Fig. 2 shows the FT-IR spectra of different PVC films in the wavenumber range from 525-715 cm-1, in which the vibration peak near 608 cm-1 was attributed to characteristic groups of the C-Cl bond [33-35]. The width of the vibration peak from the C-Cl bond on the surface of the PVC film after FA/NT modification was slightly larger than that of the control sample and the blank sample. The peak widths from large to small were C-3>C-2>S1>C-1>S0, where the peak widths for C-3, C-2 and S1 were 7 cm-1 larger than those for C-1 and S0. The differences in the vibration peaks in the wavenumbers from 715-4000 cm-1 before and after the modification were small. The reason may be that the surface coating and content of FA/NT only ranged from 0.64-3.12%.
3.3 X-ray diffraction analysis of different films
Fig.3 shows the XRD profiles of different PVC films. It can be seen from the figure that S0 and S1 had almost no characteristic diffraction peaks, and no crystalline particles existed in the samples. C-1, C-2 and C-3 generated obvious diffraction peaks in the 2θ range from 15.0~35.0°, indicating that FA/NT and PVC had a strong interaction, and they were located on the surface of PVC [6](see Figure S2). The diffraction peak intensity from large to small was C-1>C-2>C-3, indicating that the crystallinity of the sample decreased with increasing FA/NT complex content. This trend is close to the trend from the hydrogen chloride release experiment. As the content of FA/NT increased, the stability of PVC during thermal degradation decreased.
3.4 X-ray photoelectron spectroscopic analysis of different films
XPS spectra of films C-1, C-2 and C-3 are shown in Fig.4. Unlike film C-1, new Sn peaks are observed at C-2 and C-3, which are 486.3 eV and 495.7 eV, respectively.
The high resolution spectra of the C 1s regions are depicted in Fig.5(a–c). In the C 1s spectrum of the C-1, C-2 and C-3 films, the three components 1, 2 and 3 in the C ls spectra corresponded exactly to three types of carbon bonds: C–C (284.76 eV), C–O and C-N (286.39 eV) and amide C=O (289.07 eV).
As displayed in Fig.6 (a), two different types of oxygen components, O=C (532.03 eV) and O–C (533.52 eV), were present in the O 1s spectrum of the C-1 film. In the O 1s spectrum of high-resolution scans of the O 1s region of Fig.(b) C-2 and (c) C-3, although oxygen groups (O=C) still remained, the peak intensities of C-1 groups (O=C) decreased obviously. With the increase in the amount of FA/NT added, a new peak was generated near 534.35 eV, indicating that the carbonyl group was involved, and the carbonyl group was derived from DOTP and FA.
As depicted in Fig.7(c), the high-resolution Cl 2p spectrum of the C-3 film showed that there were two different chlorine components: 200.10 and 201.70 eV. In contrast, in the spectra shown in Fig.7 (a) and (b), new peaks centered at 198.35 and 198.35 eV appeared and were assigned to hydrogen bonds. The former peak corresponds to a weaker C-1 hydrogen bond.
Table 3 XPS element distributions
Sample
|
C1s (%)
|
O1s (%)
|
Cl2p (%)
|
O/C
|
Cl/C
|
C-1
|
76.14
|
19.38
|
4.48
|
0.25
|
0.06
|
C-2
|
78.10
|
15.96
|
5.94
|
0.20
|
0.08
|
C-3
|
77.37
|
14.93
|
7.70
|
0.19
|
0.10
|
The XPS binding energy data for the composite films are shown in Table 3. There were many sources of C 1s in the sample, which may be due to the increase in FA content in the FA/NT, resulting in the increase in C 1s content.
The content of O 1s in C-1, C-2 and C-3 decreased in turn, and the O/C ratio dropped from 0.25 to 0.19. The main O 1s contribution came from DOTP, followed by contributions from OT and FA. This was due to O atoms, such as those in FA and DOTP, involved in hydrogen bond formation in the PVC system, resulting in an increase in the surface oxygen atom concentration.
The content of surface Cl 2p in C-1, C-2 and C-3 increased in turn, and the Cl/C ratio increased from 0.06 to 0.10. The Cl 2p signals mainly came from the PVC resin. This result indicates that with increasing FA/NT, the surface chlorine from C-2 and C-3 films that is not involved in the formation of hydrogen bonds increases. Therefore, the 1 phr FA/NT complex produced a strong interaction with the PVC polar groups through hydrogen bonds, which was the reason for the improvement of its heat resistance.
3.5 Thermogravimetric analysis
Fig.8 shows the TG-DTG curves of the different PVC films. The carrier gas was N2, and the heating rate was 10°C min-1. It can be seen from the figure that the thermal degradation of the different PVCs underwent two stages, and the first and second stages of degradation occurred in the ranges of 201-385°C and 385-547°C, respectively. During each decomposition stage, C-1 was better than S1 and the other samples, as it withstood higher temperatures. During the first decomposition stage, when the conversion rate was 1%, the initial decomposition temperature of C-1 was 230.0°C, while the initial decomposition temperatures of S0 and S1 were 175.1 and 210.8°C, respectively. The temperature for C-1 was significantly higher than those of S0 and S1.
Table 4 The decomposition temperatures of the PVC films at different conversion rates
α
/%
|
S0
|
S1
|
C-1
|
C-2
|
C-3
|
|
|
/°C
|
|
|
1
|
175.1
|
210.8
|
230.0
|
212.7
|
198.2
|
5
|
244.1
|
250.5
|
260.0
|
248.6
|
251.3
|
10
|
264.1
|
267.7
|
272.9
|
262.7
|
265.8
|
80
|
432.1
|
435.7
|
447.7
|
451.0
|
446.6
|
90
|
477.1
|
478.6
|
495.0
|
703.5
|
511.4
|
Table 4 shows the decomposition temperatures of the PVC films for different conversion rates. When the conversion rate was 5%, the thermal degradation temperatures of C-1, C-2, C-3, S1 and S0 were 271.8, 251.3, 250.5, 248.6 and 244.1°C, respectively, showing a decreasing trend. C-1 degraded at the highest temperature of 271.8°C, and the shift to the high-temperature zone was the most obvious.
Table 5 The decomposition rates of the PVC film at different temperatures
T/°C
|
S0
|
S1
|
C-1
|
C-2
|
C-3
|
|
|
% min-1
|
|
|
200
|
0.619
|
0.168
|
0.120
|
0.209
|
0.155
|
300
|
22.75
|
19.81
|
10.85
|
10.53
|
10.85
|
400
|
0.736
|
0.603
|
0.400
|
0.379
|
0.384
|
450
|
4.334
|
2.513
|
2.210
|
2.384
|
2.289
|
500
|
1.714
|
0.602
|
0.630
|
0.585
|
0.586
|
700
|
0.797
|
0.008
|
0.006
|
0.022
|
0.022
|
Table 5 shows the decomposition rates of the PVC films at different temperatures. The decomposition rates of C-1, C-2, C-3, S1 and S0 during the first decomposition stage at 300°C were 12.55, 10.53, 10.85, 19.81 and 22.75% min-1, respectively. The decomposition rate of PVC after modification decreased significantly compared with that of the control sample and the blank sample.
Table 6 Characteristic temperatures of different PVC films
Sample
|
S0
|
S1
|
C-1
|
C-2
|
C-3
|
Stage
|
Tp/°C
|
308.1
|
302.2
|
309.5
|
304.2
|
308.5
|
1st
|
dα/dt (%min-1)
|
29.40
|
20.26
|
14.51
|
10.91
|
12.47
|
|
Tp/°C
|
464.1
|
455.8
|
464.8
|
454.5
|
456.1
|
2nd
|
dα/dt (%min-1)
|
5.55
|
2.57
|
2.67
|
2.59
|
2.61
|
|
Table 6 shows the characteristic temperatures of the different PVC films. It can be seen from the table that during the first decomposition stage, the peak temperatures of C-1, C-2, C-3, S1 and S0 were 309.5, 304.2, 308.5, 302.2 and 308.1°C, respectively, and the corresponding decomposition rates were 14.51, 10.91, 12.47, 20.26 and 29.40% min-1, respectively. The peak temperature for C-1 was 309.5°C, which was obviously in the high-temperature zone and was higher than that for S1 and S0. The decomposition rate of C-1 at the peak temperature was 39.6 and 102.6% lower than that of S1 and S0, respectively. During the second decomposition stage, the maximum decomposition rate of S0 was 5.55%/min, and the rest of the rates were similar and decreased to 2.5-2.7%/min.
3.6 TG- FTIR analysis
The gas phase 3D TG-FTIR spectra of C-1 and S0 sample pyrolysis are shown in Fig. 9, where the x coordinate is time/second, the y coordinate is wavenumbers/cm-1, and z is absorbance. Figure 6a is a TG-IR perspective of the C-1 sample.There are four stronger infrared absorption peaks at 1621 s, 1626 s, 1674 s and 1653 s, with absorbance peak intensities of 0.2562, 0.1604, 0.1498 and 0.1085, respectively.
Fig.10 shows infrared spectra of released gas at different temperatures, wherein the black curve represents C-1 and the red curve represents S0. As shown in Fig.10 (a), a and b are the infrared spectra of the gas phase at 250°C. A small amount of gas was released, but the difference was not significant. c and d are the vibration peaks at pyrolysis times of 1643 s (310.8°C) and 1624 s (307.7°C), respectively, which included the hydrogen chloride removal stage where C-1 shifted to the high temperature zone.
The vibration peak near 1264 cm-1 is attributed to esters from DOTP in C-1. Its intensity is significantly lower than that of S0, and the peak intensity decreases by 17.3%. The vibration peaks near 2950 and 2798 cm-1 are attributed to hydrogen chloride gas [36], the peaks near 1736, 1264, 1106 and 1024 cm-1 are attributed to the ester group ν(COOR) from the plasticizer DOTP [37], and 671 cm-1 is attributed to stretching of ν(C-Cl) groups [36, 38]. The peak strength of C-1 after modification decreases obviously compared with S0, which shows that the C-1 heat resistance is enhanced.
The absorption peak from Fig.10 (b) shows that upon heating from 350°C to 500°C, the vibration peaks at 2600-3100 cm-1 disappeared first, and then a weaker hydrocarbon vibration peak was generated. The vibration peaks near 1736, 1264, 1106, 1024 and 671 cm-1 almost disappeared and became weaker, indicating that DOTP gasification products were minimal and that the removal of hydrogen chloride was almost complete. The conjugated polyenes started to polymerize. Here, infrared spectra e, g and i represent C-1; f, h and j represent S0.
As shown in Fig.10 (c), the second stage of PVC thermal degradation occurred at 600°C, and the vibration peak of carbon dioxide occurred between 2318-2372 cm-1. During this stage, S0 began to release CO2, while C-1 inhibited CO2 overflow. At 700°C, the peak height of the signal from the CO2 released from S0 further increased compared with that from C-1, which also inhibited CO2 release and inhibited the degradation of the plasticizer DOTP, facilitating the recovery of DOTP. In the infrared spectra, k and m represent C-1 and l and n represent S0. Therefore, after enhancement, the intensities of the vibration peaks at 1264, 1736, 1106 and 1024 cm-1 were due to a significant decrease in DOTP, which shows that the additive had a strong interaction with DOTP, which inhibits the thermal degradation of PVC and improves the thermal stability of the C-1 film.
Scheme 1 describes the possible interactions of the FA-modified NT-reinforced PVC composite film.
FA contains two carboxyl groups, one amino group and three secondary amino groups, which easily generate intramolecular and intermolecular van der Waals forces and O or N intermolecular hydrogen bonds when interacting with NT surface polyhydroxyl groups. The excess hydroxyl groups on the NT surface easily formed hydrogen bonds with Cl atoms in the PVC molecule, which inhibited the initiation of chlorine free radicals when the PVC was heated. However, as the addition of FA/NT increased from 3 to 5 phr, the compatibility with PVC decreased.