3.1 Characterization of CDS
The structural characterization of CDS was performed by FTIR and 13C-NMR. The FTIR spectra of DDS, CDS and CNC were presented in Fig.2. Clearly, several functional groups from DDS and CNC, such as N-H(3320-3530cm-1), C=C (1620-1450cm-1), C=N(1530cm-1), can be observed in the FTIR spectra of CDS. The absorption peak of C-N(1265cm-1) appeared, and the absorption peak of C-Cl(850cm-1) disappeared, which indicated the completion of polymerization. The structures of CDS was further confirmed by 13C solid-state NMR in Fig.3. The signals at 165.75 ppm, 142.49 ppm, 136.08ppm, 128.85 ppm, 121.30 ppm were attributed to C (a, δtr-C), C (b, δ-C=C-in para-point of benzene ring), C (e, δ-C=C-in para-point of benzene ring), C (d, δ-C=C-in meta-point of benzene ring), C (c, δ-C=C-in ortho-point of benzene ring), respectively.
3.2 Flame retardancy of TPEE composites
The formula and the results of LOI values and UL-94 ratings of neat TPEE and TPEE composites are shown in Table1 and Table 2. Neat TPEE was very flammable and burned to the clamp after the first ignition, and was accompanied by severe droplets with the LOI value of 17.9. CDS was acted as both charring and blowing agents in the intumescent flame retardant system. There was hardly any improvement for the LOI values and UL-94 ratings by the addition of 20% CDS separately. It revealed that the lack of acid source for the intumescent flame retardant system is not efficient enough. When 15wt% or 20wt% of AlPi was added, the TPEE sample were extinguished in about 5 seconds after the Bunsen burner is ignited for 10 seconds, and then during the secondary combustion, droplets dripped into the fire Bunsen burner constantly. Therefore, it reaches the Ul-94 V-2 grade. In addition, compared with neat TPEE, the LOI values of TPEE/15AlPi and TPEE/20AlPi increased to 26.8 and 24.5, which showed that aluminum hypophosphite was able to play a flame retardant effect in both condensed phase and gas phase. In the gas phase, free radical capture leaded to flame suppression, and in the condensed phase, it initiated the formation of char or inorganic residues[19].
The formulation of TPEE composites and results of LOI and UL94 test of all the investigated samples were shown in Table 1 and Table 2 respectively. As the mass ratio of the acid source and the char source increased, it can be seen that when 2 wt% CDS was added into the intumescent flame retardant system, the LOI of the TPEE/18AlPi/2CDS was increased to 28.2. However, droplets were still produced during the secondary combustion and V-2 rating was reached, which meant less char agent was unable to form a thick and dense char layer to prevent melt dripping. With the increase in the proportion of CDS, TPEE/15AlPi/5CDS can stop burning during the first combustion in 2.2 seconds and the UL-94 rating was enhanced to V-0 from V-2 with inhibited dripping during the secondary combustion. However, with the further increase of the charring agent, it can be found that the LOI value of TPEE/10AlPi/10CDS was decreased to 26.2 and dripped again within 10s in the ignition process of secondary flame. The reason was that the char layer formed after expansion and combustion was too much to cover the surrounding of the sample, making the heating area larger. It can be concluded that an appropriate ratio of AlPi and CDS in the intumescent flame retardant system was particularly important for the flame retardant performance of TPEE composites.
Table 1 Composition of formulas (wt%)
Sample
|
TPEE
|
AlPi
|
CDS
|
Neat TPEE
|
100
|
0
|
0
|
TPEE/20AlPi
|
80
|
20
|
0
|
TPEE/20CDS
|
80
|
0
|
20
|
TPEE/18AlPi/2CDS
|
80
|
18
|
2
|
TPEE/15AlPi/5CDS
|
80
|
15
|
5
|
TPEE/10AlPi/10CDS
|
80
|
10
|
10
|
TPEE/15AlPi
|
85
|
15
|
0
|
Table 2 Results of LOI and UL94 test of all the investigated samples
Sample
|
LOI(%)
|
UL-94,1.6mm Bar
|
t1/t2
|
Dripping
|
Rating
|
Neat TPEE
|
17.9±0.5
|
BC
|
Yes/-
|
NR
|
TPEE/15AlPi
|
26.5±0.5
|
6.2/14.2
|
No/Yesa
|
V-2
|
TPEE/20AlPi
|
29.8±0.5
|
4.5/10.5
|
No/Yes
|
V-2
|
TPEE/20CDS
|
22.5±0.5
|
BC
|
Yes/-
|
NR
|
TPEE/18AlPi/2CDS
|
28.2±0.5
|
4.5/8.9
|
No/Yes
|
V-2
|
TPEE/15AlPi/5CDS
|
30.2±0.5
|
2.2/5.9
|
No/No
|
V-0
|
TPEE/10AlPi/10CDS
|
26.2±0.5
|
4.5/8.9
|
No/Yes
|
V-2
|
LOI: limiting oxygen index; t1: average combustion time after the first application of flame; t2, average combustion time after the second application of flame; BC: burns to clamp; NR: no rating.
b No/yes corresponds to the first/second flame application.
3.3 Charring behavior and thermal degradation of CFA and AlPi
Table 3 Results of thermogravimetric analysis under N2 atmosphere
Sample
|
T5%
|
T10%
|
Tmax
|
dw/dt(max)
|
Residue(%)
|
|
(℃)
|
(℃)
|
(℃)
|
(wt%/min-1)
|
700(℃)
|
CDS
|
336.1
|
361.3
|
403.2
|
-0.30
|
50.8
|
AlPi
|
436.6
|
451.4
|
482.8
|
-2.28
|
20.98
|
IFR
|
394.3
|
423.8
|
391.0
|
-1.22
|
32.24
|
Calculation IFR
|
409.2
|
445.6
|
396.0
|
-1.31
|
28.44
|
In order to investigate the charring behavior and thermal degradation of CDS, AlPi and IFR during heating, thermal gravimetric analysis was used. IFR was AlPi/CDS mixture whose mass fraction was 15:5 and calculation IFR was the result calculated from the experimental results of AlPi and CDS based on their percentage in the IFR system in accordance with formula (1).
Wcalculation IFR=WAlPi*15/20+WCDS*5/20 (1)
As can be seen from table.3, AlPi showed excellent thermal stability with a high char residue (20.98%) at 700℃ under N2 atmosphere. And it decomposed into one steps, starting from 436.6℃ and the maximum mass rate at 482.8℃. AlPi can catalyze dehydration and cross-linking reaction of charring agents in IFR, and serves as an acid source and CDS can be used as a charring and forming agent. The thermal degradation process of CDS was divided into two steps: the first step roughly occurs at 300 to 400°C (corresponding to the chemical reaction of dehydration and release of SO2)[20], and the second step occurred at 400 to 550°C, which may be allocated to the decomposition of macromolecular framework. And finally, CDS degraded into expanded char during the pyrolysis process with the increase of temperature, and the char residue is about 50.8% at 700°C. These results showed that CDS had good thermal degradability, which can be attributed to the presence of triazine and benzene ring.
The initial decomposition temperature (T5%) of IFR was 394.3℃ and the char residue was 32.24% at 700℃, which proved that IFR had excellent thermal stability and charring performance. Compared with Calculation IFR, the calculated value of initial decomposition temperature was 409.2 ℃, which was higher than the actual measured temperature value. This result revealed that with the incorporation of CFA into AlPi, the overall thermal degradation process changed. Meanwhile, the char residue (28.44%) of calculation IFR was lower than the IFR, which indicated that AlPi can improved the charring performance of CDS and accelerated the formation of the char layer.
3.4 Thermal degradation of TPEE and TPEE composites
Figure 6 and Figure 7 showed the TGA and DTG curves of TPEE and flame-retardant TPEE composites under N2 atmosphere. Table 4 summarized detailed data such as 5% mass loss, 10% mass loss, maximum mass loss rate, and the experimental and calculation value of char residue at 700°C. Through these data, we tried to explore the mechanism of the condensed phase and clarify the thermal stability of different formulations. The decomposition of neat TPEE started at 370.2℃, and the highest loss rate at 403.2℃ was -2.53 wt%/min-1. After the TGA test, TPEE was completely degraded into gaseous molecules, so it can be decomposed in one step with almost no residue. After AlPi was added to the TPEE matrix, the initial thermal decomposition temperature (T5%) and the maximum thermal decomposition rate temperature(Tmax) of the TPEE composite did not change much, which indicated that AlPi can cooperate with the TPEE matrix well, and the experimental value of char residue of the composite was much higher than the calculated value, which indicated that the phosphorus in AlPi can catalyze TPEE to form a char layer during the combustion process. When the intumescent flame retardant composed of CDS and AlPi was added to TPEE, the initial thermal decomposition temperature of the composite material is slightly reduced, which was due to the lower initial thermal decomposition temperature of CDS. The proportion increased gradually, and it was found that when the addition amount of the charring agent CDS was 5%, the synergistic efficiency of AlPi and CDS was the highest at this time, and the char residue of the TPEE composite material was increased to 23%, which meant the surface can form a thick and dense char layer to prevent the TPEE composite from dripping so that the flame retardancy would be improved, which was consistent with the UL-94 measurement result. Comparing all formulas from TPEE/18AlPi/2CDS to TPEE/10AlPi/10CDS, the experimental values of the char residue rate were always much higher than the calculated values, which showed that the role of the intumescent flame retardant was not a simple linear addition, but was caused by the good synergy between the acid source, the charring agent and the matrix.
Table 4. Results of thermogravimetric analysis under N2 atmosphere.
Sample
|
T5%
|
Tmax
|
dw/dt(max)
|
Residue(700℃, N2)
|
|
(℃)
|
(℃)
|
(wt%/min-1)
|
Cal
|
Exp
|
Neat TPEE
|
370.3
|
403.2
|
-2.53
|
-
|
1.68
|
TPEE/20AlPi
|
368.3
|
397.0
|
-2.17
|
5.54
|
16.50
|
TPEE/18AlPi/2CDS
|
360.5
|
402.1
|
-1.71
|
6.14
|
19.12
|
TPEE/15AlPi/5CDS
|
352.0
|
391.0
|
-1.82
|
7.03
|
23.52
|
TPEE/10AlPi/10CDS
|
341.4
|
396.0
|
-1.50
|
8.52
|
17.72
|
TPEE/15AlPi
|
368.7
|
396.0
|
-2.14
|
4.58
|
16.08
|
T5%: temperature of 5 wt% mass loss; Tmax: temperature of the maximum mass loss rate; dw/dt(max): maximum mass loss rate; Residue: weight of the residue at 700℃ after thermogravimetric analysis test; Cal: calculation value of char residue; Exp: experimental Value of char residue.
3.5 Dynamic FTIR of TPEE and TPEE composites
Through the characterization of the infrared spectrum of the TPEE composites during the heating process, the synergistic effect between flame retardants and the mechanism and form of action in the matrix can be studied. Figure.8-10 showed the infrared spectra of Neat TPEE, TPEE/AlPi and TPEE/AlPi/CDS at a heating rate of 10°C/min-1. 2962cm-1 and 2878cm-1 (-CH2, stretching vibration), 1712cm-1 (-C=O, stretching vibration), 1458cm-1 (-CH2, fracture vibration), 1410cm-1 (aromatic) ring), 1274cm-1 (-CO-O-ester), 1106cm-1 (-CH2-O-CH2-ether) and 727cm-1 (-CH, bending vibration of aromatic ring) were the characteristic peaks of TPEE. With the increase of heating temperature, the band intensity at 2956 and 2861 cm-1 (-CH2) dropped rapidly, which was caused by the scission of the α-methylene group in the soft segment, which was consistent with our previous research results[21].
For the formula TPEE/15AlPi, due to the addition of AlPi, the typical absorption bands of AlPi appeared in the spectrum, such as 1151, 1079 and 780 cm-1 (-P = O); 1018 cm-1 (PO43-); and 475cm-1 (O=PO, AlPi). Same as Neat TPEE, TPEE/15AlPi in Fig.9 was also decomposed on the soft segment due to the active a-methylene group of the ether bond. In the hard segment (C=O), due to the presence of carboxyl groups, there was a shoulder gap at 1680cm-1. As the temperature rose, due to the strong interaction between Al3+ and carboxyl groups, electron migration occurred, which also improved the thermal stability of the composite. There was no P-O-H unit (3400-3800cm-1) from the curve at 450℃, which indicated that the diethyl aluminum hypophosphite may played a flame retardant effect in the gas phase after being released. The solid phase spectrum of TPEE/AlPi showed that the interaction between TPEE and AlPi improved thermal stability and increased the residue rate, and may formed intermediate products in the gas phase to suppress flames.
The infrared spectrum of TPEE/15AlPi/5CDS during the heating process was shown in Figure.10. It can be seen that the N-H peak at 3400cm-1 gradually weakened with the increase of temperature, which indicated that the triazine charring agent CDS decomposed non-combustible gases such as ammonia and nitrogen, thereby exerting a flame retardant effect in the gas phase. In addition, C-N (1265cm-1), C=N (1500cm-1) and characteristic bands of aromatic structure 737cm-1 and 1411cm-1 appeared in the residue at 450℃, which indicated that the addition of triazine charring agent CDS can make the TPEE composites form the char layer spliced by benzene ring and triazine ring after combustion, thereby increasing the strength of char.
3.6. Morphology and chemical structure of char residues
Raman spectroscopy was widely used to study the crystal structure and molecular structure of carbon materials. In addition to analyzing the morphology of char residue at high temperature, the study of the structure of char residue was of great significance to the research of the flame retardant mechanism. Figure.11 showed the Raman spectra of TPEE / 20AlPi, TPEE / 18AlPi / 2CDS and TPEE / 15AlPi / 5CDS after calcination in a muffle furnace at 700°C for 15 minutes and it can be found that there were broad peaks of 1332 cm-1 and 1590 cm-1 in the Raman spectrum The former (D-band) was caused by the vibration of the disordered graphite sp3 hybrid atom or the swinging carbon atom on the amorphous char layer, and the latter (G-band) corresponds to the vibration of the sp2 hybrid atom of the graphite sheet. The degree of graphitization of the system can be expressed by the ratio of the peak areas of the D band and the G band (ID/IG). Generally speaking, the lower the ID/IG, the more graphitic carbon in the char layer, and the higher the quality of the char layer. The higher the degree of graphitization, the denser and stable the structure of the carbon layer. It can be seen from Figure 11 that the ID/IG ratio of TPEE/20AlPi (2.645) was higher than that of TPEE/18AlPi/2CDS (2.226), which indicated a decrease in the degree of graphitization in the residual carbon. However, compared to TPEE/18AlPi/2CDS, the ID/IG ratio of TPEE/15AlPi/5CDS (1.835) reduced, which suggested that the mass percentage of graphitized carbon in carbon was increased. The graphitized carbon formed in the combustion process was of great significance for controlling the release of heat and volatiles from the stable carbon structure at high temperatures. The ID/IG ratio followed the order of TPEE/20ALPi <TPEE/18AlPi/2CDS<TPEE/15AlPi/5CDS, which showed that TPEE/15AlPi/5CDS had the highest degree of graphitization and the best thermal stability. And this was also in line with the improvement in flame retardancy mentioned above.
Figure.12 showed the SEM image of the residual carbon after calcination in a muffle furnace at 700°C for 15 minutes. It can be seen from the figure that the char residue of the TPEE/15AlPi had many obvious defects, and the surface was irregular. Such a structure cannot effectively prevent the heat transfer from being transferred to the substrate, nor can it prevent the molten combustion material from dripping. As for Figure.12(b), due to the addition of a small amount of the charring agent CDS, the carbonization of CDS accelerated the carbonization speed of the composites, thereby forming a thick carbon layer, which can slow down the heat transfer. However, due to the lack of carbon source, it was not enough to form a dense enough char layer to prevent dripping. Further adding CDS, it can be seen from Figure.12(c) that the residue of the TPEE/15AlPi composite was dense, porous, and folded structure char layers, which can deposit char residues on the surface while blocking heat insulation. In addition, CDS can be used as a gas source after combustion, and non-combustible gases such as N2, NH2 and CO2 were generated, which played the role of inflation, thereby diluting the concentration of oxygen on the surface of the composites and reducing the back diffusion of oxygen. These reasons had also prompted the composites to pass the V-0 UL-94 test. The above results showed that the intumescent flame retardant 15CDS/5AlPi had a good flame retardant effect and can play a role in the condensed phase and the gas phase.
3.7. Proposed flame-retardant mechanism
According to the described structure characterization and char formation behavior analysis, the possible flame retardant mechanism of TPEE / 15AlPi / 5CDS composite was shown in Fig.13 and Fig.14. Firstly, the charring agent CDS decomposed at about 336°C to release incombustible gas, and then cracks to generate triazine ring structures and aromatic structures, which were easy to assemble and splice into a char layer. Subsequently, the thermal oxidative degradation of the TPEE matrix occurred selectively in the soft segment of the polyether, and at the same time, the ester bond was formed. When the temperature continued to be oxidized, the polyester segment will also be broken. The chain scission reaction passes through a six-membered ring intermediate product, -COO abstracted the H atom from the methylene group at the β position to form an oligomer with a carboxyl group and an unsaturated double bond at the end, which continued to oxidize to produce tetrahydrofuran, butadiene, benzoic acid, water, CO2, etc.[22]. Subsequently, the Al3+ provided by AlPi after the temperature raised and the carboxyl-containing oligomers produced by the degradation of TPEE formed a compound, and finally a variety of inorganic aluminum phosphate salts such as pyrophosphate were formed. This phosphate had a certain strength and can help to isolate hot oxygen exchange. In addition, the phosphate closely covered the surface of the substrate to further catalyze the carbonization of the TPEE substrate and the triazine charring agent CDS. Finally, the decomposition of CDS produced an aromatic structure that was assembled with the pyrolysis product of TPEE, and the graphitized char layer was formed on the surface of the composites to isolate heat and play a flame retardant effect.