3.1. Thermal stability
The glass transition temperature (Tg) of the modified EPs was characterized by DSC tests. In Fig. 2, EP exhibited a high Tg value of 160.7 oC. In the all modified EPs, EP/10% CS had the highest Tg, while EP10% DOPO had the lowest one, and the Tg values of CS/DOPO modified EPs were in between the above two values. With CS/DOPO supplementation, the Tg values of EP/CS/DOPO samples were all decreased. When the mass ratios of CS to DOPO were 1 : 1, 1 : 2 and 1 : 3, respectively, equivalent to the gradual increase of DOPO content, the Tg of EPs gradually decreased. Two aspects for the decrease of Tg values were concluded: 1) stereo-hindrance of DOPO structure decreased the cross-linking density of EPs (Liu et al. 2014); 2) only part of active N-H and -OH groups in CS could act as chain extenders, leading to the decrease of the crosslinking density of EPs. Meanwhile, when the mass ratio of CS/DOPO was 1: 1, 2 : 1 and 3 : 1, the Tg of EPs were 132.8 oC, 146.8 oC and 143.7 oC, respectively. It can be seen that when the mass ratio of CS to DOPO was 2 : 1, the Tg value of EP10%CS2/DOPO1 was the highest in CS/DOPO modified EPs, indicating that the EP composite formed a relatively dense cross-linking structure with epoxy resin. This is consistent with the mechanical properties of the materials.
TGA tests were further carried out to study the thermal stability and decomposition behaviors of EP and CS / DOPO modified EP samples. Fig. 3 showed the TGA curve of EP, EP / CS, EP / DOPO, and EP / CS / DOPO samples, and the corresponding results were listed in Table 2. It can be seen from Fig. 3(a, b) that under N2 atmosphere, the initial decomposition temperature (T5%) and the maximum weight loss temperature (Tmax %) of EP were 366.6 oC and 383.8 oC, respectively. The T5% and Tmax % of EP/10% DOPO and EP/10% CS samples were 285.2 oC, 385.6 oC and 366.3 oC, 385.3 oC, respectively. The T5% of all modified EPs is higher than the curing temperature of EP ranged from 100 - 150oC, indicating that DOPO, CS or CS / DOPO can meet the solidification condition. The T5% of the other samples decreased to varying degrees, but the Tmax % of the other samples did not change much. It can be inferred that the decrease of initial decomposition temperature Tg of modified materials was attributed to the addition of DOPO. It can be seen from Fig. 3 that, unlike EP, the pyrolysis process of modified materials had two rapid weight loss stages, 240 ± 10 oC and 285 ± 5 oC, respectively. The main reasons were as follows: on one hand, the thermal decomposition temperature of CS and DOPO is lower than that of EP, it can be assigned to the preferential decompose of CS and DOPO that did not react with EP over EP/CS/DOPO materials during the heating process. On the other hand, with the loading of CS and DOPO, the epoxy groups of the EPs were correspondingly reduced, thereby reducing the crosslinking density of EP more or less. Except EP/10% DOPO, the char residues at 750°C were all higher than that of EP, indicating that these EPs had superior high temperature stability and better carbonization capacity than EP.
Simultaneously, the thermal degradation process of the modified EPs was also investigated by TGA under air atmosphere. As seen in Figure 3(c, d), the thermal decomposition process of pure EP showed two stages, which was consistent with the literature (Zhang et al. 2013). The Tmax1% in the first stage appeared at 380.4°C, and the maximum mass loss rate was 9.7 wt %·min−1, which was close to that under nitrogen (383.8°C), and both were caused by the severe thermal decomposition of epoxy resin itself. Therefore, this stage was also the main stage of the thermal weight loss of epoxy resin. The Tmax2% of the second stage appeared at 576.5°C and the maximum mass loss rate was 2.9 wt %·min−1, which corresponded to the further oxidation reaction of the carbon residue at high temperature. (Liu et al. 2006) The char residue of EP was only 0.8 wt % at 800°C, indicating that EP had poor thermal stability at high temperature in air. The thermal degradation of EP/10% CS, EP/10% DOPO, EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 also had two decomposition stages, which was basically the same as that of EP. Through the analysis of specific data in Table 2, the two-stage thermal decomposition temperature of the modified EPs decreased, which can be assigned to the early decomposition of flame retardant CS/DOPO into phosphorus-containing substances. Different from EP (αmax1 = 9.7, αmax2 = 2.9), all αmax1 of the modified EPs decreased in the first stage and αmax2 increased in the second stage, where αmax1 and αmax2 represented the first and second maximum decomposition rate, respectively. The first stage was mainly contributed to the decomposition of chitosan and DOPO, which accelerated the decomposition rate of resin matrix to carbon at low temperature, while the phosphorus-containing substances decomposed by DOPO in the second stage further promoted the carbon formation.
Table 2
Thermal decomposition parameters of EP and EP/CS/DOPO under nitrogen atmosphere
Sample
|
N2
|
T5% (°C)
|
Tmax (°C)
|
Residue at 750°C (%)
|
EP
|
366.6
|
383.8
|
17.7
|
EP/10% CS
|
366.3
|
385.6
|
18.9
|
EP/10% DOPO
|
285.2
|
384.6
|
16.3
|
EP/10% CS1/DOPO1
|
326.6
|
382.4
|
19.7
|
EP/10% CS1/DOPO2
|
311.5
|
383.7
|
18.7
|
EP/10% CS1/DOPO3
|
305.5
|
382.2
|
19.3
|
EP/10% CS2/DOPO1
|
320.5
|
384.5
|
21.6
|
EP/10% CS3/DOPO1
|
335.8
|
381.3
|
20.2
|
Additionally, the thermodynamic properties of the materials were analyzed by dynamic thermodynamics (DMA). The results were shown in Figure 4 and Table 3. Tan-δ peaks of different materials were obtained from Fig. 4a, namely, the glass transition temperatures (Tg) of EP, EP/10% CS2/DOPO1 and EP/10% CS1/DOPO2 were 161.5 ℃, 147.1 ℃ and 135.7 ℃, respectively, which were basically consistent with the Tg obtained in DSC tests. The results showed that when a certain mass of CS/DOPO was added to the epoxy resin, the corresponding Tg decreased. The reason can be attributed to the decrease of the number of epoxy groups in epoxy resin due to the addition of CS/DOPO and space steric hindrance formed by DOPO, which reduced the ring-opening crosslinking of DDM and epoxy resin. The magnitude of storage modulus at Tg + 30 ℃ was E'EP/10% CS1/DOPO2 < E'EP < E'EP/10% CS2/DOPO1, indicating that the mass ratio of CS/DOPO affected E' greatly. The higher E' value of EP/10% CS2/DOPO1 was attributed to the fact that the higher CS content in EP can provide more active hydroxyl and -NH- groups to participate in the ring-opening reaction of EP, increasing the flexibility of EPs. Inversely, with the increase of DOPO content in EP, the low reactivity and rigid structure of DOPO will reduce the mobility of the chain in EP, thereby reducing the flexibility of EP materials (Xu et al. 2018). The above analyses were consistence with the crosslink density magnitude of the samples. Fig. 4b showed that the magnitude order of the storage modulus was E'EP < E'EP/10%CS1/DOPO2 < E'EP/10% CS2/DOPO1 in the temperature range of 65 – 120 ℃, and the storage modulus of the CS/DOPO modified EP samples below 120 ℃ was much greater than that of pure EP. The higher storage modulus was corresponding to the higher resistant ability to deformation of the materials. Moreover, it is generally believed that higher E′′ means higher damping energy dissipation in polymer networks, which is beneficial for the transformation of the potentially damageable vibration energy to heat. As seen in Fig. 4c, below 69 ℃, the values of E′′ of two modified Eps were lower than that of EP, and the E′′ of EP/10% CS2/DOPO1 was higher than that of EP/10% CS1/DOPO2, indicating that the increase of CS content in biomass polymer contributed to the movement of polymer chains and increased crosslink density (Jian et al. 2017).
Table 3
DMA results and thermal properties of EP samples
Samples
|
Storage modulus at
50°C (MPa)
|
Tg (oC)
|
Tan δ
|
E' (Tg+30K)
MPa
|
νea
(mol/m3)
|
EP
|
1305.5
|
161.5
|
0.71
|
27.1
|
2258.3
|
EP/10% CS1/DOPO2
|
2050.1
|
135.7
|
0.81
|
19.1
|
1752.3
|
EP/10% CS2/DOPO1
|
2004.9
|
147.1
|
0.88
|
31.5
|
2863.6
|
a νe = E'/3RT; E' equals to the storage modulus corresponding to T taken as 30°C above Tg; R is the gas constant. |
3.2. Flame retardance of EP composites
3.2.1 UL-94 tests and LOI
UL-94 rating of the EPs is determined by the length of combustion times and burning dripping at self-extinguishing (Wang et al. 2009). Limiting oxygen index (LOI) refers to the minimum oxygen concentration (volume percentage) that supports the combustion of materials. The test results of flame retardancy of EPs studied by LOI and UL-94 vertical combustion tests were summarized in Table 4. As can be seen from Table 4, all the modified EPs did not obtain V-0 rating when the total addition amount was 8%. However, when the total addition was 10%, EP/10% DOPO modified EPs passed the V-0 rating. Interestingly, when the mass ratio of CS / DOPO was 1 : 2 or 2 : 1, the modified EPs passed the V-0 rating, but the other mass ratio of CS / DOPO modified EPs only obtained the V-1 or V-2 rating. It indicated that under the premise of a certain total amount of addition with the mass ratio of 1 : 2 or 2 : 1, the synergistic effect of CS and DOPO on EPs was prominent. For LOI tests, the LOI value of EP sample was 22.6%, however, with the incorporation of CS or/and DOPO, there was a improvement on LOI values, especially with the incorporation of CS and DOPO both, the increase of LOI value was considerable. The value of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 samples reached up to 35.1% and 34.3%, respectively, and simultaneously obtained V-0 rating. According to the analyses above, it was concluded that when the total addition amount was 10% and the mass ratios of CS and DOPO were 1 : 2 and 2 : 1, EP can be endowed with good flame retardancy.
Table 4
Formulations of epoxy samples and the results of UL-94 and LOI tests
Samples
|
EP
|
DDM
|
CS/DOPO
|
CS
|
DOPO
|
LOI
|
UL-94
|
(g)
|
(g)
|
(wt/wt)
|
(g)
|
(g)
|
(%)(±0.3%)
|
Dripping
|
Rating
|
EP
|
25
|
6.5
|
-
|
0
|
0
|
22.6
|
Y
|
None
|
EP/8% CS
|
25
|
6.5
|
-
|
2.74
|
0
|
24.4
|
N
|
None
|
EP/8% DOPO
|
25
|
6.5
|
-
|
0
|
2.74
|
34.2
|
N
|
V-1
|
EP/8% CS1/DOPO1
|
25
|
6.5
|
1:1
|
1.37
|
1.37
|
31.3
|
N
|
V-2
|
EP/8% CS1/DOPO2
|
25
|
6.5
|
1:2
|
0.91
|
1.83
|
35.1
|
N
|
V-1
|
EP/8% CS2/DOPO1
|
25
|
6.5
|
2:1
|
1.83
|
0.91
|
34.3
|
N
|
V-1
|
EP/8% CS1/DOPO3
|
25
|
6.5
|
1:3
|
0.69
|
2.05
|
34.8
|
N
|
V-1
|
EP/8% CS3/DOPO1
|
25
|
6.5
|
3:1
|
2.05
|
0.69
|
32.6
|
N
|
V-1
|
EP/10% CS
|
25
|
6.5
|
-
|
3.5
|
0
|
23.3
|
N
|
None
|
EP/10% DOPO
|
25
|
6.5
|
-
|
0
|
3.5
|
30.5
|
N
|
V-0
|
EP/10% CS1/DOPO1
|
25
|
6.5
|
1:1
|
1.75
|
1.75
|
32.8
|
N
|
V-2
|
EP/10% CS1/DOPO2
|
25
|
6.5
|
1:2
|
1.17
|
2.33
|
33.7
|
N
|
V-0
|
EP/10% CS2/DOPO1
|
25
|
6.5
|
2:1
|
2.33
|
1.17
|
32.5
|
N
|
V-0
|
EP/10% CS1/DOPO3
|
25
|
6.5
|
1:3
|
0.87
|
2.63
|
34.8
|
N
|
V-1
|
EP/10% CS3/DOPO1
|
25
|
6.5
|
3:1
|
2.63
|
0.87
|
30.2
|
N
|
V-1
|
3.2.2 Cone calorimeter
CC tests, as one of the most effective bench-scale methods, were employed to assess the combustion behavior of materials. Various parameters can be obtained from the CC tests, some representative data were presented in Table 5 and the curves were illustrated in Fig. 5. As can be seen, the time for ignitions (TTI) of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 was longer than that of EP consistently. Mainly due to the presence of unreacted CS and DOPO with EP, the EP composites with CS/DOPO additives would preferentially decompose into non-combustible gases, such as H2O, NH3, CO2 and N2, diluting the combustible gases on the surface of the material in combustion. Comparison with EP, the PHRR and THR values of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 decreased, indicating that CS/DOPO increased flame retardancy of EP. FIGRA (Fire index growth rate), defined as PHRR/tP, as an important parameter for evaluating fire hazard, is calculated by the maximum value of HRR/t and always equal to PHRR/tp, which can estimate both the fire spread rate and fire scale. The lower the value of FIGRA can be considered the slower the flame spread and flame growth. It can be seen from Table 5 that the FIGRA values of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 were significantly lower than that of EP, indicating that the CS / DOPO could reduce fire hazard. In addition, it is worth noting that the PHRR, THRP and SPR values of the two modified EPs were smaller than those of EP, especially, TSP decreased by 61.5% and 70.9%, indicating that the CS / DOPO can not only reduce heat release, but also greatly enhance the smoke inhibition of EP. The possible reason was that the CS/DOPO formed a stable and dense carbon layer during combustion, which hindered the heat transfer inside the samples during ignition and reduced the flue gas release, thus achieving the quenching effect. In terms of reducing heat release, EP/10% CS1/DOPO2 had better performance. The av-EHC value of EP was 29.3 MJ·kg−1, while the av-EHC values of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 were decreased by 27.6% and 0.7%, respectively, indicating that the gas phase flame retardancy effect was more obvious when the mass ratio of CS and DOPO was 1:2. Moreover, compared with EP, the average CO2 yield decreased by 55.4% for EP/10% CS1/DOPO2 and 55.0% for EP/10% CS2/DOPO1, while the relative average CO yield only increased by 6% and 9%, respectively. The results indicated that the complete combustion degree of the modified EPs significantly reduced. The reason for the good flame retardancy of EP composites with 10% CS/DOPO additives can be explained according to the individual and synergistic functions of CS or DOPO with the same amount in the EP. It can be seen from Table 5 that EP/10% DOPO had the lowest PHRR and FIGRA values and the highest TTI value among the four samples, indicating good fire resistance. However, PSPR, TSP and av-COY values of EP/10% DOPO were the highest ones except pure EP, revealing poor smoke suppression performance. As for EP/10% CS, it exhibited the lowest PSPR and av-COY values and the highest THR and av-EHC values among the four samples, revealing better smoke suppression performance and worse heat resistance. The reverse behaviors of EP/10% CS and EP/10% DOPO endow the 10% CS/DOPO modified EP composites with good performance in almost all parameters related to the evaluation of flame retardancy. It can be contributed to the synergistic effect of CS and DOPO on EP.
Furthermore, the flame retardancy related to flame inhibition, charring, and formation of a protective barrier layer were quantify according to the calculation method reported in the literature (Tang et al. 2017). The values of the three main modes of action can be obtained from the following equations (1)-(3) and were summarized in Table S1. A significant improvement in the flame-inhibition effect 27.60% was obtained when the addition amount of CS and DOPO was 10% with the mass ratio of CS and DOPO of 1 : 2. The reason can be contributed to the obvious quenching effect caused by more PO· free radicals released during thermal decomposition by adding more DOPO. Both EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 exhibited higher barrier and protection effects, which were 20.36% and 36.52%. The better barrier and protection effects of the latter was due to the higher content of CS in it, it was also the reason why EP/10% CS2/DOPO1 can achieve V-0 rating in the UL-94 tests.
Table 5
The CC test results of EP and its flame-retardant EP samples
Sample
|
EP
|
EP/10%
CS
|
EP/10%
DOPO
|
EP/10%
CS1/DOPO2
|
EP/10%
CS2/DOPO1
|
Time for ignitions(s)
(TTI) (s)
|
59.0±2
|
46±2
|
96±2
|
81.0±2
|
87.0±2
|
Time to peaks (s)
tp (s)
|
130±3
|
105±2
|
150±2
|
130.0±3
|
130.0±3
|
Peak heat release rate (PHRR) (KW·m−2)
|
1063.1±45
|
999.8±45
|
718.3±45
|
791.9±45
|
770.4±45
|
Fire growth rate index (FIGRA)
|
8.2
|
9.5
|
4.8
|
6.1
|
5.9
|
Peak smoke produce rate (PSPR) (m2·s−1)
|
0.55±0.1
|
0.24±0.1
|
0.41±0.1
|
0.39±0.1
|
0.31±0.1
|
Total smoke release
(TSP) (m2)
|
70.53±3
|
23.66±3
|
29.53±3
|
27.18±3
|
20.56±3
|
Total heat release (THR) (MJ·m−2)
|
94.85±5
|
99.23±5
|
92.10±5
|
69.73±5
|
85.44±5
|
Average effective heat of combustion (av-EHC) (MJ·kg−1)
|
29.3±0.5
|
36.8±0.5
|
24.2±0.5
|
21.2±0.5
|
29.1±0.5
|
Average CO yield
(av-COY) (kg·kg−1)
|
0.33±0.1
|
0.19±0.1
|
0.38±0.1
|
0.35±0.1
|
0.36±0.1
|
Average CO2 yield (av-CO2Y) (kg·kg−1)
|
7.38±1
|
5.26±1
|
3.97±1
|
3.29±1
|
3.32±1
|
Char residue (%)
|
8.9
|
14.1
|
14.3
|
16.4
|
12.6
|
Total mass loss
(TML) (%)
|
91.1
|
85.9
|
85.7
|
83.6
|
87.4
|
3.3. Flame-retardant mechanism
3.3.1 Char residues analyses after CC tests
To disclose the flame-retardant mechanism in the condensed phase, the element composition, chemical structure and morphology of char residues surface were investigated by EDX, FTIR, Raman and SEM. Firstly, the elemental composition of the surface of the char residuals without crushing was studied, the results were shown in Fig. S1. In the char residue of EP/10% CS1/DOPO2, the content of carbon decreased and the content of oxygen and phosphorus increased significantly in comparison with EP, indicating that there were a large number of phosphorus-containing oxides on the surface of residual carbon. For the char residue of EP/10% CS2/DOPO1, the carbon content increased, and the content of oxygen and phosphorus was lower than those in EP/10% CS1/DOPO2, indicating that the formed char residual in the former contained more graphitized carbon than the latter, which was consistent with the results of Raman. By enlarging EDX elemental mapping images, it was found that a small amount of N atoms were dispersed in all samples. To further illustrate the changes of N atoms before and after CC tests, the N contents in the fresh EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 were calculated according to the components and relative N contents, they were 2.90% and 3.17%, respectively. The N contents in the char residues of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 can be seen relatively lower than those of P atoms. Considering that P contents were found very low, so, the N atoms must be much lower than those of P atoms, which was why EDX did not give their content.
Further EDX elemental mapping analysis was conducted on the cross-section of char residual, and the detailed distribution of elements was shown in Fig. 6. As seen in Fig. 6, all elements except carbon were increased and distributed evenly. That proved the presence of carbon, nitrogen and oxygen in the chars of EPs. In addition, it can be seen that phosphorus elements were evenly distributed in the char residuals of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1, and these phosphorus-containing substances prevented the heat release and oxygen entry.
Figure 7 showed the digital and SEM images of char residues after CC tests. As shown in Figure 7 (a1-a4), the char residuals of EP showed that there was only a small amount of char residuals left on the margin of the base plate, along with a large amount holes in its microstructure, which did not have the ability to protect the matrix from combustion. While the char residuals of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 increased by 84.3% and 41.6% when compared with EP, respectively, and exhibited more uniform carbon layer with relatively smaller holes in the microstructure. In addition, the carbon layer of EP was discontinuous and loose, while the carbon layer of the modified EPs showed a more continuous and dense structure. The dense carbon layer can be used as a protective layer to prevent the continuous combustion of the matrix. At the same time, the char residuals particles of EP were a dispersion state, while the char residuals particles of modified EPs had good agglomeration. The agglomeration of char residuals of EP/10% CS2/DOPO1 was more obvious among them.
It is known that the barrier effect of condensed phase is not only affected by its quantity, but also closely related to its structure. Raman spectroscopy is an effective technique for analyzing the internal structure of carbonaceous materials after combustion. As shown in Fig. 8, two remarkable peaks at 1360 and 1590 cm−1 in all spectra are belong to D band (vibrations of amorphous carbon atoms) and G band (vibration of sp2 −hybridized carbon atoms), respectively. Generally, the ratio of the integrated intensities of D to G band (ID / IG) is usually used to measure the degree of graphitization (Ai et al. 2018). The lower the ID/IG value, the higher the graphitization degree and the better the flame retardancy. As seen, the values of ID/IG follow the sequence of EP (3.93) > EP/10% CS1/DOPO2 (2.88) > EP/10% CS2/DOPO1 (2.65), indicating that the graphitization degree of modified EPs had significantly increased. This denser char layer can better hinder the escape of flammable gases and entrance of oxygen in the condensed phase during combustion, thus protecting the inner materials from further decomposition.
The chemical structure of char residual was studied by FTIR. As shown in Fig. 9, the main absorption peaks were observed at 2927 cm−1, 1607 cm−1 and 1080 cm−1, which belonged to the vibration of aromatic rings (Li and Yang 2014). By comparison with EP, the new peaks at 1236 and 1040 cm−1 were attributed to the stretching vibration of P=O and P-O-C, and the absorption peaks of C-P and phenol were observed at 1510 and 754 cm−1, respectively (Zhang et al. 2012a; Yang et al. 2016). The above results showed that the char residue was mainly composed of graphite-like compounds and organic phosphorus compounds.
Simultaneously, the chemical compositions of char residues for EP, EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 after CC test were further analyzed by XPS. The XPS spectra and the atom percent of C1s, N1s, O1s and P2s for the char residues of EPs were shown in Fig. 10 and Table S2, respectively. In the C1s spectrum, there were three kinds of carbon binding states in the three samples, among which the peak at 289.7 eV belongs to C=C and C=O bonds, the peak at 285.8 eV belongs to C-O and C-N bonds, and the peak at 284.7 eV belongs to C-H and C-C bonds in alipha FIGRA tic and aromatic components (Li et al. 2019; Cao et al. 2022). Moreover, it can be clearly seen that the peak at 285.8 eV of the two modified EPs increased compared with EP, suggesting that more cross-linked carbon composed of C-O and C-N was formed in the residual carbon. In the N1s spectrum, all three samples presented two peaks at 400.3 eV and 398.7 eV, wherein the former was the contribution of N-H bond, and the latter was assigned to C-N bond (Wang et al. 2010). In the O1s spectrum, similarly, all three samples had peaks at 533.3 eV and 532.3 eV, the former was attributed to the C-OH and C-O-C groups and the latter belonged to C=O (Zhang et al. 2012b; Wu et al. 2022). However, the modified EPs both had a larger peak at 533.3 eV than that of EP, which was due to the formation of C-O-P or C-O-C, P-O-P groups in the char residues of the modified EPs. In the P2s spectrum, there were two new peaks, 134.4 eV and 133.3 eV in the char residues of modified EPs (Huang et al. 2018; Sun et al. 2016), which assigned to P-O-P, P-O-C and/or PO3− group in phosphate. This further indicated the formation of phosphate-containing compounds in the char residuals, which were covered on the surface of the materials and prevented the further combustion.
3.3.2 TG-FTIR analysis of EP and modified EPs samples
In order to evaluate comprehensively the influence of the CS / DOPO on the thermal stability of EP composites, TG-FTIR spectra were applied to further disclose the decomposition behavior of the modified EPs under programmed heating process. The detailed characteristics of each TG-FTIR spectrum for EP, EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 obtained at 100 ℃, 200 ℃, 300 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 800 ℃ were shown in Fig. 10. The 3D TG-FTIR images of EP, EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 at various temperatures were shown in Fig. S2. It can be seen that the peaks of EP between 2250 cm−1 and 2500 cm−1 attributed to CO2 in EP were much stronger than other samples, indicating that epoxy produced more CO2 than other three samples. The absorption peak of phosphorus-containing substances can be clearly observed in EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1.
As shown in Fig. 11, all EPs exhibited the characteristic absorption peaks of nonflammable gases, such as H2O and CO2, which could be demonstrated by the broad bands at 3500-4000 and 2300-2390 cm−1, respectively (Wang et al. 2011a; Wen et al. 2018). It can be seen that in terms of carbon dioxide release, EP released the most. Compared with EP/10% CS1/DOPO2, EP/10% CS2/DOPO1 released more CO2, which was coincided with the results of CC tests. Furthermore, hydrocarbons can be clearly identified at the broad bands at 600-700 cm−1, 1100-1500 cm−1, and 2900-3100 cm−1 between 400°C and 800°C, as the peak at about 661 cm−1 attributed to =CH- bonds, peaks at 1180 cm−1, 1460 cm−1 and 2972 cm−1 belonged to the -CH3 bonds, the peak at 2927 cm−1 was assigned to -CH2- bonds (Wang et al. 2011b; Wu et al. 2011). In addition, some sharp peaks appeared in the range of 1550-1800 cm−1, which might be carbonyl-containing compounds such as carboxylic acids (1706 cm−1), ketones (1720 cm−1) and aldehydes (1738 cm−1) (Yan et al. 2018).
Significant difference was observed compared to EP and EP/10% CS. As shown in Fig. 11, the peak at about 1129 cm−1 for EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 was attributed to the Ph-P bond (Xu et al. 2016). Moreover, the peak at about 1260 cm−1 assigned to the stretching vibration of P=O, which was probably overlapped with the characteristic absorbance of ether bonds. A weak absorption peak at 1642 cm−1 was the stretching vibration of P-OH (Yuan et al. 2018). On the basis of the analysis above, a variety of phosphorus-containing compounds were generated during the thermal decomposition process of the modified EPs, which can increase the flame retardancy of EP significantly. It is worth noting that compared with EP/10% CS1/DOPO2, most of the absorption peaks of nonflammable gas for EP/10% CS2/DOPO1 were larger, so it was speculated that EP/10% CS2/DOPO1 was mainly gas-phase flame retardancy while EP/10% CS1/DOPO2 was mainly condensed-phase flame retardancy, which was consistent with the results of CC tests.
In order to explore the time when these EPs began to decompose into gas in the process of thermal decomposition, the curves were made with time as abscissa and infrared absorption intensity as ordinate, simultaneously the first derivative curve was drawn. As seen in the Fig. S3, the pyrolysis products of EP began to release at approximately 31.2 min. Surprisingly, the time of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 began to release pyrolysis products almost identical, which were 30.3 min and 30.2 min, respectively, a little less than that of EP. The results showed that the thermal stability of the modified EPs slightly decreased after adding CS / DOPO, which was consistent with the test results of Tg and TGA.
3.4 Mechanical properties
The mechanical properties of materials are important in some areas, so it is necessary to study the mechanical properties of modified EPs. The mechanical properties were analyzed from three indexes: tensile strength, flexural strength and izod impact strength. The results were summarized in Fig. 12. As can be seen from the figure that the tensile strength, flexural strength and izod impact strength of EP were 54 MPa, 86 MPa and 21 KJ·m−2, respectively. Compared with EP, the tensile strength, the flexural strengths of EP/10% CS and EP/10% DOPO increased by 2.3% and 7.0%, while, EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 increased by 36.0% and 38.4%, respectively. The flexural strength of CS/DOPO modified EP was significantly higher than those of EP/10% CS and EP/10% DOPO under the same addition amount in EP, which can be contributed to the synergistic effect of CS and DOPO in EP when the mass ratio was 1 : 2 or 2 : 1. The main reason was that there were -NH2 and -OH active groups in CS and P-H active bonds in DOPO. On the one hand, -NH2 and -OH groups would form intramolecular and intermolecular hydrogen bonds, increasing the compatibility of CS and DOPO with EP. On the other hand, these active groups would react with epoxy groups in EP to undergo ring-opening reaction, which enhanced the formation of cross-linking network in the curing process (Luo et al. 2021). What’s more, the π-π stacking among the aromatic rings were also conducive to improving its mechanical strength (Ma et al. 2021). The possible chemical reactions were shown in Scheme 1.