3.1. FTIR analysis of cotton fabric
The FTIR spectra of cotton fabrics before and after coating are shown in Fig 2.
As can be seen from Fig 2, the peaks at 2900 cm-1, 1640 cm-1, 1420 cm-1, 1365cm-1 and 1100 cm-1 in the uncoated cotton FTIR spectra curve which are assigned to the characteristic peaks of cellulose macromolecule of cotton. While these peaks are almost disappeared in the FTIR spectra curve of coated one. And the new peak at 1650 cm-1 which is assigned to the vibration absorption of N-H bond of PEI. The new peaks at 1160, 1060 and 983 cm-1 are attributed to the characteristic absorption peaks of P=O, O-P-C and P-O bonds. These results demonstrated that PEI and PA have been successfully applied on cotton fabric.
3.2 Morphology of cotton fabrics
The surface morphology of cotton fabrics before and after coating are shown in Fig. 3.
As can be seen from Fig. 3, the surface of uncoated cotton fabric was flat and smooth. While the surface of cotton fabrics after coating was rough, and the coating layer became thicker and thicker for the samples of C3, C4 and C5. Therefore, the SEM results also showed that PEI and PA were successfully formed the coating on cotton fabrics.
3.3. Flame retardancy and durability of coated cotton fabric
3.3.1 Flame retardancy of cotton fabrics
The results of LOI values and vertical burning test of cotton fabrics are listed in Table 2. The tensile strength, bending length and whiteness of cotton fabrics are also listed in Table 2. Fig 4 shows the digital photographs of cotton fabrics after vertical burning.
Table 2 Flame retardancy and physical properties of cotton fabrics before and after coating
Sample
|
add-on
(%)
|
LOI
(%)
|
Vertical burning test
|
Tensile strength retention
(%)
|
Bending length
(mm)
|
Whiteness
|
Char length (mm)
|
After
-flame time(s)
|
After
-glow time(s)
|
uncoated
|
-
|
18.0
|
Burn out
|
6.4
|
26.8
|
-
|
25.7
|
82
|
C1
|
25.6
|
36.7
|
108
|
0
|
0
|
69.9
|
33.2
|
75
|
C2
|
30.4
|
38.5
|
102
|
0
|
0
|
65.6
|
40.5
|
75
|
C3
|
29.3
|
34.5
|
103
|
0
|
0
|
62.7
|
40.8
|
76
|
C4
|
45.2
|
40.0
|
88
|
0
|
0
|
59.2
|
41.2
|
74
|
C5
|
51.1
|
54.0
|
80
|
0
|
0
|
37.0
|
44.6
|
73
|
As shown in Table 2 and Fig. 4, the LOI value of uncoated cotton fabric was only 18%, which was burnt out with the after-flame time of 6.4s and after-glow time of 26.8s indicating high flammability of origin cotton fabric. The LOI values of coated cotton fabrics are all over 30%, and LOI value increased steadily with the add-on of different samples. When the add-on was 45.2%, the LOI value reached 40% for C-4. As for the sample of C-5, its LOI value was 54% with the highest add-on of 51.1% among the samples in this research. The vertical burning performance of coated cotton also related with the add-on, the char length decreased with the add-on. It decreased to around 100 mm when the add-on was low than 30%, and it could continue decrease to 80 mm of C-5. Both the after-flame time and after-glow time for all the samples after coating were 0s. The flame retardancy of cotton fabric was greatly improved after the LBL assembly PEI/PA coating.
The physical properties of cotton fabric including tensile strength, softness and whiteness are very important for its practical application. Table 2 also shows the tensile strength retention, bending length and whiteness of cotton fabrics before and after coating. The tensile strength retention of coated cotton fabrics reduced linearly with the add-on and the PA concentration for the first immersion cycle. Cellulose macromolecule in cotton fabric was easily acid degradation, the strong acidic PA solution may destroy the cellulose macromolecule resulting in the decreasing of strength. PA solution concentration was lower than 2% for the first and/or second immersion cycles, the strength retention of cotton fabric was around or over 60%. While the PA solution concentration was 3% for the first immersion cycle, the strength retention was only 37%. The reason about this may be due to that the low PA concentration for the first and/or second immersion cycles to form the PEI/PA coating with gradient structure of gradual increasing PA concentration, which bring less influence of cotton fabric, and PA may form the film with PEI to protect the cellulose macromolecule to be destroyed in the high concentration of PA solution for the next immersion cycles. Therefore, the PEI/PA coating with gradient structure could form by reasonable controlling PA solutions during the LBL assembly process cycles. The bending length of coated cotton fabrics increased steadily with the add-on of coated cotton fabric indicating the softness of the coated cotton fabrics was influenced to increase the stiffness. Especially when the add-on of coated cotton fabrics higher than 30%, their softness would be influenced with the increasing of bending length higher than 60%. The whiteness of coated cotton fabric after coating was reduced comparing with uncoated one, but it hasn’t caused serious influence of the appearance. Therefore, the flame retardant cotton fabrics by PEI/PA coating endowed them with excellent flame retardancy. While the PEI/PA coating may brought out adverse influence on the physical properties of cotton fabrics, especially when the add-on of PEI/PA coating was higher than 30%.
In order to balance the flame retardancy and physical properties, the coated cotton fabrics with lower add-on of PEI/PA coating was investigated by reducing the immersion time. The concentration of PEI and PA solutions were chose as C2, while the LBL assembly process was different from the previous process. The specific process was as follow: the immersion time of cotton fabric in the PEI and PA solutions was 5 min for the first cycle, while the next immersion cycles for 1 min. The samples were named by C2-1, C2-2 and C2-3 which were repeated the cyclic LBL process by 4, 5 and 6 times. The flame retardancy, physical and mechanical properties of coated cotton fabrics were shown in Table 3.
Table 3 Flame retardancy, physical and mechanical properties of cotton fabrics
Sample
|
add-on
(%)
|
LOI
(%)
|
Vertical burning test
|
Tensile strength retention
(%)
|
Bending length
(mm)
|
Whiteness
|
Char length (mm)
|
After
-flame time(s)
|
After
-glow time(s)
|
C2-1
|
19.7
|
35.0
|
96
|
0
|
0
|
81.6
|
29.9
|
75
|
C2-2
|
24.2
|
37.5
|
92
|
0
|
0
|
78.8
|
37.5
|
74
|
C2-3
|
29.2
|
40.5
|
87
|
0
|
0
|
73.8
|
38.2
|
74
|
uncoated
|
-
|
18.0
|
Burn out
|
6.4
|
26.8
|
-
|
25.7
|
82
|
As can be seen from Table 3, the cotton fabric with lower add-on of PEI/PA coating also obtained excellent flame retardancy. All the LOI values of the coated fabrics were higher than 35%, even when the add-on of PEI/PA coating was low about 20%. And the char length of all coated fabrics were less than 100 mm with no after-flame and after-glow. And the tensile strength retention of all coated fabrics were higher than 70%, it could reach higher than 80% when the add-on was about 20%. The bending length of all coated fabrics were also greatly reduced comparing with that the fabrics were long time immersion (as shown in Table 2) . The bending length of C2-1 was just increased about 16% comparing with the uncoated one. And the whiteness of all coated fabrics were less influenced. Therefore, the cotton fabric achieved excellent flame retardancy and physical and mechanical properties when the add-on of PEI/PA coating was about 20% by the short time immersion of LBL assembly method.
3.3.2 Flame retardant durability of coated cotton fabric
The flame retardant durability is very important for the practical application of textile fabric. The LOI values of coated cotton fabrics after different washing cycles are shown in Table 4.
Table 4 LOI values of treated cotton fabric after different washing cycles
Sample
|
LOI(%)
|
0
|
5 LCs
|
10 LCs
|
20 LCs
|
C2-1
|
35.0
|
27.3
|
25.8
|
22.3
|
C2-2
|
37.5
|
31.0
|
28.3
|
24.0
|
C2-3
|
40.5
|
33.2
|
30.0
|
26.3
|
As can be seen from Table 4, the LOI values of all coated samples greatly decreased after 5 LCs, while all of them were over 27%. As for 10 LCs, the LOI values of coated cotton fabric were over 26% except for C2-1. After 20 LCs, the LOI value of C2-1 and C2-2 were 22.3% and 24.0% indicating their flame retardancy was greatly reduced. While the LOI value of C2-3 was 26.3% revealing it still possessed flame retardancy. These results demonstrated that the coated cotton fabrics possessed good flame retardant durability. The reason about this may be due to PEI and PA would form insoluble polymer film on the cotton fabric surface (Cheng, Tang, Yao, & Yang, 2019).
3.4. Thermal stability of cotton fabric
The thermal stability of cotton fabrics before and after coating were investigated by TGA both under nitrogen and air atmosphere, which were shown in Fig. 5 and Table 5, including the onset decomposition temperature (T-10% represents the temperature of 10 wt% mass loss), the temperature of maximum mass loss rate (Tmax), the mass loss rate at Tmax and the char residues at 800 °C.
Table 5 TGA data of cotton fabrics under nitrogen and air atmosphere
|
Samples
|
T-10% (°C)
|
Stage 1
|
Stage 2
|
Residue at 800 °C
(wt%)
|
Tmax
(°C)
|
The mass loss rate at Tmax (%/°C)
|
Tmax
(°C)
|
The mass loss rate at Tmax (%/°C)
|
N2
|
uncoated
|
340
|
378
|
2.50
|
-
|
-
|
0
|
C2-1
|
280
|
318
|
1.43
|
-
|
-
|
6.86
|
C2-2
|
275
|
315
|
1.26
|
-
|
-
|
19.95
|
C2-3
|
262
|
313
|
1.14
|
-
|
-
|
24.98
|
Air
|
uncoated
|
306
|
365
|
2.53
|
512
|
0.56
|
0
|
C2-1
|
266
|
302
|
1.70
|
514
|
0.26
|
1.86
|
C2-2
|
255
|
303
|
1.62
|
520
|
0.24
|
5.21
|
C2-3
|
238
|
292
|
1.30
|
530
|
0.17
|
9.36
|
As can be seen from Fig. 5a and Table 5, there are only one mass loss stage of the cotton fabrics before and after coating under nitrogen atmosphere. The onset decomposition temperature decreased from 340 °C of uncoated one to 280°C of C2-1. It continued decreased to 262°C for C2-3. The Tmax of coated cotton fabrics were also lower than the uncoated one. The lower T-10% and Tmax may be ascribed to the low decomposition temperature of PEI and PA and the promoting degradation of cotton fabric at onset decomposition stage. The mass loss rate at Tmax of coated cotton fabric greatly decreased, and the char residues at 800 °C increased from no char residue of uncoated cotton to 6.86%, 19.95% and 24.98% of C2-1, C2-2 and C2-3, respectively. These demonstrated that the flame retardant coating of cotton fabric would improve the thermal stability at higher temperature and promote the char formation under nitrogen atmosphere.
As can be seen from Fig.5b and Table 5, the cotton fabrics before and after coating both existed two main mass loss stages under air atmosphere. The first stage is mainly arisen from the dehydration of cellulose and the decomposition of levoglucosan, flammable gas and char, and the second stage may be due to the char further oxidized decomposition to form CO2 and CO. The T-10% and Tmax of coated cotton fabrics also much lower than that of uncoated one, and the reason may be the same with that under nitrogen atmosphere. While the Tmax of coated cotton fabrics at second stage were higher than uncoated one. Both the mass loss rate at Tmax of the two stages were lower than that of the uncoated one. The char residues at 800 °C under air atmosphere of coated cotton fabrics increased to 1.86%, 5.21% and 9.36% of C2-1, C2-2 and C2-3, respectively. These also revealed that the flame retardant coating on cotton fabrics could improve the thermal stability at higher temperature and promote the char formation under air atmosphere. Therefore, PEI/PA coating promoted the char formation of cotton fabric both under nitrogen and air atmosphere. And the flame retardant mechanism by this coating of cotton fabric mainly act through condensed phase flame retardant action.
The TGA results of coated cotton fabrics after 20 LCs are shown in Fig .6 and Table 6.
Table 6 TGA data of coated cotton fabrics after 20 LCs under nitrogen and air atmosphere
|
Samples
|
T-10% (°C)
|
Stage 1
|
Stage 2
|
Residue at 800 °C
(wt%)
|
Tmax
(°C)
|
The mass loss rate at Tmax (%/°C)
|
Tmax
(°C)
|
The mass loss rate at Tmax (%/°C)
|
N2
|
C2-1
|
288
|
313
|
1.89
|
-
|
-
|
0.37
|
C2-2
|
272
|
315
|
1.57
|
-
|
-
|
1.33
|
C2-3
|
263
|
312
|
1.43
|
-
|
-
|
3.04
|
Air
|
C2-1
|
275
|
305
|
1.61
|
512
|
0.12
|
0.29
|
|
C2-2
|
267
|
304
|
1.57
|
513
|
0.12
|
1.26
|
|
C2-3
|
268
|
303
|
1.49
|
513
|
0.12
|
2.80
|
As can be seen from Fig.6 and Table 6, both the thermal decompositon process of coated cotton fabrics after 20 LCs under nitrogen atmosphere and that under air atmosphere are the same with that before washing. The main difference between the coated fabrics before washing and after washing is the char residue at 800 °C, which quite decrease after 20 LCs. However, the char residues at 800 °C of coated fabrics were still higher than the uncoated one. This shows the coated fabrics still contain the PEI/PA coating on their surface confirming the PEI/PA coating possesses good washing duability, which is concidence with the LOI results of coated fabrics after washing.
3.5. Morphology of the char residues
The SEM morphology images of char residues of cotton fabrics after vertical burning were shown in Fig. 7.
As can be seen from Fig. 7, uncoated cotton fabric was burnt to form ash with no char residue, and the fiber shape was destroyed. While the coated cotton fiber shape was kept intact after vertical burning. There are also covering with char layer on the residual cotton fiber. These results also revealed that the PEI/PA coating could promote char formation. There also exist small bubbles on the char residue surface of coated cotton fabrics which may be arisen from the intumescent flame retardant (IFR) action by PEI/PA coating, and the PEI containing N element could coupled with PA to form P-N synergistic effect. Therefore, PEI and PA could construct P-N synergistic IFR system on cotton fabric surface to endow it with excellent flame retardancy.