3.1 FT-IR analysis of FR
The infrared spectrum of FR is presented in Fig. 1. 1460 cm− 1 was the bending vibration characteristic peak of C-H in -CH2, 1160 cm− 1 was the stretching vibration characteristic peak of P = O (Liu et al. 2019), 1060 cm− 1 was the stretching vibration characteristic peak of P-O-C (Wang et al. 2020), 960 cm− 1 was the stretching vibration characteristic peak of -P-O-H+ (Jia et al. 2017a). That is because the -P-O-NH4+ group in FR was thermally decomposed into -P-O-H+. In conclusion, the infrared spectrum of the synthesized product was consistent with the characteristic group of the target product, which indicates that FR has been successfully synthesized.
3.2 LOI and durability analysis of untreated cotton and treated cotton
The WG value of the cotton treated with different FR concentrations change are presented in Fig. 2(a). The WG value of the cotton treated with 100g/L, 200g/L, 300g/L and 400g/L FR before washing are 8.5%, 19.1%, 25.3% and 33.7%, respectively. After 50 LCs, their WG value is 3.7%, 8.3%, 10.3% and 12.1%, respectively. The cotton fabric treated with 400g/L FR solution achieves a large WG of 33.7%. If the concentration of FR is too high, the WG% of treated cotton will also increase, which will affect the physical properties such as handle and original comfort.
The LOI value represents the minimum oxygen concentration required to maintain the flame state when the sample is burned in oxygen and nitrogen atmosphere (Dutkiewicz et al. 2018). As shown in Fig. 2(b), the LOI values of treated cotton with 8.5%, 19.1%, 25.3%, 33.7% WG FR solution reached 26.1%, 31.7%, 40.5% and 42.0% respectively, which were higher than 17.0% of the untreated cotton. After 50 LCs, the LOI values of 25.3% and 33.7% WG FR were 30.9% and 33.9% respectively, which were higher than the LOI values of international flame retardant standard (26%-28%) (Li et al. 2015). Therefore, the cotton fabric with 25.3% WG FR or more can be regarded as durability treated cotton. At the same time, the LOI value of the treated cotton decreases with the increase of laundering cycles. That is why the unreacted FR and the covalent bond P-O-C in some FR are washed away in the washing process. FR has the active group of -P-O-NH4+, which can combine with cellulose through P-O-C covalent bond. In addition, FR can react with -OH on cellulose to form P-O-C bond at the same time. Only when the P-O-C covalent bond between FR and cellulose is completely hydrolyzed can FR be removed from the treated cotton. However, due to the polymerization of flame retardant molecules to a certain extent, it is difficult to hydrolyze all P-O-C covalent bonds. Therefore, the cotton fabric treated with flame retardant has excellent durability.
3.2 Thermal and thermo-oxidative analysis of FR, untreated cotton and treated cotton
Thermal and thermo-oxidative processes of FR, untreated cotton and treated cotton have been investigated by TGA. T5% and T10% were temperature at 5%, 10% mass loss, Tmax was the maximum thermal degradation rate temperature. Rmax was the maximum thermal degradation rate.
The TG and DTG curves and data of FR, untreated cotton and treated cotton in N2 atmosphere are presented in Fig. 3(a) and 3(b) and Table 1, For FR, T5% was 101.6 ℃, which was caused by water evaporation and oligomer decomposition. T10% was 126.8 ℃, and weight loss was 33.1%. The rapid weight loss in this stage was due to the fact that the phosphoric acid released from FR molecular fracture can promote the dehydration and carbonization of FR. Tmax of FR was 161.9 ℃, and the Rmax was 0.6%/℃. The pyrophosphoric acid produced by dehydration of phosphoric acid promotes the carbonization of FR, and then C-C, P = O, -O- break a lot, releasing volatile substances such as NH3, H2O and other small molecular products. The residual carbon rate was 21.0% at 600 ℃. These results indicated that FR has good thermal stability. For untreated cotton and treated cotton, in the initial stage, the fabric has a little weight loss, which was due to the loss of bound water due to the heating of the fabric. The main weight loss of the fabric was between 250 ℃-350 ℃, and the mass loss rate increases rapidly in this stage. This was mainly due to the depolymerization of cellulose macromolecules to produce L-glucose, which was further decomposed to form pyrolysis products and coke (Hussain et al. 2019). When the temperature continues to rise, the cellulose burning char residue will continue to dehydrate and release water and carbon dioxide, and the mass loss rate will be significantly reduced, and the char residue content will be higher and higher (Pan et al. 2018). T5% and T10% of untreated cotton are 257.5 ℃ and 280.7 ℃ respectively, while treated cotton is 184.4 ℃ and 225.9 ℃ respectively. That is because FR catalyzes the decomposition of cellulose (Shafizadeh et al. 1982).The Tmax of untreated cotton was 331.6 ℃, and the Rmax was 1.4%/℃, while the Tmax and Rmax of treated cotton were 263.6 ℃ and 1.1%/℃, respectively. That is why the treated cotton decomposes to form phosphoric acid when heated, and then polymerizes to form polyphosphoric acid, which inhibits the production of L-glucose and further improves the thermal stability. FR dehydrate the cellulose and promote the formation of char residue. The char residue was covered on the surface of the fiber to isolate heat and inhibit the further decomposition of the fiber. The mass of the char residue of treated cotton at 600 ℃ was 28.6%, while that of untreated cotton was only 18.9%, which indicates that improve the thermal stability of cotton fabric.
The thermo-oxidative processes of FR, untreated cotton and treated cotton curves and data are presented in Fig. 3(c) and 3(d) and Table.1. For FR, T5% was 92.2 ℃, T10% was 136.1 ℃,which indicated that FR began to decompose at lower temperatures in air atmosphere. Tmax of FR was 179.1 ℃, and the Rmax was 0.6%/℃. In the range of 200–500 ℃, the weight loss of FR was slow. The residual carbon rate was 35.9% at 600 ℃. For untreated cotton and treated cotton, Below 100 ℃, it belongs to the dehydration stage of water molecules in cellulose. The second degradation occurs when the temperature rises to about 200 ℃. When the temperature is kept at 262.6 ℃, the weight loss rate reaches the maximum, and the weight loss is 30.6%. The weight loss rate of the untreated cotton was the highest at 372 ℃. The weight loss was 72%. In addition, the mass of the char residue of treated cotton is 12.6% at 600 ℃ in air atmosphere, while the mass of the char residue of untreated cotton is 1.0%. Compared with the untreated cotton, the initial decomposition temperature of treated cotton decreased and the residual content increased significantly in both nitrogen and air atmospheres. The above test results show that FR can promote the thermal decomposition of cotton fabric, form a carbon slag protective layer to protect the deep fiber and reduce the decomposition rate of cellulose matrix. It also shows that FR presents a typical condensed phase flame retardant mechanism.
Table 1
TG and DTG data of FR, untreated cotton and treated cotton under N2 atmosphere and air and air atmosphere
Sample
|
T5% (℃)
|
T10% (℃)
|
Tmax (℃)
|
Rmax (%/℃)
|
Residue at 600℃ (%)
|
Untreated cotton in N2
|
257.5
|
280.7
|
331.6
|
1.4
|
18.9
|
Treated cotton in N2
|
184.4
|
225.9
|
263.6
|
1.1
|
28.6
|
Untreated cotton in air
|
153.1
|
259.8
|
301.4
|
1.1
|
1.0
|
Treated cotton in air
|
178.4
|
221.8
|
262.6
|
1.4
|
12.6
|
FR in N2
|
101.6
|
126.8
|
161.9
|
0.6
|
21.0
|
FR in air
|
92.2
|
136.1
|
179.1
|
0.6
|
35.9
|
3.3 XRD patterns of untreated cotton and treated cotton
The crystal structures of untreated cotton and treated cotton were determined by XRD analysis. As shown in Fig. 4, the diffraction peaks of treated cotton were at 14.62, 16.44, 22.18 and 34.36, corresponding to (110), (110), (200) and (004) grid planes respectively. The diffraction peaks of untreated cotton were at 14.80, 16.60, 22.56 and 34.28, corresponding to (110), (110), (200) and (004) grid planes respectively (Kwak et al. 2015; Zheng et al. 2016).The peak position and shape of untreated cotton and treated cotton were very similar, which means that the plane spacing between crystal planes does not change, and the finishing process has little effect on the structure of treated cotton. The crystal structure of cellulose was slightly expanded or layered by the dehydration reaction of FR and cotton fabric cellulose (Lee et al. 2018), and the diffraction peak intensity of treated cotton was slightly lower than that of untreated cotton. This may be due to the penetration of dicyandiamide and FR into the amorphous region of the fiber during the process of cotton fabric soaking in flame retardant finishing solution, and these small molecules were grafted in the subsequent high temperature grafting reaction in this region, the content of cellulose in the treated cotton decreases, which affects the spatial chemical structure of the fiber. That is to say, the proportion of crystalline region of treated cotton grafted with FR decreases, which leads to the decrease of diffraction peak intensity.
3.4 FT-IR patterns of untreated cotton and treated cotton
The FT-IR of untreated cotton and treated cotton are presented in Fig. 5. The absorption peaks at 3471 cm− 1 and 3488 cm− 1 were caused by the stretching vibration of O-H in untreated cotton and treated cotton respectively (Zhang et al. 2018), the treated cotton has a new strong absorption peak at 1200cm− 1, which was caused by the stretching vibration of P = O (Bosco et al. 2017), and a weak P-O-H stretching vibration absorption peak at 850cm− 1 (Mao et al. 2015). In addition, The P-O-C stretching vibration has an enhanced absorption peak at 1050 cm− 1, indicating that the flame retardant molecules have been successfully introduced into the cotton fiber (Choi et al. 2018). The absorption peak at 1625 cm− 1 was caused by the stretching vibration of C = O (Nguyen et al. 2014). That is why the glycosidic bond of cellulose was oxidized to C = O under high temperature and acid environment.
3.5 Surface morphology and elemental composition of char residue from untreated cotton and treated cotton
In order to better study the flame retardant effect of cotton fabric, the morphology of char residue in cotton fabric is more valuable than that of raw cotton. SEM and EDS were used to study the surface morphology and element composition of fabric char residue.The SEM of untreated cotton char residue with different magnification are shown in Fig. 6(a) and (b), the fiber structure was obviously damaged, and the SEM of treated cotton char residue with different magnification are shown in Fig. 6(c) and (d), the fiber structure can still remain intact, indicating that FR can effectively protect the structure of the fabric after burning, and there are obvious particles on the surface of the treated cotton after burning, indicating that FR is successfully attached to the treated cotton. As shown in Fig. 6(e) and (f), EDS shows that phosphorus is evenly distributed on the surface of the char residue of the treated cotton, and the carbon content was increased compared with that before combustion, which indicates that FR can improve the flame retardant performance of cotton fabric by promoting carbon formation.
3.6 Vertical burning analysis of untreated cotton and treated cotton
The vertical burning method is used to evaluate the burning performance of cotton fabric. Figure 7 shows the electronic photos of untreated cotton and treated cotton after vertical burning. As shown in Table 2, the treated cotton with 8.5%, 19.0%, 25.3% and 33.7% WG FR solution has carbon length equal to reach 50 mm, 42 mm, 38 mm and 33 mm. The untreated cotton was ignited immediately in the vertical burning test, the flame spread rapidly and finally completely burned out, the after-flame time(t1) and after-glow time(t2) were 7.0s and 11.2s respectively, while the flame diffusion speed of treated cotton decreased, the t1 and t2 were 0, and the treated cotton went out immediately after leaving the flame. The results show that the treated cotton has good flame retardancy. When the fire source was removed, with the increase of FR concentration, the length of carbon decreases, and the t1 and t2 are both 0. The carbon length increased after 50 LCs. When the treated cotton with 25.3% WG, the treated cotton has self extinguishing property, and passed UL-94 V-0 classification of vertical burning test. The mechanism of flame retardant is that phosphorus element can strengthen the system, which can be transformed into phosphorus/base acid in the condensed phase, thus accelerating the carbonization of cotton fabric. The carbonization layer can protect the treated cotton fabric from heat transfer and the release of combustible volatiles. Active factors such as PO· can also be released in the gas phase to terminate the free radical reaction. At the same time, small molecules such as CO2, H2O and NH3 can be produced during the burning and decomposition of treated cotton. The reduction of the contact between the treated cotton and O2 and other combustibles indicates that the treated cotton has good flame retardant properties and plays a flame retardant effect in both condensed phase and gas phase.
SEM of char residue of untreated cotton and treated cotton under different magnification are shown in Fig. 8. Figure 8(a), (b)show the char residue of untreated cotton indicated that the fiber structure was obviously destroyed, Fig. 8(c) and (d) show the char residue of treated cotton after burning was continuous and uniform, and the structure was relatively complete. That is why FR containing phosphorus decomposes into phosphoric acid during heating, and phosphoric acid forms polyphosphoric acid at high temperature, which plays the role of dehydration, thus inhibiting the formation of L-glucose and further dehydrating and carbonizing cellulose. The continuous carbon layer formed insulates the contact between internal fiber and oxygen, slows down the rate of thermal decomposition reaction (Naebe et al. 2016), and makes the structure of residual carbon relatively complete. The surface was rough with particles, which indicates that FR was successfully attached to the treated cotton.
Table 2
Vertical burning data of untreated cotton and treated cotton.
WG (%)
|
LCs
|
t1 (s)
|
t2 (s)
|
Char length(mm)
|
Self-extinguishing property
|
Untreated cotton
|
-
|
7.0
|
11.2
|
0
|
No
|
8.5
|
0
|
0
|
0
|
50
|
No
|
50
|
0
|
0
|
95
|
19.0
|
0
|
0
|
0
|
42
|
No
|
50
|
0
|
0
|
82
|
25.3
|
0
|
0
|
0
|
38
|
Yes
|
50
|
0
|
0
|
61
|
33.7
|
0
|
0
|
0
|
33
|
Yes
|
50
|
0
|
0
|
56
|
3.8 CONE analysis of untreated cotton and treated cotton
With a radiation flux of 35 kW/m2, the ignitability and combustion behavior of untreated cotton and the treated cotton with 25.3% WG are further studied by CONE. The curves of heat release rate (HRR) and THR are shown in Fig. 9(a) and (b). SEM of char residue of untreated cotton and treated cotton after CONE are shown in Fig. 9(c) and (d). It can be seen that the PHRR and THR of treated cotton were 17.9 kW/m2 and 2.8 MJ/m2 respectively, which are significantly lower than 190.3 kW/m2 and 1.8 MJ/m2 of untreated cotton.That is why FR promotes the dehydration and carbonization of cellulose macromolecules, reducing the generation of combustible gas. And the carbon layer was wrapped on the fiber surface to isolate heat and oxygen (Basak et al. 2018; Jiao et al. 2010).
It can be seen from Fig. 9(c) and (d) that the fiber structure of the untreated cotton has been destroyed after CONE, while the structure of the treated cotton was completed. The results show that FR not only improves the thermal stability of treated cotton, but also protects the internal structure of the fiber. And there are fine particles on the surface of the fiber, which is because the excessive FR molecules can not graft with the active groups in the cellulose molecules, but adhere to the surface of the fiber in the form of coating at high temperature. The relevant data of CONE are shown in Table 3. Time to ignite(TTI) and time to PHRR (TPHRR) of treated cotton were 28.7s and 30.3s, respectively, which were higher than those of untreated cotton (13.0s and 20.6s), which means that treated cotton is not easy to be ignited in the same environment, and people have a greater chance to escape from fire. The fire growth rate (FGR, FGR = PHRR/TPHRR) of treated cotton is 0.6 kW/(m2·s), while that of untreated cotton is 9.2 kW/(m2·s), which indicates that the slower the burning growth rate of treated cotton, the lower the fire risk (Wang et al. 2015). The average mass loss rate (av-MLR) decreased from 1.23 g/s to 0.55 g/s, indicating that the treated cotton released less heat in the burning process. FR can promote the formation of carbon layer of cellulose to isolate heat and make the burning difficult to spread. The ratio of CO2/CO can reflect the index of combustion efficiency of cotton fabric (Vasiljevic et al. 2015). The CO2/CO of treated cotton was 25.4%, which was lower than that of untreated cotton by 70.9%, indicating that the treated cotton has excellent flame retardancy due to incomplete combustion (Nazare et al. 2008). the mass of the char residue of treated cotton was 29.4%, and that of untreated cotton was 5.6%. It also shows that FR promotes the dehydration of cellulose into carbon and improves the flame retardancy of treated cotton. In addition, the total smoke rate (TSR) was significantly increased, because some flame-retardant gases containing NH3, H2O and CO2 were released at high temperature, which diluted the O2 concentration on the surface of the treated cotton, making the treated cotton burning insufficiently.
Table 3
CONE data of untreated cotton and treated cotton.
Radiation flux
|
Sample
|
TTI (s)
|
TPHRR (s)
|
TSR (m2/m2)
|
av-MLR (g/s)
|
FGR (kW/m2s)
|
CO2/CO
|
Residue (%)
|
35kW/m2
|
Untreated cotton
|
13.0 ± 0.5
|
20.6 ± 0.4
|
5.0 ± 0.2
|
1.23 ± 0.1
|
9.2 ± 0.5
|
70.9 ± 1
|
5.6 ± 1
|
Treated cotton
|
28.7 ± 0.6
|
30.3 ± 0.6
|
33.1 ± 1
|
0.55 ± 0.1
|
0.6 ± 0.01
|
25.4 ± 0.2
|
29.4 ± 1
|
3.9 DSC analysis of untreated cotton and treated cotton
The DSC curves of cotton fabric before and after grafting modification are shown in Fig. 10. It can be seen that the DSC curves of cotton fabric before and after graft modification have similar shapes, showing an obvious exothermic peak. There was an exothermic peak at 78.5 ℃ in the treated cotton, which was earlier than that at 79.1 ℃ in the untreated cotton, which was consistent with the results of TG analysis. For the treated cotton, an endothermic peak appeared at 150.3 ℃, which was attributed to the desorption of FR with hydrophilic groups grafted on the treated cotton, the evaporation of a small amount of water in the treated cotton and the decomposition of phosphoric acid group and amino group absorbed a certain amount of heat. Finally, a large exothermic peak appeared at 186.6 ℃ and the exothermic peak was wide, because the phosphate group and amino group were successfully introduced into the treated cotton. During the thermal decomposition of the treated cotton, the exothermic reaction of the treated cotton was inhibited and the exothermic reaction rate was slowed down. In addition, after FR is decomposed into phosphoric acid, metaphosphoric acid and other substances, these compounds in turn catalyze the dehydration and carbonization of the treated cotton, forming an expansive carbon layer on the surface of the treated cotton. This carbon layer can reduce the release of combustible gas, and isolate heat and O2.
3.10 Flexibility,whiteness and yellowness index analysis of untreated cotton and treated cotton
The flexibility, whiteness and yellowness index of untreated cotton and treated cotton are shown in Fig. 11 and Table 4. With the increase of the WG in FR, the ring height of the untreated cotton was 10.0 mm, and thus it has good flexibility. The ring height of treated cotton flexibility increased slowly, which indicates that FR has a negative effect on the flexibility of cotton fabric. The whiteness decreased slowly and the yellowness increased slightly, which may be due to the decomposition of -P-O-H+ by FR in the process of high temperature baking. Although there was a catalyst to catalyze the grafting reaction, the -P-O-H+ group obtained in the future will make the cellulose in a strong acidic environment, destroy part of the fibronectin, and make the treated cotton whiteness decreased and yellowness increased. When the treated cotton with 25.3% WG, the treated cotton has good flexibility and whiteness. But when the treated cotton reaches 33.7% WG, the ring height of treated cotton flexibility increases obviously and the whiteness of treated cotton decreases obviously.
Table 4
Whiteness and yellowness data of untreated cotton and treated cotton
WG(%)
|
Whiteness index
(%)
|
Yellowness index
(%)
|
Untreated cotton
|
90.1
|
7.5
|
8.5
|
83.2
|
16.6
|
19.1
|
81.7
|
17.1
|
25.3
|
79.4
|
18.2
|
33.7
|
70.2
|
19.1
|
3.11 Mechanical properties analysis of untreated cotton and treated cotton
Vapor transmissibility Fig. 12 (a), water absorbability Fig. 12 (b), tensile strength Fig. 12 (c) and elongation at break Fig. 12 (d) are important indices to measure the comfort of the cotton fabric. Therefore, the properties of the treated cotton were tested. There is no significant difference in vapor transmissibility and water absorbability between the treated cotton and untreated cotton, which indicates that flame retardant finishing does not affect the warp and weft density of cotton fabric. The tensile strength and elongation at break of fabric can reflect the external resistance of fabric. When the treated cotton with 25.3% WG, the tensile strength of treated cotton in warp and weft direction decreased by 18.3% and 28.4% respectively, the decrease of tensile strength was due to the high temperature treatment of the cotton fabric during the grafting reaction, which causes some damage to the strength of the cotton fiber. In addition, during the chemical grafting reaction, when the cotton fiber was in the high temperature acidic environment, part of the glycosidic bond of cellulose was oxidized into C = O group, and the number of damaged nodes in the fiber increases, which leads to the decrease of tensile strength and fracture of the treated cotton. The change of elongation at break was small, which was due to a FR molecule with multiple reactive groups can bond with multiple cellulose macromolecules. This strong and close cross-linking reaction reduces the deformation ability of the fabric under the action of external force. Therefore, it has little effect on the elongation at break of cotton fabric. But when the treated cotton reaches 33.7% WG, the tensile strength of the cotton fabric decrease obviously, therefore the best treated cotton was with 25.3% WG.