Study on thermal decomposition kinetics of N,N’-bis(5,5-dimethyl-2-phospha-2-thio-1,3-dioxan-2-yl)ethylenediamine in air

Abstract

The thermal decomposition kinetics of the N,N¢-bis(5,5-dimethyl-2-phospha-2-thio-1,3-dioxan-2-yl)ethylenediamine (DPTDEDA) in air were studied by TG-DTG techniques. The kinetic parameters of the decomposition process for the title compound in the two main thermal decomposition steps were calculated through the Friedman and Flynn-Wall-Ozawsa (FWO) methods and the thermal decomposition mechanism of DPTDEDA was also studied with the Coats-Redfern and Achar methods. The results show that the activation energies for the two main thermal decomposition steps are 128.03 and 92.59 kJ•mol^-1 with the Friedman method, and 138.75 and 106.78 kJ•mol^-1 with the FWO method, respectively. Although there are two main thermal decomposition steps for DPTDEDA in air, the thermal decomposition mechanism of DPTDEDA in the two steps are the same, i.e. f(α) =3/2(1-α)^4/3[(1-α)^-1/3-1]^-1.

Keywords

N / N′-bis(5 / 5-dimethyl-2-phospha-2-thio-1 / 3-dioxam-2-yl)ethylenediamine / thermal decomposition kinetics / activation energy / mechanism

Introduction

The fire retardants have been widely used for the materials which need to have flame retardance characteristics. In the recent years, the study on fire retardants has been spurred with the increasing worldwide flame retardance requirements. Fire retardants containing halogen (bromine or chlorine) have wide applications for their high fire retarded properties for the materials, but they can release toxic or corrosive hydrogen halide gases during the decomposition [1]. In particular, bromodiphenyl ether fire retardants can not only release toxic hydrogen bromide but also carcinogenic bromofuran during the thermal decomposition, which made the European Union, the United States and other countries or areas begin to restrict their applications [2, 3]. Thus, the research on fire retardants that are nearly non-toxic or those that do not release corrosive gases during decomposition greatly pushes the development of fire retardants containing non-halogen materials [4-8]. There are wide varieties of fire retardants containing phosphorus [9-11]. They can form phosphoric acid during thermal decomposition, which is a type of strong dehydrating agent. They can promote the fire retarded materials to dehydrate and char so as to form a layer of char residue on the surface of the fire retarded material, which can isolate the material from oxygen, so that fire retardance is achieved. The fire retardants containing nitrogen will not produce hydrogen halide and carcinogenic compounds as fire retardants containing halogen do. They have wide applications as fire retarded materials for their low smoke and environmentally friendly properties [2, 12, 13]. The new types of intumescent fire retardants are mainly composed of compounds containing phosphorus and nitrogen. They have rich acid source, carbon source and gas source, and are prospective fire retardants [14-17]. Some researches have shown that [18-21] sulfur is a kind of important fire retarded element, which has synergism with phosphorus and nitrogen. But there are few fire retardants which simultaneously contain phosphorus, sulfur and nitrogen in the same molecular structure. In the present work, TG-DTG techniques are applied to study the non-isothermal decomposition process in air of N,N¢-bis(5,5-dimethyl-2-phospha-2-thio-1,3- dioxan-2-yl)ethylenediamine (DPTDEDA) (as is shown in Scheme 1) which was synthesized by the authors of this manuscript, and the kinetics of DPTDEDA decomposition were analyzed by the Friedman method and Flynn-Wall-Ozawa (FWO) method. The kinetic parameters of the two main thermal decomposition stages were calculated, and the thermal degradation mechanism of DPTDEDA was studied with the Coats-Redfern method and Achar method.

Experiment

Synthesis and characterization of DPTDEDA

The synthesis of DPTDEDA was done according to Ref. [22], and DPTDEDA was recrystallized from a mixture with the same volume of acetone, anhydrous ethanol and water with its m.p. at 291-292°C. Elemental analysis gave: actual tested (calculated) /%, C:36.96(37.08), H:6.72(6.70), P:15.75(15.97); FTIR(KBr pellet ν/cm^-1): 3350.2(νNH), 2972.52 (νCH₃), 1473.91(νCH₂), 1043.54, 986.6 (νP–O–C), 972.45 (νP–N), 683.24 (νP=S); ¹H–NMR(CDCl₃), δ: 0.91–0.92(m, 12H, exomethyl), 3.84–4.04(m, 4H, exomethylene), 4.23–4.30(m, 8H, ring methylene), 6.52(d, 2H, NH). MS m/z: 318(M⁺).

Apparatus and test conditions

TG-DTG analysis for DPTDEDA was conducted on a NETZSCH STA 409 PG/PC Thermal Analyzer. The experiment was conducted under air at the flow rate of 20 mL/min and at various heating rates(5, 10, 15 and 20 K/min) from room temperature to 1000°C, and the sample weight was 4-5 mg.

Kinetic methods

As to the thermal decomposition reaction, the reaction rate can be usually expressed as:

(1)\mathrm{d}\alpha /\mathrm{d}t=kf(\alpha )

According to Arrhenius equation, k=Ae^-E/RT, the following equation can be achieved.

(2)\mathrm{d}\alpha /\mathrm{d}t=A{\mathrm{e}}^{-E/RT}f(\alpha )

where α is the decomposed mass fraction(%) at time t, A is the pre-exponential factor (s^-1), E is the apparent activation energy (kJ·mol^-1), R is the gas constant (J·mol^-1·K^-1), f(α) is the reaction mechanism function. Under the controlled and constant heating rate, the rate of temperature rising, i.e.β=dT/dt, thus, Eq. (2) can be changed to the follows:

(3)\mathrm{d}\alpha /\mathrm{d}T=(A/\alpha ){\mathrm{e}}^{-E/RT}f(\alpha )

Through separating the variable and rearranging with the integral or differential functions of Eq. (3), the Friedman equation [23,24] and the Flynn-wall-Ozawa(FWO) equation [25,26] can be gained respectively as follows:

(4)\ln [(\mathrm{d}\alpha /\mathrm{d}T)\beta ]=\ln [Af(\alpha )]-E/RT

(5)\lg \beta =\lg [AE/RG(\alpha )]-\mathrm{2.315}-\mathrm{0.4567}E/RT

Logging the Eq. (3), the Coats-Redfern integral equation[27,28] and Achar differential equation [29] can be gained respectively as follows:

(6)\ln [G(\alpha )/T^{\mathrm{2}}]=\ln (AR/\beta E)-E/RT

(7)\ln [(\mathrm{d}\alpha /\mathrm{d}T)/f(\alpha )]=\ln (A/\beta )-E/RT

According to Coats-Redfern equation and Achar equation, the authors selected the normal used thirty types kinetic functions G(α) and f(α) from Ref. [29], and plotted ln[G(α)/T²] and ln[(dα/dT)/f(α)] against 1/T, the regression curves could be generated by the least square method, and the kinetic parameters E, lnA and correlation coefficient r of the different mechanism functions could be obtained.

Results and discussion

The thermal decomposition of DPTDEDA

The TG and DTG curves of DPTDEDA at different heating rates in air are shown in Fig. 1. The TG and DTG curves of DPTDEDA at the heating rates of 10 K/min in air are shown in Fig. 2. As shown in Fig. 1, with the increasing heating rates, the initial decomposition temperature (T_i), the mass loss peak temperature (T_p) and the decomposition termination temperature (T_d) of the DTG curves of DPTDEDA at the different four heating rates rise correspondingly. It can be seen that the DTG curves and TG curves shift toward the high temperature zone. This may be because the heating rates can affect the temperature gradient and heat transmission between the pot and sample, outside sample and inside sample. When the heating rates is slow, the sample can have enough time to expose to specific temperature to absorb heat, which makes the initial decomposition temperature and the termination temperature lower. While with the increasing heating rates, the heat transmission rate slows down. The sample has not much time to accept heat supplied from the outer environment, which leads to higher temperature to supply heat to decompose [30]. In addition, as is shown in Fig. 1, the weight loss temperature range and the weight losing rates at the four heating rates are different, but the total weight losses are almost same when the sample is heated to the specific temperature (1000°C).

As shown in Fig. 2, the TG curve of DPTDEDA has two main weight loss stages. There is 49.66% mass loss in the first stage (267.9-377.2°C), and the temperature peak of the fastest weight loss is 281.2°C. There is a relative weak P=S double bond in the DPTDEDA structure [31,32]. In the presence of oxygen, sulfur removal reaction happens first in the first weight loss stage to produce SO₂. Then, the breaking of P—O bond and P—N bond occurs, and phosphorus forms phosphoric acid in this period followed by dehydration of phosphoric acid to form metaphosphoric acid. Then, it polymerizes to polymetaphosphoric acid which promotes neopentyl glycol to dehydrate and char. At the same time, DPTDEDA decomposes to release NH₃. There is 40.25% weight loss (506.6-667.1°C) in second stage, and the fastest weight loss temperature is 542.5°C. The weight loss of this stage is mainly due to the CO₂ release resulting from the carbon oxidization at higher temperature. The two weight loss stages are consistent with the decomposition temperature of the most polymers. There is over 25% char residue at 600°C, and 2.92% char residue left at the specific temperature (1000°C), which illustrates that DPTDEDA has good thermal stability and excellent char forming capability, and it can give excellent fire retardance for the condensation phase. There are two thermal decomposition peak in the DTG curve. The first one is narrow and sharp, which shows that the weight loss is very fast and this is consistent with the relative small temperature range of the first main weight loss of the TG curve. Compared with the first weight loss peak, the second peak is wide and passive, which shows that the weight loss is relative moderate, and this agrees with the relative wide weight loss temperature range of the second main weight loss stage of the TG curve.

Fire retardant properties

The tests on the fire retarded viscose with DPTDEDA fire retardant show that when the mass fraction of DPTDEDA in the viscose is 18% the limited oxygen index (LOI) values of the viscose can increase from 17% to over 28%. The SEM of the fire retarded viscose is shown in Fig. 3. It can be seen that the burned sample surface has many bulge bubbles, and the cross section of the sample is honeycomb like structure. It is because the SO₂ and NH₃ produced from the decomposition of DPTDEDA collide against the fire retarded system, which shows that DPTDEDA is an intumescent fire retardant.

Thermal decomposition kinetics

The Friedman method and Flynn-wall-Ozawa (FWO) method are the common methods to calculate the activation energy in the equal conversion rate method. As α<0.1 or α>0.9, the decomposition reaction is in induction or end period, which can not fully reflect the real state of the degradation, and may bring uncertainty to the judgment on the mechanism function. So, the conversion rate, α　should be selected between 0.1-0.9. The α, T, dα/dt data of the thermal decomposition of DPTDEDA are listed in Table 1. Bring the corresponding experimental data obtained from the two main weight loss stages of the thermal decomposition of DPTDEDA in air into the Eqs. (4) and (5), and calculating the activation energy of the decomposition reaction with computer, then the E values correspond to different α are listed in Table 2. As shown in Table 2, the reaction activation energies calculated with FWO method are slightly higher than those values obtained from the Friedman method. The E values attained from Friedman method appear fluctuations with the increasing α, which is due to the Friedman equation being sensitive to noise signals. However, in the FWO method, the E values of the two main decomposition stages change little with the increasing αα, which shows that the decomposition reaction represented by the respective two stages may be a one step reaction.

Mechanism speculation

According to the Coats-Redfern equation and Achar equation, the authors selected the normally used thirty types kinetic functions G(α) and f(α) from Ref. [29], and plotted ln[G(α)/T²] and ln[(dα/dT)/f(α)] against 1/T. The regression curves could be generated by the least square method and the kinetic parameters E, ln A and correlation coefficient r of the different mechanism functions could be obtained. Comparing the kinetic parameters values of E and ln A calculated from differential and integral method, selecting the closest E and lnA values and the better co-efficient r. The reaction model of the E values being nearly equal to those calculated from Friedman or FWO method is the kinetic model. In this work, the data tested from the heating rates of 5, 10, 15, 20 K/min are calculated, respectively, and the results are listed in Table 3 and Table 4. Thus, the mechanism function of the first reaction stage is f(α)=3/2(1-α)^4/3[(1-α)^-1/3-1]^-1. Taking the average values of differential and integral methods, the activation energy is E=127.93 kJ·mol^-1, and the pre-exponential factor is lg A=11.36 s^-1. While the mechanism function of the second stage is f(α)=3/2(1-α)^4/3[(1-α)^-1/3-1]^-1, activation energy is 94.27 kJ·mol^-1, and the pre-exponential factor is lg A=4.08 s^-1. Though there are two main thermal weight loss stages of DPTDEDA in air, the reaction mechanism function is the same one by calculating, i.e. f(α) =3/2(1-α)^4/3[(1-α)^-1/3-1]^-1.

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