3.1. Structural characterization and elemental analysis
The FTIR spectra of CMNC, NC, and commercial CMC samples were shown in Fig. 1(a) and the absorption bands of functional groups were summarized as shown in Table 2. All spectra were normalized in light of that for all samples, the peak intensity at around 1160 cm− 1 attributed to the asymmetric vibration of the C − O−C in β-glycosidic linkage was believed to be unchanged, which was convenient for comparing the change of characteristic peaks of samples obtained from different experiment conditions. From Fig. 1(a), there are different offsets near 1730 − 1490 cm− 1 for five CMNC samples. Those characteristic peaks of different samples were fitted and deconvoluted into two peaks: ≈1639 cm− 1 (− ONO2) and 1587 cm− 1 (COO− or COONa in − OCH2COONa), as shown in Fig. 1(b) and 1(c). From Fig. 1(b), with the increase of the degree of denitration, the number of − OCH2COONa introduced to LNNC samples increases under the same subsequent etherification reaction conditions. As seen in Fig. 1(c), the degree of etherification gradually decreases with the decrease of the degree of denitration. Under the same degree of etherification, the nitrocellulose molecular chain with high denitration degree is more likely to react with carboxymethyl groups. This may be because the sequence of − ONO2 removed from the glucose units in nitrocellulose molecule is C3 > C6 > C2, while in the process of etherification, the order of − OCH2COONa introduced into glucose ring is C2 ≈ C6 > C3(Shukla and Hill 2012; Shui et al. 2017). From the spectra of the samples in Fig. 1, the CMNC sample presents both characteristic peaks of − ONO2 and peaks of introduced − OCH2COONa, which proves the CMNC has been successfully prepared.
Table 2
Identification of prevalent bands in FTIR spectra of samples(Casaburi et al. 2018; Golbaghi et al. 2017; Zhao et al. 2016; You et al. 2016; Luo et al. 2019a; Kumar et al. 2018; Wongvitvichot et al. 2021; Wang et al. 2018; Mu et al. 2016; Wang et al. 2021).
No.
|
Absorption range/cm− 1
|
Characteristic groups
|
1
|
3375
|
−OH stretching vibration
|
2
|
2900
|
−CH2 − stretching vibration
|
3
|
1160
|
Asymmetric oxygen bridge stretching vibration
|
4
|
1587
|
COO− or COONa stretching vibration
|
5
|
821
|
−ONO2 stretching vibration
|
6
|
1273
|
−ONO2 symmetric stretching
|
7
|
1639
|
−ONO2 asymmetric stretching vibration
|
XPS experiments were employed to support the FTIR data and to further understand the chemical property and chemical bonding of the surface region of CMNC samples and their precursors. The full surface survey and individual spectra of samples were shown in Fig. 2(a). The total intensity for each sample has been normalized to unity. It can be seen that the characteristic peaks of NC, LNNC, commercial CMC and CMNC samples are C 1s, O 1s, N 1s, Na 1s, O Auger and Na Auger(Yu et al. 2013; Wang et al. 2017). Meanwhile, under the same degree of etherification, with the increasing degree of denitration, the characteristic peak intensity of Na element gradually increases and the characteristic peak of N element gradually disappears. Under the same degree of denitration, with the increasing degree of etherification, the characteristic peaks of Na element are gradually enhanced. The curve fitting with a high resolution of C 1s and O 1s spectra revealed the functional compositions were carbon and oxygen species for samples, as shown in Fig. 2(b) and (c). The C 1s peak of the CMNC (1, 2 and 3) samples were fitted to five Gaussian curves. Among them, the Gaussian peak at 283.1 eV is belonged to C − C and C − H; the Gaussian peak at 284.4 eV is assigned to the characteristic peak of C − O; the characteristic peak at 285.3 eV corresponding to the characteristic peak of C = O or O − C−O was observed(Mansur et al. 2017; Li et al. 2020a; Cui et al. 2020). The characteristic peak at 286.3 eV is correspond to the characteristic peak of C-ONO2(Lu et al. 2021); the characteristic peak at 286.6 eV is attributed to the characteristic peak of O = C − O−(Samadder et al. 2020; Liu et al. 2017). Under the same etherification conditions, the characteristic peak of C − ONO2 gradually weakened and the characteristic peak of O = C − O− gradually elevated with the increasing denitration degree. For Fig. 3(c), the O 1s peak of the CMNC sample is fitted to four Gaussian peaks. Among them, the characteristic peak at 529.3 eV is assigned to the O = C characteristic peak; the characteristic peak at 530.5 eV is belonged to the characteristic peak of O − C(Hu et al. 2018; Bai et al. 2020; Yuan et al. 2021); the characteristic peak at 531.3 eV is correspond to the characteristic peak of C − OO−(Mansur et al. 2017; He et al. 2020); the peak at 532.2 eV is attributed to the characteristic peak of O − NO2(Luo et al. 2019b). Under the same etherification conditions of CMNC (2, 4 and 5) samples, with the increase of denitration degree, the characteristic peak of C − OO− gradually increased and the characteristic peak of O − NO2 gradually weakened. The above results are consistent with the FTIR results, which confirms the − OCH2COONa groups have been introduced in nitrocellulose chemically and the CMNC samples have been prepared successfully.
The morphologies of the original NC and CMNC-3 samples were observed by SEM, as shown in Fig. 3(a) and 3(b), respectively. From the SEM images, the CMNC sample exhibits a fiber structure similar to NC and a rougher surface. Meanwhile, the local element content analysis for CMNC was performed and presented in Fig. 3(b), indicating the content of Na was 6.23 wt.%. Furthermore, uniform distribution of C, O, N and Na elements were demonstrated by the corresponding element mapping (Fig. 3c-f), indicating that the CMNC sample was prepared successfully.
The elemental analyses of NC, LNNC, CMC and CMNC samples were displayed in Fig. 4. The nitrogen content of NC and LNNC was 12.152 wt.% and 6.225 wt.%, respectively. The CMC samples do not contain nitrogen. For CMNC (1, 2 and 3) samples, the nitrogen content of the samples decreased gradually with the increase of denitration degree. With the same degree of denitration, the nitrogen content of the samples of CMNC (2, 4 and 5) gradually increased with the decrease of the degree of etherification, which could be caused by the introduction of − OCH2COONa into the NC molecular chains. Another reason for this result is that sodium hydroxide was used to provide an alkaline environment during the etherification reaction, which has a certain alkaline hydrolysis effect on NC as a strong base (Lee et al. 2019). Both NC and LNNC samples contained a small amount of Na, which may be attributed to the alkalizing step during nitrocellulose production(Urbanski et al. 1965). For CMNC (1, 2 and 3) samples, with the increase of denitration degree, the sodium content of CMNC samples increased gradually. In the CMNC (2, 4 and 5) samples, under the same denitration conditions, the Na element content gradually decreased with the decreasing degree of etherification. The results of the elemental analysis were consistent with that of FTIR and XPS, which proved forcefully that the structure construction of CMNC could be controlled precisely. Hopefully, the introduction of sodium-containing functional groups (− OCH2COONa) will provide a significant flame suppressant effect on CMNC samples.
3.2. Thermal decomposition behavior
The curves of thermogravimetric (TG) and its differential (DTG) during thermal decomposition at the heating rate of 10 ℃·min− 1 were shown in Fig. 5. Tin and Tf were the initial and ending temperature of main weight loss zone, respectively. The decomposition process of samples can be divided into three stages(Wu et al. 2016). The initial stage was from the beginning of heating to Tin, during which a slight weight loss phase and the glass transition occurred and the sample dehydrated. The yield of volatiles was extremely low at this stage(Zhang et al. 2019). The second stage started from Tin to Tf, featuring main weight loss because the thermal reaction became intensive and a large amount of volatiles were produced(Tudorachi et al. 2012). When the temperature reached T1/Tmax (temperature of the first DTG peak), the weight loss rate achieved the peak value. Then the DTG curve started to decrease with increasing temperature until the thermogravimetric process ended at Tf. The third stage was carbonization stage where the TG and DTG curves were almost flat and the residues from the second stage were carbonized(Xu et al. 2020).
The thermal characteristic parameters were presented in Table 3. The Tin, Tmax and Mr (the weight fraction of final residue) values of the CMNC sample are higher than those of the original NC sample. Combing the thermal characteristic parameters and the results of FTIR and XPS of different CMNC samples, a clear trend can be concluded that the more − OCH2COONa was introduced into the NC molecular chain, the better thermal stability of CMNC was obtained(Guo et al. 2021). Remarkably, CMNC sample exhibited two maximum decomposition rate peaks from the DTG curves. The first at near 210 ℃ is close to the maximum decomposition temperature of NC. The other is near 290 ℃, which is close to the maximum pyrolysis temperature of CMC(Wu et al. 2016). This indicated that CMNC presented similar thermal decomposition characteristics to both NC and CMC.
Table 3
Thermal behavior parameters of NC, CMC and CMNC samples with β = 10 ℃·min− 1.
Parameters
|
NC
|
CMNC-1
|
CMNC-2
|
CMNC-3
|
CMNC-5
|
Tin (℃)
|
179.5
|
155.8
|
175.8
|
188.8
|
166
|
T1/Tmax (℃)
|
205.6
|
206.3
|
222.5
|
225
|
214
|
T2 (℃)
|
-
|
296.8
|
292.5
|
280
|
261.3
|
Tf
|
245.8
|
325.2
|
320.2
|
326.7
|
385.17
|
Mr (%)
|
15.70
|
29.74
|
51.61
|
61.40
|
17.46
|
Tin: initial devolatilization temperature; T1/Tmax: temperature of the first DTG peak; T2: temperatures of the second DTG peak; Tf: temperature at the end of the DTG peak; Mr: weight fraction of the final residue. |
The TG-FTIR simultaneous analysis technique was employed to investigate the thermal degradation process of NC and CMNC-2 sample with real-time detection at a heating rate of 10 ℃·min− 1. There is apparent variation in the IR characteristic absorption peaks of the gaseous decomposition products of samples during thermal decomposition process at typical temperatures, including the temperature before decomposition (Tx, select 125 ℃ as Tx temperature), Tin, T1/Tmax, T2, and Tf, as shown in Table 3 and Fig. 6(a, b). Most gases produced during sample pyrolysis could be identified through their FTIR peaks (Table 4).
Table 4
Reported FTIR characteristic peaks from gases generated during thermal degradation(Tudorachi et al. 2012; Chai et al. 2020; Zhao et al. 2021).
No.
|
Products
|
Wavenumbers of IR absorption peaks /cm− 1
|
1
|
H2O
|
3600–4000 cm− 1
|
2
|
CO2
|
2300–2380, 660–670 cm− 1
|
3
|
HCHO
|
2700–2900 cm− 1, 1720–1740 cm− 1
|
4
|
N2O
|
2200–2300 cm− 1
|
5
|
CO
|
2109–2194 cm− 1
|
6
|
NO
|
1760–1965 cm− 1
|
7
|
NO2
|
1593–1639 cm− 1
|
8
|
HCOOH
|
1080–1128 cm− 1
|
9
|
C = O
|
1769 cm− 1
|
10
|
−OH
|
1187 cm− 1, 3200–3400 cm− 1
|
Figure 6(a) showed the real-time FTIR spectrum of NC sample. The FTIR absorption peaks of H2O, CO2 were detected at Tx and Tin. The initial characteristic gas-phase CO2 peak at 2360 cm− l was attributed to the air flow fluctuation found within the air vent inside the FTIR beam chamber (due to the presence of CO in the laboratory atmosphere). Additionally, in this temperature range, the structure of nitrocellulose was relatively stable and there was no thermal damage. When the temperature increased to T1/Tmax, the gaseous products such as H2O, HCHO, CO2, N2O, CO, NO, NO2 and HCOOH were detected. The peak at 1769 cm− 1 was attributed to the C = O stretching bands of HCHO, generated from the -CH2ONO2 group. Evolution of the HCOOH gas was due to the secondary autocatalytic reactions of NC. At the end of the decomposition process (Tf), the FTIR absorption peaks of H2O, CO2, NO and HCOOH were still easily identifiable in Fig. 6(a) (Chai et al. 2020; Zhao et al. 2021). Figure 6(b) presented the FTIR spectra of CMNC-2 at typical temperatures in the thermal decomposition. At the temperature of Tx and Tin, the peaks of H2O and CO2 were assigned to the same event with the previous discussion. At the temperature of T1/Tmax and T2, the main evolved gases were H2O, CO2, N2O, NO2, and HCOOH. The generation of nitrogen oxides were attributed to the decomposition of − ONO2 on the molecular chain during heating. This is similar to the thermal decomposition characteristics of NC(Zhao et al. 2021). At T2, the absorption bands in the FTIR spectrum were corresponding to H2O, CO2, HCHO, and CO. In addition, a band with reduced intensity at 1187 cm− 1 and a wide absorption band between 3200–3400 cm− 1, corresponding to vOH vibrations, can be attributed to ethanol fragments. The decomposition products at this temperature are similar to the decomposition process of CMC(Tudorachi et al. 2012). At Tf, H2O and CO2 were observed, which might be due to the residual organic matrix. According to the real-time detection of gas generation by TG-FTIR during thermal decomposition, CMNC exhibited a similar thermal decomposition behaviors with NC and CMC to some extent and a better thermal stability than NC, leading to potential applications in propellant with certain flame retardant properties through chemical modification. Otherwise, the chemical grafting of sodium carboxymethyl groups through partly denitration and etherification provides an inspiration of designing a new type of cellulose derivative with burn progressive, flame suppression and anti-migration properties simultaneously. It is expected to become a new type of cellulose derivative with energy containing and anti-migration properties.