FT-IR analysis of flame resistant chemical mixture

Abstract

In this study, flame retardant mixtures of decabromodiphenylethane (DBDPE) and Sb2O3 were analyzed using Fourier transform infrared (FT-IR) spectroscopy. The experimentally obtained wavenumbers of DBDPE and Sb2O3 were 1321 and 949 cm-1, respectively, whereas those obtained by theoretical calculation were 1370 and 818 cm-1, respectively. Strong correlation was observed between the mixing molar ratios and observed peak area ratios, suggesting that FT-IR analysis can be used to obtain relative amounts of the individual components of flame retardant mixture.

1. Introduction

Plastics, synthetic fibers, and resins made from petroleum are among the most commonly used materials in daily life. Since these materials are composed of carbon, hydrogen, and oxygen, they are highly flammable.¹ Therefore, flame retardants are essential substances that are included as petrochemical additives.² Flame retardants are classified into organic and inorganic types according to their composition; organic flame retardants are more commonly used than inorganic types. Among organic types, bromine-containing flame retardants such as polybrominated diphenyl ethers, hexabromocyclododecane, tetrabro-mobisphenol-A, and decabromodiphenylethane (DBDPE) are the most abundant due to their excellent heat resistance and high cost-effectiveness.³ However, regulations limiting the use of bromine-containing flame retardants continue to expand because they generate carcinogens and environmentally hazardous by-products.^4,5 Therefore, bromine-containing flame retardants are often mixed with an inorganic additive such as antimony trioxide (Sb₂O₃) to enhance flame retardancy and reduce the content of carcinogens in the combustion products.^6,7 To determine the properties of mixed flame retardants, it is important to analyze each component accurately since physicochemical characteristics can differ significantly according to composition.^8,9 Fourier Transform Infrared (FT-IR) spectroscopy has been used for the qualitative analysis of flame retardants^10,11 since it provides simple and reproducible results. In this study, flame retardant mixtures of DBDPE and Sb₂O₃ were analyzed using FT-IR spectroscopy to investigate whether this method can provide sufficiently accurate quantitative information on the components of flame-retardantmixtures.

2. Experimental

2.1. FT-IR experiments

DBDPE and Sb₂O₃ were obtained from CHEMLINK (Seoul, Korea). FT-IR spectra were obtained using an FT-IR spectrometer (Nicolet iS10, Thermo Scientific, CNU Chemistry Core Facility) using a scan number of 128, a resolution of 8, a wavenumber range of 400-4000 cm^–1, and Smart iTR Diamond ATR mode. FT-IR data acquisition was performed using the OMNIC9 software package. Mixtures of DBDPE and Sb2O₃ were prepared by changing the mixing mass ratios of the individual components from 0% to 100% in increments of 10%. Prior to FT-IR analysis, we prepared and thoroughly mixed a 1-g sample of each mixture using a vortex mixer. The areas of the FT-IR peaks were obtained using ImageJ 1.53a software (National Institutes of Health, USA).

2.2. Computational details

Geometry optimization followed by analytical vibrational frequency calculations to simulate the vibrational spectra of DBDPE and Sb2O₃ were conducted using density functional theory (DFT) employing the ωB97XD functional.¹² We used the def2-TZVP sets as basis sets.¹³ Sb atomic core electrons were represented by effective core potential (ECP). All basis sets and ECP data were obtained from the Basis Set Exchange (BSE).¹⁴ The initial molecular structure of DBDPE was obtained using the molecular drawing function implemented in the Avogadro program.¹⁵ At temperatures below 570°C, the solid phase of Sb₂O₃ is senarmonite, which is a cubic molecular crystal¹⁶ consisting of Sb₄O₆. Therefore, we removed a Sb₄O₆ structure from this crystal structure to perform geometry optimization and frequency calculations. All computations were performed using the Gaussian ¹⁶ program suite.¹⁷

3. Results and Discussion

3.1. FT-IR spectra of individual retardants

To determine whether FT-IR data for flame retardant mixtures of DBDPE and Sb2O₃ could be used to provide quantitative information on each compound, we first analyzed the individual compounds of DBDPE and Sb₂O₃. The FT-IR spectra of DBDPE and Sb2O3 are shown in Fig. 1. Unique absorption peaks were observed for each compound, at 1321 and 949 cm–1 for DBDPE and Sb₂O₃, respectively. Sb₂O₃ showed an additional broad peak around 700 cm^–1 due to Sb–O–Sb vibration;¹⁸ however, this peak was not used for subsequent quantitative analysis because its absorption had already saturated the FT-IR spectrum. To our knowledge, no previous study has reported the FT-IR spectrum of DBDPE, although FT-IR spectra have been reported for decabromodiphenylether, with a distinct peak around 1350 cm^–1. ¹⁹ Decabromodiphenylether was introduced on the market earlier than DBDPE.²⁰ The structural similarity between decabromodiphenylether and DBDPE has contributed to the closeness of the absorption peaks of decabromodiphenylether (1350 cm⁻¹) and DBDPE (1321 cm⁻¹). For Sb₂O₃, there was a report showing the absorption peak at 949 cm⁻¹.²¹

Fig. 1. Fourier transfer infrared (FT-IR) spectra of (A) decabromodiphenylethane (DBDPE) and (B) Sb₂O₃. Unique absorbance peaks were observed at 1321 and 949 cm for DBDPE and Sb₂O₃, respectively. The spectra are displayed from 400 to 2500 cm^-1 because no peaks were observed at wave numbers >2500 cm^-1.

To calculate the theoretical absorption wavenumbers for DBDPE and Sb₂O₃, we performed computational analysis using the Gaussian 16 program suite. Theoretically generated FT-IR spectra and their corresponding vibration modes for the absorption of DBDPE and Sb₂O₃ are shown in Fig. 2. The theoretically calculated wavenumber of the DBDPE peak (1370 cm^–1) was close to that obtained experimentally (1321 cm^–1). However, the theoretical wavenumber of the Sb₂O₃ peak (818 cm^–1) was significantly different from that obtained experimentally (949 cm^–1), likely because Sb₂O₃ is present as a cubic molecular crystal with a unit cell of Sb₄O₆, 16 whereas our calculations were performed using a single Sb₄O₆ molecule.

Fig. 2. Theoretical FT-IR spectra and vibration modes for (A) DBDPE and (B) Sb₂O₃. The calculated absorption peaks were 1370 and 818 cm^-1 for DBDPE and Sb₂O₃, respectively.

3.2. Comparison between mixing molar ratio and peak area ratio of flame retardant mixtures

To explore the relationship between the mixing molar ratio and peak area ratio of the flame retardant mixtures prepared using different mixing ratios of DBDPE and Sb₂O₃, the FT-IR spectra of the mixtures were obtained as shown in Fig. 3. The peak areas of DBDPE at 1321 cm^–1 and Sb₂O₃ at 949 cm^–1 are listed in Table 1. The mixing mass ratios were converted into mixing molar ratios by considering the molar mass of each compound (Table 1).

Fig. 3. FT-IR spectra of the mixtures of (A) 90% DBDPE + 10% Sb₂O₃ to (I) 10% DBDPE + 90% Sb₂O₃.

Table 1. Peak area ratios and mixing molar ratios of decabromodiphenylethane (DBDPE) and Sb₂O₃

The relationship between the peak area ratio and the mixing molar ratio is shown in Fig. 4. We observed strong linearity (R² = 0.9865), which demonstrates that the experimentally obtained peak area ratios of DBDPE and Sb₂O₃ were well represented in the FT-IR analysis, and that this method can be used to obtain the mixing molar ratios of DBDPE and Sb₂O₃.

Fig. 4. Relationship between peak area ratios and mixing molar ratios of DBDPE and Sb₂O₃.

4. Conclusions

Individual FT-IR spectra of DBDPE and Sb2O3 were experimentally obtained and compared with theoretically generated spectra. FT-IR analysis of flame retardant mixtures composed of DBDPE and Sb2O3 showed high linearity between the mixing molar ratio and peak area ratio of DBDPE and Sb2O3, demonstrating that the FT-IR data of the flame retardant mixture can be used to provide quantitative information on its components. The current result will be beneficial to those who would like to quantitatively analyze the flame retardant mixture in a simple and reproducible way using a FT-IR spectrometer.