermodynamic approach, we investigated Al2O3, SiO2, TiO2, GeO2, ZrO2, Nb2O5 and WO3 as alternative metal oxides that can be expected to act as flame retardant in the gas phase because the boiling point of the corresponding chlorides are substantially low at temperature below 750 K. Figures 10–16 show potential stability diagrams of the Al-Cl-O, Si-Cl-O, Ti-Cl-O, Ge-Cl-O, Zr-Cl-O, Nb-Cl-O and W-Cl-O systems at 573 K. There are no substances whose stable phase are gaseous metal chloride, except GeO2 at 573 K. GeO2 can be chlorinated under the equilibrium condition and potentially stable as gaseous GeCl4 at 573 K. The result indicates that only GeO2 can react with chlorinated compounds derived from modacrylic polymer and forms GeCl4 gas under the thermodegradation process.

Figure 10: The phase diagram of the Al-Cl-O systems at 573 K.
Figure 10:

The phase diagram of the Al-Cl-O systems at 573 K.

Figure 11: The phase diagram of the Si-Cl-O systems at 573 K.
Figure 11:

The phase diagram of the Si-Cl-O systems at 573 K.

Figure 12: The phase diagram of the Ti-Cl-O systems at 573 K.
Figure 12:

The phase diagram of the Ti-Cl-O systems at 573 K.

Figure 13: The phase diagram of the Ge-Cl-O systems at 573 K.
Figure 13:

The phase diagram of the Ge-Cl-O systems at 573 K.

Figure 14: The phase diagram of the Zr-Cl-O systems at 573 K.
Figure 14:

The phase diagram of the Zr-Cl-O systems at 573 K.

Figure 15: The phase diagram of the Nb-Cl-O systems at 573 K.
Figure 15:

The phase diagram of the Nb-Cl-O systems at 573 K.

Figure 16: The phase diagram of the W-Cl-O systems at 573 K.
Figure 16:

The phase diagram of the W-Cl-O systems at 573 K.

Thermal degradation property and flame retardancy evaluation for GeO2 containing modacrylic fiber

TG-MS analysis for GeO2 containing modacrylic fiber

Figure 17 indicates temperature dependence of intensities for mass numbers related to evolved gases from the modacrylic fiber containing GeO2. As seen in the figure, GeCl4 and its fragment (GeCl3) have been detected at 493–563 K. This result is consistent with thermodynamic calculation that indicates GeO2 can react with hydrogen chloride in the modacrylic polymer to form GeCl4.

Figure 17: The temperature dependence of intensities for mass numbers related to evolved gases from the modacrylic fiber containing GeO2.
Figure 17:

The temperature dependence of intensities for mass numbers related to evolved gases from the modacrylic fiber containing GeO2.

ICP-AES analysis

To understand the vaporized amount of GeCl4 under the thermal degradation process, the amount of Ge element in the fiber before and after the thermal degradation has been evaluated by ICP-AES. We also evaluated Sb2O3 as the reference. Table 1 shows the element mass % of germanium and antimony in samples before and after thermal degradation. It indicates that 58 % of germanium still remain in the modacrylic fiber after the thermal degradation, while only 5 % of antimony remain in the fiber. The results indicate that the vaporized amount of GeCl4 which is formed by the reaction of GeO2 with HCl is less than that of Sb2O3, although GeO2 can be chlorinated under the equilibrium condition and potentially stable as GeCl4 gas under the thermal degradation condition. The activation energy for chloride reaction of GeO2 is presumably higher than that of Sb2O3 because the particle size of GeO2 in the modacrylic fiber is smaller than that of Sb2O3 and the reaction area for GeO2 must be larger than that for Sb2O3 in the experiments.

Table 1:

Ge and Sb content (wt. %) in modacrylic fiber (1) before the thermal degradation (2) after the thermal degradation evaluated by ICP-AES.

Sample Ge content (wt.  %) Sb content (wt. %)
Modacrylic fiber before the thermal degradation 6.2 7.4
Modacrylic fiber after the thermal degradation 3.6 0.4

Flame retardancy evaluation of GeO2 containing modacrylic fiber

It was found that the reactivity of GeO2 for HCl was less than half of Sb2O3 under the thermal degradation process from the results by ICP-AES, while the results indicate that the vaporized molar amount of GeCl4 is more than that of SbCl3. The results suggest that GeO2 can be a candidate as gas phase flame retardant which acts as diluted gas against oxygen. Therefore, flame retardancy of modacrylic fiber containing GeO2 was evaluated by vertical flame spreads test and by LOI test. The details of the preparation method and test method are described elsewhere [15]. Flammability measured by vertical flame spread test and LOI values for each specimen which consist of modacrylic fiber containing various metal oxides and cotton are shown in Table 2. Although after-flame time and LOI value have been improved by the addition of GeO2, flame retardancy of GeO2 is inferior to that of Sb2O3 and other metal oxides which have been investigated as flame retardant or smoke suppressant in halogenated polymers.

Table 2:

Afterflame time measured by vertical flame spread test and LOI values.

Metal oxides in modacrylic fiber Afterflame time (s) LOI (%)
No metal oxide 145 24.5
GeO2 53.5 28.2
Sb2O3 0 32.7
SnO2 40.0 –
MoO3 0 35.7
Bi2O3 0 –

XRF spectrometer analysis

Experiments using XRF analysis for GeO2 containing fabric specimen after vertical frame spread test was made to investigate whether GeO2 in the modacrylic polymer could react with chlorinated compounds emitted during thermal decomposition of polymer to form germanium chloride gas. Figure 18 shows the picture and elemental maps of fabric specimens containing GeO2 and Sb2O3 after the vertical flame spread test. In element maps, white point indicates the existence of the element, while the black point indicates its absence. It is obvious that Ge element still remains in the specimen, while Sb element is absent at the flame position. The data suggests the performance of GeO2 as flame retardant in gas phase is inferior to that of Sb2O3.

Figure 18: The photos and elemental maps of fabric specimens after flame test. (a) Photo of fabric specimen containing GeO2 after flame test (left) elemental maps of Ge (middle) and Cl (right), respectively. (b) Photo of fabric specimen containing Sb2O3 after flame test (left) elemental maps of Sb (middle) and Cl (right), respectively.
Figure 18:

The photos and elemental maps of fabric specimens after flame test. (a) Photo of fabric specimen containing GeO2 after flame test (left) elemental maps of Ge (middle) and Cl (right), respectively. (b) Photo of fabric specimen containing Sb2O3 after flame test (left) elemental maps of Sb (middle) and Cl (right), respectively.

Conclusion

Modacrylic fibers containing the metal oxides that can vaporize as metal chloride under the thermal degradation process have been investigated by TG-MS analysis, and estimated the mechanism by using thermodynamics consideration. The results of the present study are summarized as follows:

  1. Under the present experimental conditions, only GeO2 can react with chloride derived from modacrylic polymer and evolve germanium chloride gas under thermal degradation process, except Sb2O3.

  2. Thermodynamic analysis indicate GeO2 and Sb2O3 can be chlorinated under the equilibrium condition and potentially stable as GeCl4 and SbCl3 gas at 573 K, while other metal oxides evaluated in the present study are not potentially stable as metal chloride gas at 573 K.

  3. The results evaluated by thermodynamic approach can explain the results of TG-MS in the present study and the knowledge from the previous researches.

  4. The measurement by ICP-AES indicates only 42 % of germanium was converted to germanium chloride and vaporized, while 95 % of antimony was converted to the chloride and vaporized.

  5. Flame retardancy of modacrylic fiber containing GeO2 was evaluated by vertical flame spread test and by LOI test, and it has been found that flame retardancy of GeO2 is inferior to that of Sb2O3 and other metal oxides.

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Received: 2015-06-16
Accepted: 2016-02-14
Published Online: 2016-04-05
Published in Print: 2017-03-01

©2017 by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  2. Research Articles
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  14. Structural and Microstructural Study on the Arc-Plasma Synthesized (APS) FeAl2O4–MgAl2O4 Transitional Refractory Compound
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https://doi.org/10.1515/htmp-2015-0140
Keywords for this article
modacrylic fiber; flame retardancy; predominance diagram; GeO2
Creative Commons
BY 4.0

Articles in the same Issue

  1. Frontmatter
  2. Research Articles
  3. Kinetics and Tribological Characterization of Pack-Borided AISI 1025 Steel
  4. A Comparative Study of Hot Deformation Behaviors for Sand Casting and Centrifugal Casting Q235B Flange Blanks
  5. Effects of Annealing Temperature on the Microstructure and Mechanical Properties of Electrodeposited Ni-Fe Alloy Foils
  6. Thermochemical Approach for Screening of Alternative Metal Oxides as a Flame Retardant of Modacrylic Fiber
  7. Hot Corrosion Behavior of Stainless Steel with Al-Si/Al-Si-Cr Coating
  8. Calculation of the Combined Heat Transfer Coefficient of Hot-face on Cast Iron Cooling Stave Based on Thermal Test
  9. The Corrosion Behavior of Stainless Steel 316L in Novel Quaternary Eutectic Molten Salt System
  10. Corrosion of Nickel-Based Alloys in Ultra-High Temperature Heat Transfer Fluid
  11. Superplastic Behaviour of AZ61-F Magnesium Composite Materials
  12. Effects of Laser Shock Processing on Fatigue Performance of Ti-17 Titanium Alloy
  13. Effect of the Platinum Electroplated Layer Thickness on the Coatings’ Microstructure
  14. Structural and Microstructural Study on the Arc-Plasma Synthesized (APS) FeAl2O4–MgAl2O4 Transitional Refractory Compound
  15. Retraction
  16. Retraction of: Mechanical and Electrochemical Characterization of Super-Solidus Sintered Austenitic Stainless Steel (316L)
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